In the production of wires and cables, copper conductor oxidation and discoloration is a major challenge. As everyone knows, copper accounts for 80% of the cost of cables, and copper is already a relatively expensive metal. Coupled with the current high processing and labor costs, the losses for cable companies once oxidation occurs are considerable. Copper oxidation is extremely common in the processing industry; often, discoloration appears within hours of cleaning and drying, and oxide slag forms several days later. Many cable companies are troubled by oxidized copper, and preventing copper oxidation has become an urgent problem for manufacturers to solve.
Taking various effective protective measures during the wire and cable production process can reduce the degree of oxidation and discoloration of the copper conductors. Appropriate measures can reduce or even avoid copper oxidation, such as: using high-quality electrolytic copper as raw material; employing suitable copper rod processing technology; using drawing oil containing antioxidants; using antioxidants in the continuous annealing process; passivating the surface of the copper wires during the stranding process; properly wrapping the copper conductors to isolate them from air during storage; using peroxide crosslinking agents in the insulation material to give the insulation a high electrode potential and keep the copper conductors in a reduced state; and minimizing the heating process of the copper wires, etc.
Specifically, preventing the copper conductors of wires and cables from oxidizing and discoloring mainly involves the following aspects: First, it is necessary to control the processing of electrolytic copper into copper rods.
Currently, there are generally two methods for processing electrolytic copper into copper rods: the upward drawing method and the continuous casting and rolling method. The upward drawing method involves adding electrolytic copper to an induction melting furnace, where it is heated by induction to melt the copper material. The molten copper flows through a narrow molten channel between two furnaces into a holding furnace for heat preservation. Then, a rod-drawing mechanism uses cooling water from the crystallizer to control the extraction of the copper rod, thus achieving copper rod production. After cooling to room temperature, the copper rod is released into the air, and finally, a winding mechanism coils the copper rod into a coil for use in the next process. The entire production process of the upward drawing method is carried out under oxygen-free conditions, ensuring the purity and oxygen-free nature of the product, preventing oxidation of the copper rod, and producing oxygen-free copper rods. Copper rods produced using the upward drawing method have low resistivity, dense structure, good processing performance, smooth appearance, round surface, no oxidation, no burrs, no cracks, no peeling, and no inclusion defects. The continuous casting and rolling process involves adding electrolytic copper to a melting furnace. The molten copper flows through a narrow trough between two furnaces into a holding furnace for heat preservation. Then, it is processed through a continuous casting machine, traction machine, rolling shear, straightening and planing machine, roughening machine, continuous rolling mill, and rod-collecting device to produce copper rods. During continuous casting, the high-temperature molten copper comes into contact with oxygen in the air, forming copper oxide on its surface. Although subsequent continuous rolling removes some of the outer oxide layer, some oxide remains rolled into the copper rod. Antioxidant-containing coolants are used in the continuous casting and rolling process to reduce surface oxidation and discoloration of the copper rod.
The control of copper rod wire drawing is crucial. Drawing large-diameter copper wires typically involves continuous drawing and annealing units, requiring processes such as drawing on a large drawing machine, annealing, cooling, drying, and winding. During continuous annealing, it is essential to ensure the proper operation of the steam protection gas device. A drawing emulsion containing antioxidants must be used, and no drawing fluid residue should remain on the copper wire surface. After drawing, the wire should be wrapped in plastic film and stored in a dry environment to minimize oxidation and discoloration. Drawing small-diameter copper wires generally involves a large drawing machine, a medium drawing machine, and then a small drawing machine. Again, a drawing emulsion containing antioxidants must be used, and no drawing fluid residue should remain on the copper wire surface. After drawing, the wire should be wrapped in plastic film and stored in a dry environment before being transferred to the next annealing process.
