**Thermoplastic Elastomers with Photo-actuating Properties**

Markéta Ilčíková, Miroslav Mrlík and Jaroslav Mosnáček

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/59647

## **Abstract**

This contribution reviews elastomeric materials with photo-actuation behavior with emphasis on thermoplastic elastomers and their composites. The principles of the photo-actuation and the main factors affecting the photo-actuation phenomena of thermoplastic elastomer materials are discussed in detail. The well-performing photoactuating systems involving both statistical and block copolymers-based thermoplas‐ tic elastomers are assessed in terms of their advantages and limitations. Methods for evaluation of photo-actuation behavior of the materials are reported as well. Finally, the utilization of the photo-actuating thermoplastic elastomers is presented.

**Keywords:** Thermoplastic polyurethane, styrene-isoprene-styrene, carbon nano‐ tubes, graphene, liquid crystals

## **1. Introduction**

MERGEFORMAT Actuation phenomenon is considered as a material's ability to undergo reversible shape changes in response to an external stimulus [1–3]. There are several trigger stimuli reported, such as the electric field, light, pH, or temperature [4–8]. The triggerresponsive materials find their employment in a wide range of applications comprising sensor, artificial muscles, etc. [1, 2]. Photo-induced actuation technologies can offer many advantages over traditional, mainly electrically driven, actuators, such as remote energy transfer, remote controllability, better scalability, low electromagnetic noise, easy construction, and capability

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

of working in harsh environments [7]. Generally, the actuating materials can be pure polymers or polymer composites. In both cases, the energy absorber-triggers and assembling structures need to be present in the materials [2]. However, the actuation may be improved by the addition of an energy trigger. As an energy absorbers, dyes [9] or carbon based fillers were reported [10, 11].

Various elastomers were investigated for their photo-actuation behavior, including liquid crystalline elastomers, poly(dimethylsiloxane), various thermoplastic elastomers (TPEs) based on polyurethane, poly(ethylene–*co*-vinyl acetate) (EVA), polystyrene-*block*-polyisoprene*block*-polystyrene (SIS), and acrylic-based block copolymers, as well as hydrophilic copolymers such as NAFION or hydrogels based on copolymers of acrylic acid and *N*-isopropyl acryla‐ mide. The principles of photo-actuation depend on the type of elastomer and type of the light absorbers.

The most common principle of photo-actuation of chemically cross-linked elastomers and/or TPEs is based on the presence of soft segments responsible for shape changes under illumi‐ nation, and hard segments responsible for returning the material to its stage before illumina‐ tion [12]. Thus, the pre-strained material containing some light absorbers absorbs the energy from the light and converts it to heat that is conducted through the material. The heat causes that the pre-strained polymer chains in the soft segments shrink, i.e., contract to form coil, and that results in shape changes of the material (Figure 1). The hard segments are formed from chemically or physically cross-linked parts, enabling the reversibility of the actuation by the energy balance between absorbed and released energy in the form of mechanical response. Therefore, sometimes the term photo-mechanical actuation is also used for the photo-actuation phenomenon [13].

**Figure 1.** Principle of photo-actuation behavior of physically or chemically cross-linked systems.

Thermoplastic polyurethanes (TPUs) materials have unique basis of the photo-actuation phenomenon. Depending on the chemical composition, the polymer chains consist of soft segments with melting point ranging from 35°C to 50°C and hard segments with melting point exceeding 100°C. During application of certain pre-strain to the material, the soft polymer chains are crystallized (Figure 2). These crystallites provide additional physical cross-linking that stabilize the polymer chains in elongated state. The actuation occurs after melting the soft crystallized segments as a recoiling of the soft polymer segments. In case of light induced actuation, the energy from the light source transported to the polymer chains must be high enough to reach the melting point of the soft segments [12].

**Figure 2.** Principle of photo-actuation behavior of thermoplastic polyurethanes [12].

of working in harsh environments [7]. Generally, the actuating materials can be pure polymers or polymer composites. In both cases, the energy absorber-triggers and assembling structures need to be present in the materials [2]. However, the actuation may be improved by the addition of an energy trigger. As an energy absorbers, dyes [9] or carbon based fillers were

Various elastomers were investigated for their photo-actuation behavior, including liquid crystalline elastomers, poly(dimethylsiloxane), various thermoplastic elastomers (TPEs) based on polyurethane, poly(ethylene–*co*-vinyl acetate) (EVA), polystyrene-*block*-polyisoprene*block*-polystyrene (SIS), and acrylic-based block copolymers, as well as hydrophilic copolymers such as NAFION or hydrogels based on copolymers of acrylic acid and *N*-isopropyl acryla‐ mide. The principles of photo-actuation depend on the type of elastomer and type of the light

The most common principle of photo-actuation of chemically cross-linked elastomers and/or TPEs is based on the presence of soft segments responsible for shape changes under illumi‐ nation, and hard segments responsible for returning the material to its stage before illumina‐ tion [12]. Thus, the pre-strained material containing some light absorbers absorbs the energy from the light and converts it to heat that is conducted through the material. The heat causes that the pre-strained polymer chains in the soft segments shrink, i.e., contract to form coil, and that results in shape changes of the material (Figure 1). The hard segments are formed from chemically or physically cross-linked parts, enabling the reversibility of the actuation by the energy balance between absorbed and released energy in the form of mechanical response. Therefore, sometimes the term photo-mechanical actuation is also used for the photo-actuation

**Figure 1.** Principle of photo-actuation behavior of physically or chemically cross-linked systems.

Thermoplastic polyurethanes (TPUs) materials have unique basis of the photo-actuation phenomenon. Depending on the chemical composition, the polymer chains consist of soft segments with melting point ranging from 35°C to 50°C and hard segments with melting point exceeding 100°C. During application of certain pre-strain to the material, the soft polymer

reported [10, 11].

116 Thermoplastic Elastomers - Synthesis and Applications

absorbers.

phenomenon [13].

The main principle of the photo-actuation of liquid crystals and liquid crystalline elastomers is based on the ordering and disordering of the structure upon light stimulation (Figure 3) [14– 16]. The material is able to be reversibly transformed between these two phases as a result of stimulation, while actuation stress is created. The magnitude of the shape changes and the stress development are strongly influenced by various factors, such as toughness of the polymer matrix, modification of the polymer matrix, or type of additive.

**Figure 3.** Principle of photo-actuation behavior of liquid crystals systems [14].

Very rare photo-actuating principle was found on the hydrogel samples [17]. In this case the actuation principle is based on the difference of swelling in the dark and upon the light and will be described more in detail later. Such changing of dark/light conditions is not that fast as in case of previous principles; however, the potential applications of such systems are very promising and the preparation of the new materials with shorter response time is highly challenging.

Here the photo-actuation of various systems and their advantages and disadvantages will be reviewed, while photo-actuating systems based on thermoplastic elastomers will be described more in detail. Analytical methods used for determination of photo-actuation behavior of various systems will be discussed as well. Finally the applicability of the photo-actuating TPE will be referred.

## **2. Photo-actuation of chemically cross-linked elastomers**

## **2.1. Liquid crystal elastomers**

Liquid crystals (LCs) are the most common materials frequently applied in order to provide the system with good actuation behavior [18]. This unique property is allowed due to the structure of the LCs that combine the mobility of the isotropic liquids and orientation order of crystalline solid [15]. By incorporation of the light-triggered materials to the LCs, the alignment can be properly controlled over large areas, and thus the materials can be effectively utilized in photonic applications such as signal processing, optical switching, or already mentioned photo-actuation [19–21].

However, it was found out that photo-chemical phase transitions disrupt the LC phase and the material turns to isotropic [16]. Thus, the research interest was further focused on the preparation of the cross-linked LCs structures. Since Finkelmann et al. showed the photocontraction of liquid crystal elastomers (LCEs) and Ikeda provided the system with photoactuating LCE films [22], the main aim of the scientific groups has been focused on the preparation of stable and well photo-actuating systems.

The main principle of LCEs utilization is the weak cross-linking of ordered macroscopical structure, which provides the benefits of orientation order of liquid crystals and elasticity of common rubbers [23, 24]. The typical case of cross-linked liquid crystalline network is LCE in the form of uniaxially oriented planar film where the gradient in light intensity through the thickness of the film causes photo-actuation [16, 25–28]. Unfortunately, the response to the external stimulus (light) is rather slow and shape changes are small due to the low thermal conductivity resulting in low energy transfer within the material. This drawback can be solved by the incorporation of light-triggered materials similarly as in the case of LCs, or by addition of fillers improving the thermal diffusivity within the whole final material.

In order to perform the LCEs with excellent photo-mechanically responsive properties, lightsensitive monomer can be used. The final materials containing mainly azobenzene derivatives belong to highly photo-actuating materials [29–31]. This material is unique due to its dynamics of *trans* to *cis* isomerization of azobenzene unit resulting in the change of absorption spectrum able to report the local rigidity of its surroundings [32]. Thus in ordered liquid crystalline network, trans-cis isomerization leads to order reduction resulting in the macroscopic contraction in the main-chain direction and an expansion in the opposite direction. Such systems exhibit considerably improved photo-actuating properties [33].

Another additive, which can considerably improve the photo-actuating properties of the LCEs, are carbon nanotubes (CNTs). CNTs are efficiently applied due to their one-dimensional shape, nanoscale diameters, large surface area, and excellent electrical and thermal properties [34, 35]. Special case of carbon nanotubes effectively applied in photo-actuating systems are singlewalled carbon nanotubes (SWCNTs) exhibiting strong absorptions in the visible and near-IR region [36]. Therefore, they efficiently convert the light energy into thermal energy providing the heat source at nanoscale and thus create the thermal pathways within the liquid crystal elastomer upon IR irradiation [37, 38]. However, the utilization of the CNTs in the case of LCEs is rather limited since the proper dispersion of CNTs in case of higher loading (above 1 wt. %) is very difficult especially without surface modification [38, 39]. This drawback can be solved by incorporation of the pyrene to both ends of the LCE chains [40]. π-π interactions between the pyrene group and CNTs can significantly improve the dispersion of the CNTs in LCEs [38].

Finally, it can be concluded that from the group of liquid crystals, mainly liquid crystal elastomers are effectively applied due to their excellent physical properties. However, in order to facilitate the photo-mechanical response of those materials, various additives can be efficiently utilized. The additives, such as azobenzene-based derivatives or carbon nanotubes provide better absorption of the light and improve thermal conductivity of the materials.

## **2.2. Other chemically cross-linked elastomers**

Very rare photo-actuating principle was found on the hydrogel samples [17]. In this case the actuation principle is based on the difference of swelling in the dark and upon the light and will be described more in detail later. Such changing of dark/light conditions is not that fast as in case of previous principles; however, the potential applications of such systems are very promising and the preparation of the new materials with shorter response time is highly

Here the photo-actuation of various systems and their advantages and disadvantages will be reviewed, while photo-actuating systems based on thermoplastic elastomers will be described more in detail. Analytical methods used for determination of photo-actuation behavior of various systems will be discussed as well. Finally the applicability of the photo-actuating TPE

Liquid crystals (LCs) are the most common materials frequently applied in order to provide the system with good actuation behavior [18]. This unique property is allowed due to the structure of the LCs that combine the mobility of the isotropic liquids and orientation order of crystalline solid [15]. By incorporation of the light-triggered materials to the LCs, the alignment can be properly controlled over large areas, and thus the materials can be effectively utilized in photonic applications such as signal processing, optical switching, or already mentioned

However, it was found out that photo-chemical phase transitions disrupt the LC phase and the material turns to isotropic [16]. Thus, the research interest was further focused on the preparation of the cross-linked LCs structures. Since Finkelmann et al. showed the photocontraction of liquid crystal elastomers (LCEs) and Ikeda provided the system with photoactuating LCE films [22], the main aim of the scientific groups has been focused on the

The main principle of LCEs utilization is the weak cross-linking of ordered macroscopical structure, which provides the benefits of orientation order of liquid crystals and elasticity of common rubbers [23, 24]. The typical case of cross-linked liquid crystalline network is LCE in the form of uniaxially oriented planar film where the gradient in light intensity through the thickness of the film causes photo-actuation [16, 25–28]. Unfortunately, the response to the external stimulus (light) is rather slow and shape changes are small due to the low thermal conductivity resulting in low energy transfer within the material. This drawback can be solved by the incorporation of light-triggered materials similarly as in the case of LCs, or by addition

In order to perform the LCEs with excellent photo-mechanically responsive properties, lightsensitive monomer can be used. The final materials containing mainly azobenzene derivatives belong to highly photo-actuating materials [29–31]. This material is unique due to its dynamics

of fillers improving the thermal diffusivity within the whole final material.

**2. Photo-actuation of chemically cross-linked elastomers**

preparation of stable and well photo-actuating systems.

challenging.

will be referred.

**2.1. Liquid crystal elastomers**

118 Thermoplastic Elastomers - Synthesis and Applications

photo-actuation [19–21].

Poly(dimethyl siloxane) (PDMS), which was often used as a main chain LCE, was effectively used as a matrix also in other systems for photo-actuation applications. This versatile material have many potential applicability in the medical field, due to good biocompatibility, low glass transition temperature, and linear elasticity over broad temperature range (-50°C–200°C) and strains [41]. Solely, PDMS has very poor mechanical response to the light because of low thermal conductivity. Hence, PDMS matrix has to be filled with carbon-based fillers. The composites based on graphene-nanoplatelet were found to exhibit improved thermal conduc‐ tivity of the samples, and thus also enhanced photo-actuation response [11, 42]. The contraction of the irradiated samples was obtained only above the certain pre-strain (above 10 %). In the case of composites containing thermally reduced graphene oxide, the pre-strain of 9% was sufficient to observe the contraction [7]. Nearly twice higher photo-actuation stress was obtained in comparison with CNTs-containing PDMS composite materials [1, 43, 44]. Thus, PDMS-based materials are very promising for their application as a photo-actuator mainly after the incorporation of the light-triggered fillers such as CNTs or graphene oxide. Those fillers significantly improve the thermal conductivity, resulting in better heat exchange within the material and thus provide the systems with promising photo-actuation performance.

Special photo-actuating materials are based on hydrogels [17]. The hydrogels were made by copolymerization of *N*-isopropyl acrylamide with various ratios of acrylic acid (AA). Benzo‐ spiropyran was used as an energy absorber in this system. The swelling of the samples in water changed with switching on/off the light. When the samples were exposed to the light the relative gel swelling was 78 %, while after switching off the light, the relative gel swelling increased up to 96%. This photo-actuation behavior was stable up to five light-dark cycles. This unique property is mainly caused by the utilization of the AA enhancing the proton transfer within the sample when exposed to light or dark and improving the swelling/ contraction behavior (Figure 4).

**Figure 4.** Schematic representation of the proton exchange taking place in hydrogels between the acrylic acid and the benzospiropyran in the dark and under irradiation [17].

## **3. Photo-actuation of thermoplastic elastomers**

TPEs having physically cross-linked structure possess several advantages compared with chemically cross-linked elastomers. The main advantages are a repeatable processability of TPEs and possibility of tuning their mechanical properties by choosing the different segments to tailor finely the required properties. TPEs are also relatively cheap compared with liquid crystal-based systems. Therefore, the TPE systems are very promising not only from laboratory or specified purposes point of view, but also in terms of large-scale industrial application.

All of the TPE photo-actuating systems utilize light-triggered additives, mainly carbon-based particles, in order to enhance their photo-actuation response [45–47]. However, the poor dispersibility of the additives is a common problem. If the light-trigged additive is not well dispersed, the photo-actuation response will be of low performance. Nevertheless, this drawback can be effectively solved by suitable surface modification either covalent [48] or noncovalent [49].

## **3.1. Poly(ethylene-***co***-vinyl acetate)**

spiropyran was used as an energy absorber in this system. The swelling of the samples in water changed with switching on/off the light. When the samples were exposed to the light the relative gel swelling was 78 %, while after switching off the light, the relative gel swelling increased up to 96%. This photo-actuation behavior was stable up to five light-dark cycles. This unique property is mainly caused by the utilization of the AA enhancing the proton transfer within the sample when exposed to light or dark and improving the swelling/

**Figure 4.** Schematic representation of the proton exchange taking place in hydrogels between the acrylic acid and the

TPEs having physically cross-linked structure possess several advantages compared with chemically cross-linked elastomers. The main advantages are a repeatable processability of TPEs and possibility of tuning their mechanical properties by choosing the different segments to tailor finely the required properties. TPEs are also relatively cheap compared with liquid crystal-based systems. Therefore, the TPE systems are very promising not only from laboratory or specified purposes point of view, but also in terms of large-scale industrial application.

All of the TPE photo-actuating systems utilize light-triggered additives, mainly carbon-based particles, in order to enhance their photo-actuation response [45–47]. However, the poor dispersibility of the additives is a common problem. If the light-trigged additive is not well dispersed, the photo-actuation response will be of low performance. Nevertheless, this drawback can be effectively solved by suitable surface modification either covalent [48] or non-

contraction behavior (Figure 4).

120 Thermoplastic Elastomers - Synthesis and Applications

benzospiropyran in the dark and under irradiation [17].

covalent [49].

**3. Photo-actuation of thermoplastic elastomers**

One of the statistical copolymers studied for its photo-actuation properties is poly(ethylene*co*-vinyl acetate) (EVA). The ethylene units in EVA provide for semi-crystalline properties of the copolymer, while the vinyl acetate units form amorphous part. The copolymerization of ethylene with vinyl acetate enhances the crystallization of the ethylene units and the crystal‐ lization degree of ethylene units increases with increasing amount of vinyl acetate in the copolymer. The final copolymer structure includes two segments: hard segment consisting of ethylene crystalline phase and soft segments represented by amorphous vinyl acetate and amorphous ethylene phase. Thanks to this unique behavior, EVA copolymers provide the materials with tunable elasticity depending on the vinyl acetate content.

The photo-actuation phenomenon has been studied on two EVA copolymers, EVATAN and LEVAPREN 500, with different content of vinyl acetate (28% and 50%, respectively) [47, 50, 51]. Generally, the selection of EVA with appropriate content of vinyl acetate can be crucial. EVA with too low vinyl acetate content will have lower melting point and loss of elastic properties can occur during photo-actuation cycles. The reason is the possible melting of the crystalline phase due to local overheating of the material after absorption of the light by carbon fillers and the release of the energy to the material in the form of the heat. On the other hand, EVA with too high vinyl acetate content can be too tough to provide good photo-actuation.

