Chemical Reactions during Vulcanization E. A. HAUSER AND J. R. BROWN’ Massachusetts Institute of Technology, Cambridge, Mass.
HE data and material of In a previous article (4) the authors prestrength of a vulcanized stock a previous article (4) will duringcure. The same is true sented data on the loss in unsaturation and be referred to directly, for the modulus. combination of sulfur during cure for four and the same nomenclature and typical low-sulfur accelerated rubber stocks terms will be used. The first Correlation of Loss in Unand for a plain rubber-sulfur compound. article presented data on four saturation with Physical Sulfur was evidently combining in excess accelerated stocks compounded Properties according to the basic formula of one atom of sulfur saturating one double given in Figure 1, but employed Since the changes in tensile bond in the rubber hydrocarbon for certain a different accelerator in each strength with time of cure are accelerators; furthermore, an excess loss stock. They were designated generally considered to be a of unsaturation was indicated in the overas stocks A (tetramethylthiuram measure of the building up of a cured samples after most of the sulfur had disulfide), B (mercaptobenzonew (vulcanized) structure in thiazole), C (zinc dimethyldiearly parts of the cure, as well combined. thiocarbamate), and D (heptalas the ability to maintain that The study has since been extended to ind e h y d e a n i l i n e ) . Stock E, a structure during overcure, the clude two more typical accelerators and a plain rubber-sulfur compound, maximum tensile strength is pure thiuram disulfide stock-i. e., withcontaining 100 parts of rubber usually considered indicative out added elementary sulfur. These new and 8 parts of sulfur, was also of the optimum physical state of the compound. Since this studied. data are presented, and a general correlaoptimum state is also the one The experimental procedure tion of the chemical reactions during vuldesired from a practical viewfor stocks G, H, and J (Figcanization and of the chemical structure point, tensile strength is generure 1) was identical to that preof vulcanized rubber is given in terms of the a,lly accepted as an important viously described, except for data obtained for the whole series of nine criterion of vulcanization. the thiuram disulfide stock J A comparison of the combined where the combined sulfur was different accelerated and unaccelerated, sulfur a t the highest tensile determined directly on acelow-sulfur stocks. strength for each stock retone-extracted samples. With veals no simificant correlation. this active vulcanizing anent it However, a plot of the total change in-unsaturation, Aut, was found necessary toextract in the cold to prevent further against the highest tensile strength for each of the stocks cure, Accordingly, for both the unsaturation and comstudied shows that the optimum tensiles for stocks containing bined sulfur determinations the samples were extracted both sulfur and accelerator are better with low AUI (Figure in the cold with the apparatus described by Lindsly (IS). The experimental data are given in Table I, and the physical 2). I n other words, a t the optimum point of each cure and chemical properties are plotted against the time of cure in (highest tensile) the accelerated stock which has lost the Figure 1. least number of double bonds will have the highest tensile The combined sulfur and unsaturation curves for stock H strength. For two of the accelerators studied, the loss of show a noticeable break around 20 to 30 minutes, just as if only 2 per cent of the double bonds existing prior to cure is a more active accelerating agent were being released a t that necessary to achieve the optimum tensile strength. time to speed up the reaction of sulfur and the loss of double This general relation in Figure 2 seems to hold in spite of bonds. The experimental values of these points were checked the wide range of time of cure necessary to reach the optimum several times to make certain that the change in trend was tensile strength. The time for maximum cure of the different definitely outside the experimental error of the measurestocks is indicated by the subscripts to theletters on the curve. ments. The accelerator in stock H, “Ureka C,” is the benStock E (100 rubber-8 sulfur) and stock J (Containing tetrazoate of mercaptobenzothiazole, in which the acidic hydrogen methylthiuram disulfide as a curing agent without added atom has been replaced by a benzoyl group. The action of sulfur) do not fall on the curve, but it must be remembered heat is evidently causing the ester to break down and liberthat their combined sulfur contents are quite different from those of other stocks, However, when equivalent atoms ate the more active mercapto accelerator after 20 to 30 minutes, This chemical evidence agrees well with the general of combined sulfur are used as a basis, the loss in unsaturaconception of the mode of action of such delayed-action tion of these two stocks can be correlated with the others. accelerators. Figure 1 again shows, in supplementing stocks Thus, A to E previously studied, that the loss in unsaturation alone, like combined sulfur, does not explain the change in tensile double bonds lost A Ut atom of combined sulfur 1 Present address, Standard Oil Development Company, Elizabeth, N. J.
