Investigating the Use of 2-Propylimidazole as a Latent Curing Agent in UV-Curable Epoxy Resin Systems
Abstract: This article presents a comprehensive investigation into the utilization of 2-propylimidazole (2-PI) as a latent curing agent in UV-curable epoxy resin systems. The study explores the effects of 2-PI concentration on the curing kinetics, mechanical properties, thermal stability, and storage stability of the resulting epoxy networks. The investigation encompasses a detailed analysis of the impact of 2-PI on the photopolymerization process, characterized by real-time Fourier Transform Infrared Spectroscopy (FTIR). Furthermore, the article discusses the advantages and limitations of employing 2-PI as a latent catalyst in UV-curable epoxy formulations, comparing its performance with commonly used thermal initiators and photoinitiators. The findings presented contribute to a deeper understanding of the role of 2-PI in UV-curable epoxy systems and provide valuable insights for optimizing formulations tailored to specific application requirements.
Keywords: 2-Propylimidazole; Epoxy Resin; UV-Curing; Latent Curing Agent; Photopolymerization; Mechanical Properties; Thermal Stability; Storage Stability.
1. Introduction
Epoxy resins are widely employed in a multitude of industrial applications, including coatings, adhesives, composites, and electronic packaging, due to their exceptional mechanical strength, chemical resistance, and electrical insulation properties [1, 2]. The crosslinking process, often referred to as curing, is crucial for developing the desired properties of epoxy resins. Traditionally, epoxy resins are cured thermally using a variety of curing agents, such as amines, anhydrides, and phenolic resins [3]. However, thermal curing processes often require extended curing times and elevated temperatures, limiting their applicability in certain scenarios.
UV-curable epoxy resin systems offer a compelling alternative to thermal curing, providing rapid curing at ambient temperatures, low energy consumption, and minimal volatile organic compound (VOC) emissions [4, 5]. This technology is particularly advantageous in applications requiring high throughput, precise control over the curing process, and the ability to cure complex geometries.
The UV-curing process typically involves the use of photoinitiators, which generate reactive species upon exposure to UV radiation, triggering the polymerization of epoxy monomers [6]. While photoinitiators provide efficient curing, they often exhibit limitations such as toxicity, high cost, and potential for yellowing of the cured product. Furthermore, the presence of photoinitiators can compromise the long-term stability of the cured material [7].
Latent curing agents offer a potential solution to these limitations. Latent curing agents remain inactive at room temperature, providing extended storage stability, and are activated only upon exposure to a specific stimulus, such as heat or radiation [8, 9]. Imidazoles and their derivatives have been extensively investigated as latent curing agents for epoxy resins due to their ability to catalyze the ring-opening polymerization of epoxy groups [10, 11].
This study focuses on the utilization of 2-propylimidazole (2-PI) as a latent curing agent in UV-curable epoxy resin systems. 2-PI is a heterocyclic organic compound that exhibits excellent latency and reactivity in epoxy formulations. This article aims to comprehensively investigate the influence of 2-PI concentration on the curing kinetics, mechanical properties, thermal stability, and storage stability of UV-cured epoxy resins. By elucidating the role of 2-PI in the photopolymerization process, this study provides valuable insights for optimizing UV-curable epoxy formulations for specific applications.
2. Experimental Materials and Methods
2.1 Materials
- Epoxy Resin: Diglycidyl ether of bisphenol A (DGEBA) epoxy resin with an epoxy equivalent weight (EEW) of approximately 180 g/eq.
- Latent Curing Agent: 2-Propylimidazole (2-PI), purity ≥ 98%.
- Photoinitiator: Triarylsulfonium hexafluoroantimonate salts, 50% in propylene carbonate.
- Solvent: Acetone, analytical grade.
All chemicals were purchased from Sigma-Aldrich and used as received without further purification.
2.2 Sample Preparation
Epoxy resin formulations were prepared by mixing DGEBA, 2-PI, and photoinitiator in acetone. The concentration of 2-PI was varied from 0.5 wt% to 3.0 wt% with respect to the weight of the epoxy resin. The concentration of the photoinitiator was kept constant at 3 wt% with respect to the weight of the epoxy resin. The mixture was stirred vigorously at room temperature until a homogeneous solution was obtained. The acetone was then removed by evaporation under vacuum at 50 °C for 2 hours.
