INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1944
(43) Koohlin, P., and Heumann, K., Ber., 15, 1736 (1882). (44)Kum, M. A,, Koeberle, K., and Berthold, E., U. S. Patent 1,960,540 (1934).
(45) (46) (47) (48) (49) (50) (51) (52) (53) (54) (55) (56) (57) (58) (59) (60) (61)
Kyrides, L. P., Ibid., 2,171,494 (1939). Ibid., 2,171,495 (1939).
Levaillant, R., and Simon, L. J., Compt. rend., 169,854 (1919). Macbeth,A. K., J. Chem. SOC.,121,1116 (1922). McKee and Salls, U. S. Patent 1,765,601 (1930). Mazzara, G., and Borgo, 9., Gazz. chim. ital., 35, 11, 563 (1905). Ibid., 36, I, 348 (1906). Mazzara, G., and Lamberti-Zanardi, M.,Ibid., 26,II, 236 (1896). Ibid., 26, 11, 399 (1896). Meybeck, J., Ann. chim., 17, 129 (1932). Moness, E., U. S. Patent 2,151,137 (1939). Nenitzesou, C. D., and Isacescu, D. A., Ber., 67, 1391 (1934). Perittoner, A,, and co-workers, Gmz. chim. {tal., 28, I, 197 (1898). Perkins, M. A., and Deinet, J., U. S. Patent 2,180,835 (1939). Perrine, T. D., Ibid., 2,216,574 (1940). Pongrats, A., and Eichler, E., Ber., 69, 1292 (1936). Raiziss,G.W., and Clemence, L. W., U. S. Patent 2.102,854(1937).
791
Reinhard, G., J . pruct. C h m . , [2] 17, 321 (1878). Reitzenstein, F., and Andre, F., Ibid., [2] 87, 101 (1913). Salibil, I., Chem.-Ztg., 35,97 (1911); Chem. Zentr., 1911, I, 1581. Silberrad, O., J. Chem. Soc., 119,2029 (1921). Ibid., 121, 1015 (1922). Ibid., 127, 2677 (1925).
Silberrad, O., Silberrad, C. A., and Parke, B., Ibki., 127, 1724 (1925).
Staudinger, H., and Bruson, H. A., Ann., 447,110 (1926). Tohl, A., and Eberhard, O., Ber., 26, 2940 (1893). Tohl, A,, and Framm, F., Ibid., 27, 2012 (1894). Trautz, M., 2.Elektrochem., 14, 534 (1908). Ullmann, F., and Conzetti, A., Bsr., 53, 826 (1920). Verein for Chemisohe Industrie in Mainz, German Patent 161.882 (1902).
Wilke, K., U. S. Patent 1,878,050 (1932). Wohl, A,, German Patent 139,552 (1901). Wohl, A., and Koch, F., Ber., 43, 3295 (1910). Wynne, W. P.,J. Chem. SOC.,61, 1042 (1892).
