Moisture-Induced Surface Roughness in TMP Sheets: Effects of coarse fibre decollapse and internal bond strength

摘要

Favourable surface properties can be attained in softwood thermomechanical pulp (TMP)-based sheets, if most of the thick-walled long fibres are collapsed. Provided this can be done, a large amount of TMP can be used in high-quality wood-containing papers, i.e., LWC or SC grades, without serious printability problems. It is known that the degree of fibre collapse during refining under standard TMP conditions is strongly related to the fibre wall thickness. However, the thickness of the thickest fibre walls cannot easily be changed by peeling actions in refining under standard TMP conditions in the CSF range of interest for news and magazine grades. Therefore, refining conditions obviously must be changed to enable the proper collapse of these fibres. Earlier studies (Norgren and Hoglund, IMPC 2003 and 2005) have demonstrated that thick-walled fibres are more easily collapsed by reject refining after preheating to temperatures well above the softening temperature of lignin, i.e., high temperature conditions, than under standard conditions. If water is applied to the surface of a TMP-based paper during coating or printing, some fibres "decollapse", i.e., revert to their original shape. Earlier studies have demonstrated that decollapse is affected by both fibre wall thickness and fibre circumference (Norman and Hoglund, IMPC 2003 and Norgren and Hoglund, IMPC 2003). The present study evaluates the effects of moisture on sheets made of different Bauer-McNett (BMN) fractions, both individually and in combination. The pulps investigated are spruce TMP reject pulp made under reference and high-temperature conditions. The smoothness of laboratory sheets made of the BMN fractions were measured with PPS and with an optical scanning instrument, after moistening the sheets in environments of different relative humidities, i.e., 50-98%, to a sheet moisture content (MC) in approximately the 5-25% range. This was done to gain a better fundamental understanding of the conditions resulting in irreversible fibre collapse and of how combinations of types of fibres with different bonding abilities influenced the surface roughness. It is well known that the coarse, stiff, long fibres from the BMN >16 fractions cause surface roughness in dry sheets. However, this study indicates that the shapes of these fibres remain relatively unchanged when moisture diffuses into the sheet structure, i.e., the degree of decollapse is low. It also indicates that sheets including both coarse fibres and fibres from the middle fractions display the highest surface roughness values at a high MC. The fibres from the middle fraction rise easily, causing the greatest change in surface roughness when moisture is added to the sheet. The trials also indicate that fines contribute somewhat to increased surface roughness; when the degree of bonding in the sheet structure is increased, there is a risk that the whole structure or fibre flocks, instead of the individual fibres, may swell.