Elsevier

Progress in Polymer Science

Volume 79, April 2018, Pages 61-97
Progress in Polymer Science

Preparation, microstructure, and microstructure-properties relationship of thermoplastic vulcanizates (TPVs): A review

https://doi.org/10.1016/j.progpolymsci.2017.11.003Get rights and content

Abstract

It is common practice to blend polymers to obtain high-performance polymer materials for new applications. Thermoplastic vulcanizates (TPVs), consisting of a high content of crosslinked rubber as a dispersed phase and a low content of thermoplastic as a continuous phase, are usually prepared by pre-blending rubber and plastic phases followed by dynamic vulcanization. They are a special class of high performance thermoplastic elastomers (TPEs) as they combine both the excellent elasticity and mechanical properties of crosslinked rubbers and good processability and recyclability of thermoplastics. As such, in the recent decades they have attracted much attention and have become the fastest growing elastomers to replace unrecyclable thermoset rubbers. This review focuses on recent progresses in TPVs, and more specifically on the following issues: (1) preparation methods of TPVs, (2) mechanisms of formation of the microstructure of TPVs; (3) relationships between the microstructure and properties, (4) review of various types of TPVs, including general TPVs, special TPVs, bio-based TPVs, and TPVs-based nanocomposites, (5) future challenges on TPVs.

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).

References (256)

  • L.F. Ma et al.

    Conductive thermoplastic vulcanizates (TPVs) based on polypropylene (PP)/ethylene-propylene-diene rubber (EPDM) blend: from strain sensor to highly stretchable conductor

    Compos Sci Technol

    (2016)
  • S. Salaeh et al.

    Dynamically cured poly(vinylidene fluoride)/epoxidized natural rubber blends filled with ferroelectric ceramic barium titanate

    Composites Part A

    (2017)
  • G. Martin et al.

    Morphology development in thermoplastic vulcanizates (TPV): Dispersion mechanisms of a pre-crosslinked EPDM phase

    Eur Polym J

    (2009)
  • C.F. Antunes et al.

    Morphology and phase inversion of EPDM/PP blends?effect of viscosity and elasticity

    Polym Test

    (2011)
  • Y. Wang et al.

    Preparation and properties of dynamically cured poly (vinylidene fluoride)/silicone rubber blends

    Polym Test

    (2013)
  • R. Rajeshbabu et al.

    Preparation of polypropylene (PP)/ethylene octene copolymer (EOC) thermoplastic vulcanizates (TPVs) by high energy electron reactive processing

    Radiat Phys Chem

    (2011)
  • K. Naskar et al.

    Influence of molecular structure of blend components on the performance of thermoplastic vulcanisates prepared by electron induced reactive processing

    Polymer

    (2016)
  • S.S. Banerjee et al.

    Design and properties of high-performance polyamide 6/fluoroelastomer blends by electron-induced reactive processing

    Eur Polym J

    (2016)
  • Y. Chen et al.

    Highly toughened polypropylene/ethyleneepropylene-diene monomer/zinc dimethacrylate ternary blends prepared via peroxide-induced dynamic vulcanization

    Mater Chem Phys

    (2013)
  • M.D. Ellul et al.

    Crosslink densities and phase morphologies in thermoplastic vulcanizates

    Polymer

    (2004)
  • M. Hernández et al.

    Influence of the vulcanization system on the dynamics and structure of natural rubber: comparative study by means of broadband dielectric spectroscopy and solid-state NMR spectroscopy

    Eur Polym J

    (2015)
  • C.F. Antunes et al.

    Morphology development and phase inversion during dynamic vulcanisation of EPDM/PP blends

    Eur Polym J

    (2011)
  • M. Van Duin et al.

    EPDM-based thermoplastic vulcanisates: crosslinking chemistry and dynamic vulcanisation along the extruder axis

    Polym Degrad Stab

    (2005)
  • M. Tian et al.

    Interfacial crystallization and its mechanism in in situ dynamically vulcanized iPP/POE blends

    Polymer

    (2014)
  • C.F. Antunes et al.

    Effect of crosslinking on morphology and phase inversion of EPDM/PP blends

    Mater Chem Phys

    (2012)
  • X. Wang et al.

    Super toughened immiscible poly(L-lactide)/poly(ethylene vinyl acetate) (PLLA/EVA) blend achieved by in situcrosslinking reaction and carbon nanotubes

    Composites Part A

    (2016)
  • S. Amin et al.

