Abstract
Here we investigate the relationship between thermomechanical properties and chemical structure of well-characterized lignin-based epoxy resins. For this purpose, technical lignins from eucalyptus and spruce, obtained from the Kraft process, were used. The choice of lignins was based on the expected differences in molecular structure. The lignins were then refined by solvent fractionation, and three fractions with comparable molecular weights were selected to reduce effects of molar mass on the properties of the final thermoset resins. Consequently, any differences in thermomechanical properties are expected to correlate with molecular structure differences between the lignins. Oxirane moieties were selectively introduced to the refined fractions, and the resulting lignin epoxides were subsequently cross-linked with two commercially available polyether diamines (Mn = 2000 and 400) to obtain lignin-based epoxy resins. Molecular-scale characterization of the refined lignins and their derivatives were performed by 31P NMR, 2D-NMR, and DSC methods to obtain the detailed chemical structure of original and derivatized lignins. The thermosets were studied by DSC, DMA, and tensile tests and demonstrated diverse thermomechanical properties attributed to structural components in lignin and selected amine cross-linker. An epoxy resin with a lignin content of 66% showed a Tg of 79 °C from DMA, Young’s modulus of 1.7 GPa, tensile strength of 66 MPa, and strain to failure of 8%. The effect of molecular lignin structure on thermomechanical properties was analyzed, finding significant differences between the rigid guaiacyl units in spruce lignin compared with sinapyl units in eucalyptus lignin. The methodology points toward rational design of molecularly tailored lignin-based thermosets.
Highlights
Lignin is one of the three main constituents of wood along with cellulose and hemicellulose
Data were processed with MestreNova (Mestrelab Research) using 90° shifted square sine-bell apodization window; baseline and phase correction were applied in both directions. 31P Nuclear magnetic resonance (NMR) samples were prepared and analyzed according to the procedure reported by Argyropolous in 1994
Kraft lignin derived from two different sources, eucalyptus and spruce, were used. The rationale for this choice was that these two species are expected to produce structurally different Kraft lignins based on their monolignol compositions
Summary
Lignin is one of the three main constituents of wood along with cellulose and hemicellulose. It is the second most abundant biopolymer and most abundant natural aromatic compound.[1,2] Its biological purpose includes provision of protection and mechanical stability to the plant cell wall and forms a biocomposite with cellulose fibrils and hemicellulose.[3]. Depending on the plant species, the monolignol composition of lignin will be different and will the structure of lignin. The plant cell is able to control the composition of the monomer feed to tune the mechanical properties of the resulting wood tissue.[4]
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