COMPOSITION OF GUM IN C R A C K E D N A P H T H A FRED K . K A W A H A R A Research and Dpcelopment Department. Amrrican 011 Co., Whitinq. I n d .
To gain information o n the mechanism of gum formation in gasoline, the kinds of oxygen and sulfur compounds in two simulated gasoline gums were determined. Gums contain thioether, dialkyl peroxide, and ether groups, as well as ester, carbonyl, acid, hydroperoxide, and hydroxyl groups. Dialkyl peroxide and ether oxygen have not been previously reported. Most of the sulfur i s thioether; hardly any disulfide, mercaptan, sulfoxide, or sulfone are present. Oxygen appears to b e the major factor in gum formation, and disulfides and trisulfides appear to b e active promoters. The high ether content in a rapidly aged gum and the high alcohol content in a naturally aged gum evidently originate in addition and abstraction reactions of alkoxy radicals, which in turn are generated from hydroperoxides, peroxides, and alkylperoxy radicals.
HE FORMATIOX of the undesirable material called gum Tpresents a serious problem in the storage of cracked naphthas and gasolines. As a n aid in reducing gum formation, information on its composition and on the mechanism of gasoline deterioration is needed. Naphtha comprises refined, partly refined. or unrefined petroleum products and liquid products of natural gas: 85Yc of which distill betxveen 347' and 464' F. Gasoline is a refined petroleum naphtha suitable for use as a carburetant in a n internal combustion engine; it normally contains considerable material boiling below the naphtha range. T h e oxidation of reactive olefins is known to be important in their deterioration (76. 73). Previous studies indicate that a n acyclic conjugated diene may be representative of the cracked-gasoline component that undergoes autoxidative deterioration (6.8)9 ) . Others have considered the reaction ( 7 7 ) of diene: mercaptan, and oxygen to be a predominant cause of gum formation in heavier fuels (24). However: evaluation of these postulations requires further information on composition, structure: and mechanisms. To elucidate the mechanism of gum formation, the types of oxygen and sulfur compounds in gum were determined.
m m . for several hoiirs. T h e product (gum I) was moderately soluble in ether hiit very soluble in ethanol and in tetrahydrofuran. Elemental analyses are shown in Table I . Desulfurization (7.5) with Raney Nickel. T h e hydrocarbon groups attached to sulfur w w e identified hy the following reaction:
RSR'
Tkvo gums were examined.
T h e first was obtained from a
This mixture was doctor-sweetened, contained no antioxidant or metal deactivator, and had a bromine number of 51 (grams per 100 grams of sample) (2). a sulfur content of 0.2476,: a n end point of 345' F., and a maleic anhydride value (20) of 7.9. For rapid aging, it was kept in txventy 55-gallon steel drums, each half filled, for 12 weeks at 60' C., and was airblown for several minutes three times a week. Its ASTM heptane-insoluble gum content ( 7 ) \vas 820 mg. per 100 ml. T h e second gum was obtained from an uninhibited mixture of 79Yc catalytically cracked naphtha, 1 5y6 virgin naphtha, and 6% butane. This doctor-sweetened mixture had a bromine value of 37 (grams per 100 grams of sample) ( 2 ) , a sulfur content of 0.13%, a n end point of 395' F . , and a maleic anhydride value (20) of 3.2. Its ASTM heptaneinsoluble gum content was 5.4 mg. per 100 ml. ( 7 ) . I t was allowed to age naturally a t ambient conditions for two years to yield a n ASTM heptane-insoluble gum content of 530 mg. per 100 ml. Isolation of Gum. l'olatile components were removed from the naphthas a t 50 C. and 0.3 mm. in a nitrogen atmosphere. Cooling of the residue to 40' C. caused a black, viscous tar to precipitate. This was macerated with n-heptane and dried at 50' C. and 0.3 min. for several hours. T h e g u m was dissolved in benzene. shaken with cold 0.5 .I-hydrochloric acid solution, washed with water, and dried a t 40' C. and 0.3
+
RII R'H NiS G u m I (4.95 grams) when dissolved in 95% ethanol a n d treatrd Lvith Raney nickel ( 1 4 ) . yielded 2040 cc. of gas. T h e results of mass spectrometric analysis of the gas, corrected for a n ethanol blank, are shown in Table 11. T h e turbid. aqueous distillate (18 i d . ) recovrred by steam distillation of the desulfurized material contained about O.3Yc (45 mg.) of monocyclic aromatics, by ultraviolet sprctrophotometry. T h e following compounds (in ethanol-water) 248.2, 254.2. and 261 m p ; were identified: benzene .,,A, 268 mp; and dialkyl benzenes. monoalkyl benzenes: , , ,A, .,,A,, 272.7 mp, 'The last named are probably meta-substituted. but may also include para-substituted benzenes.
