A Schiff base (an amine nitrogen compound) was reacted with ammonium polyphosphate (APP) achieving UL 94 V-0 (3 mm) at 17% loading in unsaturated polyester. 1,2-bis(2-aminoethoxy)ethane and terephthalaldehyde (TPA) were reacted with APP in ethyl acetate at 65°C to produce a Schiff-base modified APP. In unsaturated polyester polymer, this showed to delay the initial decomposition and promote char formation. At 17% loading, LOI (Limiting Oxygen Index) was increased from below 20 (neat polymer) to nearly 29, total smoke production was reduced by nearly 20% and UL 94 V-0 (3 mm) was achieved. Reduction in peak heat release is not as good as with 17% loading of non-modified APP, but mechanical properties are significantly better (tensile strength, elastic modulus), because of improved compatibility of the modified APP in the polyester resin, so improved dispersion.
“A Schiff base-coated ammonia polyphosphate for improving thermal and fire-retardant properties of unsaturated polyester”, J. Zhang et al., Progress in Organic Coatings 185 (2023) 107910 https://doi.org/10.1016/j.porgcoat.2023.107910