Evaluation of vessel picking tendency in printing

Bleached eucalyptus kraft pulps, Eucalyptus globulus and Eucalyptus grandis, were fractionated using a hydrocyclone (Bauer 3”) in order to enrich the vessel elements in one of the fractions. The vessel picking tendency was analyzed with a method developed at KCL. In this method, handsheets are printed with a full scale 4-colour sheet-fed offset printing machine using a commercial printing ink. The vessel picking printing test was performed on the unfractionated eucalyptus kraft pulps, the vessel-rich and the vesselpoor fractions. The picked particles were analyzed and counted using an image analyzer. The vessel picking printing test was also done on the vessel-rich fractions after a PFI mill beating at 2000 revolutions. Hydrocyclone separated the vessels according to their size and shape successfully. The microscopy analysis showed that vessels in the vessel-rich fraction were larger and more square-shaped than those in the vessel-poor pulp. The refining of the vessel-rich fraction decreased the vessel picking tendency to the same or even to a lower level than that of the unfractionated pulp.