Note: Descriptions are shown in the official language in which they were submitted.
CA 02665712 2009-04-06
Process for the production of a multilayer fiber web from cellulose fibers
Description
The invention relates to a process for the production of a multilayer fiber
web from
cellulose fibers by separately feeding in at least two different fiber
suspensions and
water into a multilayer headbox, in which they are separated by separating
elements
from one another and from water and, after leaving the nozzle mouth of the
headbox,
reach an apparatus on which a web is formed, the water being transported in
such a
way that, after emerging from the nozzle mouth, it reaches the web-forming
apparatus
in the form of a layer between two layers of fiber suspensions and thus
counteracts
mixing of the different fiber suspensions.
In order to produce multilayer papers in a paper machine, it must be equipped
with a
multilayer headbox whose outlet nozzle chambers are divided into separate flow
channels and which extend over the total machine width. Thus, it is possible
to
produce, for example, a paper consisting of three layers, passing three paper
stocks
having different compositions separately through an outlet nozzle chamber
having
three flow channels separated from one another and draining said paper stocks
on a
wire of a paper machine, cf. EP-A-0 939 842 and DE-A-10 2004 051 255.
DE-A-101 26 346 discloses a process and a stock feed system for loading a
multilayer
headbox of a paper machine, white water being fed with the aid of a single
pump and
the white water stream being divided into a plurality of white water part-
streams into
which high-viscosity stock is then metered and the part-streams thus obtained
are fed
via distributors to the multilayer headbox.
WO-A-03/048452 discloses a process for the production of a multilayer fiber
web from
at least two different fiber suspensions. The fiber suspensions and water are
fed in
each case separately to a multilayer headbox, in which they are separated from
one
another by separating elements and, after leaving the nozzle mouth of the
headbox,
reach an apparatus on which a web is formed, the water being transported in
such a
way that, after emerging from the nozzle mouth, it reaches the web-forming
apparatus
in the form of a layer between two layers of fiber suspensions and prevents
mixing of
the different fiber suspensions. The separating elements in the multilayer
headbox are
designed so that they are movable in the vertical direction and thus permit a
change in
the pressure between the fiber suspensions in the headbox.
In the literature references cited above, however, details of the fiber
suspensions are
not disclosed. With the use of a multilayer headbox for the production of
multilayer
papers, there is always the danger that mixing of the individual fiber streams
occurs
between the end of the headbox nozzle and the drainage part of the paper
machine.
PF 58514 CA 02665712 2009-04-06
2
In the production of paper which consists only of a single layer, it is known
that,
depending on paper type, different process chemicals, such as sizes, drainage
aids,
flocculants, retention aids and dry and wet strength agents or mixtures of
said process
chemicals and biocides and/or dyes and colored pigments are used, cf.
Ullmann's
Encyclopedia of Industrial Chemistry, Sixth, Completely Revised Edition, Wiley-
VCH
Verlag GmbH & Co. KgaA, Weinheim 2003, volume 25, pages 1 to 157, and optical
brighteners, compositions for increasing the volume of the paper (so-called
bulk
promoters, cf. US-B-6,273,995) and modified fibers, as described, for example,
in WO-
A-2006/048280.
Not only the composition of the process chemicals but also the time of
metering of such
products to a paper stock constitute an important feature in papermaking. The
method
and time of metering of retention and drainage aids to a paper stock has, for
example,
an effect on the retention, the drainage rate and the formation. Thus, for
example, EP-
A-0 235 893, EP-A-0 335 575, EP-A-0 310 959, US-A-4,388,150 and WO-A-94/05595
disclose a process for the production of paper using a microparticle system, a
cationic
polymer first being metered into the paper stock, the mixture then being
subjected to
the action of a shear field, bentonite or silica then being added and the pulp
thus
obtainable being drained without further action of shear forces with sheet
formation.
According to the process disclosed in DE-A-102 36 252 for the production of
paper, a
microparticle system comprising a cationic polymer and a finely divided
inorganic
component is metered into the paper stock after the last shearing stage before
the
headbox and the paper stock is then drained.
EP-A-0462 365 discloses organic microparticles which may be uncrosslinked or
crosslinked and which in each case comprise at least 1% by weight, but in
general at
least 5% by weight, of ionic comonomers incorporated in the form of
polymerized units.
The particle size of the uncrosslinked, water-insoluble microparticles is
below 60 mm,
while it is less than 750 nm for the crosslinked microparticles. The organic
microparticles are used in papermaking together with a high molecular weight
of ionic
polymer as a retention aid system which, if appropriate, may additionally
comprise
inorganic microparticles, such as bentonite or silica.
Furthermore, DE-A-10 2004 063 005 discloses a process for the production of
paper,
board and cardboard, a microparticle system comprising a cationic polymeric
retention
aid having a molar mass of at least 2 million and a finely divided inorganic
component
being used. The polymeric retention aid is metered into the paper stock at at
least two
points and the finely divided inorganic component is metered before or after
addition of
the polymeric retention aid, the paper stock being subjected to at least one
shearing
stage either before or after the addition of the retention aid. Further
microparticle
PF 58514 CA 02665712 2009-04-06
3
systems which are used as retention aids in papermaking are disclosed, for
example,
in US-A-6,103,065 and WO-A-2004/015200.
DE-A-10 2004 063 000 discloses a process for the engine sizing of paper, board
and
cardboard, where, by successive continuous addition of an aqueous dispersion
of at
least one reactive size and at least one retention aid to a paper stock stream
having
laminar flow and a consistency of not more than 2% by weight, based on dry
fibers,
and drainage of the paper stock with sheet formation, a procedure is adopted
in which
the reactive size and the retention aid are metered in with turbulent flow
into the paper
stock stream at a point which is after the last shearing stage and before the
beginning
of the drainage process.
It is the object of the invention to provide a process for the production of
multilayer
papers having improved formation, the mixing of the individual fiber streams
during the
web-forming process not taking place to the same degree as in known processes.
The object is achieved, according to the invention, by a process for the
production of a
multilayer fiber web from cellulose fibers by separately feeding in each case
at least
two different fiber suspensions and water into a multilayer headbox, in which
they are
separated by separating elements from one another and from water fed in and,
after
leaving the nozzle mouth of the headbox, reach a drainage apparatus on which a
multilayer fiber web is formed, the water being transported in such a way
that, after
emerging from the nozzle mouth, it reaches the web-forming apparatus in the
form of a
layer between two layers of fiber suspensions and thus counteracts mixing of
the
different fiber suspensions, if, for improving the retention, drainage and
formation, at
least one retention aid and/or at least one drainage aid are metered into the
fiber
suspensions and/or into the water fed in.
According to the invention, a retention aid or a drainage aid or both products
is or are
metered into fiber suspensions. The addition of these aids can be effected in
the
papermaking process before shearing of the paper stock, between two shearing
stages
or after the last shearing of the paper stock. Preferably, an aqueous solution
of at least
one retention aid and/or at least one drainage aid is metered into the paper
stock
stream at a point which is located after the last shearing stage of the paper
stock and
before the nozzle mouth of the headbox. The addition of retention aid and/or
drainage
aid to the paper stock is particularly advantageously effected with turbulent
flow of the
aqueous formulations of the process chemicals. As a result, the distribution
of these
products in the paper stock which is as uniform as possible is achieved. In
order to
produce turbulent flow, it is possible to use, for example, the apparatus
which is
described in US-B-6,659,636. It consists, for example, of a binary or multi-
material
nozzle through which water recycled from the paper machine and retention aid
and/or
drainage aid are passed in to a paper stock stream having laminar flow. The
flow
PF 58514 CA 02665712 2009-04-06
4
velocity of the paper stock stream is, for example, at least 2 m/sec and is in
general in
the range from 3 to 7 m/sec in the customary paper machines.
