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Renewable Epoxy Networks Derived from Lignin -Based Monomers: Effect of Crosslinking Density Shou Zhao, and Mahdi M. Abu-Omar ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01446 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 28, 2016

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ACS Sustainable Chemistry & Engineering

1

Renewable Epoxy Networks Derived from Lignin−Based Monomers:

2

Effect of Crosslinking Density

3

Shou Zhao† and Mahdi M. Abu−Omar†, ‡,*

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†

5

Lafayette, IN 47907

6

‡

7

480 Stadium Mall Drive, West Lafayette, IN 47907

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*

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ABSTRACT:

Brown Laboratory and Department of Chemistry, Purdue University, 560 Oval Drive, West

School of Chemical Engineering, Purdue University, Forney Hall of Chemical Engineering,

To whom correspondence should be addressed: [email protected]

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Three modification methods, which either improved molecular weight, orientation or the

11

number of functional groups, were employed to increase the crosslinking density of biobased

12

epoxy networks based on 2−methoxy−4−propylphenol (dihydroeugenol, DHE). The

13

modifications were realized through o−demethylation and phenol−formaldehyde reactions.

14

Structures of DHE−based monomers and cured networks were characterized by NMR and

15

FTIR spectroscopy. Crosslinking densities of cured networks were calculated using rubber

16

elasticity theory from dynamic mechanical analysis (DMA). Networks with higher crosslink

17

density were found to exhibit greater mechanical and thermal performance as measured by

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DMA and thermogravimetric analysis (TGA). GEDHEO−NOVO, an epoxy monomer

19

featuring all three modification processes, exhibited significant improvements in crosslinking

20

density (0.39 to 9.77 mol/dm3), α−relaxation temperature (Tα, 40 to 139 °C) and statistic

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heat−resistant index temperature (Ts, 125 to 153 °C) compared to the unmodified

22

DHEO−based networks.

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ACS Paragon Plus Environment


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KEYWORDS: Lignin, Epoxy thermoset, Crosslink density, Phenol−formaldehyde reactions

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INTRODUCTION

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The use of lignin−derived phenols to replace aromatics derived from petroleum for

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making thermoset materials has attracted increasing attention over the last decade.1,2 The

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merit of this replacement is often attributed to the sustainability of lignin over nonrenewable

6

chemicals. Furthermore, the aromatic nature of lignin provides for high performance

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thermosets with desirable mechanical properties and thermal stability. 3

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To obtain lignin−based thermosets, two strategies have been employed. The first focuses

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on integrating functionalized bulk lignin with other natural or synthetic monomers to achieve

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copolymers or blends. For example, carboxylic acid−functionalized lignin obtained by

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reacting alkali lignin with acid anhydride acts as a curing agent for epoxy networks.4 The use

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of bulk lignin without deconstruction into its phenolic components has certain economic

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advantages. However, limitations of this strategy have become evident. The reactivity of

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lignin is low because the majority of its phenolic hydroxyl groups are etherified and all para

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as well as some ortho positions of the phenolic rings are occupied.5 Also crosslinking density

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of the resulting thermosets is often compromised because of the steric hindrance introduced

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by bulk lignin.6,7

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Because of the above−mentioned limitations associated with the use of bulk lignin, use

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of well characterized lignin−derived monomers for thermosets design is most appealing and

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has received extensive attention. Thermosets including epoxy resins, polybenzoxazines, vinyl

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ester resins and cyanate ester resins developed from lignin−derived building blocks like

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eugenol,8,9 vanillin,10,11 guaiacol12 and creosol13 have been recently reported. Improved

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reactivity of lignin−derived monomers rather than bulk lignin provide thermosets which

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exhibit more satisfactory mechanical and thermal properties. Moreover, reactivity of

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lignin−derived monomers can be further increased through molecular reaction design such as

4

demethylation,14

5

formaldehyde chemistry.16

methacrylation15

or

bridging

monomers

into

oligomers

through

6

From the viewpoint of “structure determines property,” even if slightly varying

7

monomer’s chemical structure, e.g. ortho−, meta− and para−substituted bisphenols, the

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obtained polymers exhibit markedly different properties.17 Even though significant work has

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been done on lignin−based thermosets,1,2,18 differences in properties caused by various

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structural designs have not been explored systematically. In fact, if lignin−derived molecules

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can be shown to undergo reasonable structural design to achieve optimal properties, their

