Pigments and Fillers Used in Paints and Coatings

Summary

Industrial minerals used in the paints and coatings industry comprise inorganic pigments, fillers and inorganic additives. Due to its high refractive index, titanium dioxide (TiO2) is by far the most important white pigment. Before use, it is coated with Al2O3, SiO2, ZrO2 or organic compounds to eliminate light sensitivity caused by free electrons within its structure. Iron oxides account for 80% of the inorganic colored pigments used in the sector. Dolomite, CaCO3, chalk, precipitated calcium carbonate (PCC), talc, mica, kaolin, calcined kaolin, BaSO4, Blanc Fixe (synthetic BaSO4) and SiO2 are the most important fillers used in the sector.

1. Introduction

Paints and coatings, an important sector within the chemical industry, are used to provide aesthetic appearance to surfaces and to protect them. Raw materials used in paint manufacturing are divided into 4 main groups: binders, pigments and fillers, solvents and additives. Binders bond pigments and fillers to each other and to the surface, forming a film on the surface. They determine the basic properties of the paint film such as hardness/softness, durability and gloss. Pigments provide opacity and color to the paint. Fillers generally reduce the cost of paint. They also affect many properties of paint such as storage stability, flow behavior, density, gloss, surface smoothness, opacity, sanding ease, wet rub resistance and durability. Solvents facilitate the production and application of paint and evaporate after the paint is applied to the surface to complete their function. Additives, added to paint in very small quantities, play an important role in paint production, storage, application and paint film characteristics. Opacity, durability and wet rub resistance are the most important properties in construction paints and coatings. Raw material selection and usage ratios in the formulation are determined to best provide these properties. In 2013, 51 million tons of paint were consumed globally and sales generated USD 130 billion in revenue. In terms of volume, 55% of consumption was construction paints and 45% industrial paints. By sales revenue, industrial paints accounted for 57% and construction paints for 43% of 2013 figures (1, 2, 3, 8). Turkey's paint consumption in 2013 was approximately 880 thousand tons. Construction paints accounted for approximately 60% of consumption (530 thousand tons) and industrial paints for 40% (350 thousand tons). By value, 60% came from industrial paints and 40% from construction paints. The Turkish coatings and paints market was valued at approximately USD 2.2 billion. Table 1 shows the global volume proportions of the 4 main raw material groups used in paints and coatings. Globally, pigments and fillers account for approximately 25% of coating and paint costs, binders 46%, solvents 20% and additives 9%.

2. Pigments and Fillers Used in the Sector

Industrial minerals used in coatings and paints consist predominantly of inorganic pigments and fillers. Inorganic additives such as bentonites, sodium hydroxide, polyphosphates and silicic acids, used in small quantities as additives in coatings and paints, are also important industrial minerals. Bentonites, which are organically treated natural aluminum-magnesium layer silicates, are used as thickeners in paint. Synthetically produced transparent pyrogenic silicic acid at 5–50 nm particle size and the coarser precipitated silicic acid impart thixotropic properties to paints through silanol and hydrogen bonds within their structure. Silicic acid (diatomite) with an amorphous and porous structure at 2–8 μm particle size matts paints (matting agent). Matting is achieved through surface roughening of the coating. Polyphosphate serves as a dispersing agent and sodium hydroxide (NaOH) as a pH adjuster in paints and coatings. Pigments and fillers can exist in different forms such as isometric (spheres, cubes) or plate and needle forms. Pigments and fillers in spherical and cubic form are characterized by particle diameter dp, while needle and plate form pigments and fillers are characterized by effective diameter deff according to DIN 55206 standard. Powdered pigments and fillers do not exist in a single size but as a distribution of particles of different sizes. The average particle size of pigments and fillers and their particle distribution affect many properties of paints and coatings such as rheology, color, opacity, color strength and durability, so accurate determination is extremely important. However, deviations in particle size and particle distribution measured by different methods (electron microscopy, sedimentation, etc.) and instruments are natural due to different physical effects and approaches. For this reason, the measurement method and measurement conditions should be provided together with the analysis. In the sector, there are international companies such as Bayer, Eastman, Lanksess, BASF and DuPont, as well as international specialist companies such as Heubach, Kronos, Cristal, Rookwood, Solvay and Omya, and local and regional companies such as Adaçal, Esen, Niğtaş and Kaltun.

