TPUs are block copolymers typically composed of alternating hard and soft segments. The hard segments, often derived from diisocyanates and chain extenders (e.g., diols), provide rigidity and strength, while the soft segments, typically derived from macroglycols (e.g., polyester or polyether polyols), contribute flexibility and elasticity. The ratio and type of these segments can be tailored to achieve specific physical and mechanical properties, leading to a wide variety of TPU grades with varying hardness, flexibility, and chemical resistance.
1.1 TPU Structure and Properties
The microstructure of TPU significantly influences its macroscopic properties. The hard segments tend to aggregate, forming crystalline or pseudo-crystalline domains that act as physical crosslinks. These domains provide strength and dimensional stability. The soft segments, being amorphous and flexible, allow for elastic deformation. The compatibility between the hard and soft segments also plays a crucial role in determining the overall performance of the material.
Key properties of TPUs include:
- High Elasticity: TPUs can undergo significant deformation and recover their original shape.
- High Abrasion Resistance: TPUs exhibit excellent resistance to wear and tear, making them suitable for applications involving friction.
- High Tear Strength: TPUs resist tearing and propagation of cracks, ensuring durability.
- Good Chemical Resistance: TPUs demonstrate resistance to a wide range of chemicals, although the specific resistance varies depending on the TPU grade and the chemical environment.
- Low-Temperature Flexibility: Certain TPU grades maintain flexibility at low temperatures.
- Processability: TPUs can be processed using various techniques such as injection molding, extrusion, and blow molding.
1.2 Types of TPUs
TPUs can be broadly classified based on the type of polyol used in the soft segment:
- Polyester TPUs: These TPUs offer excellent abrasion resistance, oil resistance, and solvent resistance. However, they are generally more susceptible to hydrolysis compared to polyether TPUs.
- Polyether TPUs: These TPUs exhibit superior hydrolysis resistance, low-temperature flexibility, and microbial resistance. They are commonly used in applications requiring exposure to moisture or humid environments.
- Polycaprolactone TPUs: These TPUs offer a balance of properties between polyester and polyether TPUs, combining good abrasion resistance with moderate hydrolysis resistance.
- Polycarbonate TPUs: These TPUs exhibit excellent heat resistance and dimensional stability, but are generally more expensive than other types of TPUs.
The choice of TPU type depends on the specific requirements of the application, considering factors such as chemical exposure, temperature, and mechanical stress.
Ⅱ. Factors Affecting Chemical Resistance of TPU
The chemical resistance of TPU is a complex phenomenon influenced by several factors, including the TPU composition, chemical structure, temperature, concentration, and duration of exposure. Understanding these factors is essential for predicting the long-term performance of TPU in specific chemical environments.
2.1 TPU Composition
The type and ratio of hard and soft segments significantly affect the chemical resistance of TPU.
- Soft Segment Type: As mentioned earlier, polyester TPUs generally exhibit better resistance to oils and solvents, while polyether TPUs offer superior hydrolysis resistance. The specific type of polyester or polyether polyol also influences the chemical resistance. For example, adipate-based polyester TPUs may exhibit different resistance compared to sebacate-based polyester TPUs.
- Hard Segment Type: The type of diisocyanate and chain extender used in the hard segment also affects the chemical resistance. Aromatic diisocyanates, such as MDI (methylene diphenyl diisocyanate), are generally more resistant to chemicals compared to aliphatic diisocyanates, such as HDI (hexamethylene diisocyanate). The chain extender also influences the hard segment’s resistance to chemical attack.
- Additives: Additives, such as antioxidants, UV stabilizers, and plasticizers, can also impact the chemical resistance of TPU. Some additives may enhance resistance to specific chemicals, while others may reduce it.
2.2 Chemical Structure and Properties of the Attacking Medium
The chemical structure and properties of the attacking medium play a crucial role in determining its interaction with TPU.
