Progress in The Application of Halogen-free Flame Retardants in Epoxy Resins

Progress in The Application of Halogen-free Flame Retardants in Epoxy Resins

Epoxy resin has strong adhesion to metal, strong chemical resistance, electrical insulation and good mechanical properties, high thermal stability, and low manufacturing costs, can be widely used in integrated circuits, printed circuit boards, insulating encapsulated parts, conductive adhesives and other electrical and electronic materials, as well as chemical coatings, construction field.

However, as a kind of polymer material, unmodified epoxy resin has low limiting oxygen index (LOI) value and belongs to flammable material, which greatly limits the application of epoxy resin, especially in the field of epoxy resin that needs to be applied in high temperature environment.

The oxygen index of unmodified epoxy resin is about 23%, and it can be burned in the air, and it will release a lot of heat, smoke and harmful gases, and accompanied by high temperature molten droplets, which brings a great potential danger to human life and property.

Conventional epoxy resins are usually modified by adding halogenated flame retardants such as tris (chloroethyl) phosphate, tris (2,3-dibromopropyl) phosphate, tetrabromobutane, tetrabromobisphenol A, etc., to improve the flame retardant properties of the epoxy resin.

Although halogenated flame retardants have the advantages of less addition, they produce more smoke and corrosive gases when burning, as well as carcinogenic gases such as dioxin, and are therefore banned in many countries.

Based on the concept of environmentally friendly, low-toxicity and the implementation of energy saving and emission reduction under the “14th Five-Year Plan”, the modification of epoxy resin with non-halogenated flame retardants has gradually become a research hotspot.

Normally, compounds containing elements such as P, N, As, Sb, Zn, Sn, Al, Mg, B, etc. can be used to prepare halogen-free flame retardants, and the flame retardant mechanisms of the flame retardants are quite different due to the differences in the contained elements. Therefore, according to the different elements contained in the flame retardants, the halogen-free flame retardants are roughly divided into organic flame retardants and inorganic flame retardants.

As most of the flame retardants developed in recent years are synergistic flame retardants, this paper mainly categorizes the flame retardants according to the significant elements that play a role in flame retardant performance, analyzes their flame retardant performance in epoxy resins, and makes predictions on the future development trend.

1. Application of organic flame retardants in epoxy resin

Organic phosphorus flame retardant

Phosphorus flame retardant has high flame retardancy, low toxicity, low smoke and other characteristics, so the phosphorus element has become one of the important elements in the preparation of halogen-free flame retardants.

Usually, the flame retardant mechanism of phosphorus flame retardant is condensed phase flame retardant, phosphorus-containing flame retardant in the condensed phase surface formation of phosphoric acid and polyphosphoric acid. On the one hand, dehydration promotes the formation of carbon layer to reduce heat conduction; on the other hand, it can isolate oxygen to prevent the flame from continuing to spread.

In recent years, 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO, see Fig. 1(a)) is often used as a phosphorus flame retardant and is widely used.

In order to further enhance the efficiency of DOPO-based flame retardants, the lively P-H bond of DOPO is usually utilized to react with compounds with double bonds, alcohols, phenols, etc. DOPO can also be reacted with eugenol to obtain the new biobased flame retardant DOPO-GE, which can make the thermal stability of epoxy resins significantly improved and effectively enhance the flame retardancy of epoxy resins.

The bi-DOPO compounds with double hydroxyl structure have also been prepared, and when the phosphorus content is low, the EP composites can achieve V-0 rating and LOI value of 35.2%.

In addition to DOPO and its derivatives, another class of organophosphorus flame retardants, diphenylphosphine oxide (DPO, see Fig. 1(b)), also have high flame retardant efficiency.

The R&D team of YINSU Flame Retardant Company prepared the flame-retardant epoxy resin EP/DPO, and the experimental test results showed that EP/DPO is a high-performance resin with good thermal stability, high glass transition temperature and low water absorption.

After comparing the effects of DOPO derivatives and DPO derivatives in the modification of epoxy resins, it was shown that the composites modified by DPO derivatives were more flame retardant than DOPO derivatives and had high thermal stability. Therefore, the synthesis of halogen-free organophosphorus flame retardants by DPO as an intermediate has a better research prospect than that of DOPO.

