一、Formulation Technology
Low smoke halogen-free flame retardant polyolefin cable material is usually composed of polyolefin blend resin, flame retardant filler aluminium hydroxide, magnesium hydroxide and some appropriate antioxidants added to improve the thermal life. Sometimes in order to reduce the smoke emission during combustion, some smoke inhibitors, such as vanadium, nickel, molybdenum, iron, silicon and nitrogen compounds, are added. Its flame retardant mechanism is: when burning, the flame retardant filler aluminium hydroxide and magnesium hydroxide will release crystalline water and absorb a lot of heat; at the same time, the dehydration reaction will produce a large amount of water vapor, which can dilute the flammable gas, thus preventing combustion. In addition, a layer of non-fusible and non-flammable oxide hard shell will be formed on the surface of the material, which will block the channel of thermal oxygen reaction between the polymer and the outside world. The final material is flame retardant and self-extinguishing.
Low smoke halogen-free flame retardant polyolefin cable material should have good flame retardancy. Aluminum hydroxide and magnesium hydroxide in the formula must have a larger filling amount, usually more than 150 phr. A large amount of inorganic flame retardant filling will lead to material deterioration in physical and mechanical properties, electrical properties and extrusion process properties. In order to balance the flame retardancy and physical and mechanical properties of polyolefin, and make the material fully meet the technical requirements for final use, the common methods are: on the one hand, modifying and grafting polyolefin materials to improve the polarity of polyolefin materials; on the other hand, chemical modification of the surface of inorganic flame retardants, usually treated with coupling agents.
二、Performance
A large number of inorganic flame retardants give low smoke halogen-free flame retardant polyolefin cable materials flame retardant, low smoke, halogen-free, low toxicity and other characteristics. At the same time, it also makes its physical and mechanical properties, electrical properties and technological properties different from other non-flame retardant and halogen-containing flame retardant materials. Due to the different applications of low smoke halogen-free wires and cables and the production process of their supporting products, the performance requirements are also different, such as tensile strength, elongation at break, aging temperature and index, volume resistance coefficient, oil resistance, scratch and abrasion resistance, flexibility, flame retardant requirements, etc., different cables tend to be biased. In halogen-free materials technology, some of the above indicators are mutually restrictive. It is impossible to have an all-round product that can meet all the requirements of all kinds of wires and cables. The most prominent ones are the contradiction between elongation at break and flame retardancy, the contradiction between softness and thermal deformation and aging performance. All material manufacturers can do is to balance the advantages and disadvantages of some properties on the basis of meeting the basic performance requirements, so that different brands of products can adapt to different requirements of wires and cables. The performance of conventional low smoke halogen-free flame retardant polyolefin cable material is introduced below.
1、Flame retardant properties
In addition to its flame retardancy, low smoke halogen-free flame retardant cable material has the characteristics of low smoke, halogen-free, low corrosion and low toxicity compared with halogen-containing flame retardant cable material.
(1)、Oxygen Index
At present, GB/T 2406 method is mainly used to test the oxygen index of flame-retardant cable material at room temperature in order to check the flame-retardant property of the material. Although oxygen index at high temperature is also important for flame-retardant wire and cable, it is seldom tested because of the factors such as test conditions. The temperature index, another parameter closely related to the flame retardancy of materials, is seldom tested. The test method is specified in NES 715, which indicates how high the ambient temperature is, the material can be ignited in the air. The oxygen index of low smoke halogen-free cable material reaches 33-35 at room temperature, which can meet the flame retardant requirements of general cables, and its temperature index is about 300.
