Study On The Porous Formation Mechanisms Of PTFE And Preparation Of Its Elastic Laminated Membrane

Due to its excellent thermal stability,chemical inertness,low surface energy,good biocompatibility,high porosity,high strength and so on,biaxially stretched microporous PTFE membrane was widely applied into powder filtering,biomedical and functional breathable apparel areas as an ideal membrane material under harsh environment. This paper carried out a detailed study around the PTFE membrane, PTFE/TPU composite elastic membrane preparation and use of in order to solve the key scientific and technical issues.Porous structure(pore size,porosity)and mechanical properties of PTFE membrane are the two key factors deciding its applications,and the controlling of which has been the focused interest of all studies and researches.The tensile behavior,crystalline structure,porous structure, node-fibril morphological structure of PTFE membrane were characterized through measurements including IR,SEM,DSC,TMA, EDS,pore characterization,electronic material mechanical tester with heating chamber.Based on corresponding relations between the tensile behavior and the structure of PTFE membrane,the pore formation mechanism was put forward.After the study on the effects of heat setting temperature on the structure and properties of PTFE membrane, an amorphous lock-in heat setting mechanism of PTFE membrane was given in this paper.Besides,in order to avoid the properties and manufacturing defects of traditional PTFE membrane laminated fabrics, PTFE/TPU co-stretching elastic composite membrane was prepared and characterized.Following conclusions were obtained after the studies in this paper:1.The mechanical behaviors of the rolled PTFE sheet at different stretching ratios,temperatures,and rates were measured by a material tester with temperature chamber,then the relation between the mechanical behavior and porous structure,node-fibrils morphology, crystalline structure were established.The results showed the effects of stretching parameters on the membrane porous and node-fibril morphological structures were stretching rate,stretching temperature, and stretching ratio in ascending order.It was found that there was a common characteristic in the stretching stress-strain curves for the membranes which can be stretched into a porous structure:There was an obvious yield point in the stress-strain curve,and after yield,the stress kept almost in constant with the increase of stretching strain.2.Based on the above obtained inherent relation between the structure and the corresponding mechanical behavior,the concept of "porous formation starting point" was put forward,that is,the yield point of the rolled PTFE sheet was the starting point for the porous nodes-fibrils structure generation,and then a model of pore formation during stretching of PTFE membrane was given.Further,the stress at "porous formation starting point" and the stretching temperature and stretching rate relation was analyzed,then the activation energy for pore generation of the rolled sheet was obtained;relations between the strain at "pore formation starting point" and the porosity,nodes-fibrils morphological parameters were quantitatively established.3.The effects of stretching parameters on the mechanical properties and porous structure of PTFE membrane were studied,and the results indicated heating setting is the decisive factor to determine the mechanical property of the membrane.After the study on the relation between heat setting temperature and porous,node-fibril morphology, mechanical property,and size stability(shrinkage) of PTFE membrane, the mechanism of heat setting of PTFE membrane was achieved: crystalline zone melting,ribbon-like crystallite disappearance,the lock-in effect on the chain slippage on heat setting of PTFE membrane was the setting mechanism for PTFE membrane,which runs contrary to the heat setting mechanism by crystallite enhancing effects.4.A preparation process through PTFE/TPU co-stretching was obtained in order to compensate defects of PTFE membrane.The thermo-mechanical properties and tensile behaviors of PTFE and TPU, adhesion between PTFE and TPU layer,were characterized,and then the optimum conditions for PTFE/TPU co-stretching process was achieved: paste extrusion of PTFE fine powder→rolling→longitudinal stretching→TPU casting→transverse PTFE/TPU co-stretching at temperature of 120℃,rate of 20%/s→heat setting at 280℃.5.The tensile recovery behaviors of PTFE membrane,PTFE/PU coated composite membrane,PTFE/TPU elastic composite membrane and PTFE/TPU membrane laminated knitted fabric were studied,and the stress relaxation behaviors of the four materials were analyzed through Maxwell model.It was found that although the instantaneous recovery of PTFE membrane was low,the delayed recovery could amount to 100%for PTFE membranes heat set at lower temperature.Results showed the PTFE/TPU co-stretched elastic composite membrane prepared at transverse stretching temperature of 120℃,stretching rate of 20%/s,and heat setting temperature of 280℃,the recovery ratio could amount to 82%,which was very close to the elastic knitted fabric.6.Due to the low surface energy of PTFE,the PTFE membrane is apt to adsorb surfactants in detergent during washing,which will lower the water resistance of PTFE membrane.Compared with PU coated PTFE/PU membrane laminated fabric,the drop of water resistance of PTFE/TPU co-stretched membrane laminated fabrics after detergent washing was much retarded.For PU coated PTFE/PU,some tiny holes could appear owing to evaporation of the solvent during PU coating,but the cast TPU was monolithic and holes free.When the PTFE layer adsorbed surfactants in the detergent,TPU layer would function as the major water resistant component.