Selection and application of fillers in latex paint

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Selection and Application of Fillers in Latex Paints Guangzhou Tiantai Chemical Light Industry Co., Ltd. Authors: Lü Huaqi, Wang Xinchun, Hu Huanbing. It outlines the basic physical properties of common fillers used in coatings, discusses briefly how to select fillers for coating production, proposes guidelines for filler selection and quality control, and finally analyzes the development trends regarding the use of fillers in the coating industry. 1 Introduction Due to its safety, environmental friendliness, ease of application, and rapid drying time, latex paint has seen rapid adoption and development in the construction industry. As research on latex paint technology progresses, there are increasing demands regarding the performance and quality of raw materials. Technologies related to emulsions, additives, and pigments have also become more mature. However, relatively little research has been conducted on fillers, and their selection and use are often not appropriate. Additionally, due to the relatively weak production capabilities of some small and medium-sized coating manufacturers in China, there is a lack of understanding regarding the proper use of fillers. As a result, many companies use high-quality emulsions and additives but are unable to produce high-quality latex paint. Based on the current situation of filler usage in latex paint production, this article provides a brief discussion on how to select fillers in a correct and appropriate manner. 2 Common Fillers Used in Latex Paints and Their Properties There are many types of fillers used in latex paint; common ones include heavy calcium carbonate, light calcium carbonate, talcum powder, wollastonite powder, sericite powder (mica powder), kaolin, precipitated barium sulfate, bentonite, hydrated lime powder, ultra-fine aluminum silicate, and quartz powder. Calcium carbonate comes in two forms: natural and synthetic. The former is heavy calcium carbonate, commonly referred to as heavy calcium, while the latter is light calcium carbonate, commonly referred to as light calcium. Calcium carbonate is also known as chalk, double-flying powder, calcite, etc. Insoluble in water, soluble in acids, high density, prone to precipitation. The product has a low cost; its use in latex systems improves color retention. It provides moderate coverage and tends to form a white bloom. As opposed to heavy calcium, light calcium is obtained by calcining limestone at high temperatures to produce lime milk, which is then mixed with CO2 gas to induce precipitation; it is characterized by a low density and fine particles. In latex paint, it can improve color retention and suspension properties, and provides a certain level of dry coverage; its drawback is that it tends to form white frost. Talc is a hydrated magnesium silicate salt that is obtained by grinding natural talc ore. It has a hexagonal, rhombic plate-like crystal structure and a smooth texture; when used in latex paints, it improves the paint’s workability and leveling properties. Its drawback is that it tends to become dusty easily. China has abundant talc resources, with the talc in the Northeast and Guangxi regions being of the highest quality. Wollastonite powder: The main component of wollastonite powder is CaSiO3, and its particles have a needle-like, rod-like, or fibrous structure. Used in latex paints, it gives white coatings a bright hue, provides some level of dry coverage, helps improve the wear and durability of the paint film, and enhances its hardness. Kaolin is a clay whose main component is kaolinite; its chemical formula is AL2O3·2SiO2·2H2O. It is soft and easily dispersible in water. It features high whiteness, good dispersibility, and strong covering power. When used in combination with titanium dioxide, it can enhance the overall performance of coatings. Sericite powder (mica powder) is a fine-grained form of white mica; it belongs to the class of silicate minerals with a layered structure, and its crystals are flaky in shape. High elasticity, bendability, acid resistance, alkali resistance, and chemical stability. When used in latex paints, it significantly improves the weather resistance of the paint film, prevents water vapor from penetrating, stops cracking, and delays flaking; it is an important functional filler that has been widely used in recent years. The processing methods for sericite powder generally include dry and wet methods; the main difference lies in the processing techniques used. The dry method employs traditional processing techniques, that is, mechanical force is used for crushing, with wind power being utilized to achieve the desired fineness of the product ； The wet process uses water as a medium; it involves the separation of sericite from quartz and other minerals in the mineral mixture, as well as the peeling apart of mica flakes and their screening, in order to control the particle size of the final product ； The main differences between the products obtained from these two processes lie in the purity of the products, the degree to which the mica structure is preserved, and the aspect ratio of the mica particles. The differences in their role within the coating system manifest as varying contributions to the weather resistance, water resistance, and corrosion resistance of the coating film. Different types of fillers exhibit varying performance in coating systems; even for the same filler, differences in processing methods or product forms can result in variations in its performance. With the improvement of processing and application technologies for non-metallic materials in our country, many new types of fillers are now available on the market. The proper use of various fillers can enhance the properties of coatings. 3. Selection of fillers for latex coatings: Choosing the right type of filler for use in latex coatings, as well as using fillers of appropriate specifications, can significantly improve the quality of the coating. On the other hand, an inappropriate choice can lead to unnecessary problems. Different types of latex paints should be chosen based on the application area. Latex paints are divided into those for interior walls and those for exterior walls; for exterior wall paints, fillers with good weather resistance and low tendency to crumble are preferred. Materials such as sericite, barium sulfate, wollastonite, and calcined kaolin are recommended, while light calcium is generally not used ； For interior wall coatings, it is recommended to use products with high brightness such as heavy calcium carbonate, light calcium carbonate, talc, and kaolin, along with ultra-fine powders. (To be continued) Table 1: Performance and main characteristics of common filler types. Name, Main Components, Structure, Dry Cover Power, Suspension Property, Characteristics, Defects:
Calcium carbonate: Granular, Medium, Poor; Low cost, prone to sedimentation.
