Effects of veneer drying at high temperature and chemical treatments on equilibrium moisture content of plywood

Ismail Aydin

doi:10.4067/s0718-221x2014005000036

Abstract

Moisture content (MC) is one of the most important factors that can affect many physical and mechanical properties of wood and veneers. MC strongly affects the final strength and durability of joints, development of surface checks in the wood and dimensional stability of the bonded assembly. In this study, plywood panels made from Alnus glutinosa (Alder), Fagus orientalis (Beech) and Picea orientalis (Spruce) wood veneers were treated with ammonium acetate, borax and boric acid by using dipping method to test for their equilibrium moisture content (EMC). Borax and boric acid solutions were used for fire-retardancy, and ammonium acetate for reducing formaldehyde emission. Two different veneer drying temperatures (20°C and 180°C) were used to reveal the effects of veneer drying at high temperature on the hygroscopicity of plywood panels. EMC values of alder, beech and spruce panels decreased significantly with veneer drying at high temperature. Picea orientalis (Spruce) plywood panels had the highest EMC values and followed by Alnus glutinosa (Alder) and Fagus orientalis (Beech) panels. EMC values of plywood panels increased significantly for both drying temperatures after ammonium acetate, borax and boric acid treatments.

Highlights

Plywood is a very strong and durable composite panels made from three or more wood veneers bonded together with an adhesive
The highest average equilibrium moisture content (EMC) value (11,79%) was obtained for spruce plywood panels manufactured from veneers dried at 20°C, while beech panels manufactured from veneers dried at 180°C had the lowest average EMC value (9,14%)
The lowest EMC in Alder, Beech and Spruce plywood panels were obtained for untreated control, while the highest values were obtained for panels manufactured from boric acid-treated veneers for air-dried groups and borax-treated veneers for oven-dried groups (Figures 2, 3 and 4)

Summary

Introduction

Plywood is a very strong and durable composite panels made from three or more wood veneers bonded together with an adhesive. Wood and wood based materials such as veneer, plywood and lumber have a hygroscopic nature and they have the ability to release or absorb moisture to reach a moisture content that is at equilibrium with its surrounding environment. This condition is called the equilibrium moisture content (EMC). Wood has a distinct EMC for every temperature and humidity condition, and given enough time at a constant exposure, a piece of wood will reach a given EMC. The EMC condition will typically vary between 5 and 8% Local microclimates, such as coastal conditions, can change these average values somewhat (Youngquist 1999)

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