R LTHOUGH GR-S vulcanizates are quite resistant to aging B. M. STURGIS, A. A. BAUM at ordinary temperatures, their usefulness has been imJ. R. VINCENT paired by the fact that they deteriorate rapidly when subJackson Laboratory, E. 1. du Pont d e Nemours & Company, Inc., jected to elevated temperatures. This deterioration is deWilmington, D e l . pendent upon the time of heating and the temperature of heating, and is especially apparent a t temperatures in excess of I t might, on the other hand, indicate that other factors besides about 60-70" C. The effect of heat on GR-S vulcanizates is a oxidation play a considerable part in the hardening of GR-S general hardening; the modulus and hardness increase, and the vulcanizates by heat. T o determine whether the presence of PI elongation at break decreases. Four days in the 100" C. air oven larger quantity of antioxidant or of a different type of antiis sufficient to impair seriously the usefulness of many GR-S stocks. oxidant would be beneficial, a series of experiments was run in This effect of heat shows up not only on oven tests but also which additional quantities of a variety of antioxidants were during service, and is undoubtedly one of the causes contributing added to GR-S stocks. The base stock used was as follows: to the failure of GR-S tires. On the other hand, the effect of GR-S 100 Sulfur 2 Channel black 50 2-LIercaptothiazoline 1.25 heat on rubber vulcanizates is usually to produce softening, with Zinc oxide, 5 Antioxidant 1 (except where otherdecreased modulus, tensile strength, and elongation. Stearic acid 2 wise indicated) The deterioration of an elastomer by Slabs of these compounded stocks were heat is largely the result of increased 75 minutes a t cured for 15, 45, and chemical activity. This may take the 287" F. (142' C). Dumbbell strips form of oxidation, aftervulcanization, or GR-S vulcanizates undergo rapid de!yere died out,, and a portion were aged a basic change in the polymer, such as terioration when subjected to heat, both 4 days in the 100" C. oven before being i n oven tests and during service. This depolymerization or further polymerit,ested at 82" F. (28" C.). The control zation. The changes that take place in effect i s probably one of the causes of contained no added antioxidant'. rubber vulcanizates during heating are failure of GR-S tires. Tests have been Table I illustrates the magnitude of made on a number of GR-S stocks to probably a combination of oxidation, the changesinmodulus, tensilestrength, aftervulcanization, and depolymerizadetermine whether various oxidation ' and elongation in a GR-S vulcanixate tion, with oxidation playing the major factors or vulcanizing agents might mitiheated at 100" C. It also demongate the deleterious effects of heat. It is part. Cotton (1) concluded that, since strates that the antioxidants tested modern accelerated rubber mixtures found that dinitrobenzene is an effecwere ineffective in prevent.ing these containing little free sulfur after vultive vulcanizing agent for GR-S, producchanges. Even the stock containing 4 ing vulcanizates which are similar to sulcanization are nearly always softened parts of p,p'-dimethoxydiphenylamine by prolonged heating, the softening apfur vulcanizates in stress-strain propershoaed no improvement over the pears t o be due to depolymerization or ties and tear resistance. The dinitrocontrol. disaggregation of the vulcanized rubber benzene vulcanizates, however, also structure, accompanied by oxidation. possess the ability to resist deterioration EFFECT OF OXYGEN by heat to a much greater extent than The magnitude of the oxidation effect With the possibility