Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
128578~
BACKGROUND OF THE INVENTION
a) Field of the invention
The present invention is concerned with a method
for determining the fiber orientation in a sheet structure
made of fibers or containing such fibers, such as a sheet of
paper, a cardboard, a sheet of textile or fiberglass,
etc
Moreparticularly, the invention is concerned with
a method for quantitatively and locally measuring the fiber
orientation anisotropy in a fibrous structure, and with a
device for carrying out such a method.
b) Brief description of the prior art
Quantitative measurement of the fiber orientation
anisotropy in a fibrous structure is very important in some
industries, such as, in particular, the paper making industry,
in order to control and possibly adjus-t the quality of the
paper being made, especially its strength, its behaviour to
moisture, its resistance to drying stresses and its dimensional
stability.
Indeed, it is well known that numerous fac-tors such
as parameters of manufacture or designs of machinery may
substantially influence the general orientat:ion of the fibers
ina fibrous struc~ure being made. [n the paper making industry
such factors are, by way of non-restrictive examples:
- the shape oE the lips at the outlet of the head
box from which exits the pulp;
- the differential velocity between the pulp and the
belt on which it is deposited;
- the vibration of the belt; and
- the subsequent calendering.
Of course, the non-uniform distribution and/or
orientation in the machine direction (M.D.) and cross-direction
(C.D.) of the fibers in the sheet of paper that is being
made because of one or more of the above listed factors,
significantly affects the strength and general behaviour
-- 1 --
128~;~88
of -this sheet and makes it of good or poor quality.
Several methods for determining the fibre orientation
and, preferably, quantitatively mesuring the fibre orientation
anisotropy, are known in the art and some of them are commonly
used in the industry.
By way of examples, fibre orientation anisotropy
in paper can be mesured directly by incorporating to the pulp
a certain amount of dyed fibres and observing these fibres
with a microscope in the finished sheet (see CROSBY, C.M.
et al, Tappi, 103-106, March, 1981). However, such a method
is combersome and cannot be used easily in a production plant.
Another method of measuring fibre orientation
anisotropy is the one known as "zero-spantensile strength
test" (see ANCZUROWSKI, E. et al, Pulp and Paper, 112-115,
December 1983). This method which basically consists in
measuring the force necessary to tear apart a piece of paper
hold between two pairs of jaws is known to be very reliable,
and is relatively easy to apply in normal paper production by
using it on selected samples. On the other hand, this method
is destructive and statistical in nature and thus cannot be
used to find out how the fibre orientation anisotropy varies
from point to point in the sample, and how this variation
is related to other physical properties.
Several methods have also been proposed for measuring
local variations of fibre orientatlon anisotropy, based on the analysis of
the diffraction and/br scattering patterns of a visible laser light
incident on paper (see, RUDSTROW L. et al, Svensk Papperstid-
nig, 117-121, March 1970). This technique which has been
successfully developped to industrial standards, has -the
advantage of being non-destructive, but its basic principle
calls for a very sophisticated data analysis in order to
attain the requested level of precision. Moreover the visible
light diffraction depends mostly on the surface condition
and does not give any indication regarding the average anisotropy
through the whole thickness of the sample.
French laid-open patent application No 2,514,494
~28S788
MICRAUDEL SARL discloses a method of determining the fibre
orientation anlsotropy of a fibrous structure, which is also
based on the analysis of the diffraction patterns of a polarized
laser light traversing the fibrous structure. The optical
pattern of the structure, which is so obtained, is recorded
and interpreted to determine the actual distribution of the light
intensity as a function of the angular position of the plane of
polarization of the laser light, or of the structure.
In practice, such an interpretation which is not
disclosed is the French laid-open application is very complicated
and slow to carry out, -thereby making this method not usable on
line. Moreover, this method can only be used with thin paper
to avoid that too much extinction of the laser light
through the paper makes the diffractions pattern difficult or
even impossible to record and interpret.
Still another method of measuring paper anisotropy
is based on ultrasonic velocity measurements (see FLEISCHMAN,
E. H. Ph.D. thesis, The Institute of Paper Chemistry, Lawrence
University, 1981, or BAUM A.G. et al, Tappi, vol. 62, No. 5,
May, 1979). This technique is interesting in that it can be
used in line, but it usually has a spatial resolution around
3 cm and its results are dependent on many mechanical characteris-
tics of the paper sheet (young's modulus, density, etc.)
Last of a:Ll, a further method for determining the
Eibre orientation in paper is disclosed in V.S. patent 3,807,868
to VALMET OY. According to this mcthod, a polariæed laser light
beam is directed at right angles to the surface of the paper
and the intensity of the reflected light is measured by means
of two polarizers. This technique is interesting in that it
can be used on line and it gives an index value for the fibre
orientation anisotropy. However, i-t has the drawback of being a
surface measurement technique, which gives no indication regard
ing the average anisotropy through the whole thickness of the
sample.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a
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non-destructive method for quantitatively measuring the
fibre orientation anisotropy of a fibrous structure, which
method is fast and easy to carry out and can be used on
line in any industrial machine for manufacturing sheet of
fibrous material, such as a paper making machine.
Another object of the invention is to provide such
a non-destructive method which gives very precise results
that are independent of the surface condition or mechanical
characteristics of the fibrous structure, and are fully
correlated with the results obtainable with standard technique
such as the well-known zero-span tensile strength test.
A further object of the invention is to provide
such a non destructive method which makes use of a far infra-
red or submillimeter laser beam traversing the fibrous structure
whose anisotropy is to be measured, thereby giving an indication
of the average fibre orientation anisotropy through the whole
thickness of the sample.
Still another object of the invention is to provide
a device for carrying out the above non-destructive method,
which device can be implemented for use in labs or on line
on an industrial machine.
In accordance with the invention, these objects
are achieved with a method for measurlng the fibre orientation
anisotropy in a fibrous structure, comprlsing the steps of:
- directing a linearly polar:i~ed, far-infrared
laser beam having a wavelellgth preferably ranging between
50 and 2000 micrometers towards one side of the fibrous
structure whose fibre orientation anisotropy is to be measured;
- measuring the incident energy of said laser
beam before it traverses the fibrous structure;
- measuring the transmitted energy of the laser
beam on the other side of the fibrous structure with a first
laser beam energy detector;
- determining the transmission coefficient T in at
least two different orientations of the polarization plane of the
beam with respect to the flat fibrous structure, this coefficient
~28si7a8
T being the ratio of the transmitted-to-incident energies;
- provided that both reflection and scattering of
the laser beam by the fibrous structure remain small as com-
pared to absorption and, thereby, that:
-Ab
where e is 2,71828; A is the absorption coefficient of the
structure and b is the basis weight of the structure, calcu-
lating the ratio ~ of the absorption coefficients A measuredin the at least two different orientations of the polari-
zation plane of the beam; and
- using the so calculated absorption coefficient
ratio ~ as a quantitative evaluation of the amount of aniso-
tropy, the value of this absorption coefficient ratio ~ beingequal to l when no anisotropy is present in the fibrous
structure.
In accordance with a preferred embodiment of the
invention, the incident energy of the laser beam is measured
with the first laser energy detector before insertion of the
fibrous structure, part of the incident energy of the laser
beam being diverted towards a second laser beam energy
detector. This diverted incident energy measured with
said second detector, is permanently used to normalize to
the original value of the incident energy measured.
The fibrous structure may be rotated with respect
to the polarization plane of -the laser beam in order to
achieve determination of the transmission coefficient T in
at least two different orientations. Alternatively, the
polarization plane of the laser beam may be rotated with
respect to the flat fibrous structure in order to achieve
this determination. In both cases, provided that the method
be used on a fibrous structure having a known machine
direction and a known cross-direction, the transmission
coefficient T is determined in two orientations: one
lX~38
orientation being parallel to the machine direction of the
structure, the other orientation being parallel to the
cross-direction of this fibrous structure. If the machine
direction of the structure is not known, the coefficient T
will have to be determined in more than two orientations,
such as, for example, in ten different angular orientations
ranging from 0 to 90.
In accordance with another preferred embodiment
of the invention, the method may comprise the additional
steps of:
- splitting the linearly polarized laser beam
before it traverses the fibrous structure;
- modifying the angle of polarization of one of
the splitted beams before it traverses the fibrous structure;
- measuring the transmitted energy of both of
these splitted beams on the other side of the fibrous
structure; and
- using these two measurements to determine two
transmission coefficients T, each of said coefficient T
corresponding to an orientation of the polarization plane
of the laser beam.
This other embodiment is particularly useful in
the industry, as it can be carried out on-line during the
process of manufacturing the fibrous structure, and thus
used as a ~uality control tool with a possible feedback on
the fabrication parameters to correct any d:iscrepancy noted
on the course of operation.
As can now be be-tter understood, the method according
to the invention is based on the observation made by the
inventors that the transmitted-to-incident energy ratio of
a penetrating laser beam is dependent on the relative orienta-
tion of the fibres of a fibrous structure and of the plane of
polarization of the laser beam.
As can also be understood, this method is fast,
easy to carry out an non-destructive, thereby making it
usable as control means in an industrial machine, such as
a paper making machine.
lZBS~88
The invention also proposes a device for measuring
the fibre orientation anisotropy in a fibrous structure,
which device comprises:
- a far-infrared laser for directing a laser beam
towards one side of the fibrous structure whose fibre orienta-
tion anisotropy is to be measured;
- a linear polarizer for linearly polarizing the
laser beam before it reaches the fibrous structure;
- means for measuring the incident energy of the
laser beam before it traverses the fibrous structure;
- a first laser beam energy detector for measuring
the transmitted energy of said laser beam on the other side
of the fibrous structure;
- means for differently orientating the polariza-
tion plane of the beam and fibrous structure with respect
to each other; and
- processing means for determining the transmission
coefficient T in at least two different orientations of the
polarization plane of the beam with respect to the Eibrous
structure, this coefficient T being the ratio of the
transmitted-to-incident energies and then, provided that
both reflection and scattering of the laser beam by the
fibrous structure remain small as compared to absorption
and, thereby, that:
T - e ~b
where e is 2,71~2~; A is the absorpt:ion coefficient of the
fibrous structure and b is the bas.is weigh-t of the structure,
calculating the ratio ~ of the absorption coefficients A
in said at least two different orientations of the polariza-
tion plane of the beam, this ratio ~ giving a quantitative evaluation
of the amount of anisotropy present in the fibrous structure.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood upon reading
the following non-restrictive description given with reference
to the accompanying drawings, wherein:
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. ::,. ' "''
128~8~
Fig. 1 is a diagrammatic representation of a device
according to the invention for use in a laboratory;
Fig. 2 is a diagrammatic representation of
the electronic processing unit of the device of Fig. l;
Fig. 3 is a perspective representation of the main
elements of the device shown in Fig. l;
Fig. 4 is a graph obtained with the device of
Fig. 1, showing the high correlation between the measurement
technique according to the invcntion and the zero-span tensile
strength test for various paper samples;
Figs. 5(a) to 5(b) are graphs giving the
transmission coefficient versus the angle ~ between the
incident wave plane of polarization and a paper sample having
a known machine direction (M.D.), with a single sheet of
paper (a); two sheets of paper with parallel M.D. (b) and
two sheets of paper with perpendicular M.D. (c);
Fig. 6 is a graph showing the anisotropy ratio
obtained by the submillimeter mesurement (SMM) according to
the invention as a function of position for a sample made of
six adjacent paper strips; the measured zero-span angle and
strip widths for each paper strip being represented by the
line segments;
Fig. 7 is a graph showing the SMM anisotropy ratio
as a functlon oE posit:ion for a newsprint sample, the calculated
equivalent zero-span angle being shown on the righ-t axis;
Fic3. ~ :is SMM anisotropy ratio as a function of
position for a newsprint sample possibly affec-ted by a periodic
variation of fibre orientation;
Fig. 9 is a graph showing -the SMM anisotropy ratio
as a function of position for a liner board sample showing
visible surface streaks, the position of these streaks
corresponding to the shaded areas;
Fig. 10 is a graph showing the different measurement
of the SMM ratio as a function of posi-tion for the same
liner board sample;
~.2HS7~8
Fig. 11 is a diagrammate representation of a
device according to the invention for use on-line in a
paper making machine; and
Fig. 12 is a perspective representation of the
main elements of the device shown in Fig. 11, according to
a variant thereof.
DESCRIPTION OF DIFFERENT PREFERRED EMBODIMENTS
The device according to the invention as shown in
figs. 1 to 3 comprises a far infrared laser 1 producing a
beam of coherent electromagnetic radiations or waves having
a wavelength depending on the substance in the laser
cavity, the dimensions of the cavity and the way it received
its energy. By way of example, use can be made of a submil-
limetric laser 1 using methanol as active substance, which laser is pumped
by a grating tunable pulsed CO2 infrared laser 3 via a piezo-
controlled coupler 5 and Ge-lens 7. This laser produces
electromagnetic waves in the 70 to 1200 micrometres wave-
length range. However, it should be noted that other wave-
length range may be obtained, which vary from 50 to 2000
micrometers depending on the kind of far-infrared laser used
for producing the electromagnetic waves.
In the tests reported hereinafter, the laser 1
was a methanol submillimeter laser and was operated in a
pulse made at a frequency of 10 llz with a pulse duration
of about 500 microseconds. The peak power output was
approximatively 20 milliwatts while the average power level
stood around 20 microwat-ts.
The laser beam 11 can be concentrated on an area
of a few square millimeters at shorter wave length, by
means of a lens 13.
It is worth noting that the large variation or
wavelength possible with such a laser makes it very appropriate
for paper transmission measurements. Tests carried out by
the inventors have shown in the method according to the
lZ8~38
invention can be used with paper or other similar material
having a basis weight varying from 20 g/m2 to more than
2000 g/m2.
The laser beam ll emitted by the laser l is then
linearly polarized by a linear transmission polarizer 15
installed at the laser ou-tput. A beam splitter 17 is
provided between the polarizer 15 and the lens 13 to divert
part of the incident energy of the laser beam toward a
high sensitivity, laser beam energy detector 19 that can be
a Golay cell. If desired, a Fabry-Perot interferometer 21
can be provided between the beam splitter 17 and the
energy detector 19 to monitor the wavelength of the laser
beam.
Another high sensitivity, laser beam energy
detector 23 is provided to collect the transmitted energy
behind the fibrous structure which can be, for example,
a piece of paper whose fiber orientation anisotropy is to
be measured.
In one embodiment of the invention shown in fig.
3, the fibrous structure to be inspected is mounted on a
goniometer (mobile protractor) so that it may be rotated
with respect to the plane oE polarization oE the laser
beam. Alternatively, the whole measuring device can be
mounted onto a rotable frame and then rotated with respect
to the fibrous structure so as to make different measurement
at different angles between the plane ofpolarization of the
laser beam and the genera:laxis oE the fibrous structure.
The latter embodiment can be used, for example, to measure
the fibre orientation anisotropy of sheet of paper while
the same is manufactured by a paper-making machine.
The outputs of the detectors 19 and 23 are fed
to a microcomputer based data acquisition and analysis
system. This system which permits real time analysis, is
schematically illustrated in fig. 2 and will not be further
- 10 -
~Z8~i~8~3
detailed, as its basic structure is not an essential point
of invention.
As aforesaid in the "Summary in the Invention"
hereinabove, the method according to the invention is based
on the observation made by the inventors that the transmis-
sion coefficient T of a penetrating linearly polarized
laser beam is dependent on the relative orientation of the
fibres of the fibrous structure to be inspected with
respect to the plane of polarization of the laser beam.
This transmission coefficient T can be defined as being the
ratio of the transmitted-to -incident energies measured, by
way of example, by the detector 23 before and after the
fibrous structure 25 is positioned to intersect the laser
beam 11. This coefficient of transmission T is, in
practice, directly related to the basis weight "b" of the
fibrous structure by the equation:
-Ab (I)
where e is 2.7l828; b is the basis weight of the fibrous
structure and A is the absorption coefficient of this
structure.
It is noteworthy that equation tl) is valid only
if both reflcction and scattering oE the submillimetre waves
~5 by the tested structure are small enough as compared to
the absorption. Ilowever, it appears that this particular
condition is satisfied provided that use is made of a far
infrared and preferably submillimetre laser.
It is also noteworthy that equation (I) assumes
that the energy which is not transmitted is in fact
absorbed by the tested struc-ture, and that, consequently,
the transmitted energy decreases exponentially with the
basis weight of this structure.
Based on the above observation, the inventors
~28578~3
have also found that the ratio of -two different transmission
coefficients T measured with different orientations of the
wave plane of polarization, gives a quantitative evaluation
of the fibre orientation anisotropy of the fibrous structure.
In the case of a sample having a machine direction (M.D.)
and a cross-direction (C D), such as a sample of paper
industrially manufactured, the ratio of the transmission
coefficient Tm measured with the plane of polarization
parallel to the MD of the sample, to the transmission
coefficient Tc measured with the wave plane of polarization
perpendicular to the MD of the sample, that is parallel to
the CD of the sample, gives a quantitative evaluation of
the amount of anisotropy. This ratio is equal to 1 when
no anisotropy is present.
As the ratio Tm/Tc depends on the basis weight,
two superimposed samples may yield a larger relative
variation of transmission. To avoid this problem, use is
made , in accordance with the invention, of another ratio,
namely the ratio of the absorption coefficient A measured
in one orientation to the absorption coefficient A measured
in another orientation.
By way of example, provided that use is made of
a sample having a machine direction (MD), use is made, in
accordance with the present inven~ion, oE the Eollowing
ratio:
loge Tm -Amb Am
loge Tc -Acb Ac
wherein Am is the absorption coefficient in the machine
direction and Ac is the absorption coefficient in the cross-
direction.
Keeping in mind the above equation, the fibre
orientation anisotropy of a fibrous structure can be measured
as follows, using the device disclosed hereinabove.
- 12 -
128578~3
The linearly polarized laser beam 11 produced
by the laser 1 is aimed at the fibrous structure 25
which is, for example, a paper sample perpendicular to the
beam axis. The ratio of the energy transmitted by the
paper to the incident energy is measured while the sample
is rotated around the beam axis, using the goniometer 27
to do so. As aforesaid, the transmitted to-incident energy
ratio varies as the angle between the plane of polarization
of the beam and the paper machine direction of the sample.
A minimum usually occurs when this angle is 0 and the
maximum is observed at 90. The relative values of this
maximum and minimum,once corrected for basis weight, using
the above indicated exponential absorption model, is a
valuable and useful indication of the amount of fiber
orientation anisotropy in the sample.
To validate this method, comparative tests were
carried out on paper samples with and without fillers, whose
- fibre orientation anisotropy had previously been measured
with the standard, zero-span tensile strength test. The
result of the comparative tests are reported in fig. 4
and clearly shows that the anisotropy ratio d_measured with
the device according to the invention is highly correlated
to the fiber orientation anisotropy as measured with the
zero-span tensile strength test.
A limitation of the experimental procedure
described is the incorrect results it would yield if the
paper sample has a fibre orientation anisotropy which is
not symetrical about its MD. In such a case, two measure-
ments with beam polarisation first parallel, then perpen-
dicular to MD would not indicate the real level of fibre
orientation anisotropy. To overcome whis problem one can
~285788
take many measurements at each point with beam polarization
being rorated in small increments; such a procedure requires
more time, but it reveals the true fibre orientation distri-
bution.
Thus, an alternative way of measuring the
absorption anisotropy ratio ~ which is more suitable for
large paper sample or adaptable onto paper making machine
consists in rotating the polarizer 15 or the whole device
rather than the paper sample 15. Then, a subsequent
transversal displacement of a sample allows a point-to-point
anisotropy measurement. Of course, all the energies and
position values can be gathered through the microcomputer-
based data acquisition system shown in fig. 2.
When the machine direction and cross-direction
of the sample are not known to the operator, it is
necessary to measure more than two transmitted-to-incident
energy ratio T in order to determine the machine direction
of the sample prior to calculating the anisotropy ratio ~ .
By way of example, measurement of the transmitted energy
can be done onto the sample 25 while the sample angle is
varied from 0 to 90 in 10 steps. At each angle, the
transmitted-to-incident energy ratio T is determined by
averaging over 50 laser pulses ( 5 seconds) in order to in-
crease the precision. Ilowever, with a suitable stabilized
laser, a single pulse may be sufficient.
In a typical measurement run, the incident
energy of the laser beam can be measured with the
detector 23 before insertion of the sample 25, and stored
in the memory of the data acquisition system. Thereafter,
the diverted part of the incident energy of the laser beam
ll measured by the lateral laser beam energy detector 19
can be used for permanently normalizing the value of the
incident energy stored in the data acquisition system.
Figs. 11 and 12 show another embodiment of
- 14 -
128S78~3
the invention especially designed for use on an industrial
machine, such as, for example, a paper making machine.
Referring to fig. 11, this device comprises a
far infrared laser 1' producing a beam of electromagnetic
radiations 11". The beam 11" is linearly polarized by a
linear transmission polarizer 15' installed at the laser
output. A first beam splitter 29 is provided to split
the linearly polarized beam 11" before it reaches the fibrous
structure 25' to be inspected. The one splitted beam lla'
is directly aimed at a point 35 of the fibrous structure 25'
and its transmitted energy is measured by a laser beam
energy detector 23a'. The other splitted beam llb' is
reflected on a mirror 31 and passes through a polarizing
unit 33 wherein its original axis of polarization is ro-tated
by 90. The so "rotated" beam llb' is aimed at the same
point 35 of the fibrous structure 25' as the splitted beam
lla' and the transmitted energy is measured by another
energy detector 23b~ located behind the fibrous structure
25'.
As can be easily understood, the separate detectors
23a' and 23b~ both act as the first laser beam energy
detector 23 of the embodiment shown in fig. 1, and simul-
taneously measure the transmitted energies of both of the
splitted beams on the other side oE the fibrous structure
25'. The so measured transmitted energy oE each of the
splitted beam can be supplied to the data acquisition
system so that two transmission coeEEicients T are calcul-
ated, one of these coefficients T corresponding to the
machine direction of the fibrous structure while the other
coefficient T corresponds to a cross direction of this
structure.
Of course, means comprising a beam splitter
17a' or 17b! and a second laser beam energy detector 19a'
or 19b' may be used for normalizing the value of the incident
12OE~88
energy stored in data acquisition system, as was explained
hereinabove.
In a variant of this embodiment shown in fig. 12,
use can be made of two parallel lasers la' and lb' to
produce two orthogonally oriented laser beam lla' and llb',
instead of using one single laser 1' as shown in fig.
11. In this variant, both of the beams lla' and llb' may
be aimed at a same point of the fibrous structure 25' or
may be aimed at two different poin-ts spaced apart in the
machine direction of the fibrous structure 25'. In this
particular case, a timer may be incorporated so that the
transmitted energy is measured by the detector 23a' at the
same point as it was measured by the detector 23bl~ due to
the linear displacement of the structure 2S'.
EXPERIMENTAL RESULTS
1) Confirmation of the validity of the definition
of transmission coefficient T
Using an experimental device as shown in fig. 1,
different experiments were carried out with two similar
paper samples.
In the first experiment reported on fig. 5(a),
only one sample was mounted on the goniometer and the transmis-
sion-to-incident energy ratio T was determined while the
sample angle was varied from 0 to 720 in 10 steps. The
results reported in fig. 5(a) shows a cos2~ behaviour
where the amplitude, 11~ of the average transmission,
indicates that the paper is not ideal polarizer but a
polari~er nonetheless.
In a second experiment, the samples had their
machine direction parallel (see fig. 5(b)) whereas, in a
third experiment, the machine direction of the samples
were crossed at 90 (see fig. 5(c)). In the curves of fig.
5(b) and 5(c), the average transmission is much lower because
of the increased thickness. However, one can see from the
128S7B8
relative amplitude of the variations (22% of the average
transmission) that when the MD are parallel, the polarizing
effect is enhanced while it diseappears when the MD are
crossed.
2) Other tests
In the other test results reported hereinafter,
the incident and transmitted energies, were firs-t determined
without paper-sample, in both horizontal and vertical
polarization, for normalization purposes.
Then, with vertical polarization and the paper in
place, the beam pulses energy was measured while the
transverse position of the sample was varied over a 30 cm
span in 5 mm increments. Then the sample was moved back
to its exact initial position and the measurements were
repeated with horizontal polarization. The sample MD was
usually horizontal. Repeated experiments have shown that
the experimental, device used by the inventors could
measure 1~ variations in energy and 0.2 mm displacements.
In such measurements, the spatial resolution on
fibre orientation variations depends mostly on the SMM
(submillimetre) laser beam size which can be varied from a
few square millimetres to severa] square centimetres.
Experimentation led the inventors to fix their choice on a
~eometry where 90% of the beam energy was concentrated
within a 1 cm diameter circle, so that this geometry was
kept for most of the work.
A first experiment was carried out on a sample
made of narrow strips of paper mounted side by side. These
strips were cut from paper sheets whose average fibre
orientation had been previously measured with both the
zero-span and the SMM laser technique. The results obtained
by scanning this sample are shown on Fig. 6, together with
the expected values from the previous measurements. These
results confirm that the method according to the invention
- 17 -
~S~8
can detect abrupt variations of fibre orientatlons despite
the rounding caused by beam size, provided that the
variations are large enough. Average values on a small
sample may vary significantly from values obtained over
large areas.
Then, the same method using 70 microns waves, was
applied to two different newspring samples (Fig. 7-8). On
both graphs, the expected precision on the anisotropy ratio
is shown. With a difEerent wavelength, 119 microns, a liner
board sample (165 g/m ) was also tested (Fig. 9-10). On this
last graph, the results of two different measurements made
with the same sample at different times were plotted -to
illustrate the repeatability of the measuring technique.
The large variations of the SMM anisotropy
coefficient as shown on Fig. 6 are caused by the fact that
adjacent paper strips had their machine direction perpendi-
cular to each other. The rounding effect of the beam size
is clearly illustrated; it almost mask the presence of
the 6 mm wide strip # 5. This sets the lower limit of the
spatial resolution achievable with such experimental
conditions.
The results presented on Fi~. 7 and Fig. ~ were
both obtained with production newsprint scanned in machine
direction. Fourier analysis oE these anisotropy variations
should reveal any periodic pattern. At first glance, the
sample corresponding to Fig. 5 seems to be affected by a
periodic variation of fibre orientation, the periodicity
being close to 2.5 cm. This kind of information could
lead to the actual cause of such a variation and its
eventual elimination.
On the liner board sample whose fibre orientation
variations are shown on Fig. 9, one could observe, under
standard ligthing conditions, darker streaks crossing the
SMM surveyed line. The position of those streaks correspond
1285788
to the shaded areas on Fig. 8. There is some evidence of
a relation between those streaks and fibre orientation
variations, evidence that was conf:irmed in similar cases by
microscopic photography. This experimental procedure yields
fair repeatability as can be observed on Fig. 10 showing a
different measurement made later on the same area of the
same liner board sample.
As aforesaid, a major advantage of the method
according to the invention is the fact that it is non-
destructive, and inherently fast.
Another major advantage of the method according
to the invention is its repeatability that was tested by
performing the measurement on the same sample at a few weeks
intervals. The comparison of these measurements has
lS shown no significant difference between the various results.
A further major advantage of the method according
to the invention is its precision which seems to be superior
to that obtained with the well-known zero-span tensile
strength test.
Another major advantage of the method according
to the invention is that the beam traverses the fibrous
structure whose anisotropy is to be measured even when this
structure has a basis weight up to and even higher than
2000 g/m2, thereby giving an indication of the average fibre
orientation anisotropy through the whole thickness of the
sample,
In the above descriptlon, reference has been
made to energy detectors exclusively. It is however
obvious for anyone skilled in the art that use could also
be made of power detectors such as diodes, to determine the
value of the ratio T.
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