- Email: [email protected]
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WHITE PIGMENTS JUERGEN H. BRAUN * and JOHN G. DICKINSON * Pigment Consultants, Inc. 614 Loveville Road, BIdg. E-1-H Hockessin, Delaware 19707 **DuPont White Pigment & Mineral Products Chestnut Run Plaza Wilmington, Delaware 19805 Introduction Background The Path of Progress The Chloride Process Product Considerations Alternatives to Titanium Dioxide Outlook Introduction In 1985 and in this forum, Fred B. Steig^ of N L Industries, reviewed the growth of "Opaque White Pigments in Coatings" from commercial art Into a modern technology. Since then, white pigment technology continues to evolve from contributions of practitioners, theorists, engineers, scientists and businessmen. Within this orderly evolution there did occur a major development; a new manufacturing process causing profound changes in the white pigments industry but, because of proprietary concerns, the technical details were not revealed. The new process involved an engineering gamble against large odds, and progress was achieved through the cooperation of technically astute managers with innovative engineers and creative scientists. White pigment technology was changing. In the 1930's titanium dioxide began to obsolete all other white pigments. The pigment was manufactured by a process based on the digestion of titanium ores In sulfuric acid, and in the late 1940's this "sulfate" process was growing into maturity. Meanwhile, the cold war prompted interests in titanium metal for aircraft and naval applications, the metal being made from titanium tetrachloride. Titanium dioxide pigment was a potential second outlet for
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the tetrachloride, and several manufacturers tried to develop processes to make pigment from it. Only two succeeded in their ventures - in the 1950's, DuPont and in the 1960's, American Potash, later Kerr-McGee. The chlohde process, in turn, opened the door to decisive performance improvements for titanium dioxide pigments. The last three or four decades have seen the decline of the "sulfate" process and the optimization of new "chlonde" technology. The performance characteristics of titanium dioxide pigments were improved, and new grades were developed to suit the requirements of the plastics and paper Industries and specific needs within coatings applications. This progress has come in part by opportunities Inherent to chloride technology and in part by competitive pressures exerted by "chloride" into "sulfate" producers and vice versa. Background Chemical coatings are applied to surfaces to protect and to decorate. They are usually composed of two phases - continuous polymer and discontinuous pigment. The polymer component of the coating provides the protection; pigments supply aesthetics. Whether white, black or color, pigment particles hide the drabness and optical contrasts of the substrate. Hiding by thin films is essential for virtually all architectural and industrial surfaces. This is a fairly straightforward issue with either dark or heavily colored coatings, simply because blacks or dark pigments effectively absorb all light impinging on the film. White or lightly tinted systems are another matter. The specific objective of a white pigment is to scatter light (as opposed to absorption) and this task is accomplished in two ways - refraction and diffraction. In the case of TIO2, both parameters require extremely careful control of both composition and physical dimension. Titanium dioxide must be rutile phase - highest refractive index - highest refractive scattering. It must also have carefully controlled particle size since optical theory states that highest diffractive scattering occurs with particle dimensions that are approximately half that of the incident radiation to be scattered. This latter parameter, diffractive scattering implies a responsibility on the part of coatings, plastics, ink and paper manufacturers in that the pigment particle size furnished must be maintained in their finished products. The Path of Progress In white pigment technology theoretical insights developed slowly. At first, theory provided qualitative direction; now, chemical and optical theories are able to stake quantitative goals. With such help, optimization has occurred as an interplay of science with engineering technology. Periodic reviews •^''* and, more recently, a summary^ have covered most aspects of titanium technology and pigment science. But, for proprietary reasons, not much has been published about manufacturing processes.
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The Chloride Process Most of the progress of the last several decades originated from development of an innovative new process for the manufacture of titanium dioxide pigment. This "chloride" process offers dramatic environmental, decisive quality and significant economic advantages. Process development started In the 1940's and the first chloride plant began to produce in 1954. Other pigment producers and chemical manufacturers entered the technology and currently 23 chloride process plants are operating in North America, Europe, Asia and Australia. The chloride process can deal with a variety of titanium ores, rutile ore being preferred to take advantage of its low Iron content. Because rutile does not easily dissolve in sulfuhc acid, sulfate plants require ilmenite ores (FeTiOa) or titanium slag {Ti02, FesOa) whose high iron contents aggravate waste disposal problems. Raw material economics are affected decisively by local disposal options and by shifts in the price of titanium ores, some prompted by the ongoing conversion of the industry from "sulfate" to "chloride" and Ilmenite to rutile ores. The sulfate process's intermediate is an aqueous solution of titanyl sulfate that must be hydrolyzed to colloidal hydrous titania. The hydrolysis is seeded with rutile nuclei, and then calcined and crystal-grown into size optimized pigment particles. Eventually, vast quantities of iron sulfate and dilute sulfuric acid must be disposed of or recovered. By contrast, the chloride process's chemical Intermediate Is anhydrous titanium tetrachloride, a high boiling liquid which is reacted with oxygen into a titanium dioxide particulate with exclusively rutile phase and pigmentary particle size. By-product chlorine is recycled and reused in the chlorinatlon of the titanium ore. In the early years chloride pigments outperformed their sulfate counterparts by wide margins. Currently the two manufacturing processes produce a range of products somewhat more closely matched. The Chemical Reaction. Titanium dioxide can be made by an exothermic reaction, oxidation of titanium tetrachloride, carried out with a flame that generates a solid particulate from gaseous ingredients: TICl4(gas) + 02(gas) "> T I O 2 (rutlle) + 2 Cl2(gas)
