PAPER PHYSICS
Evaluation of the stress-strain properties in the thickness
direction - particularly for thin and strong papers
Christian Andersson and Christer Fellers
KEYWORDS: Delamination, Z-direction, Fracture,
energy, Stress-strain
SUMMARY: The performance of the paper in a number
of converting operations such as creasing, bending,
printing, and plastic coating put great demands on the
mechanical properties in the thickness direction of the
material. The knowledge of strength, elastic- and plastic
behavior in tension and compression in the thickness
direction is needed for a comprehensive description of the
performance of the material in these operations. In spite
of its importance, very few publications deal with the
evaluation of the entire tensile stress-strain curve of paper
in the thickness direction. A likely reason for this is the
intrinsic difficulty of testing a thin, uneven, porous,
fibrous and compressible material such as paper with
sufficient precision and testing time efficiency.
The z-directional strength test is usually performed by
fastening the paper by means of double-adhesive tape to
metal platens. The platens are fastened in a testing
machine and strained to break. The adhesion of the tape
is the limiting factors for how strong papers that can be
tested. The tape-based method also is expected to have a
lower limit in grammage due to the penetration of the
adhesive.
The aim of the present publication was to show a
procedure how to evaluate the entire stress-elongation
curve in the z-direction of papers, using a lamination
method for fastening the paper to the metal platens. From
this curve the z-strength, z-modulus, z-strain at break, zenergy at break and z-fracture energy could be extracted.
Such information is, so far, non-existing in the literature.
ADDRESSES OF THE AUTHORS: Christer Fellers
([email protected])
and
Christian
Andersson
([email protected]),
Innventia AB, Box 5604, SE-114 86 Stockholm, Sweden.
Corresponding author: Christer Fellers
The mechanical properties in the thickness direction of
paper are important for the performance in a number of
converting operations such as creasing, bending, printing,
and plastic coating. The knowledge of z-strength, elastic
modulus, strain-softening behaviour in tension and
compressibility in the thickness direction, also called the
z-direction, is needed for a comprehensive description of
the performance of the material in these operations. The
literature on the subject is briefly summarized for
instance by Girlanda and Fellers (2007). In spite of its
importance, it is noted that very few publications deal
with the evaluation of the entire tensile stress-strain curve
in the thickness direction where the elastic modulus,
strength, strain at break and the post-peak part of the
curve is recorded. A likely reason for this is the intrinsic
difficulty of testing these properties for a thin, uneven,
porous, fibrous and compressible material such as paper
with sufficient precision and testing time efficiency.
The usual testing procedure for evaluating the z-strength
is to fasten the paper by double-adhesive tape between
two circular metal platens like in the ISO method (ISO
2007). The basic problem with this technique is that the
adhesion, the penetration of adhesive into the paper and
the viscous properties of the tape limits the strength and
grammage range of the paper to be tested. Furthermore
this method is limited to the measurement of z-tensile
strength only.
The problem is clearly illustrated by Andersson (1981)
in Fig 1. Andersson compares the z-strength, using either
double adhesive tape or a glue (photo-mounting tissue, a
paper impregnated with glue that firmly fastens the paper
to the platens). Up to about 200 kPa the two fastening
methods give equal values whereas the discrepancy
increases as the paper becomes stronger. Judging from
the data, the limit of the tape seems to be around 500 kPa.
Unfortunately, many refined chemical pulps have a
higher strength value, which limits the application of the
method.
Girlanda and Fellers (2007) analyses the z-strength
using a photo-mounting tissue as adhesive. This method
solves the problem of strength limitation of the paper.
The drawback with this method is, however, the use of
heat and surface pressure which require a re-conditioning
of the papers and sometimes give a slight density
increase. The procedure also give a relatively large
penetration of the adhesive into the paper, which puts a
lower grammage limit of approximately 60 g/m2 and a
Fig 1. Z-directional tensile test. Influence of adhesive
(Andersson 1981).
Nordic Pulp and Paper Research Journal Vol 27 no.2/2012 287
PAPER PHYSICS
rather elaborate procedure for calculating the strain and
elastic modulus.
The difficulty in testing a thin, uneven, porous, fibrous
and compressible materials such as paper is mentioned in
the literature (Van den Akker 1952). Provided that the
adhesive is adhering equally deep into the paper over the
whole surface, an uneven paper will have the highest
stress concentrations in the thin sections. On the other
hand, if the adhesive only adhere to the peaks of the
paper, stress concentrations will occur in the thick
sections. Quantifications of the importance of these
effects are not previously treated in the open literature.
The aim of the present paper was to improve the
technique by Girlanda and Fellers (2007) for the
evaluation of the entire stress-elongation curve in the zdirection of paper. From those curves the z-strength, zmodulus, z-strain at break, z-energy at break and zfracture energy are extracted. Such information is, so far,
non-existing in the literature. Furthermore, the
importance of a uniform thickness profile for the results
is investigated. Limitations in grammage and strength of
the paper are specifically addressed.
Materials and Methods
Material 1. Formette Dynamique sheets of bleached
dried pine kraft
Bleached dried pine kraft pulp beaten to 25 SR. Sheets
with grammage 15 to 150 g/m2 and structural density
650 kg/m3 were made in the Formette Dynamique sheet
former (Sauret et al. 1969).
Material 2. Rapid-Köhten sheets of bleached dried
pine kraft
Bleached dried pine kraft pulp beaten to 25 SR and fines
removed. Sheets with grammage 30 to 120 g/m2 and
structural density 670 kg/m3 were made according to the
Rapid-Köhten method (ISO 1998). Single sheets or two
sheets coached together, were tested.
Material 3. FEX papers of kraft pulp
Two 105 g/m2 papers with different formation were
investigated. They were tested both untreated and surface
ground to obtain a more uniform thickness. These papers
Fig 2. The appearance of the papers at forming concentrations
of 0.55% and 1.03%. The figures were obtained by scanning
beta-ray images. Note that white denotes higher grammage and
black lower.
were manufactured from a flash dried unbleached kraft
pulp with kappa number 35. The papers were beaten to
25 SR and formed on a roll former in the Innventia
experimental paper machine FEX at 0.55 and at 1.03%
forming concentration. The structural density was 753
and 733 kg/m3 for the 0.55 and 1.03% forming
concentrations, respectively.
The grammage maps of the two papers, measured by
beta rays (Johansson and Norman 1996), is shown in
Fig 2. The effect of surface roughness was investigated.
Table 1 shows representative structural thickness profiles
of the sheets (SCAN-P88:01 2001).
Material 4. Newsprint
Grammage 45 g/m2 and structural density 563 kg/m3.
Material 5. LWC
Grammage 80 g/m2 and structural density 1300 kg/m3.
Material 6. TMP sheets
Grammage 80 to 300 g/m2, previously manufactured for
an article by Girlanda and Fellers (2007). The papers
were made from TMP pulp, CSF 210 ml and had the
structural density 484 kg/m3.
Material 7. Unprinted bank note papers
Grammage 90 g/m2, structural density 815 kg/m3. This
paper was chosen because of its extremely high zdirectional strength.
Table 1. Structural thickness profiles for the unground and surface ground papers. The total length of each diagrams is 200 mm. The
diagrams display the thickness range from 0.05 to 0.23 mm.
Forming Concentration, %
Unground paper
0.55
1.03
288 Nordic Pulp and Paper Research Journal Vol 27 no.2/2012
Ground paper
PAPER PHYSICS
Fig 3. The test piece according in the Z-test.
Statistical treatment
Five test pieces were tested in each trial unless otherwise
stated. The average and 95% confidence levels were
calculated.
Fig 4. Schematic drawing of the testing apparatus.
The Z-test
The test piece is illustrated in Fig 3. The test pieces were
prepared using a lamination method, described by
Lucisano and Pikulik (2010). The adhesive method
involved lamination of the paper between thin plastic
foils. A Lamiart-3201 pouch laminator was used to
perform the lamination. A plastic foil was placed on both
sides of the tested paper. Each foil consisted of one 0.050
mm thick, stiff polyester base layer in the middle with a
high melting temperature and two 0.070 mm thick
ethylene vinyl acetate melting layers on each side 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 at a testing
speed regulated to give desired penetration of the melting
layer into the paper. The line load used 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 to the dummy paper. 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, Lucisano and
Pikulik (2010).
Small paper samples were cut-out from the laminated
sheet, slightly larger than the metal platens. The dummy
paper side of the foil was then fastened to the metal
platens, using a strong fast curing glue (Permabond
105(C6) based on ethyl-2-cyanoacrylate). The curing of
the glue lasted for 60 minutes to make sure that the
setting time was finished. After that time the edges were
trimmed to fit the size of the platens. The papers were
laminated, conditioned and tested in 23ºC and 50% RH.
A schematic drawing of the testing apparatus is shown
in Fig 4. The rod was screwed onto the upper platen.
Successively, the lower metal platen was screwed onto
the load cell. These actions were performed without
subjecting the paper to undesired loading. The load from
the tensile tester was transferred from the lower to the
upper pin by a point-to point contact. By this
arrangement, the load is transferred to the paper in a
straight way which makes the strain distribution over the
paper surface as uniform as possible. An extensometer of
clip-on type was fastened between the lower and the
upper platen, see Fig 5. This type of extensometer was
designed to perform well at the high loading rates used.
Fig 5. The clip-on gauge arrangement.
The loading-rate was chosen such that a load of 500 kPa
was reached in 0,2 seconds, in accordance with ISO (ISO
2007)" (ISO 2007).
Nomenclature
The following nomenclature was used:
Fig 6 shows a schematic drawing of the platens and test
piece. The structural thickness t s is defined by SCAN
(2001). The melting layer thickness is the thickness
where the melting layer has penetrated the paper
structure. The effective thickness t e is the difference
between the structural thickness and twice the melting
layer thickness. This thickness was used in the
calculations of strain and elastic modulus.
z-strength  Z = tensile strength in the z-direction, the
maximum force divided by the testing area (Pa).
z-energy at break WZEAB = energy absorption up to the
maximum force (J/m2).
z- fracture energy WZFE = energy absorption to cause a
complete delamination (J/m2).
z-strain at break
z 
where
z
te
Z
,
[1]
is the elongation at the maximum force and
te
is the effective thickness.
Nordic Pulp and Paper Research Journal Vol 27 no.2/2012 289
PAPER PHYSICS
Fig 7. A typical stress-elongation curve up to the maximum
stress. Bleached spruce sulphate, 2000 PFI.
Fig 6. Schematic drawing of the platens and test piece.
z-modulus
Εz  S te ,
[2]
where S is the initial slope of the stress-elongation curve
and t e is the effective thickness.
The data up to the maximum stress was fitted to the
function   a tanh(b )  where is the stress and the
elongation and the initial slope S = ab was determined. A
typical appearance of a stress-elongation curve with a
fitted function is shown in Fig 7. The fit was found to be
excellent for all the pulps investigated.
The z-energy at break was determined by integrating the
function from zero strain to the point of maximum zstrength.
Tests showed that the z-fracture energy could be
evaluated by using the internal elongation gauge situated
in the piston of the MTS servo-hydraulic tester, a
procedure which facilitated the testing of the
comparatively large displacements used. The compliance
of the testing equipment dominates the elongation
reading in the region up to the maximum stress which
showed that the clip-on gauge in fact was needed for the
evaluation of the strains and modulus in this region.
STFI thickness gauge, structural thickness and
thickness profile
The structural thickness and thickness profile was
measured according to a SCAN procedure (SCANP88:01 2001). The structural density was calculated from
the grammage divided by the structural thickness.
Surface grinder
In order to reduce the thickness variation, the FEX papers
of kraft pulp with different formation were fastened on a
vacuum table and ground in a commercial surface
grinder. Surface grinding has been reported useful for
delamination testing, for instance by Byrd Setterholm and
Wichmann (1975).
Error estimation
The error estimation in the figures is expressed as 95%
confidence limits.
