Promoting Good Adhesion

Adhesion to the substrate you wish to bond or seal with Sikaflex ® must be verified. “Good Adhesion” occurs when adhesion to the substrate is stronger than the cohesion strength of the adhesive/sealant or substrate.

Adhesion: Physical/Chemical phenomenon producing adherence.

Adherence: Sum of all the bonding forces exerted between two surfaces in close contact. Its value is measured by the energy required to break the bond, i.e. strength of bond-line.

Substrate: Material that is bonded or sealed.

Cohesion: Sum of the forces resulting from the various bonds which tend to form between the different parts of a liquid or a solid, i.e. defines the strength of an adhesive or substrate.

Using the correct surface preparation is essential in achieving good adhesion.

The chemical bond between the substrate and the adhesive is just a few nanometers. One nanometer is a hundred million times smaller than 1 cm. 1 cm is a hundred million times smaller than 12,000 km (the diameter of the earth). The small size of the chemical bond stresses the importance of preparing the surface correctly.

In order to get contact between the chemical groups, it is important to eliminate all spacers (e.g. oil, grease, rust, etc.) To achieve a strong bond between the adhesive and substrate, the substrate must be free from loose materials such as dirt, rust or metal shims. There must be no foreign material such as oil, grease, alcohol, release agents or silicone that would prevent the adhesive from wetting the substrate. The three main steps most commonly used for preparing the surface when bonding and sealing with Sikaflex ® are the following:

• Physical Abrasion
• Cleaning/Adhesion Promoting
• Priming

Physical Abrasion

Physical abrasion is typically required for aluminum, steel, fiberglass, and some plastics. Abrading with a Scotchbrite ® pad or sandpaper removes weak layers that interfere with adhesion, i.e. oxidation layer on aluminum. Using a disk or belt sander (80x) will provide a physical interlocking of the substrate and Sikaflex ® resulting in a stronger bond.

Cleaning/Adhesion Promoting
Sika ® -Cleaner 226 and Sika ® -Aktivator are solvents containing a small amount of adhesion promoters. The function of these products consists of cleaning the surface by the action of the solvents and additionally by depositing a very thin layer of this adhesion promoter. This results in an improved bonding surface.

There is a distinction between Sika ® -Cleaner 226 and Sika ® -Aktivator however. Sika ® -Cleaner 226 contains a solvent to eliminate impurities on the surface and adhesion promoters specific to Sika ® Primers, Sikaflex ® sealants and non-direct glazing adhesives. Sika ® -Cleaner 226 is NOT compatible for direct glazing applications and can NOT be used in place of Sika ® -Aktivator.

Sika ® Aktivator also contains a solvent for eliminating impurities on the surface and adhesion promoters specific to Sikaflex ® direct glazing adhesives. Sika ® -Aktivator is also compatible with all Sikaflex ® sealants/ adhesives and Sika ® Primers.

The film thickness of Sika ® -Aktivator should be as thin as possible. Therefore, the best method of application for Sika ® -Aktivator is the “wipe on/ wipe off” process:

• If the substrate is heavily contaminated by oils or other organic substances, initial cleaning using solventsor high-pressure water cleaning with the appropriate detergents is required.
• Wipe the dry surface with a clean absorbent paper towel moistened with Sika ® -Aktivator.
• Immediately wipe the surface with a dry paper towel to remove any excess.
• Apply Sikaflex ® or Sika ® -Primer after the recommended flash-off time.

During the application of Sika ® -Aktivator, an adhesion promoter is deposited on the surface. These are small molecules having two different reactive groups. One may react with the substrate; the other reacts with Sikaflex ® giving a link between the substrate and the adhesive. This is displayed below.

If Sika ® -Aktivator is applied in a thick layer (e.g. soaked paper towel) too many layers of adhesive promoter are deposited. As these molecules cannot react with themselves, they form a separation layer. The results may be decreased quality of adhesion. The use of the “wipe on/wipe off” methods guarantees a better adhesion quality by an improved cleaning action and by avoiding too thick a layer.

The best method of application for Sika ® -Cleaner 226 is the “wipe-on” process. Sika ® -Cleaner 226 should not be wiped off.

Priming
Sika ® Primers are clear or pigmented paints that bond well to the substrate and dry to form a good adhering surface for the adhesive. It usually leaves a reactive resin and a thin film of polymer at the surface of the substrate.

Primers are commonly recommended in bonding applications for two reasons:

• Bonding applications are actually holding the joint together and must be able to withstand great dy-namic, impact, and thermal movements and stresses. Use of primers, on the recommended sub-strates, increases the durability, enabling the joint to withstand the great movements in extreme weather conditions for an extended duration.
• Adhesives are higher in viscosity and do not “wet” the surface as well as low viscous sealants.

Low viscous sealant is “wetting” the surface

High viscous adhesive is not “wetting” the surface

High viscous adhesive is now “wetting” the surface

The main functions of primers are the following:

1. Provide a reactive resin to react with the surface and adhesive.
2. Provide a clean, uniform surface to ensure proper wetting of Sikaflex ®.

An additional function of the black primers, Sika ® -Primer 209 and Sika ® -Primer 206 G+P, is to provide some protection against UV radiation on transparent substrates. The use of black primers as the sole form of UV protection is not recommended for applications that will be continuously exposed to sunlight for extended period of time. See Comment 9 for additional information.

An additional function for the primers recommended for metals, Sika ® Primer-206 G+P and Sika ® Primer-210T, is to provide some protection against corrosion. However, these primers are NOT formulated to be a complete corrosion protection system. For metallic substrates that are especially susceptible to corrosion, like steel, it is recommended to use a corrosion protective paint/primer system that is compatible with Sikaflex ®.

Below is a brief description of the primers available.

Sika ® Primers should be applied in a thin continuous coat with a clean brush, dauber, or roller. (More is not better. See page 4 for how layer thickness affects adhesion.)

The specified drying time of the primers and cleaners should be closely adhered to. It is important to wait long enough for the solvents to evaporate and the binder component to react with the atmospheric moisture. If the atmospheric temperature is less than 70°F, a longer time is required for the primer to dry.

Waiting too long, however, to apply the adhesive risks having the bonding surface contaminated with dust or debris. Therefore, it is recommended to apply Sikaflex ® within 2-3 hours of the primer application.

The bonding surface area should be isolated and not contaminated with oil, grease, perspiration, or any other foreign materials.