Note: Descriptions are shown in the official language in which they were submitted.
CA 02323341 2000-10-16
A PAPERBOARD CONTAINER HAVING ENHANCED
GREASE RESISTANCE AND RIGIDITY
AND A METHOD OF MAKING SAME
Field of the Invention
This invention pertains to press-formed paperboard containers, such as
paper plates, paper trays, paper cups, and the like. In particular, this
invention
pertains to paperboard containers having enhanced rigidity and grease
resistance and the method of making such containers.
Back_around of the Invention
In the common process, paperboard web, after drying, is passed through
a size press to apply starch sizing or other binders to both surfaces of the
web.
After smoothing the surfaces, the web surface that will be used as the top or
upper surface of a container, such as a plate or bowl, is coated with at least
one
coat of clay coating. The top layer or upper surface is then coated with at
least
one functional coating material that forms a barrier to moisture. Such a
barrier is
intended to prevent moisture from the food or drink placed in the container
from
wetting the paperboard. Many layers of functional coating material may be
placed on the upper surface. In some products, the upper surface may also be
printed. The printing ink is applied directly to the top clay coating and then
is
covered with one or more layers of functional heat releaselsealant coating
material to preclude contact between the printing ink and food placed on or in
the
container. The coated paperboard is then rolled.
To form paperboard containers from the sized and coated paperboard
stock, the paperboard web is moistened on the un-coated lower surface to
achieve a predetermined level of moisture in the paperboard. The moisture
content aids in deformation of the paperboard during press forming into the
desired container shape. The moisture is generally added by a common device
known as a flooded nip in amounts sufficient to achieve about 8-12 % moisture.
Since paperboard when dry contains about 3-6 % moisture, the flooded nip can
add up to about 6% additional moisture. While water is a preferred moistening
CA 02323341 2000-10-16
medium, some moistening solutions contain wax and surfactants, the former to
act as a lubricant in the pressing operation and the latter to assist in
moisture
penetration. An available concentrate for a moistening solution is Velvetol~
sold
by Rhone-Poulenc of Cranbury, NJ; it contains polyolefin wax and ethoxylated
surfactant.
The moistened paperboard web may be rolled and cured for up to 24
hours after which it is die-cut into blanks having the shape and dimensions
appropriate for the container to be made. Each blank is then fed into a die
press
in which mating dies, heat, and pressure work the paperboard blank to form it
into the desired container.
Typically, grease resistance in paper plates is achieved primarily through
the application of clay coatings over the porous fiber network at the board
manufacturing facility. These coatings consist of various levels of fillers,
pigments, binders, and other materials, that when combined, fill the voids in
paperboard and form a smooth, printable surface. These coatings provide some
degree of grease resistance which can be enhanced through the use of
additional top functional coating materials such as polyvinyl acetate, sarans,
and
acrylics that are FDA approved for food contact. These coatings are referred
to
as releaselsealant coatings as they serve a dual function of improving release
of
the plate from the forming die, as well as enhancing grease and moisture
resistance when applied over the clay coating. The grease resistence of prior
art
boards is still typically less than 50%. That is, typically half of the plate
bottom
will be grease stained. The paperboard for such generally accepted base
materials is on the order of 160-200 pounds per 3,000 square foot ream.
The cost of clay coated paperboard economically interferes with entry into
the light duty plate market segment with a grease resistant product.
Furthermore, feasible economic entry into this market would benefit from
lightweight paperboard in the 100-140 pounds per 3,000 square foot ream as
compared to the 160-200 pounds per 3,000 square foot ream paperboard that is
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CA 02323341 2000-10-16
typically clay coated. Lightweight clay coated board volume is relatively
nonexistent and where available sells at a premium price above conventional
weight clay coated boards. Non clay coated lightweight board is abundantly
available at low cost and is the primary choice among light duty plate
producers
who then produce plates with little or no grease resistance. Such lightweight
board forms an inferior product since it does not provide an attractive level
of
plate rigidity. In addition, the clay coating on boards can be rather brittle
and
susceptible to pleat cracking which increases the difficulty in forming
lightweight
plates from clay coated boards where high forming pressures are used.
