US2474061A - Method of producing thin microporous silica coatings having reflection reducing characteristics and the articles so coated - Google Patents

Description

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SEARCH ROOM June 21, 1949. MOULTON 2 474,061

METHOD OF PRODUCING THIN MICRO-POROUS S IL CA COATINGS HAVING REFLECTION REDUCING CHARACTER STIC AND THE ARTICLES 50 COATED Filed July 23, 1943 INVENTOR. HAROLD l2. MOULTON AT TOIZNEY Patented June 21, 1949 SEARCH ROOM METHOD OF PRODUCING THIN MICROPOR- OUS SILICA COATINGS HAVING REFLEC- TION REDUCING CHARACTERISTICS AND THE ARTICLES SO COATED Harold R. Moulton, Southbridge, Mass, assignor to American Optical Company, Southbridge, Mass., a voluntary association of Massachusetts Application July 23, 1943, Serial N0. 495,906

14 Claims.

This invention relates to new and novel means and methods of reducing surface reflections and, in some instances, producing surfaces with increased scratch resistance and of producing articles having such surfaces thereon.

This application is a continuation in part of my copending application Serial No. 452,206, filed July 24, 1942, now abandoned.

One of the principal objects of the invention is to provide novel means and methods of reducing the reflection of light impinging upon the surface of an article whereby, in the case of a transparent article, a greater percentage of incident light will traverse said article and in the case of an opaque article will reduce the reflectivity of said articles, and in some instances simultaneously increasing the scratch resistance of the surfaces of said articles.

Another object is to provide simple, efiicient and economical methods of providing articles with coatings having the above characteristics.

Another one of the principal objects of this invention is to obtain coatings of the above character which are inherently free from haze and which remain so throughout the useful life of the article on which this coating is applied.

Another object is to provide new and improved methods whereby coatings of the above character may be applied to the completed articles.

Another object is to provide new and improved means and methods of forming stable color screens and optical fllters.

Another object is to provide new and improved diffusing screens and means and method of forming the same.

Another object is to provide novel means and changes may be made in the details of construction, arrangement of parts and steps of the meth ods shown and described without departing from the spirit of the invention as expressed in the accompanying claims. I, therefore, do not wish 3 to be limited to the exact details of construction, arrangement of parts and methods shown and described as the preferred forms only have been given by way of illustration.

therein illustrates an article having a coating thereon formed according to the invention.

In the prior art, surface coatings have been produced for reducing reflections of light impinging upon the surfaces by commonly known methods.

A method which has retained a considerable commercial application is an outgrowth of vacuum evaporation technique. In this method, a thin layer, generally one quarter of a wave length thick, of a volatile salt or compound of low index is evaporated by means of heat in a vacuum and a deposit is produced upon the surface to be treated. The compounds used are customarily lithium fluoride, calcium fluoride, cryolite, or magnesium fluoride or similar compounds. These compounds produce surface layers which, when very carefully controlled as to thickness, will become quite eflicient as reflection reduction coatings. As such, they have been used in multicomponent lens systems for expensive optical parts. It has been found necessary, in order to increase the resistance to atmospheric attack and to the attack of moisture to bake these coatings at a relatively high temperature.

Another prior art method of obtaining glass surfaces having reduced reflection consists in the treatment of the surfaces with hydrofluoric acid gas or with dilute aqueous solution of hydroents of the glass are rendered soluble or volatile and are then subsequently abstracted from the Referring to the drawing, the figure shown 55 surface leaving a fllm of lower index. Other selected solvents for certain of the compounds present in the glass may be used and have been used in the prior art, such as alkalis, soluble phosphates, nitric acid and even harsh laundry soap. The use of the above described process, of course, is restricted to glass.

It, therefore, is one-of the primary objects of this invention to produce surface coatings which distinguish from the above in that they do not require high vacuum equipment or prolonged treatment, which may be applied in a simpler, more convenient and inexpensive manner and at the same time with extreme resistance to th attack of moist atmosphere and moisture itself, and in some instances more resistant to scratching and chemical attack, and which in addition may be applied to materials other than glass.

In addition to the reduction of reflection of incident light the coatings as produced serve as chemical protection for the glass or plastic sur-' faces.

In following the teachings of the invention the article I which is to be coated may be of a light transmitting nature such as glass plates, lenses, prisms or other optical elements and similar elements formed of plastics or artificial resins. In this particular instance, the element, as stated above, is to have light transmitting properties.

