There are two forms of nylon. The first is polyhexamethylene adipamide or nylon 6,6 (manufactured from two monomer units adipic acid and hexamethylene diamine, each containing 6 carbon atoms). The second is polycaprolactam or nylon 6 (synthesised from a single monomer, caprolactam).

The repeating unit of nylon 6,6 is polyhexamethylene adipamide and has a degree of polymerisation of 50-80 units (i.e. n=50 to 80).

Both have similar properties and will collectively be referred to here as 'nylon'. Nylon is sometimes referred to as polyamide by virtue of the large number of CONH (amide) groups it contain. They are highly crystalline materials (65-85 % crystalline) which accounts for many of the fibre qualities. Nylon is very strong, very tough with outstanding abrasion resistance, has excellent stretch recovery and can be spun as a very fine fibre. The high tensile strength makes nylon good for ropes, arresting cables and parachute fabric. It is easy to launder (very low water retention after spin drying and easy crease recovery)

Nylon has only fair resistance to sunlight and weathering. The imino component of the amide groups on the polymer chain can react with air under the influence of ultra violet rays from sunlight. This can lead to polymer fracture resulting in weakening of the textile material.

Acids readily hydrolyse the amide groups in nylon leading to polymer fragmentation and weakening of the fabric. This process can also lead to changes in the surface reflectance properties of the textile resulting in white fabrics appearing yellow and dulling of coloured materials. Alkalis can similarly cause hydrolysis of the polymer chain with similar results.

The long polymeric chains have a profound parallel orientation to one another. Kevlar gets its great strength from the powerful inter-polymer hydrogen bonds along with interactions between the aromatic (benzene) rings. Hydrogen bonds can form between the H on one polymer chain with an O on another. Kevlar is manufactured from 1,4-phenyl-diamine and terephthaloyl chloride. The products Kevlar and Nomex differ from each other in the points of attachment of the benzene rings within the polyamide chain. Nomex absorbs heat energy when exposed to an ignition source. The fibre will swell and thicken when heated forming a highly efficient barrier to flame & heat. This serves to reduce the heat transfer to the underlying tissue or structure. The LOI of Nomex is 28 meaning the oxygen levels (21%) normally found in the atmosphere do not readily support it's combustion. At temperatures above 427°C Nomex will simply form a very tough carbonaceous char which is the essence of its fire protective mode of action.