Preparation Of UHMWPE Porous Membrane Via Thermally Induced Phase Separation And Properties Research

The microporous membrane prepared by thermally induced phase separation method or uniaxial stretching method has been widely used in the field of Li-ion battery as a separator. The ultra-high molecular weight polyethylene (UHMWPE) microporous membrane is usually prepared by the wet method. In this thesis, the liquid paraffin (LP) was used as the diluent and the UHMWPE/LP blends were mixed at first. The preparation process and extrusion processing performance were discussed by torque rheometer, scanning electron microscope and high pressure capillary rheometer. The effects of biaxial stretching process, the secondary transverse stretching and the order of "extraction" and "stretching" on the structure and properties of microporous membrane were discussed. The results are as followed:1. In the UHMWPE/LP blends, when the swelling temperature was200℃and the shear rate was90rpm, UHMWPE could be evenly plasticized and oxidation degradation phenomenon would not occur.2. In the binary phase diagram of UHMWPE/LP blends, the dynamic crystallization temperature increased with increasing the contents of UHMWPE. Cloud point temperature was decreased first and then increased. The diagram was divided into three parts, which were homogeneous liquid phase, solid-liquid phase and liquid-liquid phase. When the UHMWPE content was10%, a typical honeycomb microporous structure was obtained, indicating the liquid-liquid separation mechanism. The typical UHMWPE spherulitic structure could be formed when the UHMWPE content was20-50%, indicating the occurrence of solid-liquid separation process induced by UHMWPE crystallization. When the UHMWPE content was higher than70%, fewer pores could be observed.3.The UHMWPE/LP blends extrusion pressure performance was discussed by high pressure capillary rheometer. The results showed that with increasing the LP contents, the pressure oscillation phenomena was first weakened and then increased. When the LP content was30%, the extrusion pressure kept stable. The pressure oscillation became weak when the extrusion temperature was improved. The blends could be stably extruded at175℃. The extrusion stability declined with the increase of UHMWPE molecular weight.4. During the first stretching and then extraction process, the stretching stress at the transverse direction was lower than that at the machine direction. The crystallinity did not change within the stretching ratio of110%and120%, but it would decrease with increasing the stretching ratio to150%. With the increase of stretching ratio from110%to150%, the Gurley value decreased from1141s to653s. The porosity was kept around56%.5. During the first stretching and then extraction process, the increase of secondary transverse stretching ratio resulted in the decrease of Gurley value. When the stretching ratio was130%in both direction, the Gurley value of the microporous membrane without secondary stretching was905s. With increasing the secondary stretching ratio from2%to10%, the Gurley value was decreased from870s to741s. When the secondary stretching ratio was4%, the crystallinity was increased.6. The orders of the "extraction" and "stretching" had a significant effect on the property of microporous membrane. The stretching stress during the first extraction and then stretching process was higher than that during the first stretching and then extraction. The membrane prepared by first "extraction" then "stretching" showed lower Gurley value. The membrane by first "extraction" then "biaxial stretching" to110%showed the Gurley value of143s, whereas that in the contrary process was1141s.