**Devulcanization of Elastomers and Applications**

## Fabiula Danielli Bastos de Sousa

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.68585

#### **Abstract**

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208 Elastomers

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In no other phase of human development was produced such amount of waste as currently. Its composition and quantity are directly related to the way the population lives, socioeconomic condition and the ease of access to consumer goods. The irregular disposition of such waste may cause harmful environmental impacts. One of the most dangerous solid wastes is the vulcanized rubber, which, besides having high natural degradation time, has many chemical additives on its formulation and the possibility to store rain water when disposed in landfills and may become a breeding place for vectors. So, recycling comes against this problem and devulcanization is a way of recycling that restores the fluidity of the rubber. One of the applications of this devulcanized rubber is in the production of polymeric blends. Devulcanization of rubbers, the application of this material in polymeric blends based on thermoplastic/recycled rubber, and the parameters involved during the processing of these materials will be addressed in this work.

**Keywords:** sustainability, recycling, devulcanization, revulcanization, polymer blends

## **1. Introduction**

Elastomers consist in a class of materials widely used today in many fields of application. Their typical properties such as high levels of elasticity and damping are what make this class so important. For this, the elastomers must, first, go through a complex process known as vulcanization [1]. Chemically, it is the process in which the molecular chains of rubber, independently, are joined by chemical bonds forming primary cross-linkings, which lead to the formation of a three-dimensional network in the material. This structural organization allows to maintain or increase the elastic properties and to reduce the plastic behavior of the material. The elastomer becomes insoluble and with greater mechanical resistance, different from nonvulcanized elastomer.

© 2017 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.

However, while the vulcanization provides improvements in the properties of the elastomers and with it the possibility of a wide use as consumer goods, it brings difficulties for recycling after the use, once the vulcanized polymer becomes a thermosetting, making it impossible for subsequent molding into another product.

Presently, a key expression is the sustainable development, which refers to the responsible search for economic and material development without damaging the human being and the environment, using natural resources wisely so that future generations will not be harmed. Among the related subjects are the waste problem, and one of the possible solutions is the recycling of recyclable materials (especially postconsumer polymeric materials) [2].

Among the most harmful polymeric materials include those that contain heavy metals fillers, plasticizers and vulcanized elastomers. The latter ones, for being nonreprocessable due to the presence of cross-linkings, may cause serious public health problems, since they may be accumulators of rain water when disposed in landfills (especially tires), becoming a place able to the proliferation of vectors, such as the *aedes aegypti* mosquito, transmitter of dengue, chikungunya and zika (and day by day the scenario is getting worse, since new diseases transmitted by the same vector are being discovered) [2]. Adding, vulcanized elastomers are materials which require long periods of time to degrade naturally due to their structure of cross-linkings, presence of stabilizers and other additives in their formulation [3], making them infusible and difficult to be reprocessed. Besides, these are considered high value-added materials, due to the range of chemical additives on the formulation, making their discard a waste. According to Imbernon and Norvez [4], "Because of the scarcity and increasing prices of natural resources, and of the growing environmental awareness, waste management has become a crucial issue in today's society."

Presently, the high growth of consumption and inadequate disposal of polymeric materials have increased problems related to solid urban residues. The ideal would be that 100% of all the polymeric materials used around the world was recycled, assisting in sustainable development. However, the real scenario is quite different for bumping into numerous difficulties, which generates serious environmental, public health, economic and governmental problems [2]. In this way, devulcanization is against this serious global problem, seeking a viable reuse way of postconsumer vulcanized elastomers.

This work aims to discuss about devulcanization of vulcanized elastomers and some possible applications of this material, with focus on the preparation of polymeric blends containing recycled elastomers (based on thermoplastic and recycled rubber), and the factors that affect the properties of the final material, with the purpose of being a useful literature serving in obtaining final materials applicable in consumer goods.

## **2. Devulcanization of elastomers**

A very prominent form of recycling is the devulcanization, which is the process of total or partial cleavage of the cross-linkings formed during the initial vulcanization [5–8]. Despite returning to the material its flow capacity, the higher the devulcanization degree, the greater the breakage of the main polymer chain links. The effect of this degradation is the significant reduction of stiffness and other mechanical properties when the material is revulcanized (second vulcanization process). Therefore, for the choice of the parameters of the devulcanization process, must be taken into account the balance between the processability and mechanical properties of the final material [9].

However, while the vulcanization provides improvements in the properties of the elastomers and with it the possibility of a wide use as consumer goods, it brings difficulties for recycling after the use, once the vulcanized polymer becomes a thermosetting, making it impossible for

Presently, a key expression is the sustainable development, which refers to the responsible search for economic and material development without damaging the human being and the environment, using natural resources wisely so that future generations will not be harmed. Among the related subjects are the waste problem, and one of the possible solutions is the

Among the most harmful polymeric materials include those that contain heavy metals fillers, plasticizers and vulcanized elastomers. The latter ones, for being nonreprocessable due to the presence of cross-linkings, may cause serious public health problems, since they may be accumulators of rain water when disposed in landfills (especially tires), becoming a place able to the proliferation of vectors, such as the *aedes aegypti* mosquito, transmitter of dengue, chikungunya and zika (and day by day the scenario is getting worse, since new diseases transmitted by the same vector are being discovered) [2]. Adding, vulcanized elastomers are materials which require long periods of time to degrade naturally due to their structure of cross-linkings, presence of stabilizers and other additives in their formulation [3], making them infusible and difficult to be reprocessed. Besides, these are considered high value-added materials, due to the range of chemical additives on the formulation, making their discard a waste. According to Imbernon and Norvez [4], "Because of the scarcity and increasing prices of natural resources, and of the growing environmental awareness, waste management has

Presently, the high growth of consumption and inadequate disposal of polymeric materials have increased problems related to solid urban residues. The ideal would be that 100% of all the polymeric materials used around the world was recycled, assisting in sustainable development. However, the real scenario is quite different for bumping into numerous difficulties, which generates serious environmental, public health, economic and governmental problems [2]. In this way, devulcanization is against this serious global problem, seeking a viable reuse

This work aims to discuss about devulcanization of vulcanized elastomers and some possible applications of this material, with focus on the preparation of polymeric blends containing recycled elastomers (based on thermoplastic and recycled rubber), and the factors that affect the properties of the final material, with the purpose of being a useful literature serving in

A very prominent form of recycling is the devulcanization, which is the process of total or partial cleavage of the cross-linkings formed during the initial vulcanization [5–8]. Despite returning to the material its flow capacity, the higher the devulcanization degree, the greater

recycling of recyclable materials (especially postconsumer polymeric materials) [2].

subsequent molding into another product.

210 Elastomers

become a crucial issue in today's society."

way of postconsumer vulcanized elastomers.

**2. Devulcanization of elastomers**

obtaining final materials applicable in consumer goods.

The literature presents several works that discuss the different methods used to devulcanize rubbers, as mechanical and chemical mechanical method [10–12], microwaves method [9, 13–19], ultrasound method [20–22], chemical method [23, 24], microbial method [25, 26], and still other methods as bioreactor and spraying for solid-state shear [27, 28]. Subsequently, when the goal is to use the recycled elastomer in the production of a blend, the role of devulcanization is to increase the interaction between the raw and recycled material, reducing the degradation of the properties of the finished product, and making it possible to increase the amount of recycled elastomer in the compound raw phase/recycled phase [29].

A well-established way of recycling vulcanized elastomers is through the production of polymeric blends, that is, physical mixes of two or more polymers that can be miscible or not. As two or more properties of the polymers can be combined, the blends have been studied widely with the aim of improving the physical properties compared to neat polymers, that is, obtain materials with additional properties, and minimal loss of original properties [30], as well as being more economically viable to unite two existing polymers to synthesize another nonexistent [31], for the creation of a new molecule.

A plethora of polymeric blends composed of elastomers can be obtained. There are basically two types of polymeric blends composed in at least one of the phases an elastomer: blends composed of two or more types of elastomers (elastomeric blends) and blends composed of a thermoplastic phase and the other elastomeric. These can be of two types: when there is a high concentration of elastomer (thermoplastic elastomers—TPEs) and when there is low concentration of elastomer (toughened plastic). In all cases, the goal is to obtain materials with desired properties, additional to the properties of the neat materials.

In addition to restore the fluidity of the rubber, devulcanization is able to chemically change the structure of the material [19]. All these changes certainly affect its revulcanization. So, revulcanization is still more complex than the vulcanization itself, since other parameters influence it. And especially in the production of dynamically revulcanized blends (blends based on thermoplastic and recycled rubber in high concentrations, in which the last phase was revulcanized during processing), beyond this, others related to processing get place, to be discussed ahead.

According to Karger-Kocsis et al. [32], among the vast number of possibilities concerning blends containing ground tire rubber (GRT), value-added application can especially be expected in thermoplastic elastomers, and rubber combinations. In the literature, several studies about elastomeric blends with satisfactory results are easily found [16, 33]. Despite the great advances obtained by this type of material, a great difficulty, or even a disadvantage, is that the rejects produced during processing are not easily recycled and reworked and may cause environmental problems due to incorrect disposal of this waste. On the other hand, rejects of TPE blends can be easily reworked, which presents some remarkable advantages, being the most important one the ability of being processed as a thermoplastic, presenting the performance of a vulcanized elastomer. However, even today with all the progress achieved in the field of science and technology, the use of recycled elastomer in this type of blend remains a major challenge. Even so, this class of polymeric blends represents a major current trend for the use of recycled elastomers and will be addressed in more details in the following section.

## **3. Thermoplastic elastomers**

Thermoplastic elastomers contain high concentrations of elastomeric phase (usually above 50%), combining the processability of thermoplastics and the functional performance of vulcanized rubbers at room temperature [34–37]. There are three distinct classes of thermoplastic elastomers, namely: block copolymer, thermoplastic/dynamically vulcanized elastomer blends—thermoplastic vulcanizates (TPVs) and ionic thermoplastic.

The unique combination of properties allows the processing of TPEs in conventional equipment used for processing of thermoplastics in processes such as injection and blow molding, film production and extruded profiles, keeping the elastomeric properties. Such behavior is attributed to their structures that contain both flexible and elastic fields of high extensibility with low glass transition temperature (Tg ), and rigid low extensibility areas with a T<sup>g</sup> and/or crystalline melting temperature (Tm) high [38].

The key advantages of TPEs include [38]: (i) their ability to become fluid with heat and then hardening with cooling gives manufacturers the possibility to produce articles with rubber behavior using equipments commonly used in the processing of thermoplastics. (ii) Little or no mixing of additives necessary for the production of TPEs. The majority is ready for manufacturing. Rubbers, however, require the mixture of all the additives. (iii) TPEs, once prepared, do not need vulcanization stage. Their processing consists of fewer stages than the processing for obtaining a vulcanized rubber. (iv) Scraps produced in the production process can be reprocessed. Scraps generated in processing of vulcanized rubbers, however, have their potential reuse limited, and the cost of its production is higher, due to the loss of material and disposal cost of scraps. (v) Thermoplastic processing consumes less total energy by having a more efficient processing and smaller time cycles.

A particular type of TPE blend, thermoplastic vulcanizate blend, known as TPV, is largely adopted when using devulcanized elastomers, due to its typical features and properties. It will be described in more details in the following sections.

