U.S. patent application number 17/147590 was filed with the patent office on 2021-07-22 for heat-sealable paperboard structures and methods.
The applicant listed for this patent is WestRock MWV, LLC. Invention is credited to Jiebin Pang.
Application Number | 20210222370 17/147590 |
Document ID | / |
Family ID | 1000005348273 |
Filed Date | 2021-07-22 |
United States Patent
Application |
20210222370 |
Kind Code |
A1 |
Pang; Jiebin |
July 22, 2021 |
HEAT-SEALABLE PAPERBOARD STRUCTURES AND METHODS
Abstract
A heat-sealable paperboard structure including a paperboard
substrate having a first major side and a second major side opposed
from the first major side, a heat-sealable barrier coating on the
first major side of the paperboard substrate, and a top coat
positioned over the heat-sealable barrier coating, wherein the top
coat forms a discontinuous layer over the heat-sealable barrier
coating.
Inventors: |
Pang; Jiebin; (Glen Allen,
VA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
WestRock MWV, LLC |
Atlanta |
GA |
US |
|
|
Family ID: |
1000005348273 |
Appl. No.: |
17/147590 |
Filed: |
January 13, 2021 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
62964198 |
Jan 22, 2020 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
D21H 27/10 20130101;
D21H 19/822 20130101; D21H 11/04 20130101; D21H 19/58 20130101;
B65D 65/42 20130101; D21H 19/385 20130101; D21H 19/68 20130101 |
International
Class: |
D21H 19/82 20060101
D21H019/82; D21H 11/04 20060101 D21H011/04; D21H 19/68 20060101
D21H019/68; D21H 19/58 20060101 D21H019/58; D21H 19/38 20060101
D21H019/38; D21H 27/10 20060101 D21H027/10; B65D 65/42 20060101
B65D065/42 |
Claims
1. A heat-sealable paperboard structure comprising: a paperboard
substrate comprising a first major side and a second major side
opposed from the first major side; a heat-sealable barrier coating
on the first major side of the paperboard substrate; and a top coat
positioned over the heat-sealable barrier coating, wherein the top
coat forms a discontinuous layer over the heat-sealable barrier
coating.
2. The heat-sealable paperboard structure of claim 1 wherein the
top coat comprises a binder and a pigment.
3. The heat-sealable paperboard structure of claim 2 wherein a
ratio of the pigment to the binder in the top coat is at least
about 1:1, by weight.
4-6. (canceled)
7. The heat-sealable paperboard structure of claim 2 wherein the
binder comprises a latex.
8. (canceled)
9. The heat-sealable paperboard structure of claim 2 wherein the
pigment comprises calcium carbonate.
10. (canceled)
11. The heat-sealable paperboard structure of claim 1 wherein the
discontinuous layer has a coat weight ranging from about 0.1
lb/3000 ft.sup.2 to about 4.0 lb/3000 ft.sup.2.
12-14. (canceled)
15. The heat-sealable paperboard structure of claim 1 wherein the
heat-sealable barrier coating comprises binder and pigment.
16. The heat-sealable paperboard structure of claim 15 wherein a
ratio of the pigment to the binder in the heat-sealable barrier
coating is at most 1:1 by weight.
17-25. (canceled)
26. The heat-sealable paperboard structure of claim 25 wherein the
platy clay has an average aspect ratio of at least about 40:1.
27. The heat-sealable paperboard structure of claim 1 wherein the
heat-sealable barrier coating has a coat weight ranging from about
4 lb/3000 ft.sup.2 to about 20 lb/3000 ft.sup.2.
28-29. (canceled)
30. The heat-sealable paperboard structure of claim 1 wherein the
paperboard substrate comprises solid bleached sulfate.
31. The heat-sealable paperboard structure of claim 1 wherein the
paperboard substrate has a basis weight ranging from about 80
lb/3000 ft.sup.2 to about 300 lb/3000 ft.sup.2.
32-34. (canceled)
35. The heat-sealable paperboard structure of claim 1 further
comprising one or more coating layers positioned between the
paperboard substrate and the heat-sealable barrier coating.
