Epoxy Resin Flame Retardants: Application Challenges and Solutions

Fig. 1 Epoxy resin should
Epoxy resins can be cured in the range of room temperature to 230°C by selecting different curing agents. The structure of the epoxy resin is different to obtain a cured product with very different properties. For example, the number of epoxy functional groups determines its functionality and cross-linking ability, hydroxyl or other polar groups determine its reactivity, and the choice of curing agent determines the thermal stability of the cured product.
Chemical Problems of Epoxy Resin

1、Classes of epoxy resin
(1) Bisphenol A-based epoxy resin
Bisphenol A epoxy resin generally has a low viscosity, room temperature can be cured, has good chemical resistance and widely used. Tetrabromobisphenol A epoxy resin has flame retardant properties and is widely used in PCB materials. Fluorinated epoxy resin also has flame retardant properties, however, due to its high cost, low Tg limits its application.
(2) Phenolic epoxy resin
Phenolic epoxy resin has good flame retardant properties, its thermal stability and flame retardancy depends on the amount of phenolic and the type of curing agent. When room temperature curing system is selected, its thermal stability is similar to that of bisphenol A epoxy resin. However, its thermal degradation stability and heat distortion stability are improved when high phenolic content and high temperature curing agent are used. Phenolic epoxy resin is mainly used in heat-resistant structural laminates, copper-clad boards, chemical-resistant long-fiber winding tubes and high-temperature adhesives.

(3) Aliphatic epoxy resin
Aliphatic epoxy resin has good UV stability, thermal stability and electrical properties.
(4) Glycidylamine Epoxy Resin
It has high reactive functional groups and cures to form a highly cross-linked structure. Therefore, the cured material has high temperature resistance, high Tg, chemical resistance, low combustibility and good mechanical properties and is widely used in the aviation industry composite materials. Due to its low viscosity at room temperature and high temperature curing properties it is used as a semi-cured sheet. It is usually toughened with thermoplastic resins such as polyethersulfone, polyetherimide, and polyetheretherketone to solve its brittleness problem. The combustibility is less than ordinary resins.
(5) Bio-based epoxy resin
Resins prepared from renewable resources such as carbohydrates, starch, proteins, lipids and oils have become a new trend.
2、Epoxy resin curing agent
Epoxy resin curing agent, also known as hardener, plays a very important role in the final performance of the cured material, its properties such as hardness, chemical resistance, heat resistance, flexibility and brittleness are related to the crosslinking density. The curing agent either reacts with the epoxy group or with the hydroxyl group. Commonly used curing agents are amines, anhydrides, phenols and thiols. Their reactivity is: phenol < acid anhydride < aromatic amine < alicyclic amine < fatty amine < mercaptan.

(1) Amine curing agentLow reactivity gets a long working time and the thermal stability of the cured substance increases. Aromatic amines form a more rigid and comparative structure, which is more thermally stable and less combustible. Fatty amines are generally used as fast curing agents at room temperature. Amines are irritating to the skin.
(2) Acid anhydrides
Acid anhydrides are less irritating to the skin than amines and are less active and exothermic. These include phthalic anhydride, hexahydro-phthalic anhydride, chlorendic anhydride, and maleic anhydride.
Test Procedures and Evaluation Criteria:
(1) LOI standards
(2) UL-94 standard
(3) Cone calorimeter method
DTA, DSC, TGA and other thermodynamic means are used to analyze and characterize in the actual R&D process.
Thermal stability and combustibility of epoxy resin curing compounds:
The thermal stability and combustibility of epoxy cured products are related to the structure of the epoxy monomer, the type of curing agent and the crosslink density. Increasing the crosslink density helps to improve the heat resistance.
Since catalytic curing agents such as BF3 do not form a thermosetting structure, they do not affect the burning properties of the resin. Reactive curing agents such as most amines, anhydrides, and phenolic resins are inserted into the structure of thermosetting crosslinked resins and have a greater effect on flammability. Epoxy resins cured with amines and phenolic resins produce more carbon than systems cured with acids or anhydrides and have lower flammability.
Thermal degradation of epoxy cures occurs in two steps: the first step is dehydrogenation to form an aromatic ring, and the second step is complete degradation by thermal oxidation.
Epoxy resin flame retardant:
1、Halogen flame retardant
Although there are environmental factors to limit the use of halogen flame retardants, but halogen flame retardants are still the most widely used flame retardants. The most widely used and cost advantageous is TBBPA, mainly used in PCB, TBBPA as a reactive flame retardant, its hydroxyl group reacts with epichlorohydrin to form glycidyl ether-type halogen-containing epoxy resin.
2、Inorganic flame retardants
(1) Metal hydroxide such as ATH, MDH
(2) Phosphorus-based flame retardants: red phosphorus, APP, MP, MPP
(3) Organic phosphorus flame retardants
(4) Silicon based flame retardant: Silica
(5) Nanoparticle flame retardant: LDH, MMT, CNT, graphene, POSS
Special Applications of Flame Retardants: Challenges
1、Composite materials: structural homogeneity, can not affect the basic nature of the
2、Electronic appliances: environmental requirements
3、Paints & Coatings: thin thicknesses
Conclusion:
(1) The influence of the structure of epoxy resin and curing agent on flame retardancy was discussed.
(2) Summarized different flame retardants and their flame retardant effects.
(3) The environmental protection requirements and future development direction of flame retardants are proposed.