U.S. patent application number 10/371698 was filed with the patent office on 2003-07-31 for process for forming a multilayer coextruded article and articles therefrom.
Invention is credited to Buehrig, Lavonna Suzanne, Gamble, Benjamin Bradford, Shelby, Marcus David.
Application Number | 20030141625 10/371698 |
Document ID | / |
Family ID | 24055000 |
Filed Date | 2003-07-31 |
United States Patent
Application |
20030141625 |
Kind Code |
A1 |
Shelby, Marcus David ; et
al. |
July 31, 2003 |
Process for forming a multilayer coextruded article and articles
therefrom
Abstract
A process for coextrusion-molding a multilayer article
comprising coextruding at a selected coextuding temperature (i) a
first outer polymer resin layer having a) a viscosity, b) a melting
temperature, and c) a degradation temperature at the selected
coextuding temperature, and (ii) a second inner polymer resin layer
having a) a viscosity, b) a melting temperature, and c) a
degradation temperature at the selected coextuding temperature,
wherein the ratio of the outer polymer resin viscosity to the inner
polymer resin viscosity at the coextuding temperature is less than
or equal to about 1 and the coextuding temperature is above the
melting temperature of the highest melting resin and below the
degradation temperature of the lowest degrading resin to form a
multilayer article.
Inventors: |
Shelby, Marcus David;
(Kingsport, TN) ; Buehrig, Lavonna Suzanne; (Gray,
TN) ; Gamble, Benjamin Bradford; (Kingsport,
TN) |
Correspondence
Address: |
NEEDLE & ROSENBERG P C
127 PEACHTREE STREET N E
ATLANTA
GA
30303-1811
US
|
Family ID: |
24055000 |
Appl. No.: |
10/371698 |
Filed: |
February 21, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10371698 |
Feb 21, 2003 |
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09516311 |
Mar 1, 2000 |
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6562276 |
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09516311 |
Mar 1, 2000 |
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09378262 |
Aug 20, 1999 |
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60097246 |
Aug 20, 1998 |
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Current U.S.
Class: |
264/173.11 ;
264/173.16 |
Current CPC
Class: |
B29C 48/12 20190201;
B29C 2948/92133 20190201; B29C 48/92 20190201; B29C 2948/92666
20190201; B29C 2948/922 20190201; B29C 2948/92809 20190201; B29C
2948/92904 20190201; B29C 48/09 20190201; B29C 48/07 20190201; B29C
2948/926 20190201; B29C 2948/92152 20190201; B29C 48/18 20190201;
B29C 45/16 20130101; B29C 2948/92695 20190201; B29C 45/1642
20130101; B29C 48/023 20190201; B29C 45/1643 20130101; B29C
2948/92266 20190201; B29C 2948/92485 20190201; B29C 2948/92704
20190201; B29C 2948/9259 20190201; B29C 2948/92761 20190201; B29C
2948/9279 20190201 |
Class at
Publication: |
264/173.11 ;
264/173.16 |
International
Class: |
B29C 047/06 |
Claims
What is claimed is:
1. A process for coextruding a multilayer article comprising
coextruding at a selected coextruding temperature: (i) a first
outer polymer resin layer comprising at least one outer layer
polymer resin having at the selected coextruding temperature: a) a
viscosity; b) a melting temperature; and c) a degradation
temperature; and (ii) a second inner polymer resin layer comprising
at least one inner layer polymer resin having at the selected
coextruding temperature: a) a viscosity; b) a melting temperature;
and c) a degradation temperature; wherein the ratio of the outer
layer polymer resin viscosity to the inner layer polymer resin
viscosity at the coextruding temperature is less than or equal to
about 1, and wherein the coextruding temperature is above the
melting temperature of the highest melting resin and below the
degradation temperature of the lowest degrading resin to form a
multilayer article.
2. The process of claim 1, wherein component (i) has an elasticity,
component (ii) has an elasticity, and the ratio of the component
(i) elasticity to the component (ii) elasticity at the coextruding
temperature is approximately the reciprocal of the viscosity
ratio.
3. The process of claim 1, wherein component (i) comprises at least
one performance polymer resin having an elasticity at the selected
coextruding temperature.
4. The process of claim 1, wherein component (ii) comprises at
least one structural polymer resin having an elasticity at the
selected coextruding temperature.
5. The process of claim 1, wherein component (ii) comprises at
least one performance polymer resin having an elasticity at the
selected coextruding temperature.
6. The process of claim 1, wherein component (i) comprises at least
one structural polymer resin having an elasticity at the selected
coextruding temperature.
7. The process of claim 1, wherein component (i) or (ii) is a
barrier resin.
8. The process of claim 1, wherein component (i) or (ii) comprises
a polyamide or a copolymer thereof, an ethylene-vinyl acetate
copolymer (EVOH), a polyalcohol ether, a wholly aromatic polyester,
a resorcinol diacetic acid-based copolyester, a polyalcohol amine,
an isophthalate-containing polyester, poly(ethylene naphthalate) or
a copolymer thereof, or a mixture thereof.
9. The process of claim 8, wherein the polyamide comprises a
partially aromatic polyamide, an aliphatic polyamide, a wholly
aromatic polyamide, or a mixture thereof.
10. The process of claim 1, wherein component (i) or (ii) comprises
a saponified ethylene-vinyl acetate copolymer (EVOH).
11. The process of claim 1, wherein component (i) or (ii) comprises
a polyester.
12. The process of claim 1, wherein component (ii) comprises a
polyester.
13. The process of claim 12, wherein the polyester is a homopolymer
or a copolymer.
14. The process of claim 1, wherein component (i) or (ii) comprises
an aromatic polyester comprising a repeat unit of terephthalic
acid, dimethyl terephthalate, isophthalic acid, dimethyl
isophthalate, dimethyl-2,6 naphthalenedicarboxylate,
2,6-naphthalenedicarboxylic acid, 1,2-, 1,3- and 1,4-phenylene
dioxydoacetic acid, ethylene glycol, diethylene glycol,
1,4-cyclohexanedimethanol (1,4-CHDM), 1,4-butanediol, neopentyl
glycol or a mixture thereof.
15. The process of claim 1, wherein component (i) or (ii) comprises
poly(ethylene terephthalate) or a copolymer thereof.
16. The process of claim 2, wherein the ratio of the outer polymer
resin viscosity to the inner polymer resin viscosity at the
coextruding temperature and the ratio of the outer polymer resin
elasticity to the inner polymer resin elasticity at the coextruding
temperature are approximately 1.
17. The process of claim 2, wherein the ratio of the outer polymer
resin viscosity to the inner polymer resin viscosity at the
coextruding temperature is less than or equal to about 1 and
greater than or equal to about 0.5 and the ratio of the outer
polymer resin elasticity to the inner polymer resin elasticity is
approximately the reciprocal of the viscosity ratio.
18. The process of claim 1, wherein component (i) comprises an
ethylene-vinyl acetate copolymer (EVOH), component (ii) comprises
poly(ethylene terephthalate) or a copolymer thereof, and the ratio
of the outer polymer resin viscosity to the inner polymer resin
viscosity and the ratio of the outer polymer resin elasticity to
the inner polymer resin elasticity at the coextruding temperature
is approximately 1.
19. A multilayer article produced by the process of claim 1.
20. The article of claim 19 in the form of a film, sheet, tube,
pipe, profile, preform, or container.
21. A process for coextruding a 5-layer article comprising
coextruding at a selected coextruding temperature: (i) two outer
polymer resin layers each comprising an outer layer polymer resin
having at the selected coextruding temperature: a) a viscosity; b)
a melting temperature; and c) a degradation temperature; (ii) two
intermediate polymer resin layers disposed between a core layer and
the two outer layers, the two intermediate resin layers each
comprising an intermediate layer polymer resin having at the
selected coextruding temperature: a) a viscosity; b) a melting
temperature; and c) a degradation temperature; and (iii) a core
layer comprising a core layer polymer resin having at the selected
coextruding temperature: a) a viscosity; b) a melting temperature;
and c) a degradation temperature; wherein at each polymer resin
interface the ratio of the outermost polymer resin viscosity to the
next innermost polymer resin viscosity at the coextruding
temperature is less than or equal to about 1 and the coextruding
temperature is above the melting temperature of the highest melting
resin and below the degradation temperature of the lowest degrading
resin to form a 5-layer article.
22. The process of claim 21, wherein at each interface of the five
layers the ratio of the outermost polymer resin viscosity to the
next innermost polymer resin viscosity at the coextruding
temperature is less than or equal to about 1 and greater than or
equal to about 0.5.
23. The process of claim 21, wherein components (i), (ii) and (iii)
have an elasticity at the coextruding temperature, and the ratio of
the outermost polymer resin elasticity to the next innermost
polymer resin elasticity at the coextruding temperature is
approximately the reciprocal of the viscosity ratio.
24. The process of claim 21, wherein component (i) comprises a
structural polymer resin.
25. The process of claim 21, wherein component (ii) comprises a
structural polymer resin.
26. The process of claim 21, wherein component (ii) comprises a
performance polymer resin.
27. The process of claim 21, wherein component (iii) comprises a
structural polymer resin.
28. The process of claim 21, wherein component (iii) comprises a
performance polymer resin.
29. The process of claim 21, wherein component (i) comprises a
structural polymer resin having a structural resin elasticity,
component (ii) comprises a structural polymer resin having a
structural resin elasticity, and component (iii) comprises at least
one performance polymer resin having a performance resin
elasticity, and at each interface of the five layers the ratio of
the outermost polymer resin elasticity to the next innermost
polymer resin elasticity at the coextruding temperature is
approximately the reciprocal of the viscosity ratio.
30. The process of claim 21, wherein components (i) and (ii)
comprise a polyester.
31. The process of claim 21, wherein components (i) and (ii)
comprise an aromatic polyester comprising repeat units of
terephthalic acid, dimethyl terephthalate, isophthalic acid,
dimethyl isophthalate, dimethyl-2,6 naphthalenedicarboxylate,
2,6-naphthalenedicarboxylic acid, 1,2-, 1,3- or 1,4-phenylene
dioxydoacetic acid, ethylene glycol, diethylene glycol,
1,4-cyclohexanedimethanol (1,4-CHDM), 1,4-butanediol, neopentyl
glycol or mixtures thereof.
32. The process of claim 21 wherein component (i) comprises
poly(ethylene terephthalate) or a copolymer thereof.
33. The process of claim 21, wherein component (ii) comprises
poly(ethylene terephthalate) regrind.
34. The process of claim 21, wherein component (iii) is a barrier
resin.
35. The process of claim 21, wherein component (iii) comprises a
polyamide or a copolymer thereof, an ethylene-vinyl acetate
copolymer (EVOH), a polyalcohol ether, a wholly aromatic polyester,
a resorcinol diacetic acid-based copolyester, a polyalcohol amine,
an isophthalate-containing polyester, poly(ethylene naphthalate) or
a copolymer thereof, or a mixture thereof.
36. The process of claim 35, wherein the polyamide comprises a
partially aromatic polyamide, an aliphatic polyamide, a wholly
aromatic polyamide, or a mixture thereof.
37. The process of claim 21, wherein component (iii) comprises a
saponified ethylene-vinyl acetate copolymer (EVOH) or
poly(m-xylylene adipamide).
