Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR MAKING MULTIPHASE POLYMERIC FILM HAVING A
LAMELLAR STRUCTURE WITH CONTROLLED PERMEABILITY AND/OR
CONTROLLED MECHANICAL PROPERTIES
1. FIELD OF THE INVENTION
The present invention relates to a process for making
polymeric films, material packaging and shaped articles like
gasoline tanks, that meet specific technical requirements
such as barrier and mechanical properties while being easy
and cheap to manufacture by extrusion.
2. BRIEF DESCRIPTION OF THE PRIOR ART
Traditionally, polymer films or shaped materials are of a
multilayer structure and are produced by coextrusion of a
polyolefin and of a condensation polymer incompatible with
the polyolefin.
The multilayer structure of the coextruded product is
achieved by combining different layers in a die before their
extrusion as a preform. The obtained preform is then blown
and molded in the form of the final product.
Polymer blending by extrusion followed by stretching at the
exit of the die was found to impart the obtained films with
enhanced barrier properties. Drawing orients the matrix
microstructure and the particles of the dispersed phase in
the direction of drawing.
Kamal et al. in US-A-5 188 784 (1993) disclose a special
sophisticated die system allowing to achieved such a post
extrusion orientation.
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Faisant and al. in Polymer, Volume 39, No. 3, 1998 disclose
the extrusion of blends of ethylene vinyl alcohol copolymer
(EVOH) dispersed in either polypropylene or in polyethylene.
The so obtained film is then stretched through a flat die.
Such a drawing is disclosed as inducing a mixture of lamellae
and fibrils of EVOH in polyolefin matrix. The polyolefin/EVOH
blends thereby prepared are compatibilized using commercial
maleic anhydride functionalized polyolefins. This approach is
found to lead to materials with 85 % decrease in the
permeability to oxygen in comparison with that of- pure
polyolefin.
Polymer blends used in such processes include two or more
polymers that are mixed physically. Compatible blends yield
polymer alloys, whereas most of the commercial blends
comprise incompatible polymers to form a dispersion of one
polymer in the other one. The behavior of polymer blend
product depends to a large extent, on the microstructure of
the blend reflecting the size and distribution of the
dispersed phase, and the nature of the interface between the
two phases.
P. Subramanian in US-A-4,410,482 (1983) discloses a method
for producing a lamellar structure from a polymer blend. In a
first step, an heterogeneous blend of polyolefin, a
condensation polymer incompatible with the polyolefin and a
compatibilizer consisting of an alkylcarboxyl-substituted
polyolefin, is prepared. The heterogeneous blend is heated
above the highest melting point of the blend constituents.
The body of the molten blend obtained by extrusion is
elongated by means of a conventional equipment.
..~__
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The polyolefin forms a continuous matrix phase while the
condensation polymer incompatible with the polyolefin
presents the form of a discontinuous distribution of the
thin, substantially two-dimensional, parallel and over-
lapping layers. The compatibilizer, namely the alkylcarboxyl-
substituted polyolefin, is present between the matrix phase
and the layers of condensation polymer to adhere these matrix
and layer together.
To carry out such multilayer extrusion process, the required
equipment is complex and expensive, and at least one adhesive
tie-layer is necessary.
The numerous works published on the matter stress the
difficulty to design a low-cost material for use as packaging
or for the manufacture of gasoline tanks, that have barrier
and mechanical properties sufficient to make them efficient.
Such a material must be easy to process at low cost and due
to recent environmental laws, it has to be recyclable.
Optical clarity, in the case of films for food packaging, is
also often required.
Plastic containers with high barrier properties are usually
made of a multi-layers material produced by coextrusion. This
is a complex and expensive technology and the final product
is not recyclable. Therefore, polymer blending appears to be
a more beneficial alternative in designing materials having
enhanced physical properties with the possibility of
recycling the final product. The addition of a small quantity
of a barrier material into a low-cost matrix material can
lead to a low-cost product with greatly improved barrier
properties.
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Polypropylene (PP) is a suitable polymer with good
mechanical and good barrier properties to H20. On the other
hand, EVOH which, as aforesaid, is a copolymer of ethylene
and vinyl alcohol, has high barrier properties to gases
such as 02 and C02 and a high resistance to hydrocarbons.
This makes EVOH an interesting candidate for food
applications. However, EVOH is an expensive material.
