Thermal Transition
The point at which the natural phenolic polymer in wood fibres transitions from a rigid, glassy state to a more pliable and rubbery condition marks a physical shift in the wood structure. Lignin softening occurs when the material is heated, particularly in the presence of moisture, during the mechanical pulping or thermomechanical processing of wood chips. This change allows the fibres to be separated with less mechanical force and less damage to the cellulose.
Structural Effect
Heat acts upon the amorphous regions of the polymer to increase molecular mobility. In a softened state, the lignin no longer holds the fibres in a stiff matrix, allowing the pulp to conform more easily during the formation of a paper sheet. This plasticity is temporary, and the material regains its rigidity once it cools down and the moisture evaporates.
Lignin softening ensures the resulting paper has higher bulk and better opacity because the fibres are less compressed during the initial pressing stages.
Energy Requirement
Reaching the softening temperature reduces the total electrical energy consumed by the refiners. Higher temperatures lead to longer, more intact fibres because the separation occurs at the interface between the cells. This produces a pulp with superior strength compared to cold mechanical refining.
