Halogen-free flame retardants mainly include the following aspects:
1. Phosphorus based flame retardants
Inorganic phosphorus flame retardants: mainly including ammonium polyphosphate, red phosphorus and so on. This kind of flame retardant will decompose into phosphoric acid, metaphosphate and other phosphoric compounds during combustion, these compounds have a strong dewater, can make the polymer surface dehydration carbonization, forming a carbon layer, so as to isolate oxygen, to achieve the purpose of flame retardant.
Organophosphorus flame retardants: mainly including phosphate esters, phosphazenes, phosphophenanthrene compounds and so on. This kind of flame retardants will decompose to produce phosphorous compounds when heated, and play a role in two ways: condensed phase flame retardancy and gaseous phase flame retardancy. They can not only reduce the temperature around the combustion, but also generate water vapor or inert gas to dilute the surrounding combustion and toxic gases, while the generated phosphoric acid and metaphosphate can be attached to the surface of the material to form a barrier layer, blocking the combustion chain reaction in the gas phase.
2. Expansive flame retardant
Intumescent flame retardant is a composite flame retardant, mainly nitrogen and phosphorus as the main components. This type of flame retardant expands to form a porous carbon foam layer when heated, which can effectively block heat and oxygen to achieve flame retardant purposes. Intumescent flame retardants are mainly composed of acid source, air source and carbon source, which interact in the combustion process to form a significant flame retardant carbon layer.
3. Silicon flame retardant
Due to the stability of Si-O-Si structure, silicon-based flame retardants exhibit the characteristics of low toxicity, anti-dripping, promoting carbon formation and smoke suppression during combustion. This type of flame retardant mainly forms an isolation layer through the condensed phase flame retardant mechanism to prevent heat exchange and contact with combustible gas, so as to achieve flame retardant effect.
4. Inorganic metal flame retardants
Inorganic metal flame retardants mainly include magnesium hydroxide, aluminum hydroxide, hydrotalcite and so on. This type of flame retardant can absorb a lot of heat during the combustion process, reduce the temperature around the burning object, and produce water vapor. The products produced by them adhere to the surface of the material to form an isolation layer, effectively isolating oxygen and achieving flame retardant purposes.
5. Nitrogen flame retardants
Nitrogen flame retardants are mainly added flame retardants, commonly used are melamine, melamine cyanuric acid (MCA) and so on. Such flame retardants can release noble gases such as nitrogen during combustion, diluting the oxygen concentration in the combustion area, thus inhibiting combustion. Nitrogen flame retardants are usually used in combination with other flame retardants to improve the flame retardant effect.
6. Bio-based flame retardants
Bio-based flame retardant is a new type of environmentally friendly flame retardant, which has the advantages of degradability and non-toxicity. Such flame retardants are usually prepared by introducing different phosphorus groups into the main chain of the vegetable oil. Biobased flame retardants can produce synergistic flame retardancy during combustion and improve the flame retardancy of materials.
7. Other new halogen-free flame retardants
Expandable graphite: This is a new type of halogen-free flame retardant, made from natural graphite after treatment. In the combustion process, it can form a tough carbon layer, isolate fuel and heat sources, while absorbing a lot of heat and releasing acid ions to promote dehydration carbonization.
Ammonium polyphosphate (APP) : As a good inorganic flame retardant, ammonium polyphosphate will quickly decompose into ammonia and polyphosphate when burned. Ammonia can dilute the oxygen concentration in the gas phase, and polyphosphate can dehydrate and carbonize the polymer to form a carbon layer, which effectively prevents combustion.
In summary, there are many kinds of halogen-free flame retardants, each with its own unique flame retardant mechanism and application advantages. In practical applications, suitable halogen-free flame retardants should be selected according to specific needs and material characteristics.
