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O R I G I N A L P A P E R

Estimation of cellulose nanofibre aspect ratio

from measurements of fibre suspension gel point

Swambabu Varanasi Rongliang He

Warren Batchelor

Received: 6 February 2013 / Accepted: 9 June 2013 / Published online: 26 June 2013

Springer Science+Business Media Dordrecht 2013

Abstract Cellulose nanofibre aspect ratio controls

the properties of sheets made from nanofibres and

processing conditions, but aspect ratio is very difficult

to measure. In this paper, aspect ratio was estimated

from the gel point of a cellulose nanofibre suspension,

the solids concentration at which the transition from a

dilute to a semi-dilute suspension occurs. Four batches

of cellulose nanofibres were tested. Two were pro-

duced from softwood fibres using ball milling. Com-

mercially produced microfibrillated cellulose material

was also used, both in as supplied form and afterremoval of the larger fibres by filtering. The average

diameter measured from SEM images of fibres ranged

from 33 to 73 nm. One sample was too heavily treated

and an average dimension could not be measured. The

gel-point was measured both from the height of a layer

of cellulose nanofibres sedimented from a dilute

suspension or from the lowest solids concentration at

which a yield stress could be measured using a vane

rheometer. The two methods were closely in agree-

ment for all samples. Aspect ratio was then calculated

using either the effective medium (EMT) or crowding

number (CN) theories. Aspect ratio calculated with an

assumed fibre density of 1,500 kg/m3, using the CN

theory ranged from 155 to 60. Use of the EMT theory

reduced the calculated aspect ratio by between 11 and

23 %. Reducing the assumed density in suspension

from 1,500 to 1,166 kg/m3 reduced the calculated

aspect ratio by 1214 %. The heavily treated sample

had by far the lowest aspect ratio.

Keywords Cellulose nanofibres Gel point Aspect ratio Yield stress Sedimentation

Introduction

Cellulose is the most common organic polymer in the

world, representing 1.59 1012 tons of total annualbiomass growth, mostly in plants(Malcolm Brown et al.

1996). Since it is formed from CO2 gas in the

atmosphere via photosynthesis in plants, it is a sustain-

able and renewable source of material. Plant fibres

consist of cellulose nanofibrilsof 24 nm in diameteras a

structural unit which is made up of cellulose crystallites

of*4 nm in diameter (Malcolm Brown et al. 1996;

Jakob et al.1995). Green properties such as biocom-

patibility and biodegradability are also expected from

S. Varanasi W. Batchelor (&)Department of Chemical Engineering, Australian Pulp

and Paper Institute, Monash University,

Melbourne, Australia

e-mail: [email protected]

S. Varanasi


R. He

Institute for Frontier Materials, Deakin University,

Geelong, VIC 3217, Australia


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these naturally-occurring raw materials, which are

significantly advantageous for biomedical and environ-

mental applications over synthetic organic and inor-

ganic nanomaterials (Czaja et al. 2006). Over the last

two decades, the synthesis of cellulose nanofibres and

their applications in various fields have gained increas-

ing attention due to their high strength and stiffness,combined with low weight, biodegradability and

renewability (Siroand Plackett2010). Cellulose nano-

fibres can be used as reinforcing fibres in bio compos-

ites, strength additives (Ahola et al. 2008), and to

produce hydrogels, anti-microbial films (Czaja et al.

2006), and high-technology devices (Nogi et al.2009).

Although there are many methods available for

production of cellulose nanofibres (Eichhorn et al.

2010), one significant area still being developed is the

characterization of nanofibre dimensions (Zhang et al.

2012). Whilst a combination of microscopic techniqueswith image analysis can provide information on cellu-

lose nanofibre widths (Dufresne et al. 2000; Taniguchi

and Okamura1998; Janarthanan and Sain 2006), it is

more difficult to determine nanofibre lengths due to

entanglements and difficulties in identifying both ends

of individual nanofibres (Henriksson et al.2008; Ishii

et al. 2011). Recently, Zhang et al. used a sedimentation

method (Zhang et al. 2012) for measuring the aspect

ratio of nanofibres based on the method proposed by

Martinez et al. (Martinez et al. 2001) for wood pulp

fibres. Using this method, the aspect ratio is estimatedbased on the gel point concentration, the threshold

consistency at which a continuous network of fibres in

suspension forms. Below the gel point concentration,

the suspension does not contribute to the mechanical

strength (Derakhshandeh et al. 2011). The measured

aspect ratio has been shown to correlate with the

concentration range over which cellulose nanofibre

sheets could be formed using filtration (Zhang et al.

2012; Varanasi and Batchelor 2013). One criticism that

can be made of this method is that in order to observe a

visible sedimented layer the fibres must form structuresthat can scatter light in the visible range, thus requiring

complexes several hundred nanometres in size. An

alternative method to probing fibres in suspension that

avoids this problem is to study the suspension rheology.

The types of rheological measurements that can be

applied to fibre suspensions will depend on the solids

content of the suspension, i.e. whether the suspension

can be classified as dilute, semi-dilute or concentrated.

One previous report (Ishii et al. 2011) of cellulose

nanofibre length estimation used visco-elastic mea-

surements of dilute suspensions and estimated the fibre

length using the theory of linear visco-elasticity of

semi-flexible rod-like polymers in dilute suspension,

estimating the fibre length as 2.2 lm for 4 nm

diameter fibres, giving a very high aspect ratio of 550.

