Note: Descriptions are shown in the official language in which they were submitted.
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COMPOSITIONS FOR USE IN THE WET-END OF PAPERMAKING
TECHNICAL FIELD
The present invention relates to novel compositions for use in the wet-end of
papermaking. In
particular, it relates to compositions comprising both starch and a non-wood
plant fiber, and to
their use for increasing the dry strength of paper products.
BACKGROUND OF THE INVENTION
During the production of paper, cellulose is mixed with water and pulped to
form a slurry (or
"cellulosic pulp"). The slurry is then applied to a screen to orient the
cellulose fibers and form a
paper web. Excess liquid is allowed to drain off. This process is referred to
as "the wet-end" of
papermaking. The web is then passed through a press section, to further reduce
water content,
followed by a drying step, typically using steam- or oil-heated drying
cylinders. The dried paper
web may then be impregnated with a sizing composition or coated before being
cut to size.
The strength of the finished paper product will depend on a number of factors
including, for
example, the use and nature of any sizing or coating compositions, the use and
nature of any
wet-end additives and, of course, the nature of the pulp itself. Virgin wood
pulp typically gives a
stronger paper than pulp obtained from recycled paper. In fact, the strength
of a paper product
will diminish with every re-pulping cycle. At the same time, there is pressure
in the industry to
move towards greater use of recycled materials, both for environmental reasons
and for cost
reasons. There is therefore a clear need to identify ways of increasing ¨ or
at least maintaining -
dry strength in paper products containing recycled materials.
One possible solution would be the use of dry strength additives in the wet-
end of the paper
production process. Dry strength additives for use in the wet-end have
typically consisted of
cationic starches. Cationic starch normally interacts with anionic cellulose
fibers in the pulp to
form complex matrices, thereby strengthening the paper web. Unfortunately,
such cationic
starches have been found to be rather ineffective when used in combination
with recycled
materials. It is indeed believed that recycled materials contain high levels
of anionic trash which
effectively "saturate" the cationic starch and therefore limit its interaction
with the anionic
cellulose fibers. Papermaking in closed circuits will lead to an accumulation
of these trash
CONFIRMATION COPY
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products together with salts which impair the electrostatic interactions
between anionic fibers
and the cationic starch. As a result, the use of cationic starches in
combination with recycled
materials has a lesser ability to effectively increase dry strength.
A number of synthetic alternatives to starch-based strength additives have
been proposed.
These have included, for instance, polyacrylamide and polyvinylamine and are
typically more
efficient, even when used in combination with recycled materials.
Unfortunately, they are also
prohibitively expensive and may be considered undesirable from an
environmental perspective.
There is therefore a clear need in the art for the development of a natural
additive which can be
used in the wet-end of papermaking to maintain or even increase dry strength
levels in paper
products even if they include recycled pulp materials and even when produced
in closed circuits.
The present invention provides such an additive.
STATEMENTS OF THE INVENTION
According to a first aspect of the present invention, there is provided a
composition for use as a
strengthening agent in the wet-end of papermaking which comprises a surface
modified non-
wood plant fiber and a starch component, wherein the plant fiber and starch
component are
preferably linked.
According to a further aspect of the present invention, there is provided a
method of producing
the above composition and a method of producing paper products with such a
composition.
Paper products obtainable by such a process are provided in another aspect of
the invention.
According to an additional aspect of the present invention, there is provided
the use of a surface
modified non-wood plant fiber to increase the dry strength of paper,
preferably paper comprising
recycled materials and/or paper produced at conductivity levels above 4 mS/cm.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a composition for use as a strengthening agent
in the wet-end of
papermaking which comprises a surface modified non-wood plant fiber and a
starch component.
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Plant Fiber
The term "non-wood plant fiber" as used herein refers to any fibrous material
which is not
derived from wood, wherein "wood" will be understood to have its common
dictionary definition
(i.e. the hard, fibrous substance which makes up most of the trunk and
branches of a tree).
Examples of suitable non-wood plant fibers (simply "plant fibers" herein) will
be apparent to a
person skilled in the art. They include, but are not limited to, cereal
fibers, seed fibers, legume
fibers, fruit fibers, algae fibers and mixtures of two or more thereof.
The plant fibers may be provided as such or in the form of a fibrous material.
