February 1950
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Discussion
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The alkyl esters of orthotitanic acid, the tetraalkyl orthotitanates, have been shown to be effective waterproofing agents for a wide variety of materials. The manner or technique of their application does not appear to be critical. Such diverse products as cotton cloth, felt, wood, and suede leather can be given a very marked water-repellent finish without change in the original physical characteristics. To account for the nonspecificity of the alkyl titanates as waterproofing agents, a hypothetical reaction mechanism has been proposed. Space does not permit an elaboration of the evidence to support this theory; however, in simplest terms one may visualize it as a hydrolysis reaction followed by dehydration. Thus, the fiber, wood, or other material treated has on its surface an adsorbed film of moisture which reacts with the watersensitive alkyl titanate to precipitate hydrated orthotitanic acid. On dehydration a t room temperature, this substance gradually changes t o various hydrated forms of titanium dioxide. It is felt that these hydrated oxides of titanium provide the ultimate water-repelling surface. However, because certain evidence has been accumulated in the course of this work which appears inexplicable in the light of such a mechanism, this postulated mechanism should be considered only as a partial explanation of the process described above. The advantages of this method of waterproofing might be itemized as follows: An effective water-repellent finish No drastic alteration in physical characteristics of substance treated-that is, color, texture, or body Unim aired air-porosity of fabrics Dry cfeaning permanence and laundering resistance Asimple method of application A list of its limitations should include: No information as to cost of materials (they are not commercially available a t present)
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Storage difficult because of water-sensitive. nature of alkyl titanates Tendency to yellow and stiffen fabrics where additional body is not desired if applied in excessively large amounts With these relative advantages and disadvantages in mind, a number of potential industrial applications which might be feasible can be visualized: Water-repellent cotton, wool, rayon, and silk fabrics for wearing apparel Waterproof fabrics for sail cloth, tent material, awnings, and other outdoor applications Water-oil selective filter cloth or cartridges Spotproof and water-resistant felt goods such as hats, handbags, and shoes Water-repellent wood for marine and outdoor use Spotproof and water-repellent suede leather goods such as jackets and shoes Weather-resistant finish for statuarv. buildine stone. and similar materials Nonhygroscopic surface for powdered products, etc. "
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All these potential uses must, of course, be predicated on the commercial availability of the titanic acid esters themselves a t a reasonable cost.
Literature Cited (1) Bischoff and Adkins, J. Am. Chem. Soc., 46,256 (1924). (2) Demarcay, Compt. rend., 80,51 (1875). (3) Gardner and Bielouss, Am. Paint and Varnish Mfrs. Assoc.,
Circ. 366,327 (1930). (4)Hancock and Stevens, J. Oil Color Chem. Assoc., 24,293(1941). (5) I. G.Farbenindustrie, A.-G., Brit. Patent 479,470(Feb. 7,1938). (6) Jennings, Wardlaw, and Way, J. Chem. SOC.,1936,637. (7) Krrtitzer, McTaggart, and Winter, Australia Degt. Munitions, Paint Notes, 2,304,348 (1947). (8) Rothrock, U. S. Patent 2,258,718(Oct. 14,1942). (9) Speer, R. J., J.Org. Chem., 14,655 (1949). RECEIVED May 16, 1949.
TITANIUM IN PORCELAIN ENAMELS G. H. Spencer-Strong and Robert F. Patrick Pemco Corporation, Baltimore, M d . *
T
HE commercial utilization of titanium, chiefly in the form of titanium dioxide, as the major opacifying agent in porcelain enamels is a comparatively recent development. This step, taking place within the last 4 or 5 years, constitutes one of the major advances in enamel technology. It makes possible the production of a porcelain enamel cover-coat or finish having extreme covering power, brilliance, and hardness together with a high degree of acid resistance. At the same time, the application weight has been reduced by almost 50%. The titaniaopacified porcelain enamels combine the more desirable properties of several enamel types. Previously, such properties were unavailable in any one material. They have, t o a large extent, eliminated the need for special types or combinations of porcelain
enamels for specific end uses. These factors have resulted in an unusually rapid acceptance of the materials by the industry and have necessitated a highly accelerated research program wherein results, previously requiring many months of investigation and study, have been demanded and obtained in days or weeks. While it was not exceptional for prior art enamels to have an active commercial life of from 5 to 10 years, the fact that the new titania-opacified enamels are often obsolete within 6 months is an indication of the rapid progress being made in this field. This development has given rise t o many problems of an unusual nature, some of which are discussed in the following pages.
