Note: Descriptions are shown in the official language in which they were submitted.
21 62668
WO 94/26973 PCT/EP94/01389
A process for controlling the sedimentation of sticky
impurities from paper stock suspensions
Field of the Invention
This invention relates to a process for controlling
the sedimentation of sticky impurities from paper stock
suspensions in paper manufacture.
Prior Art
Even when paper was invented in the second century,
the use of waste material, i.e. the technique of at least
partial recycling, played a certain role. Nowadays,
considerable significance is attributed to recycling
technology through increasing ecological awareness. In
view of the increasing production of paper, therefore,
the supply of raw materials and the avoidance of waste
are acquiring increasing significance.
By using secondary fiber stock from the recycling of
wastepaper, savings can now be made in regard to raw
materials, waste-disposal space and the energy required
for paper manufacture. Unfortunately, this technology
still involves specific difficulties.
Thus, in the processing of wastepaper, sticky
impurities, normally known as stickies, can seriously
disrupt the production process and adversely affect the
quality of the paper produced. Stickies enter the
papermaking process when the wastepaper used contains
adhesive bonds, adhesive tapes or refined products, such
as coated or laminated papers and paperboards. In
addition, however, sticky impurities can also be formed
by the resin in wood and through its interaction with
paper auxiliaries.
Where the stickies are present in compact form, they
can be mechanically removed relatively easily by means of
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sorting machines. In general, however, the stickies are
present not only in compact form, but also in dispersed
form in the pulp stock and are very difficult to remove
in this form. Recently, therefore, the increasing use of
wastepaper in paper manufacture and the restriction of
the water circuits has increasingly resulted in a higher
percentage of stickies in the circuit water.
Stickies cause a number of problems and disruptions
not only in the papermaking process, but also in the
processing of paper. On account of their stickiness,
deposits are formed on machine parts, tube walls, sieves,
wet felts, dry felts, drying cylinders, smoothing rol-
lers, calender rollers and, in addition, even on the
paper itself, resulting in web tears in the papermaking
machine and in a deterioration in paper quality through
holes, stains and marks (cf. H.L. Baumgarten, Da~ Papier,
1984, 38, No. 10A, pages V121-V125). According to H.L.
Baumgarten, stickies in industrial and institutional
publications have for years been the biggest problem in
the recycling of wastepaper. Even minimum quantities of
adhesive can cause tears in papermaking and printing
machines, so that the machines have to be stopped for
- cleaning purposes. Baumgarten states: "2 g of adhesive
at the right place in the papermaking machine can turn
several hundred kg of paper into waste" (loc. cit., page
V122, right-hand column).
Stickies have various origins. Essentially, they
emanate from the resin in wood, from auxiliaries involved
in paper manufacture, from binders for the coating of
paper and cardboard, from adhesives for the processing of
paper, from printing ink binders and from materials
involved in the processing of paper. Stickies emanating
from the resin in wood and from the adhesives used in the
processing of paper are particularly important in the
context of the problem addressed by the present inven-
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Wo 94/26973 3 PCT/EP94/01389
tion.
The resins present in chemical wood pulp and mechan-
ical wood pulp contain around 1 to 5% by weight of so-
called harmful resins, depending on the type of wood.
These resins may be present in colloidal, unbound form or
may adhere to the paper fibers. According to J. Weigl et
al., the difficulties caused by resin deposits in the
manufacture and processing of paper have steadily in-
creased in recent years for various reasons (cf. J. Weigl
10et al., Das Papier, 1986, pages V52-V62, more particular-
ly page V53, left-hand column).
The adhesives used in the processing of paper may be
divided into three groups, namely: contact adhesives,
dispersion-based adhesives and hotmelt adhesives.
15Contact adhesives are permanently tacky and per-
manently bondable products. Adhesion is achieved by
application of pressure to the surfaces of the parts to
be bonded. The basic polymers may be any of various key
chemicals in combination with corresponding additives,
for example tackifying resins, plasticizers or antioxi-
dants. Typical basic polymers are inter alia natural
rubber, butyl rubber, styrene/butadiene copolymers (SBR
-- rubber), acrylonitrile copolymers, polychloroprene,
polyisobutylene, polyvinyl ether, acrylates, polyesters,
polyurethanes, silicones.
