Curing Behavior and Properties of Sustainable Furan-Based Epoxy/Anhydride Resins

Article abstract of DOI:10.1021/acs.biomac.9b00919

The last two decades have witnessed a significant growth in using bioderived materials, driven by the necessity of replacing fossil-derived precursors, reducing the fossil fuel consumption, and lowering the global environmental impact. This is possible thanks to the availability of abundant resources from biomasses and the development of optimized technologies based on the principles of sustainability and circular economy. Herein, we report on the synthesis and characterization of new carbohydrate-derived epoxy resins. In particular, 2,5-bis[(oxiran-2-ylmethoxy)methyl]furan has been synthesized and cured with methyl nadic anhydride. The effect of different initiators was studied, in order to identify the most efficient curable formulations. A series of resins was then prepared varying the epoxide-anhydride ratios. The results gathered from physicochemical, mechanical, morphological analyses have demonstrated that the produced furan-based thermosets have the potential to be proposed as sustainable alternatives to the traditional, bisphenol A-containing epoxy resins.

Full text of DOI:10.1021/acs.biomac.9b00919

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Article
Curing behavior and properties of sustainable
furan-based epoxy/anhydride resins
Angela Marotta, Noemi Faggio, Veronica Ambrogi, Pierfrancesco Cerruti, Gennaro Gentile, and Alice Mija
Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.9b00919 • Publication Date (Web): 14 Aug 2019
Downloaded from pubs.acs.org on August 20, 2019
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Curing behavior and properties of sustainable furan-
based epoxy/anhydride resins
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1*
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Angela Marotta †, Noemi Faggio , Veronica Ambrogi , Pierfrancesco Cerruti , Gennaro
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Gentile , Alice Mija
1
Department of Chemical, Materials and Production Engineering (DICMaPI) - University of
Naples Federico II, P. le Tecchio 80, 80125 Napoli, Italy
2
Institute for Polymers, Composites and Biomaterials (IPCB) - CNR, Via Campi Flegrei 34,
8
0078 Pozzuoli (NA), Italy
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Université Côte d’Azur, Université Nice-Sophia Antipolis, Institut de Chimie de Nice, UMR
CNRS 7272, 06108 Nice Cedex 02, France
*
ambrogi@unina.it, +39 081 7682410; pierfrancesco.cerruti@ipcb.cnr.it, +39 081 8675214
KEYWORDS: Epoxy thermosets; bio-based polymers; furan-derived monomers; methyl nadic
anhydride; chemo-rheological analysis; epoxy/anhydride ratio; mechanical properties; curing
behavior.
ABSTRACT
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The last two decades have witnessed a significant growth in using bio-derived materials, driven
by the necessity of replacing fossil-derived precursors, reducing the fossil fuel consumption, and
lowering the global environmental impact. This is possible thanks to the availability of abundant
resources from biomasses and the development of optimized technologies based on the principles
of sustainability and circular economy. Herein, we report on the synthesis and characterization of
new carbohydrate-derived epoxy resins. In particular, 2,5-bis[(oxiran-2-ylmethoxy)methyl]furan
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(BOMF) has been synthesized and cured with methyl nadic anhydride (MNA). The effect of
different initiators was studied, in order to identify the most efficient curable formulations. A series
of resins was then prepared varying the epoxide-anhydride ratios. The results gathered from
physico-chemical, mechanical, morphological analyses have demonstrated that the produced
furan-based thermosets have the potential to be proposed as sustainable alternatives to the
traditional, bisphenol A (BPA)-containing epoxy resins.
Introduction
Epoxy resins are a class of thermosetting polymers extensively employed in several fields.1
2
These materials were discovered in 1936 by Pierre Castan, who was able to produce an amber
colored compound from the reaction of bisphenol A (BPA) with epichlorohydrin (ECH) that, cured
with phthalic anhydride, was employed for dental objects. Since then, epoxy resins have gained a
growing interest from both academic and industrial applications. Their success is due to the
combination of toughness, rigidity, and elevated temperature performance ascribed to the aromatic
groups present in the main polymer skeleton, the chemical inertia due to ether linkages, and the
3
,4
good adhesion properties provided by hydroxyl and epoxy groups. In addition, epoxy resins are
characterized by a high versatility, as it is possible to tune their properties thanks to a variety of
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suitable curing agents and additives that make them useful for plentiful and completely different
applications, ranging from protective coatings for canned food packaging to composites for the
aerospace industry.5
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Analogously to other petroleum derived polymer systems, in the last decades epoxy resins are
concerned regarding not only their potentially toxic effect on human and environmental health,
but also to the forecasted lack of fossil resources.
Pushed by sustainability concerns, and by the uncertainty about future availability of finite
petrochemical resources, a growing attention has been devoted in the last years to ecological, non-
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toxic compounds alternative to the traditional monomers. Therefore, the use of platform
chemicals from renewable resources for biomass-derived polymer synthesis has gradually gained
a global growing attention. So far, several agro-food sources have been indicated as precursors for
the obtainment of high value-added bio-based compounds able to replace the oil-derived ones.
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These include vegetable oils, lignin, rosin, tannins, cardanol, itaconic acid, and carbohydrates.
Among carbohydrate derivatives, furanic compounds are attracting a growing interest as
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valuable alternatives to petroleum-based molecules. They contain a heteroaromatic five-member
ring molecule that mimics the rigid, phenyl unit structure of high-performance commercial resins.
