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CHEMICAL ENGINEERING TRANSACTIONS
VOL. 66, 2018
A publication of
The Italian Association
of Chemical Engineering
Online at www.aidic.it/cet
Guest Editors: Songying Zhao, Yougang Sun, Ye Zhou
Copyright © 2018, AIDIC Servizi S.r.l.
ISBN 978-88-95608-63-1; ISSN 2283-9216
Study on Relationships between Curing Pressures and
Mechanical Properties for Epoxy Adhesive Films
Yi Xua,*, Qiang Heb, Wenfeng Yangb, Ting Sunb, Qingru Tangb
aCivil Aviation Safety Engineering Institute, Civil Aviation Flight University of China, Guanghan 618307, China
bAviation Engineering Institute, Civil Aviation Flight University of China, Guanghan 618307, China
xuyi99@yeah.net
In this paper, the effects of different curing pressures on the mechanical properties of epoxy adhesive films
were investigated. When the curing pressure increases from 0 MPa to 0.5 MPa, Nano-indentation analysis
shows that the elastic modulus of the cured adhesive films increases from 2.92 GPa to 3.49 GPa, the tensile
strength increases from 42.0 MPa to 43.7 MPa, and the breaking elongation increases from 2.61 % to 3.73 %.
It was indicated that the microscopic and macroscopic mechanical properties of the cured adhesive films were
improved. DMA results showed that, as the curing pressure increases, the Tg of the cured product does not
change significantly, but the crosslinking density is significantly improved. These results indicate that a higher
modulus of the cured product could be gained by increasing the curing pressure appropriately.
1. Introduction
High temperature curing epoxy adhesive films show high toughness, high strength, and high durability
properties (Awaja et al., 2009; Barnes and Pashby, 1998). Due to their excellence, they are indispensable
important bonding-repair materials in aerospace industry (Danielson and Berg, 1967; Archer and Mcilhagger,
2015). This technology can transmit large static and dynamic loads, and make the aircraft work reliable in a
long-term. The Redux series of phenolic-acetal high-performance structural adhesives were first used to bond
the aircraft metal structures in the UK from the 1940s. High-temperature curing structure adhesives have been
improved with the continuous development of aerospace technology. Some commercial adhesives, such as
Redux 319 (Hexcel), FM-300 (Cytec), AF311-5M (3M), and so on, had passed airworthiness certification in the
aircraft bonding maintenance due to their good bonding performance (Higgins, 2000; Comyn, 2006).
For the bonding repair, the adhesives were the medium that transferred the loading between the patch and the
repaired structures (Cagle et al., 1973; Radley, 1964). The choice for the type of adhesive, the curing process,
and the compatibility of the patch structure had significant effects on the repairing effects. Curing pressure
was an important factor affecting the final adhesive properties, while few studies were focused on this area
(Khan et al., 1964; Khan et al., 2007; Morganti et al., 2016; Wang et al., 1992). The main research on
adhesives mainly focused on further improving product quality precision, process performance, high heat
resistance and high durability. In order to design the best curing process of adhesives, some scholars studied
the curing mechanisms of adhesives through the curing dynamics (Katnam et al., 2013; Lei and Frazier, 2015;
Baker and Jones, 1988). The evaluation criteria for the performance of adhesives were generally
characterized by macroscopic mechanical properties, few studies reported the relationships between the
nano-mechanical properties and macroscopic mechanical properties. In this paper, the micro-mechanical
properties of adhesives under different curing pressures were studied by nano-indention analysis. The tensile
properties of adhesive films under various curing pressures were studied by universal testing machines.
Through the above analyses the mechanism of the influence of the curing pressure on the mechanical
properties of adhesives was proposed.
DOI: 10.3303/CET1866008
Please cite this article as: Xu Y., He Q., Yang W., Sun T., Tang Q., 2018, Study on relationships between curing pressures and mechanical
properties for epoxy adhesive films, Chemical Engineering Transactions, 66, 43-48 DOI:10.3303/CET1866008
43
2. Experimental
2.1 Material
METLBOND 1515-4M epoxy adhesive film, manufactured by German company, Cytec Industries Inc., was
adopted in the test.
2.2 Preparation of cured adhesive films
A was cured at 175 ℃ for 2 hours in a thermocompression between glass plates on which a release agent
(75-95 % aliphatic hydrocarbon and <20% dibutyl ether) had been spread with various pressures: 0 MPa,
0.075MPa, and 0.5 MPa. The 0 MPa group was cured as a non-pressurized sample for comparison, while the
0.075 MPa and 0.5 MPa samples were used during the curing processes being related to actual industrial
usage. The corresponding cured products were labeled as poly(A)-0, poly(A)-0.075, and poly(A)-0.5.
2.3 Measurements and Characterization
Universal testing machine: the films used were around 100 μm-thick and were rectangular in shape (50*10
mm). Testing of the mechanical properties of the cured resins was performed using a KQL KD-5 testing
machine according to ASTM D882 at room temperature. A gauge length of 20 mm and a crosshead speed of
2 mm/min were set for the test.
Differential scanning calorimetry (DSC) was performed on a TA Instruments Q20 model at a heating rate of
10 ℃/min under nitrogen atmosphere at a flow rate of 60 ml/min.
The dynamic mechanical properties of the cured films were obtained using TA instruments DMA Q800
dynamic mechanical analyzer (DMA) with a sample dimension of 12 mm × 9 mm × 0.2 mm in a controlled
strain tension mode with an amplitude of 20 μm and a temperature ramp rate of 5 °C/min from 40 ℃ to 300 ℃
at a frequency of 1.0 Hz.
