Edinburgh Napier University

  Wood is a natural composite polymer containing different amounts of cellulose, lignin and hemicelluloses and some other low molecular compounds (i.e. extractives). Due to its structure wood is susceptible to degradation in the surrounding environment under the action of temperature, humidity, and biological agents.

In this study, thermal treatment at low temperature (up to 140 °C) of spruce heartwood has been applied for longer periods (up to 130 days) in order to simulate the natural ageing. During this treatment, the wood material undergoes changes in the structural composition, especially concerning the amorphous compounds (hemicelluloses and lignin), which further influences its mechanical performances.

In order to identify the structural changes occurring during this treatment infrared spectroscopy, PCA and 2DCOS techniques were applied. Structural changes were further corelated with the non-destructive ultrasound measurements (which provide information concerning the mechanical properties) in an attempt to link the change in microstructure with the gradual (and linear) loss in stiffness.

The samples were periodically removed from the oven, weighed, dimensions measured, and stiffness by impact excitation in the minor axis flexural resonance mode was assessed. At the end of the 130 day period, the mass loss ranged from 1.6% at 110 °C to 13% at 140 °C and stiffness loss ranged from 4% at 110 °C to 29% at 140 °C.

The infrared spectra of the same treatment periods were recorded and they indicated modifications in the regions assigned to carbonyl and carboxyl groups’ stretching vibration and of those assigned to crystalline phase in cellulose, but also of the bands assigned to aromatic ring in lignin. More detailed information was given by principal component analysis (PCA) and two dimensional correlation spectroscopy (2DCOS).