In the present instance, the articles I referred to are particularly of the type which are adapted for use in telescope systems, field glasses, binoculars, camera lenses, periscopes or for any other uses in which the light is adapted to impinge upon a surface of the article. The surfaces 2 of said articles, in order to reduce surface reflection and to increase light transmission, are coated with a stable water insoluble coating 3 of a material having a lower index of refraction than the substrate. This is accomplished by applying a layer of the solution, which upon evaporation, deposits such a layer on the glass or material substrate. The material which forms this layer need not be present in the solution in the actual form in which it finally exists on the surface of the article but may be a decomposable compound which upon the removal of the solvent leaves a surface layer on the substrate in the desired state, for example, a solution of from 1 to 10 per cent by volume of tetraethylorthosilicate, to 60 per cent by volume ethyl acetate, 1 to 10 per cent by volume concentrated hydrochloric acid and the balance ethyl alcohol. Denatured alcohol may be substituted for the ethyl alcohol. A preferred solution is five parts of tetraethylorthosilicate, 50 parts of ethyl acetate, 50 parts of denatured alcohol and 5 parts of concentrated hydrochloric acid, mixed in the order given. This gives a clear, colorless, stable solution. An essential feature of this solution is aging for several days before use. The exact reason for this is not known but it is believed to be a partial decomposition of the tetraethylorthosilicate by the hydrochloric acid resulting in an organo-sol of either hydrated or unhydrated silica. With the composition set forth above, an aging of from two to six days is desirable. It has been found desirable for certain articles, such as lenses, especially those of steep curve, to spin the article at relatively slow speed and pour upon the upper surface thereof a small measured quantity of the solution. In this way, one surface may be done at a time and the smoothness and uniformity of the coating may be increased. In either case, the spinnin of the article is continued until the layer of solution formed thereby has evaporated sufiiciently so that no drips or runs will occur when the article is removed from the holder. Prolonged aging to two or three months produces no further change. This aging normally takes place at room temperatures but it can be accelerated slightly by working at more elevated temperatures up to 50 C.

The solution having been prepared and aged, the procedure is as follows:

The lens or other article is immersed in this liquid, removed and immediately spun in order to remove excess liquid. Over a very wide range of translational velocities the effective thickness of the resulting layer is in the range for emclent reflection reduction thereby differing from previous methods of obtaining reflection reduction in which the efficiency of the final product is greatly dependent upon the variation of thickness in the film. At this point, at least three alternative methods may be as follows:

In method 1 the article is placed in a moist atmosphere at a temperature of approximately C. for a time interval suflicient to substantially heat the entire article to said temperature after which the article may be cooled. This procedure renders the coating insoluble in acids or water and difilcultly soluble in alkaline solutions. The coating produced by this method reflects approximately 1% on glass of 1.52 index of light from each surface. In other words, for a flat piece of glass treated by this method substantially 98% of the incident light traverses it in contradistinction to the figure 91.8 which is the inherent transmission of glass of this index of refraction. If the glass, for example, is of a higher index of refraction than 1.52 the reduction of reflection is still greater. The treatment at 100 C. moist heat, serves to expedite the rendering insoluble of the surface reflection reducing coating. There is a more rapid increase in the hardness of this surface than occurs in the next two methods but the end result in all cases is substantially the same as regards hardness and insolubility. Although I have referred to the temperature as being 100 C. it is to be understood that any suitable heating temperatures may be used and been found that a wider range of temperatures may be used and produce surface coatings of substantially similar characteristics, the top limit of such temperatures being set by the softening or distortion point of the material bein treated, the lower temperature of 100 C. being preferable for use as it permits composite structures which are united by cementing or the like which are capable of withstanding this temperature but which would be susceptible to injury at higher temperatures. Lower temperatures, which are above room temperatures, are therefore preferably desirable. The temperature, therefore, must mainly be such as to produce coatings having desired characteristics without injury to the initial article. The temperatures referred to hereinafter are therefore to be considered in the light of the above.

The second method consists in holding the article at room temperature after the solution has been applied and the excess removed by any convenient means such as the spinning as set forth above. The article is held at room temperature in normally moist air for a period of approximately one minute, the time varying in-' versely with the ambient temperature. At the expiration of this exposure period the article is subjected to the action of water which may be cool or warm for any convenient period of time as it has been found that variations in the time of water treatment have substantially the same effect. This treatment somewhat increases the reflection reduction above mentioned but the article may be heated to 100 C. subsequently in order to insure permanency. It is felt that this gives a somewhat more porous surface and one which is somewhat less easy to clean.

The third method consists in simply allowing the article to stand at room temperature after the excess solution has been drained off or removed by spinning in which case the article should not be handled for some little time. About an hour has been found a desirable period.

For coating larger surfaces, it has been found that dflution of the original solution with a suitable solvent of which a mixture of equal parts of denatured alcohol and ethyl acetate have been found to produce practical results, may be used. In this case, multiple dips followed by draining, are desirable, with the article being so oriented SEARCH ROG as to drain in a different direction after each dip whereby the effectiveness of the coating is maintained substantially uniform over the article. This procedure is particularly useful in coating large sheets.

Each coating may be heat treated, as above described, before the application of the succeeding coating or the coating may be simply allowed to dry in air as described in method #3 or the final product may be treated with water as described in method #2 or any combination of these procedures may be followed either on each individual coating or on any one of the successive coatings.

