Atticus said:
tgd31968 said:
How do they deal with the fact that aromatics are less UV stable and prone to breakdown/discoloring over time? Do they add stabilizers or are the effects slow enough to meet a minimum expected life?
Or is the UV stability only an issue for aromatic isocyanates?
I know nothing about chemistry. Well, maybe a little something about EOH, but other than that, nothing.
Despite that AF, you have hit on the problem that tgd31968 (sorry, I don't know your name) mentioned concerning light stability. Apologies if readers already know this but the performance fabrics (Nomex and Kevlar - known as 'Aramids') are woven from polymer fibres (remember that polymers are simply long strings of atoms joined together in a repeating pattern, often with many hundreds of thousands of atoms or more in length) just like cotton thread in your 1940s A2 would be sewn with a natural polymer, cotton - a polymer of cellulose (made up glucose (sugar) molecules). But the big difference is that in order to improve the heat resistance (resistance to bullet impact) or tensile strength of the fibres in aramids we need to change our building block (monomer) from glucose to a monomer with rings of carbon atoms in their structure. These have more bonds holding them together and are therefore harder to break apart (molecule from molecule) by heat, force, etc.
But...I have observed that early nomex jackets do fade and change color with time. In fact, they seem to fade to a much greater degree than does nylon. My first edition CWU-45/P is now much lighter and "yellower" than it was when new. I know this because I removed an old patch receiver from its shoulder and found the fabric below to be much darker and "greener" than the rest of the jacket. Also, the fabric under the jacket's collar and pocket flaps is a whole different color than the remainder of the jacket.
But (and it's a big but) the improvement in the thermal and mechanical properties (and flame retardance, etc.) comes with a greater tendency to absorb ultraviolet radiation from sunlight (due to the carbon - phenyl- rings). The UV absorption causes the electrons (arggh...starting to get technical...knew that I couldn't keep it going
) to become more energetic. Some energy can be lost as the electrons drop back down to their lower energy (ground) state but the UV radiation also causes the polymer chains to break down into smaller chains. Some of the broken chains can lose some of the bonds that hold the chains and rings together and as these go the wavelength at which the polymers absorb radiation changes and so the apparent colour changes (it actually shifts from being higher wavelength - more reddish - to lower wavelength - more bluish). This is why your nomex jacket will have faded and yellowed.
I have also observed that, while nomex fabric seems to be durable, nomex thread is not. Many seams in my CWUs have required re-stitching, even the ones in jackets that are only a few years old.
The aramid fibre will have been spun from a wet solution (dry-jet wet spinning that orients the polymer chains and extrudes them from spinneret holes and coagulates them in a poor solvent to make the filaments). This is the yarn that is used in your thread. In the case of the fabric these filaments will have been woven into a conventional cloth. The multiple layers or will offer some protection to the sunlight and additional strength is given by the fibres running in another direction (anisotropic tensile properties) helping to hide some of the weaknesses that will have developed over time. In contrast, the spun fibres are all aligned in one plane (along the length of the fibre yarn) so any weaknesses are magnified and the thread breaks (there's no additional support from threads running at e.g. 90 degrees to the thread).
Okay, school's out now :geek: ...I'll p.m. additional source material to those who want to read more :roll: .
Cheers
Ian