Thermal degradation of polyurethane wood coating

The thermochemical properties of a solvent-borne two-component polyurethane acrylate based wood coating (PUA) and of a flame-retardant counterpart (PUA-FR) treated with tris(1-chloro-2-propyl)phosphate (TCPP) were studied using within a temperature range between 50 and 600 °C.

Data were analysed regarding structural features of the cured coating material, particularly with respect to its thermal decomposition behavior.

Four-step process

It became obvious that the thermal degradation of PUA occurred in a four-step process in the following order:

• I) Initial degradation of polyacrylate segments connected with the evaporation of the residual solvent.

• II) The cleavage of the polymer backbone of polyacrylate segments leading to the formation of the corresponding acrylate monomers.

• III) The degradation of alkyl methacrylate monomers associated with the elimination of carbon dioxide and the respective alkyl side chains.

• IVa) The dissociation of urethane bonds indicated by a rapid release of isocyanates, cyanuric acid, isocyanic acid and small proportions of hydroxyl-containing moieties;

• IVb) The decarboxylation of urethane-isocyanurate segments under the formation of primary/secondary amines and olefins.

In the case of PUA-FR, steps II) and III) were found to be mainly suppressed by TCPP at the expense of the decarboxylation of the ester side chains while degradation step IVa) was also retarded significantly.

The study has been published in Progress in Organic Coatings, Volume 157, August 2021.