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Morphology And Properties Manipulation Of Polyolefin-based Multiphase Systems Molded Via Water-assisted Injection Molding

Posted on:2014-10-24Degree:DoctorType:Dissertation
Country:ChinaCandidate:B WangFull Text:PDF
GTID:1261330422481468Subject:Mechanical design and theory
Abstract/Summary:PDF Full Text Request
Water-assisted injection molding (WAIM) is an efficient molding technology that can beused to mold hollow or partially hollow polymer parts with complicated profile. Due to thecomplexity of WAIM process, the researchers mainly focused on the optimization of WAIMequipment and processing parameters in the recent stage of its development, whereas littlework has been done on the mechanism of morphology and structure development duringWAIM. Thus, in this dissertation, the morphology and structure development mechanism ofpolyolefin during WAIM was investigated firstly, and then the mechanism of manipulating themorphology and structure via blending and nano-compositing during WAIM was revealed soas to provide theoretical guidence for molding light-weight and high-performance polyolefinparts via WAIM.The crystal morphologies of both WAIM high-density polyethylene (HDPE) andpolypropylene (PP) parts were investigated. The results showed that both WAIM HDPE andPP parts showed unique “outer layer-core layer-inner layer” hierarchical crystal morphology,which was closely related to the stress and temperature fields that the melts experiencedduring WAIM. For the WAIM HDPE parts, the molecular weight of HDPE has a significantimpact on the crystal morphology. The WAIM part of the HDPE with higher molecular weightwas dominated by oriented lamellae in the outer layer, and spherulites in both core and innerlayers; whereas banded spherulites dominanted all three layers in the WAIM part of the HDPEwith lower molecular weight, despite that the sizes of banded spherulites in both outer andinner layers were smaller than those in the core layer. For the WAIM PP parts, oriented crystalstructures were observed in both outer and inner layers, but the thickness of these orientedareas was only2%of the whole residual wall thickness; spherulites with various sizes weredominant in the rest areas of the part, among which the core layer showed spherulites withlargest sizes (about45m in diameter). The processing parameters were not effective intailoring the crystalline morphology of the WAIM PP parts, despite that lowering the melttemperature was favorable for the formation of a small amount (lower than10%) of-formcrystal.The crystal morphologies and their development mechanism in both conventionalinjection molded (CIM) and WAIM LLDPE and LLDPE/HDPE parts were investigated. Theresults showed that banded spherulites dominated both CIM and WAIM LLDPE parts;however, the crystal morphology of LLDPE could be influenced by the HDPE. At low cooling rates (such as the situation in CIM), the heterogeneous nucleation effect of the HDPE on theLLDPE hindered the growth of banded spherulites, and a low concent (10wt%) of the HDPEcould result in the banding to nonbanding morphological transition of the LLDPE; whereas athigh cooling rates (such as the situation in WAIM), the HDPE and the LLDPE crystallizedsimultaneously, a content of10wt%HDPE is not high enough to hinder the crystallization ofthe LLDPE, but increased the HDPE content to30wt%could result in the banding tononbanding morphological transition of the LLDPE.In the process of exploring an effective way to tailor the crystal morphology andstructure of the WAIM PP part, the presence of a low content (6wt%) of polymer nucleatingagent (acrylonitrile–styrene copolymer, SAN) was found to be favorable for tailoring thecrystal morphology and structure of PP matrix by varying the processing parameters duringWAIM. At a melt temperature of180or190°C, a WAIM94/6PP/SAN blend part with highcontent of-form and transcrystal can be molded. For example, the-form content in theouter and core layers were30.7and18.4%, respectively; and a totally transcrystallized matrixcan be seen near the inner layer. The formation of-form was ascribed to the-nucleatingeffect of the SAN on PP; whereas the formation of the transcrystals was ascribed to the in situfibrillation of the SAN resulted from high shear and cooling rates by high-pressure waterpenetration during WAIM. Moreover, it was found that the whole residual wall of the WAIMPP/SAN blend part was dominanted by SAN microfiber, and both impact and tensilestrengthes were improved compared to those of WAIM PP part. Consequently, the WAIMwould be expected to directly mold in-situ microfibrillar reinforced blend parts.The microstructures of the WAIM PP/halloysite nanotube (HNTs) nanocomposite partswere investigated. The results showed that the high shear rates during WAIM resulted in thepreferential orientation of the HNTs in the flow direction. Moreover, during the WAIM, theHNTs could induce the-form of the PP, increasing the crystallinity of the WAIM PP part.For example, the addition of8wt%HNTs could incease the crystallinity from35.5to43.5%.Moreover, the addition of a low content (2wt%) of HNTs was enough to ehance the thermalstability of the WAIM PP/HNTs nanocomposite part. The T5%and T10%increased22and19°C,respectively. The direct stabilizing effect of HNTs on the PP contributed largely to theincreased thermal stability of the WAIM PP/HNTs nanocomposite parts rather than theirbarrier and entrapment effects on the volatile products.
Keywords/Search Tags:Water-assisted injection molding, polyolefin, polymer blend, polymernanocomposite, rheology properties, morphology, crystallization, thermal stability
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