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Bio-based Epoxy Resins from Deconstructed Native Softwood Lignin Daniel Jason van de Pas, and Kirk M. Torr Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.7b00767 • Publication Date (Web): 06 Jul 2017 Downloaded from http://pubs.acs.org on July 7, 2017

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Bio-based Epoxy Resins from Deconstructed Native Softwood Lignin Daniel J. van de Pas* and Kirk M. Torr Scion, Private Bag 3020, Rotorua 3046, New Zealand Keywords: Lignin, Hydrogenolysis, Depolymerization, Epoxidation, Epoxy resins, Biopolymers

ABSTRACT: The synthesis of novel epoxy resins from lignin hydrogenolysis products is reported. Native lignin in pine wood was depolymerized by mild hydrogenolysis to give an oil product that was reacted with epichlorohydrin to give epoxy prepolymers. These were blended with bisphenol A diglycidyl ether or glycerol diglycidyl ether and cured with diethylenetriamine or isophorone diamine. The key novelty of this work lies in using the inherent properties of the native lignin in preparing new bio-based epoxy resins. The lignin-derived epoxy prepolymers could be used to replace 25-75% of the bisphenol A diglycidyl ether equivalent, leading to increases of up to 52% in the flexural modulus and up to 38% in the flexural strength. Improvements in the flexural strength were attributed to the oligomeric products present in the lignin hydrogenolysis oil. These results indicate lignin hydrogenolysis products have potential as sustainable bio-based polyols in the synthesis of high performance epoxy resins.

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INTRODUCTION With the growth of the bio-based economy, the chemical sector is increasingly looking towards using biomass-derived molecules and materials with new functionality, enhanced performance attributes, and low carbon footprints. This has fueled interest in replacing non-renewable polymers, or components thereof, with sustainable alternatives. Significant advances have been made in developing both bio-based thermoplastics1,2 and thermosets.3 For thermosets, aromatic intermediates are favored for their structural rigidity as they impart desirable mechanical and thermal properties to the material.4-6 The availability of petroleum-derived aromatic compounds could decline in the future as petrochemical feedstocks shift away from traditional crude oil to shale gas and lighter tight oils that contain less aromatics.7 Epoxy resins represent a major class of thermosetting resins that demand a higher price than other polymers. Epoxy resins are versatile thermosets because they can be combined with a wide range of curing agents. This enables them to be tailored to suit a diverse range of applications such as coatings, adhesives, composites, and electrical encapsulation.8 The most commonly used epoxy prepolymer is bisphenol A diglycidyl ether (BADGE), which is commercially produced from bisphenol A and epichlorohydrin. Whilst a commercial route exists for the production of bio-based epichlorohydrin9, there is considerable interest in replacing bisphenol A with biobased polyol alternatives that are equally capable of delivering a high performance resin. Alternatives for bisphenol A are also being sought on health and environmental grounds as it is a known endocrine disrupting compound that can potentially leach from BADGE-based epoxy resins used in food applications.10 A number of bio-based alternatives to bisphenol A have been investigated.3,9,11-13 Examples of bio-based epoxy prepolymers include epoxidized cardanol14, and glycidylation products of

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isosorbide15, gallic acid16 and lignin4,6,17. Lignin is the only renewable aromatic polymer that is available in larger quantities at low cost that offers a potential source of ready-made aromatic polyols. Lignin is a complex aromatic polymer derived from p-coumaryl alcohol, coniferyl alcohol (G units) or sinapyl alcohol (S units) building blocks, the proportions dictated by the type of lignocellulosic species.18 The C9 phenylpropanoid lignin units are linked together in the polymer by a variety of different linkages, the most abundant being the β-O-4 ether linkage.18 A number of strategies have been used for incorporating lignin as a polyol precursor in epoxy resins. Lignin can be blended and reacted with epoxy prepolymers such as BADGE.6,19,20 Lignin can also be reacted with epichlorohydrin to produce an epoxy prepolymer that can then be crosslinked with curing agents.4,6 However, using lignin directly can be challenging due to its poor solubility and reactivity.21 Chemical modification of lignin (e.g. phenolation, hydroxypropylation) has been effective in addressing these limitations.4,6,17 In recent years there has been renewed interest in developing epoxy resins from lignin.22-26 A new approach to producing lignin-derived polyols is to depolymerize the lignin to lower molecular weight compounds. There have been several reports on producing epoxy resins from lignin model compounds. Examples include vanillin and derivatives thereof27-29, vanillyl alcohol derivatives30, phenol31, isoeugenol32, syringaresinol33, and compounds based on propyl guaiacol and its demethylated product.34-36 Model compounds are useful in assessing the potential of lignin in materials applications, however it is important that more structurally relevant and readily available lignin streams be utilized in developing new polymers and materials. Zhao and Abu-Omar (2017), for example, combined demethylated lignin with a glycidylated derivative of dihydroeugenol to produce lignin-containing epoxy resins.37 Processed lignins such as Kraft and organosolv lignins have been depolymerized and used to produce epoxy resins.23,24,38,39 The