A process for the production of a multilayer fiber web from cellulose fibers
by
separately feeding in each case at least two different fiber suspensions and
water into
a multilayer headbox, in which they are separated by separating elements from
one
another and from water fed in and, after leaving the nozzle mouth of the
headbox,
reach a drainage apparatus on which a multilayer fiber web is formed, the
water being
transported so that, after emerging from the nozzle mouth, it reaches the web-
forming
apparatus in the form of a layer between two layers of fiber suspensions and
thus
counteracts mixing of the different fiber suspensions, is disclosed in WO-A-
03/048452
discussed in connection with the prior art. For details, reference is made to
this
publication, in particular page 2, line 2 to page 5, line 32, and the patent
claims. As
described therein, the stream of water fed in is transported by so-called
blade means
inside the headbox and enters the drainage part of the paper machine between
two
streams of fiber suspensions. The velocity of the water stream is adapted to
the
velocity of the fiber streams and is, for example, in the range from 2 to 10
m/sec,
preferably from 3 to 8 m/sec. The velocity of the water stream which separates
the fiber
streams is preferably from 1 to 25%, in general from 5 to 10%, higher than the
velocity
of the individual fiber streams.
At least two fiber suspensions which are processed to give a multilayer fiber
web have
a different composition. Thus, for example, the middle layer of a three-layer
fiber web
may consist of a cheap fiber, such as wastepaper, and top and bottom of the
fiber web
are formed from a high-quality fiber, for example bleached pine sulfate.
Different types
of multilayer paper webs can be produced through the choice of fibers and the
additional use of further process chemicals, such as engine sizes and/or
strength
agents.
With the use of different fiber suspensions which differ, for example, only in
the type of
suspended fibers or the concentration thereof, it is possible, according to
the invention,
to adopt a procedure in which the same retention aid is added to all streams
of fiber
suspensions and it is metered in such a way that at least two streams of fiber
suspensions comprise a different concentration of this retention aid. A
further variant of
the process according to the invention consists in the metering of at least
two retention
aids differing from one another into at least two streams of fiber
suspensions. These
fiber suspensions may be composed of the same or different fibers. In
addition, it is
possible for at least two streams of fiber suspensions to comprise a different
concentration of retention aid.
PF 58514 CA 02665712 2009-04-06
In another embodiment of the process according to the invention, an aqueous
solution
of a retention aid and an aqueous dispersion of at least one filler are
metered
separately from one another or as a mixture into the paper stock.
5 However, retention aid and drainage aid can also be metered into at least
one stream
of the water fed in. Thus, for example, it is possible to meter an aqueous
solution of a
retention aid and an aqueous dispersion of at least one filler separately from
one
another or as a mixture into at least one stream of the water fed in. However,
it is also
possible to adopt a procedure in which an aqueous dispersion of a filler is
metered into
at least one stream of the fiber suspension and in which the water fed in
comprises at
least one retention aid. The retention aid fed in with the water may also be
taken as an
aqueous solution from a storage vessel or metered separately into the water
fed in.
Further process variants consist in using at least one retention aid in
combination with
at least one drainage aid or in using at least one retention aid in
combination with a
fixing agent. Fixing agents are used in particular when the paper stock has a
high
cationic demand, for example a COD value of from 300 to 30 000, in general
from 1000
to 20 000, mg of oxygen/kg of the aqueous phase of the paper stock.
Examples of fixing agents are condensates of dicyandiamide and formaldehyde,
condensates of dimethylamine and epichlorohydrin, cationic polyacrylamides
having
molar masses M,, of from 1000 to 20 000, or hydrolyzed homo- and copolymers of
N-
vinylformamide having a K value of from 30 to 150, preferably from 60 to 90
(determined according to H. Fikentscher, Cellulose-Chemie, volume 13, 48-64
and 71-
74 (1932) in 5% strength by weight aqueous sodium chloride solution at a
temperature
of 25 C and a polymer concentration of 0.5% by weight). Fixing agents are
used, for
example, in an amount of from 0.02 to 2% by weight, preferably from 0.05 to
0.5% by
weight, based on dry paper stock.
All retention aids which are known for this purpose from papermaking practice
or from
the literature can be used for the process according to the invention. They
are used, for
example, in an amount of from 0.01 to 0.3, preferably from 0.01 to 0.05, % by
weight,
based on dry paper stock. The retention aid can be selected, for example, from
the
group consisting of cationic, anionic, nonionic and amphoteric polymeric
organic
compounds or a microparticle system. The most commonly used retention aids
belong,
for example, to the group consisting of polyacrylamides, polymethacrylamides,
polymers comprising vinylamine units and/or the microparticle systems.
Polyacrylamides and polymethyacrylamides may be nonionic, cationic, anionic or
amphoteric. Polymers suitable as retention aids have an average molar mass M,
of at
least 1 million, preferably at least 2 million and in particular at least 5
million. Nonionic
PF 58514 CA 02665712 2009-04-06
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polyacrylamides or polymethacrylamides are prepared, for example, by
polymerization
of N-vinylformamide, acrylamide and/or methacrylamide.
Cationic polyacrylamides are, for example, copolymers which are obtainable by
copolymerization of acrylamide and at least one di-Ci- to C2-alkylamino-C2- to
Ca-
alkyl(meth)acrylate or a basic acrylamide in the form of the free bases, the
salts with
organic or inorganic acids or the compounds quaternized with alkyl halides or
with
dimethyl sulfate. Examples of such compounds are dimethylaminoethyl
methacrylate,
diethylaminoethyl methacrylate, dimethylaminoethyl acrylate, diethylaminoethyl
acrylate, dimethylaminopropyl methacrylate, dimethylaminopropyl acrylate,
diethylaminopropyl methacrylate, diethylaminopropyl acrylate and/or
dimethylaminoethylacrylamide, dimethylaminoethylmethacrylamide,
dimethylaminopropylacrylamide, dimethylaminopropylmethacrylamide and/or
diallyldimethylammonium chloride. Said comonomers may also be copolymerized
with
methacrylamide to give cationic polymethacrylamides which comprise, for
example,
from 5 to 40 mol% of at least one cationic monomer, such as dimethylaminoethyl
acrylate or diallyldimethylammonium chloride incorporated in the form of
polymerized
units. Cationic polymethacrylamides can be prepared by copolymerization of
methacrylamide with at least one of the cationic monomers described above.
Further
cationic polymers which are used as retention aids are copolymers of N-
vinylformamide
and at least one of the cationic monomers mentioned above.
Anionic polyacrylamides are obtainable, for example, by copolymerizing
acrylamide
with at least one ethylenically unsaturated Cs- to C5-carboxylic acid, in
particular acrylic
acid or methacrylic acid, and/or a monomer comprising sulfo groups, such as
vinylsulfonic acid or styrenesulfonic acid, and/or a salt of said monomer.
Preferred salts
are the alkali metal salts, in particular the sodium salts, and ammonium
salts. Anionic
polymethacrylamides are prepared analogously thereto by polymerization of
methacrylamide with the abovementioned monomers having acid groups. The
anionic
polymers comprise, for example, from 1 to 50, preferably from 5 to 40, mol% of
at least
one anionic monomer incorporated in the form of polymerized units.
The anionic polymeric retention aids also include copolymers which are
obtainable by
copolymerization of
at least one N-vinylcarboxamide of the formula
R2
CHZ=CH-N~ (I),
CO-R
where R' and R2 are H or Cl- to C6-alkyl,
PF 58514 CA 02665712 2009-04-06
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at least one monoethylenically unsaturated monomer comprising acid groups
and/or
the alkali metal, alkaline earth metal or ammonium salts thereof and, if
appropriate,
other monoethylenically unsaturated monomers and, if appropriate, compounds
which
have at least two ethylenically unsaturated double bonds in the molecule.
A preferably used polymeric anionic compound of this group is a copolymer
which is
obtainable by copolymerization of N-vinylformamide, acrylic acid, methacrylic
acid
and/or the alkali metal or ammonium salts thereof and, if appropriate, other
monoethylenically unsaturated monomers,
the polymeric anionic compound comprising, for example,
(a) from 10 to 95 mol% of units of the formula I,
(b) from 5 to 90 mol% of units of a monoethylenically unsaturated carboxylic
acid
having 3 to 8 carbon atoms in the molecule and/or the alkali metal, alkaline
earth
metal or ammonium salts thereof and
(c) from 0 to 30 mol% of units of at least one other monoethylenically
unsaturated
monomer
incorporated in the form of polymerized units.
The compounds of this group can be modified so that they additionally comprise
at
least one compound (d) having at least two ethylenically unsaturated double
bonds in
the molecule, incorporated in the form of polymerized units. If the monomers
(a) and
(b) or (a), (b) and (c) are copolymerized in the presence of such a compound
(d),
branched copolymers are obtained. The ratios and reaction conditions should be
chosen so that polymers which are still water-soluble are obtained. In certain
circumstances, it may be necessary for this purpose to use polymerization
regulators.