T
1225
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INDUSTRIAL AND ENGINEERIA IG CHEMISTRY
VOL. 31. NO. 10
is combining in the ratio of over 2 atoms of sulfur per double bond lost, whereas H and J show a ratio nearer 1. This variation is even greater than that previously observed for stocks A and
E (4).
J us. TIMEOF CURE where U = unsaturation at given time of cure, % Aut = total loss in % unsaturation = Uunoured - U So = grams combined sulfur per 100 grams rubber 68 = mol. weight of rubber hydrocarbon unit C6Hs 32 = atomic weight of sulfur
A plot of the ratio AU~j2.1258,against the corresponding optimum tensile shown in Figure 3 reveals that all the vulcanized stocks can be correlated by the same curve. Therefore, both accelerated and unaccelerated stocks have the highest tensile strength when they have the lowest ratio of unsaturation to combined sulfur. From this it can be concluded that the relation between the loss in unsaturation and the combination of sulfur has an important bearing on the physical structure developed during vulcanization.
Relation between Loss in Unsaturation and Combined Sulfur In Figure 4, values of the unsaturation are plotted against the amount of combined sulfur for stocks G, H, and J, and compared with the theoretical linear relations of 1 or 2 atoms of sulfur combined per double bond lost, as the solid straight lines show. The data for stock G reveal that sulfur
Stock H, containing the delayed-action accelerator, reveals a definite change in the type of sulfur combination between 20 and 30 minutes of cure, where the ratio changes from 1 to about 1.2. Thus, the action is not merely an increase in the rate of sulfur addition, since the active accelerating agent is released under heat, but must be considered as an actual change in the type of reaction involving sulfur and the double bonds. Figure 4 shows that an increased loss of double bonds occurs after most of the sulfur has combined, and that this loss depends on the type of accelerator employed. This also was true of the first stocks studied. At that time it was uncertain as to whether the values for unsaturation represented the true state of the overcured compounds, or whether a loss of double bonds had occurred during the measurement of unsaturation. A study of the method of dispersing the vulcanized samples in p-dichlorobenzene revealed that the unsaturation values were not greatly affected by the time of heating or by the atmosphere of the operation, whether air or nitrogen. In the latter case, although the time for completely dispersing the overcured samples was increased about threefold in the absence of oxygen, the unsaturation values changed only to a small degree. The results for two of the overcured samples are shown in Table 11. The precision of measurement with these overcured samples was not so good as with the shorter cures whose unsaturation values usually checked to within * 0.1 per cent. It is quite possible that not all traces of oxygen were excluded from the nitrogen. If such is the case, then small amounts of oxygen would be effective, and perhaps even necessary, in dispersing the vulcanized samples in heated solvents. Staudinger (20) reported that oxygen will degrade insoluble rubber and cause it to dissolve; in an analogous
AND J Time .ofCure Combined Modulus Elongaa t 141O C. S per 100 Unsature- Tensile at tion at Btook (286O F.) G. Rubber tion Strength 500% Break Min. Grams % Lb./sq. in. % 93.9 0.00 0 a li60 iio 1665 93.2 0.74 10 935 2685 175 1.13 92.9 20 875 3210 275 92.7 1.40 30 2940 305 815 92.5 1.70 40 815 405 3780 92.2 2.01 60 815 430 3880 92.1 2.22 90 805 4500 505 91.9 2.31 120 900 175 1950 91.2 2.47 8 hr. 93.9 0.00 0 H 1240 io5 1005 91.8 0.95 10 2250 160 1020 91.1 1.29 20 200 910 2800 9 0 . 8 1 . 7 2 30 240 2800 885 89.9 2.04 40 260 905 2900 8 9 . 1 2 . 3 6 60 905 3070 260 88.4 2.45 90 250 910 2980 88.0 2.49 120 906 2210 180 2.49 87.6 8 hr. OF DATA ON STOCKS G, H, T A B LI.~ SUMMARY