Other measurements in the concentrated and semi-dilute ranges include suspension yield stress measure-

ments (Mosse et al. 2012b) to investigate the interac-

tion of cationic polymers with cellulose fibres and

shear viscosity measurements for characterizing the

dispersion of carbon nanofibre suspensions colloidal

systems (Xu et al.2005) or TEMPO oxidised cellulose

nanofibre suspensions (Lasseuguette et al. 2008).

These are a particularly sensitive probe of fibre

suspensions because in the semi-dilute and concen-

trated regimes the rheological properties are often

dominated by the interaction and entanglement of thefibres, which in turn depend upon of the structural

properties such as diameter, length and aspect ratio.

In this paper we present a new method for

estimating the gel-point of cellulose nanofibre sus-

pensions by measuring suspension yield stress as a

function of concentration. We compare the results to

that obtained by sedimentation and estimate aspect

ratio by both methods.

Theory

There are three concentration regimes for particles

dispersed within a medium. Particles in suspension can

be in dilute, semi-dilute or concentrated states. Of

interest here is the boundary between these states. If the

volume concentration is denoted by U then the connec-

tivity threshold,Uc, is the boundary between the dilute

and semi-dilute regions, the lowest volume fraction

where the particles first form a continuous network and

the rigidity threshold,Ur, is the boundary between the

semi-dilute and concentrated regions, which is thelowest volume fraction at which the particle suspension

will exhibit mechanical stability under load (Celzard

etal. 2008). The connectivity threshold is also called the

gel point (Martinez et al.2001) .

In the work here, we shall consider the application

of the semi-empirical Crowding Number (CN) and of

the more rigorously derived Effective Medium theory

(EMT) to characterise the transition between states of

fibres in suspension.

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EMT was originally developed to solve problems

such as the conductivity of a material consisting of

particles dispersed in a medium. Of most relevance is

the extension of the theory to consider spheroids as the

dispersed particles (Celzard et al. 2000) and the

successful application to predicting the conductivity

of spheroid graphene dispersed in a matrix of air. Forprolate or oblate ellipsoids, Ucis given (Celzard et al.

2000) by

Uc 9Lc 1 Lc 2 Lc 15 9Lc 1

whereLcis the depolarization factor of the particles. If

the particle is modelled as a prolate ellipsoid with

equal minor axes, a and b then (Celzard et al.2000)Lcis given by

Lc 1 e2

2e3 ln

1 e1 e

2e

2

where e is the eccentricity which ise ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1 a=c2q

where c is the major axis of the ellipsoid and when

expressed in terms of the fibre aspect ratio, A(= c/a),

yields e ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1 A2p

. The rigidity threshold is then

related to the connectivity threshold byUr= 4Uc.

The Crowding Number (CN) theory has been

developed by Kerekes and Schell, based on earlier

work by Mason. The crowding number, N, is given byKerekes and Schell (1992) as

N 2=3 U l=d 2 2=3 UA2 3where l and d are the fibre length and diameter,

respectively, of the fibres in suspension. Based on the

number of contacts developed in a suspension, the

rigidity threshold has been identified as occurring at

N = 60 (Celzard et al. 2009) from which

Ur 90=A2 4

The connectivity threshold was experimentally deter-mined to be 16 4 (Martinez et al. 2001) from

analysis of PET measurements made on dilute fibre

sedimentation experiments, from which

Uc 24=A2 5The relationship between aspect ratio, A, and Urand

Ucis shown in Fig.1.

The aspect ratio can be calculated from either the

connectivity or rigidity thresholds by the following

equations as given in Table 1, where the equations for

the EMT theory were determined by fitting the data in

Fig.1.

Finally, we are most readily able to measure the

solid fraction and not the volume fraction. If we wish

to relate the volume fraction to the solids fraction, C

(kg fibre/kg of suspension), then we may write

C qfU= qfU ql 1 U

where qf and ql are

the density of the fibres and liquid, respectively, which

if U 1 can be simplified as C qf=ql

U or

U ql=qf

C. The solids fraction forthe connectivity

and rigidity thresholds are then denoted Cc and Cr,

respectively. The appropriate fibre density to useremains an open question. The density of the cellulose

nanofibres in the dry state is approximately 1,500 kg/m3,

however it is not clear that this is applicable to a

suspension of cellulose nanofibres in water, where a

reduction of density due to an uptake of water into the

structure is likely. Recent measurements of moisture

diffusion of softwood, hardwood and cellulose nano-

fibre samples have suggested that two phases of water

exist, a slow diffusing phase tightly bound with the

cellulose fibrils and a faster diffusing phase more

loosely bound (Perkins and Batchelor 2012). Inparticular the measurements suggested that the tightly

bound phase had a maximum capacity of approxi-

mately 0.5 g water/g fibre (Perkins and Batchelor

2012). The addition of this water to the fibres would

reduce the density of the cellulose nanofibres in

suspension to (2/3)9 1,500 ? (1/3) 9 1,000 kg/m3

= 1,333 kg/m3. There exists a further complication

that the samples tested here are not fully separated. If

this is the case then they will form connected

0

20

40

60

80

100

120

140

160

180

0 0.01 0.02 0.03 0.04 0.05 0.06

AspectRaio

Phi

PhiC (EMT)

PhiC(CN)

PhiR(EMT)

PhiR(CN)

Fig. 1 Aspect ratio predicted from the connectivity (PhiC (Uc))

and Rigidity (PhiR (Ur)) thresholds using the EMT and CN

theories

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assemblies with reduced density due to the water

between the connected fibres. Given the uncertainties

in the density we will calculate aspect ratios estimated

from threshold solids concentration using three differ-

ent densities using the equations given in Table2

below.

