LEAD TITANATE D. W. ROBERTSON Titanium Pigment Company, Inc., New York, N. Y.
Lead titanate, a new paint pigment, is discussed with reference to some of its optical properties, inertness in vehicles, particle size, and behavior in films under conditions of exposure, in relation to other pigments in common use. Graphic comparisons are made between a lead titanate paint and lead-zinc paint after exposure. Uses are suggested. Lead titanate is distinguished by its ability to absorb ultraviolet light, with a visual brightness above 80 per cent, its chemical stability, high opacity in oil vehicles, and its protective action on organic films to such a marked degree that, in linseed oil, 5 per cent distensibility is retained after an exposure of 2 years at 45" south.
Compared with o r d i n a r y white pigments, lead titanate is very fine; the particles range from about 0.2 to 0.3 micron in diameter. This size is similar to titanium dioxide and less than the average particle size of the composite titanium and zinc sulfide p i g m e n t s . Lead titanate would, t h e r e f o r e , be classed as a. finely divided pigment with a comparatively uniform particle size. The particles show no indication of acicularity and are therefore not in any way similar to some types of zinc oxide or silicates, either in shape or with respect to the wide particle-size range of the latter pigments. Magnesium silicate pigments, for example, range in particle size from a small fraction of a micron to several microns in diameter and in some cases have definitely needle-shaped crystals.
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EAD TITANATE is a new pigment material which is being produced for the first time on a semi-commercial scale. It is made by reacting lead monoxide with titanium dioxide at an elevated temperature, and has the empirical formula PbTiOa, exclusive of a small percentage of impurities (in the form of lead sulfate and lead monoxide).
Physical Properties
Chemical Properties
The specific gravity of the commercial pigment (Titanox-L) is 7.3, with a bulking value of 0.016 gallon per pound. The color is a pale yellow with the following reflection characteristics: Purple-Blue 67.0
Blue-Green 82.7
Green
Yellow
Blue
83 5
84.0
85.2
WATERTANK,NEAR A SULFURIC ACID PLANT,PROTECTED BY A LEADTITANATE PAINT
Lead titanate is chemically inert in practically all types of vehicles commonly used with pigments, and the small percentage of impurities present does not materially change this characteristic in the commercial product. This property of the pigment permits its use without having to take into consideration possible reactions which might cause thickening or livering of paints. The inertness of lead titanate in paint vehicles, along with the high resistance of a lead titanate paint to chalking, offers a t least sufficient reason to doubt the idea that the ability of pigments to form soaps or similar compounds, as a result of their reaction with oil or oleoresinous vehicles, is a major factor in determining the relative durability of paints under conditions of exposure, or that it is necessarily desirable. Lead titanate is by no means a unique example. The industry is familiar with the comparatively great durability of iron oxide and carbon black paints, but lead titanate is distinguished by high brightness in comparison with any other inert pigment of similar behavior in a paint coating. Lead titanate is the only light-colored chemically inert pigment material which absorbs ultraviolet practically 100 per cent. Its behavior in ultraviolet light is therefore parallel t o
In a material of so low a reflection value a t the purple-blue end of the spectrum and a comparatively high value a t the red end of the Bpectrum, the appearance of whiteness even at low brightness is not possible. However, all ordinary tinted paints may be produced with a lead titanate base, with the possible exception of a light blue-gray. The refractive index of lead titanate is extremely highabout 2.7-and its opacifying effect in vehicles is not inconsistent with this property. For example, linseed oil paints made with lead titanate have a hiding power equivalent to about 60 per cent of a similar paint pigmented with titanium dioxide, so that for any practical purpose, in paints, a reduction with a considerable percentage of non-hiding or low-hiding pigments is possible. I By reason of technical difficulties encountered in making this determination, no exact figure can be given.
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the behavior of carbon black, and this parallelism is also apparent with respect to its behavior in oils and oleoresinous vehicles upon exposure. This characteristic is particularly noticeable with respect to the extreme durability of such coatings. Lead titanate also has the property of imparting its characteristics in appreciable degree to mixed pigment combinations where it constitutes 20 per cent or more of the mixture. As might be predicted from its behavior in ultraviolet light as well as its chemical inertness, it has a high protective action upon tints and generally reduces fading of tinted paints to a marked degree in comparison with similar paints made with any other light-colored base pigment.
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The analogy between lead titanate and carbon black continues when comparisons are made with respect to type of film failure upon exposure to t,heweather- i. e., their comparative feedom from chalking, checking, cracking, and flaking.
