Preparation of itaconic acid-modified epoxy resins and comparative study on the properties of it and epoxy acrylates

2.3.1 Structural characterization

Fourier transform infrared (FT-IR): the structure of the prepared samples was analyzed using an FT-IR spectrometer (Nicolet iS20, Thermo Scientific, USA), with a spectral range of 4,000–500 cm⁻¹. Nuclear magnetic resonance (NMR) spectrometry: for NMR analysis, 5.0–10.0 mg of the sample to be tested was accurately weighed and dissolved in 0.5 mL of deuterated chloroform. The test frequency was set at 400 MHz. Gel permeation chromatography (GPC): tetrahydrofuran at a concentration of approximately 5 mg·mL⁻¹ was employed as the test solvent. The system was operated at a pressure of 1.88 MPa, with the column oven and detector temperature set at 40°C and the mobile phase flow rate at 1.0 mL·min⁻¹.

2.3.2 Gel content test

Approximately 0.5–1.0 g of cured resin was taken, and its mass was recorded as “m ₁.” Acetone served as the solvent. The cured resin was placed in a Soxhlet extractor and extracted until a constant weight was achieved. After extraction, it was removed and dried in an oven, and its mass, denoted as “m ₂,” was measured. The gel content (GC) of the cured resin was calculated using the following formula 1:

where m ₁ is the mass of the resin before extraction (g) and m ₂ is the mass of the resin after extraction (g).

2.3.3 Dynamic mechanical analysis

Dynamic thermomechanical analysis (DMA) was performed using a DMA-Q800 analyzer. Test conditions: single cantilever beam test mode was selected, the test frequency was 2 Hz, the heating rate was 5°C·min⁻¹, and the test temperature range was 30–200°C.

2.3.4 Thermogravimetric (TGA) analysis

Thermal stability is a critical factor in assessing the suitability of epoxy resins. TGA is a widely accepted and convenient method for evaluating the thermal stability and degradation characteristics of polymer materials. A thermogravimetry/differential thermal analysis (TG/DTA) analyzer was employed to measure thermal weight loss. Test conditions: the specimens were placed within an Al₂O₃ crucible and subjected to testing in an air atmosphere, with a temperature ramp of 10°C·min⁻¹. The testing temperature range spans from 10°C to 800°C. The TG/DTA analyzer was utilized to determine the specimen temperatures.

2.3.5 Mechanical properties testing

A universal tensile testing machine was employed to assess the mechanical properties of the cured resins. Dumbbell-type sample strips were prepared in accordance with GB/T 1040.2-2006, with sample dimensions of 115 mm × 25 mm × 4 mm. In each group of five specimens tested on the universal tensile material testing machine, the speed was set at 50 mm·min⁻¹.

2.3.6 AC breakdown voltage test

For the dielectric breakdown tests, spherical electrodes were used, following IEC 60243-1-2013 standards. The specimens were subjected to a constant voltage rate of 2 kV·s⁻¹ until material breakdown occurred. To effectively prevent surface flashover caused by voltage rise during the test, square sheet specimens measuring 100 mm × 100 mm × 1 mm were placed in transparent glass containers filled with dimethylsilicon oil. Thirteen valid data values were obtained for each model, and the experimental results were statistically analyzed using a Weibull distribution.

2.3.7 Broadband dielectric spectrum testing

Dielectric properties are a fundamental consideration for dielectric materials. In this study, a broadband dielectric spectrometer was employed to measure the relative permittivity and dielectric loss angle tangent of the specimens at room temperature. The specimens were 10 mm in diameter and 1 mm in thickness, and the test frequency range spanned from 10³ to 10⁶ Hz.

2.3.8 Double bond conversion rate

The double-bond conversion rates of the light-curing coating formulations were determined using a WQF-310 FT-IR spectrometer with an OmicCure S1000 UV point light source. The resin formulations were uniformly applied to potassium bromide salt tablets, with the light intensity of the UV point light source set at 45 mW·cm⁻² during an irradiation time of 600 s. The resolution of the scan was 4 cm⁻¹. This procedure was repeated four times. The double-bond conversion rate was calculated by measuring the change in the absorption peak area of the coating samples before and after irradiation, as per the following equation:

where A ₀ represents the peak area of the carbon–carbon double bond at 810 cm⁻¹ before curing and A _t is the peak area after curing. A _ra corresponds to the peak area of the carbonyl group in the range 1,770–1,716 cm⁻¹ before curing, whereas A _rb denotes the peak area after curing.

2.3.9 Scanning electron microscopy

For the analysis of sample surface micromorphology, a Zeiss Sigma 300 scanning electron microscope was employed. The microscope was operated at an accelerating voltage of 10 kV. Its primary function was to examine the surface morphology of the material post-tensile fracture and assess the fracture characteristics.

