Effect Of Annealing And Stretching Processes On The Structure And Properties Of Polypropylene Microporous Membrane

Polypropylene is one of the most commonly used semi-crystalline polymer, and is one of the important material for li-ion battery microporous membrane preparation. Polypropylene microporous membrane has been widely used in lithium batteries. Melt-stretching method is one of the main methods for fabricating polypropylene microporous membrane. Generally, the method includes three steps:(1) production of the precursor film with a row-nucleated lamellar morphology,(2) annealing of the film to thicken the lamellae, and (3) stretching of the film at low temperature to create voids and then stretching at high temperature to enlarge the pores.In this thesis, the influence of annealing and stretching process on the structure and properties of polypropylene microporous membrane was investigated. On one hand, the evolution of the structure of PP microporous membrane with different stretching levels was studied, and then the pore-creation process during stretching of annealed PP film was proposed. On the other hand, based on hot stretching process, we have studied the formation mechanism of stable bridge structure in detail. A new preparation process without cold-stretching was proposed and its characteristics were studied under different stretching temperature and stretching rate. In addition, the important influence of temperature during annealing and heat-setting process was investigated. Differential scanning calorimetry (DSC), scanning electron microscope (SEM), wide angle X-ray diffraction, small angle X-ray scattering, pore size distribution and permeability were employed to characterize the PP samples.The experimental results showed that:1. It was found that with the increase of annealing temperature to145℃, the endotherm plateau area percent in the whole melting curves increased. The corresponding average pore size and porosity in the cold and hot-stretched microporous membrane increased, the air permeability property was improved and the connecting bridges arrangement became uniform. Further increase of the temperature to150℃lead to decrease of the plateau percent, which adversely influenced the final pore structure. There was some relationship between endotherm plateau percent and the properties of finally microporous membrane.2. The origin of plastic plateau in the stress-strain curve of annealed polypropylene film was investigated. Plastic plateau was from the unstable crystal phase, coming from the recrystallization of tie chains and amorphous chains during annealing. As the stretching ratio increased, the unstable phase gradually decreased and the plastic plateau region of tensile curve disappeared slowly. During stretching, the unstable crystal phase was converted to bridges between the lamellae. The cold-stretching had a significant impact on structure and properties of microporous membrane. When the cold-stretching ratio reached15%, the microporous film had a better microstructure and gas permeability. Further stretching to60%, the obtained microporous membrane showed worse structure, smaller pore size, lower porosity and permeability.3. Compared to the cold drawing, microporous membrane had more and longer bridges after hot drawing. With the increase of stretching ratio, the bridge would become longer during hot drawing at130℃.The samples stretched to200%had stronger bridges, corresponding to the shoulder in the right of melting peak. Some tie chains which came from disentanglement during the hot-stretching process didn’t convert to bridges during cold-drawing and would form new bridges. Meanwhile, some molecular chains were pulled out from the lamellae and also became part bridges. During the hot stretching process, some bridges would merge, and some broken bridges would attach to stable bridges. The pore and bridge structure of microporous membrane were more stable after hot drawing.4. For the fabrication of microporous membrane, the last heat-setting stage was beneficial to obtain better dimensional stability, but under higher heat-setting temperature the occurrence of some crystallization behavior may deteriorate the pore structure. It was found that after heat-setting there was a plastic plateau in the stress-strain curves. With the increase of heat-setting temperatures, the plateau became pronounced. During heat-setting, the crystallization of some remanent tie chains between separated lamellae lead to the appearance of the plateau in the left of the main melting peak. The microporous membrane heat-set under145℃showed best air permeability and dimensional stability.5. Similar microporous film was obtained by the only-hot-stretching way. Within the larger stretching rate, microporous membrane with thinner lamellae, smaller micropores and worse gas permeability could be obtained. The rate and temperature by only-hot-stretching process had the opposite effect on structure and properties. The structure and properties of microporous membrane could be effectively adjusted by stretching rate and temperature. The only-hot-stretching membrane had same characteristics:(1) thicker and more regular lamellae structure;(2) fewer bridges, but stronger;(3) good air permeability.