EVA Cable Material Low Smoke Halogen-free Flame Retardant Technology Research

EVA Cable Material Low Smoke Halogen-free Flame Retardant Technology Research

Halogen-containing flame-retardant cable materials have long dominated the cable material market due to their high flame retardancy, good processing performance, and low cost. However, cables containing halogens produce toxic and harmful gases when burned. Many people in fires die from suffocation caused by smoke and toxic gases; the hydrogen halide gases produced by combustion can also damage electronic and electrical equipment.

In 2003, the EU passed the RoHS directive, which clearly stipulated the maximum allowable value of "bromine" in electronic and electrical equipment. Since then, the use of halogen-containing cables has decreased year by year. Low-smoke halogen-free flame-retardant cables have gradually become the main force in the cable market. Many cable material manufacturers and research institutions have increased their investment in the research and development of low-smoke halogen-free flame-retardant cable materials.

Currently, the annual market demand for halogen-free flame-retardant cable materials is about 200kt, and it is expected that the demand for halogen-free flame-retardant cable materials will increase at a rate of about 10% in the next few years. By 2025, the annual demand for halogen-free flame-retardant cable materials is expected to reach around 350kt.

I. Flame-Retardant Base Material (EVA) Selection

The most critical base material for low-smoke halogen-free cable compounds is Ethylene-Vinyl Acetate copolymer (EVA). It features a low melting temperature, good flowability, polarity, and is halogen-free, allowing it to be compatible with various polymers and halogen-free flame retardants. This makes it widely used in low-smoke halogen-free flame-retardant cable compounds.

However, EVA has a Limiting Oxygen Index (LOI) of only 17% to 19%, classifying it as a flammable material. This significantly limits its application and development in flame-retardant cables. Therefore, it is crucial to modify EVA for flame retardancy to expand its range of use.

Low-smoke halogen-free cable materials are widely used in various fields, including nuclear power stations, aerospace, military industries, as well as in subways, high-speed railways, ships, high-rise buildings, and mining areas. In recent years, domestic products have captured a portion of the market in mid-to-low-end product areas, with relatively lower prices. However, for high-end products, these are mainly imported, resulting in higher prices.

In the telecommunications industry, especially with the development of fiber-to-the-home and 4G projects, the demand for low-smoke halogen-free cable materials has increased significantly, and new requirements have emerged. The base materials for low-smoke halogen-free compounds typically include EVA, polyethylene (PE), polypropylene (PP), and ethylene-propylene rubber (EPR). Apart from EVA, which contains polar groups, the others are non-polar materials or have very low polarity.

As base materials, PE, PP, and EPR have two inherent drawbacks:

• Poor Compatibility with Polar Flame Retardants: These materials have poor compatibility with halogen-free flame retardants that have strong polarity. This results in uneven dispersion of the flame retardants within the base material. When a large amount of halogen-free flame retardant is added, the mechanical properties of the material are severely compromised.

• Poor Oil and Non-polar Solvent Resistance: All three materials have poor resistance to oil and non-polar solutions, making it difficult for them to meet the requirements of cables in complex environments.

EVA is copolymerized from ethylene monomers and vinyl acetate monomers, as shown in Figure 1. The introduction of VA disrupts the regularity of the polyethylene molecules, leading to a decrease in crystallinity and an increase in polarity. This results in improved environmental stress crack resistance, compatibility with fillers, radiation crosslinking properties, oil resistance, and repeated bending performance for EVA. However, the tensile strength, hardness, melting point, and electrical insulation properties of EVA decrease, and its permeability to air and water vapor also deteriorates.

When the VA mass fraction is greater than 40%, the entire molecule exhibits a disordered arrangement with rubber elasticity, commonly referred to as ethylene-vinyl acetate rubber (EVM). When the VA mass fraction is between 5% and 40%, the material contains a small amount of crystallinity and is generally called EVA plastic.

