Note: Descriptions are shown in the official language in which they were submitted.
CA 02630743 2008-05-06
METHOD FOR MEASURING DEFORMABILITY PROPERTIES OF A FIBRE
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is the first application filed for the present invention.
MICROFICHE APPENDIX
[0002] Not applicable
TECHNICAL FIELD
[0003] The present application relates to measuring fibre deformability in
general,
and to measuring the flexibility, collapsibility, and moment of inertia of
fibres in
particular.
BACKGROUND OF THE INVENTION
[0004] Modem paper and paper board is predominantly composed of a matrix of
wood fibres. During the consolidation stage of papermaking, individual wet
fibres
are drawn and entangled together forming a web structure. The deformability of
the wet fibres used is a significant measure of the ability of the fibres to
conform to
each other by providing bonding contact in the course of dewatering, pressing,
and
drying. Fibre flexibility is a significant measure of fibre deformability.
Fibres which
are flexible are more conformable to one another, thus forming more contact
area
among fibres.
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[0005] Fibre flexibility determines the total inter-fibre contact area and the
voids in
the fibre network, and plays a dominant role in determining most paper
properties,
such as bulk, permeability, opacity, surface smoothness, and physical
strength.
[0006] The fibre flexibility of mechanical pulp, such as bleached chemi-
thermomechanical pulp (BCTMP) fibres, is more important when BCTMP fibres are
used in wood-free fine paper grades to improve paper bulk and opacity [1].
[0007] Compared with chemical pulp fibres, which usually collapse completely
during fibre processing, mechanical pulp fibres do not collapse, or collapse
only
partially depending on the papermaking process [2]. Collapsed fibres have
higher
flexibility than uncollapsed fibres, so it is important to understand how
fibre
collapsibility affects the fibre flexibility.
[0008] Among all properties of wood fibres, the elastic modulus of the fibre
is
recognized as one of the most fundamental fibre properties that affects almost
all
paper qualities and papermaking properties, such as sheet density, physical
strength, light scattering ability, smoothness, and permeability. It is the
controlling
factor that determines the deformability of the fibre wall.
[0009] There are several prior art methods for measuring the flexibility of
individual
wet fibres.
[0010] The measurement of single fibre elastic modulus is usually performed by
micro-tensile testing. The difficulties associated with this test are the
dimensions of
individual wood fibres, which are short (1-5mm) and thin (10-30um in diameter)
and require careful handling and mounting techniques in sample preparation,
and
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accurate measurements for stress and strain in a very small scale. Because of
the
heterogeneous nature, a large population of fibres needs to be tested for the
statistical analysis. Tedious and time-consuming operations in the fibre scale
become
a major drawback of this test method and make it impractical for engineering
applications.
[0011] Some existing prior art methods treat the fibre as a cantilever [3-7].
Most of
these methods are based on small deflection beam theory, which involves
measuring
the displacement of a fibre beam when applying a transverse force or bending
moment on the fibre. If the fibre is treated as a beam subject to pure elastic
deformation, the flexibility (F) of individual fibres can be defined as the
reciprocal of
its bending (also sometimes referred to as flexural stiffness) El, where E is
the elastic
modulus of the fibre wall and I is the moment of inertia of the fibre cross-
section:
F=1/ EI.
[0012] Seborg and Simmonds [8], for example, measured the stiffness of dry
fibres
by clamping individual fibres into place and then exerting a force on a fibre
using a
quartz spring to bend it like a cantilever beam. The flexural stiffness El is
determined from the slope of the load-deflection curve. The test suffers from
two
main disadvantages: (1) it is done on single fibres, making it very tedious
and
cumbersome; and (2) the clamping can damage the fibre.
[0013] James [9] calculated the fibre stiffness by measuring the resonance
frequency
of a fibre cantilever. Hydrodynamic or bending beam methods have also been
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developed for the fibre flexibility measurement by hydrodynamic forces
generated
by water flow and image analysis, so that individual fibre handling can be
avoided.
[0014] Various methods have been developed for supporting the fibres. For
example, Samuelsson [3] used a mechanical jaw to clamp fibres. Tam Doo and
Kerekes [10] supported fibre on one end of a capillary tube so that mechanical
damage to the fibre can be avoided. Like the Seborg and Simmonds method, the
Tam Doo and Kerekes method is limited to testing individual fibres.
