Michael Freemantle

[Western Research Institute photo]

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.

The compounds are classified as asphaltenes or maltenes according to their solubility in hexane or heptane. Asphaltenes are high molecular weight species that are insoluble in these solvents, whereas maltenes have lower molecular weights and are soluble. Asphalts normally contain between 5 and 25% by weight of asphaltenes and may be regarded as colloids of asphaltene micelles dispersed in maltenes.

Many of the compounds in asphalt are polar since they contain alcohol, carboxyl, phenolic, amine, thiol, and other functional groups. As a result of this polarity, the molecules self-assemble to form multimolecular clusters with molecular weights up to 100,000. The adhesion of asphalt to aggregate is also thought to depend on the polar attraction between molecules in asphalt and the polar surfaces of aggregates.

"Asphalt has a polymer-type network that is unique," Usmani says. Although not a polymer in the strict sense of the word, it is a thermoplastic material--it softens when heated and hardens upon cooling. Within a certain temperature range an asphalt is also viscoelastic, which means that it exhibits the mechanical characteristics of viscous flow and elastic deformation.

Although asphalt has been around for millions of years in crude oil, it doesn't last forever when used for paving roads. Few of us can have missed jolting over cracks and ruts in heavily trafficked roads.

A number of factors impinge on the performance of asphalt. These include its composition and the crude oil source, the type and amount of aggregate used, the presence of moisture, the method of road construction, temperature, and, of course, the volume of traffic.

Ideally, asphalt used for paving roads should remain viscoelastic in all weather conditions. However, many asphalt roads soften in summer and suffer from rutting, or permanent deformation, as it is also called. At low temperatures, neutral molecules in asphalt arrange themselves into more organized structural forms. As a result, the material hardens, becomes brittle, and cracks under the stress of heavy traffic loads. This is known as thermal and fatigue cracking.

Asphalts also lose their plasticity and therefore harden and crack or crumble when they lose their more volatile lower molecular weight constituents or when these constituents are oxidized. This process is known as aging. Moisture from rain and other sources can also invade and damage asphalts, particularly aged or oxidized asphalts because they have a larger number of polar constituents to attract water molecules.

The performance of asphalts can be improved by using various modification techniques. For example, blowing air through hot liquid asphalt removes more volatile compounds and results in a product with higher viscosity. Addition of modifiers, such as polybutadiene in the form of crumb rubber from used tires and other polymers, also stiffens asphalts.

According to Usmani, how polymer modifiers mitigate asphalt's shortcomings is not well understood. "To sustain the current usage of asphalt and develop new applications, there is a pressing need to revitalize research, development, and engineering in asphalt materials," he remarks.

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Copyright © 1999 American Chemical Society