## **4. Thermoplastic vulcanizate blends**

TPV is a type of TPE produced via dynamic vulcanization of the elastomeric phase of an immiscible blend of thermoplastic in molten state and elastomer under high shear rates [39–43]. TPVs are materials widely used in automotive [12, 44] and electronics industries, civil construction, wiring and cables, biomedical products [12, 45–47], among others. Due to the high applicability of this kind of blend, the use of recycled rubber can be useful and worth being studied.

being the most important one the ability of being processed as a thermoplastic, presenting the performance of a vulcanized elastomer. However, even today with all the progress achieved in the field of science and technology, the use of recycled elastomer in this type of blend remains a major challenge. Even so, this class of polymeric blends represents a major current trend for the use of recycled elastomers and will be addressed in more details in the following section.

Thermoplastic elastomers contain high concentrations of elastomeric phase (usually above 50%), combining the processability of thermoplastics and the functional performance of vulcanized rubbers at room temperature [34–37]. There are three distinct classes of thermoplastic elastomers, namely: block copolymer, thermoplastic/dynamically vulcanized elastomer

The unique combination of properties allows the processing of TPEs in conventional equipment used for processing of thermoplastics in processes such as injection and blow molding, film production and extruded profiles, keeping the elastomeric properties. Such behavior is attributed to their structures that contain both flexible and elastic fields of high extensibility

The key advantages of TPEs include [38]: (i) their ability to become fluid with heat and then hardening with cooling gives manufacturers the possibility to produce articles with rubber behavior using equipments commonly used in the processing of thermoplastics. (ii) Little or no mixing of additives necessary for the production of TPEs. The majority is ready for manufacturing. Rubbers, however, require the mixture of all the additives. (iii) TPEs, once prepared, do not need vulcanization stage. Their processing consists of fewer stages than the processing for obtaining a vulcanized rubber. (iv) Scraps produced in the production process can be reprocessed. Scraps generated in processing of vulcanized rubbers, however, have their potential reuse limited, and the cost of its production is higher, due to the loss of material and disposal cost of scraps. (v) Thermoplastic processing consumes less total energy by

A particular type of TPE blend, thermoplastic vulcanizate blend, known as TPV, is largely adopted when using devulcanized elastomers, due to its typical features and properties. It

TPV is a type of TPE produced via dynamic vulcanization of the elastomeric phase of an immiscible blend of thermoplastic in molten state and elastomer under high shear rates [39–43]. TPVs are materials widely used in automotive [12, 44] and electronics industries, civil construction,

), and rigid low extensibility areas with a T<sup>g</sup>

and/or

blends—thermoplastic vulcanizates (TPVs) and ionic thermoplastic.

**3. Thermoplastic elastomers**

212 Elastomers

with low glass transition temperature (Tg

crystalline melting temperature (Tm) high [38].

having a more efficient processing and smaller time cycles.

will be described in more details in the following sections.

**4. Thermoplastic vulcanizate blends**

Dynamic vulcanization is the vulcanization of the elastomeric phase in a molten mixture with other polymer(s) [48]. The process produces a cross-linked polymer dispersion in a continuous polymer matrix phase not cross-linked [38, 40, 49–52]. The continuity of the thermoplastic phase provides the thermo-plasticity and mechanical resistance necessary to blends [53], while the dynamically vulcanized rubber particles give elasticity, flexibility and stability [36, 46, 54].The process can be described as follows: after enough fusion-blend of thermoplastic and rubber, vulcanization agents are added. The vulcanization of rubber phase occurs with a continuation of the mixture. After the output of the mixer, the cold blend can be chopped, extruded, injected, molded, pelletized, etc [43, 52].

The literature presents a vast number of works showing the differences in the properties of polymer blends resulting from dynamic vulcanization, among them: improvement in mechanical properties [55], greater thermal stability [56], minor swelling of the extruded [43], better reprocessability [46, 57], increase in the service temperature [49, 55], greater weather resistance [12] among others, depending on the analyzed system. Several papers also feature improvements in mechanical properties as a result of the dynamic vulcanization, but through the use of compatibilizing agents [36, 48, 58–60] as a result of greater refinement of morphology [34, 61], in general. Among the improvements, it is also found reduced permanent elongation, increased fatigue resistance, greater stability of morphology and better chemical resistance.

TPV containing recycled elastomers, a special issue nowadays as a possible solution to the problem of solid urban residues, especially vulcanized rubbers, will be addressed in the following section.

## **5. Thermoplastic vulcanizate blends containing recycled elastomers**

The reuse, recycling and recovery of waste of cross-linked rubbers are of great scientific and technological interest. As discussed previously, there is great difficulty in recycling, as they are infusible and insoluble materials, which have difficult processing [57] due to their structure of cross-linkings. In this context, many efforts have been made regarding the preparation and characterization of polymer blends containing GTR and various thermoplastics, as an alternative to recycling [62, 63].

The properties of these materials depend on the concentration of the recycled material, as well as the adhesion among phases [64, 65]. According to Zhang et al. [66], the adhesion between the GTR and the polymer matrix is usually very weak due to the three-dimensional structure of the cross-linkings, in the case of blends, in which the GTR is just ground. Cañavate et al. [63] report that the lack of adhesion among phases is due to the large particles of GTR, their superficial characteristics and structure of cross-linkings, hindering their adsorption by molecules of the thermoplastic matrix, being that the use of only ground GTR into blends makes the processing a difficult step [67]. For Kumar et al. [68], for the production of TPVs containing recycled rubber, the addition of a raw rubber or the devulcanization (at least partial) of recycled material is prerequisites. The devulcanization improves the compatibility between GTR and the matrix [4]. However, despite all the difficulties presented by Cespedes et al., "The use of GTR is an excellent option for reducing the cost of TPVs, and GTR is an environmentally friendly alternative because of its upcycling applications" [69].

In order to improve adhesion and interaction among phases, many authors have used compatibilization techniques [48, 66, 70], devulcanization of elastomeric phase [12, 20, 57, 66, 71–74], addition of a third elastomeric phase or replacing part of recycled rubber for a raw one [53, 63, 68, 69], functionalization [75], filler addition [47, 70, 74], among others, beyond the dynamic vulcanization, which notoriously increases the adhesion and interaction among phases of the blends [20, 34, 55, 57, 68, 76]. Additionally, the dynamic vulcanization in blends containing recycled material gives them greater added value [37].

The next section will present the stages involved during the evolution of morphology of thermoplastic vulcanizate blends and some important parameters able to influence the development of the final morphology of these blends.

## **6. Evolution of the morphology of thermoplastic vulcanizate blends during processing and important parameters**

The final morphology of a blend is achieved during its processing, so it is a crucial stage in getting the final desired properties, since they are consequence of its morphology.

Many factors can change the morphology of polymer blends during processing such as temperature, residence time (processing on extruders), intensity of the mixture (speed of the extruder and setting of the screw), composition of the blend, viscosities and elasticities ratio, and interfacial tension among the phases [77]. In this way, the final morphology of immiscible polymer blends depends on the properties of the individual components, as well as processing conditions [78, 79].

Regarding the parameters related to the processing, the literature presents several works in which the processing variables are changed and analyzed [44, 55, 57, 72, 80–89].

As an important example of processing variable, temperature should be close to the Tm of the thermoplastic phase (or a little greater) [44] and be able to activate the vulcanization reaction of the elastomeric phase [46]. It is known that the behavior of vulcanization of the elastomeric phase varies according to the adopted temperature, besides that it should not be high enough so that, combined with the high shear rates involved in the process (especially in twin screw extruders), promotes high level of degradation in both phases of the blend. However, no matter the studied parameter, all of them are very important, since they are able to directly affect the final morphology of blends and, then, their final properties.

**Figure 1** shows in a schematic way the transformation of the morphology of thermoplastic vulcanizate blends during processing. According to the schema, initially a rubber-thermoplastic blend is formed, with co-continuous morphology and rubber phase non–cross-linked (stage A). In the next stage, rubber phase becomes stretched and strongly deformed due to the beginning of dynamic vulcanization. A rubber-thermoplastic blend is formed, with cocontinuous morphology as well (stage B). Due to dynamic vulcanization, rubber phase is able to break up in the following stage (stage C). So, a TPV is formed, the rubber phase is cross-linked and is dispersed in thermoplastic phase. In the lasts stages (stages D and E), rubber particles have the distribution improved on the matrix phase.

the processing a difficult step [67]. For Kumar et al. [68], for the production of TPVs containing recycled rubber, the addition of a raw rubber or the devulcanization (at least partial) of recycled material is prerequisites. The devulcanization improves the compatibility between GTR and the matrix [4]. However, despite all the difficulties presented by Cespedes et al., "The use of GTR is an excellent option for reducing the cost of TPVs, and GTR is an environmentally

In order to improve adhesion and interaction among phases, many authors have used compatibilization techniques [48, 66, 70], devulcanization of elastomeric phase [12, 20, 57, 66, 71–74], addition of a third elastomeric phase or replacing part of recycled rubber for a raw one [53, 63, 68, 69], functionalization [75], filler addition [47, 70, 74], among others, beyond the dynamic vulcanization, which notoriously increases the adhesion and interaction among phases of the blends [20, 34, 55, 57, 68, 76]. Additionally, the dynamic vulcanization in blends

The next section will present the stages involved during the evolution of morphology of thermoplastic vulcanizate blends and some important parameters able to influence the develop-

The final morphology of a blend is achieved during its processing, so it is a crucial stage in

Many factors can change the morphology of polymer blends during processing such as temperature, residence time (processing on extruders), intensity of the mixture (speed of the extruder and setting of the screw), composition of the blend, viscosities and elasticities ratio, and interfacial tension among the phases [77]. In this way, the final morphology of immiscible polymer blends depends on the properties of the individual components, as well as process-

Regarding the parameters related to the processing, the literature presents several works in

As an important example of processing variable, temperature should be close to the Tm of the thermoplastic phase (or a little greater) [44] and be able to activate the vulcanization reaction of the elastomeric phase [46]. It is known that the behavior of vulcanization of the elastomeric phase varies according to the adopted temperature, besides that it should not be high enough so that, combined with the high shear rates involved in the process (especially in twin screw extruders), promotes high level of degradation in both phases of the blend. However, no matter the studied parameter, all of them are very important, since they are able to directly affect

**6. Evolution of the morphology of thermoplastic vulcanizate blends** 

getting the final desired properties, since they are consequence of its morphology.

which the processing variables are changed and analyzed [44, 55, 57, 72, 80–89].

the final morphology of blends and, then, their final properties.

friendly alternative because of its upcycling applications" [69].

containing recycled material gives them greater added value [37].

ment of the final morphology of these blends.

ing conditions [78, 79].

214 Elastomers

**during processing and important parameters**

On the whole, the blend completely changes its morphology, from co-continuous to dispersed phase. However, so that the blend presents typical final properties for noble uses, the processing must be carefully analyzed and optimized. In the case of using an extruder, the analysis becomes more complex because of the large number of variables involved, but at the same time, it becomes a big advantage in the improvement of the technique. A good example of possible parameter change in an extruder is the number of feeders.

When producing a thermoplastic vulcanizate blend in an extruder containing two feeders, it is possible to introduce independently each phase through each one of the feeders. In the case of blends in which the elastomeric phase was previously mixed to vulcanization additives, this can be added in the second feeder, whereas, in the case of elastomeric phase not be previously mixed to additives, the two phases of the blend can be added together in the same feeder, while the additives can be added in the second one.

