Vulcanization of Elastomers. 19. The Function of Oxides in Thiuram Vulcanization

Abstract

Abstract This article is a continuation of our discussion of the role of oxides in thiuram vulcanization. The following additional results were obtained: The reaction of TETD and ZnO gives off gaseous products and is pressure dependent. By taking this into account, both the conductometric and the spectrophotometric methods of analysis furnish identical results for the DEDC yields as a function of reaction times. The reaction of TETD with Bi2O3 produces in the presence or absence of NR (independent of the temperature) approximately 82 mole per cent BiDEDC based on the initial amount of TETD. However, no vulcanization takes place. The reaction between TETD and TeO2 produces in the presence or absenee of NR (independent of the temperature) a maximum yield of Te-II-DEDC of 60 to 70 mole per cent based on the initial amount of TETD. Such a yield of DEDC is also observed during a TETD vulcanization with ZnO. Nevertheless, no normal thiuram vulcanization takes place when TETD, TeO2 and NR are heated together. When Te-IV-DEDC is dissolved in various organic solvents, it undergoes disproportionation and equimolar amounts of Te-II-DEDC and TETD are formed. The limiting value of the Te-II-DEDC formation of 60 to 70 mole per cent during the reaction of TETD with TeO5 in the presence or absence of NR can be explained by a primary formation of Te-IV-DEDC in a maximum yield of 80 mole per cent followed by a disproportionation of the Te-IV-DEDC (the same as with a corresponding reaction in the presence of Bi2O3). Of course, it is assumed that the metal oxides during their reaction with TETD give maximum yields of 80 to 90 mole per cent DEDC based on the initial amount of TETD. Furthermore, the experiments showed that thiuram vulcanization obviously is due to the reaction of TETD with ZnO in which NR participates with formation of crosslinks. This concept is supported by the identical activation energies of the TETD decrease and DEDC increase during the reaction of ZnO with TETD in the presence and absence of NR.