The Morphology Development Of EPDM/PP TPV During Dynamic Vulcanization And Its Relationship With Properties Of TPV

Thermoplastic vulcanizates (TPVs) are a group of high performance thermoplastic elastomers prepared by dynamic vulcanization, which consist of a high content of crosslinked rubber as dispersion phase and a low content of thermoplastics as continous phase. TPVs combine the excellent resilience of the conventional vulcanized elastomers, and the good processability and easy recyclability of thermoplastics. Therefore, they are becoming one of the fastest growing elastomers as a group of typical "green" polymer materials in recent years because of the requirement of environmental protection and resource saving. Ethylene-Propylene-Diene Monomer (EPDM)/Polypropylene (PP) TPVs are the most widely applied TPV, thus they have attracted considerable attention and have been widely used in industries such.as automotive, building, and electronics. It has been widely reported that the rubber phase dispersed in the plastic matrix of EPDM/PP TPV is micro-meter spherical particles, and different mechanism of the morphological evolution of TPV during dynamic vulcanization were proposed, but there still remains controversial.In our study, the minimum size of the rubber phase in the EPDM/PP blend at the early stage of dynamic vulcanization was firstly calculated to be in the range of 25 nm and 46 nm by using the critical breakup law of the viscoelastic droplets in matrix. Meanwhile, the real size of rubber phase in Thermoplastic Vulcanizate (TPV) at both the early stage and the final stage of dynamic vulcanization were observed by using Peak Force Tapping Atomic Force Microscopy (PF-AFM). The results indicated that EPDM phase indeed broke up into nano-scale particles at the early stage of dynamic vulcanization, consistent well with the calculated result. More interesting, it was firstly revealed that the micro-meter sized rubber particles commonly observed in TPV were actually the agglomeration of nano-scale rubber particles with a diameter of 40-60 nm.Based on the new understanding of the formation and agglomeration of the rubber nanoparticles in EPDM/PP TPV, we revealed a new mechanism for the morphological evolution of TPV during dynamic vulcanization. The phase inversion in TPV was dominated by the formation and agglomeration of the rubber nanoparticles rather than the elongation and breakup of the crosslinked rubber phase as previously reported. The rubber agglomerates size become larger with the increased dynamic vulcanization time, and then kept constant at the end of dynamic vulcanization. In addition, we studied the relationship between the crossl inking of the rubber phase, the formation of the rubber nanoparticles and their agglomeration, and the occurrence of the phase inversion as well as the variation of the rubber network during dynamic vulcanization.In Chapter 5, we further investigated 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. 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.Then, we further 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.Finally, inspired by the special microstructure of TPVs, we prepared carbon nanotubes (CNTs)/TPV dielectric elastomer composites with high dielectric constant (k) and low dielectric loss through constructing a dual-network formed by rubber and CNTs. The rubber network was formed by a high content of rubber nanoparticle agglomerates in the TPVs, which simultaneously promoted the formation of a CNTs network at a low content of CNTs in the matrix for the increase in k and hinder the direct connection of CNTs with one another for the decrease in dielectric loss. As a result, the CNTs/TPV composites simultaneously possess high k and low dielectric loss. Meanwhile, the elasticity of the composites was improved by CNTs because of the nano-springs of CNTs. This study provides a new simple and effective strategy to prepare high performance dielectric elastomer with high k, low dielectric loss, good mechanical properties, high elasticity, high processability and easy recyclability.