微发泡注塑成型轻质高强聚丙烯泡沫及其结构与性能研究

Lightweight and high strength polypropylene foams have very important practical application values for conserving energy and reducing emissions. The polymer foam fabricated by conventional foam injection molding has several reflects, such as low void fraction and non-uniform foam structure. Furthermore, pure polypropylene exhibits low melt strength, which leads to a poor foaming ability. These two issues are two bottlenecks restricting the preparation of lightweight and high strength polypropylene foams. Mold-opening foam injection molding and in-situ fibrillation were used in this project to solve the above two problems. However, the foaming mechanism of mold-opening foam injection molding is complicated and the formation mechanism of cell layered structure is not clear. At the same time, the structure-function relationship between the micro structure and the macro mechanical property of the polymer foam is vague. Moreover, it is hard to control the morphology of in-situ fibrillation composites. And it is unclear how micro-fibrils affect the foaming behavior of polypropylene. To this end, this project will study the influence of mold-opening foam injection molding process and material behavior on the structure and morphology of cells in depth. The regulation methods of cellular structure will be developed based on studying the cell nucleation and growth behavior. Based on the structure-function relationship between the micro structure and the macro mechanical property, high strength polypropylene foam structure will be designed. Furthermore, micro-fibrils morphology control methods will be established and the mechanism of micro-fibrils phase enhancing polypropylene foaming properties will be revealed. Accordingly, an effective modification strategy will be developed in order to improve the foaming ability of polypropylene. And then the cell structure is optimized. This project not only has important practical value for promoting vehicle lightweight, reducing energy consumption and improving energy efficiency, but also has important scientific significance for enriching polymer foaming theory.

轻质高强聚丙烯泡沫材料对于节能减排具有重要价值。传统发泡注塑制品发泡倍率低、泡孔不均和聚丙烯发泡成型困难是制约轻质高强聚丙烯泡沫制备的两大瓶颈问题。本研究采用“开合模微发泡注塑工艺”解决传统发泡注塑制品的一系列缺陷，采用“原位成纤工艺”提升聚丙烯发泡性能。但开合模微发泡注塑工艺发泡机理复杂，泡孔分层结构形成机制不清楚，制品微观结构与宏观力学性能的构性关系不清晰，微纤复合材料形态调控方法及其发泡行为不明确。为此，本项目深入研究开合模微发泡注塑工艺及材料行为对泡孔分层结构的影响规律，揭示泡孔形核长大机理，发展泡孔结构调控方法，建立制品微观结构与宏观力学性能的构性关系，设计高强度聚丙烯泡沫结构，建立微纤形态调控方法，揭示微纤相影响聚丙烯发泡行为的作用机理，形成聚丙烯高效改性方法，优化制品泡孔结构。该研究对于促进汽车轻量化、节能减排具有重要的应用价值，对于丰富聚合物发泡理论具有重要的科学意义。

本项目围绕轻质高强聚丙烯泡沫制备领域存在的发泡制品减重有限、力学强度低、表面质量差、聚丙烯熔体强度差、泡孔结构难调控等系列科学问题，采用“开合模微发泡注塑工艺”解决传统发泡注塑制品减重有限、力学强度低等一系列缺陷，采用“原位成纤工艺”提升聚丙烯发泡性能。研究团队采用“免拉伸一步法”成功制备原位聚四氟乙烯（PTFE）微纤改性聚丙烯（PP）复合材料，采用热分析、纳米力学分析、X射线衍射分析、光热诱导红外光谱等手段深入剖析了PTFE的晶体结构及表面处理工艺，揭示了PTFE伸长链晶体结构是其一步法成纤的关键核心要素；采用旋转流变仪设计了剪切实验，探索了加工过程剪切速率和剪切时间对于PTFE形态结构演变规律的影响，提出了加工剪切速率高于“临界剪切速率”是PTFE一步挤出法原位成纤的重要工艺要素。.开展了PP/PTFE复合材料结晶形态及结晶动力学研究，PTFE纤维比PTFE球状颗粒的异相形核作用更明显，但其抑制了球晶的快速生长。采用Mo方程研究了PP/F-PTFE（纤维状PTFE）和PP/S-PTFE（球状颗粒PTFE）结晶动力学，采用Kissinger方程来研究复合材料的活化能∆E，采用Dobreva方法计算成核活化能，发现了PTFE纤维的异相成核作用明显，但其复合材料PP/F-PTFE的结晶速率却低于PP/S-PTFE。采用自主研发的原位高压结晶可视化系统研究复合材料在高压CO2下的结晶行为，揭示了高压CO2条件下PTFE纤维和球状颗粒对PP/PTFE复合材料结晶形态和结晶动力学的影响，研究表明PTFE纤维在高压CO2的条件下其异相形核作用优势比大气环境下更明显，经过高压CO2处理后的PP/F-PTFE其拉伸强度提升了25%。采用开合模微发泡注塑工艺制备了轻质高强PP/PTFE复合材料，材料减重达到100%以上，力学性能获得显著改善，隔热性能优良。揭示了工艺参数对泡孔分层结构的影响规律，建立了泡孔结构模型，揭示了泡孔微观结构与制品宏观力学性能的构性关系，提出泡孔均匀度是影响泡沫材料力学性能、隔热性能的重要因素。