Plastic knowledge: Application of silicone masterbatch in cable materials
Plastic knowledge: Application of silicone masterbatch in cable materials
Silicone masterbatch pr powder contains high molecular weight silicone, and the masterbatch (granular or dry powder) with polyolefin or mineral powder as the carrier is used as a processing aid. First, it was used as a lubricant or rheology modifier. When it was found to be used as a flame retardant synergist in combination with metal hydrates, the dosage suddenly increased, and domestic manufacturers sprang up like mushrooms after rain.
The technical parameters of commercial silicone masterbatch should include the following:
(1) Silicone content: It should be clearly indicated. When used as a flame retardant synergist, the content is between 40 and 80%. When used as a rheology modifier, the content is between 5 and 50%.
(2) Carrier: The compatibility with silicone or the user’s main formula base material should be considered, and the polymer name and melting index of the carrier should be indicated for the user’s reference when designing the formula. If inorganic mineral powder is used as the carrier, the powder name should be indicated. The whiteness and fineness of inorganic powder are very important to customers, so white and micron-grade powder should be selected as much as possible.
I. Halogen-free flame retardant synergist
In recent years, the halogen-free development of flame retardant cables has been rapid. According to preliminary statistics, it has been growing at a rate of 20-50% per year. Last year (2009), China used nearly 100,000 tons of halogen-free flame retardant cable materials in the cable industry. Most of these materials are metal hydrates [Al(OH)3 or Mg(OH)2] as flame retardants, and the amount added in the formula exceeds 60%. Even so, halogen-free flame retardant materials still have the problem of poor flame retardant effect, especially low oxygen index and poor self-extinguishing property. Tests have shown that the addition of silicone in an appropriate amount can significantly improve the oxygen index, especially the Mg(OH)2 system. Tests have also shown that halogen-free flame retardant materials with an oxygen index greater than 36 can make most wires and cables with an outer diameter greater than 3mm through a single vertical combustion test. The reason is that silicone is added to the formula, which reduces the heat release rate when the wire burns and forms a silicon dioxide covering film on the combustion surface, isolating the entry of fresh air.
Compared with red phosphorus, which is also used as a flame retardant enhancer, silicone is colorless, odorless and low in smoke, while red phosphorus is purple-red and produces a lot of smoke and odor when burned. Therefore, it has been difficult to promote its use in halogen-free cable materials for a long time.
Another advantage of silicone as a flame retardant synergist is that it has internal and external lubrication (see the following section). The biggest disadvantage is that it has poor “non-flame delay”. This problem has a long history and has always been of concern to customers, and I believe it will be solved in the near future.
II. Lubricant or rheology modifier
1 Polyethylene material
As we all know, when extruding polyethylene insulated or sheathed wires and cables, the “shark skin” phenomenon often occurs, especially when extruding linear polyethylene (LLDPE) or polyethylene (ULDPE or POE) at that time. Extruded cross-linked polyethylene materials (whether peroxide or silane cross-linked) also sometimes suffer from the “shark skin” phenomenon due to insufficient consideration of the lubrication system in the material formulation. At present, the international practice is to add trace amounts of fluoropolymers to the formula, but the cost is high and the application is limited. Through experiments, adding a trace amount of higher molecular weight silicone (0.1-0.2%) to polyethylene or cross-linked polyethylene can effectively prevent the formation of shark skin. At the same time, with its lubrication effect, the extrusion torque can be effectively reduced and the motor can be prevented from shutting down due to overload. As a lubricant, silicone needs to be evenly distributed in the material due to the small amount added so that the silicone can play its role during processing. Since silicone is chemically inert, it will not interact chemically with the ingredients in the formula. It is recommended that the cable factory mix the silicone evenly during plasticizing and granulating to facilitate the use of the cable factory.
2 Halogen-free retardant
The above-mentioned materials contain a large amount of flame retardant (mineral powder), with high viscosity and poor fluidity; high viscosity makes it difficult for the motor to drive during extrusion, poor fluidity, and small glue volume during extrusion. Therefore, when the cable factory extrudes halogen-free cables, its efficiency is only 1/2~1/3 of PVC. Appropriately adding a certain amount of silicone to the halogen-free formula not only improves the flame retardant properties of the material, but also significantly improves the extrusion fluidity and reduces the viscosity of the material.