There is therefore a need for a process allowing the
preparation of films with a reduced amount of EVOH.
OBJECTS AND SUNIIKARY OF THE INVENTION
A first object of the invention is to provide a process for
making a multiphase polymeric film having a lamellar
structure with controlled permeability and/or controlled
mechanical properties.
A second object of the invention is to provide the
multiphase polymeric film with a lamellar structure
obtained by the said process for making a multiphase
polymeric film having a lamellar structure with controlled
permeability and/or controlled mechanical properties.
A third object of the invention is to provide granules
having a lamellar structure, which are obtained by
grinding the multiphase polymeric film with lamellar
structure obtained by the said process for making a
multiphase polymeric film having a lamellar structure with
controlled permeability and/or controlled mechanical
properties.
A fourth object of the invention is to provide a process
for producing a shaped article of polymeric material by
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using the granules obtained by grinding the multiphase
polymeric film according to the invention.
A fifth object of the present invention is to provide a
shaped article with improved permeability to oxygen and
improved mechanical properties.
The process according to the invention for making a
multiphase polymeric film as broadly disclosed hereinafter
comprises the steps of:
- preparing a molten blend made of
- a first polymer phase dispersed in a second polymer
phase which is a matrix polymer phase incompatible
with the said first phase; and
- a compatibilizer selected from the group consisting
of DEM, MAH, DEM-g-SEBS, MAH-g-SEBS, DEM-g-PP and
MAH-g-PP;
- extruding the molten blend through a flat die provided
with an exit,
- stretching the so-extruded blend downwards said exit at
a preselected stretching ratio to produce the said
multiphase polymeric film, and
- solidifying the extruded film sufficiently rapidly to
preserve the lamellar structure.
The invention as claimed is however restricted to the use
of DEM, DEM-g-SEBS and DEM-g-PP as compatibilizers. More
specifically, it is restricted to a process for making a
multiphase polymeric film having a lamellar structure with
controlled permeability, comprising the steps of:
- preparing a molten blend comprising:
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- a first polymer phase dispersed in a second polymer
phase which is a matrix polymer phase incompatible
with the said first phase and
- a compatibilizer selected from the group consisting
of DEM, MAH, DEM-g-SEBS, MAH-g-SEBS, DEM-g-PP and
MAH-g-PP;
- extruding the molten blend through a flat die provided
with an exit
- stretching the so-extruded blend downwards said exit at
a preselected stretching ratio to produce the said
multiphase polymeric film, and
- solidifying the extruded film sufficiently rapidly to
preserve the lamellar structure.
Preferably, the molden blend comprises:
- 60 to 90 weight percent of said matrix polymer phase,
- less than 30 weight percent of the barrier polymer, and
- 0.2 to 10 weight percent of said compatibilizer.
Granules with a lamellar structure according to the
invention, are obtained by grinding the so obtained
multiphase polymeric film.
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These granules can be used for producing shaped articles of
polymeric material.
The objects, advantages and other features of the present
invention will become more apparent upon reading of the
following non restrictive description of a preferred
embodiment thereof, given by way of example only with
reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 illustrates the modulus, yield stress and elongation
at break versus the EVOH concentration for PP/EVOH systems
blended in batch mixer: (Ml) PPhv/EVOH, 200 C, 38 rpm; (M2)
PPhv/EVOH, 230 C, 38 rpm; (M3) PPlv/EVOH, 200 C, 30 rpm; (M4)
PPlv/EVOH, 200 C, 60 rpm.
Figure 2 illustrates the modulus, yield stress and elongation
at break versus the draw ratio for extruded blends containing
10 vol.% EVOH: (P1) PPlv; (P2) PPlv/EVOH; (P3) PPlv/(PP.g.MA-
EVOH).
Figure 3 illustrates the modulus, yield stress and elongation
at break versus the draw ratio for extruded systems
containing 20 vol.% EVOH: (P4) PPlv/(HDPE-EVOH) 60/(2020);
(P5) PPlv/(HDPE-EVOH) 50/(30-20); (P6) PPlv/EVOH 80/20; (P7)
PPlv/(PP.g.MA-EVOH) 60/20-20).
Figure 4 illustrates the elastic modulus of the original
polymers and of unmodified and modified blends made of such
polymers.
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Figure 5 illustrates the blend modulus of the original
polymers and of unmodified and modified blends made of such
polymers.
