Factors causing stress cracking in halogen-free, low-smoke flame-retardant cables

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Factors causing stress cracking in halogen-free, low-smoke flame-retardant cables: A common and troublesome quality issue with halogen-free, low-smoke flame-retardant cables is stress cracking of the insulation material. Strain cracking refers to the cracking of a material resulting from the release of internal stress energy within the material, either under the influence of external forces or without such forces. The main factors causing cracking in halogen-free, low-smoke flame-retardant materials include: (1) characteristics of the polymer itself. The high content of flame-retardant fillers such as aluminum hydroxide and magnesium hydroxide reduces the strength of the polymer, while also decreasing its elongation and flexibility; as a result, its resistance to tearing and cracking is inherently low. The carriers commonly used for halogen-free materials at present are generally low-density polyethylene; after flame-retardant treatment, the hardness of such mixtures is relatively high. In particular, to achieve a flame-retardancy level that meets IEC 332-1 (single-fiber combustion) requirements, the oxygen index of the material must generally be greater than 28. At this point, the hardness of the material is usually above Shore A90. (2) Factors in the extrusion process: Due to the high filling capacity of halogen-free, low-smoke materials, issues such as the process temperature during extrusion, improper control of the screw’s compression ratio, can lead to the decomposition of hydrated oxides and the breaking of polymer bonds under high shear forces. Due to the high degree of compression, the extrusion speed of the extruder cannot reach normal levels, resulting in an excessive residence time of the material between the barrel and the screw. Under the high temperatures generated by friction, the hydrated oxides decompose prematurely, causing bubbles (microvoids) to form within the material. It **reduces** the mechanical strength and resistance to bending of the finished product after extrusion. This can be verified from two aspects: 1) By examining the cross-section of the extruded layer, micro-pores of varying densities can be observed (caused by the crystalline water after decomposition). 2) Conduct mechanical property tests on the extruded finished products. The test results show that there is significant variability in the data regarding mechanical strength and tensile elongation at break; in particular, the elongation at break is usually at relatively low levels. If halogen-free materials with an oxygen index greater than 30 are extruded improperly, their elongation at break is usually less than 100%, and in some cases it ranges from 80% to 90%. It causes the polymer to lose its high elasticity. Sudden cooling also adds to the disadvantages. I have handled a case of cable stress cracking; after analysis, it was determined that this was caused by an excessively high extrusion speed and rapid cooling during the cooling process (see Figure 1). 　　 (3) Factors related to the operating environment or storage conditions: Halogen-free, low-smoke materials based on polyethylene are generally sensitive to temperature. In environments with large temperature fluctuations, internal stresses resulting from thermal expansion and contraction often cause cracking in the interface area. For example, a user in the north stored the halogen-free, low-smoke, flame-retardant finished cables in coils outdoors, without any external packaging; a week later, it was found that on almost all of the cable coils, longitudinal cracks appeared on the side facing the sun, with the length of these cracks reaching half the circumference of the cable coil. Analysis shows that this is due to one side of the cable reel being exposed to sunlight while the other side is in shade; the large temperature difference along with bending stresses caused cracking in the material. (4) Extrusion die: The structural design of the extrusion die, as well as the drawing ratio and drawing balance, also play a crucial role in eliminating internal stresses after extrusion. Generally, semi-extrusion or semi-squeezing molds with lower pressure are used. The drawing ratio and drawing balance rate of the inner and outer dies should be controlled at 1∶2 and close to 1, respectively.