Novel reactive flame-retardant coating prepared from waste polyethylene terephthalate, phenylphosphonic acid, and novolac glycidyl ether

As global production and consumption of poly(ethylene terephthalate) (PET) becomes higher, so does the amount of waste PET. Several methods of waste PET reduction like mechanical and chemical recycling have been investigated. In this report, we selected the new chemical recycling of PET by transesterification reaction with adipic acid to regain terephthalic acid and oligo(ethylene-adipate-co-terephthalate) (OEAT). Terephthalic acid can also be recovered from PET by hydrolysis in the presence of acid or base catalysts at high temperature and pressure, then sophisticated equipment is required. The advantage of the transesterification over hydrolysis process is performing at atmospheric pressure. OEAT contains both aliphatic and aromatic units and reactive carboxylic acid end groups contributing to the flexibility and thermal stability of the prepared polymer. By ¹H NMR characterization, the molar percentages of adipate and terephthalate units in OEAT were 59.4 % and 40.6 %, respectively. The reactions of OEAT and phenyl phosphonic acid with novolac glycidyl ether have been carefully investigated separately or as mixture. The combination of OEAT and phenyl phosphonic acid enhanced the flame retardancy of the obtained coating. Structures of the prepared polymers were confirmed by FTIR and their thermal properties were investigated by DSC-TGA. A simple dicarboxylic acid (adipic acid) was used in place of OEAT for comparison. The cured coating showed higher gel content, higher flexural ultimate stress, strain at break and modulus. However, the flame test of this sample failed. Consequently, both aromatic terephthalate units of OEAT and phosphonate group of PPA contribute to the flame retardancy of the coating.