The effect of MFC on the pressability and paper properties of TMP and GCC based sheets Collin Hii, Øyvind W. Gregersen, Gary Chinga-Carrasco, Øyvind Eriksen

KEYWORDS: MFC, microfibrillated cellulose, Field

Emission SEM, Air resistance, Tensile index, Z-

directional strength, Gurley, Pressability, Fines,

Drainage, Wet pressing

SUMMARY: Different qualities of microfibrillated

cellulose (MFC) were blended with thermomechanical

pulp (TMP) and ground calcium carbonate (GCC) filler.

The addition of MFC reduced the drainage of the pulp

suspension but improved strength properties. Wet

pressing experiments showed that optimal use of MFC

and filler could enhance the strength and optical

properties without reducing the solids content after wet

pressing. Field-emission scanning electron microscopy

(FESEM) revealed that MFC adsorbed onto and

contributed to the bonding of the filler particles and

fibres. The MFC binds the filler-MFC-fines aggregates to

the fibre network and partially filled the pore network. As

a result, MFC addition increased the air resistance and

internal bonding of the sheet.

ADDRESSES OF THE AUTHORS: Collin Hii (hii@nt.ntnu.no)

Øyvind W. Gregersen

(oyvind.gregersen@chemeng.ntnu.no)

Norwegian University of Science and Technology,

NTNU, Trondheim, Norway

Gary Chinga-Carrasco (gary.chinga.carrasco@pfi.no)

Øyvind Eriksen (oyvind.eriksen@pfi.no)

Paper and Fibre Research Institute, PFI, NO-7491

Trondheim, Norway

Corresponding author: Collin Hii

The drive to reduce production cost and increasing the

opacity, brightness and surface smoothness has

encouraged the use of fillers in papermaking. However,

fillers reduce fibre bonding and paper’s strength

properties. Addition of fillers to pulp slurry without

retention aid increases the filtration resistance of the

slurry by plugging of the fibre cake (Springer,

Kuchibhotla 1992; Hubbe, Heitmann 1997). The optimal

use of retention aid has enabled high filler retention and

even filler distribution in the sheet thickness direction

(Tanaka et al. 1982). Filler addition has improved

dewatering in the wet end (Liimatainen et al 2006). Filler

usage also improves optical properties, but reduces

strength properties (Mohlin, Ölander 1986; Hjelt et al.

2008). Fillers that agglomerate to clusters could resist

pressure during wet pressing and maintain the bulk of the

sheet (Hjelt et al. 2008). However, filler particles may

also form strong agglomerates between fibres and hence

prevent fibre-fibre bonds (Hjelt et al. 2008).

Addition of kraft pulp fines counteracts negative effects

of fillers on strength properties but affects the drainage of

the pulp (Sandgren, Wahren 1960a, 1960b; Htun, Ruvo

1978; Seth 2003; Lin et al. 2007). Fines are

inhomogeneous complex particles that passes through a

200 mesh (76 μm) wire in the solid-liquid separation

method as stated in TAPPI T261 cm-00. Kraft pulp fines

possess high degree of swelling and thus enable the fines

to bind well to the fibre structure (Htun, Ruvo 1978). Pre-

mixing fillers with fines in a controlled fashion, prior to

its addition to the pulp furnish, improves strength and

optical properties of the sheet (Lin et al. 2007).

Microfibrillated cellulose

In recent years, studies have shown that microfibrillated

cellulose (MFC) can be used as strength enhancer

(Iwamoto et al. 2007; Eriksen et al. 2008; Ahola et al.

2008; Subramaniam 2008; Mörseburg, Chinga-Carrasco

2009; Zimmermann et al. 2010; Taipale et al. 2010).

Eriksen et al. (2008) found significant tensile index

increase at 4% addition of MFC to TMP handsheets

independent of the production method. Mörseburg,

Chinga-Carrasco (2009) added MFC to clay loaded

layered TMP sheets and found that the strength properties

improved. Addition of MFC also increased the air

resistance (Eriksen et al. 2008; Subramaniam 2008).

