The selection of fillers for coatings has rapidly developed due to the characteristics of latex paint, such as low odor, ease of application, quick drying, and safety and environmental protection. With the gradual deepening of research on latex paint technology, domestic technology in terms of emulsions, additives, and pigments has become increasingly mature. However, the selection of fillers is not very rigorous, and research is relatively scarce. Additionally, due to the relatively weak production technology of some small and medium-sized paint enterprises in China and insufficient understanding of filler application technology, many companies choose high-quality emulsions. This article briefly discusses how to correctly and reasonably select fillers based on the current situation of filler application in latex paint production.

1 Common Fillers Used in Latex Paint and Their Characteristics

There are many types of fillers that can be used in latex paint, with common varieties including heavy calcium carbonate, light calcium carbonate, talc powder, wollastonite powder, mica powder, precipitated barium sulfate, bentonite, gray calcium powder, ultra-fine aluminum silicate, quartz powder, etc.

1.1 Calcium Carbonate

Calcium carbonate comes in two forms: natural and synthetic. The former is heavy calcium carbonate (referred to as heavy calcium), while the latter is light calcium carbonate (referred to as light calcium). Heavy calcium is also known as whiting, double-flying powder, calcite, etc. It is insoluble in water, easily soluble in acid, has a high density, and is prone to sedimentation. Its product cost is low, and its use in latex systems can improve color retention, has a slight dry hiding power, and is prone to whitening. Light calcium is made from limestone that is calcined at high temperatures to form lime milk, which is then combined with CO2 gas and precipitated and dried. Its characteristics include low density and fine particles, which can improve color retention and have good suspension in latex paint, with a slight dry hiding power, but it is also prone to whitening.

1.2 Talc Powder

Talc powder is an inorganic salt containing hydrated magnesium silicate, made by crushing natural talc ore. Talc powder has a hexagonal plate-like crystal structure and a slippery feel. Its use in latex paint can improve the workability and leveling of the coating, but it is prone to dusting.

1.3 Wollastonite Powder

The main component of wollastonite powder is CaSiO3, and the product has a needle-like, rod-like, or refractive fibrous structure. Its use in latex paint can give white coatings a bright tone and provide some dry hiding power, while also helping to enhance the wear resistance and durability of the paint film, increasing its hardness.

1.4 Calcined Kaolin

Kaolin is a clay primarily composed of kaolinite, with the main composition being AL2O3·2SiO2·2H2O. It is soft and easily dispersible in water. Calcined kaolin is one of the functional fillers that have emerged in recent years, characterized by high whiteness, good dispersibility, and strong hiding power. Its combination with titanium dioxide can enhance the overall effect of the coating.

1.5 Mica Powder

Mica powder is a fine-grained white mica, belonging to the layered structure of silicates, with flaky crystals. It is elastic, flexible, acid-resistant, alkali-resistant, and has good chemical stability. Its use in latex paint can significantly enhance the weather resistance of the paint film, prevent moisture penetration, inhibit cracking, and delay dusting, making it an important functional filler used in recent years. With the improvement of non-metallic material processing technology and application technology in China, many fillers have emerged, and the reasonable application of various fillers can improve the performance of coatings. Below is a brief comparison of the performance of several commonly used fillers and their roles in coatings.

2 Selection of Fillers in Latex Paint

Reasonably selecting the type and specifications of fillers in latex paint can significantly enhance the quality of the coating. If the selection is inappropriate, it may lead to unnecessary troubles.

2.1 Choosing Different Varieties Based on the Application Range of the Coating

Latex paint is divided into interior wall coatings and exterior wall coatings. Generally, for exterior wall coatings, fillers with good weather resistance and low dusting are selected, such as mica, barium sulfate, wollastonite, and calcined kaolin, while light calcium is generally not used. For interior wall coatings, it is recommended to use high whiteness products such as heavy calcium, light calcium, talc powder, and kaolin, along with ultra-fine powders. Table 1 compares the product performance of several conventional fillers.

2.2 Choosing Different Fineness Based on the Type of Filler Used

Fillers such as heavy calcium, barium sulfate, talc powder, and calcined kaolin serve a dual purpose in coatings: they provide physical filling and have a certain degree of dry hiding power. It is generally recommended to use ultra-fine products, as fillers have a synergistic effect on the hiding power of titanium dioxide. When the particle size of the filler reaches a micro-fine level, close to the particle size of the titanium dioxide pigment used, it can enhance the hiding effect of titanium dioxide while improving the strength and water resistance of the paint film. The use of wollastonite and mica is aimed at enhancing the strength, weather resistance, and water resistance of the paint film. It is generally recommended to use products around 800 mesh, and if considering the effect of the paint film, products around 1250 mesh can be adopted. Products above 2000 mesh are generally not recommended.

