Vanillin-based resin for use in composite applications

Article abstract of DOI:10.1039/c2gc35672d

Lignin is an abundant, renewable material that has the potential to yield valuable, low molecular weight, single aromatic chemicals when strategically depolymerized. In order to generate a highly bio-based thermoset for use in polymer composites, a lignin-derived chemical, vanillin, was methacrylated in a two-step, one-pot synthesis to produce a vinyl ester resin (87 cP at 25 °C) with a 1:1 mole ratio of a mono-functional monomer, methacrylated vanillin, to cross-linking agent, glycerol dimethacrylate. The synthetic scheme was solventless, required little catalyst and moderate reaction temperatures while generating no by-products. Upon resin curing, a hard, transparent thermoset with a broad glass transition, T_g = 155 °C (based on the tan δ maximum), and a temperature of maximum decomposition rate, T_max, of 426 °C was produced. Overall, a potentially 100% bio-based thermoset was synthesized possessing comparable thermo-gravimetric and thermo-mechanical properties to commercial vinyl ester-based thermosets.

Full text of DOI:10.1039/c2gc35672d

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PAPER
Vanillin-based resin for use in composite applications
a
b
b
c
a
Joseph F. Stanzione III, Joshua M. Sadler, John J. La Scala, Kaleigh H. Reno and Richard P. Wool*
Received 2nd May 2012, Accepted 12th June 2012
DOI: 10.1039/c2gc35672d
Lignin is an abundant, renewable material that has the potential to yield valuable, low molecular weight,
single aromatic chemicals when strategically depolymerized. In order to generate a highly bio-based
thermoset for use in polymer composites, a lignin-derived chemical, vanillin, was methacrylated in a two-
step, one-pot synthesis to produce a vinyl ester resin (87 cP at 25 °C) with a 1 : 1 mole ratio of a mono-
functional monomer, methacrylated vanillin, to cross-linking agent, glycerol dimethacrylate. The synthetic
scheme was solventless, required little catalyst and moderate reaction temperatures while generating no
by-products. Upon resin curing, a hard, transparent thermoset with a broad glass transition, T = 155 °C
g
(based on the tan δ maximum), and a temperature of maximum decomposition rate, Tmax, of 426 °C was
produced. Overall, a potentially 100% bio-based thermoset was synthesized possessing comparable
thermo-gravimetric and thermo-mechanical properties to commercial vinyl ester-based thermosets.
Introduction
Petroleum-based vinyl ester resins are used to produce polymer
composites for a wide variety of commercial applications due to
possessing relatively high moduli, strength, and glass transition
1,2
temperatures (T s) while maintaining low weight and cost.
g
Additionally, commercial vinyl ester resins typically contain
high concentrations of a petroleum-based reactive diluent, such
as styrene, to facilitate the use of liquid molding techniques to
fabricate composite parts and to improve overall composite
performance. However, the commonly used reactive diluents
are hazardous air pollutants (HAPs) and/or volatile organic com-
Fig. 1 Chemical structures of a lignin model compound, vanillin, and
its methacrylated counterpart, methacrylated vanillin (MV).
rotational freedom of the polymer network. Vanillin is an impor-
2,3
tant aromatic compound that is used primarily by flavor
pounds (VOCs).
Moreover, styrene has recently been
5,6
and fragrance companies and pharmaceutical manufacturers.
suggested to be “reasonably anticipated to be a human carcino-
gen” by the USA’s Department of Health and Human Services
Vanillin is produced synthetically either from petroleum-derived
phenol or from lignin (nature’s most abundant aromatic
polymer), roughly 85% and 15% of the world supply,
4
through the National Toxicology Program. Furthermore, the
uncertainty of petroleum reserves and future crude oil prices
underscores the need for the development of a sustainable
supply of bio-based resins. These significant factors provide
impetus for the discovery and development of renewable vinyl
ester resins that have the same, if not better performance without
the use of hazardous reactive diluents.
6–9
respectively. Only 0.2% of the overall vanillin market utilizes
5
natural vanillin extracted from the beans of Vanilla plantifolia.
As a flavoring and fragrance ingredient, the current global
demand for synthetic vanillin is estimated to be roughly 16 000
6
tons per year. With vanillin being considered a valuable chemi-
cal, much research has been conducted in developing more sus-
tainable methods for obtaining greater quantities of vanillin from
Vanillin (Fig. 1) has been identified as a suitable bio-based
replacement for utilization in vinyl ester resins mainly due to its
aromatic character. The incorporation of aromaticity in cured
resins is known to provide structural rigidity (high moduli and
11
lignin (over 3 × 10 tons of lignin exists in the biosphere with
10
5,6,8,10–12
approximately 2 × 10 tons generated annually).
