Integrated analytical investigation of surface discoloration and blooming in <scp>NBR</scp> ‐based Rocasin insulation for solid rocket motor applications

Abstract Surface discoloration and blooming in nitrile–butadiene rubber (NBR)‐based rocket case insulation (Rocasin) used for solid rocket motors were investigated through a systematic industrial‐scale analytical study. Comparative evaluation of affected and reference insulation batches revealed that zinc oxide with calcium contamination promotes accelerator migration leading to a change in vulcanization kinetics and hence discoloration. Elemental analysis confirmed ca 5 wt% calcium impurity (in the form of carbonate) was present in zinc oxide used in the discolored Rocasin. Partial substitution of Zn 2+ sites with Ca 2+ and formation of less reactive calcium–stearate intermediates are proposed to retard sulfur crosslink precursor generation. Rheological analysis and differential scanning calorimetry demonstrated delayed cure kinetics and extended scorch safety in these formulations. Spectroscopic analysis showed surface enrichment of mercaptobenzothiazole disulfide while mechanical and thermal characterization confirmed that structural integrity remained unaffected. The discoloration phenomenon observed in the affected batches was therefore identified as a surface localized process driven by impurity‐induced modification of accelerator activation. Industrial validation using high‐purity zinc oxide restored normal cure behavior and eliminated discoloration. This study provides the first industrial‐scale impurity–process–performance correlation in Rocasin and highlights the critical importance of raw material purity in ensuring process stability and surface quality for these insulation systems. © 2026 Society of Chemical Industry.