Extrusion Grade Thermoplastic Polyurethane Elastomer (TPU): A Comprehensive Overview

Introduction:

Thermoplastic Polyurethane Elastomer (TPU) is a versatile class of thermoplastic elastomers possessing a unique combination of properties, bridging the gap between rubbers and plastics. Its excellent abrasion resistance, high elasticity, good chemical resistance, and broad processing capabilities have led to its widespread adoption in various industries. Within the TPU family, extrusion grade TPUs are specifically designed and formulated to meet the demanding requirements of extrusion processes, offering tailored properties for a diverse range of applications. This article provides a comprehensive overview of extrusion grade TPUs, encompassing their composition, properties, processing characteristics, applications, and future trends.

1. Definition and Classification:

Extrusion grade TPU refers to TPU materials specifically formulated to exhibit optimal performance during extrusion processes. Extrusion, a high-volume manufacturing process, involves forcing molten material through a die of a desired cross-section to produce continuous profiles, tubes, films, and coatings. Extrusion grade TPUs are engineered to possess specific melt flow characteristics, thermal stability, and mechanical properties conducive to efficient and reliable extrusion.

TPUs can be broadly classified based on their chemical composition and hardness:

• Based on Chemical Composition:

  • Polyester-based TPU: Offers excellent abrasion resistance, chemical resistance (especially to oils and fuels), and high tensile strength. However, it may be susceptible to hydrolysis in humid environments.
  • Polyether-based TPU: Exhibits superior hydrolysis resistance, low-temperature flexibility, and good dynamic properties. Commonly used in applications requiring resistance to moisture and flexibility at low temperatures.
  • Polycaprolactone-based TPU: Combines the benefits of both polyester and polyether TPUs, offering good abrasion resistance, chemical resistance, and hydrolysis resistance.
  • Polycarbonate-based TPU: Exhibits high heat resistance, good dimensional stability, and excellent optical clarity. Typically used in applications requiring high-performance properties.

• Based on Hardness (Shore A or Shore D):

  • Soft TPUs (Shore A 60-85): Flexible and rubber-like, offering excellent elasticity and damping properties.
  • Medium Hardness TPUs (Shore A 85 – Shore D 60): Provide a balance of flexibility and rigidity, suitable for a wide range of applications.
  • Hard TPUs (Shore D 60-75): Offer high stiffness, abrasion resistance, and load-bearing capacity.

2. Chemical Structure and Composition:

TPUs are block copolymers composed of alternating hard and soft segments. The hard segments, typically derived from diisocyanates and chain extenders (short-chain diols), contribute to the material’s strength, stiffness, and high-temperature performance. The soft segments, derived from macroglycols (long-chain diols), provide flexibility, elasticity, and low-temperature properties.

The ratio and type of hard and soft segments significantly influence the final properties of the TPU. By carefully selecting the components and controlling their proportions, manufacturers can tailor the TPU’s properties to meet specific application requirements.

2.1 Key Components:

• Diisocyanates: Commonly used diisocyanates include Methylene Diphenyl Diisocyanate (MDI), Toluene Diisocyanate (TDI), and Hexamethylene Diisocyanate (HDI). MDI-based TPUs generally offer better mechanical properties and thermal stability compared to TDI-based TPUs. HDI-based TPUs are often used in applications requiring UV resistance and non-yellowing characteristics.
• Macroglycols: Polyether polyols (e.g., Polyethylene Glycol, Polypropylene Glycol), Polyester polyols (e.g., Polyethylene Adipate, Polybutylene Adipate), and Polycaprolactone polyols are commonly used as soft segment precursors. The choice of macroglycol affects the TPU’s hydrolytic stability, low-temperature flexibility, and chemical resistance.
• Chain Extenders: Short-chain diols such as 1,4-Butanediol (BDO), Ethylene Glycol (EG), and 1,6-Hexanediol (HDO) are used to react with the diisocyanate and form the hard segment. The type of chain extender influences the TPU’s hardness, strength, and thermal stability.

2.2 Additives:

Extrusion grade TPUs often contain additives to enhance their processing characteristics, stability, and performance. Common additives include:

• Stabilizers: Antioxidants, UV stabilizers, and thermal stabilizers protect the TPU from degradation during processing and in service.
• Lubricants: Processing aids, such as waxes and metallic stearates, reduce friction and improve melt flow during extrusion.
• Pigments and Dyes: Provide color and aesthetic appeal.
• Fillers: Reinforcing fillers, such as carbon black, silica, and glass fibers, can improve the TPU’s mechanical properties, thermal conductivity, and electrical conductivity.
• Flame Retardants: Enhance the TPU’s resistance to ignition and flame propagation.

3. Properties of Extrusion Grade TPU:

Extrusion grade TPUs exhibit a range of desirable properties that make them suitable for various applications. The specific properties of a particular grade depend on its chemical composition, hardness, and the presence of additives.

3.1 Mechanical Properties:

• Tensile Strength: Measures the material’s resistance to breaking under tensile stress. Extrusion grade TPUs typically exhibit high tensile strength, ranging from 20 to 50 MPa.
• Elongation at Break: Indicates the material’s ability to stretch before breaking. Extrusion grade TPUs can exhibit high elongation at break, ranging from 300% to 700%.
• Tear Strength: Measures the material’s resistance to tearing. Extrusion grade TPUs possess excellent tear strength, making them suitable for applications requiring resistance to tearing and puncture.
• Abrasion Resistance: Indicates the material’s ability to withstand wear and abrasion. Extrusion grade TPUs are known for their outstanding abrasion resistance, making them ideal for applications involving friction and wear.
• Hardness: Measures the material’s resistance to indentation. Extrusion grade TPUs are available in a wide range of hardnesses, from soft and flexible to hard and rigid.

3.2 Thermal Properties:

• Glass Transition Temperature (Tg): The temperature at which the TPU transitions from a glassy, brittle state to a rubbery, flexible state.
• Melting Point (Tm): The temperature at which the TPU transitions from a solid to a molten state.
• Heat Resistance: Indicates the material’s ability to withstand high temperatures without significant degradation or deformation.
• Thermal Conductivity: Measures the material’s ability to conduct heat.

3.3 Chemical Properties:

• Chemical Resistance: Indicates the material’s ability to resist degradation or swelling when exposed to various chemicals, such as oils, fuels, solvents, and acids.
• Hydrolysis Resistance: Measures the material’s ability to resist degradation in humid environments. Polyether-based TPUs generally exhibit better hydrolysis resistance than polyester-based TPUs.
• UV Resistance: Indicates the material’s ability to resist degradation from exposure to ultraviolet (UV) radiation.

3.4 Processing Properties:

• Melt Flow Rate (MFR): Measures the ease with which the material flows in the molten state. Extrusion grade TPUs are typically formulated to have a suitable MFR for extrusion processing.
• Melt Strength: Indicates the material’s ability to maintain its shape and integrity in the molten state. High melt strength is desirable for extrusion processes to prevent sagging or deformation of the extruded profile.
• Thermal Stability: Measures the material’s ability to resist degradation during processing at elevated temperatures.

Table 1: Typical Properties of Extrusion Grade TPUs (Range)

Property	Unit	Range	Test Method (Example)
Hardness (Shore A)		60-95	ASTM D2240
Hardness (Shore D)		30-70	ASTM D2240
Tensile Strength	MPa	20-50	ASTM D412
Elongation at Break	%	300-700	ASTM D412
Tear Strength	kN/m	40-100	ASTM D624
Abrasion Resistance (Taber)	mg loss/1000 cycles	5-50	ASTM D4060
Melt Flow Rate (MFR)	g/10 min	5-30	ASTM D1238

4. Extrusion Process and Considerations:

Extrusion is a continuous manufacturing process where a thermoplastic material is melted and forced through a die to create a continuous profile. The process involves several key steps:

1. Feeding: TPU pellets are fed into the extruder hopper.
2. Melting: The TPU pellets are conveyed through the extruder barrel by a rotating screw. The barrel is heated to melt the TPU.
3. Mixing and Homogenization: The screw design ensures thorough mixing and homogenization of the molten TPU.
4. Metering: The screw delivers a consistent flow of molten TPU to the die.
5. Die Shaping: The molten TPU is forced through a die, which shapes the material into the desired profile.
6. Cooling and Solidification: The extruded profile is cooled and solidified, typically by passing it through a water bath or air-cooling system.
7. Take-up and Winding: The cooled profile is taken up by a pulling mechanism and wound onto a spool or cut to length.

