1. Surface Roughness

When the adhesive can well wet the surface of the adherend (contact angle θ < 90°), surface roughening is beneficial for improving the wetting degree of the adhesive liquid on the surface. It increases the density of contact points between the adhesive and the adherend, thus contributing to an increase in bonding strength. Conversely, when the adhesive has poor wetting performance on the adherend (θ > 90°), surface roughening is not conducive to the improvement of bonding strength.

2. Surface Treatment

Surface treatment before bonding is the key to successful bonding, with the aim of obtaining a strong and durable joint. Due to the presence of "weak boundary layers" formed by oxide layers (such as rust), chromium - plated layers, phosphating layers, and mold - release agents on the adherend, the surface treatment of the adherend will affect the bonding strength. For example, the surface of polyethylene can be treated with hot chromic acid to improve the bonding strength. When heated to 70 - 80°C and treated for 1 - 5 minutes, a good bondable surface can be obtained. This method is suitable for polyethylene plates and thick - walled tubes. When treating polyethylene films with chromic acid, it can only be carried out at room temperature. If treated at the above - mentioned temperatures, plasma or micro - flame treatment should be used for the surface treatment of the film. When treating the surfaces of natural rubber, styrene - butadiene rubber, nitrile rubber, and chloroprene rubber with concentrated sulfuric acid, it is expected that the rubber surface will be slightly oxidized. Therefore, the sulfuric acid should be thoroughly washed off shortly after acid application. Excessive oxidation will leave more fragile structures on the rubber surface, which is not conducive to bonding. When locally bonding the surface of vulcanized rubber, surface treatment to remove the mold - release agent should not use a large amount of solvents for washing, as it may cause the mold - release agent to diffuse onto the treated surface and hinder bonding. For the surface treatment of aluminum and aluminum alloys, it is desirable to form aluminum oxide crystals on the aluminum surface. The naturally oxidized aluminum surface is a very irregular and rather loose aluminum oxide layer, which is not conducive to bonding. Therefore, the natural aluminum oxide layer needs to be removed. However, excessive oxidation will leave a weak layer in the bonded joint.

3. Penetration

The bonded joints are often infiltrated by some other low - molecular substances under the action of the environmental atmosphere. For example, in a humid environment or underwater, water molecules will penetrate into the adhesive layer; when the polymer adhesive layer is in an organic solvent, solvent molecules will penetrate into the polymer. The penetration of low - molecular substances first causes the deformation of the adhesive layer and then enters the interface between the adhesive layer and the adherend, reducing the strength of the adhesive layer and leading to the failure of bonding. Penetration not only starts from the edge of the adhesive layer. For porous adherends, low - molecular substances can also penetrate into the adherend through the voids, capillaries, or cracks of the adherend and then invade the interface, causing defects or even failure of the joint. Penetration not only leads to a decrease in the physical properties of the joint but also causes chemical changes at the interface due to the penetration of low - molecular substances, resulting in the formation of a rusted area that is not conducive to bonding and completely invalidating the bonding.

4. Migration

Adherends containing plasticizers, such as PVC materials, are prone to migration of these small - molecule substances from the polymer surface layer or interface because of their poor compatibility with polymer macromolecules. If the migrated small molecules accumulate at the interface, they will hinder the bonding between the adhesive and the adherend, resulting in bonding failure.