Study On Molding Process Of Uhmwpe Microporous Filter Materials

Microporous material is a very critical part of filtration and separationtechnology. As a new kind of PE-type thermoplastic engineering plastics,Ultra-high-molecular-weight-polyethylene (UHMWPE) has comprehensiveexcellent properties of resistance to low temperature, abrasion resistance,impact resistance, chemical stability, etc. Due to its excellent properties,fabricating microporous materials with UHMWPE for filtration and separationis always an objective to research in academia.Currently molding methods of UHMWPE microporous filter materialsmainly include sintering inorganic particle filling nuclear track method,melting-extrusion stretching, thermally induced phase separation thermallyinduced phase separation stretching and so on. But the methods above havetheir own limitations sintering method needs long processing time whichmean low efficiency, pore diameter of product fabricated by this method is big.TIPS and TIPS-S can only fabricate product with small thickness such as membrane or sheet, and corresponding pore size is small. Inorganic particlefilling need lots of filling particles, pore diameter made by this method isusually big. Melting-extrusion stretching is also used to make product withsmall thickness. Product thickness by nuclear track method is dozens ofmicron at most, and product porosity is low, meanwhile, this method involvesstrong technical which mean complex operations. Above all, there are anumber of shortcomings in current methods for making UHMWPEmicroporous materials, and microstructure is difficult to control duringprocess, which sharply limit UHMWPE microporous materials' applicationscope.In this study, a method called loose sintering was first proposed.UHMWPE particles pack together, the surface of UHMWPE particles begin tomelt and join together at contact surface when heating, the processingcondition should be properly controlled so that the space among the particlesremains. Finally a kind of UHMWPE porous body is formed.This study first introduced face-centered cubic structure concept fromPhysical Metallurgy into polymer processing. Packing UHMWPE particleswas assumed to be arranged as face-centered cubic structure, according toprinciple of three-ball into a pore, a formular to calculate the pore size couldbe obtained. Hence, pore size of product can be predicted, on the other handprocessing parameters can be also chosen according to requirement of poresize, which realizes simple controllability on microstructure. In study, processing equipment was properly chosen and technologicalconditions were groped for steadily according to materials' DSC results. Thecompound lubricant was added to reduce sintering time so that productionefficiency was increased. The performances of samples were tested andanalyzed. The effects of processing conditions on performance of sample werealso discussed. The results showed that as UHMWPE molecular weightincreased, compressive strength increased, average pore diameter and porosityboth increased with the UHMWPE particle diameter while compressivestrength and bulk density decreased. As sintering temperature and sinteringtime determined increased, the heat was redundant to melt the particles,average pore diameter and porosity both decreased while compressive strengthand bulk density increased. Permeability's changing trend was the same as thatof average pore diameter.Moreover, fractal demension was introduced. Fractal demensioncharacterization method for pore structure uniformity was established, whichrealized quantitative characterization of microstructure's uniformity.Meanwhile it came to a conclusion that decrease of particle diameter, increaseof sintering temperature and time were all bad for microstructure's uniformity.