Shore Hardness and Thermoplastic Polyurethane Elastomers (TPU): A Comprehensive Overview

Introduction

Thermoplastic Polyurethane Elastomers (TPUs) represent a versatile class of engineering polymers bridging the gap between rubbers and plastics. Possessing both elastomeric flexibility and thermoplastic processability, TPUs find widespread application in diverse fields, from automotive components and footwear to medical devices and consumer goods. A crucial parameter defining the mechanical behavior of TPUs is their hardness, typically quantified using the Shore hardness scale. This article provides a comprehensive overview of Shore hardness in the context of TPUs, encompassing its principles, measurement techniques, influencing factors, and relationship to other material properties.

1. Understanding Shore Hardness

1.1 Principles of Shore Hardness Testing

Shore hardness is an empirical measure of a material’s resistance to indentation by a specified indenter under a controlled force. It is defined as the resistance of a material to penetration by a hardened steel pin under a specific load. Unlike fundamental material properties like Young’s modulus, Shore hardness is not directly related to stress or strain. Instead, it provides a comparative assessment of the material’s surface resistance to penetration. The Shore hardness test is particularly suitable for polymers and elastomers due to their viscoelastic nature and relatively low hardness values.

The test involves pressing an indenter of specific geometry into the material surface under a defined force. The depth of penetration is then correlated to a hardness value on a predetermined scale. Different Shore scales exist, each employing a different indenter shape and applied force, designed for materials with varying hardness ranges.

1.2 Shore Hardness Scales: A, D, and Others

The most commonly used Shore hardness scales for TPUs are Shore A and Shore D.

• Shore A: This scale is designed for softer materials, including flexible rubbers, elastomers, and soft plastics. The indenter is a truncated cone with a flat, circular tip. The hardness value ranges from 0 to 100, with higher values indicating greater hardness. Materials with a Shore A hardness of 90A or higher are generally considered to be at the upper end of the Shore A scale and may be better characterized by the Shore D scale.

• Shore D: This scale is designed for harder materials, including rigid plastics and hard rubbers. The indenter is a sharp cone. Similar to the Shore A scale, the hardness value ranges from 0 to 100, with higher values indicating greater hardness.

• Other Shore Scales: While less frequently used for TPUs, other Shore scales exist, such as Shore OO (for very soft materials like gels), Shore O (for soft materials like sponges), and Shore H (for materials with intermediate hardness).

The selection of the appropriate Shore scale depends on the expected hardness range of the TPU material. Using the wrong scale can result in inaccurate or meaningless readings.

1.3 Measurement Techniques and Standards

Shore hardness testing is typically performed using a durometer, a handheld or benchtop instrument that applies the specified force and measures the depth of indentation.

The test procedure generally involves:

1. Sample Preparation: The sample should be flat, clean, and of sufficient thickness to prevent bottoming out of the indenter. Standard thicknesses are often specified in relevant ASTM or ISO standards.
2. Instrument Calibration: The durometer should be calibrated regularly according to the manufacturer’s instructions.
3. Test Execution: The indenter is pressed perpendicularly onto the sample surface with a firm and even pressure. The hardness reading is taken after a specified dwell time, typically 1 to 15 seconds.
4. Data Recording: Multiple measurements are taken at different locations on the sample to account for material variability. The average hardness value and standard deviation are then calculated.

Common standards for Shore hardness testing include:

• ASTM D2240: Standard Test Method for Rubber Property—Durometer Hardness
• ISO 7619-1: Rubber, vulcanized or thermoplastic — Determination of indentation hardness — Part 1: Durometer method (Shore hardness)

These standards provide detailed specifications for the testing equipment, sample preparation, test procedure, and data reporting.

2. Factors Influencing TPU Hardness

The Shore hardness of TPU is influenced by several factors related to its chemical composition, morphology, and processing conditions.

2.1 Chemical Composition

• Hard Segment Content: TPUs are block copolymers consisting of hard and soft segments. The hard segments, typically composed of diisocyanates and chain extenders, provide rigidity and strength. The soft segments, typically composed of polyols, provide flexibility and elasticity. Increasing the hard segment content generally increases the Shore hardness of the TPU.

• Diisocyanate Type: Different diisocyanates, such as MDI (methylene diphenyl diisocyanate), TDI (toluene diisocyanate), and HDI (hexamethylene diisocyanate), can influence the hardness of the resulting TPU. MDI-based TPUs tend to exhibit higher hardness compared to TDI-based TPUs.

• Chain Extender Type: Chain extenders, such as 1,4-butanediol (BDO) and ethylene glycol (EG), play a crucial role in controlling the hard segment length and crystallinity. The type and concentration of chain extender can significantly impact the hardness of the TPU.