To sustain the flame, reactants must be preheated and the reaction proceeds quickly and almost completely. The chemistry is simple but engineering problems are legion; they include corrosion control, reactant mixing and flame stabilization. But the real challenges Involve control of crystal phase, crystallite size and particle aggregation. Flow Control and Mixing. The combustion generates a solid particulate that Is susceptible to accumulation on whatever surfaces confine the flame and control mixing of the ingredient gas flows. Accumulating crusts, in turn, degrade intended flow
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patterns within the reactor, resulting in a loss of control over flow and mixing. Recirculation of particulate caused by turbulence near the flame front leads to particle growth beyond the intended size. Also, titanium dioxide crusts cannot be allowed to contaminate the pigment. Crystal Phase. For outdoor durability as well as optimal hiding power in coatings and plastics applications, titanium dioxide pigment must be composed of rutile rather than anatase crystallites. Rutile has a higher refractive index than anatase, thus hides better. From a hiding perspective, a few percent of anatase in rutile pigment would not really matter but even small concentrations of anatase in the rutile degrade the outdoor durability of paint films. Thus, crystal phase of the pigment must be controlled to better than 99% rutile. Particle Size. The effectiveness with which white pigments hide is quite sensitive to particle size; primary rutile particles should be about 0.2|im in diameter. Particles below about 0.1 ^im or above about 0.5|im are wasted, the larger ones even detrimental. Thus, a narrow particle size distribution is essenti^L Size specifications were developed experimentally and confirmed theoretically. ' Computers were essential; without them, the calculations of particle size relationships to light scattering effectiveness could not have been accomplished. For reasons involving color effects as well as hiding, optimal particle size of titanium dioxide pigment differs a bit with pigment volume concentration of the intended application. For uses at low pigment concentration, as in most plastics, crystallites are made to be a bit smaller than crystallites intended for coatings. Special pigment grades are made to cater to specific size requirements that exist even within coatings applications. Aggregate Formation. The presence of aggregates of size-optimized primary particles, that is, sinter-bonded crystallites, causes several problems in the product: •
Sintered titanium dioxide crystallites, once formed, are extremely difficult to remove or grind to size.
•
Aggregates decrease the hiding power of the pigment because they scatter light with the diminished effectiveness of larger-than-optimal particles.
•
They can diminish gloss of paint films.
•
They can increase the abrasiveness of pigment used in paper applications where aggregates dull slitting knives and in the delustering of textile fibers where they erode spinnerets.
The likelihood of aggregate formation must be diminished by the design of the burner, although one can grind burner discharge in a separate process step.
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Engineering. The engineering challenges to process development were severe, where almost all problems had to be resolved at full manufacturing scale. The most serious problems involved the TiCl4/02 burner where pigment particles must be generated in their final size and crystal phase. A pilot plant was not considered useful because this burner operates at high temperatures where volume-to-surface ratios are all but impossible to scale from pilot plant to production size facilities. Heat transfer problems around the burner are further complicated because, by its nature, pigmentary titanium dioxide is a superb thermal insulator. The oxidation step, specifically its TiCU burner, is the heart of chloride pigment technology. First, oxygen and titanium tetrachloride have to be heated separately to several hundred degrees centigrade without contaminating the hot gasses by corrosion products. Then in a flame front and within milliseconds, all the basic characteristics of the pigment must be fixed: (1) The crystal phase of product must be regulated to produce rutile. (2) The size of the pigment crystallites controlled, to serve a variety of product requirements. (3) Aggregation of crystallites and the agglomeration of aggregates minimized to improve gloss performance and to reduce the costs of grinding. Failures of crystallite size and phase control cannot be corrected because crystallites cannot be ground and pigmentary titanium dioxide cannot be phase converted. Development of process engineering operations downstream from the oxidation step - wet-treatment, grinding and packaging - was continued in the traditions of conventional pigment technology. The products of the oxidation step, size optimized rutile crystallites, are "wet-treated," that is, subjected to a host of processes that are carried out in aqueous dispersion and that may include grinding operations. These operations enhance pigment performance in specific coatings, plastics and ink applications. Several of the wet-treatment processes evolved from sulfate pigment technology, other procedures were designed specifically for chloride pigments because the surface characteristics of chloride and sulfate pigments differ. Chloride pigment contains some alumina added for phase control of the crystallites, sulfate pigment some phosphate present for particle size control. After wet-treatment, pigments are filtered, dried and ground in fluid-energy mills common to chloride and sulfate plants. Product Considerations Puritv. Impurities in the pigment can degrade Its color (brightness). Ordinal^ contaminants matter but far more detrimental are impurities that can substitute for Ti ions within the lattice of the rutile crystal. In concentrations of only a few parts per million, such impurity cations - iron, chromium, nickel, vanadium and niobium among them - cause pronounced yellowing or graying of the pigment.
