1.Properties of polymers
Because the flame retardant such as aluminum hydroxide is filled with higher magnesium hydroxide, the strength, elongation and flexibility of the polymer decrease, and the tear resistance is inherently insufficient. At present, the commonly used halogen-free carriers are generally low-density polyethylene and EVA. When flame retardants are used together, the hardness of their mixture materials is high, especially reaching the flame retardancy (single combustion grade) in accordance with IEC 332-1. The corresponding oxygen index of the materials is generally greater than 28, and the hardness of the materials is often greater than Shore A90.
2.Extrusion processing of materials
Based on the high filling characteristics of halogen-free and low-smoke materials with metal hydrate as flame retardant, such as improper selection of process temperature and compression ratio of screw during extrusion, hydrated oxides will decompose and polymerize bonds under high shear. Because of the high pressure, the speed of the extruder can't reach the standard value, which causes the material to stay between the barrel and the screw for too long. At the high temperature caused by friction, the hydrated oxide decomposes ahead of time, resulting in bubbles inside the material. At the same time, the mechanical strength and bending resistance of the finished product after extrusion decreased sharply, which also aggravated its unfavorable factors.
3.Use environment and storage conditions
Halogen-free and low-smoke substances using polyethylene as base material are sensitive to temperature. When the temperature changes greatly, the internal stress caused by thermal expansion and cold contraction often leads to interface cracks. For example, if a user located in the north places the finished reel of halogen-free flame-retardant cable in the user's place and there is no outer container, after one week, it is found that almost all reels have longitudinal cracks on the sunny side, and the length has reached 1/2 of the cable reel. Through the analysis of cable tray, it is found that when the temperature difference between the two sides is large and the bending stress is large, the material will crack on both sides.
4.Factors of extrusion die
The structure type, drawing ratio and tension balance rate of extrusion die also play an important role in eliminating internal stress after extrusion. Semi-extrusion die with little pressure is usually used. The tension ratio and tension balance ratio of internal mold should be controlled between 1:2 and close to 1 respectively.
