U.S. patent application number 10/767595 was filed with the patent office on 2004-09-23 for microcellular extrusion/blow molding process and article made thereby.
This patent application is currently assigned to Trexel, Inc.. Invention is credited to Anderson, Jere R., Blizard, Kent, Okamoto, Kelvin T., Pierick, David E., Straff, Richard S..
Application Number | 20040185241 10/767595 |
Document ID | / |
Family ID | 27371287 |
Filed Date | 2004-09-23 |
United States Patent
Application |
20040185241 |
Kind Code |
A1 |
Anderson, Jere R. ; et
al. |
September 23, 2004 |
Microcellular extrusion/blow molding process and article made
thereby
Abstract
A microcellular injection blow molding system and method, and
microcellular blow molded articles produced thereby, are described.
The system is equipped to extrude microcellular material that
changes in thickness, material density, or both in the machine
direction while maintaining a constant pressure drop rate during
nucleation just prior to extrusion, providing the ability to
produce consistent uniform microcellular material independent of
material thickness. The systems and methods are particularly useful
in production of strong, thin-walled, non-liquid-permeable, opaque
containers that do not contain reinforcing agent, chromophore, or
residue of chemical blowing agent or chemical blowing agent
by-product.
Inventors: |
Anderson, Jere R.;
(Newburyport, MA) ; Straff, Richard S.;
(Livingston, NJ) ; Okamoto, Kelvin T.; (Carlsbad,
CA) ; Blizard, Kent; (Ashland, MA) ; Pierick,
David E.; (San Diego, CA) |
Correspondence
Address: |
Timothy J. Oyer
Wolf, Greenfield & Sacks, P.C.
600 Atlantic Avenue
Boston
MA
02210
US
|
Assignee: |
Trexel, Inc.
Woburn
MA
|
Family ID: |
27371287 |
Appl. No.: |
10/767595 |
Filed: |
January 29, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10767595 |
Jan 29, 2004 |
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09689320 |
Oct 12, 2000 |
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6706223 |
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09689320 |
Oct 12, 2000 |
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09241352 |
Feb 2, 1999 |
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09241352 |
Feb 2, 1999 |
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PCT/US98/27118 |
Dec 18, 1998 |
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60107754 |
Nov 10, 1998 |
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60068173 |
Dec 19, 1997 |
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Current U.S.
Class: |
428/304.4 |
Current CPC
Class: |
B29C 2948/92904
20190201; B29C 44/348 20130101; B29C 48/92 20190201; B29C
2948/92095 20190201; B29C 48/325 20190201; B29C 2948/92647
20190201; B29K 2105/041 20130101; B29C 2948/92314 20190201; B29C
2948/92514 20190201; B29K 2105/04 20130101; B29C 2948/92685
20190201; C08J 9/00 20130101; B29C 2948/92076 20190201; B29C 48/29
20190201; B29C 49/04 20130101; B29C 2948/92876 20190201; B29C 48/10
20190201; B29C 48/09 20190201; B29C 49/0073 20130101; Y10T
428/249953 20150401; B29C 2948/92695 20190201; B29C 44/08 20130101;
B29K 2105/0005 20130101; B29B 2911/14326 20130101; B29C 2948/92895
20190201 |
Class at
Publication: |
428/304.4 |
International
Class: |
B32B 003/26 |
Claims
What is claimed is:
1. An article comprising: a blow molded, foam, microcellular,
polymeric article.
2. An article comprising: an extruded, microcellular parison
suitable for blow molding.
3. An article-as in claim 1, having a void volume of at least about
10%.
4. An article as in claim 1, having a void volume of at least about
20%.
5. An article as in claim 1, having a void volume of at least about
30%.
6. An article as in claim 1, having a void volume of at least about
50%.
7. An article as in claim 1, having the appearance of an
essentially solid, white plastic article.
8. An article as in claim 1, constructed as a container for
food.
9. An article as in claim 1, constructed as a container for
milk.
10. An article as in claim 1, containing food.
11. An article as in claim 1, containing milk.
12. An article as in claim 1, constructed and arranged to contain
food.
13. An article as in claim 1, including residual chemical blowing
agent or reaction by-product of chemical blowing agent in an amount
less than that inherently found in articles blown with about 0.1%
by weight chemical blowing agent or more.
14. An article as in claim 1, including residual chemical blowing
agent or reaction by-product of chemical blowing agent in an amount
less than that inherently found in articles blown with 0.05% by
weight chemical blowing agent or more.
15. An article as in claim 1, being essentially free of residual
chemical blowing agent or free of reaction by-products of chemical
blowing agent.
16. An article as in claim 1, having less than about 0.1 percent by
weight auxiliary chromophore, constructed and arranged for
containing material subject to destruction upon exposure to
light.
17. An article as in claim 1, the article being free of a non-foam,
structurally-supporting material positioned to support the foam
article.
18. An article as in claim 1, comprising at least two blow-molded,
foam, microcellular polymeric layers.
19. An article as in claim 18, comprising at least two co-extruded
layers.
20. An article as in claim 1, comprising auxiliary non-foam,
non-structurally-supporting layer adjacent the foam article.
21. An article as in claim 1, including at least one portion having
a wall thickness of less than about 0.075 inch.
22. An article as in claim 1, wherein the article is a container
and at least 50% of the container has a wall thickness of less than
about 0.075 inch.
23. An article as in claim 1, wherein the article is a container
and at least 50% of the container has a wall thickness of less than
about 0.050 inch.
24. An article as in claim 1, wherein the article is a container
and at least 50% of the container has a wall thickness of less than
about 0.040 inch.
25. An article as in claim 1, formed of polymeric material having
melt flow of no more than about 0.2 g/10 min.
26. An article as in claim 1, including a first portion expanded to
a first extent and a second portion expanded at least 1.5 times the
first extent, the first and second portions differing in each of
thickness, material density, and cellular density by no more than
about 5%.
27. An article as in claim 1, including less than about 10 percent
by weight reinforcing agent.
28. An article as in claim 1, having an average cell size of less
than about 50 microns.
29. An article as in claim 1, having an average cell size of less
than about 30 microns.
30. An article as in claim 1, having an average cell size of less
than about 20 microns.
31. An article as in claim 1, having a maximum cell size of about
75 microns.
32. An article as in claim 1, having a maximum cell size of about
50 microns.
33. An article as in claim 1, having a maximum cell size of about
35 microns.
34. An article as in claim 1, having an average cell size of less
than about 30 microns and a maximum cell size of about 75
microns.
35. An article as in claim 1, having an average cell size of less
than about 20 microns and a maximum cell size of about 50
microns.
36. An article as in claim 1, having an average cell size of less
than about 10 microns and a maximum cell size of about 25
microns.
37. An article as in claim 1, wherein the microcellular material is
essentially closed-cell.
38. An article as in claim 1, including at least about 1% by weight
nucleating agent.
39. An article as in claim 38, wherein the nucleating agent is
talc.
40. An article as in claim 2, formed as an extruded parison
suitable for blow molding, having a first portion and a second
portion spaced from the first portion in a parison machine
direction, the first portion and second portion differing in
thickness by a factor of at least about 1.1.
41. An article as in claim 40, the first portion and second portion
differing in thickness by a factor of at least about 1.3.
42. An article as in claim 40, the first portion and second portion
differing in thickness by a factor of at least about 1.5.
43. An article as in claim 40, the first portion and second portion
differing in thickness by a factor of at least about 1.7.
44. An article as in claim 40, the parison having a first portion
and a second portion spaced from the first portion in a parison
machine direction, the first portion and second portion differing
in material density by a factor of at least about 1.1.
45. An article as in claim 44, the first portion and second portion
differing in material density by a factor of at least about
1.3.
46. An article as in claim 44, the first portion and second portion
differing in material density by a factor of at least about
1.5.
47. An article as in claim 44, the first portion and second portion
differing in material density by a factor of at least about
1.7.
48. An article as in claim 44, the first portion and second portion
differing in material density by a factor of at least about
2.0.
49. A system for microcellular blow molding, comprising: extrusion
apparatus including an extruder having an inlet designed to receive
a precursor of polymeric microcellular material, constructed and
arranged to provide a single-phase, non-nucleated solution of
polymeric material and blowing agent, and a blow molding forming
die fluidly connected to the extruder and having an outlet designed
to release a parison of microcellular material, the apparatus
including an enclosed passageway connecting the extruder inlet with
the die outlet, the passageway including a nucleating pathway
having length and cross-sectional dimensions selected to create in
a single-phase, non-nucleated solution of blowing agent and fluid
polymeric material a pressure drop at a rate sufficient to cause
microcellular nucleation; and a blow mold positionable to receive a
parison of microcellular material from the die outlet.
