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.

• 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.

• 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.

• 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:

• Bulk Polymerization: The reactants are mixed in a reactor and allowed to react without any solvent. This method is cost-effective and environmentally friendly.

• 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.

• 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]
     |
     V
[Mixing & Reaction (Bulk, Solution, or Extrusion)]
     |
     V
[Polymerization]
     |
     V
[Pelletizing or Shaping]
     |
     V
[HTPU Product]

Ⅳ. Key Properties of HTPU

HTPUs exhibit a combination of desirable properties, including:

• Excellent Hydrolysis Resistance: This is the defining characteristic of HTPU, allowing it to withstand prolonged exposure to moisture and elevated temperatures without significant degradation.

• High Tensile Strength and Elongation: HTPUs possess excellent mechanical properties, making them suitable for demanding applications.

• Good Abrasion Resistance: HTPUs exhibit superior abrasion resistance compared to many other elastomers, ensuring long service life in abrasive environments.

• Excellent Chemical Resistance: HTPUs are resistant to a wide range of chemicals, including oils, greases, solvents, and fuels.

• 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.

• Good Low-Temperature Performance: Certain HTPU formulations exhibit excellent flexibility and impact resistance at low temperatures.

• 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:

• Type of Polyol: As previously discussed, polycarbonate polyols offer the best hydrolysis resistance, followed by modified polyester and polyether polyols.

• Type of Diisocyanate: Aliphatic diisocyanates generally provide better hydrolysis resistance than aromatic diisocyanates.

• Chain Extender: The choice of chain extender can influence the morphology and crystallinity of the TPU, which in turn can affect hydrolysis resistance.

• Additives: Carbodiimide stabilizers, antioxidants, and other additives can significantly enhance the hydrolysis resistance of TPU.

• Processing Conditions: Improper processing conditions, such as excessive moisture or high temperatures, can lead to degradation of the TPU and reduce its hydrolysis resistance.

• 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:

• 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.

• Marine: HTPUs are used for marine cables, hoses, and coatings that are exposed to seawater.

• Medical Devices: HTPUs are used for catheters, tubing, and other medical devices that come into contact with bodily fluids.

• Textiles: HTPUs are used for waterproof and breathable membranes in clothing and footwear.

• 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.

• Cable Jacketing: HTPU is used for cable jacketing, especially for underground cables or cables exposed to harsh weather conditions.

• 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:

• ASTM D412: Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers—Tension

• ASTM D2240: Standard Test Method for Rubber Property—Durometer Hardness

• ASTM D4060: Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser

• 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:

• 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.

• 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.

• Smart HTPUs: The development of HTPUs with self-healing capabilities, shape memory effects, and other smart functionalities is an emerging area of research.

• Customized HTPU Formulations: Tailoring HTPU formulations to meet the specific requirements of individual applications is becoming increasingly common.

• 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

1. Hepburn, C. Polyurethane Elastomers. Springer Science & Business Media, 1992.
2. Oertel, G. Polyurethane Handbook. Hanser Gardner Publications, 1994.
3. Rosthauser, J. W., and K. Nachtkamp. "Water-dispersible polyurethanes." Advances in Urethane Science and Technology 10 (1987): 121-162.
4. Woods, G. The ICI Polyurethanes Book. John Wiley & Sons, 1990.
5. Randall, D., and S. Lee. The Polyurethanes Book. John Wiley & Sons, 2002.
6. Szycher, M. Szycher’s Handbook of Polyurethanes. CRC press, 2012.
7. Prociak, A., Ryszkowska, J., & Uram, Ł. (2016). Polyurethanes: Synthesis, modification and applications. Elsevier.
8. European Standard EN ISO 20345:2011. Personal protective equipment – Safety footwear.
9. American Society for Testing and Materials (ASTM) Standards D412, D2240, D4060, and D3137.
10. International Organization for Standardization (ISO) Standard 4892-3.

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