The change of some processing parameters was deeply studied by de Sousa et al. [61]. The authors produced dynamically revulcanized blends based on 60 wt% of devulcanized GTR (GTR5.5) and 40 wt% of high density polyethylene (HDPE), by using a twin screw extruder. Processing parameters such as screw speed and feeding mode were varied. In the production of blends using only one feeder, both HDPE and devulcanized rubber were added together. For blends produced with two feeders, the HDPE phase was added in the first feeder and the GTR phase in the second one. Vulcanization additives were previously added to the GTR phase. The authors proved the importance of knowing previously the rheological properties of the rubber phase, as well as matching these properties to the processing conditions.

**Figure 1.** Schematic morphology transformation during the dynamic vulcanization of polymeric blends. The black part represents the elastomeric phase, and the white represents the thermoplastic phase.

According to de Sousa et al. [61], overall, the effects of screw speed on the mechanical properties were not significant. Furthermore, the mechanical properties of the blends were far below those of classical TPVs, probably because of the poor compatibility and adhesion between GTR5.5 and HDPE. In general, the blends produced by using the second feeding mode obtained a finer morphology.

**Figure 2** shows some vulcanization parameters involved in the processing of dynamically revulcanized blends, as well as the scheme of a probable evolution of morphology during processing. The screw profile is for feeding mode 2, since it produced finer morphology and consequently slightly higher mechanical properties.

According to **Figure 2**, at point 1 of the extruder, there is only physical mixing among the phases and there is no revulcanization of the GTR5.5. At the beginning of the second high shear zone (point 2), the revulcanization reaction gets place (the residence time of the rubber from its introduction in the extruder to this point is about the same of ts<sup>1</sup> (scorch time of the reaction)) and, around this point, the blend presents a cocontinuous morphology, in which the elastomeric phase is stretched in the flow direction [61].

Due to the high elongational flow in this zone (use of mixing or kneading blocks), the rubber along vulcanization process can deform (due precisely to the three-dimensional structure formation of cross-linkings and consequent increase in viscosity of the blend [76]) enough for

**Figure 2.** Screw profile relative to feeding mode 2 used in the preparation of the blends, showing the schema of the possible evolution of the morphology and rheology of the elastomeric phase involved during processing. Reprinted with permission from Ref. [61]. Copyright 2017, John Wiley and Sons.

break of their particles in other smaller than the ones before [60] and phase inversion may occur. At this stage, the overall viscosity is increased (the elastomeric phase with high elasticity is stretched and breaks into smaller particles due to high shear rates, intense elongational flow and high elasticity generated by cross-linkings (point 3), resulting in high mechanical stresses [61]. The elastomeric phase deforms until it reaches a critical tension, when it breaks up into small particles [90]. The biggest changes of morphology occur in the first high-shear region, in which both phases are together [39, 91–93].

According to de Sousa et al. [61], overall, the effects of screw speed on the mechanical properties were not significant. Furthermore, the mechanical properties of the blends were far below those of classical TPVs, probably because of the poor compatibility and adhesion between GTR5.5 and HDPE. In general, the blends produced by using the second feeding mode

**Figure 2** shows some vulcanization parameters involved in the processing of dynamically revulcanized blends, as well as the scheme of a probable evolution of morphology during processing. The screw profile is for feeding mode 2, since it produced finer morphology and

According to **Figure 2**, at point 1 of the extruder, there is only physical mixing among the phases and there is no revulcanization of the GTR5.5. At the beginning of the second high shear zone (point 2), the revulcanization reaction gets place (the residence time of the rubber

reaction)) and, around this point, the blend presents a cocontinuous morphology, in which

Due to the high elongational flow in this zone (use of mixing or kneading blocks), the rubber along vulcanization process can deform (due precisely to the three-dimensional structure formation of cross-linkings and consequent increase in viscosity of the blend [76]) enough for

**Figure 2.** Screw profile relative to feeding mode 2 used in the preparation of the blends, showing the schema of the possible evolution of the morphology and rheology of the elastomeric phase involved during processing. Reprinted with

(scorch time of the

from its introduction in the extruder to this point is about the same of ts<sup>1</sup>

obtained a finer morphology.

216 Elastomers

consequently slightly higher mechanical properties.

the elastomeric phase is stretched in the flow direction [61].

permission from Ref. [61]. Copyright 2017, John Wiley and Sons.

At point 4, still under the effect of high shear rates, rubber particles break into smaller particles, and at point 5, there is a better distribution in the thermoplastic matrix. The end of the second shear zone (point 4) refers approximately to the optimum cure time (t90) of the elastomeric phase which, in the case of GTR5.5, is 44 s. The residence time of rubber from its introduction in the extruder to the end point of the reaction must be equivalent to t90, and this point should be in a high shear zone of the extruder for breakage of rubber particles in micrometric dimensions [61].

In the point 5, cross-linked rubber particles, at this moment, have a very high viscosity and elasticity, and it occurs only the distribution of the particles in the matrix, improving macroscopic homogeneity, being necessary the use of mixing elements. It must be pointed out that the dispersion process occurs instantly with the vulcanization reaction, and both processes are influencing each other. The rapid increase in the cross-linkings of rubber also leads to an increase in the surface tension of the elastomeric phase. Cross-linkings and the high superficial tension will reduce the driving force for the coalescence and, therefore, the characteristic of the particles of vulcanized rubber phase will be preserved, even in a new mixture after the completion of the cross-linking process [67, 90].

The increase of interfacial tension as a result of cross-linkings and the high elasticity of the particles produce the relaxation of the deformed structures; in an ideal case, spherical particles are formed. Due to the high viscosities ratio, elastomeric cross-linked particles can only be distributed, and no longer dispersed on a new mixing cycle [90]. In agreement, some results [94] revealed that the developed microstructure is highly affected by the type of the melt compounding process, as well as the feeding mode.

It is important to address here that, for both screw profiles used, the first mixing zone served to melt the HDPE, the second one to dynamically revulcanize the GTR and the last one to improve the distribution of the rubber particles in the HDPE. In the case of the blends produced through feeding mode 1, the high shear rate in the first mixing zone could bring about premature revulcanization of the rubber phase, since the components were added together. As the length of this zone and the corresponding residence time were short, the time for the reaction to go to completion was longer than the residence time on the zone, which probably happened in the second transport zone. Thus, the rubber domains were not satisfactorily well dispersed and distributed in the HDPE matrix. However, in the case of the blends produced through feeding mode 2, the residence time of the rubber phase inside the extruder from its introduction to the end of the second mixing zone was closer to the optimum cure time of the GTR5.5 at 180°C (43.8 s). Therefore, the mixing zone was long enough for the revulcanization reaction to go to completion and the dispersed rubber domains to have its size reduced in the HDPE matrix [61].

The blend produced by using the second feeding mode and at 250 rpm presented higher finer morphology and consequently better mechanical properties, despite the fact that the compatibility and adhesion between the phases were poor. The finer morphology is due to good match between processing conditions and rheological properties of the GTR5.5. The residence times of the GTR5.5 inside the extruder from its introduction to the respective points shown in **Figure 2** were approximately 50 and 30 s, respectively, which were very close to the values of t90and ts1 (44 and 27 s, respectively) [61].

At this point, the presentation of some equations can be useful to understand all the modification occurred to the blends during the processing. The elongation and breakage of polymeric particles suspended in another polymer under shearing flow were first studied by Taylor [95]. According to the author, two dimensionless parameters that enable the prediction of morphology in the molten state are the number of Capillarity [Eq. (1)] and the viscosities ratio [Eq. (2)]:

$$C\_{\mu} = \frac{\sigma R}{\alpha} \tag{1}$$

where σ represents the shear stress, R is the radius of the particle or drop, and α is the interfacial tension between the phases of the blend.

$$p = \frac{\eta\_d}{\eta\_m} \tag{2}$$

where ηd and ηm are the viscosities of the dispersed phase and the matrix phase, respectively.

If the value of Ca is small, interfacial forces dominate and the particles acquire the shape of ellipsoids. Above a critical value Ca crit, the particles become unstable and break down [96].

When two immiscible polymers are mixed to form a blend, in accordance with the principle of minimum dissipation of energy, it is expected that the most viscous polymer forms the dispersed phase and the less viscous forms the matrix phase. When this principle is satisfied, it means that the viscosities ratio [Eq. (2)] was the predominant factor in determining the state of dispersion of the blend [97].The viscosities ratio strongly affects the development of morphology during the reaction [52].

Additionally, in order to study the development of the phase morphology and predict the inversion phase region in immiscible polymer blends, Avgeropoulus et al. [98] developed an empirical model based on torque ratio in internal mixer and volumetric fraction of each phase [Eq. (3)]. Jordhamo et al. [99] proposed a similar equation [Eq. (4)], based on the viscosities ratio:

$$\frac{\mathcal{Q}\_1}{\mathcal{Q}\_2} \cdot \frac{T\_2}{T\_1} = X \tag{3}$$

$$\frac{\mathcal{D}\_1}{\mathcal{D}\_2} \cdot \frac{\eta\_2}{\eta\_1} = \,\,\,\,\mathbf{Y} \tag{4}$$

which results in the following variants morphologies:

X, Y > 1—Phase 1 is continuous or matrix and phase 2 is dispersed.

X, Y < 1—Phase 2 is continuous or matrix and phase 1 is dispersed.

X, Y = 1—Two phases are continuous or region of phase inversion.

Where ø1 , ø2 are volumetric fractions, T1 , T2 are measures of torques at a same temperature, and η<sup>1</sup> , η<sup>2</sup> are viscosity values for phases 1 and 2, respectively [50].

The blend produced by using the second feeding mode and at 250 rpm presented higher finer morphology and consequently better mechanical properties, despite the fact that the compatibility and adhesion between the phases were poor. The finer morphology is due to good match between processing conditions and rheological properties of the GTR5.5. The residence times of the GTR5.5 inside the extruder from its introduction to the respective points shown in **Figure 2** were approximately 50 and 30 s, respectively, which were very close to the values

At this point, the presentation of some equations can be useful to understand all the modification occurred to the blends during the processing. The elongation and breakage of polymeric particles suspended in another polymer under shearing flow were first studied by Taylor [95]. According to the author, two dimensionless parameters that enable the prediction of morphology in the molten state are the number of Capillarity [Eq. (1)] and the viscosities ratio [Eq. (2)]:

*σR*

\_\_\_*d ηm*

where σ represents the shear stress, R is the radius of the particle or drop, and α is the inter-

where ηd and ηm are the viscosities of the dispersed phase and the matrix phase, respectively. If the value of Ca is small, interfacial forces dominate and the particles acquire the shape of ellipsoids. Above a critical value Ca crit, the particles become unstable and break down [96]. When two immiscible polymers are mixed to form a blend, in accordance with the principle of minimum dissipation of energy, it is expected that the most viscous polymer forms the dispersed phase and the less viscous forms the matrix phase. When this principle is satisfied, it means that the viscosities ratio [Eq. (2)] was the predominant factor in determining the state of dispersion of the blend [97].The viscosities ratio strongly affects the development of mor-

Additionally, in order to study the development of the phase morphology and predict the inversion phase region in immiscible polymer blends, Avgeropoulus et al. [98] developed an empirical model based on torque ratio in internal mixer and volumetric fraction of each phase [Eq. (3)]. Jordhamo et al. [99] proposed a similar equation [Eq. (4)], based on the viscosities ratio:

> ∅1 ∅2 . *T*\_\_2 *T*1

∅1∅2 . *η* \_\_2 *η*1

*<sup>α</sup>* (1)

= *X* (3)

= *Y* (4)

(2)

of t90and ts1 (44 and 27 s, respectively) [61].

