Source of Water-Soluble Photooxidation Products in Asphalt John W. H. Oliver‘ and Hugh Gibson Department of P u r e and A p p l i e d Chemistry, Cniversity of Strathclyde, George Street, Glasgozu, Scotland
An asphalt was fractionated into asphaltenes, resins, dark oils, and white oils. These four fractions were labeled separately by exchanging hydrogen for tritium, and their radioactivity was measured. Experimental details are given for the tritiating technique employed. By use of the four combinations of one labeled fraction and three unlabeled fractions, four asphalts with composition and phctooxidation activity substantially the same as the original were then prepared. Thin films of these reconstituted asphalts were repeatedly photooxidized and extracted with water, and the tritium content of the water-solubles was determined, Results indicated that most of the water-soluble products of the photooxidation originated from the lower molecular-weight fractions of the asphalt with the larger dark oil fraction being more productive than the white oil fraction.
T h e rapid oxidation which takes place in the first 10 p or less (Dickinson et al., 1958; Blokker and van Hoorn, 1960) of an asphalt surface exposed to sunlight is important both in connection with the deterioration of asphalt-coated roofs and in the performance of certain types of bituminous surfacings used for pavements. The special form of sheet asphalt (low-stone content rolled asphalt) used for the surfacing of heavily trafficked roads in the United Kingdom suffers from the disadvantage that under intense traffic action the bituminous binder extrudes to the surface of the pavement unless a fairly complete layer of large precoated chips is incorporated in the surface at the time of laying. If this exposed binder is not weathered away, a surface with a low resistance to skidding is obtained. For asphaltic bitumens (derived either from crude petroleums or naturally occurring), this weathering is due to the combined action of photooxidation and leaching out of the water-soluble products of the reaction by rain water (Streiter and Snoke, 1936). I n the United Kingdom this process is sufficiently rapid in the case of “Trinidad Lake Asphalt” binder and certain asphalts of Middle East crude origin t o maintain a surface with acceptable resistance to skidding. This was not the case with certain asphalts of Venezuelan crude origin which gave low water-soluble production on photooxidation (Gibson, 1966) and formed a smooth skin on the surface of the road. The purpose of this investigation was to determine the major source in the asphalt of the water-soluble products formed by photooxidation. An asphalt of Middle East origin with acceptable performance in rolled asphalt surfacings was examined b y division into four fractions which were labeled by exchanging hydrogen for tritium. The tritium content of the water-soluble photooxidation products of the four reconstituted asphalts containing the labeled fractions was then determined. Preparation of Asphalt Fractions. T h e asphalt sample used in this investigation was a 40/50 penetration road 1 Present address, Australian Road Research Board, 60 Denmark Street, Kew, Vie., Australia 3101. T o whom correspondence should be addressed.
66 Ind.
Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 1, 1972
asphalt of Middle East origin and one of several being studied b y t h e Road Research Laboratory, United Kingdom, in full-scale road trials. The fractions required for labeling were prepared by a procedure similar to that described by Kleinschmidt (1955). The adsorbents were activated alumina and silica gel in the weight ratio of one to three. The maltene fraction, obtained by pentane precipitation of the asphaltenes from the asphalt, was added to the chromatographic column in a pentane solution. Successive fractions were eluted with the solvents pentane, methylene chloride, and methyl ethyl ketone and designated as white oils, dark oils, and resins, respectively. Material remaining on the column was desorbed with chloroform followed b y a water and acetone mixture and added t o the r e k i fraction. Results of the fractionation are given in Table I. Tritium Labeling Procedure. T h e exchange reagent employed was tritiated phosphoric acid-boron trifluoride, first reported b y Yavorsky and Gorin (1962) and now t h e subject of patent cover (Yavorsky and Gorin, 1966). T h e scaled down method of Hilton and O’Brien (1964) was more suitable for the quantities required for asphalt work. Phosphoric acid was first prepared by the slow addition of 1 ml of tritiated water (activity 200 mC/ml) to 2.75 grams of phosphorus pentoxide contained in an 8-dram glass vial. Boron trifluoride gas was introduced into the vial above the phosphoric acid while rapid stirring took place. Because of the extremely corrosive nature of the gas, the delivery line and valve were constructed from Teflon and glass and monel metal, respectively, and the stirrer bar was Teflon. The asphalt fraction t o be labeled was dissolved in 7 ml of cyclohexane (chosen since it was not susceptible to labeling) and added to the reagent. The two-phase system was then stirred for 16 hr. A recovery procedure described by Yavorsky and Gorin (1963) involves decanting the organic phase and washing it with a basic solution followed by water to remove all traces of the acidic reagent. Asphalt fractions formed a stable emulsion with this reagent. A modified procedure was adopted and consisted of filtering the contents of the glass vial through a sintered glass filter to partially break the emulsion. The solvent layer was then allowed to flow through a column of
