WO1994014088A1

WO1994014088A1 - Polymer-ceramic composite thin film that blocks ultraviolet and transmits visible light

Info

Publication number: WO1994014088A1
Application number: PCT/US1993/012219
Authority: WO
Inventors: Seshu Babu Desu; Richey Mclane Davis; Vidhu Jaikishen Nagpal
Original assignee: The Center For Innovative Technology
Priority date: 1992-12-16
Filing date: 1993-12-15
Publication date: 1994-06-23

Abstract

Polymer-ceramic composite thin films that transmit virtually all incident light in the visible spectrum, but block virtually all incident light in the ultraviolet range are disclosed. The films may be rendered microporous and so constructed as to provide controlled refractive index gradient. The films are also useful in protecting materials against U.V. damage.

Description

L ".'--a-CERAMIC COMPOSITE THIN FILM THAT BLOCKS ULTRAVIOLET

AND TRANSMITS VISIBLE LIGHT

The present invention is related generally to thin films with differential light filtering properties. More particularly, the present invention is related to polymer- ceramic composite thin films comprising ceramic particulates that have superior light transmission properties down into the ultraviolet range where the light transmission virtually drops to zero.

The formation and processing of fine ceramic particles less than 100 nm have received considerable attention in recent years due to the special optical, electronic and densification properties that these particles are assumed to possess (Sacks et al, 1984, J. Am. Ceram. Soc. 67:526; Brinker et al, Better Ceramics through Chemistry, MRS Proceedings Vol. 73, Elsevier, N.Y. 1986; Intelligent Processing of Materials, Report of an Industrial Workshop conducted by NIST, Gaithersburg, MD, Aug 31-≤ept. 1, 1988, NISTIR 89-4024) . Ceramic films have been made for ultraviolet barrier applications by various methods. However, these films differ from the present invention in that the films of the present invention are a composite of polymer-ceramic and the process for making these composite films is also distinctive and different from the prior art processes. For example, the process of the present invention is simple, inexpensive =tnd performed at low temperatures, while the prior art methodologies reguire sophisticated equipments, high temperatures and are costly. To wit, in conventionl chemical vapor deposition (CVD) technique, titanium dioxide (titania) is formed only when atmospheric moisture mixes with the vapor of hot titanium tetrachloride and is made to condense on glass surface, whereas the problem associated with reactive evaporation process is that Ti0₂ melts at 1925°C which makes it very difficult to evaporate. Furthermore, very high temperatures cause TiO to reduce to titanium monoxide and sufficient oxygen pressure is required to oxidize titanium monoxide back to titanium dioxide. Furthermore, , when the pressure is too high, the films become porous and soft. Thus, the optimal conditions for making the film are arrived at in each plant by trial and error. Similarly, in the electron beam evaporation process, transparent titania films are made by initial evaporation of pure titanium metal which is then oxidized in air by heating at 400-5000C to form Ti0₂ film.

Compared to the above-mentioned high temperature processes, sputtering is carried out at the room temperature. However, in 'order to sputter titanium dioxide onto a substrate, it is necessary to use an ion beam in a very high vacuum chamber.

It is clear from the above that high temperature processes require sophisticated equipment. Due to very high temperatures, the choice of substrates on which the films are to be cast, is limited and the properties of the films depend on a number of factors, such as starting material, oxygen pressure, rate of deposition and the substrate temperature. Moreover, these methods are generally useful for single layers and become very complicated where multilayers are required.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a relatively simple and inexpensive process that does not suffer from the limitations of the conventional methodologies and allows the formation of single or multilayer thin composite films containing ceramic particulates in a polymer matrix, without employing high temperature, high pressure or sophisticated equipments.

It is a further object of the present invention to cast the polymer-ceramic composite thin films on a variety of substrates, including those that could not withstand high temperatures required for film formation using the conventional methodologies.

It is another object of the present invention to provide multilayer films wherein each layer's composition can be ^'so controlled that films having desired refractive index gradients and selected wavelength transmission properties can be prepared.