- Polarity: Polar solvents tend to interact more strongly with polar TPUs, while non-polar solvents interact more strongly with non-polar TPUs.
- Molecular Size: Smaller molecules can penetrate the TPU matrix more easily compared to larger molecules.
- Acidity/Alkalinity: Strong acids and bases can degrade TPUs, particularly polyester TPUs, through hydrolysis or other chemical reactions.
- Oxidizing Power: Strong oxidizing agents can attack TPUs, leading to chain scission and degradation.
2.3 Temperature
Temperature has a significant impact on the chemical resistance of TPU.
- Increased Reaction Rate: Higher temperatures accelerate chemical reactions, leading to faster degradation of TPU.
- Increased Diffusion Rate: Higher temperatures increase the diffusion rate of chemicals into the TPU matrix, increasing the extent of chemical attack.
- Softening of TPU: Higher temperatures can soften TPU, making it more susceptible to deformation and swelling.
2.4 Concentration
The concentration of the attacking medium directly affects the rate of degradation. Higher concentrations generally lead to faster degradation.
2.5 Duration of Exposure
The duration of exposure is a critical factor in determining the extent of chemical attack. Longer exposure times lead to greater degradation.
2.6 Stress
Mechanical stress can accelerate the degradation of TPU in chemical environments. Stress can create micro-cracks in the TPU matrix, allowing chemicals to penetrate more easily.
Ⅲ. Chemical Resistance Testing Methods
Various standardized testing methods are used to evaluate the chemical resistance of TPU. These methods typically involve immersing TPU samples in the test chemical for a specified period at a controlled temperature and then measuring changes in physical and mechanical properties.
3.1 Standard Test Methods
Commonly used standard test methods include:
- ASTM D471: Standard Test Method for Rubber Property—Effect of Liquids. This method is widely used to evaluate the resistance of rubber and elastomers, including TPU, to various liquids. It involves immersing samples in the test liquid and measuring changes in volume, weight, and mechanical properties.
- ISO 1817: Rubber, vulcanized or thermoplastic — Determination of the effect of liquids. This standard is similar to ASTM D471 and is used to evaluate the resistance of rubber and elastomers to liquids.
- ASTM D543: Standard Practices for Evaluating the Resistance of Plastics to Chemical Reagents. This method is used to evaluate the resistance of plastics, including TPU, to chemical reagents. It involves immersing samples in the test reagent and measuring changes in weight, dimensions, and appearance.
- ISO 175: Plastics — Methods of test for the determination of the effects of immersion in liquid chemicals. This standard is similar to ASTM D543 and is used to evaluate the resistance of plastics to liquid chemicals.
3.2 Properties Measured
The following properties are typically measured to assess the chemical resistance of TPU:
- Weight Change: Indicates the absorption or leaching of chemicals.
- Volume Change: Indicates swelling or shrinkage of the TPU.
- Tensile Strength: Indicates the loss of strength due to chemical degradation.
- Elongation at Break: Indicates the loss of elasticity due to chemical degradation.
- Hardness: Indicates changes in the stiffness of the TPU.
- Appearance: Visual inspection for changes in color, surface cracking, or other signs of degradation.
3.3 Accelerated Aging Tests
Accelerated aging tests are used to predict the long-term performance of TPU in specific chemical environments. These tests involve exposing TPU samples to elevated temperatures, high concentrations of chemicals, or other aggressive conditions to accelerate the degradation process. The data obtained from accelerated aging tests can be used to estimate the service life of TPU in real-world applications.
Ⅳ. Chemical Resistance Data for Various Chemicals
The chemical resistance of TPU varies depending on the specific chemical environment. The following tables provide general guidelines for the resistance of different types of TPUs to various chemicals. It is important to note that these are general guidelines and the actual resistance may vary depending on the specific TPU grade and the test conditions.
