Mineral Filler Selection for Primer Paints

Mineral Filler Selection for Primer Coatings Primers are pigmented coatings applied to new or old surfaces prior to undercoat or topcoat application. The primary function of primers is to provide adequate adhesion to the substrate and ensure intercoat adhesion for subsequent layers. They serve as the foundation for the selected coating system and, particularly in the case of metals, provide corrosion protection. They assist in adhesion of the topcoat to the surface, increase paint durability, and can conceal or fill certain surface defects. Primer coatings also contain special pigments and resins that help seal and protect the surface, making it more resistant to moisture and mildew.

Primer coating types are as follows:

Oil-based primer: A good choice particularly for porous surfaces such as wood or concrete. It enables optimal results on metal surfaces such as pipes or ventilation holes; additional thickness helps provide corrosion protection. Latex primer: A good choice for relatively smooth surfaces such as drywall or metal. It is ideal for drywall surfaces such as walls or ceilings due to its quick-drying properties. Epoxy primer: This type of primer is best for surfaces that will be subjected to heavy wear and tear, such as garage floors or industrial machinery. Conversion primer: Applied as a primer in transition applications from solvent-based paint to water-based paint. When there is a color difference between the new paint to be applied and the previously painted surface, conversion primer should be used. Primer is a coating layer in direct contact with the substrate. Fillers affect the main properties of primers: 1. Adhesion, 2. Barrier properties, 3. Chemical resistance, 4. Corrosion resistance, 5. Gloss, 6. Pigment Volume Concentration (PVC).

1. Adhesion

Adhesion of a coating is determined essentially by the binder present in the system, possibly combined with additives that enhance specific adhesion. Some fillers with flat-plate particle shape can improve adhesion due to two effects: i. Flat particles form a barrier, thereby extending the path that molecules must travel through the coating to reach the substrate-primer interface. ii. Flat particles impart a mechanical force to the film called "anchoring" and thereby improve adhesion.

2. Barrier Properties

Permeability of a coating refers to the ability of small molecules to diffuse through the coating. Since most filler particles are solid, small molecules cannot pass through filler particles: molecules must move around the particles. A coating with low permeability to small molecules is said to have good barrier properties. Several factors govern a film's permeability: i. Adsorption of the resin system onto solid particles: Small molecules can diffuse through filler particles when adsorption (surface retention) is weak or absent. ii. Particle shape: Flat-shaped filler particles such as talc or mica act as a kind of "roof tile" by reducing permeability when they orient themselves during film formation.

3. Chemical Resistance

The chemical resistance of a filler is determined by the chemical composition of the filler. Most fillers have good resistance to chemicals and solvents. Calcium carbonate (CaCO3) is an exception with weak resistance to acids. High chemical resistance fillers should be used in metal primers.

4. Corrosion Resistance

Flat-shaped fillers can increase corrosion resistance by forming a barrier. This delays the diffusion of molecules such as oxygen, salt, and acids that can promote corrosion. Additionally, fillers with high pH values that are basic can delay corrosion because corrosion proceeds faster in acidic environments. Basic fillers such as dolomite and feldspar are frequently used in anticorrosion primers because metal corrosion progresses more slowly at high pH. This beneficial effect of basic fillers on corrosion resistance is called "passivation".

5. Gloss

Primers preferably have low gloss, indicating that the primer surface is rough. A rough primer surface improves intercoat adhesion between the topcoat and primer. Fillers with high oil absorption values and high specific surface area effectively reduce the gloss of coatings. The oil absorption value of a filler is defined as the amount of oil in grams required to make just a fluid paste from 100 grams of powder 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 solids used in a liquid system is important in terms of the fundamental properties of the resulting system. For example; i. Solid particles with high oil absorption values provide a stronger viscosity increase in a liquid system compared to solid particles with low oil absorption values. ii. Fillers with low oil absorption values should be used in systems that must have low viscosity, such as solvent-free systems or high-solids systems. Oil absorption value is determined according to the fundamental properties of filler particles: i. Smaller particles have higher oil absorption values than larger particles. ii. Particles with spherical, smooth, and closed (non-porous) surfaces have low oil absorption values. iii. Irregularly shaped particles, especially when porous, have high oil absorption values. The specific surface area of a filler is determined by particle size, particle shape, and surface morphology of particles: fillers composed of large spherical particles with smooth and closed (non-porous) surfaces have low specific surface area.

6. Pigment Volume Concentration (PVC)

PVC is defined as the volume percentage of solid particles that are pigments and fillers in film-forming systems. This means that a system's PVC must be calculated without volatile components such as water and/or solvents. Formulations are usually given in weight parts of components. Since PVC relates to volume percentage, all weights of components must be converted to volumes using the density of each component. A high-density filler will have a low effect on a system's PVC.

Conclusion

When selecting fillers for primer coatings, it is appropriate to consider the mineral's adhesion and barrier properties, resistance to chemical effects and corrosion, gloss, and Pigment Volume Concentration (PVC) values. In this study, the performance of the most commonly used minerals has been compared and practical information has been presented. Correct mineral selection can also ensure proper and appropriate application of the paint.

Sources

1.Jochum Beetsma, https://coatings.specialchem.com/selection-guide/fillers-selection-for-paintscoatings (28.09.2022) 2.Ö.Y.Toraman, Mineral Fillers Used in Paint, BoyaTürk, June-July 2016, p.2-4. 3.Ö.Y.Toraman, Mineral Fillers Used in Paint and Plastics, BoyaTürk, April/May 2014, p.34-36. 4.Ö.Y.Toraman, Selection of Filler Mineral in Paint, Turkcoat, April/May 2023, p.14-18. 5.https://www.beckers-group.com/en/knowledge/technology/primers (29.09.2022) 6.https://www.baumerk.com/tr/blog/astar-boya-nedir (24.05.2023)   Prof. Dr. Öner Yusuf Toraman Mining Engineering Department Niğde Ömer Halisdemir University