Organotin Polyurethane Two-Component Catalyst applications in cast PU elastomers

Organotin Catalyzed Polyurethane Elastomers: A Comprehensive Review of Two-Component Systems for Casting Applications

Abstract: Polyurethane (PU) elastomers are versatile materials widely employed in various industrial sectors due to their excellent mechanical properties, chemical resistance, and design flexibility. Two-component (2K) PU systems, specifically those utilized in casting applications, offer precise control over curing kinetics and final product characteristics. Organotin compounds have long been established as effective catalysts for these systems, significantly influencing the reaction between isocyanates and polyols. This article presents a comprehensive review of organotin catalysts in 2K cast PU elastomer formulations, focusing on the reaction mechanism, influence on key product parameters (e.g., pot life, demold time, hardness, tensile strength, elongation at break, and compression set), and application-specific considerations. The advantages and limitations of various organotin catalysts, alongside alternative catalytic approaches, are also discussed.

Keywords: Polyurethane Elastomer, Two-Component System, Casting, Organotin Catalyst, Reaction Kinetics, Mechanical Properties, Pot Life, Demold Time.

1. Introduction

Polyurethane elastomers are a class of polymers synthesized through the reaction of polyols (typically polyester or polyether polyols) with isocyanates, often in the presence of catalysts, chain extenders, and other additives. The resulting polymer network exhibits a unique combination of properties, ranging from soft and flexible to rigid and durable. This versatility makes PU elastomers suitable for a wide array of applications, including automotive components, footwear, industrial rollers, adhesives, coatings, and sealants [1, 2].

Two-component (2K) PU systems are particularly well-suited for casting applications. These systems involve the separate storage of isocyanate and polyol components, which are then mixed immediately before processing. The mixing ratio is precisely controlled to achieve the desired stoichiometry and, consequently, the desired material properties. 2K casting processes allow for the production of complex shapes and large parts with high precision, making them ideal for low- to medium-volume production runs [3, 4].

Catalysts play a crucial role in 2K PU elastomer systems by accelerating the reaction between isocyanates and polyols, controlling the curing kinetics, and ultimately influencing the final properties of the cured elastomer. Organotin compounds have been extensively used as catalysts in PU chemistry for decades due to their high activity and effectiveness in promoting the urethane reaction [5, 6]. However, concerns regarding the toxicity and environmental impact of certain organotin species have spurred research into alternative catalysts and modified organotin compounds with improved safety profiles [7, 8].

2. Reaction Mechanism of Urethane Formation

The fundamental reaction in PU synthesis is the addition of an isocyanate group (-NCO) to a hydroxyl group (-OH) of a polyol, resulting in the formation of a urethane linkage (-NH-COO-) [9]. This reaction is exothermic and can proceed without a catalyst, but the rate is often too slow for practical applications. Organotin catalysts accelerate the reaction through a coordination mechanism involving the isocyanate and the hydroxyl group.

The generally accepted mechanism involves the following steps [10, 11]:

1. Coordination of the organotin catalyst to the hydroxyl group of the polyol. This increases the nucleophilicity of the oxygen atom in the hydroxyl group.
2. Coordination of the organotin catalyst to the isocyanate group. This activates the isocyanate carbon, making it more susceptible to nucleophilic attack.
3. Nucleophilic attack of the activated hydroxyl group on the activated isocyanate group. This forms a transition state complex.
4. Proton transfer and regeneration of the catalyst. The urethane linkage is formed, and the organotin catalyst is released to participate in further reactions.

The exact mechanism can vary depending on the specific organotin catalyst, the nature of the polyol and isocyanate, and the reaction conditions.

3. Common Organotin Catalysts for 2K Cast PU Elastomers

Several organotin compounds are commonly used as catalysts in 2K cast PU elastomer systems. These catalysts can be broadly classified into dialkyltin dicarboxylates, trialkyltin derivatives, and monoalkyltin compounds.

3.1. Dialkyltin Dicarboxylates

Dialkyltin dicarboxylates are among the most widely used organotin catalysts in PU chemistry. They offer a good balance of activity, selectivity, and cost-effectiveness.

• Dibutyltin dilaurate (DBTDL): DBTDL is a highly active catalyst that promotes both the urethane reaction and the allophanate reaction (a side reaction that can lead to branching and crosslinking). It is effective at low concentrations and provides rapid curing. However, DBTDL has been subject to regulatory scrutiny due to its toxicity and environmental persistence.
• Dibutyltin diacetate (DBTDA): DBTDA is similar to DBTDL in its activity and selectivity. It is often used in applications where a faster cure rate is required.
• Dibutyltin maleate (DBTM): DBTM exhibits lower activity compared to DBTDL and DBTDA. It is often used in combination with other catalysts to tailor the curing profile.

3.2. Trialkyltin Derivatives

Trialkyltin derivatives are generally less active than dialkyltin dicarboxylates, but they can offer improved selectivity towards the urethane reaction.