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sustainability and viability would become more meaningful. Previous laboratory

13

experiments19 and molecular modeling20 studies have identified crosslinking density as a key

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structural parameter that determines thermomechanical properties of thermosets. To

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investigate the effects of crosslinking density on fully cured thermosets from the same

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precursor, we employed the following methods: (1) variation of monomer molecular weight,21

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(2) alteration in number of functional groups on monomer22 and (3) adjustment of chain

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orientation.23,24

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In a recent report we described a bimetallic Zn/Pd/C catalytic system to convert lignin in

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intact lignocellulosic biomass directly into two methoxyphenol products.25 One of the major

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products, 2−methoxy−4−propylphenol (or dihydroeugenol, DHE), exhibited potential as a

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building block for epoxy networks (Figure 1, Route 1),26 in which DHE was o−demethylated

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to yield propylcatechol (DHEO), and subsequently glycidylated to epoxy monomer

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(glycidylated ether of DHEO, GEDHEO). Route 1 creates two hydroxyl groups per DHE

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molecule to make it a viable epoxy monomer. However, its properties are impaired due to the

4

appearance of benzodioxen byproducts and decrease in extension caused by the ortho

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hydroxyls and three carbon propyl tail.

6

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Figure 1. Modifications of DHE−based epoxy networks through: improving molecular

8

weight and adjusting orientation of functional group from DHE to DHE−Dimer in Route (2);

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increasing number of functional group from DHE−Dimer to DHEO−Dimer via

10

o−demethylation in Route (3); and increasing molecular weight from DHEO to

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DHEO−NOVO via phenol−formaldehyde reaction in Route (4). Benzodioxane derivatives

12

produced during the glycidylation of DHEO, DHEO−Dimer and DHEO−NOVO are not

13

shown.

14

In this study, with the aim to improve the performance of the epoxy networks obtained

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from Route 1, three modified DHE−based epoxy networks with enhanced crosslinking 4


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density were prepared by varying (a) the molecular weight and orientation of functional group

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by dimerization of DHE (DHE−Dimer) via Route 2; (b) the number of functional hydroxyl

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groups by demethylation (DHEO−Dimer) in Route 3; and (c) the molecular weight through

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phenol−formaldehyde reaction (DHEO−NOVO) in Route 4. The proposed DHE−based epoxy

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networks provide insight into thermoset syntheses using lignin−derived monomers, the

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majority of which possess similar structural characteristics with DHE. More importantly, this

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study highlights how the thermomechanical properties of epoxy networks can be improved

8

through varying the crosslinking density from the same precursor.

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EXPERIMENTAL SECTION

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General. DHE o−demethylated product (DHEO), glycidylated ether of DHEO (GEDHEO)

11

and

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2−methoxy−4−propylphenol,

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tetrabutylammonium bromide and diethylenetriamine (DETA) were purchased from Aldrich

14

Chemical Co. Formaldehyde solution (37%) was obtained from Macron Fine Chemicals. All

15

chemicals were used without further puriﬁcation.

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Preparation of Polyphenols from DHE.

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Synthesis of 6, 6'−methylenebis (2−methoxy−4−propylphenol) (DHE−Dimer).

cured

epoxy

network

were

prepared

epichlorohydrin,

48%

as

previously

aqueous

described.26

hydrobromic

acid,

18

DHE (3.32 g, 0.02 mol), 37% formaldehyde solution (0.81 g, 0.01 mol), 48%

19

hydrobromic acid (15 mL) and H2O (8 mL) were stirred at room temperature for 24 h.

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Viscous oil was formed in the upper layer, while the lower aqueous layer was carefully

21

removed. The oil product was washed with water 3 times and dried under vacuum overnight

22

to yield DHE−Dimer as a viscous oil (2.86 g, 83% yield). 1H NMR (CDCl3, 400 MHz) δ: 6.72

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(s, 2H), 6.53 (s, 2H), 5.50 (s, 2H), 3.88 (s, 6H), 3.83 (s, 2H), 2.53 (t, 4H, J =7.6 Hz), 1.62 (sex,

2

4H, J =7.3 Hz), 1.01 (t, 6H, J =7.6 Hz). 13C NMR (CDCl3, 400 MHz) δ: 144.7, 143.4, 132.3,

3

131.4, 119.2, 115.9, 55.9, 34.8, 34.1, 24.2, 14.1.

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Synthesis of 6, 6'−methylenebis (4−propylbenzene−1, 2−diol) (DHEO−Dimer).

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DHEO−Dimer was prepared through o−demethylation of DHE−Dimer. In detail,

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DHE−Dimer (3.44 g, 0.01 mol) was added to 12 mL 48% aqueous hydrobromic acid. The

7

reaction mixture was refluxed at 120 °C for 20 h, cooled to ambient temperature, saturated

8

with NaCl and extracted 3 times with diethyl ether. The organic layer was dried over MgSO4

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and concentrated using rotary evaporation to obtain DHEO−Dimer as a viscous oil (2.54 g, 80%

10

yield). 1H NMR (CDCl3, 400 MHz) δ: 6.68 (m, 4H), 5.69 (s, 4H), 3.81 (m, 2H), 2.44 (t, 4H, J

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=6.6 Hz), 1.52 (sex, 4H, J =7.1 Hz), 0.89 (t, 6H, J =7.1 Hz). 13C NMR (CDCl3, 400 MHz) δ:

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143.2, 141.1, 138.4, 135.9, 121.9, 115.2, 37.3, 37.2, 24.5, 13.7.

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Synthesis of DHEO Novolac Oligomer (DHEO−NOVO).27

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DHEO (1.52 g, 0.01 mol), 37% formaldehyde solution (0.81 g, 0.01 mol), concentrated

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hydrochloric acid (4 mg) and H2O (8 mL) were mixed and refluxed at 100 °C for 6 h. Water

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and hydrogen chloride were then evaporated under reduced pressure at 80 °C. The unreacted

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DHEO was removed by washing with toluene 3 times and evaporating the toluene at 40 °C

18

under vacuum overnight to yield DHEO−NOVO as a viscous oil (1.46 g, 89% yield, based on

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an assumption that the novolac with an infinite molecular weight was obtained).

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Preparation of Glycidylated Ether of DHE−Dimer (GEDHE−Dimer), DHEO−Dimer

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(GEDHEO−Dimer) and DHEO−NOVO (GEDHEO−NOVO).

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GEDHE−Dimer was prepared by reaction of DHE−Dimer (1.72 g, 0.005 mol) and

6


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epichlorohydrin (17 g, 0.18 mol). Tetrabutylammonium bromide (0.16 g) was used as a phase

2

transfer catalyst. The mixture was heated at 60 °C for 3 h and followed by a dropwise

3

addition of 0.8 g of 50% w/w NaOH solution. The reaction was kept for another 3 h and the

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mixture was washed with acetone, filtered to remove salt and concentrated with a rotary

5

evaporator. GEDHEO−Dimer and GEDHEO−NOVO were obtained according to the same

6

procedure as GEDHE−Dimer.

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Formation of DHE−Based Epoxy Networks.

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DHE−based

epoxy

monomers,

i.e.,

GEDHE−Dimer,

GEDHEO−Dimer

and

9

GEDHEO−NOVO were respectively introduced to diethylenetriamine (DETA) with

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stoichiometric ratio of epoxy vs. −NH for curing. The mixtures were stirred for 10 min,

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degassed under vacuum to remove entrapped air and poured into molds for curing according

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to the profile: 55 °C for 2 h, 75 °C for 2 h and 95 °C for 2 h.

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Analysis Methods.

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The structural evolution from DHE to final cured epoxy was followed using 1H nuclear

15

magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy. The NMR

16

spectra were performed on a Bruker Avance ARX−400 spectrometer using deuterated

17

chloroform or d6−DMSO as solvent. FTIR analyses were conducted on a Thermo−Nicolet

18

Nexus 470 FTIR Spectrometer equipped with an ultra−high−performance, versatile

19

Attenuated Total Reflectance (ATR) sampling accessory. The spectra were scanned over a

20

wavenumber range of 400−4000 cm−1 with a resolution of 4 cm−1.

21

Differential scanning calorimetry (Perkin Elmer Jade DSC 4000) was conducted under

22

dry nitrogen atmosphere to monitor exothermic peak temperature and activation energy.

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Samples of 5−10 mg were placed in sealed aluminum pans for all DSC runs. Peak

2

temperature and enthalpy were obtained through heating the samples from 0 to 150 °C at a

3

rate of 10 °C/min. Activation energy (Ea) was calculated by the Kissinger Equation:28

4

ln ( β Tp2 ) = Ea RTp − ln ( AR Ea )

(1)

5

where R is the gas constant (8.31 J K−1 mol−1), A is the pre−exponential factor, β is the heating

6

rate, while Tp is the exothermic peak temperature. Activation energy can be obtained from the

7

slope of the linear relationship between ln (β/Tp2) and Tp−1 under different heating rates of 10,

8

15, 20 and 25 °C/min.

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Dynamic mechanical properties were characterized using a DMA 2980 (TA Instruments).

10

Rectangular specimens with dimensions of 30 mm length, 10 mm width and 2 mm thickness

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were measured in a single−cantilever mode. The measurements were conducted from 25 °C to

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180 °C at a heating rate of 3.00 °C/min and a frequency of 1 Hz. The temperature at the

13

maximum in the tan δ curve was taken as Tα.

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Thermal stability studies were carried out on a TGA Q500 (TA Instruments) under a

15

nitrogen flow of 40 mL/min. Samples (15−20 mg) were placed in a platinum pan and scanned

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from 30 to 500 °C at a ramp rate of 20 °C/min.

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RESULTS AND DISCUSSION

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Characterization of DHE−Based Polyphenols and Epoxy Monomers.

19

1

H NMR spectroscopy.

20

Structures of DHE−derived polyphenols are illustrated in Figure 2. The resonance peak

21

at δ 3.9 is assigned to −OCH3 proton of DHE (Figure 2, panel a). After DHE was

22

demethylated in aqueous HBr, DHEO (Figure 2, panel b) exhibits no peak at δ 3.9 while two

8


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hydroxyl peaks are apparent at δ 5.3 and 5.4, indicating complete removal of methoxy group.

2

In another route, DHE−based polyphenol is achieved by dimerization of DHE through

3

formaldehyde chemistry. Previous study reported Brønsted acid catalyzed formaldehyde

4

coupling occurred exclusively at the position meta to the hydroxyl group with a selectivity of

5

97%.29 The obtained bisphenol, DHE−Dimer, exhibits a methylene linkage at δ 3.8 (Figure 2,

6

panel c). DHE−Dimer is further demethylated to DHEO−Dimer in HBr solution. Figure 2,

7

panel d shows the disappearance of DHE−Dimer methoxy groups at δ 3.9 and doubling of the

8

integration of hydroxyl groups at δ 5.7. NMR spectra of DHEO−NOVO (Figure 2, panel e),

9

which is achieved through the DHEO−formaldehyde reaction, reveals a methylene linkage in

10

the region of δ 3.5−4.1. The integration ratio of H4/H2 in Figure 2, panel e is around 0.57,

11

indicating the novolac oligomer contains mainly dimers and trimers, which is consistent with

12

a reported novolac product from wood−tar creosote.16

13

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Figure 2. 1H NMR spectra of (a) DHE, (b) DHE o−demethylated product (DHEO), (c)

3

DHE−Dimer, (d) DHEO−Dimer and (e) DHEO−NOVO.

4

NMR spectra of glycidyl ethers of polyphenols are demonstrated in Figure 3. As

5

previously described, glycidylation of DHEO with epichlorohydrin results in two products

6

GEDHEO and benzodioxane with a molar ratio of 5:1. GEDHEO exhibits epoxy protons H6a,

7

H6b and H5 at δ 2.75, 2.88 and 3.37, while benzodioxane shows characteristic protons H4a,

8

H5 and H4b at δ 3.99, 4.07 and 4.23 (Figure 3, panel a). Figure 3, panel b reveals

9

glycidylation of DHE−Dimer lead to only one product (GEDHE−Dimer) with characteristic

10

epoxy protons H8a, H8b and H7 at δ 2.6, 2.8 and 3.3, respectively. As for the spectra of

11

GEDHEO−Dimer (Figure 3, panel c) and GEDHEO−NOVO (Figure 3, panel d), it is difficult

12

to assign each peak because DHEO−NOVO have different methylene linkage positions, while

10


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the intramolecular cyclization between two adjacent hydroxyls (benzodioxane products) could

2

occur in both. Thus, NMR patterns of GEDHEO and GEDHE−Dimer in Figure 3, panels a

3

and b are tentatively used to define the regions of characteristic epoxy (δ 2.6−3.0 and δ

4

3.2−3.5) and benzodioxane (δ 3.7−4.4) protons. Figure 3, panels c and d depict that there are

5

peaks showing up in both regions, which indicates DHEO−Dimer and DHEO−NOVO are

6

epoxidized.

