Low temperature cure Polyurethane Two-Component Catalyst for field applications

Low-Temperature Cure Polyurethane Two-Component Catalyst for Field Applications: A Comprehensive Review

Abstract: This article provides a comprehensive review of low-temperature cure polyurethane (PU) two-component catalysts specifically designed for field applications. It delves into the fundamental chemistry of PU formation, the challenges associated with low-temperature curing, and the diverse range of catalysts employed to overcome these limitations. The article elucidates the mechanisms of action for various catalyst types, including tertiary amines, organometallic compounds, and metal carboxylates, with a particular focus on their performance characteristics at low temperatures. Product parameters, such as gel time, pot life, cure rate, and mechanical properties of the resulting PU materials, are critically analyzed. Furthermore, the article discusses the influence of environmental factors, such as humidity and substrate temperature, on catalyst performance. By drawing upon both domestic and international literature, this review aims to provide a valuable resource for formulators, researchers, and practitioners involved in the development and application of low-temperature cure PU systems.

Keywords: Polyurethane, Two-Component Catalyst, Low-Temperature Cure, Field Application, Tertiary Amines, Organometallic Catalysts, Gel Time, Pot Life, Mechanical Properties, Environmental Factors.

1. Introduction

Polyurethane (PU) materials have achieved widespread utilization in diverse applications, including coatings, adhesives, sealants, elastomers, and foams, owing to their versatile properties, processability, and cost-effectiveness. The synthesis of PU involves the reaction between a polyol (containing hydroxyl groups) and an isocyanate component. This reaction is typically catalyzed to accelerate the process and achieve desired material properties.

Conventional PU systems often require elevated temperatures to achieve satisfactory curing rates. However, in numerous field applications, such as construction, infrastructure repair, and marine coatings, the application environment frequently presents low-temperature conditions. These low temperatures can significantly hinder the curing process, leading to prolonged cure times, incomplete reactions, and compromised mechanical performance of the final PU product. 📉

Therefore, the development and utilization of effective low-temperature cure catalysts are crucial for expanding the applicability of PU materials in field settings. This article provides a comprehensive overview of the various types of two-component catalysts employed to promote PU curing at low temperatures, focusing on their mechanisms of action, performance characteristics, and the influence of environmental factors.

2. Fundamentals of Polyurethane Chemistry

The fundamental reaction in PU synthesis is the step-growth polymerization of a polyol and an isocyanate. The reaction can be simplified as follows:

R-N=C=O + R'-OH  → R-NH-C(O)-O-R'
(Isocyanate)   (Polyol)       (Polyurethane)

This reaction proceeds via nucleophilic attack of the hydroxyl group on the electrophilic carbon of the isocyanate group. The rate of this reaction is influenced by several factors, including:

• Reactivity of the Isocyanate: Aromatic isocyanates are generally more reactive than aliphatic isocyanates.
• Reactivity of the Polyol: Primary hydroxyl groups are more reactive than secondary hydroxyl groups.
• Catalyst Type and Concentration: Catalysts accelerate the reaction rate by facilitating the nucleophilic attack.
• Temperature: Higher temperatures generally increase the reaction rate.

In addition to the primary urethane-forming reaction, other competing reactions can occur, such as:

• Isocyanate-Water Reaction: This reaction produces an amine and carbon dioxide, leading to foam formation.
• Isocyanate Dimerization and Trimerization: These reactions can lead to chain extension and crosslinking.
• Allophanate Formation: This reaction involves the reaction of a urethane group with an isocyanate group, leading to branching.
• Biuret Formation: This reaction involves the reaction of a urea group (formed from the isocyanate-water reaction) with an isocyanate group, also leading to branching.

The relative rates of these reactions are crucial for determining the final properties of the PU material. Catalysts can selectively promote certain reactions over others, allowing for the tailoring of PU properties.

