Thermal Depolymerization Challenges of PTFE:Silicone Rubber Mixtures and Composite Materials

Thermochemical depolymerization of polymer mixtures and composite materials is challenging due to non-additive degradation behavior and the emergence of new reaction pathways during pyrolysis. Both polytetrafluoroethylene (PTFE) and silicone rubber (SR) can depolymerize into monomers or low-molecular-weight oligomers when pyrolyzed individually, making them, in principle, suitable candidates for depolymerization-based recycling. Because they are frequently combined in technical applications and composites, their behavior during co-pyrolysis warrants investigation. However, the pyrolysis of PTFE:SR mixtures and composites remains poorly understood. In this study, we examine the pyrolysis behavior of PTFE:SR systems with emphasis on mass balance, product composition, and the formation of new species to address potential limitations for depolymerization-based recycling. Experiments were conducted on virgin PTFE and SR, defined polymer mixtures, and commercially relevant composites, including PTFE-lined silicone tubing and PTFE:SR septa. The results reveal a pronounced, non-linear dependence of product distribution on PTFE content. Product identification by GC-MS, NMR, and FTIR indicates cleavage of Si-O and Si-CH₃ bonds and the formation of fluorinated siloxanes as well as new per- and polyfluoroalkyl substances (PFAS). At low PTFE contents, liquid products are dominated by cyclic siloxanes (D_x). These findings show that depolymerization strategies developed for pure polymers cannot be directly applied to PTFE:SR composites. While systems with low PTFE content may be more amenable to depolymerization, higher PTFE fractions promote the formation of PFAS and difficult-to-valorize fluorinated silicon species. These products complicate selective monomer recovery and could pose significant challenges for the depolymerization-based recycling of PTFE-rich composites.