HIGH-TEMPERATURE THERMOPLASTIC ELASTOMERS FROM RUBBER–PLASTIC BLENDS: A STATE-OF-THE-ART REVIEW
ABSTRACT
This article reviews different types of high-temperature thermoplastic elastomers and thermoplastic vulcanizates from rubber-plastic blends. Preparation, structure, and properties of these materials are discussed briefly. Strategies to further improve the high-temperature performance of these materials are presented herein. A synopsis of the applications of these high-performance materials in the automotive industry is reported, pointing out the gaps to motivate potential research in this field.
I. INTRODUCTION
The emergence of thermoplastic elastomers (TPEs) has provided a new horizon in the field of polymer science and technology.1,2 Since its introduction in the market during the 1950s, TPEs have shown tremendous growth in the polymer industry.1,2 They have excellent physicomechanical properties, elasticity, processability, recyclability, and resistance to heat, chemicals, and ultraviolet radiation. These lightweight materials have a pleasant touch, luxurious feeling, and excellent price-performance ratio, all of which make them ideal candidates for enormous applications.3 The most important aspect of these materials is that these can be used for new applications because of the ease of fabrication and excellent price-performance ratio. Approximately 40% of all TPE products consumed worldwide are used in vehicle manufacturing.4–6 Most of the TPEs and thermoplastic vulcanizates (TPVs) developed so far from rubber–plastic blends have poor high-temperature performance and properties, such as low heat resistance and swelling resistance in various oils/fluids and poor compression set. The maximum operating temperature of conventional ethylene propylene diene rubber (EPDM)/polypropylene (PP) TPEs is 125 °C.7 Coran7 reported that the operating temperature of TPEs could be enhanced by using high melting point plastics. However, most of the rubbers tend to degrade at the processing temperature of such plastics. Also, many high-melting plastics have some interesting processing problems. Again, some high-temperature rubbers that withstand high processing temperature of plastics are not compatible with plastics. Poor rubber/plastic compatibility can result in poor TPV mechanical properties. In the case of compatible plastic/elastomer pairs, high TPV melt viscosity may render the product difficult to process. Addition of plasticizer to improve the TPV processability generally reduces rubber/plastic adhesion, resulting in poor TPV mechanical properties, unlike the incorporation of paraffinic oil plasticizer to PP/EPDM TPVs. Large amounts of plasticizer to TPVs from polar rubber/polar plastic combinations also results in bleed from the final product. Therefore, it is a challenge to develop chemical-, heat-, and oil-resistant TPEs and TPVs.
It may also be pointed out that polar rubber and polar plastic in the blends interact with each other at the temperature of mixing and produce molecules that may act as a compatibilizing agent. This is a positive feature and has been amply demonstrated in our earlier research.3 This is not the case with EPDM/PP blends.
Nowadays, the automotive industry has undergone dramatic changes in manufacturing technology and requirements. The complicated and sophisticated designs for automotive parts have led to increased temperature and durability requirements, dictating the development of materials having improved high-temperature properties and capability of sustaining such critical service conditions with maximum efficiency. Consequently, it is of great importance to develop high-performance materials with long-term heat and weather-resistance properties for such applications. Also, in many cases, oil/solvent resistance becomes equally important. Elastomers with increased high-temperature performance and oil resistance are required for the modern automobile because of increased underhood temperatures. Improved elastomer weatherability is also required.
In the present review, high-performance, heat-resistant TPEs and TPVs are presented with reference to their method of preparation, types, structure, properties, applications, and development trends. Special attention is paid to strategies to improve further the high-temperature performance of these materials. It may be mentioned here that this review covers all the reported high-temperature TPEs and TPVs from the blends of plastics and rubbers, although a few of these may not be on the market at present because of cost, competition, or marketing strategy. However, understanding the science behind the development of these materials is important in order to innovate materials of the future. It may also be noted that there are reports of a few potential rubber–plastics blends, but they do not present detailed measurement of TPE properties, namely, elongation, set, processability, and recycling. These are, in general, not included in this review.
II. TPEs
TPEs can be defined as a class of special type of polymers that exhibit the hybrid behavior of thermoplastics and rubbers.8–13 They can be processed like thermoplastics and at the same time can perform like rubber over a broad-use temperature range.14,15 The unique feature of TPE is the biphasic morphology, in which one phase is soft and rubbery in nature.16 On the other hand, the other phase is generally continuous, hard, and semicrystalline in nature. During processing at high temperature, the hard plastic phase melts and flows, which solidifies reversibly on cooling. Hence, the hard phase can act as physical cross-links at room temperature and provide thermoplasticity to these materials at melt processing temperature.17–19
III. TPVs
According to ASTM D 5046, a TPV is obtained when a rubber is vulcanized during simultaneous mixing with a plastic, in a process called dynamic vulcanization.20 Dynamic vulcanization was first claimed by Gessler and Haslett21 in 1962 and was then further developed by Fisher and Coran et al.22–24 The first TPV was commercialized by partially cross-linking the EPDM phase of PP/EPDM blends.22 Fisher22 maintained the thermoplastic processability of these blends by controlling the degree of vulcanization and limiting curative concentration. Significant improvement of the properties of PP/EPDM blends was achieved by Coran et al.24 by fully vulcanizing the rubber phase under dynamic shear while maintaining the thermoplasticity of the blends. Room-temperature PP/EPDM morphology is best described by 1–5 μm in diameter cross-linked, oil-swollen EPDM particulate rubber in a continuous PP matrix.25–27
IV. THE PRODUCTION METHOD OF HIGH-TEMPERATURE TPEs AND TPVs
High-temperature TPEs and TPVs are generally prepared by melt mixing of high-temperature plastics and or rubbers, which have considerable practical importance.28,29 If one follows the development and emergence of new classes of TPEs in the past few decades, it is clear that suitable melt blending of the plastics and the rubber to produce novel TPEs is the present trend. This approach is particularly attractive because so many commercial polymers with desired properties are available for blending. Also, the worldwide academic and industrial research on TPEs in the past few decades has accelerated its growth, providing a wealth of scientific information that paves the way to the introduction of new TPE products. The product properties are further improved by dynamic vulcanization (TPVs).
It may be mentioned here that PP/EPDM TPVs have been commercially successful because of the large oil content (low-cost component) of these materials. For high-temperature, oil-resistant TPVs (polar materials), high plasticizer cost and limited quantity of plasticizer that is contaminable in these products may lead to poor TPV economy, unless these are addressed.
V. CLASSIFICATION OF HIGH-TEMPERATURE TPEs AND TPVs
The conventional TPEs from rubber/plastics are made from EPDM and PP (typified by Santoprene Thermoplastic Rubber). High-temperature and high-performance TPEs can be classified based on the selection of rubbers and plastics as given below:
- 1.
High-temperature TPEs based on high-performance heat-resistant plastics (e.g., polyamide (PA), polyethylene terephthalate, fluorinated polymers, copolyesters, and other engineering plastics)
- 2.
High-temperature TPEs based on high-performance heat-resistant rubbers (e.g., acrylate rubber, silicone, fluorinated rubbers, hydrogenated nitrile rubber [HNBR], etc.) TPVs can be produced from (1) and (2) by appropriate dynamic vulcanization of the rubber phase.
- 3.
