Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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IIvvIPROVED POLYESTER COMPOSITIONS FOR MULTILAYER EXTRUSION AND
BARRIER PERFORMANCE
FIELD OF THE INVENTION
The present invention relates to improving the adhesion between a barrier
layer and a
support layer in coextruded blow-molded applications. More particularly this
invention relates
to the incorporation of a modifted polyethylene having adhesive properties
into either the,
barrier layer or the support layer, wherein the modified polyethylene is
prepared by grafting an
unsaturated carboxylic acid or derivative thereof to high-density
polyethylene. Furthermore,
the present invention also relates to modification of the rheology of base
resins, such as PET,
1 o so that they more closely match the rheology of high density polyethylene
(a preferred material
for the support layer in the coextruded blow-molded applications). A better
match in
rheological properties facilitates layer uniformity within a parison,
resulting in more consistent
final products.
BACKGROUND AND SUMMARY OF THE INVENTION
Plastics (synthetic resins) have long been used for various container
applications due
to their light weight, ready availability, relatively low cost to produce and
high strength.
Polyolefin resins have proven particularly useful for such applications. While
polyolefin
resins possess many desired properties, they are not particularly effective as
a barrier to gases
or vapors of chemicals such as hydrocarbons, alcohols, ketones, ethers, etc.
Thus, polyolefin
2 o resins by themselves are not suitable for many applications where
containment of chemical
vapors is critical for environmental or safety reasons. These applications
include fabricated
articles such as storage or transportation containers or vessels, for example,
fuel tanks,
conduits or membranes.
Accordingly, efforts have been made to improve the barrier performance of
containers
made from polyoleftns. One such effort is US-A-5,441,781 which teaches a
multilayer
container (fuel tank), such that one layer will provide the gas barrier
properties while another
(polyolefin) layer will provide the support. This reference teaches that a
third layer (an
"adhesive layer") must be used so that the barrier layer will adhere to the
support layer. The
reference teaches that the adhesive layer comprises a resin such as a modified
polyethylene
3 o prepared by grafting an unsaturated carboxylic acid or a derivative
thereof to high-density
polyethylene (HDPE).
It would be desirable to be able to eliminate this adhesive layer to simplify
manufacturing and reduce cost, yet still have a Polyolefin-based container
with adequate gas-
barrier properties.
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It has now surprisingly been discovered that when low levels of certain
adhesive
materials such as those taught in the '781 patent, are incorporated within a
resin (such as
polyethylene terephthalate, PET) exhibiting permeation barner properties to
fuel components,
and, in particular, exhibiting permeation barrier to oxygenated fuel
components such as
methanol and ethanol, then the adherence properties of the resin are improved,
while
maintaining the gas-barrier performance
Thus, one aspect of the invention is an improved resin comprising polyethylene
terephthalate and High Density polyethylene modified with malefic anhydride
(HDPE-g-MAH),
wherein the polyethylene terephthalate comprises 90 to 98 percent of the
composition, the
modified polyethylene comprises 10 to 2 percent of the composition, and the
malefic anhydride
comprises from 0.5 to 5.0% percent by weight of the modified polyethylene.
It has also been discovered that when certain other adhesive materials (for
example,
LLDPE-g-MAH) are added to the PET at the same concentrations as the
aforementioned
HDPE-g-MAH, the barrier performance of the blend is diminished. Thus, while it
is possible
l5 to achieve adhesion between PET and HDPE merely by blending in a material
that is
chemically compatible with each phase, the present invention is unique in that
adhesion can be
achieved without diminishing the barrier performance of the barrier.
This new resin can be advantageously used in multilayer structures as it will
allow the
elimination of adhesive or tie layers. Barrier layers comprised of the resin
of the invention will
2 o adhere much better to other layers, including polyolefinic support layers,
eliminating the
necessity of an adhesive or tie layer. Thus rather than the 3 or 5 layer
structures taught by the
'781 patent the resin of the current invention allows 2 or three layer
structures. Furthermore,
even if a tie layer is still used, adhesion between PET and a tie layer will
be improved if the
PET is first modified by the incorporation of high density polyethylene-
grafted-malefic
2 5 anhydride.
