Its a while since I was at uni so I may have remembered this incorrectly Kevlar and Nomex are trade names for aramids developed by DuPont one is a meta aramid the other a para (cant remember which is which). With Kevlar the filaments align better during construction making it stronger than Nomex. I think they were developed initially as a flame resistant materials for the aerospace industry. Hopefully that's something like correct.

Warren

Carbon Copies Ltd

Both fibres are remarkably similar.  Nomex is meta poly amide, kevlar is para.  This substitution pattern around the aromatic ring causes the different properties, kevlar has a greater elastic modulus making it tough whereas nomex changes its conformation with heat (ie it absorbs heat and changes chemically) hence its greater heat resistance.  If I were to speculate I'd expect a kevlar fibre to be quite straight whereas a nomex fibre would have a twist like DNA when viewed with a microscope (this is just speculation) the extra heat would be absorbed making that spiral straight.  I'd expect the fibre would be OK after exposure to heat but any matrix its in might suffer. 

There's a whole load of variants of polyamides, Nomex and Kevlar are just the dupont trade names.

Looks like I remembered some things from my time at uni.

Warren

Carbon Copies Ltd