Flame Retardants‎ > ‎

FR Mechanisims


Fire Triangle


The Fire Triangle or combustion triangle is a simple model for understanding the ingredients necessary for most fires.

The triangle illustrates a fire requires three elements: heat, fuel, and an oxidizing agent (usually oxygen). The fire is prevented or extinguished by removing any one of them. A fire naturally occurs when the elements are combined in the right mixture.

Without sufficient heat, a fire cannot begin, and it cannot continue. Heat can be removed by the application of a substance which reduces the amount of heat available to the fire reaction. This is often water, which requires heat for phase change from water to steam. Introducing sufficient quantities and types of powder or gas in the flame reduces the amount of heat available for the fire reaction in the same manner. Scraping embers from a burning structure also removes the heat source. Turning off the electricity in an electrical fire removes the ignition source.

Without fuel, a fire will stop. Fuel can be removed naturally, as where the fire has consumed all the burnable fuel, or manually, by mechanically or chemically removing the fuel from the fire. Fuel separation is an important factor in wildland fire suppression, and is the basis for most major tactics, such as controlled burns. The fire stops because a lower concentration of fuel vapor in the flame leads to a decrease in energy release and a lower temperature. Removing the fuel thereby decreases the heat.

Without sufficient oxygen, a fire cannot begin, and it cannot continue. With a decreased oxygen concentration, the combustion process slows. In most cases, there is plenty of air left when the fire goes out so this is commonly not a major factor.

The Fire Tetrahedron is an addition to the fire triangle. It adds the requirement for the presence of the chemical reaction which is the process of fire. The combustion reaction can be characterized by four components: the fuel, the oxidizing agent, heat, and an uninhibited chemical chain reaction. These four components have been classically symbolized by a four-sided solid geometric form called a tetrahedron. Fires can be prevented or suppressed by controlling or removing one or more of the sides of the tetrahedron.









Heat produces flammable gases from the pyrolysis of polymer. Then, an adequate ratio between these gases and oxygen leads to ignition of the polymer. The combustion leads to a production of heat that is spread out (Delta H1) and fed back (Delta H2). This heat feedback pyrolyses the polymer and keeps the combustion going.

To limit the establishment of this combustion circle, one (or several) ingredient has to be removed. Several techniques are available in order to break down this combustion circle. 

Flame retardants have to inhibit or even suppress the combustion process.

Depending on the polymer and the fire safety test, flame retardants interfere into one or several stages of the combustion process: heating, decomposition, ignition, flame spread, smoke process...Flame retardants can act chemically and/or physically in the condensed phase and/or in the gas phase. However, we have to remember that both of them occur during a complex process with many simultaneous reactions.






Chemical Effect Condensed Phase


In condensed phase two types of reactions can take place :

Polymer Breakdown


  •  Breakdown of the polymer can be accelerated by flame retardants. It leads to pronounced flow of the polymer which decreases the impact of the flame which breaks away. 


Charring


  •  Flame retardants can cause a layer of carbon (charring) on the polymer's surface. This occurs, for example, through the dehydrating action of the flame retardant generating double bonds in the polymer. These processes form a carbonaceous layer via cyclizing and cross-linking processes cycle



Char and intumescence formation

Intumescence

Flame retarding polymers by intumescence is essentially a special case of a condensed phase mechanism. The activity in this case occurs in the condensed phase and radical trap mechanism in the gaseous phase appears to not be involved
In intumescence, the amount of fuel produced is also greatly diminished and char rather than combustible gases is formed. The intumescent char, however, has a special active role in the process. It constitutes a two-way barrier, both for the hindering of the passage of the combustible gases and molten polymer to the flame as well as the shielding of the polymer from the heat of the flame. In spite of the considerable number of intumescent systems developed in the last 15 years, they all seem to be based on the application of 3 basic ingredients:
  • a "catalyst" (acid source),
  • a charring agent and
  • a blowing agent (Spumific).
Additives combining the last three ingredients leading to intumescent effect are commercially available. However, intumescent formulations can simply be developed and are more suitable than some commercial grades for some specific applications. Table1 below summarize usual catalyst, charring and blowing agents.

