Effect Of Viscoelasticity On Microcellular Foaming Behavior Of Polymer

Polymeric microcellular foams possess excellent properties and broad application prospects. And microcellular injection molding has attracted much attentions both in academia and industry, for its many advantages over conventional solid injection molding, such as reduced material consumption, increased dimensional stability of the molded part, and lower injection pressure. As is well known, foams with fine cell morphology, namely high cell density and low cell size, have good mechanical property. To obtain fine cell structure polymer microcellular foams, it is of great significance to investigate the foaming behavior. Microcellular injection foaming behavior is affected by the polymer viscoelastic response in the molding process. Thus the polymer viscoelasticity affect the cell morphology of foams.In this study, nano-particle filling and Chain extension and branch modification method were adopted to change the rheology behavior and its effect on cell growth and cell structure were investigated, using a injection molding machine equipped with volume-adjustable cavity. The main research content were:(1)PP/OMMT, PP/nano-SiO2 and PP/nano-CaCO3 were prepared and the OMMT was selected as finest inorganic nano-filler to improve the cell morphology of PP foams;(2) in different mould temperature, melt temperature and volume expansion ratio, the effect of viscoelastic response of PP and PP/OMMT composites on the foaming behavior were studied;(3) Chain branched poly(lactic acid) was prepared via functional group reactions, the foaming behavior of PLA and chain branched PLA was investigated in the same injection molding conditions.The result indicated that:(1) the introduction of nano-particle improved the rheological property and crystallization rate of polypropylene melt,and compared with nano-SiO2 and nano-CaCO3, PP/OMMT composites posses minimum average cell size(35.8 μm) and maximum cell density(8.56×105 cells/cm3);(2) the cell structure deteriorated in high volume expansion ratio and mould temperature and injection temperature, however the introduction of OMMT suppressed deterioration of cell structure;(3) the introduction of chain branching dramatically improved the reheological property and cell morphology of PLA obviously, and the LCB-PLA foams posses lower average cell size(54.8 μm) and higher cell density(5.31×105 cells/cm3).Based on the previous studies of bubbles growth kinetics, the cell growth process was divided into rapid growth stage and cell wall stress relaxation stage to analyze the effect of polymer viscoelasticity on the cell growth and stabilization. High viscoelasticity response suppressed cell coalescence and deterioration. In addition, the introduction of nano-particle and chain branching improved the crystallization rate of polymer melt, which contributed to the cell structure stabilization during foaming.