Examples of
suitable fibrous materials may include, for instance, cereal bran (e.g. wheat
bran), vegetable
hulls (such as soy hulls and/or pea hulls), bagasse, corn stover, straw,
switch grass, citrus fibers
(such as citrus pulp fiber), seaweed residue and mixtures of two or more
thereof. The fibrous
material will preferably comprise fibers in an amount of at least 50% by
weight, on a dry weight
basis, more preferably in an amount of at least 75% by weight. Certain fibrous
materials may
also comprise non-fibrous elements such as proteins or starch. By way of
example, soy hulls
typically comprise about 10% by weight protein and about 20% by weight starch,
in addition to
their fibrous content. Thus, according to at least one embodiment, the fibrous
material will
comprise approximately 50-85% fibers by weight, on a dry weight basis.
The plant fibers will preferably be anionic (meaning that it will preferably
have a net negative
charge). According to certain embodiments, they will have a streaming zeta
potential (SZP) of
between 0 and -30 mV, preferably of between -1 and -20 mV, more preferably of
between -1 and
-15 mV. According to another embodiment, they will preferably have an
anionicity of 10 to 2000
pg/g, more preferably of 100 to 1000 peq/g (expressed per gram of dry
substance, at pH 7).
The plant fibers will advantageously be surface modified such that their
ability to interact
(mechanically or electrostatically) with cellulose fibers in a cellulosic pulp
is increased. By way of
example only, the fibers may be modified to have a greater overall or specific
surface area.
As will be understood by a person skilled in the art, the term "surface
modification" or "surface
modified" will not be limited to modifications having an effect only on the
surface of the fibers.
Indeed, the modification(s) may also result in changes to other properties
(e.g. the internal
structure) of the fibers. Surface modification may be achieved, for example,
by milling, heat
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treatment (e.g. cooking and/or extrusion), chemical modification or any
combination thereof.
Thus, according to certain embodiments, the plant fibers may be provided in
the form of a
powder or flakes.
The composition of the present invention also comprises a starch component.
Starch Component
Although not wishing to be bound by theory, it is believed that the starch
component will
increase the plant fiber's ability to interact with the cellulosic pulp,
acting almost as an adhesive
between the plant fibers and the cellulose fibers.
The term "starch component" as used herein may refer to one or more native
starches, one or
more modified starches, one or more starch derivatives (such as a dextrin) or
mixtures thereof.
The starch component may be derived from starches of any type and any origin.
For example it
may comprise waxy and/or non-waxy starches, and it may be derived from wheat,
corn, potato,
tapioca, pea or any other available starch source, and mixtures thereof.
The starch component may have been subjected to one or more modifications,
including
chemical, enzymatic and/or heat-based modifications. Thus, the starch
component may
comprise, for instance, one or more cross-linked, etherified, esterified,
hydroxypropylated, and/or
thinned starches. Preferably, the starch component will be anionic or
cationic, i.e. having a net
negative or a net positive charge. More preferably, the starch component will
be cationic.
According to one particular embodiment, the starch component will consist of
one or more
cationic starches.
According to one possible embodiment, some or all of the starch component may
be contributed
as part of a fibrous material. As described above, the plant fibers may be
provided in the form of
a fibrous material which may contain certain non-fibrous elements such as
starch. The fibrous
side-stream recovered from wheat processing, for example, may contain up to
50% starch by
weight. If using this fibrous side-stream as the source of plant fibers, it
may not be necessary to
add any starch to the composition (or to only add a small amount of starch).
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The starch component will preferably be "activated", meaning that it will be
rendered capable of
forming a dispersion in cold water (i.e. at about 20-25 C). Preferably, the
starch component will
be solubilised, meaning that it will be formed into (or capable of forming) a
stable colloidal
dispersion. This can be achieved, for example, by cooking (heat treating,
steam treating, etc.),
by thinning (e.g. with acid), by extrusion, by pregelatinization, by roll-
drying or by any
combination thereof. According to one possible embodiment, the one or more
starch
components will comprise one or more cold water soluble cationic starches.
When used, the composition of the present invention will preferably comprise
the starch
component in an amount of 1-90% by weight, preferably of 1-50% by weight, more
preferably of
5-30% by weight, for example 20-25% by weight, based on the total dry weight
of the
composition. Ideally, the composition will comprise the plant fibers and the
starch component in
a weight ratio, based on dry weight, of from 1:20 to 20:1, preferably of from
1:1 to 10:1.
