Low smoke zero halogen flame retardant cable is a more common and troublesome quality problem it is the environmental stress cracking of insulating materials.
Caused Low-smoke halogen-free flame retardant materials environmental stress cracking has the main reasons:
1.Caused by polymer properties:
Due to the high loading of flame retardant fillers such as aluminum hydroxide and magnesium hydroxide, the strength of the polymer body is reduced, the elongation and flexibility are also reduced, and the tear resistance and cracking resistance are inherently deficient. Halogen-free materials commonly used in the carrier is generally used low-density polyethylene and EVA, The flame retardant with the mixture of high hardness of the material, in particular, to meet the IEC332-1 flame retardancy, the material hardness at this time are often greater than the Shore A90.
2.Material extrusion process:
Based on the high filling characteristics of LSZH materials with metal hydrates as flame retardants, such as the extrusion process temperature, the improper compression ratio of the screw will result in the decomposition of the hydrous oxide and the breaking of the polymer under high shear. As the compression ratio, the extruder speed can not reach the normal level, resulting in the material in the barrel and screw residence time is too long, the friction generated by the high temperature, the hydrated oxide decomposition in advance to produce bubbles in the material. So that the finished product after extrusion mechanical strength and anti-winding ability greatly reduced.
Sudden cooling also adds unfavorable factors.
3.Factors that use the environment and storage conditions:
Low-smoke halogen-free materials based on polyethylene are generally temperature-sensitive. Under the conditions of large temperature changes, the internal stress caused by thermal expansion and contraction tends to cause cracking in the interface area. For example, in the north, when a finished cable reel of halogen-free flame retardant cable is placed on the customer's site without packaging, a week later, almost all the cables on the reel were found to have longitudinal cracks on the sunny side of the cable, 1/2 circumference of the disk. This analysis is due to the cable tray one side of the sun, one side of the shade, both sides of the larger temperature difference under the effect of bending stress and material cracking.
4. Extrusion die:
Extrusion die structure type, draw ratio and stretch balance rate of extrusion is also important to eliminate the internal stress. Generally use less pressure semi-extrusion mold. The draw ratio and stretch balance ratio of the inner and outer molds should be controlled at 1: 2 and close to 1 respectively.