I'm not sure why uv deteriorates synthetics such as nylon and polypropylene. I just know it does. I did think that when it's wet the water particles in and on the rope may act like a magnifying glass and aid uv penetration. Nylon rope is denser, does not float, and is weaker when wet. Poly floats and pretty much retains it's strength.
As to why the pipe foam damaged the plastic, maybe the uv effects on the urea formaldehyde foam releasing who knows what, speeding up the normal break down of the plastic.
