Film preparation
The films were prepared by casting method. The amounts of glycerol as the plasticizer and Na–MMT nano-particle wer presented in Table 1. In this study, the faced center CCD (a = 1) at three levels (-1, 0, +1) combined with RSM have been applied through this research. Faced center CCD give us this opportunity to use Duncan test to compare significant differences in results. The films were prepared and analyzed triple times followed by Duncan test which the averages of the results were used for response surface analysis. The minimum concentration of plasticizer
was set at 25%. Glycerol amounts smaller than 10% caused the brittle films and between 10–20% the good flexibility was not observed. The content of glycerol is depend on required applications, for example the minimum amount of glycerol in order to acceptable heat seal ability was reported 30% for sago starch films
[13]. 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.