Physico-chemical aspects of filler retention in papermaking

One-pass retention of anatase, rutile, and hydrated alumina of varying isoelectric points and particle shapes, have been studied individually in papermaking regarding the effects of fibre-fines, aluminium sulphate, rates of deposition, and hydrodynamic shear. A new experimental approach has been adapted to study both the validity of the DLVO (Derjaguin, Landau, Verwey and Overbeek) theory, and the mechanism of filler retention, in papermaking.

The electrokinetic properties, and the retention behaviour, of fillers, particularly in systems of filler and fibres in the absence of aluminium sulphate, are mainly affected by soluble cellulosic materials leached from the pulp-fibres; in this respect, fibre-fines behave very similarly to the main fibres except that due to their large surface area they appear to be solely responsible for the electrokinetic modification.

Any significant filler retention in papermaking is most often due only to heteroflocculation prior to the sheet-formation. The magnitude of one-pass retention, in the absence of energy barriers to heteroflocculation, is most affected by the rate of deposition of the filler particles onto the fibres. A rotating disc, with a cellulose film in the centre, has been used to show that 100% retention of all the filler particles transported to the surface could be achieved. Similar results have been obtained by pulp free of fibre-fines; though heteroflocculation in furnishes containing fibre-fines is also similar retentions are usually lower due to the loss of filler-fibrefine floes from the sheet.

Another factor which affects the overall magnitude of the observed retention is the hydrodynamic shear during papermaking. It has been shown that the effect of shear prior to the sheet formation is much more detrimental to retention, than the relatively minor effect due to shear during the fast drainage at the wire. An attempt was made to measure the relative force of adhesion of filler particles, deposited on a cellulose film wrapped over a cylinder, using the shear at the surface of a rotating cylinder as the force for particle removal. The results show that a force > 200 pN is necessary to remove a primary particle.

It is proposed that low magnitudes of one-pass retention generally encountered, even when the electrokinetic properties of the components suggest that high retentions should be observed, are due mainly to a combined effect of the slower rates of deposition and the pre-mat-formation shear forces.