Disulfides are present in a number of biological systems. The release of radicals which results from breaking the sulphur-sulphur (S-S) in such substances, important in biological sulphide metabolism, can also provide a fire protection function in polymers. In this study, 13 different disulphide molecules were tested for flame retardancy in polypropylene (PP) films.
Results showed that the disulphides could ensure self-extinguishing of PP films at low loadings (0.5%) with a key factor being the resistance to the film extrusion processing temperatures (tested at 230°C). The flame retarding mechanism is considered to be a combination of radical release (which inhibits OH and O reactions in the flame) and catalysis of breakdown of the polymer structure, so removing the substrate from the flame. Disulphides such as diphenyl disulfide, bis(1-phenyl-1H-tetrazol-5yl)-disulfide, 2-bisbenzothiazole-2,2-disulfide and N,N-dithiobis-(phtalimide) and in particular 5,5′-dithiobis(2-nitrobenzoic acid) are identified as having strong potential as new PIN flame retardants for polypropylene.
“Disulfides – effective radical generators for flame retardancy of polypropylene”, W. Pawelec et al., Polymer Degradation and Stability, in press 2014 http://dx.doi.org/10.1016/j.polymdegradstab.2014.09.013