Lignin model compounds as bio-based reactive diluents for liquid molding resins

Article abstract of DOI:10.1002/cssc.201100687

Lignin is a copious paper and pulping waste product that has the potential to yield valuable, low molecular weight, single aromatic chemicals when strategically depolymerized. The single aromatic lignin model compounds, vanillin, guaiacol, and eugenol, were methacrylated by esterification with methacrylic anhydride and a catalytic amount of 4-dimethylaminopyridine. Methacrylated guaiacol (MG) and methacrylated eugenol (ME) exhibited low viscosities at room temperature (MG: 17 cP and ME: 28 cP). When used as reactive diluents in vinyl ester resins, they produced resin viscosities higher than that of vinyl ester-styrene blends. The relative volatilities of MG (1.05 wt % loss in 18 h) and ME (0.96 wt % loss in 18 h) measured by means of thermogravimetric analysis (TGA) were considerably lower than that of styrene (93.7 wt % loss in 3 h) indicating the potential of these chemicals to be environmentally friendly reactive diluents. Bulk polymerization of MG and ME generated homopolymers with glass transition temperatures (T_gs) of 92 and 103 °C, respectively. Blends of a standard vinyl ester resin with MG and ME (50 wt % reactive diluent) produced thermosets with T_gs of 127 and 153 °C, respectively, which are comparable to vinyl ester-styrene resins, thus demonstrating the ability of MG and ME to completely replace styrene as reactive diluents in liquid molding resins without sacrificing cured-resin thermal performance. Copyright

Full text of DOI:10.1002/cssc.201100687

DOI: 10.1002/cssc.201100687
Lignin Model Compounds as Bio-Based Reactive Diluents
for Liquid Molding Resins
Joseph F. Stanzione, III,[a] Joshua M. Sadler,[b] John J. La Scala,[b] and Richard P. Wool*[a]
Lignin is a copious paper and pulping waste product that has
the potential to yield valuable, low molecular weight, single ar-
omatic chemicals when strategically depolymerized. The single
aromatic lignin model compounds, vanillin, guaiacol, and eu-
genol, were methacrylated by esterification with methacrylic
anhydride and a catalytic amount of 4-dimethylaminopyridine.
Methacrylated guaiacol (MG) and methacrylated eugenol (ME)
exhibited low viscosities at room temperature (MG: 17 cP and
ME: 28 cP). When used as reactive diluents in vinyl ester resins,
they produced resin viscosities higher than that of vinyl ester–
styrene blends. The relative volatilities of MG (1.05 wt% loss in
18 h) and ME (0.96 wt% loss in 18 h) measured by means of
thermogravimetric analysis (TGA) were considerably lower than
that of styrene (93.7 wt% loss in 3 h) indicating the potential
of these chemicals to be environmentally friendly reactive di-
luents. Bulk polymerization of MG and ME generated homopol-
ymers with glass transition temperatures (Tgs) of 92 and 1038C,
respectively. Blends of a standard vinyl ester resin with MG and
ME (50 wt% reactive diluent) produced thermosets with Tgs of
127 and 1538C, respectively, which are comparable to vinyl
ester–styrene resins, thus demonstrating the ability of MG and
ME to completely replace styrene as reactive diluents in liquid
molding resins without sacrificing cured-resin thermal perfor-
mance.
