Application and interpretation of zero and short-span testing on nanofibre sheet materials

Transcription

1 Application and interpretation of zero and short-span testing on nanofibre sheet materials Swambabu Varanasi, Hui Hui Chiam and Warren Batchelor KEYWORDS: Weibull analysis, Fracture surfaces, SEM, Fibre dimensions, Residual span, Zero span tensile test ABSTRACT: This paper investigates the use of zerospan testing to measure the tensile strength of cellulose nanofibre sheets. The mechanical strength of cellulose nanofibre paper is a key property but tensile strength measurement requires a substantial amount test material, whereas a zero/short span test needs much less material. Sheets made from cellulose nanofibres, microfibrillated cellulose (MFC) and northern bleached softwood fibres were tested at spans ranging from 0 to 0.6 mm and tensile strength spans of 50 and 100 mm. For the cellulose nanofibres or the MFC sheets, strength was constant with span from 0 to 0.6 mm when tested dry and negligible when tested wet, except at zero span. The sheets made from the softwood fibres showed significant strength when tested wet at all spans from 0 to 0.6 mm. The results showed that for nanofibre materials, the zero or short span strength is measuring sheet tensile strength at a smaller sample length. The strength of the nanofibre sheets at 50 or 100 mm was smaller than at zero/short span due to sample size effects and Poisson contraction. The effects of starch and grammage on strength were also studied. ADDRESS OF THE AUTHORS: Swambabu Varanasi (v.swambabu@monash.edu), Hui Hui Chiam (hui.chiam@monash.edu), Warren Batchelor (warren.batchelor@monash.edu), Australian Pulp and Paper Institute, Department of Chemical Engineering, Monash University, Australia Corresponding author: Warren Batchelor Nanofibre materials are currently a topic of intense research, with particular emphasis on cellulose nanofibres (Eichhorn et al. 2010; Siró, Plackett 2010) as they are potentially a cheap, abundant nanofibre material that can be produced from a wide range of sources including wood, bacteria, sugar beet, potato, hemp and flax, wheat straw and bamboo. Cellulose nanofibres can be used as alternative for synthetic polymers in various applications because of their excellent properties, such as very high strength, low coefficient of thermal expansion and low gas permeability. Cellulose nanofibre sheets are of great interest in packaging, filter, membrane and biomedical applications. While the mechanical strength of cellulose nanofibre paper is a key property to measure, there may be issues with mechanical testing of cellulose nanofibre sheets. If the standards for paper testing are applied, the area of sample required for a single test is 15 mm wide and either 100 or 180 mm long. The average strength is calculated from at least 10 successful tests, rejecting all samples that break at the jaws or prematurely due to sample misalignment. This requires a substantial amount of test material. This can be a significant issue given that filtration times for forming range from 45 minutes (Sehaqui et al. 2010) to 3-4 hours (Nogi et al. 2009) when using a fine pore size filter and given that cellulose nanofibres are still usually made with laboratory equipment, producing a few dry grams per batch. Zero/short span testing is done with the jaws of the tester starting together or at a prescribed small span apart. The testing is rapid and the actual area tested for each zero-span test is no more than 2 mm x the tester jaw width of either 15 or 25 mm. The most recent automated tester (Gatari 2003; Pulmac 2012) uses only 210 cm x 2.5 cm strips to complete 24 tests in around 5 minutes. The test has widespread acceptance within the field of paper research as a measurement of the average fibre strength in a sheet (Van Den Akker et al. 1958), although the mechanics of the test and the relationship between fibre and sheet properties and the final result have been debated (Batchelor et al. 2006). While the test is called the zero-span test, the label is something of a misnomer. Although not measured as part of the standard test, the jaws in a zero-span tester can be instrumented to measure the displacement between the jaws during the test. Results on a Swedish never-dried unbleached softwood kraft showed that displacement between the jaws increased continuously during the test, with the final displacement at failure ranging from 33 m for a sample made from unrefined fibres to 49 m for a sample made from fibres refined for 6000 revolutions in a PFI mill (Batchelor, Westerlind 2003). For the sample made from the unrefined fibres, the span strained under the jaw was estimated to be approximately constant at 210 m for most of the test, before rising to 300 m at the end of the test. This span under the jaw that is being strained is sometimes called the residual span (Boucai 1971a; Boucai 1971b; Cowan, Cowdrey 1974). The mechanism of stress transfer under the jaws is still not clear. (Cowan 1975) postulated that transfer was entirely through frictional shear at the jaw surfaces and thus that the span under the jaws should continuously increase with the load applied during the test. However this simple model was contradicted by the measurements described above indicating a constant span under the jaws for most of the test (Batchelor, Westerlind 2003). This suggests a stick-slip type stress transfer mechanisms with a zone in from the edge of the jaw where slippage takes place and stress transfer is via friction, together with a sticking point, where stress transfer is from shear alone. Modelling of stress transfer through shear either with or without slippage (Hagglund et al. 2004) has shown that the shear transfer will produce a non-uniform stress field through the thickness of the sheet, which itself depends on the sheet mechanical properties. This non-uniform stress field will reduce sheet strength as sheet thickness Nordic Pulp and Paper Research Journal Vol 27 no.2/

2 increases with grammage. However, while there is excellent experimental evidence of the importance of this non-uniform stress field for the zero-span strength measured on very thick samples (Hagglund et al. 2004), it seems likely that the non-uniform stress field is less important for samples of grammages of 60 gsm and less. Reducing sample grammage from 60 gsm to 30 gsm has either produced no change (softwood bleached sulphite, bleached gumwood (Wink, Van Eperen, 1962), bleached softwood kraft, birch kraft and softwood TMP (Seth et al. 1989)) or small increases in strength of 5% (bleached and unbleached softwood kraft (Batchelor et al 2006, Joshi et al 2007)) and 15% (unbleached softwood kraft (Seth et al. 1989)). Further reducing sample grammage from 30 gsm to 10 gsm was observed to reduce strength by 10% (Wink, Van Eperen 1962), probably due to the effects of formation. These results suggest that a strength which is close to independent of the sample geometry can be obtained provided that the sample thickness is small compared to the residual span over which stress transfer takes place. It is the purpose of this paper to investigate the use of zero and short span tensile testing as an alternate measure of tensile strength for cellulose nanofibre sheets, requiring less time and material to measure sheet strength. Experimental method Materials and material characterisation Two nanofibre materials were tested. The starting material was micro-fibrillar cellulose (MFC) sourced from Daicell Finechem (Celish KY-100S) and was a mixture of longer, wider fibres and cellulose nanofibres. MFC was micro fibrillated by a special manufacturing process from highly refined, pure fibre raw material. This material was supplied never dried with 25% solid content. The second material consisted almost entirely of cellulose nanofibres and was prepared by separating the long fibres from the starting material by filtration. This method was described in more detail in (Zhang et al. 2012). The MFC sample was diluted to 0.5 wt% solids and filtered through a filter mesh having 100 µm opening placed on Büchner funnel. Filtrate from the filter contains the nanofibres, while the larger and wider fibres were retained on the filter fabric. Once the fabric filters became clogged, they were washed and reused. Following filtration, the filtrate was centrifuged at 5000 rpm for 15 minutes. After centrifuging, the supernatant was discarded and only the nanofibres collected at the bottom of each tube were collected. The concentrated nanofibres were then stored in never dried form at 5 0 C until required for sheet-making. Yield of the separation process was 20%. For comparison, sheets were also made from NIST reference material Northern Bleached Softwood Kraft (Ampulski 2001). Starch (Cationic Tapioca Starch DS ) sourced from Corn Products (Thailand) Co. Ltd was used as strength additive for both NIST fibres and the MFC fibres. A starch suspension was prepared with a concentration of 4 wt%. The suspension was heated and stirred continuously at 90 until it formed a uniform viscous solution. This solution was added to the suspension which is used for preparing sheets with concentration of 0.16 g of starch per g of fibres. Characterization Scanning electron microscopy (SEM) images of nanofibers were taken using Field Emission Scanning Electron Microscopy, JEOL7001F FEG SEM, in order to measure the diameter distribution of nanofibres. A drop of nanofibre suspension was cast on a metal plate, which was then coated with platinum and used for SEM analysis. Estimates of nanofibre aspect ratios were made by sedimentation experiments. This method was described in more detail in (Zhang et al. 2012). The method was adapted for nanofibre suspensions from calibration curves published for wood pulps by (Martinez et al. 2001). 