Effect of Accelerator Type on Brass-Rubber Adhesion

Abstract

Abstract A natural rubber compound accelerated with tetramethylthiuram disulfide (TMTD) was studied and compared to an N-oxydiethylene-2-benzothiazolesulfenamide (MBS) accelerated compound in order to determine the factors resulting in the poor adhesion to brass of the former. Adhesion to brass-plated steel cord was measured by using the TCAT pull-out test. Vulcanization time and temperature were varied. Also, both compounds were analyzed for crosslink density and type at different states of cure by selective cleavage of the crosslinks with thiol reagents. The poor adhesion to brass of the TMTD compound was not accounted for by cure rate or crosslink density or crosslink type. Synchronization of the rubber cure rate with the sulfidation rate of the brass surface appeared to be unimportant in determining the brass adhesion characteristics of that compound. This was investigated by testing the adhesion of the TMTD compound to brass plated steel cords of different reactivities. Cords were analyzed for copper sulfide formation by an energy dispersive x-ray (EDX) technique after pull-out and also after immersion in squalene solutions containing each accelerator with and without added free sulfur. This analysis showed that there is significant sulfidation of brass in the presence of TMTD accelerator. Atomic absorption analyses of the squalene solutions, after the cord immersions, showed relatively large amounts of copper and zinc present in the TMTD solutions as compared to the MBS solutions. This indicates that TMTD corrodes brass to a much greater extent than the MBS accelerator. From the data presented, the following conclusions can be drawn: (1) The poor adhesion to brass of TMTD accelerated compounds is not related to: (a) their rapid cure rates, (b) a lack of synchronization between cure rate and sulfidation rate, or (c) crosslink density or type. This implies that a deficiency of physical entanglements of rubber crosslinks within the copper sulfide layer is not a reason for the poor adhesion to brass of these compounds. (2) Considerable sulfidation of brass occurs in squalene solutions with TMTD indicating that lack of sulfur availability for brass sulfidation is not a valid explanation for the poor brass adhesion of TMTD compounds. Contrarily, in fact, the data suggest that excessive sulfidation of brass occurs in the presence of TMTD. (3) The corrosive nature of TMTD toward brass has been documented for cords dipped in TMTD/squalene solutions. This characteristic of TMTD likely results in a copper sulfide layer on the brass which is porous and mechanically weak. It is proposed that this direct interaction of TMTD with brass provides an explanation for the poor adhesion to brass of TMTD accelerated compounds. The mechanism by which the TMTD causes the very rapid removal of the brass into solution is not known at this time. Perhaps the TMTD or its decomposition fragments form relatively stable complexes with copper and zinc such that their removal is favored.