Introduction
Vinyl ester resins are used to produce polymer composites for
commercial applications because they have relatively high
moduli, strength, and glass transition temperatures (Tgs) while
maintaining low weight and cost.[1,2] To facilitate the use of
liquid molding techniques to fabricate large scale composite
parts, commercial vinyl ester resins typically contain high con-
centrations (>40 wt%) of a reactive diluent such as styrene
(St). In addition to lowering the resin viscosity, styrene is
a linear chain extender. This improves the overall polymer per-
formance by increasing the monomeric reactivity by delaying
the onset of gelation and reducing diffusion limitations.[2] How-
ever, styrene has been designated a hazardous air pollutant
and a volatile organic compound (VOC).[3] In 2003, with an
amendment in 2005, the Federal Environmental Protection
Agency (EPA) of the United States of America introduced legis-
lation to limit styrene emissions from composite manufactur-
ing.[1,3,4] More recently, in June 2011, the USA’s Department of
Health and Human Services through the National Toxicology
Program in their 12th Report on Carcinogens has designated
styrene as “reasonably anticipated to be a human carcino-
gen”.[5] Moreover, not only is styrene emitted during metering,
mixing, processing, and curing, but also studies have shown
that up to 40% of styrene can remain unreacted after curing
and continues to be released from composites during the re-
maining phases of their life cycle if they are not subjected to
post-curing.[6] With this significant drawback, new resins are re-
quired for the continued use of polymer composites. There-
fore, the development of nonvolatile reactive diluents with sty-
rene-like performance is gaining increasing interest. Additional-
ly, with the continued volatility of the petroleum industry and
crude oil price fluctuations, the cost of styrene will eventually
become a factor in the production of future resins if a suitable
bio-based replacement is not identified. These significant fac-
tors give impetus for the discovery and development of re-
newable, bio-based reactive diluents that have styrene-like per-
formance with minimal VOC emissions.
Previous work by La Scala et al., in conjunction with the Af-
fordable Composites from Renewable Resources (ACRES) pro-
gram at the University of Delaware, has demonstrated the po-
tential of using methacrylated fatty acids (MFAs) as styrene re-
placements. These include methacrylated lauric acid (MLau),
methacrylated hexanoic acid (MHex) and methacrylated octa-
noic acid (MOct) as styrene reducers in vinyl ester-, soybean-,
and castor oil-based polymer resins.[1,7–9] These renewable, nat-
urally occurring, methacrylated plant oil derivatives exhibit low
volatilities, low viscosities (40–80 cP at 308C), and can act as
acceptable chain extenders. These functionalized plant oil de-
rivatives can also toughen polymers through their flexible,
long alkyl side chains. Despite these benefits, styrene was still
needed at a reduced concentration to produce cured bio-
based resins with thermal and mechanical performance com-
parable to cured, commercial vinyl ester–styrene resins.[1,7]
Thus, lower viscosity, nonvolatile, renewable reactive diluents
[a] J. F. Stanzione, III, Prof. R. P. Wool
Department of Chemical Engineering and Center for Composite Materials
University of Delaware
Newark, DE 19716 (USA)
Fax: (+1) 302 831-1048
E-mail: wool@udel.edu
[b] Dr. J. M. Sadler, Dr. J. J. La Scala
Army Research Laboratory
Aberdeen Proving Ground, MD 21005 (USA)
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R. P. Wool et al.
with styrene-like performance
are still being sought, particular-
ly reactive diluents that have
similar Tgs to polystyrene when
polymerized.
Lignin is a renewable, bio-
based alternative material rich in
aromaticity with the capability of
yielding single aromatic chemi-
cals when selectively broken
down. Lignin is abundantly avail-
able with over 3ꢁ1011 t existing
in the biosphere with approxi-
mately 2ꢁ1010 t generated annu-
ally and is the second most
abundant natural raw material,
surpassed only by cellulose.[10–12]
As of 2004, the pulp and paper
industry alone produced roughly
50ꢁ106 t per year of extracted
lignin of which 95% was Kraft
lignin.[12,13] The Kraft process, in-
vented in 1879 by Carl F. Dahl, is
the treatment of wood chips
with a mixture of sodium hy-
droxide and sodium sulfide,
known as white liquor, which
separates the lignin from cellu-
lose
and
hemicellulose.[12,13]
However, only approximately
2% of the available lignin is
used commercially with the re-
mainder used as a low-value fuel
to cover the internal energy
needs of the pulp and paper in-
dustry.[14] Due to its very com-
plex, 3D aromatic structure
(Figure 1, top), lignin is largely
intractable in its unmodified
state and, therefore, is often
sought to be broken down into
Figure 1. General chemical structure of lignin (top) with a schematic for its conversion into single aromatic chemi-
cals (bottom).[18,19] Reprinted with kind permission from John Wiley & Sons (top) and Elsevier (bottom).
lower molecular weight aromatic components
(Figure 1, bottom).
