The surface treatment of plastic products mainly includes coating treatment and coating treatment.
Generally, the crystallinity of plastics is large, the polarity is small or non-polar, and the surface energy is low, which will affect the adhesion of coating. As plastic is a kind of non-conductive insulator, it can not be directly coated on the plastic surface according to a electroplating process specification. Therefore, before surface treatment, necessary pretreatment should be carried out to improve the adhesion of the coating and provide a conductive bottom layer with good adhesion for the coating.
Pretreatment of coating
The pretreatment includes the degreasing treatment of the plastic surface, that is, the oil and mold release agent cleaning the surface, and the activation treatment of the plastic surface, in order to improve the adhesion of the coating.
1. Degreasing of plastic products
Similar to the surface degreasing of metal products, plastic products can be degreased by organic solvent cleaning or alkaline water solution containing surfactant. The organic solvent is suitable for cleaning paraffin, beeswax, fat and other organic dirt from the plastic surface. The organic solvent used shall not dissolve, swell or crack the plastic. It has low boiling point, is volatile, non-toxic and non combustible.
Alkaline water solution is suitable for degreasing of alkali resistant plastics. The solution contains caustic soda, alkaline salt and various surface active substances. The most commonly used surfactant is OP series, that is, alkylphenol polyoxyethylene ether, which will not form foam or remain on the surface of plastics.
2. Surface activation of plastic products
This kind of activation is to improve the surface energy of plastics, that is to say, to form some polar groups on the surface of plastics or to coarsen them, so as to make the coating more easily wetted and adsorbed on the surface of products. There are many methods of surface activation, such as chemical oxidation, flame oxidation, solvent vapor etching and corona discharge oxidation.
Among them, the most widely used method is chemical crystal oxidation, which is commonly used as chromic acid treatment solution. Its typical formula is potassium dichromate 4.5%, water 8.0%, concentrated sulfuric acid (over 96%) 87.5%.
Some plastic products, such as polystyrene and ABS plastic, can be directly coated without chemical oxidation treatment.
In order to obtain high-quality coating, it is also useful for chemical oxidation treatment. For example, after ABS plastics are degreased, it can be etched with dilute chromic acid treatment solution. The typical treatment formula is 420g / L chromic acid and 200ml / L sulfuric acid (specific gravity 1.83). The typical treatment process is 65 ℃ 70 ℃ / 5min 10min, washed and dried.
The advantage of etching with chromic acid treatment solution is that no matter how complex the shape of plastic products is, it can be treated uniformly. Its disadvantage is that there are dangers in operation and pollution problems.
Pretreatment of coating
The purpose of coating pretreatment is to improve the adhesion between the coating and the plastic surface and to form a conductive metal substrate on the plastic surface.
The pretreatment process mainly includes mechanical coarsening, chemical oil removal, chemical coarsening, sensitization treatment, activation treatment, reduction treatment and chemical plating. The first three are to improve the adhesion of the coating, and the last four are to form a conductive metal substrate.
1. Mechanical coarsening and chemical coarsening
Mechanical coarsening and chemical coarsening are used to coarsen the plastic surface by mechanical and chemical methods respectively, so as to increase the contact area between the coating and the substrate. Generally speaking, the binding force of mechanical coarsening is only about 10% of chemical coarsening.
2. Chemical oil removal
The method of degreasing before coating on plastic surface is the same as that before coating.
3. Sensitization
Sensitization is to make the plastic surface with certain adsorption capacity absorb some easily oxidized substances, such as tin dichloride, titanium trichloride, etc. These adsorbed easily oxidized substances are oxidized during the activation treatment, and the activator is reduced to catalytic crystal nucleus, which remains on the product surface. The function of sensitization is to lay a foundation for the subsequent electroless plating of metal layer.
4. Activation
Activation is the treatment of sensitized surfaces with the aid of a solution of catalytic active metal compounds. Its essence is to immerse the product adsorbed with reductant into the aqueous solution containing the oxidant of precious metal salt, so that the precious metal ion is reduced by S2 + n as oxidant, and the reduced precious metal is deposited on the product surface as colloidal particles, which has strong catalytic activity. When the surface is immersed in the electroless plating solution, these particles become the catalytic center, which makes the reaction speed of electroless plating faster.
5. Reduction treatment
After activation treatment and cleaning with clear water, before chemical plating, the products are dipped with a certain concentration of reducing agent solution used in chemical plating, so as to reduce and remove the uncleaned activator, which is called reduction treatment. For electroless copper plating, formaldehyde solution is used for reduction treatment, and for electroless nickel plating, sodium hypophosphite solution is used for reduction treatment.
6. Chemical plating
The purpose of electroless plating is to form a conductive metal film on the surface of plastic products and create conditions for the metal plating of plastic products, so electroless plating is the key step of plastic plating.
