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.
Cationic Starch Bonding
Polymer Fixation
Wet-end chemical additives introduce modified natural polymers to the stock suspension to establish electrostatic linkages with negatively charged wood fibres and mineral fillers. Cationic starch bonding develops when quaternary ammonium groups on the starch backbone attach directly to anionic carboxyl and hydroxyl sites on cellulose fibrils. This attachment creates physical bridges across fibre-fibre interfaces during web drainage.
The resulting network improves dry tensile strength, z-directional surface cohesion and Scott bond values without requiring additional mechanical refining. It operates effectively in neutral and alkaline papermaking conditions, although excessive concentration of dissolved anionic trash in the white water system suppresses polymer retention and weakens final strength.
Strength Development
Refining mechanical pulp generates fines that increase surface area but reduce drainage speed on the forming fabric. Addition of cationic starch bonding compensates for limited refining by increasing hydrogen bonding sites across the consolidated sheet. Mill operators add cooked starch solutions at dosage rates between five and twenty kilograms per metric ton of dry pulp, typically introducing the polymer before the fan pump to ensure uniform dispersion.
Internal bond values rise steadily with dosage until available anionic sites on the fibres reach complete charge neutralization. Beyond this saturation threshold, unabsorbed polymer remains in the process water, which elevates chemical oxygen demand and destabilizes retention aid chemistry. Cohesion gains translate directly into higher burst strength and improved picking resistance during high-speed offset lithographic printing.
Charge Balance
Monitoring colloidal charge via streaming current detectors or zeta potential measurements maintains wet-end chemical stability. Excess cationic starch bonding reverses the overall furnish charge, which disperses fines, reduces filler retention and creates pinholes in lightweight paperboard grades. Accurate stoichiometric control keeps the furnish slightly anionic, ensuring that virtually all polymer chains bind permanently to the cellulose skeleton before the headbox slice.
Anionic trash catchers such as polyamines or polyDADMAC often precede starch addition to neutralize interfering pitch and dissolved wood polymers, preserving active sites for fiber-starch attachment. Precise electrostatic balance prevents foaming in the white water circuit and stabilizes dewatering rates through the press section.