In evaluating the process for making paperboard containers, the inventors
have determined that application of a latex coating on the top or outermost
upper
surface of a coated web allows for replacement of at least some, and
preferably
all, of the layers of clay coating, while achieving both rigidity similar to
that of
much heavier paperboard, as well as enhanced grease resistance. Therefore,
lightweight paperboard, i.e., less than 160 pounds per 3,000 square foot ream,
can be used while achieving good grease resistance and the desired rigidity.
In
addition, paperboard containers with superior grease resistance, as well as
superior rigidity, can be manufactured by using this method with paperboards
with weights in excess of 160 pounds per 3,000 square foot ream. In a more
preferred embodiment of the present invention, a releaselsealant coating can
be
applied over the SBR latex coating.
The object of the invention, therefore, is to achieve enhanced container
rigidity, as well as enhanced container grease resistance in an economical,
lightweight plate by application of a functional coating material in place of
the
clay coating to a lightweight paperboard. In this application, the preferred
functional coating used is an SBR latex. SBR latex is a synthetic binder
produced via polymerization of styrene and butadiene monomers in a
pressurized reaction with surfactants added for stability.
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CA 02323341 2000-10-16
Summary of the Invention
As embodied and broadly described herein, the invention is a container
press-formed of paperboard having a top surface which is substantially free of
clay coating and that is disposed for contact with material to be contained
and an
opposed bottom surface, the container comprising a paperboard layer, a size
press coating layer on the top surface and a first functional coating material
dispersed at the top surface size press coating layer, wherein the first
functional
coating is latex.
In preferred embodiments, a second functional coating material is
disposed on the top surface in quantities sufficient to form a barrier to
moisture.
The second layer can either be another layer of SBR or a releaselsealant
layer.
If the second layer is SBR, a layer of releaselsealant coating should be
applied
over it. At higher levels of SBR application, the sealant qualities of: the
releaselsealant coatings become less important than the release properties.
Preferably, the first functional coating material is a latex. In the presently
preferred embodiment, the first functional coating material is Tykote~ Base I
distributed by Reichold, Inc. of Research Triangle Park, NC. The container of
the invention preferably comprises first functional coating material at about
1-7
pounds per 3,000 square foot ream of paperboard.
The invention further contemplates a method of making the container of
the invention comprising the steps of forming the paperboard; applying first
and
second functional coating materials to the top surface size press layer;
cutting a
blank for the container from the paperboard; and press-forming the blank at a
predetermined temperature and pressure to form the container.
The accompanying drawings, which are incorporated herein and
constitute a part of this specification, illustrate an embodiment of the
invention,
and, together with the description, serve to explain the principles of the
invention.
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CA 02323341 2000-10-16
Brief Description of the Drawings
Figure 1 is a cross section of a paperboard container illustrating possible
mechanisms by which the invention improves container rigidity and grease
resistance.
Figure 2 is a cross section of a paperboard container illustrating possible
mechanisms by which the invention improves container rigidity.
Figure 3 is a micrograph of a z-fold of a prior art paper board container.
Figure 4 is a micrograph of a z-fold of a paperboard container of the
present invention.
Description of the Preferred Embodiment
Reference will now be made in detail to the present preferred embodiment
of the invention, an example of which is illustrated in the accompanying
drawings.
In accordance with the invention, the container is press-formed of
paperboard having opposed top and bottom surfaces. As illustrated in Figure 1,
the container 10 is press-formed from paperboard 12 having a top surface 14
and a bottom surface 16. The paperboard may be any cellulosic fiber web
having weight and surface characteristics known to be useful for containers.
Preferably, the paperboard has a basis weight of 100-160 pounds per 3,000
square foot ream. It is more preferred that the paperboard have a basis weight
of 130-140 pounds per 3,000 square foot ream, although the invention may
provide improved grease resistance, improved rigidity, and other improved
performance characteristics using paperboard at other weights.
