2-Ethylimidazole as a Curing Agent and Accelerator in Two-Component Epoxy Adhesive Systems: A Comprehensive Review
Abstract: This article presents a comprehensive review of the application of 2-ethylimidazole (2-EI) in two-component epoxy adhesive systems. It explores the role of 2-EI as both a curing agent and an accelerator, examining its influence on various product parameters such as curing kinetics, mechanical properties, thermal stability, and adhesion strength. Furthermore, it delves into the mechanism of action of 2-EI and compares its performance with other commonly used curing agents. The article aims to provide a rigorous and standardized overview of the current understanding and applications of 2-EI in epoxy adhesive formulations.
Keywords: 2-Ethylimidazole, Epoxy Adhesive, Curing Agent, Accelerator, Mechanical Properties, Thermal Properties, Adhesion Strength.
1. Introduction
Epoxy resins are a class of thermosetting polymers widely used in various industrial applications, including adhesives, coatings, and composites, owing to their excellent mechanical strength, chemical resistance, and adhesive properties [1]. The performance of epoxy resins is highly dependent on the curing process, which involves crosslinking the epoxy monomers with a curing agent to form a three-dimensional network [2]. Two-component epoxy adhesive systems typically consist of an epoxy resin (component A) and a curing agent (component B), which are mixed immediately prior to application. The choice of curing agent significantly affects the final properties of the cured epoxy material [3].
Imidazole derivatives, particularly 2-alkylimidazoles, have emerged as important curing agents and accelerators for epoxy resins. Among these, 2-ethylimidazole (2-EI) stands out due to its relatively low viscosity, good solubility in epoxy resins, and ability to promote rapid curing at both elevated and ambient temperatures [4]. 2-EI can function as both a curing agent in its own right and as an accelerator in conjunction with other curing agents, impacting the overall performance of the adhesive system [5]. This article aims to provide a comprehensive review of the use of 2-EI in two-component epoxy adhesive systems, covering its curing mechanism, effects on product parameters, and comparison with alternative curing agents.
2. Mechanism of Action of 2-Ethylimidazole in Epoxy Curing
The curing mechanism of epoxy resins with 2-EI involves the ring-opening polymerization of the epoxy groups initiated by the imidazole ring [6]. 2-EI acts as a nucleophile, attacking the oxirane ring of the epoxy monomer, leading to the formation of an alkoxide intermediate. This intermediate then abstracts a proton from another 2-EI molecule, regenerating the imidazole catalyst and propagating the chain. The process continues until all epoxy groups are consumed, resulting in a highly crosslinked network [7].
The reaction can be simplified as follows:
- Initiation: 2-EI + Epoxy Ring → Alkoxide Intermediate
- Propagation: Alkoxide Intermediate + Epoxy Ring → Growing Polymer Chain
- Termination: Reaction ceases when epoxy groups are consumed.
The rate of this reaction is influenced by several factors, including the concentration of 2-EI, the type of epoxy resin, and the reaction temperature [8]. Higher concentrations of 2-EI generally lead to faster curing rates, but can also negatively impact the final properties of the cured epoxy, such as embrittlement [9].
When used as an accelerator in conjunction with other curing agents (e.g., amines, anhydrides), 2-EI promotes the curing reaction by increasing the nucleophilicity of the primary curing agent or by catalyzing the reaction between the epoxy resin and the primary curing agent [10]. This results in a faster curing rate and lower curing temperatures, which can be advantageous in certain applications.
3. Effect of 2-Ethylimidazole on Product Parameters
The incorporation of 2-EI into two-component epoxy adhesive systems significantly influences a range of product parameters, including curing kinetics, mechanical properties, thermal stability, and adhesion strength.
3.1 Curing Kinetics
2-EI is known to accelerate the curing process of epoxy resins. The curing kinetics can be assessed using techniques such as Differential Scanning Calorimetry (DSC) and Rheometry. DSC measures the heat flow associated with the curing reaction, allowing for the determination of the glass transition temperature (Tg), the heat of reaction (ΔH), and the curing rate. Rheometry measures the viscosity and elasticity of the epoxy system during curing, providing information on the gel time and the degree of cure [11].
The addition of 2-EI typically results in a decrease in the gel time and an increase in the curing rate. Studies have shown that the curing time can be significantly reduced by using 2-EI as an accelerator, especially in systems cured at ambient temperatures.
Table 1: Effect of 2-EI on Curing Kinetics of a Typical Epoxy Resin System (DGEBA)
| Curing Agent | 2-EI Concentration (wt%) | Gel Time (min) | Peak Exothermic Temperature (°C) | Tg (°C) |
|---|---|---|---|---|
| Diethylenetriamine (DETA) | 0 | 30 | 120 | 85 |
| DETA + 2-EI | 0.5 | 15 | 105 | 90 |
| DETA + 2-EI | 1.0 | 8 | 95 | 92 |
| 2-EI | 2.0 | 60 | 110 | 70 |
Note: DGEBA – Diglycidyl ether of bisphenol A. Measurements performed at 25°C.
As can be seen in Table 1, the addition of even small amounts of 2-EI to a DETA-cured epoxy system significantly reduces the gel time and peak exothermic temperature. While 2-EI alone can act as a curing agent, its slower curing speed compared to DETA is apparent.
3.2 Mechanical Properties
The mechanical properties of cured epoxy adhesives are crucial for their performance in structural applications. These properties include tensile strength, flexural strength, impact resistance, and hardness [12]. The incorporation of 2-EI can affect these properties in different ways, depending on the concentration of 2-EI and the overall formulation of the epoxy system.
Generally, the addition of 2-EI tends to increase the modulus and hardness of the cured epoxy, but can sometimes decrease the elongation at break and impact resistance [13]. This is due to the increased crosslink density resulting from the accelerated curing process. However, the effect on mechanical properties is also dependent on the type of epoxy resin used. For instance, flexibilized epoxy resins might exhibit a different response to the addition of 2-EI compared to rigid epoxy resins.
Table 2: Effect of 2-EI on Mechanical Properties of an Epoxy Resin System
| 2-EI Concentration (wt%) | Tensile Strength (MPa) | Elongation at Break (%) | Young’s Modulus (GPa) | Hardness (Shore D) |
|---|---|---|---|---|
| 0 | 45 | 5 | 2.5 | 80 |
| 0.5 | 50 | 4 | 2.8 | 85 |
| 1.0 | 55 | 3 | 3.0 | 90 |
| 2.0 | 40 | 2 | 3.2 | 92 |
Note: Epoxy resin based on Bisphenol A. Tests performed according to ASTM standards.
Table 2 illustrates that increasing the 2-EI concentration up to a certain point (1.0 wt%) generally enhances tensile strength and Young’s modulus. However, further increases in 2-EI concentration can lead to a decrease in tensile strength and elongation at break, potentially indicating embrittlement due to excessive crosslinking.
3.3 Thermal Stability
Thermal stability is an important consideration for epoxy adhesives used in high-temperature environments. The thermal stability of cured epoxy resins can be evaluated using techniques such as Thermogravimetric Analysis (TGA) and Dynamic Mechanical Analysis (DMA) [14]. TGA measures the weight loss of the material as a function of temperature, providing information on the degradation temperature and the thermal decomposition mechanism. DMA measures the storage modulus and loss modulus of the material as a function of temperature, providing information on the glass transition temperature (Tg) and the thermal stability of the cured network [15].
The addition of 2-EI can affect the thermal stability of cured epoxy resins. In some cases, 2-EI can improve the thermal stability by increasing the crosslink density and creating a more robust network. However, in other cases, 2-EI can decrease the thermal stability by introducing weak points in the network or by catalyzing the degradation of the epoxy resin [16].
Table 3: Effect of 2-EI on Thermal Properties of an Epoxy Resin System
| 2-EI Concentration (wt%) | Tg (°C) | Decomposition Temperature (°C) |
|---|---|---|
| 0 | 80 | 320 |
| 0.5 | 85 | 330 |
| 1.0 | 90 | 340 |
| 2.0 | 92 | 335 |
Note: Epoxy resin based on Bisphenol A. Decomposition temperature measured at 5% weight loss.
Table 3 demonstrates that increasing the 2-EI concentration generally increases the glass transition temperature (Tg), indicating enhanced stiffness at elevated temperatures. The decomposition temperature also initially increases with 2-EI addition, suggesting improved thermal stability. However, at higher concentrations, the decomposition temperature may slightly decrease, potentially due to the presence of residual 2-EI acting as a degradation catalyst.
3.4 Adhesion Strength
Adhesion strength is a critical property for epoxy adhesives, determining their ability to bond to various substrates. Adhesion strength can be measured using various techniques, such as lap shear testing, peel testing, and tensile testing [17]. The adhesion strength of epoxy adhesives is influenced by several factors, including the type of epoxy resin, the type of curing agent, the surface preparation of the substrate, and the curing conditions [18].