3. Challenges of Low-Temperature Curing

Lowering the temperature significantly reduces the reaction rate of the isocyanate-polyol reaction. This poses several challenges for field applications:

• Prolonged Cure Times: Extended curing times can delay project completion and increase labor costs. ⏳
• Incomplete Reactions: At low temperatures, the reaction may not proceed to completion, resulting in under-cured materials with inferior mechanical properties.
• Increased Sensitivity to Moisture: Slower reaction rates allow more time for moisture to react with the isocyanate, leading to carbon dioxide generation and potential foam formation, even in systems not intended to be foams.
• Poor Adhesion: Incomplete curing can compromise the adhesion of the PU material to the substrate, leading to premature failure.
• Viscosity Increase: As the temperature decreases, the viscosity of the reactants increases, hindering mixing and application.

4. Classification of Low-Temperature Cure Catalysts

Low-temperature cure catalysts for two-component PU systems can be broadly classified into three main categories:

• Tertiary Amines: These are the most commonly used catalysts in PU systems due to their relatively low cost and effectiveness.
• Organometallic Compounds: These catalysts, typically based on tin, mercury, bismuth, or zinc, are generally more active than tertiary amines and offer faster curing rates.
• Metal Carboxylates: These catalysts, often based on zinc, bismuth, or potassium, offer a balance between activity and environmental friendliness.

5. Tertiary Amine Catalysts

Tertiary amines catalyze the urethane reaction by acting as nucleophilic catalysts. They promote the reaction by forming an intermediate complex with either the isocyanate or the hydroxyl group, increasing the reactivity of these species. The proposed mechanism involves the following steps:

1. The tertiary amine lone pair attacks the isocyanate carbon, forming a zwitterionic intermediate.
2. The hydroxyl group then attacks the carbonyl carbon of the zwitterionic intermediate, forming a urethane linkage and regenerating the tertiary amine catalyst.

Table 1: Common Tertiary Amine Catalysts and their Properties

Catalyst Name	CAS Number	Molecular Weight (g/mol)	Boiling Point (°C)	Primary Application
Triethylamine (TEA)	121-44-8	101.19	89	General purpose, relatively weak
Triethylenediamine (TEDA, DABCO)	280-57-9	112.17	174	Strong blowing and gelling catalyst
Dimethylcyclohexylamine (DMCHA)	98-94-2	127.23	160	Gelling catalyst
Bis(2-dimethylaminoethyl)ether (BDMAEE)	3033-62-3	160.26	189	Strong blowing catalyst
N,N-Dimethylbenzylamine (DMBA)	103-83-3	135.21	180-182	General purpose
1,4-Diazabicyclo[2.2.2]octane (DABCO, TEDA)	280-57-9	112.17	174	Gelling and blowing
N,N-Dimethylpiperazine	106-58-1	114.19	132	Gelling
2,2′-Dimorpholinyldiethyl ether (DMDEE)	6425-39-4	260.36	276	Strong blowing
N,N,N’,N’-Tetramethyl-1,6-hexanediamine (TMHDA)	110-18-9	172.31	194	Gelling and blowing

Tertiary amines vary in their catalytic activity, selectivity, and effect on the final PU properties. Some tertiary amines primarily promote the urethane reaction (gelling catalysts), while others primarily promote the isocyanate-water reaction (blowing catalysts). The choice of tertiary amine catalyst depends on the specific application and desired PU properties.

Advantages of Tertiary Amine Catalysts:

• Relatively low cost.
• Wide availability.
• Effective at promoting both gelling and blowing reactions.

Disadvantages of Tertiary Amine Catalysts:

• Can cause odor and VOC emissions. 👃
• Can contribute to discoloration of the PU material.
• May exhibit lower activity at very low temperatures compared to organometallic catalysts.
• Some tertiary amines can be toxic.

6. Organometallic Catalysts

Organometallic catalysts, particularly those based on tin, are highly effective at promoting the urethane reaction, even at low temperatures. The most commonly used organotin catalysts are dialkyltin dicarboxylates, such as dibutyltin dilaurate (DBTDL) and dimethyltin dineodecanoate (DMTDA).

The mechanism of action of organotin catalysts involves coordination of the tin atom with both the isocyanate and the hydroxyl group, forming a ternary complex that facilitates the urethane reaction. The tin atom acts as a Lewis acid, activating both reactants and lowering the activation energy of the reaction.

Table 2: Common Organometallic Catalysts and their Properties

Catalyst Name	CAS Number	Molecular Weight (g/mol)	Metal Content (%)	Primary Application
Dibutyltin Dilaurate (DBTDL)	77-58-7	631.56	18.7	Strong gelling
Dimethyltin Dineodecanoate	68928-76-7	507.22	23.4	Strong gelling
Stannous Octoate	301-10-0	405.12	29.1	Gelling and blowing
Bismuth Octoate	67874-70-6	670.76	31.1	Gelling
Zinc Octoate	852-85-7	351.81	18.6	Gelling

Advantages of Organometallic Catalysts:

• High catalytic activity, even at low temperatures. 🔥
• Relatively low odor.
• Can provide faster curing rates and improved mechanical properties.

Disadvantages of Organometallic Catalysts:

• Higher cost compared to tertiary amines. 💰
• Environmental concerns associated with certain metals, such as tin and mercury.
• Some organotin catalysts are toxic.
• Hydrolytic instability can lead to catalyst deactivation.

7. Metal Carboxylate Catalysts

Metal carboxylates, such as zinc octoate, bismuth octoate, and potassium acetate, are gaining popularity as environmentally friendly alternatives to organotin catalysts. These catalysts promote the urethane reaction through a similar mechanism involving coordination of the metal atom with the isocyanate and hydroxyl groups.

The catalytic activity of metal carboxylates depends on the metal, the carboxylate ligand, and the reaction conditions. Bismuth carboxylates generally exhibit higher activity than zinc carboxylates.

Advantages of Metal Carboxylate Catalysts:

• Relatively low toxicity compared to organotin catalysts. ✅
• Environmentally friendly.
• Good balance of activity and cost.

Disadvantages of Metal Carboxylate Catalysts:

• Lower catalytic activity compared to organotin catalysts.
• May require higher catalyst loadings to achieve desired curing rates.
• Potential for side reactions and discoloration.

8. Synergistic Catalyst Systems

In many applications, a combination of different catalyst types is used to achieve optimal performance. For example, a combination of a tertiary amine and an organometallic catalyst can provide a balance of gelling and blowing activity, as well as improved low-temperature cure performance.

The use of synergistic catalyst systems allows formulators to tailor the curing profile and final properties of the PU material to meet specific application requirements.

9. Influence of Environmental Factors

The performance of low-temperature cure PU catalysts is significantly influenced by environmental factors, such as:

• Temperature: Lower temperatures reduce the reaction rate and can affect the activity of certain catalysts.
• Humidity: High humidity can lead to increased isocyanate-water reaction, affecting the stoichiometry of the reaction and potentially causing foam formation.
• Substrate Temperature: The temperature of the substrate can affect the curing rate of the PU material, particularly in thin-film applications.
• Airflow: Airflow can influence the evaporation of volatile components, such as solvents and catalysts, which can affect the curing process.

Table 3: Effect of Environmental Factors on Catalyst Performance

Environmental Factor	Effect on Catalyst Performance	Mitigation Strategies
Temperature	Decreases reaction rate; some catalysts become less active.	Increase catalyst loading; use more active catalyst types; preheat components; insulate the application area.
Humidity	Increases isocyanate-water reaction; can lead to foam formation and compromised properties.	Use moisture scavengers; apply in dry conditions; use moisture-resistant formulations.
Substrate Temperature	Affects curing rate, particularly in thin films; can lead to uneven curing.	Preheat the substrate; use formulations with good wetting and adhesion properties; control the application thickness.
Airflow	Can accelerate evaporation of volatile components, affecting the curing process; can also lead to surface defects.	Minimize airflow; use formulations with low-volatility components; apply in a controlled environment; use protective coatings.

10. Product Parameters and Performance Evaluation

The performance of low-temperature cure PU systems is typically evaluated based on the following product parameters:

• Gel Time: The time required for the liquid PU mixture to transition to a gel-like state. ⏱️
• Pot Life: The time during which the PU mixture remains workable and can be applied.
• Cure Rate: The rate at which the PU material hardens and develops its final properties.
• Mechanical Properties: Tensile strength, elongation at break, hardness, and impact resistance.
• Adhesion Strength: The strength of the bond between the PU material and the substrate.
• Viscosity: The resistance of the PU mixture to flow.
• Storage Stability: The ability of the PU components to maintain their properties over time.

Table 4: Typical Performance Parameters for Low-Temperature Cure PU Systems

Parameter	Typical Range (at Low Temperature)	Measurement Method	Significance
Gel Time	5-60 minutes	Manual stirring and observation	Indicates the speed of the initial reaction; affects the workability of the system.
Pot Life	10-120 minutes	Viscosity measurement	Determines the time available for application; influences the size of batches that can be mixed and applied.
Cure Rate	24-72 hours	Hardness measurement (Shore A/D)	Affects the time required to achieve full mechanical properties; influences the speed at which the applied system can be put into service.
Tensile Strength	5-30 MPa	ASTM D412	Measures the ability of the material to withstand tensile forces; important for applications requiring structural integrity.
Elongation at Break	50-500%	ASTM D412	Indicates the flexibility and ductility of the material; important for applications where the material needs to deform without breaking.
Hardness	30-90 Shore A / 40-70 Shore D	ASTM D2240	Characterizes the resistance to indentation; influences the abrasion resistance and overall durability of the material.
Adhesion Strength	1-10 MPa	ASTM D4541 (Pull-off test)	Measures the strength of the bond between the PU material and the substrate; crucial for applications requiring long-term adhesion.
Viscosity	100-10000 mPa·s	Rotational viscometer	Affects the ease of mixing, application, and flow properties; influences the thickness of the applied layer and the ability to fill gaps and crevices.
Storage Stability	6-12 months	Viscosity and performance measurements	Indicates the shelf life of the components and the formulated system; ensures consistent performance over time and reduces waste.

11. Applications of Low-Temperature Cure PU Systems

Low-temperature cure PU systems are widely used in various field applications, including:

• Coatings: Marine coatings, industrial coatings, and architectural coatings.
• Adhesives: Construction adhesives, automotive adhesives, and aerospace adhesives.
• Sealants: Joint sealants, gap fillers, and waterproofing sealants.
• Elastomers: Potting compounds, encapsulants, and flexible molds.
• Foams: Insulation foams, spray foams, and structural foams.
• Concrete Repair: Crack injection, patching, and overlays.

12. Future Trends and Research Directions

Future research in low-temperature cure PU systems is focused on the following areas:

• Development of novel, environmentally friendly catalysts with improved activity and selectivity.
• Design of PU formulations with enhanced low-temperature flexibility and toughness.
• Development of self-healing PU materials for improved durability and longevity.
• Use of bio-based polyols and isocyanates for sustainable PU production.
• Advanced characterization techniques to better understand the curing process and structure-property relationships.

13. Conclusion

Low-temperature cure PU two-component catalysts are essential for expanding the applicability of PU materials in field applications. Tertiary amines, organometallic compounds, and metal carboxylates are the main types of catalysts employed, each with its own advantages and disadvantages. The choice of catalyst depends on the specific application requirements, desired properties, and environmental considerations. Environmental factors, such as temperature and humidity, significantly influence catalyst performance and must be carefully controlled. Continued research and development efforts are focused on creating more effective, environmentally friendly, and sustainable low-temperature cure PU systems.

14. Literature Cited

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[3] Oertel, G. (Ed.). (1993). Polyurethane handbook. Hanser Gardner Publications.

[4] Szycher, M. (1999). Szycher’s handbook of polyurethanes. CRC press.

[5] Hepburn, C. (1991). Polyurethane elastomers. Springer Science & Business Media.

[6] Chattopadhyay, D. K., & Webster, D. C. (2009). Thermal stability and fire retardancy of polyurethanes. Progress in Polymer Science, 34(10), 1068-1133.

[7] Woods, G. (1990). The ICI Polyurethanes Book. John Wiley & Sons.

[8] Ashworth, B. K., et al. "Catalysis of isocyanate reactions." Journal of Polymer Science Part A: Polymer Chemistry 26.11 (1988): 3113-3126.

[9] Blank, W. J. "New amine catalysts for polyurethane foams." Journal of Cellular Plastics 41.1 (2005): 1-19.

[10] Kumar, V., et al. "Progress in bio-based polyurethane foams: a review." Journal of Polymer Research 25.1 (2018): 1-22.

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