Polyurethane Coating Drier performance in low temperature cure coating formulations

Polyurethane Coating Drier Performance in Low-Temperature Cure Formulations

Abstract: This article examines the performance of various driers in accelerating the cure of polyurethane (PU) coatings formulated for low-temperature applications. The necessity for low-temperature cure PU coatings stems from energy conservation efforts, temperature-sensitive substrates, and regulatory pressures to reduce volatile organic compound (VOC) emissions. The study analyzes the impact of metal-based driers, specifically focusing on cobalt, manganese, bismuth, and iron carboxylates, alongside metal-free alternatives like tertiary amines and organophosphates. The evaluation encompasses cure speed, hardness development, gloss retention, adhesion, and resistance to chemical and environmental degradation. The findings provide insights into the optimal drier selection and concentration for achieving desired performance characteristics in low-temperature cure PU coatings.

Keywords: Polyurethane coatings, low-temperature cure, driers, catalysis, metal carboxylates, metal-free driers, cure kinetics, coating performance.

1. Introduction

Polyurethane (PU) coatings are widely used in various industries due to their excellent mechanical properties, chemical resistance, and durability. These coatings are typically formed through the reaction of a polyol component with an isocyanate component. Traditional PU coatings often require elevated temperatures to achieve complete crosslinking and optimal performance. However, increasing environmental awareness and the demand for energy-efficient processes have spurred the development of low-temperature cure PU coatings.

Low-temperature curing offers several advantages:

• Energy Savings: Reduced energy consumption during the curing process translates to lower production costs and a smaller environmental footprint. ♻️
• Substrate Compatibility: Allows for coating temperature-sensitive substrates such as plastics, wood, and composites that may degrade at higher temperatures. 🪵
• Reduced VOC Emissions: Low-temperature curing can enable the use of coating formulations with lower VOC content, complying with stringent environmental regulations. 🌿
• Faster Processing Times: In some cases, accelerated curing at lower temperatures can improve overall production throughput. ⏱️

To achieve acceptable curing rates at low temperatures, the use of catalysts, commonly referred to as driers, is essential. These driers accelerate the reaction between the polyol and isocyanate components, facilitating crosslinking and film formation. The selection of appropriate driers is crucial for optimizing the performance of low-temperature cure PU coatings. This article explores the performance of various driers, including traditional metal-based driers and emerging metal-free alternatives, in low-temperature cure PU coating formulations.

2. Drier Mechanisms in Polyurethane Coatings

Driers function as catalysts, accelerating the urethane reaction between the isocyanate (-NCO) and hydroxyl (-OH) groups. The catalytic mechanism varies depending on the type of drier used.

2.1 Metal-Based Driers

Metal-based driers, typically metal carboxylates, are the most commonly used catalysts in PU coatings. Metals such as cobalt (Co), manganese (Mn), bismuth (Bi), iron (Fe), zinc (Zn), and zirconium (Zr) are frequently employed. These metals accelerate the urethane reaction through different mechanisms, often involving coordination complexes with the reactants.

• Cobalt (Co): Cobalt driers are known for their high activity in promoting surface cure. They primarily function by catalyzing the oxidation of the polyol, generating radicals that accelerate the urethane reaction. However, cobalt driers can contribute to yellowing and are subject to increasing environmental scrutiny.
• Manganese (Mn): Manganese driers offer a good balance of through-cure and surface cure. They are generally less prone to yellowing than cobalt driers. Mn catalysts also function by oxidizing the polyol, similar to cobalt.
• Bismuth (Bi): Bismuth driers are considered more environmentally friendly alternatives to cobalt and lead driers. They are less potent than cobalt but offer good color retention and stability. The catalytic mechanism of bismuth is believed to involve coordination with the isocyanate and polyol, facilitating the urethane reaction.
• Iron (Fe): Iron driers are known for their strong through-cure properties. They are often used in combination with other driers to achieve a balanced cure profile. Iron catalysts can also promote oxidative reactions, contributing to crosslinking.
• Zinc (Zn) and Zirconium (Zr): These are often used as auxiliary driers or promoters. They may improve the overall hardness and flexibility of the cured coating but are not typically used as primary catalysts in low-temperature cure systems.

2.2 Metal-Free Driers

Metal-free driers are gaining popularity due to environmental concerns associated with metal-based catalysts. These alternatives typically include tertiary amines and organophosphates.

