THE USE OF FIBRE WALL THICKNESS DATA TO PREDICT HANDSHEET
PROPERTIES OF EUCALYPT PULP FIBRES
1
Iiro Pulkkinen, Research Scientist
Ville Alopaeus, Professor, Helsinki University of Technology, Chemical Engineering, Department of
Biotechnology and Chemical Technology, PO Box 6100, FI-02015, TKK, Finland.
3
Juha Fiskari, Development manager, Oy Metsä-Botnia Ab, Joutseno, FI-54120 Finland.
4
Olli Joutsimo, Development manager, Oy Metsä-Botnia Ab, Äänekoski, FI-44100, Finland.
2
SUMMARY
With modern fibre analysers, such as the FiberLab®, the whole range of fibres in a pulp mix can be measured,
providing distributions of fibre properties. Fibre dimension distributions and the calculated density of handsheets
were used to find suitable parameters to characterise the effect of fibre morphology on paper technical properties.
With these variables combined with the water retention value (WRV) data, both the effect of raw material
morphology and that of the drying shrinkage behaviour can be characterised qualitatively. Fibre wall thickness
distribution was found to be the major contributor of handsheet properties studied, such as tensile strength, sheet
density and air resistance. Fiber length, and also surprisingly fiber width had no significant effect on measured
properties. Scanning electron microscopy (SEM) was also used in order to investigate the potential of SEM in
swelling studies of the fibres and handsheets.
Keywords: FiberLab®, WRV, fibre wall thickness, morphology, distribution, drying
shrinkage, SEM
INTRODUCTION
The relationship between pulp fibre morphology and paper properties has been extensively studied over the years (17). Fibre length and strength have been shown to be particularly important for tearing resistance (7-9). The thickness
of the fibre wall has an important bearing on most paper properties, with thick-walled fibres forming bulky sheets of
low tensile but with a high tearing strength (1, 4).
It has become increasingly important to identify key pulp parameters that can be used to predict ultimate
papermaking performance on the paper machine. We have previously studied the correlations between fibre wall
thickness distributions and paper technical properties of handsheets (10).
The network structure of paper depends on the collapsibility, conformability, and flexibility of wet fibres.
These factors and the swelling of the fibres determine the densification of paper structure in wet pressing and drying.
Fibre curl affects mostly the tensile strength and the bonding ability of fibres in a fibre network. The effect of fibre
bonding has previously been studied by Retulainen (1997)(11) and Vainio (2007)(12). The high fibre curl e.g.
according to Page et al. 1985(13) affects the tensile index so that a sheet formed with curled fibres has a low tensile
index but can have a high tear strength. The low tensile index has also been explained by low fibre segment
activation (14-16).
Fibre segment activation modifies originally curly or otherwise deformed fibre segments, unable to carry the
load, into active components of the network (17-19). Activation of fibre network occurs during drying, when lateral
shrinkage of fibres is transformed into axial shrinkage of neighbouring fibres in bonded areas. In restrained drying,
the free fibre segments dry under stress and the slackness is therefore removed (17-19). Interfibre bonding and
shrinkage of fibres are the requisites for activation. The amount of drying stress needed to activate fibre segments
depends on the morphology of the fibres (11). In this paper we have investigated how fibre wall thickness can be
linked to drying shrinkage, and therefore to the development of paper technical properties.
In this study, we have characterised this by a single parameter that uses the whole mass distribution of
fibres. All fibre dimension measurements carried out in this study are made with a FiberLab® fibre analyser (20).
In this paper, the effect of fibre dimension property distributions on handsheet properties was also
examined. Previous studies have mostly concentrated on mean dimensions of fibres (21,22). In addition, SEM
images were used to characterise the swelling behaviour of handsheets when subjected to varying humidity
conditions. This further supported our suggestion to use apparent density as a basic parameter for characterising
inter-fibre bonding and network activation. The combined use of dimension distribution measurements of fibre
properties, calculated apparent density and WRV measurements provides an insight on the effect of fibre segment
activation to sheet properties.
