Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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. ~ BAC~CGROUND OF THE INVENTION I _
1. Field of the Invention
This invention pertains generally to the field of oven
heatable plastie coated paperboard eontainers and to processe~
for produeing the same. -
2. Deseription of the Prior Art
The most eommo~ containers for convenience foods whieh
are to be heated within the container are formed of thin
sheet aluminum or layers which include aluminum foil. Because
of the relative high cost of such containers and beeause
they generally eannot be used in microwave oven coo~ing, sub-
stantial efforts have been made to provide plastic eoated
paperboard eartons whieh can withstand oven heating.
Polyethylene is vften used as a coating material for
paperboard sinee it has good moisture impermeability and is
easily adhered to many types of paperboard. IHowever, poly-
ethylene and many other types of eommon pla~tie coating ,
materials do not have the resistanee to meltlng at high
temperatures required for very hot oven heating. Sueh coating
polymers must also have adequate struetural strength and
abrasion resistance, as well as being eompatible with food
products.
Polyethylene terephthalate polyester is a pa_ticularlysatisfactory coating material for oven heatable trays since
it has a high melting temperature and good structural strength,
and is compatible with and unaffected by most food produets.
However, it is well known in the art that it is d~fficult to
obtain good bonding of polyethylene terephthalate to other
mater~als and particularly to paper~oard. In the past,
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such bondin~ has been~.accomplished by the use of adhe~1ves
or primers applied over the paperboard before a hot melt
extrusion of the polymer is applied to the paperboard. The
u~e of primers and adhesives is undesirable in packaging foods
because such materialq are capable of migrating into the
contents of the food package. =
A procedure for extrusion coating polyethylene tereph- ,
thalate onto paperboard without the use of primers is shown
in U.S. Patent No. 3,924,013 to Kane, in which the paperboard -
is subjected to heating prior to being contacted with the
hot melt extrusion. While such a process may be adequate
for certain purposes, it is undesirable were;the coated paper-
board is to be die pressed into deep formed trays, since
heating the paperboard reduces it~ moisture content and em-
brittles the board to thereby make it more subject to tearing
upon die pressing. Deep pressed heatable containers are
especially preferred since they do not require the use of
adhesives or heat seals in order to form the edqe walls of the
tray. Trays formed by adhesively connecting',the sides of the
tray together or by heat sealing them together are subject
to separation at the very high temperatures of oven heating,
and the adhesive material may migrate into the food product.
Pressing allows formation of smooth radius contoured corners,
rather than sharp adhesively joined corners, which provides
good heat distribution characteristics during oven heating.
SUMM~RY OF THE INVENTION
The coated paperboard formed in accordance with the
invention is especially suited to forming deep pressed trays
which can be filled with food products and oven heated to
t peratures of 400 F. ~h polyethylene terephthal~te
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coating on the interior surface of the paperboard has a high ¦- -
degree of adhesion to the pa~erboard at initial room tempera-
tures, at the freezing temperatures at which the food is stored,
and at the 300 F. to 400 F. oven temperatures at which the
food is heated. The coating is applied to the paperboard
without the use of primers or adhesives which ~hereby
eliminates a potential source of contamination of the food.
The paperboard substrate is selected to have good resist-
ance to oven heating, low levels of contaminants which inh~bit
proper adhesion of the coating, and surface roughnes~ character-
istics which allow strong adherence of the coating to take
place. The paperboard substrate, which has a thickness in the
preferred range of .015 to .025 inch, is passed through a
corona discharge device such that the selected surface of
the paperboard receives a selected corona discharge energy
sufficient to allow adhesion of the coating to the paperboard
of at least 90 grams per linear inch. Generllly, the corona
energy density required will be at least 0.35 joule per
square inch and preferably 2 to 5 joules perlsquare inch.
Surface treatment at this energy level prepares the surfa~e
and reduces the effect of contaminants in the ~urface which
wculd tend to inhibit adhesion of the coating.