F. P. BALDWIN, L. B. TURNER, AND R. L. ZAPP Eno Laboratories, Standard Oil Development Company, Elizabeth, N. J. The most rapid sulfur vulcanization of GR-I requires the use of ultraaccelerators. Increase of accelerator concentration above certain points produces inferior original physical properties with no advantage in cure rate. A n excess of zinc oxide over that necessary to obtain maximum tensile is required to produce the maximum in modulus and vulcanizate stability. For low-temperature curing the metal salt accelerators should be used, alone or in conjunction with other accelerators to obtain fast cure rate. Reversion is apparent in GR-I vulcanizates if curing times are long enough or temperatures are hish enouclh. Decreasins the rate of vulcanization with less active accelerators delays appearance of the reversion phenomenon,
HE early recognition by the rubber industry of the rela-
7
tively low cure rate of sulfur-vulcanized Butyl rubber and its effect on factory production led to considerable work on means of increasing the cure rate. One obvious method is t o introduce more unsaturation into the polymer; but even though the cure rate of GR-I has been improved over t h a t of the early Butyl (II), i t is still considered a slow-curing polymer. With a realization of the limitations imposed upon GR-I because of its inability t o fit into production schedules designed around the use of natural rubber. this studv uresents some important aspects of the sulfur vulcanization of GR-I. Ever since the discovery and de* velopment of organic acceleretors, the search has been continued for a simple method of relating physical proper1.5 MEREAPTOBWZO1.5 BENZOTHIAZYL 21.27 ZINC DIMETHYL THIAZOLE MONOCYCLOHEXYL 10 . TETRAMETHYL ties to cure state. Early THURAM DISULFIDE DITHIOCARBAMATE 025 DIPHEMLGUAN~INE SULFENAMIDE attempts to relate combined 1200sulfur t o physical properties had little significance ( 6 , l O ) . Recently (9) methods for LBS/SQ.IN. 600 isolating definite forms of 400, combined sulfur have been 200I , developed, and i t is possible T E N S I L E 1800that these may lead to .- - - - - -- - - - -- S T R E N G T H 1600*--- , o t L \ \ I better r e l a t i o n s b e t w e e n LBS./SQ.IN. 1400chemically combined sulfur of a definite type and physELONGATio~a00~\ ical properties. -_ - - - _ I % 6oo - - Many physical tests have 400I 1 , I been utilized t o determine I TEAR 200---_*__.state of cure. By a statis0 0 : + 1I LBWIN. 100/ I tical treatment of cross. , , linked polymers (4) it is possible t o relate rubberlike elasticity and swelling capacity to concentration of cross links. I , - -I. 1530 60 I20 15 30 60 120 33 80 24030 60 120 2 4? Of these relations, the p r o p T I M E O F CURE - MINUTES A T 3 0 7 O F . erty of rubberlike elasticity in a modified form is the most Figure I. Effect of Accelerator Type on GR-I Vulcanization
-
-
-
I
-
-
xz:
‘:::;-C ;F :/;/r-
%,
O i k
.:“a,_,
\io
gkc
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1
L - L L ‘ - - -
INDUSTRIAL AND ENGINEERING CHEMISTRY
792 I
i Figure 4.
I
I
I
tion, tear, and hysteresis properties a t a given modulus. The greatest discrepancy occurs in the hysteresis of the t,et,rarnethylthiurai-ii disulfide stock and may be due t o the i'act that t h e hlocksweie given a 15-minut e "cure lead" in the press \vhich would tend t o f a v o r t,he fast-curing compound.;. Xeglecting this one dib1-rrpancy, it is obvinur t h a t the t,ime necessary t o obtain comparable cure for compotinds cont,ainjrig other than t,he ultra-accelerators tends to make factory use of these accelerat,ors prohibit,ive. 011a relative basis the compounds exhibit t,he follorvjng cure times:
--1
Effect of Tetramethylthiuram Disulfide-Sulfur Acceleration on Tensile and Modulus
convenient t o measure. For this reason the extension rnoduluq has been adopted here as a measure of state of cure. It is realized that in going from one accelerator to another this criterion is not valid for properties such as aging, creep, vulcanizate stability, et('. Further, it has been indicated (3) that some of the accelerators surh as tetramethylthiuram disulfide are capable of producing carbon-carbon bonds in addition to the carbon-sulfur-carbon bonds assumed to be formed during the sulfur vulcanization ot rubber and rubberlike polymers. However, the formation of such cross links would probably not change the general propertitii ot the vulcanizate. ROLE OF ACCELERATOR IN VULCANIZATION OF GR-I
Parts 1.0 1.27
Accelerstoi Tetriimethglthiuram disulfide Zinc dimethyl dithiocarbamate Mercaptobenzothiazole
1.5
3Iiniirea 33 3Q 129 '2 8
Thus the need for ultra-accelerators becomes appareni, and further work on acceleration was limited primarily to this type of compound. EFFECTOF SULFTX-BCCELEHAT~H. Ramo. As a basic study 0.25, 0.5, 1, 2, and in one case 4 parts of tetrarnebhylthiuratii disulfide were used in combination with sulfur dosages of 0.6) 1.2, 2.4, a.nd 4.8 parts in the sernireinforcing furnace black compound. Tensile and modulus are plotted against cure t'ime r-i,t,307" F. i n Figure 2. An increase in sulfur coilcentration up to 2.4 parts produces an incrtxase in cure rate, as judged by the increasing slept> of t h e
ACCELERATORTYPE. Although a wide variety of accelerat or6 can be utilized for the sulfur vulcanization of natural rubber and hirhlv unsaturated rubberlike polymers, the choice of accelrrat ot f o p the satisfactory sulfur vulcanization of GR-I is relatively narron . This has resulted in the almost ex1000 cl isive use of the so-called ultra1 . r ~ e l e r a t o ror s combinations of them 1600with other chemicals. This point u I IO \+as brought out in previous papel s z ( 6 , I I ) , but it is felt that the prrssi l Z o o - rtitation of more quantitative evidence is advisable. For convenience, Boor 8 the uork on the effect of accelerator< 3 has been confined t o one basic com8 400pound as follows: GR-I, 100 parts; Ainc oxide, 5 ; stearic acid, 3; sulfur, ,-._ "12; semireinfoicing furnace black, 54; accelerator as iioted. The u- ELONGATION GR-I contained 1 part of zinc stearate and 0.5 part of phenyl-p-naplit hylamine. Figure 1 shows the effect of four different accelerators on the physical properties of this GR-T 0; ' o: I a0 t $0 I l o ' lh ' TIHE or CURE -MINUTES AT 307.F. compound. With modulus as a criterion for measuring state of Figure 3. Effect of Zinc O x i d e cure, all of the compounds show Activation on Tetramethylthiessentially the same tensile, rlongauram Disulfide Acceleration
-
Vol. 36, No. 9
I
1
1600 -
2000
X
z
6
1200-
-
v)
z
a
800-
VI
-
z
g
MOD. 300%
400-
-
I
-Oo
'
R
ELONGATION
a 400
NUMBERS REFER TO PARTS OF STEARIC ACID Id0
TIME OF CURE
-
MINUTES AT 307OF.
Figure 4. Effect of Stearic A c i d on Tetramethylthiuram Disulfide Acceleration, Activated with 5 Parts of Zinc O x i d e
September, 1944
INDUSTRIAL AND ENGINEERING CHEMISTRY I
1200,
0 . 1200 3
OZ
=$
ij
800 400
200 I
II 2 2 45 90 CURE A T 30O0K-MINUTCS
Figure 5.
5
IO
20
!
