Fillers are solid particles used in paint and coating systems to:
 
- Improve properties
- Lower costs
 
Hence, there are several aspects that need to be considered when selecting a filler for coatings because the key properties of the system are controlled by the properties of the solid particles used. These properties include:
 
- Ease of production
- Application behavior
- Properties of the resulting film
- Storage stability
 
Now, let’s discuss some important system properties and understand how these system properties are influenced by the fillers used. Paint and coating systems are divided into three main categories:
 
1. Paints, which are liquid systems
2. Coatings, which form a layer on a substrate
    - Primers, the layers that are directly applied to the substrate
    - Topcoats, which are in direct contact with the surrounding environment
 
Filler Selection for Paint
 
Fillers play a significant role in the key properties of paints. Their influence is important throughout production, storage, and application. The main factors affected by fillers are as follows:
 
Paint pH 
The pH of water-based paints, for example, is important for colloidal stability. Some fillers can strongly affect the pH, while others have little to no effect. The surface composition and chemical stability of the filler particles control their effect on pH.
 
Effect of Sedimentation Rate on Filler Selection
The sedimentation rate in paints is primarily controlled by:
 
- Viscosity
- Density
- Particle size of the solids
 
Larger, denser particles settle faster than smaller, less dense ones. When considering the primary particle size of fillers, the average diameter of individual primary particles is often given, which is referred to as D-50. The particle size may be given in micrometers (µm) or nanometers (nm).
 
Paint Stability
Various types of solid particle stability in liquid systems are important, including:
 
- Sedimentation
- Dissolution
- Chemical stability
- Flocculation (the spontaneous adhesion of separate solid particles in liquid systems). This undesirable process can occur during production, storage, application, and film formation. Flocculation can be avoided by arranging repelling forces between the particles through the adsorption of specific additives, called dispersants, on the surface of solid particles.
 
Whether a dispersant will be able to adsorb onto the surface of the filler particles is determined by:
 
- The chemical composition and morphology of the dispersant molecules
- The surface composition of the solid particles
 
Most fillers are hydrophilic, meaning there are polar groups on the surface of the particles. An exception is **talc**, which is a filler with a relatively hydrophobic surface. Hydrophilic particles are easily stabilized as dispersant molecules can strongly adsorb onto the polar groups present on the surface of hydrophilic particles.
 
The dissolution and possible chemical reactions of the filler primarily occur during storage and depend on the chemical composition of the filler. Selecting fillers that have chemical resistance to the liquid environment is important; fillers must be sufficiently durable.
 
Some fillers are not inert, meaning that solid particles can, for example, be attacked by certain chemicals, solvents, acids, bases, or UV radiation. For example, calcium carbonate (CaCO₃) is a filler that slowly dissolves in an acidic environment. This means that care should be taken when using calcium carbonate in water-based systems with low pH or when the system is applied to an acidic substrate.
 
Effect on Equipment Wear
Equipment may be damaged both during production and during the use of the paint. Fillers that are highly hard with sharp edges (shape) can damage equipment through abrasion.
 
The hardness of a filler indicates how easily or difficultly the solid can undergo mechanical damage. The Mohs hardness scale is used to quantify the hardness of pigments and fillers. The hardest mineral, diamond, has a Mohs hardness of 10, while talc, a soft filler, has a Mohs hardness of 1.
 
Hard fillers, such as quartz, can abrade production equipment, especially when the particles have sharp edges.
 
Effect on Viscosity 
The viscosity of liquid systems is influenced by fillers depending on the size, shape, and surface composition of the filler particles.
 
Particles with irregular shapes increase the viscosity of a system. Fine or porous particles that absorb large amounts of resin materials cause high viscosity.
 
Filler Selection for Primers
 
A primer is a coating layer that is in direct contact with the substrate. Let’s discuss how fillers affect the key properties of primers:
 
Effect on Primer Adhesion
The adhesion of a coating is primarily controlled by the binder in the system, which may be combined with special adhesion-enhancing additives. Some fillers with a platelet shape can improve adhesion due to two effects:
 
- The platelets form a barrier, thus increasing the path that molecules need to take through the coating to reach the substrate-primer interface.
- Platelet particles provide mechanical strength known as cohesion to the film, thereby improving adhesion.
 
Effect on Barrier Properties
The permeability of a coating refers to the ability of small molecules to diffuse through the coating. Since most filler particles are impermeable solids, small molecules cannot pass through the filler particles: they must move around the particles. A coating with low permeability to small molecules is said to have good barrier properties.
 
