CHEMICAL ALTERATIONS OF HARDWOOD VENEERS DUE TO THERMAL TREATMENT

Abstract

Wood is the most popular building material in the world due to its universal versatility, although it has disadvantages - the difficulty to apply small diameter logs in construction, hygroscopicity and anisotropic swelling and shrinking. To solve these disadvantages, plywood from wood material is produced. Plywood is a material that can solve anisotropy, but it is still biodegradable by rot and stain fungi. Thermal treatment is a methodology that improves the durability of wood. In this paper aspen (Populus tremula L.), poplar (Populus x canadensis Moench) and birch (Betula pendula Roth) were treated by steam (WTT) and vacuum (TERMOVUOTO) devices under 160°C/50 min (birch and aspen), 204°C/2 h, 214°C/2 h, 217°C/3 h, 218°C/30 min (birch and poplar). Chemical changes in treated veneers were investigated by ATR-FTIR (Attenuated Total Reflection Fourier Transform Infrared Spectroscopy) in a range 2000 cm-1 – 800 cm-1. ATR-FTIR is a non-destructive methodology, which is important during manufacturing process quality control. Untreated poplar wood and aspen wood had similar ATR-FTIR spectra because both species belong to Populus genus. Untreated birch wood had higher absorption intensity peak at 1740 cm-1, which indicates the C=O bond stretching in the carboxyl group depicting more acetyl groups in birch wood than in aspen/poplar. According to spectral data, birch wood, treated in TERMOVUOTO process at 200°C for 2 hours is chemically almost identical to untreated one. WTT process causes the most significant changes in the chemistry of both in aspen and birch. Therefore, regime 160°C/50 min in water vapour is more aggressive than treatments at higher temperatures and under reduced pressure. It is expected that plywood produced from WTT treated veneers will have reduced strength in comparison with TERMOVUOTO process produced ones.