US2761797A - Method of producing conductive coating on a surface and the coated article - Google Patents

Description

United States Patent O METHOD OF PRODUCING CONDUCTIVE COAT- ING N A SURFACE AND THE COATED AR- TICL David G. Young, North Woodstock, Conn., assiguor to American Optical Company, Southbridge, Mass., a voluntary association of Massachusetts Application March 5, 1951, Serial No. 214,023

2 Claims. (Cl. 117-411) This invention relates to improvements in electrical conductive coatings and relates particularly to the provision of clear transparent, durable and stable coatings of this nature, methods of making and applying said coatings, and the production of articles having such coatings thereon.

One of the principal objects of the invention is to provide electrical conducting coatings adapted to be applied to articles normally susceptible to static charges, and method of making and applying the same whereby substantially no static charge accumulation will take place on said articles.

Another object is to provide coatings of the above character which, while retaining desirable electrical conducting characteristics, may be controlled as to their ultimate thickness whereby the said coatings will introduce desirable light modifying characteristics.

Another object is to provide coatings of the above character which may be applied either in the form of single or multiple layers whereby the electrical conductive characteristics thereof may be varied as desired while still retaining durable, stable, clear and transparent characteristics.

Another object is the provision of a transparent conductive coating of the above character which is conductively efcient after prolonged exposure to moisture and subsequent drying, and after prolonged heating at elevated temperature with high or low relative humidity.

Another object is to provide coatings of the above character which will retain their desired characteristics at elevated temperatures and at low relative humidity and which are resistant to mechanical shock.

Another object is to provide coatings of the above character which may be uniformly applied in a simple, etiicient and economical manner, which will be resistant to abrasion, discoloration, ageing and atmospheric innences.

Another object is to provide coatings of the above character which may be applied to glass, plastics or other materials either transparent or opaque in nature and which will have no detrimental effect upon said materials.

Other objects and advantages of the invention will become apparent from the following description.

ln the drawing:

Figs. l through 5 illustrate in diagrammatic form various embodiments of the invention.

The prior art teaches the use of surface coatings for reducing or increasing reflections of light, for protecting the article from abrasion and the like, or for providing conductivity. However, it is known that reflection reduction coatings must generally be transparent such as in the case of optical elements through which an observer must View. In some cases, a static charge may accumulate on the surface of the articles, which static charge is dispelled very slowly, if at all. This is a detrimental factor when, for example, a sensitive instrument is provided with a transparent cover which may pick up static.

2,761,797 Patented Sept. 4, 1956 Such static, which may be applied to the cover such as, for example, when the cover is dusted or polished, may affect the eiciency of many instruments particularly those of the type embodying sensitive needle-type indicating means. It is also known that some reection reduction coatings do inherently possess an amount of conductivity. However, in many conditions of use, such conductivity is negligible and the static will be dispelled too slowly to be satisfactory. Also, at relatively high temperatures, the conductivity characteristics generally become less effective.

Known conductive coatings of the most desirable and etiicient type were relatively opaque or lacked durability.

The present invention is directed to the provision of coatings of the above character which possess a high degree of transparency, durability and stability and which will retain their conductivity characteristics under a wider range of use than has hitherto been possible and which may or may not possess reection modifying characteristics.

A further advantage of the present invention is that the coatings may be applied in a simple, convenient and inexpensive manner.

The coatings are normally applied to transparent articles having characteristics of accumulating a static charge but may be also applied to articles of an opaque nature possessing similar characteristics. Such articles may be formed of glass, plastics, artificial resinous materials, ceramics or the like. The coatings may be in the form of single coatings or multiple coatings depending upon the light modifying characteristics and electrical conducting characteristics desired.

Coatings of the above character which have been found to produce the results desired may be formed in various ways. One such is illustrated by Fig. l and comprises a single layer.

For example, a solution containing from l to l0 per cent by volume of tetraethylorthosilicate, 1 to l0 per cent by volume of concentrated hydrochloric acid, 0 to 60 per cent by volume of ethyl acetate, and the balance of ethyl alcohol, has been found to provide reflection reduction characteristics and results in a stable water insoluble coating.

Such a coating possesses to a certain extent conductive properties, but these conductive properties deteriorate with increasing time and vary to some extent with the relative humidity of the environment.

In order to provide the coating with conductive characteristics, an ingredient is inserted in the original coating solution, which ingredient is characterized by its solubility in the solvent and by its ability to remain transparent in the finished coating. It has been found that such ingredients are those which are particularly capable of absorbing and retaining moisture at elevated temperatures.

Satisfactory ingredients for this purpose are lithium chloride and other lithium salts such as acetate, sulphate, nitrate, etc.

