Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR MEASURING THE COMPONENTS OF A COATING ON A MOVING
BASE MATERIAL
The invention relates to a rnethod for measuring components of a
coating on a moving base material by infrared measurement.
The measurement of the amount of coating on coated paper is one
of the most important paper quality measurements; in this description paper
means conventional paper and/or cardboard.
The amount of coating has conventionally been measured (US No.
5 338 361) continuously by absorption measurement of a near infrared range
(NIR). In absorption measurement of an IR range the intensity of an IR beam
absorbed by the coating or a quantity proportional thereto is determined as a
function of the wavelength of the beam; the intensity of the beam absorbed by
the IR beam measured by a wavelength corresponding to an absorption peak
of the component to be measured correlates with the amount of coating.
Measuring paper coating components by infrared measurement is known in
the art and will not be described here in greater detail.
The coating component to be measured is usually kaolin having ab-
sorption peaks in the NIR range. However, NIR measurement cannot be used
for measuring the other important coating component, calcium carbonate,
since calcium carbonate has no absorption peak in the NIR range. The total
amount of coating can then be calculated on the basis of kaolin measurement
assuming that the ratio between the amounts of kaolin and calcium carbonate
in the coating is constant. In reality, however, the ratio between the amounts
of kaolin and calcium carbonate is not always constant, but may vary. Thus,
the prior art method described above does not provide accurate results par-
ticularly in measuring the amount of calcium carbonate. In addition, the
method can only be employed when kaolin is used; when kaolin is not used as
a coating component said method cannot be employed at all.
US patent (No.) 5 455 422 describes a method in which the amount
of coating is measured by measuring, for example, the absorption peak of la-
tex at the wavelength 2.30 micrometers and the absorption peak of clay at the
wavelength 2.21 micrometers. Said patent further describes the measurement
of calcium carbonate by measuring the amount of backscattering at the wave-
length 2.09 micrometers. However, for measuring the amount of calcium car-
bonate said method is unreliable an inaccurate. The amount of calcium car-
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bonate could also be determined, for example, on the basis of kaolin
measurement assuming that the ratio between the amounts of kaolin and
calcium carbonate in the coating is constant. However, this is not always the
case and problems are created particularly if the kaolin content is low i.e.
below
approximately 20 % and the carbonate content correspondingly high i.e.
approximately 80 %.
EP publication (No.) 0 332 018 shows a method in which the amount of
kaolin in paper is measured by transmission measurement, for example, at
approximately the wavelengths 1.4 and 2.2 micrometers. However, it is very
difficult to determine by transmission measurement what the portion of the
coating in the measurement result is. Furthermore, the portion of calcium
carbonate has to be approximated in a manner described in the previous
chapter.
GB publication (No.) 2 127 541 shows how the amount of additives in
paper is measured by transmission measurement. The publication describes the
measurement of the amount of calcium carbonate by measuring the absorption
peaks at the wavelengths 11.54 micrometers and 11.77 micrometers. The
amount of coating cannot be measured by said method since the fillers in base
paper are included in the results. Furthermore, the absorption of paper can be
so
high that measurement through paper is not possible. Moreover, in its entirety
the accuracy of the measurement results is not good enough.
The present invention is directed towards elimination of the drawbacks
described above.
In a particular aspect, the present invention is directed towards the
introduction of a new method for measuring the components of a coating on a
moving base material by infrared measurement in such a manner that the
measurement is better applicable than previous measurements particularly for
determining various coating components and that the fillers on the base
material
do not cause problems in the measurement. In a further aspect, the invention
is
directed towards the introducing of a method for measuring coating components
on a moving base material, for example, paper coating components in such a
way that the measurement is not disturbed by a high absorption of the base
material, for example, paper.
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2a
The method of the invention is characterized in that the components of a
coating are determined by reflection measurement in the wavelength range 2.5-
12 pm in such a manner that the amount of calcium carbonate in the coating is
determined in the wavelength range 2.5-8 pm.
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The basic idea of the invention is that the coating components are
determined by reflection measurement of a middle infrared range. It is also es-
sential that the amount of calcium carbonate in the coating is determined in
the wavelength range 2.5-8 m.
