Microphase Formation,microstructure-properties Optimization And Rheological Behaviors Of Polyolefin Thermoplastic Vulcanizates

Thermoplastic vulcanizate(TPV)is a special kind of a rubber-plastic blending type thermoplastic elastomer prepared by dynamic vulcanization.By mixing high content(> 50%)rubber and its crosslinking agent into low content(< 40%)thermoplastic,the rubber phase is shear-induced broken up and simultaneously in situ crosslinking,which results in the formation of a large number of rubber micro-and nano-particles during melt blending.Finally,TPV consists of a high content of crosslinked rubber particles as dispersed phase and a low content of thermoplastic as continuous phase.Not only TPV exhibits the physical properties and elasticity similar to thermosetting rubber at room temperature,TPV can be processed like thermoplastic at high temperature,so that it can gradually replace thermosetting vulcanized rubber in automotive industry,electronic and electrical appliances,construction industry and other application fields as a kind of new "green" chemical materials that is easy to process and can be recycled multiple times.In order to meet the ever-increasing application needs of industrial production and daily life,people began to put forward higher requirements for TPV,such as fine phase and high performance.Therefore,the preparation and processing of high-performance TPV has become one of the most important directions of thermoplastic elastomer,and its key lies in fine phase.At present,polyolefin TPVs such as ethylene-propylene diene monomer(EPDM)/polypropylene(PP)TPV are still the most widely used TPV in industrial production and daily life,but there is still a lack of in-depth understanding of the essential influencing factors of the microphase formation(especially rubber nanoparticles and their agglomerates)of TPV,the microphase structure regulation and performance optimization,the processing rheology and so on.Therefore,this thesis focused on the essential influencing factors of microphase formation of polyolefin TPVs such as EPDM/PP TPV,the microphase structure regulation and performance optimization of polyolefin TPVs through dynamic vulcanization and post-processing,and the relationship between microphase structure and rheological behavior of polyolefin TPVs.The innovations and results are as follows:1.The formation mechanism of rubber particles during dynamic vulcanization was revealed by studying the relationship between crosslinking mechanics,break-up kinetics and coalescence kinetics of rubber phase in different blends of BIMSM/PP and BIIR/PA.The results show that the formation of rubber particles during dynamic vulcanization is the result of the interaction of crosslinking mechanics,break-up kinetics and coalescence kinetics of rubber phase,which is closely related to the viscosity ratio of rubber and plastic.For BIMSM/PP blend with the initial viscosity ratio of BIMSM to PP less than 1,the viscosity difference between BIMSM and PP becomes smaller and smaller during dynamic vulcanization,and then instead of coalescence,the matching of crosslinking and break-up of BIMSM plays a leading role that leads to the formation of the small-size BIMSM particles.For the BIIR/PA blend with the initial viscosity ratio of BIIR and PA close to 1,the viscosity difference between rubber and plastic is becoming larger and larger during dynamic vulcanization,and then the break-up of BIIR does not match with the crosslinking,and the coalescence of BIIR is dominant that leads to the formation of the large-size BIIR particles.This conclusion provides a guidance for the preparation of high-performance TPV with a fine phase by dynamic vulcanization.2.Through the orthogonal dynamic vulcanization experiments that the same EPDM blending with two different PP molecular weights and three different crosslinking systems,the mechanism and the essential factors affecting the formation of rubber nanoparticles and their agglomerates in EPDM/PP TPV were revealed.The results show that the size of the rubber nanoparticles is governed by the coupling effect of molecular weight of PP phase,and the crosslinking rate and degree of EPDM phase.The lower molecular weight of PP phase and the lower crosslinking rate and degree of EPDM phase contribute to the formation of smaller rubber nanoparticles.The size of rubber nanoparticle agglomerates is only affected by molecular weight of PP phase and the crosslinking degree of EPDM phase.The higher molecular weight of PP phase and crosslinking degree of EPDM contribute to the formation of smaller rubber nanoparticle agglomerates.This study provides guidance to optimize the microstructure of EPDM/PP TPV for the preparation of high-performance TPV products.3.Effects of shear on the anisotropic microstructure and properties of EPDM/PP TPV containing rubber nanoparticle agglomerates were studied by high-speed injection molding.Increasing the shear rate of injection molding not only makes the EPDM nanoparticle agglomerates finely break up and significantly reduces the particle size,but also more PP crystals,PP chains and EPDM nanoparticle agglomerates are oriented in the shear direction;moreover,the crystallinity of α and β crystal forms in PP was increased.The responses of these microphase structures in the anisotropy of properties are as follows: the tensile elastic modulus and tensile strength of TPV increase as a whole both in the shear direction(injection direction)and perpendicular to the shear direction,with a higher increase in the shear direction.Moreover,the tensile strength and elastic modulus of TPV in the shear direction are much higher than those perpendicular to the shear direction,while the elongation at break and elastic properties are opposite.However,the further increase of shearing will lead to the degradation of PP molecular chain,which reduce all the properties of TPV.These findings provide a method guidance for regulating the microphase structure of TPV via post-processing.4.The post-processing technology of strong shearing and interfacial encapsulation of POE was applied to regulate the microphase structure and optimize properties of EPDM/PP TPV.Polyolefin elastomer(POE)was determined as the suitable interfacial encapsulation agent for EPDM/PP TPV through the interface spreading theory.When applying high shear to break EPDM nanoparticle agglomerates,the thermodynamic stable and fine dispersion of EPDM nanoparticle agglomerates were realized by adding the interfacial encapsulation of POE,so as to improve the properties of TPV.These results were verified by experiments and theoretical calculation including the quantitative nanomechanical mapping and the reduction of interfacial tension via interfacial encapsulation;Under the action of strong shearing and interfacial encapsulation with an appropriate POE amount,the size of EPDM nanoparticle agglomerates in TPV was reduced from micron size to sub-micron size;Also,the tensile strength and the elongation at break of TPV are increased by about46% and nearly 40%,respectively,and the elastic modulus and elasticity are significantly improved due to the interfacial encapsulation of POE decreasing the interfacial tension.This post-processing provides method guidance and new ideas for fine processing TPV and performance regulation.5.The different micophase structures of rubber nanoparticle agglomerates of EPDM/PP TPV were designed by the dilution of PP matrix.The relationship between microphase structure and(non)linear viscoelasticity of TPV was quantitatively studied by dynamic oscillation scanning.It was revealed that TPV showed a yield stress fluid characteristic with weak strain overshoot behavior under strain shear flow field.The yield stress of TPV has an exponential relationship with the content of rubber nanoparticle agglomerates.When the rubber nanoparticle agglomerates changes from network structure to isolated dispersion state,the rheological behavior of TPV changes from weak strain overshoot behavior to strain thinning behavior.In addition,through the capillary extrusion rheometry,it is found that the flow of TPV presents the properties of elongational thinning and shear thinning,and the response of rubber nanoparticle aggregate phase structure in TPV to tensile flow field is more sensitive than that of shear flow field.The elongational flow characteristics and melt elasticity of TPV are mainly affected by the microphase structure of rubber nanoparticle agglomerates;The more the rubber nanoparticle agglomerates form a network structure,the larger the elongational stress or viscosity of TPV and the more significant the melt elasticity.The shear flow behavior of TPV is mainly affected by the plastic matirx.These results provide guidance for the processing of high-performance TPV.