Effect Of Annealing And Stretching On Crystalline Behavior And Microstructure Of Polypropylene Membrane

Based on stretching mechanism of hard elasticity, polypropylene microporous membrane can be prepared. Cast film technology is important for controlling the hard-elastic properties of polypropylene films. The crystallinity, lamellar thickness and orientation value determine the hard-elastic properties. Several PP resins have been chosen in this paper. Different effects on hard elastic properties of PP cast films have been studied and the suitable PP resin was selected as the research material. Cast film process including extrusion rate, drawing rate, temperature of extrusion were investigated. The influence of PP cast films which were added with HDPE, high molecular PP or a-nucleating agent on hard-elastic properties was studied. Annealing time and temperature on crystallization behavior and microstructure and the changes of crystallization and microstructure by cold stretching in different stages were also discussed in the experiment. Ultimately, explored pore-forming mechanism of cold-stretching under the optimal PP cast process and annealing conditions, Optimizing cold-stretching into holes technology. Traditional test methods and some modern analytical techniques, i.e. infrared spectroscopy (FT-IR), scanning electron microscopy(SEM) and differential Scanning Calorimeter(DSC) were used to monitor the relation on structure characteristics and hard-elastic properties of PP cast films. The results were showed below:(1) The melt flow index, melt flow ratio and crystalline rate of PP affect the properties of hard-elastic film. Lower melt flow index, higher melt flow ratio and higher crystalline rate are useful for obtaining hard-elastic film with higher elastic recovery and uniform lamellar distribution.(2) The main factors that affect the hard-elasticity of PP cast film are extrusion rate, drawing rate and roll temperature. PP cast films show good hard-elastic properties when extrusion rate is 20.0 rpm, drawing rate is 25.0rpm and roll temperature is 80℃.(3) It is found that PP cast films have good hard-elastic properties when 5.0 phr HDPE is included. The elastic recovery is increased by 11.3%;the amorphous orientation value is increased by 78.6%. The introduction of polypropylene with high molecular weight with 5.0 phr results in good hard-elastic properties. The elastic recovery have increased by 7.8% and 7.4% for unannealed and annealed samples and the orientation value of amorphous phases is increased nearly one times. The addition of 4.0 phr a-nucleating agent results in that the crystallinity of PP cast films is increased from 37.15% to 42.31% and elastic recovery is increased by 7.6%. It is meaningful that hard-elastic properties of PP1 which was added with HDPE closed to PP4, it may reduce the cost.(4) Annealing is necessary for PP cast films with excellent hard elastic properties. The results show that best annealing condition is under 145℃for 30min. Under this condition,the degree of crystallinity is increased from 39.1% to 49%, meanwhile, the elastic recovery is increased from 80.3% to 94.6%.(5) Pore-forming mechanism by cold-drawing process and effect of stretching rate on pore-forming have been showed. metastable phase formed on the surface of lamellae during annealing would disappear and becomes the bridge similar to micro-fiber between lamella. The number of pores would be fewer and pore size distribution is poor if cold stretching rate is too fast. To achieve more uniform pore size and more pores, cold stretching rate should not be too fast.