Taipela et al. (2010) found that the optimal level of MFC

addition would maintain or improve strength properties

without impeding dewatering. Synergy effects of MFC–

filler interactions can counteract the reduction in strength

properties from filler addition while improving light

scattering (Mörseburg, Chinga-Carrasco 2009).

MFC has been prepared by mechanically disintegrating

cellulose fibres through homogenization (Turbak et al.

1983; Herrick et al. 1983; Pääkkö et al. 2007; Syverud et

al. 2011), grinding (Iwamoto et al. 2005; Iwamoto et al.

2007; Eriksen et al. 2008; Abe, Yano 2009), fluidization

(Taipale et al. 2010) and combined homogenizer treat-

ment and grinding processes (Iwamoto et al. 2005).

Others have pre-treated the fibres chemically before a

mechanical disintegration (Saito et al. 2006, 2007;

Pääkkö et al. 2007; Henriksson, Berglund 2007; Wågberg

et al. 2008; Syverud et al. 2011).

MFC produced from mechanical disintegration is

heterogeneous in size and forms entangled and disordered

networks. Mechanically produced MFC, without pre-

treatment, is commonly composed of micro- and nano-

structural components, e.g. poorly fibrillated fibres, fines

and nanofibrils (Chinga-Carrasco, 2011). The nanofibrils

have diameters less than 100 nanometres and lengths in

the micrometre scale. Increasing the number of passes in

the homogenization and fluidization processes produces

more nanofibrils. MFC may thus be considered a subset

of kraft pulp fines. Taipale et al. (2010) found that increa-

sing the number of passes in a fluidization process pro-

duced MFC with more nano-sized fibrils. Enzymatic pre-

treatment reduces the fibrils diameter down to below

20 nm range (Pääkkö et al. 2007). MFC produced with

TEMPO-mediated oxidation as pre-treatment has exhi-

bited more homogeneous nanofibril diameters (< 10 nm)

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

and lengths in the micrometre scale (Saito et al. 2006,

2007; Chinga-Carrasco et al. 2011).

Wet pressing

Wet pressing has been extensively studied. Wahlström

(1960a, 1960b) found that the hydraulic pressure formed

during web compression was the main driving force for

water removal in the converging part of the press nip.

The flow resistance in the wet web generated the

hydraulic pressure. The total pressure (PT) can be defined,

using the Terzaghi principle (Terzaghi 1943), as the sum

of structural (Ps) and hydraulic pressure (Ph) as in Eq 1.

The Terzaghi principle was first applied to papermaking

by Campbell (1947).

T s hP P P [1]

Equation [1] is based on force balance. This force

balance can be stated as the stress or pressure balance by

assuming the inertial effects are not negligible and the

area over which the force is acting is equal.

Carlsson et al. (1978) revealed that water within the

fibre played an important role in press dewatering. They

found that the dewatering of fibre water started to occur

at 20-25% solid content, thus, as the web compression

progressed, the fraction of water pressed out from the

fibre wall increased. The water flow through the fibre

wall pores has significantly affected the structural

pressure when compressing the web in the press nip

(Carlsson et al. 1978; Paulapuro 1989). Studies have also

shown that the water and fibre surface interaction play a

crucial role in press dewatering (Szikla, Paulapuro 1989).

Wahlström (1990) redefined the Ps as the sum of pure

mechanical pressure that compressed the fibre network,

Pc and the pressure required to remove the water from the

fibre wall, Pf thus:

T h c fP P P P [2]

The pressure components in Eq 2 are varying both in

thickness direction and over time during wet pressing.

The swelling of mechanical pulp is largely due to fibril-

rich fines, internal fibrillation and external fibrillation.