A
EXPERIMENTAL
Materials
20 Eucalyptus grandis clones (16 4-year-old and 4 8 -year-old) were used in this study. Kraft pulps were produced
from chips representing the whole trees. Pulps were laboratory cooked to a Kappa number of 18 ± 2. Samples of the
prepared pulps were beaten with a PFI refiner up to 2,500 revolutions according to standard ISO 5264-2. Handsheets
were made according to the standard ISO 5269-1.
B
For SEM imaging, dynamic sheets of an E. grandis/E. dunnii commercial pulp were used. Dynamic sheets
were made with a dynamic sheet former (23) developed by Paprican, which had been slightly altered. The wire speed
was set to 1000 m/min. The jet speed was set to ~ 920 m/min (1.6 bar) and the feed concentration was 4 g/l. The
grammage of dynamic sheets was ~60 g/m2. The sheets were wet- pressed according to ISO 5269-1. The laboratory
sheets were dried in contact with a plate, which prevented shrinkage during drying. The drying was done in a climate
control room (23 °C and relative humidity (RH) of 50%). After drying, the handsheets were tested according to the
following standards and methods:
• apparent density EN ISO 534
• tensile properties EN ISO 1924-2
• water retention value (WRV) SCAN-C62
• air resistance EN ISO 5636-5:03
Absolute dry samples were dried overnight (16 hours) in an oven at 105°C. Handsheet conditioning to 90% RH was
carried out with an OPTIDIM hygroexpansivity meter (24).
Microfibril angle was measured with SilviScan 2 measurement apparatus (25). Samples were taken from breast
height, as a bark free sample (12 mm core from bark to pith (1 radii).
Measurements of fibre dimensions
A FiberLab® measurement equipment consists of an analyser and a sample unit. Two CCD cameras capture fibre
images. The direct results are the fibre length, fibre width and fibre wall thickness. The calculated values are the curl
index, coarseness, cross-sectional area and volume index (20). Procedures of fibre and image processing to obtain
fibre properties have been described in detail elsewhere (20).
The results used in this study are calculated from measurements of individual fibres. An average of 10,000
fibres was measured in one sample. A Fortran program calculates the characteristics of the distributions, i.e. mean,
standard deviation and skewness, for the length, width and wall thickness of the fibre.
Estimation of fibre shrinkage potential using SEM measurements
Microscopical examinations of shrinkage-swelling behaviour of pulp fibres and kraft handsheets have previously
been studied, for example, by Weise (1997)(26), Enomae and Lepoutre (1998)(27), as well as by Moss and Pere
(2006)(28).
Figure 1 presents the SEM images for the dynamic sheet cross-sections embedded in epoxy resin after
preconditioning to varying humidity conditions, and for the wet pressed sheet. Samples for SEM studies have to be
dry. The dimensions of wet fibres change if the fibres are air-dried. Freeze-drying helps in preserving the wet
dimensions. The sheets dried to absolute dryness were not freeze-dried, and were extracted to an exsiccator until
embedded in epoxy resin (Struersin Epofix) prior to the cutting of the samples. Samples were cutted perpendicularly
to the fiber direction. Grinding (Struersin Dap-V) and polishing of the sample was done according to Reme et al.
(2002)(29). Sheet thickness was measured within each frame at 30 different locations (interval of 5 µm applied). This
should smooth out the surface roughness effect in order to give us reliable results.
The sheets were investigated at three different moisture contents. The pictures were taken after wet pressing, after
oven drying of the sheets (~1-2% RH), and after conditioning to 50% RH and 90% RH. Also the sample
preconditioned to 90% RH was kept in an exsiccator prior to embedding to epoxy resin. The corresponding moisture
content of the sheets was roughly 33%, 1%, 7%, and 15%, respectively.
The results of the SEM measurements are presented in Table 1. An average of 1000 fibres per sample were
measured (40 images). A resolution of 0.14 µm was obtained for dimension measurements.
