Research on Low Smoke & Halogen-free Flame Retardant Technology for Cable Materials

Research on Low Smoke & Halogen-free Flame Retardant Technology for Cable Material

Halogen-containing flame retardant cable materials have been dominating the cable materials with its advantages of high flame retardant efficiency, good processing performance and low price. However, halogen-containing cables produce toxic and harmful gases when burned, and many people in the fire are asphyxiated by the smoke and toxic gases, which can lead to death; the hydrogen halide gases produced by combustion can also cause damage to electronic and electrical equipment.

In 2003, the European Union adopted the ROHS Directive, which clearly stipulates the halogenated elements in the “bromine” in the electrical and electronic equipment in the maximum limit value, and from then on the halogen-containing cables have been reduced year by year. Low-smoke halogen-free flame-retardant cables have gradually become the main force in the cable market, many cable manufacturers and research institutes have increased investment in research and development of low-smoke halogen-free flame-retardant cable materials.

At present, the annual market demand for halogen-free flame retardant cable material is about 200kt, and it is expected that in the next 3-5 years, the demand for halogen-free flame retardant cable material will increase at a rate of about 10%, and by 2025, the annual demand for halogen-free flame retardant cable material will reach about 350kt.

Low-smoke halogen-free cable material is the most important matrix material for ethylene vinyl acetate copolymer (EVA), which has a low melting temperature, good mobility, polarity and halogen-free, compatible with a variety of polymers and halogen-free flame retardant, widely used in low-smoke halogen-free flame retardant cable material.

Ethylene Vinyl Acetate Copolymer (EVA)

However, the limiting oxygen index value of EVA is only 17% to 19%, which is flammable, seriously restricting the application and development of EVA in flame retardant cables, so it is extremely important to carry out flame retardant modification of EVA to improve the scope of use of EVA.

EVA commonly used in cable materials

Low-smoke halogen-free cable material applications in addition to nuclear power plants, aerospace, military military industry, etc., the subway, high-speed rail, ships, high-rise buildings, mines and other fields also have a greater demand. After the development of recent years, in some low-end products, domestic products occupy a part of the market share, the price is relatively low; while in some highly sophisticated products, these products mainly rely on imports, the price is relatively high. In recent years, the communications industry, especially with the fiber to the home, 4G project to carry out the low-smoke halogen-free cable material is in great demand, but also put forward new requirements.

1. Flame retardant matrix (EVA) material selection

Low-smoke halogen-free base material is usually EVA, polyethylene (PE), polypropylene (PP), ethylene propylene rubber (EPR), etc., in addition to EVA contains polar groups, the other non-polar materials or very small polar materials. As base materials, PE, PP, and EPR have two natural defects:

①  Poor compatibility with the more polar non-halogenated flame retardants, resulting in uneven dispersion of flame retardants in the substrate, a large number of non-halogenated flame retardants added to the mechanical properties of the material will be seriously impaired;

② The 3 materials themselves are poorly resistant to oil and non-polar solutions, making it difficult to meet the application of cables in complex environments.

EVA is copolymerized from ethylene monomer and vinyl acetate monomer, and its synthesis process is shown in Figure 1. Due to the introduction of vinyl acetate VA, disrupting the regularity of the polyethylene molecule, resulting in a decrease in the crystallinity of the entire molecule and an increase in polarity, so that EVA's resistance to environmental stress cracking, compatibility with fillers, irradiation cross-linking, oil resistance, and repeated bending properties have been improved; however, EVA's tensile strength, hardness, melting point, and electrical insulating properties are consequently reduced, while resistance to air and water vapor permeation deteriorates.

When the VA mass fraction is greater than 40%, the whole molecule presents a disordered arrangement of the rubber elastic state, usually called ethylene vapour-modified (EVM) rubber; when the VA mass fraction is 5% to 40%, it contains a small amount of crystallization, usually called EVA plastic.

EVA synthesis diagram

The introduction of polar groups, EVA's electrical properties and tensile strength of varying degrees of reduction, the general situation of EVA resin can only be used as low-voltage cable insulation, as a low-smoke halogen-free sheath, in order to make up for the lack of its tensile strength, usually and PE, PP, ethylene-octene copolymer (POE) and used.

The copolymer can produce both thermoplastic plastic cables and thermosetting rubber cables, with temperature resistance levels including 70, 90, 105, 125, 150, 175 ℃; formulators can make low-smoke halogen-free flame-retardant cable compounds with performance in line with the requirements of the user (or the standard) with EVA/EVM as the base material.