Non-degraded and chemically modified lignins
have been successfully incorporated into various ma-
terials, including polymeric materials.[10,12,13,15–17] Addi-
tionally, a considerable amount of research has been
conducted into selectively breaking down all types of
lignin and multi-phenolic lignin model compounds
(LMCs) to develop new chemicals and renewable
sources of predominantly petroleum-derived chemi-
cals.[11,14,16–24] Recently, a modest yield of vanillin
(Figure 2) from Kraft lignin has been obtained using
aqueous polyoxometalates in the presence of alco-
hols to oxidatively degrade lignin.[11] Additionally, va-
nillin production by Kraft lignin oxidation using
a NaOH alkaline medium has been reported.[22] More- Figure 2. Chemical structures of lignin model compounds, their derivatives, and styrene.
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Lignin Model Compounds
over, a 70% yield of guaiacol has been achieved from the
common b-O-4 LMCs, guaiacylglycerol-b-guaiacyl and veratryl-
glycerol-b-guaiacyl ether, by catalytic hydrolysis using an acidic
ionic liquid.[19] In 2011, Zakzeski and Weckhuysen reported 22.2
and 12% yields of guaiacol from aqueous phase reforming of
a b-O-4 linked and a 5-5’ carbon–carbon-linked biphenyl
model compound, respectively, using a Pt/Al2O3 catalyst.[14]
Lastly, Sergeev and Hartwig have reported yields as high as
99% of guaiacol from hydrogenolysis reactions of representa-
tive biphenyl LMCs.[24]
4-dimethylaminopyridine (DMAP) has not been previously re-
ported in the synthesis of phenyl methacrylates. However,
methacrylations of lignins (essentially multi-phenolic macromo-
lecules) have been performed by Thielemans and Wool by
using 1-methylimidazole as the base catalyst with methacrylic
anhydride as the coreactant.[34] Despite acryl anhydrides being
less stable than acrylic acids, a significantly lower amount of
catalyst is required than for the Steglich and acid-catalyzed es-
terifications. An alternative phenolic-methacrylation technique
is a transesterification reaction involving methyl methacrylate
catalyzed by p-toluenesulfonic acid with methanol, a classified
green solvent,[35] as the byproduct. Currently, this reaction is
being investigated as a potential scalable reaction to synthe-
size MLMCs. Scheme 1 depicts the synthetic route used to gen-
erate the MLMCs.
In the work presented herein, vanillin, guaiacol, and eugenol
(Figure 2) were selected as representative LMCs for the synthe-
sis of bio-based reactive diluents as styrene replacements in
thermosetting polymers. Specifically, these LMCs were selected
based on extensive studies and the potential to obtain these
chemicals in high yields from
lignin, as well as from clove oil
in the case of eugenol.[18,20,25] To
chemically incorporate the se-
lected LMCs into vinyl ester
resins, polymerizable carbon–
carbon double bonds need to
be integrated onto the phenolic
compounds. Methacrylated ana-
logues of vanillin,[26,27] guaia-
col,[28–31]
and
eugenol[25,32]
(Figure 2) have been reported in
the literature, but, to date, have
not been studied as styrene re-
placements in polymer resins.
We report the synthesis, relative
Scheme 1. Reaction of methacrylic anhydride with a lignin model compound to form a methacrylated lignin
model compound monomer.
volatilities, viscosities, and polymer Tgs of the methacrylated
lignin model compounds (MLMCs) as well as the viscosities
and Tgs of vinyl ester resins (bismethacryl glycidyl ether of bi-
sphenol A epoxy, VE828) copolymerized with 50 wt% MLMCs.
The relative volatility of styrene and the viscosity and Tg of
VE828 with 50 wt% styrene are also reported for direct com-
parison.
Methacrylated guaiacol (MG) and methacrylated eugenol
(ME) are low viscosity liquids at 258C. Methacrylated vanillin
(MV) is a solid at 258C and was not used as a reactive diluent
to replace styrene in this work. Average yields of 85.5Æ4.5%
were obtained for the MLMC esterification reactions. The puri-
ties of the MLMCs were estimated to be ꢀ94% based on
1H NMR spectra. The impurities are believed to arise from the
methacrylic anhydride, which is typically sold with 94% purity.
