Julv, 1924
INDUSTRIAL AATDENGI.VEERING CHEMISTRY
71 1
A n Improved Rubber Aging Test Involving Oxidation under Pressure' By J. M. Bierer and C. C. Davis BOSTON WOVEN
HOSE& RUBBER eo., BOSTON, MASS.
The predominant role that oxygen plays in the deterioration of vulcanized rubber as a result of natural aging has been well established by previous investigators. B y increasing the concentration of oxygen--that is, by employing oxygen under pressure-a greatly accelerated rate of oxidation can be obtained at temperatures only slightly above normal. Preliminary experiments indicate that vulcanized rubber compounds on exposure to dry oxygen under a pressure of approximately 300 pounds per square inch at a temperature of 50 O to 60 O C. for 16 to 24 hours show a deterioration practically identical with seoeral years of natural aging. Chemical examination of vulcanized rubber after oxidation under pressure indicates that deterioration is similar to that resulting from natural aging. Oxidation of the rubber hydrocarbon occurs in each case and oxidation products are extractable with acetone.
The close relationship between the deterioration caused by oxygen under pressure and that due to aging under normal conditions i s further emphasized by the following: ( a ) The presence of moisture retards the deteriorating influence of oxygen under pressure-an eflect in accord with the observed fact that vulcanized rubber under normal conditions deteriorates much more slowly in moist than in dry air. ( b ) I n the presence of antioxidants the deteriorating action of oxygen under pressure is greatly reduced, a fact which parallels the influence of these substances under natural conditions of aging. ( e ) T h e presence in vulcanized rubber of sulfuric acid and especially of manganese and copper compounds increases enormously the deteriorating action of oxygen under pressure-a result similar to that observed in natural aging in the presence of such highly detrimental compounds.
HE predominant role that oxidation plays in the tion of oxygen by vulcanized rubber and the close relation natural aging of rubber a t ordinary temperatures has between this rate and the rate of deterioration, nor to describe long been surmised and is nom fairly well established. the contrast between aging in oxygen and in an inert gas. As far bark as 1866 Spiller1,* showed the presence of oxygen in Of most significance is their discovery that not only is the rate raw rubber which had become resinous, and Miller2 in the of deterioration proportional to the rate of oxygen absorption, same year ascribed the deterioration of vulcanized rubber to but rapid deterioration in warm oxygen gives the same oxidation. Later, Burghardt3 went so far as to say that the oxygenated products found in naturally aged rubber. It is, then, no longer questionable that the deterioration of amount of oxygen absorbed is an indication of the extent of deterioration. Shortly after, in 1885, Thompson4 exposed rubber is due chiefly to oxidation, and it probably follows that vulcanized rubber to sunlight for several months in hydrogen, the rate of deterioration depends upon the concentration of carbon dioxide, air, oxygen, and in vacuo, and found that oxygen present. This does not necessarily imply that oxidaonly the rubber in air and in oxygen suffered any change. tion is the only change occurring, for slow internal changes Then again, Henriques5in 188Ei showed by analysis that a gain are also taking place, which can be increased or retarded by in weight was due to oxidation. Ditmar16in recommending change in temperature. With our limited knowledge of the physical chemistry of his aging test, based on the gain in weight of rubber heated in oxygen a t 100' C., assumed that oxygen was responsible for rubber, it is impossible to formulate the reactions involved in aging, but a better concept may be had if the action is a t deterioration both in his test and in natural aging. least ascribed to simultaneous oxidation and degradation by BETWEEN AGINGSXD OXIDATION RELATION heat. In this connection may be mentioned the work of As evidence accumulated that ?oxygen was the active agent, Henriquesb who found, by extracting vulcanized rubber with an increasing number of investigators attempted to correlate acetone and heating the extracted product a t 125" C., for 4 the phenomena of natural aging with artificially induced hours in carbon dioxide, that a new acetone extract was ohoxidation. Peachey7 in 1912 began an elaborate series of tained. This alone will emphasize the effect of heating experiments on the behavior of both raw and vulcanized above certain temperatures. rubber in oxygen, and found that all the degradation products The relat>iveeffects of oxidation, heat, etc., will differ in a contained oxygen. Similar work by -Kirchhof8 on raw and tire tread, in a hot water bag subjected to heat and air, in a vulcanized rubber gave much the same result and indicated hose exposed to sunlight and air, or in an insulated wire exthat the process mas one of autoxidation. Further confirma- posed to all types of weather. KOtwo suffer the same comhition of the oxidation of raw rubber appeared in the work of nation of air, heat, and light, and the significance of the term G ~ r t e r who , ~ showed that products containing oxygen were "natural aging'' depends on the particular use. There is no formed when raw rubber became tacky in air or oxygen, essential difference in the action, however, and it is only a whereas 110 change occurred in inert gases. Since then Eaton question of the relative severity of the agencies a t work. and Day,lo Stevens,ll Tuttle,lZ Whitby,I3 and others have Granted, then, that deterioration is due chiefly to oxidation, worked on the basis that the deterioration of vulcanized rub- which in turn is accelerated by heat, the rate of deterioration ber is due to oxidation. But above all, Bruni,14 M a r ~ e t t i , ' ~ becomes a function of (1) the temperature, and ( 2 ) the conand their co-workers have thrown a flood of light on the prob- centration of oxygen. The degree of deterioration depends lem by a, series of most interesting experiments. There is on these factors and on the time. no space to describe in detail their work on the rate of absorpARTIFICIAL AGINGTESTS Presented before t h e Division of Rubber Chemistry B t t h e 67th Meet-
T
1
ing of the American Chemical Society, Washington, D. C., April 21 t o 26, 1924 * Numbers in text refcr t o bibliography s t end of article.
