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Atlas

Chemical composition of wood

Dry a piece of wood and take it apart, and almost everything turns out to be three polymers: cellulose, hemicelluloses and lignin. The rest is resins, fats, tannins and a little mineral matter.

Temperate wood contains 65-75% polysaccharides and 18-35% lignin; extractives are usually a few percent and ash a fraction of a percent. The proportions depend on species, position in the stem and growing conditions.

How wood is split into fractions in the lab. Numbers: % of oven-dry wood mass (Sjöström 1993).

Three polymers and everything else

Cellulose gives strength, lignin stiffness, and hemicelluloses bind the two. Together they make up more than 90% of dry wood. The rest is substances a solvent can wash out, plus ash.

The macromolecular cell wall components (cellulose, hemicelluloses, lignin) form its framework and matrix. Low molecular weight components are divided into organic extractives (terpenes and resin acids, fats and waxes, phenolic compounds) and inorganic matter that remains as ash after burning.

Softwoods and hardwoods

Softwoods have more lignin and a different set of hemicelluloses, rich in mannans. Hardwoods have less lignin but more xylan. Cellulose is about the same.

According to Sjöström, Scots pine contains 40.0% cellulose, 16.0% glucomannan, 8.9% glucuronoxylan and 27.7% lignin; silver birch 41.0% cellulose, 2.3% glucomannan, 27.5% glucuronoxylan and 22.0% lignin. Softwood lignin is guaiacyl, hardwood lignin guaiacyl-syringyl; hardwood xylans are heavily acetylated.

Elements

At the level of atoms, wood is remarkably similar across species: half carbon, almost half oxygen and a little hydrogen. There is very little nitrogen or mineral matter.

Elemental composition of wood: about 50% C, 6% H and 44% O; nitrogen usually only a few tenths of a percent or less. Ash in temperate woods is usually 0.2-0.5% and almost always below 1%, mainly calcium, potassium and magnesium. Bark contains clearly more ash and extractives than wood.

How wood is analysed

Composition is measured step by step. First a solvent washes out resins and fats. Then strong acid dissolves the carbohydrates and lignin is left behind. Separately, a mild oxidant removes lignin and leaves all the carbohydrates together.

Ground wood is extracted with organic solvents (for example acetone or ethanol-toluene). Klason lignin is the residue after treatment with 72% sulfuric acid and boiling in dilute acid. Holocellulose (total polysaccharides) is obtained by acidic sodium chlorite delignification. Alpha-cellulose is the part of holocellulose insoluble in 17.5% NaOH. Polysaccharide composition is calculated from monosaccharides after complete hydrolysis, measured by chromatography.

Why composition varies

Even within one tree the chemistry differs: heartwood holds more resin, compression wood more lignin, bark more ash. That is why table values are always averages.

Composition depends on the zone of the stem (sapwood and heartwood differ in extractives), on wood age (juvenile versus mature), on reaction wood (compression wood richer in lignin, tension wood in cellulose) and on growing conditions. Comparing species requires data obtained by the same method.

Key facts

polysaccharides
65-75%
lignin
18-35%
pine: lignin
27.7%
birch: xylan
27.5%
carbon
about 50%
ash
usually 0.2-0.5%

Sources for this chapter

  1. Sjöström E. Wood Chemistry: Fundamentals and Applications. 2nd ed. San Diego: Academic Press, 1993.
  2. Pettersen R.C. The chemical composition of wood. In: Rowell R.M. (ed.) The Chemistry of Solid Wood. Advances in Chemistry Series 207. Washington, DC: American Chemical Society, 1984, pp. 57-126.
  3. Азаров В.И., Буров А.В., Оболенская А.В. Химия древесины и синтетических полимеров: учебник. 2-е изд. СПб.: Лань, 2010.
  4. Fengel D., Wegener G. Wood: Chemistry, Ultrastructure, Reactions. Berlin; New York: De Gruyter, 1984.