Application of Extenders in the Coatings Industry and Research Progress

JI Xing-hong (Jotun Coatings (Zhangjiagang) Co., Ltd., Suzhou 215634, Jiangsu, China)

Abstract: This paper introduces the mineralogical characteristics and morphological features of commonly used extenders in the coatings industry. The key physical characterizations of extenders are pointed out and the influences of different types of extenders on coatings’ physical and chemical properties are discussed. Finally, the related research progress of extenders in the coatings industry is elaborated.

0 Foreword It is
well known that coatings are mainly a mixture system consisting of resin, pigments, fillers, auxiliaries and solvents. Among them, resins and pigments usually occupy a higher proportion in coating formulations. Resin is a film-forming substance. It combines pigments and fillers, and forms a uniform and dense coating film on the substrate. After curing, it forms a coating. The most basic function of pigments and fillers is to impart certain color, hiding power, chemical resistance and mechanical properties to the coating film. Inorganic fillers are also called physical pigments. Because the refractive index is similar to that of film-forming materials, they do not have the tinting power and hiding power of colored pigments. This can also reduce the cost of paint production. The filler can adjust the rheological properties of the coating, such as thickening, anti-settling, etc., and can also improve the mechanical strength of the coating film, such as improving wear resistance and durability. The filler can also adjust the optical properties of the coating and change the appearance of the coating film, such as matting. In addition, some fillers with specific shapes can effectively block the penetration of light, improve the weather resistance of the coating, and extend the service life of the coating film. With the continuous innovation of industrial technology, the use of fillers in the field of coatings is no longer based on cost. The development and use of functional fillers is very important to improve the performance of coatings.

1 Types and characteristics of fillers
In the field of coatings, fillers can be roughly divided into carbonate, silicate, silica, barium sulfate, and aluminum hydroxide according to different mineralogical characteristics and basic chemical composition.

1.1 Carbonates
Carbonates are the more common minerals in the earth’s crust. According to the different chemical composition and deposition factors, it can be divided into two categories: calcium carbonate and dolomite. The main differences between calcium carbonate and dolomite are shown in Table 1. Calcium carbonate was eroded by seawater during the change of sedimentary geological structure. A large amount of magnesium ions contained in seawater exchanged with calcium ions in calcium carbonate to form dolomite. As can be seen from Table 1, the density and hardness of dolomite are higher than calcium carbonate. In addition, calcium carbonate generally has a slightly yellow phase, while dolomite has a slightly blue phase.

Table 1 Main Differences between Calcium Carbonate and Dolomite

However, compared to dolomite, calcium carbonate is richer in resources and cheaper. It is the most widely used physical pigment in architectural latex paint. According to the different production processes of calcium carbonate, it is mainly divided into natural calcium carbonate and synthetic calcium carbonate. Among them, natural calcium carbonate is also called heavy calcium carbonate, which has a large particle size and a wide distribution. High-quality natural calcium carbonate products are based on calcite, with high whiteness, and can be made into powders of various mesh sizes required for coatings. Synthetic calcium carbonate is also called light calcium carbonate or precipitated calcium carbonate. Due to the finer particles, the oil absorption is greatly increased, and it is slightly alkaline. Synthetic calcium carbonate should not be used with pigments with poor alkali resistance. It can be used as a filler for water-based interior wall coatings in architectural coatings. Poor performance and rarely used in exterior coatings [1]. Dolomite is mainly used in industrial protective coatings and marine coatings. Carbonate-based fillers are mainly used as coating pigments in coatings to reduce costs, their oil absorption values ​​are low, and they provide better hiding power in high PVC coatings.

1.2 Silicates
Silicon and oxygen are the most widely distributed elements with the highest average content in the earth’s crust. In addition to the combination of silicon and oxygen to form SiO2 minerals, a large amount of silicate is also formed by combining with other cations. Silicate minerals are widely distributed in nature, accounting for about a quarter of known mineral species. The silicate mineral fillers commonly used in the coating field are shown in Table 2.