• Polyol Type: The choice of polyol, such as polyester polyols, polyether polyols, and polycarbonate polyols, affects the soft segment properties and, consequently, the overall hardness of the TPU. Polyester polyols generally lead to harder TPUs compared to polyether polyols.

2.2 Morphology

• Phase Separation: TPUs exhibit microphase separation, where the hard and soft segments segregate into distinct domains. The degree of phase separation influences the mechanical properties, including hardness. A well-defined phase separation typically results in higher hardness.

• Crystallinity: The hard segments can exhibit crystallinity, which further enhances the rigidity and hardness of the TPU. Factors such as the hard segment type, chain extender, and thermal history influence the degree of crystallinity.

• Hydrogen Bonding: Hydrogen bonding between the urethane linkages in the hard segments contributes to the intermolecular forces and influences the hardness of the TPU.

2.3 Processing Conditions

• Melt Temperature: The melt temperature during processing can affect the degree of phase mixing and the resulting morphology. High melt temperatures can lead to greater phase mixing, potentially reducing the hardness.

• Cooling Rate: The cooling rate after processing can influence the degree of crystallinity and phase separation. Slower cooling rates can promote crystallization and enhance hardness.

• Annealing: Annealing, a heat treatment process, can further improve the crystallinity and phase separation, leading to increased hardness.

2.4 Additives

• Fillers: The addition of fillers, such as silica, carbon black, and calcium carbonate, can increase the hardness of the TPU composite. The type, size, and concentration of the filler influence the extent of hardness enhancement.
• Plasticizers: Plasticizers are added to TPUs to improve flexibility and processability. However, the addition of plasticizers can reduce the hardness of the material.

3. Relationship Between Shore Hardness and Other Properties of TPU

Shore hardness is correlated with other mechanical properties of TPU, although the relationship is not always linear or straightforward.

3.1 Tensile Strength and Elongation at Break

Generally, TPUs with higher Shore hardness tend to exhibit higher tensile strength. However, the relationship between hardness and elongation at break is more complex. TPUs with very high hardness may have lower elongation at break due to their increased rigidity. TPUs with intermediate hardness often exhibit a good balance of tensile strength and elongation at break.

3.2 Tear Strength

Tear strength, the resistance to tearing, is also related to hardness. TPUs with higher hardness generally exhibit higher tear strength. However, the relationship can be influenced by other factors, such as the molecular weight and crosslinking density of the TPU.

3.3 Abrasion Resistance

Abrasion resistance, the ability to withstand wear and tear, is an important property for many TPU applications. TPUs with higher hardness tend to exhibit better abrasion resistance. The hard segments in the TPU contribute to the wear resistance by providing a strong and durable surface.

3.4 Compression Set

Compression set is a measure of the permanent deformation of a material after being subjected to a compressive force. TPUs with higher hardness generally exhibit lower compression set, indicating better elastic recovery.

3.5 Dynamic Mechanical Properties

Dynamic mechanical analysis (DMA) provides information about the viscoelastic behavior of TPUs. The storage modulus (E’) represents the elastic component, and the loss modulus (E”) represents the viscous component. TPUs with higher Shore hardness tend to have higher storage modulus and lower tan delta (E”/E’), indicating greater stiffness and lower damping.

4. Applications of TPU Based on Shore Hardness

The Shore hardness of TPU is a critical factor in determining its suitability for various applications.

4.1 Low Hardness TPUs (Shore A 60-80):

These soft TPUs are often used in applications requiring high flexibility and elasticity.

• Seals and Gaskets: Low hardness TPUs provide excellent sealing performance due to their ability to conform to irregular surfaces.
• Soft-Touch Grips: The soft and comfortable feel of these TPUs makes them ideal for grips and handles.
• Medical Tubing: Flexible medical tubing benefits from the biocompatibility and elasticity of low hardness TPUs.
• Adhesives and Sealants: The elastomeric nature of these TPUs makes them suitable for adhesive and sealant applications.

4.2 Medium Hardness TPUs (Shore A 80-95):

These TPUs offer a balance of flexibility and durability, making them suitable for a wide range of applications.

• Footwear Components: Shoe soles, midsoles, and uppers often utilize medium hardness TPUs for their abrasion resistance, flexibility, and comfort.
• Automotive Parts: Bellows, bushings, and seals in automotive applications benefit from the durability and elasticity of these TPUs.
• Protective Films: Medium hardness TPUs can be used as protective films for electronic devices and other sensitive surfaces.
• Sporting Goods: Applications such as skate wheels, protective gear, and inflatable products benefit from the balance of properties offered by these TPUs.

4.3 High Hardness TPUs (Shore D 50-70):

These rigid TPUs are used in applications requiring high strength, stiffness, and abrasion resistance.

