Abstract: Polyurethane (PU) materials are widely used in diverse applications due to their versatile properties. The performance of PU materials is significantly influenced by the crosslinking density, which can be effectively controlled by the addition of crosslinking agents. This article provides a comprehensive overview of polyurethane auxiliary agent crosslinkers, focusing on their effects on various properties of PU materials. The article covers the types of crosslinkers, their mechanism of action, influence on mechanical, thermal, chemical, and aging properties, and considerations for selection and application.
Contents:
- Introduction:
- 1.1 Overview of Polyurethane Materials
- 1.2 Importance of Crosslinking in Polyurethane
- 1.3 Role of Auxiliary Agent Crosslinkers
- Types of Polyurethane Crosslinkers:
- 2.1 Polyols
- 2.1.1 Triols
- 2.1.2 Tetrols and Higher Functionality Polyols
- 2.2 Amine Crosslinkers
- 2.2.1 Aliphatic Amines
- 2.2.2 Aromatic Amines
- 2.3 Isocyanate Crosslinkers
- 2.3.1 Polymeric MDI (PMDI)
- 2.3.2 Blocked Isocyanates
- 2.4 Epoxy Crosslinkers
- 2.5 Other Crosslinkers
- 2.5.1 Carbodiimides
- 2.5.2 Melamine Resins
- 2.1 Polyols
- Mechanism of Action of Crosslinkers:
- 3.1 Reaction with Isocyanates
- 3.2 Reaction with Polyols
- 3.3 Chain Extension and Crosslinking
- 3.4 Catalysis and Reaction Kinetics
- Effects of Crosslinkers on Mechanical Properties:
- 4.1 Tensile Strength and Elongation at Break
- 4.2 Hardness
- 4.3 Tear Strength
- 4.4 Impact Resistance
- 4.5 Abrasion Resistance
- 4.6 Compression Set
- Effects of Crosslinkers on Thermal Properties:
- 5.1 Glass Transition Temperature (Tg)
- 5.2 Heat Resistance
- 5.3 Thermal Stability
- 5.4 Flammability
- Effects of Crosslinkers on Chemical Properties:
- 6.1 Solvent Resistance
- 6.2 Water Resistance
- 6.3 Acid and Alkali Resistance
- 6.4 Hydrolytic Stability
- Effects of Crosslinkers on Aging Properties:
- 7.1 UV Resistance
- 7.2 Thermal Aging
- 7.3 Hydrolytic Aging
- 7.4 Ozone Resistance
- Selection and Application of Polyurethane Crosslinkers:
- 8.1 Factors Influencing Crosslinker Selection
- 8.2 Crosslinker Dosage and Formulation
- 8.3 Processing Conditions
- 8.4 Compatibility with Other Additives
- Applications of Crosslinked Polyurethanes:
- 9.1 Coatings and Adhesives
- 9.2 Foams
- 9.3 Elastomers
- 9.4 Thermoplastic Polyurethanes (TPUs)
- Future Trends and Developments:
- 10.1 Bio-based Crosslinkers
- 10.2 Reactive Nanoparticles as Crosslinkers
- 10.3 Smart Crosslinking Systems
- Conclusion
- References
1. Introduction
1.1 Overview of Polyurethane Materials
Polyurethanes (PUs) are a versatile class of polymers formed through the reaction of a polyol (an alcohol with multiple hydroxyl groups) and an isocyanate (a compound containing the -N=C=O functional group). The reaction is catalyzed by various compounds and can be tailored to produce materials with a wide range of properties, from flexible foams to rigid plastics. This versatility stems from the diverse selection of polyols and isocyanates, as well as the ability to control the crosslinking density within the polymer matrix.
1.2 Importance of Crosslinking in Polyurethane
Crosslinking refers to the formation of chemical bonds between polymer chains, creating a three-dimensional network structure. The degree of crosslinking significantly influences the mechanical, thermal, and chemical properties of the resulting PU material. Higher crosslinking density generally leads to increased rigidity, hardness, and resistance to solvents and heat, but can also decrease flexibility and impact resistance. Conversely, lower crosslinking density results in softer, more flexible materials with lower strength and resistance to environmental factors.
1.3 Role of Auxiliary Agent Crosslinkers
While the reaction between polyols and isocyanates inherently leads to some degree of crosslinking, the addition of auxiliary agent crosslinkers allows for precise control over the final crosslinking density and the specific properties of the PU material. These crosslinkers are typically polyfunctional compounds containing reactive groups that can react with either the polyol or the isocyanate, or both, further linking the polymer chains. They are often used to enhance specific performance characteristics such as heat resistance, chemical resistance, or mechanical strength. Selecting the appropriate crosslinker and optimizing its concentration are crucial for achieving the desired properties in the final PU product.
