Factors Affecting Scratch Resistance of Water-Based Self-Matte Resin on Leather

When applied to leather, the factors affecting the scratch resistance of water-based self-matte polyurethane resin become even more specific and demanding. Leather (especially synthetic leather and automotive leather) is a flexible substrate that frequently undergoes bending, friction, and contact with various substances during use. Therefore, scratch resistance must achieve a delicate balance with flexibility, hand feel, flex resistance, and chemical resistance. Considering leather application scenarios, below are the key factors influencing scratch resistance:

1. Resin Microstructure and Matting Mechanism: Balancing Roughness and Density

• Self-Matte vs. Added Matting Powder: Self-matte resins are the preferred choice. In leather applications, high-end products (such as automotive leather and sofa leather) favor inherent self-matte resins over reliance on added silica matting powder. This is because added matting powder tends to compromise the density of the coating film. During frequent bending and friction of leather, these "hard particles" can become stress concentration points, leading to coating chalking or scratch formation.

• Control of Surface Roughness: Self-matte resins achieve matting through microphase separation that forms an uneven surface. However, the higher the roughness, the more easily the protruding "peaks" are worn flat by abrasion, resulting in localized glossing (shine) after scratching. Therefore, balancing scratch resistance with matting effect is key: it is necessary to form as dense and rounded a microstructure as possible while maintaining the desired matte appearance, avoiding sharp protrusions.

2. Surface Lubricity and Hand Feel: Reducing the Coefficient of Friction

Leather coatings place great emphasis on hand feel, which is closely linked to scratch resistance.

• Silicone Modification (Most Effective Approach): Research and practice have shown that introducing silicone (polydimethylsiloxane, i.e., siloxane chain segments) into the polyurethane main chain or side chains is a core technology for improving leather scratch resistance. Silicone chain segments spontaneously migrate to the coating surface, forming a lubricating film that significantly reduces the coefficient of friction, making it difficult for sharp objects (such as fingernails or keys) to scratch the coating. Research from Sichuan University showed that as silicone content increases, the surface silicon element content can reach 9.26%, with a substantial improvement in abrasion resistance.

• Selection of Wax Emulsions: Adding polyethylene wax or PTFE wax to the formulation can also provide lubricity. However, on leather, hand feel is paramount — excessive or improperly selected wax can adversely affect the "skin feel" or "silky feel," requiring careful screening.

3. Crosslinking and Cohesive Strength: Resisting Deformation and Damage

Leather continuously stretches and rebounds during use, and the coating must have sufficient strength to resist these stresses.

• Introduction of Crosslinkers: Single-component thermoplastic polyurethanes have limited scratch resistance. In leather finishing, it is generally recommended to use them in combination with crosslinkers:

  • Aziridine or Carbodiimide: Suitable for single-component systems, these can significantly improve the coating's solvent resistance (e.g., alcohol resistance, sweat resistance) and scratch resistance.

  • Polyurethane-Urea Structures: By molecular design, urea bonds or rigid rings (such as pyridine rings) can be introduced to enhance hydrogen bonding interactions between molecular chains, forming physical crosslink points, thereby improving scratch resistance while maintaining flexibility.

• Hard/Soft Segment Ratio: For leather (especially soft leather), the coating needs high resilience. If too many hard segments make the coating too brittle, fine cracks will form during stretching; if too many soft segments make it too soft, permanent indentations will be easily left by hard objects. An ideal scratch-resistant coating should exhibit the characteristic of being "hard yet not brittle, flexible yet not sticky."

4. Film-Forming Density and Process

• Degree of Particle Coalescence: If the film-forming temperature of the water-based resin is inappropriate or the film-forming assistant is poorly selected, resulting in insufficient coalescence of polyurethane particles, gaps or defects will exist within the coating. Such coatings are more prone to failure at grain boundaries when scratched. For leather applications requiring flex resistance, dense film formation is particularly important.

• Coating Thickness and Number of Layers: In leather finishing, a multi-layer structure of primer, mid-coat, and topcoat is typically employed. The scratch resistance of the topcoat largely determines the final performance. Coatings that are too thin can be easily scratched through, while those that are too thick may adversely affect hand feel or lead to brittleness and cracking.

5. Differentiated Requirements by Application Scenario

Different leather products have different scratch resistance priorities, so the relative importance of influencing factors varies:

Application Scenario

Core Scratch Resistance Requirements

Key Influencing Factors

Summary

To improve the scratch resistance of water-based self-matte polyurethane on leather, the core approaches are:

1. Molecular Design Level: Introduce silicone chain segments for modification — this is the most direct and effective means to achieve "lubricating scratch resistance."

2. Formulation Level: Crosslinkers (aziridine/carbodiimide) must be used to construct a three-dimensional network, enhancing the coating's resistance to external force damage.

3. Performance Balance Level: Strictly control the relationship between matting effect (roughness) and density, ensuring that even at extremely low gloss levels (60° gloss < 2 or even < 1), the surface remains smooth and dense, avoiding the "shine after scratching" phenomenon.

(This article is for reference only. For technical questions, please consult our online engineers.)