In the case of methacrylic anhydride, Topanol A, 2-tert-butyl-
4,6-dimethylphenol, was used as the free radical inhibitor at
a concentration of 2000 ppm. It is believed that, although the
hydroxyl group of Topanol A is greatly hindered compared to
the hydroxyl groups of the LMCs, Topanol A participated in the
phenolic methacrylation reactions and was collected with the
desired product. However, methacrylated Topanol A contribut-
ed to no more than 0.2 wt% of the impurities. Despite this, fur-
ther purification was not pursued and, after concentration
under reduced pressure, the desired product was used as col-
lected.
Results and Discussion
Monomer properties
Methacrylated versions of vanillin, guaiacol, and eugenol,
which were synthesized by an acid catalyzed esterification,
a Steglich esterification, or an esterification involving methacry-
loyl chloride, have been reported in the literature.[25–29,32] The
major disadvantage of the Steglich esterification is the require-
ment of relatively large quantities of catalyst, 4-dimethylamino-
pyridine, and reagent, N,N’-dicyclohexylcarbodiimide (15.3 and
153 wt% of total reactant mass, respectively).[27] Similar to the
Steglich esterification, the reported acid-catalyzed esterifica-
tions of hydroxyl groups with an acid may require relatively
large quantities of catalyst.[33] Although the reactivity of acrylo-
yl chloride is desirable, less desirable are the hazards associat-
ed with transporting and storing such unstable chemicals. The
utilization of methacrylic anhydride with catalytic amounts of
To determine the volatilities of MG and ME relative to sty-
rene, thermogravimetric analysis (TGA) evaporation studies
were performed (Figure 3). Under the experimental conditions,
styrene almost fully evaporated in about three hours at 308C
(only 6.3 wt% remained after three hours). Conversely, 99.04
and 98.95 wt% of ME and MG, respectively, remained after
18 h at 308C (Figure 3, inset). This demonstrates that the
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a result of the slightly higher molecular weight of the ME due
to the allyl substituent. In addition, MG and ME have increased
intermolecular attractions relative to styrene due to the pres-
ence of ester linkages.[36]
As seen in Table 1, the viscosities of each VE828-reactive di-
luent resin system decreased with reactive diluent molecular
weight. The VE828–ME blend had a slightly higher viscosity
than the VE828–MG blend, while both had significantly higher
viscosities than the VE828-St blend. The observed order of
magnitude higher viscosities of the VE828-MLMC resins com-
pared to those of the VE828-St resin is attributed to the MLMC
being able to hydrogen-bond with the hydroxyl groups of
VE828 through the methacrylate group. Hydroxyl–ester hydro-
gen bonding has been shown to increase resin viscosity of
similar vinyl ester resin systems that contained methacrylic-
based reactive diluents.[1,2] However, resins containing 50 wt%
MG and ME maintain viscosities amenable to molding applica-
tions.[12]
Figure 3. TGA normalized weight as a function of time for styrene, MG and
ME at T=308C. See inset for the MG and ME evaporation behavior. The
maximum standard deviation among the styrene TGA experiments was
Æ0.17.
MLMCs have a considerably lower volatility than styrene and
a potential to be environmentally friendly and low VOC reac-
tive diluents. The crossing of the MG and ME curves at roughly
600 min and other differences between the curves are within
experimental error and not representative of the difference be-
tween the evaporation behavior of MG and ME. However, due
to the structure and molecular weight similarities of MG and
ME, the evaporation behavior of these MLMCs was anticipated
to be similar.