The corona treated paperboard is passed~into a nip formed
between a chill roll and a backup roll while a hot melt
extrusion of polyethylene terephthalate is simultaneously
passed into the nip betwe~n the corona treated side of the
paperboard and the chill roll. The hot melt extrusion
exits from the extruder at an initial temperature between
580 F. and 640 F. through an air gap before insertion into
the nip at substantially the same speed as the forward moving
paperboard, The alr gap is adjusted such that the temperature
of the extrusion at the time of contact with the paperboard
,
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( is above the melting point of the polyethylcne terephthalate
such that the extrusion will still be in a substantially fluid
state at the time that it contacts the papèrboard so as to
flow into the fibrous surface of the paperboard. At normal
ambient temperatures (65 F. to 80 F.), the air gap and paper-
board speed are preferably adjusted to provide a polymer resi-
dence time in the air gap of about 0.05 to 0.15 second~. The
chill roll is maintained at a temperature close to ambient so
as to quickly chill the extrusion coating below its glass trans-
ition temperature to a substantially non-flowing state by tfie
! time the laminate of paperboard and coating leaves the chill roll
Coated paperboard formed by the aforementioned process
has adhesion between the polyethylene terephthalate coating
and the underlaying paperboard of at least ~0 grams per inch
and preferably 200 to 500 grams per inch width. It has been
found that adhesion levels generally increase with increases
in corona energy density and in the thickness of the extrusion
coating, but that adequate adhesion can be obtained at lower
corona energy and more convenient coating thicknesses where the
paperboard surface roughness is greater than selected minimum
levels and the organic contaminants on the sùrface are below
selected ma~imum concentrations.
For forming of deep die pressed trays, it is preferred
that the moisture content of the paperboard be at least 10%
'5 by weight. Generally, the initial moisture content of the
paperboard is not substantially effected by the corona treat-
ment or extrusion process so that if adequate moisture is
present in the initial paperboard, it will be maintained through
the entire process. ~owever, where additional moisture is
0 required, the uncoated side of the paperboard can have a wetting
liqu~d applied thereto, with the entire coated paperboard
laminate being enclosed in a moisture proof wrapping for a
period of several hours to allow the moisture to reach equili-
brium distribution within the paperboard. Various
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types of paperboard substrates which have good resistance
to heating can be utilized, such as boards formed from solid
bleached sulfate pulps.
The exterior surface of the paperboard can be printed
to provide decoration and product advertising material, whlle
the polyethylene terephthalate coating itself can be pigmented
with any desired color for aesthetic enhancement as well as
concealing any browning of the paperboard that may take
place at the high oven temperatures. ~ `-
Further objects, features, and advantages of the inventionwill be apparent from the following detailed description taken
in conjunction with the accompanying drawings showing coated
paperboard material suitable for forming pressed heatable
food trays and a process for producing`the same.
BRIEF~DESCRIPTION OF THE DRAWINGS
. .
r In the drawings: ~
;i Fig. 1 is a schematic view of apparatus for treating
and coating the paperboard.
Fig. 2 is an external perspective view ~ ~f a pressed tray
formed from the coated paperboard of the invl ~ntion~
. I .
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now more particularly to the drawings, wherein
like numerals refer to like parts in both v~ews, a preferred
embodiment of an apparatus for forming the coated paperboard
; of the invention is shown generally at 10 in Fig. 1. For
exe lary purposes, a roll 11 of paperboard ~s shown whlch Is
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unwound and passed through a corona discharge device 12. The
corona discharge device 12 is shown only in schematic form in
Fig. 1, with the plates of the device being represented by
the dielectric roller 13 and the curved plate or shoe 14.