Moreover, they are easily available, as they can be produced from biomasses, such as cellulose
and hemicelluloses. These latter can be simply treated to generate hexose and pentose, that, in turn,
can be converted into hydroxymethylfurfural and furfuryl alcohol, respectively, which are
potential precursors for the synthesis of numerous polymeric systems.12-15 Recently furan derived
compounds have been used for the synthesis of bio-based epoxy resins with properties comparable
to conventional petrol-based epoxy resins.16-19
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In this study 2,5-bis(hydroxymethyl)furan (BHMF) has been used as precursor for the synthesis
of bis-epoxy monomers and cured with methyl nadic anhydride (MNA). Anhydride curing agents
have the advantage of being more sustainable over amines yet providing comparable mechanical
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performances to the final materials.20
The aim of the work was to investigate the structure-property-processing relationships of the
epoxy-based furan synthesized systems, at different epoxide-anhydride ratios, with the final
objective of obtaining furan-based resins with chemical, physical and mechanical properties
comparable to traditional phenyl analogue resins and potentially usable for coating and composite
applications.
Experimental
Materials
The commercially available bio-based epoxy precursor, namely 2,5-bis(hydroxymethyl)furan
(BHMF, 97%) was purchased from Apollo Scientific. Methyl-5-norbornene-2,3-dicarboxylic
anhydride (methyl nadic anhydride, MNA, 90%, anhydride equivalent weight AEW = 178.2 g/eq)
was provided by Sigma Aldrich and used as a curing agent. MNA was selected because it is liquid
at room temperature. 2-methylimidazole (2-MI, 99%) was purchased from Acros Organics. N,N-
dimethylbenzylamine (BDMA, ≥99%), 1-methylimidazole (1-MI, 99%) as initiators and bisphenol
A diglycidyl ether (DGEBA, epoxy equivalent weight EEW = 172-176) as reference bis-epoxide
were purchased from Sigma-Aldrich.
Anhydrous sodium hydroxide pellets (NaOH, ≥97%), ethyl acetate (EA, 99%) and hexane (Hx,
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9%) were purchased from Carlo Erba. (±)-epichlorohydrin (ECH, ≥99%), tetrabutylammonium
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bromide (TBBr, ≥99%), tetrahydrofuran (THF, 99,9%) and magnesium sulfate (MgSO ), were
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Synthesis of 2,5-bis[(oxiran-2-ylmethoxy)methyl]furan (BOMF)
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BOMF was synthesized slightly modifying the procedure described by Cho et al. A 50 wt.%
aqueous solution of NaOH (1.44 mol, 57.7 g), TBBr (1.68 mol, 3.86 g) and ECH (1.68 mol, 132
ml) were added to a double-necked round-bottomed flask equipped with a magnetic stirrer and a
condenser and put under vigorous stirring for 30 minutes at room temperature. The reaction system
was saturated with argon and the flask was then placed in a bath at 50 °C when a solution of BHMF
(0.120 mol, 15.4 g) in THF (120 ml) was added dropwise. The reaction mixture was reacted for 2
hours and subsequently poured into 600 mL of ice/water mix. Then the mixture was extracted two
times with 600 mL of EA and the organic phase was washed with water, dehydrated with MgSO4,
filtered and dried with a rotatory evaporator. To remove the ECH excess, the crude product was
washed three times with hexane and dried again. The total yield of the synthesis is 85%. The
obtained product (BOMF) was an amber-coloured viscous liquid at room temperature, with a
viscosity (η) at 30 °C of 0.032 Pa*s and an equivalent epoxy weight (EEW) of 129 g/eq, calculated
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according to the procedure proposed by F. G. Garcia et al. The chemical structure of the furan
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bis –epoxide was confirmed by H – and C NMR analysis.
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H NMR (400 MHz, CDCl ) 6.27 (s, 2H), 4.48 (q, J = 12.8 Hz, 4H), 3.74 (dd, J = 11.5, 3.1 Hz,
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1H), 3.40 (dd, J = 11.5, 5.9 Hz, 1H), 3.13 (td, J = 6.2, 2.9 Hz, 2H), 2.76 (dd, J = 6.8, 2.3 Hz, 1H),
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.58 (dd, J = 5.0, 2.7 Hz, 1H).
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C NMR (101 MHz, CDCl ) δ 151.84, 110.36, 70.75, 65.21, 50.75, 44.33.
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Synthesis of crosslinked epoxy/anhydride resins
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The chemical structure of reactants and initiators used for the different formulations are reported
in Table 1.
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Table 1. List of reactants and initiators used in the synthesis of furan-based epoxy resins, their
acronyms, chemical structures and functions
Name
Codes
Chemical
structure
Function
2
,5-bis[(oxiran-2-ylmethoxy)methyl]furan
BOMF
Epoxy monomer
O
H C
3
Methyl-5-norbornene-2,3-dicarboxylic
anhydride
O
MNA
CH3
Curing agent
Initiator
O
H3C
N
N,N-dimethylbenzylamine
BDMA
H3C
2
1
-methylimdazole
-methylimdazole
2-MI
1-MI
Initiator
Initiator
Table 2 reports the resins formulations obtained by varying the nature of initiators and the
composition expressed as R = epoxy equivalents/anhydride equivalents (EE/AE).
Table 2. Samples denomination and compositions
Sample codes
Theoretical
diepoxide/anhydride
molar ratio*
R = EE/AE
Initiator
(
0.5 wt.%**)
BOMF-MNA 1-1 (BDMA)
1/1
1.86
BDMA
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BOMF-MNA 1-1 (1MI)
BOMF-MNA 1-1 (2-MI)
BOMF-MNA 1-0.8 (2-MI)
BOMF-MNA 0.8-1 (2-MI)
DGEBA-MNA 1-1 (2-MI)
1/1
1/1
1.86
1.86
1-MI
2-MI
2-MI
2-MI
2-MI
1/0.8
0.8/1
1/1
2.33
1.49
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1.93-1.98
*
*
calculated from the theoretical molecular weights of BOMF and DGEBA
*over the total weight amount of epoxy monomer and curing agent
As a general procedure, a mixture containing the proper amounts of epoxy monomer, curing
agent and initiator was prepared by adding all the components at the same time, at room
temperature under magnetic stirring, to form a homogeneous system.
