Preparation Of Superhydrophobic Polypropylene Membrane And Its Application Membrane Distillation Properties Based On Atomic Layer Deposition

Membrane distillation（MD）is a separation technology which combines distillation with membranes,due to the utilization low-grade energy,it is expected to be popularized and applied in the fields of desalination,wastewater treatment and other fields.The hydrophobicity of the membrane used in membrane distillation is directly related to the anti-fouling and anti-wetting performance of the membrane,and also determines the stability of membrane distillation in long-term.Therefore,based on atomic layer deposition technology（ALD）,this paper studies the construction of superhydrophobic layer on the surface of polypropylene membrane,and investigates its application performance in membrane distillation to treat high-salt solution,so as to expand the application range of the membrane distillation process.Firstly,in order to construct a modified platform with certain activity on the surface of polypropylene（PP）membrane,we prepared PP/TiO₂ composite membrane by controllable deposition of TiO₂ layer on the PP microporous membrane based on atomic layer deposition technology.The results show that the titanium dioxide particles are deposited on the molecular sublayer on the surface of PP membrane,and its thickness is thin and uniform,and its porosity and average pore diameter are equivalent to those of the original membrane.The loading of titanium oxide can be accurately controlled by the number of layers of ALD.The TiO₂ layer deposited by ALD on the surface of the PP membrane is a stably bonded with the surface of the membrane.The number of ALD layers is optimized to 200 layers,and the peeling strength reaches 40 N/m,which is equivalent to the peel strength of PP membrane and the non-woven fabric supporting layer.Then,the PP/TiO₂/SiO₂/F composite membrane was prepared by grafting fluorine-containing silicone onto the surface of PP/TiO₂ composite membrane by in-situ sol-gel and coating coupling method.The results show that the water contact angle of the prepared superhydrophobic composite membrane can reach up to 156°,and the rolling angle is less than 10°.After the analysis of hydrophobic mechanism,the surface roughness of the membrane by SiO₂ nanoparticles enhance.At the same time,1H,1H,2H,2H-perfluorodecyltriethoxysilane reduces the membrane surface energy,which together leads to an increase in the hydrophobicity of the membrane.The separation performance of the membrane was evaluated,and there was no significant difference between the pore size and porosity of the unmodified PP membrane.The prepared PP/TiO₂/SiO₂/F membrane material is in high-salt（15wt.%）vacuum membrane distillation,and the results showed that the operating time is obviously improved,the flux was stable at about 13 kg/（m²·h）,and the rejection rate is Stable above 99%.Thirdly,in order to further improve the stability of PP film and nano silicon emulsion coating,the nano silicon emulsion was reacted with TiO₂ layer on the surface of PP film by photo-deposition,and the PP/TiO₂-NMES composite membrane was prepared.Hydroxyl group produced by the hydrolysis of the nano silicon emulsion with the low surface energy are condensed with Ti-OH on the TiO₂ at high temperature to form a stable coating.The results show that the contact angle of the PP/TiO₂-NMES composite film can reach 158°and the rolling angle is less than 2°.The separation performance of the membrane was evaluated.The pore size and porosity were not significantly different from those of the unmodified PP membrane.The chemically stable combination between the superhydrophobic layer and the PP membrane made its peel strength reach more than 40 N/m and rubbed 20 times.The above can still maintain a contact angle of 150°.The vacuum membrane distillation experiment of15wt.%Na Cl solution showed excellent anti-wetting ability.The running time was longer than 25h,and the flux was stable at about 15 kg/（m²·h）.To sum up,based on ALD method,two kinds of composite films with different structures were prepared on the surface of PP membrane in this paper.All contact angles are greater than 150°,which met the requirements of superhydrophobic.The reaction mechanism and the superhydrophobic effect were systematically studied.The good interfacial bonding ability between the hydrophobic layer and the PP membrane ensures the stable operating performance of the membrane.It shows excellent separation performance and anti-wetting ability in vacuum membrane distillation of high-salt solution（15 wt.%）,and this paper also provides a new idea for surface modification of inert membranes.