Improvement of wet and dry web properties in papermaking by controlling water and fiber quality

G5 Doctoral dissertation (article)


Internal Authors/Editors


Publication Details

List of Authors: Hanna Lindqvist
Publisher: Åbo Akademi University
Place: Turku
Publication year: 2013
ISBN: 978-952-12-2948-0


Abstract

Today's need for faster and more efficient processes in papermaking also
give rise to greater demands on the mechanical properties of the paper web.
Web breaks occur not only due to a low tensile strength of the wet paper
web; the reason can also be an insufficient web tension. Most problems with
the runnability of the paper machine occur during the wet pressing and/or
in the beginning of dryer section when the dry content of the paper is in the
range 30-70%. Web tension is created by a speed difference in the web
transfer between the press and dryer section. The web tension is not
constant after the straining in the open draw, but drops 20-60% due to
relaxation. Most of this relaxation occurs during the first 0.5 s after
straining.

In this work, the effects of different parameters on dewatering, wet web
strength and relaxation, as well as on the mechanical and surface properties
of the final product were investigated. A long term aim is to improve the
runnability of the paper machine and paper quality by controlling and
understanding the properties of the process water, wood fiber, and fiber
network.

By using a non-ionic surfactant at concentrations below the critical micelle
concentration (cmc), the dewatering was more efficient thereby increasing
the dry content after dewatering and wet pressing and thus increasing the
tensile strength of wet paper sheets. For dry papers, the density and the
tensile strength increased. At concentrations above cmc, the mechanical
properties for both the wet and the dry sheets deteriorated, one reason
could be that the surfactant hindered the formation of hydrogen bonding. A
lower pH during papermaking, resulted in a lower dry content, which might
be due to swelling of fibers. By using Na+ instead of Ca2+, the wet tensile
strength decreased, perhaps because the coefficient of friction was
decreased allowing the fibers to slide over each other.

The properties of the fibers were changed by using either gentle or harsh
refining. The effect of fines was studied by adding fines to the original pulp
or by removing the fines from refined pulp. The gently refined pulp
generated fines without a decrease in the fiber length. The fibers were more
stretched out and flexible after refining, which resulted in increased wet Today's need for faster and more efficient processes in papermaking also
give rise to greater demands on the mechanical properties of the paper web.
Web breaks occur not only due to a low tensile strength of the wet paper
web; the reason can also be an insufficient web tension. Most problems with
the runnability of the paper machine occur during the wet pressing and/or
in the beginning of dryer section when the dry content of the paper is in the
range 30-70%. Web tension is created by a speed difference in the web
transfer between the press and dryer section. The web tension is not
constant after the straining in the open draw, but drops 20-60% due to
relaxation. Most of this relaxation occurs during the first 0.5 s after
straining.

In this work, the effects of different parameters on dewatering, wet web
strength and relaxation, as well as on the mechanical and surface properties
of the final product were investigated. A long term aim is to improve the
runnability of the paper machine and paper quality by controlling and
understanding the properties of the process water, wood fiber, and fiber
network.

By using a non-ionic surfactant at concentrations below the critical micelle
concentration (cmc), the dewatering was more efficient thereby increasing
the dry content after dewatering and wet pressing and thus increasing the
tensile strength of wet paper sheets. For dry papers, the density and the
tensile strength increased. At concentrations above cmc, the mechanical
properties for both the wet and the dry sheets deteriorated, one reason
could be that the surfactant hindered the formation of hydrogen bonding. A
lower pH during papermaking, resulted in a lower dry content, which might
be due to swelling of fibers. By using Na+ instead of Ca2+, the wet tensile
strength decreased, perhaps because the coefficient of friction was
decreased allowing the fibers to slide over each other.

The properties of the fibers were changed by using either gentle or harsh
refining. The effect of fines was studied by adding fines to the original pulp
or by removing the fines from refined pulp. The gently refined pulp
generated fines without a decrease in the fiber length. The fibers were more
stretched out and flexible after refining, which resulted in increased wet
tensile strength and residual tension. The fines of fibril-type promoted the
formation of fiber-fiber bonds and increased the bonded area. The fines
were important for the wet tensile strength while the fiber properties, i.e.
the internal/external fibrillation, were critical for the residual tension.
Harsh refining resulted in shortening of the fiber length. The formed wet
web composed of damaged fibers contained more water, which resulted in a
decreased dry content after wet pressing. When the non-ionic surfactant
was used together with the harshly refined pulp, addition of surfactant gave
a shorter dewatering time. When the surfactant was added together with
fines to original pulp, the wet tensile strength decreased slightly. The most
likely reason was adsorption of the surfactant to the surfaces of the fines
and fibers which resulted in the particles and fibers sliding over each other
more easily.

Native galactoglucomannans (GGMs) were cationized (C-GGM),
carboxymethylated (CM-GGM), and iminated (A-GGM). Native GGMs
dispersed the fibrils on the fiber surface when added to the pulp. These
more outstretched fibrils were able to interact with fibrils on other fibers,
thereby increasing the wet tensile strength and the elastic modulus. C-GGM
acted most likely through an electrostatic mechanism, where the cationic
chain was adsorbed to negatively charged surfaces. CM-GGM was assumed
to form an ionic bond to metal ions, i.e. magnesium and calcium, present in
wood. A-GGM acted probably through the hydrophobic hydrocarbon chain
attached to the reducing end of the polysaccharide. The tail was oriented
towards hydrophobic particles; the paper containing A-GGM had the most
hydrophilic surface.

When native GGM, C-GGM and A-GGM were sprayed on freshly formed
papers and the surface was analyzed, characteristic signals were found for
each of the derivatives using Time-of-Flight Secondary Ion Mass
Spectrometry (ToF-SIMS). C-GGM penetrated though the paper, as signals
were also detected on the unsprayed side of the paper. A-GGM formed a
film on top of the fibers, and by using Fourier Transform Infrared
Spectroscopy (FTIR) it was concluded that the hydrophobic tail was
oriented close to the paper surface but not at the outmost surface.

The results obtained in this thesis form a valuable knowledge base, which
can be used for controlling runnability of the paper machine and the paper
quality. By using a readily available hemicelluloses and modifying them
using simple methods, additives with different properties and effects on
papermaking can be obtained.


Keywords

Dewatering, initial wet web strength, Non-ionic surfactant, Papermaking, Refining, Spruce galactoglucomannan

Last updated on 2019-09-12 at 02:55