218 Elastomers

*Ca* <sup>=</sup> \_\_\_

facial tension between the phases of the blend. *<sup>p</sup>* <sup>=</sup> *<sup>η</sup>*

phology during the reaction [52].

\_\_\_

\_\_\_

which results in the following variants morphologies:

X, Y > 1—Phase 1 is continuous or matrix and phase 2 is dispersed. X, Y < 1—Phase 2 is continuous or matrix and phase 1 is dispersed. X, Y = 1—Two phases are continuous or region of phase inversion.

According to Zhang et al. [57], soon after the end of the dynamic vulcanization, vulcanized rubber particles present a high surface tension, which agglomerate them. Therefore, it is necessary the application of high shear rates, which are generated due to the presence of mixing blocks in processing performed in extruder, as in point 5 (**Figure 2**). Also, Yao et al. [100] deeply studied the morphology evolution of bromo-isobutylene-isoprene rubber (BIIR)/polypropylene (PP) TPV blends. It was depicted that the dynamic vulcanization increases the compatibility among the phases, demonstrated by the increase in interfacial phase thickness and the decrease in interfacial tension. During the processing, single nanoparticles of elastomeric phase are being formed, and their agglomeration is getting lesser as dynamic vulcanization advances. Thus, Sararoudi et al. [101] concluded that the extent of agglomerations among the vulcanized rubber particles in the twin screw extruder not only depends on the rubber content, but also are controlled by a common agglomeration and disagglomeration mechanism which is, in turn, governed by the screw speed.

For the case of blends in which all phases are added together in the extruder in one feeder, studies show that as soon as the complete melting of the thermoplastic phase is reached, the blend reaches quickly its final morphology [39, 85] due to generated interfaces among phases. According to Covas et al. [102], the increase in the interfacial area raised soon after the melting of the thermoplastic, which induced chemical conversion and the evolution of morphology. Therefore, the choice of the parameters of the processing, the number of feeders to be used depending on the screw profile adopted is a factor of great importance. Van Duin and Machado [39] studied the dynamic vulcanization reaction of ethylene-propylene-diene rubber (EPDM)/HDPE blend through the withdrawal of peer-to-peer samples on twin screw extruder during processing. According to the authors, the cross-linking of EPDM phase began when the HDPE still was not fully melted, and the final morphology of blends was reached very quickly. The phase inversion occurred due to formation of cross-linkings. Machado and van Duin [103] analyzed the properties of EPDM/HDPE TPV blends and found that, the higher the content of EPDM, the greater the viscous dissipation, the higher the melting and, consequently, the greater the rate of cross-linkings.

The type of equipment can also alter the particle size distribution of rubber, in the case of morphology of dispersed phase. Studies show that blends produced on extruder tend to have smaller particle sizes compared to blends produced in internal mixer [20, 71, 104–106] due to the higher shear rate during processing in extruders and intensive flow field [105]. The elastomeric phase of the blends produced on extruder can also present greater cross-linking density, as verified by Sengupta and Noordermeer [104]. However, the distribution of sizes of particles is more uniform in the blends produced in internal mixer due to longer residence time and greater total shear stress, promoting the breakdown of particles [104]. According to Shahbikian et al. [105], that produced EPDM/PP blends in internal mixer and extruder, even with the shortest residence time for processing in extruders, the cure reaction occurred quickly, resulting in EPDM particles of sizes more heterogeneous and with greater crosslinking density. By combining the effects of time, temperature and shearing, the matrix phase acquires elasticity, and it is extruded into sheets. At the same time, there is a break of these sheets due to elongational and shear forces generated in the mixing equipment. So, there is a dynamic balance between the process of breakdown of phases and coalescence.

Among other factors mentioned, the final morphology is the result of processes of coalescence and breakage of the elastomeric phase particles (in the case of blends with the morphology of dispersed phases) during processing. In the case of TPV blends containing devulcanized rubber, the devulcanization acts on the process of break, while the dynamic revulcanization acts on the reduction of the coalescence process of particles [73].

In short, the process of devulcanization makes the rubber fluid, aiding in the process of breaking what, consequently, helps in reducing the size of the particles, increases the contact area among the phases and increases the transmission of tensions. Dynamic revulcanization helps in stabilizing the morphology by inhibition of coalescence process among the particles of the dispersed phase [73].According to Goharpey et al. [107], dynamic vulcanization can prevent the coalescence of the rubber particles from the early stage of the dynamic vulcanization.

As a conclusion, the stage of processing (dynamic vulcanization) is of extreme importance, and all aspects involved should be carefully analyzed and optimized, as they may change the final morphology of blends and, with it, completely alter their final properties [108].

## **7. Conclusion**

The irregular disposition of solid urban residues, especially vulcanized elastomers like tires, may bring together dangerous environmental impacts. In this way, a very outstanding form of vulcanized elastomers recycling is the devulcanization. Along with a variety of possible uses, the devulcanized elastomer can be used in the formation of polymeric blends. Among a vast number of polymeric blends composed of elastomers, TPV is largely adopted when using devulcanized elastomers, due to its typical features and properties. However, parameters of processing as well as the devulcanization process itself must be considered and carefully analyzed, since they are able to dictate the final morphology of the polymeric blend and, consequently, its properties as a whole.

TPV is considered a "green" polymer, since its recyclability promotes the environmental protection and resource saving. When it is composed by a recycled elastomeric phase, it aids the petroleum sources saving, saves raw materials and energy, not harming the environment, and still being a source of income for many families who survive from the collection of recyclable materials. The production of blends composed of recycled rubber is still a major challenge for the academic community, since its final properties need to justify all energy expenditures necessary for production, in addition to being economically viable. However, attention should be given to the subject, since it is a possible solution to the problem of final disposal of urban solid waste. On the other hand, it is observed that the subject is still little explored in literature, possibly due to difficulties encountered in the production and final properties obtained.

## **Author details**

sheets due to elongational and shear forces generated in the mixing equipment. So, there is a

Among other factors mentioned, the final morphology is the result of processes of coalescence and breakage of the elastomeric phase particles (in the case of blends with the morphology of dispersed phases) during processing. In the case of TPV blends containing devulcanized rubber, the devulcanization acts on the process of break, while the dynamic revulcanization

In short, the process of devulcanization makes the rubber fluid, aiding in the process of breaking what, consequently, helps in reducing the size of the particles, increases the contact area among the phases and increases the transmission of tensions. Dynamic revulcanization helps in stabilizing the morphology by inhibition of coalescence process among the particles of the dispersed phase [73].According to Goharpey et al. [107], dynamic vulcanization can prevent the coalescence of the rubber particles from the early stage of the dynamic vulcanization.

As a conclusion, the stage of processing (dynamic vulcanization) is of extreme importance, and all aspects involved should be carefully analyzed and optimized, as they may change the

The irregular disposition of solid urban residues, especially vulcanized elastomers like tires, may bring together dangerous environmental impacts. In this way, a very outstanding form of vulcanized elastomers recycling is the devulcanization. Along with a variety of possible uses, the devulcanized elastomer can be used in the formation of polymeric blends. Among a vast number of polymeric blends composed of elastomers, TPV is largely adopted when using devulcanized elastomers, due to its typical features and properties. However, parameters of processing as well as the devulcanization process itself must be considered and carefully analyzed, since they are able to dictate the final morphology of the polymeric blend and, conse-

TPV is considered a "green" polymer, since its recyclability promotes the environmental protection and resource saving. When it is composed by a recycled elastomeric phase, it aids the petroleum sources saving, saves raw materials and energy, not harming the environment, and still being a source of income for many families who survive from the collection of recyclable materials. The production of blends composed of recycled rubber is still a major challenge for the academic community, since its final properties need to justify all energy expenditures necessary for production, in addition to being economically viable. However, attention should be given to the subject, since it is a possible solution to the problem of final disposal of urban solid waste. On the other hand, it is observed that the subject is still little explored in literature, possibly due to difficulties encountered in the production and final

final morphology of blends and, with it, completely alter their final properties [108].

dynamic balance between the process of breakdown of phases and coalescence.

acts on the reduction of the coalescence process of particles [73].

**7. Conclusion**

220 Elastomers

quently, its properties as a whole.

properties obtained.

Fabiula Danielli Bastos de Sousa

Address all correspondence to: fabiuladesousa@gmail.com

Technology Development Center – CDTec, Universidade Federal de Pelotas, Pelotas, RS, Brazil

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## **Chapter 11**

## **Dielectric Elastomer Sensors**

Na Ni and Ling Zhang

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.68995

#### Abstract

Dielectric elastomers (DEs) represent a class of electroactive polymers (EAPs) that exhibit a significant electromechanical effect, which has made them very attractive over the last several decades for use as soft actuators, sensors and generators. Based on the principle of a plane-parallel capacitor, dielectric elastomer sensors consist of a flexible and stretchable dielectric polymer sandwiched between two compliant electrodes. With the development of elastic polymers and stretchable conductors, flexible and sensitive dielectric elastomer tactile sensors, similar to human skin, have been used for measuring mechanical deformations, such as pressure, strain, shear and torsion. For high sensitivity and fast response, air gaps and microstructural dielectric layers are employed in pressure sensors or multiaxial force sensors. Multimodal dielectric elastomer sensors have been reported that can detect mechanical deformation but can also sense temperature, humidity, as well as chemical and biological stimulation in human-activity monitoring and personal healthcare. Hence, dielectric elastomer sensors have great potential for applications in soft robotics, wearable devices, medical diagnostic and structural health monitoring, because of their large deformation, low cost, ease of fabrication and ease of integration into monitored structures.

Keywords: dielectric elastomers, soft capacitance sensors, electronic skin, flexible mechanical sensors, robot sensors, tactile sensors

## 1. Introduction

In recent years, the soft sensing systems have attracted considerable attention because of their potential applications in assistive soft robots, healthcare and entertainment [1]. In contrast to traditional rigid sensors, their advantageous compliant properties enable the soft sensors to safely monitor soft movements or interactions with humans. Many of these devices can be used to measure strain, pressure, force, light, humidity and temperature similar to the human

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skin, which have advantageous properties such as flexibility, stretchability, highly sensitivity and technological compatibility with a large area [2, 3].

Various transduction methods for fabricating flexible sensors have been developed [4], including piezoresistivity, capacitance, piezoelectricity, optics and wireless antennas. Current applications require highly sensitive, flexible, stretchable and low-cost devices, where capacitivebased sensors exhibit better potential for use, because of their high strain sensitivity, static force measurement and low power consumption [5]. These capacitive tactile sensors measure the magnitude of mechanical forces or strain by converting mechanical solicitations into an electrical signal. Dielectric elastomer (DE) sensors are one type of the capacitive-based sensors, which are flexible, soft and stretchable for measuring deformations, forces and pressures.

The dielectric elastomers (DEs) are a type of field-activated polymers that belong to the family of electroactive polymers (EAPs) [6]. The typical structure of a dielectric elastomer (DE) is a dielectric material sandwiched between two electrodes, which can produce a large strain response and high electromechanical efficiency from an electric field. Based on the electromechanical effect, DEs can be used as soft actuators, sensors and generators.