As an attractive bio-based monomer, vanillin has been suc-
cessfully incorporated into novel polymers for use in a wide
strength) and thermal stability (high T s) by inhibiting the
g
3,13–15
variety of commercial applications.
For example, Mialon
a
et al. have developed a renewable polyethylene terephthalate
Department of Chemical and Biomolecular Engineering, Center for
Composite Materials, University of Delaware, Newark, DE 19716, USA.
E-mail: wool@udel.edu; Fax: +(302) 831-1048; Tel: +(302 ) 831-3312
mimic that was synthesized from vanillin and acetic anhydride
13
for use as a commodity plastic. Peng et al. cross-linked chito-
san microspheres with vanillin for use as a drug-delivery vehicle
in the controlled release of resveratrol, which is a polyphenolic
phytoalexin that has beneficial effects against diseases such as
b
Army Research Laboratory, Aberdeen Proving Ground, MD 21005,
USA
c
Department of Chemical Engineering, University of South Carolina,
Columbia, SC 29208, USA
2346 | Green Chem., 2012, 14, 2346–2352
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1
4
cancer and cardiovascular disease. Moreover, Renbutsu et al.
Experimental
have synthesized 3-methoxy-4-methacryloyloxybenzaldehyde
methacrylated vanillin (MV) Fig. 1) via a Steglich esterification
Materials
(
of vanillin with methacrylic acid in the development of
UV-curable palladium-chelating chitosan derivatives for use as a
coating in electroless plating of non-conductive materials.
All reagents, 0.75 mL ampoules of dimethyl sulfoxide (DMSO-
d6), tetrahydrofuran (THF) (Optima, submicron filtered),
15
4
-dimethylaminopyridine (DMAP), phenolphthalein, (99+%),
In our laboratory, MV has been synthesized via an esterification
reaction of vanillin with methacrylic anhydride in the investi-
gation of methacrylated lignin model compounds as potential
and vanillin (99%) were obtained from Fischer Scientific and
were used as received. Methacrylic anhydride (94%, inhibited
with 2000 ppm Topanol A), and glycidyl methacrylate (≥97%,
stabilized with ∼0.0005 wt% hydroquinone monomethyl ether
3
styrene replacements in liquid molding vinyl ester resins.
However, MV was not pursued as a viable reactive diluent due
to being a solid at room temperature.
(Mequinol, MEHQ)) were purchased from Sigma Aldrich and
used as received. Compressed argon was obtained from Keen
Compressed Gas Co. (99.998%). AMC-2, which is a mixture of
In the work presented herein, an alternative, vanillin-based
vinyl ester resin for use in commercial polymer composite appli-
cations is described along with its cure behavior and polymer
properties. A two part reaction sequence to generate a vinyl ester
resin consisting of a 1 : 1 mole ratio of a mono-functional
monomer, MV, (Scheme 1, Reaction 1) and a cross-linking
agent, glycerol dimethacrylate (GDM), (Scheme 1, Reaction 2)
has been developed. The synthesis possesses several desired
green chemistry and engineering characteristics which include:
5
0% trivalent organic chromium complexes and 50% phthalate
esters, was purchased from AMPAC Fine Chemicals (Rancho
Cordova, CA) and used as received. Trigonox 239 (AkzoNobel
Polymer Chemicals), containing 45% cumene hydroperoxide,
was purchased and utilized as a free radical initiator.
Synthesis of LMC-based resin
(
(
1) the usage of a renewable, lignin-derived chemical, vanillin,
2) the usage of potential renewable reactants, methacrylic
To a 100 mL round bottom flask equipped with a magnetic
stir bar, a catalytic amount of DMAP (2 mol% of methacrylic
anhydride) was added to vanillin (10 g, 65.7 mmol). The reac-
tion vessel was sealed and purged with argon gas for an hour
to remove moisture and oxygen before adding methacrylic
anhydride (1.01 mole equivalents based on vanillin). The reac-
tion mixture was stirred at room temperature for 15 minutes and
then heated to 50 °C for a minimum of 24 hours. At the begin-
ning of the reaction, solid vanillin was suspended in methacrylic
anhydride. As the reaction progressed, all solids dissolved into
the liquid phase. After confirming that the reaction went to near
anhydride and glycidyl methacrylate, (3) no by-products,
(
(
4) neat reactions, (5) relatively small quantities of catalysts, and
1
6–20
6) moderate reaction temperatures.
The first reaction is a
substitution reaction that generates stoichiometric amounts of
MV and methacrylic acid. The methacrylic acid would be a
by-product that would need to be separated out from the MV, the
desired product of reaction 1. However, we determined that the
products of reaction 1 can be mixed with glycidyl methacrylate
to react with the methacrylic acid to produce a liquid cross-
linking agent, glycerol dimethacrylate. As a result, the series of
reactions generates no waste with a 100% mass efficiency. The
third reaction required to produce the thermoset is polymeri-
zation. Polymerization requires a small quantity of initiator,
moderate curing temperatures, and an inert atmosphere while
generating no waste. Therefore, the methacrylated vanillin–gly-
cerol dimethacrylate (MVGDM) polymer has the potential to be
a 100% bio-based and produced in a highly sustainable manner
without the need of styrene.
