Thermal Transition
Temperature range where the amorphous lignin polymers in wood transition from a hard, brittle state to a soft, rubbery condition. Understanding the lignin glass transition is essential for optimizing the mechanical pulping and calendering processes. When this threshold is reached, the fibers become easier to separate and reshape without excessive damage.
Molecular Behavior
Moisture acts as a powerful plasticizer that greatly lowers the temperature required for this phase change. In a dry state, the lignin glass transition might occur at temperatures exceeding one hundred and fifty degrees Celsius. However, under saturated conditions in a refiner, this point drops to near the boiling point of water.
Thermal shifts allow for the softening of the fiber middle lamella, which facilitates the production of high quality mechanical pulp. Heat and steam are applied specifically to reach this state before the wood chips are subjected to mechanical shear. Softened lignin can then flow and coat the fiber surfaces, which impacts the final bonding properties of the paper.
Processing Boundary
Cooling the material below this critical temperature locks the fibers into their new configuration. If the lignin glass transition is not properly managed during the press section, the resulting paper may lack the desired surface smoothness. Consistency in temperature control is therefore a requirement for uniform production.
