Single-component and fast-curing epoxy resin for liquid composite molding processes
MATEC Web of Conferences
Single-component and fast-curing epoxy resin for liquid composite molding processes
Yiru Wang 0 1
Wangshuang Liu 0
Yiping Qiu 1
Yi Wei 0
0 Donghua University, Center for Civil Aviation Composites , Shanghai, 201620 , China
1 Donghua University, College of Textiles , Shanghai 201620 , China
Development of single-component and fast-curing epoxy resins is highly desired for many industry applications. In this work, we report an epoxy system based on diglycidyl ether of bisphenol A (DGEBA) and 1-(2-cyanoethyl)-2-ethyl-4- methylimidazole (1C2E4MIM). The inductive effect of electron-withdrawing cyano group distinctly increases the latency of 1C2E4MIM without sacrificing the curing rate. The results of differential scanning calorimeter (DSC) and dynamic mechanical analysis (DMA) measurements indicate DGEBA/1C2E4MIM epoxy system can be fully cured in 15 min. The rheological, thermal and mechanical properties of DGEBA/1C2E4MIM epoxy system were studied in detail. The results show that the shelf life of this epoxy system is more than 4 days at room temperature and more than 6 months at -18 °C. The cured epoxy resins show high glass transition temperature (>155 °C), tensile strength (>80 MPa) as well as excellent moist heat resistance. Finally, carbon fiber-reinforced polymer composites (CFRPs) were fabricated using this epoxy system as matrix via vacuum assisted resin infusion (VARI) process. The mechanical properties of CFRPs, including tensile, flexural, compressive and interlaminar shear properties, were investigated.
Carbon fiber-reinforced polymer composites (CFRPs)
are increasingly used in many applications that require
high-performance light-weight structural materials. For
example, CFRPs are regarded as an ideal light-weight
substitute to mental in manufacturing electric car body.
Likewise, in commercial aircraft industry, the content of
advanced composites in Boeing 787 Dreamliner and
Airbus A350 amazingly reaches over 50 wt%. For
modern high-performance CFRPs, epoxy resins are the
most widely utilized thermosetting matrix materials
owing to their superior mechanical performance,
dimensional stability, adhesion, heat and chemical
] The traditional epoxy formulation for
CFRPs commonly includes epoxy monomer and hardener
(amines or andydrides). Owing to the limited shelf life,
the two components have to be stored separately and
mixed immediately just prior to use. Compared with
two-component epoxy system, single-component epoxy
resin can be conveniently operated without weighting and
blending processes. In addition, epoxy curing often
accounts for a large proportion of the manufacturing cycle
of CFRPs, and so fast-curing epoxy system is desired to
improve the production efficiency, which is particularly
significant in automobile industry.[
In order to reduce the molding time, short curing time
and low viscosity are active demand. Many fast curing
epoxy resin systems were studied in past years, which can
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be roughly divided into five parts, thiol curing agent,
amine curing agent, anhydride curing agent, cationic
curing agent and anion curing agent. Thiol curing agent
can not be used in liquid molding system due to low
mechanical properties, and very short pot time, and most
importantly low glass transition temperature, therefore
they are generally used in adhesion or coating[
Anhydride curing agent has to take at least 30 min to
complete the curing at 120 oC[
], which is too long to
meet the industrial production line requirements. Except
that, the curing temperature of anhydride is higher. Amine
curing agent, cationic curing agent and anion curing agent
are relatively suitable to liquid fast molding system due to
their short curing time and better mechanical properties
and thermo properties. Until now, all of epoxy curing
agent used in automobile epoxy product line are Amine
], which including aliphatic amines,
alicyclic amines, and aromatic amines. Alicyclic amines
and aromatic amines are much more expensive to
aliphatic amines. Most of aromatic amines are solid, and
have much higher viscosity when mixed with epoxy resin,
which is disadvantage to liquid molding. Except that,
aromatic amines have higher curing
temperature. Aliphatic amine curing agent is currently
used in liquid molding automobile industrial line, but
there are still many challenges, the biggest challenge is
the balance of viscosity and curing time.
Imidazoles are a class of typical epoxy hardener. Only
a catalytic amount of imidazoles can initiate the curing
reaction through nucleophilic and anionic polymerization
processes.14,15 Due to the high cross-linking density and
low content of polar groups, imidazole-cured epoxy
resins have an excellent combination of high modulus,
good heat and moisture resistance. In general, imidazoles
have intrinsic high curing reactivity towards epoxy resins,
and the curing behaviors can be facilely tailored by
introducing various substituent groups on the imidazole
rings. So far, a large amount of imidazole hardeners with
different structures has been commercialized, which
provides a generous toolbox to develop single-component
and fast-curing epoxy system with reasonable shelf life.
In this work, 2E4MIM and 1C2E4MIM were
compared as hardener of single-component and
fast-curing epoxy resin for liquid composite molding
processes.Furthermore, 1C2E4MIM was investigated in
curing behaviour, glass transition temperature (Tg) ,
tensile property and hot-wet properties. Finally, the
mechanical properties of carbon fiber reinforced
composite by 1C2E4MIM-epoxy resin was tested.