Super-TPVs, also may be termed as HOTs, high-temperature oil-resistant thermoplastic vulcanizates (when high-performance plastics and high-performance vulcanized rubbers are combined to make TPVs)
Generally, conventional TPEs and TPVs exposed to heat/oil/air may show a progressive change in their physical, chemical, and mechanical properties. On the other hand, high-temperature TPEs and TPVs are capable of operating for an extended period while still retaining their properties.
VI. HIGH-TEMPERATURE TPEs AND TPVs BASED ON HIGH-PERFORMANCE HEAT-RESISTANT PLASTICS
In the following sections, several high-temperature TPEs and TPVs (based on high-performance heat-resistant plastics) will be discussed, highlighting their preparation, several properties, and morphology.
A. PA/acrylonitrile butadiene rubber TPEs
First, high-temperature heat- and oil-resistant TPEs were developed by Coran and Patel30 in 1980 from PA/acrylonitrile butadiene rubber (NBR). PA/NBR TPEs were prepared by the melt-blending route at 220 °C. These TPEs exhibited excellent oil resistance (5.7–8.6% in ASTM No. 3 oil at 150 °C) compared with the conventional PP/EPDM TPEs (109–225% in ASTM No. 3 oil at 100 °C).30 The tensile strength of PA/NBR blends was in the range of 10–20 MPa. In addition, these TPEs have high strength at elevated temperature due to their high melting point. Recently, Gomes et al.31 have evaluated the influence of antioxidants (a combination of Irganox/Irgafos for the PA phase and Naugard 445 for the NBR phase) on the properties of the PA6/NBR blend. Better thermal protection and mechanical properties were obtained by using these antioxidants. Therefore, it could be inferred that by using a suitable antioxidant in such types of high-temperature TPEs and TPVs, it is possible to improve oxidation resistance and further high-temperature properties.
B. PA/HNBR TPEs
High-temperature TPEs were developed by Bhowmick and Inoue32 in 1993 from PA (nylon)/HNBR. They examined the structure development during dynamic vulcanization of these PA/HNBR blends and also investigated the role of chemical compatibilizers (carboxy-terminated nitrile rubber, amine-terminated nitrile rubber, and liquid carboxylated nitrile rubber) of such high-temperature TPEs and TPVs.32,33 Figure 1 exhibits a typical Debye-Bueche plot of I(q)−½ versus q2 (using the data obtained from light scattering experiments), where I(q) is the intensity of the scattered light and q is the magnitude of the scattering vector. The correlation distance (ζ) is a structural parameter obtained from the slope and the intercept of the Debye-Bueche plot as shown below33 (physically, the average distance between the consecutive particles in the morphology is indicated by the parameter):
where 〈η〉 is the mean square fluctuation of the refractive index. The morphology parameters, mean radius of the dispersed particles (R), and specific interfacial area (Ssp) were calculated from the correlation distance. It was revealed that the particle size of the dispersed rubber phase and the correlation distance of the PA/HNBR (50/50 w/w) blend decreased with increasing mixing time, attained a minimum value, and then increased (Figure 2). The decreased particle size could be due to the graft reaction between PA and the rubber and the compatibilizing action. In addition, particle size and interfacial thickness were reduced after dynamic vulcanization. A typical transmission electron microscopy photograph of a dynamically vulcanized and compatibilized PA/HNBR blend is displayed in Figure 3. To explain these phenomena, Bhowmick and Inoue32 used a model similar to that proposed by Wu34 and considered the breaking down of rubber particles, coagulation and coalescence of droplets, and droplet rupture in a matrix of PA. It was estimated that the interaction parameter and interfacial tension of dynamically vulcanized PA/HNBR blends at 250 °C were 2.8 × 10−3 and 0.24 mN/m, respectively.32 Depending on the nature and concentration of the compatibilizer, it was shown that the interfacial thickness increased (48 to 70–80 nm) and the interfacial tension decreased (0.240 to 0.209–0.190 mN/m) for reactive rubber–plastic blends.33 It should be mentioned here that there was an optimum level of the compatibilizer beyond which no significant change of interfacial parameters was observed. It was also demonstrated that the structural parameters of such reactive rubber–plastic blends were dependent on the sequence of mixing of the compatibilizer. For example, when the compatibilizer was first mixed in the PA phase, dispersed particles were found to be larger. On the other hand, mixing the compatibilizer first in rubber phase generated smaller dispersed particles.
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
C. High-performance thermoplastic alloy
Venkataswamy and Payne35 reported a high-performance thermoplastic alloy, termed TPE-4000 (a proprietary material), which was designed to meet a continuous service temperature of 177 °C with intermittent excursions to 200 °C in hot air and hydrocarbon fluids. TPE-4000 TPEA was prepared by a proprietary dynamic vulcanization process. The thermoset rubbers were silicone rubber, polyacrylate rubber, and ethylene–acrylic rubber. A polyester Hytrel G-47774 was included in this study. The properties are compared with high-temperature elastomers in Figure 4a,b. TPE-4000 has excellent performance in ASTM oil No. 3, 10W-30 oil, transmission fluid, and alkaline and acid media at ambient condition. Percentage elongation retention of TPE-4000 TPEA is equivalent to that of silicone rubber, at 150 °C.
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
D. PA 6/acrylate rubber TPEs
Jha and Bhowmick36 developed heat and oil resistance PA 6 (PA6)/acrylate rubber (ACM) TPE blends by the melt blending method. PA6/ACM TPEs exhibited two-phase morphological structure in which ACM was dispersed in the micron range (0.5–1 μm) in the PA matrix (Figure 5).36 The structure development and its relation to properties have been investigated. Figure 6 shows the temperature dependence of tanδ and storage moduli (E′) of a 40/60 (w/w) nylon 6/ACM blend mixed at different times. The tanδmax decreases progressively up to 9 min of reaction. Interestingly, there is appearance of a secondary tan δ peak in the high-temperature region with increasing level of interaction. The properties of these blends at various blend ratios are given in Table I. In the case of TPE blends with a greater amount of plastic content, the tensile strength reaches a value of 19–20 MPa and an elongation of about 100–120%. These blends, when dynamically vulcanized, display an increased elongation at break in general. However, enhanced tensile strength was obtained only for 50/50 (w/w) and 60/40 (w/w) nylon 6/ACM blend compositions. In addition, the Young's modulus and the hardness of the blends decreased slightly due to vulcanization. The blends showed excellent resistance to oil swelling at elevated temperature (e.g., 150 °C), and its service temperature range can be extended up to 175 °C.37 The mechanical properties of these blends do not deteriorate to a significant extent in the temperature range of 150–200 °C. The volume swell in ASTM oil No. 3 at 150 °C of these blends is well below 10%, which suggests its excellent hot-oil resistance.38 In addition, volume swell decreased with increasing the PA content in the blends, as given in Figure 7. The blends are reactive in nature (there is interaction between the blend components) and various key properties such as mechanical, dynamic–mechanical, oil and fuel resistance, and aging resistance of the blends are improved significantly with an increase in the extent of the reaction between the two components. The possible interfacial reaction between PA6 and ACM is given in Figure 8. In addition, there is a possibility of producing the graft polymer from the reaction of the epoxy group with the secondary amide group of PA6, as given in Figure 8. The temperature range for these reactions to occur is 190–230 °C.