In a traditional paperboard container, both top and bottom surfaces 14, 16
are size press coated. Although it is not conventional to apply size press
coating
to one side of the paperboard, it is not strictly necessary to apply coating
to both.
For example, if only one side of the paperboard bears size press coating, the
paperboard will often exhibit a tendency to curl. In addition, the paperboard
will
only achieve half the strength properties of a paperboard having a two-sided
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CA 02323341 2000-10-16
application. As depicted using dashed lines in Figure 1, the paperboard of the
container can be divided roughly into three equal cross-sectional regions. The
top region 20 proximate the top surface is defined by the depth of penetration
of
the upper size press coating from top surface 14. The bottom region 22
proximate the bottom surface is defined by the depth of size press coating
penetration from the bottom surface 16. The middle region 24, sometimes
called the fiber core, is defined between the top and bottom regions 20, 22.
In a
preferred embodiment, approximately 5 - 8 pounds of size press coating are
applied to each of the top and bottom layers per 3,000 square foot ream of
paperboard. In a more preferred embodiment, 6.5 pounds of size press coating
are applied to each of the top and bottom layers per 3,000 square foot ream of
paperboard. In traditional paperboard, the size press coating is a starch or
similar material. The size press coating layer is followed by application of
one or
more clay coating layers, as well as a sealant/release coating.
In the present invention, a container is press-farmed of paperboard having
a top surface that is substantially free of clay coating. A paperboard is
substantially free of clay coating for purposes of the present invention when
it
lacks a quantity of clay coating sufficient to materially enhance the barrier
properties of the paperboard. Barrier properties are characterized by, for
example, grease resistance. The mere presence of clay either in or as a layer
with the coated paperboard of the present invention does not materially effect
the paperboard of the present invention. For example, a paperboard could have
a clay layer, but still require a layer of latex to materially enhance the
barrier
properties of the paperboard. Clay coating layers of up to, for example, two
(2)
pounds per 3,000 square foot ream of paperboard may be added to the
paperboard without materially affecting barrier properties. Thus, containers
having such structures are considered to be part of the present invention.
In a preferred embodiment, the top surface of the paperboard has no clay
coating. As a replacement for some or all of the clay coating, a quantity of
first
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CA 02323341 2000-10-16
functional coating material 18 applied to the top surface during production of
the
container is sufficient to provide formation of a grease resistant barrier at
the top
surface. In a preferred embodiment, the first functional coating material is
applied at a rate of 1-7 pounds per 3,000 square foot ream of paperboard. The
actual preferred amount depends on board smoothness. The smoother the
board, the less first functional coating is needed. In a more preferred
embodiment, two layers of a first functional coating material are applied at a
total
rate of 1-7 pounds per 3,000 square foot ream of paperboard. In a still more
preferred embodiment, more than two layers of a first functional coating
material
are applied, having a total rate of 1-7 pounds per 3,000 square foot ream of
paperboard. If the board is very smooth, traditional hydrophilic coating
methods
may be used. In the case of rough board, it is preferred to use gravure
coating
methods with a smoothing bar. This is particularly desirable when there is no
clay on the board.
Preferably, the first functional coating material is a styrene butadiene
copolymer. In the preferred embodiment, the first functional coating material
is
primarily styrene butadiene rubber, specifically Tykote~ Base I distributed by
Reichold, Inc. of Research Triangle Park, NC.
In traditional paperboard containers, the top surface 14 is often subject to
printing. Generally, the clay coatings applied to a conventional paperboard,
as
discussed above, define a printing surface and provide partial grease
resistance
on the paperboard. Clay coatings add additional cost, as well as weight, to
traditional paperboard containers. In addition, clay coatings are generally
available on paperboards having heavy basis weights of 160-200 pounds per
3,000 square foot ream. Of note, in traditional paperboard containers,
paperboards having basis weights of 160-200 pounds per 3,000 square foot
ream are necessary to achieve the rigidity of the container desired by
consumers. Furthermore, lightweight boards with the coatings of the present
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CA 02323341 2000-10-16
invention seem to be more tolerant of high forming pressures than lightweight
clay coated boards.
