Study On The Reactive Processing, Tructure And Properties Of Polyamide/Ethylene-vinyl Acetate Rubber Blends

Polyamide (PA) is a kind of important high performance plastics withmany amide groups in the backbone, exhibiting good comprehensiveproperties and wide applications. Ethylene-vinyl acetate rubber (EVM) is akind of specialty rubber with many ester side groups and good comprehensiveproperties. Polyamide thermoplastic elastomers have good mechanicalproperties, heat resistance and chemical resistance, with wide applications inthe fields of automobile, sports equipment and electronic appliances. A studyon the ester-amide change reaction between main chains of PA and sidegroups of EVM has great theoretical and practical significance for studyingthe chemical reactions of polymers during melt processing, achieving thereactive processing of PA/EVM blends and expanding the species andpreparing methods of polyamide thermoplastic elastomers. In this study,PA/EVM blends were prepared and their reactive processing wassystematically investigated. By using dynamic vulcanization and exchangereaction, polyamide-based blend and graft copolymer thermoplasticelastomers with good comprehensive properties were successfully prepared,and their structure and properties were systematically investigated.Firstly, the reactive processing process was investigated and polyamidethermoplastic elastomers with good mechanical properties and heat oilresistance were prepared from EVM/ternary polyamide copolymer (tPA)through a dynamic vulcanization process. For EVM/tPA (70/30) blend, aphase inversion occurred during dynamic vulcanization, and the continuous EVM phase was crosslinked and evolved into the dispersed phase. Withincreasing DCP content, the mechanical properties and heat oil resistance ofEVM/tPA (70/30) blends were improved. With3.5phr DCP and5wt%ofmaleic anhydride-grafted EVM, EVM/tPA thermoplastic elastomer (TPE)with tensile strength of24MPa and elongation at break of361%wasobtained.Secondly, the transesterification crosslinking reaction of EVM withtetraethyloxysilane (TEOS) was investigated and the dynamic vulcanizationof PA/EVM blends at over200°C was achieved, providing a new method fordynamic vulcanization of PA/EVM blends at elevated temperatures andbroadening the processing temperature. As suggested from the vulcanizationreaction kinetics, the crosslinking reaction of EVM with TEOS presentedlower reaction rate constant and activation energy than the crosslinkingreaction of EVM initiated by DCP. A crystalline ternary polyamide copolymer(CTPA) was blended with EVM and TEOS was used as a crosslinking agentfor preparing thermoplastic elastomer. From the results of atomic forcemicroscopy analysis, the crosslinked EVM was the dispersed phase and CTPAwas the continuous phase. With increasing TEOS content, the crosslinkdensity increased, the EVM particle size decreased, and the mechanicalproperties and heat oil resistance were improved.Thirdly, graphene oxide (GO) reinforced EVM/CTPA TPE compositeswere prepared, and the structure and mechanical properties of the compositeswere investigated. GO nanosheets were found to be distributedhomogeneously in both EVM and CTPA phases, but not completely exfoliated.After the addition of GO, the stress at100%extension of the EVM/CTPATPE was enhanced. When the GO content was0.91wt%, the stress at100%extension of TPE was increased to8.7MPa from3.2MPa. From the results ofdifferential scanning calorimeter analysis, GO had great heterogeneousnucleation activity for CTPA, and greatly accelerated the crystallization ofCTPA, increasing the crystallization temperature by about20°C. Assuggested from the isothermal crystallization kinetics, after the addition of GO, the Avrami exponent was decreased to about2, and the crystallizationmode was changed into two-dimensional crystal growth following aheterogeneous nucleation mechanism. Therefore, low loading of GO couldgreatly increase the crystallization rate of EVM/CTPA TPE, which is helpfulfor shortening the molding time and improving the crystallization duringmolding process and is of practical significance.Fourthly, the ester-amide change reaction between main chains of PAand side groups of EVM was investigated. The kinetic calculation method ofthe exchange reaction and the theoretical characterization method wereproposed. And the effects of the exchange reaction on the morphology andmechanical properties of PA/EVM blends were investigated. During meltblending of polyamide6(PA6) and EVM, the ester-amide exchange reactiontook place, leading to the formation of PA6grafted EVM copolymer(EVM-g-PA6) and acetamide-terminated PA6. Dibutyltin oxide (DBTO)could effectively catalyze the exchange reaction. The reaction kineticparameters were calculated according to a second-order reversible reactionmechanism. The rate constant was dependent on the catalyst concentration,PA6/EVM ratio and shearing action. The reaction products were separatedfrom the blends by using selective extractions. With prolonging reaction time,the yield of EVM-g-PA6and the content of acetamide end groups in the PA6fraction both increased, and the morphology and mechanical properties werechanged. Therefore, during practical processing, the control of the exchangereaction could be used to control the structure and mechanical properties ofPA6/EVM blends.Finally, the synthesis of EVM-g-PA6copolymer was further explored bycombining ring-opening polymerization of caprolactam (CL) and theester-amide exchange reaction. The copolymer showed microphase separationmorphology, in which the PA6phase was dispersed as~200nm particles inthe continuous EVM phase. The copolymer showed tensile strength of12.7MPa and elongation at break of595%, and demonstrated a great potential forengineering thermoplastic elastomer applications. After introduction of GO into the preparation process, the EVM-g-PA6/graphene composites wereprepared. GO participated in the reactions and was thermally reducedsimultaneously. With increasing GO loading, the melting temperatures ofEVM-g-PA6decreased, and the stress at100%extension and conductivitywere greatly increased. By using the ester-amide exchange reaction betweenPA and EVM, the graft copolymer with EVM as the main chain and PA as thebranch was prepared, providing new basis for the research and developmentof new graft copolymers and elastomer materials.