The vessel assembly includes a polymeric cylinder and a circular polymeric cap for the cylinder, the cylinder being closed at one end and open at the other end to receive the cap. The open end of the cylinder has a lip that is beveled inwardly from the open end, and the circular polymeric cap has a beveled lower edge that engages the beveled lip when the cap is place upon the polymeric cylinder. For high pressure applications, a choke cylinder depends from the beveled lower edge of the cap, and has an outer diameter substantially the same as the inner diameter of the polymeric cylinder so that the choke provides a self sealing mechanism for the cylinder as pressure from a chemical reaction increases within the cylinder. A composite sleeve surrounds the polymeric cylinder, and includes at least one wound fabric layer in which the winding can be a filament or a yarn.