Flame Retardant Finishes | Mechanism and Chemicals of Flame Retardant Finishing

Last Updated on 03/05/2021

Flame Retardant Finishes in Textile: Mechanism, Chemicals and Application

Anju Singh
M.Sc. in Fabric and Apparel Science
Delhi University, India
Email: anjusingh292@gmail.com

 

Flame Retardant Finishes
A fabric can be considered flame resistant if it does not burn or does not continue to burn when subjected to a flame or heat source, with or without removal of the source. A chemical applied to a fabric to impart flame resistance is called a flame retardant. Different factors affecting flammability of textiles include type of fiber, yarn structure, fabric structure, and any chemicals / coatings applied on the fabric. Three necessary components for a fire are fuel, heat and oxygen. Flame retardant finishes improve flame resistance by masking or removing any one or more components that are required for burning.

flame retardant finishes

Some chemical flame retardants may be durable even after more than 50 laundering cycles while others may be non-durable and washed away after single laundering.

Flame retardancy is an important characteristic of textile materials in order to protect consumers from unsafe apparels. Flame retardant fabric is an important protective clothing. Firefighters and emergency personnel require protection from flames, as do floor coverings, upholstery and drapery, especially when used in public buildings. The military and airline industries also have multiple needs with regards to fire retardancy.

What is Flame Retardants (FR)?
Flame retardants (FR) are chemicals which are added to combustible materials to render them resistant to fire. They are designed to minimize the risk of fire in case of contact with a small heat source such as cigarette, candle or an electrical fault. If the material is ignited, the flame retardant will slow down combustion and prevent fire from spreading to other items.

Several types of compounds and polymers are used as FR for textile materials, including inorganic acids, acid salts and hydrates, organophosphorous and organobromine compounds, antimony salts/halogen systems etc. They may be classified into three categories:

  1. Primary FR based on phosphorous and/or halogen—the phosphorous derivatives act usually in solid or condensed phase, while halogen (chlorine or bromine) is active in gaseous phase.
  2. FR using synergists such as nitrogen and antimony for phosphorous and halogen-based FRs respectively. Synergists themselves are not flame retardants. FRs exhibiting P/N and/ or Sb/X synergism are durable in nature.
  3. Adjunctive or physical FRs includes alumina trihydrate, boron compounds, silicates, and carbonates. Their activity is mainly physical, although recently some evidence of a chemical effect has been cited. They are nondurable and used only if durability in laundering is not important.

Mechanism of Flame Retardant Finishes:

1. Explain mechanism of combustion of fire:
Combustion is an exothermic process that requires three components:

  • Heat
  • Oxygen
  • 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.

cumbostion cycle for fibers
Figure 2: Cumbostion cycle for fibers

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.

some free radical combustion reactions
Figure 3: Some free radical combustion 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. (Figure 4))

endothermic decomposition reactions
Figure 4: Endothermic decomposition reactions

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. (Figure 5)

formation of foamed glass
Figure 5: Formation of foamed glass

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. (Figure 6)

crosslinking with phosphoric acid
Figure 6: Crosslinking with phosphoric acid

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, (Figure 7)

dehydration of cellulose by strong acids
Figure 7: Dehydration of cellulose by strong acids

And prevent the formation of undesired laevoglucose, the precursor of flammable volatiles. (Figure 8)

thermal degradation of cellulose
Figure 8: Thermal degradation of cellulose

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 P/N 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)
Particularities 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)

Application of Flame Retardant Finished Fabric:
Flame retardant finishing fabrics are chemically coated fabrics which resist the fire by limiting oxygen supply. This flame retardant fabrics are used in curtains and drapes for school events and in important social gatherings. Flame retardant fabrics are used in a variety of applications like industrial work wear, uniforms for fire fighters, air force pilots, tent and parachute fabric, professional motor racing apparel etc to protect the wearer against fires, and electrical arcs etc.

References:

  1. Principles of Textile Finishing by Asim Kumar Roy Choudhury
  2. Chemical finishing of textiles by W. D. Schindler and P. J. Hauser
  3. Textile Engineering-An Introduction Edited by Yasir Nawab

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