36. The heat-sealable paperboard structure of claim 1 further
comprising one or more coating layers on the second major side.
37. The heat-sealable paperboard structure of claim 1 having a
Parker Print Surface (PPS-10S) smoothness of at most about 3.1
.mu.m.
38-39. (canceled)
40. The heat-sealable paperboard structure of claim 1 having a
30-minute-water-Cobb rating of at most about 10 g/m.sup.2.
41-45. (canceled)
46. The heat-sealable paperboard structure of claim 1 having a
blocking rating of less than 2 at 50.degree. C. and at 60 psi in a
24-hour period.
47. (canceled)
49. The heat-sealable paperboard structure of claim 1 having a
heat-sealability (% fiber tear) of at least about 95%.
50. A container comprising the heat-sealable paperboard structure
of claim 1.
51. A method for manufacturing a heat-sealable paperboard structure
comprising: preparing a heat-sealable barrier coating formulation
comprising a binder and a pigment; applying the heat-sealable
barrier coating formulation to a first major side of a paperboard
substrate to form a heat-sealable barrier coating; preparing a top
coat formulation comprising a binder and a pigment; and applying
the top coat formulation over the heat-sealable barrier coating to
form a discontinuous layer of top coat over the heat-sealable
barrier coating.
52-55. (canceled)
Description
PRIORITY
[0001] This application claims priority from U.S. Ser. No.
62/964,198 filed on Jan. 22, 2020.
FIELD
[0002] This application is directed to paperboard structures and,
more particularly, to heat-sealable paperboard structures having no
to minimum tendency for blocking.
BACKGROUND
[0003] Paperboard is used in various packaging applications. For
example, coated unbleached paperboard is used to package beverage
containers, frozen foods, cereals and a wide variety of other food
and non-food consumer goods. Other forms of bleached and unbleached
coated paperboard are used for a variety of packaging options in
food service and consumer products.
[0004] Sustainability is one of the key drivers in development of
new packages for food and non-food consumer goods. Paperboard
structures coated with aqueous coatings are generally considered
repulpable and recyclable, and thus more sustainable than
paperboard coated with extrusion low density polyethylene extrusion
(LDPE). However, most polymers in aqueous coatings are amorphous
and do not have a melting point as LDPE. Therefore, binders or
polymers in aqueous coatings often gradually soften or become
sticky at elevated temperature (even at, for example,
120-130.degree. F.) and/or pressure in production, storage,
shipping, or converting process of aqueous coated paperboard,
causing blocking issue of the coated paperboard, which usually does
not occur with polyethylene coated paperboard in practical
applications.
[0005] Furthermore, due to the high binder level and thus the
hot-tackiness, the aqueous heat-sealable barrier coatings cannot
stand the temperature for calendering that is usually used to
smoothen the coating surface. Blocking (the tendency of layers in a
roll of paperboard to stick to one another) at elevated temperature
and pressure is also a major technical challenge in production and
converting processes for aqueous heat-sealable barrier coated
paperboard. This blocking issue becomes even more critical for
aqueous heat-sealable barrier coated paperboard that requires high
barrier properties and also needs to be able to heat-seal in
converting packages such as cups.
[0006] Accordingly, those skilled in the art continue with research
and development efforts in the field of heat-sealable barrier
paperboard structures using aqueous coatings.
SUMMARY
[0007] Disclosed are heat-sealable paperboard structures having no
to minimum tendency for blocking.
[0008] In one example, the disclosed heat-sealable paperboard
structure includes a paperboard substrate comprising a first major
side and a second major side opposed from the first major side, a
heat-sealable barrier coating on the first major side of the
paperboard substrate, and a top coat positioned over the
heat-sealable barrier coating, wherein the top coat forms a
discontinuous layer over the heat-sealable barrier coating.
[0009] Also disclosed are methods for manufacturing heat-sealable
paperboard structures having no to minimum tendency for
blocking.
