adhesion accelerator

The use of adhesion accelerators and activators lowers the activation energy of vulcanization reaction to 80-125kJ/Mole from 210kJ/Mole which is necessary if we use ‘Sulphur’ alone. Adhesion accelerators and activators break sulphur chains. Accelerated sulphur vulcanization systems require only 5-15 sulphur atoms per cross-link as compared to 40-45 S atoms/crosslink for a non-accelerated sulphur vulcanization. There are many  adhesion  accelerators available for the vulcanization of rubber. That is because there is a wide range of rubber articles on the market with a wide variety of properties. For instance in a car tire alone there can be already up to eight different rubber compounds, each with specific properties. For instance the tread in a typical passenger car tire consists of a mixture of SBR (styrene-butadiene rubber) and BR (butadiene rubber). This rubber should have high abrasion resistance and high grip on both dry and wet roads. The side wall of the tire should have a high flexibility, meaning that it should resist many flexings during the running of the tire without cracking. It consists normally of a mixture of natural rubber and butadiene rubber. Inside the tire there is a rubber compound with as major function the adhesion between rubber and the steel cord of the belt. It typically consists of natural rubber with a very high sulfur level (up to 8 phr), to get a relatively stiff rubber, with sulfur promoting the adhesion with the steel cord. The basis of the tire is formed by the carcass, normally a mixture of NR (natural rubber), SBR and BR. It should have a very good adhesion to the polyester cord, used as reinforcement. And the inner side of the tire is formed by the inner liner, normally consisting of halogenated butyl rubber (IIR) For all these compounds with their different properties different
adhesion accelerators and mixtures of adhesion accelerators have to be used to obtain the required properties. A vulcanization  adhesion  accelerator is typically used in combination with sulfur as the cross-linker, and with zinc oxide and stearic acid as activators. Other additives can be added too, but for the cross-linking reaction the abovementioned ones are the most important. The various types of rubber used in the various tire compounds all have different vulcanization characteristics, like speed of cure (cure is the crosslinking reaction) and extent of cure (the number of cross-links). A typical passenger car tire is vulcanized for 10 minutes at 170 degrees C. This means that all the different compounds have to be cured to their optimum state of cure in this same 10 minutes. This is the reason why a lot of different adhesion accelerators or mixtures thereof are used in the same tire.