Effect of phosphorus based flame retardants on UL94 and Comparative Tracking Index properties of poly(butylene terephthalate)

Poly(butylene terephthalate) (PBT) is widely used for electrical and electronic devices as insulating material. For these applications PBT has to combine both good flammability and electrical properties (tracking resistance) measured, respectively, by UL94 and Comparative Tracking Index (CTI) tests. In this paper a simultaneous study of UL94 and CTI performances has been conducted on different PBT formulations with the addition of halogen-free flame retardants (diethylphosphinic acid aluminium salt (AlPi) and melamine polyphosphate (MPP)). A thermosetting resin (Ultem) has also been added. FTIR and TGA analyses have been used in order to relate the action of these additives to flammability and electrical properties. AlPi shows the best flame results achieving a V0 classification in UL94 test. On the other hand, melamine has the best performances in CTI tests. TGA and FTIR analyses show that AlPi acts in the condensed phase forming a consistent amount of char, while melamine acts in the gas phase and is not able to protect PBT during combustion.