A preferred solution for forming a transparent conductive reection reduction coating comprises approximately l0 parts of tetraethylorthosilicate, 42.5 parts of ethyl acetate, 42.5 parts of denatured or ethyl alcohol, 5 parts of concentrated hydrochloric acid, to which stock solution is added from 0.1% to 0.5% by weight of lithium stearate. Amounts of lithium stearate greater than 0.5% may be used where transparency is not essential but when used for coatings on transparent articles, this is not desirable since the coating tends to become cloudy. ,l

The foregoing formula gives a clean, colorless', stable, clear, transparent solution with the lithium steaate being soluble therein. The article to be coated may be immersed in the solution, removed and immediately spun to remove excess liquid. The resultant thickness is controlled at the time of spinning so that if eflicient reflection reduction characteristics, for instance, are desired the thickness will preferably be uniform and of an odd multiple of a quarter wave length of the incident light. Where reflection reduction is not essential the coatings may be thicker and may be formed as single or multiple layers. However, for best results the thickness should be sutiicient to introduce the required conductivity. Coatings over one-half wave length in thickness have not generally been found to be practical. The coated article is then heated slightly to accelerate the insolubilization of the coating, the temperature depending upon the heat resistance of the article, lower temperatures requiring longer heating cycles to obtain the desired results. The article is then subsequently cooled. This procedure renders the coating insoluble in acids or water and diflicultly soluble in alkaline solutions.

lf the above coating is placed on a glass article wherein the glass has an index of refraction of 1.52, the coating will reflect only about 2% as compared to approximately 4% reflection without the coating.

The conductivity of the coating, due to the inclusion of thc conductive material or hygroscopic material, is such that inclusion of only 0.1% of lithium stearate or lithium chloride, for example, will cause an electroscopc to discharge almost instantly. Even traces of the most efficient of the added ingredients will cause an electroscope to discharge but at a slower rate. Greater amounts of the added ingredient are even more conductively eficient for the presently described reflection reduction coating up to 5%, depending on the particular ingredient used. More than 5% will cause the coating to become cloudy, as stated hereinbefore, and, therefore, certain desired optical characteristics are lost.

Slight variations in the foregoing coating process may be employed with satisfactory results. For example, after spinning, the article may be held at room temperature in normally moist air for about one minute and then subjected to the action of water for any desired period of time. This results in a good reflection reduction coating and gives a somewhat more porous surface. However, the coated article should be subsequently heated.

ln the original solution mentioned above, other alkyl silicates such as tetramethylorthosilicate, or other esters of silicic acid may be used instead of tetraethylorthosilicate; also other acids such as nitric, sulphuric, hydrouosilic, uoboric, hydrouoric and hydrobromic may be substituted for the hydrochloric acid. While acids are desirable in certain cases where acidity may be undesirable, the acids may be completely eliminated and in such cases the coating-producing solution should be aged for several days. The solvents, denatured alcohol and ethyl acetate may be replaced by most organic solvents in which the silicon ester and the acid are both soluble, such as methyl acetate, methyl alcohol, and isopropyl-alcohol. Another acceptable formula is approximately 24% of ethylene glycol mono ethyl ether, of ethylene glycol mono butyl ether, 16% of butyl alcohol, 34.9% of denatured alcohol, 10% of tetraethylorthosilicate, 5% of concentrated hydrochloric acid and .1% of lithium stearate or other selected ingredient for providing increased conductivity.

lt is desirable to use conductivity increasing materials which will maintain their effectiveness at relatively high temperatures and varying humidities.

It is to be understood that substances may be included in the solution which may be subsequently leached out by suitable solvent treatment to modify the porosity of the coating. Such substances, for example, are urea, glycerine, calfein hydrochloride, calcium chloride and many others which are soluble in the solvents used in making up the solution and which remain uniformly and minutely dispersed throughout the resultant coating and which may be subsequently removed by another solvent such as water. It must be understood, however, that such leaching out must be controlled so as not to render uncontinuous the conductive chain formed by the uniformly and minutely dispersed ingredient which is included for providing the desired conductivity.

Such a coating as described above has a high intrinsic hardness and, therefore, is resistant to abrasion. This is particularly advantageous when used on plastic and resinous materials which are relatively soft.

A desirable reflection increasing coating such as illustratcd in Fig. 2 can be formed from a solution made as follows: To 75 parts of 190 proof ethyl alcohol are added slowly and with constant stirring 25 parts of titanium tetra-chloride. The reaction is rather violent and copious white fumes are evolved. There results a pale yellowish colored liquid which no longer fumes but which is rather acid and should be stored in glass. This is a master solution which upon suitable dilution will produce the results desired. To obtain surfaces of high reflectivity, the solution is diluted with an equal part of the alcohol. To this solution, in order to obtain the desired conductive properties, is added from .1% to 5% of lithium stearato or other selected ingredient to render the resulting coating increasingly conductive.

