Читать книгу Adhesives for Wood and Lignocellulosic Materials - R. N. Kumar - Страница 14

Wood is basically a fiber-reinforced polymer matrix composite with cellulose as the fiber reinforcement and lignin along with polyoses functioning as mixed polymer matrix. Wood has been found to have a hierarchical cellular structure. The functional efficiency of wood, its exceptional mechanical properties, and other unique characteristics are attributed to the distinctive structure at all its levels of hierarchy. The complex hierarchical structure of wood manifests in a wide range operating from the macro scale in the case of tree to board to growth ring (macro scale), through multilayered cell walls, composite cellulose/hemicellulose microfibrillar structure (micro scale), and down to the structure of the three main polymer components, cellulose, hemicellulose, and lignin (nanoscale).

Hierarchical solids contain basically structural elements that, in turn, have further finer structures; i.e., intricate structural features occur at different size scales.

For instance, the hierarchical nature of wood showing the size scale of each structural element within wood has been illustrated by Moon et al. [3] with typical dimensions given in parentheses:

Tree height (in meters), tree cross section (in cm), growth rings (in mm), cellular structure (in 500 μm), cell wall structure (in 25 μm), fibril-matrix structure (in 300 nm), fibril structure (10 nm), and cellulose (1 nm).

This structural hierarchy of wood can play a significant role in influencing the phenomenon of wood adhesion [3].

In order to maximize the strength and durability of adhesive bonds, one should understand the complex hierarchical structural elements of wood and their interactions with the molecules of the adhesives at various size scales.

It should be appreciated that hierarchical structural elements of wood when exposed to the molecules of adhesives during bonding interact at different size scales in an extremely unique manner not encountered in the case of other adherends. Although the concept of hierarchical model of wood has not been explicitly mentioned, the importance of practical length scale of wood composite elements in wood adhesive bonding has been recognized and reported [4].