How do changes in molecular structure affect the performance of water-based polyurethane waterproof coating during film formation?
Release Time : 2026-03-04
The film-forming process of water-based polyurethane waterproof coating is a complex physicochemical process involving significant changes in molecular structure that directly affect the final performance of the coating. Water-based polyurethane waterproof coating uses water as the dispersion medium, with polyurethane molecules uniformly dispersed as tiny particles. In the initial stage of film formation, as water gradually evaporates, the polyurethane particles initially dispersed in the water begin to move closer together, gradually reducing the distance between them. At this stage, the hydrophilic groups in the molecular structure still interact with water molecules, maintaining the stable dispersion of the particles, but the physical contact between particles gradually increases, laying the foundation for the subsequent film-forming process.
As water further evaporates, the adsorption layer on the surface of the polyurethane particles is destroyed, and the contact between particles becomes irreversible, leading to close packing. In this stage, polar groups in the molecular structure, such as urethane and urea groups, begin to play a crucial role. These polar groups can form physical interactions such as hydrogen bonds, promoting stronger adhesion between particles and making the closely packed structure more stable. This structural change based on intermolecular forces enhances the initial strength of the coating film, ensuring the formation of a continuous and complete waterproof layer.
As moisture evaporates to a certain extent, the polyurethane particles begin to deform, gradually transforming from their original spherical shape into more compact shapes such as rhomboid dodecahedrons, until the interfaces between particles gradually disappear. At this point, molecular chains begin to diffuse and permeate each other, with polyurethane molecular chains in different particles entangled and fusing together. This process causes the originally independent particles to gradually fuse into a whole, and the molecular structure transforms from a dispersed particle state to a continuous network structure. The formation of this network structure greatly improves the mechanical properties of the coating film, giving it better tensile and tear resistance, thus effectively resisting the minor deformation of the substrate and external stress.
In the later stages of film formation, the diffusion and entanglement of molecular chains further deepen, forming an even denser polymer network structure. The hard and soft segments in the molecular structure begin to undergo microphase separation. The hard segments aggregate to form microregions, acting as physical cross-linking points, enhancing the strength and hardness of the coating film; the soft segments, on the other hand, endow the coating film with good flexibility and elasticity, allowing it to adapt to the expansion and contraction of the substrate without cracking. This unique microphase separation structure of soft and hard segments is key to the high strength and good elasticity of the water-based polyurethane waterproof coating, ensuring stable waterproof performance under various environmental conditions.
Furthermore, during film formation, the chemical bonds in the molecular structure may also change. For example, the isocyanate groups in the two-component water-based polyurethane waterproof coating react with compounds containing active hydrogen to form more stable chemical bonds, further cross-linking and curing, thus further enhancing the coating's performance. This chemical cross-linking reaction not only strengthens the coating's strength and chemical resistance but also improves its water resistance and durability, effectively extending the service life of the waterproof coating.
Changes in molecular structure also affect the adhesion between the water-based polyurethane waterproof coating and the substrate. During film formation, the polar groups in the polyurethane molecules can interact with polar substances on the substrate surface, forming chemical bonds or physical adsorption forces, resulting in a tight bond between the coating and the substrate. This excellent adhesion ensures the integrity between the waterproof layer and the substrate, preventing moisture penetration at the interface, thereby improving the waterproofing effect.
During the film-forming process of water-based polyurethane waterproof coating, the molecular structure gradually transforms from a dispersed particulate state to a continuous network structure, undergoing microphase separation and forming chemical crosslinks. These changes collectively endow the coating with excellent mechanical properties, waterproof performance, durability, and good adhesion to the substrate, making it widely used in the field of building waterproofing.