Adipic acid dihydrazide (ADH) is a room temperature chemical that helps deliver several desirable properties in coatings, such as greater hardness and better chemical and mechanical resistance (like scuff and rub). ADH is used for crosslinking in a wide range of polymers, which increases the molecular weight and stiffness.
While ADH offers room temperature curing and a boost to beneficial coating properties, it has been deemed hazardous to the environment in the European Union and the United States. It is considered a skin sensitizer that causes an allergic reaction, which means products with ADH must display the exclamation point pictogram (above 0.1% in the U.S. and above 1% in the EU). ADH can also be toxic to aquatic environments, making it a challenge when trying to get environmental certifications, such as EU Ecolabels.
When used in certain applications in Europe, only contractors who have been trained in the use of special equipment to apply materials with ADH are allowed to do so. The reality is that regulations like this will continue and will likely become ever more restrictive, which is why there has been a recent push to use ADH-free crosslinking solutions in polyurethane dispersions (PUDs).
There are other crosslinking ingredients to use, but not all ADH-free solutions are created equal. The challenge is finding one that works as fast and effectively while not posing the hazards that ADH does, which might require modifying formulations.
One of the first properties to suffer is chemical resistance. To overcome this drawback, different ingredients, such as other polymers, may need to be back added to assist crosslinking and bring chemical resistance back to desired levels, or it might just take longer to dry.
Because ADH works fairly fast, it works during drying, while the film is coalescing. Some of the other crosslinking options might take longer to link. The challenge is ensuring crosslinking completes before drying does. This is counter to what conventional wisdom might be—the desire for fast drying. If the material dries before crosslinking, the performance properties will suffer.
To extend dry time long enough so that coalescence and crosslinking can continue, a tail solvent might need to be added. In extreme cases, more co-solvents could be needed to slow drying long enough to complete crosslinking.
In other cases, it might require altering the viscosity of the formulation to allow you to apply a greater film thickness with each application. Many properties, including chemical resistance, improve at higher film thicknesses. Care should be taken when doing this as flow and leveling properties may be adversely affected. Additionally, applying higher film thicknesses increase the application cost since you have less surface area coverage per volume of coating.
Going ADH Free
By removing ADH, the polymer you’re formulating becomes a safer material to handle and won’t require displaying the exclamation point pictogram or specially trained contractors to apply it, as is the case in the EU. Lubrizol has extensive experience working with formulators of all sizes, helping them formulate to their needs to achieve performance, simplicity and sustainability.
Contact your Lubrizol account manager to learn more about formulating ADH-free PUDs that are safer and easier to use.