Znd. Eng. Chem. Res. 1994,33, 2801-2809
2801
Oxidation of Roading Asphalts Philip R. Herrington,' John E. Patrick, and George F. A. Ball Works Consultancy Services Ltd. P . 0.Box 30-845, Lower Hutt, New Zealand
The oxidation of 1.0 mm films of a roading asphalt at 60,100, and 130 "C in a forced draft oven and at 60 "C under 300 psi of oxygen was studied. I n all experiments the increase in viscosity ( q ) measured at 45 "C could be fitted to a hyperbolic function of time ( t )of the form A(1og q ) = t/(a bt), where a and b are constants. The limiting viscosity A(1og q ) l l b was found to be different for each temperature. This was interpreted as being due to differences in the temperature dependence of the rates of competing oxidation reactions and differences in the availability of reactive species due to temperature-dependent structural effects. This contention was supported by infrared spectroscopy which showed that the increase in concentration of carbonyl functionalities as a function of viscosity was the same at each oxidation temperature, whereas the same was not true for sulfoxide functionalities. The rate of increase of sulfoxide concentration was greatest for asphalts oxidized at lower temperatures. Virtually no change in hydrogen type distribution measured by lH NMR was observed, except for oxidation at 130 "C where a small decrease in the proportion of aromatic protons was seen. Given the estimated total carbonyl functionality concentration present, the observed constant proportion of benzylic protons suggests that carbonyl formation during oxidation a t other than benzylic sites is more significant than supposed in the literature.
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Introduction The hardening of petroleum asphalt used in road construction due to slow autoxidation is one of the major factors limiting the life of sprayed seal surfacings, particularly on roadways with relatively low traffic volumes. As this process may take in excess of 10 years to become significant and varies according t o asphalt source, an accurate means of modeling and ultimately predicting asphalt performance would be of great value. Roading asphalt in temperate climates rarely reaches temperatures in excess of 60 "C. In the laboratory, modeling of the age hardening process is, however, usually carried out a t much higher temperatures in order to obtain data within a practical period. Enustun et al. (1990) and Bell (1989) have tabulated a large number of studies aimed at evaluating or developing asphalt or asphaltic concrete aging procedures carried out since the turn of the century. The majority of these procedures involve temperatures of 100-200 "C. Correlation of a laboratory procedure with field aging has almost exclusively been made on the basis of a limited range of physical properties such as softening point, penetration, or viscosity. Ideally a laboratory procedure should produce aged asphalts realistically modeling all physical properties of the same field-aged material. It is thus necessary to first establish that the oxidation chemistry in both cases is the same and to be cognizant of any differences; however, this is an aspect of accelerated aging procedures that has received relatively little attention. General similarities between long term aging of asphalt in the field and high-temperature laboratory aging have long been noted. These include in particular a trend toward increased average molecular weights (Yapp et al. 1991) and changes in fractional composition (Petersen, 1984, Shian et al,. 19921, namely a trend to increased levels of the more polar fractions. Petersen and co-workers (Petersen, 1971, 1975b, 1981, Dorrence, 1974) have studied in detail the formation of oxygen containing functional groups during oxidation of 15 pm films at 130 "C. This and other work related to asphalt composition and durability has been
reviewed by Petersen (1984). Ketones and sulfoxides are the principal products formed with anhydrides and carboxylic acids formed in smaller amounts. Esters and aldehydes, although suggested by previous workers (Knotnerus, 19561, do not appear to be significant oxidation products (Petersen, 1975b; Dorrence et al., 1974). Results for laboratory-aged (24 h, 130 "C, 15 pm films) and the same field-aged asphalt extracted from 11-13 year old asphaltic concrete cores were compared (Petersen, 1984). Agreement was generally good given the likely variation in air void content of the mixes and the importance of this factor in asphaltic concrete oxidation rates (Petersen, 1989). Agreement is improved if the ketonelanhydride concentration ratios are calculated (eliminating the void content variable). The implication is that the mechanism of reaction (at least in terms of ketone and anhydride formation) is similar in both laboratory aging at 130 "C and field aging of asphalt mixes. However, in a later paper (Petersen, 1989)the validity of 130 "C aging is questioned on the basis of differences in the availability of reactive species compared to in situ temperatures. In the field slow reversible molecular structuring (steric hardening) may act to sterically hinder possible reaction sites. This would have the effect of reducing the rate of reaction in the field compared to that at elevated temperatures (where steric hardening is negligible) beyond that expected from simple Arrhenius temperature dependence. It also follows that the relative importance of differing, competing oxidation reactions may also vary in a complex way. As the propensity for steric hardening is related to asphalt source, comparison of oxidation rates at high temperatures may lead to false conclusions. Given the importance of realistically modeling field aging in the laboratory, the work presented here was undertaken as the first part of a study to more fully verify the relationship of high temperatures (60- 130 "C) to field aging in particular with respect to sprayed seals. Sprayed seals differ from asphalt mixes in a number of significant ways which suggest that simple extrapolation of correlations between the oxidation
Q888-5885l94l2633-28Ql~Q4.5QlQ0 1994 American Chemical Society
2802 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 Table 1. Asphalt Physical Properties property penetration at 25 "C (dmm) softening point ("C) viscosity at 70 "C (mm2 s-l) viscosity at 135 "C (mm2 8-l) flash point ("C) density at 25 "C
186 38.8 22 000 254 > 250 1.020
ASTM method D5 D36 D2170 D2170 D92 D70
chemistry of asphaltic concrete field and laboratory aging to sprayed seals may be erroneous. Asphalt in asphalt mixes is on average present as 10-100 pm films; during its manufacture these films are held at high (160-180 "C) temperatures for periods of up to several hours and are in contact with a large (possibly catalytic) surface area of mineral aggregate. In the case of sprayed seals, thick 1-3 mm films are usual and although applied a t -160 "C the asphalt surface area to volume ratio is very small during this period (except for a fraction of a second during actual spraying). Additionally, in sprayed seals the asphalt is in contact with mineral aggregate surfaces only at low, near ambient, temperatures and the total asphalt surface area exposed to the atmosphere is negligible compared to that in asphalt mixes.
