HDPE

From PyroGuide

HDPE (high-density polyethylene) is a dense plastic polymer. It is actually very flexible in long lengths, making it ideal for use as underground piping as it can withstand the shifting ground. This makes it very safe to use for mortars. If a shell explodes in the tube(Flower pot), the HDPE absorbs much of the energy. Though destroying the tube it does not readily create shrapnel. HPDE is also very durable and mortars will last for years. Many 1.4G consumer re loadable shell kits are now being sold with HDPE mortars rather than cardboard ones.