General Definition
The physical and chemical degradation of wood when exposed to the natural elements and in some cases acid rain. (Not Biodegradation!)
Effects
Factors Affecting the Weathering of Wood
| Energy Form | Indoor Location | Degree of Effect | Outdoor Location | Degree of Effect |
|
|
severe | fire | severe |
| Thermal--slight | darkening | slight | darkening | slight |
| Light--UV and visible | color change | slight | large color changes, chemical degradation | severe |
| Mechanical | wear and tear | slight | wear and tear, wind erosion, surface roughening and defiberization | slight to severe |
| Chemical | staining, discoloration and color changes | slight to severe | surface roughening, defiberization, selective leaching, color changes and strength loss | severe |
Weathering is a surface phenomenom, cell walls on the exposed transverse surfaces are separated at the middle lamella region. Free radicals are generated at the wood surface during irradation. The interactions of the free radicals with oxygen molecules form hydroperoxides. Free radicals and singlet oxygen are important in discoloration and deterioration reactions of wood surfaces.
CHARACTERISTICS OF PHOTOCHEMICAL REACTIONS IN WOOD
1. Lignin is easily degraded by light with a wavelength <350 nm.
2. Effects of light with a wavelength of >400nm on lignin .
3. Methoxyl content of lignin is reduced.
4. Phenoxy radicals are readily produced from phenolic hydroxy groups.
5. Carbon-carbon bonds adjacent to alpha-carbonyl groups are photodissociated.
6. Compounds bearing benzoyl alcohol groups are not susceptible to photodissociation except when photosensitizers are present.
7. Alpha-carbonyl groups function as photosensitizers in the photodegradation of lignin.
PRIMARY REACTIONS OF PHOTOLYTIC DEGRADATION
Cellulose
Lignin
| Component | Exposed Wood (%) | Unexposed Wood (%) |
| Klason Lignin | 1.9 | 20.5 |
| Acid-soluble Lignin | 3.1 | 5.6 |
| Glucose | 82.2 | 49.8 |
| Xylose | 10.1 | 23.3 |
| Mannose | 1.6 | 4.2 |