TITANIUM DIOXIDE PIGMENTS Correlation between Photochemical Reactivity and Chalking A. E. JACOBSEN Titunium Division, National Lead Company, Sayrevilk, N. J . APPARATUS
Chalking behavior of titanium dioxide pigments can be attributed to a cyclic oxidation-reduction reaction involving titanium dioxide and the vehicle. Photoreduction takes place with oxidizable material; mandelic acid is a satisfactory one €or the study of photochemical reactivity of titanium dioxide. Direct correlation has been found between photochemical reactivity and chalking behavior of both anatase and rutile type pigments in alkyd type paint. A photochemically reduced rutile titanium dioxide has been identified as a-TiaOa. Reduced titanium dioxide from photoreduction is readily reoxidized in the presence of air and absence of actinic rays. The fading of tinted paints when the tinted material is inert is due to masking of this material by the chalk. In the case of oxidizable tinted material, fading may be attributed to a photoreduction-oxidation cycle involving titanium dioxide and the tinting material.
Several sources of actinic rays were used-carbon arc, highintensity ultraviolet mercury arc lamp, General Electric 5 1 type sun lamp, and direct sunlight. While these all caused photoreaction, as observed by discoloration of titanium dioxideglycerol mixtures, the s-1sun lamp was found to be satisfactory and convenient, and was adopted as standard source. The rotating table uscd in the laboratory exposure test was 22 inches in diameter and located 29 inches below the S 1 sun lamp. The samples were placed on the outer edge, and the exposure timc was varied in accordance with the resistance of the samples to the photoreduced. The apparent reflectance readings wcre made with a Hunter multipurpose reflectometer. IDENTIFICATION OF END PRODUCTS
Although Renz attributed the characteristic blue discoloration of exposed titanium dioxide-media mixtures to the formation of a lower oxide, he did not completely identify the reaction products except to report carbon dioxide formation when glycerol was the medium. This investigation waa extended to study the end products. It was found that the end mixture resulting from the photoreaction between titanium dioxide and mandelic acid contained considerable benzaldehyde which was identified qualitatively by the phenylhydrazine test. To confirm that this oxidation is not the result of direct action of actinic rays on mandelic acid, a comparative test was carried out with silica in place of titanium dioxide. The results were negative. To confirm that the reduction product ww a lower oxide, an electron &Traction pattern of a photochemically reduced oxide was taken. A rutile titanium dioxide, obtained from the pigment process preceding the final finishing, had to be used instead of a finished pigment because of technical difficulties encountered in taking t,he diffraction pattern. Glycerol was the reacting medium. The reduced oxide was identified from the A.S.T.M. x-ray card index ( I ) as a-TizOJ.
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HE characteristic free chalking of early titanium dioxide paints is well known to the paint industry (6, 6). Today
“controlled” chalking of paints is favored, and to meet these requirements, several grades of titanium dioxide (considering anatase and rutile types collectively) are being offered to the paint industry; an important difference among the grades is their relative chalking resistance (2, 8, I O ) . This paper does not int,end to discuss the question of paint formulation and the extent to which a surface should chalk for good general appearance, but rather to present evidence that the chalking and fading behavior of titanium dioxide is due to photochemical reactivity. Several explanations for the mechanics of the chalking phenomenon have been offered. One is that selective adsorption by the titanium dioxide of the vehicle decomposition products during weathering causes noncornpatibilit,y between pigment and binder, which results in the formation of a powdered or chalked surface ( 5 ) . Another is that titanium dioxide acts as a. photosensitizer for accelerating normal oxidation of the vehicle (3, 4). Still another suggests the format,ion of pertitanic acid which, in turn, oxidizes the vehicle (9). These hypotheses do not appear to explain satisfactorily the mechanism responsible for the chalking behavior of titanium dioxide paints. On the other hand, the observations by Rena ( 7 ) that titanium dioxide, when subjected to actinic rays in the presence of glycerol or other oxidizable media, is photoreduced to a lower oxide (discoloring the paste) and that the discoloration disappears with removal of the rays or in the presence of air, are of fundamental significance, since they show that titanium dioxide is a photochemically reactive material. Experiments were made which confirm R,enz’ observations. From bhese a photochemical reaction test was developed, which gives results correlating with the outdoor chalking behavior of various grades of titanium dioxide pigments. This laboratory test depends essentially upon the photochemical reduetion of titanium dioxide to a lower oxide with simultaneous oxidation of the organic medium; a suitable medium was found to he mandelic acid. Since the relative results from these test correlate with relative outdoor chalking behavior of the pigment’s, it appears that the chalking phenomenon rriay also be largely attributed to photochemical reactions involving pigment and vehicle.