A
B
C
D
Fig. 1. SEM images of dynamic handsheets made of unrefined fibres. A) abs. dry sample, B) 50% RH (~7%
MC), C) 90% RH (~15% MC) D) Wet pressed sheet (33% MC).
Calculation of wet-pressed density
In order to describe the extent of drying shrinkage potential of fibre mixtures, the concept of wet-pressed apparent
density is introduced. Wet pressed apparent density for the handsheets is calculated by dividing a handsheet into
rectangles of equal area, each representing one oval shaped fibre cross-section and a void space between fibres (Fig.
1). The fibres in the sheet are thus anisotropic and the amount of bonding between fibres is not directly included.
Fibre wall thickness distribution, fibre width distribution, and fibre length distribution measured by the FiberLab®analyser were used in the calculation of ρwet. It therefore includes the whole mass distribution of fibre material. It is
calculated as
ρ wet =
g
n
∑ (mF )i * nF
(1)
i =1
where g is the grammage of the handsheet (g/m2), mF is the mass of an each oval shaped fiber and nF is the number of
fibers in a given grammage. The mass of the fibers was calculated individually for each fiber from the fiber raw data
provided by the FiberLab® analyzer.
Each layer in a handsheet is of a thickness of two fibre wall thicknesses. An arbitrarily chosen lumen volume of
20% is included in the calculations. It is therefore assumed that most of the hardwood fibres are readily collapsed (no
visible lumen space) at the beginning of drying. This is a safe argument, since most of the softwood pulp fibres are
collapsed at the beginning of drying (30), and based on our previous measurements (31) a major portion of
eucalyptus fibers are collapsed in their wet pressed state, their shape being approximately of rectangular shaped. A
cell wall material density of 1.5 g/cm3 was used to calculate the mass of the fibres. But since we are dealing with a
relative measure of fiber properties, the value we choose does not make any difference. Fibre walls are assumed to be
intact and there are no pores. It should be further noted that the fiber dimensions obtained and presented in this paper
are for wet fibers. This is supposed to describe the swollen state of the fibres in their wet-pressed state.
Fig. 2. Cross-section of the handsheet used in calculations.
RESULTS AND DISCUSSION
Correlation analysis
Raw material morphological features (fiber length, fiber width, fiber wall thickness, apparent density) were used to
explain the variability of physical properties of the handsheets (bulk, tensile index, etc.). Table 2 shows the Pearson
correlations (32) between the measured and calculated pulp fibre properties and the handsheet properties in the unrefined and refined state. The sample size is small (n=20), so the critical value for Pearson r to make results
statistically significant (< 1%) is quite high, 0.537. The mean value of fibre wall thickness and calculated apparent
density correlated significantly with all measured properties at zero PFI revs and with all properties except light
scattering at 500, 1000 and 2,500 revs. The standard deviation of fibre wall thickness correlated significantly with
all properties except light scattering for both un-refined and refined pulps. Fibre wall thickness characteristics and
apparent density had negative correlations with tensile index, air resistance and handsheet density, and the single
greatest values found between wet pressed apparent density and handsheet properties. However, it was interesting to
see that the correlations between fiber length and especially fiber width characteristics and paper technical properties
were not so good. Microfibril angle (MFA) was measured for 12 of the samples. It correlates well with paper
technical properties, as expected, but also correlates well with fiber wall thickness index. This correlation is nothing
new, for example Hiller (1964)(33) and Courchene et al. 2006(34) observed this correlation between these two
properties. However, the correlation was positive, as opposed to the results of other authors. The reason for this could
be that MFA was measured for young wood only (4-year old clones). The measurement of MFA requires
complicated measurement procedures whereas fiber wall thickness index is easy to measure. For example Wimmer et
al. (2002)(35) do not give much credit to MFA as a predictor of handsheet properties of Eucalyptus globulus. Rather
strangely, fiber width and fiber wall thickness index did not correlate (very poor correlation). This can be due to the
assumption of circular shaped fibers with the image analysis, when fibers are “scanned” only from one side and the
level of collapse cannot be seen.
