The ridgid joiner is made from polypropylene. It's useful to make ridgid joins with push in fittings that are dead straight. Although, it is possible to make joins using short pieces of 8mm OD EVABarrier tubing, when mounting parts to a gas board or wall it will make things much neater if you use these instead. The 50mm length is long enough so even after pushing right into two Duotight 8mm fittings on both ends, you should still have an exposed length of about 3mm so you can easily take the fittings apart.