Investigation On Superhydrophobic Modification And CFD Simulation For Heat And Water Vapor Transfer Of PU Nanofiber Porous Membrane

Polyurethane(PU)porous fibrous membrane prepared by electrospinning technology pocessed a certain degree of hydrophobicity due to the increase of surface roughness.The porous structure(pore size and porosity)of electrospun membrane and the polar group(polyether or polyester terminal hydroxyl group)on the soft segment of PU endowed the membrane with a certain water vapor transmission(WVT)rate.Therefore,PU porous fibrous membrane could be used as a functional material(water repellent and permeability)for outdoor clothing.The spinning solution contianed a mixed solvent of dimethylformamide(DMF)and butyl acetate(Bu Ac)was used to prepare a smooth and non-destructive membrane.Compared with the electrospun membrane prepared by the spinning solution of DMF,the physical properties of the solvent was the influnce which cause the holes on the surface of PU porous fibrous membrane.The effects of fiber diameter,membrane pore size and porosity on the hydrophobicity,air permeability and WVT of PU porous fibrous membrane were investigated,which provided a suitable matrix for the development of superhydrophobic PU porous fibrous membrane.Superhydrophobic silica nanoparticles were prepared by hydrothermal assisted sol-gel process using a two-step synthesis route.Hydrophilic silica nanoparticles were obtained by the hydrolysis and condensation of tetraethyl orthosilicate(TEOS).Then hexadecyltrimethoxysilane(HDTMS)was added to cause the condensation reaction between the hydroxyl groups of HDTMS and silica,rendering silica particles hydrophobic.Silica nanoparticles with the HDTMS/TEOS molar ratio of 2:40 exhibited good hydrophobic property(water contact angle,WCA?156.1°).And water droplet rolled off the surface of silica powder owing to the low sliding angle of 3.6°.When hydrothermal time of TEOS decreased from 120 min to 60 min,the prepared silica powders gave rise to high water contact angles(WCAs?154.8°)and low sliding angles(SAs?5.5°).This facile preparation of superhydrophobic silica nanoparticles with short reaction time provided the advantages for potential application in industrial production.Electrospun PU fibrous membrane with porous structure could be used for water repellent and breathable materials,however,due to polar groups on the soft segment of PU,water easily infiltrates into the membrane after a period time.The hydrophobicity of PU porous fibrous membrane could be improved by grafting super hydrophobic silica particles onto the membrane to construct rough surface and low energy epidermis.PU porous fibrous membrane was chemically modified with 4,4'-methylenebis(phenyl isocyanate)(4,4'-MDI)to obtain a reactive group,which was then coupled with(3-aminoproyl)triethoxysilane(APTES).Thereafter,the treated membrane was added to the preparation solution of super hydrophobic silica particles.Herein,the particles constructed the rough surface of the membrane,and HDTMS endowed the membrane with hydrophobic epidermis.The resultant membrane exhibited significant hydrophobicity with high WCAs and low SAs to different aqueous solutions.Under cyclic stretching,the modified membrane displayed durable hydrophobicity(WCA varied between 152.7°and 154.9°,SA fluctuated between 5.0°and 6.5°)and high breathability(8.4 kg·m^-2·d^-1).Furthermore,the hydrophobic membrane was superoleophilic and had high permeation fluxes for various oils.It could effectively separate(98.5%)the dichloromethane-water mixture over 30 separation cycles.Heat and water vapor transfer of porous fibrous membrane were investigated by computational fluid dynamics(CFD)simulation in the 3D model to improve the thermal comfort of the membrane.3D models with different fiber orientations and porosity were constructed by Digimat software.The permeability was used to reflect the discrepancy in the porous structure of the model.And the coefficient of determination(R²)for the CFD simulations(apparent velocity)and experiments(air permeability)with different porosity was 0.965,which proved the accuracy of the 3D model.The influences of fiber orientation and porosity on heat and water vapor transfer were surveyed by COMSOL software.Due to the lack of temperature difference in the entire domain,heat conduction(10^-9 W/m²)and moisture convection(10^-14 mol·m^-2·s^-1)were faint.As the permeability increased from 1.002 m² to 1.200 m²,the heat convection flux increased from1236.6 W/m²to 1298.8 W/m²,and the water vapor diffusion flux increased from 0.382 mol/(m²·s)to 0.402 mol/(m²·s),which meant that increasing the permeability by adjusting the fiber orientation facilitated the transfer of heat and water vapor.When the porosity increased from 44.87%to50.15%,the heat convection flux increased from 1207.74 W/m²to 1282.44 W/m²,and the water vapor transmission rate(WVT)increased from 0.0273 mol/(m²·s)to 0.0290 mol/(m²·s).It indicated that porosity had a greater impact on heat convection and water vapor transmission than permeability,because the permeability declined with the increase of porosity.The R² for the heat convection flux and WVT of the simulations and experiments with different porosity were 0.983and 0.820,respectively,which demonstrated the validation of the simulation in heat and water vapor transfer.