Choosing the Right Filler Mineral for Paint

Selection of Mineral Fillers in Paints According to DIN 55943, mineral fillers are defined as powdered substances that are practically insoluble in the environment in which they are applied, and are used to increase volume, obtain or enhance certain technical properties, and/or modify optical properties. Filler materials are essentially natural minerals. Mineral fillers are solid particles used in paint systems. They serve two purposes: 1. To enhance properties, 2. To reduce costs. Fillers play an important role in the fundamental properties of paint. Their effects are significant during production, storage, and application stages. The properties affected by fillers are: 1. pH, 2. Sedimentation rate, 3. Stability, 4. Equipment wear, 5. Viscosity.

1. pH

As is known, pH is a unit of measurement used to grade the acidity or alkalinity of a solution. The pH of water-based paints is important, for example, in terms of colloidal stability. Knowing the pH value is particularly important for paint manufacturers in predicting its behavior in formulations. While some fillers can strongly affect pH, others have only a minor effect on it. The surface composition and chemical stability of filler minerals determine the effect of particles on pH.  

2. Sedimentation Rate

Sedimentation rate in paints is essentially related to: 1. Viscosity, 2. Density, 3. Particle size of solid particles. Larger particles with high density settle faster than smaller particles with low density. Considering the particle size of primary particles of filler materials, the median particle diameter of primary particles, commonly referred to as d50, is typically provided. Particle size can be given in micrometers (μm) or nanometers (nm).

3. Storage Stability

Different types of stability of solid particles in liquid systems are important: 1. Sedimentation, 2. Dissolution, 3. Chemical stability, 4. Flocculation – This adverse process can occur during production, storage, application, and film formation. Flocculation can be prevented by adsorbing special additives called dispersants on the surface of solid particles, thereby regulating repulsive forces between particles. Whether dispersant chemicals will be adsorbed on the surface of filler particles is determined by: 1. The chemical composition and morphology of dispersant molecules, and 2. The surface composition of solid particles. Most fillers are hydrophilic (water-absorbing), which indicates the presence of polar groups on the particle surface. Talc, a filler material with a relatively hydrophobic (water-repellent) surface, is an exception. Hydrophilic particles are easily stabilized because dispersant molecules can be strongly adsorbed on polar groups present on the surface of hydrophilic particles (can adhere to the surface). Filler dissolution occurs during storage as much as potential chemical reactions and depends on the chemical composition of the filler. It is important to select filler materials that have chemical resistance to the liquid medium. Some fillers are not inert, which means that solid particles can be attacked by certain chemicals, solvents, acids, bases, or ultraviolet radiation. For example, calcium carbonate (CaCO3) is a filler that dissolves quite slowly in an acidic environment. This means: Care must be taken when using calcium carbonate in water-based systems with low pH or if the system will be applied to an acidic substrate.

4. Equipment Wear

Equipment can be damaged both during production and during paint application. Fillers with high hardness and sharp edges (shape) can damage equipment due to wear. The hardness of a filler material is an indicator of how easy or difficult it is to mechanically damage a solid. For pigments and fillers, the Mohs hardness scale is used in measuring hardness. While the Mohs hardness of diamond, the hardest mineral, is 10, talc, a soft filler, has a Mohs hardness of 1. Hard fillers such as quartz can cause wear on production equipment, especially when particles have sharp edges.

5. Viscosity

Viscosity is the resistance that liquids exhibit to flow, or against fluidity. Flowability, on the other hand, refers to liquids not resisting flow. The viscosity of liquid systems is affected by fillers depending on: 1. Particle size, 2. Particle shape, and 3. Surface composition of filler particles. Irregularly shaped particles will increase the viscosity of a system. Small or porous particles that absorb high amounts of resin material cause high viscosity.

Conclusion

The properties of mineral fillers (pH, sedimentation rate, stability, viscosity, etc.) have positive or negative effects on paint applications; therefore, correct mineral selection becomes important.   Sources Jochum Beetsma, https://coatings.specialchem.com/selection-guide/fillers-selection-for-paints-coatings (28 September 2022) Ö.Y.Toraman, Mineral Filler Materials Used in Paints, BoyaTürk, June/July 2016, p.2-4. Ö.Y.Toraman, Mineral Filler Materials Used in Paints and Plastics, BoyaTürk, April/May 2014, p.34-36.   Prof. Dr. Öner Yusuf Toraman Department of Mining Engineering Niğde Ömer Halisdemir University