Title
From fibre surface to sheet stiffness: a comprehensive study of refining effects
Authors
NISHI K. BHARDWAJ

Received December 6, 2025
Published Volume 60 Issue 3-4 March-April
Keywords Young’s modulus, pulp refining, surface charge, specific surface area, freeness, fibre bonding

Abstract
This work studied the effects of refining on the physicochemical and mechanical properties of five pulps: three softwood kraft (KP1, KP2, KP3), one hardwood kraft (KE1) and one NSSC eucalyptus pulp (NE1). The freeness, specific surface area, surface charge, water retention value and Young’s modulus were measured at different refining levels. It was found that refining increased surface charge and the ratio of surface-to-total charge in a quite consistent way for all pulps, indicating increased accessibility of fibre surfaces rather than changes in bulk fibre chemistry. Specific surface area also showed an increase with refining, confirming external fibrillation and generation of fines. The Young’s modulus increased steadily with decreasing freeness for all the tested pulps due to increased bonding between fibres and consolidation of fibre network. KE1 reached the highest final modulus (5271 MPa), whereas KP1 and KP3 had a good stiffness development after moderate refinement. Linear regressions showed that Young’s modulus was strongly correlated with surface charge, freeness and water retention value. These results emphasized refining as one of the primary ways of optimizing stiffness in papermaking.