Although the superior quaIities of titanium dioxide as an opacification agent have been known for many years its commercial utilization as the major opacifying agent in porcelain enamels is a comparatively recent development. This paper presents a review of the development
of titanium-opacified enamels and discusses in detail their physical properties. Probably the most interesting development in connection with these enamels is their application directly to the metal without the need for an intermediate ground-coat.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Bevelopment s f Titania-Opacified Enamels Although titania-opacified enamels only recently have gained commercial importance] enamel technologists have long been cognizant of the superior opacifying properties that could be obtained by the use of titanium compounds. Patents were granted as long ago as 1907 on the use of titanium compounds as opacifying agents in enamels ( 1 7 ) . At about the same time, it was observed that the addition of titanium also increased the resistance to acid.
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Figure 1. Reflectance us. Application Weight
Although these early investigators obtained enamels having greater reflectance and acid resistance with the use of titanium compounds, they immediately realized the difficulty that was largely responsible for delaying the development of acceptable titania enamels for so many years. These enamels all had a yellowish or brownish color which made them commercially unsatisfactory as white cover-coats. The high cost of titania and its relatively high impurity content at that time also retarded the expansion of the enamels. In 1920, Landrum and Frost (8) developed some titania enamels with better results. They concluded that the discoloration was not directly proportional to the amount of titania present; rather, it seemed to be related to other chemical balances, as yet unknown. Little further work was done until Kinzie and Plunkett ( 7 ) published their study in 1935. They were able to produce a series of stable, white, acid-resistant] antimony-bearing enamels especially designed for titania. However, their results were somewhat discouraging, for they concluded that while opaque enamels could be produced with titania alone, the enamels were colored rather than white. Although few data were published in this period, a considerable amount of research was being carried on in the laboratories of the enamel frit producers, much of the emphasis being centered on the addition of titania a t the mill rather than smelting i t into the frit. The results were, however, not very encouraging. Little published information concerning these enamels became available until Tinsley (16, 16) in 1942 was able to report more favorable results. This work restimulated interest in the possibility of smelting the titania into the frit. Shortly after the war, several commercial enamel frit manufacturers were able to undertake commercial production of white titania-opacified enamels. Since that time, these enamels have been the subject of intensive research with a view to improving their properties. As a result, in the past 3 years a number of papers dealing with the more fundamental considerations involved in the development of titania enamels have been published. Substantial progress has been made, affording a prospect of further advances,
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which will undoubtedly extend the present applications of porcelain enamel finishes.
Properties of Titania-Opacified Enamels Opacification. The original purpose of incorporating titanium compounds in enamel compositions was to increaw the opacity. Superior opacity or hiding povm still remains one of the chief advantages of these enamels. The opacity in enamels containing small crystals is caused by the reflection] refraction, and diffraction of light by these crystals dispersed throughout the glassy matrix ( I ) . In general, the greater the difference between the indexes of refraction of the crystalline and glassy phases, the greater the opacity. The high opacity of titania enamels is due, in part a t least, to the relatively high index of refraction (2.55) of titania. The size, number, shape, and distribution of the crystals are also important factors in determining opacity. Figure I shows curves obtained by plotting reflectance versus application weights for several types of enamels. These curves readily show the advantage afforded by the use of titania, that of higher opacity a t lower application weights. Previously, it required an application weight of 80 grams per square foot to obtain the same hiding power as 30 grams per square foot of titania-opacified enamels. In addition, the reflectance varies less with different application weights than do other enamel types. The use of lighter application weights (thinner coats) not only affords an obvious saving in material but also results in increased resistance of the ware to failure during processing or service. Besides imparting increased reflectance, these enamels have superior hiding power. For example, when white zirconia or antimony-opacified enamels are sprayed over a red base-coat, the spectrophotometer curves show a definite increase in reflection in the red end of the visible spectrum. In other words, the red base-coat has a definite effect on the color of the finished enamel. However, similar curves of white titania-opacified enamels over a red base-coat show an almost negligible rise in the longer wave length region. The development of opacity in these enamels during the firing process is rather interesting. In enamel practice, the materials are first smelted and the resulting glass is quenched. This frit is then milled with