1
completion via H NMR, the reaction flask was cooled to room
temperature. The resulting oil was treated with glycidyl metha-
crylate (GM, 1.01 mole equivalents added based on the
methacrylic acid amount determined from titration) and AMC-2
(
0.1 wt% of the total amount of GM and MA). The reaction
mixture was stirred vigorously at room temperature for
0 minutes before elevating the temperature to 70 °C. The reac-
1
tion progress was monitored using acid number titration until an
acid number of 10 or less, corresponding to ∼3% free acid, was
achieved. An ASTM acid number titration method described in
2
La Scala et al. was performed with success.
Resin characterization
The methacrylated vanillin–glycerol dimethacrylate (MVGDM)
1
resin was characterized using H NMR (400.13 MHz, 16 scans
at 298.2 K) on a Bruker AV-400 spectrometer. Additionally,
MVGDM was characterized using Fourier transform infrared
(FTIR) spectroscopy performed on a PerkinElmer Spectrum 400
FT-IR/FT-NIR Spectrometer. At room temperature, 32 cumulat-
−1
ive scans were acquired with a resolution of 2 cm in trans-
mission mode in the mid-IR range.
Size exclusion chromatography (SEC) was employed to deter-
mine if extensive methacrylation and/or epoxy homopolymeriza-
tion occurred in the resin. A Viscotek 270max – Modular
Advanced GPC/SEC System fitted with Waters Styragel HR1
Scheme 1 Two-step, one-pot synthesis scheme to produce a resin that
contains a 1 : 1 mole ratio of methacrylated vanillin (desired mono-func-
tional monomer of Reaction 1) and glycerol dimethacrylate (desired
cross-linker of Reaction 2). The methacrylic acid produced in Reaction 1
is consumed in Reaction 2.
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and HR4 columns in series was used, operated with THF
1 mL min− ) as the mobile phase, and calibrated with
polystyrene standards. Samples were prepared by dissolving
mg of resin in 1 mL THF. Refractive index data was utilized to
1
(
2
determine the occurrence of extensive methacrylation and/or
epoxy homopolymerization.
Resin viscosity was obtained on a TA Instruments ARES-G2
rheometer. Measurements were performed isothermally at 25 °C
controlled by a Peltier plate (+/−0.1 °C error). A 20 mm 1° steel
cone with a truncation gap of 25 μm and a minimum sample
volume of 0.04 mL was utilized. The shear rate was increased
step-wise from 1 s− to 100 s collecting 21 data points to
observe any non-Newtonian behavior. At a given shear rate, the
shear stress was measured every two seconds. The shear rate,
shear stress, and viscosity were recorded when the shear rate
stabilized to within 5% tolerance for three consecutive points.
Resin was measured in triplicate and the viscosities were aver-
aged and reported.
1
−1
Resin curing
Free-radically cured MVGDM resin was prepared via bulk
polymerization by adding 1.5 wt% Trigonox 239 to the
monomer with curing at 90 °C for four hours and subsequent
post-curing at 130 °C for two hours. Prior to polymerization, the
resin with initiator was degassed. Polymerization was conducted
in an argon atmosphere, whereby, argon gas was purged through
the oven for approximately 15 minutes at the beginning of cure.
Cure kinetics
In order to determine the overall extent of polymerization of the
cured resin, cure kinetics studies were performed on a Perkin-
Elmer Spotlight 400 FTIR Microscope with a heating stage
designed to hold 13 mm diameter salt discs (heating stage con-
trolled by a BriskONE Single-Zone Digital PID Thermocouple
Temperature Controller with accuracy of ±0.25% of reading).
At 70 °C for 4 hours and 130 °C for 2 hours, 32 cumulative
1
Fig. 2 H NMR spectrum (top) of the MVGDM resin with proton
assignments. FTIR spectrum (bottom) of the MVGDM resin with two
notable peaks assigned, specifically, the hydroxyl stretch (1) and C–H
vinyl group bending (2).
−1
scans were acquired with a resolution of 2 cm in transmission
mode in the mid-IR range with spectrum taken roughly every
1
.1 minutes. A drop of resin with 1.5 wt% Trigonox 239 as the
in sample film thickness and/or component evaporation, the
vinyl group conversion was calculated by measuring the peak
absorbance intensity relative to the peak absorbance intensity of
free radical initiator was placed between two 13 mm diameter
NaCl discs (the bottom NaCl disc was 2 mm thick while the top
was 1 mm thick) prior to placement into the heating stage.
