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Various Reasons Of Copper Wire Blackening In Rubber Sheath Cable
Feb 07, 2018

Copper Wire blackening reason is caused by a variety of factors, not only the rubber formula, but also with the copper wire itself in the state, rubber processing technology, rubber vulcanization process, cable structure, sheath rubber formula, production environment and many other factors.


1 The reason analysis of the rubber hair sticking and the copper wire blackening


1.1 The reason for the copper wire itself in the 20th century 50-60 years, most domestic manufacturers are using ordinary copper rods, copper content of 99.99%, are oxygen copper rods, production methods are copper after heating by a number of pressure delayed black copper rods, after large, medium and fine will be made of copper rods a relatively fine brass wire. Because copper itself is not oxygen-free copper, in the process of copper wire surface inevitably appear oxidation. To the 20th century 80 's, the domestic introduction of oxygen-free copper rod advanced production technology, as well as the domestic development of the anaerobic copper rod production technology, so that the entire wire and cable industry are used oxygen-less copper rod, which is undoubtedly to improve the copper wire blackening problem. However, because of the processing of copper rods, especially the process of toughening and processing of good copper core storage conditions are not good, so that the copper core itself has a slight oxidation, which is one of the reasons copper wire blackening.


1.2 Reasons for the rubber formula 20th century 50, rubber insulation are made of natural rubber and styrene-butadiene rubber and formula. Because the insulating rubber is directly in contact with the copper wire, sulfur can not be used as a vulcanizing agent directly, that is, the use of very little sulfur will make copper wire black. It is necessary to use compounds that can decompose free sulfur, such as the accelerator TMTD and vulcanizing agent VA-7 mentioned earlier, as well as some vulcanization accelerators to improve the vulcanization speed and vulcanization, and to ensure the physical and mechanical properties and electrical properties of the insulating rubber. But from the elastic, strong and permanent deformation of insulating rubber, it is better to add a sulphur rubber (if copper wire blackening is not considered). Decades of practice have proved that TMTD can not solve the problem of copper wire blackening. In addition, insulating rubber to have a variety of colors, red, blue, yellow, green, black is the basic color, these colors will also promote the appearance of rubber sticky and copper wire black. The main filler in the formula is light calcium carbonate and talcum powder, because of the price of the relationship, some manufacturers in order to reduce costs, with a very cheap price of calcium carbonate and talcum powder, these filler particles coarse, free alkali content of large, impurities, so the physical mechanical properties are poor, electrical performance is not good, but also easy to cause copper wire blackening. There are also plants with active superfine calcium carbonate to improve the physical and mechanical properties of insulating rubber, and active calcium is mostly treated with stearic acid, which is also the cause of copper wire blackening. The use of vulcanizing agent VA-7 can improve the copper wire blackening, but due to the degree of vulcanization is not enough, the permanent deformation of the rubber, will cause the rubber hair sticky. In particular, after adding the promoter ZDC, the vulcanization speed is increased, in order to prevent Coke burning, the promoter DM should be added to delay the burnt time. From the structure of the Promoter ZDC, it is indirectly a metal zinc in the two connected sulfur in the tetd structure, the structure is as follows: s S h5c2‖‖h5c2 >n-c-s-zn-s-c-n< h5c2 h5c2 and tetd structural type S H5C2‖‖H5C2 >n-c-s -s-c-n< h5c2 H5c2 is very close, in the formula can not avoid and autumn lamb similar structure copper wire blackening may be a little longer, but not fundamentally resolved.


2 structure analysis from wire and cable


2.1 The catalytic aging of copper is an important reason for the rubber sticking. The former Soviet Union cable Science Research Institute has proved that copper in the vulcanization process from contact with rubber into insulating rubber, 1.0-2.0mm thickness of insulating rubber copper 0.009-0.0027%. It is well known that trace copper has a great destructive effect on rubber, which is what we usually call a heavy metal-catalyzed aging of rubber. In the insulation vulcanization process, the autumn lamb precipitates several free sulfur and the copper reaction, forms the active copper-containing group: Ch3│ch2-ch-c-ch2-││s S││CU cu in the aging, the weaker-s-s-bond breaks, forms the active copper-containing base: cu-s-, it with the rubber function, simultaneously with the oxygen function, destroys the rubber the Long-bond molecules, which make rubber soft and sticky, are combinations of low molecular chains. The French Institute of Rubber Research on the issue of Sticky reproduction also pointed out: if the rubber contains harmful metals, such as: copper, manganese and other heavy metal salts, then regardless of the type of promoter, will occur rubber sticky phenomenon.


