Influence of feedstock oxidation temperature on stability of paving asphalts

Abstract

When the oxidation temperature in the production of asphalts is increased, breakdown processes are accelerated, lowering the yield and polarity of the asphalt [1, 2]. Also, because of the high yields of gaseous products and black solar oil, environmental pollution becomes a problem. At the 4th World Petroleum Congress (1955), it was indicated that a temperature of 250~ is optimal for the oxidation of feedstocks to produce asphalt. Since that time, new technologies have been developed for rapid oxidation, yielding highly polar and more chemically inert binders at lower process temperatures. Outstanding properties were demonstrated in [2] for a plastic bitumen obtained by oxidation at a temperature far below 250~ The influence of the oxidation temperature on the thermal and thermooxidative stability of paving asphalt has received very little attention. When asphalt is held in tanks for the preparation of asphaltic concrete mixes for road construction, and during the course of the mixing process itself, the asphalt is exposed to temperatures of 140-160~ During these stages, the asphalt properties undergo their most severe changes. This period is termed the first stage of aging; it can be simulated quite accurately under laboratory conditions. The second stage of aging begins when the asphaltic concrete pavement is laid down, and it continues until the pavement fails (up to 20 years). During this time, the softening point of the asphalt changes (approximately) from 45 ~ to 70~ As a result, the road coating becomes brittle during the winter; it will not sustain dynamic stresses generated by moving vehicles, and is subject to cracking and breakdown. In studying the change of properties of paving asphalt under service conditions, extended observations under natural conditions are necessary [3]. However, hardly any such observations have been made. In the work reported here, aging tests were performed on asphalts obtained by the oxidation of one particular charge stock, from West Siberian crude, in a column-type unit; all oxidations were performed with the same air input rate, but at different temperatures (220-280~ The first stage of aging was evaluated on the basis of property changes due to thermal and thermooxidative action at 160~ The thermal action (this term is somewhat arbitrary) on the asphalt during storage in the hot state in the refinery, in the course of transportation, and upon heating in tanks at the asphaltic concrete plant was simulated by thermostating the test sample at 160_+2~ in a steel cup used for penetration tests (the specific surface area of contact between the test sample and air was 2 cm2/g). Thermooxidative action on the asphalt during the preparation of the asphaltic concrete mix was simulated in tests on a 10-/xm layer of asphalt applied to steel plates (5 x 5 cm), with a 1000 cm2/g specific surface area of contact with air. The penetration and softening point were determined at intervals on samples that had been held in penetration cups in the oven at 160~ the shearing stress at 25~ was determined on plates that were cemented together by a layer of the asphalt (tested in a cohesiometer designed by SoyuzDorNII [State All-Union Scientific-Research Street and Highway Institute]), The charge stock used in preparing the oxidized asphalts had the following physicochemical properties: nominal [Engler] viscosity 26 sec at 80~ chemical group composition (wt. %) paraffins + naphthenes 15; monocyclic, bicyclic, and polycyclic aromatics 10, 29, and 8, respectively; resins 25; asphaltenes 13. With increasing oxidation temperature in the production of the asphalts, the softening point increased at a faster rate (Fig. 1). The results of tests on samples that were practically identical in depth of oxidation at 220 ~ and 280~ (Table 1) confirmed the conclusion stated in [2] that the standard asphalt property indexes (penetration at 25~ softening point, and ductility at 25~ are independent of the oxidation temperature. The low-temperature properties (penetration at 0~ breaking point, and ductility at 0~ are appreciably better for the asphalt obtained at 220~ than for the asphalt obtained at 280~ These two asphalts differ the most significantly in their concentrations of paramagnetic centers.