Accordingly, another aspect of the invention is a multilayer plastic container
comprising two layers, one of which is a gas-barrier layer, the other of which
is a polyolefinic
support layer, wherein the barrier layer includes an amount of modified high-
density
polyethylene, wherein the modified high-density polyethylene is prepared by
grafting an
3 o unsaturated carboxylic acid or a derivative thereof to the high-density
polyethylene, the
modified high-density polyethylene being added in an amount such that the gas-
barrier layer
sufficiently adheres to the adjacent layer.
It would also be valuable to improve the adherence properties of PET in
general, so
that PET may also be more easily used in applications other than containers.
Thus, another
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aspect of the invention comprises a method of improving the adherence
properties of the
barner layer (which can be crystalline polyesters, crystalline polyamides,
crystalline
polyarylates and crystalline polyethylene-co-vinyl alcohol) resins) to
polyolefinic materials
comprising incorporating a modified polyethylene prepared by grafting an
unsaturated
carboxylic acid or a derivative thereof to the polyethylene, wherein the
modified polyethylene
is added to the polyethylene terephthalate in an amount between ~% and 10%
percent by
weight, preferably in an amount between 3% and 8% by weight. In the case of
fuel tank
applications, the polyethylene material is preferably high density
polyethylene and the
modified polyethylene is a modified high density polyethylene.
Currently, coextrusion blow-molding is the preferred method of manufacture for
multilayered fabricated articles. This method requires a sufficient
rheological match between
the constituent materials in order to promote adequate layer uniformity within
the annular
parison dye. Conventional PET, as well as other conventional polyesters, such
as
poly(butylene terephthalate), poly (ethylene naphthalate), polylactic acid,
polyester copolymers
z5 containing the terephthalate moiety, and liquid crystalline polyarylates,
exhibits fairly
newtonian behavior in the melt whereas HDPE resins behave decidedly non-
newtonianly.
Thus, combinations of PET and HDPE have heretofore resulted in coextruded
sheets and blow-
molded articles having marginal to poor layer uniformity. Accordingly, yet
another aspect of
the present invention addresses this problem by increasing the long chain
branching in the
2 0 polyesters, without the formation of significant crosslinking or gels.
DETAILED DESCRIPTION OF THE INVENTION
The improved barrier resin of the present invention comprises a base resin
which can
be crystalline polyesters, crystalline polyamides, crystalline polyarylates or
crystalline
polyethylene-co-vinyl alcohol) resins together with a minor amount of a
modified high-density
25 polyethylene (HDPE). Preferably, the HDPE is modified with unsaturated
carboxylic acid or
derivative thereof, such as malefic anhydride, acrylic acid etc. The improved
barrier resin
comprises 90 to 98 percent of the base resin, and 10 to 2 percent of the
modified polyethylene.
The modified polyethylene comprises from 0.5 to 5.0 percent by weight
(preferably 0.5 to 1.4
percent) of the unsaturated carboxylic acid or derivative.
3 0 The resin of the present invention exhibits improved adherence as compared
to
unmodified PET, while maintaining its barrier properties. Thus, the resin of
the present
invention can be advantageously used in multilayer plastic container having at
least two layers,
one of which is a gas-barrier layer, the other of which is a polyolefinic
support layer. Such
containers are described in US-A-5,441,781. Suitable polyolefinic materials
are described
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US-A-5,380,810, U.S. Pat application 08/857,817, or U.S. Pat. Application
08/857,816. The
preferred material to be used in the support layer is HDPE. Should the melt
strength need to be
improved (for example when preparing heavy items such as automobile fuel
tanks) then
methods such as those described in WO 99/10393; WO 99/10415; WO 99/10421;
WO 99/10422; WO 99/10423; WO 99/10424; WO 99/10425; WO 99/10426 or WO 99/10427
can be used to modify these polyolefinic materials in order to give them
greater melt strength.
The containers of the present invention can consist of only two layers, but
additional
layers may advantageously be used. For example, it may be desired that two
support layers
surround the barrier Iayer such that the support layers are in contact both
with the contents of
z o the container and the outside environment to which the container is
exposed.
