Classification of intumescent flame retardants-Intumescent flame retardant

1 Classification of intumescent flame retardants

    Intumescent flame retardant

 are mainly divided into phosphorus and nitrogen flame retardants and expandable graphite, and phosphorus and nitrogen flame retardants are divided into monomeric phosphorus and nitrogen flame retardants and hybrid phosphorus and nitrogen flame retardants.    

1.1 Phosphorus nitrogen flame retardant

1.1.1 Composition of phosphorus and nitrogen flame retardants

    Phosphorus nitrogen intumescent flame retardants are mainly composed of three parts: carbon source (char-forming agent), acid source (dehydrating agent) and gas source (blowing agent). The carbon source is a substance that can produce an expanded porous carbon layer, generally a carbon-rich multi-functional group (such as -OH) char-forming agent, such as pentaerythritol and its diacetal, starch, etc. Its effectiveness is related to the number of active hydroxyl groups. It should first react with the carbonization catalyst at a temperature lower than the decomposition temperature of itself or the plastic matrix. The acid source is generally a compound that releases inorganic acid under heating. The requirement for inorganic acid is high boiling point and not strong oxidizing property. It must be able to dehydrate carbon-containing polyols, but dehydration reaction should not occur before the fire occurs, so Commonly used acid sources are salts or esters. The gas source is a compound that emits inert gas when heated, generally ammonium and amide substances, such as urea, melamine, melamine, etc., which must be decomposed at a suitable temperature and produce a large amount of gas.    

    Monomeric phosphorus-nitrogen flame retardants are carbon source, acid source, and gas source coexist in the same molecule. The molecular structure generally contains free, ionizable hydrogen derivatives, so it can expand when heated. effect. Hybrid phosphorus-nitrogen flame retardant is a mixture of phosphate, polyol and nitrogen-containing compound. The most typical one is the ammonium polyphosphate/pentaerythritol/melamine (APP/PER/MEL) flame retardant system.    

1.1.2 Flame retardant mechanism of phosphorus and nitrogen flame retardants

    When the intumescent flame retardant is heated, the char-forming agent is dehydrated into char under the action of the dehydrating agent, and the charred material forms a charcoal layer with a closed structure of fluffy pores under the action of the gas decomposed by the expansion agent. The carbon layer has an amorphous carbon structure, which is essentially carbon microcrystals. Once formed, it is incombustible, and can prevent the heat conduction between the plastic and the heat source and reduce the pyrolysis temperature of the plastic. In addition, the porous carbon layer can simultaneously prevent the diffusion of the gas generated by pyrolysis and the diffusion of external oxygen to the surface of the uncracked plastic, so that the burned plastic cannot get enough oxygen and heat energy and self-extinguish, which is a typical condensed phase flame retardant mechanism. The formation process of the carbon layer is:

    (1) The acid source releases inorganic acid at a lower temperature.

    (2) The esterification reaction occurs at a slightly higher temperature and the acid release temperature, and the amine in the system can be used as a catalyst for the esterification reaction.

    (3) The system melts before and during esterification.

    (4) The water vapor generated by the reaction and the incombustible gas generated by the gas source foam the molten system. At the same time, the polyol phosphate is dehydrated and carbonized to form inorganic substances and carbon residues, and the system further expands and foams.

    (5) The system gels and solidifies to form a porous foamed carbon layer. 

 1.2 Expandable graphite

    Expandable Graphite (Expandable Graphite, EG) is a new type of halogen-free flame retardant. It is made by acidizing natural graphite with concentrated sulfuric acid, then washing, filtering, drying, and puffing at 900-1 000°C. When expandable graphite is rapidly heated to about 300°C, it can expand hundreds of times along the C-axis direction of the crystal structure. The expanded graphite changes from the original scaly shape to a low-density "worm" shape, forming a highly efficient heat and oxygen barrier. Expandable graphite forms a tough carbon layer on the surface of the plastic during the flame retardant process, thereby separating the combustibles from the heat source. During the expansion process, a large amount of heat is absorbed, which reduces the temperature of the system and releases the interlayer at the same time. The acid radical ions promote dehydration and carbonization, and combine with free radicals generated by combustion to interrupt the chain reaction. Expandable graphite is used in combination with phosphorus compounds and metal oxides, which can produce synergistic effects, and the purpose of flame retardancy can be achieved with small amounts. However, the appearance of materials modified with such flame retardants is poor, and the scope of use is limited.