A Study of the Moisture Absorption Characteristics of Vinyl Ester Polymer and Unidirectional Glass Fibre Vinyl Ester Laminates

Abstract

Vinyl esters are increasingly being used as the matrix polymer in fibre-reinforced composites for demanding large applications which experience long-term exposure to moist and wet conditions. This paper presents the results of a study of ageing due to moisture absorption in vinyl ester polymer and glass fibre–vinyl ester laminates. The moisture uptake kinetics of the two neat VE polymers, cured at different conditions, and their glass-reinforced composites has been characterised by gravimetric methods. These studies have been carried out using submersion in water at 23 °C and 50 °C and exposure to high relative humidity moisture conditions at room temperature. A dynamic mechanical analysis characterisation of the glass transition temperatures of both the aged matrix and the composite is also presented.

Full text

Citation: Thomason, J.; Xypolias, G. A
Study of the Moisture Absorption
Characteristics of Vinyl Ester Polymer
and Unidirectional Glass Fibre Vinyl
Ester Laminates. J. Compos. Sci. 2024,
8, 214. https://doi.org/10.3390/
jcs8060214
Academic Editors: Xiangfa Wu and
Oksana Zholobko
Received: 13 April 2024
Revised: 27 May 2024
Accepted: 5 June 2024
Published: 7 June 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Article
A Study of the Moisture Absorption Characteristics of Vinyl Ester
Polymer and Unidirectional Glass Fibre Vinyl Ester Laminates
James Thomason * and Georgios Xypolias
Department of Mechanical and Aerospace Engineering, University of Strathclyde, 75 Montrose Street,
Glasgow G1 1XJ, UK
*Correspondence: james.thomason@strath.ac.uk
Abstract: Vinyl esters are increasingly being used as the matrix polymer in fibre-reinforced composites
for demanding large applications which experience long-term exposure to moist and wet conditions.
This paper presents the results of a study of ageing due to moisture absorption in vinyl ester
polymer and glass fibre–vinyl ester laminates. The moisture uptake kinetics of the two neat VE
polymers, cured at different conditions, and their glass-reinforced composites has been characterised
by gravimetric methods. These studies have been carried out using submersion in water at 23
◦
C and
50
◦
C and exposure to high relative humidity moisture conditions at room temperature. A dynamic
mechanical analysis characterisation of the glass transition temperatures of both the aged matrix and
the composite is also presented.
Keywords: vinyl ester; glass fibre; composite; hydrothermal ageing; moisture absorption; glass
transition temperature
1. Introduction
The wide and expanding range of applications in which composite materials are
employed ensures an almost inevitable contact with water liquid and vapour, which can
affect both the immediate and the long-term performance of the material. The mechanisms
of water absorption, the plasticizing effect of absorbed moisture and the lowering of the
glass transition temperature are well-known processes which have been widely studied in
polymeric materials [
1
–
16
]. To a lesser extent, water absorption has also been studied in
composite materials, and it has been shown that, in general, the mechanisms of moisture
penetration are much more complex than in the case of the unreinforced matrix [7–16].
Thermoset polymers and composites exposed to humid environments will generally
experience weight gain over time as a result of moisture sorption. Moisture-induced degra-
dation is a common cause of failure for many polymeric materials and can be present from
the manufacturing stage of the material to its end of life. Moisture sorption can result in
both reversible and irreversible changes in the properties and performance of polymers and
composites [
10
]. The severity of the effect depends on the amount of moisture absorbed by
the material, but even small amounts can have a negative effect on performance [
11
]. It has
been established that moisture uptake is often accelerated by combined degradation agents,
such as thermal degradation and external loading, which may lead to irreversible changes
in the material structure and performance [
11
–
14
]. Moisture penetrates the structure of
polymeric materials following a diffusion-controlled process. In the case of composites,
moisture uptake by capillary action is also common, particularly in regions of air entrap-
ment such as voids, cracks and delaminated regions. Moreover, a small amount of moisture
may be located in the fibre/matrix interface. However, the majority of moisture absorbed
by glass fibre (GF)-reinforced composites is usually concentrated in the polymer matrix.
This imbalance in moisture sorption can lead to induced localised stress and strain fields in
the composite, which may accelerate failure [10,13,15–17].
J. Compos. Sci. 2024,8, 214. https://doi.org/10.3390/jcs8060214 https://www.mdpi.com/journal/jcs

J. Compos. Sci. 2024,8, 214 2 of 18
Vinyl ester (VE) resins are increasingly being used as the matrix polymer in fibre-
reinforced composites for demanding large applications in the marine, offshore, and civil
infrastructure sectors. VE formulations are low-viscosity at room temperature and are
highly suitable for infusion of reinforcing fabric preforms to make large parts such as
windmill blades. VE systems exhibit short fabrication cycles and ease of processability
as well as relatively low cost, high environmental durability and adequate level of cured
polymer performance in comparison to more expensive epoxies [
1
,
10
,
18
]. A recent review
of hydrothermal ageing of glass fibre-reinforced vinyl ester composites [
18
] concluded that
the growing demand for composite materials in infrastructure applications, where exposure
to environmental conditions is inevitable, makes ageing studies a necessity. However, given
the high costs and time-consuming processes involved with the in-service examination of
large composite structures, laboratory-based accelerated ageing studies are of increasing
interest in this field.
This paper presents the results of a study of ageing due to moisture absorption in
VE polymer and GF–VE laminates. The moisture uptake kinetics of a range of VE-based
materials have been investigated. This included two neat VE polymers, one a standard
commercial bisphenol-epoxy VE, and the second a developmental VE primarily based
on methyl methacrylate. The effect of the curing conditions of the VE polymer samples
was also included in this study. Furthermore, unidirectional glass reinforced laminates
were prepared with these two VE resins using two commercially available continuous
glass-fibre rovings. Ageing characteristics of these materials have been investigated using
dynamic mechanical analysis and gravimetric analysis methods under various moisture
and temperature environments.
2. Materials and Methods
2.1. Materials
Two types of unidirectional (UD) glass fibre/vinyl ester composite laminate based
on two VE resins and two glass fibre rovings were used in this investigation. The glass
fibres supplied by 3B Fibreglass (Battice, Begium) were SE3030, a 1200 tex continuous
roving containing 17
µ
m Advantex
®
fibres (boron-free E-glass), and W3030, a 2400 tex
continuous roving containing 17
µ
m HiPer-tex
®
fibres (produced from a CaO-Al
2
O
3
-
SiO
2
R-glass composition [
19
,
20
]). The VE resins (DION
®
1260 and DION
®
1273) were
supplied by Polynt Composites (Gamle Fredrikstad, Norway). DION 1260 is a commercial
bisphenol-epoxy vinyl ester with a styrene content of 48–52% by weight. DION 1273 is a
developmental resin which is a pre-accelerated, modified vinyl ester resin, primarily based
on methyl methacrylate (MMA) and styrene (<20% by weight). Material specifications
and cure conditions for the laminates are included in Table 1. The SE 3030/1260 laminate
was considered a baseline material since it consisted of commercially available matrix and
reinforcement. The W 3030/1273 laminate was a developmental composite consisting of the
DION 1273 experimental matrix and the W 3030 fibre. Composite laminates were supplied
by Polynt as 800
×
800
×
3.9 mm panels with a glass weight fraction of
71.3 ±0.2%
. These
laminates were measured as void-free within the experimental error of the standard density
method [
21
]. These were cut to appropriate specimen sizes by water-jet cutting. Prior to
ageing, the laminate specimen edges were sealed by using Araldite Epoxy, a 2-part high-
viscosity, hydrophobic adhesive. Two thin layers of coating were applied in succession to
ensure thorough sealing.
Neat resin specimens were also employed for a better understanding of the hydrother-
mal response of the matrix material on a bulk scale. Plates of DION 1260 and DION 1273
polymer cured in a closed mould were supplied by Polynt. Plates of DION 1260 polymer
were also prepared at the University of Strathclyde using open silicone rubber moulds [
21
].
Cure conditions for polymer and composite samples are given in Table 1.

J. Compos. Sci. 2024,8, 214 3 of 18
Table 1. Polymer matrix and laminate description and cure conditions.
Designation VE Resin Glass Fibre Cure 24 h at RT Post-Cure (PC) Max. PC Temp (◦C)
AF1260 DION 1260 SE3030 Sealed Mould 60
WF1273 DION 1273 W3030 Sealed Mould 60
1260, SM60 DION 1260 - Sealed Mould 60
1273, SM60 DION 1273 - Sealed Mould 60
1260, SM100 DION 1260 - Sealed Mould 100
1260, OM100 DION 1260 - Air Air 100
2.2. Ageing Conditions
Plates of 80
×
80
×
3.9 mm of all laminate and cured polymer specimens were aged,
for up to 530 days, by full immersion in deionised (DI) water at 23
◦
C and 50
◦
C and in
a humid environment of 100% RH at room temperature (20
±
2
◦
C). These are referred
to as “23
◦
C”, “50
◦
C” and “100% RH” throughout this manuscript. The selection of
immersion in water was used to simulate accelerated degradation mechanisms which
may be acting through rainwater in large infrastructure applications. However, since
the chemistry of rainwater is variable, DI water was selected in order to simplify the
nature of the degradation reactions. The temperature of 50
◦
C was used as a means of
ageing acceleration and to create an ageing environment, similar to ones present in hot and
wet climates. The increased humidity environment was chosen to replicate degradation
conditions acting in highly humid climates. All specimens were dried for at least 24 h
under vacuum prior to ageing to obtain the starting weight W
0
of the dry sample. Samples
were systematically removed from the conditioning container at various ageing times
(t) to measure their weight W(t), which was determined after any surface moisture was
removed using a dry tissue. The time-dependent increase in the weight of the sample M(t)
was calculated as W(t)
−
W
0
. Four water baths were employed for the DI water ageing,
while sealed desiccators were used for the controlled humidity conditions. The 100% RH
humidity level was achieved by placing DI water at the bottom of a desiccator and was
checked by a digital hygrometer. These ageing conditions are consistent with the previously
published study on microscale specimens [
22
]. 1260-OM100 specimens were only aged at
23 ◦C and 50 ◦C.
2.3. Diffusivity Analysis
The use of flat plate samples with a length and width to thickness ratio greater than
20 allowed the use of the classical 1-D Fickian diffusion approach for the analysis of the
weight increase with time of the ageing samples. The background to Fickian diffusion is
well-documented and will not be reproduced here. The time-dependent weight increase
M(t), as a fraction of the final equilibrium value of M
e
, in an infinite parallel-sided slab of
thickness (h) made of a polymer with diffusion coefficient (D) is given by Equation (1) [
23
]:
M(t)
Me
=1−8
π2
∞
∑
n=0
1
(2n+1)2exph−(2n+1)2(Dπ2t/h2)i(1)
The diffusion coefficient is often obtained in this type of experiment by using the initial
slope obtained from a graph of M(t)/M
e
versus t
½
. However, in this work, the full curve
has been fitted to Equation (1) to obtain D. For the laminate samples, the effect of the fibres
on the directional diffusion in the material must be considered. There are a number of
phenomena which can affect the weight increase in ageing composite materials [
8
,
12
,
13
,
24
].
However, for a thin plate of unidirectionally reinforced laminate, Shen and Springer [
25
]
have proposed that the through-the-thickness diffusion coefficient (D
c
) can be estimated
as follows:
Dc= 1−2rvf
π!Dm(2)

J. Compos. Sci. 2024,8, 214 4 of 18
where D
m
is the diffusion coefficient of the polymer matrix and v
f
is the fibre volume
fraction. Equation (2) was used for the prediction of composite laminate diffusivities for
the AF1260 and WF1273, which had measured fibre weight fractions (w
f
), measured from
ashing experiments, of 71.5% and 71.1%, respectively [
21
]. These values converted to
volume fraction values of 51.9% and 51.4%, respectively. It was assumed that the glass
fibres contributed a negligible amount to the weight gain of the ageing laminates, and so
an estimation of the expected equilibrium moisture gain of the composite laminates was
obtained by using the weight fraction of the matrix multiplied by the moisture gain of the
neat polymer at equilibrium.
2.4. Dynamic Mechanical Analysis
Dry and aged laminate and neat polymer specimens were tested by DMA in a 3-point
bending configuration using a Q800 Dynamic Mechanical Analyser (TA Instruments, New
Castle, DE, USA) and according to ASTM D5023-15 [
26
]. The input test parameters were
frequency of 1 Hz, amplitude of 50
µ
m, pre-load force of 0.1 N and a force track of 120%.
The specimen dimensions were 64
×
13
×
3.9 mm. All specimens were equilibrated at
25
◦
C for 5 min, then ramped at 2
◦
C/min up to 150
◦
C. The glass transition temperature
(Tg) assignment was conducted in agreement with ASTM D7028—07 [27].
3. Results and Discussion
3.1. Neat Polymer Moisture Uptake
3.1.1. DION 1260 Polymer
A weight gain vs. exposure time plot of the DION 1260 SM60 polymer for three
different conditions is shown in Figure 1. Diffusivity values calculated using these data
and assuming 1-D Fickian diffusion for all specimens and the maximum weight gain and
diffusivity values (calculated using Equation (1)) for the three differently cured DION
1260 polymer under the three exposure conditions are summarised in Table 2. In some
cases, it was unclear if equilibrium was reached and maximum weight gain values were
estimated from the curves obtained. These standard cure (SM60) samples exhibited a
similar Fickian-type moisture uptake behaviour when immersed at 23
◦
C in DI water and
when conditioned at 100% RH and room temperature. In both cases, an initial Fickian-like
weight gain was observed until around 66 days (
≈
40
√
h), at which point weight gain
began to stabilise at an equilibrium value, but there then followed a slow and gradual
increase for the remaining ageing period.
This further weight increase observed upon the samples reaching their Fickian equilib-
rium moisture content is indicative of a two-stage diffusion phenomenon which has been
widely observed in polymers and their composites [
12
,
24
]. It is normally attributed to the
coupling of diffusion with a viscoelastic response. More specifically, the combined effect
of coupling between diffusion and polymeric relaxation is governed by two distinct yet
interrelated phenomena: diffusion across the thickness of an area of a polymeric material
and time-dependent relaxation of the polymeric chains. For instance, moisture diffusion
until saturation in a film will occur rapidly, due to its reduced thickness. Therefore, the
establishment of a uniform distribution across the film thickness will take place before the
relaxation process starts taking place. In this case, moisture ingress is governed by the
diffusivity of an unrelaxed polymer. On the other hand, moisture diffusion in a thick slab
of a polymer will occur more slowly than relaxation, and therefore the diffusion will be
governed by the diffusivity of fully relaxed polymeric chains. However, for intermediate
slabs, the phenomena of diffusion and relaxation will occur in combination [12].
The resultant moisture-induced plasticisation of the matrix can be marked by a Tg
decrease as a function of moisture content [
28
]. At equilibrium and as presented in Table 2,
the weight gain in both conditions was found to be similar. The time to equilibrium was
between 85 (
≈
45
√
h) and 110 days (
≈
50
√
h) under both conditions. Diffusivity values
under all ageing conditions were of the same order of magnitude as values reported in the
literature [
29
]. Generally, the comparison of the two ageing conditions provides evidence

J. Compos. Sci. 2024,8, 214 5 of 18
that exposure to a highly moist environment and direct immersion in an aqueous medium
have a similar effect on vinyl ester polymer, although the moisture gain at 100% RH was
slightly lower than in water at 23 ◦C.
J. Compos. Sci. 2024, 8, x FOR PEER REVIEW 5 of 19
Figure 1. Weight gain vs. ageing time for DION 1260 SM60 polymer under different ageing condi-
tions (doed lines show fiing of Equation (1)).
This further weight increase observed upon the samples reaching their Fickian equi-
librium moisture content is indicative of a two-stage diffusion phenomenon which has
been widely observed in polymers and their composites [12,24]. It is normally aributed
to the coupling of diffusion with a viscoelastic response. More specifically, the combined
effect of coupling between diffusion and polymeric relaxation is governed by two distinct
yet interrelated phenomena: diffusion across the thickness of an area of a polymeric ma-
terial and time-dependent relaxation of the polymeric chains. For instance, moisture dif-
fusion until saturation in a film will occur rapidly, due to its reduced thickness. Therefore,
the establishment of a uniform distribution across the film thickness will take place before
the relaxation process starts taking place. In this case, moisture ingress is governed by the
diffusivity of an unrelaxed polymer. On the other hand, moisture diffusion in a thick slab
of a polymer will occur more slowly than relaxation, and therefore the diffusion will be
governed by the diffusivity of fully relaxed polymeric chains. However, for intermediate
slabs, the phenomena of diffusion and relaxation will occur in combination [12].
Table 2. Weight gain and diffusivity of DION 1260 polymers under different ageing conditions.
DION
1260
Ageing
Condition
Equilibrium
Weight Gain (%)
Diffusivity
× 10−6 (mm2/s)
Max. Ageing
Period (Days)
Max. Weight
Gain (%)
Re-dried Weight
Loss (%)
SM60
100% RH 0.49 0.80 530 0.67 −0.23
23 °C DI 0.53 0.90 530 0.70 −0.24
50 °C DI 0.76 3.00 530 0.78 −0.27
SM100
100% RH 0.57 0.90 175 0.60 −0.27
23 °C DI 0.59 1.00 175 0.60
50 °C DI 0.68 4.00 175 0.74
OM100 23 °C DI 0.95 0.52 658 1.37 −0.18
50 °C DI 1.39 2.00 389 2.60 −0.61
The resultant moisture-induced plasticisation of the matrix can be marked by a Tg
decrease as a function of moisture content [28]. At equilibrium and as presented in Table
2, the weight gain in both conditions was found to be similar. The time to equilibrium was
between 85 (≈45 √h) and 110 days (≈50 √h) under both conditions. Diffusivity values under
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 20406080100120
Weight Gain (%)
Time (√h)
50°C Water
23°C Water
100% RH
Figure 1. Weight gain vs. ageing time for DION 1260 SM60 polymer under different ageing conditions
(dotted lines show fitting of Equation (1)).
Table 2. Weight gain and diffusivity of DION 1260 polymers under different ageing conditions.
DION 1260 Ageing
Condition
Equilibrium
Weight Gain (%)
Diffusivity
×
10
−6
(mm
2
/s)
Max. Ageing
Period (Days)
Max. Weight
Gain (%)
Re-Dried Weight
Loss (%)
SM60
100% RH 0.49 0.80 530 0.67 −0.23
23 ◦C DI 0.53 0.90 530 0.70 −0.24
50 ◦C DI 0.76 3.00 530 0.78 −0.27
SM100
100% RH 0.57 0.90 175 0.60 −0.27
23 ◦C DI 0.59 1.00 175 0.60
50 ◦C DI 0.68 4.00 175 0.74
OM100 23 ◦C DI 0.95 0.52 658 1.37 −0.18
50 ◦C DI 1.39 2.00 389 2.60 −0.61
The samples submersed in water at 50
◦
C also exhibited Fickian-like behaviour but
with a significantly higher weight gain and a higher diffusivity level. The time to obtain
the higher equilibrium weight increase (35 days
≈
28
√
h) was significantly shorter when
compared to 23
◦
C and 100% RH. Although matrix relaxation was apparent at room-
temperature ageing and is nevertheless more likely to take place in higher temperatures [
30
],
only a very small post-equilibrium increase (
≈
0.02%) in sample weight could be observed in
the data in Figure 1. One possible explanation for this is that the higher water temperature
induces a much more rapid relaxation in the polymer, which means that the associated
increased moisture uptake takes place during the initial Fickian period and is essential over
by the time that Fickian equilibrium is reached.
Another possibility is that ageing at elevated temperatures induced anti-plasticisation
effects, i.e., a combination of leaching and secondary cross-linking, instead of plasticisation.
Han and Drzal [
31
] have proposed the presence of two types of bound water in thermoset net-
works; type I, which is associated with a reduction in Tg and polymer plasticisation, and Type
II, bound water which is responsible for secondary cross-linking and increased Tg. Further
claims made by Apicella et al. [
32
] suggest that high-temperature hydrothermal environments

J. Compos. Sci. 2024,8, 214 6 of 18
are associated with Tg increases in thermosets through post-curing, often accompanied by
leaching. In addition, they stated that such effects can reduce the degree of plasticisation
of the matrix (anti-plasticisation). Leaching and secondary cross-linking effects have been
previously observed by Visco et al. [
33
] for a vinyl ester matrix aged at 60
◦
C in water. Similar
effects have also been observed by Apicella et al. [
32
] for incompletely polymerised styrene-
containing unsaturated polyester. Thomason and Xypolias have observed that microdroplets
(approximately 100
µ
m in diameter) of this same vinyl ester polymer are essentially totally
degraded after only 100 h of exposure to water at 50 ◦C [22].
Despite the observation of Fickian-like plots, irreversible weight loss was confirmed
for all specimens. All specimens were re-dried upon the completion of ageing after 530 days
(
≈
120
√
h). Re-drying was conducted at 45
◦
C for 72 h, following storage in a dry desiccator
at room temperature for a minimum of 24 h. Weight losses of 0.24%, 0.27% and 0.23% were
observed for the conditions of 23
◦
C, 50
◦
C and 100% RH, respectively. Polymer matrix
ageing has been associated both with matrix leaching (matrix decomposition)—hydrolysis,
and (or) leaching polymer unreacted oligomers—not an indication of hydrolysis. A common
indicator for hydrolysis is the change in matrix colour after ageing. In this case, a mild change
in colour in the matrix was observed at 23
◦
C and 100% RH, whereas a more profound
discolouration was featured at 50
◦
C (see Figure 2). The latter observation confirms matrix
hydrolysis at 50
◦
C, while the former is an indication that hydrolytic effects had only started
to take place at room-temperature ageing. DMA measurements were carried out to further
elucidate any effects on the polymer Tg.
J. Compos. Sci. 2024, 8, x FOR PEER REVIEW 7 of 19
Figure 2. DION 1260 SM60 polymer colour changes under different ageing conditions.
Post-curing the DION 1260 matrix at a higher temperature of 100 °C did not have a
great effect on moisture uptake kinetics, as shown in Figure 3. Despite the ageing period
of the DION 1260 SM100 specimens being shorter than that of the SM60 samples, similar
equilibrium water uptake values were obtained. while the moisture gain values and dif-
fusivities were also found to be within the same range, as shown in Table 2.
Figure 3. Weight gain vs. ageing time for DION 1260 SM100 post-cured polymer under different
ageing conditions (doed lines show fiing of Equation (1)).
A notably different ageing behaviour was observed for the specimens cured in an
open mould for the first part of the cure, which allowed oxygen interaction with the curing
specimen. The average water uptake trends for specimens submerged at 23 °C and 50 °C
are shown in Figure 4. Specimens aged at 23 °C also initially followed a Fickian-like be-
haviour, reaching equilibrium after approximately 100 days (≈50 √h) and absorbing ap-
proximately 0.9% of water. Subsequently a further weight increase, which was approxi-
mately linear with time (again indicative of matrix relaxation and swelling), took place,
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 10203040506070
Weight Gain (%)
Time (√h)
23°C Water
50°C Water
100% RH
Figure 2. DION 1260 SM60 polymer colour changes under different ageing conditions.
Post-curing the DION 1260 matrix at a higher temperature of 100
◦
C did not have a
great effect on moisture uptake kinetics, as shown in Figure 3. Despite the ageing period
of the DION 1260 SM100 specimens being shorter than that of the SM60 samples, similar
equilibrium water uptake values were obtained. while the moisture gain values and
diffusivities were also found to be within the same range, as shown in Table 2.
A notably different ageing behaviour was observed for the specimens cured in an
open mould for the first part of the cure, which allowed oxygen interaction with the curing
specimen. The average water uptake trends for specimens submerged at 23
◦
C and 50
◦
C are
shown in Figure 4. Specimens aged at 23
◦
C also initially followed a Fickian-like behaviour,
reaching equilibrium after approximately 100 days (
≈
50
√
h) and absorbing approximately
0.9% of water. Subsequently a further weight increase, which was approximately linear
with time (again indicative of matrix relaxation and swelling), took place, but at a much
greater rate than shown by samples SM60 and SM100 in Figures 1and 3. The samples aged

