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Vol. 41, No. 6
DIP-EVTCMI~E
‘0
8
IS
32
24
40
48
16
64
72
T I M E , -(OURS
Figure 9. Figure 8.
Effect of Inhibition on Oxidatiori of Ethylcellulose a t 90” C.
Inhibitory Effect of Hydroquinonemonobenzyl Ether on Ethylcellulose Oxidation
haves more as a n inhibitor than as a retarder. This inhibitory effect is even greater at’a concentration of 5yo,although very little difference is observed in the oxygen absorbed over a period of 48 hours between 0.33 and 57% antioxidant concentration. On the basis of the above results, a concentration of 1% antioxidant was chosen as a standard for all the future mork. Figure 9 illustrates the effectiveness of several different conipounds in prcventing oxidation of et’hylcellulose. As can be seen, secondary aromatic amines such as diphenylamine are most effective, as illustrated by curve 5 . This same curve was obtained when substituted phenols were used such as p-menthylphenol, Belro phenol lactone, and similar compounds. I n all these cases, oxidation was completely surpressed for almost 36 hours and t.hen progressed only very slowly over the entire range of measurement. Mercaptans (thiols), on the other hand, showed somewhat peculiar results. For example, the sample containing p-nitrothiophenol as an inhibit,or (curve 4)showed a n initial rapid upt’alte of oxygen followed by a period of no absorption after oxidation was resumed. A similar behavior was shown by 2-thionaphthol. It mas postulated that this initial rapid reaction might represent Oxidation of t8hemercaptan of disulfide, and that the latter was acting as a chain breaker. T h e tcndency for aromatic disulfides t o exist as free radica,ls lends further support to such a hypothesis. However, upon the addition of di(p-nitrophenyl) disulfide to ethylcellulose, very little effect was noted, as can be seen from curve 2, Figure 9. Although cont.rary to expectations, this result might be explained by the fact that since the disulfide has poor solubility in the alcohol-benzene solvent used t o incorporate it into the ethylcellulose, it might not have been
properly diatributed in the film Primary mercaptans were found to be far more effectivo than secondary; in fact, the latter showed practically no inhibitory action. LITERATURE CITED
Bass, F. L., Rauner, L. A , , and Lipkeph, P. H. (to Dow Chemical C o . ) , U. S. Patent 2,383,361 (May 5, 1943). Berl, E., and Rueff, G., Cellulosechemie, 14, 44-7 (1933). Bolland, J. L., and Gee, G., Trans. F a r a d a g SOC.,42, 230 (1946)
Clover, J . A m . Chem. SOC.,44, 1107 (1922). Kline, G. M., Xoc. of Plastics Ind. Conference, Los Angeles, Feb. 23.1943.
Koch, 1%‘. W.(to Hercules Powder Co.), U. S. Patent 2,389,370 (Nov. 25, 1943); 2,333,577 (Nov. 2, 1943).
hlartin, Arthur, and O t t , E., “Cellulose and Cellulose Derivatives,” High Polynier Monograph, Vol. 5, p. 966, New York, Interscience Publishers. 1943. Milas, N . A., J. A m . Chem. SOC.,52, 739-63 (1930); I b i d . , 53, 221-33 (1931).
Moureu, C . , and Dufraisse, C., Chem. Revs. 3 , 113 (1927); J . Sac. Chem. I n d . , 47, 819,848 (1928).
Sharphouse, J. H., and Downing, J . , (British Celanese, Ltd.), Brit. Patent 578.286 (June 21, 1946) ; I b i d . , 580,369 (Sept. 4, 1946). Staudinger, H.. Stockand, H., and Doemisch, K. I?., M e l l i a n d Teztilber., 22, 620-1 (1941).
Tinsley. J . S. (Hercules Powder Co.), U. S. Patent 2,337,508 (Dee. 21, 1943): 2,275,708 (May 18, 1942). Ubbelohde, L., IND.EXG.CHEX,ANAL.ED.,9, 85 (1937). U. S. Treasury Dept., Bur. of Internal Revenue, appendix to Reg. No. 3, “Formulae for Completely and Specially Denatured Alcohol,” 1942. Willard and Wingler, Ann., 431, 317 (1923). RECEIVEDOctober 2, 1947. Presented before the Division of Cellulose Sociz,w, New Chemistry a t the 112th Meeting of t h e b ~ n n r c a h CXEMICAL York, N. Y.
