Main – Pu catalyst

Enhancing Crosslink Density with Bis[2-(N,N-Dimethylaminoethyl)] Ether in UV-Stable Coatings

Introduction

Ultraviolet (UV)-curable coatings have gained significant traction across various industries due to their rapid curing speed, low volatile organic compound (VOC) emissions, and excellent mechanical and chemical resistance. However, achieving optimal UV stability in these coatings remains a crucial challenge. Degradation due to prolonged UV exposure can manifest as yellowing, cracking, loss of gloss, and diminished protective performance. Enhancing the crosslink density of the coating network is a well-established strategy to improve its UV resistance by reducing polymer chain mobility and minimizing the diffusion of degradation products.

Bis[2-(N,N-Dimethylaminoethyl)] ether, often abbreviated as BDMAEE or Jeffcat ZF-10, is a tertiary amine catalyst widely used in polyurethane (PU) foam production. However, its potential as a crosslinking promoter in UV-curable coatings, especially those requiring enhanced UV stability, is increasingly recognized. This article delves into the mechanisms by which BDMAEE enhances crosslink density, its application in various UV-curable systems, and its impact on the overall performance, particularly UV stability, of the resulting coatings.

1. Bis[2-(N,N-Dimethylaminoethyl)] Ether: Properties and Mechanism

1.1. Chemical Structure and Properties

BDMAEE is a tertiary amine compound with the chemical formula C₁₂H₂₈N₂O. Its structure consists of an ether linkage connecting two dimethylaminoethyl groups. Key properties of BDMAEE are summarized in Table 1.

Table 1: Properties of Bis[2-(N,N-Dimethylaminoethyl)] Ether

Property	Value (Typical)	Unit	Reference
Molecular Weight	204.36	g/mol	[1]
Appearance	Clear, colorless liquid	–	[1]
Density (25°C)	0.84 – 0.85	g/cm³	[1]
Boiling Point	189-192	°C	[1]
Flash Point	66	°C	[1]
Vapor Pressure	< 1	mmHg (20°C)	[1]
Viscosity (25°C)	2.5-3.5	cP	[1]
Amine Value	545-555	mg KOH/g	[1]

Reference: [1] Supplier Technical Data Sheet (e.g., Huntsman, Air Products) – Note: specific values can vary slightly between suppliers.

1.2. Mechanism of Action in UV-Curable Coatings

BDMAEE acts as a catalyst to promote crosslinking reactions in UV-curable systems, particularly those based on acrylates and epoxies. Its mechanism of action can be described as follows:

Base Catalysis: BDMAEE, being a tertiary amine, acts as a nucleophilic base. It abstracts a proton from acidic groups present in the resin system or generated during the UV curing process (e.g., from carboxylic acid groups or hydroxyl groups). This proton abstraction increases the reactivity of other functional groups, such as acrylates or epoxies, towards crosslinking.
Promotion of Isocyanate Reactions (in PU Systems): In UV-curable polyurethane (PU) coatings, BDMAEE accelerates the reaction between isocyanates and hydroxyl-containing components. This is a critical step in the formation of the urethane linkages that define the PU network. The nitrogen atom in BDMAEE coordinates with the isocyanate group, increasing its electrophilicity and making it more susceptible to nucleophilic attack by the hydroxyl group.
Chain Transfer Agent (in certain acrylate systems): In some acrylate-based UV-curable systems, BDMAEE can act as a chain transfer agent, influencing the polymerization process. While not directly involved in crosslinking, its presence can lead to a more controlled polymerization and potentially higher crosslink density by affecting the chain length and branching of the polymer network.
Reaction with Photoinitiators: BDMAEE can interact with certain photoinitiators, particularly those that generate acidic byproducts upon UV exposure. This interaction can neutralize the acidic byproducts and prevent them from inhibiting the polymerization process. This indirect effect can also contribute to a higher overall crosslink density.

The specific mechanism by which BDMAEE influences crosslinking depends on the specific resin system and photoinitiator used. However, the overall effect is typically an increase in the rate and extent of crosslinking, leading to a denser and more robust coating network.

2. Application of BDMAEE in UV-Curable Coatings

BDMAEE finds application in various UV-curable coating formulations, including:

UV-Curable Polyurethane (PU) Coatings: These coatings are known for their excellent flexibility, abrasion resistance, and chemical resistance. BDMAEE plays a crucial role in accelerating the urethane reaction, ensuring rapid curing and high crosslink density.
UV-Curable Acrylate Coatings: Acrylate-based coatings are widely used in applications requiring high hardness, scratch resistance, and gloss. BDMAEE can enhance the crosslinking of acrylates, leading to improved mechanical properties and solvent resistance.
UV-Curable Epoxy Coatings: Epoxy-based coatings are valued for their excellent adhesion, chemical resistance, and electrical insulation properties. BDMAEE can promote the crosslinking of epoxies with hardeners, resulting in a denser and more durable coating.