A further object is to provide thin films of titania that transmit virtually all incident radiation in the visible spectrum but block virtually all incident radiation in the ultraviolet range.

An additional object of the present invention is to provide relatively ' non-toxic compositions of ceramic particulates, suitable for making thin films that block ultraviolet light.

A still further object is to provide thin ceramic films or membranes with controlled porosity.

Other objects and advantages will become evident from the following detailed description of the invention. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects, features and many of the attendant advantages of the understood upon a reading invention will be better of the following detailed description when considered in connection with the accompanying drawings, wherein:

Figure 1 schematically shows the process of film formation in accordance with the present invention;

Figures 2 and 3 present electron micrographs of the coated film and of the dilute suspension, respectively; and

Figures 4A and 4B show the UV/VIS transmission characteristics of Ti0₂/HPC thin films on quartz. DETAILED DESCRIPTION OF THE INVENTION

The above and various other objects and advantages of the present invention are achieved by a composition, comprising microfine ceramic particulates dispersed in a polymer solution in an aqueous medium, said composition being non-toxic and allowing formation, at normal pressure and ambient temperature, of uniform films that transmit virtually all incident radiation in the visible spectrum, but block virtually all incident radiation in the ultraviolet range.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein are preferred. Unless mentioned otherwise, the techniques employed or contemplated herein are standard methodologies well known to one of ordinary skill in the art. The materials, methods and examples are only illustrative and not limiting.

The term "microfine" particulates means sub-lOOnm particles formed at room temperature in the presence of an aqueous polymer solution, the dissolved polymer being adsorbed on the particles as they form.

The term "non-toxic" composition as used herein means that the composition is free of substances considered hazardous to the environment or to the human health at the concentrations used in the composition. The term "sophisticated" equipment as used herein means such complex or expensive equipments as high vacuum chambers, electron or laser beam apparatus, chemical vapor deposition equipments, and the like.

The term "substrate" as used herein means any article, matter, grid, device and the like on which the film of the present invention is formed or wherein the film is used. The term "transparent" or "fully transparent" as used herein means transmitting almost all incident light in the visible spectrum.

MATERIALS AND METHODS Titania Particle Formation

It is quite critical that in order to obtain films that are fully transparent, the metal oxide particle size must be carefully controlled so that the scattering elements of the porous metal oxide particles obtained in accordance with the present invention are sizably smaller than the wavelengths of the visible range. While such control of the particle size assures almost 100% transmission of the incident light in the visible spectrum, virtually zero transmission in the UV range is obtained due to total absorption of the UV by the metal oxide particles.

Titania is chosen as an illustrative example, because it is a metal oxide used extensively in coatings and ceramics. Titania particles of desired size are made in accordance with the present invention from the hydrolysis and condensation of titanium alkoxide precursors in the presence of a solution of a steric stabilizing polymer. Such titania particles are obtained from titanium ethoxide according to the reactions believed to occur as follows, although the applicants are not bound by any specific theory pertaining thereto. Ti(OC₂H₅)₄ + 4H₂0 —> Ti(0H)₄ + 4C₂H₅OH Hydrolysis

Ti(0H)₄ --> Ti0_2> x H₂0 -r (2 - x)H₂0 Condensation. When these reactions occur in a polymer solution, the dissolved polymer can adsorb onto the particles as they form, providing steric stabilization and thus limiting the titania particle size. Furthermore, the titania. particles thus formed are porous. A preferred polymer in accordance with the present invention is hydroxypropyl cellulose (HPC) manufactured by Hercules Incorporated, DE. , although other suitable polymers could also be used.

Metal oxide particles, e.g. , titanium oxide, are obtained by a sol-gel process where a metal alkoxide precursor, e.g., titanium tetraethoxide (TEOT) , is mixed in a solvent, e.g. , ethanol, and is hydrolyzed and condensed in the presence of water and polymer, e.g., HPC. The molarity ratio of water to TEOT is preferably kept greater than 5 and the polymer concentration is preferably kept greater than 0.2 g of polymer per liter of the solution.

