Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR MAHING ABRASION RESISTANT PAPER
AND PAPER AND PAPER PRODUCTS MADE BY THE PROCESS
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit of provisional application Serial No.
60/514279, filed October 24, 2003 which is incorporated herein by reference.
FIELD OF THE INVENTION
The present invention relates to papermaking and, more particularly, to
processes for making paper having improved properties such as abrasion
resistance,
decreased coefficient of friction, and increased brightness.
BACKGROUND OF THE INVENTION
The surface strength of manufactured paper products is receiving increased
attention as papermaking technology advances and the paper products produced
thereby find an ever-growing field of use. Poor surface strength has numerous
repercussions on papermaking machinery and on the products themselves. Paper
products having a low surface strength can bind or catch on rollers during the
manufacturing process causing costly delays and waste of materials. Similarly,
paper
that is used as a component of a commercial product, such as the backing paper
for
gypsum wallboard, ideally should have a high surface strength in order to
prevent
tearing or damage to the core components as well as to prevent catching or
binding on
conveyor belts during the various steps of product manufacture and
transportation.
Consequently, it would be highly desirable to be able to manufacture paper
having an
increased surface strength in order to improve abrasion resistance, especially
when the
paper is to be used as backing paper in abuse resistant wallboard products.
A variety of different solutions have been proposed to solve or minimize the
problem of abrasion resistance on the surfaces of paper. For example, U.S.
Patent No.
6,083,586 describes compositions and methods for the manufacture of material
sheets
having a starch-bound matrix, optionally reinforced with fibers and inorganic
mineral
filler.
U.S. Patent No. 6,153,040 discloses a process for reducing the rollups in
gypsum board panels when the panels are laminated. At least one face of the
gypsum
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board paper is treated with a friction reducing agent, such as a wax or wax
emulsion,
in order to reduce its coefficient of friction, resulting in the reduction of
shear force
which develops between the backing paper of a gypsum board panel and the
conveyor
belts used to carry such a panel.
The addition of cationic wet-strength polyamide resins to paper cover sheets,
especially polyamide epichlorohydrin resin, is described in U.S. Patent No.
6,489,040.
U.S. Patent No. 6,517,674 describes a process for manufacturing wear
resistant/abrasion resistant paper incorporating spacer- or separator-
particles to
minimize the amount of surface damage on the paper surface. The particles
described
and incorporated into the paper are microspheres, such as glass microspheres,
and
abrasion resistant particles of grit such as aluminum oxide or silicon
carbide.
According to the '674 patent, the particles are added to the paper fiber pulp
at the wet
end of the paper machine from a primary or secondary headbox using a curtain
slot
coater as the application device.
In the process taught in U.S. Patent No. 6,551,457, paper is produced from an
aqueous suspension containing cellulosic fibers and optional fillers. After
draining
the suspension, the obtained paper web is passed through the nip of a paper
manufacturing machine. A chemical system comprising a polymeric component and
a micro- or nano-particle component is added to the paper suspension/web. The
addition of such a mixture of components is said to improve the overall
quality and
strength of the paper product, such as its coefficient of friction.
U.S. Patent No. 6,562,444 discloses a fiber-cement and gypsum laminate
composite building material that contains an adhesive layer interposed between
the
fiber-cement sheet and the gypsum panel, so as to improve the abrasion
resistance of
the laminate. The adhesive layer is a polymeric adhesive, such as modified
starches.
U.S. Patent No. 6,568,148 discloses a covering element for building surfaces
and a method for the production of such an element. The covering element is
described as having an upper face with a support layer made up of cellulose in
which
an abrasion-resistant material, such as corundrum particles, is embedded,
thereby
providing enhanced abrasion resistance and a lowered coefficient of friction.
The literature has also reported several approaches to the problem of abrasion
resistance in papers. Zhang, et al. in Wear, Vol. 253 (2002), pp. 1086-1093
("Effect
of Particle Surface Treatment on the Tribological Performance of Epoxy Based
Nanocomposites") describes the preparation of modified nanosilica covalently
bonded
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to polyacrylamide particles, thereby increasing the interfacial interaction
between
particles and matrix, and resulting in reductions in surface abrasion.
Gurnagul, et al.
described factors affecting the coefficient of friction of paper, and
suggested that the
coefficient of friction is a function of the amount of extractives present in
or on a
paper surface (Journal ofApplied Polymer Science, Vol. 46 (1992), pp. 805-814;
"Factors Affecting the Coefficient of Friction of Paper"). According to the
article, the
amount and identity of the particles significantly effect the coefficient of
friction.
Finally, a review describing the effect of fillers on the coefficient of
friction in papers
was detailed in TAPPI Journal, Vol. 74 (1991), pp. 341-347 ("Effect of Fillers
on
Paper Friction Properties"), describing how the use of various fillers such as
kaolin,
talc, and synthetic precipitated silica in the paper manufacturing process can
effect the
coefficient of friction.
While it is known that the addition of small, hard abrasion resistant
particles
(also referred to as "grit") to the paper, or to resin mixtures which coat the
sheet, can
enhance the abrasion resistance of papers, paper products and high-pressure
laminates, their use is often accompanied by costly side effects. For example,
the use
of alumina has been reported to give wear resistance of 400 to 600 cycles.
