Oct., 1922
THE JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
951
Studies in Rubber Vulcanization’ The Relation between Chemical and Physical State of Cure of Rubber Vulcanized in the Presence of Certain Organic Accelerators By Norman A. Shepard and Stanley Krall RESEARCH LABORATORIES, FIRESTONE TIRE& R U B B E RCO., AKRON,O H I O
Hexamethylenetetramine, p-nitrosodimethylaniline,aldehyde ammonia, and the thiocarbanilide, respectioely, haoe been introduced in such quantities in a mixing consisting of 48 parts of jirst latex crepe, 48 parts of zinc oxide, and 3 parts of sulfur, that equioaleni physical states of cure (as gaged by the stress-strain relations) haoe been obtained when uulcanizedfor 60 min. at 287” F. (141.7” C.). The amounts of these accelerators required to give equioalent physical states of cure were 0.5 part hexamethylenetetramine, 0.25 part of paranitrosodimethylaniline, 0.75 part aldehyde ammonia, and 1.5 parts of thiocarbanilide, respectioely. The oulcanization coeflcients on these matched 60-min. cures, which were 0.87, 1.03, 0.98, and 1.38, respectioely, showed no uniI n only one case, that of formity in amount of combined sulfur. the p-nitrosodimethylaniline and aldehyde ammonia, did the coeficients fall closely together. In this case the digereme was only about 5 per cent. I n all the other cases, there was a wide variation in the coeficients; with the p-nitrosodimethylaniline the coeficient was 18.4 per cent grealer, with the aldehyde ammonia 12.6 per cent greater, and with the thiocarbanilide 58.6 per cent greater than with the hexamethylenetetramine, In other words, the chemical state of cure was no index to the physical state of cure. These jour organic accelerators, though all reacting with sulfur during uulcanization, do not, when heated in xylene under similar conditions, form insoluble reaction products which seriously interfere with the determination of combined sulfur.
HE RELATIVE merits of the stress-strain relations and coefficient of vulcanization as measures of state of cure have been the subject of lively discussion for many years. As far back as 1902, Weber2 stated that the fact that two different specimens of vulcanized rubber, even when produced from the same batch of crude rubber, possessed equal coefficients of vulcanization, by no means implied that the t,wo specimens also possessed t’hesame physical constants. It was his viewpoint that variations in the amount of working which a sample of rubber received and differences in the temperature and duration of the vulcanization period influenced the physical result of the vulcanization process, without exerting a corresponding influence upon the clzenzical yesult. Axelrod3 has presented figures which support Weber’s view that. the temperature of t,he vulcanization largely affects the physical properties. Upon vulcanizing to the same vulcanization coefficient, but a t two different teinperatures, Axelrod found marked differences in the load required to produce a given elongat,ion in the two vulcanizates. Scliidro~vitz,~ in working out a systematic method for t’esting rubber and placing the “optimum cure” by means of “slope,” came to the conclusion that the progress of vulcanization can be accurately expressed by a series of stressstrain curres, and the st’ate of cure of a given mixing
T
Presented before the Division of Rubber Chemistry a t the 63rd Meeting of the American Chemical Society, Birmingham, Ala., April 3 t o 7, 1922. 2 “The Chemistry of India Rubber,” p. 93. a Gummi-Ztg., 24 (1909),352; J . SOC.Chem. I n d . , 29 (19101, 34. 4 Rubbei Industry (London), 1914,212;cf. J . SOC. Chem. I n d . , 34 (1915) 1
842.
