Causes of Blackening of Copper Wire
1. The emulsified oil pool for wire drawing is small, and the return line is short and sealed, resulting in slow heat dissipation and high emulsion oil temperature.
2. Caused by annealing of the copper wire.
① The cooling water for continuous drawing and annealing is typically tap water or groundwater. Because water quality varies from region to region, the pH value in some areas can be as low as 5.5-5.0 (normally 7.0-7.5). This removes the antioxidant film in the emulsion, causing the copper wire to oxidize and turn black after annealing.
② Finished copper wire drawn on a conventional wire drawing machine is annealed on a separate annealing line without the use of an antioxidant in the cooling water. This shortens the antioxidant period and quickly leads to oxidation and blackening.
3. Some older mills still use annealing vats for annealing. Oxidation and blackening can also occur for the following reasons:
① The annealing vat nuts are not tightened properly, resulting in air leaks after flushing with carbon dioxide or high-purity nitrogen.
② The copper wire temperature after exiting the vat is too high, exceeding 30°C.
③ Inadequate maintenance of the drawing emulsion leads to a pH value that is too high. The value is too low.
These conditions are more common during high summer temperatures. Constant use of the emulsion will cause depletion, and depletion is more rapid at high temperatures. If new crude oil is not replenished promptly, the fat content will be low. Coupled with the high temperature, the emulsion temperature may exceed 45°C, which can easily lead to oxidation and blackening.
4. Another issue is the widespread use of high-speed wire drawing, which increases speeds and reduces heat dissipation time, creating space and time for oxidation. Therefore, manufacturers are advised to pay more attention to the emulsion's fat content, operating temperature, and pH value. Bacteria multiply rapidly during the spring yellow mold rain season. Use a fungicide and mildew inhibitor, and antioxidants in the summer to address oxidation and blackening.
Causes
① Excessive deformation of the finished mold
② The mold sleeve's outer periphery and front are not properly sealed.
Solutions
Add a rubber gasket to the outlet of the finished mold and then tighten the threaded mold to resolve the oil leakage issue.
A common mistake is to minimize the deformation of the finished mold. Single-mold deformation requires a minimum deformation to generate pressure greater than the metal's yield point, achieving plastic deformation, maintaining dimensional stability, and allowing the surface of the wire to exhibit the shine produced by cold drawing.
Causes of Blackening Coil
We often use a variety of products that utilize coils, such as motors, hearing aids, remote-controlled toys, wireless chargers, power switches, and computers. Blackening of coils is caused by oxidation of the copper wire. Coil wire is primarily made of copper, and all metals oxidize, resulting in the appearance of blackening.
1. Technical Reasons
Previously, most domestic manufacturers used general-purpose copper rods, which can contain up to 99.95% copper. However, even with this, oxygen is still present in the copper. Because copper is not oxygen-free, oxidation inevitably occurs on the copper surface during processing due to contact with air.
Now, with the introduction of advanced oxygen-free copper production technology in China, as well as domestically developed oxygen-free copper production technology, the entire copper wire industry has adopted oxygen-free copper, which has undoubtedly significantly improved the problem of blackening copper wire. However, due to the processing of the copper rod, particularly the annealing process, and the poor storage conditions of the finished copper wire core, the copper wire itself will still experience slight oxidation.
2. Insulation Material Issues
Insulating varnishes can be divided into five categories: impregnating varnishes, wire enamels, covering varnishes, silicon steel sheet varnishes, and anti-corona varnishes. Impregnating varnishes are used for impregnating motor and electrical coils. Impregnating varnishes fill gaps and micropores in the insulation system and form a continuous film on the surface of the impregnated material, bonding the coils together into a solid whole. This effectively improves the insulation system's integrity, thermal conductivity, moisture resistance, dielectric strength, and mechanical strength.
Secondly, it also acts as a heat dissipator. When the insulating varnish is impregnated, the dried coil can be treated as a single unit, allowing heat to be easily transferred between the inner and outer layers, thereby dissipating heat.
Currently, my country's production processes, preparation methods, and patented formulas for impregnating varnishes and insulating oils are relatively backward. Produced and processed impregnating varnishes generally only have a short-term effect and will eventually flake off and fail.
3. Usage Issues
During the use of copper wire for coils, the following common problems arise: friction and collision; slow rinsing, allowing large amounts of water to come into contact with the coil; using waste engine oil for lubrication, resulting in residue on the conductor surface and damage to the insulation layer; and oxidation of the conductor during subsequent processing.
4. Copper Wire Annealing Process
Copper wire annealing is a metal heat treatment method in which the copper wire is slowly heated to a certain high temperature, held for a period of time, and then cooled at a corresponding rate.
