Exterior grade plywood adhesives based on pine bark polyphenols and hexamine

Abstract

The aim of this work was to formulate environmental friendly adhesives for plywood production. This adhesive is based on polyphenol extracts from Pinus radiata D. Don bark, the most important by-product from the Chilean forest industry. We focused on a bio-based formulation free of formaldehyde, phenol, and isocyanates. Hexamethylenetetramine was used as hardener. The influence of pH on gel time, rheological behaviour, and pot life was evaluated. The curing process was evaluated by differential scanning calorimetry, Fourier transform infrared spectroscopy, and automatic bonding evaluation system (ABES). For the first time, exterior grade plywood (EN 314) was manufactured with pine bark extracts obtained at pilot scale. Two alternatives for adhesive application were studied: (1) wood veneers activation using a diluted solution of the bio-adhesive, and (2) pine bark extract application as dry powder on the wood veneers. Both alternatives were not reported before and demonstrated an increase in the final plywood quality, as measured by the internal bond strength (IB). On basis of this research, pine bark from Chilean forest industry is an interesting polyphenolic source for the development of adhesives free of formaldehyde and phenol.