PP/PE双层共挤复合同步拉伸成孔过程基础问题研究

The development of new energy cars is strongly dependent on the separator safety. However, the pore structure control of the PP and PE layer in the PP/PE/PP trilayer coextruded microporous separator with high safety which has been commercialized by Celgard, USA, is still a problem for us, due to the pore structure black box problem. Based on the PP/PE bilayer membrane model, the current project plans to study the fundamental problems in the three prominent processing stages of PP/PE bilayer microporous membrane, including the melt stretching-cooling, cold stretching and hot stretching. With the aid of material characteristics optimization and out-situ characterization of lamellae size and mechanical properties of bilayer co-extruded precursor film, the oriented lamellae size in the PE and PP layer, respectively, will be controlled. With the aid of advanced synchrotron radiation X-ray technique, infrared spectra and atomic microscopy, the structural evolution of PE and PP layer, respectively, during cold-stretching and hot-stretching will be followed in situ. This allows us to establish the correlation between the destruction of amorphous region and crystals and to clarify the origin of stable connecting bridges in the PE and PP layer. The quantitative size relationship among the lamellae, connecting bridges and pore will be built up. The control of pore structure during the synchronously stretching of PE and PP layer with pronouncedly different rheological and crystalline properties will be realized. This work will improve the understanding of basic scientific problems during polymer processing, including non-isothermal induced crystallization, yielding phenomenon and lamellae to fiber crystal transition. At the same time, it will also give some technical support for the domestic development of high safety microporous separator.

项目针对高安全性PP/PE/PP三层复合微孔隔膜制备过程各层成孔结构控制问题，以PP/PE双层膜为模型，针对熔体拉伸微孔隔膜制备过程关键阶段：熔体拉伸冷却、冷拉和热拉，展开多层同步拉伸基础科学问题研究。在优化材料参数并离线表征双层共挤熔体拉伸冷却预制膜结晶尺度与力学性能的基础上，实现双层共挤各层取向片晶尺度可控；采用二维SAXS/WAXS、FTIR、AFM原位跟踪研究双层共挤复合预制膜冷拉和热拉过程各层结晶结构、成孔结构、晶区以及无定形区转变基础上，明确冷拉过程无定形区与晶区破坏的竞争机制，明确热拉各层稳定架桥纤维晶来源；在量化片晶、孔洞以及纤维晶尺度基础上，实现具有不同流变与结晶性能的两种原料同步拉伸成孔的可控。项目开展工作一方面可加深非等温拉伸流动场诱导结晶、高聚物屈服破坏、片晶-纤维晶转变等高聚物加工面临的基础科学问题的认识，同时也将为国内开发高安全性微孔隔膜提供一定的技术指导。

新能源汽车的发展对采用的锂离子电池安全性提出了更高的要求。锂离子电池中，相比单层PP或者PE微孔隔膜，三层共挤微孔隔膜采用PP/PE/PP结构设计，利用PE与PP熔点差异提供了低的闭孔温度和高的破膜温度，从而体现了高安全性能。然而采用熔体拉伸工艺制备微孔膜过程，由于PP与PE原材料结晶、流变以及力学性能差异，导致三层共挤拉伸成孔过程成孔结构控制难度加大。为了明确各层在熔体拉伸成孔过程结构演变，本项目通过制备双层共挤微孔膜，首先建立了原料参数、加工参数与共挤流延预制膜各层片晶尺度之间的定量关系；建立了材料参数、流变性能与双层共挤稳定性之间的关系；明确了双层共挤预制膜在PE熔程范围内热处理过程中片晶结构演化规律及对后续双层拉伸微孔膜性能的影响；明确了双层膜拉伸过程各层微孔生长的过程；建立了拉伸过程中加工工艺-微观结构-成孔性能间的理论相关性，形成了双层共挤复合微孔膜各层拉伸成孔尺度的可控；最终采用双层膜对应的原料和加工参数，制备了三层共挤PP/PE/PP微孔膜，微孔膜性能参数与进口三层共挤微孔膜相媲美。项目研发成果为三层共挤微孔膜的工业化制备积累了相关的数据，奠定了很好的基础。.项目在执行期间共发表高水平SCI收录论文12篇，其他论文6篇，申请及授权相关专利7件，培养在站博士后1名，博士生1名，硕士生9名。