Jan., 1 9 2 1
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
negative values are obtained, owing t o t h e liquors becoming more concentrated t h a n they were originally, on account of t h e collagen abstracting water from them. 150
I20 90
8
8" 4
9"
$ 0
& -30
(3
4 -60
* -90 &
.' \8
concenffafion of Lipwin Grams PIG.
cr203 per Liter
2
We would state our belief, based upon our experience as presented in this alid earlier papers, t h a t t h e reaction between chromic sulfate solutions and hide substance is chemical and not physical, as contended by A. W. Davison.1 If t h e adsorption were a simple physical process, i. e., merely a partition of t h e chromic oxide and sulfuric acid between t h e solid hide substance phase and t h e solution phase, t h e curve should follow Freundlich's adaptation of Henry's law: CI = KCzn, which is parabolic in shape; whereas Miss Baldwin's and our experiments show t h a t in a 2-day adsorption the curve begins t o slope steeply downward after t h e concentration of t h e liquor exceeds approximately 16 g. of chromic oxide per liter in a solution of t h e composition of Cr(OH)S04, and reaches a minimum when t h e concentration of chromic oxide is 147.5 g. per liter, this minimum being maintained at a concentration of 2 0 2 g. per liter. This minimum confirms t h e prediction of Wilson and Gallun in part. T h e most concentrated chrome liquor which we used was very thick and about as concentrated as is possible t o handle; and therefore, we do not find i t possible t o test further their prediction t h a t increasing concentrations beyond this minimum would cause greater fixation of chrome. ACKNOWLEDGMENTS
Acknowledgment is made of Mr. S. B. Foster's assistance in t h e analytical work. We wish t o express our great appreciation of t h e generous support of RIessrs. A. F. Gallun and Sons Company in this investigation. J A m Leather Chem. A s s o c , 12 (1917). 2 5 8
67
THE ACTION O F CERTAIN ORGANIC ACCELERATORS TN THE VULCANIZATION OF RUBBER-11' By G. D. Kratz, A. H. Flower and B. J. Shapiro THE FALLSRUBBERCo., CUYAHDGA FALLS,OHIO
One of t h e early patentsZ for t h e use of synthetic nitrogenous organic substances in t h e vulcanization of rubber refers t o t h e dissociation constant of I X 1 0 - 8 as t h e dividing line between accelerating and nonaccelerating bases. On t h e other hand, Peacheys has pointed out t h a t certain other substances which are not basic, or b u t slightly so, are also exceedingly active as accelerators. T h e number of examples i n this class, however, is relatively small. I n t h e course of t h e experimental work described i n . this paper we have made a comparison of t h e sulfur coefficients of a t y p e mixture vulcanized with t h e assistance of a number of accelerators closely related t o aniline and for which t h e dissociation constants are known. We have also employed t h e hydrochlorides of two of these substances, relatively weak and strong bases, in order t o observe t h e effect of t h e acid portion during t h e vulcanization. T h e results obtained and t h e conclusions drawn led us t o employ t h e sulfides of ammonia as accelerators and vulcanizing agents. Briefly summarizing these results, i t was found t h a t with t h e substances tested there was apparently no direct relationship between their dissociation constants and their excess sulfur coefficients or physical properties after vulcanization. I n a closely related series, such as aniline and its methyl derivatives, t h e substance with the largest dissociation constant was found t o be t h e most active. However, t h e relative activities of t h e members of this series were not proportional t o their dissociation constants. Generally speaking, t h e activity of all of t h e substances could be traced t o t h e amino group, a n d depended t o a large extent upon whether or not substitution had taken place in this group. I n this respect, t h e y should probably be regarded as substituted ammonias, rather t h a n as the more complex derivatives of other substances. One effect of t h e basicity of two of t h e substances, methylaniline and +-toluidine, was determined with t h e hydrochlorides of these two substances. Our results showed t h a t with substances of this type, t h e first effect of t h e base is t o neutralize t h e retarding action of t h e acid formed in the decomposition of t h e salt during vulcanization. We had previously suggested this in a footnote in a former paper.4 We also found t h a t when t h e acid liberated in t h e decomposition of such a salt is neutralized by other substances in t h e mixture, t h e activity of t h e hydrochloride is very close t o t h a t of t h e free base. These results are of particular interest, as Van Heurns has shown t h a t , whereas ammonium carbonate is moderately active as an accelerator in a mixture of rubber and sulfur, 1 Presented before the Rubber Division at the 60th Meeting of the American Chemical Society, Chicago, Ill., September 6 to 10, 1920. D. R. P. 280,198 (1914). 