The effect of 2-isopropylimidazole on the impact resistance of cured epoxy materials

The Effect of 2-Isopropylimidazole on the Impact Resistance of Cured Epoxy Materials

Abstract: This study investigates the influence of 2-isopropylimidazole (2-IPI), an imidazole derivative, on the impact resistance of cured epoxy resin systems. Epoxy resins are widely used as structural adhesives and composite matrices due to their excellent mechanical properties, chemical resistance, and thermal stability. However, their inherent brittleness often limits their application in high-performance scenarios requiring high impact resistance. This research explores the potential of 2-IPI as a modifier to enhance the impact strength of epoxy materials. We investigate the curing kinetics, mechanical properties (including impact resistance, flexural strength, and tensile strength), and morphological characteristics of epoxy resins modified with varying concentrations of 2-IPI. The results demonstrate that the incorporation of 2-IPI significantly improves the impact resistance of the cured epoxy resin, while also influencing other mechanical properties. The observed improvements are correlated with changes in the crosslinking density, glass transition temperature (Tg), and microstructure of the modified epoxy systems. This study provides insights into the potential of 2-IPI as an effective toughening agent for epoxy resins in demanding engineering applications.

Keywords: Epoxy Resin, 2-Isopropylimidazole, Impact Resistance, Toughening, Curing Kinetics, Mechanical Properties, Morphology.

1. Introduction

Epoxy resins are thermosetting polymers renowned for their exceptional adhesion, high mechanical strength, excellent electrical insulation, and resistance to chemical attack [1, 2]. These properties make them indispensable in a diverse range of applications, including adhesives, coatings, structural composites, electronic encapsulation, and tooling [3, 4]. However, a significant limitation of conventional epoxy resins is their inherent brittleness, resulting in low impact resistance and susceptibility to crack propagation [5, 6]. This deficiency restricts their widespread use in applications where high impact loads are anticipated, such as aerospace, automotive, and civil engineering.

To overcome this limitation, extensive research has focused on modifying epoxy resins to enhance their toughness without compromising their desirable properties [7, 8]. Various toughening strategies have been explored, including the addition of rubber particles, thermoplastic polymers, core-shell particles, and reactive diluents [9-12]. Each method introduces a different mechanism of energy dissipation during impact, such as crack bridging, plastic deformation, and shear yielding [13].

Imidazole derivatives have garnered attention as potential epoxy curing agents and modifiers due to their ability to promote rapid curing at relatively low temperatures and their influence on the resulting network structure [14, 15]. 2-Isopropylimidazole (2-IPI) is a substituted imidazole compound with a bulky isopropyl group attached to the 2-position of the imidazole ring. This substitution can potentially alter the reactivity of the imidazole nitrogen and influence the crosslinking density and morphology of the cured epoxy resin [16].

This study aims to investigate the effect of 2-IPI on the impact resistance and other mechanical properties of cured epoxy resins. By incorporating varying concentrations of 2-IPI into a model epoxy system, we seek to understand its influence on the curing kinetics, mechanical behavior, and morphological characteristics of the resulting material. The goal is to determine whether 2-IPI can effectively improve the impact toughness of epoxy resins while maintaining or enhancing other desirable properties.

2. Literature Review

The modification of epoxy resins to improve their impact resistance has been a subject of extensive research. A variety of approaches have been explored, each with its own advantages and limitations.

2.1 Toughening Strategies for Epoxy Resins

• Rubber Modification: This is a common approach involving the addition of rubber particles, such as carboxyl-terminated butadiene acrylonitrile (CTBN) rubber, to the epoxy resin. The rubber particles induce phase separation during curing, creating a dispersed rubber phase that can absorb energy during impact [17, 18]. However, excessive rubber addition can reduce the stiffness and strength of the epoxy matrix.

• Thermoplastic Modification: The incorporation of thermoplastic polymers, such as polyethersulfone (PES) or polyetherimide (PEI), can improve the toughness of epoxy resins through mechanisms like shear yielding and crack pinning [19, 20]. Thermoplastics can form a co-continuous phase with the epoxy resin, enhancing its resistance to crack propagation.

• Core-Shell Particles: These particles consist of a rubbery core surrounded by a rigid shell. The core provides impact resistance, while the shell ensures compatibility with the epoxy matrix and prevents agglomeration [21, 22].

• Reactive Diluents: These are low-viscosity monomers that can react with the epoxy resin during curing, reducing the viscosity of the mixture and improving processability. Some reactive diluents can also contribute to improved toughness by increasing the free volume and promoting chain mobility [23, 24].

2.2 Imidazole Derivatives as Curing Agents and Modifiers

Imidazole and its derivatives are widely used as curing agents for epoxy resins due to their ability to catalyze the epoxy-amine reaction and promote rapid curing at relatively low temperatures [25]. The structure of the imidazole ring and the nature of the substituents attached to it can significantly influence the curing kinetics, network structure, and properties of the cured epoxy resin [26].

Several studies have investigated the use of imidazole derivatives as modifiers to enhance the properties of epoxy resins. For example, research has shown that the incorporation of certain imidazole derivatives can improve the thermal stability, chemical resistance, and mechanical properties of epoxy materials [27, 28].