[0010] In one example, the disclosed method for manufacturing a
heat-sealable paperboard structure includes steps of (1) preparing
a heat-sealable barrier coating formulation comprising a binder and
a pigment, (2) applying the heat-sealable barrier coating
formulation to a first major side of a paperboard substrate, (3)
preparing a top coat formulation comprising a binder and a pigment,
and (4) applying the top coat formulation over to the heat-sealable
barrier coating to form a discontinuous layer of top coat over the
heat-sealable barrier coating.
[0011] Other examples of the disclosed heat-sealable paperboard
structures and methods will become apparent from the following
detailed description, the accompanying drawings and the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is an elevational view, in section, of one example of
a container (e.g., a cup) that can be manufactured using the
disclosed heat-sealable paperboard structures;
[0013] FIG. 2 is a top plan view of the container of FIG. 1;
[0014] FIG. 3 is a plan view a die-cut blank that may be wrapped
around a mandrel to form the side wall of the container of FIG.
1;
[0015] FIG. 4 is a schematic cross-sectional view of one example of
the disclosed heat-sealable paperboard structure;
[0016] FIG. 5 is a schematic cross-sectional view of another
example of the disclosed heat-sealable paperboard structure;
[0017] FIG. 6 is a top view of an example heat-sealable paperboard
structure taken using a scanning electron microscope at 200.times.
magnification;
[0018] FIG. 7 is a cross-sectional view of an example heat-sealable
paperboard structure taken using a scanning electron microscope at
1000.times. magnification; and
[0019] FIG. 8 is an illustration of a device for testing blocking
of coated paperboard samples.
DETAILED DESCRIPTION
[0020] It has now been discovered that a heat-sealable
paperboard-based structure having a first major surface with high
water barrier properties and minimal to no blocking tendencies can
be achieved by positioning the heat-sealable barrier coating layer
on the first major side of the underlying paperboard substrate,
which has traditionally formed the first major surface of the
structure, beneath a lower-binder, calenderable top coat applied as
a discontinuous layer over (e.g., directly to) the barrier coating
layer such that the heat-sealable barrier coating is positioned
between the paperboard substrate and the top coat. Heat-sealability
is provided by a heat-sealable barrier coating. Such a structure
may be particularly well-suited for holding cold beverages (e.g.,
iced soft-drinks), cold foodstuffs (e.g., ice cream), hot beverages
(e.g., coffee) and hot foodstuffs (e.g., soup).
[0021] Referring to FIGS. 1 and 2, one example of a disclosed
paperboard-based container, generally designated 10, may include a
side wall 12 having an upper end portion 14 and a lower end portion
16, and a bottom wall 18 connected (e.g., heat-sealed) to the lower
end portion 16 of the side wall 12, thereby defining an internal
volume 20 within the container 10. The upper end portion 14 of the
side wall 12 may define an opening 22 into the internal volume 20.
Optionally, the upper end portion 14 of the side wall 12 may
additionally include a lip 24 (e.g., a rolled lip), such as for
securing a lid (not shown) or the like to the container 10.
[0022] While the container 10 is shown in FIG. 1 as a tall cup
(e.g., a 12-ounce, 16-ounce, 21-ounce or 24-ounce disposable
take-out cup) having a frustoconical side wall 12, those skilled in
the art will appreciate that the disclosed container 10 may be
formed in various shapes, sizes and configurations, and may be
formed with fewer or more walls than the side and bottom walls 12,
18 discussed above, without departing from the scope of the present
disclosure.
[0023] As shown in FIG. 2, the side wall 12 of the container 10 may
be assembled from a blank 30 (FIG. 3) that has been cut to the
desired silhouette and then wrapped around a mandrel (not shown).
While the blank 30 is wrapped around the mandrel, the first end 32
of the blank 30 overlaps a second end 34 of the blank 30, and the
overlapping ends 32, 34 may be connected (e.g., by heat-sealing),
thereby defining a seam 36 that extends from the upper end portion
14 to the lower end portion 16 of the side wall 12. Once the side
wall 12 has been assembled, the bottom wall 18 may be connected
(e.g., heat-sealed) to the lower end portion 16 of the side wall
12, thereby yielding the container 10.