For each formulation the thermoset resins were obtained by applying the following thermal
treatment: curing at T = 130 °C for t = 2 h; post-cure at T = 180 °C for t = 1 h. In the case of
BOMF-MNA 1-0.8 (2-MI), a further post-cure at T = 230 °C for t =1 h was also performed.
Methods
NMR spectra were recorded using a Bruker Advance 400 MHz spectrometer. Samples were
prepared by dissolving approximately 13 mg of the compound in 1 ml of CDCl3.
A TA Instrument DSC Q2000, equipped with Refrigerator Cooling System (RCS), was used for
calorimetric analysis (DSC), under inert nitrogen flux (30 mL/min). Samples for DSC analysis
were prepared immediately after the preparation of mixtures and dynamic measurements were
carried out at 1.5 and 10 °C/min heating rate from 25 to 200 °C. A second scan at 10°C/min to -
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0°C and a following heating scan at 10 °C to 250 °C were carried out on all the samples.
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A Thermo Scientific MARS III rheometer, interfaced with an iN20 Nicolet FT-IR
spectrophotometer by a Rheonaut temperature controller, were used to perform simultaneous
rheological and infrared analysis. Chemo-rheological tests were performed on plate–plate
geometries (20 mm diameter and 1 mm gap) at a fixed frequency () of 1.59 Hz and deformation
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(γ) in auto-strain mode of 15% ± 15%. Dynamic measurements were performed by varying the
temperature from 25 to 200 °C, at a heating rate of 1.5 °C/min. The temperature of crossover of
the values of storage (G’) and loss (G”) moduli was taken as the temperature of gelation (Tgel).
-1
Simultaneous FT-IR analysis was carried out in ATR mode in the range of 4000-400 cm with
-1
a resolution of 4 cm and 8 scans. FTIR spectroscopy was used for the quantitative analysis of the
functional groups involved in the curing process. In particular, since anhydride and ester carbonyl
absorption bands partly overlapped, quantitative analysis was done by monitoring the absorbance
area of half peaks. More specifically, anhydride consumption was evaluated by monitoring the
-1
absorbance area in the region between 1820 and 1774 cm , while ester formation was evaluated
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by integrating the region between 1730 and 1665 cm . Therefore, to evaluate the relative
conversion values (α) of ester and anhydride groups, the following equations were used.22
퐴1730,푡 ― 퐴1730,0
훼ester = 퐴1730,∞ ― 퐴1730, 0
(1)
(2)
퐴1774,푡
훼anh = 1 ― 퐴1774, 0
where A , A and Ax,∞ represent the area of the half peaks as defined above, centered at the
x,t
x,0
wavenumber x at the reaction times t, 0 and final, respectively. The conversion of epoxy groups
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was not evaluated due to the severe overlap of anhydride and epoxy absorption bands in the 950-
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50 cm wavenumber range.
Thermogravimetric analysis (TGA) was performed by using a TA Instrument TGA 500
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thermobalance. Tests were carried out from 25 to 750 °C at a heating rate of 10 °C/min;
measurements were performed under 60 mL/min nitrogen or air flux.
Swelling measurements were carried out by weighing 35 ± 5 mg of each sample before and after
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immersion in tetrahydrofuran (THF). First, the dry sample was weighed, and the value recorded
as W . Then, it was immersed in 20 mL of THF. After 24h, the sample was recovered, blotted,
d
weighed, and the value recorded as W . The swelling index SI was calculated by Equation 3, as
s
the mean value of tests carried out on 3 specimens for each sample.
푊푠 ― 푊푑
푆퐼 =
(3)
푊푑
Dynamic mechanical analysis (DMA) was performed using a Perkin Elmer Pyris Diamond
DMA analyzer in tensile mode. Experiments were carried out on specimens with a rectangular
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cross-section of about 5 x 1 mm and length of 20 mm, at a 1 Hz frequency, with temperature
ranging from -60 to 180 °C at 1 °C/min. Glass transition temperature values (T ) were evaluated
g
following the method proposed in ASTM E1640-18, and determined as the peak maximum of the
tan delta curve. DMA was used to determine the crosslinking density of the network was evaluated
by Equation 4:
퐸′ = 3휈푅푇
(4)
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where E’ is the elastic modulus in the rubbery plateau region (160 °C), ν is the crosslinking
density, R is the gas constant and T is the temperature (°K).24,25
To perform tensile tests on cured samples, an Instron 4050 dynamometer with a 100 kN load cell
was used. 3 specimens (shape and dimension according to ASTM D638-14, type V) for each
composition were tested at 20 °C and 50% relative humidity, using a 2 mm/min crosshead speed.
Engineering ultimate tensile strength (σ), and strain (ε) values, along with tensile modulus,
calculated as the slope of straight portion at the beginning of the stress-strain curve (in the stress
values range 1 to 5 MPa) are reported in Table 4.
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Results and discussion
Synthesis of BOMF
In this work, 2,5-bis[(oxiran-2-ylmethoxy)methyl]furan (BOMF) epoxy monomer was
1
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synthesized by optimizing the procedure proposed in literature. The protocol is based on the
reaction of the furan-based diol, BHMF, with a stoichiometric excess of ECH (ECH:BHMF molar
ratio 14:1) in a 50 wt.% NaOH aqueous solution and in presence of the phase transfer catalyst
TBBr at room temperature (Scheme 1).