(Oxidative Stability of Cellulose Derivatives)
ULTRAVIOLET LIGHT STABILITY OF ETHYLCELLULOSE EVAN F. EVANS ANI) LANE F. RlcBURNEY, Hercules Powder
C
ELLULOSE derivatives exposed to outdoor weathering are known t o degrade more readily than can be accounted for by simple heat-catalyzed oxidation at the prevailing temperatures. This difference is undoubtedly due largely to the ultraviolet radiation of sunlight. Accelerated weathering tests, involvirlg exposure of the sample to ultraviolet light in the presence of air, have been devised for the evaluation of cellulose derivatives. The stability of the
Company, WiZmington 99, Del.
sample I S deteiniined by measuring the pel cent ietrntion of thc original viscosity of the sample. I n the preceding paper of thiq series ( d ) , the heat stability of ethylcellulose was shown t o be related to the stability toward oxidation. Factors influencing the rate of thc heat-catalyzed oxidation were studied and a mechanism for the oxidation was discussed. This mork was extended to a n investigation of the effect of ultraviolct light on the oxidation of ethylcellulose.
June 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
1257
T h e ultraviolet light stability of ethylcellulose is shown to be a function of the ease of oxidation of ethylcellulose. The mechanism of the oxidation is essentially the same as for the heat-catalyzed reaction, but the rate is greatly accelerated. The ultraviolet-catalyzed oxidation results in the formation of peroxides and carboxyl groups with a loss in ethoxyl, little change in color, and a drop in viscosity. These degradative effects of oxidation can be reduced by the use of effective antioxidants.
Figure 1. Apparatus for Ultraviolet Light-Catalyzed Oxidation of Ethylcellulose
The control oxidations were carried out at 50" C. in the apparatus previously described (4) except t h a t a 250-ml. reaction flask was used, which was covered with metal foil t o exclude all light. I n this apparatus, the ultraviolet light had t o transverse two layers of Pyrex and one of water in order to reach the reaction chamber. Pyrex is opaque to ultraviolet radiations below about 2900A. ( 1 , 9). Thus, i t has about the same ultraviolet transmission ( 1 ) as doe3 the atmosphere of the earth for the midday summer sun, as the limit of the ultraviolet radiation of the sun a t the surface of the earth is 2920 A. The elimination of wave lengths below 2900 A. was deemed desirable in agreement with Staud ( 7 ) who cautioned against using lower wave lengths in accelerated light stability tests for cellulose acetate. I n order t o determine whether sufficient ultraviolet light of longer wave lengths was transmitted t o the reaction chamber, the photochemical decomposition of uranyl oxalate (6) containing free oxalic acid was measured in the oxidation apparatus a t 50 C. When the concentration of oxalate ion is greater than t h a t of uranyl ion, the net reaction is simply
HzCz04 +H,O
+ COz + CO
APPARATUS AND PROCEDURE
The ultraviolet light-catalyzed oxidations were carried out in the apparatus shown in Figure 1. A 250-ml. Pyrex bulb was fitted by means of a ground-glass joint with a n adapter carrying an oxygen inlet tube extending down into the bulb and a mercury seal stirrer. The second tube shown on the adapter was not used ordinarily and was closed off during the oxidation. It was employed occasionally in attempts to collect the acetaldehyde which was formed. The bulb was surrounded by a Pyrex jacket through which water was pumped continuously from a constant temperature bath maintained at 50" C. Four General Electric Company Mazda H-4 mercury vapor lamps backed by metal shields were placed symmetrically about the reaction flask at a distance of 5.5 inches from the center of the flask. Each lamp was operated from a 110-volt, alternating current line through a n individual General Electric Company Autotransformer No. 59G16. The oxygen inlet tube was connected by means of a spherical ground-glass joint to the oxygen measuring buret. The latter was constructed as described by McBurney ( 4 ) t o maintain automatically a constant pressure throughout a run. The buret was jacketed and maintained a t 50 O C. by circulating water around i t from the constant temperature bath. The apparatus was cleaned and tested for leaks before oxidation (4). The ethylcellulose sample t o be oxidized was then added through the stirrer neck while a slow stream of commercial, compressed oxygen was admitted through the oxygen inlet tube. The oxygen was the same as employed in the previous work (4). The stirrer neck was then closed and the stirring begun at a constant rate of 500 r.p.m. The oxidation flask was next shielded by metal sheets interposed between the lamps and the flask. The lamps were turned on and allowed t o reach a steady state. After 30 minutes, the shields were removed and the zero time was noted. No measurable oxidation occurred before the shields were removed.
