Syringaresinol: A Renewable and Safer Alternative to Bisphenol A for Epoxy-Amine Resins

Article abstract of DOI:10.1002/cssc.201601595

A renewable bisepoxide, SYR-EPO, was prepared from syringaresinol, a naturally occurring bisphenol deriving from sinapic acid, by using a chemo-enzymatic synthetic pathway. Estrogenic activity tests revealed no endocrine disruption for syringaresinol. Its glycidylation afforded SYR-EPO with excellent yield and purity. This biobased, safe epoxy precursor was then cured with conventional and renewable diamines for the preparation of epoxy-amine resins. The resulting thermosets were thermally and mechanically characterized. Thermal analyses of these new resins showed excellent thermal stabilities (T_{d5 %}=279–309 °C) and T_granging from 73 to 126 °C, almost reaching the properties of those obtained with the diglycidylether of bisphenol A (DGEBA), extensively used in the polymer industry (T_{d5 %}=319 °C and T_g=150 °C for DGEBA/isophorone diamine resins). Degradation studies in NaOH and HCl aqueous solutions also highlighted the robustness of the syringaresinol-based resins, similar to bisphenol A (BPA). All these results undoubtedly confirmed the potential of syringaresinol as a greener and safer substitute for BPA.

Full text of DOI:10.1002/cssc.201601595

Accepted Article
Title: Syringaresinol: a renewable and safer alternative to Bisphenol A
for epoxy-amine resins
Authors: Marine Janvier, Louis Hollande, Abdus Samad Jaufurally,
Miguel Pernes, Raphaël Ménard, Marina Grimaldi, Johnny
Beaugrand, Patirck Balaguer, Paul-Henri Ducrot, and Florent
Allais
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the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.201601595
Link to VoR: http://dx.doi.org/10.1002/cssc.201601595
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10.1002/cssc.201601595
ChemSusChem
FULL PAPER
Syringaresinol: a renewable and safer alternative to Bisphenol A
for epoxy-amine resins
Marine Janvier,[a,b] Louis Hollande,[a,c] Abdus Samad Jaufurally,[a,b] Miguel Pernes,[d] Raphaël
Ménard,[a] Marina Grimaldi,[e] Johnny Beaugrand,[d] Patrick Balaguer,[e] Paul-Henri Ducrot*[b] and
Florent Allais*[a,f]
one of the most important field of applications. Their remarkable
properties, namely excellent mechanical properties, thermal and
chemical resistance, low density and adhesion, offer a wide
range of applications including coatings, adhesives or
composites.1 Nevertheless, these thermosets suffer from several
major drawbacks. Their petrochemical sourcing, in a context of
increasing scarcity and price of fossil resources, and their
impossible recycling due to their crosslinked structures, raised
the interest towards biobased epoxy monomers. Moreover, high
concern has been recently raised towards the toxicity of
Abstract: A renewable bis-epoxy, SYR-EPO, has been prepared
from syringaresinol, a naturally occurring bisphenol deriving from
sinapic acid, using
a
chemo-enzymatic synthetic pathway.
Estrogenic activity test revealed no endocrine disruption for
syringaresinol. Its glycidylation afforded bis-epoxy SYR-EPO with
excellent yield and purity. This biobased safer epoxy precursor was
then cured with conventional and renewable diamines for the
preparation of epoxy-amine resins. The resulting thermosets were
thermally and mechanically characterized (TGA, DSC and DMA).
Thermal analyses of these new resins showed excellent thermal
stabilities (Td5% = 279-309 °C) and Tg ranging from 73 to 126 °C,
almost reaching the properties of those obtained with
bisphenol
A (BPA), precursor of diglycidyl ether of BPA
(DGEBA), the most widespread epoxy monomer. Indeed, due to
its structure similar to estrogens, BPA is recognized as an
endocrine disruptor, inducing a reduced fertility and potentially
cancers.2,3 Recently, the use of BPA-based packaging for child
products has been prohibited by the United-States Food and
Drug Administration.4 Despite the excellent properties obtained
for DGEBA-based materials, its toxicity has raised a demand for
BPA-free products and therefore increased the research of a
safer and greener alternative.5 Therefore, many works have
been focused on the valorization of renewable resources
towards epoxy resins to restrict the use of DGEBA.6,7 Numerous
alternative precursors for DGEBA-free resins have been
described: terpenes,8 linseed oil derivatives,9 sucrose soyate,10
rosin acid oligomers11. Research on isosorbide-based epoxy
also revealed high mechanical properties of the resulting cured
materials and led to the commercialization of diglycidyl ether of
isosorbide.12–14 The major drawback of these aliphatic
bioresources is the lack of rigidity compared to aromatic DGEBA,
inducing lower mechanical properties. The literature highlighted
the interest of phenolic moieties to compete with DGEBA:
several epoxy-amine resins based on phenolic extracts from
green tea tannins,15 vanillin and vanillic acid,16,17 quercetin18 or
phloroglucinol19 (an aromatic triol contained in bark of fruit trees)
have been recently reported. In opposition to the numerous
alternatives proposed for DGEBA-free resins, the design of new
biobased genuine bisphenols was more scarcely described. A
diphenolic ester obtained via condensation of levulinic acid
(biobased) and phenol (petrosourced) was studied as a BPA
alternative for epoxy thermosets applicatons.20 Hernandez et al.