50. A system as in claim 49, the nucleating pathway having length
and cross-sectional dimensions such that, when fluid polymer is
passed through the pathway at a rate of about 40 lbs fluid per
hour, a pressure drop rate in the fluid polymer of at least about
0.3 GPa/sec is created.
51. A system as in claim 49, the enclosed passageway connecting the
inlet with the outlet constructed and arranged to receive a blowing
agent that is a gas under ambient conditions and to mix the blowing
agent with the precursor to form a single-phase, non-nucleated
solution.
52. A system as in claim 49, wherein the nucleating pathway is
constructed and arranged to nucleate microcellular material at a
rate of at least about 60 lbs per hour.
53. A system as in claim 49, wherein the nucleating pathway is
constructed and arranged to nucleate microcellular material at a
rate of at least about 100 lbs per hour.
54. A system as in claim 49, wherein the nucleating pathway is
constructed and arranged to nucleate microcellular material at a
rate of at least about 400 lbs per hour.
55. A system as in claim 49, the die having an exit gap and being
constructed and arranged to vary the size of the exit gap, during
extrusion, to form an extrudate having a thickness that varies as a
function of distance from the exit gap.
56. A system as in claim 49, the die constructed and arranged to
vary the size of the exit gap without changing pressure drop rate
to which a polymeric material/blowing agent mixture passing through
the die is subjected.
57. A system as in claim 49, wherein the nucleating pathway has a
cross sectional dimension that changes along its length.
58. A system as in claim 57, wherein the pathway decreases in cross
section in a downstream direction.
59. A system as in claim 49, wherein the blow mold is constructed
and arranged to form a blow molded, foam, microcellular, polymeric
article.
60. A system comprising: an extruder constructed an arranged to
extrude polymeric foam precursor material; an accumulator,
associated with the extruder, able to receive polymeric foam
precursor material from the extruder and to accumulate a charge of
polymeric foam precursor material; and blow molding apparatus
positionable to receive a product of the accumulator, via a forming
die, and constructed and arranged to blow mold-the material to form
a blow molded foam polymeric article.
61. A system as in claim 60, wherein the die includes a nucleating
pathway having length and cross-sectional dimensions selected to
create, in a single-phase, non-nucleated solution of blowing agent
and fluid polymeric material, a pressure drop at a rate sufficient
to cause nucleation.
62. A system as in claim 60, wherein the die includes a nucleating
pathway having length and cross-sectional dimensions selected to
create, in a single-phase, non-nucleated solution of blowing agent
and fluid polymeric material, a pressure drop at a rate sufficient
to cause microcellular nucleation.
63. A system as in claim 60, further comprising a die positionable
to receive a product of the accumulator and to extrude a
microcellular polymeric parison, and the blow molding apparatus is
constructed and arranged to blow mold the parison to form a blow
molded, foam, microcellular, polymeric article.
64. A system for microcellular blow molding, comprising: an
extruder having an inlet designed to receive a precursor of
polymeric microcellular material, constructed and arranged to
provide a single-phase, non-nucleated solution of polymeric
material and blowing agent; an accumulator positionable to receive
polymeric foam precursor material from the extruder and to
accumulate a charge of polymeric foam precursor material; a blow
molding forming die fluidly connected to the accumulator and having
an outlet designed to release a parison of microcellular material;
and a blow mold positionable to receive a parison of microcellular
material from the die outlet and constructed and arranged to form a
blow molded, foam, microcellular, polymeric article, the apparatus
including an enclosed passageway connecting the extruder inlet with
the die outlet, the passageway including a nucleating pathway
having length and cross-sectional dimensions selected to create in
a single-phase, non-nucleated solution of blowing agent and fluid
polymeric material a pressure drop at a rate sufficient to cause
microcellular nucleation.
65. A method comprising: extruding microcellular polymeric foam
extrudate from an extruder die while varying the thickness of the
extrudate.
66. A method as in claim 65, comprising providing a single-phase,
non-nucleated solution of polymeric material and a blowing agent
that is a gas under ambient conditions, nucleating the single-phase
solution by subjecting the solution to a high pressure drop rate,
and extruding polymeric foam extrudate that is a product of the
single-phase solution.
67. A method as in claim 66, comprising extruding a microcellular
parison suitable for blow molding.
68. A method as in claim 67, further comprising blow molding the
parison to form a microcellular, blow-molded article.
69. A method as in claim 68, the article having a void volume of at
least about 10%.
70. A method as in claim 65, comprising establishing a stream of a
fluid, single-phase non-nucleated solution of a precursor of foamed
polymeric material and a blowing agent, continuously nucleating the
solution to form a nucleated polymeric fluid, and extruding the
polymeric foam extrudate from the nucleated polymeric fluid.
71. A method as in claim 70, the step of continuously nucleating
involving creating sites of nucleation of the blowing agent in the
stream by subjecting the stream to conditions of solubility change
sufficient to create sites of nucleation in the solution in the
absence of an auxiliary nucleating agent.
72. A method as in claim 65, comprising establishing a stream of a
fluid, single-phase non-nucleated solution of a precursor of foamed
polymeric material and a supercritical fluid blowing agent.
73. A method as in claim 70, involving creating sites of nucleation
by subjecting the stream to a pressure drop at a pressure drop rate
sufficient to create sites of nucleation.
74. A method as in claim 73, involving subjecting the stream to a
pressure drop at a pressure drop rate sufficient to create sites of
nucleation at a density of at least about 10.sup.7
sites/cm.sup.3.
75. A method as in claim 73, involving subjecting the stream to a
pressure drop at a pressure drop rate of at least about 0.3 GPa/sec
to create sites of nucleation.
76. A method as in claim 65, comprising extruding polymeric foam
extrudate into ambient conditions from an extruder die while
varying the thickness of the extrudate.
77. A method as in claim 65, involving establishing the stream of
fluid, single-phase non-nucleated solution of a precursor of foamed
polymeric material and a blowing agent by introducing, into fluid
polymeric material flowing at a rate of at least about 10 lbs./hr,
a fluid that is a gas under ambient conditions and, in a period of
less than about one minute, creating a single-phase solution of the
fluid and the polymer, the fluid present in the solution in an
amount of at least about 2% by weight based on the weight of the
solution.
78. A method as in claim 77, comprising continuously nucleating the
solution by continuously decreasing the pressure within successive,
continuous portions of the flowing, single-phase stream at a rate
which increases.
79. A method as in claim 78, wherein the concentration of the
blowing agent in the homogeneous single-phase non-nucleated
solution is at least about 5 percent, by weight, of the
solution.
80. A method as in claim 65, wherein the concentration of the
blowing agent in the homogeneous single-phase non-nucleated
solution is at least about 7 percent, by weight, of the
solution.
81. A method as in claim 65, wherein the concentration of the
blowing agent in the homogeneous single-phase non-nucleated
solution is at least about 10 percent, by weight, of the
solution.
82. A method as in claim 65, wherein the blowing agent is
supercritical carbon dioxide.
83. A method comprising: providing a polymeric foam parison; and
subjecting the parison to blow molding conditions of at least about
15 psi thereby expanding at least a portion of the parison by at
least about 50% in circumference and forming a blow-molded article,
while maintaining a relatively unchanged density by increasing the
density of the parison by no more than about 20% in going from the
parison to the blow-molded article.
84. A method as in claim 83, wherein the foam parison is of void
fraction of at least about 10%.
85. A method as in claim 83, wherein the parison is
microcellular.
86. A method as in claim 83, wherein the parison has a pre-blown
thickness of less than about 0.100 inch.
87. A method as in claims 83, further comprising extruding the
parison from a mixture of polymeric material and blowing agent, the
blowing agent present in the mixture in an amount less than about
3% by weight based on the weight of the mixture.
88. A method as in claim 87, comprising extruding the parison from
a single-phase solution of polymeric material and supercritical
blowing agent.
89. A method as in claim 88, wherein the blowing agent comprises
carbon dioxide.
90. A method as in claim 88, wherein the blowing agent comprises
nitrogen.
91. A method comprising: providing an extruded polymeric
microcellular foam parison; and subjecting the parison to blow
molding conditions.
92. A method as in claim 91, the subjecting step involving applying
pressure of at least about 1.5 bar internal of the parison.
93. A method as in claim 91, involving applying pressure of at
least about 3 bar internal of the parison.
94. A method as in claim 91, involving applying pressure of at
least about 5 bar internal of the parison.
95. A method as in claim 91, involving applying pressure of at
least 10 bar internal of the parison.
96. A method as in claim 91, involving forming a final blow-molded
product that is essentially free of a supporting, non-foam
structure, the article being essentially closed cell, having a wall
thickness of less than about 0.075 inch.
97. A method as in claim 91, comprising continuously extruding
polymeric foam extrudate and continuously subjecting the extrudate
to blow molding conditions.
98. A method as in claim 91, comprising: providing an extruded
polymeric foam parison having a first portion and a second portion
spaced from the first portion in the parison machine direction, the
first portion and the second portion differing in thickness by a
factor of at least about 1.1; and subjecting the parison to blow
molding conditions.
