The influence of furnish composition, sheet structure and pressing conditions on wet pressing performance of newsprint
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The choice of furnish in newsprint production is dictated by the raw material costs, production cost and the final quality demanded by printers. In addition to the raw material costs, energy cost is a major factor in papermaking economy. Modern newsprint machines have combinations of roll and shoe presses to increase web dryness to approximately 50% after wet pressing and thus keeping drying cost as low as possible. Thermomechanical pulp and deinked pulp are commonly used for producing newsprint nowadays. The increasing use of deinked pulp and filler in newsprint production has improved press dewatering. However, there is a lack of published research on the effects of the nip types, furnish and filler distribution in the web on sheet dewatering and sheet structure after wet pressing. The dynamic wet pressing simulator at NTNU-PFI was used for the pressing experiments. A single sided dewatering setting was used. The wet pressed samples were freeze dried in liquid nitrogen after pressing. The cross-sections of the freeze dried samples were imaged in a SEM and the cross-sectional structures were quantified. The cross-section images were split into top, middle and felt or dewatering side layers based on equal solids per layer per line length. The Euclidean distance maps generated by ImageJ software were used to quantify the distribution of fines, pores size, porosity and thickness of the sample. To understand the filler, fibre and MFC interactions, images for surface characterization were acquired using FE-SEM. 3D images for DIP and TMP samples pressed with shoe pulse were acquired using x-ray micro-(computed) tomography (X-μCT) for visual assessment of the densification across the sheet’s thickness. In addition, the surface roughness was quantified. As expected, deinked pulp (DIP) dewaters more easily than thermomechanical pulp (TMP) when pressed with a roll pulse. The in-plane moisture flow (crushing) in DIP samples started at higher peak pressure and higher solids content compared to the TMP samples. The TMP samples showed higher springback after wet pressing compared to DIP samples. When both TMP and DIP samples were designed to have a higher amount of small pores at the dewatering side’s surface layers, the samples with the coarser TMP pulp could achieve similar dryness as DIP samples when pressed with higher nip pressure. In addition, the DIP samples with the higher number of small pores at the dewatering layer also yielded lower dryness after pressing with roll pulse. The effect of filler (ground calcium carbonate, GCC) distribution in the paper’s zdirection on final dryness and sheet structure after the wet pressing with a roll pulse was also studied. The distribution of filler did not affect the maximum dryness after wet pressing (40%) for sheets containing identical (13%) amount of filler. However, the samples with more filler in the dewatering layer dewater more easily and yield the 40% maximum achievable dryness after wet pressing at a lower pressure. As a result, the samples with more filler in the dewatering layer yielded higher thickness as compared to samples with filler distributed evenly in the z-direction. In the study on wet-pressing using two different shoe press pulse profiles, the samples made from TMP and DIP yielded identical after-press solids content. More water was pressed out of the TMP samples as compared to DIP samples since TMP samples had lower initial solids content. The wet webs with more porous structure in the dewatering layer yielded higher dryness after wet-pressing. This is a major advantage, as higher dryness after wet-pressing implies a reduction of steam consumption in the dryer. This study also showed that the shoe press pulse with high peak pressure at the end yielded higher solids content and apparently higher bulk after the wet-pressing compared to the shoe press pulse with high peak pressure in the beginning. Higher filler content in newsprint increases the risk of linting at the printing press. Recent advances in technology and research in producing microfibrillated cellulose (MFC) should enable the production of MFC with acceptable energy cost in the near future. MFC can be used to improve the strength properties of the paper. Optimal selection of MFC quality and filler content can improve the optical properties while maintaining or enhancing the surface strength of the sheets. Different qualities of microfibrillated cellulose (MFC) were blended with TMP and ground calcium carbonate (GCC). The addition of MFC decreased the drainage rate of the pulp suspension but improved the strength properties. Wet pressing experiments showed that optimal use of MFC and filler could enhance the strength and optical properties without reducing the solids content after wet pressing. Field emission SEM revealed that the MFC particles adsorbed onto filler particles and fibres. The MFC binds the filler-MFC-fines aggregates to the fibre network, thus partly filled the pore network. As a result, MFC addition increased the air resistance and internal bonding of the sheet.