However,
the use of abrasion resistant particles, even microparticles or nanoparticles,
tend to
scratch and cause significant damage to highly polished caul plates and
rollers used
during the paper production process for producing both high pressure and low
pressure products. Rollers and Gaul plates scratched or otherwise damaged
through
contact with abrasion resistant materials such as described above must either
be
resurfaced or replaced at a significant cost
In view of the foregoing, it will be appreciated that there is a need for
abrasion
resistant paper, and a process for producing such abrasion resistant paper
that avoids
damage to the papermaking machinery caused by incorporation of grit into the
paper.
SUMMARY OF THE INVENTION
The present invention provides a process for making paper as well as paper
and paper products made by the process. In this papermaking process, a first
strength
agent is added to the stock suspension containing pulp and optionally other
additives
prior to its being formed into a web at the wet end of a papermaking machine.
The
web is then formed and processed into paper. A second strength agent is then
applied
to the surface of the paper. In this process, the strength agents are selected
to have
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opposite charge (or to be amphoteric). Thus, in one embodiment, for example,
the
first strength agent is a cationic dry-strength agent and the second strength
agent is an
anionic dry-strength agent.
The process of this invention can be used to make paper that is resistant to
abrasion. Embodiments of this process produce paper having other desirable
physical
properties like high optical brightness and a low friction surface. An
optically bright
paper can be obtained by applying the second strength agent in a solution that
also
contains an optical brightener. A paper having a low friction surface can be
obtained
by including a hydrophobic organo-silicone in the solution that is used to
apply the
second strength agent.
Paper made by the process is useful in a variety of paper products. In
particular, the process is useful for making abrasion resistant backing paper
for
gypsum wallboard.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Where not expressly defined, the terms used in this disclosure are intended to
be construed as those skilled in the art would understand them. The following
express
definitions are consonant with the understanding of those skilled in the art.
"Paper", as used herein, refers to a web of pulp fibers that are formed from
an
aqueous suspension on a wire or screen and held together at least in part by
hydrogen
bonding, and wluch can be made by hand or by machine. Included in this
definition
are the wide range of matted or felted webs of vegetable fiber (mostly wood)
that
have been formed on a screen from a water suspension, such as "tree paper"
manufactured from wood pulp derived from trees, "plant papers" or "vegetable
papers" which include a wide variety of plant fibers (also known as "secondary
fibers"), such as straw, flax, and rice fibers, and is broadly referred to as
"cellulose-
based paper", and Draft paper (paper manufactured by the Kraft process).
Further, the
term paper as used herein is meant to refer to products containing
substantially all
virgin pulp fibers, substantially all recycled pulp fibers, or both virgin and
recycled
pulp fibers.
"Papermaking machine", as used herein, refers to any of the papermaking
machines known in the art, all of which are suitable for use with the process
of the
present invention. Such machines include cylinder machines, fourdrinier
machines,
twin wire forming machines, FC Former machines, and modifications thereof.
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"Pulp" refers to fibers that are plant based, including but not limited to
wood
and similar "woody" plants, soy, rice, cotton, straw, flax, abaca, hemp,
bagasse,
lignin-containing plants, and the like. Such pulps include, but are not
limited to,
thermomechanical pulps, bleached thermomechanical pulps, chemi-
thermomechanical
pulps (CTMP), bleached chemi-thermomechanical pulps, and deinked bleached
thermomechanical pulps.
"Sheet", as used herein, is intended to include any substantially flat,
corrugated, curved, bent, or textured sheet made using the compositions and
methods
described herein. The sheets can have greatly varying thickness depending on
the
particular application for which the sheet is intended. That is, the sheets
can be as
thin as about 0.01 mm and as thick as 1 cm or greater, where strength,
durability,
and/or bulk are important considerations depending upon the end use of the
paper
sheet.
"Stock suspension", as used herein, refers to a mixture, or slurry, of pulp,
fillers, water, and other papermaking materials. As used herein, the term
"stock
suspension" is meant to be equivalent to the term "pulp slurry".
"Strength agent" refers to compounds that are incorporated into paper in order
to increase its resistance to tearing. "Wet-strength agents" are agents that
make paper
more resistant to tearing when the paper is wet. "Dry-strength agents" are
agents that
make the paper more resistant to tearing when the paper is dry, but are less
effective
at strengthening wet paper than wet-strength agents are. Dry-strength agents
can be
cationic, anionic or amphoteric in nature.
"Web", as used herein, refers to the continuous mat of fibers that is
deposited
on the wire or felt, drained, pressed and dried to form paper.
The present invention provides a process for making paper. The paper and
paper products made by the process may exhibit improved surface strength,
abrasion
resistant, a low friction surface and/or a high optical brightness depending
upon the
particular embodiment of the process that is followed.
The process of the present invention can be practiced on conventional
papermaking equipment. Although papermaking equipment varies in operation and
mechanical design, the processes by which paper is made on different equipment
contain common stages. Papermaking includes a pulping stage, stock preparation
stage, a wet end stage and a dry end stage.
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In pulping, individual cellulose fibers are liberated from a source of
cellulose
such as wood either by mechanical or chemical action, or both.
The liberated fibers, or pulp, is suspended in water in the stock preparation
stage. Additives such as brightening agents, dyes, pigments, fillers,
antimicrobial
agents, defoamers, pH control agents and drainage aids also may be added to
the stock
at this stage. As the term is used in this disclosure "stock preparation"
includes such
operations as dilution, screening and cleaning of the stock suspension that
may occur
prior to forming of the web. In particular, it includes feeding the pulp
stream to a fan
pump from a machine chest.
The wet end stage commences after preparation of the stock suspension. For
purposes of this disclosure, the wet end stage commences when the pulp first
contacts
a wire or felt in a papennaking machine. The wet end stage fixrther includes
such
later operations as forming of the web, draining of the web and consolidation
of the
web (pressing).
