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Pretreatment process of plastic products before plating

In recent years, the plastic metallization process has been widely used in the decorative plating of plastic parts. After the plastic parts are electroplated, they not only maintain the characteristics of light weight, strong corrosion resistance, and easy molding, but also give the metal its electrical conductivity, decorativeness, magnetic conductivity, weldability, etc. Therefore, the plastic plating products are widely used. Large aircraft, rockets, ships, small cups, buttons, glasses, etc., we can see the plastic plating. The following is a brief introduction to the surface treatment technology of plastic products before plating.


    Plastic is a non-conducting insulator and therefore cannot be coated directly on plastic surfaces in accordance with normal electroplating process specifications. Before surface treatment, necessary pretreatment should be performed. The surface treatment of plastic products includes a coating coating treatment and a plating coating treatment.


    Coating coating before treatment


    The purpose of coating pretreatment is to increase the adhesion of the coating. First of all, the degreasing treatment of the plastic surface, that is, the cleaning of the surface oil and mold release agent, and the activation of the plastic surface.


    Similar to the degreasing of the surface of the metal product, the degreasing of the plastic product can be cleaned with an organic solvent or degreasing with an alkaline aqueous solution containing a surfactant.


    Degreasing organic solvents is suitable for cleaning paraffin, beeswax, fats and other organic dirt from plastic surfaces. The organic solvent used should not dissolve, swell, or crack the plastic, and the organic solvent itself should have the characteristics of low boiling point, high volatility, non-toxicity, and non-flammability.


    Alkaline aqueous solution is suitable for degreasing alkali-resistant plastics. This solution contains caustic soda, alkaline salts and various surface active substances. The most commonly used surfactant is the OP series, alkyl phenol ethoxylates, which do not form bubbles and do not remain on plastic surfaces.


    The activation of the surface of plastic products is to increase the surface energy of the plastic, that is, to generate some polar groups on the surface of the plastic or to roughen it, so that the coating can be more easily wetted and adsorbed on the surface of the product. There are many ways to activate the surface, such as chemical oxidation, flame oxidation, solvent vapor etching, and corona discharge oxidation. One of the most widely used is the chemical oxidation treatment. The typical chromic acid treatment solution is typically 4.5% potassium dichromate, 8.0% water, and 87.5% concentrated sulfuric acid (above 96%). The advantages of etching with a chromic acid treatment solution is that regardless of the complex shape of the plastic product, it can be treated uniformly; the disadvantages are the operational hazards and the environmental pollution problems.


    The pretreatment processes include: mechanical roughening, chemical degreasing, chemical roughening, sensitization treatment, activation treatment, reduction treatment, and electroless plating. Among them, the first three are for improving the adhesion of the plating layer, and the last four are for forming a conductive metal bottom layer.


    The mechanical roughening and chemical roughening processes respectively roughen the surface of the plastic in a mechanical and chemical manner to increase the contact area between the plating and the substrate. It is generally believed that the binding force achieved by mechanical roughening is only about 10% of the chemical roughening.


    The method of degreasing the plastic coating before the coating is the same as the degreasing treatment before the coating coating.


    Sensitization is the adsorption of some easily oxidized substances on the surface of plastics with certain adsorption capacity, such as tin chloride and titanium trichloride. These adsorbed easily oxidized materials are oxidized during the activation process, and the activator is reduced to the catalytic nucleus and left on the surface of the product. The role of sensitization is to lay the foundation for the subsequent electroless plating of metal layers.


    Activation is the treatment of a sensitized surface with the aid of a solution of a catalytically active metal compound: the sensitized plastic article having the reducing agent adsorbed therein is immersed in an aqueous oxidizing solution containing a noble metal salt, and the noble metal ion is reduced as an oxidizing agent; The precious metal is colloidal particles deposited on the surface of the product and has a strong catalytic activity. When the surface is immersed in an electroless plating solution, these particles become catalytic centers, and the reaction speed of electroless plating is accelerated.

    After activation treatment and washing with clean water, the product is impregnated with a reducing agent solution for electroless plating at a certain concentration prior to electroless plating, so that the unwashed activator is reduced and removed. This is called reduction treatment chemistry. When the copper is plated, the formaldehyde solution is used for the reduction treatment, and when the electroless nickel plating is performed, the sodium hypophosphite solution is used for the reduction treatment.


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