Table 1: Chemical Resistance of Polyester TPU
| Chemical | Resistance | Notes |
|---|---|---|
| Aliphatic Hydrocarbons (e.g., Hexane) | Fair | May cause swelling and softening. |
| Aromatic Hydrocarbons (e.g., Toluene) | Poor | Causes significant swelling and degradation. |
| Chlorinated Hydrocarbons (e.g., Methylene Chloride) | Poor | Causes significant swelling and degradation. |
| Alcohols (e.g., Ethanol) | Good | Generally resistant, but prolonged exposure may cause some swelling. |
| Ketones (e.g., Acetone) | Fair | May cause swelling and softening. |
| Esters (e.g., Ethyl Acetate) | Fair | May cause swelling and softening. |
| Ethers (e.g., Diethyl Ether) | Poor | Causes swelling and degradation. |
| Mineral Oils | Excellent | Highly resistant. |
| Vegetable Oils | Excellent | Highly resistant. |
| Dilute Acids (e.g., 10% HCl) | Good | Generally resistant, but prolonged exposure to strong acids may cause degradation. |
| Dilute Bases (e.g., 10% NaOH) | Good | Generally resistant, but prolonged exposure to strong bases may cause degradation. |
| Water | Fair | Susceptible to hydrolysis, especially at elevated temperatures. |
| Salt Solutions | Good | Generally resistant. |
| Fuels (e.g., Gasoline) | Fair | May cause swelling and softening. |
Table 2: Chemical Resistance of Polyether TPU
| Chemical | Resistance | Notes |
|---|---|---|
| Aliphatic Hydrocarbons (e.g., Hexane) | Fair | May cause swelling and softening. |
| Aromatic Hydrocarbons (e.g., Toluene) | Poor | Causes significant swelling and degradation. |
| Chlorinated Hydrocarbons (e.g., Methylene Chloride) | Poor | Causes significant swelling and degradation. |
| Alcohols (e.g., Ethanol) | Good | Generally resistant, but prolonged exposure may cause some swelling. |
| Ketones (e.g., Acetone) | Fair | May cause swelling and softening. |
| Esters (e.g., Ethyl Acetate) | Fair | May cause swelling and softening. |
| Ethers (e.g., Diethyl Ether) | Poor | Causes swelling and degradation. |
| Mineral Oils | Fair | Less resistant than polyester TPU. May cause swelling and softening. |
| Vegetable Oils | Fair | Less resistant than polyester TPU. May cause swelling and softening. |
| Dilute Acids (e.g., 10% HCl) | Good | Generally resistant, but prolonged exposure to strong acids may cause degradation. |
| Dilute Bases (e.g., 10% NaOH) | Good | Generally resistant, but prolonged exposure to strong bases may cause degradation. |
| Water | Excellent | Highly resistant to hydrolysis. |
| Salt Solutions | Good | Generally resistant. |
| Fuels (e.g., Gasoline) | Fair | May cause swelling and softening. |
Table 3: Resistance of TPU to Specific Chemical Groups
| Chemical Group | Resistance of Polyester TPU | Resistance of Polyether TPU | Notes |
|---|---|---|---|
| Aliphatic Hydrocarbons | Fair | Fair | Swelling may occur. Select specific grades for improved resistance. |
| Aromatic Hydrocarbons | Poor | Poor | Significant swelling and degradation are likely. Avoid prolonged exposure. |
| Chlorinated Solvents | Poor | Poor | Similar to aromatic hydrocarbons, significant degradation can occur. |
| Alcohols | Good | Good | Generally good resistance, especially to lower molecular weight alcohols. |
| Ketones | Fair | Fair | Swelling and softening are possible. |
| Esters | Fair | Fair | Similar to ketones, swelling and softening can occur. |
| Acids (Dilute) | Good | Good | Generally resistant, but strong acids can cause degradation over time. |
| Bases (Dilute) | Good | Good | Good resistance, but prolonged exposure to concentrated bases should be avoided. |
| Oils (Mineral) | Excellent | Fair | Polyester TPUs exhibit superior resistance to mineral oils. Polyether TPUs may experience swelling. |
| Water | Fair | Excellent | Polyether TPUs are significantly more resistant to hydrolysis than polyester TPUs. |
Note: Resistance Ratings: Excellent, Good, Fair, Poor
- Excellent: No significant effect on the material.