The effect of the different type of carbon-based fillers such as MWCNTs and SWCNTs and their content in the EVATAN matrix has been investigated. In order to improve the dispersity of the CNTs within the matrix the surface of CNTs was non-covalently modified with choles‐ teryl l-pyrenecarboxylate (PyChol), based on π-π interactions between CNT surface and pyrene (Figure 5). The EVA composite containing PyChol-modified MWCNTs exhibited higher values of photo-actuation stress compared with PyChol-modified SWCNTs (Figure 5). Better light absorption of the MWCNTs enhancing the photo-mechanical response of the EVA copolymer was suggested as a possible explanation. It has also been found that MWCNT/EVA systems with 0.1 wt. % of PyChol-modified MWCNTs exhibited higher photo-actuation stress compared with the system containing 3 wt. % of PyChol-modified SWCNTs. Even though the authors did not comment on this result, the possible explanation could be that the elasticity of the CNTs/EVA systems is decreased at higher content of the filler, thus suppressing the possibility of this material actuate effectively upon light stimulation.

Similarly, incorporation of 0.1 wt. % of PyChol-modified MWCNTs into the LEVAPREN matrix provided well-dispersed systems with appropriate photo-actuation behavior upon light stimulus [51]. This study was mainly focused on the possibility of the material to provide the system with repeatable photo-actuation phenomenon. In addition the response of this material on various light intensities has been investigated. It has been shown that increasing the intensity of the light led to more pronounced changes but with slower response on the switching on/off the light.

Different methods were used for investigation of photo-actuation behavior of the systems based on EVATAN and LEVAPREN matrices; therefore, the effect of EVA matrix composition cannot be directly compared. Generally, it has been shown that both investigated EVA matrices

**Figure 5.** Schematic illustration of non-covalent modification of CNT with cholesteryl 1-pyrenecarboxylate [50].

can be utilized for photo-actuation applications, while at least for LEVAPREN, i.e., matrix with higher melting point (*T*m = 96°C and 71°C for LEVAPREN and EVATAN, respectively), also repeatable photo-actuation behavior has been proved.

## **3.2. NAFION**

Perfluorosulfonated ionomer NAFION is another statistical copolymer with properties of TPEs that has been investigated for its applicability as photo-actuating material [52, 53]. NAFION solely exhibits only poor ability of photo to mechanical energy conversion. Hence, SWCNTs were used as the light-triggered material for preparation of SWCNTs/NAFION bilayer samples providing the good photo to mechanical energy conversion. The determined actuation changes in light switching on/off cycles were 200 μm and 600 μm at light intensity of 18 mW cm-2 and 75 mW cm-2, respectively. In SWCNTs/NAFION bilayer system, the photo-actuation phenomenon is obtained by redistribution of the hydrogen ions and water molecules in the SWCNTs/NAFION interfacial region. The light establishes an electric field at the interface that promotes the hydrated hydrogen ions toward the interface. Both the SWCNTs and NAFION contribute to the mechanical action, because water molecules are depleted in the pores of NAFION near the interface and they occupy the interior of the SWCNTs leading to improved effect of contraction of the interfacial region between the SWCNTs and NAFION (Figure 6) [52].

## **3.3. Thermoplastic polyurethanes**

TPUs are special type of copolymers, where the hard and soft segments are repeatedly alternating. The hard polymer segments are represented by diisocyanates [54], such as 4,4' diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HDI), 3,3'-dimethyl-4-4' biphenyl diisocynate (TODI), etc. On the other hand, the soft segments are mainly represented by polyesters [55] or polyethers [56]. However, the real composition of the TPUs usually also includes chain extenders [57] as can be seen in Figure 7. The chain extender is mainly based

Thermoplastic Elastomers with Photo-actuating Properties http://dx.doi.org/10.5772/59647 123

**Figure 6.** Photo-actuation mechanism of the Nafion-SWCNT bilayer upon light stimulation [52].

can be utilized for photo-actuation applications, while at least for LEVAPREN, i.e., matrix with higher melting point (*T*m = 96°C and 71°C for LEVAPREN and EVATAN, respectively), also

**Figure 5.** Schematic illustration of non-covalent modification of CNT with cholesteryl 1-pyrenecarboxylate [50].

Perfluorosulfonated ionomer NAFION is another statistical copolymer with properties of TPEs that has been investigated for its applicability as photo-actuating material [52, 53]. NAFION solely exhibits only poor ability of photo to mechanical energy conversion. Hence, SWCNTs were used as the light-triggered material for preparation of SWCNTs/NAFION bilayer samples providing the good photo to mechanical energy conversion. The determined actuation changes in light switching on/off cycles were 200 μm and 600 μm at light intensity of 18 mW cm-2 and 75 mW cm-2, respectively. In SWCNTs/NAFION bilayer system, the photo-actuation phenomenon is obtained by redistribution of the hydrogen ions and water molecules in the SWCNTs/NAFION interfacial region. The light establishes an electric field at the interface that promotes the hydrated hydrogen ions toward the interface. Both the SWCNTs and NAFION contribute to the mechanical action, because water molecules are depleted in the pores of NAFION near the interface and they occupy the interior of the SWCNTs leading to improved effect of contraction of the interfacial region between the SWCNTs and NAFION (Figure 6) [52].

TPUs are special type of copolymers, where the hard and soft segments are repeatedly alternating. The hard polymer segments are represented by diisocyanates [54], such as 4,4' diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HDI), 3,3'-dimethyl-4-4' biphenyl diisocynate (TODI), etc. On the other hand, the soft segments are mainly represented by polyesters [55] or polyethers [56]. However, the real composition of the TPUs usually also includes chain extenders [57] as can be seen in Figure 7. The chain extender is mainly based

repeatable photo-actuation behavior has been proved.

122 Thermoplastic Elastomers - Synthesis and Applications

**3.2. NAFION**

**3.3. Thermoplastic polyurethanes**

on linear diols, such as ethylene glycol, 1,4 butadiene, 1,6 hexandiol, etc. This unique compo‐ sition of the TPUs enable tunability of the mechanical properties and therefore TPUs are very promising material for the photo-mechanical actuation.

**Figure 7.** Composition of the thermoplastic polyurethane elastomers containing chain extenders.

Similar to the previous matrices, TPUs solely exhibit only moderate photo-actuation perform‐ ance. This is mainly due to poor strain-induced crystallization of the soft segments upon tensile deformation. Hence, all research groups have focused their investigations on the effect of the fillers addition on the strain-induced crystallization of the soft segments upon tension and its connection with the photo-actuation performance of the TPUs composites [12, 58–60]. An impact of the CNTs (5 wt. %) and carbon black (5–20 wt. %) on the mechanical properties, strain-induced crystallization and final photo-actuation performance of TPU (Irogran PS455-203) composites has been investigated (Figure 8) [12]. For the CNT reinforced matrix the young modulus increased by factor 2–5 and considerably improved strain-induced crystallization was observed by DSC and proved by XRD measurements upon tension. Recovery of TPUs composite containing 5 wt. % carbon black, actuated by infrared (IR), was only 25%–30% of the stress achieved by heat actuation, compared to the almost 100% for CNTs containing TPUs composites. Four times higher amount of carbon black (20 wt. %) compared with CNTs (5 wt. %) was needed to obtain comparable deformation of TPU samples under IR irradiation.

**Figure 8.** The recovery rate for the samples containing SRGO particles. Reprinted with permission from Liang JJ, Xu YF, Huang Y, Zhang L, Wang Y, Ma YF, et al. Infrared-Triggered Actuators from Graphene-Based Nanocomposites. Journal of Physical Chemistry C. 2009;113(22):9921-7. Copyright © 2009 American Chemical Society [58].

It has been reported that well-dispersed graphene in MDI-based TPUs provided the system with enhanced strain-induced crystallization, while the poor dispersity of the same graphene in HDI-based TPUs led to the suppressed strain-induced crystallization [59]. Better dispersity of graphene in MDI-based TPUs compared with HDI-based TPUs was attributed to π-π interactions between graphene and aromatic ring of MDI. Further improvement of compati‐ bility between graphene and TPUs was obtained after surface modification of graphene. Increasing amount of hydroxyl groups on graphene surface by modification with methanol allowed incorporation of graphene in TPUs structure [59, 61, 62]. Such grafting of TPUs on graphene improved shape recovery (Figure 8). Contrary that it has been shown that intimately mixed graphene disturbed hydrogen bonds between hard segments of SPU balancing the reinforcing effect of graphene. The photo-actuation phenomenon of these systems has been investigated upon IR irradiation and the shape recovery after IR stimulation ranged 90%–99% of deformation.

Influence of the sulfonation of reduced graphene oxide (SRGO) (1 wt. %) on mechanical, energy-transfer and photo-actuation performance of TPU-based (Irogran PS455-203) compo‐ sites has been investigated as well [58]. The composite sample containing SRGO exhibited highest absorption in IR region (500 nm - 1000 nm) compared to neat TPU matrix and com‐ posites containing 1 wt. % of isocyanated-graphene oxide and reduced graphene oxide. The reason is considerably restored sp2 network in SRGO. Due to the increased IR absorption, the sulfonated SRGO/TPUs composites contracted faster than others and also the recovery rate was 15% faster compared with isocyanate-graphene oxide or reduced graphene oxide composites (Figure 9).

Similar to the previous matrices, TPUs solely exhibit only moderate photo-actuation perform‐ ance. This is mainly due to poor strain-induced crystallization of the soft segments upon tensile deformation. Hence, all research groups have focused their investigations on the effect of the fillers addition on the strain-induced crystallization of the soft segments upon tension and its connection with the photo-actuation performance of the TPUs composites [12, 58–60]. An impact of the CNTs (5 wt. %) and carbon black (5–20 wt. %) on the mechanical properties, strain-induced crystallization and final photo-actuation performance of TPU (Irogran PS455-203) composites has been investigated (Figure 8) [12]. For the CNT reinforced matrix the young modulus increased by factor 2–5 and considerably improved strain-induced crystallization was observed by DSC and proved by XRD measurements upon tension. Recovery of TPUs composite containing 5 wt. % carbon black, actuated by infrared (IR), was only 25%–30% of the stress achieved by heat actuation, compared to the almost 100% for CNTs containing TPUs composites. Four times higher amount of carbon black (20 wt. %) compared with CNTs (5 wt. %) was needed to obtain comparable deformation of TPU samples under IR

**Figure 8.** The recovery rate for the samples containing SRGO particles. Reprinted with permission from Liang JJ, Xu YF, Huang Y, Zhang L, Wang Y, Ma YF, et al. Infrared-Triggered Actuators from Graphene-Based Nanocomposites.

It has been reported that well-dispersed graphene in MDI-based TPUs provided the system with enhanced strain-induced crystallization, while the poor dispersity of the same graphene in HDI-based TPUs led to the suppressed strain-induced crystallization [59]. Better dispersity of graphene in MDI-based TPUs compared with HDI-based TPUs was attributed to π-π interactions between graphene and aromatic ring of MDI. Further improvement of compati‐ bility between graphene and TPUs was obtained after surface modification of graphene. Increasing amount of hydroxyl groups on graphene surface by modification with methanol allowed incorporation of graphene in TPUs structure [59, 61, 62]. Such grafting of TPUs on graphene improved shape recovery (Figure 8). Contrary that it has been shown that intimately mixed graphene disturbed hydrogen bonds between hard segments of SPU balancing the reinforcing effect of graphene. The photo-actuation phenomenon of these systems has been investigated upon IR irradiation and the shape recovery after IR stimulation ranged 90%–99%

Influence of the sulfonation of reduced graphene oxide (SRGO) (1 wt. %) on mechanical, energy-transfer and photo-actuation performance of TPU-based (Irogran PS455-203) compo‐

Journal of Physical Chemistry C. 2009;113(22):9921-7. Copyright © 2009 American Chemical Society [58].

irradiation.

124 Thermoplastic Elastomers - Synthesis and Applications

of deformation.

**Figure 9.** Photo-actuation behavior of the sample (a) pure TPU, (b) TPU with 5 wt. % GO, (c) TPU with 10 wt. % GO, and (d) TPU with 20 wt. % GO [59].

In order to obtain high actuation performance, structural uniformity, good dispersion, and high purity of carbon fillers is crucial. Properties of the carbon fillers are affected by surface defects, large bundles, impurities, the anisotropy, and a structural mixture [63]. Surface functionalization of graphene in order to obtain their good dispersion in polymer matrix results in the decrease in thermal conductivity due to structural defects [64, 65]. Therefore, a balance between the restoration of sp2 network and the dispersion of graphene is crucial for sufficient reinforcement and high thermal conductivity. Several studies have shown enhanced thermal conductivity and mechanical strength of polymer composites when hybrid graphene/ CNT nanofiller were used. The synergistic effect between well-dispersed graphene and CNTs can be tuned by weight ratio of graphene to CNT [66–70]. Expected enhancing of thermal conductivity led to the investigation of the influence of incorporation of various contents of sulfonated CNTs into the SRGO/TPUs composites on photo-actuation performance of the prepared composites [60]. IR absorption of sulfonated CNT/SRGO/TPUs composites was higher than SRGO/TPUs. The DSC results showed that the melting range of soft segment crystallites was shifted to higher temperatures with incorporation of the fillers. This temper‐ ature increase was ascribed to the heterogeneous nucleating effect of SRGO and CNT. As expected, all sulfonated CNT/SRGO/TPUs composites exhibited enhanced thermal conduc‐ tivity. TPUs composite with sulfonated CNT/SRGO ratio of 1/3 showed the best IR-actuated stress recovery of lifting in 18 s. Remarkable IR-actuated recovery delivered the mechanical stress of 1.2 MPa assigned to thermal conductivity of 1.473 W/mK and Young's modulus of 23.4 MPa. It has been concluded that a trade-off between the stiffness and efficient heat transfer, which can be controlled by synergistic effect between SRGO and SCNT, is critical for high mechanical power output of IR triggered actuators. Therefore, SRGO/SCNT/TPUs composites combining high output forces, and good cycling stability are highly suitable for development of advanced photo-actuating systems.

## **3.4. Polystyrene-based block copolymers**

The materials investigating for effective respond to the photo-stimulation include also A-B-A triblock-based TPEs. They consist of one polymer block with low glass transition temperature (*T*g) providing the actuation of the material and two blocks with high *T*g providing the elasticity and reversible shape change after switching off the light stimulus. Mechanical, and thus also the photo-actuation properties of A-B-A triblock-based TPEs can be finely tuned by choice of the monomers structure and ratio between the soft and hard blocks. Similarly, as in all previous cases, the light-triggered materials have to be added to provide the system with sufficient actuation performance. The main A-B-A triblock-based TPE investigated for photo-actuation is polystyrene-*block*-polyisoprene-*block*-polystyrene (SIS) triblock copolymer [39, 46, 71].

A reduced graphene oxide (rGO) has been successfully used as a light-triggered material in the SIS matrix [71]. Investigation of the effect of the filler content on the photo-actuation showed that the best performance was obtained for the nanocomposites containing 1.5 wt. % of rGO. The higher rGO content resulted in decreased response that was in consistence with change in the mechanical properties. Apparently, a creep has been observed during irradiation that is understandable with respect to extremely large pre-strain (up to 150%) and also due to the utilization of light source with 22 W cm-2 intensity, which could significantly contribute to the mentioned creep behavior.

Besides rGO, the MWCNTs have been investigated in SIS matrix as well. Both elongation and contraction were observed under irradiation depending on the applied pre-strain (Figure 10). Under light intensity of 1.5 W cm-2, minimal 20% pre-strain was needed to obtain 0.2% contraction of the composite [39]. The highest actuation was obtained at 40% pre-strain with 1.1% contraction. It should be mentioned that the MWCNTs were used in very low concen‐ tration (0.01 wt. %). Too high loading of neat MWCNTs disturbs physical cross-linking due to preferential interactions of MWCNTs with polystyrene blocks.

conductivity led to the investigation of the influence of incorporation of various contents of sulfonated CNTs into the SRGO/TPUs composites on photo-actuation performance of the prepared composites [60]. IR absorption of sulfonated CNT/SRGO/TPUs composites was higher than SRGO/TPUs. The DSC results showed that the melting range of soft segment crystallites was shifted to higher temperatures with incorporation of the fillers. This temper‐ ature increase was ascribed to the heterogeneous nucleating effect of SRGO and CNT. As expected, all sulfonated CNT/SRGO/TPUs composites exhibited enhanced thermal conduc‐ tivity. TPUs composite with sulfonated CNT/SRGO ratio of 1/3 showed the best IR-actuated stress recovery of lifting in 18 s. Remarkable IR-actuated recovery delivered the mechanical stress of 1.2 MPa assigned to thermal conductivity of 1.473 W/mK and Young's modulus of 23.4 MPa. It has been concluded that a trade-off between the stiffness and efficient heat transfer, which can be controlled by synergistic effect between SRGO and SCNT, is critical for high mechanical power output of IR triggered actuators. Therefore, SRGO/SCNT/TPUs composites combining high output forces, and good cycling stability are highly suitable for development

The materials investigating for effective respond to the photo-stimulation include also A-B-A triblock-based TPEs. They consist of one polymer block with low glass transition temperature (*T*g) providing the actuation of the material and two blocks with high *T*g providing the elasticity and reversible shape change after switching off the light stimulus. Mechanical, and thus also the photo-actuation properties of A-B-A triblock-based TPEs can be finely tuned by choice of the monomers structure and ratio between the soft and hard blocks. Similarly, as in all previous cases, the light-triggered materials have to be added to provide the system with sufficient actuation performance. The main A-B-A triblock-based TPE investigated for photo-actuation is polystyrene-*block*-polyisoprene-*block*-polystyrene (SIS) triblock copolymer [39, 46, 71].

