Phosphorus flame retardants have been considered to be ineffective for polycarbonates (PCs). In light of the recognition that the high efficiency of sulfonate salts is due to the base catalyzed decomposition of polycarbonate, a tertiary amine group is incorporated into a 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) derivative which was synthesized from DOPO, paraformaldehyde, and piperazine (DFPZ). DFPZ shows excellent flame retardancy and enables PC to achieve a UL-94 V0 rating at only 3%. In contrast, despite the close similarities in the structures, DPZ with a direct P–N bond synthesized from DOPO and piperazine is much less effective. It requires 10% of DPZ for PC to gain a UL-94 V0 rating. On the basis of the study of diphenyl carbonate/flame retardants and the results of thermogravimetric analysis, Fourier transform infrared analysis, inductively coupled plasma analysis, and morphology of PC/flame retardants, the base catalyzed decomposition of PC that leads to severe dripping is the key factor for the high efficiency of DFPZ.

Rigid steric hindering spiro-bisphosphates (PDMPDP and PDBPDP) were synthesized from spiro-pentaerythritol chlorophosphate and 2,6-dimethylphenol and o-tert-butylphenol respectively in order to investigate the effect of steric hindering groups on the flame retardant mechanism of bisphosphates and the effect of rigid spiro-pentaerythritol structure on the heat distortion temperature (HDT) of polycarbonate. A comparison of the steric hindering spiro-bisphosphates with the simplest phenyl spiro-bisphosphate (PDPDP) and commercial resorcinol bis(diphenyl phosphate) (RDP) on the flame retardancy of polycarbonate reveals that the presence of a thermally stable steric hindering group not only changes the efficiency but also shifts the mode of flame retardancy. In spite of low phosphorus content, PDMPDP and RDP show the highest efficiency of flame retardancy toward polycarbonate as evidenced by UL-94 combustion results with RDP exhibiting the strongest vapor phase action, PDMPDP the second, and PDPDP the least as judged from the phosphorus distribution in the vapor and the condensed phase. This is parallel to the tendency of chemical interactions between polycarbonate and flame retardants with PDPDP demonstrating the greatest interaction as proven by TGA study in air. It is suggested that the charring process of PDPDP interferes with that of polycarbonate and leads to microporous char that is seen in SEM and partially responsible for the poor flame retardancy of PDPDP. Compared to PC/RDP, PC/PDMPDP has a high HDT value.