U.S. patent application number 10/710553 was filed with the patent office on 2006-01-26 for high strength paperboard and method of making same.
This patent application is currently assigned to Sonoco Development, Inc.. Invention is credited to Jeremy E. Morin, Krishnaraju Varadarajan.
Application Number | 20060016569 10/710553 |
Document ID | / |
Family ID | 35655899 |
Filed Date | 2006-01-26 |
United States Patent
Application |
20060016569 |
Kind Code |
A1 |
Morin; Jeremy E. ; et
al. |
January 26, 2006 |
High strength paperboard and method of making same
Abstract
A method of making high strength paper or paperboard is
provided, comprising the steps of adding a high modulus filler to
the aqueous pulp slurry prior and forming the pulp slurry into
paper or paperboard. Preferably, the filler is glass fiber that has
been coated with a thermosetting resin.
Inventors: |
Morin; Jeremy E.; (Florence,
SC) ; Varadarajan; Krishnaraju; (Florence,
SC) |
Correspondence
Address: |
CLAUSEN MILLER, P.C
SUITE 1600
10S. LASALLE STREET
CHICAGO
IL
60603
US
|
Assignee: |
Sonoco Development, Inc.
Hartsville
SC
|
Family ID: |
35655899 |
Appl. No.: |
10/710553 |
Filed: |
July 20, 2004 |
Current U.S.
Class: |
162/145 ;
162/135; 162/158; 162/164.1; 162/181.6; 162/181.8 |
Current CPC
Class: |
D21F 11/00 20130101;
D21H 17/69 20130101; D21H 13/40 20130101; D21H 21/18 20130101 |
Class at
Publication: |
162/145 ;
162/164.1; 162/181.6; 162/181.8; 162/158; 162/135 |
International
Class: |
D21H 21/18 20060101
D21H021/18; D21F 11/00 20060101 D21F011/00 |
Claims
1. A method of making high strength paper or paperboard comprising
the steps of: adding a high modulus filler to an aqueous pulp
slurry to form a modified pulp slurry; and forming the modified
pulp slurry into paper or paperboard.
2. The method of claim 1 wherein the filler has a modulus of at
least 0.1 GPa.
3. The method of claim 1 wherein the filler has a modulus of at
least 3 GPa.
4. The method of claim 2 wherein the filler is selected from the
group consisting of polymers, glass fibers, clay nanoplatelets,
carbon fibers, silicon carbide fibers and alumina fibers.
5. The method of claim 2 wherein the filler has an aspect ratio of
at least 50.
6. The method of claim 1 wherein a thermosetting resin is also
added to the aqueous pulp slurry prior to the forming step.
7. The method of claim 6 wherein the resin has a glass transition
temperature higher than the service temperature.
8. The method of claim 6 wherein the resin has a glass transition
temperature of at least 85 C.
9. The method of claim 6 wherein the resin is selected from the
group consisting of melamine, PAE, phenolic resins,
phenol-formaldehyde, and anionic and cationic polymers.
10. A paperboard made according to the method of claim 2.
11. A paperboard made according to the method of claim 3.
12. A paperboard made according to the method of claim 7.
13. A paperboard made according to the method of claim 8.
14. A method of making high strength paper or paperboard comprising
the steps of: coating a filler with a resin matrix; adding the
coated filler to an aqueous pulp slurry to form a modified pulp
slurry; and forming the modified pulp slurry into paper or
paperboard.
15. The method of claim 14 wherein the filler has a modulus of at
least 0.1 GPa.
16. The method of claim 14 wherein the filler has a modulus of at
least 3 GPa.
17. The method of claim 16 wherein the filler is glass fiber.
18. The method of claim 14 wherein the resin is hydrophilic.
19. A paper or paperboard made according to the method of claim 14.
Description
FIELD OF THE INVENTION
[0001] This patent relates to a high strength paperboard and a
method of making the same by adding a high modulus filler and/or
resin to the pulp slurry prior to forming the paperboard.
DESCRIPTION OF THE RELATED ART
[0002] Paper and paperboard generally are manufactured by preparing
an aqueous pulp slurry; depositing a layer of the pulp slurry onto
a moving screen or "wire"; draining water through the screen and
away from the pulp stock, leaving a wet, weak fiber mat; and
pressing and drying the mat to form sheets ready for finishing and
cutting.
[0003] During the pulping step, various ingredients may be added to
affect the properties of the paper, including fillers such as
titanium oxide and calcium carbonate for improving the optical
properties of the paper. A number of references teach adding
polymeric or resinous materials to pulp stock to enhance wet and/or
dry strength. However, the inventors are aware of no reference that
teaches adding high modulus fillers to pulp stock to enhance the
dry strength of paper or paperboard.
[0004] Daniel, Jr. U.S. Pat. No. 2,601,598 teaches a method of
making a formed cellulosic product, such as paper and paperboard,
including the step of adding an impregnating agent to the
pulp-water slurry. A wide variety of impregnating agents are
disclosed, including a number of thermoplastic resins.
Thermoplastic phenol-formaldehyde resins are disclosed.
[0005] Rushmere U.S. Pat. No. 4,798,653 teaches a papermaking stock
comprising a two component combination of a colloidal silica sol
compound and an anionic polyacrylamide. Together these two
components increase the fines retention capability, and thus the
resistance to shear forces, during the papermaking process.
Rushmere also teaches that clays, calcium carbonate, titanium oxide
and/or recycled broke or other cellulosic waste may be added, but
doesn't teach that they increase dry strength.
[0006] Degen et al. U. S. Pat. No. 4,818,341 teaches a process for
producing paper and paperboard of high dry strength. The dry
strength enhancer is a mixture of a cationic polymer (a polymer
having a positive charge) which contains as characteristic monomers
copolymerized units of diallyldimethylammonium chloride,
N-vinylamine or an N-vinylimidazoline.
[0007] Kokko et al. U.S. Pat. No. 6,222,006 teaches a
thermosettable wet strength resin comprising a type of
polyaminamide-epichlorohydrin (PAE) resin.
[0008] Sears et al. U.S. Pat. No. 6,270,883 teaches a composite
containing cellulosic pulp fibers dispersed in a polymeric matrix
for use as a plastic substitute.
[0009] McCall et al. U.S. Pat. No. 6,322,667 teaches paper and
paperboard that have improved mechanical properties due to their
being treated with superheated steam during the drying step. McCall
teaches using clay filler for improved optical properties, but
notes that "[a]dding fillers to paper has a detrimental effect on
the strength properties."
[0010] Hjalmarson et al. U.S. Pat. No. 6,391,156 teaches a process
for making paper or paperboard including the step of using clay as
a flocculating agent (to help aggregate the cellulosic solids into
clumps). Hjalmarson does not teach adding clay to pulp stock as a
strengthening agent.
[0011] Pfohl et al. European Patent Publication No. 0146000 B1
teaches a process for making high dry strength paper including the
step of adding an aqueous polymer solution to the paper stock.
[0012] Hamaguchi et al. Japanese Patent Document No. 10025693
teaches using polyamidopolyamine-epihalohydrin resin as a wet paper
strengthening agent.
[0013] Covarrubias International Application No. WO 0188267 teaches
a method of making paper and paperboard including the step of
adding fibrous cationic alumina microparticles and a polymer to the
pulp to form a treated pulp having improved retention properties.
Secondary microparticles such as natural or synthetic hectorite,
bentonite and zeolite can also be added. Covarrubias also teaches
the use of calcium carbonate as a "filler", although its function
is not stated. Although the reference alludes to a need for
"improved strength characteristics", nowhere in the document does
Covarrubias teach that adding fibrous cationic microparticles to
the pulp actually increases dry strength of paper or
paperboard.
[0014] The primer Dry Strength Additives published by Tappi Press
(ed. Walter Reynolds, 1980), describes many wet and dry strength
additives for paper and paperboard manufacture. Most commercial dry
strength additives are hydrophilic water-soluble polymers added
internally to the aqueous pulp slurry. The book divides dry
strength agents into three categories: natural gums, natural and
modified starches, and synthetic polymers.