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Since these interstitial impurities cannot be leached from the product, offensive elements must be removed from the process intermediates, titanium tetrachloride or titanyl sulfate solution. The purification is a task that is much easier accomplished in chloride than sulfate technology. Anhydrous TiCU liquid can be purified readily by distillation and by chemical treatments whereas aqueous solutions of titanyl sulfate are quite difficult to treat effectively. Thus chloride pigments are whiter and brighter than their sulfate counterparts. Durabilitv. Titanium dioxide is a catalyst for the sunlight energized oxidation of organic polymers, and the semiconductor mechanisms involved are reasonably well understood.^''^''^ At its surface, titanium dioxide transforms the energy of ultraviolet light Into chemical energy. This chemical energy reacts with oxygen and water to generate two free radicals, hydroxyl and peroxyl: Tiv[Ti02] + 02 + H2O A)H- + H02* The free radicals can, in turn, react with and destroy almost any organic molecule: OH- + H02* + —CH2—/ CO2 + H2O As a result, paint films pigmented with unprotected titanium dioxide are said to chalk, that is, turn into dust by prolonged outdoor exposure.^^'^^ For anatase pigment, the effect is severe enough to all but preclude its outdoor use. Paint films pigmented with conventional rutile are less prone to degradation, but the chalking problems of titanium dioxide pigments were ail but resolved by chemistry developed by ller^"^ and subsequent extensions. This chemistry made it possible to encapsulate certain inorganic particulates in shells of silica glass. Today, silica encapsulated rutile pigments perform exceedingly well in even the most demanding outdoor applications. Dispersibilitv. By laws of physics, small particles stick to each other. These short-range attractions cause pigment crystals to be very sticky. They agglomerate into assemblies that are larger than intended, thus (1) become less hiding effective than individual pigment crystallites, (2) degrade gloss and (3) cause roughness of paint films. To break the agglomerates, pigment manufacturers grind their products in fluid energy mills. By a second, less intense grinding operation, pigment consumers disperse dry pigment into their appropriate media. The pigment's tendency towards spontaneous agglomeration can be reduced. Just as a coating of feathers keeps wax balls from sticking to each other, so can a hydrous alumina coating reduce the stickiness of pigment particles. Hydrous alumina coatings were advocated for titanium dioxide pigments for a variety of benefits, and predate the chloride process.^^ The explanation and optimization of their effectiveness required sophisticated techniques of modern electron microscopy. Traditionally, paint grinding, that is, pigment dispersion into paint media, is performed by paint manufacturers. In 1970, DuPont commercialized aqueous slurry
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grades of titanium dioxide pigments. For waterborne paints and where the scale of paint manufacture justifies the installation of a slurry handling system, the burden of paint grinding can now be shifted to the pigment producer. Gloss. Glossiness attracts attention. A high-gloss finish sells cars and objects intended to draw the eye of the observer. Obviously, pigment manufacturers would like to design the potential of high gloss performance into some of their products. However, highest levels of gloss can occur only on surfaces that are essentially amorphous in character, the surfaces of glasses, polished metals and clear liquids including clear paint films. Particulates in the paint film, pigments, can only degrade gloss. To be least detrimental to the gloss of its paint films, a pigment may not contain particles larger than necessary for hiding.^^ Ideally, the pigment should contain no particles that are larger than about 0.5|Lim, neither crystallites nor their assemblies into aggregates and agglomerates. Also, the pigment must be capable of packing quite densely, that is, have the lowest possible "Oil Absorption," a measure of the packing density of pigment particles into liquid dispersion. This Oil Absorption requirement is difficult to meet for durable pigment grades. Only recently has a durable version of high-gloss pigment become available. Pigment Grades. Titanium dioxide pigments are used in several industries and in many diverse applications and media. Since light scattering is their singular objective, all commercial products are appropriately particle size optimized. All rutile grades could serve the diverse needs of most industries and would do so at least moderately well. Optimal performance demands special grades designed for specific requirements: outdoor durability, high gloss, porous paint films, etc. In addition, customer convenience may be designed into the product: slurries that are intended for stir-in dispersion, drybulk products for use in plastics. Alternatives to Titanium Dioxide Classic Pigments. All the classic white pigments are now obsolete because they are neither as safe nor nearly as effective as titanium dioxide.^ For two millennia, white leads ~ basic lead carbonate and sulfate - were the only white pigments that could deliver moderately durable whiteness and brightness into a drab world of grays and earth colors. Toxicity was recognized, but accepted. Eventually, white leads were displaced by zinc whites - zinc oxide, zinc sulfide and lithopone (an equimolar composite of zinc sulfide and barium sulfate.) Zinc whites are much less toxic than lead whites but still do not hide nearly as well as titanium dioxide. A variety of composite pigments - intimate mixtures of titanium dioxide with calcium sulfate and with lithopone - were used extensively during a transition period from zinc whites to essentially pure titanium dioxide. They have effectively disappeared from the market.
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Air Hiding. Air hiding, light scattering at the interfaces of particles with air rather than polymeric binder, occurs almost everywhere. Air hiding is the optical mechanism of fog, snow, textiles, paper, chalk marks and layers of dust. Air hiding contributes significantly to hiding by porous paint films. In paint films, the advantage of air hiding is its low cost. Since porosity of the film is a prerequisite, coatings that involve air hiding have two principal disadvantages: (1) They are brittle, thus lacking in mechanical strength; (2) They do not protect because pores conduct contaminants into the coating and onto the substrate. Nonetheless, modern porous paint films serve very well in applications where chemical and mechanical assaults are infrequent, for example on interior walls and ceilings. Void Pigments. Paint films pigmented with microvoids,'' "void pigments," are far less sensitive than porous films; their air voids are individually encapsulated by polymer: •
Voids are sealed and, unlike pores, do not conduct contaminants into the film; Capsules are spherical, thus mechanically strong.