Light calcium carbonate: Granular, Medium, Good; Good suspension property, prone to forming white frost and swelling.
Wollastonite powder: CaSiO3, Needle-shaped, Fibrous, Medium, Poor; Improves film strength and scrub resistance, prone to sedimentation.
Talc powder: 3MgO.4SiO2.H2O, Flaky, Medium, Good; Improves the application properties and leveling of coatings, prone to pulverization.
Kaolin: Al2O3.2SiO2.2H2O, Flaky, Tubular, Medium, Good; Improves cover power and suspension property.
Cottony mica powder: K2O.3Al2O3.6SiO2.2H2O, Flaky, Medium, Good; Improves weather resistance, water resistance, crack resistance, and delays pulverization.
Precipitated barium sulfate: BaSO4, Granular, Medium, Poor; Improves film stain resistance, prone to sedimentation.
Quartz powder: SiO2, Granular, Medium, Poor; Improves film strength, prone to sedimentation. The fineness level should be selected based on the type of filler used. Fillers such as calcium carbonate, barium sulfate, talc powder, and calcined kaolin serve both as volume fillers and provide a certain degree of dry cover power. It is generally recommended to use ultra-fine products, as these fillers have a synergistic effect on the cover power of titanium dioxide. When the particle size of the fillers reaches a fine level, close to that of the titanium dioxide pigment used, it enhances the cover power of titanium dioxide while also improving the strength and water resistance of the coating film. Wollastonite and sericite are used to improve the strength, weather resistance, water resistance, etc. of the paint film. Products with a particle size of around 800 mesh are generally recommended; if the appearance of the paint film is a concern, products with a particle size of around 1250 mesh can be used. The use of products with a particle size of over 2000 mesh is generally not advised. Different fillers should be selected based on the required CPVC concentration. CPVC refers to the critical pigment volume concentration, which is the pigment volume concentration at which the matrix just covers the surface of the pigment particles and fills the space between them. The finer the filler, the greater its specific surface area and oil absorption value, and thus the lower the CPVC value. Generally, the PVC content in latex paint formulations does not exceed that of CPVC; otherwise, many properties of the paint film will be adversely affected. As competition in the latex paint market becomes increasingly fierce, developing latex paints with a higher PVC content has become an important research topic for various paint manufacturers, in order to reduce costs and gain a competitive advantage. Even two latex paint formulations with the same PVC value can have different CPVC values due to differences in the raw materials used and their proportions. Therefore, to achieve a high PVC value in latex paint while ensuring that its quality meets **standard requirements, the key is to have a high CPVC level in the paint, thereby minimizing the difference between its PVC and CPVC values. When designing paint formulations, high-performance latex paints use ultra-fine fillers, while lower-quality latex paints employ fillers with a relatively coarser particle size and low oil absorption values, such as heavy calcium carbonate and barite. 4. Control of filler specifications: There are a great variety of filler types and specifications available on the market, which creates confusion. Choosing high-quality and stable products is essential for ensuring the proper production of latex paints. Due to the chaos in the current market, some filler products lack standardized specifications or industry standards; it is common for substandard products to be passed off as genuine ones, which poses many difficulties for latex paint manufacturers when selecting raw materials and can result in losses. The normal parameters for fillers used in latex paint are: product purity, whiteness, particle size, residue on a 325-mesh sieve, pH value, and oil absorption. Product purity is one of the most important indicators of a product, and functional fillers are particularly important. High-quality talc powder can improve the application properties and leveling of latex paint ； High-purity aragonite can **increase the strength of the paint film ； Flaky sericite layers upon each other within the coating film, enhancing the strength and water resistance of the paint film; it also possesses a unique UV shielding capability that improves the weather resistance of the coating. Talc and wollastonite fillers are evaluated based on the SiO2 content; the higher the SiO2 level, the purer the product. For fillers such as sericite and kaolin, which are