Figure 6-a is a cross-section view of a filament of PP/EVOH
(80/20) cryogenically broken transversely to the drawing
direction (Draw ratio=20).
Figure 6-b is a cross-section view of a filament of
PP/EVOH/PP-g-MAH cryogenically broken transversely to the
drawing direction (Draw ratio=20).
Figure 6-c is a cross-section view of a filament of
PP/EVOH/PP-g-DEM cryogenically broken transversely to the
drawing direction (Draw ratio=20).
Figure 6-d is a cross-section view of a filament of
PP/EVOH/SEBS-g-MAH cryogenically broken transversely to the
drawing direction (Draw ratio=20).
Figure 6-e is a cross-section view of a filament of
PP/EVOH/SEBS-g-DEM cryogenically broken transversely to the
drawing direction (Draw ratio=20).
Figure 7-a is a cross-section view of a stretched film of
PP/EVOH(80/20) cryogenically broken along the drawing
direction (Draw ratio=10).
Figure 7-b is a cross-section view of a stretched film of
PP/EVOH/PP-g-MAH cryogenically broken along the drawing
direction (Draw ratio=10).
Figure 7-c is a cross-section view of a stretched film of
PP/EVOH/PP-g-DEM cryogenically broken along the drawing
direction (Draw ratio=10).
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Figure 7-d is a cross-section view of a stretched film of
PP/EVOH/SEBS-g-MAH cryogenically broken along the drawing
direction (Draw ratio=l0).
Figure 7-e is a cross-section view of a stretched film of
PP/EVOH/SEBS-g-DEM cryogenically broken along the drawing
direction (Draw ratio=10).
DETAILED DESCRIPTION OF THE INVENTION
As aforesaid, the process according to the invention for
making a multiphase polymeric film having a lamellar
structure with controlled permeability and/or controlled
mechanical properties, comprises four basic steps.
The first one consists of preparing a molten blend that is
made of:
- a first polymer phase dispersed in a second polymer phase
which is a matrix polymer phase incompatible with the said
first phase and
- a compatibilizer selected from the group consisting of
DEM, MAH, DEM-g-SEBS, MAH-g-SEBS, DEM-g-PP and MAH-g-PP.
The second step consists of extruding the molten blend
through a flat die provided with an exit.
The third step consists of stretching the so extruded blend
downwards the exit at a preselected stretching ratio to
produce the multiphase polymeric film.
The fourth and last step consists of solidifying the extruded
film sufficiently rapidly to preserve the lamellar structure.
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According to a particularly preferred embodiment of the
invention, the first polymeric phase consists of a barrier
polymer.
This barrier polymer is preferably selected from the group
consisting of ethylene vinyl alcohol copolymers (EVOH),
polyamides (PA), polyethylene terephthalates (PET) and
polydivinylchlorides (PVDC).
More preferably, the barrier polymer is EVOH. Such EVOH
polymer preferably has a molecular weight in the range of
20,000 to 100,000.
According to another particularly preferred embodiment of
the invention, the matrix polymer phase is a thermoplastic
matrix polymer phase which preferably consists of a
polyolefin such as polyethylene (PE) or polypropylene (PP).
According to a particularly preferred embodiment of the
invention, the molten blend comprises:
- 60 to 90 weight percent of the polyolefin polymer phase,
- less than 30 weight percent of the barrier polymer, and
- 0.2 to 10 weight percent of the compatibilizer.
More preferably, the molten blend comprises from 1 to 3
weight per cent of the compatibilizer.
According to a further particularly preferred embodiment of
the invention, the compatibilizer is selected amongst DEM-
g-SEBS, MAH-g-SEBS, DEM-g-PP and MAH-g-PP and the
concentration of DEM or MAH in said compatibilizer, ranges
from 20 to 80% weight per cent.
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The multiphase polymeric film which is obtained by the
above-defined process has a lamellar structure. By grinding
this multiphase polymeric film, one may obtain granules
having a lamellar structure.
These granules can be used to produce shaped articles of
polymeric material, which have improved mechanical and
barrier properties. These articles may be produced by
direct injection or by extrusion of a blencl of granules
obtained by the process according to the invention.
10 Shaped articles of polymeric material can also be obtained
by reprocessing granules according to the present invention
at a temperature that is lower than the melting temperature
of the barrier polymer but higher than the melting
temperature of the matrix polymer phase.
