What is HDPE poly lumber, exactly?
The plastic milk jugs are made of, turned into boards you can saw.
HDPE is high-density polyethylene: the number-2 plastic, the one milk jugs, detergent bottles and cutting boards are made from. It is tough, slightly waxy to the touch, and it absorbs essentially no water. That last property is why it makes sense outdoors.
Poly lumber is HDPE melted, blended with pigment and UV stabiliser, and pushed through a die into a solid board — the same sizes as timber, so it can be cut, drilled and bolted with the same tools. It is heavier than most wood, it does not splinter, and it has no grain, which means no direction it is weak in.
Recycled means the HDPE came from used jugs and bottles rather than from new resin. The material is identical once it is remelted; the difference is that about five hundred jugs stayed out of a landfill for every chair.
What it is not
It is not resin (moulded, hollow). It is not composite decking (wood fibre mixed with plastic, which can still absorb water). It is not PVC. It is solid polyethylene through and through.
What it does over time
Nothing much. It expands slightly in heat and contracts in cold, which the chair is built for. Over twenty years the surface shifts very slightly and very evenly in color. It does not chalk, crack, rot, splinter or peel. Why paint fails and pigment doesn't.
Questions
Is HDPE toxic or does it off-gas?
No. It is the same plastic used for food containers and cutting boards, and it is inert outdoors.
Can it be recycled again?
Yes. HDPE lumber is recyclable at the end of a very long life, which is the point of using it.