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chemical structures of processed lignins are often heavily modified compared to native lignins due to depolymerization (cleavage of C-O bonds) and repolymerization (formation of C-C bonds) reactions that occur during extraction of the lignin from biomass. Utilizing native lignin within biomass is an important emerging strategy in the biorefinery concept. In this approach, lignin is fractionated first from the lignocellulosic matrix by catalytic upstream processing to produce a solvent-soluble depolymerized lignin oil and a holocellulose-enriched residue.18,40,41 We have previously reported that mild hydrogenolysis can depolymerize lignin in situ in pine wood and an isolated native lignin into a mixture of phenolic monomers, dimers and oligomers.42,43 Mild hydrogenolysis selectively cleaves the β-O-4 and α-O-4 ether linkages and stabilizes the lignin fragments against repolymerization through catalytic reduction. In recent years, there has been increased interest in lignin hydrogenolysis as a pathway to valuable low molecular weight chemicals from lignin.40,41,44-47 Most approaches use noble metal or Ni-based catalysts and have focused on maximizing yields of monomeric products. In this regard, lignins rich in S units and with high β-O-4 contents, such as those found in hardwoods, are preferred. Mild hydrogenolysis of softwood lignins, which are mainly composed of G units, give lower yields of monomers and higher yields of oligomeric products.40,42 Research on utilizing hydrogenolysis products of native lignins to produce new bio-based materials is yet to be reported. In this study we describe the development of novel bio-based epoxy prepolymers from hydrogenolysis products derived from softwood lignin, and their use as replacements for BADGE in new epoxy thermosetting polymers. The novelty of this approach is that it takes advantage of the inherent properties of the native lignin in the wood before it is chemically altered as in conventional wood pulping processes.

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EXPERIMENTAL SECTION Materials. Ground, dry, extractive-free Pinus radiata wood was prepared as described previously.42 The Pd/C (5% Pd) catalyst, epichlorohydrin, BADGE with an epoxy equivalent weight (EEW) of 174 g/mol, glycerol diglycidyl ether (GDGE) with an EEW of 143 g/mol, phenyl glycidyl ether, diethylenetriamine (DETA) and isophorone diamine (IPDA) were purchased from Sigma-Aldrich. Hydrogenolysis. Hydrogenolysis reactions were carried out in a 450 mL limbo autoclave (Buchiglasuster, Switzerland) charged with wood (12.9 g) and Pd/C (1.03 g, 8% w/w of wood) in dioxane/water (1:1, 300 mL). The reactor was pressurized to 3.4 MPa with hydrogen and heated to 195 °C for 24 h with stirring at 750 rpm.42 After cooling, the contents of the reactor were filtered, extracted into dichloromethane and the extract was dried to constant weight to give the lignin hydrogenolysis oil product (LH, 2.9 g). This product was fractionated into a diethyl etherinsoluble fraction (LHO, ca. 50% (w/w)) and a diethyl ether-soluble fraction. Preparation

of

lignin

hydrogenolysis

epoxy

prepolymer

(LHEP).

The

lignin

hydrogenolysis product (8.22 g) was dissolved in refluxing epichlorohydrin (13.7 mL). A 20% aqueous solution of NaOH (1.40 g, 34.9 mmol) was added drop-wise to the reaction mixture over a 90 min period. During the reaction, water was removed by azeotropic distillation with epichlorohydrin using a modified dean-stark apparatus. The reaction was continued for a further 30 min, cooled, neutralized with phosphate buffer (pH 7.0, 0.3 M, 10 mL) and excess epichlorohydrin was removed. The product was extracted into dichloromethane and the solvent removed before dissolving in dioxane and treating with 20% aqueous NaOH. The final product was an amber-colored viscous oil (LHEP, ca. 88% yield w/w). The diethyl ether-insoluble

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fraction of the lignin hydrogenolysis product (LHO) was reacted in the same way to give the oligomeric lignin hydrogenolysis epoxy prepolymer (LHOEP, ca. 84% yield w/w). Chemical analysis. Analysis of the hydrogenolysis products by GCMS was performed as previously described.42 Gel permeation chromatography was performed on a PSS SDV Lux 1000Å column set eluting with tetrahydrofuran using a Knauser Smartline GPC system with UV detection at 254 nm. A calibration curve was constructed using 4-n-propyl guaiacol, dihydrodiisoeugenol, and polystyrene standards. 1H,

13

C,

31

P, and 2D HSQC NMR spectroscopy was

performed on a Bruker AVIII 400 MHz spectrometer equipped with a BBO 5 mm probe. Hydroxyl content was determined by quantitative

