|MadSci Network: Chemistry|
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
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
References: (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
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