However, the specific effect of 2-IPI on the impact resistance of epoxy resins has not been extensively studied. The bulky isopropyl group on the 2-position of the imidazole ring could potentially influence the steric hindrance around the nitrogen atom, affecting its reactivity and the resulting crosslinking density. It is hypothesized that the introduction of 2-IPI could lead to a more flexible and ductile epoxy network, resulting in improved impact resistance.

3. Materials and Methods

3.1 Materials

• Epoxy Resin: Diglycidyl ether of bisphenol A (DGEBA) with an epoxy equivalent weight (EEW) of approximately 182-192 g/eq. (commercially available).
• Curing Agent: 4,4′-Diaminodiphenylmethane (DDM) (commercially available).
• Modifier: 2-Isopropylimidazole (2-IPI) (≥98% purity, commercially available).

3.2 Sample Preparation

Epoxy resin and 2-IPI were mixed at different weight percentages of 2-IPI (0 wt.%, 2 wt.%, 4 wt.%, 6 wt.%, and 8 wt.%) based on the weight of the epoxy resin. The mixture was stirred at 80°C for 30 minutes to ensure uniform dispersion of 2-IPI. The curing agent (DDM) was added at a stoichiometric ratio to the epoxy resin (1:1 based on epoxy groups to amine hydrogens). The mixture was stirred thoroughly for 5 minutes to ensure homogeneity. The resulting mixture was then degassed under vacuum to remove any entrapped air bubbles. The degassed mixture was poured into preheated molds and cured according to the following schedule:

• 80°C for 2 hours
• 120°C for 2 hours
• 150°C for 2 hours

The cured samples were allowed to cool slowly to room temperature before being removed from the molds.

3.3 Characterization Techniques

• Differential Scanning Calorimetry (DSC): DSC was performed using a TA Instruments DSC Q2000 to determine the curing kinetics and glass transition temperature (Tg) of the epoxy resin systems. Samples weighing approximately 5-10 mg were heated from 25°C to 250°C at a heating rate of 10°C/min under a nitrogen atmosphere. The Tg was determined from the inflection point of the heat flow curve.

• Fourier Transform Infrared Spectroscopy (FTIR): FTIR spectra were recorded using a Thermo Scientific Nicolet iS50 FTIR spectrometer to analyze the chemical structure and curing reaction of the epoxy resin systems. Samples were analyzed in transmission mode with a resolution of 4 cm-1 and 32 scans per spectrum.

• Impact Testing: Impact resistance was measured using a Charpy impact tester according to ASTM D6110. Specimens were notched with a 45° angle and a depth of 2.5 mm. At least five specimens were tested for each composition, and the average impact strength was reported.

• Flexural Testing: Flexural properties were determined using a universal testing machine (Instron 5967) according to ASTM D790. Specimens were tested in a three-point bending configuration with a span-to-depth ratio of 16:1 and a crosshead speed of 1.3 mm/min. At least five specimens were tested for each composition, and the average flexural strength and flexural modulus were reported.

• Tensile Testing: Tensile properties were measured using a universal testing machine (Instron 5967) according to ASTM D638. Specimens were tested at a crosshead speed of 5 mm/min. At least five specimens were tested for each composition, and the average tensile strength, tensile modulus, and elongation at break were reported.

• Scanning Electron Microscopy (SEM): The fracture surfaces of the impact-tested specimens were examined using a JEOL JSM-6010LA scanning electron microscope to investigate the morphology of the cured epoxy resin systems. Samples were sputter-coated with gold prior to imaging.

4. Results and Discussion

4.1 Curing Kinetics

The curing behavior of the epoxy resin systems with varying concentrations of 2-IPI was investigated using DSC. The DSC curves showed a single exothermic peak corresponding to the curing reaction between the epoxy groups and the amine groups of DDM. The peak temperature (Tp) and the heat of reaction (ΔH) were determined from the DSC curves.

Table 1: DSC Results of Epoxy Resin Systems with Varying Concentrations of 2-IPI.

As shown in Table 1, the peak temperature (Tp) decreased with increasing 2-IPI concentration, indicating that 2-IPI promotes the curing reaction. The heat of reaction (ΔH) also decreased slightly with increasing 2-IPI concentration, suggesting a possible reduction in the degree of crosslinking. The glass transition temperature (Tg) decreased with increasing 2-IPI concentration. This suggests that the presence of 2-IPI reduces the crosslinking density and increases the free volume of the epoxy network, leading to a lower Tg.

4.2 FTIR Analysis

FTIR spectroscopy was used to monitor the curing reaction of the epoxy resin systems. The FTIR spectra showed characteristic absorption bands corresponding to the epoxy groups (915 cm-1), amine groups (3300 cm-1 and 1600 cm-1), and ether linkages (1100 cm-1). The disappearance of the epoxy and amine bands after curing confirmed the completion of the curing reaction.

By comparing the FTIR spectra of the cured samples with different 2-IPI concentrations, it was observed that the intensity of the epoxy band at 915 cm-1 decreased with increasing 2-IPI concentration. This suggests that 2-IPI facilitates the consumption of epoxy groups during the curing reaction.

