Softwood basic density

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). 

For almost all softwoods and low-to-medium density hardwoods increasing wood density has been ranked above all other desirable objectives in traditional wood quality improvement programmes. Basic density provides an index of wood quality to which all end users are able to relate. To the sawmiller a high density indicates the timber will be stiff and strong to the pulpmill it indicates that a given volume of wood will yield more pulp than would a low-density timber. But too high a basic density (> 600 kg m ) - more a problem with some hardwoods - is as undesirable as too low a basic density (< 400 kg m ) - more a problem with softwoods (also Paulownia and Populus sp.). For woods above 600 kg m", furniture... 

Within-tree variations in density can be highly significant. Gown and McConchie (1983) show that the density in a 24-year-old radiata pine tree can vary from 300 kg m- in the top log to greater than 450 kg m- in the outer wood of the butt log. Consequently, the drying kinetics of boards taken from the same log may be markedly different (Davis, 2001). Trends in basic density for a number of second-growth softwoods are illustrated in Fignre 40.43. 

FIGURE 40.43 Trends in basic density at breast height for commercial second-growth softwoods in British Columbia. (Adapted from Josza, L.A. and Middleton, G.R., A Discussion of Wood Quality Attributes and Their Practical Implications, Forintek Canada Special Publishing SP-34, Vancouver, British Columbia, Canada, 1994.)... 

Wood density is regarded as the easiest and most reliable measure of wood quality. Data from the drying of 106 North American hardwoods show that the volumetric shrinkage of wood increases with its basic density (Stamm, 1964). There is also a correlation between the permeability and the basic density for the sapwood of a softwood (Nijdam and Keey, 2000). Presorting a sapwood load into high-density and low-density groups can reduce the moisture variability of the final recombined kiln-dried boards. 

Wood is a composite material that is made, up basically of a mixture of three main constituents, cellulose, hemicellulose, and lignin (see Textbox 54), all of them biopolymers synthesized by the plants, which differ from one another in composition and structure (see Textbox 58). The physical properties of any type of wood are determined by the nature of the tree in which the wood grows, as well as on the environmental conditions in which the tree grows. Some of the properties, such as the density of wood from different types of trees, are extremely variable, as can be appreciated from the values listed in Table 71. No distinctions as to the nature of a wood, whether it is a hardwood or a softwood, for example, can be drawn from the value of its specific gravity. 

The first three chapters deal with particleboard, medium density fiberboard, hardwood plywood, and softwood plywood, the four most widely used wood panel products. Chapter four compares these products with other consumer products. Chapters five through seven explain the basic chemistry of formaldehyde with cellulose and wood components and provide a current understanding of the nature of liquid urea-formaldehyde adhesive resins. The next two chapters present new analytical methods that might become useful in the future. Chapters eight and eleven through sixteen explain the complex nature of the latent formaldehyde present in the products and its correlation to formaldehyde emission from wood products. Chapters fifteen and sixteen describe currently popular formaldehyde reduction methods. The last two chapters discuss the problems involved in reducing formaldehyde emission by regulating air levels or source emissions. 

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