Table 2: Typical Applications of BDMAEE in UV-Curable Coatings

Coating Type	Application Areas	Benefits of using BDMAEE
UV-Curable PU Coatings	Wood coatings, automotive coatings, textile coatings	Faster curing, improved flexibility, enhanced chemical resistance, increased crosslink density
UV-Curable Acrylate Coatings	Graphic arts, overprint varnishes, plastic coatings	Higher hardness, improved scratch resistance, better solvent resistance, increased crosslink density
UV-Curable Epoxy Coatings	Electronics, industrial coatings, floor coatings	Enhanced adhesion, improved chemical resistance, faster curing, increased crosslink density

3. Impact of BDMAEE on Coating Properties

The addition of BDMAEE to UV-curable coating formulations has a significant impact on the properties of the resulting coatings.

3.1. Crosslink Density:

The primary effect of BDMAEE is to increase the crosslink density of the coating network. This increase is a direct consequence of the mechanisms described in Section 1.2. Higher crosslink density translates to improved mechanical properties, chemical resistance, and, critically, UV stability.

3.2. Mechanical Properties:

Hardness: Increased crosslink density generally leads to higher hardness. This is because the denser network restricts the movement of polymer chains, making the coating more resistant to indentation.
Tensile Strength and Elongation: The effect on tensile strength and elongation is more complex and depends on the specific formulation. While higher crosslink density can increase tensile strength, it can also reduce elongation at break, making the coating more brittle. Careful optimization of the formulation is necessary to achieve the desired balance of these properties.
Abrasion Resistance: Higher crosslink density typically improves abrasion resistance. The denser network provides a stronger barrier against wear and tear.

Table 3: Effect of BDMAEE on Mechanical Properties (Typical Trends)

Property	Effect of Increasing BDMAEE Concentration	Explanation
Hardness	Increase	Denser network restricts chain movement, increasing resistance to indentation.
Tensile Strength	May Increase, then Plateau or Decrease	Initially increases due to stronger network, but excessive crosslinking can lead to brittleness.
Elongation at Break	Decrease	Increased crosslinking restricts chain movement, reducing the ability of the coating to stretch before breaking.
Abrasion Resistance	Increase	Denser network provides a stronger barrier against wear and tear.

3.3. Chemical Resistance:

Higher crosslink density enhances the chemical resistance of the coating. The denser network reduces the penetration of solvents, acids, and bases, protecting the underlying substrate from corrosion and degradation.

3.4. UV Stability:

The most significant benefit of using BDMAEE is the improvement in UV stability. Higher crosslink density reduces polymer chain mobility, minimizing the diffusion of degradation products formed during UV exposure. This reduces yellowing, cracking, and loss of gloss. Furthermore, a denser network can better withstand the stresses induced by UV radiation.

Table 4: Effect of BDMAEE on UV Stability (Typical Trends)

Property	Effect of Increasing BDMAEE Concentration	Explanation
Yellowing	Decrease	Reduced polymer chain mobility minimizes diffusion of yellowing degradation products.
Gloss Retention	Increase	Denser network resists surface degradation and maintains a smoother surface, preserving gloss.
Cracking	Decrease	Stronger network resists the stresses induced by UV radiation, reducing the formation of cracks.
Mechanical Strength after UV Exposure	Increase	Denser network slows down the degradation of mechanical properties upon UV exposure.

4. Factors Affecting the Performance of BDMAEE in UV-Curable Coatings

The effectiveness of BDMAEE in enhancing crosslink density and UV stability depends on several factors:

Resin System: The type of resin used (e.g., polyurethane, acrylate, epoxy) significantly affects the mechanism and extent of BDMAEE’s influence on crosslinking.
Photoinitiator: The choice of photoinitiator is crucial. Certain photoinitiators may be more compatible with BDMAEE than others, and some may even interact with BDMAEE in a detrimental way. Careful selection is essential.
BDMAEE Concentration: The optimal concentration of BDMAEE needs to be carefully determined. Too little BDMAEE may not provide sufficient crosslinking, while too much can lead to undesirable side effects, such as embrittlement or yellowing.
Curing Conditions: UV intensity, exposure time, and temperature all influence the curing process and the effectiveness of BDMAEE.
Additives: Other additives in the formulation, such as UV absorbers, hindered amine light stabilizers (HALS), and antioxidants, can interact with BDMAEE and affect its performance.

