Paper Technology International 2020 - Journal - Page 58

PAPERTECHNOLOGYINTERNATIONAL
To assess the results of interlaboratory studies of sticky
measurements according to DIN SPEC 6745, the procedure
described in [5, 6] are applied. Two main parameters are considered
for this purpose:
•
•
Repeatability is defined as “an estimated limit below
which the difference between two test results is
expected to fall 95% of the time, when the results are
obtained at a single laboratory. Repeatability reflects
the variability inherent in the experimental system used
to determine its value(s).” [5]
Reproducibility is defined “an estimated limit below
which the difference between two test results is
expected to fall 95% of the time, when the results are
obtained in different qualified laboratories for the same
homogeneous source of material.” [5]
Due to the high number of manual working steps for sticky
measurements according to ISO 15 360 - 2/ TAPPI T277, the
repeatability and reproducibility were defined as “the maximum
expected difference between two test results” and are summarized
as follows [7]:
•
Repeatability within a laboratory: 28% maximum deviation
between two results if performed by the same lab technician.
•
Note: Within a recent Voith Paper study for three
different sample series (different sticky loads), the
complete sample preparation process was conducted
three times each by different lab technicians and
the repeatability was 39%. The calculation for the
repeatability was done as described in [5,6].
•
Reproducibility (between laboratories): 101% maximum
deviation between two results.
To increase the statistical reliability of sticky measurement results
according to ISO 15 360 - 2/ TAPPI T277, especially to prove
technological performance of machinery sold, the number of
measurements is increased and for guarantee runs within Voith
Paper, typically 12 measurements are performed for every sampling
position. Furthermore, it is important that all measurements in this
context are performed by one laboratory only. Due to aging effects
of stock samples, the effort to organize logistics such that samples
from overseas can be evaluated in time is high.
Exploring a new standard – measuring stickies according to
DIN SPEC 6745 using NIR
Measuring stickies according to DIN SPEC 6745: “Paper, board
and pulps - Recycling - Determination of stickies and non-tacky
contaminants in recycled pulps and papers applying near-infrared
measurement” [4] was introduced in 2020 and thus is relatively new
for the paper industry.
The sticky measuring procedure according to DIN
SPEC 6745 is shown in Figure 2. This procedure is very straight
forward, with the only steps to prepare the stock samples for NIR
measurement being hand sheet forming and subsequent drying. The
hand sheets can be analyzed by the NIR measurement device, as
briefly described in the following:
•
•
•
•
The sheets are placed under the NIR camera on the x-y
table of the NIR sticky measurement device.
The sheets are scanned in strips under homogeneous
lightning conditions and using a NIR camera. For every
pixel, the spectral information for wave lengths of 1,320
nm to 1,900 nm is recorded.
Using a NIR imaging approach [8], neighboring
pixels that cannot be assigned to fibers or fillers are
summarized as contaminants.
A further classification is made, and these contaminants
are classified and assigned to the sticky and nontacky contaminant groups according to the spectral
information measured by the NIR camera. For example,
PVAc, acrylates and EVA are chemical substances
assigned to the sticky contaminants, whereby, PE and
PS are chemical substances assigned to the non-tacky
contaminants.
Typically, four hand sheets are formed for each stock sample
and analyzed sequentially using the NIR measurement device. A
laboratory technician needs 40 to 50 minutes to evaluate a single
stock sample according to DIN SPEC 6745; that measuring time is
almost half of the time required to use the ISO 15 360 - 2/ TAPPI
T277 approach. Since the ability to make hand sheets is widespread
within the paper industry, it thus becomes possible to prepare hand
sheets anywhere in the world in a timely manner after sample-taking
and sending the dried hand sheets to a laboratory that is able to
perform NIR macro sticky measurements. This provides several
advantages in the daily business of a paper producer and a supplier
within the paper industry.
Due to the minimized number of manual working steps, the
accuracy of results improved using the NIR measuring procedure
according to DIN SPEC 6745 compared to ISO 15 360 - 2/ TAPPI
T277. The following preliminary results for repeatability, defined as
“the maximum expected difference between two test results” [7] for
the NIR method according to DIN SPEC 6745 are for this purpose
introduced:
Figure 2: Procedure to measure stickies according to DIN SPEC 6745
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