2012-27-02-p287-294-andersson.pdf

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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

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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

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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

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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

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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

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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

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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

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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.

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 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.