Flame retardants are hot

Worldwide, between 6 and 24 million fires cause around 150,000 casualties and more than $ 400 billion damage annually. Roughly half of these take place in homes and about 15% are the result of furniture catching fire in public buildings. Casualties often belong to the most vulnerable population groups, such as children and the elderly.

Growing demand for effective flame retardants

The demand for flame retardants for a whole set of end products is increasing every year, especially in the plastics industry. The advantage of flame retardants is that they significantly increase the time to escape and/or extinguish the fire. Moreover, they can greatly reduce fire, heat and smoke development. Legislation for public buildings and public transport has already been tightened. This is being expanded with guidelines for floors, walls, textiles, furniture and electronic appliances. Most television casings were traditionally not made extra flame retardant. In Europe, electrical and electronic equipment must now meet the IEC 60335 standard as well as the GWIT (Glow Wire Ignition Test). In North America, the UL94 standard (V0, V1, V2) is used. From an environmental perspective, innovative flame retardants are being examined that are cleaner with respect to production, dosing, stability, and the formation of harmful substances in the event of a fire.

Various types of flame retardants

Over the years, various types of flame retardants have been developed. All flame retardants work by using a combination of one or more of the following parameters that are needed to stop a fire:

Reducing the temperature

Oxygen scavenging or separation of the fuel
Stopping a chemical radical reaction

Reducing the temperature

A reduction of the local temperature can, for instance, be obtained by admixing the inorganic molecule aluminium trihydrate (ATH) in the matrix of the fuel (the plastic final product). Under the influence of increased temperature (fire), this molecule splits off the water, thereby lowering the local ambient temperature. As a result, not enough energy is left in order to maintain the chain reaction (fire). This type of inorganic flame retardant may be used with products for which the mechanical properties are less important. An advantage of ATH is the relatively low price per kilogram. A disadvantage is a high dosage that is often necessary. In addition, it applies that – even if the dosage is high (50 – 70%) – compliance with a number of flame retardant standards cannot be obtained. Alternative products that have dehydrating properties at high temperatures are magnesium dihydroxide (MDH) and zinc borate. These products are used successfully mainly in thick-walled products of polyolefins and PVC.

Oxygen scavenging or separation of the fuel

Fire can also be stopped by incorporating ingredients in the matrix (product), which at an increased temperature react with the oxygen from the air so that the reaction (fire) stops. Antimony trioxide is an example of a substance that has this characteristic. The disadvantage of this product is that, just as the antimony price, it is subject to price fluctuations and that, due to environmental and health and safety reasons, it is more and more banned. An example of a product that separates oxygen from the fuel is graphite, which expands significantly at high temperatures. This creates an intermediate layer between the air (oxygen) and the fuel, thereby stopping the fire. Another product is red phosphorus. This powder is difficult to process, with a risk of fire during production, degradation, and harmful by-products that can damage the machine parts and are harmful to the lungs of employees. In the event of a fire, a product containing the ingredient red phosphorus forms a hard crust on the surface between the fuel and oxygen, thereby stopping the fire.

Stopping a radical reaction (fire)

Traditional, relatively cheap flame retardants are, among others, chlorinated and brominated systems (halogens). These systems are often combined with antimony trioxide, which has a synergistic effect. At high temperatures during a fire, these molecules fall apart to form free radicals. These radicals then react with the free radicals that come from the radical reaction, which maintains the fire. The result is that the chemical chain reaction (fire) stops. The use of brominated systems shows a slight increase. This is at the expense of the chlorinated products; this is causing the market to shrink. However, both product groups are increasingly being replaced by halogen-free alternatives, driven by technical and environmental trends. European legislation takes the lead herein, followed by Asia and North America. In Europe, the Waste Electrical and Electronic Equipment Directive (WEEE) has come into force, which distinguishes between halogenated and non-halogenated waste. This legislation is the driving force for choosing a halogen-free alternative.

Trends and application areas

There is a growing demand from the market for halogen-free systems. Also, a great deal of research is being conducted into more efficient systems in terms of dosing, price ratio and the effect on the mechanical properties of the final product. Flame retardants based on phosphate score well here. This group of products has the advantage of having both halogen-free and plasticising properties. The phosphate flame retardants are favourable in terms of price and applicable in a wide range of plastic and rubber applications. During a fire, a fire-resistant layer is formed at the interface of the fire and the polymer.