The method of the invention provides an accurate and useful
method for determining paper coating components using reflection measure-
ment of the middle infrared range 2.5-12 m. The method is particularly appli-
cable to be used for measuring paper coating components i.e. for example
measuring kaolin at the wavelength 2.5-11 m, preferably at 8-11 m and/or
measuring calcium carbonate at the wavelength 2.5-8 m, preferably at 3.0-
7.3 pm. The measurement can generally be carried out at any wavelength in
the middle infrared range, for example, at 2.5-12 m.
The method of the invention is applicable for measuring the coating
components on a moving base material also from the surface of a roller of a
paper coating machine, a roller of a paper machine and/or generally from the
surface of a metal plate.
Reflection measurement i.e. measurement where a reflection
source and a receiver are on the same side of the object to be measured, can -
be carried out using specular reflection measurement whereby the measuring
beam is directed onto the surface of the base material as an oblique beam of
parallel rays and a parallel reflected beam reflecting from the surface of the
base material is detected by a detector i.e. the intensity of the reflected
beam
is determined as a function of the wavelength.
The invention can be particularly advantageously implemented as
diffusion reflection, in which case the measuring beam is directed towards the
object to be measured and the intensity of the radiation diffusely reflecting
from the object in all directions is determined as a function of the
wavelength;
the illumination of the object can also be diffusely implemented.
Reflection measurement of the MIR range provides a very good cor-
relation for the amount of coating components, particularly kaolin and carbon-
ate and mixtures of these. The correlation is also very good for various base
papers and/or cardboards in calibrating the measurement of their coatings.
The MIR range has many advantages compared with the NIR range. In the
MIR range high absorptions are achieved for all essential coating components,
in addition the peaks are sharp and the scattering is insignificant. The sharp
peaks are a desirable property as for the function of detectors of measuring
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apparatuses based on interference filters, and the small quantity of
scattering
allows the structural variation of the coating without the calibration
suffering.
The fact that the paper industry is willing to start using carbonate-based
coat-
ings, which cannot be directly measured in the NIR range, increases the sig-
nificance of the result.
The use of the method of the invention enables the implementation
of measuring apparatuses, which can be used for on-line measurement of pa-
per and/or cardboard coating components by paper and coating machines, for
on-line measurement of coatings on rollers of paper coating machines or pa-
per machines or generally on metal plates and as paper and coating research
tools in laboratories. Particularly the use of the method of the invention en-
ables the implementation of a small-scale measuring apparatus of the coating
amount variation for laboratory use by designing the optics of the apparatus
to
suit small-scale measurement. In small-scale measurement the size of the
measuring area can be 0.1-100 mmz, preferably 0.1-10 mmz, more preferably
0.1-2 mm2, and even smaller than this. Naturally the measuring area can also
be larger, for example, of the size about 1 cm2 (n = 1-10 or larger) as is
known
from prior art. The total amount of the coating to be measured or in small-
scale
measurement the coating amount to be measured may vary, for example,
between 3-40 glm2, preferably 17-25 g/m2.
In the following the invention will be described in greater detail by
means of the preferred embodiments with reference to the accompanying
drawings, in which
Figure 1 is a diagram showing diffusion reflection measurement,
Figure 2 is a diagram showing specular reflection measurement,
Figures 3 and 4 show reflectivity (%) i.e. the intensity of light meas-
ured as a function of a wavelength at the wavelength about 2-11 m,
Figures 5 - 9 show coating amounts determined according to the
method of the invention as a function of coating amounts determined by labo-
ratory measurements and
Figure 10 shows small-scale variation of coating amount measured
by the method of the invention, the measuring field being 1 mm2.
