Friday, November 4, 2011

Plastics Help Keep Fire At Bay

What if plastics offered some of the best protection against fire? Sounds odd, and not what we’re given to think? Well, some plastics do precisely that. Fire is one of man’s greatest discoveries, but also one of his worst nightmares, for fire can destroy all in its path and be hard to control. We now know that burns are the leading cause of death in a fire. And 80% of them are due to asphyxia caused by toxic smoke. So saving lives mainly means doing something about smoke! The thing is, though, that not all types of smoke are equally dangerous. Firefighters and toxicologists class smoke into two types: asphyxiating or irritating. The former causes respiratory distress resulting in depression of the central nervous system leading to loss of consciousness, often quickly followed by death. The main asphyxiating gases are carbon monoxide (CO) and hydrogen cyanide (HCN, also called hydrocyanic acid or prussic acid). The latter type of smoke irritates body tissues and organs, causing pain and making the eyes water. The main culprits are hydrogen chloride, fluoride, bromide, nitrogen oxides and ammonia, etc. These gases are lethal only in very high concentrations very rarely found in fires. And while carbon dioxide (CO2) can be fatal in some cases, it mostly causes hyperventilation, causing toxic irritants and asphyxiants to be more rapidly absorbed. Plastic smoke no worse than any other A wide range of materials get burned. The most dangerous gases for humans (and other living creatures in the animal world) are carbon monoxide (CO) and hydrogen cyanide (HCN). CO is given off by almost anything that burns, whereas HCN is produced only through combustion of nitrogenous materials, whether natural (animal wool, duck feathers, silk...) or artificial (melamine, polyamides, polyurethanes ...). Now the problem with HCN is the “N” part – nitrogen – which you do not find in plastics like PVC, EPS (expandable polystyrene). So the short logical conclusion is that the effluents given off by some plastics are much less dangerous than those given off by natural products... Materials under the microscope Fire safety in buildings is a big concern. Regulations try to improve the properties of materials, both their fire behaviour (i.e., their propensity to fuel a fire and give off toxic smoke), and their fire resistance (the ability to withstand flames and so delay the fire spreading). Materials for use in buildings must meet tough standards. Each material is laboratory-tested and classified according to its fire behaviour and fire resistance. PVC Materials: Electric cables, pipes of all kinds, wall and floor coverings...PVC is everywhere in buildings. It is relatively flameproof, needs temperatures 150°C higher than wood to burn, and does not release HCN when burned. But it does give off an irritant gas: hydrochloric acid gas, or hydrogen chloride. In a fire, and at very low concentrations, hydrogen chloride irritates the eyes, nose and mouth giving a very early warning sign of fire so the alarm can be raised. Its concentration ratio never gets above the danger level for occupants’ health. Which is why PVC is gradually becoming a material of choice in many public buildings, including as wall and floor coverings in hospitals where with antibacterial ingredients added during manufacture it also helps fight against hospital-acquired infections. PVC was discovered over a century ago, but goes from strength to strength with every new development. The latest generation of PVC pipes and pipe fittings are intumescent and made to AFNOR standard NF Me (for “meringue-forming”)! Simply put, when exposed to the heat of a fire, the intumescent materials in this new PVC will swell to 800% or more of their original volume, forming a spongy or meringue-like foam that fills and seals off the pipe penetration, stopping the fire spreading from one room to another for up to 30 minutes. So, much more than fire resistance, it now actively helps to control fires. Fire-resistant cables Often implicated as a contributory factor in fires, electric cables are also getting a makeover. Just as well - a modern office building can have up to 200 kg/sq.m of them snaking through it. Now we have fire resistant cables (FRCs) which show little surface spread of flame and give off a little smoke so fires in hard-to-access false ceilings will spread more slowly but can be detected earlier. Also, halogenated compounds are not used in cross-linked polyethylene fire resistant cables, so, in a fire, there will only be a limited release of clear and non-corrosive, mildly asphyxiating and irritant smoke. New polyamide (PA)- and polybutylene terephthalate (PBT)-based plastics with their flame retardancy properties are now being used in the manufacture of plug sockets Composite plastics: a must in confined spaces Odd as it may sound, what sailors fear most is fire! And here again, naval architects have opted for composite materials with mind-boggling fire resistance and performance. They offer very high structural integrity, and are substantially lighter than metal or wood giving greatly enhanced weight savings, better speeds and greater fuel-efficiency. Their high-performance, easily-formable properties are bringing these materials into increasing use. Some are now even being found in Rafale fighter jet engines where temperatures reach several thousand degrees. Long gone are the days when these materials were used only in the conquest of space.

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