Composite Design And Durability Studies Of TiO₂ Based Superhydrophobic Coatings With Photocatalytic And Self-cleaning Properties

Superhydrophobic?SH?coatings have broad application possibilities,such as waterproof textiles for everyday materials,window and solar panel cleaning for industrial productions,or even drag reduction of underwater missiles for military fields,due to their special surface wettability.However,the superhydrophobicity of traditional SH coatings gradually deteriorate after being polluted by environmental organic matter?such as oil?.In recent years,SH materials with photocatalytic properties such as TiO₂-based SH coatings are very attractive since they have the ability to degrade organic matter under UV light,which would normally impair SH properties.This offers superiority in practical applicationsas it allows the surface to retain its original water repelling and self-cleaning performance.However,the design of durable photocatalytic SH surfaces via the combination of hydrophobic materials and TiO₂ is not straightforward,because TiO₂ itself will degrade the organic materials which had imparted the low surface energy,thus limiting the applications.Hence there is a need to address this drawback of existing photocatalytic SH surfacesIn this research work,robust hydrophobic and photocatalytic composites using TiO₂ have been designed through different methods,and simple coating techniques were used to fabricate the coatings.The obtained SH coatings not only demonstrate excellent multi-functionality,but also show markedly improved durability under strong UV or in bright sunlight.This research can be divided into the following aspects:?1?The hydrophobic polymer polydimethylsiloxane?PDMS?,with stable physicochemical properties,was chosen to hydrophobically modify TiO₂ by a physical blending method.A simple one-step spraying method was then used to fabricate the SH coating.The surface structure and surface energy of the final coating were optimised by adjusting the solvents used and the content ratio of PDMS and TiO₂.The results show that the superhydrophobicity originates from the hierarchical structure of the different aggregation sizes of TiO₂ nanoparticles within a robust crosslinked PDMS network,and PDMS coats the surface of TiO₂ aggregates to offer low surface energy as well as high physical and chemical stability.The obtained SH coatings are resistant to corrosion,high temperatures,and have high anti-impacting ability as well as photocatalytic properties.Importantly,they can still maintain superhydrophobicity after being subjected to a week of constant UV illumination,retaining their stability and durability.?2?TiO₂ nanoparticles were modified via a facile one-step reaction of octadecyl isocyanate?ODI?with hydroxyl groups on the surface of TiO₂ particles.The prepared ODI-TiO₂ composite particles were dispersed in solvents with a small amount of PDMS,and then the SH coating was fabricated by spray-coating or blade-coating the dispersions on various substrates.The crosslinked PDMS strengthens the bonding of the modified particles,thus increasing the mechanical properties of the SH coating.The results showed that the coatings prepared by this special composite crystal structure not only have the characteristics of photocatalytic self-cleaning,but also have the functions of oil-water separation,buoyancy enhancement and heat-healing.Their photocatalytic degradation resistance is greatly enhanced compared to that of other fluorosilane modified TiO₂ SH coatings,thus prolonging the environmental durability of the coating.?3?Combining the above network encapsulation and chemical grafting modification methods,the composite structure with TiO₂ was further chemically adapted by the addition of a diisocyanate group,in the form of toluene-2,4-diisocyanate?TDI?.The synthesized TDI/TiO₂ hybrid nanoparticles were dispersed with an environmentally-friendly waterborne PU adhesive and coated on different substrates via a scalable spray-coating method to create SH surfaces with an average water contact angle of 168°and a sliding angle of 1°.The results showed that the UV resistant durability of the final SH coatings can be effectively controlled by adjusting the content of TDI added in the modification process.With an increase in TDI concentration,the durability is greatly enhanced,and when the ratio of TDI:TiO₂ is 5:1,the superhydrophobicity of the TDI/TiO₂ hybrid network remains almost unchanged after a month of continuous UV illumination or one year in natural light.