In dispersion-based adhesives, the polymers involved
in formation of the adhesive layer are present as solid
particles in an aqueous dispersant. In the production
process, the basic monomers are first emulsified in an
aqueous phase and then polymerized therein - a technique
known as emulsion polymerization. The polymer is then
present in the form of small particles with different
particle sizes which can vary from molecularly disperse
to coarsely disperse. In general, agglomeration and
hence sedimentation of the monomer particles is counter-
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acted by adding protective colloids or emulsifiers to thesystem.
The so-called hotmelt adhesives, also known as
hotmelts, belong to the group of thermoplastics. These
materials have the property of softening on heating, so
that they become fluid. On cooling, they solidify again.
Examples of polymers used as hotmelt adhesives include
polyamides, copolyamides, polyaminoamines, saturated
polyesters and ethylene/vinyl acetate copolymers.
Stickies are divided into primary and secondary
stickies. Primary stickies are those sticky impurities
which, on account of their high resistance, are not
dispersed during wet size reduction. Accordingly, they
are present in compact form and are easy to remove.
The existence of secondary stickies emanates from
the fact that, during the recycling of wastepaper, the
sticky impurities undergo a change in their particle size
brought about by thermal, chemical and mechanical influ-
ences. This means that even impurities which are still
present in extremely coarse form at the beginning of
recycling can undergo more or less considerable size
reduction in the recycling process. In particular,
- stickies are dispersed by the processes taking place in
the hot kneading machines used in the recycling of
wastepaper. For example, stickies with a low melting
point are liquefied and then very finely dispersed.
Crumbly or fragile stickies also disintegrate into very
small particles. The particle size of the dispersed
stickies thus ranges from coarsely disperse through
colloidally disperse to molecularly disperse.
In other words, many stickies are readily disper-
sible with the result that, after the pulping step, they
are present in finely divided form and are not picked up
at the sorting stage. These substances are in danger of
forming agglomerates - also known as secondary stickies -
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in the papermaking machine under thermal, mechanical orchemical influences. It is precisely these secondary
stickies which cause problems in the further processing
of paper. For example, they are transported by the paper
webs, pass through the papermaking machine and thus
arrive at the various places where they lead to unwanted
deposits, more particularly at press felts, dry sieves,
drying cylinders, smoothing rollers. In addition, they
are of course also present in the paper itself, thus
adversely affecting its quality.
Accordingly, it is clear from the situation outlined
in the foregoing that, basically, any parameters which
promote the agglomeration of particles bring with them
the danger of formation of secondary stickies. The pH
value and the presence of certain papermaking auxiliaries
are mentioned as two very important parameters in this
regard. More specifically:
Small solid particles, which touch one another or
which are separated from one another by a very narrow
gap, attract one another through molecular interactions,
so-called Van-der-Waals forces. However, the agglomera-
tion-promoting Van-der-Waals forces are generally not
- developed in alkaline medium, i.e. the medium typical of
the recycling of wastepaper, because the particles are
surrounded by an electrical double layer which is respon-
sible for the mutual repulsion of particles carrying the
same charge. By contrast, papermaking machines are
normally operated in a neutral or mildly acidic medium,
so that the repelling negative forces are reduced.
The drainability of the paper stock suspensions
prepared using wastepaper is generally poor. According-
ly, auxiliaries known as drainage or retention aids are
often used in practice. Retention aids are understood by
the expert to be substances which bind fine fibers and
fillers to the long paper stock fibers (long fibers).
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This binding of short fibers and fillers to the long
fibers prevents the fine fibers from forming a kind of
fleece which makes the paper stock suspension difficult
to drain. In this way, retention aids improve drainage
by binding fine fibers to the long fibers.
Retention aids can be divided into three groups,
namely: inorganic products, such as aluminium sulfate or
sodium aluminate; synthetic products, such as polyethy-
lene imines, polyamines or polyacrylamides; and modified
natural products, such as cationic starch.
The way in which retention aids work is based on the
attachment of fine fibers and fillers to the paper
fibers. An important mechanism in this regard is that
polyelectrolytes of adequate chain length can bridge the
gap between two particles and, in this way, promote the
formation of agglomerates. Thus, J.L. Hemmes et al.
report that cationic polyelectrolytes, for example
cationic starch, are suitable as scavengers for anionic
impurities (Wochenblatt fur Papierfabrikation 1993, pages
163-170).
To summarize, it may be said that, according to the
present state of general specialist knowledge, a neutral
- or acidic medium on the one hand and the use of cationic
auxiliaries to improve drainage and retention on the
other hand represent conditions which promote the agglom-
eration of particles. With regard to the problem of
stickies discussed in the foregoing, this means that the
expert logically regards these conditions as favorable to
the formation of stickies.
Another key role in the control of stickies is that
played by temperature. The reason for this is that many
adhesives belong to the thermoplastics (hotmelts) of
which the tackiness increases with temperature.
In addition, it is pointed out that the manifesta-
tion of the undesirable properties of sticky impurities
WO 94/26973 2 ~ 626~8 PCT/EP94/01389
for the manufacture and processing of paper depends upon
a number of parameters which are not yet that well known
in every respect (cf. H.L. Baumgarten, loc. cit., page
V122, left-hand column). Normally harmless impurities
can even be converted into sticky impurities through the
cooperation of mechanical, chemical and thermal influen-
ces during the production process (cf. B. Brattka,
Wochenblatt fur Papierfabrikation 1990, pages 310-313).
Now, there are various known methods which seek to
counteract the manifestation of the negative properties
of sticky impurities for the process of paper manufac-
ture. In this connection, particular significance is
attributed among experts to the approach whereby the
sedimentation of stickies is suppressed by an auxiliary
so that the problems caused by the adhesive properties
are reduced to a technically acceptable level.
Thus, US 4,923,S66 describes a process in which
stickies are controlled with urea. According to the
teaching of US 3,081,219, stickies in the sulfite pulping
of wood are controlled with the aid of N-vin~1-2-Pyrroli-
done. Attempts have also been made to control stickies
by the addition of bentonites, diatomaceous earth and the
- like. This well-known approach is based on the idea of
introducing fine particles which are capable of binding
sticky impurities at their surface (cf. US 3,081,219,
column 1, lines 40-44). Another approach is based on the
addition of sequesterinq aqents, for example polyphos-
phates (cf. US 3,081,219, column 1, lines 45-50).
Finally, attempts have also been made to use various
dispersants, for example the sodium salts of sulfonated
formaldehyde/naphthalene condensates, although this leads
to disadvantages at neutral pH values and to unfavorable
interactions with cationic auxiliaries (cf. US 3,081,219,
column 1, lines 51-58). US 4,744,865 describes a process
in which the coagulation of sticky impurities is said to
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W0 94/26973 8 PCT/EP94/01389
be reduced by polymers containing methoxy groups.
Finally, according to G. Galland and F. Julien Saint
Amand, primary acrylate stickies can be removed by
flotation in alkaline medium in the presence of soap (cf.
EUR. Co Eur. Communities 14011, 1992 pages 235-243).
According to the authors, it is crucially important to
the effectiveness of their method that the alkalinity and
the soap be introduced either in the pulper or immediate-
ly after the pulper. They also state that the effective-
ness of the removal of primary stickies is increased by
the reduction in the size of the bubbles, but only at the
expense of increased fiber losses. By its very nature,
however, the Galland/Saint Amand process cannot contri-
bute anything towards solving the problem of secondary
stickies.
Description of the Invention
In overall terms, the prior art in the field in
question is extremely heterogeneous, a completely satis-
factory process for controlling stickies having still tobe developed. This applies most particularly to the
secondary stickies mentioned above, especially since the
percentage content of finely dispersed sticky impurities
responsible for the formation of secondary stickies in
the circuit water is steadily increasing.
H.L. Baumgarten's observation is still relevant
today: "A glance at the problem of "sticky impurities"
in wastepaper ..... shows that not only manufacturers of
wastepaper recycling plants but also, and in particular,
manufacturers of - mostly polymer-containing - paper
refining and paper processing auxiliaries and also the
chemical industry as the supplier of raw materials have
a responsibility to provide close support to the paper
industry." (Das Paper, 1984, No. lOA, page V124).
3S Accordingly, there is a constant need for new and alter-
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native solutions to the problem of controlling stickiesin paper manufacture.
Accordingly, the problem addressed by the present
invention was to provide a process for controlling the
sedimentation of sticky impurities which appear negative-
ly as secondary stickies in the paper machine. This
process would be generally applicable to various types of
sticky impurities, but especially to contact adhesives,
dispersion-based adhesives and hotmelt adhesives (hot-
melts). In addition, the control of stickies would notbe achieved at the expense of increased losses of fibers
and fillers.
According to the invention, the problem stated above
has been solved by a process for controlling the sedimen-
tation of stickies from paper stock suspensions in paper-
making, in which - based on oven-dry paper stock - 0.2 to
3.0% by weight of waterglass and/or 0.05 to 1.0% by
weight of a fatty acid essentially containing 12 to 22
carbon atoms or a salt thereof with monovalent to tri-
valent cations is/are added to the paper stock suspensionimmediately before the first and/or subsequent flotation
stages.
- Accordingly, the present invention relates to a
process for controlling the sedimentation of sticky
impurities (stickies) from paper stock suspensions in
papermaking, characterized in that - based on oven-dry
paper stock - 0.2 to 3.0% by weight of waterglass and/or
0.05 to 1.0% by weight of a fatty acid essentially
containing 12 to 22 carbon atoms or a salt thereof with
monovalent to trivalent cations is/are added to the paper
stock suspension immediately before the first and/or
subsequent flotation stages.
It is specifically pointed out that the time at
which components i) and/or ii) are added is a critical
feature so far as the success of the process according to
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WO 94/26973 10 PCT/EP94/01389
the invention is concerned. The fact that components i)
and/or ii) are added immediatelY before the first and/or
subsequent flotation stages implies that they enter the
flotation process without being exposed to intensive
shear forces beforehand. Corresponding shear forces
occur at various places in the papermaking process.
Examples include the wastepaper pulper and following
sorting units.
Application of the above-mentioned parameters
crucial to the invention ensures that the sticky im-
purities which are present in finely divided form in the
aqueous paper stock suspensions used and which can lead
to secondary stickies are removed from the system to a
large extent. Another advantage of the process according
to the invention is that the content of sticky impurities
is not reduced at the expense of an increased loss of
fibers and fillers. On the contrary, a reduction in the
loss of fibers and fillers is another advantage achieved
by the process according to the invention.
In one preferred embodiment of the present inven-
tion, the paper stock suspensions used are prepared from
wastepaper or from paper products containing wastepaper
-- constituents.
Basically, the type of waterglass used is not
critical. However, soda waterglass and/or potash water-
glass are preferred.
In another preferred embodiment of the present
invention, a mixture of components i) and ii) is used.
Basically, the ratio by weight between the two components
is not critical, although it is preferred to establish a
ratio by weight of 0.5:1 to 10:1. Component i) is
preferably used in an excess by comparison with component
ii), a ratio by weight of component i) to component ii)
of 3:1 to 5:1 being most particularly preferred.
In another preferred embodiment of the present
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WO 94/26973 11 PCT/EP94/01389
invention, components i) and ii) are used in combination
with an alkali metal hydroxide. The alkali metal hydrox-
ide, preferably sodium and/or potassium hydroxide, is
preferably used in a quantity of 0.05 to 2.0% by weight,
based on oven-dry paper stock. The presence of alkali
metal hydroxide provides for a further reduction in the
loss of fibers and fillers which is extremely important
to the economy of the process.
So far as the effect of component ii) is concerned,
it is crucial that it should be at least partly present
in the form of a relatively poorly soluble soap in the
particular flotation cell. This is generally achieved by
using a fatty acid containing 12 to 22 carbon atoms or
soluble salts thereof with monovalent to trivalent
cations which then form the corresponding poorly soluble
calcium soaps in situ with the water hardness present in
the system. However, if the water hardness is not suffi-
cient, the calcium soaps of fatty acids containing 12 to
22 carbon atoms may even be directly used.
It has been found that, in some cases, the effect of
the process according to the invention can be further
improved by carrying out the flotation process in the
- presence of an additional cationic flocculant or reten-
tion agent. Corresponding flocculants or retention
agents are, for example, cationic polymers, such as
polyacrylamides, polyethyleneimines, polyamidoamines, or
cationic starches and inorganic compounds, such as
aluminium sulfate.
The process according to the invention is generally
applicable to the various types of stickies. However, it
is most particularly suitable for solving the problems
caused by contact adhesives, dispersion-based adhesives
and hotmelt adhesives (hotmelts).
In principle, the process according to the invention
is suitable for controlling the sedimentation and adhe-
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sion of stickies of various kinds, i.e. differing intheir chemical and physicochemical nature. However, the
advantages of the process according to the invention are
particularly applicable to stickies based on contact
5 adhesives and hotmelt adhesives (hotmelts).
The following Examples are intended to illustrate
the invention without limiting it in any way.
E x a m p 1 e s
1. Substances and materials used
Sodium hydroxide: 50% aqueous NaOH solution.
Waterglass: "Wasserglas 37/40" (Na2SiO3), a
product of Henkel/Dusseldorf.
