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Colour changes due to heat treatment
The wood colour after the first treatment stage (hydro- thermolysis) varied from light to dark brown, caused by the formation of quinones , or the caramellisation of holocellulose components (described in Chapter 5). An increase of the treatment temperature changed the colour into a darker tinge. Most of the colour changes occurred during the hydro-thermolysis, whereas treatment of wood specimens without the first treatment stage (only the curing) resulted in a light brown colour, much lighter than after the hydro-thermolysis. The colour change also depends on the timber used and is correlated to the density of the wood since the colour is becoming darker with an increasing density.
The colour of the wood surface is slightly darker than the colour of the inner wood giving a lighter colour after planing. During heat treatment, extraction and/or diffusion of dark brown reaction products occurred and were depleted at the wood surface. Some timber species with a high resin content like Scots pine showed resin spots on the surface after treatment. Dipping treated wood specimens in water resulted in an emission of a brown component, indicating a polar characteristic. This emission has also been noticed when treated timber was painted with waterborne paint systems, resulting in light brown spots in the paint layer. This was not observed for solvent borne paint systems.
During the first microscopic analysis of several wood species after the hydro-thermolysis treatment stage it was noticed that the colour of the cell wall changes from white to brown. This colour change involved the compound middle lamella and the secondary cell wall. This is believed to be due to the formation of reaction products in the cell wall or to the diffusion of such reaction products within the cell wall.
Light microscopic analysis
Unfortunately it was not possible to prepare light microscopic slices using an embedding method. However, after the hydro-thermolysis stage the wood structure is still rather soft and not brittle, and it is not expected that this will lead to slicing artefacts. The typical effects of the hydro-thermolysis treatment on the anatomical wood structure are discussed below. Due to an apparent decrease of the visual wood quality (cracks, collaps and/or deformation), no light microscopic observations were made after the hydro-thermolysis treatment of fresh (or pre-soaked) timber.
Liquid full hydro-thermolysis treatment of shipping dry softwood
The liquid full hydro-thermolysis treatment of shipping dry Scots pine and Norway spruce reveals tangential cracks in the latewood section . Both timber species were characterized by very narrow annual rings (approximately 1 mm per annual ring) and an abrupt transition from earlywood into latewood. During treatment large stresses must have occurred between the earlywood and latewood tracheids due to differences in shrinkage/swelling behaviour, resulting in these tangential cracks. In Norway spruce specimens with wider annual rings (2.5-3 mm per annual ring) and a gradual transition from earlywood to latewood, no tangential cracks were observed .
Table of contents :
1 Introduction
1.1 Wood modification
1.2 The Plato treatment
1.3 Objectives and limitations
PART I MOLECULAR AND PHYSICAL CHARACTERISATION OF HEAT TREATED WOOD
2 Microstructural and physical aspects of heat-treated softwoods
2.1 Introduction
2.2 Materials and Methods
2.3 Results and discussion
2.4 Conclusions
3 Microstructural and physical aspects of heat-treated hardwoods
3.1 Introduction
3.2 Materials and Methods
3.3 Results and discussion
3.4 Conclusions
4 A solid-state 13C-NMR study of heat-treated wood
4.1 Introduction
4.2 Materials and Methods
4.3 Results and discussion
4.4 Conclusions
5 Chemical analysis of heat-treated softwoods
5.1 Introduction
5.2 Materials and Methods
5.3 Results and discussion
5.4 Conclusions
PART II PROPERTIES OF HEAT-TREATED WOOD
6 Durability of heat-treated wood against fungal decay
6.1 Introduction
6.2 Materials and methods
6.3 Results
6.4 Discussion
6.5 Conclusions
7 Correlation of 13C-NMR analysis with fungal decay tests: Basidiomycetes
7.1 Introduction
7.2 Materials and Methods
7.3 Results and discussion
7.4 Conclusions
8 Correlation of 13C-NMR analysis with fungal decay tests: Ground contact tests
8.1 Introduction
8.2 Materials and methods
8.3 Results and discussion
8.4 Conclusions
9 Strength properties of heat-treated softwoods
9.1 Introduction
9.2 Materials and methods
9.3 Results
9.4 Discussion
9.5 Conclusions
10 mechanical properties of heat-treated full size construction timber
10.1 Introduction
10.2 Materials and methods
10.3 Results
10.4 Discussion
10.5 Conclusions
Summary of Part II
PART III INNOVATIVE APPLICATIONS OF THERMAL MODIFICATION
11 Semi-isostatic densification of heat-treated radiata pine
11.1 Introduction
11.2 Materials and methods
11.3 Results
11.4 Discussion
11.5 Conclusions
12 Vibration welding of heat-treated wood
12.1 Introduction
12.2 Materials and methods
12.3 Results and discussion
12.4 Conclusions
13 The effect of heat treatment on the properties of particleboard
13.1 Introduction
13.2 Materials and methods
13.3 Results and discussion
13.4 Conclusions
14 Utilisation potential and perspectives
14.1 Material properties
14.2 Dimensions and wood quality
14.3 Environmental profile
14.4 Market aspects
Reference list
