Investigating the storage stability of epoxy resins containing 2-ethylimidazole

Investigating the Storage Stability of Epoxy Resins Containing 2-Ethylimidazole

Abstract: Epoxy resins are widely used in various industries due to their excellent mechanical, chemical, and electrical properties. 2-Ethylimidazole (2-EI) is a common latent hardener for epoxy resins, offering advantages such as long pot life and rapid curing at elevated temperatures. However, the storage stability of epoxy resin systems containing 2-EI is a crucial factor affecting their applicability. This article reviews the factors influencing the storage stability of 2-EI-cured epoxy resins, including resin type, 2-EI concentration, temperature, moisture content, and the presence of other additives. Furthermore, it explores various methods for characterizing storage stability, such as viscosity measurements, differential scanning calorimetry (DSC), and gel time determination. Finally, strategies for improving the storage stability of these systems are discussed, aiming to provide a comprehensive understanding of this critical aspect of epoxy resin technology.

Keywords: Epoxy resin, 2-Ethylimidazole, Storage stability, Latent hardener, Viscosity, Gel time, DSC

1. Introduction

Epoxy resins are a class of thermosetting polymers characterized by the presence of oxirane rings (epoxy groups). These resins, when crosslinked with appropriate curing agents, form rigid, durable materials widely utilized in coatings, adhesives, composites, and electronic encapsulants [1]. The versatility of epoxy resins stems from their ability to be formulated with a wide range of curing agents and additives, tailoring their properties for specific applications.

Latent hardeners are a subset of curing agents that remain relatively inactive at room temperature, providing extended pot life to the epoxy resin system [2]. Upon exposure to elevated temperatures, these hardeners initiate the curing reaction, resulting in rapid crosslinking. 2-Ethylimidazole (2-EI) is a widely employed latent hardener for epoxy resins, particularly in applications requiring long storage times and rapid curing cycles [3]. 2-EI offers several advantages, including good solubility in epoxy resins, relatively low toxicity, and the ability to catalyze both homopolymerization of the epoxy resin and reaction with other curing agents, such as anhydrides [4].

Despite its benefits, the storage stability of epoxy resins containing 2-EI remains a critical concern. Gradual advancement of the curing reaction during storage can lead to increased viscosity, reduced reactivity, and ultimately, gelation, rendering the material unusable [5]. Understanding the factors affecting storage stability and developing strategies to mitigate unwanted advancement are essential for ensuring the reliable performance of 2-EI-cured epoxy resin systems.

2. Factors Influencing Storage Stability

The storage stability of epoxy resins containing 2-EI is influenced by a complex interplay of factors. These factors can be broadly categorized as intrinsic properties of the resin and hardener, environmental conditions, and the presence of other additives.

2.1 Resin Type

The chemical structure and molecular weight of the epoxy resin significantly impact its storage stability with 2-EI. Bisphenol A diglycidyl ether (BADGE) is a common epoxy resin type, but other resins, such as bisphenol F diglycidyl ether (BFDGE) and novolac epoxy resins, are also used [6].

• Epoxy Equivalent Weight (EEW): Resins with lower EEW contain a higher concentration of epoxy groups, potentially leading to faster reaction rates with 2-EI and reduced storage stability.
• Viscosity: Higher viscosity resins can hinder the diffusion of 2-EI, potentially slowing down the reaction rate at lower temperatures and improving storage stability. However, highly viscous resins may be more susceptible to gelation if the reaction progresses.
• Purity: The presence of impurities, such as residual epichlorohydrin or unreacted bisphenol A, can act as catalysts or accelerators, affecting the reaction kinetics and storage stability [7].

2.2 2-Ethylimidazole Concentration

The concentration of 2-EI directly influences the curing rate and, consequently, the storage stability. Higher concentrations of 2-EI generally lead to faster curing but also reduced storage life [8].

• Catalytic Effect: 2-EI acts as a catalyst, promoting the ring-opening polymerization of the epoxy resin. Increasing the 2-EI concentration increases the number of catalytic sites, accelerating the reaction.
• Self-Association: At higher concentrations, 2-EI molecules can self-associate, potentially reducing their activity and affecting the curing kinetics [9]. However, this effect is less pronounced at elevated temperatures.

2.3 Temperature

Temperature is a primary factor affecting the storage stability of epoxy resins containing 2-EI. Elevated temperatures accelerate the reaction between the epoxy resin and 2-EI, leading to a decrease in storage life [10].

• Arrhenius Behavior: The reaction rate generally follows Arrhenius behavior, with the rate constant increasing exponentially with temperature. This implies that even small temperature fluctuations can significantly impact storage stability.
• Glass Transition Temperature (Tg): As the reaction progresses, the Tg of the resin mixture increases. Reaching a Tg above the storage temperature can significantly slow down the reaction rate, improving storage stability.

2.4 Moisture Content

Moisture can significantly impact the storage stability of epoxy resins, particularly those containing hygroscopic hardeners like 2-EI [11].

• Hydrolysis: Water can hydrolyze the epoxy ring, generating hydroxyl groups that can further react with 2-EI, accelerating the curing process.
• Plasticization: Moisture can act as a plasticizer, reducing the viscosity of the resin and increasing the mobility of 2-EI, potentially accelerating the reaction rate.
• Reaction with 2-EI: Water can react directly with 2-EI, forming byproducts that may influence the curing process and affect the final properties of the cured resin [12].

2.5 Additives

The presence of other additives, such as fillers, flexibilizers, and accelerators, can also influence the storage stability of epoxy resins containing 2-EI [13].

• Fillers: Some fillers can absorb moisture, affecting the water content of the resin system. Others can interact with 2-EI, either accelerating or inhibiting the curing reaction.
• Flexibilizers: Flexibilizers can reduce the viscosity of the resin, potentially increasing the mobility of 2-EI and affecting the reaction rate.
• Accelerators: The addition of accelerators, such as tertiary amines or carboxylic acids, can significantly reduce the storage stability by promoting the reaction between the epoxy resin and 2-EI [14].

3. Characterization of Storage Stability

Several methods can be employed to characterize the storage stability of epoxy resins containing 2-EI. These methods typically involve monitoring changes in the physical and chemical properties of the resin mixture over time under controlled storage conditions.

3.1 Viscosity Measurements

Viscosity measurements are a simple and widely used method for assessing the storage stability of epoxy resins. An increase in viscosity indicates advancement of the curing reaction [15].

• Method: Viscosity is typically measured using a rotational viscometer at a constant temperature. Measurements are taken at regular intervals over a specified storage period.
• Parameters: Key parameters include initial viscosity, rate of viscosity increase, and gel time.
• Limitations: Viscosity measurements are sensitive to temperature fluctuations and may not accurately reflect the extent of reaction at very early stages.

3.2 Differential Scanning Calorimetry (DSC)

DSC is a thermal analysis technique that measures the heat flow associated with phase transitions and chemical reactions. It can be used to monitor the progress of the curing reaction and assess the storage stability of epoxy resins [16].

• Method: A small sample of the resin mixture is heated at a controlled rate, and the heat flow is measured as a function of temperature. The exothermic peak corresponding to the curing reaction provides information about the reactivity of the system.
• Parameters: Key parameters include onset temperature, peak temperature, heat of reaction, and residual heat of reaction after storage. A decrease in the heat of reaction after storage indicates advancement of the curing reaction.
• Limitations: DSC requires specialized equipment and may not be suitable for highly viscous materials.

3.3 Gel Time Determination

Gel time is the time required for the resin mixture to transition from a liquid to a gel-like state. It is a practical measure of the reactivity of the system and can be used to assess storage stability [17].

• Method: A small sample of the resin mixture is heated at a constant temperature, and the time required for gelation is measured visually or using a mechanical probe.
• Parameters: Gel time is the primary parameter. A decrease in gel time after storage indicates advancement of the curing reaction.
• Limitations: Gel time determination is subjective and can be influenced by the operator’s experience. It may not be suitable for highly reactive systems with very short gel times.

3.4 Other Methods

Other methods, such as Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) spectroscopy, can also be used to monitor the chemical changes occurring during storage and assess storage stability [18, 19]. However, these methods are more complex and require specialized expertise.

Table 1: Summary of Methods for Characterizing Storage Stability

4. Strategies for Improving Storage Stability

Several strategies can be employed to improve the storage stability of epoxy resins containing 2-EI. These strategies focus on minimizing the unwanted advancement of the curing reaction during storage.

4.1 Reducing Storage Temperature

Lowering the storage temperature is the most effective way to improve storage stability. As discussed earlier, the reaction rate is temperature-dependent, and reducing the temperature significantly slows down the reaction [20].

• Refrigeration: Storing the resin mixture in a refrigerator (e.g., 4°C) can significantly extend its storage life.
• Freezing: In some cases, freezing the resin mixture may be an option, but it is important to ensure that the resin does not undergo any phase separation or degradation during freezing and thawing.

4.2 Optimizing 2-EI Concentration

Using the optimal concentration of 2-EI is crucial for achieving the desired balance between storage stability and curing performance.

• Lower Concentration: Reducing the 2-EI concentration can improve storage stability but may also increase the curing time or require higher curing temperatures.
• Inhibitors: The addition of small amounts of inhibitors, such as acids or phenols, can slow down the reaction rate and improve storage stability without significantly affecting the curing performance [21].

4.3 Controlling Moisture Content

Minimizing the moisture content of the resin mixture is essential for preventing hydrolysis and maintaining storage stability.

• Drying: Drying the resin and 2-EI before mixing can remove any residual moisture.
• Desiccants: Storing the resin mixture in a sealed container with a desiccant can help to maintain a low moisture content.
• Moisture-Resistant Packaging: Using moisture-resistant packaging can prevent the ingress of moisture during storage.

4.4 Using Stabilizers

The addition of stabilizers can help to prevent unwanted reactions and improve storage stability.

• Antioxidants: Antioxidants can prevent oxidation of the resin and 2-EI, which can lead to the formation of byproducts that accelerate the curing reaction [22].
• UV Stabilizers: UV stabilizers can protect the resin from degradation caused by ultraviolet radiation, which can also affect storage stability.
• Metal Deactivators: Metal deactivators can prevent metal ions from catalyzing the curing reaction [23].

4.5 Microencapsulation of 2-EI

Microencapsulation involves encapsulating the 2-EI hardener in a protective shell, preventing it from reacting with the epoxy resin at room temperature [24]. The shell can be designed to rupture at a specific temperature, releasing the 2-EI and initiating the curing reaction.

• Advantages: Microencapsulation can significantly improve storage stability and provide precise control over the curing process.
• Challenges: The microencapsulation process can be complex and expensive. The properties of the shell material must be carefully chosen to ensure compatibility with the resin system and proper release of the 2-EI.

Table 2: Strategies for Improving Storage Stability

5. Conclusion

The storage stability of epoxy resins containing 2-EI is a critical factor affecting their applicability in various industries. Several factors influence storage stability, including resin type, 2-EI concentration, temperature, moisture content, and the presence of other additives. Understanding these factors and employing appropriate strategies to mitigate unwanted advancement of the curing reaction are essential for ensuring the reliable performance of these systems. Viscosity measurements, DSC, and gel time determination are common methods for characterizing storage stability. Strategies for improving storage stability include reducing storage temperature, optimizing 2-EI concentration, controlling moisture content, using stabilizers, and microencapsulation of 2-EI. By carefully considering these factors and implementing appropriate strategies, it is possible to develop epoxy resin systems with excellent storage stability and reliable curing performance. Future research should focus on developing novel stabilizers and microencapsulation techniques to further enhance the storage stability of 2-EI-cured epoxy resins.

6. References

[1] Ellis, B. (1993). Chemistry and Technology of Epoxy Resins. Springer Science & Business Media.

[2] Prime, R. B. (1995). Thermosets. In Thermal Characterization of Polymeric Materials (pp. 713-775). Academic Press.

[3] Richter, C. H. F. (2000). Epoxy Resins: Chemistry and Technology. Marcel Dekker.

[4] Ochi, M., Endo, T., & Shimbo, M. (1987). Curing mechanism of epoxy resin with imidazole derivatives. Journal of Polymer Science: Part A: Polymer Chemistry, 25(2), 641-650.

[5] Mijović, J., & Wijaya, J. (1990). Effect of storage on cure kinetics of epoxy resins. Polymer Engineering & Science, 30(11), 661-667.

[6] May, C. A. (1988). Epoxy Resins: Chemistry and Technology. Marcel Dekker.

[7] Smith, J. G. (1961). The effect of impurities on the curing of epoxy resins. Journal of Applied Polymer Science, 5(15), 597-604.

[8] Schechter, L., & Wynstra, J. (1956). Glycidyl ether reactions with alcohols, phenols, carboxylic acids, and amines. Industrial & Engineering Chemistry, 48(1), 86-93.

[9] Dodiuk, H., & Goodman, S. H. (2013). Handbook of Thermoset Resins. William Andrew.

[10] Barton, J. M. (1985). The application of differential scanning calorimetry (DSC) to the study of epoxy resin curing reactions. Advances in Polymer Science, 72, 111-154.

[11] Bellenger, V., Verdu, J., & Morel, E. (1995). Influence of moisture on the ageing of epoxy resins. Polymer Degradation and Stability, 48(2), 301-308.

[12] Galy, J., Grenier-Loustalot, M. F., & Audoin, L. (1996). Influence of water on the curing kinetics of an epoxy-amine system. Polymer, 37(18), 4141-4148.

[13] Katz, H. S., & Milewski, J. V. (1987). Handbook of Fillers for Plastics. Van Nostrand Reinhold.

[14] Zahir, S. A., Labana, S. S., & Patterson, D. (1982). Accelerated curing of epoxy resins. Journal of Applied Polymer Science, 27(5), 1745-1755.

[15] Ferry, J. D. (1980). Viscoelastic Properties of Polymers. John Wiley & Sons.

[16] Brown, M. E. (2001). Introduction to Thermal Analysis: Techniques and Applications. Springer Science & Business Media.

[17] Cook, W. D. (1992). Gel point phenomena in chain polymerisation. Polymer International, 29(2), 155-164.

[18] Socrates, G. (2001). Infrared and Raman Characteristic Group Frequencies: Tables and Charts. John Wiley & Sons.

[19] Claridge, T. D. W. (2016). High-Resolution NMR Techniques in Organic Chemistry. Elsevier.

[20] Kumar, A., & Gupta, S. K. (1998). Fundamentals of Polymers. McGraw-Hill.

[21] Bauer, R. S. (1993). Epoxy Resin Chemistry. American Chemical Society.

[22] Pospíšil, J., & Nespůrek, S. (2000). Oxidation Inhibitors in Organic Materials. CRC Press.

[23] Stoyanov, A., & Rogovina, L. Z. (2004). Metal Deactivators. Smithers Rapra Publishing.

[24] Ghosh, S. K. (2006). Functional Coatings: By Polymer Microencapsulation. John Wiley & Sons.