Recently, several reviews have been published in the literature, which have detailed the properties and chemistry of dielectric elastomers [6, 7]. However, these reviews focused primarily on the development of dielectric elastomer actuators and generators. A detailed overview of the recent progress in dielectric elastomer sensors has not been reported, so this current review focuses on the recent research in dielectric elastomer sensors. In the first section, several sensing principles of dielectric elastomer sensors are introduced. In the second and third section, the materials of the dielectric and compliant electrodes are described. In the fourth section, a detailed overview is given of the recent progress regarding applications of dielectric elastomers in electronic skin, structural health monitoring, tissue elasticity measurements, self-sensing actuators and robotic technologies. Wearable dielectric elastomer sensor systems are also reviewed based on multiple physical sensors. Conclusions and future developments in practical applications of dielectric elastomer sensors are discussed in the final section.

#### 2. Principle of dielectric elastomer sensors

The capacitance of a parallel plate capacitor can be written as:

$$\mathbf{C} = \varepsilon\_0 \varepsilon\_r \frac{A}{d} \tag{1}$$

where <sup>C</sup> is the capacitance, <sup>ε</sup><sup>0</sup> <sup>¼</sup> <sup>8</sup>:<sup>854</sup> � <sup>10</sup>�<sup>12</sup>F=m. is the permittivity of the vacuum, <sup>ε</sup><sup>r</sup> is the relative permittivity, A is the electrode area and d is the dielectric distance [8]. A capacitive sensor can be designed to provide elastic deformation by sandwiching an elastomer film between two compliant conductive electrodes to form a dielectric elastomer sensor. The deformation of the dielectric elastomer produces a change in capacitance of the sensor. A simple sensing method is shown in Figure 1. When the sensor is subjected to external tension or

Figure 1. Simple measurement principle of a dielectric elastomer sensor.

skin, which have advantageous properties such as flexibility, stretchability, highly sensitivity

Various transduction methods for fabricating flexible sensors have been developed [4], including piezoresistivity, capacitance, piezoelectricity, optics and wireless antennas. Current applications require highly sensitive, flexible, stretchable and low-cost devices, where capacitivebased sensors exhibit better potential for use, because of their high strain sensitivity, static force measurement and low power consumption [5]. These capacitive tactile sensors measure the magnitude of mechanical forces or strain by converting mechanical solicitations into an electrical signal. Dielectric elastomer (DE) sensors are one type of the capacitive-based sensors, which are flexible, soft and stretchable for measuring deformations, forces and pressures.

The dielectric elastomers (DEs) are a type of field-activated polymers that belong to the family of electroactive polymers (EAPs) [6]. The typical structure of a dielectric elastomer (DE) is a dielectric material sandwiched between two electrodes, which can produce a large strain response and high electromechanical efficiency from an electric field. Based on the electrome-

Recently, several reviews have been published in the literature, which have detailed the properties and chemistry of dielectric elastomers [6, 7]. However, these reviews focused primarily on the development of dielectric elastomer actuators and generators. A detailed overview of the recent progress in dielectric elastomer sensors has not been reported, so this current review focuses on the recent research in dielectric elastomer sensors. In the first section, several sensing principles of dielectric elastomer sensors are introduced. In the second and third section, the materials of the dielectric and compliant electrodes are described. In the fourth section, a detailed overview is given of the recent progress regarding applications of dielectric elastomers in electronic skin, structural health monitoring, tissue elasticity measurements, self-sensing actuators and robotic technologies. Wearable dielectric elastomer sensor systems are also reviewed based on multiple physical sensors. Conclusions and future developments in practical applications of

C ¼ ε0ε<sup>r</sup>

where <sup>C</sup> is the capacitance, <sup>ε</sup><sup>0</sup> <sup>¼</sup> <sup>8</sup>:<sup>854</sup> � <sup>10</sup>�<sup>12</sup>F=m. is the permittivity of the vacuum, <sup>ε</sup><sup>r</sup> is the relative permittivity, A is the electrode area and d is the dielectric distance [8]. A capacitive sensor can be designed to provide elastic deformation by sandwiching an elastomer film between two compliant conductive electrodes to form a dielectric elastomer sensor. The deformation of the dielectric elastomer produces a change in capacitance of the sensor. A simple sensing method is shown in Figure 1. When the sensor is subjected to external tension or

A

<sup>d</sup> <sup>ð</sup>1<sup>Þ</sup>

chanical effect, DEs can be used as soft actuators, sensors and generators.

dielectric elastomer sensors are discussed in the final section.

The capacitance of a parallel plate capacitor can be written as:

2. Principle of dielectric elastomer sensors

and technological compatibility with a large area [2, 3].

232 Elastomers

compression, the surface of the sensor film expands, while the displacement between the two electrodes decreases, which causes an increase in capacitance.

According to the model of an ideal dielectric elastomer, assuming that the volume and permittivity remain constant, when a dielectric elastomer sensor is stretched in its plane, the change in capacitance can be derived as follows:

$$L\_{\text{stretch}} = \lambda\_1 L \tag{2}$$

$$
\lambda W\_{\text{stretch}} = \lambda\_2 W\_{\text{s}} \tag{3}
$$

$$d\_{\rm strch} = \lambda\_3 d = \frac{1}{\lambda\_1 \lambda\_2} d \tag{4}$$

where λ1, λ2, λ<sup>3</sup> are multiples of L, w, d, which are the initial dimensions of the dielectric elastomer as shown in Figure 1 [9]. Combining the above equations with the Eq. (1), the capacitance C of the dielectric elastomer can be written as:

$$\mathcal{C} = \mathbb{C}\_0 (\lambda\_1 \lambda\_2)^2 \tag{5}$$

where C<sup>0</sup> is the initial capacitance. When a uniaxial stretch is applied, the length of dielectric elastomer is <sup>λ</sup> times its initial length and the width and the thickness become <sup>1</sup>ffiffi λ <sup>p</sup> times their initial values, and the capacitance value of the deformed dielectric elastomer is then:

$$\mathbb{C} = \mathbb{C}\_0 \lambda \tag{6}$$

Hence, the capacitance change ΔC. can be expressed as:

$$
\Delta \mathcal{C} = \mathcal{C}\_0 \text{ \(\varepsilon\)}\tag{7}
$$

where ε is the strain along the stretched axis of the dielectric elastomer. Eq. (7) implies that the capacitance change is linear with the strain of dielectric elastomer sensor.

Another sensing method of the dielectric elastomer sensors can be found in the relative position change of two electrodes, which produces a capacitance change as shown in Figure 2. This sensing method is used for measuring shear forces.

Figure 2. Measurement principle of a dielectric elastomer sensor when shear force is subjected.

Figure 3. Prototype of a dielectric elastomer sensor with an air gap. Left: one elastic layer between two electrodes and right: two elastic layers between two electrodes.

To improve the sensitivity of dielectric elastomer sensors, Figure 3 shows a complex sensing method that employs a multilayer dielectric including an air gap and dielectric films. By constructing the air gap, the device is able to sensitively detect multiaxis force and pressure [10, 11]. In addition, the micro/nanostructured dielectric layer can improve the sensitivity of dielectric elastomer sensors as well.

#### 3. Materials of dielectric elastomer sensors

For the large deformation requirements of dielectric elastomer sensors, many classes of dielectric materials have been investigated, including acrylates, silicones, polyurethanes (PU), rubbers, latex rubbers, acrylonitrile butadiene rubbers, olefinic, polymer foams, fluorinated and styrenic copolymers [7]. Acrylates and silicon rubbers such as VHB, polydimethylsiloxane (PDMS) and Ecoflex have been widely used in fabricating flexible sensing devices due to their commercial availability and good performance [9, 12, 13]. The relative dielectric constant and modulus are important properties in the performance of dielectric elastomer sensors. The typical properties of a number of candidate polymers are listed in Table 1 [7, 14–17].

A commercial acrylate adhesive film, VHB, produced by the 3M Company, is commercially available and exhibits large deformation and transparency. It can be stretched more than six times its initial length [9], and the strain is nearly linear with stress to as much as three times the initial length of the film [7]. VHB acrylates have a dielectric constant that is higher than silicon rubbers.


Table 1. Properties of several dielectric elastomers [7, 14–17].

To improve the sensitivity of dielectric elastomer sensors, Figure 3 shows a complex sensing method that employs a multilayer dielectric including an air gap and dielectric films. By constructing the air gap, the device is able to sensitively detect multiaxis force and pressure [10, 11]. In addition, the micro/nanostructured dielectric layer can improve the sensitivity

Figure 3. Prototype of a dielectric elastomer sensor with an air gap. Left: one elastic layer between two electrodes and

Figure 2. Measurement principle of a dielectric elastomer sensor when shear force is subjected.

For the large deformation requirements of dielectric elastomer sensors, many classes of dielectric materials have been investigated, including acrylates, silicones, polyurethanes (PU), rubbers, latex rubbers, acrylonitrile butadiene rubbers, olefinic, polymer foams, fluorinated and styrenic copolymers [7]. Acrylates and silicon rubbers such as VHB, polydimethylsiloxane (PDMS) and Ecoflex have been widely used in fabricating flexible sensing devices due to their commercial availability and good performance [9, 12, 13]. The relative dielectric constant and modulus are important properties in the performance of dielectric elastomer sensors. The

A commercial acrylate adhesive film, VHB, produced by the 3M Company, is commercially available and exhibits large deformation and transparency. It can be stretched more than six times its initial length [9], and the strain is nearly linear with stress to as much as three times the initial length of the film [7]. VHB acrylates have a dielectric constant that is higher than

typical properties of a number of candidate polymers are listed in Table 1 [7, 14–17].

of dielectric elastomer sensors as well.

right: two elastic layers between two electrodes.

234 Elastomers

silicon rubbers.

3. Materials of dielectric elastomer sensors

Silicones have also been studied for use as dielectric elastomers [18]. Many silicone materials are produced commercially, such as Dow Cording Sylgard184. The modulus of these materials typically ranges from 0.1 to 2 MPa and their dielectric constants are generally around 3 [7]. Their elongation at break is less than that of VHB adhesive film. Dow Corning Sylgard184 is a type of polydimethylsiloxane (PDMS) that is generally used for fabricating dielectric elastomer sensors as a dielectric [19] and a substrate [10]. The advantages of PDMS include variable mechanical properties, transparency and stability over a wide range of temperatures [20]. For bonding electric materials to its surface, defining potential adhesive and non-adhesive regions can be accomplished by exposure to UV irradiation [20]. Another commercial silicone material, Ecoflex rubbers are also widely used for flexible capacitance sensors [21]. Ecoflex rubbers are platinum-catalyzed silicones that are soft, strong and "stretchy" [22]. Compared to Ecoflex, PDMS is hard and brittle as shown in Figure 4 [23].

Figure 4. Uniaxial tensile test of Ecoflex 0030 and PDMS [23].

To improve the sensitivity of dielectric elastomer sensors, it is important to develop dielectric elastomers that have a relatively high dielectric constant. There are generally three routes for enhancing the dielectric constant of an elastomer, which include addition of high permittivity inorganic particles [24, 25], addition of conductive fillers [26] and chemical design [27]. Chemical design involves polymer chain modification. Titanium dioxide (TiO2) [28] and barium titanate (BaTiO3) [29] have reportedly served as high permittivity inorganic particles for improving the dielectric constants of elastomers. Conductive fillers enhance the dielectric constant of elastomers by increasing the effective electrode area or facilitating electronic polarization, such as carbon nanotubes (CNTs) [30, 31], metal particles [26] and conductive polymers [32].