According to a preferred embodiment of the invention, the
granules with a lamellar structure can be reprocessed by
injection moulding, thermoforming or extrusion. Preferably
also, such a reprocessing requested to prepare shaped
articles can be carried out by dried injection or extrusion
of a blend of granules with another polyolefin and/or a
material compatible with polyolefin.
When the barrier polymer is EVOH and the matrix polymer
phase is PP, the reprocessing temperature is preferably in
the range from 160 to 1750C. More preferably, this
temperature is about 170 C.
When the barrier polymer is PA and the matrix polymer phase
is PP or PE, the reprocessing temperature is preferably in
the range from 180 to 215 C.
The shaped articles obtained by the process according to
the invention are characterized in that they have, in
comparison with similar shaped articles obtained according
to the same process but without compatibilizer present in
the blend:
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- a permeability to oxygen which is improved to about
100%, as compared to a similar article made of PP,
measured under 30 to 90% of relative humidity with an
apparatus Oxtront 2/60 from MOCON; and
- mechanical properties (modulus and elongation at break)
which are improved to about 30 to 120%, according to
ASTM D 882-28.
EXPERIMENTAL DATA
In the tests reported hereinafter, a blow-molding grade
polypropylene (PP) (Profax-613Dt) from Himont Canada, Inc.
was used as thermoplastic matrix phase.
EVOH was used as first polymeric phase. The EVOH that was
so used was a product sold by EVAL-CO, which contains 32
moles percent of ethylene monomer, and it is approved for
food packaging applications.
As compatibilizers, use was made of the following two
commercial copolymers obtained from Shell Canada.
1) a compolymer of styrene -ethylene -butene -styrene (SEBS)
functionalized with maleic anhydride (MAH) (Kratont
FG-1901, 2 wt.% functionalization) -- hereafter called
SEBS-g-MAH --; and
2) the same copolymer SEBS functionalized with diethyl
maleate (DEM) (Kratont G-1652, 1.1 wt.% functionali-
zation) -- hereinafter called SEBS-g-DEM --.
t trademarks
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The SEBS-g-MAH was used as such. However, the SEBS-g-DEM
originally in form of powder, was extruded and then cut into
granules before its incorporation into the blend.
As compatibilizers, use was also made of PP functionalized
with maleic anhydride (MAH) and diethyl maleate (DEM). MAH
and DEM were directly grafted onto the PP in the extruder
using dicumyl peroxide (purchased from Aldrich Canada) as
initiator. The obtained PP-g-MAH and PP-g-DEM were then used
as compatibilizers with the above PP/EVOH blend in a second
extrusion step.
During the tests, all the polymeric materials were dried for
24 hours at 80 C prior to being melt blended in a twin-screw
extruder through a flat film die (Haake Buchler Rheocord
system* 40). To mix the polymers, all components (first
polymer phase, matrix polymer phase and compatibilizer) were
starved fed into the extruder at 30 rpm under the following
temperature profile: T1=160 C, T2=180 C, T3=190 C and T4
(Die)=200 C. The extruded film was stretched 5 cm away from
the lips of the die using a controlled speed drawing machine.
The same extruder was used to functionalize the polypropylene
at a rate of 2 wt.% using a normal die. To do so, the polymer
and peroxide initiator were premixed with MAH or DEM. Then
the mixture was starved fed into the extruder. The extrudate
was cut into granules and then remixed with pure PP/EVOH
blend in a second step extrusion.
* (Trademark)
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CHARACTERIZATION
Dynamic mechanical measurements were carried out on a
Bohlin CVOt rheometer at 200 C in a linear viscoelastic
regime. The frequency range was 0.004-30 Hz. The solid
state rheological measurements were performed at 1 Hz using
a Rheometric Scientific Solid Analyzer (RSA-IIt) equipped
with a dual cantilever geometry. Temperature sweep tests
were carried out between -140 C and 160 C.
Differential Scanning Calorimetry (Perkin-Elmer, DSC-7t)
tests were performed between 50 and 200 C at a heating rate
of 10oC/min. The extruded films were cryogenically broken
along the drawing direction and the fractured surface was
coated with a 50/50 gold-platinum alloy. The morphology was
observed using a JEOL, JSM-IIIt Scanning Electron
Microscope (SEM).