In a preferred container of this invention, however, the size press coating
layer of starch or similar material is of sufficient thickness to define a
printing
layer. In one embodiment of the preferred containers, the size press coating
layer on the top surface is applied at a rate of approximately 6.5 Ibs per
3,000
square foot ream of paperboard. When a layer of this magnitude is applied, the
size press substantially fills in the interstitial spaces in the rough
paperboard
surface providing a smoother upper surface for printing as compared to a
similar
paperboard having no size press coating layer. In this embodiment the size
press coating is applied in a nip section wherein the size press coating is
forced
into the paperboard at the nip. Testing has shown that a large portion of the
size
press coating, however, remains impregnated in the paperboard such that the
amount of size press coating contributing to a smoother surface on the upper
layer is reduced. That is, if about 6.5 Ibs of size press coating is applied
per
3,000 square foot ream of paperboard, approximately 75% is located beneath
the surface of the paperboard, while only approximately 25% remains at the
surface. The 75% of the size press coating located beneath the surface of the
paperboard resides in the upper one-third of the paperboard total caliper. For
purposes of this application, we will define this ratio of size press coating
applied
to size press residing beneath the surface of the paperboard as size press
coating factor. Thus, using this method, the size press coating has a size
press
coating factor of about 1.33. While the printability of this embodiment is
greater
than a non clay coated board having a size press coating layer of 0-3 Ibs, per
3,000 square foot ream of paperboard, on the top layer, the printability is
still
inferior to that of a prior art clay coated board.
In a more preferred embodiment, printability approaching parity to that of
a clay coated board is achieved in a non clay coated board at least as
perceived
by typical consumers. In this embodiment a film forming size press coating is
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CA 02323341 2000-10-16
applied to the paperboard. A film forming starch preferred in this embodiment
has a much lower viscosity than a non-film forming starch applied at a size
press
nip. Starches typically applied at a size press nip typically have a high
viscosity
to more effectively impregnate the paperboard. One example of this type of
size
press coating is Cofilm, distributed by National Starch. In this method, the
film
forming starch is applied with a calendar box instead of in a typical size
press
nip. Using this method, it is preferred that approximately 1-6 Ibs of size
press
coating be applied to the 3,000 square foot per ream paperboard, with about
half
of the size press coating being applied to the top surface. Testing has shown
that when using a film forming size press coating, only about 33% of the size
press coating applied, impregnates the paperboard beneath the surface, thus
achieving a coating size press coating factor of approximately 3.
Improved printability when using the film forming starch was shown in
laboratory tests. In these tests, three different smoothness properties were
tested: (1) Bendtsen Roughness; (2) Sheffield Roughness; and (3) Parker Print.
As shown below, better printability, indicated by lower numbers on each of the
three factors, was seen in a paperboard coated with a film forming starch
applied
at a calendar box as compared to paperboard having starch applied at a size
press coating. Lower numbers indicate a smoother surface.
Bendtsen Sheffield
Sample Roughness Roughness Parker Print
Standard Size Press 384 168 6.40
Film Forming Size 210 139 5.97
Press
Thus, according to the present invention, less size press coating is used,
while
achieving a better printing surface. One drawback to this method is a
reduction
to the rigidity of the plate as compared to the method in which size press
coating
is deeply impregnated into the board. It should be noted, however, that as
shown below, the plate rigidity of a paper plate of the preferred invention
formed
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CA 02323341 2000-10-16
using this method can still be approximately equivalent to that found in the
heavier clay coated paperboards.
The ink layer can be applied to the size press coating layer or to the latex
layer. Thus, in one embodiment the ink is applied to the size press coating
layer
and the latex is then applied over the ink layer. In a second embodiment, the
latex layer is applied to the size press coating and then the ink is applied
to the
latex layer. One example of ink used in this application is VMT Ink supplied
by
Progressive Inks of St. Louis, Mo.
