properties. A pyrolysis is a process wherein the material is heated without the
presence of oxygen, but where char production occurs. This destructive result that
affects the properties of wood needs to be avoided during microwave drying.
Oloyede and Groombridge [11] stated that the strength reduction in wood was
60% compared to air-circulation drying when microwave energy was used for
drying. In a later article [36] they have also mentioned that if the microwave
drying of wood is controlled, the process can be much more reliably performed in
terms of quality of the final products. Machado [12] also obtained a clear loss in
compression strength parallel to the grain in clear oak wood when it was exposed
to microwaves. However, it needs to be pointed out that the microwave exposure
was not controlled and this lack of control surely affected the result [37].
The drying method, regardless of whether it is microwave drying or air-circulation
drying, has shown to have no impact on wood strength, at least in Norway spruce,
during controlled drying conditions (Paper I). Further, in Paper III the effect of
temperature level on wood hardness during microwave drying is investigated, as
well as whether the response is different from that of conventionally dried wood.
The results show that drying wood to an mc of 0.12 at drying temperatures of
60°C and 100°C does not affect wood hardness parallel or perpendicular to the
grain differently, regardless of drying method. The same can be concluded for
wood hardness perpendicular to the grain when drying proceeds to mc 0.08 at
drying temperatures of 60°C and 100°C. Nor is wood hardness parallel and
perpendicular to the grain differently affected by the drying temperature, at least
at 60°C, 100°C and 110°C, when the wood is dried by microwave heating to an
mc of 0.08 or 0.12. However, the results show that the there is a significant
difference in wood hardness parallel to the grain between the two drying methods
when the samples are dried at temperature levels of 60°C and 100°C to mc 0.08.
One possible explanation of these results is the drying technique, since the
microwave drying method moves the moisture and the extractive substance by an
internal pressure to the surface. This substance, which accumulates at the ends,
could cause the differences in the measured values. Why this difference does not appear when the wood pieces are dried to mc 0.12 could depend on the drying
time. The wood pieces dried to mc 0.08 are microwave treated much longer, and
therefore more extractives could accumulate in the ends.