All known regulators, such as, for example, thiols, secondary alcohols,
sulfites,
phosphites, hypophosphites, thio acids, aldehydes, etc., can be used (further
information can be found, for example, in EP-A-0 438 744, page 5, lines 7-12).
The
branched copolymers comprise, for example,
(a) from 10 to 95 mol% of units of the formula I,
(b) from 5 to 90 mol /o of units of a monoethylenically unsaturated monomer
comprising acid groups and/or the alkali metal, alkaline earth metal or
ammonium salts thereof,
(c) from 0 to 30 mol% of units of at least one other monoethylenically
unsaturated
monomer and
(d) from 0 to 2 mol%, preferably from 0.001 to 1 mol%, of at least one
compound
having at least two ethylenically unsaturated double bonds
incorporated in the form of polymerized units.
PF 58514 CA 02665712 2009-04-06
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Amphoteric polyacrylamides and amphoteric polymethacrylamides each comprise
units
of cationic and of anionic monomers incorporated in the form of polymerized
units. An
example of this is a copolymer of acrylamide, dimethylaminoethyl acrylate
hydrochloride and acrylic acid.
Polymers comprising vinylamine units are obtainable by hydrolysis of polymers
comprising vinylformamide units. Polyvinylamines are prepared, for example, by
hydrolysis of homopolymers of N-vinylformamide, the degree of hydrolysis
being, for
example, up to 100%, in general from 70 to 95%. High molecular weight
copolymers of
N-vinylformamide with other ethylenically unsaturated monomers, such as vinyl
acetate, vinyl propionate, methyl acrylate, methyl methacrylate, acrylamide,
acrylonitrile
and/or methacrylonitrile, can also be hydrolyzed to give polymers comprising
vinylamine units and can be used according to the invention as retention aids.
The
polymers comprising vinylamine units are cationic. In the hydrolysis of
polymers of N-
vinylformamide with acids, the salts of the polymers (ammonium salts) form,
while
polymers carrying amino groups form in the hydrolysis with bases, such as
sodium
hydroxide solution. or potassium hydroxide solution. The preparation of homo-
and
copolymers of N-vinylformamide and the preparation of the polymers having
amino or
ammonium groups and obtainable therefrom by hydrolysis are known. It is
described in
detail, for example, in US 6,132,558, column 2, line 36 to column 5, line 25.
The
statements made there are hereby incorporated by reference. Polymers
comprising
vinylamine units are preferably used as retention aids in the process
according to the
invention.
Further cationic polymeric retention aids are polydiallyldimethylammonium
chlorides
(polyDADMAC) and branched polyacrylamides, which can be prepared, for example,
by copolymerization of acrylamide or methacrylamide with at least one cationic
monomer in the presence of small amounts of crosslinking agents. Such polymers
are
described, for example, in US-A-5,393,381, WO-A-99/66130 and WO-A-99/63159.
Other suitable cationic retention aids are polyamines having a molar mass of
more than
50 000, modified polyamines which are grafted with ethylenimine and, if
appropriate,
crosslinked, polyetheramides, polyvinylimidazoles, polyvinylpyrrolidines,
polyvinylimidazolines, polyvinyltetrahydropyrines, poly(dialkylaminoalkyl
vinylethers),
poly(dialkylaminoalkyl (meth)acrylates) in protonated or in quaternized form
and
polyamidoamines obtained from a dicarboxylic acid, such as adipic acid, and
polyalkylenepolyamines, such as diethylenetriamine, which are grafted with
ethylenimine and crosslinked with polyethylene glycol dichlorohydrin ether
according to
the teaching of DE-B-24 34 816, or polyamidoamines which are reacted with
epichlorohydrin to give water-soluble condensates. Further retention aids are
cationic
starches, alum and polyaluminum chloride.
PF 58514 CA 02665712 2009-04-06
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Further suitable retention aids are so-called microparticle systems comprising
a
polymeric retention aid having a molar mass M,, of at least 1 million,
preferably at least
2 million, and a finely divided inorganic or organic component. Such systems
are
known, cf. US-A-3,052,595, EP-A-0 017 353, EP-A-0 223,223, EP-A-0 335 575, EP-
A-
0 711 371, WO-A-01/34910, US-A-6,103,065 and DE-A-102 36 252. Both components
are as a rule added separately from one another to the paper stock in the
course of the
papermaking process. Suitable organic components of the microparticle system
are the
polymers described above, for example retention aids from the group consisting
of the
polymers comprising vinylamine units, the polymers comprising vinylguanidine
units,
the nonionic, cationic and anionic polyacrylamides, polyethylenimines,
crosslinked
polyamidoamines grafted with ethylenimine, cationic starches and
polydiallyldimethylammonium chlorides.
The polymeric retention aids of the microparticle system are added to the
paper stock,
for example, in an amount of from 0.005 to 0.5% by weight, preferably in an
amount of
from 0.01 to 0.25% by weight, based on dry paper stock.
Suitable inorganic components of the microparticle system are bentonite,
colloidal
silica, silicates and/or calcium carbonate. Colloidal silica is to be
understood as
meaning products which are based on silicates, e.g. silica microgel, silica
sol,
polysilicates, aluminum silicates, borosilicates, polyborosilicates, clay or
zeolites.
Calcium carbonate may be used, for example, in the form of chalk, ground
calcium
carbonate or precipitated calcium carbonate as an inorganic component of the
microparticle system. Bentonite is understood as meaning generally sheet
silicates
which are swellable in water. These are in particular the clay mineral
montmorillonite
and similar clay minerals, such as nontronite, hectorite, saponite, sauconite,
beidellite,
allevardite, illite, halloysite, attapulgite and sepiolite. The sheet
silicates are preferably
activated before they are used, i.e. converted into a form swellable in water
by treating
the sheet silicates with an aqueous base, such as aqueous solutions of sodium
hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, ammonia
or
amines. Bentonite in the form treated with sodium hydroxide solution is
preferably used
as an inorganic component of the microparticle system. The lamellae diameter
of the
bentonite dispersed in water is, for example, from 1 to 2VLm in the form
treated with
sodium hydroxide solution, and the thickness of the lamellae is about 1 nm.
Depending
on the type and activation, the bentonite has a specific surface area of from
60 to
800 m2/g. Typical bentonites are described, for example, in EP-B-0235893. In
the
papermaking process, bentonite is added to the cellulose suspension typically
in the
form of an aqueous bentonite slurry. This bentonite slurry may comprise up to
10% by
weight of bentonite. Usually, the slurries comprise from about 3 to 5% by
weight of
bentonite.
PF 58514 CA 02665712 2009-04-06
Products from the group consisting of silicon-based particles, silica
microgels, silica
sols, aluminum silicates, borosilicates, polyborosilicates or zeolites can be
used as
colloidal silica. These have a specific surface area of from 50 to 1000 m2/g
and an
average particle size distribution of 1-250 nm, usually in the range of 40-100
nm. The
5 preparation of such components is described, for example, in EP-A-0 041 056,
EP-A-
0 185 068 and US-A-5,176,891.
Clay or kaolin is a water-containing aluminum silicate having a lamellar
structure. The
crystals have a layer structure and an aspect ratio (diameter - to - thickness
ratio) of
10 up to 30:1. The particle size is, for example, at least 50% smaller than 2
m.
Preferably used carbonates are natural calcium carbonate (ground calcium
carbonate,
GCC) or precipitated calcium carbonate (PCC). GCC is prepared, for example, by
milling and classifying processes with the use of milling assistants. It has a
particle size
of 40-95% smaller than 2 m, and the specific surface area is in the range of
6-13
m2/g. PCC is prepared, for example, by passing carbon dioxide into an aqueous
calcium hydroxide solution. The average particle size is in the range of 0.03-
0.6 m.
The specific surface area may be strongly influenced by the choice of the
precipitation
conditions. It is in the range from 6 to 13 m2/g.
The inorganic component of the microparticle system is added to the paper
stock in an
amount of from 0.01 to 2.0% by weight, preferably in an amount of from 0.1 to
1.0% by
weight, based on dry paper stock.