suitable electrolytes and floating agents, and sprayed on the metalware. After drying, the ware is fired at temperatures considerably below the smelting range, forming the familiar glassy, hard, chemically-resistant porcelain enamel finish. In the case of titania enamels, the frit as it comes from the smelter is clear, the titania compounds being completely dissolved in the glass. The smelted glass or frit is quenched so rapidly that the titania has no chance to precipitate out of solution. In other words, the frit is supersaturated with titania. However, when the frit is sprayed on the ware and fired, the titania precipitates during the firing operation. It is the small crystals of precipitated titania that give these enamels their great covering power. Most titania enamels have reaectances that range from 80 to 85% at application weights of 25 to 30 gram3 per square foot. The authors were interested in determining approximately the amount of titania remaining in solution after the firing operation. It w8s not found practical to determine the amount of titania remaining in the glass phase by a determination of the index of refraction of the glass phase because of the complex intergrowth of glass and crystals. Therefore, a series of enamel frits were prepared containing 2, 4, 6, 8, 10, 16, and 187, dissolved titania. The compositions were the same except for the titania content. Thin layers of these frits were sprinkled on 18-gage enamel stock and fired for 3 minutes at 1550" F. It was observed that those frits containing 8% or less dissolved titania showed no evidence of crystallization. Those prepared with 10% or more titania
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
February 1950
E
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Figure 2. X-Ray Patterns
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a, Titania opacifisdsnamd b . Titania introduced at the smelter
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showed recrystallization, the opacity increasing with the original amount of titania dissolved in the frit. Thus, approximately 9% titania remains in solution after firing the above enamel under these conditions. The amount of titania crystallizing is primarily dependent upon the composition and the firing treatment. It has been suggested that fluorine acts as a “center of nucleation” for the precipitation of titania, and that the presence of fluorine is necessary for satisfactory opacification. In order to determine the validity of this theory, the authors prepared two series of enamel glasses similar to the frits used in the above experiment. The two series were alike except for the fluorine content. One series contained about 3% fluorine while the other series was prepared without fluorine. The “threshold” value for titania precipitation was the same in both series, indicating that in this frit, a t least, fluorine has no appreciable effect on the initial precipitation of titania during the firing operation. Friedberg, Petersen, and Andrews (6)reported that the titania precipitated either as rutile or anatase, rutile being found in the majority of cases. From x-ray studies on a number of titania enamels, the present authors found that in all cases both anatase and rutile were present, anatase generally being the predominant form. Small amounts of a-quartz have been identified in many of these enamels. One unknown peak present in the x-ray patterns of some titania enamels checks closely with the main peak of cristobalite (devitrification of silica glass). Friedberg and Petersen ( 4 ) also found that enamels with a high anatase content had the greatest reflectance. The form of titania smelted into the batch has no apparent effect on the form of titania precipitated during the firing of the enamels. Figure 2, a, shows a portion of the x-ray pattern of a fired titania-opacified enamel. The major crystalline phase is anatase while rutile constitutes the minor phase. Figure 2, b, represents part of the x-ray pattern of the titania which was smelted into the above frit. This titania consists of rutile plus a smaller amount of anatase. Thus, while rutile was the chief form of titania introduced into the smelter, anatase was the major phase appearing after firing. These x-ray patterns indicate that the form of titania smelted into the enamel frit bears little relation t o the form of titania appearing on firing. X-ray examinations of other titania enamels and sources of titania support this observation. The form (or forms) of titania that precipitates is probably
ENAMEL 6
largely dependent upon tfie smelting and firing conditions, as well as the composition of the glass itself. Color. Closely allied to the opacifying power are the color and color stability of titania-opacified enamels. These enamels all show a characteristic low reflectance or absorption in the blue region of the visible spectrum, as can be seen from Figure 3. They differ from zirconia and antimony enamels in this respect. This absorption is largely due to the fact that both rutile and anatase, as well as titanium-bearing glasses, show a characteristic absorption in the long ultraviolet region (2). The color of titania-bearing enamels is apparently a complex function of the size, shape, type, and distribution of the titania crystals dispersed in the glassy matrix. These are in turn dependent upon such factors as the enamel composition, smelting, grinding, and firing treatments (6). The tendency toward absorption in the blue region of the visible spectrum is somewhat overcome by the tendency of the small titania crystals to scatter the blue wave lengths to a greater extent (8). Marbaker, Saunders, and Baumer (20, 2 2 ) reported that the reflectance, color, gloss, and acid resistance were materially