J. Compos. Sci. 2024,8, 214 7 of 18
at 50
◦
C also initially exhibited Fickian-like behaviour, reaching an apparent equilibrium
weight increase earlier and at a higher level compared to the sample aged at 23
◦
C. Perhaps
more notable from the results shown in Table 2is that the equilibrium water absorption
levels of the open-mould samples (OM100) were approximately double that of the sealed-
mould prepared samples (SM60, SM100). This significant increase in weight gain of
the open-mould specimens, when compared to standard sealed-mould cure, could be
caused by the interaction of oxygen with the curing polymer, as well as probable styrene
evaporation due to a difference in polymer (surface) structure [
34
–
36
]. It is noteworthy
that pronounced styrene evaporation, which was apparent in this case, can significantly
decrease the hydrophobicity of the matrix [
37
,
38
]. A parallel study on the effect of oxygen
on the curing of these two VE resins concluded that oxidation and styrene loss effects were
significantly reduced by the use of sealed-mould curing conditions [39].
J. Compos. Sci. 2024, 8, x FOR PEER REVIEW 7 of 19
Figure 2. DION 1260 SM60 polymer colour changes under different ageing conditions.
Post-curing the DION 1260 matrix at a higher temperature of 100 °C did not have a
great effect on moisture uptake kinetics, as shown in Figure 3. Despite the ageing period
of the DION 1260 SM100 specimens being shorter than that of the SM60 samples, similar
equilibrium water uptake values were obtained. while the moisture gain values and dif-
fusivities were also found to be within the same range, as shown in Table 2.
Figure 3. Weight gain vs. ageing time for DION 1260 SM100 post-cured polymer under different
ageing conditions (doed lines show fiing of Equation (1)).
A notably different ageing behaviour was observed for the specimens cured in an
open mould for the first part of the cure, which allowed oxygen interaction with the curing
specimen. The average water uptake trends for specimens submerged at 23 °C and 50 °C
are shown in Figure 4. Specimens aged at 23 °C also initially followed a Fickian-like be-
haviour, reaching equilibrium after approximately 100 days (≈50 √h) and absorbing ap-
proximately 0.9% of water. Subsequently a further weight increase, which was approxi-
mately linear with time (again indicative of matrix relaxation and swelling), took place,
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 10203040506070
Weight Gain (%)
Time (√h)
23°C Water
50°C Water
100% RH
Figure 3. Weight gain vs. ageing time for DION 1260 SM100 post-cured polymer under different
ageing conditions (dotted lines show fitting of Equation (1)).
J. Compos. Sci. 2024, 8, x FOR PEER REVIEW 8 of 19
but at a much greater rate than shown by samples SM60 and SM100 in Figures 1 and 3.
The samples aged at 50 °C also initially exhibited Fickian-like behaviour, reaching an ap-
parent equilibrium weight increase earlier and at a higher level compared to the sample
aged at 23 °C. Perhaps more notable from the results shown in Table 2 is that the equilib-
rium water absorption levels of the open-mould samples (OM100) were approximately
double that of the sealed-mould prepared samples (SM60, SM100). This significant in-
crease in weight gain of the open-mould specimens, when compared to standard sealed-
mould cure, could be caused by the interaction of oxygen with the curing polymer, as well
as probable styrene evaporation due to a difference in polymer (surface) structure [34–36].
It is noteworthy that pronounced styrene evaporation, which was apparent in this case,
can significantly decrease the hydrophobicity of the matrix [37,38]. A parallel study on the
effect of oxygen on the curing of these two VE resins concluded that oxidation and styrene
loss effects were significantly reduced by the use of sealed-mould curing conditions [39].
An interesting and very different response to hydrothermal ageing was exhibited by
the specimens exposed at 50 °C when the water submersion experiment was continued
for longer times. The data featured in Figure 4 are averages of three specimens, which all
noted a similar behaviour with each other, with only slightly varying moisture gain. The
samples initially reached weight stabilisation after 30 days (≈27 √h) of exposure at around
+1.4%, indicative of the Fickian equilibrium. However, an unexpectedly abrupt increase
in weight was observed at around 100 days (≈50 √h). The slope of this post-equilibrium
weight increase was very significantly greater than that observed with the sample aged at
room temperature. This increase can be aributed to probable more significant plasticisa-
tion and matrix relaxation of a more hydrophilic polymer. Nonetheless, leaching became
apparent and dominated sorption after an average maximum weight gain of 2.6% was
obtained, resulting in a pronounced weight loss.
Figure 4. Weight gain vs. ageing time for DION 1260 OM100 polymer at 23 °C and 50 °C (doed
lines show fiing of Equation (1)).
At the end of the water absorption measurements, the aged specimens were thor-
oughly dried and weighed in order to examine probable leaching effects. Samples aged at
23 °C were found to have lost approximately 0.18% after 658 days of ageing (≈125 √h),
whereas the samples aged at 50 °C had lost 0.6% of their weight after just 389 days (≈97
√h) of exposure. These data would seem to confirm the possibility that leaching of some
vinyl ester polymer components had occurred during the extended water aging experi-
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 20406080100
Weight Gain (%)
Time (√h)
50°C Water
23°C Water
Figure 4. Weight gain vs. ageing time for DION 1260 OM100 polymer at 23
◦
C and 50
◦
C (dotted
lines show fitting of Equation (1)).

J. Compos. Sci. 2024,8, 214 8 of 18
An interesting and very different response to hydrothermal ageing was exhibited by
the specimens exposed at 50
◦
C when the water submersion experiment was continued for
longer times. The data featured in Figure 4are averages of three specimens, which all noted
a similar behaviour with each other, with only slightly varying moisture gain. The samples
initially reached weight stabilisation after 30 days (≈27 √h) of exposure at around +1.4%,
indicative of the Fickian equilibrium. However, an unexpectedly abrupt increase in weight
was observed at around 100 days (
≈
50
√
h). The slope of this post-equilibrium weight
increase was very significantly greater than that observed with the sample aged at room
temperature. This increase can be attributed to probable more significant plasticisation and
matrix relaxation of a more hydrophilic polymer. Nonetheless, leaching became apparent
and dominated sorption after an average maximum weight gain of 2.6% was obtained,
resulting in a pronounced weight loss.
At the end of the water absorption measurements, the aged specimens were thoroughly
dried and weighed in order to examine probable leaching effects. Samples aged at 23
◦
C
were found to have lost approximately 0.18% after 658 days of ageing (
≈
125
√
h), whereas
the samples aged at 50
◦
C had lost 0.6% of their weight after just 389 days (
≈
97
√
h) of
exposure. These data would seem to confirm the possibility that leaching of some vinyl
ester polymer components had occurred during the extended water aging experiments.
Difficulty in extracting absorbed moisture was noted with both SM and OM specimens. Re-
drying open-mould specimens was particularly challenging, and rigorous drying schedules
were employed for most specimens. A decaying weight loss was recorded for all specimens,
upon heating in an air circulation oven (at 45
◦
C—below the polymer Tg) and placement
in a dry desiccator. Thus, it is challenging to provide definitive answers on weight loss.
The difficulty of re-drying the matrix was a possible indication of water molecules trapped
within micro-cavities and capillaries found in the thermoset polymer network making it
susceptible to hydrolysis. This effect has been previously studied by Han and Drzal [
31
]
and has been shown to be an indication of chemical ageing.
3.1.2. DION 1273 Polymer
The weight gain behaviour and diffusivity values of the DION 1273 SM60 polymer
were found to be relatively close to those of DION 1260, although the former absorbed
more moisture at equilibrium. Weight gain vs. time plots for DION 1273 SM60 polymer are
presented in Figure 5, while the derived diffusivity and moisture gain values are contained
in Table 3. The weight gain behaviour and diffusivity values recorded for DION 1273
at 23
◦
C and 100% RH were similar to each other, confirming the similarity of the two
conditions also observed with the DION 1260 polymers. The DION 1273 samples also
exhibited a two-stage moisture absorption process indicative of an initial Fickian absorption
behaviour later overlaid by an approximately linear (with
√
time) slow weight increase
due to polymer relaxation and swelling. Due to an increasing weight gain in all cases,
equilibrium was assumed when the weight gain reached a temporary plateau—at the
“flattest” point of the curve. The initial Fickian equilibrium levels were attained between
63 (
≈
38
√
h) and 69 days (
≈
40
√
h), significantly earlier than observed with DION 1260,
which reached equilibrium between 85 (
≈
45
√
h) and 110 days (
≈
50
√
h). The moisture
gain at this point was also significantly higher than for DION 1260 under both conditions.
Namely, 0.76% was recorded at 23
◦
C, as opposed to 0.53% found for DION 1260, and 0.67%
at 100% RH, compared to 0.49% noted for DION 1260. The post-equilibrium continuous
weight increase is indicative of matrix relaxation and swelling.
DION 1273 attained a similar weight gain pattern at 50
◦
C compared to 23
◦
C and 100%
RH but absorbed significantly more moisture. The short-term equilibrium was estimated at
around 26 days (
≈
25
√
h), also somewhat sooner than observed with the DION 1260 polymers.
Furthermore, the estimated equilibrium value of 1.09% for DION 1273 polymer was much
higher than that for DION 1260 polymers. In addition, for an ageing period of 530 days, the
maximum moisture gain obtained by DION 1273 at 50
◦
C was 1.4%, whereas a significantly
lower maximum moisture uptake of 0.78% was attained by DION 1260 (see Tables 2and 3). In

