|Título/s:||Aging in water and in an alkaline medium of unsaturated polyester and epoxy resins : experimental study and modeling|
|Autor/es:||Morales Arias, Juan P.; Bernal, Celina; Vázquez, Analía; Escobar, Mariano M.|
|Palabras clave:||Alcalinidad; Soluciones; Propiedades dinámicas; Propiedades mecánicas; Epoxi; Poliéster; Compuestos de epoxi; Agua; Elasticidad; Polímeros; Modelos matemáticos; Experimentos; Viscoelasticidad|
| Ver+/- |
Aging in Water and in an Alkaline Medium of Unsaturated
Polyester and Epoxy Resins: Experimental Study and
JUAN P. MORALES ARIAS
Instituto de Tecnologı´a en Polı´meros y Nanotecnologı´a (ITPN)—(UBA-CONICET), Facultad de Ingenierı´a, Universidad de Buenos Aires,
Las Heras 2214, Buenos Aires, Argentina
MARIANO M. ESCOBAR
Instituto Nacional de Tecnologı´a Industrial (INTI), Av. General Paz 5445, San Martı´n (1650), Buenos Aires, Argentina
CELINA BERNAL, ANAL´IA V ´AZQUEZ
Instituto de Tecnologı´a en Polı´meros y Nanotecnologı´a (ITPN)—(UBA-CONICET), Facultad de Ingenierı´a, Universidad de Buenos Aires,
Las Heras 2214, Buenos Aires, Argentina
Correspondence to: Juan P. Morales Arias; e-mail: firstname.lastname@example.org.
Received: November 17, 2015
Accepted: January 29, 2016
ABSTRACT: The change in stiffness with temperature in the presence of different media has been discussed for a long time
because the ability to predict this behavior becomes fundamental to the design of new materials and their applications. That is why,
in this work, the application of a mathematical model, which is able to predict the elastic properties of two polymers, is presented.
The study takes into account the relationship between the viscoelastic and absorption properties of these materials in alkaline
solution (Ca (OH)2, pH 12.5) and in distilled water (H2O, pH 7) as the immersion media. Diffusion coefficient values were higher
when the resins were immersed in water than in the alkaline solution. In addition, the effect of the alkaline medium was higher for
the unsaturated polyester resin (UPR). The highest decrease in modulus at the glassy state of the polymer network was observed for
the UPR immersed in the alkaline medium. The greatest reduction of the Tg value due to network plasticizing was found for the
epoxy resin (ER) in the alkaline medium. Therefore, the ER exhibited a more stable behavior after aging at moderate temperature
than the UPR. C© 2016 Wiley Periodicals, Inc. Adv Polym Technol 201 , , 21684; View this article online at wileyonlinelibrary.com.
KEY WORDS: Alkaline solution, Dynamic mechanical properties, Epoxy, Unsaturated polyester, Water diffusion
T here is an increasing use of fiber-reinforced plastics (FRPs)as an alternative to traditional materials such as wood,
metals, and ceramics due to their outstanding properties (high
rigidity, high dimensional stability, good electrical properties,
and corrosion resistance).1 In recent years, different polymers
(epoxy, phenolic, polyester, and vinyl ester resins) have been
chosen for matrices of FRP in many industrial applications such
as automotive, aerospace, and civil engineering.1–4 However, the
acceptance of polymer composites in civil engineering applica-
tions is still limited as a result of the degradation of the polymeric
Contract grant sponsor: CONICET, PIP No. 608.
Contract grant sponsor: University of Buenos Aires.
Contract grant number: UBACYT 0336.
resin in the presence of water or alkaline media. When FRPs are
to be used as replacement of steel bars for reinforcing Portland
cement, the selection of the polymeric matrix becomes essential
as the cement in contact with the resin produces a high alkaline
medium. Therefore, not only the study of the effect of water on
thermosetting resins but also that of alkaline conditions seem
to be essential for using FRP in civil applications. Among the
different thermosetting resins, polyester and epoxy have been
extensively applied in these applications mainly because of their
Figure 1 is a simplified scheme of the molecules and reac-
tions of polyester resins. Diols other than propylene glycol are
used such as various diacids or anhydrides, aswell as styrene re-
placements,which allowtuningof the polyester resin properties.
When a catalyst (usually methyl ethyl ketone hydroperoxide) is
added, a free radical polymerization reaction takes place forming
a network of polyester molecules cross-linked by styrene units.
