U.S. patent application number 10/394264 was filed with the patent office on 2003-10-02 for multilayer container resistant to elevated temperatures and pressures, and method of making the same.
This patent application is currently assigned to Continental PET Technologies, Inc.. Invention is credited to Collette, Wayne N., Krishnakumar, Suppayan M., Schmidt, Steven L..
Application Number | 20030186006 10/394264 |
Document ID | / |
Family ID | 24446502 |
Filed Date | 2003-10-02 |
United States Patent
Application |
20030186006 |
Kind Code |
A1 |
Schmidt, Steven L. ; et
al. |
October 2, 2003 |
Multilayer container resistant to elevated temperatures and
pressures, and method of making the same
Abstract
A multilayer plastic container having enhanced strength for high
temperature and pressure applications such as the pasteurization of
carbonated juice drinks. The container is commercially
cost-effective in comparison to prior art pasteurizable glass
containers, and provides all of the advantages of plastic over
glass, i.e., lightweight, shatter-resistant, etc. In a particular
embodiment, the multilayer container includes inner and outer
layers of a relatively high IV virgin PET, e.g., 0.85-0.90 dl/g,
and a core layer of post-consumer PET having a substantially lower
IV. The container has a relatively tall and slender profile, with
high orientation levels in the panel and shoulder, and an oriented
thick-walled base with feet. The base preferably has a high profile
and angled foot pads which are allowed to move outwardly under
creep. According to a method of making a multilayer preform for
such container, an enhanced injection rate and mold pressure are
utilized to enhance interlayer bonding and prevent separation of
the layers in spite of their substantial differences in IV.
Inventors: |
Schmidt, Steven L.;
(Bedford, NH) ; Krishnakumar, Suppayan M.;
(Nashua, NH) ; Collette, Wayne N.; (Merrimack,
NH) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER
LLP
1300 I STREET, NW
WASHINGTON
DC
20005
US
|
Assignee: |
Continental PET Technologies,
Inc.
|
Family ID: |
24446502 |
Appl. No.: |
10/394264 |
Filed: |
March 24, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10394264 |
Mar 24, 2003 |
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09072886 |
May 5, 1998 |
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6548133 |
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09072886 |
May 5, 1998 |
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08610810 |
Mar 7, 1996 |
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5804016 |
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Current U.S.
Class: |
428/35.7 ;
264/255; 264/513; 264/537 |
Current CPC
Class: |
B29C 2045/165 20130101;
B29C 2949/0774 20220501; B29C 2949/0773 20220501; B29C 2949/28
20220501; B29C 2949/0722 20220501; B29C 2949/3028 20220501; B29C
49/071 20220501; Y10T 428/1393 20150115; B29K 2077/00 20130101;
B29C 2949/0724 20220501; B29C 2949/3008 20220501; B29C 2949/073
20220501; B29C 2949/22 20220501; B29C 2949/3012 20220501; B29K
2105/253 20130101; B29C 2949/302 20220501; B29C 2043/527 20130101;
Y10T 428/1352 20150115; B29K 2995/004 20130101; B29C 2793/009
20130101; B29C 2949/0731 20220501; B29C 2949/0862 20220501; B65D
1/0215 20130101; B29C 2949/24 20220501; B29C 2949/3018 20220501;
B29C 2949/3016 20220501; B29C 2949/072 20220501; B29C 2949/26
20220501; B29C 2949/3009 20220501; B29K 2067/00 20130101; B29K
2667/00 20130101; B29C 2949/077 20220501; B29C 2949/3032 20220501;
B29C 49/06 20130101; B65D 1/0284 20130101; B29K 2995/0069 20130101;
B29C 2949/3036 20220501; B29C 2949/0866 20220501; B29C 2949/0733
20220501; B29C 2949/0777 20220501; B29C 2949/08 20220501; B29K
2105/26 20130101; B29C 45/1646 20130101; B29C 2949/0817 20220501;
B29L 2031/716 20130101; B29C 2949/0819 20220501; B29C 2949/0829
20220501; B29C 2949/0826 20220501 |
Class at
Publication: |
428/35.7 ;
264/513; 264/537; 264/255 |
International
Class: |
B32B 001/02; B29C
049/06; B29C 049/22 |
Claims
1. A method of making a multilayer container, the method comprising
the steps of: injecting a first thermoplastic material having a
first intrinsic viscosity (IV) into a preform mold cavity at a
first injection rate to form a first layer of a preform; injecting
a second thermoplastic material having a second IV, which differs
by at least about 0.10 dl/g from the first IV, into the mold cavity
at a second injection rate to form a second layer of the preform
adjacent the first layer; applying a pressure to the first and
second layers in the mold cavity, the injection rates and the
pressure being selected to promote layer adhesion between the first
and second layers; and blow molding a container from the preform
which can withstand a 1 meter drop onto a hard rigid surface
without layer separation.
2. The method of claim 1, wherein the second IV differs by at least
about 0.20 dl/g from the first IV.
3. The method of claim 1, wherein the first and second
thermoplastic materials are polyesters.
4. The method of claim 1, wherein the first thermoplastic material
comprises virgin polyethylene terephthalate (PET) and the first IV
is at least about 0.85 dl/g.
5. The method of claim 4, wherein the first IV is at least about
0.90 dl/g.
6. The method of claim 4, wherein the second thermoplastic material
comprises post-consumer PET (PC-PET) and the second IV is no
greater than about 0.75 dl/g.
7. The method of any one of claims 3 to 6, wherein the pressure is
at least about 9000 psi.
8. The method of claim 7, wherein the pressure is on the order of
9000 to 12,000 psi.
9. The method of claim 8, wherein at least one of the first and
second injection rates is on the order of 16-20 grams per
second.
10. The method of claim 9, wherein both injection rates are on the
order of 16-20 grams per second.
11. The method of claim 7, wherein the temperature of blow molding
is selected to reduce inter-layer shear during expansion of the
multilayer preform.
12. The method of claim 11, wherein the blow molding temperature is
on the order of 110 to 118.degree. C.
13. The method of claim 1, wherein the first IV is higher than the
second IV.
14. The method of claim 13, wherein the first material forms an
exterior preform layer and the second material forms an interior
preform layer.
