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(Gullichsen 2000). The main precursors of lignin are coniferyl alcohol, sinapyl alcohol, and p -coumaryl alcohol (Alén 2000; Vanholme et al. 2010). The origin and the type of processing of lignin influence its composition, which can vary considerably among different types of sources (Alén 2000; Domenek et al. 2013; Hult et al. 2013a; Gordobil et al. 2014). Lignins can be found in the cell walls of plants (Gullichsen 2000; Yu et al. 2016) and are classified as softwood, hardwood, and grass lignins (Alén 2000). The content of lignin in softwoods is normally higher than in hardwoods. For example, the lignin content of pine ( Pinus sylvestris ) is approximately 25% to 30% (dry weight), whereas in birch ( Betula pendula ) it is 20% to 25% (Jääskeläinen and Sundqvist 2007). Major quantities of lignin can be obtained as a byproduct of pulp production (Pouteau et al. 2004; Gandini 2008), whereas another noteworthy source of lignin is the bioethanol industry (Yu et al. 2016). A common extraction method of lignin is precipitation from black liquor by acidification (Jönsson et al. 2008). A few examples from lignin production routes can be seen in Fig. 3. Currently, most of the technical lignin from the pulp industry is burnt as fuel in recovery boilers (Pouteau et al. 2004; Gandini 2008; Domenek et. al. 2013; Hult et al. 2013a,b; Gordobil et al. 2014). However, there are serval commercial and semi-commercial facilities to produce lignin. Stora Enso, one of the largest kraft lignin producer in the world, has a lignin production of 50,000 tons annually (Upton 2018). The great availability of this complex polymer is one of the main reasons why it is such an interesting raw material (Gandini 2008). In addition, lignin has a great potential for chemical modification into specialty products (Antonsson et al. 2008; Hult et al. 2013a,b). Technical lignins are rather dark-colored, while the lignin in the wood is nearly colorless. In some applications, the dark color of lignin needs to be removed. Decolorization methods have been presented, e.g. , UV irradiation in a tetrahydrofuran (THF) solution and blocking of the free phenolic hydroxyl groups followed by oxidation (Wang et al. 2016). The formation of films with technical lignins is a great challenge due to its brittleness and rigidity. However, the required thermoplastic properties can be improved by chemical functionalization (Hult et al. 2013a,b; Li et al. , 2018). Esterification of lignin improved its moisture and oxygen resistance. A piece of paper coated with two layers (3.9 g/m 2 each) of tall oil fatty acid (TOFA) esterified softwood lignin resulted in a 70% reduction in WVTR as well as improvements in OTR (Vartiainen et al. 2014). Lignin films and coatings have been fabricated, e.g. , with a bar coater (Hult et al. 2013a), and solution casting methods (Bhat et al. 2013; Shankar et al. 2015). Possible application fields for lignin have been presented to include emulsifiers, binding dispersing agents (Jönsson et al. 2008; Watkins et al. 2015), thermosets, paints, dyes (Watkins et al. 2015), wet strength additives (Jönsson et al. 2008), chelating agents for heavy metal removal (Toledano et al. 2010; Kaewtatip and Thongmee 2013), flame retardant (Kaewtatip and Thongmee 2013), and antioxidants (Kaewtatip and Thongmee 2013; Shankar et al. 2015). Lignin and its blends have been reported to provide both gas (Hult et al. 2013a,b) and UV light barrier properties, as well as to work as an antimicrobial barrier (Yu et al. 2016; Rai et al. 2017). Due to these features, lignin-based films have been considered suitable for food packaging applications (Shankar et al. 2015; Rai et al. 2017).
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Helanto et al. (2019). “ Bio-based barriers ,” B io R esources 14(2), Pg #s to be added.
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