in mirid that the was demonstrated by Reed (5) who d o sulfur vulcanizates. Although only limited progress has been made in imshowed that little deterioration took hardening of GR-S vulcanizates by heat is not primarily an oxidation phenomeproving the heat resistance of GR-Sby place when rubber vulcanizates were using the methods formerly applied to non, a series of experiments was carried heated in an oven at 90" C. in a vacuum out in which GR-8 vulcanixates were or in purified nitrogen, and by Jones ( 2 ) rubber, the use of an entirely different method of vulcanization has enabled the who found that rubber vulcanizates heated both in air and in nitrogen from production of a more stable vulcanizate showed little change in stress-strain which the last traces of oxygen had been properties when heated 14 days a t and may point a way toward a solution removed with hexaphenylethane. of the problem. 158O F. (70' C.) in a vacuum. The For these experiments the following effect of heat in producing overvulstock was compounded a n d riired for canization or reversion in rubber is 45 minutes at 287" F.: well known. GR-S (manufacturer B, lot 1 ) 100 Stearic arid 1 Obviously, then, the changes produced by heat on GR-S Channel black 50 Sulfur 2 5 2-~,~ercaptothiaaoline 1.25 Zinc oxide vulcanizates are different from those on rubber vulcanixates. I t may be that different types of chemical changes take place Dumbbell strips were died out of this cured stock and treated in GR-S on heating, or that the three factors discussed in conas follows: Three strips were placed in a test tube, separated nection with rubber have a different order of importance in from one another by strips of filter paper. The mouth of the test tube was loosely covered with a sheet of filter paper, and producing the changes brought about by heating GR-S. this t,ube was inserted into a large glass tube which was then drawn oiit on the open end to form a tube about 3 / 8 inch in INFLUENCE OF ADDED ANTIOXIDANT diameter. Four or five grams of damp hexaphenylethane were dropped into the larger tube, which was immediately evacuated Since a considerable proportion of the effect of elevated temto a pressure of about 1 mm. of mercury and then filled with erature aging on rubber vulcanizates is due to oxidation, the purified nitrogen (containing less than 0.00001 oxygen). deleterious effects can be mitigated by the use of an antioxidant The system was evacuated and filled with nitrogen s1x times and then evacuated for 2 hours. The tube was then filled ( 5 ) . GR-S undergoes pronounced changes on heating in spite with nitrogen and sealed off. Hexaphenylethane actively comof the fact that it already contains antioxidants which are added bines with oxygen and should remove even the last traces of during its manufacture. This might indicate that the antioxygen from the tube. All of the tubes Tvere allowed to stand oxidants are not the proper type to prevent oxidation a t elefor 2 days at room temperature; duplicate tubes were aged a t vated temperatures, or are present in insufficient amount. room temperature in the dark, for 1, 2, 4,and 8 days in a 100" C.
A
r0
348
INDUSTRIAL AND
April, 1944
ENG INEERING