38. The process of claim 21, wherein component (i) comprises
poly(ethylene terephthalate) or a copolymer thereof, component (ii)
comprises poly(ethylene terephthalate) regrind and component (iii)
comprises poly(m-xylylene adipamide), and at each polymer resin
interface the ratio of the outermost polymer resin viscosity to the
next innermost polymer resin viscosity at the coextruding
temperature is less than or equal to about 1.
39. A 5-layer article produced by the process of claim 21.
40. The article of claim 39 in the form of a film, sheet, tube,
pipe, profile, preform, or container.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of, and claims the benefit
of, application Ser. No. 09/516,311, filed Mar. 1, 2000, which
status is allowed. The Ser. No. 09/516,311 application is a
continuation-in-part application of U.S. Ser. No. 09/378,262, filed
Aug. 20, 1999, now abandoned, which claims priority to provisional
patent application Serial No. 60/097,246, filed Aug. 20, 1998. U.S.
application Ser. Nos. 09/516,311, 09/378,262, and 60/097,246 are
each incorporated herein by reference in their entireties.
FIELD OF THE INVENTION
[0002] This invention relates generally to a process comprising
coinjecting or coextruding a structural polymer resin with one or
more performance polymer resins to a form a multilayer article
without melt flow defects.
BACKGROUND OF THE INVENTION
[0003] Poly(ethylene terephthalate) (PET) is an established bottle
polymer that produces rigid bottles with excellent clarity and
gloss. These containers are manufactured by a process that
comprises drying the PET resin, injection molding a preform and,
finally, stretch blow molding the finished bottle.
[0004] The injection molding of PET preforms requires the melting
of polymer pellets and the injection of the molten, viscous PET
material into a cavity, which also has a core rod. The molten PET
forms a "skin" where it comes into contact with the cold cavity
wall and core rod. This skin is composed of "frozen" PET and will
remain fairly stationary throughout the remainder of the injection
molding process.
[0005] At points extending radially inwardly away from the cavity
wall and, outwardly from the core rod, or at the points at which
the polymer does not directly contact the cavity wall or core rod,
the polymer (which is still elevated in temperature) remains a
viscous, flowing mass. This hot inner viscous material can still
flow relative to the frozen skin layer although its viscosity
increases as it continues to cool. Thus, a temperature transition
region occurs in the radial direction as well as a corresponding
melt viscosity transition (because of PET's viscosity dependence
upon temperature). Regardless of the changes in melt viscosity as a
function of radial distance from the skin, monolayer PET is, for
the most part, unaffected by the shear that develops between the
frozen skin of the PET and the molten polymer that pushes past it.
After the entire cavity has been filled using this process, the
polymer is held in the cavity until the preform has become
sufficiently cool so that it can be blown immediately into a bottle
or the preform is cool enough to be ejected. Cooled preforms that
have been ejected are stored for later reheat blow molding into the
final product.
[0006] Using this process, PET resin is used in a wide range of
applications such as carbonated soft drink, hot-filled juice
products and warm-filled foods. However, PET has insufficient
barrier to meet the desired shelf lives of products with more
demanding gas barrier needs.
[0007] In one particular application, in order to increase the gas
barrier of a PET bottle, it is possible to inject a barrier layer
into or onto a preform during the injection molding process. This
barrier layer is injected into or onto the melt flow stream of the
PET such that the barrier polymer resin flows past the skin of PET
previously injected. This "coinjection" process allows two resins
to be injected into a "multilayer" preform that can be blown to
form the final bottle product.
[0008] Unfortunately, it has been found that the coinjection of a
barrier polymer resin with PET can result in defects in the PET
preform. A commonly observed melt flow defect is small "pulls,"
frequently called chevrons because of their V shape. Chevrons are
interfacial instabilities that occur between layers. Chevrons
detract from the aesthetics of the finished article.
[0009] One barrier resin that may be used in a multilayer process
is an ethylene-vinyl acetate copolymer (EVOH) modified with various
levels of ethylene ("grades"). It is commonly known that these
"grades" of barrier resins have different melt viscosities and
melting points. Generally, it would be desirable to match both the
melt viscosity of the barrier resin and the melt temperature of the
barrier resin to the PET being used. Unfortunately, the
commercially available EVOH (regardless of the grade) has a melt
viscosity and degradation temperature far below that of
commercially available PET. In addition, heat transfer from the
hotter PET layer will further heat the EVOH above its desired
processing temperature and result in even lower melt viscosity of
the barrier resin during injection molding.
[0010] Most of the technology for coinjection is relatively new and
is just becoming commercially viable for molding multilayer
articles or preforms on a large scale. In addition, coinjection for
most practical purposes is focused almost solely on the use of PET
(or a copolymer thereof) as the structural resin for preform
molding applications. In contrast, coextrusion is a
well-established technique that is commonly applied to a wide
variety of different polymers (e.g., PET, copolyesters,
polyolefins, PVC, styrenics, nylons, etc.) and for a much wider
range of applications.
[0011] In coextrusion, multilayer film or sheet is produced as
opposed to a molded article. As with coinjection, there is one or
more "structural" layers combined with one or more "performance"
layers. The structural layers are usually (but not always) cheaper
than the performance layers and are included to keep total cost
down (since performance layers can often be expensive). Examples of
coextrusion include the use of a barrier layer in packaging film,
the use of a UV protecting layer on the outside layer of heavy
gauge sheeting for outdoor weathering protection, the use of
regrind in the center to reduce costs, the use of adhesive/sealing
layers on the outside surface, and the use of glossy and/or
pigmented layers to change the overall aesthetics of the
film/sheet. Unlike the coinjection example cited above, the
"performance" layer in coextrusion does not necessarily have to be
on the inside of the multilayer structure.
[0012] In the process of coextrusion, the various resins are first
melted in separate extruders and then brought together in a
feedblock-a feedblock being nothing more than a series of flow
channels which bring the layers together into a uniform stream.
From this feedblock, this multilayer material then flows through an
adapter and out a film die. The film die can be a traditional flat
film/sheet die (e.g., a coathanger die) or it can be an annular die
as is used in blown film. Coextrusion is also used making more
complicated shapes like profiles. When we refer to coextrusion in
this document, it is implied that all of these other coextrusion
applications are also covered in addition to traditional film/sheet
applications.
[0013] As with coinjection, coextrusion often suffers with the
problem of chevrons and other visual defects. These defects in
coextrusion and coinjection both result from high shear stresses
developing at the layer interface during flow. These stresses are a
function of the viscosities of the layers in addition to the
relative position and thickness of the layers. In fact, knowledge
gained from coextrusion can be used to help minimize the flow
defects in coinjection.
[0014] In addition, coextrusion of flat film often suffers from the
problem of poor layer distribution across the width of the sheet.
For example, if one were to take a piece of coextruded film (for
example, an A/B/A structure) and separate the layers, they might
find that one of the A layers would be much thicker near the outer
edges of the sheet, and very thin in the middle. The B layer would
be just the opposite, that is, being thin near the edges and thick
in the middle. Usually, it is desired that the layers be uniform in
thickness across the full width of the sheet so that properties
(e.g., barrier, color, stiffness, etc.) do not vary across the
width.
[0015] Up until now, correcting these two coextrusion problems
(poor layer distribution uniformity and flow defects) has really
been more of an art than science. There have been some attempts to
balance the viscosities of the resins (i.e., having a viscosity
ratio close to one) to improve layer distribution, but this has met
with only limited success. Thus, there exists a need for a process
to properly select both the resin viscosity and elasticity
parameters and the processing conditions in coextrusion such that
both the interfacial instabilities (i.e., visual defects like
chevrons) and poor layer distribution are eliminated.
[0016] In the coextrusion process according to this invention,
therefore, the "elasticity" of the various resin layers is as
important as the resin viscosity and proper balancing of both the
elasticity ratio and the viscosity ratio simultaneously is needed
in order to have a uniform layer distribution and form a multilayer
article. A process has thus been developed so that processing
conditions and resins can be optimized to eliminate these
multilayer flow problems.
[0017] Because the multilayer flow behavior is very similar for
both coinjection and coextrusion, the method can be effectively
applied for both applications. As a result, the process of the
present invention forms a high quality coinjected multilayer
article or preform as easily as it forms a multilayer coextruded
film structure.
SUMMARY OF THE INVENTION
[0018] The present invention relates to the elimination of melt
flow defects such as chevrons from coinjected and/or coextruded
articles by minimizing the interfacial stress between layers, such
as between a structural layer (e.g., PET) and a performance (e.g.,
barrier) layer, in a multilayer molded structure or article.
[0019] In addition, the present invention relates to the matching
of viscoelastic flow properties of the respective layers so that
layer distribution is maintained in a uniform fashion for
coextrusion and coinjection applications.
[0020] As embodied and broadly described herein, this invention, in
one embodiment, relates to a process for coinjection-molding a
multilayer article. The process comprises coinjecting at a selected
coinjecting temperature (i) a first outer polymer resin layer
having a viscosity at the selected coinjecting temperature, and
(ii) a second inner polymer resin layer having a viscosity at the
selected coinjecting temperature, wherein the ratio of the outer
polymer resin viscosity to the inner polymer resin viscosity at the
coinjecting temperature is less than or equal to about 2 and the
coinjecting temperature is above the melting temperature of the
highest melting resin and below the degradation temperature of the
lowest degrading resin to form a multilayer article.
[0021] In another embodiment, the present invention comprises a
process for coextruding a multilayer article comprising coextruding
at a selected coextruding temperature (i) a first outer polymer
resin layer having a viscosity at the selected coextruding
temperature, and (ii) a second inner polymer resin layer having a
viscosity at the selected coextruding temperature, wherein the
ratio of the outer polymer resin viscosity to the inner polymer
resin viscosity at the coextruding temperature is less than or
equal to about 2 and the coextruding temperature is above the
melting temperature of the highest melting resin and below the
degradation temperature of the lowest degrading resin.
[0022] In another embodiment, the present invention relates to a
process for coinjection-molding a 5-layer article comprising
coinjection-molding at a selected coinjecting temperature (i) two
outer polymer resin layers having a viscosity at the selected
coinjecting temperature, (ii) two intermediate polymer resin layers
disposed between a core layer and the two outer layers, the two
intermediate resin layers having a viscosity at the selected
coinjecting temperature, and (iii) a core layer having a viscosity
at the selected coinjecting temperature, wherein at each polymer
resin interface the ratio of the outermost polymer resin viscosity
to the next innermost polymer resin viscosity at the coinjecting
temperature is less than or equal to about 2 and the coinjecting
temperature is above the melting temperature of the highest melting
resin and below the degradation temperature of the lowest degrading
resin to form a 5-layer article.
[0023] In yet another embodiment, the present invention relates to
a process for coextruding a 5-layer article comprising coextruding
at a selected coextruding temperature (i) two outer polymer resin
layers having a viscosity at the selected coextruding temperature,
(ii) two intermediate polymer resin layers disposed between a core
layer and the two outer layers, the two intermediate resin layers
having a viscosity at the selected coextruding temperature, and
(iii) a core layer having a viscosity at the selected coextruding
temperature, wherein at each polymer resin interface the ratio of
the outermost polymer resin viscosity to the next innermost polymer
resin viscosity at the coextruding temperature is less than or
equal to about 2 and the coextruding temperature is above the
melting temperature of the highest melting resin and below the
degradation temperature of the lowest degrading resin to form a
5-layer article.
