Triazene compounds as a novel and effective class of flame retardants for polypropylene

Four triazene derivatives have been synthesized, i.e. bis-4,4′-(3,3′-dimethyltriazene)-diphenyl ether (1), bis-4,4′-(3,3′-diethyltriazene)-diphenyl ether (2), 2,2,6,6,-tetramethyl-1-phenylazo-piperidine (3) and 4-hydroxy-2,2,6,6-tetramethyl-1-phenylazopiperidine (4). Their thermal properties were determined by differential scanning calorimetry (DSC) and the fragmentation patterns were analysed by simultaneous mass spectrometry (MS) and Fourier transform infrared (FTIR) spectrometry of off-gases from a thermogravimetric analyser (TGA). The triazenes exhibited an exothermic decomposition peak at temperatures between 230 and 280 °C when the triazene units were homolytically cleaved into various aminyl, resonance-stabilized aryl radicals and different CH fragments with simultaneous evolution of elemental nitrogen. The potential of triazenes as a new class of flame retardants for polypropylene films was investigated by performing ignitability test in accordance to DIN 4102-1/B2 standard. Polypropylene samples containing very low concentration of only 0.5 wt% of any of these triazene (R–N1 = N2–N3R’R″) additives passed the test with B2 classification. Notably, no burning dripping could be detected. The average burning times are very short with exceptionally low weight losses. Based on this preliminary FR testing we have shown that the triazene compounds constitute a new and interesting family of radical generators for flame retarding of polymeric materials.