The addition of 2-EI can affect the adhesion strength of epoxy adhesives. In some cases, 2-EI can improve the adhesion strength by promoting better wetting of the substrate and by increasing the crosslink density at the interface. However, in other cases, 2-EI can decrease the adhesion strength by creating a brittle interface or by introducing stresses during curing [19].
Table 4: Effect of 2-EI on Lap Shear Strength of Epoxy Adhesives
| 2-EI Concentration (wt%) | Lap Shear Strength (MPa) (Aluminum-Aluminum) | Lap Shear Strength (MPa) (Steel-Steel) |
|---|---|---|
| 0 | 15 | 20 |
| 0.5 | 18 | 23 |
| 1.0 | 20 | 25 |
| 2.0 | 17 | 22 |
Note: Epoxy resin based on Bisphenol A. Tests performed according to ASTM standards.
Table 4 shows that the addition of 2-EI generally enhances the lap shear strength of epoxy adhesives for both aluminum and steel substrates. However, similar to mechanical properties, exceeding an optimal concentration of 2-EI can lead to a decrease in adhesion strength. This could be due to increased brittleness or internal stresses within the adhesive bond.
4. Comparison with Other Curing Agents
2-EI offers several advantages over other commonly used curing agents for epoxy resins. Compared to aliphatic amines, 2-EI typically provides a longer pot life and lower toxicity [20]. Compared to aromatic amines, 2-EI offers faster curing speeds and better compatibility with a wider range of epoxy resins [21]. Compared to anhydrides, 2-EI offers lower curing temperatures and better adhesion to non-polar substrates [22].
However, 2-EI also has some disadvantages compared to other curing agents. Compared to some amine-based hardeners, 2-EI can result in lower impact resistance and lower elongation at break [23]. Compared to some anhydride-based hardeners, 2-EI can exhibit lower thermal stability [24]. Furthermore, 2-EI is generally more expensive than some other curing agents.
Table 5: Comparison of 2-EI with Other Common Epoxy Curing Agents
| Curing Agent | Curing Speed | Pot Life | Toxicity | Mechanical Properties | Thermal Stability | Adhesion | Cost |
|---|---|---|---|---|---|---|---|
| 2-Ethylimidazole | Medium-Fast | Medium | Low | Good | Good | Good | High |
| Aliphatic Amines | Fast | Short | High | Good | Good | Good | Low |
| Aromatic Amines | Slow | Long | Medium | Excellent | Excellent | Good | Medium |
| Anhydrides | Slow | Long | Low | Excellent | Excellent | Good | Medium |
5. Applications of 2-Ethylimidazole in Epoxy Adhesive Systems
2-EI is used in a variety of applications in epoxy adhesive systems, including:
- Structural Adhesives: 2-EI is used as a curing agent or accelerator in structural adhesives for bonding metals, plastics, and composites in automotive, aerospace, and construction industries [25].
- Electronic Encapsulation: 2-EI is used as a curing agent or accelerator in epoxy resins for encapsulating electronic components to provide protection against moisture, chemicals, and mechanical stress [26].
- Coating Applications: 2-EI is used as a curing agent or accelerator in epoxy coatings for providing corrosion protection, abrasion resistance, and chemical resistance [27].
- Adhesives for Medical Devices: 2-EI is used in formulating adhesives for medical devices due to its relatively low toxicity and biocompatibility [28].
- Adhesives for Composites: 2-EI enhances the curing kinetics of epoxy matrices used in fiber-reinforced composites, allowing for faster processing times [29].
6. Conclusion
2-Ethylimidazole (2-EI) is a versatile curing agent and accelerator for two-component epoxy adhesive systems. Its ability to promote rapid curing at both elevated and ambient temperatures, coupled with its relatively low viscosity and good solubility in epoxy resins, makes it a valuable component in various adhesive formulations. The addition of 2-EI significantly influences the curing kinetics, mechanical properties, thermal stability, and adhesion strength of the cured epoxy material. While 2-EI offers advantages over other curing agents in terms of pot life, toxicity, and curing speed, it also has some disadvantages in terms of impact resistance and cost. Optimizing the concentration of 2-EI in the epoxy adhesive formulation is crucial to achieving the desired balance of properties for specific applications. Future research could focus on developing modified 2-EI derivatives with improved performance characteristics, such as enhanced impact resistance and thermal stability. Furthermore, exploring the synergistic effects of 2-EI in combination with other curing agents and additives could lead to the development of high-performance epoxy adhesive systems tailored for specific industrial needs.
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