Several factors govern the permeability of a film:
 
- Resin adsorption on the solid particles. When adsorption is weak or absent, small molecules can diffuse along the filler particles.
- Shape of the particles. Platelet-shaped filler particles, such as talc or mica, act as “roof tiles” and reduce permeability when they orient during film formation.
 
Effect on the Chemical Stability of Primer Coatings
The chemical resistance of fillers is controlled by the chemical composition of the filler. Most fillers have good resistance (= stability) to chemicals and solvents. An exception is calcium carbonate (CaCO₃), which has poor resistance to acids. Fillers with high chemical stability should be used in metal primers.
 
Effect on Corrosion Resistance
Fillers with a platelet shape can improve corrosion resistance by forming a barrier, thus delaying the diffusion of molecules that can worsen corrosion, such as oxygen, salt, and acids. Also, fillers with high pH can delay corrosion because corrosion progresses more quickly in acidic environments. Basic fillers, like dolomite and feldspar, are often used in anti-corrosive primers because metal corrosion proceeds more slowly at high pH. This beneficial effect of basic fillers on corrosion resistance is called passivation.
 
Effect on Glossiness
Primers preferably have low gloss, meaning that the surface of the primer is uneven. The rough surface of the primer improves the adhesion between the topcoat and the primer. Fillers with high oil absorption value and high specific surface area effectively reduce the glossiness of coatings.
 
Oil Absorption
The oil absorption value of a filler is defined as the amount of oil, in grams, required to create a flowing paste from 100 grams of powdered filler. For non-porous particles, the oil absorption value is directly related to the specific surface area of the material. The oil absorption value of solid materials used in a liquid system is important for the key properties of the resulting system. For example, solid particles with high oil absorption values create stronger viscosity increases in the liquid system compared to solids with low oil absorption values. Fillers with low oil absorption values should be used in systems where low viscosity is required, such as solvent-free or high-solids systems. Oil absorption value is controlled by the key properties of the filler particles:
- Smaller particles have higher oil absorption values compared to larger particles.
- Spherical and smooth particles with closed (non-porous) surfaces have low oil absorption values.
- Irregularly shaped particles have higher oil absorption values, especially when they are porous.
 
Pigment Volume Concentration (PVC)
PVC is defined as the volume percentage of solid particles, pigments, and fillers in a system that forms a layer. This means that the PVC of a system must be calculated without volatile components such as water and/or solvents. Formulations are often given in terms of weight. Since PVC is approximately a volume percentage, all the component weights must be converted to volume using the density of each component. A filler with high density will have little impact on the PVC of a system.
 
Effect on the Sandability of Primer Coatings
Primers are often sanded before the topcoat is applied. Good sandability can be achieved in primers by using fillers with low Mohs hardness (see Equipment Wear), such as talc.
 
Filler Selection for Topcoats
The topcoat is the layer that comes into direct contact with the surrounding environment. Let’s discuss how fillers affect the key properties of topcoat coatings:
 
Abrasion and Scratch Resistance in Topcoats
Resistance to mechanical damage such as abrasion or scratches can be improved in a coating by using fillers with high Mohs hardness (see Equipment Wear). Hard fillers like quartz and feldspar improve the coatings’ resistance to mechanical wear such as abrasion and scratches.
 
Barrier Properties/Permeability 
The barrier properties of a topcoat can be enhanced by using fillers that have a platy shape.
 
Chemical Resistance of Topcoats 
The chemical resistance of a topcoat is affected by the chemical composition of the fillers used, and it can be reduced if fillers lacking sufficient chemical stability are used.
 
Gloss and Matte Effect
Fillers with high oil absorption values and large specific surface areas effectively reduce the glossiness of a coating.
 
Inter-Coat Adhesion 
Inter-coat adhesion refers to the adhesion of the topcoat to the primer. Certain fillers, due to their platy shape, can improve inter-coat adhesion.
 
Mechanical Strength
Filler particles with a platy shape can improve the mechanical strength of a topcoat because the plates improve the cohesion of the layer.
 
Pigment Volume Concentration (PVC)
The density property of a filler affects the PVC of a topcoat. A filler with high density will have little effect on the PVC of a system.
 
UV Resistance
The effect of solid particles, such as fillers, on the UV resistance of a topcoat depends on the chemical composition of the particles. Most fillers are not sensitive to UV radiation in sunlight. Generally, the UV resistance of coatings is not affected by the choice of fillers used in the system. An exception is mica, which can absorb UV rays and therefore improve the UV resistance of the topcoat.