The latex functional coating is applied after application of ink to the
printing surface. When applying the functional latex coating, the latex
appears to
penetrate interstitial spaces in the size press coatings and the paperboard,
thereby enhancing grease resistance and providing the container with increased
rigidity. As seen in the examples below, the result is a 130-140 pound per
3,000
square foot ream paperboard container having a rigidity equivalent to a 160-
170
pound per 3,000 square foot ream paperboard, with enhanced grease
resistance. Thus, a desirable paperboard container is formed with an
approximately 20% reduction in paperboard weight and, a corresponding 20%
reduction in paperboard cost. As well, a further reduction in weight is
achieved
as layers of clay coating are no longer necessary.
After the addition of the latex layer, one or more release/sealant coatings
may be applied to further isolate the ink from contact with consumables. Thus,
in
a preferred embodiment, the container of the invention includes a second
releaselsealant coating material on the top surface. Preferably, any known
functional coating for the top surface of paperboard containers may be applied
in
any known manner. The second releaselsealant coating may be selected from
food-safe polymers such as nitrocellulose, Saran and styrene acrylic acid
copolymers, methyl cellulose, carboxymethyl cellulose acetate copolymer, vinyl
acetate copolymer, styrene butadiene copolymer, and styrene-acrylic copolymer.
It is preferred that the second releaselsealant coating be applied at a rate
of '/2 -1
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CA 02323341 2000-10-16
pound per 3,000 square foot ream of paperboard. There may be a number of
layers of second releaselsealant coating materials on the top surface of the
container depending on the intended uses of the container and on whether the
container's top surface is printed with an ink that must be isolated from
contact
with consumables.
In another embodiment, applicants have learned that paperboard plates
having desirable rigidity and grease resistance can be formed using unbleached
kraft paperboard. Environmentally conscious consumers often avoid the use of
paper plates due to the harsh chemicals used to bleach the paperboard used in
making paperboard plates. Applicants have found that an environmentally
friendly paperboard plate, having the desired rigidity and grease resistance,
can
be made by using Tykote Base I as a functional coating. In this embodiment, an
unbleached paperboard having a top layer of predominately hardwood fibers and
a bottom layer of predominately softwood fibers has proven to be satisfactory.
In
one commercially available grade of board, the top layer contains only a minor
amount of softwood, on the of impurities in the pulp supply, while the other
contains only minor amounts of hardwood. In this available grade, the
predominately hardwood layer typically comprises around 10 - 30% of the board
by volume. The predominately hardwood layer will most often comprise
approximately 15% of the z-directional volume of the board, with the
predominately softwood layer comprising the balance. While such a construction
will form a paper plate, it has little or no grease resistance. Addition of
about 1-7
pounds of Tykote Base I per 3,000 square foot ream of paperboard, as
discussed above, provides the desired grease resistance. Moreover, the
addition of the Tykote layer also increases the rigidity in the board. If the
predominately hardwood layer comprises substantial proportions of softwoods,
rather larger amounts of Tykote may be required to provide the desired barrier
properties.
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CA 02323341 2000-10-16
In accordance with the invention, a preferred method of manufacturing the
press-formed container of the invention comprises, before press-forming the
paperboard container having a size press coating, the step of applying to the
top
surface of paperboard a first functional coating to form a barrier to grease
as well
as to enhance rigidity, wherein the first functional coating is a latex, and
wherein
the first functional coating at least partially penetrates the size press
coating.
After application of the first functional latex coating, at least one second
releaselsealant coating may be applied.
