Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A process for the production of paper
This invention relates to papermaking and more specifically to a process for
the
production of paper in which a cationic organic polymer having an aromatic
group is
added to a papermaking stock. The process provides improved drainage and
retention.
Background
In the papermaking art, an aqueous suspension containing cellulosic fibres,
and
optional fillers and additives, referred to as stock, is fed into a headbox
which ejects the
stock onto a forming wire. Water is drained from the stock through the forming
wire so
that a wet web of paper is formed on the wire, and the web is further
dewatered and
dried in the drying section of the paper machine. Water obtained by dewatering
the stock,
referred to as white water, which usually contains fine particles, e.g. fine
fibres, fillers and
additives, is normally recirculated in the papermaking process. Drainage and
retention
aids are conventionally introduced into the stock in order to facilitate
drainage and
increase adsorption of fine particles onto the cellulosic fibres so that they
are retained
with the fibres on the wire. Cationic organic polymers like cationic starch
and cationic
acrylamide-based polymers are widely used as drainage and retention aids.
These
polymers can be used alone but more frequently they are used in combination
with other
polymers and/or with anionic microparticulate materials such as, for example,
anionic
inorganic particles like colloidal silica, colloidal aluminium-modified silica
and bentonite.
U.S. Patent Nos. 4,980,025; 5,368,833; 5,603,805; 5,607,552; and 5,858,174; as
well as International Patent Application WO 97/18351 disclose the use of
cationic and
amphoteric acrylamide-based polymers and anionic inorganic particles as stock
additives in
papermaking. These additives are among the most efficient drainage and
retention aids
now in use. Similar systems are disclosed in European Patent Application No.
805,234.
It has, however, been observed that the performance of drainage and retention
aids comprising cationic organic polymers deteriorates when used in stocks
with high
levels of salt, i.e. high conductivity, and dissolved and colloidal
substances. Higher
dosages of cationic polymer are normally required in such stocks but usually
the
drainage and retention effect obtained is still not entirely satisfactory.
These problems are
noticeable in paper mills where white water is extensively recirculated with
the introduction
of only low amounts of fresh water into the process, thereby further
increasing the accu-
mulation of salts and colloidal materials in the white water and the stock to
be dewatered.
The Invention
According to the present invention it has been found that improved drainage
and
retention can be obtained in stocks containing high levels of salt (high
conductivity) and
colloidal materials and/or in papermaking processes with a high degree of
white water
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2
closure when using a drainage and retention aid comprising a cationic organic
polymer
having an aromatic group. More specifically, the present invention relates to
a process
for the production of paper from a suspension containing cellulosic fibres,
and optional
fillers, which comprises adding to the suspension a drainage and retention aid
comprising
a cationic organic polymer, forming and dewatering the suspension on a wire,
the
process being characterised in that the cationic organic polymer has an
aromatic group
and the suspension being dewatered on the wire has a conductivity of at least
2.0
mS/cm. The present invention also relates to a process as described in the pre-
characterising clause above, the process being further characterised in that
it comprises
forming and dewatering the suspension on a wire to obtain a wet web containing
cellulosic
fibres, or paper, and white water, recirculating the white water and
optionally introducing
fresh water to form a suspension containing cellulosic fibres, and optional
fillers, to be
dewatered to form paper, wherein the cationic organic polymer has an aromatic
group and
the amount of fresh water introduced is less than 30 tons per ton of dry paper
produced.
The invention thus relates to a process as further defined in the claims.
The present invention results in improved drainage and/or retention when using
stocks having high contents of salt, and thus having high conductivity levels,
and colloidal
materials. The present invention also results in improved drainage andlor
retention when
applied to papermaking processes with extensive white water recirculation and
limited
fresh water supply andlor processes using fresh water having high salt
contents, in
particular salts of di- and multivalent cations like calcium. Hereby the
present invention
makes it possible to increase the speed of the paper machine and to use lower
dosages of
additives to give a corresponding drainage and/or retention effect, thereby
leading to an
improved papermaking process and economic benefits.
The cationic organic polymer having an aromatic group according to this
invention, herein also referred to as "main polymer", is capable of
functioning as a
drainage and retention aid {agent). The term "drainage and retention aid", as
used
herein, refers to one or more components (aids, agents, or additives) which,
when being
added to a stock, give better drainage and/or retention than is obtained when
not adding
the said one or more components. Accordingly, the main polymer provides
improved
drainage and/or retention, either when used alone or when used in conjunction
with one
or more additional stock additives. The main polymer can be linear, branched
or cross-
linked, e.g. in the form of a microparticulate material. Preferably the main
polymer is water-
soluble or water-dispersable. The aromatic group of the main polymer can be
present in the
polymer backbone or, preferably, it can be a pendent group attached to or
extending from
the polymer backbone or be present in a pendent group that is attached to or
extending
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from the polymer backbone (main chain). Suitable aromatic (aryl) groups
include those
comprising a phenyl group, optionally substituted, a phenylene group,
optionally
substituted, and a naphthyl group, optionally substituted, for example groups
having the
general formulae -C6H5, -C6H4 , -CsH3-, and -C6Hz-, e.g. in the form of
phenylene
(-C6H4-), xylylene (-CH2-CsH4-CHZ-), phenyl (-C6H5), benzyl (-CH2-C6H5),
phenethyl
(-CHzCH2-C6H5), and substituted phenyl (for example -C6H4-Y, -CsH3Y2, and -
C6HZY3)
where one or more substituents (Y) attached to the phenyl ring can be selected
from
hydroxyl, halides, e.g. chloride, nitro, and hydrocarbon groups having from 1
to 4 carbon
atoms.
