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AKD sizing: dispersion colloidal stability, spreading and sizing with pre-flocculated dispersion
FöRFATTARE
Mattsson, Rosa
DATUM
2002-02-07
INSTITUTION
Kemi och metallurgi / Kemisk apparatteknik
SAMMANFATTNING It was shown that anionic polymers and surfactants could seriously affect
the colloidal stability of the dispersions, while shear and electrolyte
concentration had a small influence. Dispersions with a high surface charge
flocculates easier than dispersions with a low surface charge in process
water from the wet-end of a linerboard paper machine. The flocculation rate
is fast and occurs on the same time scale as deposition onto fines and
fibers. The AKD agglomerates could be up to 30 µm in diameter even at high
levels of shear.
The AKD diffusion coefficient in paper sheets was estimated to be 10-11 m2/s
at a temperature above the melting point of the wax. The results indicate
that extractives do not interfere with the AKD migration. A reasonable
explanation is that migration occurs via the vapor phase.
It was also shown that sizing with flocculated AKD improved the sizing
efficiency significantly due to increased retention. This clearly indicates
that migration is efficient and that agglomerates can be used for internal
sizing provided that the flocculation process is controlled. The
conventional picture that the sizing agents must be very evenly distributed
to achieve efficient sizing could, from the viewpoint of these results, be
questioned.
The process of sizing with pre-flocculated AKD dispersions is the subject of
a patent application.
Even if this thesis is focused on AKD dispersions, the obtained knowledge
should be possible to transfer to other sizing systems. The emulsification
process for rosin and ASA is based on the same principles as for AKD.
Alkylketene dimer, AKD, dispersions are used in the paper industry to obtain
paper grades with a hydrophobic character i.e. resistance to sorption of
water. AKD is used in neutral to alkaline internal sizing, pH in the
interval of 6-9. AKD dispersions consist of wax particles emulsified in
water. The particles are electrostatically and in most cases also sterically
stabilized by cationic polyelectrolytes. The size of the particles is
usually in the range of 0.5-2 µm in diameter. The idea of having cationic
particles is that they are supposed to be deposited onto the negatively
charged fiber surfaces, heterocoagulation. An increased usage of recycled
pulp and a closure of the wet-end circuits of the paper machine will lead to
increasing levels of dissolved and colloidal substances. These substances
could be detrimental by affecting the AKD deposition onto the fiber material
and the colloidal stability i.e. induce flocculation of sizing particles.
Flocculation could lead to an inhomogeneous distribution of the sizing
agent. The aims addressed in this thesis were the following:
The colloidal stability of the dispersions was tested under shear by
measuring the particle size distribution and the microelectrophoretic
mobility. The migration of AKD through paper sheets with defined thickness
and through paper sheets impregnated with extractives was analyzed with
contact angle measurements on stored samples. The sizing efficiency of
flocculated AKD was determined by measuring 60-second Cobb and contact
angles on laboratory sheets. The retention of AKD was determined by
quantitative analysis of the AKD content in the laboratory sheets using
alkaline hydrolysis followed by solvent extraction and quantification with
capillary gas chromatography.
ISSN 1402-1757 / ISRN LTU-LIC--02/07--SE / NR 2002:07
