What’s the Best Way to Test Intumescent Coating Adhesion?

A paramount part of building design and planning, fireproofing improves building safety and longevity. Applied like paint, fire retardant intumescent coatings offer a relatively new option to protect your project. These materials shield steel and other substrates from dangerous flames, while standing up to other elements like weather.

But how can you tell which intumescent coating offers the best protection? A critical, but commonly overlooked measurement of an intumescent coating’s fire protection is its adhesion — or how well it sticks to a substrate — and its cohesion — or strength of the coating itself.

There are two ways to evaluate the adhesion and cohesion of paint: the tape test and pull-off adhesion test.

For the tape test, (ASTM D3359) cross or ribbon cuts are made into a coated surface, and a special tape is used to pull off the paint.

During the pull-off adhesion test, (ASTM D4541) which is the industry’s more definitive test, this is what occurs:

• A steel plug, called a dolly, is placed on the coated surface.

• A large amount of pressure is then applied to pull up the plug and the coating.

• Testers then determine if the failure was adhesive or cohesive.

• The amount of pull, measured in pounds per square inch, is increased until the bond eventually fails.

• The higher the number, be better the bond.

“Ideally what you want to see is an adhesive failure because that means you’re getting the maximum strength out of the intumescent coating, and the failure is not within the coating, but rather at the substrate,” explains Tony Scott, Contego International executive vice president.  In both tests, Contego received a perfect score — zero loss or 100 percent adhesion on both the cross cut and ribbon cut variants of ASTM-D3359. “You can’t get better than that,” says Scott. On the ASTM D4541, Contego showed 631 psi of bond strength. The closest competitor demonstrated half the adhesion Contego has, while most had one-third to one-fifteenth of Contego’s strength.

Fred Stack, president of Boss Construction Group, recently joined Lindsay Construction to test Contego products on a 6-inch concrete core sample. “It was amazing how Contego still adhered to all surfaces, even after the concrete was cut out,” says Stack. The core sample “looked like it was just sprayed,” adding that officials at Lindsay Construction “were very impressed.”

 In a fire, the superior adhesion in Contego’s intumescent acrylic copolymer latex paint keeps steel and other substrates from reaching failure or flashover temperatures.