3.2. Quantitative Analysis of the Curing Process Using FTIR
The curing of epoxy asphalt was analyzed by using FTIR spectroscopy to track the change in the characteristic peak (915 cm
−1) of the epoxy group in epoxy asphalt at 60 °C with changes in the curing time [
21]. FTIR tests were carried out on epoxy asphalt samples with different curing times.
Figure 3 shows that the intensity of the epoxy group peak decreased as the curing time increased and stabilized after 69 h. The concentration of epoxy groups decreased as the curing time increased, reflecting the participation of the epoxy group in the curing reaction. Therefore, the curing process of epoxy asphalt can be quantitatively analyzed by tracking the change in the concentration of epoxy groups [
20].
The two commonly used methods for FTIR quantitative analysis are peak height and peak area analyses. A vertical line is drawn between the peak vertex and the horizontal axis. The peak height is the distance between the vertex and the intersection of the vertical line and the baseline. The area enclosed by the curve of the absorbance peak and the baseline is the peak area. Peak area analysis was used in this study. To eliminate the influence of the coating thickness of the infrared samples on the absorbance peak area, an internal standard method was used to determine the variation in the concentration of characteristic groups with the curing time. The characteristic peak of a group that does not participate in the curing reaction (the benzene ring group at 830 cm
−1) was selected as a reference peak. The ratio of the peak area of the epoxy group (915 cm
−1) to that of the benzene ring group (830 cm
−1),
A915/
A830, was used to represent the variation in the epoxy group concentration [
24]. FTIR was used to quantitatively analyze the epoxy group concentration at different curing times, and then the conversion rate of epoxy group with curing time was calculated based on the epoxy group concentration to indirectly evaluate the curing degree of epoxy asphalt. The conversion rate (
α) of the epoxy groups in the curing reaction was calculated as follows:
where (
A’915/
A’830)
t and (
A915/
A830)
0 denote the aforementioned area ratio at curing times of
t and before the start of the curing reaction, respectively.
To accurately obtain the area of the absorbance peak of the chemical functional group, Peakfit (v4.12, Systat Software Inc, San Jose, CA, USA) was used in this study to fit the FTIR sample spectra [
25]. The Peakfit software performed a baseline calibration on the selected wavenumber region and then initially fitted the approximate position and number of peaks based on the first derivative of each spectral line. Multiple fittings were achieved using Gaussian peaks. After the residual was minimized, the absorbance peak area of the subpeak was quantitatively obtained. The fitting results of the absorbance peaks are shown in
Figure 4.
The value of α at each curing time was calculated using Equation (1). The curing reaction degree of epoxy asphalt,
Gα, was defined as follows:
where
αt is the conversion rate of epoxy groups at a curing time t, and
α144 is the corresponding final stable conversion rate at a curing time of 144 h.
The calculated
α and
Gα values are shown in
Table 4. It can be seen that
α increased with time: at 69 h,
α reached 0.504, and the curing reaction degree exceeded 90%;
α eventually stabilized at 0.55.
To correlate
α with the tensile strength of epoxy asphalt,
Gα was compared to the curing degree of epoxy asphalt as represented by the tensile strength,
Gs:
where
St and
S120 are the tensile strengths of epoxy asphalt at curing times
t and 120 h, respectively, where
S120 is considered the ultimate stable tensile strength. The results for
Gα and
Gs are compared in
Figure 5.
Figure 5a shows similar increasing trends for
Gα and
Gs, indicating that the strength of epoxy asphalt is mainly derived from the epoxy resin crosslinked network. Since there is no direct correspondence between
α and the curing time for the development of the tensile strength, polynomial regression was used to fit
Gα, as shown in
Figure 5b, and to obtain
Gα values at 12, 24, 36, 48, 72, 96, and 120 h (
Table 5). A correlation analysis was conducted on
Gs and
Gα. The results in
Figure 5c show that
Gα and
Gs are linearly correlated, with an
R2 greater than 0.99, indicating that
Gα and
Gs are highly correlated. Thus,
α can be used to indirectly evaluate the tensile strength and curing degree of epoxy resin.
3.3. Morphological Analysis
The curing of epoxy asphalt was further analyzed by tracking the micromorphological changes of epoxy asphalt with different curing times using LSCM. Fluorescence images of the epoxy asphalt samples cured at 60 °C for different times are shown in
Figure 6. The fluorescent and dark phases correspond to the epoxy resin and the base asphalt, respectively. The epoxy resin, the curing agent, and asphalt were mixed and then stirred by shearing to evenly disperse the three components, as shown in
Figure 6a. After mixing, the epoxy resin and the curing agent underwent a polymerization reaction, where the homogeneous phase of asphalt microparticles was polymerized into larger particles to form a two-phase system with an island-like structure, with asphalt as the dispersed phase and epoxy resin as the continuous phase, as shown in
Figure 6b–e. As the curing reaction proceeded (
Figure 6e–h), an epoxy resin crosslinked network gradually formed. The crosslinking density then continuously increased, and the large asphalt particles were gradually divided into small particles by the epoxy resin network. At 72 h, the asphalt particle diameter gradually stabilized at an average particle diameter of approximately 20 μm (
Figure 6h–i). The particles were evenly dispersed in the epoxy resin, forming a stable two-phase 3D network structure. This result is consistent with that observed by Liu et al. [
26], that is, the diameter of asphalt particles first increases and then decreases during the epoxy asphalt curing process.