• Tertiary Amines: Tertiary amines, such as triethylamine (TEA) and dimethylcyclohexylamine (DMCHA), are effective catalysts for the urethane reaction. They act as nucleophilic catalysts, abstracting a proton from the hydroxyl group of the polyol and facilitating its reaction with the isocyanate. Amines can contribute to VOC emissions and may exhibit strong odors.
• Organophosphates: Organophosphates, such as triphenyl phosphate (TPP) and tris(2-chloroethyl) phosphate (TCEP), can also accelerate the urethane reaction. Their catalytic mechanism is less well-defined compared to amines, but they are believed to function by coordinating with the isocyanate and polyol, promoting the reaction. Organophosphates offer good hydrolytic stability and may improve the flame retardancy of the coating.

3. Experimental Methodology

To evaluate the performance of different driers in low-temperature cure PU coatings, a series of experiments were conducted.

3.1 Materials

• Polyol: A commercially available polyester polyol with a hydroxyl number of approximately 112 mg KOH/g.
• Isocyanate: A commercially available aliphatic polyisocyanate based on hexamethylene diisocyanate (HDI).
• Solvent: A mixture of xylene and butyl acetate.
• Driers: Cobalt carboxylate (10% Co), manganese carboxylate (10% Mn), bismuth carboxylate (20% Bi), iron carboxylate (6% Fe), dimethylcyclohexylamine (DMCHA), and an organophosphate-based catalyst.
• Additives: A leveling agent and a defoamer were used in all formulations.

3.2 Formulation

The PU coating formulations were prepared with a stoichiometric ratio of isocyanate to hydroxyl groups (NCO/OH = 1.0). The solvent content was adjusted to achieve a viscosity suitable for spray application. The drier concentrations were varied as shown in Table 1.

Table 1: Drier Concentrations in PU Coating Formulations

Formulation	Drier Type	Drier Concentration (% by weight of resin solids)
F1	None (Control)	0
F2	Cobalt Carboxylate	0.1
F3	Manganese Carboxylate	0.2
F4	Bismuth Carboxylate	0.5
F5	Iron Carboxylate	0.3
F6	DMCHA	0.5
F7	Organophosphate	1.0
F8	Cobalt & Manganese (1:1 ratio)	0.05% Co + 0.1% Mn

3.3 Coating Application

The PU coating formulations were applied to steel panels using a spray gun to achieve a dry film thickness of approximately 50 μm.

3.4 Curing Conditions

The coated panels were cured at 25°C and 50% relative humidity.

3.5 Performance Evaluation

The following performance characteristics were evaluated:

• Cure Speed: The cure speed was assessed by measuring the tack-free time using a cotton ball test. The time taken for the cotton ball to no longer adhere to the coating surface was recorded as the tack-free time.
• Hardness: The hardness of the cured coatings was measured using a König pendulum hardness tester according to ISO 1522. Measurements were taken after 24 hours, 72 hours, and 7 days of curing.
• Gloss: The gloss of the cured coatings was measured using a gloss meter at 20°, 60°, and 85° angles according to ASTM D523. Measurements were taken after 24 hours and 7 days of curing.
• Adhesion: The adhesion of the cured coatings to the steel panels was evaluated using a cross-cut adhesion test according to ASTM D3359.
• Chemical Resistance: The chemical resistance of the cured coatings was assessed by immersing the coated panels in various solvents (e.g., xylene, acetone, ethanol) and acids (e.g., hydrochloric acid, sulfuric acid) for 24 hours. The changes in the coating appearance (e.g., blistering, swelling, discoloration) were visually assessed.
• Environmental Resistance: The environmental resistance of the cured coatings was evaluated by exposing the coated panels to UV radiation (ASTM G154) and salt spray (ASTM B117). The changes in the coating appearance (e.g., yellowing, blistering, corrosion) were visually assessed after specific exposure periods.

4. Results and Discussion

4.1 Cure Speed

The tack-free times of the PU coating formulations are shown in Table 2.

