As the whole environment is going highly technical and risky, the demand for specially treated textile such as flame resistant fabric has grown significantly. In the process of meeting with these demands, synthetic fiber has played a significant role. However, along with many advantages, these synthetic fabrics are also prone to fire. Whether it is concerned with the various electrical or electronics items in offices or dealing with highly flammable materials in other places including hotels, hospitals and even homes, the need for protective clothing is felt every where. The textile industry has found the solution by developing flame retardant finish for synthetic fibers like polyester fabrics, nylon fabrics, polypropylene fabrics etc. By applying flame retardants, fabrics are made flame resistant.
What is a Flame Retardant
Simply defined, flame retardants are materials that have the quality of inhibiting or resisting the spread of fire. Textile is highly ignitable and contribute to rapid fire spread. However, the ignitable property of a textile can be considerably reduced by any one of the three methods- by using inorganic materials such as Asbestos, Glass etc; by chemically treating the textile with Flame Retardant chemicals; and by modifying the polymer.
Categories of Flame Retardants
There are many categories in which flame retardants can be divided. The major categories among them include:
•Tetrakis (hydroxymethyl) phosphonium salts that are made by passing phosphine gas through a solution of formaldehyde and a mineral acid like hydrochloric acid. This category is mostly used as flame retardants for textiles.
•Minerals like asbestos, compounds such as aluminum hydroxide, magnesium hydroxide, antimony trioxide different hydrates, red phosphorus, and boron compounds, mostly borates. Etc.
•Synthetic materials, usually halocarbons which include organochlorines such as polychlorinated biphenyls (PCBs), chlorendic acid derivates and chlorinated paraffins; organobromines such as polybrominated diphenyl ether (PBDEs), organophosphates in the form of halogenated phosphorus compounds and others.
To make it more simple, if we talk of the types of flame retardants, they are:
•Brominated flame retardants
•Chlorinated flame retardants
•Phosphorous-containing flame retardants (Phosphate ester such as Tri phenyl phosphate)
•Nitrogen-containing flame retardants (i.e. Melamines)
•Inorganic flame retardants.
Other method of classifying Flame Retardants is to divide them as Inorganic, Organo Phosphorous, Halogenated organic and Nitrogen based compounds. Halogenated organic flame retardants are further classified as having either Chlorine or Bromine which is popularly known as Brominated Flame Retardants (BFR)
How do Flame Retardants Work
Flame retardant chemicals that are applied to fabrics are intended to inhibit or suppress the combustion process. These fire retardants interfere with combustion at different stages of the process like during heating, decomposition, ignition or spreading of flame. For understanding how flame retardants resist fire, first it should be known how a textile is heated up, catches fire and contributes in spreading it.
As with any matter, a textile fabric exposed to a heat source experiences rise in temperature. If the temperature of the fire source is high enough and the net rate of heat transfer to the fabric is great, pyrolytic decomposition of the fiber substrate occurs. The products of this decomposition include combustible gases, non combustible gases and carbonaceous char. The combustible gases mix with the surrounding air and its oxygen. The mixture ignites, yielding a flame. It happens when the composition of textile and the temperature, both are favorable. Part of the heat generated within the flame is transferred to the fabric to sustain the burning process and part is lost to the surroundings.
Now, if the textile is flame resistant then the flame retardant can act physically and/or chemically by interfering at particular stages of burning. There are different mechanisms of flame retardants.
Mechanisms of Flame Retardants
Flame retardants can act physically or chemically and sometimes both by physically and chemically interfering at particular stages of burning. The different mechanisms are:
Endothermic Degradation : Certain compounds break down endothermically when they are subjected to high temperatures. Magnesium and aluminium hydroxides are such examples. Various hydrates also act similarly. The reaction takes off heat from the surroundings, thus cooling the material.
Dilution of Fuel: Substances, which evolve inert gases on decomposition, dilute the fuel in the solid and gaseous phases. Inert fillers, eg. talc or calcium carbonate, act as diluents, lowering the combustible portion of the material, thus lowering the amount of heat per volume of material that it can produce while burning. Thus the concentrations of combustible gases fall under the ignition limit.
Thermal Shielding : A thermal insulation barrier is created between the burning and the yet-to-burn parts. Intumescent additives are sometimes applied that turn the polymer into a carbonized foam, resultantly separating the flame from the material and slowing down the heat transfer to the unburned fuel.
Dilution of Gas Phase : Inert gases, mostly carbon dioxide and water, act as diluent of the combustible gases, lowering their partial pressures and the partial pressure of oxygen, thus slowing the reaction rate. These gases are produced by thermal degradation of some materials.
Gas Phase Radical Quenching : Chlorinated and brominated materials undergo thermal degradation and release hydrogen chloride and hydrogen bromide. These react with the highly reactive H. and OH. radicals in the flame, resulting in an inactive molecule and a Cl. or Br. radical. The halogen radical has much lower energy than H. or OH. and thus has much lower potential to propagate the radical oxidation reactions of combustion. Antimony compounds tend to act in synergy with halogenated flame retardants. The HCl and HBr released during burning are highly corrosive, which has reliability implications for objects subjected to the released smoke.
Application of Flame Retardants on Textiles
Flame Retardants on fabric can be applied through conventional padding, padding with multiple dips and nips. If followed by 30 to 60 seconds dwell, it gives good results. The pH of the pad bath is optimally kept at approximately 5.0. The amount of flame retardant required depends primarily on the fabric type, application conditions, and test criteria required to be met with. Screening experiments should be conducted to determine the minimum application level for a fabric.
One of the most common processes for applying Flame Retardants on cotton fabrics is the "Precondensate"/NH3 process. One of several phosphoniums "precondensates" is applied after which the fabric is cured with ammonia. Then it is oxidized with hydrogen peroxide. Precondensate is the Tetrakis-hydroxymethyl phosphonium salt pre-reacted with urea or another nitrogenous material. The amount of anhydrous sodium acetate is approximately 4% of the amount of precondensate used. Some precondensates are formulated along with the sodium acetate. Softeners are also added along with precondensates. A critical factor in the successful application of precondensate/NH3 flame retardant is the control of fabric moisture before ammoniation. Generally, moisture levels between 10% and 20% give good results.