PHOTOREACTION IN VARIOUS M E D I A
T o study the rate of photochemical reduction of titanium dioxide when mixed with Oxidizable materials, the relative change in initial apparent reflectance (discoloration of paste) was plotted against time of exposure under the standard actinic light source. Many of the media referred to by Renz were employed-namely, glycerol, tartaric acid, mandelic acid, and stannous chloride (the latter three in solution). Other reactants included dyes in solution, linseed oil, paraffin oil, and the higher alcohols. Discoloration studies were also made on air-dried alkyd vehicle paints containing 18y0titanium dioxide by volume. The general procedure used for studying the relative rates of photoreduction of titanium dioxide using the several media was to place the smooth, semistiff mixes between thin, uniformly thick, noncorroslve, microscopic glass plates, approximately 5 X 5 inches in size and 0.04 inch thick. An initial apparent reflectance reading was made of the mix through the glass on the side to be exposed and then placed on the rotating table under the S-1 sun lamp. Readings on the exposed area were made at frequent intervals, and the change in apparent reflectance was,
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Reflecknee readings were made as soon after removal from the act,inicrays as possible and several times thereafter. The results lyhich show gradual decrease in discoloration until the reflectance is close to its original apparent reflectance appear graphically in Figure 2 . X repetition of the procedure showed that the panels discolored to t,he same degree upoii re-exposure and returned to their original rcflcctancc in the dark.
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PROCEDURE WITH MAKDELIC ACID
D€CffERSE FROM INI77AL
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Change in Reflectance with Various Media
S t a n n o u s chloride 2, Glycerol 3. Mandelic acid 4. Tartaric acid 5. Octyl alcohol 1.
6. 7. 8. 9.
Mineral oil Alkyd paint (glycerol) AA linseed oil Alkyd paint
recorded. I n the case of the paint panels one of a pair was exposed directly to the actinic rays; the other was covered with a layer of glycerol and then with a noncorrosive glass plate. The air between the glass plate was thereby completely displaced by glycerol before the panel was exposed to actinic rays.
Since the organic media arc more closcly related to the paint vehicles than stannous chloride, only thrse wcre considered for the laboratory test. Glycerol produced the fastest reaction, but other factors, such as m-orliability of the paste, made it desirable to select another. Dyes wbre unsatisfactory because of their fugitive nature. Alkyd vehicle iiivolved t'he making of paints. Mandelic acid seemed to be the most satisfactory of the media studied, arid Ihe following procedure was developed correlating relative photochemical activity of titanium dioxidemandelic acid mixtures with outdoor chalking behavior : Approximately 10 grams of pigment were made into a soft paste with 0.5 M mandelic acid solution by working the misture with a spatula on a smooth glass plate. The paste was placed between noncorrosive glass plates, 5 X 5 X 0.04 inch, and distributed by the application of pressure t o cover an area approsimately 4 inches in diameter. The cdgcs of the plates were bound with 3/*-inch cellulose adhesive tape t o prevent' drying of the paste. An initial apparent reflectance reading was made and recorded. The slide was then subjected t o actinic rays by placing it on the out,er edge of the rotating table below the S-1 sun lamp. After suitable time intervals addit'ional readings were made, and the exposure was continued to obtain a series of readings. Discoloration was plotted against time on semilog paper to obtain a graphic represent'ation of the changes which occurred (Figure I). It is customary to employ a pigment of known photoreduction behavior as a control, t o permit slight corrections due to any variations in the intensity of the light source with age of the lamp.