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
PA6/ACM blends are pseudoplastic in nature, and an increase in shear rate decreases the viscosity and increases the extrudate swell of the blends. The viscosity of the blends displays positive deviation from the average values, suggesting the reactive nature of the blend components.39 As there is interfacial reaction between the blend components, as discussed earlier, the amount of interaction increased the molecular weight during melt mixing of two polymers, which increased the viscosity deviation from the average values. The activation energy of the melt flow of the dynamically vulcanized 40/60 (w/w) PA6/ACM blend (the probable reactions between PA6 and ACM are given in Figure 8) varied in the range 8–15 kcal/mol and decreased with increasing shear rate. The blends were found to be reprocessable at 240 °C without any appreciable degradation of either phase, which suggests their applicability as TPEs. The mechanical properties of the TPE 40/60 (w/w) blend (dynamically cured) did not deteriorate to a significant extent when the samples were aged at different temperatures (150–200 °C) and times (1–7 days).40 This implies the excellent heat-resistant properties of the blends. The properties of a typical product from a 40/60 PA/ACM TPV are shown in Table II.41
E. Poly(butylene terephthalate) and acrylate rubber TPEs
Jha and Bhowmick42 also developed heat- and oil-resistant poly(butylene terephthalate) (PBT) and acrylate rubber (ACM) TPE blends using an internal mixer in the temperature range of 220–230 °C.42 PBT/ACM blends exhibited significant dynamic modulus at 150 °C (3160 MPa), which confirms their applicability at high service temperature range. Measurement of properties indicates that the service temperature can be extended up to 150 °C without much deterioration of the mechanical properties. The blends showed excellent resistance to oil swelling up to 150 °C.42 The volume swell of PBT/ACM blends is in the range of 12–25% (in ASTM oil No. 3 at 150 °C for 72 h), which suggests its excellent hot-oil resistance.42
F. PA/ethylene propylene diene terpolymer TPEs
TPVs based on PA/EPDM with excellent mechanical properties and reprocessability were prepared by dynamic vulcanization.43,44 Microscopic analysis demonstrated that EPDM particles were distributed at an average size of 1 μm in PA matrix after dynamic vulcanization.43 It was revealed that maleic anhydride (MAH) grafted ethylene propylene copolymer has a better performance in compatibilizing the PA/EPDM TPEs. It has been reported that sulfur as curative during dynamic vulcanization has given better mechanical performance of the TPVs than phenolic resin or peroxide as curative. However, the properties of sulfur-cured TPVs should vary depending on processing conditions due to sulphur-induced rubber particle agglomeration. The authors of the article compared a sulfur-cured TPV with the corresponding resin-cured TPV without using a promoter for the resin cure, as per conventional practice. In fact, sulfur cure is not desirable in TPVs as sulfur–sulfur bonds break and reform, causing agglomerations of the rubber particles in a TPV melt, resulting in unstable TPV morphology. Table III compares the properties of PA/EPDM TPVs. It was found that tensile strength and elongation at break increased with increase in PA content. On the other hand, the thermal stability of the blends significantly increased with the addition of EPDM into PA, which was further enhanced after effective compatibilization of PA/EPDM systems using EPM-g-MA.44 This could be due to improvement in the interfacial interaction between PA and EPDM, as discussed in the earlier section.
G. PA/chlorinated polyethylene TPEs
Chlorinated polyethylene/PA TPEs were prepared by melt mixing using an internal mixer.45 These TPEs have improved mechanical properties that could be due to the unique co-continuous structure. The phase separation length scale of 2–5 μm within the co-continuous phases, either in the case of spheres or open-foam-like structure, was revealed. In addition, the existence of a 3D network reminiscent of the co-continuous structure is responsible for improved tensile properties of these blends. Scanning confocal fluorescent microscopy revealed the formation of this co-continuous morphology.45
H. PA-12/ natural rubber TPEs
PA-12/natural rubber (NR) TPEs were prepared by the melt blending technique.46 Effects of blending techniques (i.e., simple and dynamic vulcanization) and types of NR (unmodified NR and epoxidized NR [ENR]) on morphology and various properties of the TPEs were investigated. Choice of NR in preparing high-temperature and high-performance blends is a matter of interest only to NR-producing countries. It was revealed that the co-continuous morphology was obtained in the simple blending method, which was transformed to dispersed morphology after dynamic vulcanization.46 Furthermore, the introduction of ENR and dynamic vulcanization caused improvement of various properties, such as mechanical, thermal, and stress relaxation, and also development of finer grain morphology than that of the blend containing the unmodified NR.46 This is attributed to the chemical interaction between the oxirane ring in ENR molecules and polar functional groups in PA molecules leading to higher interfacial adhesion. ENR-based TPV exhibited higher tensile strength and modulus than NR-based TPV. Thus, the synergistic effects of dynamic vulcanization of ENR phase and interaction between ENR and PA gave the improved properties of these materials.
I. PA/silicone rubber TPEs
Sen and Bhowmick47 attempted to prepare TPEs using PA 6 and silicone rubber (vinyl methyl silicone). However, the blends did not have adequate elongation required for TPEs. Later, PA/silicone rubber TPVs were prepared by changing the plastic to PA 12. Three structurally different peroxides, such as dicumyl peroxide, 3,3,5,7,7-pentamethyl 1,2,4-trioxepane (PMTO), and cumyl hydroperoxide were used for cross-linking the rubber phase.48 Dynamic vulcanization using PMTO showed prolonged scorch safety, better cross-linking efficiency, and higher tensile strength (26.5 MPa), elongation at break (127%), and thermal stability (Ti, Initial decomposition temperature = 432 °C) than the other two.48 It was revealed that PMTO was the superior peroxide for cross-linking of silicone rubber at high temperature. Prior to this work, in 2002 TPV-Si–based on PA/silicone rubber was commercially available from Dow Corning.49 But there is no successful product. Compression set was very poor for this material, and it found a low-temperature application in cell phone grips due to good abrasion and oil resistance.49
J. Poly(phenylene ether)–based TPEs
Gupta et al. developed a series of novel high-temperature TPEs from poly(phenylene ether) (PPE), polystyrene (PS), ethylene vinyl acetate (EVA), and styrene–ethylene butylene styrene (SEBS).50–55 The PPE-based TPEs containing SEBS, EVA, and PPE-PS have met all the key performance criteria for useful TPEs in terms of melt processability, tensile elongation, tension set, and recyclability. Mechanical properties of the blend compositions are given in Table IV. It was also revealed that the storage modulus increased with recycling and there was improvement of tensile properties. The rheological observation was in accord with the tensile property improvement upon recycling.
