Formulators Forum—Real Michael Addition in Powder Coatings

Lower temperature cure (LTC) application requires more than increasing the reactivity of the crosslinking chemistry. Formulating systems with high reactivity brings production challenges since extrusion temperatures are not far below the target cure temperature. This can also impact storage stability. A key issue is the rapid increase of melt viscosities at lower temperatures and the resulting reduced paint flow, impacting film formation and appearance.

Real (or Carbon) Michael Addition (RMA) chemistry, patented by allnex, presents new and exciting options for LTC powders. It features a nucleophilic addition reaction between an acidic C-H (the Michael donor) and an electron deficient C=C unsaturated bond (the Michael acceptor) to create a C-C bond, catalyzed by a strong base catalyst able to deprotonate the Michael donor. The RMA reaction, when catalyzed, can be very fast even at room temperature, but there is essentially zero reactivity without an active catalyst. Michael donor groups include malonates, acetoacetates, and cyanoacetates; Michael acceptor groups can be acrylates, methacrylates, fumarates, maleates, and itaconates. Binders for powder coatings containing these functional groups can be designed with various equivalent weights, functionalities, and types of backbones (polyester, urethanes, epoxy, acrylic). Combinations of donor and acceptor resins will rapidly react when exposed to a strongly basic catalyst.

The catalyst system enables control over the reactivity in time. This multi-component system consists of a catalyst precursor, an activator, and a retarder. The catalyst precursor is not basic enough to trigger the RMA reaction, but at elevated temperature, it can react with an activator component, e.g., an epoxy, to form a strongly basic adduct able to initiate the RMA reaction. However, if a carboxylic acid retarder is present, it will neutralize this species and re-form a carboxylate. The result is that until all retarder acid is consumed, no active strong base catalyst will be present, and the onset of the Michael Addition cure is delayed to that moment in the cure cycle. In this way, a delayed curing profile can be created, and the delay time controlled by the amount of acid retarder in the catalyst package, as a ‘chemical clock.’ Impacts of a delayed cure profile include mitigation of premature crosslinking during the extrusion process, and the creation of a window of high fluidity in the cure cycle, resembling flow and cure stage decoupling offered by UV-powder systems.

The appearance of powder coatings depends on application conditions such as curing temperature, heating rate and layer thickness, and on the flow and leveling properties of the powder. In addition, appearance at a given layer thickness will improve with increasing paint flow (PF). The delayed curing profile achieved through the catalyst system can maximize PF within a given curing window.

In a conventional powder coating, the crosslinking reaction starts immediately once the cure temperature is reached and conversion increases with time, reaction speed being highest at the start. This type of powder has high fluidity at the onset of cure but drops sharply due to the crosslinking reaction, resulting in increased viscosity. In RMA powders there is no crosslinking reaction during the induction time, but crosslinking conversion is rapidly achieved toward the end of the cure window. Therefore, RMA powders maintain high fluidity for a longer time and have a significantly higher PF (see right).

With the delayed curing profile, premature crosslinking during extrusion can be also avoided. Therefore, no adjustment in extrusion conditions is required for making highly reactive RMA powder coatings. In fact, preparing RMA powder coating formulations designed to be cured at 100 degrees Celsius using an extrusion temperature of 95 degrees Celsius has been routinely achieved. The powder can still have an induction time, indicating no premature Michael Addition reaction occurring. The highly reactive RMA chemistry offers very fast curing, and crosslinking can be completed within a few minutes even at 100 degrees Celsius.

RMA chemistry enables powder coatings that can deliver LTC, good workability, improved appearance, weather resistance, and matte finish options, and will open new opportunities for application to heat sensitive substrates. Further development of the technology will likely result in broad binder design options coupled with freedom in formulation to finetune performance. Functional crystalline components that can further assist in significantly reducing melt viscosity and enhancing flow are also under development.

Cal EzeAgu is technology manager Americas, PCR at allnex USA Inc.