To duplicate the effects of natural aging in a short time, at least one of these factors must be altered. Notwithstand-
INDUSTRIAL A N D ENGINEERIA-GCIIEMISTRY
712
ing tlie increa.sing evidence that oxygen is tlie active agent, artificial aging tests have tieretofore ignored or minimized this factor and have relied for their acceleration chiefly upon an undue heat effect. Thus Thompson,'O in 1890, heated stocks in an iiiciibatur, presumably in air, for a few days at GG" C., a test not muclr different from the present-day test
Val. 16, No. 7
test will not show the correct cure to avoid rapid deterioration, will not show that the stock becomes soft and tacky in tinie, and will not show that by increasing the sulfur slightly the same stock becomes very good in its aging properties. Red jar rings that became very tacky in nnc year and similar stocks that were perfectly good at the end of a year showed the following average tests after 200 liours in moving air at 70" C. : STRENGTH
PEx c,:x-T N e w After ZW Hours
I?l,ONCAT,"I,
R a d sioek Good ztwk
380 400
360
3442
--I,ss. Pi% s y in.New After 20U Hours 720 e10 710 460
Similarly, jar rings containing more sulfur, but vulcanized to such a degree that they were very stiff, did not bloom, and became hard and inelastic in 2 years, showed the following tests after 100 liours in the Geer oven and after 2 years of natural aging:
The changes brought about by t,tie Geer oven test were fiindarnentully different from those occurring in natural aging. There are many otlier instances vhcre the 70" C. I'iC. 1 --C.ILOKZXrnEK l l D M n *"K C A K R Y I N O OUT II'G,, I'I(ESSIIRI1 ovcn iest either gave no indication that a compound had a O X ~ D A T ~TEST ON short life or indicated that it had a short life when i t did not. :it, 70' C. Vl;tdiiiiir~~ff"developed a tcst consisting of ex- Iliglr-gradc Iitliarge stocks like those used in fire hose sorneposrirf! for 3 Iioors in air at 125'' C., and Lobry dc nriiynL8 timcs drop from 2500 pounds t o one-half this strength in 4 days a t 70" C., and yet do not deteriorate matcrially in serii te,?t tit 135" C., for 2 hours. 1)iimar"nppreciiltcd the influenee of oxygen whcn lie recommended his tcst in oxygen :it era1 years under natural conditions. 100" C., lrut this high temperature still overemphasized the NEW T E S T BASED OX ACCELEILlTED OXIDATION lieat effect. S o doobt the particular conrlitions that have Since natural aging is B result chiefly of oxidation, and I m ~ ~ i i imost e popular are those first advomtcd by Gcer,'* mhirlr consist essentially in lient.ingvulcanized rubber at 70" (", aging in the Gcer test a result chiefly of heat, i t mould seem in a moving current of air wliioli is coritinuoiisly renewed. reasonable that a n artificial test would more nearly approach Illis test does not, depilrt so inuch in principle from some of a natursl one if oxidation were accelerated. Tlic easiest the earlier tosts---that of Thonipson,'S for instance----as it metliod of accomplishing this is to increase the concentration docs in recognizing the influence of oxygen. It is unfortiinat,e of oxygen, in other words, i6s pressure. On this hasis an aging in a scnse tlat this k s t is so coiivenicnt, for t.liis very fact has test carried out in oxygen nuder sufficient pressure at a temincreased its popularity even in the face of its shortcomings. perature not too far above normal should much more nearly I t could he argued on theoretical grounds that the 70" C. approach nat.ura1 aging than any coriditions heretofore oven tcst is fundamentally diffrrent froin natural aging on ac- suggested. That this presumptioii is correct has been vcricoiint of the relatirely great increase in tlie heat eflect, brit fied by considerable experimentation. The exact duplication of natural aging hecornes a prtibleni of so halarrciiig tlie experimental results d l slrow this still more strikingly. I ,
SHORTCOMIXGS
OF THE
70" C. OVEN TEsr
