|MadSci Network: Chemistry|
Compounds which seem to be most effective in blocking UV rays are those which contain multiple bonds such as C=C or C=O. These compounds are effective because the multiple bonds absorb the UV energy quite effectively and can convert this energy to other forms of energy such as heat.
It so happens that I am currently working on a project that looks at the susceptibility of forest trees to UV exposure. With respect to this work we have found that, among other things, the waxy layer which coats the leaves of most plants is pretty good at filtering out UV radiation. These waxes contain a variety of chemical compounds such as fatty acids, fatty esters, long chain alkanes, and a variety of straight chain primary and secondary alcohols. These compounds occur in varying amounts depending on the specific plant and are thought to protect the leaf surface from exposure to the elements. These waxes are so effective that many companies market plant waxes (such as carnauba wax) as polishes to protect finished surfaces from the sun.
Although this type of coating would be fine for your car, I doubt that you would want to wax your fabrics. There are however, quite a few supply companies who market additives and coatings to protect materials against photodegradation. I have listed a few web sites which I thought represented the types of additives used in various manufacturing processes.
One pathway of UV degredation involves the UV rays hitting a double bond in the material of interest. The double bond (if there is sufficient light energy) can become a diradical. At this point, the most likely occurrence is that the double bond will reform releasing the energy in some other form (heat). The other thing which might happen is that if there is another double bond in close proximity the radical can react with the double bond. Unfortunately, the result of this reaction is still a diradical. These reactions are not reversible and result in degradation of the material. I noticed that a few of the compounds which are sold as UV blocking compounds are used in other instances as radical scavengers and probably serve the same purpose in preventing UV damage. They do this by simply eliminating the radicals as quickly as possible to prevent degrading reactions from taking place.
See this link for further discussion:
What I have provided here is far from a complete answer however I hope that it will provide you with some sort of starting point from which you can proceed.
Try the links in the MadSci Library for more information on Chemistry.