Azoalkanes—novel flame retardants and their structure-property relationship

A number of symmetrical and unsymmetrical azoalkanes of the general formula R(SN¼NSR and related azoxy,hydrazone as well as azine derivatives have been synthesized in order to assess their potential as novel flameretardants for polypropylene alone or in combination with commercially available flame retardants such as aluminatrihydrate (ATH), decabromodiphenyl ether (DecaBDE) and tris(3-bromo-2,2-bis(bromomethyl)-propyl)phosphate(TBBPP). The experimental results show that in the series of different sized azocycloalkanes the flame retardantefficacy decreased in the following order: R¼cyclohexyl>cyclopentyl>cyclobutyl>cyclooctanyl>>cyclododecanyl.Whereas in the series of aliphatic azoalkanes compounds the efficacy decreased in the following order:R¼n-alkyl>tert-butyl>tert-octyl. In addition, also some of the prepared azoxy, azine, and hydrazone derivativesprovide flameretardancy to polypropylenefilms atalready very lowconcentrations (0.25–1wt%). Noteworthyis thatin contrast to other halogen-free radical generators, the azoalkanes are also very effective as flame retardants inpolypropylene thick moldings. Interestingly, it was found that 4,4(-bis(cyclohexylazocyclohexyl)-methane) shows astrong synergistic effect with ATH. Thus, in the presence of 0.5wt% of azoalkane the ATH loading could be reducedfrom60to25wt%andstillUL94V-2ratingcouldbereached.Furthermore,thefiretestingdatarevealthatazoalkanesshow a synergistic effect with DecaBDE and when used in conjunction with very low loadings of TBBPP. Copyright 2010 John Wiley & Sons, Ltd.Keywords: additives; azo compound; radical (generator); non-halogenated flame retardants; poly(propylene)