U.S. patent application number 10/972730 was filed with the patent office on 2005-07-21 for process for making abrasion resistant paper and paper and paper products made by the process.
Invention is credited to Brown, Edward, Champion, Douglas, Martin, William C., Stacey, Ronald, Stephenson, Neal, Weir, Richard.
Application Number | 20050155731 10/972730 |
Document ID | / |
Family ID | 34549324 |
Filed Date | 2005-07-21 |
United States Patent
Application |
20050155731 |
Kind Code |
A1 |
Martin, William C. ; et
al. |
July 21, 2005 |
Process for making abrasion resistant paper and paper and paper
products made by the process
Abstract
In this papermaking process, a first strength agent is added to
a 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. The strength agents may be selected to have opposite
charge.
Inventors: |
Martin, William C.;
(Charlotte, NC) ; Weir, Richard; (Indian Trail,
NC) ; Stephenson, Neal; (Oxford, AL) ; Brown,
Edward; (Alexandria, AL) ; Champion, Douglas;
(Oxford, AL) ; Stacey, Ronald; (Birmingham,
AL) |
Correspondence
Address: |
Paul Johnson
Ice Miller
One, American Square
Box 82001
Indianapolis
IN
46282-0200
US
|
Family ID: |
34549324 |
Appl. No.: |
10/972730 |
Filed: |
October 25, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60514279 |
Oct 24, 2003 |
|
|
|
Current U.S.
Class: |
162/158 ;
162/135; 162/162; 162/164.1; 162/168.3; 162/175; 162/178; 162/179;
162/204; 162/205 |
Current CPC
Class: |
D21H 23/76 20130101;
D21H 17/55 20130101; D21H 17/45 20130101; D21H 17/42 20130101; D21H
21/18 20130101; D21H 17/375 20130101; D21H 21/20 20130101; D21H
17/69 20130101; D21H 21/30 20130101; D21H 11/14 20130101; D21H
17/33 20130101; D21H 27/18 20130101 |
Class at
Publication: |
162/158 ;
162/135; 162/168.3; 162/175; 162/164.1; 162/178; 162/179; 162/204;
162/205; 162/162 |
International
Class: |
D21H 021/20; D21H
021/30; D21H 017/33; D21H 017/55; D21H 017/29; D21H 017/71 |
Claims
What is claimed is:
1. A process for making paper and paper products comprising: a) a
stock preparation stage wherein a stock suspension of cellulosic
fibers is prepared, b) adding a first strength agent selected from
the group consisting of cationic dry-strength agents, amphoteric
dry-strength agents and cationic wet-strength agents to the stock
suspension, c) a wet end stage wherein the cellulosic fibers are
formed into a substantially uniform web, and d) a dry end stage
wherein the web is dried into paper and a second strength agent
selected from the group consisting of anionic dry-strength agents
and amphoteric strength agents is applied to the surface of the
paper.
2. The process of claim 1 wherein the first strength agent is a
cationic dry-strength agent.
3. The process of claim 2 wherein the cationic dry-strength agent
is selected from the group consisting of cationic polyacrylamides,
cationic natural polymers, cationic modified natural polymers,
cationic synthetic polymers, starches modified to have quaternary
ammonium functional groups, cationic celluloses, cationic natural
gums, cationic polyvinyl alcohol adducts, and combinations
thereof.
4. The process of claim 3 wherein the cationic dry-strength agent
is a cationic polyacrylamide.
5. The process of claim 2 wherein the cationic dry-strength agent
is added in an amount of from about 1 lb/t (0.5 kg/t) to about 20
lb/t (9.1 kg/t).
6. The process of claim 2 wherein the cationic dry-strength agent
has a viscosity from about 1,000 cps (1 Pa-s) to about 15,000 cps
(15 Pa-s).
7. The process of claim 2 wherein the cationic dry-strength agent
has a specific gravity of from about 1.00 to about 1.20.
8. The process of claim 1 wherein the first strength agent is a
cationic wet-strength agent.
9. The process of claim 8 wherein the cationic wet-strength agent
is selected from the group consisting of cationic acid curing
resins, cationic neutral to acid curing resins, and cationic
neutral to alkaline curing resins.
