Surface treatment of plastic products mainly includes coating coating treatment and plating coating treatment.
Generally, plastics have high crystallinity, low polarity or non-polarity, and low surface energy, which will affect adhesion of coating. Since plastic is a non-conductive insulator, it cannot be directly coated on the surface of plastic according to general electroplating process specifications. Therefore, before surface treatment, necessary pre-treatment should be carried out to improve bonding force of coating and provide a solid surface for coating. Coating provides a conductive base layer with good adhesion.

 

Pre-treatment includes degreasing treatment on plastic surface, that is, cleaning oil stains and release agents on the surface, and activation treatment on plastic surface, in order to improve adhesion of coating.

Similar to surface degreasing of metal products, degreasing of plastic products can be done by cleaning with organic solvents or by using alkaline aqueous solutions containing surfactants. Organic solvent degreasing is suitable for cleaning paraffin, beeswax, fat and other organic dirt from the plastic surface. Organic solvent used should not dissolve, swell or crack plastic. It has a low boiling point, is volatile, non-toxic and non-flammable.
Alkaline aqueous solution is suitable for degreasing alkali-resistant plastics. Solution contains caustic soda, alkaline salts and various surface active substances. The most commonly used surface-active substances are OP series, that is, alkylphenol ethoxylates, which do not form foam and do not remain on plastic surface.

This activation is to increase surface energy of plastic, that is, to generate some polar groups on the surface of plastic or to roughen it, so that coating is more easily wetted and adsorbed on the surface of workpiece. There are many methods of surface activation treatment, such as chemical oxidation, flame oxidation, solvent vapor etching and corona discharge oxidation.
Among them, chemical crystal oxidation treatment method is the most widely used. Chromic acid treatment solution is commonly used in this method. Its typical formula is 4.5% potassium dichromate, 8.0% water, and 87.5% concentrated sulfuric acid (above 96%).
Some plastic products, such as polystyrene and ABS plastics, can be directly coated without chemical oxidation treatment.
In order to obtain a high-quality coating, chemical oxidation treatment is also used. For example, after degreasing ABS plastic, it can be etched with a dilute chromic acid treatment solution. Typical treatment formula is chromic acid 420g/L, sulfuric acid ( Specific gravity 1.83) 200ml/L. Typical treatment process is 65℃70℃/5min10min, washing with water and drying.
Advantage of etching with chromic acid treatment solution is that no matter how complicated shape of plastic product is, it can be treated evenly. Disadvantage is that operation is dangerous and there are pollution problems.

 

Purpose of pre-treatment of coating is to improve adhesion between coating and plastic surface and to form a conductive metal bottom layer on plastic surface.
Pre-treatment process mainly includes: mechanical roughening, chemical degreasing, chemical roughening, sensitization treatment, activation treatment, reduction treatment and electroless plating. Among them, the first three items are to improve adhesion of coating, and the last four items are to form a conductive metal bottom layer.

Mechanical roughening and chemical roughening are to use mechanical methods and chemical methods to make plastic surface rough, so as to increase contact area between coating and substrate. It is generally believed that bonding force that can be achieved by mechanical roughening is only about 10% of that of chemical roughening.

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

Sensitization is to make some easily oxidizable substances adsorb on the surface of plastic with certain adsorption capacity, such as tin dichloride and titanium trichloride. These adsorbed easily oxidizable substances are oxidized during activation treatment, while activator is reduced to catalytic crystal nuclei and remains on the surface of product. Role of sensitization is to lay foundation for subsequent electroless plating metal layer.

Activation is accomplished by treating sensitized surface with a solution of a catalytically active metal compound. Its essence is to immerse product adsorbed with reducing agent in aqueous solution containing oxidizing agent of noble metal salt, so noble metal ion is reduced by S2+n as oxidizing agent, and reduced noble metal is deposited on the surface of product in the form of colloidal particles, which has a strong catalytic activity. When such a surface is immersed in an electroless plating solution, these particles act as catalytic centers, speeding up electroless plating reaction.

Before chemical plating, product after activation treatment and cleaning with water is dipped in a certain concentration of reducing agent solution used in chemical plating to reduce and remove unwashed activator, which 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.

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