Table 2 Common Silicate Minerals Used in Coatings

The oxytetrahedron [SiO4] 4- composed of Si and O in the basic structural unit of silicate can be surrounded by other cations in isolation, or can be connected to each other in a common angle to form various forms of silicon oxygen. The backbone is bound to other cations. The siloxane skeletons are mainly layered, frame-shaped, and needle-shaped.
In the layered silicon-oxygen skeleton, the silicon-oxygen tetrahedrons are distributed in the same plane, and are connected to each other by three corner tops to form a two-dimensionally extended planar layer. Layered silicate mineral fillers mainly include talc powder, mica powder, and kaolin. The morphology is shown in Figure 1.

Fig. 1 Morphology of Platy Silicate Minerals

Among them, talc is a magnesium silicate mineral. The layered basic unit structure is stacked on each other by a very weak Van der Waals force, and the layers are easily separated, giving it a certain degree of softness. In the field of industrial coatings, especially in primers, the introduction of talc can improve the anti-corrosion performance and crack resistance of the coating film, and can improve adhesion and abrasiveness, and can also play a role in preventing sinking and sagging. Mica powder exists in nature in the form of multilayer crystalline flakes. The horizontal arrangement in the coating can prevent ultraviolet radiation and protect the coating film, and can also prevent moisture from penetrating. In architectural coatings, the introduction of mica powder can improve the crack resistance and scrub resistance of the coating film. A small amount of mica as a special component of steel structure primer can improve salt spray resistance and durability. The chemical composition of kaolin is hydrated aluminum silicate, also known as clay, which can be divided into washed kaolin and calcined kaolin according to the production process. The comprehensive performance of calcined kaolin is better than washed kaolin. Kaolin has good anti-settling effect in coatings, good dispersion and suspension properties, and at the same time makes the coatings have good leveling, scrub resistance and weather resistance. Kaolin can provide the structural viscosity of the coating, is good for anti-sagging and storage stability, and can also be used to enhance the hiding power of the coating film. In addition, the layered structure of the filler can also improve the bending strength and internal stress of the coating film, and prevent the coating film from cracking due to bending or expansion and deformation of the substrate.

In the frame-shaped silicon-oxygen skeleton, when the silicon-oxygen tetrahedrons share four corner tops with each other, they form a three-dimensional space-like skeleton. However, this framework structure is not completely composed of silicon-oxygen tetrahedron, but is partially replaced by aluminum-oxygen tetrahedron. Therefore, an excess negative charge appears and a framework-like complex anion is formed. Feldspar powder is a mineral filler with a typical framework structure. According to different cations, feldspar powder mainly exists in three forms of sodium feldspar, potassium feldspar and anorthite. The shape of feldspar powder is shown in Figure 2. It is composed of nodular particles with corners. Compared with spherical or ordinary block fillers, it can form a denser coating film, giving the coating film high wear resistance and scratch Wipe, improve the anti-corrosion performance of the coating film. The feldspar is composed of sodium feldspar, potassium feldspar, and feldspar. Charmite is nominally sodium aluminum silicate, but potassium replaces part of the sodium. Helioclase is a six-membered ring based on silicon-oxygen tetrahedral ring angles. Half of the Si4 + in the tetrahedron is replaced by Al3 +, and the resulting charge imbalance is compensated by Na + and K +. Different from feldspar powder, Na + can exchange with H +, which makes the acid stability of nepheline reduced.

Fig. 2 Morphology of Feldspar Powder

When the oxytetrahedrons share two corner tops with each other, they form a unidirectionally extending acicular or fibrous siloxane skeleton, such as wollastonite, whose morphology is shown in Figure 3. The chemical composition of wollastonite is calcium metasilicate, and its length is 13 to 15 times the diameter. Wollastonite can increase the bright hue of white paint, and can replace part of the titanium dioxide without reducing the whiteness and hiding power of the paint. Wollastonite can also improve the leveling properties of coatings and can also be used as a good suspending agent for coatings. Wollastonite can be used in primers to provide anti-corrosion performance and improve scratch and crack resistance.

