Self Sintering Transparent Nanoporous Thin-Films For Use In Self-Cleaning, Anti-Fogging, Anti-Corrosion, Anti-Erosion Electronic And Optical Applications

Wisconsin Alumni Research Foundation (Madison, WI) inventors disclose in U.S. Patent Application 20100221513 methods of making self sintering transparent nanoporous thin-films for use in self-cleaning, anti-fogging, anti-corrosion, anti-erosion electronic and optical applications

According to inventors Marc A Anderson, Jeffrey R. Brownson, Jennifer Sanfillipo and Kevin Leonard, nanoporous thin-films also modify the physical and chemical properties of the substrates on which they are placed, such as optical, electrical, electronic, magnetic, thermal and other properties. The composition and particles may be optimized for a given application. For example, the composition and associated processes may be optimized for application to plastic and glass supports. Other suitable supports may include metals, ceramics, wood, paper, cardboard, concrete, stucco, tents, sails, shingles, and the like.

It has now been found that the thin-films of the present disclosure use ultraviolet (UV) light from the sun as an energy source to perform their functions. Specifically, the thin-films selectively absorb light in the UV range of below about 370 nm. Upon photo-excitation of the substrate member/material by incident UV radiation an electron will be excited from the valence band to the conduction band. If the energy is larger than the band gap of the material, a proton is left behind in the material (an electron `hole`). These electron-hole pairs are highly chemically active and react with surface --OH groups forming free radicals that cause the oxidative degradation of organic compounds on the substrate member. The end result of this reaction is water and carbon dioxide. 

There exists a growing interest in large-scale commercialization of optimally transparent, ceramic, inorganic oxide, thin-films that are self-cleaning, anti-microbial and/or anti-fogging.

Self-cleaning applications include windows, tiles, metals, ceramics, wood, paper, cardboard, concrete, stucco, tents, sails, shingles, lamp fixtures for tunnels, bathroom mirrors, aquariums, automotive glass, corrective eyewear, and others. Particularly, in one exemplary embodiment, there is a desire for self-cleaning solar panels and reflectors. Solar energy is a multi-billion dollar market, and countries all over the world have invested in solar collection systems. Deserts and their intense solar activity are ideal locations for these plants, however, although the sun's intensity is impressive, the dirt and dust is daunting. Power plants spend substantial amounts of money cleaning solar panels and reflectors as the accumulated dirt is detrimental to the efficiency of the system. 

The manufacturing steps include providing either a plastic or glass substrate member, pre-heating the substrate member, providing a first liquid sol comprising water, methanol, and nanoparticulate SiO2, coating the pretreated substrate member with a barrier layer of the first liquid sol, evaporating the water and methanol from the barrier layer at a temperature and for a duration sufficient to evaporate the water and methanol and insufficient to significantly thermally sinter the nanoparticulate SiO2, providing a second liquid sol comprising water, methanol, nanoparticulate TiO2, coating the barrier-coated substrate member with a top layer of the second liquid sol, and, evaporating the water and methanol from the top layer at a temperature and for a duration sufficient to evaporate the water and methanol and insufficient to significantly thermally sinter the nanoparticulate SiO2 and TiO2 to produce barrier and top coated substrate member.

The coating is made by the steps comprising providing a plastic substrate member, pretreating the plastics substrate member, providing a first liquid sol comprising water, methanol, and nanoparticulate SiO2, coating the pretreated plastic substrate member with a barrier layer of the first liquid sol, evaporating the water and methanol from the barrier layer, providing a second liquid sol comprising water, methanol, nanoparticulate TiO2, coating the barrier-coated plastic substrate member with a top layer of the second liquid sol, and, evaporating the water and methanol from the top layer to produce barrier and top coated plastic substrate member.

In one embodiment, the evaporation of the water and methanol from the barrier layer is conducted at a temperature and for a duration sufficient to evaporate the water and methanol and insufficient to significantly thermally sinter the nanoparticulate SiO2, and the evaporating the water and methanol from the top layer is conducted at a temperature and for a duration sufficient to evaporate the water and methanol and insufficient to significantly thermally sinter the nanoparticulate SiO2 and TiO2.  The first liquid sol further comprises nanoparticulate ZrO2. 

The new nanoparticulate film overcomes the problems of previous coatings such as:

Sintering at high temperature is incompatible with substrates constructed from materials such as plastics, wood, or composite materials. The substrates generally cannot withstand the high temperatures needed to sinter many oxide films.

In some cases organic polymer films have been used on substrates that are incompatible with high sintering temperatures. In some cases polymeric films have been used for some applications such as those needing anti-fogging or self cleaning properties.

Over time, photodecomposition causes such polymeric coatings to yellow. Hence, there is a need for durable thin-films constructed from inorganic ceramic oxides that harden at lower temperatures suitable for use with plastics, composites, woods or metals. Examples of suitable temperatures may include room temperature, the softening temperature, melting temperature, annealing temperature or decomposition temperature of the substrate to which the inorganic ceramic oxide films are applied.