and the amount of reaction is determined by titration of the oxalic acid with permanganate before and after irradiation. The reaction flask was filled with a solution ( 3 ) containing 4.1214 grams of uranyl oxalate (6) and 9.1060 grams of recrystallized oxalic acid per liter of solution previously warmed a t 50" C. With water a t 50" C. circulating through the jacket, the flask was irradiated by the four mercury vapor lamps. Two 5-ml aliquots of the solution were withdrawn initially and at definite time intervals. These aliquots were titrated immediately with 0.05Npotassium permanganate. The latter solution wasstandardized, immediately before use, against a 0.0500 N primary standard solution of sodium oxalate prepared from a Bureau of Standards sample. The titrations were carried out in 6 X 0.75 inch test tubes at 65 O to 75 C. in the presence of 15 ml. of 1201, sulfuric acid. The dilute permanganate solution was added slowly
Figure
.
2.
Photochemical Decomposition Uranyl Oxalate at 50' C.
of
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1258
Figure 4.
T I M E , HOW'S
Figure 3.
Rate of Oxygen Absorption by Ethylcellulose at 50' C.
with stirring until the faintest pink color observable persisted for 30 seconds. Two typical determinations are shown in Figure 2. K i t h the lamps turned off, no decomposition occurred, but when irradiated, the uranyl oxalate decomposed in a zero order reaction. Using a n average value of 0.55 for the photochemical yield of this reaction, it was found that a minimum of 7 X 1021quanta per hour of ultraviolet light entered the reaction chamber. This was considered sufficient for the purposes of this investigation. ULTRAVIOLET CATALYSIS OF T H E OXIDATION
A commercial ethylcellulose n-as chosen containing 48.2% ethoxyl and exhibiting a viscosity of 14 centipoises a t 5% concentration in 8 to 2 ratio of toluene to 2B alcohol ( 8 ) . The oxidations were carried out on the granular flake at 50" C. This temperature was selected so that the oxidation in the absence of ultraviolet light would proceed a t a measurable rate while the oxidation under ultraviolet irradlation would not be too fast t o be measured accurately. A comparison of the oxygen absorption with and without irradiation is shown in Figure 3. I n the absence of ultraviolet light, the ethylcellulose showed a n induction period of 20 hours and then began t o oxidize a t a very s l o ~ l yincreasing rate. Under ultraviolet irradiation, there was no induction period arid a rapid rate of oxidation T V ~ Squickly attained.
Color Change during Ethylcellulose Oxidat i o n a t S O o C.
This accelerating effect of ultraviolet light upon the oxidation is illustrated further in T a b l e I . The times required to effect equal absorptions of oxygen are compared for the dark and the ultraviolet-catalyzed oxidations. For example, the absorption of 403 milliatoms of oxygen per glucose unit required 451 hours or approximately 19 days in the dark. To effect this same level of oxidation under ultraviolet irradiation required only 69 hours or about 3 days. PREPARATION AND ANALYSIS OF SAMPLES
TKOseries of oxidized samples were prepared, one in the dark and the other under ultraviolet irradiation, by treating 15-gram portions of ethylcellulose described above with increasing amounts of oxygen at 50" C. Stock solutions of the oxidized samples and of the original ethvlcellulose were made UII in 9 to 1 ratios of 2B alcohol to benzenk t o contain 1.000 gram of the sample in 100.0 ml. of solution a t 25" C. Appropriate aliquots of these stock solutions were analyzed for carboxyl and peroxide contents by the procedures previously described (4). The viscosities of the stock solutions and of several dilutions of it were measured. From these data the intrinsic viscosities were obtained by graphical extrapolation (4)except that concentrations were expressed as grams per 100 ml. of solution. The per cent transmittance of the stock solutions was determined from 400 t o 700 mG wave lengths on a General Electric Company recording spectrophotometer using the pure solvent
WITH AND WITHOUT ULTRAYIOLET IRRADIATABLE I. OXIDATION TION AT 50" c.