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The following table gives figures for mandelic a$d as medium whicli are typical of the method used for the determination of change in reflectance with exposure; Figure 1 gives a graphical representation of this case: Decrease, Apparent Time, % Min. Reflectance, %
Apparent Time, Min. Reflectance,
70
Decrease,
%
Figure 1 also shorn the change in appa,rent reflectance with h i e of exposure for the various ot'her media, except the dye solut,ions. Silice the same titanium dioxide was used in all cases, the rate of photoreaction, as measured by decrease in apparent reflectance, varied with the medium. The media arranged in decreasing order of reactivity are: aqueous solution of stannous chloride, glycerol, aqueous solut,ion of mandelic acid, octyl alcohol, mineral oil, alkyd vehicle paint protected from air by glycerol, -4A varnish oil (acid No. 2-4); and alkyd vehicle paint exposed to air. REVERSAL OF PHOTOREDUCED TITANIUM DIOXIDE
Since all the mixtures as well as paint panels which had diecolored on exposure showed a gradual ret,urn to their original color after removal from the actinic rays, the reversal rate was studied for the paints when placed in the dark. I n the ca*e of the glycerol-coated paint,, the glass slide was removed and the glycerol washed off.
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DLCX'fASE fROM mI7Ia.L REfL .€CTANCE Figure 3. Change in Reflectance Curves Due to Photoreduction
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CORRELATION BETWEEN LABORATORY AND OUTDOOR T E S T S
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ance is plotted 1ogai.ithrnically against the outdoor chalking resistance of the pigments. In addition to the pigments shown in Figure 4 (marked X), this curve includes data obtained with pigments to be describcd later, Outdoor-exposure chalking values were based on the average of a number of exposures a t Miami, Fla., as shown by data for pigments A, B, C, D, and E of Table I. Shortly after the original studies were completed, high-strength rutile pigments which show variations in chalk behavior were introduced to the paint industry. Figure 4 indicates that a correlation also exists between photachemical reactivity, as described by the laboratory tests, and outdoor chhlking. These new experimental points (marked 0) lie close to the general curve. The chalking values were also based on the average of a number of exposures a t Miami (Table I, pigments F through K). DISCUSSION
The correlation between the outdoor chalking test and the photoreduction test can be explained in part on the basis that both tests involve photochemical reactions; in them the titanium dioxide oxidizes the medium in proximity with it and is itself being reduced to a lower oxide. The extent to which the reactions in both tests will progress depends upon the photoreactivity of the titanium dioxide, the resistance of the medium to oxidation, and the wave length and intensity of the impinging rays. Light reflectance studies (not reported here) indicate that both anatase and rutile varieties absorb ultraviolet light strongly and this property can be considered responsible for their photochemical reactivity. Thus far the discussion has considered only the photoreaction involving the reduction of titanium dioxide and oxidation of the medium. However, reference was made earlier to the observations of Renz that a gradual disappearance of discoloration of titanium dioxide mixtures occurred with discontinuance of the actinic rays on the exposed area. Even after marked discoloration of the major portion of titanium dioxide mixtures, areas existed around the edges of the paste (in contact with air) which were free from discoloration. The same observations were made in this work, and particular attention has been called to the sradual increase in aaoarent reflectance in the dark of the discolored &d paint TABLE I. WEEKSOF EXPOSURE FOR "CONSIDERABLE" CHALKING panels. Pigment A B C D E F G H I J K It can be assumed that, for paints exposed outJanuary 10 13 19 22 41 . . . . . . 