## 3 Plastics Used for Food Packaging
### Selected Plant-derived Polymers
#### 3.3.1 Lignocellulosic Biomass and Lignin
The thermoplastic properties of lignin were enhanced, and the water vapor and oxygen transmission rate decreased while tensile strength was not affected [150]. Polyurethanes of high transperency and flexibility for construction or packaging applications were prepared by [PERSON] et al., where petroleum based polyols could be substituted with kraft lignin up to 80 wt% [151]. In addition, demethylated lignins were also used to enhance the reaction selectivity towards polyurethane formation [152]. In a recently published study by [PERSON] et al. thermoset coatings with integrated self-healing and removal properties were investigated. They investigated a kraft lignin functionalized with carboxylic acid groups as curing agent  
Figure 4: Lignin monolignol structures: p-couraryl alcohol, coniferyl alcohol, and sinapyl alcohol forming the specific residues p-hydroxylphenyl (H), guaiacyl (G), and syringyl (S) [140]. with poly(ethylene glycol) diglycidyl ether, which resulted in a crosslinked structure [153]. [PERSON] et al. designed novel nanocomposite films of lignin and CNC that could be self-supported or used as coatings. This combination of the antibacterial properties of lignin and oxygen barrier properties of CNC films are promising as food packaging material [154]. [PERSON] et al. summarized different possibilities for bio-based paper coatings. Next to polyester and polysaccharides discussed so far, also lipids and proteins could be used [155]. Nevertheless, a full exploitation at industrial scale is not possible, due to different crystallization behaviour, brittleness, or melt instabilities that lead to difficulties in processing of these biopolymers. Blending the lipids and proteins with other biopolymers, such as lignin, may provide a route to overcoming this obstacle. Lignin cannot only be used in paper coatings but also can be used to improve the strength of paperboard. [PERSON] et al. developed a green binder system with _Salix_ lignin that was equal to the wet tear strength of the commercial vinyl acetate binder [156]. Inspired by the reinforcement principle of lignin and cellulose in wood, [PERSON] et al. developed a cellulose fiber scaffold with lignin as reinforced matrix via successive infiltration and mechanical hot-pressing treatments. The resulting composite shows a high isotropic tensile strength of 200 MPa, compared to 40 MPa of conventional cellulose paper, and [PERSON]'s modulus of 10 GPa, which is even higher than many fossil-based plastics. In addition, also the thermostability and UV-blocking performance is enhanced due to the lignin addition [157]. In conclusion, lignins are promising candidates for environmentally benign antioxidants with a great abundance [158; 159; 160]. Regarding the studies summarized in this review, lignin use as an additive seems more favorable than its copolymerization, as copolymerization usually requires at least one functionalization (such as demethylation or carboxylation). Thus, further effort is necessary to get a deeper understanding of the lignin structure and the processing conditions required to maintain and enhance the antioxidant and antimicrobial properties.