(Salmen et al. 1985; Maloney, Paulapuro 1999; Luukko,

Maloney 1999). Maloney et al. (1997) postulated that in

thermomechanical pulp (TMP) the pore closure after

pressing and drying can be due to fines coagulation.

In dynamic wet web compression, increasing the

ingoing moisture, sheet basis weight and compression

rate increased the hydraulic pressure component in both

chemical and mechanical pulp sheets (Szikla 1992).

Increase in basis weight will increase the hydraulic

pressure in the web during wet pressing and more so in

samples with high initial moisture content (Szikla 1992).

The roughness of the contacting surface during wet

pressing will induce local stress variation and affect the

water flow in a compressed fibre network (Vomhoff

1998, I’Anson, Ashworth 2000). In a grammage range

relevant to papermaking, the steady state permeability of

a web varies considerably with grammage when

compressed against a rough permeable surface (Vomhoff

1998). I’Anson, Ashworth (2000) found that at low

grammage (30 g/m2), fine contact between the surfaces

would enhance press dewatering while coarse contact

would improve dewatering for higher grammage

(60 g/m2). The high sheet permeability would be needed

for large amount of water to escape from high grammage

web.

Increasing the web temperature improves water removal

in wet pressing. This mechanism is well understood and

utilised in papermaking (Wahlström 1960b; Roya, Thorp

1982; Batty et al. 1982; Radwan, Nayar 1982; Saaristo et

al. 1984; Busker, Francik 1984; Back 1988; Patterson,

Iwamasa 1999). Increasing the web temperature reduces

water viscosity and water surface tension and thus

reduces hydraulic loads and capillary forces respectively.

Heating the web also softens the fibres, which in turn

reduces web springback and increases web

compressibility; this thus reduces rewetting and structural

loads respectively.

The press nip behaviour can be summarized into flow

controlled and pressure controlled modes (Wahlström

1969, 1979; Chang, Han 1976; Ceckler et al. 1982; Burns

1992). The flow controlled mode prevails for high

moisture sheets with high flow resistance while the

opposite holds for pressure controlled mode. Most

presses operate in the area between flow control and

pressure control mode. The press impulse, integral of

pressure over time, promotes dewatering in flow

controlled mode. The peak pressure enhances the removal

of free water in the pressure controlled mode.

During wet pressing the interaction between viscous

flow forces and fibre network can result in uneven

compaction of the web in the thickness direction, or

stratification (Wick 1982; MacGregor 1983; Burton,

Sprague 1987; Szikla, Paulapuro 1989b; Burns et al.

1992). The exit layer densification is caused by the shear

force from liquid flow (Chang 1978; Macgregor 1989,

2001; Szikla 1992). The densification of the exit layer

will increase the hydraulic pressure in the web and results

in a more flow controlled nip.

MFC has a strong water retention property and a high

specific surface area. (Herrick et al. 1983; Pääkkö et al.

2007). As a result, MFC addition into a given pulp will

reduce dewatering by increasing the hydraulic pressure,

Ph. Taipale et al. (2010) found that optimum selection of

MFC and kraft pulp components and process conditions

can enhance the strength properties without negative

effects on drainage.

There is a lack of published results on the effects of

MFC on the pressability of ground calcium carbonate

(GCC) filled TMP sheets. Hence, the purposes of this

study are:

1. Determine the optimal MFC addition in 30% filler

loaded TMP sheets to yield similar or better pressability

as compared to TMP samples with 15% filler content.

2. Determine the effects of MFC and high filler loading,

up to 30%, on paper properties.

Materials and Methods Production of microfibrillated cellulose

Never-dried Pinus radiata kraft pulp was used to produce

the MFC. The pulp fibres were homogenized with a

Rannie 15 type 12.56x homogenizer operated at 1000 bar

pressure drop. The pulp consistency during homogeni-

zation was kept at 0.5% to avoid plugging problems

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Nordic Pulp and Paper Research Journal Vol 27 no.2/2012 389

Table 1. Standards for the tests carried out in the experiment..