3 J . Soc. Chem. Ind., 36 (1917), 950. 4 Chem. b Met. Eng , LO (1919), 420. 6 Comm. of the Netherlands Government for Advising the Rubber Trade and Industry, Part 6 , 202
*
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
68
ammonium chloride is inert. The former salt decomposes into ammonia and a weak acid, t h e latter into ammonia and a strong acid, according t o t h e following reactions:
+
(hTH4)pCOa --t Z"3 HzO $- Cot NHdCI + NHI f HC1
Our final experiments, wherein we found t h a t in a closed system rubber is vulcanized by heating with ammonium polysulfide or ammonium hydrosulfide, were carried on in order t o obtain a reaction mixture of undoubted basic character, which at t h e same time would include H2S as one of t h e decomposition products. The function of H2S in connection with t h e vulcanization of rubber has long been made a subject of controversy. I n t h e present instance i t may be regarded as a very weak acid. Our results with ammonium polysulfide may be explained as due t o t h e decomposition of this substance into ammonia, hydrogen sulfide, and sulfur, the latter substance being liberated in an active (nascent) form which readily combines with t h e rubber. The analogy between our results with ammonium polysulfide, and those obtained years ago b y Gerard' with potassium tri- and pentasulfides, is taken u p in greater detail in t h e experimental part of this paper. It is equally evident, however, t h a t if this explanation is advanced in the case of ammonium polysulfide, vulcanization with ammonium hydrosulfide requires t h a t this substance decompose not into ammonia and hydrogen sulfide only, but with t h e subsequent formation of a polysdfide which liberates sulfur in the active form.2 It has been shown b y Bedford and Scott3 t h a t many of t h e more complex substances which accelerate t h e vulcanization of rubber react with sulfur, with t h e liberation of HzS and t h e formation of thiourea derivatives. I n view of our results with t h e ammonium sulfides, t h e action oi such thiourea derivatives would depend upon their ability t o enter into a subsequent reaction with t h e H2S formed, or t h e sulfur present in t h e mixture, with t h e formation of a polysulfide. Further, although t h e formation of a polysulfide in this manner would, t o a certain extent, be dependent upon t h e basicity of t h e substance originally added as t h e accelerator, i t is obvious t h a t t h e dissociation constant of t h e reaction product would be a better indication of its activity t h a n t h e dissociation constant of t h e original substance. 1 R. Hoffer, "Treatise on Caoutchouc and Gutta-percha" (trans. Brannt), H C. Baird & Co., London, 188% 2 As an aqueous solution of NHiHS was employed, the action of this substance may also be explained by its dissociaticn products. I t would dissociate with NH4+ as the cation and HS- the anion. As the HS- ion it$elf is weakly acid, there would probably be many H + and HS- ions and but few S ions in the aqueous solution. The H" and S ions in turn react to form HnS. On the other hand, (NH4)aS dissociates with "I+, the cation, and S- -, the anion. The latter, in the presence of water, dissociates with the formation of OH- and HS- ions. Thus, NH4HS dissociates with the formation of a greater number of H f ions than in the case of (NHr)aS, and consequently with a greater re-format?.. d Res. This may account for the difference in the relative activities of the two substances. The same may be true in the absence of water, as most erganic accelerators are apparently soluble in rubber, the high dielectric constant of which indicates that this substance itself may be a good dissociating medium. 8 THISJOURNAL, 12 (1920). 31.
-
-
Vol. 13, No.
