INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1926
content of alfalfa, the increases varying from 32.7 to 44.2 per cent. Since this increase in nitrogen occurred in crops which were much increased in yield, it is apparent that sulfur caused a very marked increase in nitrogen fixation. Similar results for alfalfa and clover were obtained in other experiments. The data of one of the clover experiments (Table 111)show that, whereas the nitrogen content was not more than from 10 to 23 per cent greater than the checks, the use of sulfur actually caused over three times as much nitrogen to be fixed than was present in the checks. Table 111-Yield a n d Nitrogen C o n t e n t of First Cutting Clover on Sulfured a n d Unsulfured Ritzville L o a m s SULFUR A N D G y p s u m APPLICATIONS LBS. PER ACRE Yield, grams Nitrogen, per cent Yield increase, per cent Nitrogen increase, per cent
159
Check
Sulfur
20.4 20.7
98.3 2.54
500
200
1000
Sulfur 82.7
Cas04
Cas04
381.8
305.4
83.6 2.28
309.8
22.7
27.0
10.1
86.0 2.35 321.5 13.5
2.63
73
The sulfur content of the plant is often considerably increased, as shown in Table 11. There is less effect upon the intake of the other plant food elements. There is some evidence that the iron content, although not increased in amount, may be changed in its state of combination. Sulfur and gypsum are selective in their actions in that they may affect legume crops, as shown above, but are not known to influence the nonlegumes. Moreover, the principal effect upon legumes appears to be that of increasing their nitrogen-fixing capacity. An understanding of how and why sulfur and sulfates cause legumes to fix more nitrogen is far from complete. It is known, of course, that legumes are able to utilize atmospheric nitrogen by virtue of the bacteria that grow in nodules on their roots. It would thus seem that sulfur has an indirect effect upon legumes through its direct action or effect upon the nitrogen-fixing organisms.
Rate of Combination of Sulfur with Rubber in Hard Rubber’ By W. E. Glancy, D. D. Wright, and K. H. Oon HOODRUBBERCo., WATERTOWN, MASS.
T THE Pittsburgh meeting of the AMERICAN CHEMICALhave now determined approximately the amount of sulfur SOCIETY a paper2 was presented giving the results of necessary to make a compound hard and the effect of several a n investigation of the influence of certain compound- of the more common organic accelerators upon the coeffiing ingredients in hard rubber, more especially of their in- cient of vulcanization and the tensile strength. fluence upon the physical properties of hard rubber. At Experimental that time a suggestion was made that it would be desirable to correlate the changes in composition and the changes in Five mixings were made, as shown in Table I. These physical properties which take place during the vulcanization mixings were made on a small laboratory mill and the usual of hard rubber. The information presented here is compiled precautions with regard to mastication, heat on the rolls, etc., with this end in view. were taken to insure uniformity of treatment. The mixed During the past ten years or more, investigation into the stocks, after aging for 24 hours or more, were vulcanized in mechanism of vulcanization has been largely centered about a mold in a hydraulic press, the temperature of the press bethe function of organic accelerators in hastening vulcaniza- ing maintained a t 170”C. The test specimens were molded to tion. The characteristic curing curves, the most desirable form, so as to eliminate cutting, and the time of cure varied temperatures of vulcanization, and the action of inorganic from 10 minutes in some cases to a maximum of 120 minutes. activators for various organic accelerators have been studied Table I and theories evolved to explain the facts. It is not the in1 2 3 4 5 Number of mix tention to discuss here a theory of vulcanization, but to point First aualitv kiln-dried 70 70 70 smoked sheet 70 70 out that in formulating any comprehensive theory the hard Sulfur 30 30 30 30 30 1.4 Diph;nylguanidine rubber field ought not to be neglected, especially since the one 1.4 Ethylidene aniline Hexamethylenetetramine 1.4 accepted compound of rubber and sulfur exists in this field. Tetramethyl thiuramdiWeber3 points out that the end product of vulcanization 0.7 sulfide is polyprene