The generator which provides the corona discharge voltages
between the plates 13 and 14 is not shown in Fig. 1. The -
shape of the plate or shoe 14 is preferably curved to match
the periphery of the roller 13 contacting the paper so as to
provide a substantially uniform corona field to the paper- -
board. It is preferred that the corona discharge device have
a capacity to provide corona discharge wattages of 100 to 600
watts per inch of width at 9.6 Kl~z over an alr gap of approx-
imately 0.060 inches. As explained below, the device 12 has
the capacity to treat the side lla of the paperboard facing
the curved plate 14 with a corona energy density of at least
2 to 6 joules per square inch of paperboard surface at
production speeds generally in the range of 100 to 500 ft.
per minute.
The paperboard stock provided from the roll 11 may be
formed in conventional manufacturing processes but is
preferably formed with minimal additives or impurities and
is uncoated on at least the upper surface lla thereof.
It has been found that the effect of the corona treat-
ment of the surface of the paperboard endures for a period of
at least 10 days under normal temperature and humidity conditions
following the corona treatment. Thus, although the paperboard
is shown immediately being passed into extrusion coating equip-
ment in Fig. 1, it is understood that the paperboard could ~e
rolled up after corona treatment and extrusion coated at a
lat time.
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The extrusion coating equipment shown in Fig. 1 includes
an extruder 18 which feeds the hot molten polyethylene tere-
phthalate into a sheet forming die 19. The molten extruded
film 20 exiting from the die 19 passes throuqh an air gap
and thence into a nip formed between a chrome plated chill
roll 21 and a backup roll 22. The paperboard i5 simulta-
neously passed into the nip such that the corona treated surface
lla of the paperboard comes into contact with the film in the
nip. As the molten film 20 reaches the nip, its temperature
has decreased to a temperature somewhat above the melting
point of the polyethylene terephthalate material (m.p.
approximately 480~ F.). At thi~ temperature, the film is
still su~ficiently molten that it can flow and conform to the
surface fibrous of the paperboard, while quickly cooling
below its glass transition temperature (approximately 179 F.)
and solidifying by contact with the coolex chill roll 21
which is preferably maintained at a temperature close to
ambient. The now solidified coating easily parts from the
chrome plated chill roll and allows the laminate of paper-
board and coating to be rolled up on a wind-up roll 2~.
The finished coated paperboard product ls especially
adapted to use in forming press formed one-piece trays. Such
trays are formed by placing a blank of the laminate with the
coated side up over a female die and pressing downwardly thereon
with a mating heated male die. An example of- such a tray
construction is shown in Fig. 2, wherein the finished tray
includes a bottom panel 25, integrally connected side panel 26,
and an integrally connected top flange 27. Iecause the die
forming of such trays requires the paperboard to bend and
stre h easily, it is important to the proper formatlon of
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57S~1
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the trays that the paperboard have a relatively high moisture
content, in the range of 10% by weight or more. It is noted
that in carrying out the process of the invention, the
moisture content of the board is not substantially reduced.
Furthermore, the process does not require heating of the
paperboard in any manner, which minimizes the possibility of
; oxidizing or embrittling the fibers of the paperboard, or
destroying inter-fiber bonds. If the initial paperboard, or
the roll 11 does not have sufficient moisture content, the
finished coated paperboard in the roll ~3 may have a wetting
liquid applied to the uncoated surface thereof which is allowed
to seep through the paperboard over a period of time, preferably ¦
- 10 to 24 hours. In order to minimize evaporation of the
moistened board, it is preferable to wrap the moistened board
i in a polyethylene or other moisture proof wrapping until the
paperboard is formed into trays.
