Microgels versus linear polymers for paper wet strength - Size does matter

Polyvinylamine (PVAm) microgels (∼1.5 μm diameter) were compared with the corresponding linear polymer (∼40 nm) as wet strength resins for paper. Handsheet studies showed that microgels gave substantially higher wet tensile strength than the same dosage of linear polymer. By contrast, impregnation of filter paper with polymer or gel solution gave the opposite effects - wet paper impregnated with linear polymer was stronger than the microgel treated paper. Finally, the wet delamination force for regenerated cellulose films laminated with PVAm was approximately the same for linear polymers and gels. It is proposed that microgels gave stronger handsheets because the fibres retained more microgel than linear polymer in the fibre-fibre joints. However, the microgels were too large to penetrate fibre-fibre joints during impregnation of paper sheets.