3.1. Solution Casting Method

The solution casting method, also known as the direct casting method, is one of the most commonly used to prepare polymeric films for food, pharmaceuticals, and biomedical applications (Figure 9). This method is straightforward and has been used for a long time to develop film-based polymeric systems. Hence, many authors regard this method as the conventional solution casting method. The method involves the dissolution of the polymers in suitable solvents. The polymeric solutions, which may contain various additives, such as plasticizers and crosslinkers (e.g., genipin and glutaraldehyde), are degassed. The degassing can be achieved either by applying a vacuum at a slightly elevated temperature or by ultrasonication. Then the degassed solutions are poured into molds, which are placed on a leveled surface, and subsequently dried under a controlled environment of a specific temperature and humidity. The molds are usually made up of non-sticky materials (e.g., Teflon) such that, after drying, the films can be easily peeled off from the molds. After that, the peeled-off films are cured, if needed, and stored for further use. Since this method’s preparation is relatively easy, many researchers have extensively used it to develop films as food packaging material with significant efficacy. Though the method has shown great promise in the food packaging industry, utmost care must be taken while designing films by this method. A slight deviation in the process parameters may result in batch-to-batch variations in the properties of the films [4].

A generalized method for the synthesis of chitosan films (taken from [19] under Creative Commons License).

The blends of chitosan can also be used to develop films. Blends are prepared by mixing two or more polymers to tailor the physical properties of the resultant films over the films made with pristine polymer(s). The polymeric blends may either form a homogenous matrix or a phase-separated matrix system. It has been reported that blends of chitosan with different polymers (natural or synthetic) can improve the food packaging materials’ physical, mechanical, thermal, optical, and barrier properties [1]. The natural polymers used for this purpose are classified as polysaccharides and proteins [1]. The derivatives of these polymers have also been employed. The examples of polysaccharide-based natural polymers include pectin, starches from different sources (e.g., cassava starch, purple yam starch, and potato starch), bacterial cellulose, carboxymethyl cellulose, and gum arabic [1]. Protein-based natural polymers include collagen, gelatin, fish myofibrillar protein, soy protein isolate, and zein [1]. Some of the synthetic polymers employed in conjunction with chitosan include poly (vinyl alcohol), poly (ε-caprolactone), and ethylene-vinyl alcohol [1]. These polymers are mixed with chitosan solution and homogenized before the degassing stage, and this is then used for the film casting.

The functionality of the chitosan films (pristine or blend) can further be improved by adding additives. Various additives, such as essential oils and nanoparticles, are being added to enhance the films’ antimicrobial, antioxidant, and other properties (e.g., mechanical and barrier). These components are added to the chitosan solution or chitosan blends and are subsequently homogenized to form stable emulsions or dispersions. These emulsions or dispersions are then degassed before casting into films (Figure 10).

Representative process for the development of nanocomposite films (taken from [20] under Creative Commons License).