oven; and for 7, 14, and 28 days in a 70" C. oven. After oven aging, each tube was kept a t room temperature in the dark until the 28-day period in the 70" C. oven was over. The tubes were o ened on the same day and the strips tested. As controls, sets or dumbbell strips not enclosed in tubes were aged for the same periods in the same ovens. These control strips were therefore aged in the air. Results are given in Table 11. The data for the tests carried out in nitrogen are the average of results on duplicate tests using three dumbbell strips in each test. I n every case the values for the duplicate tubes checked well. The striking point is that the hardening of the vulcanizates takes place to the same extent, whether they are heated in the presence of air or in highly purified nitrogen. I n nearly every case the modulus, tensile, elongation, and hardness figures of the strips aged in air and in nitrogen check one another within experimental error. It appears, therefore, that the hardening of GR-S vulcanizates by heat may be primarily due t o some other cause than t o the oxidation which undoubtedly takes place simultaneously when oxygen is present. It is possible that , a small amount of oxygen was present in the samples in the form of peroxides or absorbed oxygen. The evacuation technique used, together with the hexaphenylethane, should have removed most of the absorbed oxygen, however. I n any case, the results are diametrically opposed t o those obtained by Reed (6) and Jones (2)with rubber-namely, that rubber vulcanizates suffered little deterioration when heated in nitrogen or in a vacuum. It is difficult to say just how much of the hardening of GR-S vulcanizates by heat is the result of aftervulcanization. GR-S
349
CHEMISTRY
TABLE111. INFLUENCE OF ACCBLERATION ON 100' C. OVEN
AGING
Parts Used
Accelerator Butyraldehyde-aniline Mercaptobenzothiazole 2-Mercaptothiaaoline Mixed thiazyl disulfides Tetramethylthiuram monosulfide Lead dibutyl dithiocarbamate
Tensile a t Break, Lb./Sq. In. After 4 days in Original oven
Stock A B
250 325
925 1100
tention
2475 2525 2925 3100
2300 2225 2100 2275
500 520 400 510
240
0.25
3000
2125
440
230
52
0.5
3275
1775
390
180
46
Tensile a t Break, Lb./Sq. In. OrigAfter ins1 aging 2750 2050
ON
230 260
'36 % re-
1.5 1 1 1
TABLE Iv. EFFECT O F OVEN AGING STOCKS 100% Stress, Lb./Sq. In. OrigAfter inal aging
Elongation, After4 Orig- daysin inal oven
46 50 47 47
190
NONACCELERATED
Elongation a t Break, % Orig- After % ?emal aging tention 515 365
2475 2175
210 160
41 44
vulcanizates, however, appear to resist the harmful effects of aftervulcanization to a much greater extent than rubber, and the accumulated evidence indicates that aftervulcanization of the type found with rubber is not the primary factor in causing hardening. EFFECT O F ACCELERATORS
TABLE I. EFFECTOF ADDEDANTIOXIDANT"ON HEAT RESISTANCE OF GR-S
Min. Cured 15 45 75 15 45 75
l5
45
75
Days in looo C. -Antioxidant A- Antiox- Antiox- AntioxOven Control 1 part 4 parts idant B idant C idant D --Stress a t 100% Elongation. Lb ./sa. In,50 50 0 75 50 50 50 900 875 900 650 850 850 4 225 300 225 250 0 300 250 1050 1250 4 1050 I000 1125 950 325 375 425 350 0 350 350 1125 1050 1075 1125 1150 4 975 -Tensile Strength a t Break, Lb./Sq. In-.0 500 650 850 550 650 700 1050 1450 1150 1825 1350 4 1650 0 2125 2550 2550 2450 2450 2550 1850 1925 1875 4 1900 1850 2000 0 2450 2600 2475 2775 2700 2676 1850 1925 1850 1850 1825 1900 4 rElongation a t Break, % '0 495 485 510 575 520 510 4 145 120 120 210 135 155 0 440 475 485 500 525 500 4 160 145 155 155 145 165 0 395 375 390 410 425 410 4 155 155 145 160 145 155
TABLE 11. INFLUENCE OF OXYGEN ON HEATAGINGOF GR-S Aging Temp., O C. Room
Oven Aging, Days
..
100
1
100
2
100
4
100
8
70
7
70
14
70
28
-Lb. Stress Test Atmosat phere 100% Nz 400 Air 475 Nz Air Nz Air NI Air Nz Air
Nz Air Nz
Air N; Air
600
600 625 600 775 775 825 800 575 475 575 575 750 725
per Sq. In.Stress Tensile at at 200% break 1100 2250 1150 2225 1700 2175 1550 2250 1800 2275 1625 2225 1950 1926 1925 ,. 2000 1950 1550 2350 1450 2175 1650 2026 1725 2025 1850 2050 1775 2225
..
