Improving abrasion resistance in PU elastomers with Low Free TDI Trimer hardeners

Improving Abrasion Resistance in Polyurethane Elastomers with Low Free TDI Trimer Hardeners

Introduction

Polyurethane (PU) elastomers are a versatile class of polymers prized for their exceptional mechanical properties, including high tensile strength, elongation at break, and tear resistance. However, their abrasion resistance, the ability to withstand wear and tear caused by frictional forces, is often a limiting factor in many applications. This article explores the strategies for improving abrasion resistance in PU elastomers through the use of low free toluene diisocyanate (TDI) trimer hardeners. We will delve into the chemistry of these hardeners, their impact on PU elastomer properties, and the formulation techniques employed to optimize abrasion resistance.

1. Overview of Polyurethane Elastomers

Polyurethane elastomers are block copolymers composed of soft segments (polyols) and hard segments (isocyanates and chain extenders). The soft segments provide flexibility and elasticity, while the hard segments contribute to strength and rigidity. The morphology of PU elastomers, characterized by phase separation between the soft and hard segments, significantly influences their overall performance.

• 1.1 Polyol Components:

  Polyols are the long-chain, hydroxyl-terminated polymers that form the soft segment of the PU elastomer. Common types include:

  • Polyether polyols: Known for their excellent hydrolysis resistance and low-temperature flexibility. Examples include polytetramethylene glycol (PTMG), polypropylene glycol (PPG), and polyethylene glycol (PEG).
  • Polyester polyols: Offer superior mechanical properties, heat resistance, and solvent resistance. Examples include polyethylene adipate (PEA) and polycaprolactone (PCL).
  • Polycarbonate polyols: Provide excellent weatherability, chemical resistance, and abrasion resistance.

• 1.2 Isocyanate Components:

  Isocyanates react with polyols and chain extenders to form the urethane linkage, which constitutes the hard segment. Common isocyanates include:

  • Toluene diisocyanate (TDI): Historically widely used due to its low cost and reactivity. However, its toxicity has led to a shift towards alternative isocyanates.
  • Methylene diphenyl diisocyanate (MDI): Offers improved safety and processing characteristics compared to TDI.
  • Hexamethylene diisocyanate (HDI): Aliphatic isocyanate, resulting in excellent UV resistance.
  • Isophorone diisocyanate (IPDI): Aliphatic isocyanate, contributing to good weatherability and flexibility.

• 1.3 Chain Extenders:

  Chain extenders are low molecular weight diols or diamines that react with isocyanates to further build the hard segment and increase the polymer’s molecular weight. Common chain extenders include:

  • 1,4-Butanediol (BDO): A widely used diol chain extender that contributes to good mechanical properties.
  • Ethylene glycol (EG): A simple diol chain extender.
  • Diamines: Such as 4,4′-methylenebis(2-chloroaniline) (MOCA), although its use is restricted due to toxicity concerns.

2. TDI Trimer Hardeners: Structure and Properties

TDI trimer hardeners are isocyanurate derivatives of TDI. Isocyanurates are cyclic trimers of isocyanates, formed through a trimerization reaction. This process reduces the concentration of free TDI, thereby mitigating its associated health risks. Low free TDI trimers contain a very small percentage of unreacted TDI.

• 2.1 Chemical Structure:

  The isocyanurate ring structure significantly enhances the thermal stability and chemical resistance of the PU elastomer. The presence of three isocyanate groups per trimer molecule increases the crosslink density, leading to improved mechanical properties.

• 2.2 Properties of Low Free TDI Trimer Hardeners:

  Property	Typical Value	Unit	Test Method
  NCO Content	22-24	%	ASTM D2572
  Viscosity (25°C)	1000-3000	mPa·s	ASTM D2196
  Free TDI Content	<0.5	%	GC
  Functionality (NCO groups/mol)	3	–	Calculated
  Appearance	Clear to Pale Yellow Liquid	–	Visual Inspection

• 2.3 Advantages of Using Low Free TDI Trimer Hardeners:

  • Reduced Toxicity: Significantly lower concentration of free TDI compared to conventional TDI, minimizing health hazards.
  • Improved Thermal Stability: The isocyanurate ring imparts enhanced thermal stability to the resulting PU elastomer.
  • Enhanced Chemical Resistance: The cyclic structure contributes to improved resistance against solvents and chemicals.
  • Increased Crosslink Density: The trifunctional nature of the trimer increases crosslinking, leading to improved mechanical properties.

