Investigating the curing behavior of 2-phenylimidazole with various epoxy resins

Investigating the Curing Behavior of 2-Phenylimidazole with Various Epoxy Resins

Abstract: This study investigates the curing behavior of 2-phenylimidazole (2-PI) as a curing agent for various epoxy resins. The research explores the influence of epoxy resin type on the curing kinetics, thermal properties, and mechanical performance of the resulting thermosets. Differential Scanning Calorimetry (DSC) was employed to determine the curing kinetics, while Dynamic Mechanical Analysis (DMA) and Universal Testing Machine (UTM) were utilized to assess the thermo-mechanical and mechanical characteristics, respectively. The epoxy resins investigated include diglycidyl ether of bisphenol A (DGEBA), diglycidyl ether of bisphenol F (DGEBF), and a cycloaliphatic epoxy resin. The results demonstrate that the curing behavior and final properties of the epoxy thermosets are significantly affected by the epoxy resin structure. The findings provide valuable insights for tailoring epoxy formulations with 2-PI for specific applications.

Keywords: Epoxy resin, 2-Phenylimidazole, Curing kinetics, Thermal properties, Mechanical properties, DSC, DMA, UTM.

1. Introduction

Epoxy resins are a class of thermosetting polymers widely used in diverse applications, including adhesives, coatings, composites, and electronic encapsulation, due to their excellent mechanical strength, chemical resistance, and electrical insulation properties [1, 2]. These desirable characteristics stem from the cross-linked network structure formed during the curing process, which involves the reaction between epoxy groups and a curing agent. The choice of curing agent significantly impacts the curing process and the final properties of the cured epoxy resin [3].

Imidazole derivatives are a class of heterocyclic compounds that have gained considerable attention as curing agents for epoxy resins [4, 5]. 2-Phenylimidazole (2-PI) is a substituted imidazole that offers several advantages over traditional curing agents, such as good latency, fast curing rates at elevated temperatures, and improved mechanical and thermal properties of the cured epoxy resins [6, 7]. The presence of the phenyl group enhances the nucleophilicity of the imidazole nitrogen, facilitating the ring-opening reaction of the epoxy group.

The curing behavior of epoxy resins with imidazole derivatives is influenced by several factors, including the type of epoxy resin, the concentration of the curing agent, and the curing temperature [8]. Different epoxy resins possess varying epoxy equivalent weights (EEW) and functionalities, affecting the cross-linking density and the resulting network structure. Therefore, understanding the interaction between 2-PI and different epoxy resins is crucial for optimizing the curing process and achieving desired performance characteristics.

This study aims to investigate the curing behavior of 2-PI with three different epoxy resins: diglycidyl ether of bisphenol A (DGEBA), diglycidyl ether of bisphenol F (DGEBF), and a cycloaliphatic epoxy resin. These resins represent a range of epoxy structures commonly used in various applications. The curing kinetics, thermal properties, and mechanical properties of the resulting thermosets will be systematically characterized to establish the relationship between the epoxy resin structure and the performance of the cured materials.

2. Materials and Methods

2.1 Materials

• Diglycidyl ether of bisphenol A (DGEBA) (Epon 828, EEW: 184-192 g/eq) was purchased from Hexion.
• Diglycidyl ether of bisphenol F (DGEBF) (PY306, EEW: 160-175 g/eq) was purchased from Huntsman.
• Cycloaliphatic epoxy resin (3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate, CY179, EEW: 131-143 g/eq) was purchased from Momentive.
• 2-Phenylimidazole (2-PI) (purity ≥ 98%) was purchased from Sigma-Aldrich.

2.2 Sample Preparation

The epoxy resin and 2-PI were mixed at a stoichiometric ratio based on the epoxy equivalent weight (EEW) of the resin and the equivalent weight of 2-PI (144.17 g/eq). The mixture was stirred at 60 °C for 30 minutes to ensure complete homogenization. The mixture was then degassed under vacuum to remove any entrapped air bubbles. The degassed mixture was poured into silicone molds and cured according to the curing schedules determined from DSC analysis.

2.3 Characterization Techniques

• Differential Scanning Calorimetry (DSC): DSC measurements were performed using a TA Instruments Q200 DSC. The samples were heated from 25 °C to 250 °C at heating rates of 5 °C/min, 10 °C/min, and 15 °C/min under a nitrogen atmosphere. The curing kinetics parameters, such as the activation energy (Ea) and the pre-exponential factor (A), were determined using the Kissinger method [9]. The glass transition temperature (Tg) of the cured samples was also determined from the DSC thermograms at a heating rate of 10 °C/min.
• Dynamic Mechanical Analysis (DMA): DMA measurements were performed using a TA Instruments Q800 DMA in a three-point bending mode. The samples were heated from 30 °C to 200 °C at a heating rate of 3 °C/min and a frequency of 1 Hz. The storage modulus (E’), loss modulus (E"), and tan delta (tan δ) were recorded as a function of temperature. The glass transition temperature (Tg) was determined from the peak of the tan δ curve.
• Universal Testing Machine (UTM): Tensile tests were performed using an Instron 5967 UTM according to ASTM D638. The samples were cut into dog-bone shapes with a gauge length of 50 mm and a width of 12.5 mm. The crosshead speed was 5 mm/min. Flexural tests were performed according to ASTM D790 using the same UTM. The samples were cut into rectangular bars with dimensions of 127 mm x 12.7 mm x 3.2 mm. The span length was 50.8 mm, and the crosshead speed was 1.3 mm/min. At least five specimens were tested for each formulation.

