Preparation And Properties Of Polypropylene Microporous Membrane By Melt-stretching Method

Melt-stretching method is an important method to manufacture PP microporous membrane. During the melt-stretching process, the precursor film with row-nucleated crystalline structure is obtained by melt stretching bewteen the die and the chill roll. Physical properties of raw materials such as molecular weight, molecular weight distribution, isotacticity, degree of branching and processing conditions including air cooling, melt-draw ratio, annealing conditions, stretching conditions, heat-setting process determine the initial lamellar morphology of precursor film and influence the structure and properties of stretched microporous membrane.In this paper, the main influencing factors on the initial lamellar morphology and stretched pore structures during polypropylene microporous membrane manufacturing were studied. For the melt-stretching process, we focused on the effect of roll temperature on the structure and properties of PP microporous membrane. The formation of initial pores during room-temperature stretching process of polypropylene annealed film with row-nucleated crystalline structure was discussed. The relationship between recrystallization, secondary crystals and stable pore structure was built up during heat-setting process. Finally, the interaction between tie chains retraction and recrystallization, secondary crystals was investigated.The experimental results showed that:1. The influence of roll temperature on the structure and properties of PP film and stretched microporous membrane was studied. It was found that with an increase of roll temperature from 60? to 120?, the strain-hardening behavior turned apparently. Long period, crystalline thickness, the elastic recovery, orientation degree and the tie molecule volume fraction of PP precursor film were increased. The lamellae thickness was increased significantly and crystalline region was improved perfectly after annealing. The annealing resulted in small difference of properties of final microporous membrane prepared at different roll temperature. The roll temperature during extrusion-cast method was broad from 80? to 120? and it can be applied for the fabrication of PP microporous membrane.2. The pore formation during cold stretching was follwed by small angle X-ray scattering (SAXS), wide angle x-ray diffrecation (WAXS), temperature modulated differential scanning calorimetry (TMDSC) and scanning electron microscope (SEM). The result showed that the disappearance of daughter crystal and melt-recrystallizated part was the main reason for initial pores formation. The initial bridges connecting the separated lamellae could be observed by 10% stretching at room temperature. During stretching process, the daughter crystal and recrystallizated crystal within the crystalline region were stretched and converted to the initial bridges. At the stretching ratio of 30%, which was situated at the transition point from plastic plateau to strain-hardening region in the stress-strain curve of annealed film, uniform initial connecting bridges and pores appearred, at the same time showing maximum second order signal in SAXS patterns. Except for the stretching of daughter crystal, the decrease of tie chains and secondary crystals within the amorphous region converted to the formation of more connecting bridges. Higher stretching ratio to 50% lead to the stretching of mother lamellae and would induce lamellae deformation and lower structure periodicity.3. The secondary crystallization and melting recrystallization behavior were always accompaning with heat-setting process. With an increase of heat setting temperature, recrystallzation and secondary crystallization enthalpy were apparently increased. After two minutes heat setting, the separated lamella were already supported by connecting bridges, air permeability and porosity were improved greatly. The pore structure became stable by decrease of recrystallization enthalpy. After ten minutes, connecting bridges became longer and stronger, which lead to uniform arrangement of lamellae structure and pores. Melt-recrystallization enthalpy continued to decrease, while the secondary crystallization enthalpy showed little change, the formation of stable pore structure was mainly dominated by melt-recrystallization. Extending the heat-setting time to fifteen minutes, recrystallization enthalpy increased and the secondary crystallization enthalpy was decreased. The formation of stable pore structure was mainly dominated by secondary crystallization. The best heat setting time was from 5min to 15min.4. The tie chains relaxation and crystallization was competive. With retraction ratio increases, the molecular chains transformed from orientation to disorder.Melting-recrystallization turned apparently while secondary crystallization became weak. Long period of lamellae was increased and the average length of pore structure was decreased. The porosity, air permeability and crystalline orientation degree of corresponding stretched microporous membrane were worse. The formation of stable pore structure was mainly dominated by melting-recrystallization during retraction heat setting. At the retraction ratio of 5%, the stretched microporous membrane showed best properties.