described fully biobased bisphenols obtained from guaiacaol
and vanillyl alcohol and their application towards epoxy resins.21
Recently, biscreosol22 was transformed into biscyclocarbonate
for the synthesis of polyurethanes23, or into biscyanate for
cyanate ester resin24. Vanillin-based bisphenols obtained via a
chemo-enzymatic process has been described as a platform of
high interest for the design of novel rigid bio-based polymers.25
DiGlycidylEther of Bisphenol
A (DGEBA), extensively used in
polymer industry (Td5% = 319 °C and Tg = 150 °C for DGEBA-IPDA
resins). Degradation studies in NaOH and HCl aqueous solutions
also highlighted the robustness of the syringaresinol-based resins,
similar to bisphenol
A (BPA). All these results undoubtedly
confirmed the potential of syringaresinol as a greener and safer
substitute to BPA.
Introduction
Among the thermoset polymers market, epoxy resins constitute
[a]
[b]
Dr M. Janvier, L. Hollande, A.S. Jaufurally, Dr R. Ménard, Prof F.
Allais
Chaire ABI
AgroParisTech
CEBB 3 rue des Rouges Terres 51110 Pomacle, France
E-mail: florent.allais@agroparistech.fr
Dr M. Janvier, A.S. Jaufurally, Dr P.-H. Ducrot
Institut Jean-Pierre Bourgin
INRA/AgroParisTech/CNRS/Université Paris-Saclay
Route de Saint-Cyr 78026 Versailles, France
E-mail: ducrot@inra.fr
[c]
[d]
L. Hollande
UMR1145 GENIAL
INRA/AgroParisTech
1 avenue des Olympiades 91744 Massy, France
M. Pernes, Dr Johnny Beaugrand
UMR614 FARE
URCA/INRA
Vanillin-based
bisphenol
also
found
applications
in
2 Esplanade Roland Garros 51686 Reims, France
M. Grimaldi, Dr P. Balaguer
Institut de Recherche en Cancérologie de Montpellier
208 rue des Apothicaires 34298 Montpellier, France
Prof. F. Allais
polycarbonate thermoplastics.26 An eugenol-based bisphenol
was studied for high-performance thermosetting resins27 and
polycarbonates28.
[e]
[f]
UMR 782 GMPA
In this context, an interesting alternative envisaged is the
valorization of lignin, the main bioresource for aromatic
compounds, highly abundant (30% of the organic carbon in the
biosphere29) and available as a by-product of paper industry
without competition with food production. Our team recently
described new ferulic acid-based bis- and trisphenols as
potential substitutes to BPA, obtained via an efficient chemo-
INRA/AgroParisTech/CNRS/Université Paris-Saclay
Avenue Lucien Brétignières 78850 Thiverval-Grignon, France
E-mail: florent.allais@agroparistech.fr
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/cssc.201601595
ChemSusChem
FULL PAPER
enzymatic process coupling ferulic acid derivative to different
biobased diols (for example isorbide dihydroferulate IDF in Fig.
1).30 The endocrine activity test showed no endocrine disruption
for these phenols. The evaluation of these safer lignin-based
bis- and trisphenols for epoxy resin formulation revealed a wide
range of mechanical properties (Tg ranging from 32 to 85 °C,
upon curing with various diamines)31 and an attractive recycling
opportunity32. Furthermore, these bis- and tris-phenols were
valorized for the synthesis of linear (thermoplastics) crosslinked
(thermoset) non-isocyanate polyurethanes, highlighting a wide
range of Tg (17-72 °C).33 Unfortunately, the aliphatic chain of the
ester moiety in IDF is flexible and provides lower mechanical
properties compared to DGEBA. The search of a biobased
bisphenol able to compete with BPA is not over yet.
In this study, the access to a fully renewable diepoxy monomer
(SYR-EPO) from biobased syringaresinol via its glycidylation in
presence of biobased epichlorhydrine (EPICEROL® from
glycerol by SOLVAY®)38 is described. In order to ensure the
relevance of syringaresinol as a safe replacement of BPA, its
estrogenic activity has been studied. Aiming to synthesize fully
biobased epoxy thermosets, the diglycidyl ether of
syringaresionol SYR-EPO is then cured with biobased diamines,
namely decane diamine (DA10) and difurfurylamine (DIFFA),
respectively obtained from ricin oil and furfural. Moreover, SYR-
EPO is crosslinked with isophorone diamine (IPDA) and
compared with the DGEBA-IPDA system as a reference of
current use. Thermal and mechanical properties of the resulting
thermosets have been determined by TGA, DSC and DMA.