Prior to pressing the containers, a moistening solution is applied to the
bottom surface of the paperboard. The moistening solution used can be water
or, preferably, a solution of 50 parts water and one part Velvetol~ as sold by
Rhone-Poulenc of Cranbury, NJ. The moistening solution used, therefore,
contains water, a surfactant, and a wax. ,
r
The enhanced rigidity and grease resistance of lightweight paperboard
containers made in accordance with the method of the invention was first
confirmed in a laboratory evaluation. A comparison of paper plates made in
accordance with the method of this invention having paperboard weights from
126 to 176 pounds per 3,000 square foot ream (samples 1 to 6) were compared
with paper plates made with clay coated paperboard having a paperboard weight
of 162 pounds per 3,000 square foot ream (sample 7), as well as with clay
coated paper plates having a paperboard weight of 124 pounds per 3,000
square foot ream (sample 8). In addition, a non clay coated paper plate having
a
film forming starch applied and having a paperboard weight of 143 pounds per
3,000 square foot ream was also compared (sample 9). In the laboratory
evaluation, the 126 and 176 pounds per 3,000 square foot ream non clay coated
paperboards were coated with 13 pounds of size press per 3,000 square foot
ream of paperboard (6.5 pounds per 3,000 square foot ream on each of the
upper and lower surfaces), four layers of ink, approximately 2-3 pounds of
Tykote Base I SBR latex per 3,000 square foot ream of paperboard, a barrier
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CA 02323341 2000-10-16
base release/sealant coating, and a barrier top releaselsealant coating. The
162 pounds per 3,000 square foot ream clay-coated paperboard had two layers
of clay coating, four layers of ink, a barrier base releaselsealant coating,
and a
barrier top releaselsealant coating. Sample 8 had at least one layer of clay
coating. Sample 9 was formed similarly to Samples 1-6 in that 2-3 pounds of
Tykote Base I SBR latex was applied per 3,000 square foot ream of paperboard;
however, the board was coated with approximately 4-6 pounds of film forming
size press per 3,000 square foot ream of paperboard (approximately 2-3 pounds
per 3,000 square foot ream on each of the upper and lower surfaces).
The paperboards were then used to make 9" paper plates under standard
press conditions. The paperboard was unrolled; no blocking (adhesive sticking)
was encountered. Blanks were cut, scored and press-formed from the
paperboard at 55 strokes per minute. The press temperature was 300°F.
The
plates formed thereby were then tested for dry rigidity using the Plate
Rigidity
Tester, Model ML4431-2. The plate tester used conforms to the standard FSK
tester, except that it is modified to simulate how a consumer would hold the
plate
with one hand, whereas the thumb is placed on the upper surface brim area and
the fingers are placed on the lower surface near the center of the plate. The
following table provides the test results for the dry rigidity test.
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CA 02323341 2000-10-16
Quantity Basis Weight Dry Mean
Sample of Lbs13,000 Square Foot Ream Rigidity'
Plates
1 116 126 176
2 20 136 204
3 12 143 209
4 24 148 226
5 24 153 262
6 20 176 260
7 67 162 175
8 10 124 98
9 20 143 -191
'Grams per'/:' deflection
The test data summarized in the table above establishes that removal of
the clay coating and application of a Tykote Base I to the upper surface of a
lightweight paperboard achieves at least equivalent dry rigidity as a more
expensive heavier weight paperboard having two layers of clay coatings.
Sample 1, having a paperboard weight of 126 pounds per 3,000 square foot
ream with no clay coating and coated with Tykote Base I achieves a
substantially
equivalent dry rigidity as compared to a 162 pound per 3,000 square foot ream
clay coated paperboard (Sample 7). The data also shows that paper plates
made with heavier paperboards, 136 to 176 pounds per 3,000 square foot ream,
achieve substantially higher rigidity than paper plates fabricated with a clay
coated board. Sample 9, the non clay coated paperboard with film forming
starch, shows that when higher quality printing is desired, lightweight
paperboard
paper plates can be manufactured while still maintaining adequate rigidity. In
comparing Sample 9 to Sample 3, wherein both paperboards had a weight of
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CA 02323341 2000-10-16
143 pounds per 3,000 square foot ream, there is approximately a 10% reduction
in rigidity when using the film forming size press. This reduction is due, as
noted
above, to the absence of size press coating imbedded in the paperboard of
Sample 3. A comparison of Sample 8 (124 pounds per 3,000 square foot ream
clay coated paperboard) to Sample 1 (126 pounds per 3,000 square foot ream
non clay coated paperboard) shows the substantial improvement in rigidity by
adding a first functional layer of Tykote Base I in the absence of clay
coating.
