[JGraham]

Hi all. I was wondering if anybody could tell me why the strength of nylon is degraded by exposure to sunlight. As I understand it, it's the ultra-violet light that weakens the material, but what actually goes on to make this the case?

[Alberto_Kravina]

AFAIR the UV light forms oxygen radicals which form instable peroxides with the polymer chain. These Peroxides form Aldehydes (which are more stable than the peroxide) by breaking the chain, so the strength of the polymer becomes weaker and weaker

[JGraham]

So the light breaks off the double-bonds with the oxygen atoms, two of which steal two hydrogens from the polymer chain, forming peroxide, this peroxide then breaks the chain (at any specific point? or just randomly?), leaving the broken ends each with a O=CH- group, right?
So how come this only hapens with UV light? Or is it just that the higher energy of the light accelerates the process? So this would happen to other nylons, like Kevlar for example?

[Alberto_Kravina]

So the light breaks off the double-bonds with the oxygen atoms, two of which steal two hydrogens from the polymer chain, forming peroxide, this peroxide then breaks the chain (at any specific point? or just randomly?), leaving the broken ends each with a O=CH- group, right?
So how come this only hapens with UV light? Or is it just that the higher energy of the light accelerates the process? So this would happen to other nylons, like Kevlar for example?

I think that the peroxides form on the methylene groups of nylon, I don't think that the keto groups break, but check me on this

Something like this:
                                            O - O
                                             |     |
-[CH₂-CH₂-]-    ----Ox----->   -CH-CH-  ---------> 2  -CHO

The UV light has a high energy, and it also accellerates the formation of Oxygen radicals (O₂ -UV-> 2 O ).

[JGraham]

Thanks very much, it all begins to make sense! So the weakening occurs because of the increaced presence of oxygen radicals in the air. Are there any O radicals in the air when it's not exposed to UV light? So this wouldn't be a factor (or rather, as much of a factor) on something like Kevlar, where the chain is made up of benzene rings as opposed to just straight carbon chains, I guess - right?

[Alberto_Kravina]

So the weakening occurs because of the increaced presence of oxygen radicals in the air.

As far as I can tell - yes, although I think that the UV light can weaken the C-C or C-H bond "directly"

Are there any O radicals in the air when it's not exposed to UV light?

Maybe Visible light can form O radicals as well, but not as much as UV light would forum

So this wouldn't be a factor (or rather, as much of a factor) on something like Kevlar, where the chain is made up of benzene rings as opposed to just straight carbon chains, I guess - right?

I think that Kevlar is not as "reactive" with O radicals as nylon 'cause there aren't any -CH₂- chains, however the oxygen radicals (and UV light as well) could "attack" the amide groups....