affected by the type clay and electrolyte used in the mill additions. The preparation of colored titania enamels is more difficult than in the case of previous types. It is harder to obtain strong colors by the addition of coloring oxides and stains to the enamel. This effect is generally due either to the strong hiding power of the titania crystals or to a reaction between the titania and the coloring oxides. Equally as important as the color of enamels is that of color stability. It is essential that the color of an enamel remain stable over a rather wide firing range with various application weights. This is necessary in order that normal temperature gradients present in commercial furnaces will not cause appreciable color differences in the finished product. Since titania enamels have very high covering power, they are quite stable with respect t o color over wide ranges of application weights. However, the problem of maintaining color stability with different firing treatments is more complex. Friedberg, Fischer, and Petersen (8) found that increasing the firing temperature increased the size of the opacifying crystals. As the size of the crystals increased, the tendency to scatter the blue wave lengths was decreased, and the color tended t o change from a bluish-white to a, creamy-white. They found that with increased firing temperature, the rutile crystals evidenced a greater increase in size than did the anatase crystals. The change in color in those enamels, containing both rutile and anatase, is attributed to the fact that a t lower temperatures the small anatase crystals predominate, while a t higher temperatures the larger rutile crystals constitute the major phase. They conclude, therefore, that in order to obtain color stability the particle size of the crystalline phase must be rigidly controlled.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Experiments conducted by the authors substantiate some of the observations of Friedberg, Fischer, and Petersen. Data obtained with a General Electric XRD-3 X-Ray Diffraction Unit show that both the anatase and rutile crystals generally increase in size with an increase in firing temperature (in the range of noma1 firing temperatures). The relative crystal size was obtained by using the “half-value breadth” convention-Le., the greater the width of the peak a t half the intensity, the smaller the crystals. I n the enamels studied the rutile crystals increased in size a t a faster rate than the anatase crystals. Comparison of spectrophotometer and x-ray data indicated that, in general, where there is a color variation, the color of titania-opacified enamels changed from a blue-white to a cream color with increased firing temperature. The firing range extended from 50” below to 50’ above the normal maturing temperature for each enamel. Since the size of the crystals increased a t the same time, it would seem that the crystal size plays an important role in determining the color. Experiments showed that the titania crystals in enamels which exhibited superior color stability changed very little in size with increasing temperature. Enamels with poor color stability, conversely, showed large increases in crystal size with increasing firing temperatures. Acid Resistance. The advantage of adding titanium compounds to the enamel batch in order to increase the resistance to acids was recognized many years ago. Small amounts of titanium compounds were used for many years to increase acid resistance before they were generally used as opacifying agents. When substituted for part of the silica, they enhance the acid resistance while simultaneously lowering the firing temperature. The high resistance is due to the high ratio of silica and titania to the other batch constituents. Titania-opacified porcelain enamels, according to the Porcelain Enamel Institute acidresistance test, range from Class B to Class AA, with most of them Class A. Abrasion Resistance. Another property of practical importance is abrasion resistance. Abrasion resistance depends on a number of factors such as hardness, structure, and homogeneity. The bubble structure-Le., the amount, size, and distribution of the bubbles-is an important factor in determining abrasion resistance. The fact that titania enamels generally have fewer bubbles is partly due to the lower proportions of alkalies and fluorine used in their preparation. Since titania enamels have a relatively low bubble structure, they stand up well in the Porcelain Enamel Institute gouge-resistance test and show good resistance to scratching. Their resistance as determined by the Taber abraser is also superior to most other enamels (12). Thermal Shock Resistance. The thermal shock properties are of primary importance to the manufacturers of kitchenware. Previously, the hollow ware producers found it necessary to use special enamels to obtain the desired resistance to thermal shock. However, due to the superior thermal shock resistance of titania-opacified enamels, virtually any of these enamels may be used with satisfactory results. This increased resistance is due chiefly to the decreased thickness of the cover coat with its consequent greater flexibility. The fact that these enamels often have a relatively low coefficient of expansion, thus placing the enamel finish in a state of compression, also contributes to its increased resistance.
Application As viewed above, the chief advantages of titania enamels are their superior opacity or hiding power, coupled with extreme hardness and excellent acid resistance. Thus, these enamels were &st used where their greater opacity enabled the manufacturer to eliminate a second cover-coat, and where their chemical durability made a special acid-resistant coat unnecessary (6).