Finally, a degradation study has been carried out to assess the
resistance of these biobased thermosets to drastic chemical
treatments.
Committed to offering a genuine renewable alternative to BPA
(comparable thermo-mechanical properties), we are now taking
into consideration syringaresinol (SYR) (Fig. 1), a naturally
occuring bisphenol present in plants such as Syringa patula34 or
Magnolia Thailandica35, as a promising candidate. Our team
indeed recently reported on an efficient selective chemo-
enzymatic process for the synthesis of syringaresinol, resulting
from the oxidative coupling of two molecules of sinapyl alcohol,
starting from syringaldehyde.36 With a cis-fused bis-furanic
moiety and two aromatic rings providing high stiffness (Fig. 1),
syringaresinol shows interesting structural similarities with BPA,
and has been used as a precursor for the synthesis of novel
α,ω-dienes monomers. Their polymerization via ADMET led to
polymers with molecular weight up to 14.1 kg.mol-1, excellent
thermal stabilities (257-360 °C) and tunable Tg (18-70 °C)
depending on the structure of the diene monomer.37
Results and Discussion
Synthesis of syringaresinol (SYR) and its functionalisation
to epoxy precursor SYR-EPO
Syringaresinol (SYR) was prepared from syringaldehyde, a
biobased starting material that can be obtained through the
oxidation of lignins,39 in a 62% overall yield via a 3-step process
(Scheme 1), including the oxidative enzymatic dimerization of
sinapyl alcohol as key step.36 However, we observed the high
importance of O2 dissolution for the enzyme-catalyzed
dimerization: depending on the scale, the magnetic stirring was
not always sufficient to ensure a sufficient concentration of O2 in
the reaction medium. While scaling-up the reaction to 10 g of
sinapyl alcohol, mechanical stirring was combined with O2
bubbling for a better efficiency of the enzyme. To determine the
duration of the reaction, the conversion of sinapyl alcohol was
followed by 1H NMR (see supporting information) in order to
quench the reaction after complete conversion and before
degradation of syringaresinol into 2,6-dimethoxybenzoquinone.36
The purity of the resulting crude product was sufficient to
engage it in the functionalization step without further purification.
Thereafter, the functionalization of bisphenol SYR towards
epoxy precursor SYR-EPO was attempted through glycidylation
using epichlorohydrine. This method consists of forming a
phenolate that is able to attack each one of the carbons of
epichlorohydrine (see the mechanism in Supporting Information),
two of them leading to the epoxy ring opening. Thus, a basic
treatment is necessary to close the open intermediates obtained.
The glycidylation was first tested following a classical procedure
(excess of epichlorohydrin under alkaline conditions) but led to
less than two epoxy functions per syringaresinol moiety. The use
of a catalytic amount of triethylbenzylammonium chloride17 as
phase transfer catalyst allowed the access to SYR-EPO as a
white solid with a 88% yield and with epoxy functionality of 2.0
(according to 1H NMR spectrum, see supporting information).
Figure 1. BPA and two potential alternatives, IDF from ferulic acid and SYR
from syringaldehyde.
OH
O
H
OMe
OH
OMe
OMe
O
O
Jaufurally et al. 36
MeO
HO
MeO
MeO
(a)
(b)
O
O
O
O
MeO
OMe
MeO
OMe
OMe
O
O
OH
Syringaldehyde
OH
Sinapyl alcohol
MeO
SYR
SYR-EPO
(a) laccase from Trametes versicolor (9 mL/h), CH3CN, citrate phosphate buffer pH 5 (C 0.05 M), 50 °C (93%); (b) TEBAC (0.2 eq), epichlorhydrine (20 eq), 80 °C, 2 h then NaOH (8 eq), r.t., 2 h (88%)
Scheme 1. Chemo-enzymatic synthesis of diglycidyl ether of syringaresinol (SYR-EPO).
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10.1002/cssc.201601595
ChemSusChem
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Endocrine binding affinity test
disruption for DGEBA,32 and thus confirming the importance of
the free phenols for the toxicity. Based on these results
demonstrating syringaresinol innocuousness, SYR can be
considered as a genuine safer BPA replacement.
According to the recognized toxicity of BPA due to its ability to
disrupt endocrine activity, estrogenic activity evaluation was first
conducted to ensure the elaboration of a viable alternative.
Bisphenol derivatives (SYR, IDF and BPA) and their bisepoxy
derivatives (SYR-EPO, IDF-EPO and DGEBA) were evaluated.
BPA is known to bind to and activate estrogen receptor Erα, as
a partial agonist of Erα. ERα is a member of the nuclear
hormone receptor family, which activity is regulated by the
steroid and estrogen sex hormone 17β-estradiol (E2).
