Halogen-free flame retardants mainly include the following types:
First, phosphorous flame retardants
Inorganic phosphorus flame retardants: such as ammonium polyphosphate, red phosphorus and melamine salt. Inorganic phosphate-based flame retardants will decompose into metaphosphate and other substances during combustion, promoting the formation of carbon layer, and generating free radical catcher to capture free radicals in the air, terminating the combustion chain reaction, and playing a flame retardant role.
Organophosphorus flame retardants: such as phosphate ester flame retardants, phosphatonitrile and phosphoheterophene compounds. Organophosphorus flame retardants have the advantages of low smoke, non-toxic, low halogen, halogen free, etc., which is in line with the development direction of flame retardants. In recent years, a large number of new organophosphorus flame retardants have been developed at home and abroad, and some have been industrialized.
Second, intumescent flame retardant
Intumescent flame retardant is a compound flame retardant based on nitrogen and phosphorus, which is expanded by heat, hence its name. It is mainly composed of acid source, gas source and carbon source. During combustion, these three components interact to form a thick layer of carbon foam, which blocks heat and oxygen, achieving the effect of smoke suppression and drip suppression.
Third, silicon flame retardant
Silicon-based flame retardants have low toxicity, anti-dripping, carbonization, smoke suppression and high flame retardancy during combustion due to the stability of Si-O-Si structure. Silicon-based flame retardants form an isolation layer mainly through the flame retardant mechanism of condensed phase to prevent heat exchange and contact with combustible gases.
Fourth, inorganic metal flame retardants
Inorganic metal flame retardants are mainly magnesium hydroxide, aluminum hydroxide, hydrotalcite and so on. This type of flame retardant will absorb a lot of heat during the combustion process to reduce the temperature around the burning object, while producing water vapor. The resulting product adheres to the surface of the material to play an isolating role, so as to achieve a flame retardant effect.
Fifth, nitrogen flame retardants
Nitrogen flame retardants are mainly additive, and commonly used nitrogen flame retardants are melamine, melamine cyanuric acid (MCA) and so on. Nitrogen flame retardants decompose into a large amount of gas during the polymer combustion process to dilute the combustible gas content in the air and take away part of the heat, playing a flame retardant role in the gas phase.
Sixth, bio-based flame retardants
Because of its advantages in green recycling, such as biodegradability and non-toxic, bio-based flame retardants have become a hot spot in future flame retardants research. Such flame retardants are usually prepared by introducing different phosphorus groups into the main chain of the vegetable oil.
Seventh, other new halogen-free flame retardants
Expandable graphite: a new type of halogen-free flame retardant, prepared from natural graphite by treatment. In the flame retardant process, it can form a tough carbon layer on the surface of the polymer, separate the combustible from the heat source, and reduce the temperature of the system with a large amount of heat absorption, and release acid ions to promote dehydration carbonization.
Ammonium polyphosphate (APP) : inorganic flame retardant with good performance, low cost, low toxicity, good thermal stability and other advantages. At high temperatures, APP quickly breaks down into ammonia and polyphosphate, which can dilute the oxygen concentration in the gas phase, thus preventing 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 can be selected according to specific needs and material characteristics.