Catalyst
(Acid source)
Charring agentsBlowing agents
(Spumific)
Ammonium salts Phosphates, polyphosphatesPolyhydric compoundsAmines/amides
Sulfates 
Halides
Starch 
Dextrin 
Sorbitol Pentaerythritol, monomer, dimer, trimer 
Phenol-formaldehyde resins 
Methylol melamine
Urea 
Urea-formaldehyde resins 
Dicyandiamide
Melamine
Polyamides
Phosphates of amine or amideOthers Charring 
Products of reaction of urea or Guanidyl urea with phosphoric acids 
Melamine phosphate 
Product of reaction of ammonia with P2O5
Polymers (PUR, PA, …) 
Organophosphorus compounds
Tricresyl phosphate
  
Alkyl phosphates
Haloalkyl phosphates
  

Chemical Effect Gas Phase

The flame retardant or their degradation products stop the radical mechanism of the combustion process that takes place in the gas phase. The exothermic processes, which occur in the flame, are thus stopped, the system cools down, the supply of flammable gases is reduced and eventually completely suppressed. The high-reactive radicals HO· and H· can react in the gas phase with other radicals, such as halogenated radicals X· resulted from flame retardant degradation. Less reactive radicals which decrease the kinetics of the combustion are created.




Flame inhibition studies have shown that the effectiveness decreases as follow: H
I>HBr>HCl>HF.


mechanism of action of halogenated flame retardants

Brominated compounds and chlorinated organic compounds are generally used because iodides are thermally unstable at processing temperature and effectiveness of fluorides is too low. The choice depends on polymer type. The behaviour of the halogenated fire retardant in processing conditions (stability, melting, distribution, etc…) and/or effect on properties and long-term stability of the resulting material are among the criteria that have to be considered. Moreover it is particularly recommended to use an additive that produces halide to the flame at the same range of temperature of polymer degradation into combustible volatile products. Then, fuel and inhibitor would both reach the gas phase according to the "right place at the right time" principle. 

The most effective fire retardant (FR) polymeric materials are halogen-based polymer (PVC, CPVC, FEP, PVDF...) and additives (CP, TBBA, DECA, BEOs...). However the improvement of fire- performance depends on the type of fire tests i.e. the application. 

They perfectly illustrate the previously described chemical modes of action. Severe perturbations of the kinetic mechanism of the combustion lead to incomplete combustion. 

Synergism with Antimony trioxide (Sb2O3)
To be efficient the trapping free radicals needs to reach the flame in gaz phase. Addition of antimony trioxide allows formation of volatile antimony species (antimony halides or antimonyoxyhalide) capable to interrupt the combustion process by inhibiting H* radicals via a serie of reactions proposed bellow. This phenomenon explains the synergistic effect between halogenated compounds and Sb2O3. For most applications, these two ingredients are present in the formulations.


Physical Effects

Formation of a protective layer


The additives can form a shield with low thermal conductivity, through an external heat flux, that can reduce the heat transfer Delta H2 (from the heat source to the material). It then reduces the degradation rate of the polymer and decreases the "fuel flow" (pyrolysis gases from the degradation of the material) that feeds the flame.

Phosphorous additives may act the same way. Their pyrolysis leads to thermally stable pyro- or polyphosphoric compounds which form a protective vitreous barrier. The same mechanism can be observed using boric acid based additives, zinc borates or low melting glasses.




formation of protective layer inhibiting, combustion and volatiles

Cooling effect 

The degradation reactions of the additive can influence the energy balance of combustion. The additive can degrade endothermally which cools the substrate to a temperature which is below the one required for sustaining the combustion process. Different metal hydroxides follow this principle and its efficiency depends on the amount incorporated in the polymer. 

Dilution

The incorporation of inert substances (e.g. fillers such as talc or chalk) and additives (which evolve as inert gases on decomposition) dilutes the fuel in the solid and gaseous phases so that the lower ignition limit of the gas mixture is not reached. In recent work, the isolating effect of a high amount of ash (resulting from certain silica-based fillers) has been shown in fire-retarded systems. Moreover, it highlights also an opposite effect as thermal degradation of the polymer in the bulk is increased by heat conductivity of the filled material.