Which diamine reacts faster with phenyl glycidol ether?
When measuring epoxy hardening rates, the curing profile curve suggests an elegant change from liquid to solid as the system cures. What occurs chemically is much more complex. With a primary amine curative, the initial amine-epoxide reaction plays a big role in establishing the overall curing rate. However, in-situ chemical and physical changes also influence the overall kinetics of the curing mechanism. These factors include the kinetics of the formed secondary amine, the catalytic effects of the tertiary amine, and the effects of the secondary hydroxyl group resulting from the initial epoxy ring opening reaction. Fortunately for us, lots of literature already exists on the kinetics of epoxy curing reactions. Despite this complexity, people generally accept that the nucleophilicity of the amine plays an important role in determining the reaction kinetics in amine-epoxide reactions.
Short chain unhindered primary diamines typically set the benchmark for fast reactivity. Methods to make a primary diamine a stronger nucleophile typically adds steric hindrance negating any increase in nucleophilicity. If an epoxy formulation requires curing speed above what an unhindered primary diamine offers, formulators turn to accelerators or catalysts. These additives include phenol, Mannich base derivatives, tertiary amine bases, or strong acids. These methods may be undesirable due to toxicity, corrosiveness, compatibility with the rest of the formulation, or deleterious effects on the final physical properties. An ideal solution would be to increase the nucleophilicity of the amine without adding too much steric hindrance to offset the increased nucleophilicity.
In 1999 Air Products and Chemicals, Inc. published a paper in the Journal of Polymer Science investigating the use of N-Methyl secondary amines to accelerate amine epoxy reaction (Reference Below). They studied the potential to increase the reactivity of primary amines by converting them to the corresponding N-methyl secondary amines. Their experiments measured relative rates of reaction of a series of amines with phenyl glycidol ether (PGE) under pseudo-first-order conditions – a 10-fold excess of PGE.
What they found was a consistent increase in reactivity rate when going from a primary aliphatic amine to the corresponding secondary N-methyl amine. For example, N,N’-dimethylethylenediamine (DMEDA) was 50% more reactive than ethylenediamine (EDA).
This research suggests that the electron donating effects of the added methyl group increases the nucleophilicity of the amine and outweighs the added steric hindrance from the methyl groups.
To see how these laboratory kinetic results translated to formulated epoxy systems, the study team tested partially methylated polyamine mixtures in a standard liquid epoxy resin (Epon® 828) coating. The researchers used partially methylated polyamines because of the inherent difficultly in synthesizing the pure N,N’-dimethyl secondary amine products. The results indicate that even the partially methylated polyamine mixtures had an acceleration effect on the Thin-Film Set Time (TFST) at 5°C and 25°C compared to the corresponding non-methylated amine. The authors go on to note that the epoxy system with the partial methylation also gave a smoother surface. Their final finding surprisingly showed that the gel time was unaffected by the methylated amine even though total curing was faster. This technology could maintain an applicator’s valuable working time while decreasing total cure time.
Unfortunately, commercially available sources for such amines are limited. The N,N’-dimethyl secondary diamine derived from hexamethylenediamine (HMD) is commercially available and registered in many parts of the world. A similar product based on Dytek® A is not registered at this time, but we would expect it to have a lower viscosity than the HMD analog and to give a very different curing profile and different cured epoxy physical properties.
Selectively adding a methyl group to a primary amine is difficult, but we think this is an interesting technology because partially methylated mixtures are still effective. Pure secondary methylamines may not be required, which would make this technology more accessible. This is another technology our Aminovation Lab™ is currently investigating.
To discuss this chemistry in more detail please contact us at Dytek@INVISTA.com or post your comments directly in the comments section.
Reference: Marsella, J. A. and Starner, W. E. (2000), Acceleration of amine/epoxy reactions with N‐methyl secondary amines. J. Polym. Sci. A Polym. Chem., 38: 921-930.