In addition, styrene molecules sometimes react directly with
Advances in Polymer Technology, Vol. , No. , 201 , DOI 10.1002/adv.21684
C© 2016 Wiley Periodicals, Inc. 21684 (1 of 11)
37 2 8
FIGURE 1. Simplified scheme of the molecules and reactions of polyester resins.
FIGURE 2. Simplified scheme of the molecules and reactions of epoxy resins.
other styrene molecules and polyester molecules react with other
polyester molecules withdifferent reaction constants,5 where the
unsaturated in the polymer chain means they contain carbon–
carbon multiple bonds (in this case double bonds) that are nec-
essary for the polymerization/cross-linking reaction and finally
form unsaturated polyester resins (UPRs).
On the other side, an epoxy resin (ER) network can be the
result of the reaction between the hydrogen of the amine and
the oxirane group of the ER. For this, primary amino groups
react first, followed by the reaction of secondary amino groups
formed after the reaction of the primary amino groups as well
as secondary amino radicals, which are embedded in the struc-
ture of the molecule. The corresponding rate of reaction of pri-
mary amino groups is greater than that of the secondary amino
groups because of the higher reactivity of the primary amines
with respect to the secondary ones (having two hydrogen sus-
ceptible assets to be transferred to the oxirane group to become
the hydroxyl group). Figure 2 shows a simplified scheme of the
reactions of the ER and amine to form a network, where the
cross-linking points are formed by the amine compound.6
Several intrinsic factors influence the viscoelastic properties
of thermosetting resins and their solvent absorption behavior:
free volume,7,8 polymer network density,9 and the hydrophilic
groups of the polymer network,10–16 as well as cross-linking den-
sity and chemical composition.17 These intrinsic factors strongly
affect solvent sorption behavior in terms of diffusion coefficients,
equilibrium value or capacity, and solubility.
Degradation of thermosetting resins duringwater absorption
can induce physical and chemical processes. First, the water is
attracted to areas of air entrainment or voids. Thus, these areas
can collect water over time. Even microscopic cracks and voids
allow the easy penetration of water into the resin via capillary
action and diffusion. After an initial period of seeking out and
filling of cracks and voids, water begins to swell the resin,18
resulting in an increase in the free volume and a decrease in the
glass transition temperature, as well as the water with double
hydrogen bonds can act as a physical cross-linking agent.19
Over the past decades, significant efforts have been made
to study the aging of UPRs. Zhijun et al. 20 have investigated
this phenomenon by immersing the resin in water and UV for
different time intervals. Hydrolytic and oxidative reactions pro-
duced by hydroxyl and carbonyl groups during the aging pro-
cess, wettability of UPR from changes in the chemical structure,
and swelling of the polymer have been studied. Bela´n et al. 21
have also analyzed four unsaturated polyester networks with
chain ends modified by isocyanate or dicyclopentadiene. The
UPRs were immersed in distilled water at 100°C or exposed
in humid atmosphere at 70 and 50°C. These authors indicated
that water absorption was dependent on the ester nature and
the reactivity toward water, as well as on the ester concentra-
tion. They pointed out that the replacement of acids by non-
catalytic and nonreactive species could have a stabilizing effect.
Bifunctional molecules could even have suppressed chain ends,
thus having a double effect: acidic suppression and a decrease
in the yield of small molecules per chain scission. A compari-
son of the sample spectra in their initial state and during aging
showed a decrease in the absorption band at 1295 cm−1 represen-
tative of the CO group of the aliphatic ester. The reference band
21684 (2 of 11)
FIGURE 3. Scheme of the reaction of ester and water.
chosen, located at 700 cm−1, was representative of the styrene
benzene rings (–CH aromatic ring) and did not change during
aging, as styrene could not be hydrolyzed. When miscibility is
high, esterification starts to competewith hydrolysis (Fig. 3). The
kinetic constant for the hydrolysis seems to increase rapidly with
T above Tg.
On the other hand, Coniglio et al.22 have studied the cyclic
water absorption–desorption of ERs. They showed that the hy-
drothermal aging can lead tomatrix cracking after an immersion
time at temperatures ranging from 20 to 85°C (absorption tests)
and drying in a furnace between 60 and 85°C (desorption tests).
They explained the effect of water absorption into cracks and the
expansion of the resin with temperature, resulting in the squeez-
ing of water out and the reduction of void spaces. Moisture
and temperature together can induce irreversible damage in the
polymeric network in the form of microvoids. These microvoids
contribute to the moisture absorption properties of ERs.23–27
Prolonged hydrothermal aging above a threshold tempera-
ture leads to irreversible damage of the resin such as oxidation,28
microcavity growth,29 and polymer network relaxation,30,31 and,
as a consequence, strongly affects the mechanical properties.
To understand the phenomenon of water absorption in
polymers, it is important to know that the absorbed water is
composed of free water and bound water. The free water is
formed by water molecules that are able to move independently
through gaps and cracks, whereas the bound water is composed
of those water molecules that are limited to the polar groups of
To analyze the effect of solvent absorption, different experi-
ments have been performedby several authors inwhich samples
are immersed in a liquid or solution and the variation of mass
uptake as a function of time is measured. From these exper-
iments, the absorption and effective diffusion coefficients can
be estimated. Fick’s law describes the diffusion in matter or
energy into a medium in which initially there is no (chemical
or thermal) equilibrium. Fick’s law tells that the diffusive flux
through a surface (J, mol/cm2s) is directly proportional to the
concentration gradient (C). The coefficient of proportionality is
called diffusion coefficient (D, cm2/s). An extensive description
and comparison among several models have been published by
Masaro and Zhu32 In addition, Liang et al.33 have studied the
water diffusion into an ER. They found that the driving force
for the diffusion resulted in energy released by the hydrogen
bonds, whereas the transportation rate was essentially related
to the local chain mobility as well as to the dissociation of water
molecules from the epoxy network.
The aim of the present work was to study the viscoelastic
behavior of UPR and ER, when they were exposed to distilled
water (H2O) at pH 7 and to an alkaline solution of (Ca(OH)2)
at pH 12.50 (simulated pore concrete solution). The alkaline so-
lution simulated the case when these polymers are used as ma-
trices of FRP employed instead of steel bars as reinforcement
of concrete. A temperature of 50°C was chosen to accelerate the
degradation behavior of the resins without high evaporation
that could produce mechanical damage of the samples. To the
authors’ knowledge at the time of writing, few published papers
showed the effect of aging in water and in an alkaline solution
on the viscoelastic properties of UPR and ER.
Materials and Methods
A commercial UPR (type orthophthalic), catalyzed with 1
wt% of methyl ethyl ketone (supplied by Resinplast Tigre S.R.L,
Buenos Aires, Argentina), and a commercial ER (diglycidyl ether
of bisphenolAwithn= 0.14), catalyzed with tetraethyleneamine
(70%, Huntsman Chemical imported by Distraltec S.A., Buenos
Aires, Argentina), were chosen for this study.
Samples of UPR and ER were prepared in glass plates of
10 cm × 10 cm. To avoid problems with the stripping of the
piece, both the base and the lid were placed on Teflon adhesive
paper. Samples were cut off in plates (6× 6× 0.25 cm) according
to ASTM D570 standard recommendations. The following tem-
perature ramp was used to cure the resins: 40°C for 2 h, 60°C for
2 h, 80°C for 8 h, 120°C for 1 h, 160°C for 1 h, and 180°C for 1 h.
Absorption tests were carried out in accordance with ASTM
D570 standard. Samples were dried for 1 h at 50°C to reach a
constant initialmass. Then, some samples were immersed in dis-
tilled water and other samples in a saturated calcium hydroxide
solution (referred to as alkaline solution in the following text).
A thermostatic bath at 50°C was used for absorption tests to ac-
celerate the process of absorption and to keep the temperature
constant. To record the absorption, samples were removed from
the containers and cleaned with a soft dry paper before weight
measurement. After this, samples were immersed again in wa-
ter or in the alkaline solution. Samples weight was measured
at different times: 30 min, 1 h, 24 h, and 48 h. The measure-
ment was continued until saturation was reached. Percentage of
absorption was calculated as follows:
% Absorption = Qt = Mt − M0M0 × 100 (1)
whereM0 is the initial sample weight at time t = 0 andMt is the
sample weight after absorption at time t.
A Fourier transform infrared (FTIR) spectroscope (IRAffinity-
1 FTIR; Shimadzu, Japan) was used to characterize ER and
UPR (KBr pellets). Each spectrum was collected between
4400 and 450 cm−1 with 40 scans at a spectral resolution of
Viscoelastic properties of the resinswere determinedby using
adynamic mechanical analyzer (DMA8000; Perkin Elmer, USA).
Tests were performed on rectangular bars (30 mm× 8 mm× 2.5
mm) in the simple beam configuration with a span length of
12.55 mm. Temperature was scanned from ambient temperature
to 180°C with a ramp rate of 2°C/min. Oscillation amplitude
was set at 0.05 mm at a constant frequency of 1 Hz. Reported
results are the average of three replicates.
Samples surface was analyzed using a Zeiss Supra 40 Field
emission scanning electron microscope operated at 5 kV and
different magnifications (500, 2000, and 7000×). Samples were
21684 (3 of 11)
FIGURE 4. Percentage of absorption in water and in the alkaline solution versus time (seg1/2) at 50°C. (a) Unsaturated polyester resin. (b) Epoxy
Absorption Results for Unsaturated Polyester and Epoxy Resins
Immersed in Water and in the Alkaline Solution at 50°C
Sample Medium Qt (%) Time (seg1/2)
UPR Water 1.2 2015.1
UPR Alkaline solution 0.9 2179.9
ER Water 2.6 2015.1
ER Alkaline solution 3.1 2179.9
polished with emery paper of different sizes between 320 and
1000 to obtain a surface suitable for viewing the content of voids.
Results and Discussion
Figure 4 shows the rate of absorption of the samples im-
mersed in water and in the alkaline solution and equilibrium
values of absorption are listed in Table I for the different
It was observed that the absorption behavior of UPR in both
media (water and alkaline solution) was significantly different
than that of ER, which exhibited much higher percentage of ab-
sorption values. This can be due to the higher amount of polar
groups (secondary OH− groups), which were present in the ER
at the initial state. Polymerization of epoxywith the hydrogen of
the amine produces a secondary OH− and this makes the epoxy
networksmorehydrophilic.34,35 In addition, after 200hof immer-
sion in the alkaline solution, the weight of UPR decreased with
time. In the caseof polyester films,Guet al.36 have alsofoundthat
the mass loss increased with increasing exposure time in an alka-
line solution. They attributed this phenomenon to the hydrolysis
of ester groups and the subsequent leaching of low molecular
mass and water soluble fragments of the polyester into the al-
kaline medium.37–40 The leaching can be due to molecules with
very lowmolecular weight such as styrene of dimmers or due to
the hydrolytic reactions that produce higher quantities of OH−
groups and acid end-chains groups. Therefore, in the UPR in-
vestigated here, there is a superimposed effect of the hydrolytic
reaction and the water diffusion mechanism. This behavior was
not observed in the epoxy system.
Absorption data are useful for estimating the diffusion coef-
ficient of the fluid within the polymers and, thus, to determine
the rate of absorption. The diffusion is calculated assuming a
one-dimensional diffusion flow, and that the material is homo-
geneous and inert to the fluid. To calculate the diffusion coef-
ficient, the second Fick’s law equation is employed in Eq. (2).
J = D× ∇C (2)
where J is the diffusive flux, D is the diffusion coefficient, and
C is the concentration gradient.
According to the literature, there are twomainmechanisms of
absorption in materials that are associated with a diffusion coef-
ficient. The first corresponds to a diffusion of solvent molecules
between the polymer chains and the second is the transport of
solvent molecules by bubbles in the matrix, which are formed
duringmixing,41 or air included and cracks created into the poly-
mer during the absorption process.
These two mechanisms can be distinguished theoretically by
the shape of the absorption curve as a function of time, repre-
sented byEq. (3)2,3,8 and normally used for describing the Fickian
and non-Fickian behavior. To understand the mechanism of ab-
sorption, diffusion canbe fittedwith the empirical Eq. (4) derived
from the equation of transport phenomena.8
Q = QtQ∞ = k t
log Q = log k + n log t (4)
where Q is absorption and Qt and Q are the weight gain at
the time “t” and at infinity, respectively, k is a constant that
depends on the structural characteristics of the polymer and
its interaction with the solvent, and n determines the mode of
transport or diffusion within the polymer.
21684 (4 of 11)
FIGURE 5. Log absorption as a function of log t (s) in water and in the alkaline solution at 50°C: (a) Unsaturated polyester resin. (b) Epoxy resin.
k and n Coefficients for Unsaturated Polyester and Epoxy Resins
Immersed in Water and in the Alkaline Solution at 50°C
Sample Medium k (1/sn) n
UPR Water 2.4 0.44
UPR Alkaline solution 3.2 0.61
ER Water 2.7 0.48
ER Alkaline solution 3.4 0.61
For the diffusion mechanisms operating within the resin, the
coefficients (n, k) are obtained from the slope and the intercept
of the plot of log Q as a function of log t, respectively. Figure 5
shows the fit of experimental data for the investigated resins
immersed in water and in the alkaline solution at 50°C.
The transport mechanisms in polymers can be described by
the following cases where the value of the coefficient n gives an
idea of the diffusion behavior within the resins. Table II presents
the values of k and n for each sample.
Case 1 or Fickian diffusion: For values of n = 0.5, the diffusion
rate is much lower than the mobility of the polymer segments.
The balance within the polymer is rapidly achieved and main-
tained regardless of the time.1,2,4
Case 2: In this case, transport is characterized by a non-Fickian
behavior, where the diffusion is faster than the relaxation rate
of the polymer. If n is less than 0.5, then transport is termed as
Case 3: Intermediate behavior between cases 1 and 2. If n is
between 0.5 n 1, then the behavior is called anomalous trans-
port, where rate of diffusion and relaxation are comparable.4,7
For the UPR and ER investigated here, n values for the diffu-
sion of water were 0.44 and 0.48 respectively, so water transport
in these resins obeys a pseudo-Fickian behavior.
For both resins immersed in the alkaline solution, the same
value of n was found (0.61), suggesting an anomalous diffusion
behavior.42 Anomalous diffusion can be attributed to two phe-
nomena: the chemical interaction between the diffusing solution
and the polymer, and the relaxation of the polymer structure.43
Another possibility is that the fluid penetrates into the interior
of the resin causing damage and cracking.30
Effective Diffusion Coefficient Values for the Unsaturated Polyester
and Epoxy Resins Immersed in Water and in the Alkaline Solution
Sample Medium Deff (mm2/s)
UPR Water 4.36E–09
UPR Akaline solution 4.02E–11
ER Water 1.95E–08
ER Alkaline solution 4.77E–10
The most important parameter that can be obtained from
Fick’s law is the diffusion coefficient D, which represents the
ability of solventmolecules to penetrate inside the compound. In
our case, we used an effective diffusion coefficient (Deff), which
represents a process not involving pure diffusion, but which
takes into account other effects, such as the existence of voids,
and the fact that the fluid is inert to the material. The effective
diffusion coefficient can be calculated from Eq. (5).44,45
pi × M2 × a 2
where M = Mt /M , M is the weight of the sample when
saturation is reached andMt is the weight of the sample at time
t. Deff is the effective diffusion coefficient, t is the time, and a is
the sample thickness.
Table III shows the effective diffusion coefficient values for
each resin in water and in the alkaline media at 50°C. It was ob-
served that the effective diffusion coefficient values were higher
for samples immersed in water probably because the ions of
Ca++ of the alkaline solution prevented its entry into the poly-
mer. The following reaction can occur within a polymer having
L−OH + Ca++ + OH− L−O− Ca++ + H2O
Where L is the polymer, Ca++ is the calcium ion, and OH is the
Similar equilibrium absorption (Qt) and diffusion coefficient
values were obtained by Mei35 for UPR and ER immersed in
21684 (5 of 11)
FIGURE 6. SEM micrographs showing unsaturated polyester samples: (a) Before immersion, (b) after immersion in water, (c) after immersion in the
alkaline solution, and (d) closer view of a sample after immersion in the alkaline solution.
distilled water at 50°C for 2500 h. He obtained Qt values of 1.8
and 3% and diffusion coefficient values of 3.7 × 10−9 and 2.4 ×
10−8 mm2/s for UPR and ERs, respectively.
Effect of the Exposure to Water and
Alkali Solution on the Polymer
To investigate the effect of hydrolysis on the morphology of
UPR and ER, samples were examined by a scanning electron mi-
croscope (SEM; Carl Zeiss Microscopy GmbH, Germany) before
and after immersion in water and in the alkaline solution.
