Preparation, microstructure, and microstructure-properties relationship of thermoplastic vulcanizates (TPVs): A review
Introduction
A thermoplastic elastomer (TPE) comprises two or more polymer phases, with one phase that is hard at room temperature but becomes fluid at high temperature, while other, discontinuous phases are soft and elastic at room temperature [1]. Thus, TPEs combine the high elasticity of traditional vulcanized rubber and the good processability and recyclability of thermoplastics. The substitution of traditional vulcanized rubber by TPEs can enhance productivity, and save energy and resources [2], [3], [4]. Since the first development of TPE in the 1960s, TPEs have been widely used in footwear, wire insulation, medical devices, sporting goods and adhesives, etc. [5], [6]. There are two typical kinds of TPEs [7]. One is a thermoplastic block copolymer, such as an ABA block copolymer, including poly(styrene-b-butadiene-b-styrene) triblock copolymer (SBS) [8], poly(styrene-b-isoprene-b-styrene) triblock copolymer (SIS) [9], and poly(styrene-b-(ethylene-co-butylene)-b-styrene) triblock copolymer (SEBS) [10], etc., and (AB)n multi-block copolymers, including copolyamides (COPA), thermoplastic polyurethanes (TPU) [11], and copolyesters (COPE), etc. [7], [12]. The other kind, the focus of this review, comprises thermoplastic blends, among which the most important being thermoplastic vulcanizates (TPVs) [5], [7], [13].
TPVs, composed of a high content of crosslinked rubber as the dispersed phase and a low content of thermoplastic as the continuous phase, are prepared by dynamic vulcanization (DV), a special polymer reactive blending technique [7], [13]. Unlike thermoplastic block copolymers, both the plastic and rubber phases in TPVs are usually commercially available, and thus do not require design and synthesis of new polymers. Various kinds of high performance and high-value added TPVs composed of different polymer blends may be prepared by using DV techniques, including the use of “green” materials and methods. Therefore, TPVs have attracted considerable attention in recent years, and widely used in industries such as automotive, building, and electronics [7], [13], [14], [15]. In recent years, TPVs have become the fastest growing elastomers to replace unrecyclable petroleum-based thermoset rubbers because of the requirements of environmental protection and resource saving [16] (Table 1).
Gessler proposed the idea of DV in 1962, and introduced TPVs in 1972 [7], [13]. In 1973, Fisher produced ethylene-propylene-diene monomer rubber (EPDM)/polypropylene (PP) TPVs containing a partially vulcanized EPDM phase to maintain processability [13]. In 1978, Coran and Patel et al. [17] performed extensive studies on EPDM/PP TPVs that were fully vulcanized under dynamic shear, resulting in high crosslinking density in the rubber phase and significantly improved properties of the resultant TPVs. In the early 1980s, they carried out extensive studies on TPVs based on various components [5]. Those studies resulted in the first commercialization of EPDM/PP TPVs product named “Santoprene” TPE by Monsanto (ExxonMobil AES) in 1981 [18]. Later, many new families of general TPVs were explored, including blends of ethylene octene copolymer (EOC) and PP [19], [20], natural rubber (NR) and PP [21], [22], epoxidized natural rubber (ENR) and PP [23], NR and high density polyethylene (HDPE) [24], [25], [26]. Meanwhile, various kinds of special purpose TPVs, such as nitrile butadiene rubber (NBR)/PP TPVs [27], [28], [29], isobutylene-isoprene rubber (IIR)/PP TPVs [30], and IIR/polyamide (PA) TPVs [31], and silicone rubber (SiR)/PA TPVs [32], have attracted much attention because they have special properties, such as good oil resistance, good gas barrier properties or high temperature resistance, and find some special applications [14], [24], [30], [33], [34], [35], [36], [37]. In recent years, novel special TPVs, such as oil resistant ethylene-vinyl acetate rubber (EVM)/poly(vinylidene fluoride) (PVDF) TPVs [18] and carboxylated acrylonitrile butadiene rubber (XNBR)/polyamide 12 (PA 12) TPVs [38], and high temperature resistant SiR/PVDF TPVs [39], bio-based TPVs, such as poly(lactic acid) (PLA)/NR TPVs [40], PLA/ethylene-co-vinyl acetate (EVA) TPVs [41] and poly(butanediol-lactate-sebacate-itaconate) bioelastomer (PLBSI)/PLA TPVs [2], and functional TPVs nanocomposites with good conductivity, have attracted attention for a wide range of industrial applications of TPVs, such as strain sensors and stretchable conductors [14], [32], [38], [42], [43], [44].
