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Wood as renewable resource

Wood has been used for thousands of years, serving for many kinds of human activities. As the main component and act as construction material in trees, wood serves as transporting material between root and leaves. Through vascular cambium, wood is formed. Constructed by interconnected complex biopolymer composites of cellular structure inside, it has high mechanical strength at low weight level. Wood is known as a biodegradable, unique, recyclable, and versatile material. As a biodegradable material, wood can be decomposed by certain organisms such as fungi, termites, and bacteria. As the unique and versatile material, wood has unique physical properties that can be useful for many applications such as a thermal isolator, shock absorber, air pollution absorber, aesthetic, natural, etc. Through its chemical properties, wood can be used for many applications such as for producing many kinds of organic chemicals (cellulose, furfural, acetic acid, benzene-based chemicals, etc.), paper, bioenergy, and nanotechnology applications (nanocellulose and its derivatives). Some wood products and its derivatives can be recyclable, wood particles or flakes from sawmill wood industry or wood-based furniture can be used for making particleboard, fiberboard, paper, paper boards, etc.
Unlike fossil fuels, wood is also known as a renewable material. The renewable sources become one of the most efficient and effective solutions for environmental and resource crisis recently and in the future. The so-called “renewable” for wood as a product from the forest can be qualified if the rate of its utilization/harvesting is the same or lower than the rate of its growth. A good implementation of this role would have sustainable results that can serve for many years to come. On the way to minimize the rate of its exploitation/harvesting, some technologies are needed to recycle well the used wood-based products to become the newest products or for other certain applications. Other techniques, including an expansion of its serving time during utilization, are needed through some improvements of its properties corresponding to its destined applications.

Anatomical structure of wood

Cross section of wood

The cross section of wood (the transverse plane section) figures out the information about the features from the pit-to-bark (radial direction) and from the circumferential direction (tangential direction). The radial plane provides information about longitudinal changes in the stem from the pit to the bark along the radial system. Whereas, the tangential plane provides information about the tangential dimension of features (Rowell 2012).
Bark consists of two main parts, an inner bark and an outer bark. The inner bark comprises a secondary phloem/living phloem (transfers nutrients from the leaves to the rest of the tree) and cork cambium. Cork cambium and outer bark layers are also dubbed together as periderm. The outer bark functions as the protector for the inner bark (phloem) from outside attacks, disease, and drought (Rowell 2012).
Vascular cambium is a thin layer lies between the inner bark and the wood (secondary xylem). It is responsible for producing wood to the inside, and the bark (secondary phloem) to the outside of the tree (Larson 1994). Vascular cambium creates the axial and radial systems in the tree by means of two types of cells: the ray initial and the fusiform initial. Fusiform initial gives rise in the axial system, whereas ray initial gives rise in the radial system. Thus, there is a direct and continuous connection between the vascular cambium, the inner bark, and the most recently formed wood (Rowell 2012).
The sapwood is part of the tree where parenchyma cells (the living cell at maturity and metabolically active) is located, found both in softwood and hardwood. Sapwood is usually a lighter-colored wood adjacent to the bark. Heartwood is an older xylem that has been penetrated by gums and resins. Heartwood is usually the darker-colored wood located in the interior of sapwood, denser and more resistant than sapwood. Pith is the remaining of the initial growth trunk before the formation of wood (Rowell 2012).
Vascular cambium produces the wood at a time through its one layer of cell divisions, but in general many woods are formed approximately together in time, as known that woods comprise a large group of cells. The compilation of those produced cells over a period of time interval is called growth ring. Earlywood (springwood) cells are formed at the beginning of the growth ring. Whereas, latewood (summerwood) is the cell in the latter portion of the wood increment.

Structural difference between softwoods and hardwoods

Softwood structure is relatively simple. Axial tracheid is mostly constructed in the axial/vertical system. They serve for both conductive and mechanical functions in wood. Axial parenchyma is another cell type that sometime exist. In some softwood species, there is structure called resin canals. Compared to hardwoods, it is structurally more complicated than softwood. Constructed by various kinds of fibrous elements, vessel elements in various size and arrangements, and parenchyma cells in various patterns and abundance are composed in the axial system of the hardwoods (Table 1).

