The more widespread applicability of photopolymerization-based three-dimensional (3D) printing is limited by the availability of light-curable resins, most of which are based on fossil-derived compounds. We developed a biobased lignin-derivable resin by utilizing methacrylated derivatives of vanillin, vanillyl alcohol, and eugenol as aromatic monomers. Lignin nanoparticles (LNPs) were incorporated as functional fillers that enhance print resolution and material properties. The crosslinking degree, and thereby the tensile properties, was modulated through the use of mono-or dimethacrylated vanillin derivatives in the resin formulation. The LNPs acted as UV absorbers, conferring better control of the photopolymerization process by preventing light penetration across unintended layers, leading to enhanced print resolution. The LNPs showed excellent dispersion stability due to their size and morphology. The inclusion of up to 2 wt% of LNPs improved the ductility of the 3D printed nanocomposites through toughening mechanisms enabled by the rigid nanoparticles. Finally, exploiting the differences in crosslinking degree of the resin formulations, a multi-material model featuring both soft and rigid domains was fabricated. This study demonstrates a simple but effective strategy for the design of biobased photocurable resins with tailorable mechanical properties that are suitable for high-resolution and multi-material 3D printing.

Additive manufacturing (AM) has led to the immense design freedom and fast production rates of wood-polymer composites (WPCs). However, vat photopolymerization-based AM techniques such as digital light processing (DLP) have yet to be exploited for the fabrication of complex and high resolution WPCs. This work demonstrates a microwave-assisted strategy for the rapid hydrophobization and functionalization of wood flour with meth-acrylate groups, in order to enhance its compatibility and interfacial interaction with novel wood-monomer derived photopolymer resins. The efficiency of this approach was exemplified in the inclusion of a high loading amount (10 wt%) of methacrylated wood flour to a biobased resin consisting of methacrylated eugenol and vanillin, without compromising the resin's dispersion stability and printability. Ultimately, the development of the resin culminated in the successful DLP 3D printing of wood flour-reinforced biobased composites that exhibited increases of 617%, 482%, and 31% in tensile strength, Young's modulus, and elongation at break, respectively, compared to resins without wood flour.

Lignin is the most abundant aromatic biopolymer, with a potential to serve as a building block of rigid and thermally stable bio-based materials. However, it is still underutilized because of the heterogeneous and not fully understood chemical structure. Here, technical softwood Kraft lignin is refined in to narrow-dispersity and relatively low molar mass fractions by microwave-assisted processing, followed by microwave-assisted allylation and further application in lignin-based thermosets. This microwave processing is carried out under non-catalyzed conditions using a low boiling point solvent and elevated pressure. The properties of the retrieved fractions are investigated by 31P-NMR, heteronuclear single quantum coherence spectroscopy-NMR, SEC, differential scanning calorimetry, and thermogravimetric analysis. The extraction yield of the selected lignin fraction is around 25%, with the number-average molar mass (Mn), weight-average molar mass (Mw), and dispersity (Đ) significantly reduced. The chemically modified lignin is characterized by 31P NMR and FTIR, which provides evidence of the introduction of the allyl moieties. The analyses demonstrate that 90 ± 3% of the hydroxyl groups in fractionated lignin are successfully allylated. Subsequently, the allylated lignin is cross-linked through thermally induced thiol-ene chemistry to produce lignin-based thermosets. The final thermosets exhibit a storage modulus of 4050 ± 60 MPa and a Tg of 105 ± 5°C.

The heterogeneous nature of lignin poses significant obstacles to its practical use in material applications. Common fractionation methods employ harsh processing conditions that further exacerbate lignin's structural complexity. We propose a microwave (MW)-assisted approach for a mild organosolv extraction of structurally-intact lignin from spruce wood. The efficient energy transfer enabled by microwave irradiation facilitates the rapid extraction of lignin in 5, 10, and 20 minutes, ensuring a low level of process severity. Comparison of the 10 minutes MW-extracted lignin products with a cyclic-extracted (CE) organosolv lignin revealed that equivalent amounts of β-O-4 linkages were preserved in both processes. This is indicative of the promising potential of MW-extraction as a biomass pretreatment method for the rapid extraction of more native-like lignin. Finally, we demonstrate the utilization of both MW- and CE-extracted lignins as property-enhancing fillers in a biobased photocurable resin for digital light processing (DLP). The more native-like structures of the mildly-extracted lignins proved beneficial for functionalization with reactive methacrylate moieties, enabling the mechanical reinforcement of DLP 3D printed thermosets with improved toughness after the incorporation of only 1 wt% of the lignins. Compared to the resin without lignin, the tensile strength was improved by 15 and 41% and elongation at break by 79 and 75% in the presence of methacrylated MW- and CE-lignins, respectively. This highlights the potential of MW and CE strategies to effectively process and modify lignin, thereby enhancing its utilization in targeted material applications.

The chemical diversity and structural complexity of lignin, an abundant biopolymer found in vascular plants, present a multitude of opportunities for the modification and fine-tuning of its properties to suit downstream demands. In this thesis, microwave-assisted strategies were explored as efficient and environment-friendly pathways for lignin valorization towards value-added material applications. First, the mild microwave-assisted organosolv extraction of lignin from spruce wood was demonstrated. The effective deconstruction of lignocellulosic structures by microwave irradiation led to the rapid extraction of structurally intact lignin with preserved β-aryl ether linkages and minimal condensation. The high structural quality of the obtained lignin was visibly manifested in its significantly lighter color relative to most technical lignins, hence improving its suitability for material applications wherein the color of the end product is important. Next, a green approach to convert lignosulfonate to carbonaceous products was implemented via microwave-assisted hydrothermal carbonization. This resulted in the synthesis of carbon spheres that served as precursors for nanographene oxide (nGO)- type carbon dots, which were characterized as having abundant oxygen- containing functional groups. The nGO-type carbon dots were utilized as building blocks in the development of porous composites for the adsorption of metal ions and cationic dye pollutants. Lastly, microwave-assisted esterification was employed to both hydrophobize alkali lignin and to functionalize the microwave-extracted organosolv lignin with methacrylate units to facilitate their utilization for 3D printing applications. Through this microwave-assisted approach, high degrees of substitution were achieved after a short reaction duration without the need for additional solvents or catalysts. Effective hydrophobization was exemplified in the enhanced thermal stability and compatibility of the hydrophobized lignins in polylactide (PLA), thereby enabling the melt-processing of up to 50 wt% of lignin in PLA thermoplastic blends for fused filament fabrication. Also, successful functionalization of microwave-extracted organosolv lignin with reactive methacrylate moieties enabled it to partake in network formation within a photocurable resin for digital light processing. This ultimately resulted in 3D printed thermosets with improved tensile strength (by 15%) and elongation at break (by 79%) relative to unfilled resin, after the incorporation of just 1 wt% lignin.

Lignins komplexa struktur med dess mångfald av kemiska bindningar möjliggör modifiering och finjustering av dess egenskaper. I denna avhandling presenteras strategier för miljövänliga och effektiva mikrovågsassisterade metoder som syftar till att öka lignins värde som material. Först utvecklades en mild mikrovågsassisterade organosolv-extraktion av lignin från gran. Mikrovågsbestrålning ledde till en effektiv sönderdelning av lignocellulosa och en snabb extraktion av lignin med en välbevarad struktur, med mycket liten kondensation. Den höga kvalitén syntes på dess betydligt ljusare färg jämfört med tekniska ligniner, därmed ökar användbarheten i applikationer där färgen på slutprodukten är viktig. Därefter utvecklades en metod för att omvandla lignosulfonat till kolprodukter genom mikrovågsassisterad hydrotermisk karbonisering. De syntetiserade kolsfärerna användes sedan för tillverkning av nanografenoxidkolprickar med många syreinnehållande funktionella grupper. Kolprickarna användes därefter som byggstenar i porösa kompositer för adsorption av tungmetalljoner och katjoniska färgämnesföroreningar. Slutligen användes mikrovågsassisterad esterifiering för att hydrofobisera alkali-lignin och för att funktionalisera mikrovågsextraherad organosolv-lignin för användning i 3D-printing. På kort tid uppnåddes höga substitutionsgrader med hjälp av den mikrovågsassisterade funktionaliseringen utan att ytterligare lösningsmedel eller katalysatorer användes. En effektiv hydrofobisering var tydlig genom en ökad termisk stabilitet och kompatibilitet med polylaktid, vilket möjliggjorde extrudering av filament och påföljande 3D-printing av blandningar med upp till 50 vikt% lignin. Genom funktionalisering av mikrovågsextraherad organosolv-lignin med reaktiva metakrylatgrupper kunde lignin inkorporeras som en egenskapshöjande del av 3D-printade och UV-härdade nätverk. Material med endast 1 vikt% lignin hade en högre draghållfasthet (15%) och förlängning vid brott (79%) jämfört med nätverk utan lignin.

