The role of lignin in wood strength

The process of wood formation is influenced by genetic and environmental factors. The resulting cellular morphology as well as chemical composition determine wood properties and consequently wood material quality as desired by the end-user. One of the major polymeric compounds playing a key role with respect to important wood properties is lignin. It is widely assumed that this incrusting material`s main function is to provide stiffness to the cellulose framework. However, a lack of experimental evidence concerning the role of lignin for the mechanical properties of wood is still present. As breeding programmes related to the pulp and paper sector often focus on a reduction of lignin content, the need for more experimental evidence is given. Therefore, the investigations described in this proposal will be focused on the significance of lignin to the strength and elasticity of wood. Special emphasis will be laid on the different behaviour of dry wood compared to wood in a living tree.

In an effort to question textbook knowledge on the role of lignin, one of the main wood constituents, in wood strength, experiments were performed in tension, compression, and shear. As it is often claimed that lignin gives compression strength, a main emphasis was put on this property both on the micro and macro levels. In wood science, an analogy frequently used when discussing the structure of wood cell walls is reinforced concrete. Due to their high tensile strength and fibrous structure, cellulose microfibrils are compared to reinforcing steel, while the cell wall matrix consisting of lignin is compared to compression resistant concrete. In the current project it was found that, regarding the mechanical significance of variable lignin content, a strict distinction must be made between developing and mature wood tissue. Lignification of the already existing cellulose-hemicellulose network is the final step in cell wall synthesis and completes maturation. By comparing the elastic modulus and hardness of developing, not yet fully lignified cell walls with mature, fully lignified cell walls, it was observed that increasing lignin content improves both stiffness and mechanical strength of the developing cell wall. For comparison, the axial compression strength of mature wood tissue with highly variable lignin content was also studied. An analysis of the compression failure mechanism in wood cell walls by means of failure criteria developed for fibre reinforced composite plates showed that fibre deviations in the vicinity of certain wood anatomical features such as rays are critical to compression failure, whereas cell wall structure, and in particular lignin content, did not exhibit any significant effect on axial compression strength. Therefore, it is not necessary to consider variability in lignin content as a quality parameter in the structural use of solid wood. In addition to compression strength, a significant part of the project was dedicated to the little studied shear behaviour of wood, also with an emphasis on structure-property relationships. For this purpose, a new test method was developed. Model calculations showed that microstructural features like lignin content do affect shear modulus and strength. This effect is readily observable when wood with highly variable lignin content is tested, e.g. normal and compression wood, but becomes less significant in commercial wood samples.