Elongationat Break,
%
320 320 230 265 230 255 195 200 195 195 265 255 225 230 210 230
Hardness Shore'A 68 66 72 69 72 72 74 73 76 77 70 71 74 72 74 72
GR-S does not easily overcure in the same sense that rubber does. Neal (S) pointed out that even such an active accelerator as tetramethylthiuram monosulfide does not readily cause an overcure, and that there is little change in modulus, tensile strength, or elongation a t break, even on prolonged curing. If the effects produced by heat on GR-S vulcanizates are largely due t o aftercure, a considerable difference should be expected, depending on whether slow or active accelerators are used. To determine if such a difference exists, a series of GR-S stocks was compounded in which accelerators of different activity were used; their activities in G R S are rated by Neal (3) as follows: Butyraldehyde-aniline Mercaptobenzothiaaole 2-Merca tothiaqoline Mixed &as 1 disulfides Tetramethyghiuram monosulfide Lead dibutyl dithiocarbamate
Slow and weak Moderate Moderate Moderate Rapid Extremely active
The base stock used was: 100 parts GR-S (manufacturer D, lot I), 50 channel black, 5 zinc oxide, 2 sulfur, and accelerator as indicated. These stocks were cured for 30 minutes a t 307"F. (153OC.) and were tested both before and after agingina 100' C. oven (Table 111). These results indicate that the activity of the accelerator has little effect on the retention of properties of a GR-S vulcanizate after oven aging, as measured in this case by the retention of elongation. I n general, the deleterious effect of heat can best be followed by measuring changes in modulus, elongation, and hardness, since the tensile is apt to be erratic. It has also been demonstrated that similar changes take place when GR-S vulcanizates containing no organic accelerator are heated in air for 4 days a t 100" C. This is shown by the data (Table IV) on the following two compounds, which were vulcanized for 60 minutes a t 287" F.: Stock
A
B
100 50 5 5
100
...
50 5
..
5
INDUSTRIAL AND ENGINEERING CHEMISTRY
350
One of the outstanding classes of vulcanizing agents for this purpose consists of the aromatic polynitro compounds. Of these, mdinitrobenzene is one of the most effective and practical. Aromatic compounds containing only one nitro group are weak vulcanizing agents. Trinitrobenzene and 2,4-dinitrochlorobenzene are good vulcanizing agents but are more dangerous to handle. Vulcanizates prepared with dinitrobenzene appear to possess physical properties which are in nearly every case ae good as, or superior to, those of sulfur vulcanizates. Typical properties afforded by dinitrobenzene are illustrated by the following series of stocks:
This evidence that accelerators have little effect on the changes accompanying the heat aging of GR-S may offer an indication that aftervulcanization is not the primary cause of these changes.
TABLE v. EFFECTO F
Stook
Parts Sulfur
c D E
3 2 1.25
LOW
SULFURO N HE.%?. AGINGO F GR-S
Elongation at Stress a t 300%, Tensile at Break, Break, % Lb./Sq. In. Lb./Sq. In. _______ % reOrigAfter OrigAfter Orig- After teninal aging inal aging inal aging tion 2050 1825 1525
.. .. ,.
3025 2925 3200
2375 2100 2225
430 400 400
210 190 210
49 47 52
EFFECT OF SULFUR CONTENT
A second point to be considered is the relation of the amount of sulfur present in a stock to its rate of change of physical properties when heated. The following stocks containing, respectively, 3, 2, and 1.25 parts of sulfur were compounded and vulcanized for 30 minutes a t 307" F. Test pieces were testcd before and after 4-day aging at 100' C.: C
D
100 50 5 3 1
100
Stock GR-S (manufacturer D , lot 1) Channel black Zinc oxide Sulfur 2-Mercaptothiazoline Butyraldeh yde-aniline
...
50 5 2 1
...
E 100
50 5 1.25 1
0 5
The stock containing only 1.25 parts of sulfur (Table V) showed Neal and Ottenhoff ( 4 ) show that a decrease in sulfur from 2 parts to 1 gives some immovement in the resistance of GR-S vulcanizates to deterioration in the 121" C. air oven. This improvement is small, however, compared to the magnitude of the change. These results seem to show that aftervulTABLE canization may play a part, but probably not a major one, in the hardening of GR-S vulcanizates at elevated temperatures. There is good indication, therefore, that some factor Stook other than oxidation or aftervulcanization has a conF siderable, and perhaps major influence in bringing about the changes in modulus, tensile strength, elongation, and hardness that accompany the heat aging of GR-S vuiG canizates. This other factor must be tied up with the instability of the GR-S polymer, or a t least of the vulH canized polymer; and the changes on heating must therefore consist in part of a further polymerization, cyclization, or other similar change in the polymer. only a slight superiority over the others.
