Crosslinking of epoxy-modified phenol novolac (EPN) powder coatings: Particle size and adhesion

These studies were undertaken to examine holy particle size of epoxy phenol novolac (EPN) powder coatings may affect adhesion to metal substrates. Par ticle sizes of 21 and 83 mu m diameter were utilized. DSC analysis shows th at the activation energies of crosslinking for the 21 mu m particle size is 41 kJ/mol and 58 kJ/mol for 83 mu m particle size which is attributed to t he effect of particle size, and time-temperature-particle size (TTPS) param eters are used to describe powder-liquid-solid from transformation process. Although, the TTSP term represents a combination of intrinsic and extrinsi c properties. We believe that this is the TTPS term that adequately describ es the processes in which, in order for crosslinking reactions to occur, pa rticles must initiate the flow. Quantitative attenuated total reflectance ( ATR) Fourier transform infrared (FTIR) spectroscopic analysis was used to f ollow crosslinking processes by monitoring the decrease of oxirane concentr ation, and showed that for thermal cure at 185 degrees C for 20 min, the ox irane concentration decreases at a similar rate for 21 mu m and 83 mu m par ticle sizes. The results of pull-off adhesion measurements from an Al subst uate show that when the 21 mu m particle size is crosslinked for 10 min at 110, 140, and 170 degrees C, adhesion is consistently higher than for the s ame coating system at 83 mu m particle size. This difference is attributed to the finite time required for powder particles to reach a proper melt vis cosity, followed by reactions of functional groups leading to crosslinking. Extended cure times to 120 min for the 83 mu m particle resulted in adhesi on similar to the 21 mu m particle size.