Dual-Templating Approach for Engineering Strong, Biodegradable Lignin-Based Foams

Technical lignins are generated as byproducts from the wood pulping industry. Although their estimated annual production amounts to approximately 70 million tons, their exploitation as value-added products remains insignificant. Yet, the diversity in the molecular structure and surface chemistry of technical lignins and their intrinsic role as mechanical support of plants may be an asset to consider in the engineering of plant-inspired materials such as biofoams. Valorization of lignins into solid foams, however, rarely accounts for more than 45–50 wt % of lignins because of their brittle nature. Herein, we report a strategy to develop fully biodegradable lignin-based foams of high stiffness, strength, and toughness that are comparable to, or in some cases exceed, the performance of petroleum-derived foams. A dual-templating approach using ice and cellulose nanofibrils (CNFs) as templates was selected to control the porous architecture of the foams made by the assembly of lignin and cellulose in the cell walls. Foams with varying lignin-to-CNF weight ratios showed enhanced structural and mechanical integrity compared with neat lignin and CNF foams. For 80–90 wt % of lignin, a significant increase (+50%) in the foams’ compressive performance was observed. Varying the degree of sulfonation of lignin and in turn its chemical interaction with cellulose enabled the generation of biodegradable composite foams with tunable compressive strength. The greater the colloidal stability of the lignin-CNF suspension, the higher the foams’ compressive performance. This study thus discusses an engineering approach for the valorization of technical lignins into sustainable foams that have potential as packaging materials and sandwich panels, in which high stiffness, strength, and toughness per unit weight are required

Analysis of the Porous Architecture and Properties of Anisotropic Nanocellulose Foams: A Novel Approach to Assess the Quality of Cellulose Nanofibrils (CNFs)

Cellulose nanofibrils (CNFs) are a unique nanomaterial because of their abundant, renewable, and biocompatible origin. Compared with synthetic nanoparticles, CNFs are commonly produced from cellulose fibers (e.g., wood pulp) by repetitive high-shear mechanical disintegration. Yet, this process is still highly demanding in energy and costly, slowing down the large-scale production and commercialization of CNFs. Reducing the energy consumption during fibers fibrillation without using any chemical or enzymatic pretreatments while sustaining the CNF quality is challenging. Here, we show that the anisotropic properties of the CNF foams are directly connected to the degree of nanofibrillation of the cellulose fibers. CNFs were produced from wood pulps using a grinder at increasing specific energy consumptions. The anisotropic CNF foams were made by directional ice templating. The porous architecture, the compressive behavior of the foams, and the CNF alignment in the foam cell walls were correlated to the degree of fibrillation. A particular value of specific energy consumption was identified with respect to the highest obtained foam properties and CNF alignment. This value indicated that the optimal degree of fibrillation, and thus CNF quality, was achieved for the studied cellulose pulp. Our approach is a straightforward tool to evaluate the CNF quality and a promising method for the benchmarking of different CNF grades