Analysis of surface treatment process before plating of plastic products

1. Pretreatment of coating coating

Pretreatment includes degreasing of the plastic surface, ie cleaning of the oil and release agent on the surface, and activation of the plastic surface in order to increase the adhesion of the coating.

(1) Degreasing of plastic products

Similar to the degreasing of metal products, the degreased plastic products can be cleaned with an organic solvent or degreased with an alkaline aqueous solution containing a surfactant. Organic solvent degreasing is suitable for cleaning paraffin, beeswax, fat and other organic soils from plastic surfaces. The organic solvent used should not dissolve, swell or crack the plastic. It has low boiling point, is volatile, non-toxic and non-combustible.

The alkaline aqueous solution is suitable for degreasing of alkali-resistant plastics. The solution contains caustic soda, an alkaline salt, and various surface active substances. The most commonly used surface active material is the OP series, an alkyl phenol ethoxylate, which does not form a foam and does not remain on the plastic surface.

(2) Activation of the surface of plastic products

This activation is to increase the surface energy of the plastic, that is, to form some polar groups on the surface of the plastic or to roughen it to make the coating more wettable and adsorbed on the surface of the part. There are many methods for surface activation treatment, such as chemical oxidation, flame oxidation, solvent vapor etching, and corona discharge oxidation. The most widely used one is chemical crystal oxidation treatment. This method is commonly used as a chromic acid treatment liquid. Its typical formula is 4.5% potassium dichromate, 8.0% water, and 87.5% concentrated sulfuric acid (96% or more).

Some plastic products, such as polystyrene and ABS plastics, can be coated directly without chemical oxidation. In order to obtain high-quality coating coating, it can also be treated with chemical oxidation. 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 ( The specific gravity is 1.83) 200ml/L. The typical treatment process is 65 ° C 70 ° C / 5 min 10 min, washed with water and dried.

The advantage of etching with chromic acid treatment solution is that even if the shape of the plastic product is complicated, it can be treated uniformly, and the disadvantage is that the operation is dangerous and there is a problem of pollution.

2. Pretreatment of plating coating

The purpose of the coating pre-treatment is to increase the adhesion of the coating to the plastic surface and to form a conductive metal underlayer on the plastic surface. The pretreatment processes mainly include: mechanical roughening, chemical degreasing, chemical roughening, sensitization treatment, activation treatment, reduction treatment, and electroless plating. The first three are to improve the adhesion of the coating, and the last four are to form a conductive metal underlayer.

(1) Mechanical roughening and chemical coarsening

Mechanical roughening and chemical roughening treatment are mechanically and chemically thickened to increase the contact area between the coating and the substrate. It is generally believed that the bonding force that can be achieved by mechanical roughening is only about 10% of chemical coarsening.

(2) Chemical degreasing

The method of pre-treatment degreasing of the plastic surface coating is the same as the pre-treatment degreasing method of the coating.

(3) sensitization

Sensitization is to adsorb some easily oxidizable substances such as tin dichloride and titanium trichloride on the surface of plastics with certain adsorption capacity. These adsorbed oxidizable species are oxidized during the activation treatment, and the activator is reduced to a catalytic nucleus which remains on the surface of the article. The role of sensitization is to lay the foundation for subsequent electroless plating of metal layers.

(4) Activation

Activation is by treating the sensitized surface with a solution of a catalytically active metal compound. The essence is that the product adsorbing the reducing agent is immersed in an aqueous solution of an oxidizing agent containing a precious metal salt, so that the noble metal ion is reduced as S2 by n as an oxidizing agent, and the reduced precious metal is deposited on the surface of the product as colloidal particles, which is strong. Catalytic activity. When such a surface is immersed in an electroless plating solution, these particles become a catalytic center, and the reaction speed of the electroless plating is accelerated.

(5) reduction processing

The product after being activated and washed with water is immersed in a certain concentration of a reducing agent solution for electroless plating before electroless plating to remove the unwashed activator, which is called reduction treatment. . When electroless copper plating is carried out, it is reduced by a formaldehyde solution, and when electroless nickel plating is treated with a sodium hypophosphite solution.

(6) Electroless plating

The purpose of electroless plating is to create a layer of conductive metal film on the surface of plastic products to create conditions for plating metal layers on plastic products. Therefore, electroless plating is a key step in plastic plating.