Softwoods extractives

Fig. 31. AOX formation in chlorination (C) and extraction (E) stages for 30 kappa softwood kraft pulp chlorinated at 4% consistency in mixer, using...

Softwoods are generally more resistant to acids than are hardwoods because they have high lignin and low hemiceUulose contents. In general, heartwood is more resistant to acids than sapwood, probably because of heartwood s higher extractive content and slower movement of Hquid into the heartwood. For these reasons, the heartwood of certain conifers has been widely used in the chemical industry. 

Wood contains a small proportion (usually less than 5%) of components which are extractable by organic solvents such as ethanol or dichloromethane. The proportion of these extractives varies in hardwoods and softwoods and also between species. Although many of these substances are removed during the chemical pulping process, some may still be retained in the final sheet of paper. Their chemical composition is very varied, and they include alkanes, fatty alcohols and acids (both saturated and unsaturated), glycerol esters, waxes, resin acids, terpene and phenolic components. The proportion which remains in pulp and paper depends upon the pulping process used. In general, acidic components such as the resin and fatty acids are relatively easily removed by alkali by conversion to their soluble... 

The resin acids present in pulps, particularly those from softwood, have also been found to be significantly toxic to aquatic organisms. The amount in wood varies greatly between species (Table 10.4). Between 0.3 and 3.6 kg/tonne is extracted during pulping. 

Nuopponen, M., Vuorinen, T., Jamsa, S. and Viitaniemi, P. (2003). The effects of a heat treatment on the behaviour of extractives in softwood studies by FTIR spectroscopic methods. Wood Science and Technology, 37(2), 109-115. 

The basic structure of all wood and woody biomass consists of cellnlose, hemicelluloses, lignin and extractives. Their relative composition is shown in Table 2.4. Softwoods and hardwoods differ greatly in wood stmctnie and composition. Hardwoods contain a greater fraction of vessels and parenchyma cells. Hardwoods have a higher proportion of cellulose, hemicelluloses and extractives than softwoods, but softwoods have a higher proportion of lignin. Hardwoods ate denser than softwoods. 

Hemicelluloses are quite difficult to extract from cell walls of softwoods (9,10) and are usually destroyed or depolymerized during the chemical pulping of these raw materials. However, other hemicelluloses, primarily xylans, can be extracted by cold, dilute sodium hydroxide from grasses and many hardwoods in very high yields (9,77). These xylans are deacetylated in an alkaline medium and are for the most part insoluble (hemicellulose A). A partially water soluble fraction (hemicellulose B) has also been... 

Kraft Lignin. A softwood kraft lignin (KL) was isolated from a partly evaporated, industrial kraft black liquor by precipitation through the addition of dilute sulfuric acid as described elsewhere (7). The lignin was thereafter fractionated by successive extraction with organic solvents (7). The KL fraction used in the present investigation was the second of five fractions obtained (propanol soluble - methylene chloride insoluble). 

This paper is a report on preliminary studies evaluating the suitability of bark extracts from four West Coast softwood species as bonding agents for particleboard. The barks investigated include white fir (Abies concolor), ponderosa pine (Pinus ponderosa), Douglas fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla) ... 

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. 

The exterior durability of softwood veneer species in this country has been demonstrated. There have been some difficulties encountered in the long term exterior durability of some Asian veneer species when bonded with phenolic resins. Extractives interfering with the cure of the resin directly or the resin bonded to the extractives rather than the wood cause failures along the glue line. These species shrink and swell more than native softwoods. Stresses are greater and breaks in the wood hydrogen... 

Table 28.1 compares the chemical composition of extractive-free hardwoods and softwoods. As cellulose and the hemicelluloses are both polysaccharides, it is obvious that the polysaccharide of wood is by far the dominant component, making up approximately 70 percent of both hardwoods and softwoods. 

Schell, D., Nguyen, Q., Tucker, M., and Boynton, B., Pretreatment of softwood by acid-catalyzed steam explosion followed by alkali extraction. Appl Biochem Biotechnol 1998, 70-2, 17- 24. 

Bjorkman A, Person B (1957) Studies on finely divided wood Part 2 The properties of lignins extracted with neutral solvents from softwoods and hardwoods Sven Papperstidn 60 158-169... 

Thioacidolysis of extractive-free softwood meal or softwood MWL essentially leads to the guaiacyl structures listed in Table 6 4 2 and referred to previously in the section on thioacidolysis of hardwood lignins When subjected to thioacidolysis, compression wood additionally gives rise to significant amounts of p-hydroxyphenyl QC-, monomeric compounds (Lapierre et al 1987)... 

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