Coal Tar and Asphalt Coatings

Abstract

Bituminous coal, a very complex chemical mixture, decomposes into simpler components when heated in retorts without air above 700 ° C (1292 ° F). Gas, aqueous vapor, and coal tar are driven off, leaving coke as residue. The coat tar is dehydrated and heated in stills to yield oil and coal tar pitch. Depending on the source of the coal tar and the amount of heat applied, pitches of different characteristics are obtained. When used as bases for superior coatings, coal tar pitches are reprocessed, and any corrosion-accelerating substances are removed. Various types of coal tar pitches are then blended together. The outstanding quality of coal tar paints is their extremely low permeability, their high electrolytic resistance, and their remarkable resistance to the disintegrating action of water. There are hardly any materials, old or new, that are as water resistant as properly compounded coal tar coatings. They will not be affected by mineral oil but may be dissolved by vegetable and animal oil, grease, and detergents, if they are in direct contact with them. Their resistance to weak mineral acids, alkalis, salts, brine solutions, and other aggressive chemicals is good. Furthermore, coal tar paints give more value per dollar than any other protective coating. This fact should not be overlooked when selecting paint for a certain job. Coal tar paints are made by dissolving the processed pitch or blend of pitches in suitable solvents. Skill and experience are essential in compounding coal tar paints because similar physical characteristics of raw materials do not necessarily mean similar behavior of the finished product under exposure. The raw materials selected for the blending, the degree of refining, and the addition of other modifiers, often in small quantities, decide the final merit of the coating. There are five main types of coal tar coatings: