Effect of calcium crosslinking and glycerol plasticizing in alginate films

Packaging wastes creates big environmental problems, because of using non-biodegradable polymers for food packaging applications. These materials do not decompose for years and cause environmental polluiton. Producer companies usually choice the non-biodegradable polymers for their cheapness. However, natural polysaccharides like alginate can be a good choice for food and health applications. Thanks to their edible, biodegradable and biocompatible nature, alginate films have found a vast field of applications ranging from food packaging, especially edible food packaging, to biomedical applications such as wound dressing. Alginate is a natural polymer, extracted from brown seaweed. It is accesible and cheap material. Pure alginate films are not suitable for packaging applications since they are water soluble and brittle with poor tensile properties. To improve its physical properties, different plasticizers and covalent or ionic crosslinking are applied and films with sufficient water vapor permeability, mechanical and film forming properties are obtained. Polysaccharide chains have many hydroxide groups along the chain. Hydrogen bonds give extra strength to the system. Glycerol is a small covalent molecule, which has three hydroxide groups. When glycerol is added to the alginate, hydroxides of glycerol replace the hydrogen bonds of the system and relive the tension leading to an increase in ductility. Sodium alginate is a polyelectrolyte, namely a poly-salt that has a high mean molecular weight. When a polyelectrolyte is dissolved in water, salt groups dissociate, upon which positively and negatively charged chain members and counter ions appear. In the case of sodium alginate, alginic acid residue (-) and sodium ion (+) appear. When calcium chloride is added to the system, Ca2+ ions attract negatively charged alginic acid residues in the chain and create an egg-box structure. Beyond a certain amount of glycerol, hydroxide groups suppress the calcium-alginic acid interaction and deteriorate the tensile properties of the system. External Ca crosslinking imposes a considerable amount of anisotropy and inhomogeneity into the film structure. Calcium crosslinking has prominent effect on tensile properties. In this work, alginate – glycerol films were prepared by solvent cast method and externally crosslinked by calcium chloride. For film preparation, alginate is dissolved in water (1%) mixing vigorously for 2-3 hours at room temperature until a homogenous solution was obtained. 0, 10, 20, 30% glycerol (w/w alginate) is added. Films were cast on polystyrene petri dishes with 8.5 cm diameters and dried in an air-ventilated oven at 60oC overnight. Dry films were crosslinked with 0.5, 1, 1.5 and 2% calcium chloride/ethanol solutions. Films were first immersed in calcium chloride/ethanol solution 30 minutes, then the surface alcohol was wiped off with a paper towel and wet films were immersed in Ca2+ containing 50% ethanol- 50% water solution for another 30 minutes. At the end of this period films were wiped off again and immersed in Ca2+ containing 100% water and soaked for another 30 minutes. Wet films were dried at room temperature for 1-2 hours and kept at room temperature (25oC), 50% humidity. Films with a wide spectrum of properties were prepared by varying glycerol and calcium concentrations. For example, solubilities ranging from completely water-soluble to nonsoluble were obtained, while tensile properties could be tuned from hard and brittle to soft and flexible. Low and high swelling films were prepared with low and high crosslinking ratios, respectively. Calcium chloride crosslinking strongly affected all physical properties of the films. SEM pictures clearly showed the crosslinked, egg-box structures in the films. Their UV spectra, water vapor permeabilities and swelling profiles in water, acetic acid and citric acid were investigated. Mechanical properties were discussed in detail. Properties of alginate films such as tensile strength, % elongation, water vapor permeability and swelling depend on both calcium chloride and glycerol concentrations. A suitable set of parameters for a given application can be achieved by varying both concentrations. Here we report results of a parametric investigation of the effect of both concentrations on these properties. SEM pictures clearly show continuous smooth alginate appearance in uncrosslinked 0% glycerol films, 0% glycerol – 0% Ca and 30% glycerol – 0% Ca. Films containing %0 glycerol and 0% Ca, show the brittle structure. Fractures of the film caused by lack of glycerol. Crosslinked films clearly show the egg-box structure. The sizes of the boxes increased slightly with increasing the glycerol concentration. Sizes of the boxes were 160-200nm in 0% glycerol - 2% Ca films but in 30% glycerol - 2% Ca films sizes of the cells increased to 240-300nm. We can see the difference between uncrosslinked films and crosslinked films clearly from SEM images. If we look at UV absorbances, we can say that all films have absorption in the 200-250 nm band and a minor absorption beyond 300 nm. Films do not have absorption in the visible region which is important in packaging applications. The uncrosslinked pure film has the lowest absorption as expected and as the crosslinking was decreased absorption increased. Decreasing in the egg box sizes with crosslinking causes less light transition. The crosslinked films were soaked in calcium salt containing ethanol and water-ethanol and water baths. Glycerol is water soluble and in the presence of glycerol containing films, a part of glycerol is solubilized during crosslinking and correlation of absorption with Ca concentration is not as clear as in pure alginate films. When we investigate results of water vapour permeability experiments, we can say that, if Ca crosslinking degree increased, water vapour permeability decreased. With high crosslinking degree, the pores (egg-box models) become smaller. The results are all expected. Uncrosslinked pure alginate films easily dissolve in water but crosslinked alginate films swell but not dissolve. Mechanical experiments show that all samples that had glycerol, a clear pattern was that increasing calcium chloride increased tensile strength in the samples, although it also decreased plastic behavior, resulting in more brittle samples. For 0% glycerol, the differences in strain and tensile strength were minimal. Overall highest tensile strength was seen at 20% glycerol and 1.5% calcium chloride. Crosslinking increased strength in all samples, strain increased in all samples except 2% calcium, where high crosslinking lead to brittle films, decreasing strain values. Addition of glycerol also increased tensile strength, although for higher calcium values such as 1.5% Ca and 2% Ca, 30% Glycerol showed a decrease in strength. When swelling results take into consideration, experiments show that increasing Ca concentration makes film more swelled. The results of 10%, 20% and 30% glycerol containing 0.5% and 2% calcium crosslinked films in water, 1M acetic acid and 1 M citric acid are examined. Limit swelling in water and acetic acid are around 50%-70% and in citric acid are around 110%-120%. Films were observed for several weeks and were stable for the whole period. As calcium content increased, swelling decreased and as glycerol content increased, swelling increased. For example, swelling values in water decreased from 66 for 10% glycerol - 0.5% Ca to 51 for 10% glycerol – 2% Ca. Swelling increased from 66 to 68 to 80 as the glycerol content increased from 10 to 20 to 30 percentages. Films reach limit swelling value in 5 minutes after insertion into the water, in 100-110 minutes after insetion into the acetic acid solution and 30 minutes after insertion into the citric acid soluiton. Swelling decreased as the calcium content increased. The films swelled the most in citric acid. The properties of alginate films can easily be modified by calcium crosslinking and glycerol addition. Films with desirable water vapor permeability, swelling and mechanical properties, for applications from wound dressing to food packaging can be obtained by appropriate amounts of these additives. Even though previous studies also discuss glycerol containing alginate films and external calcium crosslinking, it is hard to compare their results with the present study due to differences in the parameters. The same trend is the increase in tensile strength and decrease in strain with calcium crosslinking was observed partly in this work too. However, increasing both glycerol and calcium beyond a certain limit resulted in weaker films. Thus, the effects of glycerol plasticizing and external calcium chloride crosslinking in alginate films were investigated for a wide parameter range and the films were evaluated as candidates for packaging and pharmaceutical applications.