Theoretical Study on Epoxy Flame-Retardant Self-Extinguishing Insulation Powder Coatings

Theoretical Study on Epoxy Flame-Retardant Self-Extinguishing Insulation Powder Coatings

In recent years, the state has vigorously implemented green and environmentally friendly policies. Powder coatings, thanks to their environmental friendliness and functionality, are being applied in more and more different industrial sectors. Each industry has different performance requirements for them. Among them, in the field of new energy vehicles, when the vehicle is fast-charging, the on-board battery will accumulate a large amount of thermal energy in a short time. If the battery management system fails and the heat cannot be dissipated in time, it may cause damage to the battery cells, or even spontaneous combustion and explosion. It may also cause the battery to malfunction during charging and generate accidents. Vehicle collisions can also cause the battery to burn or spontaneously combust upon damage. Therefore, its protective layer must possess flame-retardant properties so that, in the event of an explosion, it can slow down the spread of flames and heat, providing a fire insulation effect, thus buying the driver more time to escape. In addition, during the operation and charging/discharging of new energy vehicles, current is generated. To ensure normal operation, the protective layer must also have high-voltage insulation properties to prevent the coating from being broken down by high-voltage current.

I. Flame-Retardant Classification of Powder Coatings
Powder coatings, mainly composed of resin, curing agent, filler, additives and other raw materials, need to achieve their flame-retardant properties through experimental exploration of each component from different dimensions. There are two flame-retardant methods for powder coatings: one is additive flame retardancy, and the other is reactive flame retardancy. Additive flame retardancy refers to relying on the material's own flame-retardant characteristics to complete the flame-retardant protection of the coating, so that it has flame-retardant properties. This is achieved by adding various flame-retardant substances to the powder coating that do not participate in the curing reaction. Reactive flame-retardant method refers to introducing flame-retardant structures into the powder coating. Through monomers or structures in the raw materials participating in the polymerization reaction, flame-retardant groups are ultimately introduced into the powder coating, thereby improving the flame-retardant properties of the powder coating.

1. Additive Flame Retardancy
This flame-retardant method does not directly participate in the reaction. Instead, it relies on its own flame-retardant characteristics to complete the flame-retardant properties of the coating. Therefore, it is mainly composed of fillers and additives, while resin and curing agent are not included in this category.

• Research on Filler Flame Retardancy

At present, the common flame-retardant fillers on the market mainly include aluminum hydroxide, magnesium hydroxide, etc. However, their particles are generally at the micron level, unable to achieve high filler loading effects, with low flame-retardant efficiency, and also affecting the physical and mechanical properties of the material. Therefore, by enhancing the interfacial interaction force between flame-retardant fillers, it is possible to disperse them more evenly in the powder coating, thereby more effectively improving the mechanical properties of the blended material. Addition of specific nanoscale flame-retardant fillers into the powder coating for filler flame retardancy will greatly reduce filler loading and effectively enhance flame-retardant efficiency.

Research uses nanoscale aluminum hydroxide, magnesium hydroxide, and silica powder for comparative experiments to explore the effect of flame-retardant fillers on powder coating performance.

From the test data in Table 1, it can be seen that powder coatings with added nanoscale fillers have excellent mechanical properties and can meet the requirements. Among them, powder coatings with added magnesium hydroxide and aluminum hydroxide have the best flame-retardant properties, both achieving V-0 level flame-retardant effect. Powder coatings with silica powder alone as filler cannot meet the flame-retardant requirements.

Because aluminum hydroxide begins to decompose at 200~300°C, after decomposition, it will produce water vapor due to dehydration reaction, causing an endothermic reaction on the surface coating, reducing the temperature of the coating surface. At the same time, the generated water vapor dilutes the concentration of combustible gases and oxygen, so it has good thermal insulation flame-retardant effect.

The decomposition temperature of magnesium hydroxide is higher, reaching 340~490°C. Its flame-retardant mechanism is similar to aluminum hydroxide, but magnesium hydroxide can promote carbonization of the coated surface. The formed carbonized layer can isolate the contact between oxygen and the metal surface. Aluminum hydroxide does not have this effect. Therefore, under the same conditions, the flame-retardant effect of magnesium hydroxide is significantly better than that of aluminum hydroxide.

Since silica powder is composed of SiO2, the Si—O bond in SiO2 structure forms a silicate-like structure in the cured product, generating chemical forces, giving the coating better solvent resistance and insulation properties, while also having a certain flame-retardant effect. However, experimental data shows that the flame-retardant effect of silica powder is poor and does not meet V-0 level flame-retardant requirements.

To explore the combination of flame retardancy and insulation properties, magnesium hydroxide, aluminum hydroxide and silica powder are compounded to explore the best powder coating formula that combines flame retardancy and insulation. The data is shown in Tables 2 and 3.

Experimental results indicate that after compounding magnesium hydroxide, aluminum hydroxide and silica powder, the coatings can achieve both flame retardancy and insulation voltage resistance characteristics. Among them, when the compounding ratio of magnesium hydroxide to silica powder is 2:1, the comprehensive performance of the coating is the best, able to meet both flame retardancy and insulation voltage resistance requirements.

