Fibre length effect on fibre suspension flocculation and sheet formation

Fibre suspension flocculation was studied in a flow loop system, simulating the flow conditions in a modern paper machine headbox. The fibre suspensions after the headbox were accelerated by a secondary contraction, mimicking flow accelerations that may take place during forming. Fibre suspensions were studied using an optical detection method. Images were taken by a high speed video camera with transmitted infrared laser light pulse illumination separately before and after the secondary contraction. The mean fibre floc size was evaluated by power spectrum analysis in MID and CID respectively. A series of twelve pulp samples, four different commercial bleached softwood kraft pulps refined to three different levels, were used in the experiments. The fibre physical properties and formation number for paper sheets produced on a pilot paper machine with these pulp samples were characterised. Refining shortened the fibre length, thus decreased the fibre floc size in flowing suspensions and improved the formation in paper sheets. A series of mixtures at different ratios of bleached softwood and hardwood kraft pulps were also used in the experiments. The fibre physical properties of the mixtures were also characterised. The fibre length increases when increasing the ratio of softwood pulp in the mixture, thus fibre floc size in the suspension increases.