40
I
CURE A T 320°C-MINUTES
Modulus at 500% Elongation with Zinc Dibutyl Dithiocarbamate Acceleration
193
to obtain maximum tensile (Figure 3), but the modulus and elongation curves show that a low state of cure and an unstable cure results. The instability of the vulcanizate, as noted by the change in modulus over the cure range, persists a t I part zinc oxide concentration and is discussed in detail in ft later section. When no zinc oxide is used, the maximum tensile obtained is low and the modulus deteriorates over the entire time range. With increasing amounts ot zinc oxide (from 2 to 10 parts), state of cure as measured by modulus is increased, and maximum moduli are obtained with about 5 parts of activator. By increasing the activator to 10 parts, little or no improvement results. The results of considerable work relative to the effect of fatty acids on the activity of zinc oxide have been published ( 7 , 8, 14). I n general, the conclusions reached are that fatty acids function by converting insoluble zinc oxide to soluble zinc stearate. However, Figure 4 indicates that the addition of stearic acid t o this compound has no effect on vulcanization rate but produces slightly lower moduli, possibly through plasticizing action. It is possible that the small amount of zinc stearate present in GR-I may be sufficient t o render additional stearic acid relatively ineffective; however, the authors believe that the presence of zinc oxide as such is necessary when dealing with this type of acceleration. ACCELERATOR TIME-TEMPERATURE RELATIONS.Because of the complexity of the vulcanization reaction, any time-temperature relations established must be rather arbitrary. The practice has been to relate the logarithm of cure time t o temperature in
modulus curves in the early stages of cure. An increase over 2.4 parts produces little or no change in this respect and results in profuse blooming of the cured samples. In all but the 4.8-partsulfur stock an increase in cure state and rate of cure accompanies incremental increases in accelerator concentration up to 1 part; but in going from 1 to 2 parts accelerator, there is a tendency toward reducTable 1. Effect of Various Accelerators on Vulcanization Time-Temperature tion in cure state (decreased modulus) with Relations little change in cure rate. With 4.8 parts of sulfur there is an increase in cure state over the whole range of increasing accelerator content. With respect to tensile strength, 1.2 parts 1380- 350-1000 1370- 270-1060 1650- 410-860 1510- 270-940 260 55 of sulfur are sufficient to obtain maximum 110 1730- 510- 860 1800- 490- 860 1930- 740-760 1800- 580-810 values. The most rapid-curing compounds 220 1860- 740- 780 2010- 810- 780 2010-1160-700 1840- 830-710 440 1950-1210680 1970-1270680 1500-1430-600 1900-1150-680 (those with high sulfur and accelerator) show 280 25 1600- 390- 880 1330- 210-1060 1630- 340-840 1610- 450-870 the greatest decrease in tensile, after a maxi50 1820- 670- 800 1720- 450- 860 1900- 630-820 1900- 730-790 mum has once been obtained, with increase in 100 1940-1010- 720 2120- 860- 740 1920-1000-710 2020-1060-710 200 1820-1310- 660 1810-1200- 700 1720-1340-610 1800-1310-650 oure time. The tensile plateaus may be in300 11 1710- 410- 900 1180- 170-1100 1590- 280-930 1720- 400-920 creased either by the use of low sulfur and 22 1830- 790- 800 1770- 430- 900 1820- 570-830 2020- 700-820 high accelerator, high sulfur and low accelera48 1830-1130- 680 1860- 800- 800 1870- 970-680 1880-1130-710 90 1840-1060- 750 1720-1280- 640 1760-1270-640 1710-1280-620 tor, or with both low sulfur and accelerator. 320 5 1620- 400- 880 1360- 180-1020 1500- 270-930 1740- 460-870 Over-all, then, for the proper balance of cure 10 1920- 780- 800 1710- 390- 910 1780- 560-800 2000- 850-770 rate and initial physical properties care should 20 1870-1030- 710 1820- 740- 790 1980- 840-760 2110-1260-700 40 1560-1240- 590 1950-1110-750 1790-1190-670 1850-1460-590 be taken in the choice of accelerator-sulfur combinations. It should be kept in mind that the values shown here are not criteria for all compounds and will vary with pigment type and concentration. t ROLEOF ACTIVATORS.Metallic oxides, and specifically zinc oxides, have long played a role in the sulfur vulcanization of unsaturated polymers. With GR-I, zinc oxide activation is ' necessary for proper vulcanization. Not only is sufficient zinc oxide needed t o form a metal salt if a nonmetallic dithiocarbamate is used ( I , B), but an excess over this amount is required t o w obtain maximum moduli. This is in accord with observations made on natural rubber with dithiocarbamate acceleration (1%'). SELENIUM DIETHYL Figure 3 shows the effect of increasing DITHIOCARBAMATE amounts of zinc oxide on the physical properties over a cure range a t 307" F. I n this series no stearic acid was used in the compound which 2.5 2.4 2.3 2.5 2.4 2 included 54 parts of SRF black, 2 parts of sulfur, 7 x 103 and 1 part of tetramethylthiuram disulfide. Figure 6. Vulcanization Time-Temperature Relations Only about 0.5 part of zinc oxide is needed
