topic of PEF production and application, there is no information on enzymatic hydrolysis of PEF. Nevertheless, especially for bio‐based PEF, biotechnological processing for functionalization and chemical recycling of monomers could have a large potential to replace harsh chemicals [84].
1.3.8 Poly(ɛ‐caprolactone)
Poly(ɛ‐caprolactone) (PCL) is a biodegradable aliphatic polyester derived from the chemical synthesis of crude petroleum [91]. It is obtained from ROP of ɛ‐caprolactone in presence of metal alkoxides and also through the polycondensation of 6‐hydroxyhexanoic acid. PCL is commercially available under the trade names CAPA (Solvay, Belgium), Tone (Union Carbide, USA), Celgreen (Daicel, Japan), and many others [92].
Some features of PCL include biodegradability, good solubility, flexibility, low Tm (approximately 60 °C) and a Tg of around −60 °C, and easy processing. PCL is used in flexible packaging materials in the form of films or coatings for extending the shelf life of food products. However, due to its high price and long biodegradability cycles, PCL is commonly blended with other biopolymers, such as chitosan and starch [93]. Among them, starch has been proposed as a reinforcing agent to improve the mechanical strength of PCL [94]. There are also several studies on the hydrolysis and biodegradability of PCL. The degradation process of PCL takes place through hydrolysis, thereby leading to molecular fragmentation or chain scission [94]. Moreover, enzymes and fungi easily biodegrade PCL. However, to improve the degradation rate, several copolymers with lactide or glycoside have been developed [94].
1.3.9 Thermoplastic Starch
Starch is a natural polysaccharide that can be obtained from a great variety of crops such as cassava and corn. It is considered as one of the most promising biopolymers for food packaging due to its availability, biodegradability, and low price [95]. Starch is composed of a mixture of two polymers, namely amylose and amylopectin. Amylose is a linear polysaccharide composed entirely of D‐glucose units joined by α‐1,4‐glycosidic linkages. Amylopectin is a branched‐chain polysaccharide composed of glucose units linked primarily by α‐1,4‐glycosidic bonds but with occasional α‐1,6‐glycosidic bonds, which are responsible for the branching. Relative percentages of amylose and amylopectin in starch are in the range 10–30% amylose and 70–90% amylopectin. The starch having a crystallinity between 20% and 40% is termed as semicrystalline in which the amorphous region of starch contains amylose and the branching points of amylopectin [96]. Native dry starch has a limited range of applications because of its high brittleness, high viscosity, retrogradation, insolubility in cold water, and poor melt processability because its Tg is probably above the decomposition point so it does not soften and flow [97]. BIOTEC® (Emmerich, Germany) has three product lines of TPS that include Bioplast® granules for injection molding, Bioflex® for film applications, and Biopur® as foamed starch [98]. In any case, films developed from native starch typically show moderate oxygen barrier properties but poor moisture barrier and mechanical properties, which limit their applications in food packaging [99].
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
