U.S. patent number RE32,270 [Application Number 06/574,668] was granted by the patent office on 1986-10-28 for polyester coated paperboard for forming food containers and process for producing the same.
This patent grant is currently assigned to James River-Norwalk, Inc.. Invention is credited to Lee J. Murray, Jr..
United States Patent |
RE32,270 |
Murray, Jr. |
October 28, 1986 |
Polyester coated paperboard for forming food containers and process
for producing the same
Abstract
A coated paperboard product and a process for producing the same
which includes corona discharge treatment of a paperboard surface
and subsequent extrusion of molten polyester thereon. The resulting
product has a very high degree of adhesion between the paperboard
and polyester layers, and is capable of being utilized for forming
pressed food trays which can be subjected to oven cooking
temperatures.
Inventors: |
Murray, Jr.; Lee J. (Appleton,
WI) |
Assignee: |
James River-Norwalk, Inc.
(Norwalk, CT)
|
Family
ID: |
27076454 |
Appl.
No.: |
06/574,668 |
Filed: |
January 27, 1984 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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243359 |
Mar 13, 1981 |
|
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|
|
104554 |
Jan 8, 1980 |
|
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Reissue of: |
891053 |
Mar 28, 1978 |
04147836 |
Apr 3, 1979 |
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Current U.S.
Class: |
428/335; 156/221;
156/224; 156/244.13; 156/244.17; 156/244.23; 229/5.84; 427/525;
428/34.2; 428/481 |
Current CPC
Class: |
B32B
27/10 (20130101); D21H 19/28 (20130101); Y10T
428/3179 (20150401); Y10T 428/1303 (20150115); Y10T
156/1043 (20150115); Y10T 156/1048 (20150115); Y10T
428/264 (20150115) |
Current International
Class: |
B32B
27/10 (20060101); D21H 19/28 (20060101); D21H
19/00 (20060101); B32B 027/10 (); B29C 019/04 ();
B05D 003/06 () |
Field of
Search: |
;428/481,35,480,482,500,335,349 ;427/39 ;162/135
;156/221,224,272,274,244.17,244.23,380,244.13 ;229/3.5R,30
;220/457,458,459 ;204/165,168 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Herbert; Thomas J.
Attorney, Agent or Firm: Aguele; William A. Whaley; Thomas
H.
Parent Case Text
This application is a continuation of application Ser. No. 243,359,
filed Mar. 13, 1981, now abandoned, which was, in turn, a
continuation of application Ser. No. 104,554, filed Jan. 8, 1980,
now abandoned. .Iaddend.
Claims
.[.We claim.]. .Iadd.I claim.Iaddend.: .[.1. A process for
producing coated paperboard material suitable for forming pressed
heatable food trays, comprising the steps of:
(a) passing a web of paperboard material through a corona discharge
device at a corona energy density level sufficient to subject the
paperboard to an energy density of at least 0.35 joules per square
inch of paperboard surface;
(b) passing the corona treated paperboard into a nip formed between
a chill roll and a backup roll while simultaneously passing a hot
melt extrusion of polyethylene terephthalate into the nip between
the corona treated side of the paperboard and the chill roll, the
hot melt extrusion exiting from the extrusion die at an initial
temperature of between 580.degree. F. and 640.degree. F. through an
air gap before insertion into the nip, the air gap distance being
selected with respect to the speed of the paperboard and the
ambient and chill roll temperatures such that the temperature of
the extrusion at the time of contact with said paperboard is above
its melting temperature and such that it is chilled below its glass
transition temperature at the time that it leaves the chill roll to
thereby facilitate separation therefrom..]. .[.2. A process for
producing coated paperboard material suitable for forming pressed
heatable food trays, comprising the steps of:
(a) passing a web of paperboard material through a corona discharge
device to corona treat a surface of the paperboard to a selected
energy density;
(b) passing the corona treated paperboard into a nip formed between
a chill roll and a backup roll while simultaneously passing a hot
melt extrusion of polyethylene terephthalate into the nip between
the corona treated side of the paperboard and the chill roll, the
thickness of the extrusion coating on the paperboard and the corona
energy density on the paperboard surface being selected such that
the adhesion between the extrusion coating and the paperboard is at
least 90 grams per linear inch width as
measured at a 180.degree. pull angle at 5 inches per minute..].