Fig. 3 Morphology of Wollastonite

1.3 Silica
Silica (SiO2) is also very extensive in nature. It can be divided into two categories: natural and artificial. Natural products include crystalline silica, namely quartz sand, which is mainly used in the preparation of architectural real stone paints. There is also amorphous natural silica, diatomaceous earth. Due to its lower density and porosity, it is commonly used in interior wall coatings to absorb and eliminate odors. In addition, diatomite has an ionic effect, which can decompose water molecules into positive and negative ions, thereby generating strong oxidation and playing a certain sterilizing effect. Artificial products include precipitated silica and synthetic fumed silica. The uniform dispersion of the precipitated silica in the coating film can produce a slightly rough surface, which makes the light diffusely reflected and has a strong extinction effect. Synthetic fumed silica, also known as white carbon black, has a thickening effect and exhibits a certain thixotropy in coatings. In aqueous acrylic systems, the introduction of silica may reduce the aging resistance of the coating film. This is because the metal ion impurities contained in the silica will cause the photo-oxidative degradation of the coating film under the condition of ultraviolet light irradiation, and the silanol group in the silica channel will also promote the photo-degradation reaction.

1.4 Barium
sulfate Barium sulfate usually also exists in two forms, namely natural barite powder and precipitated barium sulfate. Barium sulfate is an inert substance with high chemical stability, high density, good resistance to acids and alkalis, light and heat. Barite powder is mainly used in the primer in the coating industry. With its low oil absorption and low paint consumption, it can be made into a thick film primer. And filling, leveling and anti-permeability are good, can increase the hardness and wear resistance of the coating film. In general, the performance of precipitated barium sulfate is better than natural products. It has high whiteness, fine texture and anti-frost. The disadvantage is that it has a high density and is prone to precipitation.

1.5 Aluminum hydroxide
As a flame retardant filler, aluminum hydroxide has good thermal stability, and has three major functions: flame retardant, smoke elimination and filling. It is the most important inorganic filler in fireproof coatings. Its flame retardant principle is that aluminum hydroxide releases water at high temperatures, an endothermic reaction occurs, and water evaporation consumes additional energy. After the decomposition of aluminum hydroxide, a barrier layer can be formed, which can slow the flow of oxygen and the generation rate of other gases. The generated aluminum oxide residue is deposited on the surface to isolate oxygen and achieve the effect of suppressing combustion. In addition, the low absorption of aluminum hydroxide by ultraviolet light makes it very suitable for UV curing coating systems.

2 Physical properties of
fillers There are many varieties and specifications of fillers. The use of high-quality and stable products is very important to ensure the performance of coatings. In the selection of fillers, in addition to their chemical composition and mineral morphology, characteristics such as particle size and distribution, hardness, oil absorption value, and aspect ratio of fillers must also be considered as important indicators.

2.1 Particle size and distribution
The particle size and distribution of the filler can directly affect the performance of the coating and ultimately affect the quality of the product. Therefore, the efficient and accurate determination of the particle size and distribution of filler samples has important guiding significance for both production and technological research and development.
Common methods for determining the particle size of minerals include sieving, sedimentation, light scattering, and microscopy. The sieving method is a relatively traditional particle size measurement method. The sieve is arranged from top to bottom in order from large to small pores, and the sample is divided into several particle sizes through the sieve through a rocking and shaking method. Weigh them separately to find the particle size distribution represented by the mass fraction. The sieving method has the advantages of simple equipment and easy operation, but has poor accuracy, takes a long time, and is difficult to measure for agglomerated particles.

