Download Evaluation of the Scott bond test method

PAPER PHYSICS
Evaluation of the Scott bond test method
Christer Fellers, Sören Östlund and Petri Mäkelä
KEYWORDS: Delamination, Stress-Strain Properties,
Energy Consumption, Z-Direction Strength
SUMMARY: The Scott bond test is the most commonly
used test method for quantifying the delamination
resistance of paper and board. The objective of this
investigation was to validate the hypothesis that the Scott
bond value would be dominated by the total energy under
the force elongation curve in a z-directional tensile test.
The investigation comprised three types of hand sheets
with comparatively low strength values. Three test
methods were used to obtain the energy for delamination:
1) Z-test, a z-directional tensile test, 2) Scott bond test,
and 3) Simulated Scott bond test, a Scott bond type of test
performed in a hydraulic tensile tester.
The test data were expressed as a correlation between
the failure energy obtained from the Z-test and the other
two tests. The results showed that the Scott bond test
gave slightly higher values than the Z-test for the weakest
paper, but that the value tended to be much higher for the
stronger papers. On the other hand the Simulated Scott
bond test tended to give lower values than the Z-test.
High speed photography was used to reveal several
energy consuming mechanisms in the Scott bond test that
can explain why this test gave higher values than the Ztest. The lower values from the Simulated Scott bond
values are more difficult to explain. At this stage we can
suggest that the failure mechanism is different if the
paper is delaminated by pure tension or by a gradual
delamination as in the Scott bond test.
ADDRESSES OF THE AUTHORS: Christer Fellers
([email protected]): Innventia AB, Box
5604, SE-114 86 Stockholm, Sweden.
Sören Östlund ([email protected]): KTH, Royal Institute of
Technology, Department of Solid Mechanics, SE-100 44
Stockholm, Sweden.
Petri Mäkelä ([email protected]): Innventia
AB, Box 5604, SE-114 86 Stockholm, Sweden. Present
address: Tetra Pak Packaging Solutions AB, Ruben
Rausings gata, SE-221 86 Lund, Sweden.
Corresponding author: Christer Fellers
Failure of paper and paperboard in the thickness direction
is a recurrent problem in many converting processes and
end-use situations. The interfibre bond strength, can be
measured either indirectly by mechanical testing of whole
sheets such as in peel tests (Skowronski and Bichard
1987; Skowronski 1991), the z-toughness test (Lundh and
Fellers 2001), the wheel delamination test (Girlanda and
Fellers 2006) and the Scott bond test, e.g. Reynolds
(1974), or directly by testing of individual fibre-fibre
cross test pieces such as first reported by Mayhood,
Kallmes and Cauley (1962) and later by for example
Schniewind, Nemeth and Brink (1963), Mohlin (1974)
and Stratton (1991). There are advantages and
disadvantages with both types of methods. In the indirect
methods the effects of papermaking variables are
considered, while modifications on fiber and bond levels,
respectively, are more difficult to capture. Direct methods
are better suited for such investigations, but test piece
preparation does not necessary resemble papermaking
conditions and there is also in general a large scatter in
the results.
The term delamination is often used to describe
macroscopic failure in the thickness direction due to
interfibre bond failure. Delamination may be caused by
out-of-plane normal and shear loading and combinations
of these. The numerical value describing the phenomenon
may fundamentally be expressed in terms of stress, for
example the critical stress value obtained from tensile
testing in the thickness direction, or energy, for example
the value obtained from a Scott bond test, but are in some
cases expressed in terms of various runnability
parameters. In this paper, the relation between the Scott
bond value and the energy consumed in the Z-test will be
investigated.
The objective was to validate the hypothesis that the
value measured in a Scott bond test would be dominated
by the total energy under the force elongation curve in the
Z-test. This means, implicitly, that it is assumed that the
failure in a Scott bond test is dominated by the normal,
opening, mode of loading. The three test methods that
were used to obtain the delamination energy in this work
were
 Z-test
 Scott bond test.
 Simulated Scott bond test.
Different versions of tests for z-directional tensile
strength exist. The Z-test is a version developed at
Innventia with the main goal to be able to test the zdirectional properties of strong and stiff papers
(Andersson and Fellers 2012).
The Scott bond test is the most commonly used test
method for quantifying the delamination resistance of
paper and board. This test method is widely used for
product control purposes although its relation to
deformation and failure in converting and end-use is not
completely clear. An attempt to theoretically examine the
Scott bond test method is presented by Isaksson et al.
(2010). Their investigation illustrates an influence of
shear and a dependence on loading rate through the value
of the yield stress.
For the purpose of this investigation, a custom-made
test set-up that mimics the Scott bond test was developed.
