| Mechanical properties of a plastic product depend on chain structure of polymer. Furthermore, they are greatly affected by molecular aggregations which include crystallization and orientation, as well asphase morphology for blended plastic system. Molecular aggregations are induced under complex temperature and external force conditions during process. Studies of molecular aggregations formation and its development are important to improve basic theory of polymer processing and exploit potentialities of conventional polymer. Generally speaking, the number of process parameters for optimizing is limited, and the ranges of parameters for adjusting are restricted for conventional injection molding(CIM). Compare to CIM, Sequential co-injection molding(SCIM) involving more parameters makes it possible to control molecular aggregations of polymer. Therefore, in this thesis, isotatic polypropylene(i PP) and its composites were used to prepare SCIM parts. In the process, the formation and developmentof microstructure which includes crystalline morphologies, orientation and phase morphology were discussed, and their effects on the mechanical properties of SCIM parts were investigated.The mainresults are as follows:(1) Study ofcrystallization morphology and mechanical properties of i PP SCIM parts was performed. Compare to CIM, SCIM samples have a richer hierarchy. According to crystallization morphologies of i PP, a SCIM productwas divided into four layers: shear layer, transition layer, layer of cylindrulite and core layer. And each layer exhibits peculiar crystalline morphology: shear layer showed shish-kebab, transition layer presented deformed spherulite, layer of cylindrulite featured cylindritic structures, core layer comprised spherulitic structure. For high orientation of molecular chain and lamella, SCIM producthad stronger Young’s modulus and tensile strength which are improved 16.6% and 7% respectively, however, breaking elongation of SCIM part decreases.(2) Crystallization morphology and mechanical properties of SCIM rod which involves undercooled melt skin material during processing were studied. In optimized delay-time, skin melt which has been injected into mould cavity firstly become undercooled melt. Shear imposed on skin melt which was applied by filling flow of core melt induce shish-kebab with α-lamellae in shear region between skin material and core material. Then, epitaxial growth of γ-lamellae based on(010) crystal face appeared under specific condition, and angle between α-lamellae and γ-lamellae is ~40o. Young’s modulus and tensile strength of SCIM part have improved to 60.7% and 25.6%. Breaking elongation of SCIM part also decreases.(3) Crystallization morphology and mechanical properties of i PP/TMB4 SCIM rod were investigated in the presence of TMB4, a self-assembled nucleating agent. In cross section which is perpendicular to the direction of flow, the arrangement of β-i PP, α-i PP, β-i PP alternately from skin to core forms sandwich structure. And polywood-like structure is formed for in the section which parallels to flow direction, main chain perpendicular to and parallels to and then perpendicular to flow direction. In tensile tests, Young’s modulus and tensile strength as well as breaking elongation increase synchronously.(4) Blended system of i PP/PC SCIM parts and their mechanical properties were researched. Deformation of dispersed phase(PC) is larger than those in CIM part, which is due to that the morphologies of PC phase which locate in SCIM parts are fibrous, rod or smaller ellipsoid for higher deformation. With the increase of core melt injection rate, the deformation of dispersed phase increases, and aspect ratio of dispersed phase becomes larger. As melt temperature increases, the dispersed phase is prone to deform, break, and become into fiber or smaller ellipsoidal distribute in the matrix material uniformly. Results of tensile tests show that Young’s modulus, tensile strength and breaking elongation of SCIM parts increase. |
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