Void hiding has also been demonstrated as an aqueous slurry of beads, each composed of polymer, voids and titanium dioxide. The voids enhanced the scattering effectiveness of the pigment. Other Substances. To serve as a white pigment, a substance must satisfy stringent requirements. It must have an extreme refractive index, be colorless, chemically inert, stable, and nontoxic; it must be available as microscopic particulate. Only a few substances have refractive indices high enough for a pigment, most of them are compounds of titanium, zinc, and lead. Titanium dioxide Is uniquely qualified because it Is nontoxic and rutile and anatase have the highest refractive indices of all colorless substances. Brookite, the third titanium dioxide phase has properties and stability characteristics that fall between rutile and anatase but promise no advantage over either. It can be manufactured in pigmentary particle size. Since an extreme refractive index Is the essential feature of a white pigment, titanium dioxide has no direct competition. Could Hyperbaric phases of titanium dioxide qualify? Yes, novel titanium dioxide phases made at extreme pressures and temperatures, are likely to combine higher densities with higher refractive Indices. A pigment made from such a hypothetical titanium dioxide phase would be more hiding effective than rutile, but such pigment will not be available in the foreseeable future. Hyperbaric syntheses are incredibly costly and can make only milligram batches.
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Outlook Given the unique optical characteristics of rutile, titanium dioxide pigments are here to stay through the foreseeable future. No other substance or light scattering technology is on the horizon, let alone on the drawing board. Titanium ores are plentiful. However, by-product disposal of pigment manufacture in a manner that is safe to the environment has become a worldwide issue. No longer is it acceptable to ocean dump by-product acids and acidic iron compounds. Deep well disposal is restricted severely, limiting deep wells to sites of exceptional geology. Minor but noxious contaminants In the ore have become major disposal problems. Pigment costs are now driven by waste problems and disposal costs are increasing quickly, much faster than selling prices. Progress in pigment development is likely to continue but, for good reasons, probably less rapidly than in the past: For some time now, pigment grades have proliferated to meet specific requirements within the coatings, plastics, paper and even ink industries. Lately a consolidation of pigment grades is underway to offset inventory and distribution costs of grade proliferation. Because of the urgency of disposal problems, the focus of pigment research and development has turned to waste disposal. The shift comes at the expense of long term research, application studies and product developments. Meanwhile, the white pigments industry can take pride in the fact that the developments of the last fifty years have resulted in products that leave relatively modest room for improvement within the theoretical limits imposed by physics and chemistry.
Literature Cited ^ Steig, F. B., Jr., "Opaque White Pigments in Coatings," ACS Symposium Series 285, Applied Polymer Science, 2d ed., edit. R. W. Tess and G. W. Poehlein, (94 references) 1985. ^ Barksdale, J., "Titanium," The Ronald Press Company, New York, 2d ed., (691 pages) 1966. ^Patton, T. C, "Pigment Handbook," Vol. I, John Wiley & Sons, New York, pp. 1-108, 1973. "^Braun, J. H., A. Baldins, R. E. Marganski, "Ti02 Pigment Technology - A Review," Prog. Organic Coatings, 20 [2], 105-138, (200 references) 1992.
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^Braun, J. H., "White Pigments," Monograph in the Federation Series on Coatings Technology, Federation of Societies for Coatings Technology (USA), (43 pages) 1993. ^Ross, W. D., "Theoretical Computation of Light Scattering Power: Comparison between Ti02 and Air Bubbles," J. Paint TechnoL, 43 [563], 50-66, (30 references) 1971 ^Ross, W.D., "Theoretical Computation of Light Scattering Power of Ti02 and MIcrovoids," l&EC Product Research & Development, 13[3], 45-49, (12 references) 1974. ^Ross, W.D., "Kubelka-Munk Formulas Adapted for Better Computation," J. Paint TechnoL, 39 [511], 515-521, (8 references) 1967. ^Sullivan, W. F., "Weatherability of Titanium-Dioxide-Containing Paints," Progr. Org. Coatings, 1, 157-203 (238 references) 1972. ^°Braun, J. H., "Titanium Dioxide's Contribution to the Durability of Paint Films," Progr. Org. Coatings, 15, 249-260, (9 references) (1987); "Titanium Dioxide's Contribution to the Durability of Paint Films - II. Prediction of Catalytic Activity." J. Coatings Technology, 62 [785], 37-42, (15 references) 1990. ^^Diebold, M.P., "The Causes and Prevention of Titanium Dioxide Induced Photodegradation of Paints, Parts I and II," Surface Coatings International 1995 [6], 250-256, and 1995 [7], 294-299 (76 references) 1995. ^^Kampf, G., W. Papenroth and R. Holm, "Degradation Processes in Ti02 Pigmented Paint Films on Exposure to Weathering," J. Paint TechnoL, 46 [598], 56-63, (10 references) (1974). ^^/oltz, H. G., G. Kampf, H. G. Fitzky, "Surface Reactions on Titanium Dioxide Pigments in Paint Films during Weathering," Prog. Org. Coatings, 2, 223-235, (41 references) (1973/4). ^"^iler, R. A., "The Chemistry of Silica," John Wiley & Sons, New York, (866 pages) 1979. ^^Farup, P., US Patent 1,368,392, "Titanium dioxide pigment coated with hydrous alumina," (1921) ^^Braun, J. H., "Gloss of Paint Films and the Mechanism of Pigment Involvement," J. Coatings Technology, 63 [799], 43, (10 references) 1991. ^^Braun, J. H. and D. P. Fields, "Gloss of Paint Films: II. Effect of Pigment Size," J. Coatings Technology, 66 [828], (9 references) 1994.