Oxygen index, as the most commonly used combustion test in the world, can not be used as an index to judge the flame retardancy of materials. The evaluation of material self-extinguishing seems to be more appropriate to measure the flame retardancy of materials. For example, if the oxygen index of polyvinyl chloride or halogen-containing flame-retardant polyolefin reaches about 30, it can pass the VW-1 combustion test in UL standard as a wire with an insulation cross section of 0.5mm2, while the oxygen index of low smoke and halogen-free flame-retardant polyolefin material may not meet the requirements of VW-1 combustion even if it reaches above 34. In addition, low smoke halogen-free flame retardant polyolefin cable material, high oxygen index product self-extinguishing is not necessarily good. For example, the oxygen index of halogen-free materials with calcium carbonate and polysiloxane as flame retardants can be higher than that of polyolefin/mineral hydroxide systems with oxygen index of 32-34, even up to 36-38. Some foreign users have begun to use the V-0 grade of UL 94 to test the self-extinguishing property of halogen-free materials as a supplement to the oxygen index. Nowadays, more and more attention has been paid to the pyramidal calorimeter method, which is more close to the actual combustion performance. This method can quantitatively evaluate the ignition time, heating rate, total calorific value and other combustion parameters of materials during combustion. Experiments show that halogen-free materials with higher heating rate are more likely to spread heat to the surrounding area and expand the combustion range, i.e. they are easy to ignite and have poor self-extinguishing performance, while halogen-free materials with higher oxygen index may not necessarily have lower heating rate than halogen-free materials with lower oxygen index. Although there are some limitations in simply using oxygen index to evaluate the flame retardancy of materials, because the test method is simple and feasible, and the difference of flame retardancy of the same kind of materials can be relatively indicated in most cases, so oxygen index is usually used as an evaluation index of flame retardancy of materials.
(2)、Smoke density
Usually the finished cables are tested by bundle combustion method of GB/T 17651 to evaluate the low smoke performance of cables. The transmittance of low smoke halogen-free cables is more than 60% which meets the requirements. For materials, the method of GB/T8323 is generally used to test the maximum smoke density Dm of flameless and flameless combustion, in order to evaluate the smoking performance of materials. The smoke emission of low smoke halogen-free cable material is much lower than that of halogen-containing flame-retardant cable material. Its index is generally flame Dm < 100, flameless Dm < 200, while halogen-containing flame-retardant cable material has flame and flameless Dm more than 300.
(3)、Halide Content
At present, IEC 754-1 and MIL-C-24643A are two methods for the determination of halogen and hydrohalogen gas content. The former is not suitable for halogen content less than 5 mg/g, that is to say, it is not suitable for halogen content determination of halogen-free materials. The latter is to use the_-ray principle to measure the content of halogen elements in the system. The accuracy of the measurement is 0.2%. Strictly speaking, this method can not completely determine whether the tested materials contain halogen at all. Therefore, in many halogen-free materials standards, halogen content is not used as an evaluation index, but rather the halogen-free property is characterized by PH value and conductivity. Only in BS, IEC and other standards, the release of hydrogen halide in Halogen-free system is determined to be less than 5 mg/g, which is used as the boundary point for assessment.
(4)、Corrosive
IEC published IEC 754-2 in 1991. The acidity of the released gases during the combustion of cable materials was measured by measuring the PH value and conductivity. The halogen-free and low corrosion criteria were defined as PH (> 4.3) and conductivity (< 10 u s/mm). The standard of IL-C-24643A stipulates that the content of halogen less than 0.2% is the halogen-free and low corrosive standard for the gas released during the combustion of cable materials. The PH value of the low smoke halogen-free cable material is generally 4.5-6.0, and the conductivity is 0.7-5 ugs/mm.
(5)、Toxicity index
Nowadays, the toxicity index of halogen-free materials is mainly determined by NES 713, i.e. gas analysis method. The British Navy stipulates that the toxicity index of halogen-free materials is not more than 5 in the Navy Engineering Standard NES 518-1983 "Code for Reducing Fire Risk of Cable Sheaths". This index is also commonly used in China, but some halogen-free insulating products are also required to be no more than 3 abroad.
2、Physical, mechanical and electrical properties
The physical and mechanical properties 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 due to the filling of a large number of inorganic flame retardants.
(1)、Tensile strength and elongation at break decreased. The tensile strength of halogen-free cable material is about 10-14 Mpa and the elongation at break is about 150-250%.
(2)、The anti-aging property has been reduced. The aging resistance of halogen-free cable materials is greatly affected by the addition of inorganic flame retardants. Thermoplastic halogen-free flame-retardant cable material is generally assessed by the change rate of tensile strength and elongation at break not more than (+30%) after aging at 100 C *168hr.