Nano-indentation instrument: NHT2, the berkovich diamond Mitsubishi cone indenter was pressed into the
samples with the loading rate of 60 mN / min from 0 mN to 30 mN, the maximum depth of indentation was
2900 nm. Hold for 10 s to eliminate the creep effect before unloading, and then unload at a rate of 60 mN /
min. The loading-time curves please see Figure 1.
Figure 1: Loading-time curves from Nano-indention analysis.
3. Results and Discussion
3.1 Curing behavior of the cured adhesive films
The DSC curve of uncured A was shown in Figure 2(a). The onset temperature Tonset and the peak
temperature Tpeak were 134 ℃ and 159 ℃ respectively. The total heat enthalpy was 268 J/g. After curing for 2
hours at 175 ℃, the exothermic peaks disappeared, and the residual heat enthalpy became zero. These
results indicated that the cured samples were cured completely and adding curing pressure had little effects
on the curing of the adhesive films.
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Figure 2: DSC curves of A (a), poly(A)-0 (b), poly(A)-0.075 (c), and poly(A)-0.5 (d)
3.2 Nano-indentation test and analysis
Loading and displacement curves of the films under different curing pressures were shown in Figure 3. The
load displacement curves of the three samples were easily reproducible. According to the computational
model proposed by Waugh (2016), the hardness and elastic modulus of the samples could be derived from
equations 1 and 2.
p
iT
A
P
H m ax (1)
p
r
A
S
E
2
(2)
Where β related to the shape of the indenter, for berkovich indenter, β=1.034. Ap was the projection contact
area.
The average hardness of adhesive films under different pressures was shown in Figure 4. The average
hardness of poly (A)-0.5, poly (A)-0.575 and poly (A)-0.5 were 185.7 MPa, 206.2 MPa, and 243.9 MPa.
The hardness rates increased by 11.0% and 31.3% respectively within the increasing curing pressure.
Meanwhile, the elastic modulus increased from 2.92 GPa to 3.49 GPa within the increasing curing pressure.
It was indicated that the cross-linking densities of cured adhesive films increased when the curing pressure
increased, moreover, the strength of cured adhesive films also increased.
(a) poly(A)-0
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(b) poly(A)-0.075
(c) poly(A)-0.5
Figure 3: The load-displacement curves
Figure 4: Elastic modulus and hardness of the cured adhesive films under different curing pressures
3.3 Tensile property analysis
The tensile properties of the cured adhesive films were shown in Table 1. The tensile strengths of poly(A)-0
and poly(A)-0.5 were 42.0 MPa and 43.7 MPa respectively. The breaking elongations of poly(A)-0 and
poly(A)-0.5 were 2.61 % and 3.73 % respectively.
46
The tensile properties of the cured products slightly improved when the curing pressure increased from 0 MPa
to 0.5 MPa. This phenomenon may be related to the cross-linked structures of the polymers, as the degree of
cross-linking increased with increasing cure pressure (see below table).
Meanwhile, the rules of change on the macro-mechanical properties were also consistent with those of the
micro-mechanics analysis for the three cured adhesive films.
Table 1: Tensile properties of cured adhesive films prepared under different cure pressures
Sample Maximum force (N) Tensile strength (MPa) Breaking elongation (%)
poly(A)-0.5 225.5 43.7 3.73
poly(A)-0 224.1 42.0 3.10
3.4 DMA Analysis
Figure 5: DMA curves of the cured adhesive films under different curing pressure
The DMA curves of the cured adhesive films under different curing pressure were shown in Figure 5. The E’
values of poly(A)-0.5, poly(A)-0.075, poly(A)-0 at room temperature (40 oC) were 2.55 GPa, 1.94 GPa and
1.77 GPa. It was shown that the higher the curing pressure, the higher the E ' values of the cured products.
These three cured adhesive films had only one peak temperature from the Tanσ curves, and the Tpeak was
192 ℃.
In order to further explain the difference, the following equation (3) for the rubbery plateau storage modulus
(Tg+40 °C) was used to anlysis the cross-link density (Ran et al., 2011; Xu et al., 2017; Chang et al., 2018):
dcross-link =E′/2(1+γ)RT (3)
Where E′ is the storage modulus (dyn/cm2) of the polymer at temperature T, R was the gas constant, and γ
was Poisson’s ratio which was assumed to be 0.5 for incompressible networks. dcross-link was the cross-link
density (mol/cm3) of the polymer. The cross-link density of the poly(A)-0, poly(A)-0.075 and poly(A)-0.5 were
calculated to be 3.0 × 10-3 mol/cm3, 4.0 × 10-3 mol/cm3, and 4.8 × 10-3 mol/cm3. The results showed that, to
some extent, the increased curing pressure could improve the cross-linking densities of the cured products.
4. Conclusions
In this paper, the effects of different curing pressures (0 MPa, 0.075 MPa, and 0.5 MPa) on the properties of
METLBOND 1515-4 epoxy adhesive film were investigated. The results showed that the macro-mechanical
properties and micro-mechanical properties of the cured products were improved as the curing pressure
increased. The DMA results showed that the cross-linking densities of the cured products increased
substantially within the increasing curing pressure. That was because the application of curing pressure
promoted the collision of epoxy precursors in the probability of collision, thus increasing the cross-linking
densities of the cured adhesive films.
Acknowledgements
This research was financially supported by the Doctoral Research Foundation of Civil Aviation Flight
University of China (No. BJ 2016-01); Youth Foundation of the Civil Aviation Flight University of China (No.
Q2018-43).
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