Viscoelasticity of In Situ Lignin as Affected by Structure

The viscoelastic properties of various wood species under water-saturated conditions have been studied by mechanical spectroscopy. Tests were made at temperatures from 20° to 140°C at frequencies ranging from 0.05 to 20 Hz. It is shown, contrary to earlier data, that hardwood lignins have lower softening temperatures than softwood lignins. For the hardwood lignins the softening process also has a lower apparent activation energy. Increased cross-linking of the lignin, achieved by heating in an acid environment, raises the softening temperature and increases the apparent activation energy of the softening process. In all cases the softening follows a WLF type of behavior, indicating that under wet conditions the viscoelastic properties of the lignin govern the properties of the wood fiber. The differences noted between softwood and hardwood lignin are discussed in terms of structural parameters.