(3)、The thermal deformation becomes larger. Due to the large proportion of EVA with high VA content in halogen-free flame-retardant cable material, the thermal deformation resistance of halogen-free flame-retardant cable material is poor, and the deformation rate of 80 4hr is generally less than 50%.
(4)、Softness is poor. Generally, the hardness of halogen-free flame retardant polyolefin cable material Shaw A exceeds HA90 and Shaw D exceeds HD40.
(5)、Trauma resistance is poor, and it is easy to get scars after scratching by external force. The reduction degree of this property is closely related to the formulation, and the halogen-free cable materials of different varieties vary greatly.
(6)、The moisture resistance is poor. Due to the addition of inorganic flame retardants such as magnesium hydroxide and aluminium hydroxide, halogen-free flame retardant cable materials are very easy to absorb moisture. If there is no vacuum aluminum foil packaging, its storage life should not exceed one month under normal conditions, otherwise it will affect the use of moisture absorption. It is found that the volume resistivity of halogen-free flame retardant cable material will decrease by 60-80% after one day's immersion.
(7)、Volume resistivity and electrical breakdown strength decreased. Compared with halogen-containing flame retardant polyolefin cable material, the decrease is not obvious, but it is more obvious compared with non-flame retardant polyolefin cable material.
(8)、The dielectric constant and dielectric loss angle tangent TG delta decrease sharply.
三、Testing standard
1.Halogen free：
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 Navy Standard); TI (Toxicity Index), up to 5
The toxicity of gases is assessed by NES713 method. The toxicity index is produced under specified combustion conditions by testing.
1、Halogen acid (HCL) release during combustion is less than 5 mg/g - IEC 60754-1
2、PHY205404.3 --------------------------IEC60754 -2
3、Conductivity less than 10 US/mm --------------------IEC60754-2
4、Transmittance (smoke density) > 60%--------------- IEC 61034-1
Meet the above four requirements can be called low smoke halogen-free materials
Note: IEC60754-1, IEC60754-2 are halogen-free standards, IEC61034-1 is low smoke standard.
Low smoke halogen-free flame retardant polyolefin cable material is usually composed of polyolefin blend resin, flame retardant filler aluminium hydroxide, magnesium hydroxide and some appropriate antioxidants added to improve the thermal life. Sometimes in order to reduce the smoke emission during combustion, some smoke inhibitors, such as vanadium, nickel, molybdenum, iron, silicon and nitrogen compounds, are added. Its flame retardant mechanism is: when burning, the flame retardant filler aluminium hydroxide and magnesium hydroxide will release crystalline water and absorb a lot of heat; at the same time, the dehydration reaction will produce a large amount of water vapor, which can dilute the flammable gas, thus preventing combustion. In addition, a layer of non-fusible and non-flammable oxide hard shell will be formed on the surface of the material, which will block the channel of thermal oxygen reaction between the polymer and the outside world. The final material is flame retardant and self-extinguishing.
Low smoke halogen-free flame retardant polyolefin cable material should have good flame retardancy. Aluminum hydroxide and magnesium hydroxide in the formula must have a larger filling amount, usually more than 150 phr. A large amount of inorganic flame retardant filling will lead to material deterioration in physical and mechanical properties, electrical properties and extrusion process properties. In order to balance the flame retardancy and physical and mechanical properties of polyolefin, and make the material fully meet the technical requirements for final use, the common methods are: on the one hand, modifying and grafting polyolefin materials to improve the polarity of polyolefin materials; on the other hand, chemical modification of the surface of inorganic flame retardants, usually treated with coupling agents.
