Investigating the storage stability of epoxy resins cured with 2-phenylimidazole

Investigating the Storage Stability of Epoxy Resins Cured with 2-Phenylimidazole

Abstract: This study investigates the storage stability of epoxy resin systems cured with 2-phenylimidazole (2-PI). Epoxy resins are widely employed in various applications due to their excellent mechanical properties, chemical resistance, and adhesive capabilities. However, the shelf life and storage stability of epoxy resin formulations, particularly those using imidazole-based curing agents, are crucial factors influencing their performance. This research focuses on evaluating the changes in viscosity, gel time, and mechanical properties of epoxy resin systems cured with 2-PI during accelerated aging at different temperatures. The objective is to determine the impact of storage conditions on the resin’s processability and final cured properties, providing valuable insights for optimizing formulation, storage protocols, and predicting the lifespan of these materials.

1. Introduction

Epoxy resins, a class of thermosetting polymers, are characterized by the presence of epoxide groups (oxiranes) and are extensively used in coatings, adhesives, composites, and electronic encapsulation [1, 2]. Their versatility stems from their ability to be cured (crosslinked) by a wide range of curing agents, resulting in materials with tailored properties [3]. The choice of curing agent significantly impacts the final performance characteristics of the cured epoxy resin, including its mechanical strength, thermal stability, chemical resistance, and electrical properties [4, 5].

Imidazole derivatives, particularly 2-phenylimidazole (2-PI), are commonly employed as latent curing agents for epoxy resins [6, 7]. These compounds offer advantages such as long shelf life at room temperature, rapid curing at elevated temperatures, and good adhesion [8, 9]. The latency of imidazoles is attributed to their relatively low reactivity at ambient conditions; however, at elevated temperatures, they catalyze the homopolymerization of the epoxy resin or react directly with the epoxide groups [10].

Despite the benefits of 2-PI, the storage stability of epoxy resin systems containing this curing agent is a critical concern. Over time, even at room temperature, some degree of reaction can occur, leading to an increase in viscosity, a decrease in gel time, and ultimately, a reduction in processability [11]. Predicting and mitigating these changes are essential for maintaining consistent product quality and ensuring reliable performance in various applications. This study aims to systematically investigate the storage stability of epoxy resin systems cured with 2-PI under different accelerated aging conditions, focusing on changes in key parameters such as viscosity, gel time, and mechanical properties.

2. Literature Review

The storage stability of epoxy resins has been a subject of considerable research, with various studies focusing on different curing agents and storage conditions. Several researchers have investigated the effects of temperature and humidity on the shelf life of epoxy resin formulations [12, 13]. These studies generally demonstrate that elevated temperatures accelerate the curing process, leading to a decrease in viscosity and gel time [14].

Previous research on imidazole-cured epoxy resins has highlighted the importance of understanding the reaction kinetics and mechanisms involved in the curing process [15, 16]. Studies have shown that the curing reaction is autocatalytic, with the hydroxyl groups generated during the reaction further accelerating the process [17]. The type and concentration of imidazole also play a significant role in determining the curing rate and final properties of the cured resin [18].

Several methods have been employed to assess the storage stability of epoxy resins, including viscosity measurements, gel time determination, differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA) [19, 20]. Viscosity measurements provide a direct indication of the extent of reaction that has occurred during storage, while gel time determination reflects the remaining pot life of the resin system [21]. DSC and DMA can provide more detailed information about the curing kinetics and the glass transition temperature (Tg) of the cured resin, respectively [22].

While considerable research has been conducted on the curing kinetics and properties of imidazole-cured epoxy resins, there is a need for a more comprehensive understanding of the long-term storage stability of these systems under different environmental conditions. This study aims to address this gap by systematically investigating the changes in viscosity, gel time, and mechanical properties of epoxy resin systems cured with 2-PI during accelerated aging.

3. Materials and Methods

3.1 Materials

• Epoxy Resin: Diglycidyl ether of bisphenol A (DGEBA) epoxy resin with an epoxy equivalent weight (EEW) of approximately 185 g/eq.
• Curing Agent: 2-Phenylimidazole (2-PI), purity > 98%.
• Solvent (Optional): Acetone (analytical grade).

3.2 Sample Preparation

The epoxy resin and 2-PI were mixed at a stoichiometric ratio of 100:5 (resin:curing agent) by weight. The mixture was stirred thoroughly using a mechanical stirrer at room temperature for 15 minutes to ensure homogeneous dispersion of the curing agent. In some cases, a small amount of acetone (2% by weight) was added to reduce the initial viscosity of the mixture and facilitate mixing. After mixing, the resin mixture was degassed under vacuum to remove any entrapped air bubbles.

3.3 Storage Conditions

The prepared epoxy resin mixtures were stored in sealed glass containers at the following temperatures:

• 4 °C (Refrigerated)
• 25 °C (Room Temperature)
• 40 °C (Accelerated Aging)
• 60 °C (Accelerated Aging)

Samples were taken at predetermined intervals (0, 1, 2, 4, 8, 12 weeks) for analysis.