Experimental

Material

Cellulose nanofibres were prepared using two different

methods. In the first method, nanofibres were prepared

as reported previously (Varanasi et al. 2012; Varanasi

and Batchelor 2013). Micro fibrillated cellulose (MFC)

supplied from DAICEL Chemical Industries Limited

(grade Celish KY-100G) was used as the starting

material for preparing cellulose nanofibres as well as

being tested. The material contains a mixture of

nanofibres and larger fibres and particles. The large

fibres and particles were filtered from the MFC sample

using a solids concentration of 0.5 kg/m3 and two

fabric filters with 100 lm openings to retain the larger

fibres on the fabric. The filtrate was centrifuged at

5,000 rpm for 20 min. After centrifuging, the super-

natant was discarded and only the nanofibres at the

bottom of tubes were collected. The yields of the

filtration process before and after centrifugation were

21.6 and 20 %, respectively. More details are given in

Zhang et al. (2012). The starting material was also used

for measurements. In this paper, the original sample

and the sample prepared by filtration have been

labelled as MFC and NF, respectively.

In the second method, cellulose nanofibres were

prepared using ball milling with a SPEX 8000 shaker

mill. Firstly, cellulose pulp sheet (NIST reference

material 8495) was cut into 5 9 5 cm pieces and

soaked in deionised water overnight in the refrigerator

to prepare 1 wt% solid suspension. The wet cellulose

pieces were then shredded using a conventionalkitchen blender and then stirred at 70 C overnight.

Then 20 g of 1 wt% cellulose pulp suspension, 45 g of

cerium-doped Zirconium balls (0.5 mm in diameter)

and 20 mL of deionised water were then placed in a

70 ml polypropylene container and milled using Spex

8000 ball mill for 60 and 90 min, for the NIST60 and

NIST90 grades, respectively. The final suspension was

filtered using a polyester mesh (opening size 125

micron) to remove the zirconium balls and any larger

remaining fibres. The filtrate was then stored in the

refrigerator and used as required. Results from earlierexperiments to manufacture cellulose nanofibres from

this starting material have been reported in Zhang et al.

(2012).

Diameter distribution

SEM Images of MFC and Nanofibre samples were

taken using a JEOL SEM (JSM-7001F FEGSEM) and

NIST60 and NIST90 were taken using Supra 55 V

SEM. Samples were highly diluted and a drop of

suspension was cast on a metal plate, air dried and thenplatinum coated. Coated samples were imaged using

SEM. The magnification ranged from 3,000 to 80,000.

Diameter distribution of MFC, NF and NIST60

samples were measured from SEM images. NIST90

samples only contained a few fibres and square

particles so the diameter distribution was not mea-

sured for this sample.

From each image, the width of each nanofibre

observable in the image was measured using ImageJ.

The fibre diameters measured in each image were then

sorted into bins of 10 nm size and normalized tocounts/m2, by dividing by the total area of the image.

Table 1 Equations to calculate aspect ratio using the EMT

and CN theories

Uc (EMT) Ur(EMT) Uc (CN) Ur(CN)

A 2:52U0:58c A 5:64U0:58r A 4:90U0:5c A 9:49U0:5r

Table 2 Equations used

for calculating aspect ratio

from solids fraction, C

Assumed density,

qf(kg/m3

)

EMT, Cc EMT, Cr CN, Cc CN, Cr

1,500 A 3:19C0:58c A 7:13C0:58r A 6:0C0:5c A 11:61C0:5r1,333 A 2:98C0:58c A 6:66C0:58r A 5:66C0:5c A 10:95C0:5r1,166 A 2:76C0:58c A 6:17C0:58r A 5:29C0:5c A 10:24C0:5r

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Fractions were calculated by dividing the individual

bin counts/m2 with total counts/m2. In the event that a

bin had fibres measured at different magnifications,

the highest value of counts/m2 was used. Further

details of the method are given in Zhang et al. (2012).

Yield stress measurement

Yield stress was measured on fibre suspensions using

the Haake Rotovisco RV20 system with four vanes of

38 mm (diameter) by 76 mm (length). This method

was described in more detail in Mosse et al. (2012a, b).

The suspension with known concentration was placed

in a beaker and the vane fully immersed. The maximum

torque, quoted from the user manual, measurable by

the instrument was 0.049 Nm at 100 % full scale.

Rheometer can be operated at three different scales, 1,

10 and 100 % torque. Measurements were carried out

using the 1 % scale. Maximum measurable torque

value at 1 % scale is 4.9 9 10-4 Nm. The instrument

output is in volts. The smallest subdivision is 1/1,000 of

the maximum measureable torque and thus the lowest

possible torque value that can be measured is

4.9 9 10-7 Nm. All the values discussed in the results

are within the 1 % scale range. Beaker dimensions are

72 mm diameter and 92 cm length, and the beaker

diameter is much larger than the vane diameter,

minimising the chance of slip at the wall. Prior to each

measurement series at a given solids concentration, the

pulp suspension was stirred thoroughly using the vane

rotating at 500 rpm (the highest speed setting),

removing any air bubbles in the pulp and ensuring that

the pulp was evenly distributed throughout the beaker.