The main polymer can be selected from homopolymers and copolymers prepared
from one or more monomers comprising at least one monomer having an aromatic
group,
suitably an ethyfenically unsaturated monomer, and the main polymer is
suitably a vinyl
addition polymer. The term "vinyl addition polymer", as used herein, refers to
a polymer
prepared by addition polymerization of one or more vinyl monomers or
ethyienically
unsaturated monomers which include, for example, acrylamide-based and acrylate-
based
monomers. Suitable main polymers include cationic vinyl addition polymers
obtained by
polymerizing a cationic monomer or a monomer mixture comprising a cationic
monomer
represented by the general formula (I):
CHZ = C - R, RZ (I)
O=C-A,-B,-N+-Q X-
l
R3
wherein R, is H or CH3; Rz and R3 are each or, preferably, an alkyl group
having from 1 to 3
carbon atoms, usually 1 to 2 carbon atoms; A, is O or NH; B, is an alkylene
group having
from 2 to 8 carbon atoms, suitably from 2 to 4 carbon atoms, or a hydroxy
propylene group;
Q is a substituent containing an aromatic group, suitably a phenyl or
substituted phenyl
group, which can be attached to the nitrogen by means of an alkylene group
usually having
from 1 to 3 carbon atoms, suitably 1 to 2 carbon atoms, and preferably Q is a
benzyl group
(- CH2- CsHs); and X- is an anionic counterion, usually a halide like
chloride. Examples of
suitable monomers represented by the general formula (I) include quaternary
monomers
obtained by treating dialkylaminoalkyl (meth)acrylates, e.g.
dimethylaminoethyl (meth)-
acrylate, diethylaminoethyl (meth)acrylate and dimethylaminohydroxypropyl
(meth)acrylate,
and dialkylaminoalkyl (meth)acrylamides, e.g. dimethylaminoethyl
(meth)acrylamide,
diethylaminoethyl (meth)acrylamide, dimethylaminopropyl (meth)acrylamide, and
diethyl-
aminopropyl (meth)acrylamide, with benzyl chloride. Preferred cationic
monomers of the
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general formula (I) include dimethylaminoethylacrylate benzyl chloride
quaternary salt and
dimethylaminoethylmethacrylate benzyl chloride quaternary salt.
The main polymer can be a homopolymer prepared from a cationic monomer
having an aromatic group or a copolymer prepared from a monomer mixture
comprising a
cationic monomer having an aromatic group and one or more copolymerizabie
monomers.
Suitable copolymerizable non-ionic monomers include monomers represented by
the
general formula (II):
CH2 = C - R4 RS (II)
O=C-AZ-BZ-N
Rs
wherein R4 is H or CH3; R5 and Rs are each H or a hydrocarbon group, suitably
alkyl, having
from 1 to 6, suitably from 1 to 4 and usually from 1 to 2 carbon atoms; Az is
O or NH; BZ is
an alkylene group of from 2 to 8 carbon atoms, suitably from 2 to 4 carbon
atoms, or a
hydroxy propylene group or, alternatively, A and B are both nothing whereby
there is a
single bond between C and N (O=C-NRSRs). Examples of suitable copolymerizable
monomers of this type include (meth)acrylamide; acrylamide-based monomers like
N-alkyl
(meth)acrylamides and N,N-dialkyl (meth)acrylamides, e.g. N-n-
propylacrylamide, N-
isopropyl (meth)acrylamide, N-n-butyl (meth)acrylamide, N-isobutyl
{meth)acrylamide and
N-t-butyl (meth)acrylamide; and dialkylaminoalkyl (meth)acrylamides, e.g.
dimethylamino-
ethyl (meth)acrylamide, diethylaminoethyl (meth)acrylamide,
dimethylaminopropyl
(meth)acrylamide and diethylaminopropyl (meth)acrylamide; acrylate-based
monomers like
dialkylaminoalkyl (meth)acrylates, e.g. dimethylaminoethyl (meth)acrylate,
diethylamino-
ethyf (meth)acrylate, t-butylaminoethyl (meth)acrylate and
dimethylaminohydroxypropyl
acrylate; and vinylamides, e.g. N-vinylformamide and N-vinylacetamide.
Preferred
copolymerizable non-ionic monomers include acrylamide and methacrylamide, i.e.
(meth)acrylamide, and the main polymer is preferably an acrylamide-based
polymer.