The control of the copper wire annealing process is crucial. After multiple drawing and stretching processes, the crystal structure of the copper wire changes, resulting in lattice distortion and dislocations that generate internal stress. This manifests as copper wire hardening, reduced elongation, and increased conductor resistivity. Annealing involves slowly heating the copper wire to a specific temperature, holding it for a sufficient time, and then gradually cooling it. The purpose is to improve the copper wire's plasticity and toughness, rearrange the crystal lattice, and eliminate internal stress. Currently, annealing methods include can annealing and water-sealed tubular continuous annealing. The can annealing process involves placing the copper wire in a can, sealing the can, evacuating it to -0.1 MPa, holding it for a certain time, then purging it with nitrogen or carbon dioxide to 0.2-0.4 MPa, heating it to a specified temperature, holding it for a certain time, then suspending the can in air for cooling, and finally suspending it in water for cooling to room temperature before releasing the gas and removing it from the furnace. It is essential to ensure that the copper wire does not come into contact with air throughout the high-temperature can annealing process and cools to room temperature before being removed from the furnace, effectively preventing oxidation.
Water-sealed continuous tubular annealing involves running copper wire directly through a high-power heating tube, controlling the heating temperature and wire running speed to achieve the annealing purpose. The inlet end is open, using steam generated by the cooling water to expel air from the tube, ensuring the copper wire undergoes high-temperature annealing without oxidation. The outlet end uses a water seal to prevent air from entering. It is particularly important to note that an antioxidant must be added to the cooling water. The antioxidant forms a uniform passivation film on the copper wire surface, maintaining its luster and preventing oxidation and discoloration. The annealed copper wire is wrapped in plastic film and stored in a dry environment.
Control of the copper wire bundling and stranding process (hereinafter referred to as the stranding process): Before stranding the copper wires, if the copper wires have not undergone passivation treatment with an antioxidant, this treatment can be performed during the stranding process. The antioxidant formula uses a 0.2% - 0.4% benzotriazole alcohol solution. The antioxidant is dissolved in industrial alcohol and dripped into the stranded wires using an infusion tube. The amount dripped is just enough for the alcohol to wet the copper wires; dripping too much will cause it to fly onto the equipment and result in waste. At the same time, the alcohol solution is volatile and can lubricate and cool the stranded wires, preventing the surface oxidation of the compressed conductor caused by temperature rise during the drawing process. After the stranded conductor is unwound, it is wrapped in plastic film and stored in a dry environment.
Controlling the Insulation Tank Vulcanization Process: Traditional tank vulcanization processes are generally suitable for natural styrene-butadiene rubber insulation, using thiuram (TMTD) vulcanized insulating rubber, and employing both antioxidants MB and DNP. During vulcanization, antioxidant MB purifies the copper surface, forming a relatively stable golden-yellow protective film. Antioxidant DNP acts as a copper inhibitor, forming a stable chelate with active copper ions. MB and DNP have a synergistic effect. In production, polyester film or cotton paper (cable insulating paper) is used as an insulating layer for the conductor, both ends of the insulated core are sealed before vulcanization, and appropriate vulcanization tank pressure and vulcanization time are selected. After vulcanization, a golden-yellow protective film forms on the outermost surface of the bundled conductor, while the inner layer of the bundled wire turns red, and in severe cases, dark brown. While antioxidant passivation treatment improves the appearance quality of the stranded copper wire after primary vulcanization, secondary vulcanization inevitably leads to discoloration of the copper wire.
Control of the thermoplastic insulation process. Under normal circumstances, thermoplastic insulation has no effect on copper wire oxidation. However, if inferior raw materials are used in the cable material, although routine testing indicators may pass, long-term use will damage the copper wire. Plasticizers in PVC insulation, such as DOP, DOS, diethyl phthalate, trioctyl trimellitate, and chlorinated paraffin, can release free acid ions, which can corrode the copper wire, causing oxidation and blackening, severely affecting cable performance. Avoid using cable materials containing epoxidized soybean oil, as these materials will develop an oily surface over time, causing copper wire oxidation and discoloration, affecting cable product quality.