Microfine metal oxide particles can also be obtained from the dried polymer-ceramic composite film by either removing the adsorbed polymer from the film (e.g., by dissolving the polymer in a suitable solvent) or by sintering away the polymer from the dried polymer-ceramic composite film. Film Formation

Thin films of titania/HPC were made by spin-coating silicon and guartz wafers with suspensions of porous titania particles grown in the presence of the polymer HPC dissolved in aqueous ethanol. The film making procedure is shown in Fig. 1. Once the titania suspensions are cast and the ethanol evaporates, thin transparent films of titania and HPC result. A typical film composition contains about 48:52 weight percent of Ti0₂:HPC, although the ratio can be changed according to the particular optical property desired in the film. The ratio may range from 95:5 to 5:95 percent by weight.

Some of the advantages of forming ceramic films by the method of the present invention are: (i) relatively non-toxic reagents are required, including titanium tetraethoxide, ethanol, water, and HPC wnich is a polymer commonly used in pharmaceutical applications; (n) spin coating is simple and does not require complex or expensive equipment; and (iii) coating is carried out at room temperature (22-24° C) .

It is noted that the film thickness can be controlled by the number of layers deposited, as would be known to a skilled artisan. Of course, the films can be cast on a variety of substrates, including those which could not withstand the high temperatures required for the conventional film formation techniques described above. Moreover, multilayer films can be made easily. Additionally, by controlling each layer's composition, the layer's refractive index can be controlled, resulting in films with preferred refractive index gradients. Such films have very useful optical properties. For example, narrow band pass filters which pass light at only selected wavelengths could be fabricated by this process. These filters, wnen made by conventional process, are much more complicated since each layer must have matching ε .esses (generated by differences in thermal expansion coefficients of film layers and the substrates) , uniformity, chemical and temperature compatibility, and the like. Furthermore, the polymer in which the titania particles are dispersed can be selected to control film properties. For example, a polyimide which is chemically stable at very high temperatures (in excess of 250°C) can be used instead of HPC to obtain films that could function at high temperatures. Titania/HPC Film Optical Properties

After the films are ma . light transmission through them is measured as a function of wavelength. Two distinctive features were seen that have important implications: (i) Ultraviolet filter behavior and (ii) interference patterns. These are discussed separately below. UV Filter Behavior

In the visible range, between 450-700 nm, the films showed almost 100% transmission, i.e., they were virtually completely transparent. However, in the ultraviolet range, below wavelengths jof .400 nm, the transmission dropped off sharply, dropping to zero for wavelengths less than 300 nm. Thus, the films act as filters for a major portion of the UV spectrum. The total impermeability of UV radiation below the wavelength of 300 nm results from the fact that titania strongly absorbs UV radiation. The films are transparent in the visible part of the spectrum because the microfine titania particles made in accordance with the present invention scatter virtually no visible light.

Films have been made by spin coating the suspension with a starting titania/HPC weight ratio of 48/52 onto other substrates. Figure 2 is a TEM micrograph of a coat of film cast on a copper-grid. For comparison, a TEM micrograph of the titania/HPC suspension diluted with ethanol and coated onto a copper grid without spinning is shown in Figure 3. The dark regions in both Figures 2 and 3 are titania particles. In Figure 3 , the suspension has small titania particles about lOOnm in diameter. The dark regions in Figure 2 are bigger than in Figure 3 suggesting that the titania particles in the spun-coated films formed large aggregates due to the spin casting process. Interference Patterns

