Study Of Microcellular Foaming Of Structured Melt Reinforced Isotactic Polypropylene

In recent years,with the rapid development of science and technology,higher requirements have been focused on the innovations of polymer processing and materials related fields.Isotactic polypropylene?iPP?foamed materials had attracted the attentions of industry and scientific researchers,due to their cost saving,good mechanical property,light weight,and good insulation property,etc.However,the linear molecular structure feature of the iPP results in low melt strength,which is easy to be broken and merged the cells neighbored during the foaming process,these could result in poor foaming quality.In order to solve this problem,this article enhances the melt strength of iPP by adding high-melt-strength polypropylene?HMSPP?,and incorporating with the"three-stage temperature control"method to introduce the structured melt in the matrix.Batch foaming under supercritical carbon dioxide?Sc-CO₂?was used to produce the foamed samples,the crystallization and rheological properties of iPP/HMSPP were characterized.The foaming performances were also analyzed.The influences of different foaming parameters on the foaming behaviors of the polymers were investigated.Moreover,the effect of microcellular structure on the mechanical properties of the foamed materials was further studied.?1?IPP/HMSPP composites was prepared by melt blending and compounding.The addition of HMSPP with the branched structure enhances the melt strength of the composites.The branched structure of HMSPP could also change the crystallization behaviors of iPP,which could help to introduce the crystals during the annealing process,furthermore,partially melted ordering structure?or structured melt?were induced in the subsequent temperature increasing stage.With the improving of the melt strength and cell nucleation sites through this way,a cell density of up to?10¹¹cells/cm³ was achieved,using the batch foaming,under Sc-CO₂ conditions.Rheological testing results show that the elastic modulus of iPP-20HMSPP reaches to?10⁶ Pa after annealing,which has an obvious increase,compared with the value of?10⁴ Pa of pure iPP without annealing treatment.The non-isothermal crystallization process was observed by using a polarizing microscope,and it was found that the crystal size of iPP-20HMSPP is much more smaller than that of iPP,and the number of crystals was greatly increased.With the addition of HMSPP,its long-chain branched structure can refine the crystal grains.The partially ordering melted structure can play the role of heterogeneous nucleation effect during the cell nucleation process,and generate more cells nucleus to increase the cell density.?2?With a fixed HMSPP content of 20 wt.%?iPP-20HMSPP?,different foaming parameters were used to study the foaming of the blends.The changing of the matrix hardness under different temperature and pressure conditions will affect the solubility and diffusion rate of the compressed CO₂,which can determine the nucleation and growth of the cells.Introducing the structured melt into the matrix,relatively uniform microcellular foams were produced at around the nominal melting point of iPP?160°C?,with the cell density of 5.4×10¹¹ cells/cm³ and average cell diameter of 2.7?m.This study also explores the foaming window of iPP/HMSPP,the range of its foaming temperature and pressure is greatly increased in the presence of the structured melt,compared with pure iPP.In a broadened microcellular foaming temperature range of about 8°C,quite fine and ideal microcellular foamed iPP-20HMSPP were produced,which can provide a significant guidance to the potential industrial processing application.?3?The effects of cell size and size distribution on the mechanical properties of microcellular foamed iPP/HMSPP were also studied.The smaller cell size and narrower size distribution help to reduce the effect of stress concentration around the cells suffered the destroy,thus the foamed material has a stronger ability to resist external forces.The impact strength of foamed iPP-20HMSPP is increased by 120%,compared to iPP foam,and the increment of tensile strength is 33%.Through the foaming procedure in this paper,the ideal microcellular foamed materials produced with the improved mechanical properties generally have the cell size of less than 3?m,and more uniform cell morphology and structure.