0 5 10 20
0.00 0.12 0.21 0 0 .. 3 36 5
40 30 00 90 120
0 .33 0.31 0.32
93.9 93.6 93.4 93.0 92.6 92 2 .. 5 0 9
1875 2650 2700 2710 2710
390
io5 160 250 300 345 305
965 940 850 815 810 790
9 1 . 35
2760 2720
340 335
790 785
OCTOBER,1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
case traces of oxygen were shown to be necessary in milling to reduce the rubber to the plastic state (6, 6). Since the breakdown of rubber on the mill causes no change in measurable unsaturation, the assumption seems permissible that traces of oxygen will also cause the dispersion of vulcanized rubber without a measurable loss in unsaturation. TABLE11. EFFECT OF OXYGENON MEASUREMENT OF UNSATTJRATION Time
Stock
Cure
Atmosphere
Min.
of Heating Min.
Umaturation
%
1227
of the molecule by oxygen. In the case of the simple rubbersulfur compound stock E (100 rubber-8 sulfur) infrared absorption spectra on a thin film show no particular change after 3-hour heating a t 140' C. (284' F.). After 10 hours, however, the presence of oxygen is definitely established in the form of C=O, hydroxyl, and ether linkages, as well as a degradation in the hydrocarbon structure. This seems to be in good agreement with the infrared work of Stair and Coblentz (19) on the aging of natural rubber, and with the large excess loss in unsaturation observed for the 10-hour cure of stock E, which was accompanied by a decided loss of tensile strength from the optimum value of the 4-hour cure. Likewise, van Rossem and Dekker (18) recently showed that oxidation products may be developed during vulcanization as well as during aging.
Type of Sulfur Combination Correlation of Loss in Unsaturation and Combined Sulfur A general consideration of all the accelerated stocks studied reveals that in vulcanization sulfur is combining with the rubber in a definite ratio of the number of atoms of sulfur combined per double bond lost, during the early parts of the cure. After most of the sulfur has combined, an additional loss of double bonds takes place due to polymerization by heat under the influence of accelerators, or some similar effect caused by oxygen. These two reactions involving the unsaturation of the rubber molecule are dependent upon the type of accelerator employed and in general may be correlated with the optimum physical properties of the vulcanizates. Accelerators showing ratios of atoms of sulfur combined per double bond lost of 2 or more, produce higher optimum tensile strengths than accelerators showing ratios nearer 1. Also, the excess loss of unsaturation in the latter part of the cure is greater in most cases for those accelerators having a low ratio. The particular type of sulfur combination characterized by a high accelerator ratio is evidently con-
J ~ o
2500 E 4 HR. 6
Ah
3500
4000
4500
5000
OPTIMUM TENSILE STRENGTH,LB. / SQ.lN
FOR EXAMPLE AloMEANS THE IO-MINUTE CURE OF STOCK A
I I
3000
I FOR EXAMPLE AioMEANS THE IO-MINUTE CURE OF STOCK A
TENSILE STRENGTH FIQURE 3. AUt/2.125S, us. OPTIMUM
I
I
I
tributing to the vulcanized structure to produce higher tensile strengths. However, the excess loss of unsaturation occurring with accelerators having a low ratio shows no correlation with the change in tensile strength during overcure and therefore cannot be definitely associated with changes in the vulcanized structure.