The wood cell wall

The tree species, the location at where the trees growth, and the location at where wood samples are taken in the tree would influence the content of the wood cell wall components. Hardwoods are different from softwoods, sapwood from heartwood, and earlywood from latewood. In the latewood, glucomannans contents is higher than in the earlywood, but glucoronoarabinoxylans contents are higher in the earlywood. On the other hand, extractives are higher in the heartwood than in the sapwood. There are three main regions in the cell wall, the middle lamella, the primary wall, and the secondary wall (Figure 3). The cell wall has the main constituents: cellulose microfibrils (specify the organization and distribution), hemicellulose, and a matrix (pectin in the primary walls and lignin in the secondary walls) (Rowell 2012).
The primary cell wall (P) is composed of microfibrils with largely random orientation (microfibril angle is between 0 – 90 degrees) and enables for the expansion of the cell to take place as cell growth. It is the first layer to be laid down when the cell is formed. Afterwards, the secondary cell wall is formed and organized by its sub-layers (S1, S2, and S3), displaying different pattern and specific orientation corresponding to their microfibrils (Figure 3). The secondary wall occupies the greatest portion of the cell wall and plays the greatest effect on the cell properties and consequently to the wood macroscopic properties. It is composed of many lamellae of associated microfibrils exhibiting a helical twisted pattern (Hill 2006).
The middle lamella and primary wall are composed mainly by lignin and a small amount of cellulose and hemicellulose. In secondary walls, from the first layer (S1) to the second layer (S2) and third layer (S3), the lignin content decreases towards the lumen. The total amounts of lignin is located in S2, even though the highest lignin concentration is located in middle lamella and primary wall (Rowell 2012). Since the S2 layer is the thickest layer of the cell wall, it has about 80% of the cell wall by weight. Therefore, it dominates the wood properties integrally, such as mechanical performance and swelling (Salmén and Burgert 2009).
The arrangement of the polymers in each layer of the secondary cell wall shows different orientation in some degree of order. Imbedded in the matrix of lignin and hemicellulose, the cellulose microfibrils have different inclination in relation with the fiber axis (microfibril angle, MFA) in the different layers. MFA of S2 layer is lower than MFA of S1 (50 – 70 degrees) and S3 (70 – 90 degrees) layers. In the normal wood, MFA of S2 layer is 5 – 30 degrees (Rowell 2012). In the secondary cell wall, cellulose microfibrils are arranged in an undulating aggregate structure with diameter reaching up to 30 nm. The amorphous cell wall polymers (hemicellulose and lignin) fill the spacing between those aggregates (Salmén and Burgert 2009). Hemicelluloses present much lower degree of orientation than cellulose (Salmén et al. 2012). They do not bond covalently with the cellulose fibril surface, but mostly by hydrogen bonds, which then some physical entanglements from non-crystalline regions might support and tighten all of the components (Salmén and Burgert 2009). On the other hand, lignin does not bond to the cellulose directly, but they make some covalent bonds with hemicelluloses to form lignin-carbohydrate complex. Thus, hemicelluloses play the main role in conserving the cell wall assembly (Salmén and Burgert 2009). The illustration of the cell wall assembly can be seen in Figure 4.
microfibril (MFA) of the secondary wall (S2), the concentric lamellar arrangement of cellulose aggregates interspaced by lamella matrix, the undulating cellulose aggregate structure and the variability of cellulose aggregate sizes, and also the arrangement of the matrix component from glucomannan closest to the cellulose microfibrils and to condensed lignin followed by the xylan (more highly substituted xylan in hardwoods) associated to a more non-condensed lignin (Salmén and Burgert 2009);

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The chemical compositions of wood