WHITE PIGMENTS JUERGEN H. BRAUN * and JOHN G. DICKINSON * Pigment Consultants, Inc. 614 Loveville Road, BIdg. E-1-H Hockessin, Delaware 19707 **DuPont White Pigment & Mineral Products Chestnut Run Plaza Wilmington, Delaware 19805 Introduction Background The Path of Progress The Chloride Process Product Considerations Alternatives to Titanium Dioxide Outlook Introduction In 1985 and in this forum, Fred B. Steig^ of N L Industries, reviewed the growth of "Opaque White Pigments in Coatings" from commercial art Into a modern technology. Since then, white pigment technology continues to evolve from contributions of practitioners, theorists, engineers, scientists and businessmen. Within this orderly evolution there did occur a major development; a new manufacturing process causing profound changes in the white pigments industry but, because of proprietary concerns, the technical details were not revealed. The new process involved an engineering gamble against large odds, and progress was achieved through the cooperation of technically astute managers with innovative engineers and creative scientists. White pigment technology was changing. In the 1930's titanium dioxide began to obsolete all other white pigments. The pigment was manufactured by a process based on the digestion of titanium ores In sulfuric acid, and in the late 1940's this "sulfate" process was growing into maturity. Meanwhile, the cold war prompted interests in titanium metal for aircraft and naval applications, the metal being made from titanium tetrachloride. Titanium dioxide pigment was a potential second outlet for
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the tetrachloride, and several manufacturers tried to develop processes to make pigment from it. Only two succeeded in their ventures - in the 1950's, DuPont and in the 1960's, American Potash, later Kerr-McGee. The chlohde process, in turn, opened the door to decisive performance improvements for titanium dioxide pigments. The last three or four decades have seen the decline of the "sulfate" process and the optimization of new "chlonde" technology. The performance characteristics of titanium dioxide pigments were improved, and new grades were developed to suit the requirements of the plastics and paper Industries and specific needs within coatings applications. This progress has come in part by opportunities Inherent to chloride technology and in part by competitive pressures exerted by "chloride" into "sulfate" producers and vice versa. Background Chemical coatings are applied to surfaces to protect and to decorate. They are usually composed of two phases - continuous polymer and discontinuous pigment. The polymer component of the coating provides the protection; pigments supply aesthetics. Whether white, black or color, pigment particles hide the drabness and optical contrasts of the substrate. Hiding by thin films is essential for virtually all architectural and industrial surfaces. This is a fairly straightforward issue with either dark or heavily colored coatings, simply because blacks or dark pigments effectively absorb all light impinging on the film. White or lightly tinted systems are another matter. The specific objective of a white pigment is to scatter light (as opposed to absorption) and this task is accomplished in two ways - refraction and diffraction. In the case of TIO2, both parameters require extremely careful control of both composition and physical dimension. Titanium dioxide must be rutile phase - highest refractive index - highest refractive scattering. It must also have carefully controlled particle size since optical theory states that highest diffractive scattering occurs with particle dimensions that are approximately half that of the incident radiation to be scattered. This latter parameter, diffractive scattering implies a responsibility on the part of coatings, plastics, ink and paper manufacturers in that the pigment particle size furnished must be maintained in their finished products. The Path of Progress In white pigment technology theoretical insights developed slowly. At first, theory provided qualitative direction; now, chemical and optical theories are able to stake quantitative goals. With such help, optimization has occurred as an interplay of science with engineering technology. Periodic reviews •^''* and, more recently, a summary^ have covered most aspects of titanium technology and pigment science. But, for proprietary reasons, not much has been published about manufacturing processes.
White Pigments
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The Chloride Process Most of the progress of the last several decades originated from development of an innovative new process for the manufacture of titanium dioxide pigment. This "chloride" process offers dramatic environmental, decisive quality and significant economic advantages. Process development started In the 1940's and the first chloride plant began to produce in 1954. Other pigment producers and chemical manufacturers entered the technology and currently 23 chloride process plants are operating in North America, Europe, Asia and Australia. The chloride process can deal with a variety of titanium ores, rutile ore being preferred to take advantage of its low Iron content. Because rutile does not easily dissolve in sulfuhc acid, sulfate plants require ilmenite ores (FeTiOa) or titanium slag {Ti02, FesOa) whose high iron contents aggravate waste disposal problems. Raw material economics are affected decisively by local disposal options and by shifts in the price of titanium ores, some prompted by the ongoing conversion of the industry from "sulfate" to "chloride" and Ilmenite to rutile ores. The sulfate process's intermediate is an aqueous solution of titanyl sulfate that must be hydrolyzed to colloidal hydrous titania. The hydrolysis is seeded with rutile nuclei, and then calcined and crystal-grown into size optimized pigment particles. Eventually, vast quantities of iron sulfate and dilute sulfuric acid must be disposed of or recovered. By contrast, the chloride process's chemical Intermediate Is anhydrous titanium tetrachloride, a high boiling liquid which is reacted with oxygen into a titanium dioxide particulate with exclusively rutile phase and pigmentary particle size. By-product chlorine is recycled and reused in the chlorinatlon of the titanium ore. In the early years chloride pigments outperformed their sulfate counterparts by wide margins. Currently the two manufacturing processes produce a range of products somewhat more closely matched. The Chemical Reaction. Titanium dioxide can be made by an exothermic reaction, oxidation of titanium tetrachloride, carried out with a flame that generates a solid particulate from gaseous ingredients: TICl4(gas) + 02(gas) "> T I O 2 (rutlle) + 2 Cl2(gas)
