## 3 Plastics Used for Food Packaging
### Selected Biodegradable Synthetically Manufactured Polymers
#### 3.2.2 Polymers Produced by Microorganisms  
Here, we discuss the most prominent representatives of polymers produced by microorganisms: PHA and PHB and corresponding copolymers or composites. In the group of PHAs, more than 100 known bio-derived polymers exist. The most common ones are PHB and corresponding copolymers such as poly(3-hydroxybutyrate-co-3-hydroxy-valerate) (PHBV) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) (Figure 3) [110; 128]. In general, PHAs can be used to coat paper or paperboard to produce water-resistant surfaces, making the coated material completely biodegradable. PHB-coated paperboard has been used for packaging of ready meals, while PHBV-coated paperboard has been used for dry products, dairy products, and beverages [129]. In addition to functionalizing the surface of fiber-based materials, PHAs can also functionalize paper and board's grease resistance and sealability [110]. PHAs involve a range of biodegradable thermoplastic polymers that are produced through fermentation by different microorganisms [99]. These polymers are characterized by thermomechanical properties that are similar to synthetic polymers such as PP [130]. PHAs can be processed into different products including films, trays, and coatings on other bio-based materials (e.g., paperboard) [110]. Initially, PHAs were used to make everyday articles like shampoo bottles. Moreover, they are used to produce carrier bags, containers and paper coatings, biodegradable bags, and lids. Currently, their use in terms of packaging applications is restricted as they are not transparent [106,110]. Their gas and water vapor permeability offer opportunities to be applied as food packaging materials. Copolymerization as well as blending are used to improve physical-mechanical properties of PHAs [110]. Several studies have shown that PHB, PHBV, and PHBHx films are promising materials for food packaging due to their good barrier properties. The oxygen permeability of PHAs is comparable to PET and PLA. The values are much lower compared to conventional polymers such as PE and PP. The water vapor permeability of PHAs is similar to materials such as PET and PLA but slightly higher than more apolar polymers such as PE and PP. The carbon dioxide permeability of PHAs is higher compared to PET but substantially lower than for common packaging materials such as PP and PE (Table 2) [131,132,133,134,135,136]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Permeability**} & \multicolumn{3}{c}{**Mechanical Properties**} \\ \cline{2-7}
**Film** & **O2** & **CO2** & **H2O Vapor** & **Tensile Strength** & **Elongation at Break** & **References** \\ \cline{2-7}  & \multicolumn{3}{c}{**[cm\({}^{3}\).mm/(m\({}^{2}\)-d\(\cdot\)atm)]**} & **[g-mm/(m\({}^{2}\)-d\(\cdot\)atm)]** & **[MPa]** & **[\%]** & & \\ \hline PHB & 2–11.4 & 3–28.9 & 1–5 & 35–40 & 3–8 & [110,137] \\ (P3 HB) & (23 \({}^{\circ}\)C, 0\% RH) & (23 \({}^{\circ}\)C, 0\% RH) & (unknown conditions) & 35–40 & 3–8 & [110,137] \\ PHBV & 4.9–16.7 & 146 & 1.5 & & & \\ (P3 HB-co-3 HV) & (25 \({}^{\circ}\)C, 0\% RH) & (25 \({}^{\circ}\)C, 0\% RH) & (38 \({}^{\circ}\)C, 90\% RH) & 38 & NR & [110,137] \\ PHBHx & 8.3 & 54 & 1.42 & & & \\ (23 \({}^{\circ}\)C, 0\% RH) & (23 \({}^{\circ}\)C, 0\% RH) & (23 \({}^{\circ}\)C, 0\% RH) & 20 & 850 & [132,137] \\ \hline \hline \end{tabular}  
\end{table} 
Table 2: Barrier and mechanical properties of bio-based polymers produced by microorganisms. Figure 3: Overview of several poly(hydroxyl alkanoate)s (PHA) used for food packaging applications: (**a**) general structure of PHAs with residues R1/R2 as alkyl chains ranging from 1-13 carbons, (**b**) poly(hydroxyl butyrate) (PHB), (**c**) Poly(3-hydroxybutyrate-co-3-hydroxyalerate) (PHBV), and (**d**) poly(3-hydroxybutyrate-co-3-hydroxybenzoate) (PHBHHx). [PERSON] et al.