A reduced graphene oxide (rGO) has been successfully used as a light-triggered material in the SIS matrix [71]. Investigation of the effect of the filler content on the photo-actuation showed that the best performance was obtained for the nanocomposites containing 1.5 wt. % of rGO. The higher rGO content resulted in decreased response that was in consistence with change in the mechanical properties. Apparently, a creep has been observed during irradiation that is understandable with respect to extremely large pre-strain (up to 150%) and also due to the utilization of light source with 22 W cm-2 intensity, which could significantly contribute to

Besides rGO, the MWCNTs have been investigated in SIS matrix as well. Both elongation and contraction were observed under irradiation depending on the applied pre-strain (Figure 10). Under light intensity of 1.5 W cm-2, minimal 20% pre-strain was needed to obtain 0.2% contraction of the composite [39]. The highest actuation was obtained at 40% pre-strain with 1.1% contraction. It should be mentioned that the MWCNTs were used in very low concen‐ tration (0.01 wt. %). Too high loading of neat MWCNTs disturbs physical cross-linking due to

preferential interactions of MWCNTs with polystyrene blocks.

of advanced photo-actuating systems.

126 Thermoplastic Elastomers - Synthesis and Applications

the mentioned creep behavior.

**3.4. Polystyrene-based block copolymers**

**Figure 10.** Photoactuation behavior of SIS samples with various contents of graphene particles. Reprinted with permis‐ sion from Ansari S, Neelanchery MM, Ushus D. Graphene/Poly(styrene-b-isoprene-b-styrene) Nanocomposite Optical Actuators. Journal of Applied Polymer Science. 2013;130(6):3902-8. Copyright © 2013 Wiley Periodicals, Inc. [71].

In order to prevent the negative interactions of CNTs with SIS matrix resulted in deteriorating of the mechanical properties, the surface of CNTs was modified. Complex investigation of the structural, mechanical, and photo-actuation performance of CNT/SIS composites with both neat MWCNTs and covalently modified MWCNTs has been reported for filler loading of 0.15 wt. % to 3 wt. % [46, 72]. In order to tailor preferential interactions of MWCNTs with individual blocks of SIS matrix, the surface of MWCNTs was modified either with cholesteryl groups or with short polystyrene chains (Figure 11). In the former case the preferential interactions with polyisoprene phase, and in later case the preferential interactions with polystyrene phase were expected. The dynamic mechanical analysis (DMA) in wide temperature range (from -100°C to 150°C) has been used to investigate the specific interactions of selectively modified carbon nanotubes with individual blocks of SIS. The shift in *T*g and activation energy of glass transition was compared. Addition of neat MWCNT resulted in the shift of *T*<sup>g</sup> of both polyisoprene and polystyrene phase to lower temperatures, while the influence was more pronounced for polystyrene phase. The cholesteryl-modified MWCNT shifted the *T*<sup>g</sup> of both polyisoprene and polystyrene phases to higher temperatures, while the shift for polyisoprene phase was more significant. The highest shift in *T*g of polystyrene phase was observed in composites containing polystyrene-modified MWCNTs. Contrary to neat MWCNTs, in the case of polystyrenemodified MWCNTs the *T*g shifted to higher temperatures. The activation energies of glass transitions followed a similar trend (see Table 1). In all investigated composites, an increase of MWCNTs concentration from 1 wt. % to 3 wt. % led to the deterioration of the mechanical properties. The highest actuation stresses were generated by composite containing low content of neat MWCNTs. However, the photo-actuation was irregular and fast drop of the baseline was observed as a result of negative interactions of neat MWCNTs with polystyrene phase and loss of the elastic properties. The most stable and reversible response was obtained in composite containing polystyrene-modified MWCNTs (Figure 12). In this case, however, the actuation stresses were only half of the stresses obtained for composites containing either neat MWCNTs or cholesteryl-modified MWCNTs [72]. The reason is not effective energy transfer from polystyrene-modified MWCNTs, preferentially localized in hard polystyrene phase, to soft polyisoprene phase responsible for actuation changes. These extensive studies showed that the best photo-actuation could be expected in the case of selective localization of MWCNT in the soft phase of triblock thermoplastic elastomers with effective energy transfer only to soft phase polymer chains.

**Figure 11.** Covalent modification of multiwalled carbon nanotubes either by cholesteryl groups or with polystyrene chains [72].

In order to investigate the MWCNTs/SIS composites for their applicability in tactile displays, the Braille-like elements were prepared by thermoforming and their photo-actuation behavior has been investigated [46, 47]. In all experiments, blister expansion was observed as a result of low pre-strain induced during the thermoforming process. Regardless, the actuating response increased with intensity of the light source. The fastest response was observed in the composites containing polystyrene-grafted carbon nanotubes.

Another polystyrene-based triblock copolymer investigated for its photo-actuation properties is polystyrene-*block*-poly(vinylmethylsiloxane)-*block*-polystyrene triblock copolymer [45]. In order to introduce light triggered groups into the matrix, the poly(vinylmethylsiloxane) block was covalently modified by attaching of the pendant azobenzene groups (Figure 13). This material exhibited two *T*g (23°C and 100°C for azobenzene-modified silane block and poly‐ styrene block, respectively). Such narrow difference between the *T*<sup>g</sup> provides very narrow

MWCNTs or cholesteryl-modified MWCNTs [72]. The reason is not effective energy transfer from polystyrene-modified MWCNTs, preferentially localized in hard polystyrene phase, to soft polyisoprene phase responsible for actuation changes. These extensive studies showed that the best photo-actuation could be expected in the case of selective localization of MWCNT in the soft phase of triblock thermoplastic elastomers with effective energy transfer only to soft

**Figure 11.** Covalent modification of multiwalled carbon nanotubes either by cholesteryl groups or with polystyrene

In order to investigate the MWCNTs/SIS composites for their applicability in tactile displays, the Braille-like elements were prepared by thermoforming and their photo-actuation behavior has been investigated [46, 47]. In all experiments, blister expansion was observed as a result of low pre-strain induced during the thermoforming process. Regardless, the actuating response increased with intensity of the light source. The fastest response was observed in the

Another polystyrene-based triblock copolymer investigated for its photo-actuation properties is polystyrene-*block*-poly(vinylmethylsiloxane)-*block*-polystyrene triblock copolymer [45]. In order to introduce light triggered groups into the matrix, the poly(vinylmethylsiloxane) block was covalently modified by attaching of the pendant azobenzene groups (Figure 13). This material exhibited two *T*g (23°C and 100°C for azobenzene-modified silane block and poly‐ styrene block, respectively). Such narrow difference between the *T*<sup>g</sup> provides very narrow

composites containing polystyrene-grafted carbon nanotubes.

phase polymer chains.

128 Thermoplastic Elastomers - Synthesis and Applications

chains [72].

**Figure 12.** Changes in photo-actuation stress for composites containing either 0.15 wt. % or 3 wt. % of neat MWCNT or cholesteryl-modified multiwalled carbon nanotubes (MWCNT) (MWCNT-chol) or polystyrene-modified MWCNT (MWCNT-PS) during irradiation for 10 s (region I) and for 30 s (region II). Reprinted with permission from Ilcikova M, Mosnacek J, Mrlik M, Sedlacek T, Csomorova K, Czanikova K, et al. Influence of surface modification of carbon nano‐ tubes on interactions with polystyrene-b-polyisoprene-b-polystyrene matrix and its photo-actuation properties. Poly‐ mers for Advanced Technologies. 2014;25(11):1293-300. Copyright © 2014 John Wiley & Sons, Ltd. [72].


**Table 1.** Glass transition temperatures (*T*g), obtained at various frequencies from DMA measurements and corresponding calculated activation energies of glass transitions of individual phases for pure polystyrene-*block*polyisoprene-*block*-polystyrene (SIS) and pure poly(methyl methacrylate)- *block*-poly(butyl acrylate)-*block*-poly(methyl methacrylate) (MBM) matrices and their composites containing 1 wt. % of neat or modified MWCNTs. NA stays for Not Available, since *T*g of PMMA phase was not observable.

application window for these materials, even though they provide very promising photoactuation performance with reversible strain of 3.1% and tensile strength 25.7 kPa.

**Figure 13.** Schematic illustration of triblock copolymer TPEs containing pendant azobenzene groups [45].

## **3.5. Acrylic block copolymers**

Acrylic-based TPEs are triblock copolymers consisting of one middle soft polyacrylate and two hard polymethacrylate blocks. These materials were just very recently investigated as photoactuators with very promising results [48, 73]. Their main advantage compared with the styrene-based triblock TPEs is the higher UV stability, better mechanical properties at elevated temperatures, and variability of their properties depending on the choice of (meth)acrylate monomers structure.

The photo-actuation phenomenon has been investigated on poly(methyl methacrylate)-*block*poly(butyl acrylate)-*block*-poly(methyl methacrylate) triblock copolymer (PMMA-*b*-PBA-*b*-PMMA) filled with MWCNTs as a light-triggered material. In addition to neat MWCNTs, also MWCNTs grafted with either poly(butyl acrylate) homopolymer (MWCNT-*g*-PBA) or poly(butyl acrylate)-*block*-poly(methyl methacrylate) diblock copolymer (MWCNT-*g*-PBA-*b*-PMMA) have been incorporated into the PMMA-*b*-PBA-*b*-PMMA matrix [48, 73]. The modi‐ fication of MWCNTs was focused on the improvement of interactions with soft PBA phase, stiffening of the soft phase, and efficient heat transfer from MWCNTs to the soft PBA phase. Composites containing 1 wt. % of neat MWCNTs or MWCNT-*g*-PBA were prepared by solution mixing with polymer matrix, while 1 wt. % MWCNT-*g*-PBA-*b*-PMMA composite was prepared during *in situ* polymerization of PMMA-*b*-PBA-*b*-PMMA matrix (Figure 14) [48]. Therefore, the composites based on PMMA-*b*-PBA-*b*-PMMA containing 1 wt. % MWCNT-*g*-PBA-*b*-PMMA were prepared. It has been proved by electron microscopies that significantly better dispersity with more homogeneous distribution of the MWCNTs was obtained in the

composite prepared *in situ* during polymerization. The modification of MWCNTs led to enhancement of both G´ and G´´ in wide range of temperatures (up to 260°C) compared to pure matrix, while the enhancement was most pronounced in the case of MWCNT-*g*-PBA-*b*-PMMA composite. **7** 1 Poly(ethylene-co-vinyl acetate) Poly(ethylene-*co*-vinyl acetate) **7** 2-3 poly(ethylene-co-vinyl acetate) poly(ethylene-*co*-vinyl acetate) **8** 17 (Figure6) (Figure 6)

**5** 15 without covalent modification without surface modification

application window for these materials, even though they provide very promising photo-

actuation performance with reversible strain of 3.1% and tensile strength 25.7 kPa.

**Figure 13.** Schematic illustration of triblock copolymer TPEs containing pendant azobenzene groups [45].

Acrylic-based TPEs are triblock copolymers consisting of one middle soft polyacrylate and two hard polymethacrylate blocks. These materials were just very recently investigated as photoactuators with very promising results [48, 73]. Their main advantage compared with the styrene-based triblock TPEs is the higher UV stability, better mechanical properties at elevated temperatures, and variability of their properties depending on the choice of (meth)acrylate

The photo-actuation phenomenon has been investigated on poly(methyl methacrylate)-*block*poly(butyl acrylate)-*block*-poly(methyl methacrylate) triblock copolymer (PMMA-*b*-PBA-*b*-PMMA) filled with MWCNTs as a light-triggered material. In addition to neat MWCNTs, also MWCNTs grafted with either poly(butyl acrylate) homopolymer (MWCNT-*g*-PBA) or poly(butyl acrylate)-*block*-poly(methyl methacrylate) diblock copolymer (MWCNT-*g*-PBA-*b*-PMMA) have been incorporated into the PMMA-*b*-PBA-*b*-PMMA matrix [48, 73]. The modi‐ fication of MWCNTs was focused on the improvement of interactions with soft PBA phase, stiffening of the soft phase, and efficient heat transfer from MWCNTs to the soft PBA phase. Composites containing 1 wt. % of neat MWCNTs or MWCNT-*g*-PBA were prepared by solution mixing with polymer matrix, while 1 wt. % MWCNT-*g*-PBA-*b*-PMMA composite was prepared during *in situ* polymerization of PMMA-*b*-PBA-*b*-PMMA matrix (Figure 14) [48]. Therefore, the composites based on PMMA-*b*-PBA-*b*-PMMA containing 1 wt. % MWCNT-*g*-PBA-*b*-PMMA were prepared. It has been proved by electron microscopies that significantly better dispersity with more homogeneous distribution of the MWCNTs was obtained in the

**3.5. Acrylic block copolymers**

130 Thermoplastic Elastomers - Synthesis and Applications

monomers structure.

**Figure 14.** Schematic illustration of covalent modification of MWCNT surface with PBA-*b*-PMMA diblock copolymer *in situ* during synthesis of PMMA-*b*-PBA-*b*-PMMA matrix [48]. **<sup>12</sup>**11 23.4MPa 23.4 MPa

DMA analysis reveals that the incorporation of neat MWCNTs affected the *T*g of PBA phase only negligibly (-33.3°C compared with -33.7°C for pure matrix). In the MWCNT-*g*-PBA composite the *T*<sup>g</sup> increased to -30.8°C and the most pronounced shift was obtained in the case of MWCNT-*g*-PBA-*b*-PMMA composite (-28.7°C) [73]. The activation energies of glass transition of the PBA phase increased in the same order (see Table 1). Similarly, significantly improved photo-actuation ability was obtained in the case of MWCNT-*g*-PBA-*b*-PMMA composites (Figure 15). Under all investigated conditions the reversible contraction with the highest absolute values of photo-actuation was observed in this composite. It exhibited the actuation contraction of 280 μm and 400 μm at light power of 6.6 mW and 18.5 mW, respec‐ tively, after 30 s of irradiation. These values correspond to 3.2% and 4.5% change in sample length, respectively. Since both MWCNT-*g*-PBA and MWCNT-*g*-PBA-*b*-PMMA were found to interact preferentially with soft PBA phase, the big difference between the photo-actuation behavior of these two composites can be assigned to much better dispersity and more homo‐ geneous distribution of the later one resulting in more effective heat transfer to the matrix. **12** 25 polystyrene-block-polyisoprene-blockpolystyrene polystyrene-*block*-polyisoprene-*block*polystyrene **13** 9 0.15wt. %-3 wt. % 0.15 wt. % to 3 wt. % **15** 40 cholesteryl modified cholesteryl-modified **15** 43 1wt. % 1 wt. % **16** 22 in situ *in situ*  **17** 1 during in situ *in situ* during Please delete old Figure 15 and insert this corrected one:

**17** Figur e 15

**<sup>17</sup>**23-27 **Figure 15.** Comparison of actuation length change of neat matrix, composite containing 1 **Figure 15.** Comparison of actuation length change of neat matrix (a), composite containing **Figure 15.** Comparison of actuation length change of neat matrix (a), composite containing 1 wt. % of MWCNT-g-PBA (b) and MWCNT-*g*-PBA-*b*-PMMA (c), pre-load: 0.05N, irradiation for 10 s, light power of 6.6 mW [73].

1 wt. % of MWCNT-*g*-PBA (b) and MWCNT-*g*-PBA-*b*-PMMA (c), pre-load: 0.05N, irradiation

for 10 s, light power of 6.6 mW [73].

wt. % of MWCNT, MWCNT-*g*-PBA and MWCNT-*g*-PBA-*b*-PMMA, pre-load: 0.05N, irradiation for 10 s, light power of 6.6 mW. Reprinted with permission from Ilcikova M, Mrlik M, Sedlacek T, Slouf M, Zhigunov A, Koynov K, et al. Synthesis of Photoactuating Acrylic Thermoplastic Elastomers Containing Diblock Copolymer-Grafted Carbon Nanotubes.

Acs Macro Letters. 2014;3(10):999-1003. Copyright © 2014 American Chemical Society

2

## **4. Methods for determination of photo-actuation behavior**

## **4.1. Setups for measuring of photo-actuation performance**

Several different methods have been developed to measure photo-actuation ability of TPE materials. A setup frequently used for the investigation of photo-actuation behavior is the dynamometer [11, 50]. In this case, the sample in the form of the stripe is fixed in the upper holder and in the bottom holder. On the bottom holder, the weight of various values is mounted. Thus, the certain pre-load is applied (Figure 16). The pre-loaded sample is then exposed to photo-stimulation and the change in the length is measured. The main advantage is the relatively easy construction of the setup; however, if the materials with different mechanical properties are subjected to the certain pre-load (depending on used weights), different pre-strain is achieved. Hence, the adjusting to certain pre-strain is rather impossible using various weights. Also, the accuracy of the length change measurement is not precise [11].

**Figure 16.** Dynamometer setup used for the investigation of photo-actuation behavior [11].

The same principle of applying certain pre-load has been used in the case of samples measured with the thermo-mechanical analyzer (TMA) [48, 73]. Compared to the previous setup, this device is very accurate and a certain value of pre-strain can be finely tuned. The TMA device is able to collect the data each 0.1 s depending on the settings and is able to record the change in the length automatically with very high precision usually in nanometers scale.

A very often utilized device for the investigation of photo-actuation behavior of TPE-based actuators in the form of the stripes is the universal tensile testing machine [12]. This machine allows to define a certain level of the pre-strain and thus a certain degree of alignment of polymer chains can be achieved. In the case of TPU samples, strain-induced crystallization also takes place and is fixed. When the material reaches equilibrium, it is able to respond on application of the light stimulus and to exhibit photo-actuation phenomenon [12].

The most precise device for the photo-actuation investigation is dynamic mechanical analysis (DMA) in iso-strain tensile mode (Figure 17) [50, 72]. The sample in the form of the strip is fixed into the clamps and the certain pre-strain is set. After stabilization of sample stress while keeping the same clamp distance, photo-stimulation is started. This device very precisely calculates the position of the clamps and records the load resulting from the contraction or expansion of the sample. According to these values, the device automatically provides the certain level of the actuation displacement (in the range of μm) of the sample, as well as the actuation stress (in the range of kPa) as the most important values for the evaluation of the photo-actuation phenomenon.

