METHOD OF INCREASING FILLER CONTENT IN PAPERMAKING
Cross-Reference to Related Applications
None.
Statement Regarding Federally Sponsored Research or Development
Not Applicable.
Background of the Invention
This invention relates to a method of increasing the strength of a paper mat of
fibers produced in a papermaking process. Paper mat comprises water and solids and is
commonly 4 to 8% water. The solid portion of the paper mat includes fibers (typically cellulose
based fibers) and can also include filler. Increasing the strength of the paper mat would allow
one to increase the proportion of the solids that is filler content. This is desirable because it
reduces raw materials costs, reduces energy needed in the papermaking process, and increases the
optical properties of the paper. Prior Art discloses paper mat having a solid portion of between
10% and 40% filler. The Prior Art however also discloses that increasing the filler content
coincides with a loss in strength in the resulting paper.
Fillers are mineral particles that are added to paper mat during the papermaking
process to enhance the resulting paper's opacity and light reflecting properties. Some examples
of fillers are described in US Patent Number 7,211,608. Fillers include inorganic and organic
particles or pigments used to increase the opacity or brightness, or reduce the cost of the paper or
paperboard sheet. Some examples of fillers include one or more of: kaolin clay, talc, titanium
dioxide, alumina trihydrate, barium sulfate, magnesium hydroxide, pigments such as calcium
carbonate, and the like. Previous attempts to increase the filler content in paper without losing
paper strength are described in British Patent GB 2016498, and US Patent Numbers 4,710,270,
4,181,567, 2,037,525, 7,211,608, and 6,190,663.
Calcium carbonate filler comes in two forms, GCC (ground calcium carbonate)
and PCC (precipitated calcium carbonate). GCC is naturally occurring calcium carbonate rock
and PCC is synthetically produced calcium carbonate. Because it has a greater specific surface
area, PCC has greater light scattering abilities and provides better optical properties to the
resulting paper. For the same reason however, PCC filled paper mat produces paper which is
weaker than GCC filled paper.
Paper strength is a function of the number and the strength of the bonds formed
between interweaved fibers of the paper mat. Filler particles with greater surface area are more
likely to become engaged to those fibers and interfere with the number and strength of those
bonds. Because of its greater surface area, PCC filler interferes with those bonds more than
GCC.
As a result, papermakers are forced to make an undesirable tradeoff. They must
either choose to select a paper with superior strength but inferior optical properties or they must
select a paper with superior optical properties but inferior strength. Thus there is a clear need for
a method of papermaking that facilitates a greater amount of filler in the paper, a paper that has a
high opacity, and a filled paper that has a high degree of strength.
Brief Summary of the Invention
At least one embodiment of the invention is directed towards a method of
papermaking having an increased filler content. The method comprises the steps of: adding a
first flocculating agent to an aqueous dispersion in an amount sufficient to mix uniformly in the
dispersion without causing significant fTocculation of the filler particles,
adding a second flocculating agent to the dispersion after adding the first flocculating agent in an
amount sufficient to initiate flocculation of the filler particles in the presence of the first
flocculating agent, the second flocculating agent being of opposite charge to the first flocculant,
combining the filler particles with the paper fiber stock, treating the combination with at least one
strength additive, and forming a paper mat from the combination. The paper fiber stock
comprises a plurality of fibers and water, and the initiated flocculation enhances the performance
of the strength additive in the paper mat.
At least one embodiment of the invention is directed towards this method in which
the strength of the paper made by the papermaking process is increased by an amount greater than
the sum of: the strength enhancement provided by the preflocculation process using the first and
second flocculating agents and the strength enhancement provided by the strength additive by
itself.