Figures 6 and 7 are SEM micrographs of the surface of UPR
and ER samples, respectively. A nonuniformdistribution of bub-
bles can be observed for both types of samples immersed in wa-
ter. This observation can be analyzed from a chemical point of
view. In general, solvent uptake is the first step of the degrada-
tion process, which is influenced by bulk polarity, microstruc-
ture, cross-linking density, and heterogeneity of the sample. Pre-
vious studies of organic coatings by SEM showed that water
did not diffuse into a sample uniformly along the boundaries
of the polymer structural units, and then penetrated itself into
the polymer structure.46 Osmotic pressure also plays a role in
the intrusion of water in bulk polymers and their subsequent
chemical and mechanical failure.30 After entering the resin, wa-
ter attacks the “hydrophilic” regions by hydrolysis, swelling, or
dissolution. These hydrophilic regions are presumably regions
that contain low molecular mass/low cross-linked materials.
They take up a large amount of water, have a low resistance to
ion transport, and are susceptible to water attack.47
Bubbles were also observed on the exposed surface of sam-
ples immersed in the alkaline solution (Figs. 6 and 7). For the
ER, no significant differences were found between the morphol-
ogy of the samples immersed in water and those exposed to the
alkaline medium. In the case of the UPR, a significantly differ-
ent morphology was observed. A few larger and more irregular
bubbles containing pits were found in this resin after immer-
sion in the alkaline medium (Fig. 6d). The occurrence and the
development of these pits are believed to be due to the higher
degradation of UPR in thatmedium. These topographic changes
are consistent with the mass loss data presented in Fig. 1 and
might have some correlation with the microvoids reported by
Abeysinghe et al.38 for degraded polyester samples. These au-
thors found that the pits were caused by solvent contact, which
increased with immersion in some specific solvents.
To investigate the spectral changes during aging of epoxy
samples (Fig. 8), the aromatic peak at 830 cm−1 was taken
as the reference peak since aromatic groups were expected to
be unreactive with water and stable at the temperature un-
der study.48 During exposure, an incremental increase in the
broad absorbance around 3423 cm−1 was observed. This band is
21684 (6 of 11)
FIGURE 7. SEM micrographs showing epoxy samples: (a) Before immersion, (b) after immersion in water, and (c) after immersion in the alkaline
FIGURE 8. FTIR for the epoxy resin immersed in water and in the
attributed to the OH− stretching of hydroxyl groups, which in-
creases due to the hydrolysis reaction. The ether bonds are the
most sensitive bonds to hydrolysis in the epoxy network, lead-
ing to the formation of phenol groups and increasing the amount
of OH− groups.
For the unsaturated polyester sample, the band located at
700 cm−1 was taken as the reference band. This is representative
of the styrene benzene rings (–CH aromatic ring) and does not
change during aging, as styrene cannot be hydrolyzed.21 Dur-
ing exposure, an incremental increase in the broad absorbance
around 3416 cm−1 was observed. This band is attributed to the
OH stretching of hydroxyl groups that appears as the hydrolysis
reaction proceeds. The peak centered at 1727 cm−1 is attributed
to C=O stretching of the ester groups and the multiple strong
absorption bands in the 1250–1000 cm−1 region are assigned
to C–O stretching vibrations. The bands peaking at 1600, 1580,
1492, and 1453 cm−1 are due to C–H in-plane bending of the
benzene ring. It was observed that the ester linkage at 1727 cm−1
increased in water and in the alkaline solution (Fig. 9). For the
alkaline solution, there was no apparent increase in the intensity
of the carboxylate ions. For water, in contrast, there was an in-
crease in the intensity of the carboxylate ions indicating the high
carboxylic group content as a consequence of the hydrolysis.
Also, the band of 3517 cm−1 corresponding to the OH− groups
of the alcohols increased with the hydrolysis in water and in
the alkaline solution. The peak at 1268 cm−1 corresponding to
C–O stretch also showed an increase in peak height due to the
hydrolysis. The highest increase in OH− and carboxylic groups
was observed in water for each peaks analyzed. The complex
formed between the OH− end chain in unsaturated polyester
network and Ca++ ions could produce the less availability of
water to react with the ester groups of the polymer.
21684 (7 of 11)
FIGURE 9. FTIR for the unsaturated polyester resin immersed in
water and in the alkaline solution.
