All composite parts, including carbon fiber parts, are usually painted if exposed to sunlight. Clear coats are paints and contain UV absorbers. Carbon fiber is not affected by UV but the matrix resins are. Carbon fiber parts will start to develop surface fuzz due to surface resin degradation after long exposure to sunlight even though the deeper layers are unaffected because carbon is opaque to light. Aramid, and most amber colored polymers, are pretty sensitive to UV degradation. UV inhibitor can be added to matrix resins. Dry aramid cables are sometimes used in high performance yacht rigging. These cables have a black plastic jacket. Coating individual filaments will not work because the coating is too thin to provide protection.

Compositepro,
I know little about carbon fibers, but am interested in your comment that "Clear coats are paints and contain UV absorbers". No clear coat that I am aware of, including the ones that claim to be UV resistant, provide the amount of protection against the sun that is provided by tinted acrylics.

I am not very familiar with tinted acrylics as they are not used in the aerospace industry or on composite parts. Almost all paints used in aircraft applications are urethanes which crosslink by reacting with moisture. I believe this maybe due to the requirements for solvent resistance. Clearcoats for composites are also mostly urethanes and have quite a bit of UV protection, but certainly do not provide 100% protection forever. UV inhibitors are consumed with time so recoating is required periodically. Mineral pigments can last forever in UV but not the binder resins, in much the same ways as carbon fiber. Carbon black provides pretty good UV protection but composite parts are usually painted white because they lose strength in hot sun.

Thanks for the explanation.

Dik

My point was that paint, whether acrylic or urethane, protects better against UV if it is pigmented. Clear polyurethane gives a good finish on wood, but put it out in the sun, and it doesn't last long.

Hokie... from what the explanation was, it's the binder holding the pigment that breaks down... and, you can get clear polyurethane coatings that have excellent UV resistance. I had always thought that pigments provided a 'UV Screen', but apparently they aren't necessary. Without pigments or 'anti-gloss' additives (high gloss), coating systems usually last longer.

Dik

dik,

My experience is the opposite. If you know of a clear polyurethane coating with actual excellent UV resistance (as opposed to advertised), I would like to know about it. Where I live in Queensland, the UV exposure is extreme, and no clear coating will last more than 2 years, while tinted acrylics last 10 years.