The surface treatment of plastic products mainly includes coating treatment and coating treatment. General plastic crystallinity is large, the polarity is small or no polarity, the surface energy is low, which will affect the coating coating adhesion. Since the plastic is a non-conductive insulator, it is not possible to coat the surface directly on the plastic surface in accordance with an electroplating process. Therefore, prior to surface treatment, the necessary pretreatment should be carried out to improve the coating adhesion and coating The coating provides a conductive substrate with good adhesion.
1, coating pretreatment
Pretreatment includes degreasing of the plastic surface, ie cleaning of the surface of the oil and release agent, as well as the activation of the plastic surface, the purpose is to improve the coating adhesion.
1, the removal of plastic products
Similar to the surface degreasing of metal products, plastic products can be used to clean the organic solvent or alkaline solution containing alkaline solution degreasing. Organic solvent degreasing is suitable for cleaning paraffin, beeswax, fat and other organic soils from plastic surfaces. The organic solvents used should not dissolve, do not swell, do not crack, and have low boiling point, volatile, noncombustible and nonflammable.
Alkaline aqueous solution is suitable for oil removal of alkali-resistant plastic. The solution contains caustic soda, alkaline salts and various surface active substances. The most commonly used surfactant is the OP series, the alkylphenol polyoxyethylene ether, which does not form a foam and does not remain on the plastic surface.
2, the activation of plastic products on the surface
This activation is to improve the surface energy of the plastic, that is, in the plastic surface to generate some polar groups or to coarsen, so that the coating is more easy to wet and adsorbed on the surface of the workpiece. Surface activation treatment of many methods, such as chemical oxidation, flame oxidation, solvent vapor etching and corona discharge oxidation and so on. One of the most widely used chemical oxidation process, this method is commonly used chromic acid treatment solution, the typical formula for potassium dichromate 4.5%, 8.0% of water, concentrated sulfuric acid (96% or more) 87.5%.
Some plastic products, such as polystyrene and ABS plastic, etc., can not be directly coated with chemical coating. In order to obtain high-quality coating coating, but also useful chemical oxidation, such as ABS plastic in the degreasing, can be used more dilute chromic acid treatment solution erosion, the typical treatment formula for chromic acid 420g / L, sulfuric acid Specific gravity 1.83) 200ml / L. Typical treatment process is 65 ℃ 70 ℃ / 5min10min, washed and dried.
The advantage of using chromic acid treatment solution erosion is that no matter how complex the shape of the plastic product, can handle evenly, the disadvantage is that the operation is dangerous and has pollution problems.
2, coating pretreatment of the coating
The purpose of the coating pretreatment is to increase the adhesion of the coating to the surface of the plastic and to form a conductive metal substrate on the surface of the plastic. Pretreatment processes include: mechanical roughening, chemical degreasing, chemical coarsening, sensitization, activation, reduction and electroless plating. The first three are in order to improve the adhesion of the coating, the latter four is to form a conductive metal substrate.
1, mechanical coarsening and chemical coarsening
Mechanical roughening and chemical roughening are the use of mechanical methods and chemical methods to make the plastic surface thicker to increase the contact area between the coating and the substrate. It is generally believed that the mechanical coarsening can achieve the binding force is only about 10% of chemical coarsening.
2, chemical degreasing
The surface treatment of the plastic surface coating is the same as that of the coating before treatment.
3, sensitization
Sensitization is to have a certain adsorption capacity of the plastic surface adsorption of some easily oxidized substances, such as tin dichloride, titanium trichloride and so on. These adsorbed, readily oxidizable materials are oxidized during activation and the activator is reduced to a catalytic nucleus that remains on the surface of the article. The role of sensitization is to lay the foundation for the subsequent chemical plating of the metal layer.
4, activation
Activation is the treatment of the sensitized surface by means of a solution of the catalytically active metal compound. In essence, the product in which the reducing agent is adsorbed is immersed in the aqueous solution of the oxidizing agent containing the noble metal salt, and the noble metal ions are reduced by S2 + n as the oxidizing agent. The reduced precious metal is colloidal particles deposited on the surface of the product, Of the catalytic activity. When the surface is immersed in the electroless plating solution, these particles become the catalytic center, so that the chemical plating reaction speed.
5, restore processing
The product after the activation and the washed with water is subjected to electroless plating and the product is impregnated with a reducing agent solution for a certain concentration of electroless plating so as to remove and remove the uncooked activator, which is called a reduction treatment The Chemical copper plating, with formaldehyde solution reduction treatment, electroless nickel plating with sodium hypophosphite solution reduction treatment.
6, electroless plating
The purpose of electroless plating is to create a layer of conductive metal film on the surface of plastic products, to create conditions for plastic products to plating metal layer, so the chemical plating is a key step in plastic plating.