Quantification of curing, hardness development, and degradation in epoxy and polyurethane coatings

Epoxy and polyurethane coatings are widely applied to protect metal substrates against corrosion. For adequate anticorrosive properties, these thermoset organic coatings should have a low permeability to corrosive chemicals and excellent mechanical properties. However, an inadequate curing after coating application or a later in-service degradation by aggressive chemicals can considerably deteriorate the coating performance.

In the present project, we investigated the curing and hardness development of organic coatings under insufficient ventilation conditions, as well as the ability of methanol to degrade coating barrier and mechanical properties. The first investigation on the curing behaviour of organic coatings involved a single layer polyurethane coating, where the curing and hardness evolution of the coating were followed under freely evaporating and solvent evaporation-suppressed conditions.

Using a gravimetric approach, the solvent evaporation rate was quantified, while the pendulum hardness test allowed evaluation of the coating hardness and Fourier-transform infrared spectroscopy (FTIR) the monitoring of the degree of curing. The average coating glass transition temperature was estimated by dynamic mechanical analysis (DMA).

As expected, due to solvent build-up in a closed exposure chamber, the amount of residual solvent in the coating increased under the evaporation-suppressed conditions. Higher segmental mobility, caused by the residual solvent, boosted the curing reaction rate, which, conversely, reduced the solvent evaporation rate with an associated higher amount of residual solvent.

The latter was observed to reduce the coating hardness and glass transition temperature due to a plasticizing effect. The so-called Kelley-Bueche equation successfully simulated the coating glass transition temperature as a function of the polyurethane volume fraction, which verified the additivity of free volume of the residual solvent to the coating matrix.

Using the same characterization methods as the single-layer polyurethane coating, the curing of a two-layer epoxy-polyurethane coating system was mapped from the aspects of solvent evaporation, isocyanate conversion, and hardness evolution. Under the evaporation-suppressed conditions, the amount of residual solvents in the epoxy-polyurethane coating system increased.

Compared to the single-layer coating system, interlayer solvent migration took place and had a great influence on the hardness development of the polyurethane topcoat. In particular, the residual 1-butanol in the epoxy primer migrated to the topcoat and reacted with the isocyanate reactants. The reduced crosslinking density, as a consequence of this undesired reaction, and the plasticizing effect of residual solvents, together resulted in an insufficient hardness development of the epoxy-polyurethane system.

Additionally, a kinetic study on the reactivity of 1-butanol with the isocyanate crosslinker showed a two orders of magnitude higher reaction rate than for the polyol reactant in the polyurethane system, which verified the adverse effects of the 1-butanol migration. Furthermore, 2-butanol, with a lower reactivity towards isocyanates, was substituted for 1-butanol in the epoxy primer, which significantly improved the hardness development of the epoxy-polyurethane coating system.

In addition to the curing study, methanol degradation of novolac epoxy and polyurethane coatings was investigated using methanol absorption and desorption experiments, supported by DMA studies for monitoring of the mechanical properties. Also, permeation cells were used to evaluate the coating barrier properties (i.e., breakthrough time and permeation rates of methanol).

During methanol absorption, physical degradation, i.e., leaching of certain coating ingredients and interaction of methanol with the coating network via hydrogen bonds, was evidenced, and the bonding of methanol classified into two types. The first, Type I bound methanol, refers to a single hydrogen bond forming to the coating network, whereby methanol performs as a plasticizer, which decreases the storage modulus and the glass transition temperature.

The second, Type II bound methanol, takes place when two hydrogen bonds form to the coating network, resulting in so-called physical crosslinking. Generally, Type I bound methanol boosted the segmental mobility and contributed to the leaching of a plasticizer, benzyl alcohol, from the novolac epoxy coatings and residual solvents (i.e., naphtha and xylene) from the polyurethane coating.

Following methanol desorption, epoxy novolac and polyurethane coatings both exhibited significant increases in their glass transition temperature, which was attributed to an increased effective crosslinking density from Type II bound methanol. In addition, a gradual decline in permeability of methanol was observed over time for the three coatings.

These enhanced (and unexpected) barrier properties result from a combination of effects from the forming of Type II bound methanol and the leaching process. In summary, the curing, hardness evolution, and interlayer molecular migration in epoxy and polyurethane coatings were quantified under evaporation-suppressed conditions.

The results give insights into the negative effects of the limited ventilation conditions and provide guidelines for how to optimize ventilation conditions and coating formulations to achieve adequate curing. Moreover, the methanol degradation study elucidates the mechanisms of small hydroxyl group-containing solvents interacting with the coating network, as well their influence on the coating mechanical and barrier properties.