.[.3. The process of claim 2 wherein the thickness of the extrusion
coating on the paperboard is between 0.5 mil and 1.5 mils and the
corona energy density applied to the paperboard is between about
0.35 joule per square inch and 5 joules per square inch, and
wherein the paperboard is selected such that the corona treated
surface thereof has a surface roughness above a minimum level and a
concentration of organic contaminants below a maximum level such
that the adhesion between the extrusion coating and the paperboard
is at least 90 grams per linear inch as measured at a 180.degree.
pull angle at 5 inches per minute..]. .[.4. The process of claim 3
wherein the hot melt extrusion exits from the extrusion die at an
initial temperature of between 580.degree. F. and 640.degree. F.
through an air gap before insertion into the nip, the air gap
distance being selected with respect to the speed of the paperboard
and the ambient and chill roll temperatures such that the
temperature of the extrusion at the time of contact with the
paperboard is above its melting temperature and such that it is
chilled below its glass transition temperature at the time that it
leaves the chill roll to thereby facilitate separation
therefrom..]. .[.5. The process of claims 1 or 2 wherein the
paperboard is selected such that the corona treated surface has a
surface roughness as determined by the Bendtsen test at 5 Kg of at
least 100..]. .[.6. The process of claims 1 or 2 wherein the
paperboard is selected such that the corona treated surface thereof
has a level of impurities as detected by an iodine stain test on a
Macbeth Ms-2000 Spectrophotometer relative to a white plate
standard of less than 25..]. .[.7. The process of claims 1 or 2
wherein the paperboard is formed of solid bleached sulfate pulp..].
.[.8. The process of claims 1 or 2 wherein the paperboard has a
moisture content of at least 6% by weight..]. .[.9. The process of
claims 1 or 4 wherein the chill roll is maintained at a surface
temperature between 60.degree. F. and 120.degree. F..]. .[.10. The
process of claims 1 or 2 wherein the hot melt of polyethylene
terephthalate has a particulate color pigment intermixed
therewith..]. .[.11. The process of claims 1 or 2 including the
additional step of applying water to the uncoated surface of the
coated paperboard laminate and storing the moistened paperboard in
a moisture proof container for 24 hours to allow moisture
throughout the paperboard to reach at least 10% by weight..].
.[.12. A coated paperboard product made
in accordance with the processes of claims 1 or 2..]. .Iadd.13. A
process for producing coated paperboard material suitable for
forming pressed heatable food trays, comprising the steps of:
(a) passing a web of paperboard material through a corona discharge
device at a corona energy density level sufficient to subject the
paperboard to an energy density of at least 0.35 joules per square
inch of paperboard surface;
(b) passing the corona treated paperboard into a nip formed between
a chill roll and a backup roll while simultaneously passing a hot
melt extrusion of polyethylene terephthalate into the nip between
the corona treated side of the paperboard and the chill roll to
produce a thickness of the extrusion coating on the paperboard of
between 0.5 mil and 1.5 mils, the hot melt extrusion exiting from
the extrusion die at an initial temperature of between 580.degree.
F. and 640.degree. F. through an air gap before insertion into the
nip, the air gap distance being selected with respect to the speed
of the paperboard and the ambient and chill roll temperatures such
that the temperature of the extrusion at the time of contact with
said paperboard is above its melting temperature of about
480.degree. F. and such that it is chilled below its glass
transition temperature of about 179.degree. F. at the time that it
leaves the chill roll to thereby facilitate separation therefrom,
said paperboard being selected such that the corona treated surface
has a surface roughness, as determined by the Bendtsen test at 5
Kg, of at least 100, and a level of impurities, as detected by an
iodine stain test (.DELTA.L) on a Macbeth MS-2000 Spectrophotometer
relative to a white plate standard, of less than
25. .Iaddend. .Iadd.14. The process of claim 13 wherein said level
of impurities, so detected, is about 15 to 25. .Iaddend. .Iadd.15.
The process of claim 13, wherein said air gap distance and said
paperboard speed are selected to provide a residence time of said
extrusion in said air gap of from about 0.05 second to about 0.15
second. .Iaddend.
.Iadd. In a process for producing coated paperboard suitable for
forming pressed oven-heatable food trays, the steps comprising:
passing a web of paperboard through a corona discharge device at a
corona energy density level sufficient to subject said surface of
said paperboard to an energy density of at least 0.35 joule per
square inch thereof; passing the corona-treated paperboard into a
nip formed between a chill roll and a backup roll, while
simultaneously passing into the nip between the corona-treated side
of said paperboard and the chill roll a hot melt extrusion, in a
thickness of between 0.5 and 1.5 mils, of a polyethylene
terephthalate resin having a melting point temperature of about
480.degree.; causing said hot melt extrusion to exit from the
extrusion die at an initial temperature of from about 580.degree.
F. to about 640.degree. F., and to pass, before insertion into said
nip, through an air gap distance so selected with respect to the
speed of said paperboard and the ambient and chill roll
temperatures that said extrusion is, at the time of contact with
said paperboard, above said melting point temperature and in a
substantially fluid state, so as to flow into said fibrous surface
of said paperboard; and quickly chilling said extrusion below the
glass transition temperature of said resin of about 179.degree. F.
at the time that it leaves the chill roll, said paperboard being
selected such that the corona treated surface has a surface
roughness, as determined by the Bendtsen test at 5 Kg, of at least
100, and a level of impurities, as detected by an iodine stain test
(.DELTA.L) on a Macbeth MS-2000 Spectrophotometer relative to a
white plate standard, of less than 25, the level of adhesion
between said extrusion coating and said paperboard so attained
being at least 90 grams per linear inch width, as measured at a
180 degree pull angle at 5 inches per minute. .Iaddend. .Iadd.17. A
coated paperboard product made in accordance with the process of
claims 13, 14, 15 or 16. .Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains generally to the field of oven heatable
plastic coated paperboard containers and to processes for producing
the same.
2. Description of the Prior Art
The most common containers for convenience foods which 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 because they generally cannot be used
in microwave oven cooking, substantial efforts have been made to
provide plastic coated paperboard cartons which can withstand oven
heating.
Polyethylene is often used as a coating material for paperboard
since it has good moisture impermeability and is easily adhered to
many types of paperboard. However, polyethylene and many other
types of common plastic coating materials do not have the
resistance to melting at high temperatures required for very hot
oven heating. Such coating polymers must also have adequate
structural strength and abrasion resistance, as well as being
compatible with food products.
Polyethylene terephthalate polyester is a particularly satisfactory
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 products. However, it is well
known in the art that it is difficult to obtain good bonding of
polyethylene terephthalate to other materials and particularly to
paperboard. In the past, such bonding has been accomplished by the
use of adhesives or primers applied over the paperboard before a
hot melt extrusion of the polymer is applied to the paperboard. The
use of primers and adhesives is undesirable in packaging foods
because such materials are capable of migrating into the contents
of the food package.
A procedure for extrusion coating polyethylene terephthalate onto
paperboard without the use of primers is shown in U.S. Pat. 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 paperboard is to be die pressed into deep formed
trays, since heating the paperboard reduces its moisture content
and embrittles 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 edge 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.
SUMMARY 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 temperatures of 400.degree.
F. The polyethylene terephthalate coating on the interior surface
of the paperboard has a high degree of adhesion to the paperboard
at initial room temperatures, at the freezing temperatures at which
the food is stored, and at the 300.degree. F. to 400.degree. F.
oven temperatures at which the food is heated. The coating is
applied to the paperboard without the use of primers or adhesives
which thereby eliminates a potential source of contamination of the
food.
The paperboard substrate is selected to have good resistance to
oven heating, low levels of contaminants which inhibit proper
adhesion of the coating, and surface roughness characteristics
which allow strong adherence of the coating to take place. The
paperboard substrate, which has a thickness in the preferred range
of 0.015 to 0.025 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.
Generally, the corona energy density required will be at least 0.35
joules per square inch and preferably 2 to 5 joules per square
inch. Surface treatment at this energy level prepares the surface
and reduces the effect of contaminants in the surface which would
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
between 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.degree. F. and 640.degree. F. through an
air gap before insertion into the nip at substantially the same
speed as the forward moving paperboard. The air gap is adjusted
such that the temperature of the extrusion at the time of contact
with the paperboard is above the melting point of the polyethylene
terephthalate such that the extrusion will still be in a
substantially fluid state at the time that it contacts the
paperboard so as to flow into the fibrous surface of the
paperboard. At normal ambient temperatures (65.degree. F. to
80.degree. F.), the air gap and paperboard speed are preferably
adjusted to provide a polymer residence time in the air gap of
about 0.05 to 0.15 seconds. The chill roll is maintained at a
temperature close to ambient so as to quickly chill the extrusion
coating below its glass transition temperature to a substantially
non-flowing state by the 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 90 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 .Iadd.in a range of from about 0.5 mil to about 1.5
mils .Iaddend.where the paperboard surface roughness is greater
than selected minimum levels and the organic contaminants on the
surface are below selected maximum concentrations.
For forming of deep die pressed trays, it is preferred that the
moisture content of the paperboard be at least 10% by weight.
Generally, the initial moisture content of the paperboard is not
substantially effected by the corona treatment or extrusion process
so that if adequate moisture is present in the initial paperboard,
it will be maintained through the entire process. However, where
additional moisture is required, the uncoated side of the
paperboard can have a wetting liquid 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 equilibrium distribution within the paperboard. Various
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, while 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 invention will 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
In the drawings:
FIG. 1 is a schematic view of apparatus for treating and coating
the paperboard.
FIG. 2 is an external perspective view of a pressed tray formed
from the coated paperboard of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now more particularly to the drawings, wherein like
numerals refer to like parts in both views, a preferred embodiment
of an apparatus for forming the coated paperboard of the invention
is shown generally at 10 in FIG. 1. For exemplary purposes, a roll
11 of paperboard is shown which is 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 paperboard. 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 KHz over an air gap of approximately 0.060 inches. As
explained below, the device 12 has the capacity to treat the side
11a 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 11a thereof.
It has been found that the effect of the corona treatment 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 equipment in FIG.
1, it is understood that the paperboard could be rolled up after
corona treatment and extrusion coated at a later time.
The extrusion coating equipment shown in FIG. 1 includes an
extruder 18 which feeds the hot molten polyethylene terephthalate
into a sheet forming die 19. The molten extruded film 20 exiting
from the die 19 passes through an air gap and thence into a nip
formed between a chrome plated chill roll 21 and a backup roll 22.
The paperboard is simultaneously passed into the nip such that the
corona treated surface 11a 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.degree. F.). At this temperature, the film is
still sufficiently molten that it can flow and conform to the
surface fibrous of the paperboard, while quickly cooling below its
glass transition temperature (approximately 179.degree. F.) and
solidifying by contact with the cooler 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 paperboard and coating to be rolled up
on a wind-up roll 23.
The finished coated paperboard product is 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.
Because the die forming of such trays requires the paperboard to
bend and stretch easily, it is important to the proper formation of
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 23 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 in a
polyethylene or other moisture proof wrapping until the paperboard
is formed into trays.
High adhesion of the polyester coating to the paperboard is
desired, preferably being a minimum of 90 to 150 grams per inch as
measured transversely at a 180.degree. pull angle and 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 maintained 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 adhesion are the degree of penetration
of the polyethylene terephthalate into the paperboard, the
roughness of the paperboard surface being coated, and the presence
of chemical additives or contaminants in the paperboard. Generally,
it has been found that the crystallinity of the laminated
polyethylene terephthalate, and the commercial source of the
polymer, do not substantially affect the adhesion of the coating to
the paperboard.
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 plastic polyethylene terephthalate into the
paperboard surface at elevated temperatures 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 mechanical 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 a 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 organic compounds on thin-layer chromatography plates.
The technique is effective in detecting materials such as 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 2 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
paperboard 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 lightness-darkness (.DELTA.L) of the sample versus the
white plate standard was read on the Spectrophotometer. The iodine
stain test is a test of relative concentrations of contaminants,
and exact test readings may be expected to vary with changes in
test equipment and whiteness standard.
It 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 stain test, but rather
apparently neutralizes the effect of the contaminants where their
concentration is initially low. It 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 of the
bonding, are that the additives and contaminants are oxidized in
the presence of the corona or that the corona produces active sites
for adhesion on the cellulose fibers of the paperboard 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 following 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 characteristics, with each run
of paperboard being passed through the corona device (Pillar Model
Components AB 1326-3(-) 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
with resin obtained from Goodyear under the designation 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 exit melt temperature of
640.degree. F. The extruded film passed through an air gap of
approximately 2 inches and into contact with the corona treated
paperboard surface. The chrome plated chill roll was maintained at
a temperature of 60.degree. F. The results of these tests are given
in Table 1 below. In this table, the base board thickness and the
polyester thickness were determined by measurement after separation
of the polyester from the board, except where separation could not
be obtained without fiber tearing, in which case nominal
theoretical thickness are provided based on the expected thickness
of the polyester coating.
TABLE 1
__________________________________________________________________________
Adhesion Instron. Board Poly- polyester Basis Base Pendt- ester: to
board Weight, Board sen thick- 180.degree. ang., lbs/rm Corona-
Iodine thick- rough- ness 5"/min., Sam- 24 .times. 36 .times.
joules/ Stam ness ness at (mils), grams/25, ple 500' sq. in.
(-.DELTA.L) mils 5 Kg Supplier 4 mm width
__________________________________________________________________________
1 199 3.41 37 16.0 188 1.31 10-25 Goodyear 2 232 2.81 15 18.5 123
1.34 55-225 Eastman 3 258 2.11 20 21.1 351 1.36 120-380 Goodyear 4
256 3.61 20 21.8 351 1.50 125-375 Eastman 5 231 2.81 15 18.5* 123
1.50* CNS Eastman 6 191 3.73 25 15.0 94 1.51 20-110 Eastman 7 193
3.73 25 13.8 94 1.67 75-140 Eastman 8 193 3.73 25 14.0 94 1.78
75-110 Eastman 9 211 2.76 37 16.6 188 2.14 175-275 Eastman
__________________________________________________________________________
CNS = Could not separate *Estimated value
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 1 and 9 that a very large
increase in adhesion was obtained by increasing the thickness of
the polyester coating to slightly over 2 mils from approximately
1.3 mils for paperboard having similar surface characteristics.
Although different polyester suppliers were utilized for these two
tests, 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
yielded similar adhesion results. The foregoing test results are
exemplary of data which indicates that, for polyester coating of a
thickness of 1.5 mils or less, it is highly preferred that the
Bendtsen roughness at 5 Kg. (TAPPI standard T-479) be at least 100
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 provide substantial enhancement of adhesion. It is also seen
from this data that adequate adhesion may be obtained by increasing
the thickness of the extrusion coating which apparently increases
penetration of the hot melt into the paperboard. However, coating
thickness greater than approximately 1.5 mils are undesirable since
the stiffness of the coating interferes with die press forming of
trays.
EXAMPLES 10-14
The following examples illustrate the effect of varying levels of
corona treatment on board surfaces having the 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.degree. F., exiting from the extrusion die through an air gap
of 41/2 inches before contact with the the paperboard surface. The
chrome plated chill roll was maintained at a temperature of
60.degree. 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(-).
With no corona treatment of the paperboard surface, the adhesion of
the polyester to paperboard using an Instron tester at a
180.degree. angle, 5 inches per minute, yielded adhesion
fluctuating 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 minute continuously through the
corona treater to the extrusion coating equipment.
TABLE 2 ______________________________________ Corona level
Adhesion, Instron, -Sample joules per polyester to board
180.degree. Identification square inch angle, grams/25 4 mm width
______________________________________ 10 0.35 90-320 11 0.74
90-320 12 1.81 230-490 13 5.05 230-453 14 3.26 230-680
______________________________________
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 approximately 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 accordance with the process set forth for Examples 10-14, except
that the melt temperature was lowered from 640.degree. F. to
590.degree. 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 carrier.
The resulting coating had a thickness of approximately 1 mil and
good adhesion, as measured on the Instron tester at 180.degree., 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 side 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.degree. F. Upon removal
from the freezer, the foil was removed and the tray was heated in
an electric oven at 375.degree. F. for 35 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 come within the
scope of the following claims .
* * * * *