Preparation And Characterization Of PVDF Microporous Membrane With Highly Hydrophobic Surface By Dual Bath Method

Hydrophobic polyvinylidene fluoride microporous membranes, which adapted to therequirement of gas-liquid membrane contactor, have been prepared by a dual bath methodwith soft nonsolvent as the first bath, and hard nonsolvent （water） as the second one. Thecasting solution was composed of polyvinylidene fluoride, N, N-dimethylacetamide andsome suitable additives.The effects of first coagulant, such as ethanol/water, ethanol and isopropanol, on themembrane morphology and performance were explored by scanning electronic microscopy,atomic force microscopy, X-ray diffraction, and analysis of gelation kinetics. It was foundthat the precipitation rate of casting solution was lower, so that the solid-liquid demixingprocess was promoted and the membrane surface was formed by micro-nano compositedstructure, which caused the amplify of surface roughness, crystallinity, and hydrophobicity.The experimental results showed that the water contact angle of PVDF membrane, whichwas fabricated by isopropanol-water dual bath method, achieved as high as130o.Modified TiO₂nanoparticles, PEG-400, and TEOS were added into membrane solutionand the effects of membrane structure and performance were discussed. It was shown that,membrane flux was dramatically increased without significant transform in membranehydrophobicity when modified TiO₂nanoparticle was used as additive, but the mechanicalstrength of membranes were reduced. The number of microporous on the membrane surfacewas significantly increased while PEG was used as additive. The appearance of macroporouson membrane surface and bottom leaded to a slightly increase of membrane flux, anddecrease both in surface hydrophobicity and mechanical strength. The number ofmicroporous on both membrane surface and bottom were significantly increased whileTEOS was used as additive, fibrous lamella structure was appeared on membrane surface,the flux of PVDF membrane was substantially increased, the hydrophobicity of membranesurface was increased in a slight scope, and the mechanical properties of membrane werereduced. The gas flux of membrane was increased with maintaining surface hydrophobicproperties and reducing the intensity in small amplitude when added PEG-TEOS compositedadditives into casting solutions, the water contact angle of membrane surface was over130o.To further optimize the performance of highly hydrophobic PVDF membrane, Single factor experiments aided orthogonal test were adopted to investigate the effect of membraneformation conditions on membrane performance, such as PVDF content, first bathimmersing time, and system temperature. The optimized conditions of membrane formationwere determined and the typical membrane sample was investigated in repeated VMDprocess. When the downstream vacuum degree of VMD system was0.096MPa, the feedtemperature was60oC, and feed flow rate was60L/h, the permeation flux was extremely ashigh as24.2kg/（m²·h） and the rejection rate was over99.9%.