Inter-Ply Bond Strength Development in Recycled Fiber Formations under Variable Dewatering Pressures
Optimizing shoe press peak pressure and wet-end couch solids maximizes inter-ply bond strength in multi-ply recycled fiber formations.
Schopper-Riegler Drainage
Freeness Index
Empirical testing procedures quantify the dewatering speed and drainage resistance of dilute aqueous pulp suspensions under controlled laboratory conditions. Schopper-riegler drainage measures the rate at which water separates from a two-gram per liter fibre suspension through a standardized phosphor-bronze wire mesh under gravity. The resulting value, expressed in degrees Schopper-Riegler, provides an inverse index of pulp drainage speed, indicating the degree of mechanical refining and fibrillation applied to the furnish.
The measurement evaluates stock slurry behavior in laboratory apparatuses, stopping at the boundary of dynamic multi-axial paper machine forming fabrics.
Refining Index
Mechanical refining fibrillates cellulose fibers, delaminates cell walls and generates microscopic fines to improve sheet consolidation. As refining progresses, schopper-riegler drainage values rise from approximately twelve to fifteen degrees for unrefined chemical pulps up to sixty to eighty degrees for heavily refined specialties such as greaseproof papers. The testing apparatus splits drained filtrate through two distinct orifices, directing fast drainage through a large bottom orifice and slower overflow through a calibrated side measuring cylinder.
A higher volume discharged through the side orifice indicates elevated drainage resistance, yielding a higher Schopper-Riegler degree. Unrefined chemical pulp drains rapidly through the bottom orifice, producing low values that indicate free-draining pulp with low internal fiber swelling.
Drainage Boundary
Stock preparation operators balance drainage indices against mechanical strength and machine runnability requirements. Excessive schopper-riegler drainage values slow down dewatering on the paper machine forming table, requiring reduced machine speeds, increased vacuum levels or elevated steam consumption in drying cylinders. Conversely, excessively low drainage values indicate insufficient fiber fibrillation, resulting in weak fiber bonding, poor formation and reduced tensile properties in finished folding boxboards.
Papermakers monitor this index continuously to adjust refiner plate gaps and specific edge loading, stabilizing stock dewatering behavior ahead of the headbox. Consistent pulp freeness ensures predictable web consolidation on commercial fourdrinier and multi-ply board machines.