• Trimethyltin hydroxide (TMTH): TMTH is a mild catalyst that is often used in moisture-curing PU systems.
• Tributyltin oxide (TBTO): TBTO has been largely phased out due to its high toxicity.

3.3. Monoalkyltin Compounds

Monoalkyltin compounds are considered to be less toxic than dialkyltin and trialkyltin derivatives. They are often used in applications where environmental and health concerns are paramount.

• Monomethyltin trichloride (MMTC): MMTC is a highly reactive catalyst that can promote rapid curing.
• Monomethyltin tris(2-ethylhexyl thioglycolate) (MMT): MMT is a less volatile and more stable alternative to MMTC. It is often used in applications where long pot life is desired.

3.4. Product Parameters of Common Organotin Catalysts

The following table summarizes the key product parameters of the commonly used organotin catalysts.

Catalyst	Chemical Formula	Activity	Selectivity	Toxicity	Application
DBTDL	(C4H9)2Sn(OOC(CH2)10CH3)2	High	Moderate	High	General purpose PU elastomers, coatings, adhesives
DBTDA	(C4H9)2Sn(OOCCH3)2	High	Moderate	High	Fast-curing PU elastomers, rigid foams
DBTM	(C4H9)2Sn(OOCCH=CHCOOH)	Moderate	Moderate	High	PU elastomers with improved flexibility, coatings
TMTH	(CH3)3SnOH	Low	High	Moderate	Moisture-curing PU systems, sealants
MMT	CH3Sn(SCH2COOC8H17)3	Moderate	High	Low	Low-toxicity PU elastomers, coatings, adhesives, flexible foams

4. Influence of Organotin Catalysts on PU Elastomer Properties

The type and concentration of organotin catalyst significantly influence the properties of the resulting PU elastomer. These properties include pot life, demold time, hardness, tensile strength, elongation at break, and compression set.

4.1. Pot Life and Demold Time

• Pot Life: Pot life refers to the time period during which the mixed 2K system remains workable and can be poured or processed. Organotin catalysts, particularly highly active ones like DBTDL and DBTDA, reduce pot life by accelerating the urethane reaction. A shorter pot life can be advantageous for rapid processing but can also pose challenges for complex geometries or large castings. The catalyst concentration is a primary factor in determining pot life. Lower catalyst concentrations extend the pot life, while higher concentrations shorten it.
• Demold Time: Demold time is the time required for the cured elastomer to develop sufficient strength to be removed from the mold without deformation. Organotin catalysts shorten demold time by accelerating the curing process. The relationship between catalyst concentration and demold time is inverse; higher concentrations lead to shorter demold times.

4.2. Mechanical Properties

• Hardness: The hardness of a PU elastomer is a measure of its resistance to indentation. Organotin catalysts can influence hardness by affecting the crosslinking density of the polymer network. Highly active catalysts, which promote both the urethane and allophanate reactions, can lead to increased crosslinking and higher hardness.
• Tensile Strength: Tensile strength is the maximum stress that a material can withstand before breaking under tension. Organotin catalysts can influence tensile strength by affecting the molecular weight and uniformity of the polymer chains.
• Elongation at Break: Elongation at break is the percentage of elongation that a material can undergo before breaking under tension. Organotin catalysts can influence elongation at break by affecting the flexibility and chain mobility of the polymer network.
• Compression Set: Compression set is a measure of the permanent deformation that a material undergoes after being subjected to a compressive load. Organotin catalysts can influence compression set by affecting the elastic recovery of the polymer network.

4.3. Factors Affecting Catalyst Performance

Several factors can affect the performance of organotin catalysts in 2K cast PU elastomer systems:

• Catalyst Concentration: Catalyst concentration is a critical parameter that directly affects the reaction rate and the resulting elastomer properties. Optimizing the catalyst concentration is essential to achieve the desired balance of pot life, demold time, and mechanical properties.
• Temperature: Temperature significantly influences the reaction rate. Higher temperatures accelerate the urethane reaction, while lower temperatures slow it down. The catalyst concentration should be adjusted accordingly to compensate for temperature variations.
• Moisture: Moisture can react with isocyanates, leading to the formation of urea linkages and the evolution of carbon dioxide. This can result in foaming and defects in the cured elastomer. It is crucial to use dry ingredients and to minimize moisture exposure during processing.
• Impurities: Impurities in the polyol or isocyanate components can interfere with the catalytic activity of the organotin catalyst. It is important to use high-quality raw materials and to ensure that they are free from contaminants.

5. Application-Specific Considerations

The selection of an appropriate organotin catalyst for a 2K cast PU elastomer system depends on the specific application requirements. Some key considerations include:

• Desired Pot Life and Demold Time: For applications requiring long pot life, less active catalysts like MMT are preferred. For applications requiring rapid curing, highly active catalysts like DBTDL or DBTDA are more suitable.
• Desired Mechanical Properties: The desired hardness, tensile strength, elongation at break, and compression set will influence the choice of catalyst. The catalyst concentration can be adjusted to fine-tune these properties.
• Environmental and Health Concerns: For applications where environmental and health concerns are paramount, low-toxicity catalysts like MMT are preferred.
• Processing Conditions: The processing temperature, mixing method, and mold design can all influence the performance of the catalyst.

6. Advantages and Limitations of Organotin Catalysts

Organotin catalysts offer several advantages in 2K cast PU elastomer systems:

• High Activity: Organotin catalysts are highly effective in accelerating the urethane reaction, leading to rapid curing and short demold times.
• Versatility: Organotin catalysts can be used with a wide range of polyols and isocyanates.
• Cost-Effectiveness: Organotin catalysts are relatively inexpensive compared to some alternative catalysts.

However, organotin catalysts also have some limitations:

• Toxicity: Certain organotin compounds, particularly dialkyltin and trialkyltin derivatives, are toxic and can pose environmental and health risks.
• Hydrolytic Instability: Some organotin catalysts are susceptible to hydrolysis, which can lead to deactivation and loss of catalytic activity.
• Side Reactions: Highly active organotin catalysts can promote side reactions, such as the allophanate reaction, which can lead to undesirable properties in the cured elastomer.

7. Alternative Catalytic Approaches

Due to concerns regarding the toxicity and environmental impact of organotin catalysts, researchers have explored alternative catalytic approaches for PU synthesis. These include:

• Amine Catalysts: Amine catalysts are widely used in PU foam applications. They promote the reaction between isocyanates and hydroxyl groups, as well as the reaction between isocyanates and water (blowing reaction). However, amine catalysts can also cause undesirable side reactions and can lead to odor problems.
• Metal Carboxylates: Metal carboxylates, such as zinc carboxylates and bismuth carboxylates, are less toxic than organotin catalysts and can offer good catalytic activity. However, they may require higher concentrations to achieve comparable curing rates.
• Enzymes: Enzymes can be used as biocatalysts for PU synthesis. They offer high selectivity and can operate under mild reaction conditions. However, enzymes are often expensive and can be sensitive to temperature and pH.
• Metal-Free Catalysts: Research is ongoing to develop metal-free catalysts for PU synthesis. These catalysts could offer a more sustainable and environmentally friendly alternative to organotin catalysts.

8. Future Trends

The future of organotin catalysts in 2K cast PU elastomer systems is likely to be shaped by several key trends:

• Development of Low-Toxicity Organotin Catalysts: Research efforts will continue to focus on developing organotin catalysts with improved safety profiles, such as monoalkyltin compounds and sterically hindered dialkyltin compounds.
• Combination of Catalysts: The use of catalyst blends, combining organotin catalysts with other types of catalysts (e.g., metal carboxylates or amines), will become more prevalent to tailor the curing profile and achieve specific performance requirements.
• Encapsulation of Catalysts: Encapsulation of organotin catalysts can improve their stability, reduce their volatility, and control their release into the reaction mixture.
• Increased Use of Alternative Catalysts: As concerns about the toxicity and environmental impact of organotin catalysts continue to grow, the use of alternative catalysts, such as metal carboxylates, enzymes, and metal-free catalysts, will likely increase.

9. Conclusion

Organotin catalysts have been instrumental in the development and widespread use of 2K cast PU elastomers. They offer high activity, versatility, and cost-effectiveness. However, concerns regarding their toxicity and environmental impact have spurred research into alternative catalysts and modified organotin compounds with improved safety profiles. The selection of an appropriate organotin catalyst for a specific application requires careful consideration of the desired pot life, demold time, mechanical properties, environmental and health concerns, and processing conditions. Future trends will likely focus on the development of low-toxicity organotin catalysts, the use of catalyst blends, the encapsulation of catalysts, and the increased use of alternative catalytic approaches. The optimal catalyst choice will balance performance requirements with environmental and safety considerations to ensure the sustainable production of high-performance PU elastomers for various applications.

10. Tables

Table 1: Typical Properties of PU Elastomers Prepared with Different Catalyst Types

Property	DBTDL Catalyzed	MMT Catalyzed	Bismuth Catalyzed	Amine Catalyzed
Pot Life (minutes)	5-10	30-45	15-20	2-5
Demold Time (minutes)	15-30	60-90	45-60	10-20
Hardness (Shore A)	70-90	60-80	65-85	75-95
Tensile Strength (MPa)	20-30	15-25	18-28	22-32
Elongation (%)	300-500	400-600	350-550	250-450

Table 2: Influence of Catalyst Concentration on PU Elastomer Properties (Using DBTDL)

DBTDL Concentration (%)	Pot Life (minutes)	Demold Time (minutes)	Hardness (Shore A)
0.01	20	60	65
0.05	8	25	75
0.10	4	15	85

11. References

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