27 38 ...... doors, the same type of photoreduction and reoxiFebruary 6 12 16 20 31 6 . . . 27 40 ...... dation occurs as was observed in the laboratory. March 4 . . . 18 17 36 ... 12 28 46 13 42 April 6 10 ... 18 36 . . . 12 . . . . . . 18 42 These observations further indicate that a photoMay 18 37 ... I1 . . . .39. . 13 14 44 chemical reduct,ion and reoxidation of the titanium June .7 . . .. .. .. .. .. .. 22 36 . . . 11 28 38 July . . . . . . . . . 19 ... 7 15 32 39 13 40 dioxide in air may proceed simultaneously and August . . . . . . . . . . . . 37 15 34 50 20 35 September 5 11 18 26 38 16' 15 32 50 19 42 that the process may be considered as a cyclic October 5 14 20 26 35 .... . . 15 35 43 17 ... oxidation-reduction phenomenon. In actual pracNovember 8 15 18 21 36 15 33 43 24 39 December 8 17 17 25 47 . . . . . . 29 39 18 29 tice it is reasonable to expect that during t'he life Average 6 . 6 1 3 . 1 18.0 2 1 . 3 3 7 . 1 7 . 7 13.4 3 0 . 5 4 3 . 3 16.9 ,39.0 of a paint such cyclic reactions take place innumerable times during weathering.
Figure 3 shows characteristic curves obtained for a series of titanium dioxide pigments with different chalk resistances. In general, the discoloration-time relations do not give straight lines for all pigments. For some there is an induction period during which the pigment resists photoreduction, followed by a noticeable increase in the rate of reaction. Therefore, if such information were to be correlated with outdoor chalking data, it was necessary to select a point on each of the discolorationtime curves which could be determined with reasonable accuracy and yet not carried too far beyond the induction period. The point corresponding to interceptions of these curves and the ordinate erected at 6% change in initial apparent reflectance was chosen for correlation with outdoor exposure. Comparisons were made on the basis of weeks of exposure of the paint panels to develop a "considerable" degree of chalking. (Medium-oillength fatty acid-modified alkyd paints were used, containing 18% pigment by volume. The paints were tinted with carbon black to give an apparent reflectance of about 35%. The panels were baked at 125" C. for 2 hours.) Figure 4 shows such a correlation in which the exposure time in minutes necessary to produce a 6% decrease in apparent reflect-
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Chalkin!: of titanium oxide-bearing paint films may be cxplained more specifically as a breakdown of the film caused by disintegration of t,he organic binder due to t,he cyclic reduction and oxidation of titanium compounds present in t,he paint film. Partial reduction of titanium dioxide in the presence of t,he organic binder is a photochemical reaction taking place in daylight (particularly in sunlight). Oxygen made available by the reaction oxidizes t>he complex organic compounds to simpler ones, such as soluble acids and carbon dioxide. The unstable, reduced titanium oxide then reverts to the dioxidc by reaction with air, while the organic reaction cannot be reversed. By this process the continuity of the film is gradually dcstroyed so that moist'ure can enter and leach out soluble constituents. This gives a film with unprotected surface pigment which, when removed by mechanical abrasion, gives a po.i?-der kn0a.n 3s chalk. The rate of chalking, of course, depends upon the reactivity of the titanium dioxide, the nature of the vehicle employed in the paint, and the severity of exposure conditions. While this paper has been essentially concerned with the phenomenon of chalking, fading should also be mentioned, since this also is essentially the result of phot>ochemicalreaction. I n the ease of the inert type of t,inting material, fading is due to this material being masked by the titanium dioxide chalk. On the other hand, the accelerated fading of paints when the tinting mat,erial is susceptible to oxidation can be explained on the basis previously given for decomposition of an oxidizable vehicle-that is, through a photoreduction and oxidation cycle involving bitanium dioxide and the tinting material.