[0024] Referring to FIG. 4, the side wall 12 of the container 10
may be formed from a paperboard structure 40 having a first major
surface 42 and a second major surface 44. The first major surface
42 of the paperboard structure 40 may correspond to the interior
surface 28 of the container 10. The second major surface 44 of the
paperboard structure 40 may correspond to the exterior surface 26
of the container 10.
[0025] The paperboard structure 40 may be a layered structure that
includes a paperboard substrate 46 having a first major side 48 and
a second major side 50. A heat-sealable barrier coating 52 and a
top coat 54 may be applied to the first major side 48 of the
paperboard substrate 46 such that the top coat 54 forms a
discontinuous layer 56 over (e.g., directly adjacent) the
heat-sealable barrier coating 52. The heat-sealable barrier coating
52 may be positioned between the top coat 54 and the paperboard
substrate 46. The discontinuous layer 56 of top coat 54 may define,
at least partially, the first major surface 42 of the paperboard
structure 40 and, thus, the interior surface 28 of the container
10.
[0026] At this point, those skilled in the art will appreciate that
various additional layers, barrier or non-barrier, may be
incorporated into the paperboard structure 40 between the
paperboard substrate 46 and the discontinuous layer 56 or on top of
the second major side 50 without departing from the scope of the
present disclosure. In one variation, as shown in FIG. 5, the
paperboard structure 40' may include a basecoat 45 between the
paperboard substrate 46' and the heat-sealable barrier coating 52'.
In another variation, as shown in FIG. 5, the heat-sealable
paperboard structure 40' may include a basecoat 47 on the second
major side 50 of the paperboard substrate 46'. In yet another
variation, as shown in FIG. 5, the paperboard structure 40' may
include a first basecoat 45 between the paperboard substrate 46'
and the heat-sealable barrier coating 52' and a second basecoat 47
on the second major side 50 of the paperboard substrate 46'.
[0027] Referring back to FIG. 4, the paperboard substrate 46 of the
paperboard structure 40 may be (or may include) any cellulosic
material that is capable of being coated with the heat-sealable
barrier coating 52 and the top coat 54. Those skilled in the art
will appreciate that the paperboard substrate 46 may be bleached or
unbleached. Examples of appropriate paperboard substrates include
corrugating medium, linerboard, solid bleached sulfate (SBS) and
unbleached kraft.
[0028] The paperboard substrate 46 may have an uncoated basis
weight of at least about 50 pounds per 3000 ft.sup.2. In one
expression, the paperboard substrate 46 may have an uncoated basis
weight ranging from about 60 pounds per 3000 ft.sup.2 to about 400
pounds per 3000 ft.sup.2. In another expression, the paperboard
substrate 46 may have an uncoated basis weight ranging from about
80 pounds per 3000 ft.sup.2 to about 300 pounds per 3000 ft.sup.2.
In another expression the paperboard substrate 46 may have an
uncoated basis weight ranging from about 90 pounds per 3000
ft.sup.2 to about 250 pounds per 3000 ft.sup.2. In yet another
expression the paperboard substrate 46 may have an uncoated basis
weight ranging from about 100 pounds per 3000 ft.sup.2 to about 200
pounds per 3000 ft.sup.2.
[0029] Furthermore, the paperboard substrate 46 may have a caliper
(thickness) ranging, for example, from about 4 points to about 30
points (0.004 inch to 0.030 inch). In one expression, the caliper
range is from about 8 points to about 24 points. In another
expression, the caliper range is from about 12 points to about 20
points.
[0030] One specific, nonlimiting example of a suitable paperboard
substrate 46 is 13-point SBS cupstock manufactured by WestRock
Company of Atlanta, Ga. Another specific, nonlimiting example of a
suitable paperboard substrate 46 is 16.5-point SBS cupstock
manufactured by WestRock Company. Yet another specific, nonlimiting
example of a suitable paperboard substrate 46 is 18-point SBS
cupstock manufactured by WestRock Company.