Combinations of an organic polymer having a molar mass M, of at least 2
million and a
mixture of a finely divided inorganic component and a finely divided organic
component
are also suitable as a microparticle system, the two components being metered
independently of one another, either simultaneously or in succession. A
suitable finely
divided organic component having an anionic charge is described, for example,
in WO-
A-98/29604. At least one finely divided crosslinked copolymer of acrylamide
and at
least one monoethylenically unsaturated anionic monomer is preferably used as
the
finely divided, organic component of the microparticle system.
Examples of microparticle systems are combinations of cationic polymers, such
as
cationic starch, and finely divided silica or of cationic polymers, such as
cationic
polyacrylamide, and bentonite.
Instead of a retention aid or in combination with a retention aid, it is also
possible to
meter flocculants into at least one stream of a fiber suspension and/or into
at least one
stream of the water fed in. Those flocculants which are effective only at a
relatively high
temperature may be of particular interest here. An example of this is methyl
cellulose,
which is ineffective as a flocculant at about 20 C and acts as a flocculent at
75 C, cf.
PF 58514 CA 02665712 2009-04-06
11
G.V. Franks, Journal of Colloid and Interface Science 292, 598-603 (2005). A
further
example of a thermally sensitive flocculant is poly(N-isopropylacrylamide).
The
thermally sensitive flocculants can be metered in the process according to the
invention, for example, into at least one stream of the water fed in, which
has a
temperature in the range in which the flocculant is effective. The stream of
fibers may
have, for example, a temperature in the range from 20 to 45 C while the stream
of the
water fed in may have a temperature in the range from 60 to 75 C.
It is also possible to use pH-sensitive flocculants in the process according
to the
invention. An example of such a flocculant is the polysaccharide chitosan. It
is
ineffective as a flocculant, for example, at pH 4.5 but flocculates as soon as
the pH is
increased to, for example, 8.
Preferred drainage aids are polymers comprising ethylenimine units. Such
polymers
have already been mentioned above in the case of the cationic retention aids.
They act
both as retention aids and as drainage aids. Since the draining effect of this
class of
compounds is more pronounced than the retention effect, they are referred to
in the
present context as drainage aids. This class of compounds includes in
particular
polyethylenimines, which are obtainable by polymerization of ethylenimine in
aqueous
solution in the presence of acidic catalysts, such as mineral acids, or
halogen
compounds, such as methylene chloride, carbon tetrachloride, ethylene chloride
or
tetrachloroethane, and crosslinked ethylenimine-grafted condensates of a
polyamidoamine and a dicarboxylic acid. Such products are sold under the trade
mark
Polymin by BASF, Ludwigshafen. They are used in papermaking, for example, in
an
amount of at least 0.01 % by weight, in general in the range from 0.1 to 0.3%
by weight.
Depending on the fibers used in each case for the production of multilayer
papers, a
polyacrylamide and/or a polymer comprising vinylamine units are metered into
at least
one stream of a fiber suspension, and a polymer comprising ethylenimine units
is
metered into a stream of another fiber suspension.
In order to prepare, for example, a three-layer fiber web,
(a) a retention aid from the group consisting of polyacrylamides,
polymethacrylamides, polymers comprising vinylamine units, microparticle
systems and mixtures thereof is metered into the stream of the fiber
suspension
which forms the top of the fiber web,
(b) a drainage aid from the group consisting of the polymers comprising
ethylenimine units and/or a retention aid from the group consisting of the
polymers comprising vinylamine units, cationic, anionic, nonionic and
amphoteric
polyacrylamides, polymethacrylamides and mixtures thereof are metered into the
stream of the fiber suspension which forms the middle layer of the fiber web,
and
PF 58514 CA 02665712 2009-04-06
12
(c) a retention aid from the group consisting of the polyacrylamides,
polymethacrylamides, polymers comprising vinylamine units, microparticle
systems and mixtiures thereof is metered into the stream of the fiber
suspension
which forms the bottom of the fiber web.
In the production of a three-layer fiber web, preferably
(a) a polymer comprising vinylamine units is metered as a retention aid into
the
stream of the fiber suspension which forms the top of the fiber web,
(b) a polymer comprising ethylenimine units is metered as a drainage aid into
the
stream of the fiber suspension which forms the middle layer of the fiber web,
and
(c) a polymer comprising vinylamine units is metered as a retention aid into
the
stream of the fiber suspension which forms the bottom of the fiber web.
Paper webs having improved formation are obtained, for example, when, in the
production of a three-layer fiber web, a higher concentration of retention aid
is used in
the stream which forms the middle fiber web than in the streams which form the
top
and bottom of the fiber web. The amount of retention aid which is used for the
middle
layer of a three-layer fiber web is, for example, > 0.01 % by weight, in
general from
0.015 to 0.3% by weight, based on dry paper stock. For the production of the
top and
the bottom of a three-layer fiber web, the amounts of retention aid are in
general <
0.01 % by weight, for example in the range of from 0.001 to 0.009% by weight,
based
on dry fiber.
According to another process variant the addition of a retention aid or of a
filler is
effected with the aid of the water fed in. Thus, at least one retention aid
and at least
one thickener are metered into the stream of the water fed in. However, these
additives
can be mixed in the desired ratio in a storage container and transported
therefrom into
the nozzle chamber of the headbox. Suitable thickeners are, for example, high
molecular weight polyacrylamides or high molecular weight polycarboxylic acids
having
molar masses M, of at least 1 million, preferably at least three million. The
thickeners
are preferably crosslinked polyacrylamides or crosslinked polycarboxylic acids
which
swell very strongly in aqueous medium. An example of a known thickener is a
crosslinked polyacrylic acid. Suitable crosslinking agents are, for example,
methylenebisacrylamide, glycol diacrylate, butanediol diacrylate, butanediol
dimethacrylate, pentaerythrityl triacrylate, trimethylolpropane triacrylate,
pentaerythrityl
triallylether or triallylamine The amounts of thickener added to the water
are, for
example, from 0.001 to 10% by weight, preferably from 0.01 to 1% by weight. An
increase in the viscosity of the water is achieved thereby, with the result
that the
danger of mixing of the different fiber streams during the drainage process is
reduced.
PF 58514 CA 02665712 2009-04-06
13
In a further process variant, the paper stock, preferably the low-consistency
stock,
and/or the stream of water fed in comprise at least one suspended filler.
Suitable fillers
are the finely divided inorganic substances usually used in papermaking, e.g.
titanium
dioxide, ground calcium carbonate (marble), precipitated calcium carbonate,
chalk, talc,
montmorillonite, dolomite or clay. Filler can be used, for example, in an
amount of up to
40% by weight, in general in the range from 5 to 30% by weight, based in each
case on
dry paper stock. The fillers are used, for example, in the form of an aqueous
pumpable
slurry which comprises a dispersant, such as polyacrylic acid having a molar
mass MW
of from 5000 to 12 000. The fillers can also be added by addition to the paper
stock
during the preparation of the pulp. The finely divided inorganic fillers
generally lead to
an increase in the basis weight of the filler-containing paper compared with a
filler-free
paper.
However, it is also possible to increase the volume of the paper by, for
example,
adding thermally expandable microparticles directly to the paper stock during
the
papermaking or, according to the invention, metering them as an aqueous
suspension
together with a retention aid and/or a drainage aid into the paper stock
stream at a
point which is after the last shearing stage of the paper stock and before the
nozzle
mouth of the headbox. In a further variant of the process according to the
invention, an
aqueous suspension of thermally expandable microparticles can also be metered
into
the stream of the water fed in, which counteracts mixing of the fiber streams.
The
microparticles are used, for example, in an amount of from 2 to 50% by weight,
preferably from 5 to 45% by weight, based on dry paper stock.
Thermally expandable microparticles are known. They have, for example, a mean
particle diameter from 17 to 35 m. They are prepared by polymerization of
ethylenically unsaturated monomers in the presence of a blowing agent and, if
appropriate, further substances, such as silica, bentonite, clay, organic
suspending
media, such as methylcellulose, carboxymethylcellulose or polyvinyl alcohol,
starch or
oxides and hydroxides of aluminum, calcium, magnesium or barium. The blowing
agent
content of the microparticles is from 17 to 40% by weight, cf. US-A-
2006/0102307. The
microparticles described therein have a shell comprising a polymer of
vinylidine
chloride, acrylonitrile and methyl methacrylate. Further data on expandable
microparticles can be found in the publications US-A-3,615,972, US-A-
3,945,956, US-
A-5,536,756, US 6,235,800, US 6,235,394, US 6,509,384 and EP-A-0 486,080. On
drying of papers which comprise expandable microparticles, an increase in the
volume
of the paper occurs.