Tensile properties of the molded samples (dumbbells) of the
original polymers and their blends were measured using a
standard Instront universal-testing machine (Model 5565)
according to ASTM D 882-88 procedure. The polymer sample
(dumbbell) was clamped by two pneumatic grips of serrated
clamps to prevent slipping of the specimens. The tensile
measurements were carried at a constant cross head speed of
0.33 mm. s-1 using a 50 kg force (490 N) load cell at room
temperature. Each test was repeated at least five times and
mean values and standard deviations were calculated.
The permeability of the extruded films to oxygen gas was
directly measured with a Moccon apparatus Ox-Tran 2/60t at
room temperature under different relative humidities.
t trademarks
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COMPARATIVE EXAMPLES
In a previous work (Faisant et al. Polymer, Volume 39, No. 3,
1998) improvements in the mechanical properties of PP/EVOH
blends were observed when use is made of a commercial PP-g-
MAH compatibilizer as additive and the blend films are drawn
at the exit of extruder die. The so measured, improved
mechanical properties of the blends are reported in Figures 1
to 3.
To clarify the effect of stretching, barrier properties of
stretched films were compared with those of blends prepared
in a two blades batch-mixer. These blends are characterized
by a nodular morphology and by a slightly deformed nodular
morphology when molded in the form of films by compression
molding. The results are reported in Table 1. The measured
permeabilities of the batch mixer samples are relatively
high. Upon drawing, the permeability of the blends decreases
(Table 2) . Up to 85% improvement in barrier properties with
respect to that of pure matrix (PP) was found. The
improvement in barrier properties is due to the lamellar
structure induced by drawing.
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TABLE 1: Barrier properties of PP/EVOH systems blended on a
batch mixer
T Measured Pp2
Sample Vol.% mixing
(oC) (rim) [ (mm em-3/ (m2 daY-1 atm-1) l
PPlv 70.9
PPhv 64.3
EVOH 0.09
PPlv/EVOH 83.5/12.5 200 30 81.7
83.5/12.5 200 60 58.1
75/25 200 30 69.9
75/25 200 60 65.6
83.5/12.5 200 200 59.9
PPhv/EVOH 83.5/12.5 200 200 75.0
83.5/12.5 230 230 63.9
75/25 200 200 233.0
75/25 200 200 109.2
75/25 230 230 73.5
TABLE 2: Barrier properties of PP/EVOH extruded films
Permeability
Draw
Sample Vol.% ratio [mm CM-3/ (m2 ciay-1 atm-1) l
Measured
PPhv 64.3
PPlv 70.9
EVOH 0.09
PPhv/(PPlv-EVOH) 80/(10-10) 3.4 22.4
PPlv/EVOH 90/10 2.8 22.1
80/20 3.4 12.4
PP1v/PP.g.MA/EVOH 80/10/10 2.8 23.1
80/10/10 8.7 25.5
PP1v/(PP.g.MA-EVOH) 80/(10/10) 2.8 30.1
80/(10/10) 8.7 25.0
60/(20/20) 3.2 9.5
PPLv/(HDPE-EVOH) 60/(20/20) 3.4 51.6
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The conclusion of this previous work is as follows.
Non-expensive extrusion-drawing technique induces desired
properties of extruded polymer films. First of all,
stretching of films of polymer blends in between die and
rolls gives rise to orientation of the matrix and the
dispersed phase. This, in turn, improves the mechanical
properties of the extruded film. Secondly, the stretched
films have a special microstructure that is a mixture of
fibrils and lamellae. Appearance of lamellae of the dispersed
phase (EVOH, high barrier copolymer) substantially decreases
the barrier properties of the stretched films.
EXAMPLES ACCORDING TO THE INVENTION
In order to demonstrate the advantage of the present
invention, the same blending strategy and polymers were used
except that the above mentioned new compatibilizers were
used.
Then, it was found that the new compatibilizers improve the
mechanical properties of the original blend due to their
effectiveness as interfacial emulsifiers and to their own
good mechanical properties. Replacing the strong, MAH,
compatibilizer by a medium one, DEM, increases the
interfacial tension while ensuring good adhesion at the
interface between the matrix and the dispersed phase. Such a
less efficient compatibilizer avoids the decrease of EVOH
particle size, which leads to longer and wider structured
lamellae and thus higher barrier performances.