2. Types of Polyurethane Crosslinkers
Polyurethane crosslinkers can be broadly categorized based on their chemical structure and the type of reactive groups they contain.
2.1 Polyols
Polyols are the primary reactants in polyurethane synthesis and contribute significantly to the crosslinking process, particularly when using polyols with a functionality greater than two.
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2.1.1 Triols: Triols, such as glycerol and trimethylolpropane (TMP), contain three hydroxyl groups and are commonly used to increase the crosslinking density in PU systems. They offer a balance between flexibility and rigidity.
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2.1.2 Tetrols and Higher Functionality Polyols: Polyols with four or more hydroxyl groups, such as pentaerythritol and sucrose-based polyols, significantly increase the crosslinking density, leading to harder and more rigid PU materials. They are often used in applications requiring high strength and chemical resistance.
2.2 Amine Crosslinkers
Amine crosslinkers contain one or more amine groups (-NH2 or -NHR) that react rapidly with isocyanates.
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2.2.1 Aliphatic Amines: Aliphatic amines, such as ethylenediamine (EDA) and diethylenetriamine (DETA), react very quickly with isocyanates, leading to rapid curing and high crosslinking density. They are often used in fast-setting PU systems. However, their high reactivity can also lead to processing challenges.
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2.2.2 Aromatic Amines: Aromatic amines, such as methylene dianiline (MDA) and diamino diphenylmethane (DADPM), are less reactive than aliphatic amines due to the electron-withdrawing effect of the aromatic ring. This slower reactivity allows for better control over the curing process. They often impart improved thermal and chemical resistance to the final PU product.
2.3 Isocyanate Crosslinkers
Isocyanate crosslinkers contain multiple isocyanate groups and react with polyols to form urethane linkages, increasing the crosslinking density.
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2.3.1 Polymeric MDI (PMDI): PMDI is a mixture of diphenylmethane diisocyanate (MDI) oligomers with varying numbers of isocyanate groups. It is a widely used isocyanate crosslinker in rigid PU foams and elastomers due to its high functionality and cost-effectiveness.
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2.3.2 Blocked Isocyanates: Blocked isocyanates are isocyanates that have been reacted with a blocking agent, such as caprolactam or methyl ethyl ketoxime (MEKO). The blocking agent prevents the isocyanate group from reacting at room temperature, providing stability and extending the pot life of the PU formulation. At elevated temperatures, the blocking agent is released, regenerating the isocyanate group and allowing it to react with the polyol, initiating crosslinking.
2.4 Epoxy Crosslinkers
Epoxy crosslinkers contain one or more epoxy groups (oxirane rings) that can react with hydroxyl groups or amine groups in the PU system. They are often used to improve the adhesion and chemical resistance of PU coatings and adhesives. Examples include bisphenol A diglycidyl ether (BADGE) and epoxidized soybean oil.
2.5 Other Crosslinkers
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2.5.1 Carbodiimides: Carbodiimides (-N=C=N-) react with carboxylic acids and water, and can be used as crosslinkers in waterborne PU systems to improve hydrolytic stability and adhesion.
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2.5.2 Melamine Resins: Melamine resins, such as hexamethoxymethylmelamine (HMMM), contain multiple methoxymethyl groups that can react with hydroxyl groups in the PU system at elevated temperatures, forming ether linkages and increasing the crosslinking density. They are commonly used in PU coatings to improve hardness, scratch resistance, and chemical resistance.