250 ml of nanofibre suspensions were decanted into measuring cylinders. The suspension was agitated to suspend the fibres completely in the cylinder, and then the fibres were allowed to settle. Once the fibres settled down completely, the height of sediment in the cylinder was measured. Sheet preparation Two sets of sheets were prepared from each material, one without any additives, while the second sheet used cationic starch to improve the bonding between the fibres. Sheets were made both at 30 gsm and 60 gsm to investigate the effect of the test conditions on the results (Batchelor et al. 2006). A British Hand Sheet Maker was used to prepare non-woven nanofibre sheets from both MFC and nanofibres. A solids concentration of 0.5 wt% was used for both 60 gsm and 30 gsm sheets. The full volume of the forming suspension was mixed thoroughly and carefully poured into the forming chamber after which vacuum was applied to enhance the filtration. The nanofibre sheets formed by filtration were placed between blotting papers and dried using an automatic sheet dryer at 91. ing took around 10 minutes. MFC sheets were placed between the blotting papers and dried at room condition under restraint to avoid shrinkage. After drying, nanofibre sheets were placed in a conditioned room maintained at 23 and 50% RH for at least 24 hours before testing. A Moving Belt Sheet Former (Raisanen et al. 1995; Xu, Parker 2000) was used to prepare square (22x22 cm 2 ) non-woven sheets from the NIST reference material. A solids concentration of 0.1 wt% was used make both 60 gsm and 30 gsm sheets. The sheet former uses a moving belt with machined holes running over a stationary vacuum box to simulate the type of vacuum pulses typically found in industrial sheet forming. First, the suspension was poured into a chamber sealed at the bottom with plastic sheeting, with a fixed filter underneath the sheeting. The moving belt was then started and the plastic sheeting was rapidly removed to expose the suspension to vacuum. Sheets were dried under ambient conditions for 24 hours and were then conditioned at 23 and 50% RH for 24 hours before testing. 344 Nordic Pulp and Paper Research Journal Vol 27 no.2/2012

3 Mechanical tests A Pulmac Troubleshooter was used for zero/short-span testing. All samples were cut into 2 cm wide 0.5 cm long samples with the length being the testing direction. Three samples were needed for each test; one sample was placed in the central clamping area of the tester and two other samples were placed under the two back steps of the clamping jaws to ensure proper jaw alignment under pressure. After each test, the samples were removed and replaced. This procedure was undertaken to minimise the amount of sample required for testing. Before the zero/short-span testing, optimum clamping pressures were determined. It is the pressure which minimizes sample slippage from under the jaw without inducing excessive fibre damage. It was determined by plotting the clamping pressure curve which showed an increasing average sample failure load with increasing clamping pressure up to a plateau region. Optimum clamping pressure is in the early plateau region. Further increasing clamping pressure will damage fibres and reduce the failure load. An optimum clamping pressure of 60 psi was used for softwood samples and 70 psi was used for both MFC and nanofibre samples. For the wet zero/short-span testing, the dry samples were thoroughly soaked in a tray of water and placed between blotting papers to remove excessive water before testing. The testing procedures were the same as the dry testing. Spans of 0, 0.1, 0.2, 0.4 and 0.6 mm were used for both dry and wet tests. The breaking load was calculated from the calibration supplied with the instrument of failure load (kg force) = (P-P 0 ) 0.362, where P is the internal pressure pushing the jaws apart at sample s failure, psi, and P 0 is the pressure required to overcome the zero load from the jaw spring at any given span, and is the instrument