Morphological analysis of the blends indicated a unique microstructure, wherein EVA domains were dispersed in a mainly co-continuous matrix comprising the blend of PPE-PS/SEBS, as shown in Figure 9.50 With the increase in PPE-PS content, tensile strength, modulus, and hardness increased, whereas percentage elongation at break and tension set decreased. Elongation at break increased significantly with increasing SEBS content in the blend. Graphical optimization showed that an optimum set of superior mechanical properties (tensile strength, elongation at break, and modulus) could be achieved at a VA content of 50% and with low MW SEBS due to the better compatibility of the blend.51 Statistical analysis by “design of experiments” revealed several-trend analysis indicating the dependence of responses over categorical and mixture variables. Because of enhanced compatibility, superior mechanical properties were found with a VA content of EVA ∼50% with low SEBS molecular weight.54 Tension set of the quaternary blend was relatively low (<20%). Thermogravimetric analysis showed that the onset of degradation temperature of SEBS/EVA/PPE-PS: 60/10/30 was ∼317 °C. It was noted that the developed quaternary TPEs showed better thermal stability than SANTOPRENE general purpose grade thermoplastic rubber, which is known to be thermally stable up to 260 °C. Significant improvement in mechanical properties upon repeated recycling was an important finding of these TPEs as most of the common TPEs are reported to show lower mechanical properties upon recycling. Improvement in mechanical properties during recycling was obtained mainly due to cross-linking of EVA, which is evident from the higher storage modulus as a manifestation of restrained flow by network formation. The novel PPE-based TPEs showed superior heat aging performance. Figure 10 displays the results of aging at different times and temperatures.52 It is apparent that significant degradation occurs at higher temperature and longer time. However, the continuous use temperature (the temperature at which the product sustains its properties up to 1000 h) of the PPE-based TPE is found to be ∼151 °C. Compared with SANTOPRENE thermoplastic rubber general purpose grade, with a recommended continuous use temperature of ∼135 °C, the PPE-based TPE exhibits superior heat performance due to the presence of a high Tg component such as PPE. An effort has been made to understand the in-depth mechanism of degradation of the quaternary blend with a detailed time-temperature program. Interestingly, the mechanical properties were marginally improved upon aging at 120 °C compared with 80 °C for 2000 h. This was possibly due to better microstructure development of the quaternary blend. EVA undergoes cross-linking, and thus, the morphology of EVA changed from lamellar to discrete domains. However, a significant decrease in tensile strength and elongation at break was found upon 2000 h of exposure at 140 °C and greater. This was attributed mainly to the degradation of PPE-PS and EVA of the quaternary blends.53
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
As part of the development of the current PPE-based TPE in the high-heat TPE platform, the emphasis was given to the heat aging performance followed by the lifetime prediction of the material. The resulting TPE can withstand ∼131 °C for 100 000 h.55 Looking at the application perspective, accelerated weathering performance was analyzed for the developed TPE in both exterior and interior accelerated weathering protocols. Change in mechanical properties was found to be independent of the type of pigments upon weathering exposure. In brief, PPE-based TPEs (SEBS/EVA/PPE-PS) at certain proportions have been prepared through twin screw extrusion and injection molding technique. These exhibited good mechanical integrity, elastic recovery, stable morphology, and excellent recyclability along with good thermal stability.
K. Polyoxymethylene/ethylene propylene diene terpolymer TPEs
Polyoxymethylene (POM)/ EPDM TPEs with and without compatibilizer (MAH-grafted EPDM) were prepared by melt mixing in a twin screw extruder via dynamic vulcanization.56 Compatibilized dynamically vulcanized POM/EPDM TPEs exhibited a significant increase in impact strength (78 J/m) and thermal stability (degradation temperature = 372 °C).56 This could be due to the formation of the interface by using compatibilizer, which increased the miscibility between the two phases. Morphology of the blends revealed improved interaction between the blend components in the presence of compatibilizer, which reduced the interfacial tension and led to finer phase domains. Therefore, the enhanced interfacial adhesion is responsible for better mechanical and thermal properties.
L. Thermoplastic polyurethane/NR TPEs
High-performance NR/thermoplastic polyurethane (TPU) TPEs were prepared by dynamic vulcanization technique.57 NR/TPU-based thermoplastic NR (TPNR) exhibited better tensile strength and thermal stability when compared with commercial TPVs with similar hardnesses. In addition, the ENR/TPU TPNRs showed low compression set.57 These thermoplastic NRs (TPNR) showed improved oil resistance (oil swelling = 12%) in comparison with commercial TPVs (oil swelling = 40%) with similar hardness.57 Consequently, NR/TPU-based TPNR could be useful for industrial applications in which heat and oil resistance are desired. However, these would be interesting only to limited NR-producing countries.
M. PA/butyl rubber TPEs
PA-12 was blended with butyl rubber, bromobutyl rubber, and chlorobutyl rubber and dynamically vulcanized.58,59 Tensile strength and elongation at break are higher for chlorobutyl-containing blends in comparison with those made from other butyl rubbers. With increasing PA content in PA/CIIR blends, normalized swelling index (rubber phase only) decreased and reduction was close to being linear from 0 to 20% PA content.58 Morphological analysis revealed lower particle size for halogenated rubber specimens, suggesting better compatibility in blends with these rubbers, and compatibility order was found to be chlorobutyl > bromobutyl > butyl rubber.58 Maiti et al60 prepared TPE from brominated poly(isobutylene-coparamethyl styrene)–nylon 6 by the melt blending method. They used various organoclays such as hexadecyltrimethylammonium bromide and octadecyl amine–modified sodium montmorillonite to make TPE–clay nanocomposites (TPN). It was revealed that when nanoclays were incorporated in the rubber phase of TPN, a significant improvement in tensile properties such as tensile strength, elongation at break, and modulus was observed. On the other hand, the mechanical reinforcement was comparatively poorer when the nanoclay was added to the plastic phase due to partial destruction of the crystallinity. It was demonstrated that the reactive compatibilization of brominated poly(isobutylene-co-p-methylstyrene) (BIMS) and PA (PA6) blends occurs through instantaneous reactions between the benzylic bromine of BIMS and backbone amide and chain-end amine of PA6.61
N. Polycarbonate-based TPEs
Choudhury and Bhowmick have developed thermostable insulating TPEs from rubber polycarbonate blends.62 They used rubbers having varying polarity, for example, acrylic rubber (ACM), nitrile rubber (NBR), HNBR, chlorinated polyethylene (CPE), and EPDM. It was revealed that PC/HNBR and PC/CPE were found to have the best processability, highest tensile strength, and lowest surface energy mismatch compared with other PC/rubber compositions.62 On the other hand, PC/EPDM and PC/HNBR have excellent thermal stability. It should be noted that all PC/rubber blends gave good electrical insulation characteristics.
