Study On The Preparation Of PSF Ultrafiltration Membrane With Density Gradient Structure

Polysulfone(PSF) ultrafiltration membranes were prepared from a series of casting solutions with different composition by phase inversion process. NMP, DMF and DMAc were used as casting solvent, and organic alcohol (ethyl alcohol, ethylene glycol and glycerol) and organic acid (formic acid, acetic acid and propionic acid) were used as additives. The surface and cross-section structures of membranes were characterized by scanning electron microscopy. The flux of pure water and rejection of lysozyme were determined by an apparatus designed for our laboratory. The properties(C_p, η₀ and ΔE)of casting solutions were measured by falling ball viscometer and cloud point titrator. The effect of properties of casting solutions on the structure and performance of membranes was investigated. The ultrafiltration membranes with gradient density structure could be obtained by controlling the properties of casting solutions. Further more, the membrane formation mechanism was investigated.Experimental results showed that the size of the polymer chain and the degree of cloud point was NMP>DMAc>DMF in the three solvents. With the same PSF concentration, the zero shearing viscosity was NMP>DMF>DMAc, and the flow activation energy was DMF>DMAc>NMP. The cross-section structures of membranes prepared from DMF, which was used as solvent, were sponge-like with density gradient. However, when DMAc and NMP were used as solvents, the cross-section structures of membranes were finger-like. The order of pure water flux was DMF>DMAc>NMP and the rejection was NMP >DMAc> DMF.In the phase inversion process for preparation of ultrafiltration membrane, the kinds and content of addictive were important factors for controlling membrane structure. The cloud point and zero shearing viscosity of casting solutions decreased when the ethyl alcohol content increased, while the flow activation energy firstly decreased and then increased. When ethylene glycol or glycerol were used as additives, the cloud point and zero shearing viscosity of casting solutions decreased, and the flowactivation energy increased with the increase of ethylene glycol or glycerol content. The effect of different alcohol on the structures and performance of membranes was similar. When the alcohol content in casting solutions increased, the amount and aperture of pore on surface increased, the amount of finger-like pore on cross-section decreased. At the same time, the flux of pure water increased, the rejection decreased. With the further increase of alcohol, the structures of cross section gradually changed from finger-like to sponge-like.When organic acid was used as additive and the acid content increased, the cloud point of casting solutions decreased, the zero shearing viscosity and the flow activation energy increased(except propionic acid 2% ). According to the same amount of carboxyl, the longer the carbon chain was, the lower the property of non-solvent of acid was. The effect of different acid on the structure and performance of membranes was similar. The amount and aperture of pore on surface increased, and the amount of finger-like pore on cross-section decreased with the increase of acid content in casting solutions. At the same time, the flux of pure water increased and the rejection decreased. The cross-section structure of membranes was sponge-like with density gradient when content of acid was appropriate.