In the table below, the 162 pound per 3,000 square foot ream clay coated
plates (Sample 1 ), the 126 pound per 3,000 square foot ream non clay coated
plates (Sample 2), and the 162 pound per 3,000 square foot ream noncoated
plates (Sample 3) were also tested for wet rigidity using the same tester as
used
to determine dry rigidity. A wet rigidity test differs from a dry rigidity
test. in that
prior to testing the rigidity of the plate, a heated substance, such a$ baked
beans
r
heated to 150°F, is placed on the eating surface for 10 minutes, then
removed.
Of note, wet rigidity is generally more important to the consumer than dry
rigidity, as paper plates are often used to contain wet products such as baked
beans. The following table provides the test results.
Quantity Base Weight Wet Mean
Sample of Lbs13,000 Square Foot ReamZ
Plates Rigidity
1 45 162 99
2 49 126 126
3 30 162 168
zGrams per %4' deflection
The test data, summarized in the table above, establishes that a non clay
coated lightweight paperboard paper plate, coated with Tykote Base I, achieves
superior wet rigidity as compared to a heavyweight clay coated paperboard
paper plate.
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CA 02323341 2000-10-16
Similar improvements using unbleached paperboard have also been
observed. Using identical test methodology as described above, unbleached,
non clay coated paperboard having two layers of top release/sealant coatings
(Sample 1 ) was compared with unbleached paperboard having a 2 - 4 pounds of
Tykote Base I per 3,000 square foot ream layer of paperboard and one layer of
top releaselsealant coating. As shown in the table below, paper plates formed
from the same basis weight unbleached paperboard having a Tykote Base I
layer exhibited greater dry rigidity and wet rigidity as compared to paper
plates
formed of unbleached paperboard having no Tykote layer.
Quantity Dry Mean Wet Mean Visual Grease
Sample of Plates Rigidity Rigidity Failure
3 3
1 30 123 106 1-2%
2 30 112 83 100%
3Grams per %4' deflection
A visual grease failure test was performed on various plates to determine
the amount of penetration of grease through the top layer of the paper plates.
In
the visual grease plate failure test, a 3 mm layer of dyed oil, heated to
approximately 150°F, is placed in the upper surface of a plate for 20
minutes.
The oil is then removed and the plates are observed on the reverse side or
back
to determine the percentage on the bottom surface area of the plate that has
been penetrated by the dye. Sample 1 was a 136 pound per 3,000 square foot
ream paperboard having a 2-3 pounds of Tykote Base I layer (per 3,000 square
foot ream) and one layer of top releaselsealant coating. Samples 2-5 were
commercially available paper plates having paperboard weights of approximately
180 pounds per 3,000 square foot ream, an undetermined number of clay
coating layers, an undetermined number of layers of top functional coatings,
and
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CA 02323341 2000-10-16
at least two layers of ink. Samples B, R, I, and A are competitive products
currently available from a variety of manufacturers.
BASIS WEIGHT VISUAL GREASE
SAMPLE LBS13,000 Square Foot FAILURE
Ream
1 136 0%
B 183 50%
R 186 100%
I 175 100%
A 180 100%
The above results indicate an enhanced grease resistance achieved by
applying Tykote to the top surface of the paperboard as compared torcommercial
paperboards having clay coating. And, as such, the above results establish
that
a lightweight plate can be fabricated having at least equivalent rigidity and
enhanced grease resistance as compared to a heavier weight commercial
paperboard plate, at a substantially reduced cost.
Testing of the invention suggests that the overall rigidity of a paperboard
plate is not only related to shape, board physical properties, and forming
efficiencies, but is also related to the rigidity of the pleats found around
the
edges of the plate. Pleats are formed in the shape of a "z" fold around the
edges
of the plate when the paperboard is press formed, as shown in Fig. 2. The
rigidity of the pleats in a conventional plate is largely due to the repair of
the
internal delineation caused by the scoring process with contribution from
bonding
of adjacent surfaces. As shown in Fig. 2, when the pleats are formed, portions
of the top surface are folded back on themselves, and portions of the bottom
surface are likewise folded back on themselves. When the plates are press
formed, heat is transferred into the paperboard from the heated dies. Not
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wishing to be bound by theory, this heat can activate the cross linking of
compatible materials in the surface layers, allowing the surfaces to rebond.