J. Compos. Sci. 2024,8, 214 9 of 18
addition to the overall higher moisture equilibrium values observed with DION 1273 polymer,
the slope of the post-equilibrium linear increase in sample weight observed in Figure 5was
significantly higher than that obtained with the DION 1260 samples in Figures 1and 3. Hence,
it seems reasonable to conclude that DION 1260 polymer is more hydrophobic compared
to DION 1273 polymer. One possible reason for this is that DION 1260 has a much higher
styrene content than DION 1273, and styrene is known to increase the hydrophobicity of vinyl
ester polymers [
38
]. Moreover, the two resins consist of two different primary monomers,
methyl-methacrylate (MMA)—DION 1273, and bisphenol-epoxy—DION 1260, which in turn
produce a matrix with polymer networks entirely different from each other. N’Diaye et al. [
40
]
have reported that although methyl-methacrylate polymers (PMMA) are hydrophobic, they
have been found to absorb up to 2% of water and can undergo swelling and plasticisation, as
did DION 1273 under all ageing conditions.
J. Compos. Sci. 2024, 8, x FOR PEER REVIEW 9 of 19
ments. Difficulty in extracting absorbed moisture was noted with both SM and OM spec-
imens. Re-drying open-mould specimens was particularly challenging, and rigorous dry-
ing schedules were employed for most specimens. A decaying weight loss was recorded
for all specimens, upon heating in an air circulation oven (at 45 °C—below the polymer
Tg) and placement in a dry desiccator. Thus, it is challenging to provide definitive answers
on weight loss. The difficulty of re-drying the matrix was a possible indication of water
molecules trapped within micro-cavities and capillaries found in the thermoset polymer
network making it susceptible to hydrolysis. This effect has been previously studied by
Han and Drzal [31] and has been shown to be an indication of chemical ageing.
3.1.2. DION 1273 Polymer
The weight gain behaviour and diffusivity values of the DION 1273 SM60 polymer
were found to be relatively close to those of DION 1260, although the former absorbed
more moisture at equilibrium. Weight gain vs. time plots for DION 1273 SM60 polymer
are presented in Figure 5, while the derived diffusivity and moisture gain values are con-
tained in Table 3. The weight gain behaviour and diffusivity values recorded for DION
1273 at 23 °C and 100% RH were similar to each other, confirming the similarity of the two
conditions also observed with the DION 1260 polymers. The DION 1273 samples also ex-
hibited a two-stage moisture absorption process indicative of an initial Fickian absorption
behaviour later overlaid by an approximately linear (with √time) slow weight increase
due to polymer relaxation and swelling. Due to an increasing weight gain in all cases,
equilibrium was assumed when the weight gain reached a temporary plateau—at the
“flaest” point of the curve. The initial Fickian equilibrium levels were aained between
63 (≈38 √h) and 69 days (≈40 √h), significantly earlier than observed with DION 1260,
which reached equilibrium between 85 (≈ 45 √h) and 110 days (≈50 √h). The moisture gain
at this point was also significantly higher than for DION 1260 under both conditions.
Namely, 0.76% was recorded at 23 °C, as opposed to 0.53% found for DION 1260, and
0.67% at 100% RH, compared to 0.49% noted for DION 1260. The post-equilibrium con-
tinuous weight increase is indicative of matrix relaxation and swelling.
Figure 5. Weight gain vs. ageing time for DION 1273 SM60 polymer under different ageing condi-
tions (doed lines show fiing of Equation (1)).
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 20406080100120
Weight Gain (%)
Time (√h)
50°C Water
23°C Water
100% RH
Figure 5. Weight gain vs. ageing time for DION 1273 SM60 polymer under different ageing conditions
(dotted lines show fitting of Equation (1)).
Table 3. Weight gain and diffusivity of DION 1273 SM60 polymers under different ageing conditions.
Ageing
Condition
Equilibrium
Weight Gain (%)
Diffusivity
×10−6(mm2/s)
Max. Ageing Period
(Days)
Max. Weight Gain
(%)
Re-Dried Weight Loss
(%)
100% RH 0.67 0.9 530 0.97 −0.19
23 ◦C DI 0.76 1.0 530 1.05 −0.18
50 ◦C DI 1.10 3.8 530 1.4 −0.42
All aged DION 1273 SM60 polymer samples were re-dried at the termination of
the aging experiments to determine any overall weight loss which occurred during their
submersion. The drying conditions used were identical to those used for the DION 1260
SM60 samples (see Section 3.1.1). Values are tabulated in Table 3. At 23
◦
C and 100% RH, the
final weight loss of DION 1273 was slightly less than that of DION 1260, despite the former
absorbing more moisture. However, at 50
◦
C, DION 1273 noted a significantly higher
weight loss than DION 1260, indicative of the susceptibility of the matrix to hydrothermal
attack at elevated temperatures. Pronounced discolouration, indicative of chemical ageing,
was also observed for all DION 1273 samples under all ageing conditions (see Figure 6).

J. Compos. Sci. 2024,8, 214 10 of 18
J. Compos. Sci. 2024, 8, x FOR PEER REVIEW 11 of 19
Figure 6. DION 1273 SM60 polymer colour changes under different ageing conditions.
3.2. Laminate Moisture Uptake
Figure 7 presents a weight gain vs. time plot for the AF1260 laminates under different
ageing conditions with the doed lines showing the fiing of the 1-D Fickian diffusion
model in Equation (1). The 80 × 80 × 3.9 mm specimen geometry was selected to allow
direct comparison with the results for the neat polymer plates presented in the previous
section. Similarly, Figure 8 shows the results for the long-term moisture absorption meas-
urements on the WF1273 laminates. In both cases, a similar paern of weight increase with
time can be observed, as seen in the respective polymer matrices. Under all conditions,
the composites exhibited an initial Fickian-like weight increase approaching a plateau
equilibrium level followed by a further slow linear (with sqrt time) weight increase at
longer times. Composite diffusivity and weight gain values obtained experimentally are
compared in Table 4 with diffusivity and weight gain values predicted from the neat pol-
ymer matrix ageing values. The average matrix weight fraction of 0.29 was used for the
evaluation of the predicted equilibrium moisture gain. The same weight fraction was con-
verted to a volume fraction value of 0.52 for use in Equation (2). In all cases, the experi-
mentally obtained diffusivity and weight gain values were higher than the theoretical pre-
dictions. In most cases, the measured and predicted Fickian equilibrium weight gain val-
ues were relatively close. Given the uncertainty in the measured values generated by the
possible overlap of the second, slower, observed moisture absorption phenomenon of
weight uptake in the polymers, it seems reasonable to state that the predicted and meas-
ured values were equivalent within the experimental error. However, the diffusivity val-
ues showed a very different behaviour. In this case, the measured composite diffusion
coefficients were significantly greater (approximately ×4) than those predicted by Equa-
tion (2).
Three possible reasons for such deviations have been suggested by Karbhari and
Zhang [41]. These include relaxation of elastic forces induced by the cross-linked network
after the initial weight-gain plateau, and (or) wicking at the interface, and (or) the pres-
ence of manufacturing voids and the likelihood of them being increased by ageing. Fur-
thermore, the degradation of the edge sealant during ageing is a common issue in the
ageing of composites. According to the literature, simplistic 1-D diffusion models can lead
to erroneous results, especially when removal of the sealant occurs. More complex diffu-
sion models may thus be employed to understand the full effect of ageing when changing
specimen geometry [24].
Figure 6. DION 1273 SM60 polymer colour changes under different ageing conditions.
3.2. Laminate Moisture Uptake
Figure 7presents a weight gain vs. time plot for the AF1260 laminates under different
ageing conditions with the dotted lines showing the fitting of the 1-D Fickian diffusion
model in Equation (1). The 80
×
80
×
3.9 mm specimen geometry was selected to allow
direct comparison with the results for the neat polymer plates presented in the previous
section. Similarly, Figure 8shows the results for the long-term moisture absorption mea-
surements on the WF1273 laminates. In both cases, a similar pattern of weight increase with
time can be observed, as seen in the respective polymer matrices. Under all conditions, the
composites exhibited an initial Fickian-like weight increase approaching a plateau equilib-
rium level followed by a further slow linear (with sqrt time) weight increase at longer times.
Composite diffusivity and weight gain values obtained experimentally are compared in
Table 4with diffusivity and weight gain values predicted from the neat polymer matrix
ageing values. The average matrix weight fraction of 0.29 was used for the evaluation
of the predicted equilibrium moisture gain. The same weight fraction was converted to
a volume fraction value of 0.52 for use in Equation (2). In all cases, the experimentally
obtained diffusivity and weight gain values were higher than the theoretical predictions.
In most cases, the measured and predicted Fickian equilibrium weight gain values were
relatively close. Given the uncertainty in the measured values generated by the possible
overlap of the second, slower, observed moisture absorption phenomenon of weight up-
take in the polymers, it seems reasonable to state that the predicted and measured values
were equivalent within the experimental error. However, the diffusivity values showed a
very different behaviour. In this case, the measured composite diffusion coefficients were
significantly greater (approximately ×4) than those predicted by Equation (2).
Three possible reasons for such deviations have been suggested by Karbhari and
Zhang [
41
]. These include relaxation of elastic forces induced by the cross-linked network
after the initial weight-gain plateau, and (or) wicking at the interface, and (or) the presence
of manufacturing voids and the likelihood of them being increased by ageing. Furthermore,
the degradation of the edge sealant during ageing is a common issue in the ageing of com-
posites. According to the literature, simplistic 1-D diffusion models can lead to erroneous
results, especially when removal of the sealant occurs. More complex diffusion models
may thus be employed to understand the full effect of ageing when changing specimen
geometry [24].

