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Optimisation of fibre suitable for coated paper production
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posted on 21.02.2017by Banham, Paul Wayne
Pilot plant coating trials compared a coating basesheet made from P.radiata (radiata pine) TMP with several European basesheets. The pine basesheet compared favourably when coated using a metered film press coater though more fibre rising occurred with it during coating. The coated sheet also required more severe calendering conditions to meet the gloss target. It was suggested that the P.radiata TMP used in the trial basesheet would benefit from additional processing that would promote stripping of material from the long fibre wall in order to make it more collapsible and promote improved strength and surface roughness. The TMP would also benefit from processes that would generate more split fibres.
The properties of TMP originating from the Australasian mills of Norske Skog were examined and compared to TMP from spruce. Some additional measurements of %split fibres and bendability were made on the pulps at PFI. A Fiberlab fibre analyser was used extensively to measure fibre dimensions.
Factor analysis of pressing experiments aided in identifying the P.radiata pulps that approached the quality of spruce pulp used for coating and SC grades of paper.
Pulps were fractionated and the dimensions of the fibres, the specific surface area determined from sedimentation measurements and the physical and optical properties of each of the fractions were measured. Specific surface area, tensile index, roughness and sheet density were all significantly correlated and could be used as measures of fibre development. The long fibre fraction was identified as being the poorest quality fibre and most in need of development.
Mill screening operations were examined to look at their selectivity in removing the long fibre fraction from the rest of the pulp so that it could be further processed to promote fibre collapse and flexibility. High consistency screening was compared with low consistency screening and it was concluded that a multistage low consistency screening system was more selective than a single high consistency screen.
When refined in a low consistency refiner the properties of the whole pulp improved but the long fibre component showed no detectable improvement in density, tensile or wall thickness. High consistency refining of screen rejects improved the whole pulp properties and the properties of the long fibre with significant improvements in specific surface area and reduction in the fibre wall thickness.
Improved long fibre screen selectivity followed by high consistency refining of the long fibre is a strategy for improving the quality of the fibre for a coating basesheet.