31

P NMR spectroscopy as described

previously.43 The EEW of the epoxy resins was determined by potentiometric titration following ASTM Standard D1652-04 scaled down to analyze 20 mg samples. Total chlorine content was determined by the Campbell Microanalytical Lab (Otago University, New Zealand).48 Hydrolysable chlorine content was determined by titration following ASTM Standard D1726-03 scaled down to analyze 25-50 mg samples. The viscosity of the products, as 40% solutions in diethylene glycol butyl ether, were measured at 25 °C using a Brookfield cone and plate viscometer. Preparation of cured epoxy specimens. LHEP or LHOEP were mixed with BADGE or GDGE in the desired proportions to give a total mass of 0.6 g. Phenyl glycidyl ether (60 mg) was added as a reactive diluent to all mixtures, unless indicated otherwise, and the components were mixed thoroughly with warming to ca. 40 °C. The EEW of the individual epoxy resin components was used to calculate the equimolar amount of curing agent required to cure the resin. The curing agent, either DETA or IPDA, was added and mixed thoroughly before centrifuging at 4,000 rpm for 1 min to remove air bubbles. The mixture was warmed to ca. 40 °C

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and transferred into an aluminum mold (40 x 5.7 x 0.9 mm), which was fixed between glass plates. Resins containing DETA or IPDA were cured at 40 °C and 80 °C, respectively for 24 h followed by a post-cure at 120 °C and 150 °C, respectively for 24 h. Thermal analysis and flexural testing. Glass transition temperature was determined by differential scanning calorimetry on a Q1000 instrument (TA Instruments, USA) by subjecting cured epoxy resins to a heat-cool-heat cycle from 0 °C to 200 °C at 10 °C/min under a nitrogen atmosphere. Cure behavior was determined using the same instrument by heating freshly prepared resins from 30 °C to 200 °C at 5 °C/min under a nitrogen atmosphere. Cured epoxy resins were analyzed on a Q500 thermogravimetric analyzer (TA Instruments, USA) to determine mass loss as a function of temperature by heating to 500 °C at 10 °C/min under a nitrogen atmosphere. The statistic heat-resistant index temperature (Ts) was calculated using T5% and T30% (temperature at 5% and 30% weight loss) according to the following equation: = 0.49 % + 0.6 % − %

(1)

Cured epoxy resins were conditioned at 23 °C and 50% relative humidity for at least 24 hrs. Three-point bending was performed using an RSA-G2 DMTA instrument (TA Instruments, USA). The support span length was 15 mm (span to thickness ratio: 16-17). The specimens were tested to failure, where possible. Average values and 95% confidence intervals were determined from at least four replicates, except for LHOEP/BADGE 1:9 and the IPDA-cured specimens that were tested in duplicate due to limited amounts of sample.

RESULTS AND DISCUSSION Hydrogenolysis. Native lignin in Pinus radiata wood was depolymerized by mild hydrogenolysis to a complex mixture of aromatic polyols composed of monomers, dimers and oligomers as described previously.42 In this study, scaling-up the reaction six-fold in a 450 mL Page 7 of 33


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reactor gave the lignin hydrogenolysis oil product (LH) in a yield of 78% w/w on starting lignin. Consistent with previous findings, the main monomers in the oil product were dihydroconiferyl alcohol (DCA, 4-(3-hydroxypropyl)-2-methoxyphenol) and 4-propyl guaiacol (PG, 4-propyl-2methoxyphenol) constituting approximately 22% and 3% w/w, respectively (Figure 1).42 Various dimers and oligomers of DCA and PG represented approximately 75% of the oil product. The oligomers ranged in degree of polymerization from 3-7 as determined by gel permeation chromatography (Figure 2). LH was fractionated by solubility in diethyl ether. The diethyl etherinsoluble fraction (LHO), which was enriched in oligomers (Figure 2B), made up 50% w/w of LH and was used along with LH to prepare epoxy resins. The weight average molecular weight (Mw) for LH and LHO was 667 and 898 g/mol, respectively. Both LH and LHO had a polydispersity index (PDI) of 1.5.

Figure 1. Approach used to prepare cured epoxy resins from lignin hydrogenolysis products.

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Figure 2. Molecular weight profiles of lignin hydrogenolysis products and their epoxy derivatives. Quantitative

31

P NMR spectroscopy is an important analytical technique for quantifying the

different types of hydroxyl groups in lignin (Figure 3).43,49. A high number of phenolic hydroxyl groups is generally advantageous as they can be reacted with epichlorohydrin to generate epoxide functionality, i.e. glycidyl ethers (Figure 1). The LH and LHO products contained 4.1 and 4.4 mmol/g of total phenolic hydroxyl groups, respectively (Table 1). This compares with 1.1 mmol/g for enzymatic mild acidolysis lignin isolated from P. radiata wood42 and 2.3 mmol/g for a depolymerized lignin that was used to produce epoxy resins.50 The hydrogenolysis reaction cleaves β-O-4 ether linkages in the native wood lignin to liberate new phenolic hydroxyl groups. 31

