It is possible to classify (tentatively) three degrees of the wood modification which depend on the MW processing conditions:
- Low Degree - rupturing wooden cell pore membranes, resin melting replacement in channels, partly rupturing tyloses (in hardwood species). Figure 1 shows ruptured tyloses in Paulownia wood after MW processing.
- Moderate Degree - rupturing wooden cell pore membranes, resin boiling and replacement, destroying tyloses (in hardwood species) and rupturing ray cells. Figure 2 shows ruptured ray cells in Radiata pine and Blue Gum wood.
- High Degree - rupturing wooden cell pore membranes, resin boiling and replacement, destroying tyloses (in hardwood species), rupturing ray cells, rupturing main cell (traheids, libriform, parenchyma) walls and walls of vessels, and formation of cavities being primarily in radial-longitudinal planes. Figure 3 and 4 shows wood structure after high degree of MW modification.
Figure 1. Paulownia wood after MW treatment. 1 - wood fiber cells, 2 - vessels, 3 - tyloses in the vessel are broken.
Figure 2. Ruptured ray cells after Moderate degree MW modification.
a - Radiata pine (tangential cut), 1 - rays with ruptures cells, 2 - traheids, 3 - ray with not ruptured cells;
b - Blue Gum (cross cut), 1 - vessels, 2 - narrow white lines are original rays, 3 - wider white lines are ruptured rays.
Figure 3. Mountain Ash wood structure after High degree of MW modification.
Figure 4. Messmate sample after intensive MW modification.
Left side - natural wood,
Right side - High degree MW modified wood - Torgvin
Low degree of MW modification can increase wood permiability in the range of 1.1 - 1.5 times. Moderate degree of modification can increase the permeability of thousand fold. While a high degree of modification can convert wood into a highly porous material with numerous cavities mainly in the radial - longitudinal plane permeability also increases dramatically compared to natural wood. This porous material was named "Torgvin". Torgvin can be produced with modified and unmodified zones by irradiating selected areas of a sample. A Low degree of MW modification only slightly changes wood properties which is difficult to measure taking into consideration wood properties variability. Moderate and High degree of MW modification of the wood structure dramatically changes the physical and mechanical properties of wood.