Hydrolysis Resistant Thermoplastic Polyurethane Elastomer (HTPU): A Comprehensive Overview
Ⅰ. Introduction
Thermoplastic Polyurethane Elastomers (TPUs) are a versatile class of polymers known for their exceptional abrasion resistance, high elasticity, good tensile strength, and resistance to oils and greases. These properties make them suitable for a wide range of applications, including automotive parts, footwear, adhesives, and medical devices. However, conventional TPUs are susceptible to hydrolysis, a degradation process accelerated by exposure to moisture and elevated temperatures. This limits their use in humid environments or applications requiring prolonged contact with water.
Hydrolysis resistant TPUs (HTPUs) have been developed to address this limitation. These specialized TPUs are formulated with specific chemical structures and additives to enhance their resistance to hydrolytic degradation, thereby extending their service life and broadening their application scope. This article provides a comprehensive overview of HTPUs, covering their chemical composition, manufacturing process, key properties, applications, and future trends.
Ⅱ. Chemical Composition and Structure
The fundamental structure of TPU consists of alternating hard and soft segments. The hard segments, typically derived from diisocyanates and chain extenders (e.g., short-chain diols), provide rigidity and strength. The soft segments, usually derived from polyols (e.g., polyester polyols, polyether polyols, or polycarbonate polyols), contribute to elasticity and flexibility.
The hydrolysis resistance of TPU is primarily determined by the type of soft segment used. Ester-based TPUs are inherently susceptible to hydrolysis due to the presence of ester linkages in the soft segment, which are easily cleaved by water. Ether-based TPUs offer improved hydrolysis resistance compared to ester-based TPUs, but they can still degrade under prolonged exposure to harsh conditions.
HTPUs are typically formulated using polycarbonate polyols or specifically modified polyester or polyether polyols.
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Polycarbonate Polyols: These polyols contain carbonate linkages (-O-CO-O-) in their backbone, which are significantly more resistant to hydrolysis than ester linkages. TPUs based on polycarbonate polyols exhibit superior hydrolytic stability, making them ideal for applications in humid environments.
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Modified Polyester Polyols: Modification of polyester polyols can involve incorporating steric hindrance around the ester linkage or using specific diacids and diols that improve hydrolysis resistance.
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Modified Polyether Polyols: While generally better than polyester polyols, specific polyether polyols, or their blends with additives, can be formulated to further enhance hydrolysis resistance.
The choice of diisocyanate and chain extender can also influence hydrolysis resistance. Aliphatic diisocyanates, such as hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI), generally offer better UV resistance and hydrolysis resistance compared to aromatic diisocyanates, such as methylene diphenyl diisocyanate (MDI) and toluene diisocyanate (TDI). However, aromatic diisocyanates are often preferred for their cost-effectiveness and enhanced mechanical properties.
The use of appropriate additives, such as carbodiimide stabilizers and antioxidants, can further enhance the hydrolysis resistance of TPU.
Table 1: Comparison of Soft Segment Types in TPU
| Soft Segment Type | Chemical Structure | Hydrolysis Resistance | Cost | Mechanical Properties | Typical Applications |
|---|---|---|---|---|---|
| Polyester Polyol | -[CO-R-CO-O-R’-O]- | Low | Low | Good | General purpose, adhesives |
| Polyether Polyol | -[R-O-R’-O]- | Medium | Medium | Good | Automotive, cable jacketing |
| Polycarbonate Polyol | -[O-CO-O-R]- | High | High | Excellent | Medical devices, demanding environments |
| Modified Polyester Polyol | -[Modified Ester Linkages]- | Medium to High | Medium | Good | High-performance applications |
| Modified Polyether Polyol | -[Modified Ether Linkages]- | Medium to High | Medium | Good | High-performance applications |
Ⅲ. Manufacturing Process
The manufacturing process of HTPU is similar to that of conventional TPU and typically involves the reaction of a diisocyanate, a polyol, and a chain extender. The reaction can be carried out using various methods, including:
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Bulk Polymerization: The reactants are mixed in a reactor and allowed to react without any solvent. This method is cost-effective and environmentally friendly.