Polymer glass transition temperature
Polystyrene has a relatively high Tg (ꢁ1008C).[37] The aromatic
side chain of styrene is known to provide structural rigidity,
thermal stability, limited “free volume”, and brittleness through
p-bond stacking. According to a new theory of the glass transi-
tion entitled the Twinkling Fractal Theory (TFT), the glassiness
of polystyrene at room temperature is attributed to the rigid
aromatic side chains rapidly slowing down the twinkling fractal
dynamics and, thus, enhancing vector percolation of solid frac-
tal clusters and ultimately fractal cavitation.[38,39] The enhance-
ment of vector percolation and fractal cavitation occurs de-
spite a considerable amount of liquid-like clusters still existing,
approximately 40% at 298 K.[38,39] The liquid-like clusters exhibit
characteristics of “free volume”, but due to the closeness of
the aromatic side chains to the polymer backbone they lack
the normal alkyl chain-like mobility. Thus, they contribute to
the relatively high onset temperature of vector percolation, Tcv,
and fractal cavitation at Tg =1008C.[38–40]
To elucidate the ability of MG and ME to replace styrene as
reactive diluents in resins, MG, ME, and styrene were blended
(50 wt%) with a standard vinyl ester resin, VE828. The viscosi-
ties of these blends, as well as the neat viscosities of MG and
ME, were evaluated. The results are shown in Table 1, where
Table 1. Viscosities of the neat MLMCs and VE828 resins. Styrene viscosi-
ty was obtained from literature (at 308C).[1]
System
Viscosity at 258C [cP]
The similarities between the MLMCs and styrene are evident
through the aromatic ring and the presence of a reactive ter-
minal carbon–carbon double bond. Additionally, it is interest-
ing to note the similarities between the MLMCs and methyl
methacrylate, which has a Tg of approximately 1048C when ho-
mopolymerized.[37] Despite the fact that there are alkyl sub-
stituents on the phenol groups that increase the bulkiness of
polymer side chains, the MLMCs, when polymerized, are ex-
pected to possess Tgs close to that of polystyrene (PS) and
poly(methyl methacrylate) (PMMA) and exhibit twinkling dy-
namics similar to that of PS and PMMA.
MG
ME
St
17.1Æ1.2
27.9Æ1.3
0.7
937Æ10
1148Æ36
16.4Æ1.7
1/1 VE828–MG
1/1 VE828–ME
1/1 VE828–St
the viscosities at 258C are listed with standard deviations. In-
cluded in Table 1 is the viscosity of styrene at 308C as reported
in the literature.[1] For all samples, the viscosities exhibited
Newtonian behavior. The neat MG and ME monomers have
higher viscosities than that of styrene (0.7 cP at 308C). This is
attributed to the presence of bulkier substituent groups on
the phenyl ring of the neat MLMC monomers relative to just
the vinyl group on styrene as van der Waals attractions are
proportional to molecular weight.[35] Accordingly, the neat ME
monomer has a slightly higher viscosity than that of MG as
Prior to methacrylation, eugenol contains a terminal allyl
group in the para position relative to the hydroxyl group on
the aromatic ring. However, eugenol itself is not easily poly-
merized due to the presence of a phenolic hydroxyl group,
which acts as a free radical scavenger, and the allylic nature of
the propene substituent group.[25,32] The reactivity of the allyl
group of eugenol, which determines whether ME behaves as
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Lignin Model Compounds
a monofunctional reactive diluent or as a cross-linker in vinyl
ester resin systems, is currently under investigation.
degree of rigidity and lowers molecular mobility during poly-
merization. Allylic monomer reactivity in the presence of
methyl methacrylate has been reported to be four orders of
magnitude lower than that of the comonomer.[41] Additionally,
Rojo et al. have reported thermosetting-type behavior of PME,
which is attributed to pendant allyl groups contributing to
grafting and cross-linking when polymerization reaches high
conversion.[25,32] Intentionally breaking PMG and PME speci-
mens, PME exhibited greater qualitative brittleness compared
to PMG indicating a potential degree of cross-linking. This is
consistent with the fact that increased cross-linking is known
to increase the breadth of the glass transition.[36] Currently, we
are investigating the reactivity of the allyl group relative to the
acrylic groups in the curing of vinyl ester resins by performing
in situ FTIR cure kinetics studies.
The Tgs of poly(methacrylated guaiacol) (PMG), poly(metha-
crylated eugenol) (PME), and the cured vinyl ester resins were
determined by means of differential scanning calorimetry
(DSC). The DSC thermograms and the measured Tgs are shown
in Figure 4. All of the Tgs of the VE828 cured resins are above
Conclusions
In this work, lignin model compounds—vanillin, guaiacol, and
eugenol—were methacrylated by esterification with methacryl-
ic anhydride and a catalytic amount of DMAP. MG and ME,
both low viscosity and nonvolatile liquids at 258C, were shown
to be viable, bio-based candidates to completely replace sty-
rene as reactive diluents in liquid molding resins. Bulk polymer-
izations of MG and ME generated polymers with Tgs very simi-
lar to those of polystyrene and poly(methyl methacrylate).
