Proper coating application starts with proper pre-treatment

Before coating a substrate, it is treated through manual cleaning, sanding, surface activation. Let’s look at the possibilities. 

Blasting or sanding and degreasing or cleaning are steps in a painting or coating process that are commonly used on parts prior to the actual painting phase. Primer coatings are also often sanded or softened to ensure proper adhesion of the top layer.

In addition to manual cleaning and roughening of the substrate, there are also industrial pre-treatment techniques that can be applied in a continuous line. This may include mechanical pre-treatment, such as blasting, laser cleaning, thermal stripping and dry ice blasting, or chemical pre-treatment (degreasing, staining, phosphating, chromating) done in spray tunnels, blasting equipment or immersion baths. 

Surface activation

However, some substrates require additional treatment in order to activate the surface. In these types of activation, new groups are formed on the surface, which allow good wetting, bonding and adhesion.

Flame treatment and flame blasting

Flame blasting refers to the cleaning of steel by means of heat. When flame treatment is applied to plastic, the surface is brought into contact with a natural gas flame containing unburned oxygen. By setting the correct oxygen volume, oxygen-rich groups are implanted in the polymer chains to activate the surface. The advantage is the low investment cost, but for many applications the technique cannot be used because the heat of the flame may cause damage to the surface. 

Plasma and corona treatment

In these treatments, the air is charged by applying a high-voltage field between two electrodes. The ionised air will then, through radical reactions, form new groups on the surface of the treated component (plasma activation) or oxidise contaminants.

For a corona treatment, a cloud of ionised air is formed between two electrodes in which or through which the surface to be treated is moved.

For a plasma treatment, plasma is generated by one electrode and a torch is created by means of a process gas or air flow, which means that the plasma density is higher than in a corona treatment. The gas in which plasma is made depends on the application.

Problems with detachment?

Both corona and plasma treatment are applied to plastics, where the surface energy is increased to allow a good spread of adhesives, inks and lacquers. Plastics with an apolar surface and low surface energy such as polyethylene and polypropylene in particular need to be pre-treated.

Correct coordination between process parameters, plasma treatment time and choice of adhesives, inks and coatings are crucial to achieve ideal adhesion.

A plasma process can be applied both atmospherically and in a vacuum.

Sirris has an Easytreat BC20 corona system, a multi-torch Tigres Plasma pretreater MEFV7/56 and a low-pressure plasma chamber, which allows us to pre-treat both flat and formed parts.

A robot application, in which a plasma device is placed on a robot, is also possible.

Sirris can demonstrate the different techniques and assist you in determining the right parameters.

Characterisation

The degree of cleaning and effect of pre-treatment can be checked in different ways. An adhesion test after application of the paint or coating is the most outspoken technique, but prior to further treatment the change in surface energy can be determined by contact angle measurements or by the use of dyne solutions with different surface tension.

Want to find out what a pre-treatment can mean for your product? Contact the Smart Coating Lab

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