Reliability of FibreLab® measurements
FibreLab measurements were made in duplicate for each pulp sample. By using a Mann-Whitney U test (36), the
variation of the mean values of fibre wall thickness duplicates was approximately ± 0.06 microns (95% confidence
interval). Even though an average of 10000 fibres per distribution were measured, this is probably an
underestimation, since the resolution of measurements of fibre wall thickness is 0.74 microns. The variation of mean
value of fibre width was estimated to be ± 0.2 microns.
Based on the results obtained in this study there seems to be a correlation between the fibre wall thickness
characteristics measured with FibreLab® and handsheet properties. It should be noted, that the measurement are
made for swollen fibres, and that the measured wall thickness is not the true wall thickness of the fibres. The
variation of the ρwet was estimated to be roughly ± 2.5% based on the measurement inaccuracy of fibre width and
fibre wall thickness.
Richardson et al. (2003)(37) studied the measurement of fibre cross-section dimensions, and were conserned
about the measurement accuracy of FiberLab® measurements. The measurents were compared against other more
“reliable” measurement techniques. The statement made, that fiber width has a strong influence on fiber wall
thickness index, was not verified by our results observed in this study.
The swelling ability of fiber walls
Fibers swell under the influence of water at their amorphous regions (38). Fibers do not swell in their width, but
swelling causes and increase in the fiber wall thickness in the direction towards the fiber lumen (39,40). Swelling of
the cell wall relates to the WRV (39,40). WRV has also been found to correlate with fiber saturation point (FSP)
measured by the solute exclusion method (41). FSP measures the pore water inside fiber walls. Based on Figure 3,
WRV (42) decreases with increasing mean fibre wall thickness. This is probably due to the water retained by
capillarity inside the lumens. When fibres contain more total volume (lower wall thickness) of lumen, they have
higher WRV values. Thinner walled samples are more flexible and collapse easily in the centrifuge. Figure 3 shows
that also the width of the fibre wall thickness distribution correlates well with WRV.
2.0
2.00
2
1.8
1.80
1.7
WRV (g/g)
WRV (g/g)
2
R = 0.5992
R = 0.6426
1.9
1.6
1.5
1.4
1.60
1.40
1.3
1.2
2.00
2.50
3.00
3.50
4.00
4.50
Fiber wall thickness (µm)
1.20
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
2.4
2.5
Standard deviation of fiberwall thickness (µm)
Fig. 3. WRV as a function of mean value (left) and standard deviation (rigth) of fibre wall thickness as a
function of water retention value.
Figure 4 shows the difference between ρwet and measured sheet density as a function of WRV.
1.20
1.00
2
R = 0.7167
density (g/cm3)
0.80
0.60
2
R = 0.8807
0.40
0.20
0.00
1.40
1.50
1.60
1.70
1.80
1.90
2.00
2.10
WRV (g/g)
Fig. 4. Density of the handsheets (triangles) and calculated ρwet (squares) as a function of WRV.
Figure 4 show that in the wet pressed state there still is a higher density in the sheet containing more thick-walled
fibres (low WRV). The difference in curves of Figure 4 therefore has to come from somewhere else. It can be
deduced that the difference in drying shrinkage potential is a greater force than fibre collapsibility (as the collapse
degree is similar for all of the fibers) in determining how fibre properties will develop. This translates into smaller
effect of wet pressing on thin-walled fibres. In this study, the drying shrinkage potential is considered as a property
that keeps fibres in fibre-fibre contact, i.e. what is the breaking tendency of hydrogen bonds.
