In much the same way we got to explore PVC as a millable interior trim, we have begun working with solid, high-density polyethylene (HDPE). We're using it for thresholds on the exterior doors, sills on the shower pans, bathroom counters, and cabinet doors.
We bought ¾" thick Seaboard HDPE, a Vycom product, from Hamilton Marine. We bought it by the sheet, which runs 54x96" but thinner variants are available in smaller sizes.
Fishermen use Seaboard for their cabinetry and lobster because it's impervious to water, weather, age, mole, and algae. It's enormously durable, it doesn't warp, it doesn't fade or bleach in the sun, and it's easy to clean.
We've been using KOMA PVC for trim in the Little Easy but it's air-entrained, basically a rigid plastic foam. There's no question that it lacks the durability for thresholds, door sills, and the like, so we wanted something else for those applications. High-density polyethylene seemed like it would fit the bill, and we liked the commercial fishing connection.
There are important considerations in dealing with the stuff, though. It's hard to find in thicker grades and it's wicked expensive, for a start. It's density makes it tremendously heavy - 135 pounds per ¾" sheet - which can mean prior planning for transport and storage. It's also very slick, which means it's hard to maintain a grip on but also means it's probe to sliding off of things; you need to be especially careful about placement.
That slickness is called low-energy in material science, and it means that the product in its native state will not take caulk or paint. There appear to be a few caulks that will adhere to sanded HDPE surfaces, but nothing that will stick to bare, unsanded HDPE. We haven't yet seen a paint that will do the job but we don't need one for this application; they may well be out there.
Finally, and something we didn't see coming, Seaboard comes with a granulated surface, so if you need a smooth surface you've got some sanding to do.
Sanding HDPE isn't super easy; I would guess it takes twice the effort of wood sanding. We've been using 40, 60, 80, 120, 220, 320, 400, and 2000 grit sandpaper, primarily with random-orbit power sanders. If we were going to be sanding a huge amount of Seaboard we would figure out which sanding disc or discs we could do without (the 80 grit, maybe), but we're very happy with the satiny smoothness we're getting.
Cutting Seaboard is super easy. With an 80 tooth fine finish blade and a good fence on your table saw (we have the former but not the latter) you could have perfectly surfaced edges needing no further work. We've used circular, table, and jig saw so far and it's been easy to get square, clean cuts. We've also run a ⅝ roundover bit on the handheld router and don't need to do anything to clean up that surface.
From a health perspective, HDPE seems less problematic than PVC. The residue from sanding and cutting operations is a little more granular, and higher density, do it's quicker to fall to the floor, and less likely to get kicked back up. Consequently less of it is finding its way into our nose and lungs than PVC or wood dust. It would still be foolhardy not to mask up but we spent a day without in order to see how it would affect us. We are not/not medically qualified for anything, so wear proper personally protective equipment (PPE) or talk to your doctor about risks inherent in working with polyethylene.