Chrome Plating Parts-Surface treatment process of plastic products before electroplating
Generally, plastics have greater crystallinity, less or no polarity, and low surface energy, which will affect the adhesion of the coating. Since plastic is a non-conductive insulator, it cannot be directly plated on the surface of the plastic according to the electroplating process specifications. Therefore, necessary pre-treatment should be carried out before surface treatment to improve the adhesion of the coating and provide a better coating The coating provides a conductive bottom layer with good bonding force.
Pre-treatment of coating
Pre-treatment includes degreasing the plastic surface, that is, cleaning the oil stains and release agents on the surface, and activating the plastic surface to improve the adhesion of the coating.
1. Degreasing plastic products
Similar to the surface degreasing of metal products, the degreasing of plastic products can be cleaned with organic solvents or alkaline aqueous solutions containing surfactants. Organic solvent degreasing is suitable for cleaning paraffin, beeswax, fat, and other organic dirt from plastic surfaces. The organic solvent used should not dissolve, swell, or crack the plastic. It has a low boiling point and is volatile, non-toxic, and non-flammable.
The alkaline aqueous solution is suitable for degreasing alkali-resistant plastics. The solution contains caustic soda, alkaline salts, and various surface-active substances. The most commonly used surface-active substances are OP series, and alkylphenol polyoxyethylene ethers, which do not form foam and do not remain on the plastic surface.
2. Activation of the surface of plastic products
This kind of activation is to increase the surface energy of the plastic, that is, to generate some polar groups or roughen the surface of the plastic, so that the paint can be more easily wetted 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 the chemical crystal oxidation treatment method. 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 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 can be etched with a relatively dilute chromic acid treatment solution. The typical treatment formula is 420g/L chromic acid, sulfuric acid ( Specific gravity 1.83)200ml/L. The typical treatment process is 65℃70℃/5min10min, washed with water, and dry.
The advantage of etching with chromic acid treatment liquid is that it can treat the plastic products evenly no matter how complex the shape is. The disadvantage is that the operation is dangerous and there are pollution problems.
Pre-treatment of plating coating
The purpose of coating pre-treatment is to improve the adhesion between the coating and the plastic surface and to form a conductive metal bottom layer on the plastic surface.
The pre-treatment processes 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 roughening and chemical roughening
Mechanical roughening and chemical roughening treatments use mechanical methods and chemical methods to roughen the plastic surface respectively to increase the contact area between the coating and the substrate. It is generally believed that the bonding force achieved by mechanical roughening is only about 10% of that of chemical roughening.
2. Chemical degreasing
The method of degreasing the plastic surface before coating is the same as the method of degreasing before coating.
3. Sensitization
Sensitization is to adsorb some easily oxidized substances, such as tin dichloride, titanium trichloride, etc., on the surface of plastics with a certain adsorption capacity. These adsorbed easily oxidizable substances are oxidized during the activation treatment, and the activator is reduced to catalytic crystal nuclei and remains on the surface of the product. The function of sensitization is to lay the foundation for subsequent electroless plating metal layers.
4. Activation
Activation is carried out by treating the sensitized surface with a solution of a catalytically active metal compound. Its essence is to immerse the product with an adsorbed reducing agent into an aqueous solution of oxidant containing precious metal salt, so the precious metal ions are reduced by S2+n as the oxidant, and the reduced precious metal is deposited on the surface of the product in the form of colloidal particles, which has strong catalytic activity. When such a surface is immersed in an electroless plating solution, these particles become catalytic centers, speeding up the electroless plating reaction.
5. Restoration processing
Before electroless plating, the products that have been activated and washed with clean water are immersed in a certain concentration of reducing agent solution used in electroless plating to reduce and remove the unwashed activator. This is called reduction treatment. When 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 to create conditions for electroplating metal layers on plastic products. Therefore, electroless plating is a key step in plastic electroplating.
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