15. The method of claim 1, further including: injecting a third
thermoplastic material at a third injection rate to form a layer
adjacent one of the first and second layers, the third material
having a third IV which differs by at least 0.10 dl/g from the IV
of the material of the adjacent one of the first and second
layers.
16. The material of claim 15, wherein the first and second
materials form at least a sidewall-forming portion of the preform,
and the third material is included in a base-forming region of the
preform.
17. The method of claim 16, wherein the first and third materials
have a higher IV than the second material.
18. A biaxially-oriented multilayer expanded preform container
having a first layer of a first thermoplastic material having a
first intrinsic viscosity (IV), and a second layer adjacent to the
first layer of a second thermoplastic material having a second IV
which differs by at least about 0.10 dl/g from the first IV, which
container can withstand a 1 meter drop onto a hard rigid surface
without separation of the first and second layers.
19. The container of claim 18, wherein the second IV differs by at
least about 0.20 dl/g from the first IV.
20. The container of claim 18, wherein the first and second
thermoplastic materials are polyesters.
21. The container of claim 18, wherein the first thermoplastic
material comprises virgin polyethylene terephthalate (PET) and the
first IV is at least about 0.85 dl/g.
22. The container of claim 21, wherein the second thermoplastic
material is post-consumer PET (PC-PET), and the second IV is no
greater than about 0.75 dl/g.
23. The container of claim 18, wherein the container when filled
with a pressurized liquid of 2.5 volumes, sealed and then exposed
to an elevated temperature of 75.degree. C. for 10 minutes,
undergoes an overall volume change of no greater than about 3%.
24. The container of claim 23, wherein the overall volume change is
no greater than about 2%.
25. The container of claim 1, wherein the first IV is higher than
the second IV.
26. The container of claim 25, wherein the first material forms an
exterior preform layer and the second material forms an interior
preform layer.
27. The container of claim 18, further including: injecting a third
thermoplastic material at a third injection rate to form a layer
adjacent one of the first and second layers, the third material
having a third IV which differs by at least 0.10 dl/g from the IV
of the material of the adjacent one of the first and second
layers.
28. The container of claim 27, wherein the first and second
materials form at least a sidewall-forming portion of the preform,
and the third material is included in a base-forming region of the
preform.
29. The container of claim 28, wherein the first and third
materials have a higher IV than the second material.
30. The container of claim 20, having a generally cylindrical panel
portion with a height-to-diameter ratio on the order of 2.0 to 3.0,
a panel wall thickness on the order of 0.25 to 0.38 mm, and an
average planar stretch ratio in the panel portion on the order of
13.0 to 14.5, and a base having a substantially hemispherical
bottom wall and a plurality of legs, wherein the bottom wall has a
thickness on the order of 0.60 to 2.5 mm.
31. The container of claim 30, wherein the bottom wall extends from
about .theta.=60.degree. to .theta.=90.degree. from a vertical
centerline of the container.
32. The container of claim 31, wherein each leg has an angled foot
pad which is disposed at an angle of about 5 to 10.degree. with a
flat surface on which the container rests.
33. The container of claim 32, wherein the angled foot pads as
formed are disposed at about 60 to 75% of the panel diameter.
34. The container of claim 30, wherein the container has an
outwardly protruding and substantially rounded shoulder section
above the panel section.
35. The container of claim 30, wherein the first thermoplastic
material is virgin PET and forms exterior inner and outer layers,
and the second thermoplastic material is post-consumer PET and
forms an interior core layer between the inner and outer
layers.
36. The container of claim 35, wherein the second thermoplastic
material comprises on the order of 30 to 60% of a total weight of
the container.
37. The container of claim 36, wherein the first thermoplastic
material comprises on the order of 40 to 70% of the total weight of
the container.
38. The container of claim 18, having a relatively tall and slender
profile, a shoulder portion and a panel portion with an average
planar stretch ratio at least on the order of 13.0, and a footed
base including a substantially hemispherical bottom wall wherein
the bottom wall has a thickness greater than that of the panel
portion.
39. The container of claim 38, wherein the bottom wall has a base
profile of .theta.=60.degree. to 90.degree., where .theta. is an
angle that a radius, defining the substantially hemispherical
bottom wall, extends from a vertical centerline of the
container.
40. A multilayer preform for blow molding a container, the preform
having a first layer of a first polyester material having a first
intrinsic viscosity (IV), and a second layer adjacent the first
layer of a second polyester material having a second intrinsic
viscosity (IV) which differs by at least about 0.20 dl/g from the
first IV, the multilayer preform being injection molded without
separation of the first and second layers.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to multilayer plastic
containers for pressurized products which may be exposed to
elevated temperatures and pressures, e.g., during pasteurization,
and wherein the multiple layers are resistant to layer
separation.
BACKGROUND OF THE INVENTION
[0002] Juice drinks are normally filled by one of three basic
sterilization processes:
[0003] hot fill
[0004] pasteurization
[0005] aseptic fill.
[0006] Hot filling is not suitable for carbonated juice drinks due
to the inability to maintain carbon dioxide (CO.sub.2) in solution
at elevated temperatures. Aseptic filling of carbonated drinks is
possible, but has certain disadvantages which include requiring
high levels of capital investment, operating maintenance, and
expertise. As a result, pasteurization is the preferred
sterilization approach for carbonated juice drinks.
[0007] Prior art pasteurizable beverage containers are typically
made of glass, because glass can withstand the extended high
temperatures and high internal pressures of the pasteurization
cycle. FIG. 1 illustrates graphically, as a function of time, the
increasing internal temperature and pressure during a
pasteurization cycle of a 16-ounce glass container, which has been
filled with a juice product carbonated at 2.5 volumes; "2.5
volumes" means that the volume of carbon dioxide at 0.degree. C.
under 1 atmosphere is 2.5 times the volume of the liquid. The
typical pasteurization cycle, as shown in FIG. 1, includes five
steps:
[0008] (1) immersion in bath 1, having a bath temperature of about
43.degree. C., for about 12.5 minutes in order to raise the
container and contents up to about the bath-1 temperature;
[0009] (2) immersion in bath 2, having a bath temperature of about
77.degree. C., for the time from 12.5 to 21 minutes in order to
raise the container and contents up to about the bath-2
temperature;
[0010] (3) immersion in bath 3, having a bath temperature of about
73.degree. C., for the time from 21 to 31.5 minutes in order to
hold the container and contents at about the bath-3
temperature;
[0011] (4) immersion in bath 4, having a bath temperature of about
40.degree. C., for the time from 31.5 to 43 minutes in order to
lower the container and contents down to about the bath-4
temperature; and
[0012] (5) immersion in quench bath 5 for the time from 43 to 60
minutes in order to cool the container and contents down to about
10.degree. C.
