Broadleaf/hardwood tree species differ from hardwoods not only in having generally shorter fibres but also in having developed special cells for transportation of water: vessel elements, which link to each other forming long vertical vessels in the stem. Hardwood fibres are good for a number of paper grades, i.e. printing paper. The vessel elements may, however, cause problems in papermaking and printing. When paper is recycled, vessel elements will also appear also in DIP.
Without special treatment, the vessel elements stay rather intact on passage through the processes of pulp and paper production. On the paper surface, they may appear as a "flake", which may loosen and destroy the print quality, or its surface may not take ink as the fibres around, making the print look non-uniform.
At Innventia, efficient methods are available for the analysis of vessel elements and their consequences all along the production chain:
in wood with SilviScan;in pulp with systems for characterisation of fibres and vessel elements;on paper surfaces with microscopy and image analysis.
Data from such measurements show that the size, character and number of vessel elements differ a lot between tree species and they also vary systematically within trees. Due to this and depending on processing, the problems differ among pulps. For example, birch has a comparably high number of vessel elements, but they are not a problem, whereas severe difficulties may occur when using Eucalyptus or Acacia pulps.
The variations in number, size and properties of vessel elements within and between trees and how they relate to growth processes is a more or less virgin field of research, as efficient measurement methods have been missing until recently.