| The study of Polyolefin/CaCO3 composite porous membranes which were prepared by three-layer co-squeezing casting method was seldomly found in domestic and international literatures. In this dissertation, the composition, structures and process of Polyolefin/CaCO3 composite porous membranes which were prepared by three-layer co-squeezing casting method were investigated. Experiments and Intermediate productive tests indicated that polypropylene is a suitable substrate for micro-porous membranes, and calcium carbonate (600-mesh) is a high performance-to-price ratio porogen. There are mainly five parts as follows:(1)Surface morphology and surface modification of calcium carbonate were investigated. Calcium carbonate was modified by stearic acid and borate ester coupling agent, respectively. Compared with neat porogen particles, the modification effect of borate ester coupling agent is better than that of stearic acid, especially when the content of the boric additive is 4% of calcium carbonate weight or 2% of composite material. Meanwhile, the modification mechanisms were inquired, demonstrating that both stearic acid and boric acid ester are combined to the surface of calcium carbonate through chemical bonds as well as physical cladding.(2) Study on the fracture surface of LDPE/CaCO3, HDPE/CaCO3 and PP/CaCO3 composites showed that polypropylene has the best compatibility to CaCO3 among the polyolefin we surveyed. Meanwhile, mechanisms between calcium carbonate and polypropylene are inquired, demonstrating that calcium carbonate is well dispersed in polypropylene resin with well-defined branched chain density, porogen could cause a spherulite diminishing and induce formation ofβcrystal phase, and 600-mesh calcium carbonate/PP showed excellent tensile strength and dimensional stability.(3) Rheological behavior and crystal characteristics of PP/CaCO3 composites were investigated. The results showed that PP/CaCO3 composites is the typical pseudo-plastic fluid, that glass transition temperature decreases as calcium carbonate created more free volume to propylene macro-molecule chains, and the calcium carbonate modified by borate ester coupling agent caused crystallization peak shift to high temperature.(4) Effects of technical parameters on pore density and pore size were surveyed. The results demonstrated that when cooling too fast, the particles have no enough time to disperse well, creating inorganic island phase and coarse-touching porous membranes, while crystal diminishes with high nucleation density, forming dense cortical surface on porous membranes; when extruding too quickly, there are more crystal defects, calcium carbonate detached from the matrix, diminished pore and rougher porous membranes; last but not the least, post-heat treatment caused crystal rearrangement, crystal defects-revisiting, radius reduction of inorganic island phase and pore density increased.(5) Non-isothermal crystallization kinetics and crystallization behavior of organic and inorganic composite porous membranes were investigated by means of DSC. The results demonstrated that calcium carbonate could be employed as the heterogeneous nucleating agent, especially inducing formation ofβcrystal phase; with acceleration of heat rate, fast-growingβcrystal volume fracture was close to a crystal and Tg, Tb, Tp shifted to high temperature region, displaying obvious kinetics characteristics; porous membranes' activation energy of glass transition, onset crystallization and apparent crystallization peak were 55.38 kJ/mol,1167.45kJ/mol,588.46kJ/mol, respectively, which are higher than those of neat polypropylene membranes. Furthermore, according to the characteristic temperature and the onset crystallization temperature, such Polyolefin/CaCO3 composite porous membranes are deemed to represent illustrious thermal stability. |
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