Oxygen barrier layer high flame retardant power cable Photo: Anhui Huayu Cable

In view of EVA and halogen-free flame retardants have good compatibility and extrusion performance, the cable industry uses EVA as the base material to prepare high flame retardant oxygen barrier (usually refers to the ultimate oxygen index of 40% or more).

2. Low smoke halogen-free flame retardants for EVA cable materials

EVA cable material commonly used halogen-free flame retardants, including metal hydroxide, intumescent, boron, silicon and other flame retardants, different flame retardants and their flame retardant mechanisms are different.

2.1 Metal hydroxide flame retardants for EVA cable materials

As non-halogen flame retardants, ATH and MH have the triple functions of flame retardant, smoke elimination and filling, and they are widely available and inexpensive, occupying a dominant position in non-halogen flame retardants. Metal hydroxide is decomposed into metal oxide and water vapor by heat, the water vapor can not only take away the heat, but also dilute the oxygen concentration in the air, and the metal oxide plays the role of isolating oxygen and preventing the heat from being emitted.

However, the efficiency of metal hydroxide flame retardant is relatively low, compared with halogenated flame retardant, to achieve the same flame retardant effect, the amount of additive is several times or even a dozen times of halogenated flame retardant, at the same time, the compatibility of metal hydroxide flame retardant and polyolefin is relatively poor, it is necessary to modify its surface, reduce the surface energy of the powder to increase the amount of filler of flame retardant and improve the mechanical properties of composite materials.

The miniaturization of metal hydroxide can increase the contact area between the powder and polyolefin, which in turn can alleviate the decrease in mechanical properties due to a large number of flame retardants, thus improving the flame retardant efficiency.

Compound with ATH and MH, add to EVA, make full use of ATH (200 ℃) and MH (330 ℃) different decomposition temperature, so as to achieve different decomposition temperature gradient flame retardant effect when EVA cable material burning.

There are also researchers with compatibilizers to improve the proportion of flame retardants in polyolefins, compatibilizers can alleviate the mechanical properties due to the addition of flame retardants caused by the reduction of the situation, which only needs to be added to the cable material mixed compatibilizers, do not increase the additional processes, and the effect is very obvious.

2.2 Intumescent flame retardants for EVA cable materials

The composition of intumescent flame retardant (IFR) is mainly based on phosphorus and nitrogen, the phosphorus material is decomposed by heat to generate acid to promote the carbonization of polymers to form a carbon isolation layer; nitrogen material is decomposed by heat to generate the gas so that the carbon isolation layer has a honeycomb pattern. The honeycomb carbon layer can play the role of oxygen barrier, heat insulation, and at the same time can prevent the melt drop.

However, the price of intumescent flame retardant is expensive compared with metal hydroxide, and the price of common intumescent flame retardant is 3 to 5 times of metal hydroxide. Intumescent flame retardants are mainly used in high value-added special cable materials, the dosage is relatively small, the development of relatively inexpensive intumescent flame retardants is one of the future development direction.

Compared with metal hydroxide flame retardant, intumescent flame retardant will produce more smoke when burning, in the halogen-free flame retardant cable materials and metal hydroxide flame retardant will generally be used for compounding, on the one hand, intumescent flame retardant flame retardant efficiency is relatively high, you can reduce the amount of metal hydroxide; on the other hand, metal hydroxide has a very good smoke suppression effect; the use of the two flame retardant compounding to achieve a better low-smoke halogen-free flame retardant effect. The combination of the two flame retardants can achieve better low smoke and halogen free flame retardant effect.

In addition, the expansion type flame retardant also includes expandable graphite (EG), the efficiency of using this flame retardant alone is low, but it is easy to be used in combination with other flame retardants.

Intumescent flame retardants

2.3 Boron flame retardants for EVA cable materials

Boron flame retardant in the burning can form a glass-like protective layer to protect the polyolefin carbon layer is not destroyed, and to prevent the volatile combustibles from escaping. One of the water-containing zinc borate can release water molecules when heated, take away the heat, which reduces the ignition point of combustion, and play a flame retardant effect.

Expandable Graphite

2.4 Silicon flame retardant for EVA cable material

Silicon flame retardant is a green flame retardant, with low toxicity, anti-melting drops and smoke and other characteristics. Silicon in the combustion, silicon residue in the condensed phase, the formation of glassy inorganic carbonized layer, play a role in thermal insulation and oxygen barrier, so as to achieve synergistic flame retardant synergistic effect.

