Mechanism of Flame Retardant Finishes

Anju Singh
Pursuing M.Sc. in Fabric and Apparel Science
Delhi University, India

1. Explain mechanism of combustion of fire:
Combustion is an exothermic process that requires three components
  1. Heat
  2. Oxygen
  3. A suitable fuel

  • When left unchecked, combustion becomes self catalyzing and will continue until the oxygen, the fuel supply or the excess heat is depleted. 
  • When heat is applied, the fiber’s temperature increases until the pyrolysis temperature, Tp, is reached. At this temperature, the fiber undergoes irreversible chemical changes, producing non flammable gases (carbon dioxide, water vapor and the higher oxides of nitrogen and sulfur), carbonaceous char, tars (liquid condensates) and flammable gases (carbon monoxide, hydrogen and many oxidisable organic molecules). 
  • As the temperature continues to rise, the tars also pyrolyse, producing, more non- flammable gases, char and flammable gases. Eventually, the combustion temperature, Tc, is achieved. At this point, the flammable gases combine with oxygen in process called combustion, which is a series of gas phase free radical reactions. 
  • These reactions are highly exothermic and produce large amounts of heat and light. The heat generated by the combustion process provides the additional thermal energy needed to continue the pyrolysis of the fiber, thereby supplying one more flammable gases for combustion and perpetuating the reaction. The burning behavior of textiles is determined more by the speed or rate of heat release than by the amount of this heat.
2. What are the various approaches used for disruption of combustion cycle:
The various methods for disruption of combustion cycle are:

A. To provide a heat sink on or in the fiber by use of materials that thermally decomposes through strongly endothermic reactions. If enough heat can be absorbed by these reactions, the pyrolysis temperature of the fiber is not reached and no combustion takes place. Examples of this method are the use of aluminium hydroxide or ‘alumina trihydrate’ and calcium carbonate as fillers in polymers and coatings. (FIG.8.3)
B. To apply a material that forms an insulating layer around the fiber at temperatures below the fiber pyrolysis temperature. Boric acid and its hydrated salts function in this capacity. Fig 8.4
When heated, these low melting compounds release water vapor and produce foamed glassy surface on the fiber, insulating the fiber from the applied heat and oxygen.

C. To achieve flame retardancy is to influence the pyrolysis reaction to produce less flammable volatiles and more residual char. This ‘condensed phase’ mechanism can be seen in the action of phosphorous- containing flame retardants which, after having produced phosphoric acid through thermal decomposition, crosslink with hydroxyl-containing polymers thereby altering the pyrolysis to yield less flammable by-products. FIG* 8.5
But there is also other explanation for the first step of this dehydration, including single esterification without cross linking, for example, of the primary hydroxyl group in the C-6position of the cellulose units. The phosphorous esters catalyze the dehydration,
And prevent the formation of undesired laevoglucose, the precursor of flammable volatiles.

Compare Condensed and Gas Phase Mechanism for Flame Retardancy:
Type of mechanism
Condensed phase
Gas phase
Type of chemistry involved
Pyrolysis chemistry

Flame chemistry

Typical type of synergism
Sb/Br or Sb/Cl
Effective for fiber type

Mainly cellulose, also wool, catalyzing their dehydration to char
All kinds of fibers, because their flame chemistry is similar (radical transfer reactions)
Very effective because dehydration and carbonization decrease the formation of burnable volatiles

Fixation with binder changes textiles properties such as handle and drape, preferably for back coating for example of furnishing fabrics and carpets
Application process
If for durable flame retardancy then demanding multi step process
Relatively simple, standard methods of coating, but viscosity control is important
      Environment toxicity
With durable flame retardancy, formaldehyde emission during curing and after finishing ,phosphorous compounds in  the waste water
Antimony oxide and organic halogen donators (DBDPO and HCBC) are discussed as problems (for eg.Possibility of generating poly halogenated dioxins and furans)


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