The test set-up has the same geometry as the original
Scott bond test method, but it is designed so that it can be
mounted in a hydraulic tensile tester, which makes it
possible to perform controlled Scott bond tests at
different loading rates. This test is named Simulated Scott
bond test in the sequel.
The investigation comprised three types of hand sheets
with comparatively low strength values.
Nordic Pulp and Paper Research Journal Vol 27 no.2/2012 231
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Materials and methods
Materials
Three types of 150 g/m2 hand sheets according to Table 1
were manufactured with the use of 0.02% Percol 292
retention aid. The structural thickness was measured
according to SCAN-P88:01 2001. The structural density
was calculated as the grammage divided by the structural
thickness. The explicit stiffness and strength properties of
these materials are not of particular interest in the present
investigation, and it is sufficient to note that they
represent three materials with considerable different
stiffness and strength properties.
Z-test
This z-directional tensile test was developed at Innventia
and is described in detail by Fellers and Andersson
(2012). The procedure for testing is described briefly as
follows.
The paper test piece was laminated between thin plastic
foils using a Lamiart-3201 pouch laminator, Fig 1. Each
foil consisted of one 0.050 mm thick, stiff, polyester base
layer with high melting temperature in the middle and
two 0.070 mm thick ethyl-vinyl acetate surface layers
with a melting temperature of 78ºC. Additionally, a
15 g/m2 dummy paper was placed on the outside of each
foil, to provide backing for the subsequent gluing. The
line-load of the laminator rolls was set by the
manufacturer and was not specifically determined. The
melting layers of each foil melted and adhered to the
paper test piece and the dummy papers. By proper choice
of lamination speed it is possible to make the melting
layer to be fastened only to the outermost parts of the
paper with controlled penetration.
Paper samples were cut from the laminate and were
glued to metal platens using strong fast curing glue
(Permabond 105-C6 based on ethyl-2-cyanoacrylate).
The curing lasted for 60 minutes to make sure that the
setting was finished. Then the edges were trimmed to fit
the size of the platens. The papers were laminated,
conditioned and tested at a climate of 23ºC and 50% RH.
The tensile properties of the laminate without the paper
(glue-dummy paper-foil-dummy paper-glue) were
evaluated. The strength and elastic modulus in the
thickness direction, ZD was 7280 ± 900 kPa and
1360 ± 200 MPa, respectively. This should be compared
with data for paper, which are in the order of 250 to 2000
kPa and 10 to 200 MPa, respectively (Girlanda, Fellers
2007).
Based on these data, the laminate was assumed to be
infinitely strong compared to the paper for the strength
evaluation. Regarding the evaluation of the modulus and
strain at break in ZD, the elongation of the plastic foil
must be taken into account for thin papers. The
comparatively high grammage of 150 g/m2 used in this
investigation ensured that the elongation of the plastic
foil could be considered negligible for the evaluation of
these properties.
A schematic drawing of the testing apparatus is shown
in Fig 2. The rod was screwed onto the upper platen.
Successively, the lower metal platen was screwed onto
232 Nordic Pulp and Paper Research Journal Vol 27 no.2/2012
the load cell. These actions were performed without
subjecting the paper to undesired loading.
The loading was performed by means of an MTS servohydraulic testing machine. The load was applied under
displacement control in order to enable measurement of
the post-peak behavior of the material. The loading rate
was chosen in such a way that a nominal stress of 500
kPa was reached in 0.2 seconds, according to ISO (ISO
2007).
Scott bond test
In the Scott bond test, a right angle L-shaped metal
bracket is fastened to the surface of board by doublesided tape. Then the L-bracket is hit by a pendulum
causing delamination in the test piece, Fig 3. The method
is standardized for instance by TAPPI (T 569 pm-00) and
is described in the literature, e.g. (Reynolds 1974,
Blockman and Wikstrand 1958). The energy required to
split the paper is estimated from the position reached by
the pendulum after impacting the L-bracket. Its simplicity
makes the Scott bond test a common tool for the
evaluation of the delamination resistance of paper-board.
However, this test method presents some inherent
limitations.
The dynamic force of the pendulum produces a complex
and unknown combination of shear and tensile stresses in
the test piece. The delamination resistance is intended to
Table 1. Materials
Material
Bleached pine sulphate, beaten
2000 PFI revolutions
TMP
CTMP
Structural density
[kg/m3]
545
254
226
Fig 1. The test piece according to the Z-test method
Fig 2. Schematic drawing of the testing apparatus (Andersson
and Fellers 2012).
PAPER PHYSICS
Fig 3. Schematic drawing of the Scott bond test (TAPPI T 569
pm-00).