2.1 Inorganic Pigments

Although there are many white pigments such as zinc oxide, zinc sulfide and antimony oxide, rutile titanium dioxide (TiO2) is the most important white pigment predominantly used in paint due to its high refractive index. A significant portion of the world's titanium reserves (ilmenite Fe+2TiO3) is located off the eastern and western coasts of Australia. It is produced by two different methods: sulfate and chloride processes (4). TiO2 pigment obtained via the chloride process is bluish white, while pigment from the sulfate process is yellowish white. The average particle size of high-quality titanium dioxide pigment is 0.3 μm (0.2–0.4 μm). Due to free electrons within its structure, TiO2 pigments are sensitive to sunlight (photocatalytic), which negatively affects the resin surrounding the particle and leads to paint chalking. For this reason, the surfaces of TiO2 pigments are coated with various materials such as Al2O3, SiO2, ZrO2 and organic components. Depending on the type of coating, TiO2 exhibits basic (Al2O3), acidic (SiO2) or neutral (Al2O3/SiO2) properties. High-coating-ratio TiO2 such as DuPont Ti Pure R 931, with coating ratios up to 20%, is preferred in matte paints due to its high opacity performance. For neutral surface-conditioned titanium dioxide types, nonionic additives are recommended as the best choice; for Al2O3-coated basic surfaces, anionic; and for silica-conditioned acidic types, cationic additives. Generally, the isoelectric point (IEP) of each pigment is used as the basis for determining the most suitable additive (dispersing agent). Uncoated and nano-sized anatase TiO2 is used in self-cleaning photocatalytic paints for the decomposition of organic contaminants, odor removal and antibacterial properties. The wide band-gap energy of anatase TiO2 (H=3.2 eV) is highly suitable for photochemical conversion of solar energy. When the coated surface is irradiated with UV rays (λ<388nm), electrons in the valence band of TiO2 are excited to the conduction band. This creates electron-hole pairs in the conduction band and positive holes in the valence band. For the surface to exhibit good photocatalytic activity, recombination of these hole pairs must be prevented. This can largely be achieved by synthesizing nano-TiO2 with a large surface area and extremely pure anatase crystal form. In 2011, the largest TiO2 producers were DuPont, Cristal, Huntsman, Kronos, Tronox and Sachtleben. Approximately one-third of the global annual 5.5–6 million tons of TiO2, 60% of which is used in the coatings and paints sector, is produced by DuPont (1.25 million tons/year) and Cristal (0.85 million tons/year). Turkey's annual TiO2 consumption, approximately 60–65 thousand tons of which is used in the paints and coatings sector, totals approximately 100 thousand tons. Iron oxides (yellow PY 42, red PR 101, brown PBr 6 and black PBk 11) account for 80% of inorganic colored pigments. Iron oxides are considerably more economical than other inorganic pigments (cobalt green, cobalt blue, nickel-titanium yellow, chromium-titanium yellow, iron blue, ultramarine). The use of pigments containing lead and cadmium (molybdate orange, chromium yellow, etc.) is declining due to their toxic properties. Like all other inorganic pigments, iron oxide pigments typically have particle sizes in the range of 0.1–10 μm. Nano-sized iron oxides with particle sizes smaller than 0.1 μm are used in transparent wood stains. To prevent corrosion on metal surfaces such as aluminum, zinc and steel, phosphate-containing pigments (zinc, aluminum, calcium, barium, aluminum-zinc phosphates) and zinc dust are used. As the use of chromate and lead-containing anticorrosive pigments declines due to toxicity and ecological concerns, they are being replaced by phosphate-containing anticorrosive pigments.

