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Flame retardants, it is often spoken of in correspondence with polymeric materials that have exactly this property and which, in English, are indicated with the term of "Flame retardant" which also explains the abbreviation used now in the way rather universal FR.
Also for the insiders it is challenging to navigate the world of flame retardants which are numerous and with different characteristics, let alone for those who do not have to deal with this kind of substances every day. No one therefore claims in an article to tell everything there is to know about polymers FR but to understand what they are for and how they work, because the various types are based on different mechanisms of action.
Flame retardants: what they are
THE flame retardants they intervene during the combustion process that involve a certain type of chemical reaction and interrupt it. They do this in several ways, most notably with 5 essentially recognized possible mechanisms of action: development of heat, gas phase, char, intumescence, dripping.
Let's see what happens during combustion. A chemical oxidation-reduction reaction is triggered which develops heat, there is a "fuel" that oxidizes and a "comburent", oxygen, which is reduced. In this process, heat and electromagnetic radiation, it may happen that a light radiation, a flame, is also emitted, but it is not an essential condition for to be able to talk about combustion.
There combustion takes place step by step, after its initiation, consisting of heating, we move on to decomposition and priming, then to propagation and branching which is the moment when the combustion expands and accelerates.
There combustion ends with the formation of stable and no longer combustible species. THE flame retardants they can come into play in several stages. Usually when a combustion occurs between a polymer and oxygen, carbon dioxide and water are produced, as well as by-products such as nitrogen oxides or carbon monoxide.
There are polymers that burn more easily, such as PE and PP, others like the PA and PVC, or the PTFE, which take much longer, which resist the combustion process.
Halogenated flame retardants
Halogenates are flame retardants based on gas phase mechanism, in this category we find brominated polystyrene, for example, and they are that category that causes in PVC that flame resistance we talked about. THE halogenated flame retardants sThey are appreciated for their versatility, for their low cost and availability, for their ability to maintain mechanical properties but, on the other hand, they can cause environmental damage.
Flame retardants hydroxides
Metal hydroxides are gods flame retardants which act with the mechanism that we have called "heat development", both in the condensed phase and in the vapor phase. In this case hydroxides are those additives which cool the material and dilute the combustible gases.
Among the most used hydroxides for this purpose we find that of magnesium and those of aluminum, in general are substances that fortunately do not create many environmental problems, it does not costnot a lot and they are not difficult to find, however, they must be used in very high dosages and compromise the mechanical properties unlike the halogenates seen above.
Halogen-free flame retardants
Phosphorus compounds are useful flame retardants that exploit the char mechanism. In this category we find red phosphorus or some organophosphates. During combustion these substances cause one to form Carbonaceous "crust" which is completely inert with respect to combustion and which hinders and limits the meeting between oxygen and the polymer. In this way slows down the reaction underlying the combustion.
Phosphorus compounds work even when used in non-massive doses, they are halogen-free and they don't cost much, but they compromise mechanical properties and are hygroscopic.
Flame retardants for polymers: new
On the market, innovation is increasingly pressing and also in this sector the innovations follow one another, to try to respond to requests from clients. Usually they relate to the need to always have flame retardants less toxic, more biodegradable, more thermally stable and compatible with the polymers in which they are inserted. And, the law of the market that never fails, must cost as little as possible.
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