The surface treatment of plastic products mainly includes coating treatment and electroplating coating treatment. Generally, the crystallinity of plastics is large, the polarity is small or nonpolar, and the surface energy is low, which will affect the adhesion of the coating. Because the plastic is a non-conductive insulator, it could not be directly coated on the plastic surface according to the electroplating process specification. Therefore, before the 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
Pretreatment includes degreasing treatment of plastic surface. It is cleaning the oil stain , release agent on the surface and activation treatment of plastic surface, in order to improve the adhesion of coating.
①. Degreasing of plastic products
Similar to the surface degreasing of metal products, plastic products can be cleaned with organic solvent or degreased with alkaline aqueous solution containing surfactant. Organic solvent deoiling is suitable for cleaning paraffin, beeswax, fat and other organic dirt from the surface of plastics. The organic solvent used should be insoluble, non swelling, non cracking, low boiling point, volatile, non-toxic and non combustible.
Alkaline aqueous solution is suitable for degreasing of alkali resistant plastics. The solution contains caustic soda, alkaline salts and various surfactants. The most commonly used surfactant is OP series, also known as APEO, which will not form foam or remain on the surface of plastics.
②. Surface activation of plastic products
This activation is to improve the surface energy of plastics, that is to form some polar groups on the surface of plastics or coarsen them, so that the coatings can be wetted and adsorbed on the surface of parts more easily. There are many methods of surface activation treatment, such as chemical oxidation, flame oxidation, solvent vapor etching and corona discharge oxidation. The most widely used is the chemical crystal oxidation treatment, which is commonly used in chromic acid treatment solution. It’s typical formula is potassium dichromate 4.5%, water 8.0%, concentrated sulfuric acid (more than 96%) 87.5%.
Some plastic products, such as polystyrene and ABS plastics, can be coated directly without chemical oxidation treatment. In order to obtain high-quality coating, chemical oxidation treatment can also be used. For example, ABS plastic can be etched with dilute chromic acid treatment solution after degreasing. The typical treatment formula is chromic acid 420g / L, sulfuric acid (specific gravity 1.83) 200ml / L. The typical treatment process is 65 ℃ 70 ℃ / 5min, 10min, washed and dried.
The advantage of etching with chromic acid treatment liquid is no matter how complicated the shape of the plastic product is, it can be treated uniformly. The disadvantage is that the operation is dangerous and there are pollution problems.
2. Pre-treatment of coating
The purpose of the pre-treatment of the coating is to improve the adhesion of the coating to the plastic surface and to form a conductive metal bottom layer on the plastic surface. The pre-treatment procedures mainly include: mechanical roughening, chemical degreasing, chemical roughening, sensitization treatment, activation treatment, reduction treatment and electroless plating. The first three items are to improve the adhesion of the coating, and the last four items are to form a conductive metal bottom layer.
①. Mechanical coarsening and chemical coarsening
Mechanical roughening and chemical roughening are respectively mechanical and chemical methods to make the surface of the plastic thicker to increase the contact area between the coating and the substrate. It is generally believed that the binding force that can be achieved by mechanical roughening is only about 10% of that of chemical roughening.
②. Chemical degreasing
On the plastic surface, the degreasing method before electroplating coating is same with the degreasing method before coating.
Sensitization is the adsorption of some easily oxidizable substances on the plastic surface with adsorption capacity, such as stannous chloride and titanium chloride. These adsorbed easily oxidizable substances are oxidized during the activation treatment. The activator is reduced to a catalytic nucleus and remains on the surface of the product. The role of sensitization is to lay the foundation for the subsequent electroless plating of the metal layer.
Activation is to treat the sensitized surface with a solution of a catalytically active metal compound. The principle is to immerse the product adsorbing the reducing agent in the aqueous solution containing the oxidizing agent of the precious metal salt, then the precious metal ion is used as the oxidizing agent to be reduced by S2+n, and the reduced precious metal is deposited on the surface of the product in the form of colloidal particles. It has strong catalytic activity. When such a surface is immersed in the electroless plating solution, these particles become the catalytic center, which accelerates the reaction speed of the electroless plating.
⑤. Reduction processing
For the activated and cleaned products, before electroless plating, the products are immersed with a certain concentration of reducing agent solution, in order to reduce and remove the unwashed activator. This is called reduction treatment. For electroless copper plating, use formaldehyde solution for reduction treatment, and for electroless nickel plating, use sodium hypophosphite solution for reduction treatment.
⑥. Electroless plating
The purpose of electroless plating is to generate a conductive metal film on the surface of plastic products, which creates conditions for the electroplating of the metal layer of plastic products. Therefore, electroless plating is a key step in plastic electroplating.