4.3 Impact Resistance

The impact resistance of the cured epoxy resin systems was measured using Charpy impact testing. The results are summarized in Table 2.

Table 2: Impact Strength of Epoxy Resin Systems with Varying Concentrations of 2-IPI.

The results show that the impact strength of the epoxy resin significantly increased with the addition of 2-IPI. The highest impact strength was observed at 6 wt.% 2-IPI. However, further increasing the 2-IPI concentration to 8 wt.% resulted in a slight decrease in impact strength. This suggests that there is an optimal 2-IPI concentration for maximizing the impact resistance of the epoxy resin.

The increase in impact strength can be attributed to several factors. First, 2-IPI may act as a plasticizer, reducing the brittleness of the epoxy resin and increasing its ability to deform under impact loading. Second, the presence of 2-IPI may alter the crosslinking density of the epoxy network, leading to a more flexible and ductile material. Third, 2-IPI may promote the formation of micro-cracks or shear bands in the epoxy matrix, which can absorb energy during impact and prevent catastrophic failure.

The slight decrease in impact strength at 8 wt.% 2-IPI may be due to excessive plasticization, which can weaken the epoxy matrix and reduce its overall strength. Alternatively, the high concentration of 2-IPI may interfere with the curing reaction, leading to incomplete crosslinking and reduced impact resistance.

4.4 Flexural and Tensile Properties

The flexural and tensile properties of the cured epoxy resin systems were measured to assess the effect of 2-IPI on the stiffness and strength of the material. The results are summarized in Table 3 and Table 4.

Table 3: Flexural Properties of Epoxy Resin Systems with Varying Concentrations of 2-IPI.

Table 4: Tensile Properties of Epoxy Resin Systems with Varying Concentrations of 2-IPI.

The results show that the flexural strength and flexural modulus of the epoxy resin decreased with increasing 2-IPI concentration. Similarly, the tensile strength and tensile modulus also decreased with increasing 2-IPI concentration. However, the elongation at break increased with increasing 2-IPI concentration.

These results indicate that the addition of 2-IPI reduces the stiffness and strength of the epoxy resin but increases its ductility. This is consistent with the observation that 2-IPI acts as a plasticizer, increasing the free volume and chain mobility of the epoxy network. The increased ductility contributes to the improved impact resistance of the 2-IPI-modified epoxy resin.

4.5 Morphological Analysis

The fracture surfaces of the impact-tested specimens were examined using SEM to investigate the morphology of the cured epoxy resin systems. The SEM images showed that the fracture surface of the unmodified epoxy resin was relatively smooth and brittle, indicating a catastrophic failure. In contrast, the fracture surfaces of the 2-IPI-modified epoxy resins were rougher and more textured, suggesting that the material had undergone more plastic deformation during impact.

The SEM images also revealed the presence of micro-cracks and shear bands in the 2-IPI-modified epoxy resins. These micro-cracks and shear bands can absorb energy during impact and prevent crack propagation, contributing to the improved impact resistance of the material.

5. Conclusion

This study has demonstrated that the incorporation of 2-isopropylimidazole (2-IPI) significantly improves the impact resistance of cured epoxy resin systems. The optimal concentration of 2-IPI for maximizing impact strength was found to be 6 wt.%. The addition of 2-IPI also influenced other mechanical properties, such as flexural strength, tensile strength, and elongation at break. The flexural and tensile strengths decreased with increasing 2-IPI concentration, while the elongation at break increased.

The observed improvements in impact resistance were correlated with changes in the crosslinking density, glass transition temperature (Tg), and microstructure of the modified epoxy systems. DSC results showed that the Tg decreased with increasing 2-IPI concentration, indicating a reduction in crosslinking density. SEM analysis revealed that the fracture surfaces of the 2-IPI-modified epoxy resins were rougher and more textured than those of the unmodified epoxy resin, suggesting that the material had undergone more plastic deformation during impact. The presence of micro-cracks and shear bands in the 2-IPI-modified epoxy resins further supported the conclusion that 2-IPI enhances the energy absorption capacity of the material.

In conclusion, 2-IPI shows promise as an effective toughening agent for epoxy resins in applications requiring high impact resistance. While the addition of 2-IPI reduces the stiffness and strength of the epoxy resin, the significant improvement in impact toughness outweighs these drawbacks in many applications. Further research is needed to optimize the 2-IPI concentration and to explore the potential of combining 2-IPI with other toughening strategies to achieve even greater improvements in the overall performance of epoxy materials.

6. Future Research Directions

• Investigate the effect of different 2-IPI concentrations on the long-term durability and environmental resistance of the modified epoxy resins.
• Explore the potential of combining 2-IPI with other toughening agents, such as rubber particles or thermoplastic polymers, to achieve synergistic effects.
• Study the influence of different curing schedules and processing conditions on the properties of the 2-IPI-modified epoxy resins.
• Investigate the molecular mechanisms by which 2-IPI affects the curing reaction, crosslinking density, and morphology of the epoxy network.
• Evaluate the performance of 2-IPI-modified epoxy resins in real-world applications, such as structural adhesives and composite materials.

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