10. The process of claim 8 wherein the cationic wet-strength agent
is added in an amount of from about 1 lb/t (0.5 kg/t) to about 20
lb/t (9.1 kg/t).
11. The process of claim 1 wherein the second strength agent is an
anionic dry-strength agent.
12. The process of claim 11 wherein the anionic dry-strength agent
is selected from the group consisting of anionic polyacrylamides,
anionic natural starches, and anionic carboxymethylcellulose.
13. The process of claim 12 wherein the anionic dry-strength agent
is an anionic polyacrylamide.
14. The process of claim 11 wherein the anionic dry-strength agent
is added in an amount of from about 5 lbs/t (2.3 kg/t) to about 25
lbs/t (11.3 kg/t).
15. The process of claim 11 wherein the anionic dry-strength agent
has a specific gravity from about 1.00 to about 1.20.
16. The process of claim 11 wherein the anionic dry-strength agent
has a viscosity from about 1,000 cps (1 Pa-s) to about 15,000 cps
(15 Pa-s).
17. The process of claim 1 wherein the second strength agent is
applied to the surface of the paper by a technique selected from
the group consisting of: a) immersing the paper in a solution of
the second strength agent in a calender waterbox, b) applying a
solution of the second strength agent to the paper with a size
press, and c) spraying a solution of the second strength agent on
the paper using a sprayboom.
18. The process of claim 17 wherein the solution of the second
dry-strength agent is applied to the paper in a calender
waterbox.
19. The process of claim 17 wherein the solution further contains
at least one optical brightener.
20. The process of claim 19 wherein the at least one optical
brightener is selected from the group consisting of azoles,
biphenyls, chelating agents, coumarins, furans, ionic brighteners,
naphthalimides, pyrazenes, stilbenes, tetrasulfonated stilbenes,
hexasulfonated stilbenes, salts thereof, and combinations
thereof.
21. The process of claim 19 wherein the at least one optical
brightener is added in an amount of from about 0.1 lbs/MSF to about
0.5 lbs/1000 square feed.
22. The process of claim 19 wherein the paper product has an
optical brightness with a lightness value, L*, greater than about
0.89 after application of the solution.
23. The process of claim 1 further comprising adding at least one
biocide to the stock suspension.
24. The process of claim 17 wherein the solution further contains
at least one hydrophobic organo-silicone.
25. Paper made by the process of claim 1.
26. The process of claim 1 further comprising incorporating the
paper into a paper product.
27. A paper product made by the process of claim 26.
28. A paper product of claim 27 that is drywall (wallboard) facing
paper applied to the drywall by a conventional gypsum wallboard
industry manufacturing process or by lamination onto the drywall
after it is manufactured.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of provisional
application Ser. No. 60/514279, filed Oct. 24, 2003 which is
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] 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
[0003] 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.
[0004] 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. Pat. 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.
[0005] U.S. Pat. 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 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.
[0006] The addition of cationic wet-strength polyamide resins to
paper cover sheets, especially polyamide epichlorohydrin resin, is
described in U.S. Pat. No. 6,489,040.
[0007] U.S. Pat. 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.
[0008] In the process taught in U.S. Pat. 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.
[0009] U.S. Pat. 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.
[0010] U.S. Pat. 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.
[0011] 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 to polyacrylamide particles, thereby increasing
the interfacial interaction between particles and matrix, and
resulting in reductions in surface abrasion. Gumagul, 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 of
Applied 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.
[0012] 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 caul 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
[0013] 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
[0014] 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 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.
[0015] 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.
[0016] 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
[0017] 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.
[0018] "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 which 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 Kraft 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.
[0019] "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.
[0020] "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.
[0021] "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.
[0022] "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".
[0023] "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.
[0024] "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.
[0025] 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.
[0026] 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.
[0027] In pulping, individual cellulose fibers are liberated from a
source of cellulose such as wood either by mechanical or chemical
action, or both.
[0028] 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.
[0029] 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
papermaking machine. The wet end stage further includes such later
operations as forming of the web, draining of the web and
consolidation of the web (pressing).
[0030] 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.
[0031] 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.
[0032] 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.
[0033] In addition to a size press and calender waterbox, the dried
paper can be coated by spray coating using a sprayboom.
[0034] 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.
[0035] 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, carries 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, multi-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.
[0036] 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.
