The influence of polyvinylamine microgels on paper strength

Bleached softwood kraft pulp fibre suspensions were treated with six polyvinylamine (PVAm) microgels with average particle size ranging from 1.5 to 38pm, and based on two levels of crosslinking monomer. Handsheets were prepared and tested. PVAm microgels increased wet and dry paper strength compared to a linear PVAm (MW 950 kDa) when the dosage was above 0.2 wt%. The smallest microgel with the lower crosslink density gave the best performance among the six microgels. It is proposed that microgels are more effective at improving paper strength than are the corresponding linear polymers because more of the large gels are retained in the fibre-fibre joints. In contrast, the gels cannot penetrate existing fibre-fibre joints so the microgels are not effective as impregnation agents for increased strength. The details of microgel composition are important. Microgels with the lowest crosslinker content (highest swelling) and the smallest size (∼ 1.5 μm) gave stronger paper than gels which were more crosslinked or larger (>10 μm).