O. PA 6/fluoroelastomer TPEs
All the TPE blends (TPEs and TPVs) prepared from the rubber–plastics blends, as discussed above, have microstructure morphology, in which the rubber phase is dispersed in the plastic matrix in the micron range (0.5–3.0 μm). Banerjee and Bhowmick played with the morphology of PA/fluoroelastomer TPEs and TPVs recently by changing the processing conditions and came up with systems in which nanostructured rubber particles were dispersed in the plastic matrix.63–68 From the structural point of view, it is well known that the shape and size of the dispersed rubber domains in the plastic matrix play significant roles on the processability and properties of TPEs and TPVs. Attempts were made in the past to reduce the size of the dispersed rubber particles, which could lead to a significant improvement of properties of TPEs and TPVs, but this was met with limited success. The nanostructured TPE blends based on interactive PA 6 (PA6)/fluoroelastomer (Viton A, a copolymer of vinylidene fluoride and hexafluoropropylene) with outstanding properties were developed by batch mixing followed by high shear micro-injection molding (shear rate = 500–700 s−1).63–67 It was demonstrated that a low torque ratio (0.34) of rubber/plastic, high mixing speed, long mixing time, and high shear rate of micro-injection molding were the factors responsible for developing the nanostructured morphology of the PA6/FKM TPE blend.63,64 PA6 and FKM are interactive in nature. A nanometric interface was evident in such types of interactive blend. Therefore, the interaction between the blend components and nanometric interface has a strong role in enhancing the physicomechanical properties of the blends. Nanometer-sized rubber particles (60–80 nm) observed in these TPEs and TPVs after micro-injection molding of the samples are shown in Figure 11. The results were explained with the help of a viscoelastic drop break-up mechanism.64
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
As shown in Table V, these TPEs have a very high value of tensile strength (27.0–50.0 MPa) due to formation of nanostructure morphology (Figure 11), and a significant improvement was obtained after dynamic vulcanization.63,64 The tension set was good. Tensile strength, modulus at 100% elongation, and elongation at break of 40 PA 6/60 fluoroelastomer TPE were 27.0 MPa, 24.8 MPa, and 175%, which changed to 34.5 MPa, 33.1 MPa, and 130% after dynamic vulcanization of the same 40 PA 6/60 fluoroelastomer TPE.64 It would be interesting to compare the properties of fluoroelastomer rubber with fluoroelastomer/PA TPEs/TPVs. Tensile strength, modulus at 100% elongation, and elongation at break of cured fluoroelastomer were 2.50 MPa, 1.68 MPa, and 175%, which enhanced to 4.12 MPa, 2.53 MPa, and 180% when fluoroelastomer was filled with 4 wt% Cloisite 10A.69 In the case of MT black reinforced fluoroelastomer, tensile strength and modulus were found to be 9.9 MPa and 4.6 MPa, respectively.70 Thus, it could be inferred that a significant improvement in tensile strength and modulus was obtained in fluoroelastomer-based TPEs/TPVs as compared with the cured and filled fluoroelastomer. However, there was no significant change in elongation at break in both the cases. PA6/FKM TPEs and TPVs showed excellent reprocessability with high mechanical properties. The nanostructures were formed gradually by breakdown of particles, as shown clearly in the field emission scanning electron microscope images. The rubber particles were large initially (500 nm; Figure 12) and were reduced to a smaller size (300 nm to 70 nm), especially after injection molding (Figure 12). Figure 12f shows the corresponding torque versus mixing time curve, which also indicate the positions on the curve at which the samples were taken and analyzed. Figure 13 demonstrates how these properties were developed during mixing and relationships between the properties and diameter of the rubber particles with and without dynamic vulcanization. Larger is the diameter, lower is the strength and modulus. These TPEs have a smooth extrudate surface without any sign of melt fracture, which could be due to the fine nanostructure morphology of the blends.66
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
PA6/FKM blends (TPEs and TPVs) are pseudoplastic in nature. These TPEs have a nanometric interface (4–5 nm) that has a strong influence on functional properties. Using advanced PeakForce quantitative nanomechanical AFM technique, it was demonstrated that such a type of interface has intermediate value (200 MPa) of elastic modulus in between the rubber (50 MPa) and plastic (700 MPa) phases (Figures 14–15).67 An idea about the interface could be obtained from the adhesion force and elastic modulus measurements (Figures 14–15). In addition, it was revealed that thin rubber ligaments of ∼10 nm width in the thermoplastic matrix are interconnected, which significantly contributed to the rubber elasticity in TPEs and TPVs ( Figure 16).67 Real possibilities of carbon dioxide (CO2) laser cutting on PA6/FKM TPE blends were also demonstrated.68 Several process parameters such as cutting speed, power, specimen thickness, kerf width, melted transverse area, melted volume, and heat-affected zone were measured and analyzed.68 CO2 laser cutting will be a new technique in the TPE industry. It was revealed that heat-resistant rubbers are more suitable for CO2 laser technology. It might be possible to improve the quality of the finished surface by using heat-resistant polymers.
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
VII. HIGH-TEMPERATURE TPEs BASED ON HIGH-PERFORMANCE HEAT-RESISTANT RUBBERS
In the following sections, several high-temperature TPEs (based on heat-resistant rubbers) will be discussed, highlighting their preparation, several properties, and morphology.
A. PP/nitrile rubber TPEs
Coran et al.71 developed oil-resistant TPEs by dynamic vulcanization from PP-NBR compositions (Geolast). They used phenolic resin curative to technologically compatibilize the blend components.71 It was revealed that there is formation of in situ graft copolymer. The probable scheme of this reaction is shown in Figure 17. Significant improvement of mechanical properties was obtained in technologically compatibilized blend as compared with unvulcanized blend. The hot oil resistance of compatibilized PP/NBR blends is excellent. However, low-temperature performance of these blends is poor. It is important to note that the oil resistance of PP/NBR blends is superior to PP/EPDM TPVs. On the other hand, mechanical properties of both TPVs are comparable.
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
B. Polyolefin/acrylate rubber TPEs
PP/acrylic elastomer blends having good strength and heat resistance were developed by Du Pont de Nemours in 1987.72 These blends were prepared by reactive processing using a combination of PP, ethylene-butyl acrylate-glycidyle methacrylate, and an ionomer (ethylene-butyl acrylate-acrylic acid Zn salt methacrylate). PP/acrylate rubber TPVs are commercially available from Kraburg.73 PP/acrylate rubber TPVs could be used under the hood because of their high thermal stability up to 140 °C. Other probable applications are seals, cable bushings, and water tank covers. Soares et al.74 prepared the TPE based on PP and acrylic rubber (ACM) by the melt blending process and gave special attention to the compatibilization and dynamic vulcanization. The compatibilized blends (with MAH-functionalized PP/triethylene tetramide [TETA] system) exhibited excellent improvement in mechanical properties, oil resistance (5.8–8.1% in ASTM oil No. 3 at 100 °C for 22 h), elasticity, and decrease in damping characteristic.74 The dynamically vulcanized blends showed superior mechanical performance.
It should be noted here that the finer morphology of PP/ACM blends (Figure 18) was achieved for compatibilized blends, which could be due to a decrease in the interfacial tension, better interfacial adhesion, or cross-linking of the rubber particles imparted by TETA in combination with PP-g-MA. Specially designed acrylate elastomer (Sunigum) was also used for developing a soft PP-based TPE.75 Ethylene methyl acrylate-glycidyl methacrylate/ethylene-octene elastomer was used as a compatibilizer.
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
C. Polyolefin/silicone rubber TPEs
Polyolefins (high-density polyethylene/PP)/silicone rubber TPEs were prepared using a compatibilizer (a polymer comprising both olefin- and silicone-containing moieties).76 Polyolefins/silicone rubber TPEs provide superior resistance to organic oils (naturally occurring oils, such as vegetable oils and animal fats, and other hydrocarbon oils), particularly at high temperatures and especially at over 140 °C, compared with those based on comparable PP-EPDM compositions.76 The high-temperature performance of PP in PP/silicone rubber TPEs could be due to the presence of silicone rubber, a layer of which can bloom out on the surface. In addition, the polyolefins/silicone rubber TPEs have significant advantages such as resistance to flexibility, lubricity, hydrocarbon chemicals, dielectric properties, water repellence, and relative inertness to ozone, corona, and other extreme weather environments, as compared with TPEs in general, due to the presence of silicone rubber.76 TPEs based on silicone rubber greatly expand their applicability because of the relative ease of processing and product design flexibility possessed by such compositions.