In the dry end stage, the web is dried and may be subjected to additional
processing like size pressing, calendering, spray coating of surface
modifiers,
printing, cutting, corrugating and the like.
Of relevance to the present invention, a size press is a device for applying a
solution to the paper. It includes a pair of squeeze rolls which are moistened
with the
solution sought to be applied. The size press typically is situated between
drying
sections to allow removal of excess moisture. Size presses are typically used
to apply
surface sizing to improve the water resistance of the paper and improve ink
absorption.
A calender stack is a series of solid rolls, usually made of steel or iron
through
which the dry paper is passed in a serpentine manner. Pressure applied to the
paper as
it passes between rolls in the calender stack can improve surface smoothness,
increase
gloss, make the caliper of the paper more uniform and decrease porosity: Of
relevance to the present invention, a nip (or multiple nips) between calender
rolls may
be flooded in a "waterbox" application. The calender waterbox may be used to
apply
coatings to the paper for a variety of purposes, such as to increase water
resistance,
reduce curl and improve gloss.
In addition to a size press and calender waterbox, the dried paper can be
coated by spray coating using a sprayboom.
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Three general types of papermaking machines that are routinely used in the
papermaking industry are differentiated by the way that they form the web. In
a
fourdrinier papermaking machine, the web is formed by delivering a ribbon of
stock
suspension to a porous belt known to those skilled in the art as the "wire"
from a
headbox. The headbox is a tank positioned above or beside the wire. The wire
is
drawn between a "breast roll" and a "couch roll" and is typically driven by
the couch
roll. The headbox is typically positioned above the wire near the breastroll.
The web
is delivered from the headbox to the wire through a narrow opening in the
headbox
that is known to those skilled in the art as the "slice." As the wire travels,
the web is
drawn towards the couch roll. While in transit, water drains from the pulp
through the
porous wire under the effect of gravity and typically with the assistance of
tube rolls,
hydrofoils and/or suction boxes. From the wire, the web is passed to the
pressing
section of the paper machine. The web typically has a consistency of from
about 12%
to about 25% before pressing. In the pressing section, the web is squeezed
between
press rolls to eliminate more water. From the pressing section, the partially
dried web
is passed to the drying section. There, the web is dried, typically to a
moisture
content of from about 4% to about 12% by passing over heated dryer cans,
although
many papermachines in the gypsum industry dry to 0% to about 1% moisture
content
for greater dimensional stability.
Another common papermaking machine is the cylinder machine. The stock
suspension is fed into one or more vats. In each vat, there is a horizontally
disposed
cylinder having a wire around its circumference. The cylinder is partially
immersed
in the stock suspension. The cylinder is rotated. As it does so, the wire
picks up
fibers, carnes them out of the stock suspension and delivers them to a "pick-
up felt ."
The pick-up felt is a porous belt that travels synchronously with the
cylinder. In a
multiple cylinder machine, mufti-ply paper can be made by supplying a
different
stock suspension to each vat. The web is then transferred from the pickup felt
to the
pressing section and then to the drying section.
In another common design, the stock suspension is sprayed between two
converging wires. Such twin wire formers accelerate the removal of water
making
them well suited for high speed machines.
It has been found that adding a cationic dry-strength agent prior to the wet
stage of the papermaking process and an anionic dry-strength agent during the
dry
stage of the papennaking process yields paper having an increased surface
strength.
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Accordingly, the present invention provides a process for making paper and
paper products comprising the steps of (1) preparing a stock suspension of
cellulosic
fibers, (2) adding a first strength agent to the stock suspension, (3) forming
the
cellulosic fibers into a substantially uniform web and (4) drying the web into
paper
and applying a second strength agent to the surface of the paper. The first
strength
agent is either a cationic dry-strength agent, an amphoteric dry-strength
agent, or a
cationic wet-strength agent, with cationic dry-strength agents being
preferred. The
second strength agent is either an anionic dry-strength agent or an amphoteric
dry-
strength agent, with anionic dry-strength agents being preferred.
Cationic dry-strength agents useful in practice of the present invention
include, but are not limited to, cationic polyacrylamides, natural polymers,
modified
natural polymers, synthetic polymers, starches modified to have quaternary
ammonium functional groups, celluloses, natural gums, polyvinyl alcohol, and
any
number of commercially available compounds having Bipolar functional groups
that
allow for the formation of hydrogen bonds. Preferred cationic dry-strength
agents are
cationic polyacrylamides and cationic synthetic polymers. As those skilled in
the art
appreciate, a cationic polyacrylamide can be made by co-polymerization of
acrylamide with another acrylic monomer having a quaternary ammonium
substituent
thereon, such as (CH3)3N+CHZCH20C(O)CHCH2. An example of a commercially
available cationic polyacrylamide is Nalco 997, available from Nalco Chemical
Company (Naperville, IL).
Anionic dry-strength agents useful in practice of the present invention
include,
but are not limited to, anionic polyacrylamides, natural starches, and
carboxymethylcellulose (CMC). The most preferred anionic dry-strength agents
are
anionic polyacrylamides. As those skilled in the art appreciate, an anionic
polyacrylamide can be made by co-polymerization of acrylamide with an anionic
acrylic monomer such as sodium acrylate. An example of a commercially
available
anionic polyacrylamide is Nalco 1044, available from Nalco Chemical Company
(Naperville, IL).