Another category of materials was obtained by dispersing DEM
modified blends in a matrix of low viscosity polypropylene
(PPlv) at temperatures lower than the melting temperature of
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EVOH. This type of organic/organic composites offers the
possibility of tailoring with either extrusion or injection
molding a wide range of materials with adjustable properties.
RESULTS AND DISCUSSION
RHEOLOGY
Rheological measurements at 200 C and 30 rad/s showed that
the viscosity of the matrix (PP) is close to that of EVOH.
Therefore, a viscosity ratio
71 dispersed phase
Tl matrix
very close to unity is expected for these polymers at 200 C
and 30 rad/s. Close viscosity of the phases ensures better
dispersion of the EVOH in the PP matrix.
SOLID STATE PROPERTIES
Elastic modulus of the original polymers and of the
unmodified and modified blends are reported in Figure 4. As
can be seen, EVOH is always more elastic than PP. The
elasticity of 80/20 blend of these polymers is not very
different from that of the PP matrix. On addition of DEM-g-PP
and MAH g-PP, the resulting blends become more elastic at
very low temperature whereas they show a slightly smaller
elasticity at high temperatures. Functionalized PP do not
show any significant change in the solid state properties of
the blend in the temperature range that is important for food
preservation (-20 to 50 C).
Addition of functionalized SEBS compatibilizers to the blends
results in a decrease of the blends' modulus. This elasticity
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reduction is more pronounced for SEBS-g-MAH modified blend.
As seen in Figure 5, the glass-transition of PP is close to
0 C, which could result in brittleness of the packaging film
at food preservation temperatures. A lower elasticity would
make a packaging film more flexible (ductile) at low
temperatures, which would make the polymer film more
resistant to mechanical shocks and forces.
DIFFERENTIAL SCANNING CALORIMETRY (DSC)
The results of the DSC tests carried out on the starting
materials and their blends are summarized in Table 3. The
extent of the interaction between the compatibilizer and the
EVOH phase can be estimated using DSC data. The melting point
of the original PP is slightly decreased upon its
functionalization or its mixing with EVOH polymer. In
contrast, melting of the EVOH copolymer remains almost
unchanged. A slight decrease in the melting point of the EVOH
phase is observed for PP-g-MAH compatibilized blends while
PP-g-DEM seems to be ineffective. As expected, anhydride
functional groups are chemically more active than ester
groups. Therefore, a higher rate of interaction is expected
for PPS-g-MAH. The same increase is observed for
functionalized SEBS copolymers. It is worth noting that the
melting point of the SEBS-g-MAH modified blend has the lowest
melting temperature.
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TABLE 3: Melting point of different phases of the blends
Polymer or Blend Tm(oC) PP Tm(oC)EVOH
phase phase
Original PP 164.53 0.20
Original EVOH 182.02 2.26
PP-g-MAH 161.68 1.33
PP-g-DEM 162.77 1.14
PP/EVOH (80/20) 162.03 0.57 183.00 0.12
PP/EVOH/PP-g-MAH (81.5/16.5/2) 162.62 0.34 182.35 0.19
PP/EVOH/PP-g-DEM (81.5/16.5/2) 162.40 0.68 183.02 0.09
PP/EVOH/SEBS-g-MAH (81.5/16.5/2) 162.35 0.18 181.01 0.04
PP/EVOH/SEBS-g-DEM (81.5/16.5/2) 161.87 0.06 182.81 0.06
FILM PROCESSING
After the components of a blend are mixed in the extruder and
enter the extruder's die, the dispersed phase domain has a
spherical shape. Leaving the die, the materials are subjected
to an external force applied by the drawing machine (rolls).
Film stretching occurs over a distance separating the die
from rolls. The ability of an extruded film to be stretched
is a very important factor in film making processes. Such a
stretching orients matrix and dispersed phase. The oriented
structure reduces the gas permeability of the drawn film by
aligning and overlapping the barrier components of the film.
Consequently, a polymer blend having a larger ability to be
stretched should give a less permeable film.
To check this assumption, different blends were stretched at
different speeds of the drawing machine. Such experiments
showed that the maximum attainable draw ratio for the blend
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and polymer-g-DEM modified blends is 20. Over this value, the
extruded films disintegrate under the force applied by
drawing machine. However, this maximum ratio was enhanced up
to 40 for polymer-MAH modified blends. This is mainly due to
5 the smaller size of EVOH inclusion in the case of MAH
modified blends, which reduces the barrier properties of the
blends.