Suitable copolymerizable cationic monomers include the monomers represented
by the general formula (111):
CHz=C-R, RB (III)
O=C-As-B3-N+-R,o X_
R9
wherein R, is H or CH3; Re, R9 and R,o are each H or, preferably, a
hydrocarbon group,
suitably alkyl, having from 1 to 3 carbon atoms, usually 1 to 2 carbon atoms;
A3 is O or NH;
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WO 99/55965 PCT/SE99/00677
B3 is an alkylene group of from 2 to 4 carbon atoms, suitably from 2 to 4
carbon atoms, or a
hydroxy propylene group, and X- is an anionic counterion, usually
methylsulphate or a
halide like chloride. Examples of suitable cationic copolymerizable monomers
include acid
addition salts and quaternary ammonium salts of the dialkylaminoalkyl
(meth)acrylates and
5 dialkylaminoalkyl (meth)acrylamides mentioned above, usually prepared using
acids like
HCI, HZS04, etc., or quaternizing agents like methyl chloride, dimethyl
sulphate, etc.; and
diallyldimethylammonium chloride. Preferred copolymerizable cationic monomers
include
dimethylaminoethyl (meth)acrylate methyl chloride quaternary salt and
diallyldimethyl-
ammonium chloride. Copolymerizable anionic monomers like acrylic acid,
methacrylic acid,
various sulfonated vinyl addition monomers, etc. can also be employed and,
preferably, in
minor amounts.
The main polymer according to this invention can be prepared from a monomer
mixture generally comprising from 1 to 99 mole%, suitably from 2 to 50 mole%
and
preferably from 5 to 20 mole% of cationic monomer having an aromatic group,
preferably
represented by the general formula (I), and from 99 to 1 mole%, suitably from
98 to 50
mole%, and preferably from 95 to 80 mole% of other copolymerizable monomers
which
preferably comprises acrylamide or methacrylamide ((meth)acrylamide), the
monomer
mixture suitably comprising from 98 to 50 mole% and preferably from 95 to 80,
mole% of
(meth)acrylamide, the sum of percentages being 100.
The main polymer can also be selected from polymers prepared by condensation
reaction of one or more monomers containing an aromatic group. Examples of
such
monomers include toluene diisocyanates, bisphenol A, phthalic acid, phthalic
anhydride,
etc., which can be used in the preparation of cationic polyurethanes, cationic
polyamide-
amines, etc.
Alternatively, or additionally, the main polymer can be a polymer subjected to
aromatic modification using an agent containing an aromatic group. Suitable
modifying
agents of this type include benzyl chloride, benzyl bromide, N-(3-chloro-2-
hydroxypropyl)-
N-benzyl-N,N-dimethylammonium chloride, and N-(3-chloro-2-hydroxypropyl)
pyridinium
chloride. Suitable polymers for such an aromatic modification include vinyl
addition
polymers. If the polymer contains a tertiary nitrogen which can be quaternized
by the
modifying agent, the use of such agents usually results in that the polymer is
rendered
cationic. Alternatively, the polymer to be subjected to aromatic modification
can be cationic,
for example a cationic vinyl addition polymer.
Usually the charge density of the main polymer is within the range of from 0.1
to
6.0 meqvlg of dry polymer, suitably from 0.2 to 4.0 and preferably from 0.5 to
3Ø
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The weight average molecular weight of synthetic main polymers is usually at
least about
500,000, suitably above about 1,000,000 and preferably above about 2,000,000.
The upper
limit is not critical; it can be about 50,000,000, usually 30,000,000 and
suitably 25,000,000.
The main polymer of this invention may be in any state of aggregation such as,
for
example, in solid form, e.g. powders, in liquid form, e.g. solutions,
emulsions, dispersions,
including salt dispersions, etc. Examples of suitable main polymers for use in
this invention
include those described in U.S. Patent Nos. 5,169,540; 5,708,071; and European
Patent
Applications 183,466; 525,751 and 805,234. When being added to the stock, the
main polymer is suitably in liquid form, e.g. in the form of an aqueous
solution
or dispersion.
The main polymer can be added into the stock to be dewatered in amounts which
can vary within wide limits depending on, inter alia, type of stock, salt
content, type of salts,
filler content, type of filler, point of addition, etc. Generally the main
polymer is added in an
amount that give better retention than is obtained when not adding it. The
main polymer
is usually added in an amount of at least 0.001 %, often at least 0.005% by
weight, based
on dry stock substance, whereas the upper limit is usually 3% and suitably
1.5% by weight.
In a preferred embodiment of this invention, the main polymer is used in
conjunction with an additional stock additive, thereby forming a drainage and
retention
aid comprising two or more components, usually referred to as drainage and
retention
aids. The term "drainage and retention aids", as used herein, refers to two or
more
components (aids, agents or additives) which, when being added to a stock,
give better
drainage andlor retention than is obtained when not adding the components.