Preferably, the starch component and non-wood plant fibers will be linked. The
term "linked" as
used herein may refer to a direct or indirect link, whether through
absorption, electrostatic
interaction, chemical bond or any other means, which allows at least part of
the starch
component to be retained on the surface of some or all of the plant fibers. In
a preferred
embodiment, at least part of the cationic starch component will be linked to
the anionic plant
fibers through a direct electrostatic link. Alternatively, an anionic starch
may be linked to the
anionic plant fibers through a cationic bridge (e.g. a cationic trivalent or
divalent metal ion such
as calcium or aluminium, or cationic starch).
Advantageously, the composition of the present invention will comprise up to
100% by weight,
based on total dry weight, of the plant fibers and starch component.
Preferably, it will comprise
80-100% by weight of the plant fibers and starch component, more preferably 90-
100% by
weight, based on total dry weight.
Other Optional Ingredients
The composition of the present invention may comprise one or more further
optional ingredients.
These will preferably be selected from wet-end additives well known to a
person skilled in the
art. They may include, for instance, hardeners, flowability improvers,
lubricants, antifoamers,
releasing agents, optical brighteners, preservatives, yellowing inhibitors,
ultraviolet absorbers,
antioxidants, insolubilisers, antistatic agents, pH regulators, water-
resisting agents, wet strength
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agents, sizing agents, dewatering aids, grease and oil resistance additives
and combinations of
two or more thereof.
The amount of each of these additives to be included, if at all, will be
determined in accordance
with standard practice and with the desired properties of the final paper
product in mind.
Advantageously, the composition of the present invention will not need to
include ¨ and will
therefore preferably not include ¨ any synthetic dry strength additives. In
fact, the composition of
the present invention will preferably include less than 5%, preferably less
than 2%, more
preferably less than 1% by weight, based on total dry weight, of any synthetic
additives, wherein
"synthetic additives" will be understood as referring to non-naturally
occurring chemical additives
such as polyacrylamide, polyvinylamine, melamine resins, urea formaldehyde
resins and so on.
The composition of the present invention will preferably be provided in the
form of a powder.
Alternatively, it may also be provided in the form of a slurry or of an
aqueous composition. As
such, it may have a total dry substance of between 10% and 98% by weight,
preferably of
between 50% and 98% by weight, more preferably of between 70% and 95% by
weight, based
on the total weight of the composition.
Strengthening Agent
The composition of the present invention is intended for use as a
strengthening agent. In
particular, when used in the manufacture of a paper product, it can be used to
maintain or
increase the product's dry strength (as measured for example by standard CMT,
SCT and/or
Burst tests). It is also believed that compositions of the present invention
will contribute to a
good or improved wet strength, that is to a good or improved strength for the
wet paper web
during the papermaking process, as described in more detail below.
Advantageously, the composition of the present invention may be used to
increase the wet
and/or dry strength of paper products containing recycled materials,
especially when compared
to cationic starch alone. The compositions of the present invention are
believed to perform at
least as well as synthetic agents such as polyacrylamide and/or
polyvinylamine. The
composition of the present invention may also be used to increase the wet
and/or dry strength of
paper products produced at conductivity levels above 4 mS/cm.
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A further advantage of the composition of the present invention is that it is
not detrimental to
other important properties of the papermaking process such as retention (the
retention of
cellulose fibers on the web) or dewatering (the ability of water in the paper
pulp to be easily
removed). In some instances, it is even believed that the composition of the
invention may
contribute to an improvement in these properties (i.e. increased retention and
increased
dewatering).
Process of Producing the Composition
The present invention further provides a process for the production of the
above composition. In
particular, the present invention provides a process for the production of a
composition for use
as a strengthening agent in the wet end of paper making which comprises:
(a) providing a composition comprising a plant fiber and a starch component;
and
(b) surface modifying at least the plant fiber;
or:
(a) providing a composition comprising a plant fiber;
(b) surface modifying the plant fiber; and
(c) mixing the modified plant fiber composition with a starch component.