A vanillin-based photocurable resin was designed for circularity by incorporation of imine functionalities through a Schiff-base reaction between the aldehyde function of vanillin and amino group of ethylene diamine. Sufficient flexibility was provided by a short aliphatic segment introduced by reaction of vanillin with ethylene carbonate, while photocurability was obtained by subsequent methacrylation. The cured thermoset had good solvent resistance, relatively high glass transition temperature (similar to 75 degrees C) and good thermal stability with an onset of degradation above 300 degrees C. Due to the crosslinked structure and imine linkages, the thermoset expressed malleability, self-healing and thermal reprocessability. Furthermore, it could be chemically recycled by immersing in ethylene diamine, which activated transimination. The obtained oligomeric product with amineterminal groups could be utilized for production of new thermoset films. Tensile testing illustrated similar elastic modulus for mechanically and chemically recycled thermosets, while a slight increase was observed for the self-healed samples, ascribable to a completion of the curing during the post-processing. At the same time elongation and stress at break slightly decreased. Finally, the suitability of the resin for the production of 3D objects by means of digital light processing (DLP) 3D printing was demonstrated.

Removal of contaminants from wastewater is one key element for the development of sustainable society. Here, innovative “all-lignocellulose” derived photo-curable hydrogel nanocomposites based on methacrylated carboxymethyl cellulose (M-CMC) and lignosulfonate-derived carbonaceous products were successfully designed. The carbon products were synthesized by microwave-assisted hydrothermal carbonization (MAHC), followed by oxidation and methacrylation. This yielded nano-graphene oxide (nGO) and methacrylated nGO (M-nGO). The structure of the carbon products was confirmed by several spectroscopic techniques. The photo-curing process, mechanical properties, swelling degree, adsorption efficiency towards cationic contaminants and recycling efficiency of the produced hydrogel nanocomposites containing different amounts of nGO or M-nGO were evaluated. Rapid photo-curing was demonstrated for all studied compositions. However, the shielding effect caused by the addition of aromatic nGO increased the time required for reaching gel point (8.5–19.5 s, instead of 4.8 s for pure M-CMC). This was partially compensated by the addition of M-nGO, that could participate in the photo-curing process. The photo-cured nanocomposites, M-CMC/nGO and M-CMC/M-nGO, demonstrated good mechanical properties, extremely high swelling degrees, outstanding adsorption capacity (up to 350 and 145 mg/g for MB and Cu(II) adsorption, respectively) and very good recyclability for at least 3 cycles. The designed “all-lignocellulose” derived hydrogel nanocomposites are, thus, promising candidates for wastewater purification to ensure access to clean water. 

Carbon-based adsorbents possess exceptional adsorption capability, making them an ideal platform for the remediation of environmental contaminants. Here, we demonstrate carbonized lignosulfonate (LS)-based porous nanocomposites with excellent adsorption performance towards heavy metal ions and cationic dye pollutants. Through microwave-assisted hydrothermal carbonization, a green approach was employed to carbonize lignosulfonate to carbon spheres. The LS-derived carbon spheres were then oxidized into nanographene oxide (nGO) carbon dots. A facile two-step procedure that involved the self-assembly of nGO and gelatin into a hydrogel precursor coupled with freeze-drying enabled the construction of three-dimensional (3D) free-standing porous composites without the use of organic solvents or chemical crosslinking agents. The favorable pore structure and abundance of surface functional groups on the nGO/gelatin porous composite proved to substantially facilitate the adsorption of Cu(II) in comparison to conventionally-used activated carbon. Further enhancement of adsorption performance was achieved by introducing additional surface functional groups through a non-covalent functionalization of the porous composite with lignosulfonate. The presence of negatively-charged sulfonate groups increased the Cu(II) equilibrium adsorption capacity (66 mg/g) by 24% in comparison to the non-functionalized nGO/gelatin counterpart. Both functionalized and non-functionalized composites exhibited significantly faster adsorption rates (40 min) compared to many graphene- or GO-based adsorbents reported in literature. In addition to the adsorption of heavy metal ions, the composites also demonstrated good adsorption capacity towards cationic dyes such as methylene blue. This paves the way for a high value-added application of lignin in environmental remediation and opens up new possibilities for the development of sustainable materials for adsorption and water purification.