**Figure 17.** DMA setup for investigation of the photo-actuation performance [50].

**4. Methods for determination of photo-actuation behavior**

**Figure 16.** Dynamometer setup used for the investigation of photo-actuation behavior [11].

The same principle of applying certain pre-load has been used in the case of samples measured with the thermo-mechanical analyzer (TMA) [48, 73]. Compared to the previous setup, this device is very accurate and a certain value of pre-strain can be finely tuned. The TMA device is able to collect the data each 0.1 s depending on the settings and is able to record the change

A very often utilized device for the investigation of photo-actuation behavior of TPE-based actuators in the form of the stripes is the universal tensile testing machine [12]. This machine allows to define a certain level of the pre-strain and thus a certain degree of alignment of polymer chains can be achieved. In the case of TPU samples, strain-induced crystallization also takes place and is fixed. When the material reaches equilibrium, it is able to respond on

The most precise device for the photo-actuation investigation is dynamic mechanical analysis (DMA) in iso-strain tensile mode (Figure 17) [50, 72]. The sample in the form of the strip is fixed into the clamps and the certain pre-strain is set. After stabilization of sample stress while keeping the same clamp distance, photo-stimulation is started. This device very precisely

in the length automatically with very high precision usually in nanometers scale.

application of the light stimulus and to exhibit photo-actuation phenomenon [12].

Several different methods have been developed to measure photo-actuation ability of TPE materials. A setup frequently used for the investigation of photo-actuation behavior is the dynamometer [11, 50]. In this case, the sample in the form of the stripe is fixed in the upper holder and in the bottom holder. On the bottom holder, the weight of various values is mounted. Thus, the certain pre-load is applied (Figure 16). The pre-loaded sample is then exposed to photo-stimulation and the change in the length is measured. The main advantage is the relatively easy construction of the setup; however, if the materials with different mechanical properties are subjected to the certain pre-load (depending on used weights), different pre-strain is achieved. Hence, the adjusting to certain pre-strain is rather impossible using various weights. Also, the accuracy of the length change measurement is not precise [11].

**4.1. Setups for measuring of photo-actuation performance**

132 Thermoplastic Elastomers - Synthesis and Applications

Some photo-actuation studies were performed directly for the possible application of the materials in the development of new types of tactile displays. In these studies, the Braille-like elements were prepared instead of stripes and thus, also, different methods for evaluation of their photoactuation behavior have been developed. Atomic force microscopy (AFM) was one of the utilized devices to detect the movement of the Braille-like element (Figure 18). In this method, the AFM tip is placed on the top of the element and after exposition to the light the AFM records the tip movement up to several μm [46, 47]. This technique is very useful for the investigation of the photo-actuation kinetics; however, the actuation stress cannot be calcu‐ lated and the actuation displacement investigation is limited by maximal possible displace‐ ment of AFM tip.

**Figure 18.** AFM setup for investigation of the photo-actuation performance of the samples in the form of Braille-like elements.

The second device utilized for the investigation of the Braille-like elements is scanning electron microscopy (SEM) [47, 49]. In this case, the Braille-like element is placed into the SEM evacu‐ ated chamber and then the samples are irradiated through the self-modified setup. The maximal change in the Braille-like element height is recorded. Again, this method does not allow the calculation of the actuation stress, but it can provide good information of the Braillelike element photo-actuation performance.

## **4.2. Preparation of samples for measuring of photo-actuation performance**

In most of the set-ups used for the measurement of photo-actuation, the tested samples were in the form of strips. The polymer films for strips can be simply prepared by casting from the solution. However, very often, carbon base fillers are used as light-triggered materials that have to be incorporated into the TPE matrix. In order to provide the photo-actuator with promising behavior, the additive should be well dispersed and homogenously distributed in the TPE matrix. There were two methods reported for preparation of photo-actuating TPE composites in the literature. The first method is solution mixing, where the filler is dispersed in the proper solvent, while very often the ultrasonication is needed, and TPE matrix is afterwards added to the additive dispersion either in the form of solid or solution. When TPE is added as a solid, the mixture has to be mixed for a sufficiently long time to completely dissolve the matrix [47–51, 72]. Sonication and/or high shear mixing can be applied in order to disperse the filler well [46, 72].

The second method is based on the preparation of *in situ* composites during synthesis of polymer matrix in the presence of a filler [48, 73]. This method allows direct modification of filler surface during the polymerization process and can provide composites with much better dispersed filler. After polymerization, the polymer/filler mixture has to be precipitated to remove unreacted monomer and then re-dispersed in a solvent for casting process. In both methods, finally, the homogenous mixture is casted onto the Teflon dishes or molds of specified shape [50, 51] and the solvent is evaporated at elevated temperature and reduced pressure. Then the samples for photo-actuating investigation is cut to obtain the regular shape of stripes not exceeding dimensions of length 30 mm, width 10 mm, and thickness 1 mm. The thickness of the tested stripes was usually in the range of 0.3 mm–0.8 mm [47–51, 72].

In the case of Braille-like elements, thermoforming using a specific mold was used (Figure 19). Thermoforming allows not only preparation of the sample with required shape, but also obtaining certain pre-strain and orientation of the polymer chains needed for achieving the photo-actuation behavior of the material [46, 47].

**Figure 19.** Thermoforming process of the Braille-like elements. Reprinted with permission from Ilcikova M, Mrlik M, Sedlacek T, Chorvat D, Krupa I, Slouf M, et al. Viscoelastic and photo-actuation studies of composites based on poly‐ styrene-grafted carbon nanotubes and styrene-b-isoprene-b-styrene block copolymer. Polymer. 2014;55(1):211-8. Copy‐ right © 2013 Elsevier Ltd. [46].

## **5. Applicability of photo-actuating TPE materials**

allow the calculation of the actuation stress, but it can provide good information of the Braille-

In most of the set-ups used for the measurement of photo-actuation, the tested samples were in the form of strips. The polymer films for strips can be simply prepared by casting from the solution. However, very often, carbon base fillers are used as light-triggered materials that have to be incorporated into the TPE matrix. In order to provide the photo-actuator with promising behavior, the additive should be well dispersed and homogenously distributed in the TPE matrix. There were two methods reported for preparation of photo-actuating TPE composites in the literature. The first method is solution mixing, where the filler is dispersed in the proper solvent, while very often the ultrasonication is needed, and TPE matrix is afterwards added to the additive dispersion either in the form of solid or solution. When TPE is added as a solid, the mixture has to be mixed for a sufficiently long time to completely dissolve the matrix [47–51, 72]. Sonication and/or high shear mixing can be applied in order

The second method is based on the preparation of *in situ* composites during synthesis of polymer matrix in the presence of a filler [48, 73]. This method allows direct modification of filler surface during the polymerization process and can provide composites with much better dispersed filler. After polymerization, the polymer/filler mixture has to be precipitated to remove unreacted monomer and then re-dispersed in a solvent for casting process. In both methods, finally, the homogenous mixture is casted onto the Teflon dishes or molds of specified shape [50, 51] and the solvent is evaporated at elevated temperature and reduced pressure. Then the samples for photo-actuating investigation is cut to obtain the regular shape of stripes not exceeding dimensions of length 30 mm, width 10 mm, and thickness 1 mm. The

thickness of the tested stripes was usually in the range of 0.3 mm–0.8 mm [47–51, 72].

In the case of Braille-like elements, thermoforming using a specific mold was used (Figure 19). Thermoforming allows not only preparation of the sample with required shape, but also obtaining certain pre-strain and orientation of the polymer chains needed for achieving the

**Figure 19.** Thermoforming process of the Braille-like elements. Reprinted with permission from Ilcikova M, Mrlik M, Sedlacek T, Chorvat D, Krupa I, Slouf M, et al. Viscoelastic and photo-actuation studies of composites based on poly‐ styrene-grafted carbon nanotubes and styrene-b-isoprene-b-styrene block copolymer. Polymer. 2014;55(1):211-8. Copy‐

**4.2. Preparation of samples for measuring of photo-actuation performance**

like element photo-actuation performance.

134 Thermoplastic Elastomers - Synthesis and Applications

to disperse the filler well [46, 72].

photo-actuation behavior of the material [46, 47].

right © 2013 Elsevier Ltd. [46].

The biocompatible grades of TPU open opportunity for utilization of photo-thermal activated actuators for *in vivo* medical applications. One of the proposed actuator was a photo-thermally expandable vascular stent for treatment of arterial stenosis. The polymer-based stents were designed to replace the frequently used metallic stents due to enhanced flexibility and possible drug elution [1]. Engineering aspects related to the application of photo-actuating thermoset TPU as materials for intravascular laser activated devices were reported. There were two type of devices designed. The interventional ischemic stroke devices and micro-grippers based on releasing embolic coils. The crucial parameter of suitable TPU stents was addressed to *T*g that has to be low enough to actuate at the lowest laser power, but it should be high enough not to self-expand at body temperature. The suitable materials were determined as those actuated in the range of 65°C to 80°C. The crucial issue was determined as attaining these temperatures in the actuators in flow conditions and coupling of the light from the diffusing fiber through the blood into the actuator.

Later on, laser actuated intravascular thrombectomy device based on thermoplastic TPU has been designed [2, 3]. The prototype of self-expanding stent was composed of TPU crimped over a light diffuser attached to the end of the optical fiber connected to the IR laser diode [3]. The optical fiber had a dual function, first as a transport vehicle and second as a light energy delivery. The authors used the MM5520 phase separated TPU with *T*gs of -25°C and 55°C. The material was first crimped above *T*g, i.e., above 55°C, and cooled down. The primary shape was achieved after heating the material to 40°C–45 °C. The modulus turned from 800 MPa to 1.4 MPa during the shape change. The requirements for the moduli of the expandable polymer stents lie in the range from 800 MPa to 7 GPa in the glassy state. The applications such as neural stenting require low mechanical strength and the moduli ranges from 400 MPa in the glassy state. However, in this case, the low expansion force is generated [4].

The TPU was considered as a promising candidate for vascular stents, as the requirements for mechanical properties, deployment, and biocompatibility are met. Nevertheless, there are still some aspects to face, such as those related to the prevention of photo-thermally induced injury of arterial tissue.

Recently, the photo-thermally responsive TPEs have been utilized in the development of tactile displays for blind or visually impaired people [5]. The everyday activities of the population of the whole world can be greatly improved with the implementation of at least one Braille-like element in various devices. For wide industrial application, the cost of raw material and processing belong to decisive factors. From this point of view, the TPE poses advantage over still costly LCs or chemically cross-linked elastomers, which were recently studied for the same application as well.

Another potential application in plastic motors has emerged recently [9]. The photo-thermally activated localized contraction of scrolled strip caused rolling of the sample upon irradiation by UV and visible light. The investigated material was based on LCE with aligned structure. Even though TPE actuators have not been tested in this pioneer publication, it can be assumed that TPEs with oriented structure can be suitable candidates for such application as well. Similarly, even though so far TPE photo-actuators have not been investigated for other applications, these materials that have responses in visible or near-infrared (NIR) wavelength region have various potential applications in telecommunication, thermal imaging, remote sensing, thermal photovoltaics, solar cells, sensors, etc.

## **6. Conclusions**

The photo-actuating phenomenon is mostly influenced by the chemical structure of matrix, light absorbers, and fillers improving mechanical properties and thermal conductivity of the final material. Thus, photo-actuating materials with good performance are mostly composites consisting of elastomers and carbon-based fillers. TPEs having physically cross-linked structures possess several advantages compared to chemically cross-linked elastomers. The advantages of TPEs compared to chemically cross-linked elastomers are repeatable processa‐ bility, tuning their mechanical properties by slight changes in composition and chemical structure, and price. Good photo-actuation response has been shown for TPE composites mainly based on thermoplastic urethanes, poly(ethylene-*co*-vinyl acetate), polystyrene-*block*polyisoprene-*block*-polystyrene and poly(methyl methacrylate)-*block*-poly(butyl acrylate) *block*-poly(methyl methacrylate) matrices. Carbon nanotubes and graphene are appropriate fillers providing good optical, mechanical, and thermal properties at quite low loading. Good dispersity and homogeneous distribution of the filler in the TPE matrix is crucial for achieving materials with good photo-actuation behavior. Therefore, surface modification of the fillers to improve compatibility with TPE matrix is encouraged. In block copolymers-based TPEs, preferential interactions of fillers with soft phase should be tailored to maximize the heat energy transfer to the soft phase, which is responsible for the actuation changes of the material. The photo-actuation phenomenon has been utilized in various smart applications. Poly(eth‐ ylene-*co*-vinyl acetate) and polystyrene-*block*-polyisoprene-*block*-polystyrene have been used in the development of tactile displays, and the biocompatible thermoplastic polyurethanes have been utilized for fabrication of vascular stents. The applicability of these materials is, however, much broader than published so far.

## **Acknowledgements**

The authors thank the Centre of Excellence SAS for Functionalized Multiphase Materials (FUN-MAT), Grant agency VEGA 2/0112/13, Slovak Research and Development Agency APVV through Grant APVV-0109-10 and project SAS-MOST JRP 2014-9 "Synthesis of welldefined novel copolymers by use of living polymerization methods and advanced chroma‐ tography technique".

## **Author details**

Even though TPE actuators have not been tested in this pioneer publication, it can be assumed that TPEs with oriented structure can be suitable candidates for such application as well. Similarly, even though so far TPE photo-actuators have not been investigated for other applications, these materials that have responses in visible or near-infrared (NIR) wavelength region have various potential applications in telecommunication, thermal imaging, remote

The photo-actuating phenomenon is mostly influenced by the chemical structure of matrix, light absorbers, and fillers improving mechanical properties and thermal conductivity of the final material. Thus, photo-actuating materials with good performance are mostly composites consisting of elastomers and carbon-based fillers. TPEs having physically cross-linked structures possess several advantages compared to chemically cross-linked elastomers. The advantages of TPEs compared to chemically cross-linked elastomers are repeatable processa‐ bility, tuning their mechanical properties by slight changes in composition and chemical structure, and price. Good photo-actuation response has been shown for TPE composites mainly based on thermoplastic urethanes, poly(ethylene-*co*-vinyl acetate), polystyrene-*block*polyisoprene-*block*-polystyrene and poly(methyl methacrylate)-*block*-poly(butyl acrylate) *block*-poly(methyl methacrylate) matrices. Carbon nanotubes and graphene are appropriate fillers providing good optical, mechanical, and thermal properties at quite low loading. Good dispersity and homogeneous distribution of the filler in the TPE matrix is crucial for achieving materials with good photo-actuation behavior. Therefore, surface modification of the fillers to improve compatibility with TPE matrix is encouraged. In block copolymers-based TPEs, preferential interactions of fillers with soft phase should be tailored to maximize the heat energy transfer to the soft phase, which is responsible for the actuation changes of the material. The photo-actuation phenomenon has been utilized in various smart applications. Poly(eth‐ ylene-*co*-vinyl acetate) and polystyrene-*block*-polyisoprene-*block*-polystyrene have been used in the development of tactile displays, and the biocompatible thermoplastic polyurethanes have been utilized for fabrication of vascular stents. The applicability of these materials is,

The authors thank the Centre of Excellence SAS for Functionalized Multiphase Materials (FUN-MAT), Grant agency VEGA 2/0112/13, Slovak Research and Development Agency APVV through Grant APVV-0109-10 and project SAS-MOST JRP 2014-9 "Synthesis of welldefined novel copolymers by use of living polymerization methods and advanced chroma‐

sensing, thermal photovoltaics, solar cells, sensors, etc.

136 Thermoplastic Elastomers - Synthesis and Applications

however, much broader than published so far.

**Acknowledgements**

tography technique".

**6. Conclusions**

Markéta Ilčíková<sup>1</sup> , Miroslav Mrlík<sup>1</sup> and Jaroslav Mosnáček2\*

\*Address all correspondence to: upolmosj@savba.sk

1 Center for Advanced Materials, Qatar University, Doha, Qatar

2 Polymer Institute, Slovak Academy of Sciences, Bratislava, Slovakia

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## **Thermoplastic Resins used in Dentistry**

Lavinia Ardelean, Cristina Maria Bortun, Angela Codruta Podariu and Laura Cristina Rusu

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/60931

## **Abstract**

Thermoplastic materials such as polyamides (nylon), acetal resins, epoxy resins, polystyrene, polycarbonate resins, polyurethane and acrylic thermoplastic resins were introduced in dentistry as an alternative to classic resins, which have major disadvantages such as the toxicity of the residual monomer, awkward wrapping system and difficult processing.

Indications for thermoplastic resins include partial dentures, preformed clasps, partial denture frameworks, temporary or provisional crowns and bridges, full dentures, orthodontic appliances, anti-snoring devices, different types of mouth guards and splints. Some flexible myofunctional therapy devices, used for orthodontic purposes, may also be made of thermoplastic silicone polycarbonate-urethane.

The main characteristics of thermoplastic resins used in dentistry are as follows: they are monomerfree and consequently nontoxic and nonallergenic, they are injected by using special devices, they are biocompatible, they have enhanced esthetics and they are comfortable to wear.

**Keywords:** Thermoplastic resins, injection devices, metal-free removable partial dentures

## **1. Introduction**

Continuous development and progress of polymer's industry with applications in general and dental medicine was of great importance for the health domain. Using various types of resins for restorations in the oral cavity is beneficial from childhood till geriatric period [1-4].

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Thermopolymerizable acrylic resins were first used in dental technique in 1936, this being a great step forward. Acrylic resins are also known as polymethylmethacrylate or PMMA. These synthetically obtained materials can be modeled, packed or injected into molds during the plastic phase and become solid after chemical polymerization [5, 6]. Thermopolymerisable acrylic resins have many disadvantages as increased porosity, high water retention, volume variations and irritating effect due to the residual monomer, awkward wrapping system and difficult processing. Because of these, once polymers developed, alternative materials such as polyamides (nylon), acetal resins, epoxy resins, polystyrene, polycarbonate resins etc. [7-9] came on the market.

The main characteristics of the thermoplastic resins used in dentistry are as follows: they are monomer-free and consequently nontoxic and nonallergenic, they are injected by using special devices, they are biocompatible, they have enhanced esthetics and they are comfortable to wear [10].

## **2. Types of thermoplastic resins used in dentistry**


The classification of resins according to DIN EN ISO 1567 is presented in Table 1:

**Table 1.** The classification of resins according to DIN EN ISO 1567

Among the technologies for manufacturing removable complete and partial dentures we distinguish: heat-curing, self-curing, injection, light-curing, casting and microwave use [11].

Thermoplastic resins may be classified by their composition, as acetal resins, polycarbonate resins (belonging to the group of polyester resins), acrylic resins and polyamides (nylons).

