No abstract

The synthesis of novel epoxy resins from lignin hydrogenolysis products is reported. Native lignin in pine wood was depolymerized by mild hydrogenolysis to give an oil product that was reacted with epichlorohydrin to give epoxy prepolymers. These were blended with bisphenol A diglycidyl ether or glycerol diglycidyl ether and cured with diethylenetriamine or isophorone diamine. The key novelty of this work lies in using the inherent properties of the native lignin in preparing new biobased epoxy resins. The lignin-derived epoxy prepolymers could be used to replace 25-75% of the bisphenol A diglycidyl ether equivalent, leading to increases of up to 52% in the flexural modulus and up to 38% in the flexural strength. Improvements in the flexural strength were attributed to the oligomeric products present in the lignin hydrogenolysis oil. These results indicate lignin hydrogenolysis products have potential as sustainable biobased polyols in the synthesis of high performance epoxy resins.

Producing the next generation of thermoset polymers from renewable sources is an important sustainability goal. Hydrogenolysis of pinewood lignin was scaled up for the first time from lab scale to a 50 L pilot-scale reactor, producing a range of depolymerized lignin oils under different conditions. These lignin hydrogenolysis oils were glycidylated, blended with bisphenol A diglycidyl ether, and cured to give epoxy thermoset polymers. The thermal and mechanical properties of the epoxy polymers were assessed by differential scanning calorimetry, thermogravimetric analysis, flexural testing, and dynamic mechanical thermal analysis. Replacing up to 67% of the bisphenol A epoxy with the lignin oil epoxies resulted in cured epoxy polymers with improvements of up to 25% in flexural stiffness and strength. Considerable scope exists in simplifying and scaling up the hydrogenolysis process to produce depolymerized lignins that can substitute established petrochemicals in the quest for renewable high-performance thermoset polymers.

Sequential extraction method was applied to lignins from hardwood and softwood isolated by kraft and VTT organosolv processes. Solvent extraction was found to fractionate lignin according to the molecular weight: small molecular weight lignin is dissolved in the organic solvents and the lignin with higher molecular weight is enriched into the residue. Isolated acetone fractions of lignin are more homogeneous with narrow molecular weight distributions. Based on the 31 P NMR results, both total hydroxyl content and the content of phenolic hydroxyl units are higher in the acetone fraction than in the residue. Pyrolysis-GC/MS of all lignins showed that p-hydroxy phenols are enriched to the residues. Preferential dissolution of syringyl type lignin in acetone was observed for hardwood kraft lignin, whereas the opposite behavior was seen for the hardwood organosolv lignin. Glass transition temperatures of all acetone soluble fractions were notably low compared to starting and residue lignins, which gives possibilities for future applications as a material with specific properties.

Lignin is the most abundant source of renewable ready-made aromatic chemicals for making sustainable polymers. However, the structural heterogeneity, high polydispersity, limited chemical functionality and solubility of most technical lignins makes them challenging to use in developing new bio-based polymers. Recently, greater focus has been given to developing polymers from low molecular weight lignin-based building blocks such as lignin monomers or lignin-derived bio-oils that can be obtained by chemical depolymerization of lignins. Lignin monomers or bio-oils have additional hydroxyl functionality, are more homogeneous and can lead to higher levels of lignin substitution for non-renewables in polymer formulations. These potential polymer feed stocks, however, present their own challenges in terms of production (i.e., yields and separation), pre-polymerization reactions and processability. This review provides an overview of recent developments on polymeric materials produced from lignin-based model compounds and depolymerized lignin bio-oils with a focus on thermosetting materials. Particular emphasis is given to epoxy resins, polyurethanes and phenol-formaldehyde resins as this is where the research shows the greatest overlap between the model compounds and bio-oils. The common goal of the research is the development of new economically viable strategies for using lignin as a replacement for petroleum-derived chemicals in aromatic-based polymers.

Biobased epoxy thermoset polymers were prepared from lignin hydrogenolysis oils produced from native hardwood lignin. Native lignin in Eucalyptus nitens and Eucalyptus saligna wood was reacted in situ under Pd-catalyzed mild hydrogenolysis conditions to give depolymerized lignin oils in yields up to 98 wt %. Reacting these lignin oils with epichlorohydrin produced biobased epoxy resins. Blending these resins with nonrenewable bisphenol A diglycidyl ether (BADGE) in different proportions, and curing with diethylenetriamine, produced a series of epoxy thermoset polymers with varying biobased content. Up to 67% of the BADGE could be replaced with hardwood lignin-derived epoxy resins while achieving superior or equivalent mechanical properties to the BADGE control polymer. Comparing the performance of lignin-based epoxy polymers from eucalyptus and pine wood provided insights into the advantages and disadvantages of using hardwood versus softwood native lignins in the quest for high performance biobased thermoset polymers.

Succinate esters of three lignin materials were prepared by reactive extrusion. Reactive extrusion was developed as a facile route to chemically modify lignin with the benefits of being a scalable, solvent-free, and economic process. Kraft lignin and crude and purified enzyme saccharification lignins were reactively extruded with varying amounts of succinic anhydride. The resulting products were characterized chemically and thermally. Succinylation occurred preferentially at the aliphatic hydroxyl groups of kraft lignin, with phenolic substitution occurring at higher amounts of succinic anhydride addition. The degree of substitution was dependent on the relative amounts of succinic anhydride and lignin substrate. Thermal stability of the products decreased with increasing levels of substitution, primarily due to the loss of succinate groups on heating. The degree of substitution influenced the glass transition temperature, which reflected the melt flow and solubility of the products.

Mild hydrogenolysis has been compared with thioacidolysis as a method for degrading lignins in situ and in isolated form before analysis by gas chromato graphy/ mass spectrometry and quantitative 31 P nuclear magnetic resonance (NMR) spectroscopy. Both degradation methods gave similar levels of β-aryl ether-linked phenylpropane units that were released as monomers. Degradation by hydrogenolysis generally gave lower levels of total phenylpropane units when analyzed by 31 P NMR, especially in the case of lignins with high levels of condensed units. Overall, these results indicate that mild hydrogenolysis could offer an alternative to thioacidolysis for probing lignin structure.