Blends of a standard vinyl ester resin (VE828) with MG and ME
produced thermosets with Tgs that were comparable to those
of commercial vinyl ester–styrene based thermosets. Overall,
both MG and ME were successfully utilized as low viscosity,
nonvolatile reactive diluents in liquid molding resins with MG
acting solely as a monofunctional monomer and ME acting as
a monofunctional monomer with the potential ability to cross-
link through the allyl group.
Figure 4. DSC thermograms of VE828 cured resins that contain 50 wt% of
a reactive diluent along with DSC thermograms of PMG and PME. Thermo-
grams are offset for clarity.
1208C and the Tgs of PMG and PME are 92 and 1038C, respec-
tively. The 1:1 VE828–MG Tg is lower than the 1:1 VE828–St Tg,
whereas the 1:1 VE828–ME Tg is higher than that of 1:1 VE828–
St. Wesslꢂn et al. reported a PMG (number average molecular
weight, Mn,=7600 gmolÀ1) Tg of 1108C determined by means
of DSC,[29] whereas Rojo et al. reported a low-conversion
(<10 wt%) PME (Mn =8.1ꢁ104 gmolÀ1) Tg of 968C measured
by DSC at a heating rate of 108CminÀ1 [32]
PMG and PME were
.
not soluble in THF indicating the preparation of high molecu-
lar weight polymers. Despite the discrepancies between the
homopolymer Tgs reported in this paper and those reported
previously, the facts that PMG and PME have similar Tgs to
those of PS and PMMA and that the cured vinyl ester resins
have similar Tgs to those that contain styrene are encouraging.
Glass transition broadness increased with reactive diluent
molecular weight (styrene<MG<ME). This may be a result of
an increased amount of relaxation modes present in the poly-
mer, which according to TFT, arises when a broad distribution
of solid fractal clusters twinkles into the liquid upon heating
exists.[38–40] With increasing resin viscosity, molecular mobility is
suppressed during cure leading to a greater chance of an ir-
regular cross-linked network, and, in turn, a broader distribu-
tion of solid fractal clusters and relaxation dynamics. Unlike
styrene, MG and ME contain methacrylate groups that are ca-
pable of hydrogen bonding with the hydroxyl groups of
VE828. Hydrogen bonding may influence the relaxation dy-
namics of the cross-linked network and the ability of the solid
fractal clusters to transition into the liquid. Furthermore, the ir-
regularity of the cross-linked network may further increase due
to the ability of ME to cross-link, which imparts a higher
Experimental Section
Materials
Deuterated dimethyl sulfoxide ([D6]DMSO), 4-dimethylaminopyri-
dine (DMAP), vanillin (99%), guaiacol (99 +%), and eugenol (99%)
were purchased from Fisher Scientific and used as received. Metha-
crylic anhydride (94%, inhibited with 2000 ppm Topanol A), CDCl3,
2-butanone peroxide (MEKP), and styrene (inhibited with 10–
15 ppm 4-tert-butylcatechol) were purchased from Sigma Aldrich
and used as received. Compressed argon was purchased from
Keen Compressed Gas Co. (99.998%). Trigonox 239 (AkzoNobel
Polymer Chemicals), containing 45% cumene hydroperoxide, was
purchased and utilized as a free radical initiator.
The vinyl ester cross-linking monomer was prepared as described
in the literature.[1,2] Epon 828 was converted to the bismethacryl
glycidyl ether of bisphenol A epoxy (Hexion Specialty Chemicals,
Inc.) using standard literature procedures and is referred to as vinyl
ester 828 (VE828). Characterization methods as described by La S-
cala et al. were performed and similar results were obtained.[1]
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Synthesis of methacrylated lignin model compounds
Monomer and resin viscosity
Monomer and resin viscosities were obtained by using a TA Instru-
ments ARES-G2 rheometer. Measurements were performed isother-
mally at 258C controlled by a Peltier plate (+/À 0.18C error). A
20 mm 18 steel cone with a truncation gap of 25 mm and a mini-
mum sample volume of 0.04 mL was utilized. The shear rate was
increased stepwise from 1 to 100 sÀ1 and 21 data points were col-
lected to observe any non-Newtonian behavior. At a given shear
rate, the shear stress was measured every two seconds. The shear
rate and viscosity were recorded when the shear rate stabilized to
within 5% tolerance for three consecutive points. Each monomer
and resin viscosity was measured three times, and the viscosities at
50 sÀ1 were averaged and reported.