[0015] Kuhn et al. [6] developed a device that bends fibres by a T-junction
tube when
fibres in water flow out of a capillary. The fibre deformation is observed by
a
microscope and the force is calculated according to hydrodynamic theory. The
Kuhn method is a direct measure of the flexibility of a fibre and may give
flexibility
results that are higher than expected [6].
[0016] Conformability testing as opposed to directly measuring flexibility is
another
typical method for fibre flexibility measurement. This method was first
proposed by
Mohlin [4]. In this method, a fibre is wet pressed onto a thin glass fibre
(diameter =
60mm) that is fixed on a glass slide. The wet fibre arcs over the glass fibre
and then
is allowed to dry. The non-contact span, or freespan, length of the fibre is
determined to calculate the fibre flexibility according to the beam deflection
theory.
Since only a conventional light microscope is required, and it can provide a
numerical measure in an engineering unit, this method has commonly been used
for
fibre flexibility measurement [11-13]. No pressure, however, is applied to the
fibre
CA 02630743 2008-05-06
when taking the measurement and most likely does not approximate what happens
in a paper structure of such fibres.
[00171 Steadman and Luner [7] have sought to improve upon the Mohlin method.
In the Steadman method, the stiffness (flexibility) of individual wet fibres
is
determined from the elastic modulus (E) and the moment of inertia (1) of the
fibre
wall. This method is advantageous because it does not need to handle
individual
fibres. In the Steadman method, a wire of 254m diameter was used as the
support
wire for forming the fibre arc over it. A larger wire will lead to a larger
arc, which
will be easier to identify with a conventional microscope, but a large wire
will also
increase the deflection ratio.
[0018] In the Steadman method, fibres are wetted and pressed onto a thin
support
wire that is fixed on a glass slide. The fibre and the support wire are
approximately
90 degrees to one another such that when pressed onto the wire, the fibre
forms an
arch-like span over the wire as it deforms. The fibre is then allowed to dry
and the
sections of the fibre in contact with the slide become adhered to the glass
slide. The
length of the section of the span not in contact with the glass slide,
referred to as the
non-contact span or freespan length, is measured from above using a
conventional
light microscope with incident lights, under which the optical contact zone of
the
fibre and the glass slide appears in dark, whereas the non-contact zone
appears in
light, thus the freespan length is measured. The freespan length measurement
is
then used in the calculation of flexibility according to the following
formula:
F=1/EI=72d/PWS4
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Where E=modulus of elasticity (Nm-2)
I = moment of inertia (m4)
d = wire diameter (m)
P = pressing pressure (Nm z)
W = projected fibre width (m)
S = mathematical estimate of the loaded span (m)
[0019] The fibre at which the distance between fibre surface and the glass
slide is less
than half of the wavelength of the light (usually assumed as 550nm) appears in
dark
even if they are not contacted physically due to light interference;
therefore, the
freespan length is usually under-measured. Since the fibre thickness is not
uniform
and a fibre does not collapse uniformly along the fibre length, the thickness
of the
fibre cross-section affects the freespan length used for the stiffness
calculation, which
is neglected in this method as the conventional Iight microscope only
generates
images from the top view.
[0020] Since the moment of inertia of a fibre cross-section cannot be measured
using
a conventional light microscope (LM), the Steadman method has only been used
for
measuring fibre flexibility but not for measuring the elastic modulus. The
elastic
modulus can be solved only if the moment of inertia of the fibre is known but
prior
art methods do not yield the moment of inertia.
[0021] As discussed above, in the Steadman method, a LM is employed to observe
pulp fibres. In recent years, confocal laser scanning microscopes (CLSM) have
been
used in pulp and paper research as an alternative to LMs for imaging fibres.
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However, CLSMs have not been used to take optical sections of fibres. Even
where
CLSMs have been used to image fibre cross-sections, the images have been of
the
cross-sectional surfaces of fibres which have been physically cut into cross-
sections.
SUMMARY OF THE INVENTION
[00221 According to one aspect of the present invention, there is provided a
method
for measuring a property of a fibre which involves providing a fibre, wetting
the
fibre, deforming the fibre in its wet state, acquiring an optical section
image of the
deformed fibre, making a measurement on the image, and calculating the
property
using the measurement.
