Figure 4. Chemical structure of amylose and amylopectin in the starch granule [38].
Starch-based polymers are members of the biodegradable polymer family, and their source is from renewable natural resources like corn, potato, wheat, rice, and cassava
[39]. Starch is a naturally occurring carbohydrate that is made of glucose units joined by α-1,4 and α-1,6 glycosidic bonds, and these two different forms of polymer chains are called amylose and amylopectin. Amylopectin contains α-1,4 glycosidic bonds, which are connected to the α-1,6 glycosidic bonds. On the other hand, amylose consists of a long straight chain containing only 1,4-glycosidic linkages
[40,41][40][41]. Various types of physical or chemical modifications are applied to improve its properties
[42]. Thermoplastic starch (TPS) is produced by mixing the starch-based polymer with other biopolymers, such as poly (lactic acid), poly (butylene adipate-co-terephthalate), poly(ε-caprolactone), and poly (vinyl alcohol). Adding these biopolymers enhances the mechanical properties, thermal stability, water resistance, and biodegradability of starch-based polymers. Thermoplastic starch can be reshaped through extrusion, injection, moulding, etc.
[43]. Chemical alteration of starch-based biopolymers is another method to improve their properties, and it is normally achieved by reaction with the hydroxyl group in the starch molecule
[44]. The resulting derivatives have different physicochemical characteristics in comparison with the original starch, while retaining their inherent biodegradability. Therefore, adding or changing some groups or chains on the hydroxyl parts can make different biodegradable starch-based materials that serve different purposes
[43]. Compared to cellulose and other polysaccharides, starch has a diverse origin, different molecular weights, and distinct functional characteristics. Starch shows better processability and compatibility with other biopolymers. These qualities are highly relevant to its position within the biodegradable polymer sectors, highlighting its usefulness in supporting environmentally concerned enterprises
[45].
3.4. Cellulose-Based Polymers (CBP)
5. Cellulose-Based Polymers (CBP)
Cellulose is one of the most common and natural polymers. It can be processed into various shapes and forms, like powders, films, gels, etc.
[17]. Cellulose can also be chemically modified to produce cellulose-derivative biopolymers, such as cellulose acetate and cellulose nitrate, with desirable properties for cladding and various applications
[17,46][17][46]. Plastic or synthetic polymers can be melted and reshaped, but cellulose cannot be melted or reshaped this way because of the strong connection between its molecules and hydrogen bonds
[47]. Thermoplastics can be created by adding additional components to cellulose derivatives in a solvent state. However, this method can be costly because of using chemical agents such as cellulose ester
[48]. There are different methods of making cellulose-based biopolymers, such as the hot-pressing process
[49[49][50],
50], directed deformation assembly process
[51], and bacterial cellulose synthesis process
[52]. In the hot-pressing process, cellulose molecular chains are guided to align in various directions but in a fixed dimension. As a result of this alignment, the cellulose undergoes a change in its shape and structure, ultimately leading to deformation
[49,50][49][50]. The bacterial cellulose (BC) process is conducted with various bacterial species, such as acetobacter, agrobacterium, rhizobium, sarcina, and others. This complex procedure takes place under many environmental and growth circumstances, which help to produce the result
[52]. It has excellent properties, such as biodegradability, biocompatibility, thermal stability, and good mechanical and barrier properties. Wood pulp, cotton, bacterial cellulose, etc., are the most common sources of cellulose-based polymers
[46,53][46][53].
3.5. Polyhydroxybutyrate (PHB)
6. Polyhydroxybutyrate (PHB)
PHB is a biodegradable polymer produced from renewable and sustainable sources like food waste. It has a great capacity to break down in specific biological conditions, and these factors make it a strong substitute for artificial polymers such as PVC, PP, and PE
[18]. However, the production cost of PHB is relatively higher than that of petrochemical-based plastics. Recent research has identified methods for lower PHB manufacturing costs, including increased bacterial strains, simplified fermentation, and improved recovery methods
[54,55][54][55]. PHB is biosynthesised and accumulated by a number of specialised bacterial strains such as Alcaligenes eutrophus, Bacillus megaterium, Pseudomonas oleovorans, strains using a variety of organic substances that are found naturally and unutilised (wasted) as carbon sources. PHB can be blended or combined with other polymers or fillers like wood, metal, glass, etc, to improve its properties and performance, especially to form hybrid cladding systems
[56,57,58][56][57][58]. The formulation of PHB can be divided into two main steps: the production of PHB by microorganisms and the extraction of PHB
[59]. For the production of PHB, specific bacteria need to be selected and grown in a nutrient environment for 24 h at 37 °C. According to one report, around 5% of thriving bacteria was added to the 50 mL of modified mineral salt and left to grow for 72 h at 37 °C with gentle agitation (120 rpm)
[60]. After the bacteria multiplied, these were separated from the liquid, the dried bacterial clumps were collected, and they were soaked in sodium hypochlorite to release the PHB materials. A filter was used to remove unwanted components from PHB material
[61].
