Reducing Fire Hazards of Reinforced Plastics

Since the Kings Cross disaster 12 years ago, fire safely Issues regarding reinforced plastics have been heating up. There has been a lot of pressure to minimise risk posed by materials used within enclosed spaces (the Channel Tunnel being a good example).

Reinforced plastics have been specified for electrical insulation purposes for many years, but in the past decade, awareness of fire safety has been raised to the point where the burning behaviour of materials, particularly those that are to be used in an enclosed situation has become the primary concern. It's no longer a case of out of sight, out of mind.

In reality, most reinforced plastics can be designed to have a degree of fire resistance - many additives are available which inhibit burning, but unfortunately, many of these compounds give off very toxic fumes (nitrous oxides, cyanide, halogenated compounds etc.) and in a fire situation, whilst the inhibited material is unlikely to be the cause of the fire, it can become a major contributing factor to the conflagration.

Another unfortunate fact is that there seem to be almost as many different 'flammability' tests as there are materials available and the majority of these are difficult if not impossible to relate to one another and in truth do not in themselves give a true indication of what will happen in a real fire.
"One of the continuing issues in a fire laboratory is to try to scale up from a bench test to a realistic fire scenario, but this isn't at all easy given all the different complexities of a fire" says Peter Briggs, manager of research and special projects at Warrington Fire Research.

In an ideal world, specifiers would need to consider the overall heat in a fire situation, in addition to the ventilation and the smoke escape, as all these can have a massive impact on the results of a flammability test. But in reality, no one can truly predict what will happen.

Certainly with regard to small scale monitoring tests, the UL 94 method has in the past been pretty much a universal test adopted by manufacturers, not least because its simple and cheap to perform. At best however, this test will only indicate how a particular material compares with other similar materials in terms of ignition and burning under identical conditions. A footnote to the test in the current IEC Materials Standard IEC 60893, which describes synthetic resin bonded laminates really sums it up: "The small scale laboratory test used in this standard for assigning a flammability category is primarily for monitoring the consistency of production of laminates. The results so obtained should not in any circumstances be considered as an overall indication of the potential fire hazards presented by these laminates under actual conditions of use."

Other tests, which together give an indication of how a product will behave, are now finding more widespread use. These include Oxygen Index, heat release tests, smoke emission, surface spread of flame, small-scale ignition tests and toxicity index. Together, these tests give us a better understanding of the potential risks.

As previously mentioned, many additives have been developed specifically to inhibit burning and many of these are very effective indeed in preventing ignition. Often this is sufficient for an insulating material, which may itself be housed within a sealed unit or may actually be present in such relatively small quantity compared to the surroundings as to present an insignificant risk. However in an enclosed area or where larger quantities of material are involved, this may not be the case.

One option which is often overlooked is the use of materials which do not burn at all such as ceramics, but often this is not a practical consideration as insulating parts are often subject to shock /impact and/or flexural stress which when coupled with high short run production costs weigh heavily against this choice.

Surprisingly however, there are composite materials, which have been around for a long time which are inherently flame retardant and which give of relatively little in terms of toxic by products when exposed to fire.

The best of these has to be silicone glass, which is now finding extensive use as an insulating material in underground situations. Silicone itself is inherently flame retardant and in combination with glass exhibits excellent properties. Similarly phenolic glass laminates also exhibit extremely good properties under the same test conditions. Both of these materials are approved by London Underground to specification SE 970 and are finding ever more widespread use in critical locations.

Of course, there is always one drawback and as you would expect, the materials are more expensive than for example polyesters or commercial SRBP laminates, but what price can you put on safety?

Mr Andy Pye
Magpye Publishing Ltd
pyeline@compuserve.com
0181 319 0214

New Materials International Nov/Dec 1999 Vol 14, No 158 Pyeline Publishing
Business Innovations and Innovations from the leading edge of the Materials World Editor: Andy Pye Tel: 0181 319 0214 Fax: 0181 333 6690 email: pyeline@compuserve.com www.newmaterials.com