The autoxidation of alkenyl succinimides - Mimics for polyisobutenyl succinimide dispersants

Short chain alkenyl succinimides (ASIs) were synthesised and used as high purity chemical models to investigate the autoxidative degradation at 170 oC of polyisobutenyl succinimide dispersants (PIBSIs), a significant additive in automotive engine lubricants. Degradation products were characterised by GC-EI-MS and quantified by GC-FID. The rate of autoxidation of ASIs in a model lubricant, squalane, was also investigated. Although this is a complex molecule containing many possible sites of radical attack, all of the autoxidation products identified result from attack at the double bond or the adjacent allylic hydrogen atoms, which indicates the controlling influence of the double bond in the degradation of alkenyl succinimides, and therefore of commercial polyisobutenyl succinimide dispersants. The observed site-selective cleavage of the ASI structure, and by analogy PIBSI dispersants, would yield products that both reduce dispersancy and promote the formation of insoluble products that could have a detrimental effect on lubricant performance.