热拉伸比对熔融挤出拉伸法制备聚丙烯微孔膜的影响

Effect of Hot Draw Ratio on PP Microporous Membranes Obtained by Melt-Extrusion-Stretching Method

通过固定冷拉伸比,改变热拉伸比制备了具有不同微孔结构的聚丙烯微孔膜,通过扫描电子显微镜(SEM)观测了微孔的形貌,通过差示扫描量热法(DSC)测试了微孔膜的片晶厚度,研究了热拉伸过程中微孔形成过程以及微孔尺寸和数量,微孔膜孔隙率随热拉伸比的变化情况。结果表明,冷拉伸过程主要是片晶间微裂纹的形成过程,微裂纹均匀的分布在试样内部,但由于不同微裂纹的抗拉伸性能不同,在热拉伸阶段首先被拉开的是较薄弱的微裂纹,形成部分微孔。当微孔的尺寸足够大,微孔中的"桥结构"拉伸至较大尺寸提供足够的张力以维持微孔不再变形。随着热拉伸的继续进行,"桥结构"能将力传递给带状微裂纹之间的堆积片晶,将这些堆积片晶之间的次薄弱缺陷拉开,继续形成微孔。

Polypropylene microporous membranes with different microporous structures were prepared by fixing cold stretching ratio and changing hot draw ratios.The morphology of micropores was observed by scanning electron microscope(SEM).The thickness of lamellae was measured by differential scanning calorimetry(DSC).The formation process of micropores,the size and number of micropores,and the change of porosity with hot stretching ratios were studied.The results show that the cold stretching process is mainly the formation process of intergranular microcracks,and the microcracks are evenly distributed in the sample.However,due to the different tensile resistance of microcracks,the weakest microcracks are firstly separated in the hot stretching stage,forming some micropores.When the size of the micropores is large enough,the“bridge structure”in the micropores is stretched to a larger size to provide enough tension to maintain the micropores from deformation.With the hot stretching going on,“bridge structure”can transfer the force to the stacking lamellae between the banded microcracks,and pull off the sub weak defects between the stacked lamellae,and continue to form micropores.