The fractional conversion of monomer carbon–carbon double
bonds (CvC) to polymer carbon–carbon single bonds (–C–C–),
α, was calculated by measuring the absorbance intensity of a
peak (i.e. a group that is affected by the reaction) relative to an
−1
the hydroxyl stretch of GDM at 3510 cm
bottom). The conversion as a function of time data was fitted
(see Fig. 2
22
to an empirical autocatalytic kinetic model in order to determine
23
the reaction rate constant, k, and reaction order, m.
internal standard (i.e. a group that is not affected by the reaction)
2
Polymer properties
(eqn (1)):
ꢀ
ꢁꢀ
ꢁ
Thermo-gravimetric behaviour of the uncured and cured
MVGDM resin was measured using a TA Instruments Q500
Thermogravimetric Analyzer (TGA). Approximately 10 mg of
sample was placed in a platinum pan and heated to 650 °C at
ABSðtÞ
ꢀ1
ABSðt ¼ 0Þ
ABSðtÞ
ꢀ1
3510 cm
9
47 cm
α ¼ 1 ꢀ
ð1Þ
ABSðt ¼ 0Þ
ꢀ1
ꢀ1
9
47 cm
3510 cm
In the MVGDM resin, both MV and GDM possess methacry-
late groups that appear at 947 cm , specifically corresponding
to the out-of-plane bending of C–H bonds in the vinyl group.
−
1
−1
−1
10 °C min in a N atmosphere (40 mL min balance gas flow
2
−1
2,21
rate and 60 mL min sample gas flow rate).
Since the decrease in absorbance intensity can result from not
only polymerization but also physical changes, such as changes
Thermo-mechanical properties of the MVGDM polymer
were measured using dynamic mechanical analysis (DMA).
2348 | Green Chem., 2012, 14, 2346–2352
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Rectangular samples with approximate dimensions of 35 × 12 ×
3
3
mm were tested using a TA Instruments Q800 DMA in single
cantilever geometry. The samples were tested at 1 Hz with a
deflection of 7.5 μm while ramping the temperature from −50 to
−1
2
50 °C at a rate of 2 °C min . The point at which the modulus
in the rubbery plateau began to increase with increasing tempera-
ture was used to calculate the molecular weight between
cross-links, M . M was estimated using the theory of Rubber
C
C
Elasticity and the polymer density (measured using Archimedes’
principle) (eqn (2)):
E ¼ 3RTρ=MC
ð2Þ
Fig. 3 FTIR spectra of the MVGDM during curing as a function of
reaction time at 70 °C.
where E is the rubbery modulus, R is the ideal gas constant, T is
the absolute temperature, and ρ is the sample density.
2
,24,25
−
1
of wavenumber from 1350 to 700 cm . As seen in Fig. 3, the
Results and discussion
47 cm− peak associated with the out-of-plane bending of C–H
bonds in vinyl groups decreased with increasing time.
1
9
21,22
Resin characterization
Fur-
thermore, the appearance of a left shoulder increased with
Upon completion of the two-part, one-pot synthesis, the resin
was a brownish-green, transparent liquid. The colour is attributed
to the AMC-2 catalyst used in the resin synthetic scheme.
MVGDM resin exhibited no epoxy homopolymerization
increasing time. Brill and Palmese have shown that the
1
9
47 cm− includes contributions from underlying peaks of
absorbances associated with the monomers and cured reaction
22
−1
products. Upon deconvolution, the 947 cm peak was shown
to be a combination of three absorbances with the out-of-plane
bending of C–H bonds in vinyl groups being the prominent
(confirmed by SEC) and possessed an acid number of 9.00 ±
0
.01 indicating that less than 3% free acid remained. The resin
was Newtonian and had a viscosity of 87.3 ± 0.5 cP at 25 °C; a
viscosity that is slightly lower than the recommended lower limit
for conventional resin transfer molding processes but, neverthe-
22
peak. This fact, which results in a slight difference between the
fitted and unfitted prominent peak heights corresponded to an
underestimation of the final fractional conversion of double
bonds of about 5%. The left shoulder that increasingly appears
with increasing cure time at 954 cm− (an identified underlying
7
less, a viscosity that is highly usable. Additionally, the low vis-
cosity allows for potential resin blending with other higher
viscosity, bio-based cross-linkers, such as acrylated epoxidized
soybean oil (AESO) and maleinated acrylated epoxidized
soybean oil (MAESO) as well as commercially available
1
22
peak) is believed to be from the backbone mode of GDM.
