Our previous article examined the possibility of using N-methyl secondary amines as reactive accelerators in epoxy systems. Despite some interesting potential benefits, making these molecules in a pure form cost effectively presents a synthetic organic challenge at a commercial scale.
Accelerators are compounds added to epoxy systems to increase the curing rate, especially when slow reacting amines like polyetheramines form the foundation of an epoxy formulation. Accelerators do exactly what one would expect, but accelerators can also affect properties of the final cured epoxy. Even though this sounds intuitively obvious, the small menu of the relatively well-known accelerators suggest that formulators generally tolerate any unintended consequences on final epoxy properties in favor of desired curing rate.
Commonly used accelerators include benzyl dimethylamine (BDMA), tris(dimethylaminomethyl) phenol (DMP-30), nonylphenol, benzyl alcohol, and triethanolamine (TEA). These accelerators are non-reactive, which means they do not become chemically incorporated into the cured epoxy. As a result, they can migrate or bloom to the surface causing appearance or adhesion issues. In some cases, these molecules act as plasticizers reducing glass transition temperature (Tg). Some even possess odor, color, or regulatory disadvantages.
An alternative approach to modify reactivity would be to replace a non-reactive accelerant with a very fast reacting curative. Some epoxy formulators use Dytek® A amine as an accelerator that becomes permanently bonded into the cured epoxy. This approach not only reduces gel time significantly but can improve properties, such as increase Tg, which may be desirable in some applications.
The following table summarizes data for an Epon® 828 resin cured with various amines. It shows how Dytek® A simultaneously reduces gel time while increasing Tg.
|Amine||Pot Life [hr]||Gel Time [hr]||Onset Tg [°C]||Midpoint Tg [°C]|
|100% Dytek A||0.67||2.32||113.2||117.4|
|50% Dytek® A / 50% D230||0.96||3.20||93.5||97.6|
As a reactive accelerator, the four active (N-H) groups of Dytek® A amine participate in cross-linking reactions with epoxy groups. This means that the amount of Dytek® A amine in the formulation influences stoichiometry, unlike a non-reactive accelerator. When reactive accelerators are used the amount of amine and epoxy in the formulation must be adjusted to maintain proper stoichiometry.
Dytek® A amine can be used in combination with non-reactive accelerators to achieve acceleration, minimize adverse effects of non-reactive accelerators, and eliminate the need to adjust amine-epoxy ratio. Once a base formulation is developed with Dytek® A amine, small amounts of non-reactive accelerators may be added, if necessary, to further tune reactivity without the need to adjust stoichiometry. In this case, relatively small amounts of non-reactive accelerators are used and the impact on final epoxy properties can be minimized.
If you want to discuss this chemistry or learn more about the other projects the Dytek® Aminovation Lab™ is working on, we will be at the 2019 Thermoset Resin Formulators Association annual meeting in Charleston, SC April 8-9.