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

are two major types of wood: softwood (from needle-bearing trees) and hardwood (from broad-leaved trees). The terms softwood and hardwood are misnomers and have little to do with hardness or density. Many softwoods are actually much harder than many hardwoods [5].

      In both softwoods and hardwoods, the most noticeable features are the light outer ring and the dark inner core. Thus, the wood in the trunk of the tree is typically divided into two zones, each of which serves an important function distinct from the other. The actively conducting portion of the stem in which parenchyma cells (explained later) are still alive and metabolically active is referred to as sapwood [6].

      A looser, more broadly applied definition is that sapwood is the band of lighter-colored wood adjacent to the bark. Heartwood is the darker-colored wood found to the interior of the sapwood. As the tree grows larger in diameter, cells closer to the center of the tree, which are no longer required for these activities, die and are converted to heartwood.

      In the living tree, sapwood is responsible not only for conducting sap but also for storage and synthesis of biochemicals. An important storage function is the long-term storage of photosynthesis products. The primary storage products of photosynthates are starch and lipids. Living cells of the sapwood are also the agents of heartwood formation. Biochemicals must be actively synthesized and translocated by living cells [6]. For this reason, living cells at the border between heartwood and sapwood are responsible for the formation and deposition of heartwood chemicals, one important step leading to heartwood formation [7].

      These chemicals are collectively named as extractives and are known for protecting the wood. Extractives also play a significant role in the adhesive bonding of wood. Extractives are formed by parenchyma cells at the heartwood–sapwood boundary and are then exuded through pits into adjacent cells [7]. In this way, dead cells can become occluded or infiltrated with extractives despite the fact that these cells lack the ability to synthesize or accumulate these compounds on their own. When these chemicals oxidize, the heartwood darkens, and the border between sapwood and heartwood becomes more distinct. Heartwood formation differs markedly between species, even within trees of the same species. These changes, especially the chemical changes, account for much of the difficulty and unpredictability in the bonding of heartwood [5].

       1.3.1.1 Growth Rings and Ring-Porous and Diffuse-Porous Wood

Wood cells are produced by the vascular cambium, the only living part of the tree, at the boundary between bark and sapwood, one layer of cell divisions at a time. These collections of cells produced together over a discrete time interval are known as growth increments or growth rings. Cells formed at the beginning of the growth increment are called earlywood cells, andcells formed in the latter portion of the growth increment are called latewood cells (Figure 1.1). Springwood and summerwood were terms formerly used to refer to earlywood and latewood, respectively, but their use is no longer recommended [8].

Figure 1.1 Earlywood and latewood [5].

      The growth rings are usually prominent because of cyclical variation in color or porosity. These variations are in turn due to the formation of different types of cells and wood structures during different parts of the growing season. The lighter-colored (less dense) and more porous cell tissue of earlywood is formed early in the growing season. The porous earlywood cells are largely responsible for the movement of liquid and nutrients within the tree. The darker (more dense) and less porous cell tissue of the latewood, formed later in the growing season, is largely responsible for mechanically supporting the tree [5] (Figure 1.1).

      Large differences between the earlywood and latewood porosity and density in some species like oak and southern pine often cause difficulty in bonding [5].

      Hardwoods may be divided into ring-porous and diffuse-porous woods. Diffuse-porous woods have vessels of roughly the same radial diameter throughout the growing season. In the diffuse porous wood, the pores are distributed evenly throughout the wood.

       1.3.1.2 Wood Cells

      A living plant cell consists of two primary domains: the protoplast and the cell wall. The protoplast is the sum of the living contents that are bounded by the cell membrane. The cell wall is a non-living, largely carbohydrate matrix extruded by the protoplast to the exterior of the cell membrane. The plant cell wall protects the protoplast from osmotic lysis and often provides mechanical support to the plant at large [9, 10].

In wood, the ultimate function of the cell is borne solely by the cell wall. This means that many mature wood cells not only do not require their protoplasts, but indeed must completely remove their protoplasts prior to achieving functional maturity. For this reason, a common convention in wood literature is to refer to a cell wall without a protoplast as a cell. Although this is technically incorrect from a cell biological standpoint, this convention is common in the literature [6].

      In the case of a mature cell in wood in which there is no protoplast, the open portion of the cell where the protoplast would have existed is known as the lumen (plural: lumina). Thus, in the wood cells, there are two domains: the cell wall and the lumen.

      Wood cells are microscopic, long, thin, hollow tubes, like soda straws with their ends pinched shut. Most longitudinal cells that are parallel to the longitudinal axis or grain direction of the tree trunk are meant either for support or for the movement of fluids in the living tree. Some special cells are organized into tissue called rays that lie perpendicular to the longitudinal axis of the tree trunk and along its radii. Ray cells are responsible for the production and storage of amorphous materials of complex chemical nature. The rays are also the pathway for lateral movement of fluids in the tree [5].

      There are two basic types of cells—prosenchyma and parenchyma. Softwoods and hardwoods have different types of prosenchyma and parenchyma cells. Prosenchyma cells are generally the strong woody cells responsible for mechanical support and the movement of fluids in the living tree. Parenchyma cells are responsible for the production of chemicals and for the movement and storage of food. The real differences between softwoods and hardwoods are in the size, shape, and diversity of these two types of cells [5].

      The structure of softwoods is characterized by relatively few types of prosenchyma and parenchyma cells compared to hardwoods as a result of their lower position on the evolutionary scale. One type of prosenchyma cell, the longitudinal tracheid, constitutes approximately 90–94% of the volume of softwood wood. Tracheids perform both the support and fluid movement for the tree. Earlywood tracheids are generally of large diameter and thin walled. Earlywood cells are specifically adapted to moving fluids through large openings (bordered pits) that connect adjoining cells. Latewood tracheids, which are generally smaller in diameter, are thicker walled, have smaller pits, and are specifically adapted for strength. The remaining 10% of softwood consists of longitudinal parenchyma cells, ray tracheids, and ray parenchyma cells. Generally, parenchyma cells play a secondary strength role, but they are important for adhesive bonding as paths for adhesive penetration. Moreover, the chemicals contained by the cells affect adhesion and adhesive cure.

In comparison to softwoods, the structure of hardwoods is characterized by a greater diversity of cell types and functions. One notable difference is that one type of specialized prosenchyma cells is responsible for mechanical support, and another type of specialized prosenchyma cells is responsible for fluids movement. Support is provided by two types of small-diameter thick-walled prosenchyma cells called libriform fibers and fiber tracheids. Fluid movement is provided by medium-