The final state of waterborne polyurethane is a solid film. During the process from emulsion to solid film, accompanied by the evaporation of water and the concentration of water dispersion, the latex particles are close to each other. Before the solid content reaches 74%, the dispersion particles can move in the continuous phase of water. Under the influence of water/air interfacial tension, the particles fuse with each other, and finally diffuse through the molecular chain between the dispersion particles, resulting in a homogeneous film with certain mechanical properties.From the basic principle of film forming, the key to the film forming performance of polymer emulsion lies in the deformability of latex particles and the molecular chain diffusion movement. Polyurethane emulsion particles are an open water swelling structure, latex particles contain a certain amount of water, with strong deformation ability, and the characteristics of polyurethane particles swelling can cause particles to produce a very large surface area, with the swelling of small particles, to promote the polyurethane emulsion has a strong film forming ability. Even at a lower film forming temperature, it still has a high film hardness.

In addition, when the film is dry, the moisture in the emulsion particles will help soften the polymer, making it easy to deform and form a film. Because the polyurethane molecule is composed of soft and hard segments, its special molecular structure determines that the polyurethane molecular chain has a strong diffusion movement ability, and the glass transition temperature of the soft segment is about 60~30℃, low glass transition temperature will reduce the minimum film forming temperature of the coating film, which can significantly help the film formation of the emulsion. Therefore, polyurethane emulsion has excellent film forming properties.

In addition, when the film is dry, the moisture in the emulsion particles will help soften the polymer, making it easy to deform and form a film. Because the polyurethane molecule is composed of soft and hard segments, its special molecular structure determines that the polyurethane molecular chain has a strong diffusion movement ability, and the glass transition temperature of the soft segment is about 60~30℃, low glass transition temperature will reduce the minimum film forming temperature of the coating film, which can significantly help the film formation of the emulsion. Therefore, polyurethane emulsion has excellent film forming properties.

The main medium of waterborne polyurethane is water, the volatility of water is lower than that of general organic solvents, and the evaporation heat of water is larger than that of organic solvents. The biggest drawback is that aqueous polyurethane adhesives dry slowly compared to solvent-based ones. However, the lowest film forming temperature of waterborne polyurethane is about 0℃, and because it does not contain emulsifier, it can form a film with luster, uniformity and good toughness when dried at room temperature. The drying of emulsion glue is related to the relative humidity of the air, if the air humidity is small and the temperature is high, it is beneficial to the drying of the film. If the film is formed on a smooth surface, drying too fast may cause uneven film and cracks. If it is dried on the porous substrate, the water is absorbed by the substrate, so that the concentration of the adhesive layer resin is increased, the drying is relatively fast, and it is easy to produce a adhesive force in a short time.