Analysis: Causes of blackening of copper wire in rubber sheath cables

The reason for the blackening of the copper wire is caused by many factors, not only the problem of the rubber formula, but also the state of the copper wire itself, the rubber processing technology, the rubber vulcanization process, the structure of the cable, the sheath rubber formulation, and the production environment. There are many factors involved.

1. Analysis of the reasons for the blackening of rubber hair and copper wire

1.1 Causes of copper wire itself In the 1950s and 1960s, most domestic manufacturers used ordinary copper rods with a copper content of 99.99%, all of which were aerobic copper rods. The production method was based on multi-channel heating of copper ingots. After the rolling, a black copper rod was prepared, and after the large, middle, and small pulls, the copper rod was made into finer copper wire. Because copper itself is not oxygen-free copper, oxidation of the surface of the copper wire is inevitable during processing. In the 1980s, domestic advanced oxygen-free copper rod production technology and domestically developed oxygen-free copper rod production technology made the entire wire and cable industry use oxygen-free copper rods, which is undoubtedly improved. Black wire problem. However, due to the processing of copper rods, in particular the mastery of the toughening process and the poor storage conditions of the processed copper cores, the copper core itself has been slightly oxidized, which is one of the reasons for the blackening of copper wires.

1.2 The reason for the rubber formula In the 1950s, rubber insulation was formulated with natural rubber and styrene-butadiene rubber. Since the insulating rubber is in direct contact with the copper wire, it is not possible to use sulfur directly as a vulcanizing agent, that is, using less sulfur also makes the copper wire black. Some compounds that can decompose free sulfur must be used, such as the accelerator TMTD mentioned above and the vulcanizing agent VA-7, and some vulcanization accelerators must be added to increase the vulcanization speed and vulcanization degree to ensure the physical and mechanical properties of the insulating rubber. And electrical performance. However, from the perspective of the elasticity, strength, and permanent deformation of the insulating rubber, it is not as good as a rubber added with sulfur (if the copper wire is not considered black). Decades of practice have confirmed that TMTD cannot solve the blackening problem of copper wire.

In addition, the insulating rubber must have a variety of colors. Red, blue, yellow, green, and black are the basic colors. The appearance of these colors also promotes the stickiness of the rubber and the blackening of the copper wire. The main fillers in the formulation are light calcium carbonate and talcum powder. Due to the price, some manufacturers use calcium carbonate and talc powder, which are particularly inexpensive, in order to reduce the cost. These fillers have a large content of coarse particles, free alkali, and impurities. Many, so the physical and mechanical properties are relatively poor, poor electrical properties, but also easily cause black copper wire. Other factories use active ultra-fine calcium carbonate to improve the physical and mechanical properties of the insulating rubber. Most of the active calcium is treated with stearic acid. This acid is also responsible for the blackening of the copper wire. The use of vulcanizing agent VA-7 can improve the blackening of the copper wire, but due to the insufficient degree of vulcanization, the permanent deformation of the rubber may cause the rubber to become sticky. In particular, after adding the accelerator ZDC, the vulcanization speed is increased. In order to prevent scorch, an accelerator DM is added to retard the scorch time. From the perspective of the structure of the accelerator ZDC, it is an indirect metal zinc in the two connected sulfurs in the TETD structure. The structural formula is: SS H5C2‖‖H5C2>NCS-Zn-SC-NN-CSSCN

2, from the wire and cable structure analysis

2.1 Catalytic aging of copper is an important cause of rubber stickiness. The former Soviet Union’s Institute of Cable Science has demonstrated that copper penetrates from the contact with rubber into the insulating rubber during the vulcanization process. The insulated rubber of 1.0-2.0mm thickness contains 0.009-0.0027% of copper. . As we all know, trace copper has a great destructive effect on rubber, which is what we commonly call the aging of heavy metals on rubber. In the insulating vulcanization process, thiuram precipitates some free sulfur and reacts with copper to form an active copper-containing group: CH3│CH2-CH-C-CH2-|│SS││Cu Cu During aging, weak-SS - The bond breaks to form an active copper-containing group: Cu-S-, which acts on the rubber and acts simultaneously with oxygen to destroy the rubber's long-chain molecules, making the rubber soft and viscous, a combination of low-molecular chains. The French Rubber Research Institute also pointed out that if the rubber contains harmful metals, such as copper, manganese and other heavy metal salts, then rubber stickiness will occur regardless of the type of accelerator.

2.2 Migration of sulphur in rubber sheath cables to the surface of insulating rubber and copper wire Soviet scientists applied radioisotopes to confirm the possibility of sulfur diffusion in cable jacket rubber. In natural rubber-based vulcanizates, the diffusion coefficient of free sulfur is about 10-6 cm2/s at a temperature of 130-150°C. In a continuously vulcanized production plant, when the vulcanized sheath rubber is used, the temperature is between 185-200°C, and this diffusion coefficient is even greater. Due to the diffusion of free sulfur in the rubber sheath, the structure of Qiulam rubber is changed, and polysulfide bonds may be formed. These polysulfides are chemically decomposed and combined to achieve migration, ie "chemical expansion." As a result of the migration, not only the structure of the insulating rubber can be changed, the heat resistance thereof is reduced, but also sulfur reacts with the copper surface to form copper sulfide and cuprous sulfide, resulting in blackening of the copper wire. In turn, copper sulfide and cuprous sulfide accelerate the aging of the rubber, which in turn leads to stickiness.