Examples
of suitable stock additives of this type include anionic microparticulate
materials, e.g.
anionic organic particles and anionic inorganic particles, water-soluble
anionic vinyl
addition polymers, low molecular weight cationic organic polymers, aluminium
compounds, and combinations thereof. In a preferred aspect of this embodiment,
the
main polymer is used in conjunction with an anionic microparticulate material,
notably
with anionic inorganic particles. In another preferred aspect of this
embodiment, the main
polymer is used in conjunction with anionic inorganic particles and a low
molecular weight
cationic organic polymer. In yet another preferred aspect of this embodiment,
the main
polymer is used in conjunction with anionic inorganic particles and an
aluminium
compound.
Anionic inorganic particles that can be used according to the invention
include
anionic silica-based particles and clays of the smectite type. It is preferred
that the anionic
inorganic particles are in the colloidal range of particle size. Anionic
silica-based particles,
i.e. particles based on Si02 or silicic acid, are preferably used and such
particles are usually
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supplied in the form of aqueous colloidal dispersions, so-called sols.
Examples of suitable
silica-based particles include colloidal silica and different types of
polysilicic acid. The silica-
based sols can also be modified and contain other elements, e.g. aluminium
andlor boron,
which can be present in the aqueous phase and/or in the silica-based
particles. Suitable
silica-based particles of this type include colloidal aluminium-modified
silica and aluminium
silicates. Mixtures of such suitable silica-based particles can also be used.
Drainage and
retention aids comprising suitable anionic silica-based particles are
disclosed in U.S. Patent
Nos. 4,388,150; 4,927,498; 4,954,220; 4,961,825; 4,980,025; 5,127,994;
5,176,891;
5,368,833; 5,447,604; 5,470,435; 5,543,014; 5,571,494; 5,573,674; 5,584,966;
5,603,805;
5,688,482; and 5,707,493.
Anionic silica-based particles suitably have an average particle size below
about
50 nm, preferably below about 20 nm and more preferably in the range of from
about 1 to
about 10 nm. As conventional in silica chemistry, the particle size refers to
the average size
of the primary particles, which may be aggregated or non-aggregated. The
specific surface
area of the silica-based particles is suitably above 50 mZ/g and preferably
above 100 m2lg.
Generally, the specific surtace area can be up to about 1700 mZlg and
preferably up to
1000 mZlg. The specific surface area can be measured by means of titration
with NaOH in
known manner, e.g. as described by Sears in Analytical Chemistry 28(1956):12,
1981-1983
and in U.S. Patent No. 5,176,891. The given area thus represents the average
specific
surface area of the particles.
In a preferred embodiment of the invention, the anionic inorganic particles
are
silica-based particles having a specific surface area within the range of from
50 to 1000
m2lg, preferably from 100 to 950 mzlg. Sols of silica-based particles these
types also
encompass modified sots like aluminium-containing silica-based sots and boron-
containing
silica-based sots. Preferably, the silica-based particles are present in a sol
having an S-
value in the range of from 8 to 45%, preferably from 10 to 30%, containing
silica-based
particles with a specific surface area in the range of from 300 to 1000 m2lg,
suitably from
500 to 950 mzlg, and preferably from 750 to 950 m2lg, which sols can be
modified with
aluminium andJor boron as mentioned above. For example, the particles can be
surface-
modified with aluminium to a degree of from 2 to 25% substitution of silicon
atoms. The S-
value can be measured and calculated as described by Iler & Dalton in J. Phys.
Chem.
60(1956), 955-957. The S-value indicates the degree of aggregate or microgel
formation
and a lower S-value is indicative of a higher degree of aggregation.
In yet another preferred embodiment of the invention, the silica-based
particles are
selected from poiysilicic acid and modified polysilicic acid having a high
specific surface
area, suitably above about 1000 m2lg. The specific surface area can be within
the range of
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from 1000 to 1700 m2lg and preferably from 1050 to 1600 m2/g. The sots of
modified
polysilicic acid can contain other elements, e.g. aluminium andlor boron,
which can be
present in the aqueous phase and/or in the silica-based particles. In the art,
polysilicic acid
is also referred to as polymeric silicic acid, polysilicic acid microgel,
polysilicate and
polysilicate microgel, which are all encompassed by the term polysilicic acid
used herein.
Aluminium-containing compounds of this type are commonly also referred to as
poly-
aluminosilicate and polyaluminosilicate microgel, which are both encompassed
by the
terms colloidal aluminium-modified silica and aluminium silicate used herein.
Clays of the smectite type that can be used in the process of the invention
are
known in the art and include naturally occurring, synthetic and chemically
treated materials.