290 Nordic Pulp and Paper Research Journal Vol 27 no.2/2012
Results and discussion
Laminate properties
The stiffness of the laminate (glue-dummy paper-foildummy paper-glue) was evaluated. The strength was
7280 ± 900 kPa and elastic modulus 1360 ± 200 MPa.
This should be compared with data for paper, which are
in the order of 250-2000 kPa and 10-200 MPa
respectively (Girlanda, Fellers 2007). Based on these
data, the laminate was considered to be infinitely strong
and stiff compared to the paper for strain and elastic
modulus determination.
Laminator speed
The next goal of the investigation was to find a suitable
laminator speed that minimized the penetration of the
polyester into the tested paper. The target was to be able to
test a paper of grammage 30 g/m2. Fig 8 shows the zstrength versus speed of the laminator for Material 1.
Two 15 g/m2 sheets were placed on top of each other,
defining the tested paper (lower curve). At a speed of
around 12 mm/s, the strength reduced to zero. The
comparable strength for the 30 g/m2 sheet (upper curve)
formed a plateau from around 15 mm/s.
On the basis of these results a temperature of 140°C and
a speed of 17 mm/s were initially used as the standard
speed in future testing. It was then assumed that sheets of
30 g/m2 and higher could be tested. An investigation on
the sensibility of the laminator speed was performed on
the reasonably thin and weak commercial paper,
Material 4, newsprint of 45 g/m2. For this grammage the
strength was independent on the laminator speed in the
range 5 to 20 m/s.
Conditioning time
The effect of conditioning time was investigated on the
solid bleached carton board. The strength, obtained 1
hour after lamination, which was the fastest point
possible due to the gluing time, was 309 ± 4 kPa. The
strength after 24 hours was 309 ± 7 kPa. Considering that
the strength is moisture dependent, the conclusion was
that the very short exposure time for heat in the laminator
did not reduce the moisture content in the paper to any
significant degree and that the paper consequently had
retained its moisture content and remained in equilibrium
PAPER PHYSICS
Z-strength, kPa
2500
2000
30 g/m2
1500
1000
500
0
2x15=30 g/m2
0
5
10
15
20
Laminator speed, mm/s
Fig 8. The z- strength versus laminator speed for Formette
Dynamique sheets of bleached dried pine kraft pulp.
Fig 9. The z-strength versus grammage for Formette Dynamique
sheets of bleached dried pine kraft pulp.
Fig 10. The z-stress-elongation curves for Formette Dynamique sheets of bleached dried pine kraft pulp. Representative curves for
two grammage levels are shown. Left curve 60 g/m2. Right curve 120 g/m2.
Table 2. The effect of surface unevenness of the FEX papers
of kraft pulp on the z-strength.
Forming
Original
concentration paper,
kPa
0.55%
739 ± 8
1.03%
795 ± 34
Surface
ground paper,
kPa
641 ± 29
705 ± 34
Difference,
%
-13
-11
or had quickly been reconditioned. The consequence was
that the papers could be tested soon after the gluing
procedure.
Effect of surface unevenness
The effect of surface unevenness was investigated for
Material 3, FEX papers of kraft pulp. The papers were
tested in their original shape and after surface grinding to
reduce thickness variations.
The results of the investigation are given in Table 2.
The grinding did not improve the strength. A slight
decrease was in fact found. It is possible that grinding
damaged the surface and that the layer was not able to
anchor well in the surface structure in this test. It was
concluded that surface grinding was not necessary for
obtaining reliable data according to these test methods.
The mentioned concern of Van den Akker seemed not to
be justified (Van den Akker 1952).
Material 1. Sheets of different grammage
Formette Dynamique sheets of bleached dried pine kraft
were tested with a laminator speed of 17 m/s. The results
are given in Fig 9. The results show that the z-strength
was independent of grammage down to a grammage of
30 g/m2, which was the targeted lower grammage.
An important feature with the present method was that
stable stress-elongation curves were possible to obtain.
Representative curves for the 60 g/m2 paper (left curve)
and 120 g/m2 paper (right curve) are shown in Fig 10.
Note specifically that the z-energy at break up to the
maximum stress is only a fraction of the total z-fracture
energy.
