Improves Adhesion Increases Bond

Adhesion

Concrete sealers and coatings must exhibit good adhesion in order to be effective. Adhesion is the strength of bonds forming between one material and another. Adhesive failure on the other hand can result from low adhesion between a coating and the substrate.

Good adhesion results when the following occurs:

1. The coating molecules flow freely over the substrate to make contact and adhere also called adsorption.
2. Chemical bonds are formed between the coating and the substrate.
3. The coating film penetrates the roughness on the substrate surface, resulting in mechanical interlocking once the coating dries.

All three do not need to occur in order for proper bond and adhesion. Depending on the coating, substrate and application method, different options could work.

Adsorption

The adsorption theory states, “That adhesion results from molecular contact between two materials and the surface forces that develop” (sciencedirect.com). According to this theory, when applying a concrete coating, the coating film must make molecular contact with the substrate surface. The process of establishing continuous contact between a coating and the substrate surface is known as “wetting.” Good wetting results when the coating flows into the valleys and crevices on the substrate surface; poor wetting results when the adhesive bridges over the surface irregularities. Also, if the substrate is contaminated with dirt, debris, oil or grease, the contaminant becomes a weak boundary layer which can easily fail and weaken the entire coating system. This is why surface preparation is extremely important prior to the installation of a concrete coating. Common surface preparation methods are chemical etching or mechanical scarification.

Chemical Bonding

Coating systems are formulated with binders that can chemically bond to a substrate. These strong and durable bonds are generally the result of close contact or adsorption on the surface followed by a chemical reaction. Chemical bonding is most common with adhesion promoters or primers. These products provide a “molecular bridge” between the substrate and the molecules in the coating. One end of the adhesion primer molecule has functionality that will react with the coating, and the other end has a functionality that will react with the substrate. A strong and durable bond forms as the primer cures. Silanes are an example of a widely used adhesion primer. They promote adhesion, improve moisture resistance, and reduce the potential of corrosion.

Mechanical Interlocking

Another way adhesion occurs is by a coating filling pores, holes, and micro-cracks in the substrate. When the coating hardens, it is held on mechanically. This is why mechanical scarification is a common surface preparation method for coating adhesion. In order for a coating to properly perform, it must penetrate the surface irregularities and roughness acquired by scarification and lock on mechanically to the substrate. The surface roughness aids in adhesion by increasing the total contact area giving “teeth” to the substrate and stopping the coating from lifting.

How is adhesion measured?

Adhesion tests verify that both the surface preparation and coating application are within specification. Adhesion testing of a coating quantifies the strength and bond between concrete and the subsequent coating, or between different coating layers. In order to perform adhesion testing a common testing method is pull off elcometer testing. Elcometers accurately measure the strength of the bond between the coating and the substrate.