Effects of pressurised high consistency refining on sheet density

Abstract

This study investigates the possibility of decreasing the sheet density of paper made from unbleached kraft pulp for the middle layer in board production. The objective was to evaluate how treatments in a compressive screw equipment pressure expansion (PREX) plug screw feeder, atmospheric (HCatm) and pressurised high-consistency refining (HCpress refining) affect both fibre and paper properties and, in particular, how fibre curl affects sheet density.The plug screw pre-treatments and HC refining were performed under atmospheric and steam-pressurised conditions at 100 degrees C, 125 degrees C and 175 degrees C. Even though fibre curl increased considerably during plug screw treatment and HCatm and HCpress refining, the density of the paper sheets did not decrease, as had been expected. A possible explanation for this result may be that the fibres become more flexible during curl treatment, resulting in increased sheet density.In order to obtain fibres that were both very curly and stiff, three ways of improving the fibre stiffness were tested: 1) lowering the pH during refining, 2) drying the fibres (hornification) and 3) treating the fibres with 1,2,3,4-ButaneTetraCarboxylic Acid (BTCA). If the fibres are dried (hornified), there is a lowering effect on sheet density. However, fibre curl does not have any lowering effect on sheet density. If the pH is decreased during plug screw treatment, the sheet density may be reduced but, again, the fibre curl does not have any positive effect on sheet density. Fibres treated with BTCA had a marked lowering effect on sheet density. With this treatment, the fibre curl does have a positive effect on sheet density. The BTCA-treated fibres were also used to lower sheet density. A mixture of 15% by weight BTCA- treated fibres resulted in an approximately 9% density reduction.