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Solution Polymerization: The reactants are dissolved in a solvent, and the reaction is carried out in solution. This method allows for better control of the reaction temperature and viscosity.
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Reactive Extrusion: The reactants are fed into an extruder, where they react and are simultaneously shaped into the desired form. This method is highly efficient and suitable for large-scale production.
The key to manufacturing HTPU lies in the careful selection and control of the raw materials and reaction conditions. It is crucial to use high-quality raw materials with low moisture content to minimize the potential for hydrolysis during processing. The reaction temperature, catalyst concentration, and mixing speed must be carefully controlled to ensure complete and uniform reaction.
Flowchart 1: General HTPU Manufacturing Process
[Diisocyanate] + [Polyol] + [Chain Extender] + [Additives]
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V
[Mixing & Reaction (Bulk, Solution, or Extrusion)]
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V
[Polymerization]
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V
[Pelletizing or Shaping]
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V
[HTPU Product]Ⅳ. Key Properties of HTPU
HTPUs exhibit a combination of desirable properties, including:
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Excellent Hydrolysis Resistance: This is the defining characteristic of HTPU, allowing it to withstand prolonged exposure to moisture and elevated temperatures without significant degradation.
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High Tensile Strength and Elongation: HTPUs possess excellent mechanical properties, making them suitable for demanding applications.
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Good Abrasion Resistance: HTPUs exhibit superior abrasion resistance compared to many other elastomers, ensuring long service life in abrasive environments.
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Excellent Chemical Resistance: HTPUs are resistant to a wide range of chemicals, including oils, greases, solvents, and fuels.
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Good Flexibility and Elasticity: HTPUs retain their flexibility and elasticity over a wide temperature range, allowing them to be used in applications requiring dynamic performance.
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Good Low-Temperature Performance: Certain HTPU formulations exhibit excellent flexibility and impact resistance at low temperatures.
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Processability: HTPUs can be processed using various techniques, including injection molding, extrusion, and blow molding.
Table 2: Typical Properties of HTPU (Values are indicative and vary based on specific formulation)
| Property | Test Method | Unit | Typical Value |
|---|---|---|---|
| Tensile Strength | ASTM D412 | MPa | 30-50 |
| Elongation at Break | ASTM D412 | % | 300-600 |
| Hardness | ASTM D2240 | Shore A/D | 70A – 75D |
| Tear Strength | ASTM D624 | kN/m | 40-80 |
| Abrasion Resistance | ASTM D4060 (Taber Abraser, CS-17 wheel, 1000 cycles) | mg loss | 10-50 |
| Hydrolysis Resistance | ASTM D3137 (95°C, 95% RH) | % Retention of Tensile Strength after 7 days | 80-95 |
| Density | ASTM D792 | g/cm³ | 1.1-1.3 |
| Glass Transition Temperature (Tg) | DSC | °C | -50 to -20 (Varies with soft segment) |
Note: Hydrolysis resistance testing is critical for HTPU. ASTM D3137 is a common method, but other accelerated aging tests are also employed, including exposure to high temperature and humidity, immersion in water at elevated temperatures, and steam autoclave testing. The specific test conditions are tailored to the intended application.
Ⅴ. Factors Affecting Hydrolysis Resistance
Several factors can influence the hydrolysis resistance of TPU, including:
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Type of Polyol: As previously discussed, polycarbonate polyols offer the best hydrolysis resistance, followed by modified polyester and polyether polyols.
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Type of Diisocyanate: Aliphatic diisocyanates generally provide better hydrolysis resistance than aromatic diisocyanates.
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Chain Extender: The choice of chain extender can influence the morphology and crystallinity of the TPU, which in turn can affect hydrolysis resistance.
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Additives: Carbodiimide stabilizers, antioxidants, and other additives can significantly enhance the hydrolysis resistance of TPU.