Fig 5. Close-up of the experimental set-up of the Simulated
Scott bond test. The left picture shows the situation when the
piston hits the L-bracket and the right picture shows the
situation where the piston has been reversed after having
caused delamination of the test piece.
Fig 4. Experimental set-up for the Simulated Scott bond test.
Fig 6. Representative total stress-elongation curve for paper.
be equal to the energy lost when the paper is split into
two halves. However the energy may also be lost in
plastic dissipation of the whole material (Reynolds 1974)
and by oscillations of the pendulum after impact. We will
use the term Scott bond value for describing the result
from a Scott bond test.
Simulated Scott bond test
The original Scott bond test is based on the measurement
of the energy lost in a dynamic impact test. As pointed
out by Isakson et al. (2010) the test is difficult to analyze
in detail. In order to take a first step to sort out the failure
mechanism, the Scott bond test was simulated using a
custom-built set-up that was mounted in an MTS servohydraulic tensile tester, Fig 4. The force was measured by
a load cell and the displacement by the position of the
piston. A metal ball was fastened at the end of the piston
to provide well-defined contact to the L-bracket. Note
that the paper was fastened to the L-bracket by means of
the same lamination- and gluing technique that was used
for the Z-test. Hereby the results for these two tests can
be compared without considering the influence of the
tape. Fig 5 shows a close-up of the experimental set-up of
the Simulated Scott bond test. The left picture shows the
situation when the piston hits the L-bracket and the right
picture shows the situation where the piston has been
reversed after having caused delamination of the test
piece. The loading rate was chosen to be the same as in
the Z-test.
Fig 7. The high-speed photography of the failure of a paper in
the Scott bond test.
Nomenclature
The Z-fracture energy, Wz (J/m2) is the absorbed energy
required to cause complete delamination. A typical stresselongation curve, including the behavior in the post peak
region is shown in Fig 6. The Z-fracture energy was
determined by integrating the area under the curve from
zero strain to the strain where complete delamination had
occurred. The contribution to the area under the curve
from reversible elastic energy, which is of the order of
the area below the curve to the left of the peak load in Fig
6, was to a very good approximation, neglected.
Results and discussion
Fracture behavior of the paper in the Scott bond test
Fig 7 shows the typical failure pattern for the papers in
this investigation.
In order to illustrate how different sources contribute to
the consumed energy in the Scott bond test, an extremely
strong paper was used. A high speed camera was used to
capture the movements of the parts of the apparatus. In
Nordic Pulp and Paper Research Journal Vol 27 no.2/2012 233
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Fig 9. The support is actually is lifted from the stable position.
Fig 8. The left figure shows the moment where the pendulum
hits the L-bracket and the right figure the final position of the
pendulum after it had bounced backwards.
E
D
A
B
C
Fig 10. Results from a test where the support was secured by a
grip, to prevent it from lifting. The following energy-consuming
effects were noticed. A - Shear of the tape and the paper. B The paper delaminated. C - The tape was released locally from
the support. D - The vertical leg of the L-bracket was bending. E
- The pendulum was oscillating after impact.
this way the aim was to exaggerate possible energyconsuming mechanisms. The maximum recordable
energy is 729 J/m2. This value corresponds to the case
where the pendulum stops in the lowest point. The left
part of Fig 8 shows the moment where the pendulum hits
the L-bracket. Since the available energy was not high
enough to cause delamination of the extremely strong
paper, the pendulum bounced backwards and reached a
final position as in the right part of Fig 8. This position
corresponds to about 300 J/m2, an energy that was
consumed during the test in spite of the fact that no
delamination of the test piece had occurred. Apparently a
substantial amount of energy was consumed in the system
without breaking the test piece.
A close-up high-speed movie was shot of the area
around the L-bracket. Fig 9 shows that the support
actually is lifted from the stable position, which naturally
consumes a lot of energy.
In the next test, the support was secured by a grip, to
prevent it from lifting. Some other energy-consuming
effects were noticed, see Fig 10.
A. Shear deformation in the tape and the paper.
B. The paper delaminated.
C. The tape was released locally from the support.
D. The vertical leg of the L-bracket was bending.
E. The pendulum was oscillating after impact.
234 Nordic Pulp and Paper Research Journal Vol 27 no.2/2012
Fig 11. Comparison of the fracture energy from the three
investigated tests and the three different materials.
Results from the testing
The energy required to complete a total delamination was
recorded for the three paper grades using the three test
methods. The results are presented in Fig 11. The
horizontal axis shows the values from the Z-test while the
vertical axis shows the values from the Scott bond test
and the Simulated Scott bond test respectively. A one-toone line is displayed in the graph. The red curves were
drawn to represent the trend for each test.