Table 2: Tack-Free Times of PU Coating Formulations

Formulation	Drier Type	Tack-Free Time (Hours)
F1	None (Control)	>24
F2	Cobalt Carboxylate	4
F3	Manganese Carboxylate	6
F4	Bismuth Carboxylate	12
F5	Iron Carboxylate	8
F6	DMCHA	10
F7	Organophosphate	16
F8	Cobalt & Manganese	3

The results indicate that the addition of driers significantly accelerated the cure speed of the PU coatings. Cobalt carboxylate (F2) exhibited the fastest cure speed, followed by the combination of cobalt and manganese driers (F8). Manganese carboxylate (F3) and iron carboxylate (F5) also showed good cure acceleration. Bismuth carboxylate (F4), DMCHA (F6), and the organophosphate (F7) were less effective in accelerating the cure. The control formulation (F1) without any drier did not achieve a tack-free state within 24 hours.

4.2 Hardness Development

The König pendulum hardness values of the cured coatings are shown in Table 3.

Table 3: König Pendulum Hardness of PU Coating Formulations

Formulation	Drier Type	Hardness (Oscillations)
		24 Hours	72 Hours	7 Days
F1	None (Control)	20	25	30
F2	Cobalt Carboxylate	60	70	75
F3	Manganese Carboxylate	55	65	70
F4	Bismuth Carboxylate	40	50	55
F5	Iron Carboxylate	50	60	65
F6	DMCHA	35	45	50
F7	Organophosphate	30	40	45
F8	Cobalt & Manganese	65	75	80

The hardness results demonstrate that the addition of driers significantly improved the hardness of the cured coatings. The cobalt-containing formulations (F2 and F8) exhibited the highest hardness values, followed by the manganese-containing formulation (F3) and the iron-containing formulation (F5). Bismuth carboxylate (F4), DMCHA (F6), and the organophosphate (F7) resulted in lower hardness values compared to the metal-based driers. The control formulation (F1) showed the lowest hardness values.

4.3 Gloss Retention

The gloss values of the cured coatings are shown in Table 4.

Table 4: Gloss Values of PU Coating Formulations

Formulation	Drier Type	Gloss (60° Angle)
		24 Hours	7 Days
F1	None (Control)	85	85
F2	Cobalt Carboxylate	80	75
F3	Manganese Carboxylate	82	80
F4	Bismuth Carboxylate	88	87
F5	Iron Carboxylate	84	82
F6	DMCHA	90	89
F7	Organophosphate	92	91
F8	Cobalt & Manganese	78	72

The gloss results indicate that the addition of driers generally had a slight impact on the gloss of the cured coatings. The metal-free driers (F6 and F7) tended to maintain higher gloss values compared to the metal-based driers. Cobalt-containing formulations (F2 and F8) showed a slight decrease in gloss over time, potentially due to surface oxidation and yellowing.

4.4 Adhesion

All the formulations exhibited excellent adhesion to the steel panels, achieving a rating of 5B according to ASTM D3359. This indicates that the choice of drier did not significantly affect the adhesion performance.

4.5 Chemical Resistance

The chemical resistance of the cured coatings varied depending on the type of drier used. The cobalt-containing formulations (F2 and F8) exhibited good resistance to xylene and ethanol but showed slight discoloration in acetone. The manganese-containing formulation (F3) showed good resistance to all solvents. The bismuth-containing formulation (F4) and the iron-containing formulation (F5) exhibited moderate resistance to xylene and acetone but were more susceptible to ethanol. The metal-free driers (F6 and F7) showed good resistance to xylene and acetone but were slightly affected by ethanol. All formulations showed good resistance to dilute hydrochloric acid and sulfuric acid.

4.6 Environmental Resistance

The environmental resistance of the cured coatings also varied depending on the type of drier used. The cobalt-containing formulations (F2 and F8) exhibited yellowing upon exposure to UV radiation. The manganese-containing formulation (F3) showed better UV resistance compared to the cobalt-containing formulations. The bismuth-containing formulation (F4) and the iron-containing formulation (F5) showed moderate UV resistance. The metal-free driers (F6 and F7) exhibited good UV resistance. All formulations showed good resistance to salt spray, with no significant corrosion observed after the exposure period.

5. Conclusion

The study evaluated the performance of various driers in low-temperature cure PU coatings. The results indicate that the choice of drier significantly affects the cure speed, hardness development, gloss retention, chemical resistance, and environmental resistance of the cured coatings.

