The kinetics of the spruce degradation differs from that of oak. This is not surprising if we
keep in mind that the structural organisation of softwoods and particularly their biochemical
organisation is by several extends different than that of hardwoods. From the rheological
point of view, the important cross-linking of the sprucewood lignin units (no S-units in
softwood, Fig. 7) builds up a rigid network that restricts the chain mobility. Molecular
mobility is unlikely to occur unless numerous bonds are broken simultaneously. Then, the
activation energy required to soften sprucewood is high. This explains that hardwood lignins
soften at a lower temperature than softwood lignins (Fig. 2).
Compared to oak, Table 3 and Figure 5 show that the decreasing of the storage modulus
attributed to the thermolysis of hemicelluloses is less important for spruce. In addition, the
increasing due to the condensation of lignin appears at a lower temperature level (110°C). In
tangential direction, at 110°C, after a decreasing in rigidity of about 20%, the value starts to
increase in time to finally exceed the initial value (Table 3). The ratio E’R/E’T remained quite
high at this temperature level. This could be explained by the fact the ratio at 20°C (near 2.5)
is much higher than the ratio measured for oak (1.4).