The filler may be selected from the group consisting of calcium carbonate, kaolin
clay, talc, titanium dioxide, alumina trihydrate, barium sulfate, and magnesium hydroxide. The
paper fiber may be cellulose fiber. The method may further comprise the step of shearing the
dispersion to obtain a pre determined floc size. The filler floes may have a median particle size
of 10-100 mm. The first and second flocculating agents may have an RSV of at least 2 dL/g. The
first flocculating agent may be anionic. The strength additive may be glyoxylated
Acrylamide/DADMAC copolymer. The ratio of strength additive relative to the solid portion of
the paper mat may be 0.3 to 5 kg of strength additive per ton of paper mat. The first flocculating
agent may be a copolymer of acrylamide and sodium acrylate. The strength additive may be a
cationic starch. The strength additive and the second flocculating agent may carry the same
charge.
The second flocculating agent may be selected from the list consisting of
copolymers of acrylamide with DMAEM, DMAEA, DEAEA, DEAEM. The second flocculating
agent may be in quaternary ammonium salt form made with a salt selected from the list consisting
of dimethyl sulfate, methyl chloride, benzyl chloride, and any combination thereof. The filler
may be anionically dispersed and a low molecular weight, cationic coagulant is added to the
dispersion to at least partially neutralize its anionic charge prior to the addition of the first
flocculating agent. The second flocculating agent may have a charge, which is opposite to the
charge of the first flocculating agent. The filler floes may have a median particle size of 10-100
mih. The filler may be selected from the group consisting of calcium carbonate, kaolin clay, talc,
titanium dioxide, alumina trihydrate, barium sulfate and magnesium hydroxide. The low
molecular weight composition may be a cationic coagulant, the first flocculating agent may be an
anionic flocculent, the second flocculating agent may be a cationic flocculent, and both
flocculants may have a molecular weight of at least 1,000,000
Brief Description of the Drawings
A detailed description of the invention is hereafter described with specific
reference being made to the drawings in which:
FIG. 1 is a graph showing the improved strength of paper made according to the
invention.
Detailed Description of the Invention
For purposes of this application the definition of these terms is as follows:
"Coagulant" means a composition of matter having a higher charge density and
lower molecular weight than a flocculant, which when added to a liquid containing finely divided
suspended particles, destabilizes and aggregates the solids through the mechanism of ionic charge
neutralization.
"DMAEM"means dimethylaminoethylmethacrylate as described and defined in
US Patent 5,338,816.
"DMAEA" means dimethylaminoethylacrylate as described and defined in US
Patent 5,338,816.
"DEAEA" means diethylaminoethyl acrylate as described and defined in US
Patent 6,733,674.
"DEAEM" means diethylaminoethyl methacrylate as described and defined in US
Patent 6,733,674.
"Flocculant" means a composition of matter having a low charge density and a
high molecular weight (in excess of 1,000,000) which when added to a liquid containing finely
divided suspended particles, destabilizes and aggregates the solids through the mechanism of
interparticle bridging.
"Flocculating Agent" means composition of matter that when added to a liquid,
destabilizes and aggregates colloidal and finely divided suspended particles in liquid into floes.
"GCC" means ground calcium carbonate, which is manufactured by grinding
naturally occurring calcium carbonate rock
"PCC" means precipitated calcium carbonate which is synthetically produced.
"Preflocculation" means the modification of filler particles into agglomerates
through treatment with a particular flocculating agent selected on the basis of the size distribution
and stability of the floe that the flocculating agent will form.
In the event that the above definitions or a definition stated elsewhere in this
application is inconsistent with a meaning (explicit or implicit) which is commonly used, in a
dictionary, or stated in a source incorporated by reference into this application, the application
and the claim terms in particular are understood to be construed according to the definition in this
application, and not according to the common definition, dictionary definition, or the definition
that was incorporated by reference.
At least one embodiment of the invention is a method of making paper, which is
strong, has a high filler content, and has superior optical properties. In at least one embodiment
of the invention the method of papermaking comprises the steps of: providing filler material, pretreating
at least some of the filler material by preflocculation leading to a decrease in the
adsorption of a strength additive on the filler material, and adding both the preflocculated filler
blend and the strength additive to the paper mat.