[0015] The first synthetic polymer specifically designed for dry
strength without imparting wet strength was an acrylamide based
anionic (negative charged) polymer (polyacrylamide resin)
introduced in 1955. Other synthetic polymers used as dry strength
additives include carboxymethyl cellulose (CMC),
"melamine-formaldehyde" (MF), urea-formaldehyde (UF) and DAS.
[0016] Although adding dry and wet strength agents to paper and
paperboard is well known, it has been heretofore unknown to add
high modulus fillers either alone or coated with resin to the pulp
slurry prior to forming the paperboard to obtain a high strength
paperboard.
[0017] Thus it is an object of the present invention to enhance the
modulus and strength of paperboard by adding a high modulus filler
or a filler coated with resin to the pulp slurry prior to forming
the paperboard.
[0018] Another object of the invention is to provide a high
strength paperboard for use in composite containers that can
replace metal cans.
[0019] A further object of the invention is to provide a high
strength paperboard that can be used in the manufacture of
containers for holding foodstuffs such as ground coffee.
[0020] Further and additional objects will appear from the
description, accompanying drawings, and appended claims.
SUMMARY OF THE INVENTION
[0021] The present invention is a high strength paperboard and
method of making same. In a first embodiment of the invention, a
high modulus filler (Ef>0.1 GPa and preferably Ef>3 GPa) is
added to the aqueous pulp slurry prior to forming the modified pulp
slurry into paper or paperboard. The addition of the high modulus
filler increases the strength or modulus of the resulting
paperboard.
[0022] In another aspect of the invention, a high modulus filler
and a resin having a high glass transition temperature are added to
the aqueous pulp slurry before the forming step. The resin promotes
adhesion between the paper fibers and the high modulus filler to
improve load transfer.
[0023] In the preferred embodiment of the invention, glass fibers
are coated with a hydrophilic resin and the coated filler is added
to the aqueous pulp slurry. This may be accomplished by using a
secondary reactor/mixer in which the high modulus filler is mixed
and/or agitated with the resin matrix. The mixing/agitation enables
the resin to wet or chemically react with the surface of the glass
fibers, thus rendering the surface of the filler hydrophilic. The
hydrophilic surface of the filler facilitates even distribution of
the filler throughout the aqueous pulp slurry and improves the
adhesion of the glass fibers to the paper fibers. After the coating
step, the coated fibers are added to the aqueous pulp slurry.
DEFINITIONS
[0024] In the description that follows, a number of terms are used.
In order to provide a clear and consistent understanding of the
specification and claims, the following definitions are
provided:
[0025] Aspect ratio: As used in paperboard making, the ratio of the
length of the major axis (length) to the minor axis (diameter) of a
fiber or other cylindrical filler.
[0026] Dry strength: The ability of a material to resist bursting
stresses in its dry state.
[0027] Electrical glass (a.k.a. E-glass): The most commonly used
and most economical glass fiber. Structural or S-type glass has
slightly higher strength and corrosion resistance.
[0028] Filler: Any of a number of materials added to paper to
enhance its properties. Examples include glass fibers, clay
nanoplatelets and whiskers.
[0029] Glass transition temperature (Tb): The temperature at which
a polymer becomes hard and brittle.
[0030] Hydrophilic: Water soluble.
[0031] Hydrophobic: Water repelling.
[0032] Isotropic: Having uniform properties in all directions.
[0033] Machine direction (MD): The direction in which the greater
number of fibers of a sheet of paper tend to be aligned. Paper
tends to be stronger in the machine direction. Also called the
"grain direction." The direction at cross angles to the machine
direction is the cross direction (CD).
[0034] Modulus (a.k.a. tensile elastic modulus or Young's Modulus
(E)): The force required to elongate (stretch) a material. Often
measured in pounds force per square inch (psi) or in gigapascals
(GPa (10.sup.9 Pa)), where 1 psi=6.895.times.10.sup.-6 GPa.
[0035] Orthotropic: Having at least two orthogonal planes of
symmetry where material properties are independent of direction
within each plane.