SYSTEM FOR CARRYING OUT HIGH PRESSURE OXIDATION TEST ~
___ _ _ ~ ~ _ _ _ -
It. is a simple matter to compouiid a rubber stock which will in time become either soft and tacky or progressively liarder a.11~1 more brittle. Htiwever, it has iiot yet been found possible to make these same stocks become soft and tacky or harder and stiffer in tlic Gecr oven test. This needs no new experimoitsl evidence, for it is stated by Geer and Evans20 that samples are not hardened as in actual aging, hut are clofinitely umkened. They do not nientiiin w1ietht.r or not t.he in natural aging, but it can easily acetone ext.ract increase be shown tliat, wlierees its increase in iiatrir:~Iaging is an index of the extent of det,erioratioq in the Ceer test the extract does not change materially. Moreover, t.liere are many importmt mechanical rubber goods with st.ocks having t.ensile strengths below 1000 pounds per square inch. The (her test is admittedly unreliable for such "cheap" st.ooks, and yet it ma.y IE just as important to foresee the aging of tliese stocks as those of higher grade. Suppose, for exaiirple, that a nonblooniing jar ring must he manufactured. the aualitv of which is indicated hv a tensile strength of only 700 po~indsper square inch. The Geer
I u
V&E
I
1'10
2
INDUSTRIAL A N D ENGINEERING CHEMISTRY
July, 1924
concentration of oxygen and the temperature that the rate of oxidation and the internal changes due to heat cause the same relative amount of deterioration that they do in natural aging.
INITIAL EXPERIMENTS Initial experiments were made by placing typical rubber goods in an oven at 70" C. but inside an ordinary calorimeter bomb containing oxygen a t 500 pounds pressure. It was immediately discovered that deterioration was so rapid that several years of natural aging could be duplicated in a few hours. For example, it had previously been found that a certain red jar ring compound, vulcanized to a stiff, "tight" feeling with sulfur low enough not to bloom, had become dead and very tacky after about a year of natural aging. The Geer test had given no indication that the stock would not age well. A few hours a t 70" C. in oxygen a t 500 pounds put the rings in the same tacky condition in which they were found after natural aging for a year or more, whereas rings of a different rubber compound which were good after a year were much more resistant. Likewise, a fire hose tube containing considerable sulfur, which had been overcured and had become hard, short, and cracked in 2 or 3 years, became hard and brittle in oxygen under pressure for a few hours. TEMPERATURE, PRESSURE, AND
Such surprising results led to a more systematic investigation of the influence of the concentration of oxygen, the temperature, and the time. To this end the following three experimental compounds were studied: Parts Smoked sheets., 100 Sulfur . . . . . . . . . . . 4 Diphenylguanidine 0.5 Zinc oxide., ..... 4 Precipitated whiting.. 50 Cures Minutes O c. 20 142 30 142 50 142
.
.....
(') Parts Smoked sheets., 100 Sulfur 5 Hex4 1 Zinc o x i d e . , . . . . 5 Precipitated whiting . . . . . . . 50 Cures c. Minutes 30 142 45 142 75 142
.......... ........
line of tubing leading to the oxygen supply. After charging the bomb with samples, the system is connected through the control valve with the compressed oxygen and the pressure raised to the required amount. After the necessary pressure is reached, the control valve is closed, the valve on the tank then closed, and the tubing leading to the compressed oxygen tank disconnected. When the test is completed, the control valve is opened and the pressure in the bomb is thus released, after which the bomb may be disconnected and removed from the oven.
(3) Parts Smoked sheets.. . . . 100 Sulfur., 6 Litharge., . . . . . . . . 10 Precipitated whit1ng ............. 50
400 LBS.
4 000
40
3000
30
2000
20 h
u
too0
0
20
TIMEFACTORS
713
10 P
P
30 40 50 60 70 Ti?M P€RA TURE
1600 LBS. 4000
ul 2
?I
40
3
T
3000
30?8
2000
20
..........