DINITROBENZENE VULCANIZATES
If this is the case, some method must be sought to prevent or inhibit these basic changes in the polymer. I t is possible that the sulfur-vulcanized polymer is inherently unstable, and better results might be obtained by vulcanizing GR-S in an entirely different manner. A vulcanizate might result which would be more stable and less readily undergo further polymerization or cyclization on being heated. I n order to determine if GR-S can be satisfactorily vulcanized by other methods than by sulfur, a study was made of over a thousand compounds. A number have been found t o be vulcanizing agents for GR-S. These new types of agents often produce vulcanizates whose p r o p erties differ from those obtained with the conventional use of sulfur and accelerators. Some of these vulcanizates have superior resistance to deterioration by heat.
Vol. 36, No. 4
Stock
F
G
H
GR-S (manufacturer B , lot 5' Channel black Dinitrobenzene Litharge Zinc oxide Sulfur Mixed thiazgl disulfides 2-Mercaptothiazoline
100
100 50
100 50
50 3 5
3
10
4 10
. . . . . . . . . . . . . . . . . . . . . . . . . . .
I
3
100
100
50
...
,..
5 2 1.5
. . . . . . . . . . . .
50
.. 5 2
....
1 .Z.i
Litharge appears to be quite specific in stocks vulcanized with dinitrobenzene. Although cures can be obtained with other metal oxides, comparable results to those obtained with litharge have not been observed. Table VI illustrates the range of moduli resulting from variation of the amounts of dinitrobenzene and litharge used. Although the tensiles obtainable by the use of dinitrobenzene are usually slightly lower than those obtained with an optimum sulfur-accelerator cure, the elongations, hardness, tear resistance a t both room and elevated temperatures, and tensiles at elevated temperatures are similar to those of sulfur cures. Often the tear resistance of vulcanizates cured with dinitrobenzene is superior to that of sulfur-cured G R S . The heat build-up obtained on dinitrobenzene-cured vulcanizates is nearly as good as that with sulfur-cured vulcanizates. Thuq the heat build-ups of 30- and 60-minute cures a t 298" F. (148" C.)
VI.
PHYSICAL PROPERTImS O F I h N I T R O B E N Z E N E \TUI.C!.4NlZ.4TES
Stirs
Cure Min.
F. 287 298 287 298 287 298
I
287
J
287
30 60 30 45 30 60 30 45 30 60 30
45 30 60 30 60
Tensile at Hreak, 30070, - Lb./Sq In Lb./ OrigAt Sq. In. inal 70' C. 750 1150 1025 1150 1350 1600 1625 1700 1625 2025 1850 2050 1050 1700 1500 2150
2325 2425 2400 2450 2400 2450 2550 2450 2425 2450 2450 2375 2675 2875 2625 3025
1525 1325 1550 1650 1725 1675 1725 1500 1675 1625 1650 1550 1575 1825 1700 1850
ElonShore Eation Duromat eter Tear ResistBreak, Hardance % ness 82' F. 70' C. 640 525 555 500 500 430 465 400 435 350 370 345 545 415 585 500
55 58 56 57 62 64 64 65 65 66 65 68 57 62 59 64
290 245 425 295 205 140 215 175 215 195 195 I75 160 130 215 185
175 230 185 205 215 235 225 215 250 145 175 145 165 245 185 270
TABLE VII. HEATRESISTANCE OF DINITROBENZENB VULCANIZATES AFTER4 DAYSIN 100' C. O V ~ N
Stock Vulcanizing Agent K Dinitrobenzene L Dinitrobenzene M
N 0 P
Original Properties Properties after Aging ElonInRetenStress Tensile gation Tensile crease tion of at at at at in elonga300% break break break moduli tion 900 910 880 910
Mercaptobenzothiazole Butyraldehyde-aniline 2-Mercaptothiazoline 4butyraldehyde-aniline 1550 Zinc diethyl dithiocarbamate 2210
2190 2160 2520 2500
530 520 550 530
1650 18'50 1520 2430
148 175 347 369
68 65 31 49
2650
420
1860
230
50
2210
300
1540
211
53
)IE N G I N E E R IN G C H E M I S T R Y I N D U S T R I A L &D
April, 1944 TABLEVIII.