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Processing Conditions: Improper processing conditions, such as excessive moisture or high temperatures, can lead to degradation of the TPU and reduce its hydrolysis resistance.
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Environmental Conditions: The severity of the exposure environment, including temperature, humidity, and pH, can significantly affect the rate of hydrolysis.
Table 3: Factors Influencing Hydrolysis Resistance of TPU
| Factor | Influence | Mitigation Strategies |
|---|---|---|
| Polyol Type | Polycarbonate > Modified Polyester/Polyether > Polyester | Select appropriate polyol based on application requirements. |
| Diisocyanate Type | Aliphatic > Aromatic | Consider aliphatic diisocyanates for demanding applications. |
| Chain Extender | Influences morphology and crystallinity | Optimize chain extender selection for improved resistance. |
| Additives | Can significantly enhance resistance | Incorporate carbodiimide stabilizers and antioxidants. |
| Processing Conditions | Improper conditions can accelerate degradation | Control moisture content and temperature during processing. |
| Environmental Conditions | High temperature, humidity, and pH accelerate hydrolysis | Design for environmental conditions and consider protective coatings. |
Ⅵ. Applications of HTPU
HTPUs are used in a wide variety of applications where resistance to hydrolysis is critical. These applications include:
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Automotive: HTPUs are used for seals, gaskets, hoses, and other automotive parts that are exposed to moisture and heat. Specifically, under-the-hood applications benefit greatly.
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Marine: HTPUs are used for marine cables, hoses, and coatings that are exposed to seawater.
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Medical Devices: HTPUs are used for catheters, tubing, and other medical devices that come into contact with bodily fluids.
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Textiles: HTPUs are used for waterproof and breathable membranes in clothing and footwear.
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Industrial Applications: HTPUs are used for seals, gaskets, hoses, and conveyor belts in industrial settings where exposure to moisture and chemicals is common. This includes applications in wastewater treatment plants.
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Cable Jacketing: HTPU is used for cable jacketing, especially for underground cables or cables exposed to harsh weather conditions.
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Agriculture: HTPU is utilized in agricultural applications, such as irrigation tubing and liners, where resistance to moisture and soil chemicals is crucial.
Table 4: HTPU Applications and Corresponding Benefits
| Application | HTPU Benefits | Specific Examples |
|---|---|---|
| Automotive | Hydrolysis resistance, chemical resistance, abrasion resistance | Seals, gaskets, hoses, CVJ boots |
| Marine | Seawater resistance, UV resistance, flexibility | Cables, hoses, coatings, inflatable boats |
| Medical Devices | Biocompatibility, sterilization resistance, hydrolysis resistance | Catheters, tubing, wound dressings |
| Textiles | Waterproofness, breathability, durability | Waterproof membranes, footwear components |
| Industrial | Chemical resistance, abrasion resistance, hydrolysis resistance | Seals, gaskets, conveyor belts, hydraulic hoses |
| Cable Jacketing | Moisture resistance, electrical insulation, durability | Underground cables, marine cables, power cables |
| Agriculture | Chemical resistance (fertilizers, pesticides), UV resistance, flexibility | Irrigation tubing, pond liners, greenhouse films |
Ⅶ. Testing and Standards
The hydrolysis resistance of TPU is typically evaluated using accelerated aging tests, such as ASTM D3137. This test involves exposing the TPU sample to high temperature and humidity (e.g., 95°C and 95% RH) and measuring the change in its mechanical properties over time. Other relevant standards include:
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ASTM D412: Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers—Tension
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ASTM D2240: Standard Test Method for Rubber Property—Durometer Hardness
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ASTM D4060: Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser
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ISO 4892-3: Plastics — Methods of exposure to laboratory light sources — Part 3: Fluorescent UV lamps
These standards provide guidelines for evaluating the mechanical properties, abrasion resistance, and weathering resistance of TPU.