The percent transmission versus wavelength curves for the titania/HPC films showed local maxima and minima due to interference patterns (Figures 4A and 4B) . These occur when the films have extremely uniform structure so that light reflected between the top and bottom of the film undergoes constructive and destructive interference. Films exhibiting these characteristic interference patterns can be used to construct narrow band pass filters that transmit light only of selected wavelengths. The data in Figures 4A and 4B clearly show that pure HPC film is transparent to the ultra-violet (UV) light, whereas the film containing titania completely blocks the UV radiation and transmits more than 95% incident light in the visible region, thus acting as an optical filter. Role of HPC in Film Formation

Without being bound to any specific theory, it is believed that the HPC polymer serves two functions: (i) it acts as a colloid stabilizer in the suspension state, resulting in the formation of microfine particles, and (ii) it acts as a matrix in the dry film state, encasing the titania particles and serving to fill cracks that may form during drying. It is essential that the films be free of cracks for good optical transmission properties. Light must not be able to "leak" through cracks in the films. Cracks are easily repaired by heating the film slightly above the glass transition temperature (Tg) of the polymer. It may be pointed out, however, that the polymer hydroxpropylcellulose (HPC) has a glass transition temperature (T ) between 100-150°C. Hence, the films must be used below 100°C to have mechanical integrity. As mentioned herein supra. films that can be used at higher temperatures can also be made by dispersing titania particles in a polvmer solution where the polymer has greater thermal stability, such as polyimide and the like. The titania/polymer dispersions are then used to form a film as described herein.

Of course, new and inexpensive narrow band pass filters that transmit light only of selected wavelengths are made by spin coating films with multiple layers in a "sandwich" structure that results in a controlled refractive index gradient. Such narrow band pass filters are quite useful where selective transmission of specific wavelength of light is reguired, such as in spectrophotometric instruments and the like.

Thin Ceramic Films or Membranes with Controlled Porositv:

It is noted that by sintering the titania/HPC films to burn out the HPC polymer, a porous titania film is obtained where the porosity is controlled by the size of the titania particles and the original titania/HPC ratio. Such a film would be porous on two length scales - the porosity of the titania particles themselves, and the porosity resulting from the burnout of the HPC polymer between different titania particles. Such films with controlled porosity would be useful in the development of microsensors for specific chemical species and in the development of catalysts where the location of active catalytic groups is well controlled (Sol- Gel Science: The Physics and Chemistry of Sol-Gel Processing; eds. Brinker and Ξcherer, Academic Press Inc., 1990; Chapter 14: Applications, page 852).

Furthermore, if the films are supported on macroporous substrates, controlled pore-size films may be used as microfilters (50 nm to 1 micron pore size) , membranes (pore size < 50 nm) , or ultrafilters (pore size about 2 nm) . Compared to conventional organic polymer membranes, microporous membranes derived by this process offer several advantages: (i) they cin be sterilized; (ii) they do not swell or shrink in contact with liquids; and (iii) they are much more abrasion resistant. If still greater abrasion resistance is desired, a thin layer of abrasion resistance material can be coated to obtain enhanced abrasion resistance. In addition to the porous nature of the sintered films, it is noted that the films sintered below 800 C showed virtually 100% transmission in the visible range while totally absorbing UV radiation below about 320 nm. The percent transimission versus wavelength curves showed local maxima and minima due to interference patterns, proving that these sintered films have an extremely uniform structure that is analogous to the uniform structure of the titania/HPC films.

Various Utilities of the Invention UV filters for optics and packaging materials. There is a great need for inexpensive UV filters for coating lenses and other optical components in instruments. UV filters also have great utility in the packaging industry for protecting goods in storage or .transit against UV-induced deterioration. The thin films of the present invention, i.e., (1) titania/HPC, and (2) sintered titania, fulfill these needs. Narrow Bank Pass Filters. As mentioned above, new and inexpensive narrow band pass filters that transmit light only of selected wavelengths are made by constructing coatings with multiple layers in a "sandwich" structure that result in a controlled refractive index gradient. Antireflection Coatings for Photovoltaic Cells and Passive Solar Collectors. By making transparent layered films as described above, the amount of light incident on the surface of a solar collector can be enhanced, thus trapping more incident radiation in the collector and thereby raising its efficiency.