J. Compos. Sci. 2024,8, 214 11 of 18
J. Compos. Sci. 2024, 8, x FOR PEER REVIEW 12 of 19
Figure 7. Weight gain vs. ageing time plot for AF1260 composite specimens under different ageing
conditions (doed lines show fiing of Equation (1)).
Table 4. Experimental vs. predicted composite diffusivity and weight gain values.
Composite
Reference
Ageing
Condition
Equilibrium
Prediction (%)
Measured
Equilibrium (%)
Diffusivity
Prediction
(10−6 mm2/s)
Measured
Diffusivity
(10−6 mm2/s)
AF1260
100% RH 0.14 0.16 0.16 0.60
23 °C DI 0.15 0.20 0.17 1.50
50 °C DI 0.22 0.32 0.57 2.20
WF1273
100% RH 0.19 0.20 0.17 0.75
23 °C DI 0.22 0.22 0.19 0.90
50 °C DI 0.32 0.35 0.75 2.20
Figure 8. Weight gain vs. ageing time plot for WF1273 composite specimens under different ageing
conditions (doed lines show fiing of Equation (1)).
0
0.1
0.2
0.3
0.4
0 20406080100
Weight Gain (%)
Time (√h)
50°C Water
23°C Water
100% RH
0
0.1
0.2
0.3
0.4
0.5
0 20406080100
Weight Gain (%)
Time (√h)
50°C Water
23°C Water
100% RH
Figure 7. Weight gain vs. ageing time plot for AF1260 composite specimens under different ageing
conditions (dotted lines show fitting of Equation (1)).
J. Compos. Sci. 2024, 8, x FOR PEER REVIEW 12 of 19
Figure 7. Weight gain vs. ageing time plot for AF1260 composite specimens under different ageing
conditions (doed lines show fiing of Equation (1)).
Table 4. Experimental vs. predicted composite diffusivity and weight gain values.
Composite
Reference
Ageing
Condition
Equilibrium
Prediction (%)
Measured
Equilibrium (%)
Diffusivity
Prediction
(10−6 mm2/s)
Measured
Diffusivity
(10−6 mm2/s)
AF1260
100% RH 0.14 0.16 0.16 0.60
23 °C DI 0.15 0.20 0.17 1.50
50 °C DI 0.22 0.32 0.57 2.20
WF1273
100% RH 0.19 0.20 0.17 0.75
23 °C DI 0.22 0.22 0.19 0.90
50 °C DI 0.32 0.35 0.75 2.20
Figure 8. Weight gain vs. ageing time plot for WF1273 composite specimens under different ageing
conditions (doed lines show fiing of Equation (1)).
0
0.1
0.2
0.3
0.4
0 20406080100
Weight Gain (%)
Time (√h)
50°C Water
23°C Water
100% RH
0
0.1
0.2
0.3
0.4
0.5
0 20406080100
Weight Gain (%)
Time (√h)
50°C Water
23°C Water
100% RH
Figure 8. Weight gain vs. ageing time plot for WF1273 composite specimens under different ageing
conditions (dotted lines show fitting of Equation (1)).
Table 4. Experimental vs. predicted composite diffusivity and weight gain values.
Composite
Reference
Ageing
Condition
Equilibrium
Prediction (%)
Measured
Equilibrium (%)
Diffusivity
Prediction
(10−6mm2/s)
Measured
Diffusivity
(10−6mm2/s)
AF1260
100% RH 0.14 0.16 0.16 0.60
23 ◦C DI 0.15 0.20 0.17 1.50
50 ◦C DI 0.22 0.32 0.57 2.20
WF1273
100% RH 0.19 0.20 0.17 0.75
23 ◦C DI 0.22 0.22 0.19 0.90
50 ◦C DI 0.32 0.35 0.75 2.20

J. Compos. Sci. 2024,8, 214 12 of 18
3.3. Dynamic Mechanical Analysis
3.3.1. DMA of the VE Polymer Matrices
The DMA results for the storage and loss moduli of SM60 vinyl ester polymers of
DION 1260 and DION 1273 are compared in Figure 9. Both polymers exhibited a single
glass transition in the temperature range studied. The glass transition temperature (as
characterised by the peak in the loss modulus curve) was found to be 101
◦
C for DION 1260
compared to a much lower value of 77
◦
C for DION 1273. The Tg values for all polymer and
composite samples after aging are summarised in Table 5. DION 1260 SM60 and SM100
polymers exhibited an approximate 10
◦
C Tg reduction for all aging times and conditions
listed in Table 2. Figure 10 compares the DMA loss modulus curves for DION1260 SM60
polymer after 180 days of aging under the three different conditions used in this study.
The Tg depression due to the exposure to, and absorption of, moisture for these polymers
can clearly be observed by the peak in loss modulus shifting to lower temperature. It is
interesting to note in Figure 11 that the reduction in Tg exhibited by these samples appears
to be fully reversible upon re-drying the aged samples.
J. Compos. Sci. 2024, 8, x FOR PEER REVIEW 13 of 19
3.3. Dynamic Mechanical Analysis
3.3.1. DMA of the VE Polymer Matrices
The DMA results for the storage and loss moduli of SM60 vinyl ester polymers of
DION 1260 and DION 1273 are compared in Figure 9. Both polymers exhibited a single
glass transition in the temperature range studied. The glass transition temperature (as
characterised by the peak in the loss modulus curve) was found to be 101 °C for DION
1260 compared to a much lower value of 77 °C for DION 1273. The Tg values for all poly-
mer and composite samples after aging are summarised in Table 5. DION 1260 SM60 and
SM100 polymers exhibited an approximate 10 °C Tg reduction for all aging times and
conditions listed in Table 2. Figure 10 compares the DMA loss modulus curves for
DION1260 SM60 polymer after 180 days of aging under the three different conditions used
in this study. The Tg depression due to the exposure to, and absorption of, moisture for
these polymers can clearly be observed by the peak in loss modulus shifting to lower tem-
perature. It is interesting to note in Figure 11 that the reduction in Tg exhibited by these
samples appears to be fully reversible upon re-drying the aged samples.
Figure 9. DMA results for un-aged DION 1260 and DION 1273 polymers.
Table 5. DMA-determined (loss modulus peak) Tg of polymers and composites.
Sample Glass Transition Temperature (°C)
0 Days
(=0 √h)
30 Days
(≈27 √h)
90 Days
(≈46 √h)
180 Days
(≈66 √h)
270 Days
(≈80 √h)
Re-dried
(≈112 √h)
1260 SC RH 92 90 101
1260 SC 23 101 92 90 89.5 101
1260 SC 50 90 90.5 90 102
1260 PC RH 94 93
1260 PC 23 99 94 93
1260 PC 50 93 92 93
1273 SC RH 83
1273 SC 23 77 82
1273 SC 50 88
AF1260 RH 107 102 100.5
AF1260 23 102 100
AF1260 50 98 98
0
100
200
300
400
500
600
1
10
100
1000
25 50 75 100 125 150
Loss Modulus E'' (MPa)
Storage Modulus E' (MPa)
Te mp e ra tu r e ( °C)
1260 E'
1273 E'
1260 E''
1273 E''
Figure 9. DMA results for un-aged DION 1260 and DION 1273 polymers.
Table 5. DMA-determined (loss modulus peak) Tg of polymers and composites.
Sample
Glass Transition Temperature (◦C)
0 Days
(=0 √h)
30 Days
(≈27 √h)
90 Days
(≈46 √h)
180 Days
(≈66 √h)
270 Days
(≈80 √h)
Re-Dried
(≈112 √h)
1260 SC RH 92 90 101
1260 SC 23 101 92 90 89.5 101
1260 SC 50 90 90.5 90 102
1260 PC RH 94 93
1260 PC 23 99 94 93
1260 PC 50 93 92 93
1273 SC RH 83
1273 SC 23 77 82
1273 SC 50 88
AF1260 RH 107 102 100.5
AF1260 23 102 100
AF1260 50 98 98
WF1273 RH 89 82 80 79
WF1273 23 81 79 78
WF1273 50 77 77 77.5