The use of thermoplastic resins in dental medicine is continuously growing. The material is thermally plasticized and no chemical reaction takes place. The injection of plasticized resins into a mold represents a new technology in manufacturing complete and removable partial dentures [12].

At present, due to successive alterations in the chemical composition, thermoplastic materials are suitable for manufacturing removable partial dentures with no metallic components, resulting in the so-called "metal-free removable partial dentures" [13].

Indications for thermoplastic resins include removable partial dentures, preformed clasps [14], partial denture frameworks, temporary or provisional crowns and bridges, complete dentures, orthodontic appliances, anti-snoring devices, different types of mouth guards and splints. Some flexible myofunctional therapy devices, used for orthodontic purposes, may also be made of thermoplastic silicone polycarbonate-urethane.

## **2.1. Thermoplastic acetal**

Thermopolymerizable acrylic resins were first used in dental technique in 1936, this being a great step forward. Acrylic resins are also known as polymethylmethacrylate or PMMA. These synthetically obtained materials can be modeled, packed or injected into molds during the plastic phase and become solid after chemical polymerization [5, 6]. Thermopolymerisable acrylic resins have many disadvantages as increased porosity, high water retention, volume variations and irritating effect due to the residual monomer, awkward wrapping system and difficult processing. Because of these, once polymers developed, alternative materials such as polyamides (nylon), acetal resins, epoxy resins, polystyrene, polycarbonate resins etc. [7-9]

The main characteristics of the thermoplastic resins used in dentistry are as follows: they are monomer-free and consequently nontoxic and nonallergenic, they are injected by using special devices, they are biocompatible, they have enhanced esthetics and they are

**2. Types of thermoplastic resins used in dentistry**

The classification of resins according to DIN EN ISO 1567 is presented in Table 1:

Type 1 Thermopolymerizable resins (>65°C) Group 1: bicomponent powder and liquid

Type 2 Autopolymerizable resins (<65°C) Group 1: bicomponent powder and liquid

Type 3 Thermoplastic resins Monocomponent system grains in cartridges

Among the technologies for manufacturing removable complete and partial dentures we distinguish: heat-curing, self-curing, injection, light-curing, casting and microwave use [11].

Thermoplastic resins may be classified by their composition, as acetal resins, polycarbonate resins (belonging to the group of polyester resins), acrylic resins and polyamides (nylons).

The use of thermoplastic resins in dental medicine is continuously growing. The material is thermally plasticized and no chemical reaction takes place. The injection of plasticized resins into a mold represents a new technology in manufacturing complete and removable partial

Group 2: monocomponent

Group 2: bicomponent powder and casting liquid

**Type Class (manufacturing) Group (presentation form)**

Type 4 Light-cured resins Monocomponent system Type 5 Microwave cured resins Bicomponent system

**Table 1.** The classification of resins according to DIN EN ISO 1567

came on the market.

146 Thermoplastic Elastomers - Synthesis and Applications

comfortable to wear [10].

dentures [12].

Thermoplastic acetal is a poly(oxy-methylene)-based material, which as a homopolymer has good short-term mechanical properties but as a copolymer has better long-term stability [15]. Due to its resistance to wear and fracture, combined with a certain amount of flexibility, acetal resin is an ideal material for preformed clasps for partial dentures, single-pressed unilateral partial dentures, partial denture frameworks (Figure 1), provisional bridges, occlusal splints and implant abutments, artificial teeth for removable dentures and orthodontic appliances [16].

Because of their resistance to occlusal wear, acetal resins are also well suited for maintaining vertical dimension during provisional restorative therapy. Acetal is not translucent and does not match the esthetic appearance of thermoplastic acrylic and polycarbonate [17].

**Figure 1.** Removable partial denture with acetal frame and clasps

## **2.2. Thermoplastic polyamide (nylon)**

Thermoplastic polyamide (nylon) is a resin derived from diamine and dibasic acid monomers. Versatililty is one of its characteristics and makes it suitable for various applications. Nylon exhibits high flexibility, physical strength, heat and chemical resistance. It can be easily modified to increase stiffness and wear resistance. Because of its excellent balance of strength, ductility and heat resistance, nylon is an outstanding candidate for metal replacement applications [18]. Nylon is mainly used for tissue supported removable dentures. Its stiffness makes it unsuitable for usage as occlusal rests or denture elements that need to be rigid [7, 13]. Because it is flexible, it cannot maintain vertical dimension when used in direct occlusal forces. Adjustment and polishing is difficult but provides excellent esthetics due to its semitranslucency [19, 20].

Nylon is specially indicated for patients allergic to methyl metacrylate, being monomer-free, lightweight and impervious to oral fluids [21]. Some may also be combined with a metal framework (Figure 2).

**Figure 2.** Removable partial denture of polyamide combined with metal

Comparative properties of thermoplastic acetal and polyamide, the two types of resins suitable for manufacturing removable partial dentures, are shown in Table 2.


**Table 2.** Comparative aspects of acetalic and polyamidic thermoplastic resins

## **2.3. Thermoplastic polyester**

Thermoplastic polyester resins are also used in dentistry. They melt between 230°C and 290°C and the technology implies casting into molds. Polycarbonate resins are particularly polyester

materials. They have good fracture strength and flexibility, but the wear resistance is lower than acetal resins. Polycarbonates have a natural translucency and finishes very well, which recommends them for temporary restorations, but they are not suitable for partial denture frameworks [22]. **2.3 Thermoplastic polyester** Thermoplastic polyester resins are also used in dentistry. They melt between 230ºC and 290ºC and the technology implies casting into molds. Polycarbonate resins are particularly polyester materials. They

#### **2.4. Thermoplastic acrylate** have good fracture strength and flexibility, but the wear resistance is lower than acetal resins. Polycarbonates have a natural translucency and finishes very well, which recommends them for

exhibits high flexibility, physical strength, heat and chemical resistance. It can be easily modified to increase stiffness and wear resistance. Because of its excellent balance of strength, ductility and heat resistance, nylon is an outstanding candidate for metal replacement applications [18]. Nylon is mainly used for tissue supported removable dentures. Its stiffness makes it unsuitable for usage as occlusal rests or denture elements that need to be rigid [7, 13]. Because it is flexible, it cannot maintain vertical dimension when used in direct occlusal forces. Adjustment and polishing is difficult but provides excellent esthetics due to its

Nylon is specially indicated for patients allergic to methyl metacrylate, being monomer-free, lightweight and impervious to oral fluids [21]. Some may also be combined with a metal

Comparative properties of thermoplastic acetal and polyamide, the two types of resins suitable

polioximetylen very good very high medium good very good

Thermoplastic polyester resins are also used in dentistry. They melt between 230°C and 290°C and the technology implies casting into molds. Polycarbonate resins are particularly polyester

very high

very good very good

**Resin type Main substance Resistance Durity Flexibility Esthetics Biocompatibility**

semitranslucency [19, 20].

148 Thermoplastic Elastomers - Synthesis and Applications

framework (Figure 2).

Acetalic resin

Polyamidic resin

**2.3. Thermoplastic polyester**

**Figure 2.** Removable partial denture of polyamide combined with metal

for manufacturing removable partial dentures, are shown in Table 2.

diamine good high medium or

**Table 2.** Comparative aspects of acetalic and polyamidic thermoplastic resins

Thermoplastic acrylate consists of fully polymerized acrylate, its base component being methyl-metacrylate, the special blend of polymers giving it the highest impact rating of any acrylic. This material was developed for manufacturing complete dentures. It is not elastic, but its flexibility makes it practically unbreakable. The material has long-term stability, its surface structure being dense and smooth. Due to the absence of residual monomer its biocompatibility is very good. The denture has very good long-term adaptability because water retention is limited. You can bounce such denture off the floor without cracking the base [7, 21, 23]. temporary restorations, but they are not suitable for partial denture frameworks [22]. **2.4 Thermoplastic acrylate**  Thermoplastic acrylate consists of fully polymerized acrylate, its base component being methylmetacrylate, the special blend of polymers giving it the highest impact rating of any acrylic. This material was developed for manufacturing complete dentures. It is not elastic, but its flexibility makes it practically unbreakable. The material has long-term stability, its surface structure being dense and smooth. Due to the absence of residual monomer its biocompatibility is very good. The denture has

very good long-term adaptability because water retention is limited. You can bounce such denture off

#### **2.5. Presentation form and injection** the floor without cracking the base [7,21,23].

3).

Thermoplastic materials can be polymerized or prepolymerized and they can be found in granular form, with low molecular weight, already wrapped in cartridges that eliminate dosage errors (Figure 3). **2.5 Presentation form and injection**  Thermoplastic materials can be polymerized or prepolymerized and they can be found in granular form, with low molecular weight, already wrapped in cartridges that eliminate dosage errors (Figure

**Figure 3.** (a, b) Cartridges of different thermoplastic resins (c) The granular aspect of the material

(200°C\_ 250°C). Thermal plasticization takes place in special devices afterward the material is injected under pressure into a mold, without any chemical reactions. After heating, the metallic cartridges containing thermoplastic grains are set in place into the injecting unit and the plasticized resin is forced into the mold at a pressure of 6-8 bars. Pressure, temperature and injecting time are automatically controlled by the injecting unit. Dentures obtained using this technology have excellent esthetics and good compatibility [7,12,13,22]. Injecting thermoplastic resins into molds is not a common technology in dental laboratories because the need of expensive equipment and this could be a disadvantage. The special injection devices we use are Polyapress (Bredent) and R-3C (Flexite) injectors (Figure 4). They exhibit a high rigidity despite their low molecular weight. Their plasticizing temperature is low (200°C-250°C). Thermal plasticization takes place in special devices afterward the material is injected under pressure into a mold, without any chemical reactions. After heating, the metallic cartridges containing thermoplastic grains are set in place into the injecting unit and the plasticized resin is forced into the mold at a pressure of 6-8 bars. Pressure, temperature and injecting time are automatically controlled by the injecting unit. Dentures obtained using this technology have excellent esthetics and good compatibility [7, 12, 13, 22].

Figure 3. (a, b) Cartridges of different thermoplastic resins (c) The granular aspect of the material

They exhibit a high rigidity despite their low molecular weight. Their plasticizing temperature is low

Injecting thermoplastic resins into molds is not a common technology in dental laboratories because the need of expensive equipment and this could be a disadvantage.

The special injection devices we use are Polyapress (Bredent) and R-3C (Flexite) injectors (Figure 4).

molding device (Bredent) (b) The R-3C injector (Flexite)

**3. Prosthetic devices made of thermoplastic resins Figure 4.** (a) The Polyapress injection-molding device (Bredent) (b) The R-3C injector (Flexite)

#### free and consequently nontoxic and nonallergenic, they are injected by using special devices, they are biocompatible, they have enhanced esthetics and they are comfortable to wear. **3. Prosthetic devices made of thermoplastic resins** a. b. Figure 4. (a) The Polyapress injection\_ molding device (Bredent) (b) The R-3C injector (Flexite)

Figure 4. (a) The Polyapress injection\_

partial edentations, with removable partial dentures without metallic framework, or combining the metallic framework with thermoplastic resin saddles, using different thermoplastic resins, selected according to their indications and manufacturing technology (Figure 5). The main characteristics of thermoplastic resins used in dentistry are as follows: they are monomer-free and consequently nontoxic and nonallergenic, they are injected by using special devices, they are biocompatible, they have enhanced esthetics and they are comfortable to wear. **3. Prosthetic devices made of thermoplastic resins**  The main characteristics of thermoplastic resins used in dentistry are as follows: they are monomerfree and consequently nontoxic and nonallergenic, they are injected by using special devices, they are

The main characteristics of thermoplastic resins used in dentistry are as follows: they are monomer-

Our experience with thermoplastic resins for dental use involves solving several different cases of

Our experience with thermoplastic resins for dental use involves solving several different cases of partial edentations, with removable partial dentures without metallic framework, or combining the metallic framework with thermoplastic resin saddles, using different thermo‐ plastic resins, selected according to their indications and manufacturing technology (Figure 5). biocompatible, they have enhanced esthetics and they are comfortable to wear. Our experience with thermoplastic resins for dental use involves solving several different cases of partial edentations, with removable partial dentures without metallic framework, or combining the metallic framework with thermoplastic resin saddles, using different thermoplastic resins, selected according to their indications and manufacturing technology (Figure 5).

of a metal one [16], consequently the main connector, the clasps and the spurs need to be oversized [12]. Injection was carried out using the R-3 C digital control device that has five preset programs, as Figure 5. Different combinations between thermoplastic resins, with or without metal **Figure 5.** Different combinations between thermoplastic resins, with or without metal

**3.1 Removable partial dentures with acetal framework** 

well as programs that can be individually set by the user. The types of thermoplastic resins we used for manufacturing different types of removable partial dentures are acetal resins and polyamides of different flexibilities. The types of thermoplastic resins we used for manufacturing different types of removable partial dentures are acetal resins and polyamides of different flexibilities.

The acetal resin has optimal physical and chemical properties and it is indicated in making frameworks and clasps for removable partial dentures, being available in tooth color and pink [12]. Experimentally, in some cases, we combined an acetal resin frames with classic acrylic resins for the saddles (Figure 4). However, the resistance values for the acetal resin framework do not reach those of a metal one [16], consequently the main connector, the clasps and the spurs need to be oversized

## **3.1. Removable partial dentures with acetal framework**

well as programs that can be individually set by the user.

Injecting thermoplastic resins into molds is not a common technology in dental laboratories

The special injection devices we use are Polyapress (Bredent) and R-3C (Flexite) injectors

a. b.

The main characteristics of thermoplastic resins used in dentistry are as follows: they are monomerfree and consequently nontoxic and nonallergenic, they are injected by using special devices, they are

a. b.

Our experience with thermoplastic resins for dental use involves solving several different cases of partial edentations, with removable partial dentures without metallic framework, or combining the metallic framework with thermoplastic resin saddles, using different thermoplastic resins, selected

The main characteristics of thermoplastic resins used in dentistry are as follows: they are monomer-free and consequently nontoxic and nonallergenic, they are injected by using special devices, they are biocompatible, they have enhanced esthetics and they are comfortable to

The main characteristics of thermoplastic resins used in dentistry are as follows: they are monomerfree and consequently nontoxic and nonallergenic, they are injected by using special devices, they are

Our experience with thermoplastic resins for dental use involves solving several different cases of partial edentations, with removable partial dentures without metallic framework, or combining the metallic framework with thermoplastic resin saddles, using different thermo‐ plastic resins, selected according to their indications and manufacturing technology (Figure 5).

Our experience with thermoplastic resins for dental use involves solving several different cases of partial edentations, with removable partial dentures without metallic framework, or combining the metallic framework with thermoplastic resin saddles, using different thermoplastic resins, selected

Figure 5. Different combinations between thermoplastic resins, with or without metal The types of thermoplastic resins we used for manufacturing different types of removable partial

The acetal resin has optimal physical and chemical properties and it is indicated in making frameworks and clasps for removable partial dentures, being available in tooth color and pink [12]. Experimentally, in some cases, we combined an acetal resin frames with classic acrylic resins for the saddles (Figure 4). However, the resistance values for the acetal resin framework do not reach those of a metal one [16], consequently the main connector, the clasps and the spurs need to be oversized [12]. Injection was carried out using the R-3 C digital control device that has five preset programs, as

Figure 5. Different combinations between thermoplastic resins, with or without metal

The types of thermoplastic resins we used for manufacturing different types of removable partial

The types of thermoplastic resins we used for manufacturing different types of removable

The acetal resin has optimal physical and chemical properties and it is indicated in making frameworks and clasps for removable partial dentures, being available in tooth color and pink [12]. Experimentally, in some cases, we combined an acetal resin frames with classic acrylic resins for the saddles (Figure 4). However, the resistance values for the acetal resin framework do not reach those of a metal one [16], consequently the main connector, the clasps and the spurs need to be oversized

biocompatible, they have enhanced esthetics and they are comfortable to wear.

**Figure 4.** (a) The Polyapress injection-molding device (Bredent) (b) The R-3C injector (Flexite)

according to their indications and manufacturing technology (Figure 5).

biocompatible, they have enhanced esthetics and they are comfortable to wear.

**3. Prosthetic devices made of thermoplastic resins**

dentures are acetal resins and polyamides of different flexibilities.

according to their indications and manufacturing technology (Figure 5).

**3.1 Removable partial dentures with acetal framework** 

well as programs that can be individually set by the user.

**3.1 Removable partial dentures with acetal framework** 

dentures are acetal resins and polyamides of different flexibilities.

**Figure 5.** Different combinations between thermoplastic resins, with or without metal

partial dentures are acetal resins and polyamides of different flexibilities.

molding device (Bredent) (b) The R-3C injector (Flexite)

molding device (Bredent) (b) The R-3C injector (Flexite)

because the need of expensive equipment and this could be a disadvantage.

Figure 4. (a) The Polyapress injection\_

**3. Prosthetic devices made of thermoplastic resins** 

Figure 4. (a) The Polyapress injection\_

150 Thermoplastic Elastomers - Synthesis and Applications

**3. Prosthetic devices made of thermoplastic resins** 

(Figure 4).

wear.

The acetal resin has optimal physical and chemical properties and it is indicated in making frameworks and clasps for removable partial dentures, being available in tooth color and pink [12]. Experimentally, in some cases, we combined an acetal resin frames with classic acrylic resins for the saddles (Figure 4). However, the resistance values for the acetal resin framework do not reach those of a metal one [16], consequently the main connector, the clasps and the spurs need to be oversized [12]. Injection was carried out using the R-3 C digital control device [12]. Injection was carried out using the R-3 C digital that has five preset programs, as well as programs that can be individually set by the user. control device that has five preset programs, as

Figure 6. Acetal framework and clasp and removable partial dentures with acetal framework and clasps **Figure 6.** Acetal framework and clasp and removable partial dentures with acetal framework and clasps

The maintenance, support and stabilizing systems used are metal-free ones. The maintenance, support and stabilizing systems used are metal-free ones.

The significant aspects of the technical steps in the technology of removable partial dentures made of thermoplastic materials are described. The significant aspects of the technical steps in the technology of removable partial dentures made of thermoplastic materials are described.