Experimental Section Materials. Experiments were carried out using a Safaniya (Heavy Arabian) 180/200 penetration grade, straight run vacuum distilled asphalt. Physical properties are given in Table 1. Model compounds used in the infrared study were "analysed reagent" grade or better (>97% purity) and were used as received. Dichloromethane, n-heptane, toluene (MallinchkrodtAustralia Ltd, Sydney, Australia), and tetrahydrofuran (BDH New Zealand Ltd, Palmerston North, New Zealand) were >99.7% purity and dried over molecular sieve (Aldrich Chemical Co Ltd, Dorset, England). Oxidation Experiments. Films of asphalt (1.00 f 0.07 mm) were prepared by allowing the necessary quantity of asphalt to flow into a precisely machined depression in a stainless steel disk (40 mm i.d.1. The films were oxidized at 130 f 0.3 "C, 100 f 0.3 "C or 60 f 0.3 "C in a forced draft oven. Samples (three or four films) were removed at intervals, and the asphalt was scraped from the holders, combined, and mixed in a 5 mL beaker at 130-150 "C under a nitrogen atmosphere (this process took 2-3 min). Asphalt films were also oxidized a t 60 k 0.1 "C under 300 psi of oxygen (99.7%)in a Parr Model 4767 pressure vessel (Parr Instrument Company, Moline, IL) mounted in an oil bath. The bomb was pressurized twice to > 300 psi to remove ambient air before finally being filled to 300 psi. The oxidized asphalts were stored in airtight containers at 4 "C until analysis. Viscosity Measurements. Asphalt viscosities at 45 & 0.1 "C were measured using a Cannon cone and plate viscometer according to ASTM D3205-86. The instrument's original system was replaced with a rotational potentiometer and in-house software to calculate viscosities. Plots of log shear rate versus log viscosity were used to calculate viscosities at a shear rate of 0.01 s-l. Reported values are the average of two or three replicate determinations. 'H NMR. Proton NMR spectra were measured at 250 MHz on 10% w/v asphalt solutions in CDCl3 with TMS as internal standard.
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0
5 10 15 20 Oxidation time (hrs)(x loo)
Figure 1. Effect of oxidation time on asphalt viscosity: oxidation at 130 "C, +; 100 "C,0;60 "C, 60 300 psi of oxygen, 0.
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Infrared Spectra. Infrared spectra were determined for 4% w/v solutions (CH2C12 or THF) in a 1mm KBr liquid cell (calibrated pathlength 0.969 mm). All spectra were recorded on a purged (dry C02 free air) Digilab FTS-7 FTIR instrument with a DTGS detector by coadding 100 interferograms obtained a t 2 cm-l resolution. W Visible Spectra. W visible absorbance data were measured on a Philips SP6 model single beam spectrophotometer using solutions of about 0.05% (w/ v) asphalt in CH2C12. Molecular WeightdElemental Analysis. Carbon, hydrogen, nitrogen, and sulfur contents were determined by the microanalytical facility at Otago University. Number-average molecular weights (M,) were determined by vapor pressure osmometry (VPO) at 37 "C in CHC13. Three differing concentrations were measured and the results extrapolated to zero concentration to obtain the reported values. Intermolecular associations between polar species in solution can, in some cases, give rise to erroneously high molecular weights being measured by VPO. This is a particular problem during studies on isolated petroleum asphaltenes but is likely to be much less significant for the whole asphalts used here. The experimental conditions were selected to minimize possible errors due to this effect. In practice, the measured molecular weights were relatively independent of concentration. Results and Discussion Effect of Oxidation on Asphalt Viscosity. Hardening of the asphalt due to evaporation of volatile components was discounted as thermogravimetric experiments using a nitrogen purge and a heating rate of 5 "C/min from 35 to 850 "C showed weight loss of volatiles to be negligible (