Cures Minutes 15 25 50
IO
c. 142 142 142
These stocks contain three standard accelerators of different types and a t the same time more nearly resemble commercial stocks than a ~ m p l erubber-sulfur mix in that they are accelerated mixes and with their content of inert filler have a porosity more nearly like the majority of rubber compounds. Incidentally such compounds could be calendered accurately and gave extremely uniform results in mechanical testing. EXPERIMENTAL CONDITIONS The t h e e stocks were mixed in 100-pound batches, calendered, and cured in a 30-foot press to finish about 0.080 inch thick. Each stock was given the three different cures tabulated above. All mechanical tests were made with 1/4-inch dumb-bell test pieces with 1-inch marks, which were broken on a Schopper machine 24 hours after treatment. To investigate the influence of oxygen, heat, and time, nurnerom tests were made in a special Emerson calorimeter bomb (Figs. 1 and Z), the capacity of which was 1100 cc. An examination of Fig. 2 will show the system used in the experiments. The pressure bomb with the samples ir closed and placed in a constant-temperature oven. T o the bomb is permanently attached a rigid pipe long enough to reach the outside of the oven. To this can be attached by hand by means of a flange a line of flexible tubing which leads to a gage for reading the pressure, and thence by a heavy brass pipe to the control valve. In this line is also a safety plug. The control valve can be connected with an independent
'20
30
FIG.3-DETERIORATION AND
50 60 70 TEMPERATURE 40
AT DIBPERENT TSMPERATURES F O R A
0 GIVENTIME
PRESSURE OF OXYGEN. 16 HOURS
Should an explosion occur, a wave cannot travel back to the oxygen tank. Furthermore, the operator is a t no time near the bomb when it is under pressure, for the flexible tubing may be of any length and the control valve as far away as desirable from the oven. Thus the control valve can be located in a laboratory and the oven many feet away outside, protected by an iron case packed with sand bags or any other suitable method of protection. In this connection warning should be given of the disastrous effects likely to occur under certain conditions. During the earlier part of the experimentation, a relatively large amount (approximately 100 grams) of fine-ground vulcanized rubber which had been successively extracted with acetone and water and dried was heated a t 70" C. in oxygen initially under 1200 pounds per square inch pressure. Approximately 2.6 hours after starting the test, the charge exploded and completely wrecked the bomb, the oven, and the furniture and walls of the 15 X 10-foot room in which the apparatus waslocated. Such a phenomenon was due, no doubt, to the oxidation increasing to such a rate that enough heat was developed to cause spontaneous combustion. I n all tests few enough test pieces were oxidized at one time to insure a great excess of oxygen over that necessary to oxidize the rubber samples to a point where they were beyond
714
I-VDUSTRIAL -4VD ESGINEERI,VC: CHE;MIXTRY
Vol. 16, s o . 7
(2) An examination of Figs. 6 and 7 will show that, starting a t atmospheric pressure, deterioration increases ,rapidly a t first, then more slowly as the pressure increases. From a practical point of view it is not necessary to use very high pressures, and, as will be shown in experiments which follow, a pressure of 300 pounds per square inch is convenient and safe. This is close to one hundred times the concentration of atmospheric oxygen. Similar data were obtained for the stock containing hexamethylenetetramine. Without including the results in detail, it may be stated that under all experimental conditions the tensile strength and the elongation showed the same characteristic changes as did those of the litharge and diphenylguanidine stocks. The hexa stock a t all cures was more sensitive to oxygen and to heat than the diphenylguanidine stock, but was relatively more nearly like it than it was like the litharge stock. (3) The changes in the acetone extract and in the tensile strength with the time of heating a t a given temperature and pressure suggest autocatalysis (Figs. 8 and 9). Autocatalysis, where a catalyst is either formed or increased in amount during a reaction, is always represented by characteristic DAYS IN OVEN AT 70°C. S-shaped curves of a form similar to those suggested by the FIG.4-DETERIORATION IN A N O V E N A T 70' c. CONTINUOUS RENEWAL OF graphs. AIR. GEEK TEST For the sake of brevity the data obtained for the change in ultimate elongation under the various experimental conditions The changes in physical quality evidenced by the changes in ultimate elongation and strength, and the chemical changes are not included complete in graphical form. Fig. 7 , however, indicated by the acetone extract are shown graphically for is representative of the changes that occur. It is obvious that the litharge and for the diphenylguanidine stocks vulcanized such changes are essentially the same as those occurring in the tensile strength and this analogy is applicable throughout at the intermediate cure. the experiments. The results obtained for the three experimental stocks in a EFFECTSOF TEMPERATURE, TIME,AND CONCENTRATION OF state of over- or undercure indicate that a stock is most reOXYGEN sistant to oxidation under pressure a t one specific cure and ,4n examination of the graphs will show most clearly that an overcure renders a vulcanized rubber compound (1) the effect of increasing the temperature for a given particularly susceptible to oxidation. This in turn would pressure and time; (2) the effect of increasing