Due to its special molecular structure, polyurethane emulsion can be mixed or cross-linked modified with a variety of resins to effectively control the composition and structure of the coating, thus making the coating film formed by waterborne polyurethane emulsion have excellent properties such as good adhesion, hardness, softness and good electrical insulation, which can be widely used in coating, adhesive, furniture paint, printing and dyeing auxiliaries, leather finishing and other industries.
However, in water-based polyurethane emulsions, the surface tension of polyurethane emulsions is generally 40-60 dynes, and the surface tension of the substrate is generally below 40 dynes. According to the surface chemistry theory, it is known that if the surface tension of the liquid is lower than the surface tension of the substrate, the liquid is easy to achieve the desired effect of wetting the substrate. The most common method to improve wettability is to add substrate wetting agent to the emulsion, these active substances can be effectively adsorbed at the interface, Baise wetting agent, fully reduce the surface tension of the liquid coating, so as to wet those substrates that are not easily wetted.
Redistribution of bubble size. It is caused by the different radii of curvature of the bubbles, resulting in different pressures of the gas in the bubbles. There are 2 bubbles, the radius of curvature of the large bubble is R. The radius of curvature of the small bubble is r, the atmospheric pressure is p, and the surface tension of the liquid is o. Then the pressure in the large bubble is p+4a/R, and the pressure in the small bubble is p + 4o/r. Since r<R, the pressure in the small bubble is higher than the pressure in the large bubble, and the pressure difference is 4o (] One if two bubbles are separated, each remains stable. If the two bubbles are close, their common interface film is not ventilated, and the bubbles will remain intact. In fact, such a situation does not exist, and the gas always grows from small bubbles. The low pressure side of the large bubble from the high pressure side diffuses quickly or slowly through the interfacial membrane, causing the small bubbles to become smaller and larger, and the difference in the radius of curvature of the two bubbles becomes larger and larger, finally causing the bubble to burst
The thinning of the bubble film thickness is the result of drainage and evaporation.
A container is full of bubbles. After a certain period of time, a liquid layer appears at the bottom of the container. This is caused by gravity and curved surface pressure. Under the action of gravity, the liquid in the liquid film continuously flows downward, making the thickness of the liquid film gradually thinner.
Under the action of surface tension, the liquid film between two adjacent bubbles should have the smallest area to be stable. Therefore, the foam The bubbles in Fig. 3-6 Plateau are not spherical, but are polyhedral in shape. The liquid film on the surface of the polyhedron is flat. At the junction of the liquid film at the apex of the polyhedron, the liquid film is curved, which is called the Plateau boundary. The Laplace formula for the basf polyethylene wax pressure difference of the curved liquid surface:
4p (3-6) where ▲p–additional pressure on the curved surface; surface tension of the liquid; R radius of curvature of the curved surface.
Leave a message
We’ll get back to you soon