Currently used low-smoke halogen-free flame-retardant cable materials usually fall into two categories: polyolefin and ethylene-propylene. Among them, low-smoke halogen-free flame-retardant polyolefin cable materials are the mainstream. This article will focus on low-smoke halogen-free flame retardant polyolefin cable materials. Make an analysis of the formula technology, performance, extrusion equipment and process.
Low-smoke halogen-free flame-retardant polyolefin cable materials are usually composed of polyolefin blend resin plus flame-retardant fillers aluminum hydroxide, magnesium hydroxide and some appropriate amounts of antioxidants added to improve heat-resistant life. Sometimes in order to reduce the amount of smoke produced when burning, some smoke inhibitors are added, such as vanadium, nickel, molybdenum, iron, silicon, and nitrogen compounds. Its flame retardant mechanism is: when burning, the flame retardant fillers aluminum hydroxide and magnesium hydroxide will release crystal water and absorb a large amount of heat; at the same time, the dehydration reaction will generate a large amount of water vapor, which can dilute the flammable gas, thereby Preventing combustion, in addition, a layer of non-melting and non-combustible oxide hard shell will be formed on the surface of the material, blocking the channel for the polymer to react with external hot oxygen, and ultimately the material will be flame retardant and self-extinguishing.
Low-smoke halogen-free flame-retardant polyolefin cable materials must have good flame retardancy. The formula must have a large filling amount of aluminum hydroxide and magnesium hydroxide, usually more than 150 parts, and a large amount of inorganic flame retardants Filling will inevitably cause the material to greatly deteriorate in terms of physical and mechanical properties, electrical properties and extrusion process performance. In order to solve the balance between its flame retardancy and physical and mechanical properties, so that the material can fully meet the technical requirements of final use, commonly used methods are: on the one hand, modifying and grafting polyolefin materials to improve the polyolefin material’s extreme On the other hand, the surface of the inorganic flame retardant is chemically modified, usually treated with a coupling agent.
The use of a large amount of inorganic flame retardants gives low-smoke, halogen-free, flame-retardant polyolefin cable materials the characteristics of flame retardancy, low smoke, halogen-free, and low toxicity. It also makes it consistent with physical and mechanical properties, electrical properties, and process properties. There are differences between other non-flame retardant and halogen-containing flame retardant materials. Due to the different use occasions of low-smoke halogen-free wires and cables and the production processes of their supporting products, the performance requirements are also different, such as tensile strength, elongation at break, aging temperature and indicators, volume resistivity, oil resistance, scratch resistance, etc. In terms of performance, flexibility, flame retardant requirements, etc., different cables tend to have different emphasis. In halogen-free material technology, some of the above indicators are mutually restrictive. It is impossible to have an all-in-one product that can meet the requirements of all types of wires and cables mentioned above. The most prominent one is: the relationship between elongation at break and flame retardancy. Contradiction, contradiction between softness and thermal deformation and aging properties. All material manufacturers can do is to balance the advantages and disadvantages between certain properties on the basis of meeting basic performance requirements, and use different brands of products to adapt to wires and cables with different requirements. The following is an introduction to the properties of conventional low-smoke halogen-free flame-retardant polyolefin cable materials.
1. Flame retardant properties
In addition to being flame retardant, low-smoke halogen-free flame-retardant cable materials also have the characteristics of low smoke, halogen-free, low corrosion, and low toxicity compared with halogen-containing flame retardant cable materials.
At present, the GB/T 2406 method is mainly used to test the oxygen index of flame-retardant cable materials at room temperature to evaluate the flame-retardant performance of the material. Although the oxygen index at high temperature is also important for flame-retardant wires and cables, due to the test Conditions and other factors are generally rarely tested. Another parameter closely related to the flame retardancy of materials – temperature index – is rarely tested. The test method is specified in NES 715, which indicates how high the ambient temperature rises before the material can ignite in the air. The oxygen index of low-smoke halogen-free cable materials reaches 33-35 at room temperature, which can meet the general cable flame retardant requirements, and its temperature index is about 300.