In the above example, tetraethylorthosilicate has been mentioned as has hydrochloric acid. denatured alcohol and ethyl acetate. It is, of course, understood that other alkyl silicates, such as tetramethylorthosilicate, or other esters of sillcic acid may be used instead of tetraethylorthosilicate and it is also to be understood that other acids instead of hydrochloric acid may be used with equal effectiveness such as nitric acid, sulphuric acid, hydrobromic acid, and the solvents used, denatured alcohol and ethyl acetate, have been selected as being the convenient ones readily commercially available and cheap. Other solvents, however, it has been found, may be used. The sole solvent may be denatured alcohol, for example, but it is found that the ethyl acetate assists in giving a uniform layer. In this particular instance, for example, methyl acetate, methyl alcohol, amyl acetate, isopropylalcohol, or in fact most organic solvents may be used in which both the silicon ester and the acid are both soluble. It is to be understood also that the organo-sol of silica may be separately prepared and purified and used in suitable solvents.

It has been found that other organic solvents than those enumerated may be used to advantage; for example:

Ethylene glycol mono ethyl ether approximately 24%; ethylene glycol mono butyl ether approximately 10%, butyl alcohol approximately 16%, denatured alcohol approximately 40%, tetraethylorthosilicate approximately 5%, and concentrated hydrochloric acid approximately The relative proportions of these various ingredients may be varied in accordance with the method of application as lower speed of rotation during or subsequent to the application to the article to be coated will require a lower concentration of the active ingredient; namely the tetraethylorthosilicate. The high boiling solvents, such as ethylene glycol mono ethyl ether, ethylene glycol mono butyl ether and butyl alcohol may have their proportions varied so as to control the rate of evaporation of the solvent. In the case of high atmospheric humidity a reduction in the amount of the denatured alcohol and its replacement by higher boiling solvents results in a more uniform haze-free coating.

Another composition which may be used advantageously consists of ethylene glycol mono ethyl ether approximately 94% by volume, tetraethylorthosilicate approximately 5% by volume, concentrated hydrochloric acid approximately 1% by volume. This particular composition is particularly resistant to the effects of atmospheric humidity. It has been found also that a composition consisting of approximately 72% denatured alcohol, 7% tetraethylorthosilicate approximately 20% ethylene glycol monoethyl ether and approximately 1% hydrochloric acid gives excellent coatings of high permanence and good efficiency. Substantially 50% of the ethyl alcohol may be replaced by ethyl acetate if desired.

It is believed that there has been produced a 5 micro-porous insoluble abrasion resistant layer of silicon dioxide on the surface of the article, the pores being smaller in dimension than a wave length of light. This micro-porous silica layer has an effective index of refraction due to its porosity of less than 1.4 thus more nearly fulfilling the requirements for a layer whose index of refraction is equal to the square root of the index of refraction of the substrate than do silicon dioxide layers produced by evaporation and the like. The relative proportions under the conditions of use are such as to produce a film of this index and of such thickness that the reflected light from one surface of the layer is approximately one half wave out of phase with the reflected wave from the other surface 0f the layer, or any number of full waves plus one half wave out of phase.

It is to be understood also that there may be incorporated in the solution substances which retain their solubility in suitable solvents after the body of the film itself has been rendered insoluble. Such substances when subsequently leached out by suitable solvent treatment modify the porosity of the surface reflection reducing layer and give an additional control of the resultant effective index of refraction of said layer. It has been found that substances, such as urea, paraphenylenediaminedihydrochloride, glycerine, caffein hydrochloride, magnesium chloride, calcium chloride and in general other compounds soluble in the solvents used in making up the solution and which remain uniformly and minutely dispersed throughout the resulting coating and which after the coating itself has become set and insoluble may subsequently be removed by a solvent such as water leaving, when leached out, a coating which is considerably more microporous than the coating which does not contain the soluble constituents. This gives an additional reduction of the refractive index of the resulting finished coating.

While these coatings in their finished form are of great stability, before the heating operation or before the coatings have stood too long it is possible to remove a coating which has been damaged or which for some reason was defective, by treatment with dilute alkali in which instance the article can be recoated without damage.

.In the solution there is a compound which is capable of being decomposed and depositing a layer of silica, such a compound being tetraethylorthosilicate or other analogous compounds and also in the solution there is a compound capable of decomposing the decomposable silicon compounds, this decomposing agent being the acid referred to above. In the dilute solution, as applied, it is believed that the two reacting materials do not interact rapidly or completely but that as the solvent is removed the concentrations of the decomposable substance and the decomposing agent build up until concentrations are reached at which the reaction takes place rapidly. At this time and not before, the coherent film or layer of silica is deposited on the surface of the article to which the solution has been applied, the film subsequently being hardenedeither by aging or by treatment with moist heat as described above.

