Preparation And Properties Of Polyethylene Hard Elastic Film And Microporous Membrane

Polyethylene (PE) is commonly used for preparing hard elastic material, and is one of the most significant material for preparing li-ion battery microporous membrane. Generally speaking, melt-stretching method is one of the main methods for fabricating PE microporous membrane. Generally, the method includes three steps:(1) preparation of the precursor film with row-lamellar structure, (2) annealing the precursor film to thicken the lamellae (such as annealing temperature, annealling time), and (3) stretching the film at low temperature to create voids and then stretching at high temperature to enlarge the pores.In this thesis, the influence of melt draw ratio, annealing time, stretching ratio and the content of linear-low-density-polyethylene (LLDPE) on the structure and properties of PE microporous membrane was investigated. We established the developing relationship of structure and properties between precursor film and the final microporous membrane. Differential scanning calorimetry (DSC), scanning electron microscope (SEM), small angle X-ray scattering (SAXS), pore size distribution and permeability were employed to characterize the PE samples.The experimental results showed that:1. With increasing melt draw ratio (MDR), the elastic recovery of polyethylene precursor film and annealed film is increasing, but when MDR is beyond 134, it decreases a little. The corresponding air permeability of final microporous membrane changes with the elastic recovery. When MDR is beyond 80, polyethylene precursor film shows apparent hard-elastic behavior. When MDR为134, the final stretching microporous membrane shows the best air permeability and hot-shrinkage resistance capability.2. With increasing annealing time of up to 3h, the lamellar thickness increases and the arrangement of lamellae structure becomes uniform. The corresponding final microporous membrane shows higher porosity, higher pore size, uniform pore size distribution and lower Gurley value. When the annealing time is further increased to 4h, since the annealing temperature of 125℃ is near the melting point, the longer time under so high temperature deteriorates the uniform structure, leading to the decrease of lamellar thickness and porosity, resulting in the increase of Gurley value.3. During the stretching under room temperature, the lamellae separation initiates the appearance of connecting bridges and pores. Initial pores appear at the stretching ratio of 45%. Higher stretching ratio induces more lamellae separation, but at the same time also leads to the deformation of lamellae structure. During the hot stretching, with increasing the stretching ratios, better air permeability property is obtained and more connecting bridges are formed. The pore formation process in PE microporous membrane is similar to that in PP microporous membrane. The difference is that during the preparation of PE microporous membrane, the contribution of new crystals formed during annealing and the main lamellae separation to the initial connecting bridges cannot be distinguished.4. The microporous membrane of high-density polyethylene (HDPE) and linear-lower-density polyethylene (LLDPE) blend was prepared based on melt-stretching mechanism and the cocrystallization behavior was ascertained. It was found that compared with that without LLDPE, the main melting peak point with 12 wt% of LLDPE was moved to lower temperature. HDPE/LLDPE blend is miscible in only one crystalline phase due to the formation of cocrystallization. The porosity and Gurley value of stretched microporous membrane were decreased by 43.9% and increased by 143.0%, respectively. Adding 5 wt% LLDPE to HDPE is appropriate to obtain final microporous membrane with better puncture resistance property but the porosity merely reduce by 7.1%.