Based on the theory of mechanical interlocking and chemical bonding, researchers have found that surface modification of substrates can change the crystal morphology and surface morphology of materials, increase the surface roughness of substrates, and enhance the mechanical interlocking effect at the coating/substrate interface; Changing the chemical composition of the surface, increasing the polarity and surface energy of the material, removing weak boundary layers, improving the wettability of the material surface, enhancing the chemical bonding and intermolecular forces between the coating/substrate, thereby improving the adhesion performance of the coating.

The surface treatment methods of substrates can be simply divided into mechanical treatment and chemical treatment. Common mechanical surface treatment methods include sandblasting, polishing, phosphating, etc. Mechanical treatment is a destructive processing method applied to the surface of a substrate, which increases the surface roughness of the substrate and enhances the mechanical interlocking between the coating and the substrate, thereby increasing adhesion. Surface chemical treatment includes surface oxidation, surface grafting, surface coating, etc. The surface oxidation method mainly includes plasma treatment, acid oxidation treatment, flame oxidation treatment, etc.

Plasma treatment

Plasma treatment mainly uses plasma generated by glow discharge, corona discharge and other methods to treat the surface of the substrate. The highly active particles in the plasma act on the surface of the substrate, causing etching and the formation of free radicals on the substrate surface. The active particles combine with the free radicals to introduce new functional groups or form new cross-linked structures on the substrate surface.

Researchers have proposed three mechanisms for plasma enhanced coating adhesion performance: plasma treatment leads to an increase in material surface energy, promoting adsorption between materials; The etching effect of plasma on the material surface promotes the mechanical interlocking effect at the material interface and the reaction between the free radicals generated by plasma radiation on the material surface and the adhesive, resulting in chemical bonding and enhanced bonding strength between materials.

The Effect of Plasma Treatment on the Adhesion of Organic Silicon Coatings on PC Material Surface

To investigate the effect of substrate plasma treatment on the adhesion between organic silicon coatings and PC substrates, plasma treatment was performed on the PC substrate before coating preparation. The specific steps of plasma treatment are as follows: place the PC sample on the stage, and then place the stage horizontally in the plasma cleaning machine processing chamber. Set the plasma treatment power to 180W, the treatment time to 150s, and the treatment gas to be oxygen. After the plasma treatment is completed, take out the sample and coat it within half an hour. Two types of organosilicon sol, STC2200 and STC2560, were used in this experiment. The organosilicon sol was prepared by the sol gel method.

As shown in Figure 1, adhesion scratch tests were conducted on two types of organic silicon coatings, STC2200 and STC2560, according to the adhesion testing standard ASTMD3359. After the tests, there was no coating peeling off at the scratch edges of both coatings, indicating that the adhesion of both coatings was 5B.

Figure 1 Surface morphology of PC/Silicone after adhesion test

The above grid method can only qualitatively test the adhesion of two types of organic silicon coatings, and cannot be quantitatively tested. Therefore, the tensile shear method is used to test the adhesion of the two coatings separately. The tensile shear method is a commonly used method for testing the adhesion of film substrates. Its testing principle is to peel off the film layer from the substrate by mechanical shear stress through a stretching machine, and record the shear stress at the time of coating adhesion failure as the coating adhesion strength under tensile shear. Conduct tensile and shear tests on samples of PC/STC2200, PC/STC2560, and samples coated with the above two coatings after plasma treatment on PC substrates. The test results are shown in Figure 2. The interfacial shear strength of the PC/STC2200 sample is 7.27 MPa, and the interfacial shear strength of the PC/STC2560 sample is 4.24 MPa. It can be seen that the interfacial shear strength between STC2200 coating and PC substrate is greater than that between STC2560 and PC, which may be related to the high degree of crosslinking of epoxy groups in STC2560 coating. Therefore, when STC2560 is subjected to surface tensile shear, the shear force is easily transmitted to the membrane substrate interface through the cross-linked organic molecular network, leading to easy adhesion failure of the coating.

Figure 2 PC/Silicone: Interface Shear Strength

The interfacial shear strength between the two coatings and the PC substrate after plasma treatment was 8.47 MPa and 6.12 MPa, respectively, both of which were higher than those without plasma treatment. This indicates that appropriate plasma treatment can help improve the interfacial adhesion between the organic silicon coating and the substrate. This is because plasma treatment of PC generates - OH and - COOH on its surface, which easily form chemical bonds with the coating, thereby enhancing the adhesion of the film substrate.