## 4. Compliant and stretchable electrodes

Much like the dielectrics, good stretching ability and flexibility in the electrodes are necessary to enable the dielectric elastomer sensors to be used in soft robots, healthcare and entertainment. They must have the ability to exhibit large deformation (bend, fold, twist, compress, and stretch) while maintaining a high level of conductive performance, integration and reliability [33]. Of the work performed on stretchable and flexible electrodes, two main types of conductors, electrical conductors and ionic conductors, have been identified as the most desirable for modification of the electrodes based on their conductance.

## 4.1. Electrical conductors

Sensors employing electrical conductors (e.g., carbon grease, metal films and carbon nanotubes) as electrodes develop signals based on the movement of electrons in the material. The electronic conductors used as compliant electrodes in sensors mainly include carbon-based electrodes [11, 34], metallic thin films [35], composites of conducting materials and elastomers (or rubber fiber mats) [36], conducting films (indium-tin oxide (ITO)-coated poly (ethylene terephthalate (PET)) [37] and liquid conductors [38].

Carbon-based electrodes are widely used for dielectric elastomer sensors because they are compliant, easy to fabricate and exert a low impact on the stiffness of the dielectric [7]. Carbon-based electrodes are usually fabricated from carbon black, graphite [39], carbon nanotubes (CNTs) [34] and single-walled carbon nanotube (SWNT) [1, 40] as loose particles or mixed with the matrix to form a viscous media (carbon grease) [11] or mixed into an elastomer (conductive elastomer) [41]. Goulbourne's experiments showed that the performance of carbon grease was better than that of graphite power, graphite spray or silver grease [42]. Lipomi et al. [34] developed a skinlike pressure and strain sensor composed of carbon nanotubes (CNTs) as compliant electrodes. The advantage of these sensors was that they were transparent compared to the electrodes of sensors fabricated using other carbon-based electrodes such as carbon grease.

In other compliant electrodes composed of a single material (e.g., Au [43], Al [19] and Pt [44], AgNWs [45, 46]), metallic thin films are generally deposited on elastomers by sputtering and using an E-beam evaporator [7]. These electrodes are more suitable for use as pressure sensors [19, 43] or normal and shear force sensors [44], compared to strain sensors [40]. This is because they are thin, highly conductive and can be patterned [7], but their hardness affects the stretching of the capacitance sheets.

Conductive elastomers (composites of conducting materials and an elastomer) offer the advantages of integration into the dielectric layer, robustness and durability. For example, Laflamme et al. [41, 47] reported on a soft, large surface area capacitor network employed in structural health monitoring that was composed of carbon black dispersed in poly-styrene-co-ethyleneco-butyleneco-styrene (SEBS) solution. Experimental testing demonstrated the improved robustness and stable capacitance of the device after it was cut or punched [47]. Cai et al. [36] designed an excellent, durable capacitance strain sensor based on CNTs-doped Dragon skin 30 (Smooth-on, Inc., USA) that could measure strains of up to 300% over thousands of cycles. An elastomeric capacitive sensor array was constructed with conductive PDMS (CNT-PDMS) using micro-contact printing techniques [10]. These devices were stable under various elastic deformations. In additional to these carbon-based electrodes, CNT fabrics were reported that employed a capacitor geometry grid as multimodal skin sensors [2]. In addition to CNTs, elastic conductors composed of silver nanowires (AgNWs) and elastomers have been used for improving mechanical robustness of sensors that can detect stains of up to 50% [48, 49].

Other electrodes used in capacitive sensors include conductive textiles composed of conductive particles and fabrics. A commercial, stretchable conductive textile, termed Electrolyca (Mindsets Ltd, UK), was composed of woven silver with nylon elastic fibers employed as electrodes in a flexible capacitance-sensing element integrated in a measured structure [4]. Another conductive textile, Zelt fabric (Mindsets Ltd, UK), made of soft copper-/tin-coated woven fabrics, was assembled into a dielectric film for use in soft tactile sensors [5]. Park et al. [50] fabricated stretchable, thin-film electrodes using silver nanoparticles and rubber fiber mats. The electrodes retained high conductivity at 140% strain and were compatible with various substrates and could be deployed over large areas.

A conductive film has been reported, which was composed of indium-tin oxide (ITO)-coated poly (ethylene terephthalate) (PET) film and exhibited good performance when its bending radius was greater than 8 mm [51]. This ITO/PET film was suitable for use as a pressure sensor with microstructured elastomeric dielectric film [37, 51, 52].

In addition to the use of compliant electrical conductors, conductive liquids have also been employed based on liquid, metal-filled channels [38, 53, 54] or microliquid metal droplets [55]. This technical approach is beneficial to use in an array of soft tactile sensors.

## 4.2. Ionic conductors

To improve the sensitivity of dielectric elastomer sensors, it is important to develop dielectric elastomers that have a relatively high dielectric constant. There are generally three routes for enhancing the dielectric constant of an elastomer, which include addition of high permittivity inorganic particles [24, 25], addition of conductive fillers [26] and chemical design [27]. Chemical design involves polymer chain modification. Titanium dioxide (TiO2) [28] and barium titanate (BaTiO3) [29] have reportedly served as high permittivity inorganic particles for improving the dielectric constants of elastomers. Conductive fillers enhance the dielectric constant of elastomers by increasing the effective electrode area or facilitating electronic polarization, such as carbon nanotubes (CNTs) [30, 31], metal particles [26] and conductive

Much like the dielectrics, good stretching ability and flexibility in the electrodes are necessary to enable the dielectric elastomer sensors to be used in soft robots, healthcare and entertainment. They must have the ability to exhibit large deformation (bend, fold, twist, compress, and stretch) while maintaining a high level of conductive performance, integration and reliability [33]. Of the work performed on stretchable and flexible electrodes, two main types of conductors, electrical conductors and ionic conductors, have been identified as the most

Sensors employing electrical conductors (e.g., carbon grease, metal films and carbon nanotubes) as electrodes develop signals based on the movement of electrons in the material. The electronic conductors used as compliant electrodes in sensors mainly include carbon-based electrodes [11, 34], metallic thin films [35], composites of conducting materials and elastomers (or rubber fiber mats) [36], conducting films (indium-tin oxide (ITO)-coated poly (ethylene terephthalate

Carbon-based electrodes are widely used for dielectric elastomer sensors because they are compliant, easy to fabricate and exert a low impact on the stiffness of the dielectric [7]. Carbon-based electrodes are usually fabricated from carbon black, graphite [39], carbon nanotubes (CNTs) [34] and single-walled carbon nanotube (SWNT) [1, 40] as loose particles or mixed with the matrix to form a viscous media (carbon grease) [11] or mixed into an elastomer (conductive elastomer) [41]. Goulbourne's experiments showed that the performance of carbon grease was better than that of graphite power, graphite spray or silver grease [42]. Lipomi et al. [34] developed a skinlike pressure and strain sensor composed of carbon nanotubes (CNTs) as compliant electrodes. The advantage of these sensors was that they were transparent compared to the electrodes of

In other compliant electrodes composed of a single material (e.g., Au [43], Al [19] and Pt [44], AgNWs [45, 46]), metallic thin films are generally deposited on elastomers by sputtering and using an E-beam evaporator [7]. These electrodes are more suitable for use as pressure

desirable for modification of the electrodes based on their conductance.

sensors fabricated using other carbon-based electrodes such as carbon grease.

polymers [32].

236 Elastomers

4.1. Electrical conductors

(PET)) [37] and liquid conductors [38].

4. Compliant and stretchable electrodes

There are instances when compliant electrodes must meet the stretch requirements of a sensor as well as the biocompatibility and transparency requirements [9]. Sensors employing ionic conductors can develop signals using these ions. Ionic conductors have been studied for use in applications that demand greater stretch and greater transparency than electronic conductors, such as polyacrylamide hydrogels [56–58] and ionogels composed of ionic liquid and polyacrylic acid [59]. Recent work has demonstrated that these materials can be used as the electrodes of transparent sensors and actuators for artificial muscles, skin, axons and kinesthetic sensing [60–64]. A transparent, capacitive sensor termed ionic skin was first developed using a dielectric elastomer covered with ionic electrodes, which were composed of a polyacrylamide hydrogel with NaCl [9]. To solve the problem of water loss from the hydrogel, high-retention hydrogel conductors [58] were applied in a highly stretchable and transparent actuator [60]. This was then formed into a highly stretchable electroluminescence [65] and an ionic cable that exhibited high-speed and long-distance signal transmission [66]. The highretention hydrogel conductors were very stretchable, with strains that exceeded 1000% as shown in Figure 5. The hydrogel conductor was also very tough. We poked a knife at the hydrogel conductor (Figure 6a). Figure 6b showed it remained intact after being poked [67].

For use in sensors, a good compliant electrode should exhibit low hysteresis of resistance versus strain, a well-controlled surface resistance and limited degradation with load cycling [7].

Figure 5. The hydrogel conductor from an original state to a stretched state.

Figure 6. (a) Poking at the hydrogel conductor with a knife and (b) the ionic conductor remaining intact after being poked.

## 5. Applications of dielectric elastomer sensors

Compared to traditional rigid sensors (resistance strain gages, piezoelectric ceramics sensors, etc.), dielectric elastomer sensors exhibit advantages, including low cost, large deformation (the strain even exceeds 100%) and fast response time, and are easily integrated into monitored structures [5, 50, 52]. They have been used extensively as physical sensors, such as pressure sensors [52], strain sensors [1, 36, 68], normal and shear force sensors [5, 34, 43, 53]. In addition, for simulating multifunctional human skin, multimodal sensors have been made for measuring pressure, temperature and humidity [2, 69]. These outstanding features make these sensors potentially useful in soft robots, human-machine interfaces, human-activity monitoring, personal healthcare and structural health monitoring (Figure 7).

#### 5.1. Flexible and stretchable sensors as capacitive electronic skin

electrodes of transparent sensors and actuators for artificial muscles, skin, axons and kinesthetic sensing [60–64]. A transparent, capacitive sensor termed ionic skin was first developed using a dielectric elastomer covered with ionic electrodes, which were composed of a polyacrylamide hydrogel with NaCl [9]. To solve the problem of water loss from the hydrogel, high-retention hydrogel conductors [58] were applied in a highly stretchable and transparent actuator [60]. This was then formed into a highly stretchable electroluminescence [65] and an ionic cable that exhibited high-speed and long-distance signal transmission [66]. The highretention hydrogel conductors were very stretchable, with strains that exceeded 1000% as shown in Figure 5. The hydrogel conductor was also very tough. We poked a knife at the hydrogel conductor (Figure 6a). Figure 6b showed it remained intact after being poked [67]. For use in sensors, a good compliant electrode should exhibit low hysteresis of resistance versus strain, a well-controlled surface resistance and limited degradation with load cycling [7].

5. Applications of dielectric elastomer sensors

Figure 5. The hydrogel conductor from an original state to a stretched state.

poked.

