82
Gelatin is a valuable raw material for the packaging of food products due to its
film-forming ability. However, the gelatin film deteriorates when exposed to light
and/or oxygen. Lignin can be used to overcome this disadvantage. Núñez-Flores
et al. (2013) incorporated lignin at non-cytotoxic concentrations into the gelatin
films. The prepared antioxidant and light barrier properties of the film improved.
This indicates that lignin-gelatin film has great potential as a food coating to prevent
the lipid oxidation induced by UV rays.
5.5.6 Applications of Lignin as a Thermal Oxidation Stabilizer
When it comes to the performance of lignin in polymers, the relationship between
the antioxidant properties of lignin and the thermal stability of lignin/polymer mixtures should be emphasized. Lignin can act as a thermal oxidation stabilizer in composites (Sliwa et al. 2012), and its antioxidant effect plays a key role in improving
thermal stability. The addition of lignin to composites can increase their oxidation
induction times and initial degradation temperatures, i.e. lignin can act as a protective barrier against thermal decomposition (Morandim-Giannetti et al. 2012).
Sadeghifar and Argyropoulos (2015) investigated and evaluated the correlation
between the antioxidant properties of lignin fractions and the thermal stability of
polyethylene (PE)/lignin mixtures by means of induction oxidation temperature
(OIT). OIT of PE/lignin mixtures increased with the amount of phenolic OH in
lignin. In other words, the results revealed that the phenolic OH groups in lignin
were extremely important in determining the thermal-oxidative stability of their
fractions and mixtures with PE. Chen et al. (2018b) also explored the structural
effect of natural KL on the thermal-oxidative stability of KL/PP. Based on the correlation analysis between KL structures and their antioxidant performances in KL/
PP films, the factors that affected the thermal stability of KL/PP films can be organized in the following order of importance: non-condensed phenolic OH groups >
average Mw > aliphatic OH groups ≈ total phenolic OH groups > condensed phenolic OH groups > polydispersity. The antioxidant activity of lignin in the KL/PP
mixture was affected by its compatibility with PP (Chen et al. 2018b). To improve
its solubility in PP, lignin can be modified by butylation reactions. According to Ye
et al. (2016), the compatibility of KL/PP mixture increased with the degree of lignin
butylation. However, extensive butylation alters the antioxidant properties (thermaloxidative stability) of the mixture. Thus, the conflict between the compatibility and
antioxidant capability must be resolved. Aminolysis provides an effective approach
to treat this problem. As shown in Fig. 5.9, with pyrrolidine, the phenolic ester in
lignin can be unblocked much faster than the aliphatic ester. Ye et al. (2018) reported
that after the selective aminolysis of acetylated KL, phenolic OH group content of
lignin increases and its aliphatic OH groups were strongly blocked by the acetate
group. This allowed a good dispersion of lignin into PP, and therefore, improves the
thermal anti-oxidation properties without affecting the mechanical properties of the
mixtures (Ye et al. 2018). Gadioli et al. (2014) prepared PP composites using
Z. Qin et al.
Gelatin is a valuable raw material for the packaging of food products due to its
film-forming ability. However, the gelatin film deteriorates when exposed to light
and/or oxygen. Lignin can be used to overcome this disadvantage. Núñez-Flores
et al. (2013) incorporated lignin at non-cytotoxic concentrations into the gelatin
films. The prepared antioxidant and light barrier properties of the film improved.
This indicates that lignin-gelatin film has great potential as a food coating to prevent
the lipid oxidation induced by UV rays.
5.5.6 Applications of Lignin as a Thermal Oxidation Stabilizer
When it comes to the performance of lignin in polymers, the relationship between
the antioxidant properties of lignin and the thermal stability of lignin/polymer mixtures should be emphasized. Lignin can act as a thermal oxidation stabilizer in composites (Sliwa et al. 2012), and its antioxidant effect plays a key role in improving
thermal stability. The addition of lignin to composites can increase their oxidation
induction times and initial degradation temperatures, i.e. lignin can act as a protective barrier against thermal decomposition (Morandim-Giannetti et al. 2012).
Sadeghifar and Argyropoulos (2015) investigated and evaluated the correlation
between the antioxidant properties of lignin fractions and the thermal stability of
polyethylene (PE)/lignin mixtures by means of induction oxidation temperature
(OIT). OIT of PE/lignin mixtures increased with the amount of phenolic OH in
lignin. In other words, the results revealed that the phenolic OH groups in lignin
were extremely important in determining the thermal-oxidative stability of their
fractions and mixtures with PE. Chen et al. (2018b) also explored the structural
effect of natural KL on the thermal-oxidative stability of KL/PP. Based on the correlation analysis between KL structures and their antioxidant performances in KL/
PP films, the factors that affected the thermal stability of KL/PP films can be organized in the following order of importance: non-condensed phenolic OH groups >
average Mw > aliphatic OH groups ≈ total phenolic OH groups > condensed phenolic OH groups > polydispersity. The antioxidant activity of lignin in the KL/PP
mixture was affected by its compatibility with PP (Chen et al. 2018b). To improve
its solubility in PP, lignin can be modified by butylation reactions. According to Ye
et al. (2016), the compatibility of KL/PP mixture increased with the degree of lignin
butylation. However, extensive butylation alters the antioxidant properties (thermaloxidative stability) of the mixture. Thus, the conflict between the compatibility and
antioxidant capability must be resolved. Aminolysis provides an effective approach
to treat this problem. As shown in Fig. 5.9, with pyrrolidine, the phenolic ester in
lignin can be unblocked much faster than the aliphatic ester. Ye et al. (2018) reported
that after the selective aminolysis of acetylated KL, phenolic OH group content of
lignin increases and its aliphatic OH groups were strongly blocked by the acetate
group. This allowed a good dispersion of lignin into PP, and therefore, improves the
thermal anti-oxidation properties without affecting the mechanical properties of the
mixtures (Ye et al. 2018). Gadioli et al. (2014) prepared PP composites using
Z. Qin et al.