Table 1: Common Polyurethane Crosslinkers and Their Characteristics
| Crosslinker Type | Examples | Reactive Groups | Reactivity with Isocyanates | Effect on PU Properties | Applications |
|---|---|---|---|---|---|
| Triols | Glycerol, TMP | -OH | Moderate | Increased crosslinking, balanced flexibility/rigidity | Flexible foams, coatings |
| Tetrols | Pentaerythritol | -OH | High | High crosslinking, high rigidity, good chemical resistance | Rigid foams, high-performance coatings |
| Aliphatic Amines | EDA, DETA | -NH2 | Very High | Rapid curing, high crosslinking, potential for brittleness | Fast-setting adhesives, coatings |
| Aromatic Amines | MDA, DADPM | -NH2 | Moderate | Improved thermal/chemical resistance, slower curing | Elastomers, high-performance coatings |
| PMDI | Polymeric MDI | -NCO | Moderate | High crosslinking, rigidity, good thermal stability | Rigid foams, elastomers |
| Blocked Isocyanates | Caprolactam-blocked isocyanate | -NCO (blocked) | Temperature-dependent | Stable at room temperature, delayed curing | One-component coatings, adhesives |
| Epoxy Resins | BADGE, Epoxidized Soybean Oil | Epoxy | Reacts with -OH/-NH2 | Improved adhesion, chemical resistance | Coatings, adhesives |
| Carbodiimides | – | -N=C=N- | Reacts with -COOH/H2O | Improved hydrolytic stability, adhesion | Waterborne PU systems |
| Melamine Resins | HMMM | Methoxymethyl | Reacts with -OH (heat) | Increased hardness, scratch resistance, chemical resistance | Coatings |
3. Mechanism of Action of Crosslinkers
The mechanism of action of polyurethane crosslinkers depends on the type of reactive groups they contain and their interaction with the polyol and isocyanate components of the PU system.
3.1 Reaction with Isocyanates
Amine crosslinkers and polyols react directly with isocyanates. The reaction between an amine and an isocyanate produces a urea linkage (-NH-CO-NH-), while the reaction between a hydroxyl group (from a polyol) and an isocyanate produces a urethane linkage (-NH-CO-O-). The rate of reaction depends on the reactivity of the amine or hydroxyl group and the steric hindrance around the isocyanate group. Aliphatic amines are generally more reactive than aromatic amines, while primary hydroxyl groups are more reactive than secondary hydroxyl groups.
3.2 Reaction with Polyols
Epoxy crosslinkers can react with hydroxyl groups in the polyol component, forming ether linkages and crosslinking the polymer chains. The reaction typically requires a catalyst, such as a tertiary amine or an acid. Melamine resins also react with hydroxyl groups at elevated temperatures, forming ether linkages.
3.3 Chain Extension and Crosslinking
Difunctional crosslinkers primarily act as chain extenders, increasing the molecular weight of the polymer chains but not necessarily forming a three-dimensional network. However, polyfunctional crosslinkers with three or more reactive groups can form multiple linkages between polymer chains, leading to the formation of a crosslinked network structure. The degree of crosslinking is determined by the functionality of the crosslinker and its concentration in the PU formulation.
3.4 Catalysis and Reaction Kinetics
The reactions involved in polyurethane crosslinking are often catalyzed by tertiary amines or organometallic compounds, such as dibutyltin dilaurate (DBTDL). These catalysts accelerate the reaction between the isocyanate and the polyol or crosslinker, allowing for faster curing and improved control over the crosslinking process. The type and concentration of the catalyst can significantly influence the final properties of the PU material.
4. Effects of Crosslinkers on Mechanical Properties
The mechanical properties of polyurethane materials are strongly influenced by the crosslinking density, which is directly affected by the type and concentration of crosslinkers used.
4.1 Tensile Strength and Elongation at Break
Tensile strength, the ability of a material to withstand tensile stress before breaking, generally increases with increasing crosslinking density up to a certain point. However, excessive crosslinking can lead to brittleness and a decrease in elongation at break, the amount of strain a material can withstand before breaking. The optimal crosslinking density for tensile strength and elongation at break depends on the specific application requirements.
4.2 Hardness
Hardness, the resistance of a material to indentation, typically increases with increasing crosslinking density. Higher crosslinking restricts chain movement and deformation, leading to a harder material.
4.3 Tear Strength
Tear strength, the resistance of a material to tearing, is also influenced by crosslinking density. Optimal tear strength is usually achieved at an intermediate crosslinking density, as excessive crosslinking can lead to brittleness and reduced resistance to crack propagation.
4.4 Impact Resistance
Impact resistance, the ability of a material to withstand sudden impact without fracturing, generally decreases with increasing crosslinking density. Higher crosslinking reduces the ability of the material to absorb energy through deformation, making it more prone to cracking under impact.
4.5 Abrasion Resistance
Abrasion resistance, the ability of a material to resist wear from friction, can be improved by increasing the crosslinking density. Higher crosslinking provides a harder and more durable surface that is less susceptible to abrasion.
4.6 Compression Set
Compression set, the permanent deformation of a material after being subjected to compressive stress, is reduced by increasing the crosslinking density. Higher crosslinking provides greater resistance to permanent deformation under compression.
