Papermaking effects
Forming of the paper web has an effect on the roughness coming from the distribution of fines and fillers in the z-direction. High concentrations of fillers or fines in the surface layers of paper give low roughness. Wire marks that may remain in paper in forming, wet pressing and drying also affect macro roughness. The marking by wires occurs by local compression of the wet paper. Usually, the strongest wire marks come from forming wires. These are particularly evident in wood-containing papers. Even weak wire marks can be visible to the naked eye because of their periodic texture.
In the drying section, the hot drying cylinders reduce roughness. The smooth cylinder surface reproduces on the paper because hot and moist fibres deform easily. The largest decrease in roughness occurs before reaching 60% solids content1. Drying shrinkage makes fibres crimpy and therefore increases roughness (shrinkage roughening), as shown in Figure 1. The base paper (uncalendered) is typically unsuitable as such for most applications.
Figure 1. Bendtsen roughness of uncalendered newsprint vs. drying shrinkage2
In the papermaking and specifically in the finishing process, roughness is naturally controlled by calendering. A breaker stack, machine calender, multi-nip or supercalender can produce the smooth surface required. Pigment sizing or coating also reduces roughness because the small particles fill voids on the base paper surface.
In calendering, surface roughness decreases proportionately more than thickness. In principle, strong calendering could remove all roughness, but this is usually not possible because of the simultaneous loss of paper thickness and bulk. If calendered under constant conditions, the order of papers usually does not change. A rougher paper remains rougher also after calendering.
In practice, there is usually a target level of roughness to which the paper must be calendered. The corresponding loss of thickness depends on the furnish composition. For example, Figure 2 shows how coarse latewood kraft fibres gave higher density than earlywood fibres at the same PPS, although the opposite was true for uncalendered paper. The corresponding densities were 440 and 280 kg/m3 at zero fines content 3. Beating of the latewood fibres increased uncalendered density to 430 kg/m3 but only had a mild effect on the calendered density.
Figure 2. Density of softwood handsheets calendered to a constant 4-μm PPS roughness with different amounts of added TMP fines (a) or kraft fines (b) using unbeaten earlywood, unbeaten latewood, and beaten latewood fibres 3
The thickness variation of uncalendered paper reflects formation. There may exist a close linear correlation between local grammage and thickness 2. In calendering, thicker areas are compressed more than thin areas. Thin areas therefore remain rough, and thick areas lose porosity and opacity. Soft nip calendering alleviates these negative effects. Compared to hard nip calendering, soft nip calendering gives more uniform roughness and porosity but less uniform thickness, as shown in Figure 3.
Figure 3. Effect of hard and soft nip calendering on the lateral variation of roughness, porosity, and thickness
If the nip load in calendering increases, the shape of the surface height distribution at micro level first changes but then remains constant when the nip load increases further 1. The distribution does not become markedly skewed. However, increasing calendering makes optical roughness clearly skewed, as shown in Figure 4. The highest fibre surfaces become completely smooth while surface troughs close to fibre crossings and the like remain unchanged 3.
Figure 4. Distribution of surface heights at different levels of calendering measured with a confocal scanning laser microscope at 200 nm lateral resolution. Optical roughness values given by3 RRMS.