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Scheme 1. Reaction scheme for the glycidylation of BHMF.
First, an intermediate chlorohydrin species formed by the reaction between ECH and NaOH in
presence of the TBBr. This intermediate was then reacted with BHMF in THF solution at 50 °C
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for 2 h, under argon. The so-obtained BOMF was characterized by FTIR-ATR, H and C NMR
analyses to confirm the successful addition of glycidyl moieties and the degree of purity of the
synthesized BOMF (Figure S1). By analyzing the NMR spectra (Figure S1b-c) performed on
BOMF no relevant amounts of by-products were noticed, and only the presence of some residual
ECH, entrapped in the viscous BOMF and hardly removable through evaporation, was detected.
Therefore, the purification step via chromatography resulted unnecessary, and it was replaced by
repeated extractions with ethyl acetate and successive washing with water and n-hexane. The
absence of this chromatographic purification step was of great advantage, reducing solvent
consumption and workout time. According to the obtained results a yield of 85% was achieved.
Curing reaction of BOMF epoxy with MNA anhydride
The process of epoxy crosslinking involves the use of appropriate curing agents. There is a
variety of curing agents commercially available, mostly including amines, anhydrides,
dicarboxylic acids. The choice depends on the processing conditions, such as curing time, pot life,
viscosity, etc. and the end-use requirements in terms of chemical, mechanical and thermal
4
resistance. Among these curing agents, anhydrides have the advantage of being more sustainable
2
6
than amines providing good mechanical performances of the final materials. To improve the
system reactivity, tertiary amines are often used as initiators to promote the reaction at opportune
temperature conditions.27,28 In particular, imidazole derivatives are widely used initiators for
4
anhydride-epoxy resins. In the present work, MNA was selected as curing agent since,
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analogously to BOMF, it is liquid at room temperature. Therefore, the two components were easily
mixed yielding a homogeneous blend. Moreover, the effect of three amine initiators, namely
BDMA, 1-MI and 2-MI (0.5 wt.% of initiator with respect to the total weight of epoxy+anhydride)
on the curing behavior of epoxy+anhydride reaction mixtures with R = 1.86 was evaluated.
Generally, the crosslinking reaction of epoxies with anhydrides proceeds through an initiation
step involving the nucleophilic attack of the nitrogen of the initiator to either the oxirane ring or
the anhydride (Scheme 2) that generates an intermediate zwitterion containing a quaternary
nitrogen cation and an active oxyanion.29,30 The latter reacts with the anhydride or the epoxy group
giving rise to a carboxylate or alkoxide anion, respectively, which initiates the chain-wise
0
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polymerization.24,31-34 The formed anion propagates the chain growth through an alternating
copolymerization anionic mechanism.35
Propagation
Initiation
Scheme 2. Mechanism of epoxy-anhydride polymerization reactions initiated by tertiary amines
As summarized in Table 2, BOMF-MNA 1-1 systems with 0.5 wt.% of BDMA, 1-MI or 2-MI
were prepared, obtaining the samples labelled as BOMF-MNA 1-1 (BDMA), BOMF-MNA 1-1
(1-MI) and BOMF-MNA 1-1 (2-MI), respectively. The curing behavior of these mixtures was
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studied by DSC carried out at a heating rate of 10 °C/min. Data gathered are reported in Table S1
and Figure S2. Summarizing, when 2-MI was used as initiator, a sharp peak associated with the
highest reaction enthalpy was registered in the DSC curing curve, suggesting an increased
reactivity and function conversion. Therefore, 2-MI was chosen as the most suitable initiator for
the cure of BOMF MNA 1-1 resins.
1
1
1
1
1
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Another key factor in the optimization of the curing process is the epoxide-anhydride ratio,
which can affect the reactivity, and in turn the performance of the final product. Typically, an
excess of epoxy is used, with an epoxy/anhydride equivalent ratio ranging from 1.1 to 2.0.31,36,37
In this work, in order to get a detailed insight on the effect of stoichiometry on the curing process
and the final properties of the thermosets, different formulations were prepared, by selecting three
epoxy/anhydride equivalent ratios (EE/EA), corresponding to R values = 1.49, 1.86, and 2.33
(Table 2).
The study on curing process of epoxy/anhydride reacting mixtures at different R values was first
performed through DSC. Total enthalpy of reaction (ΔH ) and peak temperature (T ) values
tot
p
obtained by dynamic DSC experiments (Figure 1a) at 1.5 °C/min on the three formulations BOMF-
MNA 1-1 (2-MI), BOMF-MNA 1-0.8 (2-MI), BOMF-MNA 0.8-1 (2-MI) are reported in
Table 3.
Table 3 Total enthalpy of reaction (ΔH ), peak (T ), onset (Tonset) and glass transition (Tg)
tot
p
temperature values calculated from dynamic DSC experiments.
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ΔH [J/g]
T [°C]
Tonset [°C]
T [°C]*
tot
p
g
BOMF-MNA 1-1 (2-MI)
BOMF-MNA 1-0.8 (2-MI)
BOMF-MNA 0.8-1 (2-MI)
265
219
198
119
125
125
95
104
97
46
5
64
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*
Tg was determined from the DSC second heating scan.
