**4. Alternative resins and technologies for denture manufacturing**

Our experience in denture manufacturing includes a variety of alternative resins and manufacturing technologies such as self-curing acrylics manufactured by casting, different thermoplastic resins manufactured by injection, light-cured diacrylic and urethane-based resins and poly(ether ether ketone) (PEEK) high-performance polymers manufactured by milling.

### **4.1. Self-curing acrylics manufactured by casting**

matically converted into pg/mL for each of the sample tested. Determination of TNF-alpha concentration in the first case revealed slightly increased values of this cytokine (50.48–50.7 pg/mL). In the second case, the values were higher (90.3–90.9 pg/mL) and in the third case the values were the highest (100.4–107 pg/mL), which indicate certain inflammation. The results obtained with TNF-alpha test are eloquent for cytotoxic detection of early signs of allergy and inflammation by measuring the endotoxins from the patient serum. It is always recommended to associate multiple types of clinical tests and these should always be histologically

Three acrylic resin samples were used, each one being harvested from a different full denture base. The samples were preconditioned at 80°C for 2 h for removing moist. The weight of the samples is S1: 0.7863, S2: 0.05638 and S3: 0.8421 g. A Petri box is sterilized at 150°C for 1 h and then weighed in the analytical scale with high precision. The samples are positioned in the Petri box and weighed again very accurately. They are kept in the oven for 10 h at 150°C, and weighed again, very precisely. The weight difference is given by the amount of existing residual monomer: S1: 0.7864 g, S2: 0.5640 g, S3: 0.8422 g. In all these three cases, no significant weight loss was noticed. These results conclude that almost no residual monomer was found

In order to verify the above results, we have used the bromine index method which determines the percentage of monomer in the sample, based on the amount of bromine added to the double bond (C=C) links. All the three samples were found to be free from the residual

confirmed [16].

**3.2.** *In vitro* **testing**

14 Acrylic Polymers in Healthcare

*3.2.1. Volatile-component content method*

**Figure 15.** Results reading for TNF alpha test.

in the three full dentures [14].

*3.2.2. Bromine index method*

Full-denture casting represents one choice to classic heat-curing acrylic dentures. Self-curing acrylic resins suitable for casting belong to type 2, group 2 of acrylic resins (**Table 1**). For polymerization, temperatures below 65°C are used. The acrylic paste was previously prepared in a texture suitable for casting and then poured in a special flask. The mould is made up of either reversible hydrocolloid or silicone (**Figure 16**), compared to the classic plaster mould used for investing heat-cured acrylic dentures. The casting system has the following advantages: the reversible hydrocolloid can be reused and the polymerization time is shorter. There are wide colour ranges (10 colours) of the acrylics and minimal adjustments are required. The most common errors when using this technology are consequence of bubbles forming when pouring the mould. This causes porosity of the mucosal surface and lack of substance due to the fast setting of the resin. Therefore, great skill is required when pouring the mould. Porosity may also occur due to improper preparing of the acrylate [4, 18].

**Figure 16.** The mould made up of (a) reversible hydrocolloids and (b) silicone, before pouring the acrylic paste.

#### **4.2. Thermoplastic resins manufactured by injection**

Compared to classical self- or heat-curing acrylates, thermoplastic resins have a number of advantages as follows: a very good long-term performance, maintaining their size and colour in time, stability, resistance to deformation, wear and solvents, very good tolerance due to the absence or reduced quantity of residual monomer, responsible for allergies in a lot of patients, no porosity which prevents development of microorganisms and deposits [4].

The advantages of the injecting system lie in the fact that the resin is delivered in a cartridge (**Figure 17a**) which eliminates dosage errors, guaranteeing long-term stability of the shape, reduced contraction, as well as mechanical resistance with ageing. The disadvantages are mainly the consequence of the high cost of the injection device (**Figure 17b**) and of the materials to be used.

The processing technology implies the thermal plasticization of the material, in the absence of any chemical reaction and injection of the plasticized resins into a mould [3].

Advantages of thermoplastic resins include removable partial dentures, preformed clasps, partial denture frameworks, temporary prosthetic restorations, full dentures, orthodontic appliances, anti-snoring devices, mouth guards and splints.

Thermoplastic resins include acetal, polycarbonate (polyesters group), acrylates and polyamides (nylons) [19].

Metal-free removable partial dentures made up of thermoplastic materials represent a modern alternative solution to the classical metal framework dentures. These have the advantages of being lightweight, flexible and much more comfortable to the patient. These are also biocompatible, non-irritant, sure, non-toxic, biologically inert, superior aesthetics and also offer quality static and dynamic stability [5]. The clasps are made up of the same material as the denture base or are readily-made from the same material. Though the mechanical resistance is most important, the first choice for manufacturing the framework is an acetal resin. The removable partial dentures with acetal resin framework are the most laborious to manufacture. The acetal framework was being manufactured first, followed by the acrylic saddles and artificial teeth (**Figure 18**).