Table 1.
Analysis of Gasoline Gums
aied ( m o l . mt. 282 f 70)"
Experimental
50: 50 mixture of coker and catalytically cracked naphthas.
+
NL(H*)
---+
Component
Hydrogen Carbon Nitrogen Sulfur Oxygen icalrd. by difference) Ash
aqed ( m o l . wt.
.?2.? =k 72)" 8 91
8 13 61 55 0 12
75 35 0 25 0 74 1 4 75 o 9n
1 41
28 79 0 31
Modtjipd 'iienzirs ebullmscopic method (in b m z m e )
Table II.
Gaseous Products from Raney Nickel Desulfurization of Rapidly Aged Gum" iMtllimoles Gas ~. -. ~~
.Mt/Iimolrs
Component
E,thylene Ethane Propane n-Butane Butenes n-Pentane Isopentane Pentenes Benzene. alkylhenzene. and dialkylhenzrne
VOL. 4
'
'
0 88 5 26
30 90 1 89 0 37 0 85 0 19 0 19
32 00
NO. 1
M A R C H 1965
7
A portion of I was dissolved in 95% ethanol and steamdistilled. T h e distillate (18 ml.) contained not more than 0.0270 of total aromatics, by ultraviolet spectrophotometry. Residue 11, remaining after the steam distillation, was extracted several times with ether. Removal of solvents a t 40’ C. a n d 25 mm. left a heavy tarlike residue (3.94 grams) containing 0.56 weight 76 sulfur. Meerwein-Ponndorf Reduction a n d Reductions with Sodium a n d Lithium Aluminum H y d r i d e . A portion of I1 was treated Liith aluminum isopropoxide (26) to reduce the carbonyl groups to alcohols. Reduction \vas complete, as indicated by a negative dinitrophenylhydrazine test. T h e product was then treated with aqueous N a O H to hydrolyze the ester groups, and finally with aqueous H C l to convert carboxylate salts to carboxylic acids. T h e product I11 had a hydroxyl number of 179 (mg. of KOH per gram) and a n acid number of 66 (mg. of K O H per gram). Test for Dialkyl Peroxides. A portion of 111 was treated with sodium (73). T h e product (IV) had a hydroxyl number of 223 a n d a n acid number of 95. Another portion of 111 was treated with lithium aluminum hydride (7. 72). Analysis of the product (V) showed: hydroxyl number. 215; acid number, 6 7 ; C, 66.67%; H, 9.50yo; K,0.12‘30; S: 0.5670. Discussion
Since only a few per cent of the sulfur in the gums is disulfide, and only a trace of mercaptan and no sulfoxide or sulfone linkages are present, most of the sulfur must be thioether (Table 111). Since the sulfur content of each gum is about six times that of the original naphtha, sulfur compounds are undoubtedly active in gum formation. T h e low disulfide content indicates that the disulfide and polysulfide formed from mercaptans in the doctor-sweetening process are transformed during autoxidation. [Autoxidation with heptane-insoluble di-n-butyl trisulfide increased the A S T M gum content for cyclopentadiene and for styrene 5.5 and 2.3 times, respectively. Samples containing 2 weight % of compound in heptane were autoxidized ( 8 )a t 110’ F. for 2 weeks. T h e trisulfide, alone, gave negligible gum when autoxidized in heptane.] Presumably, they are cleaved by various oxygenated or hydrocarbon radicals to form a thioether and a mercaptyl radical : RSSR
+ R’.
+ RSR’
+ RS.
Reaction of mercaptyl radical with olefin forms more thioether (4)1 RS.
i l + C=C ,
~
I
1
RS-C-C. I
1
+ R’H
I +
I&EC
ROOH
+ R’.
2 ROO.
+
+
+ R’H
ROO.
2 RO.
+
0 2
Metals catalytically decompose hydroperoxide to the alkoxy radical :
+ FeT2
ROOH
+ Fef3
+ OH- + R O .
or to peroxy radicals (70) : ROOH
+ Fe+3
Table 111.
+
Fe+’
+ H T + ROO.
Composition of Gasoline G u m
G u m Concentration, W t . % R a p idly .\-at ur ally aged aged
Component
Hydroperoxide oxygen ( 2 5 ) Carbonyl oxygen (27)’ Ester oxygen Hydroxyl oxygen* Acid oxygen Peroxide oxygen Ether oxygen (calcd. by difference)a Disulfide sulfur Mercaptan sulfur (23) Sulfoxide sulfur
0.64 3.12 8.13 2.84 2.21 1.06 12.50
0.004 2.43 3.94 8 96 1.01 0.33
0 06
0.05 0 00 Trace
Trace 0.00
...
a Determined after hydroperoxide reduction. After reduction of hydroperoxide and carbonyl group. Infrared absorption spectra of rapidly aged gum treated with lithium aluminum hydride show bands at 8.94 and 7.99 microns. Former band indicates presence of a secondary alcohol or an ether linkage and latter indtcates carbonvl or unsaturated ether.