I
I n a previous paper' we have suggested t h a t t h e activity of certain nitrogenous substances may be interpreted on t h e basis of a change in valency of t h e nitrogen, with t h e nitrogen functioning as a sulfur carrier. This suggestion was made t o assist in correlating t h e nitrogen content with t h e activity of t h e substances employed, although, as pointed out in t h e above paper, results obtained b y others already indicated t h a t t h e sulfur is not necessarily attached t o t h e nitrogen. While our present results show t h a t vulcanization may be effected b y polysulfide formation, they do n o t exclude t h e possibility of t h e active nitrogen group acting as a catalyst. EXPERIMENTAL PART
The same general method of procedure was pursued in t h e course of this work as was previously reported in P a r t I. The rubber was good quality, first latex, pale crepe, and the same lot was employed for all mixtures. All of t h e mixtures, t h e composition of which is shown at t h e head of t h e various tables, were mixed and vulcanized as before. Physical tests were. made on a Scott testing machine of t h e vertical type. Sulfur estimations were made b y our method, previously described in detail.2 The accelerators were purified, and melted or boiled a t t h e temperature shown in t h e tables, All of t h e accelerators were compared on a mplecularly equivalent basis, 0.01 g. molecule of t h e accelerator being added for each I O O g. of rubber in t h e mixture. EXPT. r-This experiment was carried on in order t o ascertain t h e relative accelerating effect of t h e homologs of aniline and other closely related bases, and also t o compare t h e excess sulfur coefficients with t h e dissociation constants of t h e substances originally added as accelerators. The results obtained, together with the physical constants of t h e substances employed as accelerators, are shown in Table I. It is evident from this table t h a t with aniline a n d its methyl derivatives, or in t h e case of t h e two phenylenediamines, t h e substance with t h e largest dissociation constant produces t h e greatest excess coefficient of vulcanization. I t is also apparent t h a t this relationship is confined t o more or less closely related substances only, a n d t h a t , as a general rule, t h e dissociation constant is not a reliable guide t o the activity of a substance as a n a c ~ e l e r a t o r . ~ Excess sulfur coefficients of equal magnitude (3.0) were obtained from p-toluidine, p-benzidine and m phenylenediamine. It is interesting t o note t h a t t h e subtraction of t h e excess sulfur coefficient of any one of these substances from t h a t obtained for p-phenylenediamine (5. 2) leaves a figure very close t o t h e excess obtained for aniline (2.4). Further, although t h e mixtures vulcanized with t h e assistance of t h e three substances in question were found t o have t h e same excess sulfur coefficient, all of them had widely differTHIYJOURNAL, 12 (1920), 317. * I n d i a Rubber World, 61 (1920), 356. S The dissociation constants given in Table I are taken from the LandoltBarnstein tables and are not strictly comparable, in that they were not all determined by the same method. 1
Jan., 1921
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
69
TABLE I First Latex Pale Crepe.. ........... 100 Sulfur.. ......................... 8.1 Accelerator ....................... x Vulcanized for 90 Min. a t 148" C. Determined x==
M. P. or B. P. of
Dissociation Constant K a t 15' to 18' C
FORMULA SUBSTANCE 0.01 G. Mol. Accelerator' Control. . . . . . . . . . . . . . . . . . . Aniline. . . . . . . . . . . . . . . . . . . 0:93 iii. 1 3 . so io-10 1.07 Methylaniline. . . . . . . . . . . . . 192.0 2.55 x 10-19 1.21 192.5 Dimethplaniline. . . . . . . . . . . . 2.42 X 10-10 1.07 45.0 +Toluidine. ............... 1.60 x 10-9 1.08 m-Phenylenediamine . . . . . . . . 62.6 I .35 x 10-12 1.08 140.6 $-Phenylenediamine. . . . . . . . 2.48 X 10-1* * 1.84 126.2 $-Benzidine. ............ 7.40 x 10-13 2 1.08 Phenylhydrazine. . . . . . . . . . . 240.0 1.60 X 10-8 1.84 126.0 Hvdrazobenzene" . . . . . . . . . . I AU. m. p. are below, and b. p. above the temperature of vulcanization. 2 Figure applieq t o second "K
'x'
.
9
e n t physical properties. These substances may be regarded as aniline in which hydrogen of t h e benzene ring has been replaced b y radicals. 01% t h e other hand, (methylaniline), phenylhydrazine, and hydrazobenzene may be regarded as aniline i n which t h e hydrogen of t h e amino group has been replaced. T h e difference i n t h e activity of these two types of accelerators has already been mentioned in P a r t I in connection with t h e phenylguanidines. As in t h e previous instance, t h e same excess coefficient was obtained for (methylaniline), phenylhydrazine a n d hydrazobenzene, b u t t h e value was much smaller ( 0 . 7 5 ) t h a n before. The excess value found for these three substances when subtracted from t h a t obtained for $-phenylenediamine gives a figure equal t o about twice t h a t obtained with aniline. Here, also, t h e physical properties of t h e three mixtures were greatly diflerent.