Diglycidyl ether of bisphenol-A (DGEBA) with an
epoxy equivalent weight of 182~192 (NANYAEPOXY
(1C2E4MIM, cas:23996-25-0) purity 99% from
Huaweiruike company. 2-ethyl-4-methylimidazole
(2E4MIM, cas:931-36-2) purity 99% from Huaweiruike
company. T700 carbon fiber plain weave cloth (Zhongfu
Shenying Carbon Fiber Co.,Ltd). Peel ply, infusion net, vacuum bag, seal salve, sealing tape, ventilated felt, pipe thread, guid pipe.
2.2 Sample preparing
2.2.1 Preparing of the resin system
Mix the hardener and epoxy resin by parts with
speed 200 r/min at ambient temperature for 5 min. Then
degas the mixture in vacuum drying oven for 15 min at 45
2.2.2 Preparing of the neat resin sample
Pure the disgassed mixture into the aluminum mold
cured 20 min at 120 oC for level of 4 wt % 2E4MIM and
4 wt %, 6 wt % 1C2E4MIMIM as the hardener. And
cured 40 min at 120 oC for level of 2 wt % 1C2E4MIM as
hardener. The water absorption sample was obtained by
putting the cured resin into deionized water for 80 days at
70 oC. The hot-wet property sample was obtained via
putting the cured sample into a deionized water at 100 oC
for 48 h.
2.2.3 Preparing of the carbon fiber composites
Lay up the plain wave carbon fiber cloth (30 cm ×30 cm) on a glass plate, put the infusion net (29 cm ×29 cm) on the clothes at the centre, the inlet and outlet pipe
respectively in each side of the carbon fiber cloth. The
ventilated felt was put on the outlet side. The outlet pipe
was connected with vacuum pump, with a safety resin
trap between them. The vacuum bag was put on the top
sealed by sealing tape. Heating the degassed mixture to
different temperature inject into the carbon fiber
preform. Subsequently cured 30 min at 120 oC.
2.3.1 Pot life
The viscosity was obtained by Dynamic Shear
Rheometer (DSR) (Rheometer: MARS60) at 25 oC, 1.592
2.3.2 Curing behaviour
The curing time was tested by DSR and Differential
Scanning Calorimeter (DSC) (NETZSCH
DSC214). Isothermal DSC studies were performed by
rapidly ramping the sample from 0 oC to 120 oC, with the
rate of 100 oC/min, holding for various periods of
time. Dynamic scans at 10 oC/min over a temperature
range from 25 oC to 220 oC was used to determine the
total heat of reaction.
2.3.3 Mechanical properties
The Tg and hot-wet glass transition temperature
(HW-Tg) were measured by dynamic mechanical analysis
(DMA)(TA Q800) at a heating rate of 10 oC/min. The neat
resin mechanical properties were tested according to
ASTM by Instron 2360. The tensile properties and
flexural properties were tested per ASTM D 638 and
ASTM D 790. To evaluate the CFRPs mechanical
properties, the T700 plain weave carbon fabric reinforced
composite mechanical properties were tested according to
ASTM. The casting time and mechanical properties at
difference casting temperature have been measured.
3 RESULTS AND DISCUSSIONS
3.1 Comparison of 1C2E4MIM and 2E4MIM as hardener
3.1.1 Pot life of 1C2E4MIM and 2E4MIM as the hardener
3.1.2 The DMA result of 2E4MIM and 1C2E4MIM as the hardener
3.1.3 Flexural properties of 2E4MIM and 1C2E4MIM as the hardener
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3.2 Comparison of different 1C2E4MIM level
3.2.1 Curing behaviour of different 1C2E4MIM level
The increasing of 1C2E4MIM level have no impact on
initial curing temperature, but significant impact on heat release
and curing time (table 1). The increase of 1C2E4MIM level
sharply increased the heat release and decreased curing time
3.2.2 The DMA result of different 1C2E4MIM level
When increased the 1C2E4MIM load, the Tg improved
significantly while 1C2E4MIM concentration under 4 wt %, but
not that remarkable while 1C2E4MIM concentration above 4
wt % (figure 5), therefore the 4 wt % was investigated as the
3.2.3 Tensile properties of different 1C2E4MIM level
The tensile strength did not increase much when
1C2E4MIM content was above 4 wt % , while the tensile modular decreased when 1C2E4MIM increasing (figure 6).
3.2.4 CFRPs mechanical properties
The CFRPs mechanical properties with 4 wt % level of
1C2E4MIM based and T700 plain weave carbon fiber
reinforced composite showed high mechanical properties, which
could meet the load bearing structure requirements.
Tensile property Flexural property Compress property
Strength Modular Strength Modular Strength Modular
(M Pa) (G Pa) (M Pa) (G Pa) (M Pa) (G Pa)
632 69 662 55 310 60
Short beam shear strength (M Pa)
The 2E4MIM and 1C2E4MIM were investigated as
fast cure thermal latent epoxy resins, which could be used
in automobile production line. The Stern effect and
coordination effect on formidable ring of 1C2E4MIM
bring better thermal latency and longer pot life. The
higher cross-linking with epoxy resin created higher Tg,
bending strength. It could be deduced that the latency of
the electron deficiency group on formidable ring which
could react with epoxy group can both improved the
latency and properties. Then the properties of different
content of 1C2E4MIM were investigated. The when the
content below the 4 wt %, the Tg sharp fell, and curing
time last long, when the content above 4 wt %, the Tg
climbed very slow, but the heat of reaction were increased
and got too high. The hot-wet properties of
1C2E4MIM-Epoxy resin were excellent. And the
mechanical properties of T700 carbon fiber reinforced
1C2E4MIM-Epoxy composites result to be auspicious.
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