Two definitions are introduced to quantify the dispersion of asphalt particles in the epoxy resin crosslinked network: homogeneity (also known as overall uniformity) and dispersion. Homogeneity refers to the amplitude of concentration fluctuations of the dispersed phase, and dispersion refers to the size distribution range of the particulate phase. The image analysis program ImageJ was used to determine and statistically analyze the asphalt particle sizes in epoxy asphalt [
27].
Figure 7 shows the asphalt particle size distribution in epoxy resin for different curing times. In the initial curing stage, the distribution range of the asphalt particle diameters increased from 0–50 μm to 0–100 μm. After 12 h, the particle size distribution range gradually decreased and returned to a stable normal distribution in the 0–40 μm range at 96 h. As the curing reaction proceeded, the diameter distribution changed from narrow to wide, then retracted, and stabilized at 0–40 μm, indicating that the asphalt particles tended to become evenly distributed in the epoxy resin as the curing time increased.
According to Liu et al. [
23], the changes in the dispersity of the asphalt particles with the curing time were determined by calculating
Dn,
Dw, and
PDI of the asphalt particles using the following equations:
where
ni is the number of particles with a diameter
Di. The values of
Dn,
Dw, and
PDI of the asphalt particles for different curing times are given in
Table 6. In the initial curing stage,
Dn of the asphalt particles increased with the curing reaction time. After 12 h,
Dn gradually decreased and stabilized at approximately 18 μm. The
PDI first increased and then decreased, indicating relatively low uniformity of the epoxy asphalt dispersion in the initial curing stage. The formation of the crosslinked network resulted in the gradual division of large asphalt particles into small particles, increasing the dispersion uniformity.
The evolution of the microscopic structural characteristics of the 3D epoxy resin crosslinked network during the curing process was investigated by performing SEM on epoxy asphalt samples (after asphalt etching) with different curing times.
Figure 8 shows the SEM images of the epoxy resin for different curing times (12 h, 24 h, 36 h, 48 h, 72 h, and 96 h) at 300x magnification. The etched epoxy asphalt contained many pores of various sizes, indicating that the asphalt was wrapped by the epoxy resin network and that the epoxy resin network was filled with asphalt. As shown in
Figure 8a, the crosslinked epoxy asphalt network did not form before 12 h. The crosslinked network gradually formed after 24 h, as shown in
Figure 8b. The remaining pores had irregular shapes. In the earlier LSCM images, the pores were approximately round, showing that the pore shape depends on the curing degree of the epoxy resin. At the early curing stage, the epoxy resin had a low degree of crosslinking and was relatively soft; the sample pores after etching were large and deformed into irregular shapes at room temperature during drying. After 36 h (when
α reached approximately 0.36), the crosslink density of epoxy resin continuously increased, the pores became smaller and denser, and the epoxy resin formed a relatively stable and compact 3D network skeleton (
Figure 8c–f), in agreement with the LSCM images. These phenomena reflect that in the early curing stage, the initially epoxy resin network was weak and easily deformable. As the curing reaction proceeded, the continuous increase in the crosslink density of the epoxy resin strengthened the 3D network structure, while also lowering the ductility of the epoxy asphalt. These results are consistent with the continuous increase in the tensile strength and the decrease in the elongation at break of the epoxy asphalt with increasing curing time.
A microscopic morphology analysis showed that the epoxy asphalt curing process is dominated by the reaction of the epoxy resin oligomer. In this process, heterogeneous microgels are first generated in the system, then gradually form large gels (
Figure 8a), and finally form a gel-like polymer (
Figure 8b). As the reaction continues, the crosslink density continuously increases, forming a highly crosslinked polymer with a 3D network structure, as shown in
Figure 8f. From a molecular chemistry perspective, the epoxy asphalt curing process is essentially the polymerization of epoxy monomers into single chains that are repolymerized into a crosslinked network, which gradually densifies [
28,
29]. During the curing reaction of epoxy asphalt, the epoxy monomers are polymerized into single chains, and the single chains are repolymerized into crosslinked network. The molecular weight of the epoxy resin continuously increases, and the compatibility between the epoxy resin and the thermoplastic asphalt decreases, which promotes phased development. The dispersed asphalt particles are surrounded by a crosslinked network of epoxy resin, forming a two-phase 3D network with each other. A schematic of this process is shown in
Figure 9. The results of the micromorphological changes that are shown in
Figure 7 and
Figure 8 are consistent with this process.
In summary, an analysis of the test data shows that the main mechanism of epoxy asphalt strength formation is the curing of epoxy asphalt into a stable 3D network. The curing process can be divided into three stages: an initial stage, an intermediate stage, and a late stage. In the initial stage, Gα is less than 27%, no network exists, the reactivity is high, and the material strength increases rapidly. In the intermediate stage, Gα is between 27% and 65%, a 3D network starts to form, and the network skeleton is relatively soft and easy to deform. Thus, the epoxy asphalt should not be disturbed to avoid affecting the increase in the epoxy resin strength. In the late stage, Gα is greater than 65%, and the skeleton gradually densifies into a stable two-phase 3D crosslinked epoxy resin network containing asphalt particles. This dense uniform 3D network in the cured epoxy asphalt increases the deformation of the epoxy resin. The asphalt fills in and protects the epoxy resin against aging, and the crosslinked network formed by the polymerization of the epoxy resin changes the thermoplasticity of the asphalt, resulting in a high strength and good thermosetting properties.