Hardwood volume

Economic Aspects. The hardwood and decorative plywood industry has decreased in size and production significantly in the past few years. In 1994, there were an estimated 100 mills operating in the United States having a production volume of 1.135 x 10 m (2). The doUar value of this production is extremely difficult to estimate because of the very wide range of prices for the products. 

In hardwoods, morphological structural elements in longitudinal series comprise the segmented structure termed vessel . Vessels, which are exposed in transverse section, constitute about 10-46% of the stem volume in deciduous hardwoods and are cells of relatively large diameters (50-300 p.m). Vessels have in short the appearance of open vertical tubes within the wood structure because their end walls have partially dissolved. By comparison, the hardwood vessel diameter can be as much a 10 times the diameter of a softwood fiber. 

The two types of wood differ, however, in their nature and structure. The main structural characteristic of the hardwoods (which are botanically known as angiosperms, plants that flower to pollinate for seed reproduction) is that in their trunks or branches, the volume of wood taken up by dead cells, varies greatly, although it makes up an average of about 50% of the total volume. In softwoods (from the botanical group gymnosperms, which do not have flowers but use cones for seed reproduction) the dead cells are much more elongated and fibrous than in hardwoods, and the volume taken up by dead cells may represent over 90% of the total volume of the wood. 

In Quercus alba the rays represent 28% of the wood volume and in other oak species 19-32% in most other hardwoods the rays occupy about 15% but only 8% of the wood volume in the conifer Sequoia sempervirens (13). The large rays of the oak are so spaced and numerous that a molecule of water diffusing through the side of a barrel must cross five or more large rays if it exits on a straight path or follow a much extended path if it is to go around the rays interposed. These large rays no doubt contribute to the strength and bendability of oak as well as to its dimensional stability and relative impermeability. 

Figure 5. Cross-sectional and longitudinal surfaces of a ring-porous hardwood. In the cross-sectional view (X) the largest diameter cells are springwood vessels whereas the smaller cells with obvious openings are sum-merwood vessels. Smaller diameter thick-walled fibers constitute most of the remaining volume. Transversely oriented food-storing cells can be seen on the radial surface (arrow). 40X (Courtesy of N. C. Brown Center for Ultrastructural Studies, S.U.N.Y. College of Environmental Science and Forestry)...

However, specifically it refers only to those cell types found in hardwoods which meet the above definition. Fibers range in length from 0.7 mm to 3 mm with an average slightly less than 2 mm for domestic species. In diameter, an average of less than 20 ym can be expected. The percentage of the volume of wood occupied by fibers varies considerably. In sweetgum, fibers constitute only... 

Considerable variation in the amount of transverse and longitudinal parenchyma exists among hardwood species. For example, basswood has approximately the same as softwoods, that is about 10%, while some oak species approach k0% parenchyma. As in softwoods, the parenchyma are usually brick-shaped cells although some variations of this shape occurs. The rays, composed of transverse parenchyma, range from one to thirty-plus cells wide. The ray illustrated in Figure 18 is seven cells wide. Thus the higher parenchyma volume is due to wider rays and the additional presence of axial parenchyma which is rather rare in softwood species. 

Many wood species, both hardwoods and softwoods, are used for particleboard however, the density of the particleboard should be higher than the density of the raw material to efficiently utilize the adhesive system. The compression of the particles, which is required for consolidation into the finished product, enhances the particle-particle contact, producing more inter-particle adhesive bonds as well as reducing the total void volume in the panel. With wood of density higher than the finished particleboard, the compression of the particles is lower and the resultant reduced interparticle contact and higher void volume adversely influence the physical and mechanical properties of the parti cleboard. 

After the veneer has been properly unitized and dried, it is transported to the gluing operation. It is here that the greatest proportion of chemicals other than water are involved in the plywood process. To better understand the adhesive resins involved, perhaps it is best to review the quantities of softwood and hardwood plywood manufactured in relation to the adhesive needs required. The 1972 - 1973 era were years of peak production in the United States for plywood. Data will be extracted from various reports for presentation purposes. Hardwood and softwood plywood production is normally reported in different manners and it is difficult to compare. While the values reported are not intended to be exactly accurate, they will give some comparison, relatively speaking, and should give some concept of the volume of plywood produced and in turn the volume of adhesives used. 

Hardwoods contain several cell types, specialized for different functions (Fig. 1-9). The supporting tissue consists mainly of libriform cells, the conducting tissue of vessels with large cavities, and the storage tissue of ray parenchyma cells. In addition, hardwood contains hybrids of the above-mentioned cells which are classified as fiber tracheids. Although the term fiber is frequently used for any kind of wood cells, it more specifically denotes the supporting tissue, including both libriform cells and fiber tracheids. In birch these cells constitute 65 to 70% of the stem volume. 

Hardwood rays consist exclusively of parenchyma cells. The ray width varies in the tangential direction. In aspen wood the rays form one row, in birch wood and oak wood 1 -3 and 1 -30 rows, respectively. The height varies from one up to several hundred tiers. The rays account for 5-30% of the stem volume. 

Methanol. As is the case with ethanol, the concept of producing methanol from wood is not new. Methanol obtained from the destructive distillation of wood represented the only commercial source until the 1920s. The yield of methanol from wood by this method is low, only about 1-2 percent or 20 L/metric ton (6 gal/ton) for hardwoods and about one-half that for softwoods. With the introduction of natural gas technology, the industry gradually switched to a synthetic methanol formed from a synthesis gas (syngas) produced from reformed natural gas. Two volumes of H2 and one volume of CO are reacted in a catalytic converter at pressures of 1500-4000 psi to produce methanol. Presently, 99 percent of the methanol produced in the United States is derived from natural gas or petroleum. 

In the isolation of hemicelluloses from plant materials containing relatively large amounts of hemicelluloses and small amounts of insoluble pectic materials, as in hardwoods, the powdered material is first extracted with organic solvents and hot water. The hemicelluloses are then removed by two extractions with a cold 4% solution of sodium hydroxide. The material is made neutral with dilute hydrochloric acid and washed with water. It is then extracted successively with a hot. 05 N solution of hydrochloric acid and a cold 5% solution of ammonium hydroxide. This procedure removes only a part of the hemicelluloses and pectic materials from a wood since lignin protects the underlying hemicelluloses from the alkaline solution or is combined with them. - To overcome this difficulty the wood is suspended in water and treated with chlorine gas. - - The subsequent addition of a large volume of... 

Stoltze et al. find that the gasification of hardwood is 2-3 times slower than straw, probably due to the different char structure and composition. However, since the density of the hardwood char is 5 times higher than the one of straw, in a volume basis the reactivity of wood char is double than of straw. The direct consequence of this fact is that the gasifiers for wood char only require half the volume of a straw gasifier. 

Figure 1. Commercial timber is obtained from angiosperms (hardwoods) or from gymnosperms (softwoods). The wood tissue is made up largely of dead, hollow plant cells that are arranged to form a composite material of substantial void volume.

In hardwoods, parenchyma cells may be organized into numerous vertical strands and can occupy up to 25-50% of the wood volume (2). However, such strands usually occupy a relatively small percentage of the total wood volume in conifers (1-2%). In many softwoods and hardwoods the vertical strand parenchyma are essentially absent (2). 

Hardwoods contain a substantial volume of fiber cells, but the distinguishing feature of angiosperm xylem is the occurrence of vessels. The vessels are seen on the wood cross section as holes or pores... 

Hardwood fibers, because of the presence of vessels, occupy a proportionally smaller volume of wood tissue than softwood fibers... 

The parenchyma-cell content of hardwoods is, on the average, much greater than that of softwoods. This situation is a result of the wider rays (1-50 cells) and greater ray volume of hardwoods, and also the relatively high proportion of longitudinal parenchyma (2). Additionally, the rays are all parenchyma—no ray tracheids. 

The volume ratio of vessels to fibers and fiber wall thickness are two important factors influencing the hardness and density of different hardwood species and the permeability of these woods to liquids and gases. Wood grain is also partly a function of these two parameters. 

In softwoods, basic density is strongly related to the volume proportion of latewood and its average fiber wall thickness. However, hardwood basic density depends not only on fiber wall thickness but also involves the volume ratio of fibers to vessels. Native commercial woods fall mostly in the basic density range of 0.35-0.65 g/cm, although native species can be as low as 0.21 g/cm (corkwood) and as high as 1.04 g/cm (black ironwood) (2). 

The reduced vessel volume of tension wood, together with thickened fiber walls, can lead to a higher than normal basic density. This general situation, coupled with a difference in wood chemistry, could cause a variable response of such tissue to both chemical and physical treatments or to microbial degradation when compared to normal hardwood xylem. 

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