Hanguang Wu

New understanding of microstructure formation of the rubber phase in thermoplastic vulcanizates (TPV)

The breakup of the rubber phase in an ethylene-propylene-diene monomer (EPDM)/polypropylene (PP) blend at the early stage of dynamic vulcanization is similar to that in an unvulcanized EPDM/PP blend because of the low crosslink density of the EPDM phase. In this work, the minimum size of the rubber phase in the unvulcanized EPDM/PP blend was first calculated by using the critical breakup law of viscoelastic droplets in a matrix. The calculated results showed that the minimum size of the rubber phase in the unvulcanized blend was in the nanometer scale (25-46 nm), not the micrometer scale as reported in many works. Meanwhile, the actual size of the rubber phase in the thermoplastic vulcanizate (TPV) at both the early stage and the final stage of dynamic vulcanization was observed by using peak force tapping atomic force microscopy (PF-AFM). The results indicated that the EPDM phase indeed broke up into nanoparticles at the early stage of dynamic vulcanization, in good agreement with the calculated results. More interestingly, we first revealed that the micrometer-sized rubber particles commonly observed in TPV were actually the agglomerates of rubber nanoparticles with diameters between 40 and 60 nm. The mechanism for the formation of rubber nanoparticles and their agglomerates during dynamic vulcanization was then discussed. Our work provides guidance to control the microstructure of the rubber phase in TPV to prepare high performance TPV products for a wide range of applications in the automobile and electronic industries.

Effect of additives on the morphology evolution of EPDM/PP TPVs during dynamic vulcanization in a twin-screw extruder

Ethylene-Propylene-Diene Monomer/Polypropylene thermoplastic vulcanizates (EPDM/PP TPVs) have been widely used as a kind of typical “green” elastomer because of their excellent mechanical properties and recyclability. The industrial TPVs always contain various types of additives, which influence the viscosity ratio of EPDM and PP and the morphology of TPVs. This work studied the morphology evolution of EPDM/PP TPVs with various amounts of curing agents, fillers, and plasticizer during dynamic vulcanization in a twin-screw extruder, which provides much more complicated dynamic vulcanization process than haaker rheometer. The results show that the increased curing agents content leads to the faster morphology evolution of TPV because it enhances the cross-linking speed and the viscosity of EPDM. The increased fillers content leads to the later breakup of EPDM and the bigger size of the rubber aggregation because it enhanced the modulus of EPDM and weakens the interfacial interaction between EPDM and PP. In addition, the increase in the plasticizer content leads to the earlier breakup of EPDM and the larger size of the rubber phase in TPV. Our work firstly demonstrates the morphology evolution of industrial EPDM/PP TPV, and thus can provide a guidence for the industrial production of high-performance EPDM/PP TPVs.

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

In this study, the isothermal/nonisothermal crystallization behavior of polypropylene (PP) in acrylonitrile butadiene rubber (NBR)/PP thermoplastic vulcanizates (TPVs) prepared with three different processing methods, the compatibility effect therein, and the mechanism involved were studied. We concluded that the vulcanized NBR particles in TPVs act as heterogeneous nucleation centers and increase the number of nuclei. The crystallization rate of PP thereby increases and the growth of PP spherulites is restrained because of the isolation of vulcanized NBR particles. Since the addition of compatibilizer improves the compatibility of NBR and PP, the smaller and uniformly dispersed NBR particles are obtained, resulting in more and smaller PP crystals as well as higher crystallization rate, compared with Ultra-fine fully vulcanized NBR particles (UFNBR)/PP TPV and NBR/PP TPV without compatibilization. The isothermal crystallization kinetics of PP in TPVs obeys the Avrami equation, whereas the nonisothermal crystallization kinetics is well described by the equation of Mo et al.

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

We studied the microstructure, morphological evolution and the corresponding mechanism, and the properties of bromo-isobutylene-isoprene rubber (BIIR)/polypropylene (PP) thermoplastic vulcanizates (TPVs). Interestingly, a large number of single rubber nanoparticles were observed in the crosslinked BIIR/PP blends, ascribed to the improvement of compatibility between the BIIR and PP with increasing dynamic vulcanization (DV) time, as demonstrated by the increase in interfacial phase thickness and the decrease in the interfacial tension. Most of these single nanoparticles agglomerated as the DV proceeded, leading to the deterioration of the rubber network. Another interesting observation was that the size of rubber agglomerate decreased as the DV proceeded, leading to the strengthening of the rubber network. Importantly, the as-prepared BIIR/PP TPV exhibits good processability, high elasticity and good mechanical property. The relationship between the unique morphology and properties were studied. Our study provides guidance for the preparation of high-performance BIIR/PP TPV for its industrial applications such as medical bottle stoppers.

