Wood, chemical constituents

Research on the chemical nature of hardwood xylem has revealed that the walls of fibers and ray cells contain lignin of one type, syringyl, while vessel walls together with the surrounding middle lamella are rich in lignin of a second type, guaiacyl, which is the same type found in softwood xylem (9, 10). This basic chemical difference between hardwoods and softwoods could, in some instances, potentially influence phenomena that depend on the chemical nature or reactivity of wood tissue at the cellular level. Further details on the nature and distribution of wood chemical constituents are found in Chapter 2. 

The two most important chemical constituents of wood are cellulose and lignin. Through the efforts of many investigators over the past... 

Chemical modification of wood is defined as the reaction of a chemical reagent with the wood polymeric constituents, resulting in the formation of a covalent bond between the reagent and the wood substrate. 

Many of the physical, chemical and biological properties of wood can be understood by referring to the polymeric chemical constituents. In many cases of wood modification, these polymeric components are modified to some extent. The three structural polymeric components of the wood cell wall are cellulose, hemicellulose and lignin. There are many excellent texts describing the structure and function of these components, and only a brief account is given here. 

Wood density is the indicator of nearly all wood properties and of wood quality. Relatively dense species are strong, as one might expect from their large cell wall mass. Wood species also vary in features not related to density, such as color, percentage and arrangement of cell types, and minor chemical constituents. Therefore, even woods of the same density are likely to differ, with the properties varying more than the densities. 

As we know, from earliest times up to the present days, the substances used as vehicles for writing have been numerous. The papers of one hundred years ago were basically made from rags of cotton and linters. The principal chemical constituent of these sources is cellulose, a polymeric carbohydrate composed of long linear chains of /3-linked anhydroglucopyranose. The chains in cellulose are composed of as many as ten thousand glucose units (degree of polymerization). The composition of wood fibers is quite different from that of cotton, as can be seen from Table I. The chemical constituents of modern papers are compli-... 

Table II. Chemical Constituents of Wood and Pulp Fibers...

Chemical Characteristics of Paper Documents. From the earliest times up to the present day, the substances used as vehicles for writing have been numerous. Ancient paper documents were basically made from rags of cotton and linters cellulose is the major chemical constituent (I). Modern papers, however, are made of wood fibers, which usually are composed of cellulose, hemicelluloses, and lignin (I). In addition, for most of the modern papers, fillers, sizing agents, and other additives are used to improve paper properties (I). 

Table 7.7.2. Chemical constituents of the pyroly sate of holocellulose (A), a xylan fraction (B), and technical xylan (C) from beech wood [64], (The origin from lignin of certain compounds is also indicated in the table.)...

Another reason for predicting low emissions is that the small amount of residual formaldehyde that might be present in the prepared resin is diminished even farther by reactions which occur when the resin cures. Phenolic resins are cured under heat and pressure in a hot-press, usually under highly alkaline conditions. Curing temperatures are usually in the range of 130-220 C. Under these conditions, unreacted formaldehyde continues to react with phenol to form larger phenol formaldehyde polymers. Also, some formaldehyde reacts with various chemical constituents in the wood. Moreover, some formaldehyde is probably converted to methyl alcohol and formic acid by way of the Cannizzaro reaction (J ). ... 

Ishikawa, T, Seki, M., Nishigaya, K., Miura, Y., Seki, H., Chen, I.-S., and Ishii, H. (1995) Studies on the chemical constituents of Xanthoxylum nitidum (Roxb.) D. C. (Fagara nitida Roxb.). III. The chemical constituents of the wood. Chem. Pharm. Bull. 43(11), 2014-2018. 

The authors also examined coal and wood particles within the sediment. They found a larger concentration of PAH within these particles compared to the silica particles. They also determined that while organic carbon molecules were present uniformly throughout these particles in mixtures of aliphatic and aromatic (cyclic) forms, the PAH was concentrated at the surfaces of these particles. The conclusions derived from this powerful combination spectroscopies will allow a more successful approach to remediation of the Milwaukee Harbor. These results could also have a larger impact, since they show the feasibility of identifying chemical constituents and their distributions in sediments which are important in planning remediation efforts in different environs. 

The most significant information in the gross analytical studies is the fact that lignin, even though altered, is the chemical constituent present in highest amounts in the most degraded woods. 

Extensive works conducted by different authors utilizing GC and GC/MS sometimes lead to different results which indicate tfui difficulties of carrying out accurate detailed analysis of the chemical constitupyrolysis oils. Examples of incomplete or even contradictory results can be found in the literature (3 4) and this paper in the analysis of vacuum pyrolysis oils. Other researchers have studied the functional group distribution in l>yrolysis oil (8), Althoiigh those techniques are long and tedious, they will lead to useful information about wood oil chemistry. 

In this chapter no distinction is made between a real chemical modification (acetylation), where covalent bonds between the reagent and the chemical constituents of wood are formed, and the impregnation modification, where monomers diffuse into the wooden cell wall and become polymerized there, like Hill suggested in his definitions (see above), because it was shown that DMDHEU and furfuryl alcohol not only swell the cell wall and polymerize there but also have the potential to form some covalent bonds. ... 

Meier H. 1964. General chemistry of cell walls and distribution of the chemical constituents across the wall. In Zimmermann M.H. (ed.) The Formation of Wood in Forest Trees. Academic Press, New York, pp. 137-151. 

Ishii H, Ohida H, Haginiwa J 1972 The chemical constituents of Xanthoxylum inerme Koidz, (Fagara boniensis Koidz). Isolation of the chemical constituents from bark and wood. Yakugaku Zasshi 92 118-128... 

Harun J, Labosky P Jr 1985 Chemical constituents of five northeastern barks. Wood Fiber Sci 17 274-280... 

Labosky P 1979 Chemical constituents of four southern pine barks. Wood Sci 12 80-85... 

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