Practical aspects of using flame retardants

While the benefits of using flame retardants appear self-evident, it is important to ensure that the retardants themselves are safe.

There are two main aspects to consider:

  1. Are the materials the correct ones for the situation and conditions of use?
  2. Are the flame-retardant materials themselves safe?

Flame retardants consist primarily of halogens (brominated or chlorinated), phosphorus and inorganic types. While their applications vary depending on the types of plastics, parts, finished products and desired functions, they can be broadly categorised as shown in Figure X, based on their composition and usage. 

There are two main approaches to flame retarding. The first is to use flame retardants. These are incorporated into plastics and rubber products during manufacturing, or are applied as a treatment to the surface of fibres and paper. These do not render materials totally non-combustible; rather, they will burn (ignite) for a short time when heated by fire, but the flame will not spread. The material extinguishes when separated from the flame source (self-extinguishing).

The second approach uses flame retardant promoters. This relies on chemical substances that are not flame retardant in their own right, but can enhance the effectiveness of other flame retardants such as halogen compounds. 

Ensuring the materials used to manufacture flame retardants are safe

While the importance of flame retardants in improving product safety is clear, it is equally important that the materials themselves are safe and non-toxic. 

The term ‘flame retardant’ is a description of the function of a chemical, rather than a substance itself. In reality, there are a wide range of substances used – more than 200 – to provide this function, sometimes alone on their own, but often in combination. However, there are only a small number of substances that dominate use; bromine, phosphorous, nitrogen, and chlorine along with a several mineral-based substances.

Using the correct flame retardants 

The decision on which materials are the correct ones for each situation is governed by the fire safety standards. Fire safety standards have seen flame retardation become an integral part of a huge range of products. These standards are constantly under review and subject to continuous improvement, as regulators learn from experience. 

The application of these regulations has proved highly effective. For example, the introduction of a European fire safety standard for audio, video and similar electronic apparatus – standard EN 60065 – stipulates that these devices now have to be designed in such a way that avoids the risk of spontaneous ignition and minimises the spread of fire wherever possible. The use of flame retardants has allowed manufacturers to replace older, potentially more flammable materials with lightweight and inexpensive plastics with improved fire resistance. Given the increasing numbers of electronic devices in all settings, this represents a considerable contribution to safety in the home, office and transport. Similar advances have been seen in making foam-filled furniture and textiles. 

Many manufacturers pursue standards that are above and beyond those required for compliance with existing fire safety standards and deploy them in areas where their use is not yet prescribed. In addition, innovation to improve the effectiveness – and the safety – of flame retardants continues.


The Voluntary Emission Control Action Program (VECAP)

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The Voluntary Emission Control Action Program (VECAP) is a product stewardship scheme for the management of chemicals throughout the value chain, which goes beyond regulatory and legislative requirements. It was launched in 2004 by the International Bromine Council (BSEF) and the UK Textile Finishers Association to reduce emissions of the brominated flame retardant Deca-BDE. In 2015, it was expanded to include all powder brominated flame retardants (BFRs) produced by VECAP member companies.

VECAP’s original concept – as a tool to control emissions during handling and use of BFRs – has evolved into a comprehensive system of chemicals management, which can be applied to a much wider range of processes and raw materials.

Through VECAP, the industry reiterates its voluntary commitment to take responsibility for the environmentally sound management of chemicals within the context of a European Framework for Corporate Social Responsibility (CSR). The programme adds to the industry’s call for a solid CSR strategy as per the latest European public consultation on the Commission’s work in the area of CSR. The brominated flame retardants industry seeks to reduce the environmental footprint of its value chain and ensures resources are used as efficiently and sustainably as possible.

The science behind flame retardation

In order to understand how flame retardants work, it is helpful to understand how materials catch fire and burn. For solid materials such as modern plastic polymers to catch fire, it is rarely enough for them to simply be exposed to a naked flame; they are generally quite stable.

First, the material needs to be broken down by the heat of the flame; this produces (potentially) flammable gases. When these are mixed with oxygen, they can start a series of exothermic radical chain reactions that release further flammable gases. Where this becomes self-sustaining, then there is fire. However, without this process and the absence of these conditions, there will be no fire; the material will smoulder or even self-extinguish. 

Gas Phase — combustion quenching

In the gas phase, halogenated flame retardants work by substituting the high-energy radicals with low-energy counterparts– the so-called ‘quenching’ effect. This will work to slow down the reaction and prevent it from becoming an established fire. Although found in many halogens, the ability of bromine to provide this quenching effect is particularly pronounced, as is released active bromine atoms into the gas phase before the material reaches its ignition temperature.

This is why, of the different halogens, bromine-based fire retardants are the most common. They offer high effectiveness for common plastics in a range of applications and can be incorporated into the raw material for the polymer without any major impact on its properties.

Thermal shielding — solid phase

When heated, phosphorus-containing flame retardants release an acid, which causes the material to form a glassy layer – a so-called ‘char’. This creates a barrier that prevents potential fuel from reaching the flame and the heat from reaching the material. There are a wide range of phosphorus-based flame-retardants available. Phosphorus-based compounds can be chemically bound to the plastic molecules during the polymerisation process.

Gas Phase — dilution

At the simplest level, gas dilution works by releasing inert gases – principally nitrogen – into the area of combustion. By diluting the flammable gas / oxygen mix, it prevents the initiation of a chain reaction. In addition, the nitrogen is believed to encourage the formation of crosslinked molecular structures that promote ‘char’ formation.

Most nitrogen-based flame retardants use melamine, which is usually found in polyurethane foams and nylon polyamides.

Endothermic decomposition

There are a range of inorganic flame retardants, of which the most common are metal hydrates, principally aluminium hydroxide and magnesium hydroxide. These operate by a process known as endothermic decomposition, meaning that as they reach high temperatures, they absorb energy, cooling the surrounding area and slowing the pyrolytic process. They also release inert gases – usually water vapour – inhibiting combustion.

Although effective, these materials need to be present in large quantities or be used in combination with other types of flame retardants, such as bromine or nitrogen.