Impact of antioxidant addition on the thermal decomposition behavior of polyisoprene rubber during low-temperature pyrolysis

The pyrolysis of used rubber is a promising approach for resource recovery; however, the selective recovery of isoprene remains a challenge due to extensive thermal decomposition and secondary reactions. This study proposes a co-pyrolysis process aimed at enhancing the retention of the isoprene skeleton in the recovered liquid rubber during low-temperature pyrolysis. A cross-linked polyisoprene rubber reinforced with carbon black was co-pyrolyzed with phenolic antioxidants, namely tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane, denoted as AO-1010, and n-octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, denoted as AO-1076, to inhibit radical-induced structural degradation. The isoprene skeleton retention rate decreased with increasing pyrolysis temperature for all samples. However, antioxidant-treated rubbers (AO-1010 and AO-1076) showed higher retention at 300 °C compared to untreated rubber, indicating that antioxidants can suppress thermal degradation and preserve the isoprene structure at elevated temperatures. For untreated rubber, the isoprene recovery rate increased from approximately 12% at 200 °C to a peak of 53% at 270 °C, followed by a sharp decline to 34% at 300 °C, suggesting intensified secondary reactions at higher temperatures. In contrast, rubbers treated with antioxidants AO-1010 and AO-1076 exhibited improved stability at elevated temperatures. At 300 °C, the AO-1010- and AO-1076-treated rubbers maintained higher isoprene skeleton recovery rates of 50% and 47%, respectively. Additionally, antioxidant treatments significantly reduced the molecular weight of the rubber from Mw 340,000 to approximately 35,000 at 270 °C. These findings demonstrate that co-pyrolysis with antioxidants can better preserve the isoprene skeleton and increase liquid product yields at low temperatures. This approach offers a promising strategy for improving the efficiency and selectivity of isoprene recovery. • Co-pyrolysis with AO-1010/1076 aids to maintain isoprene skeleton in liquid rubber. • TG/DTG shows AO-1010 degrades in steps, AO-1076 in a single stage. • AO-1010/1076-treated rubber retained 84% and 77% isoprene skeleton, respectively. • A high antioxidant ratio reduced isoprene retention due to excessive radical scavenging. • A 15:1 rubber/AO ratio maintains isoprene-like untreated polyisoprene rubber.