Test methods Standards

Closed water sheet making ISO 5269-1:2005 Sample conditioning ISO 187:1900 Laboratory wet disintegration ISO 5263:1995 Determination of dry matter ISO 638:1978 Determination of stock consistency ISO 4119:1995 Freeness ISO 5267-2:2001 Grammage ISO 536:1995 Thickness and apparent density ISO 534:1998 Ash content 525°C ISO 1762:2001 Air permeance (Gurley) ISO 5636-5:2003 Tensile strength-constant elongation ISO 1924-2:1994 Z-strength T541 om-99 Light scattering ISO 9416:1998

Table 2. TMP freeness (ml), pulp coarseness (μg/m) and WRV(g/g). The values are reported as average ±1 standard deviation.

Parameters Values

Freeness (ml) 126 ±10 Coarseness (µg/m) 177 ±7 WRV (g/g) 1.33 ±.07 Ash (%) 0.5 ±0.1

during the homogenization operation. The fibrillated

materials were collected after 3 and 5 passes. Although

the MFC material collected after 5 passes has a larger

fibrillation degree than the MFC collected after 3 passes,

both materials are composed of a major fraction of

nanofibrils (diameter < 100 nm). A detailed description

of the fibrillated material applied in this study is given by

Syverud et al. (2011) and Chinga-Carrasco et al. (2011).

Materials

A newsprint grade TMP, MFC and commercial newsprint

grade GCC filler were the materials used for sample

making. The TMP was based on Norway spruce. Medium

cationic, low to medium molecular weight fixation aid

(Nalco N74528 EU PLUS) and low cationic charge, ultra

high molecular weight flocculant (Nalco Nor Floc. VP4)

were used to retain and to fix fillers and fines onto the

fibres. 1 kg/ton of fixation aid and 0.5 kg/ton flocculant

were used to make the samples. The fixation aid

neutralized the charge in the furnish slurry to improve

filler retention. The flocculant retained the fines and

fillers onto the fibres by bridging mechanisms. The

materials were pre-mixed before the sheet making in the

following sequence, TMP – Filler – MFC - 0.5 kg/ton

fixation aid - 0.25 kg/ton flocculant - 0.5 kg/ton fixation

aid - 0.25 kg/ton flocculant. The TMP, filler and MFC

were mixed in a container and the subsequently diluted to

0.35% concentration. The fixation aid and flocculant

were added into the diluted furnish mixture. The time

delay between the addition of the fixation aid and

flocculant was 30 seconds. The fixation aid and

flocculant were pre diluted to 0.5 g/l and 0.25 g/l

respectively before addition into the furnish mixture. The

furnish mixture was stirred rigorously with a stirrer at

1250 rpm during the addition of the retention chemicals.

Fluffy flocs were seen forming after 500 ml of the furnish

mixture was shaken rigorously in a 1 litre bottle and let to

settle in a 1 litre graduated cylinder.

All the tests carried out in the experiment followed the

standards as illustrated in Table 1. Table 2 shows the

TMP pulp coarseness, freeness, water retention values

and ash content. The pulp coarseness was measured with

FiberMaster. The water retention value (WRV) for all the

furnish mixtures were performed by centrifuging the pulp

sample for 17 minutes at 4500 rpm and at 23ºC.

Hand sheets making

80 g/m2 hand sheets for optical properties, strength and

physical testing were made using a conventional

handsheet former with closed water circulation. In each

sample series, 12 sheets were made to ‘fill’ the white

water system with fines and filler until the basis weight of

the handsheet is stable. The standard for handsheet

making with closed water circulation is stated in Table 1.

Eight series of handsheets, as illustrated in Table 3, were

made to study the effects of MFC and fillers on physical

and optical properties. The internal bonding and z

directional strength measurements were carried out using

Zwick Roell SMART.PRO material testing machine.

