Flame retardants halogen-free systems including phosphorus additives

There are three essential conditions to be met if a polymer, once ignited, is to continue burning. There must be a supply of heat to the bulk polymer, a generation of fuel (typically volatile decomposition products) and there must be a flame. Halogen-based systems act by a well-documented flame poisoning mechanism in the vapour phase. The alternative halogen-free systems, which encompass a wide variety of additives, tend to act by mechanisms which disrupt heat flow and the supply of fuel to the flame. Here the mechanisms are not always understood in great detail but two broad types of flame retardant action can be defined. 

First there are additives which act to remove heat by endothermic decomposition and/or the generation of copious quantities of inert gases to dilute the combustible polymer degradation products. Materials such as alumina trihydrate (ATH) and magnesium hydroxide, which in toimage terms are by far the most widely used halogen-free flame retardants, work in this way. These additives are more fully described in the section Flame retardants inorganic oxide and hydroxide systems.  

The second type of flame retardant action involves the formation of char and this is most often accomplished by phosphorus-containing additives. Char formation is a process which occurs mostly in the condensed phase and has several benefits. A good char layer is difficult to ignite and acts as a physical barrier. It hinders the escape of pol)nner degradation 

In assessing the relative performance of different flame retardant additives in plastics a number of small scale tests have been developed. These are carried out under well-defined conditions and attempt to measure reduced rates of burning or flame propagation and/or increased resistance to ignition. It should be noted that the performance of a flame retardant plastic in small scale tests does not necessarily reflect the hazards present under actual fire conditions. 

The most widely quoted small-scale test is the Underwriters Laboratory UL-94 Vertical Burning Test, in which materials are classified as V-0, V-1, V-2 or are non-rated (NR). In the test the bottom of a vertically-mounted test strip is exposed to a flame twice, each time for 10 seconds duration. 

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Flame retardants halogen-free systems (including phosphorus additives)... 

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