J. Compos. Sci. 2024,8, 214 13 of 18
J. Compos. Sci. 2024, 8, x FOR PEER REVIEW 14 of 19
WF1273 RH 89 82 80 79
WF1273 23 81 79 78
WF1273 50 77 77 77.5
This implies that the majority of the Tg reduction can be aributed to plasticisation—
reversible degradation [11,28,31], especially for specimens aged at 23 °C and 100% RH.
This is in correlation with the moisture sorption behaviour of the specimens, which exhib-
ited a continuously increasing weight paern, indicative of matrix relaxation through
plasticisation. However, narrowing of the loss modulus peak upon ageing under either of
the three ageing conditions may be a hint of secondary cross-linking. In particular, as
shown in Figure 10, the narrowing of the loss modulus peak followed the intensity of the
ageing conditions. It was the broadest at 100% RH and the narrowest at 50 °C. Such
changes may be indicative of anti-plasticisation effects starting to take place after room-
temperature ageing, and being more apparent at a higher temperature. Moreover, these
were denoted by the weight loss of the samples upon ageing and their discolouration,
which was mildest at 23 °C and 100% RH and more amplified at 50 °C, which is in line
with the observation regarding anti-plasticisation effects. Any increased cross-linking in
these samples did not result in a significant Tg increase, but anti-plasticisation effects may
be nonetheless counteracted by subsequent moisture-induced degradation. It is possible
that further ageing would have resulted in amplified anti-plasticisation effects and a
higher Tg.
Figure 10. DMA loss modulus curve for DION 1260 polymer aged for 180 days under different con-
ditions.
0
100
200
300
400
500
25 50 75 100 125 150
Loss Modulus E'' (MPa)
Te mp er at u re ( °C)
Dry
23°C
50°C
RH
Figure 10. DMA loss modulus curve for DION 1260 polymer aged for 180 days under different conditions.
J. Compos. Sci. 2024, 8, x FOR PEER REVIEW 15 of 19
Figure 11. DMA loss modulus curves for aged and re-dried neat DION 1260 polymer specimens.
Gravimetric and DMA results for DION 1260 SM60 vinyl ester polymer can be sum-
marised as follows. For specimens aged at 23 °C and 100% RH, plasticisation/relaxation
was the dominating moisture-induced effect. This was manifested by the continuous in-
crease in the weight of the specimens and was confirmed by a Tg decrease as a function
of ageing. The majority of degradation was indeed physical and was confirmed upon re-
drying the specimens and measuring their Tg, which remained close to the Tg of the un-
aged specimens. However, chemical ageing did take place in the form of leaching. Leach-
ing effects were apparent, as indicated by the discolouration and weight loss of the aged
specimens. Leaching seemed to have introduced secondary cross-linking, indicated by the
narrowing of the loss modulus curve. The laer effect, along with discolouration, was
more pronounced for the specimens aged at 50 °C. The weight uptake vs. ageing behav-
iour of the plate aged at 50 °C resembled Fickian diffusion, and relaxation effects were not
apparent. Despite the aainment of a Fickian curve, DMA results during ageing showed
a notable Tg decrease—plasticisation, whereas DMA results after ageing and re-drying
indicated a small Tg increase and narrowing of the loss modulus curve, which may imply
cross-linking. It can thus be concluded that competing ageing effects took place at 50 °C.
Similar degradation did indeed take place under all ageing conditions, but evidently at a
slower rate, which is expected considering that 50 °C is an accelerated ageing environ-
ment.
The Tg of the un-aged DION 1273 SC60 polymer was measured by DMA to be 77 °C.
This was notably lower than that of DION SC60 (101 °C). Due to the developmental nature
of the DION 1273 resin, there was insufficient material available to carry out a full DMA
ageing study, and so DMA was only carried out on un-aged samples and upon the com-
pletion of ageing (530 days) and re-drying. The results for the DMA loss modulus Tg val-
ues for these aged and re-dried samples are also presented in Table 5. In all cases, the aged
and re-dried specimens exhibited a higher Tg than the un-aged samples. Tg values of 82
and 83 °C were found after ageing at 23 °C and 100% RH, and 88 °C after ageing at 50 °C.
This Tg increase upon ageing is a clear indication of moisture-induced cross-linking. Sec-
ondary cross-linking has been previously associated with anti-plasticisation effects
[29,42,43]. In this case, these were probable, indicated by the pronounced discolouration
of the specimens and their weight loss through leaching. Nevertheless, the DION 1273
polymer samples underwent a continuous weight increase during ageing, indicative of
relaxation through plasticisation, which is a form of physical ageing. It is thus likely that
0
100
200
300
400
500
25 50 75 100 125 150
Loss Modulus E'' (MPa)
Te mp er at u re ( °C)
Dry
23°C
50°C
RH
Figure 11. DMA loss modulus curves for aged and re-dried neat DION 1260 polymer specimens.
This implies that the majority of the Tg reduction can be attributed to plasticisation—
reversible degradation [
11
,
28
,
31
], especially for specimens aged at 23
◦
C and 100% RH. This
is in correlation with the moisture sorption behaviour of the specimens, which exhibited
a continuously increasing weight pattern, indicative of matrix relaxation through plasti-
cisation. However, narrowing of the loss modulus peak upon ageing under either of the
three ageing conditions may be a hint of secondary cross-linking. In particular, as shown
in Figure 10, the narrowing of the loss modulus peak followed the intensity of the ageing
conditions. It was the broadest at 100% RH and the narrowest at 50
◦
C. Such changes may
be indicative of anti-plasticisation effects starting to take place after room-temperature
ageing, and being more apparent at a higher temperature. Moreover, these were denoted
by the weight loss of the samples upon ageing and their discolouration, which was mildest
at 23
◦
C and 100% RH and more amplified at 50
◦
C, which is in line with the observation
regarding anti-plasticisation effects. Any increased cross-linking in these samples did not
result in a significant Tg increase, but anti-plasticisation effects may be nonetheless coun-
teracted by subsequent moisture-induced degradation. It is possible that further ageing
would have resulted in amplified anti-plasticisation effects and a higher Tg.

J. Compos. Sci. 2024,8, 214 14 of 18
Gravimetric and DMA results for DION 1260 SM60 vinyl ester polymer can be sum-
marised as follows. For specimens aged at 23
◦
C and 100% RH, plasticisation/relaxation
was the dominating moisture-induced effect. This was manifested by the continuous in-
crease in the weight of the specimens and was confirmed by a Tg decrease as a function
of ageing. The majority of degradation was indeed physical and was confirmed upon
re-drying the specimens and measuring their Tg, which remained close to the Tg of the
un-aged specimens. However, chemical ageing did take place in the form of leaching.
Leaching effects were apparent, as indicated by the discolouration and weight loss of the
aged specimens. Leaching seemed to have introduced secondary cross-linking, indicated by
the narrowing of the loss modulus curve. The latter effect, along with discolouration, was
more pronounced for the specimens aged at 50
◦
C. The weight uptake vs. ageing behaviour
of the plate aged at 50
◦
C resembled Fickian diffusion, and relaxation effects were not
apparent. Despite the attainment of a Fickian curve, DMA results during ageing showed
a notable Tg decrease—plasticisation, whereas DMA results after ageing and re-drying
indicated a small Tg increase and narrowing of the loss modulus curve, which may imply
cross-linking. It can thus be concluded that competing ageing effects took place at 50
◦
C.
Similar degradation did indeed take place under all ageing conditions, but evidently at a
slower rate, which is expected considering that 50
◦
C is an accelerated ageing environment.
The Tg of the un-aged DION 1273 SC60 polymer was measured by DMA to be 77
◦
C.
This was notably lower than that of DION SC60 (101
◦
C). Due to the developmental
nature of the DION 1273 resin, there was insufficient material available to carry out a
full DMA ageing study, and so DMA was only carried out on un-aged samples and
upon the completion of ageing (530 days) and re-drying. The results for the DMA loss
modulus Tg values for these aged and re-dried samples are also presented in Table 5.
In all cases, the aged and re-dried specimens exhibited a higher Tg than the un-aged
samples. Tg values of 82 and 83
◦
C were found after ageing at 23
◦
C and 100% RH,
and 88
◦
C after ageing at 50
◦
C. This Tg increase upon ageing is a clear indication of
moisture-induced cross-linking. Secondary cross-linking has been previously associated
with anti-plasticisation effects [
29
,
42
,
43
]. In this case, these were probable, indicated by
the pronounced discolouration of the specimens and their weight loss through leaching.
Nevertheless, the DION 1273 polymer samples underwent a continuous weight increase
during ageing, indicative of relaxation through plasticisation, which is a form of physical
ageing. It is thus likely that competing relaxation and secondary cross-linking effects were
apparent, whereby relaxation dominated but was reversed upon re-drying the specimens.
3.3.2. DMA of the GF-VE Laminates
The DMA results for the loss moduli glass transition temperatures of AF1260 and
WF1273 laminates are summarised in Figure 12 (with values presented in Table 5). Both
composite systems exhibited a similar pattern of Tg reduction with moisture ageing, which
plateaued out after approximately 90 days (
≈
46
√
h). This was after the moisture absorp-
tion level attained the Fickian equilibrium level indicated in Figures 7and 8. For the AF1260
laminate, the Tg was approximately 6
◦
C higher than that of the neat matrix. However,
for the WF1273 laminate, the increase was much greater, at 12
◦
C. The addition of rein-
forcements has been frequently observed to result in an increase in the measured polymer
matrix Tg [
44
–
47
]. This is generally thought to be a reflection of the restriction of motion
of the polymer matrix molecules in the vicinity of the fibre surface due to the interfacial
interactions between fibre and polymer. A direct measure of the interfacial interaction
in these two fibre-matrix systems has been reported. Single-fibre micro-bond testing of
the interfacial shear strength of the WF1273 system was reported at 43.5 MPa, which was
significantly higher than the 35.3 MPa found for the AF1260 system [21,22].