High adhesion of the polyester coating to the paperboard
i~ desired, preferably being a minimum of 90 ta 150 grams
per inch as measured transversely at a 180 pull angle and
l at a 5 inch per minute rate, or to the point were fiber
tearing in the paperboard occurs. 90 grams per inch adhesion
is the minimum acceptable level at which adhesion is main-
tained during die pressing, and a minimum of 150 grams per
inch is preferred to prevent spontaneous delamination if the
coated board is die cut. The factors most influencing ad~e~ion
are the degree of penetration of the polyethylene terephthalate
into the paperboard, the roughness of the paperboard surface
belng coated, and the presence of chemical additives or
contaminants in the paperboard. Generally, it has been found
that the crystallinity of the laminated polyethylene tere-
phthalate, and the commercial source of the polymer, do not
- substantially affect the adhesion of the coating to the
paperboard,
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11,457~1
The adhesive peel strength of the coating depends on both
the mechanical and chemical aspects of the paperboard. The
mechanical factors of the paperboard include the roughness of
the paperboard surface and the fiber tearing strength of the
paperboard in a direction toward its surface. These mechanical
features affect the flow of molten or plastlc polyethylene -
terephthalate into the paperboard surface at elevated tempera-
tures and pressures as well as the spreading of peel forces
over a wider area by the pulling of fibers. The roughness of
the paperboard surface is the major contributor to the mechan-
ical aspects of the final adhesion of the coating, and the
roughness of the surface with the coating in situ increases
with increases in the application weight of the coating.
Additionally, less significant conditions which affect the
flow of the extrusion into the paperboard are the polymer
temperature at the time of contact with the paperboard, the
laminating pressure at the nip between the back-up roll
and the chill roll, and the contact time above the polymer
melting point during laminating.
Chemical additives and contaminants in the paperboard
also have been found to have à substantial effect on the
strength of adhesion which is obtained. The strength of
adhesion improves with decreasing concentrations of organic
contaminants or additives, which can be measured quantitatively
by the adsorption of an iodine stain applied to the paper.
A positive relationship was found between the intensity of
an iodine stain developed on the paperboard and the level
of adhesion that could be developed when polyethylene
terephthalate was extrusion laminated to the paperboard. The
test is similar to one commonly used to detect the presence
of Drganic compounds on thin-layer chromatography plates.
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The techniq~e is effective in detecting materials such as I -
oils~ waxes~ and certain paperboard additives such as wax
and rosin size.
The stain test was carried out utilizing a Macbeth
MS-2000 Spectrophotometer, a ceramic white plate standard
provided with the Spectrophotometer, iodine crystals (Fisher
Catalog No. I-36), and a rectangular developing tank (Fisher
Catalog No. 5-718-16). The tests were conducted on paperboard
which had been cut to sections of approximately ~ inches by
6 inches. 1 gram of iodine solid was emplaced in a glass
exposure vessel which was covered for three hours to allow
the iodine vapors to reach an equilibrium level. The paper-
board samples were placed standing up in the exposure vessel
and the vessel was covered for three hours to allow the iodine
stain to develop. The samples were then removed and allowed
to stand for three minutes to reduce excess iodine vapors,
and the change in liqhtness-darkness (~ L) of the sample versus 1l
the white plate standard was read on the Spectrophotometer.
The iodine stain test is a te~t of relative concentrations
of contaminants, and exact test readinqs may be expected
to vary with changes in test equipmen~ and whiteness standard.
I~ has been observed that the corona treatment of the
paperboard surfaces does not decrease the concentration of
additives and contaminants, as measured by the iodine sta~n
test, but rather apparently neutralizes the effect of the
contaminants where their concentration is inltially low.
I~ is theorized that the corona treatment produces bonding
sites on the additives and contaminants so that the polyester
coating can bond thereto. Other possible, although less
likely explanations for the enhancement o~ the ~onding, are
that the additives and contaminants are ox~dized in the
pres~ ce of the corona or that the corona produces act~ve ¦
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; sites for adhesion on the cellulose fibers of the paperbonrd
itself. While the corona treatment of the paperboard surface
provides increased adhesion of the polyester coating on
paperboard substrates in general, optimum adhesion is obtained
where the paperboard substrate meets preferred conditions
of roughness and sufficiently low levels of contaminants.
The effect of these factors are set forth in the follow~ng
¦ examples which are illustrative of the invention.