aluminum silicates, control is carried out using the SiO2 and Al2O3 contents. Calcium carbonate is determined by its CaCO3 content. Whiteness is one of the criteria for customers when choosing fillers; fillers such as heavy calcium carbonate, light calcium carbonate, and kaolin, which rely on their dry covering power, are particularly important ； At the same time, the hue of the filler should be considered, with a cyan hue being the best. Particle size: In the market, it is generally determined based on the mesh number, but this method is not very scientific. Moreover, many manufacturers pass off products with a lower mesh number as those with a higher one. It is generally recommended that customers request a particle size distribution chart when selecting raw materials; using the median particle size for determination might be more scientific and useful. The main parameters of fillers can be effectively controlled, which is highly beneficial for the stability of product quality. When selecting functional fillers, it is also necessary to understand the product manufacturing process; for example, there are significant differences in the properties of products produced by high-temperature firing or electric firing methods for kaolin products, and similar differences exist among products made using dry or wet processing methods for sericite powder. When choosing fillers, customers must pay close attention to relevant specifications, gain a good understanding of the manufacturers, and ensure the batch stability of the raw materials. It is also recommended not to change the product specifications or manufacturers arbitrarily. 5. Functionalization of fillers in latex paints: As research and development in latex paints continue to advance, consumers have higher expectations regarding the functions of these products. The pursuit of high performance and multiple functions in latex paints has become a trend in the coating industry. For example, interior wall latex paint is environmentally friendly and non-toxic, with antibacterial and antifungal properties ； The aging resistance of exterior wall coatings, self-cleaning function, etc. Traditional fillers serve as a matrix for the skeletal structure in coatings, and they have little effect on improving the functional properties of the coatings ； In recent years, some domestic and international latex paint manufacturers have begun to study the functions of certain fillers in order to improve the paints or endow them with new properties, and have achieved certain results. Due to differences in their chemical composition and crystal structure, functional fillers require different processing and application techniques. By improving these processing techniques, it is possible to enhance the specific properties of the fillers or endow latex paints with new functions; a notable example is the contribution of flaky sericite powder to the weather resistance of coating films ； Adsorptive fillers improve the storage stability of coatings ； The improvement of properties such as antibacterial and weather resistance in coatings due to nano-fillers, as well as the widespread use of various new functional fillers. Flaky sericite powder exhibits a crystal polarizing effect as well as an optical interference effect due to the interlayer crystalline water; it has a strong absorption and shielding effect against ultraviolet rays. In latex paints, this powder not only improves the mechanical properties of the coating but also enhances its resistance to aging and ultraviolet damage, prevents cracking and powdering, and helps maintain the color of the paint over time. Nanofillers are generally prepared through synthetic processes, or by grinding non-metallic powders with nanoscale structures such as nano-calcium carbonate and nano-ZnO. When used in coating systems, they endow the coatings with antibacterial and antifungal properties while also improving the weather resistance of the coating film, thereby **enhancing the overall quality of the coatings. 6. Conclusion: Latex paint products are developing rapidly, but competition is also intensifying. To remain competitive in the market, quality and cost remain eternal priorities. The cost and quality of coatings are closely related to the choice of raw materials; we cannot focus only on emulsions, titanium dioxide, and additives – we must also pay attention to the selection and control of fillers. Fillers account for 20-40% of the components in latex paint; high-quality and stable fillers are essential for producing superior products. As research in coating technology continues to advance, more new functional materials will undoubtedly be incorporated into coating systems.