Rational Design and Scalable Synthetic Strategies for Expediting Flame Retardant Innovations

This doctoral thesis addresses the limitations of current flame retardants, which are often toxic, persistent, and environmentally unsustainable, by developing new flame-retardant families through rational design and scalable synthesis. The goal is to create safer, more sustainable alternatives to meet rising fire safety regulations and public health concerns, thereby accelerating innovation in polymeric material fire safety, leading the intellectual property rights holder to consider commercialization.
A series of novel silyl-and siloxyamines have successfully been designed and synthesized via facile synthetic routes. Upon thermal dissociation, the N-Si and N-O-Si compounds were shown to yield aminyl, silyl and oxygen-centered radicals that provided fire-proofing effect in polypropylene (PP) films alone. Whereas in combination with traditional phosphorus flame retardants, like spirocyclic phosphonate AFLAMMIT® PCO 900, a profound synergistic effect was observed. The most effective compounds, incorporating imide synthons like imide phthalate or pyromellitic diimide, offer enhanced thermal and hydrolytic stability compared to other N-Si/N-O-Si derivatives.
A strategy to further enhance flame retardant properties of polypropylene (PP) multifunctional mono-component flame retardants was designed by integrating sulfenamide and phosphate moieties into single molecules. The prepared flame retardants were heated at 260°C in the presence and absence of 9, 10-dihydroantracene and the formed decomposition products were analysed using GC/MS, LC/MS, 13C NMR and 31P NMR. The results indicate that the sulfenamide bond dissociates before the phosphorus moiety for four samples. Whereas the phosphorus moiety dissociated first in case of two of the samples. All the PP film samples containing any of the different multifunctional flame retardants at the low loading of 0.5 wt% passed the DIN4102 B2 small scale fire test. Noteworthy is that bis(1-((4-((diphenoxyphosphoryl)-oxy)phenyl)thio)-2,2,6,6-tetramethylpiperidin-4-yl)carbonate and bis(1-((4-((bis(2,6-dimethyl-phenoxy)phosphoryl)oxy)phenyl)thio)-2,2,6,6-tetramethyl-piperidin-4-yl)-carbonate samples reached the UL-94 V2 classification as standalone flame retardants even at a loading of only 4 wt%, which is a significant achievement.
A comparative study of the efficacy and mechanism of flame retardancy between sulfenamides, sulfinamides and sulfonamides were conducted by synthesis of such model flame retardant compounds. The results reveal, that sulfenamides and sulfinamides have the specific feature of generating highly reactive aminyl and sulfenyl radicals during pyrolysis. These radicals contribute to flame retardancy through two primary actions: triggering rapid degradation of polymer chains in the solid phase and by quenching active fire-propagating radicals in the flame zone. In contrast, the sulfonamides thermally decomposed via a non-radical mechanism and their main flame retardancy effect was instead attributed to increased charring of polymeric materials. At this low loading of flame retardants (< 5 wt%) the radical generating flame retardants are superior to char forming, ditto. This work shows that by slightly altering the substitution pattern of the flame retardant one may change the flame retardant mechanism.