Min. of Cure a t Stock 287’F.
% 90
120
R
8
*
AGINGOF DINITROBENZBNE VULCANIZAT~S IN AIR PRESSURE HEATTEST Stress st 100% Lb./Sq. In. Original Aged 175 975 1050 275 375 1225 1200 400
Shore Tensile a t Break, Durometer Hardness Lb./Sq. In. origOrigAged inal inal Aged 59 79 1950 975 64 80 1650 2850 68 81 2450 1700 68 82 2450 1550
-
Elongation a t
-Break, % % reoriginal
Aged
500 445 335 310
100
100 100 100
20 22 30 32
30 60 90 120
100 150 200 225
575 750 750 775
825 1550 2150 2350
325 975 1250 1425
54 58 61 63
74 76 77 77
600 485 490 475
200
140 145
140
40 29 29 31
30 60 90 120
200 225 225 275
350 400 375 425
2300 2400 2400 2350
1650 1525 1550 1475
61 63 61 62
66 68 68 69
580 500 470 395
300 285 260 230
52 57 55 58
Stress a t 100% Tensile st Break, Lb./Sq. i n . Lb./Sq. In. OrigOriginal Aged ins1 Aged 25 450 1025 650 25 375 825 650 75 375 650 700 75 350 825 750 25 375 925 575 75 ... 700 ... 75 350 650 650 76 350 650 725 100 875 50 1000 75 900 850 75 75 100 900 900 100 100 900 750
Stock T
U
Min. Cured 30 45 60 90 30 45 60
V
90 30 45 60 90
Shore Durometer Hardness 62 62 61 61 59
Channel black Zinc oxide aoid
B$te$
61 59
47 47 48 48
595 560 500 490
Aged 125 135 145 160 130
ibi
160 455 375 340 310
of stock H (measured after 20-minute flexing on a Goodrich flexometer using a stroke) were 54’ and 52’ C., respectively. Figures for the same cures on sulfur-cured stock J were 50’ and 47’ C. I n regard to heat resistance, however, vulcanizates of G R S cured with dinitrobenzene are outstandingly superior. Where sulfur vulcanizates usually retain less than 50% of their original elongation after 4 days in a 100’ C. oven, dinitiobenzene vulcanizates retain 65-70%. At the same time the dinitrobenzene vulcanizates show a much smaller increase in modulus and hardness after oven aging. Data (Table VII) on the following GR-S compounds illustrate these facts: Stock GR-S (manufacturer B, lot 1) Channel black Litharge Djnitrcbenzene Zinc oxide Stearic acid Sulfur Mercaptobenzothiazole Butyraldehyde-aniline 2-Mercaptothiazoline Zinc diethyl dithiocsrbamate
K L M 100 100 100 50 50 50 5 5 3 3 5
N O 100 100 50 50
100 50 5
1 2 1.25 0.2
2-Meroa tothiaaoline Butyf alcfehyde-aniline Litharge Dinitrobenzene
.... ....
R 100 50 5 1 1.5 0.85 0.15
.... .*..