The master model is poured of class IV hard plaster, using a vibrating table (Figure 7). The master model is poured of class IV hard plaster, using a vibrating table (Figure 7).

muco-osseous tissues are marked. The abutments undercuts are measured and the engagement of the

After the parallelograph analysis is carried out, the future frame design is drawn, including all extensions of saddles, major connector, retentive and bracing arms of the clasps, occlusal rests and minor connectors of Ackers circumferential clasps on abutment teeth. The design starts with the saddles, following the main connector, the retentive and opposing clasp arms, the spurs and the

of the cast is chosen and recorded so that a favorable path of insertion is obtained. To record the position of the cast tripod marks are used. The contour heights on the abutment teeth and the retentive **Figure 7.** Casting the master model

terminal third of the retentive arms of the clasps is established.

secondary connectors of the Ackers circular clasps [12, 13].

the framework has to be spaced from the gingival tissue. [12].

In order to assess its retentiveness and to determine the place where the active arms of the clasp are placed a parallelograph analysis is made (Figure 8). The abutment teeth are selected and the position of the cast is chosen and recorded so that a favorable path of insertion is obtained. To record the position of the cast tripod marks are used. The contour heights on the abutment teeth and the retentive muco-osseous tissues are marked. The abutments undercuts are measured and the engagement of the terminal third of the retentive arms of the clasps is established.

After the parallelograph analysis is carried out, the future frame design is drawn, including all extensions of saddles, major connector, retentive and bracing arms of the clasps, occlusal rests and minor connectors of Ackers circumferential clasps on abutment teeth. The design starts with the saddles, following the main connector, the retentive and opposing clasp arms, the spurs and the secondary connectors of the Ackers circular clasps [12, 13].

After designing the framework, the master model is prepared for duplication, including foliation and deretentivisation (Figure 9). At the beginning, blue wax plates are used as spacers in regions where the framework has to be spaced from the gingival tissue. [12]. After designing the framework, the master model is prepared for duplication, including foliation and deretentivisation (Figure 9). At the beginning, blue wax plates are used as spacers in regions where

Figure 8. Parallelograph analysis

**Figure 8.** Parallelograph analysis

clasps arms. The block-out wax meets the spacing wax in a smooth joint. In order to duplicate the master model, a vinyl-polysiloxane silicone placed in a flask is used. After its **Figure 9.** Deretentivisation of the model

setting, the duplicate model is poured (Figure 10), using class IV hard plaster.

a. b.

Figure 10. (a) Duplication of the model (b) Casting of the duplicate model using class IV hard plaster

Block-out wax is applied between teeth cervices and gingival margin of the drawing repre‐ senting the clasps arms. The block-out wax meets the spacing wax in a smooth joint. In order to duplicate the master model, a vinyl-polysiloxane silicone placed in a flask is used. After its setting, the duplicate model is poured (Figure 10), using class IV hard plaster. Figure 9. Deretentivisation of the model Block-out wax is applied between teeth cervices and gingival margin of the drawing representing the clasps arms. The block-out wax meets the spacing wax in a smooth joint. In order to duplicate the master model, a vinyl-polysiloxane silicone placed in a flask is used. After its

After designing the framework, the master model is prepared for duplication, including foliation and deretentivisation (Figure 9). At the beginning, blue wax plates are used as spacers in regions where

Figure 8. Parallelograph analysis

the framework has to be spaced from the gingival tissue. [12].

In order to assess its retentiveness and to determine the place where the active arms of the clasp are placed a parallelograph analysis is made (Figure 8). The abutment teeth are selected and the position of the cast is chosen and recorded so that a favorable path of insertion is obtained. To record the position of the cast tripod marks are used. The contour heights on the abutment teeth and the retentive muco-osseous tissues are marked. The abutments undercuts are measured and the engagement of the terminal third of the retentive arms of the clasps is

After the parallelograph analysis is carried out, the future frame design is drawn, including all extensions of saddles, major connector, retentive and bracing arms of the clasps, occlusal rests and minor connectors of Ackers circumferential clasps on abutment teeth. The design starts with the saddles, following the main connector, the retentive and opposing clasp arms,

After designing the framework, the master model is prepared for duplication, including foliation and deretentivisation (Figure 9). At the beginning, blue wax plates are used as spacers

the spurs and the secondary connectors of the Ackers circular clasps [12, 13].

in regions where the framework has to be spaced from the gingival tissue. [12]. After designing the framework, the master model is prepared for duplication, including foliation and deretentivisation (Figure 9). At the beginning, blue wax plates are used as spacers in regions where

Figure 8. Parallelograph analysis

Figure 9. Deretentivisation of the model

Block-out wax is applied between teeth cervices and gingival margin of the drawing representing the

In order to duplicate the master model, a vinyl-polysiloxane silicone placed in a flask is used. After its

Figure 10. (a) Duplication of the model (b) Casting of the duplicate model using class IV hard plaster

clasps arms. The block-out wax meets the spacing wax in a smooth joint.

setting, the duplicate model is poured (Figure 10), using class IV hard plaster.

a. b.

the framework has to be spaced from the gingival tissue. [12].

152 Thermoplastic Elastomers - Synthesis and Applications

**Figure 8.** Parallelograph analysis

**Figure 9.** Deretentivisation of the model

established.

**Figure 10.** (a) Duplication of the model (b) Casting of the duplicate model using class IV hard plaster

Figure 10. (a) Duplication of the model (b) Casting of the duplicate model using class IV hard plaster

setting, the duplicate model is poured (Figure 10), using class IV hard plaster.

The elements of removable partial denture's wax pattern are as follows (Figure 11a): the main connector, made of red wax (so that its thickness is twice as normal), the saddles and the Ackers circular clasps, made of blue wax. Injection bars are also required for those areas of the framework that are not visible in the finite piece. A large central shaft is necessary in order to connect with the main connector, through which the initial injection takes place. Unlike the pattern of a metallic framework, the patterns of the metal-free framework have to be 50% thicker (clasps, occlusal rests and main connector) [12, 13]. The elements of removable partial denture's wax pattern are as follows (Figure 11a): the main connector, made of red wax (so that its thickness is twice as normal), the saddles and the Ackers circular clasps, made of blue wax. Injection bars are also required for those areas of the framework that are not visible in the finite piece. A large central shaft is necessary in order to connect with the main connector, through which the initial injection takes place. Unlike the pattern of a metallic framework, the patterns of the metal-free framework have to be 50% thicker (clasps, occlusal rests

Spruing the framework is performed using minor sprues of 2.5 mm calibrated wax connected to one major sprue (Figure 11a). and main connector) [12, 13]. Spruing the framework is performed using minor sprues of 2.5 mm calibrated wax connected to one major sprue (Figure 11a).

Figure 11. (a) Wrapping the wax pattern frame of the removable partial denture (b) Insulation of the **Figure 11.** (a) Wrapping the wax pattern frame of the removable partial denture (b) Insulation of the investment

[12,13].

by the user. The pressure developed is 6\_

pressure is according to procedure demands (7.2\_

investment

Surface tension reducing solution is applied and the wax pattern is then invested in a vaseline insulated aluminum flask (Figure 11b). Class III hard stone is used as an investment. The gypsum paste is poured into one of the two halves of the flask and the duplicated model containing the framework pattern with sprues attached is centrally dipped base-face down. After setting, the gypsum surface is insulated and the second half of the flask is assembled. Class III hard stone is once more prepared and the flask is submerged in warm water in a thermostatic container. The two halves of the flask are disassembled and the wax is boiled out using clean hot water. The surface of the mold is then insulated and treated with a light-curing transparent varnish in order to obtain a shining aspect

Injection is carried out with the R-3C (Flexite) injector (Figure 4b), which does not take up much space as it can be mounted on a wall as well. The device has the following parameters: digital control, preset programs for different types of thermoplastic resins and programs that can be individually set

Before starting injection, the valves of carbon dioxide tank are checked to make sure the injecting

checked (15 minutes at 220°C). The selected cartridge (quantity and color) is introduced into one of the two heating cylinders and the preheating process is then activated (Figure 12). After preheating ends, the two halves of the flask are assembled and fastened. Early assembling of the flask is not indicated because water vapor condensation might occur inside the mold, with negative effects on the

The injection process takes only 0.25 seconds and it is initiated by pressing the key on the control panel. The pressure is automatically kept constant for one minute so that setting contraction is compensated. This stage is indicated with the sign "----" on the screen. The cartridge is separated and the flask is then released and pulled out. In order to achieve optimal

7.5 Bar). Preheating temperature and time are also

8 Bar [23].

quality of the injected material. The flask is inserted and secured in the injecting unit.

quality of the material, the flask is left to cool slowly for 8 hours [12,13].

Surface tension reducing solution is applied and the wax pattern is then invested in a vaseline insulated aluminum flask (Figure 11b). Class III hard stone is used as an investment. The gypsum paste is poured into one of the two halves of the flask and the duplicated model containing the framework pattern with sprues attached is centrally dipped base-face down. After setting, the gypsum surface is insulated and the second half of the flask is assembled. Class III hard stone is once more prepared and the flask is submerged in warm water in a thermostatic container. The two halves of the flask are disassembled and the wax is boiled out using clean hot water. The surface of the mold is then insulated and treated with a light-curing transparent varnish in order to obtain a shining aspect [12, 13].

Injection is carried out with the R-3C (Flexite) injector (Figure 4b), which does not take up much space as it can be mounted on a wall as well. The device has the following parameters: digital control, preset programs for different types of thermoplastic resins and programs that can be individually set by the user. The pressure developed is 6-8 Bar [23].

Figure 12. Schedule of "G" program of injecting the thermoplastic material (a) Start (b) Heating (c) Injecting (d) Cooling **Figure 12.** Schedule of "G" program of injecting the thermoplastic material (a) Start (b) Heating (c) Injecting (d) Cool‐ ing

Before investment removal, screws are loosened and the flask is gently disassembled (Figure 13). Before starting injection, the valves of carbon dioxide tank are checked to make sure the injecting pressure is according to procedure demands (7.2-7.5 Bar). Preheating temperature and time are also checked (15 minutes at 220°C). The selected cartridge (quantity and color) is

Figure 13. Disassembling the framework of the acetal resin removable partial denture (a) The framework is still in the flask (b) Disassembling is complete

a. b.

introduced into one of the two heating cylinders and the preheating process is then activated (Figure 12). After preheating ends, the two halves of the flask are assembled and fastened. Early assembling of the flask is not indicated because water vapor condensation might occur inside the mold, with negative effects on the quality of the injected material. The flask is inserted and secured in the injecting unit.

Surface tension reducing solution is applied and the wax pattern is then invested in a vaseline insulated aluminum flask (Figure 11b). Class III hard stone is used as an investment. The gypsum paste is poured into one of the two halves of the flask and the duplicated model containing the framework pattern with sprues attached is centrally dipped base-face down. After setting, the gypsum surface is insulated and the second half of the flask is assembled. Class III hard stone is once more prepared and the flask is submerged in warm water in a thermostatic container. The two halves of the flask are disassembled and the wax is boiled out using clean hot water. The surface of the mold is then insulated and treated with a light-curing

Injection is carried out with the R-3C (Flexite) injector (Figure 4b), which does not take up much space as it can be mounted on a wall as well. The device has the following parameters: digital control, preset programs for different types of thermoplastic resins and programs that

Figure 12. Schedule of "G" program of injecting the thermoplastic material (a) Start (b) Heating (c) Injecting (d) Cooling

**Figure 12.** Schedule of "G" program of injecting the thermoplastic material (a) Start (b) Heating (c) Injecting (d) Cool‐

Before starting injection, the valves of carbon dioxide tank are checked to make sure the injecting pressure is according to procedure demands (7.2-7.5 Bar). Preheating temperature and time are also checked (15 minutes at 220°C). The selected cartridge (quantity and color) is

Figure 13. Disassembling the framework of the acetal resin removable partial denture (a) The framework is still in the flask (b) Disassembling is complete

Before investment removal, screws are loosened and the flask is gently disassembled (Figure 13).

a. b.

ing

transparent varnish in order to obtain a shining aspect [12, 13].

154 Thermoplastic Elastomers - Synthesis and Applications

can be individually set by the user. The pressure developed is 6-8 Bar [23].

The injection process takes only 0.25 seconds and it is initiated by pressing the key on the control panel. The pressure is automatically kept constant for one minute so that setting contraction is compensated. This stage is indicated with the sign "----" on the screen. The cartridge is separated and the flask is then released and pulled out. In order to achieve optimal quality of the material, the flask is left to cool slowly for 8 hours [12, 13]. Figure 12. Schedule of "G" program of injecting the thermoplastic material (a) Start (b) Heating

Beforeinvestmentremoval, screwsareloosenedandtheflaskisgentlydisassembled(Figure13). (c) Injecting (d) Cooling

Before investment removal, screws are loosened and the flask is gently disassembled (Figure 13).

Figure 13. Disassembling the framework of the acetal resin removable partial denture (a) The **Figure 13.** Disassembling the framework of the acetal resin removable partial denture (a) The framework is still in the framework is still in the flask (b) Disassembling is complete flask (b) Disassembling is complete

The sprues are cut off using low-pressure carbide and diamond burs to avoid overheating the material. Finishing and polishing is performed using soft brushes, ragwheel and polishing paste (Figure 14).

Disassembling the framework of the future removable partial denture is followed by matching it to the model, processing and finishing this component of the removable partial denture (Figure 15).

Once the framework is ready, the artificial teeth are set up. Wax patterns of the saddles are constructed by dropping pink wax over the framework. Teeth set up starts with the most mesial tooth, which is polished until it esthetically fits onto the arch [24].

After properly setting of all the teeth are the wax pattern is invested in order to obtain the acrylic saddles. An impression of the wax pattern placed on the master model is made by using a putty condensation silicone.

The sprues are cut off using low\_

Finishing and polishing is performed using soft brushes, ragwheel and polishing paste (Figure 14).

Finishing and polishing is performed using soft brushes, ragwheel and polishing paste (Figure 14).

pressure carbide and diamond burs to avoid overheating the material.

pressure carbide and diamond burs to avoid overheating the material.

Figure 14. (a) Tools used for processing the acetal framework (b) Tools used for finishing and polishing the acetal framework (c) Special polishing paste Disassembling the framework of the future removable partial denture is followed by matching it to **Figure 14.** (a) Tools used for processing the acetal framework (b) Tools used for finishing and polishing the acetal framework (c) Special polishing paste Disassembling the framework of the future removable partial denture is followed by matching it to the model, processing and finishing this component of the removable partial denture (Figure 15).

Figure 15. (a) Matching the acetal framework to the model (b) The finished acetal framework **Figure 15.** (a) Matching the acetal framework to the model (b) The finished acetal framework

a. b.

Figure 15. (a) Matching the acetal framework to the model (b) The finished acetal framework

Once the framework is ready, the artificial teeth are set up. Wax patterns of the saddles are

After setting, impression is detached, wax is removed, and the teeth, framework and the master model are thoroughly cleaned. Openings are being cut on the lateral sides of the impression and the teeth are set in the corresponding places inside the impression [13]. After insulating the master model, the framework is placed and the impression set in its original place. The acrylic component of the denture is wrapped as usual, using rectangular flasks and a class II plaster (Figure 16a). Once the framework is ready, the artificial teeth are set up. Wax patterns of the saddles are constructed by dropping pink wax over the framework. Teeth set up starts with the most mesial tooth, which is polished until it esthetically fits onto the arch [24]. After properly setting of all the teeth are the wax pattern is invested in order to obtain the acrylic saddles. An impression of the wax pattern placed on the master model is made by using a putty condensation silicone. After setting, impression is detached, wax is removed, and the teeth, framework and the master model are thoroughly cleaned. Openings are being cut on the lateral sides of the impression and the teeth are constructed by dropping pink wax over the framework. Teeth set up starts with the most mesial tooth, which is polished until it esthetically fits onto the arch [24]. After properly setting of all the teeth are the wax pattern is invested in order to obtain the acrylic saddles. An impression of the wax pattern placed on the master model is made by using a putty condensation silicone. After setting, impression is detached, wax is removed, and the teeth, framework and the master model are thoroughly cleaned. Openings are being cut on the lateral sides of the impression and the teeth are

Self-curing acrylic resin is prepared and poured inside the impression through the lateral openings. The cast is introduced into a heat-pressure-curing unit setting a temperature of 50°C and a pressure of 6 bars for 10 minutes to avoid bubble development. Once the resin is cured, the impression is removed [12, 13]. Burs, brushes, ragwheels and pumice are used to remove the excess, to polish and finish the removable partial denture (Figure 16b). set in the corresponding places inside the impression [13]. After insulating the master model, the framework is placed and the impression set in its original place. The acrylic component of the denture is wrapped as usual, using rectangular flasks and a class II plaster (Figure 16a). set in the corresponding places inside the impression [13]. After insulating the master model, the framework is placed and the impression set in its original place. The acrylic component of the denture is wrapped as usual, using rectangular flasks and a class II plaster (Figure 16a).

The result is a consistent removable partial denture with no macroscopic deficiency even in the thinnest 0.3-0.5 mm areas of clasps, which means the technology is effective. finish the removable partial denture (Figure 16b). The result is a consistent removable partial denture with no macroscopic deficiency even in the thinnest 0.3\_ 0.5 mm areas of clasps, which means the technology is effective. Self\_ curing acrylic resin is prepared and poured inside the impression through the lateral openings. The cast is introduced into a heat-pressure-curing unit setting a temperature of 50°C and a pressure of 6 bars for 10 minutes to avoid bubble development. Once the resin is cured, the impression is

Figure 16. (a) Wrapped wax pattern with teeth (b) Partial dentures made of acetal resin and acrylic

#### **3.2. Removable partial dentures made of different types of polyamides 3.2 Removable partial dentures made of different types of polyamides**  removed [12,13]. Burs, brushes, ragwheels and pumice are used to remove the excess, to polish and finish the removable partial denture (Figure 16b).

medium-low flexibility polyamide [12] (Figure 17).

