Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HEAT-SEALABLE PAPERBOARD STRUCTURES AND METHODS
PRIORITY
[0001] This application claims priority from U.S. Ser. No. 62/964,198 filed on
January 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 F) and/or
pressure in
production, storage, shipping, or converting process of aqueous coated
paperboard, causing
blocking issue of thc coated paperboard, which usually does not occur with
polyethylene
coated paperboard in practical applications.
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[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 stcps of (1) preparing a heat-scalable 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
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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 200x magnification;
[0018] Fig. 7 is a cross-sectional view of an example heat-sealable paperboard
structure
taken using a scanning electron microscope at 1000x magnification; and
[0019] Fig. 8 is an illustration of a device for testing blocking of coated
paperboard
samplcs.
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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
shapcs, sizes and configurations, and may bc 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, thc sidc wall 12 of thc containcr 10 may bc
assembled from a
blank 30 (Fig. 3) that has been cut to the desired silhouette and then wrapped
around a
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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
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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 ft2. In one expression, the paperboard substrate 46 may
have an
uncoated basis weight ranging from about 60 pounds per 3000 ft2 to about 400
pounds per
3000 ft2. In another expression, the paperboard substrate 46 may have an
uncoated basis
weight ranging from about 80 pounds per 3000 ft2 to about 300 pounds per 3000
ft2. In
another expression the paperboard substrate 46 may have an uncoated basis
weight ranging
from about 90 pounds per 3000 ft2 to about 250 pounds per 3000 ft2. In yet
another
expression the paperboard substrate 46 may have an uncoated basis weight
ranging from
about 100 pounds per 3000 ft' to about 200 pounds per 3000 ft'.
[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, Georgia. 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.
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[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 ft2, 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 ft2, 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
ft2, 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 bindcrs arc also contemplated, such as styrcnc-
butadicnc 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
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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 (CaCO3) pigment. As one example, the CaCO3 pigment
may
be a coarse ground CaCO3 with a particle size distribution wherein about 60
percent of the
particles are less than 2 microns. As another example, the CaCO3 pigment may
be a fine
ground CaCO3 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 ft2, 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 ft2, 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 ft2, as dried. In yet
another expression,
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the top coat 54 may be applied at a coat weight of about 0.5 to 2.0 pounds per
3,000 ft2, 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
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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 (CaCO3) pigment. As one example, the CaCO3 pigment can be a coarse
ground
CaCO3 with a particle size distribution wherein about 60 percent of the
particles are less
than 2 microns. As another example, the CaCO3 pigment can be a fine ground
CaCO3
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
othcr paperboard structurcs may bc used to form thc bottom wall 18, such as
when
printability of the bottom wall 18 is of little or no concern.
EXAMPLES
[0049] Experiments wcrc conductcd to evaluate thc 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.
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TABLE 1
Name Pigment Description
CL-1 BARRISURFTM XP (IMERYS Platy clay with high
aspect ratio
Kaolin, Georgia)
CC-1 HYDROCARB 60 (Omya AG Coarse ground CaCO3
(particle
of Oftringen, Switzerland) size 60% <2 micron)
CC-2 HYDROCARB 90 (Omya AG) Fine ground CaCO3
(particle size
90% <2 micron)
TABLE 2
Name Binder Description
SA-1 CARTASEAL SCR (Archroma, Water based acrylic
emulsion
Reinach, Switzerland) polymer, Tg of 30 C
TABLE 3
Barrier Coating Formulation (in Parts)
BC-1
CaCO3 (CC-1) 65
CaCO3 (CC-2)
Clay (CL-1) 35
Binder (SA-1) 400
TABLE 4
Top Coat Formulation (in Parts)
TC-1
CaCO3 (CC-1)
CaCO3 (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
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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 5
Example Control 1 2 3 4
Barrier Coat Weight
9.8
(1b/3000ft2)
Top Coat Weight
0 0.5 1.0 1.8 3.0
(1b/3000ft2)
PPS (pm) 3.3 3.0 2.7 2.2
2.0
H20 Cobb (g/m2-30min) 4.2 3.2 3.5 3.3
3.4
Coffee Cobb (g/m2-30min) 13.8 10.1 8.5 9.4
8.1
Blocking Rating
2.0 1.9 1.6 1.1
0.8
(50 C/60psi/24h)
Heat-Sealability ( /0 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 'I'APP1 standard
'1555. 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
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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/m2, with many below 4 g/m2.
[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
C water
with 90 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 C coffee Cobb rating of less than 15 g/m2 after 30 minutes, with
most less than 10
g/m2 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 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.
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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 cach condition, with cach
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 1000psi is achieved by reducing the lower surface area of 224
contacting the stack
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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 "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.
CA 03165612 2022- 7- 21