P NMR spectroscopy also gives information on the chemical environment of the phenolic

hydroxyl groups, namely whether these groups present in C9 phenylpropanoid units are: i) bonded to H at the 5-position (uncondensed) or ii) bonded to another C9 unit at the 5-position, such as β-5, 5-5 or 4-O-5 linked units (condensed) (Figure 3 and 4, Table 1).43 The phenolic hydroxyl groups in the monomers, DCA and PG (Figure 1), analyze as uncondensed hydroxyl groups, whereas the dimers and oligomers of DCA and PG could contain both uncondensed and condensed phenolic hydroxyl groups. Consequently, the oligomeric fraction of the hydrogenolysis oil (LHO) contained a greater proportion of condensed phenolics compared to Page 9 of 33


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the whole oil (LH) (Table 1). In this study, molecules with at least two phenolic hydroxyl groups (i.e. dimers and oligomers) were required to make crosslinking epoxy resins.

Figure 3. Quantitative

31

P NMR spectrum of lignin hydrogenolysis oil after reaction with the

phosphitylating agent.

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Table 1. Hydroxyl group content of the lignin hydrogenolysis products and their epoxy derivatives by quantitative 31P NMR spectroscopy. Sample

Phenolic OH (mmol/g)

Acidic OH (mmol/g)

β-5

4-O-5

5-5

Uncondensed guaiacyl (G)

p-Hydroxy phenyl

Total

LH

0.53 (0.008)a

0.20 (0.003)

0.76 (0.012)

2.56 (0.019)

0.06 (0.007)

4.11 (0.06)

0.14 (0.005)

LHEP

0.07

0.01

0.02

0.03

0.001

0.13

0.003

% reacted

87

95

98

99

99

97

98

LHO

0.81

0.34

1.17

1.95

0.10

4.36

0.16

LHOEP

0.16

0.02

0.05

0.13

NDb

0.37

0.003

% reacted

80

93

96

93

100

92

98

a

Standard deviations in parentheses for LH analyzed in triplicate; b not detected.

Figure 4. Examples of condensed and uncondensed structures in lignin hydrogenolysis products.

Glycidylation reaction. The main product from the reaction of LH with epichlorohydrin was confirmed as the expected glycidyl ether by 1H, 13C and 2D HSQC NMR spectroscopy. Signals in the HSQC spectrum of the lignin hydrogenolysis epoxy prepolymer (LHEP) (Figure 5), labelled as 1, 2 and 3, are characteristic of a glycidyl ether group connected to a lignin guaiacyl Page 11 of 33


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unit. The 1H and 13C chemical shifts of the glycidyl signals were consistent with those reported in the literature.28

Figure 5. Partial HSQC NMR spectrum of LHEP showing glycidyl ether functionality.

Quantitative 31P NMR spectroscopy of LHEP and the oligomeric epoxy prepolymer (LHOEP) showed that 97% and 92% of the phenolic hydroxyl groups were derivatized, respectively (Table 1). The condensed β-5 phenolic hydroxyl group was less reactive compared to the uncondensed phenolic hydroxyl group (Table 1). This could be due to steric hindrance of the reaction. Similarly, Fache and co-workers (2016) found that the phenolic hydroxyl groups in syringylbased (S-type) model compounds, with a methoxy group at the 5-position, gave lower conversions on glycidylation compared to guaiacyl-based (G-type) lignin models.27 Interestingly, the other condensed phenolic hydroxyl groups (e.g. 5-5 and 4-O-5 linked) were not affected to the same degree as the phenolic hydroxyl groups in β-5 structures (Table 1), suggesting factors other than steric hindrance influence the reactivity. Page 12 of 33


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Both LHEP and LHOEP had higher molecular weights than the lignin hydrogenolysis starting materials, LH and LHO (Figure 2). Potential oligomerization reactions between newly-formed glycidyl groups and unreacted phenolic hydroxyl groups, which would lead to higher molecular weight resins, were minimized by using an excess of epichlorohydrin in the reaction. Indeed, the Mw for LHEP and LHOEP increased by only 60-70% to 1084 and 1514 g/mol (Table 2), respectively, compared to the starting materials. When compared against commercial BADGE resins, the average molecular weight of LHEP was intermediate between typical liquid and solid BADGE resins. The molecular weight of LHOEP was equivalent to a solid BADGE resin with a degree of polymerization of 2.8 The PDI for the LHEP and LHOEP products was 1.8 (Table 2), which was similar to that of low molecular weight BADGE resins.51 Higher PDI values have been reported for epoxy resins produced from depolymerized Kraft (PDI 3.5), organosolv (PDI 2.8) and hydrolysis (PDI 4.6) lignins.39,50 Polydispersity is a measure of the molecular weight distribution. In this study, a low PDI was desirable as it gave a more molecularly-homogenous prepolymer which was reflected in the ability to process the resin.

Table 2. Chemical properties of the epoxy prepolymers. Resin

Mn (g/mol)

Mw (g/mol)

PDI

EEW (g/eq)

Hydrolysable chlorine (%)

Total chlorine (%)

Viscosity (cP)a

LHEP

610

1084

1.8

359 (17)b

0.1 (0.04)

1.0 (0.03)

55

LHOEP

837

1514

1.8

452 (1)

0.3 (0.04)

1.7 (0.02)

132

a

As 40% solutions in diethylene glycol monobutyl ether; b standard deviations are given in parentheses.

