Evaluation of the Scott bond test method Christer Fellers, Sören Östlund, and Petri Mäkelä

KEYWORDS: Delamination, Stress-Strain Properties, Energy Consumption, Z-Direction StrengthSUMMARY: 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 Z-test. 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 (christer.fellers@innventia.com): Innventia AB, Box 5604, SE-114 86 Stockholm, Sweden.Sören Östlund (soren@kth.se): KTH, Royal Institute of Technology, Department of Solid Mechanics, SE-100 44 Stockholm, Sweden.Petri Mäkelä (petri.makela@tetrapak.com): 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 FellersFailure 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 resembles 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 z-directional 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.

Materials and methodsMaterialsThree 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-testThis 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 Etyl-Vinyl Acetat surface layers with a melting temperature of 78 ºC. 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 and 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

the load cell. These actions were performed without subjecting the paper to undesired loading.

The loading was performed by means of an MTS servo-hydraulic 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 500 kPa should be reached in 0.2 seconds, according to ISO (ISO 2007).

Scott bond testIn the Scott bond test, a right angle L-shaped metal bracket is fastened to the surface of board by double-sided 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. MaterialsMaterial Structural density

[kg/m3]Bleached pine sulphate, beaten 2000 PFI revolutions

545

TMP 254CTMP 226

Fig 1. The test piece according to the Z-test method

Fig 2. Schematic drawing of the testing apparatus (Andersson and Fellers 2012).

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.

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 testThe 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 servo-hydraulic 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 4. Experimental set-up for the Simulated Scott bond test.

Fig 6. Representative total stress-elongation curve for paper.

Fig 7. The high-speed photography of the failure of a paper in the Scott bond test.

NomenclatureThe Z-fracture energy, Wz (J/m2) is the absorbed energy required to cause complete delamination. A typical stress-elongation 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 discussionFracture behavior of the paper in the Scott bond testFig 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

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.

A

C

BD

E

A

C

BD

E

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 energy-consuming 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.

Results from the testingThe 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-to-one line is displayed in the graph. The red curves were drawn to represent the trend for each test.

Fig 9. The support is actually is lifted from the stable position.

A

C

BD

E

A

C

BD

E

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.

Fig 11. Comparison of the fracture energy from the three investigated tests and the three different materials.

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.

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 discussionThe 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 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 2-3 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. Figure 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.

Fig 12. Comparison of the fracture energy from Scott bond and Simulated Scott bond tests and the three different materials.

Force

Wire

Line load

Lowerwheel

Upperwheel

Paper

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).

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 15. The failure pattern obtained in the Z-toughness test (Lundh and Fellers 2001)

ConclusionsThe 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 supportThe 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.

AcknowledgementsThe financing companies of the Paper Mechanics cluster within the Innventia Research Program 2009-2011 are gratefully acknowledged.

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TAPPI (T 569 pm-00) Internal bond strength (Scott type).