Fiber Softening
Thermomechanical processes involve the use of heat and moisture to soften the natural polymers that bind cellulose fibers together in wood. Through lignin plasticization, the rigid structure of the wood becomes more pliable, allowing the fibers to be separated without excessive breakage. This state is reached during the steaming stage of pulp production.
Temperature Threshold
The transition occurs when the wood is heated above its glass transition temperature, which decreases as the moisture content increases. Achieving lignin plasticization ensures that the mechanical energy used in refining is directed toward separating the fibers rather than fracturing them. This preserves the length and strength of the individual fibers.
Refining Efficiency
Operating at the correct temperature reduces the amount of electrical power required to produce a ton of pulp. If the temperature is too low, the lignin remains brittle and the resulting pulp will contain more shives and damaged fibers. If the temperature is too high, the fibers may become too soft and lose their structural integrity.
This balance is critical for producing high-quality mechanical pulp for use in lightweight coated papers. The process also influences the final color and brightness of the pulp. Careful control of the residence time in the steamer is a requirement.
Steam pressure must be held within narrow limits to maintain stability.