3.1. Moisture Absorption Tests on AG-80 Resins
The saturated moisture absorption tests were conducted in strict accordance with the ASTM D5229/D5229M-14 standards [
11]. Initially, the specimens were placed in an environment where the temperature was maintained at 22 °C and the relative humidity was 58%. After that, they were transferred to another environment with a temperature of 71 °C and a relative humidity of 85%. The specimens were kept in this environment until moisture absorption equilibrium was achieved.
Through the water-immersion moisture absorption tests, the moisture absorption levels of the two resins at different time points were measured. Fick’s diffusion law [
12] is expressed as follows:
where
and
represent the moisture contents of the specimen at time
and at equilibrium, respectively;
is the diffusion coefficient; and
is the specimen thickness. By plotting
against
, the moisture absorption curve of the specimen was obtained (as shown in
Figure 1). From the horizontal extension line and the linear part of the curve,
and
could be determined, respectively, and the values are listed in
Table 1.
This difference results from their distinct molecular structures, as supported by the analysis of infrared spectra.
Figure 2 shows the infrared spectra of AG-80 resin, which indicate that BA9916-II shows significantly lower transmittance (more than 20% lower) than 5228A at approximately 1717 cm
−1, 3360 cm
−1, and 3465 cm
−1. The absorption peak at 1717 cm
−1 is attributed to the stretching vibration of the carbonyl (C=O) group. This implies that BA9916-II likely has a higher content of carbonyl-containing functional groups such as ketones or conjugated carbonyls. Meanwhile, the absorption peaks at 3360 cm
−1 and 3465 cm
−1 are related to the stretching vibrations of N-H or O-H groups, suggesting that BA9916-II contains more alcohol hydroxyl, phenolic hydroxyl, amine, or amide structures.
5228A has a relatively low density of polar functional groups. Polar groups such as hydroxyl (-OH), amino (-NH2), and carbonyl (C=O) groups exhibit strong affinity towards water molecules through hydrogen-bonding interactions. With a lower density of these polar groups, as also evidenced by the relatively higher transmittance in the corresponding infrared regions, there are fewer sites available for water molecule adsorption, resulting in a lower saturated moisture absorption rate. Additionally, the molecular chains of 5228A exhibit higher cross-linking density and a more compact packing arrangement. A high degree of cross-linking restricts the mobility of molecular chains and reduces the free volume within the material, thereby impeding the diffusion of water molecules and limiting moisture absorption.
In contrast, BA9916-II has more polar groups, as indicated by the strong infrared absorption in the regions related to polar functional groups. These additional polar groups provide more sites for hydrogen bonding with water molecules. Moreover, it has more flexible chains, which provide less hindrance to the movement of water molecules. This combination facilitates water diffusion and leads to higher moisture absorption. These results are consistent with previous studies on similar epoxy resins, such as the moisture absorption behavior reported by Xu et al. [
4] for T800/high-temperature epoxy composites.
3.2. DMA Tests on AG-80 Resins
Four loss factor curves were obtained (in
Figure 3), representing dry 5228A resin, wet 5228A resin, dry BA9916-II resin, and wet BA9916-II resin. Each curve exhibits a single peak characteristic that represents the glass transition process of the materials. The peak temperature of the dry 5228A resin curve is 219.90 °C and has a maximum loss factor of approximately 1.2. For the wet 5228A resin, the peak temperature rises to 222.22 °C, while the loss factor decreases to around 0.95. The dry BA9916-II resin curve has a peak temperature of 236.88 °C and a loss factor of about 0.7. In the case of the wet BA9916-II resin, the peak temperature drops to 229.33 °C, and the loss factor reaches the lowest value of approximately 0.6.
In the storage modulus curves (in
Figure 4), the onset temperature of the slope change reflects the starting point when the material’s properties begin to change significantly. The onset temperature of the dry 5228A resin is 203.15 °C. The wet 5228A resin shows two slope changes at 161.79 °C and 206.87 °C, respectively. Both the dry and wet BA9916-II resins have an onset temperature of 180.36 °C. Notably, for the storage modulus, the dry-state 5228A resin has a value around 10
6 kPa, with a significant increase in the wet state. In contrast, for BA9916-II resin, the change in storage modulus between the dry and wet states is not obvious, only showing a change in the onset temperature.
