The Importance of Adhesion

Obvious adhesion issues

Achieving 100% adhesion with a coating system is an extremely important task as well as a necessary step that leads to ideal results. The entire performance of a coating system relies heavily on its adhesion. Any less than 100% adhesion to the substrate or between any of the layers or coats (inter-coat adhesion) will often result in poor performance of the coating. Peeling is a common and obvious sign of adhesion failure. However, peeling is not the only negative or obvious result of poor adhesion. The chemical resistance of a coating is also negatively affected by poor adhesion. Many coatings rely on 100% adhesion to achieve the optimal amount of chemical resistance and performance.

Coatings within a coating system should be formulated to adhere to each other as well as the substrate. Most coatings have a recoat window, which allows the subsequent coat to stick or adhere to the previously applied coating without the need for additional preparation processes. Poor inter-coat adhesion will lead to coating failure. Referring to technical data sheets (TDSs) is crucial to avoid adhesion failure. Applying coatings outside of the stated recoat windows will require some type of mechanical or chemical preparation process to ensure full adhesion.

Adhesion failure - blistering

Coatings within a coating system should be formulated to adhere to each other as well as the substrate. Most coatings have a recoat window, which allows the subsequent coat to stick or adhere to the previously applied coating without the need for additional preparation processes. Poor inter-coat adhesion will lead to coating failure. Referring to technical data sheets (TDSs) is crucial to avoid adhesion failure. Applying coatings outside of the stated recoat windows will require some type of mechanical or chemical preparation process to ensure full adhesion.

TDS with recoat window.

Adhesion can also fail when coatings do not cure properly. Epoxy coatings are known for amine bloom and amine blush issues. These phenomenon occur when coatings are applied outside of the recommended environmental parameters – such as high humidity or high dew point situations. If the first coat of product does not appear to have cured or dried properly (such as having a hazy or dull appearance), contact your manufacturer’s representative and explain the situation prior to applying a subsequent coat.

Prior to beginning a coating project, it is recommended to test the concrete for moisture. Moisture within a concrete slab will cause adhesion issues down the road if the proper system is not used to treat the moisture or accommodate for the moisture issue. There are several visible signs that there may be excessive moisture in a slab. However, there is really no way to know for sure without performing a moisture test. A simple plastic sheet test (see right photos) will tell you if there is an alarming amount of moisture in a slab.

Plastic sheet test

However, more scientific tests such as calcium chloride (see bottom right photo) or in situ probe testing (see bottom left photo) will yield a “moisture number” such as the number of pounds per 1000 SF / 24 hours or the relative humidity of the slab. These readings can then be used to select the best product or system.

Insitu probe test
Insitu probe test
Calcium Chloride test

Once the coating system is selected, preparation procedures recommended by the manufacturer must be followed to ensure 100% adhesion. Creating the proper concrete surface profile (CSP) starts with referring to the coating’s TDS. TDSs should include recommended preparation procedures. Lack of proper preparation procedures is the number one cause of coating failure (https://ultradt.com/why-coatings-fail/) It’s been said that a coating is only as good as the floor or substrate it is being applied to.

Once prepared, the surface should be cleaned. This could include sweeping, vacuuming, scrubbing, degreasing, or a host of other methods. Grease, oils, dust, and other contaminants should be removed prior to applying a coating. Attaining a properly profiled and clean surface is critical to a product’s adhesion and resulting performance.

Visual grease, oil or other contaminants
Crass hatch test

There are multiple methods for testing adhesion. A relatively cheap and easy way to test adhesion is by using a cross-hatch test (see photo below). This method involves cutting a grid through the coating using a sharp knife, razor blade, or cross-hatch tester. The checkerboard type of grid is then abraded with a very-fine maroon conditioning pad followed by a cleaning process. Once dry, adhesion tape is applied to the grid and then pulled off. If any of the lines are distorted or the full squares are removed through the pulling of the tape, adhesion is not 100%. Some cross-hatch adhesion testers will create 100 squares

Distorted lines = signs of poor adhesion

The number of squares remaining after the pulling of the tape is recorded as the percentage of adhesion.  For example, if 85 squares remain (or 15 pulled off with the tape), the adhesion of the coating or system is roughly 85%.

Another method of adhesion testing is utilizing an automatic adhesion tester (see right photo). Little pawn-type pieces called dollies are glued to the top of the coating and allowed to dry or cure for 24 hours. Once cured, a device is attached to the dolly and air pressure is applied so that the dolly is slowly being pulled in an upward motion. A PSI reading is provided so that, when the dolly eventually pops off of the surface, the PSI can be documented. Additionally, the floor surface and the bottom of the dolly can be inspected to see if the eventual failure point is due to concrete failure or inter-coat adhesion somewhere within/between the various coats that were applied.

Technician using an adhesion tester
Positest automatic adhesion tester

Rectifying adhesion issues is often a costly endeavor. To ensure a peeling issue is fully resolved, complete removal of the entire system is recommended by using chemical removers, grinders, or shot blasting equipment. Occasionally, coating failures are confined to areas with contamination or areas where preparation mistakes or shortcuts were made. Spot patches or fixes can be performed by removing the failing product to the point that the peeling or flaking stops. Patches or repairs are usually visible upon completion. However, they may be hidden by a well-trained contractor that knows how to blend in the patch and/or how to use expansion joints or floor seams as start/stop points of a repair.

In conclusion, contractors should always carefully read a product’s TDS and ask the manufacturer a lot of questions before beginning. Preparation, mixing, and application instructions must be precisely followed to ensure a successful outcome and a satisfied customer.

Travis Negaard
CEO – Ultra Durable Technologies