EXAMPLES 1~9
Polyethylene terephthalate coatings were applied to
corona treated paperboard in accordance with the process of
the invention set forth above at varying corona treatment
levels. The paperboard was provided from four separate types
of solid bleached sulfate paperboard having different surface
i characteristics, with each run of paperboard being passed
through the corona device (Pillar Model Components AB 1326-
3l-) and AB 1418-4(-)) and the extrusion coater at the rate
of 175 ft. per minute. Polyethylene terephthalate obtained
¦ from Eastman as Eastman 6857 resin was used to coat 7 samples
of paperboard, while 2 samples of paperboard were coated
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with resin obtained from Goodyear under the ds~ign~t~on
Goodyear VPE-5792, to determine if the source of supply
of the polyester affected adhesion. The polyester resin was
thoroughly dried, and then heated in the extruder to an
i exit melt temperature of 640~ F. The extruded film passed
through an air gap of approximately 2 inches and into contact
with the corona treated paperboard surface. q~e-chrome plated
chill roll was maintained at a temperature of 60 F. The
results of these tests are given in Table 1 below. In this
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table, the base board thic~ness and the polyester thickness I -
were determined by measurement after separation of the poly-
ester from the board,, except where separation could not be
obtained without fiber tearing, in which case nominal theo-
retical thickness are provided based on the expected thickness
of the polyester coating.
TABLE 1
Adhesion,
Instron,
Board Poly- polyester
Basis Base Bendt- ester: to board
Weight, Board sen thick- 180 ang.lbs/rm Corona- Iodine thick- rough- ness 5"/min.,
Sam- 24x36x joules/ Stain ness ness at (mils), grams/25.
lS ple 500' sq. in. (-~ L) mils 5 Kg Supplier 4mm width
1 199 3.41 37 16.0188 1.31 10-25
Goodyear
2 232 2. al 15 18.5123 1.34 ` 55-~25
Eastman
3 258 2.11 20 21.1351' 1.36 120-380
Gcodyear
4 256 3.61 20 21.8351 1.50 125-375
, Eastman
231 2.81 15 18.5123 1.50 CNS
Eastman
6 191 3.73 25 15.094 1.51 20-110
Eastman
7 193 3.73 25 13.894 1.67 75-140
Eastman
- 30 8 193 3.73 25 14.094 ~.78 75-11
- I Eastman
9 211 2.76 37 16.6188 2.14 175-275
Eastman
CNS = Could not separate
* Estimated value i
l ~, :
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Since similar tests without corona treatment yielded
very low to no adhesion of polyester coating to paperboard
for all of the above samples, the test results indicate that
corona treatment provides some additional adhesion under virtually
all conditions. However, it is noted from a comparison of
samples l and 9 that a very large increase in adhesion wa~
obtained by increasing the thickness of the polyester coating
to slightly over 2 mils from approximately l.3 mils for paper- ¦
board having similar surface characteristics. Although different¦
polyester supp~iers were utilized for these two te~ts, the effect
of the source of polyester is discounted, particularly in
comparing the results of samples 3 and 4 wherein coating of two
different sources of polyester on similar surfaces yi~lded
similar adhesion results. The foregoing test results are
exemplary of data which indicates that, for polyester coatlng
of a thickness of l.5 mils or less, it is highly preferred
that the ~endtsen roughnes~ at S Kg. (TAPPI standard ~-479)
be at least lOO for the paperboard surface, and that the
contamination level of the paperboard surface as measured
by the foregoing iodine stain response test be approximately
25 or less. Under such board surface conditions, corona
treatment above minimal levels may be expected to pro~-ide
substantial enhancement of adhesion. It is also seen from
this data that adequate adhesion may be obtained by increasin~
the thickness of the extrusion coating which apparently
increases penetration of the hot melt into the paperboard.
However, coating thickness greater than approximately l.5 mils
are undesirable since the stiffness of the coating interferes
with die press forming of trays.