a t a given time by the corresponding curve. Stevens,& on the other hand, while admitting that the coefficient of vulcanization can be taken only as an approximate guide to the condition or degree of vulcanization, favors this factor, rather than the physical properties, as a measure of state of cure, since the tensile properties of a vulcanized rubber are not constant, but vary with the age of the specimen, while the coefficient of vulcanization suffers little change under similar conditions; furthermore, according to Stevens, the coefficient of vulcanization is an index to the stability of the physical properties. Spenceo takes a similar stand as regards the value of the coefficient. Eaton and Grantham’ utilize the physical properties almost exclusively in their work on the variability of plantation rubber, though their figures8 for combined sulfur show a very marked relationship between physical and chemical properties. De Vries,g though not finding such large differences as do Schidrowitz and Goldsbrough,’O agrees with the latter that, the chemical process of the combination of rubber and sulfur is in itself quite independent of the physical process which determines the position of the stress-strain curve. De Vries’ arguments are based on a series of age tests conducted a t 75” C. from which he concludes that it is possible to bring the rubber to the “correct” or any chosen standard of cure (as judged by the curve) with other and quite arbitrary coefficients of vulcanization, He, therefore, is of the opinion that the mechanical properties are the more important, and that the curve is a better gage for judging the properties than is the chemical condition as expressed by the coefficient of vulcanization. Stevens11 aptly replies to this that De Vries has only shown that under diferent condiiions different coefficients of vulcanization may be obtained for the same rubber, giving the same stress-strain curve. Stevens is willing to admit the value of an arbitrary “standard curve” in placing state of cure, provided the results can be correlated to rubber correctly cured in the manufacturing sense, and “it is here that the coefficient is of great value.” From this review of the literature it appears that there is considerable confusion concerning the term “state of cure.” I t is the opinion of the writers that to cure two samples of rubber to the same chemical state of cure is one thing, while to vulcanize to equivalent physical state of cure may be quite another. The physical stateof cure is vital for the performance of vulcanized rubber in service, provided of course that the stability of the physical condition is certain. It is here that the chemical state of cure unquestionably plays a major part. It would appear, from what has already been published, that only under standardized conditions of mixing, curing, and testing is there a close relationship between the chemical and physical states of cure. J . SOL Chem Ind , 36 (1916),872 J , 62 (1916),861 I n d , 34 (1915),989, 36 (19161,715. 8 A g r . Bull. Federated M a l a y States, 27 (19181,139. 9 I n d m Rubber J . , 63 (1917), 101. 10 Ibzd , 61 (1916),505. 11 Ibid., 63 (1917),220. 6
e I n d z a Rubber 7 J . Soc Chem
952
T H E JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY
I
I
I
I
I
I
I
I
'
I 800
1
700
I I I I I I I
I 1 I
Vol. 14, No. 10
I
'I
I
I
I lBo0 700
600
600
500
500
400
300
400
-t
300
200
200
100 34 32 3000 28
IC
24 22 ZOO0
/6 I4 (2 lo00 8
6
4
2
0
Fro. 3
FIG.1 500
34 32 3000 28 26 24 2 2 ZOOo/8 /6 / 4 12
/000
6
6
4
2
800
700
700
BOO
600
500
5oG
400
400
300
300
zoo /oo
200
0
100 3 4 32 3000 28 26 24
22 2000 /8
16
/4
/2 /DO0 8
6
4
2
0
FIG.2
Fro. 4
With the advent of organic accelerators the relationship between the vulcanization coefficient and physical properties came to be more and more questioned. Cranor,12 in curing smoked sheet compounded with 6 per cent of sulfur and 1 per cent zinc oxide, without accelerator and in the presence of hexamethylenetetramine and of dimethylammonium dimethyl-dithiocarbamate, respectively, found that when cured to practically the same coefficient of vulcanization (2.85, 2.83, and 2.68, respectively), the loads required to produce an elongation of 700 per cent were 700, 2070, and 2910, respectively. The periods of cure in these cases were all of different length: 130 min. a t 292" F., 50 min. a t 292" F., and 5 min. a t 295" F., respectively. Furthermore, in spite