the pressure for indicate that a method might be developed for detecting an a given temperature and time, and (3) the effect of continued overcure by a relatively great increase of the rate of oxidaheating a t a giv.en temperature and pressure. tion under pressure above a definite time of cure. (1) The effect of temperature is most striking (Fig. 3). There is a temperature range, differing for the various stocks, I I within which they become disproportionately sensitive under treatment with oxygen. Thus, in the case of the litharge stock heated for 16 hours under 1600 pounds of oxygen, the OXIDATION UNDER I deterioration caused by increasing the temperature from 20" ' PRESSURE to 50" C. is small. From 50" to 55" C. an extraordinary effect is evident, for the stock becomes sensitive to deterioration out of all proportion to its resistance a t lower temperaG€ER TEST tures. At a lower oxygen concentration a similar though less pronounced phenomenon occurs. Each type of compound appears to have its peculiar range of temperature above which it becomes relatively very sensitive to deteriorating influences. This effect is well shown by a loss in physical quality, which in turn is corroborated by an increase in acetone extract. This is probably not chiefly a heat effect as in the longer heating of the Geer test, for the acetone extract, which does not increase greatly in the Geer test, is nearly the same value for a stock deteriorated to a certain physical state whether this deterioration is brought about a t 50", BO", or 70" C. The 0 . 20 30 40 acetone extract is the same when the litharge stock has yo ACE T O N E EXTRACT dropped in tensile strength from 3000 to 1000 pounds under 400 pounds pressure of oxygen, whether this was the result of FIG. RELATION BETWEEN TENSILE STRENGTH A N D ACETONEEXTRACT. TEMPERATURE, A N D 16 hours a t 60" C. or of 8 hours a t 70" C. This is clearly LITHARGE STOCK FOR ANY OXYGENCONCENTRATION, TIMEOF TREATMENT seen by the graphs showing the deterioration with time. In contrast, Fig. 4 shows the deterioration in an oven with INCREASE I N ACETONE EXTRACT WITH DETERIORSTIOS continuous renewal of air a t 70" C., and Fig. 5 shows thereThe following data show that the increase in acetone extract lation between the acetone extract and the decreasing strength obtained in the ordinary oven test a t 70" C., compared with accompanying oxidation under pressure conforms to that taking place during natural aging, whereas the laearly conthat obtained by oxidation under pressure.
testing. Here the previous work of bIarzettiljmay i;e :ec a!led. He found that by the time 1 per cent of oxygen was absorbed deterioration was far advanced. Though it will be shown that more oxygen than this is absorbed under pressure if this absorption is based on a gain in weight, the amount is still only a small fraction of the weight of the rubber. I n studying the influence of the oxygen concentration, the temperature, and the time, the three experimental stocks were subjected to pressures of dry oxygen up to 1600 pounds per square inch, temperatures from 20" to 70" C., and periods of time up to 24 hours.
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--
/o
July, 1024
INDUXTRlAL A N D ENGINEERING CHElMISTRY
715
40 30
0
G
400
800
-0
I600
E00
OXYGEN
z
R
-
400 LBS
800 P€R.
/200
16CIo 0
SQ.
400
1N.
I200
20
E 3
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$
0 2
3
/600 -
T
70°C.
6 0 "6.
50 '6.
800
h
2
$!
FIG.6-EFFECT OF DIFFERENT PRESSURES OF OXYGEN. 16 HOURS L,
2 3 E3
p $2 3
5 F ts
800 600
2
-400
P 3 P
200
G
i?
0
400
800
/200
/600 0
800 1200 1600 OXYGEN- LBS. PER SQ.IN.
50 "C. FIG.7-EFFECT
60°C. OF
DIFFERENT PRESSURES
New 2 years old 4 years old
After oxidation 2oo hours (300 in Geer Ibs.,Oven 60' C., 16 hours)
New 3 years old
After 100 hours in Geer oven
New 2 years old After 100 hours in Geer oven
Akron Stock
Jar Ring (1) (2) (3) 370 710 3 3 Very tacky 12 0
(1) 310
"
lj0
360
very tacky 9 6 Hose Tube (2) (3) 550 2080 2 2 450 1250 5 5 jZo 'Oo0 Hose Tube (2) (1) 570 2000 480 1200 500 1050
iio
(2) 1100
(3) 4 8
960
5 7
8oo
230 950 5 3 Hose Tube (3) 570 2100 l 8 520 1950 1 7 520 " O 0 Hose Tube (1) (2) 590 1950 580 1760
520
1150
1200
f600
8
ELONGATION. 16 HOURS
deterioration after 9 years, whereas a similar sample containing no antioxidant became hard and brittle. Very recently, moreover, Pelizz01a2~has reported that the addition of pyrogallol to certain rubber compounds increases the resistance to aging. The influence of antioxidants on rubber subjected to oxygen under pressure was determined by incorporating numerous antioxidants in the litharge stock and in the diphenylguanidine stock used in the earlier experimentation. Twenty such compounds-including benzidine, tolidine, dianisidine, potassium iodide, tetramethylammonium iodide, hydroquinol, resorcinol, p-aminophenol, a-naphthol, /?-naphthol,pyrogallol, "XX," etc.-were tested in varying percentages. The results compounds-including benzidine, tolidine, dianisidine, potasindicate that all these compounds retard the oxidation of rubber when treated with oxygen under pressure in the same way that they retard its oxidation under natural conditions of aging. The results indicate, furthermore, that the greater the amount present, the more effective is the retardation. The phenols, in general, retard vulcanization also, and each determines the optimum cure of the stock containing it. The following data show (1) the per cent ultimate elongation, and ( 2 ) the tensile strength in pounds per square inch before and after treatment of the litharge stock with oxygen a t 300 pounds per square inch for 16 hours a t 60" C.