Table 2: Effect of Crosslinking Density on Mechanical Properties
| Property | Low Crosslinking Density | Moderate Crosslinking Density | High Crosslinking Density |
|---|---|---|---|
| Tensile Strength | Low | Moderate to High | Moderate |
| Elongation at Break | High | Moderate | Low |
| Hardness | Low | Moderate | High |
| Tear Strength | Low | High | Moderate |
| Impact Resistance | High | Moderate | Low |
| Abrasion Resistance | Low | Moderate | High |
| Compression Set | High | Moderate | Low |
5. Effects of Crosslinkers on Thermal Properties
The thermal properties of polyurethane materials, such as glass transition temperature, heat resistance, and thermal stability, are significantly affected by the crosslinking density.
5.1 Glass Transition Temperature (Tg)
The glass transition temperature (Tg) is the temperature at which a polymer transitions from a hard, glassy state to a soft, rubbery state. Increasing the crosslinking density generally increases the Tg, as the crosslinks restrict chain movement and require more energy to overcome the intermolecular forces.
5.2 Heat Resistance
Heat resistance, the ability of a material to withstand elevated temperatures without degradation, is improved by increasing the crosslinking density. Higher crosslinking provides greater resistance to chain scission and thermal decomposition.
5.3 Thermal Stability
Thermal stability, the ability of a material to maintain its properties over time at elevated temperatures, is also enhanced by increasing the crosslinking density. The crosslinked network structure provides greater resistance to creep and deformation at high temperatures.
5.4 Flammability
Flammability, the ease with which a material ignites and burns, can be influenced by crosslinking. In general, higher crosslinking density can reduce flammability by increasing the char yield and reducing the release of volatile flammable compounds. However, the chemical nature of the crosslinker itself also plays a significant role.
6. Effects of Crosslinkers on Chemical Properties
The chemical resistance and stability of polyurethane materials are significantly influenced by the crosslinking density and the chemical nature of the crosslinker.
6.1 Solvent Resistance
Solvent resistance, the ability of a material to resist swelling or dissolution in organic solvents, is improved by increasing the crosslinking density. The crosslinked network structure restricts the penetration of solvent molecules into the polymer matrix.
6.2 Water Resistance
Water resistance, the ability of a material to resist absorption or degradation in water, is also enhanced by increasing the crosslinking density. Higher crosslinking reduces the number of hydrophilic groups (such as urea and urethane linkages) that are exposed to water.
6.3 Acid and Alkali Resistance
Acid and alkali resistance, the ability of a material to resist degradation in acidic or alkaline environments, can be influenced by the crosslinking density and the chemical stability of the crosslinker. Some crosslinkers may be more susceptible to hydrolysis or degradation in acidic or alkaline conditions than others.
6.4 Hydrolytic Stability
Hydrolytic stability, the ability of a material to resist degradation in the presence of water, is a critical property for polyurethane materials used in humid environments. Certain crosslinkers, such as carbodiimides, can improve hydrolytic stability by reacting with water and preventing the hydrolysis of the urethane linkages.
7. Effects of Crosslinkers on Aging Properties
The aging properties of polyurethane materials, such as UV resistance, thermal aging, hydrolytic aging, and ozone resistance, are influenced by the crosslinking density and the chemical nature of the crosslinker.
7.1 UV Resistance
UV resistance, the ability of a material to resist degradation from ultraviolet radiation, can be improved by adding UV stabilizers and by using crosslinkers that form stable linkages that are less susceptible to UV degradation.
7.2 Thermal Aging
Thermal aging, the degradation of a material due to prolonged exposure to elevated temperatures, can be minimized by using crosslinkers that form thermally stable linkages and by adding antioxidants to the PU formulation.
7.3 Hydrolytic Aging
Hydrolytic aging, the degradation of a material due to prolonged exposure to moisture, can be improved by using crosslinkers that enhance hydrolytic stability, as discussed in Section 6.4.
7.4 Ozone Resistance
Ozone resistance, the ability of a material to resist cracking and degradation from ozone exposure, is particularly important for elastomers used in outdoor applications. Certain crosslinkers and additives can improve ozone resistance by reacting with ozone and preventing its attack on the polymer chains.
8. Selection and Application of Polyurethane Crosslinkers
Selecting the appropriate crosslinker and optimizing its concentration are crucial for achieving the desired properties in the final PU product.
8.1 Factors Influencing Crosslinker Selection
- Desired Properties: The primary factor influencing crosslinker selection is the desired properties of the final PU material. For example, if high tensile strength and hardness are required, a highly functional crosslinker, such as PMDI or a tetrol, may be chosen. If flexibility and elongation are desired, a lower functionality crosslinker, such as a triol, may be more appropriate.