powdered sodium carbonate supported b y a sintered glass disc. Any organic material adhering t o the sodium carbonat’e was eluted with methylene chloride. The organic phase was then washed with water which was subsequently tested for acidity, and the process repeated until all traces of acidic reagent had been removed. Contacting of the asphaltene and resin fractions with the highly acidic tritiating reagent resulted in the formation of high-molecular-weight cyclohexane insoluble material. Solutions of these fractions were consequently contacted with tritiated phosphoric acid only and recovered b y the same procedure. Krenkler analyses (Krenkler and Eilhard, 1951), molecular weights, and carbon and hydrogen contents were done on all fractions before and after labeling. Results indicated that the procedures had produced no significant degradation. Data on the activities of the fractions subjected t o tritium exchange treatments are given in Table 11. Some comments on tritium exchange reactions with asphalt fractions are given in the Appendix. Photooxidation Conditions. T h e photooxidation apparatus consisted of a n aluminum cylinder, 37 cm in diameter, enclosing a high-pressure mercury vapor lamp surrounded b y a tubular borosilicate glass envelope which transmitted radiat’ioiis of 3000 A and above. Rectangular aluminum sample plates were arranged around the inside wall of the cylinder, and with the forced draught cooling used, the plates were maintained a t a temperature of 40 f 1°C. Preparation of Water-Solubles. Four reconstituted asphalts were prepared b y combining a labeled fraction with t h e three complementary unlabeled fractions in t h e correct proport’ions. T h e asphalts with a labeled fraction and the untreated asphalt were deposited on specially prepared aluminum plates from a carbon disulfide solution. After evaporation of the solvent, the deposited films were warmed and painted with a brush to evenly cover an area of 100 cm2, resulting in film thicknesses ranging from 55-120 p . T h e films were exposed in the photooxidation apparatus for 25 hr, soaked for 1 h r in enamel trays containing distilled water and after drying returned to bhe apparatus for further exposure. After four such cycles the water containing the accumulated water-soluble products was filtered into weighed evaporating basins. The trays were then cleaned and refilled with fresh distilled water. Exposure periods for the two runs were slightly different. The water-soluble products from each plate were recovered b y evaporation of the water at room temperature, dried in a dessicator, and weighed. The rat’e of production of water-soluble material for the two runs with the labeled asphalts and the unlabeled asphalt is shown in Figure 1. Labeling of the fractions has not signifi-
CUMULATIVE TIME OF PHOTO.OXIDATION
(Hours)
Figure 1 . Production rate of water-soluble material by labeled asphalts
Table 1. Characteristics of Asphalt Fractions Wt
%
Fraction
C,
%
H,
%
C IH ratio
Mol wta
Asphaltenes 2 8 . 1 83.88 8.10 10.35 3100 Resins 80.78 14.3 9.39 8.60 1197 Dark oils 43.7 83.79 9.99 8.40 695 6.33 512 White oils 13.4 85.88 13.57 Xumber average molecular weights with vapor pressure osmometer. Q
cantly affected the rate of production of water-soluble material. Determination of Tritium Content. Tritium content of t h e asphalt fractions and water-soluble material was measured b y liquid scintillation counting. Only a singlechannel, single-photomultiplier instrument was available for R u n 1. T h e counting head of this instrument was placed in a refrigerated cabinet a t - 15OC. A three-c hannel automatic instrument equipped with coincidence circuitry was used for R u n 2. T h e asphalt samples were counted in toluene with the scintillation phosphors 2,5-diphenyloxazole (PPO) and 1,4-bis2-(5-phenyloxazolyl) benzene (POPOP) (Turner, 1967). More elaborate solvent systems were examined to assay t h e
Table II. Tritium labeling of Asphalt Fractions Asphalt fraction treated
Tritiating reagent
Xsphaltenes Resins Dark oils White oils
THzPOi THzP04 T”QP04. BF3 TH?POi.BF3
Reagent toa sample ratio Run 1 Run 2
Specific activity, m c 1g Run 1 Run 2
Comparativeb activity, mC/g Run 1 Run 2
0.70 0.71 0.08 0.65
0.52 1.60 7.03 0.25
0.74 2.25 88.95 0.38
0.69 0.68 0.10 0.67
0.41 1.55 10.05 1.68
0.60 2.29 96.67 2.52
Recovery, wt % Run 1 Run 2
60 72 76 86
62 80 81 86
a Curies of reagent per gram of sample (1 curie is amount of radioactive material producing 3.7 x 1010 disint,egration/sec).* Specific activitylreagent ratio.