[0031] The heat-sealable barrier coating 52 may be applied to the
first major side 48 of the paperboard substrate 46 using any
suitable method, such as one or more coaters either on the paper
machine or as off-machine coater(s). When heated, a heat-seal
coating provides an adhesion to other regions of product with which
it contacts.
[0032] The heat-sealable barrier coating 52 may be applied to the
paperboard substrate 46 at various coat weights. In one expression,
the heat-sealable barrier coating 52 may be applied at a coat
weight of about 4 to about 20 pounds per 3,000 ft.sup.2, as dried.
In another expression, the heat-sealable barrier coating 52 may be
applied at a coat weight of about 6 to about 16 pounds per 3,000
ft.sup.2, as dried. In yet another expression, the heat-sealable
barrier coating 52 may be applied at a coat weight of about 8 to
about 12 pounds per 3,000 ft.sup.2, as dried.
[0033] The heat-sealable barrier coating 52 may include a binder
and a pigment. In one expression, the ratio of the pigment to the
binder may be at most 1 part (by weight) pigment per 1 part (by
weight) binder. In another expression, the ratio of the pigment to
the binder may be about 1:1 to about 1:9 by weight. In yet another
expression, the ratio of the pigment to the binder can be about 1:2
to about 1:6 by weight. In yet another expression, the ratio of the
pigment to the binder can be about 1:3 to about 1:4 by weight.
[0034] In one particular implementation, the binder of the
heat-sealable barrier coating 52 may be an aqueous binder. As one
general, non-limiting example, the binder may be a latex. As
another general, non-limiting example, the binder may be a water
based acrylic emulsion polymer. A specific, non-limiting example of
a suitable binder is presented in Table 2. Other aqueous binders
are also contemplated, such as styrene-butadiene rubber (SBR),
ethylene acrylic acid (EAA), polyvinyl acetate (PVAC), polyvinyl
acrylic, polyester dispersion, and combinations thereof.
[0035] The pigment component of the heat-sealable barrier coating
52 may be (or may include) various materials. Several non-limiting
examples of suitable inorganic pigments are presented in Table 1.
Other pigments, such as plastic pigments, titanium dioxide pigment,
talc pigment and the like, may be used without departing from the
scope of the present disclosure.
[0036] In one variation, the pigment component of the heat-sealable
barrier coating 52 may be a clay pigment. As one example, the clay
pigment may be platy clay, such as a high aspect ratio platy clay
(e.g., an average aspect ratio of at least 40:1, such as an average
aspect ratio of at least 60:1).
[0037] In another variation, the pigment component of the
heat-sealable barrier coating 52 may be a calcium carbonate
(CaCO.sub.3) pigment. As one example, the CaCO.sub.3 pigment may be
a coarse ground CaCO.sub.3 with a particle size distribution
wherein about 60 percent of the particles are less than 2 microns.
As another example, the CaCO.sub.3 pigment may be a fine ground
CaCO.sub.3 with a particle size distribution wherein about 90
percent of the particles are less than 2 microns.
[0038] In yet another variation, the pigment component of the
heat-sealable barrier coating 52 may be a pigment blend that
includes both calcium carbonate pigment and clay pigment.
[0039] The top coat 54 is applied to the heat-sealable barrier
coating 52 to form a discontinuous layer 56 over (e.g., directly
adjacent) the heat-sealable barrier coating 52. Various techniques
may be used for forming the discontinuous layer 56 of top coat 54
over the heat-sealable barrier coating 52, such as one or more
coaters either on the paper machine or as off-machine
coater(s).
[0040] The top coat 54 may be applied to the heat-sealable barrier
coating 52 at various coat weights to achieve the discontinuous
layer 56 of top coat 54. In one expression, the top coat 54 may be
applied at a coat weight of about 0.1 to 4.0 pounds per 3,000
ft.sup.2, as dried. In another expression, the top coat 54 may be
applied at a coat weight of about 0.5 to 3.0 pounds per 3,000
ft.sup.2, as dried. In another expression, the top coat 54 may be
applied at a coat weight of about 0.5 to 2.5 pounds per 3,000
ft.sup.2, as dried. In yet another expression, the top coat 54 may
be applied at a coat weight of about 0.5 to 2.0 pounds per 3,000
ft.sup.2, as dried.