3.6. Polybutylene Adipate Terephthalate (PBAT)
7. Polybutylene Adipate Terephthalate (PBAT)
PBAT’s speciality lies in its ability to break down naturally in the environment, and the reason behind that is its ester linkage. The ester bond breaks down through the water and enzymes in the environment
[62]. PBAT has good qualities of both synthetic and bio-based polymers. Although it is produced from regular petrochemicals such as purified terephthalic acid (PTA), butanediol, and adipic acid, it is biodegradable
[19]. PBAT shows great water resistance properties, and its manufacturing process is much easier
[63]. It has the perfect characteristics to create flexible films just like regular plastic materials
[62]. However, PBAT is not strong enough for certain uses and is also more costly than other biopolymers
[64].
In the PBAT synthesis process, adipic acid and 1,4-butanediol are added to a stainless-steel reactor in a fixed mole ratio. Stirring is used to bring the reactants’ temperature up, and distillation is used to get rid of the water that is produced during the reaction
[65]. After 1–2 h, tetrabutylorthotitanate (TBOT) is added to the mixture at room temperature. Under vacuum, the reaction temperature is elevated for 4h, and dimethyl-terephthalate (DMT), 1,4-butanediol, and TBOT are added in a specific ratio. The temperature needs to be maintained, and the process takes around 20h under a high vacuum
[65].
3.7. Polybutylene Succinate (PBS)
8. Polybutylene Succinate (PBS)
Polybutylene succinate (PBS) is a semi-crystalline polymer which is produced through a direct reaction of succinic acid and 1,4-butanediol. It is environmentally friendly because it breaks down more rapidly than traditional petrochemical plastics and leaves no toxic materials behind
[66]. However, polybutylene succinate (PBS) is much more expensive compared to petrochemicals such as polystyrene (PS), polyamides (PAs), polyethylene terephthalate (PET), and polyethylene (PE), because it involves special processing and combining succinic acid and 1,4-butanediol in the process
[67].
Polybutylene succinate (PBS) can be produced in various ways, including polycondensation of succinic acid and 1,4-butanediol, where monomers come from fossil-based or renewable sources. This method has potential in the case of enhanced thermal and mechanical properties
[68]. In the fermentation process, microorganisms can be used to make succinic acid. To create bio-based PBS, numerous microorganisms have been examined and put to the test
[69]. In the chemical synthesis process, chemically synthesised aliphatic polyesters with high molecular weights can improve the properties of PBS. For instance, poly (butylene succinate-co-ethylene succinate) is synthesised through direct polycondensation in the presence of N35 catalyst
[70].
3.8. Polycaprolactone (PCL)
9. Polycaprolactone (PCL)
Polycaprolactone (PCL) is a member of the biodegradable synthetic polyesters group which has proven incredibly valuable in various applications
[71]. In the 1930s, researchers produced polycaprolactone (PCL) polymer materials from the ε-caprolactone monomer for the first time with a cyclic polymerisation process facilitated by initiators (
Figure 5). Polycaprolactone (PCL) is non-toxic in nature, which makes it safe for living organisms
[72]. It has a great ability to break down naturally in the environment, and it also has excellent stability and compatibility with biological systems
[73]. Additionally, PCL shows great crystallinity, which means it is in solid form at room temperature and is strong. It gets thick and sticky in its liquid state
[74].
Figure 5. Ring-opening polymerisation process of polycaprolactone (PCL) [75].
PCL is mostly produced through a ring-opening polymerisation process by linking ε-caprolactone molecules together. This method was invented in the 1930s. Different types of catalysts are used in this process, such as stannous octoate, which is used to speed up the polymerisation process
[75]. Different mechanisms, including anionic, cationic, coordination, and radical processes, influence the polymerisation process. These methods influence the resulting polymer, in terms such as molecular size, distribution, the composition of end groups, and the chemical structure
[75]. Polycaprolactone (PCL) is a versatile and promising biopolymer, and mixing it with other polymers changes its properties
[76]. It is a synthetic polymer that breaks down slowly in nature over time. It is also hydrophobic. Its interesting properties make it quite intriguing for the preparation of long-term implantable devices
[77].