Application of this solution to an article is by dipping, immer-sing, and subsequent spinning or allowing to drain, or similar methods, the solvents being evaporated from the solution, leaving upon the surface of the article a uniformly thick coating of the nature desired. Following this, the article is baked from 50 to 200 C. or more and results in a coating which, for example, on 1.523 index glass produces reflections of the order of 15% where the reflectivity of the glass is normally about 4%.

The thickness of this coating is controlled by varying the concentration of the active material in the master solution, by varying the dilution of the master solution, or by varying the rate of withdrawal of the article from the solution after immersion, or by varying the speed of rotation during spinning. Surface coatings of the nature described resulting from the above solution or modifications thereof are extremely permanent and resistant to abrasion, weathering, and chemicals.

A coating such as the alcohol-titanium coating described immediately above may, if desired, be applied over a coating such as the siliceous coating described hereinbefore. This as illustrated in Fig. 3 provides avery efficient reflection increasing coating having good conductivity, permanence, clear transparency, and abrasion rcsistance.

In some cases, if it is desired to produce a conducting coating whose low reflection properties are greater than those produced by using a solution of tetraethylorthosilicate alone, it is possible to deposit a low index layer in superimposition with the high index layer as illustrated in Fig. 4. This can be done by applying a high index layer, such as the titanium oxide layer, which may or may not contain the materials for increasing conductivity, of approximately 1A wave length thickness directly on the substrate and then applying to this high index layer, a siliceous layer containing materials for increasing the electrical conductivity of a thickness of substantially 1A wave length, the two coatings functioning cooperatively to reduce the reflection of light to a very marked degree.

The thickness of the high index undercoat is not as critical as the thickness of the low index outer coat but by suitable dilution of the solution used in producing the outer coat and observation of the color of the reflected light a few trials will allow the adjustment of the outer solution to produce the most effective coating which for visible light is that whose reflected color at normal incidence is purple. A coating which is too thin is reddish and a coating which is too thick appears blue.

This coating should also be baked as described above.

It has been found that for articles whose index of refraction is 1.52 or more for maximum efiiciency the index of refraction of the high index undercoating should be lower than that of the articles. This is accomplished by mixing the two solutions in varying proportions, the coating resulting from such mixtures being lower in index as the proportion of the tetraethylorthosilicate in the resulting solution is increased. For example, a mixture containing 75 parts of ethyl alcohol, 3 parts of titanium tetra chloride, 18 parts of ethyl acetate, 2 parts of tetraethylorthosilicate and 2 parts of hydrochloric acid, when applied to glass in such a manner as to produce a coating substantially 1A wave length in thickness after baking has an index of refraction lower than that produced by the solution of titanium tetrachloride alone. Upon the subsequent application on this coating of the low index layer resulting from a solution containing the tetraethylorthosilicate and having the thickness of 1/4 wave length substantially the same reflectivity is obtained for materials having indices of refraction which may vary from 1.52 to 1.70.

By applying these coatings in the reversed order (all of the coatings or the outer coating alone having the conductivity-increasing materials incorporated therein), surfaces having increasing reilectivity over that produced by the single high index layer alone may be produced.

By repeating the process of forming alternate low and high index coatings of the proper thickness the amount of reflectance may be increased. Thus, it is possible, by building up coating of alternating low index and high index, to obtain very high reflectances, of the order of 90% for a particular wave length. For example, using a coating consisting of eight layers alternating low index and high index as illustrated in Fig. 5 it has been possible to produce a glass article having a reflectance of over 90% at a wavelength of 500 millimicrons. The actual location of the maximum will depend upon two factors, the physical thickness of each coat and within limits the index of refraction of each coat, the controlling factor being the optical thickness which is the thickness divided by the index of refraction of the resulting layer.

Any of the aforementioned types of solutions may serve as a carrier for a coloring agent if desired.

An article having a conductive coating of the character described hereinbefore when mounted so as to have a lead or ground in engagement therewith, Will be found to be entirely free of static charges. Any static which is normally accumulative on the surfaces of the article in the absence of any conductive coating will be promptly conducted away.

The conductivity of the coating should be such that when the coating is grounded a static charge will be dispelled almost instantly.