constant. testing was performed using an Instron Tester (Model 5566). Samples were cut into strips of size 140 mm 15 mm. Both dry and wet tensile tests were conducted at 50 and 100 mm span. Preparation of samples for a wet tensile test was the same as for a wet zero/short-span test. Results for all zero/short-span and tensile tests are presented as tensile index, i.e. the force per unit width divided by the grammage (g/m 2 ) of the sample. Cut strips from short span tests and tensile tests were coated with 1 nm thick platinum to observe the fracture surfaces using a JEOL7001F FEG SEM. Samples were loaded vertically. Fracture surfaces were imaged at 10 inclinations. The surfaces of the sheets were also imaged to measure the MFC and nanofibre diameters. The measurements were performed using ImageJ. Results and discussion Fibre characterisation Figs 1 and 2 show the SEM images and distributions of fibre diameter for MFC and nanofibre samples, respectively. Minimum diameter, maximum diameter, mean diameter, and standard, number of fibres measured (count) and bin width are reported for each material. Fig 1 shows that MFC sample contains a few clumps and large fibres. These clumps and large fibres were mostly removed through filtration. However, a few larger fibres remained in the nanofibre sample due to inefficiency in the filtration. The majority of the nanofibre sample contains fibres of width less than 100 nm and the mean diameter of nanofibres sample is 51 nm. Table 1 shows the mean diameter, the aspect ratio of MFC, nanofibres and NIST softwood, as well as the fibre length either measured (NIST sample) or the calculated from the diameter and aspect ratio (MFC and nanofibre samples). As it was impossible to measure the length of nanofibres from SEM image, aspect ratio was estimated using sedimentation method reported in (Zhang et al. 2012) and adapted from (Martinez et al. 2001). Fig 1 SEM image and diameter distribution for MFC sample. Fig 2 SEM image and diameter distribution of Nanofibres Nordic Pulp and Paper Research Journal Vol 27 no.2/

4 Table 1 Diameter and Aspect ratios of three materials used Sample Mean Diameter (µm) Aspect Ratio Mean length (µm) Micro-fibrillar cellulose Cellulose Nanofibre NIST softwood (Joshi et al. 2011) Mechanical test results The results for the NIST softwood sheets are shown in Fig 3. The data including means and standard s are given in Table A1, while the sheet density data are listed in Table A4. There are a number of points to note in this figure. The zero-span wet strength is less than dry strength for all samples. There are three competing explanations in the literature. One explanation (Cowan, Cowdrey 1974) is that the reduction with wetting is caused by a residual span under jaws, i.e. a distance in from the jaw edge over which the load is transferred into the sample. These authors extrapolated dry and wet curves versus span to determine the negative span at which they cross and called this the residual span. This compares to (Gurnagul, Page 1989) where the reduction in strength is due to plasticising effect of water on the fibres, which reduces their strength. This mechanism was based on the observation that the difference between dry and rewetted zero-span strength increases with decreasing yield. A third explanation (Mohlin, Alfredsson 1990) is that fibre defects such as kink or curl increase the dry strength compared to the wet because the bonds around a defect allow for stress to be transferred around the defect. It is possible that all three mechanisms are contributing. zero and short span strength linearly decreases as function of span for all samples. This is consistent with the analysis of (Batchelor 2003; Boucai 1971b; Michie 1963), where it was shown that the wet zero-span strength decreases approximately linearly with span, provided that the span is much less than the length of the fibre. In the case of a distribution of fibre lengths, such as is typical with wood pulp samples, it can be shown that the slope of the decrease with span is linearly related to the average length of load-bearing element in the sheet (Batchelor 2003). The reduction in wet short-span strength with span is because the major contribution to the wet zero and short span strength is from fibres which are gripped by both jaws and thus will be loaded to failure. As the span increases, this fraction of fibres will decrease and thus reduce the overall strength. The assumption is that wet fibres that are not gripped by both jaws will be unable to transfer stress between the jaws through the fibre-fibre bonds. The wet tensile strength at