Most important properties of all TPVs including mechanical property, elasticity and rheological property, are controlled by their microstructure [16], [45], [46]. To obtain high elasticity of TPVs, a high content (>50 wt.%) of rubber phase with a high crosslinking degree (CD) is required, leading to the formation of a continuous rubber phase in the rubber phase/plastic phase (R/P) premix before DV. Nevertheless, a continuous plastic phase is required to achieve good processability and easy recyclability of TPVs. Therefore, the key to prepare TPVs is realizing the phase inversion of the rubber phase from a continuous phase (in the premix) to a dispersed phase (in the TPVs) [47], [48]. On the other hand, a fine dispersed rubber phase is required to achieve good mechanical properties of TPVs. Although the crosslinked rubber domains in TPVs constitute the dispersed phase, TPVs usually exhibit as good elasticity as that of the crosslinked rubber. This is mainly attributed to the high content (>50 wt.%) of the rubber phase with a high CD in TPVs. Thus, the mechanisms of formation of the microstructure of TPVs, and the microstructure-property relationships of TPVs have been widely studied in the past several decades to provide guidance for preparing high-performance TPVs through controlling their microstructure [16], [45], [46].
In the present review, we focus on the latest development in preparations methods, formation mechanism and influencing factors of the microstructure, and microstructure-property relationship of TPVs. To the best of our knowledge, there is no systematic review on these aspects of TPVs, although there were general reviews on TPVs years ago [5], [7], [13]. This review will address the following topics: (1) preparation methods of TPVs; (2) formation mechanisms and influencing factors of the microstructure of TPVs; (3) relationships between microstructure and properties of TPVs; (4) various types of TPVs including general purpose TPVs, special TPVs, bio-based TPVs, and TPVs-based nanocomposites; (5) conclusions and future perspectives. Our goal is not only to help potential readers better understand TPVs but also provide them with guidance to produce high performance and high-valued added TPVs.
Section snippets
Preparation methods of TPVs
Polymer blends can be prepared by solution blending, latex blending or melt bending, whereas TPVs are usually prepared by melt blending [7]. Compared with traditional polymer blends, the preparation of TPVs is more complex because of the simultaneous mixing of various compositions, and crosslinking and breakup of the rubber phase. In most cases, TPVs are prepared by using conventional chemical crosslinking [33], [49], [50], [51], [52]. Three feeding procedures are usually followed to prepare
Formation mechanisms and influencing factors of microstructure of TPVs
The properties of TPVs depend on their microstructure. Thus, in the past decades, much attention has been paid to the mechanisms of formation of their microstructure and factors that influence the formation of the microstructure. The most important progresses on this subject are summarized below.
Stress-strain behavior
Mechanical properties of TPVs are usually characterized by their stress-strain behavior. The stress-strain behavior is almost the same for all TPVs, which is a combination of the deformation behavior of the thermoplastics at low strains (<50%) and that of the elastomers at high strains, as schematically shown in Fig. 12 [14], [15], [18], [158], [159]. At low strains, the stress significantly increases with increasing strain. Thus, the elastic modulus of TPVs is similar to that of the plastic
Various types of TPVs
Based on the properties, function, application and source of raw materials, TPVs are classified as the following four types. First, the most conventional TPVs with good elasticity, good processibilty and easy recyclability are classified as general purpose TPVs, such as EPDM/PP TPVs and EOC/PP TPVs. Second, TPVs with extra special properties, such as good oil resistance, good gas barrier properties or high temperature resistance are classified as special TPVs, such as SiR based TPVs and EVM
Conclusions and perspectives
As a special class of high performance TPEs, TPVs have attracted a tremendous interest over the past decades and have become the fastest growing elastomers that replace unrecyclable thermoset rubbers. To achieve good mechanical properties, high elasticity, and good processability and recyclability, TPVs are usually prepared by premixing of a high content of rubber and a low content of thermoplastics through melt blending and subsequent dynamic vulcanization by introducing chemical curing
Acknowledgements
We gratefully acknowledge Mr. Yueqing Hua (Central Research Institute, Wanhua Chemical Group Co., Ltd), Mr. Heng Liu (Jiang Su Heng Rui Medicine Co., Ltd.) and Mr. Jian Sheng (Zhejiang JuHua Novel Materials Research Institute Co., Ltd.) for bibliographic search. This work was supported by National Key Research & Development Plan (2017YFB0307003) and National Natural Science Foundation of China (Grant No. 51525301, 51673014 and 51521062).
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