Hydroxymethylated resorcinol (HMR) primer to improve the performance of wood-adhesive bonds-A review

A hydroxymethylated resorcinol (HMR) primer was developed in the 1990s to improve the performance of epoxy-bonded wood lamellas for use in moist conditions. Its chemical composition resembles a strongly diluted resorcinol-formaldehyde adhesive, which is applied before the actual adhesive. The effectiveness of HMR priming is proven for different wood species and adhesive types based on delamination tests and shear strength measurements. However, the primer is not used in the industrial mass production of glued laminated timber. The reason can be found in additional process steps, therefore costs, and the fact that certain combinations of currently broadly available wood species and adhesives yield proper bonds without the additional use of a primer. Nevertheless, due to ongoing changes in the availability of wood species, it becomes increasingly important to consider the use of currently less-used wood species, including the corresponding adaptations in the production process. Since it is known that the HMR primer improves the bond performance, but is not used due to necessary process changes, it would be interesting to integrate the primer's functionality into existing adhesive systems. This review summarizes and structures the available information on the HMR primer from previous research. Furthermore, that information is critically discussed and a model of functionality is introduced. Finally, it is evaluated if the existing knowledge about the HMR primer is sufficient to transfer its mode of action into existing adhesive systems or which questions need to be addressed to do so.