EFFECT OF AXTIOXIDANTS There has been a rapidly growing interest in the application to rubber of "antioxidants," compounds which inhibit or retard the oxidation of much larger amounts of other substances. These have attained their prominence chiefly through the work of Moureu and Dufraisse,*lwho have shown that many compounds, particularly phenols, when present in very small amounts inhibit the oxidation of organic compounds and even of oils, resins, rubber, and many other technically important substances. Perhaps the first successful experiment with rubber is due to Helbronner and BernsteinZ2in France, who report that the addition of 2.5 per cent of hydroquinol t o vulcanized rubber prevented any
800
70 "6.
OF O X Y G E N ON ULTIMATE
stant extract in the Geer test is a t variance with natural effects. The last four compounds show, furthermore, that there is no reliable comparison between the changes in quality after natural aging and those brought about by the Geer oven test. The first stock is the red jar ring already mentioned; the second, a stock from a well-known manufacturer of Akron; and the third, fourth, fifth, and sixth, high-grade fire hose tubes containing litharge, two of which deteriorated and became dry, and the other two remained in good condition. The data represent in order (I) the per cent ultimate elongation, ( 2 ) the tensile strength in pounds per square inch, and (3) the per cent acetone extract. CONDITION
400
400
ADDITION
None
1%
Benzidine
1%
Tolidine
1% "XX"
I E"
{ ~~,9'" { Before { After
r P b O Stock(1) (2) 720 2800 400 1000 710 2900 540 2200 710 2650 600 2100 720 2700 650 2300
Without further elaboration, such data will indicate the influence of antioxidants in general on the oxidation of rubber. EFFECT OF OXIDIZING AGENTS The effect of manganese and copper compounds and sulufric acid on rubber is the opposite of that of antioxidants in natural aging. Especially, therefore, should this be true
Vol. 16, No. 7
INDUSTRIAL, A N D ENGINEERING CHEMISTRY
716
when vulcanized rubber containing such compounds is subjected to a high concentration of oxygen. T o verify this supposition, the diphenylguanidine stock was mixed separately with 0.25 per cent potassium permanganate, copper sulfate, and sulfuric acid. The stocks were vulcanized a t the intermediate cure and were then subjected to oxygen a t 300 pounds per square inch for 16 hours at 60" C. The physical tests obtained before and after such treatment are summarized below: (1) P e r cent Ultimate Elongation (2) Tensile Strength (Lbs. per Sq. In.) before a n d after Treatment --Before--AfterAGENTADDED (1) (2) (1) (2) 690 3200 680 3000 Nonea 700 3000 670 2200 HzS04 710 3000 Sticky paste KMnOr 720 3250 Sticky paste CuS04.5HzO a T o the control stock containing no oxidizilig agent the same volume of water was added as t h a t used t o incorporate the three agents in solution.
Potassium permanganate, copper sulfate, and sulfuric acid gave the same characteristic effects in a few hours as many months of natural aging. The stocks containing the manganese or the copper compound became the same sticky mass observed under natural conditions. Moreover, the sulfuric acid stock showed considerable, though correspondingly less, deterioration. It is interesting to note that free sulfuric acid was probably absent, since the stock contained both zinc oxide and whiting. The same relative effects were obtained, therefore, as in natural aging.
(1)Per cent Ultimate Elon ation (2) Tensile Strength (Lbs. per In.) before a n d after Treatment Diohenvlmanidine Litharge Stock stock
8,.
~
(1) 650 460
Before D r y oxygen Wet oxygen
680
(1) 690 640 680
(2) 3300 3100 3250
It has already been found by Stevensz4 that vulcanized rubber deteriorates far more slowly in a moist atmosphere than it does in a dry one, owing to a protective action against oxidation. The results show that under these conditions accelerated oxidation under pressure is also retarded by water vapor, serving in turn to show that the new test further conforms to the phenomena of natural aging.