[0024] Additional advantages of the invention will be set forth in
part in the detailed description, including the figures, which
follows, and in part will be obvious from the description, or may
be learned by practice of the invention. The advantages of the
invention will be realized and attained by means of the elements
and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing general description
and the following detailed description are exemplary and
explanatory of preferred embodiments of the invention, and are not
restrictive of the invention, as claimed.
BRIEF DESCRIPTION OF THE FIGURES
[0025] FIG. 1 is a plot of a frequency sweep to determine dynamic
viscosity data for EVOH at 180EC.
[0026] FIG. 2 is a schematic diagram of velocity and shear stress
profiles in a polymer resin flow channel.
[0027] FIG. 3 is a plot of .lambda.(A)/.lambda.(B) versus
.eta.(A)/.eta.(B) illustrating the optimum operating region for
eliminating chevrons and balancing layer thickness.
[0028] FIG. 4 is a plot of .lambda.(A)/.lambda.(B) versus
.eta.(A)/.eta.(B) for an A/B/A coextrusion of a PE (polyethylene)
cap layer onto a PETG (polyethylene terephthalate-G) core layer.
The points refer to the different combinations of processing
temperatures as outlined in Example 1.
[0029] FIG. 5 is a plot of .lambda.(A)/.lambda.(B) versus
.eta.(A)/.eta.(B) for an A/B/A coextrusion of a PE (polyethylene)
cap layer onto a PETG (polyethylene terephthalate-G) core layer.
Extrusion temperatures for both layers were held constant at 235EC
and the PE melt index varied from 0.9 to 3.2.
[0030] FIG. 6 is a plot of .lambda.(A)/.lambda.(B) versus
.eta.(A)/.eta.(B) for an A/B/A coextrusion of a PC (polycarbonate)
cap layer onto a PET (polyethylene terephthalate) core layer.
Extrusion temperatures for both layers were different.
DETAILED DESCRIPTION OF THE INVENTION
[0031] The present invention may be understood more readily by
reference to the following detailed description of the invention,
including the appended figures referred to herein, and the examples
provided therein. It is to be understood that this invention is not
limited to the specific processes and conditions described, as
specific processes and/or process conditions for processing molded
articles as such may, of course, vary. It is also to be understood
that the terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting.
[0032] It must also be noted that, as used in the specification and
the appended claims, the singular forms "a," "an" and "the"
comprise plural referents unless the context clearly dictates
otherwise. For example, reference to processing or forming an
"article," "container" or "bottle" from the process of this
invention is intended to comprise the processing of a plurality of
articles, containers or bottles.
[0033] Ranges may be expressed herein as from "about" or
"approximately" one particular value and/or to "about" or
"approximately" another particular value. When such a range is
expressed, another embodiment comprises from the one particular
value and/or to the other particular value. Similarly, when values
are expressed as approximations, by use of the antecedent "about,"
it will be understood that the particular value forms another
embodiment.
[0034] Overview
[0035] The present invention relates to the elimination of melt
flow defects such as chevrons from coinjected and/or coextruded
articles by minimizing the interfacial stress between layers, such
as between a structural layer (e.g., PET) and a performance (e.g.,
barrier) layer, in a multilayer molded structure or article.
Preferably, these melt flow defects may be minimized while
obtaining and retaining the preferred physical properties in the
resulting article.
[0036] In addition, the present invention relates to the matching
of viscoelastic flow properties (i.e., both the viscosity and
elasticity properties) of the respective layers so that layer
distribution is maintained in a uniform fashion for coextrusion and
coinjection applications.
[0037] In one embodiment, it is possible to inject a barrier layer
into a preform during the injection-molding process. The barrier
layer is injected into the melt flow stream of PET (structural
layer) such that the barrier resin flows past the skin (preferably
on the inside) of the PET previously injected. Preferable barrier
resins used in the multilayer process include EVOH modified with
various levels of ethylene ("grades") and poly(m-xylylene
adipamide) (MXD6). This "coinjection" process allows two resins to
be injected into a "multilayer" preform that can be blown to form
the final bottle product. However, it is commonly known that these
"grades" of barrier resins have different melt viscosities and
melting points. When wide differences in melt viscosity occur
between PET and the barrier resin, the potential for formation of
visible melt flow defects increases.
[0038] In addition, the viscosity ratio of the two polymers also
plays an important role in the formation of visible defects. One
example of a visible flow defect is a v-shaped chevron. Chevrons
are interfacial instabilities that occur between layers when the
shear stress at the interface exceeds a critical value. It is,
therefore, desirable to match the melt viscosity of the barrier
resin with the PET resin to eliminate visible defects, particularly
chevrons.
[0039] To eliminate the visible defects, it is necessary to develop
a set of processing conditions and carefully select a grade of PET
or other structural polymer resin such that the melt viscosity and
processing temperatures can be matched as closely as possible to
the performance or barrier polymer resin. By more closely matching
these parameters, the shear stress at the interface between layers
can be reduced thereby producing a higher quality, more resilient
and defect-free multilayer container. For example, a CHDM-modified
PET with dramatically reduced I.V. may be selected so that it can
be processed in such a manner that preforms and the resulting
bottles can be produced without visual defects.
[0040] One embodiment of the present invention comprises minimizing
the stress by balancing the structural polymer layer/performance
polymer layer viscosity ratio in the coinjection and coextrusion
processes. For example, the ratio of the viscosity of the
structural polymer resin divided by the viscosity of the
performance polymer resin is preferably less than or equal to about
2, and more preferably less than or equal to about 1 to form a
multilayer shaped article without melt flow defects. Most
preferably, the ratio of the structural resin melt viscosity to the
performance resin melt viscosity is less than or equal to about 1
and greater than or equal to about 0.5. Widely differing viscosity
ratios can lead to distortion of the shape of the interface as well
as high interfacial stresses (the latter causing chevrons).
[0041] In the coextrusion of films and sheets, interfacial
instabilities are more likely when the outer (cap) layer is very
thin, typically less than 10% of the total thickness because the
shear stresses during coextrusion are highest near the outer
surface. To overcome this, and to minimize the stress and eliminate
chevrons, the outer (cap) layer may have a slightly lower viscosity
than the inner (core) layer. However, the viscosity ratio is still
close to one. However, for a coinjection process, the situation is
reversed. The outer layer(s) preferably comprises at least one
suitable structural polymer resin with a very thin inner (core)
layer(s) of a performance polymer resin. While stresses are
naturally much lower in the middle of the flow channel where,
typically, the performance resin layer flows, it is still possible
for them to become high enough for chevrons to occur.
[0042] For purposes of this invention, when forming a multilayer
article by a coinjection or coextrusion process, the outer or
outermost resin layer of a multilayer material is defined as the
layer that is closest to the surface of the metal wall(s), which
define the flow channel. The metal wall(s) may typically include
the surfaces of the mold wall and core pin, for example. Each
subsequent layer toward the center line of the flow channel is
considered an inner layer relative to the layer closer to the
wall(s). In other words, for every interface of two or more layers,
an outer layer is defined as the closest to the surface of any
wall.
[0043] In contrast to coextrusion, it is preferable in coinjection
if the inner preferably performance polymer resin has a viscosity
that is equal to or slightly higher than the viscosity of the
structural polymer resin (with the viscosity ratio still being
within the above-defined ranges). Currently, for example, the
viscosity of EVOH is much less than that of PET at the processing
temperatures used, which greatly contributes to the formation of
chevrons.
[0044] Both viscosity and elasticity will vary with temperature.
Therefore, it is important to select processing conditions which
(a) most closely match the optimum viscosity and elasticity
temperatures of the selected resins and (b) are within known resin
constraints, such as the melting point and degradation temperature
for each of the selected polymers. In fact, according to the
processes of this invention, the coinjecting or coextruding
temperature should be above the melting temperature of the highest
melting polymer resin used, which may or may not be the structural
or performance polymer resin. Further, the coinjecting or
coextruding temperature should be below degradation temperature of
the lowest degrading polymer resin, which may or may not be the
structural or performance polymer resin.
[0045] It should be appreciated that for many
structural/performance resin combinations there will be more than
one set of suitable processing temperatures within the ranges of
the present invention. This is highly desirable as it allows for
optimization on varying molding and extruding equipment. Thus, it
is desirable to choose a set of optimum elasticity and viscosity
temperatures that are within the polymer resin set constraints and
are reasonably close together or matched, and provide an elasticity
and viscosity ratio within the range of the present invention.
[0046] Viscoelastic Parameters
[0047] In order to apply the processes described herein, it is
first necessary to define the viscoelastic properties of each layer
as a function of temperature. The viscoelastic properties are
viscosity and elasticity. Both of these parameters decrease with
increasing temperature and at varying rates depending on the type
of polymer (polymer I.V., M.sub.w, etc.).
[0048] The viscosity is simply the ratio of the shear force exerted
by a fluid divided by the applied shear rate. More viscous fluids
like honey or oil have a greater resistance to flow than less
viscous fluids like water. In contrast, "elasticity" is a measure
of the "memory" or "rubberiness" of a fluid. A highly elastic
fluid, after deformed, will try to return to its original
undeformed shape once the stress is removed. A rubber band is an
extreme example of a highly elastic material. In contrast, a
material with no elasticity (e.g., a purely viscous fluid like
water) will have no memory of its original shape and will not try
to "snap back" to its original shape after the stress is
removed.
[0049] Polymers fall in between the two extremes of purely elastic
(e.g., a rubber band) and purely viscous, with the degree of
elasticity depending on such things as the molecular weight, degree
of chain entanglement, etc. Polymers also have what is known as a
"fading memory". In other words, the polymer's memory of a stress
event will gradually decrease with increasing time. So if one
applies a stress to a polymer with a fading memory and waits a long
time before releasing, the polymer will have little or no "snap
back" because its memory of the event is gone. The time that it
takes for most of the memory to fade is called the "relaxation
time" and is denoted as .lambda.. For purposes of this invention,
relaxation time is used as a measure of "elasticity."
[0050] Elasticity is important in coextrusion or coinjection
processing in particular because every time a polymer undergoes a
change in flow condition (which results in a change in stress), the
polymer retains some memory of that stress which affects its flow
further downstream. For example, the hot runners that connect the
extruder and injection mold often have a 90 degree elbow in the
piping to change the direction of the flow. This elbow also imparts
a different stress to the polymer as it goes around the bend. If
the time it takes for the polymer to flow from the bend to the gate
is less than the relaxation time .lambda., then the stresses in the
bend will still be remembered by the resin which consequently can
affect the flow in the mold. In some cases, this can lead to the
polymer preferentially and non-symmetrically filling up one side of
the preform mold. This affects cooling behavior and possibly even
layer distributions for multilayer preforms. For coextrusion, the
imparted stresses occur when the layers are first brought together
in the feedblock and continue at different points as the polymer
flows through the adapter and then into the die. Because of the
differences in elasticity between the different layers in a
multilayer flow, the elastically induced stresses at the interface
will cause gradual rearrangement of the interface as it flows down
the channel.