1 N U U C T R I T . L A N D E N G I N E E R I N G CHEMISTRY
794 2.5 ZINC OXIDE
r
5.0 ZINC OXIDE
am 600--
I
SLABS CURED 30h20'F. 0-AGED IN PRESS AT 3.20-F. +-AGE0 IN STEAM AT 320%
1200-
-ELONGATION-
400
- MODULUS
I
600
* 200
AT 300°hE
.-+
J
030 90 150 210 270
0.5 SULFUR
be measured most accurately vias chosen, and the time required at each temperature to reach this predetcrmined modulus was ascertained. These time points were plotted as shown in Figure 6, and the slopes of the lines determined. There is considerable difference in slope between the metal salt accelerators and the others : Accelerator Tetramethylthiuram disulfide Tetramethylthiuram monosulfide Zinc dibutyl dithiocarbamate Selenium diethyl dithiocarbamate
-
Slope
6.79 X 6.66 X 4.83 x 4.72 X
10-3 10-3
10-3 10-3
I
Y. --t
030 90 150 210 270 030 90 150 210 270 TIME OF AGING MINUTES
-
1.0 SULFUR
,
I
I
400-
20.0 ZINC OXIDE
BASE FORMULA GR-I 100.0 FINE THERMAL BLK. 72.0 SULFUR 2.0 TMTDS 1.0
-TENSILE-
600-
2
10.0 ZINC OXIDE
Vol. 36, No. 9
030 9'0 150 210 270
1.5 SULfUR
2.0 SULFIJR _?
1800
-
51200-
-s
I
Therefore, by adjustment of accelerator concentration, curing at low temperatures can best be accomplished by the use of metallic dithiocarbamates either alone or in conjunction with other accelerators. Compounds containing both a thiuram and a metallic dithiocarbamate have been shown l o be applicable to these low-temperature c u m . VULCANIZATE STABILITY
I n the vulcanization of natural rubber the phenomenon of reversion is well known. In TMTDS GR-I reversion is obtained only after long cure times. However, since reversion is wholly relative in nature, this is not surprising. EFFECT OF ZINCOXIDE. As pointed out earlier, 400 very little activator is necessary to obtain maximum tensile values (a criterion used to judge best cure). However, it was shown that a small quantity is insufficient to produce ultimate stability of the vulcanizate. TIME OF AGING - MINUTES The upper graphs of Figure 7 present thc tenFigure 7. Effect OF Zinc O x i d e and OF Sulfur on GR-I Vulcanizate Stability sile, modulus, and elongation data obtained in an accelerated reversion test. Compounds containing 2.5, 5, 10, and 20 parts of zinc oxide and accelerated with 1 part of tetramethylthiuram disulfide were cured degrees Fahrenheit. This method of expression should be confor 30 minutes at 320" F. Whole tensile slabs were then aged fined to rather narrow temperature ranges; further, it does not both in the press and in the steam digester for various times at bring out the differences in scorch characteristics among certain 320" F. Judging reversion by modulus decrease, the compound accelerators having essentially the same cure rate a t a definite curcontaining the lowest amount of zinc oxide shows the greatest ing temperature. I n this paper relative cure state is confined rate of reversion. Furthermore the steam-aged slabs show lower to state of cure as indicated by extension modulus, and the moduli than the press-aged slabs. The data indicate that the relation of log time to l / T o K. is used. reversion tendency is reduced with incremental increase of zinc The accelerators examined were tetramethylthiuram disulfide oxide up t o 5 parts. An increase beyond this point does not have and monosulfide, zinc dibutyl dithiocarbamate, and selenium an appreciable effect in this compound. diethyl dithiocarbamate. The curing temperatures encompassed The fact that an excess of zinc oxide over and above that necesthe range 260320' F. with cure times adjusted accordingly. sary to activate the accelerator is found necessary t o decrease the Tensile, modulus, and elongation data are shown in Table I. reversion tendency of these compounds suggests that hydrogen Moduli for each of the compounds were plotted as illustrated in sulfide (or in general a mercaptan) is a t least partly responsible. Figure 5; a modulus point on the curve where the changes could Weyl (IS) states that hydrogen sulfide can decompose disulfides as follows: R-S-S-R + H2S 4 2RSH S Table 11. Effect of Hydrogen Sulfide on Crude and Vulcanized GR-I It is also known that monosulfides can undergo Properties of Crude Polymer Properties of Vulcanized Polymer" similar reaction: Tensile. Mndulua Vol. swell Staudinger Intrinsic lb./sq. in. a t 300% in cycloR-S-R HZS + 2RSH BASE FORMULA
GR-I
ZINC OXIDE
FINE THERMAL
100.0
BLK.