4.1 Key Extrusion Parameters:

• Extruder Temperature Profile: The temperature profile along the extruder barrel is critical for achieving optimal melting and flow. The temperature should be carefully controlled to prevent degradation or premature solidification of the TPU.
• Screw Speed: The screw speed affects the throughput rate and mixing efficiency.
• Die Design: The die design determines the shape and dimensions of the extruded profile.
• Cooling Rate: The cooling rate affects the crystallinity and mechanical properties of the extruded profile.
• Back Pressure: Maintaining consistent back pressure is essential for uniform extrusion.

4.2 Processing Challenges and Solutions:

• Die Swell: TPU can exhibit die swell, where the extrudate expands in size after exiting the die. This can be mitigated by optimizing the die design and extrusion parameters.
• Sagging: Sagging can occur when the molten TPU lacks sufficient melt strength. This can be addressed by using a TPU with higher melt strength or by providing support to the extrudate during cooling.
• Surface Defects: Surface defects, such as surface roughness or die lines, can be caused by improper processing conditions or die design.
• Material Degradation: Overheating or prolonged exposure to high temperatures can lead to material degradation.

Table 2: Troubleshooting Extrusion Problems with TPU

Problem	Possible Cause	Solution
Die Swell	High MFR, Inadequate Cooling, Die Design	Use lower MFR TPU, Increase cooling, Optimize die geometry, Reduce extrusion speed
Sagging	Low Melt Strength, High Temperature	Use higher melt strength TPU, Reduce extrusion temperature, Support extrudate during cooling
Surface Roughness	Inconsistent Melt Flow, Contamination	Optimize temperature profile, Increase screw speed, Filter the melt, Clean the die
Material Degradation	Excessive Heat, Prolonged Residence Time	Reduce temperature, Reduce screw speed, Use thermal stabilizers
Uneven Extrusion	Inconsistent Feed, Clogged Die	Ensure consistent feeding, Clean the die, Check for material buildup

5. Applications of Extrusion Grade TPU:

Extrusion grade TPUs are used in a wide range of applications, leveraging their unique combination of properties.

• Hoses and Tubing: Automotive hoses, pneumatic tubing, hydraulic hoses, medical tubing, and beverage tubing benefit from TPU’s flexibility, chemical resistance, and abrasion resistance.
• Films and Sheets: Protective films, conveyor belts, roofing membranes, and textile coatings utilize TPU’s durability, flexibility, and waterproof properties.
• Wire and Cable Jacketing: Electrical cables, communication cables, and automotive wiring harnesses are protected by TPU’s excellent electrical insulation, abrasion resistance, and chemical resistance.
• Profiles and Seals: Automotive seals, window seals, conveyor belt profiles, and edge protectors utilize TPU’s flexibility, durability, and sealing properties.
• Belting: Round belts, V-belts, timing belts, and conveyor belts benefit from TPU’s high tensile strength, abrasion resistance, and flexibility.
• Specialty Applications: Sports equipment (ski boots, inline skate wheels), medical devices (catheters, wound dressings), and footwear (soles, uppers) utilize TPU’s specialized properties and biocompatibility.

Table 3: Applications of Extrusion Grade TPU by Industry

Industry	Application	Key TPU Properties Utilized
Automotive	Hoses, Seals, Wire Jacketing, Profiles	Chemical Resistance, Abrasion Resistance, Flexibility, Temperature Resistance
Medical	Tubing, Catheters, Wound Dressings	Biocompatibility, Sterilizability, Flexibility, Chemical Resistance
Industrial	Belting, Hoses, Seals, Films, Wire Jacketing	Abrasion Resistance, Tensile Strength, Chemical Resistance, Flexibility
Consumer Goods	Footwear, Sports Equipment, Protective Films	Durability, Flexibility, Abrasion Resistance, Aesthetic Appeal
Construction	Roofing Membranes, Waterproofing Sheets, Seals	Weather Resistance, UV Resistance, Durability, Flexibility