The sedimentation method is used to test the particle size distribution of particles by different sedimentation rates in the liquid. According to Stokes’ law, the sedimentation speed of a particle is proportional to the square of the equivalent particle size. For the finer particles, the gravity sedimentation method requires a longer sedimentation time, so the centrifugal sedimentation method is usually used to accelerate the sedimentation speed, thereby shortening the measurement time and improving the measurement accuracy. The advantage of the sedimentation method is that it is simple to operate and does not require high environmental requirements, so it is widely used. However, the disadvantages of the sedimentation method are the effect of particle re-agglomeration and the Brownian motion of ultrafine particles, which will greatly affect the measurement results.
The principle of light scattering method is that the particles will be diffracted and scattered according to the light irradiated by the light, and then imaged by a Fourier lens on multiple photodetectors. The particle size of the particles is calculated by measuring the scattered light energy distribution and the corresponding diffraction angle Trail distribution. The advantages of light scattering method are high measurement accuracy, fast speed, easy operation and high repeatability. However, the laser particle size analyzer is performed on the assumption that the particles are spherical, and the diameter of the irregular particles measured is larger than that of spherical particles of the same volume.

Microscopy is a method of measuring particle size with the help of a microscope eyepiece micrometer. It can be used to directly observe and measure the particle size characteristics of individual particles. Depending on the particle size, an optical microscope and an electron microscope can be selected. For ultrafine particles with a particle size smaller than 1 μm, the high resolution of the electron microscope is more suitable. Microscopy is highly reliable and intuitive, but it has high requirements for sample preparation, complicated operations, and cannot accurately reflect the true distribution of particle size. It is not suitable for industrial quality and production control.
Therefore, each particle size determination method may produce different results. When comparing particle size data for different minerals, first make sure that the same measurement method is used. For layered fillers, the value of the sieving method is usually between the light scattering method and the sedimentation method, and the more the product tends to be layered, the greater the difference between the light scattering method and the sedimentation method. For block fillers, the values ​​given by the three methods are similar. Compared with the particle size, the particle size distribution width of the filler can better reflect the performance of the filler particles. The particle size of the filler is usually expressed as D98 and D50, but a finer filler does not mean better performance. The particle size distribution width index (SF = D50 / D20) is also very important. When SF is less than 2, it means that the particle size distribution width is narrow. Werner compared the performance of calcium carbonate with two different particle size distribution widths in coatings. The results show that narrower distribution of fillers is beneficial to improve the gloss retention and hiding power of latex paint films. It can shorten the application of anticorrosive primers. Drying time of the film, thus reducing the risk of flash rust during drying.

2.2 Hardness
Hardness is an important physical index of mineral fillers and directly affects the mechanical properties of the final coating film. Figure 4 shows the Mohs hardness index of different fillers. Mohs hardness is a standard for representing the hardness of minerals. The surface of the mineral being tested is scribed by a pyramidal diamond drill with a notch method, and the depth of the scratch is measured to represent the Mohs hardness. Among them, the filler with the highest hardness is silica, especially quartz-like crystalline silica, which can reach about 7. High hardness filler can improve the hardness, abrasion resistance and scratch resistance of the coating film. The hardness of feldspar powder and wollastonite is slightly lower than that of silicon dioxide, which can be reasonably substituted for silicon dioxide. In contrast, the hardness of talc and kaolin is low, which gives the coating film a good sandability.

Fig. 4 Comparison of Moh's Hardness of Different Extenders

2.3 Oil absorption value The
oil absorption value is also called the resin adsorption amount, which represents an index of the amount of resin absorbed by the filler. In practical applications, most fillers use the index of oil absorption value to roughly predict the demand for fillers for resin. The oil absorption value is usually expressed as the mass of linseed oil required for 100 g pigment and filler, that is, the minimum amount of oil used per 100 g of pigment and filler to achieve complete wetting. Table 3 lists the oil absorption value and density of various fillers.