5. Formulation Considerations and Optimization

Formulating UV-curable coatings with BDMAEE requires careful consideration of the factors mentioned above. The following guidelines can help optimize the formulation:

Resin Selection: Choose a resin system that is compatible with BDMAEE and suitable for the desired application. Consider the functional groups present in the resin and their reactivity with BDMAEE.
Photoinitiator Selection: Select a photoinitiator that is compatible with both the resin system and BDMAEE. Avoid photoinitiators that generate acidic byproducts that can be neutralized by BDMAEE, as this can reduce its effectiveness as a crosslinking promoter.
BDMAEE Concentration Optimization: Perform a series of experiments to determine the optimal concentration of BDMAEE. Start with a low concentration and gradually increase it, monitoring the effect on crosslink density, mechanical properties, and UV stability. Techniques such as Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) can be used to assess crosslink density.
Additive Selection: Incorporate UV absorbers and HALS to further enhance UV stability. These additives work synergistically with BDMAEE to protect the coating from UV degradation. Antioxidants can also be added to prevent thermal oxidation during the curing process.
Curing Condition Optimization: Optimize the curing conditions to ensure complete curing and maximum crosslink density. Adjust the UV intensity, exposure time, and temperature as needed.
Testing and Evaluation: Thoroughly test and evaluate the performance of the coating, including mechanical properties, chemical resistance, and UV stability. Use standardized test methods to ensure accurate and reliable results.

6. Challenges and Future Trends

While BDMAEE offers significant benefits in enhancing crosslink density and UV stability, there are also some challenges associated with its use:

Yellowing: In some formulations, high concentrations of BDMAEE can contribute to yellowing of the coating, especially upon UV exposure. This can be mitigated by using lower concentrations of BDMAEE, incorporating UV absorbers and HALS, and selecting a photoinitiator that minimizes yellowing.
Odor: BDMAEE has a characteristic amine odor, which can be objectionable in some applications. Using encapsulated BDMAEE or incorporating odor masking agents can help reduce the odor.
Migration: BDMAEE can migrate out of the coating over time, especially in flexible coatings. This can lead to a reduction in performance and potential health and environmental concerns. Using higher molecular weight amine catalysts or chemically bonding the catalyst to the resin can help prevent migration.

Future trends in the use of BDMAEE in UV-curable coatings include:

Development of New BDMAEE Derivatives: Researchers are developing new derivatives of BDMAEE with improved properties, such as lower odor, reduced yellowing, and enhanced compatibility with various resin systems.
Combination with Nanomaterials: Combining BDMAEE with nanomaterials, such as silica nanoparticles or carbon nanotubes, can further enhance the mechanical properties, UV stability, and other performance characteristics of the coating.
Use in Waterborne UV-Curable Coatings: Waterborne UV-curable coatings are gaining popularity due to their low VOC emissions. BDMAEE can be used in these coatings to enhance crosslinking and improve performance.
Development of "Smart" UV-Curable Coatings: BDMAEE can be incorporated into "smart" UV-curable coatings that respond to external stimuli, such as temperature or pH. This can be used to create coatings with self-healing properties or other advanced functionalities.

7. Conclusion

Bis[2-(N,N-Dimethylaminoethyl)] ether (BDMAEE) is a valuable additive for enhancing the crosslink density and UV stability of UV-curable coatings. Its ability to promote crosslinking reactions in various resin systems, particularly polyurethanes, acrylates, and epoxies, makes it a versatile tool for formulators. By carefully optimizing the formulation and curing conditions, BDMAEE can be used to create high-performance UV-curable coatings with excellent mechanical properties, chemical resistance, and UV stability. While challenges such as yellowing and odor need to be addressed, ongoing research and development are leading to new and improved BDMAEE derivatives and applications, paving the way for even more advanced UV-curable coating technologies. The continued exploration of BDMAEE’s potential will undoubtedly contribute to the development of more durable, sustainable, and high-performing coatings for a wide range of industries.

Literature Sources (Fictitious Examples – Replace with Actual Citations)

[1] Smith, A. B., & Jones, C. D. (2010). UV-Curable Coatings: Principles and Applications. Wiley-VCH.

[2] Brown, E. F., et al. (2015). The effect of tertiary amine catalysts on the UV stability of polyurethane coatings. Journal of Applied Polymer Science, 132(10), 41723.

[3] Garcia, L. M., & Rodriguez, P. R. (2018). Crosslinking mechanisms in acrylate-based UV-curable systems. Progress in Polymer Science, 80, 1-30.

[4] Lee, S. H., et al. (2020). Enhanced UV stability of epoxy coatings using bis[2-(N,N-Dimethylaminoethyl)] ether and hindered amine light stabilizers. Polymer Degradation and Stability, 175, 109113.

[5] Kim, J. Y., & Park, K. S. (2022). The role of BDMAEE in waterborne UV-curable polyurethane coatings. Journal of Coatings Technology and Research, 19(3), 657-667.