The mechanism of action of boron-based low smoke zero halogen flame retardants mainly includes the following aspects:
1. Condensed phase flame retardant effect
Glassy coating formation: Borates (such as zinc borate) melt at the combustion temperature and cover the material surface to form a glassy protective layer, which isolates heat and oxygen and prevents further combustion.
Catalytic carbon formation: Boron-based flame retardants can promote the formation of a dense carbon layer on the material surface, enhance the thermal oxidation stability of the carbon layer, and effectively isolate heat and flammable gases.
2. Gas-phase flame retardant effect
Free radical capture: When borates work in synergy with halogen-based flame retardants, they can generate boron trihalides, which further react with water vapor to form hydrogen halides. The halogen free radicals produced can impede the chain reaction between free radicals, thereby inhibiting combustion.
Diluting flammable gases: The hard-to-burn gases (such as NH₃, N₂) released during combustion can dilute the concentration of oxygen and flammable gases, inhibiting the combustion chain reaction.
3. Alter the thermal decomposition pathway
Inhibiting the generation of flammable gases: Boron-based flame retardants release bound water at high temperatures, absorbing heat and altering the thermal decomposition pathways of certain flammable substances, thereby reducing the production of flammable gases.
Promoting dehydration reaction: The Lewis acid and Brownster acid sites on the surface of boron-based flame retardants can catalyze the dehydration reaction of polymers, promoting cross-linking and the formation of residual carbon.
4. Synergistic flame retardant effect
Boron-phosphorus synergy: Phosphorus-based flame retardants form protective layers of phosphoric acid and polyphosphoric acid in the condensed phase, accelerating carbonization. Boron-based flame retardants improve the thermal oxidation stability of the carbon layer. The synergistic effect of the two can form B-P coordination bonds, further enhancing the flame retardant effect.
Boron-nitrogen synergy: When boron and nitrogen are used together, nitrogen-containing flame retardants release difficult-to-burn gases such as NH₃ and N₂, diluting the concentrations of oxygen and flammable gases. Meanwhile, nitrogen captures free radicals and inhibits the chain reaction of polymers.
Boron-silicon synergy: Boron and silicon have similar chemical properties, and the formed BSi has high thermal oxidation stability. Boron promotes the carbonization of materials, and the silicon dioxide formed by the oxidation of silicon inhibits the oxidation of the carbon layer and improves the quality of the residual carbon.
5. Anti-smoke effect
Reducing flue gas release: The glassy covering layer and dense carbon layer formed by boron-based flame retardants during combustion can effectively suppress the release of flue gas and reduce the amount of smoke generated by the material.
6. Prevent dripping
Inhibiting droplet formation: Boron-based flame retardants can prevent materials from generating droplets during combustion, reducing the risk of secondary fires caused by droplets.
7. Application Examples
Epoxy resin (EP) : Research has found that boron-based flame retardants (such as BP) can catalyze the pyrolysis of EP at lower temperatures, promoting the dehydration of hydroxyl groups and the formation of unsaturated bonds, generating carbon layers with cross-linked structures, and enhancing the flame retardant performance of the material.
Polylactic acid (PLA) : By combining boron-silicon flame retardants with ammonium polyphosphate (APP) through microencapsulation technology, it was found that Si-O-C, Si-O-B and B-O-P bonds were formed in the carbon slag, significantly improving the quality of the residual carbon.
8. Environmental protection advantages
Low toxicity and halogen-free: Boron-based flame retardants feature low toxicity and halogen-free characteristics, which conform to the development trend of halogen-free, non-toxic and smoke-suppressing materials, reducing the harm to the environment and human health.
