Chemical and ultrastructural aspects of thermally modified wood with emphasis on durability
Sammanfattning: The Termovuoto (thermo-vacuum) process is an environmental friendly industrial approach to modify wood by combining efficient vacuum drying and thermo-treatment. In this thesis, chemical and ultrastructural aspects of two softwoods (spruce and fir for 4 h at 160−220℃) and two hardwoods (ash and beech for 3 h at 190−220℃) thermally modified using the Termovuoto process were studied by light- and electron microscopy with emphasis on durability. Histochemical staining indicated an increasing amount of acidic groups in thermally modified woods (TMWs), particularly in the compound middle lamella (CML) including middle lamella cell corner (MLcc) (CMLcc) regions of TMWs treated at 220℃ (TMW220℃). TEM observations showed significantly increased KMnO4 staining intensity of lignin in TMW220℃, presence of electron dense particles in CMLcc regions of softwood TMW220℃, and large lignin aggregates and disordered lamellar structure in the fibre S2 layer of hardwood TMW220℃. The durability of TMWs against two white rot (Phlebia radiata, Pycnoporous sanguineus)-, two brown rot (Postia placenta, Gloeophyllum trabeum)- and three soft rot (Chaetomium globosum, Phialophora mutabilis, Phialophora malorum) fungi was evaluated by the soil block test. For brown- and white-rot fungi, Termovuoto treatment showed considerable improvement in durability class (i.e. class 1−3) for soft- and hardwoods at 220℃ against all fungal species tested. Softwood TMWs showed an overall lower decay resistance than hardwood TMWs, among which ash TMWs showed greater durability than beech TMWs. For soft rot fungi, softwood TMWs were more durable than hardwood TMWs, irrespective of fungal species. Ash showed lower durability than beech in untreated reference wood, while ash TMWs showed greater durability than beech TMWs during one year decay test. Behavior of thermal modification (TM) differed significantly between ash (ring-porous hardwood) and beech (diffuse-porous hardwood) against brown-, white- and soft-rot fungi, indicating importance of the native wood anatomy. Decay patterns and morphological changes of TMWs were examined by light- and electron microscopy. The white rot fungus P. sanguineus did not show significant differences in characteristic features of decay in tracheids and fibres of TMWs compared to those in untreated reference. However, the delignification process in tracheids and fibres by P. sanguineus was delayed in TMWs, particularly at high treatment temperatures as evidenced by narrower transition zones from delignified and lignified areas than untreated reference. The soft rot fungus P. mutabilis produced typical soft rot Type-I cavities in fibres of hardwood TMWs at low temperature (190−200℃). However, soft rot cavity formation was greatly inhibited and/or delayed in fibres at high treatment temperatures (i.e. 210−220℃). Ash TMW200℃ showed a radial-like distribution of electron dense materials in cavities and lack of fibrillar-like materials within degraded fibre walls, which differed from reference. The fungal durability of Termovuoto TMWs differed in terms of treatments, wood and fungal species. The Termovuoto process did not change the patterns of decay caused by white-, brown- and soft-rot fungi used, but rather slowed down the decay process at certain treatment temperatures for certain wood species. Understanding of the decay patterns in TMWs is essential for further optimization of the Termovuoto process for improving the durability of specific wood species for specific purposes.
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