Where it is desired or intended that the starch component be cationic or
anionic, cationisation or
anionisation of the starch component may be performed prior to or during
contact with the plant
fiber. Thus, for example, where it is desired that the starch component be
cationic, the process
of the present invention may comprise the following steps:
(a) providing a composition comprising a plant fiber and a starch component;
(b) surface modifying at least the plant fiber; and
(c) cationizing the starch component (wherein b and c may be performed in any
order);
or:
(a) providing a composition comprising a plant fiber and a cationic starch
component; and
(b) surface modifying at least the plant fiber;
or:
(a) providing a composition comprising a plant fiber;
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(b) surface modifying the plant fiber; and
(c) mixing the modified plant fiber composition with a cationic starch
component;
or:
(a) providing a composition comprising a surface modified plant fiber and a
starch component;
and
(b) cationizing the starch component.
Whether performed together or separately, activation of the plant fiber
(through surface
modification) and/or starch component may comprise, by way of example only:
heat treatment
(e.g. cooking, jet cooking, dry or semi-dry heat treatment, extrusion, roll-
drying...), mechanical
treatment (e.g. dry milling or wet milling), chemical treatment (e.g.
oxidation) and/or, for the
starch component at least, pregelatinization. Preferably, the plant fiber
and/or starch component
will both be cooked. The plant fiber will preferably also be milled. If
milled, the plant fiber will
preferably be milled to an average particle size (D50) in the range of 30-500
m, preferably in the
range of 30-2001Jm, more preferably in the range of 30-100 m. According to one
particular
embodiment, cooking may consist of jet cooking, e.g. at a temperature in the
range of 80-180 C
or of 100-140 C.
The process of the present invention will preferably further comprise the step
of forming a link (or
allowing a link to form) between the plant fiber and starch component. As
described above, this
may be a direct or indirect link. Preferably, the plant fiber and starch
component with be linked
by electrostatic interaction. Thus, according to one possible embodiment, the
process of the
present invention will include the step of contacting the plant fiber and
starch component in
water. This will advantageously be done prior to use of the composition in the
wet-end of
papermaking, i.e. in clean water, to prevent the starch component from
interacting with non-
fibrous materials. Thus, "clean water" will be understood as having its
ordinary meaning, that is:
it does not refer to waste water or water with high levels of trash (i.e.
white water). It need not
refer to sterile or de-ionized water either (although of course such "hyper-
clean" waters could be
used). Instead it will typically refer to fresh water or simple "tap water".
Thus, according to one particular embodiment of the present invention, there
is provided a
process for the production of a composition for use as a strengthening agent
in the wet end of
paper making which comprises:
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(a) providing a surface modified plant fiber and a cationic, preferably cold
water soluble, starch
component; and
(b) mixing the plant fiber and starch component in clean water such that the
plant fiber and
starch component become linked, preferably by electrostatic interaction.
According to a preferred embodiment, the starch component and plant fiber will
be brought into
contact in clean water and then cooked to encourage the formation of links
between these two
components (i.e. to encourage them to interact). Thus, for example, according
to a preferred
embodiment, the present invention provides a process for the production of a
composition for
use as a strengthening agent in the wet end of paper making which comprises:
(a) providing a plant fiber and a cationic starch component;
(b) mixing the plant fiber and starch component in clean water; and
(c) heating the aqueous mixture of step (b), preferably at 80-180 C.
Preferably, the plant fiber and starch component will be brought into contact
at high
concentrations. By "high concentrations" it is meant that the plant fiber and
starch component
will preferably be mixed at 1-30% dry solids, more preferably at 5-20% dry
solids, although, of
course, in certain embodiments, the dry solids could be much higher. Without
wishing to be
bound by theory, it is believed that such concentrations will encourage
interactions between the
plant fibers and starch component. Of course, if necessary before use, the
composition may be
diluted and/or further optional ingredients may be added to it.
Products and Methods of Production
Compositions obtainable by this process, together with their use as
strengthening agents and
paper products made with them, are all part of the present invention. In
particular, the present
invention provides a method of producing a paper product comprising the steps
of:
(a) bringing a composition as described above (or produced according to one of
the above
processes) into contact with a cellulosic pulp; and
(c) producing a paper product from the pulp obtained in step (a).
The term "cellulosic pulp" refers to an aqueous suspension of cellulosic
fibers as typically used
in the paper industry for the production of paper products. It will be
understood as including any
type of pulp suitable for use in the manufacture of paper products including,
for instance, paper
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sheets, board (cardboard or corrugated board), packaging or case materials and
so on. It may
include virgin wood pulp, pulp from recycled materials, mechanical pulp, etc.