Self\_

After setting, impression is detached, wax is removed, and the teeth, framework and the master model are thoroughly cleaned. Openings are being cut on the lateral sides of the impression and the teeth are set in the corresponding places inside the impression [13]. After insulating the master model, the framework is placed and the impression set in its original place. The acrylic component of the denture is wrapped as usual, using rectangular flasks and a class II

After properly setting of all the teeth are the wax pattern is invested in order to obtain the acrylic saddles. An impression of the wax pattern placed on the master model is made by using a putty

After setting, impression is detached, wax is removed, and the teeth, framework and the master model are thoroughly cleaned. Openings are being cut on the lateral sides of the impression and the teeth are set in the corresponding places inside the impression [13]. After insulating the master model, the framework is placed and the impression set in its original place. The acrylic component of the denture

Once the framework is ready, the artificial teeth are set up. Wax patterns of the saddles are constructed by dropping pink wax over the framework. Teeth set up starts with the most mesial tooth,

Figure 15. (a) Matching the acetal framework to the model (b) The finished acetal framework

Figure 15. (a) Matching the acetal framework to the model (b) The finished acetal framework

Once the framework is ready, the artificial teeth are set up. Wax patterns of the saddles are constructed by dropping pink wax over the framework. Teeth set up starts with the most mesial tooth,

After properly setting of all the teeth are the wax pattern is invested in order to obtain the acrylic saddles. An impression of the wax pattern placed on the master model is made by using a putty

After setting, impression is detached, wax is removed, and the teeth, framework and the master model are thoroughly cleaned. Openings are being cut on the lateral sides of the impression and the teeth are set in the corresponding places inside the impression [13]. After insulating the master model, the framework is placed and the impression set in its original place. The acrylic component of the denture

a. b.

a. b.

**Figure 15.** (a) Matching the acetal framework to the model (b) The finished acetal framework

which is polished until it esthetically fits onto the arch [24].

which is polished until it esthetically fits onto the arch [24].

pressure carbide and diamond burs to avoid overheating the material.

pressure carbide and diamond burs to avoid overheating the material.

Finishing and polishing is performed using soft brushes, ragwheel and polishing paste (Figure 14).

Finishing and polishing is performed using soft brushes, ragwheel and polishing paste (Figure 14).

a. b. c.

a. b. c.

Figure 14. (a) Tools used for processing the acetal framework (b) Tools used for finishing and polishing the acetal framework (c) Special polishing paste

Figure 14. (a) Tools used for processing the acetal framework (b) Tools used for finishing and polishing the acetal framework (c) Special polishing paste

Disassembling the framework of the future removable partial denture is followed by matching it to the model, processing and finishing this component of the removable partial denture (Figure 15).

**Figure 14.** (a) Tools used for processing the acetal framework (b) Tools used for finishing and polishing the acetal

Disassembling the framework of the future removable partial denture is followed by matching it to the model, processing and finishing this component of the removable partial denture (Figure 15).

Self-curing acrylic resin is prepared and poured inside the impression through the lateral openings. The cast is introduced into a heat-pressure-curing unit setting a temperature of 50°C and a pressure of 6 bars for 10 minutes to avoid bubble development. Once the resin is cured, the impression is removed [12, 13]. Burs, brushes, ragwheels and pumice are used to remove

the excess, to polish and finish the removable partial denture (Figure 16b).

is wrapped as usual, using rectangular flasks and a class II plaster (Figure 16a).

is wrapped as usual, using rectangular flasks and a class II plaster (Figure 16a).

plaster (Figure 16a).

condensation silicone.

condensation silicone.

The sprues are cut off using low\_

156 Thermoplastic Elastomers - Synthesis and Applications

The sprues are cut off using low\_

framework (c) Special polishing paste

Making polyamide resin removable partial dentures does not require so many intermediary steps as those made of acetal resins. The steps are similar to those followed for acrylic dentures, but with thermoplastic materials the injecting procedure is used. The clasps are made of the same material as the denture base, when using superflexible polyamide or ready-made clasps, in the case of using medium-low flexibility polyamide [12] (Figure 17). Making polyamide resin removable partial dentures does not require so many intermediary steps as those made of acetal resins. The steps are similar to those followed for acrylic dentures, but with thermoplastic materials the injecting procedure is used. The clasps are made of the same material as the denture base, when using superflexible polyamide or ready-made clasps, in the case of using The result is a consistent removable partial denture with no macroscopic deficiency even in the thinnest 0.3\_ 0.5 mm areas of clasps, which means the technology is effective. **3.2 Removable partial dentures made of different types of polyamides**  Making polyamide resin removable partial dentures does not require so many intermediary steps as those made of acetal resins. The steps are similar to those followed for acrylic dentures, but with

thermoplastic materials the injecting procedure is used. The clasps are made of the same material as

Figure 17. Polyamide removable partial denture with (a) pre-formed clasps (b) clasps made of the same material as the denture base **Figure 17.** Polyamide removable partial denture with (a) pre-formed clasps (b) clasps made of the same material as the denture base same material as the denture base

Figure 17. Polyamide removable partial denture with (a) pre-formed clasps (b) clasps made of the

Using flexible polyamide is indicated in cases of retentive dental fields (Figure 18). Using flexible polyamide is indicated in cases of retentive dental fields (Figure 18). Using flexible polyamide is indicated in cases of retentive dental fields (Figure 18).

When manufacturing polyamidic dentures, the support elements blend in with the rest of the denture, as they are made of the same material [25, 26].

When manufacturing polyamidic dentures, the support elements blend in with the rest of the denture, Figure 18. Removable partial dentures made of a super-flexible polyamide (a) The model with retentive tuberosity embedded in the flask (b) The denture immediately after unwrapping (c, d) The flexible removable partial denture **Figure 18.** Removable partial dentures made of a super-flexible polyamide (a) The model with retentive tuberosity em‐ bedded in the flask (b) The denture immediately after unwrapping (c, d) The flexible removable partial denture

flexible removable partial denture

Figure 19. Medium\_ low flexibility thermoplastic polyamide denture **Figure 19.** Medium-low flexibility thermoplastic polyamide denture Figure 19. Medium\_

as they are made of the same material [25,26].

The superflexible polyamide resin is extremely elastic, virtually unbreakable, monomer-free, lightweight and impervious to oral fluids (Figures 18, 20, 21). The medium-low flexibility polyamide is a half-soft material mainly indicated for removable partial dentures. It offers superior comfort, good esthetics and no metallic taste (Figures 19 and 20). Polishing and adjusting is easy, it can be added to or relined in both dental practice and laboratory. In certain cases we used preformed clasps made of The superflexible polyamide resin is extremely elastic, virtually unbreakable, monomer-free, lightweight and impervious to oral fluids (Figures 18, 20, 21). The medium-low flexibility polyamide is a half-soft material mainly indicated for removable partial dentures. It offers superior comfort, good esthetics and no metallic taste (Figures 19 and 20). Polishing and adjusting is easy, it can be added to or relined in both dental practice and laboratory. In certain cases we used preformed clasps made of nylon. These clasps have the same composition as The superflexible polyamide resin is extremely elastic, virtually unbreakable, monomer-free, lightweight and impervious to oral fluids (Figures 18, 20, 21). The medium-low flexibility polyamide is a half-soft material mainly indicated for removable partial dentures. It offers superior comfort, good esthetics and no metallic taste (Figures 19 and 20). Polishing and adjusting is easy, it can be added to or relined in both dental practice and laboratory. In certain cases we used preformed clasps made of nylon. These clasps have the same composition as the polyamidic resin used for denture

low flexibility thermoplastic polyamide denture

the polyamidic resin used for denture manufacturing, and they are heated in order to adapt (Figure 17a). This kind of clasp can be used for dentures with metal framework, or in associ‐ ation with injected thermoplastic resins [12, 23]. Another option we used was making the clasps of the same thermoplastic resin as the saddles or from acetal resin. nylon. These clasps have the same composition as the polyamidic resin used for denture manufacturing, and they are heated in order to adapt (Figure 17a). This kind of clasp can be used for dentures with metal framework, or in association with injected thermoplastic resins [12,23]. Another option we used was making the clasps of the same thermoplastic resin as the saddles or from acetal

nylon. These clasps have the same composition as the polyamidic resin used for denture manufacturing, and they are heated in order to adapt (Figure 17a). This kind of clasp can be used for dentures with metal framework, or in association with injected thermoplastic resins [12,23]. Another option we used was making the clasps of the same thermoplastic resin as the saddles or from acetal

**Figure 20.** (a) Medium-low flexibility polyamide partial dentures (b) Superflexible polyamide partial dentures Figure 20. (a) Medium-low flexibility polyamide partial dentures (b) Superflexible polyamide partial

Figure 21. (a) The superflexible polyamide denture (b) The final flexibility test

dentures

**3.3 Kemeny**-**type removable partial dentures made of acetal Figure 21.** (a) The superflexible polyamide denture (b) The final flexibility test

resin.

resin.

a. b.

a. b.

c. d.

c. d.

as they are made of the same material [25,26].

158 Thermoplastic Elastomers - Synthesis and Applications

as they are made of the same material [25,26].

Figure 19. Medium\_

**Figure 19.** Medium-low flexibility thermoplastic polyamide denture

Figure 19. Medium\_

Figure 18. Removable partial dentures made of a super-flexible polyamide (a) The model with retentive tuberosity embedded in the flask (b) The denture immediately after unwrapping (c, d) The flexible removable partial denture

When manufacturing polyamidic dentures, the support elements blend in with the rest of the denture,

Figure 18. Removable partial dentures made of a super-flexible polyamide (a) The model with retentive tuberosity embedded in the flask (b) The denture immediately after unwrapping (c, d) The flexible removable partial denture

**Figure 18.** Removable partial dentures made of a super-flexible polyamide (a) The model with retentive tuberosity em‐ bedded in the flask (b) The denture immediately after unwrapping (c, d) The flexible removable partial denture

When manufacturing polyamidic dentures, the support elements blend in with the rest of the denture,

The superflexible polyamide resin is extremely elastic, virtually unbreakable, monomer-free, lightweight and impervious to oral fluids (Figures 18, 20, 21). The medium-low flexibility polyamide is a half-soft material mainly indicated for removable partial dentures. It offers superior comfort, good esthetics and no metallic taste (Figures 19 and 20). Polishing and adjusting is easy, it can be added to or relined in both dental practice and laboratory. In certain cases we used preformed clasps made of

The superflexible polyamide resin is extremely elastic, virtually unbreakable, monomer-free, lightweight and impervious to oral fluids (Figures 18, 20, 21). The medium-low flexibility polyamide is a half-soft material mainly indicated for removable partial dentures. It offers superior comfort, good esthetics and no metallic taste (Figures 19 and 20). Polishing and adjusting is easy, it can be added to or relined in both dental practice and laboratory. In certain cases we used preformed clasps made of nylon. These clasps have the same composition as

The superflexible polyamide resin is extremely elastic, virtually unbreakable, monomer-free, lightweight and impervious to oral fluids (Figures 18, 20, 21). The medium-low flexibility polyamide is a half-soft material mainly indicated for removable partial dentures. It offers superior comfort, good esthetics and no metallic taste (Figures 19 and 20). Polishing and adjusting is easy, it can be added to or relined in both dental practice and laboratory. In certain cases we used preformed clasps made of nylon. These clasps have the same composition as the polyamidic resin used for denture

low flexibility thermoplastic polyamide denture

low flexibility thermoplastic polyamide denture

## **3.3. Kemeny-type removable partial dentures made of acetal**

As an experiment, we managed partial reduced edentations with acetal Kemeny-type dentures (Figures 22 and 23) as an alternative to fixed partial dentures, mainly in order to test the physiognomic aspect, having the advantage of a minimal loss of hard dental substance, located only at the level of the occlusal rims, in case of posterior teeth.

 a. b. Figure 21. (a) The superflexible polyamide denture (b) The final flexibility test **3.3 Kemeny**-**type removable partial dentures made of acetal**  Figure 22 shows wax patterning aspects and manufacturing a molar unidental Kemeny denture of acetal resin, while Figure 23 shows the way in which a frontal bidental edentation can be managed. The effectiveness of the technology is ensured by making artificial teeth of the same material.

the level of the occlusal rims, in case of posterior teeth.

As the material is not translucent, it is mainly suitable for dealing with lateral edentations. It can, however, be used temporarily, in the frontal area as well, in those clinical cases where short-term esthetic aspect is irrelevant [12, 23, 27]. The effectiveness of the technology is ensured by making artificial teeth of the same material. As the material is not translucent, it is mainly suitable for dealing with lateral edentations. It can, however, be used temporarily, in the frontal area as well, in those clinical cases where short-term esthetic aspect is irrelevant [12,23,27]. Figure 22 shows wax patterning aspects and manufacturing a molar unidental Kemeny denture of acetal resin, while Figure 23 shows the way in which a frontal bidental edentation can be managed. The effectiveness of the technology is ensured by making artificial teeth of the same material. As the material is not translucent, it is mainly suitable for dealing with lateral edentations. It can,

however, be used temporarily, in the frontal area as well, in those clinical cases where short-term

Figure 22 shows wax patterning aspects and manufacturing a molar unidental Kemeny denture of acetal resin, while Figure 23 shows the way in which a frontal bidental edentation can be managed.

physiognomic aspect, having the advantage of a minimal loss of hard dental substance, located only at

As an experiment, we managed partial reduced edentations with acetal Kemeny-type dentures (Figures 22 and 23) as an alternative to fixed partial dentures, mainly in order to test the physiognomic aspect, having the advantage of a minimal loss of hard dental substance, located only at

Figure 22. Kemeny dentures: (a) Unimolar denture wax pattern (b) Denture made of acetal resin

Figure 22. Kemeny dentures: (a) Unimolar denture wax pattern (b) Denture made of acetal resin

**Figure 22.** Kemeny dentures: (a) Unimolar denture wax pattern (b) Denture made of acetal resin a. b.

Thermoplastic resins are also indicated for manufacturing antisnoring devices, different types of Figure 23. Kemeny-type frontal denture **Figure 23.** Kemeny-type frontal denture

#### manufactured acetalic resin splints (Figure 24) which turned out to be a viable solution because it matches the color of the teeth and thereby represents a temporary postoperative esthetic choice **3.4 Splints made of acetal resin 3.4. Splints made of acetal resin**

[18,23]. Thermoplastic resins are also indicated for manufacturing antisnoring devices, different types of mouthguards and splints. Parodonthotic teeth after surgery need immobilization. We experimentally manufactured acetalic resin splints (Figure 24) which turned out to be a viable solution because it matches the color of the teeth and thereby represents a temporary postoperative esthetic choice [18,23]. Thermoplastic resins are also indicated for manufacturing antisnoring devices, different types of mouthguards and splints. Parodonthotic teeth after surgery need immobilization. We experimentally manufactured acetalic resin splints (Figure 24) which turned out to be a viable solution because it matches the color of the teeth and thereby represents a temporary postop‐ erative esthetic choice [18, 23].

mouthguards and splints. Parodonthotic teeth after surgery need immobilization. We experimentally

## **3.5. Mouth guards**

Mouth guards are dental appliances that can be manufactured using thermoplastic resins. The most satisfactory mouth protectors are custom-made mouth guards. This type of mouth guards is designed by the dentist. They adapt well and provide good retention and comfort. Being custom-made they interfere the least with speaking and have virtually no effect on breathing

Mouth guards are dental appliances that can be manufactured using thermoplastic resins. Figure 24. Thermoplastic acetal splint **Figure 24.** Thermoplastic acetal splint

feel (Figure 25).

As the material is not translucent, it is mainly suitable for dealing with lateral edentations. It can, however, be used temporarily, in the frontal area as well, in those clinical cases where

Figure 22 shows wax patterning aspects and manufacturing a molar unidental Kemeny denture of acetal resin, while Figure 23 shows the way in which a frontal bidental edentation can be managed. The effectiveness of the technology is ensured by making artificial teeth of the same material. As the material is not translucent, it is mainly suitable for dealing with lateral edentations. It can, however, be used temporarily, in the frontal area as well, in those clinical cases where short-term

Figure 22. Kemeny dentures: (a) Unimolar denture wax pattern (b) Denture made of acetal resin

Figure 22. Kemeny dentures: (a) Unimolar denture wax pattern (b) Denture made of acetal resin

Figure 23. Kemeny-type frontal denture

Thermoplastic resins are also indicated for manufacturing antisnoring devices, different types of mouthguards and splints. Parodonthotic teeth after surgery need immobilization. We experimentally manufactured acetalic resin splints (Figure 24) which turned out to be a viable solution because it matches the color of the teeth and thereby represents a temporary postoperative esthetic choice

Thermoplastic resins are also indicated for manufacturing antisnoring devices, different types of mouthguards and splints. Parodonthotic teeth after surgery need immobilization. We experimentally manufactured acetalic resin splints (Figure 24) which turned out to be a viable solution because it matches the color of the teeth and thereby represents a temporary postoperative esthetic choice

Thermoplastic resins are also indicated for manufacturing antisnoring devices, different types of mouthguards and splints. Parodonthotic teeth after surgery need immobilization. We experimentally manufactured acetalic resin splints (Figure 24) which turned out to be a viable solution because it matches the color of the teeth and thereby represents a temporary postop‐

Mouth guards are dental appliances that can be manufactured using thermoplastic resins. The most satisfactory mouth protectors are custom-made mouth guards. This type of mouth guards is designed by the dentist. They adapt well and provide good retention and comfort. Being custom-made they interfere the least with speaking and have virtually no effect on breathing

Figure 23. Kemeny-type frontal denture

Figure 22 shows wax patterning aspects and manufacturing a molar unidental Kemeny denture of acetal resin, while Figure 23 shows the way in which a frontal bidental edentation can be managed. The effectiveness of the technology is ensured by making artificial teeth of the same material. As the material is not translucent, it is mainly suitable for dealing with lateral edentations. It can, however, be used temporarily, in the frontal area as well, in those clinical cases where short-term

As an experiment, we managed partial reduced edentations with acetal Kemeny-type dentures (Figures 22 and 23) as an alternative to fixed partial dentures, mainly in order to test the physiognomic aspect, having the advantage of a minimal loss of hard dental substance, located only at

As an experiment, we managed partial reduced edentations with acetal Kemeny-type dentures (Figures 22 and 23) as an alternative to fixed partial dentures, mainly in order to test the physiognomic aspect, having the advantage of a minimal loss of hard dental substance, located only at

short-term esthetic aspect is irrelevant [12, 23, 27].

a. b.

a. b.

**Figure 22.** Kemeny dentures: (a) Unimolar denture wax pattern (b) Denture made of acetal resin

esthetic aspect is irrelevant [12,23,27].

