Fortunately, starch can be modified by processing with plasticizers, grafting with vinyl monomers, and blending with other polymers. Plasticizers increase both the flexibility and processability of starch, which exhibits thermoplastic behavior and takes the name of TPS when plasticized by relatively low levels, in the 15–30 wt% range, of molecules that are capable of hydrogen bonding with the starch hydroxyl groups, mainly water, glycerol, and sorbitol. TPS can readily flow at elevated temperature and pressure and it can be extruded to give both foams or shaped into solid articles by injection molding. TPS products with different viscosity, water solubility, and water absorption have been prepared by altering the moisture/plasticizer content, amylose/amylopectin ratio of the raw material, and the temperature and pressure in the extruder [98]. A great deal of research has been performed on the plasticization of TPS using glycerol [100], sorbitol [101], urea or formamide [102], dimethyl sulfoxide [103], and low‐MW sugars [104]. Figure 1.6 shows different packaging articles obtained from starch, ranging from food trays to cups.
Unfortunately, the properties of neat TPS materials are not still good enough for most packaging applications. For example, the properties of films made of water‐ and glycerol‐plasticized TPS are poor at high humidity, present poor dimensional stability, and become brittle as water is lost. Fortunately, TPS can be blended with other polymers and fillers to improve the mechanical properties and also attained water resistance [105]. For instance, TPS/PLA blends show chemical resistance, improved flexibility and toughness, and low cost [106]. The use of TPS in biopolymer blends is particularly relevant to obtain materials with high elongation at break properties in food packaging [107].
Figure 1.6 Biodegradable packaging articles based on starch.
1.3.10 Cellulose and Derivatives
Cellulose is the most abundant natural polymer on earth. The major source of cellulose is certainly wood, which contains 40–50 wt%, being the fundamental component of the cell walls of plants and natural fibers. Cellulose is a linear naturally occurring polymer composed of 1,4‐linked‐β‐D‐anhydroglucopyranose units that are covalently linked via acetal functions between the equatorial –OH group of C4 and the C1 carbon atom [108]. Neat cellulose is, however, unsuitable for film production because it is highly crystalline and also insoluble in water due to the strong intra‐ and intermolecular hydrogen bonding between the individual chains and its highly crystalline structure [109]. Therefore, cellulose is usually dissolved in a mixture of sodium hydroxide and carbon disulfide and recast into sulfuric acid. This chemical treatment results in the production of the so‐called cellophane film, which has good mechanical properties. However, it is often coated with nitrocellulose wax or PVDC to improve its moisture sensitiveness. Coated cellophane is then used for baked goods, fresh products, processed meat, cheese, and candy though it is not heat sealable due to its non‐thermoplastic nature [110].
Alternatively, cellulose can be chemically modified to produce water‐soluble cellulose ester or ether derivatives by either esterification or etherification, respectively, of individual hydroxyl groups on the polysaccharide backbone. Commercial derivatives of cellulose include cellulose acetate, ethyl cellulose, hydroxylethylcellulose, and hydroxyl‐propyl cellulose, among others [111]. Figure 1.7 summarizes some of the hydrophilic and hydrophobic cellulose derivatives categorized according to their pH‐responsive behavior and chemistry. Steps involved in making these thermoplastic materials include, first, making cellulose derivatives biopolymers in powder form and, thereafter, extruding them in the presence of different additives and plasticizers such as citrates. Although the gas and moisture barrier properties of cellulose acetate are not optimal for food packaging, this film is excellent for products demanding high moisture as it allows respiration and reduces fogging [110]. Mazzucchelli (Castiglione Olona, Italy) and Planet Polymer (California, USA) manufacture biodegradable plastics under the trade names of BIOCETA® and EnviroPlastic® Z, respectively, based on cellulose acetate. BIOCETA® has been applied for the manufacture of biodegradable packaging films, retractable films, and tubes [98].
1.3.11 Proteins
Protein‐based films have lately become a hot research topic due to their film‐forming capacity and cohesiveness, low cost, and biodegradability features. Proteins present good barrier against oxygen and aroma, among others gases. However, they also show high water vapor permeability due to their hydrophilic nature [112, 113]. Protein films have been developed from gelatin, corn zein, wheat gluten (WG), soy protein (SP), casein, and whey protein [114–116].
Figure 1.7 Cellulose derivatives categorized based on their pH‐responsive behavior and chemistry.
1.3.11.1 Gelatin
Gelatin is a water‐soluble protein that is prepared by thermal denaturation of collagen in the presence of dilute acid (gelatin type A) or alkali (gelatin type B). Collagen is found in animal skins and bones such as connective tissues, skin, and bones (see Figure 1.8) [117]. Its structure is triple helix, being stabilized by the formation of hydrogen bridges between the chains through amino and carboxyl groups. When collagen is denatured, the triple helix breaks and the polypeptide chains adopt a random configuration, forming gelatin composed mainly of glycine, proline, and 4‐hydroxyproline residues [118–120]. Then, gelatin results in a heterogeneous mixture of single or multi‐stranded polypeptides, each with extended left‐handed proline helix conformations and containing between 300 and 4000 amino acids. Gelatin is primarily used as a gelling agent forming transparent elastic thermoreversible gels on cooling below 35 °C. It can be used as a valuable biopolymer in tissue engineering applications. Moreover, the gelification properties of gelatin, its capacity of forming and stabilizing emulsions, and its adhesive properties and dissolution behavior make this biopolymer a potential polymer for the manufacture of bio‐based films [121]. However, its poor mechanical properties, especially in the wet conditions, limit its application as a packaging material, [122, 123]. Many techniques, including vapor cross‐linking, orientation, and use of fillers such as hydroxyapatite nanoparticles (nHAs) and tricalcium phosphate (TCP), have been developed to reinforce gelatin‐based films [124].
1.3.11.2 Wheat Gluten
WG is a protein complex having two major components, known as glutenin and gliadin. It is composed of proteins containing water‐insoluble and ethanol‐soluble prolamins and water‐ and ethanol‐insoluble glutelins (70–80 wt%) in combination with small amounts of wheat oils, starch, and insoluble hemicellulose [125]. The gliadins are mainly monomeric single chain polypeptides, whereas the glutenins are polymeric and disulfide linked polymeric chains [126]. These components are responsible for the physical and chemical properties of WG and confer it with higher viscoelastic properties compared with other plant proteins [127,