In an alternative embodiment, either the cationic dry-strength agent or the
anionic dry-strength agent, or both, is substituted by an amphoteric dry-
strength agent,
such as amphoteric starches. Amphoteric compounds useful in practice of the
present
invention have a ratio of anionic groups to cationic groups of from about 0.1:
1.0 to
about 1.0: 1Ø Preferably, the amphoteric compounds have a ratio of anionic
groups
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to cationic groups of about 1.0: 1Ø For example, ratios of anionic groups to
cationic
groups in amphoteric compounds suitable for use with the present disclosure
include
ratios of about 0.1:1.0, about 0.2:1.0, about 0.3:1.0, about 0.4:1.0, about
0.5:1.0, about
0.5:1.0, about 0.6:1.0, about 0.7:1.0, about 0.8:1.0, about 0.9:1.0, about
1.0:1.0, and
ratios that fall between any two of these ratios.
In yet another alternative embodiment, the cationic dry-strength agent is
substituted by a cationic wet-strength agent. Wet-strength agents are
typically
thermosetting resins that are added to the stock suspension, web or paper in
order to
impart wet-strength to the paper product. They also often contribute to the
dry-
strength of the paper. Wet-strength agents are often cationic thermosetting
resins, and
are typically added to the stock prior to being sent to the paper machine. By
thermosetting, it is meant that upon drying and/or heating, the wet-strength
resins
form a substantially insoluble, and water-resistant, network which can
withstand
wetting of the paper, thus contributing to the wet-strength of the paper.
Generally
speaking, wet-strength agents are polymeric, polar enough to be soluble or
substantially dispersible in water, cationic so as to be substantive to pulp,
and
reactive/thermosetting. The types of wet-strength agents useful in the
practice of the
present invention include acid-curing resins, neutral to acid curing resins,
and neutral
to alkaline curing resins. Useful acid-curing, or formaldehyde-based, resins
include
urea-formaldehyde (UF) resins, melamine-formaldehyde (MF) resins, and other
resins
which can be used at a system pH between about pH 4 and pH 5. Neutral to acid
curing resins that are useful as wet-strength agents in the practice of the
present
invention include dialdehyde starch (DAS), polyacrylamide-glyoxal (PAMG)
resins,
and aldehyde-modified starches. Neutral/alkaline curing resins that are useful
as wet-
strength agents polyamide-epichlorohydrin resin (PAE), resins containing at
least one
epoxide functional group, and derivatives of the reaction of epichlorohydrin
with a
polyamine resin.
The cationic, anionic and amphoteric dry-strength agents, as well as the wet
strength agents, preferably have a specific gravity of from about 1.00 to
about 1.20,
and more preferably a specific gravity of from about 1.01 to about 1.10. Most
preferably, the specific gravity is from about 1.02 to about 1.08. The dry-
strength
agents preferably have a viscosity of from about 1,000 cps (1 Pa-s) to about
15,000
cps (15 Pa-s), and more preferably of from about 2,000 cps (2 Pa-s) to about
14,000
cps (14 Pa-s).
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The cationic dry-strength agents added prior to the wet end (e.g., fed to the
liner thick stock) can be added in an amount of from about 1 lbs/ton (of total
paper)
(0.5 kglt) to about 40 lbs/t (9.1 kg/t), and more preferably from about 5
lbs/ton (2.3
kg/t) to about 15 lbs/t (6.8 kg/t). For example, the cationic dry-strength
agents added
prior to the wet end of the manufacturing process can be added in an amount of
about
1 lb/t (0.5 kg/t), about 2 lb/t (0.9 kg/t), about 3 lb/t (1.4 kg/t), about 4
lb/t (1.8 kg/t),
about 5 lb/t (2.3 kg/t), about 6 lb/t (2.7 kg/t), about 7 lb/t (3.2 kg/t),
about 8 lb/t (3.6
kg/t), about 9 lb/t (4.1 kg/t), about 10 lb/t (4.5 kg/t), about 15 lb/t (6.8
kg/t) and about
20 lb/t (9.1 kg/t), as well as in ranges between any two of these values. When
the
cationic dry-strength agent is Nalco 997, it is preferably added at a rate of
about 10
lbs/ton dry.
The anionic dry-strength agents are added at the dry end (e.g., in the
calender
waterbox) in an amount of from about 5 lbs/ton (of liner plies) (2.3 kg/t) to
about 25
lbs/ton (11.3 kglt), and more preferably from about 6 lbs/t (2.7 kg/t) to
about 20
lbs/ton (9.1 kg/t). The dry-strength agents added at the dry end of the
manufacturing
process can be added in an amount of from about 5 lb/t (2.3 kg/t), about 6
lb/t (2.7
kg/t), about 7 lb/t (3.2 kg/t), about 8 lb/t (3.6 kg/t), about 9 lb/t (4.1
kg/t), about 10 lb/t
(4.5 kg/t), about 15 lb/t (6.8 kg/t), about 20 lb/t (9.1 kg/t), and about 25
lb/t (11.3
kg/t), as well as in ranges between any two of these values. When the anionic
dry-
strength agent is Nalco 1044, it is preferably added at a rate of about 2
lbs/ton dry.
The dry-strength agents can be added in one portion, or in increments over a
predetermined period of time. For example, the cationic dry-strength agent can
be
added prior to the wet end of the papermaking machine in substantially one
portion,
or charge. Preferably, the cationic dry-strength agent is added to the wet end
incrementally in predetermined amounts over a period of time.
