Pretreatment includes degreasing the plastic surface, i.e. cleaning the grease and release agent surface, activating the plastic surface to improve the adhesion of the coating.
1. Degreasing of Plastic Products
Similar to surface degreasing of metal products, degreasing of plastic products can also be cleaned with organic solvents or alkaline aqueous solutions containing surfactants. Organic solvent degreasing is suitable for removing organic dirt on plastic surface such as paraffin, beeswax, grease and so on. Organic solvents used should not dissolve, expand or crack plastics because of their low boiling point, volatility and non-flammability.
Alkaline aqueous solution is suitable for degreasing of alkali-resistant plastics. The solution contains caustic sodium, alkaline salts and various surfactants. The commonly used surface active material is OP series, that is, alkylphenol polyoxyethylene ether, it does not form foam, nor does it stay on the plastic surface.
2. Surface Activation of Plastic Products
The purpose of this activation is to improve the surface energy of plastics, that is, to form some polar groups on the surface of plastics products or to be coarsened, so that coatings can be more easily wetted and adsorbed on the surface of products. Surface activation methods include chemical oxidation, flame oxidation, solvent vapor etching and corona discharge oxidation.
Among them, chemical crystal oxidation treatment method has been widely used, and chromic acid treatment solution has been widely used. Typical formulations are potassium dichromate 4.5%, water 8.0%, concentrated sulfuric acid (over 96%) 87.5%.
Some plastics, such as polystyrene and ABS plastics, can be directly coated without chemical oxidation treatment.