Wood‐derived phenol novolaks and their wood/epoxy biocomposites

Abu‐Omar

2017

135

An approach to lignin−based epoxy networks from both organosolv lignin and lignin derived phenol (dihydroeugenol) are developed using multiple chemical modifications including demethylation, phenolation and phenol−formaldehyde reaction. Structures of lignin incorporated novolac polyphenols (LINPs) and epoxy networks (LINENs) were characterized using proton nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy. Compared to a common synthesis route in which lignin was epoxidized prior to blend with comonomers (LBEN), LINEN derivatives exhibited improved crosslink density (ρ), α−relaxation temperature (T α ) and storage modulus in glassy region (E g ') as obtained from dynamic mechanical analysis (DMA), and increased thermal stability measured by thermogravimetric analysis (TGA). This study widens the routes of lignin modification.Renewable epoxy networks derived from both lignin and lignin derivatives are economically and ecologically attractive.However, steric hindrance and poor dispersion of modified lignin impair covalent crosslinking. 12,20 Another chemical modification is to make covalent links prior to epoxidation and curing. For example, LDPs can be grafted onto lignin through phenolation, substituting lignin aliphatic hydroxyl by ortho/para−bound LDPs. The grafted LDPs do not only improve lignin's compatibility, they also introduce additional sites (ortho and para), which can be used in building up cross−linked networks.In this study, organosolv lignin and LDPs are deployed through chemical modifications as illustrated in Figure 1. First, lignin's aromatic methoxy groups are deprotected, 21 followed by addition of deprotected dihydroeugenol (DHEO), which can be obtained from catalytic depolymerization of lignin (CDL). 22 As depicted in Figure 1, deprotected lignin (DL) is reacted with excess amount of DHEO under acid catalysis to yield phenolated lignin (PL). Formaldehyde solution is subsequently added to make a deprotected lignin incorporated novolac polyphenol (DLINP). The latter is reacted with epichlorohydrin to yield epoxy monomer that can be cured with diethylenetriamine (DETA) to make deprotected lignin incorporated novolac epoxy network (DLINEN). A comparison between the proposed synthesis routes with direct epoxidation of lignin (or deprotected lignin, DL) followed by blending with epoxidized DHEO novolac oligomer (DLBEN) is presented in Figure 2.

Section: Resultssupporting

confidence: 87%

“…The integration ratio of H d /H b is around 0.57, indicating the novolac oligomer contains mainly dimer and trimer. This is in agreement with the novolac product of wood-tar creosote, 24 The structures of DHEO-NOVO and LINP-5 were also examined using infrared (IR) spectroscopy. Glycidyl Ether of Lignin-Based Polyphenols.…”

Section: ■ Results and Discussionsupporting

confidence: 54%

Synthesis of Renewable Thermoset Polymers through Successive Lignin Modification Using Lignin-Derived Phenols

Abu‐Omar

2017

135

“…Recent catalytic depolymerization techniques can convert lignin to various value-added phenolic monomers including phenols and aldehydes. − Lignin-derived phenol monomers (LDPMs) have been extensively studied as BPA alternatives. Because cross-linkable epoxy monomers require at least two epoxides per molecule, special efforts have been taken to increase the number of functional hydroxyl groups of LDPMs through (1) conversion of other reactive groups like methoxy, double bond, or aldehyde to hydroxyl groups, − and (2) coupling repeated LDPMs using bridging reagents. − Despite the abundance of LDPMs and their coupling methods, lignin-derived aldehyde monomers (LDAMs) are often neglected as renewable bridging reagents (akin to acetone in the preparation of BPA). A few examples include the work of Foyer et al, in which a synthesis route of formaldehyde-free resol resin based on vanillin was reported. , However, the hydroxyl group of vanillin had to be protected prior to polymerization because the hydroxyl of vanillin was deprotonated preferentially over phenols.…”

Section: Introductionmentioning

confidence: 99%

Renewable Epoxy Thermosets from Fully Lignin-Derived Triphenols

Huang

Whelton

et al. 2018

A series of fully renewable triphenols (TPs) with various number of methoxy group substituents (n = 0–6) were synthesized using lignin-derived phenols (guaiacol and 2,6-dimethoxyphenol) and aldehydes (4-hydroxybenzaldehyde, vanillin, and syringaldehyde). The structural evolution from TPs to epoxy thermosets was followed by nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy. Thermomechanical properties of the resulting epoxy thermosets were investigated by dynamic mechanical analysis (DMA), tensile analysis (TA), and thermogravimetric analysis (TGA). Increasing the content of methoxy groups decreased the glass transition temperature (132–118 °C) and glassy modulus (2.7–2.2 GPa). Thermal stability of high-methoxy-content thermosets was reduced due to electron-donating effects and higher oxygen content. Conversions and isolated yields of TPs significantly decreased as the number of methoxy substituents increased, which markedly determined the feasibility of TPs as precursors for polymers. This work widens the synthesis route of fully lignin-derived polyphenols, yielding polymers with thermomechanical properties comparable to bisphenol A (BPA) based materials. Evaluation of methoxy substitution provides insight for the selection of lignin-derived monomers.

“…Improved reactivity of lignin-derived monomers rather than bulk lignin provides thermosets that exhibit more satisfactory mechanical and thermal properties. Moreover, reactivity of lignin-derived monomers can be further increased through molecular reaction design such as demethylation, methacrylation or bridging monomers into oligomers through formaldehyde chemistry …”

Section: Introductionmentioning

confidence: 99%

Renewable Epoxy Networks Derived from Lignin-Based Monomers: Effect of Cross-Linking Density

Abu‐Omar

2016

155

123

Three modification methods, which either improved molecular weight, orientation or the number of functional groups, were employed to increase the cross-linking density of biobased epoxy networks based on 2methoxy-4-propylphenol (dihydroeugenol, DHE). The modifications were realized through o-demethylation and phenol−formaldehyde reactions. Structures of DHE-based monomers and cured networks were characterized by NMR and FTIR spectroscopy. Cross-linking densities of cured networks were calculated using rubber elasticity theory from dynamic mechanical analysis (DMA). Networks with higher cross-link density were found to exhibit greater mechanical and thermal performance as measured by DMA and thermogravimetric analysis. GEDHEO-NOVO, an epoxy monomer featuring all three modification processes, exhibited significant improvements in cross-linking density (0.39 to 9.77 mol/dm 3 ), α-relaxation temperature (T α , 40 to 139 °C) and statistic heat-resistant index temperature (T s , 125 to 153 °C) compared to the unmodified DHEO-based networks.