Effect of crystalline phase formed by compound flame retardant on the flame retardancy and ceramization of polyethylene composites

Ceramic polyolefin composites have the capability to transform into hard ceramics when exposed to fire conditions. During the ceramization process, the formation of new crystalline phase plays a crucial role in enhancing flame‐retardant and ceramifiable properties. Consequently, ceramic polyolefin composites show great potential for the applications in fire‐resistant wires and cables. In this article, the incorporation of the compound flame retardant consisting of ammonium polyphosphate/melamine cyanurate/zinc borate (APP/MCA/ZB) was found to enhance the flame retardancy and ceramization of polyethylene/wollastonite fiber/phosphate glass frits (PE/WF/PGF) composites. The results indicated that ceramifiable flame‐retarding PE composites with compound flame retardant exhibited superior flame retardancy compared to pure PE and PE composites with a single flame retardant. Specifically, the limiting oxygen index (LOI) was significantly increased to 26.8%, and the vertical combustion test rating in UL‐94 (test for flammability of plastic materials for parts in devices and appliances) reached V‐0. During the heating process, ZB thermally decomposed to produce 2ZnO ⋅ 3B2O3, which reacted with CaSiO3 to form a silicate glass intermediate phase (CaO ⋅ SiO2 ⋅ 2ZnO ⋅ 3B2O3). APP thermally decomposed to produce (HPO3)n, which reacted with 2ZnO ⋅ 3B2O3 to form a phosphate glass intermediate phase (nP2O5 ⋅ 2ZnO ⋅ 3B2O3). These two glass phases experienced a eutectic reaction with WF, ultimately producing the formation of a new crystalline phase of calcium zinc phosphate (CZP, Ca19Zn2(PO4)14). This newly formed CZP phase made sintered ceramics more compact and had higher flexural strength. The flexural strength of ceramic residues after sintering was 11.68 MPa, meeting the requirements for practical applications.