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67
I
j
I
I
a
I
1
I
. . w
1
0-
v
4SPHALTEHES
Y
I I I
Figure 2. Percentage yield of water-soluble material from four asphalt fractions
highly polar photooxidation products. The mobt satisfactory mixture for use on the single photomultiplier instrument contained ethylene glycol monoethyl ether, 1,4-dioxane, naphthalene, PPO, and toluene in the proportions mggested by Baxter et al. (1964). For the counter equipped with coincidence circuitry, a n alcohol and toluene mixture incorporating the phosphors PPO and POPOP gave higher counting efficiencies. I n both cases the water-soluble samples were redissolved in a suitable volume of water before addition to the scintillation mixture. The efficiency of counting was determined by using the internal standard method for all samples (Hayes, 1956). Tritiated n-hexadecane was employed as standard. T h e strong color of the asphalt and water-soluble samples gave severe quenching of the tritium emissions. Phosphorescence (Davidson and Feigelson, 1957) of the counting vials required dark adaptation when the single channel instrument was used. Chemiluminescence (Bousquet and Christian, 1960) was exhibited b y the water-soluble material and gave a spuriously high background count rate reduced by the addition of 25 p1 of 35% hydrogen chloride solution t o each sample as suggested b y Herberg (1958). Evaluation of Results. T h e internal standard method (Hayes, 1956), used t o measure t h e activity of each sample of water-soluble material, asbumes t h a t the disintegrations from a measured quantity of added standard material are detected with t h e same efficiency as those of the sample. T h e weighed samples were diluted prior to counting, and the specific activity of each sample was calculated from the equations:
A
=
S=
C - B ___. Q (counts/sec) D - C A.E ~
l T ! l
(counts/sec/mg)
where A
=
sample activity
C = count rate of sample B = background count rate from control sample D = count rate of sample with standard added Q = absolute count rate of measured quantity of standard added S = specific activity of sample E = dilution factor = weight of water-soluble sample before dilution, mg 68
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Table 111. Average Yields of Water-Soluble Material Based on Unit Weight of Fraction ( F values) Average F values,
%
Asphalt fraction
Run 1
Run 2
Mean
Xsphaltenes Resins Dark oils White oils
4.4 17.6 33.0 45.0
6.0 20.9 33.8 39.4
5.2 19.2 33.4 42.2
Since the weights of water-soluble material obtained from the four tritiated asphalt samples in a single oxidation period were dight,ly cliff erent, all calculations for these materials were based on unit \veight (1 mg) of water-soluble material recovered. When converting from specific activity to weight of watersoluble material originating from the labeled fraction, correction is required for the difference in t'racer cont'ent between the asphalt fraction and the mater-soluble material. This mas made by using hydrogen as a model for the tritium behavior. Thus, the weight of water-soluble material originat'ing from each labeled fraction is
where X and Y are the hydrogen contents of the water-soluble material and labeled fraction, reqpectively, and H is the specific activity (counts/sec,'mg) of the fraction. Summing the TV values for all four labeled fractions yielded the total weight of water-soluble material detected by the radioactive tracer technique. The amount of water-soluble material originating from one labeled fraction expressed as a percentage of this total is given by
p
=
T i7 . 100% zTV (all fractions)
Results are also expresqed on the basis of unit weight of fraction (Table 111).Where K is the percentage of each fraction obtained by the Kleirischmidt separation, '
P(fraction) ___K(fraction) .loo% P C K ( a l 1 fractions)
