Persistent Free Radicals (PFRs) Formed in Pyrolyzed Synthetic CHO-Type Polymers: Novel Raw Material for PFR and Better Understanding for Its Formation

Abstract

Three types of synthetic polymer; polyvinyl alcohol (PVA), polyethylene terephthalate (PET), and poly ether ether ketone (PEEK) were pyrolyzed to investigate (1) if persistent free radicals (PFRs) can be obtained, and (2) the effect of polymer type on PFR density. Cellulose, a representative natural polymer, was used for comparison. All the four types of polymers gave PFRs, which were quantified by electron paramagnetic resonance (EPR). For achieving substantial pyrolysis, PVA, PET, PEEK and cellulose were pyrolyzed at 773 K, 873 K, 973 K and 773 K, respectively. All the polymers gave PFRs, and the order of the PFR density observed was cellulose ≅ PVA > PET > PEEK: inverse trend of the pyrolysis temperature. The most likely explanation for the trend above can be found in the well-known fact that PFR density obtained from biomass pyrolysis increases with pyrolysis temperature first, then decreases with temperature at higher temperatures. Since carbonization and PFR formation occurred in parallel with progressive pyrolysis, the PFR formation mechanism may be common among these four polymers: (1) radical formation by main chain scission, and (2) radical stabilization by aromatics and/or polyenes in the char.

Keywords

• Persistent free radicals
• Pyrolysis of polymers
• PVA
• PET
• PEEK