• Industrial Wheels and Rollers: The high load-bearing capacity and abrasion resistance of these TPUs make them suitable for industrial wheels and rollers.
• Hard Cases and Enclosures: The rigidity and impact resistance of these TPUs make them ideal for hard cases and enclosures for electronic devices and tools.
• Automotive Exterior Parts: Bumpers, trim, and other exterior parts can utilize high hardness TPUs for their durability and weather resistance.
• Cable Jacketing: The abrasion resistance and chemical resistance of these TPUs make them suitable for cable jacketing applications.

5. Property Modification and Control

The hardness of TPU can be modified and controlled through various techniques to tailor the material to specific application requirements.

5.1 Compounding with Additives

• Fillers: Adding fillers like silica, carbon black, talc, or calcium carbonate can significantly increase the hardness and stiffness of TPU. The particle size, shape, and concentration of the filler influence the degree of hardness enhancement.
• Reinforcing Fibers: Incorporation of reinforcing fibers such as glass fibers, carbon fibers, or aramid fibers can dramatically improve the strength and stiffness of TPU, leading to a substantial increase in hardness.
• Plasticizers: Plasticizers can be added to reduce the hardness and increase the flexibility of TPU. However, the use of plasticizers can also compromise other properties, such as tensile strength and chemical resistance.

5.2 Blending with Other Polymers

Blending TPU with other polymers, such as polycarbonate (PC), acrylonitrile butadiene styrene (ABS), or polyvinyl chloride (PVC), can modify its hardness and other properties. The miscibility and compatibility of the polymers influence the resulting properties of the blend.

5.3 Chemical Modification

• Crosslinking: Introducing crosslinks into the TPU structure can increase its hardness, strength, and solvent resistance. Crosslinking can be achieved through chemical reactions or irradiation.
• Chain Extension: Modifying the chain extender type or concentration can affect the hard segment length and crystallinity, influencing the hardness of the TPU.
• Surface Modification: Surface modification techniques, such as plasma treatment or chemical grafting, can alter the surface hardness and other surface properties of TPU.

5.4 Processing Optimization

Optimizing the processing conditions, such as melt temperature, cooling rate, and annealing, can influence the morphology and crystallinity of the TPU, affecting its hardness.

6. Conclusion

Shore hardness is a critical parameter for characterizing the mechanical behavior of Thermoplastic Polyurethane Elastomers (TPUs). It provides a comparative measure of the material’s resistance to indentation and is closely related to other important properties such as tensile strength, tear strength, abrasion resistance, and compression set. The hardness of TPU is influenced by its chemical composition, morphology, processing conditions, and the addition of fillers or other additives. By understanding the factors that influence Shore hardness, engineers and designers can select the appropriate TPU material for a given application and tailor its properties to meet specific performance requirements. From flexible seals and gaskets to rigid industrial wheels and rollers, TPUs with varying Shore hardness values play a crucial role in a wide range of industries, showcasing their versatility and adaptability as engineering materials. Continued research and development in TPU materials and processing techniques will further expand their applications and enhance their performance in demanding environments. The careful selection and control of Shore hardness are essential for maximizing the benefits of TPU’s unique combination of elastomeric flexibility and thermoplastic processability.

References

1. Oertel, G. (Ed.). (1985). Polyurethane Handbook. Hanser Publishers.
2. Hepburn, C. (1992). Polyurethane Elastomers. Elsevier Science Publishers.
3. Saunders, J. H., & Frisch, K. C. (1962). Polyurethanes: Chemistry and Technology. Interscience Publishers.
4. Ashworth, V., & Beevers, R. B. (1973). Dynamic mechanical properties of segmented polyurethanes. Polymer, 14(8), 433-439.
5. Seymour, R. W., & Cooper, S. L. (1973). Properties of segmented polyurethane elastomers. Rubber Chemistry and Technology, 46(1), 23-51.
6. Blackwell, J., Nagarajan, M. R., & Hoitink, T. B. (1982). Structure-property relationships in segmented polyurethanes. Polymer Engineering and Science, 22(17), 1069-1079.
7. Chattopadhyay, D. K., & Webster, D. C. (2009). Polymer Synthesis. Progress in Polymer Science, 34(2), 134-177.
8. ASTM D2240, Standard Test Method for Rubber Property—Durometer Hardness, ASTM International, West Conshohocken, PA, 2015, https://www.astm.org
9. ISO 7619-1, Rubber, vulcanized or thermoplastic — Determination of indentation hardness — Part 1: Durometer method (Shore hardness), International Organization for Standardization, Geneva, Switzerland, 2010.
10. Brydson, J. A. (1999). Plastics Materials (7th ed.). Butterworth-Heinemann.
11. Mark, J. E. (Ed.). (1996). Physical Properties of Polymers Handbook. American Institute of Physics.
12. Billmeyer, F. W. (1984). Textbook of Polymer Science. John Wiley & Sons.

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