A typical process for the manufacture of a paper product having increased
surface strength in accordance with the present invention is as follows. A
suspension
of pulp and fibers is prepared and additives, as necessary, are added in. A
cationic
dry-strength agent or agents can be added at this point. The pulp is 'formed',
or
applied to the wire at a consistency suitable to give good formation. That is,
the stock
is applied such that an even distribution of fibers results, allowing for the
generation
of a paper product of uniform thickness. This is accomplished by circulating
the
stock suspension into a headbox so that the stock is delivered as a
substantially
uniform web of pulp onto the wire through the slice at a velocity
substantially
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equivalent to that of the wire. An optional secondary headbox can be provided
to
deliver a top coat of higher-quality fiber onto the primary paper product
sheet as it
moves down the production line.
Following deposition of the stock suspension from the headbox onto the
moving wire, the web is carried over rolls (such as breast rolls, table rolls,
and couch
rolls) and suction boxes, and off the table. As the paper web is transported
on the
wire, the sheet loses water by drainage and through the suction boxes, and
optionally
through foils, lovacs, vacuum units, and the like.
Water is further removed from the web by pressing and drying. Drying can be
accomplished through the use of drying devices such as dryer cans (hollow,
revolving,
steam-filled drums), dryer felts, steam control systems, pocket ventilation
systems,
dryer hoods, Yankee dryers drums, impulse drying, combinations thereof, and
the
like. The choice of type of drying means will generally depend upon the
machine
and/or the type of paper product being manufactured. Sizing, defoamers, and
the like
can be added using one or more size presses located between dryer sections.
The paper then passes through a waterbox-equipped calender stack, where an
anionic dry-strength agent or agents are added. Optionally, a hydrophobic
organo-
silicone compound in combination with the anionic dry-strength agent and an
optical
brightener are fed into the waterbox, and are consequently applied to the
paper as it
passes through the calender stack waterbox.
As further illustrated in Example 1, which follows, paper and paper products
made by the process of this invention exhibit improved surface strength.
Normal
gypsum facing paper will lose ~ .009" in 100 to 200 cycles of abrasion while
paper
made according to our process loses only 0.000 to 0.005" after 1000 cycles.
Surface
strength was measured using a modification of the procedure specified in ASTM
D
4977-98b.
One particular embodiment of the inventive process yields a paper with a
surface having a low coefficient of friction. This embodiment includes the
steps of
(1) preparing a stock suspension of cellulosic fibers, (2) adding a first
strength agent
to the stock suspension, (3) forming the cellulosic fibers into a
substantially uniform
web and (4) drying the web into paper and applying a solution containing a
second
strength agent and a hydrophobic organo-silicone compound to the surface of
the
paper.
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Preferred hydrophobic organo-silicones are described in U.S. Patent No.
3,389,042, the disclosure of which is hereby incorporated by reference in its
entirety.
Commercially available silicones that are especially preferred for use in the
present
invention are RE-29, GE-OSI and SM-8715 available from Dow Corning Core.
(Midland, MI). The hydrophobic organo-silicone is preferably added in solution
with
the anionic dry-strength agent that is fed into the waterbox. Previously, due
to the
high cost of silicone, surface sizing was done prior to a silicone coating,
and its use as
a sizing agent was deterred by its cost (Duraiswamy, C, et al., "Effect of
Starch Type
on the Silicone Hold-Out of Release Papers, 2000 Coating Conference
Proceedings",
TAPPI Jourhal, 2001, Vol. 84(3)). However, it has been found that addition of
silicone in combination with an anionic dry-strength agent in the waterbox
creates a
synergistic effect, wherein the silicone imparts some sizing while the dry-
strength
agent enhances the strength of the surface of the paper product. Of course,
silicone
sizing agents also can be added in any conventional manner during the
papermaking
process.
The hydrophobic organo-silicone is preferably added in an amount of from
about 1 lb/ton (0.5 kg/t) to about 10 lb/ton (4.5 kg/t), and more preferably
from about
1 lb/ton (0.5 kg/t) to about 5 lb/ton (2.3 kg/t), and most preferably from
about 1 lb/ton
to about 3 lb/ton (0.5 -1.5 kg/t).
Another particular embodiment of the inventive process yields a paper with a
bright surface. Surfaces with an L* value of 89 or above can be obtained. This
embodiment includes the steps of: (1) preparing a stock suspension of
cellulosic
fibers, (2) adding a first strength agent to the stock suspension, (3) forming
the
cellulosic fibers into a substantially uniform web and (4) drying the web into
paper
and applying a solution containing a second strength agent and a brightener to
the
surface of the paper. Compounds useful as brightening agents in practice of
the
present invention include but are not limited to azoles; biphenyls; chelating
agents
such as ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic
acid
(DTPA), hydroxyethylethylenediaminetriacetic acid (HEDTA) and nitrilotriacetic
acid (NTA) and other compounds that are capable of chelating heavy metals that
catalyze color-forming reactions. Useful optical brighteners further include
coumarins; furans; ionic brighteners, including anionic, cationic, and anionic
(neutral)
compounds, such as the Eccobrite~ and Eccowhite~ compounds available from
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Eastern Color & Chemical Co. (Providence, Rn; naphthalimides; pyrazenes;
stilbenes, such as the Leucophor~ range of optical brighteners available from
the
Clariant Corporation (Muttenz, Switzerland), and Tinopal~ from Ciba Specialty
Chemicals (Basel, Switzerland); salts of such compounds including but not
limited to
alkali metal salts, alkali earth metal salts, transition metal salts, organic
salts (e.g.,
cyclohexyl and citric acid salts), and ammonium salts of such brightening
agents; and
combinations of one or more of the foregoing agents.