An important criterion for epoxy resins is the epoxide content, which is commonly referred to as the epoxy equivalent weight (EEW), expressed as the weight of resin to obtain one equivalent

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epoxide group.8 The lower the EEW the greater the potential cross-linking density in the cured epoxy resin. The EEW values for LHEP and LHOEP were 359 g/eq and 452 g/eq, respectively (Table 2). This translates to approximately one epoxide group per 1.5 lignin C9 units in the LHEP resin and one epoxide group per 1.9 lignin C9 units in the LHOEP resin, for an estimated molecular weight of a glycidylated C9 lignin unit of 238 g/mol. The theoretical EEW values expected for LHEP and LHOEP were 300 g/eq and 307 g/eq, respectively, based on the quantitative 31P NMR results (Table 1). The difference between the actual and theoretical values was possibly due to side reactions that can occur during the glycidylation reaction.8 Commercial BADGE resins can have a wide range of EEW values, from 174 up to 6000 g/eq.8 BADGE used in this study had an EEW of 174 g/eq. The EEW values for LHEP and LHOEP were lower than reported for epoxy resins made from depolymerized softwood Kraft lignin (768 g/eq) and hardwood organosolv lignin (537 g/eq),39 indicating more cross-linking is possible with lignin hydrogenolysis products from native softwood lignin. Chlorine content is an important quality indicator for epoxy resins.8 High chlorine values can result in corrosivity issues in cured resins, therefore low values are desirable. The total chlorine content of LHEP and LHOEP was 1.0% and 1.7%, respectively (Table 2). The hydrolysable chlorine content of LHEP and LHOEP was 0.1% and 0.3%, respectively (Table 2). Hydrolysable chlorine content provides a measure of the presence of chlorohydrin intermediates (Figure 6), from incomplete dehydrohalogenation that can reduce the potential for the resin to crosslink with curing agents. Chlorine content is rarely reported in publications on bio-based epoxy resins. Zhao and Abu-Omar (2015) reported a hydrolysable chlorine content of 0.5% and a total chlorine content of 3.4% for an epoxy functionalized lignin model compound.34 A low level of hydrolysable chlorine was achieved in this study by applying a post-reaction alkali treatment.52

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For glycidylation reactions, optimized conditions are needed to achieve low levels of total chlorine. This is difficult to achieve at a small scale. Undesirable side reactions can lead to bound chlorides which are not readily saponified with NaOH solutions.8 In commercial resins, typical hydrolysable and total chlorine contents range from 0.02 – 0.1% and 0.1 – 0.2%, respectively.8

Figure 6. Main glycidylation reaction pathway of lignin hydrogenolysis products showing chlorohydrin intermediate.

Cured epoxy resins containing BADGE. Blending and curing. The viscosities of LHEP and LHOEP as 40% solutions in diethylene glycol butyl ether were 55 and 132 cP, respectively (Table 2). These viscosities were higher than for a typical liquid BADGE resin, but lower than for a solid BADGE resin with a degree of polymerization of 2.8 Adding 9% of a reactive diluent was helpful in reducing the viscosity of the epoxy resins so that they could be mixed with the curing agent and remain workable at 40 °C. Reactive diluents are typically used to reduce the viscosity of epoxy resins without severely impacting on their overall properties53, and are used at levels up to 20% w/w.8 Using this system gave workable resins, whereas other researchers have incorporated solvents or used elevated temperatures (70-130 °C) in order to effectively process lignin-based epoxy resins.24,27,28,33,39

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We first investigated DETA for curing the lignin-based epoxy prepolymers as it is used commercially as an aliphatic curing agent for curing epoxy resins at ambient temperature.8 Cured epoxy resins produced from LHEP and DETA resulted in specimens that were brittle and easily broken when removed from the mold. The brittleness was attributed to the oligomer molecules in the resin that contained multiple glycidyl ether groups. These are analogous to multifunctional epoxy novolac resins based on phenol formaldehyde novolacs that have high cross-linking densities, which can result in increased brittleness.8 It is common for commercial epoxy resins to be formulated with multiple components to give optimum processability and performance.8 Therefore, we investigated the potential to formulate LHEP and LHOEP with BADGE. DETAcured epoxy resin specimens suitable for mechanical testing could be prepared by blending LHEP with BADGE in mass ratios of up to 3:1, and blending LHOEP with BADGE in ratios of up to 1:3 (Figure 7). Preparation of specimens for property testing using higher proportions of LHEP and LHOEP was difficult due to processing limitations, except for a thermal analysis specimen of the 100% LHEP epoxy resin. Hence, LHEP was tested as a partial replacement for BADGE while LHOEP was tested as a potential performance enhancing additive. The cured epoxy resins were all transparent, although those containing LHEP and LHOEP were amber in color (Figure 7). The color is unlikely to be a major limitation for most epoxy resin applications.