Phosphorus flame retardants can effectively improve the flame retardancy of epoxy resins, and at the same time will increase the amount of carbon residue of the material, reduce the amount of smoke released when the material is burned, and is one of the important halogen-free flame retardants. Comprehensive comparison of several types of phosphorus flame retardants LOI value and limiting oxygen index, see Table 1.

Table 1 Flame retardant properties of phosphorus-based flame retardants for epoxy resin

Note: * LOI value, the amount of limiting oxygen index; ** UL-94 grade, the material fire rating, the same below.

Nitrogen-based flame retardants

Nitrogen-based flame retardants usually include dicyandiamide, bis(urea), guanidine salt, melamine and its salt, etc. The flame retardant mechanism is gas-phase flame retardant. When nitrogen flame retardant burns, it will produce nitrogen gas, such as nitrogen, ammonia, etc., which can dilute the concentration of oxygen, and at the same time, the gas will take away a lot of heat when it escapes.

Melamine is used as a cost-effective nitrogen source for nitrogen-based flame retardants. PPy-MAPP was obtained by microencapsulating melamine-coated ammonium polyphosphate (MAPP) using polypyrrole (PPy), and the flame retardant was applied to epoxy resin. Compared with MAPP with the same amount of additive, the epoxy resin added with PPy-MAPP has better flame retardant and smoke suppression properties.

The R&D team of YINSU Flame Retardant Company synthesized melamine phenyl hypophosphite (MABP). Due to the addition of MABP, the flame retardancy of the epoxy resin and the densification of the carbon layer of the condensed phase were effectively improved, while the smoke release rate of the epoxy resin was reduced.

Nitrogen-based flame retardants, despite their environmental friendliness, have poor compatibility and limited flame retardant effect, and usually need to be used in conjunction with other flame retardants.

Phosphorus-Nitrogen Synergistic Flame Retardant

In the current research, most of them are based on nitrogen and phosphorus elements, while adding other elements to synthesize synergistic flame retardants in order to achieve better flame retardant effect, generally adding sulfur, silicon, boron and metal elements for synergistic modification. For example, intumescent flame retardant is a composite flame retardant mainly composed of nitrogen and phosphorus.

The R&D team of YINSU Flame Retardant Company has synthesized a reactive silicon-phosphorus-nitrogen flame retardant PmDOH by utilizing nitrogen source, DOPO and p-hydroxybenzaldehyde, and the carbon residual of the modified epoxy resin gradually increases and generates a more dense and continuous structural carbon layer during the combustion process.

Chitosan-based expansion flame retardant is a new type of composite flame retardant, due to the simultaneous addition of N, P elements of both cohesive phase and gas-phase flame retardant effect, can give the epoxy resin a certain flame retardant properties.

Comprehensive comparison of several types of nitrogen and phosphorus-nitrogen synergistic flame retardants on the flame retardancy of epoxy resin LOI value and limiting oxygen index, see Table 2.

Table 2 Nitrogen and phosphorus-nitrogen synergistic flame retardants on the flame retardancy of epoxy resin.

Other new flame retardants

At present, many people began to study the use of new substances as raw materials for the preparation of halogen-free flame retardants, of which the POSS class of flame retardants is the most prominent.

Polyhedral Oligosilsesquioxane (POSS) is a new type of nano-hybridized material with a typical chemical formula of RSiO1.5, which produces more residual carbon when burned, and at the same time produces a thermally stable silicon-carbon layer that migrates to the surface of the EP as a protective layer to prevent combustion.

The R&D team of YINSU Flame Retardant Company prepared a new type of phosphorus-silicon flame retardant (KAPP-OGPOS), and experimentally measured that the amount of carbon residue of the modified epoxy resin is 3 times of that of the pure epoxy resin, and it can reduce the release of heat, smoke, CO, and CO2 by 79%, 83%, 80%, and 82.5%, respectively. It provides a new idea for modifying epoxy resin.

The use of POSS and DOPO synthesized flame retardants subsequently introduced into the epoxy system can be obtained POSS/D-bp/DGEBA hybrid composites. It was demonstrated that the peak heat release rate of the composites was significantly reduced and the glass transition temperature was enhanced, while the mechanical properties were also improved.

The Si-O bond in the POSS structure can effectively improve the mechanical properties, but the high cost of POSS is the main factor restricting its development.

