Luksin, Michael Jan

Polystyrene and styrene copolymers belong to the group of easily flammable standard plastics. [1] Currently, a polybrominated SBS copolymer is used as a flame retardant for flame-retardant styrene polymers in applications within the construction and building sector. [2] Brominated compounds exhibit high persistence in the environment, which means they degrade slowly and can remain in ecosystems for long periods. Another risk arises under thermal stress, as these compounds release gaseous hydrogen bromide (HBr), which is highly reactive and strongly corrosive to metals. Inhalation of HBr can cause severe burns to the respiratory tract. Due to these material properties, there is currently significant interest in developing alternative flame-retardant systems with improved risk profiles. In this context, halogen-free flame retardant combinations could potentially be attractive candidates. The aim of this study was therefore to develop, characterize, and test novel halogen-free phosphorus- and sulfur-containing flame-retardant formulations (P-S flame retardant systems) for foamed polystyrene. Additionally, the developed formulations were to be comparatively evaluated with regard to their flame-retardant efficiency. Various methods were used in this process, including a newly developed screening test for polystyrene foams, the standardized fire test DIN 4102 (B2) [3] for applications in construction and building, as well as the quantitative Limiting Oxygen Index (LOI). Testing the fire behavior of various phosphorus-organic flame retardants in polystyrene foams according to DIN 4102 (B2) revealed that the variation in the number of oxygen atoms directly bonded to the phosphorus atom in the flame retardant had no measurable impact on its efficiency—provided the co-flame retardant was a di- or oligosulfide. Rather, the decisive factor for effective fire protection in polystyrene was the phosphorus content (as a percentage) in the flame retardant. Further investigations using thermogravimetric analysis (TGA), gel permeation chromatography (GPC), scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM-EDX), and thermogravimetric analysis combined with mass spectrometry (TGA-MS) provided insights into the mechanism of action of the P-S flame retardant system in both the solid and gas phases. TGA investigations combined with GPC showed a significant influence of poly(tert-butylphenol) disulfide (FR-S1) on the thermal stability of polystyrene. In TGA experiments, the presence of the sulfur-containing co-flame retardant FR-S1 led to a significantly accelerated degradation of the polystyrene molecular weight under heat input compared to both reference polystyrene alone and polystyrene in combination with FR P1. Fire tests according to DIN 4102 (B2) showed increased burning drips in foams containing FR-S1, which also indicates a reduction in polymer viscosity due to molecular weight degradation. SEM-EDX investigations revealed an accumulation of phosphorus and oxygen on the surface of the burned polystyrene foam bodies. Gas phase analysis using TGA-MS detected sulfur indicators such as hydrogen sulfide and dihydrogen sulfide as potential thermal degradation products. This suggests at least partial gas-phase activity of the co-flame retardant FR-S1. The cooperative effect of the phosphorus- and sulfur-containing flame retardants was then specifically used to synthesize tailor-made flame retardants from elemental sulfur and various organic comonomers. Initially synthesized highly sulfur-rich flame retardants did not show sufficient flame-retardant efficiency in polystyrene on their own. As a result, styrene copolymers were subsequently synthesized that covalently incorporated both sulfur and phosphorus functionalities into the polymer. Foam bodies made from these terpolymers (CP3–CP7) exhibited self-extinguishing behavior in initial fire tests according to DIN 4102 (B2) and passed the test. However, this positive characteristic was compromised by undesirable burning drips.

Luksin, Michael Jan

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