Note: Descriptions are shown in the official language in which they were submitted.
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A PROCESS FOR THE PRODUCTION OF PAPER
Field of the Invention
The present invention relates to a process for the production of paper in
which papermaking additives are introduced into a cellulosic stock before it
is ejected from
a headbox onto a wire and dewatered to form a web of paper.
Backaround of the Invention
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 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.
Retention agents are usually introduced into the stock in order to increase
adsorption of fine particles, e.g. fine fibres and filler particles, onto the
cellulosic fibres so
that they are retained with the fibres on the wire. A wide variety of
retention agents are
known in the art, examples of which include anionic, non-ionic, amphoteric and
cationic
organic polymers of different molecular weights, inorganic materials, and
combinations
thereof. Due to incomplete retention, the water obtained by dewatering the
stock and the
wet web, referred to as white water, contains fine particles not being
retained on the wire
and the white water is usually recirculated in different flow circuits.
In paper machines having a dilution headbox, white water is used to dilute the
stock within the headbox. Hereby the flow of high consistency stock is diluted
with a low
consistency flow originating from the white water. The headbox can have a
series of mixing
sections or dilution lines distributed over the width of the headbox. White
water is injected
into the mixing sections to locally control the stock dilution thereby forming
a variable
consistency profile leaving the slice opening at a constant volume flow. The
dilution
headbox design provides better control of paper properties; by adjusting the
amount of
dilution, i.e. the ratio of high consistency flow to low consistency flow, at
a plurality of points
of the dilution headbox across the machine, the basis weight of the web can be
controlled
in an improved manner and rendered essentially uniform in a cross machine
direction.
However, notably when using high performance retention agents, it has been
experienced
that the papermaking process and the properties of the paper produced are
still not
completely satisfactory, which has been attributed to inadequate pitch
deposition control.
Problems caused by pitch build-up on papermaking machinery and formation
of pitch globules in the final paper in the production of all types of paper
has previously
been recognized. Pitch generally refers to emulsified hydrophobic organic
compounds.
Pitch can be defined as the sticky, resinous materials that are released from
wood during
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the pulping process. Pitch has also come to include sticky materials which
arise from
components of coated broke and recycled fibres, such as adhesives, and are
offien
referred to as stickies and tackles. In paper mill process waters, pitch
exists as unstable,
colloidal dispersions of hydrophobic particles. Therefore, typical papermaking
process
conditions, such as hydrodynamical and mechanical shear forces and abrupt
changes of
temperature as well as chemical environment and equilibrium, may cause the
colloidal
pitch particles to agglomerate within the cellulosic suspension or deposit on
the surfaces of
the wire or other equipment. This may lead to quality defects in the finished
product, such
as formation of spots or holes and a poor quality paper surface, and shortened
equipment
life, runnability problems, paper machine downtime and, ultimately, lost
profit for the mill.
These problems are magnified in paper mills with high level of process water
closure, such
as extensive white water recirculation.
Summary of the Invention
The present invention is generally directed to a process for the production of
paper which comprises:
(i) providing a main aqueous flow containing cellulosic fibres;
(ii) introducing one or more retention components into said main aqueous flow
to form a
main aqueous flow containing one or more retention components;
(iii) providing a diluting aqueous flow;
(iv) introducing a low molecular weight cationic organic polymer into said
diluting aqueous
flow to form a diluting aqueous flow containing a low molecular weight
cationic organic
polymer, said low molecular weight cationic organic polymer having a weight
average
molecular weight up to 5,000,000;
(v) introducing said diluting aqueous flow containing a low molecular weight
cationic
organic polymer into said main aqueous flow containing one or more retention
components
to form a resulting aqueous flow; and then
(vi) ejecting said resulting aqueous flow onto a wire and dewatering said
resulting aqueous
flow to form a web of paper.
