For any material that isn't explicitly rated for pressure, you'll want to reinforce the outside of the tube in such a way that even a catastrophic failure of the tube would not produce flying shrapnel. Thin pieces of plastic are difficult to spot on an X-ray image (notice how
only the sides of the PVC pipes are visible in
this real-world example), so you
really don't want shards of that stuff getting embedded in you!
If you take that thought and then go do a
bit of
research, however, you'll find that the only things that will reliably stop shrapnel from an exploding t-shirt cannon (essentially a pipe bomb) are going to be outright bulletproof-- sufficiently thick polycarbonate, metal, Kevlar, that sort of thing.
All told, then, you're probably best off if you make sure that the tube is rated for pressure just like the rest of the components in the system.