Triethylamine—often abbreviated as TEA or Et3N—is a colorless, volatile liquid used in creating polyurethane dispersions (PUDs). It is commonly employed in organic synthesis as a base for a variety of applications, such as a catalytic solvent in chemical syntheses, a corrosion inhibitor, and a curing and hardening agent for polymers.
In PUDs, TEA functions as a counter ion to the acid in the polymer backbone, allowing for the formation of polymer particles in water. It accomplishes this by ionizing the carboxylic acid in the polyurethane prepolymer. The TEA neutralizes the acid in the prepolymer, forming an ionized acid-base pair that makes it dispersible—without being ionized, it would not have the required affinity for water necessary for creating a polymeric dispersion.
However, TEA has recently faced challenges and regulatory pressure due to its hazardous vapors. Even short-term exposure to the vapors can cause severe irritation to the eyes and skin. In addition to its human health concerns, TEA is considered a volatile organic compound (VOC) and has an unpleasant, potent odor that is sometimes described as the smell of expired fish.
While TEA does offer some unique advantages for the production of PUDs, TEA-free PUD formulations can be just as effective with the right formulation changes—without hazards to human health.
One alternative to TEA is dimethylethanolamine (DMEA). It requires a bit more attention in the processing of PUDs because DMEA has a reactive alcohol attached (ethanol substituent). That means it needs to be added to the water phase, unlike TEA that can be added directly to the prepolymer. DMEA also has a higher boiling point (less volatile) than TEA, so it takes longer to leave the coating, which can reduce the chemical resistance of the coating when tested. DMEA can provide a colloidally satable particle size and dispersion; in large part due to its ability not to shock the prepolymer when its added to the water. However, it is still a relatively undesirable VOC with a poor odor profile.
Inorganic or metal bases, such as sodium and potassium hydroxide, can be used as a TEA alternative for ionizing acid substituents on polymers. However, these materials can be difficult to work with and can negatively impact final PUD qualities. Because metals such as sodium form such strong bases, they tend to shock or agglomerate the prepolymer as it is added to water, resulting in a gritty/“dirty” dispersion of poor quality that contains polymer that is not stable in water and result in sediment in the container. Moreover, metal bases such as sodium are permanent or non-volatile and may be expected to negatively impact coating performance in terms of poorer water resistance. However, they will not contribute to the VOC emissions of the coating, improving the sustainability of the product.
Another alternative is ammonia, which is commonly used to adjust the pH or create ionized acrylic emulsions. However, it is difficult to use to prepare PUDs due to its high level of reactivity with isocyanates. Without taking special precautions, ammonia will react with the isocyanate functional prepolymer during the dispersion process and convert the isocyante into a urea. The resulting urea will not react further to build the molecular weight of the prepolymer. Polymers need to be a certain molecular weight to function properly and the use of ammonia will likely result in a lower molecular weight polymer. If the molecular weight becomes too low for the final product, it can impact final performance, possibly to such an extent that the product no longer behaves like a solid, but more like a liquid. In other words - a tacky material that is not useful as a protective coating. Thus, ammonia, or any amine with a hydrogen attached, would be problematic to use to disperse an anionic water dispersible polyurethane containing active isocyanate for creating a high molecular weight polymer.
Nonionic dispersing groups can be used to make a PUD and avoid using TEA or the need for any other counter ion. However, these types of polymers are typically inferior to anionic PUDs as protective coatings. This is due to the permanance of the water soluble nonionic dispersing material, as well as the relatively large amounts needed to produce polymeric dispersion of a particle size required for a stable dispersion in water. This results in a coating with significantly higher sensitively to water and other polar solvents compared to that obtainable from an anionic dispersion.
Synthesis without TEA can be more challenging, but from an application standpoint, it doesn’t change how the PUR could be used. However, properties such as chemical resistance, hardness and film formation can be negatively impacted should the formulation or composition not be as robust as that when using TEA-based PUDs.
From an applications standpoint, formulating a TEA-free PUD is no different than formulating one that contains the material. However, should a formulator change from a TEA-containing PUD to a TEA-free alternative, they should reevaluate other raw material choices, particularly co-solvent selection. Solvent choice is integral to film formation, which in turn will impact coating performance, i.e. hardness, barrier properties, and chemical resistance. If a decrease in performance in these areas is observed, solvent loading should be reoptimized to restore performance.
When evaluating a TEA free PUD for the first time, normal formulation methodology should be followed. Materials should be screened individually for compatibility and effectiveness with the resin. The best raw materials from each screening study should then be combined into an initial coating formulation for testing. Based on those results, the formula is refined (optimized) and retested. This iterative process continues until the desired performance is obtained.
Where can you turn when formulating a TEA-free PUD for performance, simplicity and sustainability? Lubrizol can help with a starting point, collaborating on tailor-made solutions for your specific needs or just providing advice. Lubrizol has many years of experience delivering high-performing, easy-to-apply products to formulators of all sizes that are key ingredients for durable water-borne coatings.
Contact your Lubrizol account manager to learn more about options for TEA-free PUD formulations.