The pulp may be
bleached or unbleached and it may or may not include recycled materials.
Preferably, the pulp will comprise recycled materials (i.e. recycled cellulose
fibers). Recycled
materials may include any kind of recovered, waste or scrap paper products
("waste products")
which are re-pulped for further use. Examples of suitable waste products may
include, for
example, mill broke, pre-consumer waste and/or post consumer waste. The
recycled materials
may or may not be de-inked, bleached or treated in any other way before use
and they may
include materials that have already been recycled one or more times. According
to one possible
embodiment, the pulp may consist entirely of recycled materials. Preferably,
it will comprise at
least 50% recycled materials by weight, more preferably at lest 75% by weight.
Without wishing to be bound by theory, it is believed that the composition of
the present
invention is particularly useful in the production of paper products
comprising recycled materials.
In particular, it is believed that the composition of the present invention
will provide better wet
and dry strength properties in the production of paper products comprising
recycled materials
than cationic starches alone (and at least equivalent properties to their
synthetic alternatives),
without negatively affecting water drainage, retention or other important
parameters.
According to one particular embodiment, the composition of the present
invention will be used
such that the composition of the present invention is brought into contact
with the cellulosic pulp
in an amount of 0,2 to 20%, preferably 0,5 to 10%, more preferably 2-6% by
weight, based on
the dry weight of the pulp. Expressed in a different way, paper products of
the present invention
will preferably comprise surface modified plant fibers and cellulose fibers in
a weight ratio of
1:500 to 1:4, more preferably of 1:100 to 1:20, more preferably of 1:60 to
1:30.
The present invention will now be described in more detail by way of the
following, non-limiting
examples.
EXAMPLES
A number of tests were carried out using 100% recycled OCC paper with a
conductivity value of
4.7 mS/cm. In each case, the starch and the fibrous material were cooked
together and then
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mixed with the pulp. Paper was then made with a Kemira pilot paper machine
(type: fourdrinier
paper machine; production width: 30 cm; headbox concentration: 0.25-0.5%,
circular distributor;
wire section with vacuum foils; double press, double felted press section; oil
heated drying
cylinders; machine speed: 2.5 m/min) and tested.
Trial type Blend Total Resulting CMT SCT Burst
Breaking
ratio addition starch Index Index (in Strength
Length (in
amount (in Nm/g) Index (in m)
Nm2/g) kPam2/g)
-
Blank - - 0.81 13.7 1.310 2952
Starch (ref) - 1% 1% 0.84 14.5 1.400 3171
Wheat fiber 1:4 1.25% 1% 0.94 15.8 1.520 3293
/Starch 4:1 5% 1% 0.95 15.7 1.620 3631
Soy fiber 1:4 1.25% 1% 0.87 13.8 1.370 3350
/Starch 3:2 2.5% 1% 0.91 13.6 1.430 3395
4:1 5% 1% 0.96 14.5 1.490 3619
Further tests were performed, this time with 100% recycled OCC paper with a
conductivity value
of 11 mS/cm (i.e. in a polluted system). In each case, the starch and the
fibrous material were
cooked together and then mixed with the pulp.
Trial type Blend Total Resulting CMT SCT Burst
Breaking
ratio addition starch Index Index (in Strength
Length (in
amount (in Nm/g) Index (in m)
Nm2/g) kPam2/g)
Blank - - - 0.82 12.8 1.317 3002
Starch (ref) - 1% 1% 0.89 12.9 1.400 3074
Wheat fiber 1:4 1.25% 1% 0.92 13.6 1.340 3299
/Starch 4:1 5% 1% 0.95 13.9 1.570 3568
In all cases, the starch used was of the type C*Bond 05946 available from
Cargill Incorporated,
with a degree of substitution of 0.042, the wheat fiber was milled wheat bran
with an average
particle size of 43pm and the soy fiber was milled soy hulls with an average
particle size of
75pm.
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METHODS
CMT is measured according to the following standard method: DIN EN IS07263.
SCT is measured according to the following standard method: DIN 54518
Burst Strength is measured according to the following standard method: Mullen
DIN 53141 Part
1 (Tappi 403-0M-85)
Tear Strength (Breaking Load and Breaking Length) is measured according to the
following
standard method: DIN EN ISO 1924-2
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