In the examples given, hydrochloric acid is the 75 decomposing agent which reacts with tetraethyl- .applied in a single surface. coating.

acre-eel orthosillcate to form silicon dioxide. If these two ingredients are mixed in concentrated form the mixture becomes hot and solidifies in a few minutes. The solvent or solvents used in the disclosed solutions serve to prevent jelling and produce a relatively permanent solution useable for a period of time and one which has suitable evaporation characteristics. The solvent used may be, of course, simple or complex, that is to say, it may consist of a single chemical compound, such as ethyl acetate, alcohol or in fact any solvent in which the reacting substances can be dissolved. The solvent may be a complex solvent consisting of two or more suitable solvents mixed in any desired proportion within the range of workability in which case the relative evaporation rates of the solvents may differ and the process of building up the concentrations of the decomposable substance and the decomposition agent may be further controlled.

When surface coatings of the type set forth herein are applied to materials such as polymeric methyl methacrylate and the like, cellulose acetate, cellulose nitrate, cellulose acetate-butyrate,

polystyrene and other resinous or plastic materials the coating has a considerably higher intrinsic hardness than the inherent surface hardiness of the article itself and therefore imparts an increased resistance to scratching and abrasion. In addition, the inert chemical nature of the resulting coating and its freedom from cracks, and

ing which is a superficial cracking of the surface of a stressed plastic part by exposure to vapors of a solvent or to the solvent itself.

Although we have specified that the articles to be treated are of a transparent nature it is to be understood that any article of opaque or transparent material might be similarly coated; for example, an article of suitable metal might be provided with surface coatings of the above character in which instance the article will be rendered less visible or more resistant to chemical injury and, of course, the same statements apply to opaque glasses, glazes, paints, plastics or in fact any substance crystalline or non-crystalline.

Although thegiven proportions are set forth above, it is to be understood" that the proportions of the various ingredients might be varied in accordance with the procedure followed and in order to produce a thicker coating the content of the silica ester would be increased and to produce tially increased or decreased proportionately. The concentration of the various ingredients varies according to the number of surface coatconcentrations may be formed relatively weak as compared with concentrations which are to be .65 thinner coatings the proportions would be de- "creased with the remaining ingredients substan- .ings applied. If a plurality of surface coatings ,are to be applied, it is to be understood that the 8 The coating compositions set forth above do not depend upon fortuitous decomposition of the decomposable silicon compounds by atmospheric moisture or by moisture or other substances diifusing out of the material to which the coating is applied, but the decomposition agent is present in the solution in controlled concentration and in such a form that until the coating has been concentrated by evaporation after application to the article the reactions do not take place and the solution is completely stable until such time as this evaporation takes place. This is one of the main distinctions of the present invention over known prior art.

The siliceous coatin formed by the above described methods may also serve as the carrier and/or protector of coloring matters, such as dyes, pigments and the like. In general, spirit soluble dyes should be used in suitable quantities to produce the desired tinctorial effect in the finished film and may be dissolved in the solution used in forming such film. Upon evaporation of the solvent and the completion of the hardening of the film the dyestufi or pigment remains in permanent form uniformly dissolved in or scat.- tered throughout the film. Likewise, white finely divided pigments, such as titanium dioxide or other similar white pigments having a high index of refraction may be suspended in the film-form'- ing solution and upon the removal of the solvent and completion of the formation of such film the particles remain therein and serve as light diffusing means. The film containing either the dyes or the pig'- ments may also be used for modifying the color of articles in general, mineral, metallic, or organic, whereby a surface color of high permanence may be applied to the article.

Incorporation of fluorescent compounds, either in solution or as finely divided particles in the coating solution, produce fluorescence when irradiated with radiation of suitable wave length. 'In the prior art, such fluorescent pigments have made use of organic binders, such as lacquers, ,varnishes, etc. which suffer from the serious drawback of limited transparency to the radiation and lack of heat stability and/or chemical stability. The siliceous coating having these fluorescent substances incorporated in it is "obviously free from these drawbacks having "great heat stability and high chemical permanency.

Certain inorganic compounds, such as cerium compounds, possessing the property of absorbing ultra-violet radiation may be incorporated in the original solution and upon evaporation of the solution remain in the siliceous layer.

Certain organic compounds, not generally con- "sidered as dyestuffs, such as diphenyl, quinin, anthracene and naphthalene, for example, may be dissolved in the original solution and produce 'upon evaporation of the solvents films havin ultra-violet absorption.

Coatings of the above character are particularly advantageous when used on articles having relatively smooth or highly polished optical sur- -faces such, for example, as exist on lenses or other optical elements. In the case of lenses 'which have highly polished optical surfaces thereon such surface coatings increase the trans.- mission of light and thereby greatly increase the efficiencyv of said optical elements.

It is to be understood that when applicant prefers, to acids throughout the specification and SEARCH Rom:

in the claims as a general statement, it is intended to mean any acid with the exception of hydrofluoric acid, or to mean an acid which is inert as to its effect on silica.

The compositions of the solutions render it possible to apply said coatings with great ease and with great uniformity and speed without the requirement of special coating equipment and further distinguishes from most prior art coating technique in that the solutions are particularly adaptable to wide area coverage with ease and simplicity and are exceedingly transparent and particularly adaptable for coating optical elements because of the uniformity of coating which may be obtained with such composition and which will have little or no altering effect upon the desired refractive characteristics of said optical elements. In addition, the formation of such a silica film or layer is not dependent upon the chance occurrence in the atmosphere of a decomposing agent which, of course, is not controllable and produces inconsistent results. The solutions of the present invention positively control the decomposing characteristics and provide means for obtaining positive, accurate and rapid results under definite control.