BRIEF DESCRIFTION OF THE DRAWINGS
Figure 1 is a schematic of fibres being prepared for mounting on a glass slide
according to the present invention;
Figure 2 is a schematic of fibres on a glass slide according to the present
invention;
Figure 3 is a schematic cross-section of a fibre deformed on a glass fibre
according to
the present invention;
Figure 4 is a series of images of optical sections of a fibre taken in the x-y
plane
according to the present invention;
Figure 5 is a 3D image of a fibre reconstructed from the images of Fig. 4;
Figure 6 is a transverse optical section according to the invention of a fibre
in the XZ
plane;
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Figure 7 is a cross-sectional optical section according to the invention of a
fibre taken
in the YZ plane;
Figure 8 is a binarized image of the fibre of Fig. 6;
Figure 9 is a skeletonised image of the fibre of Fig. 8 with a neutral bending
plane;
Figure 10 is a binarized image according to the invention of a collapsed fibre
taken
in the YZ plane;
Figure 11 is a binarized image of an optical section according to the
invention of a
fibre taken in the YZ plane;
Figure 12 is an optical section according to the invention of a wet bleached
kraft
pulp (BKP) fibre taken in the XZ plane;
Figure 13 is an optical section according to the invention of a wet BCTMP
fibre taken
in the XZ plane;
Figure 14 are bar graphs of distribution of measured flexibility according to
the
present invention;
Figure 15 is a schematic cross-section of a fibre non-symmetrically deformed
on a
glass fibre according to the present invention;
Figure 16(a) is an optical section according to the invention of a Spruce BKP
fibre
taken in the YZ plane without wet pressing;
Figure 16(b) is an optical section according to the invention of a Birch BCTMP
fibre
taken in the YZ plane without wet pressing;
Figure 16(c) is an optical section according to the invention of a Birch BCTMP
fibre
taken in the YZ plane after wet pressing;
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Figure 16(d) is an optical section according to the invention of an Aspen
BCTMP
fibre taken in the YZ plane without wet pressing;
Figure 16(e) is an optical section according to the invention of an Aspen
BCTMP
fibre taken in the YZ plane after wet pressing;
Figure 17 is a graph according to the invention showing the effect of fibre
collapsibility on fibre moment of inertia;
Figure 18(a) is a graph according to the invention showing the effect of fibre
wall
thickness on fibre collapsibility;
Figure 18(b) is a graph according to the invention showing the effect of fibre
wall
thickness on moment of inertia;
Figure 19(a) is a graph according to the invention showing the effect of fibre
wall
elastic modulus on fibre collapsibility;
Figure 19(b) is a graph according to the invention showing the effect of fibre
wall
elastic modulus on flexibility;
Figure 20 is a graph according to the invention showing the relationship
between
measured fibre flexibility and collapsibility;
Figure 21 is a graph according to the invention showing the effect of wet
pressing on
measured flexibility values;
Figure 22 is a table according to the invention showing median values of
flexibility
of fibres;
Figure 23 is a table according to the invention comparing freespan length and
deflection height measurement; and
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Figure 24 is a table according to the invention comparing fibre collapsibility
and
fibre-wall elastic modulii.
DETAILED DESCRIPTION OF THE INVENTION
[0023] According to one embodiment, the present invention relates to a method
of
taking optical sections of wet pulp fibre in order to directly observe the
shape and
the cross-sectional geometry of the wet fibres once they have been deformed by
a
pressing pressure. Measurements of various dimensions of the fibres are made
using the optical section images and used for calculating the flexibility,
collapsibility, moment of inertia, and in turn, the elastic modulus of the
fibre wall.
The elastic modulus of wood fibres is also important for the production and
application of wood fibre in composite materials as a reinforced component.
[0024] Referring to Figure 1, in one embodiment of the present invention,
fibres are
prepared for observation in a manner similar to the set-up method using in the
Steadman method. A glass fibre 4 with a diameter of about 10um is fixed on a
microscope glass slide 5. The glass fibre 4 serves as the support for the
fibres 2 in the
same way as the support wire in the Steadman method. Pulp fibres 2 are stained
with a proper fluorescent dye and suspended in water. The fibre suspension
(not
shown) is swirled and then drained through a filter paper 3 and the pulp
fibres 2 are
deposited on the filter paper 3. The pulp fibres 2 are then wet pressed onto
the glass
fibre 4 and the glass slide 5 together with blotting paper 1 at a controlled
pressure
(P) and for a period of time, sufficient for the pulp fibres 2 to adhere to
the glass
slide 5. Referring to Figure 2, when the filter paper 1 is removed, at least
some of the
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fibres 2 should be positioned spanning the glass fibre 4 at a substantially
perpendicular angle to the glass fibre 4.
