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Mixing is a process that every type of rubber or plastic must go through. However, in the process of mixing, it is indeed a discipline. What are the reasons for the blackening of mixed rubber or copper wire? Copper wire is the core item of wires and cables, and rubber is the core item of the rubber industry. Below, Li will explain in detail the reasons for the blackening of mixed rubber.

The reason for the blackening of copper wire in rubber sheathed cables is caused by various factors, not only the formula of the rubber, but also the state of the copper wire itself, rubber processing technology, rubber vulcanization technology, cable structure, sheath rubber formula, production environment and many other factors.

1. Analysis of the causes of rubber sticking and copper wire blackening

1.1 The reason for the 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 to heat copper ingots and roll them through multiple passes to produce black copper rods, which were then pulled into relatively thin copper wires through large, medium, and small pulls. Because copper itself is not oxygen free copper, oxidation is inevitable on the surface of copper wire during processing. In the 1980s, advanced production technology for oxygen free copper rods was introduced in China, as well as domestically developed production technology for oxygen free copper rods, which enabled the entire wire and cable industry to use oxygen free copper rods. This undoubtedly improved the blackening problem of copper wires. However, due to the processing of copper rods, especially the mastery of toughening technology, and the poor storage conditions of processed copper wire cores, the copper wire cores themselves have undergone slight oxidation, which is also one of the reasons for the blackening of copper wires.

1.2 Reasons for Rubber Formula In the 1950s, natural rubber and styrene butadiene rubber were used in combination for rubber insulation. Due to the direct contact between the insulating rubber and the copper wire, sulfur cannot be directly used as a vulcanizing agent, and even using very little sulfur can cause the copper wire to turn black. It is necessary to use compounds that can decompose free sulfur, such as the accelerator TMTD and vulcanizing agent VA-7 mentioned earlier. At the same time, some vulcanizing accelerators should be used to improve the vulcanization speed and degree, ensuring the physical, mechanical, and electrical properties of the insulating rubber. But in terms of the elasticity, strength, and permanent deformation of insulating rubber, it is not as good as rubber with sulfur added (if copper wire blackening is not considered). Decades of practice have proven that TMTD cannot solve the problem of copper wire blackening. In addition, insulating rubber should come in various colors, with red, blue, yellow, green, and black being the basic colors. The appearance of these colors can also cause the rubber to become sticky and the copper wire to turn black. The main fillers in the formula are lightweight calcium carbonate and talc powder. Due to price constraints, some manufacturers use particularly cheap calcium carbonate and talc powder to reduce costs. These fillers have coarse particles, high content of free alkali, and many impurities, resulting in poor physical and mechanical properties, poor electrical performance, and easy blackening of copper wires. Some factories use activated ultrafine calcium carbonate to improve the physical and mechanical properties of insulating rubber, and most activated calcium is treated with stearic acid, which is also the reason for the blackening of copper wire. The use of vulcanizing agent VA-7 can improve the blackening of copper wire, but due to insufficient vulcanization, the permanent deformation of the rubber is large, which can cause the rubber to become sticky. Especially after adding the accelerator ZDC, the vulcanization rate was increased. In order to prevent burning, the accelerator DM was also added to delay the burning time. From the structure of accelerator ZDC, it can be seen that a metal zinc is connected between two connected sulfur atoms in the TETD structure, with the structural formula: SSH5C2 ∥∥ H5C2>N-C-S-Zn-S-C-NN-C-S-C-N

2. Analysis of Wire and Cable Structure

2.1 Catalytic aging of copper is an important reason for rubber sticking. The former Soviet Cable Science Research Institute has proven through experiments that copper infiltrates into the insulating rubber from the contact area with the rubber during the vulcanization process. Insulation rubber with a thickness of 1.0-2.0mm contains 0.009-0.0027% copper. As is well known, trace amounts of copper have a great destructive effect on rubber, which is commonly referred to as the catalytic aging of rubber by heavy metals. During the insulation vulcanization process, some free sulfur is released from the thiuram and reacts with copper to form active copper containing groups: CH3 │ CH2-CH-CH2- │ SS │ CuCu. During aging, weaker - S-S - bonds break, forming active copper containing groups: Cu-S -, which interact with rubber and oxygen, breaking the long bond molecules of rubber and making it soft and sticky. It is a combination of low molecular chains. The French Rubber Research Institute also pointed out that if rubber contains harmful metals such as copper, manganese, and other heavy metal salts, regardless of the type of accelerator, rubber sticking will occur.

