Phenolated larch‐bark formaldehyde adhesive with multiple additions of sodium hydroxide

This paper aims to evaluate the effect of multiple additions of sodium hydroxide (NaOH) on the properties of bark‐phenol‐formaldehyde (BPF) adhesives, and to lay the foundations for further studies on bark utilisation.

Synthetic technologies that used multiple additions of NaOH were developed for the production of BPF adhesives. Differential scanning calorimetry (DSC), gel permeation chromatography (GPC) and plywood bond were used to evaluate properties of the PF and BPF adhesives.

The number of NaOH additions had an important effect on many BPF adhesive properties, such as gel time, free formaldehyde content in adhesive, thermosetting peak temperature, molecular weight distribution, as well as the wet shear strength and free formaldehyde release of the bonded plywood panels. The study determined that a two‐step process for adding NaOH offers a prospective synthetic technology for BPF adhesive production. This technology made it possible to use 28.6 per cent bark by weight and resulted in plywood with properties comparable with those of plywood bonded with a commercial PF adhesive. However, BPF adhesives prepared with more than two NaOH additions were fast‐curing.

BPF adhesives are very complex systems with many unknown variables, such as the chemical structures of bark derivatives from phenolation and adhesive synthesis. To further improve the curing rate and adhesion of BPF, future investigations should be based on a two‐addition process or attempt to increase the amount of NaOH in the second addition.

The BPF adhesive prepared with two NaOH additions and 28.6 per cent bark was comparable with a commercial PF adhesive in terms of adhesive properties and plywood bond quality. These results indicate that this technology shows potential for commercial applications.

Synthetic technologies using multiple additions of NaOH were developed to produce BPF adhesives. The BPF with two additions of NaOH seemed to be comparable with a commercial PF adhesive.