[0051] Therefore, the longer the relaxation time, the more rubbery
(as opposed to liquid-like) the polymer will behave. The elasticity
ratio should fall roughly within the same range as the viscosity
ratio. However, it is preferred to have the elasticity ratio be
approximately or equal to the reciprocal of the viscosity ratio to
offset or balance the tendency of the mismatched polymer
viscosities, which may create non-uniform layers. One method of
quantifying the reciprocal value and for defining a range that is
acceptable in the processes of this invention is to require
that
[0052] -0.2<log 10(elasticity ratio)+log 10(viscosity
ratio)<0.2
[0053] In this range, the variation in layer thickness is less than
about 25% from center to outer edge for a coextruded structure. In
the above equation, if the elasticity ratio is the exact reciprocal
of the viscosity ratio, then the above term equals 0. However, this
value may range from -0.2 to 0.2 with a more preferred range from
-0.1 to 0.1, which gives about +/-10% on the thickness
variation.
[0054] Thus, in one embodiment of the present invention, the ratio
of the elasticity of the structural polymer resin to the elasticity
of the performance polymer resin is preferably between 0.5 and 2,
and more preferably equal to or slightly greater than 1, the more
preferred case stemming from the fact that the preferred viscosity
ratio is slightly less than 1. As the elasticity ratio begins to
deviate excessively from the reciprocal of the viscosity ratio,
layer thickness non-uniformity will become severe. Also, the die
swell of each polymer as it exits the injector nozzle and enters
the mold is a strong function of its relaxation time. Therefore, a
balanced elasticity ratio between the structural (e.g., PET) and
performance (e.g., EVOH barrier) resins will lead to similar die
swells that improve flow and layer uniformity.
[0055] The melt viscosity, elasticity, and processing temperatures
for commercially available EVOH barrier resins, for example, have
been predetermined. It is, therefore, desirable to develop a set of
processing conditions and carefully select a grade of PET such that
the melt viscosity, elasticity, and processing temperatures can be
matched as closely as possible to the performance resin in order to
eliminate flow defects (such as chevrons) from occurring and to
keep the layer thickness distribution uniform. Thus, the shear
stress at the interface between layers can be reduced producing a
higher quality, more resilient and defect-free multilayer article
or container.
[0056] Polymer melt viscosities are known to be proportional to
M.sub.w.sup.3.4, where M.sub.w.sup.3.4 is the weight average
molecular weight. Because M.sub.w is directly related to I.V., melt
viscosity is therefore directly proportional to I.V.sup.5.1, where
I.V..sup.5.1 is measured in 60/40 phenol/tetrachloroethane at
25.degree. C. Thus, by reducing the I.V. of the PET resin, for
example, the melt viscosity can also be decreased.
[0057] Polymer relaxation times will also decrease with decreasing
I.V., so it is important to try to balance both the viscosity and
elasticity ratios at the same time. Usually this involves a
tradeoff in that both ratios may not be made to exactly equal 1.
Thus, for coextrusion, it is not necessary for both the viscosity
and elasticity ratios to be the same, but it is preferable if the
two ratios offset one another (the elasticity ratio should be the
reciprocal to the viscosity ratio). In other words, if the
elasticity ratio is slightly greater than one, then the viscosity
ratio should be slightly less than one. This would lead to uniform
interfaces between layers in coextrusion applications and it is
likely that the same behavior holds true for coinjection. Having
both ratios significantly less than one or both significantly more
than one may lead to problems with layer uniformity.
[0058] The melt viscosity and elasticity of polyesters may be
altered by modifying the polymer compositions, lowering the I.V. of
the polyester, and/or by the careful selection of processing
conditions for both polymers. Thus, those skilled in the art could
readily produce polymers having the desired viscosity and
elasticity ratios using the process of the present invention. For
example, a CHDM-modified PET with a dramatically reduced I.V. can
be coinjected with EVOH under appropriate processing conditions in
such a manner that preforms and the resulting bottles can be
produced without visual defects such as chevrons or other flow
anomalies.
[0059] Estimation of the Viscosity and Elasticity (or Relaxation
Time)
[0060] Estimation of the viscoelastic parameters for a resin
requires the appropriate Theological test data. In the present
invention, frequency sweeps on a cone and plate rheometer are used
to obtain dynamic viscosity information on the polymer melt. This
test, which is well known in the art, provides a complex viscosity
.eta.*, a storage modulus G' and a loss modulus G", all as a
function of the oscillation frequency co.
[0061] An example set of data is shown in FIG. 1 for EVOH at
180.degree. C. For purposes of the detailed description, the
complex viscosity .eta.* is approximately the same as the steady
shear viscosity .eta.. Similarly, the oscillation frequency is
approximately the same as the shear rate for a steady shear test.
The storage modulus G' is a direct measure of the "rubberiness" of
the polymer, whereas G" is related to the amount of viscous
dissipation (similar to the viscosity).
[0062] For coextrusion/coinjection optimization, the relaxation
time .lambda. and the zero shear viscosity .eta..sub.o are
extracted. One method of extraction is to fit all of the data to
one of many constitutive equations available in the literature.
However, an easier method is to estimate the parameters
graphically. The zero shear viscosity .eta..sub.o can be estimated
by extrapolating .eta.* to very small values of .omega. (this has a
value of 93020 poise in FIG. 1). For purposes of this description,
we will refer to the fitted value of .eta..sub.o as simply .eta.,
although it should be understood by the reader that the true steady
shear viscosity .eta. is really frequency (or shear rate)
dependent. To estimate .lambda., the frequency .omega.* where G'
and G" intersect must be found. The relaxation time .lambda. can
then be approximated as 1/.omega.* where, for this EVOH example, we
get a value of 0.006 S.
[0063] Because both .lambda. and .eta. vary with temperature, it is
important to repeat this dynamic viscosity measurement at different
temperatures. At least 3 sweeps for each resin are usually
performed. The parameters .eta. and .lambda. can both be curve fit
to an Arrhenius type of equation with activation energy E.sub.a
having the form:
[0064] .eta.=A exp(Ea/RT)
[0065] .lambda.=B exp(Ea/RT)
[0066] where A and B are front factors, T is temperature and R is
the gas constant. Once values of A, B, and Ea/R are fitted, the
viscosity ratio and elasticity ratio for any two polymers at any
given set of melt temperatures can be calculated.
[0067] Factors for Forming Coinjected or Coextruded Articles
[0068] Some factors to consider when forming a multilayer article
that has minimal interfacial instabilities (i.e., chevrons, wavy
lines, etc.) according to this invention include, but are not
limited to the following:
[0069] 1. Interfacial instabilities occur when the shear stress at
the interface between two layers exceeds a certain critical value
(this value depends on the resins involved). Thus, by keeping the
stress at the interface to a minimum during coinjection or
coextrusion, the instabilities can be eliminated.
[0070] 2. Shear stresses increase as the shear rate increases.
Because of the shape of the velocity profile in the flow channel,
the shear rate tends to be a maximum near the wall (this can be the
die wall in a coextruded structure or the mold wall in coinjection)
and zero near the center of the flow channel (see FIG. 2). As a
result, shear stresses are highest near the wall and lowest at the
center. Thus, the closer the interface is to the wall surface, the
more likely the interfacial stress will exceed the critical stress
such that visual defects form. Interfaces near the center of the
flow channel are unlikely to develop flow instabilities because the
stress is low. Thus, where the application allows (not all do), it
is preferred to have the interface as close to the center as
possible.
[0071] 3. Shear stresses are also increased when the outermost
layer is at a higher viscosity than the next innermost layer.
Therefore, by maintaining viscosities where the outer layer is at a
lower viscosity than an inner layer, the stresses are minimized and
the instabilities are minimized. The closer the interface is to the
mold wall, the lower the viscosity of the more outer layer should
be.
[0072] 4. Shear stresses can also be reduced by lowering the flow
rate of polymer through the die (or the rate at which it is
injected into the mold). However, lowering the flow rate implies a
reduction in line speed, which is not economically attractive.
[0073] 5. Coextrusion often involves having the "performance" layer
as a thin cap layer on the outside of the sheet (e.g., for UV
block, higher gloss, heat sealing, etc.). As a general rule,
whenever this cap layer is less than about 10% of the total sheet
thickness, the interfacial stress is likely to be high enough to
cause instabilities. Thin cap layers are also believed to cause
layer "wavy line" oscillations that start in the feedblock as
opposed to the die. They occur when the angle of impingement of the
flow channels is too high. They can usually be alleviated by
bringing the layers together more gradually (i.e., a smaller
impingement angle) and by keeping the outer layer viscosity low
just as with the regular flow instabilities. Thus methods for
reducing regular flow instabilities described herein also help to
eliminate the wavy lines.
[0074] 6. During coextrusion, the interfacial instabilities are
most likely to start to form in the die land region just before
reaching the die lips. This is because the flow channel is
narrowest there and the stresses are higher. In contrast, the wavy
lines mentioned previously usually start at the impingement point
in the feedblock or coinjection channel.
[0075] 7. Unlike coextrusion, coinjection rarely involves that the
"performance layer" be added as a thin cap layer on the outer edge.
Thus, it would seem that the problems of instabilities should be
minor. However, coinjection involves much higher shear rates than
coextrusion, so the interfacial stresses can be significantly high,
even far removed from the mold wall. In addition, the polymer is
being rapidly cooled from the wall surface inward, which
effectively narrows the flow channel as polymer solidifies and thus
raises the stresses even further. As mentioned before, the closer
the coinjected barrier layer is to the centerline, the less likely
that instabilities will form.
[0076] Detailed Description of the Embodiments
[0077] 1. Coinjection
[0078] In one embodiment for forming a multilayer article by a
coinjection process according to this invention, the ratio of
viscosity for an outermost layer (A), which is closest to the mold
wall and typically called a "cap" layer, over the next innermost
layer (B), which is closer to the center and typically called a
"core" layer, should be less than or equal to (#) about 2. In other
words, .eta.(A)/.eta.(B) #2 where .eta. is viscosity, .eta.(A) is
the outermost layer viscosity and .eta.(B) is the next innermost
layer viscosity. A more preferred embodiment is where
.eta.(A)/.eta.(B) is greater than or equal to about 0.5 and less
than or equal to about 1 (0.5 #.eta.(A)/.eta.(B) #1).
[0079] For most, but not all coinjection applications, the
structural resin (e.g., PET) will be resin A (the outermost layer)
and resin B will be the inner barrier layer since it is near the
center. In a similar manner to coextrusion, if the barrier layer is
in the center of the wall, then .eta.(A)/.eta.(B) is preferred to
be closer to 1. As the barrier layer location moves closer to the
wall, then .eta.(A)/.eta.(B) should get smaller, preferably
approaching 0.5.
[0080] However, other numerous multilayer embodiments are
contemplated by this invention. For example, a multilayer article
of this invention may be prepared from a coinjection of 5 layers
where (starting from one side) the layers are arranged as follows:
PET/EVOH/PET regrind/EVOH/PET. In this example, the
(PET)/.eta.(EVOH) #2 and .eta.(EVOH)/.eta.(PET regrind) #2.
Preferably, each ratio is from 0.5 to 1.