Porcelain fire-resistant flame retardant materials (containing silicon flame retardant at high temperatures to generate a hard ceramic shell) in the role of the porcelain agent, cable combustion can be automatically porcelain, the formation of a hard shell, play a role in heat insulation, isolation of oxygen. The higher the general ignition temperature, the harder the ceramic shell, the denser the surface, the better the fire-resistant effect.

Porcelain refractory materials and other flame retardant compound use, can achieve efficient fire-resistant effect. At present, ceramicized flame retardant polyolefin and ceramicized silicone cables have been widely used in high-rise buildings, rail transportation, ships and other important projects, is currently an important development direction of flame retardant fire-resistant cables.

Ceramizable flame retardant cables

GB/T19666-2019 “Flame-retardant and fire-resistant wire and cable or fiber optic cable” standard has increased the low toxicity performance requirements of low-smoke halogen-free flame-retardant cables, and the concentration of nitrogen oxides (NOX <90mg/m3) and hydrogen cyanide (HCN <55mg/m3) is also stipulated, which puts forward a higher requirement for low-smoke halogen-free flame-retardant cable materials, therefore, the In the future, low smoke halogen-free flame retardant cables can be carried out from the following 3 aspects:

(1) EVA and halogen-free flame retardant has good compatibility, but also has good processing performance, has become the most important low-smoke halogen-free cable flame retardant matrix materials. eva due to the introduction of polar groups, reducing its tensile strength and insulating properties, the preparation of low-smoke halogen-free sheaths, need to be and with high tensile strength of PE, PP and other polymers, in order to achieve the standard requirements of the cable.

EVA insulation properties are relatively low, limiting its use as a medium and high voltage cable range, how to improve the insulation properties of EVA-based low-smoke halogen-free flame-retardant cable materials, is an important direction of development of low-smoke halogen-free flame-retardant cable materials.

(2) Metal hydroxide as EVA-based low-smoke halogen-free flame retardant cable material is one of the most important flame retardant, surface modification and particle size ultrafine is the development trend, China's low-end inorganic flame retardant modification can meet the requirements of conventional halogen-free flame retardant cable material, and cheap; high-end metal hydroxide flame retardant or rely on imports are dominated by the expectation of a breakthrough in the domestic high-end flame retardant modification technology as soon as possible for China's low smoke halogen-free flame retardant cable material of high quality and comprehensive. Halogen-free flame retardant cable materials for China's low-smoke comprehensive development of high-quality help.

(3) Flame retardant compounding is the basic idea of flame retardant use of halogen-free flame retardant cable materials, but the design of the formula needs to take into account the latest toxicity requirements of low-smoke halogen-free cable. Expansion type flame retardant dosage must be controlled within a reasonable range, otherwise, even if the flame retardant performance is qualified, but low smoke and low toxicity performance does not necessarily meet the standard requirements, which is the new version of the low-smoke halogen-free flame retardant cable standard implementation of the vast number of scientific research workers must face the issue.

Conclusion

In the current cable material market's continuous pursuit of low-smoke halogen-free flame retardant technology, YINSU Flame Retardant Company has successfully developed a series of new low-smoke halogen-free flame retardants by virtue of its innovative R & D capability. These products include microcapsule-coated red phosphorus FRP-950X, piperazine PPAP-15, and expandable graphite EG, all of which have highly efficient flame retardant properties.

The application of microencapsulation technology not only improves the thermal stability of red phosphorus, but also reduces its potential impact on processing equipment and the environment. As a new type of phosphorus-nitrogen compound, piperazine PPAP-15 exhibits excellent flame retardancy and low smoke emission. Expandable graphite EG can rapidly expand to form a dense carbon layer when exposed to heat, effectively isolating oxygen and heat and enhancing the flame retardant effect of the material. In addition, YINSU Flame Retardant has developed ceramicized fire retardants, which form a hard ceramic layer at high temperatures to provide additional fire protection. The low additive properties of these innovative flame retardants not only meet the flame retardant requirements of cable materials, but also help to reduce the overall cost of flame retardancy, providing a cost-effective solution for the cable material industry. As technology continues to advance and market demand grows, these products from YINSU Flame Retardant are expected to occupy a significant position in the cable material market in the future.