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. 2023 Nov 9;11(46):16442-16452.
doi: 10.1021/acssuschemeng.3c03030. eCollection 2023 Nov 20.

Ductile, High-Lignin-Content Thermoset Films and Coatings

Alice Boarino  1 , Justine Charmillot  2 , Monique Bernardes Figueirêdo  2 , Thanh T H Le  1 , Nicola Carrara  1 , Harm-Anton Klok  1
Affiliations

Ductile, High-Lignin-Content Thermoset Films and Coatings

Alice Boarino et al. ACS Sustain Chem Eng. .

Erratum in

Abstract

In the context of transitioning toward a more sustainable use of natural resources, the application of lignin to substitute commonly utilized petroleum-based plastics can play a key role. Although lignin is highly available at low cost and presents interesting properties, such as antioxidant and UV barrier activities, its application is limited by its low reactivity, which is a consequence of harsh conditions normally used to extract lignin from biomass. In this work, the use of glyoxylic acid lignin (GA lignin), rich in carboxylic acid groups and hence highly reactive toward epoxy cross-linkers, is presented. GA lignin, which is directly extracted from biomass via a one-step aldehyde-assisted fractionation process, allowed the preparation of thermoset films and coatings via a simple reaction with sustainable poly(ethylene glycol) diglycidyl ether and glycerol diglycidyl ether cross-linkers. This allows one to prepare freestanding films containing up to 70 wt % lignin with tunable mechanical properties and covalently surface-attached coatings containing up to 90 wt % lignin with high solvent resistance. Both films and coatings display antioxidant properties and combine excellent UV barrier activity with high visible transparency, which is attractive for applications in sustainable food packaging.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Scheme 1
Scheme 1. Schematic Representation of the Structure of GA Lignin
Figure 1
Figure 1
(A) Reaction of GA lignin with PEGDE and GDE. (B) Carboxylic acid and phenol group content in GA lignin before and after the reaction with PEGDE and GDE (50 wt % of GA lignin and 50 wt % of cross-linker), as determined by 31P NMR spectroscopy.
Figure 2
Figure 2
(A) Sol fractions of PEGDE and GDE cross-linked GA lignin films containing 50, 60, or 70 wt % lignin. (B) Sol fraction of PEGDE and GDE cross-linked GA lignin films plotted against the molar ratio of cross-linker epoxy groups and functional groups in GA lignin.
Figure 3
Figure 3
Stress–strain curves of GA lignin/PEGDE and GA lignin/GDE films containing 50, 60, or 70 wt % lignin.
Figure 4
Figure 4
(A) Photographs of GA lignin/PEGDE and GA lignin/GDE films placed on top of a printed text to highlight their visible transparency (EPFL logo used with permission). (B) Transmittance through the films in the visible range (660 nm). (C) Transmittance through the films in the UV range (280 nm). (D) Magnification of plot (C) in the y-axis range 0–0.02%.
Figure 5
Figure 5
Antioxidant activity of PLA (green), as well as GA lignin/PEGDE (red) and GA lignin/GDE (blue) films containing 50, 60, and 70 wt % GA lignin, as measured by the DPPH assay.
Figure 6
Figure 6
Color change (ΔE*ab) of broccoli florets stored under sunlight covered by a 50 wt % GA lignin/PEGDE (red), 50 wt % GA lignin/GDE (blue), PLA (green), or a polyethylene (PE) food wrapping film (gray) or not covered (black). ΔE*ab represents the color change of the broccoli compared to the first day of the experiment (e.g., day 1). Statistical analysis was performed with Student’s t-test (*p < 0.05, ** p < 0.01, ***p < 0.001).
Scheme 2
Scheme 2. Preparation of Covalently Surface-Attached PEGDE and GDE Cross-linked GA Lignin Coatings
Figure 7
Figure 7
(A) Photographs of GA lignin/PEGDE, GA lignin/GDE, soda lignin/PEGDE, and soda lignin/GDE coatings on activated fused silica, overlapped on a printed text to highlight their visible transparency (EPFL logo used with permission). (B) Visible light transmittance (at 660 nm) of the different PEGDE and GDE cross-linked GA lignin and soda lignin surface coatings. (C) UV light transmittance (at 280 nm) of the different PEGDE and GDE cross-linked GA lignin and soda lignin surface coatings. (D) Magnification of plot (C) in the y-axis range 0–0.02%.
Figure 8
Figure 8
Antioxidant activity of PEGDE (red) and GDE (blue) cross-linked GA lignin-based, surface-attached coatings containing 50, 70, and 90 wt % GA lignin.
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