Copper wire annealing can reduce hardness, improve machinability, eliminate residual stress, stabilize dimensions, reduce deformation and cracking, refine grain size, adjust structure, and eliminate structural defects. However, if the temperature exceeds 50°C before exiting the can during production, the specified pumping time is insufficient, the SO₂ content is high, and the shielding gas is impure, insufficient annealing can occur, and the copper wire will easily turn black over time.
The blackening of copper wire in coils is caused by a variety of factors, not just the four aforementioned issues, but also the condition of the copper wire itself, the coil processing technology, the vulcanization process, the coil structure, the formula, and the coil production environment.
Causes of Blackening of Copper Wire in Rubber-Sheathed Cables
The blackening of copper wire in rubber-sheathed cables is caused by a variety of factors, not just the rubber formulation. It also relates to the condition of the copper wire itself, the rubber processing and vulcanization process, the cable structure, the sheath rubber formulation, and the production environment.
1. Analysis of the Causes of Rubber Stickiness and Blackening of Copper Wire
1.1 Causes of the Copper Wire Itself
In the 1950s and 1960s, most domestic manufacturers used ordinary copper rod with a copper content of 99.99%. This was oxygen-containing copper rod. The production method involved heating the copper ingot, then rolling it through multiple passes to produce black copper rod. This rod was then drawn through large, medium, and small drawbars to create relatively fine copper wire. Because copper itself is not oxygen-free, oxidation inevitably occurs on the copper wire surface during processing.
In the 1980s, China introduced advanced production technology for oxygen-free copper rod, as well as domestically developed oxygen-free copper rod production technology. This led to widespread use of oxygen-free copper rod throughout the wire and cable industry, undoubtedly improving the problem of blackening copper wire. However, due to the processing of the copper rod, particularly the difficulty in mastering the annealing process, and the poor storage conditions of the processed copper wire core, the copper wire core itself has already been slightly oxidized, which is one of the reasons for the blackening of the copper wire.
1.2 Rubber Formula
In the 1950s, rubber insulation was formulated using a combination of natural rubber and styrene-butadiene rubber. Because the insulating rubber comes into direct contact with the copper wire, sulfur cannot be used directly as a vulcanizing agent. Even a small amount of sulfur will cause the copper wire to turn black. Compounds that can decompose free sulfur, such as the aforementioned accelerator TMTD and vulcanizing agent VA-7, must be used. Vulcanization accelerators are also needed to increase the vulcanization speed and degree to ensure the physical, mechanical, and electrical properties of the insulating rubber. However, in terms of elasticity, strength, and permanent deformation, the insulating rubber is inferior to rubber infused with sulfur (ignoring the blackening of the copper wire). Decades of practice have proven that TMTD is ineffective in resolving the problem of copper wire blackening.
Insulating rubber is available in a variety of colors, with red, blue, yellow, green, and black being the basic colors. These colors can also cause the rubber to become sticky and the copper wire to darken. The main fillers in the formula are light calcium carbonate and talc. Due to price constraints, some manufacturers use particularly inexpensive calcium carbonate and talc to reduce costs. These fillers have coarse particles, high free alkali content, and high impurities, resulting in poor physical and mechanical properties, poor electrical performance, and a tendency to cause the copper wire to darken.
Other manufacturers use activated ultrafine calcium carbonate to improve the physical and mechanical properties of insulating rubber. However, activated calcium carbonate is often treated with stearic acid, which also contributes to the blackening of the copper wire. The use of vulcanizing agent VA-7 can improve the blackening of the copper wire, but insufficient vulcanization results in significant permanent deformation of the rubber, which can cause the rubber to become sticky. The addition of accelerator ZDC, in particular, increases the vulcanization rate. To prevent scorching, accelerator DM is also added to slow the scorch process.
The structure of the accelerator ZDC is a metallic zinc intermediate two linked sulfur atoms in the TETD structure. The formula is S S H5C2 ‖ ‖ H5C2 > N-C-S-Zn-S-C-N < H5C2 H5C2 . This is very similar to the TETD structure (S S H5C2 ‖ ‖ H5C2 > N-C-S-S-C-N < H5C2 H5C2). The formula cannot avoid a similar structure to thiuram. While the blackening of copper wire may take slightly longer, it does not fundamentally solve the problem.
2. Analysis from the Structure of Wires and Cables
2.1 Catalytic aging of copper is a major cause of rubber stickiness.
Tests conducted by the former Soviet Cable Research Institute have shown that copper penetrates into the insulating rubber from the contact point during vulcanization. Insulating rubber with a thickness of 1.0-2.0mm contains 0.009-0.0027% copper. It's well known that trace amounts of copper can severely damage rubber, often referred to as heavy metal-catalyzed rubber aging.