Thin Ceramic Films or Membranes with Controlled Porosity. As noted above, it is possible to sinter the titania/HPC films to burn out the HPC polymer, thus leaving a porous titania film where the porosity is controlled by the size of the titania particles and the original titania/HPC ratio. Such porous films can be applied in the development of microsensors for specific chemical species and in the development of catalysts where the location of active catalytic groups is well controlled.

It may be noted that while an embodiment of the present invention has been exemplified by spin-coating, which is a batch process, continuous processes suitable for large scale production, such as dip coating and spray coating, are adaptable as would.be suggested to the skilled artisan. For very large substrates, such as windowpanes, spray coating is preferred. Of course, film properties including transmission versus wavelength, film thickness, cracks, film uniformity, etc. , will have to be adjusted as a function of processing conditions. However, the governing factors which control these properties in dip coating and spray coating are well documented (Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; eds. Brinker and Scherer, Academic Press Inc., 1990; Chapter 13: Film Formation, page 788) and given the guidance and directions noted herein, manufacturing processes can be easily scaled up for large scale applications.

It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.

Claims

1. Porous, microfine, metal oxide particles dispersed in a layer of a polymer, said particles being of a size that fail to scatter virtually any incident radiation in the visible wavelength.

2. The particles of Claim 1 having a colloidal layer of a polymer adsorbed on said particles.

3. The particles of claim 1, wherein said metal oxide is titania.

4. The particles of Claim 1, wherein said polymer is hydroxypropyl cellulose (HPC) .

5. The particles of claim 1, being a composite of titania- HPC.

6. A method of preparing a film of microfine metal oxide particles, comprising the sequential steps of:

(a) forming a dispersion of metal oxide particles in a solution of a polymer at room temperature;

(b) spin coating the dispersion formed in step (a) on a substrate; (c) drying- the spin coated dispersion on said substrate; and

(d) obtaining a crack-free film of said metal oxide particles.

7. The method of claim 6, wherein said polymer is hydroxypropyl cellulose.

8. The method of claim 6, wherein said metal oxide is titania.

9. A composition, comprising microfine ceramic particulates dispersed at room temperature in a polymer solution, said composition being nontoxic and allowing formation, at normal pressure and ambient temperature, of films that transmit virtually all incident radiation in the visible spectrum, but block virtually all incident radiation in the ultraviolet range.

10. The composition of Claim 9, wherein said particulates are of titania and said polymer is a polyimide.

11. The composition of Claim 9, wherein said particulates are of titania and said polymer is hydroxypropyl cellulose (HPC) .

12. The composition of claim 11, wherein the ratio of titania to HPC is in the range of about 95:5 to about 5:95 percent by weight.

13. A film that transmits virtually all radiation incident on the film in the visible spectrum, but blocks virtually all radiatio incident on the film in the ultraviolet range.

14. The film of Claim 13 being a single or multiple layered structure.

15. The film of Claim 14 being a multiple layered structure.

16. The film of Claim 15, wherein said multiple layered structure is so constructed that only light of selected wavelength is transmitted through the film.

17. The film of Claim 14 having a specific refractive index gradient.

18. The film of claim 13 composed of microfine titania particles of less than 100 nm diameter, dispersed in a colloidal matrix of hydroxypropyl cellulose (HPC) .

19. The film of claim 18, wherein the ratio of titania to HPC ranges from about 95:5 to 5:95 percent by weight.

20. The film of claim 18, wherein said film is sintered to remove said colloid matrix leaving micropores in the film.

21. A substrate having the film of claim 13.

22. A method of protecting food or any other material against U.V.-induced spoilage or damage, comprising the step of blocking U.V. light from reaching said food or material by utilizing the film of claim 13.

23. The method of claim 6, further comprising the step of sintering the film obtained in step (d) to recover microfine metal oxide particles.

24. Microfine metal oxide particles obtained by the method of claim 23.