Table 5: Common Testing Standards for HTPU
| Standard | Description | Properties Measured |
|---|---|---|
| ASTM D3137 | Standard Test Method for Hydrolytic Resistance of Rubber | Retention of tensile strength and elongation after accelerated aging in high humidity and temperature |
| ASTM D412 | Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers—Tension | Tensile strength, elongation at break, modulus |
| ASTM D2240 | Standard Test Method for Rubber Property—Durometer Hardness | Hardness (Shore A or Shore D) |
| ASTM D4060 | Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser | Abrasion resistance (weight loss) |
| ISO 4892-3 | Plastics — Methods of exposure to laboratory light sources — Part 3: Fluorescent UV lamps | Resistance to UV degradation (assessed by changes in mechanical properties) |
Ⅷ. Market Trends and Future Directions
The market for HTPUs is expected to grow steadily in the coming years, driven by the increasing demand for durable and reliable materials in various industries. Key trends in the HTPU market include:
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Development of Bio-based HTPUs: There is a growing interest in developing HTPUs from renewable resources to reduce reliance on fossil fuels and minimize environmental impact.
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Nanocomposite HTPUs: The incorporation of nanoparticles, such as carbon nanotubes and graphene, can further enhance the mechanical properties, thermal stability, and barrier properties of HTPUs.
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Smart HTPUs: The development of HTPUs with self-healing capabilities, shape memory effects, and other smart functionalities is an emerging area of research.
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Customized HTPU Formulations: Tailoring HTPU formulations to meet the specific requirements of individual applications is becoming increasingly common.
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Increased Focus on Sustainability: Efforts are being made to develop more sustainable HTPU production processes, including reducing waste and using environmentally friendly raw materials.
Table 6: Future Trends in HTPU Development
| Trend | Description | Potential Benefits |
|---|---|---|
| Bio-based HTPUs | HTPUs derived from renewable resources | Reduced reliance on fossil fuels, lower carbon footprint |
| Nanocomposite HTPUs | HTPUs reinforced with nanoparticles | Enhanced mechanical properties, thermal stability, and barrier properties |
| Smart HTPUs | HTPUs with self-healing, shape memory, or other smart functionalities | Extended product lifespan, improved performance in dynamic applications |
| Customized HTPU Formulations | HTPUs tailored to specific application requirements | Optimized performance, improved efficiency |
| Sustainable HTPU Production | Environmentally friendly production processes | Reduced waste, lower environmental impact |
Ⅸ. Conclusion
Hydrolysis resistant TPUs represent a significant advancement in polymer technology, offering a combination of excellent mechanical properties, chemical resistance, and, most importantly, resistance to hydrolytic degradation. These materials are essential for applications in demanding environments where exposure to moisture and elevated temperatures is unavoidable. Ongoing research and development efforts are focused on further enhancing the performance and sustainability of HTPUs, paving the way for new and innovative applications in the future. The development of bio-based HTPUs and nanocomposite HTPUs, coupled with a growing emphasis on sustainability, promises to further expand the role of HTPUs in various industries.
Ⅹ. References
- Hepburn, C. Polyurethane Elastomers. Springer Science & Business Media, 1992.
- Oertel, G. Polyurethane Handbook. Hanser Gardner Publications, 1994.
- Rosthauser, J. W., and K. Nachtkamp. "Water-dispersible polyurethanes." Advances in Urethane Science and Technology 10 (1987): 121-162.
- Woods, G. The ICI Polyurethanes Book. John Wiley & Sons, 1990.
- Randall, D., and S. Lee. The Polyurethanes Book. John Wiley & Sons, 2002.
- Szycher, M. Szycher’s Handbook of Polyurethanes. CRC press, 2012.
- Prociak, A., Ryszkowska, J., & Uram, Ł. (2016). Polyurethanes: Synthesis, modification and applications. Elsevier.
- European Standard EN ISO 20345:2011. Personal protective equipment – Safety footwear.
- American Society for Testing and Materials (ASTM) Standards D412, D2240, D4060, and D3137.
- International Organization for Standardization (ISO) Standard 4892-3.