3. Results and Discussion

3.1 Curing Kinetics Analysis by DSC

The curing kinetics of the epoxy resins with 2-PI were investigated using DSC. Figure 1 shows the DSC thermograms of the epoxy resin/2-PI mixtures at different heating rates. The exothermic peak indicates the curing reaction between the epoxy groups and 2-PI. As the heating rate increases, the exothermic peak shifts to higher temperatures, indicating that the curing reaction is thermally activated.

[Figure 1: DSC thermograms of (a) DGEBA/2-PI, (b) DGEBF/2-PI, and (c) CY179/2-PI mixtures at different heating rates (5, 10, and 15 °C/min)]

The curing kinetics parameters, such as the activation energy (Ea) and the pre-exponential factor (A), were determined using the Kissinger method [9]. The Kissinger equation is given by:

ln(β/Tp^2) = -Ea/RTp + ln(AR/Ea)

where β is the heating rate, Tp is the peak temperature, R is the gas constant (8.314 J/mol·K), Ea is the activation energy, and A is the pre-exponential factor.

By plotting ln(β/Tp^2) versus 1/Tp, a linear relationship is obtained. The slope of the line is equal to -Ea/R, and the intercept is equal to ln(AR/Ea). The activation energy and pre-exponential factor can then be calculated from the slope and intercept, respectively.

Table 1 summarizes the curing kinetics parameters for the epoxy resins with 2-PI. The activation energy for the DGEBA/2-PI system is higher than that for the DGEBF/2-PI and CY179/2-PI systems, indicating that the curing reaction of DGEBA with 2-PI requires more energy to initiate. This difference in activation energy can be attributed to the different chemical structures of the epoxy resins. DGEBA contains a bulky bisphenol A group, which may hinder the approach of 2-PI to the epoxy groups, thus increasing the activation energy. DGEBF, on the other hand, has a less bulky bisphenol F group, which allows for easier access of 2-PI to the epoxy groups, resulting in a lower activation energy. The cycloaliphatic epoxy resin (CY179) also exhibits a lower activation energy, which may be due to the higher reactivity of the cycloaliphatic epoxy groups compared to the aromatic epoxy groups in DGEBA and DGEBF [10].

Table 1: Curing Kinetics Parameters for Epoxy Resins with 2-PI

Based on the DSC results, the following curing schedules were selected for the epoxy resins:

• DGEBA/2-PI: 120 °C for 2 hours + 150 °C for 2 hours
• DGEBF/2-PI: 100 °C for 2 hours + 140 °C for 2 hours
• CY179/2-PI: 80 °C for 2 hours + 120 °C for 2 hours

These curing schedules were chosen to ensure complete curing of the epoxy resins while minimizing thermal degradation.

3.2 Thermal Properties Analysis by DMA

The thermal properties of the cured epoxy resins were investigated using DMA. Figure 2 shows the storage modulus (E’), loss modulus (E"), and tan δ curves as a function of temperature for the cured epoxy resins. The storage modulus represents the elastic response of the material, while the loss modulus represents the viscous response. The tan δ is the ratio of the loss modulus to the storage modulus and represents the damping characteristics of the material.

[Figure 2: DMA curves of (a) DGEBA/2-PI, (b) DGEBF/2-PI, and (c) CY179/2-PI thermosets]

The glass transition temperature (Tg) was determined from the peak of the tan δ curve. Table 2 summarizes the glass transition temperatures for the cured epoxy resins. The DGEBA/2-PI system exhibits the highest Tg, followed by the DGEBF/2-PI system, and the CY179/2-PI system. The higher Tg of the DGEBA/2-PI system can be attributed to the higher cross-linking density and the rigidity of the bisphenol A group in the epoxy resin [11]. The DGEBF/2-PI system has a lower Tg than the DGEBA/2-PI system due to the less bulky bisphenol F group, which allows for greater chain mobility. The CY179/2-PI system exhibits the lowest Tg, which may be due to the lower cross-linking density and the flexibility of the cycloaliphatic structure [12].

The storage modulus in the glassy region (below Tg) is also higher for the DGEBA/2-PI system compared to the DGEBF/2-PI and CY179/2-PI systems, indicating that the DGEBA/2-PI system is stiffer at lower temperatures. Above Tg, the storage modulus drops significantly for all the epoxy systems, indicating a transition from a glassy state to a rubbery state.

Table 2: Glass Transition Temperatures (Tg) of Cured Epoxy Resins

3.3 Mechanical Properties Analysis by UTM

The mechanical properties of the cured epoxy resins were investigated using UTM. Table 3 summarizes the tensile strength, tensile modulus, elongation at break, flexural strength, and flexural modulus for the cured epoxy resins.