Eqn (1) was used to calculate α, the fractional conversion of
CvC to –C–C–, for the MVGDM resin as a function of time
7
petroleum-based vinyl esters. This resin blending, in turn, gives
(shown in Fig. 4). In general, the polymerization reaction
rise to the possibility of producing highly bio-based thermosets
that possess a wide range of tailored physical properties.
follows typical behavior associated with free-radical curing,
including the presence of an initial inhibition period and a
period of accelerated propagation that is followed by a reduction
1
H NMR spectrum of the MVGDM resin (Fig. 2, top) possessed
peaks indicative of methacrylated vanillin (MV) (3-methoxy-
22
−1
in cure rate due to vitrification. After curing at 70 °C for four
hours, a fractional conversion of 0.66 ± 0.06 was achieved
(Fig. 4). The fractional conversion increased to 0.78 upon sub-
sequent post-curing at 130 °C for two hours (Fig. 4). Since the
fractional conversions were calculated using unfitted peaks, the
reported values, based on the work of Brill and Palmese, could
4
-methacryloyloxybenzaldehyde, 220.21 g mol ) and glycerol
dimethacrylate (GDM) (2-hydroxy-1,3-propanediyl bismethacry-
late, 228.23 g mol−1). MV 1H NMR ([D ]DMSO): δ = 9.99
6
(
(
s, 1H), 7.62–7.41 (bm, 3H), 6.31 (t, 1H), 5.95 (t, 1H), 3.87
1
s, 3H), 2.01 ppm (s, 3H). GDM H NMR ([D ]DMSO): δ =
6
6
.07 (m, 2H), 5.70 (t, 2H), 4.11 (m, 5H), 1.89 ppm (m, 6H).
22
be underestimated by approximately 5%. Prior to post-curing,
the 66% conversion is in agreement with reported conversions of
Peaks associated with glycidyl methacrylate, for example, the
peaks related to vinyl protons (6.02 and 5.73 ppm), are present
2
1
similar vinyl ester resins that contain methacrylate groups. The
in the H NMR spectrum but are hardly detectable. Additionally,
a peak at 907 cm−1, which is associated with epoxy group
seemingly low conversion is partially attributed to the inherent
2
low reactivity of methacrylate groups with themselves. In
vibrations, was not present in the FTIR spectrum of the
MVGDM resin (Fig. 2, bottom). These results, along with a
computed MV to GDM molecular ratio of 1.00 ± 0.002 based on
addition, due to the chemical structure differences between MV
and GDM, particularly the aromaticity of MV and the hydroxyl-
ated alkyl backbone of GDM, there is potential for micron or
sub-micron domain formations which can affect reactivity and
1
the H NMR spectrum, indicates near complete conversion of
2
6
glycidyl methacrylate to glycerol dimethacrylate.
21
ultimately the extent of cure. However, since both monomers
contained methacrylate groups, were in a roughly 1 : 1 mole
ratio, and polymerization was carried out free radically, it is
anticipated that the polymer network formed in a predominantly
Cure kinetics
2,21,22
FTIR spectra versus cure time in the region of interest are shown
random fashion.
Moreover, the polymerization rate may
in Fig. 3 where the normalized intensity is plotted as a function
have decreased early and rapidly due to entrapment of unreacted
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Fig. 4 Fractional conversion as a function of time for the cure of the
MVGDM resin at 70 °C for four hours. Autocatalytic kinetic model fit
shown as the solid, black line. After a post-cure at 130 °C for two hours,
fractional conversion reached 78%.
species. This may have been a result of GDM having the poten-
tial to hydrogen bond with carbonyl groups present on MV and
other GDM molecules, which can lead to mobility limitations.
Since the polymerization followed the typical behavior associ-
ated with free-radical curing, the empirical autocatalytic kinetic
model was able to fit the experimental data with a sum of the
−5
squares error of 5.0 × 10 , as shown by the best fit curve in
Fig. 4. A reaction order, m = 0.63, and a reaction rate constant,
k = 0.175 min−1, were obtained from the kinetic model and are
Fig. 5 TGA weight (left y-axis) and the derivative of the weight (right
y-axis) as a function of temperature for cured MVGDM (top) and
uncured MVGDM resin (bottom).
similar to values reported in the literature for the curing of vinyl
22
ester resins.
Table 1 Thermo-gravimetric and thermo-mechanical properties as well
as the density of the MVGDM cured resin
Polymer properties
After curing with 1.5 wt% Trigonox 239, the resin became a
transparent, hard polymer. In order to elucidate the thermo-gravi-
metric behavior and thermo-mechanical properties of the
polymer, TGA and DMA experiments, respectively, were per-
formed. Fig. 5 shows the TGA thermograms and the derivatives
of the thermograms of the cured MVGDM resin (top) and
uncured MVGDM resin (bottom). From the TGA data, thermal
stability factors, including the initial decomposition temperature,
IDT, the temperature of 50% weight loss, T50%, the temperature
of maximum decomposition rate, Tmax (tallest peak of the ther-
mogram derivative), and the decomposition activation energy,
ED, for the cured MVGDM resin were determined and are pres-
ented in Table 1. ED was calculated using the Horowitz and
IDT
244 ± 3.6 °C
405 ± 0.4 °C
T
T
50%
max
426 ± 1.4 °C 1
110.9 kJ mol−
3.6 ± 0.3 GPa
17.4 ± 0.5 MPa
1.284 ± 0.002 g cm
825 ± 26 g mol−
155 ± 1.7 °C
E
D
E′ at 25 °C
E′ at 237.5 °C
ρ at 25 °C
−3
1
M
C
g
T (tan δ max)
31
poly(phenyl methacrylate) thermograms.
