Influence of Curing Agents on Rubber-to-Textile and Rubber-to-Steel Cord Adhesion

Abstract

Abstract The results presented in this paper show that the dosage of sulfur greatly influences rubber-to-textile and rubber-to-steel cord adhesion. As far as rubber-to-textile adhesion is concerned, the pull-out strength is reduced when the sulfur level falls below 2 phr, the extent of the reduction depending on the type of fiber and the bonding process. With steel cord the lower limit for sulfur is as high as 3 phr. This is discouraging because it shows that the use of efficient and semi-efficient vulcanization in the manufacture of goods reinforced with fabrics or steel cord is, to say the least, highly problematical. On the other hand, it is often desirable to reduce the proportion of sulfur in order, for instance, to increase resistance to aging and reversion and to reduce compression set and heat build-up. The choice of accelerators also has an important influence on adhesion. A sufficiently long scorch time is required to allow complete wetting of the reinforcing material by the rubber compound. If curing starts before complete wetting of the textile or steel cord has occurred, adhesion cannot be built up. For example, unsatisfactory or no rubber-to-textile and rubber-to-steel cord adhesion is obtained, when ultra-accelerators are used. As the scorch time increases, adhesion improves. Best results are obtained with sulfenamides, but sulfenamides showing increasing scorch time give no improvement in rubber-to-textile bonding. However, DCBS gave much better results for rubber-to-steel cord adhesion than the other sulfenamides and thus appears particularly suitable. Although the results given by the other sulfenamides can be somewhat improved by the addition of a retarder, pull-out strength values do not reach those given by DCBS. Any decrease of scorch time of a compound deteriorates adhesion whether scorch time is reduced by compounding ingredients or by heat history imposed by processing. MBI, used as an antioxidant, which reduces scorch time drastically, causes complete loss of rubber-to-steel cord adhesion and also impairs rubber-to-textile adhesion. Most of the antioxidants, however, have a beneficial effect on mbber-to-steel cord adhesion. For zinc-plated steel cord antioxidants of the amine type give results inferior to those of the phenolic type. To obtain optimal rubber-to-textile and rubber-to-steel cord adhesion the compounder primarily has to consider the efficacy of the bonding agents but in addition he has to take into account the effects which may be exerted by compounding ingredients and processing conditions.