Preparation Of LDPE Microporous Membrane By Thermally Induced Phase Separation Method

As one of the most widely used general synthetic resin, low density polyethylene (LDPE) has a prominent advantage of chemical resistance and high performance price ratio. LDPE is the ideal membrane material, especially suitable for microfiltration process under the harsh conditions. Unfortunately, LDPE is difficult to be prepared by nonsolvent induce phase separation (NIPS) or melt spinning method because of its poor solubility at room temperature and low crystallization. Thermally induced phase separation (TIPS) is a simple and novel microporous membrane preparation technology, which is suitable to parpare polyolefin porous membranes because the film forming process is conducted by the principle that a polymer can be dissolved in a certain solution at higher temperature and phase separatated at lower temperature.In this paper, LDPE microporous membranes, LDPE/zeolite blend microporous membranes and LDPE/silica blend microporous membranes were prepared by TIPS method. Then the membrane morphology, mechanical and thermal properties were characterized. The effects of preparation parameters on the membrane flux were also studied. Meanwhile we investigated the variation of membrane morphologies and separation performances with the incorporation of different inorganic particles, which provided theoretical data for the manipulation of membrane structures and improvement of membrane properties. Researches and conclusions were as follows:Firstly, LDPE microporous membranes were prepared by TIPS and the effect of polymer concentration and cooling rate on the microstructures and properties of microporous membranes were studied. The results showed that porosity of LDPE microporous membranes declined from 72.26% to 11.31% with the increasing of polymer concentration, connectivity got bad, permeable performance did not improve, but the tensile strength increased from 1.11MPa to 5.11MPa and the elongation at break from 39.5% to 142.5%. With the increasing of the cooling bath temperature, the tensile strength decreased from 4.3MPa to 0.58MPa and the elongation at break from 120% to 25%.Secondly, LDPE/zeolite blend microporous membranes were prepared by TIPS, by incorporating of hydrophilic and hydrophobic zeolite, respectively. The effect of zeolite content, polymer concentration and cooling rate on microstructures and properties of microporous membranes were also studied. Compared with ZSM-5 zeolite, 4A zeolite has a good effect to microstructures and flux of microporous membranes due to interaction effects of diluent, polymer and inorganic particles. With the increasing of 4A zeolite content from 2 wt% to 10 wt%, microporous structures of blend microporous membranes were improved, pore size increased and porosity reduced. Pure water flux increased from 16.6L/m2h to 34.2L/m2h, tensile strength from 1.58MPa to 1.74MPa, the elongation at break from 42.8% to 55.6 and melting temperature from 98.32oC to 103.1oC, but the contact angle decreased from 130.5 oC to 122.6 oC. The crystallinity increased firstly and then decreased. With the increasing of diluent concentration or cooling bath temperature, pore size and water flux enlarged. Finally, LDPE/SiO₂ blend microporous membranes were prepared by TIPS, by incorporating of hydrophilic and hydrophobic SiO₂, respectively. and SiO₂ content, polymer concentration and cooling rate on microstructures and properties of microporous membranes were studied. The results showed that hydrophilic SiO₂ has a centain degree of reunion, but the film toughness and permeability were higher than hydrophobic LDPE/SiO₂ membrane. With the increasing of hydrophilic SiO₂ concentration from 1 wt% to 10 wt%, pore connectivity and protiforium of blend microporous membranes became good, pore size decreased. Pure water flux increased from 10.98 L/m2h to 167.4 L/m2h, tensile strength from 1.42 MPa to 2.48 MPa, melting temperature from 101.2 oC to 109.1 oC, but the contact angle decreased from 119.5 oC to 103.2 oC. The elongation at break and crystallinity increased firstly and then decrease. With the increasing of diluent concentration or cooling bath temperature, pore size and water flux were enlarged.