From the foregoing description it will be seen that simple, eflicient and economical means and methods have been provided for acomplishing all of the objects and advantages of the invention.

Having described my invention, I claim:

1. An article of the character described comprising a substrate having a thin transparent surface reflection reducing coating of substantially pure silica formed on the surface thereof, said coating being microporous and consisting of the dried silica product of a tetra-alkylorthosilicate selected from the group consisting of tetra-ethyl-orthosilicate, tetra-methyl-orthosilicate and mixtures thereof hydrolyzed in situ by an acid inert to silica and the substrate, said coating having a substantially uniform optical thickness of an odd multiple of a quarter wave length of the incident light and a substantially uniform effective index of refraction less than that of massive silica, said coating being firmly adherent to the substrate and being resistant to abrasion.

2. An article of the character described comprising a piece of light transmitting material having a thin transparent surface reflection reducing coating of substantially pure silica formed on asurface thereof, said coating being microporous and consisting of the dried silica product of tetra-ethyl-orthosilicate hydrolyzed in situ by an acid inert to silica and the light transmitting material, said coating having a substantially uniform optical thickness of an odd multiple of a quarter wave length of the incident light and a substantially uniform effective index of refraction less than that of massive silica, said coating being firmly adherent to the piece of light transmitting material and being resistant to abrasion.

3. An article of the character described comprising a substrate having a thin transparent surface reflection reducing coating formed on a surface thereof, said coating being microporous and consisting of the dried silica product of a tetra-alkyl-orthosilicate selected from the group consisting of tetraethylorthosilicate, tetramethyl-orthosilicate and mixtures thereof hydrolyzed in situ by an acid inert to silica and the substrate. and containing dispersed there- 10 through inert substances possessing light transmission altering characteristics other than that possessed by the dried silica product, said coating having a substantially uniform optical thickness of an odd multiple of a quarter wave length of the incident light and a substantially uniform effective index of refraction less than that of massive silica, said coating being firmly adherent to the substrate and being resistant to abrasion.

4. The method of producing a surface refiection reducing coating on the surface of an article comprising the steps of applying to said surface a liquid solution consisting of controlled amounts of tetra-alkyl-orthosilicate selected from the group consisting of tetraethylorthosilicate, tetramethylorthosilicate and mixtures thereof and a mineral acid inert to silica as a decomposing agent for the tetra-alkyl-orthosilicate, the tetra-alkyl-orthosilicate being decomposable by said acid to produce silica, said tetra-alkyl-orthosilicate and acid being dissolved in a volatile organic solvent miscible with both, said solvent being selected from the group consisting of mono ethyl ether of ethylene glycol, mono butyl ether of ethylene glycol, ethyl acetate, methyl acetate, amyl acetate, ethyl alcohol, methyl alcohol, isopropyl alcohol, butyl alcohol, and mixtures thereof, the proportion of acid embodying approximately 1 to 10% of the solution, the combined proportions of acid and solvent embodying at least of the solution, and the proportion of tetra-alkyl-orthosilicate embodying substantially the balance of the solution, the tetra-alkyl-orthosilicate being held in the solution in a substantially undecomposed state until the concentration of the tetra-alkylorthosilicate and the acid is increased by evaporation of the solvent sufliciently to permit the acidimdecomposeihe tetra-alkyl orthosillcate, causing the liquid solutiontoform on said surface of the article a liquid layer of a uniform thickness which will contain an amount of tetra-alkyl-orthosilicate and acid sufiicient to produce on decomposition of the tetra-alkyiorthosilicate a. coating of silica having an optical thickness of an odd multiple of a quarter wave length of the incident light, and evaporating the solvent from said liquid layer so as to bring about the decomposition of the tetra-alkylorthosilicate by the acid on the surface of the article and. the production of the silica coating in situ on said surface and to the desired optical thickness.

