Citation: Daassi, R.; Durand, K.; Rodrigue, D.; Stevanovic, T. Optimization of the Electrospray Process to Produce Lignin Nanoparticles for PLA-Based Food Packaging. Polymers 2023, 15, 2973. https://doi.org/10.3390/ polym15132973 Academic Editor: Swarup Roy Received: 6 June 2023 Revised: 29 June 2023 Accepted: 5 July 2023 Published: 7 July 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Optimization of the Electrospray Process to Produce Lignin Nanoparticles for PLA-Based Food Packaging Rodrigue Daassi 1,2 , Kalvin Durand 1,2 , Denis Rodrigue 2 and Tatjana Stevanovic 1, * 1 Renewable Materials Research Centre (CRMR), Institute of Nutrition and Functional Foods (INAF), Université Laval, Quebec City, QC G1V 0A6, Canada; rodrigue.daassi.1@ulaval.ca (R.D.); kalvin.durand.1@ulaval.ca (K.D.) 2 Chemical Engineering Department, Université Laval, Quebec City, QC G1V 0A6, Canada; denis.rodrigue@gch.ulaval.ca * Correspondence: tatjana.stevanovic@sbf.ulaval.ca; Tel.: +656-2131-7337 Abstract: The development of new processing methods is required in order to meet the continuous demand for thinner films with excellent barrier properties for food packaging and other applications. In this study, rice husk organosolv lignin nanoparticles were prepared using the electrospray method, which were applied to produce polylactic acid (PLA)-based films for food packaging. The effect of the following electrospray parameters has been investigated: lignin concentration (LC) ranging from 5–50 mg/mL, flow rate (FR) from 0.5–1 mL/min, applied voltage from 10–30 kV, and tip- to-collector distance (TCD) from 10–25 cm, on the morphology, size, polydispersity index (PDI), and Zeta potential (ZP) of lignin nanoparticles (LNPs). The response surface methodology with a Box-Behnken design was applied to optimize these parameters, while dynamic light scattering (DLS) and scanning electron microscopy (SEM) analyses were used to characterize the controlled LNPs. The results showed that the LNPs shape and sizes represent a balance between the solvent evaporation, LC, applied voltage, TCD and FR. The application of optimal electrospray conditions resulted in the production of LNPs with a spherical shape and a minimal size of 260 ± 10 nm, a PDI of 0.257 ± 0.02, and a ZP of −35.2 ± 4.1 mV. The optimal conditions were achieved at LC = 49.1 mg/mL and FR = 0.5 mL/h under an applied voltage of 25.4 kV and TCD = 22 cm. Then, the optimized LNPs were used to improve the properties of PLA-based films. Three types of PLA-lignin blend films were casted, namely lignin/PLA, LNPs/PLA and PLA-grafted LNPs. PLA-grafted LNPs exhibited a more uniform dispersion in PLA for lignin contents of up to 10% than other composite samples. Increasing the lignin content from 5% to 10% in PLA-grafted LNPs resulted in a significant increase in elongation at break (up to four times higher than neat PLA). The presence of PLA-grafted lignin led to a substantial reduction in optical transmittance in the UV range, dropping from 58.7 ± 3.0% to 1.10 ± 0.01%, while maintaining excellent transparency to visible light compared to blends containing lignin or LNPs. Although the antioxidant capacity of unmodified lignin is well-known, a substantial increase in antioxidant capacity was observed in LNPs and PLA-grafted LNP films, with values exceeding 10 times and 12 times that of neat PLA, respectively. These results confirm the significant potential of using studied films in food packaging applications. Keywords: electrospray; lignin nanoparticles; response surface methodology; poly(lactic acid) composite; film; food packaging; antioxidant 1. Introduction In recent years, the demand for eco-friendly food packaging with improved properties has increased. Polylactic acid (PLA) has been considered as a promising biopolymer for food packaging due to its renewability, biodegradability, and excellent mechanical properties [1,2]. However, its poor barrier properties against oxygen, water vapor, and UV light, as well as its limited thermal stability and its low elongation at break (mechanical Polymers 2023, 15, 2973. https://doi.org/10.3390/polym15132973 https://www.mdpi.com/journal/polymers