By Ingrid K. Meier, Carrie E. Hamilton, Andrew W. Wang, Gauri S. Lal, and Khalil Yacoub, Evonik Corporation, USA


With the enactment of the Clean Air Act of 1970, the use of waterborne coatings began to increase; however, the high surface tension of water (72 mN/m) required the addition of surfactants to enable water-based coatings to wet most substrates, many of which have surface energies below 50 mN/m. Most surfactants available at that time were primarily designed for use in detergents, cleaners, and emulsion polymerization; while they could lower the equilibrium surface tension of waterborne coatings, these products tended to stabilize foam and could not maintain low surface tension when the coatings were applied under dynamic conditions.1 Thus, the need for low-foam dynamic wetting agents emerged to facilitate the transformation from solvent-based to waterborne coatings.

The earliest low-foam dynamic wetting agents were based on acetylenic diol chemistry.1-4 These surfactants’ chemical structures differ significantly from those of most traditional surfactants. To achieve excellent dynamic wetting, these molecules need to be able to move quickly to newly created interfaces. This requires the surfactants to be lower in molecular weight and to have hydrophobic groups that prevent strong micelle formation or interaction with other components of the coating formulation.

Subsequent research and development produced additional non-acetylenic low-foam dynamic wetting agents, but they also had relatively low molecular weights and short, often branched, hydrophobes.5-10 Such structures appear to be necessary for achieving the required coatings properties, yet their low molecular weights preclude these substances from meeting the United States Environmental Protection Agency (U.S. EPA) definition of low-risk polymer under the Toxic Substances Control Act (TSCA). As a result, significant (potentially prohibitive) time and testing costs are likely to be required to register a new dynamic wetting agent in the United States or other countries with similar inventories.

During the 1970s through the 1990s, it became apparent that many surfactants that performed well in their intended applications had the potential for adverse environmental, health, and safety (EH&S) effects. Several surfactants were found to have reached high levels in waterways, persist in the environment due to slow or minimal biodegradation, harm or kill aquatic organisms, or have the potential to be endocrine disruptors. Therefore, regulatory agencies began to require more diligent assessments of new chemicals and particular attention began to be paid to those with the potential to have adverse effects on aquatic life.

In 1981, the U.S. EPA began using quantitative structure-activity relationships (QSARs) to model chemicals’ potential ecotoxicity behavior. The U.S. EPA and Syracuse Research Corporation then worked together to develop computerized versions of these models known as the Estimation Programs Interface Suite™ for Microsoft® Windows (EPI Suite™), which the Office of Pollution Prevention and Toxics (OPPT) relies upon for risk assessment development when actual data are lacking. These models can be downloaded from the U.S. EPA’s website,11 and regulatory experts may use them to better understand how the EPA might view a new substance’s risk profile in the absence of data. The first version of the EPI Suite™ provided estimated physical, chemical, and environmental properties based upon the simplified molecular input line entry system (SMILES) notation for a molecular structure, and the current version, EPI Suite™ v4.11, provides similar information using improved models.

Once EPI Suite™ became available, the Product Safety group within what has now become part of Evonik’s Specialty Additives division began to use the new tool to understand how the U.S. EPA might view the hazards of the chemicals they evaluated using these models. Initially, comparisons between predicted and actual ecotoxicity data for existing chemicals were made. However, it was soon realized that there was also the opportunity to use EPI Suite™ to predict the ecotoxicity profile of experimental prototypes. Over the next few years, it became clear that surfactants predicted to have superior environmental profiles (i.e., faster, more complete biodegradation and less toxicity to aquatic organisms) required less time and money to register in the United States and Japan than those with potential EH&S red flags. Therefore, changes were made to the company’s new product development work process to reduce both the time and cost required to develop and commercialize new substances, including new low-foam dynamic wetting agents, in the future.

Continue reading in the March-April digital issue of CoatingsTech.