INDUSTRIAL AND ENGINEERING CHEMISTRY
2448
Vol. 44, No. 10
I 0.2 0.4 0.6 SULFIDE SULFUR, GRAMS/100 GRAMS OF RUBBER
Figure 8. Effect of Accelerator Content on Relationship between Retractive Force and Sulfide Sulfur Compounds contain sulfur 0.50 8 , 1.00 0 or 1.50 8 zinc oxide 10.0, lauric acid 10.0, and variable amount: of hlBT p& 100 grams of rubber. Cured 64 hours at 110' C.
froni linearity occur, particularly a t the higher temperatures and toward the end of the vulcanization reaction. Insufficient data are available a t this time t o indicate the relationship between sulfide sulfur and retractive force for coinpounds vulcanized a t temperatuie>beloTv 90" C. The fe\T points given in Table I indicate that a t curing temperatures below 90 C. a larger amount of zinc sulfide is formed for a given number of cross links than is formed a t 90" C. The linear relationship which exists between retractive force and sulfide sulfur under the temperature conditions which lead to the maximum yield of cross links suggests that under these conditions the formation of sulfide sulfur is closely associated with the formation of cross links. Under optimum conditions this relationship may even be stoichiometric. However, because the retractive force for a given amount of sulfide sulfur increases viith decreasing temperature of vulcanization from 110" t o 90" C., it is difficult to determine whether a one t o one relationship is approached a t 90" C. or xhether a one to one relationship is obtained a t a higher temperature-e.g., 110" C. In the latter case more than one cross link would be formed per atom of sulfide sulfur a t the lower temperatures of vulcanization. This problem will be considered in greater detail in a later publication. EFFECT O F LAURIC ACID, Z I N C OXIDE, AND 2-MERCBPTOBEKZOTHIAZOLE COSCENTRATIOS. I n Figure 3 the yield of cross links during vulcanization is shown t o be a function of the lauric acid concentration. The relationship between sulfide sulfur and retractive force for these compounds is shown in Figure 7 . Even though variation in fatty acid concentration gives wide variations in the stress a t 200% elongation, sulfide sulfur appears t o be in all cases closely associated with the formation of cross links. All the compounds were cured 64 hours at 100" C. The relationship between sulfide sulfur and retractive force in stocks containing a given amount of lauric acid and variable zinc oxide concentration is confused because of the softening action of free fatty acid. As a result the effect of zinc oxide concentration on sulfide sulfur formation \Till not be discussed. The effect of accelerator content on the relationship between sulfide sulfur and retractive force is shown in Figure 8. These data indicate that although there is a measure of correlation between sulfide sulfur and retractive force, it is not close. This conclusion is in general agreement with that of Hull, Olsen, and France (4),who found that although retractive force increased with increasing concentration of accelerator, no change occurred in the formation of sulfide sulfur. EFFECT OF SULFUR CONCENTRATION. In Figure 6, it is shown that, when sufficient zinc oxide, lauric acid, and 2-mercaptobenxothiazole are present to give a maximum yield of cross links, a
0.1
0.2
0.3
0.4
SULFIDE SULFUR, GRAMSJIOO GRAMS OF RUBBER
Figure 9. Effect of Sulfur Concentration on Relationship between Retractive Force and Sulfide Sulfur Compounds contain variable amounts of sulfur, zinc oxide 10.0, lauric acid 10.0, and MBT 2.0 parts per 100 grams of rubber and were cured 64 hours a t 100' C. Refer t o Table 111. 0 Compounds contain sulfur 1.0, zinc oxide 10.0, lauric acid 10.0. and MBT 2.0 parts per 100 grams-of rubber and were cured variable times at 100' C. Refer to Table I.
linear relationship exists between stress a t 200% elongation arid the sulfur added to the compound. The variable stress a t 200% elongation recorded in Table I11 is a result of variable sulfur content. Time of vulcanization is the same for all. Variable retractive force has also been obtained for a compound of given sulfur content by variation in time of vulcanization. These data are recorded in Tables I and 11. The relationship between sulfide sulfur and retractive force for the two series of compounds cured a t 100' C. is shown in Figure 9. It is apparent that when sufficient quantities of vulcanization reactants are present and 1% hen vulcanization is carried out a t a temperature as lovi- as 100 O C., the linear relationship between sulfide sulfur and retractive force is independent of either time of cure or initial concentration of sulfur. No conclusion can be drawn a t this time as t o u-hether a niolecule of zinc sulfide is formed for each cross link formed during vulcanization but there is strong circumstantial evidence that this may be nearly the case n hen a maximum yield of cross links is obtained. LITERATURE CITED
(1) Armstrong, R. T., Little, J. R., a n d Doak, K. W., IKD.ESG. CHEM., 36, 628 (1944). (2) Farmer. E. H., "Advances i n Colloid Science," Vol. 11, p. 355, New York, Interscience Publishers, 1946. (3) Flory, P. J., Chem. Rem., 35, 63 (1944). (4) Hull, C . M., Olsen, S.R., and France, VV. G., 1x0. EYQ.CHCM., 38, 1282 (1946). ( 5 ) Wall, F. T., J . Chem. Phus., 10, 132, 485 (1942).
RECEIVED for review February 14, 1952. ACCEPTED hIaY 1 6 , 1962. Contribution No. 120 from the General Laboratories of the U. S.Rubber Co.. Passaic, N. J.
Correction In the article "Solid-Liquid Extraction Calculations" [J. A. Grosberg, IXD.ESG.CHEX.,42, 154-62 (1950)], the ordinate of Figure 2 should be w instead of 1/w. J. 8.GROSBERG 12 FRANCES ST.,YEOVILLE
J O H A X S E S B U R G , SOCTH
AFRICA