The effect of the drainage aids can be enhanced by using them together with at
least
one surface-active agent. Preferred surface-active agents are alkoxylation
products of
alcohols and amines. An example of this is Sursol VL (BASF
Aktiengesellschaft,
Ludwigshafen). The amounts of surface-active agent are, for example, from 0.01
to
PF 58514 CA 02665712 2009-04-06
14
10% by weight, based on dry paper stock. Further agents of this category are
quaternized alkanolamine-fatty acid esters, which are described, for example,
in US-A-
2006/0196624, or polyaminoamides, as described, for example, in Nordic Pulp
and
Paper Research Journal 2003, 18, 188-193.
In a further configuration of the process according to the invention, at least
one
retention aid is used in combination with an engine size. Preferred engine
sizes are
reactive size, in particular C12- to C22-alkylketene dimers, C5- to C22-alkyl-
and/or C5- to
C22-alkenylsuccinnic anhydrides, C12- to C36-alkyl isocyanates or mixtures of
said
compounds. It is also possible to use rosin size as an engine size. The
aqueous
reactive size dispersions are stabilized, for example, with the aid of
cationic starch, cf.
EP-B-0 353 212, EP-B-0 369 328 and EP-B-0 437 764. Both cationic and in
particular
anionic aqueous dispersions of at least one C12- to C22-alkyldiketene are used
as sizes.
Such dispersions are disclosed, for example, in WO-A-00/23651, pages 2 to 12.
For
the preparation of size dispersions, the reactive sizes are usually heated to
a
temperature which is above their melting point and then emulsified in water
under the
action of shear forces.
Liquid alkenylsuccinic anhydrides can be emulsified even at room temperature.
For
example, the customary homogenizers are used for emulsification. In order to
stabilize
the dispersed sizes in the aqueous phase, dispersants are used. Thus, for
example, at
least one ionic dispersant is used for the preparation of anionic size
dispersions, for
example a dispersant from the group consisting of the condensates of
(a) naphthalenesulfonic acid and formaldehyde,
(b) phenol, phenolsulfonic acid and formaldehyde,
(c) naphthalenesulfonic acid, formaldehyde and urea and
(d) phenol, phenolsulfonic acid, formaldehyde and urea.
The anionic dispersant may be present in the form of the free acids, the
alkali metal
salts, alkaline earth metal salts and/or the ammonium salts. The ammonium
salts may
be derived both from ammonia and from primary, secondary and tertiary amines;
for
example, the ammonium salts of dimethylamine, trimethylamine, hexylamine
cyclohexylamine, dicyclohexylamine, ethanolamine, diethanolamine and
triethanolamine are suitable. The condensates described above are known and
are
commercially available. They are prepared by condensation of said
constituents, it also
being possible to use the corresponding alkali metal, alkaline earth metal or
ammonium
salts instead of the free acids. For example, acids, such as sulfuric acid, p-
toluenesulfonic acid and phosphoric acid, are suitable as a catalyst in the
condensation. Naphthalenesulfonic acid or the alkali metal salts thereof are
condensed
with formaldehyde, preferably in the molar ratio from 1:0.1 to 1:2 and in
general in the
molar ratio of from 1:0.5 to 1:1. The molar ratio for the preparation of
condensates of
PF 58514
CA 02665712 2009-04-06
phenol, phenolsulfonic acid and formaldehyde is likewise in the abovementioned
range,
any desired mixtures of phenol and phenolsulfonic acid being used instead of
naphthalenesulfonic acid in the condensation with formaldehyde. Instead of
phenolsulfonic acid, it is also possible to use the alkali metal and ammonium
salts of
5 phenolsulfonic acid. The condensation of the abovementioned starting
material can, if
appropriate, additionally be carried out in the presence of urea. For example,
from 0.1
to 5 mol of urea, based on naphthalenesulfonic acid or on the mixture of
phenol and
phenolsulfonic acid, are used per mole of naphthalenesulfonic acid or per mole
of the
mixture of phenol and phenolsulfonic acid.
The condensates have, for example, molar masses in the range from 800 to 100
000,
preferably from 1000 to 30 000 and in particular from 4000 to 25 000. Salts
which are
obtained, for example, on neutralization of the condensates with lithium
hydroxide,
sodium hydroxide, potassium hydroxide or ammonia are preferably used as
anionic
dispersants. The pH of the salts is, for example, in the range from 7 to 10.
Other suitable anionic dispersants are amphiphilic copolymers of
(i) hydrophobic monoethylenically unsaturated monomers and
(ii) hydrophilic monomers having an anionic group, such as monoethylenically
unsaturated carboxylic acids, monoethylenically unsaturated sulfonic acids,
monoethylenically unsaturated phosphonic acids or mixtures thereof.
Suitable hydrophobic monoethylenically unsaturated monomers (i) are, for
example,
olefins having 2 to 150 carbon atoms, styrene, a-methylstyene, ethylstyrene, 4-
methylstyrene, acrylonitrile, methacrylonitrile, esters of monoethylenically
unsaturated
Ca- to Cs-carboxylic acids and monohydric alcohols, amides of acrylic acid or
methacrylic acid with C,- to C24-alkylamines, vinyl esters of saturated
monocarboxylic
acids having 2 to 24 carbon atoms, diesters of maleic acid or fumaric acid
with
monohydric Cl- to C24-alcohols, vinyl ethers of alcohols having 3 to 24 carbon
atoms or
mixtures of said compounds.
The amphiphilic copolymers comprise, as hydrophilic monomers (ii), for
example,
monoethylenically unsaturated C3- to Clo-carboxylic acids or anhydrides
thereof, 2-
acrylamido 2-methylpropanesulfonic acid, vinylsulfonic acid, styrenesulfonic
acid,
vinylphosphonic acid, salts of said monomers or mixtures thereof as
hydrophilic
monomers having an anionic group incorporated in the form of polymerized
units.
Particularly preferred are aqueous size dispersions which comprise, as anionic
dispersants, amphiphilic copolymers of
PF 58514 CA 02665712 2009-04-06
16
(i) a-olefins having 4 to 12 carbon atoms, styrene or mixtures thereof as
hydrophobic monomers and
(ii) maleic acid, acrylic acid, methacrylic acid, monoesters of maleic acid
and
alcohols having 1 to 25 carbon atoms or alkoxylation products of such
alcohols,
monoamides of maleic acid, salts of said monomers or mixtures of these
compounds as hydrophilic monomers having an anionic group
incorporated in the form polymerized units and have a molar mass MW of from
1500 to
100 000.
Preferably used anionic dispersants are copolymers of maleic anhydride with C4-
to
C12-olefins, particularly preferably Ca-olefins, such as 1-octene and
diisobutene.
Diisobutene is very particularly preferred. The molar ratio of maleic
anhydride to olefin
is, for example, in the range from 0.9:1 to 3:1, preferably from 0.95:1 to
1.5:1. These
copolymers are preferably used in hydrolyzed form as aqueous solution or
dispersions,
the anhydride group being present in opened form and some or all of the
carboxyl
groups being neutralized. The following bases are used for the neutralization:
alkali
metal bases, such as sodium hydroxide, potassium hydroxide, sodium carbonate,
potassium carbonate, alkaline earth metal salts, such as calcium hydroxide,
calcium
carbonate, magnesium hydroxide, ammonia, primary, secondary or tertiary
amines,
such as triethylamine, triethanolamine, diethanolamine, ethanolamine,
morpholine, etc.
If the amphiphilic copolymers are not sufficiently water-soluble in the form
of the free
acid, they are used in the form of water-soluble salts; for example, the
corresponding
alkali metal, alkaline earth metal and ammonium salts are used. The molar mass
MW of
the amphiphilic copolymers is, for example, from 800 to 250 000, in general
from 1000
to 100 000 and is preferably in the range from 3000 to 20 000, in particular
from 1500
to 10 000. The acid numbers of the amphiphilic copolymers are, for example,
from 50
to 500, preferably from 150 to 300 mg KOH/g of polymer.
