Fibre rising and surface roughening in lightweight coated paper - An environmental scanning electron microscopy study

Printing with water-based ink may suffer from fibre rising and associated surface roughening due to the wetting involved. These detrimental changes were simulated in commercial lightweight coated (LWC) paper by wetting and drying in the environmental scanning electron microscope (ESEM) sample chamber. Most fibres in the basesheets [made from thermomechanical pulp (TMP) and groundwood] were flat and ribbon-like. They swelled upon wetting and shrank upon drying, but remained flat. These changes were reversible. Other fibres were hollow and tube-like. They also expanded upon wetting, but showed different behaviour upon drying: (1) flattening and collapse; or (2) retention of the tube shape and, in some cases, movement of loose fibre ends. Some fibre ends rose out of the sheet surface upon drying. Rewetting returned the fibre ends to a position largely within the sheet surface. Further drying resulted in rising again. Isolated groundwood and TMP fibres with tube shape showed the same types of contrasting behaviour, that is, collapse upon drying or retention of the tube shape. From these observations we conclude that fibre rising is limited to comparatively stiff fibres that maintain their shape upon wetting and drying. Rising occurred during drying, not during wetting and swelling. Fibre rising might be avoided by eliminating the tube-like fibres, or by further refining to reduce their stiffness to an acceptable level. As evidence of this, we observed that some of the TMP fibres collapsed upon drying instead of showing rising. Fibre rising was thus observed here in fibres which retain their tube-like shape (despite refining and calendering) and not, as suggested in the literature, in ribbon-like fibres returning to an uncollapsed tube-like shape when wetted. Fibre rising was less common in the sheets than expected from the strong gloss reduction and surface roughening of the LWC paper seen upon wetting (outside the ESEM). In other words, gloss reduction and roughening may be caused in part by processes other than fibre rising. Roughening might also result from the association of flat and tube-like fibres in the sheet. Tube-like fibres swell radially (vertically) much more than do flat, collapsed fibres, which leads to thin zones in the coating over the raised fibres and coating cracks along their edges. These types of damage of the coating were quite common. This mode of sheet roughening does not require fibre rising in the strict sense, that is, rising of fibre ends out of the sheet surface, but the tube-like fibres causing it might show fibre rising at their tails to some extent.