3. Impact of TDI Trimer Hardeners on Abrasion Resistance

Abrasion resistance is a complex property influenced by multiple factors, including the hardness, toughness, and tear strength of the PU elastomer. TDI trimer hardeners can significantly enhance abrasion resistance through several mechanisms:

• 3.1 Increased Hardness:

  The higher crosslink density imparted by the trimer hardener leads to a harder PU elastomer. Increased hardness generally correlates with improved abrasion resistance, as the material is more resistant to indentation and scratching.

• 3.2 Enhanced Cohesive Strength:

  The increased crosslinking strengthens the intermolecular forces within the polymer matrix, resulting in higher cohesive strength. This enhanced cohesive strength makes the material more resistant to the removal of surface particles during abrasion.

• 3.3 Improved Tear Strength:

  The presence of the isocyanurate ring and the increased crosslink density contribute to improved tear strength. Higher tear strength reduces the likelihood of crack propagation during abrasive wear, leading to enhanced abrasion resistance.

• 3.4 Optimized Phase Separation:

  The choice of polyol and the compatibility between the soft and hard segments play a crucial role in phase separation. TDI trimer hardeners can influence the morphology of the PU elastomer, leading to optimized phase separation and improved mechanical properties, including abrasion resistance.

4. Formulation Techniques for Optimizing Abrasion Resistance

Achieving optimal abrasion resistance requires careful consideration of the formulation, including the choice of polyol, chain extender, additives, and processing conditions.

• 4.1 Polyol Selection:

  The choice of polyol significantly impacts the overall properties of the PU elastomer.

  • Polyester Polyols: Typically provide superior abrasion resistance compared to polyether polyols due to their higher mechanical strength and toughness. However, they are more susceptible to hydrolysis. Polycaprolactone (PCL) based polyester polyols are particularly favored for their abrasion resistance.
  • Polyether Polyols: Offer excellent hydrolysis resistance and low-temperature flexibility. While generally less abrasion resistant than polyester polyols, they can be modified to improve their performance.
  • Polycarbonate Polyols: These polyols deliver the best abrasion resistance among the three types, combined with outstanding chemical resistance and weatherability. They are often used in demanding applications.

  Polyol Type	Abrasion Resistance	Hydrolysis Resistance	Cost	Applications
  Polyester Polyol	High	Low	Moderate	Wheels, rollers, seals, coatings
  Polyether Polyol	Moderate	High	Low	Flexible foams, adhesives, sealants
  Polycarbonate Polyol	Very High	Moderate to High	High	High-performance coatings, rollers, mining equipment, military applications

• 4.2 Chain Extender Selection:

  The choice of chain extender influences the hardness and modulus of the hard segment.

  • 1,4-Butanediol (BDO): A common chain extender that provides good mechanical properties and abrasion resistance.
  • Other Diols: Short-chain diols like ethylene glycol (EG) can increase hardness but may reduce elongation.
  • Diamines: Can be used to achieve high hardness and reactivity, but their toxicity often limits their application.

• 4.3 Additives:

  Various additives can be incorporated into the PU formulation to further enhance abrasion resistance.

  • Fillers: Inorganic fillers, such as silica, alumina, and carbon black, can improve hardness and abrasion resistance. The particle size and dispersion of the filler are critical for optimal performance.
  • Lubricants: Internal lubricants, such as silicone oils and fatty acid esters, can reduce the coefficient of friction and improve abrasion resistance.
  • Crosslinking Agents: Additional crosslinking agents can further increase the crosslink density and enhance mechanical properties.
  • UV Stabilizers: These are especially important when using aliphatic isocyanates like HDI or IPDI to prevent degradation from sunlight.

• 4.4 Processing Conditions:

  Proper processing conditions, including mixing, casting, and curing, are essential for achieving optimal properties.

  • Mixing: Thorough mixing of the components is crucial to ensure homogeneity and complete reaction.
  • Casting: Bubble-free casting is important to avoid defects that can weaken the material.
  • Curing: Proper curing temperature and time are necessary to achieve complete crosslinking and optimal mechanical properties.

5. Applications of Abrasion Resistant PU Elastomers

Abrasion resistant PU elastomers are used in a wide range of applications where wear and tear are significant concerns.

• 5.1 Industrial Applications:

  • Wheels and Rollers: Used in forklifts, conveyor systems, and printing presses.
  • Coatings: Applied to floors, pipelines, and machinery to protect against abrasion and corrosion.
  • Linings: Used in chutes, hoppers, and mixers to reduce wear from abrasive materials.
  • Seals and Gaskets: Provide durable sealing in harsh environments.

• 5.2 Consumer Applications:

  • Shoe Soles: Provide excellent wear resistance and durability.
  • Sporting Goods: Used in skateboard wheels, rollerblade wheels, and other high-wear applications.
  • Protective Gear: Used in knee pads, elbow pads, and other protective equipment.

