Surface treatment process of plastic products before plating

The surface treatment of plastic products mainly includes coating treatment and plating coating treatment.

Generally, the crystallinity of plastics is large, the polarity is small or non-polar, and the surface energy is low, which affects the adhesion of the coating. Since plastic is a non-conductive insulator, it cannot be plated directly on the plastic surface according to a plating process specification. Therefore, before surface treatment, necessary pretreatment should be carried out to improve the bonding force of coating and provide a conductive substrate with a good bonding force for plating.


Pre-treatment for coating

The pre-treatment includes degreasing treatment of the plastic surface, i.e. cleaning the surface oil and release agent, and activation treatment of the plastic surface, in order to improve the adhesion of the coating being coated.


1. Degreasing of plastic products


Similar to the metal products surface degreasing, plastic products degreasing can be cleaned with organic solvents or an alkaline aqueous solution containing surfactants to remove oil. 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, non-cracking plastic, its own low boiling point, easy to evaporate, non-toxic, and non-flammable.


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, i.e. alkylphenol polyoxyethylene ether, which does not form foam and does not remain on the plastic surface.


2. Activation of the surface of plastic products


This activation is to improve the surface energy of the plastic, that is, to generate some polar groups on the surface of the plastic or to roughen it, in order to make the coating easier to wet and adsorb on the surface of the parts. There are many methods of surface activation, such as chemical oxidation, flame oxidation, solvent vapor etching and corona discharge oxidation.

The most widely used is the chemical crystal oxidation treatment method, this method is commonly used in chromic acid treatment solutions, its typical formula is 4.5% potassium dichromate, 8.0% water, concentrated sulfuric acid (more than 96%) 87.5%.


Some plastic products, such as polystyrene and ABS plastics, can also be coated directly without chemical oxidation treatment.

In order to obtain a high-quality coating, chemical oxidation treatment is also used, such as ABS plastic after degreasing, which can be used more dilute chromic acid treatment solution etching, the typical treatment formula is chromic acid 420g / L, sulfuric acid (specific gravity 1.83) 200ml / L. The typical treatment process is 65 ℃ 70 ℃ / 5min10min, water washing, drying.


The advantage of etching with chromic acid treatment solution is that the plastic products can be treated evenly no matter how complicated the shape of the product is, and the disadvantage is that the operation is dangerous and has pollution problems.

Pre-treatment for coating


The purpose of pretreatment is to improve the adhesion of the plating to the plastic surface and to form a conductive metal base layer on the plastic surface.

The pre-treatment processes include: mechanical roughening, chemical degreasing, chemical roughening, sensitization, activation, reduction and chemical plating. The first three of them are to improve the adhesion of the plating layer, and the last four are to form a conductive metal base layer.


1. Mechanical roughening and chemical roughening


Mechanical roughening and chemical roughening treatments are used to coarsen the surface of plastic by mechanical and chemical methods respectively, in order to increase the contact area between the plating and the substrate. It is generally believed that the bonding power achieved by mechanical roughening is only about 10% of that of chemical roughening.


2. Chemical degreasing


The method of removing oil before coating plastic surface is the same as that of removing oil before coating.


3. Sensitization


Sensitization is to make certain adsorption capacity of the plastic surface adsorption of some easy to oxidize substances, such as tin dichloride, titanium trichloride, etc.. These adsorbed easy-to-oxidize substances are oxidized during the activation treatment, and the activator is reduced to catalytic nuclei, which remain on the surface of the product. The role of sensitization is to lay the foundation for the subsequent chemical coating of the metal layer.


4. Activation


Activation is the treatment of the sensitized surface with the help of a solution of catalytically active metal compounds. In essence, the product with an adsorbed reducing agent is immersed in an aqueous solution of oxidizing agent containing precious metal salts, whereupon the precious metal ions are reduced by S2+n as an oxidizing agent, and the reduced precious metal is deposited as colloidal particles on the surface of the product, which has strong catalytic activity. When such a surface is immersed in the chemical plating solution, these particles become the catalytic center, making the reaction speed of chemical plating faster.


5. Reduction treatment

After activation and washing with water, before chemical plating, the product is impregnated with a certain concentration of the reducing agent solution used in chemical plating in order to remove the unwashed activator reduction, which is called reduction treatment. In the case of chemical copper plating, formaldehyde solution is used to reduce the treatment, and in the case of chemical nickel plating, sodium hypophosphite solution is used to reduce the treatment.

6. Chemical plating


The purpose of chemical plating is to generate a conductive metal film on the surface of plastic products to create the conditions for plating metal layers on plastic products, so chemical plating is the key step of plastic plating.