The z-fracture energy, the area under the z- stresselongation curve is shown in Fig 11. The z-fracture
energy is increasing with increasing grammage.
Nordic Pulp and Paper Research Journal Vol 27 no.2/2012 291
PAPER PHYSICS
Z-strength, kPa
2000
Grammage
1500
1x30=30 g/m2
1x60=60 g/m2
1000
2x30=60 g/m2
2x60=120 g/m2
500
0
0
20
40
60
80
100
120
140
Grammage, g/m2
Fig 11. The z-fracture energy versus grammage for Formette
Dynamique sheets of bleached dried pine kraft pulp.
Fig 12. The z-strength versus grammage for Rapid-Köhten
sheets of bleached dried pine kraft pulp.
Z-modulus, MPa
Z-strength. kPa
20
400
Adhesive penetration = 0 g/m2
18
350
16
300
14
250
12
10
200
8
150
Adhesive penetration = 30 g/m2
6
100
4
50
0
2
0
50
100
150
200
Grammage, g/m
250
300
350
2
Fig 13. The z- strength versus grammage for TMP sheets.
Material 2. Rapid Köhten sheets of bleached dried
pine kraft
Papers of different grammage, manufactured according to
the Rapid-Köhten method were tested. The results are
given in Fig 12. For these strong Rapid Köhten papers, it
was difficult to obtain sufficient adhesion between the
paper and the laminate at the recommended speed of
17 mm/s. The speed was therefore reduced to 13 mm/s.
The consequence was that the 1x30 g/m2 sheet got
somewhat higher values than the other sheets due to
penetration of the melting layer. The z-strength was
however independent of grammage for the remaining
sheets and further more equal for the couched sheets and
the sheets formed in one operation.
Material 6. TMP sheets at different grammage
This set of papers were a series of different grammage,
previously manufactured for a paper by Girlanda and
Fellers (2007). The papers were made from a TMP pulp,
CSF of 210 ml and a structural density of 484 kg/m3. In
this set of papers, the grammage range was higher than in
the previous trials and this in combination with a
moderate strength made it possible to evaluate the desired
mechanical properties.
The stress-strain curves were stable and the evaluation
of z-strength, z-elastic modulus, z-strain at break and z292 Nordic Pulp and Paper Research Journal Vol 27 no.2/2012
0
0
50
100
150
200
250
300
350
Grammage, g/m2
Fig 14. The z-modulus versus grammage for TMP sheets
evaluated with two different assumptions of adhesive
penetration.
fracture energy was straight forward, Figs 13 to 16. The
strength was independent of grammage, which indicate a
rather uniform structure. The z-fracture energy increased
linearly with grammage with an intercept on the y-axis.
Any interpretation of this intercept, except the obvious
fact that it takes certain energy to break the paper even at
a small grammage, is not at hand at this stage of the
research.
The z-strain at break and z-modulus were evaluated
under two different assumptions regarding the thickness
to be used in the calculations, Eq 1 and Eq 2. In one case
it was assumed that no adhesive penetration took place
whereas in the next case it was assumed that the
penetration in fact was equal to the thickness of a paper
of 30 g/m2, which was given by the calibration procedure.
The data showed that the properties were essentially
independent of grammage with the 30 g/m2 penetration
assumption.
Material 7. Bank note paper
A bank note paper was chosen in this trial because of its
extremely high z-strength. In this trial, we were forced to
use a low laminator speed of 5.3 mm/s to ensure a
sufficient adhesion. The strength of two papers tested was
PAPER PHYSICS
Z-fracture energy, J/m2
Z-strain at break, %
10
140
9
Adhesive penetration = 30 g/m2
120
8
100
7
6
80
5
Adhesive penetration = 0 g/m2
4
60
3
40
2
20
1
0
0
50
100
150
200
250
Grammage, g/m
300
350
2
2968 and 3053 kPa. The fracture plane was situated in the
middle of the paper in both cases and there was no sign of
failure between the paper and metal platens. The trial
confirmed the ability of the method to test very strong
papers.