Cause of Excess Lass in Unsaturation In the case of accelerated stocks there is no direct evidence as to whether the increased loss of double bonds occurring after most, of the sulfur has combined is due to polymerization by heat under the influence of accelerators or degradation
Chemical tests for all such compounds were applied to the stocks tested. The vulcanized samples were dispersed in pdichlorobenzene as in the measurement of unsaturation and diluted with carbon tetrachloride to assure a liquid solution. The use of nitrous acid indicated the absence of mercaptans (16) and thiophenes (1%').Sodium arsenite and potassium cyanide gave no indication of disulfides (11). Isatin in sulfuric acid showed no evidence of mercaptans (8) or thiophenes (1). Sodium nitroprusside gave no evidence of mercaptans (1) and, when modified as in Grote's reagent, was entirely negative in revealing the presence of -SH, =C===S, or -S-Scompounds (10). Finally, attempts to detect mercaptans with sodium plumbite (9) were entirely negative. Although the applications of these chemical tests were not altogether satisfactory in some cases, owing to precipitation of the rubber from solution, it is felt that there is at least not much probability of these forms of sulfur combination being present in vulcanized rubber to any great extent. The combination of sulfur with substances other than the rubber hydrocarbon was previously shown to be small enough to be neglected. Further tests for zinc sulfide in the compounded stocks by the method of Stevens (81) showed it to be less than 0.1 per cent of sulfur on the rubber, which agrees well with the work of Brazier and Ridgway (3).
VOL. 31, NO. 10
INDUSTRIAL AND ENGINEERING CHEMISTRY
1228
Although previous work has shown the methyl iodide reaction of Meyer and Hohenemser (1.4) to be invalid in a quantitative sense, it does indicate that sulfur has combined with the hydrocarbon, either by addition or as a bridge between adjacent double bonds (Figure 5 , a and b). Both of 94
accelerators is the reaction of hydrogen sulfide to form dithiocarbamic acid (a good accelerator) and free sulfur, as pointed out by Bedford and Gray (2). Experimental evidence as to the type of sulfur combination in vulcanization is noticeably lacking. However, the existence of more than one type of combination finds additional support in the work of Midgley (16) and Williams (28) who fractionated vulcanized rubber into portions of varying sulfur content. The fact that the type of sulfur combination is evidently controlled by the accelerator present can be explained on the basis of the relative catalytic control over one or more of the reactions taking place. For example, accelerators having a high ratio of atoms of sulfur combined per double bond lost might favor the dehydrogenation reaction in preference to simple addition or bridging.
Conclusions
0
0.5
1.0
1.5
2.0
2.5
3.0
0.1
0.2
0.3
0.4
0.5
0.6
94 93
Direct addition or bridging of sulfur seems to occur a t the double bonds of the rubber in sulfur vulcanization without accelerators (Figure 5, a and b). In accelerated stocks the fact that sulfur combines in excess of one atom of sulfur saturating one double bond of the rubber hydrocarbon suggests that sulfur may also combine in a dehydrogenation reaction
92
91
0
COMBINED SULFUR (GMS I 1 0 0 GMS RUBBER1
FIGURB 4.