;:7
+
Test Condition Control Heated 2 hr. a t 255' F. in Ha9 at 50 lb./sq. in. Heated 2 hr. at 255' F.
in vacuo
mol. wt. 40,600
viscosity 1.30
40,600 40,300
1.30 1.28
(elongatlon, %)
elongation
hexane, %
1620 (630) 1640 (820)
620 150
227 377
1510 (590)
660
220
Basic compound accelerated with 1 part tetramethylthiuram disulfide, and cured 60 mlnutes a t 807O F.
+
Therefore it is possible for hydrogen sulfide to destroy the sulfur cross links formed whether they are sulfide or disulfide type. I n support of this proposed mechanism, specimens of crude and vulcanized GR-I were heated for 2 hours a t 265" F., both in
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1944
?95
shown is relatively unaffected by changes in sulfur concentreTHIURAM THIURAM DITHIOCARBAMATE I DISULFIDE MONOSULFIDE tion over the time-temperature I range used here. BASE FORMULA 1600 EFFECTOF A C C E L E R A T O R GR-I 100.0 ZINC OXIDE 5.0 2 1400TYPE. It was pointed out earlier FINE THERMAL BLK.72.0 - . SULFUR 2.0 that factory usage of accelerators other than ultra-accelerators is probably undesirable; but the 800effect of such accelerators on the -ELONGATION reversion of GR-I are included here as a matter of record. The 400accelerated reversion test was 800-MODULUS AT 300%Eextended t o compounds containing several accelerator types, U and the results are shown in Figure 8. The modulus curves 2000 ' I " ' I I I I I I l ' I I I f ' I I ' I I I l l f l ' l show that the use of less potent 030 80 IS0 ?IO 270 0 30 90 150 210 a70 0 30 90 150 210 270 0 30 BO I50 ?IO 270 0 x) 90 I50 210 270 accelerators decreases but does TIME OF AGING I N MINUTES not eliminate reversion tendency. Figure 8. Effect of Accelerator Type on GR-I Vulcanizate Stability During GR-I vulcanization, many reactions are probably occurring; but as an oversimplified picture, the vulcanization process may be thought of as sealed vacuum tubes and in the presence of hydrogen sulfide a t 50 consisting of two major types of reactions. One would be the pounds per square inch pressure to ensure adequate permeation. formation of a three-dimensional network, and the other would The results and control data are presented in Table 11. The hybe degradation of both the individual polymer chain and the drogen sulfide treatment had no noticeable effect on the crude network. Therefore the appearance of reversion as indicated polymer, as shown by the Staudinger molecular weight and inby modulus decrease will depend to a large extent upon the ratio trinsic viscosity data. The vulcanizate, however, when treated of the rates of these general reactions to one another. in this manner shows a high degree of reversion, as indicated by Of probable interest is the wavy appearance of some of the the decrease in modulus and the increase in swelling capacity. tensile curves in this reversion study. This may be explained Heating in vacuo increased the modulus and decreased the by an orientation mechanism. Theoretically maximum tensile swelling capacity, and both these effects can reasonably be atshould be obtained when maximum chain length is obtained contributed to continued cure. current with maximum orientation. Thus the ideal vulcanWe would expect, therefore, that some mercaptans may behave izate from the