[00251 Figure 3 shows a cross-section of the glass fibre 4 with a pulp fibre 2
spanning
the glass fibre 4.
[0026] A CLSM is then used to image the fibres 2. The basic imaging mode of
CLSM
is an XY plane or section of the sample of the focal plane. The major
difference
between CLSM and conventional LM is that CLSM allows only the signals from the
focal plane to be recorded, so the image formed is only a pIane, not the
entire sample
object, while in LM, signals from above and below the focal plane can be
recorded.
Therefore, the CLSM image is crisper and is of higher resolution. By changing
the
focal plane along the height direction, a series of focal planes, also called
optical
sections, can be imaged as shown in Figure 4. With suitable image processing
software, these optical sections can be stacked up to construct a 3D image of
the
object, in this case a fibre 2 as shown in Figure 5. The glass fibre 4 is not
shown in
Figure 5.
[0027] For fibre flexibility measurements, the freespan length (L) and the
deflection
height (d) are measured. The freespan length is the length along the x-axis of
the
non-contact section of the fibre span i.e. the section not in contact with the
glass
slide. The freespan length is L1 + L2 in Figure 5.
[0028] The deflection height d is measured in the z-axis as described in more
detail
below.
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[0029] In order to measure the freespan length and the deflection angle, the
CLSM is
used to take an optical section (also referred to as a single line scan) of
the fibre in
the XZ plane. From the XZ plane, the transverse view of the fibre deformation
can
be obtained, which provides the same information as the 3D image, and from the
YZ
plane, the cross-sectional view of the fibre can be obtained, which can be
used to
determine the collapsibility of the fibre and to determine the moment of
inertia of
the fibre wall.
[0030] An example of a transverse optical section of a fibre 2 in the XZ plane
is
shown in Fig. 6. An example of a cross-sectional optical section in the YZ
plane is
shown in Fig. 7. As described in more detail below, binary images of these
optical
sections are generated from which measurements can be made.
Materials and Sample Preparation
[0031] The method according to one embodiment of the invention is now
described
with reference to the analysis of four commercial pulps: bleached Spruce kraft
pulp
(BKP), Aspen CTMP, Aspen BCTMP and Birch BCTMP, obtained from two
Canadian paper mills. Aspen CTMP and Aspen BCTMP are taken from the same
production line. The Aspen BCTMP were further refined by a PFI mill at 4%
consistency to 3000 revolutions and at 10% consistency to 4000 revolutions,
denoted
as LCR and MCR, respectively. The Canadian Standard Freeness (CSF) of LCR and
MCR are 236mL and 268mL, respectively.
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[0032] Acetone washed glass fibres were deposited on glass microscope slides
(Fisher brand precleaned microscope slide) as support wires for the pulp
fibres prior
to depositing pulp fibres on them. Glass fibres (0.5g, CDS Analytical 1001-
0345) are
suspended in 1L of distilled water and drained onto a piece of filter paper
(Fisher
brand Q8) by a TAPPI standard handsheet former. The suspension was swirled
before draining so that it was spinning while draining down, thus glass fibres
became oriented approximately in parallel close to the edge of the fiIter
paper. Then
the glass fibres were transferred onto the microscope slides by placing and
gently
tapping the filter paper onto the slides.
[0033] To enhance the fluorescence intensity, pulp fibres (0.3g o.d) were
stained in
20m1 0.1% Safranin-O for 24 hours at room temperature, and then diluted to
0.03%
consistency and drained onto a filter paper in the same manner as was done for
the
glass fibres. The filter paper with fibres was placed on two pieces of dry
blotting
paper, and then pressed onto eight glass microscope slides at 34OkPa by a
standard
handsheet press (Labtech) for 5 minutes. Prior to pressing, the glass slide
and the
filter paper were arranged in a way so that the pulp fibres and the glass
fibres cross
each other perpendicularly. The actual pressures on fibre samples are
calculated
based on the projected fibre area. Slides are dried in air and kept under
TAPPI
standard conditions before CLSM imaging. It should be noted that not all the
fibres
cross each other perpendicularly. A pulp fibre was measured only when it
crossed a
glass fibre at a perpendicular angle, i.e., 90 degrees t 10 degrees. About 30%
to 50%
of fibres form almost perpendicular crossings. Since there are about a
thousand
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CA 02630743 2008-05-06
pulp fibres on a single glass slide, sufficient perfect crossings can be found
for the
measurements to be carried out.