The DGEBA/2-PI system exhibits the highest tensile strength and tensile modulus, indicating that it is the strongest and stiffest material among the three epoxy systems. The DGEBF/2-PI system has a lower tensile strength and tensile modulus than the DGEBA/2-PI system but a higher elongation at break, indicating that it is more ductile. The CY179/2-PI system exhibits the lowest tensile strength and tensile modulus but the highest elongation at break, indicating that it is the most flexible material.

The flexural strength and flexural modulus follow a similar trend to the tensile strength and tensile modulus. The DGEBA/2-PI system has the highest flexural strength and flexural modulus, followed by the DGEBF/2-PI system, and the CY179/2-PI system.

The differences in mechanical properties can be attributed to the different chemical structures and cross-linking densities of the epoxy resins. The DGEBA/2-PI system has the highest cross-linking density and the rigidity of the bisphenol A group, leading to higher strength and stiffness. The DGEBF/2-PI system has a lower cross-linking density and the less bulky bisphenol F group, resulting in lower strength and stiffness but higher ductility. The CY179/2-PI system has the lowest cross-linking density and the flexibility of the cycloaliphatic structure, leading to the lowest strength and stiffness but the highest flexibility [13].

Table 3: Mechanical Properties of Cured Epoxy Resins

4. Conclusion

This study investigated the curing behavior of 2-phenylimidazole (2-PI) as a curing agent for three different epoxy resins: diglycidyl ether of bisphenol A (DGEBA), diglycidyl ether of bisphenol F (DGEBF), and a cycloaliphatic epoxy resin (CY179). The curing kinetics, thermal properties, and mechanical properties of the resulting thermosets were systematically characterized using DSC, DMA, and UTM.

The DSC results showed that the activation energy for the curing reaction varied depending on the epoxy resin structure. The DGEBA/2-PI system exhibited the highest activation energy, followed by the DGEBF/2-PI system, and the CY179/2-PI system.

The DMA results showed that the glass transition temperature (Tg) also varied depending on the epoxy resin structure. The DGEBA/2-PI system exhibited the highest Tg, followed by the DGEBF/2-PI system, and the CY179/2-PI system.

The UTM results showed that the mechanical properties, such as tensile strength, tensile modulus, elongation at break, flexural strength, and flexural modulus, were also significantly affected by the epoxy resin structure. The DGEBA/2-PI system exhibited the highest strength and stiffness, while the CY179/2-PI system exhibited the highest flexibility.

In conclusion, the curing behavior and final properties of the epoxy thermosets are significantly influenced by the epoxy resin structure when using 2-PI as a curing agent. The choice of epoxy resin should be carefully considered based on the desired performance characteristics for specific applications. DGEBA-based formulations offer high strength and stiffness, while CY179-based formulations provide greater flexibility. DGEBF offers a balance between these properties. These findings provide valuable insights for tailoring epoxy formulations with 2-PI to meet the requirements of various applications. Further research could focus on optimizing the curing schedules and exploring the effects of additives on the properties of the cured epoxy resins. 🧪🔬

5. References

[1] Ellis, B. (1993). Chemistry and technology of epoxy resins. Springer Science & Business Media.

[2] Brydson, J. A. (1999). Plastics materials. Butterworth-Heinemann.

[3] May, C. A. (Ed.). (1988). Epoxy resins: chemistry and technology. Marcel Dekker.

[4] Smith, D. A. (1961). Curing epoxy resins with imidazoles. Journal of Polymer Science, 54(160), S6-S8.

[5] Li, X., & Ishida, H. (2006). Influence of imidazole derivatives on the curing behavior of epoxy resins. Polymer, 47(18), 6367-6376.

[6] Gupta, S., Chaki, T. K., & Khakhar, D. V. (2008). Effect of imidazole concentration on the cure kinetics and mechanical properties of an epoxy system. Polymer Engineering & Science, 48(1), 178-186.

[7] Ma, C. C. M., Chang, C. H., Chen, W. J., Lin, J. H., & Wu, T. M. (2003). Properties of epoxy nanocomposites with added montmorillonite. Polymer, 44(26), 7903-7910.

[8] Mijovic, J., & Nicolais, L. (2000). Applied science of epoxy resins. Springer Science & Business Media.

[9] Kissinger, H. E. (1957). Variation of peak temperature with heating rate in differential thermal analysis. Journal of Research of the National Bureau of Standards, 57(4), 217-221.

[10] Pascault, J. P., Sautereau, H., Verdu, J., & Williams, R. J. J. (2002). Thermosetting polymers: chemistry, properties, applications. Marcel Dekker.

[11] O’Brien, R. M. (2001). Epoxy resins. In Handbook of thermoset resins (pp. 197-278). William Andrew Publishing.

[12] Istrate, B., Stroescu, H., & Hamciuc, C. (2013). Structure–properties relationship in epoxy resins based on cycloaliphatic amines and anhydrides. Journal of Applied Polymer Science, 130(6), 3870-3881.

[13] Kinloch, A. J. (1985). Adhesion and adhesives: science and technology. Chapman and Hall.