standpoint of tensile strength would exist when a like hydrogen sulfide, according to the following equations: continuous molecule is produced in which the polymer units are hooked up, say, in an end to end fashion. Any increase in sulfur RS-R' R-SH S R S 4 - R R'SH linkages over this point of maximum chain length would cause rigidity and reduce the degree of orientation. R4-R R'SH + R S - R ' R-SH Thus it is felt that up t o the point of maximum tensile effective molecular weight is being increased. After this point additional According t o this reasoning, zinc oxide would reduce this tendlinkages cause an increase in modulus but a decrease in tensile ency toward reversion by reacting with the hydrogen sulfide t o by virture of their effect on the mobility of the polymer units form zinc sulfide, with respect t o one another. As reversion progresses, whether HB ZnO + ZnS H10 cleavage occurs in the chain proper or a t the points of cross linkage, the effect is t o remove some of the excess linkages; and as or in the case of mercaptans t o form a metal salt, expected, modulus decreases and tensile increases. The decrease in tensile observed after the second peak can be assumed t o be a PRSH ZnO Zn(S-R)2 H20 progressively effective decrease in over-all molecular weight. 2.0 MERCAPTO-
1.5 TRIBUTYLIDENEANILINE
1.3 ZINC DIBUTYL
BENZOTHIAZOLE 0.5 DIPHENYLGUANIDINE
+
-
-
+ +
+
+
+
+
+
+
+
Either hydrogen sulfide or a mercaptan may possibly be generated by many of the reactions taking place during vulcanization. It is assumed that the degradation which takes place in the presence of a large excess of zinc oxide may be attributed t o degradation of the chain proper by an oxidative process or some free radical mechanism. It seems reasonable to conclude that, in the vulcanization of GR-I with this accelerator type and concentration, the major portion of the cross links formed is composed ofsulfur links, since carbon-carbon links would be relatively stable under the conditions of these tests. I n a similar test the EFFECT OF SULFUR CONCENTRATION. amount of sulfur was varied, and the data obtained are the lower graphs of Figure 7. The compounds show characteristics with regard t o rate of reversion. Percentage modulus decrease from the highest t o the lowest point of each curve is practically constant. This leads t o the conclusion that the reversion characteristic of GR-I compounds accelerated as
LITERATURE CITED
(1) Bedford and Sebrell. J. IND. ENO.CIIIM;,14, 25 (1922). (2) Clark, LeTourneau, and Ball, Rubber Chem. Tech., 16, 621-33 (1943). (3) Farmer, Ibid., 15, 774-9 (1942). (4) Flory and Rehner, J. Chem. Phys., 11, 612-26 (1943). (5) Hardman and White, IND.ENQ.CEEM.,19, 1037 (1927). (6) Haworth and Bald (7) Martin and Davey (8) Sebrell and Vogt, IND. ENQ.CHIM.,16, 792 (1924). (9) Selker and Kemp, Ibid., 36, 16 (1944). (IO) Shepard and Kroll, fbid., 14, 961 (1922). (11) Thomaa, Lightbown, Sparks, Frolioh, and Murphree, Ibid., 32, 128 1. (12) Twiss r. and Thomas, J . SOC.Chem. I d . , 41, 81T (1922). (13) Weyl, oden der Organische Chemie", p. 243. (14) Whitby and Evans, J. Soo. Chem. Znd.,47, 122T (1928). PRESENTED before the spring meeting of the Division of Rubber Chemistry, AYERICAN CHEMICAL SOCIITY,in New York, N. Y.
