BITUMINOUS COATINGS The most wide& used fami& of protective coatings i s stiZZ growing n the field of protective coatings the consumer today
I has an extremely wide and constantly growing choice of excellent materials to cope with almost any need. Among them is a group of engineering materials whose known history of useful application probably surpasses that of any of the others. In the world of coatings they are indeed the “Plain Janes.” They are not available in two dozen assorted colors, but like Henry Ford’s Model T, come in just one-black-though they are capable of limited pigmentation. They may be described as almost entirely utilitarian materials, seldom decorative, and yet they are being used at a rate estimated at 25 million tons annually. These materials are the bituminous coatings. With such a rate of usage, there must be much that can be said for them. There is. They are highly water resistant, have excellent adhesive properties, can be quite resilient and ductile, are durable in many applications without any modifications and can be compounded to further increase this property, have a versatility matched by few, if any, similar engineering materials, offer all of the above properties plus numerous others and at a cost that is quite a bargain in today’s burgeoning coatings market. The bulk of the bituminous coatings are derived from asphalt or from coal tar. Both are thermoplastic materials and may therefore be applied in the molten state. Though somewhat cumbersome, this method does have the advantage that the application is fully ready for its intended use as soon as it has cooled. But for many uses, especially those that may be called “field” uses, there is a distinct need for coatings that may be applied cold, without the need for special equipment, or the element of haste (because of cooling) or the danger inherent in handling molten pitch. In the four articles that follow, the hot-applied and cold-applied products of both the asphalt-based and coal tar-based coatings are described. I t is known that asphaltic bitumens were used as far back as 8000 years ago. An actual specimen of asphaltic foundation coating is still in existence today in Iraq4000 years after it was applied. I’ve often wondered what the original contractor’s guarantee was for this job. All of the early asphalts were simply seepages of more or less viscous materials. The natural or “mined” asphalts today, such as the famous Trinidad deposits, gilsonite, and so forth, constitute a small but useful segment of the industry, but the vast bulk of today’s asphalts are of petroleum origin. They are in plentiful supply, of high and consistent quality, and can be tailored to a wide variety of uses. They are in no sense man-made
asphalts, for they are present as such in the original crude petroleum, and the process of separating them is purely physical, usually by distillation, either atmospheric or vacuum. Those ’ancient seepages, as well as today’s mined apshalts, are also products of a distillation process, but one which nature, never in a hurry, has taken centuries to complete. Once this separation has been effected, there are many ways in which asphalts can be modified. Probably the most used (as well as most useful) modification is air blowing, usually referred to (incorrectly) as oxidation. By blowing finely divided air through the asphalt at elevated temperatures, both physical and rheological changes are made to occur. Broadly, these may be considered to be the result of dehydrogenation and condensation of unsaturated linkages. The air-blowing process has been expanded by use of “catalysts,” which further improve the temperature susceptibility, penetration index, ductility, and other desirable characteristics. Coal tar- and coal tar pitch-base coatings probably date back to the first production of these black viscous materials in gas plants and by-product coke ovens. It can safely be said that they have been manufactured and used for at least 100 years. Early in their production they found wide use as water- or moisture-proofing paints. There is good reason for their continued popularity in the marine and water works industries. The first application of coal ‘tar enamel to steel pipe for potable water service, made in New York in 1914, is still in operation today. In 1913, an early form of coal tar enamel was used to protect the gates, locks, and penstocks of the Panama Canal. Examination after 35 years of service showed them to be in perfect condition. In the 1930’s, AWWA-type enamels were used extensively in water lines in many large scale projects, particularly in the Far West. These installations are still giving trouble-free service and the coal tar enamels are virtually unchanged after service of over 30 years. The high aromaticity of high temperature coal tar pitch (about 95y0)accounts for many of its unusual physical and chemical properties to make it the preferred raw material for a wide variety of applications. High temperature coal tar ’pitch is practically inert to the action of water and neither absorbs nor transmits it. High temperature coal tar pitch is highly resistant to attack by bacteria and fungi. These properties make it eminently suitable for roofing, waterproofing, coating of buried steel to protect it from the corrosive action of wet soil, lining of water pipes, and so forth.
W. F. Fair, Jr.,
Chairman
