As the third component, vulcanizing agent is used to assist crosslinking agent and accelerant. Their function is to give the compound acceptable processability and ensure the vulcanizate to obtain the best performance.
In sulfur / accelerant curing system, zinc oxide must be added. The zinc oxide used has many different particle sizes, purity and effectiveness. If the amount of zinc oxide is not enough, not only the physical and mechanical properties (modulus, compression) are damaged, but also the anti reversion performance is poor. With the increase of zinc oxide content, the anti reversion property increased rapidly at first, but then increased very slowly. In NR (conventional sulfur dosage + subsulfonamide accelerant), when zinc oxide is more than 4-5 phr, the resistance to reversion will not increase.
Because zinc oxide is insoluble in rubber, its activity cannot be fully exerted when it is used alone. Instead, zinc soap (complex of stearic acid and zinc) which can dissolve in rubber must be used together with stearic acid to participate in vulcanization reaction.
Stearic acid can activate the double bond of rubber molecules and accelerate the formation of cross-linking bond. In some curing systems, it can improve the cross-linking density of vulcanizate. In the basic promoters or their combined promoters, they usually delay the sulfur point. In addition, stearic acid can improve the dispersion effect of filler, and also reduce the tendency of sticking roller. If the rubber is filled with a large amount of reinforcing carbon black, the amount of stearic acid can be up to 3 parts to help dispersion. However, when the stearic acid content is high, the adhesion between rubber and metal will be weakened.
It is worth noting that the solubility of zinc stearate produced by zinc oxide and stearic acid during vulcanization is relatively small in cis-1,4-polybutadiene rubber, so the excessive use of stearic acid in the compound mixed with cis-1,4-polybutadiene rubber will cause frost spray.
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