Dynamic Mechanical Analysis
Thermal and mechanical behaviors of UPR and ER exposed
to the two media described above were also analyzed. Immer-
sion in different media produces swelling and plasticization
of the matrix, which lead to a loss of mechanical and thermal
Figure 10 presents the storage modulus and tan δ values as
a function of temperature for the different samples after immer-
sion in water and in the alkaline solution at 50°C and Table IV
shows glass transition temperature values, determined from the
maximum of the tan δ peak. It was observed that glass transi-
tion and storage modulus values decreased as a result of the
hydrolysis. Storage modulus value for the UPR was found to
be more affected by hydrolysis than that of the epoxy, because
of the OH− bridges between the polar groups of the polyester
and water, and chain scission. In addition, the ER Tg value was
the most affected property in this process, because of the exis-
tence of secondary OH− groups in this network. This is also the
main reason of the degradation of ERs. The water acts as bridge
between the OH− groups in some part of the epoxy network,
but also acts as lubricant and produces higher mobility of these
chains. Two peaks can be seen in Fig. 10b, indicating bimodal
chain lengths. However, these OH− bridges are the reason for
the more stable storage modulus of the ER. The height of tan
δ decreased for both resins after immersion in the two media.
However, the epoxy network exhibited a higher drop of tan δ,
indicating a harder network after immersion due to their chain
bridges with water.
The decay of the storage modulus with temperature was
found to be more pronounced for the UPR. Gu et al.36 have
studied the degradation of polyester and reported that the base-
catalyzed hydrolysis of this resin was a heterogeneous process
and produced voids that increased in number and size with
exposure time. The alkaline solution acted as a catalyst of the
hydrolytic reactions, and this was the reason why the glassy
modulus for unsaturated polyester in the alkaline solution de-
creasedmore than under watermedium. The hydrolytic reaction
produced acid ends in the polymer; hence, the pH could change
during the absorption under alkaline medium.
In addition, thewidth of the tan δ peakwashigher for theUPR
than the ER, indicating that the polyester network is formed by
a wide chains length distribution. Partini and Pantani demon-
strated that the degradation rate increased with increasing poly-
dispersity in the resin.49 For the polyester, they also suggested
that the controlling mechanism was the chemical hydrolytic re-
action rather than water diffusion.
Modeling of Viscoelastic Properties
To understand the viscoelastic behavior of thermosetting
polymers, the mechanical response is driven by the movement
of the smallest chain segments (only a fewmonomers long). The
number and strength of the links involved in the process of re-
laxation can be associated with aWeibull parametermi andwith
the relaxation of a given point.50
E(t) = E1 exp
Using the time–temperature relationship proposed by
Aklonis and MacKnight51 for instantaneous response and in-
troducing a conversion constant (7):
E(t) = E1 exp
where T1 is the characteristic temperature of the region and E1 is
a reference value of the modulus for the region. For each relax-
ation, newWeibull parametersm1 appear. These parameters are
related to the bond failures and to the strength of the intermolec-
ular bonds that is required for a relaxation to occur. Finally, as
we have different mechanisms, it is possible to sum different
relaxations in a single Eq. (8).
i = 1
Ei × exp
The general equation can be represented with different coef-
ficients. The coefficients (Ei and Ti) can be obtained by different
means, experimental or analytical. For the case of a material
that does not undergo any change before reaching the rubbery
state, its behavior is described by Eq. (9), for a material with
two changes by Eq. (10), and for the case in which the material
undergoes three changes, Eq. (11) can be applied.52
E = E3 × exp
E = (E2 − E3) × exp
+ E3 × exp
21684 (8 of 11)
FIGURE 10. Storage modulus and tan δ values as a function of temperature after immersion in water and in the alkaline solution at 50°C. (a)
Unsaturated polyester and (b) epoxy resins.
Values of Modulus E′ at 50°C, Tg, Percentage Decrease in Modulus, and E′ at 180°C for the Unsaturated Polyester and Epoxy Resins
Sample Medium m1 m2
Glassy (GPa) at
Modulus Tg (°C)
Decrease in Tg
UPR Neat 1.56 6.22 1.86 – 119 – 19.3
UPR Water 2.78 10.85 1.60 14.3 114 4.2 15.9
UPR Alkaline solution 2.42 10.88 1.34 16.1 111 6.7 11.0
ER Neat 2.50 20.98 1.63 – 134 – 25.5
ER Water 2.08 16.71 1.52 6.4 117 12.7 34.9
ER Alkaline solution 2.28 15.86 1.55 5.1 115 19.0 30.7
E = (E1 − E2)× exp
+ (E2 − E3)
+ E3 × exp
To describe the transition, it is necessary to consider several
elements. As a result, a model with two elements that allows to
describe our experimental curve regions can be proposed. For
each element, we define a Weibull parameter mi showing the
behavior of the material in each region, as indicated in Eq. (12).
