Introducing a polysiloxane coating on flame retardant to realize durable UV resistance and flame retardancy of polypropylene

The applications of polypropylene (PP) are severely limited by its flammability and poor resistance to UV aging. Melamine hydrogen bromide (MHB) and aluminum hypophosphite (AHP) are widely used as synergistic flame retardants. Hindered amine light stabilizers (HALS) are typically used as anti-UV agents for polyolefins. However, the direct combination of acidic MHB and basic HALS induces antagonistic interactions, leading to a deterioration in material performance. In this work, antagonism is mitigated by coating MHB with polysiloxane (Si-MHB). Moreover, the polysiloxane layer can improve the compatibility between the flame retardant and PP, thereby enhancing flame retardancy and mechanical properties. PP composites containing Si-MHB, AHP, and HALS derivative were prepared. After 60 h of UV exposure, the surface of PP/Si-MHB/AHP/HALS119 remained relatively smooth with some shallow cracks. The water contact angle was maintained at 92°, and the carbonyl index decreased to 10.45, pointing out that the degree of photodegradation was significantly lower than that observed in the control samples. Besides, the limiting oxygen index (LOI) of PP/Si-MHB/AHP/HALS119 reached up to 26.5 %. After 60 h of UV irradiation, LOI decreased to 24.8 % and still maintained UL-94 V-0 rating, the tensile strength and impact strength decreased by 7 % and 10.2 %, which was a significantly lower reduction compared to the control samples. The above results demonstrate that the coating polysiloxane can effectively mitigate the antagonistic interactions between MHB and HALS119, effectively enhancing the UV aging resistance, flame retardancy, and mechanical properties of PP composites.