[0036] Paperboard: A broad term for a class of paper, the other
class being paper (specific term), that is generally heavier,
thicker and more rigid than paper, although the distinction is not
a sharp one. Paperboard is used in the manufacture of tubes, cones,
cores, packaging forms, containers and container partitions, among
other items. When used herein, the term paperboard includes paper
and vice versa.
[0037] Pascal (Pa): Unit of pressure. 1 Pa=0.000145 lbf/square
inch.
[0038] Polymer: A chemical compound or mixture of compounds,
generally having a high molecular weight, formed by
polymerization.
[0039] Pulp (a.k.a. pulp slurry, pulp stock): The mixture of fiber,
water and other components extracted by chemical or mechanical
means from plant material, mostly wood.
[0040] Resin: a. Any of various solid or semisolid amorphous
fusible flammable natural organic substances. Examples include
melamine, polyamide-polyamine-epichlorohydrin (PAE), polyvinyl
acetate (PVA) and phenols. b. Any of a large class of synthetic
products that have some of the properties of natural resins. c. Any
of various products made from natural or synthetic resins or
natural polymers.
[0041] Tensile strength: The ability to resist being ruptured when
subjected to pulling.
[0042] Thermoplastic: Capable of becoming softened when heated and
then rehardened when cooled. Any of numerous organic materials that
are thermoplastic.
[0043] Thermosetting: Capable of becoming permanently rigid when
heated or cured.
[0044] Wet strength: The ability of a material to resist bursting
stresses in its wet state.
[0045] Whiskers: Short synthetic or organic fibers. Examples
include carbon, silicon carbide and alumina.
THE INVENTION
[0046] The invention is a high strength paperboard and method of
making same. As used herein, strength may refer to compressive
strength, tensile strength (i.e. modulus), tear resistance, folding
endurance, etc.
[0047] The Young's modulus of the composite paperboard of the
invention (paperboard with added filler and/or resin, Ec) may be
calculated as: Ec=K Ef Vf+K Em Vm (1)
[0048] where:
[0049] K=efficiency parameter .about.1/5 to 3/8, depending on the
randomness of the matrix throughout the composite
[0050] Ef=modulus of the filler
[0051] Vf=volume of filler
[0052] Em=modulus of resin matrix
[0053] Vm=volume of resin matrix
[0054] Paperboard is an orthotropic material, and thus its
mechanical properties are dependent on the direction in which they
are measured. A sheet of paperboard may be thought of as having
three orthogonal directions: machine direction (MD), cross
direction (CD) and the direction orthogonal to the plane of the
paperboard (ZD). Paperboard fibers tend to be aligned in the MD
direction during the manufacturing process. Paperboard typically
has a modulus (E) of about 5.5-8.3 GPa (800,000-1,200,000 psi) in
the machine direction (MD), 1.5-2.7 GPa (220,000-400,000 psi) in
the cross direction (CD) and 0.3-1.7 GPa (44,000-250,000 psi) in
the z direction (ZD).
[0055] In a first embodiment of the invention, a high modulus
filler (Ef>0.1 GPa and preferably Ef>3 GPa) is added to the
aqueous pulp slurry prior to forming the modified pulp slurry into
paper or paperboard. The addition of the high modulus filler
increases the strength or modulus of the resulting paperboard.
[0056] A number of high modulus fillers may be used for this
purpose, including but not limited to polymers (E=0.1 to 5 GPa),
glass fibers (E=70 to 110 GPa), clay nanoplatelets (E=110 GPa), and
various high strength whiskers.
[0057] The aspect ratio of the filler should be at least 50 to
provide for efficient load transfer. Aspect ratio determines the
efficiency of the load transfer from the filler matrix to the paper
fibers. The larger the aspect ratio, the more efficient will be the
load transfer.
[0058] In a second embodiment of the invention, a thermosetting
resin having a high glass transition temperature is added to the
paper pulp to improve the mechanical properties of the paperboard.
Possible thermosetting resins include melamine, PAE, phenolic
resins, phenol-formaldehyde, and anionic and cationic polymers.