In regards to an ultra-structural level, wood is considered as a three-dimensional biopolymeric composite, built up of interconnected networks of cellulose, hemicelluloses, and lignin with small quantities of extractives and minerals. Based on its dry weight, wood is composed mainly of polysaccharides (65 – 75%) and combined with lignin (18 – 35%). In total, an elemental composition of wood consists approximately of carbon (50%), hydrogen (6%), oxygen (44%), and trace amounts of minerals. The polysaccharides are mainly composed of cellulose (40 – 45%) and hemicellulose (15 – 25%), also a small amount of starch and pectin. The chemical composition is different between the wood species. In general, softwoods have higher cellulose content (40 – 45%), higher lignin (26 – 34%), and lower pentosan (7 – 14%) content compared to hardwoods (cellulose 38 – 49%, lignin 23 – 30%, and pentosans 19 – 26%) (Rowell 2012). The three major polymers of wood cell walls are represented briefly below.

Table of contents :

I.1. Wood as renewable resource
I.2. Anatomical structure of wood
I.2.1 Cross section of wood
I.2.2 Structural differences of softwoods and hardwoods
I.2.3 The wood cell walls
I.3. The chemical composition of wood
I.3.1 Cellulose
I.3.2 Hemicellulose
I.3.3 Other minor polysaccharides
I.3.4 Lignin
I.3.5 Extractives
I.4. Natural properties of wood
I.4.1 The interaction wood-water
I.4.1.1 Fiber saturation point
I.4.1.2 Equilibrium moisture content
I.4.1.3 Sorption isotherm
I.4.1.4 Dimensional instability
I.4.1.5 Water repellency
I.4.2 Mechanical properties of wood
I.4.3 Wood degradation by biological agents
I.4.3.1 White rot
I.4.3.2 Brown rot
I.4.3.3 Soft rot
I.4.3.4 Tunneling bacteria
I.4.3.5 Insect
I.4.3.6 Marine organism
I.5. The need for wood modification
I.5.1 Biocide-based wood treatments
I.5.2 Wood Modification
I.5.2.1 Thermal wood modification
a. Structural defects (micro and macro)
b. Changes in chemical composition and ultra-structure of thermally modified wood
1) Hemicellulose
2) Cellulose
3) Lignin
4) Extractive
c. Mass loss by thermal modification
d. Water related properties
e. Dimensional changes
f. Mechanical properties
g. Biological durability
I.5.2.2 Chemical wood modification
I.5.2.3 Impregnation modification
a. Wood modification with polyglycerol-maleic anhydride
b. Wood modification with polyglycerol-glycidyl methacrylate
c. Wood modification based on combination of chemical polymer and thermal modification
d. Wood modification based on in-situ esterification of citric acid and sorbitol
II.1. Introduction
II.2. Results
II.2.1 Paper 1
a. Introduction
b. Paper 1 “Comparison of different treatments based on glycerol or polyglycerol additives to improve properties of thermally modified wood”
c. Conclusion
II.2.2 Paper 2
a. Introduction
b. Paper 2 “Non-biocide antifungal and anti-termite treatments based on combinations of thermal modification with different chemical additives”
c. Conclusion
II.2.3 Paper 3
a. Introduction
b. Paper 3 “Resistance against Subterranean Termite of Beech Wood impregnated with different derivatives of Glycerol or Polyglycerol and Maleic Anhydride followed by Thermal Modification- A Field Test Study”
c. Conclusion
II.3. Conclusion
III.1. Introduction
III.2. Results
III.2.1 Paper 4
a. Introduction
b. Paper 4 “Wood Modification Based on Glycerol maleate and Maleic Anhydride under Opened and Closed System”
c. Conclusion
III.2.2 Paper 5
a. Introduction
b. The Up-scaling treatments
c. Paper 5 “Properties of Thermo-chemically Modified Wood base on Vinylicglycerol and Vinylicpolyglycerol Derivative under Oven heating (OHT) in Opened System and Heat Pressurized Steam (HPS) in Closed system”
d. Conclusion
III.3. Conclusion
IV.1. Introduction
IV.2. Results
IV.2.1 Paper 6
a. Introduction
b. Paper 6 “Beech wood modification based on in-situ esterification with sorbitol and citric acid”
c. Conclusion
IV.3. Conclusion


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