Printing wood
As a pressure timber or Buchs (wood) refers to the reaction of wood of conifers that occurs mainly on the bottom obliquely standing trunks or branches and is characterized by the buildup of pressure forces as well as often a reddish tint.
construction
| component | Printing wood | Normal wood |
|---|---|---|
| lignin | 35… 41 | 22… 32 |
| Cellulose | 29… 31 | 38… 47 |
| Galactoglucomannan | 8… 9 | 15 ... 18 |
| Galactan | 9… 11 | - |
| Xylan | 7… 12 | 8… 12 |
| Others | approx 4 | approx. 2 |
The chemical and anatomical structure of pressure wood shows numerous differences to normal logs:
- In the case of printing wood, the proportion of lignin and galactan is increased, but the cellulose and galactoglucomannan content is reduced.
- Intercellular spaces often occur in the cell corners between the tracheids .
- Both the central lamella and the primary wall are less lignified than in normal wood.
- The secondary wall 1 is considerably thicker in pressure wood than in normal wood and can even reach the thickness of the secondary wall 2 in the cell corners.
- The microfibril angle, i.e. H. the angle of the microfibrils to the longitudinal axis of the cell in the secondary wall 2 is extremely large. It fluctuates between 30 ° and 50 °. Within the secondary wall 2, a distinction is made between an outer and an inner area in the case of pressure wood. The outer part has a lower cellulose content and a higher lignin content than the rest of the secondary wall 2. The inner area is traversed by helical radial gaps.
- Usually the tertiary wall is missing in the pressure wood.
- The cell wall thickness of the pressure wood tracheids exceeds that of normal wood (also that of latewood ). In contrast, values that are 20 to 40% lower are observed in the case of cell length.
Printing wood as a guide fabric
As an active alignment fabric, pressure wood enables the stabilization or reorientation of the trunk and branch areas that are exposed to external stress ( earth's gravitational field , snow load, wind). This is due to growth tensions that arise during cell differentiation in the cambium . Two phenomena are responsible for their formation: The crystallization of cellulose fibrils, which causes an axial contraction, and the storage of lignin between the cellulose fibrils, which leads to the formation of compressive stresses perpendicular to the direction of the fibrils and tensile stresses parallel to it. As a result of the flat increase in fibrils in the cell walls of compression wood, these growth tensions lead to the formation of compressive tensions in the direction of the fibers.
Physical Properties
The chemical and anatomical characteristics of printing wood affect its physical properties in a variety of ways:
- Compressed wood differs macroscopically from normal wood in its darker, reddish color and is therefore also referred to as redwood. This is due to the high lignin content of pressure wood and the thick walls of the tracheids.
- The density of the wood increases as the pressure wood increases.
- Due to the high lignin content, printing wood has a lower fiber saturation point .
- Compressed wood has a significantly higher longitudinal shrinkage, but a lower degree of shrinkage in the radial and tangential direction compared to normal wood.
- When subjected to longitudinal tensile loads, the strength and modulus of elasticity of pressure wood is up to 65% lower than that of normal wood, and the elongation at break (in the fiber-saturated state) can be increased by a factor of five or more. The main reason for this is the larger microfibril angle.
- Under pressure, the strength of compression wood is generally higher than that of normal wood. The causes are the increased lignin concentration and condensation and the thick walls of the cells.
Use of wood
The presence of pressure wood usually results in undesirable properties. When felling , the saw may jam , when cutting the wood into sawn timber , the saw block may bend or the saw may slide off. Because of its hardness and brittleness , printing wood is generally considered difficult to work with. The lower load capacity limits the suitability as construction timber. In addition, wood with a high proportion of pressure wood works hard. Pressure wood occurs in the grown structure in close proximity to normal wood and so the deviating swelling and shrinkage dimensions lead to a discarding of the sawn wood when the wood moisture content changes. If the grown wood structure dissolves in the course of the production of cellulose or wood-based materials , the influence of the pressure wood properties decreases. Advantageous use properties arise only in individual cases.
literature
- Rosenthal, M .; Bäucker, E. (2012): Pressure wood - reaction wood of softwoods. Selected properties and essential differences to normal wood fabric.
- Timell, TE (1986): Compression Wood in Gymnosperms. Springer, Berlin, Heidelberg, New York, Tokyo
Individual evidence
- ↑ Explanation of the "characteristics of wood" on Proholz.at
- ↑ Rosenthal, M. (2009): Development of a biologically inspired, three-dimensionally deformable veneer made of printing wood. Diss., TU Dresden
- ^ Richard Kenneth Bamber: A general theory for the origin of growth stresses in reaction wood: How trees stay upright. In: IAWA Journal. January 1, 2001, Volume 22, Issue 3: pp. 205–212 ( PDF ; 3.1 MB, English). Not available on March 10, 2019.