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Unfortunately the reactions which take place in the weathering of a paint, or in the process of fading are complicated, but i t should he possible when all of the end products have been identified t o set up complete reactions on the basis of the photochemical theory. ACKNOWLEDGMENT
The author wishes to acknowledge the valuable assistance given by C. A. Kumins (now associated with the Interchemical Corporation) in part of this work. LITERATURE CITED
din. Soc. for Testing Materials (card index), X-Ray Diffraction for Chemical Analysis, 1941. Anderson, W. B., OAicial Digest Federation Paint & Vamish P r o d i ~ c t i o nClubs, No. 207, 3 3 2 (1941). Goodeve, C. F., and Kiwhener, J. A , , Trans. Faradau Soc., 34, 5 7 0 (1938).
Ibid., 34, 9 0 2 (1938). Hoek, C . P. van, Farben-Z., 36, 267 (1930). Kempf, R., I b i d . , 36, 20 (1930). Rem, C., H e h . Chim. Acta, 4, 961 (1921). Robertson, D. W., and Lutz, J. A., Paint Znd. Mag., 55, 232, 234, 236 (1940).
Ruben, B. A. Org. Chem. Ind. ( U.S.S.R.) , 7, 223 (1940). Vannoy, W. G., Oficial Dkest Federation Paint & Varnish Production Clubs, No. 235, 177 (1944). RECEIVED October 31, 1947. Presented before the Division of Paint, Varnish, and Plastics Chemiqtry a t the 112th Meeting of the AMERICAN CHEMICAL SOCIETY, New York, N. Y .
TOXIC HYDROCARBONS Properties of Hydrocarbons a s Related to Their Wood-P res e wing Value J. A. VAUGHAN Southern If/ood Preseruing Company, Atlarttcr, Gu. Research conducted for the past ten or mnre years, in an attempt to determine causes underlying the toxicity of certain hydrocarbons, has led to the development of controls which may be employed in producing toxic hydrocarbons from any source of hydrocarbon material. I t has further determined that the presence of aromatic hydrocarbons and an established specific gravity-boiling point relationship may be employed as a means of predicting toxicity toward fungi.
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HROUGHOUT the years the wood-prescrving induutry has been faced with cycles of creosote shortage and owing to one condition or another, has been forced frequently to change ideas as to what was wanted or would be accepted as coal tar crzosote. The current specification is broad enough in its provisions t,o meet all normally produced coal tar creosotes, and it is possible to have a number of creosotes, all of which meet the specification, but which vary from each other to such a marked degree that their physical characteristics and toxic values are markedly different. I t is even possible to visualize a coal t,ar distillate which will meet. the specification in every detail and yet be of doubtful value as a practical preservative for wood. As the iodustry has developed and progressed, definite efforts have been made to improve treated products by obtaining deeper and better penetration and more uniform distribution of the pre-
servative throughout the treatable portion of the wood, anti to improve the appearance and general cleanliness of the treated materials, especially poles, croesarmq, and lumber. UNDERLYING REASONS FOR TOXICITY OF HYDROCARBONS
Early in the thirties the research department of the Southern Wood Preserving Company established a long range project (6, 6) for the purpose of investigating various hydrocarbon oils as wood preservat.ivcs and establishing, if possible, the underlying reasons for toxicity and other wood-preserving qualities of these oils. It was hoped that this study might add to previous ( 3 ) work of a similar nature and that it might be a means of devcloping a process whereby hydrocarbon-wood preserving oils could be produced to a specification t,hat would be such as to assure a wood preservative of predictable and acceptable value. The iiiitial stagcs of this proiect were confined to the st.udy of coal tar, coal tar distillates, water gas tar, water gas-t,ar distillates, and Some of the highly aro~nat.icpetroleum residuals and distillatea. Without going too deeply into t,he detailed {:hemica1 structure of the materials under in~est~igation, because of their extremely complex nature, the work resolved itself into a study of the basic general chemical struct'ure, the physical properties, and the relation of these to such mat as toxicity toward wood-dest,roying organisms. permanenee of material, and permanence of toxicity.
