From the early days of plastics, biaxial orientation has been studied as a means of changing properties [la]. Draw in two directions, rather than just one, has been widely studied with different thermoplastics (polyethylene [1–24], polypropylene [25–451, poly(ethy1ene terephthalate) [12, 46–71], and many others [72–861. The mechanical properties of tensile modulus, strength, and impact particularly of semicrystalline thermoplastics are increased by orientation. For the former, the measurement and use properties are generally in tension, and for the last, impact in the transverse direction. Optical clarity is also commonly enhanced by planar draw processes, and permeation through thickness can be changed dramatically.