Preflocculation is a process in which, material is treated by two flocculating agents
in a manner that optimizes the size distribution and stability of the floes under a particular shear
force prior to its addition to the paper stock. The particular chemical environment and high fluid
shear rates present in modern high-speed papermaking require filler fiocs to be stable and shear
resistant. The floe size distribution provided by a preflocculation treatment should minimize the
reduction of sheet strength with increased filler content, minimize the loss of optical efficiency
from the filler particles, and minimize negative impacts on sheet uniformity and printability.
Furthermore, the entire system must be economically feasible. Examples of preflocculation
methods applicable to this invention are described in US Published Application 2009/0065162
Al and US Application 12/431356.
It has been known for some time that adding strength additives to paper mat
increases the strength of the resulting paper. Some examples of strength additives are described
in US Patent Number 4,605,702. Some examples of strength additives are cationic starches,
which adhere to the cellulose fibers and tightly bind them together.
Unfortunately it is not practical to add large amounts of strength additives to
compensate for the weakness that results from using large amounts of filler in paper mat. One
reason is because strength additives are expensive and using large amounts of additives would
result in production costs that are commercially non-viable. In addition, adding too much
strength additive negatively affects the process of papermaking and inhibits the operability of
various forms of papermaking equipment. As an example, in the context of cationic starch
strength additives, the cationic starch retards the drainage and dewatering process, which
drastically slows down the papermaking process.
Adding filler to the paper mat reduces the effectiveness of the strength additive.
Because filler has a much higher specific surface area than fiber, most of the strength additives
added into the papermaking slurry go to filler surfaces, and therefore there is less strength
additive available to bind the cellulose fibers together. This effect is more acute with PCC
compared to GCC because PCC has a much higher surface area and is able to adsorb more
strength additive. One method of addressing this situation is by pre-treating the filler material
with a coagulant as described in US Application 12/323976. Another method involves using
preflocculation instead of a coagulant.
In at least one embodiment the filler content in the paper is increased by the
following method: An aqueous dispersion of filler materials is formed and the filler materials are
preflocculated before being added to a paper fiber stock. A first flocculating agent is added to the
dispersion in an amount sufficient to mix uniformly in the dispersion without causing significant
flocculation of the filler particles. A second flocculating agent is then added following the first
flocculating agent, in an amount sufficient to initiate flocculation of the filler material in the
presence of the first flocculating agent, the second flocculating agent being of opposite charge to
the first flocculating agent. A paper mat is formed by combining the preflocculated filler material
with the fiber stock and treating this combination with the strength additive. The preflocculation
of the filler material enhances the performance of the strength additive. The fiber stock
comprises fibers, fillers, and water.
In at least one embodiment, the fibers are predominantly cellulose based. In at
least one embodiment the flocculated dispersion is sheared to obtain a particularly desired
particle size.
While pre-treating filler particles is known in the art, prior art methods of pretreating
filler particles are not directed towards affecting the adhesion of the strength additive to
the filler particles with two flocculants. In fact, many prior art pre-treatments increase the
adhesion of the strength additive to the filler particles. For example, US Patent Number
7,2 11,608 describes a method of pre-treating filler particles with hydrophobic polymers. This
pre-treatment however does nothing to the adhesion between the strength additive and the filler
particles and merely repels water to counterbalance an excess of water absorbed by the strength
additive. In contrast, the invention decreases the interactions between the strength additive and
the filler particles and results in an unexpectedly huge increase in paper strength. This can best
be appreciated by reference to FIG. 1.
FIG. 1 illustrates that a paper produced from a paper mat that includes PCC filler
tends to become weaker as more PCC filler is added. When a large amount of PCC is added
(over 25%), the addition of a strength additive adds little strength to the paper. Paper made from
preflocculated PCC filler combined with a strength additive however increases the strength of the
paper to a degree that it is stronger than paper having 10% less PCC that is not preflocculated.
Even more surprising was the fact that paper containing preflocculated PCC without a strength
additive was almost as strong as the paper with the strength additive.
As a result, at least two conclusions can be reached, 1) the strength agent is more
effective in increasing sheet strength with preflocculated filler than with untreated filler and 2)
there is a synergistic effect from the combination of strength agent and filler preflocculation
which makes it superior to the additive effects of the sum of the strength agent alone plus the
filler preflocculation alone. As a result, preflocculation of the PCC filler material leads to the
production of paper that is unexpectedly strong.
