Role of dicumyl peroxide on the morphology and mechanical performance of polypropylene random copolymer in microinjection molding

Abstract

The purpose of this work is to systematically investigate the effects of dicumyl peroxide (DCP) on the microstructural evolution and mechanical properties of polypropylene random copolymers (PPRs) during the microinjection process. Polarized light microscopy, differential scanning calorimetry, X-ray diffraction, and scanning electronic microscopy measurements were employed to characterize the morphology evolution of the PPR microparts with DCP. A hierarchical structure was found in the PPR microparts with DCP. Specifically, with the individual addition of organic peroxide, the orientation parameter of the PPR microparts decreased pronouncedly and the formation of skin layer was suppressed, while the formation of core layer was promoted.This was ascribed to the distribution of shear rate in the microchannel, which was determined by the wall ship effect in the filling stage and the relaxation behavior in the cooling stage. A mechanism was proposed to explain the distinctive filling behavior and molding characteristics of PPR with DCP in microinjection molding. Microinjection molding possesses many advantages, such as high precision, low cost, and high productivity. Therefore, many microsystem technology-related products, such as micropumps, microgears, optical grating elements, and microheat exchangers, are widely used in many industrial fields. 2,3 During μIM, the interfacial effects of wall slip and surface tension become more pronounced as compared with that of conventional injection molding (CIM). 4The μIM of polymers requires extremely high injection velocity, high mold temperature, and high melt temperature to prevent premature freezing or the occurrence of short shots. Therefore, under such a stringent condition, high shear stress is inevitably produced in the micromold cavities. 5 When the wall shear stress exceeds the critical stress (σ c1 ), the melts slip over the solid surfaces of the microchannel upon which the phenomenon of wall slip occurs. 2The resulting interfacial effects on the morphology and properties of polymers in μIM have since attracted the intense attention of many researchers.Crystalline polymers in microchannels, such as polypropylene, exhibit hierarchical structures and mechanical properties that are strongly dependent on the temperature, velocity profile, and shear stress during the μIM. Chains subjected to a higher shear rate and higher shear stress generally induce the formation of either highly oriented or parallel lamellar stack structures. 6,7 On the contrary, chains with lower shear rates and lower shear stresses induce the formation of a spherulitic structure. The distribution of oriented structures greatly influences the hierarchical Until now, the application of polypropylene resins in microinjection production is limited due to the extremely severe molding condition. A "controlled-rheology polypropylene" technology provides an approach to improve the processing ability of polypropylene resin in μIM, which is performed using a peroxide-initiated scission (β-scission) reactio...

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