of the moderate coefficient obtained in each case, Cranor stated that the 50-min. "hexa" cure was somewhat overvulcanized and that the 5-min. "carbamate" cure was much overcured. Similar results have been obtained when vulcanizing samples to the same coefficients with p-nitrosodimethylaniline.la Viewing his results from the standpoint of constant physical properties, Cranor has shown that when the three stocks mentioned above were cured to the point where a load of 1000 lbs. per sq. in. was required to produce an elongation of 700 per cent, the respective coefficients were approximately 4.9 for the unaccelerated stock, 1.8 for the hexamethylenetetramine stock, and 0.9 for the stock containing the dimethylamine addition product. The coefficients of vulcanization were thus no index to the physical state of cure in this case. In order to throw further light upon the relation between the chemical and physical states of cure in the case of accelerated stocks, the writers have investigated the behavior of hexamethylenetetramine, plnitrosodimethylaniline, aldehyde ammonia, and thiocarbanilide, when introduced in such quantities that equivalent physical states of cure are obtained when cured for the same length of time a t the same temperature. I n this way each stock was subjected to identical heat conditions, which, from the above review of the work of pre-
vious investigators, appears to be of paramount importance if a fair comparison of stress-strain data and vulcanization coefficients is to be made. In matching the physical states of cure of these stocks, no attempt was made to work with the so-called "optimum cure." The cure selected was somewhat below that of maximum tensile strength (cf. the control stock A, Fig. 5 ) and was judged to be a good commercial cure. EXPERIMENTAL PROCEDURE COMPOUNDING-Aselected batch of massed first latex crepe was used throughout the work and the mixing consisted of 48 parts of rubber, 48 parts of zinc oxide, and 3 parts of sulfur, to which was added the required amount of accelerator necessary to give, when cured for 60 min. a t 287" F. (141.7" C.), a vulcanizate having stress-strain relations which would match those produced by using 0.5 part of hexamethylenetetramine (see Curve A-60, Fig. 1). The selection of a formula with so high a percentage of zinc oxide (48 per cent by weight, 14 per cent by volume) may appear an unfortunate choice. The writers selected this for two reasons. In the first place, it was essential to work in the presence of zinc oxide, in order to bring out the full activity of a t least two of the accelerators under consideration. In the second place, from the standpoint of comparing the results with certain actual compounds used in factory practice, it was desired to use a somewhat highly compounded stock. The writers are aware that this procedure, by flattening out the stress-strain curves and bringing those for successive cures nearer together, made the selection of identical stress-strain curves more difficult, a t the same time complicating the analysis for combined sulfur. The organic accelerators were all carefully tested for purity. The hexamethylenetetramine was the chemically pure material; the p-nitrosodimethylaniline melted a t 83" to 84" C.;14 the aldehyde ammonia, melting a t 93" to 95"C.,16 was SO free from the resinous decomposition products with which it is commonly contaminated that it was easily ground to a
Indaa Rubbev W o r l d , 6 1 (1919),137 Van Rossem, "Communications of the Netherland Govt. Inst. (Delft)," Part V I , p. 213.
Schraube gives melting point of pure material as 85O C. Melting point variously given in the literature: Beilstein, 70' to 80'; Aschan, 96' t o 98' C .
12
13
14
16
THE JOURhTAL OF INDUSTRIAL A X D ENGIhTEERISG CHEMISTRY
Oct., 1922
953 800
700
600
600
500
500
400
400
300
300
200
200
/oo 34 32 30W 20 26 24 22 2000 18 JG
/4
12 lo00 8
6 4
/oo
0
2
34 3 2 3GOO 28 26 24
22 200018 2GOO/8
FIG.5 I
1
l
I
I
/B
/4 12 /OOG 8
6
4
2
I
I
I
0
FIG. 7 800
l
I
700 600 500
400
3 00 200
roo FIG.6
Fro. 8
fine powder; and the thiocarbanilide was twice recrystallized from alcohol and melted a t 149" to 150" C.16 Three trial experiments were made before the amounts of these three accelerators were arrived at, which were necessary to give the same stress-strain curve in 60 min. at 287" F. as 0.5 part of hexamethylenetetramine. The complete list of compounds investigated is given in Table I.