238 Elastomers

Compared to traditional rigid sensors (resistance strain gages, piezoelectric ceramics sensors, etc.), dielectric elastomer sensors exhibit advantages, including low cost, large deformation

Figure 6. (a) Poking at the hydrogel conductor with a knife and (b) the ionic conductor remaining intact after being

Recently, flexible and stretchable strain sensors have been developed that can be worn as electronic skin (e-skin) for soft robots, wearable devices and human motion monitoring in medicine, including diagnosis development, rehabilitation assistance and activity monitoring [3]. These flexible and stretchable capacitive strain sensors consisted of a dielectric polymer film sandwiched by two compliant electrical conductors. Cohen et al. [68] proposed a highly elastic strain sensor composed of dielectric elastomers sandwiched between CNT percolation electrodes. These sensors can be stretched to 100% of their original size over thousands of cycles with a 3% variability, which has resulted in the demonstration of useful applications in a robotics context for transduce joint angles. A highly stretchable and transparent capacitive strain sensor based on CNT elastic electrodes has been proposed [36]. The highly sensitive strain sensors can measure strains up to 300% with excellent durability, stability and fast response, which have potential applications in wearable smart electronics as demonstrated by experiments in glove and respiration monitoring. Yang et al. [1] made a recoverable motion sensor based on the surface-modified CaCu3Ti4O12 (S-CCTO) nanoparticles involving a self-healing

Figure 7. Applications of the dielectric elastomer sensors in soft robots, human-machine interfaces, human-activity monitoring, personal healthcare and structural health monitoring.

polymer matrix. The composites had a high dielectric permittivity of 93 at 100 Hz. These authors' work showed the benefits of the electrical and mechanical self-healing properties of their motion sensors.

Similar to the concept of human skin, the flexible and stretchable dielectric elastomer sensors can be used as soft strain gauges but can also be used for measurements of pressure and threeaxis forces. Pressure sensors have been fabricated by sandwiching a soft dielectric between two sets of flexible electrodes, such as metal thin films [43], CNTs [34], AgNW-based [45], liquid electrodes [53]. Dielectric elastomer sensors were designed as skin-like tactile sensors, which could measure pressure produced by the human-body activity. These pressures included a low-pressure regime (<10 kPa) produced by intrabody pressure (intraocular pressure and intracranial pressure), a medium-pressure regime (<100 kPa) generated by wave, vibration or pulses (blood pressure, respiration, phonation, heart, radial artery, jugular venous) and a highpressure regime (>100 kPa) produced in the foot [52, 68]. Extremely robust pressure sensors were fabricated using flexible polyurethane foams and Au thin films that could measure normal pressures from 1 to 100 kPa for applications as artificial skin and in wearable robotics [43]. Transparent and stretchable pressure and strain sensor arrays have been developed to detect pressures of up to 1 MPa using nanomaterial electrodes. Potential applications include prosthetic limbs, bandages, robotics and touch screens [34, 43]. Li et al. [53] proposed highly deformable tactile sensing arrays based on low modulus platinum-catalyzed silicone polymer (EcoFlex00-30) with embedded liquid metal microfluidic (eutectic gallium indium) arrays. The wearable tactile sensors could be stretched greater than 400% and could conform to curved objects or soft biological tissues.

However, the incompressibility and viscoelasticity of rubbers limit their sensitivity for use in pressure sensors [52]. By constructing an air gap and compressible dielectric layers between two electrodes, the performance of the pressure sensors can be improved [70]. Zhang et al. [21, 71] designed a compressive soft sensor with an air gap and theoretically demonstrated that the air gap could improve the sensitivity of the pressure sensors. These authors'results showed that when the thickness of the air gap was large, the detection range was large but the resolution was small.

Based on the structure containing an air gap, three-axial force sensors were designed using a 2D overlap with a larger top electrode on four bottom electrodes [5, 11]. In this structure, Zhang et al. [11] investigated the effect of the geometry (rectangle and circular sector) of the electrodes on the sensitivity, linearly, hysteresis and detection range of the device using simulations and experimentation. Their results showed that the rectangular strategy improved the performance of the sensor over that of a circular design (four circular sector bottom electrodes). Viry et al. [5] developed a highly compressible and sensitive capacitive three-axial force sensor that was fabricated with a top textile electrode and four bottom textile electrodes sandwiching a fluorosilicone film and an air gap. This device reportedly could detect pressures of up to 190 kPa (estimated up to 400 kPa) and minimal weights of less than 10 mg. Hence, the application of the sensor was extensive, including biomimetic touch, heartbeat monitoring and foot pressuredistribution monitoring.

The micro/nanostructured dielectrics have been widely developed to improve the sensitivity and response and/or relaxation time of the pressure sensors [72]. Mannsfeld et al. [52] proposed a type of organic thin film that consisted of a regularly structured dielectric layer with organic field-effect transistors (OFETs) for use as pressure sensors. The design of this device resulted in faster response and relaxation times (<<1s), and higher sensitivity (0.55 kPa<sup>1</sup> ) than the unstructured film (0.02 kPa). Using a device with microstructured rubber dielectric layers, Schwartz et al. [37] further developed highly flexible transistor devices with high sensitivity (8.4 kPa<sup>1</sup> ), high stability and low power consumption that were used as an electronic skin for humanmachine interfaces [37]. Various microstructures have been studied for use as the dielectric layer for flexible capacitive pressure sensors, including pyramids [19], microhairs [73] and microspheres [74]. Tee et al. [19] found that the pyramidal structures with relative small sidewall angles were the optimum shapes for rendering improved sensitivity, compared to structures with relative large sidewall angles and square cross-sectional structures. In addition, when the structures were spaced further apart, the sensitivity increased. The authors also demonstrated that these pressure sensors could be used for blood pulse monitoring and next-generation force sensing track pads with high sensitivity, easy manufacturing and low cost [19]. The design of microhair structures for flexible pressure sensors can enhance the signal-to-noise ratio and enable signal detection of the deep-lying internal jugular venous pulses for healthcare monitoring [73]. Li et al. [74] proposed a pressure sensor composed of polystyrene microspheres (dielectric layer) and Au electrodes with a surface micropattern similar to lotus leaves. The devices facilitated measurements over a wide dynamic response range (0–50N) in addition to high sensitivity (0.815 kPa<sup>1</sup> ) and a fast response time. This device was suitable for use over a wide pressure-measuring range for applications in wearable healthcare devices, patient rehabilitation and biomedical prostheses. In other devices, a dielectric layer of the pressure sensor respectively employed porous PDMS [51], porous silicone foam [75, 76] or graphene oxide (GO) foam [77]. In particular, Chen et al. [51] developed a microstuctured elastomeric dielectric film with air voids (porous PDMS) integrated with ITO/PET electrodes, which was highly sensitive and fast, capable of measuring pressures from 1 to 250 kPa and durable under loads larger than 1 MPa. Experiments demonstrated that these advantages of this device readily let it for use as a smart insole and a wrist pulse monitoring sensor.

#### 5.2. Ionic skin

polymer matrix. The composites had a high dielectric permittivity of 93 at 100 Hz. These authors' work showed the benefits of the electrical and mechanical self-healing properties of their motion

Similar to the concept of human skin, the flexible and stretchable dielectric elastomer sensors can be used as soft strain gauges but can also be used for measurements of pressure and threeaxis forces. Pressure sensors have been fabricated by sandwiching a soft dielectric between two sets of flexible electrodes, such as metal thin films [43], CNTs [34], AgNW-based [45], liquid electrodes [53]. Dielectric elastomer sensors were designed as skin-like tactile sensors, which could measure pressure produced by the human-body activity. These pressures included a low-pressure regime (<10 kPa) produced by intrabody pressure (intraocular pressure and intracranial pressure), a medium-pressure regime (<100 kPa) generated by wave, vibration or pulses (blood pressure, respiration, phonation, heart, radial artery, jugular venous) and a highpressure regime (>100 kPa) produced in the foot [52, 68]. Extremely robust pressure sensors were fabricated using flexible polyurethane foams and Au thin films that could measure normal pressures from 1 to 100 kPa for applications as artificial skin and in wearable robotics [43]. Transparent and stretchable pressure and strain sensor arrays have been developed to detect pressures of up to 1 MPa using nanomaterial electrodes. Potential applications include prosthetic limbs, bandages, robotics and touch screens [34, 43]. Li et al. [53] proposed highly deformable tactile sensing arrays based on low modulus platinum-catalyzed silicone polymer (EcoFlex00-30) with embedded liquid metal microfluidic (eutectic gallium indium) arrays. The wearable tactile sensors could be stretched greater than 400% and could conform to curved

However, the incompressibility and viscoelasticity of rubbers limit their sensitivity for use in pressure sensors [52]. By constructing an air gap and compressible dielectric layers between two electrodes, the performance of the pressure sensors can be improved [70]. Zhang et al. [21, 71] designed a compressive soft sensor with an air gap and theoretically demonstrated that the air gap could improve the sensitivity of the pressure sensors. These authors'results showed that when the thickness of the air gap was large, the detection range was large but the

Based on the structure containing an air gap, three-axial force sensors were designed using a 2D overlap with a larger top electrode on four bottom electrodes [5, 11]. In this structure, Zhang et al. [11] investigated the effect of the geometry (rectangle and circular sector) of the electrodes on the sensitivity, linearly, hysteresis and detection range of the device using simulations and experimentation. Their results showed that the rectangular strategy improved the performance of the sensor over that of a circular design (four circular sector bottom electrodes). Viry et al. [5] developed a highly compressible and sensitive capacitive three-axial force sensor that was fabricated with a top textile electrode and four bottom textile electrodes sandwiching a fluorosilicone film and an air gap. This device reportedly could detect pressures of up to 190 kPa (estimated up to 400 kPa) and minimal weights of less than 10 mg. Hence, the application of the sensor was extensive, including biomimetic touch, heartbeat monitoring and foot pressure-

The micro/nanostructured dielectrics have been widely developed to improve the sensitivity and response and/or relaxation time of the pressure sensors [72]. Mannsfeld et al. [52] proposed a

sensors.

240 Elastomers

objects or soft biological tissues.

resolution was small.

distribution monitoring.

Ionic conductors have attracted attention in capacitive tactile sensing applications, due to their high transparency and conductivity under large deformation [61–63]. Nie et al. [61] fabricated a capacitive pressure sensor utilizing an ionic gel matrix. This transparent film sensor had a high sensitivity (3.1 nF/kPa) because the electrical double layer produced an ultra-high unitarea capacitance. The sensor also exhibited excellent mechanical and thermal stability and a rapid response. The applicability of this sensor as a wearable device was successfully demonstrated so that it was later incorporated into the consumer electronic devices including a smart watch, augmented reality glasses and a custom fingertip-mounted tactile sensor. Sun et al. [9] developed a highly stretchable and transparent capacitive sensing sheet called ionic skin, which was composed of a dielectric and two ionic conductors. These sensors can detect strains over a wide range (from 1 to 500%) and pressures as low as 1 kPa. The working principle of the sensor was reported in detail in the literature [9]. A hybrid ionic-electronic circuit was formed by connecting the ionic conductors to electronic conductors for transmitting electrical signals. When a low voltage was applied between the two electrodes, electrochemical reactions did not occur, and no electrons or ions crossed the interface between the electrode and the ionic conductor. This allowed electrical double layers to form at the interface, similar to a capacitor CEDL, which was in series with a capacitor Cm formed by the ionic conductors and the dielectric (Figure 8). As a result, the capacitance C measured between the two electrodes can be written as C ¼ Cm=ð2Cm=CEDL þ 1Þ. The capacitance of the dielectric was much less than the electrical double layer since the separation (nanometer) of the charges in the electrode and in the ionic conductor was much less than the distance between the charges separated by the dielectric with a thickness. Consequently, the measured capacitance C was dominated by the capacitance of the dielectric Cm. According to this principle, a transparent capacitive sensing film was fabricated in our laboratory for structural health monitoring.

