Cohesion and thermal expansion coefficient of paint film

2019-08-15

Cohesion and thermal expansion coefficient of paint film
The force caused by the cohesion of molecules of the same substance is called cohesion.The more cohesive the coating, the worse the adhesion.In the drying process of paint, with the increase of solvent volatilization and crosslinking degree, the cohesion between film forming material molecules increases, the film produces shrinkage phenomenon, and finally leads to the reduction of film adhesion.Therefore, the purpose of improving the adhesion can be achieved by adopting a method of reducing the cohesion of the paint film. Commonly used methods are:
1.Reduce the thickness of the coating, reduce the cohesive force, and improve the adhesion strength of the paint film to the substrate.
2.Add appropriate pigment to the paint to reduce the cohesion of the paint film and improve the adhesion of the paint film to the substrate.This is an important reason why the adhesion of paint to varnish is generally better.


The difference in thermal expansion coefficient between the paint film and the substrate also affects the adhesion of the paint film.It is well known that as temperatures change, all materials undergo different degrees of volumetric contraction and expansion.When the coating is applied to the surface of the substrate, the bonding point between the coating and the surface to be coated will be damaged to varying degrees due to the influence of thermal expansion and contraction.Generally speaking, the coefficient of thermal expansion of the paint film is significantly larger than the coefficient of thermal expansion of the substrate. Therefore, when the temperature changes, the degree of expansion or contraction of the paint film is larger than that of the plate, which causes corresponding deformation of the paint film, resulting in wrinkles and moiré. Etc., reducing the adhesion of the paint film.The smaller the thermal expansion coefficient of the coating, the better the adhesion of the coating film. For example, the thermal expansion coefficient of the epoxy resin is smaller than that of other resins, so the adhesion of the epoxy resin paint film is good.


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