[0037] 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 papermaking process
yields paper having an increased surface strength.
[0038] 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.
[0039] 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 dipolar
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
(CH.sub.3).sub.3N.sup.+C- H.sub.2CH.sub.2OC(O)CHCH.sub.2. An
example of a commercially available cationic polyacrylamide is
Nalco 997, available from Nalco Chemical Company (Naperville,
Ill.).
[0040] 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, Ill.).
[0041] 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 1.0:1.0. Preferably, the amphoteric
compounds have a ratio of anionic groups to cationic groups of
about 1.0:1.0. 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.
[0042] 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.
[0043] 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).
[0044] 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 kg/t) 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.
[0045] 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
kg/t), 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.
[0046] 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.
[0047] 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 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.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] 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
.about.0.009" 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.
[0052] 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.
[0053] Preferred hydrophobic organo-silicones are described in U.S.
Pat. 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 Corp.
(Midland, Mich.). 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 Journal, 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.
[0054] 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).
[0055] 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),
hydroxyethylethylenediaminetri- acetic 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.RTM. and Eccowhite.RTM. compounds
available from Eastern Color & Chemical Co. (Providence, R.I.);
naphthalimides; pyrazenes; stilbenes, such as the Leucophor.RTM.
range of optical brighteners available from the Clariant
Corporation (Muttenz, Switzerland), and Tinopal.RTM. 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.
[0056] 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.
[0057] 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.
[0058] 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.
[0059] 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 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.
[0060] 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
(TiO.sub.2), such as anatase or rutile; clay, or kaolin, consisting
of hydrated SiO.sub.2 and Al.sub.2O.sub.3; 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.
[0061] 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.
[0062] 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.
[0063] 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.
[0064] 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.
[0065] 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 (Al.sub.2(SO.sub.4).sub.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, Mich.); fluorochemicals of the general structure
CF.sub.3(CF.sub.2).sub.nR, wherein R is anionic, cationic or
another functional group, such as Gortex.TM.; alkylketene dimer
(AKD), such as Aquapel.RTM. 364, Aquapel.RTM. 752, Hercon.RTM. 70,
Hercon.RTM. 79, Precis.RTM. 787, Precis.RTM. 2000, and Precis.RTM.
3000, all of which are commercially available from Hercules,
Incorporated (Willmington, Del.); 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.
[0066] 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.
[0067] Microorganisms such as bacteria, algae, yeasts, and fungi
are a common problem associated with the papermaking process, often
occurring 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.
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
alkyl, 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-cyanovi- nyl)-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.
[0068] Biocides are typically added to the stock suspension in an
amount ranging from about 0.1 to about 2.0 lbs/ ton of paper.
Optimal usage will depend upon the process variables of a given
papermachine (primarily degree of closure and incoming raw
materials.
[0069] 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 anionic polyacrylamide, or
cationic starch or PAM with colloidal silica, as well as
combinations of such compounds.
[0070] 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.
[0071] 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.
[0072] 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).
[0073] 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.
EXAMPLES
[0074] 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
[0075] 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 cationic 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.
[0076] 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.
1TABLE 1 Dry-strength Dry-strength Abrasion Test Results.sup.4
Agent Added Agent Added Caliper Prior to Wet end.sup.2 at Dry
end.sup.3 Number Reduction Run (lb/ton) (lb/ton) of Cycles (inches)
control.sup.1 0 0 200 0.009 1 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 .sup.1Ordinary
gypsum board cover sheet. .sup.2The amount of cationic dry-strength
agent added to the stock prior to the wet end of the process.
.sup.3The amount of anionic dry-strength agent added to the stock
at the dry end of the process. .sup.4The abrasion test was
performed following generally the procedure of ASTM D4977-98b.
[0077] 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
[0078] Paper was produced according to the process described in
Example 1, with the addition of an optical brightener,
Leucophor.RTM. 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
lightness/darkness of the color; a* refers to the chromaticity on
the red/green axis; and b* refers to chromaticity on the
blue/yellow axis.
2 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
[0079] 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* .about.0 and
b* is approaching 0) paper product. That is, L* is approaching 100
(ideal), while a* and b* are both approaching zero, the point of
ideal optical brightness (pure white).
[0080] 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.
* * * * *