Furthermore, while the improved adherence of the resins of the present
invention allow
tie-layers to be eliminated in most cases, in certain applications, superior
adherence between
the layers may be desired, in which case the use of a tie layer may still be
preferred. It should
be appreciated that just as the resins of the present invention improve the
adherence of the
~.5 barrier layer to a support layer, it will also improve the adherence of
the barrier layer to a tie
layer. Preferred tie layers to be used in the present invention include those
described in the
'781 patent.
The multilayer containers, which are an example of the present invention, may
be
produced by any means known in the art. This includes blow molding as well as
coextruding
2 o sheets followed by thermoforming with or without welding of the two or
more parts to form
the containers. Blow molding methods, are generally preferred. For example,
resins for each
layer can be separately plasticized in two or more extruders, introduced into
the same die,
laminated in the die while leveling each thickness to prepare a parison having
the appearance
of being one-layered. The parison can then be inflated in a mold by
application of inner
25 pressure of air so that the parison is brought into contact with the mold
and cooled.
In coextrusion blowmolding, it is advantageous that the various layers have
similar
rheological properties. To this end, it has been discovered that by increasing
long chain
branching within polyester material used as the base barrier material, typical
polyesters will
have rheology which is more similar to HDPE. This is advantageous whether or
not the barrier
3 0 material includes the modified polyoleEn to improve the adhesiveness. Base
polyesters which
can be altered in this way include PET, poly (butylene terephthalate), poly
(ethylene
naphthalate), polylactic acid, polyester copolymers containing the
terephthalate moiety, and
liquid crystalline polyarylates.
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Long chain branching can be promoted by incorporating multifunctional monomers
within the initial polymerization, or by post reactor modiEcation such as
reactive extrusion
with a mufti-functional branching agent. These processes are generally known
in the art (see
for example, US-A-5,536,793; US-A-5,556,926; US-A-5,422,381; US-A-5,362,763,
and
US-A-5,422,381). Potential branching agents known in the art include
trimellitic anhydride,
trimesic anhydride, phthalic anhydride, pyromellitic dianhydride (PMDA) and
any monomers
containing 3 or more hydroxyl groups. Reactive extrusion using PMDA is a
preferred method
of promoting long chain branches. The branching agent should be added at a
level to avoid
significant cross linking and/or gel formation. Less than 1% by weight of the
branching agent
is preferred.
Optionally, additives which are good nucleating agents may be used to promote
the
crystallization of the branched polyester, to help compensate for the fact
that crystallization of
branched materials are generally less thermodynamically favored compared to
linear materials.
Suitable nucleating agents are well known in the art (see, for example, US-A-
4,572,852;
US-A-5,431,972; US-A-5,843,545; or US-A-5,747,127).
Thus, a particularly favored embodiment of the present invention comprises a
multilayered article comprising at least a barrier layer and a support layer.
The support layer is
preferably HDPE, and the barrier layer comprises polyethylene terephthalate
with long chain
branching with a relatively small amount of HDPE to which a small amount of
malefic
2 o anhydride has been grafted. The article in this particularly favored
embodiment is prepared by
coextrusion blow molding. Such an article would be especially well suited for
use as a fuel
tank compatible for use with oxygenated fuels.
Further, it has been discovered that the barrier properties of the barrier
layer are
largely dependent upon the percent crystallinity (X~) ofthe polymer which
makes up the
2 5 barrier layer. When using PET as the barrier layer, it is preferred that
the polymer in the
finished container exhibit greater than 8 per cent, more preferably 21 percent
and most
preferably 34 percent crystallinity, and preferably no more than 50 per cent,
more preferably
no more than 40% as measured by Differential Scanning Calorimetry. It is
expected that other
barner resins will exhibit similar relationship between barrier properties and
amount of
3 o crystallinity. Crystallinity of these barrier resins can be altered by
those means known in the
art, such as controlling the cooling rate and or annealing.
