Measuring Reinforcement
How should we measure Reinforcement?
Some studies have
shown that the reinforcement potential of LWC base
paper is proportional to the average
fiber length and tear strength at 70 Nm/g tensile
index and inversely proportional to fiber coarseness.
For SC papers the reinforcement potential has been
theorized to be described simply by the ratio of
fibre length to coarseness. However, there is no
general accepted way for measuring the reinforcement
potential.
For simplicity, industry has traditionally
measured the strength properties of pure chemical
pulps and
combined them together to evaluate the reinforcement
potential. The strength properties have traditionally
dealt with tear strength, tensile strength and elastic
modulus, and more recently with fracture energy and
fracture toughness. From the paper physics perspective
the relevant parameter to measure would be one that
is in-plane since that is the direction that paper
most frequently fails in. Elmendorf or Brecht-Imset
tear strength is an out-of-plane test, and in recent
years the consensus has grown that such tests are
of limited usefulness in predicting paper failure behaviour.
The
use of tear strength in some mills producing LWC
and SC is declining in popularity. As a measure
of
reinforcement it may have been more appropriate
when the mechanical pulp had high levels of shives
and
most of the paper machines had open draws. For
most papermakers
these days are long gone. In many cases the mills
that still measure reinforcement on the basis
of tear strength
will actually be over-using softwood kraft with
consequent impacts on formation and cost. These mills
are often
able to realise further optimisation opportunities
if they optimise paper properties on the basis
of an in-plane test such as tensile or fracture toughness.
A very interesting area of research today concerns
the activation of fibre segments within the paper
structure. The basis for this approach is that paper
should not
be considered as an inert composite material, but
rather as a network where forces from bonding, drying,
shrinkage
have acted on the fibres. Fibre elements that are
not sharing in load bearing are unable to contribute
to
the strength of the sheet, regardless of how well
bonded they may be to other fibres. This field is an
exciting
one and will probably reveal new opportunities to
optimise furnishes in the near future. |