The present invention is further directed to a process for the production of
paper on a paper machine containing a dilution headbox, the process
comprising:
(i) introducing one or more retention components into a main aqueous flow
containing
cellulosic fibres, and feeding the obtained main aqueous flow into the
dilution headbox;
(ii) introducing low molecular weight cationic organic polymer having a weight
average
molecular weight up to 5,000,000 into a diluting aqueous flow and feeding the
obtained
diluting aqueous flow into the dilution headbox;
(iii) mixing the obtained main aqueous flow with the obtained diluting aqueous
flow in the
headbox to form a resulting aqueous flow; and
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(iv) ejecting the resulting aqueous flow onto a wire and dewatering the
resulting aqueous
flow to form a web of paper.
The invention is also directed to a process for the production of paper from
an aqueous suspension containing cellulosic fibres, and optional filler, which
comprises
introducing one or more retention components into the suspension followed by
introducing
into the suspension a low molecular weight cationic organic polymer having a
weight
average molecular weight up to 5,000,000, and thereafter forming and draining
the
suspension on a wire.
Detailed Description of the Invention
According to the present invention it has been found that pitch problems can
be reduced by the introduction of additives into a stock in a certain manner
before it is
dewatered on a wire to form the web of paper. This finding is particularly
applicable to
papermaking processes where paper is produced on a paper machine with a
dilution
headbox. It has also been found that pitch deposition in the papermaking
system can be
better controlled according to the present invention. It has further been
found that the
process of this invention renders possible production of paper with improved
properties.
Dilution headboxes generally can be described as devices comprising at least
one inlet for a first partial volume flow, at least one inlet for a second
partial volume flow, at
least one section for mixing the partial volume flows to form a mixture volume
flow, and at
least one outlet for ejecting the mixture volume flow. Preferably the dilution
headbox
comprises a plurality of such inlets, sections and outlets across its working
width.
Examples of suitable dilution headboxes include those disclosed in U.S. Pat.
Nos.
4,909,904; 5,196,091; 5,316,383; 5,545,293; and 5,549,793.
The term "main aqueous flow", as used herein, refers to the main flow of
stock containing cellulosic fibres, and optional filler, entering the headbox
which has a high
consistency (hereafter HC), i.e. a high solids content, HC stock, thereby
representing the
high consistency flow (hereafter HC flow). The consistency of the HC flow can
be within the
range of from 0.1 % to 3.5% by weight, suitably from 0.3% to 2.2% and
preferably from
0.4% to 1.9%. The term "diluting aqueous flow", as used herein, refers to the
aqueous flow
which is used to dilute the HC flow and which, in relation to the HC flow, has
a low
consistency (LC), i.e. a low solids content, LC stock, thereby representing
the low
consistency flow (hereafter LC flow). The consistency of the LC flow can be
within the
range of from 0-1.5% by weight, suitably 0.002-0.9%, and preferably 0.005-0.8%
with the
proviso that the consistency of the LC flow is lower than that of the HC flow.
Preferably, in
the headbox, the HC flow is mixed and diluted with the LC flow, for example
just before the
turbulence generator, to form a resulting flow which is discharged onto the
wire for
dewatering. The volume ratio of HC flow to LC flow can be within the range of
from 99:1 to
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50:50, suitably from 97:3 to 60:40, preferably from 95:5 to 75:25 and
typically about 85:15.
As conventional in dilution headbox designs, the volume ratio of HC flow to LC
flow
preferably is varying at a plurality of points of the headbox across its width
in order to
adjust the amount of dilution, thereby enabling better control of the basis
weight cross
profile of the paper web formed. Preferably the partial volume flows, i.e, the
HC flow and
the LC flow, are mixed in the headbox to form a resulting HC/LC mixture volume
flow which
is ejected from the headbox and which is essentally constant in a cross-
machine direction.
The aqueous LC flow used for dilution can be selected from fresh water,
white water and other types of aqueous flows that are recycled in the process.
The diluting
LC flow may contain fibre fines and filler, and it may be treated by means of
any
purification step before being fed into the headbox. Examples of suitable
steps that can be
used for purifying or clarifying aqueous flows of these types include
filtration, flotation,
sedimentation, anaerobic and aerobic treatment. Preferably, the LC flow is
white water
containing, for example, cellulosic fines, extractives and other materials
released from
wood during the pulping process as well as filler and other additives
introduced into the HC
flow but not retained on the wire. The white water used is preferably obtained
by
dewatering the stock and/or the wet web on the wire, and it may be clarified
as mentioned
above before being fed into the dilution headbox. The LC flow usually has a
composition
that is different from that of the HC flow. When filler is used in the process
the filler content
of the LC flow usually differs from that of the HC flow; the LC flow normally
has a higher
filler content, expressed as percentage of the dry substance of the flow, than
the HC flow.
