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. 2022 Oct 3:10:1025076.
doi: 10.3389/fbioe.2022.1025076. eCollection 2022.

Multifunctional lignin-poly (lactic acid) biocomposites for packaging applications

Esakkiammal Sudha Esakkimuthu  1 , David DeVallance  1   2 , Ievgen Pylypchuk  3 , Adrian Moreno  3 , Mika H Sipponen  3
Affiliations

Multifunctional lignin-poly (lactic acid) biocomposites for packaging applications

Esakkiammal Sudha Esakkimuthu et al. Front Bioeng Biotechnol. .

Abstract

Lignin is the most abundant aromatic biopolymer with many promising features but also shortcomings as a filler in polymer blends. The main objective of this work was to improve the processability and compatibility of lignin with poly (lactic acid) (PLA) through etherification of lignin. Commercial kraft lignin (KL) and oxypropylated kraft lignin (OPKL) were blended with PLA at different weight percentages (1, 5, 10, 20, and 40%) followed by injection molding. Low lignin contents between 1 and 10% generally had a favorable impact on mechanical strength and moduli as well as functional properties of the PLA-based composites. Unmodified lignin with free phenolic hydroxyl groups rendered the composites with antioxidant activity, as measured by radical scavenging and lipid peroxidation tests. Incorporating 5-10% of KL or OPKL improved the thermal stability of the composites within the 300-350°C region. DSC analysis showed that the glass transition temperature values were systematically decreased upon addition of KL and OPKL into PLA polymer. However, low lignin contents of 1 and 5% decreased the cold crystallization temperature of PLA. The composites of KL and OPKL with PLA exhibited good stabilities in the migration test, with values of 17 mg kg-1 and 23 mg kg-1 even at higher lignin content 40%, i.e., well below the limit defined in a European standard (60 mg kg-1). These results suggest oxypropylated lignin as a functional filler in PLA for safe and functional food packaging and antioxidant applications.

Keywords: composites; lignin; matrix; modification; packaging; polylactic acid; polymer.

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Conflict of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

FIGURE 1
FIGURE 1
(A) Schematic reaction of the oxypropylation of kraft lignin. (B) Distribution of aliphatic hydroxyl, carboxylic acid and phenolic hydroxyl groups in kraft lignin and Oxypropylated lignin. Error bars show the standard deviation from the average values obtained from two replicates of the quantitative 31P NMR experiments. (C) 31P NMR spectra of Oxypropylated kraft lignin (top) and kraft lignin (bottom).
FIGURE 2
FIGURE 2
Injection molded PLA/OPKL composites specimens (top-left) and the SEM micrographs of neat PLA, PLA/KL, and PLA/OPKL tensile-fractured surfaces with different lignin and oxypropylated lignin contents. The scale bars in SEM images: 10 μm.
FIGURE 3
FIGURE 3
TG and the corresponding DTG curves of (A,B) PLA/KL and (C,D) PLA/OPKL composites materials.
FIGURE 4
FIGURE 4
DSC curves of (A) first heating scan, (B) cooling scan and (C) second heating scan. (A–C): Continuous lines—PLA + X % of KL and Dashed lines—PLA + X % of OPKL. (X is 1, 5, 10, 20, and 50%).
FIGURE 5
FIGURE 5
Mechanical properties of KL/PLA and PLA/OPKL, (A) Tensile strength, and (B) Elongation at break.
FIGURE 6
FIGURE 6
Reduction of ABTS•+ radical of Neat PLA, PLA/KL and PLA/OPKL at 734 nm.
FIGURE 7
FIGURE 7
Antioxidant results of neat PLA, PLA/KL and PLA/OPKL with respect to the control at 234 nm in UV.
FIGURE 8
FIGURE 8
Migration study results, (A) Absorption of samples (control, neat PLA, PLA/KL and PLA/OPKL), and (B) overall migration of lignin content in 10% ethanol/water.
See this image and copyright information in PMC

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