Packaging Technology丨Surface Pretreatment Technology of Plastic Products

Introduction: The manufacturing process of plastic products mainly includes four key processes: mold forming, surface treatment, printing, and assembly. Surface treatment is an indispensable key part. In order to improve the bonding strength of the coating and provide a good conductive base for the plating, the pre-treatment process is indispensable.

Surface pretreatment of plastic products
Mainly includes coating treatment and plating treatment. Generally, plastics have a large degree of crystallinity, small polarity or no polarity, and low surface energy, which will affect the adhesion of coating. Since plastic is a non-conductive insulator, it cannot be directly plated on the plastic surface according to the general electroplating process specifications. Therefore, before surface treatment, necessary pretreatment should be carried out to improve the bonding strength of the coating and provide a conductive bottom layer with good bonding strength for the plating.

Pretreatment of coating

Pretreatment includes degreasing of plastic surface, i.e. cleaning the oil and release agent on the surface, and activating the plastic surface, in order to improve the adhesion of the coating.

Degreasing of plastic products. Similar to degreasing of metal products, degreasing of plastic products can be done by cleaning with organic solvents or degreasing with alkaline aqueous solutions containing surfactants. Degreasing with organic solvents is suitable for cleaning paraffin, beeswax, fat and other organic dirt from the plastic surface. The organic solvent used should not dissolve, swell or crack the plastic, and it has a low boiling point, is volatile, non-toxic and non-flammable. Alkaline aqueous solutions are suitable for degreasing alkali-resistant plastics. The solution contains caustic soda, alkaline salts and various surfactants. The most commonly used surfactant is the OP series, i.e. alkylphenol polyoxyethylene ether, which does not form foam and does not remain on the plastic surface.

2、Surface activation
This activation is to improve the surface properties of plastics, that is, to generate some polar groups on the plastic surface or to roughen it so that the coating can be more easily wetted and adsorbed on the surface of the workpiece. 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 chemical crystal oxidation treatment, which often uses chromic acid treatment liquid, and 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 high-quality coating, chemical oxidation treatment is also used. For example, after degreasing, ABS plastic can be etched with a dilute chromic acid treatment liquid. Its typical treatment formula is 420g/L chromic acid and 200ml/L sulfuric acid (specific gravity 1.83). The typical treatment process is 65℃70℃/5min10min, water washing, and drying. The advantage of etching with chromic acid treatment liquid is that no matter how complex the shape of the plastic product is, it can be treated evenly. The disadvantage is that the operation is dangerous and there are pollution problems.
Pretreatment of coating coating

The purpose of pretreatment of coating coating is to improve the adhesion of the coating to the plastic surface and to form a conductive metal bottom layer on the plastic surface. The pretreatment process mainly includes: mechanical roughening, chemical degreasing, chemical roughening, sensitization treatment, activation treatment, reduction treatment and chemical 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 treatment are to make the plastic surface rougher by mechanical methods and chemical methods respectively to increase the contact area between the coating and the substrate. It is generally believed that the bonding force that can be achieved by mechanical roughening is only about 10% of that of chemical roughening.

2、Chemical degreasing
The degreasing method for pretreatment of plastic surface coating is the same as the degreasing method for pretreatment of coating.

Sensitization is to adsorb some easily oxidized substances, such as tin dichloride, titanium trichloride, etc., on the surface of plastics with certain adsorption capacity. These adsorbed easily oxidized substances are oxidized during activation treatment, and the activator is reduced to catalytic crystal nuclei and remains on the surface of the product. The role of sensitization is to lay the foundation for the subsequent chemical plating metal layer.

Activation is to treat the sensitized surface with the help of a solution of catalytically active metal compounds. Its essence is to immerse the product adsorbed with the reducing agent in an aqueous solution containing an oxidant of a precious metal salt, so that the precious metal ions are reduced by S2+n as an oxidant, and the reduced precious metal is deposited on the surface of the product in the form of colloidal particles, which has a strong catalytic activity. When this surface is immersed in a chemical plating solution, these particles become catalytic centers, which accelerates the reaction rate of chemical plating.

5、Reduction treatment
Before chemical plating, the products that have been activated and washed with clean water are immersed in a certain concentration of reducing agent solution used in chemical plating to reduce and remove the unwashed activator. This is called reduction treatment. When chemical copper is plated, formaldehyde solution is used for reduction treatment, and when chemical nickel is plated, sodium hypophosphite solution is used for reduction treatment.

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

Post time: Jun-13-2024
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