Table 3 Specific Densities and Oil Adsorptions of Common Extenders

The oil absorption value of a filler reflects the combined effects of many of its physical properties, such as particle shape, particle size distribution, and specific surface area. The finer the particles of the filler, the larger the surface area, the narrower the distribution, and the higher the oil absorption value. Needle particles have a higher porosity, and their oil absorption values ​​are usually higher than spherical particles. There is also a certain relationship between the oil absorption value and the density of the powder. The powder with a higher density generally has a lower oil absorption value. For example, the oil absorption values ​​of barium sulfate and titanium dioxide are relatively low. In addition, an important parameter in coatings is also closely related to the oil absorption value, namely the critical pigment volume concentration (CPVC).
In the vicinity of CPVC, the properties of coatings will change drastically, so it is often used as a reference value for formula design. In solvent-based systems, the relationship between CPVC and oil absorption value can be expressed as CPVC = 1 / [1+ (OA × r) /93.5], where OA represents the oil absorption value of pigment and filler, and r represents the density of pigment and filler. The higher the oil absorption value, the lower the CPVC. In the water-based system, since the base material particles and pigment particles form a close stack together during the film formation process, the base material particles are deformed to form a continuous coating film. Therefore, CPVC is also closely related to the properties of the emulsion. Formula for calculation. But no matter what kind of system, the oil absorption value is an important factor affecting the CPVC of the coating, so it also has a direct impact on the performance of the coating film.

2.4 Length to diameter ratio
As mentioned earlier, fillers can be expressed in three basic shapes. Lump fillers such as calcium carbonate, feldspar powder, and feldspar; layered fillers such as talc, mica powder, and kaolin; acicular fillers such as wollastonite. For the layered and acicular particles, the aspect ratio can be further characterized. As shown in FIG. 5, the aspect ratio of the acicular particles is the ratio of the average particle length L to the average diameter D. For layered particles, the aspect ratio is the ratio of the average diameter D of the equal-area circles on the layer to the average thickness T of the layer. The barrier effect of layered filler with high aspect ratio is better. The aspect ratio of most fillers is in a lower range (less than 10), and the aspect ratio of wollastonite can reach 13-15.

Fig. 5 Calculation of the Aspect Ratio

Although fillers with small particle size and high specific surface area help to improve the durability of the coating film, they also increase the amount of resin and reduce the CPVC level, so the amount of such fillers is limited. The strengthening of the coating film and the balance of the amount of resin can be controlled by the combination of fillers. For example, wollastonite can be used as a substitute for layered silicate, because its needle-like structure has a relatively low specific surface area, which can still provide good mechanical properties at the same time of high usage. As shown in Figure 6, particles with a high aspect ratio tend to aggregate in a stacking manner, resulting in more gaps. Therefore, choosing needle-shaped or layered high aspect ratio fillers with wider particle distribution can fill these gaps instead of Resin, which improves CPVC without sacrificing the mechanical strength of the coating. For high aspect ratio fillers with narrow particle distribution, it can be used with finer particle size fillers, such as calcium carbonate, to achieve the same purpose.