RESULTS IN THE ABSENCEOF OXYGEN There is always the possibility in any such test that both the physical effect of the high pressure and the effect of heating even for BO short a time may in themselves cause physical degradation of the rubber. T o determine the changes due to causes other than the action of oxygen-in other words, whether or not there is a constant involved in all the testscontroI tests were made in the absence of oxygen. This was accomplished by subjecting both the litharge and the diphenylguanidine stocks to 300 pounds per square inch of carbon dioxide for 16 hours a t 60" and a t 70" C. The results were as follows : (1) Per cent Ultimate Elongation (2) Tensile Strength (Lbs. per Sq. In.) before and after Treatment Diphenylguanidine PbO Stock Stock (1) (2) (1) (2) 650 3050 690 3300 640 2600 680 3200 460 1300 640 3100 640 2400 670 3100 80 700 620 2600
EFFECT OF WET AND DRYOXYGEN The more diverse the conditions under which the high pressure oxidation test can be found to conform with natural aging, the more likely is it a true measure of deterioration under normal conditions. To investigate still another condition where it is known that different aging effects are obtained, stocks were submitted to oxygen under 300 pounds per square inch pressure for 16 hours a t 60" C. in dry oxygen and in oxygen saturated with water vapor. I n this experiment both the litharge and the diphenylguanidine stock (intermediate cures) were tested.
(2) 3050 1300 2300
Before treatment 600 C .
700 C.
{ EGgen { :?;pen
T h a t little deterioration occurred in the absence of oxygen in the case of the diphenylguanidine stock, and that considerably more occurred with the litharge stock points to two facts. I n the first place, the sensitivity of the litharge stock above 50" C. is evidently due partly to a specific effect other than oxidation. Since the diphenylguanidine stock did not show
4000
3000
2000
loo0 0 0
4
8
/2
/6 20 24
0
4
8
12
16
20 24
0
4
8
HOURS
HOURS
FIQ.8-DETERIORAT 'ION
20 24
-x
s
70°C.
60°C.
50°C.
12 16 HOURS
WITH TIMEOF TREATMENT.OXYGEN400 ,BS. PER SQ. IN. L
4000
40 2
3000
30
E: 2
2000
1000
4
8
I2 /6 20 HOURS
4
24
12 /6 20 24 HOURS
0
4
8
/2 /6 20 24 HOURS
60 "6.
50 "C. PXG.DETERIORATION
8
WITH
OXYGGN 1600 LBS. PER SQ.I N . TIMEOF T R E A T M E N T .
70°C.
O'
July, 1924
INDUSTRIAL A N D ENGINEERINGiCHEMISTRY
this change, deterioration was negligible in carbon dioxide. On the other hand, the fact that neither stock deteriorated to any such extent as in the oxidation test indicates again that oxidation is the predominant factor involved.
GASEOUS OXIDATIONPRODUCTS The presence of gaseous products, such as carbon dioxide or sulfur dioxide and water, was investigated by passing the total volume of gas in the bomb, after treating the litharge stock with 300 pounds of oxygen for 16 hours at 60" C., through an absorption train containing successively calcium chloride, soda lime, and calcium chloride. A control test was made without the rubber present. Based on the rubber content of the stock oxidized, only 0.19 per cent of COZ, SOr, or so3 and no HzO were recovered by absorption. With this very small recovery of COz, SO2, or 503 the gain in weight of the stock during oxidation was 5.5 per cent, based on the rubber content. Since no visible products in liquid or solid form were found in the oxidation chamber, and since so little COZ, SOZ, or SO3 was recovered, almost the complete reaction involved is represented by the gain in weight. PRESENCE OF OXYGENIN THE PRODUCTS Since oxidation is the chief phenomenon involved, and since so little carbon dioxide was recovered from the gaseous products, it is almost certain that compounds containing oxygen are present in the solid product, Furthermore, since it has been shown that the acetone extract is a n approximate index of the deterioration, it is of interest to determine (1) whether substances containing oxygen are present in the acetone extract, :tnd if so ( 2 ) whether the acetone extract contains all the oxidation products. T o determine this in a preliminary way, the original stock, the oxidized stock before extraction, the oxidized stock after extraction, and the acetone extract itself were tested for levulinic aldehyde (also certain dialdehydes, cliketones, and ketone-aldehydes) by the pyrrole reaction. This test was made in a manner similar to that first recommended by BruniZ5-i. e., the sample was heated with ammonium acotate, a pine wood splinter moistened with concentrated hydrochloric acid held in the ascending fumes, and the appearance of a crimson color noted. Both the litharge and the diphenylguanidine stocks were tested in the form of (1)the original vulcanized stocks, ( 2 ) the same stocks after oxidation under pressure, (3) the same stocks after oxidation and after extraction with acetone, and (4)the acetone extracts of the oxidized stocks. A negative reaction was obtained with the original stocks and with the oxidized stocks after extraction with acetone. A positive reaction was obtained with the oxidized stocks before extraction, and with the acetone extract itself. These results show that the original stocks contain no oxygen compounds responding to the pyrrole reaction, that such conipounds are formed during the high pressure oxidation, and that these compounds are completely extractable with ace1one. To correlate this phenomenon with natural aging, the red jar rings and the fire hose tubes, already described as having deteriorated badly under normal conditions, were subjected to the pyrrole test. A positive test (intense crimson color) was obtained for each stock in the deteriorated condition, whether this deterioration was the result of natural aging or of oxidation under pressure. A negative test was obtained with the same compounds newly vulcanized. BIBLIOQRAPHY
1-J. Chem. SOL.(London), 18, 44 (1865). 2 - I b i d , 18, 273 ( 1 8 6 5 ) . 3-J. Soc. Chem. I n d , 2 , 119 ( 1 8 8 3 ) .