0
0
0
0
0
0
0
.30
.25
.20
.15
.10
.05
.00
a)
BOMF-MNA 1-1 (2-MI)
BOMF-MNA 1-0.8 (2-MI)
BOMF-MNA 0.8-1 (2-MI)
40
60
80
100
120
140
160
180
200
Temperature (°C)
20
20
40
60
-2.000
2
00
c)
b)
-1.500
175
-
1.000
50
1
-
0.5000
.000
.500
.000
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.000
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1
100
1900
1850
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1750
1700
1650
1150
1100
1050
1000
950
900
850
800
750
-1
Wavenumber (cm )
-1
Wavenumber (cm )
Figure 1. (a) DSC thermograms on curing of BOMF MNA 1-1 (2MI), BOMF MNA 1-0.8 (2-MI)
and BOMF MNA 0.8-1 (2-MI) at 1.5 °C/min of. Evolution of BOMF-MNA 1-1 (2-MI) ATR FT-
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-1
IR spectra in (b) 1900–1650 cm , and (c) 1150–750 cm wavenumber regions, collected during
curing at 1.5 °C/min.
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All the systems exhibited a single exothermic peak related to the curing reaction. BOMF-MNA
1
-1 (2-MI) displayed lower onset and peak temperatures and higher reaction enthalpy with respect
to those of BOMF-MNA 1-0.8 (2-MI) and BOMF-MNA 0.8-1 (2-MI). This outcome can be related
to the more balanced stoichiometry of the reacting mixture, which favored the chain growth
through an alternating copolymerization mechanism where anhydride and epoxide are consumed
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8
at comparable rate and etherification almost does not take place. On the other hand, excess of
either epoxy or anhydride monomer resulted in a lower conversion degree, giving rise to reduced
values of curing enthalpies. A second heating ramp was performed on all samples, to check the
occurrence of any residual post-cure reactions, and to calculate the T values (Table 3). BOMF-
g
MNA 0.8-1 (2-MI) showed the highest T value, which together with the lower reaction enthalpy
g
suggested that the rigid structure of the anhydride caused the reaction to be diffusion-controlled
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resulting in early vitrification, as also demonstrated by rheological characterization reported in
the following.
On the other hand, BOMF-MNA 1-0.8 (2-MI) displayed a dramatically lower T value, due to
g
the presence of unreacted epoxy monomer. Indeed, the occurrence of homopolymerization of the
residual epoxy was evidenced by an upward curvature of the DSC thermogram above 200 °C
(Figure S3), indicating the onset of a second exothermic peak.40,41 The T value of the system cured
g
up to 300 °C was ~60 °C, which was comparable to that of BOMF-MNA 0.8-1 (2-MI).
The temperature-dependent conversions of the BOMF-MNA reaction mixtures in presence of 2-
MI were also monitored by collecting FT-IR spectra in ATR mode upon heating the sample at a
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controlled rate (1.5 °C/min) on the rheometer stage. In this way, the chemical evolution of the
three analyzed systems could be monitored alongside their viscoelastic behavior. ATR FT-IR
spectra of the epoxy monomer (BOMF) and curing agent (MNA) are reported in Figure S4, while
Figure 1b-c shows the evolution of the FT-IR spectra collected upon curing BOMF-MNA 1-1 (2-
MI) during rheological dynamic measurements, in the spectral regions 1900-1650 and 1150–750
0
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-
1
cm .
The complete disappearance of the peak characteristic of the reacting functional groups, as well
as the appearance of the characteristic peak of the C=O stretching of ester carbonyl at 1735 cm-1
-1
confirmed the occurrence of cure. In particular, the peaks at 1850 and 1770 cm were used to
follow the curing reaction as they gradually disappeared over time due to the opening of the
anhydride group. Unfortunately, direct monitoring of epoxy ring peak was not possible due to the
-1
overlapping of both the epoxy and anhydride characteristic vibrations in the 954 – 875 cm region
(Figure 1c).
As for the forming species, ester characteristic groups are present both in the 1210–1160 cm-1
-1
(
ether C-O-C stretching) and 1735–1750 cm (carbonyl C=O stretching) spectral regions.
-1
Carbonyl ester stretching at 1735 cm was monitored to determine the kinetics of ester group
4
2
formation because of the fewer overlaps in the analyzed region (Figure 1b). From the figure, the
peak area increase was accompanied by a shift to lower wavenumbers, indicating the progressive
increase of polar interactions between ester carbonyl groups and hydroxyl groups progressively
formed upon epoxide ring opening (Figure S5).43,44
The curves of absolute conversion of characteristic groups involved in the cure reaction, evaluated
from ATR FT-IR spectra for the three epoxy/anhydride systems, are reported in Figure S6. For all
samples, at temperatures below 90 °C, the conversion rate of anhydride groups was higher than
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that of esters, and this difference was even more significant for BOMF-MNA 0.8-1 (2MI), where
an excess anhydride was used. This evidence was due to both initiation and propagation processes
in which anhydride is involved, while the ester group is only generated by propagation
1
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32
mechanism. Above 90 °C, the conversion of both groups proceeded roughly at the same rate,
3
8
confirming the occurrence of the alternating copolymerization mechanism. Figure 2 shows the
conversion of anhydride (Figure 2a) ad ester (Figure 2b) groups for all the three systems. When
anhydride was the limiting reagent, its consumption rate was faster, and maximum conversion was
reached at 120 °C. Anhydride excess instead resulted in a slower rate, and a temperature as high
as 134 °C was needed to completely convert the anhydride groups. Similarly, as concerns the ester
formation, for BOMF-MNA 1-0.8 (2-MI) and BOMF-MNA 1-1 (2-MI) the reaction proceeds with
comparable kinetics, while BOMF-MNA 0.8-1 (2-MI) reacted slower than the other samples.