Epoxy-amine resin formulation and preparation
Amines are common hardeners for epoxy resins and can react
in a two-step mechanism to form a crosslinked network (Scheme
2). According to this mechanism, we supposed that secondary
amines formed would react contrary to less reactive hydroxyls,
formed upon epoxy ring-opening. Consequently the theoretical
epoxy-amine ratio is 2:1, and this stoichiometric ratio was
chosen for all formulations in order to obtain high crosslinking
rates.
a)
120
The chemical structures of all epoxy precursors and amine
hardeners are reported in Fig. 3. Three epoxy precursors were
compared: lately synthesized SYR-EPO, previously studied IDF-
EPO31,32 and commercial reference DGEBA.
100
80
60
40
20
0
Epoxy precursors
Curing agents
H2N
NH2
10
O
O
O
O
DA10
DGEBA
b)
120
E2
BPA
100
80
60
40
20
0
O
O
O
SYR
H2N
O
IDF
IDF-EPO
SYR-EPO
O
O
O
O
O
O
O
1,E-13
1,E-11
1,E-09
1,E-07
1,E-05
1,E-03
O
O
[ligand] (M)
O
H2N
DIFFA
IDF-EPO
Figure 2. (a) Estrogenic activity of the bisphenols and their corresponding
epoxides at 10-5 M and E2 at 10-8 M; (b) estrogenic activity as a function of the
concentration.
OMe
O
NH2
MeO
O
O
O
O
OMe
For this test, the estrogenic activities were measured with ERα
and benchmarked against E2 (Fig. 2 (a)). In Fig. 2 (b), the sex
hormone E2 shows a 100% activity at about 5 x 10-10 M. For
BPA, estrogenic activity is observed for concentrations higher
than 10-7 M, and about 60% activity is observed at 10-5 M. In
contrast, both biobased macrobisphenols IDF and SYR
exhibited a low activity, comparable to the control (~10%), even
with increased concentration. The lack of estrogenic disruption
for IDF and SYR is coherent with their structure: both bisphenols
display a longer spacer between the two phenolic moieties in
comparison with BPA, this distance having a key role in the
disruption mechanism.40 The bisepoxy derivatives SYR-EPO,
IDF-EPO and DGEBA, showed no endocrine disruption, in
O
H2N
MeO
IPDA
SYR-EPO
Figure 3. Chemical structures of the epoxy precursors and curing agents used
for the resins formulation.
accordance with
a previous study showing no endocrine
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10.1002/cssc.201601595
ChemSusChem
FULL PAPER
O
O
O
O
OH
O
O
O
O
H
H2N
N
H2N
NH2
O
O
O
step 1
step 2
OH
O
O
O
O
O
HO
OH
HO
OH
N
H2N
N
N
OH
O
O
O
O
O
Scheme 2. Crosslinking mechanism for epoxy-amine materials.
Two biobased amines (DA10 and DIFFA) and one extensively
used fossil-based amine (IPDA), presenting different rigidities,
were chosen as hardeners. The structure of IDF-EPO, with its
flexible aliphatic ester bound, was believed to be less rigid than
the bicyclic structure of SYR-EPO, forecasting higher
mechanical properties with the latter. Contrary to DGEBA, IDF-
EPO and SYR-EPO monomers were solids at room temperature
and rapid crystallization from the melt was observed. For the
formulations, IDF-EPO and SYR-EPO had to be molten and
rapidly mixed with the corresponding amine hardeners. All resins
were rapidly blended in molten state at 120 °C to warrant
formulation homogeneity (in the case of solid DA10) and prevent
bubbles. To ensure that syringaresinol epoxy monomer has a
good reactivity towards aliphatic diamines, a DSC analysis of an
equimolar mixture of SYR-EPO and IPDA was carried out (Fig.
4). An exothermic peak was observed around 110 °C
corresponding to the epoxy-amine reaction, which proved a
reactivity of SYR-EPO comparable to DGEBA. The delayed
onset of cure with SYR-EPO is more likely due to its higher
melting point.
In order to ensure chain mobility and to obtain optimal
crosslinking content, a curing temperature program was defined
to slow down the crosslinking reaction. The formulations based
on SYR-EPO were cured first at Tfus < T ≈ TDSC crosslinking peak (2
hours at 120 °C) then at T > TDSC crosslinking peak (16 hours at 160 °C
then 1 hour at 180 °C). This processing guaranteed the
homogeneity of materials as shown in Fig. 5. Interestingly, the
SYR-EPO-based resins showed good translucency and just a
slight orange coloration. Nevertheless, the resin cured with
DIFFA displayed an intense coloration and low translucence, as
the starting amine used is dark brown.
15
10
5
Figure 5. Aspect of the cured thermosets.
The DSC analyses of post-cured resins showed no residual
exothermic peak above or below glass transition corresponding
to further curing reactions (see Supporting Information). This
result indicated that the degree of thermoset polymer cure was
high and that further curing reactions that might occur due to
remaining oxirane moieties were low since they were below the
detection limit of DSC experiments. Further comparison of the
FT-IR spectra of precursor SYR-EPO and cured resins showed
0
50
100
150
200
-5
Temperature (°C)
-10
DGEBA / IPDA
SYR-EPO / IPDA
a dramatic decrease in the epoxy vibrational band at ∼ 910 cm−1
Figure 4. DSC analysis of an equimolar mixture of DGEBA or SYR-EPO with
IPDA, before melt mixing.