[0013] The temperature curve 12 shows that the container and
contents remain above 70.degree. C. for roughly 10 minutes (in bath
3), during which time the internal pressure increases significantly
to about 140 psi (1.times.10.sup.6 N.multidot.m.sup.-2). This
10-minute hold period at a temperature of about 70 to 75.degree. C.
provides effective sterilization for most carbonated beverage
products, including those containing 100% fruit juice. A glass
container can withstand these temperatures and pressures without
deformation.
[0014] In contrast, a conventional polyester carbonated soft drink
(CSD) container made of polyethylene terephthalate (PET), and
filled with a carbonated product, would undergo significant volume
expansion (creep) when exposed to the elevated temperatures and
pressures of the pasteurization process. An exemplary curve 16 of
modulus versus temperature for biaxially-oriented PET is shown in
FIG. 2. The modulus (an indicator of strength under pressure)
decreases with increasing temperature; thus creep increases with
increasing temperature. This data shows the tensile properties of a
sample taken from a cylindrical panel section of a disposable CSD
container made of PET (0.80 IV resin). The panel section was
oriented at a planar stretch ratio of about 13:1; the testing was
conducted on an Instron machine according to ASTM D638. For this
prior art CSD container, the drop in strength at elevated
temperatures would result in excessive volume expansion and
physical distortion under normal pasteurization conditions,
resulting in an unacceptable drop in the fill point and/or base
roll out (instability).
[0015] PET (homopolymer or copolymer) resin used for disposable CSD
containers has a glass transition temperature (T.sub.g) on the
order of 65-70.degree. C. It is known that increasing the molecular
weight (i.e., chain length of PET molecules) of the resin, which
effectively increases T.sub.g, can significantly strengthen the
resulting biaxially-oriented container so as to resist or diminish
creep at elevated temperatures. Intrinsic viscosity (IV) is used in
the PET container industry as a standard measure of PET chain
length. Known disposable CSD containers (freestanding, monolayer
PET containers) have been produced from resins with IVs in the
range of 0.70 to 0.85 dl/g. Increasing the IV beyond 0.85, and
preferably beyond 0.90, has produced a freestanding monolayer PET
container that can be successfully pasteurized at 70-75.degree. C.
for products carbonated at up to four volumes.
[0016] Although a higher molecular weight (higher IV) PET can
provide enhanced strength at elevated temperatures, use of such
high IV PET is difficult to justify economically because of its
cost premium. For example, 0.90 or higher IV PET resins cost 20-30%
more per Unit weight, than 0.80 IV PET.
[0017] FIG. 3. is a graph of modulus versus temperature, similar to
FIG. 2, but with three curves 20, 22, 24 to illustrate the
influence of IV on the modulus/temperature relationship.
Biaxially-oriented PET samples were taken from the panel sections
of containers oriented at a planar stretch ratio of 12.0-12.5 for
three different resin IVs, namely, 0.74, 0.80, and 1.00. These
curves show that for example, at a modulus of 3.times.E.sup.6 psi
(20,690.times.10.sup.6 N.multidot.m.sup.-2), there is a temperature
difference of 40.degree. F. (22.2.degree. C.), i.e., 160-120,
between the 0.74 IV sample and the 1.00 IV sample. Thus, increasing
the IV produces a desirable increase in strength at elevated
temperatures, but again at a cost premium.
[0018] There is an ongoing need for a plastic container able to
withstand the elevated temperatures and pressures of pasteurization
and other high temperature applications, and wherein the container
can be manufactured commercially at a price competitive with that
of glass containers.
SUMMARY OF THE INVENTION
[0019] The present invention is directed to a multilayer container,
which can withstand elevated temperatures and pressures (e.g., the
pasteurization process) without significant creep and which is
commercially cost-effective. For example, in a preferred embodiment
the container undergoes an overall volume increase of no greater
than about 3.0%, and more preferably no greater than about 2.0%,
compared to the as-molded container volume. The invention is also
directed to a method of making the container and to multilayer
preforms which are expanded to form containers.
[0020] In one embodiment, a two-material, three-layer (2M, 3L)
container structure includes exterior inner and outer layers of
virgin polyethylene terephthalate (PET) homopolymer or copolymer,
and an interior core layer of post-consumer PET (PC-PET). PC-PET is
available at a 15-25% cost advantage, as compared to 0.80 IV virgin
PET resin; the cost difference is even greater with virgin PET
above 0.80 IV. This savings enables production of a container with
30-60% PC-PET by total container weight, and the remaining 70-40%
of 0.85 (or higher) IV virgin PET, that is cost-competitive with
existing glass containers for pasteurization. The higher IV outer
layers have a higher T.sub.g for enhanced thermal resistance, while
the lower IV core provides the necessary wall thickness for
strength at a reduced cost.
[0021] An unexpected problem arises when preforms are produced with
polymers of substantially different IVs, i.e., a difference (delta)
of at least 0.10 dl/g, such as a multilayer structure of 0.73 IV
PC-PET and 0.85 IV virgin PET. In an IV delta range of 0.10 to
0.20, one or more layers may separate when the container is dropped
from a height of one meter onto a hard rigid surface (e.g.,
concrete). Still further, if the IV delta exceeds 0.20, layer
separation may occur in the preform, immediately following removal
from the injection mold.
[0022] Layer separation is an important commercial issue for CSD
containers which are stored for extended periods of time. Carbon
dioxide gas may permeate through the container sidewall into a
delaminated layer region, creating a pressurized gas pocket; over
time, the pocket may expand to a significant size, rendering the
container visually unacceptable.