In addition to the HC flow and the LC flow entering the headbox as described
above, there can be at least one additional flow entering the headbox in
accordance with
the present invention. The additional flow is preferably a flow that contains
water alone.
The additional flow may also be a flow of stock or pulp, the consistency
andlor composition
of which differs from that of the HC flow.
The retention components) to be introduced into the HC flow according to
this invention may be a single retention agent or a retention system, for
example any of
those defined hereinafter. The single component can be any component
functioning as a
retention agent, preferably a cationic polymer such as, for example, any of
those defined
herein. In this embodiment, the amount of the component introduced into the
main
aqueous flow should be sufficient so as to give better retention than is
obtained when not
adding the component. The
In a preferred embodiment of this invention, there is used a retention system.
The term "retention system", as used herein, refers to two or more components,
or agents
which, when being added to a stock, give better retention than is obtained
when not adding
the two or more components, or agents. The components of retention systems are
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preferably selected from two or more organic polymers and one or more organic
polymers in
combination with aluminium compounds and/or inorganic microparticles.
In a preferred embodiment of the invention, there is used a microparticle
retention system. The term "microparticle retenfiion system", as used herein,
refers to a
5 retention system comprising a microparticulate material, or microparticles,
such as, for
example, anionic inorganic particles, cationic inorganic particles and organic
microparticles, as defined herein. The microparticulate material is used in
combination with
at least one further component, usually at least one organic polymer, herein
also referred to
as a main polymer, preferably a cationic, amphoteric or anionic polymer.
Anionic
microparticles are preferably used in combination with at least one amphoteric
and/or cationic
polymer, whereas cationic microparticles are preferably used in combination
with at least one
amphoteric and/or anionic polymer. Preferably the microparticles are anionic
inorganic
particles. It is further preferred that the microparticles are in the
colloidal range of particle
size. The retention system, e.g. systems comprising microparticles, can
comprise more
than two components; for example, it can be a three- or four-component
retention system.
Such suitable additional components include, for example, aluminium compounds
and low
molecular weight cationic organic polymers. Usually retention systems,
including
microparticle retention systems, also give better dewatering than is obtained
when not
adding the components, and the systems are commonly referred to as retention
and
dewatering systems.
In another preferred embodiment of the invention, there is used a retention
system comprising one or more cationic organic polymers and one or more
anionic organic
polymers. Suitably such a retention system includes a cationic organic polymer
having one
or more aromatic groups and/or an anionic organic polymer having one or more
aromatic
groups, as defined herein.
Anionic inorganic particles that can be used according to the invention
include
anionic silica-based particles and clays of the smectite type. Anionic silica-
based particles,
i.e. particles based on SiO~ or silicic acid, including colloidal silica and
different types of poly-
silicic acid and polysilicates, are preferably used. Anionic silica-based
particles are usually
supplied in the form of aqueous colloidal dispersions, so called sots.
Suitable silica-based
sols according to the invention may also contain other elements, for example
nitrogen,
aluminium and boron. Such elements may be present as a result of modification
using
organic nitrogen-containing organic compounds, aluminium-containing compounds
and
boron-containing compounds, respectively. These compounds may be present in
the
aqueous sol and/or in the silica-based particles. Retention and dewatering
systems
comprising suitable anionic silica-based particles are disclosed in U.S. Pat.
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;
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5,470,435; 5,543,014; 5,571,494; 5,584,966; 5,603,805; and 6,379,500, which
are all hereby
incorporated herein by reference.