99. A method as in claim 98, the first portion and the second
portion differing in thickness by a factor of at least about
1.3.
100. A method as in claim 98, the first portion and the second
portion differing in thickness by a factor of at least about
1.5.
101. A method as in claim 98, the first portion and the second
portion differing in thickness by a factor of at least about
1.7.
102. A method as in claim 98, comprising: providing an extruded
polymeric foam parison having a first portion and a second portion
spaced from the first portion in the parison machine direction, the
first portion and the second portion differing in material density
by a factor of at least about 1.1; and subjecting the parison to
blow molding conditions.
103. A method as in claim 98, comprising: providing an extruded
parison of polymeric material of melt flow no more than about 0.2
g/10 min; and subjecting the parison to blow molding
conditions.
104. A method comprising: providing a polymeric microcellular foam
parison; and without heating the parison subjecting the parison to
blow molding conditions.
105. A method as in claim 104, wherein the parison is an extruded
polymeric microcellular foam parison.
106. A method comprising: extruding a polymeric foam extrudate from
a extruder die in a machine direction while varying the temperature
of the extrudate exiting the die thereby forming an extrudate
having a first portion and a second portion spaced from the first
portion in the machine direction, the first portion and the second
portion differing in material density by a factor of at least about
1.1.
107. A method as in claim 106, comprising successively varying the
temperature of the extrudate exiting the die via a cold gas
stream.
108. A device comprising: a polymer forming die including an inlet
at an upstream end thereof constructed and arranged to receive a
single-phase, homogeneous solution of polymeric fluid and blowing
agent that is a gas under ambient conditions, an outlet at a
downstream end thereof, defining a die gap, for releasing foamed
polymeric material, and a fluid pathway connecting the inlet with
the outlet, the fluid pathway including a nucleating pathway, the
die constructed and arranged to vary the width of the die gap
during extrusion while maintaining a constant nucleating pathway
gap.
109. A device as in claim 108, the nucleating pathway having length
and cross-sectional dimensions that, when fluid polymer is passed
through the pathway at a rate greater than 40 lbs fluid per hour,
creates a pressure drop in the fluid polymer of at least about 0.3
GPa/sec.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/689,320, filed Oct. 12, 2000, which is a
divisional of U.S. patent application Ser. No. 09/241,352, filed
Feb. 2, 1999, which is a continuation-in-part of PCT application
Ser. No. PCT/US98/27118, filed Dec. 18, 1998, which claims priority
to U.S. patent application Ser. No. 60/107,754 filed Nov. 10, 1998,
and U.S. patent application Ser. No. 60/068,173 filed Dec. 19,
1997.
FIELD OF THE INVENTION
[0002] The present invention relates generally to extrusion blow
molding, and more particularly to a technique for extrusion blow
molding of microcellular polymeric material.
BACKGROUND OF THE INVENTION
[0003] Polymeric extrusion blow molding is a known process in which
a molten polymeric material is extruded from an extruder die as a
parison (an essentially cylindrical polymeric sleeve). The parison
is placed in a mold and, typically while still warm enough to be
soft and moldable, is subjected to significant gas pressure
internal of the cylinder and expanded against the mold. Many common
articles such as beverage bottles, motor oil bottles,
pharmaceutical packaging, cosmetic packaging, and the like are
manufactured using this technique.
[0004] In many cases, a parison is extruded so as to have differing
thickness along its length. Thicker portions may correspond to
locations where the article needs to be reinforced to a relatively
greater extent, or to provide for expansion in some regions to a
greater extent than in other regions (in the blow-molding formation
of, for example, a plastic detergent bottle), while maintaining an
essentially constant thickness in the molded article.
[0005] Foamed polymeric materials are well known, and can be
produced by introducing a physical blowing agent into a molten
polymeric stream, mixing the blowing agent with the polymer, and
extruding the mixture into the atmosphere while shaping the
mixture. Exposure to atmospheric conditions causes the blowing
agent to gasify, thereby forming cells in the polymer.
Alternatively, a chemical blowing agent can be added and caused to
react in the molten polymeric stream, resulting in the generation
of gas that forms cells in the polymer. In both cases, nucleating
agents are normally used to control cell size and uniformity.
[0006] U.S. Pat. No. 4,444,702 (Thomas, et al.) describes a system
for producing tubular extruded parisons of thermoplastic material,
the wall thickness of the extruded parison being varied during
extrusion.
[0007] U.S. Pat. No. 3,939,236 (Hahn) describes a technique
involving extruding a cellular polymeric tubular parison, then blow
molding the parison.
[0008] U.S. Pat. No. 3,225,127 (Scott) describes a process
involving extruding molten plastic containing a foaming agent
through an annular orifice to form a foamed parison, then placing
the parison in a blow mold cavity and expanding the parison within
the mold.
[0009] U.S. Pat. No. 4,874,649 (Daubenbuchel, et al.) states that
major difficulties exist in extrusion blow molding of foam articles
in which a preform that has already been foamed is expanded.
Daubenbuchel, et al. state that foamed material of a preform that
is still in a thermoplastic condition has regions that exhibit
different strength and expandability values over the length and
periphery of the preform, with the result that weak points are
formed under the effect of internal pressure within the preform,
and that in many circumstances these weak points cause the wall of
the preform or the molded article produced therefrom to tear open,
giving rise to wastage. Daubenbuchel, et al. purportedly solve this
problem by co-extruding a multi-layer thermoplastic preform in
which at least one layer is non-foamable. Using a non-foamable
layer purportedly allows the preform to be expanded, after the
material has been foamed, without giving rise to the danger of
forming weak points or holes through the wall of the article. When
the layer of non-foamable material is arranged on the outside of
the article, an article is produced having a smooth exterior
surface. Daubenbuchel, et al. also describe blow-molding expansion
of the preforms at a pressure on the order of 1 bar, or less than
around 0.5 bar, which they characterize as markedly lower than in
the case of conventional extrusion blowing process, to avoid
bubbles or pores in the foamed material from being compressed.
[0010] While processes for the extrusion blow molding of foamed
polymeric material are known, a need exists for simplified
processes for production of extruded blow-molded products having
good physical qualities. It is an object of the invention,
therefore, to provide extrusion blow-molded foam articles of good
physical properties, and techniques for producing these articles.
It is another object to provide relatively thin-walled extruded,
blow-molded foam articles and techniques for producing these
articles that involve controlling foam uniformity and density.
SUMMARY OF THE INVENTION
[0011] The present invention provides a series of articles,
systems, devices, and methods associated with foam, blow-molded
articles.
[0012] In one aspect, the invention provides an article. In one
embodiment, an article is provided comprising a blow-molded, foam,
microcellular, polymeric article.
[0013] In another embodiment, the invention provides an extruded,
microcellular parison suitable for blow-molding.
[0014] In another aspect, the invention provides systems. One
system includes extrusion apparatus having an extruder with an
inlet designed to receive a precursor of polymeric microcellular
material, constructed and arranged to provide a single-phase,
non-nucleated solution of polymeric material and a blowing agent. A
blow-molding forming die is fluidly connected to the extruder and
has an outlet designed to release a parison of microcellular
material. The apparatus includes an enclosed passageway connecting
the extruder inlet to a blow molding forming die outlet. The
passageway includes a nucleating pathway having length and
cross-sectional dimensions selected to create, in a single-phase,
non-nucleated solution of blowing agent and fluid polymeric
material, a pressure drop at a rate sufficient to cause
microcellular nucleation. A blow mold also is included, and is
positionable to receive a parison of microcellular material from
the die outlet.
[0015] In another embodiment, a system is provided that includes an
extruder constructed and arranged to provide a polymeric foam
precursor material, and an accumulator associated with the
extruder. The accumulator is able to receive polymeric foam
precursor material from the extruder and to accumulate a charge of
polymeric foam precursor material. Blow molding apparatus also is
provided in this system, and is positionable to receive a product
of the accumulator, via a forming die. The blow molding apparatus
is constructed and arranged to blow mold the material to form a
blow-molded foam polymeric article.
[0016] In another embodiment a system that includes a combination
of some aspects described above as provided. The system includes an
extruder having an inlet to receive a precursor of polymeric
microcellular material that is constructed and arranged to provide
a single-phase non-nucleated solution of polymeric material and a
blowing agent. An accumulator is provided and is positionable to
receive polymeric foam precursor material from the extruder and to
accumulate a charge of the polymeric foam precursor material. A
blow-molding forming die is fluidly connected to the accumulator
and has an outlet designed to release a parison of microcellular
material. A blow mold is positionable to receive a parison of
microcellular material from the die outlet and is constructed and
arranged to form a blow-molded, foam, microcellular, polymeric
article. The apparatus includes an enclosed passageway connecting
the extruder inlet with the die outlet, the passageway including a
nucleating pathway defined above.
