Lignocellulosic fibers are biomaterials composed of cellulose, hemicellulose, and lignin. The content of the components depends on the wood species. Table 14.3 shows a general chemical composition (cellulose, hemicelluloses and lignin con­tent) as well as the fiber length in hardwoods and softwoods.

In general, hardwoods species present slightly higher cellulose content and sig­nificantly lower lignin content. Hemicellulose content between the two is variable depending on the wood species.

The hollow cellulose fibrils are heterogeneously embedded within a matrix of hemicellulose and lignin. Bonds between carbohydrates (hemicellulose and cellu­lose) and lignin and between hemicellulose and cellulose are present within the ce­menting matrix. The forming cellulose-hemicellulose network comprises the main structural component of the fiber cell. On the other hand, lignin-carbohydrate bonds (presented as benzyl esters, benzyl ethers, and phenyl glycosides) increase the stiff­ness of the cellulose-hemicellulose composite.

With respect to structure, the fibrils possess a thin primary wall (S1) first formed during cell wall biogenesis that is engirded by a larger, more voluminous secondary wall (S2) made up of three layers with a middle layer determining the load-bearing capacity of the wood fiber. This middle layer can be characterized as a rope-like structure of helically wound microfibrils from long chained cellulose macromol­ecules. An important property controlling the ultimate mechanical strength of the wood cell architecture is the angle betwixt the fiber axis and the microbrils known as the MFA (microfibril angle) that spans a gamut of values depending on the wood species.