Nitrogen-based flame retardants
Nitrogen-based flame retardants, this article focuses on melamine and its derivatives, some of which can be used alone or are the main components of intumescent flame retardants. These flame retardants are halogen-free, low-smoke, heat- and light-stable, with good flame retardant efficiency and low cost. The problem with them is that the plastics they are used to flame retard are difficult to process and poorly dispersed in the base material. The requirements for particle size and particle size distribution are more stringent and sometimes co-efficient agents are required. In addition to melamine, there are also melamine cyanurate (MCA), phosphates, borates, guanidine salts, dicyandiamide salts, etc. These flame retardants are used in PA, PU and PO. They can be used in PA, PU, PO, PET, PS, PVC and epoxy resins, and are also suitable for textiles, paints, wood and paper.
One of the main uses of nitrogen-containing compounds as flame retardants is in combination with phosphorus-containing flame retardants to obtain a synergistic phosphorus-nitrogen effect. Mixtures of nitrogen-containing compounds with phosphorus-containing compounds have a higher flame-retardant effect on cellulose than the sum of the flame-retardant effects of the individual compounds. Nitrogen promotes the phosphorylation of cellulose by phosphoric acid and thus contributes to the swelling flame-retardant effect. Therefore, the active components of many intumescent flame retardants are phosphorus and nitrogen.
Melamine (MA) is non-flammable, easily sublimated when heated, and decomposes when heated sharply. The reasons why it is an excellent flame retardant are:
- Decomposition reactions occur at 250 to 450°C, absorbing large amounts of heat and giving off ammonia to form a variety of condensates;
- The ability to influence the melting behavior of the material and accelerate its charring into coke.
Melamine is inexpensive, non-corrosive, non-irritating to the skin and eyes, and not a mutagen. The disadvantage is that it produces toxic cyanide when decomposed at high temperatures.
Melamine is a widely used flame retardant, particularly as a component of intumescent flame retardants and for flame retardant polyurethane and triazine resins. Used in combination with liquid phosphate esters, melamine is widely used to flame retard polyurethane foams.
Melamine cyanurate is produced by reacting melamine with cyanuric acid, referred to as MCA. It is a lubricant and flame retardant, a white crystalline powder, odourless and tasteless. It is very stable under 300°C and starts to sublimate around 350°C, but does not decompose, its decomposition temperature is about 440~450°C. MCA contains high nitrogen content, very easy to absorb moisture, high temperature dehydration into charcoal, burning nitrogen gas, dilute the concentration of oxygen and polymer decomposition generated by gas. Moreover, the generation of gas and thermal convection take away part of the heat, thus providing a flame-retardant function.
MCA is widely used in rubber, nylon, phenolic resins, epoxy resins, acrylic emulsions and other olefin resins. MCA is used as a flame retardant for epoxy resins. The oxygen index is increased from 24% to 28% to 46% to 48% and it is particularly suitable for flame retardant electrical components and parts. It is added to vinyl acetate emulsions, acrylate emulsions and rubber emulsions to make flame-retardant coatings with excellent smear adhesion and smoothness, and no coloring contamination.
MCA is a triazine flame retardant with excellent properties and its use or mixing with other flame retardants is much more effective than using melamine alone.
Melamine phosphonates can be prepared from melamine and monophenylphosphonic acid. The disadvantage is that it is difficult to prepare and to prevent it from foaming when blending with polyolefins, degassing is carried out. This flame retardant can also cause colouring contamination. It is widely used as a flame retardant for polyolefins and polystyrene.
Melamine pyrophosphate can be produced by treating melamine with phosphoric acid and then heating it at 250-270°C. There are many flame retardants for fibres, textiles and some plastics, but the flame-retardant effect on polyurethane plastics is not satisfactory, while melamine pyrophosphate can be used effectively to flame retard polyurethane plastics. Flame retardant polypropylene by its reaction product with pentaerythritol or its reaction product with pentaerythritol and phosphoric acid can also form a non-flammable, non-dripping charred coking material.
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