[zalo]

Mixing graphite into HDPE appears to make it immune to UV photo-degradation.

There’s even a company that capitalizes on this by making durable boat hulls with it: https://tidemanboats.com/

Not sure how it holds up in plasphalt, but it could be worth exploring graphite additives.

[iancmceachern]

Exactly. It's called uv stabilization. When spec'ing a resin for a product a designer can choose to go with a UV stabilized resin, which is ok in the sun. Basically they add dirt, titanium oxide, other oxides to the plastic to mechanically block the uv waves.