160 Thermoplastic Elastomers - Synthesis and Applications

esthetic aspect is irrelevant [12,23,27].

**3.4 Splints made of acetal resin** 

**Figure 23.** Kemeny-type frontal denture

**3.4 Splints made of acetal resin** 

erative esthetic choice [18, 23].

**3.5. Mouth guards**

**3.4. Splints made of acetal resin**

[18,23].

[18,23].

the level of the occlusal rims, in case of posterior teeth.

the level of the occlusal rims, in case of posterior teeth.

[28]. Custom-made mouth guards may be classified into two types: the vacuum mouth guard and the pressure-laminated mouth guard. The most satisfactory mouth protectors are custom-made mouth guards. This type of mouth guards is designed by the dentist. They adapt well and provide good retention and comfort. Being custom-made they interfere the least with speaking and have virtually no effect on breathing [28]. Custom-made **3.5 Mouth guards**  Mouth guards are dental appliances that can be manufactured using thermoplastic resins.

The vacuum mouth guard is manufactured using a model of the upper arch. The model is casted using an impression. The thermoplastic mouth guard material, usually a polyethylene vinyl acetate (EVA) copolymer, is adapted over the model with a special vacuum machine. The vacuum mouth guard is then trimmed and polished to allow for proper tooth and gum adaptation. All posterior teeth should be covered and muscle attachments should be unim‐ pinged. Using a vacuum machine single-layer mouth guards are manufactured. More and more, multi- ple-layer mouth guards (laboratory pressure-laminated) are preferred to the single-layer vacuum ones. The laboratory pressure-laminated mouth guard, also made from a stone cast, is a custom-made multiple-layered mouth guard that is considered the state-ofthe-art mouth guard in for years. It can be made by laminating two or three layers of material to achieve the necessary thickness. Lamination is defined as the layering of mouth guard material using high heat and pressure machines. The mouth guard material should be biocompatible, have good physical properties, and last for at least 2 years [29]. mouth guards may be classified into two types: the vacuum mouth guard and the pressure-laminated mouth guard. The vacuum mouth guard is manufactured using a model of the upper arch. The model is casted using an impression. The thermoplastic mouth guard material, usually a polyethylene vinyl acetate (EVA) copolymer, is adapted over the model with a special vacuum machine. The vacuum mouth guard is then trimmed and polished to allow for proper tooth and gum adaptation. All posterior teeth should be covered and muscle attachments should be unimpinged. Using a vacuum machine singlelayer mouth guards are manufactured. More and more, multi- ple-layer mouth guards (laboratory pressure-laminated) are preferred to the single-layer vacuum ones. The laboratory pressure-laminated mouth guard, also made from a stone cast, is a custom-made multiple-layered mouth guard that is considered the state-of-the-art mouth guard in for years. It can be made by laminating two or three layers of material to achieve the necessary thickness. Lamination is defined as the layering of mouth guard material using high heat and pressure machines. The mouth guard material should be biocompatible, have good physical properties, and last for at least 2 years [29]. We manufactured laminated custom-made mouth guards for 3 layers, with the inner layer made of a thermoplastic polyurethane which increases discoloration resistance and creates a soft inner surface The most satisfactory mouth protectors are custom-made mouth guards. This type of mouth guards is designed by the dentist. They adapt well and provide good retention and comfort. Being custom-made they interfere the least with speaking and have virtually no effect on breathing [28]. Custom-made mouth guards may be classified into two types: the vacuum mouth guard and the pressure-laminated mouth guard. The vacuum mouth guard is manufactured using a model of the upper arch. The model is casted using an impression. The thermoplastic mouth guard material, usually a polyethylene vinyl acetate (EVA) copolymer, is adapted over the model with a special vacuum machine. The vacuum mouth guard is then trimmed and polished to allow for proper tooth and gum adaptation. All posterior teeth should be covered and muscle attachments should be unimpinged. Using a vacuum machine singlelayer mouth guards are manufactured. More and more, multi- ple-layer mouth guards (laboratory pressure-laminated) are preferred to the single-layer vacuum ones. The laboratory pressure-laminated mouth guard, also made from a stone cast, is a custom-made multiple-layered mouth guard that is considered the state-of-the-art mouth guard in for years. It can be made by laminating two or three layers of material to achieve the necessary thickness. Lamination is defined as the layering of mouth guard material using high heat and pressure machines. The mouth guard material should be

Figure 25. Laminating the custom-made mouth guard. Custom made mouth guard **Figure 25.** Laminating the custom-made mouth guard. Custom made mouth guard

biocompatible, have good physical properties, and last for at least 2 years [29].

We manufactured laminated custom-made mouth guards for 3 layers, with the inner layer made of a thermoplastic polyurethane which increases discoloration resistance and creates a soft inner surface feel (Figure 25). Figure 25. Laminating the custom-made mouth guard. Custom made mouth guard **3.6 Myofunctional therapy devices** 

## **3.6. Myofunctional therapy devices**

Controlling dentofacial growth interferences is an important issue. The negative effects of mouth breathing, abnormal lip and tongue function and incorrect swallowing patterns on craniofacial development in the mixed dentition period is well known. Correcting these myofunctional habits improves craniofacial growth and decreases the severity of malocclu‐ sion [12].

Myofunctional therapy retrains the muscles of swallowing, synchronizes the swallowing movements obtaining a normal resting posture of the tongue, lips, and jaw. Myofunctional therapy may be rescheduled before, during or after orthodontic treatment [30]. The most typical age range for this type of therapy is between 8 and 16 years.

The main objective of the myofunctional appliances is to eliminate oral dysfunction and to establish muscular balance. These appliances play a certain role in orthodontics because they are simple and economical. The selection of the cases needs to be thorough and the specialist needs to be well trained in their use.

The universal size products, suitable for children between 6 and 11 years old (mixed dentition stage), allow implementing the orthodontic treatment earlier and at lower cost. These are made of a flexible thermoplastic silicone polycarbonate-urethane, a ground-breaking copolymer that combines the biocompatibility and biostability of conventional silicone elastomers with the processability and toughness of thermoplastic polycarbonate-urethanes. This type of appli‐ ances has good in vitro and in vivo stability. Its strength is comparable to traditional polycar‐ bonate-urethanes, and the biostability is due to the silicone soft segment and end groups. Various fabrication techniques may be used in order to obtain to different. Additional surface processing after fabrication is not needed [12].

## **4. Errors in manufacturing thermoplastic resins dentures**

Errors might occur when manufacturing thermoplastic resins dentures: the insufficient pressure at injection, which leads to lack of substance, poor polishing, or too thick saddles being some of the causes (Figure 26). These errors lead to deficiencies of the denture, which might be unusable because of esthetic deficiencies, occlusal dysmorphia, exaggerated elastic‐ ity, and decubitus areas [31].

## **5. Conclusions**

Thermoplastics used in dentistry have known a great diversification in the last years. Proc‐ essing principles are similar to the injecting technology of chemoplastics, the main difference consisting in their chemical composition, liquefying temperature of grains, injecting pressure and the fact that thermoplastic resins are monocomponent.

Errors might occur when manufacturing thermoplastic resins dentures: the insufficient pressure at injection, which leads to lack of substance, poor polishing, or too thick saddles being some of the

Controlling dentofacial growth interferences is an important issue. The negative effects of mouth breathing, abnormal lip and tongue function and incorrect swallowing patterns on craniofacial development in the mixed dentition period is well known. Correcting these myofunctional habits

Myofunctional therapy retrains the muscles of swallowing, synchronizes the swallowing movements obtaining a normal resting posture of the tongue, lips, and jaw. Myofunctional therapy may be rescheduled before, during or after orthodontic treatment [30]. The most typical age range for this

The main objective of the myofunctional appliances is to eliminate oral dysfunction and to establish muscular balance. These appliances play a certain role in orthodontics because they are simple and economical. The selection of the cases needs to be thorough and the specialist needs to be well trained

The universal size products, suitable for children between 6 and 11 years old (mixed dentition stage), allow implementing the orthodontic treatment earlier and at lower cost. These are made of a flexible thermoplastic silicone polycarbonate-urethane, a ground-breaking copolymer that combines the biocompatibility and biostability of conventional silicone elastomers with the processability and toughness of thermoplastic polycarbonate-urethanes. This type of appliances has good in vitro and in vivo stability. Its strength is comparable to traditional polycarbonate-urethanes, and the biostability is due to the silicone soft segment and end groups. Various fabrication techniques may be used in order

to obtain to different. Additional surface processing after fabrication is not needed [12].

**4. Errors in manufacturing thermoplastic resins dentures** 

improves craniofacial growth and decreases the severity of malocclusion [12].

**3.6 Myofunctional therapy devices** 

type of therapy is between 8 and 16 years.

in their use.

**3.6. Myofunctional therapy devices**

162 Thermoplastic Elastomers - Synthesis and Applications

needs to be well trained in their use.

processing after fabrication is not needed [12].

ity, and decubitus areas [31].

**5. Conclusions**

sion [12].

Controlling dentofacial growth interferences is an important issue. The negative effects of mouth breathing, abnormal lip and tongue function and incorrect swallowing patterns on craniofacial development in the mixed dentition period is well known. Correcting these myofunctional habits improves craniofacial growth and decreases the severity of malocclu‐

Myofunctional therapy retrains the muscles of swallowing, synchronizes the swallowing movements obtaining a normal resting posture of the tongue, lips, and jaw. Myofunctional therapy may be rescheduled before, during or after orthodontic treatment [30]. The most

The main objective of the myofunctional appliances is to eliminate oral dysfunction and to establish muscular balance. These appliances play a certain role in orthodontics because they are simple and economical. The selection of the cases needs to be thorough and the specialist

The universal size products, suitable for children between 6 and 11 years old (mixed dentition stage), allow implementing the orthodontic treatment earlier and at lower cost. These are made of a flexible thermoplastic silicone polycarbonate-urethane, a ground-breaking copolymer that combines the biocompatibility and biostability of conventional silicone elastomers with the processability and toughness of thermoplastic polycarbonate-urethanes. This type of appli‐ ances has good in vitro and in vivo stability. Its strength is comparable to traditional polycar‐ bonate-urethanes, and the biostability is due to the silicone soft segment and end groups. Various fabrication techniques may be used in order to obtain to different. Additional surface

Errors might occur when manufacturing thermoplastic resins dentures: the insufficient pressure at injection, which leads to lack of substance, poor polishing, or too thick saddles being some of the causes (Figure 26). These errors lead to deficiencies of the denture, which might be unusable because of esthetic deficiencies, occlusal dysmorphia, exaggerated elastic‐

Thermoplastics used in dentistry have known a great diversification in the last years. Proc‐ essing principles are similar to the injecting technology of chemoplastics, the main difference consisting in their chemical composition, liquefying temperature of grains, injecting pressure

typical age range for this type of therapy is between 8 and 16 years.

**4. Errors in manufacturing thermoplastic resins dentures**

and the fact that thermoplastic resins are monocomponent.

Figure 26. Errors that might occur when manufacturing dentures from thermoplastic resins (a) Lack of substance (b) Poor polishing **Figure 26.** Errors that might occur when manufacturing dentures from thermoplastic resins (a) Lack of substance (b) Poor polishing

Processing technology is based on the thermal plasticization of the material, in the absence of any chemical reaction. The technology of injection molding is not widely used in dental technique labs yet, as it requires special devices, but has opened new perspectives in the technology of total and partial removable dentures. **5. Conclusions**  Thermoplastics used in dentistry have known a great diversification in the last years. Processing principles are similar to the injecting technology of chemoplastics, the main difference consisting in

Solving partial edentations with metal-free removable partial dentures represents a modern alternative solution to classical metal framework dentures, having the advantage of being lightweight, flexible and much more comfortable for the patient. Metal-free removable partial dentures made of thermoplastic materials are biocompatible, nonirritant, sure, nontoxic, biologically inert, with superior esthetics, which make them rapidly integrate in dentomaxil‐ lary structure. They offer quality static and dynamic stability. their chemical composition, liquefying temperature of grains, injecting pressure and the fact that thermoplastic resins are monocomponent.

> The clasps are made of the same material as the denture base or ready-made clasps from the same material may be used. Where the mechanical resistance of the structure comes first, the choice is an acetal resin for making the framework. Superflexible polyamide resin is especially indicated for retentive dental fields, which would normally create problems with the insertion and disinsertion of the removable partial dentures. The removable partial dentures with acetal resin frame are the most laborious to manufacture, requiring most working steps. Manufac‐ turing the acetal framework is first, followed by the acrylic saddles and artificial teeth. A removable partial denture with an acetal resin frame is rapidly integrated into the dentomax‐ illary system and accepted by the patient. Such a removable denture is a comfortable solution for the partial edentulous patient, achieving the principles of static and dynamic maintenance and stability. These types of partial dentures are not bulky, the frameworks being 0.3-0.5 mm thin, and clasps are flexible and esthetic.

> A particular advantage of a removable partial denture made of acetal resin applies to patients with large oral defects as a result of a maxillectomy procedure, who are due to have postop‐ erative radiotherapy and need to have the density of the defect restored to ensure standardized radiation distribution. Different types of boluses may be used for restoration but a stent is

usually needed as a support. Traditional metal-clasp retained stents are discarded in such cases as the clasps cause backscatter of the radiation beams. Acetal resin is a radiolucent material suitable for making a stent with clasps or even a removable partial denture to retain the bolus.

In the case of Kemeny-type acetalic dentures, the artificial teeth are made of the same material and in the same step as the rest of the denture. Because it is not translucent, its first indication is lateral edentations but it can be used for short periods, in the frontal area as well, if shortterm esthetic aspect is not important.

Thermoplastic resins have several advantages: long-term performance, stability, resistance to deformation, resistance to wear, excellent tolerance, resistance to solvents, absence or low quantity of allergy-inducing residual monomer, and lack of porosity, thus preventing the development of microorganisms and deposits, all of which, together with maintaining size and color in time are very important characteristics, presenting a high degree of flexibility and resistance, permitting the addition of elastomers for increased elasticity or reinforcement with fiberglass, in order to increase their physical splinter quality; some of them can also be repaired or rebased.

The advantages of using the molding-injection system lay in the fact that the resin is delivered in a cartridge, thus excluding mixture errors with long-term shape stability, reduces contrac‐ tion, and gives mechanical resistance to ageing.

As this class of materials, as well as the processing devices, has been continuously perfected, their future applicability in dental medicine will keep spreading.

Most probably, further chemical development of elastomeric and polymeric materials will enlarge the domain of clinical applications of thermoplastics in dentistry.

## **Author details**

Lavinia Ardelean1\*, Cristina Maria Bortun2 , Angela Codruta Podariu3 and Laura Cristina Rusu1

\*Address all correspondence to: lavinia-ardelean@umft.ro

1 Department of Technology of Dental Materials and Devices in Dental Medicine, "Victor Babes" University of Medicine and Pharmacy, Timisoara, Romania

2 Department of Dentures Technology, "Victor Babes" University of Medicine and Pharma‐ cy, Timisoara, Romania

3 Department for Preventive Dentistry, Community Dentistry and Oral Health, "Victor Babes" University of Medicine and Pharmacy, Timisoara, Romania

## **References**

usually needed as a support. Traditional metal-clasp retained stents are discarded in such cases as the clasps cause backscatter of the radiation beams. Acetal resin is a radiolucent material suitable for making a stent with clasps or even a removable partial denture to retain the bolus.

In the case of Kemeny-type acetalic dentures, the artificial teeth are made of the same material and in the same step as the rest of the denture. Because it is not translucent, its first indication is lateral edentations but it can be used for short periods, in the frontal area as well, if short-

Thermoplastic resins have several advantages: long-term performance, stability, resistance to deformation, resistance to wear, excellent tolerance, resistance to solvents, absence or low quantity of allergy-inducing residual monomer, and lack of porosity, thus preventing the development of microorganisms and deposits, all of which, together with maintaining size and color in time are very important characteristics, presenting a high degree of flexibility and resistance, permitting the addition of elastomers for increased elasticity or reinforcement with fiberglass, in order to increase their physical splinter quality; some of them can also be repaired

The advantages of using the molding-injection system lay in the fact that the resin is delivered in a cartridge, thus excluding mixture errors with long-term shape stability, reduces contrac‐

As this class of materials, as well as the processing devices, has been continuously perfected,

Most probably, further chemical development of elastomeric and polymeric materials will

1 Department of Technology of Dental Materials and Devices in Dental Medicine, "Victor

2 Department of Dentures Technology, "Victor Babes" University of Medicine and Pharma‐

3 Department for Preventive Dentistry, Community Dentistry and Oral Health, "Victor

, Angela Codruta Podariu3

and

term esthetic aspect is not important.

164 Thermoplastic Elastomers - Synthesis and Applications

tion, and gives mechanical resistance to ageing.

Lavinia Ardelean1\*, Cristina Maria Bortun2

\*Address all correspondence to: lavinia-ardelean@umft.ro

Babes" University of Medicine and Pharmacy, Timisoara, Romania

Babes" University of Medicine and Pharmacy, Timisoara, Romania

their future applicability in dental medicine will keep spreading.

enlarge the domain of clinical applications of thermoplastics in dentistry.

or rebased.

**Author details**

Laura Cristina Rusu1

cy, Timisoara, Romania


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## *Edited by Chapal Kumar Das*

Thermoplastic elastomers (TPEs), commonly known as thermoplastic rubbers, are a category of copolymers having thermoplastic and elastomeric characteristics. A TPE is a rubbery material with properties very close to those of conventional vulcanized rubber at normal conditions. It can be processed in a molten state even at elevated temperatures. TPEs show advantages typical of both rubbery materials and plastic materials. TPEs are a class of polymers bridging between the service properties of elastomers and the processing properties of thermoplastics. Nowadays, the best use of thermoplastics is in the field of biomedical applications, starting from artificial skin to many of the artificial human body parts. Apart from these, thermoplastic elastomers are being used for drug encapsulation purposes, and since they are biocompatible in many cases, their scope of applications has been broadened in the biotechnological field as well. The present book highlights many biological and biomedical applications of TPEs from which the broader area readers will benefit.

Thermoplastic Elastomers - Synthesis and Applications

Thermoplastic Elastomers

Synthesis and Applications

*Edited by Chapal Kumar Das*

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