The migration of sulfur to insulating rubber and copper wire surface in 2.2 rubber sheathed cables the possibility of sulfur diffusion in cable sheath rubber was confirmed by the use of radioisotope by former Soviet scientists. The diffusion coefficient of free sulfur is about 10-6cm2/s at 130-150 ℃ temperature in the vulcanized rubber base of natural rubber. Continuous vulcanization production plant, vulcanization sheath rubber, temperature between 185-200 ℃, this diffusion coefficient is greater. Because of the diffusion of free sulfur in the rubber sheath, the structure of the autumn lamb rubber is changed, and the sulfur bond may be formed. These sulfur compounds migrate through chemical decomposition and combination, i.e. "chemical diffusion". As a result of the migration, not only can change the structure of insulating rubber, reduce its heat resistance, and sulfur and copper surface reaction, forming copper sulfide and cuprous sulfide, resulting in copper wire blackening. In turn, copper sulfide and cuprous sulfide accelerate the aging of rubber, and lead to the occurrence of sticky phenomenon.


3 The reasons of processing technology


3.1 Processing of rubber materials in the insulating formula based on natural and styrene-butadiene rubber, natural rubber needs to be molded to improve the plasticity of rubber. Some manufacturers in order to yield, with the mixer plastic refining, but also to add a small amount of chemical plasticizer-promoter m to improve plasticity. If the temperature is not well controlled by the rubber filter, 140 ℃ or higher temperatures are present, when the rubber is placed on the mill slowly through the drum, and the top of the glue due to heat and promoter m of the same effect, it will be found that the rubber surface as if coated with a layer of oil, is actually rubber molecules in the promotion of chemical plasticizer under the chain is more serious , resulting in a relatively soft and sticky small molecular weight rubber. Although the insulating rubber was later mixed with styrene butadiene rubber, these small molecular weight of natural rubber is evenly dispersed in the rubber material, these plastic materials squeezed in the copper wire for continuous vulcanization, then may not see what the problem, but has been for the rubber sticky copper wire buried a hidden danger, that is to say, These small molecular weight of natural gum will first appear local copper-bonded phenomenon. The process of insulating rubber plus vulcanizing agent and accelerator is also very important. Some small factories in the mixer on the vulcanizing agent, that is, will be equipped with vulcanizing agent cans, in the middle of the drum into a lot, and less on both sides. When the vulcanizing agent to eat in the eraser, the number of turn triangle less, will make vulcanizing agent in the rubber distribution uneven. In this way in the continuous vulcanization, containing vulcanizing agent more place is very easy to appear copper wire blackening phenomenon, in the black place of a long time, there will be rubber sticky wire phenomenon.


3.2 Insulation Rubber vulcanization reasons some enterprises in order to pursue production, the continuous vulcanization tube is only 60 meters long, the steam pressure is 1.3Mpa, and the vulcanization speed to open to 120 meters/min, so that the insulation rubber in the tube stay time only 30 seconds. The rubber itself is the heat of the bad conductor, insulation core surface temperature is greater than 190 ℃, when the temperature of heat to contact with copper wire in the inner rubber, but also by the copper wire endothermic, copper heating up to the inner rubber temperature close to the vulcanized rubber wire core has been out of the vulcanization tube. This layer of rubber temperature is relatively low, about 170 ℃, stay only a few seconds out of the vulcanization tube, into the cooling and receiving line, insulating rubber will be vulcanization insufficient. In order to achieve sufficient vulcanization. The dosage of accelerator tmtd (used as vulcanizing agent) up to 3.4%, excessive vulcanizing agent, in the vulcanization process released more free sulfur, in addition to cross-linked rubber molecules, there are excess free sulfur. This is why the copper surface is blackened.


In short, to solve the problem of copper wire blackening, the difficulty is still large, from the copper wire to the rubber every process must be taken seriously in order to achieve better results. The choice of gum seed and the adoption of vulcanization system are still the crux of the problem. The solution of this problem needs to go through the test of time.


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