The amphiphilic copolymers are used, for example, in amounts of from 0.05 to
20,
preferably from 0.5 to 10, % by weight, based on the reactive size, as an
anionic
dispersant for the preparation of the size dispersions. The amphiphilic
copolymers are
preferably used in amounts of from 0.1 to 2, in particular from 0.6 to 1, % by
weight,
based on the size to be dispersed. With the use of amphiphilic copolymers
alone as
dispersants, aqueous size dispersions which are formaldehyde-free and have a
long
shelf life are obtained.
In order to prepare aqueous, anionic size dispersions, for example, an aqueous
solution of at least one condensate or of at least one amphiphilic copolymer
can be
initially taken and the size dispersed therein at temperatures of, for
example, from 20 to
1 00 C, preferably from 40 to 90 C. The size is preferably added in the form
of a melt
PF 58514 CA 02665712 2009-04-06
17
and is dispersed with vigorous stirring or shearing. The resulting dispersion
is cooled in
each case. In this way, it is possible to prepare, for example, aqueous,
anionic size
dispersions which comprise from 6 to 65% by weight of an alkyldiketene or from
0.1 to
65% by weight of alkylsuccinic anhydride as a size in dispersed form. Highly
concentrated size dispersions which comprise, for example, from 25 to 60% by
weight
of alkyldiketene as a size in the presence of from 0.1 to 5.0% by weight of a
condensate of naphthalenesulfonic acid and formaldehyde or of at least one
condensate from (b), (c) and/or (d) in dispersed form are preferred.
Further preferred size dispersions comprise from 25 to 60% by weight of an
alkyldiketene as a size and from 0.1 to 5.0% by weight of an amphiphilic
copolymer of
(i) from 95 to 50% by weight of isobutene, diisobutene, styrene or mixtures
thereof
and
(ii) from 5 to 50% by weight of acrylic acid, methacrylic acid, maleic acid,
monoesters of maleic acid and/or mixtures thereof or a water-soluble salt of
such
a copolymer.
Such highly concentrated size dispersions have a relatively low viscosity, for
example
in the range from 20 to 100 mPa.s (measured using a Brookfield viscometer and
at a
temperature of 20 C) in the preparation of the aqueous dispersions, the pH is,
for
example, from 2 to 8 and preferably in the range from 3 to 4. Aqueous, anionic
size
dispersions having a mean particle size of the sizes in the range of from 0.1
to 3,
preferably from 0.5 to 1.5, m are obtained.
The anionically dispersed reactive sizes can, if appropriate, additionally
comprise at
least one cationic dispersant, but the amount of the cationic dispersant
should be
chosen so that the dispersion as a whole carries an anionic charge. A
preferred
cationic dispersant is cationic starch.
According to another process variant, the addition of a retention aid and/or
of a filler is
effected in combination with an engine size and/or a strength agent with the
aid of the
. water fed in. Thus, at least one retention aid, a fixing agent and an engine
size and/or a
strength agent and, if appropriate, at least one thickener are metered, for
example, into
the stream of water fed in. However, these additives can be mixed in the
desired ratio
in a storage container and transported therefrom into the nozzle chamber of
the
headbox.
According to another process variant, the addition of a binder is effected
alone or in
combination with a filler, a retention aid, a fixing agent, an engine size
and/or a strength
agent, preferably with the aid of the water fed in, or it is metered into the
paper stock.
PF 58514 CA 02665712 2009-04-06
18
Thus, at least one binder and optionally a filler, a retention aid, a fixing
agent, an
engine size, a strength agent and, if appropriate, at least one thickener are
preferably
metered into the stream of the water fed in. However, these additives can be
mixed in
the desired ratio in a storage container and transported therefrom into the
nozzle
chamber of the headbox. Binders produce, for example, better binding of
fillers to the
paper fibers and, if they are metered into the fiber streams which form the
outer layers
of the paper, improve the printability of the paper. With the aid of the
binders, it is also
possible to improve the barrier properties of paper, for example against the
penetration
of fats, oils and water and against the passage of gases, in particular oxygen
or air.
Suitable synthetic binders are, for example, polymers which are composed of at
least
40% by weight of so-called main monomers selected from C,- to C2o-alkyl
(meth)acrylates, vinyl esters of saturated carboxylic acids comprising up to
20 carbon
atoms, vinylaromatics having up to 20 carbon atoms, ethylenically unsaturated
nitriles,
vinyl halides, vinyl ethers of alcohols comprising 1 to 10 carbon atoms,
aliphatic
hydrocarbons having 2 to 8 carbon atoms and one or two double bonds or
mixtures of
these monomers.
Particularly suitable synthetic polymers are polymers which are obtainable by
free
radical polymerization of ethylenically unsaturated compounds (monomers).
The binder is preferably a polymer which comprises at least 40% by weight,
preferably
at least 60% by weight, particularly preferably at least 80% by weight, of so-
called main
monomers.
The main monomers are selected from Cl-C2o-alkyl (meth)acrylates, vinyl esters
of
saturated carboxylic acids comprising up to 20 carbon atoms, vinylaromatics
having up
to 20 carbon atoms, ethylenically unsaturated nitriles, vinyl halides, vinyl
ethers of
alcohols comprising 1 to 10 carbon atoms, aliphatic hydrocarbons having 2 to 8
carbon
atoms and one or two double bonds or mixtures of these monomers.
For example, alkyl (meth)acrylates having a Cl-C,o-alkyl radical, such as
methyl
methacrylate, methyl acrylate, n-butyl acrylate, ethyl acrylate and 2-
ethylhexyl acrylate
may be mentioned. Mixtures of the alkyl (meth)acrylates are also particularly
suitable.
Vinyl esters of carboxylic acids having 1 to 20 carbon atoms are, for example,
vinyl
laurate, vinyl stearate, vinyl propionate, vinyl versatate and vinyl acetate.
Suitable vinylaromatic compounds are vinyl toluene, a- and p-methylstyrene, a-
butylstyrene, 4-n-butylstyrene, 4-n-decylstyrene and preferably styrene.
Examples of
nitriles are acrylonitrile and methacrylonitrile.
PF 58514 CA 02665712 2009-04-06
19
The vinyl halides are ethylenically unsaturated compounds substituted by
chlorine,
fluorine or bromine, preferably vinyl chloride and vinylidene chloride.
For example, vinyl methyl ether or vinyl isobutyl ether may be mentioned as
vinyl
ethers. Vinyl ethers of alcohols comprising 1 to 4 carbon atoms are preferred.
Ethylene, propylene, butadiene, isoprene and chloroprene may be mentioned as
hydrocarbons having 2 to 8 carbon atoms and one or two olefinic double bonds.
Preferred main monomers are Cl-C,o-alkyl (meth)acrylates and mixtures of the
alkyl
(meth)acrylates with vinylaromatics, in particular styrene (polymers having
these main
monomers are referred to for short as polyacrylates), or, alternatively,
hydrocarbons
having 2 double bonds, in particular butadiene, or mixtures of such
hydrocarbons with
vinylaromatics, in particular styrene (polymers having these main monomers are
referred to for short as polybutadienes).
In the case of mixtures of aliphatic hydrocarbons (in particular butadiene)
with
vinylaromatics (in particular styrene), the ratio may be, for example, from
10:90 to
90:10, in particular from 20:80 to 80:20.
In addition to the main monomers, the polymer may comprise monomers having at
least one acid group (acid monomer for short), for example monomers having
carboxyl,
sulfo or phosphonic acid groups. Carboxyl groups are preferred. For example,
acrylic
acid, methacrylic acid, itaconic acid, maleic acid or fumaric acid may be
mentioned.
Further monomers are, for example, monomers comprising hydroxyl groups, in
particular Cl-C,o-hydroxyalkyl (meth)acrylates, and (meth)acrylamide.