The Scott bond test gave slightly higher values than the
Z-test at small energies, but the curve tended to bend
upwards for higher energies. The Simulated Scott bond
test showed the opposite trend with slightly lower values
than the Z-test at small energies, while the curve bended
downwards for higher energies.
Final discussion
The investigation showed that the Scott bond test gave
slightly higher values than the Z-test at small energies,
but that the Scott bond values tended to be much higher
for the stronger papers. It is possible that the higher
loading rate in the Scott bond test is partly responsible for
higher energy values than those obtained at the much
lower loading rates used in the other two methods
(Isaksson et al. 2010). The reason for the progressively
higher values for higher energies is also to be found in
the several energy consuming mechanisms in the Scott
bond test revealed by high speed photography.
To further illustrate these mechanisms, the values from
the Scott bond and Simulated Scott bond tests are
PAPER PHYSICS
compared in Fig 12, which clearly show that the Scott
bond value accelerated the stronger the paper was. This
likely indicates that increasing contributions from
oscillations of the pendulum, lifting of the support,
shearing of the tape, shear deformation of the tape and
bending of the L-bracket. Furthermore, it is not unlikely
that the mode of failure of the paper material
(contributions from normal and shear loading,
respectively) will be different for materials of different
strength and also a function of loading rate.
Previous investigations in the literature support the
results in this investigation. Fig 13 shows the relation
between the energies obtained by the Scott bond test and
the Wheel delamination test (WDT) for various carton
boards (Girlanda and Fellers 2006). The WDT measures
the energy for delamination in a progressive peeling type
of deformation mode similar to the Simulated Scott bond
test described in this investigation. The Scott bond test
gives values that are much higher than the WDT.
In another investigation, the Scott bond test is related to
the Z-toughness test (ZTT), Fig 14, (Lundh and Fellers
2001). The ZTT also characterizes the energy for
delamination in a progressive type of failure mode. Also
in this case the Scott bond values are much higher than
the values from the ZTT. The relation between the two
tests varied considerably depending on the paper grade.
The trend of the Simulated Scott bond test data, giving 23 times lower energy values than the Scott bond test SBT,
agrees well with the results from the studies for the wheel
delamination test and the Z-toughness test.
The lower values of the Simulated Scott bond test
compared to the energy under the stress-strain curve in
the Z-test is more difficult to explain. Fig 15 shows the
failure pattern in the Z-toughness test (Lundh and Fellers
2001). A complicated failure pattern is noticed,
comprising fiber bridging, local fiber bridging and two
crack fronts. It is possible that the crack in a progressive
type of fracture test, like in the Simulated Scott bond test,
will create stress concentrations and that the crack will
find the path of lowest energy. This is in contrast to the
Z-test where the whole structure is loaded
simultaneously, a case where less stress concentrations
may be created, which will lead to higher delamination
energy. More research is needed, e.g. by model
experiments to describe and understand the failure
mechanism for different structures.
It is imperative that the delamination resistance of paper
is analyzed by a relevant testing method. In the analyses
of how different papermaking parameters affect
delamination resistance one must ask the question
whether a particular converting operation requires
strength or energy to break of the material and which
mode of failure that is acting. Fundamentally, it is a
question whether one can accept an almost complete
delamination, where energy to break may be relevant, or
if one cannot accept a visible delamination failure such as
in offset printing, where strength may be more relevant.
Fig 12. Comparison of the fracture energy from Scott bond and
Simulated Scott bond tests and the three different materials.
Upper
wheel
Line load
Paper
Lower
wheel
Force
Wire
Fig 13. The Scott bond test related to the Wheel delamination
test (WDT) (Girlanda and Fellers 2006).
F
F
Fig 14. The Scott bond test related to the Z-toughness test
(Lundh and Fellers 2001).
Fig 15. The failure pattern obtained in the Z-toughness test
(Lundh and Fellers 2001)
Nordic Pulp and Paper Research Journal Vol 27 no.2/2012 235
PAPER PHYSICS
Conclusions
The Scott bond test gave higher energy values than those
obtained by the Z-test. The reason is that in the Scott
bond test the loading rate is considerable higher and
several non-desired energy-consuming mechanisms are
acting:
 Shear energy consumption in the in the tape and
paper
 Elastic deformation of the L-bracket
 Vibrations of the pendulum
 Non rigid support of the platens
 Local release of the tape from the support
The Simulated Scott bond test gave much lower fracture
energy values than those obtained by the Z-test. These
results are more difficult to explain. At this stage one
may suggest that the failure mechanism is different if the
paper is delaminated by pure tension than when it was
gradually delaminated as in the Scott bond test, starting at
one end of the paper and progressing to the other end.
Acknowledgements
The financing companies of the Paper Mechanics cluster within
the Innventia Research Program 2009-2011 are gratefully
acknowledged.
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