[0081] Thus, to form the above 5 layered article, .eta.(PET)
#.eta.(EVOH) #.eta.(PET regrind). Unfortunately, this is not always
easy since the regrind PET is usually at a lower I.V. (and thus
lower viscosity) than the regular PET. Nevertheless, this is the
optimum condition that will give the best preform (or coextruded
film) with no chevrons and/or instabilities. Further, since the
viscosities are temperature dependent in that the viscosity
decreases with increasing temperature, the various processing
temperatures to help achieve the conditions above may be changed
(e.g., PET regrind may run colder to increase its viscosity).
[0082] It must also noted that having the viscosity ratio of
.eta.(A)/.eta.(B) of less than or equal to 2, more preferably of
less than or equal to about 1, helps reduce pumping pressures
because the less viscous fluid near the wall is serving as a
lubricant.
[0083] In another embodiment, a multilayer article of this
invention may be prepared from a coinjection of 5 layers where
(starting from one side) the layers are arranged as follows:
PET/MXD6/PET regrind/MXD6/PET. MXD6 is poly(m-xylylene adipamide)
and acts as the performance layer with barrier properties in the
5-layer structure. In this example, the .eta.(PET)/.eta.(MXD6) #2
and .eta.(MXD6)/.eta.(PET regrind) #2. Preferably, each ratio is
from 0.5 to 1.
[0084] In yet another embodiment, a multilayer article of this
invention may be prepared from a coinjection of 5 layers where
(starting from one side) the layers are arranged as follows:
PET/PET regrind/MXD6/PET regrind/PET. In this example, the
.eta.(PET)/.eta.(MXD6) #2 and .eta.(MXD6)/.eta.(PET regrind) #2.
Preferably, each ratio is from 0.5 to 1. EVOH may also be the
barrier or performance layer in this embodiment thereby forming a
5-layer article arranged as follows: PET/PET regrind/EVOH/PET
regrind/PET. Again, the .eta.(PET)/.eta.(EVOH) #2 and
.eta.(EVOH)/.eta.(PET regrind) #2.
[0085] Another embodiment involves coinjecting a multilayer article
such that the viscoelastic properties of the resins (i.e.,
viscosity and elasticity) are properly balanced to achieve the best
layer thickness uniformity. In particular, the resins are chosen so
that, at given processing/melt temperatures, both the viscosity
ratio .eta.(A)/.eta.(B) and the relaxation time ratio
.lambda.(A)/.lambda.(B) are approximately less than or equal to
about 2 or otherwise balanced according to this invention. Most
preferably, it is desired that the elasticity ratio is
approximately the reciprocal of the viscosity ratio. If this
condition is not met, then layer rearrangement will occur in the
adapter and the thickness distribution will be altered.
[0086] An example of this is two polymers having different
viscoelastic properties. As the two resins flow together down a
channel, resin A gradually wraps around and "encapsulates" resin B.
The longer the channel, the greater the degree of encapsulation. In
coextrusion, this encapsulation generally occurs in the adapter
that connects the feedblock to the die. Therefore it is important
to keep the adapter length short to minimize this encapsulation.
Once the encapsulated polymer reaches the die, it fans out into the
full sheet width essentially "locking in" whatever distorted shape
was present at the end of the adapter.
[0087] To optimize the conditions to eliminate both the visual
defects and the poor layer distribution, it is usually (but not
always) important that both sets of conditions are simultaneously
met. These "operating windows" are probably easier understood when
shown graphically. FIG. 3 plots the relaxation time ratio of
.lambda..sub.cap/.lambda..sub.core versus the viscosity ratio of
.eta..sub.cap/.eta..sub.core where the "cap" resin denotes the
outermost cap layer "A" and the "core" resin denotes the innermost
core layer "B." For any given resins and processing temperatures,
this will produce an "operating point" somewhere on the graph. For
this operating point to satisfy the instability criterion of
.eta.(A)/.eta.(B) #2 and more preferably 0.5 #.eta.(A)/.eta.(B) #1,
it should fall to the left of the "y-axis" as denoted by the shaded
area. For the flow to maintain layer uniformity, it is most
preferable that the conditions fall approximately along a 45-degree
line (from the upper left-hand corner to the lower right hand
corner). The shaded ellipse in FIG. 3 depicts this region. The
closer the operating point to this diagonal line, the more uniform
the layer structure. As the operating point moves further away in
either direction, the layer distribution becomes poorer, as
depicted in the diagram.
[0088] If both uniform layer distribution and elimination of visual
defects are to be obtained simultaneously, then the operating point
must fall in the upper left hand quadrant along the 45 degree line
(where the two operating regimes intersect). This is therefore the
true optimum processing point for most coextrusion and/or
coinjecting applications.
[0089] For coinjection applications, it is still desirable to meet
these same criteria although the reciprocal balancing of the
viscosity and elasticity ratios is for a slightly different reason.
In coinjection, the problem of poor layer distribution across the
width does not exist since it is a symmetrical annular flow
pattern. However, as described earlier, by balancing the
viscoelastic flow properties, we can help to minimize the chance of
layer distortion as it flows around any bends or elbows in the
connecting piping and gate.
[0090] 2. Coextrusion
[0091] There are four items preferred for a coextruded article or
structure:
[0092] (1) Good layer thickness distribution across the width of
the sheet;
[0093] (2) The absence of interfacial flow instabilities (e.g. wavy
lines, chevrons, etc.);
[0094] (3) Good layer adhesion; and
[0095] (4) Minimal curling/warping of the final film/sheet.
[0096] The method presented herein only addresses the first two
items as good adhesion is more a matter of the chemistry
differences between two, three or five resin layers. Poor adhesion
is often corrected by a tie layer. Similarly, curling is related
primarily to roll cooling conditions and is rarely a "fatal
flaw".
[0097] To understand where items (1) and (2) come into play
requires an understanding of a typical coextrusion feedblock and
die setup for a flat film coextrusion. Depending on the feedblock
plate configuration, two or three resins could be brought together
to form a 2-layer A/B structure, a 3-layer A/B/A structure or a
5-layer A/B/C/B/A or structure. The 3-layer A/B/A structure, being
symmetric (or "balanced") about the center plane, is the easiest to
make. Non-symmetric structures are more prone to curling and
warping due to the differences in thermal expansion and relaxation
during cooling. Fortunately, for the purposes of eliminating flow
instabilities and balancing the layer thickness, it usually does
not matter whether or not the structure is symmetric.
[0098] Many coextruded multilayer embodiments are contemplated by
this invention. For example, a multilayer article of this invention
may be prepared from coextruding 5 layers where (starting from one
side) the layers are arranged as follows: PET/EVOH/PET
regrind/EVOH/PET. In this example, the .eta.(PET)/.eta.(EVOH) #2
and .eta.(EVOH)/.eta.(PET regrind) #2. Preferably, each ratio is
from 0.5 to 1.
[0099] To form the above 5 layered article, .eta.(PET) #.eta.(EVOH)
#.eta.(PET regrind). Unfortunately, this is not always easy since
the regrind PET is usually at a lower I.V. (and thus lower
viscosity) than the regular PET. Nevertheless, this is the optimum
condition that will give the best coextruded film with no chevrons
and/or instabilities. Further, since the viscosities are
temperature dependent in that the viscosity decreases with
increasing temperature, the various processing temperatures to help
achieve the conditions above may be changed (e.g., PET regrind may
run colder to increase its viscosity).
[0100] In another embodiment, a multilayer article of this
invention may be prepared from coextruding 5 layers where (starting
from one side) the layers are arranged as follows: PET/MXD6/PET
regrind/MXD6/PET. MXD6 is poly(m-xylylene adipamide) and acts as
the performance layer with barrier properties in the 5-layer
structure. In this example, the .eta.(PET)/.eta.(MXD6) #2 and
.eta.(MXD6)/.eta.(PET regrind) #2. Preferably, each ratio is from
0.5 to 1.
[0101] In yet another embodiment, a multilayer article of this
invention may be prepared from coextruding 5 layers where (starting
from one side) the layers are arranged as follows: PET/PET
regrind/MXD6/PET regrind/PET. In this example, the
.eta.(PET)/.eta.(MXD6) #2 and .eta.(MXD6)/.eta.(PET regrind) #2.
Preferably, each ratio is from 0.5 to 1. EVOH may also be the
barrier or performance layer in this embodiment thereby forming a
5-layer article arranged as follows: PET/PET regrind/EVOH/PET
regrind/PET. Again, the .eta.(PET)/.eta.(EVOH) #2 and
.eta.(EVOH)/.eta.(PET regrind) #2.
[0102] Laver Rearrangement and Encapsulation
[0103] The different layers are brought together inside the
feedblock although how they are brought together depends on the
type of feedblock (e.g., Welex, Dow, Cloeren, etc). After
impingement, the layers flow side by side through the adapter and
into the die. The adapter can have any of a number of different
cross-sectional shapes although circular and rectangular are the
most common. It is in the adapter where problems with layer
thickness uniformity usually arise. If the viscosity (or as will be
discussed later, the "elasticity") of A is lower than B then it
will tend to wrap around or "encapsulate" B as it flows down the
adapter. Similarly, if B has a lower viscosity than A, then it will
try to encapsulate A. To keep this encapsulation to a minimum, it
is usually recommended that the ratio of viscosities for A and B be
kept less than about 2 (or greater than about 0.5). This general
rule works in many instances but fails in many others. As will be
discussed in the next section, this failure resulted because
elasticity effects, which have previously been neglected, are as
important than the viscosity effects. Thus, it becomes important to
balance the elasticity and viscosity ratios simultaneously.
[0104] Interfacial Instabilities
[0105] Whereas layer rearrangement occurs primarily in the adapter
region, interfacial instabilities occur primarily in the die where
shear rates are higher (100 to 1000 s.sup.-1 in the die versus 10
to 30 s.sup.-1 in the adapter). When the shear stress at the
interface between layers gets above a certain critical value, flow
instabilities occur. These instabilities result in the wavy lines,
chevrons, and ripples that are unacceptable for most end-use
applications.
[0106] There are three main factors that contribute to high
interfacial stresses and thus to flow instabilities. These are (a)
thin cap layers (<10% total thickness), (b) cap layers having a
viscosity which is higher than the next innermost layer and (c)
overall throughput rates that are too high. In other words,
interfacial instabilities are most likely to occur when thin cap
layers are present, particularly if the viscosity of the cap layer
is higher than the core layer. This is because shear stresses tend
to be higher near the outer wall of the adapter or flow channel.
For interfaces near the center of the adapter (e.g., a 50/50 A/B
structure), it is very rare for flow instabilities to occur because
the stresses are already low. Even for thin cap layers, if the cap
layer viscosity is kept lower than the core layer, then interfacial
instabilities can usually be eliminated. This is important since
thin cap layers are very common.
[0107] Finally, as noted in item (c) above, another simple way to
eliminate flow instabilities is to reduce the overall throughput
rate through the die. This will reduce the interfacial shear stress
although it may result in economically unacceptable line speeds.
Reduction in throughput rate should only be used as a "tweaking"
adjustment if problems arise on the line. It is better to properly
select the resins in the initial design phase so that higher
throughput rates can be maintained.
