Note: Descriptions are shown in the official language in which they were submitted.
CA 022~1347 1998-10-07
ILE, D'~ , T~ r~ AF~?"'
Tt'.. r"L.~
[67190/963245]
METHOD FOR DETERMINING THE MECHANICAL PROPERTIES OF PAPER AND
AN ASSOCIATED ARRANGEMENT
The invention relates to a method for determining the
mechanical properties of paper, in particular for measuring
the fiber bonds in paper. In addition, the invention relates
to the associated arrangement for carrying out the method,
using a spectrometer having a beam source, an optical system,
and a detector.
In the manufacture of paper and/or cardboard, it is necessary
to constantly monitor the mechanical properties for quality
assurance. These properties are a function of the kind and
number of the fiber bonds. For this purpose, the goal is the
direct measurement of the fiber bonds in the paper during
manufacture, which would permit an online adjustment of the
production process.
Thus far, direct measurement of the fiber bonds has not been
possible. No sensor is known with which direct measurements
can be made as to how well and how solidly the individual
cellulose fibers in the paper and/or cardboard are bonded to
each other. However, a series of measuring methods has been
proposed for monitoring the strength of paper, these methods
directly or indirectly relating to the fiber bonds in the
cellulose. Nevertheless, the measurements themselves are
influenced by the strength of the individual fibers, so that
no direct correlation with the desired measuring result
exists.
To date, laboratory analyses in the paper factory have been
conducted in an ongoing manner, for example, at the paper-
making machine, regarding the strength of the paper, in orderto assure a predetermined quality standard. For this purpose,
samples are taken from the moving paper web and are analyzed
in the laboratory for properties such as strength, resistance
to tearing, gas pressure, etc. Such measurements generally are
time- consuming and require qualified personnel.
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An online sensor has been already proposed, which, on the
moving paper web, measures the rise in the stress-strain curve
using the propagation speed of ultrasound in the paper. The
sensor is made up of an ultrasound source and two detectors at
varying distances. From the difference in propagation times of
the ultrasound impulses at the two detectors, the paper
strength can be determined. This value is generally determined
by the rise in the curve in the stress-strain diagram.
In the journal publications "The Paper" (1991), p. 45-51 and
"The Paper" (1993), p. 695-702, the p~ssibilities and limits of
the FTIR spectroscopy in characterizing cellulose have already
been reported on. In this context, in particular,
spectroscopic and electron microscopic analyses of the fine
structure of cellulose have been compared with each other. No
practical consequences can be drawn from this.
Furthermore, in Tappi Journal (1992), p. 147-149, assertions
are made to the effect that measurements can be obtained by
means of optical infrared measurements of the lignin content
of cellulose pulp, i.e., cellulose having a pulpy consistency.
The object of the invention is thus to indicate a method and
to create the associated arrangement which can be used for
measuring the fiber bonds in paper.
The object according to the invention is achieved through the
use of infrared spectroscopy, the presence of hydroxyl and/or
carboxyl groups, joined to each other on the fiber surface,
being optically determined and evaluated as a measure for the
mechanical properties of the paper. In the associated
arrangement, an evaluating unit is present, in which a
baseline correction of the spectra is carried out, and,
subsequently, on the basis of bands in the spectrum, the
mechanical properties are determined, in particular the
presence of fiber bonds.
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Thus the invention makes available for the first time a sensor
which meets the requirements of actual practice, which with
the help of infrared spectroscopy (IR) is capable of directly
measuring the fiber quality in paper. IR spectroscopy is, on
the one hand, a generally known method for producing chemical
characterizations of materials, and it has already been
proposed, as was mentioned at the beginning, for analyzing
cellulose. Now, however, using IR spectroscopy, the fiber
quality of paper and/or cardboard can be determined directly.
Within the scope of the invention, the non-trivial reflection
has been confirmed that the mechanical properties of paper are
determined mainly through the quality of the cellulose fibers.
Fiber quality is a measure for the fibers' property of forming
a stable interconnection with other fibers through solid
bonds, and thus it assures the mechanical properties of the
paper. According to the prevailing theory, the strength of
this interfiber bonding is determined by the concentration on
the fiber surface of hydroxyl groups and/or carboxyl groups,
which are linked to each other by hydrogen bridge bondings.
Fibers may be prevented from bonding by undesirable
impurities. In this case, free OH groups are present which are
not saturated through bonds. The greater the concentration of
saturated OH groups, the greater the strength of the paper.
