The effects of cationic starch wet-end addition on the mechanical and optical properties of clay loaded papers are discussed. It is shown that massive strength improvements can be achieved at high filler loadings with high starch additions. The properties of super-filled paper structures with filler loadings up to 90% are also reported. Cationic starch wet-end addition is superior to starch impregnation applications (e.g. size press) on highly filled structures. This behaviour is understood from the effect of wet-end starch addition on sheet consolidation (sheet density improvement). Wet-end starch addition increases the drying stress built up during sheet consolidation. This increase is generally much higher for filled papers than for papers of zero filler content.
The effects of fillers and wet-end starch addition on the intensity of stress concentrations in paper structures have also been investigated. It was found that an increase in filler level increases the stress concentration, whereas starch addition leads to a decrease. Addition of starch may actually bring the stress concentration intensity in a highly loaded sheet down to the level of a sheet with no filler at all. On the basis of the results, a mechanism of lubrication by which wet-end cationic starch addition improves the strength properties of filled papers is proposed .