Oxygen Absorbed, R/Iilliatorns/ Glucose Unit 6
29 58 115 173 230 288 345 403 460
Time Required
Time Required n i t h
i n Dark, Hr.
Ultraviolet, Hr.
35
4
71)
11
404 451 (19 d a y s ) 494
OXYGEN
Figure 5.
Chemical Changes Accompanying Ethylcellulose Oxidation a t SOo C.
Figure 6.
A 8 5 0 R B E D , VlLLlATOUS/
18 29 38 47 56 62 69 (3 days)
..
GLUCOSE U Y I T
Ethoxyl Loss upon Oxidation of Ethylcellulose a t 50 ' C.
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1949
Figure 7 .
Viscosity Degradation on Oxidation of Ethylcellulose at 50' C.
mixture in the reference cell. As a measure of color, one thousand times the ratio of the per cent transmittance at 420 mp t o t h a t a t 680 mp was employed. On this basis, 1000 is colorless, and lower values indicate increasing color. EFFECT O F ULTRAVIOLET IRRADIATION ON CHEMICAL AND PHYSICAL CHANGES
Neither oxidation in the dark nor oxidation under ultraviolet irradiation had any appreciable effect upon the color of the ethylcellulose as shown in Figure 4. The samples oxidized in t h e dark showed no change in color. The one point, which does not fall on the curve, was unaccountably hazy. The presence of haze interferes with the determination of color by the method used and probably accounts for the low value obtained. Oxidation under ultraviolet irradiation seemed to effect a slight bleaching. This phenomenon is sometimes observed on exposure of ethylcellulose to outdoor weathering. The formation of peroxides and carboxyls upon oxidation is shown in Figure 5 . A maximum peroxide content, which was observed (4) for the oxidation at 90" C., was not reached in the dark oxidation at 50 O C. Apparently the ratio of formation to decomposition of the peroxides is so greatly altered a t the lower temperature t h a t the maximum concentration does not occur until much greater amounts of oxygen have been absorbed. Oxidation under ultraviolet irradiation produced peroxides with similar, but slightly lower, amounts present for the same amount of oxygen
0
do
Figure 8.
80
I20
160
200 240 T I M E , HOURS
280
320
360
Oxidation of Ethylcellulose a t 50" C.