The loss modulus curves also have peak characteristics (in
Figure 5), where the peak temperature corresponds to the activated state of the internal molecular motion of the material. The dry 5228A resin has a peak temperature of 219.90 °C, with a loss modulus of approximately 190,000 kPa. For the wet 5228A resin, the peak temperature drops to 169.94 °C, and the loss modulus is about 150,000 kPa. The dry BA9916-II resin has a peak temperature of 180.36 °C, and the wet BA9916-II resin has a peak temperature of 148.81 °C, with both having a loss modulus of around 160,000 kPa.
In the dry state, the
of BA9916-II resin (236.88 °C) is significantly higher than that of 5228A resin (219.90 °C). This indicates that the molecular segments of BA9916-II resin require more energy for movement. Its molecular structure may be more rigid, with a higher degree of cross-linking, more hydrogen bonds, or other strong intermolecular forces [
13]. After moisture absorption, the
of 5228A resin shows a slight increase. This is due to the formation of hydrogen bonds between water molecules and polar groups, which enhances intermolecular forces. However, the decrease in loss factor is due to the lubricating effect of water molecules, which reduces direct friction between molecular chains. In contrast, the
of BA9916-II resin decreases, suggesting that water molecules act as a plasticizer, weakening the intermolecular forces and facilitating the movement of molecular segments.
The loss factor of dry 5228A resin is higher than that of BA9916-II resin, indicating that 5228A resin exhibits higher energy dissipation during deformation. This is because the internal friction between its molecular chains is relatively large, resulting in the movement of molecular chains being more disordered. After moisture absorption, the loss factors of both resins decrease. This is because the presence of water molecules reduces direct friction between molecular chains, thereby decreasing energy dissipation.
In the storage modulus curves, the onset temperature of dry 5228A resin is higher than that of dry BA9916-II resin, which is consistent with the difference reflected in the loss factor curves. This further demonstrates that 5228A resin starts to show significant property changes at a higher temperature, indicating better thermal stability. The wet 5228A resin shows two slope changes, which may imply that there are two different structural change processes inside the resin after moisture absorption. The first change may be related to the interaction between water molecules and some polar groups, while the second change may correspond to the overall change in the movement of molecular segments. In contrast, the onset temperature of BA9916-II resin remains the same in both dry and wet states, indicating that moisture absorption has little effect on the starting temperature at which its properties begin to change.
The significant increase in the storage modulus of 5228A resin after moisture absorption suggests that water molecules may interact with the resin matrix by enhancing the material’s ability to store elastic energy. The interaction between water molecules and the resin is complex. In 5228A resin, it is manifested as enhancing the intermolecular forces to a certain extent and increasing the storage modulus. However, this does not mean that the overall rigidity is enhanced; it only changes in terms of elastic energy storage. For BA9916-II resin, this interaction is mainly reflected in changing the forces between molecular chains, increasing its flexibility, and having a minor effect on elastic energy storage. For BA9916-II resin, the minimal change in storage modulus indicates that its molecular structure is relatively less affected by moisture absorption in terms of elastic energy storage. The change in onset temperature, nevertheless, still shows that moisture can influence the temperature at which the material’s viscoelastic properties start to change.
In the loss modulus curves, in the dry state, the peak temperature of the loss modulus of 5228A resin is higher than that of BA9916-II resin, which again proves that the molecular segments of 5228A resin require more energy to move. After moisture absorption, the peak temperatures of the loss modulus of both resins decrease, which is consistent with the change trend of
after moisture absorption, indicating that water molecules promote the movement of molecular segments. The peak loss modulus of dry 5228A resin is higher than that of the wet state and also higher than that of BA9916-II resin. This indicates that the internal molecular motion of dry 5228A resin is more intense during the glass transition process, resulting in greater energy dissipation. The loss moduli of dry and wet BA9916-II resins are similar, suggesting that moisture absorption has a minor effect on its energy dissipation during the glass transition process. The observed
values are higher than those of conventional DGEBA-based epoxy resins [
14], indicating the superior thermal stability of AG-80 epoxy resin.
