Polyaspartic polyureas are fairly new in the marketplace and have great potential for decorative concrete because of their many unique qualities. Contractors are currently using them as clear coats over finishes such as stamped, stained, colored, and stenciled concrete, and as pigmented coatings, which includes the addition of decorative media. There is also some experimentation with applications on exterior slab-on-grade decorative work.
Aspartic esters were invented and patented by Bayer Corp. in the 1990s. Aspartic resins followed and the manufacturer found that films could be formed which were very dense and shiny, leading to the creation of polyaspartic aliphatic polyureas. Now several companies have introduced formulations that contractors can use commercially. Their formulations share several common properties but there are also benefits and cautions unique to each product on the market.
Polyureas are 2-component materials. The system works when a resin is mixed with a catalyst, usually a polyisocyanate, which causes a reaction and develops a polymer compound. The first to be marketed were aromatic polyureas but installing them required expensive spraying equipment that mixed parts A and B together at the spray tip, with hardening of the material following a few seconds later. Aromatic polyureas are also susceptible to UV degradation at the membrane's surface.
Within the past couple of years several companies have introduced polyaspartic polyurea products with remarkable features compared to other coating materials:
- They can be applied at temperatures as low as -30°F and as high as 140°F, depending on the product. In hot and cold climates contractors can install them all year long.
- When cured they can tolerate temperatures as high as 350°F.
- They are UV light stable, providing “water clear” finishes.
- Polyureas have much greater abrasion and impact resistance than either epoxy or urethane coatings.
- They do not blush or whiten.
- Curing is quick and work can be turned over to owners for use within a couple hours after application.
- High ambient humidity doesn't restrict their application, although some products set more quickly in high humidity.
- They don't stain, they resist fat and oil penetration, and concrete is protected from mild acids.
- Polyureas have excellent wetting and penetrating characteristics, which means they form excellent bonds to concrete.
There are different thoughts among formulators about the preparation needed for concrete applications. The type of application also matters. Jack Bracco, president of Flexmar Coatings, New Kensington, Pa., says that the excellent “wetting” ability of polyaspartic polyureas and the way they penetrate concrete makes low profile preparation for decorative concrete possible.
Some contractors profile their slabs with grinders using 40 to 80 grit diamond pads. Lonnie Salyers, a technical representative for Sherwin Williams, Cleveland, says that heavier polyurea coatings for industrial floors should be profiled with shotblasting equipment. He adds that for decorative concrete receiving a thin film coating, thoroughly cleaning with a power washer may be adequate. Art Weiss, technical supervisor for Versa Flex, Kansas City, Mo., says that his company recommends removing the cement paste from the surface and neutralizing all soluble salts. They recommend following the guidelines set forth by professional associations like the International Concrete Repair Institute (ICRI).
Joe Sheehan, owner of Garage Floors 1, Oakdale, Minn., an application contractor, says he's developed a system for preparing slabs by diamond grinding with a dry vacuum pick-up to achieve an 80 grit profile. “I know that my preparation is good when the primary coat absorbs completely into the surface,” he says. Hugh Monteiph, owner of Ameridream, Ventura, Calif. specializes in water-based stain finish decorative work. He says he applies a citric acid wash and pressure washes with a turbo tip to prepare his slabs before staining. Neither contractor reports any scaling, debonding, or other problems involving polyurea coatings to date.
Moisture content in slabs?
Polyureas don't transmit water vapor so they are considered waterproof. Most manufacturers interviewed said that aspartic ester polyureas “wet out” and penetrate concrete surfaces to achieve a superior bond. This bonding ability may prevent moisture vapor transmission issues. Manufacturers agree however, that the moisture content of concrete shouldn't exceed 3 pounds per 1000 square feet using the calcium chloride test at the time of application to ensure that the primer application will penetrate the surface properly.
Sheehan says he installs three applications; a primer (which penetrates into the surface), a “build coat” containing the decorative application if it's more than a clear coat, and a top coat. Most of his work is 20 to 25 mils in thickness when he includes vinyl chips in his finishes. Weiss says that 6 to 8 mil thicknesses are normal and will provide “hiding capacity” when pigments are added.
Polyaspartic polyureas are sold as 100% solids material, with solvents added, or with recommendations about adding solvents. With the addition of solvents the material is less viscous and there is increased penetration into a slab. Pot life is also increased.
Another advantage of polyaspartic polyureas is that the material doesn't outgas. Concrete still traps air though, and it can escape into a coating after it's applied. It's best to apply coatings when ambient temps are declining. But because they set so quickly, there is less likelihood of trapping air in polyureas.
In terms of application, Salyers warns that humidity accelerates setting time. When his product is applied at 40% humidity the setting time might be 30 to 40 minutes. But at 90% humidity it can decrease to 15 minutes.