bath. This procedure was necessary to prevent reducing the temperature of the curing bath when inserting molds. After vulcanization, the molds were immediately plunged into a tank of cold water. The slabs produced were approximately 0.07 in. thick. PHYSICAL TEsTING-Dumb-bell strips were cut from the vulcanized slabs 0.25 in. wide a t their central portion. The stress-strain data were obtained on the Scott testing machine by two observers. Strips of this thickness permitted the employment of the inner scale on the machine so that the load could be easily read to 0.4 lb. Two strips of each cure were broken except where otherwise stated. ANALYTICAL METHOD-The combined-sulfur figures and vulcanization coefficients were determined in the following manner: 1-g. samples, creped on a tight cold mill, were extracted with acetone for 16 hrs. in an Underwriters extraction apparatus. After air-drying for about 15 min., the acetone-extracted samples were digested for 8 hrs. in a 5 per cent alcoholic potash solution, followed by a 16-hr. extraction with alcohol. After another air-drying, the samples were covered in a small flask with a mixture of 20 cc. ether and 20 cc. concentrated (aqueous) hydrochloric acid and allowed to stand a t ordinary temperature for l hr. with occasional shaking. After a washing with hot water a final extraction was made with boiling water for 4 hrs. This procedure is similar to that of Kelly,17the ether-hydrochloric acid treatment being that proposed by Stevens.'* The
TABLE I CONTROL EXPT.1 A B C D First latex crepe. ... 48 48 48 48 Zinc oxide ........ 48 48 48 48 Sulfur ............. 3 3 3 3
EXPT.2 E -~ F 48 48 48 48 3 3
.
Hexamethylene- tetramine.. 0.5 Aldehyde ammonia. Thiocarbanilide. +Nitrosodimethylaniline
. ,.. . . . . . . .. .. .... ...... ..
.. ..
. . 1: 0 .. .. . . .. 0.22
0:5
0:;
..
. . 1.25 .. ..
G -
EXPT.3
48 48 3
H ~~
48 48 3
I -
48 48 3
:: .. 0.i5.. 1 :..5 0.25 .. ..
MIxrNG-The stocks were mixed on a small experimental mill. After milling for 10 min. to break down the rubber, the mixture of zinc oxide, sulfur, and accelerator was incorporated during the course of 4 min. and the mixing was continued for an additional 8 min. in order to obtain a uniform product. The mill rolls were cooled to approximately the same temperature before mixing each batch and kept the same distance apart for all mixes. After mixing, the stocks passed directly to a small calender where they were calendered to a thickness of approximately 0.1 in.
TABLE I1
PERCENTAGE
ELONGATION 200 300 400 500 600 Break
c
A
235 470 825 1410 2240 (637) 2770
- ~ _ _ LOAD-LBS. _ B
C
45-Man. Cure 166 213 302 457 510 790 865 1300 1375 2080 (676) 1840 (620) 2310
PER
D
205 465 810 1340 2060 (650) 2460
SQ.IN -__-A
324 605
inio
1720 2680
(656) 3380
7
B
D
C 60-Mzn. Cure
212 392 670 1110 1770 (640) 2180
284 508 845 1380 2120 (637) 2460
(637)
290 610 1025 1620 2600
..
c ~ , curing ~ w&s~conducted ~ at~2 8 7 ~T (141.70 ~ c . )~ sulfur was determined in the extracted samples by the method in a paraffin bath regulated to within o,50 F. The molds of Waters and Tuttle,lg by oxidizing with nitric acid-bromine were allowed to remain in a preheating bath at approximately , mixture and fusing with potassium nitrate-sodium carbonate 17 THIS JOURNAL, 14 (19221,196. 320" F. for exactly 1min. before they were placed in the curing 18 I n d i a
15
Lellman, 150.5' C.; Bamburger, 153' C.
19
Rubber J . , 60 (1915),187. Bur. Standards, Reprint 174.
954
T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEiMIXTRY
mixture. It was precipitated and weighed as barium sulfate in the usual manner.
used in Fig. 4 were run in triplicate. The results, which are given in Table VII, show that these triplicates in every case checked to within 0.03 per cent of combined sulfur.