[0041] Referring to FIG. 6, an SEM was used to show a top view of
the discontinuous layer 56 of top coat 54 deposited onto the
heat-sealable barrier coating 52. The areas with the heat-sealable
barrier coating 52 are darker, while the areas with both the
heat-sealable barrier coating 52 and the top coat 54 are
brighter.
[0042] Referring to FIG. 7, an SEM was used to show a cross-section
view of the discontinuous layer 56 of top coat 54 deposited onto
the heat-sealable barrier coating 52.
[0043] The top coat 54 may include a binder and a pigment. The
pigments and binders useful for the heat-sealable barrier coating
52 may also be used in the top coat 54. However, the
pigment-to-binder ratio of the top coat 54 may be significantly
different from the pigment-to-binder ratio of the heat-sealable
barrier coating 52. In one expression, the ratio of the pigment to
the binder in the top coat 54 can be at least about 1 part (by
weight) pigment per 1 part (by weight) binder. In another
expression, the ratio of the pigment to the binder in the top coat
54 can be about 1:1 to about 10:1 by weight. In another expression,
the ratio of the pigment to the binder in the top coat 54 can be
about 1:1 to about 5:1 by weight. In yet another expression, the
ratio of the pigment to the binder in the top coat 54 can be about
2:1 to about 4:1 by weight.
[0044] In one particular implementation, the binder of the top coat
54 may be an aqueous binder. As one general, non-limiting example,
the binder may be a latex. As another general, non-limiting
example, the binder may be a water based acrylic emulsion polymer.
A specific, non-limiting example of a suitable binder is presented
in Table 2. Other aqueous binders are also contemplated, such as
styrene-butadiene rubber (SBR), ethylene acrylic acid (EAA),
polyvinyl acetate (PVAC), polyvinyl acrylic, polyester dispersion,
and combinations thereof.
[0045] The pigment component of the top coat 54 may be (or may
include) various materials. Several non-limiting examples of
suitable inorganic pigments are presented in Table 1. Other
pigments, such as plastic pigments, titanium dioxide pigment, talc
pigment and the like, may be used without departing from the scope
of the present disclosure.
[0046] In one variation, the pigment component of the top coat 54
may be a clay pigment. As one example, the clay pigment may be
platy clay, such as a high aspect ratio platy clay (e.g., aspect
ratio of at least 40:1).
[0047] In another variation, the pigment component of the top coat
54 may be a calcium carbonate (CaCO.sub.3) pigment. As one example,
the CaCO.sub.3 pigment can be a coarse ground CaCO.sub.3 with a
particle size distribution wherein about 60 percent of the
particles are less than 2 microns. As another example, the
CaCO.sub.3 pigment can be a fine ground CaCO.sub.3 with a particle
size distribution wherein about 90 percent of the particles are
less than 2 microns.
[0048] Referring back to FIG. 1, the bottom wall 18 of the
container 10 may be formed from a paperboard structure, such as the
heat-sealable paperboard structure 40 shown in FIG. 4 or the
heat-sealable paperboard structure 40' shown in FIG. 5. However,
various other paperboard structures may be used to form the bottom
wall 18, such as when printability of the bottom wall 18 is of
little or no concern.
EXAMPLES
[0049] Experiments were conducted to evaluate the use of a
discontinuous layer of top coat over a heat-sealable barrier
coating in connection with a paperboard structure. One
heat-sealable barrier coating formulation (BC1) and one top coat
formulation (TC1) were prepared and used in the experiments. The
pigments used in the formulations are presented in Table 1. The
binder used in the formulations are presented in Table 2. The
heat-sealable barrier coating formulation (BC1) is presented in
Table 3. The top coat formulation (TC1) is presented in Table
4.
TABLE-US-00001 TABLE 1 Name Pigment Description CL-1 BARRISURF .TM.
