Kraft Lignin/Tannin as a Potential Accelerator of Antioxidant and Antibacterial Properties in an Active Thermoplastic Polyester-Based Multifunctional Material

Abstract

This research focuses on key priorities in the field of sustainable plastic composites that will lead to a reduction in CO2 pollution and support the EU’s goal of becoming carbon neutral by 2050. The main challenge is to develop high-performance polyphenol-reinforced thermoplastic composites, where the use of natural fillers replaces the usual chemical additives with non-toxic ones, not only to improve the final performance but also to increase the desired multifunctionalities (structural, antioxidant, and antibacterial). Therefore, poly (lactic acid) (PLA) composites based on Kraft lignin (KL) and tannin (TANN) were investigated. Two series of PLA composites, PLA-KL and PLA-TANN, which contained natural fillers (0.5%, 1.0%, and 2.5% (w/w)) were prepared by hot melt extrusion. The effects of KL and TANN on the PLA matrices were investigated, especially the surface physicochemical properties, mechanical properties, and antioxidant/antimicrobial activity. The surface physicochemical properties were evaluated by measuring the contact angle (CA), roughness, zeta potential, and nanoindentation. The results of the water contact angle showed that neither KL nor TANN caused a significant change in the wettability, but only a slight increase in the hydrophilicity of the PLA composites. The filler loading, the size of the particles with their available functional groups on the surfaces of the PLA composites, and the interaction between the filler and the PLA polymer depend on the roughness and zeta potential behavior of the PLA-KL and PLA-TANN composites and ultimately improve the surface mechanical properties. The antioxidant properties of the PLA-KL and PLA-TANN composites were determined using the DPPH (2,2′-diphenyl-1-picrylhydrazyl) test. The results show an efficient antioxidant behavior of all PLA-KL and PLA-TANN composites, which increases with the filler content. Finally, the KL- and PLA-based TANN have shown resistance to the Gram-negative bacteria, E. coli, but without a correlation trend between polyphenol filler content and structure.

Effect of Micro- and Nano-Lignin on the Thermal, Mechanical, and Antioxidant Properties of Biobased PLA–Lignin Composite Films

“…By contrast, the addition of either L or NL in the PLA matrix enhanced the antioxidant activity, as could be deduced from the higher reduction rate of DPPH• over time. The composites with higher L/NL content showed better antioxidant activity, in agreement with the literature [ 30 , 43 , 74 ]. In the case of the PLA–L composites, the PLA–L composite with 5 wt% L loading yielded 74% residual DPPH content reduction after 8 h, while the PLA–L composites with lower L loadings (0.5–2.5 wt%) showed poor antioxidant activity.…”

“…In addition, PLA–L films showed excellent antimicrobial activity, reducing the viability of Gram-positive bacteria tenfold. Similar results were obtained by Cresnar et al [ 30 ] and by Domenek et al [ 31 ], who prepared PLA–L films by melt-mixing using Kraft and Soda lignin, respectively. Both studies also reported greater antioxidant activity with increasing lignin content.…”

Synthesis and Characterization of Poly(lactic acid) Composites with Organosolv Lignin

“…The visual appearance of the films prepared by compression moulding ( Figure 2 ) shows a macroscopically uniform distribution of the lignin in the PLA matrix without any visible particles or aggregates. In contrast, the PLA/kraft lignin composites, prepared without the use of masterbatch directly in the melt compounder, resulted in a visibly heterogenous polymer [ 14 ].…”

“…In previous work, by our group, kraft lignin was added to a PLA matrix, via melt mixing, and the composites had both antioxidant and antibacterial activity, an increased hardness and Young’s modulus, accelerated crystallization, and a slightly improved thermal stability [ 14 , 15 , 16 ]. In this work, another type of biorefinery lignin, the so-called organosolv lignin, was evaluated as a filler in PLA.…”

Colloidal Solutions as Advanced Coatings for Active Packaging Development: Focus on PLA Systems

“…For instance, Pušnik et al studied the changes in surface properties, antibacterial activity, and mechanical properties of PLA films by adding metal-based nanoparticles, namely, Ag, ZnO, and TiO 2 [ 38 ]. In another work by the same group, the improvement of properties was investigated by the addition of wood fibres, kraft lignin and tannin [ 43 ]. Giulia Fredi et al demonstrated successfully that mixing PLA with different poly(alkylene furanoates) would improve the mechanical properties and gas barrier properties, making it more suitable for packaging [ 44 ].…”