Relationship between biaxial orientation and oxygen permeability of polypropylene film

Biaxially oriented polypropylene (BOPP) films were produced by simultaneous and sequential biaxial stretching to various balanced and unbalanced draw ratios. The BOPP films were characterized in terms of density, crystallinity, refractive index, oxygen permeability and dynamic mechanical relaxation behavior. It was found that the density and crystallinity of BOPP films decreased as the area draw ratio increased. Sequential stretching led to a slightly lower density than simultaneous stretching to the same draw ratio. Moreover, sequential stretching produced lower orientation in the first stretch direction and higher orientation in the second stretch direction compared to simultaneous stretching. The study confirmed the generality of a one-to-one correlation between the oxygen permeability of BOPP films and the mobility of amorphous tie chains as measured by the intensity of the dynamic mechanical β-relaxation. Moreover, the study established the correlation for commercially important sequentially drawn BOPP films with an unbalanced draw ratio. Finally, the chain mobility in the stretch direction was found to depend on the final stress during stretching.