[0108] Elasticity in Coextrusion
[0109] Elasticity plays a role in coextrusion. Interestingly, the
effect of elasticity on layer uniformity is probably more important
than viscosity, which may explain why using only viscosity ratios
to predict flow behavior rarely works.
[0110] The first step in understanding the effects of elasticity is
to define exactly what is meant by "elasticity". Elasticity is the
rubbery like behavior of the fluid--the ability for the melt to
have a memory. On one extreme are materials that are purely viscous
with no elasticity. Examples include water, glycerin, air, etc. On
the other extreme are materials that are purely elastic with no
significant viscosity. Examples of purely elastic materials include
rubber bands, most metals, and solids in general. Polymer melts
fall somewhere in between, being both viscous and elastic at the
same time (i.e., viscoelastic).
[0111] One of the easiest methods for quantifying the viscous and
elastic portions of a polymer melt is to measure the dynamic
viscosity (or dynamic modulus) using a cone and plate rheometer.
This method is well known to those skilled in the art and need not
be described in detail herein. It is emphasized that all of these
properties are functions of the effective shear rate.
[0112] As discussed above, elasticity and relaxation time are
synonymous. The elasticity ratio should preferably be from about
0.5 to about 2 for coextrusion to typically be successful.
Similarly, if the cap layer relaxation time is lower than the core
layer, it will try to encapsulate the core (just as if the cap
layer viscosity was lower than the core layer). Likewise, if the
cap layer relaxation time is higher than the core, then the core
layer will try to encapsulate the cap layer. In effect, it is just
as important to balance the elasticities as it is to balance the
viscosities.
[0113] After defining the two key parameters for each resin, the
results may be combined and predictions about the flow may be made.
It turns out that interfacial instabilities and layer
uniformity/rearrangement can be treated independently during the
analysis. This is preferable since there are times when some
interfacial instabilities are allowed (e.g., in opaque sheet)
although uniform layer distribution across the width of the sheet
is critical. It is also emphasized that "good layer distribution"
really depends on the application. For most applications, it is
desirable to have a constant thickness of each layer across the
entire width of the sheet (this minimizes edge trim). However,
there are some applications where it is more desirable to have the
cap layer completely encapsulate the core (in an A/B/A structure),
especially when the core layer may pose some sort of hazard (even
on the outer edges). One example of this might be where
post-consumer recycle is the core layer (or some other non-FDA
resin) in a food-contact application.
[0114] The steps for performing the coextrusion analysis for
predicting flow are listed below:
[0115] 1. Test each resin for a dynamic viscosity sweep (cone and
plate rheometer) at 3 or more different reasonable temperatures
(i.e., don not use temperatures where the resin can not be
processed or will degrade excessively).
[0116] 2. Determine .eta..sub.o and .lambda. for each resin and at
each temperature.
[0117] 3. Curve fit .eta..sub.o and .lambda. versus T using an
Arrhenius plot for each resin to determine activation energies and
front factors. One skilled in the art would understand without
description how to fit .eta..sub.O and .lambda. versus T in an
Arrhenius plot and as such a detailed description of this technique
is not necessary. Although not required, this will make
extrapolation of the results to different temperatures easier in a
later part of the analysis.
[0118] 4. Calculate the viscosity ratio and elasticity ratio
(.eta..sub.oA/.eta..sub.oB and .lambda..sub.A/.lambda..sub.B) as a
function of melt temperature for each resin pair. By convention,
the numerator of each resin represents the "outermost" layer and is
the one closest to the wall. For a two-layer coextrusion structure
(A/B) the outermost layer is usually taken to be the one that is
thinnest.
[0119] 5. Determine whether interfacial instabilities are a problem
for a given melt temperature. If the cap layer is thin (less than
10%) and .eta..sub.oA/.eta..sub.oB greater than or equal to 2, then
interfacial instabilities will likely occur. It is most preferable,
therefore, to keep .eta..sub.oA/.eta..sub.oB less than or equal to
1 to prevent these instabilities.
[0120] 6. Determine layer uniformity. If .eta..sub.oA/.eta..sub.oB
and .lambda..sub.A/.lambda..sub.B are both greater than 1, then the
core layer will encapsulate the cap. The degree increases the
further away from 1 the ratios get. If both are less than one, then
the cap will encapsulate the core. For uniform layer distribution,
both ratios should be close to 1. Also, if
.eta..sub.oA/.eta..sub.oB is greater than or equal to about 1 and
.lambda..sub.A/.lambda..sub.B is less than or equal to about 1 (or
vice-versa), then the encapsulation effects will offset and the
layers will be roughly uniform.
[0121] Below is a discussion of some of the above steps in more
detail.
[0122] Step 1: Dynamic Viscosity
[0123] Each polymer in the coextruded article or structure should
be tested via standard dynamic viscosity sweeps using a cone and
plate (or parallel plate rheometer) (See FIG. 1). This is a
standard test. At least three or more temperatures for each resin
should be run although these temperatures should represent
"typical" extrusion temperatures. For example, for PET,
temperatures of 260, 280 and 300EC can be used. Below 260EC, the
polymer would not melt and above 300EC, degradation becomes
significant.
[0124] Step 2: Determination of .eta..sub.o and .lambda. for Each
Resin
[0125] As described previously, values for .eta..sub.o and .lambda.
should be extracted from the dynamic viscosity data for each
temperature and for each resin. The parameter .eta..sub.o is the
.eta.* viscosity at low shear rates. The relaxation time .lambda.
is equal to l/w* where w* is the angular velocity where G' and G"
intersect. For many resins, G' and G" will intersect "on the page"
and within the plot range for w*. For some resins, however, one
will have to extrapolate G' and G" off of the page in order to
determine an intersection point.
[0126] Step 4: Calculation of the Viscosity and Elasticity
Ratios
[0127] Once the values of .eta..sub.o and .lambda. are calculated
for each resin and at different temperatures, it is possible to
determine the viscosity and elasticity ratios as a function of melt
temperature. It is often assumed that the two resins are extruded
at the same melt temperature. Even if they are melted and processed
at different temperatures, they will usually equilibrate to some
average temperature within the feedblock and adapter so the
constant temperature assumption is reasonable. This is particularly
true if one layer is a very thin compared to other layers. This
ratio calculation should repeated for each resin pair interface in
the film.
[0128] Step 5: Determination of the Onset of Interfacial
Instabilities
[0129] Interfacial instabilities will usually only occur in thin
cap layers (or in thin die layers if they are close to the outer
edge) when the viscosity of the cap layer is higher than the core
layer (elasticity is not a significant factor here). The general
rule of thumb then is that interfacial instabilities will occur
when .eta..sub.oA/.eta..sub.oB is greater than or equal to 2. Thus,
to prevent the instabilities, .eta..sub.oA/.eta..sub.oB is
preferably less than or equal to 2. How much lower than 2 really
depends on the throughput rate and the thickness of the cap layer
A. For very thin cap layers and/or high line speeds,
.eta..sub.oA/.eta..sub.oB is less than or equal to 2 and should
most preferably be less than about 1. Having a low viscosity cap
layer serves as a sort of lubricant which minimizes pumping
pressures as well as eliminating interfacial instabilities. The
disadvantage of having a low viscosity cap layer is that it will
tend to encapsulate the core resin.
[0130] Step 6: Determination of Layer Uniformity
[0131] Proper determination of layer uniformity across the width of
the sheet requires knowledge of both the viscosity and elasticity
ratios. Also the type of coextrusion where uniformity may be a
problem is for flat film dies whereas annular dies (e.g., blown
film, pipe, preform molding) will not exhibit the same across the
width variability.
[0132] If the calculations for the initial resins result in
unacceptable operating conditions, it is still possible to correct
the problem. A number of ways to do this are described below. These
techniques apply to coinjection and coextrusion.
[0133] Changing the Molecular Weight (or I.V.)
[0134] The first, and most logical method is to change the
molecular weight (or I.V.) of one of the resins. This is because
.eta..sub.o is proportional to M.sub.w.sup.3.4 (or .eta..sub.o is
proportional to I.V..sup.5.1 ). The relaxation time .lambda. also
follows the same M.sub.w (or I.V.) dependence. So, for example, by
increasing the I.V./M.sub.w of one of the resins, we change both
.eta..sub.oA/.eta..sub.- oB and .lambda..sub.A/.lambda..sub.B in a
similar manner. Thus, if we increase the cap layer M.sub.w, both
.eta..sub.oA/.eta..sub.oB and .lambda..sub.A/.lambda..sub.B will
increase. Similarly, decreasing the cap layer M.sub.w will cause
the operating point to decrease. Changing the core layer viscosity
(resin B), has a similar effect although the directions are
reversed. Often the choice of whether to vary the cap or core layer
molecular weight is restricted by what resin formulations are
commercially available.
[0135] Adding a Branching/Crosslinking Agent
[0136] Changing the M.sub.w, or I.V. of one of the resins causes
both .eta..sub.oA/.eta..sub.oB to change in the same direction.
There are situations where this is not desirable and it is
preferred to change the elasticity and viscosity ratios
independently. Adding a brancher/crosslinking primarily affects the
elasticity and is therefore a nice method for varying
.lambda..sub.A/.lambda..sub.B without significantly altering
.eta..sub.oA/.eta..sub.oB. For polyesters, this brancher might be a
typical multifunctional branching agent like trimellitic anhydride
(TMA) or pyromellitic dianhydride (PMDA). For polyethylene,
blending in LDPE (assuming either LLDPE or HDPE is being used) can
increase the branching.
[0137] Changing the Melt Temperatures
[0138] Up until now, it has generally been assumed that the two
resins are at the same melt temperature. This is not an
unreasonable assumption since some thermal equilibration will occur
in the feedblock, adapter and die. Still it is possible to run the
polymers at slightly different melt temperatures (usually 25EC is
considered the maximum temperature differential). Running a polymer
at a slightly different temperature has the same effect as if the
M.sub.w had been changed. For example, if the cap layer is at a
slightly hotter temperature, its elasticity and viscosity are
reduced relative to the nominal melt temperature. This shifts the
operating point along the same diagonal line associated with
changing M.sub.w/I.V.
[0139] If different temperatures, are used, it is necessary to
modify (5) slightly since viscosities and relaxation times must be
extracted for each polymer at the appropriate temperature. The
simplest approach is to use (4), plugging in the appropriate
temperatures for each resin, and then manually calculating the
ratios. While changing melt temperatures has the same effect as
changing I.V./M.sub.w, the effects are not as significant.
Therefore, varying the melt temperature should only be used as an
online "tweaking" adjustment.
[0140] Changing the Feedblock Design
[0141] Cutting metal is always considered a last resort and is
usually on applied when layer uniformity is unacceptable and no
other modification seems to work. Typically, as with a Welex block,
a flow plate is altered so that when the resins impinge on one
another there is some compensation for the encapsulation. For
example, if the cap layer is encapsulating the core, the flow plate
is modified so that when the layers first impinge, the core layer
is partially wrapped around the cap by an equal amount. As the
resins flow towards the die, the cap layer will still try to flow
around the core.