Thin-walled fibres have a naturally narrow wall thickness distribution as Figure 3 shows. Fibre network
consisting of thin-walled fibres also have a narrow drying stress distribution, therefore resulting in increased elastic
modulus and tensile strength (12, 43) when the sheet is dried. If a perfectly uniform fibre network is considered,
where stress forces of neighbouring fibres are cancelled, there will be a more dense network structure in the x-yplane with a lower change in dimensions, as described by situation C in Figure 5. Decreasing the fibre wall thickness
and taking the whole distribution into account, (distribution A of Fig. 5) the situation would be slightly different.
Even though the mean value is lower than in situation C, there would be a slight heterogeneity in the forces
transferring between adjacent fibres, resulting in a higher change in sheet dimensions. Fibres with a wider force
distribution do not draw fibres close to each other, which result in a loose fibre network. The thick-walled fibres with
wide wall thickness distribution (situation B) have a wide stress distribution with a different amount of fibre
shrinkage between adjacent fibres, resulting in a higher change in sheet dimensions but the resulting fibre network
would not be as dense as with situations A and C. The forces between fibres are not cancelled, and fibre network
faces heterogeneous shrinkage, resulting in lower strength values of the fibre network and decreased activation of the
sheet.
frequency
C
B
A
increasing fibre wall thickness
Fig. 5. Schematic fibre wall thickness distributions.
The effect of fibre wall thickness on handsheet properties
Figure 6 shows a handsheet density for different refining levels as a function of ρwet. The calculated wet-pressed
density describes the pulp fibres in their swollen wet-pressed state. Fibres with low apparent density are thin-walled
and produce dense sheets with improved bonding and fibre segment activation that resulted in higher tensile index
values (Fig. 7). The correlations were linear, independent on the refining level of the fibers for handsheet density and
for the tensile index. Wet-pressed density values after refining being therefore dependent on the original mass
distribution of un-refined fibres.
Less coarse fibres are known to form more closed network structures in a sheet. Additionally, the calculated
apparent density is therefore expected to have strong correlation with sheet porosity indicated by air resistance. There
is almost a linear relationship for un-refined fibres and a strongly exponential behaviour of air resistance as a
function of apparent density for fibres that are highly refined (Fig. 8).
hansheet density (g/cm3)
1.0
0.8
2
R = 0.791
2
R = 0.808
2
R = 0.808
0.6
2
R = 0.823
0.4
0.75
0.8
0.85
ρ F (g/cm3)
0.9
0.95
1
Fig. 6. Handsheet density as a function of wet-pressed density for different PFI refining levels (0= diamonds,
500= crosses, 1000= triangles, 2,500= squares).
140
Tensile Index (Nm/g)
120
100
R2 = 0.603
R2 = 0.649
80
2
R = 0.649
60
R2 = 0.699
40
20
0
0.75
0.8
0.85
ρ F (g/cm3)
0.9
0.95
1
Fig. 7. Tensile Index as a function of apparent density for different levels of refining (0= diamonds, 500=
crosses, 1000= triangles, 2,500=squares).
200
R2 = 0.8162
180
A ir resistance (s)
160
140
120
100
R2 = 0.8915
80
60
R2 = 0.8784
40
20
R2 = 0.8434
0
0.75
0.8
0.85
0.9
0.95
1
ρ F (g/cm3)
Fig. 8. Air resistance as a function of apparent density for different levels of refining (0= diamonds, 500=
crosses, 1000= triangles, 2,500= squares).
Table 1. Results of the SEM analysis of the fibres.
Abs. Dry
50%RH
90%RH
Wet Pressed
2.33
0.60
2.19
0.60
2.28
0.62
2.38
0.67
59.75
24.43
52.80
23.12
61.21
26.21
62.22
26.50
13.57
3.57
13.19
4.82
14.53
4.29
13.72
4.20
Fibre wall thickness
average
standard deviation
Fiber wall area
average
standard deviation
Fiber diameter
average
standard deviation
Table 2. Pearson correlation coefficients for the pulp properties fibre wall thickness (CWT) mean value and
standard deviation and apparent density and four handsheet properties at different PFI refining levels;
coefficients are significant at p<1%, 2-tailed, n=20.