5. The method of producing a surface reflection reducing coating on the surface of an article comprising the steps of applying to said surface a liquid solution consisting of controlled amounts of tetra-ethyl-orthosilicate and a mineral acid inert to silica as a decomposing agent for the tetra-ethyl-orthosilicate, the tetra-ethyl-orthosilicate being decomposable by said acid to produce silica, said tetra-ethyl-orthosilicate and acid being dissolved in a volatile organic solvent miscible with both, said solvent being selected from the group consisting of mono ethyl ether of ethylene glycol, mono butyl ether of ethylene glycol, ethyl acetate, methyl acetate, amyl acetate, ethyl alcohol, methyl alcohol, isopropyl alcohol, butyl alcohol, and mixtures thereof, the proportion of acid embodying approximately 1 to 10% of the solution, the proportion of tetraethyl-orthosilicate embodying approximately 1 to 10% of the solution, and the proportion of solvent embodying substantially the balance of the solution, the tetra-ethyl-orthosilicate being held in the solution in a substantially undecomposed state until the concentration of the tetraethyl-orthosilicate and the acid is increased by evaporation of the solvent sufficiently to permit the acid to decompose the tetra-ethyl-orthosilicate, causing the liquid solution to form on said surface of the article a liquid layer of a uniform thickness which will contain an amount of tetra-ethyl-orthosilicate and acid sufficient to produce on decomposition of the tetra-ethylorthosilicate a coating of silica having an optical thickness of an odd multiple of a quarter Wave length of the incident light, evaporating the solvent from said liquid layer so as to bring about the decomposition of the tetra-ethyl-orthosilicate by the acid on the surface of the article and the production of the silica coating in situ on said surface and to the desired optical thickness, subjecting the coated article to a moist atmosphere of approximately 100 C. for a period of time sufficient to heat the coated article to said temperature, and then cooling.

6. The method of producing a surface reflection reducing coating on the surface of an article comprising the steps of applying to said surface a liquid solution consisting of controlled amounts of tetra-alkyl-orthosilicate selected from the group consisting of tetraethylorthosilicate, tetramethylorthosilicate and mixtures thereof, a mineral acid inert to silica as a decomposing agent for the tetra-alkyl-orthosilicate and a substantially involatile water soluble substance inert in the presence of the acid, the tetra-alkylorthosilicate, the solvent to be used and the material of the article, the tetra-alkyl-orthosilicate being decomposable by said acid to produce silica, said tetra-alkyl-orthosilicate, acid and water soluble substance being dissolved in a volatile organic solvent miscible with each, said solvent being of the group selected from mono ethyl ether of ethylene glycol, mono butyl ether of ethylene glycol, ethyl acetate, methyl acetate, amyl acetate, ethyl alcohol, methyl alcohol, isopropyl alcohol, butyl alcohol, and mixtures thereof, the proportion of acid embodying approximately 1 to of the solution, the combined proportions of acid and solvent embodying at least 90% of the solution, and the proportion of tetra-alkyl-orthosilicate embodying approximately 1 to 10% of the solution, and the water soluble substance the balance of the solution, the tetra-alkyl-orthosilicate being held in the solution in a substantially undecomposed state until the concentration of the tetra-alkyl-orthosilicate and the acid is increased by evaporation of the solvent sufiiciently to permit the acid to decompose the tetra-alkyl-orthosilicate, causing the liquid solution to form on said surface of the article a liquid layer of a uniform thickness which will contain an amount of tetra-alkylorthosilicate and acid sufficient to produce on decomposition of the tetra-alkyl-orthosilicate a coating of silica having an optical thickness of an odd multiple of a quarter wave length of the incident light, evaporating the solvent from said liquid layer so as to bring about the decomposition of the tetra-alkyl-orthosilicate by the acid on the surface of the article and the production of the silica coating in situ on said surface and to the desired optical thickness, said water soluble substance remaining stable through said evaporation of the solvent and being minutely dispersed through the silica coating produced, and washing said coated article with water so as to dissolve the water soluble substance and 12 remove said substance from the coating thereby increasing its porosity.

7. The method of producing a surface reflection reducing coating on the surface of an article comprising the steps of applying to said surface a liquid solution consisting of controlled amounts of tetra-alkyl-orthosilicate selected from the group consisting of tetraethylorthosilicate, tetramethylorthosilicate and mixtures thereof, a mineral acid inert to silica as a decomposing agent for the tetra-alkyl-orthosilicate and a relatively involatile coloring material substantially inert in the presence of the acid, the tetra-alkyl-orthosilicate, the solvent to be used and the material of the article, the tetra-alkylorthosilicate being decomposable by said acid to produce silica, said coloring material, tetraalkyl-orthosilicate and acid being dissolved in a volatile organic solvent miscible with each, said solvent being of the group selected from mono ethyl ether of ethylene glycol, mono butyl ether of ethylene glycol, ethyl acetate, methyl acetate, amyl acetate, ethyl alcohol, methyl alcohol, isopropyl alcohol, butyl alcohol, and mixtures thereof, the proportion of acid embodying approximately 1 to 10% of the solution, the combined proportions of acid and solvent embodying at least of the solution, the proportion of tetra-alkyl-orthosilicate embodying approximately 1 to 10% of the solution and the coloring material embodying substantially the balance of the solution, the tetra-alkyl-orthosilicate being held in the solution in a substantially undecomposed state until the concentration of the tetraalkyl-orthosilicate and the acid is increased by evaporation of the solvent sufliciently to permit the acid to decompose the tetra-alkyl-orthosilicate, causing the liquid solution to form on said surface of the article a liquid layer of a uniform thickness which will contain an amount of tetra-alkyl-orthosilicate and acid sufficient to produce on decomposition of the tetra-alkylorthosilicate a coating of silica having an optical thickness of an odd multiple of a quarter wave length of the incident light, and evaporating the solvent from said liquid layer so as to bring about the decomposition of the tetra-alkylorthosilicate by the acid on the surface of the article and the production of the silica coating in situ on said surface and to the desired optical thickness, said coloring material remaining stable through said evaporation and being dispersed through the resultant silica coating.