either 50 or 100 mm is extremely small, indicating that the wet fibre-fibre bonds are extremely weak in these samples. The dry zero and short span data shows some reduction with span, particularly at 0.6 mm for the 60 gsm samples with or without starch addition. The effect of span is much smaller than when testing wet as the fibre-fibre bonds are much stronger dry than wet and those fibres that are only gripped by one jaw can still transfer load through their bonds to other fibres. The data for the 30 gsm sheets is generally lower than the data for the 60 gsm sheets. As discussed in the introduction, this contradicts previous data where zerospan strength increased (Batchelor et al. 2006; Joshi et al. 2007) or stayed constant (Wink, Van Eperen 1962; Seth et al. 1989) with decreasing grammage. However this trend does match that observed for tensile strength (I'Anson, Sampson 2007) which increases with grammage up to maximum at 60 gsm, due to formation effects being more important at lower grammage. Although it should be noted that our data showed no effect of grammage for the tensile strength at either 50 and 100 mm spans. Starch addition increases dry paper tensile strength, consistent with many previous studies. The effect is approximately the same at either 50 or 100 mm spans. However, no effect of starch on dry zero span strength could be observed. There may have been some effects at the longest short span of 0.6 mm, but more testing would be needed to conclusively show this. There is no evidence of starch improving the wet strength at any span. All this data is consistent with the starch improving dry, but not the wet, fibre-fibre bond strength and having no effect on fibre strength. Thus starch has the largest effect for long span tensile tests, where fibre-fibre bonding is important and possibly some effects at short-span where fibre-fibre bonding is only important for those fibres which are gripped by one jaw but not two jaws. Fig 4 shows the results for the tensile testing of the MFC and nanofibre sheets. The data including means and standard s are given in Tables A2 and A3, while the sheet density data are listed in Table A4. There are some significant differences between these results and the NIST softwood results shown in Fig 3. The first interesting point to note is the wet zero-span strength for all samples is over 30 Nm/g, for all samples except for the 30 gsm MFC sheets. Two explanations could be considered. Firstly, this behaviour might be evidence that these samples contain large enough fibres to bridge the residual span under the jaws required to load the sample, such as occurs with the NIST sample. It is not known what this span is, although displacement measurements on a softwood sample suggested a span of between m (Batchelor, Westerlind 2003). However, none of the SEM and sedimentation measurements for these samples show evidence of fibres that long. In addition, the nanofibre sheets have exactly the same strength as the MFC sheet despite the nanofibres having been processed from the MFC sample to remove the larger fibres. The most likely explanation is that pressure applied by jaws increases the normal force at fibre-fibre contacts, and thus greatly increases the fibre-fibre friction when under load. This process will significantly improve the bonding between the fibres, 346 Nordic Pulp and Paper Research Journal Vol 27 no.2/2012

5 which would be weaker than when dry but sufficient to give the sheet some strength. Fig 3 Zero, short and tensile span measurements for softwood sheets. The error bars show the 95% confidence intervals of the measurements Fig 4 Zero, short and tensile span measurements for micro fibrillated cellulose and nanofibres sheets. This evidence suggests that two of the three current explanations (Cowan, Cowdrey 1974; Gurnagul, Page 1989) as to why the wet-zero- span strength is less than the dry zero-span strength could both be incomplete. The wet zero-span strength will include significant contributions from friction from fibre-fibre bonds for fibres that are held by one jaw only. This will contribute to the wet zero-span of all samples and may change with pulp yield. It also cannot be assumed that the difference between zero and short-span strength is due to fibre length effects, raising doubts about the extrapolation method proposed by (Cowan, Cowdrey 1974) to determine residual span or using the zero and short span data to infer information about the fibre or load-bearing element lengths (Batchelor 2003; Boucai 1971b; Michie 1963). At all other spans, except zero, wet tensile strength is close to zero. The