[0017] In another aspect, the invention provides a forming die
device. The die includes an inlet at an upstream end constructed
and arranged to receive a single-phase, homogeneous solution of
polymeric material and a blowing agent that is a gas under ambient
conditions, and an outlet at a downstream end thereof, defining a
die gap, for releasing foamed polymeric material. A fluid pathway
connects the inlet with the outlet and includes a nucleating
pathway. The die is constructed and arranged to vary the width of
the die gap during extrusion while maintaining a constant
nucleating pathway gap.
[0018] In another aspect, the invention provides a series of
methods. In one embodiment, a method is provided that involves
extruding polymeric foam extrudate from an extruder die while
varying the thickness of the extrudate.
[0019] In another embodiment, a method is provided that includes
providing an extrudate polymeric microcellular foam parison and
subjecting the parison to blow molding conditions.
[0020] In another embodiment, a method is provided that involves
extruding a polymeric foam extrudate from an extruder die in a
machine direction while varying the temperature of the extrudate
exiting the die. An extrudate thereby is formed having a first
portion and a second portion spaced from the first portion in the
machine direction, the first portion and second portion differing
in material density by a factor of at least 1.1.
[0021] In another embodiment, a method is provided that involves
subjecting a foam polymeric parison to relatively severe
blow-molding conditions while maintaining relatively constant
density in the parison. A parison can be subjected to blow-molding
conditions of at least about 15 psi thereby expanding at least a
portion of the parison at least about 50% in circumference. This
takes place while the density of the parison remains relatively
constant, in particular the density is increased by no more than
about 20%.
[0022] Other advantages, novel features, and objects of the
invention will become apparent from the following detailed
description of the invention when considered in conjunction with
the accompanying drawings, which are schematic and which are not
intended to be drawn to scale. In the figures, each identical or
nearly identical component that is illustrated in various figures
is represented by a single numeral. For purposes of clarity, not
every component is labeled in every figure, nor is every component
of each embodiment of the invention shown where illustration is not
necessary to allow those of ordinary skill in the art to understand
the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a schematic illustration of an injection blow
molding system of the invention.
[0024] FIG. 2 illustrates a multihole blowing agent feed orifice
arrangement and extrusion screw.
[0025] FIG. 3 is a schematic illustration of a die for the
injection blow molding system of FIG. 1.
[0026] FIG. 4 is a schematic illustration of the die of FIG. 1,
adjusted to extrude relatively thicker microcellular material.
[0027] FIG. 5 is a schematic illustration of another embodiment of
the die of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
[0028] Commonly-owned, co-pending U.S. patent application Ser. No.
08/777,709 "Method and Apparatus for Microcellular Polymer
Extrusion", filed Dec. 20, 1996, commonly-owned co-pending
International patent application Ser. No. PCT/US 98/27118, filed
Dec. 18, 1998, and commonly-owned, co-pending International patent
application serial no. PCT/US 97/15088, filed Aug. 26, 1997 are
incorporated herein by reference.
[0029] The various embodiments and aspects of the invention will be
better understood from the following definitions. As used herein,
"nucleation" defines a process by which a homogeneous, single-phase
solution of polymeric material, in which is dissolved molecules of
a species that is a gas under ambient conditions, undergoes
formations of clusters of molecules of the species that define
"nucleation sites", from which cells will grow. That is,
"nucleation" means a change from a homogeneous, single-phase
solution to a mixture in which sites of aggregation of at least
several molecules of blowing agent are formed. Nucleation defines
that transitory state when gas, in solution in a polymer melt,
comes out of solution to form a suspension of bubbles within the
polymer melt. Generally this transition state is forced to occur by
changing the solubility of the polymer melt from a state of
sufficient solubility to contain a certain quantity of gas in
solution to a state of insufficient solubility to contain that same
quantity of gas in solution. Nucleation can be effected by
subjecting the homogeneous, single-phase solution to rapid
thermodynamic instability, such as rapid temperature change, rapid
pressure drop, or both. Rapid pressure drop can be created using a
nucleating pathway, defined below. Rapid temperature change can be
created using a heated portion of an extruder, a hot glycerine
bath, or the like. A "nucleating agent" is a dispersed agent, such
as talc or other filler particles, added to a polymer and able to
promote formation of nucleation sites from a single-phase,
homogeneous solution. Thus "nucleation sites" do not define
locations, within a polymer, at which nucleating agent particles
reside. "Nucleated" refers to a state of a fluid polymeric material
that had contained a single-phase, homogeneous solution including a
dissolved species that is a gas under ambient conditions, following
an event (typically thermodynamic instability) leading to the
formation of nucleation sites. "Non-nucleated" refers to a state
defined by a homogeneous, single-phase solution of polymeric
material and dissolved species that is a gas under ambient
conditions, absent nucleation sites. A "non-nucleated" material can
include nucleating agent such as talc. A "polymeric
material/blowing agent mixture" can be a single-phase,
non-nucleated solution of at least the two, a nucleated solution of
at least the two, or a mixture in which blowing agent cells have
grown. "Essentially closed-cell" microcellular material is meant to
define material that, at a thickness of about 100 microns, contains
no connected cell pathway through the material. "Nucleating
pathway" is meant to define a pathway that forms part of
microcellular polymeric foam extrusion apparatus and in which,
under conditions in which the apparatus is designed to operate
(typically at pressures of from about 1500 to about 30,000 psi
upstream of the nucleator and at flow rates of greater than about
10 pounds polymeric material per hour), the pressure of a
single-phase solution of polymeric material admixed with blowing
agent in the system drops below the saturation pressure for the
particular blowing agent concentration at a rate or rates
facilitating rapid nucleation. A nucleating pathway defines,
optionally with other nucleating pathways, a nucleation or
nucleating region of a device of the invention. "Reinforcing
agent", as used herein, refers to auxiliary, essentially solid
material constructed and arranged to add dimensional stability, or
strength or toughness, to material. Such agents are typified by
fibrous material as described in U.S. Pat. Nos. 4,643,940 and
4,426,470. "Reinforcing agent" does not, by definition, necessarily
include filler or other additives that are not constructed and
arranged to add dimensional stability. Those of ordinary skill in
the art can test an additive to determine whether it is a
reinforcing agent in connection with a particular material.
[0030] In preferred embodiments, microcellular material of the
invention is produced having average cell size of less than about
50 microns. In some embodiments particularly small cell size is
desired, and in these embodiments material of the invention has
average cell size of less than about 30 microns, more preferably
less than about 20 microns, more preferably less than about 10
microns, and more preferably still less than about 5 microns. The
microcellular material preferably has a maximum cell size of about
100 microns or preferably less than about 75 microns. In
embodiments where particularly small cell size is desired, the
material can have maximum cell size of about 50 microns, more
preferably about 35 microns, and more preferably still about 25
microns. A set of embodiments includes all combinations of these
noted average cell sizes and maximum cell sizes. For example, one
embodiment in this set of embodiments includes microcellular
material having an average cell size of less than about 30 microns
with a maximum cell size of about 50 microns, and as another
example an average cell size of less than about 30 microns with a
maximum cell size of about 35 microns, etc. That is, microcellular
material designed for a variety of purposes can be produced having
a particular combination of average cell size and a maximum cell
size preferable for that purpose. Control of cell size is described
in greater detail below.
[0031] The present invention provides systems and techniques for
extrusion blow molding of microcellular and other polymeric foam
material, and microcellular parisons suitable for blow molding,
that is, parisons that can be subjected to blow molding conditions
as described herein to produce articles as described herein. In
particular, the invention provides techniques for production of
lightweight, strong microcellular articles that can be blow molded
to form microcellular polymeric blow molded parisons that can have
particularly thin walls. It is a feature that articles of the
invention can be produced that are free of a non-foam,
structurally-supporting material positioned to support the foam
article. This means that where a plastic bottle, for example, is
produced, the walls of the bottle can be composed entirely of the
microcellular foam material, without an auxiliary layer of solid
supporting plastic.
[0032] The invention involves the discovery that microcellular
material overcomes problems associated with certain prior art
techniques, in particular, problems in blow molding associated with
the inherent relative weakness of conventional thermoplastic
polymer foams. Microcellular material of the present invention
surprisingly can be blow molded at relatively high pressures, in
particular a pressure of at least about 1.5 bar internal of a
microcellular parison, in some cases at least about 2.5 bar, in
some cases at least about 5 bar, in some cases at least about 7
bar, and in some cases still at least about 10 bar internal of the
parison. This strength is achieved even in microcellular parisons
including at least some portion having a void volume of at least
about 5%, preferably at least about 10%, preferably at least about
20%, more preferably at least about 30%, and in some cases as high
as at least about 50% or at least about 70%, even without
reinforcing agents, and while forming final microcellular foam
products having thin walls, in particular at thicknesses described
below. In this regard, microcellular blow molded articles are
produced having less than about 10% reinforcing agent by weight,
more preferably less than about 5% reinforcing agent, more
preferably still less than about 2%, and in particularly preferred
embodiments the articles of the invention are essentially free of
reinforcing agent.
