Research Article pubs.acs.org/journal/ascecg
Biorenewable Epoxy Resins Derived from Plant-Based Phenolic Acids Guozhen Yang, Brian J. Rohde, Hiruy Tesefay, and Megan L. Robertson* Department of Chemical and Biomolecular Engineering, University of Houston, 4726 Calhoun Road, Houston, Texas 77204-4004, United States S Supporting Information *
ABSTRACT: Plant-derived phenolic acids are attractive substitutes for petroleum sources for the derivation of polymers, due to their rigid aromatic rings and chemical groups amenable to functionalization. Difunctional phenolic acids were investigated as replacements for the diglycidyl ether of bisphenol A (DGEBA) in anhydride-cured epoxy resins. Functionalization of each phenolic acid was carried out through allylation, followed by epoxidation. Epoxy resins were synthesized through reaction of either epoxidized salicylic acid (ESA) or epoxidized 4-hydroxybenzoic acid (E4HBA) with the curing agent methylhexahydrophthalic anhydride (MHHPA) (catalyzed by 1-methyl-imidazole, 1-MI). The MHHPA anhydride curing agent (catalyzed by 1-MI) was chosen due to the resulting high conversion and advantageous high polymer glass transition temperature. ESA and E4HBA had similar curing behavior to that of DGEBA when cured with MHHPA. A two-step protocol was developed to avoid monomer evaporation and polymer vitrification during curing. ESA- and E4HBA-based epoxy resins exhibited comparable tensile moduli and strengths relative to a conventional DGEBA-based epoxy resin. E4HBA- and DGEBA-based epoxy resins (with para placement of functional groups) fractured at comparable elongation at break values, which were higher than that of the ESAbased epoxy resin (with ortho placement of functional groups). Epoxidized difunctional phenolic acids were found to be nontoxic and renewably sourced replacements for DGEBA in epoxy resins, producing epoxy resins of high modulus, high glass transition temperature, and elongation at break (in the case of E4HBA) comparable to a conventional DGEBA-based epoxy resin. KEYWORDS: Renewable resource polymers, Phenolic acids, Epoxy resins, Thermosets, Sustainability
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INTRODUCTION Epoxy resins are widely applied in composites, coatings, adhesives, automotive components, and other applications, due to their superior chemical, electrical, and heat resistance, adhesion, and mechanical properties.1 Nowadays epoxy resins also play an important role in wind power, a renewable energy source and attractive alternative to fossil fuel.2 Traditional epoxy resins are derived from petroleum, which produces harmful environmental impacts when processed. Additionally, there are potential health impacts from residual monomers and additives in polymers, and traditional epoxy resins are derived from bisphenol A, a chemical that has received much attention due to negative health consequences.3 Therefore, it is a worthwhile goal to fabricate sustainable epoxy resins from renewable, nontoxic feedstocks. Prior studies have investigated sustainable replacements for traditional epoxy resin components.4−6 The incorporation of vegetable oils into epoxy resins has been a recent focus in the literature.7,8 The vegetable oil-containing epoxy resins exhibit a higher fracture toughness and impact strength,9−11 with a corresponding decrease in the glass transition temperature,12 due to a decrease in the cross-link density and increase in chain flexibility. Epoxidized natural rubber is another soft material that can be incorporated into epoxy resins, yet adversely impacts the thermal and mechanical properties.13 © XXXX American Chemical Society
Other raw material sources are also appropriate for the derivation of epoxy resins. Isosorbide, a glucose-derived molecule, has attracted recent attention due to its rigid structure and the presence of hydroxyl groups which are amendable to conversion to the epoxide groups required for the epoxy resin synthesis. Though isosorbide-based epoxy resins have desirable attributes,14−16 they also exhibit significant water-uptake relative to conventional epoxy resins.15,17 Furans (also derived from plant sugars and polysaccharides) can be functionalized with carboxylic acids which can be converted to epoxides or amines (such as 2,5-furan-dicarboxyl acid).18 Additionally, rosins, obtained from sources such as pine trees and other conifers,19,20 lignin,21−23 cellulose,24 and other plantsourced molecules,25−27 can be used to synthesize epoxy resin components. Here, difunctional epoxidized phenolic acids are explored as replacements for DGEBA in epoxy resins. Phenolic acids are produced by a diverse array of plant sources, including agricultural waste products such as skins, seeds, and leaves of fruits and vegetables.28−31 Epoxidized phenolic acids are of particular interest in epoxy resin applications, as their rigid aromatic rings are anticipated to produce polymers of high Received: June 14, 2016 Revised: August 25, 2016
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DOI: 10.1021/acssuschemeng.6b01343 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering modulus and glass transition, similar to DGEBA-based epoxy resins. Their functional groups (hydroxyl and carboxyl groups) are readily converted to epoxide groups. Few studies in the literature have examined phenolic acids as sources for epoxy resin components. Prior studies probed the physical behavior of epoxy resins derived from epoxidized gallic acid (containing four epoxide groups).32−36 An array of multifunctional epoxy monomers derived from 4-hydroxybenzoic acid were previously explored.37 Additionally, salicylic acid has been employed as an accelerator or modifier in epoxy resin syntheses with traditional components.38−41 In this paper, dif unctional epoxidized phenolic acids were selected to mimic the functionality of the traditional DGEBA monomer and replace DGEBA in epoxy resins. We present the synthesis and thermal and mechanical properties of epoxy resins derived from two difunctional epoxidized phenolic acids: epoxidized salicylic acid and epoxidized 4-hydroxybenzoic acid. Salicylic acid is found in a wide range of fruits and vegetables (and their waste products),42 and 4-hydroxybenzoic acid is found in coconut husks.43 The physical properties of these biorenewable epoxy resins are benchmarked to that of a conventional DGEBAbased epoxy resin, based on our control experiments using DGEBA at similar curing conditions. This work explores strategies for developing sustainable epoxy resins with thermal and mechanical behaviors that are competitive to traditional materials.