Preferably, the brightening agent is added to the paper in an amount of from
about 0.01 wt. % to about 90 wt. %. More preferably, paper contains from about
0.1
wt. % to about 50 wt. % brightening agent. For example, the optical brightener
can be
added in an amount of from about 0.1 lbs/1000 sq. ft of paper to about 0.5
lbs/1000
sq. ft of paper. In accordance with this particular embodiment of the
inventive
process, the brightener is added to the solution of the second strength agent
and
applied simultaneously therewith to the paper during the dry stage of the
papermaking
process. Of course, brightening agents also can be added in any conventional
manner
during the papermaking process.
The paper and paper products manufactured according to the inventive process
can also optionally contain other additives useful in improving one or more
properties
of the finished paper product, assisting in the process of manufacturing the
paper
itself, or both. These additives are generally characterized as either
functional
additives or control additives.
Functional additives are typically those additives that are use to improve or
impart certain specifically desired properties to the final paper product and
include but
are not limited to brightening agents, dyes, fillers, sizing agents, starches,
and
adhesives. Control additives, on the other hand, are additives incorporated
during the
process of manufacturing the paper so as to improve the overall process
without
significantly affecting the physical properties of the paper. Control
additives include
biocides, retention aids, defoamers, pH control agents, pitch control agents,
and
drainage aids. Paper and paper products made using the process of the present
invention may contain one or more functional additive and/or control additive.
Pigments and dyes impart color to paper. Dyes include organic compounds
having conjugated double bond systems; azo compounds; metallic azo compounds;
anthraquinones; triaryl compounds, such as triarylmethane; quinoline and
related
compounds; acidic dyes (anionic organic dyes containing sulfonate groups, used
with
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organic~cations such as alum); basic dyes (cationic organic dyes containing
amine
functional groups); and direct dyes (acid-type dyes having high molecular
weights
and a specific, direct affinity for cellulose); as well as combinations of the
above-
listed suitable dye compounds. Pigments are finely divided mineral that can be
either
white or colored. The pigments that are most commonly used in the papermaking
industry are clay, calcium carbonate and titanium dioxide.
Fillers, are added to paper to increase opacity and brightness. Fillers
include
but are not limited to calcium carbonate (calcite); precipitated calcium
carbonate
(PCC); calcium sulfate (including the various hydrated forms); calcium
aluminate;
zinc oxides; magnesium silicates, such as talc; titanium dioxide (Ti02), such
as
anatase or rutile; clay, or kaolin, consisting of hydrated Si02 and A1z03;
synthetic
clay; mica; vermiculite; inorganic aggregates; perlite; sand; gravel;
sandstone; glass
beads; aeorgels; xerogels; seagel; fly ash; alumina; microspheres; hollow
glass
spheres; porous ceramic spheres; cork; seeds; lightweight polymers; xonotlite
(a
crystalline calcium silicate gel); pumice; exfoliated rock; waste concrete
products;
partially hydrated or unhydrated hydraulic cement particles; and diatomaceous
earth,
as well as combinations of such compounds.
The average diameter of the filler particles is typically less than about 5
microns, although sizes up to 200 microns can be used depending upon the
thickness
of the finished paper sheet. Generally, however, the average particle size
diameter of
the filler particles is typically from about 0.001 microns to about 100
microns, and
more typically from about 0.01 microns to about 50 microns in diameter.
Fillers are typically added to the pulp suspension in amounts of from about 1
wt. % to about 70 wt. %, and more typically from about 5 wt. % to about 40 wt.
%,
and most typically from about 10 wt. % to about 30 wt. %, based on total dry
weight
of the starting pulp stock.
Fillers typically have an index of refraction from about 1.50 to about 3.00,
and
more typically from about 1.53 to about 2.80. Indices of refraction of fillers
include
about 1.50, about 1.51, about 1.52, about 1.53, about 1.54, about 1.55, about
1.56,
about 1.57, about 1.58, about 1.59, about 1.60, about 1.61, about 1.62, about
1.63,
about 1.64, about 1.65, about 1.70, about 1.75, about 1.80, about 1.90, about
2.00,
about 2.10, about 2.20, about 2.30, about 2.40, about 2.50, about 2.60, about
2.70,
about 2.80, about 2.90, about 3.00, and ranges between any two of these
values.
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Fillers typically have a specific gravity of from about 1.50 to about 4.5, and
more typically from about 1.50 to about 4.2, and most typically from about
2.50 to
about 2.70.
Sizing agents are added to the paper during the manufacturing process to aid
in the development of a resistance to penetration of liquids through the
paper. Sizing
agents can be internal sizing agents or external (surface) sizing agents, and
can be
used for hard-sizing, slack-sizing, or both methods of sizing. More
specifically,
sizing agents include rosin; rosin precipitated with alum (Al2(S04)3); abietic
acid and
abietic acid homologues such as neoabietic acid and levopimaric acid; stearic
acid and
stearic acid derivatives; ammonium zirconium carbonate; silicone and silicone-
containing compounds, such as RE-29 available from GE-OSI and SM-8715,
available from Dow Corning Corporation (Midland, MI); fluorochemicals of the
general structure CF3(CF2)"R, wherein R is anionic, cationic or another
functional
group, such as GortexTM; alkylketene dimer (AKD), such as Aquapel~ 364,
Aquapel~ 752, Hercon~ 70, Hercon~ 79, Precis~ 787, Precis~ 2000, and Precis~
3000, all of which are commercially available from Hercules, Incorporated
(Willmington, DE); and alkyl succinic anhydride (ASA); emulsions of ASA or AKD
with cationic starch; ASA incorporating alum; starch; hydroxymethyl starch;
carboxymethylcellulose (CMC); polyvinyl alcohol; methyl cellulose; alginates;
waxes; wax emulsions; and combinations of such sizing agents.