Fiber lengthmean (mm)
Parameter
Fiber lengthmean (mm)
Fiber lengthstdv. (mm)
Fiber widthmean (mm)
Fiber widthstdv. (mm)
Fiber lengthstdv. (mm)
Fiber widthmean (mm)
Fiber widthstdv. (mm)
CWTmean
CWTstdv.
app. dens.
hemicellulose
MFA
1
0.94
0.61
n.s
0.78
0.89
0.77
0.50
0.85
0.94
1
0.61
n.s
0.85
0.84
0.84
n.s
0.74
0.61
0.61
1
0.67
0.61
0.84
n.s
0.54
0.54
n.s
n.s
0.67
1
n.s
n.s
n.s
n.s
0.42
TI (Nm/g)
density (kg/m3)
light scatt. (m2/kg)
air res. (s)
-0.64
-0.66
n.s
n.s
-0.68
-0.65
n.s
n.s
n.s
n.s
n.s
n.s
-0.65
-0.65
n.s
n.s
TI (Nm/g)
density (kg/m3)
light scatt. (m2/kg)
air res. (s)
-0.56
-0.65
n.s
-0.66
-0.55
-0.62
n.s
-0.62
n.s
n.s
n.s
n.s
TI (Nm/g)
density (kg/m3)
light scatt. (m2/kg)
air res. (s)
-0.58
-0.57
n.s
-0.65
-0.61
-0.54
n.s
-0.61
TI (Nm/g)
density (kg/m3)
light scatt. (m2/kg)
air res. (s)
-0.58
-0.55
n.s
-0.59
-0.60
-0.53
n.s
-0.54
CWTmean
CWTstdv
0.78
0.85
0.61
n.s
1
0.88
0.97
n.s
0.84
0.89
0.84
0.84
n.s
0.88
1
0.87
0.54
0.86
app. dens. Hemicellulose
0.77
n.s
0.84
n.s.
n.s
0.54
n.s
n.s
0.97
n.s
0.87
0.54
1
n.s
n.s
1
-0.76
0.66
MFA
0.85
0.74
0.54
0.42
0.84
0.86
0.92
0.66
1
PFI=0
-0.79
-0.83
-0.77
-0.75
-0.79
-0.75
n.s.
-0.74
-0.83
-0.88
-0.72
-0.82
n.s.
n.s.
n.s.
n.s.
-0.76
-0.56
-0.52
-0.72
n.s
n.s
n.s
n.s
-0.76
-0.82
n.s.
-0.69
-0.78
-0.74
n.s.
-0.76
-0.79
-0.88
n.s.
-0.76
n.s.
n.s.
n.s.
n.s.
-0.80
-0.61
n.s
-0.64
n.s
n.s
-0.52
n.s
n.s
n.s
n.s
n.s
-0.77
-0.82
n.s.
-0.69
-0.70
-0.73
n.s.
-0.75
-0.78
-0.87
n.s.
-0.75
n.s.
n.s.
n.s.
n.s.
-0.78
-0.67
n.s.
n.s
n.s
n.s
n.s
n.s
n.s
n.s
n.s
n.s
-0.74
-0.81
n.s.
-0.63
-0.70
-0.74
n.s.
-0.73
-0.75
-0.86
n.s.
-0.68
n.s.
n.s.
n.s.
n.s.
-0.76
-0.56
n.s
n.s
PFI=500
PFI=1000
PFI=2500
Characterisation the swelling of fibres and handsheets
The cellulose/hemicellulose-ratio of the samples varied between 2.11 and 4.21. The xylose, mannose and arabinose
content were used to describe the amount of hemicellulose in the pulp. The hemicellulose has an effect on the
flexibility of fibres and thus the bonding ability, important for tensile strength development (44). However, in this
study, no correlation between tensile index and cellulose/hemicellulose-ratio was observed (Table 2).
