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R.P. Kibblewhite. Effect of refined softwood: eucalypt pulp mixtures on paper properties. In Products of Papermaking, Trans. of the Xth Fund. Res. Symp. Oxford, 1993, (C.F. Baker, ed.), pp 127–157, FRC, Manchester, 2018.


Fibre property, refining requirement, and handsheet strength and optical property interrelations are examined for a eucalypt and several softwood market kraft pulps and blends. Market krafts included in the study are radiata pine pulps of low and medium coarseness, a benchmark pulp from the interior region of British Columbia, and a eucalypt pulp from Brazil. Eucalypt: softwood blends are in proportions of 100:0, 50:50, 80:20, and 0:100, and effects of separate and co-refining are assessed using -a laboratory scale Escher Wyssre finer which is considered to be indicative of commercial scale refining operations.

For the softwood pulps, refining at the low 1 Ws/m specific edge load has minimal effects on fibre shortening, fibre collapse, and wall expansion and delamination. Under these conditions fibres are neither rapidly rewetted nor made flexible. The converse occurs with refining at 3 and 5 Ws/m.

Tensile strengths are relatively high and softwood fibre walls are slow to respond with refining at low specific edge load. Such effects are consistent with the retention of fibre stiffness and length, and the development of high bonding potential. The high bonding potential is presumably developed through selective fibre surface disruption, wetting, and molecular and micro level fibrillation. Light-scattering coefficient/tensile index relations are independent of specific edge load and indicate mutual compensatory responses for these handsheet properties.

For eucalypt: softwood blend proportions of about 80:20, tear/tensile relationships (reinforcement properties) and light-scattering coefficients (optical properties) are roughly the same and independent of the origin or type of softwood used in the investigation. Such results are to be expected since there are only 2-3% by number of softwood fibres included in the 80:20 eucalypt: softwood furnish blends. For 50:50 eucalypt: softwood blends the effects of using softwoods of different fibre quality are also relatively small.

With co-refining reinforcement properties are decreased, and optical properties can be increased depending on specific edge load. It is envisaged with co-refining that the small number of softwood fibres present in the 80:20 eucalypt: softwood softwood blends (<3%) receive disproportionate levels of the refining, and tear strengths decrease forgiven tensile strengths and energy inputs. Also, such an explanation is consistent with the possibility that light scattering coefficients can increase with co-refining. Thus, softwood fibres can be expected to be more refined and hardwood fibres less refined for given energy inputs with co-refining than with separate refining before pulp blending.

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