Characterizing Fibres

Characterizing the papermaking properties of reinforcing pulps

Many paper makers and pulp producers evaluate pulps by measuring their response to the application of mechanical energy delivered by a Valley beater, a Lampen ball mill or a PFI mill. This practice continues today, despite the realization that the response of a pulp to these “ideal” beating treatments is completely different from the response that is seen when the pulp is refined in the paper mill! Pulp typical property sheets often provide information obtained from one of these laboratory evaluation methods. In 2000, Canfor Pulp decided to break with tradition and revised its typical property sheets to reflect the refined response of our fibre. This has proven to be extremely popular with our customers. We use a 30cm (12 in) Sprout Bauer laboratory disc refiner operating at a specific edge load of 1.7 Ws/m. We still provide data based on a PFI mill, but only as supplemental data for those few customers who require it for goods inward quality control purposes.

The most frequently used basis for evaluating reinforcing pulps is by examining their strength characteristics. Many papermakers and pulp producers evaluate pulps by comparing their strengths at a particular freeness or at a set of constant freeness’s. In part this reflects the need for the pulp to deliver its strength potential on the paper machine without impairing the drainage rate of the paper on the wire. Unfortunately it also results in many pulps being improperly refined, and therefore being used in less than optimal ways. This is especially likely when pulps with very different refining characteristics are compared, such as a coastal BC hemlock with a BC interior Douglas fir, or a Scandinavian pine with a Radiata pine.

When asked by customers, we recommend that pulps be compared using a set of critical pairs of relevant properties. In most cases tensile strength is one of these parameters, and this is natural as it closely reflects development of fibre to fibre bonding, and developing bonding is one of the reasons why pulp is refined. Most frequently we recommend plotting tear index against tensile strength. Using this approach it is possible to compare pulps at some constant strength level, or to compare the strength potential of separate shipments.

Similarly if tensile strength is plotted against sheet density it is possible to compare the different density responses of different pulps. In this figure we see the response of three different pulps to refining, and while each can achieve a breaking length of 7km, they do so at very different sheet densities.

Other commonly used critical pairs are:
  • Tensile strength against specific energy applied
  • Tensile strength against porosity
  • Tensile strength against SR or CSF

In general we only suggest using freeness or SR as one of the parameters in these critical pairs comparisons if some other pairs of properties are also being considered. Only in this way will it be possible to obtain a complete picture of the characteristics of a pulp.

We hope that you have found the discussion on this page helpful. If you would like to further information on evaluating pulps, please send an email to us at

Characterization strategies for papermaking pulps, Jan-Erik Levlin, Hannu Paulapuro, Paperi ja Puu – Papper och Tra 5/1987