Chapter 4
Which plastics are flame-retardant?
Examining the best flame-retardant plastics and the tough requirements they have to meet
Preventing fires is a challenge wherever plastics are used. Flame-retardant plastics are the answer. Recent flame-retardant systems meet ever tougher environmental requirements.
Nearly all plastics are inherently flammable. The burning behaviour of plastic parts at ignition and thereafter is, however, not a property of the material but of the component or appliance of which it is a part.
The burning process depends on many variables (for example, the flammability of the plastic and its heat of combustion; the nature, duration and intensity of the source of heat) and on the concrete situation (for example, supply of air and flame spread at the point of burning).
The consequences of fire, such as smoke development and the toxicity and corrosiveness of the combustion gases, must also be kept in mind.
Flame-retardants are incorporated into plastics so they can meet fire regulations. They serve to slow down combustion or prevent it altogether.
The processing and in-use characteristics of flame-retardant plastics can be significantly different from those of the unmodified material.
Flame-retardants can be classified according to their chemical composition and the way they work:
- Halogenated flame-retardants work in the gaseous phase by interrupting the process of combustion by chemical means. They are mostly very effective, compatible with most engineering plastics and relatively inexpensive. Their use is decreasing for environmental reasons (in fire they liberate halogenated gases).
- Phosphorous flame-retardants which form a protective layer on the burning surface through carbonisation, sometimes in combination with foaming. This insulating layer works by reducing the supply of oxygen at the point of burning. Phosphorous flame-retardants do not harm the environment and are particularly effective in nylons and polyesters, but they require extra care in processing. The amount of additive needed is relatively small, so plastics with phosphorous flame-retardants have good mechanical and electrical properties.
- Inorganic flame-retardants such as aluminium hydroxide Al(OH)3 or magnesium hydroxide Mg(OH)2. In a fire they decompose, releasing water that dilutes the combustion gases and cools the point of burning. These mineral-based flame-retardants are inexpensive and environmentally harmless. The large quantities that need to be added may,
however, substantially deteriorate the mechanical properties of flame retarded plastics.
Liquid crystal polymers play a special role in fire prevention. Unlike other plastics, they are inherently flame-retardant and thus need no flame-retardant additives. Depending on the application, it may be possible and even desirable to combine different flame-retardants.
Recent research shows that, with a small dose of nanoadditives, the amount of flame-retardant needed is substantially reduced without impairing its effectiveness. As a result, the application properties of plastic parts are improved.
Flammability testing
The Underwriters’ Laboratories UL 94 method for testing plastics’ flammability is accepted worldwide and is included in the CAMPUS database.
In this test, a test bar is exposed to a Bunsen burner flame under specific conditions. The classification thus obtained (see table below) is an important criterion in the choice of a plastic. There are also other flammability tests, generally specific to certain industries.
When used in electrical or electronic applications, plastics have to meet the requirements of the IEC 60335 glow-wire test for domestic equipment. The updated and more stringent version results in improved fire safety for electrical equipment operated without supervision.
Safety in use
In the automotive industry, US Federal Motor Vehicle Safety Standard (FMVSS ) 302 and ISO 3795 state that, after exposure to a flame, the rate of combustion of plastics used for interior parts may not exceed 110mm per minute.
Bodywork parts must self-extinguish after removal of the flame (DIN 53438, Part 1). Regarding flammability and smoke generation, plastic parts used in aircraft have to meet the strictest requirements of all.
For applications in buildings, plastics are classified according to DIN 4102 as either B1 (difficult to ignite), B2 (normally ignitable) or B3 (easy to ignite), and their ability to withstand fire is determined according to ISO 834-1, using in situ parts under realistic burning conditions.
The new electronically commutated motors by Wellington Drive of Auckland, New Zealand, is a marvel of adaptability. They are used in equipment for heat recovery and air dehumidification, as well as in “white goods”. Zytel® FR 15 nylon, used for the motor enclosure, offers the high degree of passive fire safety that is a prerequisite for such a broad range of applications.
The more severe requirements of the glow-wire test have influenced the choice of materials for connectors, switches and relays in all types of domestic appliances.
Flame-retardant types of Crastin® polybutylene terephthalate (PBT), Thermx® polycyclohexylene diethyl-terephthalate (PCT) and Zytel® HTN high-performance polyamide meet the requirements for shorter after-burn times even after higher glow-wire temperatures.
Certain types of Zytel® nylon incorporate new flame-retardant systems that fulfil the requirements for flame retardancy without halogens and red phosphorus.
If the application calls for flame retardancy, high strength with low wall thickness and good process-ability, the choice of material is practically restricted to Zenite® liquid crystal polymer (LCP).
An example of its use is the coil bobbins and stator encapsulation of the torque motors made by Sonceboz SA of Switzerland. As no flame retardant reduces the material’s strength, the thin-walled coil bobbins withstand the mechanical stresses to which they are subject during winding.
Testing the combustion behaviour of plastics and plastics components depends on the application for which they are intended. The result is that in equipment such as electrical appliances and motor vehicles, plastics contribute substantially to passive fire protection.
Recently developed flame-retardant systems are very effective, are environmentally safe and produce plastic parts with good physical properties.
| Flammability testing: 20mm high flame | ||||||
|---|---|---|---|---|---|---|
| Horizontal position of specimen | Vertical position of specimen | |||||
| Duration of flame application | 30 sec | 1. Duration of flame application | 10 sec | 10 sec | 10 sec | |
| Rate of burning of specimen: | Duration of burning after flame application | < 30 sec | < 30 sec | < 10 sec | ||
| Up to 3mm thick | < 75mm/min | 2. Duration of flame application | 10 sec | 10 sec | 10 sec | |
| From 3mm to 13 mm thick | < 40mm/min | Duration of burning after flame application | < 60 sec | < 60 sec | < 30 sec | |
| Dripping of flaming droplets | Permitted | No | No | |||
| Complete combustion | No | No | No | |||
| Combustibility class | HB | Combustibility class | V-2 | V-1 | V-0 | |
| Table 1 – Classification of flammability of plastics according to UL94 | ||||||
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The examples in this series of articles are intended to illustrate underlying principles and to explain the main influencing factors.
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