At least some of the fillers encompassed by this invention are well known and
commercially available. They include any inorganic or organic particle or pigment used to
increase the opacity or brightness, reduce the porosity, or reduce the cost of the paper or
paperboard sheet. The most common fillers are calcium carbonate and clay. However, talc,
titanium dioxide, alumina trihydrate, barium sulfate, and magnesium hydroxide are also suitable
fillers. Calcium carbonate includes ground calcium carbonate (GCC) in a dry or dispersed slurry
form, chalk, precipitated calcium carbonate (PCC) of any morphology, and precipitated calcium
carbonate in a dispersed slurry form. The dispersed slurry forms of GCC or PCC are typically
produced using polyacrylic acid polymer dispersants or sodium polyphosphate dispersants. Each
of these dispersants imparts a significant anionic charge to the calcium carbonate particles.
Kaolin clay slurries also are dispersed using polyacrylic acid polymers or sodium polyphosphate.
In at least one embodiment, the strength additive carries the same charge as the
second flocculating agent. Strength additives encompassed by the invention include any one of
the compositions of matter described in US Patent 4,605,702 and US Patent Application
2005/0161 181 Al and in particular the various glyoxylated Acrylamide/DADMAC copolymer
compositions described therein. An example of a glyoxylated Acrylamide/DADMAC copolymer
composition is product^ Nalco 64170 (made by Nalco Company, Naperville, Illinois).
In at least one embodiment, the fillers used are PCC, GCC, and/or kaolin clay. In
at least one embodiment, the fillers used are PCC, GCC, and/or kaolin clay with polyacrylic acid
polymer dispersants or their blends. The ratio of strength additive relative to solid paper mat can
be 3kg of additive per ton of paper mat.
In at least one embodiment, the effectiveness of the synthetic strength additive is
independent of or despite the presence of some, low amounts, or no amount of starch in the paper
mat. In prior art disclosures, it is known that adding between 10 to 20 lbs of starch per ton of
paper mat increases the strength of the resulting paper. The addition of materials in such large
amounts however is cumbersome and less than ideal. The use of synthetic strength additives in
contrast allows similar strength performance to be achieved while requiring the addition of far
less strength additive material to the paper mat. In at least one embodiment the synthetic strength
additive is cationic or anionic or contains both cationic and anionic functional groups.
Unfortunately synthetic strength additives are known to be far more expensive
than starch. In some processes the cost of using bulky large amounts of starch may be less
expensive than smaller and more easily manageable amounts of synthetic strength additives. The
combination of the strength adding effects of synthetic strength additives in low dosages
combined with the prefiocculation allows unexpected degrees of strength to be observed than
would otherwise be expected with such low dosages of strength additives and in the absence of
large amounts or any amount of starch.
EXAMPLES
The foregoing may be better understood by reference to the following
example, which is presented for purposes of illustration and is not intended to limit the
scope of the invention.
A furnish was produced containing 25% pine softwood and 75%
eucalyptus hardwood. Both the softwood and hardwood were reslushed from dry lap.
The filler used was Albacar HO PCC obtained from Specialty Minerals Inc. The filler
material preflocculation was performed with the dual flocculant approach described in
example 14 of US Application 12/431356. During the handsheet preparation, 6 lb/ton
strength additive (Nalco 641 14, a glyoxalated Acrylamide/DADMAC copolymer
available from Nalco Company, Naperville, IL, USA) was added. The results are
displayed in FIG. 1.
While this invention may be embodied in many different forms, there are shown in
the drawings and described in detail herein specific preferred embodiments of the invention. The
present disclosure is an exemplification of the principles of the invention and is not intended to
limit the invention to the particular embodiments illustrated. All patents, patent applications,
scientific papers, and any other referenced materials mentioned herein are incorporated by
reference in their entirety. Furthermore, the invention encompasses any possible combination of
some or all of the various embodiments described herein and incorporated herein.
