Fibre Yield
High-yield wood pulp produced by mechanically breaking down wood chips under elevated temperature and steam pressure retains a high proportion of lignin while softening the matrix to preserve individual fibre length. Thermo-mechanical pulp functions as the primary high-bulk filler in newsprint and lightweight coated paper grades where stiffness and opacity outweigh tear resistance. Steam pre-treatment softens the middle lamella before disc refiners tear the wood apart, which avoids the extensive chemical digestion required for kraft pulp but leaves short broken fines mixed with long structural elements.
Residual lignin content prevents natural bonding capability, requiring high pressure in the paper machine press section to force the rigid fibres into direct contact. Surface roughness remains high unless heavy calendering flattens the sheet, which damages bulk and reduces bending stiffness.
Refining Energy
Electrical power consumption during secondary disc abrasion determines the final drainage resistance and tensile strength of the stock supplied to the paper machine wet end. Specific energy input dictates how many fibres split internally to expose microfibrils rather than simply cutting into short unusable fragments. Mills balance motor load against production throughput because excessive refining generates fines that plug the wire mesh and slow down dewatering speeds.
High freeness values indicate coarse drainage properties suited for thick packaging boards, whereas low freeness results in dense sheets prone to drying wrinkles. Steam generated inside the refiner housing requires continuous venting to maintain stable plate clearance and prevent uncontrolled fiber damage.
Opacity Retention
Light scattering coefficients depend on the bulky three-dimensional network formed by rigid unbleached or peroxide-bleached fibres within the finished paper structure. High scattering efficiency allows lighter basis weights to maintain sufficient print opacity without show-through on double-sided catalogue and magazine runs. Photo-induced yellowing occurs rapidly when ultraviolet light strikes the exposed lignin, causing severe brightness reversion in unsealed storage environments.
Accelerated aging tests measure this discoloration to establish allowable limits for retail catalogues and directory printing papers. Lignin preservation provides stiffness at lower grammage, which reduces transport fuel consumption for delivered print products.