In the case of the polybutadienes, particularly preferred polymers are
accordingly
composed of
from 10 to 90% by weight, preferably from 20 to 70% by weight, of aliphatic
hydrocarbons having two double bonds in particular butadiene,
from 10 to 90% by weight, preferably from 30 to 80% by weight, of
vinylaromatic
compounds, in particular styrene,
from 0 to 20% by weight, preferably from 0 to 10% by weight, of acid monomer
and
from 0 to 20% by weight, preferably from 0 to 10% by weight, of further
monomers
or alternatively, in the case of the polyacrylates, of
from 10 to 95% by weight, preferably from 30 to 95% by weight, of Cl- to C,o-
alkyl
(meth)acrylates,
PF 58514 CA 02665712 2009-04-06
from 0 to 60% by weight, preferably from 0 to 50% by weight, of vinylaromatic
compounds, in particular styrene,
from 0 to 20% by weight, preferably from 0 to 10% by weight, of acid monomer
and
from 0 to 20% by weight, preferably from 0 to 10% by weight, of further
monomers.
5
Both the polybutadienes and the polyacrylates preferably comprise acid
monomers as
comonomers, preferably in an amount of from 1 to 5% by weight. The maximum
amount of the above aliphatic hydrocarbons in the case of the polybutadienes
or of the
alkyl (meth)acrylates in the case of the polyacrylates decreases
correspondingly by the
10 minimum amount of the acid monomers.
In a preferred embodiment, the preparation of the polymers is effected by
emulsion
polymerization and the polymer is therefore an emulsion polymer. However, the
polymers can, for example, also be prepared by solution polymerization and
15 subsequent dispersing of the polymer solution in water.
In the case of the emulsion polymerization, ionic and/or nonionic emulsifiers
and/or
protective colloids or stabilizers are usually used as surface-active
compounds.
20 The surface-active substance is used, for example, in amounts of from 0.1
to 10% by
weight, based on the monomers to be polymerized.
Water-soluble initiators for the emulsion polymerization are, for example,
ammonium
and alkali metal salts of peroxodisulfuric acid, e.g. sodium peroxodisulfate,
hydrogen
peroxide or organic peroxides, e.g. tert-butyl hydroperoxide.
So-called reduction-oxidation (redox) initiator systems are also suitable.
The amount of the initiator is in general from 0.1 to 10% by weight,
preferably from 0.5
to 5% by weight, based on the monomers to be polymerized. It is also possible
to use a
plurality of different initiators in the emulsion polymerization.
In the polymerization, it is possible to use regulators, for example in
amounts of from 0
to 0.8 part by weight, based on 100 parts by weight of the monomers to be
polymerized, by which the molar mass is reduced. For example, compounds having
a
thiol group, such as tert-butyl mercaptan, thioglycolic acid ethyl acrylic
ester,
mercaptoethanol, mercaptopropyltrimethoxysilane or tert-dodecyl mercaptan, are
suitable.
The emulsion polymerization is effected as a rule at from 30 to 130 C,
preferably from
to 90 C. The polymerization medium may consist either only of water or of
mixtures
of water and liquids miscible therewith, such as methanol. Preferably, only
water is
PF 58514 CA 02665712 2009-04-06
21
used. The emulsion polymerization can be carried out either as a batch process
or in
the form of a feed process, including the step or gradient procedure. The feed
process
in which a part of the polymerization batch is initially taken, heated to the
polymerization temperature and prepolymerized and the remainder of the
polymerization batch is then fed to the polymerization zone, usually via a
plurality of
spatially separated feeds, one or more of which comprise the monomers in pure
or in
emulsified form, continuously, stepwise or with superposition of a
concentration
gradient, while maintaining the polymerization, is preferred. A polymer seed
may also
be initially taken in the polymerization, for example for better adjustment of
the particle
size.
The manner in which the initiator is added to the polymerization vessel in the
course of
the free radical aqueous emulsion polymerization is known to the average
person
skilled in the art. It may be either completely initially taken in the
polymerization vessel
or used continuously or stepwise at the rate of its consumption in the course
of the free
radical aqueous emulsion polymerization. Specifically, this depends on the
chemical
nature of the initiator system as well as on the polymerization temperature.
Preferably,
a part is initially taken and the remainder is fed to the polymerization zone
at the rate of
consumption.
For removal of the residual monomers, initiator usually added even after the
end of the
actual emulsion polymerization, i.e. after the monomer conversion of at least
95%.
The individual components can be added to the reactor in the feed process from
above, at the side or from below through the reactor bottom.
In the emulsion polymerization, aqueous dispersions of the polymer, as a rule
having
solids contents of from 15 to 75% by weight, preferably from 40 to 75% by
weight, are
obtained.
Particularly suitable binders are also mixtures of different binders, for
example also
mixtures of synthetic and natural polymers. Aqueous polymer dispersions which
are
composed of at least 60% by weight of butadiene or mixtures of butadiene and
styrene
or aqueous dispersions of polymers which comprise at least 60% by weight of Cl-
to
C2o-alkyl (meth)acrylates or mixtures of C,- to C2o-alkyl (meth)acrylates with
styrene
incorporated in the form of polymerized units are preferably used as binders.
Other suitable binders are polymers which comprise N-vinylformamide and/or
vinylamine units and have an average molar mass M,, of at least 10 000. These
polymers may be present as an aqueous dispersion or as a solution in water.
They are
prepared, for example, by polymerization of N-vinylformamide alone or in the
presence
of at least one other nonionic, cationic and/or anionic monomer. The homo- and
PF 58514 CA 02665712 2009-04-06
22
copolymers of N-vinylformamide which can be prepared in this manner can be
hydrolyzed in a polymer-analogous reaction with elimination of formyl groups
from the
vinylformamide units incorporated in the form of polymerized units, with
formation of
amino groups. The hydrolysis is preferably effected in an aqueous medium in
the
presence of at least one acid, such as hydrochloric acid or sulfuric acid,
enzymatically
or in the presence of bases, such as sodium hydroxide solution or potassium
hydroxide
solution. The vinylformamide units may be completely or only partly
hydrolyzed. Thus,
for example in the case of complete hydrolysis of homopolymers of N-
vinylformamide,
polyvinylamines are obtained.
Suitable anionic monomers are, for example, monomers comprising acid groups.
Examples of these are acrylic acid, methacrylic acid, maleic acid, fumaric
acid, itaconic
acid, vinylphosphonic acid, acrylamido-2-methylpropanesulfonic acid,
styrenesulfonic
acid, allylacetic acid, crotonic acid and ethacrylic acid. The anionic
monomers can be
used in the polymerization in the form of the free acids or in a form partly
or completely
neutralized with alkali metal, alkaline earth metal and/or ammonium bases. The
sodium
salts or potassium salts of the acids are preferred. Both the unhydrolyzed
copolymers
of N-vinylformamide with anionic monomers and the partly or completely
hydrolyzed
copolymers of N-vinylformamide and anionic monomers, which are described, for
example, in DE-A-103 15 363, cf. in particular page 5, line 39 to page 12,
line 39, can
be used as binders for modifying inorganic pigments.
N-vinylformamide can also be copolymerized with cationic monomers, such as
dialkylaminoalkyl (meth)acrylates and/or diallyldimethylammonium chloride. The
basic
monomers are preferably used in the form of the salts with mineral acids or in
a form
partly or completely quaternized with alkyl halides or with dimethyl sulfate.
In the case
of the copolymerization of N-vinylformamide with anionic and/or cationic
monomers,
nonionic monomers, such as methyl acrylate, ethyl acrylate, methyl
methacrylate, vinyl
acetate, acrylamide and/or methacrylamide, can, if appropriate, additionally
be used.
Both the hydrolyzed cationic copolymers and the unhydrolyzed cationic
copolymers
can be used as binders for modifying the inorganic pigments. It is also
possible to use
amphoteric polymers which are obtainable, for example, by copolymerization of
N-
vinylformamide, dimethylaminoethyl acrylate methochloride and acrylic acid or
which
form as a result of complete or partial hydrolysis of the vinylformamide units
of these
copolymers. The polymers which comprise vinylformamide and/or vinylamine units
and
are used for modifying pigments preferably have an average molar mass M, of at
least
20 000. In general, the average molar masses of the copolymers are in the
range from
30 000 to 5 million, in particular from 50 000 to 2 million. The molar masses
are
determined, for example, with the aid of static light scattering at pH 7.6 in
a 10 mmolar
aqueous sodium chloride solution.