• Research on Additive Flame Retardants

From an environmental perspective, halogen-free flame-retardant additives were selected for research, because halogen elements release harmful gases at high temperatures, are volatile and permeable, and easily cause pollution to the surrounding environment through gaseous, liquid, and solid states.

Phosphorus flame retardants are currently the most frequently used halogen-free flame-retardant additives on the market. Their diverse structures and high flame-retardant efficiency have attracted much attention. According to different structures, inorganic phosphorus flame retardants can be divided into ammonium polyphosphate, red phosphorus, etc., while organic phosphorus flame retardants are divided into various flame retardants, such as phosphate esters, phosphates, etc. Due to differences in oxidation states and coordination bonds, phosphorus flame retardants generally exhibit different flame-retardant mechanisms. Usually, high-oxidation phosphorus flame retardants mainly act by condensing the flame-retardant phase. Phosphoric acid and its derivatives decompose upon heating, causing dehydration and carbonization of polymers, forming a carbonized layer to block oxygen, minimizing heat transfer during combustion, thereby achieving flame-retardant and thermal insulation effects. Low-oxidation phosphorus flame retardants mainly act through gas-phase flame retardancy. Upon thermal decomposition, they produce a large number of phosphorus-based free radicals, which, through chain reactions during quenching, inhibit flame propagation, terminating combustion of OH· and H· radicals, thus achieving flame retardancy. However, at the same time, smoke release will also increase. Currently, phosphorus flame retardants are at a relatively high price level. These additives have strong flame-retardant effects, but when used alone, they will affect the mechanical properties of powder coatings. Therefore, currently, more often, while reducing the amount of phosphorus flame retardants, synergistic additives and phosphorus flame retardants are compounded to achieve flame-retardant properties and enhance their mechanical properties.

2. Reactive Flame Retardancy

• Research on Resin Flame Retardancy

Epoxy resin is a key material that enables powder coatings to have flame-retardant and thermal insulation effects, playing a huge role in flame-retardant insulating powders. The epoxy and hydroxyl groups in epoxy resin have good insulation properties, thus having strong reactivity. After reacting with the curing agent, a cross-linked network structure is rapidly formed, thereby achieving insulation properties. Flame-retardant resin is not the widely used bisphenol A epoxy resin on the market, but rather a corresponding flame-retardant resin prepared by introducing flame-retardant elements such as phosphorus, nitrogen, and silicon into the epoxy resin.

Nitrogen-containing groups have flame-retardant self-extinguishing properties, with excellent characteristics such as temperature resistance and arc resistance. Introducing nitrogen-containing groups into epoxy resin for modification and optimization enables the coating formed with curing agents and other raw materials to have excellent flame-retardant, temperature-resistant, and pressure-resistant properties.

Phosphorus has good heat resistance and flame-retardant properties. After introducing 1%~10% phosphorus content into epoxy resin, the flame-retardant and temperature-resistant properties are excellent. Coatings prepared with flame-retardant resin generate phosphorus-containing oxyacids upon heating, which can catalyze compound dehydration and carbonization, reducing the rate of mass loss of the material, decreasing the generation of combustibles, and achieving flame retardancy through the formed carbonized layer.

Using organic silicon to modify epoxy resin reduces its internal stress while also promoting the toughness of epoxy resin, optimizing high-temperature resistance and other properties. Organic silicon polymers containing epoxy groups can all be considered organic silicon epoxy resins. When their molecular weights differ, the dispersion state of the organic silicon phase will exhibit different changes, fundamentally altering their properties. Therefore, the key to using this resin is to properly address compatibility issues.

l Research on Curing Agent Flame Retardancy

Introducing phosphorus-containing groups into amine curing agents ensures that the active hydrogen in the amine curing agent enables complete curing of the product. After curing, the product contains two flame-retardant elements, namely P and N. During gas-phase and condensed-phase flame retardancy, these two flame-retardant elements can exert synergistic effects, achieving certain flame-retardant effects. At the same time, due to the presence of P, N, and C elements forming an intumescent flame-retardant system, it also achieves flame-retardant effects on composite materials.

Another common epoxy curing agent is anhydride curing agent. By introducing phosphorus or other flame-retardant elements into the curing agent molecule, it is then melt-blended with raw materials such as epoxy resin to form flame-retardant powder coatings. Research shows that when phosphorus content in the system reaches 1.5%, it passes UL94 V-0 level flame retardancy.

II. Conclusion
Currently, flame-retardant powder coatings are developing rapidly, and their flame-retardant properties can be achieved from different raw material stages. To achieve functional compounding of flame retardancy and insulation, it is necessary to start from the structure and mechanism of raw materials, exploring the synergistic effects between various raw materials to achieve functional compounding. This paper explores the structure and characteristics of each raw material system, providing a theoretical basis for performance compounding. However, actual application conditions need to be determined based on laboratory results.

Seeking a ready-to-use, non-halogen route to both flame retardancy and high-voltage insulation can turn to the generation of organophosphorus additives offered by Guangzhou Yinsu Flame Retardant. The Yinsu YS-22E, YS-M220G and YS-9003 are all white-powder, phosphorus-based, halogen-free systems that have been surface-activated for nano-scale dispersion in epoxy powders. They work by forming an intumescent char barrier while maintaining the dielectric integrity of the coating.