During the insulation vulcanization process, thiuram releases some free sulfur that reacts with copper to form active copper-containing groups (CH3 | CH2-CH-C-CH2- | | S S | | Cu Cu). During aging, the weaker -S-S- bonds break, forming active copper-containing groups: Cu-S-. These groups react with the rubber and oxygen, breaking down the long bonds within the rubber, making it softer and stickier, forming low-molecular-weight chains. The French Rubber Research Institute, while studying the recurrence of stickiness, also noted that if the rubber contains harmful metals, such as copper, manganese, and other heavy metal salts, stickiness will occur regardless of the type of accelerator used.
2.2 Migration of Sulfur to the Insulating Rubber and Copper Wire Surfaces in Rubber-Sheathed Cables
Using radioisotopes, scientists in the former Soviet Union confirmed the possibility of sulfur diffusion in cable sheath rubber. In natural rubber-based vulcanizates, the diffusion coefficient of free sulfur is approximately 10⁻⁶ cm²/s at temperatures between 130°C and 150°C. In continuous vulcanization plants, where sheath rubber is vulcanized at temperatures between 185°C and 200°C, this diffusion coefficient is even higher.
Diffusion of free sulfur in the rubber sheath alters the structure of the thiuram rubber, potentially forming polysulfide bonds. These polysulfide compounds migrate through chemical decomposition and combination, a process known as "chemical diffusion." This migration not only alters the insulating rubber structure and reduces its heat resistance, but also reacts with the copper surface to form copper sulfide and cuprous sulfide, causing the copper wire to darken. In turn, copper sulfide and cuprous sulfide accelerate rubber aging and lead to stickiness.
3. Processing Factors
3.1 Rubber Processing Factors
In insulation formulations based on a combination of natural rubber and styrene-butadiene rubber, the natural rubber requires plasticization to improve its plasticity. To meet production targets, some large manufacturers use internal mixers for plasticizing and also add a small amount of a chemical plasticizer (accelerator M) to enhance plasticity. If the plasticizing temperature and the temperature during straining of the raw rubber are not properly controlled, exceeding 140°C, the rubber, when slowly passed through the rollers on the open mixer, will be exposed to the combined effects of heat, oxygen, and accelerator M. This can create a surface that appears to be coated with oil. This is actually due to the chemical plasticizer's facilitation of severe chain scission in the rubber molecules, resulting in a softer and stickier, lower molecular weight rubber.
Although this low molecular weight natural rubber is later mixed with styrene-butadiene rubber to produce insulating rubber, evenly dispersing it in the compound may not be a problem when extruded onto copper wire for continuous vulcanization. However, this creates a potential risk of rubber sticking to the copper wire: localized adhesion of the low molecular weight natural rubber to the copper wire.
The process for adding vulcanizers and accelerators to insulating rubber is also crucial. Some small factories add vulcanizer on an open mixing mill. This involves pouring the vulcanizer from a can into the center of the drum, with more vulcanizer in the center and less on the sides. As the vulcanizer absorbs into the rubber, the triangular rolls are infrequently repeated, resulting in uneven distribution of the vulcanizer throughout the rubber. Consequently, during continuous extrusion vulcanization, areas with high levels of vulcanizer are prone to blackening of the copper wire. Over time, these blackened areas can also cause the rubber to adhere to the copper wire.
3.2 Issues with Insulating Rubber Vulcanization
To meet production targets, some companies use continuous vulcanization tubes that are only 60 meters long, with a steam pressure of 1.3 MPa and a vulcanization speed of 120 m/min. This results in the insulating rubber remaining in the tube for only 30 seconds.
Rubber itself is a poor conductor of heat. When the surface temperature of the insulating core exceeds 190°C, the heat is transferred to the inner rubber layer in contact with the copper wire, where it is absorbed by the copper wire. By the time the copper wire reaches a temperature close to that of the inner rubber layer, the vulcanized rubber core has already emerged from the vulcanization tube. As a result, the inner rubber layer remains relatively cool, at approximately 170°C, and remains there for only a few seconds before exiting the vulcanization tube, entering the cooling and winding process. This results in insufficient vulcanization of the insulating rubber. To achieve adequate vulcanization, the accelerator TMTD (used as a vulcanizing agent) is used at a high dosage of 3.4%. This excess vulcanizing agent releases a large amount of free sulfur during the vulcanization process, which, in addition to crosslinking the rubber molecules, also produces excess free sulfur. This is the cause of the blackening of the copper wire surface.
