Masterbatch (granular or dry powder) containing high molecular weight silicone, with polyolefin or mineral powder as a carrier, is used as a processing aid. Initially, it was used as a lubricant or rheology modifier. When it was discovered to be used in combination with metal hydrates as a flame retardant synergist, 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: Should be clearly stated. When used as a flame retardant synergist, the content should be between 40% and 80%.
When used as a rheology modifier, the content should be between 5% and 50%.
(2) Carrier: Compatibility with silicone or the user's main formulation substrate should be considered, and the polymer name and melt index of the carrier should be specified for the user's reference when designing the formulation. If inorganic mineral powder is used as the carrier, the powder name should be specified. The whiteness and fineness of the inorganic powder are very important to customers, so white and micron-sized powders should be selected as much as possible. 1. Halogen-Free Flame Retardant Synergist
In recent years, the development of halogen-free flame-retardant cables has been rapid. Preliminary statistics show an annual growth rate of 20-50%. Last year (2009), China used nearly 100,000 tons of halogen-free flame-retardant cable materials in the cable industry. These materials mostly use metal hydrates [Al(OH)3 or Mg(OH)2] as flame retardants, with an addition amount exceeding 60% in the formulation. Even so, halogen-free flame-retardant materials still suffer from poor flame-retardant effects, especially low oxygen index and poor self-extinguishing properties. Experiments show that adding an appropriate amount of silicone significantly improves the oxygen index, especially with the synergistic effect of the Mg(OH)2 system.
Experiments also show that halogen-free flame-retardant materials with an oxygen index greater than 36 can ignite most wires and cables with an outer diameter greater than 3mm through a single vertical combustion test. This is because the addition of silicone to the formulation reduces the heat release rate during wire combustion and forms a silica coating on the burning surface, isolating fresh air from entering.
Compared to red phosphorus, which is also used as a flame retardant synergist, silicone is colorless, odorless, and produces low smoke, while red phosphorus is purplish-red and produces a large amount of smoke and odor when burning, thus hindering its widespread use in halogen-free cable materials for a long time.
Another advantage of silicone as a flame retardant synergist is its internal and external lubrication (see later section). Its major drawback is poor flame retardancy. This problem has been a long-standing concern for customers and is expected to be resolved in the near future.
II. Lubricants or Rheology Modifiers
1. Polyethylene Materials
It is well known that a "sharkskin" phenomenon often occurs when extruding polyethylene insulated or sheathed wires and cables, especially when extruding linear polyethylene (LLDPE) or polyethylene (ULDPE or POE). The "sharkskin" phenomenon also sometimes occurs when extruding cross-linked polyethylene materials (whether peroxide cross-linked or silane cross-linked) due to insufficient lubrication in the material formulation. Currently, the international practice is to add trace amounts of fluoropolymers to the formulation, but this is costly and has limited application.
Experiments have shown that adding a trace amount of high molecular weight silicone (0.1-0.2%) to polyethylene or cross-linked polyethylene can effectively prevent the formation of sharkskin-like coatings. Simultaneously, its lubricating effect effectively reduces extrusion torque, preventing motor shutdown due to overload.
Silicone, as a lubricant, requires very small amounts and must be evenly distributed throughout the material to function effectively during processing. Since silicon is chemically inert, it will not react with other components in the formulation. It is recommended that cable manufacturers uniformly mix silicone during plasticizing and granulation for optimal use.
2. Halogen-Free Flame Retardant
The aforementioned materials contain a large amount of flame retardant (mineral powder), resulting in high viscosity and poor flowability. High viscosity makes it difficult for the motor to drive during extrusion, and poor flowability leads to a small amount of adhesive extruded. Therefore, the efficiency of halogen-free cable extrusion is only 1/2 to 1/3 that of polyvinyl chloride. Adding a suitable amount of silicone to the halogen-free formulation not only improves the flame retardant properties of the material but also significantly improves extrusion flowability and reduces viscosity.