Each sample was mixed for a minimum of 2 min until

the suspension was completely clear. Five runs were

made for each sample at a given solids concentration.

Before each run, the sample was mixed for 10 s at

500 rpm, before being allowed to stand for 30 s.

Following this, a measurement was taken by initiating

shear flow at a rotation rate of 0.65 rpm, and recording

the torque measured as a function of time.

Yield stress is the maximum stress sustained by the

suspension at the onset of continuous motion. The

peak torque in each start-up of shear flow experiment

was converted to yield stress using following equation

(Dzuy and Boger1985):

Tm pD3

2

H

D 1

3

sy 6

where Tm is the maximum torque measured, D is the

diameter of the vane,His the height of the vane, and syis the yield stress.

Sedimentation

Estimates of nanofibre suspension gel point were madeby 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 Marti-

nez et al. (Martinez et al. 2001). 250 ml of nanofibre

suspensions with solids concentration (=solids frac-

tion 9 suspension density) ranging from 0.5 to 5 kg/m3

were decanted into measuring cylinders. The suspen-

sion was agitated to suspend the fibres completely in the

cylinder, andthen the fibres were allowed to settle. Once

the fibres settled down completely, the height ofsediment in the cylinder was measured. A minimum

of 48 h was allowed for the sedimentation process.

Results and discussion

Fibre cross-section measurements

Figures2and3show the SEM images and distribution

of fibre diameter of the MFC sample, while the

corresponding data for the NF sample derived fromthe MFC sample are given in Figs. 4 and5. Figures2

and 3 show that the MFC sample contains nanofibres as

well as a few large fibres. The mean fibre diameter was

73 nm. These largefibres were mostly removed through

filtration, which reduced the mean diameter to 53 nm.

Figures6and7show that NIST60 sample contains

uniform nanofibres with average diameter of 33 nm.

The milling time of 60 min has produced a very

uniform sample with low fibre diameter. Figure8

shows an image of the NIST90 sample. This was milled

for 90 min. The increased milling time has clearlybroken down the nanofibres as observed in Fig. 8, as

the sample consists mostly of irregularly shaped

particles of low aspect ratio with only a few fibres.

Sedimentation data to calculate suspension gel

point

The sedimentation data obtained for all four samples is

shown in Fig. 9. For each sample, the sedimentation

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behaviour was measured of fibre suspensions with

different dilute concentrations. A graph of initial solid

concentration (C0) versus the ratio of sediment height

(hs) to initial suspension height (H0) was then plotted

Fig. 2 SEM image of MFC sample

Fig. 3 Diameter distribution of MFC sample

Fig. 4 SEM image of Nanofibres sample

Fig. 5 Distribution of fibre diameters for the nanofibre sample

Fig. 6 SEM Image of NIST60 sample

Fig. 7 Distribution of fibre diameters for the NIST60 sample

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and fitted with a quadratic equation. The linear term of

the fit then gives the gel concentration, gc. The resultsof the fits, fitting statistics and confidence intervals are

given in Table3.

Finally, it is interesting to consider whether the

rigidity threshold can be applied to this data. As

discussed previously, the rigidity threshold occurs at a

volume concentration that is 3.75 times (Crowding

number) or 4 times (EMF theory) (Celzard et al.2009)

the connectivity threshold. The fits to the data show

that these concentrations are only achieved at solids

concentrations well above what would be required to

produce Hs/H0 = 1, i.e. the lowest concentration

where the suspension does not sediment, suggesting

that the rigidity threshold is not applicable to sedi-

mentation data.

Yield stress measurements

Yield stress is one of the most important rheological

properties of semi-dilute or concentrated fibre suspen-

sions. Unless it is exceeded, flow does not take place.

Yield stress is the maximum stress reached when strain

is increased to initiate flow, after which stress

decreases. The maximum stress is called the ultimate

shear strength and used as a measure of apparent yield

stress (Liddel and Boger1996). A yield stress occurs

because the fibres form a continuous network, wherebyfibres are restrained from moving because of contacts

with other fibres (Derakhshandeh et al. 2011). The

corollary is that a dilute fibre suspension will display no

yield stress. In the work here yield stress is measured as

a function of solids concentration in order to determine

the transition from a dilute fibre suspension to a semi-

dilute fibre suspension, i.e. the solids concentration at

which a continuous network first forms.

There are many methods available to measure yield

stress. We used a vane rheometer, whereby the torque

was measured as a function of time. If the fibresuspension has a yield stress at that concentration, then

the torque value reaches maximum and then decreases

(Derakhshandeh et al. 2011). If the fibre suspension

does not have a yield stress at that concentration,

torque profile of fibre suspension will be like the

torque profile of water. This maximum torque value is

called the peak torque in the rest of discussion.