Discussion
The results shown in Figure 2 indicate that on a weight basis the oil fractions produced over 80% of the water-soluble material. This agrees with the work of Kleinschmidt and Snoke (1959) who exposed films of asphalt of different thicknesses to light and water treatment. Fractional analysis, supported by a correlation between film thickness and loss of weight of the samples, led these authors to suggest that the dark and white oils were selectively degraded and that migration of these components from within the film to the surface occurred. Table 111 shows the results expressed on a basis of unit weight of fraction, and production from the white oils is greater than that from the dark oils. The white oils coiisist almost entirely of saturated structures suggesting that aliphatic entities in the asphalt may be responsible for the production of most of the water-soluble materials. Support for this view is given by Xackenzie (1969) who found little trace of aromatic material in the infrared spectra of the water-solubles.
The approach of pliot,ooxidiaing reconstituted asphalts used i i i this work simulates natural \Teathering in the seii3e that, iiiteraction effects betweeii fractions are not preveiited. rlYius, while the oil fractions are the principle precursors of the water->oluble material, the asphalteiie and resin fractions could well play aii ini1,ortant role in the photooxidation reactioiiq leading to its forniat,ioii. Appendix
Tritium Exchange with Asphalt Fractions. T h e tritium eschaiigc reageiii phosphoric acid-boron tiifluoride was ext eiisively studied by Yavorsky and Gorin (1963) who fouiid t h a t exchange took place Iietween t h e reagent a I i d a ro nin t i c h y (1 rnge n at o iiis and a 1.;o those h j-drn g e n 3 attached to carhoii atoms adjacent to a tertiary carbon atom. However, in the latter case the reaction was slower. From nuclear magnetic resonance studies on deuterated cumeiies, they fouiid that the tert,iary hydrogeii position was not’ itself labeled but appeared to act as a gate to permit tritium tagging on adjacent, carbon atoms. 111 this work, to increase the tritiating reagent’ t o sample ratio for some fractions (Table 11) several vials, each contaiiiiiig 200 m C of reagent,, were used, aiid the sample was distributed evenly among them. The fractions were combined for the recovery operation. When this is taken into account by coilsidering the comparative activit’ies (Table 11), there is acceptable duplication between the two runs with the exception of the white oils. This fraction had the least t’eiideiicy to emulsify aiid required high-speed stirring to obtain good contact with the trit’iatiiig reagent phase. By comparison of the asphaltene and resin fractions (Table 11), between three and four times as much t’ritium was exchanged into the resins as into the asphalteiies. This indicates a higher proport’ion of exchangeable hydrogen positions in the re$ins, and since aromatic hydrogens label much more rapidly than those adjacent to a tert,iary carbon atom, the resin fraction probably contains more aromatic hydrogens per unit weight than the asphaltene fraction. Much more t’ritium was exchanged with the dark oils than the white oils. As the white oils have a negligible aromatic content as indicated by their infrared spectrum, a slow exchange would only be possible for the limited number of hydrogen positions adjacent to a tertiary carbon atom whereas much more rapid exchange could take place with the aromatic hydrogens in the dark oils. KO comparison can be made between samples treated with the phosphoric acid-boron trifluoride reagent and those treated with phosphoric acid alone since the exchange rate is greatly reduced if the boron t,rifluoride is omitted (Yavorsky and Gorin, 1963). KO attempt was made to obtain complete recovery of any fraction as this would have increased the risk of splash or spillage of radioactive material. T h e asphaltene fraction formed the most stable emulsions and gave the lowest recovery figure. A test with inactive material gave a recovery of 8870 for this fraction. The precipitation of the asphaltene and resin fractions from cyclohexane solution b y the strong acid phosphoric acid-boron trifluoride complex appears t’o be a similar process to the precipitation observed when a crude oil was treated with per-