2.2 Fillers

Materials such as kaolin (china clay), calcined kaolin, talc, mica, dolomite, chalk, calcium carbonate, precipitated calcium carbonate (PCC), modified calcium carbonate, barium sulfate (barite, blanc fixe) and SiO2 are used as fillers in paints and coatings. Fibrous or sheet-form fillers reduce the risk of cracking in coatings. Fillers, like pigments, can be processed to make them easier to disperse. The color and purity of fillers vary depending on where they are found in nature; for example, talc can be white, light gray, gray-brown, or even green-brown. Through synthetic production, the purity and brightness of fillers is increased and particle size is reduced. In the paints and coatings sector, 85% of fillers are used in construction paints and 15% in industrial paints. Fillers constitute up to 15% of the raw materials used in glossy, silky matte, semi-matte and exterior facade paints, while their usage rate reaches up to 50% in matte paints. Undoubtedly, the most important filler group in coatings and paints is the carbonate group. Found in nature as chalk (amorphous), calcite and aragonite. Synthetically, it is produced (PCC) from the reaction of lime milk with CO2. Alkaline carbonates have low hardness and acid resistance. Whiteness is L > 80 in amorphous chalk, L > 85 in calcite and L > 95 in PCC. Due to low acid resistance and absorption, chalk is recommended only for interior surfaces. If the CaCO3 content in chalk is at least 96%, it is defined as Type KA, and if the CaCO3 content is at least 90%, it is defined as Type KB. The low binder requirement, high resistance to external conditions and high compatibility with other pigments of CaCO3 with trigonal crystal structure according to DIN 55918 Type C explains its high usage level. Calcites coated with stearic acid are required for different applications due to their hydrophobic properties.

Precipitated calcium carbonate PCC with trigonal crystal structure (like calcite) or rhombic crystal structure (like aragonite) is whiter and finer than natural calcite.

TiO2 in matte paints can be partially replaced with PCC. Dolomite, which is more resistant to acids and harder than calcite with its trigonal-rhombohedral crystal structure, has a chemical composition of calcium-magnesium-carbonate CaCO3.MgCO3. Among sulfates, BaSO4, found in nature as Schwerspat, finds application in paints and coatings. Chemical inertness, low oil absorption and high density are its most distinctive properties. Synthetically, it is produced as pure Blanc Fixe at 0.5–4 μm particle size. Blanc Fixe forms a good (tight) film with sheet-form fillers such as talc and kaolin. Micronized Blanc Fixe at 30–60 nm particle size is used in high-quality paints. The properties of silicate fillers differ from other fillers. Chemically inert talc, Mg3[Si4O10(OH)2], is a magnesium silicate-hydrate in plate form. High oil absorption positively affects the rheological properties of paint and hydroxyl groups positively affect adhesion in primers (5). With a Mohs hardness of 1, it provides sanding ease in primers and fillers. Kaolin (China Clay, ASP), aluminum-silicate-hydrate Al2O3.2SiO2.2H2O, can be produced finer than talcs. It has high binder requirement and high resistance to external conditions. Through calcination, crystal water is removed to produce a much harder and whiter product (Calcined Kaolin). Mica, found in nature as muscovite K2O.Al2O3.6SiO2.2H2O. It has high resistance to chemical and external conditions. It is used where chemical, heat and UV resistance and insulating properties are required. It prevents cracking in paints and coatings. Disadvantages include high price, dark color and difficulty in dispersion. Quartz (SiO2), due to its high hardness (Mohs hardness 6–7), is ground and used as a filler in floor coatings and road marking paints. Due to their properties and economy, calcium carbonate accounts for 80% of fillers used in paints and coatings. To meet sector demand, 45 different grades of calcite are produced between 0.5 μm and 7 mm. Narrow particle distribution calcium carbonate CaCO3-ID (d50/d20 < 2, isodiametric) offers many advantages in paint compared to very fine (ultra fine) calcium carbonate CaCO3-UF (6, 7). With particle sizes of 0.1–0.5 μm with no very fine particles, gaps remaining in the film during drying increase opacity in matte paints and make dispersion easier. Since TiO2 can be partially substituted, paint raw material costs decrease.

3. Conclusion

Industrial mineral producers should work in cooperation with the paints and coatings sector they serve to develop products of the required quality and guarantee standard quality production. To achieve this goal, qualified technical personnel, fast and quality technical service and a well-equipped paints and coatings application laboratory are required. Functional fillers such as calcined kaolin, precipitated calcite PCC, modified CaCO3 (containing a certain amount of air voids within its structure), CaCO3-ID and narrow particle distribution very fine kaolin can be partially substituted for TiO2, providing significant cost advantages in paints and coatings. Dr. Tahir Altunbulduk / Chemical Engineer / Board Member, Technological and Technical Affairs / İshakol Paints and Chemical Industry Inc.

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