8. The method of producing a surface reflection reducing coating on the surface of an article comprising the steps of applying to said surface a liquid solution consisting of controlled amounts of tetra-alkyl-orthosilicate selected from the group consisting of tetraethylorthosilicate, tetramethylorthosilicate and mixtures thereof, a mineral acid inert to silica as a decomposing agent for the tetra-alkyl-orthosilicate and a relatively involatile material substantially inert in the presence of the acid, the tetra-alkyl-orthosilicate, the solvent to be used and the material of the article, said inert material when dispersed in the resultant coating having light transmission altering characteristics other than that possessed by silica, the tetra-alkyl-orthosilicate being decomposable by said acid to produce silica, said light altering material, tetra-alkyl-orthosilicate and acid being dissolved in a volatile organic solvent miscible with each, said solvent being of the group selected from mono ethyl ether of ethylene glycol, mono butyl ether of ethylene glycol, ethyl acetate,

QQTMSOQH methyl acetate. amyl acetate, ethyl alcohol, methyl alcohol, isopropyl alcohol, butyl alcohol; and mixtures thereof. the proportion of acid embodying approximately 1 to 10% of the solution. the combined proportions of acid and solvent embodying at least 90% of the solution, the propor tion of tetra-alkyl-orthosilicate embodying ap proximately 1 to 10% of the solution and the light altering material embodying substantially the balance of the solution, the tetra-alkyl-orthosilicate being held in the solution in a substantially undecomposed state until the concentration of the tetra-alkyl-orthosilicate and the acid is increased by evaporation of the solvent sufficiently to permit the acid to decompose the tetra-alkyl-orthosilicate, causing the liquid solution to form on said surface of the article a liquid layer of a uniform thickness which will contain an amount of tetra-alky .orthosilicate and acid sufficient to produce on decomposition of the tetra-alkyl-orthosilicate a coating of silica having an optical thickness of an odd multiple of a quarter wave length of the incident light, and evaporating the solvent from said liquid layer so as to bring about the decomposition of the tetra-alkylorthosilicate by the acid on the surface ofthe article and the production of the silica coating in situ on said surface and to the desired optical thickness, said light altering material remaining stable through said evaporation and being dispersed through the resultant silica coating.

9. The method of producing a surface reflection reducing coating on the surface of an article comprising the steps of applying to said surface a liquid solution consisting of by volume approximately 7% of tetra-ethyl-orthosilicate, approximately 1% hydrochloric acid, decomposing agent for the tetra-ethyl-orthosilicate, the tetra-ethylorthosilicate being decomposable by said acid to produce silica, approximately 20% of the mono ethyl ether of ethylene glycol and approximately 72% denatured alcohols, said tetra-ethyl-orthosilicate and acid being dissolved in said organic solvents, the tetra-ethyl-orthosi1icate being held in the solution in a substantially undecomposed state until the concentration of the tetra-ethylorthosilicate and the acid is increased by evaporation of the solvents sufiiciently to permit the acid to decompose the tetra-ethyl-orthosilicate, causing the liquid solution to form on said surface of the article a liquid layer of a uniform thickness which will contain an amount of tetraethyl-orthosilicate and acid sufficient to produce on decomposition of the tetra-ethyl-orthosilicate a coating of silica having an optical thickness of an odd multiple of a quarter wave length of the incident light, and evaporating the solvents from said liquid layer so as to bring about the decomposition of the tetra-ethyl-orthosilicate by the acid on the surface of the article and the produc tion of the silica coating in situ on said surface and to the desired optical thickness.

10. The method of producing a surface reflection reducing coating on the surface of an article comprising the steps of applying to said surface a liquid solution consisting of by volume approximately of tetra-ethyl-orthosilicate, approximately 5% of concentrated hydrochloric acid as a decomposing agent for the tetra-ethyl-orthosilicate, the tetra-ethyl-orthosilicate being decomposable by said acid to produce silica, approximately 24% ethylene glycol mono ethyl ether, approximately 10% ethylene glycol mono butyl ether, approximately 16% butyl alcohol and approximately 40% denatured alcohol, said tetra- SEARCH m 14 ethyl-orthosilicate and acid being dissolved in said organic solvent, the tetra-ethyl-orthosilicate being held in the solution in a substantially undecomposed state until the concentration of the tetra-ethyl-orthosilicate and the acid is increased by evaporation of the solvent sufficiently to permit the acid to decompose the tetra-ethyl-orthosilicate, causing the liquid solution to form on said surface of the article a liquid layer of a uniform thickness which will contain an amount of tetraethyl-orthosilicate and acid sufiicient to produce on decomposition of the tetra-ethyl-orthosilicate a coating of silica having an optical thickness of an odd multiple of a quarter wave length of the incident light, and evaporating the solvent froni said liquid layer so as to bring about the decomposition of the tetra-ethyl-orthosilicate by the acid on the surface of the article and the production of the silica coating in situ on said surface and to the desired optical thickness.