[0142] However, another approach to minimizing the amount of
encapsulation is to shorten the adapter length. A long adapter
provides more time for the resins to rearrange before reaching the
die. The Cloeren multi-manifold die takes this approach to an
extreme since the layers are literally brought together inside the
die with no real adapter to speak of. The Cloeren die is expensive
but useful when viscosity ratios (or elasticity ratios) are
extreme. Layer rearrangement may still want to occur, but is not
given enough time to actually happen.
[0143] 3. Structural Layer
[0144] In accordance with the present invention, and in a preferred
embodiment, the structural layer comprises one or more polymers
that provide the mechanical and physical properties required of a
package material or article. Suitable polymers comprise, but are
not limited to, any polyester homopolymer or copolymers that are
suitable for use in packaging, and particularly food packaging. The
more preferred polyester is PET, including PET regrind.
[0145] Suitable polyesters useful in the present invention are
generally known in the art and may be formed from aromatic
dicarboxylic acids, esters of dicarboxylic acids, anhydrides of
dicarboxylic esters, glycols, and mixtures thereof. Suitable
partially aromatic polyesters are formed from repeat units
comprising terephthalic acid, dimethyl terephthalate, isophthalic
acid, dimethyl isophthalate, dimethyl-2,6 naphthalenedicarboxylate,
2,6-naphthalenedicarboxylic acid, 1,2-, 1,3- and 1,4phenylene
dioxydoacetic acid, ethylene glycol, diethylene glycol,
1,4-cyclohexane-dimethanol, 1,4-butanediol, and neopentyl glycol
mixtures thereof.
[0146] Preferably, the structural polyesters comprise repeat units
comprising terephthalic acid, dimethyl terephthalate, isophthalic
acid, dimethyl isophthalate, and/or
dimethyl-2,6-naphthalenedicarboxylate. The dicarboxylic acid
component of the polyester may optionally be modified with one or
more different dicarboxylic acids (preferably up to about 20 mole
%). Such additional dicarboxylic acids comprise aromatic
dicarboxylic acids preferably having 8 to 14 carbon atoms,
aliphatic dicarboxylic acids preferably having 4 to 12 carbon
atoms, or cycloaliphatic dicarboxylic acids preferably having 8 to
12 carbon atoms. Examples of dicarboxylic acids to be comprised
with terephthalic acid are: phthalic acid, isophthalic acid,
naphthalene-2,6-dicarboxylic acid, cyclohexanedicarboxylic acid,
cyclohexanediacetic acid, diphenyl-4,4'-dicarboxylic acid, succinic
acid, glutaric acid, adipic acid, azelaic acid, sebacic acid,
mixtures thereof and the like.
[0147] In addition, the glycol component may optionally be modified
with one or more different diols other than ethylene glycol
(preferably up to about 20 mole %). Such additional diols comprise
cycloaliphatic diols preferably having 6 to 20 carbon atoms or
aliphatic diols preferably having 25 to 20 carbon atoms. Examples
of such diols comprise: diethylene glycol, triethylene glycol,
1,4-cyclohexanedimethanol, propane-1,3-diol, butane-1,4-diol,
pentane-1,5-diol, hexane-1,6-diol, 3-methylpentanediol-(2,4),
2-methylpentanediol-(1,4), 2,2,4-trimethylpentane-diol-(1,3),
2-ethylhexanediol-(1,3), 2,2-diethylpropane-diol-(1,3),
hexanediol-(1,3), 1,4-di-(hydroxyethoxy)-b- enzene,
2,2-bis-(4-hydroxycyclohexyl)-propane, 2,4-dihydroxy-1,1,3,3-tetra-
methyl-cyclobutane, 2,2-bis-(3-hydroxyethoxyphenyl)-propane,
2-bis-(4-hydroxypropoxyphenyl)-propane, hydroxyethyl resorcinol,
mixtures thereof and the like. Polyesters may be prepared from two
or more of the above diols.
[0148] The resin may also contain small amounts of trifunctional or
tetrafunctional comonomers such as trimellitic anhydride,
trimethylolpropane, pyromellitic dianhydride, pentaerythritol, and
other polyester forming polyacids or polyols generally known in the
art.
[0149] 4. Performance Layer
[0150] At least one layer in a multilayer article of the present
invention is a performance layer and provides the resulting article
with improved physical properties. Such properties comprise, but
are not limited to, barrier to migration (gas, vapor, and/or other
small molecules), barrier to harmful light (ultraviolet light) and
mechanical properties such as heat resistance.
[0151] In one embodiment, a multilayer article of the present
invention where the performance layer is a barrier layer displays
improved CO.sub.2 and/or O.sub.2 barrier compared to an article of
unmodified PET homopolymer. In other embodiments, all of the layers
are modified to display improved properties. Suitable materials for
the barrier layers of the present invention comprise polyamides,
ethylene-vinyl acetate copolymer (EVOH), polyalcohol ethers, wholly
aromatic polyesters, resorcinol diacetic acid-based copolyesters,
polyalcohol amines, isophthalate containing polyesters, PEN and its
copolymers and mixtures thereof. Barrier materials may be used neat
or may be modified to further improve barrier, such as with the
addition of platelet particles, preferably layered clay material,
such as those available from Nanocor, Southern Clay Products, Rheox
and others.
[0152] Suitable polyamides comprise partially aromatic polyamides,
aliphatic polyamides, wholly aromatic polyamides and mixtures
thereof. By "partially aromatic polyamide," it is meant that the
amide linkage of the partially aromatic polyamide contains at least
one aromatic ring and a nonaromatic species.
[0153] Suitable polyamides preferably have a film-forming molecular
weight. Wholly aromatic polyamides preferably comprise, in the
molecule chain, at least 70 mole % of structural units derived from
m-xylylene diamine or a xylylene diamine mixture comprising
m-xylylene diamine and up to 30% of p-xylylene diamine and an
aliphatic dicarboxylic acid having 6 to 10 carbon atoms. These
wholly aromatic polyamides are further described in Japanese Patent
Publication Nos. 1156/75, 5751/75, 5735/75 and No. 10196/75, and
Japanese Patent Application Laid-Open Specification No.
29697/75.
[0154] Polyamides formed from isophthalic acid, terephthalic acid,
cyclohexanedicarboxylic acid, meta- or para-xylylene diamine, 1,3-
or 1,4-cyclohexane(bis)methylamine, aliphatic diacids with 6 to 12
carbon atoms, aliphatic amino acids or lactams with 6 to 12 carbon
atoms, aliphatic diamines with 4 to 12 carbon atoms, and other
generally known polyamide forming diacids and diamines can be used.
The low molecular weight polyamides may also contain small amounts
of trifunctional or tetrafunctional comonomers such as trimellitic
anhydride, pyromellitic dianhydride, or other polyamide-forming
polyacids and polyamines known in the art.
[0155] Preferred partially aromatic polyamides comprise:
poly(m-xylylene adipamide), poly(hexamethylene isophthalamide),
poly(hexamethylene adipamide-co-isophthalamide), poly(hexamethylene
adipamide-co-terephthala- mide), and poly(hexamethylene
isophthalamide-co-terephthalamide). The most preferred partially
aromatic polyamide is poly(m-xylylene adipamide).
[0156] Preferred aliphatic polyamides comprise poly(hexamethylene
adipamide) and poly(caprolactam). The most preferred aliphatic
polyamide is poly(hexamethylene adipamide). Partially aromatic
polyamides are preferred over the aliphatic polyamides where good
thermal properties are crucial.
[0157] Preferred aliphatic polyamides comprise polycapramide (nylon
6), poly-aminoheptanoic acid (nylon 7), poly-aminonanoic acid
(nylon 9), polyundecane-amide (nylon 11), polyarylactam (nylon 12),
polyethylene-adipamide (nylon 2,6), polytetramethylene-adipamide
(nylon 4,6), polyhexamethylene-adipamide (nylon 6,6),
polyhexamethylene-sebacami- de (nylon 6,10),
polyhexamethylene-dodecamide (nylon 6,12),
polyoctamethylene-adipamide (nylon 8,6),
polydecamethylene-adipamide (nylon 10,6),
polydodecamethylene-adipamide (nylon 12,6) and
polydodecamethylene-sebacamide (nylon 12,8).
[0158] Suitable polyalcohol ethers comprise the phenoxy resin
derived from reaction of hydroquinone and epichlorohydrin as
described in U.S. Pat. No. 4,267,301 and U.S. Pat. No. 4,383,101.
These materials can also contain resorcinol units and may in fact
be all resorcinol units as opposed to hydroquinone units for the
aromatic residue.
[0159] Suitable wholly aromatic polyesters (frequently called LCPs)
are formed from repeat units comprising terephthalic acid,
isophthalic acid, dimethyl-2,6-naphthalenedicarboxylate,
2,6-naphthalenedicarboxylic acid, hydroquinone, resorcinol,
biphenol, bisphenol A, hydroxybenzoic acid, hydroxynaphthoic acid
and the like.
[0160] Diacetic resorcinol copolymers are described in U.S. Pat.
No. 4,440,922 and U.S. Pat. No. 4,552,948 and consist of
copolyesters of terephthalic acid, ethylene glycol and a modifying
diacid from 5 to 100 mol % in the composition replacing
terephthalate units. The modifying diacid is either
m-phenylenoxydiacetic acid or p-phenylenoxydiacetic. Either one of
these diacids can be employed either by themselves or as mixtures
in preparation of copolyesters for this invention.
[0161] Suitable polyalcohol amines comprise those derived from
reaction of either resorcinol bisglycidyl ether with an alkanol
amine, such as ethanolamine, or hydroquinone bisglycidyl ether with
an alkanol amine. Mixtures of these bisglycidyl ethers can
obviously also be used in preparation of a copolymer.
[0162] Suitable isophthalate-containing polyesters comprise
polyesters comprising repeat units derived from at least one
carboxylic acid comprising isophthalic acid (preferably at least 10
mole %) and at least one glycol comprising ethylene glycol.
[0163] Suitable poly(ethylene naphthalate) (PEN) and PEN copolymers
comprise polyesters comprising repeat units derived from at least
one carboxylic acid comprising naphthalene dicarboxylic acid
(preferably at least 10 mole %) and at least one glycol comprising
ethylene glycol.
[0164] The most preferred performance layer for barrier is a
saponified ethylene-vinyl acetate copolymer (EVOH). The saponified
ethylene-vinyl acetate copolymer is a polymer prepared by
saponifying an ethylene-vinyl acetate copolymer having an ethylene
content of 15 to 60 mole % up to a degree of saponification of 90
to 100%. The EVOH copolymer should have a molecular weight
sufficient for film formation, and a viscosity of generally at
least 0.01 dL/g, especially at least 0.05 dL/g, when measured at
300.degree. C. in a phenol/water solvent (85 wt. %:15 wt. %).
[0165] Conventional processes all of which are well known in the
art, and need not be described here can make the polymers of the
present invention.
[0166] Although not required, additives normally used in polymers
may be used, if desired. Such additives comprise colorants,
pigments, carbon black, glass fibers, impact modifiers,
antioxidants, surface lubricants, denesting agents, UV light
absorbing agents, metal deactivators, fillers, nucleating agents,
stabilizers, flame retardants, reheat aids, crystallization aids,
acetaldehyde reducing compounds, recycling release aids, oxygen
scavenging materials, or mixtures thereof, and the like.
[0167] All of these additives and many others and their use are
known in the art and do not require extensive discussion.
