ken56 wrote:
My understanding is ABS is more UV resistant, PVC is not.
The truth is that ALL plastic (pipe) is subject to UV damage given enough time and substantial UV exposure. Pipe manufacturers add UV protectants to the plastic design mix (binding agents) before the extrusion process. Those additives can range from simple carbon black, (soot) to Calcium Carbonate, to colored pigments, to in some cases, fine grained sand (silt) as well as regrind (recycled) plastic from failed previous manufacturing runs. The purpose of the additive(s), which are usually solid at room temperature, is to provide a physical barrier that blocks the UV rays from penetrating into the wall of the pipe and limits the UV damage to the surface.
In plastic pipe, the additives are usually cheaper than the binders so the tendency is to add as much additive as possible and minimize the binder content. That's why some plastic pipe seems very durable and some pipe easily fractures like glass.
So, Why ABS instead of PVC? Because in the location where the plastic is used, of all a available approved products for a particular application, ABS is currently the cheapest.
Chum lee