A set of intrinsically flame retardant, halogen-free styrenic copolymers: Synthesis, characterization, processing, and properties

Halogen-free, intrinsically flame retardant copolymers with glass transition temperatures in the range of 93- 97 degrees C were synthesized from styrene, elemental sulfur, and organophosphorus comonomers in a facile radical bulk polymerization process. The copolymers were characterized using multidetector size exclusion chromatography (SEC), elemental analysis (EA), thin layer chromatography (TLC), UV/Vis spectroscopy, and 1H, 13C, and 31P nuclear magnetic resonance (NMR) spectroscopy. SEC revealed number average molar masses (Mn) ranging from 14- 18 kg/mol, and weight average molar masses (Mw) from 41- 62 kg/mol. The phosphorus contents ranged from 0.1- 0.6 wt.%, while the sulfur contents were 0.4- 0.7 wt.%, with covalently bound sulfur being predominantly present in the lower molar mass fractions. Structural details of the formed sulfur-containing moieties in the copolymers were elucidated with homonuclear correlation spectroscopy (COSY) and 1H/13C heteronuclear single-quantum correlation spectroscopy (HSQC) techniques, indicating the presence of PS-substituted 1-phenyl- ethyl sulfanyl units with a mono-, di- or trisulfidic arrangement. Thermogravimetric analyses (TGA) showed slightly decreased decomposition onset temperatures for all copolymers compared to that for polystyrene, and no or only minimal formation of residual masses above 500 degrees C. Selected copolymers were extruded and injection molded on a laboratory scale. UL 94 vertical flame testing demonstrated a good flame retardant performance for most copolymer types, alongside with short after-flaming times and intensive melt dripping. Increased melt flow caused by polymer decomposition, probably initiated by the sulfur moieties in the copolymer structures, and cooling are considered the main condensed-phase flame retardant processes involved. Two copolymers of the set display a UL 94 V-0 classification.