## 5.3. Flexible and large-surface dielectric elastomer sensors applied in structural health monitoring

Detecting and locating damage to bridges, roads, buildings and other structures is necessary to ensure a long lifespan of the national infrastructure. Detecting defects and providing warnings in time can prevent the collapse of structures. Soft film sensors employing functional materials have been proposed for monitoring the condition of structures. These sensors include resistance-based, piezoelectric, antenna, vacuum and capacitance-based strain sensors. Some of these technologies are unsuitable for use on a large scale, because their complex manufacturing processes make them too expensive.

In the recent developments in structural health monitoring, Laflamme et al. [78] first proposed a sensing technique for damage localization using a layer of commercial dielectric polymer (DEAP, Danfoss PolyPower) on the surface of a monitored structure with a same reference

Figure 8. A schematic diagram of additional electric charges accumulated when a voltage is applied between the electrical conductors [9].

capacitance. The next step entailed fabricating soft elastomeric capacitors (SECs) that were inexpensive and useable on large surface areas [79]. The robust and static characterization of the sensors was demonstrated through testing [47]. Static characterization, dynamic monitoring, localization of fatigue cracks and the distribution of thin film sensor arrays on structures were studied using the referenced SECs [80–83]. The electrodes of these sensors were composed of SEBS containing carbon black particles, and the sensors were not transparent. The damaged parts of the subject structure are invisible when the sensors were placed on the surface.

occur, and no electrons or ions crossed the interface between the electrode and the ionic conductor. This allowed electrical double layers to form at the interface, similar to a capacitor CEDL, which was in series with a capacitor Cm formed by the ionic conductors and the dielectric (Figure 8). As a result, the capacitance C measured between the two electrodes can be written as C ¼ Cm=ð2Cm=CEDL þ 1Þ. The capacitance of the dielectric was much less than the electrical double layer since the separation (nanometer) of the charges in the electrode and in the ionic conductor was much less than the distance between the charges separated by the dielectric with a thickness. Consequently, the measured capacitance C was dominated by the capacitance of the dielectric Cm. According to this principle, a transparent capacitive sensing film was

5.3. Flexible and large-surface dielectric elastomer sensors applied in structural health

Detecting and locating damage to bridges, roads, buildings and other structures is necessary to ensure a long lifespan of the national infrastructure. Detecting defects and providing warnings in time can prevent the collapse of structures. Soft film sensors employing functional materials have been proposed for monitoring the condition of structures. These sensors include resistance-based, piezoelectric, antenna, vacuum and capacitance-based strain sensors. Some of these technologies are unsuitable for use on a large scale, because their complex manufacturing

In the recent developments in structural health monitoring, Laflamme et al. [78] first proposed a sensing technique for damage localization using a layer of commercial dielectric polymer (DEAP, Danfoss PolyPower) on the surface of a monitored structure with a same reference

Figure 8. A schematic diagram of additional electric charges accumulated when a voltage is applied between the electrical

fabricated in our laboratory for structural health monitoring.

monitoring

242 Elastomers

conductors [9].

processes make them too expensive.

We have developed a transparent, capacitive sensor that is inexpensive and can easily detect cracks in large-scale infrastructures [67]. The sensor was attached to a monitored surface by a bonding layer (Figure 9). The sensor consisted of a transparent dielectric elastomer sandwiched between two transparent and stretchable ionic electrodes. The transparent capacitive sensor could be used to monitor large areas of a structure to detect potential damage. The robustness of this sensor was demonstrated by tests (Figure 10a and b), particularly on the over-reinforced beam. Tests conducted on the over-reinforced concrete beam demonstrated that the sensor was capable of detecting small cracks (Figure 10c and d).

### 5.4. Cantilever sensors (force, torque and displacement) for robotic technologies

In the previously described applications of dielectric elastomer sensors, most of these sensors were in the shape of a flat sheet. This is not suitable for measuring other mechanical parameters such as concentrated force and displacement. The cantilever beam sensors are a class of force sensors that are used extensively for weight measurement, environmental monitoring, biological monitoring and gas detection. Most of these devices are rigid sensors that are unsuited for soft robotics applications. There have been few reports in the literature about soft displacement or force dielectric elastomer sensors based on the flexible beam. Lucarotti et al. [4] proposed a strategy to sense-bending angle and force in a soft body based on beam configuration, which was integrated with two capacitive sensing elements for distinguishing between convex and concave bending. In their analysis of the device, they imposed a bending and/or an external force to a soft beam. The results confirmed the superiority of this sensing strategy for use in soft robotics.

Figure 9. A schematic of a dielectric covered with ionic conductors deployed over the surface of a monitored structure by using a bonding layer [67].

Figure 10. (a) An illustrative photo of four cuts on the transparent capacitive sensor, (b) relative change in capacitance when cuts were made, (c) a photograph of cracks formed under the sensor and d) relative change in capacitance of the sensor and load against time [67].

We have proposed a dielectric elastomer cantilever beam sensor for measuring force or displacement [84]. A dielectric membrane sandwiched between compliant electrodes was pasted to the surface of a soft uniform strength cantilever beam by the bonding layer, which made the sensing device as shown in Figure 11. The concentrated force at the free end induced a change in the capacitance of the dielectric membrane. Therefore, the applied force could be determined from the change in the capacitance of the sensor. The beam of uniform strength was used to determine that the deformation of dielectric membrane was homogeneous. The results of the experiments showed the change in capacitance almost increased linearly with the increasing of the force from 0 N to 2 mN, which was applied to the end of the beam. As the results indicate, the dielectric elastomer cantilever beam force sensor appeared to work well. Because the force was linear with the deflection at the free end of the cantilever beam, the proposed device can be used as a displacement sensor.

#### 5.5. Multimodality of dielectric elastomer sensors

The tactile sense of the human skin not only includes physical responses such as strain, pressure, shear and torsion but also temperature and humidity. To mimic the human skin, the electronic skin must be designed to sense multimodality consisting of physical, biological and chemical stimuli. The integration of flexible and stretchable multiple sensors into wearable platforms has been developed for applications in human-activity monitoring, human-machine interfaces and personal healthcare [2, 3, 69, 85]. To assess these parameters, multimodal

Figure 11. A sketch of the dielectric elastomer cantilever beam force sensor [84].

dielectric elastomer sensors have been tested. Kim et al. [2] described a highly sensitive and multimodal capacitive sensor composed of Ecoflex and carbon nanotube microyarns. The sensor was capable of measuring pressures as low as 0.4 Pa, temperature or humidity gradients and biological variables with different dipole moments in a single pixel. The measurements of mechanical deformation, humidity and biological variables were realized by the change in capacitance. The measurement of temperature was accomplished by determining a change in resistance. Ho et al. [69] developed a transparent and stretchable multimodal electronic skin sensor matrix. This sensor could measure pressure, thermal and humidity simultaneously using independent electrical signals. Graphene (chemical vapor deposition) electrodes sandwiched PDMS, which formed capacitive sensors for measuring pressure and strain. The graphene oxide (GO) and reduced graphene oxide (rGO) were used as impedance humidity sensor and resistive thermal sensor, respectively. The pressure, humidity and thermal sensors were integrated into a layer-by-layer geometry. In addition, excellent sensitivity of the sensors was demonstrated by monitoring moving hot air, breathing and finger touching.

In addition to the previously mentioned applications of dielectric elastomer sensors, other applications have also been developed. For example, a soft capacitive tactile sensor consisting of two sensing elements with different stiffness was reported for measuring tissue elasticity [86]. The results of tests using this device showed that the sensor could detect tissue elasticity from 0.1 to 0.5 MPa. Dielectric elastomer sensors have also been used as self-sensing actuators, such as pneumatic actuators [62], dielectric elastomer actuators [87], coaxial actuators [88] and McKibben actuators [39].

## 6. Conclusions and outlook

We have proposed a dielectric elastomer cantilever beam sensor for measuring force or displacement [84]. A dielectric membrane sandwiched between compliant electrodes was pasted to the surface of a soft uniform strength cantilever beam by the bonding layer, which made the sensing device as shown in Figure 11. The concentrated force at the free end induced a change in the capacitance of the dielectric membrane. Therefore, the applied force could be determined from the change in the capacitance of the sensor. The beam of uniform strength was used to determine that the deformation of dielectric membrane was homogeneous. The results of the experiments showed the change in capacitance almost increased linearly with the increasing of the force from 0 N to 2 mN, which was applied to the end of the beam. As the results indicate, the dielectric elastomer cantilever beam force sensor appeared to work well. Because the force was linear with the deflection at the free end of the cantilever beam, the

Figure 10. (a) An illustrative photo of four cuts on the transparent capacitive sensor, (b) relative change in capacitance when cuts were made, (c) a photograph of cracks formed under the sensor and d) relative change in capacitance of the

The tactile sense of the human skin not only includes physical responses such as strain, pressure, shear and torsion but also temperature and humidity. To mimic the human skin, the electronic skin must be designed to sense multimodality consisting of physical, biological and chemical stimuli. The integration of flexible and stretchable multiple sensors into wearable platforms has been developed for applications in human-activity monitoring, human-machine interfaces and personal healthcare [2, 3, 69, 85]. To assess these parameters, multimodal

proposed device can be used as a displacement sensor.

5.5. Multimodality of dielectric elastomer sensors

sensor and load against time [67].

244 Elastomers

In this chapter, the primary sensing principles and materials used in flexible and stretchable dielectric elastomer sensors have been described along with the efforts to develop dielectric elastomer sensing devices for applications in soft robotics, structure health monitoring, electronic or ionic skin for human-activity and personal healthcare. In addition, developments in our own laboratory on dielectric elastomers were presented, including cantilever force (displacement) sensors for soft robotics and transparent sensors used in structural health monitoring.

Highly sensitive, flexible and stretchable dielectric elastomer mechanical sensors have been developed for soft robots and personal healthcare; however, a tunable measurement range is needed for different positions on the human body or on robots. For practical applications of dielectric elastomer sensors, the performance of the sensors must be further improved to increase their durability and minimize drift and interference. The multiple dielectric elastomer sensors need be developed to improve their sensitivity in the measurement of temperature, humidity, chemical and biological stimuli. For future development, a sensor-integrated wearable platform with low-power consumption will be needed, which is equipped with a wearable power generator with high output efficiency as well as a power-storage device with large capacity [3].

## Author details

Na Ni and Ling Zhang\*

\*Address all correspondence to: zhangl@mail.xjtu.edu.cn

State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi'an Jiaotong University, Xi'an, China

## References


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laboratory on dielectric elastomers were presented, including cantilever force (displacement)

Highly sensitive, flexible and stretchable dielectric elastomer mechanical sensors have been developed for soft robots and personal healthcare; however, a tunable measurement range is needed for different positions on the human body or on robots. For practical applications of dielectric elastomer sensors, the performance of the sensors must be further improved to increase their durability and minimize drift and interference. The multiple dielectric elastomer sensors need be developed to improve their sensitivity in the measurement of temperature, humidity, chemical and biological stimuli. For future development, a sensor-integrated wearable platform with low-power consumption will be needed, which is equipped with a wearable power generator with high output efficiency as well as a power-storage device with large

State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi'an Jiaotong

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