[0023] It has been found that the injection molding and/or blow
molding process conditions can substantially diminish or completely
eliminate the problem of layer separation for IV deltas on the
order of 0.10 or more. More specifically, the rate of injection and
amount of pressure applied in the preform mold are increased to
insure higher levels of layer bonding. For example, a standard
injection molding process for low-IV PET (i.e., 0.70 IV) may
utilize an injection rate of 10-12 grams/second, and a mold
pressure on the order of 7,000 psi (50.times.10.sup.6
N.multidot.m.sup.-2) In contrast, the injection rate for molding
multilayer virgin PET/PC-PET preforms is increased to about 16-20
grams/second (a 50% or greater increase) and the mold pressure to
about 9,000 psi (60.times.10.sup.6 N.multidot.m.sup.-2) (about a
30% increase). In a preferred process, the virgin PET is injected
at about 16-20 grams/second at a melt temperature of about
275-300.degree. C., and the PC-PET is injected at the same rate at
a melt temperature of about 265-290.degree. C. The mold is then
packed (to fill any void space created by shrinkage) at a pressure
of about 9000-12,000 psi (60.times.10.sup.6 to 85.times.10.sup.6
N.multidot.m.sup.-2), for about 2-3 seconds, and then held (in the
mold) at a pressure of about 6000 psi (40.times.10.sup.6
N.multidot.m.sup.-2) for about 13-15 seconds before ejection. Still
further, the blow molding temperature is preferably about
110.degree. C., to minimize inter-layer stresses during
blowing.
[0024] It is hypothesized that increasing the IV delta between the
virgin PET and PC-PET alters the melt solubility of the materials
sufficiently to reduce molecular migration and chain entanglement
at the layer boundary, thus decreasing layer adhesion. The enhanced
injection rate and pressure overcomes this problem. The exact mold
temperature, injection rate, pressure and hold time will vary
depending upon the specific polymers used and preform wall
thicknesses.
[0025] The present invention includes multilayer preforms and other
injection-molded articles, as well as various containers, including
bottles and cans, made from such preforms. The neck finish of the
container may be amorphous, biaxially oriented, an insert molded
with a high T.sub.g polymer and/or crystallized, depending on the
particular wall thickness and/or applications.
[0026] These and other advantages of the present invention will be
more particularly described in regard to the following detailed
description and drawings of select embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a graph illustrating the changes in internal
temperature and pressure over a typical pasteurization cycle, for a
prior art 16-oz glass container, filled with a juice product
carbonated at 2.5 volumes.
[0028] FIG. 2 is a graph of modulus versus temperature for a 0.80
IV PET biaxially-oriented container sample.
[0029] FIG. 3 is a graph similar to FIG. 2, illustrating the change
in modulus with temperature for three different IVs.
[0030] FIG. 4 is a vertical cross-section of a multilayer preform
useful in making a container according to one embodiment of the
present invention.
[0031] FIG. 5 is a schematic illustration of a two-material,
three-layer metered sequential co-injection apparatus for making
the preform of FIG. 4.
[0032] FIG. 6 is a graph of pressure versus time, illustrating by
way of example the enhanced injection rate and pressure in a
preform mold according to the present invention.
[0033] FIG. 7 is a vertical cross-section of a blow-molding
apparatus for making an exemplary container of the present
invention.
[0034] FIG. 8 is a side elevational view of a multilayer
pressurized polyester container made according to FIG. 7, which can
withstand the elevated temperatures and pressures of the
pasteurization process.
[0035] FIG. 9 is a horizontal cross-section taken along line 9-9 of
FIG. 8, showing the multilayer sidewall of the container.
[0036] FIG. 10 is a vertical cross-section taken along line 10-10
of FIG. 8, showing one foot of the container base.
[0037] FIG. 11 is an enlarged fragmentary cross-section of a
crystallized neck finish and cap, according to one embodiment.
[0038] FIG. 12 is an enlarged fragmentary cross-section of an
amorphous neck finish and cap, according to another embodiment.
[0039] FIG. 13 is a schematic sectional view through a preform used
in an alternative embodiment to make a can.
[0040] FIG. 14 is a schematic sectional view of an intermediate
article made from the preform of FIG. 13, including as a lower
portion a can which is biaxially-oriented up through the finish,
and an upper portion which is removed and discarded.
[0041] FIG. 15 is a schematic sectional view through a preform
according to another embodiment having a neck finish insert and
multilayer body- and base-forming portions.
[0042] FIG. 16 is a graph of glass transition temperature (T.sub.g)
and melting temperature (T.sub.m) for various "low-PEN"
compositions of PET and PEN, useful in moderate-temperature
applications.
[0043] FIG. 17 is a graph of T.sub.g and T.sub.m for various
"high-PEN" compositions of PET and PEN, useful in high-temperature
applications.
DETAILED DESCRIPTION
[0044] According to a first embodiment, an injection-molded
multilayer preform and method of making the same are illustrated in
FIGS. 4-6. The preform may be expanded to form a multilayer
pasteurizable carbonated beverage container as illustrated in FIGS.
7-10.
[0045] FIG. 4 shows a substantially cylindrical preform 30 (defined
by vertical center line 32) which includes an upper neck portion or
finish 34 integral with a lower body-forming portion 36. The neck
portion includes an upper sealing surface 31 which defines the open
top end of the preform, and an exterior surface having threads 32
and a lowermost flange 35. Below the neck finish is the
body-forming portion 36 which includes a flared shoulder-forming
portion 37, increasing (radially inwardly) in wall thickness from
top to bottom, a cylindrical panel-forming section 38, having a
substantially uniform wall thickness, and a thickened base-forming
section 39, which is thicker than the panel-forming section. The
bottom end 40 of the preform is substantially hemispherical and may
be thinner than the upper base-forming portion.
[0046] This preform has a two-material, three-layer (2M, 3L)
structure and is substantially amorphous and transparent. The
multiple preform layers comprise, in serial order: outer layer 42
of virgin PET, core layer 43 of PC-PET, and inner layer 34 of
virgin PET. The virgin PET is a homopolymer, or low copolymer with
for example 2% isophthalic acid modifier, having an intrinsic
viscosity of about 0.90 dl/g. The PC-PET has an intrinsic viscosity
of about 0.70.
[0047] This particular preform is designed for making a 1.0 liter
pasteurizable carbonated beverage container (as shown in FIG. 8).
