Larry Good, chemical engineer for Specialty Concrete Products, West Columbia, S.C., defines coatings as products that penetrate into concrete, and sealers as products that form films on top. For this reason, solvent urethane and epoxies, which are very hard coatings, have long been considered the best class of products. Good says they are available as aromatic or aliphatic sealers. Aromatic sealers are tougher and achieve better bonds, but aliphatic are resistant to UV radiation and don't yellow over time. Aliphatic resins are therefore better for use on decorative concrete.
In the past few years there has been much research to develop waterborne polyurethane dispersion (referred to as PUDs) products that can handle the alkaline environments of concrete. Several companies now have such a product on the market, and many more are testing products that they hope to have available next year. They fall into two categories: single- and double-component materials. By law, the urethane resin content has to be only 11 % of the total in order to be labeled as urethane.
Jim Esseg, Western technical director for Crossfield Products, Rancho Dominguez, Calif., says that a current favorite technology is called “Inner Penetrating Network” (IPN) urethane and acrylic resins. The two resins are polymerized together, rather than cold blended afterwards. Combining urethanes and acrylics provides the advantages and disadvantages of both resins and a significant cost advantage over a two-component system. There is greater water-vapor transmission than with just urethane, the film is harder than acrylic but softer than 100% urethane, and there is greater resistance to oil penetration than with acrylic sealers. Some IPN products are recommended for exterior use and some aren't.
According to Jack Bracco, a partner in Dual Cure Technology, Lewis Center, Ohio, two-component waterborne urethane coatings are thermoplastic setting so they don't soften under warm conditions the way single-component systems (thermoplastic films) do. The molecules of two-part urethanes “crosslink” as they harden, making the resulting films much harder than single-component systems. Lindy Ausburne, technical service manager for Golden Look, Waco, Texas, says that two-part systems provide a very rich “wet look.” Colored surfaces look more vibrant. And higher mill thickness buildups are possible, with less potential for haziness compared with typical one-part systems. Two-part systems also have greater chemical and abrasion resistance, making them a viable sealer for concrete countertops and food preparation areas.
Slab preparation for both single-and two-component systems are simple. Elaborate preparation, such as the shotblasting required for an epoxy coating, isn't necessary. Ausburne says that slab preparations for both single- and two-component systems require a clean surface, free of contaminants, dry, and having a certain degree of profile—meaning that a hard troweled slab wouldn't be a good candidate. Paul Porreca, the owner of Americrete, Temecula, Calif., adds that there should be no curing compounds on concrete that receives Americrete's single-component product.