The introduction of polar groups leads to varying degrees of reduction in EVA's electrical performance and tensile strength. Typically, EVA resins are only used for the insulation of low-voltage cables. When used as a low-smoke halogen-free sheath, to compensate for the insufficient tensile strength, EVA is often combined with PE, PP, and ethylene-octene copolymer (POE).

The blends can be used to produce both thermoplastic plastic cables and thermosetting rubber cables, with temperature ratings including 70, 90, 105, 125, 150, and 175°C. Formulators can design low-smoke halogen-free flame-retardant cable materials based on EVA/EVM that meet the performance requirements of users (or standards).

Given the good compatibility and extrusion performance of EVA with halogen-free flame retardants, the cable industry uses EVA as a base material to prepare highly flame-retardant oxygen barrier layers (typically referring to a limiting oxygen index of over 40%).

II. Low-Smoke Halogen-Free Flame Retardants for EVA Cable Materials

Commonly used halogen-free flame retardants for EVA cable materials include metal hydroxides, expandable, boron-based, and silicon-based flame retardants. Each type of flame retardant has a different flame retardancy mechanism.

• Metal Hydroxide Flame Retardants for EVA Cable Materials

Aluminum trihydroxide (ATH) and magnesium hydroxide (MH) are commonly used as halogen-free flame retardants. They possess three key functions: flame retardancy, smoke suppression, and filling. These materials are abundant and cost-effective, dominating the market for halogen-free flame retardants.

When heated, metal hydroxides decompose into metal oxides and water vapor. The water vapor not only removes heat but also dilutes the oxygen concentration in the air. The metal oxides act as a barrier to oxygen and prevent heat dissipation.

However, the efficiency of metal hydroxide flame retardants is relatively low compared to halogen-containing flame retardants. To achieve the same level of flame retardancy, the amount of metal hydroxide required is several times, or even more, than that of halogen-containing flame retardants. Additionally, metal hydroxide flame retardants have poor compatibility with polyolefins. Surface modification is necessary to reduce the surface energy of the powder, thereby increasing the flame retardant loading and improving the mechanical properties of the composite material.

The micronization of metal hydroxides increases the contact area between the powder and polyolefins, which can mitigate the decrease in mechanical properties caused by the high loading of flame retardants, thus improving flame retardancy efficiency.

ATH and MH can be blended and added to EVA, taking advantage of their different decomposition temperatures (ATH at 200°C and MH at 330°C) to achieve a gradient flame retardancy effect during the combustion of EVA cable materials.

Furthermore, researchers have used compatibilizers to increase the proportion of flame retardants in polyolefins. Compatibilizers can alleviate the reduction in mechanical properties caused by the addition of flame retardants. They can be added during the mixing of cable materials without additional processes, and the effects are quite significant.

• Intumescent Flame Retardants for EVA Cable Materials

Intumescent flame retardants (IFRs) primarily consist of phosphorus and nitrogen compounds. The phosphorus-based substances decompose upon heating to generate acids that promote the carbonization of polymers, forming a carbon barrier layer. The nitrogen-based substances decompose to produce gases that give the carbon barrier a honeycomb structure. This honeycomb carbon layer serves to isolate oxygen and heat, and also prevents dripping.

However, compared to metal hydroxide flame retardants, IFRs are more expensive, with the cost of typical IFRs being 3 to 5 times that of metal hydroxides. IFRs are mainly used in high-value specialty cable materials, where their usage is relatively limited. Developing IFRs with lower prices is one of the future development directions.

When compared to metal hydroxide flame retardants, IFRs produce more smoke during combustion. In halogen-free flame retardant cable materials, IFRs are generally used in combination with metal hydroxide flame retardants. On one hand, the higher flame retardancy efficiency of IFRs can reduce the amount of metal hydroxide required. On the other hand, metal hydroxides have excellent smoke suppression effects. The combination of these two types of flame retardants can achieve better low-smoke halogen-free flame retardancy.

In addition to the above, IFRs also include expandable graphite (EG). While EG is less effective when used alone, it can easily be combined with other flame retardants.