It is also to be understood that although the carrier solutions mentioned hereinbefore are for forming coatings which are capable of modifying light reflections from the surfaces of the articles to which they are applied, any available coating solutions can be used which contain a solvent in which the added ingredient is soluble and wherein the added ingredient may be substantially uniformly dispersed. However, in accordance with this invention the added ingredient preferably should possess clear transparent characteristics which will not affect the light transmission and reflection characteristics of the original solutions. The added ingredient should also be capable of conducting at relatively high temperatures and in varying humidities as described.

lt is further to be understood that although the articles referred to hereinbefore on which the conductive coatings are applied have been formed of dielectric materialsin general, metals or synthetic minerals such as mica, quartz, and in general materials of extremely low electrical conductivity can be provided with the presently described coatings. In certain cases metals having dielectric coatings thereon, for example, lacquers, enamels and anodic coatings such as aluminum oxide on aluminum articles by chemical and electrical treatments may also be advantageously provided with conductive coatings of the character described.

In providing conductive coatings wherein light modifying characteristics are unimportant, critical control of the thicknesses of the coatings is unnecessary, it being necessary that the thickness be merely suicient to introduce the conductivity required and controlled so that the coatings will tenaciously adhere to the surfaces 'on which they are applied.

From the foregoing description, it will be seen that all of the objects and advantages of the invention have been accomplished.

While the novel features of the invention have been described and are pointed out in the annexed claims, it will be understood that various changes in the formulae and other details of the invention can be made without departing from the scope of the invention, and it is intended therefore that all matter contained in the foregoing description be interpreted as illustrative and not in a limiting sense.

Iclaim:

l. A coated article of the character described comprising a substrate having a normally static chargeable `surface and a thin transparent, durable, stable electrical r`charge conductive coating thereon, said coating being formed of a material selected from the group consisting of silicon dioxide, titanium oxide, and mixtures thereof .having substantially uniformly dispersed therein from one to two-tenths of one per cent lithium chloride, said coating being of a porous nature such as to more readily permit moisture vapor to gain access to said dispersed lithium chloride so that, when properly grounded, the coating will be increasingly conductive to electrical charges, said coating further being resistant to abrasion, shock, discoloration and aging under varying heat and atmospheric influence.

2. The method of producing a conductive coating on the surface of -an article comprising the steps of apply- ,ing to said surface a liquid solution embodying from about 1 to 10 per cent by Volume of tetraethylortho- 1silicate, about .11 to 10 per cent by volume of concentrated hydrochloric acid, about one to two-tenths of one per cent lithium chloride, and the balance a readily volatile, water miscible organic solvent, causing the liquid solution to form on said surface of the article a liquid layer of a substantially uniform thickness, and evaporating the solvent from said liquid layer whereby the resultant layer will be of a porous nature and will have uniformly dispersed therein lithium chloride, the proportion of said lithium chloride being so controlled that its presence in the coating formed will substantially unatfect the light transmission characteristics thereof.

References Cited in the file of this patent UNITED STATES PATENTS 1,570,077 Pitman Ian. 19, 1926 2,327,460 Rugeley Aug. 24, 1943 2,346,483 Goss Apr. 11, 1944 2,366,516 Geffecken et al. Jan. 2, 1945 2,412,496 Dimmick Dec. 10, 1946 2,428,357 Cohen et al. Oct. 7, 1947 r 2,466,119 Moulton et al. Apr. 5, 1949 6') 2,478,385 Geist-.r Aug. 9, 1949 2,519,722 Turner Aug. 22, 1950 2,522,531 Mochel Sept. 19, 1950 2,601,124 Moulton June 17, 1952 FOREIGN PATENTS 473,943 Great Brita-in Oct. 22, 1937

Claims (1)

1. A COATED ARTICLE OF THE CHARACTER DESCRIBED COMPRISING A SUBSTRATE HAVING A NORMALLY STATIS CHARGEABLE SURFACE AND A THIN TRANSPARENT, DURABLE, STABLE ELECTRICAL CHARGE CONDUCTIVE COATING THEREON, SAID COATING BEING FORMED OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF SILICON DIOXIDE, TITANIUM OXIDE, AND MIXTURES THEREOF HAVING SUBSTANTIALLY UNIFORMLY DISPERSED THEREIN FROM ONE TO TWO-TENTHS OF ONE PER CENT LITHIUM CHLORIDE, SAID COATING BEING OF A POROUS NATURE SUCH AS TO MORE READILY PERMIT MOISTURE VAPOR TO GAIN ACCESS TO SAID DISPERSED LITHIUM CHLORIDE SO THAT, WHEN PROPERLY GROUNDED, THE COATING WILL BE INCREASINGLY CONDUCTIVE TO ELECTRICAL CHARGES, SAID COATING FURTHER BEING RESISTANT TO ABRASION, SHOCK, DISCOLORATING AND AGING UNDER VARYING HEAT AND ATMOSPHERIC INFLUENCE.

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