WOOD CHEMICAL COMPOSITION BASICS AND TUTORIALS

CHEMICAL COMPOSITION OF WOOD BASIC INFORMATION
What Are The Chemical Composition Of Wood?

Wood is composed of cellulose, lignin, hemicellulose, extractives, and ash-producing minerals. Cellulose accounts for approximately 50 percent of the wood substance by weight (USDA-FS, 1999).

The exact percent is species dependent. It is a linear polymer (aliphatic carbon compound) having a high molecular weight. The main building block of cellulose is sugar: glucose.

As the tree grows, linear cellulose molecules arrange themselves into highly ordered strands, called fibrils.

These ordered strands form the large structural elements that compose the cell walls of wood fibers. Lignin accounts for 23% to 33% of softwood and 16% to 25% of hardwood by weight.

Lignin is mostly an intercellular material. Chemically, lignin is an intractable, insoluble, material that is loosely bonded to the cellulose. Lignin is basically the glue that holds the tubular cells together.

The longitudinal shear strength of wood is limited by the strength of the lignin bounds.


Hemicelluloses are polymeric units made from sugar molecules. Hemicellulose is different from cellulose in that it has several sugars tied up in its cellular structure.

Hardwood contains 20% to 30% hemicellulose and softwood averages 15% to 20%. The main sugar units in hardwood and softwood are xylose and monnose, respectively.

The extractives compose 5% to 30% of the wood substance. Included in this group are tannins and other polyphenolics, coloring matters, essential oils, fats, resins, waxes, gums, starches, and simple metabolic intermediates.

These materials can be removed with simple inert neutral solvents, such as water, alcohol, acetone, and benzene. The amount contained in an individual tree depends on the species, growth conditions, and time of year the tree is harvested.

The ash-forming materials account for 0.1% to 3.0% of the wood material and include calcium, potassium, phosphate, and silica.

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