7 8

Figure 3. 1H NMR spectra of glycidylation products of DHE−based polyphenols. (a)

9

GEDHEO, (b) GEDHE−Dimer, (c) GEDHEO−Dimer and (d) GEDHEO−NOVO. Regions of

10

characteristic epoxy (δ 2.6−3.0 and δ 3.2−3.5) and benzodioxane protons (δ 3.7−4.4) are

11

highlighted. Benzodioxane derivatives of (c) and (d) are not shown in the figure.

12

FTIR spectroscopy.

13

The structures of polyphenols and corresponding epoxy monomers are further supported

14

by FTIR analysis. Figure S1, panel A reveals characteristic absorption bands of DHEO,

11



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DHE−Dimer, DHEO−Dimer and DHEO−NOVO appear at around 3355 cm−1 (O−H

2

stretching), 2864 cm−1, 2936 cm−1 and 2960 cm−1 (alkyl C−H stretch), and 1604 cm−1, 1516

3

cm−1, and 1445 cm−1 (aromatic C−C bond). After glycidylation of polyphenols with

4

epichlorohydrin, an epoxy ring band at 912 cm−1 and a C−O−C ether linkage at 1028 cm−1 are

5

observed (Figure 4). Meanwhile, OH bands of the glycidyl ethers decrease, indicating the

6

hydroxyl groups are consumed in the glycidylation process. The FTIR results are consistent

7

with the 1H NMR analysis confirming formation of the epoxy ring. This conclusion is further

8

supported by the FTIR spectra of the cured epoxy networks in Figure S1, panel B. When

9

epoxy monomers are cured with DETA, epoxy peaks are opened while hydroxyl groups are

10

generated concurrently. This process is reflected by the IR results, in which the epoxy band at

11

912 cm−1 of all cured networks disappear while the intensity of OH bands at around 3355 cm−1

12

increases.

13 14

Figure 4. FTIR spectra of GEDHEO, (b) GEDHE−Dimer, (c) GEDHEO−Dimer and (d)

15

GEDHEO−NOVO. FTIR bands corresponding to 912 cm−1 are attributed to epoxy groups.

16

Determination of Crosslinking Density Using Rubber Elasticity Theory.

17

A fundamental difference between low− and high−performance epoxy networks is the

12


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1

crosslinking density.30 Crosslinking densities (ρ) for cured DHE−based networks are

2

calculated from the equilibrium storage modulus in the rubber region over the α−relaxation

3

temperature (Tα) according to the rubber elasticity theory in Equation 2,31−33

ρ = E ' (φ RT )

4

(2)

5

where E’ is the storage modulus at Tα+30 °C. ϕ is the front factor (approximated to 1 in the

6

Flory theory32,34), while R and T are the gas constant and absolute temperature at Tα+30 °C,

7

respectively.

8

Table 1. Dynamic mechanical properties and crosslink density (ρ) of DHE−based epoxy

9

networks. Tα is α−relaxation temperature and E30’ is the storage modulus at 30 °C.

10

11

Epoxy networks from GEDHEO GEDHE−Dimer GEDHEO−Dimer GEDHEO−NOVO

Tα (°C) 40 70 84 139

E30’ (MPa) 159 551 1441 1703

E’ at Tα+30 °C (MPa) 1.13 4.31 10.56 35.91

ρ (mol/dm3) 0.39 1.39 3.28 9.77

12

Table 1 reveals the crosslinking density follows the order: DHEO < DHE−Dimer
GEDHEO (84.2 °C) > GEDHEO−Dimer (82.1 °C) >

14

GEDHEO−NOVO (72.9 °C). The shift of DSC exothermic peak to lower temperature

15

indicates increased overall activity of epoxy groups, which is derived from catalytic effects or

16

favorable configuration.39 GEDHE−Dimer has the highest peak temperature, which could be

17

caused by the phenomenon that no catalytic benzodioxane byproduct is observed for

16


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1

GEDHE−Dimer from the NMR spectra in Figure 3, panel b. As for GEDHEO,

2

GEDHEO−Dimer and GEDHEO−NOVO, certain amounts of benzodioxane with hydroxyls

3

that can catalyze the curing reactions in these mixtures, resulting in decreased peak

4

temperatures compared to GEDHE−Dimer. Meanwhile, the activation energy (Ea) decreases

5

from DHEO to DHEO−NOVO, which may be attributed to improved configuration of the

6

modified monomers.

