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Where would we be without that black sticky stuff called asphalt? We walk, cycle, and drive cars on it. The aircraft we fly in take off from and land on it. And sometimes it sticks to our shoes.
"About 70 billion lb of asphalt is used annually in the U.S. alone, and asphalt usage will grow dramatically in Asia during the next 10 years," notes Arthur M. Usmani, chief scientific officer of Usmani Development Co., Indianapolis, in the preface of his book "Asphalt Science and Technology" (New York: Marcel Dekker, 1997). He adds that asphalt-containing materials find application not only in paving and road construction, but also in roofing, coatings, adhesives, and batteries.
The widespread use of asphalt relies on its remarkable waterproofing and binding properties. The hard surfaces of roads, for example, depend on the ability of asphalt to cement together aggregates of stone and sand. Most asphalts are also perfect absorbers of light. That's why they are black.
The American Society for Testing & Materials defines asphalt as a dark brown to black cementitious material in which the predominating constituents are bitumens that occur in nature or are obtained in petroleum processing. Bitumen is a generic term for natural or manufactured black or dark-colored solid, semisolid, or viscous cementitious materials that are composed mainly of high molecular weight hydrocarbons. The term includes tars and pitches derived from coal.
"Almost all asphalt used today is derived from the bottom of the barrel--that is, the last cut in the petroleum refinery after naphtha, gasoline, kerosene, and other fractions have been removed from crude oil," Usmani tells C&EN. "Very little is produced from other natural sources."
Asphalts are highly complex and not well-characterized materials containing saturated and unsaturated aliphatic and aromatic compounds with up to 150 carbon atoms. Their composition varies depending on the source of crude oil. Many of the compounds contain oxygen, nitrogen, sulfur, and other heteroatoms. Asphalt typically contains about 80% by weight of carbon; around 10% hydrogen; up to 6% sulfur; small amounts of oxygen and nitrogen; and trace amounts of metals such as iron, nickel, and vanadium. The molecular weights of the constituent compounds range from several hundred to many thousands.