84
this problem: 1) by chemical modifications (e.g. esterification, acetylation, and
alkylation) and 2) by organic solvent fractionation, which can selectively separate
the lignin fractions with low polydispersity by changing the solvent polarity. Lignin
also has little compatibility with non-polar polymers such as PE, due to a large
number of polar groups into lignin structure (Yeo et al. 2015). Poor compatibility
between the polymer and lignin can restrict its antioxidant activity (Avelino et al.
2019). Poor compatibility can also have a negative influence on the mechanical
properties of the composites (Dehne et al. 2016). Some strategies to improve the
compatibility of lignin with polymers have been those previously indicated.
Although other alternatives can also be highlighted such as the addition of coupling
agents (e.g. maleic anhydride-grafted PP and ethylene-vinyl acetate) or lignin grafting of with non-polar chains (Ye et al. 2016). Modified lignin has been shown to
have good compatibility and dispersibility with the polymers and can be used for
many purposes, such as making biomedical, packaging and thermoplastic materials,
as well as asphalt and cosmetics.
5.7 Lignin as Raw Material for the Production
of Antioxidants
From the point of view of the chemical structure, lignin is composed of phenolic
units and is mainly degraded to phenolic compounds with low Mw under hydrothermal or pyrolysis conditions (Kang et al. 2015; Larson et al. 2017). Some of the
phenolic products derived from the lignin decomposition have high antioxidant
activities and can be used for the production of commercial antioxidants (Kang
et al. 2015). This means lignin can be used as a source for the production of antioxidants. Kang et al. (2015) reported that AL products obtained after hydrothermal
liquefaction at 320 °C for 30 min. showed better antioxidant capacities than the
original AL. Cesari et al. (2019) found that eight phenolic compounds derived from
the depolymerization of lignin, namely guaiacol, m-, o- and p-cresol, phenol, pyrocatechol, syringol and vanillin, exhibited a good radical scavenging and reducing
powers. Larson et al. (2017) extracted phenols from the dichloromethane-soluble
portion of the pyrolyzate. Dimeric fractions such as α- and ω-dicatechols and
diguaiacols are the main products responsible for their high antioxidant properties.
The high antioxidant capacities of these lignin-derived products suggest they could
be potentially used as commercial antioxidants.
5.8 Conclusions and Future Perspectives
A clear understanding of the structure-antioxidant activity relationship will contribute to the production of lignin with high antioxidant capacity, an effective chemical
modification and a wider use. Lignin has already found many applications in a wide
variety of fields, but some of its properties still undermine its use. These
Z. Qin et al.
this problem: 1) by chemical modifications (e.g. esterification, acetylation, and
alkylation) and 2) by organic solvent fractionation, which can selectively separate
the lignin fractions with low polydispersity by changing the solvent polarity. Lignin
also has little compatibility with non-polar polymers such as PE, due to a large
number of polar groups into lignin structure (Yeo et al. 2015). Poor compatibility
between the polymer and lignin can restrict its antioxidant activity (Avelino et al.
2019). Poor compatibility can also have a negative influence on the mechanical
properties of the composites (Dehne et al. 2016). Some strategies to improve the
compatibility of lignin with polymers have been those previously indicated.
Although other alternatives can also be highlighted such as the addition of coupling
agents (e.g. maleic anhydride-grafted PP and ethylene-vinyl acetate) or lignin grafting of with non-polar chains (Ye et al. 2016). Modified lignin has been shown to
have good compatibility and dispersibility with the polymers and can be used for
many purposes, such as making biomedical, packaging and thermoplastic materials,
as well as asphalt and cosmetics.
5.7 Lignin as Raw Material for the Production
of Antioxidants
From the point of view of the chemical structure, lignin is composed of phenolic
units and is mainly degraded to phenolic compounds with low Mw under hydrothermal or pyrolysis conditions (Kang et al. 2015; Larson et al. 2017). Some of the
phenolic products derived from the lignin decomposition have high antioxidant
activities and can be used for the production of commercial antioxidants (Kang
et al. 2015). This means lignin can be used as a source for the production of antioxidants. Kang et al. (2015) reported that AL products obtained after hydrothermal
liquefaction at 320 °C for 30 min. showed better antioxidant capacities than the
original AL. Cesari et al. (2019) found that eight phenolic compounds derived from
the depolymerization of lignin, namely guaiacol, m-, o- and p-cresol, phenol, pyrocatechol, syringol and vanillin, exhibited a good radical scavenging and reducing
powers. Larson et al. (2017) extracted phenols from the dichloromethane-soluble
portion of the pyrolyzate. Dimeric fractions such as α- and ω-dicatechols and
diguaiacols are the main products responsible for their high antioxidant properties.
The high antioxidant capacities of these lignin-derived products suggest they could
be potentially used as commercial antioxidants.
5.8 Conclusions and Future Perspectives
A clear understanding of the structure-antioxidant activity relationship will contribute to the production of lignin with high antioxidant capacity, an effective chemical
modification and a wider use. Lignin has already found many applications in a wide
variety of fields, but some of its properties still undermine its use. These
Z. Qin et al.