Torque values as function of time at different

concentrations are shown in Figs.10, 11, 12 and 13

for MFC, NF, NIST90 and NIST60 suspensions,

respectively. In Fig.10, three replicates of torque

Fig. 8 SEM Image of NIST90 sample

Fig. 9 Sedimentation data

Table 3 Gel point solids

concentrations from

sedimentation method and

the standard errors of the fit

given in brackets

Sample Fitting equation Gel point solids

concentration (kg/m3

)

Solids mass

fraction

MFC y = 2.03x2? 1.78x, R

2= 0.98 1.78 (0.54) 0.00178

NF y = 1.98x2? 2.30x, R

2= 0.99 2.30 (0.27) 0.00230

NIST90 y = 5.65x2? 9.35x, R

2= 0.99 9.35 (0.50) 0.00935

NIST60 y = 1.59x, R2= 0.99 1.59 (0.02) 0.00159

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values as function of times for solids concentration of

2.5 kg/m3 are shown. These show that results were

consistent and reproducible. The deviation between

runs for a given sample was at most 10 %. There were

several interesting points from Figs. 10,11,12and13.

Figures10 and11contain measurements made at 0 %

solids concentration (distilled water only)and show thatthe vane rheometer has a small level of intrinsic drift.

All measurements showed a small initial increase

associated with commencement of rotation of the vanes

followed by a drift downwardsas the rotation continued.

The maximum peak torque observable when measuring

water alone was approximately 7.2 9 10-6 N m. The

effect of testing below and above the gel point can be

seen very clearly in Fig.11. The test at a solids

concentration of 2 kg/m3 was essentially indistinguish-

able from the water curve indicating that the fibre

suspension was dilute and the fibres did not form acontinuous network. Increasing the solids concentration

to 2.5 kg/m3 however produced an observable yield

stress with torque increasing to a yield stress of

2.5 9 10-5 N m after 10 s before flattening out. Some

other cases are less clear cut. For example the curve for

the MFC sample at 1.0 kg/m3 solids concentration

showed a similar shape to other solids concentrations

which displayed a yield stress but the maximum torque

reached was similar to that produced by water alone.

The first unambiguous evidence of a yield stress was

only observed when the solids concentration wasincreased to 1.5 kg/m3. An observable peak torque

was observed at minimum concentration of 2.5 kg/m3

for NF suspension, 10 kg/m3 for NIST90 suspension

and 1.5 kg/m3 for NIST60 suspension.

For the MFC and NF samples, Cc was determined

by fitting the data of yield stress versus concentration,

as described below. However, there was insufficient

sample for the NIST90 and 60 samples, so for these

two samples, Cc was determined as the lowest solids

fraction at which a yield stress could be measured.

For the MFC and NF samples, yield stress mea-

surements were carried for the samples at a range of

0

0.00005

0.0001

0.00015

0.0002

0.00025

0.0003

0.00035

0.0004

0.00045

0 10 20 30 40

Torque

(N.m

)

Time (sec)

2.5

2.5

2.5

1.5

1.0

0.5

0

Solids

Concentration

(kg/m3)

Fig. 10 Torque measured from instrument as function of time

for MFC sample and three individual measurement for solids

concentration 2.5 kg/m3

0

0.000005

0.00001

0.000015

0.00002

0.000025

0.00003

0 5 10 15 20 25

Torque(N.m

)

Time (sec)

2.5

2.0

0

Solids

Concentration

(Kg/m3)


for NF sample

0.000000

0.000005

0.000010

0.000015

0.000020

0.000025

0.000030

0.000035

0.000040

0 5 10 15 20 25

Torque(N.m

)

Time (sec)

10

8.0

6.0

SolidsConcentration

(Kg/m3

)


for NIST90 Sample

0.000000

0.000005

0.000010

0.000015

0.000020

0.000025

0 2 4 6 8

Torque(N.m

)

Time (sec)

2.5

1.5

Solids

Concentration(Kg/m3)


for NIST60 sample

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different concentrations at which peak torque was

observed. Yield stress was calculated from peak

torques at different concentrations higher than the

minimum concentration as mentioned in the experi-

mental procedure for MFC and NF suspensions and

plotted against concentration in Figs.14 and 15.

Increasing suspension concentration increases yieldstress because the increase in the number of fibre

contacts and entanglements, increases the strength of

the fibre network formed. The dependency of the yield

stress values on fibre weight percentage (=solids

concentration 9 100/suspension density) has been

correlated using a power-law model sy aCbm, whereCmis the fibre weight percentage. The resulting fitted

constants are shown in the Table 4. These values are

consistent with literature values for wood pulp fibre

suspensions, which for a ranged from 1.8\ a\ 24.5

while values of b ranged from 1.69\ b\ 3.02

(Kerekes et al. 1985). Many key variables contributing

to these differences were not measured or reported

(Derakhshandeh et al. 2011). For example, Dalpke and

Kerekes (2005) reported that a and b values depend

upon aspect ratio (Dalpke and Kerekes 2005). The

fitting constants for MFC and NF suspensions are

within the range of previously reported results. Yield

stress value reported for MFC suspension at concen-

tration of 5 kg/m3 is 0.7 Pa which is close to the values

reported in Karppinen et al. (2011).

It can be inferred from Figs. 10,11,12and13that

minimum yield stress values for MFC sample is

between the solids concentrations of 11.5 kg/m3, for

NF suspension between 2.0 and 2.5 kg/m3

, forNIST90 suspension between 8.0 and 10 kg/m3, for

NIST60 suspension 1.251.5 kg/m3. Assuming lowest

possible yield stress can be measured as

1.84 9 10-5 N m torque or a yield stress of 0.11 Pa,

then the corresponding concentration values were

found from the power law fitting equation.