• Boron-based Flame Retardants for EVA Cable Materials

Boron-based flame retardants form a glass-like protective layer during combustion, which protects the polyolefin carbon layer from being destroyed and prevents the escape of volatile combustible materials. Zinc borate, a common boron-based flame retardant, releases water molecules when heated, which helps to remove heat and lower the ignition point, thereby achieving a flame-retardant effect.

• Silicon-based Flame Retardants for EVA Cable Materials

Silicon-based flame retardants are environmentally friendly and possess low toxicity, anti-drip, and smokeless characteristics. When organic silicon burns, the silicon remains in the condensed phase, forming a glassy inorganic carbonized layer that acts as a barrier to heat and oxygen, thus enhancing the flame-retardant effect.

Ceramifiable fire-resistant flame retardant materials (which form a hard ceramic shell under high temperatures due to the presence of silicon flame retardants) can automatically vitrify during cable combustion, creating a hard shell that provides insulation and oxygen isolation. The higher the ignition temperature, the harder and more dense the ceramic shell becomes, resulting in better fire resistance.

Combining ceramifiable fire-resistant materials with other flame retardants can achieve highly effective flame retardancy and fire resistance. Currently, ceramifiable flame-retardant polyolefins and ceramifiable silicone cables are widely used in important projects such as high-rise buildings, rail transit, and ships, representing a significant development direction for flame-retardant and fire-resistant cables.

III. Development Directions for Low-Smoke Halogen-Free Flame Retardant Cable Materials

The GB/T19666—2019 standard for flame-retardant and fire-resistant wires and cables has added requirements for the low toxicity of low-smoke halogen-free flame retardant cables, specifying the concentrations of nitrogen oxides (NOX < 90 mg/m³) and hydrogen cyanide (HCN < 55 mg/m³). This sets higher standards for low-smoke halogen-free flame retardant cable materials. Therefore, future development of low-smoke halogen-free flame retardant cable materials can focus on the following three aspects:

• EVA and Halogen-Free Flame Retardants: EVA has good compatibility with halogen-free flame retardants and excellent processing properties, making it the most important flame-retardant base material for low-smoke halogen-free cable materials. However, the introduction of polar groups in EVA reduces its tensile strength and insulation performance. When preparing low-smoke halogen-free sheaths, it is necessary to combine EVA with high-tensile-strength polymers such as PE and PP to meet cable standards. The relatively low insulation performance of EVA limits its use in medium and high voltage cables. Improving the insulation performance of EVA-based low-smoke halogen-free flame retardant cable materials is an important direction for development.

• Metal Hydroxide Flame Retardants: As one of the most important flame retardants for EVA-based low-smoke halogen-free cable materials, surface modification and ultrafine particle size are the development trends. China's mid-to-low-end inorganic flame retardant modifications can meet the requirements of conventional halogen-free flame retardant cable materials and are cost-effective. However, high-end metal hydroxide flame retardants still rely mainly on imports. It is hoped that domestic high-end flame retardant modification technology will break through soon to support the high-quality development of China's low-smoke halogen-free flame retardant cable materials.

• Flame Retardant Blending: Blending flame retardants is a basic approach for halogen-free flame retardant cable materials. However, when designing formulations, it is necessary to consider the latest toxicity requirements for low-smoke halogen-free cables. The use of intumescent flame retardants must be controlled within a reasonable range; otherwise, even if the flame retardancy is qualified, the low-smoke and low-toxicity performance may not meet the standard requirements. This is a challenge that researchers must face after the implementation of the new low-smoke halogen-free flame retardant cable standard.

IV. Conclusion

YINSU Flame Retardant Company has developed a special flame retardant FRP-950X for low-smoke halogen-free cables, which is not only applicable to PE, but also applicable to EVA as well as cables mixed with PE and EVA and cable compounds by virtue of its high content and low additive amount, effectively maintaining the original elasticity and toughness of raw materials and providing a more efficient and environmentally friendly flame retardant solution for the wire and cable industry.