U
vti. S, AND
H,
FOR
J
STOCKSG, I
-c-s-c-
these types would produce a ratio of 1 atom of sulfur per double bond lost, as was found in the plain rubber-sulfur compound. If this ratio is to be greater than 1,a n additional reaction must be taking place where sulfur can combine without loss of unsaturation. Such a possibility would be, for instance, a dehydrogenation reaction (Figure 5, c). Although hydrogen sulfide is known to be given off during vulcanization (17)of pure rubber-sulfur compounds, no such data exist for accelerated stocks. I n the present case it is not likely that much hydrogen sulfide was lost during the cure since the total sulfur remained practically constant in all .stocks. If any dehydrogenation did occur, the hydrogen .sulfide must have remained in the sample for the most part. It is possible that i t could be oxidized back to free sulfur available for further vulcanization, but such a reaction would require enough oxygen to remove all the displaced hydrogen as water. If 1 gram of sulfur per 100 grams of rubber were combined in a dehydrogenation reaction, the oxygen required to combiiie with the displaced hydrogen would be only 10.5 gram. Although this is more than the free oxygen dissolved in pure gum rubber [according to Williams and Neal ('7') dissolved oxygen is about 0.02 per cent for normal conditions], oxygen may have entered in during milling to form peroxides or other agents which might attack the hydrogen sulfide formed. [Cramer (7) recently found an increase in oxygen content of 0.12 per cent after %%minute milling.] A further possibility is the absorption of oxygen by the rubber buring cure in the molds. It is well known that stocks will blister due to gas formation when not enough pressure is applied during cure, but in a press cure it is hard to predict just what happens to possible gaseous by-products. Reactions of hydrogen sulfide with unsaturated hydrocarbons do not seem to have been studied much in the literature, so that possible reactions of this type cannot be predicted. Another possibility in the case of thiuram disulfide
I
c-c-s-c-c I I I @ l C
I
c-s-c c-c
II
1
II
c-c
+
H2S
' 0 ' I
I
I
I
-c-c-
c-c-c-c
'01 FIGURE 5. REACTIONS IN VULCANIZATION
(Figure 5, c), and that this type of sulfur linkage is most effective in producing high tensile strengths. The excess loss in unsaturation on overcure indicates that direct polymerization without sulfur (Figure 5 , d), as well as reactions of oxygen a t the double bonds, may occur after most of the sulfur has combined, but that these chemical changes are not definitely associated with changes in the vulcanized structure as measured by tensile strength. I n the vulcanization of a simple rubber-sulfur compound, infrared absorption data indicate that oxygen is combining with the rubber during overcure to produce a degradation in the hydrocarbon structure which is accompanied by a decided loss in tensile strength.
Acknowledgment The authors would like to express their appreciation to the
R. T. Vanderbilt Company for the preparation and physical testing of the vulcanized samples used in this investigation, and to R. B. Barnes of the American Cyanamid Company for carrying out the infrared absorption measurements.
OCTOBER, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
Literature Cited Bauer, F. W., Ber., 37, 3128 (1904). Bedford, C. W., and Gray, H., IND.EXG.CHEM.,15, 720 (1923). Brazier, S. A,, and Ridgway, L. R., J. SOC.Chem. Ind., 47, 351T (1928). Brown, J. R.,and Hauser, E. A,, IND. ENG. CHEM.,30, 1291 ( X938\. B&se: W.F., Ibid., 24, 140 (1932). Cotton, F. H., Trans. Inst. Rubber Ind., 6, 501 (1931). Cramer, H. I., private communication, Sept. 17, 1938. Denighs, G., Compt. rend., 108, 350 (1889). Faragher, W. F.,Morrell, J. C., and Monroe, G. S., IND.ENQ. CHEM..19. 1281 (1927). Grote, I . 'W.,'AnaZyst, 56,' 760 (1931). Gutmann, A., Ber., 56, 2365 (1923). Liebermann, C., Ibid., 20, 3231 (1887).
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(13) Lindsly, C. H., IND.ENQ.CHEM.,Anal. Ed., 8 , 179 (1936). (14) Meyer, K. H., and Hohenemser, W., Helv. Chim. Acta, 18, 1061 (1935). (15) Midgley, T., Heme, A. L., Shepard, A. F., and Renoll, M., J. Am. Chem. SOC.,56, 1325 (1934). (16) Rheinboldt, H . , Ber., 60, 184 (1927). (17) Rossem, A. van, India Rubber J . , 92, 845 (1936). 119'1 Rossem, A. van, and Dekker, P., Ibid., 95, 682 (1938). Stair, R., and Coblentz, W. W., Bur. Standards J . Research, 15 295 (1936). (20) Staudinger, H., India Rubber J . , 95, 646 (1938). (21) Stevens, H. P., AnaZyst, 40, 275 (1915). (22) Williams, I., India Rubber J. (London Conf. Suppl.). May, 1938.
i;;;
PRESENTEDbefore the North Jersey Section of the American Chemical Society.
