Some manufacturers use ordinary copper in the production of rubber cables, because copper itself is not oxygen-free copper, and the surface of the copper wire is inevitably oxidized during the processing. Due to the processing of the copper rod, especially the mastery of the toughening process and the poor storage conditions of the processed copper wire core, the copper wire core itself has been slightly oxidized, which is one of the reasons for the blackening of the copper wire.

The insulation of rubber cables is formulated with natural rubber and styrene-butadiene rubber. Since the insulating rubber is in direct contact with the copper wire, sulfur cannot be used directly as a vulcanizing agent, and even a small amount of sulfur will make the copper wire black. Some compounds that can decompose free sulfur must be used, such as accelerator TMTD, vulcanizing agent VA-7, and some vulcanization accelerators must be used to improve the vulcanization speed and degree of vulcanization, and ensure the physical, mechanical and electrical properties of the insulating rubber.

However, in terms of elasticity, strength and permanent deformation of insulating rubber, it is not as good as rubber with sulfur. After decades of practice, it has been confirmed that TMTD cannot solve the blackening problem of copper wire. In addition, the insulating rubber should have various colors. Red, blue, yellow, green and black are the basic colors. The appearance of these colors will also make the rubber sticky and the copper wire black. The main fillers in the formula are light calcium carbonate and talcum powder. Due to the price, some manufacturers use calcium carbonate and talcum powder which are especially cheap in order to reduce costs. These fillers have coarse particles, large free alkali content and impurities. Therefore, the physical and mechanical properties are relatively poor, the electrical properties are not good, and it is easy to cause the copper wire to turn black. Some factories use active ultrafine calcium carbonate to improve the physical and mechanical properties of insulating rubber, and most active calcium is treated with stearic acid, which is also the reason for the blackening of copper wires.

The use of vulcanizing agent VA-7 can improve the blackening of copper wires, but due to insufficient vulcanization, the permanent deformation of the rubber is large, which will cause the rubber to become sticky. Especially after adding the accelerator ZDC, the vulcanization speed is increased. In order to prevent scorch, the accelerator DM is also added to delay the scorch time.