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Oxidized asphalts stored at elevated temperatures for extended periods of time tend to “fallback” or decrease in softening point and increase in penetration. An analytical method, utilizing a floating glass lid technique, is presented to measure this tendency in the laboratory. The fallback characteristics of oxidized asphalts prepared from two fluxes suitable for processing into prepared roofing grade materials were studied. The rate of fallback was determined to be rapid during the initial storage, followed by a period during which the asphalt was resistant toward further fallback. The asphalts were found to be relatively stable below 475 F, but the fallback tendencies increased substantially above this temperature. Highly oxidized asphalts were more susceptible to fallback than were asphalts from intermediate stages of blowing. Fallback obtained in plant storage approximated that anticipated from laboratory data. The evidence presented indicates that asphalt that has been subjected to fallback can be reprocessed to the original grade.
Wronka, J. A.
Cities Service Research and Development Co., Product Development Laboratory, Cranbury, N. J.