RESULTS
TABLE VI1
The stress-strain curves obtained in the three trial experiments are shown in Figs. 1, 2, and 3, and the data given in Tables 11,111, and IV. Figs. 1 and 2 show that the amounts of the three accelerators in these cases were insufficient, while Fig. 3 shows that the proportions used in Expt. 3 produce almost identical stress-strain relations when the stocks are cured for 60 min. a t 287" F. (141.7' C.). .-----LoAD-LBs. A
ELONGATION
PER
E
S Q ,IN.----
A G H
372 602 1005 1660 2620 (662) 3300
600 Break
308 540 900 1520 2380 (675) 3080
F
340 550 850 1380 2180 (650) 2580
200 300 400 500
7---
A
248 455 788 1325 2090 (637) 2480
600
Break
G H 45-Min. Cure 263 266 560 464 960 805 1535 1420 2360 2170 (600) 2360 (650) 2720
TABLI?IV -LOAD-LBS. I
302 605 1030 1670 2650 (637) 3080
---A
ELONGATION 200
G 60-Min. Cure 320 655 1100 1780 2760 (620) 3000 (630)
290 610 1020 1740 2760
300
400 500 600 Break
(650)
TABLEV
LOAD-LBS. PER SQ.IN.
..
H
I
298 560 955 1610 2480 2940
298 575 975 1580 2540 (630)
..
7 -
ELONGATION 200 300 400 500
600 Break
200 300 400 500 600
Break
A 400
682 1150 1820 (637) 2180 274 525 885 1460 2290 (637) 2790
G
___
H 3 0 - M i n . Cure
170 410 680 1125 1800 (637) 2050 45-Mzn Cure 292 270 610 520 1030 895 1470 1620 2340 (575) 2230 (619) 2500 60-Min. Cure 244 428 714 1130 1785 (637) 2070
o:ie
0.11 0.61
PER SQ.
-
IN.
A
G
330 620 1070 1820 2740 (606) 2840
--7
H
I
315 595 1010 1740 2590
332 662 1070 1750 2580
6 0 - M i n . Cure
386 685 1130 1870 2660 (605) 2720
(644)
..
(625)
..
2 1 The writers are aware that, in changing from a colloidal medium (rubber) to a noncolloidal medium (xylene), the amounts of the various reaction products obtained from any given accelerator and sulfur may be differently proportioned. I t would hardly be expected, however, that the nature of the reaction products would be changed.
TABLE VI LOAD-LBL. PER SQ. IS. I A
342 765 1290 2135
io7 1040 1680 2590 (644)
..
(620)
290 695 1160 1860 (631)
.. ..
..
.. ..
418 855 1390 2210 (600)
2900 137)
ik:4 12.6 68.6
20 Peachey, Brit. Patent 113,570 (1918); Dubosc, India Rubber World, 18 (1919),248; Bedford and Scott, THIS JOURNAL, 12 (1920),33; 13 (1921), 1034.
The stress-strain data for these four stocks, over a range of cures from 30 to 120 min. (see Table VI), are plotted in Figs. 5, 6, 7, and 8. These are introduced in this report merely to give a more complete picture of the general characteristics of these stocks. The combined sulfur determinations on the 60-min. cures
PERCENTAGE
0.87 1.03 0.98 1.38
throw some light on this point in the case of the four accelerators under consideration, they were heated with sulfur and zinc oxide under conditions approaching as closely as possible those employed during the actual vulcanization. The same proportions of zinc oxide, sulfur, and accelerator, as employed in compounding the above stocks (A, G, H, and I), with xylene21 (boiling point 141 O C.) replacing the rubber, were heated for 60 min. a t 287' F. (the time and temperature actually employed in the vulcanization), At the end of this time, the xylene was distilled off under reduced pressure
stress-strain curves plotted in Fig. 4 (data in Table V) were obtained. PERCBNTAGE
.
Increase in Coefficient over That of A Actual Per cent
Since it has been repeatedly observed, both in this laboratory and elsewhere,ZO that the organic accelerators used in these experiments react quite readily with sulfur a t or below the temperature here employed for vulcanization, it was necessary to determine whether such reaction products are completely dissolved during the extractions with acetone, alcoholic potash, and ether-hydrochloric acid, which precede the determination of true combined sulfur. Kelly17 believes that part of these reaction products are removed by the alcoholic potash solution and the remainder by the ether-hydrochloric acid; however, he submits no proof of this, and later qualifies the above assertion by stating that, in the case of some accelerators, it is not a t all certain that the sulfur reaction products can be entirely removed in this way. In order to
In order to check up the results plotted in Fig. 3, new batches of stocks A, G, H, and I were mixed and a sufficient number of the 60-min. slabs cured so that four test strips could be cut. By averaging the four individual results, the
PERCENTAGE ELONGATION
Coefficient of Vulcanization
0.44 0.41 0.41 0.42 0.50 0.51 0.49 0.50 0.47 0.48 0.47 0.47 0 . 6 6 0.65 0 . 6 6 0.66
I
6 0 - M i n . Cure
200 300 400 500
Combined Sulfur Per cent Sample 1 2 3 Av.