11. The method of producing a surface reflection reducing coating on the surface of an article comprising the steps of applying to said surface a liquid solution consisting by volume of approximately 1 to 10% of tetra-ethyl-orthosilicate and approximately 1 to 10% of a mineral acid inert to silica as a decomposing agent for the tetra-ethylorthosilicate, the tetra-ethyl-orthosilicate being decomposable by said acid to produce silica, said tetra-ethyl-orthosilicate and acid being dissolved in the mono ethyl ether of ethylene glycol, said mono ethyl ether of ethylene glycol embodying substantially the balance of the solution, the tetra-ethyl-orthosilicate being held in the solution in a substantially undecomposed state until the concentration of the tetra-ethyl-orthosilicate and the acid is increased by evaporation of the mono ethyl ether of ethylene glycol from the solution sufiiciently to permit the acid to decompose the tetra-ethyl-orthosilicate, causing the liquid solution to form on said surface of the article a liquid layer of a uniform thickness which will contain an amount of tetra-ethyl-orthosilicate and acid sufficient to produce on decomposition of the tetra-ethyl-orthosilicate a coating of silica having an optical thickness of an odd multiple of a quarter wave length of the incident light, and evaporating the mono ethyl ether of ethylene glycol from said liquid layer so as to bring about the decomposition of the tetra-ethyl-orthosilicate by the acid on the surface of the article and the production of the silica coating in situ on said surface and to the desired optical thickness.

12. The method of producing a surface reflection reducing coating on the surface of an article comprising the steps of applying to said surface a liquid solution consisting by volume of approximately 5% tetra-ethyl-orthosilicate and approximately 1% of a mineral acid inert to silica as a decomposing agent for the tetra-ethyl-orthosilicate, the tetra-ethyl-orthosilicate being decomposable by said acid to produce silica, said tetra-ethyl-orthosilicate and acid being dissolved in the mono ethyl ether of ethylene glycol, said mono ethyl ether of ethylene glycol embodying approximately 94% of the solution, the tetraethyl-orthosilicate being held in the solution in a substantially undecomposed state until the concentration of the tetra-ethyl-orthosilicate and the acid is increased by evaporation of the mono ethyl ether of ethylene glycol from the solution sufficiently to permit the acid to decompose the tetra-ethyl-orthosilicate, causing the liquid solution to form on said surface of the article a liquid layer of a uniform thickness which will contain an amount of tetra-ethyl-orthosilicate and acid suflicient to produce on decomposition of the tetra-ethyl-orthosilicate a coating of silica having an optical thickness of an odd multiple of a quarter wave length of the incident light, and evaporating the mono-ethyl ether of ethylene glycol from said liquid layer so as to bring about the decomposition of the tetra-ethyl-orthosilicate by the acid on the surface of the article and the production of the silica coating in situ on said surface and to the desired optical thickness.

13. An article of the character described comprising a piece of of light transmitting material having a thin transparent surface reflection reducing coating of substantially pure silica formed on a surface thereof, said coating being microporous and consisting of the dried silica product of tetramethyl-orthosilicate hydrolyzed in situ by an acid inert to silica and the light transmitting material, said coating having a substantially uniform optical thickness of an odd multiple of a quarter wave length of the incident light and a substantially uniform effective index of refraction less than that of massive silica, said coating being firmly adherent to the piece of light transmitting material and being resistant to abrasion.

14. An article of the character described comprising a substrate having a thin transparent surface reflection reducing coating formed on a surface thereof, said coating being micro-porous and consisting of the dried silica product of a tetraalkyl-orthosilicate selected from the group consisting of tetra-ethyl-orthosilicate, tetra-methyl- 16 orthosilicate and mixtures thereof hydrolized in situ by an acid inert to silica and the substrate. and containing inert coloring substances dispersed therethrough, said coating having a substantially uniform optical thickness of an odd multiple of a quarter wave length of the incident light and a substantially uniform effective index of refraction less than that of massive silica, said coating being firmly adherent to the substrate and being resistant to abrasion.

HAROLD R. MOULTON.

REFERENCES CITED The following referenlces are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,809,755 King et al June 9, 1931 2,118,898 Price May 31, 1938 2,220,862 Blodgett Nov. 5, 1940 2,329,632 Marsden Sept. 14, 1943 2,347,733 Christensen May 2, 1944 2,356,553 Weissenberg Aug. 22, 1944 2,366,516 Geficken et al Jan. 2, 1945 2,371,611 Dimmick Mar. 20, 1945 2,384,209 Sukumlyn Sept. 14, 1945 2,428,357 Cohen Oct. 7, 1947 2,432,484 Moulton Dec. 9, 1947 FOREIGN PATENTS Number Country Date 538,273 Great Britain 1941

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