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EXPERIMENTAL DETAILS
Materials. All chemicals were purchased from Sigma-Aldrich unless otherwise noted below. Two phenolic acids were used in this study: salicylic acid (SA, ≥ 99%, FG/Halal/Kosher) and 4-hydroxybenzoic acid (4HBA, 99%, ReagentPlus). The chemical structures of both phenolic acids are shown in Figure 1. Multiple curing agents and
Figure 2. Chemical structures of curing agents used in this study. Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra were recorded on a Thermo Scientific Nicolet 4700 spectrometer in transmission mode as well as with an attenuated total reflection (ATR) stage (containing a germanium crystal). The OMNIC Series software was used to follow selected peaks at 1.928 cm−1 resolution using 32 scans. FTIR spectra were collected on epoxidized monomers and epoxy networks. Monomer Synthesis. We have reported the procedures for allylation of SA and 4HBA in a prior publication.44 Phenolic acid (10.0 g, 72.4 mmol) was dissolved into 340 mL of DMF in a 1000 mL glass round-bottom flask equipped with a rubber septum and a magnetic stirring bar. The temperature was maintained at 0 °C using an ice bath. K2CO3 (22.0 g, 159 mmol) was added to the flask (the molar ratio of K2CO3 to phenolic acid was 2.20 to 1.00). After 3 min of stirring, allyl bromide (19.3 g, 159 mmol) was added dropwise with a syringe (the molar ratio of allyl bromide to the phenolic acid was 2.20 to 1.00). The solution was stirred at room temperature for 48 h. Distilled water (340 mL) was added, and the product was isolated by extracting with ethyl acetate (3×), washing with saturated brine, drying over MgSO4, and concentrating in vacuo, followed by drying in a vacuum oven at 50 °C, until the NMR peaks associated with DMF (7.96 ppm, 2.94 ppm, 2.78 ppm) were not observed. Characterization data for allylated SA and 4HBA were reported in ref 44. The epoxidation of allylated phenolic acid (SA and 4HBA) was conducted following literature procedures for conversion of the allyl groups to epoxide groups.32 Allylated phenolic acid (5.00 g, 22.9 mmol) was dissolved into 500 mL of chloroform in a 1000 mL glass round-bottom flask equipped with a rubber septum and a magnetic stirring bar. mCPBA (31.6 g, 183 mmol) was added to the flask (molar ratio of mCPBA to allylated phenolic acid was 8.00 to 1.00). The solution was stirred at 40 °C for 24 h. Next, the solution was washed with an equivalent volume of a 10% (wt/v) Na2SO3 aqueous solution and recovered using a separatory funnel. The organic phase was then washed with an equivalent volume of a saturated NaHCO3 aqueous solution, and recovered using a separatory funnel. Finally, the organic phase was washed with an equivalent volume of distilled water, and
Figure 1. Chemical structures of phenolic acids used in this study: (a) salicylic acid (SA) and (b) 4-hydroxybenzoic acid (4HBA). catalysts were used: methylhexahydrophthalic anhydride (MHHPA, Huntsman, Aradur HY 1102, ≥99%), 1-methyl-imidazole (1-MI, Huntsman, Accelerator DY 070), nadic methyl anhydride (NMA, ≥95%), 2,4,6-tris(dimethylaminomethyl) phenol (Ancamine K54, Air Products,
