The surface treatment of plastic bottle products mainly includes coating treatment and coating treatment. Generally, the crystallinity of plastics is large, the polarity is small or non-polar, and the surface energy is low, which will affect the adhesion of coating. As plastic is a kind of non-conductive insulator, it can not be directly coated on the plastic surface according to a electroplating process specification. Therefore, before surface treatment, necessary pretreatment should be carried out to improve the adhesion of the coating and provide a conductive bottom layer with good adhesion for the coating.
1. pretreatment of coating
The pretreatment includes the degreasing treatment of the plastic surface, that is, the oil and mold release agent cleaning the surface, and the activation treatment of the plastic surface, in order to improve the adhesion of the coating.
(1) degreasing of plastic bottle products. Similar to the surface degreasing of metal products, the plastic bottle products can be cleaned with organic solvent or degreased with alkaline water solution containing surfactant. The organic solvent degreasing is suitable for cleaning paraffin, beeswax, fat and other organic dirt from the plastic surface. The organic solvent used shall not dissolve, swell or crack the plastic, with low boiling point, volatile, non-toxic and non combustible.
Alkaline water solution is suitable for degreasing of alkali resistant plastics. The solution contains caustic soda, alkaline salt and various surface active substances. The most commonly used surfactant is op series, i.e. alkylphenol polyoxyethylene ether, which will not form foam or remain on the plastic surface.
(2) activation of the surface of plastic bottle products. This kind of activation is to improve the surface energy of plastics, that is to say, to form some polar groups on the surface of plastics or to coarsen them, so as to make the coating more easily wetted and adsorbed on the surface of products. There are many methods of surface activation, such as chemical oxidation, flame oxidation, solvent vapor etching and corona discharge oxidation. Among them, the most widely used method is chemical crystal oxidation, which is commonly used as chromic acid treatment solution. Its typical formula is potassium dichromate 4.5%, water 8.0%, concentrated sulfuric acid (over 96%) 87.5%. Some plastic bottle products, such as polystyrene and ABS plastic, can also be coated directly.
2. pretreatment of coating
The purpose of coating pretreatment is to improve the adhesion between the coating and the plastic surface and to form a conductive metal substrate on the plastic surface. The pretreatment process mainly includes mechanical coarsening, chemical oil removal, chemical coarsening, sensitization treatment, activation treatment, reduction treatment and chemical plating. The first three are to improve the adhesion of the coating, and the last four are to form a conductive metal substrate.
(1) mechanical coarsening and chemical coarsening. Mechanical coarsening and chemical coarsening are used to coarsen the plastic surface by mechanical and chemical methods respectively, so as to increase the contact area between the coating and the substrate. Generally speaking, the binding force of mechanical coarsening is only about 10% of chemical coarsening.
(2) chemical oil removal. The method of degreasing before coating on plastic surface is the same as that before coating.