Modified lignin-PLA biocomposites for food packaging applications
The consumption of polymers has increased considerably over the past few decades; packaging now uses 42% of all polymer materials worldwide. The widespread use of polymers affects the environment, including land and waterways. Polymers are used heavily in food packaging because they are more convenient than traditional glass and metal packaging.
Polyethylene (PE), polypropylene (PP) and polylactic acid (PLA) are the most commonly used polymers in food packaging. Sustainable and bio-based polymers such as PLA are generally more expensive than non-sustainable PP and PE-based packaging materials. PLA has the potential to produce more sustainable packaging systems because it is a biopolymer made from natural resources. It is widely used in plastic films, bottles and biodegradable medical devices. However, due to its poor thermal stability, slow crystallization, and high cost, PLA is not suitable for food storage applications that require longer shelf life. Research conducted by InnoRenew CoE aims to overcome the limitations of PLA by combining biopolymers such as lignin.
Compatible and biodegradable
Lignin can be obtained as a low-cost by-product of agricultural and forest biorefineries. It contains several different functional groups (hydroxyl, carbonyl and carboxyl). However, lignin has some compatibility issues with PLA due to the polar functional groups present in this form. Modification of these polar groups in lignin can significantly reduce compatibility issues with PLA matrices and enhance the mechanical and physicochemical properties of PLA without compromising biodegradability.
Recent work in the MSCA-PACK-NIN project proposes a modification reaction, oxypropylation of lignin hydroxyl groups, to enhance the hydrophobicity and plasticity of lignin polymers. The researchers produced modified lignin/PLA composites with different weight percent components through mixing and injection molding processes. They examined the thermal degradation properties and glass transition temperature of the composites produced, as well as cold crystallization and melting temperatures. They optimized the weight percent of modified lignin in the composite based on the resulting properties. The picture shows the injection-molded PLA-modified lignin composite samples and SEM micrographs of the selected composites after tensile tests of pure PLA, PLA lignin and PLA-modified lignin.
Future research
To further develop these composites, future research will include additional testing in support of food packaging applications, including antioxidant and degradation studies. The combined results of this study have been submitted for peer-reviewed publication. Part of the research was carried out in the SUSMATCHEM group at Stockholm University, Sweden, with secondment opportunities admitted under the MSCA-PACK-NIN program.
