Synthesis and Characterization of Bio-based Epoxy Resins Derived from Vanillyl Alcohol

Abstract

Vanillyl alcohol, a lignin derived aromatic diol, is a potential platform chemical for the production of renewable epoxy thermosets. A bio-based bisphenolic analogue, bisguaiacol (BG), was synthesized via electrophilic aromatic condensation of vanillyl alcohol and guaiacol, from which diglycidyl ether of BG (DGEBG) was prepared. In addition, three single aromatic diglycidyl ethers were synthesized from vanillyl alcohol (DGEVA), gastrodigenin (DGEGD), and hydroquinone (DGEHQ). All epoxies were characterized via 1H NMR, 13C NMR, FTIR, MS, and GPC. Dynamic mechanical analyses of the epoxies by themselves or blended with a commercial epoxy resin and cured with a diamine were conducted to determine the effect of the methoxy and methylene moieties on polymer properties. The thermomechanical results indicate that the methoxy lowers the glass transition temperature (T g) yet increases the glassy storage modulus at 25 °C, while the methylene spacer between the aromatic ring and the epoxide further lowers the T g in cured epoxy–amine systems.

Preparation and Characterization of Highly Bio‐Based Epoxy Amine Thermosets Derived from Lignocellulosics

“…Multiple titration trials were performed for each monomer species. EEW values for each purified epoxy monomer are summarized in Table and are in agreement with those found in literature …”

“…The EPON828‐DFDA thermoset is expected to have the highest T g based on this information from previous results. The slight difference in the T g of the DGEBA‐DFDA sample compared with results from a previous study by Hu et al may be due to instrumental differences and sample mounting variation on the DMA . Likewise, the combination of methylene spacer between the aromatic ring and epoxy moiety as well as the presence of the methoxy functionality in DGEVA results in cured resins with the lowest T g .…”

“…After curing, the oxirane and amine signals are observed to decrease (due to chemical reaction with each other), while a broad hydroxyl absorption around 6500–7000 cm −1 is seen to increase as residual hydroxyl groups are formed during curing . The area under the oxirane absorption at 4530 cm −1 resolves clearly from other neighboring signals and was used to quantify the extent of cure according to Equation , where peaks ≈6000 cm −1 were used as internal references . Within the limits of NIR spectrometry, all resins were >99% cured with respect to the epoxy functionality.…”

“…The lower T g of all the cured resins containing lignin‐derived epoxy monomers compared to the EPON828‐containing cured resin is likely explained by a combination of factors regarding molecular structures. A comprehensive structure–property relationship study previously performed by Hernandez et al determined that the methylene spacer between two aromatic rings (as in DGEBG‐containing resins) and between aromatic ring and epoxy functionality (as in DGEVA‐ and DGEGd‐containing resins) decreases T g and modulus while the presence of methoxy substituents was shown to decrease T g but increase glassy modulus. The modulus increase observed in species containing methoxy groups is suspected to be caused by hydrogen bonding between hydroxyls formed during epoxy and amine reaction and the methoxyl oxygen.…”

“…Previous work by Hernandez et al has shown that a lignin‐derived bisphenolic epoxy resin synthesized via condensation of vanillyl alcohol and guaiacol can produce cured resins with good thermomechanical properties, such as moderate T g 's and high moduli at 25 °C . Other potentially lignin‐derived phenols, such as hydroquinone, vanillyl alcohol, and gastrodigenin, demonstrated respectable thermomechanical properties when used in the epoxy and amine resins.…”