Zulfiqar et al., the first stage is believed to be depolymerization
initiated at unsaturated chain ends and the second stage is de-
polymerization by random scission. Referring to Fig. 5 top, at
330.5 °C, which is the temperature where the second derivative
of the TGA thermogram is equal to zero indicating the transition
from the first thermal decomposition stage to the second, 78 ±
0.06% of the polymer remains. This result is in agreement with
the FTIR cure study result of reaching 78% conversion of mono-
meric CvC to polymeric –C–C– after post-curing. Therefore,
for the MVGDM cured resin, the first stage is believed to be a
According to
2
7,28
31
Metzger integral method.
possess approximately Tmaxs and T50%s ≥ 400 °C.
Typical cured vinyl ester resins
2
9,30
The
results show that the MVGDM polymer is thermally stable up to
about 244 °C (the IDT) with Tmax = 426 °C and T50% = 405 °C.
In addition, the MVGDM cured resin exhibited a two stage
thermal decomposition (first stage between 200 °C and 330.5 °C
and the second stage between 330.5 °C and 650 °C) (Fig. 5 top).
This two stage behavior was also observed by Zulfiqar et al. in
2350 | Green Chem., 2012, 14, 2346–2352
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combination of depolymerization initiated at unsaturated chains
ends, evaporation of unreacted monomer, and degradation of
potential oligomers. Evaporation of unreacted monomer was
confirmed via TGA of uncured MVGDM resin (Fig. 5 bottom).
Most of the uncured resin evaporated near 200 °C (Fig. 5
bottom) with most of the evaporation occurring prior to the first
thermal decomposition stage of the cured resin (MVGDM). This
suggests that the first thermal decomposition stage of the cured
MVGDM is largely attributed to depolymerization initiated at
unsaturated chain ends and degradation of potential oligomers
and weakly attributed to monomer evaporation. The depolymeri-
zation initiated at unsaturated chains ends is most likely occur-
ring at GDM molecules that are partially connected to the
polymer network. Like Zulfiqar et al., the second stage is
believed to be largely attributed to depolymerization by random
scission. Due to the abundance of methacrylate groups, anhy-
dride ring formation may also occur during thermal treatment,
which, in turn, interferes with polymer chains unzipping to
monomers causing appreciable charring (12.61 ± 0.04 °C char
content at 625 °C). It is also noted that thermally initiated free
radical polymerization most likely occurred simultaneously with
evaporation during the TGA experiment of the uncured resin.
This is based on the presence of a plateau near 300 °C in the
uncured resin thermogram (Fig. 5 bottom).
Thermo-mechanical properties were measured using DMA
and are shown in Fig. 6, where the storage modulus (E′) and tan
δ are plotted as a function of temperature. Typical cured styrene-
Table 2 Thermo-mechanical properties of a typical vinyl ester resin
2
(VE828) with 45 wt% styrene
E′ at 35 °C
MC
2.7 GPa
432 g mol−
1
T
g
(E′′ max)
145 ± 1.7 °C
Tan δ, also known as the damping factor or loss tangent, is the
ratio of the loss modulus (E′′) to E′ where E′′ represents
20,32
the energy lost as heat during a loading cycle.
The width of
the tan δ peak reflects polymer network heterogeneity with
32,35
a broader peak implying a more heterogeneous polymer.
According to a new theory of the glass transition entitled the
twinkling fractal theory (TFT), an increased amount of relaxation
modes present in the polymer network arises when there exists a
broad distribution of solid fractal clusters that twinkle into the
36,37
liquid upon heating, thus, exhibiting heterogeneity.
The tan
δ peak in Fig. 6 is considered a broad peak indicating the for-
mation of a rather heterogeneous polymer network that contains
a very broad distribution of relaxation modes and twinkling solid
fractal clusters. Thus, a glass transition region occurs over a wide
temperature range with a broad distribution of solid fractal clus-
31
ters vector percolating until eventual fractal cavitation upon
36,37
further cooling.
This result may be attributed to the for-
mation of highly cross-linked regions, regions that may contain
32,35
unreacted monomer pools, and regions in between.