The preform 30 has a height of about 150 mm, and an outer diameter
in the panel-forming section 38 of about 23.8 mm. The total wall
thickness of the panel-forming section 38 is about 4.1 mm, and the
thicknesses of the various preform sidewall layers are: outer layer
42 and inner layer 44 are each about 1.2 mm thick, and core layer
43 is about 1.7 mm thick. For pasteurizable carbonated beverage
containers of about 0.3 to 1.5 liters in volume, having a panel
wall thickness of about 0.25 to about 0.38 mm, and filled at about
2.0 to 4.0 volumes, a preferred average planar stretch ratio for
the panel section 38 is on the order of 13.0 to 14.5. The planar
stretch ratio is the ratio of the average thickness of the preform
panel-forming portion 38 to the average thickness of the container
panel 86 (in FIG. 8), wherein the "average" is taken along the
length of the respective preform and container portions. The
average panel hoop stretch is preferably about 4.0 to 4.5 and the
average panel axial stretch about 3.0 to 3.2. This produces a
container panel 86 with the desired biaxial orientation and visual
transparency. The specific panel thickness and stretch ratio
selected depend on the dimensions of the bottle, the internal
pressure, and the processing characteristics (as determined for
example by the intrinsic viscosity of the particular materials
employed).
[0048] A suitable multilayer preform injection molding apparatus 50
is shown in FIG. 5. The apparatus provides a sequential
introduction of two melt streams in a metered fashion and
includes:
[0049] "A" extruder 51
[0050] melt channel from "A" extruder 52A
[0051] melt channel from "B" extruder 52B
[0052] valve cam 53
[0053] "B" extruder 54
[0054] melt valve 55
[0055] shot pot 56
[0056] ball check 57
[0057] preform mold 58
[0058] preform 59
[0059] gate 60
[0060] The "A" extruder 51 is charged with 0.90 IV virgin PET resin
which has been dried to below 50 ppm moisture content. The virgin
PET resin is melted in a screw and barrel, at a barrel temperature
of 285.degree. C. The melt is plasticized at 300 psi
(2.07.times.10.sup.6 N.multidot.m.sup.-2) and 25 RPM.
[0061] The "B" extruder 54 is charged with 0.70 IV PC-PET which has
been dried down to 100-150 ppm. The PC-PET is melted in a screw and
barrel, at a barrel temperature of 275.degree. C. The melt is
plasticized at 270 psi (1.86.times.10.sup.6 N.multidot.m.sup.-2)
and 35 RPM.
[0062] The process sequence starts once the "A" extruder 51 is
fully charged, and the shot pot 56 is fully charged with material
from the "B" extruder 54. First, the "A" extruder 51 comes forward
injecting virgin PET (for the inner and outer layers) until about
60% of the preform weight has been injected into the mold 58. The
"A" injection is preprogramed to stop at this point, thus giving a
predetermined metered virgin PET shot. The melt valve 55 extends
fully to a position which provides clearance for the valve cam 53
to shift. The valve cam 53 then shifts to the "B" position and the
melt valve 55 is retracted until it rests against the valve cam 53.
In this position, the melt channel 52A for the "A" extruder 51 to
the preform mold 58 is blocked, but the melt channel 52B for the
shot pot 56 to the preform mold 58 is opened. The shot pot 56
extends pushing the PC-PET melt (for the core layer) through the
melt valve 55 filling the preform mold 58. The ball check 57
prevents the flow of melt back into the "B" extruder 54. When the
shot pot 56 is empty, the melt valve 55 again extends fully for
enough time that the valve cam 53 can shift back to the "A"
position. The melt valve 55 then pulls back until it rests again on
the valve cam 53. In this position, the melt channel 52B from the
shot pot 56 to the preform mold is blocked, but the melt channel
52A from the "A" extruder 51 to the preform mold 58 is opened. The
"A" extruder 51 again comes forward and packs the mold against
shrinkage of the preform 59 and clears the PC-PET from the gate 60.
After packing, the mold pressure is partially reduced and held
while the preform cools. The "A" extruder 51 plasticizes material
for the next shot, and the "B" extruder 54 plasticizes material for
the next shot, pushing it through the melt channel 52B and into the
shot pot 56. The machine is now ready for the next cycle.
[0063] FIG. 6 is a graph of pressure versus time showing the
difference between a standard injection cycle 64 and the enhanced
injection cycle 66 of the present invention. The standard curve 64
is for a 2-material, 3-layer preform structure including a first
shot of about 0.70 IV virgin PET resin, and a second shot of about
0.70 IV PC-PET resin. In the standard process, each of the polymer
melts are injected into the mold at a rate of about 10-12
grams/second, a packing pressure of about 7500 psi
(50.times.10.sup.6 N.multidot.m.sup.-2) is applied for about four
seconds, and the pressure is then dropped to about 4500 psi
(30.times.10.sup.6 N.multidot.m.sup.-2) for the next 15 seconds,
after which the pressure is released and the preform is ejected
from the mold. The second curve 66 shows the enhanced process of
this invention. Both materials (0.90 virgin PET and 0.70 PC-PET)
are injected at a rate of about 16-20 grams/second, the packing
pressure is held at about 10,500 psi (70.times.10.sup.6
N.multidot.m.sup.-2) for about three seconds, the pressure is
dropped to about 6,000 psi (40.times.10.sup.6 N.multidot.m.sup.-2)
for the next 15 seconds, and then the pressure is released and the
preform ejected from the mold. Increasing the pressures (above
previous levels) is believed to force higher levels of interlayer
bonding, which may include chain entanglement, hydrogen bonding,
low-level interlayer crystallization and layer penetration. The
increased pressure holds the preform against the cold mold wall to
solidify the preform without haze (i.e., loss of transparency), at
the minimum possible cycle time. Reduction of the hold time may be
desirable to avoid pushing a solidified gate into a molten preform
base, which would result in plastic deformation and weakness in the
gate area. In addition, it is believed that faster injection rates
yield higher melt temperatures within the injection cavity,
resulting in increased polymer mobility which improves migration
and entanglement during the enhanced pressure portion of the
injection cycle. As an additional option, increasing the average
preform temperature (e.g., in this example to 115.degree. C.)
and/or decreasing the temperature gradient through the preform wall
(e.g., in this example to less than 5.degree. C. temperature
difference), may further reduce layer separation by minimizing
shear at the layer boundaries during container inflation.
[0064] FIG. 7 illustrates a stretch blow-molding apparatus 70 for
making a container from the preform 30. More specifically, the
substantially amorphous and transparent preform body section 30 (of
FIG. 4) is reheated to a temperature above the glass transition
temperatures of the PET and PC-PET layers, and then positioned in a
blow mold 71. A stretch rod 72 axially elongates (stretches) the
preform within the blow mold to ensure complete axial elongation
and centering of the preform. The thickened base-forming region 39
of the preform resists axial deformation compared to the panel- and
shoulder-forming portions 38 and 37; this produces greater axial
elongation in the resulting panel and shoulder portions of the
container. A blowing gas (shown by arrows 73) is introduced to
radially inflate the preform during axial stretching in a customary
manner to match the configuration of an inner molding surface 74 of
the blow mold. The formed container 80 is substantially transparent
but has undergone strain-induced crystallization to provide
increased strength (to withstand carbonation and the increased
pressure during pasteurization).
[0065] FIG. 8 shows a 1.0 liter pasteurizable multilayer beverage
bottle 80 made from the preform of FIG. 4. The tubular body-forming
portion 36 of the preform has been expanded to form a substantially
transparent, biaxially-oriented container body 81. The upper thread
finish 34 has not been expanded, but is of sufficient thickness or
material construction to provide the required strength. The bottle
has an open top end 82 and receives a screw-on cap (see FIGS.
11-12). The expanded container body 81 includes:
[0066] (a) an upper flared shoulder section 83 with an
outwardly-protruding profile, and which generally increases in
diameter from below the neck finish flange 35 to a cylindrical
panel section 86; it is preferable to provide a rounded
(hemispherical) shoulder 83 because this shape maximizes the
biaxial orientation and minimizes the applied stress levels. Higher
orientation and lower stress will lower the volume increase due to
creep at elevated temperatures; this will minimize any drop in the
fill level if there is creep during pasteurization; also, it is
preferable to provide a small transition radius 84 between the neck
finish 34 and shoulder 83 to minimize the unoriented area at the
top of the shoulder (an unoriented area may be prone to creep);
[0067] (b) the substantially cylindrical panel section 86
preferably has a relatively tall and slender configuration, i.e., a
height to diameter ratio on the order of 2.0 to 3.0, in order to
minimize the stress in the sidewall (and minimize creep);
relatively shallow transition regions 87 and 88 are provided at the
upper and lower ends of the panel 86, respectively; larger
transition areas would be more likely to expand (straighten) during
pasteurization and cause a volume increase (fill level drop); for
the same reason, preferably no ribs are provided in the panel
section 86;
[0068] (c) a footed base 90 has a substantially hemispherical
bottom wall 92 and for example, five legs 91 which extend
downwardly from the bottom wall to form five foot pads 93 on which
the container rests; the legs 91 are symmetrically disposed around
the container circumference; in addition, it is preferable to
provide a high depth base, i.e., close to a hemispherical base, in
order to maximize strength and resistance against creep; it is also
preferable to provide an angled foot pad which can move outwardly
under creep and yet remain within the diameter of the
container.
[0069] The panel-forming section 38 of the preform may be stretched
at an average planar stretch ratio on the order of 13.0 to 14.5;
the virgin PET layers of the resulting panel section 86 have an
average crystallinity on the order of 20% to 30%, and preferably on
the order of 25% to 29%. The shoulder 83 undergoes an average
planar stretch ratio of about 10.0 to 12.0; the virgin PET layers
of the resulting shoulder 83 have an average crystallinity,of about
20% to 25%. The hemispherical bottom wall 92 in the base undergoes
an average planar stretch of about 5.0 to 7.0 and the virgin PET
layers have about 5% to 15% average crystallinity; the legs and
feet undergo an average planar stretch of about 13.0 to 14.0, and
the virgin PET layers have about 20% to 26% average crystallinity.
The core PC-PET layer has somewhat less crystallinity in each
respective region.
[0070] FIG. 9 shows a cross-section of the panel wall 86, including
inner layer 95 of virgin PET, core layer 96 of PC-PET, and outer
layer 97 of virgin PET. In this embodiment, the relative percent by
total weight of the various layers in the panel section are about
30% for inner layer 95, about 40% for core layer 96, and about 30%
for outer layer 97.
[0071] The preferred features of the footed container base are
shown more clearly in FIG. 10. As a basis of comparison, a known
five-foot PET disposable carbonated beverage container
(non-pasteurizable) has a relatively low base profile (.theta.of
about 45.degree.). In contrast, the present base preferably has a
relatively high base profile on the order of 60.degree. or better.
FIG. 10 shows in solid lines a base having a full hemisphere A
where .theta.=90.degree., and in dashed lines a truncated
hemisphere B where .theta.=60.degree., .theta. being the angle that
the radius R, defining the hemispherical bottom wall 92, extends
from the vertical centerline (CL) of the container body. The
relative heights of the base are illustrated as H.sub.A for the
full hemi, and H.sub.B for the truncated hemi. It is preferable to
provide a base height between H.sub.B and H.sub.A, and more
preferably where .theta. is greater than 65.degree..
[0072] In addition, it is preferable to provide an angled foot pad.
The foot pad extends between points G and K on the leg 91 (for
.theta.=90.degree.), or 91' (for .theta.=60.degree.). The foot pad
is preferably spaced a distance L.sub.F from the vertical
centerline CL to a point G which is vertically aligned with a
center point of radius R.sub.G. Radius R.sub.G forms the outer edge
of the foot pad. The foot pad forms an angle .alpha. with a
horizontal surface 102 on which the base rests. Preferably, L.sub.F
is on the order of 0.32R to 0.38R, and .alpha. is on the order of
5.degree. to 10.degree., to allow each foot pad and leg to move out
under creep, and yet remain within the diameter of the
container.
[0073] FIG. 11 is an enlarged cross-section of an opacified neck
finish enclosure according to one embodiment. More specifically,
the unoriented neck finish 110 has been thermally crystallized
(opacified) by for example, high-temperature exposure; this
increases the strength and enhances its resistance to the increased
temperature and pressure of pasteurization. The heat-treated area
may extend just below the flange 111. A cap 116 has an annular ring
117 of a resilient material (e.g., plastisol or other thermoplastic
elastomer) which seals an upper surface 112 of the neck finish. If
there is any deformation of the neck finish during pasteurization,
the liner 117 deforms to ensure a tight seal and prevent
leakage.
[0074] In an alternative embodiment shown in FIG. 12, a
substantially amorphous and unoriented neck finish 120 is provided,
i.e., it has not been crystallized. In this case, the amorphous
neck finish is provided with a laminated foil liner 124, which lies
within an inner surface of a cap 126, and which may, for example,
be heat sealed or adhesively sealed to an upper surface 122 of the
neck finish. Again, if there is any deformation of the neck finish,
the liner 124 ensures a tight seal to prevent leakage.
[0075] In yet another embodiment, a relatively wide mouth container
such as a can is formed according to the present invention. The can
may be formed from a preform according to the process described in
U.S. Pat. No. 4,496,064 to Beck et al., which issued Jan. 29, 1985,
and which is hereby incorporated by reference in its entirety. FIG.
13 shows a preform 142 (from the Beck patent) which includes a
support flange 144, a thin upper body portion 15 which flares into
a thick generally cylindrical main body portion 146, and a
generally hemispherical bottom portion 148. The Beck process
enables a high degree of biaxial orientation to be obtained in all
portions of the resulting container, e.g., can, so that the
container may have economical thin walls while having the desired
strength characteristics. In this case, the preform is expanded to
form an intermediate article 150, which includes a lower portion
152 in the form of the desired container, and an upper portion 154.
The lower portion includes a cylindrical body 132, concave bottom
134, tapered shoulder 136, mouth 138, and annular flange 130. The
upper portion is severed from the flange 130 at port 164 (as by
cutting or laser trimming, and discarded or ground and the material
reused). It is not necessary to thermally crystallize or otherwise
reinforce the upper end of the container, because the biaxial
orientation provides the necessary strength. A method of trimming
the expanded preform to remove the upper unoriented portion is
described in U.S. Pat. No. 4,539,463 to Piccioli et al., which
issued Sep. 3, 1985, and is hereby incorporated by reference in its
entirety. In a typical PC-PET/PET can application, the IVs of
adjacent layers may be about 0.6 and 0.8 dl/g; in a PC-PET/PET
bottle application, they would more typically be on the order of
0.7 and 0.9 dl/g.
[0076] Yet another method for providing a multilayer expanded
preform container with a crystallized neck finish is described in
U.S. Ser. No. 08/534,126, entitled "Preform And Container With
Crystallized Neck Finish And Method Of Making The Same," which was
filed Sep. 26, 1995 by Collette et al., and which is hereby
incorporated by reference in its entirety. As described therein, an
indexer (e.g., rotary or oscillatory) has two faces, each with a
set of preform molding cores, and simultaneously positions the two
core sets in two different sets of preform molding cavities. In the
first set of cavities (first molding station), a high T.sub.g
amorphous or crystallized neck portion is formed on one set of
cores, while in the other set of cavities (second molding station)
a plurality of amorphous body-forming portions are formed on the
other set of cores. The cores are sequentially positioned in each
of the first and second molding stations. By simultaneously molding
in two sets of cavities, an efficient process is provided. By
molding the neck and body-forming portions separately in different
cavities, different temperatures and/or pressures may be used to
obtain different molding conditions and thus different properties
in the two preform portions. For example, as shown in FIG. 15, in
one embodiment a polyester preform (for making a hot-fillable
container has a crystallized neck portion 180 of CPET, a
terephthalic polyester with nucleating agents which render the
polymer rapidly crystallizable during injection molding. CPET is
sold by Eastman Chemical Company, Kingsport, Tenn. The body-forming
portion 181 is a two-material, three-layer (2M, 3L) structure,
including inner and outer layers of virgin polyethylene
terephthalate (PET), and a core layer of for example post-consumer
PET (PC-PET). The base-forming portion 182 is similar to the
body-forming portion, but may include a core layer 183 of virgin
PET in at least the bottom part and possibly extending through to
the exterior of the preform. Alternatively, the core layer 183 in
the base may be of a higher T.sub.g polymer to enhance the thermal
stability of the resulting container base; this is particularly
useful with champagne-type container bases. The higher T.sub.g
polymer may be injected via a third extruder. Numerous alternative
high-glass transition (T.sub.g) polymers may be used in place of
CPET, such as arylate polymers, polyethylene naphthalate (PEN)
homopolymers, copolymers or blends, polycarbonates, etc. As for the
body-forming portion, numerous alternative polymers and layer
structures are possible, incorporating PEN, ethylene/vinyl alcohol
(EVOH) or MXD-6 nylon barrier layers, oxygen scavenging polymers,
etc. The container is useful in a variety of applications,
including refillable, pasteurizable, and hot-fillable
containers.
[0077] Although particular embodiments of the present invention
have been described, various modifications will be readily apparent
to a person skilled in the art and are included herein.
[0078] For example, one or more layers of the preform and
container, or portions thereof, can be made of various other
polymers, such as polyolefins (e.g., polypropylene and
polyethylene), polyvinyl chloride, polyarcylate, etc. Suitable
polyesters include homopolymers, copolymers or blends of
polyethylene terephthalate (PET), polybutylene terephthalate (PBT),
polypropylene terephthalate (PPT), polyethylene napthalate (PEN),
and a cyclohexane dimethanol/PET copolymer, known as PETG
(available from Eastman Chemical Company, Kingsport, Tenn.).
Polyesters based on terephthalic or isophthalic acid are
commercially available and convenient. The hydroxy compounds are
typically ethylene glycol and 1,4-di-(hydroxy methyl)-cyclohexane.
In general, the phthalate polyester may include polymer linkages,
side chains, and end groups not related to the formal precursors of
a simple phthalate polyester previously specified. Conveniently, at
least 90 mole percent will be terephthalic acid and at least 90
mole percent an aliphatic glycol or glycols, especially ethylene
glycol.
[0079] Post-consumer PET (PC-PET) is prepared from PET plastic
containers and other recyclables that are returned by consumers for
a recycling operation, and has now been approved by the FDA for use
in certain food containers. PC-PET is known to have a certain level
of I.V. (intrinsic viscosity), moisture content, and contaminants.
For example, typical PC-PET (having a flake size of one-half inch
maximum), has an I.V. average of about 0.66 dl/g, a moisture
content of less than 0.25%, and the following levels of
contaminants:
[0080] PVC: <100 ppm
[0081] aluminum: <50 ppm
[0082] olefin polymers (HDPE, LDPE, PP): <500 ppm
[0083] paper and labels: <250 ppm
[0084] colored PET: <2000 ppm
[0085] other contaminants: <500 ppm
[0086] PC-PET may be used alone for in one or more layers for
reducing the cost or for other benefits.
[0087] Also useful as a high-oxygen barrier layer is a packaging
material with physical properties similar to PET, namely
polyethylene naphthalate (PEN). PEN provides a 3-5.times.
improvement in barrier property and enhanced thermal resistance, at
some additional expense. Polyethylene naphthalate (PEN) is a
polyester produced when dimethyl 2,6-naphthalene dicarboxylate
(NDC) is reacted with ethylene glycol. The PEN polymer comprises
repeating units of ethylene 2,6 naphthalate. PEN resin is available
having an inherent viscosity of 0.67 dl/g and a molecular weight of
about 20,000 from Amoco Chemical Company, Chicago, Ill. PEN has a
glass transition temperature T.sub.g of about 123.degree. C., and a
melting temperature T.sub.m of about 267.degree. C. PET and PEN may
be blended or copolymerized in various amounts as shown in FIGS.
16-17. In the ranges of about 0-20% PEN and 80-100% PEN, the
material is crystalline, while from about 20-80% PEN the material
is substantially amorphous.
[0088] The structures of PET and PEN are shown below: 1
[0089] Suitable polyamides (PA) include PA6, PA6,6, PA6,4, PA6,10,
PA11, PA12, etc. Other options include acrylic/amide, amorphous
nylon, polyacrylonitrile (PAN), polystyrene, crystallizable nylon
(MXD-6), polyethylene (PE), polypropylene (PP), and polyvinyl
chloride (PVC).
[0090] The multilayer preform/container may also include one or
more layers of an oxygen barrier material such as ethylene/vinyl
alcohol (EVOH), PEN, polyvinyl alcohol (PVOH), polyvinyldene
chloride (PVDC), nylon 6, crystallizable nylon (MXD-6), LCP (liquid
crystal polymer), amorphous nylon, polyacrylonitrile (PAN) and
styrene acrylonitrile (SAN).
[0091] The intrinsic viscosity (I.V.) effects the processability of
the resins. Polyethylene terephthalate having an intrinsic
viscosity of about 0.8 is widely used in the carbonated soft drink
(CSD) industry. Polyester resins for various applications may range
from about 0.55 to about 1.04, and more particularly from about
0.65 to 0.85 dl/g. Intrinsic viscosity measurements of polyester
resins are made according to the procedure of ASTM D-2857, by
employing 0.0050.+-.0.0002 g/ml of the polymer in a solvent
comprising o-chlorophenol (melting point 0.degree. C.),
respectively, at 30.degree. C. Intrinsic viscosity (I.V.) is given
by the following formula:
I.V.=(ln(V.sub.Soln./V.sub.Sol.))/C
[0092] where:
[0093] V.sub.Soln. is the viscosity of the solution in any
units;
[0094] V.sub.sol. is the viscosity of the solvent in the same
units; and
[0095] C is the concentration in grams of polymer per 100 mls of
solution.
[0096] The blown container body should be substantially
transparent. One measure of transparency is the percent haze for
transmitted light through the wall (H.sub.T) which is given by the
following formula:
H.sub.T=[Y.sub.d.div.(Y.sub.d+Y.sub.s)].times.100
[0097] where Y.sub.d is the diffuse light transmitted by the
specimen, and Y.sub.s is the specular light transmitted by the
specimen. The diffuse and specular light transmission values are
measured in accordance with ASTM Method D 1003, using any standard
color difference meter such as model D25D3P manufactured by
Hunterlab, Inc. The container body should have a percent haze
(through the panel wall) of less than about 10%, and more
preferably less than about 5%.
[0098] The preform body-forming portion should also be
substantially amorphous and transparent, having a percent haze
across the wall of no more than about 10%, and more preferably no
more than about 5%.
[0099] The container will have varying levels of crystallinity at
various positions along the height of the bottle from the neck
finish to the base. The percent crystallinity may be determined
according to ASTM 1505 as follows:
% crystallinity=[(ds-da)/(dc-da)].times.100
[0100] where ds=sample density in g/cm.sup.3, da=density of an
amorphous film of zero percent crystallinity, and dc=density of the
crystal calculated from unit cell parameters. The panel portion of
the container is stretched the greatest and preferably has an
average percent crystallinity of at least about 15%, and more
preferably at least about 20%. For primarily PET polymers, a 25 to
29%. crystallinity range is useful in the panel region.
[0101] Further increases in crystallinity can be achieved by heat
setting to provide a combination of strain-induced and
thermal-induced crystallization. Thermal-induced crystallinity is
achieved at low temperatures to preserve transparency, e.g.,
holding the container in contact with a low temperature blow mold.
In some applications, a high level of crystallinity at the surface
of the sidewall alone is sufficient.
[0102] As a further alternative, the preform may include one or
more layers of an oxygen-scavenging material. Suitable
oxygen-scavenging materials are described in U.S. Ser. No.
08/355,703 filed Dec. 14, 1994 by Collette et al., entitled "Oxygen
Scavenging Composition For Multilayer Preform And Container," which
is hereby incorporated by reference in its entirety. As disclosed
therein, the oxygen scavenger may be a metal-catalyzed oxidizable
organic polymer, such as a polyamide, or an anti-oxidant such as
phosphite or phenolic. The oxygen scavenger may be mixed with
PC-PET to accelerate activation of the scavenger. The oxygen
scavenger may be advantageously combined with other thermoplastic
polymers to provide the desired injection molding and stretch blow
molding characteristics for making substantially amorphous.
injection molded preforms and substantially transparent
biaxially-oriented polyester containers. The oxygen scavenger may
be provided as an interior layer to retard migration of the oxygen
scavenger or its byproducts, and to prevent premature activation of
the scavenger.
[0103] Although certain preferred embodiments of the invention have
been specifically illustrated and described herein, it is to be
understood that variations may be made without departing from the
spirit and scope of the invention as defined by the appended
claims.
* * * * *