These types of partial dentures have thin frameworks, with flexible and aesthetic clasps [20].

Thermoplastic polyamide (nylon) is a versatile material, with high flexibility, physical strength, heat and chemical resistance. The super flexible polyamide is extremely elastic, virtually unbreakable, lightweight and impervious to oral fluids. The medium-low flexibility polyamide is a half-soft material which offers superior comfort, good aesthetics and could be

**Figure 17.** (a) The cartridge of thermoplastic material and (b) the injection unit MG-Newpress (Quattroty).

**Figure 18.** Partial denture with acetal framework, clasps and acrylic saddles.

in time, stability, resistance to deformation, wear and solvents, very good tolerance due to the absence or reduced quantity of residual monomer, responsible for allergies in a lot of patients,

The advantages of the injecting system lie in the fact that the resin is delivered in a cartridge (**Figure 17a**) which eliminates dosage errors, guaranteeing long-term stability of the shape, reduced contraction, as well as mechanical resistance with ageing. The disadvantages are mainly the consequence of the high cost of the injection device (**Figure 17b**) and of the materials

The processing technology implies the thermal plasticization of the material, in the absence of

Advantages of thermoplastic resins include removable partial dentures, preformed clasps, partial denture frameworks, temporary prosthetic restorations, full dentures, orthodontic

Thermoplastic resins include acetal, polycarbonate (polyesters group), acrylates and polyam-

Metal-free removable partial dentures made up of thermoplastic materials represent a modern alternative solution to the classical metal framework dentures. These have the advantages of being lightweight, flexible and much more comfortable to the patient. These are also biocompatible, non-irritant, sure, non-toxic, biologically inert, superior aesthetics and also offer quality static and dynamic stability [5]. The clasps are made up of the same material as the denture base or are readily-made from the same material. Though the mechanical resistance is most important, the first choice for manufacturing the framework is an acetal resin. The removable partial dentures with acetal resin framework are the most laborious to manufacture. The acetal framework was

being manufactured first, followed by the acrylic saddles and artificial teeth (**Figure 18**).

**Figure 17.** (a) The cartridge of thermoplastic material and (b) the injection unit MG-Newpress (Quattroty).

These types of partial dentures have thin frameworks, with flexible and aesthetic clasps [20]. Thermoplastic polyamide (nylon) is a versatile material, with high flexibility, physical strength, heat and chemical resistance. The super flexible polyamide is extremely elastic, virtually unbreakable, lightweight and impervious to oral fluids. The medium-low flexibility polyamide is a half-soft material which offers superior comfort, good aesthetics and could be

no porosity which prevents development of microorganisms and deposits [4].

any chemical reaction and injection of the plasticized resins into a mould [3].

appliances, anti-snoring devices, mouth guards and splints.

to be used.

16 Acrylic Polymers in Healthcare

ides (nylons) [19].

used for removable partial dentures. Super flexible polyamide is especially useful for retentive dental fields, which would normally create problems with the insertion and disinsertion of the removable partial dentures. The clasps are made up of the same material as the denture base in the case of super flexible polyamide dentures. In the case of medium-low flexibility polyamide dentures, ready-made clasps are an option. Metal clasps may also be used [21] (**Figure 19**).

Polycarbonate resins are particularly polyester materials. These have good fracture strength and flexibility, a natural translucency, but the wear resistance is lower than acetal resins and are not recommended for partial denture frameworks. The finishing is very good, which makes them suitable for temporary prosthetic restorations.

Thermoplastic acrylate has the highest impact rating of any acrylic, has long-term stability, its surface structure being dense and smooth. This material was developed for manufacturing complete dentures. It is not elastic, but its flexibility makes it practically unbreakable as one can bounce such a denture off the floor without cracking the base. The biocompatibility is very good due to the absence of residual monomer. The denture has very good long-term stability because water retention is limited.

**Figure 19.** Removable partial denture made up of superflexible polyamide with metal clasps.

#### **4.3. Light-cured diacrylic and urethane-based resins**

Diacrylic composite resins are complex materials. Initially elaborated as aesthetic restorative materials, these developed a lot. The advantages of prosthetics are as follows: veneering of metal-polymeric fixed dentures, single-tooth or temporary crowns, inlays, onlays, epitheses, repairing damaged porcelain veneers, artificial teeth, base of removable dentures and repairing removable dentures. Compared to acrylics, these have a lower shrinkage during poly-merization and have superior physico-mechanical and chemical resistance. Diacrylic composite resins can be made using self-, heat or light-curing, in some cases several curing methods being combined. Light-curing diacrylic resins are successfully used in dental laboratories, especially for veneering, having the advantage of prolonged handling time. In addition, these physico-chemically adheres to the metallic framework, have good colour stability in time and a special aesthetic effect, in part due to the wide selection of shades available for veneering. These can be easily repaired after fixing in the oral cavity, if needed [1].

