Thermochemical and isoconversional kinetic analysis of a polyester–epoxy hybrid powder coating resin for wood based panel finishing

Powder coating is an established technology especially for the surface finishing of metallic substrates for example in the automotive industry. Moreover, powder technology holds also great promises for the coating of non-conventional substrates like plastics or wood due to the lack of solvents and good recoverability. Here, low-temperature curing resins are required and especially mild processing conditions are demanded by the substrates. Advanced characterization methods need to be established that allow the precise balancing of the processing conditions required for adequate melting, flowing and curing of the powder with the process conditions that can be tolerated by the temperature-sensitive substrates. In the present contribution it is shown that differential scanning calorimetry (DSC) in combination with isoconversional kinetic analysis (ICKA) provides great potential for this purpose. DSC is a standard thermo-chemical method that can be successfully used to study both the melting and curing processes of powder coatings and to determine, for example the glass transition temperature of the cured coating directly from the measured thermograms. However, still more information can be extracted from the enthalpy signals when more sophisticated methods of data post-treatment and analysis are employed. Isoconversional kinetic analysis techniques such as the Kissinger–Akahira–Sunose (KAS) or the advanced Vyazovkin (VA) approaches allow calculating the time-dependencies of physical and chemical processes at various temperatures based on the estimates of activation energies which are obtained from DSC raw data. These analyses allow for example to calculate the time required for a certain degree of cross-linking in the coating after processing the coating under specified curing conditions. In the present contribution the application of ICKA of DSC measurements for the analysis of the flowing and curing behaviour of a powder coating based on a polyester–epoxy hybrid resin is illustrated and the potential of this approach to predict optimal curing times for arbitrary curing temperatures is demonstrated. This is especially useful when temperature-sensitive substrates like wood-based panels are coated. Additionally, the potential to relate the thermo-chemical properties of the powder coating to the surface properties of the coated substrates is discussed.