In prior art paperboard plates, the top surface is coated with size press,
followed by two layers of clay coating and releaselsealant coatings. The
bottom
surface of the paperboard is coated only with size press. Therefore, when a
"z"
fold is formed, a release/sealant coating bond is formed on the top surface,
while
a size press coating to size press coating bond is formed on the lower
surface.
Laboratory testing has shown that the bonds formed on the bottom surface are
substantially stronger than those formed on the top surface, thereby
indicating
that the combination of clay coating and releaselsealant coating inhibits
bonding.
This is expected, as release/sealant coating is a material that one of
ordinary
skill in the art would not expect to bond to itself. Size press coating, on
the other
hand, is soluble and rebonds to itself, causing increased strength in the
pleat
bonds, which results in increased plate rigidity from the pleats. Thus,
removal of
the clay coating from the top surface apparently allows for increased bonding
and, hence, a more rigid plate.
Figures 3 and 4 depict micrographs taken of the fold in a paperboard plate
having clay coating and a paperboard plate without clay coating. As noted in
comparing the two figures, on the upper surface of the plate having the clay
coating do not appear to be bonded to each other (Fig. 3). On the other hand,
(as shown in Figure 4), bonding is complete on both the upper and lower
surfaces.
However, as noted above, the removal of the clay coating leaves the
paperboard plate with little or no grease resistance. The addition of a latex
first
coating, such as Tykote, to the paperboard not only restores the grease
resistance lost by removal of the clay coating, but enhances the plate's
rigidity as
compared to plates having clay coating. Moreover, the rigidity of the Tykote
coated plate is greater than those plates having no clay coating and no
Tykote.
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The following table compares pleat tensile strength in lightweight
paperboard plates having no clay coating with heavyweight paperboard pleats
having clay coating. Sample 1 is a 126 pound per 3,000 square foot ream
paperboard with no clay coating and no Tykote Base I. Sample 2 is a 126 pound
per 3,000 square foot ream paperboard with no clay coating, but with a Tykote
Base I upper layer. Sample 3 is a 162 pound per 3,000 square foot ream
paperboard with clay coating. The tensile strength of the pleats was tested
using
an Instron~ testing machine and image analysis techniques. Sample pleats
were preconditioned and conditioned according to TAPPI standards. The pleats
were then cut from the plates using a razor blade. Although a quarter inch
sample is desired, the true width of each sample is measured so that accurate
load (Id/in of pleat) can be calculated. Dots are inked on the edges of the
sample on opposite sides of the pleat to provide reference marks to measure
displacement. The samples are then pulled using an Instron~ set with a
crosshead speed of 0.025 in/min. Two video cameras are used to record the
testing and provide input to the image analysis. One camera shows the sample
under tension while the other camera provides the load values from the
Instron~.
The two camera inputs are combined using a picture in picture function. The
video is then played through a digitizing function and OptimasT"' image
analysis
software is used to track the movement of the dots.
Quantity Basis Weight
Sample of Pleats Lbs13,000 Spuare Foot ReamPleat Strength
4
1 11 126 10.6
2 10 126 14.0
3 12 162 4.7
° Pounds of force
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The results show that the clay coating appears to inhibit the strength of the
pleat.
In addition, the results indicate an even greater improvement in pleat
strength
when using Tykote Base I as an upper layer.
Therefore, application of Tykote allows for the use of light weight
paperboard and removal of the clay coatings, achieving a less expensive light
weight paperboard plate having equivalent rigidity and enhanced grease
resistance.
It is understood that the invention is not confined to the particular
construction and arrangement of parts and the particular processes described
herein but embraces such modified forms thereof as come within the scope of
the following claims.
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