(see Supporting Information). This qualitative analysis confirmed
the diminution of oxirane moiety amount.
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10.1002/cssc.201601595
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Characterization of the syringaresinol-based epoxy-amine
resins
differences observed between the three epoxides. For instance,
the molar mass of IDF-EPO and SYR-EPO being higher to
DGEBA, the final curing agent concentration and thus the
theoretical crosslinking density are reduced. Moreover, while
both IDF-EPO and SYR-EPO present a rigid bicyclic core and
two aromatic rings, the aliphatic segment of IDF-EPO provides
higher flexibility whereas, in the case of syringaresinol, the rigid
fused rings are directly linked to the aromatic rings, resulting in
higher rigidity. Interestingly, with a Tg of 126 °C, SYR-EPO IPDA
resin comes closer to the commercial DGEBA IPDA reference
(Tg = 150 °C) with nearly competitive mechanical properties.
Thermal characterization
Thermal Gravimetric Analysis Thermal stability of cured
thermosets was studied by thermal gravimetric (TGA) analysis
under nitrogen flow (Fig. 6). Td5% was defined as the
temperature at which the thermoset lost 5%w of its initial mass;
w%char corresponds to the relative amount of stable residue at
high temperature (700 °C). Table 1 sums up the values of Td5%
and w%char of the thermosets prepared.
The resins obtained with IDF-EPO and SYR-EPO exhibited
lower thermal stability than with DGEBA. When comparing the
different IPDA-cured resins, IDF-EPO led to a Td5% around 30 °C
lower than DGEBA, which is supposed to be caused by the
thermolysis of the ester function from the biobased epoxy
precursor, occurring around 300 °C.31 Similarly, in the case of
SYR-EPO cured with IPDA, the Td5% is around 20 °C lower than
DGEBA. The lower Td5% observed with IDF-EPO and SYR-EPO
compared to DGEBA might also be due to the presence of
electro-donating methoxy substituents on the phenol as
previously discussed by Harvey et al.41 The lowest Td5% is
obtained with DIFFA, which might be explained by the initiation
of the degradation by the rupture of the curing agent segment.31
Concerning the high temperature stable char content, higher
char content were obtained for both SYR-EPO (24%) and IDF-
EPO (12%) compared to DGEBA (7%), up to 3 times more for
SYR-EPO. This parameter would be particularly interesting for
the development of flame retardants.
150
160
126
140
120
100
80
60
40
20
0
102
85
IDF-EPO
SYR-EPO
DGEBA
73
DA10 DIFFA
IPDA
Figure 7. Tg for epoxy-amine resins based on IDF-EPO, SYR-EPO and
DGEBA.
Thermo-mechanical characterization
100
80
60
40
20
0
Dynamic Mechanical Analysis (DMA) Mechanical properties of
the cured resins were evaluated by DMA. DMA temperature
scans (Fig. 8) gave values of the glassy modulus at 50 °C
(E’glassy), rubbery storage modulus at 200 °C (E’rubbery) and alpha
transition temperatures (Tα) related to glass transition and
determined from the peak of loss modulus vs temperature. The
results are given in Table 1. In accordance to the values of Tg
assessed by DSC, the mechanical transition temperatures (Tα)
of the resins based on SYR-EPO are lower than that of DGEBA-
based resin. For IPDA-based resins, Tα observed was 9 °C
lower for SYR-EPO (157 °C) compared to DGEBA (166 °C). As
previously discussed, the rigidity of both the epoxy precursor
and the curing agent influences the mobility of the network and
the resulting mechanical transitions. Compared to DGEBA, the
50
250
450
650
Temperature (°C)
SYR-EPO DA10
SYR-EPO IPDA
DGEBA IPDA
SYR-EPO DIFFA
IDF-EPO IPDA
elastic moduli of SYR-EPO resins were comparable at the
!"#!!"#
glassy plateau (ꢀ′
≈ ꢀ′!"#$%
!"#$$% ), proving good mechanical
!"#$$%
Figure 6. Thermograms of cured resins (10 °C.min-1 under N2 flow).
properties of this biobased alternative. The elastic moduli of
SYR-EPO resins were 25% lower at the rubbery plateau
!"#!!"#
(ꢀ′
< ꢀ′!"#$%
!"!!"#$ ) compared to DGEBA, in accordance
!"##$!%
Differential Scanning Calorimetry Analysis (DSC) Since
applications of epoxy resins usually required high thermal and
mechanical properties, high glass transition temperatures (Tg)
are targeted. Differential Scanning Calorimetry (DSC) analyses
were carried out to determine the Tg values of the different cured
resins studied (Fig. 7). The curing of SYR-EPO with hardeners
presenting various rigidities led to a wide range of Tg (from 73 to
126 °C). The most flexible aliphatic amine DA10 led to the
lowest Tg (73 °C). The aromatic rings of DIFFA conferred higher
rigidity and induced higher Tg (102 °C). Finally the cyclic
configuration of IPDA reduces the mobility of the curing agent
and the resulting more rigid network showed the highest Tg
(126 °C). When comparing the curing of the three resins (IDF-
EPO, SYR-EPO and DGEBA) with IPDA, the following trend is
observed for the Tg: DGEBA (150 °C) > SYR-EPO (126 °C) >
IDF-EPO (85 °C). Different parameters might explain the
with its lower theoretical crosslinking content.
