Mineral Fillers, Pigments and Additives Used in Various Paints: GCC/PCC and Talc
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Introduction
As is well known, paints are chemical compositions that form a dry film layer on surfaces when applied to various surfaces for protection or decorative purposes. Countless products in our daily lives involve paints of various kinds.
These products appear on the walls of our homes, furniture surfaces, motorcycles, automobiles, bridges, and in our office cabinets and factory machines and storage tanks. Paints can generally be classified into 3 groups according to their application:
- Architectural/decorative paints (interior/exterior facade and wood paints) (Construction Paints),
- Industrial paints (automotive original equipment manufacturing (OEM) and powder coatings) (Industrial Paints),
- Specialty paints (maintenance, marine, traffic or road marking, automotive and aerosol paints).
Figure 1. Basic components of paint
For many paint types, the use of the following mineral fillers and auxiliary pigments provides benefits:
- Ground calcium carbonate (GCC),
- Precipitated calcium carbonate (PCC),
- Talc.
Ground calcium carbonates (GCC), also known as ground limestone, are generally the most economical filler mineral and have very high brightness. However, they do not contribute significantly to film integrity and their chemical composition makes them susceptible to acid attack.
Precipitated calcium carbonate (PCC) is a synthetic form of calcium carbonate and is widely used in some parts of the world, particularly in latex or emulsion paints.
Talc improves the moisture barrier properties of exterior paints and semi-transparent or opaque paints due to its flat/platelet particle shape and hydrophobic (water-repellent) nature. It is not as bright as calcium carbonates and has a higher cost. Talc is also used in pre-dispersed liquid colorants and paint coloring systems that support the suspension and color development of primary color pigments in colorants.
When selecting a filler pigment, consideration should be given to both end-user demands and cost constraints (i.e., selling price). The performance of a specific filler in paint depends on:
- Chemical composition,
- Particle size,
- Particle shape,
- Surface chemistry,
- Color,
- Whiteness or brightness.
Most decorative paints contain a combination of fillers of different types and sizes to achieve the targeted balance of application, appearance, durability and cost properties.
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Architectural Paints
Architectural paints are used in the interior and exterior parts of structures such as houses, apartment buildings and office buildings. Other applications for architectural paints include commercial paints, decorative paints, building paints and DIY (do-it-yourself) paints. These may include semi-transparent and opaque paints for wood.
These are the paints that people see, use and apply most frequently. To meet the decorative and functional needs of their users, house and commercial construction paints are formulated between flat or matte gloss finishes and eggshell and semi-gloss ranges.
Exterior architectural paints and wood paints must be formulated to withstand wind, rain and direct sunlight. Interior paints are generally used in a more favorable environment, but in places where surface defects are more easily noticed, touch-up and washability are important.
The color of the paint is determined by the primary pigment used. If the paint is white or light-colored, the filler will usually be titanium dioxide (TiO2); in darker paint shades, it may be iron oxide, carbon black or some bright-colored organic pigment.
In addition to the primary color pigments described above, decorative paints also contain fillers or auxiliary pigments such as GCC, talc and kaolin. The primary functions of these minerals are:
- To increase or maximize the effectiveness of more expensive color pigments,
- To reduce the amount of use required to achieve the desired color,
- To reduce the use of resins and/or solvents by filling the volume in the paint.
Table 2. Mineral fillers and particle size used in architectural paints
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Industrial Paints
Industrial paints generally refer to paints applied in the factory to goods manufactured as part of the production process. Such paints are also known as original equipment manufacturing (OEM) paints or product coatings.
Examples include paints used in automobiles, furniture and
metal structural products. These paints are applied in controlled environments using strict quality control procedures. Unlike architectural paints, industrial paints will also contain auxiliary pigments for additional reasons besides cost reduction.
These formulations turn to minerals that add some functionality or performance to the paint. When an OEM formulation contains a dispersant, talc will generally be selected. Talc is used in primer paints because it provides easy sanding and good adhesion. Talc also helps create a moisture barrier and improves exterior durability.
Generally, calcium carbonates are not used in industrial paints, with the exception of nano-type PCCs. Coated and uncoated nano-PCCs are used, but the selection depends on the system. PCC is most commonly used in paint as a dispersant for titanium dioxide.
Small, narrow PCC particles help create space for individual TiO2 particles. This distribution of the primary white pigment can increase matteness and reduce cost.
Unlike GCC, which has only one particle shape, PCCs can achieve many particle shapes and smaller sizes that cannot be obtained by grinding alone. Needle-shaped aragonites and clustered skenohedral calcites are typical crystal types used as dispersants to increase opacity and porosity.