Samples for wet pressing

The 60 g/m2 samples for the wet pressing experiment

were made using a FiberXpress apparatus (Fig 1). The

pulp slurry was dewatered through membrane and felt at

1 MPa for 60 seconds. The membrane used for

dewatering the slurry was 50 micrometres thick with 41

micrometres openings and 31% open area. The felt

placed underneath the membrane gave support and

enabled the forming of 60 g/m2 samples. The membrane

maximised the retention of fines and fillers. Eight series

of samples, as depicted in Table 3, were made to study

the effects of MFC and fillers on dryness after wet

pressing. Samples with 50 mm diameters were formed

and conditioned sandwich with wet blotters to maintain

dryness close to 20%.

Wet Pressing

A dynamic wet pressing simulator (Fig 2) was used to

carry out the pressing experiment. A single sided dewate-

ring set up was used for the experiment. The design

details of the dynamic wet pressing simulator were

similar to the unit located in the paper laboratory of Aalto

University (Saukko 2007). The pressing nip consisted of

a top solid metal plate with polished surface and a 90 mm

diameter sintered porous bottom plate. 3 eddy current

distance sensors are located in the bottom plate, each

separated 120º from the two others. During wet pressing,

the water from the wet web flows into the porous plate.

The porous plate has low resistance to flow. When

saturated with water, the sintered porous material

generated only 0.1 MPa hydraulic pressure when pressed

with 4 ms roll pulse at 3 MPa peak pressure.

A single roll press pulse of 8 ms pulse length was used

in the wet pressing experiment. The top and bottom plates

had to be in closed position before a stable and repeatable

roll pulse could be generated. This inevitably would

cause some water to flow from the sample to the porous

plate. After pressing, the sample will stick onto the

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

Table 3. Furnish mixtures and labels.

Label TMP (%) Ash (%) MFC (%) MFC (# of passes) Drainage (s)

T100 100 0 0 0 23.9 ±1.0 F15T85 85 15 0 0 33.2 ±1.4 5MFC5F15 77.5 17.5 5 5 37.6 ±1.4 5MFC5F30 66.1 28.9 5 5 51.5 ±3.2 5MFC2.5F15 82.5 15 2.5 5 53.6 ±4.3 5MFC2.5F30 64.9 32.6 2.5 5 49.9 ±4.5 3MFC2.5F15 80.3 17.2 2.5 3 43.6 ±1.2 3MFC2.5F30 65.5 32 2.5 3 35.0 ±3.5

Table 4. Dewatering data for the different sample series. The maximum pressure is set at 3.5 MPa. The values are reported as average±1 standard deviation. The labels for the different furnish mixes are explained in Table 3.

Label Ash (%) WRV (g/g) Pre solids (%) Solids (%)

T100 0.5 ±0.1 1.33 ±0.07 13.9±0.1 33.0±0.3 F15T85 15.0 ±0.1 0.97 ±0.03 15.5±0.1 36.3±1.2 5MFC5F15 17.5 ±0.8 0.94 ±0.03 13.9±0.6 36.4±1.4 5MFC5F30 28.9 ±0.5 0.95 ±0.02 14.7±0.2 36.3±0.2 5MFC2.5F15 15.0 ±0.1 1.04 ±0.03 14.2±0.4 35.9±0.3 5MFC2.5F30 32.6 ±0.3 0.90 ±0.02 16.3±0.1 38.9±0.5 3MFC2.5F15 17.2 ±0.1 1.07 ±0.03 15.7±0.1 33.6±0.8 3MFC2.5F30 32.0 ±0.2 0.93 ±0.03 18.2±0.5 36.5±1.2

Fig 1. The FiberXpress used to make 60 g/m2 samples.

Fig 3. Average solids content (%) after wet pressing vs. pressure (MPa) during wet pressing for 15% filler and 85% TMP mixture pressed at 25ºC and 50ºC. The error bars are 1 standard deviation.

smooth polished surface of the top plate as the nip opened

thus minimising rewetting. 20 ms after the nip opened, a

vacuum was generated in the bottom plate to clean and

dry the porous plate.

