Surface Preparation for Adhesive Bonding

Surface Preparation is one of the most critical steps in achieving a high bond strength and improving the performance of an adhesive.
For an effective bond, the adhesive must completely wet the surface of each substrate being joined together.

Why use surface pretreatment?

Most surfaces are covered with layers of contaminants, for example, dust, oils, grease and oxide.
These contaminants adversely affect the adhesion and wetting of the adhesive and if they are not removed properly before applying the adhesive, it can result in bond failure.

Surface pretreaments are designed to remove all types of contaminants found on the surface of the substrate, making it possible for the adhesive to wet the actual surface rather than its apparent surface.

It can activate and increase the surface energy of the substrates and the active bonding area to improve the adhesion of the adhesive.


Surface preparation – degrease, abrade, degrease

A simple method to prepare surfaces is to degrease, abrade and degrease. This method is suitable for most industrial applications; however, the type of pretreatment will depend on the substrate type.

Degrease

Degreasing the surface using isopropanol or acetone can remove any traces of oil, dust and grease residue from the outer layer.


Abrade

Mechanical abrasion methods such as sandblasting or wire brushing can remove the layer of oxides, rust and paint on the surface.

Grit blasting can be used to remove surface deposits on metal surfaces. The two main preferred grits are fused alumina and silicon carbide. Fused alumina is used as an abrasive for aluminium alloys and stainless steels. Silicon carbide is used on special alloys that are likely to react adversely with any residual fused alumina at temperatures they may be exposed to under service conditions, or when the materials to be abraded are either soft or extremely hard.

Abrading a surface will increase the surface area allowing the adhesive to form a strong and durable bond.


Degrease

A further degrease and clean may be required to remove any loose particles from the abrading process.

The adhesive should be applied immediately after the surface treatment to ensure the surface does not become recontaminated. All surfaces must be dry before the bonding process.


Water break test –  test for a clean surface

The water break test can be performed to check for sufficient cleanliness on the surface.  It allows the user to check the presence of hydrophobic contaminants and residue such as oil or oil-based coatings and paints.

The cleaned surface is placed in a vertical position and sprayed with drops of distilled water to wet the surface.

If the surface is clean, the water will spread out over the surface due to high surface energy. A large contact angle is formed due to its attraction to the surface.  This shows the surface is free of contamination and the surface treatment was successful.

It is important to consider that even though the water break test can be used for most substrates, there are certain composite materials that even when clean, may not be wetted by distilled water but will be wetted by the adhesive.


Surface preparation for different substrates

The following surface preparation steps are recommended for preparing most substrates for bonding (for industrial use only). For specific recommendations, please refer to the technical data sheets.

METALS

  1. Wipe the surface with acetone using a clean cloth
  2. Lightly abrade the surface by sandblasting or using fine grit sandpaper to remove surface deposits.
  3. Wipe surface with acetone to remove any loose particles and debris
  4. Allow surface to dry before applying the adhesive

PLASTICS

  1. Lightly abrade the surface by sandblasting, fine grit sandpaper
  2. Wipe the surface with isopropyl alcohol or using an impregnated wipe like Wipemaster Wipe 9 IPA Wet Wipes to remove loose particles and debris.
  3. Allow the solvent to evaporate before applying the adhesive.

GLASS

  1. Wipe the surface using a clean cloth with a solvent containing 0.5wt% silane adhesion promoter.
  2. Allow the solvent to evaporate before applying the adhesive.

Chemical Pretreatments

Some substrates have low surface energy which make it difficult to bond. As a result, they may require a chemical or electrolyte pretreatment to change the properties of the surface to improve the adhesion. Examples include: metals such as copper, stainless steel, and aluminium.

Pretreatments include etching metals with acid to remove the surface oxidation layer and replace it with an oxide layer which is mechanically and chemically compatible with the adhesive.

Anodising is a pretreatment for aluminium and titanium alloys. The process involves depositing a porous oxide layer on top of the oxide layer formed after etching. The porous oxide layer enables the adhesive to penetrate the pores to form a strong bond.


Other Pretreatments

Some thermoplastics such as polypropylene (PP), acetal, polyether ether ketone (PEEK) or polyoxymethylene (POM) require special pretreatment such as Plasma treatment or Corona treatment. The low surface energy of these plastics and non-porous surface makes it difficult to bond.

Plasma treatment is performed in a vacuum chamber, where the air is pumped out prior to letting a gas or mixture of gases in at low pressure. Electrical power is then applied. Plasma treatment can be used to remove any foreign contaminants present on the surface of a substrate making it suitable for further processing.

Corona treatment uses a high frequency beam or corona discharge on the substrate. This results in a break up of the oxygen molecules into their atomic form. The atoms are then free to bond with the molecule ends in the substrate being treated.

These surface treatments increase the surface energy of the substrate to provide better adhesion capability and bonding strength.

For more detailed explanations regarding surface preparation and pretreatment, please contact us.

The information in this article is intended only as a guide.

 

Twitter
LinkedIn

IMPROVEMENTS IN YOUR PRODUCTION PROCESSES

We look forward to optimize your applications through our technologies and advise you without any commitment.

Product Search

TDS / MSDS

To request any type of technical sheet or safety sheet.