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.