S 100 50
... .. ... .., ...5 3
Table VI11 shows that the dinitrobenzene vulcanizates retained their properties after aging to a much greater extent than either of the sulfur vulcanizates. This resistance to heat aging of G R S vulcanized with dinitrobenzene is of extreme importance for a large number of uses. Tests were also run on lightly loaded stocks with similar results. Slabs of the following stocks were vulcanized a t 287’ F., and strips were tested before and after 4 days aging in a 100’ C. oven:
7 365 310 270 295 380 285 270 275
..
Q
Stook
Elongation a t Break, ’ir,
7 insf Aged inal 46 48 49 49 48 49 49 51 34 36 37 39
a t 260’ F. (127’ C.) with 80 pounds air pressure. The following stocks were cured and tested before and after 16 hours under these aging conditions; in stock R both tpe sulfur and acceleration were reduced to obtain an original hardness comparable to that of the dinitrobenzene stock and with the hope of obtaining improved heat resistance: GR-S(manufacturer D, lot 1)
TABLE IX. HEAT RESISTANCE OF LIQRTLY LOADBD G R S STOCKS
-
c
tention
351
P 100 50
............. . . . .. . I. . . .51 . . .5..... . . .51. . 2 2 2 2 . 1.5 . . . . . . . . . . ... 2 0.15 ... . . . . . . 0,85 ... .............. 0.5
These compounds were vulcanized for 30 minutes a t 298’ F., and were tested before and after 4 days in a 100’ C.oven. In this series K and L were check determinations. These tests show that dinitrobenzene vulcanizates retain niuch more of their original properties after oven aging than do sulfur vulcanizates. Even though stocks 0 and P were cured tighter than K and L and had much higher original moduli, the increase in modulus was still greater after aging than that of the dinitrobenzene vulcanizates. Dinitrobenzene vulcaniaates also show considerable superiority to sulfur vulcanizates, in the air pressure heat test conducted
0 retention 34 44 54 54 34
bi
58 76 67 68 63
Stock
GR-S (manufacturer B, lot Channel black Zinc oxide P-33 black Stearic acid Sulfur 2-Mercaptothiazoline Butyraldehyde-aniline Litharge Dinitrobenzene
1)
T 100 15 5
.... 1
2 1.25 0.2
.... ....
u ....
100 15
.... ....
3
100
5 30 1 2 1.25 0.2
v
. ... ... .. ... ... 5
Table IX shows that in a 15-part channel black stock, dinitrobenzene will produce vulcanizates having equal tensile strength to a sulfur-cured stock, but much higher elongation and lower hardness. I n spite of the high original elongations, the dinitrobenzene vulcanizates retain a higher percentage of elongation after heat aging than the sulfur vulcanizates. Stock V also shows very little increase in modulus and a better retention of tensile strength and hardness after aging. Attempts to improve the heat resistance of ordinary sulfurcured GR-S vulcanizates by the addition of dinitrobenzene prior to vulcanization resulted in failure. Two parts of dinitmbenzene were added to GR-S stocks vulcanized with a variety of accelerators; dinitrobenzene seemed to have no effect on the rate of cure or physical properties of the resulting vulcanizates. This indicates that dinitrobenzene is effective in producing heat resistance in GR-S only when it acts as the vulcanizing agent, and that the accelerated sulfur cure takes precedence over the dinitrobenzene cure and takes place almost to the total exclusion of the nitro cure. The latter observation is borne out by the fact that small amounts of sulfur inhibit the dinitrobenzene cure. LITERATURE CITED
(1) Cotton, in Davis and Blake’s “Chemistry and Technology of Rubber”, p. 528, New York, Reinhold Pub. Corp., 1937. (2) Jones, IND. ENQ.CHEM.,17, 871 (1925).
(3) Neal, Rubber Age (New York), 53, 31 (1943). (4)
Ned and Ottenhoff, IND. ENQ.CHEM.,36, 352 (1944).
(5) Reed, Ibid.,21. 316 (1929) PRnslNTnD before the fall meeting of the Division of Rubber Chemistry, AMERICAN CHEMICAL SOCIETY, New York, N. Y., 1943.