Further properties and advantages of the invention emerge in
the following description of the Figures of exemplary
embodiments on the basis of the drawing in connection with
further dependent claims. Specifically,
Figure 1 shows an infrared spectrum of paper containing waste
paper,
Figure 2 shows the infrared spectrum of Figure 1 after a
baseline correction,
Figure 3 shows an infrared spectrum of wood-free paper, and
Figure 4 shows a block diagram having associated evaluation
units.
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Infrared spectroscopy is a standard procedure in tne chemical
industry for characterizing materials. For measuring paper,
the following specifically should be considered, taking into
account the remarks made at the beginning as to the
composition of cellulose fibers:
The resonant vibration frequency of free OH groups is in the
range of approx. 3700 cm~l. If two OH groups are joined via
hydrogen bridge bondings, the OH groups are hindered in their
vibration and the resonant frequency is shifted to the range
of approx. 3200 cm~1 to 3400 cm1 Therefore, by means of
spectroscopic measurement, it can be directly determined
whether OH groups are free or bound. By evaluating the
spectrum of paper, it can additionally be determined
quantitatively how many OH groups are contributing to the
fiber bonds and how many OH groups are free. The more OH
groups are bound to each other, the greater the strength of
the paper.
For the practical application, an infrared spectrometer should
be used, covering the range from 3500 cm1 to 3800 cm~1. A
conventional spectrometer, which is not depicted here in
detail, includes an optical system which guides the light from
a beam source and a detector. If appropriate, a so-called
Fourier-transform spectrometer can also be used, with which
the signals are emitted directly in processed form.
At an appropriate place, a paper sample is introduced into the
spectrometer and is either irradiated, by the infrared beam
(IR), or the diffuse reflection of the IR beam is measured. If
the sample is irradiated, which is only possible with papers
but not with cardboard, a transmission measurement occurs. A
reflection is also equally possible. In measuring the diffuse
reflection, on the other hand, the directly reflected light is
screened out and the diffusely reflected light is focussed
using concave mirrors and directed to the detector. For normal
use, the spectral resolution of the spectrometer must be
CA 022~1347 1998-10-07
better than 10 cm-1.
Figure 1 shows an IR spectrum 1 in the range of approx. 3500
cm~ to 3800 cm1, the IR spectrum having been obtained from
paper having a high waste paper content. First of all, using a
processing unit, a baseline correction is carried out on a raw
spectrum of this type, having a structure that can be
interpreted as bands, by means of which a corrected spectrum 2
according to Figure 2 is obtained. Latter spectrum 2 is
characterized by significant bands. A spectrum of this type
can be processed for the purpose of furt'-er evaluation by a
Fast Fourier Transformation (FFT).
The latter bands in the spectrum indicate the oscillations of
the free OH groups, i.e., of those OH groups which have not
undergone binding. These bands arise when not all OH groups
participate in the bonds, when the fibers are thus not
optimally bonded to each other.
Figure 3 shows a spectrum 3 of high-quality wood-free paper,
without a baseline correction already having been carried out.
However, it can still clearly be seen that none of the bands
depicted above are to be seen. This means that the fibers are
optimally bonded to each other.
In the corresponding block diagram according to Figure 4, a
unit 10 contains an IR spectrum which was measured at the
paper, for example, spectrum 1. Eleven depicts a unit for Fast
Fourier transformation (FFT), which is followed by a unit 12
for mathematical preprocessing. In the latter unit 12, the
signals are smoothed out and the already-mentioned baseline
correction is carried out. After the signals are deconvoluted,
it is possible to determine the line width and the intensities
as well as to carry out a component analysis.
In Figure 4, unit 15 contains a mathematical model, "Paper
Quality." This model correlates the results of the spectral
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analysis and the associated evaluation with the criteria of
paper quality or the like. Unit 15 controls unit 16, which
acts to determine the measures to be undertaken for assuring
the paper quality.
In infrared measurement, the sensor can be installed directly
at the moving paper web. This makes possible a rapid online
control of the paper quality. It is also possible to build the
sensor into a measuring frame which moves in a transverse
direction over the paper web, just as the measuring frame was
advantaseously proposed as transverse to the paper web, for
example, for measurements such as surface thickness and/or
humidity content. For a transmission measurement, the beam
source can be positioned in the upper part of the measuring
frame and the detector in the lower part. On the other hand,
in the case of measurements of the diffuse reflection, both
the beam source as well as the detector are in the upper or in
the lower part of the measuring frame.
Apart from the described online measurement, an offline sensor
would also provide a significant savings in time as opposed to
laboratory measurements. In this case, a sample is taken from
the paper web and is measured in the laboratory in an IR
spectrometer. Measurements of this type can be carried out in
a few minutes.