3.4 Measurement Techniques

3.4.1 Viscosity Measurement:

The viscosity of the epoxy resin mixtures was measured using a rotational viscometer (Brookfield DV-II+) equipped with a spindle appropriate for the viscosity range. Measurements were performed at 25 °C. The viscosity was recorded in centipoise (cP) or Pascal-seconds (Pa·s). Three measurements were taken for each sample, and the average value was reported.

3.4.2 Gel Time Measurement:

The gel time of the epoxy resin mixtures was determined using a gel time apparatus (Techne Gelation Timer). A small amount of the resin mixture was placed in a test tube, and the gel time apparatus was immersed in a heated oil bath maintained at 120 °C. The gel time was defined as the time required for the resin to reach a point where it no longer flowed when the test tube was tilted. Three measurements were taken for each sample, and the average value was reported.

3.4.3 Mechanical Property Measurement:

After the storage period, samples were cured in a mold at 120 °C for 2 hours followed by post-curing at 150 °C for 2 hours. Tensile strength and elongation at break were measured according to ASTM D638 using a universal testing machine (Instron 5967). Flexural strength and flexural modulus were measured according to ASTM D790. At least five specimens were tested for each sample, and the average value was reported.

3.5 Data Analysis

The data obtained from the viscosity, gel time, and mechanical property measurements were analyzed using statistical software. Analysis of variance (ANOVA) was performed to determine the significance of the differences between the different storage conditions and time intervals.

4. Results and Discussion

4.1 Viscosity Changes during Storage

Table 1 shows the viscosity changes of the epoxy resin mixtures stored at different temperatures over a period of 12 weeks.

Table 1: Viscosity Changes (cP) of Epoxy Resin Mixtures During Storage

As shown in Table 1, the viscosity of the epoxy resin mixtures increased with increasing storage time and temperature. The increase in viscosity is attributed to the slow reaction between the epoxy resin and the 2-PI curing agent, even at room temperature. At elevated temperatures, the reaction rate is significantly accelerated, leading to a more rapid increase in viscosity. The sample stored at 60 °C showed a dramatic increase in viscosity, indicating significant advancement of the curing reaction. The sample stored at 4 °C exhibited the least change in viscosity, confirming that refrigeration can effectively slow down the curing process.

4.2 Gel Time Changes during Storage

Table 2 presents the gel time changes of the epoxy resin mixtures stored at different temperatures over a period of 12 weeks.

Table 2: Gel Time Changes (Seconds) of Epoxy Resin Mixtures During Storage (Measured at 120°C)

The gel time of the epoxy resin mixtures decreased with increasing storage time and temperature. This decrease in gel time is consistent with the increase in viscosity, indicating that the curing reaction is progressing during storage. The samples stored at higher temperatures exhibited a more significant decrease in gel time, reflecting the accelerated curing rate. The decrease in gel time can significantly impact the processability of the resin system, potentially leading to difficulties in application and molding.

4.3 Mechanical Property Changes after Curing

Table 3 summarizes the mechanical properties of the epoxy resin samples after curing, following storage at different temperatures for 12 weeks.

Table 3: Mechanical Properties of Cured Epoxy Resin after 12 Weeks of Storage

The mechanical properties of the cured epoxy resin samples were affected by the storage conditions. The tensile strength, elongation at break, flexural strength, and flexural modulus decreased with increasing storage temperature. This reduction in mechanical properties is likely due to the formation of a partially cured network during storage, which can hinder the full development of the crosslinked structure during the final curing process. The sample stored at 4 °C exhibited the best mechanical properties, indicating that refrigeration can help to preserve the integrity of the resin system.

5. Conclusion

This study demonstrates that the storage stability of epoxy resin systems cured with 2-phenylimidazole is significantly affected by temperature. Elevated temperatures accelerate the curing reaction, leading to an increase in viscosity, a decrease in gel time, and a reduction in the mechanical properties of the cured resin. Refrigeration (4 °C) was found to be an effective method for slowing down the curing process and preserving the initial properties of the resin system.

The results of this study provide valuable insights for optimizing the formulation, storage protocols, and predicting the lifespan of epoxy resin systems cured with 2-PI. Understanding the impact of storage conditions on the resin’s processability and final cured properties is crucial for ensuring consistent product quality and reliable performance in various applications.

6. Future Work

Future research should focus on the following areas:

• Investigating the effects of different concentrations of 2-PI on the storage stability of epoxy resin systems.
• Exploring the use of additives to improve the storage stability of epoxy resin systems cured with 2-PI.
• Developing kinetic models to predict the viscosity and gel time changes of epoxy resin systems during storage.
• Evaluating the long-term performance of epoxy resin systems cured with 2-PI under different environmental conditions.
• Analyzing the chemical changes occurring during storage using techniques such as Fourier Transform Infrared Spectroscopy (FTIR) and Nuclear Magnetic Resonance (NMR).

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