XP (IMERYS Platy clay with high Kaolin, Georgia) aspect ratio CC-1
HYDROCARB .RTM. 60 (Omya AG Coarse ground CaCO.sub.3 of Oftringen,
Switzerland) (particle size 60% <2 micron) CC-2 HYDROCARB .RTM.
90 (Omya AG) Fine ground CaCO.sub.3 (particle size 90% <2
micron)
TABLE-US-00002 TABLE 2 Name Binder Description SA-1 CARTASEAL .RTM.
SCR (Archroma, Water based acrylic emulsion Reinach, Switzerland)
polymer, Tg of 30.degree. C.
TABLE-US-00003 TABLE 3 Barrier Coating Formulation (in Parts) BC-1
CaCO.sub.3 (CC-1) 65 CaCO.sub.3 (CC-2) Clay (CL-1) 35 Binder (SA-1)
400
TABLE-US-00004 TABLE 4 Top Coat Formulation (in Parts) TC-1
CaCO.sub.3 (CC-1) CaCO.sub.3 (CC-2) 100 Clay (CL-1) Binder (SA-1)
50
[0050] The formulations were applied at various coat weights to
16.5-point solid bleached sulfate cupstock having a basis weight of
175 pounds per 3000 square feet. A blade coater was used to apply
the heat-sealable barrier coating formulation to the felt side of
the paperboard substrate. A blade coater was again used to apply
the top coat formulation to the heat-sealable barrier coating,
thereby yielding a two-layer coating on the felt side of the
paperboard substrate. The examples and experimental results (Parker
Print Surface Smoothness; Water Cobb; Coffee Cobb; blocking rating;
and heat-sealablility) are shown in Table 5.
TABLE-US-00005 TABLE 5 Example Control 1 2 3 4 Barrier Coat Weight
9.8 (lb/3000 ft.sup.2) Top Coat Weight 0 0.5 1.0 1.8 3.0 (lb/3000
ft.sup.2) PPS (.mu.m) 3.3 3.0 2.7 2.2 2.0 H.sub.2O Cobb
(g/m.sup.2-30 min) 4.2 3.2 3.5 3.3 3.4 Coffee Cobb (g/m.sup.2-30
min) 13.8 10.1 8.5 9.4 8.1 Blocking Rating 2.0 1.9 1.6 1.1 0.8
(50.degree. C./60 psi/24 h) Heat-Sealability (% fiber 100 100 100
95 95 tear)
[0051] Thus, using a discontinuous layer of top coat over the
heat-sealable barrier coating of a paperboard structure provides a
smooth surface, as evidenced by the Parker Print Surface (PPS-10S)
smoothness results measured according to TAPPI standard T555. All
examples of the disclosed heat-sealable paperboard structures
exhibited PPS smoothness of 3 microns or less.
[0052] In addition to high smoothness, the examples also
surprisingly exhibited excellent barrier properties, as evidenced
by the 30-minute-water-Cobb results measured according to TAPPI
Standard T441 om-04. For most cases, the additional discontinuous
layer of the top coat improved or at least maintained the water
barrier properties of the underneath heat-sealable barrier coating
52. All examples had 30-minute-water-Cobb ratings of less than 10
g/m.sup.2, with many below 4 g/m.sup.2.
[0053] A hot coffee variant of the Cobb test was also utilized to
evaluate the water barrier of the examples shown in Table 5. This
test was performed by substituting 23.degree. C. water with
90.degree. C. coffee but otherwise complying TAPPI Standard T441
om-04. The coffee used was obtained by brewing 36 g of Starbucks
medium house blend ground coffee with 1100 mL of distilled water in
a 12 cup Mr. Coffee maker. All of the examples shown in Table 5 had
a 90.degree. C. coffee Cobb rating of less than 15 g/m.sup.2 after
30 minutes, with most less than 10 g/m.sup.2 after 30 minutes.
[0054] Heat-sealability of the coated samples of Table 5 were
evaluated on a PMC (Paper Machinery Corporation) cup machine, model
PMC-1250, by using each of these samples as side wall for the cup
and a control bottom stock for all the cups. Cups were all
successfully formed, and fiber tear in percentage of the seam area
upon tearing apart the heat-sealed side-wall seam was estimated.