400
1259
absorbed. It is possible that the ultraviolet light catalyzes their decomposition, The two curves showing the carboxyl content of the samples are very similar except slightly more carboxyls are present in the ultraviolet-oxidized samples. The ethoxyl loss, which occurs upon oxidation of ethylcellulose is shown in Figure 6. Oxidation under ultraviolet irradiation caused a loss of ethoxyl which was linear with respect t o the amount of oxygen consumed. I n the absence of ultraviolet light, the ethoxyl loss paralleled the other curve except for a slightly faster initial rate. The reason for this initial drop is not known. As oxidation proceeded, the viscosity of the ethylcellulose decreased. Figure 7 shows the per cent retention of the original intrinsic viscosity and intrinsic fluidity change, A[+], for the samples. The intrinsic viscosity fell off a t a decreasing rate upon oxidation i n the dark such t h a t the intrinsic fluidity change increased in direct proportion t o the oxygen consumed. With oxidation under ultraviolet irradiation, the intrinsic viscosity fell off at a slower initial, but constantly increasing, rate, and the intrinsic fluidity change increased with increasing oxidation. The data shown in Figures 5, 6, and 7 are in general agreement with the mechanism proposed ( 4 ) for the oxidation of ethylcellulose by oxygen at 90' C. According to this mechanism, the initial attack of the oxygen is upon the ethoxyl groups to form hydro-. peroxides. These peroxides then decompose resulting in the loss of ethoxyl. Concomitantly, there is a drop in viscosity. The general similarity of data for the samples oxidized with and without ultraviolet light indicates that there was no essential difference in the mechanism of the oxidation, except perhaps for the mode of chain scission. The only striking difference between the two types of oxidation is the rate at which they take place. This conclusion is similar t o t h a t expressed by Milas ( 5 ) for the autoxidation of simple ethers under ultraviolet irradiation. T h e acceleration of the ethylcellulose oxidation under ultraviolet light cannot be due t o the formation of ozone or to the activation of the oxygen, since oxygen does not absorb the longer wave lengths of the ultraviolet which are transmitted by Pyrex glass. The ultraviolet light must initiate new oxidation chains by the formation of increased amounts of free radicals, perhaps by the photochemical decomposition of peroxides or of aldehydes. T h a t the oxidation of ethylcellulose both with and without ultraviolet proceeds by essentially the same mechanism is further substantiated by the data shown in Figure 8. An ethylcellulose sample was oxidized for 3 days under ultraviolet irradiation. The ultraviolet light was then removed and the oxidation allowed t o continue in the dark. Immediately upon the cessation of the irradiation, the rapid oxidation decreased to a slower rate which was essentially the same rate at which a sample, oxidized in the dark to a n equal degree, had attained.
T I M E , HOURS
Figure 9. Effect of Stabilizers on Ethylcelhlose Film Oxidation a t 50' C. under Ultraviolet Light
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INDUSTRIAL AND ENGINEERING CHEMISTRY
T h e ultraviolet-catalyzed oxidation of ethylcellulose should be subject to control bv use of inhibitors or antioxidants in view of the free radical oxidation. The effect of scveral antioxidants on oxidation of ethylcellulose under ultraviolet irradiation is shown in Figure 9. These oxidations were carried out on ground films containing the desired antioxidants prepared as described previously (4).The rapid oxidation exhibited by the control sample was reduced by the addition of the antioxidants. Cetyl mercapt a n (thiol), which is a n excellent stabilizer in the absence of ultraviolet light ( 4 ) , was not very effective. Menthylphenol and hydroquinonemonobenzyl ether, however, were effective antioxidants. I n agreement with these observations, it is generally recommended t h a t stabilizers be used with ethylcellulose which may be subject to heat or sunlight in the presence of air. The danger of degradation of ethylcellulose due to extensive oxidation is thereby reduced and the useful life of the ethylcellulose is greatly increased.
Vol. 41, No. 6
LITERATURE CITED
Corning Glass Works Catalog, “Glass Color Filters,” p . 15, Form C-206-9-39, Corning, N . Y . Hodgman, C. D., “Handbook of Chemistry and Physics,” p. 2215, 29th ed., Cleveland, Chemical Rubber Publishing Co., 1945. Leighton, W. G., and Forbes, G . S., J . Am. Chem. Soc., 52, 3139 (1930). McBurney, L. F., ISD. ESG.CHEM.,41, 1251 (1949). Milas, N . A , , J.Am. Chem. Soc., 5 3 , 2 2 1 (1931). Noyes, W. A , and Leight,on, P. A., “The Photochemistry of Gases,” p. 83, New York, Reinhold Publishing Corp., 1941. Staud, C . J., Paint, Oil, Chem. Rec., 89, No. 18, 8 (1930). U. S. Treasury Dept., Bur. of Internal Revenue, Appendix t o Reg. h-o. 3, “Formulae for Completely a n d Specially Denatured Alcohol,” 1942. RECEIVEDOctober 2 . 1947.
Presented before t h e Division of Cellulosr SOCIETY. New Chemistry at t h e 112th Meeting of t h e . ~ M ? E R I C A NCHEMICAL York, N . Y .