Starch has many uses in papermaking. For example, it functions as a retention
agent, dry-strength agent, surface sizing agent. Starches include but are not
limited to
amylose; amylopectin; starches containing various amounts of amylose and
amylopectin, such as 25% amylose and 75% amylopectin (corn starch) and 20
amylose and 80% amylopectin (potato starch); enzymatically treated starches;
hydrolyzed starches; heated starches, also known in the art as "pasted
starches";
cationic starches, such as those resulting from the reaction of a starch with
a tertiary
amine to form a quaternary ammonium salt; anionic starches; ampholytic
starches
(containing both cationic and anionic functionalities); cellulose and
cellulose derived
compounds; and combinations of these compounds.
Microorganisms such as bacteria, algae, yeasts, and fungi are a common
problem associated with the papermaking process, often occurnng around the
paper
manufacturing machines themselves and producing slimes that can result in
pitted
paper products, corrosion damage to the machines, or even breaks in the paper
web.
CA 02543609 2006-04-24
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The growth of microorganism can be inhibited with biocides. Biocides used in
papermaking include thiazoles and thiazolidinones such as isothiozolin, 3-
chloroisothiazolidinone, 2-methyl-4-isothiazolin-3-one, 5-chloro-4-
isothiazolin-3-one,
and 1,2-bensiothiazolin-3-one; quaternary ammonium salts containing allcyl,
aryl, or
heterocyclic substituents; aldehydes capable of acting as crosslinking agents,
such as
glutaraldehyde, formaldehyde, and acetaldehyde; alcohols and diols such as 2-
bromo-
2-nitropropane-1,3-diol (NBG 88, available from Nova BioGenetics, Inc.,
Atlanta,
GA); amides, and especially halogenated propionamides such as
dibromopropionamide (NBG 20, available from Nova Biogenetics, Inc.);
carbamates
such as monoalkyl carbamates; chlorine compounds, including both inorganic and
organic chemicals that either contain chlorine or can split off chlorine and
are
commonly employed in the paper industry, including but not limited to alkali
hypochloride, alkali earth hypochloride, chlorine, and chlorine dioxides;
cyanates
such as methylene bis-thiocyanate and disodium cyanodithioimido carbonate;
gases
such as ozone or chlorine which are capable of being bubbled into a slurry of
pulp;
peroxides such as hydrogen peroxide (e.g. 35% solution); sulfides such as
tetramethylthiuram disulfide; salts such as sodium chloride, sodium peroxide,
and
sodium hydrogen sulfite; sulfones such as phenyl-(2-chloro-2-cyanovinyl)-
sulfone
and phenyl-(1,2-dichloro-2-cyanovinyl)-sulfone; organic acids such as benzoic
acid,
ascorbic acid, formic acid, sorbic acid, p-hydroxybenzoic acid, and mixtures
thereof;
and silicate such as sodium hexafluorosilicate, and mixtures and combinations
of the
above.
Biocides are typically added to the stock suspension in an amount ranging
from about 0.1 to about 2.0 lbsl ton of paper. Optimal usage will depend upon
the
process variables of a given papermachine (primarily degree of closure and
incoming
raw materials.
Retention and drainage aids affect the amount of pulp that is retained on the
wire and hence incorporated into the paper. Retention and drainage aids
include
polyamines, such as polyethylenimine (PEI) and poly(diallyldimethylammonium
chloride (DADMAC); high molecular weight polyacrylamides (e.g., those with a
molecular weight greater than 500,000); polyethyleneoxide (PEO); starch; gums;
alum; aluminum-containing polymers; wood fibers; and dual component systems
containing both cationic and anionic agents, such as polyethyleneimine (PEI)
and
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anionic polyacrylamide, or cationic starch or PAM with colloidal silica, as
well as
combinations of such compounds.
Defoamers, compounds used to destabilize and break apart existing foams also
can be added to the stock suspension, web or paper. Defoamers are typically
used to
control the foaming that results when air or other entrained gases mixes in
with the
stock suspension, especially one of the ingredients of the suspension is a
surfactant.
Defoamers are usually added late in the papermaking process, near to the
origination
of the foam. Defoamers include but are not limited to aliphatic chemicals such
as
kerosene; fuel oils; hydrophobic oils, such as vegetable oils; hydrophobic
particles
such as hydrocarbon or polyethylene waxes; fatty alcohols; fatty acids; fatty
esters;
hydrophobic silica; ethylenebisstearamide (EBS) suspended in oil,
hydrocarbons, or a
water emulsion; alkylpolyethers; silicon oils such as polydimethylsiloxanes;
oligomers of ethylene oxide or polypropylene oxide attached to an alcohol,
amine, or
organic acid, the oligomer having a degree of polymerization from about 3 to
about 8;
as well as combinations of these compounds. Typically such defoamers are added
in
an amount of from about 0.01 wt.% to about 1.0 wt. %, and more typically from
about
0.01 wt. % to about 0.5 wt. %, based upon total weight of the pulp mixture.
Additives for the control of pH can also be optionally added to the pulp
suspension so as to buffer the overall pH and thereby reduce corrosion of the
machines and minimize fungal and bacteria growth. Typical pH control agents
include sulfuric acid, carbon dioxide gas bubbled into the slurry, organic
buffering
agents, and combinations thereof.
