Study On The Method Of Superhydrophobic Modified Polyvinylidene Fluoride Hollow Fiber Membrane

Polyvinylidene fluoride(PVDF)has been extensively used because of its excellent chemical stability and fouling resistance.Because of its exceptional physical and chemical properties,PVDF is an ideal membrane distillation material for the membrane separation industry.However,the hydrophobic properties of PVDF are limited.It causes the restriction of PVDF membranes in membrane distillation application.In this work,a hydrophobization treatment for porous polyvinylidene fluoride(PVDF)hollow fiber membranes using an etching method combined with an ultrafiltration coating process was achieved.Firstly,DMAc(a solvent for PVDF)and anhydrous ethanol(a non-solvent for PVDF)were mixed at various volume ratios to prepare etchants.PVDF particle was subjected to an etching process to coarsen its surface and then coated onto the PVDF hollow fiber membrane surface via ultrafiltration to construct a super-hydrophobic surface with a micro/nanostructure.The optimal etching conditions for the PVDF particles were as follows:solubility parameter of the etchant,25.6(J/cm3)1/2;etching time,60 min at 25 ℃.Etched PVDF particles were coated onto the PVDF hollow fiber membrane surface via ultrafiltration to construct a super-hydrophobic surface with a micro/nanostructure.The solubility parameter of the dispersant was adjusted to achieve a specific PVDF-dissolving capacity.The dispersant had a swelling effect on both base membrane and PVDF particles.After the swollen particles were bonded to the swollen base membrane via solvation,a robust particle-coating layer formed on the membrane surface.When the solubility parameter of the dispersant was 25.87(J/cm3)1/2,the dispersant didn’t cause damages to the PVDF hollow fiber membrane structure.With the coating amount of 18 g/m2,the contact angle of the modified membrane was 163.8°,reaching super-hydrophobic and the membrane pores were not blocked.The super-hydrophobic coating on the outer surface of PVDF membranes was obtained by a particle coating process.A lotus-leaf-like structure with multi scale roughess on the membrane surface was observed by SEM.In the DCMD experiments,it was not until the particles coated layer wetted that SDBS could wet the surface of base membrane leading to through-wetting.Because the particles coated layer extended the path of wetting which provided a wetting depth indirectly,the anti-wetting property and critical wetting depth of modified membrane increased.The water in the hot side feed solution evaporated on the outer surface of the membrane to become water vapor.The water vapor reached the cold side through the pores of the membrane and condensed into liquid water in the inner side of the membrane to produce a flux.The surface roughness of a membrane affected the surface area where the liquid evaporated,which insignificantly affected the membrane distillation flux.The modified membrane could resist hydrophilization for 180 min,which was longer than the original membrane(40 min).In addition,after the membrane surface was modified,the pure water DCMD flux increased from 26.0 to 29.9 kg·m-2·h-1 and the critical wetting depth increased from 22.5 to 37.5 μm.It illustrated that the anti-wetting property of membrane increased significantly after the etching and ultrafiltration process.