Paper is a heterogeneous and porous network constructed from fibrous material, fillers and different chemicals. In the papermaking process, fibres tend to form flocks which lead to differences in the local basis weight of paper. Even so, in paper network, there is a high probability of finding similar basis weight values at short distances [108]. The basis weight distribution is therefore not totally random. The structure of the paper web affects the uni- and multiaxial mechanical properties of paper web, which affects how paper can resist the forces affecting the web during papermaking processes and end use [3].
5.1 Fibre orientation
In machine-made papers, more fibres are aligned in the machine direction than perpendicular to it. Fibre orientation refers to this anisotropy in the structure of paper [108]. The target fibre orientation level for each paper grade and for each paper machine is determined by the requirements of the final product and the demands of the process. Fine paper grades, which are the main research target in this thesis, are typically produced using low fibre orientation (which is controlled with the jet/wire ratio). This is because fibres mainly shrink in the cross direction and too high orientation could therefore cause adverse effects, such as reduced CD dimensional stability, and increase the wrinkling during printing. Local variations in fibre orientation also have a strong effect on the cockling of paper. An increased orientation also leads to higher tensile strength and tensile stiffness of the final product in the machine direction and tear energy in the cross direction. However, at the same time, tensile strength and stiffness in the cross direction decrease along with dimension stability. On the other hand, increased fibre orientation results in higher MD tensile stiffness of wet paper, which facilitates the press-to-dryer transfer and therefore improves the stability of the running web in the open draw [109].
In addition to fibre orientation, dryness of the wet web after wet pressing has a significant effect on wet web behaviour in press-to-dryer transfer and at the beginning of the dryer section [110].
5.2 Effect of wet pressing
Wet pressing consolidates the wet web by removing water from it. The dryness of the web when entering the press section is typically about 20%. At this dryness level, water is already expelled from fibre walls. In modern press sections, the last press nip typically operates in the dryness range of 40-50% [110]. Water removed from the paper web during wet pressing contains particles originating from the web. However, the amount of removed particles (mainly fines and fillers) is quite small and therefore this rarely affects the z-direction materials distribution in paper to such an extent that it would have a significant effect on paper properties [111, 112]. Due to wet pressing, some pores in the fibre wall are closed, causing fibre hornification. Hornification in wet pressing has also been called “wet hornification” [113]. Wet pressing increases the average density of paper and it can have a significant effect on the z-direction density distribution [114]. The change in density induced by wet pressing is greatly affected by the properties of the used furnish. Pulps with low bonding ability or high stiffness result in low density [110]. The increase in the sheet density in wet pressing affects many of the mechanical properties of dry paper. Web consolidation improves fibre bonding which results in increased tensile strength, burst strength, and z-directional delamination energy. On the other hand, the opacity, stiffness, and compressibility of the paper reduce when the intensity of wet pressing increases.
The wet pressing is dependent on at least the following parameters [110]:
Nip pressure and pressure distribution, nip residence time,
temperature,
properties of the web.
The mechanical properties of the wet web, such as tensile stiffness and tensile strength, are known to increase rapidly with increasing dryness [115]. Figure 29 shows that increased dryness after the press section increases the paper machine production speed still giving an acceptable runnability (amount of web breaks) with a wood-free paper grade.
Figure 29. Speed vs. dryness after press [115]. The production speed of paper machine giving an acceptable runnability with a wood-free paper grade increases with increasing dryness (the actual dryness values are not presented).
However, in some cases (especially for wood-free grades), it is not the efficiency of dewatering in wet pressing that limits the increase of production speed, but the strong market requirements for high bulk. Figure 30 shows how the increased dryness of wet web caused by more intensive wet pressing reduces the bulk of the end product. Because of this, a higher production speed of wood-free paper grades cannot practically be achieved by increasing dryness through more intensive wet pressing [115].
Figure 30. Bulk after press vs. dryness after press [115]. Bulk of paper (actual values are not presented) reduces with more intensive wet pressing.
According to Paulapuro [110], very little can be done to optimise the wet pressing variables or press configuration in a way that higher wet web stiffness and strength could be achieved at a given dryness. There is more potential for optimising pulp composition, networks structure or use of chemicals to improve the mechanical properties of wet web.
However, it is not common that paper mills use chemicals that are especially designed to improve the mechanical properties of wet web. Therefore, it is important to identify the main factors that affect the mechanical properties of wet web and to understand how the papermaking chemicals used today influence wet web properties. In addition, it would be valuable to discover what kind of chemicals and adding strategies could be used to improve wet web mechanical properties.