CLSM operation
[0034] Image scanning was carried out with a Leica TCS-SP2 confocal laser
scanning
microscope. A dry objective lens (HC FLOUTAR 50x) with a numerical aperture of
0.8 is used for imaging transverse and cross-section of fibres that were wet
pressed
on a glass slide. An excitation wavelength of 514nm from an Ar laser is used.
The
pinhole size is set at the optimum value by Leica Confocal Control software.
The
emission light collected by detector (PMT) is set from 525nm to 760nm. The
gain
and offset of PMT are automatically adjusted for each fibre by software to
ensure a
constant image quality. The CLSM is operated in XZ scanning mode to obtain
both
a transverse and a cross-sectional image. Scanning step size in Z direction is
0.12um.
An oil immersion lens (HCX OLAPO CS 63 x 1.4) was used for imaging fibre cross-
section before wet pressing.
Image processing
[0035] To improve the accuracy of the measurement and avoid subjective errors,
image processing is performed with the image processing toolbox in Matlab 7.0
(Mathworks Inc.). CLSM images are smoothed using lowpass filtering and then
converted into binary format (see Figures 8, 9 and 10). The threshold for
binarization was determined automatically by the double peak histogram method
[14,15]. All measurements were carried out on the binarized images.
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Freespan length (L) and deflection height (d)
[0036] Since the fibre thickness is not uniform along the fibre length and a
fibre does
not collapse uniforrnly along the fibre length, the thickness of the fibre
cross-section
may affect the deflection height and freespan length. In this example, a
neutral
bending plane 10 is defined as the symmetric centre in fibre thickness along
the fibre
length (see Figure 9). The neutral bending plane 10 is located by applying a
skeletonization operation to the fibre transverse images [16]. Since fibre
edges on
the binarized image are usually not smooth, several "open" operations are
required
before skeletonization so that the object can be smoothed and isolated pixels
removed. The deflection height (d) is defined as the vertical distance between
the
highest and the lowest pixels on the neutral bending plane. The freespan
length is
defined as the horizontal distance between the highest pixel and the lowest
pixel at
the start point of the horizontal segment of the neutral bending plane. In
Figure 9,
the freespan length as measured by a conventional LM is indicated by L', and
the
freespan length as measured by a CLSM according to the method the invention is
indicated by L.
Fibre collapsibility (AR) and moment of inertia (I)
[00371 With reference to Figure 10, fibre collapsibility was measured as the
aspect
ratio of the fibre cross-section dimension in Eq. 1 according to Jang [17]:
AR= D= " (1~
Dmax
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where Dm. is the fibre thickn.ess (shortest Feret diameter) and D. is the
fibre width
(longest Feret diameter), which were obtained from a binarized fibre cross-
sectional
image. The cross-sectional images were taken from the fibre on the top of the
glass
fibre 4 (support wire). The main reason that this portion of the fibre was
chosen for
collapsibility measurement is that this portion of the fibre was subjected to
the
maximal stress, and it is consistent if the same spot was chosen for all the
fibres
measured throughout the example. In an alternate embodiment, the cross-section
could be extracted from part of the freespan region, but not the part which is
in
contact with the glass slide since the part in contact with the glass slide
may not
contribute much to the deformation process of the fibre under stress.
[0038] With reference to Figure 11, because of the irregular shape of the
fibre cross-
section, the moment of inertia (1) of fibre with regard to the neutral bending
plane
was calculated based on the relative location of each pixel (Eq. 2) [18].
(2)
3
(12 +A'z2)
where a and b are the width and height of the pixel, respectively, A is the
area of a
pixel and z is the distance of pixel i to the neutral bending plane. The fibre
wall
thickness was only measured on the fibres without wet pressing from the fibre
cross-sectional images following Jang's procedure [14, 19].
[0039] Typical images of the transverse view of the fibre deformation acquired
with
CLSM XZ scanning mode are shown in Figure 12. From these images, fibre
deformation height and freespan length are measured. Fibre flexibility was
calculated based on the Steadman method by assuming that the fibres are
subjected
to only pure bending (Eq. 3):
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Flexibility = 1 = 72d (3)
El qL4
where E and I are the elastic modulus and the moment of inertia of the fibre
wall,
respectively, d is the deflection height, L is the freespan length and q is
the pressing
load on the fibre in N/m.