To sustain the flame, reactants must be preheated and the reaction proceeds quickly and almost completely. The chemistry is simple but engineering problems are legion; they include corrosion control, reactant mixing and flame stabilization. But the real challenges Involve control of crystal phase, crystallite size and particle aggregation. Flow Control and Mixing. The combustion generates a solid particulate that Is susceptible to accumulation on whatever surfaces confine the flame and control mixing of the ingredient gas flows. Accumulating crusts, in turn, degrade intended flow
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patterns within the reactor, resulting in a loss of control over flow and mixing. Recirculation of particulate caused by turbulence near the flame front leads to particle growth beyond the intended size. Also, titanium dioxide crusts cannot be allowed to contaminate the pigment. Crystal Phase. For outdoor durability as well as optimal hiding power in coatings and plastics applications, titanium dioxide pigment must be composed of rutile rather than anatase crystallites. Rutile has a higher refractive index than anatase, thus hides better. From a hiding perspective, a few percent of anatase in rutile pigment would not really matter but even small concentrations of anatase in the rutile degrade the outdoor durability of paint films. Thus, crystal phase of the pigment must be controlled to better than 99% rutile. Particle Size. The effectiveness with which white pigments hide is quite sensitive to particle size; primary rutile particles should be about 0.2|im in diameter. Particles below about 0.1 ^im or above about 0.5|im are wasted, the larger ones even detrimental. Thus, a narrow particle size distribution is essenti^L Size specifications were developed experimentally and confirmed theoretically. ' Computers were essential; without them, the calculations of particle size relationships to light scattering effectiveness could not have been accomplished. For reasons involving color effects as well as hiding, optimal particle size of titanium dioxide pigment differs a bit with pigment volume concentration of the intended application. For uses at low pigment concentration, as in most plastics, crystallites are made to be a bit smaller than crystallites intended for coatings. Special pigment grades are made to cater to specific size requirements that exist even within coatings applications. Aggregate Formation. The presence of aggregates of size-optimized primary particles, that is, sinter-bonded crystallites, causes several problems in the product: •
Sintered titanium dioxide crystallites, once formed, are extremely difficult to remove or grind to size.
•
Aggregates decrease the hiding power of the pigment because they scatter light with the diminished effectiveness of larger-than-optimal particles.
•
They can diminish gloss of paint films.
•
They can increase the abrasiveness of pigment used in paper applications where aggregates dull slitting knives and in the delustering of textile fibers where they erode spinnerets.
The likelihood of aggregate formation must be diminished by the design of the burner, although one can grind burner discharge in a separate process step.
White Pigments
487
Engineering. The engineering challenges to process development were severe, where almost all problems had to be resolved at full manufacturing scale. The most serious problems involved the TiCl4/02 burner where pigment particles must be generated in their final size and crystal phase. A pilot plant was not considered useful because this burner operates at high temperatures where volume-to-surface ratios are all but impossible to scale from pilot plant to production size facilities. Heat transfer problems around the burner are further complicated because, by its nature, pigmentary titanium dioxide is a superb thermal insulator. The oxidation step, specifically its TiCU burner, is the heart of chloride pigment technology. First, oxygen and titanium tetrachloride have to be heated separately to several hundred degrees centigrade without contaminating the hot gasses by corrosion products. Then in a flame front and within milliseconds, all the basic characteristics of the pigment must be fixed: (1) The crystal phase of product must be regulated to produce rutile. (2) The size of the pigment crystallites controlled, to serve a variety of product requirements. (3) Aggregation of crystallites and the agglomeration of aggregates minimized to improve gloss performance and to reduce the costs of grinding. Failures of crystallite size and phase control cannot be corrected because crystallites cannot be ground and pigmentary titanium dioxide cannot be phase converted. Development of process engineering operations downstream from the oxidation step - wet-treatment, grinding and packaging - was continued in the traditions of conventional pigment technology. The products of the oxidation step, size optimized rutile crystallites, are "wet-treated," that is, subjected to a host of processes that are carried out in aqueous dispersion and that may include grinding operations. These operations enhance pigment performance in specific coatings, plastics and ink applications. Several of the wet-treatment processes evolved from sulfate pigment technology, other procedures were designed specifically for chloride pigments because the surface characteristics of chloride and sulfate pigments differ. Chloride pigment contains some alumina added for phase control of the crystallites, sulfate pigment some phosphate present for particle size control. After wet-treatment, pigments are filtered, dried and ground in fluid-energy mills common to chloride and sulfate plants. Product Considerations Puritv. Impurities in the pigment can degrade Its color (brightness). Ordinal^ contaminants matter but far more detrimental are impurities that can substitute for Ti ions within the lattice of the rutile crystal. In concentrations of only a few parts per million, such impurity cations - iron, chromium, nickel, vanadium and niobium among them - cause pronounced yellowing or graying of the pigment.