SCANNING ELECTRON MICROSCOPY (SEM)
The microstructure of the extruded filaments and films of the
10 blends were studied using scanning electron microscopy. The
SEM micrographs of filaments drawn at a draw ratio of 20
(Figures 6-a to 6-e) and films drawn at a draw ratio of 10
(Figures 7-a to 7-b) are reported. The filaments were
cryogenically broken transversely to the drawing direction
15 whereas the films were broken along the drawing direction.
Such procedure provides two different dimensions of the
oriented dispersed phase.
In Figure 6-a some lamellae and round cross-section of the
dispersed phase (EVOH) are observable. The diameter of the
20 fibrils ranges between 1 and 3 m. A large number of
cavities, smoothness of the interface and separated lamellae
are indication of a poor interfacial adhesion. With the
addition of polymer-g-DEM modifiers, no significant change in
the morphology of the blend is observable (Figures 6-c and
6-e). Addition of polymer-g-MAH modifiers drastically changes
the state of the dispersion of the EVOH phase in the PP
matrix (Figures 6-b and 6-d). The diameter of the EVOH
fibrils is reduced to less than 0.5 m and no lamella is
observed. The EVOH domain size is also finer for the SEBS-g-
MAH modified blend. A very important point here is that the
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addition of a very strong compatibilizer such as MAH
dramatically reduces the interfacial tension and thus makes
the stretching of the dispersed particles difficult to
achieve. Instead, it is more pertinent to select a lesser
efficient compatibilizer such as DEM that ensures adhesion at
the interface but does not reduce the size of particles.
In Figures 7-a to 7-e, the morphology of the blends along to
the direction of drawing are reported. In Figure 7-a, the
fibrils of EVOH in PP matrix are observable (draw ratio=10).
In Figures 7-c and 7-e, the microstructure of polymer-DEM
modified blends are shown (draw ratio = 10). In both cases
the large domain size of the dispersed phase is easily
observable, but a good interfacial adhesion seems to be
absent.
Lamellar morphology reduces permeability. Addition of SEBS-g-
MAH and PP-g-MAH to the blend results in intimately mixed
mixtures (Figures 7-b to 7-d, draw ratio = 10). Very small
fibrils of EVOH phase are observable whereas lamellae are
absent.
To have a lamellar morphology where use is made of PP-g-DEM
and SEBS-g-DEM modified blends, one may simply reduce the
percentage of these compatibilizers or use a compatibilizer
having lower degree of functionalization. This allows the
generation of lamellar structure with good adhesion at the
interface between such lamellae and the matrix, which
ultimately results in higher barrier properties.
MECHANICAL PROPERTIES
Tensile properties of the starting components and the blends
are summarized in Table 4. The original polymers (PP and
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EVOH) are characterized by high tensile modulus and high
elongation at break. Upon functionalization, the tensile
properties of the matrix (PP) are deteriorated in the case of
PP-g-DEM. However, in the case of PP-g-MAH, the modulus is
increased and the elongation at break is decreased.
As shown in Table 4, addition of PP-g-MAH to the blend
results in higher modulus but lower elongation at break. In
the case of PP-g-DEM, both the modulus and elongation at
break are increased. In the case of SEBS-g-MAH, the modulus
of the blend substantially decreases but the elongation at
break strongly increases as compared to the original blend.
In the case of SEBS-g-DEM, the modulus of the blend is of the
same magnitude as that of the original blend while the
elongation at break is increased by 600% as compared to that
of the original blend.
These observations along with the observed barrier properties
emphasize that DEM-grafted polymers are the best additives to
ensure a certain adhesion while permitting to obtain lamellar
structures and enhanced barrier properties.
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TABLE 4: Tensile properties of the blends
Young modulus Elongation
Polymer or Blend (Mpa) at break
M
Original PP 932.31 76.09 504.0 77.1
Original EVOH 2467.87 224.97 309.7 19.4
PP-g-MAH 1010.99 99.04 5.96 1.06
PP-g-DEM 756.95 79.56 5.10 1.36
PP/EVOH (80/20) 769.76 106.97 20.69 6.4
PP/EVOH/PP-g-MAH (81.5/16.5/2) 1174.42 69.33 11.9 2.3
PP/EVOH/PP-g-DEM (81.5/16.5/2) 943.00 179.05 27.0 9.9
PP/EVOH/SEBS-g-MAH
(81.5/16.5/2) 476.62 76.09 153.3 40.7
PP/EVOH/SEBS-g-DEM
(81.5/16.5/2) 754.96 64.33 124.9 5.4
BARRIER PROPERTIES
Barrier properties of the new blends are reported in Table 5.
