Fire Retardance in Poly(butylene terephthalate). The Effects of Red Phosphorus and Radiation‐Induced Cross‐Links

The combustion performance of poly(butylene terephthalate) (PBT) can improved by the addition of red phosphorus provided it is intermolecularly cross-linked upon irradiation with 60 Co γ-rays in the presence of triallyl cyanurate (TAC). At a content of 3 or 4 wt.-% the latter significantly promotes cross-linking in the presence of air. From combustion tests with samples containing red phosphorus (P red ) and having been γ-irradiated in the presence of TAC it turned out that an improved fire resistance of PBT is achieved if the red phosphorus content is at least 12.5 wt.-%. In this case test samples were self-extinguishing and the UL 94 rating corresponded to V-1. Product analysis and thermal gravimetric analysis revealed that P red stimulated aromatization and charring. These processes involve the reaction of P red with the polymer. 31 P NMR spectroscopy revealed that the residue contained chemically bonded phosphorus.