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

We successfully prepared bromo-isobutylene–isoprene rubber (BIIR)/polyamide 12 (PA 12) thermoplastic vulcanizate (TPV) by dynamic vulcanization (DV), and studied the microstructure and properties of BIIR/PA 12 TPV toward recyclable inner liners for green tires, to reduce fuel consumption and carbon emissions. The as-prepared BIIR/PA 12 TPV exhibits good mechanical properties, good elasticity, easy processability and good gas barrier properties. More importantly, our BIIR/PA 12 TPV still exhibits good properties after recycling several times. The mechanism for the formation of the phase morphology and morphological evolution during dynamic vulcanization, and the microstructure–property relationship of BIIR/PA 12 TPV were thoroughly studied to provide guidance for the preparation of high-performance BIIR/PA 12 TPVs toward tire inner liners. As the DV proceeded, the size of the dispersed BIIR particles and the thickness of the PA 12 ligaments decreased, leading to the strengthening of the rubber network. Although the rheological properties slightly deteriorated, the elasticity, the mechanical properties and the gas barrier properties of the TPVs were obviously improved as the DV proceeded.

A novel dielectric elastomer by constructing dual-network structure of carbon nanotubes and rubber nanoparticles in dynamically vulcanized thermoplastic elastomer

Thermoplastic vulcanizates (TPVs), as a special class of high-performance thermoplastic elastomers (TPEs), consist of a high content (60–80 wt%) of crosslinked rubber particles as the dispersed phase and a low content of a thermoplastic as the matrix. In this study, inspired by the special microstructure of TPVs, we prepared carbon nanotubes (CNTs)/TPV dielectric elastomer composites with a high dielectric constant (k) and low dielectric loss by constructing a dual network formed by rubber and CNTs. The rubber network was formed by a high content of agglomerates of rubber nanoparticles in the TPVs, which simultaneously promoted the formation of a CNTs network at a low content of CNTs in the matrix, to increase the value of k, and hindered the direct connection of CNTs with one another, to decrease the dielectric loss. As a result, the CNTs/TPV composites simultaneously possessed a high value of k and low dielectric loss. Moreover, the elasticity of the composites was improved by the CNTs because of the nanosprings of CNTs. This study provides a new simple and effective strategy for preparing a high-performance dielectric elastomer with a high value of k, low dielectric loss, good mechanical properties, high elasticity, high processability and easy recyclability.

Effect of Rubber Nanoparticle Agglomeration on Properties of Thermoplastic Vulcanizates during Dynamic Vulcanization

We previously reported that the dispersed rubber microparticles in ethylene-propylene-diene monomer (EPDM)/polypropylene (PP) thermoplastic vulcanizates (TPVs) are actually agglomerates of rubber nanoparticles. In this study, based on this new understanding of the microstructure of TPV, we further revealed the microstructure-properties relationship of EPDM/PP TPV during dynamic vulcanization, especially the effect of the size of rubber nanoparticle agglomerates (dn), the thicknesses of PP ligaments (IDpoly) and the rubber network on the properties of EPDM/PP TPV. We were able to simultaneously obtain a high tensile strength, elongation at break, elastic modulus, and elasticity for the EPDM/PP TPV by the achievement of a smaller dn, a thinner IDpoly and a denser rubber network. Interestingly, the effect of dn and IDpoly on the elastic modulus of EPDM/PP TPV composed of rubber nanoparticle agglomerates is different from that of EPDM/PP TPVs composed of rubber microparticles reported previously. The deformation behavior of the TPVs during stretching was studied to understand the mechanism for the achievement of good mechanical properties. Interestingly, the rubber nanoparticle agglomerates are oriented along the tensile direction during stretching. The TPV samples with smaller and more numerous rubber nanoparticle agglomerates can slow down the development of voids and cracks more effectively, thus leading to increase in tensile strength and elongation at break of the EPDM/PP TPV.

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

Thermoplastic vulcanizates (TPVs) combine the excellent elasticity of conventional vulcanized rubbers and the easy processability and recyclability of thermoplastics. In this study, we successfully prepared novel heat and oil-resistant TPVs based on ethylene-vinyl acetate rubber (EVM) and poly(vinylidene fluoride) (PVDF) by dynamic vulcanization (DV). The phase morphology, morphological evolution, and the properties of the EVM/PVDF TPVs were studied, and the microstructure–property relationship during DV was revealed. Interestingly, a large number of EVM rubber nanoparticles are observed in the EVM/PVDF TPVs during DV, and these nanoparticles assemble into oriented EVM fibers and EVM bundles during DV. As the DV further proceeds, the oriented EVM fibers and EVM bundles agglomerate with one another more densely and further assemble into EVM spherulites, leading to an increase in the thickness of the PVDF ligaments and the deterioration of the rubber network. More importantly, our EVM/PVDF TPVs show good mechanical properties, high elasticity, good processability, excellent heat-and oil resistance, and good recyclability. This study provides guidance for the preparation of new TPVs that can replace the traditional thermosetting rubbers in automotive and oil pipeline areas.