3.1.1 Infrared characterization of resins

As illustrated in Figure 4, the absorption peaks at 810 and 1,634 cm⁻¹ in the EA are attributed to C═C stretching vibrations, whereas the absorption peak at 1,508 cm⁻¹ confirms the presence of the benzene ring within bisphenol A EA. For the epoxy resin E51, subsequent to the reaction with IA, a reduction in the intensity of the epoxy group absorption peaks was observed, indicating a decrease in epoxy group content. Additionally, a new carbonyl peak was observed at 1,720 cm¹. The broad peak at 3,480 cm⁻¹ corresponds to the hydroxyl group resulting from the reaction between the epoxy group and carboxylic acid to form hydroxyl groups (–OH). Furthermore, a distinctive absorption peak of the carbonyl–carbon double bond was observed at 1,606 cm⁻¹. These findings collectively signify the successful occurrence of a ring-opening reaction between the epoxy resin and IA (15).

Figure 4

Infrared spectra of EIA and EA.

3.1.2 NMR characterization of resins

As shown in Figure 5(a), the benzene ring in the bisphenol A structure appeared at 6.8 and 7.1 ppm, the protons on the epoxy group appeared at 2.8, 2.9, and 3.3 ppm, and the methyl proton peak on the bisphenol A appeared at 1.6 ppm. When the epoxy resin was reacted with IA, the ¹H NMR spectrum of EIA showed that the C═C bond appeared at 6.35 and 5.70 ppm, which indicated that the C═C double bond was successfully attached in the epoxy resin, suggesting the successful synthesis of the EIA epoxy resin. Figure 5(b) shows the ¹H NMR spectrum of EA, which can be seen as the active hydrogen on the hydroxyl group appeared at 1.63 ppm, H on the bisphenol A benzene ring appeared at 6.80 and 7.12 ppm, and CH ₂ ═CH– appeared at 6.47 and 5.89 ppm, while CH₂═CH – appeared at 6.17 ppm.

Figure 5

NMR hydrogen spectra of (a) EIA and (b) EA.

3.1.3 Gel chromatogram analysis

The relative molecular mass (Mn) and molecular weight distribution index (PDI) were measured using GPC. As depicted in Figure 6, EA exhibited an average relative molecular mass of 2,301 with a molecular weight distribution index of 1.362. Upon subjecting epoxy resin E51 to a chain expansion reaction with IA, a notable increase in both the relative molecular mass (Mn) and molecular weight distribution index (PDI) was observed, reaching 3,447 and 1.171, respectively. These results confirm that IA effectively expanded the chain of epoxy resin E51 (16).

Figure 6

Gel permeation chromatograms of EIA and EA.

The EA showed an average relative molecular mass of 2,301 and molecular weight distribution index of 1.362, and the relative molecular mass (Mn) and molecular weight distribution index (PDI) of the EIA were 3,447 and 1.171, respectively. The molecular weight of EIA was significant, with a narrow distribution.

3.2.1 Double bond conversion rate

The double-bond conversion rate serves as a measure of the curing rate and extent of curing for the cured samples. Figure 7 illustrates the relationship between the double-bond conversion rate of EIA and the irradiation time. During the UV curing of both EIA and EA, a specific quantity of DPGDA reactive diluent was incorporated. To assess the double-bond conversion rate and extent of conversion, real-time infrared spectroscopy was employed to monitor changes in peak areas at the carbon–carbon double bond positions of EIA (1,606 cm⁻¹) and EA (1,634 cm⁻¹). The analysis of the variation curve of the double-bond conversion rate for EIA with UV irradiation time shows that the double-bond conversion rate of the EIA system reached 34.70% within 10 s. However, the final double-bond conversion rate remained relatively low due to the presence of fewer reactive double bonds in EIA, particularly within the structure of IA. In contrast, the overall double-bond conversion of EA peaked at 99.98% within the same 10 s timeframe. This high overall double-bond conversion can be attributed to the positioning of the double bonds at the two ends of the EA structure, rendering them more reactive in the reaction compared to EIA.

Figure 7

Variation of double bond conversion rate of EIA formulation system with irradiation time.

3.2.2 Gel content test

The gel content was employed as a measure of the crosslinking density of the cured material. The gel contents of both EA and EIA after curing are presented in Figure 8. These gel contents are divided into four categories: EA-UV, EIA-UV, EA-UV-heat, and EIA-UV-heat. The former two represent test data following UV curing without additional heat treatment, whereas the latter two correspond to dual curing utilizing both UV and heat. Specifically, the gel content for EA-UV was 66.39%, whereas for EIA-UV, it reached 73.12%. This higher gel content in EIA-UV can be attributed to the greater presence of carbon–carbon double bonds in EIA, stemming from UV exposure. Following heat curing, the gel content for EIA-UV-heat significantly rose to 87.78%, surpassing that of EA. This substantial increase can be primarily attributed to the ring-opening curing of the epoxy groups within EIA during heat treatment, which effectively enhanced the crosslinking density of the cured material.