It should be understood that crystallinity can be affected by certain fuel
components,
such as methanol. Methanol is known to disrupt hydrogen bonding of EVOH and
thereby
reduce the barrier performance of EVOH. In the case of PET, however, we have
discovered,
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that methanol can cause solvent-induced crystallization which raises the level
of crystallinity
and therefore further improves the barrier performance. The hydrogen bonding
in EVOH is
also known to be disrupted by moisture, whereas the barrier performance of PET
is not
effected by moisture. This has particular consequences in the overall
construction and design
of multilayer fuel container structures. EVOH should be precluded from being
in direct
contact with a fuel layer which contains moisture or methanol. PET, on the
other hand, does
not exhibit the same drawbacks, and can be in direct contact with the fuel.
It is also generally known that in addition to the amount of crystallinity,
the
morphology of the crystals is another factor in improving the barrier
resistance properties of
the resin, but this effect is minor in comparison to effect related to the
level of crystallinity.
EXAMPLES
In the Examples the following terms shall have the indicated meanings:
"PET1" is conventional PET (LighterTM L90A from The Dow Chemical Company),
having an inherent viscosity of 0.77, determined at 0.5% concentration (w/v)
and 23°C in
phenol/1,2-dichlolobenzene solution (60/40 by weight).
"PET2" is a modified PET prepared by reactively extruding PET1 with 0.45 % by
weight pyromellitic dianhydride (PMDA), followed by solid state advancement
for 14 hours at
a temperature of 196°C. GPC-DV was used to analyze the resulting
polymer and it was
determined that PET2 exhibited an increase in weight average molecular weight
(from 46 to
2 0 135 kg/mol), a broader polydispersity index (from 1.9 to 5.3) as compared
to PET!. PET2 had
an inherent viscosity of 2.28, determined at 0.5% concentration (w/v) arid
23°C in phenol/1,2-
dichlolobenzene solution (60/40 by weight).
"PET3" is a nucleated PET (VersatrayTM 12822 from Eastman Chemical Company),
having an inherent viscosity of 0.89, determined at 0.5% concentration (w/v)
and 23°C in
2 5 phenol/tetrachloroethane solution (60/4,0 by weight).
EXAMPLES 1-4 °
The following examples were prepared to demonstrate the improved cohesiveness
of
multilayer articles where the barrier layer includes a modified polyolefin
according to the
present invention. The multilayer bottles were prepared on a Bekum BM-502 Blow
Molding
3 o machine, running at a production rate of approximately 42 pounds per hour.
Bottle weight was
approximately 60 g (total shot weight 85-90 g). The PET barrier layer was the
inner layer, and
in all cases exhibited a melt temperature of approximately 254°C. The
support layer in each
case was HDPE (LupolenTM 4261A HDPE obtained from BASF). The tie layer if
present was
ADMERTM SF-700, an EVA base adhesive obtained from Mitsui Petrochemicals.
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The
results
of
these
evaluations
are
shown
in
table
I.
Table
I
EXAMPLE BARRIER LAYER TIE LAYER RESULT
1 PET1 yes Good adhesion
2 PET2 yes Better adhesion than
in
Example 1
3 PET1 None Delamination within
an
hour
4 PET2 None No delamination even
after 2 weeks
EXAMPLES 5-8
The following examples were prepared to demonstrate the improved processing
characteristics obtained by using a polyester material having long chain
branching wherein the
amount of long chain branching in the polyester material is selected such that
the rheology of
the polyester material more closely matches the rheology of a support layer,
according to the
presentinvention.
1o The melt viscosity of HDPE (LupolenTM 4261A HDPE obtained from BASF), PET1,
PET2 and PET3 were then characterized using a Rheometrics RMS800 equipped with
a
parallel plate fixture and conEgured to operate in the linear viscoelastic
regime. The data is
reproduced in figure 1 and indicates that PET2, exhibits similar rheology to
HDPE, and
substantially different than PETl or PET3.
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DMS Rheology ~ 270°-C
0.1 1 10 100
shear rate [1 /s]
Figure 1
EXAMPLES 9-13 - Permeation Testing
The permeability of Fuel CM15 through free standing films of the barrier
materials is
measured at 41°C (+/-1°C) using the following procedure. A test
film, 4 inch diameter disk
with a thickness between 1 and 100 mil, is mounted between the two chambers of
the test cell.