[0033] It also has been surprisingly found that microcellular foam
parisons of the invention can be blow molded under relatively
severe conditions without a significant change in density in the
material. Specifically, a foam parison of the invention can be
subjected to blow-molding conditions of at least about 15 psi, or
18 or 20 psi or other pressures described above, thereby expanding
at least a portion of the parison by at least about 50% and forming
a blow-molded article while maintaining a relatively constant
density in the material, specifically, increasing the density of
the parison by no more than about 20% in going from the parison to
the blow-molded article. In preferred embodiments at least a
portion of the parison is expanded by at least about 75%, 100%,
150%, 200%, 300%, or at least about 400% in circumference while the
density of the parison is increased by no more than about 15%, 10%,
8%, 5% or preferably 3%.
[0034] Without wishing to be bound by any theory, it is believed
that the microcellular material of the invention is particularly
suitable to the relatively harsh conditions of blow molding because
the cells of the invention, of very small size, are not easily
crushed or otherwise distorted. It is believed that as the size of
the cells decreases, the force required to cause collapse of an
individual cell significantly increases.
[0035] The die of the invention can be shaped and controlled to
produce blow-molded articles that have sections with differing
thicknesses and sections with differing void volume. For example, a
blow-molded, square-shaped bottle can be formed that has sections
defining its corners that are thicker than remaining portions of
the bottle wall. The thicker portions can, e.g., have a void volume
of 50% and the thinner wall a void volume of about 10%. These
thicker regions are reinforcing regions. Reinforcing regions also
can be provided at corners that define the boundary between the
bottle wall and the bottle bottom, or the bottle wall and bottle
top, or vertical corners, or all of these.
[0036] It is a feature of the present invention that strong,
thin-walled articles can be produced that are opaque without the
use of opacifiers. This is because polymeric foam diffracts light,
thus it is essentially opaque and has a white appearance. It is a
feature of the invention that microcellular foams are more opaque,
and uniformly so, than conventional foams. This is a significant
advantage in connection with articles constructed and arranged to
contain material that is subject to destruction upon exposure to
light, such as food containers. Such material can involve food
consumable by animals such as humans, containing vitamins that can
be destroyed upon exposure to light. In a preferred embodiment the
invention provides microcellular blow-molded milk containers, as it
is particularly known that vitamins in milk can be lost upon
exposure to fluorescent light. Milk bottle container producers are
reported to introduce pigments into milk bottles, typically high
density polyethylene milk bottles, so as to protect milk from
vitamin-destroying light. However, pigmented polymeric material is
less amenable to recycling. The present invention provides, in one
embodiment, thin, opaque, blow-molded containers that include less
than about 1% by weight auxiliary opacifer, preferably less than
about 0.05% by weight auxiliary opacifer, and more preferably still
material that is essentially free of auxiliary opacifer. "Auxiliary
opacifer", in the present invention, is meant to define pigments,
dies, or other species that are designed specifically to absorb
light, or talc or other materials that can block or diffract light.
Those of ordinary skill in the art can test whether an additive is
an opacifer. Microcellular blow molded articles of the invention
have the appearance of essentially solid, white, plastic articles,
which offers significant commercial appeal.
[0037] Material of the present invention is, in preferred
embodiments, blown with a physical blowing agent such as an
atmospheric gas, in particular carbon dioxide, and thus in this
embodiment does not require the added expense and complication of
formulating a polymeric precursor to include a chemical blowing
agent, that is, a species that will react under extrusion
conditions to form a blowing agent. Since foams blown with chemical
blowing agents inherently include a residual, unreacted chemical
blowing agent after a final foam product has been produced, as well
as chemical by-products of the reaction that forms a blowing agent,
material of the present invention in this set of embodiments
includes residual chemical blowing agent, or reaction by-product of
chemical blowing agent, in an amount less than that inherently
found in articles blown with 0.1% by weight chemical blowing agent
or more, preferably in an amount less than that inherently found in
articles blown with 0.05% by weight chemical blowing agent or more.
In particularly preferred embodiments, the material is
characterized by being essentially free of residual chemical
blowing agent or free of reaction by-products of chemical blowing
agent. That is, they include less residual chemical blowing agent
or by-product that is inherently found in articles blown with any
chemical blowing agent.
[0038] One advantage of embodiments in which a chemical blowing
agent is not used or used in very minute quantities is that
recyclability of product is maximized. Use of a chemical blowing
agent typically reduces the attractiveness of a polymer to
recycling since residual chemical blowing agent and blowing agent
by-products contribute to non-uniformity in the recyclable material
pool.
[0039] As mentioned, the present invention provides for
blow-molding of relatively high void-volume articles having thin
walls, in some embodiments. In particular, the articles of the
invention have a wall thickness less than about 0.100 inch, more
preferably less than about 0.075 inch, more preferably less than
about 0.050 inch, more preferably still less than about 0.040 inch,
and in some cases as low as 0.025 inch, 0.015 inch, or 0.010 inch
or less.
[0040] In one set of embodiments the invention represents the
solution of problems associated with the extrusion of polymeric
foam parisons having a variety of conventional cell sizes, in
addition to microcellular parisons, for blow molding, that must be
varied in thickness or density. In this set of embodiments the
invention provides techniques for producing a polymeric foam
parison, which can be microcellular, that varies in thickness,
and/or varies in material density, along its length. Specifically,
the preferred extruded polymeric foam parison has a first portion
and a second portion spaced from the first portion in the parison
machine direction, the first portion and the second portion
differing in thickness by a factor of at least about 1.1. In other
embodiments the first and second portions differ in thickness by
factors of at least about 1.3, 1.5, or 1.7. The first and second
portions can differ in material density by a factor of at least
about 1.1, and in other embodiments by a factor of at least about
1.3, 1.5, or 1.7. The parison is suitable for blow-molding to
produce an article including a first portion expanded to a first
extent and a second portion expanded at least 1.5 times the first
extent, the first and second portions, after expansion, differing
in each of thickness, material density, and cellular density by no
more than about 5%. In this technique, a polymeric extrusion die is
provided that is constructed and arranged to subject a flowing,
single-phase solution of molten polymeric material and physical
blowing agent that is a gas under atmospheric conditions to a
consistent pressure drop rate while varying the annular gap at the
die exit to facilitate production of a microcellular polymeric foam
parison that varies in thickness along its length. The die is
effective in this task by providing the physical separation of
nucleation from shaping. That is, nucleation occurs in a consistent
manner (an essentially constant pressure drop rate) upstream of
shaping, thus differential shaping does not effect cell size, cell
density, or material density, substantially. Alternatively or in
addition, the parison can be subjected, during extrusion, to
differing temperature resulting in differential material density as
a function of position in the machine direction.
[0041] Referring now to FIG. 1, an extrusion blow molding system 6
of the present invention is illustrated schematically. System 6
includes an extruder 8 fluidly connected to a blow-molding
extrusion die 10, and a blow mold 11 positionable to receive a
parison of microcellular material from the outlet of the die. Blow
mold 11 can be a conventional mold, and is not described in detail
here except to say that foam parisons of the invention can be blow
molded without heating, thus mold 11 need not include auxiliary
heating systems. That is, a foam parison of the invention,
preferably a microcellular foam parison, can be extruded and then
blow molded in mold 11 without applying heat to the parison in the
mold. Extruder 8 includes a barrel 32 having a first, upstream end
34, and a second, downstream end 36 connected to die 10. Mounted
for rotation within barrel 32 is a screw 38 operably connected, at
its upstream end, to a drive motor 40. Although not shown in
detail, screw 38 includes feed, transition, gas injection, mixing,
and metering sections.
[0042] Positioned along barrel 32, optionally, are temperature
control units 42. Control units 42 can be electrical heaters, can
include passageways for temperature control fluid, and or the like.
Units 42 can be used to heat a stream of pelletized or fluid
polymeric material within the barrel to facilitate melting, and/or
to cool the stream to control viscosity and, in some cases, blowing
agent solubility. The temperature control units can operate
differently at different locations along the barrel, that is, to
heat at one or more locations, and to cool at one or more different
locations. Any number of temperature control units can be
provided.
[0043] Barrel 32 is constructed and arranged to receive a precursor
of polymeric material. As used herein, "precursor of polymeric
material" is meant to include all materials that are fluid, or can
form a fluid and that subsequently can harden to form a
microcellular polymeric article. Typically, the precursor is
defined by thermoplastic polymer pellets, but can include other
species. For example, in one embodiment the precursor can be
defined by species that will react to form microcellular polymeric
material as described, under a variety of conditions. The invention
is meant to embrace production of microcellular material from any
combination of species that together can react to form a polymer,
typically monomers or low-molecular-weight polymeric precursors
which are mixed and foamed as the reaction takes place.