Network properties
Direct measurements of network properties are time-consuming and laborious. For this reason, the effect of fibre
wall thickness and fibre width (both of them are included in apparent density) on network properties were studied by
using FibreLab® data and SEM pictures.
Table 1 and Figure 9 show that the fibre wall thickness increased rapidly when the sample was oven dried
from 50% RH, and increased almost linearly when the humidity was changed from 50% to 90% RH (an
approximately 15% moisture content of the sheet). At the wet pressed state, the fibre wall thickness seems to follow
the linear trend seen in Figure 9. This phenomenon of Figure 9 seen under low moisture contents is more evident
with thin-walled fibres (31). The effect of surface forces acting on thin-walled fibres could be strong enough to result
in an initial decrease in fibre wall thickness when the moisture is increased.
In very dry fibres, there are less inter-fibre hydrogen bonds present (only the ones keeping cellulose
molecules together) and fibres are not subjected to any contractive forces (stresses). The point where the fibre wall
starts to swell could not be determined based on the measurement points available. Figure 10 shows the percentage
of un-collapsed fibres out of the whole fibre population. The differences are again very small, thus indicating that
fiber shape is more or less un-changed after wet-pressing thus indicating that fiber network activation is not affected
by the collapse degree of the fibers. After a certain amount of water present (~85% dry matter content), the number
of un-collapsed fibres starts to increase slightly. This is roughly the region where the fibre-fibre bonding sites starts
to swell (45). Figure 11 presents the measured sheet thickness based on the SEM images. The sheet thickness started
to increase somewhere between 50% RH and 90% RH%. Nissan et al. (46) have shown how an elastic modulus can
be modeled solely based on effective fibre bonds per unit volume and how the moisture content and temperature
affect the elastic modulus values. Based on this theory, the elastic modulus appears to be independent of its network
structure. However, the distributions of fibre properties determine how large forces (mostly capillary) are present to
keep the fibres in contact. This is determined mainly by the wall thickness values of the fibres (pore size), so in
theory one would expect thin walled fibres (or an isotropic set of fibres) to stay in contact for the longest. The void
space in the sheet area increases with increasing moisture after the initial period when dried-in stresses are released.
When the drying stresses are released, sheet expansion occurs mainly in the sheet plane.
Fiber wall thickness, µm
2.5
2.4
2.3
2.2
2.1
2.0
0
5
10
15
20
25
30
35
Moisture content, %
Fig. 9. Fibre wall thickness as a function of varying moisture content (error bars 95% confidence interval).
Uncollapsed fibres, %
22
21
20
19
18
17
16
2
5
15
33
Moisture content, %
Fig. 10. The number of uncollapsed fibres as a function of varying moisture content.
Effective sheet thickness, µm
90
85
80
75
70
65
60
2
5
15
33
Moisture content, %
Fig. 11. Effective sheet thickness as a function of varying moisture content (error bars 95% confidence interval).
CONCLUSION
Fibre dimension distributions and calculated density of handsheets were used to find suitable parameters to
characterise the effect of fibre morphology on paper technical properties. Using these variables combined with the
WRV data, both the effect of raw material morphology and the effect of drying shrinkage behaviour can be
characterised qualitatively. A theory was presented to explain the behaviour of the activation of a fibre network
structure between wet-pressing and dry handsheet. Networks consisting of thin-walled fibres will result in improved
bonding through a narrower drying stress distribution than those consisting of thick-walled fibres. Fibre wall
thickness distributions affected the whole range of handsheet properties, such as tensile strength, sheet density and
air resistance. Fibres with low wall thickness and narrow fibre wall thickness distribution had higher strength
properties, density of the handsheets and air resistance. The amount of hemicelluloses did not correlate with fibre
properties or the WRV value. The swelling behaviour of the handsheets using SEM imaging under various humidity
conditions was also studied. Overall, the shape of the fibers seemed to be un-affected by the varying moisture
content, further indicating that the changes in fiber shape are not responsible for the development of sheet properties.
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