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Figure 7. Cured epoxy resin specimens.

Thermal properties. The glass transition temperature (Tg) values of the DETA-cured epoxy resins produced from LHEP/BADGE blends were lower than that of the BADGE control resin, with a range of 68-80 °C (Table 3). Higher proportions of LHEP resulted in a lower Tg, which was consistent with the 100% LHEP resin having the lowest Tg of 53 °C. This trend was the same for the cured LHOEP/BADGE resins, although the differences relative to the BADGE resin were less pronounced due to the lower proportions of LHOEP used in the blends. The Tg of the cured BADGE resin was similar to that reported previously (117 °C vs. 119 °C).54 The lower Tg of the cured blends could be due to several factors. BADGE is a rigid molecule with the aromatic rings joined by a methylene bridge (Figure 1), whereas LHEP and LHOEP contain dimers and oligomers with both rigid (e.g. 5-5) and flexible (e.g. β-5) linkages (Figure 4). The flexible structures are likely to reduce the Tg. Methoxy groups that are associated with the lignin structures are known to reduce the Tg of cured epoxy resins.30 By combining a depolymerized lignin with an annulation reaction prior to glycidylation, Kaiho and co-workers (2016) were able to produce epoxy resins with Tg values as high as 134 °C.55

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Table 3. Thermal analysis data for epoxy resin blends containing BADGE and cured with DETA. Resin

Tg (°C)

T5% (°C)

Ts (°C)

BADGE

117 (0.7)a

328 (0.8)

169 (0.2)

LHEP/BADGE 1:1

80

289

161

LHEP/BADGE 2:1

70

270

156

LHEP/BADGE 3:1

68

258

151

LHEP

53

236

144

LHOEP/BADGE 1:9

111

325

169

LHOEP/BADGE 1:3

104

311

165

a

95% confidence intervals are given in parentheses for the BADGE resin based on four replicates.

The cured LHEP/BADGE and LHOEP/BADGE resins were less thermally stable than the BADGE resin (Table 3). The initial decomposition temperature (T5%) was lowest for specimens containing the highest proportion of LHEP and LHOEP, and was consistent with the 100% LHEP resin having the lowest T5% of 236 °C. The statistic heat-resistant index temperature (Ts) is characteristic of the thermal stability of the cured resins.35 The Ts values had the same trend as for the T5% values (Table 3). The reduction in thermal stability is likely due to the presence of methoxy groups on the aromatic ring as they are known to decrease thermal stability by means of electron donation to the aromatic ring.56 The thermal results suggest that cured epoxy resins containing LHEP and LHOEP may be less suitable than BADGE-based resins in high temperature applications. The onset curing temperature (To) and the peak curing temperature (Tp) for the BADGE resin was 67 °C and 91 °C, respectively. The LHEP/BADGE 1:1 and 2:1 blends had a reduced To of

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46 °C and 43 °C, and a reduced Tp of 80 °C and 78 °C, respectively. This curing behavior is attributed to the presence of primary aliphatic hydroxyl groups associated with the DCA moieties (Figure 1) in LHEP. Hydroxyl groups are known to accelerate the rate of cure of epoxy resins with amine curing agents8, and have been implicated in promoting the curing reaction of epoxy resins produced from lignin model compounds and other depolymerized lignins.34,39 Mechanical properties. The measured mechanical properties of the LHEP/BADGE and LHOEP/BADGE blends cured with DETA were superior to the BADGE control resin (Figure 8). The cured LHEP/BADGE 2:1 blend resulted in flexural modulus and strength values that were 52% and 28% greater, respectively, than BADGE alone. Increasing the proportion of LHEP in the blend from 67% to 75% resulted in a decline in both the flexural modulus and, to a greater degree, the flexural strength. The flexural strength values of replicate specimens of the cured LHEP/BADGE 3:1 blend were highly varied (ranging from 80 to 159 MPa), which was attributed to increased brittle behavior. The results indicated there was no advantage in testing cured LHEP/BADGE blends at ratios lower than 1:1 (Figure 8). The cured LHOEP/BADGE 1:3 blend resulted in a flexural modulus and strength that was 26% and 38% greater, respectively, than BADGE alone. The increase in the flexural modulus was greater than for the LHEP/BADGE 1:1 blend and the flexural strength was the highest of all the specimens measured at 163 MPa. An increase in the flexural strength was observed even when the proportion of LHOEP to BADGE was only 10% compared to the BADGE resin (140 vs. 118 MPa).

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Figure 8. Flexural properties of epoxy resin blends containing BADGE and cured with DETA (error bars are 95% confidence intervals of at least four replicates).