[0044] Preferably, a thermoplastic polymer or combination of
thermoplastic polymers is selected from among amorphous,
semicrystalline, and crystalline material including polyaromatics
such as styrenic polymers including polystyrene, polyolefins such
as polyethylene and polypropylene, fluoropolymers, crosslinkable
polyolefins, and polyamides.
[0045] Typically, introduction of the pre-polymeric precursor
utilizes a standard hopper 44 for containing pelletized polymeric
material to be fed into the extruder barrel through orifice 46,
although a precursor can be a fluid prepolymeric material injected
through an orifice and polymerized within the barrel via, for
example, auxiliary polymerization agents. In connection with the
present invention, it is important only that a fluid stream of
polymeric material be established in the system.
[0046] Immediately downstream of the downstream end 48 of screw 38
in FIG. 1 is a region 50 which can be a temperature adjustment and
control region, auxiliary mixing region, auxiliary pumping region,
or the like. For example, region 50 can include temperature control
units to adjust the temperature of a fluid polymeric stream prior
to nucleation, as described below. Region 50 can include instead,
or in addition, additional, standard mixing units (not shown), or a
flow-control unit such as a gear pump (not shown). In another
embodiment, region 50 can be replaced by a second screw in tandem
which can include a cooling region.
[0047] Microcellular material production according to the present
invention preferably uses a physical blowing agent, that is, an
agent that is a gas under ambient conditions. However, chemical
blowing agents can be used and can be formulated with polymeric
pellets introduced into hopper 44. Suitable chemical blowing agents
include those typically relatively low molecular weight organic
compounds that decompose at a critical temperature or another
condition achievable in extrusion and release a gas or gases such
as nitrogen, carbon dioxide, or carbon monoxide. Examples include
azo compounds such as azo dicarbonamide.
[0048] In embodiments in which a physical blowing agent is used,
along barrel 32 of extruder 30 is a port 54 in fluid communication
with a source 56 of a physical blowing agent. Any of a wide variety
of physical blowing agents known to those of ordinary skill in the
art such as hydrocarbons, chlorofluorocarbons, nitrogen, carbon
dioxide, and the like, and mixtures, can be used in connection with
the invention and, according to a preferred embodiment, source 56
provides carbon dioxide, or nitrogen, or a mixture thereof as a
blowing agent. Supercritical fluid blowing agents are preferred,
particularly supercritical carbon dioxide and/or nitrogen. Where a
supercritical fluid blowing agent is used, a single-phase solution
of polymeric material and blowing agent is created having viscosity
reduced to the extent that extrusion and blow-molding is readily
accomplished even with material of melt flow no more than about 0.2
g/10 min. A pressure and metering device 58 typically is provided
between blowing agent source 56 and port 54. Device 58 can be used
to meter the blowing agent so as to control the amount of the
blowing agent in the polymeric stream within the extruder to
maintain a level of blowing agent at a level, according to one set
of embodiments, between about 1% and 15% by weight, preferably
between about 3% and 12% by weight, more preferably between about
5% and 10% by weight, more preferably still between about 7% and 9%
by weight, based on the weight of the polymeric stream and blowing
agent. In other embodiments very low levels of blowing agents are
suitable, for example less than about 3%, less than about 2%, or
less than about 1.5% by weight blowing agent. These blowing agent
levels can find use, in some instances, where a nucleating agent is
used.
[0049] The systems and methods of the invention allow formation of
microcellular material without use of a nucleating agent. But such
agents can be used and, in some embodiments, polymeric material
including a nucleating agent such as talc is blow molded. It has
been discovered, in accordance with the invention, that polymeric
material including a filler such as talc adds to the ability to
make thicker parts at higher pressures, and improves cell
structure. Although not wishing to be bound by any theory, it is
believed that use of a nucleating agent such as talc reduces the
amount of blowing agent such as carbon dioxide or nitrogen needed,
thus the material will have a higher viscosity (since carbon
dioxide or nitrogen reduces viscosity in such material). Therefore,
the size of nucleating pathways and exit gaps can be increased
while maintaining similar extrusion conditions otherwise, resulting
in thicker parts. In addition, a nucleating agent such as talc adds
to the viscosity of molten polymeric material inherently, allowing
formation of thicker parts. In this embodiment of the invention
nucleating agent such as talc can be added in an amount of at least
1%, or 2%, or 4%, 5.5% or even 7% or more.
[0050] In some embodiments carbon dioxide is used in combination
with other blowing agents such as nitrogen, and in other
embodiments carbon dioxide is used alone with no other blowing
agents present. In other embodiments carbon dioxide can be used
with other blowing agents so long as the other blowing agents do
not materially alter the blowing process. When nitrogen is used,
similarly it can be used alone, in combination with another blowing
agent that adds to or changes the blowing agent properties, or in
combination with another agent that does not materially change the
blowing process.
[0051] The pressure and metering device can be connected to a
controller (not shown) that also is connected to drive motor 40
and/or a drive mechanism of a gear pump (not shown) to control
metering of blowing agent in relationship to flow of polymeric
material to very precisely control the weight percent blowing agent
in the fluid polymeric mixture.
[0052] The described arrangement facilitates a method that is
practiced according to several embodiments of the invention, in
combination with blow molding. The method involves introducing,
into fluid polymeric material flowing at a rate of at least about
10 lbs/hr., a blowing agent that is a gas under ambient conditions
and, in a period of less than about 1 minute, creating a
single-phase solution of the blowing agent fluid in the polymer.
The blowing agent fluid is present in the solution in an amount of
at least about 2.0% by weight based on the weight of the solution
in this arrangement. In preferred embodiments, the rate of flow of
the fluid polymeric material is at least about 40 or 60 lbs/hr.,
more preferably at least about 80 lbs/hr., and in a particularly
preferred embodiment greater than at least about 100 lbs/hr., and
the blowing agent fluid is added and a single-phase solution formed
within one minute with blowing agent present in the solution in an
amount of at least about 3% by weight, more preferably at least
about 5% by weight, more preferably at least about 7%, and more
preferably still at least about 10% (although, as mentioned, in a
another set of preferred embodiments lower levels of blowing agent
are used). In these arrangements, at least about 2.4 lbs per hour
blowing agent, preferably CO.sub.2, is introduced into the fluid
stream and admixed therein to form a single-phase solution. The
rate of introduction of blowing agent is matched with the rate of
flow of polymer to achieve the optimum blowing agent
concentration.
[0053] Although port 54 can be located at any of a variety of
locations along the barrel, according to a preferred embodiment it
is located just upstream from a mixing section 60 of the screw and
at a location 62 of the screw where the screw includes unbroken
flights.
[0054] Referring now to FIG. 2, a preferred embodiment of the
blowing agent port is illustrated in greater detail and, in
addition, two ports on opposing top and bottom sides of the barrel
are shown. In this preferred embodiment, port 154 is located in the
gas injection section of the screw at a region upstream from mixing
section 60 of screw 38 (including highly-broken flights) at a
distance upstream of the mixing section of no more than about 4
full flights, preferably no more than about 2 full flights, or no
more than 1 full flight. Positioned as such, injected blowing agent
is very rapidly and evenly mixed into a fluid polymeric stream to
promote production of a single-phase solution of the foamed
material precursor and the blowing agent.
[0055] Port 154, in the preferred embodiment illustrated, is a
multi-hole port including a plurality of orifices 164 connecting
the blowing agent source with the extruder barrel. As shown, in
preferred embodiments a plurality of ports 154 are provided about
the extruder barrel at various positions radially and can be in
alignment longitudinally with each other. For example, a plurality
of ports 154 can be placed at the 12 o'clock, 3 o'clock, 6 o'clock,
and 9 o'clock positions about the extruder barrel, each including
multiple orifices 164. In this manner, where each orifice 164 is
considered a blowing agent orifice, the invention includes
extrusion apparatus having at least about 10, preferably at least
about 40, more preferably at least about 100, more preferably at
least about 300, more preferably at least about 500, and more
preferably still at least about 700 blowing agent orifices in fluid
communication with the extruder barrel, fluidly connecting the
barrel with a source of blowing agent.
[0056] Also in preferred embodiments is an arrangement (as shown in
FIG. 2) in which the blowing agent orifice or orifices are
positioned along the extruder barrel at a location where, when a
preferred screw is mounted in the barrel, the orifice or orifices
are adjacent full, unbroken flights 165. In this manner, as the
screw rotates, each flight, passes, or "wipes" each orifice
periodically. This wiping increases rapid mixing of blowing agent
and fluid foamed material precursor by, in one embodiment,
essentially rapidly opening and closing each orifice by
periodically blocking each orifice, when the flight is large enough
relative to the orifice to completely block the orifice when in
alignment therewith. The result is a distribution of relatively
finely-divided, isolated regions of blowing agent in the fluid
polymeric material immediately upon injection and prior to any
mixing. In this arrangement, at a standard screw revolution speed
of about 30 rpm, each orifice is passed by a flight at a rate of at
least about 0.5 passes per second, more preferably at least about 1
pass per second, more preferably at least about 1.5 passes per
second, and more preferably still at least about 2 passes per
second. In preferred embodiments, orifices 154 are positioned at a
distance of from about 15 to about 30 barrel diameters from the
beginning of the screw (at upstream end 34).
[0057] Referring now to FIG. 3, a die 10 of the invention is
illustrated schematically in cross-section and includes an annular
outer die body 26 surrounding an inner die body 24 which, in turn,
surrounds an inner mandrel 31. The die includes a fluid inlet 12,
constructed and arranged to receive a single-phase, homogeneous
solution of polymeric fluid and blowing agent that is a gas under
ambient conditions, defined by the junction of the outlet of
extruder 30 and a sidewall entrance of the die. Fluid inlet 12
communicates with an annular ring-like void 18 between the outer
die body and inner die body that is in fluid communication with an
annular channel 20 defined as a gap between the inner die body 24
and outer die body 26. Channel 20 fluidly communicates with an
annular section 28 of the die that is of greater width than that of
channel 20. Section 28 communicates, in turn, with a narrowed
annular portion 29 defining a nucleating pathway having a gap 22
that is of a dimension that creates a rapid pressure drop
facilitating nucleation of the single-phase solution fed to the
die. At its downstream end nucleating pathway 29 fluidly
communicates with an exit 32 of the die having a gap 34. Nucleating
pathway 29, as illustrated, has an essentially constant
cross-sectional dimension along its length. The pathway can change
in cross-sectional dimension along its length as well, for example
decreasing in cross-sectional dimension in a downstream direction
for particularly high pressure drop rates, as disclosed in U.S.
patent application Ser. No. 08/777,709 and International patent
application Ser. No. PCT/U.S. 97/15088, referenced above. Where the
pathway decreases in cross-sectional dimension in a downstream
direction, a single-phase solution can be continuously nucleated by
experiencing continuously decreasing pressure within successive,
continuous portions of the flowing, single-phase stream at a rate
which increases.
[0058] Die 10 is constructed such that inner die body 24 can move
axially relative to outer die body 26. Inner die body 24 can move
from an upstream position as illustrated in FIG. 3 to a downstream
position in which it almost fills a region indicated as 25. Thus,
when inner die body 24 is positioned in an upstream position as
illustrated in FIG. 3, region 25 defines an accumulator.
[0059] In operation, a single-phase solution 23 of polymeric
material and blowing agent is fed from extruder 30 to the die 10,
first into annular ring 18, then through channel 20, accumulator 25
(to the extent that inner die body 24 is positioned upstream) and
section 28 of the die as a single-phase, non-nucleated solution, is
nucleated through a rapid pressure drop occurring at nucleating
pathway 29, and is extruded at exit 32 as a parison suitable for
blow molding. When it is desired to use the accumulating feature of
die 10, exit 32 can be closed (described below) and non-nucleated,
single-phase solution 23 of polymeric material and blowing agent
can be fed from extruder 30 into accumulator 25 while inner die
body 24 moves in an upstream direction. A load can be applied to
inner die body 24 in a downstream direction, during this procedure,
to maintain in accumulator 25 an essentially constant pressure that
maintains the polymer/blowing agent solution in a non-nucleated,
single-phase condition. Then, exit 32 can be opened and inner die
body 24 driven in a downstream direction to nucleate and extrude a
microcellular parison. This feature allows for an extruder to be
run continuously while parison extrusion occurs periodically.
[0060] While polymeric material nucleated in nucleating pathway 29
can include nucleating agent in some embodiments, in other
embodiments no nucleating agent is used. In either case, the
pathway is constructed so as to be able to create sites of
nucleation in the absence of nucleating agent whether or not
nucleating agent is present. In particular, the nucleating pathway
has dimensions creating a desired pressure drop rate through the
pathway. In one set of embodiments, the pressure drop rate is
relatively high, and a wide range of pressure drop rates are
achievable. A pressure drop rate can be created, through the
pathway, of at least about 0.1 GPa/sec in molten polymeric material
admixed homogeneously with about 6 wt % CO.sub.2 passing through
the pathway of a rate of about 40 pounds fluid per hour.
Preferably, the dimensions create a pressure drop rate through the
pathway of at least about 0.3 GPa/sec under these conditions, more
preferably at least about 1 GPa/sec, more preferably at least about
3 GPa/sec, more preferably at least about 5 GPa/sec, and more
preferably still at least about 7,10, or 15 GPa/sec. The nucleator
is constructed and arranged to subject the flowing stream to a
pressure drop at a rate sufficient to create sites of nucleation at
a density of at least about 10.sup.7 or, preferably, 10.sup.8
sights/cm.sup.3. The apparatus is constructed and arranged to
continuously nucleate a fluid stream of single-phase solution of
polymeric material and flowing agent flowing at a rate of at least
20 lbs/hour, preferably at least about 40 lbs/hour, more preferably
at least about 60 lbs/hour, more preferably at least about 80
lbs/hour, and more preferably still at least about 100, 200, or 400
lbs/hour.
[0061] Die 10 is constructed such that mandrel 31 can move axially
relative to the remainder of the die. This allows for exit 32 to be
closed, if desired, by moving mandrel 31 in an upstream direction
so as to seal the inner die lip against the outer die lip.
[0062] Referring now to FIG. 4, die 10 is illustrated with mandrel
31 extended distally such that exit 32 includes a gap 33 that is
significantly widened relative to gap 34 as illustrated in FIG. 3.
This can be effected while maintaining a constant gap 22 in
nucleating section 29 of the die. Thus, nucleation of the
single-phase polymer/blowing agent fluid stream takes place at a
constant pressure drop rate while the die can produce a parison
that varies in thickness. A controller actuates the mandrel such
that exit 32 widens and narrows to produce a parison having varied
thickness as desired. A microcellular product varying in thickness
in a machine direction while having essentially uniform
microcellular structure as is produced using die 10 are described
above.
[0063] The invention also allows co-extrusion of foam or
microcellular foam articles. Although a die for extrusion of such
an article with two or more layers is not illustrated, it can be
clearly understood with reference to FIG. 3. A multi-layer
extrusion die, in one embodiment, includes co-axial, separate,
pathways defining nucleating sections that feed together into a
single exit 32. That is, the die includes a nucleating section 29
as illustrated in FIG. 3, and an additional nucleating section
spaced radially outwardly from nucleating section 29 and fed by a
separate section similar to section 28. Simultaneous, separate
nucleation of separate layers is followed by joining of the
nucleated layers slightly before or at gap 32 where combination of
the layers and shaping and ejection of the layers takes place.
[0064] According to another aspect of the invention a microcellular
polymeric parison is extruded that differs in material density
along its length. In this embodiment the parison can differ in
thickness along its length, as well. This can be accomplished using
the system illustrated in FIG. 5 in which the die portion of an
extruder 70 is provided that is similar to extruder 30 of FIG. 1.
Die portion of extruder 70 need not necessarily include a mandrel
that is movable axially during extrusion to produce a parison of
varying thickness, but includes an air ring 52 for subjecting the
parison, during extrusion, to varying conditions of cooling. The
air ring can subject different portions of the parison to different
cooling conditions, thus reducing cell growth in certain portions
of the parison relative to other portions. In a similar manner,
selected sections of the internal surface of the parison can be
cooled by passing air through a channel 60 formed in mandrel 31
between an inner mandrel part 61 and an outer mandrel part 62.
Internal air cooling can be used alternately or in conjunction with
external air cooling via air ring 52. The resulting parison can be
blow molded and can be created such that some sections are
relatively higher in material density than others. Sections
subjected to different cooling immediately post-extrusion
experience different cell growth and therefore different
density.
[0065] The system of FIG. 5 can be used also to produce a
blow-molded article having increased density at locations where
greater strength is required. For example, in a plastic beverage
container including a threaded mouth for receiving a screw-on cap,
the threaded mouth might desirably be made of higher material
density for added strength than the remainder of the bottle.
[0066] It is one feature of the present invention that the
microcellular extruded parison of the invention is better able to
withstand blowing conditions than many prior art foam parisons.
This is because of the greater resistance of smaller cells the
pressure exerted during blowing. Many prior art foams will exhibit
cell collapse when exposed to blow molding conditions. However, as
cell size decreases, greater pressure is required to cause cell
collapse.
[0067] In one embodiment of the invention, a microcellular parison
is co-extruded with an auxiliary polymeric layer that can be
internal of or external of the microcellular parison, or both. The
auxiliary material can be foam or non-foam and can be added to
create a particular appearance (for example when a colored article
is desired, a microcellular foam core can be covered with a
colored, co-extruded layer). Also, a co-extruded layer may be added
to provide good printability on an article or to provide a
particular surface texture. Other characteristics such as chemical
compatibility, and the like are contemplated. In some cases, a
co-extruded layer may be used, internally or externally of a
microcellular parison core, to isolate the core from internal
contents of the article, or external environment. This can be
useful to increase the use of recycled material in the core. The
auxiliary, co-extruded layer, in preferred embodiments, is not
necessary for structural support. That is, the microcellular
parison could be blow-molded and would provide adequate structural
support on its own, and the co-extruded layer is for purposes of
surface modification only. In one embodiment, an auxiliary
non-foam, non-structurally-supporting layer is provided adjacent
the foam article. This layer can be designed for specific barrier
properties (for example, for compatibility with material to be
contained in the article, Federal regulation requirements,
etc.).
