双向拉伸外场下聚丙烯电容膜微观结构演化机理研究

Biaxially oriented polypropylene (BOPP) capacitor film is the key material to determine the capacity and safety performance of the capacitor. However, the domestic enterprise has not yet completely broken through the processing technology barriers. To improve the capacity as well as safety of the capacitor, forming the structure with evenly distributed micropores in surface but without micropores (defect) inside the film is the key issues to be solved during the biaxial stretching process. The line speed can be 300 m/min when producing capacitor film, which is a successive process of rapid evolution of multi-scale structures such as lamellae, fibril crystal and micropore under high speed, high temperature and biaxial stretching. In view of the characteristics of the capacitor film processing, the project is intended to in situ track the full-scale structural evolution kinetics of β-α crystal transition, α-crystal melt recrystallization, crystalline slip, lamellae-fibril crystal transition and micropore formation during biaxial stretching, with combination of time-resolved synchrotron radiation X-ray scattering technology and biaxial stretching equipment as well as constant width film stretching device developed by the applicant. Combining the nonlinear mechanical behavior, the morphological phase diagram of BOPP capacitor film is constructed in two-dimensional biaxial strain–temperature space. Also, the physical mechanism and processing windows of the micropore formation in the surface and interior of the film are ascertained, respectively, providing both accumulation of theoretical basis and optimization program of the processing parameters for producing BOPP capacitor film with high capacity and safety.

双向拉伸聚丙烯（BOPP）电容膜是决定电容器容量和安全性能的关键材料，国内尚未完全突破其加工技术壁垒。双向拉伸过程中形成表面含有均匀分布的微孔、内部不含微孔（缺陷）的结构是提高电容器容量、安全性需解决的关键问题。薄膜加工线速度达300m/min，是高速、高温、双向拉伸外场同时作用下片晶、纤维晶、微孔等多尺度结构快速演化的连续过程。针对电容膜加工的特点，本项目拟采用高时间分辨同步辐射X射线散射技术，与申请人负责研制的薄膜双向拉伸装置和恒幅宽薄膜拉伸装置联用，原位实时跟踪电容膜双向拉伸过程中β-α晶型转变、α晶熔融再结晶、晶面滑移、片晶-纤维晶转变和微孔形成等全尺度结构演化动力学。结合同步检测的非线性力学行为，构建BOPP电容膜在双向拉伸应变-温度二维空间中的形态相图，探明薄膜内部与表面成孔分别的物理机制和加工参数区间，为高容量、高安全性BOPP电容膜的加工奠定理论基础和提供工艺参数优化方案。

薄膜双向拉伸加工是一个高速、高温、双向拉伸外场耦合作用下片晶、纤维晶、微孔等多尺度结构快速演化的过程。例如双向拉伸聚丙烯（BOPP）电容膜在双向拉伸过程中会形成表面含有微孔而内部不含微孔的结构，是电容器容量、安全性能的决定因素。探明薄膜内部与表面结构演化机理和加工—结构关系是指导高容量、高安全性电容膜加工的基础。本项目针对BOPP膜加工特点，设计研制了与同步辐射X射线联用的立式薄膜双向拉伸装置，并且完成了薄膜高温双向拉伸装置的升级改造，为BOPP薄膜双向拉伸机理研究提供必需的平台基础，并利用搭建好的双拉装置研究了α-PP在宽温域范围内单向拉伸过程中结构演化，构建了温度—应变二维空间内的结构相图。同时通过量子级联红外吸收单元与薄膜双向拉伸装置联用，原位研究了β-PP双向拉伸过程中分子链取向结构变化趋势，为探明BOPP电容膜的结构演化物理机制奠定基础。在此基础上，系统地研究了多步拉伸工艺对β-PP微孔膜性能的影响，为实现其在高端动力电池领域的应用提供新的工艺思路。此外，本项目还利用已搭建的薄膜双向拉伸设备平台，研究了聚乙烯（PE）/增塑剂体系在纵向及横向双向拉伸过程中的微观晶体结构演化及微孔演变过程，为BOPP及PE膜的加工奠定理论基础。