Stability of phenolic antioxidants in the presence of sulfonic acid: Model compound studies for moisture-crosslinked polyethylene

Alkoxysilane-functionalized polyethylene can be crosslinked in the presence of acid catalysts, including Brønsted acids, to yield durable pipe and cable insulation materials. However, Brønsted acids can also react with additives present in the formulation such as phenolic antioxidants, resulting in reduced additive efficacy and undesired reaction products. To provide guidelines for the selection of stable phenolic antioxidants in the presence of Brønsted acids, reactions of 4 model alkylphenol compounds in the presence of p-dodecylbenzenesulfonic acid (DBSA) in dodecane were studied as a solution model for polyethylene-based systems. The results point to increasing stability towards DBSA of phenolic compounds when: (i) alkyl groups have less substitution (tertiary carbon compared to quaternary carbon), (ii) they have alkyl-substitution in the para position, and (iii) they have a reduced number of alkyl substituents around the phenol ring. To confirm the trends observed with the model compounds, commercially available antioxidants were also evaluated under the same conditions. Antioxidants with alkyl or isocyanurate bridges had reaction rates comparable to butylated hydroxytoluene (BHT) and followed the trend outlined in model alkylphenols with regards to alkyl group substitution. By comparison, the sulfur-bridged antioxidant stood out as rapidly consumed under the reaction conditions. Therefore, selection of phenolic antioxidants for formulations containing strong Brønsted acids should generally favor phenolic compounds with alkyl-substituted para positions and limited number of alkyl substituents in non-para positions, and without quaternary carbon substituents. Moreover, stability of alternate sulfur-containing antioxidants should be assessed if used with sulfonic acids, given the rapid decomposition of the sulfur-bridged antioxidant observed in this work.