As the most commonly used combustion test in the world, the oxygen index cannot be used as the only indicator to judge the flame retardancy of materials. The assessment of the self-extinguishing properties of materials seems to be a more appropriate measure of the flame retardant properties of materials. For example, as long as the oxygen index of polyvinyl chloride or halogen-containing flame-retardant polyolefin reaches about 30, it can be used as a wire with an insulating cross-section of 0.5mm2 to pass the VW-1 combustion test in the UL standard, while low-smoke halogen-free flame-retardant polyolefin Even if the oxygen index of olefin materials reaches above 34, it may not be able to meet the VW-1 combustion requirements. In addition, the same low-smoke halogen-free flame-retardant polyolefin cable materials, products with a high oxygen index may not necessarily have better self-extinguishing properties. For example: halogen-free materials using calcium carbonate and polysiloxane as flame retardants can have a higher oxygen index, even reaching 36-38, with appropriate base materials, but the self-extinguishing properties are likely to be not as good as those with an oxygen index of 32. -34 polyolefin/mineral hydroxide system. Some foreign users have begun to use the V-0 rating in UL 94 to assess the self-extinguishing properties of halogen-free materials as a supplement to the oxygen index assessment index. Now in terms of flame retardant indicators, a cone calorimeter method that is closer to actual combustion performance has attracted more and more attention. This method can measure combustion parameters such as ignition time, heating rate, and total calorific value when the material is burning. Conduct a quantitative evaluation. Experiments have shown that halogen-free materials with a higher heating rate are more likely to spread heat to the surroundings and are more likely to expand the combustion range, that is, they are easy to delay combustion and have poor self-extinguishing properties. However, halogen-free materials with a high oxygen index may not necessarily heat up faster than The one with low oxygen index generates heat slowly. Although there are certain limitations in simply using oxygen index to assess the flame retardancy of materials, the test method is simple and easy to implement, and in most cases it can relatively indicate the difference in flame retardancy of the same type of materials. , so it is not a bad idea to usually use the oxygen index as an assessment index of the flame retardancy of materials.
Finished cables generally use the bundled burning method of GB/T17651 to test the light transmittance of the cable after burning to evaluate the low-smoke performance of the cable. The light transmittance of low-smoke halogen-free cables reaches more than 60% and meets the requirements. For materials, the method of GB/T8323 is generally used to test the maximum smoke density Dm during flame and flameless combustion to evaluate the smoking performance of the material. The smoke emission of low-smoke halogen-free cable materials when burning is much lower than that of halogen-containing flame-retardant cable materials. Its indicators are generally flame Dm≦100 and flame-free Dm≦200, while halogen-containing flame-retardant cable materials have flame and flame-free cable materials. Dm is above 300.
(3) Halogen content
Currently, there are two methods, IEC 754-1 and MIL-C-24643A, used to determine the content of halogen and hydrohalic acid gases. The former is not suitable for occasions where the halogen acid gas content is less than 5 mg/g, that is, it is not suitable for the determination of the halogen content of halogen-free materials. The latter uses the x-ray principle to measure the content of halogen elements in the system, and the test accuracy is 0.2%. In a strict sense, Even this method cannot completely determine whether the material being tested does not contain any halogen. Therefore, many halogen-free material standards do not use halogen content as an assessment indicator, but rather use PH value and conductivity to characterize their halogen-free properties. Only BS, IEC and other standards set the indicator of hydrogen halide release in halogen-free systems at ≦5 mg/g as the cut-off point for assessment.
IEC published IEC 754-2 in 1991, which tests the acidity of gases released when cable materials are burned by measuring the pH value and conductivity, and stipulates that the measured pH ≧ 4.3 and conductivity ≦ 10 μs/mm are halogen-free and low-corrosion standards. The IL-C-24643A specification stipulates that the gas released when the cable material is burned has a halogen content of ≦0.2% as a halogen-free and low-corrosive standard. The pH value of low-smoke halogen-free cable materials is generally 4.5-6.0, and the conductivity is 0.7-5μs/mm.
