The Influence of Solvent Content in Liquefied Wood and of the Addition of Condensed Tannin on Bonding Quality

Liquefied wood (LW) is a promising natural material that can be used as a part of the adhesive formulation. However, adhesive bonds made of LW only, have low durability. The aim of this study was, therefore, to increase the durability of adhesive bonds containing LW. LW was obtained with liquefaction of black poplar wood in ethylene glycol (EG) as the solvent and sulphuric acid (SA) as the catalyst. An optimal time of 120 minutes and a wood/EG mass ratio of 1:3 was defi ned for liquefaction at 180 °C. After liquefaction, the EG was evaporated in order to achieve a low solvent content LW with a fi nal mass ratio of 1:1. A hydroxyl number for 1:3 and 1:1 LW was determined in order to examine the reduction of hydroxyl groups. Four different adhesive mixtures were prepared: LW with a mass ratio of 1:1 (LW1:1 ), LW with a mass ratio of 1:3 (LW1:3 ), LW with a mass ratio of 1:1 and added condensed tannin (CT) (LW1:1 /CT), and LW with a mass ratio of 1:3 and added CT (LW1:3 /CT). The solid beech wood lamellas, which were bonded with these adhesive mixtures, were tested directly after bonding, and later on, after 7, 30 and 50 days. The test results indicated greater bonding shear strength in the case of LW1:1 compared to LW1:3. The addition of CT did not contribute to essentially higher shear strength values. The adhesive mixtures LW1:1 and LW1:1 /CT (uncured and cured) were analyzed using FT-IR spectroscopy. No significant differences were observed between the cured LW1:1 and the LW1:1 /CT samples.