Moreover, a gradual decrease in ester absolute conversion curve was noted after the maximum was
reached, due to the strong convolution of the ester and anhydride bands and the progressive change
in polarity of the reacting systems.43,44
Anhydride group
a) 1.0
Ester group
b) 1.0
0
0
0
0
0
.8
.6
.4
.2
.0
0
0
0
.8
.6
.4
BOMF-MNA 1-1 (2-MI)
BOMF-MNA 1-0.8 (2-MI)
BOMF-MNA 0.8-1 (2-MI)
0.2
BOMF-MNA 1-1 (2-MI)
BOMF-MNA 1-0.8 (2-MI)
BOMF-MNA 0.8-1 (2-MI)
0
.0
4
0
60
80
100 120 140 160 180 200
4
0
60
80
100 120 140 160 180 200
Temperature (°C)
Temperature (°C)
Figure 2. Comparison of the absolute conversion of (a) anhydride and (b) formation of ester groups
for the three formulations.
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The evolution of the viscoelastic properties of the reacting mixtures was simultaneously
monitored through dynamic rheological tests. Rheology was first used to determine the resin
vitrification point. In particular, tan peak of curing resins upon a dynamic temperature sweep test
0
1
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was used as criterion to determine the vitrification temperature. Figure S7 shows the change in
tan for the three systems upon curing. The vitrification temperatures obtained for the three
systems were 113, 119, and 123 °C for BOMF-MNA 0.8-1 (2-MI), BOMF-MNA 1-1 (2-MI), and
BOMF-MNA 1-0.8 (2-MI), respectively, which confirmed the occurrence of premature
vitrification due to the excess of anhydride as detected by DSC.
Further, a comparison between thermal, FTIR and rheological data was carried out. Figure 3
shows the evolutions of elastic (G’) and viscous (G”) moduli with temperature, along with the
ester formation evaluated from FTIR analyses and the overall conversion evaluated from DSC, for
the three reacting systems. The comparison of G’ and |η*| evolution of the three mixtures is
displayed in Figure S8.
BOMF-MNA 1-1 (2-MI)
BOMF-MNA 1-0.8 (2-MI)
a) 1E7
1.0
b) 1E7
1000000
100000
10000
1000
1.0
0.8
0.6
0.4
0.2
0.0
BOMF-MNA 0.8-1 (2-MI)
G'
G"
G'
G"
c)
1E7
000000
100000
10000
1.0
G'
G"
1000000
1
Ester conversion
DSC conversion
Ester conversion
DSC conversion
0.8
Ester conversion
DSC conversion
0
.8
.6
1
00000
0000
000
00
1
0.6
0
1
1000
0
.4
.2
1
100
0.4
100
10
10
10
0
0.2
1
1
1
0.1
0.0
200
0.1
0.1
0.0
200
40
60
80
100
120
140
160
180
40
60
80
100
120
140
160
180
200
40
60
80
100
120
140
160
180
Temperature (°C)
Temperature (°C)
Temperature (°C)
Figure 3. Elastic (G’) and viscous (G”) moduli, compared with overall conversion evaluated from
DSC and ester formation conversion from FTIR spectroscopy of (a) BOMF-MNA 1-1 (2-MI), (b)
BOMF-MNA 1-0.8 (2-MI) and (c) BOMF-MNA 0.8-1 (2-MI) reaction mixtures heated at 1.5
°
C/min.
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For all the systems, the initial moduli and viscosity values were low and constant up to around
120 °C for BOMF-MNA 1-1 (2-MI) and BOMF-MNA 0.8-1 (2-MI), and 125 °C for BOMF-MNA
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-0.8 (2-MI). Above these temperatures the rheological parameters increased significantly,
reaching a plateau value when the curing process was complete, and all systems showed a
predominantly viscoelastic behavior. Except for BOMF-MNA 1-0.8 (2-MI), the rheological
measurements were interrupted at temperatures below 200 °C because the rheometer maximum
stress was reached.
In particular, for BOMF-MNA 0.8-1 (2-MI) the critical stress value was reached already at 140
°C (153 °C in the case of BOMF-MNA 1-1 (2-MI)), as anhydride excess caused vitrification of
the reacting system. Therefore, a higher temperature was required to ensure complete anhydride
conversion. Conversely, for BOMF-MNA 1-0.8 (2-MI) a steady increase in the rheological
properties was measured above the plateau temperature, likely due to the homopolymerization
process occurring at higher temperatures. Nonetheless, the G’ value at 200 °C (1.32 MPa) was
lower than that of BOMF-MNA 1-1 (2-MI) (1.56 MPa) and BOMF-MNA 0.8-1 (2-MI) (1.84
MPa). Similarly, the latter showed significantly higher G’’ and η* ultimate values. By comparing
ester formation and rheological properties, it is noticed that the reactant stoichiometry had a
significant impact on the evolution of network viscoelasticity. For BOMF-MNA 0.8-1 (2-MI), G’
was low and constant until the ester group formation, as well as DSC conversion, reached about
30% (Figure 3). In the case of epoxy/anhydride stoichiometric ratio, the change in the rheological
properties started when 85% of ester groups formed, corresponding to about 65% DSC conversion.
For the system with an epoxy excess, the mechanical response was detected only after ester
formation reached the plateau value, and the cure reaction as detected by DSC was almost complete
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(
α=0.7). This difference was reflected also in the properties of BOMF-MNA 1-0.8 (2-MI) cured
sample, that had significantly lower T in comparison with the other two formulations.
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Characterization of thermosets and comparison with the DGEBA/anhydride thermoset
system
Once the curing process was optimized, thermoset resin samples were prepared and
characterized in their thermal and chemical stability, mechanical and dynamic mechanical
properties. The oven curing protocol was the following: cure at T = 130 °C for t = 2 h and post-
cure at T = 180 °C for t = 1 h. The same protocol was used for the reference sample DGEBA-
MNA 1-1 (2-MI). After curing, BOMF-MNA 1-1 (2-MI) and BOMF-MNA 0.8-1 (2-MI) appeared
to be stiff materials, whereas BOMF-MNA 1-0.8 (2-MI) was still in a rubbery state. Therefore, a
second curing step was employed for the latter formulation, including an additional post-curing
treatment at T = 230 °C for t = 1 h. After post-cure, also BOMF-MNA 1-0.8 (2-MI) turned to be
rigid due to the homopolymerization of the excess epoxy groups.27,35 The latter sample was
labelled as BOMF-MNA 1-0.8 (2-MI)-pc230.