To a 100 mL round-bottomed flask equipped with a magnetic stir
bar, a catalytic amount of DMAP (2 mol% of methacrylic anhy-
dride) was added to a lignin model compound (LMC, 20 g). Prior to
adding methacrylic anhydride (1.2 LMC equivalents, adjustable to
as low as 1.01), the flask was sealed and subsequently purged with
argon gas for an hour to remove moisture and oxygen from the re-
action vessel. For the first three hours with stirring, the reaction
progressed at room temperature. The flask was placed in a 458C
silicone oil bath for a minimum of 24 h. The reaction mixture was
then cooled to room temperature and diluted with methylene
chloride (150 mL, ethyl acetate was also used as a greener substi-
tute). To remove unreacted methacrylic anhydride and methacrylic
acid product, the organic phase was washed repeatedly with a sa-
turated sodium bicarbonate aqueous solution (150 mL) until
carbon dioxide no longer evolved. The organic phase was then
washed with 1.0m NaOHaq (150 mL), 0.5m NaOHaq (150 mL), 1.0m
HClaq (150 mL), and water (150 mL); dried over sodium sulfate; and
then concentrated under reduced pressure.
Thermogravimetric evaporation study
The evaporation behavior of MG, ME, and styrene was measured
by using a TA Instruments Q500 TGA. Approximately 50 mg of
sample was placed in a platinum pan and held isothermally at
308C in a nitrogen atmosphere (40 mLminÀ1 balance gas flow rate
and 60 mLminÀ1 sample gas flow rate). Evaporation studies of
methacrylated LMCs (MLMCs) and styrene were performed for 18
and 3 h, respectively. Experiments were conducted three times to
ensure reproducibility. The evaporation of MV was not measured
as it is a solid at room temperature and we believe that the subli-
mation rate would be orders of magnitude lower than the evapo-
ration rates of styrene, MG, and ME.
1
Products were characterized by means of H NMR (400.13 MHz, 16
scans at 298.2 K) and 13C NMR spectroscopy (100.6 MHz, 32 scans
at 298.2 K) by using a Bruker AV-400 spectrometer, and the spectra
showed peaks in agreement with the expected chemical shifts. Ad-
ditionally, the products were characterized by FTIR spectroscopy
performed on a Perkin Elmer Spectrum 400 FT-IR/FT-NIR Spectrom-
eter. At room temperature, 16 cumulative scans were acquired
with a resolution of 4 cmÀ1 in transmission mode in the mid-IR
range.
Methacrylated vanillin (MV, 3-methoxy-4-methacryloyloxybenzalde-
hyde, mp=55.48C; in agreement with literature values):[26,42]
1H NMR ([D6]DMSO): d=9.98 (s, 1H), 7.62–7.40 (bm, 7.62–7.40 (bm,
3H), 6.30 (s, 1H), 5.95 (t, 1H), 3.86 (s, 3H), 2.00 ppm (s, 3H).
13C NMR ([D6]DMSO): d=192.58, 164.74, 152.05, 144.76, 135.60,
135.07, 128.98, 124.28, 124.13, 112.32, 56.57, 18.50 ppm. Notable
FTIR spectral peaks of MV: carbonyl wagging vibrations associated
Resin cure
Homopolymers of MG, ME, polymerized MG (PMG) and ME (PME),
were prepared by bulk polymerization by adding 1.5 wt% MEKP to
the monomer with curing at 908C for 4 h and subsequent post-
curing at 1308C for two hours. Polymerizations were conducted in
an argon atmosphere, whereby argon gas was purged through the
oven for approximately 15 min at the beginning of cure. This purg-
ing technique was used for all subsequently described polymeri-
zations. MV was not cured as it is a solid at standard temperature
and pressure, thereby eliminating it as a reactive diluent candidate.
However, curing of resins containing MV is currently under investi-
gation.
with the methacrylate and acetal groups, n˜ =1745 and 1702 cmÀ1
respectively; terminal C=C wagging vibration, n˜ =1636 cmÀ1; termi-
nal C=CH2 bending vibration, n˜ =947 cmÀ1
,
.
