MadSci Network: Chemistry

Re: What kind of fabrics are fire retardant and what are their charecteristics

Date: Wed Mar 1 09:53:12 2000
Posted By: Narayan Variankaval, Grad student, Polymers/Textile and Fiber Engineering, Georgia Tech
Area of science: Chemistry
ID: 951058689.Ch

Man-made fibers and fabrics are used in a growing variety of applications ranging from clothing to carpets, blankets and many industrial uses. The flammability of these fabrics is critical not only to everyday clothing but also for specialized applications as those encountered in the space industry.

Before moving on you should understand first that flammability is different from actually decomposition of the fabric. Flammability is a property of fabrics, which measures the ability of the fabric to burn and propagate a flame or fire. Most fabrics decompose when they are exposed to fire. However, many of them do not actually burn and propagate a fire (classic example is nylon - I will talk about it later). Thus, the aim of using flame-retardants is to prevent the propagation of a fire. This is very important to bear in mind.

There are usually five methods used to impart flame retardance to a fabric

  1. Finishing the fabrics with flame retardant chemicals. Both reactive and non-reactive flame-retardants can be used. The reactive ones will react with the fabric at some stage during the application.

  2. Addition of a flame retardant to a polymer (of which the fabric is made) before it is made into a fiber form.

  3. Copolymerization - i.e. polymerizing a flame retardant polymer with the polymer used to make the fabric.

  4. Synergism - i.e. the polymer contains some groups like chlorine, bromine and fluorine which give inherent flame retardancy to the polymer. These elements are also the basis for flame retardant additives added to the polymer to make the final fabric flame retardant.

  5. Chemical modification to the polymer like chlorination of polyester, cyanoethylation of cellulose based materials (like cotton) etc. For example, suppose you have a polyester fiber that is going to go into making a shirt that has to be flame retardant. Using chemical methods, the polyester is chlorinated by suitable means and this increases the fire resistance of the fabric.
Now, what determines whether a fabric is flame retardant or not. It depends, as I have told you previously, mostly on the chemical composition of the polymer used to make that fabric. In general, combustibility increases with increase in hydrogen content. A decrease in hydrogen content will also decrease the height of the flame. As before, the presence of elements like nitrogen, chlorine, bromine and fluorine also make the polymer flame retardant.

Now, the way the fabric is constructed also affects the flame retardance at least in terms of burning rate. Proper fabrication goes hand - in - hand with adding flame retardant additives to the fabric. Thermoplastics like polyester, nylon etc. have an increased burning rate with increased thickness. However, other polymers like polybenzimidazole show decreasing burning rate with increasing thickness. Weave patterns also affect the burning rate. Suppose you have twisted fibers in the yarn used to make the fabric. These fibers have low air-to-fiber ratios and the amount of air trapped in the fabric will affect the thermal conductivity. The heat transfer necessary for flame propagation is less in twisted yarns resulting in a reduced burning rate.

Now let us move on to specific examples.

(1) Take rayon fibers. These will burn upon immediate contact with a flame. Upon leaving the flame it will support combustion and will continue to burn very rapidly. So rayon is mostly used in only undergarments that usually do not come in direct contact with flames. To improve flame retardancy, additives containing both phosphorous and nitrogen are usually added. For example, derivatives of phosphonitrilic chloride are added to viscose rayon to impart flame retardancy. Sulfur based flame retardant systems can also be used with rayon.

(2) Cellulose acetate based fibers/fabrics - To impart flame retardancy, haloalkylphosphates ( ie. Containing a halogen, an alkyl group like methyl, ethyl etc and phosphorous), such as tris(chloroethyl)phosphate have been used to increase flame retardancy.

(3) Nylon - Nylon 66 is actually less flammable than cotton, rayon, wool or silk. If a flame is applied to a nylon fabric, the material melts and tends to drop away. Its ignition temperature is around 530C. The fire risk of nylon is low since it melts to hard glassy brown-Grey bead. Additives added to nylon to increase flame retardance are as follows

Chemical	minimum effective amount needed to prevent flaming

Thiourea						3%
Ammonium thiocyanate					6%
Ammonium bromide					9%
Ammonium sulfate					11%
If a flame retardant lowers the melting temperature of nylon, it is a good flame retardant. The flame retardant is also effective if it is hydrophilic i.e. Likes water.

(3) Nomex - This is a DuPont fiber and has been described as "flameproof". If a single strand is lit with a match, it will flame brightly for a few seconds and then goes out. Nomex is usually self-extinguishing but tend to shrink when exposed to a flame. A sin other cases additives can also be added to increase flame retardancy.

These are but some examples of flame retardant fabrics and how retardance can be achieved. If you need more information, you can read up several articles that I have listed below. If you have any additional questions you can ask me. Hope this was helpful

(1) Man-Made Fibers: Flame Retardance and Flame Retardants, Michelle M. 
Gauthier, R.D. Deanin, C.J. Pope, Polymer-Plastics Technology and 
Engineering, Volume 16, Number 1, Pages 1-39, 1981
(2) Clothing Against Heat and Flame by R. Halls in the magazine Apparel 
International, Volume 29, Number 8, Pages 28+, 1998

Current Queue | Current Queue for Chemistry | Chemistry archives

Try the links in the MadSci Library for more information on Chemistry.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2000. All rights reserved.