Surface preparation is often expressed as surface pretreatment or surface treatment.
Surface pretreatment for painting can be approached in two main ways.
1. Surface cleanliness
When we think about the surface preparation, there is a famous well-known standard for it. It is ISO 8501(Preparation of steel substrates before application of paints and related products - Visual assessment of surface cleanliness).
And its title said 'surface cleanliness'. Because this is an important factor in surface preparation, and most surface preparation methods are used to make the surface clean.
2. Improving Adhesion
In the painting preparation process, the surface pre-treatment through the cleaning or post-cleaning process usually involves a process to increase the adhesion between the coating and the substrate. One way to increase adhesion is to increase the surface area physically and chemically, such as by shot/grit blasting or conversion coating.
Anyway, surface treatment for painting determines about 60-80% of the paint quality.
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| Spray type process line of Surface Preparation |
Let's take a look at surface pretreatment methods that are mainly used in powder coating.
1. Shot/Grit Blasting
Abrasive blasting is a more appropriate and broad term, but in the field, sandblasting and shot blasting are also used.
Short or grit is a term that describes the form of media used for blasting.
Sandblasting use sand to blast, so it is called sandblasting.
Abrasive blasting is an effective method that has been used for a long time.
The media particles are collided with the surface of the material by using air pressure or centrifugal force to remove rust or foreign substances on the surface. And a profile is formed on the surface due to the collision, which increases the surface area and thereby affects the adhesion.
2. Degrease
In powder coating and liquid coating field, the surface is prepared by just removing foreign substances such as oil and grease on the surface with solvent or detergent, to be coated.
Most of the metal is contaminated with oil because the processing oil or lubricating oil is usually used when the metal goes through the forming process.
Such the oils can decrease the adhesion between the surface of the material and the coating film, or cause defects such as cratering.
Degreasing methods includes using alkaline detergent, acid solution and solvent.
3. Conversion Coating
Conversion coating is slightly difficult to get the concept of it. But it is easy if you think of the plating on the material surface with chemical reaction. The method combines the surface of the material with other metals to improve the coating adhesion and corrosion resistance.
Conversion coating is largely divided into Phsphate and Chromate. Phosphating conversion coating is mainly used for iron(steel) materials, and chromate is used for aluminum(alloy) materials.
In recent years, various conversion coating agents such as Zirconium and non-chrome have come out, and these various conversion coating agents are being developed to reduce environmental pollution, improve adhesion and corrosion resistance.
4. Acid Cleaning
Acid treatment, called pickling, is a method of removing rust or mill scale from surfaces using a strong acid solution.
The preparation methods is used alone or also in combination.
For example, the surface preparation process for the automotive wheel is simply like below.
Blasting -> Degreasing -> Rinsing -> Acid Cleaning -> Rinsing -> Conversion Coating -> Rinsing -> DI water Rinsing
Continuously surface preparation methods are applied in various ways according to environmental regulations, materials to be coated, and pretreatment efficiency, and are being developed in various ways.
And we will see them in detail later.
LINK: Korean Version of This Post([도장 전처리]분체도장에 주로 사용되는 전처리방법들)