Fuel CM15 (mixture comprising 42.5/42.5/15 volume % of
toluene/isooctane/methanol, 95
mL) is added to the upper chamber, layering on top of the test specimen film,
and helium
flowing at 10 mL/min is passed through the lower chamber. As fuel permeates
through the
1 o barrier film into the lower chamber, it is swept in a helium stream from
the test cell and
through an injector loop of a gas chromatograph (GC). At a specific time
interval, the contents
of the injector loop are injected onto the front end of a 25 m, 0.53 mm ID,
Chrompack
Poraplot-U capillary column operating at 140°C using a helium flow of
10 mL/min as the
carrier gas. The GC separates, identifies by retention time, and quantifies
the fuel components
which have permeated through the specimen film. The date and time of the
injection,
permeant identities and peak raw area counts of the permeated components are
stored in a
computer file for further analysis. Using a multiport valve, 16 helium sample
streams are
monitored by the GC; each stream is tested for fuel component content at an
eight-hour
interval. Fifteen of the 16 sample streams are connected to specimen film
permeation cells.
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The sixteenth stream is from a gas cylinder containing a reference mixture of
50 ppm each of
toluene, isooctane, and methanol, with a make up of helium. The reference gas
data is used to
calibrate the GC raw area counts data to determine the ppm levels of the fuel
components in
the sample streams from the permeation cells.
The specimen test films were prepared by compression molding using a 6 inch by
6
inch by 5 mil thick mold in a Pasadena Hydraulics, Inc. Press. The EVOH
material was
EvaITM FlOIA, with 32 mol percent ethylene. The EVOH was compression molded
using the
following conditions: 1) melt resin in the mold for 4 minutes at 1000 pounds
applied pressure
at 210°C; 2) press resin for 6 minutes at 40,000 pounds applied
pressure at 210°C; and 3) cool
the mold slowly, over one hour, to 50°C under 40,000 pounds applied
pressure. The PET
resins were molded under similar conditions except that in step 1, the mold
was heated to
280°C.
Fuel barrier properties were measured on thin film specimen of several
materials.
These evaluations produced the following results, as shown in Table II.
Table II.
Permeability,
(g*mil/m2*day),
@ 41 C
Material 400 Hours 900 Hours 3500 Hours
EVOH 50
PET3 4fi
PET1 7~ 15
PET2 3~ 10
t The permeation was not yet at steady state when this measurement was taken,
the permeation
was still slowly increasing.
As shown in Table II, the EVOH reached steady state permeation in 400 hours
and the
2 o experiment was stopped. At 900 hours the permeations associated with the 3
PET samples had
not yet reached steady state, though the permeations were all roughly an order
of magnitude
lower than the steady state permeation of EVOH. The permeation experiment for
PET3 was
discontinued at this time. The permeations in PETl and PET2 came to steady
state after 3500
hours. Contrary to expectations, the PET2 material has lower permeability than
PET1. PET2,
being long-chain branched, was not expected to crystallize as efficiently as
PETl. It is
believed, though, that the branching in PET2 performs like a homogeneous site
of nucleation,
similar to the heterogeneous nucleation in PET3, as shown in the 900 hour
permeation data.
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EXAMPLES 14-17
The effect of the level of crystallinity on the fuel barrier properties of PET
were
evaluated according to the following procedure. Samples of PET2 were prepared
having
varying levels of crystallinity (X~). Examples 14-16 were prepared by melt,
quench and then
annealing the material at 130°C for 10, 20 or 30 seconds, respectively.
Example 17 was
prepared by melt followed by a slow cool. The crystallinity levels were
estimated using DSC.
Permeability measurements were then conducted as in the Examples 9-13, and the
permeability
rates after 350 hours are reported in Table III:
Table III
Sample X~ Permeability rate after 350 hours
_ ( *mil)/m2*day) __ _ ' _
14 2 unmeasurably high
15 ~ 12
16 21 6
17 34 4
s0
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