I
i I
i 1
RS-C-C-H
1
Table IV.
;
PRODUCT RESEARCH A N D DEVELOPMENT
Analysis of Reduced G u m
Hydroxyl and Acid Oxygen. W t . yc Rapidly Aged G u m .Yaturally Aged G u m Acid Hydroxyl Acid Hydroxyl
+ R’.
T h e R S . could not have originated in the light-induced homolytic scission of disulfide (22), because the gums were formed in the dark. T h e virtual absence of mercaptan indicates that R S . reacts almost not a t all with a paraffin molecule to yield R S H a n d a n alkyl radical. In I , the gum produced a t 60’ C.: and 111) its reduction product, the oxygenated functional groups that were directly determined account for no more than half of the oxygen. Consequently. a substantial proportion of oxygen must be in the form of ether or dialkyl peroxide (Table I V ) . T h e increase in hydroxyl number \\.hen it is treated with LiA1H4 or sodium indicates that 111 contains about 1% dialkyl peroxide. I n the gum produced a t ambient temperature the picture is very different. T h e oxygenated functional groups that were directly determined account for all of the oxygen. T h e absence of ether \vas confirmed by infrared. T h e reduction 8
+ .OH
ROOH+RO. or via the alkylperoxy radical (77):
RS-C-C.
I
+
product of this gum shoiis such a small increase in hydroxyl when treated with LiAlH4 or Na: that the presence of dialkyl peroxide is uncertain. T h e proportions of total oxygen present as carbonyl, acid, and ester are approximately the same in the t\vo gums. Ho\vever? the distributions of hydroxyl, ether, and dialkyl peroxides are different. T h e gum produced a t 60’ C. contains about 1070 of its oxygen as alcohol, about 407, as ether. and a small proportion as dialkyl peroxide; that produced a t ambient temperature contains 50 to 60Y0 as alcohol. and little or none as ether or dialkyl peroxide (Table V ) . A hydroperoxide mechanism of oxidation can account for these products and distributions. Hydroperoxide forms alkoxy radical either directly (3):
,Material free of hydroperoxides, carbonyls, esters, epoxides After treatment with sodium After treatment with lithium aluminum hydride Table V.
3.8
5.1
1.3
4.8
5.4
6 4
3.3
5.2
3.8
6.2
1.3
5 1
Distribution of Oxygen
Functional OxygenlTotal Oxygen. c“ Rabidly .Vaturally aged gum aged gum
Carbonyl Ester
Hydroxyl Acid Peroxide Ether
10 28 9 7 2 42
15 24 56 6
Labile dialkyl peroxide
IS
literature Cited
scissioned thermally (78):
ROOR
+
2 RO.
.4t the lower temperature alkoxy radical reacts only by abstracting hydrogen from hydroperoxide or allylic or tertiary carbons (73).to form alcohol :
RO.
+ R’H
+
ROH
+ R’.
But a t the higher temperature the reaction with olefin to form ether (which has a higher activation energy) can take place (5)
RO.
+\C-C /
+
/
R-0-C-C.
I
~l
\
Conjugated dienes undergo this reaction (70) with particular ease. Dialkyl peroxide is known to form in the autoxidation of dimethylbutadiene (8). Therefore its presence in the gum produced a t 60’ C. probably results from the reaction of alkylperoxy radical \cith the conjugated dienes of coker naphtha :
ROO.
‘ I / +\ C=C--C=C /
+
ROO-(!-C=C-C. I
\
~l I
But since the naphtha that was autoxidized a t ambient temperature contained very little conjugated diene, it should not be expected to form dialkyl peroxide. T h e high ether value in rapidly aged gum and the high hydroxyl value in naturally aged gum both demonstrate that alkoxy radicals are involved. Dialkyl peroxides confirm the participation of alkylperoxy radicals. Thus the hydroperoxide mechanism involving alkylperoxy and alkoxy radicals can account €or major products and distributions. Disulfides and trisulfides appear to be active promoters of g u m formation as chain transfer agents.