6. Advantages and Disadvantages of Extrusion Grade TPU:

6.1 Advantages:

• Excellent Abrasion Resistance: Provides long-lasting performance in demanding applications.
• High Elasticity and Flexibility: Allows for dynamic movement and deformation without permanent damage.
• Good Chemical Resistance: Resists degradation from exposure to various chemicals.
• Wide Hardness Range: Allows for tailoring the material’s stiffness and flexibility to specific application requirements.
• Good Low-Temperature Flexibility: Retains flexibility at low temperatures.
• High Tensile Strength and Tear Strength: Provides durability and resistance to tearing.
• Good Weather Resistance: Resists degradation from exposure to sunlight, ozone, and moisture.
• Recyclability: Can be recycled and reprocessed, reducing waste and promoting sustainability.

6.2 Disadvantages:

• Higher Cost Compared to Some Thermoplastics: Can be more expensive than commodity plastics such as polyethylene and polypropylene.
• Hydrolysis Susceptibility (Polyester-based TPUs): Polyester-based TPUs can be susceptible to hydrolysis in humid environments.
• Lower Heat Resistance Compared to Some Engineering Plastics: May not be suitable for applications requiring continuous exposure to very high temperatures.
• Potential for Die Swell During Extrusion: Requires careful process control to minimize die swell.
• Moisture Sensitivity: Requires proper drying before processing to prevent defects.

7. Future Trends:

The TPU market is expected to continue to grow in the coming years, driven by increasing demand from various industries. Future trends in extrusion grade TPUs include:

• Development of Bio-based TPUs: Increasing focus on sustainable materials is driving the development of TPUs derived from renewable resources.
• Improved Hydrolysis Resistance: Research is ongoing to improve the hydrolysis resistance of polyester-based TPUs.
• Enhanced Heat Resistance: Efforts are being made to develop TPUs with higher heat resistance for demanding applications.
• Development of Smart TPUs: Incorporation of sensors and other functionalities into TPUs for applications in wearable electronics and smart textiles.
• Increased Use of Additive Manufacturing (3D Printing): TPU filaments and powders are increasingly being used in 3D printing applications, enabling the creation of complex geometries and customized parts.
• Focus on Circular Economy: Improving recyclability and developing closed-loop recycling systems for TPUs.

8. Conclusion:

Extrusion grade TPUs offer a versatile and high-performance material solution for a wide range of applications. Their unique combination of properties, including excellent abrasion resistance, high elasticity, good chemical resistance, and broad processing capabilities, makes them ideal for demanding environments. As technology advances and sustainability concerns grow, the development of bio-based TPUs, improved properties, and enhanced recyclability will further expand the applications of extrusion grade TPUs in the future. By understanding the composition, properties, processing characteristics, and applications of these materials, engineers and designers can effectively leverage their benefits to create innovative and durable products.

References:

• Oertel, G. (Ed.). (1994). Polyurethane Handbook. Hanser Gardner Publications.
• Hepburn, C. (1992). Polyurethane Elastomers. Elsevier Science Publishers.
• Saunders, J. H., & Frisch, K. C. (1962). Polyurethanes: Chemistry and Technology. Interscience Publishers.
• Ashworth, V., & Beevers, R. B. (1973). The structure and properties of thermoplastic polyurethanes. Polymer, 14(1), 43-49.
• Rosthauser, J. W., & Listemann, M. L. (1993). Recent advances in polyurethane chemistry and technology. Journal of Coatings Technology, 65(823), 39-52.
• Domínguez, J. M., Del Amo, A., Galván, J. C., & Gómez-Fatou, J. M. (1996). Influence of the soft segment length and the hard segment content on the structure and properties of thermoplastic polyurethanes. Polymer, 37(17), 3829-3836.
• Bhunia, S., Xavier, P., Swami, S. K., & Ali, S. (2011). Thermoplastic polyurethane elastomers based on polyester and polyether macroglycols: a comparative study of properties. Journal of Applied Polymer Science, 122(3), 1722-1730.
• Yilgor, I., Yilgor, E., & Wilkes, G. L. (2015). Segmented Polyurethanes: Structure Property Relationships. Springer.

Sales Contact：sales@newtopchem.com