D. Polyolefin/styrenic elastomer TPEs
A high-performance TPV (STPVs) consisting of the hydrogenated styrenic block copolymer in a PP matrix was invented by Teknor Apex.77 This TPV has superior long-term elastic recovery compared with conventional TPVs. For example, it exhibits 5% deterioration of compression set in long-term testing at 125 °C, versus 20% to 50% deterioration with respect to conventional TPVs.77 In addition, STPVs have improved resistance to hot oils and solvents, excellent long-term compression set and up to 20% higher tensile strength, and improved adhesion with nylon as compared with conventional TPVs.77 STPVs have targeted their applications in overmolded high-performance grips, seals, and diaphragms.77
E. BIMS rubber/various plastics–based TPEs
Venkataswamy78 developed novel TPVs based on the compatibilized plastic phase. The plastic phase in TPVs from PP and olefinic rubber was compatibilized with PA 6 using maleated PP. The TPVs based on EPDM and BIMS rubbers generated triblend TPVs with good elastomeric properties. All of these vulcanizates have improved upper service functionality in comparison with PP-based TPVs.
F. Acrylate plastics/rubber–based TPEs
Kader and Bhowmick79 reported novel oil- and heat-resistant TPEs from the blends of acrylate rubber or fluororubber and an acrylic plastic derived from the multifunctional acrylates. Among various multifunctional acrylates used (these monomers polymerize in situ while curing with rubber to produce the continuous plastic matrix), trimethylol propane triacrylate (TMPTA) showed optimum improvement in the mechanical properties of the blends before and after thermal aging of the 50/50/30 (w/w) ACM/FKM/TMPTA blends. The solubility parameter of these blends was in the range of 8 to 25 (cal/cm3)1/2. It was revealed that with increasing TMPTA level in the 50/50/30 (w/w) ACM/FKM/TMPTA blends, the modulus, tensile strength, tear strength, and storage modulus increased (Table VI). However, elongation at break and tanδ value decreased.
G. Polyolefin/fluoroelastomer TPEs
Nanostructured PP/fluoroelastomer (FKM) thermoplastic elastomeric blends, in which FKM was dispersed in the nanometer range (50−70 nm) into the continuous PP matrix, were recently developed using melt blending route.80 Development of fine rubber domain morphology is due to the high shear rate of micro-injection molding. Because of development of unique nanostructured morphology, these blends exhibited outstanding mechanical properties (tensile strength = 19.5–31.0 MPa), oil swelling resistance (8–10% ASTM oil No. 3 at 100 °C for 72 h), and thermal stability (Tmax, the temperature at which the rate of thermal degradation is maximum = 493–496 °C), which increased after dynamic vulcanization.80 A typical stress–strain curve is shown in Figure 19. Tensile strength and Young's modulus of 40 PP/60 FKM (w/w) TPE were 19.5 MPa and 140 MPa, respectively, which increased to 24.0 MPa and 160 MPa for TPV. The higher thermal stability of these materials is demonstrated in Figure 20. In addition, it was shown that a small amount of FKM (5 wt%) significantly increased the thermal stability of PP (as given in Figure 20), which has huge importance in the industry. It was demonstrated that these PP/FKM TPEs can be used in the automotive industry, particularly as a liner material for car hoses due to unique properties, such as superior flexibility, recyclability, outstanding oil swelling resistance, and thermal stability.
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
VIII. SUPER-TPVs (S-TPVs)/HIGH-TEMPERATURE OIL-RESISTANT TPVs (HOTs)
TPVs with the name super-TPVs were made from the combination of high-performance plastics and high-performance vulcanized rubbers (discussed in the forthcoming section) in 2002,81–88 although prior to these investigations, there were reports of such TPVs (with no mention of the term super-TPVs, as discussed earlier). These materials may also be abbreviated as HOTs (high-temperature oil-resistant thermoplastic vulcanizates). These TPVs have been designed to withstand very severe environmental conditions such as heat, exposure to oil and fuel, and so forth, for applications in various automotive parts and have inherent advantages of thermoplastic processing.82 In addition, these bridge the gap between the specialty and engineering plastics and the rubbers. Super-TPVs/HOTs have several targets in automotive applications such as fuel vent hose, high-performance body plugs, spark plug boots, soft-touch interiors, electrical insulation, glazing seals, electrical insulation, and so on,81–88 as shown in Figure 21. Some examples of s-TPVs (HOTs) and their applications are discussed below:
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
A. Super-TPVs based on acrylic elastomer
Acrylic elastomer–based s-TPVs consists of modified ethylene-acrylate rubber in a copolyester matrix, unveiled by DuPont, also called engineering TPV (ETPV).89 DuPont™ ETPV can be processed in a variety of thermoplastic processing equipment such as injection molding, two-component injection molding, extrusion, blow molding, and melt casting to make high-performance parts. Depending on the grades, the range of processing temperature is 245–255 °C and 220–230 °C for injection molding and extrusion, respectively.89 It should be noted that all of the grades have good melt stability and a sharp melting point. ETPVs have excellent resistance to hot oil, grease, and many chemicals. For example, the standard and heat-stabilized ETPVs sustain oil resistance at 150 °C for more than 1000 h and 3000 h, respectively. ETPV (both standard and heat-stabilized) can provide grades in a broad hardness range (60–90 Shore A). In a recent report, it was mentioned that DuPont ETPVs can offer excellent resistance to engine fluids (−40 to 160 °C).89 ETPVs are fully recyclable and have no extractable additives. The tensile strength of various DuPont ETPVs after aging is shown in Figure 22. The various physical properties DuPont ETPV 60A01L NC010 are given in tabular form in Table VII. The probable applications of ETPV include truck air-brake hose, spark plug boots, fuel vent hose, ignition seals, ducting, body plugs, and CVJ boots.89
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
Zeon's s-TPV can withstand hot-oil immersion at 150 °C over 3000 h and also survive 175 °C temperature spikes. Commercially available TPV from Zeon is based on nylon/acrylate rubber.90 Zeon's nylon/acrylate rubber–based TPV is commercially available because of thermoplastic processability and melt adhesion to nylon, which allows it to be overmolded on to nylon. The injection molding process parameters of Zeotherm materials are given in Table VIII.91 Zeotherm TPVs are excellent at resisting the most aggressive automotive fluids, including engine oil, transmission fluid, and lubricating greases, and they have superior fluid resistance to conventional TPEs and copolyesters. The fluid resistance of Zeotherm TPVs is shown in Table IX.91
Zeotherm TPVs consist of vulcanized polyacrylate (ACM) rubber dispersed in PA thermoplastic matrix.90 The properties of Zeotherm TPVs depend on grades and PA content. Table X shows the physical properties of available Zeotherm TPV grades. These TPVs have superior long-term heat resistance compared with conventional TPVs and copolyester. The amount of PA content influences the processing in these TPVs. In this type of TPV, melt viscosity is more dependent on shear than temperature. It was revealed that lower viscosity is obtained at higher shear rates, which enhances the flow of this material.
