Why do epoxy resins need flame retardants?
Epoxy resin is a kind of organic polymer material with good mechanical properties, strong corrosion resistance, good thermal stability, good electrical insulation, easy processing, good bonding performance and stable structure. Therefore, epoxy resin is widely used in the fields of construction, machinery, electronics and aerospace.
However, the oxygen index of epoxy resin is only 19.8, which is an extremely flammable material. In a fire accident, epoxy resin is often the most flammable part of the composite material, and it continues to burn after a fire with a huge amount of smoke. There are roughly two methods of making epoxy resin flame retardant, namely "composite type" and "structural type". Herein, we discuss in detail how to prepare flame-retardant epoxy resins.
Composite Flame Retardant Epoxy Resin
Composite flame retardant epoxy resins add various flame retardants that do not participate in the curing reaction, so that the material has flame retardant properties. This process is simple and convenient, with low cost and a wide range of raw materials. Common additive flame retardants used in epoxy resins include:
- Inorganic flame retardants
The representative metal hydroxides (aluminum hydroxide, magnesium hydroxide) are stable, inexpensive and low-toxic as flame retardants, and also have flame retardant and smoke suppression effects. Examples of other inorganic flame retardants include borides, antimony trioxide, and red phosphorus, etc. Due to the large amount of inorganic flame retardants, it is a better choice to use them through compounding. For example, the combined use of red phosphorus and aluminum hydroxide can improve the flame retardant effect of epoxy resin, while greatly reducing the amount of aluminum hydroxide.
- Organic halogen flame retardants
The addition amount of organic halogen flame retardants is relatively small, and the flame retardant efficiency is high, especially brominated flame retardants. Examples include tetrabromobisphenol A (TBBPA), decabromodiphenyl ether (DecaBDE), and chlorinated paraffins. However, the possible toxicity and environmental hazards in use have led to them being replaced by other halogen-free flame retardants.
Halogenated flame retardants used for PWB [1]
- Organophosphorus flame retardants
Organophosphorus flame retardants have both flame retardant and plasticizing functions, and have low toxic and side effects. It can be further divided into compounds such as phosphates, phosphonates, and phosphazenes. Examples include ammonium polyphosphate (APP), aluminum diethylphosphinate, and melamine polyphosphate.
Structural Flame Retardant Epoxy Resin
On the one hand, the use of a large proportion of added flame retardants will affect the mechanical properties of the material. On the other hand, during the curing and use process, the migration of the flame retardants will lead to a gradual decrease in the flame retardant effect, showing an unstable flame retardant state. Therefore, it is considered to use reactive monomers or curing agents with flame retardant effect to prepare epoxy resins, so as to obtain structural flame retardant epoxy resins.
- Flame-retardant functional monomer
The structural flame-retardant epoxy resin synthesized from flame-retardant functional monomers exhibits good flame-retardant properties because the molecular structure contains a large amount of flame-retardant elements such as halogen, silicon, and phosphorus. For example, if tetrabromobisphenol A is used instead of ordinary bisphenol A as the reaction raw material of epoxy resin, brominated epoxy resin with high molecular weight can be prepared, which has the characteristics of good stability and high flame retardancy.
- Flame retardant curing agent
Elements with flame retardant functions such as halogen, silicon, and phosphorus can be introduced into the molecular structure of common curing agents. Examples include dichloromaleic anhydride, tetrabromophthalic anhydride, phosphorus-containing amines, phosphoric acid containing amino groups, and amides of phosphoric acid.
Reference
- Rakotomalala M, et al. Materials, 2010, 3(8): 4300-4327.
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