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10.1002/cssc.201601595
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weight loss. Interestingly, biobased IDF-EPO and SYR-EPO
offer complementary behaviors regarding hydrolysis resistance.
2
1,00E+09
1,00E+08
1,00E+07
1,00E+06
1,8
1,6
1,4
1,2
1
Alkaline medium
100
80
60
40
20
0
0,8
0,6
0,4
0,2
0
1,00E+05
40
0
20
40
60
80
100
120
90
140
190
240
Time (h)
Temperature (°C)
SYR-EPO DA10
SYR-EPO IPDA
SYR-EPO DIFFA
DGEBA IPDA
Acidic medium
100
80
60
40
20
0
Figure 8. DMA curves of cured SYR-EPO and DGEBA resins.
Chemical degradation study of the cured resins With
mechanical properties, chemical resistance is one of the most
interesting properties of epoxy-amine resins.
A chemical
degradation study was thus carried out to assess the resistance
of IPDA-cured resins based on SYR-EPO, IDF-EPO and
DGEBA. Samples were immersed in alkali (NaOH 3M) or acidic
(HCl 3M) solutions at 60 °C. Mass loss was determined as a
function of the incubation time (Fig. 9). As described
previously,32 resins based on IDF-EPO showed a rapid decrease
in mass upon incubation with a linear behavior, due to the
presence of hydrolysable ester moieties. The cleavage of the
ester bonds of IDF-EPO during the degradation allows a
potential chemical recycling and thus provides an attractive end-
of-life option. On the contrary, resins based on SYR-EPO
showed hydrolysis resistance comparable to DGEBA, with no
0
10
20
30
40
50
60
Time (h)
DGEBA IPDA SYR-EPO IPDA IDF-EPO IPDA
Figure 9. (Top) Chemical degradation in NaOH (3M) at 50 °C (Bottom)
Chemical degradation in HCl (3M) at 50 °C
Table 1. Properties of the cured resins (ATG, DSC, DMA).
Resin
Td5% (°C) *
303
w%char (at 700 °C) *
Tg (°C)**
73
Tα (°C)***
102
E’glassy (MPa) at 50 °C***
E’rubbery (MPa) at 200 °C***
SYR-EPO/DA10
SYR-EPO/DIFFA
SYR-EPO/IPDA
IDF-EPO/IPDA
DGEBA/IPDA
23.1
29.4
24.0
12.1
7.4
880
1112
758
14
10
12
931
16
288
102
126
85
108
298
157
289
9931
200031
319
150
166
783
* determined by TGA (10 °C.min-1, N2 flow); ** determined by DSC on the second heat-cool-heat cycle (10 °C.min-1, N2 flow); *** determined by DMA
(frequency 1 Hz, amplitude 6 µm, 3 °C.min-1)
choice candidate for the replacement of DGEBA in thermoset
formulations. The curing of SYR-EPO with different biobased
Conclusions
amines was thus investigated and provided fully renewable
epoxy-amine resins whose tunable thermo-mechanical
Syringaresinol, a renewable bisphenol obtained at the multigram
properties were benchmarked against the commonly used
scale via an efficient chemo-enzymatic pathway from lignin-
derived syringaldehyde, proved – for the first time - not to be an
endocrine disruptor, confirming its interest as a genuine safer
alternative to BPA. This bisphenol was then efficiently
glycidylated to provide a bisepoxy precursor (SYR-EPO) in
excellent yield and purity and whose high rigidity makes it a
DGEBA/IPDA resin. Analyses revealed that the thermoset
resulting from the curing of SYR-EPO with IPDA displayed
optical, thermal and thermo-mechanical properties (up to Tg
=
126 °C and Td5% = 298 °C) almost competing with that of
DGEBA (Tg = 150 °C and Td5% = 319 °C). The chemical
resistance of the SYR-EPO/IPDA and DGEBA/IPDA resins was
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10.1002/cssc.201601595
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previously described in the literature. Detailed procedures are reported in
Supporting Information.
then studied in acidic or alkaline aqueous media and revealed a
high resistance towards hydrolysis for both thermosets.