Examples of suitable smectite clays include montmorillonite/bentonite,
hectorite, beidelite,
nontronite and saponite, preferably bentonite and especially such bentonite
which after
swelling preferably has a surface area of from 400 to 800 mz/g. Suitable clays
are disclosed
in U.S. Patent Nos. 4,753,710; 5,071,512; and 5,607,552
Anionic organic particles that can be used according to the invention include
highly
cross-linked anionic vinyl addition polymers, suitably copolymers comprising
an anionic
monomer like acrylic acid, methacrylic acid and sulfonated or phosphonated
vinyl addition
monomers, usually copolymerized with nonionic monomers like (meth)acrylamide,
alkyl
(meth)acrylates, etc. Useful anionic organic particles also include anionic
condensation
polymers, e.g. melamine-sulfonic acid sots. Water-soluble anionic vinyl
addition polymers
that can be used according to the invention include copolymers comprising an
anionic
monomer like acrylic acid, methacrylic acid and sulfonated vinyl addition
monomers, usually
copolymerized with nonionic monomers like acrylamide, alkyl acrylates, etc.,
for example
those disclosed in U.S. Patent Nos. 5,098,520 and 5,185,062,
Low molecular weight (hereinafter LMW) cationic organic polymers that can be
used according to the invention include those commonly referred to and used as
anionic
trash catchers (ATC). ATC's are known in the art as neutralizing and/or fixing
agents for
detrimental anionic substances present in the stock and the use thereof in
combination with
drainage andlor retention aids often provide further improved drainage and/or
retention.
The LMW cationic organic polymer can be derived from natural or synthetic
sources, and
preferably it is an LMW synthetic polymer. Suitable organic polymers of this
type include
LMW highly charged cationic organic polymers such as polyamines,
polyamidoamines,
polyethyleneimines, homo- and copolymers based on diallyldimethyl ammonium
chloride,
(meth)acrylamides and (meth)acrylates. In relation to the molecular weight of
the main
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polymer, the molecular weight of the LMW cationic organic polymer is
preferably lower; it is
suitably at least 2,000 and preferably at least 10,000. The upper limit of the
molecular
weight is usually about 700,000, suitably about 500,000 and usually about
200,000.
Aluminium compounds that can be used according to the invention include alum,
aluminates, aluminium chloride, aluminium nitrate and polyaluminium compounds,
such as
polyaluminium chlorides, polyaluminium sulphates, polyaluminium compounds
containing
both chloride and sulphate ions, polyaluminium silicate-sulphates, and
mixtures thereof.
The polyaluminium compounds may also contain other anions than chloride ions,
for
example anions from sulfuric acid, phosphoric acid, organic acids such as
citric acid and
oxalic acid.
Components of drainage and retention aids according to the invention can be
added to the stock in conventional manner and in any order. When using
drainage and
retention aids comprising a main polymer and an anionic microparticulate
material,
notably anionic inorganic particles, it is preferred to add the main polymer
to the stock
before adding the microparticulate material, even if the opposite order of
addition may be
used. It is further preferred to add the main polymer before a shear stage,
which can be
selected from pumping, mixing, cleaning, etc., and to add the anionic
particles after that
shear stage. When using an LMW cationic organic polymer or an aluminium
compound,
such components are preferably introduced into the stock prior to introducing
the main
polymer, optionally used in conjunction with an anionic microparticulate
material.
Alternatively, the LMW cationic organic polymer and the main polymer can be
introduced
into stock essentially simultaneously, either separately or in admixture, for
example as
disclosed in U.S. Patent No. 5,858,174. The LMW cationic organic polymer and
the
main polymer are preferably introduced into the stock prior to introducing an
anionic
microparticulate material.
The drainage and retention aids) according to the invention can be added to
the
stock to be dewatered in amounts which can vary within wide limits depending
on, inter alia,
type and number of components, type of stack, salt content, type of salts,
filler content, type
of filler, point of addition, degree of white water closure, etc. Generally
the aids) are added
in amounts that give better drainage andlor retention than is obtained when
not adding
the components. The main polymer is usually added in an amount of at least
0.001%,
often at least 0.005% by weight, based on dry stock substance, and the upper
limit is
usually 3% and suitably 1.5% by weight. Similar amounts are suitable for water-
soluble
anionic vinyl addition polymers, if used. When using an anionic
microparticulate material in
the process, the total amount added is usually at least 0.001 % by weight,
often at least
0.005% by weight, based on dry substance of the stock, and the upper limit is
usually 1.0%
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and suitably 0.6% by weight. When using anionic silica-based particles, the
total amount
added is suitably within the range of from 0.005 to 0.5% by weight, calculated
as Si02 and
based on dry stock substance, preferably within the range of from 0.01 to 0.2%
by weight.
When using an LMW cationic organic polymer in the process, it can be added in
an amount
5 of at least 0.05%, based on dry substance of the stock to be dewatered.
Suitably, the
amount is in the range of from 0.07 to 0.5%, preferably in the range from 0.1
to 0.35%.
When using an aluminium compound in the process, the total amount introduced
into the
stock to be dewatered depends on the type of aluminium compound used and on
other
effects desired from it. It is for instance well known in the art to utilize
aluminium
10 compounds as precipitants for rosin-based sizing agents. The total amount
added is usually
at least 0.05%, calculated as AIz03 and based on dry stock substance. Suitably
the amount
is in the range of from 0.5 to 3.0%, preferably in the range from 0.1 to 2.0%.
