application and interpretation of zero and short-span ... · application and interpretation of zero...

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 zero-

span 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 ([email protected]), Hui Hui Chiam

([email protected]), Warren Batchelor

([email protected]),

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

210m 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

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Nordic Pulp and Paper Research Journal Vol 27 no.2/2012 343

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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 50C

until required for sheet-making. Yield of the separation

process was 20%.

For comparison, sheets were also made from NIST

reference material 8495- Northern Bleached Softwood

Kraft (Ampulski 2001).

Starch (Cationic Tapioca Starch DS 0.03-0.04) 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 nano-

fibers were taken using Field Emission Scanning Electron

Microscopy, JEOL7001F FEG SEM, in order to measure

the diameter distribution of nanofibres. A drop of nano-

fibre 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 cat-

ionic 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 Standard 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 . Drying 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 cm2)

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 sus-

pension 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.

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344 Nordic Pulp and Paper Research Journal Vol 27 no.2/2012

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-P0) ×0.362, where P is the internal pressure pushing

the jaws apart at sample’s failure, psi, and P0 is the

pressure required to overcome the “zero” load from the

jaw spring at any given span, and 0.362 is the instrument

constant.

Tensile testing was performed using an Instron Tensile

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/m2) 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 deviation, 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

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Table 1 Diameter and Aspect ratios of three materials used

Sample Mean Diameter (µm)

Aspect Ratio

Mean length (µm)

Micro-fibrillar cellulose 0.073 147 10.73 Cellulose Nanofibre 0.051 138 7 NIST softwood (Joshi et al. 2011)

27.8 72 2000

Mechanical test results

The results for the NIST softwood sheets are shown in

Fig 3. The data including means and standard deviations

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.

Wet 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 zero-

span 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 deviations 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 300-350 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,

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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 zero-

span 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.

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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.

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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, Zp, would be lower than the zero-

span strength without Poisson contraction, Znp. If the

strain at failure is assumed to be the same in the two

cases then the two are related by 21p npZ Z

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 8 /9p npZ Z . Thus the zero and

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

Fig 8 Tensile index from zero and short span tests multiplied by (8/9) and Tensile 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 LP0989823 and LP0990526. 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.

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Appendix

Table A1 Mean values and standard deviations of Tensile Index (TI) data for sheets from NIST samples Span (mm)

30 gsm NIST 60 gsm NIST 60 gsm NIST with starch

Dry Tensile Index (Nm/g)

Standard deviation

Wet Tensile Index (Nm/g)

Standard deviation

Dry TI Nm/g

Standard deviation


Standard deviation


Standard deviation


Standard deviation

0 107.65 16.21 85.65 10.30 118.56 12.31 95.59 15.00 121.02 16.04 116.11 13.79

0.1 98.24 8.80 87.63 7.81 118.02 11.83 89.14 8.56 129.20 11.72 87.42 10.77

0.2 79.92 13.89 60.47 7.51 116.04 13.14 78.49 12.06 110.98 12.15 82.05 11.81

0.4 65.37 11.64 50.91 5.50 98.65 7.57 66.27 6.71 127.02 9.05 60.23 9.26

0.6 64.75 15.40 42.80 5.71 82.20 7.80 49.94 8.23 89.50 5.89 48.68 9.35

50 24.70 5.35 1.08 0.15 25.87 5.37 1.10 0.20 46.21 6.08 1.21 0.13

100 18.04 3.95 0.98 0.15 18.52 1.56 0.94 0.05 46.97 6.35 1.15 0.10

Table A2 Mean values and standard deviations of Tensile Index data for sheets from Nanofibre samples

Span (mm)

Dry Tensile Index (N.m/g)

Standard deviation

Wet Tensile Index (N.m/g)

Standard deviation

0 104.24 3.15 30.35 6.68

0.1 108.10 4.83 0.00 3.14

0.2 98.51 6.15 0.00 3.62

0.4 104.79 5.07 2.56 0.00

0.6 111.17 9.94 3.49 3.29

50 92.85 7.90 4.11 1.61

100 78.88 8.39 3.79 1.21

Table A3 Mean values and standard deviations of Tensile Index data for sheets from MFC samples Span (mm)

30 gsm MFC 60 gsm MFC 60 gsm MFC with starch


Standard deviation


Standard deviation


Standard deviation


Standard deviation


Standard deviation


Standard deviation

0 93.35 13.07 10.46 10.54 124.40 13.84 33.79 3.69 116.10 10.23 35.34 3.69

0.1 89.82 10.74 0.00 3.98 108.11 5.53 12.82 2.75 112.70 14.30 3.40 2.75

0.2 93.49 12.70 3.63 0.00 117.06 8.21 1.19 2.13 126.94 11.63 0.39 2.13

0.4 97.60 14.78 3.61 0.00 126.39 9.98 5.11 0.88 126.39 5.95 3.54 0.88

0.6 121.82 4.78 0.00 0.00 124.86 2.95 12.14 1.40 122.61 8.16 11.48 1.40

50 78.52 9.01 4.26 1.64 81.14 5.46 10.77 9.50 84.04 16.58 10.92 2.64

100 64.16 7.95 5.44 0.83 82.86 5.23 16.92 11.35 73.84 4.20 4.29 2.90

Table A4 Apparent densities for sheets made from MFC, Nanofibres and NIST

Material Apparent Density (Kg/m3)

60 gsm MFC 783.73

60gsm MFC with starch 830.06

30gsm MFC 660.45

60 gsm Nanofibres 806.57

60 gsm NIST 516.69

30 gsm NIST 477.05

60 gsm NIST with starch 582.08

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