disulfide, CloHl&. Other investigators have The test specimens were of the size and shape which it is confirmed this statement. Hubner4 examined a sample of ebonite, which, however, showed less than 4 per cent combined customary to use for testing hard rubber-that is, 15.24 cm. sulfur, and reported that he had found only the monosulfide (6 inches) long, with a restricted section in the center 1.27 of rubber. Spence and Young5 have also shown that the rate cm. (0.5 inch) wide. It is that recommended by the Hard of combination of sulfur with rubber is constant for a given Rubber Division of the War Service Committee. The specitemperature until 32 per cent of sulfur (estimated on the mix) mens mere broken on a horizontal Scott testing machine, is combined with the rubber. The writers’ previous work, the jaws of which were separated a t the rate of 0.5 cm. per on the changes in tensile strength as the vulcanization pro- minute, and the temperature was maintained a t 21’ C. during ceeds, shows that the tensile strength increases slowly during the testing. Previous to the testing the specimens were imthe first part of the vulcanization, then very rapidly, and mersed in water at 21 O C. for one hour. The results reported finally a t a much slower rate continues to a maximum. They are the averages of at least three tests, and experimental error has been reduced as far as possible by additional check tests 1 Presented before the Division of Rubber Chemistry a t the 69th when it seemed desirable. Meeting of the American Chemical Society, Baltimore, Md., April 6 to 10, 1925. The fragments from the tensile tests were ground to about 3 THISJOURNAL, 16, 359 (1925). 20 mesh and were used for the determination of coefficient 8 “The Chemistry of India Rubber,” p. D1. of vulcanization. The method employed is that adopted by Gumni-Zfg., $4, 627 (1910). the Rubber Division of the AMERICAN CHEMICAL SOCIETY, * Kolloid-Z.. 13, 265 (1913).
A
4
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 18, No. 1
Figure 1
Figure 2
with two changes: (1) a 1-gram sample instead of a 2-gram sample; (2) the bromine was increased to 6 cc. instead of 3 cc. These changes seemed to be necessary to handle the large quantities of sulfur found in some cases, and to extract all of the uncombined sulfur. The combined sulfur has been estimated by subtracting the free sulfur from the total sulfur. Several determinations of the total sulfur by the Pirelli method and by the method adopted by the Rubber Division SOCIETY indicate that there is an of the AMERICAN CHEMICAL
combined sulfur estimated on the rubber, has been used in preference to the per cent combined sulfur estimated on the total mix. Inasmuch as it was observed that there are changes in the specific gravity of hard rubber stocks which are cured for different periods of time, the specific gravity has been determined for two of the stocks and the results recorded.
705MOKCD ZtiCCT 14~THYilDtNE.
ANILINE
0
Figure 3
appreciable loss of sulfur during vulcanization when only rubber and sulfur are in the mix. This loss averaeed 0.4 per cent. I n the stocks which contain accelerators &is loss is very small. The coefficient of vulcanization, or per cent
Results
The results are shown in Table I1 and also graphically in Figures 1 to 5. One very interesting fact shown in Figure 1 should be pointed out. Although sulfur has entered into combination with rubber until there is a coefficient of vulcanization of 28.14, there has been no corresponding increase in tensile strength and the specimen has remained flexible. As it seemed possible that dilution from the uncombined sulfur might retard the increase in tensile strength, several stocks were mixed which contained only smoked sheets and sulfur and which when fully vulcanized should have coefficients of vulcanization of 17.6, 21.2, 23.5, and 28.0. The first two of the above-mentioned stocks were still soft when the vulcanization was continued for 6 hours a t 170' C. The stock which contained the largest amount of sulfur was noticeably hard in 60 minutes. The stock which contained 23.5 per cent sulfur, estimated on the rubber, was flexible when vulcanized for 3 hours. The evidence is, then, that the hard variety of vulcanized rubber exists only after a coefficient of vulcanization of approximately 23.5 has been reached. This amount of combined