Compared to our previous studies on IDF-EPO that shows both
lower mechanical properties (upon curing with IPDA, Tg = 85 °C
and Td5% = 289 °C) and degradability properties, SYR-EPO is
undoubtedly a better alternative to IDF-EPO in terms of chemical
resistance and thermo-mechanical properties while being non-
toxic towards the endocrine system. This study thus confirms the
great potential of syringaresinol as a greener and genuine safer
alternative to BPA. To further explore the potentiality of
syringaresinol in polymers, the carbonation of SYR-EPO and its
application towards non-isocyanate poly(hydroxyurethane)s
(NIPUs) has been investigated and will be reported in due
course.
Synthesis of diglycidyl ether of syringaresinol (SYR-EPO)
OMe
O
γ
O
O
O
6
β
7
4
1
8
O
O
α
OMe
O
2
3
O
5
SYR (3 g, 11.9 mmol, 1 eq) was dissolved in epichlorohydrine (11.3 mL,
239 mmol, 20 eq). Triethyl benzyl chloride (TEBAC) (327 mg, 2.39 mmol,
0.2 eq) was added and the suspension was stirred for 2 hours at 80 °C.
The reaction medium was cooled down and NaOH (5M) (11.4 mL, 95.6
mmol, 8 eq) was added. The biphasic solution was vigorously stirred for
4 hours then extracted with ethyl acetate (3x100 mL). The organic layers
were washed with brine (80 mL), dried over anhydrous MgSO4, filtered
and concentrated. The crude product was purified on silica gel using
cyclohexane/ethyl acetate (40/60) as eluent to give diglycidyl ether of
syringaresinol (SYR-EPO) as a white solid (3.3 g, 88%). 1H NMR (300
MHz, CDCl3, δ): 2.64 (dd, 2H, J = 2.7 and 4.8 Hz, H8a, H8a’), 2.82 (app. t,
2H, J = 4.8 Hz, H8b, H8b’), 3.11 (m, 2H, Hβ, Hβ’), 3.38 (m, 2H, H7, H7’),
3.87 (s, 12H, H5, H5’), 3.91-4.02 (m, 4H, Hɣ1, Hɣ1’, H6a, H6a’), 4.14 (2 dd,
2H, J = 1.1 Hz and 4.0 Hz, H6b, H6b’), 4.31 (m, 2H, Hɣ2, Hɣ2’), 4.76 (d, 2H,
J = 4.2 Hz, Hα, Hα’), 6.55 (s, 4H, H2, H2’). 13C NMR (75 MHz, CDCl3, δ):
44.7 (C8, C8’), 50.5 (C7, C7’), 54.3 (Cβ, Cβ’), 56.2 (C5, C5’), 71.9 (Cɣ, Cɣ’),
74.1 (C6, C6’), 85.9 (Cα, Cα’), 102.7 (C2, C2’), 136.1 (C1, C1’), 137.0 (C4,
C4’), 153.4 (C3, C3’). Mp: 115.3 °C. UV (nm, λmax): 205. FT-IR (neat): νmax
2920 (C-Harom), 1400-1600 (C=Carom), 1000-1300 (C-O-C), 911 (C-Ooxiran).
HRMS: (TOF MS, ES+): m/z calcd for C28H34O10Na [M + Na]: 553.2050;
found: 553.2059.
Experimental Section
Materials and methods
Syringaldehyde, malonic acid, aniline, pyridine, DIBAL-H, TEBAC and
Trametes versicolor laccase were purchased from Sigma-Aldrich;
epichlorhydrine and decane diamine were purchased from Acros
Organics; isophorone diamine was purchased from Chemical Industry
Co; furfurylamine was purchased from Alfa Aesar. Reagents were used
as received. Solvents were purchased from ThermoFisher Scientific.
Dichloromethane was dried on a mBraun SPS 800 system. Deuterated
chloroform (CDCl3) was purchased from Euriso-top. Evaporations were
conducted under reduced pressure at temperature below 40 °C. Column
chromatographies were carried out with an automated flash
chromatography (PuriFlash 4100, Interchim) and pre-packed
INTERCHIM PF-30SI-HP (30 µm silica gel) columns using a gradient of
cyclohexane and ethyl acetate for the elution. FT-IR analyses were
performed on Cary 630 FT-IR with ATR. UV analyses were performed on
Cary 60 UV-Vis from Agilent technologies, by dissolving the samples in
acetonitrile. Melting points were measured on a Mettler Toledo MP50
Melting Point System at 2 °C.min-1. NMR analyses were recorded on a
Bruker Fourier 300. 1H NMR spectra of samples were recorded in CDCl3
at 300 MHz, chemicals shifts were reported in parts per million (CDCl3
residual signal at δ = 7.26 ppm). 13C NMR spectra of samples were
recorded at 75 MHz (CDCl3 signal at δ = 77.16 ppm). HRMS were
recorded by the PLANET platform at URCA on a Micromass GC-TOF.
Thermo-gravimetric analyses (TGA) were recorded on a Q500, from TA.