t h e type described is directly traceable t o t h e amino group, and particularly t o t h e first amino group in t h e benzene nucleus. EXPT. 11-In view of t h e results of Expt. I , they should be analogous t o those of ammonia or ammonium salts. From a consideration of t h e work of Van Heurn' it seemed possible t h a t certain other substances, or their reaction products, active as accelerators, might decompose with t h e formation of a (relatively) strong base and a weak acid in an analogous manner; or t h a t some substances, which are not ordinarily classed as accelerators, owing t o their decomposition into a weak base and strong acid, might be active if t h e acid so formed was neutralized by another constituent of t h e mixture. Aniline sulfate and p-toluidine hydrochloride, when employed in the presence of zinc oxide, are examples of t h e latter type. TABLEI1 First Latex Pale Crepe., . . . . ., . . . . . . 100 Zinc Oxide.. ...................... 0 Sulfur. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Accelerator . . . . . . . . . . . . . . . . . . . . . . . . x x = 0.01 e . Mol. of Substance Vulcanized for 90 Min. at 148" C.
/loo
6tono 5 -4
2
"
-PHYSICALPROPERTIES' I ens ' i 1e Excess Strength Final Sulfur Lbs. per Sq. In. Length Coefficient a t Break a t Break (2.581) 1229 1090 2.400 2005 910 0.612 1665 1050 1938 0.250 1060 2476 2.987 920 1933 2.986 830 5.248 430 193 3.056 1464 810 0.751 1052 1080 1140 2165 0.777 3 Does not have basic properties.
900
Q
$800 r,
700
Sulfur Coefficient MIXTURE Rubber-Sulfur Control.. . . 2.789 Zinc Oxide Control.. . . . . . . 2.538 R-S Control HC1.. . . . . . 0.652 ZnO Control HCI.. . . . . 2.491 $-Toluldjne. . . . . . . . . . . . . .5.568 $-Toluidine Z n O . . . . . . . . 5.371 +Toluidine Hvdrochloride 2.308 &Toluidine Hydrochloride ZnO:. 3.990 Methylaniline.. . . . . . . . . . . . 3.193 Methylaniline 4- ZnO. . . . . . 2.750 Methylaniline Hydrochlo. ride .................... 1.012 Methylaniline Hydrochlo. ride Z n O . . . . . . . . . . . . 2.012
++
sa 600 F 500 3
+
+
400 TENSILE S T R P N G T ~IN LBS PER SQ IN.
FIG 1
The discrepancy in t h e physical properties of mixtures vulcanized t o t h e same sulfur coefficient by means of different accelerators is of especial interest a n d has been made t h e subject of a subsequent paper. As our present results are based on one cure only, we are not warranted in drawing many conclusions from those recorded here. A comparison of t h e results given in Table I, with t h e stress-strain curves shown in Fig. 1, however, shows t h a t these differences are most evident at, or near, t h e point of break.l T h e above results indicate t h a t , irrespective of whether or not an interaction between t h e accelerat o r and other substances in t h e mixture takes place during vulcanization, t h e activity of substances of 1 $-Phenylenediamine was so greatly over-cured t h a t concordant results could not be obtained and no curve is given for this substance.
..............
+
Sulfur Coefficient Over Control
..,
(+)
.. .. .. ...
2: 7i9
2.833
...
1.460 0.404 0.217
... ...
100 100 8.1 5
Sulfur Coefficient Under Control (-)
1265
1140
5 64
1250
1783 2476 1824 1070
820 920 640 1210
2485 1665 2237
757 1050 800
1.777
530
1150
0.526
1731
840
ii
0: 1 2.137 0.047
...
0:4i(1
... ... ...
I n Table I1 are given t h e results obtained with molecularly equivalent quantities of p-toluidine and p-toluidine hydrochloride, in t h e presence and absence of zinc oxide. Two control mixtures were employed, one with and t h e other without zinc oxide, no accelerator being added in either case. The excess sulfur coefficients (+ or -) shown in t h e third and fourth columns of this table were obtained by t h e subtraction of t h e coefficients of their respective controls, depending upon whether or not they contained zinc oxide. F r o m t h i s table it is evident t h a t z i n c o x i d e itselj e x erts a slight r e t a r d i n g a c t i o n a n d t h a t hydrochloric acid 1 LOG.