chloric acid by Nicksic and Jeff ries-Harris (1968). These workers fouiid that t.heir precipit’ate could be converted to normal aqihalteiies by dispersion in toluene followed by extensire extraction with aqueous potassium hydroxide. Several explanations for the precipitation effect were considered, b u t 110 definite coiiclusions were drarrn. Neasurement of the tritium activity of the mater-soluble material present’ed problems, aiid in view of the new developments in t,his field, a preferred approach to the counting of these materials would he to burn them t’o water which could then be incorporat,ed in t’he scintillation mixture. The simplicity of the tritium exchange method used, the fast reaction rates obtained, and the absence of labeled byproducts suggest that t,his method might find wider application in the field of petroleum chemistry. Because of the low energy of the beta particles emitted by tritium, no special shielding is required, and work a t tracer level can be carried out in a normal chemical laboratory provided certain precaut’ions are taken (Evans, 1966). Acknowledgment
The authors are grateful to P. D. Ritchie for provision of research facilities. Literature Cited
Baxter, J. A , , Fanning, I,. E., Swart7, H. A , , Znt. J . A p p l . Radiat. Zsolopes, 15, 415 (1964). Blokker, P. C., van Hoorn, H., W o r l d Petrol. Congr., Proc., 5th, LY.Y., 1959, 6,417 (1960). Boiiiquet, W. F., Christian, J. E., Anal. Chem., 32, 732 (1960). Ilavidson, J. I)., Feigelson, P., Znt. J . A p p l . Radiat. Isotopes, 2, 1 (1957). Dirkinyon, E. J., ?jichola>, J. H., Boas-Traube, S.,J . A p p l . Cheni. (London). 8. 673 (1958). Evans, G. A , , “Trifiiim and Its Compounds,” Biitt erworths, London, England, 1966, pp 70-98. Gibson, H., llinistrv of Transport, U.K., RRL Rept. So. 37, Harmondworth, 1966. Hayes, F. X., Znt. J . A p p l . R’adiaf.Isotopes, 1, 46 (1956). Herberg. 11. J.. Science. 128. 199 (1958). Hilton,-B. I]., O’Brien, R’. I)., J . Agr. Food Chem., 12, 236 (1964). Kleinsrhmidt, L. R., J . Res. .I at. Bur. Stand., 54, 163 (1955). Kleirischmidt, L. R., Snoke, €1. 11.)J . Xes. S a t . Bur. Stand., 63C, 31 (1959). Krenkler, K., Eilhard, J., Bztzmen, Teere, Asphalt?, Pech?, 2, ,59, 85, 105 (19513. llackenzie, I).’ A,, PhD thesis, University of Rtrathclyde, Glasgow, Scotland, 1969. Kicksic, S. W.j Jeffries-Harris, 31. J., J . Znsl. Petrol., 54, 107 11968) - .. ,. Streiter, 0. G., Snoke, H. R., J . Res. S a t . Bur. Stan,d., 16, 481 (1936). Turner, J. C., “Sample Preparation for Liquid Scintillation Counting,” Whitefriars Press Ltd., London, England, 1967, pp 8-1 1. Yavorsky, P. I f . , Gorin, E., J . dmer. Chem. SOC.,84, 1071 (1962). Yavorsky, P. M., Gorin, E., U.S. At. Energy Comm., Rept. SYO-10178, June 1963. Yavorsky, P. ll.,Gorin, E., (to U.S.At. Energy Comm.), U.S. Patent 3,230,261 (January 18, 1966). ~
__
RECEIVED for review June 4, 1971 ACCEPTEDSeptember 17, 1971 Presented at the Division of Petroleum Chemistry, 161st AIeeting, ACS, Los Angeles, Calif., March-April 1971. The authors are grateful to the Science Research Council for a maintenance grant. Work done by J.W.H.O. was in partial fulfillment of the requirements for a PhD degree from the University of Strathrlydt..
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 1 , 1972
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