How can water-based polyurethane waterproof coating improve its balance between water resistance and flexibility through molecular structure design?
Release Time : 2026-02-09
Water-based polyurethane waterproof coating, as an environmentally friendly polymer material, boasts a core advantage in achieving a balance between water resistance and flexibility through molecular structure design. This balance requires a multi-dimensional synergistic effect, including adjusting the ratio of hard and soft segments, optimizing the content of hydrophilic groups, controlling crosslinking density, selecting polyols, matching isocyanate types, designing molecular chain extension and branching, and introducing functional monomers.
The ratio of hard and soft segments is a key factor affecting the performance of waterborne polyurethane. Hard segments, formed by the reaction of isocyanates and chain extenders, impart strength and water resistance to the material; soft segments, composed of polyols, determine the material's flexibility and elasticity. By adjusting the ratio of hard and soft segments, water resistance can be improved while maintaining flexibility. For example, increasing the hard segment content improves the material's resistance to water penetration, but excessive increases can lead to decreased flexibility; conversely, appropriately increasing the soft segment ratio enhances the material's elasticity, but a significant reduction in water resistance must be avoided. Therefore, the optimal ratio of hard and soft segments needs to be determined experimentally to achieve a synergistic improvement in both properties.
The content of hydrophilic groups directly affects the water resistance and stability of waterborne polyurethane. Hydrophilic groups such as carboxyl and sulfonic acid groups are crucial for the dispersion of waterborne polyurethane in water, but excessive introduction can lead to increased water absorption and decreased water resistance. Therefore, the content of hydrophilic groups should be minimized while ensuring emulsion stability. Furthermore, the water resistance of the material can be further improved by introducing polyether polyols with stronger water resistance or by synthesizing polyester polyols from long-chain diacids and diols.
Controlling the crosslinking density is an important means of balancing water resistance and flexibility. Crosslinking in water-based polyurethane waterproof coatings can enhance the material's cohesion, improve water resistance and mechanical strength, but excessive crosslinking can lead to brittleness and decreased flexibility. Therefore, precise control of the crosslinking density is necessary by selecting appropriate crosslinking agents and processes. For example, combining internal and external crosslinking can improve water resistance while maintaining flexibility. Internal crosslinking is achieved by introducing trifunctional raw materials or reactive functional groups, while external crosslinking is achieved by adding crosslinking agents to form a crosslinked structure during film formation.
The choice of polyol has a decisive influence on the performance of waterborne polyurethane. Water-based polyurethane waterproof coatings using polyether polyols exhibit superior water resistance compared to polyester polyols due to the ether bonds' resistance to hydrolysis; however, their mechanical strength and adhesion are relatively lower. Therefore, a balance between water resistance and mechanical strength can be achieved by blending polyethers with polyester polyols, or by selecting high-performance polyethers such as tetrahydrofuran ether diol. Furthermore, the molecular weight and number of functional groups of the polyol must be considered comprehensively; excessively high molecular weight or too many functional groups can lead to increased material hardness and decreased flexibility.
The type of isocyanate directly affects the hard segment structure and properties of waterborne polyurethane. Aromatic isocyanates such as TDI and MDI are highly reactive and can form rigid hard segments, improving the material's water resistance and mechanical strength, but they are prone to yellowing. Aliphatic isocyanates such as HDI and IPDI possess excellent light stability and chemical resistance, but have lower reactivity. Therefore, it is necessary to select the appropriate isocyanate type based on the application scenario, or achieve complementary properties through blending.
The extension and branching design of the molecular chain can further optimize the performance of waterborne polyurethane. By introducing long-chain extenders or branching agents, the length and degree of branching of molecular chains can be increased, improving the flexibility and elasticity of water-based polyurethane waterproof coatings. Simultaneously, the branched structure can enhance the material's cohesion, improving water resistance and mechanical strength. Furthermore, the extension of molecular chains can reduce the viscosity of the emulsion, improving application performance.
The introduction of functional monomers can endow waterborne polyurethanes with special properties. For example, the introduction of siloxane segments or fluorinated monomers can significantly improve the material's water resistance, weather resistance, and chemical resistance; the introduction of epoxy groups can enhance the material's cohesion and mechanical strength through crosslinking reactions. The introduction of these functional monomers requires comprehensive consideration of their compatibility with the host material to ensure stable performance improvement.