J. Compos. Sci. 2024,8, 214 15 of 18
J. Compos. Sci. 2024, 8, x FOR PEER REVIEW 16 of 19
competing relaxation and secondary cross-linking effects were apparent, whereby relaxa-
tion dominated but was reversed upon re-drying the specimens.
3.3.2. DMA of the GF-VE Laminates
The DMA results for the loss moduli glass transition temperatures of AF1260 and
WF1273 laminates are summarised in Figure 12 (with values presented in Table 5). Both
composite systems exhibited a similar paern of Tg reduction with moisture ageing,
which plateaued out after approximately 90 days (≈46 √h). This was after the moisture
absorption level aained the Fickian equilibrium level indicated in Figures 7 and 8. For
the AF1260 laminate, the Tg was approximately 6 °C higher than that of the neat matrix.
However, for the WF1273 laminate, the increase was much greater, at 12 °C. The addition
of reinforcements has been frequently observed to result in an increase in the measured
polymer matrix Tg [44–47]. This is generally thought to be a reflection of the restriction of
motion of the polymer matrix molecules in the vicinity of the fibre surface due to the in-
terfacial interactions between fibre and polymer. A direct measure of the interfacial inter-
action in these two fibre-matrix systems has been reported. Single-fibre micro-bond test-
ing of the interfacial shear strength of the WF1273 system was reported at 43.5 MPa, which
was significantly higher than the 35.3 MPa found for the AF1260 system [21,22].
Under the range of ageing conditions studied, the Tg change of the AF1260 composite
was relatively small, with the greatest effect observed for samples submerged at 50 °C.
Moreover, the trends in Tg for the AF1260 composite tracked the changes observed in the
1260 matrix polymer, so presumably the composite changes may also be aributed to
moisture-induced plasticisation. The Tg depression upon ageing was slightly greater for
WF1273 than that of AF1260. The fact that all of the aged composite samples maintained
a significantly higher Tg compared to the unreinforced polymer may be taken as some
evidence that the level of interfacial fibre–polymer interaction, which resulted in the
higher Tg observed in the composites, was not significantly degraded by the long-term
exposure to moisture over the term of these measurements.
Figure 12. DMA-determined Tg values for laminates aged under different conditions.
4. Conclusions
In conclusion, this study enabled a comparison between two vinyl ester matrices and
their composites:
70
80
90
100
110
0 20406080
Tg (°C)
Exposure Time (√h)
A1260 RH
A1260 23°C
A1260 50°C
W1273 RH
W1273 23°C
W1273 50°C
Figure 12. DMA-determined Tg values for laminates aged under different conditions.
Under the range of ageing conditions studied, the Tg change of the AF1260 composite
was relatively small, with the greatest effect observed for samples submerged at 50
◦
C.
Moreover, the trends in Tg for the AF1260 composite tracked the changes observed in
the 1260 matrix polymer, so presumably the composite changes may also be attributed to
moisture-induced plasticisation. The Tg depression upon ageing was slightly greater for
WF1273 than that of AF1260. The fact that all of the aged composite samples maintained
a significantly higher Tg compared to the unreinforced polymer may be taken as some
evidence that the level of interfacial fibre–polymer interaction, which resulted in the higher
Tg observed in the composites, was not significantly degraded by the long-term exposure
to moisture over the term of these measurements.
4. Conclusions
In conclusion, this study enabled a comparison between two vinyl ester matrices and
their composites:
•
DION 1260—a baseline VE, and DION 1273—a developmental VE, polymers under
long-term wet environments. A comparison between different curing conditions for
DION 1260 was also included.
•AF1260—a baseline laminate and WF1273—a developmental laminate.
Generally, the DION 1260 matrix was found to be less hydrophilic than DION 1273.
Overall, wet ageing at 23
◦
C and moisture exposure (100% RH at room temperature) had
a similar effect on both matrices and were found to be milder than wet ageing at 50
◦
C,
which was characterised by a higher moisture gain and more pronounced degradation.
Submersion in DI water at 50
◦
C was used as a means of accelerating ageing. However, the
degradation mechanisms activated by this elevated temperature in combination with the
direct water contact were irreversible in most cases. Chemical degradation in the forms of
hydrolysis, leaching and secondary cross-linking was induced by the elevated temperature
environment, which was not always apparent in room-temperature conditioning. It is
possible that such effects may be evident when ageing composites in humid environments
at average normal outdoor temperatures in the long term or in warmer climates. This study
further validated that oxygen intrusion in the polymerising vinyl ester medium and styrene
loss can alter the chemistry of the polymer. Different uptake kinetics were exhibited by
DION 1260 open-mould specimens, which were characterised by a higher moisture gain
and a more anomalous water uptake behaviour. On the other hand, the effect of a higher

J. Compos. Sci. 2024,8, 214 16 of 18
cure temperature of 100
◦
C did not have a great effect on the uptake kinetics of the DION
1260, provided it was cured in a sealed mould.
DMA measurements showed Tg depression for both polymers tested in their wet state.
However, Tg was fully recovered upon re-drying the specimens, and most of the reduction
was attributed to plasticisation. For re-dried specimens, there was a hint suggesting
secondary cross-linking, primarily for elevated temperature conditioning. DMA indicated
plasticisation of DION 1260 open-mould specimens aged at 23
◦
C, suggested by broadening
of the loss modulus shoulder, which was also shifted at lower temperature values. On
the other hand, depression of the loss modulus shoulder was evident in DION 1260 open-
mould specimens aged at 50
◦
C, indicative of secondary cross-linking of the lower-Tg,
presumably styrene-rich phase of vinyl ester. Secondary cross-linking was also indicated
for re-dried DION 1273 polymer, as denoted by a notable Tg increase.
It was found that the Tg of AF1260 laminate in its un-aged state was 6
◦
C higher than
that of its neat polymer matrix, whereas the Tg of WF1273 was 12
◦
C higher than that
of its neat polymer matrix. Although a definitive answer cannot be provided on what
may be causing such a Tg increase when adding glass fibres to the VE polymers, it can be
presumed that this is an interfacial effect. It is possible that certain chemical reactions take
place between the glass, the sizing and in turn the matrix, which result in an increase in
Tg. This has been shown to correlate with a higher measured interfacial strength in the
WF1273 system. Under the range of ageing conditions studied, the change in the Tg of
the AF1260 composite was relatively small, with the greatest effect observed for samples
submerged at 50
◦
C. Moreover, the trends in Tg for the AF1260 composite tracked the
changes observed in the 1260 matrix polymer, which were attributed to moisture-induced
plasticisation. The Tg depression upon ageing was slightly greater for WF1273 than that
of AF1260. The fact that all of the aged composite samples maintained a significantly
higher Tg compared to the unreinforced polymer was taken as some evidence that the level
of interfacial fibre–polymer interaction, which resulted in the higher observed Tg in the
composites, was not significantly degraded by the long-term exposure to moisture over the
term of these measurements.
Author Contributions: Conceptualization, J.T.; methodology, G.X. and J.T.; validation, J.T. and G.X.;
formal analysis, G.X. and J.T.; investigation, G.X.; resources, J.T. and G.X.; data curation, J.T. and G.X.;
writing—original draft preparation, J.T.; writing—review and editing, J.T. and G.X.; visualization, J.T.
and G.X.; supervision, J.T.; project administration, J.T.; funding acquisition, J.T. All authors have read
and agreed to the published version of the manuscript.
Funding: This research was partially funded by the DACOMAT project from the European Union’s
Horizon 2020 research and innovation programme 5 under GA No. 761072.
Data Availability Statement: Data is unavailable due to privacy or ethical restrictions.
Acknowledgments: The authors acknowledge Polynt Composites Ltd. and 3B Fibreglass for supply-
ing the materials used in this study.
Conflicts of Interest: The authors declare no conflicts of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or
in the decision to publish the results.
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