(1) Am. SOC. Testing Materials; Philadelphia; Pa.; Method D-381-52T. (2) Ibid., Method D-1159. (3) Bell, E. R., Raley, F. F., Seubold, F. H., Jr., Vaughn, LG. E.. Discussions I’araday Soc. 10, 242 (1951). (4) Burkhardt, G. N., Trans. Faraday Sac. 30, 18 ( (5) Gray, P.. \Villiarns, A.; Ibid., 55, 760 (19! , (6) Hardy, C. T., Kothrock, H. S., J . A m . Chern. Soc. 80, 5306 (1958). ( 7 ) Hochstein, F. A , . Ibid., 71, 305 (1949). (8) Kawahara, F. K.: Ibid., 79, 1447 (1957). (9) Kern. N’., Stallrnann, J.. Makromo!. Chem. 7, 199 (1951). (10) Kharasch, M. S.: Arimoto, F., Nudenberg, \G.; J . Org. Chem. 16, 1.556 (1951). (11) Kharasch, M. S., Nudenberg, W., Mantell, G. J., Ibzd., 16, 524 (1951). (12) Matic. M., Sutton, D. ‘4.. J . Chem. Sac. 1952, 2679. (13) Milas. N. A , . Surgenor, D. M., J . A m . Chem. Soc. 68, 205, 643 (1946). (14) Mozingo. K.:org. Syn. 21, 15 (1941). (15) Mozingo, R., \Volf. D. E.; Harris, S. A , , Folkers, K . , J . Am. Chrm. Soc. 65, 1013 (1943). (16) Nixon. ’4. C.; ”Autoxidation and Antioxidants,” LV, 0. Lundberg, ed.. Vol. 11, Chap. 17: Interscience, New York. 1962. (17) Raley. J. H., Porter, L. M.: Rust, F. F.. Vaughn, It’. E.: J . A m . Chem. Soc. 73, 15 (1951). (18) Kaley: J . H.; Rust, F. F., Vaughn, W. E.. Ibid., 70, 1336 (1948). (19) Kosen\vald, K. H., “Chemistry of Petroleurn Hydrocarbons,” B. T. Brooks, ed., Vol. 2, Chap. 34, Reinhold, New York? 1955. (20) Shell Standardization Committee, Method 247/22. (21) Smith, D. M., Mitchell, J.. Jr., Ana!. Chem. 22, 750 (1950). (22) Snyder, H. R., Steward. J. M., Allen, R. E.: Dearborn, R.J.. J . A m . Chem. Sac. 68, 1422 (1946). (23) Tamela, M. \V,:Ryland, L. B.: .4na!. Chem. 8, 16 (1936). (24) Thaler, \V., Oswald, A. A , , Griesbaum, K., Chem. Eng. h’ezcs 41, 40 (Oct. 28, 1963). (25) Lt‘alker; D. C., Conway, H. S., Anal. Chem. 25, 923 (1953). (26) LVilds, A. L., or!. Reactions 2, 203 (1944).
RECEIVED for review August 13, 1964 ACCEPTED December 21. 1964
A CAST FLEXIBLE ESTER-URETHANE
PO LY M ER C L I FF0
R D H
.
S PA I T H , Materials Engineering, Kansas City Diuision, The Bendix Carp.: Kansas City: .WO.
This report describes a urethane casting resin based on a linear dihydroxyl e-caprolactone polyester and pphenylene diisocyanate. Prepolymer S-857 casting formulations reviewed here demonstrate a high degree Several physical properties may b e varied, to make the materials adaptable to many applications. The over-all properties of the cured polymers are comparable to those of most other cast urethane elastomers, in some instances even better. Outstanding features are miscibility at room temperature, easy degassing, and long pot life. The hydrolytic, thermal, and solvent resistance of the anhydrous 1,4-butanediol formulation make it suitable for many mechanical and electrical applications,
of versatility.
EARLY
lost amid the remarkable growth of polyurethane
N foams. solid urethane elastomers now appear ready for greatly increased usage Commercial interest in these elastomeric products has shonn rapid increase over the past fe\L years. This can best be exhibited by the growth in sales for solid urethane polymers, from some 6,000,000 pounds in 1960 to a predicted 75,000.000 pounds by 1965 (76). Cast urethane elastomers are expected to cut into a number of applications no\\ reserved for either plastics or conventional rubbers. as these elastomers possess many unique capabilities-
for example. use in the potting of electronic components, because of excellent thermal cycling properties; or industrial pallet wheels, because of good abrasion resistance. I n some cases, these solid urethanes \vi11 even move into areas where metals are traditionally employed. particularlywhere not required to bear a load ( 7 ) . This is true for man!gear, bearing. and bushing applications, because the urethane elastomers combine high abrasion resistance \vith resilience. vibration damping. and solvent resistance. Their low coefficient of friction makes possible bearings which require virtuallyVOL. 4
NO. 1
MARCH 1965
9
