Abstract
The amount of various types of sulfur in gum mix accelerated with MBT were determined and the changes of these values with extension of cure were observed.
Preliminary study of the method which was originally used by Kambara and Ohkita (R. C. T. 25 209 (1952)) for the determination of thioketone-type combined sulfur in pure rubber-sulfur mix, using a H2O2-glacial acetic acid mixture (H2O2-HAc), was made as to its applicability to MBT accelerated compound. Experiments on the reactions of sulfer in compounds likely to be found in the vulcanizates, such as ZnS, MBT, MBTS, Zn salt of MBT, toward H2O2-HAC oxidation showed that the ZnS-type sulfur was completely converted and slightly more than one-half equivalent of the total sulfur in a mole of MBT-Zn salt was converted into sulfates, while MBT and MBTS were much less reactive. Considering these reactions and recent reports on the amount of MBT and its transformation products formed during vulcanization, the amount of sulfur within the value obtained by the H2O2-HAc oxidation after acetone extraction (SH2O2) due to the MBT originally added was negligible and consequently, the sulfur combined to rubber and removable by the H2O2-HAC treatment as sulfates, expressed as S2, may be given approximately as the difference of SH2O2 and ZnS-sulfur.
This Sα first increased and then decreased after passing through a maximum, which was about one-third of the total sulfur used in the mix, with extension of curing time. On the other hand, organically combined sulfur which was not removable by the oxidation treatment (Sβ) increased steadily with extention of cure, thereby, revealing that transformation of Sα, to Sβ occurred.
These changes of Sα and Sβ coincide with the case of pure rubber-sulfur vulcanizates observed by the forementioned workers.
As the mutual relation of changes in the amount of the different types of sulfur were identical for vulcanizates cured at 20, 40, and 60 psi steam pressure, irrespective of curing temperature, it seems that vulcanization reaction within this curing range were analogous in relation to sulfur. However, the re were no constant relation between Sβ and network density, derived from data on swelling, modulus or elongation at constant stress.
These latter results showed that higher network density was obtained at the lower curing temperature, revealing that thermal scission not involving sulfur changes occured at higher temperatures.
