Preparation Of Polypropylene Hard Elastic Film And Its Stretching Behavior To Form Pores

Melt-stretching method is an important way to manufacture polypropylene(PP)microporous membrane.During the melt-stretching process,the precursor film with row-nucleated crystalline structure was obtained by melt-stretching between the die and the chill roll.Then annealing was carried out to improve the crystal structure arrangement.The room-temperature stretching was used to induce initial pores and then hot stretching was carried out to enlarge the pore and form the connecting bridges between the separated lamellae.Finally,the heat-setting was carried out to improve the dimensional stability.The 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 the first section of this paper,three isotactic polypropylene(iPP)resins with different molecular weight distribution were chosen to prepare cast films via an extrusion casting process.The structure and properties of extrusion cast films were characterized by means of SEM,FTIR,DSC,2D-SAXS and tensile test to investigate the influence of high molecular weight component and low molecular weight tail.In the second section,we used in-situ small angle x-ray scattering(SAXS)to follow the hot-stretching process of annealed PP films.The transformation of microstructure such as lamella separation,void growth and bridge crystal appearance during stretching was studied.The experimental results showed that:1.The cast films were prepared using three PP resins with different molecular weight distribution.Then,the influence of high molecular weight component and low molecular weight tail on the structure and properties of extrusion cast films were tested by means of SEM,FTIR,DSC,2D-SAXS and tensile test.The results indicated that the existence of high molecular weight species would not alter the thickness of amorphous region but enhance the elastic recovery and the orientation degree of crystalline and amorphous,at the same time lower the crystallinity,thinner the crystalline thickness as well as decrease the lateral lamellar size.Additionally,the cast film containing a higher molecular weight component showed higher tie chain fraction and higher entanglement density in the amorphous region,however,smaller activation volume,indicating a stronger interplay between the amorphous entanglement density and the stability of crystal blocks.During the cycle loading of cast films,the tie molecular chain of cast film which was prepared with the resin containing a higher molecular weight component was more difficult to be destroyed and with the increase of the cycle loading times,the elastic recovery decreased more slowly.We also found that the existence of the low molecular weight tail resulted in higher crystalline and amorphous region thickness together with larger lateral lamellar dimension in cast film.It also led to higher crystallinity and elastic recovery but lower the content of tie chains and crystalline orientation degree.In fact,the low molecular weight tail showed little influence on the amorphous orientation degree,while enlarged the amorphous region thickness and decreased the entanglement density.At the same time,the activation volume of the cast film prepared using PP with low molecular weight tail was larger,indicating a weaker interplay between the amorphous entanglement density and the stability of crystal blocks,which was easier to suffer yield deformation.During the cycle loading of cast films,the tie molecular chain of the film prepared with the resin containing a low molecular weight tail was easier to be destroyed and with the increase of the cycle loading times,the elastic recovery decreased rapidly.The Gurley value of microporous membranes prepared by PP with higher content of low molecular weight component was lower,which showed better permeability.2.The combination of SAXS and tensile device was used to follow the hot-stretching process to study the transformation of micro-structure such as lamellae separation,void growth and bridge crystal appearance of two annealed PP film with different micro-structure.The two annealed films(PP1 and PP3)showed nearly lamellar uniformity,crystalline and amorphous region thickness,but PP3 showed smaller activation volume as well as smaller molecular entanglement density,higher tie chain fraction,higher crystalline and amorphous orientation degree.When the annealed film was stretched to 6% under 130?,the initial void appeared in PP1 but there was no apparent signal from connecting-bridge crystal,while such structure appeared at the stretching ratio of 10% for PP3.During this stretching process,the amorphous region was expanded under the tensile stress,leading to the significant increase of long periods of PP1 and PP3,while the bending or tilt deformation of lamellae crystalline during stretching caused the decrease of lamellae lateral size.When the films were stretched to 20%,both PP1 and PP3 formed one kind of pre-oriented structure like straight molecule chain and PP1 formed more.When the hot stretching ratio was increased to 40%,the tie chain structure and melt-recrystallization of PP1 and PP3 began to transform into bridge crystal structure,and the molecule chain of PP1 was more easily to be extracted and melt-recrystallized,forming more bridge crystals.The length of bridge crystal in PP3 and the gap between them were larger,and it grew faster with the increase of stretching ratio.As the hot stretched ratio was increased from 20% to 100%,the long period of PP3 increased gradually while the lateral size of lamellae almost unchanged.During this process,the lamellae that acted as the main force-receiving units were separated obviously,so the lamellae separation was more complete and the pores were expanded faster,leading to longer length of bridge crystal and larger lamellae long period.However,the lamellae long period of PP1 was nearly unchanged and the lateral dimension was reduced gradually.The separation of the lamellae cluster was dominant during stretching and the lamellae were more susceptible to bend deformation.The molecule chain was also more easily to be extracted and melt-recrystallized to form bridge crystal structure.