The gel points in Table 5calculated from the yield

stress measurements are in excellent agreement with

those obtained from the sedimentation data and given

in Table3. The NF sample has lower aspect ratio

compared to MFC although the NF sample has a loweraverage diameter than the MFC sample. There are

likely to be two factors contributing to this. Firstly, the

highest aspect ratio nanomaterial may have been

removed with the supernatant after centrifugation. In

addition, the higher aspect ratio nanofibres may have

been retained preferentially on the fibres retained on

the filter mat. The gel-points measured for the MFC,

NF and NIST60 samples are also in good agreement

with previous measurements from the literature. Gel

0 0.5 1 1.50

2

4

6

8

10

12

14

16

18

Fibre weight percentage

Yields

tress(Pa)

Experimental

Kerekes et.al.(1985)

Fig. 14 Effect of fibre weight percentage on yield stress for

MFC suspension

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

5

10

15

20

25

30

35

40

Fibre weight percentage

Yield

stress

(Pa)

Experimental

Kerekes et.al.(1985)

Fig. 15 Effect of fibre weight percentage on yield stress for NF

suspensions

Table 4 Power-law fitting constants for MFC and NF samples

Sample Power law fitting constants

a b

MFC 6.70 2.34

NF 6.34 2.57

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points estimated by sedimentation for three nanofibre

samples derived from the same precursor material as

the NIST90 and 60 samples, ranged from 1.76 to

5.37 kg/m3 (Zhang et al. 2012) while the gel-point

estimated from the change of slope of viscosity as a

function of solids concentration of a MFC sample

prepared by TEMPO oxidation of sulphite pine pulpwas approximately 2 kg/m3 (Lasseuguette et al. 2008).

The data of Ishii et al. (2011) showed a boundary

between dilute and semi-dilute suspensions of nano-

fibre prepared by TEMPO oxidation of bleached

hardwood kraft pulp was between 0.1 and 0.2 kg/m3.

They estimated the boundary based on the terminal

relaxation time of dilute suspensions. They have done

measurements at 0.1 kg/m3 and 0.2 kg/m3 but inter-

mediate concentrations were not tested.

Calculated aspect ratios

Table6 shows the aspect ratios calculated from all

four samples using the values ofCc calculated from the

sedimentation and yield stress measurements, using

the EMT and CN theories at several different densities

of the fibres in suspension, as given in Table 2.

The Table6 shows firstly that estimated aspect

ratio using either sedimentation or yield stress mea-

surements still has considerable sources of uncer-

tainty. We do not know the correct density of the fibresin suspension to use, and neither the model of a

straight cylindrical fibre used in the CN theory or of a

spheroid used in the EMT theory completely repre-

sents the reality of the fibres in suspension. The basic

Crowding Number theory is based on single averages

only. Kropholler and Sampson (2001) reported that

mean Crowding Number of fibre with a distribution of

lengths is higher than that for fibres of uniform lengths

equal to mean by a factor that is dependent only on the

coefficient of variation of fibre length. The crowding

number does not depend too strongly on coefficient of

variation of the length distribution. EMT is actually

based on inclusions with same aspect ratio and shape

while allowing an arbitrary distribution of lengths. No

method is available to calculate length distribution

from average aspect ratio measurement.However it is encouraging that the data set is quite

consistent between samples and with the different

theories and fibre densities used. From the data the

ranking order of aspect ratio is NIST60 having the

largest aspect ratio followed by the MFC, the NF and

the NIST90 samples, independent of the fibre density

or the theory used. There is also excellent qualitative

agreement between the SEM images of the different

samples and the calculated aspect ratios. The exces-

sive treatment applied to produce fibre fragmentation

in the NIST90 samples is observable both in the SEMimage shown in Fig. 8as well as producing sedimen-

tation curves and yield stress measurements that are

substantially different to the other three samples. The

aspect ratio estimated for the nanofibre prepared by

Ishii et al. (2011) based on the method reported here

assuming gel point concentration as 0.2 kg/m3 is 440,

when the fibre density is assumed to be 1,500 kg/m3.

This compares well with the aspect ratio of 550

estimated by Ishii et al. (2011) from terminal relax-

ation times. They reported that nanofibres are dis-

persed without entanglement.It is also encouraging that the two independent

methods of measuring Cc are also in excellent

agreement, despite sedimentation relying on the

formation of structures large enough to scatter visible

light, while the yield stress measurement relies on the

establishment of a continuous network of fibres. The

results suggest that either method could be chosen

depending on equipment availability and the rapidity

with which the measurements are required.

There still remains the question as to the meaning of

the aspect ratio estimated here. As is observable in theSEM images of the fibres in Figs. 2, 4 and 6, the

nanofibres are often not fully separated and often form

network type structures. It is open question as to

whether fibre lengths calculated by dividing aspect

ratio by average fibre diameter are physically

reasonable.

However, as discussed in the theory, the statistics of

fibre contacts in suspension depend only on aspect

ratio. The aspect ratio controls both the connectivity

Table 5 Gel points for MFC,NF, NIST90 and NIST60 sam-

ples from yield stress measurements

Suspension Gel point concentration

of suspension (kg/m3

)

Solids mass

fraction (kg/kg)

MFC 1.76 1.76E-03

NF 2.4 2.4E-03

NIST90 10 10E-03

NIST60 1.5 1.5E-03

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(gel-point) and rigidity thresholds. We have demon-

strated previously a method to rapidly form cellulose

nanofibre sheets (Varanasi and Batchelor2013) using

commercial woven filters. We have shown that almost

100 % retention is possible if the sheets are formed at

solids concentrations above the gel-point. The contin-

uous network of nanofibres, whether individual orpresent as networks, prevents the nanofibres from

being pulled through the 125 lm filter openings under

the applied vacuum. Sheets were unable to be formed

through filtration when the initial solids content was

less than the gel-point (Varanasi and Batchelor2013).

