The surface pretreatment of plastic products mainly includes coating coating treatment and plating coating treatment. Generally, the crystallinity of plastic is large, the polarity is small or non-polar, and the surface energy is low, which will affect the adhesion of the coating. Since plastic is a non-conductive insulator, it cannot be coated directly on the surface of the plastic according to a plating process specification. Therefore, before the surface treatment, the necessary pretreatment should be performed to improve the adhesion of the coating and the coating The coating provides a conductive bottom layer with good adhesion.
Pre-treatment of coating
The pre-treatment includes degreasing treatment of the plastic surface, that is, oil stain and mold release agent for cleaning the surface, and activation treatment of the plastic surface, the purpose is to improve the adhesion of the coating.
Degreasing of plastic products. Similar to degreasing the surface of metal products, degreasing of plastic products can be cleaned with organic solvents or degreasing with an alkaline aqueous solution containing a surfactant. Organic solvent degreasing is suitable for cleaning paraffin, beeswax, fat and other organic dirt from the plastic surface. The organic solvent used should be insoluble, non-swelling, and cracking of the plastic.
The alkaline aqueous solution is suitable for degreasing of alkali-resistant plastics. The solution contains caustic soda, alkaline salts and various surface-active substances. The most commonly used surface-active substance is the OP series, namely alkylphenol polyoxyethylene ether, which does not form foam and does not remain on the plastic surface.
02 surface activation
This activation is to improve the surface properties of the plastic, that is, to generate some polar groups on the surface of the plastic or roughen it to make the coating easier to wet and adsorb on the surface of the product. There are many methods for surface activation treatment, such as chemical oxidation method, flame oxidation method, solvent vapor etching method and corona discharge oxidation method. Among them, the chemical crystal oxidation treatment method is most widely used. This method commonly uses a chromic acid treatment solution. Its typical formula is 4.5% potassium dichromate, 8.0% water, and 87.5% concentrated sulfuric acid (more than 96%).
Some plastic products, such as polystyrene and ABS plastic, can also be directly coated without chemical oxidation treatment. In order to obtain a high-quality coating, chemical oxidation treatment is also available. For example, after degreasing ABS plastic, it can be etched with a dilute chromic acid treatment solution. Its typical treatment formula is 420 g / L chromic acid and Specific gravity 1.83) 200ml / L. The typical treatment process is 65 ℃ 70 ℃ / 5min10min, washed with water and dried. The advantage of using chromic acid treatment liquid for etching is that no matter how complex the shape of the plastic product, it can be processed uniformly. The disadvantages are that the operation is dangerous and the problem of pollution.
Pre-treatment of plating coating The purpose of the plating coating pre-treatment is to improve the adhesion of the plating layer to the plastic surface and to form a conductive metal base layer on the plastic surface. The pretreatment process mainly includes: mechanical roughening, chemical degreasing, chemical roughening, sensitization treatment, activation treatment, reduction treatment, and chemical 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. 01 Mechanical and chemical roughening
The mechanical roughening and chemical roughening treatments respectively use a mechanical method and a chemical method to thicken the plastic surface to increase the contact area between the plating layer and the substrate. It is generally believed that the binding force that can be achieved by mechanical roughening is only about 10% of chemical roughening.
02 Chemical degreasing
The method of degreasing the plastic surface before coating and coating is the same as the method of degreasing the coating before coating.
Sensitization is the adsorption of some easily oxidizable substances, such as tin dichloride and titanium trichloride, on a plastic surface with a certain adsorption capacity. These adsorbed oxidizable substances are oxidized during the activation treatment, and the activator is reduced to a catalytic crystal nucleus and remains on the surface of the product. The role of sensitization is to lay the foundation for subsequent electroless metal plating.
Activation is performed by treating the sensitized surface with a solution of a catalytically active metal compound. The essence is to immerse the product adsorbed with the reducing agent into the aqueous solution of the oxidant containing the precious metal salt, so the precious metal ion is reduced by S2 + n as the oxidant. The reduced precious metal is deposited on the surface of the product as colloidal particles, which has a 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.
05 reduction processing
Before the electroless plating, the product after the activation treatment and washing with water is impregnated with the reducing agent solution used in the electroless plating at a certain concentration to reduce and remove the unwashed activator. This is called reduction treatment. . For electroless copper plating, use a formaldehyde solution for reduction treatment; for electroless nickel plating, use a sodium hypophosphite solution for reduction treatment.
06 Chemical plating
The purpose of electroless plating is to create a conductive metal film on the surface of plastic products, creating conditions for plating metal layers on plastic products. Therefore, electroless plating is a key step in plastic electroplating.