Thermoforming of ordered polypropylenes

Abstract Uniaxial molecular ordering of polyolefin billets by a process of roll‐drawing produces clear, transparent sheets having greatly increased strength and modulus in the draw direction whereas the strength and modulus in the transverse direction remain essentially unaltered. Such sheets may be laminated together by means of ultrasonic welding, electromagnetic induction heating or simple pressure lamination using adhesive tie layers to produce multiply laminates with controlled biaxial properties. These self‐reinforced laminates have mechanical properties which compare favorably with advanced aramid or carbon fiber composites. Conventional thermoforming techniques may be employed to convert these highly ordered sheets into shaped products having potential commercial value. In these investigations examples are presented to illustrate the methods of laminating and thermoforming of ordered polypropylene sheets. Examples are also presented to demonstrate the feasibility of pressure forming of ordered polypropylene laminates into more complex three‐dimensional shapes without loss of their mechanical properties. The techniques described in this paper may be applied to other crystalline polymers to create a new spectrum of self‐reinforced plastics capable of approaching the limits of performance.