Formation aids promote the dispersion of fibers throughout the slurry. The
addition of such compounds can lead to improvements in product formation, as
well
as improved headbox consistencies. Formation aids include linear, water
soluble
polyelectrolytes of high molecular weight, such as anionic polyacrylamides;
and
natural gums such as locust bean gum, karaya gum, and guar gum, as well as
mixtures
and combinations thereof. These formation aids are typically used at a volume
of .
from about 1 lb/ton (0.5 kg/t) of stock suspension solution to about 10 lb/ton
(4.5
kg/t), and more preferably from about 2 lb/ton (0.9 kg/t) to about 6 lb/ton
(2.7 kg/t).
Having thus described the present invention with reference to certain
preferred
embodiments, it is further illustrated by the examples which follow. These
examples
are provided for illustrative purposes only and are not intended to limit in
any way the
invention which is defined by the claims which follow the examples.
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EXAMPLES
The abrasion resistance, indentation resistance, and impact resistance of the
paper product produced by the processes of the present invention can be
determined
by methods and modifications of methods used in such standard industry tests
as
ASTM D 4977-98b (Standard Test Method for Granule Adhesion to Mineral Surfaced
Roofing by Abrasion), ASTM D 5420 (Impact Resistance of Flat, Rigid Plastic
Specimen by Means of a Striker by a Falling Weight (Gardner Impact), or other
suitable abrasion or impact tests.
EXAMPLE 1
A stock suspension for the outer liner plies of the paper was prepared from
recycled wastepaper. The grades of waste paper were flyleaf, sections, and
envelope
cuttings. This stock suspension was pumped from the machine chest to the fan
pump.
A metering pump accurately fed the catiouc dry-strength agent into a flow of
dilution
water which was then fed into the liner thick stock prior to the fan pump, The
dilution
water was used to help mix the dry-strength with the thick stock. The dry-
strength
agent was fed before the addition of retention aid, ASA, and defoamer.
The anionic dry-strength agent was blended in a tank with other ingredients
(silicone, optical brightner, water). The solution was mixed until all
ingredients were
thoroughly dispersed. The solution was pumped to a run tank, which feeds to
the
calender waterbox with the overflow from the waterbox returning to the run
tank to
maintain a flooded nip.
Table 1
Abrasion Test Results
Dry-strength Agent Dry-strength Agent Number Caliper
Added Prior to Wet end2 Added at Dry end3 of Cycles Reduction
Run (lb/ton) (lb/ton) (inches)
control 0 0 200 0.009
10.0 6.5 1000 0.001
2 10.0 6.5 1000 0.001
3 10.0 6.5 1000 0.001
4 10.0 6.5 1000 0.002
Ordinary gypsum board cover sheet.
Z The amount of cationic dry-strength agent added to the stock prior to the
wet end of the process.
3 The amount of anionic dry-strength agent added to the stock at the dry end
of the process.
4 The abrasion test was performed following generally the procedure of ASTM
D4977-98b.
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As can be seen in Table 1, gypsum board facing paper made according to the
process of the invention lost only 0.001 to 0.002 inches of surface material
after 1000
cycles of abrasion. In contrast, normal gypsum facing paper will lose about
0.009
inches of surface material after only two hundred cycles. Thus, this example
illustrates the improvement in surface strength that can be realized with the
process.
EXAMPLE 2
Paper was produced according to the process described in Example 1, with the
addition of an optical brightener, Leucophor~ BCW Liquid, T-26 Liquid, or T-4
Liquid (Clariant Corporation, Muttenz, Switzerland) to the solution
circulating
between the run tank and waterbox in the amounts shown in Table 2. Optical
brightness was determined using CIE Lab values, as measured on a profilometer
wherein L* refers to the value relating to the lightnessldarkness of the
color; a* refers
to the chromaticity on the red/green axis; and b* refers to chromaticity on
the
blue/yellow axis.
Table 2
Quantity of Optical
Brightener
Example lbs/1000 sq. ft Gal/batch L* b* a*
Control 0.0 0.0 87.55 -0.04 3.87
1 0.1 15.0 88.23 -0.08 3.05
2 0.2 30.0 88.57 0.09 2.30
3 0.3 40.0 88.69 -0.02 2.03
4 0.2 30.0 89.37 -0.10 1.81
5 0.2 30.0 89.47 -0.02 1.84
6 0.2 30.0 89.64 -0.01 1.74
As can be seen in Table 2, the addition of an optical brightener in the
waterbox, along with the anionic dry-strength agent yielded a paper product
having a
marked improvement in optical brightness. While the control paper product
contained
no optical brightener and had a brightness (L*) of 87.55, the addition of an
optical
brightener such as Leucophor in the waterbox (e.g., run 6) results in a
markedly
brighter (L*=89.64 , a* ~0 and b* is approaching 0) paper product. That is, L*
is
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approaching 100 (ideal), while a* and b* are both approaching zero, the point
of ideal
optical brightness (pure white).
While the compositions and methods of this invention have been described in
terms of preferred embodiments, it will be apparent to those of skill in the
art that
variations may be applied to the compositions and/or methods and/or processes
and in
the steps or in the sequence of steps of the methods described herein without
departing from the spirit and scope of the invention. More specifically, it
will be
apparent that certain agents which are chemically related can be substituted
for the
agents described herein while the same or similar results would be achieved.
All such
similar substitutes and modifications apparent to those skilled in the art are
deemed to
be within the spirit and scope of the invention.