Cottonseed Hulls as an Industrial Raw Material D. M. MUSSER
AND
R. F. NICKERSON
Mellon Institute, Pittsburgh, Penna.
I
N An' average year the United States produces about a million and a quarter tons of cottonseed hulls as a byproduct of the cotton crop. During the past few years unusually large crops have increased that figure. Although the utilization of agricultural products in general as industrial raw materials has been promoted extensively in recent, years, the possible uses of cottonseed hulls have received but scant attention. As one part of a broad research program to develop new uses for the cotton plant, the available literature and all other sources of information on this low-priced waste product have been searched for chemical facts and ideas regarding practical applications. In collating this knowledge, we have maintained a constantly critical attitude.
Production The hull of the cottonseed is the horny, external covering which encloses the seed kernel or meat. During the growth season of the plant the hulls contain the cells which give rise to the cotton fibers or hairs. After the cotton has been picked, the seeds are ginned out of the lint, but in most American varieties the ginned seeds are covered with an adherent fuzz consisting mostly of short fibers. Subsequent processing by delinting machines removes the greater part of this fuzz, which is then marketed as cotton linters. The hard, dark brown hull is cracked off and removed from the kernels in a series of operations. It is frequent practice to pass the hulls so obtained through a beater, which breaks them down to hull bran and facilitates the removal of hull fibers, the very short cotton which escaped the delinting processes. Several methods have been reported for removing the fibers from the hull bran. Dorner (9)obtained a patent on a process for powdering the hulls and separating them from the hull fibers with an air blast. Chetverikov (7) effected the separation by boiling the hulls for several hours with a solution of sodium hydroxide. Kao and Yu (26) separated the linthull mixture by the action of hydrogen chloride gas. More
recently Earle (10) patented a method employing froth flotation for dividing the fibers and hulls. As cotton is picked, the cottonseeds represent about two thirds by weight of the crop, or about a half ton of seeds for each bale of lint. Harrington (19) found the ratio of hulls to meats in nine common varieties to be approximately 1 to 1, with very little variation. Thus the hulls constitute approximately one third by weight of the total crop. Woolrich and Carpenter (66) give an excellent discussion of the production and processing of cottonseed.
Chemical Composition Harrington (19) reported the following data on the composition of hulls from seventy-three varieties of cotton: Water Protein Fat N-Free Ext.a Fiber Ash 41.2 2.4 8.18 1.62 0.12 34.8 Minimum, % 9.97 5.18 1.05 42.1 47.9 2.8 Maximum, yo a Nitrogen-free extract ie for the most part pentosans. Exoept for some minor variations these data agree with the results compiled by MoBryde (51).
Analytical determinations on cottonseed hull bran have been carried out in our study and have been treated in detail elsewhere (36). The standard methods for wood technology have been applied in the analyses for the following constituents: cellulose by the procedure of Cross and Bevan (8); lignin by the method of Ritter, Seborg, and Mitchell (43); furfural, pentose, and pentosans according to the standard A. 0. A. C. procedures (4); hydrolysis number by the method of Hawley and Fleck (20);and methoxyl, acetic acid, ethersoluble material, one per cent sodium hydroxide-soluble material, cold-water soluble material, hot-water soluble material, ash, and moisture as suggested by Schorger (46). The hull bran analyzed was first freed as completely as possible from hull fibers and then ground to 40 mesh. Samples of this material were then used for the following determinations, given in per cent on an oven-dry basis:
