U.S. patent application number 11/454570 was filed with the patent office on 2006-12-21 for foam-paperboard laminates, articles incorporating same and methods of making the same.
This patent application is currently assigned to Fort James Corporation. Invention is credited to Gregory J. Anderson, Rana Shehadeh, Anthony J. Swiontek, Dean P. Swoboda.
Application Number | 20060286325 11/454570 |
Document ID | / |
Family ID | 37156052 |
Filed Date | 2006-12-21 |
United States Patent
Application |
20060286325 |
Kind Code |
A1 |
Swoboda; Dean P. ; et
al. |
December 21, 2006 |
Foam-paperboard laminates, articles incorporating same and methods
of making the same
Abstract
The present invention relates to foam-paperboard laminates
(foam-paperboard laminates) prepared using an in situ foaming
process. The foam-paperboard laminates of the present invention are
formed by extruding LDPE polymer onto a moisture containing
paperboard to provide a LDPE-coated paperboard material. Upon
heating of the LDPE-coated paperboard, the moisture in the
paperboard causes steam to act as a blowing agent for the LDPE and
a LDPE foam is obtained. The foam is adhered to the paperboard by
way of physical adhesion of the polymer to the paperboard. The
foam-paperboard laminates of the present invention exhibit
insulating and cushioning properties. The foam-paperboard laminates
are suitable for use, for example, in insulating beverage cups,
food service containers, packaging material, and in other products
where laminates can be useful.
Inventors: |
Swoboda; Dean P.; (DePere,
WI) ; Shehadeh; Rana; (Atlanta, GA) ;
Swiontek; Anthony J.; (Neenah, WI) ; Anderson;
Gregory J.; (Appleton, WI) |
Correspondence
Address: |
PATENT GROUP GA030-43;GEORGIA-PACIFIC CORPORATION
133 PEACHTREE STREET, N.E.
ATLANTA
GA
30303-1847
US
|
Assignee: |
Fort James Corporation
Atlanta
GA
30303
|
Family ID: |
37156052 |
Appl. No.: |
11/454570 |
Filed: |
June 16, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60691404 |
Jun 17, 2005 |
|
|
|
60705605 |
Aug 4, 2005 |
|
|
|
Current U.S.
Class: |
428/36.5 |
Current CPC
Class: |
B32B 2038/0084 20130101;
B32B 27/10 20130101; B32B 5/18 20130101; B32B 27/32 20130101; B32B
2266/025 20130101; B32B 2250/44 20130101; D21H 27/34 20130101; B32B
3/04 20130101; B32B 29/007 20130101; B32B 2317/12 20130101; B32B
2307/304 20130101; Y10T 428/1376 20150115; B32B 2323/046 20130101;
B65D 81/3874 20130101; B32B 2439/70 20130101; B32B 2307/581
20130101; D21H 19/70 20130101; D21H 27/10 20130101; B32B 2305/022
20130101; B29C 44/12 20130101; B32B 2307/744 20130101; B32B
2305/022 20130101; B32B 2323/046 20130101 |
Class at
Publication: |
428/036.5 |
International
Class: |
B65D 39/00 20060101
B65D039/00 |
Claims
1) A LDPE foam-paperboard laminate comprising: a) a first layer
comprising a foam prepared from LDPE having a melt index of from
about 8.0 to about 20.0 g/10 min as measured by ASTM 1298; b) a
second layer comprising paperboard, wherein the laminate is
prepared in an in situ process.
2) The LDPE foam-paperboard laminate of claim 1, wherein the foam
layer comprises foam cells, wherein at least about 90% of the foam
cells have an aspect ratio of 2.0 or greater.
3) The foam-paperboard laminate of claim 1, wherein the laminate
comprises a third layer comprising an occlusive layer disposed on a
surface of the paperboard opposite to the foam layer.
4) The foam-paperboard laminate of claim 1, wherein the LDPE has a
melt index of from about 10.0 to about 20.0 g/10 min.
5) The foam-paperboard laminate of claim 1, wherein the LDPE is
present on the paperboard at less than about 30 pounds per ream of
paperboard.
6) The foam-paperboard laminate of claim 1, wherein the LDPE is
present on the paperboard at less than about 25 pounds per ream of
paperboard.
7) The foam-paperboard laminate of claim 2, wherein the foam layer
has a caliper of less than about 15 mils.
8) The foam-paperboard laminate of claim 2, wherein the foam layer
has a caliper of less than about 10 mils.
9) The foam-paperboard laminate of claim 2, wherein at least about
95% of the foam cells have an aspect ratio of 2.0 or greater.
10) The foam-paperboard laminate of claim 1, wherein the foam layer
comprises foam cells, wherein the foam cells have an average MD
ratio of less than about 1.5.
11) An insulated container prepared from the foam-paperboard
laminate of claim 1.
12) A packaging material prepared from the foam-paperboard laminate
of claim 1.
13) A slip-resistant material prepared from the foam-paperboard
laminate of claim 1.
14) A LDPE foam-paperboard laminate comprising: a) a first layer
comprising a foam prepared from LDPE having a melt index of from
about 8.0 g/10 min to about 20.0 g/10 min as measured by ASTM 1298;
b) a second layer comprising paperboard, wherein the foam layer
comprises foam cells, wherein at least about 90% of the foam cells
have an average MD ratio of less than about 1.5, and wherein the
laminate is prepared in an in situ process.
15) A method of making a foam paperboard laminate comprising: a)
providing paperboard having an outer and an inner layer, wherein
the paperboard has a moisture content of from about 4 to about 10%;
b) providing a melted LDPE having a melt index of at least about
8.0 g/10 min as measured by ASTM 1298; c) applying the melted LDPE
to the outer surface of the paperboard using an extruder, thereby
providing a LDPE-coated paperboard material; and d) heating the
paperboard material at from about 220.degree. F. to about
330.degree. F. for at least about 30 seconds thereby resulting in
release of the moisture from the paperboard in the form of steam,
wherein the amount of steam is sufficient to provide a LDPE foam,
and wherein the foam is adhered to the paperboard surface, thereby
providing a foam-paperboard laminate.
16) The method of claim 15, wherein an occlusive layer is applied
to an inner surface of the paperboard material prior to heating of
the paperboard material.
17) The method of claim 16, wherein the occlusive layer comprises a
polyolefin.
18) The method of claim 16, wherein the occlusive layer comprises
LDPE, HDPE or a blend thereof.
19) The method of claim 15, wherein the melted LDPE is applied to
the paperboard at less than about 30 pounds per ream.
20) The method of claim 15, wherein the melted LDPE is applied to
the paperboard at less than about 25 pounds per ream.
21) The method of claim 15, wherein the extruder has an air gap
distance of from about 9 to about 20 inches.
22) The method of claim 15, wherein the extruder has a die gap
distance of from about 10 to about 25 mils.
23) The method of claim 15, wherein the LDPE is melted at from
about 575 to about 625.degree. F.
24) The method of claim 15, wherein the extrusion step is conducted
at a web speed of greater than about 325 feet per minute.
25) The method of claim 15, wherein the web speed is from about 350
feet per minute to about 900 feet per minute.
26) The method of claim 15, wherein the heating step is conducted
for from about 45 to about 180 seconds.
27) The method of claim 15, wherein the heating step is conducted
at from about 250 to about 325.degree. F.
28) The method of claim 15, wherein the foam layer comprises foam
cells, wherein at least about 90% of the foam cells have an aspect
ratio of 2.0 or greater.
29) The method of claim 15, wherein the foam layer comprises foam
cells, wherein at least about 95% of the foam cells have an aspect
ratio of 2.0 or greater.
30) The method of claim 15, wherein the foam layer comprises foam
cells and wherein at least about 90% of the foam cells have an MD
ratio of greater than about 1.5.
31) The method of claim 28, wherein the foam layer has a caliper of
about 15 mils or less.
32) The method of claim 28, wherein the foam layer has a caliper of
about 10 mils or less.
33) The method of claim 15, wherein the LDPE is applied at a coat
weight of from about 18 to about 27 pounds per ream.
34) A method of making a LDPE foam-paperboard laminate comprising:
a) providing paperboard having an inner and an outer surface,
wherein the paperboard has a moisture content of from about 4 to
about 10%; b) providing a melted LDPE having a melt index of at
least about 5.0 g/10 min as measured by ASTM 1298; c) applying the
melted LDPE to the outer surface of the paperboard using an
extruder, wherein the LDPE is applied at a web speed of at least
about 325 feet per minute, thereby providing a LDPE-coated
paperboard material; d) heating the LDPE-coated paperboard material
at from about 220.degree. F. to about 330.degree. F. for at least
about 30 seconds thereby providing moisture release from the
paperboard in the form of steam, wherein the amount of steam is
sufficient to result in foaming of the LDPE, and wherein the foam
is adhered to the paperboard surface the paperboard material,
thereby providing a foam-paperboard laminate comprising foam cells,
wherein at least about 90% of the foam cells have an aspect ratio
of about 2.0 or greater.
35) The method of claim 34, wherein the LDPE has a MI of at least
about 8.0 g/10 min as measured by ASTM 1298.
36) The method of claim 34, wherein an occlusive layer is applied
to the inner surface of the paperboard prior to the heating
step.
37) The method of claim 36, wherein the occlusive layer comprises a
polyolefin.
38) The method of claim 36, wherein the occlusive layer comprises
LDPE, HDPE or a blend thereof.
39) The method of claim 34, wherein the foam layer has a caliper of
about 15 mils or less.
40) The method of claim 34, wherein the foam layer has a caliper of
about 10 mils or less.
41) The method of claim 34, wherein the extruder has an air gap
distance of from about 9 to about 20 inches.
42) The method of claim 34, wherein the extruder has a die gap
distance of from about 10 to about 25 mils.
43) The method of claim 34, wherein the melt temperature of the
LDPE is from about 575 to about 625.degree. F.
44) The method of claim 34, wherein the LDPE is applied to the
paperboard at from about 18 to about 27 pounds per ream.
45) The method of claim 34, wherein the web speed is from about 350
feet per minute to about 900 feet per minute.
46) The method of claim 34, wherein the heating is conducted for
from about 45 to about 180 seconds.
47) The method of claim 34, wherein the heating is conducted at
from about 250 to about 325.degree. F.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional
Application Nos. 60/691,404, filed Jun. 17, 2005 and 60/705,605,
filed Aug. 4, 2005. The disclosures of each of these applications
are incorporated herein in their entireties by this reference.
FIELD OF THE INVENTION
[0002] The present invention relates to foam-paperboard laminates
prepared using an in situ foaming process. The foam-paperboard
laminates of the present invention are formed by extruding LDPE
polymer onto a moisture-containing paperboard to provide a
LDPE-coated paperboard material. Upon heating of the LDPE-coated
paperboard, the moisture in the paperboard causes steam to act as a
blowing agent for the LDPE and a LDPE foam is obtained. The foam is
adhered by way of physical adhesion of the polymer to the
paperboard. The foam-paperboard laminates of the present invention
exhibit insulating and cushioning properties. The foam-paperboard
laminates are suitable for use in, for example, insulated beverage
cups, food service containers, packaging materials, and in other
products where insulating and/or cushioning laminate materials can
be useful.
BACKGROUND OF THE INVENTION
[0003] Foam-paperboard laminates prepared using in situ foaming
processes are known for use in insulated beverage cups that are
sold commercially as PerfecTouch.RTM. by the assignee of the
present invention. The basic technology used to prepare the foam
surface of the cups is disclosed in U.S. Pat. No. 4,435,344 to
Iioka, the disclosure of which is incorporated herein in its
entirety by this reference.
[0004] To make this product, generally, the beverage cup is
fabricated to include a bottom panel and a sidewall, both of which
are made of paperboard material. Prior to application of the bottom
panel, LDPE is extruded to one surface of the sidewall material. A
blend of LDPE and HDPE is extruded to the other side of the
sidewall material. Foaming of the LDPE-coated outer sidewall
surface is carried out in situ by placing the unfoamed beverage cup
in an oven and heating it above the melting point of the outer LDPE
coating. The moisture within the paperboard causes steam to form
and, since steam occupies more volume than liquid water, pressure
is created, thus providing a foaming action on the LDPE-coated
outer cup surface. The HDPE/LDPE layer on the inside of the cup
prevents steam from escaping toward the interior of the beverage
cup to result in preferential foaming of the outer layer. Cups made
in this manner exhibit superior insulating properties, especially
with respect to hot liquids such as coffee, tea, and the like. The
cups are also suitably used for cold beverages.
[0005] Refinements in the manufacturing of the in situ foaming
process are disclosed in U.S. Pat. No. 5,993,705 to Grishchenko et
al. (incorporated by reference herein). In the '705 patent,
containers are conveyed through an oven on a conveyor to cause a
foamable material on the container to foam and become a heat
insulative layer. The containers are supported on respective
holders of the conveyor which prevent the containers from
contacting one another while the outer coatings on the container
walls are foamed. See also U.S. Pat. No. 6,328,557 to Grishchenko
et al. (incorporated by reference herein).
[0006] While PerfecTouch insulated cups have been manufactured for
a number of years, recently this product has experienced
significantly increased demand. In attempting to meet this demand,
it was determined that using existing processes and materials,
suitable foam qualities could not be obtained using web speeds of
greater than about 300 feet per minute.
[0007] It was thus determined that it would be desirable to develop
improved methods to conduct an in situ foaming process more
efficiently. Still further, it was determined that it would be
desirable to obtain suitable foam quality using less polymer than
was possible using previous processes and materials. The present
invention meets these objectives, as well as others.
SUMMARY OF THE INVENTION
[0008] In one aspect, the present invention provides
foam-paperboard laminates prepared from LDPE having a MI of greater
than 8 to about 20 g/10 min, as measured by ASTM 1298. In a further
form, the foam aspect of the foam-paperboard laminates of the
present invention is prepared by one or more methods of reducing
the orientation of an extruded LDPE coated on the paperboard. The
foam is obtained by first extruding the LDPE onto the paperboard to
provide a LDPE-coated paperboard material. The material is then
heated to provide steam release from the paperboard. The steam
operates as a blowing agent and causes the LDPE to foam in situ.
Upon completion of the foaming process, the LDPE foam is physically
adhered to the paperboard to provide a foam-paperboard laminate.
The foam-paperboard laminates of the present invention are suitable
for use in insulated beverage cups, packaging materials, as well as
many other products. Using LDPE as specified herein, it has been
found possible to manufacture foam-paperboard laminates at
significantly higher speeds than previously allowable. Moreover,
the characteristics of the foam prepared in accordance with the
present invention are significantly improved over foams prepared
using prior art methods.
[0009] Additional advantages of the invention will be set forth in
part in the description that follows, and in part will be obvious
from the description, or may be learned by practice of the
invention. The advantages of the invention will be realized and
attained by means of the elements and combinations particularly
pointed out in the appended claims. It is to be understood that
both the foregoing general description and the following detailed
description are exemplary and explanatory only and are not
restrictive of the invention, as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The patent or application file contains at least one drawing
executed in color. Copies of this patent or patent application
publication with color drawing(s) will be provided by the Office
upon request and payment of the necessary fee.
[0011] The invention is described in detail below with reference to
the Figures, wherein like numbers designate similar parts and
wherein:
[0012] FIG. 1 is a schematic view in elevation and section of an
insulated beverage cup prepared from a foam-paperboard laminate of
the present invention;
[0013] FIG. 1a is a detail of the insulated beverage cup of FIG. 1
showing the various layers schematically in the area where the
bottom panel is joined to the paperboard sidewall.
[0014] FIG. 2 is a schematic diagram illustrating coating of
paperboard with a LDPE using a slit die apparatus.
[0015] FIG. 2a is a schematic view of a foam-paperboard laminate in
accordance with an aspect of the present invention comprising an
occlusive layer on an interior side, and a LDPE foam layer on the
other side.
[0016] FIG. 3 is a photomicrograph in section of a foam-paperboard
laminate where LDPE having a melt index of 5.7 g/10 min was applied
to the paperboard at a web speed of 200 ft/min.
[0017] FIG. 4 is a photomicrograph in section of a foam-paperboard
laminate where LDPE having a MI of 5.7 g/10 min was applied to the
paperboard at a web speed of 450 ft/min, where the extrusion
conditions were not optimized.
[0018] FIG. 5 is a photomicrograph in section of a foam-paperboard
laminate where LDPE having a MI of 5.7 g/10 min was applied to the
paperboard web speed of 200 ft/min.
[0019] FIG. 6 is a photomicrograph in section of a foam-paperboard
laminate where LDPE having a MI of 5.7 g/10 min was applied to the
paperboard at a web speed of 450 ft/min where the extrusion
conditions were not optimized.
[0020] FIG. 7 is a plot of polymer foam caliper versus coat weight
in pounds/ream of LDPE after foaming for 5.7 MI LDPE for various
melt temperatures and extrusion speeds.
[0021] FIG. 8 is a plot of polymer foam caliper versus coat weight
in pounds/ream of LDPE after foaming for 5.7 MI LDPE for various
melt temperatures at an extrusion speed of 300 fpm.
[0022] FIG. 9 is a plot of polymer foam caliper versus coat weight
in pounds/ream of LDPE after foaming for 5.7 MI LDPE for various
melt temperatures at an extrusion speed of 450 fpm.
[0023] FIG. 10 is a plot of polymer foam caliper versus coat weight
in pounds/ream of LDPE after foaming for 5.7 MI for various
extrusion speeds.
[0024] FIG. 11 is a plot of polymer foam caliper versus coat weight
in pounds/ream of LDPE after foaming for 5.7 MI LDPE at various
extrusion speeds.
[0025] FIG. 12 is a plot of polymer foam caliper versus coat weight
in pounds/ream of LDPE after foaming for 4.5 MI LDPE at various
extrusion speeds.
[0026] FIG. 13 is a plot of polymer foam caliper versus coat weight
in pounds/ream of LDPE after foaming for 12.0 MI LDPE at various
extrusion speeds.
[0027] FIG. 14 is a plot of average polymer foam caliper in mils
versus LDPE melt index for various MI LDPE at a web speed of 300
fpm.
[0028] FIG. 15A is a photomicrograph of a foam-paperboard laminate
produced from 5.7 MI LDPE at a web speed of 450 feet per minute
using a 40 mil die gap and 5'' air gap.
[0029] FIG. 15B is a further view of the foam-paperboard laminate
of FIG. 15A.
[0030] FIG. 16A is a photomicrograph of a foam-paperboard laminate
produced from 5.7 MI LDPE at a web speed of 450 feet per minute
using a 20 mil die gap and 9'' air gap.
[0031] FIG. 16B is a further view of the foam-paperboard laminate
of FIG. 16A.
[0032] FIG. 17A is a photomicrograph of a foam-paperboard laminate
produced from 5.7 MI LDPE at a web speed of 450 feet per minute
using a 20 mil die gap, 13'' air gap and a coat weight of 22 pounds
per ream.
[0033] FIG. 17B is a further view of the foam-paperboard laminate
of FIG. 17A.
[0034] FIG. 18A is a photomicrograph of a foam-paperboard laminate
produced from 13.7 MI LDPE at a web speed of 450 feet per minute
using a 20 mil die gap, 13'' air gap and a coat weight of 24 pounds
per ream.
[0035] FIG. 18B is a further view of the foam-paperboard laminate
of FIG. 18A.
[0036] FIG. 19A is a photomicrograph of a foam-paperboard laminate
produced from 4.5 MI LDPE at a web speed of 450 feet per minute
using a 20 mil die gap, 9'' air gap and a coat weight of 22 pounds
per ream.
[0037] FIG. 19B is a further view of the foam-paperboard laminate
of FIG. 19A.
[0038] FIG. 19A is a photomicrograph of a foam-paperboard laminate
produced from 12.0 MI LDPE at a web speed of 450 feet per minute
using a 20 mil die gap, 13'' air gap and a coat weight of 26 pounds
per ream.
[0039] FIG. 20B is a further view of the foam-paperboard laminate
of FIG. 20A.
[0040] FIGS. 21A and 21B are laser-scanned surface images of
various foam-paperboard laminates. FIGS. 22A and 22B are angular
plots of isotropy for the samples of FIGS. 21A and 21B.
[0041] FIG. 23 is a plot of polymer foam caliper for 13.7 MI LDPE
at various melt temperatures and extrusion speeds at an air gap of
11'' and a die gap of 20 mils.
[0042] FIG. 24 is a plot of polymer foam caliper for various MI
LDPE at various extrusion speeds.
[0043] FIG. 25 is a plot of polymer foam caliper for 5.7 MI LDPE at
various oven temperatures and residence times.
[0044] FIG. 26 is a plot of polymer foam caliper for 12.0 MI LDPE
at various oven temperature and residence times.
[0045] FIG. 27 is a plot of polymer foam caliper for 13.7 MI LDPE
at various oven temperatures and residence times.
[0046] FIG. 28 is a plot of polymer foam caliper for 5.7 MI LDPE at
various oven temperatures and residence times, where the oven
temperatures are higher than those in FIG. 25.
[0047] FIG. 29 is a plot of polymer foam caliper for 12.0 MI LDPE
at various oven temperatures and residence times, where the oven
temperatures are higher than those in FIG. 26.
[0048] FIG. 30 is a plot of polymer foam caliper for 13.7 MI LDPE
at various oven temperatures and residence times, where the oven
temperatures are higher than those in FIG. 27.
[0049] FIG. 31 is a comparison of polymer foam caliper visual
characteristics and caliper for 12.0 MI LDPE at various oven
temperatures and residence times.
DETAILED DESCRIPTION OF THE INVENTION
[0050] The present invention may be understood more readily by
reference to the following detailed description of the invention
and the examples provided herein. Before the present invention is
disclosed and described, it is to be understood that the aspects
described below are not limited to specific methods or materials
discussed, as such may, of course, vary. It is also to be
understood that the terminology used herein is for the purpose of
describing particular aspects only and is not intended to be
limiting.
[0051] As used herein terminology has its ordinary meaning.
Exemplary definitions as to terminology used in this patent are
given below.
[0052] Often, ranges are expressed herein as from "about" one
particular value, and/or to "about" another particular value. When
such a range is expressed, another embodiment includes from the one
particular value and/or to the other particular value. Similarly,
when values are expressed as approximations, by use of the
antecedent "about," it will be understood that the particular value
forms another embodiment. It will be further understood that the
endpoints of each of the ranges are significant both in relation to
the other endpoint, and independently of the other endpoint.
[0053] "Melt index" is measured according to ASTM 1298 and has the
units g/10 mins. For brevity, melt index may be abbreviated herein
as "MI." The stated melt index may also be presented without the
units; however, it should be understood that when presented without
units, the stated melt index has the units g/1 0 mins and the melt
index is measured in accordance with ASTM 1298.
[0054] "Melt temperature" refers to the temperature at the
extrusion die used to apply a coating to paperboard.
[0055] "Per ream" means per 3000 square feet of paperboard, which
is a common measurement used by one of ordinary skill in the
art.
[0056] Unless otherwise specified, an ASTM test method referred to
means the version in effect as of Jun. 1, 2005 unless specifically
stated otherwise. Each ASTM referenced in this application is
incorporated herein in its entirety by this reference.
[0057] Foam thickness of a foam-paperboard laminates is determined
by measuring overall caliper of a foam-paperboard laminates. As
used herein, the foam thickness is presented as a total thickness
(in mils) of the foam layer, the paperboard layer and the occlusive
layer on the inner surface of the paperboard. The paperboard used
for all examples herein was about 15 mils. The thickness of the
occlusive layer was negligible. Accordingly, a thickness reported
as, for example, about 25 mils, will have a paperboard thickness of
about 15 mils and a LDPE thickness of about 10 mils. Overall
caliper is generally measured on at least 5 locations on a sample
and averaged.
[0058] "LDPE" means "Low Density Polyethylene." LDPE is also known
as "high pressure polyethylene" because it is typically produced at
pressures ranging from 82-276 MPA (800-2725 atm). LDPE is generally
produced in either a tubular or stirred autoclave reactor.
Traditionally, LDPE has been defined as a homo-polymer having a
density from about 0.915 and 0.940 g/cm.sup.3; however comonomers
are sometimes used in LDPE products (products having a density
above 0.940 g/cm.sup.3 are considered HDPE).
[0059] HDPE means "High Density Polyethylene." HDPE is defined by
ASTM D 1248-84 as a product of ethylene polymerization with a
density of 0.940 g/cm.sup.3 or higher. This range includes both
homo-polymers of ethylene and its copolymers with small amounts of
alpha-olefins.
[0060] Further information concerning polyethylene polymers may be
found in the Kirk-Othmer Encyclopedia of Chemical Technology,
4.sup.th edition, Volumes 17 and 19 (Wiley 1996).
[0061] "Mils" means thousandths of an inch.
[0062] "SEM" means scanning electron micrograph.
[0063] "FPM" means feet per minute.
[0064] As would be recognized by one of ordinary skill in the art,
foam-paperboard laminates not made using an in situ process are
generally made by first foaming LDPE to provide a foam structure.
This foam structure is then separately adhered, such as with an
adhesive, to the paperboard to provide a laminate. Such foaming
uses a traditional blowing agent, such as a gas (e.g. a hydrocarbon
gas or CO.sub.2) or chemical blowing agent to prepare the foam.
Significantly more control of the foaming process is found with
such a foam manufacturing method.
[0065] In contrast, the primary mechanism for producing foam in an
in situ process is the creation of steam by way of evaporation of
water from the paperboard. This in situ method has been found by
the inventors herein to require a careful balance between LDPE
properties, the extruded coating properties and the paperboard
properties to provide a foam-paperboard laminates having a
satisfactory foam as discussed further herein where the foam is
suitably adhered to the surface of the paperboard without
application of a separate adhesive.
[0066] In order to prepare the foam aspect of the foam-paperboard
laminate of the present invention, LDPE is extruded onto a
paperboard material having a certain amount of moisture therein.
The foam-paperboard laminate is then placed into an oven or other
type of heating system. During heating, the polymer softens and the
moisture in the paperboard turns into steam. The steam causes the
LDPE coating to soften and the steam acts as a blowing agent to
deform the LDPE coating to provide foam cells.
[0067] The inventors herein have determined that the properties of
the LDPE suitable to provide satisfactory foam formation are
significantly different from the properties required for general
applications of extruded LDPE coated paperboard structures, such as
those used in packaging applications. In particular, the inventors
herein have determined that the properties of the extruded LDPE
coating are highly significant to obtaining satisfactory LDPE
foams. The properties of the LDPE coating in the present invention
are determined by the properties of the LDPE itself, and also by
processing parameters as discussed further herein.
[0068] As noted previously, the assignee of the present invention
has manufactured insulated beverage cups using an in situ foaming
process for some time. However, when sales volume for this product
increased recently, considerable difficulty was experienced in
obtaining higher volumes of foam-paperboard laminates that are the
major component of the insulated beverage cups. Initial attempts to
increase the volume of cups manufactured were somewhat
unsuccessful. That is, when the extrusion speed was increased while
keeping all other variables the same (e.g. LDPE MI, extrusion
parameters, oven temperature etc.), unsatisfactory product quality
resulted. Through experimentation, which is described herein in
detail, it was determined that the polymeric properties needed to
obtain satisfactory LDPE foam properties did not match the
properties understood to result in good extruded LDPE coatings.
[0069] As would be understood by one of ordinary skill in the art,
low melt index LDPE polymers are used to prepare extruded coatings;
higher melt index polymers, which are generally "softer" polymers
and exhibit better flow properties, are used for injection molding
applications. One of ordinary skill in the art would normally not
seek to use a higher melt index polymer for extrusion coating
because the resulting coating would be thought to be too soft to
provide an article with useful properties.
[0070] While the foam-paperboard laminates of the present invention
incorporate an extruded LDPE polymer coating, further processing of
this extruded coating is necessary to provide the foam aspect of
the foam-paperboard laminates. Prior art would thus dictate that a
lower melt index polymer should be used to prepare an extruded LDPE
coating. However, when run at high extrusion speeds (which was
necessary to meet the increased volume for insulated beverage
cups), low melt index LDPE did not provide suitable foaming. The
resulting foam, for example, exhibited low aspect ratio and poor
adhesion.
[0071] The investigation conducted by the inventors herein showed
that the properties desired to provide a good extrusion coating had
to be balanced with polymer properties to provide suitable foam.
These properties were further influenced by the limitations of the
in situ foaming process, in which the blowing agent effectiveness
is a function of the amount of moisture in the board. The adhesion
of the extruded coating during and after the foaming process was
also relevant to the resulting foam quality, which added an
additional variable to the objective of obtaining good results. Put
simply, the inventors herein determined that there were multiple
variables that affected the ability to obtain foam-paperboard
laminates as described and claimed herein.
[0072] The present invention provides foam-paperboard laminates
prepared using an in situ foaming process. Generally, in this
process, a LDPE polymer is extruded onto a paperboard material
having a suitable amount of moisture to provide an LDPE-coated
paperboard material. This material is then placed into an oven,
whereby the moisture in the paperboard turns to steam. This steam
then acts as a blowing agent for the LDPE to provide LDPE foam.
Upon completion of the foaming process, the foam is adhered to the
paperboard surface to provide the foam-paperboard laminates of the
present invention.
[0073] The inventors herein have found that in order to obtain the
foam-paperboard laminates of the present invention, the extruded
LDPE coating must comprise a coating that is "soft" enough to
provide foam with a suitable aspect ratio. After foaming, the
extruded LDPE foam must also be suitably adhered to the paperboard
surface to provide good foam quality.
[0074] It has been surprisingly determined that when the
foam-paperboard laminates of the present invention are prepared
from an extruded LDPE as discussed herein, along with a paperboard
material having a suitable amount of moisture included therein, the
inventive foam-paperboard laminates can be obtained using
significantly higher speed operations than that possible
previously. Moreover, the foam aspect of the foam-paperboard
laminates of the present invention is of a better quality and
consistency than obtainable using the prior art in situ foaming
process disclosed in U.S. Pat. No. 4,435,344 (previously
incorporated by reference), the disclosure of which is incorporated
herein in its entirety by this reference. The improved quality of
the foam obtained herein allows the foam-paperboard laminates to
provide better insulation properties. Further, the laminate surface
is more consistent in appearance and quality. The foam-paperboard
laminates of the present invention are therefore more aesthetically
pleasing than foam-paperboard laminates obtainable previously.
Still further, since the foam aspect of the foam-paperboard
laminates of the present invention is improved in consistency, it
is expected that the cushioning properties of the foam-paperboard
laminates of the present invention will also be improved.
[0075] In view of the above, the inventors herein determined that
to obtain suitable foam using the in situ process, it was necessary
to provide an LDPE coating wherein the molecular orientation of the
polymer was minimized. However, in the attempt to increase the
manufacturing volume of foam-paperboard laminates so as to provide
insulated beverage cups, it was found that speeding up of the web
speed without modification of any other variables resulted in
increased orientation of the LDPE, which, in turn, reduced the
ability of the LDPE to foam and/or suitably adhered to the surface
of the paperboard after foaming. It was thus determined that in
order to provide a suitable foam on a foam-paperboard laminates, it
would be necessary to ensure that the LDPE coating was not oriented
to a degree that would prevent the coating from suitably foaming
under the conditions of the in situ foaming process, which is in
itself limited.
[0076] In a first aspect, the inventors determined that, in
contrast to the low melt index LDPE typically used to prepare
extruded LDPE coatings, foam-paperboard laminates could be suitably
prepared from LDPE polymers having higher melt indices. This was
found to be due to the fact that the in situ foaming process was
adversely affected by the use of low melt index polymer when a high
speed process was conducted as a result of the increased
orientation of the LDPE when a high speed extrusion process was
conducted.
[0077] Without being bound by theory, it is believed that various
properties of the LDPE that are specific to a particular LDPE
polymer (such as melt index) can influence the amount of residual
strain of the LDPE coating prior to foaming. Still further, it is
also believed that the parameters of the extrusion process
influence the amount of residual strain of the LDPE coating prior
to foaming. It is believed that such higher residual strain
inhibits foaming (e.g., the LDPE is too oriented to allow foaming
under the conditions of the in situ foaming process).
[0078] Since it was determined that a higher melt index gives a
better quality foam under the in situ foaming process, in one
aspect, the present invention pertains to foam-paperboard laminates
prepared from LDPE having a MI of from greater than 8.0 to about 20
g/10 min, as measured by ASTM 1298. In a further aspect, the LDPE
of the present invention consists essentially of a MI of greater
than 8.0 to about 20 g/10 min, as measured by ASTM 1298. LDPE
having such MI's have been found to orient less substantially when
the extrusion process is conducted at speeds greater than about 350
feet per minute. As discussed, this lesser orientation is believed
to result in the formation of a higher foam quality in the in situ
process of the present invention.
[0079] Still further, foam-paperboard laminates of the present
invention are prepared from a LDPE having a MI of greater than 8.0,
8.5, 9.0, 9.5 or 10.0 g/10 min., as measured by ASTM 1298. Yet
further, the foam-paperboard laminates of the present invention are
prepared from a LDPE having a MI of about 8.5, 9.0, 10.0. 11.0.
12.0, 13.0, 14.0, 15.0, 16.0, 17.0, 18.0, 19.0 or 20.0 g/10min, as
measured by ASTM 1298, where any of the stated values can comprise
an upper or a lower endpoint, as appropriate. Such polymers are
available, for example, from Westlake Chemical (Houston, Tex.).
[0080] In preparing the foam-paperboard laminates of the present
invention, a melted LDPE having a MI conforming to the specified
range set out herein is extruded onto a paperboard substrate. The
extrusion melt temperatures suitable for the present invention are
discussed in more detail herein.
[0081] It is also believed that other polyethylene materials that
exhibit suitable molecular orientation to allow preparation of a
foam that can be used in the invention herein. In this regard,
substantially linear ethylene polymers may be used as the extruded
polyethylene material. Such polymers are disclosed in U.S. Pat.
Nos. 5,272,236 and 5,278,272, the disclosures of which are herein
incorporated in their entireties by this reference. Suitable
examples of substantially linear ethylene polymers are available
from Dow Global Technologies (Freeport, Tex.).
[0082] A particularly suitable paperboard substrate for use in the
present invention is solid bleached sulfate ("SBS"). This
paperboard is available as "cupstock" or "platestock" from
Georgia-Pacific Corporation (Atlanta, Ga.). SBS exhibits good
surface properties (e.g., smoothness) such that the inventors
herein have found the adhesion between the foam and the paperboard
to be exceedingly good.
[0083] Other types of paperboard are also currently thought to be
suitable to prepare the foam-paperboard laminates of the present
invention, as long as such paperboard products comprise suitable
amounts of moisture to act as a blowing agent for the LDPE (as
discussed in more detail herein) and has suitable surface
properties to provide adequate adhesion between the foam and the
paperboard surface. For example, coated unbleached Kraft paperboard
could be used in the present invention. Still further, recycled
paperboard (either or both of pre- or post-consumer recycled) could
also be suitably be used.
[0084] In one aspect, the LDPE contains no added adhesive material,
such as a maleic anhydride graft copolymer or other type of
adhesive polymer. As such, the adhesion of the LDPE to the
paperboard occurs by way of adhesion of the LDPE coating to the
paperboard. This is a physical attachment of the extruded LDPE to
the surface of the paperboard by the LDPE component itself. To
provide suitable adhesion, it is believed that the paperboard
surface must be somewhat rough to provide adequate points of
attachment of the LDPE. Some surface roughness is believed to
provide more surface area for contact of the LDPE coating to the
paperboard surface. However, it is believed that board surface
roughness is only one factor that influences adhesion of the LDPE
to the paperboard surface.
[0085] Although the extruded LDPE coating is attached to the
paperboard surface firmly, it will be appreciated that when foam is
generated, the LDPE will be attached to the paperboard surface in a
markedly different manner than a LDPE extrusion coating. That is,
when an extruded coating of LDPE is applied to a surface to provide
a laminate (such as in a coated packaging material), substantially
the entire inner surface of the LDPE will be coextensive with the
corresponding surface of the paperboard. When foam is prepared from
this LDPE coating, the LDPE will be attached to the paperboard in a
much less extensive manner because the air voids defining the foam
cells represent a loss of contact for the LDPE coating.
[0086] In further aspects, the adhesion of the LDPE to the
paperboard surface can be improved by applying a surface treatment
to the paperboard surface prior to extrusion of the LDPE onto the
paperboard surface. For example, the paperboard can be subjected to
a corona treatment prior to extrusion of the LDPE. Chemical
treatments that provide oxidation effects to the paperboard surface
can also be used to promote adhesion. Still further, a tie layer
can be applied to the paperboard surface to improve the adhesion of
the LDPE to the paperboard. Suitable tie layer materials can be
identified by one of ordinary skill in the art without undue
experimentation.
[0087] It is currently believed by the inventors herein that the
thickness of the paperboard material is not particularly
significant to the resulting foam qualities. Thus, it is believed
that paperboard having a wide range of thicknesses (calipers) can
be used to prepare the foam-paperboard laminates of the present
invention. As non-limiting examples, it is believed that paperboard
of from about 10 to about 50 mils, or from about 15 to about 30
mils can suitably be used herein. The thickness of the resulting
foam-paperboard laminates of the present invention will be dictated
in large part by the thickness of the paperboard used, since the
thickness of the foam-paperboard laminates comprises the sum of the
thickness of the foam and the paperboard used.
[0088] Still further, the basis weight of the paperboard suitably
used in the present invention is not believed to significantly
effect the properties of the resulting foam-paperboard laminates.
As such, the basis weight of the paperboard used is generally
dictated by the desired end use for the foam-paperboard laminates.
For example, if the foam-paperboard laminate is to be used to
prepare beverage cups, a desirable basis weight for the paperboard
is from about 100 to about 220 pounds per ream or from about 140 to
about 180 pounds per ream. If the foam-paperboard laminate is to be
used to prepare disposable plates or containers, a desirable basis
weight for the paperboard is from about 100 to about 280 pounds per
ream or from about 160 to about 220 pounds per ream. Suitable
further paperboard basis weights can be determined by one of
ordinary skill in the art without undue experimentation.
[0089] While the paperboard thickness and basis weight are not
currently believed to be significant to the properties of the
foam-paperboard laminates of the present invention, the inventors
herein have determined that the paperboard used must comprise a
suitable amount of moisture in the paperboard prior to heating so
as to allow enough steam to escape from the paperboard, so as to
operate as a blowing agent for the LDPE to result in foaming. In
some aspects, the amount of moisture is highly significant to the
qualities of the resulting foam. For SBS paperboard, it has been
found that the amount of moisture in the board should be from about
4 to about 10%, as measured by the weight of the board. Still
further, the amount of moisture in the board can be from about 5 to
about 7%, as measured by the weight of the board. Yet further, the
amount of moisture can be from about 4, 5, 6, 7, 8, 9 or 10% as
measured by weight of the board, where any value can form an upper
or a lower endpoint, as appropriate.
[0090] Any suitable extrusion equipment can be used to coat the
paperboard; for example, one suitable coating apparatus is an Egan
34 (Egan, Sommerville, N.J.) extrusion coater provided with a 36
inch wide EDI die having a multi-layer configuration with a screw
feeding a Cloeren combining block.
[0091] Prior to preparing the foam aspect of the foam-paperboard
laminates of the present invention, LDPE having the properties set
forth above is extruded onto a paperboard having the features
discussed previously. A number of parameters have been found to be
significant to provide foam-paperboard laminates having good foam
qualities. Various extrusion parameters are believed to
significantly affect the resulting orientation of the LDPE coating
prior to foaming.
[0092] In particular, the inventors herein have determined that the
significant extrusion parameters comprise at least: die gap, air
gap (h), rotational speed i.e., speed of the polymer exiting the
die (v.sub.0) and web speed (v.sub.1). These parameters have been
found to affect foaming level through the elongational strain,
.epsilon., that the polymer experiences upon extrusion, which is
believed to be due to machine direction ("MD") molecular
orientation.
[0093] Without being bound by theory, the inventors herein believe
that in order to improve foam caliper and foam cell quality, the
elongational strain of the LDPE polymer must be minimized. As noted
previously, elongational strain can be minimized through use of a
LDPE having a MI of greater than about 8.0 when the extrusion
process is conducted at speeds of greater than about 350 feet per
minute. However, it has been found to be possible to reduce the
amount of elongational strain when modifying several extrusion
parameters. As such, it was found to be possible to prepare good
foams using the in situ foaming process with LDPE having MI's of as
low as about 5.7. Further, these extrusion parameters were found to
be useful even when using LDPE having MI's of greater than about
8.0. Ratio of Web Speed to Rotational Speed ( v 1 v 0 ) .times. :
##EQU1## The ratio of screw rotational speed to web speed has been
found to affect the draw-down of the LDPE as it is exits the
extruder. In relation to the foam aspect of the foam-paperboard
laminates of the present invention, it has been found that
increasing this ratio (that is extruding a higher volume of LDPE
onto the paperboard surface while keeping web speed constant) will
increase the flexural stiffness of the LDPE layer which, in turn,
will create more resistance to foaming. Thus, the screw rotational
speed must be considered in the present invention so as to obtain a
suitable coat weight at a desired web speed to obtain good quality
LDPE foams.
[0094] Die Gap and Air Gap: The inventors have also determined that
reducing die gap reduces the draw down ratio of the LDPE and,
consequently, the elongational strain of the LDPE. Without being
bound by theory, it has been determined that increasing the air gap
allows more time for molecular relaxation prior to reaching the
extruder nip. This is believed to result in less orientation in the
polymer, which, in turn, results in better foaming under the
conditions of the in situ foaming process.
[0095] In accordance with this discovery, the air gap (the distance
between the extruder nip and the web) can be from about 9 to about
20 inches, or from about 10 to about 13 inches. Still further, the
air gap can be from 9, 10, 11, 12, 13, 14, 15, 17 or 20 inches,
where any value can serve as an upper or lower endpoint, as
appropriate.
[0096] The extruder die gap can be from about 10 to about 25 mils
or from about 13 to about 17 mils, where any value can serve as an
upper or lower endpoint, as appropriate. Yet further, the extruder
die gap can be from about 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
20, 22 or 25 mils, where any value can serve as an upper or lower
endpoint, as appropriate.
[0097] These stated die gaps and/or air gap parameters were found
to result in good foam qualities when used with LDPE having MI's of
as low as about 5.7 at web speeds of greater than about 350 feet
per minute. This is in contrast to prior art die gaps and air gaps
used for the in situ process which did not provide a good quality
foam with LDPE's having MI's of as low as about 5.7 when the web
speed was greater than about 350 feet per minute.
[0098] Without being bound by theory, it is believed that a smaller
die gap can reduce the propensity of the LDPE polymer chains to
align during the extrusion process, thereby generally increasing
the number of amorphous regions in the LDPE extruded coating. This,
in turn, is believed to result in a "softer" LDPE extruded coating,
which improves the foamability and adhesion of the LDPE polymer
when subjected to the conditions of the in situ process.
[0099] Polymer thickness: It has been found by the inventors herein
that increasing the thickness of LDPE extruded on paperboard
decreases the draw down ratio (when all other variables are kept
constant), which has been found to reduce residual polymer stress
in the extruded LDPE coating prior to foaming. It has been also
found by the inventors herein that thicker LDPE coatings exhibit
higher flexural stiffness, which generally inhibits foam formation.
These two parameters should be considered when preparing the
foam-paperboard laminates of the present invention.
[0100] Without being bound by theory, it is believed that these
opposing factors of decreasing draw down ratio and higher flexural
stiffness provides a specific coating thickness at which maximum
foam caliper can be obtained under the conditions of the in situ
foaming process. Significantly, in the in situ foaming process,
there is a maximum amount of blowing agent action that is provided
by the moisture contained within the paperboard. Put simply, there
is an upper limit of coating thickness to which the steam blowing
agent can effectively operate given the amount of moisture present
in the paperboard. The upper polymer thickness that is feasible
varies with the properties of the LDPE used and with the level of
extruded LDPE extruded coating adhesion to the paperboard
surface.
[0101] For a lower stiffness LDPE (for example, a LDPE having a MI
of greater than about 8.0 g/10 min), the extruded coat weight can
be less than about 30 pounds per ream to obtain suitable foam. In
particular, the coat weight can be less than about 27 pounds per
ream. Yet further, the coat weight is from about 15 to about 27
pounds per ream. Still further, the coat weight is at least about
15, 17, 19, 21, 23, 25 or less than about 27 pounds per ream, where
any value can serve as an upper or lower endpoint, as appropriate.
Note that the coat weight values assume that the polymer is evenly
applied to the surface of the paperboard.
[0102] These stated values for the polymer thickness for LDPE
having the stated MI values are significantly different than prior
art in situ LDPE foaming processes. In particular, the only known
commercial applications of the in situ foaming process is conducted
by the assignee of the present invention in that PerfecTouch.RTM.
beverage cups are made from this process. However, prior to the
improvements of the present invention, it believed that a LDPE coat
weight of 30 pounds per ream or greater was necessary to obtain
suitable foaming. (This process was conducted using web speeds of
less than about 300 feet per minute.)
[0103] In the present invention, it has been found that
significantly lower LDPE coat weight can be used to obtain an
excellent foam using the in situ foaming process of the present
invention even when using speeds of greater than about 350 feet per
minute. The lower polymer weight results in considerable cost
savings in the present invention. Further, the lower polymer weight
results in considerable energy savings.
[0104] Extrusion Melt Temperature: It is believed that increasing
melt temperature increases LDPE temperature at the extruder nip.
Such higher LDPE temperature is believed to result in better
adhesion of the LDPE extruded coat to the paperboard surface. This
is further thought to improve adhesion. In addition, at higher
temperatures, it is believed that there is an increase in polymer
chain relaxation and a reduction in elongational strain.
[0105] Without being bound by theory, it is believed that these
features would be expected to improve the ability of the LDPE
coating to foam due to the improvements in the adhesion of the
polymer coat prior to foaming. The melt temperature of the LDPE can
be from about 550 to about 650.degree. F. or from about 580 to
about 620.degree. F.
[0106] Web Speed: A significant aspect of the present invention is
that using LDPE having the specified characteristics and methods of
the present invention, the web speed can be run significantly
faster than in the prior art, while still obtaining an excellent
foam qualities in the foam-paperboard laminates of the present
invention. In accordance with this aspect of the invention, the web
speed is greater than about 325 feet per minute. Yet further, the
web speed is greater than about 350 feet per minute. Still further,
the web speed is from about 350 to about 900 feet per minute. The
web speed can be about 325, 350, 400, 450, 500, 550, 600, 650, 700,
750, 800, 850 or 900 feet per minute, where any value can be used
as an upper or lower endpoint, as appropriate.
[0107] Due to processing difficulties seen in the manufacture of
PerfecTouch beverage cups, it was surprising that the web speed
could be greater than 300 feet per minute. Using prior art LDPE and
in situ foaming methodology, it was thought to be necessary to keep
the web speed significantly lower. The higher speeds of one aspect
of the present invention make it possible to manufacture the
foam-paperboard laminates of the present invention in significantly
higher volumes, which greatly increases the industrial utility of
the foam-paperboard laminates of the present invention. It is
believed that by varying the parameters of the extrusion coating
process, less polymer chain alignment will result in the coating,
which, in turn, will result in a "softer" coating that will allow
better foamability.
[0108] As discussed, foam for use to prepare LDPE foam is typically
prepared by adding chemical blowing agent or physical blowing
agent. The only known use of the in situ process herein is the
PerfecTouch.RTM. beverage cups process conducted by the assignee of
the present invention. In this process, the blowing agent for the
LDPE coating results from the creation of steam (evaporation of
water from the paperboard). To provide the foam of the
foam-paperboard laminates of the present invention, there must be
enough moisture in the paperboard to create vapor pressure below
the extruded LDPE surface to cause deformation of the coating,
which manifests in the finished foam-paperboard laminates as foam.
However, the inventors herein have determined that excessive
moisture in the paperboard can increase the time required for the
paperboard to suitably heat and, in turn, for steam to form. This
wastes energy because, as would be appreciated, if more heat is
applied to the system, more energy is required.
[0109] With regard to the foaming process itself, the LDPE-coated
paperboard material having the stated amount of moisture are placed
in an oven to provide the foam. The oven temperatures can be from
about 200 to about 330.degree. F. Yet further, the oven
temperatures can be from about 245 to about 275.degree. F.
[0110] The residence time (that is, the time the LDPE-coated
laminate is subjected to heat so as to provide the foam) can be
from about 30 to about 120 seconds. Yet further, the residence time
can be 60 to about 90 seconds. In order to provide a high speed
generation of the foam, it can be desirable to decrease the
residence time of the LDPE-coated paperboard material. Keeping all
variables the same, the residence time can be decreased by
increasing the oven temperature. However, as discussed elsewhere
herein, while higher oven temperatures result in a quicker
formation of steam and, accordingly, a faster activation of the
coating, if the LDPE-coated paperboard material is heated too
quickly, the resulting foam quality often will be unsuitable. Thus,
the oven temperature must be moderated so as to provide suitable
quality foam.
[0111] After application of the LDPE-coating to the paperboard,
this material can be used immediately to prepare the
foam-paperboard laminate of the present invention. Alternatively,
the LDPE-coated paperboard can be stored for later preparation of
the foam-paperboard laminate as long as the paperboard retains or
is provided with suitable moisture prior to the foaming
process.
[0112] It should be noted that the above-stated percentages of
moisture are relevant when the paperboard surface opposite the
LDPE-coated paperboard surface is provided with an occlusive layer.
For example, when insulated beverage cups are prepared from the
foam-paperboard laminates of the present invention, an occlusive
layer of a polyolefin, such as a blend of LDPE and HDPE, can be
extruded onto one side of the paperboard. This HDPE-coated inner
surface will become the side of the cup that is in contact with the
liquid during use. During the heating process in which the foam is
formed, the HDPE layer prevents the moisture in the paperboard from
exiting the paperboard through the HDPE-coated surface. A pressure
build up also happens at this time. As such, when the inner surface
of the paperboard is so coated with an occlusive layer (which can
also be other polymeric material, wax, etc.), the previously stated
amount of moisture (e.g. about 4 to about 10% by weight of the
board) has been found to provide suitable foaming.
[0113] The individual foam cells comprising the foam-paperboard
laminates of the present invention can be from about 40 to about
150 square microns on average. Still further, the individual foam
cells comprising the foam-paperboard laminate of the present
invention can be from about 40, 50, 60, 70, 80, 90, 100, 110, 120,
130, 140, 150, 160, 170, 180, 190 or200 square microns on average,
where any value can form an upper or a lower end point, as
appropriate. Yet further, at least about 90% or at least about 95%
of the individual foam cells conform to the specified size range.
It would be understood that small foam cells are not thought to be
undesireable, as long as the aspect ratios were as set forth
herein. However, it is believed that the in situ foaming process of
the present invention would not generally allow the formation of
very small foam cells due to limitations resulting from the
characteristics of the foam blowing agent. It is expected that foam
cells falling outside of the specified range would fall on the high
end of the stated range.
[0114] In accordance with the foam-paperboard laminates of the
present invention, more than about 90% of the foam cells comprise
an elongate structure extending away from the paperboard, wherein
their aspect ratio is greater than about 2, with less than about
10% of the cells are in the form of large cells having that have
aspect ratios of less than 2. "Aspect ratio" means the ratio of the
height of the foam cells divided by the average horizontal span of
the cell over a single sample of foam-paperboard laminate. Thus, an
aspect ratio of greater than about 2 means that the height of the
foam cells is at least about 2 times the width of the cells. Still
further, 5% or less of the foam cells have aspect ratios of less
than about 2.
[0115] Further suitable foams can have a regular microstructure
wherein the cells have an average MD ratio of less than about 1.5
or less than about 1.25. MD ratio is the length of the cell in the
machine direction as observed by surface SEM, divided by the length
of the cell in the cross machine direction as observed by surface
SEM.
[0116] Unrestrained MD linear thermal shrinkage resistance at
110.degree. C. is determined in accordance with ASTM test method,
ASTM 2732-03, except that thermal shrinkage resistance at
110.degree. C. is reported as a percent of the original length of
the specimen. For example, a specimen having a 100 mm length prior
to immersion in the heated bath and a MD length of 90 mm after
immersion for 10 seconds has an unrestrained MD linear thermal
shrinkage resistance of about 90%. This parameter is believed
significant in connection with foam quality of foams produced from
the extruded LDPE as will be appreciated from the discussion which
follows hereinafter. It will be appreciated also from the data that
follows that the shrinkage resistance is sensitive to the speed at
which the board web is traveling in the machine direction as well
as other variable as the LDPE coating is applied. In general, a MD
shrink value of from about 80% to up to about 100% provides a good
quality foam (where 100% represents no shrinkage and, therefore,
substantially no orientation in the extruded LDPE coating).
[0117] A sample of foam-paperboard laminate for shrink resistance
testing may be obtained by placing a Kevlar or other LDPE coating
on the paperboard prior to extrusion so that a foam sample can
readily separate from the paperboard. Alternatively, a sample may
be removed from coated board by sulfuric acid solution. If a sample
is to be obtained by way of sulfuric acid solution, a MD specimen
is cut using a 6 inch by 1/2 inch die cutter. The paperboard is
split in half exposing the paper fibers and only the half that has
the extruded LDPE is retained. The sample is placed in an 800 ml
beaker covered with 72 percent H.sub.2SO.sub.4 aqueous solution and
let stand for five to eight hours with occasional stirring. The
sample is washed with tap water and dried prior to shrink
resistance testing.
[0118] The foam surface of the foam-paperboard laminates of the
present invention can be printed with ink or mineral oil, for
example, in order to provide designs which are attractive and can
mimic, for example, embossments or debossments. In this respect,
attention is drawn to the following U.S. patents (the disclosures
of which are incorporated herein in there entireties by this
reference): U.S. Pat. No. 5,490,631 to Iioka et al.; U.S. Pat. No.
5,725,916 to Ishii et al.; U.S. Pat. No. 5,766,709 to Geddes et
al.; U.S. Pat. No. 5,840,139 to Geddes et al.; U.S. Pat. No.
6,030,476 also to Geddes et al.; U.S. Pat. No. 6,139,665 to
Schmelzer et al.; U.S. Pat. No. 6,308,883 also to Schmelzer et al.;
and U.S. Pat. No. 6,319,590 to Geddes et al.
[0119] The foam-paperboard laminates of the present invention are
suitable for use in a number of different applications. A
significant use is for insulating beverage cups sold commercially
by the assignee of this invention as PerfecTouch. The processes and
methods used to prepare such cups are disclosed in detail in the
following U.S. patents (the disclosures of which are incorporated
herein in their entireties): U.S. Pat. No.6,129,653 to Fredericks
et al.; U.S. Pat. No. 6,416,829 to Breining et al.; U.S. Pat. No.
6,482,481 to Fredericks et al.; U.S. Pat. No. 6,565,934 also to
Fredericks et al.; U.S. Pat. No. 6,663,927 to Breining et al.; U.S.
Pat. No. 6,676,586 to Breining et al.; U.S. Pat. No. 6,703,090 to
Breining et al.; and United States Patent Application Publication
No.: US 2004/0126517 also to Breining et al.
[0120] As disclosed in the referenced PerfecTouch-related patents,
sealing of the cups can be facilitated by doping of the LDPE layer
with an amount of HDPE, where the inner layer is coated with HDPE
to serve as an occlusive layer. Inasmuch as the occlusive coating
and the foamable coating are both LDPE-based, the two coatings will
adhere well to one another when melt-bonded. The modified LDPE
containing HDPE will not foam under conditions utilized to foam the
LDPE outer coating. Such doping is also believed to be feasible
when the foam-paperboard laminates of the present invention are to
be converted into, for example, packaging materials, such as boxes
or other types of structures having edges or surfaces that are
adhered together. However, when sealing the foam-paperboard
laminates, an upper limit of MI is believed to exist in that if the
LDPE polymer is too soft, it could be difficult to obtain a good
seal on the cup. Other forms of sealing, such as by separate
application of adhesives, are also believed to have utility in the
present invention.
[0121] The invention as used in insulated beverage cups is
illustrated and explained further in connection with FIGS. 1, 1a
and the Figures following.
[0122] There is shown in FIGS. 1 and 1a a paperboard cup 10 which
includes a bottom panel 12 as well as a sidewall 14 and a curled
brim 16. Bottom panel 12 has a polymer occlusive layer 18 which may
be a blend of LDPE and HDPE, for example, as well as paperboard
layer 20. The bottom side of paperboard layer 20 of panel 12 does
not have a occlusive layer since it is not typically foamed.
Looking at a finished cup prepared from the foam-paperboard
laminates of the present invention, it will be appreciated from
FIG. 1a that the various layers are particularly concentrated in
the area of attachment of the sidewall and bottom panel. This
structure becomes even more complex in the area of a seam as well
be appreciated by one skilled in the art. In suitable form, the
occlusive layer 18 is predominantly a blend of LDPE/HDPE and the
foam layer 22 of sidewall 14 consists essentially of LDPE such that
the two layers will readily melt bond when they come in contact at
a seam (not shown).
[0123] In regards to the structure of the foam-paperboard
laminates, sidewall 14 includes a foamed layer 22 which comprises
an in situ foamed LDPE coating. Sidewall 14 also comprises a
paperboard layer 24 as well as a occlusive layer 26 on its inner
surface. Layer 26 is suitably a modified LDPE which incorporates
about 10% by weight of HDPE. Any suitable polymer composition,
suitably one which does not foam under conditions used to foam the
in situ foamed coating on the outside of the cup, may be used as
the occlusive layer.
[0124] In addition to insulated beverage cups, the foam-paperboard
laminates of the present invention have a number of utilities. For
example, the foam-paperboard laminates of the present invention can
be used as containers for take-out food items, such as hamburger or
sandwich "clamshells." The foam-paperboard laminates of the present
invention can also be used as a substitute for polystyrene foam
take out containers. The insulating qualities of the
foam-paperboard laminates of the present invention are believed to
be suitable for use in hot food applications. Moreover, since the
foam-paperboard laminates of the present invention comprise
primarily paperboard material, they are compostable. In the
foodservice area where foam waste is generally undesirable, the
foam-paperboard laminates of the present invention would be quite
desirable.
[0125] The foam-paperboard laminates provide utility for types of
packaging where insulation is desirable. For example, the
foam-paperboard laminates of the present invention can be used to
package frozen food, such as ice cream, vegetables, dinners,
pizzas, meats and the like. Further, since the foam-paperboard
laminates are heat resistant, the foam-paperboard laminates can be
used to heat the frozen food for end use. The foam-paperboard
laminates can also be used for refrigerated food where insulation
is desirable, such as a wrapper for butter, cheese and the
like.
[0126] The insulating character of the foam-paperboard laminates
also make them suitable for use as insulating sleeves for hot
beverages. Such an example is a hot cup sleeve.
[0127] The foam-paperboard laminates are also grease and fat
resistant due to the polymeric nature of the coating, which also
makes these materials suitable for packaging products such as
butter, cheese and the like.
[0128] The foam-paperboard laminates can also be used as an
insulating sleeve or box to assist in transporting frozen or
refrigerated goods. For example, the insulating sleeve or box can
be used to transport hot and cold items from the grocery store to a
residence by a consumer or the like.
[0129] Still further, the cushion characteristics of the present
invention make the foam-paperboard laminates of the present
invention suitable for use in packaging materials where cushioning
is desirable. For example, the foam-paperboard laminates can be
used as cushioning for fragile items, such as plates or other
dishware. The foam-paperboard laminates can also be used as
cushioned shipping envelopes, such as for transporting CD's or
photos or the like.
[0130] The foam-paperboard laminates can also be used as a
cushioning or insulating material inside of a packaging material.
For example, when the foam material is positioned inside a box, the
cushioning material will provide protection against scuffing or
marring of easily damaged products.
[0131] The foam-paperboard laminates of the present invention can
also be used in any application where a fairly thin foam laminate
made from plastic has been used previously. Since the
foam-paperboard laminates of the present invention are compostable,
it would be expected that these inventive materials will have
utility when it is desirable to reduce plastic waste. When coated
with a barrier material, such as HDPE, the foam-paperboard
laminates can be expected to function well as disposable products
even in moist environments. To this end, the foam-paperboard
laminates of the present invention can be used as shoe inserts,
sandals or the like.
[0132] Yet further, the since the foam aspect of the
foam-paperboard laminates of the present invention is textured, the
foam-paperboard laminates of the present invention can be useful in
applications where it is desirable to increase the slip-resistance
of an item. To this end, the foam-paperboard laminates can be used
in preparing slip-resistant beverage cartons or in other
applications where grippability is desirable.
[0133] Still further, the foam-paperboard laminates of the present
invention can be used as foodservice containers for greasy foods,
such as popcorn or fried chicken. The textured surface of the
container assists in holding thereof if the outer surface of the
container becomes greasy. Further, the coated container is
resistant to grease, thus making the container substantially
impermeable to the greasy food inside the container.
[0134] The foam-paperboard laminates of the present invention has
an appearance that can be described as "pearlescent" or
"opalescent." Such an appearance lends itself to premium packaging
materials where aesthetic desirability is highly valued. To this
end, the foam-paperboard laminates of the present invention are
suitable for use in "high end" packaging materials where insulation
or cushioning are not readily needed. Such potential applications
include perfume and/or jewelry packaging.
[0135] The foam-paperboard laminates can also be used as a
disposable cutting board due to the cut resistance of the foam. The
foam-paperboard laminates can also be used as a disposable trivet
due to its heat resistant qualities. The foam-paperboard laminates
can also be used to store knives safely. Yet further, the
foam-paperboard laminates can be used as shelf liner, place mat,
wall covering, acoustic barrier for wall or floor, floor mat and
polishing material.
[0136] With respect to the layers of the foam-paperboard laminates
of the present invention, additional components besides the primary
polymer materials can be added; such as stabilizers, nucleants,
fillers, compatible blended in polymers and so forth, so long as
the additional ingredients do not change the basic and novel
characteristics of the foam-paperboard laminates, that is, its
ability to be foamed in situ. Likewise, polymers such as LDPE or
HDPE may contain amounts of co-monomers in addition to ethylene
which is the predominant monomer.
[0137] The foam-paperboard laminates used to form sidewall 14
includes paperboard layer 24, barrier layer 26 and a foamable
coating layer 22a. The foam-paperboard laminates can be produced on
an apparatus shown schematically in part in FIG. 2. The structure
of the foam-paperboard laminates prior to foaming is shown in FIG.
2a.
[0138] With regard to the extrusion process, there is shown in FIG.
2 a coating station 30 including a coating nip 32 defined between a
support roll 34 and a chill roll 36. Coating station 30 also
includes an extruder 38 feeding a slit die 40 which has a die gap
or width indicated at 42. The die is located at height 44 (also
referred to as air gap 44) above the center of nip 32 as shown in
the diagram.
[0139] In order to coat paperboard 24, a web 50 of paperboard 24 is
fed to the nip in the machine direction as shown by arrows 25 while
a curtain 32 of molten LDPE is fed from die 42 on to the board.
Chill roll 36 cools the polymer as it adheres to the board. The
curtain 52 thus becomes a foamable LDPE layer 22a as shown in FIG.
2a.
EXAMPLES
[0140] The following Examples are put forth so as to provide those
of ordinary skill in the art with a complete disclosure and
description of how the present invention is practiced, and
associated processes and methods are constructed, used, and
evaluated, and are intended to be purely exemplary of the invention
and are not intended to limit the scope of what the inventors
regard as their invention. Efforts have been made to ensure
accuracy with respect to numbers (e.g., amounts, temperature, etc.)
but some errors and deviations should be accounted for. Unless
indicated otherwise, parts are parts by weight, temperature is as
specified or is at ambient temperature, and pressure is at or near
atmospheric.
[0141] A. Preparation of Insulated Beverage Cups from
Foam-Paperboard Laminates of the Present Invention
[0142] Utilizing generally the apparatus and procedures described
above, a series of extrusion-coated LDPE paperboard samples were
prepared and foamed in situ at a temperature of about 130.degree.
C. for from about 1 minute to about 2 minutes. The paperboard used
had a thickness of about 15 mils. The paperboard was
extrusion-coated on one side with 90% LDPE/10% HDPE at about 5
pounds per ream. This coating had a thickness of about 0.4 mils.
The other side of the paperboard was extrusion-coated with LDPE at
from about 18 and about 35 pounds per ream using different web
speeds, air gaps, die gaps, melt temperatures and polymers of
different melt indices as set out in more detail below.
TABLE-US-00001 TABLE 1 Typical Physical Properties of WESTLAKE EC
476 LDPE ASTM Property Value Method Melt index (gm/10 min.) 13.7 D
1238 Density (gm/cc) 0.9165 D 1505 Ultimate tensile (psi) 1,300 D
638 Elongation (%) 450 D 638 Tensile modulus (psi) 14,000 D 1709
Vicat softening (.degree. C.) 87 D 1525
[0143] TABLE-US-00002 TABLE 2 Typical Physical Properties of
WESTLAKE EC 478 LDPE ASTM Property Value Method Melt index (gm/10
min.) 4.5 D 1238 Density (gm/cc) 0.923 D 1505 Ultimate tensile
(psi) 1,600 D 638 Elongation (%) 450 D 638 Tensile modulus (psi)
36,000 D 1709 Vicat softening (.degree. C.) 100 D 1525
[0144] TABLE-US-00003 TABLE 3 Typical Physical Properties of
WESTLAKE EC 479 LDPE ASTM Property Value Method Melt index (gm/10
min.) 5.7 D 1238 Density (gm/cc) 0.921 D 1505 Ultimate tensile
(psi) 1,450 D 638 Elongation (%) 475 D 638 Tensile modulus (psi)
20,000 D 1709 Vicat softening (.degree. C.) 92 D 1525
[0145] TABLE-US-00004 TABLE 4 Typical Physical Properties of
WESTLAKE EC 482 LDPE ASTM Property Value Method Melt index (gm/10
min.) 12.0 D 1238 Density (gm/cc) 0.9183 D 1505 Ultimate tensile
(psi) 1,400 D 638 Elongation (%) 500 D 638 Tensile modulus (psi)
20,000 D 1709 Vicat softening (.degree. C.) 88 D 1525
[0146] Details and results appear in the Tables below and FIGS.
3-20B annexed hereto. Foam calipers are reported in the total of
foam, paperboard and occlusive layer. TABLE-US-00005 TABLE 5
Extruded LDPE Paperboard Composites-Experimental Parameters PE
Speed Air Weight Melt TMI Polymer (ft per Gap (pds. per Temp Foam
MD Extruder Extruder Extruder MI min) (inches) ream) (.degree. F.)
(mils) Shrink % RPM Amps PSI 4.5 300 9 26.7 577 27.3 85 125 200
2310 4.5 300 9 25.3 589 25.7 86 125 200 2160 4.5 300 9 35.5 606
19.1 91 200 247 2530 4.5 450 9 22.5 603 22.8 86 188 230 2480 4.5
450 13 22 608 19 -- 188 232 2490 4.5 300 9 25.4 598 25.9 81 126 191
2090 4.5 300 9 35.2 610 19 68 200 240 2480 4.5 450 9 21.9 610 21.9
76 188 223 2430 5.7 300 9 24.8 582 28.7 69 125 186 2170 5.7 300 9
23.5 591 29.8 -- 125 185 2560 5.7 300 9 32.4 600 26.7 -- 200 225
2450 5.7 300 13 23.9 588 27.2 74 125 186 2750 5.7 300 13 25.6 574
28.3 79 125 187 2200 5.7 300 13 24 588 27 79 125 189 2610 5.7 300
13 26 602 29 -- 200 225 2440 5.7 450 9 22.9 593 27.5 64 188 219
2510 5.7 450 9 18.1 596 27.9 69 188 223 3010 5.7 450 9 24 601 27.6
-- 200 235 2460 5.7 450 13 21.5 592 30.7 74 188 215 3150 5.7 450 13
22.3 589 31.1 70 188 219 2530 5.7 450 13 19.3 600 29.6 72 188 215
2930 5.7 300 9 21.4 591 27.3 79 125 184 2040 5.7 300 9 31.6 598
18.8 79 200 235 2440 5.7 300 13 24.5 592 27 80 125 183 2030 5.7 450
13 25.4 578 27.2 79 188 220 2590 5.7 450 9 25.1 584 27.8 70 188 217
2510 12.0 450 9 22.2 596 32.8 85 190 193 2030 12.0 300 9 23.2 589
31 82 126 157 1670 12.0 300 9 34.6 595 24.58 80 200 207 2040 12.0
450 13 25.8 598 33.1 81 190 192 2030 12.0 300 13 23.8 589 30.8 93
126 163 1650 12.0 300 13 35 597 22.3 81 200 209 2030 12.0 450 13
24.4 577 29.9 76 190 196 2140 12.0 300 13 26.7 567 30.3 91 126 162
1830 12.0 450 9 24.2 581 31.1 77 190 198 2120 12.0 300 9 24.2 572
30.4 81 126 164 1790 13.7 300 9 25.1 566 29.8 77 126 149 1790 13.7
300 9 21.7 585 29.9 91 126 146 1610 13.7 300 9 28 592 30.5 85 200
198 2010 13.7 300 13 25 567 30.9 98 126 149 1780 13.7 300 13 23.3
580 34.2 74 126 152 1670 13.7 300 13 31.7 590 28.9 83 200 201 2040
13.7 450 9 22.1 576 16 71 190 185 2100 13.7 450 9 19.9 593 32.8 71
190 181 1940 13.7 450 13 23.2 577 30.5 75 190 187 2100 13.7 450 13
24.1 590 30.9 78 190 187 1980 13.7 450 9 22.6 589 32.6 74 190 187
1940 13.7 300 9 22.8 587 33.3 88 126 149 1590 13.7 300 9 34.9 593
27.6 84 200 200 1960 13.7 450 13 23.8 590 32.6 81 190 187 1960 13.7
300 13 24.5 588 32.2 97 126 147 1600 13.7 300 13 37.2 595 27.8 86
200 196 1960 13.7 450 13 24.9 575 32.9 91 190 187 2090 13.7 300 13
25.7 566 32.3 92 126 149 1760 13.7 450 9 23.5 577 32.2 82 190 187
2050 13.7 300 9 24 567 32 78 126 150 1730
[0147] TABLE-US-00006 TABLE 6 Foam Caliper of 5.7 MI LDPE With
Different Web Speeds and Melt Temperatures 11'' Air Gap, 20 mil Die
Gap Caliper after Foaming (mils) Poly Weight 450 fpm 450 fpm 300
fpm 300 fpm (pounds/rm) 600.degree. F. 570.degree. F. 600.degree.
F. 570.degree. F. 28.5 26.3 27.6 26.6 27.4 27 24.2 27.6 22.7 27.1
19.9 25.9 17.2 22.5 14.4 20.2 29.5 26.8 28.8 26 24 25.2 20.6 23
18.3 21.8 14.8 19.8 28.1 29.5 26.5 29.1 23 27.6 20.6 26.1 17.7 24.1
15.6 21.4 30.3 29.4 27.3 28.6 25.1 27 22.4 24.9 19.6 23.5 17.2
21.5
[0148] TABLE-US-00007 TABLE 7 Foam Caliper 5.7 MI LDPE With
Different Melt Temperatures 11'' Air Gap, 20 mil Die Gap, 300 fpm
Web Speed Caliper after Foaming (mils) Trial 2 Trial 3 Poly Weight
Trial 1 300 fpm 300 fpm (pounds/rm) 300 fpm 600.degree. F.
600.degree. F. 570.degree. F. 23.5 29.8 23.9 27.2 24 27 24.8 28.7
25.6 28.3 26 29 32.4 26.7 21.4 27.3 31.6 18.8 24.5 27 28.1 29.5
26.5 29.1 23 27.6 20.6 26.1 17.7 24.1 15.6 21.4 30.3 29.4 27.3 28.6
25.1 27 22.4 24.9 19.6 23.5 17.2 21.5
[0149] TABLE-US-00008 TABLE 8 5.7 MI LDPE With Different Melt
Temperatures 11'' Air Gap, 20 mil Die Gap, 450 fpm Web Speed
Caliper after Foaming (mils) Poly Weight Trial 1 Trial 2 Trial 2
450 fpm (pounds/rm) 450 fpm 600.degree. F. 450 fpm 600 F.
570.degree. F. 18.1 27.9 19.3 29.6 21.5 30.7 22.3 31.1 22.9 27.5 24
27.6 25.4 27.2 25.1 27.8 28.5 26.3 27.6 26.6 27.4 27 24.2 27.6 22.7
27.1 19.9 25.9 17.2 22.5 14.4 20.2 29.5 26.8 28.8 26 24 25.2 20.6
23 18.3 21.8 14.8 19.8
[0150] TABLE-US-00009 TABLE 9 5.7 MI LDPE at Different Extrusion
Speeds Poly Weight Caliper after Foaming (mils) (pounds/rm) 300 fpm
450 fpm 18.1 27.9 19.3 29.6 21.4 27.3 21.5 30.7 22.3 31.1 22.9 27.5
23.5 29.8 23.9 27.2 24 27 24 27.6 24.5 27 24.8 28.7 25.1 27.8 25.4
27.2 25.6 28.3 26 29 31.6 18.8 32.4 26.7
[0151] TABLE-US-00010 TABLE 10 Foam Caliper of 13.7 MI LDPE Poly
Weight Caliper after Foaming (mils) (pounds/rm) 300 fpm 450 fpm
19.9 32.8 21.7 29.9 22.1 16 22.6 32.6 22.8 33.3 23.2 30.5 23.3 34.2
23.5 32.2 23.8 32.6 24 32 24.1 30.9 24.5 32.2 24.9 32.9 25 30.9
25.1 29.8 25.7 32.3 28 30.5 31.7 28.9 34.9 27.6 37.2 27.8
[0152] TABLE-US-00011 TABLE 11 Foam Caliper of 4.5 MI LDPE Poly
Weight Caliper after Foaming (mils) (pounds/rm) 450 fpm 300 fpm
21.9 21.9 22 19 22.5 22.8 25.3 25.7 25.4 25.9 26.7 27.3 35.2 19
35.5 19.1
[0153] TABLE-US-00012 TABLE 12 Foam Caliper of 12.0 MI LDPE Caliper
after Poly Weight Foaming (mils) (pounds/rm) 300 fpm 450 fpm 22.2
32.8 23.2 31 23.8 30.8 24.2 31.1 24.2 30.4 24.4 29.9 25.8 33.1 26.7
30.3 34.6 24.58 35 22.3
[0154] TABLE-US-00013 TABLE 13 Comparison of Foam Caliper for
Various MI Polymers Average Caliper after Melt Index Foaming
Polymer (g/10 min) (mils) 478 4.5 24.6 479 5.7 28 482 12 30.9 476
13.7 31.3
[0155] In Table 13, the LDPE polymer was applied at 25 pounds per
ream at 450 fpm.
[0156] Photomicrographs of foamed composites appear in FIGS. 3
through 6. In FIG. 3 a foam-paperboard laminate is shown that was
coated as noted above and subsequently foamed at approximately
130.degree. C. in an oven for about I to 2 minutes. The foamable
LDPE used in FIGS. 3 and 4 was 5.7 g/10 min MI LDPE. The sample in
FIG. 3 was coated at 200 ft/min, whereas the sample shown in FIG. 4
was coated at a web speed of 450 ft/min. It will be appreciated
from the photomicrographs that the foam of FIG. 3 had a regular
foam structure wherein the cells have a like horizontal span and a
relatively high aspect ratio (an average aspect ratio of greater
than about 2.5 as measured across a cross-section of the
foam-paperboard laminate at an area defined by 100.times. or
500.times. on an SEM). The aspect ratio referred to here is the
height of the cell away from the paperboard divided by its average
width (horizontal or parallel to the board).
[0157] In FIG. 4 it is seen that the paperboard that was extrusion
coated with LDPE at higher speed without optimized extrusion
conditions resulted in foam having an irregular pattern. The foam
appears as large macrovoids which appear to comprise agglomerated
or merged cells that have are quite large and have a low aspect
ratio. These macrovoids have horizontal spans much larger than the
remainder of the foam.
[0158] FIGS. 5 and 6 are photomicrographs of samples of LDPE with a
MI of 5.7. The sample of FIG. 5 was coated at 200 ft/min prior to
foaming, whereas the sample in FIG. 6 was coated at a web speed of
450 ft/min. Here it is seen that even at the high speeds of 450
ft/min, the foam prepared from a MI of 12.0 had a generally regular
structure consisting essentially of higher aspect ratio foam
cells.
[0159] There is shown in FIG. 7 a plot of caliper after foaming in
mils versus the LDPE coat weight in pounds per ream at different
coating speeds for the 5.7 MI polymer utilizing an air gap of 11
inches and 20 mil extruder die gap. It is seen in FIG. 7 that the
5.7 MI LDPE deposits coated at 300 ft/min generally had a higher
caliper after foaming than did the paperboard coated at 450 ft/min
at a coat weight above about 22 pounds per ream. It is also seen in
FIG. 7 that a decrease in melt temperature during coating adversely
affected caliper at any LDPE coat weight.
[0160] FIG. 8 is another plot of caliper of the foam-paperboard
laminates after foaming in mils versus extruded LDPE coat weight in
pounds per ream. Here it is also seen that lower melt temperatures
also decreased caliper generally, all other things being equal.
[0161] FIG. 9 is another plot of caliper after foaming versus
polymer coat weight in pounds/reams for the 5.7 MI LDPE. It is
confirmed in FIG. 9 that the higher melt temperatures produced a
higher caliper, up to a LDPE coat weight of about 28 pounds per
ream.
[0162] FIG. 10 is another plot of caliper after foaming in mils
versus LDPE extrusion coat weight in pounds per ream for 5.7 MI
LDPE at optimized die gap and air gap conditions. Here it is seen
that relatively high calipers are achieved with low coat weight.
This is an unexpected and very useful result; especially with
increasing polymer costs.
[0163] FIG. 11 is a plot of caliper after foaming for the 13.7 MI
LDPE. Here it is seen that the effect of coat weight on foam
caliper is more significant. In particular, the higher melt index
polymer produced surprisingly high caliper in situ foam-paperboard
laminates at lower coat weights. Further, significantly, this
higher melt index polymer foamed well when coating at web speeds of
up to about 450 fpm were used. Previous experiments had shown that
higher web speeds resulted in a degradation of foam height and
quality.
[0164] FIG. 12 is a plot of caliper after foaming of the
foam-paperboard laminates versus coat weight for a 4.5 MI LDPE
tested. Here is seen that the calipers were quite low as opposed to
the higher melt index polymers, regardless of coat weight.
[0165] FIG. 13 is another plot of caliper after foaming of the
foam-paperboard laminates versus coat weight for a LDPE having a MI
of about 12.0. Here it is seen that high calipers were achieved
with coat weights below 25 pounds per ream at web speeds of 300
feet per minute and 450 feet per minute.
[0166] FIG. 14 is a plot of average caliper after foaming of the
foam-paperboard laminates versus polymer melt index in g/10 min.
Here it is seen that the average caliper after foaming increased
substantially as the melt index of the LDPE was increased at
similar and preferred process conditions. The LDPE coat weight of
this example was 25 pounds per ream.
[0167] The influence of polymer coat weight selection and coating
conditions on the characteristics of the in situ foams later
produced by heating of the fabricated container is further
appreciated by considering FIGS. 15A-20B.
[0168] FIG. 15A is a surface SEM of a foamed composite wherein the
foamed coating was produced from 5.7 MI LDPE extrusion coated a
coat weight of 30 pounds per ream. The LDPE was applied to the
board at a web speed of 450 feet per minute using a 40 mil die gap
and 5'' air gap. It is seen in FIG. 15A that the foam cells have
considerable MD ratio, that is, are elongated in the machine
direction when viewed from top to bottom in the photograph.
[0169] FIG. 15B (comparative) is a view in section of the foam
layer of FIG. 15A, wherein the cross machine direction is from left
to right and wherein it is seen that the foam is irregular, with
large foam cells of low aspect ratio.
[0170] FIG. 16A is a surface SEM of a foamed composite sample using
the same 5.7 MI LDPE as in FIGS. 15A and 15B at the same coat
weight and coating speed, however, the air gap was increased to 9''
and the die gap reduced to 20 mils. Here there is seen much less MD
ratio (top to bottom in the photograph) elongation of the foam
cells. FIG. 16B is an SEM in section of the foam of FIG. 16A (cross
machine direction left to right) and it is seen that the foam
microstructure is considerably more regular.
[0171] FIGS. 17A through 20B further illustrate the influence of
polymer selection and coat weight on the resulting foam-paperboard
laminates. FIGS. 17A, 18A, 19A and 20A are surface SEMs wherein the
machine direction of the paperboard web is from top to bottom,
while FIGS. 17B, 18B, 19B and 20B are SEMs of foam sections,
wherein the cross machine direction of the coated web is from left
to right. Polymers utilized in each case and coating conditions are
indicated in Table 14 below. In all cases the die gap was 20 mils.
TABLE-US-00014 TABLE 14 Extruded LDPE Coating Comparison Figures
Polymer Coat weight Air Gap Web Speed 17A, 17B 5.7 MI LDPE 22 13''
450 fpm (comparative) pounds/ream 18A, 18B 13.7 MI LDPE 24 13'' 450
fpm pounds/ream 19A, 19B 4.5 MI LDPE 22 9'' 450 fpm (comparative)
pounds/ream 20A, 20B 12.0 MI LDPE 26 13'' 450 fpm pounds/ream
[0172] In FIGS. 17A and 17B, it is seen that good quality coatings
are produced with the 5.7 MI LDPE using a large air gap, a small
die gap and a coating speed of 450 feet per minute. Likewise, it is
seen in FIGS. 18A and 18B, that good quality coatings are produced
under similar conditions with the 13.7 MI LDPE. On the other hand,
it is seen in FIGS. 19A and 19B, that under these conditions, the
4.5 melt index LDPE would not foam properly; there were large areas
of unfoamed polymer. Finally, it is seen in FIGS. 20A and 20B, that
the 12.0 MI LDPE produced excellent in situ foamed coatings from
extruded coatings coated at 450 fpm.
[0173] The present invention was tested at a commercial facility in
order to confirm that manufacturing speeds could be increased in
accordance with the invention. Composite production speeds were
increased to 400 fpm. Air gap was increased, die gap and coat
weight were decreased in order to achieve the incremental
production.
[0174] Without intending to be bound by any theory, it is believed
that adhesion of the coating to the paperboard as well as polymer
orientation, coat weight and viscosity in the melt influence the
foaming properties of the polymer coating of foam-paperboard
laminates prepared from the in situ process.
[0175] It has been reported in literature that polymer orientation
is highly correlated to elongational strain; see Toft, N. and
Rigdahl, M., "Influence of Extrusion Coating Conditions on
Structure and Tensile Properties of Polyethylenes, International
Journal of Polymeric materials, 53:809-828, (2004); which notes
that [0176] Increasing web speed (v.sub.1) during extrusion
increases % shrink in the MD direction. This increase in % shrink
is related to polymer orientation which is created at higher web
speeds. [0177] Polymer orientation is highly correlated to
elongational strain (.epsilon.), which is a function of air gap
(h), extruder rotational speed (v.sub.0) and web speed (v.sub.1).
The relationship can be expressed as follows:
.epsilon.=(v.sub.1/h)(ln(v.sub.1/v.sub.0)) It is seen in the data,
Table 5 in particular, that increasing the web speed during
extrusion increases the percent shrink in the MD direction. This
increase in shrink, or decrease in shrink resistance, is believed
to be related to polymer orientation, which is created at higher
web speeds. In any event, the shrink resistance has been found by
the inventors herein to correlate generally with foam caliper.
[0178] The melt temperature of the LDPE has been found to strongly
influence ultimate foam quality, reinforcing the qualitative
observation that coatings that were strongly adhered to the
paperboard tended to produce better foams. Methods of promoting
adhesion that might be employed include increasing coat weight,
increasing melt temperature or treating the paperboard surface (by
corona treatment, for example). While a precise threshold of
adhesion for superior foaming may vary depending on the materials
and conditions employed, it is believed that the more contact
points between the foamed coating and the paperboard substrate, the
better the foam quality and less material required for a given
thickness. More contact points likely exist after foaming of the
LDPE if the coating is strongly adhered prior to foaming. Unfoamed
coating adhesion exceeding so called "fiber tear" strength is
preferred, wherein the will not separate from the substrate without
removing substantial amounts of fiber. If a peeled coating from
paperboard contains fiber from the board surface which is readily
observed, then adhesion between the coating and the board is
probably adequate for good foaming.
[0179] Melt index and coat weight results suggest that resistance
to flow in the polymer coating as it is foamed in situ plays an
important role as well. Higher melt index foams are unexpectedly
superior, when all other variables are kept constant. Thinner
coatings (especially thinner higher melt index coatings) tend to
foam better than like thicker coatings, producing thicker coatings
at lower coat weight. Here again is an unexpected result suggesting
effects of melt rheology are important and may override any adverse
effect on adhesion by lowering the coat weight, provided that
adhesion is adequate. The coat weight effect on in situ foaming may
be related to stiffness of the unfoamed coating that is strongly
related to caliper. Bending stiffness is generally proportional to
the caliper of a coating to the third power. Thus, coat weights of
less than about 25 pounds per ream are possible with weights of
from about 20 to about 25 pounds per ream being suitable to obtain
good quality foam (that is, high aspect ratio foam with good
adhesion). This coat weight is markedly below the coat weight of
the prior art in situ process where a coat weight of around 30
pounds per ream were specified.
[0180] B. Laser Topography of Beverage Cups Prepared from the
Foam-Paperboard Laminates
[0181] The surface of cups having in situ formed LDPE foamed
coatings prepared as discussed above were laser-profiled using the
Taylor Hobson--Talysurf CLI 1000 Scanning Laser Profilometer fitted
with a 10 mm triangulation gauge. A surface 5 mm by 5 mm was
collected using a lateral resolution of 5 .mu.m in both the MD and
CD directions and a gauge resolution of 0.17 .mu.m. The samples
were oriented with the X axis corresponding to the machine
direction (stock curvature in the cross-machine direction).
[0182] After data collection, each sample dataset was processed
with standard software as follows: 1.) missing data was filled
using a technique employing a morphological dilation operation and
substitution based on interpolation of a smoothed shape calculated
from the neighborhood of the missing points; 2.) stock curvature
was removed by fitting a 2nd order polynomial to the surface; 3.)
the Z-axis was inverted in order to present the data a positive
image; 4.) The dataset was spatially filtered (median 3 by 3
kernel) to eliminate ultra-fine scale texture.
[0183] The texture aspect ratio, Str, assumes values between 0 and
1, with 1 representing an isotropic surface. "Isotropy" as that
term is used herein refers to the laser Str value measured as
described above, or may be expressed in percent (Str.times.100%).
For example, a sample with an Str value of 0.75 has an isotropy of
0.75 or 75%. Texture direction, Std, assumes values between -90 and
90 with 0 aligned with the cross-machine direction. This parameter
has meaning when Str assumes values less than 0.5.
[0184] Planarity was assessed by the magnitude of the developed
surface, Sdr. This parameter will assume a value of 0% for a flat
surface: the more convoluted the surface, the higher the value of
Sdr. The density of peaks, SPc, measures the number of peaks per
square millimeter that extend above C2 and below C1. Both C2 and C1
are expressed relative to the surface mean plane. Default values of
0.01 mm and 0.001 mm were used for C2 and C1, respectively. This
parameter measures the uniformity of the surface relative to the
specified thresholds C2 and C1.
[0185] Details as to particular samples and results appear in Table
15 below. TABLE-US-00015 TABLE 15 Laser Imaging Data Coating
Planarity Speed MI Air Gap Die Gap Coatweight Str Std SPc Sdr
Sample ID (fpm) (g/10 min) (inches) (mils) (lb/Ream) (Isotropy)
(degrees) (peaks/mm2) (percent) 1_130 200 5.7 5 20 30 0.486 26.5
9.3 1287 11 450 5.7 13 20 22 0.537 -63.5 28.7 1733 11_130 450 5.7 5
40 30 0.225 -85.0 14.1 1055 13_130 450 5.7 9 20 30 0.398 63.5 20.1
1434 15_NTC_2 300 5.7 11 20 24 0.474 63.5 15.7 1192 17_130 200 10 5
20 30 0.491 -82.0 19.0 1034 19 300 13.7 9 20 25 0.511 85.0 12.2
1447 21_NTC_2 300 5.7 11 20 24 0.463 86.0 19.4 1258 27_130 450 10 5
20 30 0.309 -63.5 15.5 1199 29_130 450 10 9 20 30 0.507 63.5 14.6
1525 3 300 5.7 9 20 32 0.539 -63.5 33.4 1512 3_NTC_2 450 5.7 11 20
24 0.369 86.0 16.7 1264 32 450 13.7 13 20 24 0.626 63.5 9.56 1192
39 300 4.5 9 20 27 0.523 71.5 16.4 1438 46 450 4.5 9 20 22 0.249
-85.0 13.9 358 49 300 12 9 20 35 0.590 63.5 17.8 1282 50 450 12 13
20 26 0.639 -63.5 8.59 1267 9_NTC_2 450 5.7 11 20 24 0.366 78.5
18.8 1164
[0186] It is seen in Table 15 that the laser profilometer analyses
are consistent with observations made from the photomicrographs.
Generally, satisfactory foams had Str (isotropy) values of from
about 0.4 and 0.75. Values of greater than about 0.45 or 0.5
provide especially good foam. Str values of 0.55 and more were
achieved consistently. Good coatings typically had a 0.01/0.001 mm
peak density of less than 20 peaks/mm.sup.2 of surface area.
Particularly good foams had 0.01/0.001mm peak densities of less
than about 15 peaks/mm.sup.2 with between 5 and 10 peaks/mm.sup.2
being exhibited by some especially suitable extruded LDPE
coatings.
[0187] Laser profiling of the cup surface is further appreciated by
reference to FIGS. 21A through 21B. FIG. 21A is a laser scanned
surface image of the in situ foam surface of sample 46 (4.5 MI, 450
fpm) showing large, irregular cells. FIG. 21B is an angular plot
showing that isotropy is quite low.
[0188] FIG. 22A is a laser scanned surface image of the foam of
sample 50 (MI 12, 450 fpm) showing a much finer microstructure than
that of FIG. 21A. FIG. 22B is an angular plot showing a much more
isotropic surface than is the case in FIG. 21B; confirming the
results seen in Table 15 and the accompanying photomicrographs.
[0189] C. Examination of Extrusion Parameters Effect on Properties
of Foam-Paperboard Laminates
1--Effect of Processing Parameters on Foaming
[0190] Experiments were conducted to determine the influence of air
gap, die gap, extrusion speed and LDPE type on foaming level. The
dependent parameter measured was foam caliper (as measured by total
laminate thickness) while the independent variables were: [0191]
Die gap: 20 mils and 40 mils [0192] Air gap: 5.25'' and 9.25''
[0193] Extrusion Speed: 200 fpm and 450 fpm [0194] Polymers: LDPE
polymer having MI of 5.7 (EC479, Westlake Chemical, Houston, Tex.)
and LDPE polymer having MI of 10.0 (EC471, Westlake)
[0195] The properties of the polymer used in this experiment are
listed in Table 16. TABLE-US-00016 TABLE 16 Analysis of 471 and EC
479 LDPE (Westlake Chemical) Westlake Westlake Property Units
EC471AA EC479AA Melt Index.sub.2.16 g/10 min 10.0 5.7 Melt
Index.sub.21.6 g/10 min 342.0 226.3 MFR.sub.(21.6/2.16) -- 34.2
39.7 Density g/cc 0.9181 0.9215 Initial Tension 34 63 Extrusion
Coefficient 340 359 DSC Melt Point .degree. C. 106.8 110.7 Heat of
Fusion J/c 105.8 115.1 Number Avg. Molecular 21800 21200 Weight
Weight Avg. Molecular 361700 253600 Weight Molecular Weight 16.6
12.0 Distribution
[0196] It was observed by the inventors that the higher melt index
polymer generally provided increased foam caliper, all other
variables being equal.
2--Effect of Extrusion Melt Temperature and Coat Weight on Foam
Caliper
[0197] FIG. 23 shows foam caliper as a function of coat weight for
5.7 MI LDPE (Westlake EC 479) polymer at two different extrusion
speeds and two different melt temperatures. The results confirmed
that increasing melt temperature had a positive effect on foam
caliper. The inventors herein therefore determined that increasing
the temperature at which the polymer exits the die (extrusion melt
temperature) improved foam caliper by increasing micro-level
adhesion as well as allowing for molecular relaxation. However, the
melt temperature needs to be below the decomposition temperature of
the polymer.
[0198] Foam caliper increased as LDPE coating thickness (as
measured by coat weight) increases. However, foam caliper reached a
maximum at a certain thickness and decreased at higher LDPE coating
thickness. This is believed to be the result of two opposing
forces: the lower the coating thickness, the lower the flexural
stiffness of the coating and the easier it is to deform. However,
as coating thickness was decreased, the draw down ratio increased,
thus resulting in higher residual stress in the extruded LDPE
coating and reducing foam caliper.
[0199] 3--Effect of Polymer Properties on Foam Caliper
TABLE-US-00017 TABLE 17 Effect of polymer properties on final
extruded LDPE coating properties INVENTORS' COATING INCREASING
INCREASING INCREASING EXPERIMENTAL PROPERTY MELT INDEX MWD DENSITY
OBSERVATIONS Tensile DECREASE -- INCREASE The lower the better for
Properties bubble formation. The higher the better for other
extrusion applications Impact DECREASE DECREASE -- Not important
for bubble Resistance formation, but important for common extrusion
applications Tear Strength DECREASE DECREASE DECREASE Not important
for bubble formation, but important for common extrusion
applications Heat Seal/ DECREASE -- INCREASE Important for cup
forming, Hot Tack but not bubble formation Temperature Heat Seal/
DECREASE INCREASE DECREASE Important for cup forming, Hot Tack but
not bubble formation Range Drawdown INCREASE INCREASE -- Very
important for bubble Ability formation, not as much for common
extrusion applications Melt Strength DECREASE INCREASE -- Very
important in bubble formation, not as important in common extrusion
applications ESCR DECREASE DECREASE -- Not important for bubble
formation, but important for common extrusion applications
[0200] The inventors herein investigated the LDPE extruded coating
properties significant to foam formation. These were found to
include: lower stiffness, higher draw ability and higher melt
strength.
[0201] It should be noted that when extruding LDPE for standard
packaging grade LDPE extruded coating applications (that is, where
the LDPE is not foamed), molecular orientation is desirable since
it increases properties such as stiffness, tear resistance and
tensile strengths well as improves oxygen and moisture vapor
transmission rates. These properties are not of interest, and are
actually undesirable, to prepare the foam-paperboard laminates of
the present invention. It was found that increase of molecular
orientation resulted in lower foam caliper.
[0202] Based on the above information, it was determined that it
would be beneficial to increase the MI of the LDPE polymer to
improve the resulting foam properties. To this end, two previously
unexamined LDPE polymers having higher MI were selected for further
investigation TABLE-US-00018 TABLE 18 Analysis of Westlake 482 and
476 LDPE Polymers Westlake Westlake Property Units EC482AA EC476AA
Melt Index.sub.2.16 G/10 min 12.0 13.7 Melt Index.sub.21.6 G/10 min
391.2 453.4 MFR.sub.(21.6/2.16) -- 32.6 33.1 Density G/cc 0.9183
0.9180 Initial Tension 28 28 Extrusion Coefficient 336 384 DSC Melt
Point .degree. C. 106.2 104.9 Heat of Fusion J/c 115.4 114.8 Number
Avg. Molecular 18200 18000 Weight Weight Avg. Molecular 291900
289700 Weight Molecular Weight 16.0 16.1 Distribution
[0203] Based upon the inventors' previous determination of the
somewhat counterintuitive effects of melt index and molecular
weight distribution on foam caliper, the inventors investigated the
effect of these parameters on foam caliper at equal speeds than the
polymer used earlier. The inventors determined that using these
higher MI polymers, higher foam calipers could be obtained at
higher extrusion speeds using less polymer, as will be described
further herein.
4--Effect of Polymer Type and Thickness on Foam Caliper
[0204] To test the effects of polymer type at different polymer
thickness on foam caliper, the LDPE having the melt indices set
forth in Table 18 above were tested. FIG. 24 shows that for the
12.0 MI LDPE and the 13.7 MI LDPE polymers, web speed did not
influence foam caliper as seen with the 5.7 MI LDPE polymer having
the lower MI. In addition, 13.7 MI LDPE polymer provided a higher
foam caliper at equivalent web speed and extruded coat thickness
when compared to the lower MI 12.0 MI LDPE polymer.
[0205] After obtaining these preliminary results, the inventors
proceeded to investigate both the 12.0 MI LDPE and 13.7 MI LDPE
polymers and to test these polymers in a manufacturing environment.
Converting trials at a Georgia-Pacific extrusion facility validated
the lab scale experimental results. The 12.0 MI LDPE polymer
produced target foam caliper of about 30 mil (paperboard and foam)
at only 25 pounds/ream of LDPE while the 13.7 MI LDPE polymer
produced target foam caliper with only 20 pounds/ream of LDPE. In
sum, these manufacturing scale results indicated that it was
possible to obtain good foam qualities through use of a lighter
extruded coat weight of LDPE.
5--Effect of Oven Temperature and Residence Time on Foaming
[0206] In the manufacture of PerfecTouch insulated beverage cups
using LDPE having a MI of 5.7, it was previously determined that
the residence time and oven temperature had an effect on foam
caliper and appearance, however, at that time it was not found
possible to predict the influence of such factors on the resulting
foam. At lower oven temperatures, it was observed that foam did not
always meet thickness and quality targets.
[0207] The inventors examined the effects of oven parameters on
resulting foam caliper in order to gain a better understanding of
the foaming mechanism inside the oven and to identify a process to
produce consistent foam appearance.
[0208] Based on how the higher MI LDPE's were found to behave, the
inventors hypothesized that shorter residence time and /or oven
temperature could be used to produce foam of a targeted caliper. In
order to test this hypothesis, 5.7 MI, 12.0 MI and 13.7 LDPE
polymers were extruded at 25 pounds per ream on SBS. The resulting
foam caliper was analyzed under various oven temperatures and
residence times.
[0209] Initially, the oven residence time tested ranged from 60
seconds to 180 seconds and the temperature ranged from 235.degree.
F. to 265.degree. F. (Previous manufacturing conditions were
255.degree. F. oven temperature and residence time of 120 seconds
using 5.7 MI LDPE). Results of R&D foam caliper measurements
for the three polymers are shown in FIGS. 25 to 27.
[0210] At lower oven temperatures (235.degree. F.), long residence
times >180 seconds are needed to initiate extruded polymer
deformation for all types of LDPE examined. At 245.degree. F.,
polymer deformation and bubble formation is observed only for the
"softer" polymer (MI 12.0). A temperature of 245.degree. F. was too
low to produce optimal foam caliper.
[0211] Increasing the temperatures to 255.degree. F. and
265.degree. F. resulted in a significant increase in bubble height
in as quickly as 90 seconds. The 13.7 MI LDPE resulted in higher
foam caliper than the 12.0 MI LDPE and 5.7 MI LDPE at similar
conditions.
[0212] FIGS. 25-27 show that it is possible to decrease oven
residence time if oven temperature is correspondingly increased.
However, it was necessary to understand the limitations of
increasing oven temperature and its affects on foam caliper and
appearance.
[0213] To do so, the experiments were repeated for oven
temperatures as high as 300.degree. F., the results of which are
shown in FIGS. 28-30. FIGS. 28-30 should be interpreted with care.
The 13.7 MI LDPE polymer reached higher foam caliper than the 12.0
MI LDPE polymer, which had higher caliper than the previously used
LDPE polymer having a MI of 5.7. It also appeared that all LDPE
polymers tested reached a maximum caliper at a certain residence
time, which depended on the type of polymer and the temperature of
the oven. For example, for the 12.0 MI LDPE polymer, the maximum
caliper at the lower oven temperatures of 265.degree. F. and
275.degree. F. was reached after 60 seconds. This time is reduced
to 45 seconds for the 285.degree. F. and 300.degree. F. oven
temperature.
[0214] From these results, it might appear that oven residence time
can be significantly reduced if the oven temperature was increased.
However, the inventors herein found that foam appearance and
insulation levels can be adversely affected by an increase in
temperature.
[0215] In particular, it was determined that higher oven
temperatures can cause abrupt increase in vapor pressure (that is,
the steam forms and releases from the paperboard too quickly) under
the polymer coating surface. This, in turn, resulted in a loss of
foam quality. In addition, higher temperatures were found to
decrease the viscosity of the polymer to result in coalescence of
the bubbles under the surface or to the bursting of the
bubbles.
[0216] These conclusions can be seen in the SEMs presented in FIG.
31. The lower foam quality can be seen when comparing the 60 second
micrograph with the 120 second photomicrograph at the 285.degree.
F. and at 300.degree. F. oven temperatures.
[0217] While the invention has been described in connection with
numerous examples, modifications to those examples within the
spirit and scope of the invention will be readily apparent to those
of skill in the art. In view of the foregoing discussion, relevant
knowledge in the art and references including co-pending
applications discussed above in connection with the Background and
Detailed Description, the disclosures of which are all incorporated
herein by reference, further description is deemed unnecessary.
* * * * *