- Reactivity: The reactivity of the crosslinker with the isocyanate and polyol components must be considered. Fast-reacting crosslinkers, such as aliphatic amines, can lead to rapid curing and processing challenges, while slow-reacting crosslinkers, such as aromatic amines, allow for better control over the curing process.
- Compatibility: The crosslinker must be compatible with the other components of the PU formulation, including the polyol, isocyanate, catalysts, and additives.
- Cost: The cost of the crosslinker is also a factor to consider, particularly for large-scale applications.
8.2 Crosslinker Dosage and Formulation
The optimal dosage of the crosslinker depends on the desired crosslinking density and the functionality of the crosslinker. It is typically expressed as a percentage of the total weight of the polyol and isocyanate components. The formulation should be carefully balanced to ensure that the crosslinker reacts completely with the isocyanate and polyol, avoiding any unreacted crosslinker in the final product.
8.3 Processing Conditions
The processing conditions, such as temperature and mixing speed, can significantly influence the crosslinking process. Elevated temperatures can accelerate the reaction between the crosslinker and the isocyanate or polyol, while vigorous mixing ensures that the crosslinker is evenly dispersed throughout the PU formulation.
8.4 Compatibility with Other Additives
The compatibility of the crosslinker with other additives, such as catalysts, surfactants, fillers, and pigments, must be considered. Incompatible additives can interfere with the crosslinking process and lead to undesirable properties in the final PU product.
9. Applications of Crosslinked Polyurethanes
Crosslinked polyurethanes are used in a wide range of applications due to their versatile properties.
9.1 Coatings and Adhesives: Crosslinked PUs are used in coatings and adhesives to provide excellent adhesion, durability, chemical resistance, and weatherability.
9.2 Foams: Crosslinked PUs are used in both flexible and rigid foams for applications such as cushioning, insulation, and packaging.
9.3 Elastomers: Crosslinked PUs are used in elastomers for applications such as tires, seals, and gaskets, where high elasticity, abrasion resistance, and tear strength are required.
9.4 Thermoplastic Polyurethanes (TPUs): TPUs are a type of polyurethane elastomer that can be processed like thermoplastics. They contain both hard and soft segments, with the hard segments providing crosslinking through physical associations rather than chemical bonds.
10. Future Trends and Developments
The field of polyurethane crosslinkers is constantly evolving, with ongoing research focused on developing new and improved crosslinkers that offer enhanced performance and sustainability.
10.1 Bio-based Crosslinkers:
There is growing interest in developing bio-based crosslinkers derived from renewable resources, such as vegetable oils, lignin, and sugars. These bio-based crosslinkers offer a more sustainable alternative to traditional petroleum-based crosslinkers.
10.2 Reactive Nanoparticles as Crosslinkers:
Reactive nanoparticles, such as silica nanoparticles functionalized with amine or epoxy groups, are being explored as potential crosslinkers for polyurethanes. These nanoparticles can provide enhanced mechanical properties, thermal stability, and chemical resistance.
10.3 Smart Crosslinking Systems:
Smart crosslinking systems that respond to external stimuli, such as temperature, pH, or light, are being developed for applications such as self-healing materials and controlled release systems.
11. Conclusion
Polyurethane auxiliary agent crosslinkers play a critical role in determining the properties of polyurethane materials. By carefully selecting the appropriate crosslinker and optimizing its concentration, it is possible to tailor the mechanical, thermal, chemical, and aging properties of PU materials to meet the specific requirements of a wide range of applications. Ongoing research and development efforts are focused on developing new and improved crosslinkers that offer enhanced performance, sustainability, and functionality.
12. References
(Note: The following list includes example references in a standard scientific format. Please replace these with actual references used in your research.)
- Hepburn, C. Polyurethane Elastomers. 2nd ed. London: Elsevier Applied Science Publishers, 1992.
- Oertel, G. Polyurethane Handbook. 2nd ed. Munich: Hanser Publishers, 1994.
- Randall, D., & Lee, S. (2002). The polyurethanes book. John Wiley & Sons.
- Szycher, M. Szycher’s Handbook of Polyurethanes. 2nd ed. Boca Raton: CRC Press, 1999.
- Prociak, A., Ryszkowska, J., & Uramowski, P. (2016). Polyurethane foams: properties, modifying and application. Industrial Chemistry Library.
- Ashida, K. (2006). Polyurethane and related foaming systems. CRC press.
- Billmeyer, F. W. (1984). Textbook of polymer science. John Wiley & Sons.
- Saunders, J. H., & Frisch, K. C. (1962). Polyurethanes chemistry and technology: Chemistry. Interscience Publishers.
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