2.3. Density analysis
The apparent density of films was defined as the density of the
film with air on the surface and/or inside twists. This was measured
immediately after adding the films in water without any
mixing. The true density of films was also determined after removal
of the entrapped air from the films in water by mixing at
300 rpm under vacuum condition. The density of films was measured
used by specific gravity bottle method (ISO, 2004), using
IWAKI, 420 specific gravity bottle for samples of 0.1–0.5 g of film.