Comparison of the z-test and the z-test according to ISO
In a recently developed ISO method the loading rate was
calibrated to reach 500 kPa in 0.2 seconds (2007). If only
strength is required for a given paper grade it would be
beneficial to use the much simpler ISO method instead of
the more elaborate z-test. The advantages and limitation
of the z-test has been investigated thoroughly in this
report and we now seek the properties of the ISO method.
Table 3 gives results for various papers. The ISO
method failed to give reliable results for the strong LWC
paper and the extremely strong bank note paper, most
likely due to the limitation of the strength of the doubleadhesive tape. For the 30 g/m2 paper and newsprint the
two methods gave almost identical result.
In summary, the z-test (ISO) (2007) gave comparable
results to the present z-test for moderately strong papers
independent of grammage.
Table 3. Comparison of the z-strength and the newly developed
ISO/NP 15754 standard, z-test(ISO).
z-test,
kPa
886 ± 57
z-test(ISO),
kPa
846±56
Difference,
%
-5
590 ± 12
601±14
2
1229 ± 47
957±27
-22
1100±17
Failure
between
tape and
paper
-
2968
0
50
100
150
200
250
300
350
Grammage, g/m2
Fig 15. The z-strain at break versus grammage for TMP sheets
evaluated with two different assumptions of adhesive
penetration.
Paper material 1:
30 g/m2
Paper material 4:
Newsprint, 45 g/m2
Paper material 5:
LWC, 80 g/m2
Paper material 7:
Bank note paper,
90 g/m2
0
Fig 16. The z-fracture energy versus grammage for TMP
sheets.
Final discussion
A previous method (Girlanda, Fellers 2007) for
evaluation of z-properties was further developed for the
purpose of this investigation. The main difference from
the previous method was that a new lamination and
gluing technique was used, thus, enabling the testing of
papers of much lower grammage. The target for the
lowest grammage was set to 30 g/m2, a goal that was
fulfilled by regulating the speed of the paper through the
laminator for two 15 g/m2 papers.
It was then possible to test most papers down to a
grammage 30 g/m2. For stronger papers at lower
grammage great care must be taken to ensure that the
penetration of adhesive is adequately large, but not too
large. The balance was delicate. However, for papers of
slightly higher grammage, such as 45 g/m2, the laminator
speed was not critical.
The gluing technique produced a very stiff and strong
bond between the paper and the loading system. The
calculation of z-strain at break and z-modulus depended
on the estimation of the melting layer penetration. When
calculating strain and elastic modulus of a particular
paper, the precision of the calculation depends
fundamentally on the effective thickness, not affected by
the melting layer. One important goal of the investigation
was therefore to determine the melting layer thickness.
The procedure is described as follows. The lamination
conditions were performed in such a way that the melting
layer just barely penetrates a 15 g/m2 paper. The effective
thickness of the test piece will then be considered to be in
the range between the effective thickness and the
effective thickness minus the thickness of two 15 g/m2
papers of the same pulp. This condition was shown to
work for moderately strong papers. With the knowledge
of the glue stiffness and penetration, the stress-strain
curve of the paper could be evaluated.
For stronger papers it was found that a lamination speed
of half the value used for weaker papers had to be used
for ensuring a sufficient adhesion to the metal platens.
The melting layer penetration was estimated to be less
than 2 x 20 g/m2 based on the independence of lamination
Nordic Pulp and Paper Research Journal Vol 27 no.2/2012 293
PAPER PHYSICS
speed for a newsprint paper with a grammage around
40 g/m2.
This investigation dealt with z-strength values ranging
from small to extremely high values. For practical
reasons it was considered too elaborate to make
handsheets of different grammage for all the pulps and
perform lamination studies of different speeds in order to
find the adhesive layer penetration for each pulp and
beating level. A more pragmatic approach was taken by
assuming that the penetration was 2 x 20 g/m2. The true
penetration would be higher than 2 x 15 g/m2 but less
than 2 x 20 g/m2. In this way the error in the evaluated
elastic modulus and strain at break for the 150 g/m2
papers, was estimated to be around half the difference in
grammage, i.e. 5/150 = 3%.
