Plastic containers having improved physical properties are prepared from a composite billet having an outer peripheral area composed of a thermoplastic resin and an inner central core portion composed of the thermoplastic resin modified with a material which imparts improved physical properties such as reduced gas permeability or increased stiffness or rigidity to the resin. The composite billet is molded into a container wherein the billet core is formed into the sidewalls and bottom of the container and the peripheral portion is formed into a flexible thermoplastic flange surrounding the open end of the container which can be doubled seamed to a metal end closure without fracture.