Analysis of Wire and Cable Structures

The migration of sulfur from rubber sheathed cables to the surfaces of insulating rubber and copper wires was confirmed by Soviet scientists using radioactive isotopes. In vulcanized rubber based on natural rubber, the diffusion coefficient of free sulfur is about 10-6cm2/s at a temperature of 130-150 ℃. In a continuous vulcanization production plant, when vulcanizing the sheath rubber, the temperature is between 185-200 ℃, and the diffusion coefficient is even greater. Due to the diffusion of free sulfur in the rubber sleeve, the structure of the thiuram rubber is altered, which may result in the formation of polysulfide bonds. These polysulfide compounds migrate through chemical decomposition and combination, known as' chemical diffusion '. As a result of migration, not only can the structure of the insulating rubber be changed, reducing its heat resistance, but sulfur reacts with the copper surface to form copper sulfide and cuprous sulfide, leading to blackening of the copper wire. On the other hand, copper sulfide and cuprous sulfide accelerate the aging of rubber, leading to the occurrence of stickiness.

3. Reasons for processing technology

3.1 The reason for rubber processing is that in insulation formulas based on the combination of natural rubber and styrene butadiene rubber, natural rubber needs to be plasticized to improve its plasticity. Some large factories use internal mixers for molding in order to increase production, and also add a small amount of chemical plasticizer - accelerator M to improve plasticity. If the temperature control during plasticizing and rubber filtration is not good, resulting in high temperatures above 140 ℃, when the raw rubber is slowly passed through the drum on the open mill, and the accumulated rubber on top is subjected to the simultaneous action of thermal oxygen and accelerator M, it will be found that the rubber surface seems to be coated with a layer of oil. In fact, the rubber molecules are more severely broken under the promotion of chemical plasticizers, producing relatively soft and sticky smaller molecular weight rubber. Although later mixed with styrene butadiene rubber to produce insulating rubber, these small molecular weight natural rubber were evenly dispersed in the rubber material. After being extruded and continuously vulcanized on copper wires, there may not be any problems at that time, but it has already laid a hidden danger for the rubber to stick to copper wires. That is to say, these small molecular weight natural rubber will first experience local adhesion to copper wires. The process of adding vulcanizing agents and accelerators to insulating rubber is also very important. Some small factories add vulcanizing agents to the refining machine by pouring the cans containing vulcanizing agents into the middle of the drum, with many in the middle and fewer on both sides. When the vulcanizing agent is ingested into the rubber, the frequency of flipping the triangle is relatively small, which will cause uneven distribution of the vulcanizing agent in the rubber material. In this way, during continuous vulcanization of extruded packaging, it is easy for copper wires to turn black in areas with a high content of vulcanizing agents. Over time, there may also be rubber sticking to the copper wires in the blackened areas.

3.2 Reasons for vulcanization of insulating rubber: Some companies pursue production by continuously vulcanizing pipes that are only 60 meters long, with a steam pressure of 1.3 Mpa, and a vulcanization speed of 120 meters per minute. As a result, the residence time of the insulating rubber in the pipe is only 30 seconds. Rubber itself is a poor conductor of heat, and the surface temperature of the insulated wire core is greater than 190 ℃. When the temperature is transferred to the inner layer of rubber in contact with the copper wire, it is absorbed by the copper wire. When the copper wire heats up to a temperature close to that of the inner layer of rubber, the vulcanized rubber wire core has already come out of the vulcanization tube. In this way, the temperature of the inner layer rubber is relatively low, about 170 ℃. After only a few seconds, it will exit the vulcanization tube and enter the cooling and winding process, causing insufficient vulcanization of the insulating rubber. In order to achieve sufficient sulfurization. The dosage of accelerator TMTD (used as a vulcanizing agent) is as high as 3.4%, and excessive vulcanizing agent releases a lot of free sulfur during the vulcanization process. In addition to supplying cross-linked rubber molecules, there is also excess free sulfur. This is the reason that causes the copper wire surface to turn black.

In short, solving the problem of copper wire blackening is still difficult, and every process from copper wire to rubber must be taken seriously in order to achieve good results. The selection of rubber types and the adoption of vulcanization systems are still the key issues. The solution to this problem will take the test of time.