The leveling property of waterborne polyurethane topcoat directly affects the surface quality of the coating. Insufficient leveling can easily lead to defects such as orange peel, pinholes, and brush marks, reducing its decorative and protective properties. Adjusting its leveling property requires a comprehensive approach involving multiple dimensions, including formulation design, application techniques, and environmental control, to achieve a smooth and even coating.
Formulation optimization is key to improving leveling property. The leveling property of waterborne polyurethane topcoat is significantly affected by the resin molecular structure, type, and proportion of additives. Introducing low surface tension components, such as silicone leveling agents or acrylic leveling agents, can reduce the interfacial tension between the coating and the substrate, promoting uniform spreading of the coating on the surface. Simultaneously, adjusting the glass transition temperature (Tg) of the resin and selecting a resin with a lower Tg can enhance the fluidity of the coating at low temperatures and prolong the leveling time. Furthermore, adding an appropriate amount of thickener can balance the coating viscosity, preventing sagging due to excessively low viscosity or leveling difficulties due to excessively high viscosity.
Application techniques play a decisive role in leveling property. When spraying, select a spray gun with an appropriate nozzle diameter based on the paint viscosity and control the spraying pressure and distance. Insufficient pressure or excessive distance will result in inadequate paint atomization, forming a granular surface; excessive pressure or excessive distance may cause dripping or dry spraying. When roller or brush coating, keep tools clean to avoid affecting the smoothness of the coating due to residual impurities. The application speed should be uniform to avoid uneven coating thickness, leading to inconsistent leveling time. For complex structures, segmented application is necessary to ensure sufficient leveling time for each segment.
Ambient temperature and humidity are key factors affecting leveling. Low temperatures slow paint flow, prolonging drying time and resulting in insufficient leveling; high temperatures may cause the paint to dry too quickly, hindering solvent evaporation and causing pinholes or craters. High humidity hinders water evaporation in water-based paints, easily causing whitening or blistering; low humidity may cause the paint to dry too quickly, affecting leveling performance. Therefore, the ambient temperature during construction should be controlled between 15-35℃, and the relative humidity should be maintained between 40%-70%. Airflow should be regulated using ventilation equipment to avoid abnormal local temperature and humidity.
Substrate treatment indirectly affects leveling properties. The surface roughness, cleanliness, and surface energy of the substrate all affect paint adhesion and spread. If the substrate surface has oil, dust, or an oxide layer, it will hinder direct contact between the paint and the substrate, leading to decreased leveling properties. Therefore, the substrate must be thoroughly cleaned before construction, removing surface impurities through methods such as sanding, acid washing, or solvent wiping. Corona treatment or applying a primer can increase the substrate's surface energy and enhance paint wetting properties.
The selection and proportioning of additives must be precisely matched to the paint system. Leveling agents, defoamers, wetting agents, and other additives must work synergistically according to the paint type and construction requirements. For example, leveling agents can improve paint flowability, but excessive addition may cause pinholes; defoamers can eliminate air bubbles during construction, but improper selection may cause surface defects. Therefore, the optimal amount of additives needs to be determined experimentally, and additives with good compatibility with waterborne polyurethane should be prioritized to avoid affecting leveling properties due to additive migration or precipitation.
The storage and transportation of coatings are also crucial. Long-term storage may cause the components in the coating to separate or precipitate, affecting the stability of leveling properties. Therefore, the coating should be stirred regularly during storage to prevent sedimentation, and the storage temperature should be controlled between 5-35℃ to prevent additive inactivation due to excessively high temperatures or freezing of the coating due to excessively low temperatures. During transportation, severe vibrations should be avoided to prevent separation of coating components.
Post-application curing is the final step in ensuring leveling properties. Sufficient leveling time should be allowed after coating application to avoid premature drying or human interference. For large-area coatings, segmented curing can be used to ensure that each section of the coating completes leveling before drying. Furthermore, during the curing period, the coating should be kept away from water, dust, or other contaminants to prevent external factors from damaging the leveling effect.