(5) Toxicity index
The toxicity index is currently mainly detected using NES 713, which is the gas analysis method. The British Navy stipulates in the naval engineering standard NES 518-1983 “Specification for Cable Sheathing to Reduce Fire Hazard” that the toxicity index of halogen-free materials is not greater than 5. In China, it is usually also This indicator is adopted, but foreign countries also have requirements of no more than 3 for some halogen-free insulation products.
2. Physical, mechanical and electrical properties
Due to the large amount of inorganic flame retardant filling, the physical, mechanical and electrical properties of low-smoke halogen-free flame-retardant polyolefin cable materials are lower than those of non-flame-retardant and halogen-containing flame-retardant polyolefin cable materials. This is mainly reflected in the following aspects: aspects:
(1) Tensile strength and elongation at break are reduced. The tensile strength of halogen-free cable materials is around 10-14Mpa, and the elongation at break is around 150-250%;
(2) The anti-aging performance is reduced. The addition of a large amount of inorganic flame retardants greatly affects the anti-aging properties of halogen-free cable materials. Thermoplastic halogen-free flame-retardant cable materials are generally evaluated based on the change rate of tensile strength and elongation at break not exceeding ±30% after aging at 100℃×168hr;
(3) Thermal deformation becomes larger. Due to the large proportion of high VA content EVA in halogen-free flame retardant cable materials, its heat deformation resistance is poor. Generally, the deformation rate at 80℃×4hr is not more than 50% as the assessment index;
(4) The softness is poor. Generally, the Shore A hardness of halogen-free flame-retardant polyolefin cable materials exceeds HA90, and the Shore D hardness exceeds HD40.
(5) The trauma resistance is poor, and it is prone to scars after being scratched by external force. The degree of degradation of this performance is closely related to the formula, and different varieties of halogen-free cable materials vary greatly.
(6) Poor moisture resistance. Due to the addition of inorganic flame retardants magnesium hydroxide and aluminum hydroxide, halogen-free flame-retardant cable materials are easily hygroscopic. If there is no vacuum aluminum foil packaging, the storage period is best not to exceed 1 month under normal conditions, otherwise it will be affected by moisture absorption. use. Comparative experiments have found that after one day of immersion in water, the volume resistivity of halogen-free flame-retardant cable materials will drop by 60 to 80%.
(7) Volume resistivity and electrical breakdown strength have decreased. Compared with halogen-containing flame retardant polyolefin cable materials, the decrease is not very obvious, but compared with non-flame retardant polyolefin cable materials, the decrease is more obvious.
(8) The dielectric constant ε and dielectric loss tangent tgδ decrease sharply.
IEC 60754-1:Halogen Content HCL<0.5%;
IEC 60754-2:PH>4.3, Conductor<10us/mm
2. Low smoke:
IEC 61034-1, IEC 61034-2, light transmittance>=60%; low halogen requires smoke density Dm<300, minimum light transmittance>30%;
3. Low toxicity:
NES 713 (British Naval Standard); TI (Toxicity Index), maximum not exceeding 5
The toxicity of the gas is evaluated by the NES713 method. The toxicity index refers to the test produced under specified combustion conditions.
1. The amount of halogen acid (HCL) released during combustion is less than 5 mg/g —-IEC60754-1
2. PH value ≥ 4.3 ————————–IEC60754-2
3. Conductivity ≤10μS/mm ————————–IEC60754-2
4. Light transmittance (smoke density) ≥ 60% ————— IEC 61034-1
Materials that meet the above four requirements can be called low-smoke halogen-free materials.
Note: IEC60754-1 and IEC60754-2 are halogen-free standards, and IEC61034-1 is a low-smoke standard.