Table of Contents
Although paint is often considered to be a relatively simple commodity, it is in fact a complex composite material. Considerable effort is required to design a coating which is easily manufactured, applies easily by a variety of techniques, dries evenly, covers thoroughly, has a pleasing appearance, resists physical or chemical attack, and is cost efficient. Natural and synthetic minerals play a major role in determining paint performance, directly or indirectly affecting most properties of a paint system. They provide the hiding power of the paint and contribute to the color, durability, protective qualities, texture, gloss and application characteristics of the coating. Variations in mineral quality can profoundly affect batch to batch uniformity of paint lots. In this paper, we will discuss the general uses of minerals in paint, how mineral quality and property variations affect paint performance and how mineral use has evolved over time. Possible future trends in mineral usage will also be examined.
The Uses of Minerals in Paint
Paint consists of a film forming material (a polymer or a resin), solvent or a dispersing medium (in latex paints), pigments, and additives which protect the material from deterioration before and after application (e.g., biological attack), help control flow and leveling (rheology), and stabilize it during storage and application. Natural and synthetic minerals are used as pigments or additives, and sometimes perform both roles. Large quantities of minerals are used in paint For example, an estimated 773 million kg (1.7 billion lbs) of extender pigments were consumed by the paint industry in 1990 (Rauch Associates, 1990). More types of extenders are used in coatings than in any other product. Only the paper industry consumes a larger tonnage of extenders (Stoy and Brown, 1982).
Extender minerals can be used to modify the film’s appearance and texture. For example, extenders can change the reflectance of a film by imparting different optical properties to the bulk film or by changing its surface roughness. They are also used to adjust the sheen and gloss of paints to achieve the desired targets, whether in fiat, satin, semi-gloss, or gloss formulations. Specialty versions of hydrous kaolin may even be used to replace some binder without loss of gloss, reducing cost and removing some of the volatile organics (e.g., coalescents) which cause environmental concerns. When gloss and sheen adjustment is the primary concern, diatomaceous or very fine synthetic silicas with high binder demands are the materials of choice. The proper choice of extender packages can be used to minimize differences in film appearance when paint is applied by several techniques, such as airless spray and brush, in the same area.
Extender minerals also play a crucial role in strengthening the paint film. Hard minerals such as micro crystalline silica increase the mechanical strength of the film, improving abrasion and mar (burnishing) resistance in the film. Calcined kaolins achieve similar results by helping to provide a dense, low sheen surface. This strengthening, often described as scrub resistance, allows marks and smudges to be scrubbed from the film surface without unduly degrading the finish. The extender package can also be designed to improve resistance to stain and dirt uptake.
Pigments and extenders significantly influence the rheological properties of paint (Wicks, Jr., 1986). For example, larger particle grades of silica can contribute to reducing brush drag during application, and can contribute to controlling sag resistance and leveling of brush marks (Illinois Minerals Co., 1987). Extenders can also improve film uniformity and consistency. They do this in part by helping to maintain the paint suspension, preventing settling. Talcs and kaolin clays are particularly useful in this respect (Madson, 1967). The amounts and types of pigments and extenders also influence the drying rate of the film, speeding the initial drying rate of the film due to the amount of solvent and binder adsorbed on the surfaces of the pigments (Wicks, Jr., 1986).
Effects of Mineral Properties on Paint Performance
The properties of individual minerals are central to their performance in paint. The natural and engineered properties of individual minerals will determine which minerals fit specific paint applications, with the application defining the mineral property variances which can be tolerated. Important properties of minerals used in paint include color, particle size and distribution, surface area, oil and/or water absorption, impurity mineral or element content, grit (typically the percentage of particles larger than 45 micrometers), pH, density, and surface treatments. These properties, and the paint properties which they affect, are summarized in Table 2. As can be seen from this table, a single mineral property can affect several paint properties. Note also that multiple mineral properties are sometimes involved in a single paint performance property such as oil/water absorption.
Particles larger than 45 micrometers (325 mesh grit) are detrimental to most finishes, particularly in gloss or thin film systems (Stoy and Brown, 1982). A dry paint film will typically range from 25-75 micrometers in thickness. Excessive amounts (typically >0.5 wt% for extenders) of particles larger than 45 micrometers will produce unsightly bumps or roughness in a typical dry paint film. For very thin films, even smaller grit particles cannot be tolerated. Paint containing excessive grit requires an expensive filtering or milling (in the case of some solvent-based paints) process to be made usable, if it can be made so at all. In the worst cases, the product must be scrapped.
Achieving Improved Mineral Quality for Paint Production
Quality is assuming an increasingly central role throughout industry. The coatings industry is no exception. A major driving force for increased quality emphasis is the need to produce performance-based products at more competitive prices. The driving forces of cost and performance are pushing formulation technology to the limits of raw materials capabilities. In order to design in the necessary consistency, the paint industry is expecting tighter specifications on raw materials. The whole regime of specifications itself is being reevaluated as information requirements become more precise.
Excessive variation of one or more mineral properties can cause paint to perform poorly, or fail altogether, as noted in examples of paint performance problems in the previous section. Lack of raw material consistency can also cause manufacturing problems which require batch adjustments to bring the paint into specification, and potentially yielding inconsistent performance to the customer. The goal of the paint manufacturer is to provide the customer with a high quality product which has no noticeable lot to lot variation. Proper raw material test methods and specification limits are one key to achieving this goal.
It is difficult to overstate the impact environmental issues have on the paint industry. In all likelihood, it is only the beginning. Legislated and self-imposed regulations now drive much of the technology which focuses on reducing harmful contaminants and on developing water-based systems which help reduce volatile organic emissions. The regulations will also alter the mix of minerals which are used by the industry. For example, to meet volatile organic content requirements, the paint industry is reformulating solvent-based coatings towards water-borne, high solids, and powder coatings (Anonymous, 1992). This reformulation is causing a shift in the types and grades of extenders used by the paint industry.
Future Trends in Mineral Use in Paint
Developments in mineral technology for use in paint are driving the industry to higher levels of mineral processing and the use of sophisticated ceramic technology in efforts to develop materials with better consistency, higher cost- effectiveness, and lower environmental and health impact. In the extender area, efforts are proceeding in a number of areas. Additional processing to control particle size distribution and impurity levels is receiving more consideration (or at least a switch to more tightly controlled materials). Some efforts, particularly in the synthetic and processed mineral areas are also aimed at controlling particle shape. This is particularly important in the colored pigment arena, where particle morphologies influence the color of the material and sometimes cause stability problems in colorants and paints.
Efforts are also focused on the development of structurally modified minerals, such as structured kaolin clay, to produce better TiO2 extenders with improved dispersion, opacity and tint strength. Other efforts are aimed at surface treating extender minerals with organic and/or inorganic materials to enhance brightness and opacity while altering the particle surface characteristics for better dispersion and suspension stability.