wet frictional force under pressure suggested for the zero-span tests will not apply at short span as failure will occur in the short span, rather than under the jaws. In addition, there is no observable difference between results at short span and results for tensile measurements. It should be noted that uncertainties of the zero and short span results are relatively high as the pressure needed to drive jaws apart without a sample is most of the measured pressure at failure. Given that wet short-span and tensile strength measurements give similar results for MFC and nanofibre samples, then the mechanisms of stress transfer in the samples could be similar. That is, in the short-span test there are no fibres bridging between jaws, just as in the 50 and 100 mm tensile tests. The scale is different but the mode of loading is the same where force is transferred through the fibre-fibre bonds to apply load to the fibres. Thus short span strength is measuring sheet tensile strength, but at a smaller span. This hypothesis is also supported by the dry zero and short span and tensile strength data. Although there are some fluctuations in the data, there is no reduction in strength as span increases for zero/short-span strength. Strength is approximately constant, in contrast to the NIST results shown in Fig 3, which showed a reduction in strength when testing with a short-span of 0.4 and 0.6 mm, compared to the zero-span strength. In addition, the reduction in strength from a short-span of 0.6 mm to a span of 50 mm is much less for these samples than for the NIST softwood samples where the tensile strength at 50 mm span is much less than half that of the zero-span strength for all samples. All of this suggests that in zerospan testing, as in short-span testing, the fibres from the MFC or nanofibre sheets are never gripped by both jaws and thus that load always has to be transferred through fibre-fibre bonds to load a sample to failure. SEM results Figs 5 and 6 show little quantitative difference between the appearances of the nanofibre sheet fracture line tested at any span. In each figure there is the occasional larger fibre that has been pulled out across the fracture line but otherwise the fracture line appears very sharp. Part of the reason for this is that many of the fibres must be too small to be separately observable in these images. Figs 5 and 6 show little quantitative difference between the fracture lines of samples tested either wet or dry or with different spans. The implications for the failure mechanism of these samples deserve further study. Fig 7 shows microscope images of the fracture lines of the samples made from the NIST softwood fibres. There are only a few fibres, mostly not in the direction of loading, that have been pulled-out from one side of the fracture line for the zero-span test. In contrast the fracture line of the test at the 0.1 mm span shows many fibres that have been pulled out. Nordic Pulp and Paper Research Journal Vol 27 no.2/

6 Fig 5 Fracture line images from sheets made from Nanofibres. top left from dry zero span test, top right from dry 0.6 span test, bottom left from wet zero span test, bottom right from wet 0.1 span test Fig 6 Fracture line images of dry and wet tensile test with 50 mm gap between jaws for sheets made from Nanofibres. Fig 7 Fracture line images of zero span and short span (0.1 mm) test for sheets made from NIST reference material, tested dry. 348 Nordic Pulp and Paper Research Journal Vol 27 no.2/2012

7 Discussion and analysis All the evidence so far points to the conclusion that for nanofibre sheets, the zero-span test is measuring the tensile strength, but at a greatly shortened length scale. There then remains the question as to why the measurements at different length scales give different values. The first point is that the tensile samples with dimensions 100 mmx15mm are subject to Poisson contraction under tensile loading, whereas the zero and short span measurements are not, given the span under the jaws is much smaller than the sample width. If the sample width was greatly reduced to allow Poisson contraction then the measured zero-span strength with Poisson contraction, Z p, would be lower than the zerospan strength without Poisson contraction, Z np. If the strain at failure is assumed to be the same in the two 2 cases then the two are related by Zp Znp 1 where is the Poisson ratio (Page 1969). This assumption has been used in the derivation of the Page equation for tensile strength (Page 1969). Experimentally Poisson s ratio for dry paper is approximately 1/3 (Page 1969), which yields Z 8 Z / 9. Thus the zero and p short-span strength should be multiplied by 8/9 to properly compare the strength values with each other. The results of this data correction of the zero/short-span strength and comparison with the tensile strength are shown in Fig 8. Fig 8 shows that there is a maximum reduction of 27% in strength between a short span of 0.6 mm and a tensile span of 50 mm for all MFC and nanofibre samples while the corresponding minimum reduction in strength for the NIST sample was 42% for one sample, while for the other two samples the strength was more than halved. This is consistent with the predominant contribution to the load in the zero/short-span test of the NIST softwood sample being the direct loading of fibres to failure, while for the MFC and nanofibre sample the contributions of both bond and fibre strength are important. There then remains the question of why there is a reduction in strength with span. One piece of further evidence is that there is for all samples, except the 30 gsm NIST sample, a small additional reduction in strength between 50 mm and 100 mm spans. These results are then explainable via Weibull failure analysis (see eg (Gregersen et al. 1998; I'Anson, Sampson 2007; Uesaka et al. 2001; Wathen et al. 2006)) for applications in the testing of paper), where local areas in a sample have a strength distribution. Failure at the weakest point in the sample means larger samples have lower strength (Gregersen 1998). Conclusion It has been established that zero/short span test can be used instead of standard tensile test to measure the tensile strength for nanofibre sheets. This is because there are no fibres bridging between and directly gripped by both jaws, just as in case of tensile tests. The zero/short-span test cannot be used as alternative method for measuring np Fig 8 index from zero and short span tests multiplied by (8/9) and index from tensile test. tensile strength for the NIST softwood sample because fibres are longer than distance between jaws. Starch addition increased dry paper tensile strength for sheets made from the NIST softwood fibres but had no effect on strength for the sheets made from the MFC sample. Acknowledgements Warren Batchelor would like to acknowledge many valuable discussions about the zero and short span test with Dr. Bo Westerlind from the SCA Research Centre and Mid-Sweden University. We acknowledge the financial support of the Australian Research Council, Australian Paper, Nopco Australasia, Norske Skog, SCA Hygiene Australasia and Visy through Linkage Project Grants LP and LP Swambabu Varanasi would like to acknowledge the support of Monash University through a Monash Graduate Scholarship. The authors would like to acknowledge the facilities used with the Monash Centre for Electron Microscopy. Authors would like to thank Javindu Hathurusinghe for helping us with experiments. Literature Ampulski, R.S. (2001): Report of Investigation Reference Materials 8495 Northern Softwood Bleached Kraft 8496 Eucalyptus Hardwood Bleached Kraft, National Institute of s and Technology Gaithersburg, MD. Batchelor, W.J. (2003): Determination of Load-Bearing Element Length in Paper Using Zero/Short Span Testing, TAPPI Journal 2(8), 3-7. Batchelor, W.J. and Westerlind, B.S. (2003): Measurement of short span stress-strain curves of paper, Nordic Pulp & Paper Research Journal 18(1), Batchelor, W.J., Westerlind, B.S., Hagglund, R. and Gradin, P. (2006): Effect of test conditions on measured loads and displacements in zero span testing, TAPPI Journal 5(10), 3-8. Boucai, E. (1971a): Zero-Span Test and Fibre Strength, Pulp and Paper Magazine of Canada 72(10), Boucai, E. (1971b): The zero-span test: its relation to fibre properties, In International Paper Physics Conference, Mont Gabriel, QC, pp Cowan, W.F. (1975): Short span tensile analysis, Pulmac Instruments, Montreal, Canada. Nordic Pulp and Paper Research Journal Vol 27 no.2/

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9 Appendix Table A1 Mean values and standard s of (TI) data for sheets from NIST samples 30 gsm NIST 60 gsm NIST 60 gsm NIST with starch Span (mm) TI Nm/g Table A2 Mean values and standard s of data for sheets from Nanofibre samples Span (mm) (N.m/g) (N.m/g) Span (mm) Table A3 Mean values and standard s of data for sheets from MFC samples 30 gsm MFC 60 gsm MFC 60 gsm MFC with starch Table A4 Apparent densities for sheets made from MFC, Nanofibres and NIST Material Apparent Density (Kg/m 3 ) 60 gsm MFC gsm MFC with starch gsm MFC gsm Nanofibres gsm NIST gsm NIST gsm NIST with starch Nordic Pulp and Paper Research Journal Vol 27 no.2/