Transparency of recycled polypropylene film

Turbidity of quenched polypropylene films was measured as a function of the film thickness and number of times film was recycled. Turbidity τ is defined by τ = (1/d) ln (I_o/I_t), where I_o and I_t are the intensity of the incident and transmitted lights, respectively, and d is the thickness of the sample. We assume here that most of the attenuation of light is due to scattering from superstructure in the film, such as spherulites, since no characteristic absorption bands are present in the wavelength region studied in this work. Turbidity varied sigmoidally with film thickness. It remained constant when the film thickness was lower than ca. 400 μm and then increased with film thickness and reached a plateau around 800 μm. When the film preparation was repeated, turbidity increased exponentially with the number of recycles. The spherulite size, however, was an invariant against the number of recycles and was dependent upon film thickness. The variation of turbidity with film thickness and the number of recycles is discussed.