3.3. Development of a Hygro-Thermal Constitutive Model for AG-80 Resin
In dynamic mechanical analysis (DMA) of resin materials, understanding the viscoelastic behavior under specific conditions is crucial. Given that the tests were conducted at a fixed frequency of 1 Hz, this section presents a generalized Maxwell-based constitutive model that accounts for the influence of temperature and moisture absorption on the resin’s mechanical properties. The generalized Maxwell model serves as the foundation for describing the viscoelastic characteristics of the resin. It comprises multiple Maxwell elements connected in parallel, in which each Maxwell element consists of a spring, representing elastic behavior, and a dashpot, representing viscous behavior, connected in series.
At a fixed angular frequency
rad/s, corresponding to a frequency of 1 Hz, the storage modulus
and loss modulus
of the resin can be described by the following equations:
where
is the instantaneous elastic modulus,
is the spring modulus of the
th Maxwell element, and
is the relaxation time of the
th Maxwell element.
To incorporate the effects of moisture absorption, we assume that the relationship between the modulus, moisture content, and temperature is as follows. The instantaneous elastic modulus
and the spring moduli
of each Maxwell element are functions of moisture content
and temperature
. They can be expressed as follows:
where
and
are the instantaneous elastic modulus and the spring modulus of the
th Maxwell element in the dry state, respectively.
and
are functions of moisture content
and temperature
. Polynomial forms can be used to represent, for example, the following functions:
The coefficients need to be determined by fitting the experimental data.
Regarding the relationship between relaxation time, moisture content, and temperature, the relaxation time
of the
th Maxwell element can be similarly expressed as follows:
where
is the relaxation time of the
th Maxwell element in the dry state, and
is a function of moisture content
and temperature
. The following polynomial form can also be used:
The least-squares method is applied to fit the 5228A resin constitutive model. First, experimental data of storage modulus, loss modulus, and loss factors are obtained from DMA tests under different temperature and moisture conditions. An objective function is defined to minimize the squared differences between experimental and predicted results. Initial values are assigned for unknown model parameters. An optimization algorithm iteratively adjusts these parameters until the objective function converges, yielding the fitted parameters. This behavior aligns with findings from Rooney et al. [
15], who reported similar temperature-dependent viscoelastic properties in 3D-printed resin composites.
3.4. Validation of Constitutive Model via 5228A/CCF300 DMA Tests
Following the establishment of the constitutive model for 5228A resin that accounts for temperature and moisture absorption effects, the model is extended to composite materials. A simple rule-of-mixtures approach is adopted to estimate the viscoelastic properties of the composites. This method offers a simple approach to combine the properties of the individual components to predict the overall behavior of the composite.
The storage modulus of the composite is calculated as a weighted sum of fiber and resin contributions. It is calculated as , where is the fiber volume fraction, is the modulus of fiber, and is the resin storage modulus determined by the resin-based model.
Since fibers do not contribute to energy dissipation, the composite’s loss modulus is primarily derived from the resin matrix and is expressed as , with being the resin loss modulus.
To assess the validity and predictive accuracy of the proposed 5228A resin constitutive model, DMA tests were conducted on four 5228A/CCF300 composite specimens with dimensions of 35 mm × 10 mm × 2 mm and a fiber volume fraction of 16.6667%. Tensile tests indicated that the 5228A resin had a tensile modulus of 3.22 GPa. Two specimens were in a dry state and two in a wet state. The CCF300 carbon fibers in these composites exhibited a tensile modulus of 235 GPa. The analysis of the resulting DMA curves serves as a basis for comparing the model predictions with experimental results. The DMA curves of one dry-state and one wet-state specimens are shown in
Figure 6 and
Figure 7.
The comparison results between the model predictions and the measured values are shown in
Figure 8. As can be seen from this figure, for the 5228A resin, the storage modulus
decreases with the increase in temperature. The model prediction results are consistent with the experimental results. The viscoelastic model derived from the generalized Maxwell model can well reflect the properties of the material as an elastic solid and a viscous fluid.