The absence of methyl, ethyl, propyl and butyl groups in urethane-based resin composition does not generate contact allergies. The light-curing Eclipse Resin System (Dentsply-DeguDent) allows a rapid manufacturing of full dentures, eliminating some time-consuming intermediate steps, like investing and heat-curing. The light-curing resins of Eclipse Resin System contain aliphatic urethane dimethacrylate-urethane oligomers (UDMA) as base monomers and acrylic copolymers, an inorganic submicronic silica filling, a light-curing initiating system and additives. The system consists of three types of resins, which can be handled like wax (base plate, set-up and contour resins). The light-curing protocol was made available by the producer in many variants, which correspond to different technical procedures. The system is extremely efficient, a complete denture base may be ready in 30 min, after master model complete setting. The 'wax-up' is practically made on the denture's polymerized base and, after checking it, the rest of the pattern (saddles) was light-cured. Thereafter, the denture was finished [6] (**Figure 20**).

**Figure 20.** (a) Base plate resin before light-curing, (b) teeth mounting using set-up resin, (c) contour resin processed using the warm air gun and (d) light-curing unit in use.

## **4.4. High-performance polymers, manufactured by milling**

**4.3. Light-cured diacrylic and urethane-based resins**

18 Acrylic Polymers in Healthcare

was finished [6] (**Figure 20**).

using the warm air gun and (d) light-curing unit in use.

Diacrylic composite resins are complex materials. Initially elaborated as aesthetic restorative materials, these developed a lot. The advantages of prosthetics are as follows: veneering of metal-polymeric fixed dentures, single-tooth or temporary crowns, inlays, onlays, epitheses, repairing damaged porcelain veneers, artificial teeth, base of removable dentures and repairing removable dentures. Compared to acrylics, these have a lower shrinkage during poly-merization and have superior physico-mechanical and chemical resistance. Diacrylic composite resins can be made using self-, heat or light-curing, in some cases several curing methods being combined. Light-curing diacrylic resins are successfully used in dental laboratories, especially for veneering, having the advantage of prolonged handling time. In addition, these physico-chemically adheres to the metallic framework, have good colour stability in time and a special aesthetic effect, in part due to the wide selection of shades available for

veneering. These can be easily repaired after fixing in the oral cavity, if needed [1].

The absence of methyl, ethyl, propyl and butyl groups in urethane-based resin composition does not generate contact allergies. The light-curing Eclipse Resin System (Dentsply-DeguDent) allows a rapid manufacturing of full dentures, eliminating some time-consuming intermediate steps, like investing and heat-curing. The light-curing resins of Eclipse Resin System contain aliphatic urethane dimethacrylate-urethane oligomers (UDMA) as base monomers and acrylic copolymers, an inorganic submicronic silica filling, a light-curing initiating system and additives. The system consists of three types of resins, which can be handled like wax (base plate, set-up and contour resins). The light-curing protocol was made available by the producer in many variants, which correspond to different technical procedures. The system is extremely efficient, a complete denture base may be ready in 30 min, after master model complete setting. The 'wax-up' is practically made on the denture's polymerized base and, after checking it, the rest of the pattern (saddles) was light-cured. Thereafter, the denture

**Figure 20.** (a) Base plate resin before light-curing, (b) teeth mounting using set-up resin, (c) contour resin processed

Poly(ether ether ketone) (PEEK) is a high performance polymer used in dentistry since 2002. PEEK is a biocompatible thermoplastic material, with superior properties such as mechanical properties, resistance to wear and fracture and elasticity comparable to bone. PEEK for dental use may be optimized by adding ceramic 0.3–0.5 μm particles. It is resistant to high temperature, good stability, easy to be polished properly, insoluble in water and ideal for allergic patients. The material (grains) may be injected at 400°C or milled (disks) using a CAD/CAM system (**Figure 21a**). It is indicated for crowns and bridges (replacing the metal framework), abutments and removable partial dentures including precision attachments [22, 23].

A removable partial denture framework made up of Juvora PEEK (Invibio) (including clasps), using the Exocad CAD and Coritec 450i (imes-icore) CAM, weights only 1.36 g (**Figures 21b** and **22a**). The saddles are made of acrylate having the entire denture weights only 3.36 g (**Figure 22b**).

**Figure 21.** (a) PEEK milling disk and (b) PEEK framework on a weighting scale.

**Figure 22.** (a) PEEK framework and (b) finished denture.
