PCB007 Magazine

PCB-Mar2015

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10 The PCB Magazine • March 2015 erwise they are considered as fillers. Reactive flame retardants are chemically bound to the polymer molecule by incorporating them into the polymer backbone or by grafting them onto the backbone as branches. As reactive flame re- tardants are chemically bound to the host poly- mer they are prevented from bleeding out and thus generally exert greater flame retardancy than additive compounds due to their greater availability throughout the life cycle of the polymer into which they are incorporated. II. Why do we need flame retardants? On average there are more than 4,500 fatalities annual- ly in the EU-27 as a result of fires; this accounts for 2% of all fatal injuries [1] . Fires de- velop from inception through build-up until a stage where the total thermal radiation from the fire-plume, hot gases and hot compartment bound- aries cause the radiative igni- tion of all exposed combus- tible surfaces within the com- partment. This sudden and sus- tained transition of a growing fire to a fully developed fire is called flashover [2] . At this point the radiation of energy to the con- tents of the room raises all the contents to their ignition temperature whereby the contents of the room suddenly and simultaneously ignite. It is estimated that in a domestic dwelling fitted with working fire alarms on all levels where the occupants are asleep upstairs and a fire starts on the main level of the residence the occupants have about three minutes to escape if they are to have any chance of survival [3] . The presence of flame retardants in otherwise combustible materials has two possible effects; • The flame retardant may prevent the fire fom developing altogether or; • The flame retardant may slow down the build-up phase of the fire thus delaying the onset of flash over thus extending the escape time window. In either case, the flame retardant serves its primary purpose of reducing the risk of fire re- lated fatalities. The efficacy of the use of appropriate flame retardants can be seen by way of example from the introduction of "The Furniture and Furnish- ings Fire Safety Regulations" in the UK in 1988. These imposed a fire resistance requirement on all upholstered furniture supplied in the UK. Between 1988 and 2002 a Govern- ment commissioned report esti- mated that the "Furniture and Furnishings Fire Safety Regu- lations" played a direct role in saving 1,150 lives and pre- venting 13,442 injuries [4] . III. Flame Retardant Mechanisms Flame retardants fulfil their purpose primarily by either physical or chemical action. Physical action can be subdivided into three modes; 1. Cooling—An endother- mic process is triggered by ad- ditives cooling the substrate to a temperature below that re- quired for sustaining the combus- tion process. 2. Formation of protective layer—The com- bustible layer is shielded from the gaseous phase with a solid or gaseous protective layer. The oxygen required for the combustion pro- cess is excluded and heat transfer is impeded. 3. Dilution—Fillers are incorporated which evolve inert gases on decomposition diluting the fuel in the solid and gaseous phase so that the lower ignition limit of the gas mixture is not exceeded. Chemical action can be subdivided into two modes: 1. Reaction in the solid phase—The flame retardant causes a layer of carbon to form on the polymer surface. This can occur through dehydration of the flame retardant forming a carbonaceous layer by cross-linking. The carbo- Feature as reactive flame retardants are chemically bound to the host polymer they are prevented from bleeding out and thus generally exert greater flame retardancy than additive compounds due to their greater availabil- ity throughout the life cycle of the polymer into which they are incorporated. " " FIRE RETARDANCy: WHAT, WHy, AND HOW continues

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