Paal-Knorr kinetics in waterborne polyketone-based formulations as modulating cross-linking tool in electrodeposition coatings

Highlights: • Waterborne electrodeposition coatings were developed by using a solvent-free and catalyst-free Paal-Knorr curing chemistry • Electrodeposition bath stability and surface homogeneity were improved by employing sterically hindered di-amines for the curing reaction • Uniform and pin-hole free surfaces were obtained on several metal substrates • Electrodeposition bath formulations showed stable performance even after being re-used 21 and 56 days later. Novel cationic resins were synthesized for a direct application in automotive industry. A previous patent protocol with lower bath toxicity (i.e. no catalyst neither harmful compounds were employed), as compared to the current industrial methods based on polyurethane chemistry, was implemented. This includes the use of aliphatic polyketones as cross-linking agents for epoxy-based resins in electro-deposition (EPD) processes. Cross-linking takes place through the Paal-Knorr reaction of the amino groups on the epoxy-based materials with 1,4-dicarbonyl groups present on the polyketone backbone. The reaction kinetics can be finely tuned by the steric hindrance on both components. This was preliminarily investigated by using 2,5,8-nonanetrione as model compound. The used amines (i.e. 4-aminopiperidine (AP) and 4-aminomethyl piperidine (AMP)) display higher steric hindrance with respect to the primary ones used in the original protocol. Application of these components during electro-deposition coating yields the desired high-quality coating layers on several metallic substrates as shown by optical microscopy and image processing. All the produced coatings scored higher adhesion and chemical resistance rating than the amine-based ones used in the previous protocol (i.e. diethylenetriamine, DETA). Afterwards, the same electro-deposition baths showed to have stable performance even after being re-used 21 and 56 days later.