STOCK
TABLEI11 PERCENTAGE
Vol. 14, No. 10
(581)
..
..
... .
7
~
G
H 7 5 - M z n . Cure 460 338 822 638 1340 1110 2140 1780 2750 (562) 2f40 (625) 3100 90-Min. Cure 565 321 960 670 1490 1100 2310 1800 2730 (575) (612) 2880 120-Min. Cure
I 347 750 1210 1940 3100 (612)
.. .. ..
.. .. ..
..
..
(525)
.. ..
(600)
..
..
,.
(600)
..
..
T H E JOURiNAL OF IXDUSTRIAL A N D ESGINEERING CHE.VIXTRY
Oct., 1922
and the resulting mass given the same extraction treatment as the rubber stocks, i. e., with acetone, alcoholic potash, and ether-hydrochloric acid. The insoluble residue a t the end of this treatment was collected on a Gooch crucible, washed thoroughly with hot water, and dried at, 50” C , to constant weight. A blank was also run to determine the hydrochloric acid-insoluble in the zinc oxide. The percentages of insoluble residue, after correcting for the zinc oxide-insoluble, are given in Column 1 of Table VIII, together with the figures showing what this insoluble material would amount to in the actual vulcanizates (Column 3).
ACCELERATOR Hexamethylenetetramine +Nitrosodimethylaniline Aldehyde ammonia.. Thiocarbanilide.. .
. . .. . . . . ..
TARLEVI11 Insoluble -~ Accelerator Residue Formed in Stock Per cent Per cent 4.3 0.5 11.0 0.25 6.S 0.75 2.0 1.5
Insoluble‘ in Stock Per cent 0.02 0.03 0.04 0.03
STOCK A G H I
1 Owing t o the small amounts obtained in each case, the presence of sulfur was only proved qualitatively; the figures cannot, therefore, be used as correction factors for the vulcanization coe5cients; but since the amounts present in each stock are so small and so nearly identical, the effect on the coefficient is almost negligible.
DISCUSSION OF RESULTS The results for combined sulfur recorded in Table VI1 indicate that for accelerated stocks, even when the quantity of accelerator is so chosen that equivalent stress-strain relations are obtained in the same time of cure, the vulcanization coeficients show no corresponding uniformity. This lack of uniformity in the coefficients cannot be ascribed to sulfur retained in chemical combination by the respective accelerators, as the figures in Table VI11 show. I n other words, then, the 17ulcanization coefficient of an accelerated stock is no criterion of the physical state of cure. It is interesting in this connection to note the comments of Twiss22 along this line: The chemical action of sulfur on the rubber induces the physical alterations which constitute the advantage to be gained by vulcaniz,ation, but the chemical and physical processes are not necessarily strictly proportionate and some accelerators influence one more than the other. In the presence of certain accelerators the physical or mechanical alteration is disproportionately rapid, and the tensile strength attains its maximum at a coefficient of vulcanization well below the normal value.