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 m2/g and
preferably above 100
m2/g. Usually, the specific surface area is up to about 1700 m~/g and
preferably up to 1000
m~/g. The specific surface area is 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. Pat. No. 5,176,891, after appropriate removal of or adjustment for any
elements or
compounds present in the sample that may disturb the titration like aluminium
and boron
species. 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 m~/g
and preferably from 100 to 950 m~/g. Preferably, the anionic inorganic
particles are present in
a silica-based sol having an S-value in the range of from 8 to 45%, preferably
from 10 to
35%, containing silica-based particles with a specific surface area in the
range of from 300 to
1000 m~lg, suitably from 500 to 950 m2/g, which parfiicles can be non-
aluminium-modified or
aluminium-modified, suitably surface-modified with aluminium. The S-value is
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 anionic inorganic
particles are selected from polysilicic acid, optionally reacted with
aluminium, having a high
specific surface area, suitably above about 1000 m~/g. The specific surface
area can be
within the range of from 1000 to 1700 m~/g and preferably from 1050 to 1600
m~/g. 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 polysilicic acid is commonly referred to as
polyaluminosilicate and poly-
aluminosilicate microgel, which are both encompassed by the term polysilicic
acid 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 which after
swelling
preferably has a surface area of from 400 to 800 m2/g. Suitable clays are
disclosed in U.S.
Pat. Nos. 4,753,710; 5,071,512; and 5,607,552, which are hereby incorporated
herein by
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reference, the latter patent disclosing mixtures of anionic silica-based
particles and smectite
clays, preferably natural bentonites. Cationic inorganic particles that can be
used include
cationic silica-based particles, cationic alumina, and cationic zirconia.
Suitable organic polymers for use as a retention agent or part of a retention
system
according to this invention can be anionic, non-ionic, amphoteric, or cationic
in nature, they
can be derived from natural or synthetic sources and they can be linear,
branched or cross
linked, e.g. in the form of microparticles. Preferably the polymer is water-
soluble or water
dispersable.
Examples of suitable cationic polymers include cationic polysaccharides, e.g.
starches, guar gums, celluloses, chitins, chitosans, glycans, galactans,
glucans, xanthan
gums, pectins, mannans, dextrins, preferably starches and guar gums, suitable
starches
including potato, corn, wheat, tapioca, rice, waxy maize, barley, etc.;
cationic synthetic
organic polymers such as cationic chain-growth polymers, e.g. cationic vinyl
addition
polymers like acrylate-, acrylamide-, vinylamine-, vinylamide- and allylamine-
based polymers,
and cationic step-growth polymers, e.g. cationic polyamidoamines, polyethylene
imines,
polyamines and polyurethanes. Cationic starches and cationic acrylamide-based
polymers
are particularly preferred cationic polymers, both as single retention
components as well as in
retention systems with and without anionic inorganic particles. Examples of
suitable cationic
organic polymers having one or more aromatic groups include those disclosed in
WO
02/12626. The weight average molecular weight of the cationic organic polymer
can vary
within wide limits depending on, inter alia, the type of polymer used, and
usually it is above
2,000,000, more often above 3,000,000, suitably above 5,000,000. The upper
limit is not
critical; it can be about 600,000,000, usually 150,000,000, and suitably
100,000,000.
Examples of further suitable cationic polymers that can be introduced into the
HC
flow according to the invention include cationic organic polymers having a low
molecular
weight. Such cationic organic polymers include those commonly referred to as
anionic
trash catcher (hereafter ATC). The weight average molecular weight of the ATC
cationic
organic polymer is usually at least 2,000, suitably at least 10,000 and
preferably at least
50,000, and it is usually up to 2,000,000 and often up to 1,500,000. Suitable
ATC's include
linear, branched and cross-linked polymers, usually highly charged, which can
be derived
from natural and synthetic sources. Examples of suitable ATC's include low
molecular weight
degraded polysaccharides, e.g. those based on starches, guar gums, celluloses,
chitins,
chitosans, glycans, galactans, glucans, xanthan gums, pectins, mannans,
dextrins,
preferably starches and guar gums, suitable starches including potato, corn,
wheat, tapioca,
rice, waxy maize, barley, etc.; cationic synthetic organic polymers such as
cationic chain-
growth polymers, e.g. cationic vinyl addition polymers like acrylate-,
acrylamide-, vinylamine-,
vinylamide- and allylamine-based polymers, for example homo- and copolymers
based on
diallyldialkyl ammonium halide, e.g. diallyldimethyl ammonium chloride, as
well as (meth)-
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acrylamides and (meth)acrylates; and cationic step-growth polymers, e.g.
cationic polyamido-
amines, polyethylene imines, polyamines, e.g, dimethylamine-epichlorhydrin
copolymers,
and polyurethanes.