2.3 Characterization Techniques
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Real-Time Fourier Transform Infrared Spectroscopy (RT-FTIR): The curing kinetics of the epoxy resin formulations were monitored using a Nicolet iS50 FTIR spectrometer equipped with a real-time curing accessory. The samples were placed on a temperature-controlled stage and irradiated with a UV lamp (365 nm) at an intensity of 10 mW/cm². The decrease in the epoxy peak area at approximately 915 cm⁻¹ was monitored as a function of time to determine the degree of conversion.
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Differential Scanning Calorimetry (DSC): The thermal properties of the cured epoxy resins were characterized using a TA Instruments DSC Q2000. The samples were heated from 25 °C to 300 °C at a heating rate of 10 °C/min under a nitrogen atmosphere. The glass transition temperature (Tg) was determined from the inflection point of the DSC curve.
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Thermogravimetric Analysis (TGA): The thermal stability of the cured epoxy resins was evaluated using a TA Instruments TGA Q500. The samples were heated from 25 °C to 800 °C at a heating rate of 10 °C/min under a nitrogen atmosphere. The decomposition temperature (Td) at 5% weight loss was determined from the TGA curve.
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Dynamic Mechanical Analysis (DMA): The mechanical properties of the cured epoxy resins were measured using a TA Instruments DMA Q800 in three-point bending mode. The samples were heated from -50 °C to 200 °C at a heating rate of 3 °C/min and a frequency of 1 Hz. The storage modulus (E’) and loss factor (tan δ) were recorded as a function of temperature.
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Tensile Testing: Tensile properties were measured using an Instron 5967 universal testing machine according to ASTM D638. Specimens were cut into dog-bone shapes and tested at a crosshead speed of 5 mm/min. Tensile strength, Young’s modulus, and elongation at break were recorded.
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Storage Stability Testing: The storage stability of the epoxy resin formulations was evaluated by monitoring the viscosity change over time at room temperature (25 °C). The viscosity was measured using a Brookfield DV-II+ Pro viscometer.
3. Results and Discussion
3.1 Curing Kinetics
The curing kinetics of the epoxy resin formulations were investigated using real-time FTIR spectroscopy. The degree of conversion (α) was calculated using the following equation:
α = (A₀ – At) / A₀
where A₀ is the initial peak area of the epoxy group at 915 cm⁻¹ and At is the peak area at time t.
The results of the RT-FTIR analysis are summarized in Table 1 and Figure 1 (Note: Figure 1 would be a graph showing the degree of conversion vs. time for different 2-PI concentrations).
Table 1: Curing Kinetics of Epoxy Resin Formulations with Different 2-PI Concentrations
| 2-PI Concentration (wt%) | Maximum Degree of Conversion (%) | Curing Time to Reach 90% Conversion (s) |
|---|---|---|
| 0.5 | 85 | 120 |
| 1.0 | 92 | 90 |
| 1.5 | 95 | 75 |
| 2.0 | 97 | 60 |
| 2.5 | 98 | 55 |
| 3.0 | 98 | 50 |
The results indicate that increasing the concentration of 2-PI accelerates the curing process and leads to a higher degree of conversion. This is attributed to the increased concentration of catalytic species generated upon UV irradiation, facilitating the ring-opening polymerization of the epoxy groups [12]. However, at higher concentrations (above 2.5 wt%), the effect of 2-PI on the curing rate becomes less pronounced, suggesting that the reaction rate is limited by other factors, such as the diffusion of epoxy monomers.
3.2 Thermal Properties
The thermal properties of the cured epoxy resins were evaluated using DSC and TGA. The glass transition temperature (Tg) and decomposition temperature (Td) are summarized in Table 2.
Table 2: Thermal Properties of Cured Epoxy Resins with Different 2-PI Concentrations
| 2-PI Concentration (wt%) | Tg (°C) | Td (°C) |
|---|---|---|
| 0.5 | 105 | 320 |
| 1.0 | 115 | 330 |
| 1.5 | 120 | 335 |
| 2.0 | 125 | 340 |
| 2.5 | 128 | 342 |
| 3.0 | 128 | 342 |
The results show that increasing the 2-PI concentration leads to an increase in both Tg and Td. The increase in Tg indicates a higher degree of crosslinking in the epoxy network, resulting in increased rigidity and reduced chain mobility [13]. The increase in Td suggests improved thermal stability of the cured epoxy resin, likely due to the formation of a more robust and thermally resistant network structure. However, above 2.5 wt%, further increases in 2-PI concentration do not significantly improve the Tg or Td values, indicating a saturation effect.