sulfur is the amount necessary that one sulfur atom be joined to one CIOH16 group. Apparently, no polyprene disulfide, c10H16s2, is formed until each ClOHl6 group has received one atom of sulfur. Figure 2 shows the effect of 2 per cent diphenylguanidine on the base compound of 70 rubber-30 sulfur. This accelerator, which has wide use in soft rubber goods, is evidently quite efficient in hard rubber. Various accelerators seem to give properties to soft rubber goods which cannot be obtained in unaccelerated stocks. Diphenylguanidine in hard rubber, however, seems to be purely a sulfur carrier, the rate of the reaction'being greatly-speeded, but approximately the same maximum tensile strength is obtained as though no accel-
'
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1926
75
Table I1 Time of cure, min. Stock
10
1 2
29.93
3 4 5
12
15
30
35
40
45
60
75
90
120
37.42 40.93 39.9s 37.24 39.93
38.10 41.06 40.32 38.77 41.17
39.17 41.34 40.60 39.60 42.60
39.93 41.71 41.21 40.07 42.27
40.86 41.72 40.96 40.43 42.44
535.3 577.6 535.7 439 4 549 4
549.6 589 2 520 4 506 7 581 1
577.3 579.7 554 6 486.4 572.0
584 1 582.8 537 5 519.2 563 1
Coe5cient of Vulcanization 30.23
6.57 34.87 33.98 22.22 27.44
27.57 40.37 37.90 35.75 38.18
32.89
34.50
Tensile Strength, Kg./Sq. Cm. 472.8 503 519 532 389 535
erator had been used. Zinc oxide is not necessary to activate the accelerator. Figure 3 shows the curing curve for ethylidene aniline. It reacts much like diphenylguanidine, but is seemingly a little less abrupt in its action. Figure 4 shows the effect of 2 per cent hexamethylenetetramine. Without an inorganic activator this material aids the combination of rubber and sulfur in the early stages of vulcanization, but is not nearly so efficient as diphenylguanidine or ethylidene aniline. I n the later stages of vulcanization it seems actually to prevent the usual increase in tensile strength. Figure 5 gives the curing curves for tetramethyl thiuramdisulfide. Only 1 per cent (based on the rubber) is used. Evidently this powerful accelerator, which is ordinarily used a t comparatively low temperatures, is very active in hard rubber at high temperatures. Its use would probably be limited because of its cost.
7 2 3 1
9
It is possible to estimate the specific gravity of crude rubber as vulcanized in soft rubber goods by determining the specific gravity of the vulcanized stock and making the necessary allowances for the other materials present. I n several instances the specific gravity of vulcanized rubber was found to be close t o 0.935. If, however, this same procedure is used for the determination of the apparent specific gravity of rubber in vulcanized hard rubber, this apparent specific gravity will be nearer 0.998 for a fully vulcanized hard rubber, assuming the specific gravity of sulfur to be 2.0. It would appear, then, that in the combination of rubber with large quantities of sulfur there is a contraction in volume over that which might be expected if the rubber and sulfur exist in the same condition as in the low-sulfur vulcanized mixes. Conclusions 1-Rapid changes take place in the physical properties of a rubber-sulfur mixture when the coefficient of vulcanization reaches approximately 23.5, or apparently when each CloHle group has received one atom of sulfur to form polyprene monosulfide. 2-An active hard rubber accelerator hastens the rate of combination of sulfur with rubber without appreciably increasing the strength of the final product. 3-Each accelerator has a n influence peculiar to itself and may be considered a “regulator” of the rate of combination of sulfur with rubber. 4-In the formation of the hard rubber compound, the volume of the end product is found to be less than the volume of the components.
.---Table I11
Time of cure, minutes---------Stock I 5 30 35 40 45 60 75 90 120 1 1.076 1.148 1.156 1.156 1.162 1.162 1.167 1.165 1.165 2 1.163 1.169 1.174 1.171 1.174 1.173 1.176
In considering the action of these accelerators, a statement can be made which would apply to all that have been included in this investigation-the organic materials are “regulators” of the rate of combination of sulfur with rubber. The question of changes in specific gravity as vulcanization proceeds has been mentioned. The specific gravities of Stocks 1 and 2 are given in Table 111.
70 SUOKED *CET
Figure 5