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Since these interstitial impurities cannot be leached from the product, offensive elements must be removed from the process intermediates, titanium tetrachloride or titanyl sulfate solution. The purification is a task that is much easier accomplished in chloride than sulfate technology. Anhydrous TiCU liquid can be purified readily by distillation and by chemical treatments whereas aqueous solutions of titanyl sulfate are quite difficult to treat effectively. Thus chloride pigments are whiter and brighter than their sulfate counterparts. Durabilitv. Titanium dioxide is a catalyst for the sunlight energized oxidation of organic polymers, and the semiconductor mechanisms involved are reasonably well understood.^''^''^ At its surface, titanium dioxide transforms the energy of ultraviolet light Into chemical energy. This chemical energy reacts with oxygen and water to generate two free radicals, hydroxyl and peroxyl: Tiv[Ti02] + 02 + H2O A)H- + H02* The free radicals can, in turn, react with and destroy almost any organic molecule: OH- + H02* + —CH2—/ CO2 + H2O As a result, paint films pigmented with unprotected titanium dioxide are said to chalk, that is, turn into dust by prolonged outdoor exposure.^^'^^ For anatase pigment, the effect is severe enough to all but preclude its outdoor use. Paint films pigmented with conventional rutile are less prone to degradation, but the chalking problems of titanium dioxide pigments were ail but resolved by chemistry developed by ller^"^ and subsequent extensions. This chemistry made it possible to encapsulate certain inorganic particulates in shells of silica glass. Today, silica encapsulated rutile pigments perform exceedingly well in even the most demanding outdoor applications. Dispersibilitv. By laws of physics, small particles stick to each other. These short-range attractions cause pigment crystals to be very sticky. They agglomerate into assemblies that are larger than intended, thus (1) become less hiding effective than individual pigment crystallites, (2) degrade gloss and (3) cause roughness of paint films. To break the agglomerates, pigment manufacturers grind their products in fluid energy mills. By a second, less intense grinding operation, pigment consumers disperse dry pigment into their appropriate media. The pigment's tendency towards spontaneous agglomeration can be reduced. Just as a coating of feathers keeps wax balls from sticking to each other, so can a hydrous alumina coating reduce the stickiness of pigment particles. Hydrous alumina coatings were advocated for titanium dioxide pigments for a variety of benefits, and predate the chloride process.^^ The explanation and optimization of their effectiveness required sophisticated techniques of modern electron microscopy. Traditionally, paint grinding, that is, pigment dispersion into paint media, is performed by paint manufacturers. In 1970, DuPont commercialized aqueous slurry
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grades of titanium dioxide pigments. For waterborne paints and where the scale of paint manufacture justifies the installation of a slurry handling system, the burden of paint grinding can now be shifted to the pigment producer. Gloss. Glossiness attracts attention. A high-gloss finish sells cars and objects intended to draw the eye of the observer. Obviously, pigment manufacturers would like to design the potential of high gloss performance into some of their products. However, highest levels of gloss can occur only on surfaces that are essentially amorphous in character, the surfaces of glasses, polished metals and clear liquids including clear paint films. Particulates in the paint film, pigments, can only degrade gloss. To be least detrimental to the gloss of its paint films, a pigment may not contain particles larger than necessary for hiding.^^ Ideally, the pigment should contain no particles that are larger than about 0.5|Lim, neither crystallites nor their assemblies into aggregates and agglomerates. Also, the pigment must be capable of packing quite densely, that is, have the lowest possible "Oil Absorption," a measure of the packing density of pigment particles into liquid dispersion. This Oil Absorption requirement is difficult to meet for durable pigment grades. Only recently has a durable version of high-gloss pigment become available. Pigment Grades. Titanium dioxide pigments are used in several industries and in many diverse applications and media. Since light scattering is their singular objective, all commercial products are appropriately particle size optimized. All rutile grades could serve the diverse needs of most industries and would do so at least moderately well. Optimal performance demands special grades designed for specific requirements: outdoor durability, high gloss, porous paint films, etc. In addition, customer convenience may be designed into the product: slurries that are intended for stir-in dispersion, drybulk products for use in plastics. Alternatives to Titanium Dioxide Classic Pigments. All the classic white pigments are now obsolete because they are neither as safe nor nearly as effective as titanium dioxide.^ For two millennia, white leads ~ basic lead carbonate and sulfate - were the only white pigments that could deliver moderately durable whiteness and brightness into a drab world of grays and earth colors. Toxicity was recognized, but accepted. Eventually, white leads were displaced by zinc whites - zinc oxide, zinc sulfide and lithopone (an equimolar composite of zinc sulfide and barium sulfate.) Zinc whites are much less toxic than lead whites but still do not hide nearly as well as titanium dioxide. A variety of composite pigments - intimate mixtures of titanium dioxide with calcium sulfate and with lithopone - were used extensively during a transition period from zinc whites to essentially pure titanium dioxide. They have effectively disappeared from the market.
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Air Hiding. Air hiding, light scattering at the interfaces of particles with air rather than polymeric binder, occurs almost everywhere. Air hiding is the optical mechanism of fog, snow, textiles, paper, chalk marks and layers of dust. Air hiding contributes significantly to hiding by porous paint films. In paint films, the advantage of air hiding is its low cost. Since porosity of the film is a prerequisite, coatings that involve air hiding have two principal disadvantages: (1) They are brittle, thus lacking in mechanical strength; (2) They do not protect because pores conduct contaminants into the coating and onto the substrate. Nonetheless, modern porous paint films serve very well in applications where chemical and mechanical assaults are infrequent, for example on interior walls and ceilings. Void Pigments. Paint films pigmented with microvoids,'' "void pigments," are far less sensitive than porous films; their air voids are individually encapsulated by polymer: •
Voids are sealed and, unlike pores, do not conduct contaminants into the film; Capsules are spherical, thus mechanically strong.