To determine the pressure set point where the nip was

reasonably flow controlled for wet pressing samples at

50ºC, F15T85 samples were wet pressed with 5 different

Fig 2. The dynamic wet pressing simulator for the wet pressing experiments. (A) Pressing cones with internal heating element (B) Bottom plate (C) Top plate (D) Distance sensor.

peak pressures at 25ºC. The solids content versus

pressure response curve was constructed (Fig 3). The

pressure at 25ºC when the maximum solids content first

occurred was used as set point for wet pressing the other

sample series at 50ºC. The wet pressing at 50ºC was done

by heating the top and bottom plates to 50ºC with heating

elements attached inside the pressing cones.

At around 3.5 MPa the solids content flattened out

(Fig 3). The maximum pressure represents the setting

with the maximum fraction of flow control until some

type of sheet crushing problem is encountered (crush

point). Fig 3 shows that the pressing pressure to achieve

maximum solids content for F15T85 sample wet pressed

at 25ºC is at around 3.5 MPa. Thus 3.5 MPa was used as

pressure set point for pressing samples at 50ºC. As

expected, wet pressing with 3.5 MPa at 50ºC yields

higher solids content, 36%, as compared to the 33.5% at

25ºC.

29

30

31

32

33

34

35

36

37

38

1 2 3 4 5 6

Pressure (MPa)

So

lid

s c

on

ten

t (%

)

average 25 ºC average 50 ºC

A

A

B

C

D

Controlled press pulse

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Nordic Pulp and Paper Research Journal Vol 27 no.2/2012 391

Fig 4. Field emission SEM cross sectional image of 5MFC5F30. The white areas correspond to the GCC fillers.

Field-Emission SEM for structural visualization

After wet pressing, the samples were frozen in liquid

nitrogen and freeze-dried at -92.8ºC and 615 Pa for 24

hours using Heto LyoPro 6000 freeze dryer. The freeze-

dried samples from each series were cut into 10 mm X 20

mm. The samples were coated with a conductive layer of

gold. Digital images were acquired at 100x and 10000x

magnifications, with a field-emission SEM (Zeiss Ultra

field-emission SEM), in secondary electron imaging

(SEI) mode. The resolution of the applied microscope is

approximately 1 nm. The nanofibrils of the MFC material

used in this study have diameter of approximately 20 nm

(Chinga-Carrasco et al. 2011). The used equipment and

magnification provide sufficient resolution to detect the

different scales of the fibrillated materials.

Mixture design of experiment

A mixture design of experiment (Cornell, 2002; Meyers,

Montgomery 2002) with replicates (6 runs with replicate

for each run) was used to estimate the quadratic mixture

model for Gurley (s/100ml) in relation to MFC, filler and

TMP fractions. The analysis of variance calculates the

significance of the linear and the interaction components

over the region covered by the experimental data. The

100% TMP and 85% TMP with 15% filler mixture were

used as the zero points for fitting the quadratic mixture

models.

Results and discussion As expected, the addition of fixative and retention aid

yields an even distribution of filler particles in the sheets

thickness direction (Fig 4).

The drainage time (Table 3) was measured while

draining the slurry during handsheet making using a PFI

handsheet former. The samples used to measure the

WRV (Table 4), the pre solids content (Table 4) and for

wet pressing experiment were made by dewatering the

furnish mixtures with FiberXPress at 1 MPa for 60

seconds.

Tables 3 and 4 show the following trends:

1. The pure TMP sample yields the shortest drainage

time and highest water retention values (WRV).

2. Increasing the ash content reduces the amount of

fines and fibres in the sample and thus reduces

WRV. This increases both the pre pressing solids

content (Pre solids) and the solids content after wet

pressing (Solids). The low WRV indicates less

swelling water in the sample due to less TMP

fraction and therefore easier to dewater during sheet

forming and wet pressing.

