The main reasons for environmental stress cracking of halogen-free low-smoke Flame Retardant materials are as follows:
(1) Caused by the characteristics of the polymer: due to the high content of flame-retardant fillers such as aluminum hydroxide and magnesium hydroxide, the strength of the polymer is reduced, the elongation and flexibility are also reduced, and the tear resistance and crack resistance are inherently insufficient . Currently, the carriers of halogen-free materials are usually low-density polyethylene and EVA. After being flame-retardant, the hardness of the mixture is higher, especially reaching the flame-retardancy of iec332-1 (single burning level, the corresponding oxygen index of the material is generally greater than 28), and the hardness of the material is often greater than Shore a90.
(2) Material extrusion process: Based on the high filling characteristics of metal hydrate flame-retardant, halogen-free and low-smoke materials. If the process temperature and screw compression ratio are not properly selected during the extrusion process, the hydrous oxide will decompose and the polymer bond will break under high shear. Due to the large compression ratio, the speed of the extruder cannot reach the normal level, resulting in a long residence time between the barrel and the screw. At high temperatures caused by friction, hydrated oxides decompose prematurely, creating bubbles (micropores) in the material. The mechanical strength and bending strength of extruded products are greatly reduced. The sudden cooling also increases the disadvantages.
(3) Use environment and storage conditions: Generally speaking, polyethylene-based halogen-free low-smoke materials are more sensitive to temperature. In an environment with large temperature changes, internal stress caused by thermal expansion and cold shrinkage often leads to cracks in the interface zone. For example, the northern user placed the finished halogen-free flame-retardant cable reel on the user site without outer packaging. A week later, it was found that almost all the cables on the positive side of the reel had longitudinal cracks, and the length reached 1/2 of the circumference of the cable reel. After analysis, the reason is that one side of the cable tray is sunny and the other side is cool. Under the action of a large temperature difference and bending stress, the material cracks.
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