The process of this invention is preferably used in the manufacture of paper
from a
suspension containing cellulosic fibers, and optional fillers, i.e. a stock,
which has a high
conductivity. Usually, the conductivity of the stock that is dewatered on the
wire is at least
2.0 mSlcm, suitably at least 3.5 mS/cm, preferably at least 5.0 mS/crn and
most preferably
at least 7.5 mS/cm. Conductivity can be measured by standard equipment such
as, for
example, a WTUV LF 539 instrument supplied by Christian Berner. The values
referred to
above are suitably determined by measuring the conductivity of the cellulosic
suspension
that is fed into or present in the headbox of the paper machine or,
alternatively, by
measuring the conductivity of white water obtained by dewatering the
suspension. High
conductivity levels mean high contents of salts (electrolytes), where the
various salts can
be based on mono-, di- and multivalent cations like alkali metals, e.g. Na+
and K+, alkaline
earths, e.g. Ca2+ and Mg2+, aluminium ions, e.g. AI3+, AI(OH)2+ and
polyaluminium ions, and
mono-, di- and multivalent anions like halides, e.g., CI-, sulfates, e.g. SOa2-
and HS04 ,
carbonates, e.g. C032- and HC03 , silicates and lower organic acids. The
invention is
particularly useful in the manufacture of paper from stocks having high
contents of salts of
di- and multivalent cations, and usually the content of di- and multivalent
cations is at least
200 ppm, suitably at least 300 pm and preferably at least 400 ppm. The salts
can be
derived from the stock preparation stage, i.e. from the materials used to form
the stock, e.g.
water, celfulosic fibres and fillers, in particular in integrated mills where
a concentrated
aqueous fibre suspension from the pulp mill normally is mixed with water to
form a dilute
suspension suitable for paper manufacture in the paper mill. The salt may also
be derived
from various additives introduced into the stock, from the fresh water
supplied to the
process, etc. Further, the content of salts is usually higher in processes
where white water
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is extensively recirculated, which may lead to considerable accumulation of
salts in the
water circulating in the process.
The present invention further encompasses papermaking processes where white
water is extensively recirculated (recycled), i.e. with a high degree of white
water closure,
for example where from 0 to 30 tons of fresh water are used per ton of dry
paper produced,
usually less than 20, suitably less than 15, preferably less than 10 and
notably less than 5
tons of fresh water per ton of paper. Recirculation of white water obtained in
the process
suitably comprises mixing the white water with cellulosic fibres and/or
optional fillers to form
a suspension to be dewatered; preferably it comprises mixing the white water
with a
suspension containing cellulosic fibres, and optional ~Ilers, before the
suspension enters
the forming wire for dewatering. The white water can be mixed with the
suspension before,
between, simultaneous with or after introducing the components of drainage and
retention
aids, if used; and before, simultaneous with or after introducing the main
polymer. Fresh
water can be introduced in the process at any stage; for example, it can be
mixed with
celluiosic fibres in order to form a suspension, and it can be mixed with a
suspension
containing cellulosic fibres to dilute it so as to form the suspension to be
dewatered, before,
simultaneous with or after mixing the stock with white water and before,
between,
simultaneous with or after introducing the components of drainage and
retention aids, if
used; and before, simultaneous with or after introducing the main polymer.
Further additives which are conventional in papermaking can of course be used
in
combination with the additives) according to the invention, such as, for
example, dry
strength agents, wet strength agents, optical brightening agents, dyes, sizing
agents like
rosin-based sizing agents and cellulose-reactive sizing agents, e.g. ketene
dimers and
succinic anhydrides, etc. The cellulosic suspension, or stock, can also
contain mineral fillers
of conventional types such as, for example, kaolin, china clay, titanium
dioxide, gypsum,
talc and natural and synthetic calcium carbonates such as chalk, ground marble
and
precipitated calcium carbonate.
The process of this invention is used for the production of paper. The term
"paper", as used herein, of course include not only paper and the production
thereof, but
also other cellulosic fibre-containing sheet or web-like products, such as for
example board
and paperboard, and the production thereof. The process can be used in the
production of
paper from different types of suspensions of cellulose-containing fibres and
the
suspensions should suitably contain at least 25% by weight and preferably at
least 50% by
weight of such fibres, based on dry substance. The suspension can be based on
fibres
from chemical pulp such as sulphate, sulphite and organosolv pulps, mechanical
pulp such
as thermomechanical pulp, chemo-thermomechanical pulp, refiner pulp and
groundwood
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pulp, from both hardwood and softwood, and can also be based on recycled
fibres,
optionally from de-inked pulps, and mixtures thereof.
The invention is further illustrated in the following Examples which, however,
are
not intended to limit the same. Parts and % relate to parts by weight and % by
weight,
respectively, unless otherwise stated.
Example 1 (Comparison)
Drainage performance was evaluated by means of a Dynamic Drainage
Analyser (DDA), available from Akribi, Sweden, which measures the time for
draining a
set volume of stock through a wire when removing a plug and applying vacuum to
that
side of the wire opposite to the side on which the stock is present.
The furnish used was based on 70% by weight of pulp of bleached birch/pine
sulphate (60/40) refined to 200°CSF and 30% by weight of ground marble.
Stock volume
was 800 ml, consistency 0.3% and pH about 8.