3. Increasing the fibrillation of the MFC, from 3 pass to

5 pass during homogenization, increases the drainage

time.

4. The samples that contain 5% 5 passes MFC and 29%

ash can be wet pressed to similar solids content

compared to F15T85.

Fig 5 shows that the tensile index and apparently the

strain at break decrease with increasing ash content for

mixture with 3MFC2.5%. Suprisingly no changes are

seen in either tensile index or strain at break in 5MFC5%

and no change in strain at break is observed in TMP

samples when ash content increases. The samples that

contain higher concentration of MFC with higher filler

content appear to be more brittle than samples made with

100% TMP.

Fig 6, 7 and 8 show the field emission SEM images for

F15T85, 3MFC2.5F30 and 5MFC5F30 samples

respectively. The MFC with higher number of passes

yields more fibrillated material. Increasing the 5 passes

MFC concentration enhances the MFC adsorption onto

the filler particles (Fig 8). The increased concentration

of more fibrillated MFC (5MFC5%) yields tensile index

close to 30 kNm/kg even at 29% ash content (Fig 5).

Increasing the ash content increases the density but

reduces the tensile index (Fig 9). The 5MFC5F30 (29%

ash content) shows the highest sheet density (490 kg/m3)

while the tensile values are still reasonably high, 30

kNm/kg. The higher amount of highly fibrillated,

swollen and flexible MFC in the 5MFC5 presumptively

fills more crevices between fibre and filler particles. The

large coverage caused by an increased amount of

nanofibrils adsorbs more filler particles and binds them

to the fibre network (Fig 8).

As expected, the tensile index for all samples correlated

well (r2=85%) with the water retention values (Fig 10),

similar to what other researchers have found (Stone et al.

1968; Htun, Ruvo 1978; Scallan, Carles 1972; Scallan

1977).

The increase in filler content increases the light

scattering coefficients as expected (Fig 11). The

5MFC5F30 sample yields a light scattering coefficient of

75 m2/kg, and registers a tensile index of 30 kNm/kg.

Sheets with MFC and 15% ash content exhibits higher

z-direction strength when compared to TMP sheets with

15% ash content (Fig. 12). 5MFC2.5F30 and

3MFC2.5F30 yields similar z strength as F15T85.

5MFC5F30 samples yields highest z direction strength at

511 kPa.

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

Fig 5. Tensile index vs. strain at break. The ash content is labelled beside each data point. The error bars are 95% confidence intervals

Fig 6. FESEM image for the surface of F15T85 samples. The surface images were acquired at low (Left) and high (Right) magnification.

Fig 7. FESEM images for the surface of 3MFC2.5F30 samples. The surface images were acquired at low (Left) and high (Right) magnification.

Fig 8. FESEM images for the surface of 5MFC5F30 samples. The surface images were acquired at low (Left) and high (Right) magnification.

15%

0%

17.5%

28.9%

15%

32.6%

17.2%

32%

20

25

30

35

40

45

1.2 1.4 1.6 1.8 2 2.2

Strain at break (%)

Te

ns

ile

In

de

x (

kN

m/k

g)

no MFC

5MFC5%

5MFC2.5%

3MFC2.5%

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Fig 9. Tensile index vs. density. The ash content is labelled beside each data point. The error bars are 95% confidence intervals.

Fig 10. Tensile index versus water retention value (WRV). The error bars are 95% confidence intervals.

Subramaniam (2008) found that increasing filler and

microfines in chemical pulp handsheet improved internal

bonding strength. Nanko, Ohsawa (1989) observed that

fibrillar fines filled gaps between fibres in wet pressed

sheets. The more fibrillated 5MFC5% contains more

nanofibrils and larger specific surface area than

3MFC2.5% and 5MFC2.5%. The MFC readily adsorbs

onto the fillers and fibres and improves packing and

bonding (Fig. 7 and 8) during wet pressing. This

increases the z strength of 5MFCF30 as compared to

F15T85 (Fig. 12).