These results imply that the lignin-derived oligomers play a key role in improving the flexural modulus and strength of the cured epoxy resin blends. For the LHEP/BADGE blends, monomers in the lignin hydrogenolysis oil with single glycidyl groups potentially act more as reactive diluents, contributing little to the mechanical properties of the cured resins. In this regard, choice of the lignin feedstock for hydrogenolysis is important as softwood native lignins give higher yields of oligomeric products compared to hardwood lignins.40,42 Lignin monomers can be used to make a crosslinked epoxy polymer if, for example, they contain at least two phenolic hydroxyl groups that can be glycidylated.34 The presence of methoxy groups in the lignin hydrogenolysis products is likely to contribute to the superior mechanical properties of the cured LHEP/BADGE and LHOEP/BADGE blends. Hernandez and co-workers (2016) found that methoxy groups improved the modulus of cured epoxy resins derived from vanillyl alcohol30, and hydrogen bonding between methoxy groups and hydroxyl groups formed during curing has been

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implicated in improving mechanical strength.4 Nonetheless, other factors such as monomer to oligomer ratio and cross-linking densities can also influence mechanical properties. Other researchers have prepared cured epoxy resins based on monomers that can be derived from lignin, such as vanillin27,28, vanillyl alcohol30, isoeugenol32 and propyl guaiacol34,36, however no comparable information is provided on mechanical properties. In related work, Ferdosian and co-workers (2016) measured mechanical properties of glass fiber-reinforced plastics prepared using BADGE blended with epoxy resins made from depolymerized softwood Kraft and hardwood organosolv lignins.39 They also observed improvements in flexural modulus and strength, but mainly at a low blend ratio of 1:3 lignin-based epoxy to BADGE. In our work, we were able to increase both the flexural modulus and strength in higher blend ratios up to 2:1 lignin-based epoxy to BADGE. Alternative curing agent. Selected epoxy resin blends were cured with IPDA, a cycloaliphatic curing agent commonly used in industry, to investigate the effect of a different curing agent. These cured epoxy resins had higher Tg values compared with those cured with DETA (Table 4). Cycloaliphatic curing agents are known to enhance the thermal properties of cured resins compared to aliphatic amine curing agents.8 As with DETA, blending LHEP and LHOEP with BADGE resulted in cured epoxy resins with lower Tg values relative to the BADGE control resin. The flexural modulus of the LHEP/BADGE 1:1 blend resin was similar when cured with either curing agent (Table 4). This was also true for the BADGE control resin. The LHEP/BADGE 1:1 blend that was cured with IPDA no longer had a flexural strength performance advantage over the BADGE resin, compared to curing with DETA. Curing BADGE with IPDA has previously been reported to increase flexural strength compared to curing with DETA.54 The flexural properties of the IPDA-cured LHOEP blend were similar to the IPDA-

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cured BADGE resin, and reduced compared to the DETA-cured LHOEP blend (Table 4). Overall, these results indicated that blending LHEP and LHOEP with BADGE, and curing with IPDA, gave mechanical properties that were similar to the BADGE control resin.

Table 4. Effect of curing agent (DETA, IPDA) on thermal and mechanical properties of BADGE and blended cured resins. Resin

DETA

IPDA

Tg (°C)

Flexural modulus (GPa)

Flexural strength (MPa)

Tg (°C)



BADGE

117 (0.7)a

3.1 (0.03)

118 (1)

162 (0.2)

2.9 (0.2)

146 (6)

LHEP/BADGE 1:1

80

3.7

129

100

3.8

141

LHOEP/BADGE 1:3

104

3.9

163

135

3.1

155

a

95% confidence intervals are given in parentheses for the BADGE resin based on at least three replicates.

Cured epoxy resins containing glycerol diglycidyl ether. LHEP was blended with GDGE to produce epoxy resins containing entirely bio-based components. GDGE has been used as an epoxy prepolymer in lignin-containing epoxy resins.22,57 The addition of the reactive diluent was not required for the LHEP/GDGE blends as the viscosity was sufficiently low to cast specimens at 40 °C (e.g. Figure 7). The Tg values of the blends that were cured with DETA were similar to the GDGE control resin (Table 5), and approximately 60 °C lower than the BADGE control resin. The cured GDGE resin had reduced thermal stability compared to the BADGE resin and addition of LHEP decreased this further (Table 5), as was seen with the LHEP/BADGE blends. These results suggest that LHEP/GDGE epoxy resin blends would be restricted to lower temperature applications.

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Table 5. Thermal and mechanical properties of epoxy resin blends containing GDGE and cured with DETA. Resin

Tg (°C)

T5% (°C)

Ts (°C)



GDGE

58

260

139

2.8 (0.6)a

95 (8)

LHEP/ GDGE 1:1

59

242

139

3.7 (0.1)

121 (2)

LHEP/ GDGE 2:1

60

220

136

4.0 (0.2)

140 (7)

a

95% confidence intervals are given in parentheses.

