Five years ago I wondered the same thing as I planned a swimming raft and outdoor table. And I can now answer with complete confidence that composite deck lumber is a great material for outdoor furniture.
‘Composite’ is the current term for plastic lumber because today’s versions are made from a 50-50 blend of wood fibers and plastic. This yields a better product than the all-plastic lumber that originally came to market. Composites are lighter, better looking, offer better foot traction and are easier to work than true plastic lumber.
Most composites are hollow extrusions, and that won’t do for building your chairs. What you really need is a solid composite that’s sized like wood lumber, and cuts, drills and routes like the real thing. You’ll find that fresh ends exposed during construction look just like the rest of the board. The only thing to keep in mind is strength. Composites aren’t as strong as wood, and they tend to bend over time if they’re not supported properly. Be sure to work extra supports into your design in load-bearing areas of your chairs. An extra cross member in the seat and back area will do just fine.