Examples of suitable anionic organic polymers according to the invention can
be selected from step-growth polymers, chain-growth polymers, polysaccharides,
naturally
occurring aromatic polymers and modifications thereof. Examples of suitable
anionic step
growth polymers include anionic benzene-based and naphthalene-based
condensation
polymers, preferably naphthalene-sulphonic acid based and naphthalene-
sulphonate based
condensation polymers; and addition polymers, i.e. polymers obtained by step-
growth
addition polymerization, e.g. anionic polyurethanes. Examples of suitable
anionic chain-
growth polymers include anionic vinyl addition polymers, e.g. acrylate- and
acrylamide-based
polymers comprising anionic or potentially anionic monomers like (meth)acrylic
acid and
paravinyl phenol (hydroxy styrene). Examples of suitable naturally occurring
aromatic
polymers and modifications thereof, i.e. modified naturally occurring aromatic
anionic
polymers, according to the invention include lignin-based polymers, preferably
sulphonated
lignins, e.g. lignosulphonates, kraft lignin, sulphonated kraft lignin, and
tannin extracts.
Examples of other suitable anionic organic polymers having one or more
aromatic groups
include those disclosed in WO 02/12626. The weight average molecular weight of
the
anionic polymer can vary within wide limits dependent on, inter alia, the type
of polymer
used, and usually it is at least about 500, suitably above about 2,000 and
preferably above
about 5,000. The upper limit is not critical; it can be about 600,000,000,
usually
150,000,000, suitably 100,000,000 and preferably 10,000,000.
The term "step-growth polymer", as used herein, refers to a polymer obtained
by
step-growth polymerization, also being referred to as step-reaction polymer
and step-reaction
polymerization, respectively. The term "chain-growth polymer", as used herein,
refers to a
polymer obtained by chain-growth polymerization, also being referred to as
chain reaction
polymer and chain reaction polymerization, respectively.
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
confiaining both chloride and sulphate ions, polyaluminium silicate-sulphates,
and mixtures
thereof. The polyaluminium compounds may also contain other anions, for
example anions
from phosphoric acid, sulphuric acid, organic acids such as citric acid and
oxalic acid.
Preferred retention systems according to the invention comprise:
(i) anionic silica-based particles in combination with cationic starch,
cationic guar gum
or cationic acrylamide-based polymer, optionally in combination with anionic
organic
particles and/or ATC and/or aluminium compound;
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(ii) anionic silica-based particles in combination with anionic chain-growth
polymer,
preferably anionic acrylamide-based polymer in combination with cationic
organic
polymer and/or ATC;
(iii) bentonite in combination with cationic acrylamide-based polymer,
optionally in
combination with ATC and/or aluminium compound;
(iv) cationic polysaccharide, preferably cationic starch, in combination with
anionic step-
growth polymer, preferably anionic naphthalene-based condensation polymer;
optionally in combination with ATC and/or aluminium compound;
(v) cationic polysaccharide, preferably cationic starch, in combination with
naturally
occurring aromatic anionic polymer and modifiations thereof, preferably
sulphonated
lignin, optionally in combination with ATC and/or aluminium compound;
(vi) cationic chain-growth polymer, preferably cationic acrylamide-based
polymer, in
combination with anionic step-growth polymer, preferably anionic naphthalene-
based
condensation polymer, optionally in combination with ATC and/or aluminium
compound; and
(vii) cationic chain-growth polymer, preferably cationic acrylamide-based
polymer, in
combination with naturally occurring aromatic anionic polymer and modifiations
thereof, preferably sulphonated lignin, optionally in combination with ATC
and/or
aluminium compound;
(viii) cationic chain-growth polymer, preferably cationic acrylamide-based
polymer, in
combination with ATC; and
(ix) cationic chain-growth polymer, preferably cationic acrylamide-based
polymer, in
combination with anionic organic particles.