7 8 9

10

Table 2. DSC curing data for different DHE−based epoxy monomers/DETA systems

11

exhibiting onset curing temperature (Ti), peak curing temperature (Tp) and activation energy

12

(Ea). Sample GEDHEO GEDHE−Dimer GEDHEO−Dimer GEDHEO−NOVO

13

Ti (°C) 34.5 51.4 42.7 39.6

Tp (°C) 84.1 91.7 82.1 72.9

Ea (kJ/mol) 54.9 45.9 44.0 41.9

Effect of Crosslinking Density on Thermal Stability.

17



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1 2

Figure 7. Thermogravimetric analysis thermograms of DHE−based epoxy networks as a

3

function of temperature.

4

Figure 7 demonstrates all epoxy networks show a one−step degradation profile, which is

5

attributed to the decomposition of cross−linked polymer network.40 Thermal stability of the

6

networks improves with increased crosslinking density. This is reflected in the change of

7

onset degradation temperature (expressed as Td5, temperature at 5% weight loss) from 193 °C

8

of GEDHEO based networks to 235, 245 and 297 °C of networks from DHE−Dimer,

9

DHEO−Dimer and DHEO−NOVO, respectively. This phenomenon is explained by that

10

polymer chains in highly−crosslinked networks are more constrained, leading to lower

11

mobility during thermal expansion.20 Meanwhile, the more tortuous pathway in the

12

highly−crosslinked network postpones the decomposed products to diffuse out and heat to

13

flow into the underlying materials. The statistic heat−resistant index temperature (Ts),41,42

14

which is calculated based on Td5 and Td30 (temperature at 30% weight loss) (Equation 3), is

15

characteristic of the thermal stability of cured resins. As can be seen in Table 3, Ts and Td50

16

(temperature at 50% weight loss) increase from DHEO to DHEO−NOVO based networks,

17

supporting the positive role of crosslinking density on thermal stability. Besides, Char500

18


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1

(char formed at 500 °C) of modified networks is also observed to be higher than the

2

DHEO−based resin.

Ts = 0.49 [Td 5 + 0.6(Td 30 − Td 5 ) ]

3

(3)

4

Table 3. Thermogravimetric data of Td5, Td30, Td50 (temperature at 5%, 30% and 50% weight

5

loss), Ts (statistic heat−resistant index temperature) and Char500 (char residue at 500 °C) of

6

DHE−based epoxy networks. Epoxy networks from GEDHEO GEDHE−Dimer GEDHEO−Dimer GEDHEO−NOVO

Td5 (°C) 193 235 245 297

Td30 (°C) 296 334 338 331

Td50 (°C) 321 351 361 362

Ts (°C) 125 144 147 153

Char500 (%) 3.3 10.9 10.6 10.1

7 8

Effect of Crosslinking Density on the Overall Performance of DHE−Based Epoxy

9

Networks

10

To describe more clearly the role of molecular weight, orientation and number of

11

functional groups on crosslinking density, and further investigate the influence of crosslinking

12

density on the overall performance, Tα, E30’, and Ts of DHE−based epoxy networks are

13

compared in Figure 8. Highlighted (in yellow) areas corresponding to crosslink densities of

14

0.39, 1.39, 3.28 and 9.77 mol/dm3 represent the performances (Tα, E30’, Ts and ∆H) of

15

networks from DHEO, DHE−Dimer, DHEO−Dimer and DHEO−NOVO, respectively. Dotted

16

boxes (A, B and C) between the highlighted areas reflect related modification methods.

17

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Figure 8. Performances (Tα, E30’, and Ts) of DHE−based epoxy networks as a function of

3

crosslink density as modified through adjusting molecular weight, orientation and number of

4

epoxy group. Highlighted areas correspond to the performances of DHE−based networks

5

while dotted boxes between the highlighted areas reflect related modification methods.

6

Figure 8 reveals mechanical and thermal performances of DHE−based networks improve

7

with increased crosslinking density. It is important to note that performances increase

8

proportionally with crosslinking density, except a relatively higher slope in box (A) which is

9

attributed to the role of molecular weight and orientation. The improvement in molecular

10

weight and orientation can be easily realized by reacting DHE and aqueous HBr at ambient

11

temperature and it is an effective way to increase crosslink density, mechanical (75% and 247%

12

increase in Tα and E30’) and thermal (15% increase in Ts) properties as demonstrated in Figure

13

8. As stated above, for DHE−Dimer, the methylene bonds between two DHE molecules are

20


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1

considered as apriori crosslinks. In another word, by making DHE−Dimer, 50% of DHE

2

molecules has already been linked even before the curing reaction. Moreover, the advantages

3

of DHE−Dimer especially evident in the case of DHE−based monomer, because the undesired

4

benzodioxane byproduct in GEDHEO is avoided and the obtained GEDHE−Dimer is more

5

stretched.

