Rosin is one of the original naturally derived binders used in decorative coatings. It is an amber-to -brown, hard, brittle material that is a by-product of distilled turpentine. Reacting rosin with lime creates limed rosin; with glycerin, it creates ester gum; and with pentaerythritol, it creates pentaresin. A solution of limed rosin and mineral spirits, called gloss oil, was once popular in low cost general utility varnishes, floor paints, and barn paints. Rosin is a thermoplastic material usually supplied as a solid, flake, or as a solution in a compatible solvent. Alone, this binder has relatively poor drying time and resistance to solvents, chemicals, and water. Varnishes were once made from rosin mixed with various oils until the development of synthetic alkyd resins.
Shellac is a thermoplastic material dissolved in alcohol. Lac is a hard, resinous secretion from the scale insect Laccifer lacca, found largely in Southeast Asia and in parts of India. Separated through heat, solvent, and filtering methods, it is then dried into sheets and pulverized into flakes. Shellac coatings are generally thermoplastic materials dissolved in alcohol and range in color from a light yellow, which is preferred, to a deep orange, although often bleached white. They’re used for sealing knots and in 'alcohol-based' primers and may be thinned with denatured alcohol.
Silicones are most notable for their heat resistance and water-repellent properties. When the first silicone oil was made in the 1870s, its insensitivity to both high and low temperatures was noted, but the first silicone polymers were not invented until 1943. In the 1950s silicones were developed commercially for the aerospace and electronics industries but rapidly found applications in many fields, especially construction. Silicone oils are generally monomeric and therefore non-drying. They’re useful as mold release agents and thanks to their very low surface tension, they make excellent water repellents. Polymeric silicones in paint are used as modifiers (co-polymers) for improving weatherability and durability of alkyds. Pure polymeric silicone resins are used as the primary binder in many heat-resistant coatings. Silicone elastomers are used as adhesives, elastomeric wall and roof coatings, and caulking compounds where the retention of flexibility and resistance to weathering and water are important. Silicone resins are available in solid, solution, liquid, and emulsion forms. Pure silicone resins cure by heat but co-polymers and blends can cure by oxidation, solvent evaporation (thermoplast), or thermoset reaction. Silicone resins are typified by excellent chemical resistance and physical properties.