Fig. 6 Stocking of High-aspect-ratio Extenders

3 Research progress of fillers in coatings
The filler plays a supporting role in the coating, can impart certain mechanical properties to the coating film, and can also adjust the rheological and optical properties of the coating. Therefore, fillers have been extensively studied in various coating systems.
Kang Sibo and others selected feldspar powder, aluminum tripolyphosphate, and mica powder as the main pigments and fillers to prepare a low-temperature curing solvent-free epoxy heavy-duty anticorrosive coating. Among them, feldspar powder can improve the flexibility of the coating, increase the resistance to media penetration, mica powder has excellent heat resistance and elasticity, enhance the coating’s mechanical properties and resistance to high and low temperature alternation. Wu Linlin and others compared the effects of calcium carbonate, wollastonite, kaolin, aluminum hydroxide, and talc on the thermal insulation effect, carbon layer quality, and swelling height of fire-resistant coatings. The results show that kaolin and talc have the same effect as wollastonite, but the performance is not as good as wollastonite, and calcium carbonate is not suitable for fire-resistant coatings. The effect of wollastonite and aluminum hydroxide is better. Wollastonite plays a skeleton role, and aluminum hydroxide plays a role in suppressing smoke and cooling. Igwebike-Ossi et al. Compared the application of kaolin and calcium carbonate in latex paints, and the results showed that kaolin had better hiding power, anti-settling, and paintability than calcium carbonate systems. Zhang Xinyu studied the effect of fillers on the tensile properties of polymer emulsion architectural waterproofing coatings. It was found that the tensile strength of waterproofing coating films prepared with precipitated barium sulfate as filler was low, while the waterproofing coating films prepared with kaolin as filler were more flexible difference. This is because when the filler has a high oil absorption value, the polymer emulsion is in a dispersed phase, the filler is in a continuous phase, and the coating film is somewhat
rigid. Using quartz powder or heavy calcium as a filler, a polymer emulsion architectural waterproof coating with a large margin of elongation at break and tensile strength performance can be obtained. Liang et al. [14] studied the effect of filler types on the water permeability of high PVC exterior wall latex paint. The results show that the heavy calcium carbonate with lower oil absorption alone can be densely packed in the coating film, has less pores, and has better water permeability. Kaolin, mica, and wollastonite increase the porosity of the coating film, which makes the water permeability of the coating film rapidly deteriorate.
In addition, nanocrystallization of fillers is also a hot topic of current research. In powder coatings, the introduction of nanometer calcium carbonate can reduce the density of powder coatings and increase the spraying area. At the same time, it has good charging performance and improves the powder coating rate. However, the nano-filler has a high surface energy, is in a thermodynamically unstable state, and is extremely easy to agglomerate. And because of its high polarity, nano-fillers are difficult to disperse in organic media, and it is easy to form surface defects, which leads to a decline in coating film performance. Therefore, the surface treatment of the filler appears very important, that is, an inorganic film or a polymer film of a certain thickness is formed on the surface of the filler particles. This surface treatment can adjust the acid-base balance and surface hydrophobicity of the filler, thereby reducing the agglomeration between filler particles and improving the grinding dispersibility in the coating.

4 Future Trends In the
future, research and development of inorganic fillers in the coatings field will mainly be carried out from three aspects, namely, stable, functional and environmentally friendly. For the development of stable fillers, the first is to strictly control the differences between batches from the production process to ensure the stability of downstream coating products. In addition, it is necessary to combine different coating systems and application sites to improve the quality stability of the filler itself. For example, outdoor coatings need to consider the application of surface coating technology from the perspective of weather resistance and chemical stability. In fire-resistant coatings, treatment methods such as ultra-fine miniaturization, high purification, or composite technology can improve the thermal stability of fillers. For functional fillers, in the field of architectural coatings, the whiteness of the filler is improved through the screening of the original ore and the optimization of the production process, and the optimal particle size and distribution are obtained from the perspective of the hiding power of the coating film. In the field of industrial protective coatings, the contribution of fillers’ acid resistance, alkali resistance, salt spray resistance and scratch resistance is achieved. For example, the use of the barrier structure characteristics of high aspect ratio sheet fillers enables better protection of the coating film. The continuous upgrading of the current environmental protection regulations has also promoted the transformation of fillers into environmentally friendly directions. Traditional fillers containing asbestos and heavy metals will be abandoned. In addition, the partial replacement of titanium dioxide obtained from high energy consumption is also an important direction for the development of fillers in the future. This is a new way to achieve low carbon emissions and sustainable development.

5 Conclusion
With the continuous development of coating technology, the performance requirements for its components are also getting higher and higher. For the use of inorganic fillers, it is not only used as a supporting frame for coating films, but also for researching and developing its contribution to various properties of coating films. Therefore, it is very important to understand the mineralogical characteristics of inorganic fillers and to seek the purpose of reasonable and efficient utilization.

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