Methacrylated guaiacol (MG, 2-methoxyphenyl methacrylate, mp=
À59.98C)[42]:1H NMR ([D6]DMSO): d=7.28–6.95 (bm, 4H), 6.28 (s,
1H), 5.89 (t, 1H), 3.76 (s, 3H), 2.00 ppm (s, 3H). 13C NMR
([D6]DMSO): d=165.17, 151.38, 139.82, 135.49, 128.13, 127.42,
123.32, 121.03, 113.22, 56.15, 18.55 ppm. Notable FTIR spectral
peaks of MG: carbonyl wagging vibration, n˜ =1737 cmÀ1; terminal
C=C wagging vibration, n˜ =1641 cmÀ1; terminal C=CH2 bending vi-
VE828 resins containing 50 wt% MLMC monomers were prepared.
VE828–St resin (50 wt% St) was blended for use as a standard
equivalent to commercial resins. In an argon atmosphere, resins
were cured at 908C using Trigonox 239 (1.5 wt% of the total resin
mass) for 4 h and then post-cured at 1808C for 2 h.
bration, n˜ =945 cmÀ1
.
Methacrylated eugenol (ME, 4-allyl-2-methoxyphenyl methacrylate,
mp=À58.28C):[42] 1H NMR ([D6]DMSO): d=7.03–6.76 (bm, 3H), 6.25
(t, 1H), 6.03–5.93 (bm, 1H), 5.86 (t, 1H), 5.14–5.05 (bm, 2H), 3.73 (s,
3H), 3.38 (d, 2H), 1.98 ppm (s, 3H). 13C NMR ([D6]DMSO): d=
165.27, 151.13, 139.29, 138, 137.91, 135.51, 128.08, 123.06, 120.73,
116.48, 113.32, 56.12, 39.96, 18.58 ppm. Notable FTIR spectral peaks
of ME: carbonyl wagging vibration, n˜ =1741 cmÀ1; terminal C=C
wagging vibrations associated with the methacrylate and allyl
groups, n˜ =1638 cmÀ1; terminal C=CH2 bending vibrations associat-
Polymer glass transition temperature
Thermo-physical behavior of PMG, PME, and the VE828 cured
resins was measured by using differential scanning calorimetry
(DSC). A TA Instruments Discovery DSC was used with 40 mL alumi-
num pans as sample holders with sample masses of approximately
ed with the methacrylate and allyl groups, n˜ =946 and 916 cmÀ1
respectively.[25,32,43–45]
,
10 mg. With a continuous nitrogen purge at a rate of 50 mLminÀ1
,
the samples were cooled to À908C and then heated to 2258C at
a rate of 108CminÀ1. The cooling–heating cycle was repeated three
times to eliminate any pre-existing thermal history. The third heat-
ing cycle was used for glass transition temperature acquisition.
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Lignin Model Compounds
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Acknowledgements
The authors gratefully acknowledge the U.S. Army Research Lab-
oratory for financial support under the Strategic Environmental
Research Development Program (SERDP) WP-1758 and through
the Cooperative Agreement W911NF-06-2-001. In addition, the
authors would like to thank Dr. Michael Mackay of the Materials
Science and Engineering Department at the University of Dela-
ware for usage of his DSC and TGA.
Keywords: lignin · renewable resources · resins · styrene ·
sustainable chemistry
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Published online on && &&, 0000
ChemSusChem 0000, 00, 1 – 8
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www.chemsuschem.org
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FULL PAPERS
J. F. Stanzione, III, J. M. Sadler,
J. J. La Scala, R. P. Wool*
&& – &&
Lignin Model Compounds as Bio-
Based Reactive Diluents for Liquid
Molding Resins
Glassy lignin makes good resins:
Lignin model compounds have been
methacrylated and utilized as reactive
diluents in a vinyl ester-based resin
without the need to include the tradi-
tionally used reactive diluent styrene.
The glass transition temperatures of the
cured resins are comparable to those
containing styrene, demonstrating the
ability of these lignin model compounds
to completely replace styrene as reac-
tive diluents in liquid molding resins
without sacrificing thermal perfor-
mance.
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 8
ÝÝ
These are not the final page numbers!