    Thermoplastic elastomeric (TPE) materials and their use in outdoor electrical insulation

    Rev Adv Mater Sci

    (2011)
  • J.G. Drobny

    Handbook of thermoplastic elastomers

    (2014)
  • M. Van Duin

    Recent developments for EPDM-based thermoplastic vulcanisates

    Macromol Symp

    (2006)
  • K. Naskar

    Thermoplastic elastomers based on PP/EPDM blends by dynamic vulcanization

    Rubber Chem Technol

    (2007)
  • J. Bai et al.

    A simple approach to preparation of polyhedral oligomeric silsesquioxane crosslinked poly(styrene-b-butadiene-b-styrene) elastomers with a unique micro-morphology via UV-induced thiol-ene reaction

    Polym Chem

    (2014)
  • Y. Zhao et al.

    Largely improved mechanical properties of a poly(styrene-b-isoprene-b-styrene) thermoplastic elastomer prepared under dynamic-packing injection molding

    Ind Eng Chem Res

    (2014)
  • X. Lu et al.

    Mechanical and structural investigation of isotropic and anisotropic thermoplastic magnetorheological elastomer composites based on poly (styrene-b-ethylene-co-butylene-b-styrene)(SEBS)

    Rheol Acta

    (2012)
  • R.P. Quirk et al.

    Thermoplastic elastomers

    (2004)
  • R.R. Babu et al.

    Recent developments on thermoplastic elastomers by dynamic vulcanization

    Adv Polym Sci

    (2011)
  • P. Yao et al.

    Microstructure and properties of bromo-isobutylene–isoprene rubber/polyamide 12 thermoplastic vulcanizate toward recyclable inner liners for green tires

    RSC Adv

    (2016)
  • N. Ning et al.

    Unique microstructure of oil resistant nitrile butadiene rubber/polypropylene dynamically vulcanized thermoplastic elastomer

    RSC Adv

    (2017)
  • H. Wu et al.

    New understanding of morphology evolution of thermoplastic vulcanizate (TPV) during dynamic vulcanization

    ACS Sustain Chem Eng

    (2015)
  • R.S. Ismail et al.

    Development of novel polar thermoplastic vulcanizates based on ethylene acrylic elastomer and polyamide 12 with special reference to heat and oil aging

    J Appl Polym Sci

    (2015)
  • N. Ning et al.

    Novel heat and oil-resistant thermoplastic vulcanizates based on ethylene-vinyl acetate rubber/poly(vinylidene fluoride)

    RSC Adv

    (2016)
  • K.L. Walton

    Metallocene catalyzed ethylene/alpha olefin copolymers used in thermoplastic elastomers

    Rubber Chem Technol

    (2004)
  • N. Tortorella et al.

    Morphology and mechanical properties of impact modified polypropylene blends

    Polym Eng Sci

    (2008)
  • S. Varghese et al.

    Natural rubber–isotactic polypropylene thermoplastic blends

    J Appl Polym Sci

    (2004)
  • A. Joseph et al.

    Nonisothermal thermophysical evaluation of polypropylene/natural rubber based TPEs: effect of blend ratio and dynamic vulcanization

    Polym Eng Sci

    (2009)
  • C. Nakason et al.

    Hermoplastic elastomers based on epoxidized natural rubber and high-density polyethylene blends: effect of blend compatibilizers on the mechanical and morphological properties

    J Appl Polym Sci

    (2008)
  • C. Nakason et al.

    Thermoplastic elastomer based on high-density polyethylene/natural rubber blends: rheological, thermal, and morphological properties

    Polym Adv Technol

    (2008)
  • J. George et al.

    Failure properties of thermoplastic elastomers from polyethylene/nitrile rubber blends: effect of blend ratio, dynamic vulcanization, and filler incorporation

    J Appl Polym Sci

    (2006)
  • B.G. Soares et al.

    Influence of curing agent and compatibilizer on the physicomechanical properties of polypropylene/nitrile butadiene rubber blends investigated by positron annihilation lifetime technique

    J Appl Polym Sci

    (2006)
  • M. Tian et al.

    Dramatic influence of compatibility on crystallization behavior and morphology of polypropylene in NBR/PP thermoplastic vulcanizates

    J Polym Res

    (2012)
  • P. Yao et al.

    Properties and unique morphological evolution of dynamically vulcanized bromo-isobutylene-isoprene rubber/polypropylene thermoplastic elastomer

    RSC Adv

    (2016)
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