二、Performance
A large number of inorganic flame retardants give low smoke halogen-free flame retardant polyolefin cable materials flame retardant, low smoke, halogen-free, low toxicity and other characteristics. At the same time, it also makes its physical and mechanical properties, electrical properties and technological properties different from other non-flame retardant and halogen-containing flame retardant materials. Due to the different applications of low smoke halogen-free wires and cables and the production process of their supporting products, the performance requirements are also different, such as tensile strength, elongation at break, aging temperature and index, volume resistance coefficient, oil resistance, scratch and abrasion resistance, flexibility, flame retardant requirements, etc., different cables tend to be biased. In halogen-free materials technology, some of the above indicators are mutually restrictive. It is impossible to have an all-round product that can meet all the requirements of all kinds of wires and cables. The most prominent ones are the contradiction between elongation at break and flame retardancy, the contradiction between softness and thermal deformation and aging performance. All material manufacturers can do is to balance the advantages and disadvantages of some properties on the basis of meeting the basic performance requirements, so that different brands of products can adapt to different requirements of wires and cables. The performance of conventional low smoke halogen-free flame retardant polyolefin cable material is introduced below.
1、Flame retardant properties
In addition to its flame retardancy, low smoke halogen-free flame retardant cable material has the characteristics of low smoke, halogen-free, low corrosion and low toxicity compared with halogen-containing flame retardant cable material.
(1)、Oxygen Index
At present, GB/T 2406 method is mainly used to test the oxygen index of flame-retardant cable material at room temperature in order to check the flame-retardant property of the material. Although oxygen index at high temperature is also important for flame-retardant wire and cable, it is seldom tested because of the factors such as test conditions. The temperature index, another parameter closely related to the flame retardancy of materials, is seldom tested. The test method is specified in NES 715, which indicates how high the ambient temperature is, the material can be ignited in the air. The oxygen index of low smoke halogen-free cable material reaches 33-35 at room temperature, which can meet the flame retardant requirements of general cables, and its temperature index is about 300.
Oxygen index, as the most commonly used combustion test in the world, can not be used as an index to judge the flame retardancy of materials. The evaluation of material self-extinguishing seems to be more appropriate to measure the flame retardancy of materials. For example, if the oxygen index of polyvinyl chloride or halogen-containing flame-retardant polyolefin reaches about 30, it can pass the VW-1 combustion test in UL standard as a wire with an insulation cross section of 0.5mm2, while the oxygen index of low smoke and halogen-free flame-retardant polyolefin material may not meet the requirements of VW-1 combustion even if it reaches above 34. In addition, low smoke halogen-free flame retardant polyolefin cable material, high oxygen index product self-extinguishing is not necessarily good. For example, the oxygen index of halogen-free materials with calcium carbonate and polysiloxane as flame retardants can be higher than that of polyolefin/mineral hydroxide systems with oxygen index of 32-34, even up to 36-38. Some foreign users have begun to use the V-0 grade of UL 94 to test the self-extinguishing property of halogen-free materials as a supplement to the oxygen index. Nowadays, more and more attention has been paid to the pyramidal calorimeter method, which is more close to the actual combustion performance. This method can quantitatively evaluate the ignition time, heating rate, total calorific value and other combustion parameters of materials during combustion. Experiments show that halogen-free materials with higher heating rate are more likely to spread heat to the surrounding area and expand the combustion range, i.e. they are easy to ignite and have poor self-extinguishing performance, while halogen-free materials with higher oxygen index may not necessarily have lower heating rate than halogen-free materials with lower oxygen index. Although there are some limitations in simply using oxygen index to evaluate the flame retardancy of materials, because the test method is simple and feasible, and the difference of flame retardancy of the same kind of materials can be relatively indicated in most cases, so oxygen index is usually used as an evaluation index of flame retardancy of materials.
(2)、Smoke density
Usually the finished cables are tested by bundle combustion method of GB/T 17651 to evaluate the low smoke performance of cables. The transmittance of low smoke halogen-free cables is more than 60% which meets the requirements. For materials, the method of GB/T8323 is generally used to test the maximum smoke density Dm of flameless and flameless combustion, in order to evaluate the smoking performance of materials. The smoke emission of low smoke halogen-free cable material is much lower than that of halogen-containing flame-retardant cable material. Its index is generally flame Dm < 100, flameless Dm < 200, while halogen-containing flame-retardant cable material has flame and flameless Dm more than 300.
