Optimizing mechanical and optical properties of nanocomposite films based on polyvinyl alcohol for food packaging

Introduction: Among the different types of polymers used for packaging and coating, polyvinyl alcohol (PVA), given its very enviable properties, has been used in various industrial applications. It is used for instance as controlled release in pharmaceutical elements, paper, textile and food supplement coating due to its good physical properties, chemical resistance, thermostability, film-forming capability, efficiency and biodegradability. The aim of this work was to examine the combined effect of montmorillonite (MMT) platelets and titanium oxide (TiO2) spherical nanoparticles on the physical and mechanical properties of PVA/ TiO2/MMT nanocomposites, and to determine the optimal combination to provide good properties, using response surface methodology (RSM).   Materials & methods: PVA, PVA/TiO2, PVA/MMT and PVA/ TiO2/MMT nanocomposite films were prepared by the solution casting method. For each sample, 1.8 g of PVA was dissolved in 50 mL deionized water and maintained for 24 h at room temperature. The mixture was then heated to 90˚C and stirred using a magnetic stirrer up to 3 h to ensure the complete dissolution of PVA, followed by cooling down the solution to room temperature. Various amounts of TiO2 nanoparticles (1 and 2 w% on a dry basis) were added to deionized water and agitated with a stirrer for 12 h at 500 rpm. This method was also used for MMT (2 and 4 w% on a dry basis). The nanoparticle suspension was subjected to ultrasonic homogenization for 20 min to ensure a good dispersion. The 50 mL nanoparticle suspension was added to the PVA solution drop by drop during a period of 5 min while maintaining intense stirring (1000 rpm). Mixing was continued and glycerol (30 w% based on the polymer) was added. Vacuum with a rotary vacuum pump was applied to remove air bubbles from the solution. The solution was poured into a 15-cm internal diameter Petri dish with a perfectly flat bottom and carefully aligned horizontally. Homogeneous films were peeled off after drying in an air oven at 40˚C for 72 h. Scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD) were performed for characterizing the morphology of nanocomposite films. The effect of these two nanoparticles on physical and mechanical properties, was evaluated by response surface methodology (RSM). A three-level factorial design was used to define the test points for the series of experiments. Among the various design alternatives suggested by theoretical algorithm, the selected design consisted of 13 experiments including five replicate central points used for variance calculation. Furthermore, PVA film data were analyzed using the Design-Expert program (Version 7.0, Stat-Ease Inc., Minneapolis, Minnesota) to find the optimum combination of constituents for the best properties.   Results and discusions: X-ray diffraction patterns showed that the nanoparticles were well dispersed in the polymer matrix of PVA/ TiO2 and PVA / MMT films with layered microstructure. In addition, the linear effect of MMT nanoparticles and the interaction of TiO2 and MMT on tensile strength were significant. The linear, quadratic and interaction effects of both nanoparticles on Young's modulus were also significant. In general, the optimum values of TiO2 and MMT were 1% and 4% respectively for mechanical properties. The presence of both nanoparticles had a significant effect on transparency and ΔE. Results of nanocomposite films indicated that the film with 2% TiO2 and 4% MMT has higher WI and actually is darker than other samples. By analyzing different results with response surface method, the nanocomposite film with 0.5% TiO2 and 4% MMT was proposed as optimum combination for mechanical and physical properties