Viscoelasticity of Air-Dried or Thermo-Treated Woods

Viscoelasticity of air-dried or thermo-treated Cedrus deodara and Alnus cordata has been evaluated by means of Dynamic Mechanical Analysis (DMA) in the bending mode at 30 °C. E1, E2 and tand have been investigated by frequency-strain spectra. Mechanical properties have been explored by means of creep, stress-relaxation experiments and stress–strain plots at different strain-rates. Relaxation times and Young’s modulus E have been evaluated. The morphology of the samples has been examined by means of optical microscopy. Rheological differences either among air-dried woods, or between airdried and thermo-treated woods, have been observed. In particular, air-dried wood (and especially Cedrus) shows a significant plastic behaviour. In contrast, the thermo-treatment makes the materials mainly elastic and structurally stable. The plasticity of the air-dried woods seems to be due to the shear of the hemicellulose interface and to plasticizers of lignin matrix; the increased elasticity of the thermo-treateds would be due to the additional lignin crosslinking, and loss of hemicellulose and plasticizers. Airdried woods, especially the softwood, have a thermoplastic-like behaviour, where the non-bonded intermolecular interactions prevail, in contrast, the thermo-treated woods are thermosetting-like due to the higher degree of crosslink of the polymeric matrix. The study demonstrates the possibility of characterizing wood through viscoelasticity and dynamic mechanical analysis.