ASPHALT AND ASPHALTENE

Neither One is a Single Material

 

Asphalt and asphaltenes are names that show up in articles and papers discussing paving and roofing materials. Especial with people not very familiar with technical field, discussions often sound like each is a single well define material such as salt or water. However that is far from the fact. Some may even feel that asphaltenes are something in the way that needs to be isolated or corralled. Yet they are vital in controlling the properties of an asphalt. Also researchers may reach conclusions on an asphalt from a particular crude source and believe that those conclusions pertain to all asphalts.

Asphalt.

Asphalt is the part of crude oil that is left when all the other hydrocarbons have been removed. There are two main ways of separating the asphalt from the gasoline, kerosene and oils; distilling, and solvent extraction.

Source. The properties of a particular unmodified asphalt are controlled by the source of the crude oil. The differences can be profound. In California there are three crude sources that produce entirely different asphalts: California Valley, Coastal and LA Basin. Within those broad designations are subgroups such as the coastal crudes; Santa Maria and San Ardo. A specification can be developed such that it can be met by asphalts from all three sources however they will perform differently. There are some asphalts that have very poor cold temperature performance and others that perform very badly in hot weather.

Distillation. In the distillation of crude oil, one pipe goes into the distillation towers, and a number of pipes come out. Each tower system is designed for a particular crude or crude blend and there are pumps removing the products. What is left over is asphalt on the bottom of the tower also. Some crude oils have no asphalts while others may contain as much as 65% asphalt. If any one of the storage tanks gets full, the refinery has to shut down.

Propane Extraction. The other method is to extract the non-asphalt portion with propane.

Asphaltene

One of the components of asphalt is the asphaltenes. Here we have two problems: the misconception that asphaltenes are significantly different than other asphalt components, and the basic definition. While some methods define asphaltenes as n-pentane insoluble material, other methods use hexane or heptane or even iso-octane as the solvent. n-Pentane will produce the largest amount. Because certain asphaltenes are precipitated by a solvent doesn’t mean that there aren’t still other materials in the asphalt that are very similar to asphaltenes. Asphaltenes give body to the asphalt. If the asphaltenes are completely solvated, the asphalt won’t perform well. On the other hand, if they are in a second phase, again the asphalt may cause problems. In some cases, the asphaltenes will be at least solvated sufficiently at ambient temperatures for a single phase to be present, however they may form two phases in cold conditions, resulting in cracking in winter.

Modification

The addition of polymer modifiers further complicates the situation. Adding a polymer to any asphalt will result in two phases no matter how well the asphaltenes are solvated. When polymer modification was young problems with phase separation was a problem that had to be resolved. It can be seen that with a wide range of properties in asphalts, polymer modification can be more of an art than a science. One question I have is how well modified asphalts will perform in low temperatures even though they pass all of the low temperature test. For pavements to resist low temperature cracking the binder must be able to stress relax faster than thermal stresses build up. If the binder becomes more like a plastic with a yield force necessary, the pavement will crack.

Robert L. Dunning, chemistdunning@gmail.com, www.petroleumsciences.com

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