Adhesive Chemistry and Moisture in Concrete Floors
Due to the numerous regulations developed over the last 20 years, flooring adhesives have been reformulated to reduce or eliminate VOCs. Most adhesive formulations include a binder, solvent, fillers, pigments, and additives. The primary source of VOCs in historical adhesive formulations is the solvent. The primary purposes of the solvent are to control viscosity during application, and when dissipated, facilitate the adhesive to setup and cure. Once the adhesive is applied, the solvent is not needed and evaporates into the air, which is necessary for completion of the chemical reaction of other components in the adhesive, resulting in cure. Historically, the solvents used in flooring adhesives have been organic liquids that typically contain VOCs. The evaporation of these VOC – containing solvents into the air during adhesive application is what compelled the reformulations under the various standards previously discussed.
However, these traditional VOC-containing adhesives had been tested and optimized over time, leading to proven, long-term durability. As the industry transitioned away from VOC-based solvent adhesives, new adhesive formulations were developed with either water-based solvents or without a dedicated solvent (i. e., 100% solids adhesives). Unfortunately, reformulating the adhesives has led to some unanticipated durability issues, especially those which are found with concrete substrate applications.
Concrete floor slabs are difficult substrates on which to apply floor finishes. Concrete inherently contains moisture. Water is an integral ingredient in all concrete, both for hydration of the cement and workability of the concrete during placement. The water that is not used for the hydration of the cement remains in the concrete after curing is complete and slowly dries to equilibrium with the surrounding environment when conditions are favorable for evaporation. Cured concrete can also reabsorb moisture from the surrounding environment, either
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above or below the slab, after curing is complete. Ground water below slabs-on – grade or slabs-on-ground, rainwater and spills onto the concrete, and cleaning and maintenance procedures can all increase the amount of moisture in concrete floor slabs.
Concrete can take a very long time to dry. Many studies have been done to determine the drying time of concrete, and these studies have identified many variables that contribute to concrete’s ability to dry. Even under the most ideal conditions, normal concrete can take several months to reach a moisture content that will not negatively affect floor adhesives. Once a floor finish with low permeability (e. g., most adhered resilient floor finishes) is applied to the concrete, moisture in the concrete can no longer dry into the ambient air provided that bottom side evaporation is not possible. Instead, the residual moisture in the concrete will tend to collect between the floor finish and the concrete. Depending on the moisture levels in the concrete, significant amounts of moisture can collect at this interface, where it is in direct contact with the adhesive.
Concrete also has a naturally high (alkaline) pH. The typical pH of freshly placed concrete is about 12 to 13. The pH of cured concrete will decrease over time through carbonation if it is exposed to the carbon dioxide in the air, but the pH of carbonated concrete is still around 9. Since the hydroxide ions that create the alkaline condition only exist in water, the high pH of concrete alone is not necessarily a concern. However, when the hydroxide ions are combined with the high levels of moisture that are potentially present in concrete floor slabs, the concrete becomes an unsuitable surface for floor finishes, especially those installed with the newer, water-based adhesives.
The most common flooring adhesives to come out of the new low – or no – VOC reformulations are water-based acrylic emulsions. Because these adhesives are designed with water as their solvent, they are susceptible to reemulsification when exposed to water from other sources, such as moisture within a concrete floor slab. The degradation of the adhesive is commonly exacerbated by high pH levels and contact with very alkaline moisture from the concrete substrate. Because the older solvent-based adhesives did not have this inherent tendency to dissolve in water, they were much less susceptible to moisture and pH related deterioration.
The following case studies illustrate moisture – and pH-related degradation of water-based flooring adhesives.