B. High-performance TPSiV-based on TPU and cross-linked silicone rubber
Dow Corning has developed TPSiV TPE from TPU and cross-linked silicone rubber. Silicone rubber has been dispersed into TPU by the process of dynamic vulcanization, which results in a stable droplet-matrix morphology.92 This TPSiV is fully processable and recyclable like a thermoplastic. The key physical properties of the TPSiV® 4000 series are given in Table XI. This TPSiV has combined benefits of excellent chemical and heat resistance properties from the silicone rubber and good abrasion resistance from polyurethane with a soft hand feel. TPSiV is found to be useful in applications in seals for waterproof smartphone designs, earphones, camera grips, and soft-touch overmolding on substrates.
C. High-performance TPVs based on TPU foam
It is well known that the limitation of first-generation TPV foams has restricted their potential applications in body seals and glazing seals.85,86 The current foaming technology and the use of supercritical blowing agents provide very fine particle dispersion in extruded profiles. Trexel MuCell®, Nishikawa Rubber, and Jyco have suggested that this foaming technology could accelerate the penetration of TPV into the body seals sector due to fine particle dispersion in extruded profiles. Recently, BounCell-X has invented new and refined TPU foam for the footwear industry using the combined benefits of TPU chemistry with the efficiency of foam injection molding.93 They have shown that this low-density (0.30 g/cm3), recyclable TPU foam offers the better performance in terms of higher production efficiency and improved sustainability when compared with EVA and rubber. Also, this new and refined TPU foam helps the footwear industry to reduce waste, improve quality, and increase the efficiency in production cycles.93
IX. STRATEGIES TO IMPROVE THE HIGH-TEMPERATURE PERFORMANCE FURTHER
A. Microstructured TPE versus nanostructured TPE
High-temperature functional properties (such as thermal stability, swelling resistance, compression set, etc.) of TPEs may be enhanced by transforming the morphology from microstructure to nanostructure. All the earlier reports on TPEs and TPVs from rubber–plastic blends showed that the dimension of the dispersed rubber phase was in the micrometer range. Recently, it was reported that the rubber phase could be dispersed in nanoscale in the continuous thermoplastic matrix by adopting high-shear micro-injection molding.63,64 The properties of these materials are found to be unique because of the nanostructure of the dispersed rubber phase and the nanometric interface. It was revealed that even the nanostructured PP/fluoroelastomer (FKM) blends exhibit outstanding oil swelling resistance and thermal stability. Volume swell of nanostructured PP/FKM blends was 8–10% (in ASTM oil No. 3 at 100 °C for 72 h), which was the lowest in this type of TPE.80
The flow behavior of polymer melts during different processing methods is one of the crucial parameters for good finish and dimensional tolerance of the product. Therefore, to determine the overall processability, analysis of the rheological behavior of polymer melt is very important. The melt viscoelastic properties of PA6/FKM blends revealed that the nanostructured TPE (the average dimension of the dispersed rubber phase, D ∼90 nm) displayed higher complex viscosity than the microstructured TPE (D ∼0.35 μm) in the entire frequency range, as shown in Figure 23.66 It was estimated that the complex viscosity of the nanostructured blend was 1140 Pa.s, while that of the microstructured blend was 706 Pa.s at an angular frequency of 1 rad/s.66 Nanostructured TPE has a higher surface area, which leads to an increase in the interaction between the blend components. Because of the higher surface area, nanostructured TPE can lead to an increase in vapor barrier properties. Therefore, nanostructured high-performance TPEs can play a major role in the predicted growth of improved fuel system components with reduced vapor permeability to meet the challenges of new fuel emission requirements.
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83786
B. Dynamic vulcanization technique
Dynamic vulcanization is a process in which the rubber is vulcanized in the presence of the molten thermoplastic under shear forces and the rubber particles are dispersed in the thermoplastic matrix, even at high rubber content. Dynamic vulcanization in general has the following improvements in comparison with its unvulcanized counterparts: improved ultimate mechanical properties, reduced permanent tension set, improved fatigue resistance, greater resistance to attack by fluids (e.g., hot oils), improved high-temperature utility, greater stability of phase morphology in the melt, greater melt strength, improved reliability of thermoplastic fabrication, and reduced die swell of the extruded profiles.94–97 The improvement of these properties can readily be understood as a result of cross-linking of the rubber phase and the generation of a preferred morphology having 1–2 μm rubber particles in the continuous plastic matrix. It is important to note that optimization of curative concentration as well as the choice of the right cross-linking system are essential to obtain maximum improvement of properties. For example, in the case of PA/acrylate rubber blends, volume swell decreased sharply with increasing curative concentration. A further increase in curative concentration had no effect on the improvement of volume swell. Also, a high amount of curatives can lead to high production cost.
C. Incorporation of nanoparticles in TPEs
The incorporation of nanofillers is one of the most effective approaches to increase the high-temperature functional properties of TPEs. The level of nanofiller dispersion and interaction with the surrounding phases affects the structure and properties of these TPE nanocomposites. Thakker and Goettler87 reported the effect of nanoclays in TPV formulations and found better processability (e.g., in blow molding and profile extrusion; due to better flowability of nanoclay filled TPVs), increased modulus, and reduced tanδ values of TPE nanocomposites. Jha et al.40 showed that the addition of a low amount of silica improved the swelling resistance of the PA6/ACM TPVs. The introduction of filler (silica) reduces the volume swell of the rubber phase in the TPEs, as it preferentially reinforces the rubber phase. Hence, the overall volume swell of the blends in oil and fuel is reduced after incorporation of nanofiller in TPEs and TPVs. The silica nanoparticle in the PPE-based TPEs was found to improve the tensile strength, percentage elongation at break, and tension set due to the silica nanoparticle reinforcement of the dispersed elastomeric phase.55 Shear loss modulus (G″) of PPE-based TPEs with 2.5% silica nanoparticles (sample C) and with 0.3% triethoxysilane (sample E) along with the control sample (sample A) was measured at a function temperature, as shown in Figure 24.98 It was revealed that sample E exhibited the lowest G″. Lower G″ in sample E compared with the control (sample A) suggests a development of a less viscous and more elastic component. The greater elastic nature of sample E could be due to cross-linking formation. Higher G″ in sample C compared with the control (sample A) revealed an increase in permanent deformation or loss factor in the material. This would also suggest almost no cross-linking formation in sample C. Morphology studies showed that silica nanoparticles were present mainly in the elastomeric phase. Organomodified siloxanes can be effectively used to provide multiple functions in TPE development and formulations, such as improved mechanical and surface properties, low odor, and flame retardancy, and improved scratch resistance.99 Nanofiller-reinforced TPEs prepared through several routes have shown superior mechanical properties and hence hold promise in the area of high-heat TPE platform. Preliminary findings suggest that the use of nanoparticles has potential advantages in performance and processing in TPV formulations. The major benefit of the nanoparticle reinforcement (e.g., silica nanoparticle) in TPEs/TPVs is the recyclability with retention of mechanical properties along with higher elastic recovery and lower tension set.