The flexural modulus and strength of the cured epoxy resins were improved with increasing proportions of LHEP in the LHEP/GDGE blends (Table 5). The cured LHEP/GDGE 2:1 blend resulted in flexural modulus and strength values that were 43% and 47% greater, respectively, than the GDGE control resin, and were intermediate between the modulus and strength values obtained for the cured LHEP/BADGE 1:1 and 2:1 blends. The LHEP/GDGE 2:1 blend produced a cured resin with a 29% greater flexural modulus and 19% greater flexural strength than the BADGE control resin. This indicates that it is possible to make 100% bio-based epoxy resins that, when cured, have superior mechanical properties compared to BADGE resins. Summary of mechanical performance. In summary, the flexural properties were very promising, with the cured LHEP/BADGE and LHOEP/BADGE blends performing better than the industry standard BADGE resin. LHEP has potential as a bio-based partial replacement for BADGE. Alternatively, LHEP used in combination with GDGE warrants further investigation as a complete replacement for BADGE. LHOEP appears to have potential as a bio-based

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performance enhancing additive for BADGE-based resins. More extensive formulation and property testing is needed to fully assess the potential of these lignin hydrogenolysis products in developing novel bio-based epoxy resins.

CONCLUSIONS Mild hydrogenolysis of native softwood lignin produces a polyol product that can be used to prepare novel bio-based epoxy resins. Reacting this lignin-based polyol with epichlorohydrin gave an epoxy prepolymer that, when blended with BADGE or GDGE, resulted in cured epoxy resins with a greater flexural modulus and strength to those made from BADGE alone. This is the first reported example of a deconstructed native softwood lignin being used to produce concept epoxy resins with superior mechanical properties to the industry standard. Further work is needed to improve and scale-up the lignin hydrogenolysis process, and to optimize the glycidylation and resin formulation chemistry. The promising performance of these concept epoxy resins suggests that with this further work, a viable and fully bio-based epoxy resin with properties comparable to the bisphenol A-derived industry standard may be realized.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Phone: +64-7-343-5899 Fax: +64-7-348-0952 ACKNOWLEDGMENTS

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This research was supported by the New Zealand Ministry of Business Innovation and Employment via Scion Core funding. The authors wish to acknowledge Ibrar Hussain for technical assistance with lignin hydrogenolysis. REFERENCES 1. Chen, G. Q.; Patel, M. K. Plastics derived from biological sources: present and future: a technical and environmental review. Chem. Rev. 2012, 112, 2082-2099. 2. Wang, C.; Kelley, S. S.; Venditti, R. A. Lignin-Based Thermoplastic Materials. ChemSusChem 2016, 9, 770-783. 3. Raquez, J. M.; Deléglise, M.; Lacrampe, M. F.; Krawczak, P. Thermosetting (bio)materials derived from renewable resources: A critical review. Prog. Polym. Sci. 2010, 35, 487-509. 4. Koike, T. Progress in development of epoxy resin systems based on wood biomass in Japan. Polym. Eng. Sci. 2012, 52, 701-717. 5. Llevot, A.; Grau, E.; Carlotti, S.; Grelier, S.; Cramail, H. From Lignin-derived Aromatic Compounds to Novel Biobased Polymers. Macromol. Rapid Commun. 2016, 37, 9-28. 6. Upton, B. M.; Kasko, A. M. Strategies for the Conversion of Lignin to High-Value Polymeric Materials: Review and Perspective. Chem. Rev. 2016, 116, 2275-2306. 7. Bruijnincx, P. C.; Weckhuysen, B. M. Shale gas revolution: an opportunity for the production of biobased chemicals? Angew. Chem. Int. Ed. Engl. 2013, 52, 11980-11987. 8. Pham, H. Q.; Marks, M. J. Epoxy Resins. In Ullmann's Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2000.

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55. Kaiho, A.; Mazzarella, D.; Satake, M.; Kogo, M.; Sakai, R.; Watanabe, T. Construction of the di(trimethylolpropane) cross linkage and the phenylnaphthalene structure coupled with selective β-O-4 bond cleavage for synthesizing lignin-based epoxy resins with a controlled glass transition temperature. Green Chem. 2016, 18, 6526-6535. 56. Harvey, B. G.; Guenthner, A. J.; Lai, W. W.; Meylemans, H. A.; Davis, M. C.; Cambrea, L. R.; Reams, J. T.; Lamison, K. R. Effects of o-Methoxy Groups on the Properties and Thermal Stability of Renewable High-Temperature Cyanate Ester Resins. Macromolecules 2015, 48, 3173-3179. 57. Ismail, T. N. M. T.; Hassan, H. A.; Hirose, S.; Taguchi, Y.; Hatakeyama, T.; Hatakeyama, H. Synthesis and thermal properties of ester-type crosslinked epoxy resins derived from lignosulfonate and glycerol. Polym. Int. 2009, 59, 181-186.

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Biomacromolecules

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Title: Bio-based Epoxy Resins from Deconstructed Native Softwood Lignin Authors: Daniel J. van de Pas and Kirk M. Torr

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Biobased Epoxy Resins from Deconstructed Native ... - ACS Publications

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