- Good: Minor effect on the material, acceptable for many applications.
- Fair: Moderate effect on the material, may limit the application.
- Poor: Significant effect on the material, not suitable for the application.
Important Considerations:
- The data provided in these tables are general guidelines and should not be considered definitive.
- The actual chemical resistance of a specific TPU grade may vary depending on the manufacturer, formulation, and test conditions.
- It is essential to consult the manufacturer’s data sheets for specific chemical resistance data for the TPU grade being considered.
- Testing under actual service conditions is recommended to ensure the suitability of the TPU for a particular application.
Ⅴ. Applications of TPU Based on Chemical Resistance
The chemical resistance of TPU is a crucial factor in determining its suitability for various applications.
5.1 Automotive Industry
- Fuel Lines and Seals: Polyester TPUs are often used in fuel lines and seals due to their excellent resistance to fuels and oils.
- Hoses and Tubing: Polyether TPUs are used in hoses and tubing requiring resistance to hydrolysis and low-temperature flexibility.
- Automotive Interior Parts: TPUs are used in interior parts such as dashboards and door panels due to their abrasion resistance and aesthetic appeal.
5.2 Footwear Industry
- Outsoles: TPUs are widely used in outsoles of athletic shoes and work boots due to their abrasion resistance, tear strength, and flexibility. The choice between polyester and polyether TPU depends on the specific application and environmental conditions.
- Midsoles: TPUs are used in midsoles for cushioning and support.
5.3 Medical Industry
- Catheters and Tubing: Polyether TPUs are often used in medical catheters and tubing due to their biocompatibility, flexibility, and resistance to hydrolysis and sterilization.
- Medical Devices: TPUs are used in various medical devices requiring biocompatibility and chemical resistance to bodily fluids and cleaning agents.
5.4 Industrial Applications
- Hoses and Seals: TPUs are used in industrial hoses and seals requiring resistance to oils, solvents, and chemicals.
- Protective Coatings: TPUs are used as protective coatings for various surfaces requiring abrasion resistance and chemical resistance.
- Conveyor Belts: TPUs are used in conveyor belts requiring abrasion resistance and resistance to chemicals and oils.
5.5 Consumer Goods
- Mobile Phone Cases: TPUs are used in mobile phone cases due to their abrasion resistance, flexibility, and impact resistance.
- Watch Straps: TPUs are used in watch straps due to their comfort, durability, and resistance to sweat and chemicals.
Ⅵ. Strategies for Enhancing TPU Chemical Resistance
Several strategies can be employed to enhance the chemical resistance of TPU.
6.1 Selection of Appropriate TPU Grade
The most important step is to select the appropriate TPU grade for the specific application, considering the chemical environment and other performance requirements. This involves carefully evaluating the chemical resistance data provided by the manufacturer and selecting a TPU grade that is known to be resistant to the chemicals that it will be exposed to.
6.2 Use of Additives
Additives can be used to enhance the chemical resistance of TPU.
- Antioxidants: Antioxidants can protect TPU from oxidative degradation, which can be accelerated by exposure to certain chemicals.
- UV Stabilizers: UV stabilizers can protect TPU from degradation caused by ultraviolet radiation, which can also weaken its resistance to chemicals.
- Hydrolysis Stabilizers: Hydrolysis stabilizers can protect polyester TPUs from hydrolysis, which is the degradation of the polymer chain by water.
- Fillers: Certain fillers, such as mineral fillers, can improve the chemical resistance of TPU by reducing the permeability of the material.
6.3 Surface Treatments
Surface treatments can be used to create a barrier layer on the TPU surface, preventing chemicals from penetrating the material.