Gurley versus TMP, filler and MFC (Quadratic mixture models)

The Gurley is modelled with quadratic mixture models in

relation to ash content, TMP and MFC (3 and 5 passes)

fractions (Table 3).

At 95% confidence level, the addition of MFC is found

to significantly increase Gurley or air permeance

(P<0.05). The quadratic mixture model showed that the 5

passes MFC fraction has the most positive effect in

increasing the Gurley. In mixture that contains 5 pass

MFC, the model shows significant effects from

interaction components, MFC-ash content (P<0.05) and

MFC-TMP (P<0.05). The analysis of variance shows that

the error component contributes less than 1% of the

detected total variations detected.

Fig 11. Light scattering (m2/kg) versus ash content (%). The error bars are 95% confidence intervals.

Fig 12. Z direction strength (kPa) vs. Gurley (s/100ml). The ash content is labelled beside each data point. The error bars are 95% confidence interval.

High Gurley or air resistance indicates that the MFC

formed close contact with the fibre network during

handsheet pressing and drying resulting in pore network

blockage (Subramaniam 2008). Nanko, Ohsawa (1989)

also found that fibrillar fines filled gaps between fibres in

wet pressed sheets. Air permeability of paper describes

both porosity and the network structure of the sheet

(Taipale et al. 2010). As confirmed by the structures seen

in Fig 7 and 8 the MFC collected after 5 passes during a

homogenization process contains more nanofibrils.

Larger amount of nanofibrils fills more pores and

enhances bonding. This results in a densely packed

structure after drying, which increases air resistance.

15%

0%

17.2%

28.9%

32.6%

15%

32%

17.2%

20

25

30

35

40

45

390 410 430 450 470 490 510

Density (kg/m3)

Te

ns

ile

in

de

x (

kN

m/k

g)

no MFC 5MFC 5%

5MFC 2.5% 3MFC 2.5%

R2 = 0.85

0

5

10

15

20

25

30

35

40

45

50

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

WRV (g/g)

Te

ns

ile

in

de

x (

kN

m/k

g)

R2 = 0.93

40

45

50

55

60

65

70

75

80

85

0 5 10 15 20 25 30 35

Ash content (%)

Lig

ht

sc

att

eri

ng

(m

2/k

g)

15%

0%

17.5%

28.9%

32.6%

15%

32%

17.2%

300

350

400

450

500

550

0 10 20 30 40 50 60 70 80

Gurley (s/100ml)

Z d

ire

cti

on

str

en

gth

(k

Pa

)

no MFC 5MFC 5% 5MFC 2.5% 3MFC 2.5%

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

Conclusions Optimal selection of MFC quality and filler content

may maintain or improve strength properties without

affecting the pressability of the sheet in the wet end.

Filler-MFC-fibre interactions simultaneously improve

both the light scattering and the strength properties of a

given sheet. Increasing both filler and more fibrillated

MFC concentration increased the density and the air

resistance. MFC readily adsorbs onto the filler particles

and fibres thus binding the fillers effectively with the

fibre network. MFC produced through higher number

of passes, 5 passes in this study, at 5% concentration

improved the z direction strength even at 30% filler

loading. This study shows the potential use of MFC and

filler for engineering sheet structures with optimal

properties, i.e. strength, light scattering and air

resistance, without impeding dewatering at press.

Acknowledgements

Tony Lehto and Jani Yli-Alho from Kenttäviiva Oy Automaatio for designing, constructing and commissioning of the dynamic wet pressing simulator. The Research Council of Norway, Norske Skog, PFI, Voith, News International, Omya and Sun Chemical for funding this work through the grant no. 187990 - ENergy efficient PAPer production of wood containing paper for next generation printing presses.

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