The
heterogeneity of the polymer network captured by the broadness
of the tan δ coincides with FTIR cure study and TGA results that
revealed the presence of unreacted CvC and a two stage
thermal decomposition profile, respectively, which suggested the
presence of a heterogeneous polymer network. The designation
diluted vinyl ester resins possess E′s ≥ 2.5 GPa at 25 °C and T s
g
2
≥
120 °C. The stiffness of a viscoelastic material is represented
by E′ and is proportional to the energy stored during a loading
32
cycle. E′ at 25 °C was measured to be 3.6 ± 0.3 GPa (Table 1),
which is comparable to typical, cured, commercial vinyl ester
of a specific temperature as T is difficult to provide due to the
g
2
,33,34
resins (Table 2).
Referring to Fig. 6 and Table 1, the
broadness of the glass transition region. However, in using the
−1
rubbery E′ was ∼17.4 MPa, indicating a M of 825 g mol as
per the theory of Rubber Elasticity. The calculated M is roughly
.5 times greater than the anticipated MC of 224.22 g mol
C
maximum peak intensity of the tan δ curve, T = 155 ± 1.7 °C,
g
C
which is comparable to cured commercial vinyl ester resins
−1
3
.
1,2,30
(Table 2).
This difference is most likely a result of GDM monomers incom-
pletely connected to the polymer network.
The extent of polymer chain segmental mobility at the temp-
erature of maximum tan δ peak intensity is reflected by the inten-
sity value with higher values indicating higher energy loss and
more viscous behavior, whereas lower values indicate higher
32
elastic behavior. Having a maximum tan δ peak intensity value
less than 0.2 is reflective of high elastic behavior and the pres-
ence of a highly cross-linked polymer network, which is
expected with a mono-functional monomer to cross-linker ratio
of 1 : 1. Additionally, the viscous behavior may be further
restricted and the elastic behavior enhanced due to the ability of
the polymer chains to hydrogen bond with each other, which
may influence the relaxation dynamics of the cross-linked
network and the ability of the solid fractal clusters to transition
3,36,37
into the liquid upon heating.
Conclusions
In this work, a low viscosity polymer resin consisting of a
: 1 mole ratio of MV to GDM was synthesized using a two-
1
step, one-pot synthesis. The synthesis utilized a renewable,
lignin-derived chemical, vanillin, and potential bio-based reac-
tants, methacrylic anhydride and glycidyl methacrylate.
Fig. 6 Storage modulus (E′) and tan δ of the MVGDM polymer as a
function of temperature.
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Additionally, it generated no by-products, required small quan-
tities of catalyst and moderate reaction temperatures, and was
solventless. Resin curing generated a hard, transparent thermoset
7 R. P. Wool and X. S. Sun, Bio-based Polymer and Composites, Elsevier
B.V., 1st edn, 2005.
8
9
T. Voitl and P. Rudolf von Rohr, ChemSusChem, 2008, 1, 763–769.
J. H. Lora and W. G. Glasser, J. Polym. Environ., 2002, 10, 39–48.
that possessed a broad glass transition, T = 155 °C (based on
the tan δ maximum), and E′ at 25 °C = 3.6 GPa. An overall frac-
tional conversion of monomeric CvC to polymeric –C–C– of
10 A. Gandini, Green Chem., 2011, 13, 1061–1083.
1 J. E. Holladay, J. F. White, J. J. Bozell and D. Johnson, Top Value-Added
g
1
Chemicals from Biomass Volume II - Results of Screening for Potential
Candidates from Biorefinery Lignin, Pacific Northwest National Labora-
tory, Richland, WA, 2007.
0
.78 was elucidated from a FTIR cure study while thermo-gravi-
metric analysis revealed a two stage thermal decomposition
profile with a Tmax = 426 °C and T50% = 405 °C. Overall, a
potentially 100% bio-based resin was synthesized for use in
polymer composite applications and, when cured, possessed
comparable thermo-gravimetric and thermo-mechanical proper-
ties to commercial vinyl ester resins while eliminating VOC/
HAP components and emissions. However, optimization of cure
and post-cure temperatures and durations as well as initiator con-
centration is recommended to improve overall conversion, thus,
reducing the concentration of unreacted monomer that could
potentially leech into the environment.
12 J. D. P. Araújo, C. a. Grande and A. E. Rodrigues, Chem. Eng. Res. Des.,
2
010, 88, 1024–1032.
3 L. Mialon, A. G. Pemba and S. A. Miller, Green Chem., 2010, 12,
704–1706.
1
1
14 H. Peng, H. Xiong, J. Li, M. Xie, Y. Liu, C. Bai and L. Chen, Food
Chem., 2010, 121, 23–28.
1
1
1
5 E. Renbutsu, S. Okabe, Y. Omura, F. Nakatsubo, S. Minami, H. Saimoto
and Y. Shigemasa, Carbohydr. Polym., 2007, 69, 697–706.