[0068] The production of blow-molded microcellular polymeric
articles in accordance with the invention is surprising since
desirable characteristics for polymers for blow molding are
different from those characteristics desired in typical extrusion
processes. For blow molding, typically high-molecular-weight,
high-viscosity polymers are needed to withstand, successfully, blow
molding conditions. In contrast, in standard extrusion it is
desirable to use lower-molecular weight, lower-viscosity polymers
for high throughput. Thus, extrusion blow molding includes an
inherent dichotomy that adds even more complication when foams are
used. For controlled foaming, higher-molecular weight,
higher-viscosity polymers are favored to prevent uncontrolled
foaming resulting in open-celled material.
[0069] The present invention provides successful high-throughput
microcellular polymeric extrusion blow molding since
higher-molecular weight polymers can be used while reducing
viscosity via supercritical fluid blowing agent incorporation.
Relatively high molecular weight polymers are reduced in viscosity
via the supercritical fluid blowing agent for high-throughput
extrusion, yet at extrusion and gasification of the blowing agent
the high-molecular weight polymer provides the strength needed for
well-controlled microcellular foaming. Therefore, as noted above,
extrusion and blow molding of foam polymeric material, preferably
microcellular foam polymeric material, can be accomplished with
material of melt flow of no more than about 0.2 g/10 min,
preferably no more than about 0.12 g/10 min, more preferably no
more than about 0.1 g/10 min.
[0070] The function and advantage of these and other embodiments of
the present invention will be more fully understood from the
examples below. The following examples are intended to illustrate
the benefits of the present invention, but do not exemplify the
full scope of the invention.
EXAMPLE 1
System
[0071] A tandem extrusion line including a 21/2 mm 32:1 L/D single
screw primary extruder (Akron Extruders, Canal Fulton, Ohio) and a
3 36:1 L/D single screw secondary extruder (Akron Extruders, Canal
Fulton, Ohio) was arranged in a right angle configuration. A
volumetric feeder capable of supplying up to 30 lb/hr was mounted
in the feed throat of the primary extruder such that compounded
talc additive pellets could be metered into the primary extruder.
An injection system for the injection of CO.sub.2 into the
secondary was placed at approximately 8 diameters from the inlet to
the secondary. The injection system included 4 equally spaced
circumferential, radially-positioned ports, each port including 176
orifices, each orifice of 0.02 inch diameter, for a total of 704
orifices. The injection system included an air actuated control
valve to precisely meter a mass flow rate of blowing agent at rates
from 0.2 to 12 lbs/hr at pressures up to 5500 psi.
[0072] The screw of the primary extruder was specially designed
screw to provide feeding, melting and mixing of the polymer/talc
concentrate followed by a mixing section for the dispersion of
blowing agent in the polymer. The outlet of this primary extruder
was connected to the inlet of the secondary extruder using a
transfer pipe of about 24 inches in length.
[0073] The secondary extruder was equipped with specially designed
deep channel, multi-flighted screw design to cool the polymer and
maintain the pressure profile of the microcellular material
precursor, between injection of blowing agent and entrance to a
gear pump (LCI Corporation, Charlotte, N.C.) attached to the exit
of the secondary. The gear pump was equipped with an integral
jacket for heating/cooling and sized to operate at a maximum output
of 250 lb/hr with a rated maximum discharge pressure of 10,000
psi.
[0074] The system was equipped, at exit from the gear pump, with a
die adapter and a vertically mounted blow molding die (Magic
Company, Monza, Italy). The die adapter was equipped with taps for
measurement of melt temperature and pressure just prior to entry
into the die. The blow molding head included a conventional spider
type flow distribution channel and a die adjustment system that
allowed movement of the die relative to the fixed position tip
providing a variety of exit gaps depending on the chosen
tooling.
[0075] A two-piece bottle mold was mounted in a fixture for the
hand molding of sample bottles as a secondary process. One half of
the mold was mounted stationary in the fixture with the other half
mounted on linear slides. Quick acting clamps mounted on the
stationery half of the mold provided the mechanism to clap the mold
shut. A short section of steel tubing sharpened to a point attached
to a 0-50 psi regulator using a length of flexible hose provided
the blow system. Mold diameter varied from approximately 1 inch in
the cap area to 2 to 3 inches in the body of the bottle. The
overall cavity length of the bottle mold was approximately 10
inches.
EXAMPLE 2
Parison and bottle formation
[0076] High density polyethylene (Equistar LR 5403) pellets were
introduced into the main hopper of extrusion line described in
example 1 and a precompounded talc concentrate (50% talc in a HDPE
base) was introduced in the additive feeder hopper. The tooling
attached to the blow molding head included a die with a 0.663 inch
exit diameter and 6.2.degree. taper angle and a tip of 0.633 inch
exit diameter and 2.degree. taper angle. The combination of this
tip and die provides an 8.2.degree. included convergence angle.
[0077] The extruder and gear pump rpm were adjusted to provide an
output of approximately 210 lb/hr at speeds of approximately 78 rpm
on the primary, 32 rpm on the secondary and 50 rpm of the gear
pump. Secondary barrel temperatures were set to maintain a melt
temperature of approximately 315.degree. F. at entrance to the die.
The additive feeder was set to provide an output of approximately
11 lb/hr resulting in a 2.7% by polymer weight talc in the
material. CO.sub.2 blowing agent was injected at a nominal rate of
3.3 lb/hr resulting in a 1.6% by polymer weight blowing agent in
the material.
[0078] The above conditions produced a parison that was 0.045 inch
thick by approximately 1.3 inches in diameter at a density of 0.74
gm/cc. Based on a nominal solid material density of 0.95 gm/cc, the
achieved density reduction is 23%.
[0079] Sample bottles were produced in the following manner: A
parison of approximately 16 inches in length was extruded, manually
removed from the extruder and immediately positioned in the mold.
The mold halves were quickly closed and clamped. With the air
regulator set to 20 psi, the sharpened tube was then used to pierce
the parison at the top of the mold and introduce the air into the
ID of the parison now closed at end of the mold.
[0080] The above conditions produced a bottle of 0.015 inch thick
by approximately 2.5 inches in diameter at a density of 0.70
gm/cc.
EXAMPLE 3
Parison and bottle formation
[0081] High density polyethylene (Equistar LR 5403) pellets were
introduced into the main hopper of an extrusion line described in
example 1 and a precompounded talc concentrate (50% talc in a HDPE
base) was introduced in the additive feeder hopper. The tooling
attached to the blow molding head included a die with a 0.675 exit
diameter and 4.0.degree. taper angle and a tip of 0.633 exit
diameter and 2.degree. taper angle. The combination of this tip and
die provided a 6.0.degree. included convergence angle.
[0082] The extruder and gear pump rpm were adjusted to provide an
output of approximately 180 lb/hr at speeds of approximately 66 rpm
on the primary, 30 rpm on the secondary and 40 rpm of the gear
pump. Secondary barrel temperatures were set to maintain a melt
temperature of approximately 310.degree. F. at entrance to the die.
The additive feeder was set to provide an output of approximately
18 lb/hr resulting in a 5.3% by polymer weight talc in the
material. N.sub.2 blowing agent was injected at a nominal rate of
0.6 lb/hr resulting in a 0.33% by polymer weight blowing agent in
the material.
[0083] The above conditions produced a parison that was 0.080 inch
thick by approximately 1.2 inches in diameter at a density of 0.69
gm/cc. Based on a nominal solid material density of 0.95 gm/cc, the
achieved density reduction is 29%.
[0084] Sample bottles were produced in the following manner: A
parison of approximately 16 inches in length was extruded, manually
removed from the extruder and immediately positioned in the mold.
The mold halves were quickly closed and clamped. With the air
regulator set to 40 psi, the sharpened tube was then used to pierce
the parison at the top of the mold and introduce the air into the
ID of the parison now closed at end of the mold.
[0085] The above conditions produced a bottle of 0.037 inch thick
by approximately 2.0 inches in diameter at a density of 0.79
gm/cc.
[0086] Those skilled in the art would readily appreciate that all
parameters listed herein are meant to be exemplary and that actual
parameters will depend upon the specific application for which the
methods and apparatus of the present invention are used. It is,
therefore, to be understood that the foregoing embodiments are
presented by way of example only and that, within the scope of the
appended claims and equivalents thereto, the invention may be
practiced otherwise than as specifically described.
* * * * *