It should be noted that, when using this method, we

were readily able to form sheets using filtration from

the MFC, NF and NIST60 samples, which all have

high aspect ratios, but that it proved impossible to

form a sheet using the NIST90 sample, which had a

very low aspect ratio.

Conclusion

Two simple methods were developed for measuring

suspension gel point for cellulose nanofibres based on

either yield stress measurements or sedimentation.

Aspect ratios were calculated using effective medium

theory (EMT) and Crowding number (CN) theory.

Both theories showed reasonable agreement with each

other. Four different types of cellulose nanofibres were

prepared and tested. Aspect ratios were calculated for

assumed density of fibre in suspension ranging from

1,166 to 1,500 kg/m3. Aspect ratios of MFC, NF,

NIST90 and NIST60 are 142, 125, 62 and 150 at

density of 1,500 kg/m3. Suspension gel point andAspect ratio calculated from either the sedimentation

or suspension yield stress methods are quite consistent

between samples and with the different theories and

fibre densities used.

Acknowledgments The authors would like to acknowledge

the facilities used with the Monash Center for Electron

Microscopy. The authors would like to thank Wade Mosse

and Mae-Gene Yew for assisting us with rheological

measurements and Liyuan Zhang and A.Prof. Takuya Tsuzuki

for helping us to develop the method for separating nanofibres

from MFC sample. We acknowledge the financial support of theAustralian Research Council, Australian Paper, Nopco

Australasia, Norske Skog, SCA Hygiene Australasia and Visy

through Linkage Project Grants LP0989823 and LP0990526.

Swambabu Varanasi thanks Monash University for a MGS and

FEIPRS Scholarship.

References

Ahola S, Osterberg M, Laine J (2008) Cellulose nanofibrils

adsorption with poly(amideamine) epichlorohydrin

Table 6 Average aspect ratio values for calculated from gel point measurements from sedimentation and yield stress measurements

and standard errors are reported as range in brackets

Assumed

density

(kg/m3

)

Average aspect ratio

MFC Nanofibre NIST90 NIST60

Sedimentation

CN 1,500 142 (28/18) 125 (8/7) 62(2/2) 150(1/1)

1,333 134 (27/17) 118 (7/6) 58 (2/2) 142 (1/1)

1,166 125 (25/16) 110 (7/6) 55 (2/1) 133 (1/1)

EMT 1,500 125 (29/18) 108 (8/7) 48 (2/1) 134 (1/1)

1,333 117 (27/17) 101 (7/6) 45 (1/1) 125 (1/1)

1,166 108 (25/15) 93 (7/6) 41 (1/1) 116 (1/1)

Yield stress

CN 1,500 143 121 60 155

1,333 135 115 57 146

1,166 126 107 53 137

EMT 1,500 126 105 46 1391,333 118 98 43 129

1,166 109 90 40 120

Cellulose (2013) 20:18851896 1895

1 3

8/10/2019 10.1007-s10570-013-9972-9-Printed copy

12/12

studied by QCM-D and application as a paper strength

additive. Cellulose 15(2):303314. doi:10.1007/s10570-

007-9167-3

Celzard A, Mareche JF, Payot F (2000) Simple method for

characterizing synthetic graphite powders. J Phys D Appl

Phys 33(12):1556

Celzard A, Fierro V, Pizzi A (2008) Flocculation of cellulose

fibre suspensions: the contribution of percolation and

effective-medium theories. Cellulose 15(6):803814. doi:

10.1007/s10570-008-9229-1

Celzard A, Fierro V, Kerekes R (2009) Flocculation of cellulose

fibres: new comparison of crowding factor with percolation

and effective-medium theories. Cellulose 16(6):983987.

doi:10.1007/s10570-009-9314-0

Czaja WK, Young DJ, Kawecki M, Brown RM (2006) The

future prospects of microbial cellulose in biomedical

applications. Biomacromolecules 8(1):112. doi:10.1021/

bm060620d

Dalpke B, Kerekes RJ (2005) The influence of fibre properties

on the apparent yield stress of flocculated pulp suspensions.

J Pulp Pap Sci 31(1):3943

Derakhshandeh B, Kerekes RJ, Hatzikiriakos SG, BenningtonCPJ (2011) Rheology of pulp fibre suspensions: a critical

review. Chem Eng Sci 66(15):34603470. doi:10.1016/

j.ces.2011.04.017

Dufresne A, Dupeyre D, Vignon MR (2000) Cellulose micro-

fibrils from potato tuber cells: processing and character-

ization of starch-cellulose microfibril composites. J Appl

Polym Sci 76(14):20802092

Dzuy NQ, Boger DV (1985) Direct yield stress measurement

with the vane method. J Rheol 29(3):335347

Eichhorn S, Dufresne A, Aranguren M, Marcovich N, Capadona

J, Rowan S, Weder C, Thielemans W, Roman M, Ren-

neckar S, Gindl W, Veigel S, Keckes J, Yano H, Abe K,

Nogi M, Nakagaito A, Mangalam A, Simonsen J, Benight

A, Bismarck A, Berglund L, Peijs T (2010)Review: currentinternational research into cellulose nanofibres and nano-

composites. J Mater Sci 45(1):133. doi:10.1007/s10853-

009-3874-0

Henriksson M, Berglund LA, Isaksson P, Lindstrom T, Nishino

T (2008) Cellulose nanopaper structures of high toughness.

