Investigation of sintering properties and ceramifying process of amorphous silica/mica-based ceramifiable polyethylene composites

Ceramifiable polyolefin-based composites are promising materials for fire-resistant cables. Many studies have been focused on improving the ceramifiable properties by developing various composite matrixes with optimum inorganic fillers. Few studies have been carried out to compare the sintering properties of ceramifying powder (CP) and ceramifiable polymer-based composites directly. In this work, a low-temperature sintering CP, formulated with mica, amorphous silica and glass frits, and filled into polyethylene (PE) matrix to prepare a ceramifiable PE composites (CPE). The sintering properties, including the flexural strength, apparent porosity and volume variation, were tested at calcination temperatures from 600 to 900 degrees C. Results showed that the CP had better sintering properties than the CPE at the same calcined temperatures, but the gap between the two continued to decrease as the temperature increased. The negative effect of PE matrix decomposition on the ceramifying process was more pronounced in the low-temperature ranges (600 and 700 degrees C). As the temperature reached above 700 degrees C, the sintering of the CP was accelerated, and the sintering properties of composite residue increased significantly. The ceramifying process of the CPE was investigated by thermalgravimetric analysis, X-ray diffraction and scanning electron microscopy. The analyses demonstrated that the induction of the CP improved the thermal stability of the CPE, and played the main role in the structure evolution of the composite residue. During the ceramifying process, the glass frits melted at low temperatures, bridging mica and amorphous silica. As the temperature increased to 800 and 900 degrees C, mica had a eutectic reaction with the glass liquid phase, facilitating the change of a bridging structure to the continuous island structure responsible for the superior mechanical and fire-resistant properties of the CPE.