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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 ([email protected])
Øyvind W. Gregersen
Norwegian University of Science and Technology,
NTNU, Trondheim, Norway
Gary Chinga-Carrasco ([email protected])
Øyvind Eriksen ([email protected])
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)
PAPER PHYSICS
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
PAPER PHYSICS
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
PAPER PHYSICS
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
PAPER PHYSICS
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.
PAPER PHYSICS
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%
PAPER PHYSICS
Nordic Pulp and Paper Research Journal Vol 27 no.2/2012 393
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%
PAPER PHYSICS
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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