Lignin-Based Epoxy Resins: Unravelling the Relationship between Structure and Material Properties

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 31 P 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.

Choosing the Right Lignin to Fully Replace Bisphenol A in Epoxy Resin Formulation

“…In addition, the average numbers of epoxy groups ( ) in each macromolecule [epoxy content (mol g −1 )× M n ] are given in Table 2. The reactivities of hydroxy (OH) functional groups in lignin toward ECH, in decreasing order, are phenolic‐OH>carboxylic acid>aliphatic‐OH [30] . It has been reported that the phenol epoxidation mechanism has three steps [51] .…”

“…These attributes cause lignin to have lower reactivity toward ECH than BPA and possibly result in resin with lower homogeneity. Lignin can be incorporated into epoxy resin via three different methods: 1) blending with petroleum‐based epoxy resin, [24,25] 2) modification of lignin followed by epoxidation, [23,26–30] and 3) epoxidation of unmodified lignin [16,31,32] . Although many studies have focused on utilizing lignin in epoxy resin, [23,27,33,34] they mostly used modified lignin (fractionated or lignin monomers) [23,27,29] .…”

Toward a Consolidated Lignin Biorefinery: Preserving the Lignin Structure through Additive‐Free Protection Strategies

“…The lignin fraction could be used directly as a polymer precursor for material synthesis or catalytically depolymerized to platform monomers. Based on their functionality, structure, low DP n and demonstrably narrow Đ , these lignins might be suitable as polymer precursors for the synthesis of thermosetting resins, as shown in recent studies . In those cited studies, oligomeric fractions were shown to be preferable to polymeric fractions for the synthesis of homogeneous materials, due to their mutual solubility with other chemical components used in the synthesis.…”

Biochar as an Effective Filler of Carbon Fiber Reinforced Bio-Epoxy Composites

“…They may occur in the form of powder, microparticles or nanoparticles, or long and short fibers [2][3][4]. The use of such modifiers as biochar [5][6][7], carbon nanotubes [8][9][10], basalt powder [11][12][13][14], lignin [15,16], and ground waste from the agricultural [17][18][19] and food industries [20][21][22] in polymer composites has been the subject of many scientific papers in recent years. A significant change in the properties of the polymer matrix is obtained by using additives of various shapes in the form of particles and fibers.…”