6

The number of epoxy groups increases from GEDHE−Dimer to GEDHEO−Dimer. This

7

modification, even though brings about certain amount of benzodioxane derivatives, still

8

provides evident improvement for the networks as seen in Figure 8, box (B). For example, the

9

crosslink density exhibits a 136% increase compared to the DHE−Dimer counterparts. The

10

effect of dimerization reaction can also be evident when DHEO and DHEO−Dimer based

11

networks are compared. As depicted in Figure 8, the dimerization of DHEO to DHEO−Dimer

12

essentially consists of improvements of molecular weight, orientation and number of

13

functional group. All these improvements contribute to the significant increase (741%) of

14

crosslink density from DHEO to DHEO−Dimer based networks.

15

DHEO−NOVO based network is also featured with continuous improvements of

16

molecular

weight,

orientation

and

number

of

functional

17

phenol−formaldehyde oligomerization reaction, the molecular weight of DHEO−NOVO

18

(average molecular weight between dimer and trimer) is slightly higher than DHEO−Dimer as

19

mentioned above. The effect of molecular weight increase from DHEO−Dimer to

20

DHEO−NOVO based networks can be estimated through the comparison in Figure 8, box (C).

21

A 198% increase in crosslinking density from DHEO−Dimer to DHEO−NOVO is observed.

22

This might imply that a slight increase in the molecular weight can evidently promote

21


group.

After

the


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

crosslinking density. Overall, it can be concluded that performance parameters of DHE−based

2

epoxy networks improve with increased crosslinking density. Oligomerization of DHEO can

3

create simultaneous modifications of molecular weight, orientation and number of functional

4

groups, which lead to the highest performance parameters among all investigated networks.

5

CONCLUSIONS

6

Renewable biobased epoxy networks become more relevant if the mechanical and thermal

7

properties are improved. DHE, a renewable building block monomer derived from lignin, has

8

unfavorable structural characteristics if used without modification. However, modifications of

9

DHE−based epoxy monomers through improving (1) molecular weight, (2) orientation and (3)

10

number of epoxy groups via dimerization and oligomerization reactions are found to increase

11

the crosslinking density of cured thermosets, while enhanced crosslinking density improves

12

thermal and mechanical properties. Oligomerization of DHEO is an effective way of

13

improving performances due to its simultaneous modifications of molecular weight,

14

orientation and number of functional groups. The modified renewable DHE−based epoxy

15

networks exhibit sufficiently desirable performance parameters to potentially replace

16

petroleum−based thermosets.

17

ACKNOWLEDGEMENTS

18

This research was supported in part by the Center for direct Conversion of Biomass to

19

Biofuels (C3Bio), an Energy Frontiers Research Center (EFRC) funded by the U.S.

20

Department of Energy, Office of Science, Office of Basic Energy Sciences under award no.

21

DE−SC000097 and by Purdue University.

22

Authors’ Information

22


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1

Current address: Department of Chemistry and Biochemistry, and Department of Chemical

2

Engineering, University of California, Santa Barbara, CA 93106.

3

Corresponding author: [email protected]

4

Supporting Information

5

FTIR, 3−D models of epoxy monomers and DSC analysis.

6

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For Table of Contents Use Only

27



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1 2

Renewable Epoxy Networks Derived from Lignin−Based Monomers:

3

Effect of Crosslinking Density

4

Shou Zhao† and Mahdi M. Abu−Omar†, ‡,*

5

†

6

Lafayette, IN 47907

7

‡

8

480 Stadium Mall Drive, West Lafayette, IN 47907

Brown Laboratory and Department of Chemistry, Purdue University, 560 Oval Drive, West

School of Chemical Engineering, Purdue University, Forney Hall of Chemical Engineering,

9 10

Synopsis:

11

Mechanical and thermal properties of lignin−based epoxy thermosets can be significantly

12

improved through chemical modifications.

28


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Renewable Epoxy Networks Derived from Lignin ... - ACS Publications

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