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4-Ibid., 4, 710 (1885). 5-Chem. Ztg., 19, 235, 382 ( 1 8 9 5 ) . 6-Gummi-Zlg., 20, 6 2 8 , 945 ( 1 9 0 6 ) . 7 - J . SOC.Chem. Ind., 31, 1103 ( 1 9 1 2 ) ; 32, 179 ( 1 9 1 3 ) ; 37, 55 ( 1 9 1 8 ) . S-KoZZod-Z., 13, 4 9 (1913). 9-caOUtChouC b gUlta-pef'Lha, 12,8724 (1916). IO-J. Sac. Chem. I n d . , 88, 339 (1919). 11-Ibid., 31, 251 (1920). 12-Rubber Age, 8 , 271 (1921). 13-India Rubber J . , 63, 742 (1922). 14--lbid., 63, 4 1 5 , 814 ( 1 9 2 2 ) . 15-Giorn. chim. ind. applicata, 5, 122 ( 1 9 2 3 ) ; Rubber Age, 13, 4 3 3 (1923). 16-Chem. N e w s , 6 2 , 192 (1890). 17-Movskoi Sbovnik, 1892, 5 7 ; J . SOC.Chem. Ind.. 11, 929 (1892). 18-Chem. Zlg., 18, 329 ( 1 8 9 4 ) . 19-India Rubber Would, BS, 127 ( 1 9 1 6 ) . 20-India Rubber J . , 61, 1163 ( 1 9 2 1 ) ; Rubber A g e , 11, 345 ( 1 9 2 2 ) . 21-Compf. Rend., 169, 1068 ( 1 9 1 9 ) ; 170, 26 ( 1 9 2 0 ) ; 174, 258 ( 1 9 2 2 ) ; 176, 127 ( 1 9 2 2 ) ; 176, 6 2 4 , 797 ( 1 9 2 3 ) ; Bull. SOC. chim., 81, 1152 ( 1 9 2 2 ) ; British Patents 141,361 a n d 181,365. 22-Comfit. rend.,177,204 ( 1 9 2 3 ) . (See Compt. rend., 176, 127 ( 1 9 2 2 ) , f o r bibliography on oxidation inhibitors, beginning with the work of Berthollet in 1797.) 23-Giorn. chim. z n d . applzcutu, 6 , 59 (1924). 24-J. Soc Chem. I n d . , 31, 251 ( 1 9 2 0 ) ; I n d i a Rubber J . , 61, 310 ( 1 9 2 1 ) . 25-India Rubber J . , 63, 415 (1922).
Decomposition of Anthraquinone by Heat''z By Harry F. Lewis and Sherman Shaffer CORNEU COLLEGE, MT. VERNON, I A .
NTHRAQUINONE has been prepared commercially for many years b y the oxidation of anthracene, the oxidizing agent being dichromate and sulfuric acid. The crude oxidation product contains such volatile impurities as anthracene and phenanthrene, etc., and nonvolatile organic chromium compounds. This crude product may be purified by sublimation. I n this process the necessary heat may be obtained either by superheated steam or directfired kettles. The temperature of sublimation varies from 200' to 350' C., and the temperature in case of local overheating may go much higher. I n this connection, it is interesting to know the rate of decomposition of anthraquinone a t elevated temperatures under differing conditions.
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METHODS The anthraquinone used was of high purity, as indicated both by analysis and the melting point of 285.5' C. (cor.). The absence of anthracene was shown by the lack of charring with 15 per cent oleum. A sample weighing 4 grams was placed in a Pyrex bomb tube, sealed either at atmospheric pressure and room temperature, or evacuated a t room temperature and the pressure determined with a manometer before sealing, or, in the experiments carried out under water vapor pressure, the required amount of water added to obtain the desired vapor pressure and the tubes evacuated and sealed. The heating was carried out in a Carius furnace so constructed as to insure even heat, and the temperatures were determined with a standard thermometer. The reaction products were removed mechanically and tested for anthraquinone by the following modification of the zinc reduction method: The sample was finely ground, a 0.2-gram sample weighed and added to 50 cc. of 5 per cent sodium hydroxide solution. The temperature was raised Received December 29, 1923. A portion of the thesis submitted t o Cornel1 College by Mr. S h a f f e r in partial fulfilment of the requirements for the degree of master of arts. 1
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