Microstructure,Formation Mechanism And Microstructure-Property Relationship Of Specialty Thermoplastic Vulcanizates

Thermoplastic vulcanizate (TPV) is a special kind of thermoplastic elastomer prepared by dynamic vulcanization (DV), resulting in high-content (50-80%) dynamically vulcanized rubber particles dispersed in low-content (20-50%) continuous plastic matrix. TPV, which combines the high elasticity of traditional crosslinked rubbers and the process ability and recyc lability of thermoplastics, has become one of the most important kinds of "green" novel chemical materials. Thus, TPV has attracted much attention and has been developed rapidly. The key to prepare TPV by DV is that how to obtain high crosslinking degree in the rubber phase during mechanical blending and to break up the rubber phase simultaneously, resulting in the phase inversion of the rubber phase and the plastic phase. However, the unique microstructure of TPV can not be obtained by traditional simple blending, which leads to continuous rubber phase due to its high content and dispersed plastic phase due to its low content. The mechanical property, the elasticity and the processability of TPV depend on its unique microstructure, thus, it is significant to study the micro structure, the formation mechanism and the microstructure-property relationship in order to control the morphology and to prepare high-performance TPV.Ethylene-propylene-diene monomer (EPDM)/polypropylene (PP) TPV is the most widely used and successfully commercialized TPV, and the formation mechanism of its microstructure and microstructure-property relationship has been systematically and deeply studied. Recently, specialty TPVs, which show special properties and functions such as heat resistance, solvent resistance, gas barrier, flame resistance, electrical, medical, et al, have become the focus of research and development. However, studies of specialty TPVs on the formation mechanism of their microstructure and the microstructure-property relationship are lacking. Thus, four kinds of different TPVs with good application prospect and special properties were selected, including bromo-isobutylene-isoprene (BIIR)/PP TPV for medical bottle stoppers, BIIR/polyamide (PA) TPV for gas barrier tire inner liners, oil resistant nitrile butadiene rubber (NBR)/PP TPV and oil resistant ethylene-vinyl acetate copolymer (EVM)/PA TPV. And the microstructure, the formation mechanism and the microstructure-property relationship of miscible/immiscible dynamic vulcanized blending systems, including nonpolar rubber/nonpolar plastic, nonpolar rubber/polar plastic, polar rubber/nonpolar plastic, polar rubber/polar plastic, were studied to provide guidance for the controlling of the morphology and the preparation of high-performance TPV. The innovative work and results are as following:1. Different dynamic vulcanized blending systems have different micro structures and formation mechanisms. For nonpolar rubber/ nonpolar plastic (BIIR/PP) TPV, the dispersed BIIR particles (350 nm) are actually agglomerates of rubber nanoparticles (40 to 100 nm) formed by the breakup and in situ vulcanization of BIIR nanodroplets at the early stage of the DV, and a large number of single BUR nanoparticles are also observed. The phase inversion is dominated by the formation and agglomeration of rubber nanoparticles. For reactive compatibilized polar rubber/nonpolar plastic (NBR/PP) TPV, the dispersed NBR particles (3 to 8 μm) are actually agglomerates of NBR microparticles (1.7 μm), and many PP domains are found embedded in the dispersed crosslinked NBR phase (due to the reactive compatibilization) resulting in special microstructure. The phase inversion is consistent with that in BIIR/PP TPV. For nonpolar rubber/polar plastic (BIIR/PA) TPV and polar rubber/polar plastic (EVM/PA) TPV, the dispersed rubber particles (BIIR particles:2 μm; EVM particles:3 to 7 μm) are formed by the breakup of crosslinked rubber phase, and the phase inversions are dominated by the increase in viscosity of the rubber phase due to crosslinking and the breakup of crosslinked rubber phase. Besides, some PA domains are also found embedded in the dispersed crosslinked BUR phase.2. Different dynamic vulcanized blending systems behave different microstructure-property relationships. For BIIR/PP TPV, as the DV proceeded, most of the single BIIR nanoparticles agglomerated leading to the deterioration of the rubber network, while the size of the rubber agglomerates decreased leading to the strengthening of the rubber network. The competition leads to the slight changes in the mechanical property, the elasticity and the rheological property. For BIIR/PA TPV, NBR/PP TPV and EVM/PA TPV, as the DV proceeded, the size of the dispersed rubber particles and the thickness of the plastic ligaments decreased, leading to the increase in the density of rubber particles and the strengthening of the rubber network, thus leading to the improvement in the mechanical property and the elasticity and the slight deterioration in the rheological property. The gas barrier property of the BIIR/PA TPV and the oil resistance of the NBR/PP TPV and the EVM/PA TPV are also improved. Besides, the BIIR/PA TPV still exhibits good properties after recycling for several times.3. Compared the size of the dispersed rubber phase in our TPVs with the theoretical critical size of the broke-up rubber phase in the corresponding blending systems. The results show that the theoretical critical size of the broke-up rubber phase is small in the blending system with good compatibility between the rubber phase and the plastic phase while it is big in the blending system with bad compatibility between the rubber phase and the plastic phase. When the viscosities of the rubber phase and the plastic phase are equal or close, the size of the dispersed rubber phase in TPV is close to the theoretical critical size of the broke-up rubber phase in the corresponding blending systems. When the viscosity of the rubber phase is smaller than the plastic phase, the size of the dispersed rubber phase in TPV is bigger than the theoretical critical size of the broke-up rubber phase in the corresponding blending systems because the coalescence of the rubber phase dominated between the competition of the coalescence and breakup in the rubber phase.4. The effects of the crosslinking degree and the addition of plastic izer in the rubber phase and the processing conditions during DV on the breakup of the rubber phase were studied based on the formation mechanism of the BIIR/PA TPV. The results show that, low crosslink degree and high content of the plasticizer in the rubber phase, leading to low cohesive energy and modulus of the rubber phase, facilitate the breakup of the rubber phase. And low blending temperature, leading to high viscosity of the plastic matrix, also facilitates the breakup. However, high rotor speed, leading to high shear heating and shear thinning resulting in the decrease in the viscosity of the plastic matrix, and low rotor speed, leading to low shear, both are adverse to the breakup.5. Based on the above results, guidance was provided to the cooperative enterprise for the successfully development of BIIR/PP TPV for medical bottle stoppers, which has met the requirements of the national standard (YBB00042005) and has passed the certification of the drug master files (DMF) of the United States Food And Drug Administration (FDA) and the highest class of the United States Pharmacopoeia (USP CLASS VI). Oil resistant NBR/PP TPV was also successfully developed and the industrialization of the two TPVs was both realized for the first time in China. Moreover, BIIR/PA TPV for heat resistant, high gas barrier tire inner liners is developing.