These differences in PCC crystal morphology and size can cause large differences in how a particular PCC performs in different paint formulations.
Many PCC products for paint are available in "aqueous" form that provides dispersion of the produced particles and does not require a production step to redistribute dried powders. PCC, very similar to GCC, finds its largest application in interior and exterior architectural paints.
In powder coatings, some is used in fine and ultra-fine nano grades, particularly with small, narrow particle size distributions. In addition to standard, uncoated hydrophilic grades, some PCCs can be coated with stearic acid. These products increase hydrophobicity, dispersibility and compatibility in solvent-based or 100% solid paints.
The degree to which PCC is accepted as a standard raw material for paint formulations varies from region to region. While PCCs are common in decorative paints in Europe, South America and Asia, they are less prevalent in North America.
Selection is usually dependent on the relative availability of other mineral minerals in the region and relative costs.
On the other hand, talc is one of the most widely used minerals in paint. It is used in exterior and interior architectural paints, semi-transparent and opaque wood paints, specialty paints and industrial/OEM (original equipment manufacturing) applications. Talc is a true functional dispersant mineral.
Its layered structure, hydrophobic (water-repellent) surface and inert chemical composition help create paints that provide true moisture barriers. Its layered structure provides a strong, flexible film. Talc has the lowest hardness of industrial minerals. This softness makes talc a particularly good additive in sanding primers.
Talc ground to a very fine size of d50=1 micron can be used in industrial paints to control sheen and gloss. Talc helps keep pigments suspended and dispersed in suspension and maximizes the development of coloring intensity from the primary pigments used in colorants.
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Specialty Paints
The specialty paints group captures a product with a wide range of products that fall outside of architectural paints or industrial OEM (original equipment manufacturing) paints. In this category are road markings or traffic paints, maintenance paints for bridges and storage tanks, other metal protective uses, roof and aerosol paints.
Specialty paints are primarily protective paints rather than appearance paints. Since many specialty applications require very high performance, additives are selected primarily for their functionality and secondarily for cost effectiveness. Metal protective maintenance paints can effectively use the barrier properties of talc.
However, there are other situations, such as traffic paints, that must meet minimum performance standards within tight cost parameters.
In road marking paints that use fillers intensively, the ability to reduce overall formulation cost is very important in terms of selecting a coloring pigment compared to other pigments. Generally, GCC, a more economical mineral, is used.
In many specialty paints, such as aerosols, there may be only one dispersant system that completely complements the specific raw material set used in the formulation. GCC (also known as ground limestone or chalk) is the most commonly used dispersant mineral in paint applications.
It is available worldwide and is present near most paint production facilities. It is bright and has a wide variety of particle size distributions (PSD). It is the most economical of all common filler minerals. The widest use for GCC is as a filler in architectural or decorative paints.
While primarily used in interior paints, it is also included in some exterior formulations. It tends to be a filler for paints sold at a low price point and/or where longevity is not the primary characteristic. High-brightness calcium carbonates are selected for ceiling paints and other applications and are formulated above the critical pigment volume concentration (CPVC).
By combining two or more classes of different particle sizes to change the particle size distribution of the filler package, manufacturers can adjust the surface smoothness of interior paints. GCC can be used in exterior building paints as long as the pigment volume concentration (PVC) is not very high.
GCCs are also economical dispersants included in road marking and traffic paint formulations while reducing costs while meeting the optical properties of the finished film.
Unlike high aspect ratio platelet talcs, calcium carbonates contribute relatively less to film formation and integrity with round or nodular particle shape and low aspect ratio, and increase film porosity.
For these reasons, GCC use in industrial, marine and metal protective maintenance paints is limited.
Prof. Dr. Öner Yusuf Toraman
Industrial Raw Materials and Building Materials
Application and Research Center
Sources
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2. http://www.dynkimya.com/haberler/detay/TR/1/Boya-Nedir
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4. Ö.Y. Toraman, Mineral Fillers Used in Paints and Plastics, Boyatürk, April-May 2014, p. 34-40.
5. Ö.Y. Toraman, Mineral Fillers Used in Paints, Boyatürk, June-July 2016, p. 38-42.
6. Paints, Coatings and Solvents, Edited by Dieter Stoye, VCH Publishers, Inc., NY (USA), 1993.
7. S. Koltka, E. Sabah, Paint Industry and Synthetic (Precipitated) Calcium Carbonate (PCC), 8th International Industrial Raw Materials Symposium, Proceedings Book, p. 47-56, 29-30 November 2012, Istanbul.
