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

Generally speaking, plastics with high crystallinity, low polarity or non polarity, and low surface energy can affect the adhesion of coatings. Due to the fact that plastic is a non-conductive insulator, it cannot be directly coated on the surface of plastic according to electroplating process procedures. Therefore, before surface treatment, necessary pre-treatment should be carried out to improve the adhesion of the coating and provide a conductive substrate with good adhesion for the coating.

The pre-treatment of the coating includes degreasing treatment of the plastic surface, which involves cleaning the surface with oil stains and release agents, as well as activation treatment of the plastic surface to improve the adhesion of the coating.

One. Degreasing of plastic products

Similar to degreasing of metal products. The degreasing of plastic products can be cleaned using organic solvents or alkaline aqueous solutions containing surfactants. Organic solvent degreasing is suitable for cleaning organic dirt such as paraffin, beeswax, grease, etc. on plastic surfaces. The organic solvents used do not dissolve, expand or crack plastic, and have low boiling point, volatility, non-toxicity, and non flammability.

Alkaline aqueous solution is suitable for degreasing alkali resistant plastics. The solution contains caustic soda, alkali salts, and various surface active substances. The most commonly used surfactant is OP series, namely Alkylphenol polyoxyethylene ether, which will not form foam or remain on the plastic surface.

2。 Surface activation of plastic products

This activation is aimed at improving the surface energy of plastics, that is, generating some polar groups on the plastic surface or making them thicker, making the coating easier to wet and adsorb on the surface of the parts. There are various methods for surface activation, such as chemical oxidation, flame oxidation, solvent vapor etching, and corona discharge oxidation. Among them, the most commonly used method is chemical crystal oxidation, commonly used as a chromic acid treatment solution. Its typical formula is potassium dichromate 4.5%, water 8.0%, and concentrated sulfuric acid (over 96%) 87.5%.

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

In order to obtain high-quality coatings, it is also suitable for chemical oxidation treatment. For example, after degreasing ABS plastic, it can be etched with a dilute chromic acid treatment solution. The typical treatment formula is 420gL chromic acid and 200mL sulfuric acid (specific gravity 1.83). The typical treatment processes are 65 ℃, 70 ℃ for 5 minutes, 10 minutes, washing, and drying.

The advantage of etching with chromic acid treatment solution is that no matter how complex the shape of plastic products is, they can be uniformly processed. Its disadvantage is that there are hazards and pollution issues during operation.

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 pre-treatment process mainly includes mechanical roughening, chemical oil removal, chemical roughening, sensitization treatment, activation treatment, reduction treatment, and chemical plating. The last three are to improve the adhesion of the coating, and the last four are to form a conductive metal substrate.

One. Mechanical roughening and chemical roughening

Two methods, mechanical roughening and chemical roughening, are used to roughen the plastic surface to increase the contact area between the coating and the substrate. Generally speaking, the binding force of mechanical roughening is only about 10% of that of chemical roughening.

2。 Chemical degreasing

The degreasing method for plastic surfaces after coating is the same as after coating.

III. Sensitization is the process of making plastic surfaces with a certain adsorption capacity absorb some easily oxidizable substances, such as tin dichloride, titanium trichloride, etc. These easily adsorbed oxidizing substances are oxidized during the activation process, and the activator is reduced to catalytic crystal nuclei, which remain on the surface of the product. Sensitization lays a foundation for subsequent electroless plating of metal layer.

Four. activation

Activation is the treatment of sensitized surfaces with a solution of catalytic active metal compounds. Its essence is to immerse the product adsorbed by the reducing agent into an aqueous solution containing a noble metal salt oxidant, reducing the noble metal ions in S2+N as the oxidant. The reduced noble metal is deposited as colloidal particles on the surface of the product, which has strong catalytic activity. When the surface is immersed in the chemical plating solution, these particles become catalytic centers, accelerating the reaction rate of the chemical plating.

5. Restoration processing

After chemical plating, the product after activation treatment and clean water cleaning is soaked in a certain concentration of reducing agent solution for chemical plating to reduce and remove the uncleaned activating agent, which is called reduction treatment. Chemical copper plating is treated with formaldehyde solution reduction, while chemical nickel plating is treated with sodium hypophosphate solution reduction.

6。 The purpose of chemical plating is to form a conductive metal film on the surface of plastic products, creating conditions for electroplating a metal layer on top of plastic products. Therefore, chemical plating is a key step in plastic electroplating.