Due to the short exposure for heat in the lamination
procedure, the paper needed no excessive conditioning
time beyond the curing time for the glue. The stiffness of
the system made it possible to achieve stable crack
propagation and it was possible to calculate the z-fracture
energy consumed during crack propagation which was
increasing with increasing grammage. No published data
have been presented previously in the literature on the
determination of the z-fracture energy and its dependence
on grammage. However the results are reasonable
considering that the fracture energy may not consist only
of the energy for crack propagation along a specific
fracture zone but also on the fracture energy from local
cracks and plastic deformation which shall increase with
the grammage of the sheet.
The strong bond made it possible to test papers with
very high strength. The strongest paper tested in this
investigation was a bank note paper of 3053 kPa. No
attempt was, however, made to find the limit in this
respect even if it may be speculated that the limit would
be given by the strength of the joint between the plastic
foil and the metal platens, which was measured to be in
the order of 7000 kPa.
The limit for the previously used SCAN method (1998)
had been approximately 500 kPa (1981). The higher
speed of the ISO method (2007) seems to have resulted in
possibilities to test significantly stronger papers. Judging
from the present limited tests, the limit now seems to be
around 1000 kPa.
Conclusions
 An improved method for z-directional testing using a
lamination technique to control the penetration of a
melting layer into the paper was presented. The
melting layer produces a very strong bond to the
paper, around 7000 kPa.
 Testing of very strong papers was possible.
 Testing of very thin papers, down to 30 g/m2 was
possible.
294 Nordic Pulp and Paper Research Journal Vol 27 no.2/2012
 The z-modulus, z-strain at break and z-fracture energy
was possible to evaluate.
 The z-strength, z-modulus and z-strain at break were
independent of grammage, whereas the z-fracture
energy to separate the paper completely was
increasing with grammage.
 The z-strength was equal for sheets formed in one
operation and two sheets couched together.
 The newly developed ISO method gave comparable
results as the z-test at least for moderately strong
papers.
Acknowledgements
The financial contribution and enthusiastic support from
Aracruz, Billerud, Eka Chemicals, Holmen, Korsnäs, MetsäBotnia, Mondi Packaging Paper, M-real, Peterson, Stora Enso,
Södra, Tetra Pak and Voith is greatly appreciated.
Literature
Andersson, M. (1981): Aspects of Z-Strength in Pulp
Characterization, Svensk Papperstidning 84(6), R34-R42.
Byrd, V. L., Setterholm, V. C. and Wichmann, J. F. (1975):
Methods for Measuring the Interlaminar Shear Properties of
Paper, Tappi J. 58(10), 132-135.
Girlanda, O. and Fellers, C. (2007): Evaluation of the Tensile
Stress-Strain Properties in the Thickness Direction of Paper
Materials, Nord. Pulp Paper Res. J. 22(1), 49-56.
ISO (1998): ISO 5269-2. Pulps - Preparation of Laboratory
Sheets for Physical Testing- Part 2: Rapid-Köhten Method.
ISO (2007): ISO/NP 15754 Paper and board - Determination of
z-directional tensile strength.
Johansson, P.-Å. and Norman, B. (1996): Methods for
evaluating formation, print unevenness and gloss variations
developed at STFI. Proceedings TAPPI 1996 Process and
Product Quality Control Conference, Cincinnati, TAPPI Press,
Atlanta. 139-145.
Lucisano, M. F. C. and Pikulik, L. (2010): Sheet splitting with
a heat seal pouch lamination technique. Innventia report No.
71.
Sauret, G., Trinh, H. J. and Lefebre, G. (1969): Versuche zur
herstellung mehrlagiger industriekartons im laboratorium, Das
Papier 23(1), 8-12.
SCAN (2001): P88:01. Paper and board, Structural thickness
and structural density.
SCAN (1998): P80:98. Paper and Board, Z-Directional Tensile
Strength.
Van den Akker, J. A. (1952): Instrumentation Studies. LXIX.
General Discussion of the Measurement of the Adhesion and
Cohesion, Tappi J. 35(4), 155A-162A.