Pyrimidine derivatives as hardness stabilizers

A vulcanizable composition comprising a sulfur vulcanizable SBR rubber, a sulfur vulcanizing agent, an accelerating agent selected from the group consisting of sulfenamide accelerators, other than thiazole sulfenamides, and a hardness stabilization agent comprising a pyrimidine derivative of the formula:

• Where X=H, R1 through R4, NR3R4, OR5, SR5, SO2R6, M, (SO3)zM (M=metal ion), and n and z can be the same or different and 1, 2 or 3, depending on whether the respective valence of X and M is 1, 2 or 3, R1 through R4 are the same or different and selected from the group consisting of the substituents H, halogen, OH, NH2, alkyl, cycloalkyl, aryl, alkylaryl, aralkyl, with the substituents alkyl, cycloalkyl, aryl, alkylaryl and aralkyl optionally having additional functional groups selected from the group consisting of NH2, OH, substituted amino, substituted hydroxyl, halogen, and carbonyl containing group, when R3 and/or R4 are one of the substituents alkyl, cycloalkyl, aryl, alkylaryl and aralkyl, they may be in the same constituent in a ring together with N to form a heterocyclic group, R5 is selected from the group consisting of the substituents H, alkyl, cycloalkyl, aryl, alkylaryl and aralkyl, or a radical derived from a carbon based heterocyclic group containing at least one of S or N, or both S and N, R6 is selected from the group consisting of the substituents alkyl, cycloalkyl, aryl, alkylaryl, aralkyl, H, OH, OM, NH2, NR3R4, and OR5, the amount of accelerating agent in said composition being greater than about 0.6 phr, and the amount of hardness stabilization agent being at least about 0.5 phr.