What flame retardants are available for polyamides?

Why do polyamides need flame retardants?

Polyamide (PA), also known as nylon, refers to a polymer containing amide groups (-NHCO-) in the main molecular chain, which is prepared by polycondensation reaction of diamine and dibasic acid. The main varieties of PA are PA6, PA66, PA610, PA1010, PA11, PA12, etc., which generally have the advantages of good mechanical properties, wear resistance, chemical resistance, and oil resistance. PAs are commonly used in a variety of applications including textiles, carpets, electrical insulation, automotive parts, and more.

However, the flammability of PA is the biggest obstacle to its application. In order to improve the flame retardant properties of PA, adding flame retardants is a very important step in the manufacturing process. These flame retardants function through various mechanisms, such as interrupting the combustion process, generating non-flammable gases, or forming a protective layer on the surface of the material.

Nitrogen Flame Retardants for Polyamides

The main component of nitrogen-based flame retardants is melamine, which has the advantages of low toxicity, low corrosion, high flame-retardant efficiency, and no conflict with additives. It can make PA reach UL 94 V-0 level when used alone. Nitrogen-based flame retardants mainly include melamine cyanurate (MCA), melamine polyphosphate (MMP), and guanidine sulfamate (GAS).

Phosphorous Flame Retardants for Polyamides

Phosphorous compounds can function through various mechanisms, including the formation of a protective char layer, gas phase extinguishment, and catalytic effect on the decomposition of radicals. Phosphorus-based flame retardants include organic phosphorus compounds such as ammonium polyphosphate (APP), red phosphorus, and phosphorus-nitrogen compounds like melamine polyphosphate (MPP). Phosphorus-based flame retardants are considered less toxic and have lower environmental impact compared to halogenated flame retardants.

Inorganic Flame Retardants for Polyamides

Common inorganic flame retardants include aluminum hydroxide (ATH), magnesium hydroxide (MDH), and zinc borate. These act as flame retardants by releasing water vapor during combustion, cooling down the material and inhibiting the flame. In addition, in recent years, nanotechnology has driven the development of flame-retardant PA, and research in this area is increasing. For example, nano clay materials have shown potential as flame retardants for polyamides. They provide excellent barrier properties and can also enhance the mechanical properties of the material.

Schematic for polymer/clay nanocomposite. [1]

Compound Flame Retardants for Polyamides

There are a series of problems in the use of flame retardants alone, such as poor compatibility between nitrogen-containing and phosphorus-containing flame retardants and the matrix, and difficulty in distribution; large additions of inorganic flame retardants reduce the mechanical properties of materials, and form a discontinuous carbon layer after combustion. In order to reduce the probability of the above problems, various flame retardants can be compounded and applied to flame-retardant PA. After compounding, the cost and pollution of flame retardants are reduced, and the flame-retardant efficiency is improved. Common compound systems are phosphorus-containing flame retardants and inorganic materials.

Flame Retardant Polyamides

Reference

1. Giuseppe Cavallaro, et al. 7-Flame retardant potential of clay nanoparticles, 2020, 169-184.