Figure 8

Gel content of EIA and EA after UV curing and UV/heat curing processes.

3.4.1 Dynamic mechanical analysis

The crosslink density (ν _e) and glass transition temperature (T _g) of the cured systems were determined through DMA. Figures 9 and 10 depict the DMA curves for the EIA and EA curing systems, respectively. The crosslinking density (ν _e) was calculated using the formula ν _e = E′/3RT, yielding values of 19,710.72 and 8,287.79 mol·m⁻³ for EIA and EA, respectively. Notably, the network in EIA exhibits a significantly higher E at T _g + 40°C, and ρ is notably greater compared to the EA system. The figure also reveals that the glass transition temperatures for EIA and EA are 83°C and 70°C, respectively. This discrepancy can be attributed to the fact that the glass transition temperature of the epoxy network is determined by its crosslinking density and the chemical structure of the chain segments. A higher crosslink density or more rigid chain segments result in a higher T _g value. Moreover, EIA boasts a greater molecular weight than EA, leading to more molecular entanglement points, stronger intermolecular forces, and consequently higher T _g values.

Figure 9

DMA curves of EIA cured system.

Figure 10

DMA curves of EA curing system.

3.4.2 TGA of resins

The thermal stability of a resin plays a pivotal role in evaluating its operational durability. As evident from Figure 11, the initial decomposition temperatures for a 5% weight loss of EIA and EA were measured at 323.86°C and 273.61°C, respectively. Subsequently, the temperatures at which the maximum decomposition rates occurred were recorded at 430.53°C for EIA and 378.12°C for EA. The residual mass fractions were found to be 7.6% and 3.9%, respectively, indicating the degradation of the polymer backbone structure throughout the process. Notably, while the curing network of EA disintegrated earlier and more thoroughly than that of EIA, the overall distinction remains relatively modest. This can be attributed to the presence of a substantial number of ester bonds in the molecules of EIA and EA polymers, which are susceptible to thermal cleavage. Additionally, the chain structure of EIA, to some extent, constrains its potential application scope. Therefore, its heat resistance must be enhanced through subsequent structural modifications.

Figure 11

Comparison of TG for EIA and EA.

3.5.1 Breakdown field strength analysis

AC breakdown voltage tests were performed on the EIA and EA curing systems. The Weibull distribution curves representing their breakdown field strengths are presented in Figure 12. For EIA and EA, the AC breakdown field strengths, with a breakdown probability of 63.2%, were measured at 40.15 and 35.58 kV·mm⁻¹, respectively. Notably, these breakdown field strengths in EIA were higher than those in EA. This difference can be attributed to several factors. First, the lower cross-linking density of the curing resin in EA leads to the rapid expansion of voids within the molecules under voltage, ultimately resulting in material breakdown. Furthermore, the lower degree of cross-linking and the expansive nature of the ester group within EA contribute to its heightened susceptibility to water adsorption (17). Water adsorption on insulating materials can induce significant damage to their insulation properties as polar water molecules tend to conduct electricity.

Figure 12

Breakdown field strength Weibull distribution for EIA and EA.

3.5.2 Analysis of dielectric properties

The AC dielectric spectral curves of the EIA and EA curing systems are depicted in Figure 13. The figure shows that the fundamental component of the dielectric constant in both the EIA and EA curing systems gradually diminishes as the frequency rises. The difference between the dielectric constants of the EA and EIA curing systems is not substantial. In these systems, electronic polarization and atomic polarization contribute minimally compared to dipole polarization, which has a short relaxation time and primarily governs the high-frequency band dielectric constant of the polymer. The complex, non-uniform arrangement of high-frequency dipoles results in a reduction in permittivity. Both EIA and EA polymers contain molecular chains with O–H (hydroxyl), C–O (ether bond), and C–H bonds, alongside multiple polar ester bonds. The permanent dipole moments associated with polar bonds induce potential traps in their vicinity, and the relaxation time constant is determined by the trap depth of these polar bonds. The dielectric loss factor is a fundamental property of insulating materials, characterizing the extent of dielectric loss in a material following the application of an electric field. It stems from two sources: dielectric loss due to leakage current and dielectric loss due to polarization. At low frequencies, various polarization mechanisms have sufficient time to respond, resulting in minimal polarization loss. Consequently, dielectric loss mainly arises from leakage current. However, under high-frequency conditions, the dipoles within the dielectric fail to keep pace with the rapid changes in the electric field frequency, leading to relaxation and an increase in the dielectric loss of the composite. At this stage, the dielectric loss factor is contingent on the alignment of polar chain segments. When polarization loss predominates, the total dipole moment of the molecules drives the dielectric loss peaks observed in both the EIA and EA polymer molecules, owing to the prevalence of numerous ester bonds within their structures (18,19).

Figure 13

(a) Dielectric constant versus frequency for the EIA and EA cured systems. (b) Dielectric loss factor versus frequency for the EIA and EA cured systems.