[0059] Most of these resins have a glass transition temperature
higher than that of polyvinyl acetate (Tb=85 C), a resin currently
used in paperboard production. The use of resins having glass
transition temperatures higher than the service temperature (so
that the resins will always be in their glassy state) can lead to
increased stiffness in the paperboard. Most polymeric resins above
their glass transition temperature have tensile elastic moduli of
about 3 GPa or about 435 ksi (kilopounds force per square inch). As
noted earlier, paperboard typically has a Young's modulus of about
5.5-8.3 GPa in the MD, 1.5-2.7 GPa in the CD and 0.3-1.7 GPa in the
ZD. Consequently, adding a thermosetting resin having a high glass
transition temperature, and preferably higher than 85 C, without a
high modulus filler, will increase the modulus of a paperboard in
the CD and ZD directions.
[0060] In a third embodiment of the invention incorporating both of
the aforementioned aspects, a high modulus filler and a resin
having a high glass transition temperature are added to the aqueous
pulp slurry before the forming step. The resin promotes adhesion
between the paper fibers and the high modulus filler to improve
load transfer. Therefore, the resin should have a hydrophilic end
(to adhere to the paper fibers) and a hydrophobic end (to adhere to
the filler). The high modulus filler and the resin may be added
separately or at the same time during the paper manufacturing
process.
[0061] In a fourth, preferred embodiment of the invention, a high
modulus filler (preferably glass fibers) is primed (coated) with a
resin that is hydrophilic (to adhere to the paper fibers), and then
the coated filler is added to the aqueous pulp slurry. This may be
accomplished by using a secondary reactor/mixer in which the high
modulus filler is mixed and/or agitated with the resin matrix. The
mixing/agitation enables the resin to wet or chemically react with
the surface of the filler, thus rendering the surface of the filler
hydrophilic which facilitates even distribution of the filler
throughout the aqueous pulp slurry and improves the adhesion of the
filler to the paper fibers. After the coating step, the coated
filler is added to the aqueous pulp slurry.
[0062] In foregoing four embodiments, because the filler and/or
resin are dispersed throughout the paperboard (instead of just
coating a surface), they improve the out-of-plane strength of the
paperboard, which is typically low.
EXAMPLES
[0063] In the examples that follow, the paperboard modulus is
measured in the MD direction.
Example 1
[0064] Demonstration that adding a high modulus filler to the pulp
slurry increases the board modulus In a first example, E-glass
fiber filler having a modulus of 72 GPa was added to an aqueous
pulp slurry in varying concentrations prior to forming the
paperboard.
[0065] The results are summarized in Table 1 below: TABLE-US-00001
TABLE 1 Effect of E-glass Filler Volume on Paperboard Modulus
Paperboard Modulus Paperboard Modulus Volume % E-glass filler (psi)
(Gpa) 0 406,500 2.80 5 454,000 3.13 10 425,400 2.93 15 442,700 3.05
20 508,900 3.51
[0066] As shown in Table 1, the modulus in the MD direction was
greatest (3.51 GPa) at 20 volume % E-glass. This represented a 25 %
increase in modulus in the MD direction over the unmodified
paperboard. However, the observed modulus was not as high as would
be expected from equation (1) at lower filler concentrations. It is
expected that increases in modulus would be observed at filler
concentrations greater than 20 %.
Example 2
[0067] Demonstration that adding resin coated glass fibers to the
pulp slurry increases the paperboard modulus
[0068] In a second example, E-glass fibers (Ef=72 GPa) were wetted
with a resin matrix (Em=5.5-11.7 GPa) to form coated glass fibers,
then the coated glass fibers were added to an aqueous pulp slurry
prior to forming the paperboard. The volume of filler was
approximately 20% and the volume of resin was approximately 10%.
The modulus of the composite paperboard increased 20% in the MD
(from 0.9 GPa to 1.1 GPA) and 100% in the CD (from 0.25 GPa to 0.5
GPa).
[0069] Other modifications and alternative embodiments of the
invention are contemplated which do not depart from the spirit and
scope of the invention as defined by the foregoing teachings and
appended claims. It is intended that the claims cover all such
modifications that fall within their scope.
* * * * *