Adhesives for Wood and Lignocellulosic Materials. R. N. Kumar

plants (trees, grass, etc.), algae (Valonia, Cladophora, etc.), and even in some animals (tunicates), and it can also be synthesized by some bacteria (Acetobacter xylinum). Around 40% of the dry weight of wood consists of cellulose. Cellulose is a linear polymer built up of D-anhydroglucose units linked together by β-(1-4)-glycosidic bonds. The degree of polymerization (DP) is normally 9000–10,000 glucose units, but DP values as high as 15,000 glucose units have been reported [18]. Most of the cellulose found in wood fibers has approximately the same molecular size, i.e., a very low polydispersity [18].

The cellulose molecule is linear and it is therefore capable of forming strong intra- and intermolecular hydrogen bonds and aggregated bundles of molecules. In the literature, these bundles of cellulose molecules, as has been already mentioned, have been given many different names such as elementary fibrils, microfibrils, protofibrils, etc. [12–14]. The term cellulose microfibrils will be used in this book. These cellulose microfibrils have crystalline and amorphous regions.

      1.4.2 Hemicelluloses

      Hemicelluloses are a group of heterogeneous polymers that play a supporting role in the fiber wall. Twenty to thirty percent of the dry weight of wood consists of hemicelluloses. The hemicellulose polymers are built up of several different monomers, such as mannose, arabinose, xylose, galactose, and glucose. Some acidic sugars like galacturonic acid and glucuronic acid are also constituents of hemicelluloses. One, two, or several types of monomers usually build up the backbone of hemicellulose polymers. Most of the hemicelluloses also have short branches containing types of sugars other than those of the backbone. The degree of polymerization for the hemicelluloses is between 100 and 200 [11].

      Softwoods contain about 20–25% glucomannans. Acetyl groups and galactose residues are attached to the polymer chain. The hydroxyl groups at the C(2) and C(3) positions in the chain are partly substituted by O-acetyl groups. Galactose units are also attached to the chain as α-(1-6)-linkages. Hence, softwood mannans can be designated as O-acetyl-galactoglucomannans [11]. Thus, the galactoglucomannans of softwood have a backbone of (1-4) linked by β-D-glucose and β-D-mannose units in the main chain with α-D-galactose linked to the chain through (1-6)-bonds. An important structural feature is that the hydroxyl groups at C(2) and C(3) positions in the chain units are partially substituted by O-acetyl groups.

It has a backbone of (1-4)-β-D-xylose, where most of the xylose residues have an acetyl group at C(2) or C(3). About every 10th xylose unit also has a 4-O-methyl-α-D-glucuronic acid residue linked by a (1-2)-bond (Figure 1.5). The backbone substitution and degree of branching can vary considerably between hemicelluloses of the same category [19].

      1.4.3 Lignin

      Lignin is a heterogeneous three-dimensional polymer that constitutes approximately 30% of the dry weight of wood. Lignin limits the penetration of water into the wood cells and makes wood very compact.

Lignin is the second most abundant and important organic substance in the plant world. Lignin increases the mechanical strength and stiffness properties to such an extent that huge plants such as trees with heights of even more than 100 m can remain upright.

Lignin is a complex polymer of phenylpropane units (monolignols) joined through many different linkages. The three monolignols are p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Figure 1.4). The monolignols in various proportions are the building blocks for the 3-D structure of native lignin in higher plants [19]. Monolignols copolymerize mainly by radical polymerization.

Figure 1.4 The three monolignols.

      Thus, the monolignols are the building blocks of the lignin macromolecule. Lignin is therefore defined as an amorphous polyphenolic material arising from an enzyme-mediated dehydrogenative polymerization (DHP) of three phenylpropanoid monomers, coniferyl, synapyl, and p-coumaryl alcohols.

      The following nomenclature of radicals (Figure 1.5) and the building units from which lignin is derived should be kept in mind to understand scientific publications on lignin:

Figure 1.5 Radicals and units—nomenclature.

      Lignin’s functions in the tree are as follows (Wang Wood Chemistry Class):

      1 Support for mechanical strength

      2 Antioxidant for protection

      3 Sealant and reinforcing agents, bonding cellulose and hemicellulose together

      4 Cross-linker to cross-link he polymeric carbohydrates

      The so-called lignin–carbohydrates complex defines the type of covalent bonds existing between lignin and hemicelluloses. These are mainly benzyl ester and benzyl ether linkages between the side chains of xylans and phenyl glycosidic linkages with the main chain of glucomannans [11].

1.4.3.1 Lignin Isolation

      Isolation of lignin in an unchanged form (native lignin) is rendered difficult due to its complex structure and its location within the cell wall. Hence, determination of the exact chemical structure of lignin is therefore difficult [11]. All methods of isolation have the disadvantage of either fundamentally changing the native structure of lignin or releasing only a part of it in a relatively unchanged condition.

      There are two methods by which lignin can be isolated from extractive free wood [19]:

      1 As an insoluble residue after hydrolytic removal of the polysaccharides.

      2 Alternatively, lignin can be hydrolyzed from wood or converted into soluble derivative.

      According to method (1) the polysaccharides can be removed in the following procedures:

      1 Klason lignin: Klason lignin is obtained after removing the polysaccharides from extractives-free wood by hydrolysis with 72% sulfuric acid.

      2 Cellulolytic enzymes may be used to dissolve polysaccharides from finely divided wood meal leaving behind lignin as residue. This lignin is called cellulolytic enzyme lignin (CEL). This method is tedious but the CEL retains its original structure essentially unchanged.

      Bjőrkman lignin, also called milled wood lignin (MWL), has been widely used for structural studies. Wood meal is ground in a ball mill either without any solvent or with a solvent such as toluene, which is a non-swelling solvent. The lignin can then be extracted by using a mixture of dioxane, water, and HCl (dioxane lignin). Lower temperature extraction minimizes structural changes of lignin.

      A number of researchers have tried the dioxane method in lower temperature (and consequently lower yields) to minimize structural changes in extracted lignins [4].

      For the extraction of lignins, a modified dioxane method and ionic liquid and comparative molecular weight (MW) and structural studies by chromatography and ¹³C NMR spectroscopy techniques were