6 P. T. Anastas and J. C. Warner, Green Chemistry: Theory and Practice,
Oxford University Press, 1998.
7 Biorefineries - Industrial Processes and Products: Status Quo and Future
Directions, ed. M. Kamm, B. Gruber and P. R. Kamm, Wiley-VCH
Verlag GmbH & Co. KGaA, Weinheim, Germany, 2012.
18 R. T. Mathers, J. Polym. Sci., Part A: Polym. Chem., 2011, DOI:
0.1002/pola.24939.
1
1
2
9 K. Nagai, Appl. Catal. A: General, 2001, 221, 367–377.
0 E. Santacesaria, R. Tesser, M. Di Serio, L. Casale and D. Verde, Ind.
Eng. Chem. Res., 2010, 49, 964–970.
Acknowledgements
2
2
2
2
1 S. Ziaee and G. R. Palmese, J. Polym. Sci., Part B: Polym. Phys., 1999,
The authors gratefully acknowledge the U.S. Army Research
Laboratory for financial support under the Strategic Environ-
mental Research and Development Program, project WP-1758,
and through the Cooperative Agreement W911NF-06-2-001. In
addition, the authors would like to thank Dr. Thomas H. Epps of
the Chemical and Biomolecular Engineering Department at the
University of Delaware for usage of his SEC. The authors would
like to also thank Dr. Michael Mackay of the Materials Science
and Engineering Department at the University of Delaware for
usage of his TGA.
37, 725–744.
2 R. P. Brill and G. R. Palmese, J. Appl. Polym. Sci., 2000, 76, 1572–
1582.
3 P. W. K. Lam, H. P. Plaumann and T. Tran, J. Appl. Polym. Sci., 1990, 41,
3
043–3057.
4 G. R. Palmese and R. L. McCullough, J. Appl. Polym. Sci., 1992, 46,
863–1873.
1
25 L. H. Sperling, Introduction to Physical Polymer Science, John Wiley &
Sons, Inc., Hoboken, N.J., 4th edn, 2006.
2
6 P. Juntuek, C. Ruksakulpiwat and P. Chumsamrong, J. Appl. Polym. Sci.,
011, 122, 3152–3159.
27 S.-J. Park and F.-L. Jin, Polym. Int., 2005, 54, 705–709.
2
2
2
3
8 H. H. Horowitz and G. Metzger, Anal. Chem., 1963, 35, 1464–1468.
9 A. Chatterjee, J. Appl. Polym. Sci., 2009, 114, 1417–1425.
0 T. Glodek, S. Boyd, I. Mcaninch and J. Lascala, Compos. Sci. Technol.,
Notes and references
2008, 68, 2994–3001.
1
2
3
4
J. J. La Scala, J. A. Orlicki, C. Winston, E. J. Robinette, J. M. Sands and
G. R. Palmese, Polymer, 2005, 46, 2908–2921.
J. J. La Scala, J. M. Sands, J. A. Orlicki, E. J. Robinette and
G. R. Palmese, Polymer, 2004, 45, 7729–7737.
J. F. Stanzione III, J. M. Sadler, J. J. La Scala and R. P. Wool, Chem-
SusChem, 2011, Accepted cssc.201100687.
“Report on Carcinogens.” United State. Department of Health and
Human Services. Public Health Services. National Toxicology Program.
31 S. Zulfiqar, M. Zulfiqar and K. T, Polym. Degrad. Stab., 1987, 17,
327–339.
32 J. Park, J. Eslick, Q. Ye, A. Misra and P. Spencer, Dental Mater., 2011,
27, 1086–1093.
33 J. J. La Scala, M. S. Logan, J. M. Sands and G. R. Palmese, Compos.
Sci. Technol., 2008, 68, 1869–1876.
34 T. F. Scott, W. D. Cook and J. S. Forsythe, Eur. Polym. J., 2008, 44,
3200–3212.
1
2th edn, (2011).
C. Brazinha, D. S. Barbosa and J. G. Crespo, Green Chem., 2011, 13,
197.
35 A. R. Kannurpatti, J. W. Anseth and C. N. Bowman, Polymer, 1998, 39,
2507–2513.
36 J. F. Stanzione III, K. E. Strawhecker and R. P. Wool, J. Non-Cryst.
Solids, 2011, 357, 311–319.
37 R. P. Wool, J. Polym. Sci., Part B: Polym. Phys., 2008, 46, 2765–
2778.
5
6
2
E. A. Borges da Silva, M. Zabkova, J. D. Araújo, C. A. Cateto,
M. F. Barreiro, M. N. Belgacem and A. E. Rodrigues, Chem. Eng. Res.
Des., 2009, 87, 1276–1292.
2352 | Green Chem., 2012, 14, 2346–2352
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