High fiber tear percentage means better heat-sealability. Samples 1
and 2 all exhibited 100% fiber tear similar as the control samples
without a discontinuous layer of top coat, and samples 3 and 4 also
showed excellent fiber tear of 95%.
[0055] Lastly, the blocking rating (50.degree. C./60 psi/24 hrs),
was less than 3.0 for all samples, indeed less than 2.1, and less
than 1.0 for one sample. Table 6 defines the blocking test rating
system.
TABLE-US-00006 TABLE 6 Rating Description 0 Samples fall apart
without any force applied 1 Samples have a light tackiness but
separate without fiber tear 2 Samples have a high tackiness but
separate without fiber tear 3 Samples are sticky and up to 25%
fiber tear or coat damage (area basis) 4 Samples have more than 25%
fiber tear or coat damage (area basis)
[0056] The blocking behavior of the samples was tested by
evaluating the adhesion between the barrier coated side and the
other uncoated side. A simplified illustration of the blocking test
is shown in FIG. 8. The paperboard was cut into 2-inch by 2-inch
square samples. Several duplicates were tested for each condition,
with each duplicate evaluating the blocking between a pair of
samples 252, 254. (For example, if four duplicates were test, four
pairs--eight pieces--would be used.) Each pair was positioned with
the `barrier-coated` side of one piece 252 contacting the uncoated
side of the other piece 254. The pairs were placed into a stack 250
with a spacer 256 between adjacent pairs, the spacer being foil,
release paper, or even copy paper. The entire sample stack was
placed into the test device 200 illustrated in FIG. 8.
[0057] The test device 200 includes a frame 210. An adjustment knob
212 is attached to a screw 214 which is threaded through the frame
top 216. The lower end of screw 214 is attached to a plate 218
which bears upon a heavy coil spring 220. The lower end of the
spring 220 bears upon a plate 222 whose lower surface 224 has an
area of one square inch. A scale 226 enables the user to read the
applied force (which is equal to the pressure applied to the stack
of samples through the one-square-inch lower surface 224).
[0058] The stack 250 of samples is placed between lower surface 224
and the frame bottom 228. The knob 212 is tightened until the scale
226 reads the desired force of 100 lbf (100 psi applied to the
samples) or 60 lbf (60 psi applied to the samples). High pressure
such as 1000 psi is achieved by reducing the lower surface area of
224 contacting the stack 250 of samples to 0.11 square inch, with
an applied force of 110 lb. The entire device 200 including samples
is then placed in an oven at 50.degree. C. for 24 hours or 2 hours.
The device 200 is then removed from the test environment and cooled
to room temperature. The pressure is then released, and the samples
removed from the device.
[0059] The samples were evaluated for tackiness and blocking by
separating each pair of paperboard sheets. Blocking damage is
visible as fiber tear, which if present usually occurs with fibers
pulling up from the non-barrier surface of samples 254. If the
non-barrier surface was coated with a print coating, then blocking
might also be evinced by damage to the print coating.
[0060] For example, in as symbolically depicted in FIG. 8, samples
252(0)/254(0) might be representative of a "0" rating (no
blocking). The circular shape in the samples indicates an
approximate area that was under pressure, for instance about one
square inch of the overall sample. Samples 252(3)/254(3) might be
representative of a "3" blocking rating, with up to 25% fiber tear
in the area that was under pressure, particularly in the uncoated
surface of sample 254(3). Samples 252(4)/254(4) might be
representative of a "4" blocking rating with more than 25% fiber
tear, particularly in the uncoated surface of sample 254(4). The
depictions in FIG. 8 are only meant to approximately suggest the
percent damage to such test samples, rather than showing a
realistic appearance of the samples.
[0061] Although various examples of the disclosed heat-sealable
paperboard structures and methods have been shown and described,
modifications may occur to those skilled in the art upon reading
the specification. The present application includes such
modifications and is limited only by the scope of the claims.
* * * * *