E = (E1 − E2)× exp
The reference temperature and modulus can be indepen-
dently measured or calculated. The reference temperature cor-
responds to the inflection point of the transition rather than
temperature (Ti), and the reference modulus (Ei), on the other
hand, is taken as the average value for each region (Fig. 11).
Ei represents the instantaneous storagemodulus at the begin-
ning of each region. E1 is the instantaneous storage modulus at
the beginning of the glassy transition and E2 is the instantaneous
storage modulus at the beginning of the rubbery region.
Changes in the modulus in different regions (Ei) are the most
important mechanisms of the relaxation processes. These values
FIGURE 11. Reference values for model calculations.
dependon the chemistry of thepolymer (stiffness of thematerial,
molecular weight, crystallinity, and the degree of cross-linking).
From the glassy state to the region of rubber, the primary links
of the molecules remain intact. However, the secondary bonds
(hydrogen, dipole, and Van der Waals) will be altered by the
molecular movements during temperature increase in the re-
laxation process. When reaching the rubbery state, there are no
longer secondary bonds, and thus, the degradation of the mate-
The Weibull parameters (mi) (Table IV) are continuous
probability distributions corresponding to the behavior of the
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FIGURE 12. Modeling of storage modulus at the rubbery state after immersion in water and in the alkaline solution at 50°C. (a) Unsaturated
polyester resin and (b) epoxy resin. (a1) Closer view of the unsaturated polyester resin in the rubbery state. (b1) Closer view of the epoxy resin in the
breaking of bonds in the material. However, these parameters
will depend on the degree of molecular motion (cross-linking,
molecular weight, and crystallinity). If the movement of the
molecular chains is severely restricted in precise locations (for
cross-linking, etc.), we expect mi will be very low (approaching
a Boltzmann distribution). However, if mobility becomes more
significant, the value of mi will increase.48
Above Tg, the mobility of the polymer chains increases and
the system reaches the ‘‘rubbery’’ state. The storage modulus
at the rubbery state (T = T + 50 K) can be related to the rub-
berymodel and is directly proportional to the cross-linking den-
sity. Figure 12 shows the different behaviors obtained for the
unsaturated polyester and the epoxy network in the rubbery
state and Table IV lists E′rubber and Weibull parameters values
obtained. The values of m1 and m2 represent the behavior of
the polymer in the glassy and rubbery states, respectively. For
the UPR, immersion in water and the alkaline solution led to
a decrease in the rubbery modulus (E′rubber) and an increase
in the m2 parameter and, consequently, to a decrease in the
cross-linking density and an increase in the molecular weight
between the cross-linking points, indicating the presence of
chain scission. For the ER, the immersion in both media pro-
duced an increase in the rubbery modulus (E′rubber) and a de-
crease in the m2 parameter as a result of chain bridges between
the OH− from the water and the secondary OH− of the epoxy
-It was observed that diffusion coefficient values were higher
in both resins (UPR and ER) when immersed in water than
in the alkaline solution. However, the effect of the alkaline
solution was found to be more pronounced on the UPR. These
results were also confirmed from SEM observations of exposed
-After 200 h at 50°C, the absorbed mass decreased for the UPR
due to the loss of low molecular weight compounds (i.e., non-
reacted styrene and short length chains formed during the
-For a resin to be used in civil applications, the decrease in the Tg
value due to degradation is not a critical factor if the polymer
is maintained in the glassy state and the modulus decay is
not significant. The greatest decrease in glassy modulus was
found for the UPR immersed in the alkaline medium, whereas
the highest decrease in the Tg value was observed for the ER
in the same medium. Therefore, the most stable behavior was
exhibited by the ER after immersion in water at a moderate
-It was also found that the value of n in the transport equa-
tion for the diffusion of water in unsaturated polyester and
epoxywas 0.44 and 0.48, respectively, which indicated pseudo-
Fickian behavior. On the other hand, in the case of the
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specimens immersed in the alkaline solution, the value of n
was 0.61 that represented an anomalous transport within these
-In addition, the values of m2 for the unsaturated polyester
in both media increased and the rubbery storage modulus
(E′rubber) decreased, indicating a decrease in the cross-linking
density and an increase in the molecular weight between the
cross-linking points, and consequently the presence of chain
scission. For the ER, on the other hand, the rubbery modulus
(E′rubber) increased and them2 parameter decreased, confirming
the existence of chain bridges between the OH− from thewater
and the secondary OH− of the epoxy network.
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