Conductivity of the stock was adjusted to 0.47 mS/cm by addition of sodium
sulphate. The stock was stirred in a baffled jar at a speed of 1500 rpm
throughout the
test and chemicals additions were conducted as follows: i) adding cationic
polymer to the
stock following by stirring for 30 seconds, ii) adding anionic inorganic
particles to the
stock followed by stirring for 15 seconds, iii) draining the stock while
automatically
recording the drainage time.
The polymers used for in the test series were P1) a cationic copolymer
prepared
by polymerisation of acrylamide (90 mole%) and
acryloxyethyldimethylbenzylammonium
chloride (10 mole%) and having an average molecular weight of about 6,000,000;
and
P2) a cationic copolymer prepared by polymerisation of acrylamide (90 mole%)
and
acryloxyethyltrimethylammonium chloride (10 mole%) and having an average
molecular
weight of about 6,000,000. The polymers P1 and P2 were dissolved in water and
used as
0.1 % aqueous solutions.
The anionic inorganic particles used were silica-based particles of the type
disclosed in U.S. Patent No. 5,368,833. The sol had an S-value of about 25%
and
contained silica particles with a specific surface area of about 900 m2lg
which were
surface-modified with aluminium to a degree of 5%. The silica-based particles
were
added in an amount of 1.0 kglton, calculated as SiOz and based on dry stock
system.
Table 1 shows the drainage time at various dosages of P1 and P2, calculated as
dry polymer on dry stock system.
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Table 1
Test Polymer SiOz ConductivityDewatering
No. Dosage Dosage time
[s]
[kg/t] [kglt] [mSlcm] P1 P2
1 0 0 0.47 18.4 18.4
2 1 1 0.47 12.5 10.6
3 1.5 1 0.47 6.9 5.6
4 2 1 0.47 4.9 4.3
Example 2 (Comparison)
Dewatering and retention effect was evaluated by means of the DDA used in
Example 1 in combination with a nephelometer. First pass retention was
evaluated by
measuring the turbidity of the filtrate, the white water, obtained by draining
the stock.
The furnish used was based on 56% by weight of peroxide bleached TMPISGW
pulp (80/20), 14% by weight of bleached birch/pine sulphate pulp (60140)
refined to
200° CSF and 30% by weight of china clay. To the stock was added 40 g/I
of a colloidal
fraction, bleach water from an SC mill, filtrated through a 5 ~.m screen and
concentrated
with an OF filter, cut off 200,000. Stock volume was 800 ml, consistency 0.14%
and pH
was adjusted to 4.0 using dilute sulphuric acid. The conductivity was adjusted
by addition
of calcium chloride (60 ppm Ca2+), magnesium sulphate (18 ppm Mgz+) and sodium
bicarbonate (134 ppm HC03 ).
The polymers and anionic inorganic particles according to Example 1 were
similarly used in this test series. Two dosages of polymers were used, 1 kg/t
and 2 kg/t,
respectively, calculated as dry polymer on dry stock system. Table 2 shows the
dewatering and retention effect at various dosages of silica-based particles,
calculated as
Si02 and based on dry stock system.
Table 2
TestPolymer Si02 ConductivityDewatering Turbidity
No. dosage Dosage time [NTU]
[s]
[kglt] [kglt] [mSlcm] P1 P2 P1 P2
1 1 0 1.375 21.2 18.7 63 55
2 1 1 1.375 17.2 16.1 67 60
3 1 2 1.375 21.2 18.6 66 57
4 2 0 1.375 15.2 14.2 47 45
5 2 1 1.375 11 9.9 47 47
6 2 2 1.375 11.4 10.8 45 50
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Examale 3
In this test series, the dewatering and retention effect was evaluated
according
to the procedure descibed in Example 2.
The furnish used was the same as used in Example 2. Stock volume was 800 ml
and pH about 7. The conductivity was adjusted by addition of calcium chloride,
thus
simulating a very high electrolyte content and a high degree of white water
closure.
The polymers and anionic inorganic particles according to Example 1 were
similarly used in this test series.
Table 3 shows the dewatering and retention effect at various dosages of silica-
based particles, calculated as Si02 and based on dry stock system.
Table 3
TestPolymer Si02 ConductivityDewatering Turbidity
time
No. Dosage Dosage [s] [NTU]
(kglt] [kglt] [mSlcm] P1 P2 P1 P2
990 ppm Ca2+
1 2 0 5.5 14.2 19.2 42 64
2 2 1 5.5 10.8 13.9 41 43
3 2 2 5.5 7.7 9.5 35 36
4 2 3 5.5 7.3 8.9 32 39
1300 ppm
Ca2+
5 2 0 7.0 16.2 23.0 46 50
6 2 1 7.0 10.0 17.1 40 45
7 2 2 7.0 7.5 13.6 36 44
8 2 3 7.0 7.7 11.7 34 44
1930 ppm
Ca2+
9 2 0 10.0 18.7 22.0 44 58
10 2 1 10.0 11.6 23.3 39 52
11 2 2 10.0 8.2 15.8 36 53
12 2 3 10.0 8.0 15.4 41 47
Example 4
In this test series, the dewatering effect was evaluated with a "Canadian
Standard Freeness Tester" which is the conventional method for characterising
drainage
according to SCAN-C 21:65. All additions of chemicals were made in a "Britt
Dynamic
Drainage Jar" with blocked outlet at a stirring speed of 1000 rpm during 45
seconds
according to the procedure of Example 1 and the stock system was then
transferred to
the Freeness apparatus. Here the smallest hole in the bottom of the Freeness
tester was
blocked and the time for 400 ml of furnish to filtrate through the screen was
measured.