Therefore, only a limited number will be referred to, it being
understood that any of these compounds can be used in any
combination of the layers so long as they do not hinder the present
invention from accomplishing its objects.
[0168] Shaped, multilayer articles according to this invention
comprise film, sheet, tubing, pipe, profiles, preforms and
containers such as bottles, trays, cups and the like.
EXAMPLES
[0169] The following examples and experimental results are
comprised to provide those of ordinary skill in the art with a
complete disclosure and description of particular manners in which
the present invention can be practiced and evaluated, and are
intended to be purely exemplary of the invention and are not
intended to limit the scope of what the inventors regard as their
invention. Efforts have been made to ensure accuracy with respect
to numbers (e.g., amounts, temperature, etc.); however, some errors
and deviations may have occurred. Unless indicated otherwise, parts
are parts by weight, temperature is in .degree. C. or is at ambient
temperature, and pressure is at or near atmospheric.
Example 1
[0170] Coinjection of Various Grades of PET with EVOH as the
Barrier Layer
[0171] Three different grades of PET were evaluated for use in a
coinjection trial with EVALCA EVOH grade F-101. The PET resins are
tabulated below in Table 1. Each has a different I.V. and the level
of CHDM copolymer modification varied slightly between them as
shown. The higher the I.V. of the resin, the greater the viscosity
and elasticity (the copolymer modification has only a minor
effect).
1TABLE 1 PET Sample I.V. (dL/g) Copolymer Modification Level #1
0.71 Moderate #2 0.77 High #3 0.80 High
[0172] A 5-layer structure was coinjected consisting of
PET/EVOH/PET/EVOH/PET. The EVOH layer was relatively close to the
outside of the structure (i.e., near the wall) so interfacial
stresses are expected to be high. Barrel temperatures for the PET
samples were nominally 285EC whereas the barrel temperatures for
the EVOH were 185EC. However, heat transfer calculations show that
because the EVOH layer is so thin, and is surrounded by the hotter
PET, that it quickly reaches a temperature of approximately 170EC.
This temperature was used to estimate .eta. and .lambda.. The
operating points for each of these three PET resins with PET were
determined according to this invention. Only Resin #1 is predicted
to have both good uniform layer distribution and no
instabilities/chevrons.
[0173] An injection molding trial was held using samples #1 and #2.
Previous attempts with sample #3 had already shown that it would
not work without causing chevrons. Under identical molding
conditions, parts molded using sample #1 were free of chevrons
whereas the parts molded with sample #2 had a visible flow defects
in the form of chevrons.
Example 2
[0174] Determination of an Optimum Coextrusion Window for an
Cap/Core/Cap Layer Coextrusion of polyethylene onto PETG 6763
[0175] A thin layer of polyethylene (Eastman Chemical Company CM
27057-F (2.0 MI)) was coextruded onto both sides of a PETG 6763
sheet. PETG 6763 is a copolyester commonly used in the film and
sheeting industry and has an IV=0.76 dl/g (as measured in 60 wt.
%/40 wt. % phenol/tetrachloroethan- e at 25.degree. C.). The total
thickness was approximately 40 mils with the polyethylene cap
layers being 10% of the thickness each. The film width was
approximately 20 inches. The fact that the cap layers are thin
makes interfacial instabilities a distinct possibility. The
polyethylene (PE) had a melt index of 2.0. Dynamic viscosity
measurements were performed at 220, 240 and 260.degree. C. for the
PETG and 230, 250 and 270.degree. C. for the polyethylene and the
values for .eta. and .lambda. extracted at each temperature. These
values were plotted along with the Arrhenius curve fit. Based upon
the plot, the viscosity of the PETG is higher at lower temperatures
but becomes lower above about 235.degree. C. Because both of these
resins can be processed over a wide range of melt temperatures
(from about 200.degree. C. to 300.degree. C.), it was desired to
find the best set of temperatures to optimize the process.
[0176] To test the method for optimizing conditions, a designed
experiment was performed around the PE and PETG melt temperatures.
The run conditions were as follows in Table 2:
2TABLE 2 Run # T(Polyethylene) T(PETG) 1 210.degree. C. 210.degree.
C. 2 210.degree. C. 260.degree. C. 3 260.degree. C. 260.degree. C.
4 260.degree. C. 210.degree. C. 5 235.degree. C. 235.degree. C.
[0177] This provided a spread of temperatures covering a range of
possible processing conditions. The viscosity ratio and elasticity
ratio (PE over PETG since PE is the outermost layer) was determined
for each of the runs above according to the method outlined in the
detailed description (See FIG. 4). Runs 1 and 5 will probably give
the best layer uniformity since they are closest to the layer
thickness optimum. As one moves to the lower left in FIG. 4 (e.g.
run 4), it is predicted that the thickness of the cap layer will
get thicker near the edges and thinner near the middle. As one
moves to the upper right hand corner of FIG. 4 (e.g. run 2), just
the opposite is predicted to occur.
[0178] Because the PE has poor adhesion to the PETG, it was
possible to peel apart the layers and measure the thickness across
the width. The cap layer thickness was found to go from being thick
on the edges to thick in the middle as one moves from the lower
left hand corner to the upper right hand corner in FIG. 4.
Similarly, there is a crossover in the thickness distribution at
the same location that the optimum process condition is predicted
(between run 1 and run 5). Based on the model, it is predicted that
the optimum run conditions be when both extruders are set at about
220EC. This prediction is verified by the experimental data.
Example 3
[0179] Determination of the Optimum Grade of Polyethylene to be
Coextruded with PETG at 235EC
[0180] In this example, the extrusion temperature was fixed at
235EC for both the PE and the PETG. Otherwise, the coextrusion
equipment and conditions were the same as Example 2. Three
different PE's were evaluated including Eastman Chemical Company
PE's: CM-27053-F (0.9 MI), CM-27057-F (2.0 MI) and CM-27058-F (3.2
MI). The higher the melt index (MI), the lower the molecular weight
of the resin. This lowering of the molecular weight also
correspondingly causes a decrease in both the viscosity and
elasticity.
[0181] The operating points for these three resins coextruded onto
PETG at 235EC are shown in FIG. 5. In addition, a small insert
graph at each point shows the thickness distribution of the PE cap
layer for each value of MI. As with changing the melt temperature
in Example 2, changing the MI for the PE from high to low causes
the layer distribution of the cap layer to go from being heavy on
the outer edges, to heavy in the middle. The optimum value of MI
for uniform layer distribution is predicted to be around 2.2 to
2.4. Nevertheless, the 2.0 melt index sample, which was closest to
the optimum area, was also the best looking sample. In addition,
the 0.9 MI sample had small chevrons present as predicted by the
model.
Example 4
[0182] Coextrusion of Eastman PET 9921 with a 5% Cap Layer of
MAKROLON 2608 Polycarbonate
[0183] A multilayer structure of PET with a thin cap layer of
polycarbonate (PC) was coextruded. The polycarbonate added surface
gloss and also helps to stiffen the polymer since it softens at a
higher temperature (the glass transition temperature of PET is 80EC
and for polycarbonate it is 150EC. Because the cap layer is thin,
the formation of interfacial instabilities is a significant
problem.
[0184] FIG. 6 shows the operating points for different extrusion
temperatures (the PET and PC temperatures were set the same). Below
a processing temperature of 275EC, instabilities are predicted to
occur. Extrusion trials on a 2.5" extruder with a 24" film die
confirmed this. The processing temperature had to be set at 290EC
or higher in order to eliminate the chevrons. This corresponds on
the plot to a viscosity ratio around 0.75 as would be expected for
a relatively thin cap layer. Layer uniformity was generally good as
predicted by the model.
Example 5
[0185] Five Layer Coextrusion of PET and MXD6
[0186] A 5 layer A/B/C/B/A coextrusion test was setup using small
Killion laboratory scale extruders to simulate what occurs in a
five-layer coinjection process and also determine where best to
place the various layers. The PET resins used in the experiment
were Eastman PET 9921 (0.80 IV) and Eastman PET 20007 (0.72 IV) and
the nylon was MXD6 6007. The Eastman PET 20007 was selected to
represent an IV typical of regrind material. It is the lowest
viscosity resin of the three resins at 280.degree. C. Similarly,
the MXD6 6007 has the highest viscosity of the three resins at
280.degree. C. Therefore, the ideal structure to minimize chevrons
based on the viscosity criterion would be to have the Eastman PET
20007 as the outer (A) layer, the PET 9921 as the intermediate (B)
layer and the MXD6 as the core (C) layer.
[0187] As part of the experiment and to test this hypothesis, the
various resins were placed in the A, B, and C layers in various,
but not exhaustive combinations as shown in Table 3 below. The B
and C extruders were 1" extruders although their maximum outputs
were different (C had a maximum RPM of 57 whereas B had a maximum
RPM of 107). The A extruder was a 1.25" extruder. All extruders,
the feedblock and die were set at 280.degree. C. The screw RPM for
each extruder, which roughly correlates with the throughput rate,
is also shown in Table 3. Chevrons occurred when the A cap layer
thickness was very small (<10%) and only runs in this thickness
range are shown below.
[0188] The best films (runs 10 and 11) occurred when the higher
viscosity MXD6 was at or near the center of the film. The worst
chevron-filled films occurred when the MXD6 was the outer cap layer
(layer A). This is in agreement with the predictions of the
viscosity criterion. It further suggests that a 5 layer coinjected
preform consisting of PET/regrind/MXD6/regrind/PET might be more
stable than the current PET/MXD6/regrind/MXD6/PET structure,
particularly if a lower IV PET is used for the structural resin
(compare, for example, runs 11 and 9).
3TABLE 3 Run# Layer A Layer B Layer C Summary of Film Quality 6
MXD6 20007 20007 chevrons on the surface (12 rpm) (100 rpm) (57
rpm) towards the middle of the film 7 MXD6 9921 20007 mild chevrons
but larger (12 rpm) (100 rpm) (57 rpm) than in run 6 8 MXD6 9921
9921 severe chevrons (12 rpm) (100 rpm) (57 rpm) 7 repeat MXD6 9921
20007 chevrons a little bit (12 rpm) (100 rpm) (57 rpm) worse than
in original run 7 9 20007 MXD6 20007 chevrons formed and (15 rpm)
(100 rpm) (57 rpm) disappeared in a periodic fashion 10 20007 MXD6
MXD6 minimal if any chevrons (15 rpm) (100 rpm) (57 rpm) 11 20007
20007 MXD6 good film, no chevrons (15 rpm) (100 rpm) (57 rpm) 9
repeat 20007 MXD6 20007 fairly clear with few (15 rpm) (100 rpm)
(57 rpm) chevrons
[0189] Throughout this application, various publications are
referenced. The disclosures of these publications in their
entireties are hereby incorporated by reference into this
application in order to more fully describe the state of the art to
which this invention pertains.
[0190] It will be apparent to those skilled in the art that various
modifications and variations can be made in the present invention
without departing from the scope or spirit of the invention. Other
embodiments of the invention will be apparent to those skilled in
the art from consideration of the specification and practice of the
invention disclosed herein. It is intended that the specification
and examples be considered as exemplary only, with a true scope and
spirit of the invention being indicated by the following
claims.
* * * * *