Fundamental aspects of wood as a component of thermoplastic composites

In order to utilize wood‐based particles and fibers effectively as fillers or reinforcements in thermoplastic composites, a fundamental understanding of the structural and chemical characteristics of wood is required. Wood is the secondary xylem of trees, shrubs, and woody lianas (vines). Although the physiological function of wood is similar among different groupings of trees, significant differences are found in the basic anatomical structure of the broadest groupings of gymnosperms (coniferous trees or “softwoods”) and angiosperms (broad‐leaved trees or “hardwoods”). In addition, there are anatomical differences in wood structure among the various species of trees. These structural differences may have effects on the use of these materials in composites. Wood cell walls comprise three major organic constituents, namely, cellulose hemicelluloses, and lignin. In addition to these structural polymers, numerous other organic materials (“extractives”) may be present within the wood. The chemical composition of wood varies between species. The basic characteristics of anatomy and structure combine to impart variations in permeability, bulk chemistry, and surface chemistry. Characterization of particle size and shape, as well as surface tension characteristics as indicators of wettability, becomes important as we try to understand how these biopolymeric materials will behave when introduced into synthetic polymer systems.