• 5.3 Mining and Construction:

  • Screening Media: Used in vibrating screens to separate aggregates and minerals.
  • Hydrocyclones: Used to separate solids from liquids in mining operations.
  • Pipe Linings: Protect pipelines from abrasion caused by slurry transport.

6. Testing Methods for Abrasion Resistance

Various standardized testing methods are used to evaluate the abrasion resistance of PU elastomers.

• 6.1 Taber Abrasion Test (ASTM D4060):

  This test uses a rotating abrasive wheel to wear away the surface of the material. The weight loss after a specified number of cycles is used as a measure of abrasion resistance.

  Parameter	Description
  Abrasive Wheels	CS-17, H-18, etc.
  Load	500g, 1000g, etc.
  Number of Cycles	1000, 5000, etc.
  Measurement	Weight loss (mg)
  Interpretation	Lower weight loss indicates better abrasion resistance

• 6.2 Akron Abrasion Test (ASTM D5963):

  This test involves rubbing a rotating abrasive wheel against the surface of the material under a specified load. The volume loss is measured to determine the abrasion resistance.

• 6.3 DIN Abrasion Test (DIN 53516):

  Similar to the Akron test, this method uses a rotating abrasive wheel to wear away the surface of the material. The volume loss is measured, providing a measure of abrasion resistance.

• 6.4 Martens Abrasion Test (EN ISO 12947-2):

  This test measures the resistance of a material to abrasion caused by rubbing against a standard abrasive cloth under a controlled load.

7. Future Trends and Developments

The field of PU elastomers is continuously evolving, with ongoing research focused on developing new materials and technologies to further enhance abrasion resistance.

• 7.1 Nanomaterials:

  Incorporating nanomaterials, such as carbon nanotubes, graphene, and nanosilica, into the PU matrix can significantly improve mechanical properties and abrasion resistance.

• 7.2 Bio-based Polyols:

  The use of bio-based polyols derived from renewable resources is gaining increasing attention due to environmental concerns. Research is focused on developing bio-based polyols with comparable or superior performance to traditional polyols.

• 7.3 Self-Healing Materials:

  The development of self-healing PU elastomers that can repair damage caused by abrasion is a promising area of research.

• 7.4 Surface Modification Techniques:

  Surface modification techniques, such as plasma treatment and chemical grafting, can be used to improve the abrasion resistance of PU elastomers without significantly altering their bulk properties.

Conclusion

Improving abrasion resistance is crucial for expanding the applications of PU elastomers. The use of low free TDI trimer hardeners offers a viable strategy for enhancing abrasion resistance while mitigating the toxicity associated with conventional TDI. Careful consideration of the formulation, including the choice of polyol, chain extender, additives, and processing conditions, is essential for achieving optimal performance. With ongoing research and development in materials and technologies, PU elastomers will continue to play an increasingly important role in applications requiring high abrasion resistance. 🛡️

Literature References

1. Oertel, G. (Ed.). (1993). Polyurethane Handbook: Chemistry-Raw Materials-Processing-Application. Hanser Gardner Publications.
2. Hepburn, C. (1992). Polyurethane Elastomers. Elsevier Science Publishers.
3. Saunders, J. H., & Frisch, K. C. (1962). Polyurethanes: Chemistry and Technology. Interscience Publishers.
4. Rosthauser, J. W., & Nachtkamp, K. (1987). Water-Borne Polyurethanes. Advances in Urethane Science and Technology, 10, 121-162.
5. Randall, D., & Lee, S. (2002). The Polyurethanes Book. John Wiley & Sons.
6. Szycher, M. (1999). Szycher’s Handbook of Polyurethanes. CRC Press.
7. Krol, P. (2007). Polyurethanes: synthesis, properties and applications. Progress in Materials Science, 52(6), 915-1015.
8. Chen, L., et al. (2016). Abrasion resistance of polyurethane elastomers: A review. Journal of Applied Polymer Science, 133(48), 44289.
9. Xiao, X., et al. (2020). Enhancing the abrasion resistance of polyurethane elastomers by incorporating nanosilica. Polymer Testing, 82, 106320.
10. Zhang, Y., et al. (2018). Preparation and properties of bio-based polyurethane elastomers derived from castor oil. Industrial Crops and Products, 125, 478-486.
11. ASTM D4060, Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser.
12. ASTM D5963, Standard Test Method for Rubber Property—Abrasion Resistance (Rotary Drum Abrader).
13. DIN 53516, Testing of rubber and elastomers; determination of abrasion resistance.
14. EN ISO 12947-2, Textiles – Determination of the abrasion resistance of fabrics by the Martindale method – Part 2: Determination of specimen breakdown.