In the process of the invention, the retention components) is/are introduced
into the HC flow which is to be mixed with the LC flow, preferably in the
headbox, thereby
introducing the components) into the resulting aqueous flow in the dilution
process. When
using a retention system comprising more than one component, the components
can be
added to the stock flow in conventional manner, preferably at different points
and in any
order. When using a retention system comprising anionic inorganic particles
and a cationic
polymer, it is preferred to add the cationic polymer to the HC stock flow
before adding the
microparticulate material, even if the opposite order of addition may be used.
When using
a retention system comprising cationic and anionic organic polymers, it is
preferred to add
the cationic polymer to the HC stock flow before adding the anionic polymer,
even if the
opposite order of addition may be used. It is further preferred to add fihe
first component,
e.g. the cationic polymer, before a shear stage, which can be selected from
pumping, mix-
ing, cleaning, etc., and to add the second component, e.g. the anionic
inorganic
microparticles or organic polymer, after that shear stage. When using a low
molecular
weight cationic organic polymer as an ATC, it is preferably introduced into
the HC stock flow
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prior to or simultaneous with other retention component(s). When using an
aluminium com-
pound, it is preferably introduced into the HC stock flow prior to or
simultaneous with other
retention component(s).
The components of the retention system are introduced into the stock to be
5 dewatered in amounts which can vary within wide limits depending on, inter
alia, type and
number of components, type of stock, type of filler, filler content, ~ point
of addition, etc.
Generally the components are added in amounts that give better retention than
is obtained
when not adding the components. When using anionic inorganic particles as a
microparticulate material, the total amount added is usually at least 0.001 %
by weight, often
10 at least 0.005% by weight, based on dry substance of the stock. The upper
limit is usually
1.0% and suitably 0.6% by weight. When using anionic silica-based particles,
the total
amount is suitably within the range of from 0.005 to 0.5% by weight,
calculated as SiO~ and
based on dry stock substance, preferably within the range of from 0.01 to 0.2%
by weight.
Organic polymers, e.g, cationic and anionic polymers, are usually added in
total amounts of
at least 0.001 %, often at least 0.005% by weight, based on dry stock
substance. The upper
limit is usually 3% and suitably 1.5% by weight. When using a low molecular
weight cationic
organic polymer as an ATC, it can be introduced into the HC stock flow in an
amount of at
least 0:01 %, based on dry stock substance, suitably the amount is in the
range from 0.05%
to 0.5%. When using an aluminium compound in the process, the total amount
introduced
into the stock to be dewatered is dependent 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
compounds as precipitants for rosin-based sizing agents. The total amount
added is usually
at least 0.05%, calculated as AI2O3 and based on dry stock substance. Suitably
the amount is
in the range of from 0.05 to 2.8%, preferably in the range from 0.1 to 2.0%.
According to the present invention, a low molecular weight cationic organic
polymer is introduced into the LC flow to be mixed with the HC flow,
preferably in the dilution
headbox. Suitable low molecular weight (hereafter LMW) cationic organic
polymers include
linear, branched and cross-linked polymers, usually highly charged, which can
be derived
from natural and synthetic sources. Examples of suitable LMW cationic organic
polymers
include LMW degraded polysaccharides, e.g. those based on starches, guar gums,
celluloses, chitins, chitosans, glycans, galactans, glucans, xanthan gums,
pectins, mannans,
dextrins, preferably starches and guar gums, suitable starches including
potato, corn, wheat,
tapioca, rice, waxy maize, barley, etc.; LMW cationic synthetic organic
polymers such as
cationic chain-growth polymers, e.g. cationic vinyl addition polymers like
acrylate-, acryl-
amide-, vinylamine-, vinylamide- and allylamine-based polymers, for example
homo- and
copolymers based on diallyldialkyl ammonium halide, e.g. diallyldimethyl
ammonium chloride,
as well as (meth)acrylamides and (meth)acrylates; and LMW cationic step-growth
polymers,
e.g. cationic polyamidoamines, polyethylene imines, polyamines, e.g.
dimethylamine-
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epichlorhydrin copolymers, and polyurethanes. The weight average molecular
weight of the
LMW cationic organic polymer is usually at least 100,000, suitably at least
500,000 and
preferably at least 1,000,000, and it is usually up to 5,000,000, suitably up
to 3,000,000 and
preferably up to 2,000,000. lJsually, in case a cationic organic polymer is
added to the HC
flow as a retention agent or part of a retention system, the weight average
molecular weight
of the LMW cationic organic polymer added to the LC flow is lower than that of
the cationic
organic polymer added to the HC flow.