Thermal and thermo-oxidative behavior of the resins was evaluated by thermogravimetric
analysis under both inert (N ) and oxidative (air) atmosphere. The percent weight vs. temperature
2
curves for the three selected systems in both N and air are reported in Figure S9. The relevant
2
thermal parameters are listed in Table S2. BOMF-MNA 0.8-1 (2-MI) exhibited the highest thermal
and thermal oxidative stability among the bio-based systems (T = 256 °C and 271°C in nitrogen
d5
and air, respectively), whereas post-cure at 230 °C significantly improved the resistance of BOMF-
MNA 1-0.8 (2-MI as T values increased from 242 °C to 252 °C in nitrogen and from 242 °C to
d5
271 °C under oxidative atmosphere. Overall, degradation temperatures of BOMF-based samples
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were about 15 °C lower than those of DGEBA/anhydride resin.45,46 However, the former exhibited
higher char yields both in nitrogen and air, which could be advantageous in terms of flame
retardancy properties.
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On the same samples, swelling tests in THF were carried out, to get an insight on their chemical
stability. The results obtained are reported in Table 4.
Table 4. Swelling index (SI), tensile modulus (E’), ultimate tensile stress (σ) and strain (ε) values
of the investigated thermoset systems.
Sample
SI
E’ [MPa]
1467 ± 46
293 ± 72
σ [MPa]
32.8 ± 3.6
5.9 ± 1.6
ε [%]
4.0 ± 0.2
65.7 ± 20.7
4.5 ± 1.2
6.4 ± 0.6
2.6 ± 0.6
BOMF-MNA 1-1 (2-MI)
BOMF-MNA 1-0.8 (2-MI)
BOMF-MNA 0.8-1 (2-MI)
BOMF-MNA 1-0.8 (2-MI) pc230
DGEBA/MNA-2-MI
45.2 ± 8.6
91.9 ± 10.7
47.1 ± 8.9
55.9 ± 4.3
45.5 ± 11.6
1724 ± 77
1201 ± 111
1326 ± 159
48.9 ± 7.0
44.3 ± 1.7
20.4 ± 1.2
BOMF-MNA 1-1 (2-MI) and BOMF-MNA 0.8-1 (2-MI) exhibited SI values (45.2 and 47.1 %,
respectively) comparable with that of DGEBA-MNA 1-1 (2-MI) (SI = 45.5 %). On the other hand,
BOMF-MNA 1-0.8 (2-MI), due to the incomplete epoxy conversion, showed a higher SI that, upon
post-curing at 230 °C, significantly decreased yielding a value of 55.9%.
Mechanical properties of epoxy resins prepared with different epoxy/anhydride ratios were
evaluated by means of tensile tests. The stress-strain curves for all resins are reported in Figure ,
while tensile modulus, and ultimate tensile stress and strain values are reported in Table .
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BOMF-MNA 1-1 (2-MI)
70
60
50
40
30
20
10
0
BOMF-MNA 1-0.8 (2-MI)
BOMF-MNA 0.8-1 (2-MI)
DEGBA-MNA 1-1 (2-MI)
BOMF-MNA 1-0.8 (2-MI) pc230
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10 20 30 40 50 60 70 80 90
Strain (%)
0
2
4
6
8
10
12
14
16
Strain (%)
Figure 4. Representative stress-strain curves of the investigated thermoset samples.
Tensile tests performed at room temperature revealed satisfactory mechanical properties for the
proposed bio-based epoxy resins, comparable to those reported for linoleic derived bio-based
epoxies, which typically show elongation at break values <5%, and tensile strength and modulus
values of about 45 MPa and 1650 MPa, respectively.47 In particular, evaluating the tensile
properties recorded for BOMF-MNA 1-1 (2-MI) and for BOMF-MNA 0.8-1 (2-MI), it can be
evidenced that increasing the relative amount of the MNA curing agent, higher tensile modulus
and ultimate tensile stress values were obtained, thus suggesting a possible increase of the
crosslinking density at higher MNA contents. It is interesting to note that both BOMF-MNA 1-1
(
2-MI) and for BOMF-MNA 0.8-1 (2-MI) exhibited better tensile properties with respect to the
corresponding DGEBA-based sample. As concerning the mechanical behavior of the BOMF-
MNA 1-0.8 (2-MI) resin, in agreement with the results obtained by swelling index tests, the
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properties of this sample changed dramatically with the curing process. In fact, when the sample
was cured at 180 °C, it resulted rubbery, with a very low modulus and ultimate stress, and a high
elongation at break. With the supplementary post-cure at 230 °C, a significant increase in both
tensile modulus and ultimate strength was evidenced. This phenomenon can be attributed to a
higher cross-linking density achieved with the post-treatment at 230 °C.