The stock was taken from a closed mill using waste paper. Consistency was
0.14%,
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conductivity 8.0 mS/cm and pH about 7. Table 4 shows the dewatering effect at
various
dosages of silica-based particles, calculated as Si02 and based on dry stock
system
Table 4
Test Polymer SiOz ConductivityDewatering
No. dosage Dosage time
[s]
[kglt] [kglt] [mSlcm] P1 P2
1 0.6 0 8.0 100.4 103.2
2 0.6 0.25 8.0 66.4 92.5
3 0.6 0.5 8.0 58.3 85.8
4 0.6 0.75 8.0 50.0 76.0
5 0.6 1 8.0 44.6 79.2
5 Example 5
In this test series, the dewatering effect was evaluated as in Example 3,
except
that both sodium acetate (550 ppm Na') and calcium chloride (1300 ppm Ca2')
was used
to adjust the conductivity.
The polymers and anionic inorganic particles according to Example 1 were
10 similarly used in this test series.
Table 5 shows the dewatering effect at various dosages of silica-based
particles,
calculated as Si02 and based on dry stock system.
Table 5
Test Polymer Si02 ConductivityDewatering
No. dosage dosage time
[s]
[kglt] [kglt] [mSlcm] P1 P2
1 2 1 2.5 16.1 18.2
2 1 3 10.0 10.7 14.7
3 2 3 10.0 6.8 13.5
4 3 3 10.0 5.3 14.0
5 2 1 10.0 9.7 20.4
6 2 2 10.0 7.9 14.8
15 Example 6
In this test series, the dewatering and retention effect was evaluated as in
Example 3, using a combination of sodium acetate (550 ppm Na+) and calcium
chloride
(1300 ppm Ca2+) to adjust the conductivity.
The polymers according to Example 1 were similarly used in this test series.
The
anionic microparticulate material used was hydrated suspension of powdered Na
bentonite in water. The bentonite had a surface charge of about 0.33 meq/g and
a
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swelling ability of 41 ml (2 g}. The bentonite particles were added in an
amount of 8.0
kg/ton, calculated as dry bentonite on dry stock system
Table 6 shows the dewatering and retention effect at various dosages of P1 and
P2, calculated as dry polymer on dry stock system.
Table 6
TestPolymer Bentonote ConductivityDewatering Turbidity
No. Dosage dosage time [NTU]
[s]
[kglt] [kglt] [mSlcm] P1 P2 P1 P2
1 1 8 10.0 13.6 18.5 41 47
2 2 8 10.0 10.8 20.6 29 41
3 3 8 10.0 8.48 24.8 20 36
4 4 8 10.0 7.42 26.6 18 36
Example 7
In this test series, the dewatering effect was evaluated as in Example 6,
except
that sodium chloride was used to adjust the conductivity.
The polymers and bentonite according to Example 6 were similarly used in these
tests. The bentonite particles were added in an amount of 8.0 kg/ton,
calculated as dry
bentonite on dry stock system. Table 7 shows the dewatering and retention
effect at
various dosages of P1 and P2, calculated as dry polymer on dry stock system.
Table 7
Test Polymer BentoniteConductivityDewatering
time
No. Dosage dosage [s]
[kglt] [kglt] [mSlcm] P1 P2
550 ppm
Na+
1 2 8 2.5 15.3 17.5
2 3 8 2.5 11.9 14.1
3 4 8 2.5 8.6 9.8
4 5 8 2.5 6.8 8.2
3320 ppm
Na+
5 2 8 10.0 12.7 15.5
6 3 8 10.0 9.4 12.5
7 4 8 10.0 6.9 10.9
8 5 8 10.0 5.6 10.0
Example 8
In this test series, the dewatering effect was ealuated as in Example 3,
except
that zink chloride was used to adjust the conductivity. The polymers and
anionic inorganic
particles according to Example 1 were similarly used in these tests.
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Table 8 shows the results of the dewatering tests at various dosages of silica-
based particles, calculated as as Si02 and based on dry stock system.
Table 8
Test Polymer SiOZ ConductivityDewatering
No. Dosage dosage time
Is]
[kglt] [kglt] [mSlcm] P1 P2
700 ppm
Zn2+
1 2 0 2.4 13.6 22.7
2 2 1 2.4 7.9 8.5
3 2 2 2.4 5.5 5.6
1400 ppm
Znz+
4 2 0 4.5 18.0 28.0
2 2 4.5 8.3 11.4