Biomacromolecules 9(6):15791585. doi:10.1021/bm800

038n

Ishii D, Saito T, Isogai A (2011) Viscoelastic evaluation of

average length of cellulose nanofibers prepared by

TEMPO-mediated oxidation. Biomacromolecules 12(3):

548550. doi:10.1021/bm1013876

Jakob HF, Fengel D, Tschegg SE, Fratzl P (1995) The ele-

mentary cellulose fibril in picea abies: comparison of

transmission electron microscopy, small-angle X-ray

scattering, and wide-angle X-ray scattering results. Mac-

romolecules 28(26):87828787. doi:10.1021/ma00130

a010

Janarthanan S, Sain M (2006) Isolation of cellulose microfibrils

an enzymathic approach. Bioresources 1(2):176188

Karppinen A, Vesterinen A-H, Saarinen T, Pietikainen P, Sep-

pala J (2011) Effect of cationic polymethacrylates on the

rheology and flocculation of microfibrillated cellulose.

Cellulose 18(6):13811390. doi:10.1007/s10570-011-

9597-9

Kerekes RJ, Schell CJ (1992) Characterization of fibre floccu-

lation regimes by a crowding factor. J Pulp Pap Sci 18(1):

J32J38

Kerekes RJ, Soszynski RM, Tam Doo PA (1985) The floccu-

lation of pulp fibres.In: Proceedings of the 8th fundamental

research symposium, Oxford, England. Mechanical Engi-

neering Publications, pp 265310

Kropholler HW, Sampson WW (2001) The effect of fibre length

distribution on suspension crowding. J Pulp Pap Sci

27(09):301305

Lasseuguette E, Roux D, Nishiyama Y (2008) Rheological

properties of microfibrillar suspension of TEMPO-oxi-

dized pulp. Cellulose 15(3):425433

Liddel PV, Boger DV (1996) Yield stress measurements with

the vane. J Nonnewton Fluid Mech 63(23):235261. doi:

10.1016/0377-0257(95)01421-7

Malcolm Brown R Jr, Saxena IM, Kudlicka K (1996) Cellulose

biosynthesis in higher plants. Trends Plant Sci

1(5):149156. doi:10.1016/S1360-1385(96)80050-1

Martinez DM, Buckley K, Jivan S, Lindstrom A, Thiruvenga-

daswamy R, Olson JA, Ruth TJ, Kerekes RJ (2001)

Characterizing the mobility of papermaking fibres duringsedimentation. In: Baker CF (ed) The science of paper-

making, transactions of the 12th fundamental research

symposium, Oxford. The Pulp and Paper Fundamental

Research Society, Bury, Lancaster, UK, pp 225254

Mosse WKJ, Boger DV, Garnier G (2012a) Avoiding slip in

pulp suspension rheometry. J Rheol 56(6):15171533

Mosse WKJ, Boger DV, Simon GP, Garnier G (2012b) Effect of

cationic polyacrylamides on the interactions between cel-

lulose fibers. Langmuir 28(7):36413649. doi:10.1021/

la2049579

Nogi M, Iwamoto S, Nakagaito AN, Yano H (2009) Optically

transparent nanofiber paper. Adv Mater 21(16):

15951598. doi:10.1002/adma.200803174

Perkins EL, Batchelor WJ (2012) Water interaction in papercellulose fibres as investigated by NMR pulsed field gradi-

ent. Carbohydr Polym 87:361367. doi:10.1016/j.carbpol.

2011.07.065

Siro I, Plackett D (2010) Microfibrillated cellulose and new

nanocomposite materials: a review. Cellulose 17(3):

459494. doi:10.1007/s10570-010-9405-y

Taniguchi T, Okamura K (1998) New films produced from mi-

crofibrillated natural fibres. Polym Int 47(3):291294. doi:

10.1002/(sici)1097-0126(199811)47:3\291:aid-pi11[3.0.

co;2-1

Varanasi S, Batchelor W (2013) Rapid preparation of cellulose

nanofibre sheet. Cellulose 20(1):211215. doi:10.1007/

s10570-012-9794-1

Varanasi S, Chiam HH, Batchelor WJ (2012) Application and

interpretation of zero and short-span testing on nanofibre

sheet materials. Nord Pulp Pap Res J 27(2):343352

Xu J, Chatterjee S, Koelling KW, Wang Y, Bechtel SE (2005)

Shear and extensional rheology of carbon nanofiber sus-

pensions. Rheol Acta 44(6):537562. doi:10.1007/

s00397-005-0436-5

Zhang L, Batchelor W, Varanasi S, Tsuzuki T, Wang X (2012)

Effect of cellulose nanofiber dimensions on sheet forming

through filtration. Cellulose 19(2):561574. doi:10.1007/

s10570-011-9641-9

1896 Cellulose (2013) 20:18851896

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