042: Oleic acid mixture "Olinor 042", a
product of Henkel/Dusseldorf.
2. Determination of the dichloromethane extract
2.1. Principle of the method
The dichloromethane extract was used as an indirect
measure for determining the percentage content of sticky
impurities in paper suspensions. The dichloromethane
extract is obtained by filtering a sample of the paper
stock suspension to be tested, drying the residue and
determining the dichloromethane-soluble constituents
present therein - essentially sticky impurities - by
extraction.
2.2. Analytical aids
a) Round filters: Before use, the round filters used
were dried to constant weight in a heating cabinet at a
temperature of (103 + 2)C by the method according to DIN
54359 and were weighed after cooling in an exsiccator.
35 b) Flat-bottomed flask: Before use, the flat-bottomed
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flask used was dried to constant weight in a drying
cabinet at (105 + 2)C by the method according to DIN
54354 and was weighed after cooling in exsiccator.
2.3. Procedure
A 500 ml sample of a thoroughly mixed paper stock
suspension was removed and filtered through a filter
consisting of a 15 cm diameter Buchner funnel, a large
suction bottle and a round paper filter. After filtra-
tion, the filtrate was visually examined for clouding.If any clouding was discernible, the filtrate was refil-
tered through the same filter. The round filter was
dried together with the stock cake filtered off in a
heating cabinet and then weighed.
The dried stock cake was then transferred together
with the round filter to a Soxhlet extractor with a
ground-in condenser and a connected 500 ml flat-bottomed
flask with a ground neck. After addition of 400 ml of
dichloromethane, the stock cake was extracted with heat-
ing for 6 hours. After extraction, the extract solution
was concentrated by distillation to such an extent that
it was still just liquid. The cake was then dried to
-- constant weight in a drying cabinet at a temperature of
(105 + 2)C and then weighed as described in 2.2. The
dichloromethane extract DCM in ~ (based on the dry weight
of the sample) was calculated as follows from the results
obtained:
m4-m3
DCM = 100
m2-m
where
m1 = weight of the round filter in g
m2 = weight of the round filter with stock cake in g
m3 = weight of the empty flask in g
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m4 = weight of the flask with residue in g.
3. Test procedure
In a test arrangement in which a Sulzer-Escher Wyss
type CF flotation cell was integrated, wastepaper was
disintegrated with 1% of waterglass, 0.5% of sodium
hydroxide, 0.5% of hydrogen peroxide and 0.33% of Olinor
042 (all percentages based on oven-dry paper stock) in a
pulper at a pulp consistency of 12 "otro" (= oven-dry).
After coarse sorting, a volume corresponding to 200 kg of
oven-dry paper stock was pumped into a vat and, after
determination of the stock consistency, was diluted with
circuit water to a stock consistency of 1.3%.
The additives to be tested were then added, a sample
of the stock was removed and was then floated for 30
minutes during which the paper stock suspension circu-
lated from the vat through the flotation cell to an
adjoining second vat and then back again to the first
vat. On completion of the flotation process, all the
Z0 stock was pumped back into the first vat and another
sample was taken. The dichloromethane extract of the
stock samples obtained was determined before and after
-- flotation, as described above, in addition to which the
total losses of filler and fibers was determined from the
pulp consistency and the vat level.
The values obtained for the reduction of the dichlo-
romethane extract (DCM extract) and the loss of fibers
and filler (total loss) are set out in Table 1.
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Table 1
Additive introduced ) Reduction of Total loss3)
DCM extract
(%) (%)
None 33 18
1% Waterglass2)39 17
1% Waterglass 57 14
1% Waterglass
+ 0.07% 042 65 11
1% Waterglass
+ 0.5% NaOH
+ 0.07% 042 64 8
The addition was made immediately before flotation
as described above
2) In this case, the addition was made in the pulper
for comparison purposes
3) Loss of fibers and filler
2.3 Discussion of the results
-- It is clear from Table 1 that a considerable reduc-
tion in the DCM extract and, at the same time, a distinct
reduction in the total loss of fibers and filler are
achieved by the process according to the invention. Com-
parison of the additions of 1% of waterglass respectively
in the pulper and immediately before flotation clearly
shows that the required reduction in the DCM extract is
only achieved where the addition is made immediately
before flotation (process according to the invention).
In addition, Table 1 shows that the total loss of fibers
and filler is further reduced in the presence of addi-
tional alkali metal hydroxide for otherwise substantially
the same DCM extract value.