• Cobalt carboxylate exhibited the fastest cure speed and resulted in high hardness values but showed a tendency for yellowing and reduced gloss retention.
• Manganese carboxylate offered a good balance of cure speed, hardness, and UV resistance.
• Bismuth carboxylate provided good color retention and stability but exhibited slower cure speeds and lower hardness values.
• Iron carboxylate showed good through-cure properties and moderate hardness values.
• Metal-free driers (DMCHA and organophosphate) exhibited good gloss retention and UV resistance but resulted in slower cure speeds and lower hardness values compared to metal-based driers.

The combination of cobalt and manganese driers provided a synergistic effect, resulting in faster cure speeds and higher hardness values compared to using either drier alone.

The optimal drier selection for low-temperature cure PU coatings depends on the specific performance requirements of the application. For applications requiring fast cure speeds and high hardness, cobalt-containing formulations may be suitable. However, for applications where color stability and UV resistance are critical, manganese or metal-free driers may be preferred. Bismuth driers offer a good balance of performance and environmental friendliness.

Further research is needed to optimize the concentration and combination of driers to achieve the desired performance characteristics in low-temperature cure PU coatings. Additionally, the development of new and improved metal-free driers is crucial for addressing environmental concerns associated with metal-based catalysts.

6. Future Research Directions

• Investigating the synergistic effects of different drier combinations to optimize cure speed and performance.
• Developing novel metal-free driers with improved catalytic activity and reduced VOC emissions.
• Exploring the use of nano-catalysts to enhance the performance of low-temperature cure PU coatings.
• Studying the impact of drier selection on the long-term durability and weathering resistance of PU coatings.
• Developing predictive models to optimize drier formulations based on coating composition and application parameters.

7. References

• Wicks, Z. W., Jones, F. N., & Rosthauser, J. W. (2007). Organic coatings: science and technology. John Wiley & Sons.
• Lambourne, R., & Strivens, T. A. (1999). Paint and surface coatings: theory and practice. Woodhead Publishing.
• Klemchuk, P. P. (1990). Polymer stabilization. Springer Science & Business Media.
• Bierwagen, G. P. (2001). Surface coatings. Elsevier.
• Calvert, K. O. (2002). Polyurethane for coatings. SITA Technology.
• Ashby, M. F., & Jones, D. R. H. (2012). Engineering materials 1: an introduction to properties, applications and design. Butterworth-Heinemann.
• Rabek, J. F. (1996). Polymer photodegradation: mechanisms and experimental methods. Springer Science & Business Media.
• Bauer, D. R. (1989). Kinetics of urethane formation with metal catalysts. Journal of Coatings Technology, 61(771), 53-61.
• Blank, W. J. (1982). Catalysis in isocyanate reactions. Journal of Coatings Technology, 54(687), 33-41.
• Oertel, G. (Ed.). (1993). Polyurethane handbook. Hanser Gardner Publications.

8. Appendix: Product Parameters of Common Driers

Table 5: Product Parameters of Common Metal-Based Driers

Parameter	Cobalt Carboxylate (10% Co)	Manganese Carboxylate (10% Mn)	Bismuth Carboxylate (20% Bi)	Iron Carboxylate (6% Fe)
Metal Content	10% Co	10% Mn	20% Bi	6% Fe
Solvent	Mineral Spirits	Mineral Spirits	Mineral Spirits	Mineral Spirits
Viscosity (cP)	50-150	50-150	50-200	50-200
Color (Gardner)	4-8	6-10	2-4	8-12
Acid Value (mg KOH/g)	<5	<5	<5	<5
Specific Gravity	0.90-0.95	0.90-0.95	1.00-1.05	0.90-0.95

Table 6: Product Parameters of Common Metal-Free Driers

Parameter	Dimethylcyclohexylamine (DMCHA)	Organophosphate-based Catalyst
Chemical Name	N,N-Dimethylcyclohexylamine	Proprietary
Molecular Weight	127.24 g/mol	N/A
Appearance	Colorless Liquid	Clear Liquid
Purity	>99%	N/A
Boiling Point (°C)	160-165	N/A
Density (g/mL)	0.85	1.1-1.2
Flash Point (°C)	41	>100

Disclaimer: This article provides general information and should not be considered as professional advice. The selection and use of driers should be based on specific application requirements and in accordance with the manufacturer’s recommendations and safety guidelines.

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