The minimum amount of clay was set to zero. The clay amount more than 5% caused the casting solution very viscous which release of bubbles was impossible without using a vacuum apparatus. The specified amount of Na–MMT determined according to Table 1, was added to 99.5 g of distillated water under rigorous mixing for 20 min at 80 ?C. Then, the resulted aqueous suspension homogenized for 20 min at 80 ?C via sonication method to make a uniform solution. Then proper amount of glycerol according to Table 1 with 5 g of pristine corn starch was added and mixed by
stirrer for 10 min and sonicated for 20 min at room temperature. The final mixture placed in water bath (80 ± 0.2 ?C) under mild stirring for 30 min. The gelatinized solution was gently mixed in order to release bubbles. The casting solution was poured into polystyrene petri dishes and placed on a flat surface in the laboratory
(23 ± 2 ?C, 52 ± 3% RH). To study the film thickness, 22.000 g of the cooled casting solution was poured into polystyrene petri dishes with internal diameter of 6.1 cm. The thickness of each film was measured at fifteen different locations with a hand-held micrometer (Mitutoyo, Japan). Average of each five random points was considered as one datum. It is worth mentioning each sample was prepared three times and their average results were reported. After 36 h, films were carefully separated and stored in poly ethylene bags. All films were prepared in triplicates. The prepared samples were designated with label such as G30C2.5, where G and C referred to glycerol and clay, respectively and the number 30 indicated 30% glycerol and 2.5 referred to 2.5% Na–MMT.