The coefficients recorded in Table VI1 are all remarkably low. Other investigators, however, have recorded similar results.23 Kelly has given figures for a thiocarbanilide stock, having a combined sulfur content of only 0.84 and a coefficient of 0.99; the physical properties of this stock, however, are not recorded. Cranor found a stock to have satisfactory physical properties with a vulcanization coefficient of 1.09; he was working, however, with the very active dimethylammoniuin dimethyl-dithiocarbamate, and the curing time was only 3 min. Looking a t the results from another angle, they indicate that an accelerator has a specific action in vulcanization, aside from its influence on the chemical combination between rubber and sulfur. Many of the published figures on the properties of accelerated stocks show abnormally high tensile properties for low coefficients of vulcanization. However, in most cases, it is difficult to draw any conclusion as to any specific effect of the accelerator on the vulcanization process, since the duration of the curing period has been so variable. Many have ascribed these abnormal properties entirely to the J . SOC.Cham. I n d . , 39 (1920), 125T. Cranor, LOC.czt., Whitby and Smith, paper presented before the Section of Rubber Chemistryat the 62nd Meeting of the American Chemical crt. Society, New York, N. Y , September 6 t o 10, 1921, Kelly, LOC. 2%
955
shortening of the curing period through the influence of the accelerator. By the tabulation of Gottlob’s24 results obtained with Vulkazit125SeidlZ6has shown that the shorter the time of vulcanization, the higher the tensile strength corresponding to a definite combined sulfur content. Seidl states that, parallel with the strengthening of the rubber through the taking u p of sulfur, a depolymerization of the rubber takes place under the influence of the heat which exerts its harmful effect to a lesser extent, the shorter the time of heating. Aside from a theoretical interest, the results recorded in the present paper have a practical application, since for several commercially applicable accelerators, the quantities necessary for the production of equivalent physical states of cure have been derived. The problem of matching the physical properties of a stock in the presence of different accelerators, but under identical temperature conditions and duration of curing period, is one often confronting the rubber technologist, for shortening of the period of cure is far from the only consideration in connection with organic accelerators. Toxicity, volatility, miscibility, effect on range of cure, influence on aging proper tie^,^' and tendency to produce “scorching,” all have an important bearing in the selection of accelerators for specific purposes. Furthermore, where a given stock, of definite stress-strain relations, has proved through years of experience in service its applicability for a specific purpose, the introduction of a new accelerator should not disturb these stress-strain relations unless exhaustive practical tests have proved the desirability of making such a change in these factors. Hence the value, in making comparative studies of accelerators, of matching physical “state of cure” and studying the relative behavior of stocks which contain equivalent accelerating quantities of accelerator. The equivalent quantities which have been determined in . the course of this work indicate that 1 part of hexamethylenetetramine is equivalent in accelerating power to 1.5 parts of aldehyde ammonia, 3 parts of thiocarbanilide, and 0.5 part of p-nitrosodimethylaniline, respectively, in the type o j stock here tested. It does not follow, for example, that these exact reIationships will hold for a stock in which the sulfur content is sharply reduced. As Twiss2* has remarked, “the possibility must always be borne in mind that the activity of an accelerator may possibly be influenced by the proportion of free sulfur simultaneously present.” Recent publications on the mechanism of the action of organic accelerator^^^ would indicate that the concentration of sulfur is an important factor in their activation. Furthermore, it must not be understood that these equivalent quantities have been worked out to the limit of accuracy. Averaged stress-strain data indicate that the errors in curing and testing run upwards to 5 per cent, while the deviations of the three 60-min. stress-strain curves from the control (the 60-min. hexamethylenetetramine curve) average 7 per cent (cf. Fig. 4 and Table V). Further refinements of the equivalent quantities, though desirable from a theoretical standpoint, have not yet been made; from the standpoint of practical application, the above results are sufficiently accurate I n a recent article entitled “The Relative Activity of Certain Accelerators in the Vulcanization of Rubber,” Endres30 arrived a t somewhat different conclusions regarding the relative equivalent accelerating quantities of some of these same I
Gummz-Ztg., 30 (1916), 303. Probably aldehyde ammonia. 26 Gummz-Zlg., 34 (1920), 797. 27 Ditmar, Z. angew. Chem., 34, Aufsatztell (1921), 465; C. A . , 16 (1922), 174. 28 J . SOL. Chem. Ind., 39 (1920). 125T. 29 Bedford and Sebrell, THISJ O U R N A L , 13 (1921), 1034; Bruni and Romani, Indza Rubber J . , 62 (1921), 63. 3 0 Caoutchouc b gulta-peucha, 18 (1921), 11089. 24 26