The LMW cationic organic polymer is usually added to the LC flow in an
amount of at least 0.01 %, based on dry substance of the stock to be
dewatered. Suitably, the
amount is in the range of from 0.05 to 1.0%, preferably in the range from 0.1
to 0.5%.
In a preferred embodiment of this invention, subsequent to introducing the LC
flow containing the LMW cationic organic polymer into the HC flow containing
one or more
retention components to form the resulting aqueous flow, no further retention
components
are introduced into the resulting aqueous flow. The formation of the resulting
aqueous flow
preferably takes in the dilution headbox, but may also take place outside the
headbox.
The process of this invention is applicable to all papermaking processes and
cellulosic suspensions, and it is particularly useful in the manufacture of
paper from a stock
that has a high conductivity. In such cases, the conductivity of the stock
that is dewatered on
the wire is usually at least about 1.5 mS/cm, suitably at least 3.5 mS/cm, and
preferably at
least 5.0 mS/cm. Conductivity can be measured by standard equipment such as,
for
example, a WTW LF 539 instrument supplied by Christian Berner. The values
referred to
above are determined by measuring the conductivity of the resulting aqueous
flow that is
ejected onto the wire to be dewatered. High conductivity levels mean high
contents of salts
(electrolytes) which are usually derived from materials used to form the
stock, 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 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 recycled, or recirculated, 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. Fresh water can be introduced in the
process at any
stage; for example, fresh water can be mixed with cellulosic fibres in order
to form a
suspension, and fresh water can be mixed with a thick suspension containing
celluiosic fibres
to dilute it so as to form a thin suspension that is fed into the headbox as a
high consistency
flow.
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The process according to the invention is used fior the production of paper.
The
term "paper", as used herein, of course include not only paper and the
production thereof, but
also other web-like products, such as for example board and paperboard, and
the production
thereofi. The process can be used in the production of paper from different
types of
suspensions ofi cellulosic fibres, and the suspensions should suitably contain
at least 25%
and preferably at least 50% by weight of such fibres, based on dry substance.
The
suspensions can be based on fibres from chemical pulp, such as sulphate and
sulphite pulp,
thermomechanical pulp, chemo-thermomechanical pulp, organosolv pulp, refiner
pulp or
groundwood pulp from both hardwood and softwood, or fibers derived from one
year plants
like elephant grass, bagasse, flax, straw, etc., and can also be used for
suspensions based
on recycled fibres. The invention is preferably applied to processes for
making paper from
wood-containing suspensions. The suspension also contain mineral fillers of
conventional
types, such as, for example, kaolin, clay, titanium dioxide, gypsum, talc and
both natural and
synthetic calcium carbonates, such as, for example, chalk, ground marble,
ground calcium
carbonate, and precipitated calcium carbonate. The stock can of course also
contain
papermaking additives of conventional types, such as wet-strength agents,
stock sizes, such
as those based on rosin, ketene dimers , ketene multimers, alkenyl succinic
anhydrides, etc.
Suitably the invention is applied on paper machines producing wood-
containing paper and paper based on recycled fibres, such as SC, LWC and
different
types of book and newsprint papers, and on machines producing wood-free
printing and
writing papers, the term wood-free meaning less than about 15% of wood-
containing
fibers. The invention is also applicable for the production of board on single
layer machines
as well as on machines producing paper or board in multilayered headboxes, and
on
machines with several headboxes, in which one or more of the layers
essentially consist of
recycled fibres. In machines using multi layer headboxes, or several
headboxes, in which
one or more of the layers are produced with a headbox of the dilutiori type,
the invention
can be applied to one or more of these layers. Suitably the invention is
applied on paper
machines running at a speed of from 300 to 2500 m/min and preferably from 1000
to 2000
m/min.
