Biaxially oriented polypropylene crystallization

Figure 2.41. Avrami plots of the crystallization isotherms for biaxially oriented polypropylene after erasing the thermal history of the samples by heating them to 250 °C, which is much higher than the equilibrinm melting point of polypropylene...

Biaxial orientation effects are important in the manufacture of films and sheet. Biaxially stretched polypropylene, poly(ethyleneterephthalate) (e.g., Melinex) and poly(vinylidene chloride) (Saran) produced by flat-film extrusion and tentering are strong films of high clarity. In biaxial orientation, molecules are randomly oriented in two dimensions just as fibers would be in a random mat the orientation-induced crystallization produces structures which do not interfere with the light waves. With polyethylene, biaxial orientation often can be achieved in blown-film extrusion. 

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