X. SUMMARY AND FUTURE DIRECTIONS
In this review, different types of high-temperature TPEs, TPVs, and super-TPVs (HOTs) and their preparation, morphology, various properties, and applications were discussed briefly. Strategies to improve the high-temperature performance of TPEs were presented. Considering recent progress, as discussed in this review, there is lot of scope for further development of high-temperature TPEs and super-TPVs/HOTs. On one side, novel new materials and applications would be explored, which are still challenging today. On the other side, there is a need to improve particular properties of existing materials, which influence applications of these systems. These would be the driving force for future developments of high-temperature TPEs, TPVs, and super-TPVs/HOTs and their applications.
Although new TPEs and TPVs are in continuous demand and being developed routinely in this industry, theoretical aspects have been little understood. When two polymers are mixed together, the Gibbs free energy of mixing (ΔGM) of the system is given by100:
where φ is the volume fraction, N is the degree of polymerization, and χ is the Flory's interaction parameter. What happens to ΔGM, when different ingredients are added? Can these TPEs be analyzed on a molecular level? In our recent work, we have made an attempt to reveal various aspects, which are currently under publication.101 Also, processing of high-temperature TPE blends, interaction, and role of various ingredients and fabrication of their products need further in-depth study.
— An example of Debye-Bueche plot of I(q)−1/2 versus q2 for a 50:50 dynamically vulcanized nylon/HNBR premixed with 3 phr carboxyl terminated butadiene and 0.9 phr cross-linker blend (mixing time of 10 min at 250 °C).33 Copyright 2003. Reproduced with permission from John Wiley & Sons Inc.
— Time variation of morphology parameters during melt mixing at 250 °C for (○) 50:50 nylon/HNBR premixed with 0.9 phr cross-linker; (Δ) 50:50 nylon/HNBR premixed with 0.9 phr cross-linker and 1 phr LXNBR; (□) 50:50 nylon/HNBR premixed with 0.9 phr cross-linker and 3 phr LXNBR. HNBR, hydrogenated nitrile rubber; LXNBR, liquid carboxylated nitrile rubber.33 Copyright 2003. Reproduced with permission from John Wiley & Sons Inc.
— Transmission electron micrograph of a 50:50 blend of nylon/HNBR premixed with 0.9 phr cross-linker and 3 phr carboxyl terminated butadiene (8 min mixing time at 250 °C).33 Copyright 2003. Reproduced with permission from John Wiley & Sons Inc.
— Scanning electron micrograph of cryofractured nylon-6/ACM, 50/50 (w/w) blend, after extracting the ACM phase by chloroform, followed by sputter coating with gold. Note the presence of ACM as the dispersed phase of dimension 0.5–1 μm in the matrix of nylon-6, X3000.36 Copyright 1997. Rubber Division, American Chemical Society, Inc.
— Temperature dependence of tanδ and E′ of nylon-6/ACM (40/60) blends interacted for different times.36 Copyright 1997. Rubber Division, American Chemical Society, Inc.
— Equilibrium volume swelling of dynamically vulcanized nylon-6/ACM blends in ASTM oil No. 3 at 150 °C versus weight fraction of the nylon-6 phase in the blends.38 Copyright 2003. Reproduced with permission from John Wiley & Sons Inc.
— Possible reaction schemes of nylon-6/ACM blends.36 Copyright 1997. Rubber Division, American Chemical Society, Inc.
— Morphology of the quaternary blends.50 Copyright 2007, Rubber Division, American Chemical Society, Inc. (a) Batch E: SEBS/EVA (VA: 50%)/PPE (0.41 dl g-1 IV)-PS: 60/15/25. (b) Batch D: SEBS/EVA (VA: 50%)/PPE (0.41 dl g-1 IV)-PS: 45/30/25. (c) Batch B: SEBS/EVA (VA: 50%)/PPE (0.33 dl g-1 IV)-PS: 60/10/30. (d) Batch C: SEBS/EVA (VA: 50%)/PPE (0.46 dl g-1 IV)-PS: 60/15/25.
— (a) Tensile strength. (b) Elongation at break at different temperatures (80 °C, 120 °C, 140 °C, and 170 °C) exposed up to 2000 h for batch C SEBS/EVA/PPE.53 Copyright 2008. Rubber Division, American Chemical Society, Inc.
— Field emission scanning electron microscope images micrographs of (a) TPE and (b) TPV. The insets to (a) and (b) are the AFM images of TPE and TPV, respectively.65 Copyright 2015. Reproduced with permission from Springer.
— Field emission scanning electron microscope images of 40 PA6/60 FKM (w/w) TPVs (with vulcanizing agents) at different stages of mixing: (a) S1, (b) S2, (c) S3, (d) S4, (e) after injection molding of final stage, S4 (IM). (f) Torque versus time curve showing S1, S2, S3, and S4 from where the samples were taken and analyzed.64 Copyright 2015. Reproduced with permission from Elsevier Ltd.
— (a) Tensile strength and Young's modulus versus time of mixing. (b) Tensile strength versus rubber particle diameter in TPEs and TPVs.64 Copyright 2015. Reproduced with permission from Elsevier Ltd.
— (a, b) Magnified AFM phase images of TPE and TPV. (c) TEM image of TPE. (d–h) Typical force versus tip sample separation curve of (d) TPE, (e) TPV, (f) plastic phase in TPE and TPV, (g) interface in TPE and TPV, and (h) rubber phase in TPE and TPV.67 Copyright 2015. Reproduced with permission from John Wiley and Sons.
— (a, b) Representative maps of adhesion obtained by PeakForce QNM imaging modes. The images show (a) TPE and (b) TPV, (c) the average adhesion force profile of TPE and TPV, and (d) the average adhesion force and elastic modulus profiles.67 Copyright 2015. Reproduced with permission from John Wiley and Sons.
— (a,b) High magnified AFM phase images of (a) TPE and (b) TPV. (c) Idealized scheme of connectivity of the rubber phase in the continuous plastic matrix through ligaments for thermoplastic elastomeric blends (TPE and TPV).67 Copyright 2015. Reproduced with permission from John Wiley and Sons.
Probable scheme of formation of compatibilizing graft copolymer.71
— SEM micrographs of PP/ACM (50:50 parts) blends: (a) unvulcanized/uncompatibilized, (b) unvulcanized/compatibilized with PP-g-MA/TETA (5:0.5), (c) vulcanized/uncompatibilized, (d) vulcanized/compatibilized with PP-g-MA/TETA (5:0.5).74 Copyright 2008. Reproduced with permission from eXPRESS Polymer Letters.
— Tensile stress versus tensile strain plots of PP/FKM blends.80 Copyright 2015. Reproduced with permission from the American Chemical Society.
— (a) Weight (%) versus temperature of PP, FKM, and their blends (TPE and TPV). (b) Derivative weight versus temperature of PP, FKM, and their blends (TPE and TPV). (c) Weight (%) versus temperature of PP and PP having 5 wt. % FKM. (d) Derivative weight versus temperature of PP and PP having 5 wt. % FKM.80 Copyright 2015. Reproduced with permission from the American Chemical Society.
— Some typical super-TPVs materials.89
Tensile strength of DuPont ETPVs after aging in air at 150 °C.89
Absolute value of complex viscosity versus angular frequency of micro- and nanostructured blends at reference temperature 240 °C.66 Copyright 2015. Reproduced with permission from John Wiley & Sons Inc.
Shear loss modulus of silica filled PPE-based TPEs.98
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