(3)、Halide Content
At present, IEC 754-1 and MIL-C-24643A are two methods for the determination of halogen and hydrohalogen gas content. The former is not suitable for halogen content less than 5 mg/g, that is to say, it is not suitable for halogen content determination of halogen-free materials. The latter is to use the_-ray principle to measure the content of halogen elements in the system. The accuracy of the measurement is 0.2%. Strictly speaking, this method can not completely determine whether the tested materials contain halogen at all. Therefore, in many halogen-free materials standards, halogen content is not used as an evaluation index, but rather the halogen-free property is characterized by PH value and conductivity. Only in BS, IEC and other standards, the release of hydrogen halide in Halogen-free system is determined to be less than 5 mg/g, which is used as the boundary point for assessment.
(4)、Corrosive
IEC published IEC 754-2 in 1991. The acidity of the released gases during the combustion of cable materials was measured by measuring the PH value and conductivity. The halogen-free and low corrosion criteria were defined as PH (> 4.3) and conductivity (< 10 u s/mm). The standard of IL-C-24643A stipulates that the content of halogen less than 0.2% is the halogen-free and low corrosive standard for the gas released during the combustion of cable materials. The PH value of the low smoke halogen-free cable material is generally 4.5-6.0, and the conductivity is 0.7-5 ugs/mm.
(5)、Toxicity index
Nowadays, the toxicity index of halogen-free materials is mainly determined by NES 713, i.e. gas analysis method. The British Navy stipulates that the toxicity index of halogen-free materials is not more than 5 in the Navy Engineering Standard NES 518-1983 "Code for Reducing Fire Risk of Cable Sheaths". This index is also commonly used in China, but some halogen-free insulating products are also required to be no more than 3 abroad.
2、Physical, mechanical and electrical properties
The physical and mechanical properties 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 due to the filling of a large number of inorganic flame retardants.
(1)、Tensile strength and elongation at break decreased. The tensile strength of halogen-free cable material is about 10-14 Mpa and the elongation at break is about 150-250%.
(2)、The anti-aging property has been reduced. The aging resistance of halogen-free cable materials is greatly affected by the addition of inorganic flame retardants. Thermoplastic halogen-free flame-retardant cable material is generally assessed by the change rate of tensile strength and elongation at break not more than (+30%) after aging at 100 C *168hr.
(3)、The thermal deformation becomes larger. Due to the large proportion of EVA with high VA content in halogen-free flame-retardant cable material, the thermal deformation resistance of halogen-free flame-retardant cable material is poor, and the deformation rate of 80 4hr is generally less than 50%.
(4)、Softness is poor. Generally, the hardness of halogen-free flame retardant polyolefin cable material Shaw A exceeds HA90 and Shaw D exceeds HD40.
(5)、Trauma resistance is poor, and it is easy to get scars after scratching by external force. The reduction degree of this property is closely related to the formulation, and the halogen-free cable materials of different varieties vary greatly.
(6)、The moisture resistance is poor. Due to the addition of inorganic flame retardants such as magnesium hydroxide and aluminium hydroxide, halogen-free flame retardant cable materials are very easy to absorb moisture. If there is no vacuum aluminum foil packaging, its storage life should not exceed one month under normal conditions, otherwise it will affect the use of moisture absorption. It is found that the volume resistivity of halogen-free flame retardant cable material will decrease by 60-80% after one day's immersion.
(7)、Volume resistivity and electrical breakdown strength decreased. Compared with halogen-containing flame retardant polyolefin cable material, the decrease is not obvious, but it is more obvious compared with non-flame retardant polyolefin cable material.
(8)、The dielectric constant and dielectric loss angle tangent TG delta decrease sharply.
三、Testing standard
1.Halogen free：
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 Navy Standard); TI (Toxicity Index), up to 5
The toxicity of gases is assessed by NES713 method. The toxicity index is produced under specified combustion conditions by testing.
1、Halogen acid (HCL) release during combustion is less than 5 mg/g - IEC 60754-1
2、PHY205404.3 --------------------------IEC60754 -2
3、Conductivity less than 10 US/mm --------------------IEC60754-2
4、Transmittance (smoke density) > 60%--------------- IEC 61034-1
Meet the above four requirements can be called low smoke halogen-free materials
Note: IEC60754-1, IEC60754-2 are halogen-free standards, IEC61034-1 is low smoke standard.