- Coatings: Coatings can be applied to the TPU surface to provide a barrier against chemicals.
- Plasma Treatment: Plasma treatment can modify the surface of TPU, making it more resistant to chemicals.
6.4 Design Considerations
The design of the TPU component can also influence its chemical resistance.
- Minimizing Stress: Minimizing stress on the TPU component can reduce the risk of chemical attack.
- Avoiding Sharp Corners: Avoiding sharp corners can reduce stress concentrations, which can make the TPU more susceptible to chemical attack.
- Increasing Thickness: Increasing the thickness of the TPU component can increase its resistance to chemical penetration.
Ⅶ. Future Trends in TPU Chemical Resistance
Future trends in TPU chemical resistance are focused on developing new TPU grades with improved resistance to a wider range of chemicals, as well as developing new additives and surface treatments to enhance the chemical resistance of existing TPU grades. Research is also being conducted on the development of bio-based TPUs with improved chemical resistance.
Specifically, future trends include:
- Development of New TPU Chemistries: Research is ongoing to develop new TPU chemistries that offer improved chemical resistance compared to existing TPU grades. This includes exploring new types of polyols, diisocyanates, and chain extenders.
- Development of Nanocomposites: Nanocomposites, which are TPUs reinforced with nanoscale fillers, are being developed to improve the chemical resistance and mechanical properties of TPU.
- Development of Self-Healing TPUs: Self-healing TPUs, which can repair themselves after being damaged, are being developed to improve the durability and service life of TPU components in harsh chemical environments.
- Increased Focus on Sustainable Solutions: The development of bio-based TPUs with comparable or superior chemical resistance to traditional petroleum-based TPUs is a growing area of focus.
Ⅷ. Conclusion
Understanding the chemical resistance of Thermoplastic Polyurethane Elastomer (TPU) is paramount for selecting the right material for specific applications and ensuring long-term performance. Factors like TPU composition, the chemical structure of the attacking medium, temperature, concentration, exposure duration, and stress significantly impact chemical resistance. Polyester TPUs generally offer better resistance to oils and solvents, while polyether TPUs excel in hydrolysis resistance. Chemical resistance testing, following standards like ASTM D471 and ISO 1817, helps evaluate material behavior. By considering these factors and utilizing appropriate testing methods, engineers and designers can make informed decisions, optimizing the use of TPU in diverse industries. Future trends focus on developing improved TPU grades, nanocomposites, self-healing capabilities, and sustainable bio-based alternatives, further expanding the application possibilities for this versatile material.
Ⅸ. References
(Note: These references are provided as examples and should be replaced with actual cited literature. Do not include external links.)
- Hepburn, C. Polyurethane Elastomers. Springer Science & Business Media, 1992.
- Oertel, G. Polyurethane Handbook. Hanser Gardner Publications, 1994.
- Saunders, J. H., and K. C. Frisch. Polyurethanes: Chemistry and Technology. Interscience Publishers, 1962.
- ASTM D471, Standard Test Method for Rubber Property—Effect of Liquids. ASTM International.
- ISO 1817, Rubber, vulcanized or thermoplastic — Determination of the effect of liquids. International Organization for Standardization.
- Randall, D., and S. Lee. The Polyurethanes Book. John Wiley & Sons, 2002.
- Ashida, K. Polyurethane and Related Foams: Chemistry and Technology. CRC Press, 2006.
- Prociak, A., Ryszkowska, J., and Uram, K. Polyurethane Thermoplastic Elastomers: Synthesis, Properties and Applications. Elsevier, 2016.
- Domínguez-Candela, I., et al. "Chemical resistance and long-term performance of thermoplastic polyurethane elastomers." Polymer Degradation and Stability 96.1 (2011): 1-10.
- Jia, L., et al. "Effect of chemical structure on the chemical resistance of thermoplastic polyurethanes." Journal of Applied Polymer Science 130.5 (2013): 3641-3648.
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