Characterising 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
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 freenessess. In part this reflects
the need for the pulp to deliver its strength potential
on the papermachine 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 pairs1 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 Temap.
1) Characterization strategies for
papermaking pulps, Jan-Erik Levlin, Hannu Paulapuro,
Paperi ja Puu – Papper och Tra 5/1987 |
