Crosslinking of Epoxy-Modified Phenol Novolac (EPN) Powder Coatings: Particle Size and Adhesion

By B.D. Pennington, J.C. Grunlan, M.W. Urban

These studies were undertaken to examine how particle size of epoxy phenol novolac (EPN) powder coatings may affect adhesion to metal substrates. Particle sizes of 21 and 83 µm diameter were utilized. DSC analysis shows that the activation energies of crosslinking for the 21 µm particle size is 41 kJ/mol and 58 kJ/mol for 83 µm particle size which is attributed to the effect of particle size, and time-temperature-particle size (TTPS) parameters are used to describe powder-liquid-solid film transformation process.

Although, the TTSP term represents a combination of intrinsic and extrinsic properties. We believe that this is the TTPS term that adequately describes the processes in which, in order for crosslinking reactions to occur, particles must initiate the flow. Quantitative attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopic analysis was used to follow crosslinking processes by monitoring the decrease of oxirane concentration, and showed that for thermal cure at 185°C for 20 min, the oxirane concentration decreases at a similar rate for 21 µm and 83 µm particle sizes.

The results of pull-off adhesion measurements from an Al substrate show that when the 21 µm particle size is crosslinked for 10 min at 110, 140, and 170°C, adhesion is consistently higher than for the same coating system at 83 µm particle size. This difference is attributed to the finite time required for powder particles to reach a proper melt viscosity, followed by reactions of functional groups leading to crosslinking. Extended cure times to 120 min for the 83 µm particle resulted in adhesion similar to the 21 µm particle size.