No matter what coating method is used for coating, there is a process of flowing and drying to form a film, and then gradually form a flat, smooth and uniform coating film. Many other defects or incompleteness will occur during or after construction of the coating, and to avoid these defects requires the addition of corresponding additives under the guidance of theory. Surface tension and the theory of flow induced by surface tension are fundamental to understanding why these defects form. In order to form a flat, smooth and uniform paint film, leveling agents, thixotropic agents, pigment dispersants, and defoamers are commonly used in latex paints.
Surface Tension
Surface tension theory can be used to understand problems that arise during paint film formation. The surface tension pushes the wet paint film to flow. On the one hand, it tries to reduce the surface area of the wet paint film (from rough to smooth), and on the other hand, the low surface tension liquid flows to cover the surface with high surface tension (the coating should be controlled so that the paint The surface tension of the substrate is low, the substrate is high, and the wet paint film automatically levels on the substrate surface). Shrinkage is caused by the high surface tension of the coating and the low substrate, and the inability of the wet paint film to level off on the substrate surface.
The leveling of the paint film is not good, there will be brush marks when brushing, orange peel when spraying, and rolling marks when rolling. But coatings that level too well are usually prone to sagging. The spatter falls on the wet paint film, if the surface tension of the spatter is lower than that of the wet film, the paint will flow to the surroundings, resulting in shrinkage cavities; if it is higher than that of the wet film, it will cause the adjacent wet paint film with low surface tension to flow over, Produces orange peel.
(1) The concept of surface tension
Molecules on the surface of an object experience different forces than those in the bulk. Molecules in the bulk, the force is symmetrical. And the molecules on the surface are attracted by the molecules in the body without the counterbalance force, that is, what they receive is the force pulled into the body, and this force is the surface tension. The role of surface tension has always been to try to make this unbalanced state of force tend towards a balanced state.
Surface molecules have higher free energy, which means more energy is required to remove surface molecules per unit area. Surface tension is also expressed in terms of surface free energy, which is the minimum energy required to form a surface per unit area.
When the system reaches equilibrium, that is, when the surface free energy of the system reaches the minimum, the surface area of the liquid per unit volume becomes the minimum, such as making the liquid quotient into a spherical shape, which is the result of surface tension. Surface tension is the driving force for the leveling of the paint film. There are two reasons: ① When painting, the surface tension of the paint should be low, and the surface tension of the substrate should be high. When forming a paint film, the paint covers the substrate, reducing the total surface free energy of the system. ②The higher the surface tension of the paint, the easier the paint film will flow
The surface has the lowest surface tension, followed by the methyl group. The surface tension of polydimethylsiloxane is low because the siloxane chain is soft and can easily rotate on the main chain to allow a large number of methyl groups to be oriented on the surface. The surface tension of the methylene group is greater than that of the methyl group, so the surface tension of the linear fat increases as the chain lengthens. The order of increasing surface tension is: aliphatic chain, aromatic ring, ester, ketone, alcohol, water. The surface tension of water is the highest, and adding a small amount of surfactant to water can reduce the surface tension of water, because the hydrophobic part of the surfactant has a low surface tension, and is oriented at the interface between water and air, extending into the air. Adding polydimethylsiloxane to an organic solvent can reduce the surface tension because the methyl groups are oriented on the solution surface.
(2) Dynamic surface tension
Strong agitation during construction will cause molecules with different surface tensions to mix evenly, and the distribution of surface tension in the system will be unbalanced, so a temporary surface tension gradient will be caused during coating, and molecules with different surface tensions will move, and different Molecules move at different speeds. Surfactants with short hydrocarbon chains or branched hydrocarbon chains are better at reducing the surface tension of the system because they can quickly diffuse to reach equilibrium. It takes longer for the polymer to reach diffusion equilibrium. If the molecular weight of the polymer is medium and the main chain is flexible, it will reach the surface equilibrium quickly, such as polydimethylsiloxane (silicone oil) and low molecular weight polyoctyl acrylate, which are used to reduce the surface of the coating when it forms a film. tension.