The above disclosure is intended to be illustrative and not exhaustive. This
description will suggest many variations and alternatives to one of ordinary skill in this art. All
these alternatives and variations are intended to be included within the scope of the claims where
the term "comprising" means "including, but not limited to". Those familiar with the art may
recognize other equivalents to the specific embodiments described herein which equivalents are
also intended to be encompassed by the claims.
All ranges and parameters disclosed herein are understood to encompass any and
all subranges subsumed therein, and every number between the endpoints. For example, a stated
range of "1 to 10" should be considered to include any and all subranges between (and inclusive
of) the minimum value of 1 and the maximum value of 10; that is, all subranges beginning with a
minimum value of 1 or more, (e.g. 1 to 6.1), and ending with a maximum value of 10 or less,
(e.g. 2.3 to 9.4, 3 to 8, 4 to 7), and finally to each number 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 contained
within the range.
This completes the description of the preferred and alternate embodiments of the
invention. Those skilled in the art may recognize other equivalents to the specific embodiment
described herein which equivalents are intended to be encompassed by the claims attached hereto.
Claims
1. A method of papermaking having an increased filler content, the method comprising the
steps of:
adding a first flocculating agent to an aqueous dispersion in an amount sufficient to mix
uniformly in the dispersion without causing significant flocculation of the filler particles,
adding a second flocculating agent to the dispersion after adding the first flocculating
agent in an amount sufficient to initiate flocculation of the filler particles in the presence of the
first flocculating agent, the second flocculating agent being of opposite charge to the first
flocculant,
combining the filler particles with the paper fiber stock,
treating the combination with at least one strength additive selected from the group
consisting of synthetic strength additives, and
forming a paper mat from the combination,
the paper fiber stock comprises a plurality of fibers and water, and
the initiated flocculation enhances the performance of the strength additive in the paper
mat, wherein the strength additive is not starch.
2. The method of claim 1 in which the strength of the paper made by the papermaking
process is increased by an amount greater than the sum of: the strength enhancement provided by
the preflocculation process using the first and second flocculating agents and the strength
enhancement provided by the strength additive by itself.
3. The method of claim 1 wherein the filler is selected from the group consisting of calcium
carbonate, kaolin clay, talc, titanium dioxide, alumina trihydrate, barium sulfate, and magnesium
hydroxide.
4. The method of claim 1 in which paper fiber is cellulose fiber.
5. The method of claim 1 further comprising the step of shearing the dispersion to obtain a
pre determined floe size of between 10 and 100 microns.
6. The method of claim 1 in which the first and second flocculating agents have an RSV of
at least 2 dL/g.
7. The method of claim 1 wherein the first flocculating agent is anionic.
8. The method of claim 1 in which the ratio of strength additive relative to the solid portion
of the paper mat is 0.3 to 5 kg of strength additive per ton of paper mat.
9. The method of claim 1 wherein the first flocculating agent is a copolymer of acrylamide
and sodium acrylate.
10. The method of claim 1 wherein the filler is anionically dispersed and a low molecular
weight, cationic coagulant is added to the dispersion to at least partially neutralize its anionic
charge prior to the addition of the first flocculating agent.
11. The method of claim 1 in which the strength additive and the second flocculating agent
carry the same charge.
12. The method of claim 1 wherein the second flocculating agent is selected from the list
consisting of copolymers of acrylamide with DMAEM, DMAEA, DEAEA, DEAEM.
13. The method of claim 12 in which the second flocculating agent is in quaternary
ammonium salt form made with a salt selected from the list consisting of dimethyl sulfate, methyl
chloride, benzyl chloride, and any combination thereof.
14. The method of claim 1 in which the second flocculating agent has a charge, which is
opposite to the charge of the first flocculating agent.
15. The method of claim 14 wherein the low molecular weight composition is a cationic
coagulant, the first flocculating agent is an anionic flocculant, the second flocculating agent is a
cationic flocculent, and both flocculants have a molecular weight of at least 1,000,000.