Comparison with Lead-Zinc Paint These characteristics are well illustrated in Figure 1 where a graphic comparison is made, after exposure, between a paint pigmented with 100 per cent lead titanate and one pigmented with 60 per cent basic-carbonate white lead and 30 per cent zinc oxide with 10 per cent extender. These exposures were made s t 45O south so that the results are considerably accelerated. The combination of white lead, zinc oxide, and extender is used for comparison because it is typical of a large gallonage of exterior house paint, covering sales over a period of the past several years, and also because the behavior of this particular paint is well known to all technologists in the industry. Although the lead-zinc-extender paint started to chalk in 1 month and to crack and flake after 3 or 4 months, reaching complete failure at 14 months, the lead titanate paint did not chalk until 9 months and after 14 months has shown only slight chalking and no checking, cracking, or flaking whatever. This film is still perfectly sound after a 3-year exposure a t 45" south. Jacobsenz has shown some of the essential characteristics of paint films after various periods of exposure. In his work there appeared to be a direct relationship between the retention of the film's distensibility after exposure and the relative durability of the paint. His results indicated that any ordinary paint-for example, one composed of lead, zinc, and extender as indicated in Figure 1-loses all distensibility after exposure of 8 to 11 weeks, vertical exposure south; and a t the time his paper was published, a lead titanate film retained 7.5 per cent distensibility after 78 weeks of exposure at 45" south. For all ordinary types of ppint the rate of loss of distensibility a t 45" exposure is so rapid that it would not be practical to make this kind of film test on paints exposed a t 45' south. Since Jacobsen's work was reDorted, the lead titanate film (applied as a single doat) has reached an age of 2 years a t 45" south, and still retains 5 per cent distensibility. A graphic relationship between a 45' exposure of lead titanate and a vertical exposure of leadand-zinc paint is shown in Figure 2 .
Lead Titanate as a Research Tool
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TITAXATE PAISTIs U ~ E DFOR STHCCTUR9L P R O l E C T I O S
The unique properties of lead titanate, together with the unusual behavior of paints pigmented with it, indicate that, in addition to its practical possibilities, it will be an interesting research tool and may lead to a better fundamental knowledge of those major factors which contribute to the behavior of pigmented films upon exposure to weather. The writer's experience with lead titanate indicates that a considerable amount of popular theory with regard to exterior paint performance must be modified or rejected. For example, the necessity for multiple pigment combinations in order to introduce a wide range of particle sizes and shapes has no place in an explanation of the behavior of a lead titanate paint and probably not of other paints. The idea that cracking a Jacobsen, A. E., Federation of Paint & Varnish Production Clubs, Oficial Digest, May, 1935.
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218 W.L.,
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55
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35 30 25
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FIGURE1. COMPARISON OF PAINTPIGMENTS
is prevented by a distribution of strains in the film as a result of a differential wetting of pigment particles of varying size and composition does not agree well with the observed behavior of lead titanate paints. Past experience has shown that extenders have no value from a durability standpoint when used with such pigments as carbon black and red lead. We have, perhaps, assumed that these cases were not comparable to white paints because of the difference in color. This great difference in color, however, no longer enters into the picture when lead titanate is used, and neither do extenders increase the durability of a paint pigmented with this new material. We have apparently been confused or misled by our observations of the behavior of paints containing a high percentage of extender pigments along with the usual opaque white pigments and assumed a direct relationship between these extender pigments and the organic film, when, as a matter of fact, the effects were apparently due to dilution and consequent reduced activity of pigment materials other than extenders. Inertness and protection of the organic film from the action of ultraviolet light as a consequence of the optical properties of the pigment are apparently two major factors which are a part of the fundamental picture and which contribute to long film life. Notable examples, other than lead titanate, are carbon black and iron oxide. Lead titanate offers a striking demonstration of this conception and also suggests that neither blackness nor extreme fineness are the reasons for the durability of paints made with carbon pigments. I n other words, carbon black has a counterpart in performance in a pigment which has a total brightness ranging above 80 per cent.
FIGURE 2. COMPARATIVE DISTENSIBILITY OF PIGMENTS
Uses Many uses suggest themselves for a pigment with the characteristics of lead titanate; it will undoubtedly find a place in the formulation of exterior house paint tints for the purpose of increasing durability and giving better control of type of failure and tint retention. Its properties and behavior in linseed oil also suggests possible advantages as a pigment constituent for exterior primers on wood. It is also apparent that it will be a useful material for finishing coats on steel structures, such as bridges, gas holders, and other industrial units where long life and protection from corrosion are essential factors. Lead titanate also has rust-inhibitive properties to a marked degree when applied as the first coat on iron and steel. Comparisons with red lead, for example, have been made over a 4year period, and for this period lead titanate appears to be equal to red lead as a rust inhibitor. This period of test is not sufficiently long nor are the tests sufficiently numerous for making an arbitrary statement with respect to this property, but it is, a t least, indicative of some rust-inhibitive value. In enamels and other gloss finishes lead titanate contributes not only to long life but to gloss and color retention. Therefore, we may expect a development of new systems of industrial finishing for automobiles, trucks, oil tanks, railroad equipment, and other surfaces where extreme durability and protection are the major considerations and the necessity for whiteness does not exist. RECEIWDAugust 6 , 1935. Presented before the Division of Paint and Varnish Chemistry at the 90th Meeting of the American Chemical Society, San Francisco, Calif., August 19 to 23,1935.
G A T H E R I NAND Q PREPARINQ RuBBER IN JUNQLE
(?OurbfJsy,Firastons TWO & Rubber Co.
