In this paper, the viscoelastic properties of wood under water-saturated conditions have been
investigated from 10°C to 135°C using the WAVET*
apparatus.
Dynamic mechanical tests of wood under water-saturated conditions revealed a transition in
the vicinity of 70 °C to 105°C depending on the wood species, the wood directions and the
frequency. This transition is related to the glass transition of wet in situ lignins. In these tests,
it is expected that the temperature level solely acts as a mean of activation of the viscoelastic
properties. Nevertheless, other tests, performed at constant temperature, proved that, from 80-
90°C, the temperature also alters the material itself. The drop of the rigidity of samples could
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be explained by the degradation of the hemicelluloses. However, above 110-120°C and after
several hours of treatment, the tendency is reversed: the storage modulus increases with the
treatment duration. In addition, the exposure of wood samples to severe temperature (135°C)
for several hours dramatically changes the viscoelastic properties measured at room
temperature. This change seems to be consistent with the condensation of lignins as described
in the literature for this temperature level.
Dynamic mechanical analysis at high temperature under water-saturated conditions allows the
region of thermal activation and thermal degradation to be mapped depending of the wood
species and directions. The collected data and parameters could be used to predict and
simulate the behaviour of green wood in drying, forming and peeling processes. The unique
ability of WAVET
to assess the mechanical degradation during the treatment could also be
applied to define the best parameters for the heat treatment of wood in liquid water
(temperature/ time pathway). The present interest to produce ethanol from biomass is
certainly a good motivation for this application.