[0040] It was observed that almost all BKP fibres were collapsed and solid
fibre
walls were imaged (Figure 12). In comparison, mechanical pulp fibres were not
completely collapsed, which can be seen from the lumen area appearing dark
between the fibre walls (Figure 13). The shapes of the deformation of the two
types
of fibres were also distinguishable. The shape of the deformation of the BKP
fibres
appeared straight, resembling a shear deformation, but that of the BCTMP
appeared
more like a bending deformation. This confirms Waterhouse and Page's finding
[20]
that shear contribution can be substantial in the Steadman and Luner method.
In
this embodiment, only bending deformation is considered based on the Steadman
and Luner method. To limit the shear contribution and make the results
comparable
to other hardwood pulp fibres, the pressure used for BKP fibres was reduced to
220kPa, and the resultant deflection ratio of it was about 20%, close to that
of
hardwood pulp samples.
[0041] Various types of pulp fibres were measured using the method. About 40 -
50
fibres of each pulp sample were measured. It can be seen from Figure 22 that
the
deflection ratios, which are the ratios of the deflections in fibre thickness
direction to
the freespan lengths, are about 20% or below. According to Lawryshyn and Kuhn
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CA 02630743 2008-05-06
[21], when the small deflection theory is used, as in the Steadman method, the
error
introduced can be controlled to about 5% when the deflection ration is less
than 20%.
Due to the heterogeneity of pulp fibres, the flexibility values of each pulp
distribute
in a wide range (Figure 14). Therefore, the median value of flexibility for
each
sample is presented in Figure 22. The measured flexibility values of each pulp
sample were also compared using analysis of variance test (One-way ANOVA) with
SPSS (SPSS Inc., USA). The significance between any two samples is less than
0.001,
which indicates that this method is able to differentiate different types of
fibres
effectively with a sample size of about 50.
Freespan length by CLSM and LM
[0042] One advantage of using CLSM is that CLSM can accurately identify the
physical contact points from the transverse view of the fibre span, and hence
the
exact freespan length can be measured. With introducing the concept of
"neutral
bending plane" as discussed foregoing, the accuracy of the freespan length
measured is even greater. Another advantage is that the deflection height can
be
measured directly other than being assumed to be the diameter of the support
wire.
As reported by Lowe et al. [22], in some cases the overlaying fibre may
conform to
the support fibre by overlapping. Figure 9 illustrates the difference in
measured
freespan length by a CLSM and a light microscope (LM) for a perfect span
shape.
Due to light interference between the glass slide and the fibre, LM is only
able to
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identify the optical contact points so the measured freespan length is "L'"
measured
with a LM in comparison with "L" measured with a CLSM.
[00431 Figure 23 shows the difference in the average freespan length measured
with
CLSM and LM (Leica DM4500 microscope). For thick-walled and partially
collapsed
fibres (BCTMP and CTMP), the freespan measured by a CLSM (L) is up to 35%
larger. However, for flexible and thin-walled fibres, the freespan measured
with a
LM is greater than that measured with a CLSM. According to the illustration in
Figure 9, L is always larger than L', but this is only for a perfect
deformation shape.
In reality fibres do not always deform like that, and the deformation is not
symmetrical about the central point or the support wire. This is probably due
to the
non-uniformity of fibre wall structure. With CLSM, only the perfect
deformation on
the right side is measured since the shape can be seen. With LM, the freespan
lengths of both sides are measured without seeing the deformation shape. In
some
cases, as shown in Figure 15, the first contact point (A) can be missed due to
irregular deformation shape and the second contact point (B) is taken as the
contact
point, thus, the left half OB is much larger than the real freespan OA. It was
observed in this example that about 50% BCTMP fibres formed irregular
deformation; only perfect span shape was measured for calculating the fibre
flexibility. The rationale behind this is that if irregular deformation is
formed, that
means the operation or the result does not comply with the beam deflection
theory,
so the measurement or calculation would be invalid. This is another benefit of
using
CA 02630743 2008-05-06
CLSM. A light microscope cannot identify the irregular shape so all kinds of
deformation were measured which lead to errors in the measurements.
Fibre collapsibility and moment of inertia
[0044] Since CLSM can image the fibre cross-section directly, the
collapsibility and
the moment of inertia can be obtained when the fibre flexibility is measured.
Therefore, additional information on how the fibre collapsibility affects the
fibre
flexibility can be revealed. Figure 16 shows typical cross-section images of
fibres
obtained with CLSM. Thin-walled Spruce BKP fibres collapsed completely without
wet pressing (Figure 16a). The thick walled mechanical fibre (Birch BCTMP
fibres)
only collapse slightly after pressing (Figure 16c) compared to before pressing
(Figure
16b). Aspen BCTMP fibres originally collapsed partially before wet pressing
(Figure
16d). After pressing, they collapsed almost completely (Figure 16e). The
aspect
ratios (AR) listed in Figure 24 give quantitative information of the
collapsibility of
the different types of fibres during flexibility measurement.
10045] Once fibres collapse, the thickness of the fibre cross-sections reduce
greatly,
thus reducing the moment of inertia of the fibre. Therefore, the
collapsibility of
fibres affects the flexibility of the fibres through reducing the moment of
inertia.
Figure 9 shows the relationship between fibre moment of inertia and the fibre
collapsibility. When the fibre collapsibility is small, as for the BCTMP
fibres, a small
change of fibre collapsibility can cause significant change in fibre moment of
inertia.
21
CA 02630743 2008-05-06
It can also be seen in Figure 24 that both bleaching and refining increased
significantly the fibre collapsibility of Aspen fibres.
[0046] It can be seen from Figure 16 that fibre collapsibility is essentially
determined
by the fibre wall thickness. Thin-walled BKP fibres collapse completely
without any
wet pressing. The thick-walled Birch fibres collapse only partially even after
pressing. Figure 16 illustrates how the fibre wall thickness affects the fibre
collapsibility (AR) and, in turn, affects the fibre moment of inertia.
Fibre-Wall Modulus
[0047] With CLSM, the moment of inertia can be calculated with Equation 3.
This
makes it possible to measure the longitudinal elastic modulus of the fibre
wall. It
can be seen from Figure 24 that the elastic modulus obtained in this study is
within
the range of 1.4 GPa - 17.2 GPa for the softwood BKP and hardwood BCTMP. The
result is comparable to the elastic modulus of wet pulp fibres obtained with
micro-
tensile test, which is on the order of 10 GPa [20, 23]. For Spruce BKP, the
measured
elastic modulus of 1.4 GPa is slightly lower than 4.3 GPa reported by
Ehrnrooth [23]
for Spruce kraft pulp fibres. In addition to the difference in the types of
fibres, the
contribution of shear deformation to measured flexibility may lead to a lower
E
calculated from Eq, 3 due to the pure bending assumption, according to
Waterhouse
and Page [20].
[0048] Both bleaching and refining altered the elastic modulus of the fibre
wall
significantly (Figure 24). The elastic modulus of Aspen CTMP has been reduced
22
CA 02630743 2008-05-06
from 17.2 GPa to 2.3 GPa by bleaching and is then further reduced to 0.7 GPa
and 0.2
GPa by LC refining and MC refining, respectively. It is interesting to find
that the
elastic modulus does not much affect the fibre collapsibility. As shown in
Figure 24,
different Aspen fibres have almost the same AR but completely different E,
from 0.2
GPa to 17.2 GPa. This, on the other hand, further confirms that fibre wall
thickness
is the predominant factor in determining the fibre collapsibility.
[0049] In general, collapsed fibres are more flexible. For Aspen fibres, the
flexibility
is mainly determined by the elastic modulus, and the collapsibility has little
effect
since it does not change much. Bleaching and mechanical treatment altered
slightly
the collapsibility but improved significantly the flexibility. This new
understanding
may have significant impact on the use of BCTMP fibres in wood-free fibre
paper
grades and multi-ply board grades. In both cases, the major objective is to
increase
paper bulk by adding BCTMP fibres in the furnish. However, adding too much
BCTMP may reduce the paper strength. Bleaching does not increase the fibre
collapsibility, which means paper bulk can be maintained, but bleaching can
increase fibre flexibility through decreasing the elastic modulus of the fibre
wall,
thus increasing the bonded area among fibres. Therefore, the manufacturer may
adjust the pulp properties by modifying the bleaching process in BCTMP
manufacturing.
[0050] The following references are referred to in this application and are
incorporated herein by reference:
23
CA 02630743 2008-05-06
1. NILSSON, B., LARS WAGBERG and GRAY, D., "Conformability of wet pulp
fibres at small Length Scales". 12th Fundamental Research Symposium, p. 211
(2001)
2. JANG, H.F., "A theory for the transverse collapse of wood pulp fibres".
12th
Fundamental Research Symposium p.193 (2001)
3. SAMUELSSON, L.G., "Measurement of the stiffness of fibres". Svensk.
Papperstidn 15(1):S41-S46 (1963)
4. MOHLIN, U-K., "Cellulose fibre bonding Part 5: Conformability of pulp
fibres".
Svensk. Papperstidn 78(11):412-416 (1975)
5. KEREKES, R.J. and TAM DOO, P. A., "Wet fibre flexibility of some major
softwood species pulped by various processes". J. Pulp Paper Sci. 11:60-61
(1985)
6. KUHN, D.C.S., LU, X., OLSON, J.A. and ROBERTSON, A.G., "Dynamic wet fibre
flexibility measurement device". J. Pulp Paper Sci. 21(l):337 (1995)
7. STEADMAN, R. and LUNER, P., "The effect of wet fibre flexibility of sheet
apparent density ". 8th Fundamental Research Symposium p.211 (1981)
8. SEBORG, C.O. and SIMMONDS, F.A., "Measurement of stiffness in bending of
single fibres". Paper Trade Journal 113(1):49-50 (1941)
9. JAMES, W.L., "A method for studying the stiffness and internal friction of
individual fibres. Wood Sci. 6(1):30-38 (1973)
10. TAM DOO, P. A. and KEREKES, R.J., "Method to measure wet fibre
flexibility".
Tappi 64:113-116 (1981)
11. ZHANG, M., HUBBE, M.A., VENDITTI, R.A. and HEITMANN, J.A., "Effects of
sugar addition before drying on the wet flexibility of redispersed kraft
fibres". J.
Pulp Paper Sci. 30:29-34 (2004)
24
CA 02630743 2008-05-06
12. DELGADO, E., LOPEZ-DELLAMARY, F.A., ALLAN, G.G., ANDRADE, A.,
CONTRERAS, H., REGLA, H. and CRESSON, T., "Zwitterion modification of fibres:
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30:141-144
(2004)
13. KARNIS, A., "Mechanism of fibre development in mechanical pulping". J.
Pulp
Paper Sci. 20(1):280-288 (1994)
14. THE MATHWORKS INC., "Matlab Reference Manuel". (2004)
15. OTSU, N., "A threshold selection method from gray-level histograms". IEEE
Transactions on Systems, Man and Cybernetics SMC-9:62-6 (1979)
16. HARALICK, R.M. and Linda, G.S., Computer and robot vision, Addison-Wesley,
(1992)
17. JANG, H.F. and SETH, R.S., "Characterization of the collapse behaviour of
papermaking fibres using confocal microscopy". In the proceedings of the 84th
Annual Meeting of the Technical Section of Canadian Pulp and Paper
Association, p.
205 (1998)
18. MARK, R.E., Handbook of Physical Testing of Paper II, Marcel Dekker, NY,
(2002)
19. JANG, H.F., ROBERTSON, A.G. and SETH, R.S., "Measuring fibre coarseness
and wall thickness distributions with confocal microscopy". In the proceedings
of
78th Annual Meeting of the Canadian Pulp and Paper Association, Montreal,
Quebec, Canada, p. 189 (1992)
20. WATERHOUSE, J.F. and PAGE, D.H., "The Contribution of Transverse Shear to
Wet Fibre Deformation Behavior". Nordic Pulp and Paper Research Journal 19:89-
92
(2004)
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21. LAWRYSHYN, Y.A. and KUHN, D.C.S., "Large deflection analysis of wet fibre
flexibility measurement techniques". J. Pulp Paper Sci. 22(1):423-431(1996)
22. LOWE, R. RAGAUSKAS, A. and PAGE, D.H. "Imaging fibre deformations". In
Advances in Paper Science and Technology, Proc. 13th Fundamental Research
Symposium (S.J. I'Anson, ed.), pp. 921, FRC, Manchester, 2005.
23. EHRNROOTH, E.M.L. and KOLSETH, P., "The tensile testing of single wood
pulp fibres in air and water". Wood Fibre Sci. 16(4):549-566 (1984)
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