利用片状填料调控聚丙烯线收缩率的机理研究

Study on the Mechanism of the Flake-shaped Filler Tailored Linear Shrinkage of Polypropylene

添加无机粒子是有效降低聚丙烯(PP)成型收缩率的一条重要途径,但目前对其作用机制的理解还很局限.本研究以滑石粉(talc)填充的等规PP(i-PP)为例,通过理论计算与实验相结合的方法,研究了无机粒子对i-PP注塑制品线收缩率的影响规律和机理.结果表明,当talc含量较低时(<6vol%),注塑制品在面内(即平行模流方向MD和垂直模流方向TD)和面外(即厚度方向ND)的线收缩率均随talc含量的增加而线性降低,与理论预测基本一致.然而,当talc含量较高时(>6vol%),收缩率的变化趋势显著偏离理论曲线,面内线收缩率急剧降低,而面外线收缩率则基本保持不变.进一步分析发现,由于注塑制品中的片状talc倾向于沿MD-TD平面方向取向,并诱导i-PP片晶垂直于其表面生长,因此致密的talc片层网络可促使i-PP结晶形成垂直于MD-TD平面高度取向的晶体,最终导致制品的面内线收缩率显著低于理论值(这对大尺寸低收缩薄壁制品的开发非常有利),而面外收缩率则高于理论值.研究结果打破了通过填料体积占比减小PP复合材料成型收缩的传统认识,从结晶的角度揭示了基于填料诱导的取向结晶调控PP制品线收缩的新机理,为低收缩PP材料的开发提供了可行思路.

The addition of inorganic fillers has been deemed as an effective strategy for reducing the mold shrinkage of polypropylene(PP).However,the current understanding on the underlying mechanisms is still unclear.In this work,taking talc filled isotactic PP(i-PP)as an example,the effect and mechanisms of inorganic particles on the linear shrinkage of injection-molded i-PP products were investigated theoretically and experimentally.The results indicate that,at low contents(<6 vol%)of talc,the linear shrinkage of the injection-molded products in both in-plane(i.e.,along the machine direction MD and transverse direction TD)and out-of-plane directions(i.e.,along the thickness direction ND)is found to decrease linearly with increasing talc content,which is consistent with the theoretical prediction.However,at high contents(>6 vol%)of talc,the change trend of the linear shrinkage deviates significantly from the theoretical curves.The in-plane shrinkage decreases remarkably,while the out-of-plane shrinkage remains almost unchanged.Further analysis indicates that the talc plates in the injection-molded products show a strong tendency to align along the MD-TD plane and induce the growth of i-PP lamellae perpendicular to their surfaces.Accordingly,the dense talc network promotes the formation of highly oriented i-PP lamellae perpendicular to the MD-TD plane,ultimately leading to a much lower in-plane linear shrinkage but a higher out-of-plane shrinkage as compared to the theoretical values. Achieving low in-plane linear shrinkage is particularly beneficial for the development of large-sized thin-walled products. Our work not only breaks through the current knowledge (the shrinkage of composites is decreased based on the increased volume fraction of fillers) and gives a new insight on the role of the filler-induced orientation crystallization in tailoring the shrinkage of PP products, but also provides a solution for the development of low-shrinkage PP materials.