In Figure 1 a parallel measuring beam B, is directed using a lens 1
obliquely to a paper 2 to be measured, from which a parallel reflected beam B2
is directed using a lens system 3 to a detector 4. The measuring beam B, may
consist of light rays having various wavelengths, for example, 2-12 m. The
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CA 02279904 1999-08-10
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detector 4 determines the intensity of the beam B2 as a function of a
different
beam wavelength L The paper 2 may be stationary, for example, in laboratory
measurement, or it may be in motion, for example in a paper machine. When
the paper 2 moving in the paper machine is being measured the measuring
5 apparatuses are preferably located in a measuring frame traversing in a
cross
direction in relation to the direction of the paper 2 in which case the
measure-
ment can be performed at the width of the entire paper 2. The illumination of
the paper by the measuring beam B, and the detection of the reflected beam
BZ as a function of the wavelength X of the light are known in the art, and
will
therefore not be described in more detail here. In measuring the MIR range
specular optics can preferably be used, the specular optics being known per
se and therefore not described in more detail here.
Figure 2 is a diagram showing specular reflection measurement. An
incoming beam B, is directed, for example, as a beam of parallel rays at an
oblique angle towards the paper 2 and the beam B, of parallel rays reflected
from the paper is detected in a manner known in the art using the lens system
3 and the detector 4, the intensity of the reflected beam is detected as a
func-
tion of the wavelength k of the light.
Figures 3 and 4 graphically show the results of specular reflection
measurement performed as shown in Figure 2 compared with the reflectivity of
the base paper, a reflection spectrum of kaolin coated paper in Figure 1 and
carbonate coated paper in Figure 2 being shown in relation to the spectrum of
the base paper. The kaolin peak is shown in Figure 1 at the wavelength 8.5 -
10 m and the carbonate peak in Figure 2 at the wavelength 6.5 - 7 m. The
peak of styrene butadiene used as a binder is seen around 3 m (Figure 4).
The peaks are very pronounced since the reflectivity of coated paper at the
wavelengths of the peaks of the pigments is 5-11 times greater than the re-
flectivity of base paper.
A particularly advantageous measuring method is to measure the
amount of calcium carbonate by measuring the size of the absorption peaks
located in its wavelength range 2.5-8 m. Most preferably the size of an ab-
sorption peak or peaks located in the wavelength range 3 -7.3 m is meas-
ured. The absorption peaks of calcium carbonate are located, for example, at
the wavelengths about 5.55 m and 3.95 m.
It is further preferable to measure the amount of kaolin by measur-
ing the size of the absorption peaks located in its wavelength area 2,5 11 m.
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An absorption peak of kaolin is located, for example, at the wavelength about
2.7 m.
A series of coating amount measurements of kaolin and carbonate
coated papers was performed in order to find out how useful the measuring
method is. The measurements showed that the measurement peak response
increased to such an extent while the measurement angle of the coating
amount increased that a choice had to be made between measurement dy-
namics and penetration depth. As a compromise the measurements were
performed as diffusion reflection measurements, whereby the calibration
showed a better result than specular reflection measurement on account of a
good signal-to-noise ratio. Diffusion measurement is not either as sensitive
to
distance as specular reflection measurement.
The coating amount measurements were performed by a Bomem
FTIR spectrometer using a commercial diffusion reflection accessory (Harrick,
The Praying Mantis). 'During the measurement a sample was moved over a
measurement sample opening using an electric motor, the measuring spot
being about 3 mm in diameter. Corresponding components were determined
from the measured papers using laboratory measuring methods. In Figures 5- -
9 the component amounts determined by the method of the invention are
shown on y-axis and the corresponding component amounts determined by
laboratory determinations on x-axis. Figure 5 shows the measurement of kao-
lin coating coated on wood-containing base paper, Figure 6 the measurement
of kaolin coating coated on wood-free base paper, Figure 7 the measurement
of carbonate coating coated on wood-containing base paper, Figure 8 the
measurement of carbonate/kaolin coating coated on wood-containing base
paper and Figure 9 the measurement of kaolin coating coated on wood-
containing and wood-free base paper. A standard deviation (SEP) of the de-
terminations performed by the method of the invention and a standard devia-
tion (SEC) of the determinations performed by laboratory methods were cal-
culated from the results; the number of factors describes the number of vari-
ables used in the determinations performed by laboratory methods.
Figure 10 shows the variation of the measured small-scale coating
amount and the deviation of coating amount measurement when the meas-
urement was repeated twice from the same points of 1 mm in diameter.
The preferred embodiments are meant to illustrate the invention
without limiting it in any way.
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