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DMA analysis was performed to determine the viscoelastic properties and crosslinking density
of the synthesized systems. Figure 5 shows the comparison of E’, E” and tan δ curves of the
different BOMF-MNA systems, while in Figure S10 the same DMA parameters are displayed for
each single system as well as for the DGEBA-based resin. Values of moduli in both glassy and
rubbery regions are reported in Table 5.
a) 1E10
b) 1E9
1
E8
E7
1E9
1
1E8
1
000000
100000
10000
BOMF/MNA 1-1
1E7
BOMF/MNA 1-1
BOMF/MNA 1-0.8
BOMF/MNA 0.8-1
DGEBA/MNA 1-1
BOMF/MNA 1-0.8 pc230
BOMF/MNA 1-0.8
BOMF/MNA 0.8-1
DGEBA/MNA 1-1
BOMF/MNA 1-0.8 pc230
1
000000
-60 -40 -20
0
20
40
60
80 100 120 140 160 180
-60 -40 -20
0
20
40
60
80 100 120 140 160 180
Temperature (°C)
Temperature (°C)
c) 0.7
BOMF/MNA 1-1
BOMF/MNA 1-0.8
BOMF/MNA 0.8-1
DGEBA/MNA 1-1
0.6
0.5
0.4
0.3
0.2
BOMF/MNA 1-0.8 pc230
0.1
0.0
-60 -40 -20
0
20
40
60
80 100 120 140 160 180
Temperature (°C)
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Figure 5. (a) Storage modulus, (b) loss modulus and (c) tan δ evaluated by DMA analysis of the
investigated thermoset samples.
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Table 5. Storage modulus (E’), loss modulus (E’’), glass transition temperature (T ) and
g
crosslinking density () of the investigated thermoset systems.
Sample
E’
E’’
Tg

[103
mol/cm ]
[MPa]
[MPa]
[°C]
3
@
@
@
@
-20°C
160°C -20°C 160°C
BOMF-MNA 1-1 (2-MI)
BOMF-MNA 1-0.8 (2-MI)
BOMF-MNA 0.8-1 (2-MI)
BOMF-MNA 1-0.8 (2-MI) pc230
DGEBA/MNA-2-MI
3564
3378
3759
3290
3590
11.0
6.9
147
212
102
122
124
0.14
0.09
0.21
0.44
3.69
50
29
1.02
0.64
1.57
1.29
1.63
16.9
14.0
17.6
70
54
152
All samples exhibited glassy storage moduli close to 3.5 GPa, which were comparable with that
of the DGEBA-based sample, and higher than E’ values previously reported for furan-based
epoxies cured with aliphatic diamines (about 3 GPa).16,19 Such a high stiffness has been attributed
to the hydrogen bonding between oxygen atoms on furanic monomer and hydroxyl groups formed
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in the cured networks. Moreover, storage modulus values appeared to be well correlated with the
anhydride content, as E’ of BOMF-MNA 0.8-1 (2-MI) was the highest, both in glassy and in
rubbery state. On the other hand, BOMF-MNA 1-0.8 (2-MI) showed higher loss modulus in the
glassy region, however, in the rubbery state also E” increases with the anhydride amount. As can
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be noticed, for BOMF-MNA 1-0.8 (2-MI) a secondary relaxation (β-relaxation) is present at -40
°
C, due to the motion of short segments, because of the reduced crosslinking density due to the
unreacted epoxide excess. Post curing at 230 °C caused the β-relaxations signal to disappear. The
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glass transition temperature (T ) was determined as the peak maximum of the tan delta curve.
g
Values ranging from 29 (BOMF-MNA 1-0.8 (2-MI) to 70 °C (BOMF-MNA 0.8-1 (2-MI) were
noted, which confirm that increasing the anhydride content yields a stiffer material. Further, post
curing of (BOMF-MNA 1-0.8 (2-MI) resulted in a significant increase of the T (54 °C). The
g
obtained values are comparable with those reported for furan-based epoxy cured with aliphatic and
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aromatic diamines, indicating that anhydrides are viable building blocks for the synthesis of
renewable high-performance epoxies endowed with satisfying thermo-mechanical properties.
Finally, values of crosslinking degree (ν) evaluated by eq. 4 are reported in Table 5. By
increasing the anhydride amount the increase of the crosslinking density was confirmed, yielding
3
a maximum value of 1.57 mol/cm for BOMF-MNA 0.8-1 (2-MI), comparable with that of
DGEBA-MNA 1-1 (2-MI).
4
. Conclusions
In the present work,
bis(hydroxymethyl)furan
a
furan-based diepoxide monomer starting from 2,5-
(BHMF) was synthesized, the 2,5-bis[(oxyran-2-
ylmethoxy)methyl]furan (BOMF). Methyl nadic anhydride (MNA) was used as curing agent in
three different epoxy/anhydride ratios in presence of 2-methyl imidazole (2-MI), chosen as the
best performing initiator.
The reaction kinetics was studied through spectroscopic FTIR coupled with chemo-rheological
analyses. The kinetic evaluation by FT-IR were done following the effective consumption of
anhydride group upon curing together with the formation of carbonyl peaks characteristic of the
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ester group formation on cured resin. The degree of cure was evaluated monitoring the relative
area of these peaks. Rheological tests gave the viscoelastic properties of resins undergoing the cure
process. The epoxy/anhydride ratio is a parameter that affects strongly not only the kinetic of
functions evolution but also the vitrification phenomenon. For the stoichiometric system the
change in the rheological properties started at 85% of ester groups formed.
0
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The thermal stability of cured resins tested by TGA show results comparable with the
DGEBA/MNA thermoset: T = 256 °C and 271°C in nitrogen and air, respectively.
d5
The T of networks before post curing are ranging from 29 to 70°C, in function of
g
epoxy/anhydride ratio, showing that we can modulate the application by adapting the system
formulation.
Overall results demonstrated that the synthesized furan-based resins have good chemical-
physical properties and can be proposed as effective alternatives to traditional, bisphenol A-based
resins. Moreover, by changing the relative formulations and the thermal conditions used to cure
the systems, the features of the final materials can be modulated depending on the characteristics
required for the specific application.
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nd
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