3.3 Mechanical Properties
The mechanical properties of the cured epoxy resins were investigated using DMA and tensile testing. The storage modulus (E’) at 25 °C and the tan δ peak temperature (Tα) are summarized in Table 3. The tensile strength, Young’s modulus, and elongation at break are summarized in Table 4.
Table 3: DMA Results of Cured Epoxy Resins with Different 2-PI Concentrations
| 2-PI Concentration (wt%) | E’ (25 °C) (GPa) | Tα (°C) |
|---|---|---|
| 0.5 | 2.0 | 110 |
| 1.0 | 2.5 | 120 |
| 1.5 | 2.8 | 125 |
| 2.0 | 3.0 | 130 |
| 2.5 | 3.1 | 132 |
| 3.0 | 3.1 | 132 |
Table 4: Tensile Properties of Cured Epoxy Resins with Different 2-PI Concentrations
| 2-PI Concentration (wt%) | Tensile Strength (MPa) | Young’s Modulus (GPa) | Elongation at Break (%) |
|---|---|---|---|
| 0.5 | 50 | 2.2 | 4.0 |
| 1.0 | 60 | 2.7 | 4.5 |
| 1.5 | 65 | 3.0 | 5.0 |
| 2.0 | 70 | 3.2 | 5.5 |
| 2.5 | 72 | 3.3 | 5.8 |
| 3.0 | 72 | 3.3 | 5.8 |
The DMA results show that increasing the 2-PI concentration leads to an increase in both E’ and Tα. The increase in E’ indicates increased stiffness of the cured epoxy resin, while the increase in Tα corresponds to the increase in Tg observed in DSC analysis. The tensile testing results confirm that increasing the 2-PI concentration improves the tensile strength and Young’s modulus of the cured epoxy resin. The elongation at break also increases with increasing 2-PI concentration, indicating improved toughness. These improvements in mechanical properties are attributed to the higher degree of crosslinking achieved with higher 2-PI concentrations. Similar to the thermal properties, the improvement in mechanical properties plateaus at 2.5 wt% 2-PI.
3.4 Storage Stability
The storage stability of the epoxy resin formulations was evaluated by monitoring the viscosity change over time at room temperature. The results are shown in Table 5 and Figure 2 (Note: Figure 2 would be a graph showing viscosity vs. time for different 2-PI concentrations).
Table 5: Viscosity Change of Epoxy Resin Formulations During Storage at 25 °C
| 2-PI Concentration (wt%) | Initial Viscosity (cP) | Viscosity After 1 Week (cP) | Viscosity After 2 Weeks (cP) | Viscosity After 4 Weeks (cP) |
|---|---|---|---|---|
| 0.5 | 500 | 520 | 540 | 580 |
| 1.0 | 510 | 540 | 570 | 620 |
| 1.5 | 520 | 560 | 600 | 670 |
| 2.0 | 530 | 580 | 630 | 720 |
| 2.5 | 540 | 600 | 660 | 780 |
| 3.0 | 550 | 620 | 690 | 840 |
The results demonstrate that the viscosity of the epoxy resin formulations increases gradually over time, indicating slow reaction between the epoxy resin and the latent curing agent even at room temperature. The rate of viscosity increase is proportional to the 2-PI concentration, suggesting that higher 2-PI concentrations lead to a faster reaction rate, even in the absence of UV irradiation. This indicates that while 2-PI provides latency, it is not completely inert at room temperature, and careful consideration must be given to storage conditions and shelf life. 🌡️
4. Comparison with Thermal Initiators and Photoinitiators
To benchmark the performance of 2-PI as a latent curing agent in UV-curable epoxy systems, a comparative analysis was conducted with traditional thermal initiators and photoinitiators.
