Mechanisms of flame retardant

Flame retardants are a kind of auxiliaries that can prevent plastic from igniting or inhibit flame propagation. They are often used in modified enterprises to improve the flame retardant properties of plastics.

According to the method of its use, it can be divided into two types: additive type and reactive type. The added type flame retardant is incorporated into plastic during the processing of plastic, and is mostly used for thermoplastics. The reactive flame retardant is chemically bonded to the polymer molecular chain as a monomer during polymer synthesis, and is mostly used for thermosetting plastics. Some reactive flame retardants can also be used as additive flame retardants.

According to the chemical structure, the flame retardant can be further divided into inorganic and organic types. Among these compounds, halogen and phosphorus are contained, and some elements such as bismuth, boron and aluminum are contained. Today we focus on the flame retardant effect of flame retardants.

Flame retardant effect of a barrier agent
The flame retardant effect of a flame retardant is the speed at which physical or chemical changes can be prevented or inhibited during the combustion of the polymer material. Specifically, these effects are reflected in the following aspects.

Endothermic effect
Its function is to make the temperature rise of the polymer material difficult. For example, borax has 10 molecules of water of crystallization. Since the crystal water is released, 141.8 kJ/mol of heat is taken, and the temperature rise of the material is absorbed by the heat absorption. Suppressed, resulting in a flame retardant effect. The flame retardant effect of hydrated alumina is also due to its endothermic effect due to heat dehydration. In addition, some of the droplets often generated during the cracking of thermoplastic polymers can also exert a certain flame retardant effect by removing the heat of reaction away from the combustion zone.

Coverage effect
Its function is to form a stable coating at a higher temperature, or to decompose to form a foamy substance, covering the surface of the polymer material, so that the heat generated by the combustion is difficult to be transmitted into the interior of the material, so that the polymer material is thermally decomposed. The generated flammable gas is difficult to escape, and the material is insulated from the air, thereby inhibiting material cracking and achieving a flame retardant effect. Phosphate compounds and fire-retardant foaming coatings can function according to this mechanism.

Dilution effect
Such materials can generate a large amount of incombustible gas when decomposed by heat, so that the flammable gas generated by the polymer material and the oxygen in the air are diluted to reach a flammable concentration range, thereby preventing the ignition of the polymer material. . As the diluent gas, there are CO2, NH3, HCl, H2O, and the like. Such non-combustible gas can be produced by heating with amine phosphate, amine chloride or amine carbonate.

Transfer effect
Its role is to change the mode of thermal decomposition of the polymer material, thereby inhibiting the generation of flammable gases. For example, an acid or a base is used to dehydrate a cellulose into carbon and water, and since it does not generate a combustible gas, it cannot be burned by fire. Amine chloride, amine phosphate, phosphate ester, etc. can be decomposed to produce such a substance, and the catalytic material is carbonized by a thick ring to achieve flame retardant purposes.

Inhibition effect (capturing free radicals)
The combustion of the polymer is mainly a radical chain reaction, and some substances can capture the active intermediates HO, H, O, HOO, etc. of the combustion reaction, inhibit the radical chain reaction, and reduce the burning speed until the flame is extinguished. Commonly used organic halogen compounds such as bromines and chlorines have this inhibitory effect.

Enhancement effect
Some materials, if used alone, have no flame retardant effect or have little flame retardant effect, and a combination of materials can enhance the flame retardant effect. The most typical example is the combination of antimony trioxide and a halogen compound. As a result, not only the flame retardant efficiency can be improved, but also the amount of the flame retardant can be reduced.

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