Structure–Property Studies on a New Family of Halogen Free Flame Retardants Based on Sulfenamide and Related Structures

A wide variety of molecules containing S–N or S–N–S cores were synthesized, and their flame retardant properties in polypropylene (PP), low density polyethylene (LDPE) and polystyrene (PS) were investigated. In addition, polymers or oligomers bearing the sulfenamide functionality (SN) were also synthesized. It was shown that this radical generator family based on sulfenamides is very versatile in terms of structural modifications, and the thermal decomposition range can be easily adjusted by changing the R groups attached to the core. The thermal stabilities of the different sulfenamides were examined by thermogravimetric analysis (TGA). Radicals generated by the homolytic cleavage of the S–N or S–N–S bonds at an elevated temperature can effectively interact with the intermediate products of polymer thermolysis and provide excellent flame retardant properties. The choice of most suitable SN-structure varies depending on the polymer type. For polypropylene DIN 4102-1 B2 and UL94 VTM-2 classifications were achieved with only 0.5 to 1 wt % of sulfenamide, and, in some cases, no flaming dripping was observed. Also for LDPE thin films, sulfenamides offered the DIN 4102-1 B2 rating at low dosage. In the case of polystyrene, the very stringent UL94 V-0 classification was even achieved at a loading of 5 wt % of sulfenamide

Disulfides – Effective radical generators for flame retardancy of polypropylene

The potential of thirteen aliphatic, aromatic, thiuram and heterocyclic substituted organic disulfide derivatives of the general formula R-S-S-R’ as a new group of halogen-free flame retardants (FR) for polypropylene films have been investigated. According to DIN 4102-1 standard ignitibility test, for the first time it has been demonstrated that many of the disulfides alone can effectively provide flame retardancy and self-extinguishing properties to polypropylene (PP) films at already very low concentrations of 0.5 wt%. In an effort to elucidate the mechanism of the thermal decomposition of disulfide derivatives the fragmentation patterns of the evolved gases from a thermogravimetric analyzer (TGA) have been analyzed by simultaneous mass spectrometry (MS) and Fourier transform infrared spectrometry (FTIR). The main decomposition products initiated by homolytic scission of the S-S bond and/or scission of the C-S bond were identified as thiols, aliphatic and aromatic hydrocarbons, isothiocyanates (depending on the disulfide structures) with further evolution of elemental sulfur and sulfur dioxide at temperatures of above 300 oC and 450 oC, respectively. Based on this preliminary study, we have shown that disulfides represented by e.g. diphenyl disulfide (1), 5,5'-dithiobis(2-nitrobenzoic acid) (2), bis(1-phenyl-1H-tetrazol-5yl)-disulfide (4), 2-bisbenzothiazole-2,2′-disulfide (6) and N,N-dithiobis-(phtalimide) (10) constitute a new halogen-free family of additives for flame retarding of polypropylene