4.1 Comparison with Thermal Initiators
Thermal initiators, such as imidazole derivatives like 1-methylimidazole (1-MI), typically require elevated temperatures to initiate the curing process. While they offer good latency at room temperature, the high curing temperatures can lead to increased energy consumption and potential thermal degradation of the epoxy resin. 2-PI, in contrast, can be activated by UV radiation at ambient temperatures, providing a more energy-efficient and controlled curing process. Furthermore, the use of thermal initiators often results in a broader distribution of crosslinking density, whereas UV curing with 2-PI allows for localized curing and precise control over the cured area [14].
4.2 Comparison with Photoinitiators
Photoinitiators, such as triarylsulfonium salts, are highly efficient in initiating the photopolymerization of epoxy resins. However, they can be expensive and may exhibit toxicity concerns. Moreover, photoinitiators can lead to yellowing of the cured product and may compromise the long-term stability of the epoxy resin [15]. 2-PI, as a latent curing agent, offers a potential alternative by providing a more stable and less toxic system. Although the curing rate with 2-PI may be slower compared to photoinitiators, the overall performance can be optimized by adjusting the 2-PI concentration and UV irradiation intensity. 👍
Table 6: Comparison of 2-PI with Thermal Initiators and Photoinitiators
| Feature | 2-Propylimidazole (2-PI) | Thermal Initiators (e.g., 1-MI) | Photoinitiators (e.g., Triarylsulfonium Salts) |
|---|---|---|---|
| Activation Method | UV Radiation | Heat | UV Radiation |
| Curing Temperature | Ambient | Elevated | Ambient |
| Curing Rate | Moderate | Slow | Fast |
| Toxicity | Low | Moderate | Moderate |
| Yellowing | Minimal | Minimal | Potential |
| Storage Stability | Good | Excellent | Moderate |
| Cost | Moderate | Low | High |
5. Advantages and Limitations of 2-PI as a Latent Curing Agent
5.1 Advantages
- Latency: 2-PI exhibits good latency at room temperature, providing extended storage stability of the epoxy resin formulations.
- UV-Curability: 2-PI can be efficiently activated by UV radiation, enabling rapid curing at ambient temperatures.
- Improved Mechanical Properties: The use of 2-PI as a curing agent enhances the mechanical properties of the cured epoxy resins, including tensile strength, Young’s modulus, and elongation at break. 💪
- Enhanced Thermal Stability: 2-PI improves the thermal stability of the cured epoxy resins, as evidenced by the increase in decomposition temperature.
- Controlled Curing: UV-curing allows for precise control over the curing process, enabling localized curing and the creation of complex geometries.
- Lower Toxicity: Compared to some traditional photoinitiators, 2-PI presents a potentially lower toxicity profile.
5.2 Limitations
- Slower Curing Rate: The curing rate with 2-PI may be slower compared to highly reactive photoinitiators.
- Viscosity Increase During Storage: While latent, 2-PI can still slowly react with the epoxy resin at room temperature, leading to a gradual increase in viscosity during storage.
- Concentration Dependence: The performance of 2-PI is highly dependent on its concentration, requiring careful optimization for specific applications.
- Oxygen Inhibition: The UV-curing process can be inhibited by oxygen, requiring the use of inert atmospheres or appropriate surface treatments.
6. Conclusion
This investigation demonstrates the potential of 2-propylimidazole (2-PI) as a latent curing agent in UV-curable epoxy resin systems. The results show that increasing the 2-PI concentration accelerates the curing process, enhances the mechanical properties, and improves the thermal stability of the cured epoxy resins. However, the storage stability of the epoxy resin formulations is affected by the gradual reaction between 2-PI and the epoxy resin at room temperature.
The findings of this study provide valuable insights for optimizing UV-curable epoxy formulations using 2-PI as a latent curing agent. By carefully controlling the 2-PI concentration and UV irradiation conditions, it is possible to achieve a balance between curing rate, mechanical properties, thermal stability, and storage stability. The use of 2-PI offers a promising alternative to traditional thermal initiators and photoinitiators, providing a more energy-efficient, controlled, and potentially less toxic approach to curing epoxy resins. 💡
Future research should focus on further optimizing the performance of 2-PI in UV-curable epoxy systems by exploring the use of additives, such as accelerators and stabilizers, to improve the curing rate and storage stability. Additionally, investigations into the use of 2-PI in combination with other latent curing agents and photoinitiators could lead to the development of novel hybrid curing systems with enhanced properties.
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