2.3 Choosing Different Fillers Based on CPVC Concentration Requirements

Critical Pigment Volume Concentration (CPVC) refers to the pigment volume concentration when the base material just covers the surface of the pigment particles and fills the space of the pigment particle accumulation. The higher the fineness of the filler, the larger its specific surface area and oil absorption value, resulting in a smaller CPVC. Typically, the PVC of latex paint formulations does not exceed the CPVC; otherwise, it will affect many physical properties of the paint film. As competition in the latex paint market becomes increasingly fierce, developing high PVC latex paint has become an important research topic for various latex paint manufacturers to reduce costs and enhance competitive advantages. Two latex paint formulations with the same PVC value may have different CPVC values due to differences in raw materials and proportions used. Therefore, to achieve a high PVC value in latex paint while ensuring quality meets national standards, the key is to achieve a high CPVC, thereby minimizing the gap between PVC and CPVC values. In designing coating formulations, high-performance latex paints use ultra-fine fillers, while lower-grade latex paints choose relatively coarse fillers with low oil absorption values, such as heavy calcium and barite.

3 Control of Filler Indicators

There are many types and specifications of fillers, and compared to the chaos, choosing high-quality and stable products is essential to ensure the quality of latex paint. The indicators to consider when using fillers in latex paint include: product purity, whiteness, particle size, residue on a 325 mesh sieve, pH value, and oil absorption. Purity is one of the most important indicators of a product, especially for functional fillers. High-quality talc can improve the application performance and leveling of latex paint; high-purity wollastonite can significantly enhance the strength of the paint film; flaky mica can layer in the coating to enhance the strength and water resistance of the paint film, while also providing unique UV shielding capabilities to improve the weather resistance of the coating. For talc and wollastonite fillers, the SiO2 content is used as a standard; the higher the SiO2 content, the purer the product. For fillers like mica and kaolin, the SiO2 and Al2O3 content is controlled; for calcium carbonate, the CaCO3 content is used as a standard. Whiteness is one of the criteria for customers to choose fillers, especially important for fillers like heavy calcium, light calcium, and kaolin that utilize dry hiding power; the hue of the filler should also be considered, with a bluish hue being optimal. In the market, particle size is generally determined by mesh number, which is not very scientific, and some manufacturers may misrepresent low mesh products as high mesh. It is generally recommended that customers request a particle size distribution chart when selecting raw material products, as determining based on median particle size is more valuable. When selecting functional fillers, it is also necessary to understand the production process of the products, as there can be significant performance differences between kaolin products produced by coal burning and electric burning processes, and also between mica powder products produced by dry and wet methods. Customers must strengthen their understanding of the indicators and control over the manufacturers, ensuring the stability of raw material batches, and it is also advised not to arbitrarily change the specifications and manufacturers of the products used.

Functionalization of fillers for latex paint

As the research and development level of latex paint continues to improve, consumers have higher demands for product functionality. The high performance and multifunctionality of latex paint have become the development trend in the coatings industry. For example, interior wall latex paint is environmentally friendly, non-toxic, and has antibacterial and anti-mold properties; exterior wall coatings have aging resistance and self-cleaning functions, etc. Traditional fillers mainly serve as a skeletal structure in the coating and have little effect on improving the functionality of the coating. In recent years, some latex paint manufacturers both domestically and internationally have begun to research certain functional fillers to improve or impart new functions to coatings, achieving certain results.

Functional fillers differ in their chemical composition and crystal structure, leading to variations in processing and application technologies. Through advancements in processing technology, the characteristics of fillers can be improved or new functions can be imparted to latex paint. Key representatives include flaky mica powder contributing to the weather resistance of the coating; adsorbent fillers enhancing the storage stability of the coating; and the enhancement of antibacterial and weather resistance properties of coatings through the application of nano-fillers, as well as the widespread use of some new functional fillers. Flaky mica powder possesses crystal birefringence and the light interference effect of interlayer crystallization water, providing strong absorption and shielding against ultraviolet rays. In latex paint, it not only improves the mechanical properties of the coating but also enhances the aging resistance and UV resistance of the coating, preventing cracking and delaying chalking, while also maintaining the color of the coating for a long time. Nano-fillers are generally synthesized or processed from non-metallic powders with nano-structures, such as nano-calcium carbonate and nano-ZnO, applied in coating systems to provide antibacterial and anti-mold effects while enhancing the weather resistance of the paint film, significantly improving the overall quality of the coating.

Conclusion

The development of latex paint products is rapid, but competition is also becoming increasingly fierce. How to maintain an invincible position in the market is an eternal theme of quality and cost. The cost and quality of coatings are closely related to the selection of raw materials; attention should not only be paid to emulsions, titanium dioxide, and additives, but also to the selection and control of fillers. Fillers account for 20% to 40% of latex paint components, and high-quality, stable fillers are essential for producing quality products. With the continuous in-depth research of coating technology, Hanlong Chemical will undoubtedly have more and more new functional materials applied in coating systems.