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

Abstract

Bio-based poly(lactic acid) (PLA) composite films were produced using unmodified soda micro- or nano-lignin as a green filler at four different contents, between 0.5 wt% and 5 wt%. The PLA–lignin composite polymers were synthesized by solvent casting to prepare a masterbatch, followed by melt mixing. The composites were then converted into films, to evaluate the effect of lignin content and size on their physicochemical and mechanical properties. Differential scanning calorimetry (DSC), supported by polarized light microscopy (PLM), infrared spectroscopy (FTIR-ATR), X-ray diffraction (XRD), and transmission electron microscopy (TEM) were employed to investigate the PLA crystallization and the interactions with Lignin (L) and Nanolignin (NL). The presence of both fillers (L and NL) had a negligible effect on the glass transition temperature (chain diffusion). However, it resulted in suppression of the corresponding change in heat capacity. This was indicative of a partial immobilization of the PLA chains on the lignin entities, due to interfacial interactions, which was slightly stronger in the case of NL. Lignin was also found to facilitate crystallization, in terms of nucleation; whereas, this was not clear in the crystalline fraction. The addition of L and NL led to systematically larger crystallites compared with neat PLA, which, combined with the higher melting temperature, provided indications of a denser crystal structure in the composites. The mechanical, optical, antioxidant, and surface properties of the composite films were also investigated. The tensile strength and Young’s modulus were improved by the addition of L and especially NL. The UV-blocking and antioxidant properties of the composite films were also enhanced, especially at higher filler contents. Importantly, the PLA–NL composite films constantly outperformed their PLA–L counterparts, due to the finer dispersion of NL in the PLA matrix, as verified by the TEM micrographs. These results suggest that bio-based and biodegradable PLA films filled with L, and particularly NL, can be employed as competitive and green alternatives in the food packaging industry.

Nano‐Strategies for Lignin Biomaterials toward Cancer Therapy

“…In a study, Sofia and colleagues looked into using unmodified lignin and nanolignin as fillers in PLA‐based composite films. [ 120 ] Because of the finer dispersion of nanolignin in the PLA matrix, PLA‐nanolignin outperformed PLA‐lignin biocomposites in all characteristic studies. The PLA‐lignin composites (5% lignin) reduced residual DPPH content by 74% after 8 h, while PLA‐nanolignin composites (5% nanolignin) reduced residual DPPH content by 62%, indicating that PLA‐nanolignin has better antioxidant properties.…”

Effect of Filler Content on the Morphology and Physical Properties of Poly(Lactic Acid)-Hydroxyapatite Composites

“…In Figure 8 , the XRD spectrum of the PLA/HAp composite, loaded at 5 wt%, displays two sharp diffraction peaks, the first one is located at 2θ = 16.6° and the other at 19.8°, which correspond respectively to the plans (110)/(200) and (203)/(113). According to the literature [ 16 , 45 ], this indicates the growth of new crystals in the PLA matrix promoted by the HAp particles acting as a heterogeneous nucleating agent [ 45 ]. However, at 10 and 15 wt% filler content, this phenomenon is not observed in the PLA composites, due probably to the presence of filler aggregates which disturb the crystallization growth.…”

“…Although PLA has many good properties, its low tenacity and slow crystallization speed limit its use in other industrial applications requiring a higher mechanical resistance [ 8 ]. To overcome these drawbacks, one of the most common approaches consists of adding either organic or inorganic fillers to the PLA, including organo-modified layered silicates [ 9 ], hydroxyapatite [ 10 ], talc [ 11 ], cellulose [ 12 ], carbon nanotubes, metallic oxides and others to improve the functional characteristics of the materials [ 13 , 14 , 15 , 16 ].…”

Lignin-containing biodegradable UV-blocking films: a review

“…Via combining solvent casting and melt mixing, nano-lignin was more finely dispersed in PLA; this maintained outstanding mechanical properties while lignin content was increased and UV resistance was improved. 127 In addition, the microwave-assisted functionalization (MAF) approach was revealed as an effective technique to produce functionalized wood fibers (FWFs) in small amounts with exceptional precision and high yields (approaching 99%). This process imparted remarkable compatibility with the PLA matrix, leading to substantial enhancements in the mechanical properties of the resulting composite.…”