Structure And Properties Of Ultra High Molecular Weight Polyethylene / High Density Polyethylene Microporous Membrane Via Thermal Induced Phase Separation

Lithium-ion batteries is useful in dailylife, in which the separator is an important part. Separator's main role is to provide short circuit protection and to avoid direct contact between positive and negative electrodes . The separator is usually used by the polyethylene(PE) and polypropylene(PP) composite membrane. The Properties of ultra-high molecular weight polyethylene (UHMWPE) is superior to PE and PP. Meanwhile, despite the melting point of UHMWPE is around 130oC, but at higher temperatures, it can maintain the original shape. it is expected the use of UHMWPE as a lithium ion battery separator films. However, UHMWPE is difficult to process, high melt strength is not conducive to sshutdown protection, hence it need to modification.In this paper, viscoelastic and liquid-solid phase separation were investigated, by using UHMWPE/HDPE as matrix, liquid paraffin (LP) as the solvent. A thermally induced phase separation method was adopted to prepare UHMWPE/HDPE microporous membranes. The effects of different processes on the structure and the performance of microporous membranes were characterized.The result shows that addition of HDPE can reduce the viscosity, as well as elastic. Processing performance can be improved by adding HDPE.Isothermal crystallization and non-isothermal crystallization process of UHMWPE/HDPE/LP blends was studied by DSC. Only liquid-liquid phase separation was observed during the cooling process of UHMWPE/HDPE/LP system.In the study of isothermal crystallization, UHMWPE crystallization in LP was accompanied by heterogeneous nucleation and two-dimensional growth mechanism. UHMWPE has higher nucleation density and crystallization is controlled by nucleation, therefore it hard to form a Spherulite. HDPE has lower nucleation density, three-dimensional spherulite growth can be occured in the process of crystallization. Addition of HDPE decreases the nucleation density. In the study of non-isothermal crystallization, HDPE decreases not only the crystallization rate, but also reduced the L-S separation temperature as well as crystallization temperature, which is also due to lower nucleation density caused. Activation energy of isothermal crystallization and non-isothermal crystallization both increased with the adding of HDPE, indicating that addition of HDPE is not conducive to crystal formation.The morphology of membrane was greatly affected by cooling rates, especially at a low cooling rate, HDPE could change the morphology of membrane and get the pores larger. Besides, the blend of UHMWPE and HDPE was used as base polymeric material and liquid paraffin as solvent. The experimental study showed that the UHMWPE/HDPE membrane with pores in a uniform distribution and about 1 um can be prepared. The UHMWPE/HDPE membranes was able to shut-down at 135oC, but was not melt-down at 160oC. This feature shows a better heat-resistance property than membranes fabricated from PP ,PE and their multilayer composites and it is predicted that UHMWPE/HDPE membrane could make lithium-ion battery safer when it is used as separator.The results show that with the ratio of UHMWPE: HDPE = 80:20, microporous membranes can be prepared by TIPS. Membranes can be expected to replace the current PP/PE/PP microporous membranes, using as a lithium battery separator. Its mechanical properties and heat resistance are better than PP, PE membrane. At high temperature, it is hard to deformation, but also has a lower shutdown temperature. So it can ensure the safe use of lithium-ion battery.