Void hiding has also been demonstrated as an aqueous slurry of beads, each composed of polymer, voids and titanium dioxide. The voids enhanced the scattering effectiveness of the pigment. Other Substances. To serve as a white pigment, a substance must satisfy stringent requirements. It must have an extreme refractive index, be colorless, chemically inert, stable, and nontoxic; it must be available as microscopic particulate. Only a few substances have refractive indices high enough for a pigment, most of them are compounds of titanium, zinc, and lead. Titanium dioxide Is uniquely qualified because it Is nontoxic and rutile and anatase have the highest refractive indices of all colorless substances. Brookite, the third titanium dioxide phase has properties and stability characteristics that fall between rutile and anatase but promise no advantage over either. It can be manufactured in pigmentary particle size. Since an extreme refractive index Is the essential feature of a white pigment, titanium dioxide has no direct competition. Could Hyperbaric phases of titanium dioxide qualify? Yes, novel titanium dioxide phases made at extreme pressures and temperatures, are likely to combine higher densities with higher refractive Indices. A pigment made from such a hypothetical titanium dioxide phase would be more hiding effective than rutile, but such pigment will not be available in the foreseeable future. Hyperbaric syntheses are incredibly costly and can make only milligram batches.
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Outlook Given the unique optical characteristics of rutile, titanium dioxide pigments are here to stay through the foreseeable future. No other substance or light scattering technology is on the horizon, let alone on the drawing board. Titanium ores are plentiful. However, by-product disposal of pigment manufacture in a manner that is safe to the environment has become a worldwide issue. No longer is it acceptable to ocean dump by-product acids and acidic iron compounds. Deep well disposal is restricted severely, limiting deep wells to sites of exceptional geology. Minor but noxious contaminants In the ore have become major disposal problems. Pigment costs are now driven by waste problems and disposal costs are increasing quickly, much faster than selling prices. Progress in pigment development is likely to continue but, for good reasons, probably less rapidly than in the past: For some time now, pigment grades have proliferated to meet specific requirements within the coatings, plastics, paper and even ink industries. Lately a consolidation of pigment grades is underway to offset inventory and distribution costs of grade proliferation. Because of the urgency of disposal problems, the focus of pigment research and development has turned to waste disposal. The shift comes at the expense of long term research, application studies and product developments. Meanwhile, the white pigments industry can take pride in the fact that the developments of the last fifty years have resulted in products that leave relatively modest room for improvement within the theoretical limits imposed by physics and chemistry.
Literature Cited ^ Steig, F. B., Jr., "Opaque White Pigments in Coatings," ACS Symposium Series 285, Applied Polymer Science, 2d ed., edit. R. W. Tess and G. W. Poehlein, (94 references) 1985. ^ Barksdale, J., "Titanium," The Ronald Press Company, New York, 2d ed., (691 pages) 1966. ^Patton, T. C, "Pigment Handbook," Vol. I, John Wiley & Sons, New York, pp. 1-108, 1973. "^Braun, J. H., A. Baldins, R. E. Marganski, "Ti02 Pigment Technology - A Review," Prog. Organic Coatings, 20 [2], 105-138, (200 references) 1992.
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^Braun, J. H., "White Pigments," Monograph in the Federation Series on Coatings Technology, Federation of Societies for Coatings Technology (USA), (43 pages) 1993. ^Ross, W. D., "Theoretical Computation of Light Scattering Power: Comparison between Ti02 and Air Bubbles," J. Paint TechnoL, 43 [563], 50-66, (30 references) 1971 ^Ross, W.D., "Theoretical Computation of Light Scattering Power of Ti02 and MIcrovoids," l&EC Product Research & Development, 13[3], 45-49, (12 references) 1974. ^Ross, W.D., "Kubelka-Munk Formulas Adapted for Better Computation," J. Paint TechnoL, 39 [511], 515-521, (8 references) 1967. ^Sullivan, W. F., "Weatherability of Titanium-Dioxide-Containing Paints," Progr. Org. Coatings, 1, 157-203 (238 references) 1972. ^°Braun, J. H., "Titanium Dioxide's Contribution to the Durability of Paint Films," Progr. Org. Coatings, 15, 249-260, (9 references) (1987); "Titanium Dioxide's Contribution to the Durability of Paint Films - II. Prediction of Catalytic Activity." J. Coatings Technology, 62 [785], 37-42, (15 references) 1990. ^^Diebold, M.P., "The Causes and Prevention of Titanium Dioxide Induced Photodegradation of Paints, Parts I and II," Surface Coatings International 1995 [6], 250-256, and 1995 [7], 294-299 (76 references) 1995. ^^Kampf, G., W. Papenroth and R. Holm, "Degradation Processes in Ti02 Pigmented Paint Films on Exposure to Weathering," J. Paint TechnoL, 46 [598], 56-63, (10 references) (1974). ^^/oltz, H. G., G. Kampf, H. G. Fitzky, "Surface Reactions on Titanium Dioxide Pigments in Paint Films during Weathering," Prog. Org. Coatings, 2, 223-235, (41 references) (1973/4). ^"^iler, R. A., "The Chemistry of Silica," John Wiley & Sons, New York, (866 pages) 1979. ^^Farup, P., US Patent 1,368,392, "Titanium dioxide pigment coated with hydrous alumina," (1921) ^^Braun, J. H., "Gloss of Paint Films and the Mechanism of Pigment Involvement," J. Coatings Technology, 63 [799], 43, (10 references) 1991. ^^Braun, J. H. and D. P. Fields, "Gloss of Paint Films: II. Effect of Pigment Size," J. Coatings Technology, 66 [828], (9 references) 1994.