About 10 mg of each sample was heated at 10 °C.min-1 from 30 to
800 °C under nitrogen flow (60 mL.min-1). Differential scanning
calorimetry (DSC) thermograms were obtained using a DSC TA Q20,
Epoxy Equivalent Weight (EEW) The synthesis of epoxides with
epichlorohydrin invariably leads to generation of some oligomers
that can be difficult to remove by common purification techniques.
Epoxy equivalent weight (EEW) for SYR-EPO, IDF-EPO and
DGEBA was determined by 1H NMR according to the equations
previously described by Garcia et al.46 The EEW corresponds to the
weight of resin in grams that contains one equivalent of epoxy.
Calculations are detailed in Supporting Information and results for
each epoxy precursor are reported in Table 2.
Table 2. Epoxy equivalent weight of SYR-EPO, IDF-EPO and DGEBA
Epoxy precursor
SYR-EPO
EEW (g.mol-1)
under inert atmosphere (N2), with
a calibration using indium, n-
octadecane and n-octane standards. For each sample, about 10 mg
were weighed in a pan which was then sealed and submitted to 3
heat/cool/heat cycles: heating from 30 °C to 250 °C at 10 °C.min-1,
cooling from 250 °C to -50 °C at 20 °C.min-1. Glass transition
temperatures (Tg) were determined at the inflexion value in the heat
capacity jump. Dynamical Mechanical Analyses (DMA) were performed
266
310
187
IDF-EPO
DGEBA
on
a DMA Q800 from TA Instrument. The DMA samples had a
Epoxy-amine resins formulation Considering that one primary
amine of the curing agent can react with two epoxy moieties, the resins
were formulated with a 2:1 epoxy-amine ratio. SYR-EPO (800 mg, 2 eq)
was molten at 120 °C, then diamine (1 eq) was quickly added and the
mixture was vigorously stirred for 1 min at 120 °C. In the case of the solid
diamine DA10, an additional stirring time at 120 °C was necessary to get
a complete dissolution of the amine in the epoxy precursor. The liquid
and homogeneous mixture was then poured into a silicon mold. Finally,
the formulation was cured in an oven, following this thermal curing
program: 2 hours at 120 °C, 18 h at 160 °C, 1 hour at 180 °C. For
DGEBA and IDF precursors, the temperature curing program chosen
was already described: 2 hours at 80 °C, 18 h at 110 °C, 1 hour at 140
°C.31 The different compositions of epoxy-amine resins are depicted in
Table 3. The theoretical crosslink density mentioned herein is defined as
the molar amount of crosslinks (i.e., the amount of nitrogen atoms from
the curing agent) for a formulation of 100 g of resin.
rectangular geometry (length: 20 mm, width: 10 mm, thickness: 1 mm).
Uniaxial stretching of samples was performed while heating at a rate of
3 °C.min-1 from 25 to 250 °C, keeping frequency at 1 Hz. Deformation
was kept at 0.1% (amplitude of 6 µm) to stay in the linear viscoelastic
region. The storage modulus (E’) and tan δ curves as a function of the
temperature were recorded and analyzed using the TA Instruments
Universal Analysis 2000 software. E’ is the elastic response and is
related to the mechanical energy per cycle when the sample is deformed.
E’’ is the viscous response and is related to the dissipated energy per
cycle upon deformation. The loss factor corresponds to the angle
between in-phase and out-of phase components of the modulus in the
cyclic motion and it is defined as tan δ = E’’/E’. The temperatures Tα of
the relaxation process, corresponding to the relaxation of the networks
starting to coordinate large-scale motions, were determined as the
temperatures at the peak maximum of the tan δ curves. For degradation
study, samples were freeze-dried with an Alpha 1-2 LD Plus system from
CHRIST, supplied with a Vacuubrand RZ2.5 vacuum pump, after 20
hours at -20 °C.
Synthesis of monomers The syntheses of ethyl sinapate,42-44 sinapyl
alcohol,36 syringaresinol (SYR),36 isosorbide diferulate (IDF-EPO),31
difurfurylamine (DIFFA)45 have been carried out following procedures
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10.1002/cssc.201601595
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FULL PAPER
Table 3. Composition of the formulated thermosets and their theoretical
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FULL PAPER
A natural and safe alternative: A
bisepoxide monomer prepared from
syringaresinol, a natural bisphenolic
starting material showing no
endocrine disruption, is reacted with
different diamines to afford
Marine Janvier, Louis Hollande,
Abdus Samad Jaufurally, Miguel
Pernes, Raphaël Ménard, Marina
Grimaldi, Johnny Beaugrand, Patrick
Balaguer, Paul-Henri Ducrot* and
Florent Allais*
renewable epoxy-amine resins. The
thermal and mechanical properties
of these resins are compared to
diglycidyl bisphenol A (DGEBA)-
IPDA epoxy resin for potential
substitution in advanced material
applications.
Page No. – Page No.
Syringaresinol: a renewable and
safer alternative to Bisphenol A for
epoxy-amine resins
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