(Oxidative Stability of Cellulose Derivatives)
HEAT STABILITY QF CELLULOSE ACETATE EVAN F. EVANS AND LANE F. RICBURNEY, Hercules Powder
CLL‘
Company, Wilmington 99, Del.
TLOSE acetate is T h e heat stability of cellulose acetate has been shown to havior observed using this be a function of its ease of oxidation. During oxidation, static system. Aftcr the inthe most stable of the duction period, a rapid oxidacellulose derivatives under volatile products were formed, an orange-yellow color developed, and the viscosity decreased. The per cent comtion set in, which soon denormal conditions of aging. creased owing t o the displacebined acetic acid when determined by saponification inSamples have been reported creased as the oxidation progressed but evidence is prement of the oxygen from the (3) to have retained their sented that these results do not measure the actual reaction flask by the volatile initial properties after storamount of combined acetic acid. The value of various products evolved. Droplets age for 25 years. At elevated heat stability tests have been discussed briefly in the light of liquid with the odor of acetemperatures, however, celluof these data. tic acid ivere observed t o collose acetate may suffer degralect upon the cooler portions dation. but i t is more stable of the apparatus. In order t o measure the rate of oxidation, i t (3,7 , 9, 18, $O), under comparable conditions, than cellulose was necessary accordingly to remove the volatile products as soon nitrate or cellulose ethers. A variety of heat stability tests have as they were formed. This was accomplished by circulating the been described (2, 17, 18) which measured the acidic or volatile gas in the reaction flask through suitable absorption tubes in H products formed or the temperature a t 13 hich yellowing or closed system. charring occurred. I n present-day commercial practice, however, it is customary (12) to measure the amount of discoloration or the per cent loss of the original viscosity upon heating the cellulose acetate at a temperature in the range of 160” t o 210” C. for a definite period of time. I n nearly all these tests, the cellulose acetate is heated in a limited, but unknown and probably variable, amount of air. The effect of heating on the physical and chemical properties of cellulose acetate has been studied only in relatively few instances v 5 ,o a0 30 43 50 60 7c T I M E , HOURS (7, 18, 16, $0) and then only a s a function of the time of heating. Wehr (go), however, demonstrated qualitatively that the loss of Figure 1. Oxidation of Cellulose viscosity on heating in thc prcscncc of air was duc t o an osidative Acetate at 160” C. without Absorption of Reaction Gases degradation. Similarly, the changes resulting from heating ethyl cellulose in air (oxygen) have been shomm (6, 11) to be due to the oxidation of ethyl cellulose. The present investigation was, The apparatus was modified to include a pump and glass check therefore, undertaken to study the oxidation of cellulose acetate valves by which the gases in thk reaction flask were pumped through a n absorption tube and back into the flask. A photoby oxygen under the influence of heat. graph of the pump, valves, and absorption tube is shown in Figure 2 as well as the oxvgen buret and pressurr-regulating device deAPPARATUS AND PROCEDURE scribed previously ( 1 1 ) . The pump was constructed from a 20-ml. hvpodermic syringe similar to a pump described by Olson and A commercial sample of secondary cellulose acetate containing Spurr ( 1 4 ) . The mercury seal recommended by them mas not 52.5% combined acetic acid was selected a t random for orienting used. The plunger was lubricated with dibutyl phthalate which purposes. It could not be oxidized, a t least a t a measurable rate, formed a n effective seal. Even after several years’ usage, no leakage occurred around the plunger. The pump was driven by n. under the conditions which mere sufficient for the rapid oxidation 32-r.p.m., constant-speed electric motor supplied by the Merkleof ethylcellulose (5, 11). Oxidation was obtained a t 145” C. only Korff Gear Company of Chicago. The length of t h r stroke of the after a n induction period of 160 hours (approximately 1 week), plunger, although adjustable within limits, was arbitrarily set to and at 160” C. after a n induction period of 12 hours using the give a displacement of 10 ml. per stroke. The absorption tube was filled a t first with Drierite, Ascarite, and Drierite in that order apparatus described by McBurney ( 1 2 ) . Figure 1 shows the be-