The invention is further illustrated in the following example which, however,
is
not intended to limit the same. Parts and % relate to parts by weight and % by
weight,
respectively, unless otherwise stated.
Example
The process of this invention was tested using different LMW cationic organic
polymers as additive to the LC stock.
Paper was produced from a cellulosic suspension on a paper machine utilizing
a dilution headbox to make SC grades. Retention agents were added to the HC
stock; first
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0.8 kg/ton based on dry furnish of dimethylamine-epichlorhydrin copolymer with
a weight
average molecular weight of about 1 million and then 0.36 kg/ton based on dry
furnish of
cationic polyacrylamide with a weight average molecular weight of 4.6 million.
The LC stock
was obtained by draining the stock.
500 ml of LC stock was added to a Dynamic Drainage Jar and mixed at 1000
rpm for 15 seconds, and then LMW cationic organic polymer was added to the
stock and
mixed for 30 seconds. For blank tests, the LC stock was added to a Dynamic
Drainage Jar
and mixed at 1000 rpm for 45 seconds without the addition of LMW cationic
organic polymer.
The obtained LC stock was then drained and the filtrate was collected and
passed through a
1 micron filter. An Ocean Optics S2000 UV spectrophotometer with a fast
scanning rate was
used to measure the UV absorption as a representation of the pitch content of
the filtered
fraction.
Several LMW cationic organic polymers were tested at the same dry dosage (4
kg/ton based on LC stock dry substance, corresponding to.about 2 kg/ton based
on total dry
substance reel tonnage) and the results outlined below.
LMW-1 was a dimethylamine-epichlorhydrin copolymer with a weight average
molecular weight of about 120,000;
LMW-2 was a dimethylamine-epichlorhydrin copolymer with a weight average
molecular weight of about 1,000,000;
LMW-3 was a polydiallyldimethylammonium chloride with a weight average
molecular weight of about 680,000; and
LMW-4 was a polydiallyldimethylammonium chloride with a weight average
molecular weight of about 1,800,000.
Compared to the blank test, all the processes according to the invention
showed a reduction in UV absorbtion. The most effective process according to
the invention
was the one employing the polydiallyldimethylammonium chloride with a weight
average
molecular weight of about 1,800,000.
The tests are summarised in Table 1, showing UV absorbance at different
wavelengths for the processes according to the invention and the blank
corresponding to the
prior art.
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Table 1: UV absorbance at difFerent wavelengths.
UV Ab sorbance
Wavelength Blank LMW-1 LMW-2 LMW-3 LMW-4
nm
241.21 2.68 2.656 2.683 2.636 2.647
244.96 2.707 2.744 2.707 2.671 2.657
248.7 2.705 2.61 2.603 2.499 2.454
252.44 2.586 2.427 2.403 2.296 2.243
256.18 2.555 2.345 2.352 2.228 2.156
259.91 2.575 2.376 2.37 2.255 2.181
263.64 2.607 2.454 2.441 2.328 2.293
267.37 2.692 2.545 2.567 2.488 2.438
271.1 2.728 2.666 2.649 2.588 2.575
274.82 2.71 2.682 2.66 2.642 2.623
278.54 2.687 2.706 2.666 2.64 2.633
282.26 2.654 2.653 2.639 2.621 2.596
285.97 2.634 2.59 2.582 2.534 2.496
289.68 2.476 2.331 2.318 2.212 2.164
293.39 2.05 1.838 1.823 1.715 1.661
297.09 1.672 1.463 1.453 1.359 1.31
300.8 1.433 1.239 1.23 1.144 1.101
304.49 1.284 1.104 1.096 1.017 0.98
308.19 1.172 1.005 0.998 0.927 0.892
311.88 1.073 0.916 0.91 0.844 0.813
315.57 0.97 0.826 0.817 0.757 0.73
319.26 0.868 0.733 0.727 0.673 0.648
322.94 0.768 0.643 0.638 0.589 0.567
326.62 0.672 0.558 0.553 0.509 0.49
330.3 0.576 0.473 0.468 0.431 0.415