2. Application of inorganic flame retardants in epoxy resin

Aluminum flame retardant

In the flame retardant market, the main substance used in aluminum flame retardants is hydrated alumina, which has a condensed phase flame retardant effect. When it is thermally decomposed, it will be transformed into AlO(OH), and will absorb the heat of combustion, reduce the combustion temperature, and promote the polymer combustion to occur when the thick ring carbonization.

The advantage of aluminum-based flame retardants is that they are lower in cost but higher in addition. To enhance the flame retardant efficiency and improve compatibility with the matrix, Al2O3 can be modified.

A new thermosetting EP (Al2O3-PTDOB/EP) was obtained by modifying the epoxy resin using alumina added to hyperbranched flame retardant (PTDOB). It was concluded that the modified epoxy resin not only improved the flame retardant property, but also improved the thermal conductivity of the material due to the addition of Al2O3.

FR/EP composites were obtained by co-modifying the epoxy resin with a novel intumescent flame retardant and aluminum hydroxide (ATH), which showed that the modified material could generate a stable carbon layer, which could play the roles of preventing the release of combustible gases, heat insulation and oxygen barrier.

Magnesium flame retardant

Magnesium flame retardant is mainly magnesium hydroxide. When it is subjected to high temperature, it will be thermally decomposed into magnesium oxide and water, at the same time, the generated water will absorb a large amount of heat and cool down the substrate in order to achieve the effect of flame retardant.

The R&D team of YINSU Flame Retardant Company has synthesized an inorganic curing agent TRPAM-MEL using magnesium hydroxide and other substances, and experiments have shown that it exhibits excellent flame retardant properties when applied to epoxy resin systems.

The use of magnesium hydroxide microcrystalline whiskers (mw-MH) as a super reinforcing material added to the epoxy resin found that, due to the clogged network structure, magnesium hydroxide microcrystalline whiskers and the traditional industrial magnesium hydroxide compared with the superior fire resistance.

Boron flame retardant

Boron flame retardants are characterized by low toxicity and smoke suppression. The traditional boron flame retardant is mainly composed of borate as raw material, when it burns, it will form a glass body to cover the surface to prevent combustion, but also generate water to absorb heat to achieve the effect of flame retardant.

At present, there are also researchers using boron nitride and boron two-dimensional materials for epoxy resin flame retardant research. Using polyglycol methacrylate (PGMA) chain and DOPO reaction to form a branched structure of flame retardant functionalized BNNS.

The results show that BNNS inhibits heat and mass exchange by forming a continuous and dense barrier layer, while suppressing the generation of flammable gases through synergistic enhancement.

The R&D team of YINSU Flame Retardant Company synthesized a new phosphorus-free PBA-Salen-Ni flame retardant, so that the residual carbon generated from the combustion of the modified epoxy resin possesses a special porous structure, which effectively prevents the diffusion of the smoke transfer, and therefore the smoke suppression of the epoxy resin is better improved.

Comprehensive comparison of several types of inorganic flame retardants on epoxy resin flame retardant properties of LOI value and limiting oxygen index, see Table 3.

Table 3 Summary of flame retardant properties of epoxy resin by inorganic flame retardants

Inorganic flame retardants are more stable and inexpensive, but they have a greater impact on the physical and mechanical properties of the material, and often need to be modified.

CONCLUSION

From a comprehensive point of view, phosphorus-based flame retardants have a better modification effect on epoxy resin, and the amount of additives needed to achieve the same flame retardant effect is smaller compared with other types of flame retardants. The synergistic effect of nitrogen and phosphorus elements makes the flame retardants have various flame retardant mechanisms, and the effect is outstanding for improving the flame retardant performance of epoxy resin. Most of the inorganic type flame retardants have the dual function of flame retardant and thermal conductivity, and there are also organic-inorganic hybridized flame retardants under continuous research and development. Overall the flame retardant effect can be optimized by designing acid source, gas source and carbon source.

Epoxy resin is one of the important polymer materials, and with the research of halogen-free flame retardants will inevitably expand the application field of epoxy resin. Therefore, when designing flame retardants, in addition to considering the excellent flame retardancy, it is also necessary to modify epoxy resin from the direction of mechanical properties and thermal conductivity, so as to make it both multifunctional.