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EXAMPLES 10-14
The following example~ illustrate the effect of varying
levels of corona treatment on board surfaces having the I -
preferred surface characteristics. The paperboard of sample 5
above was utilized. The paperboard in all samples was run
through the extrusion equipment at a rate of 175 ft. per
minute and coextruded with Goodyear VPE 5792 polyethylene
terephthalate at an extrusion temperature of 640 F., exit~ng
from the extrusion die through an air gap of 4 1/2 ~nches
before contact with the the paperboard surface. The chrome plated
chill roll was maintained at a temperature of 60 F. and the
nip pressure between the chill roll and the backup roll was
145 pli. The corona device was a Pillar model components
AB 1326-3(-) and AB 1418-4(-).
i With no corona treatment of the paperboard surface, the
adhesion of the polyester to paperboard using an Instron tester
at a 180 angle, 5 inches per minute, yielded adhesion fluctuatin~
between 0 and approximately 90 grams per inch width. Samples
10-13 summarized in the table below were performed by first
corona treating one surface of the paperboard to the energy
density stated in the table, storing the paperboard for 10
days, and then extruding the polyester onto the treated
surface thereof under the foregoing conditions. Sample 14 was
obtained by running the paperboard at a rate of 175 ft. per
i minute continuously through the corona trea~er to the extrusion
coating equipment.
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TAsLE 2
Corona level Adhesion, Instron,
Sample joules per polyester to board 180
Identification square inch anqle, qrams~25.4mm width
0.35 90-320
11 0.74 90-320
12 1.81 230-490
13 5.05 230-453
14 3.~6 230-680 ,
Substantially enhanced adhesion is thus obtained with
corona treatment levels as low as 0.35 joules per square inch,
and without regard to whether the corona treatment is applied
immediately before extrusion coating or after an intervening
period of time. It is seen that optimum adhesion is obtained
with corona treatment levels of approximately 2 to 5 joules
per square inch. It is noted however, that enhancement of
the adhesion does take place at corona levels as low as
0.35 joules per square inch.
'
EXAMPLE 15
,
The paperboard specified above in Examples 10-14 was
passed through the corona treater at a corona level of approxi-
mately 5 joules per square inch at 175 ft. per minute and
directly into the extrusion coating apparatus. A hot melt
was prepared consisting of a uniform misture of 80~ by weight
Eastman 6857 polyethylene terephthalate and 20% by weight
particulate Ampacet 11171 white concentrate pigment. Extrusion
of the melt onto the paperboard was carried out in accordan~e
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with the process set forth for Examples 10-14, except that the
melt temperature was lowered from 640 F. to 590 F. to form
an acceptable melt curtain with the blend. The required lowering
of the melting temperature was due to the presence of low
density polyethylene present as a pigment carr~er. The resulting~
coating had a thickness of approximately 1 mil and good adhesion,l =
as measured on the Instron tester at 180, of approximately
300 to 600 grams per inch adhesion. The uncoated side of the
laminate was moistened with water and a wetting agent, wrapped
in polyethylene and stored for 24 hours, and then formed on
a die press into a tapered tray having a top flange, similar to
that shown in Fig. 2. The tray was filled with 10 ounces of
spagetti and beef sauce, and a film lid of 92 gauge polyester
coated on one siae with Adcote 1189-36 adhesive was applied
and heat sealed to the top of the tray. The filled tray was
covered with aluminum foil and frozen 3 days at 0 F. Upon
removal from the freezer, the foil was removed and the tray
was heated in an electric oven at 375 F. for 3~ minutes.
Upon removal from the oven, the temperature of the product
was checked and the contents were removed from the tray. The
tray was examined for adhesion of the coating and scorching
of the board. No delamination of the coating from the board
was observed. There was slight to moderate scorching of the
flange but no scorching of the tray at the area in contact
with the product, and no observable scorching of the board
in the areas covered by the pigmented polyester coating.
It is understood that the invention is not confined to
the particular embodiments described herein as illustrative,
but embraces all such modified forms thereof which oome
within the scope of the following claims.
