December, 1931
INDUSTRIAL AiYD ENGINEERING CHEMISTRY
rate of cure and tensile properties of the compounds, as illustrated in Figure 8. Presumably the accelerator, or more specifically the reaction product of the accelerator, dimethyldithiocarbamic acid or a polysulfide of it, is in equilibrium with zinc and lead as their corresponding dimethyldithiocarbamates. With increments of zinc oxide, the concentration of zinc dimethyldithiocarbamate is increased by mass action and the fast cure of the zinc derivative becomes effective, until in high-volume zinc oxide stocks (30 to 40 volumes) a large amount of the dimethyldithiocarbamic acid is present as the zinc salt, and ultimately tensile values are obtained which approach those of a stock containing only the zinc accelerator. The mass action mechanism whereby increments of zinc oxide in a low-litharge stock have increased the rate of cure with tetramethylthiuram monosulfide is also demonstrated in Figure 9, which shows that equal volumes of zinc oxides of increasingly fine particle size increase the rate of cure by virtue of the greater amount of zinc oxide surface available for influencing the equilibrium. Order of Adding Ingredients
The data that have been shown were obtained on stocks in which the accelerator was milled in thoroughly before the addition of pigment. The zinc oxide was added after the accelerator, and the sulfur last. It was found necessary to standardize this procedure, especially for stocks containing free lead oxide, since the order of mixing makes a considerable difference, as shown by the results in Figure 10. When the lead content was lowered, the order of mixing had less effect, as shown in Figure 11. I n order to eliminate the mechanical effect of mixing on the dispersion of the accelerator, tests were made in which the time of mixing after the addition of the accelerator was the same, but this appeared to be a secondary consideration. Since the accelerator required only a fraction
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of a minute to add, the time that the accelerator and pigment were mixed together was also practically constant. The explanation of the variability in rate of cure with different orders of adding the ingredients is the same as has been advanced throughout this paper-namely, the solubility of the lead compound of the accelerator. When lead oxide is mixed into rubber it reacts with the fatty acid present and forms lead soaps. These lead soaps migrate to particles of the thiuram accelerator and react with them, forming insoluble coatings around the accelerator particles. When rubber deficient in fatty acid content is used instead of normal rubber, relatively less lead goes into solution and the accelerator disperses satisfactorily even when added after the oxide, giving a curing curve which compares favorably with the curve for the accelerator added first, using rubber, as shown in Figure 10. Effect of Soaps on Set-Up Cure
The addition of stearic acid exaggerates the effect by enabling the formation of more soluble lead and thereby coating the thiuram particles more thoroughly (Figure 12). The larger amount of soluble lead, when stearic acid is added, absorbs more hydrogen sulfide and thereby develops a delayed cure, but the order of adding the ingredients becomes increasingly important. The large number of variables that affect the cure when the thiurams are used makes it more difficult to obtain laboratory check tests using these materials than with many other accelerators, and work that is done in this field must be carried out under carefully standardized conditions. Literature Cited (1) Bedford, C. W., and Sebrell, L. B., J. IND.ENG.CHEX.,14, 25 (1922). (2) “Du Pont Chemicals for the Rubber Industry,” Bulletin No. 9, E. I. du Pont de Nemours & Company. (3) Maximoff, A., Caoutchouc 6’gutfa-oercha, is, 18 (1921).
Behavior of Rubber under Repeated Stresses1’* W. L. Holt BUREAUOF STANDARDS, WASHINGTON, D. C.
This paper describes a simple and convenient apHIS p a p e r describes been stretched several hunparatus for obtaining a graphical record of the tensile some of the effects of dred per cent and caused to properties of rubber under a variety of conditions of repeated stressing on vibrate while under tension. stressing. Data are given showing the effect of rethe tensile properties of rubAfter this treatment his data peated stretching and the speed of stretching on the ber. The fact that the propshow t h a t s t r e s s - s t r a i n stress-strain properties of typical rubber compounds. erties of rubber are influenced curves, up to certain elongaThe recovery of rubber from strain is considered, and it by stretching or other met i o n s , c a n be o b t a i n e d is observed that complete recovery does not take place. chanical t r e a t m e n t is not w i t h o u t a p pr e c i a b 1e hysConclusions are drawn regarding the practical use of new. Data have been given teresis. stress-strain curves in evaluating rubber compounds. by Buasse and Carn’Bre (a), In the present paper some The present paper does not consider the retraction cycle S h e d d a n d I n g e r s o l (9), of the work of previous inof the stress-strain curve, nor the energy relations inS c h w a r t z (Y), B e a d l e and vestigators has been repeated, volved: No theoretical explanation is offered at this Stevens ( I ) , Scoble ( 8 ) , e x c e p t that m o d e r n comtime for phenomena described. Gurney and Travener (6),and pounds have been employed others. All of these authors and tests conducted at relapoint out that changes take place in the physical properties of tively high speeds. By the use of apparatus, which was derubber on stretching. Williams ( 1 1 ) has suggested stretching veloped and is described, extension and retraction curves can rubber samples a few times to reduce “plastic flow” on subse- be made autographically in less than one second, although for quent tests. Gerke (4) has described “equilibrium stress- most purposes a somewhat slower speed is recommended. Restrain curves” which were obtained from samples which had peated stress strain curves up to 100 or more successive cycles can be recorded with facility. The behavior of different types , 1 Received September 10, 1931. Presented before the Division of Rubber Chemistry at the 82nd Meeting of the American Chemical Society, ofl’ubber under repeated stress is illustrated in this paper, and the recovery of rubber from strain is considered. Buffalo, N. Y.,August 31 to September 4, 1931. Published by permission of the Director, U. S. Bureau of Standards. The way in which the stress-strain curve of a sample of
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