Also suitable as binders are ethylene copolymer waxes which comprise
PF 58514 CA 02665712 2009-04-06
23
(A') from 20.5 to 38.9% by weight, preferably from 21 to 28% by weight, of at
least
one ethylenically unsaturated carboxylic acid,
(B') from 60 to 79.4% by weight, preferably from 70 to 78.5% by weight, of
ethylene
and
(C') from 0.1 to 15% by weight, preferably from 0.5 to 10% by weight, of at
least one
ethylenically unsaturated carboxylate incorporated in the form of polymerized
units.
The ethylene copolymer waxes described above have, for example, a melt flow
rate
(MFR) in the range from 1 to 50 g/10 min, preferably from 5 to 20 g/10 min,
particularly
preferably from 7 to 15 g/10 min, measured at 160 C and a load of 325 g
according to
EN ISO 1133. The acid number is usually from 100 to 300 mg KOH/g of wax,
preferably from 115 to 230 mg KOH/g of wax, determined according to DIN 53402.
They have a kinematic melt viscosity n of at least 45 000 mm2/s, preferably of
at least
50 000 mmz/s. The melting ranges of the ethylene copolymer waxes are, for
example,
in the range from 60 to 110 C, preferably in the range from 65 to 90 C,
determined by
DSC according to DIN 51007.
The melting ranges of the ethylene copolymer waxes may be broad and may have a
temperature interval of at least 7 to not more than 20 C, preferably at least
10 C and
not more than 15 C.
The melting points of the ethylene copolymer waxes can, however, also have a
small
variation and may be in a temperature interval of less than 2 C, preferably
less than
1 C, determined according to DIN 51007.
The density of the waxes is usually from 0.89 to 1.10 g/cm3, preferably from
0.92 to
0.99 g/cm3, determined according to DIN 53479.
The ethylene copolymer waxes are, for example, alternating copolymers or block
copolymers or preferably random copolymers.
Ethylene copolymer waxes obtained from ethylene and ethylenically unsaturated
carboxylic acids and, if appropriate, ethylenically unsaturated carboxylic
esters can
advantageously be prepared by free radical copolymerization under high
pressure
conditions, for example in high-pressure autoclaves which are equipped with a
stirrer,
or in high-pressure tubular reactors. The preparation in high-pressure
autoclaves which
are equipped with a stirrer is preferred. Such high-pressure autoclaves are
known per
se and a description is to be found in Ullmann's Encyclopedia of/ndustrial
Chemistry,
PF 58514 CA 02665712 2009-04-06
24
5th edition, keyword: Waxes, vol. A28, page 146 et seq., Verlag Chemie,
Weinheim,
Basel, Cambridge, New York, Tokyo, 1996. In them, the length/diameter ratio is
predominantly in the ranges from 5:1 to 30:1, preferably from 10:1 to 20:1.
The high-
pressure tubular reactors which can likewise be used are also described in
Ullmann's
Encyclopedia oflndustrialChemistry, 5th edition, keyword: Waxes, vol. A28,
page 146
et seq., Verlag Chemie, Weinheim, Basel, Cambridge, New York, Tokyo, 1996.
Suitable pressure conditions for the polymerization are from 500 to 4000 bar,
preferably from 1500 to 2500 bar. Conditions of this type are also referred to
below as
high-pressure. The reaction temperatures are in the range from 170 to 300 C,
preferably in the range from 195 to 280 C. The polymerization can also be
carried out
in the presence of a regulator. The abovementioned waxes are described in
detail, for
example, in WO-A-04/108601, page 2, line 38 to page 12, line 10.
According to another variant of the process according to the invention, the
addition of
an unstabilized engine size alone or in combination with a filler, a retention
aid, a fixing
agent and/or a strength agent is effected with the aid of the water fed in.
Thus, at lest
one unstabilized engine size and optionally a filler, a retention aid, a
fixing agent, a
strength agent and at least one thickener are metered into the stream of the
water fed
in. However, these additives can be mixed in the desired ratio in the storage
container
and transported therefrom into the nozzle chamber of the headbox.
Unstabilized engine sizes are to be understood as meaning the above-described
engine sizes which contain no stabilizers or very small proportions of
stabilizers. If the
engine size dispersion is metered into the stream of the water fed in,
directly after
emulsification according to the above-described variant of the process
according to the
invention, the engine sizes can be emulsified on site.
The aqueous dispersions of an engine size can, if appropriate, be used
together with a
cationic, synthetic polymer acting as a fixing agent and promoter, fixing
agent and
promoter being metered as a mixture with at least one engine size or
separately
therefrom into the paper stock. The metering is generally effected into the
low-viscosity
stock. For optimum metering, engine size and, if appropriate, promoter are
metered in
combination with a retention aid into the paper stock stream at a point which
is after the
last shearing stage of the paper stock and before the nozzle mouth of the
headbox.
The cationic polymers used as fixing agent and promoter for engine sizes can
be
metered into the paper stock, for example, before or after the last shearing
stage.
Examples of cationic polymers of this type are polymers comprising vinylamine
units,
polymers comprising vinylguanidine units, polyethylenimines, polyamidoamines
grafted
with ethylenimine and/or polydiallyldimethylammonium chlorides. The amount of
fixing
agents is, for example, from 0.02 to 2.0, preferably from 0.05 to 0.5, % by
weight,
based on dry paper stock.
PF 58514
CA 02665712 2009-04-06
In another embodiment of the invention, at least one retention aid is used in
combination with a strength agent for paper. Known strength agents for paper
are, for
example, urea-formaldehyde resins which increase not only the wet strength but
also
5 the dry strength of the paper (cf. EP-A-0 123 196 and US-A-3,275,605),
melamine-
formaldehyde resins (cf. DE-B-10 90 078) or other commercially available
products, for
example polyamidoamines crosslinked with epichlorohydrin (cf. the Luresin
brands,
BASF Aktiengesellschaft, Ludwigshafen).
10 In order to be able to prepare multilayer webs from cellulose fibers, it is
possible to start
from all fibers customary in papermaking. The cellulose fibers are first
suspended in
water for the preparation of a paper stock. Suitable cellulose fibers are, for
example,
fibers obtained from mechanical pulp and all annual plants. Mechanical pulp
includes,
for example, groundwood, thermomechanical pulp (TMP), chemothermomechanical
15 pulp (CTMP), pressure groundwood, semi-chemical pulp, high-yield pulp and
refiner
mechanical pulp (RMP) and wastepaper. Chemical pulps which can be used in
bleached or in unbleached form, such as sulfate, sulfite and soda pulps, and
fibers
obtained from wastepaper are also suitable. Unbleached chemical pulps, which
are
also referred to as unbleached kraft pulp, are preferably used. Said fibers
can be used
20 alone or as a mixture. The use of kraft pulp and of TMP and CTMP is
particularly
preferred. The pH of the cellulose fiber slurry is, for example, from 4 to 8,
preferably
from 6 to 8. The consistency of the pulp which is drained in the paper machine
is not
more than 2% by weight and is in general in the range from 0.5 to 1.0% by
weight,
based on dry paper stock.
The process chemicals usually used in papermaking can also be added in the
usual
amounts to the paper stock, for example the abovementioned fixing agents,
sizes, dry
and wet strength agents, biocides and/or dyes. The paper stock is processed in
each
case according to the invention to give a multilayer fiber web by draining it
on a wire
with web formation. The webs thus produced are dried. Draining of the paper
stock and
drying of the webs are part of the papermaking process, which is carried out
continuously.
In the process according to the invention, for example, it is possible to
produce papers
which are composed of 2, 3, 4, 5 or more layers. Three-layer papers are
preferred. It is
in fact possible here to produce good-quality papers economically by using
cheap
fibers for the middle layer and fibers of better quality for the top and
bottom of the
three-layer paper. Thus, bleached fibers, such as bleached fibers of birch
sulfate
and/or pine sulfate can be used, for example, for the formation of the top and
bottom of
a three-layer paper while fibers from wastepaper, TMP and/or groundwood are
suitable
for the middle layer of the three-layer paper.
PF 58514 CA 02665712 2009-04-06
26
The multilayer papers produced according to the invention are suitable, for
example, as
printing and writing papers, copying papers, inkjet papers, cardboard and
board and for
the packaging of liquids and the production of folding boxes and corrugated
board and
cardboard.