A number of phosphate flame retardants also have other properties, for example, low smoke formation during a fire. This is an added bonus since many casualties are not caused by the fire itself but by the often toxic smoke. The phosphate flame retardants are compatible with polar plastics such as PC-ABS blends, PS, PPO-PS blends, TPU, PVC, natural and synthetic rubbers, phenol resin, unsaturated polyester resin and polyurethane. Without the use of phosphate flame retardants, many of these raw materials could not be used, for example, in public transport and the construction industry. New products have been developed for the construction industry that combines the thermal insulation properties of polyurethane foam with fire resistance. Other application areas for phosphate products are mainly cars, electronics, cables and PVC.

The phosphate-containing flame retardants have a relatively high phosphorus content, thereby obtaining good results at low dosages. An innovative variant is based on reactive alkyl phosphates. These additives can be incorporated into the chemical structure, so they are more resistant to ageing processes and migration to the surface. The halogen-free molecule dimethyl pyrazole phosphate (DMPP) is already being used as an alternative for brominated flame retardant systems in rigid polyurethane foam.

Good results are obtained when this product is compared with the halogen-containing alternative trichloropropylphosphate (TCPP). DMPP enables the flame-retardant standard to be reached at a lower concentration. Aryl phosphates are also suitable for thermoplastics. They are used in, for example, electronic circuit boards, flexible PVC and other polymers. In addition, the flame-retardant plasticiser diphenyl octyl phosphate (DPO) is used as a gelling agent, and DPO is also resistant to weathering. Furthermore, aryl alkyl phosphate is especially suitable for use if, in the event of a fire, a product may only produce little smoke. DPO is therefore widely used in PVC, polyurethane, NBR and SBR, and in other polymers. Moreover, with soft PVC, good resistance to saponification is obtained.

Flame retardants in masterbatches

Flame retardant masterbatches are available for virtually all thermoplastics, such as PP, PE, PS, elastomers, polyesters, PA, PC, PC-ABS and ABS. The use of masterbatches reduces the amount of active material being handled. Traditionally, compounding companies often process powders and liquids. In their pure form, these ingredients are often hazardous to humans and the environment, while the masterbatches based on these raw materials are much easier and safer (dust-free!) to process. The advantage of masterbatches is that the various additives and flame retardants can be combined in a single concentrate and that they are safely stored in the matrix of the carrier material of the masterbatch. For example, the dosing system can be easily and rapidly cleaned at the change of a production run. This improves the efficiency and speed of production. A novelty is the development of a series of halogen-free masterbatches for polycarbonate (PC). The advantages of these masterbatches are that they do not affect properties such as the impact strength and flow (MFI) of PC. Furthermore, the masterbatches are economically viable to deploy due to the low dosage. With a dosage of 2%, the UL-94 V0 standard can be met at a thickness of 3.2 mm. This does not substantially affect the transparency of the finished product. This masterbatch meets the German standard DIN 57472 for halogen-free flame retardants.

Another interesting development is a halogen-free flame retardant masterbatch for glass-filled PBT. The masterbatch meets the German standard DIN 57472, the EC packaging standard 2002/95/EC for electrical equipment (RoHS) and the EC End of Life Vehicles Directive. Upon the addition of 27% masterbatch with 30% glass-filled PBT, the UL-94 V0 standard is met at a thickness of 1.6 mm, in which the mechanical properties are hardly affected. This product is interesting for numerous electrical and electronic applications.

Ready-to-use flame retardant engineering plastics and compounds
Over the years, many types of flame retardant engineering compounds have been developed. The advantage of this is the great reproducibility of the fire behaviour of the final product. Many commercial flame retardant plastics have a UL-94 yellow card certification. Thus, the processors of these plastics can give an assurance that their application achieves the flame retardant standard. Their customers do not then need to incur high costs to certify their end products with an outside body. The processor should, however, realise that adding additives to UL-94 compliant plastics may change the flame-retardant properties. This is often overlooked. An example is the colouring of these materials with masterbatches, after which the UL-94 standard may no longer be met. When using flame retardant compounds, the processor no longer needs to admix harmful raw materials or masterbatches separately. The compound can be directly processed. This promotes a fast and efficient way of working. Production changes proceed smoothly, whereby the amount of waste is limited.

In conclusion

Flame retardants are hot. Research into innovative flame retardant systems that are cheaper, better, more efficient and more environmentally friendly, has produced a range of new products and alternatives. The trend is that the market for halogen-containing products is shrinking in favour of halogen-free flame retardant systems, such as phosphate products. The choice of the proper type of flame retardant is strongly dependent on the polymer and on the wishes and requirements to be met by the final product. In close collaboration with customers, suppliers and research laboratories, new innovative products are being developed in application areas that previously seemed inaccessible.