Titania-opacified enamels are widely used today throughout the industry in such wide applications as stoves, refrigerators, signs, reflectors, architectural products, sanitary ware, and hollow ware.
Vol. 42, No. 2
Probably the most interesting development of these enamels is their application directly to the metal without an intermediate ground-coat. Conventional enameled ware consists of a groundcoat which is fired directly on the ware, over which a cover-coat is then applied and fired. The advantages of a single coat from an economic point of view are obvious. Previously, the chief difficulty in preparing a white one-coat enamel was to obtain a finish free from surface defects and having a satisfactory bond with the base metal. This has been overcome in titania enamels by using special steels and a nickel flash (IS, 14). I n the preparation of one-coat enamels, more care in the preparation of the metal surface prior to the application of the enamel must be exercised. Rigid control of the whole enamel process is also necessary. The thinner coat will naturally disclose many metal defects which ~ o u l dbe hidden by a thicker two-coat application. Lannon (9) reports that ranges coated with these direct-to-steel enamels enjoy an even better record in service than the conventional two-coat enamels. Where there is sufficient bond between the enamel and the metal, a thinner coat is more resistant to failure, since a thinner coat is more elastic than a thicker coat. The single-coat enamels are about 0.005 inch thick. It may be noted that zirconia enamels may also be applied direct to special steel; however, a relatively thick coat must be applied to obtain the requisite opacity. This thicker coat is much more susceptible to failure. Antimony enamels in directto-steel applications have exceptional adherence, but the reaction with the metal gives rise to objectionable spots.
Future Trends While prediction is often risky, several trends in the development of enamel technology are evident. The present trend in titania-opacified enamels is toward lower maturing temperatures, higher opacity a t lower application weights, and increased application of the direct-to-steel finishes. Meanwhile, research on titania enamels, both for one-coat and two-coat applications, is being continued in the laboratories of the commercial frit producers and in several university laboratories. Much research work will be required before many of the problems incident to these enamels can be solved; nevertheless, substantial progress has been made, and the use of titanium compounds has materially improved existing enameled products. The future of these porcelain enamels appears bright and it is believed that they will eventually make possible an extension of the field of application of this finish.
Literatnre Cited Andrews, A. I., “Enamels,” p. 46, Champaign, Ill., Twin City Printing Co., 1935. Boncke, R., Dietzel, A., and Pralow, TV., SprechsaaZ, 74, 372 (1941). Friedberg, A. L., Fischer, R. B., and Petersen, F. A., J. Am. Ceram. Soc., 31, 246 (1948). Friedberg, A. L., and Petersen, F. A., “A Systematic Study of Simple Titanium-Bearing Porcelain Enamels,” paper presented before the Enamel Division of thP American Ceramic Society, Cincinnati, Ohio, April 1949. Friedberg, A. L., Petersen, F. A , and Andrews, A. I., J . Am. Ceram. Soc., 30, 261 (1947). King, B. W., Ceram. I n d . , 52 (2), 66 (1949). Kinzie, C . J., and Plunkett, J. A,, J . Am. Ceram. Soc., 18, 117 (1935). Landrum, R. D., and Frost, L. J.,Ibid., 3.316 (1920). Lannon, J. L., Better Enameling, 19 ( l l ) , 6 (1948). Marbaker, E. E., Saunders, H. S., and Baumer, L. N., “Effect of Some Electrolytes on the Color Value of Enamels Made from a Standard Titania-Opacified Frit ,” paper presented before the Enamel Division of the American Ceramic Society, Cincinnati, Ohio, April 1949. Marbaker, E. E., Saunders, H. S.,and Baumer, L. N., J . Am. Ceram. Soc., 31, 260 (1948). Petersen, F. A., I b i d . , 30, 94 (1947). Stolte, N. H., Enamelist, 25, 19 (1948). Swarta, J. C . , Better Enameling, 19 ( l l ) , 12 (1948). Tinsley, 8.G., Ann. PTOC. Inst. Gitreous Enamellers, 7 , 99-128 (1941-42). Tinsley, S. G., Ceram. Ind., 38, 36 (1942). West, C. J., J . Am. Ceram. Soc., 4 , 47 (1921). R E C E I Y ~September D 26, 1949.
