Azoalkanes-novel flame retardants and their structure-property relationship.

A number of symmetrical and unsymmetrical azoalkanes of the general formula R′−N = N−R and related azoxy, hydrazone as well as azine derivatives have been synthesized in order to assess their potential as novel flame retardants for polypropylene alone or in combination with commercially available flame retardants such as alumina trihydrate (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 retardant efficacy decreased in the following order: R = cyclohexyl > cyclopentyl > cyclobutyl > cyclooctanyl >> cyclododecanyl. Whereas in the series of aliphatic azoalkanes compounds the efficacy decreased in the following order: R = n-alkyl > tert-butyl > tert-octyl. In addition, also some of the prepared azoxy, azine, and hydrazone derivatives provide flame retardancy to polypropylene films at already very low concentrations (0.25-1 wt%). Noteworthy is that in contrast to other halogen-free radical generators, the azoalkanes are also very effective as flame retardants in polypropylene thick moldings. Interestingly, it was found that 4,4′-bis(cyclohexylazocyclohexyl)-methane) shows a strong synergistic effect with ATH. Thus, in the presence of 0.5 wt% of azoalkane the ATH loading could be reduced from 60 to 25 wt% and still UL94 V-2 rating could be reached. Furthermore, the fire testing data reveal that azoalkanes show a synergistic effect with DecaBDE and when used in conjunction with very low loadings of TBBPP. Copyright © 2010 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]