The permeability of the extruded and batch mixed films to
oxygen gas were measured under two different conditions, that
is
02 31%
relative humidity = and
N2 Dry
02 90%
relative humidity =
N2 90%
The effect of film stretching on barrier properties of non-
compatibilized blend is also presented in Table 5.
As can be seen, upon stretching, the permeability to oxygen
of PP is decreased by 75-79% under two different humidity
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conditions. A comparison between the barrier properties of
non-stretched and stretched films made of a non-
compatibilized blend, show 15-25% and 75-79% improvements,
respectively. This comparison shows the effect of stretching
process on microstructure and barrier properties of films.
It is also shown that compatibilization with DEM leads to
materials with enhanced barrier properties. A 97% decrease in
PP permeability to oxygen was obtained when the blend is
compatibilized by PP-g-DEM. The decrease reached even 100% in
the case of SEBS-g-DEM.
These observations confirm the beneficiary effect of
tailoring the polymers in barrier polymer films. Among the
two grafted polymers used as compatibilizers, SEBS-g-DEM show
higher mechanical properties and the lowest permeability to
oxygen.
TABLE 5: Barrier properties of the blends
Permeability Permeability Changes in barrier
mm.cm3/(m2.day.atm) mm.cm3/(m2.day.atm) properties
Polymer or Blend Draw 02 31 02 90 cf.PP
ratio Rh% = Rh% =
N2 dry N2 90 cf. original blend
Condition A Condition B A B
Original PP B 80.7 76 >
Original EVOH B -0.0001 0.023
B 61.4 64.5 +24%, +15%
0% 0%
PP/EVOH (83.5/16.5)
+75%, +79% Ln
5* 20.6 16.2 +66% +75%
PP/EVOH/PP-g-MAH +71%, +62%
(81.5/16.5/2) 5 23.8 29.1 -16% -81%
PP/EVOH/PP-g-DEM +97%, +93%
(81.5/16.5/2) 5 2.7 5.4 +87% +67%
PP/EVOH/SEBS-g-MAH +60%, +51%
(81.5/16.5/2) 5 32.1 37.2 -56% -130%
PP/EVOH/SEBS-g-DEM +99.9%, +95.5%
(81.5/16.5/2) 5 0.07 3.4 +99,7 % +79%
B = Non stretched film, + Improvement, -= Deterioration, *= Original blend
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ORGANIC COMPOSITE MATERIALS WITH LAMELLAE STRUCTURE
Another category of materials with enhanced mechanical and
barrier properties was obtained by dispersing at 30-40wt% of
PP/EVOH/PP-g-MAH or PP/EVOH/SEBS-g-DEM in a low viscosity
polypropylene (PPlv) at temperatures lower than the melting
point of EVOH. These polymer/polymer composite materials were
obtained by a second extrusion or injection molding at
temperatures lower than (Tm=185 C) where the EVOH is in the
rubbery state. Morphological analysis showed that the EVOH
lamellae were kept almost intact, thereby imparting the
extruded film with enhanced barrier properties. In fact, the
permeability of these films is of about 0.2 at 31 RH and 5.1
for 90 RH (RH relative humidity) and the mechanical
properties were found, within experimental errors, almost
unmodified.
This embodiment offers a useful procedure for preparing
materials with a lamellar structure. The PP/EVOH/PP-g-DEM and
PP/EVOH/SEBS-g-DEM can be tailored separately and then
combined afterwards in the desired proportion with a
compatible matrix at temperatures ensuring the preservation
of lamellar structure. The strategy can be used with other
barrier materials such as PA (polyamide) or PBT (polybutylene
therephtalate) combined or not with EVOH. These materials
offer even more flexibility for the temperature of processing
since their melting temperatures are both higher than 220 C.
A first extrusion is carried out at high temperature (for
instance 250 C) and is followed by a stretching. Such
procedure generates the requested lamellar structure. Then, a
second dispersion of the blend via extrusion or injection
molding in a material having low melting temperature such as
polyolefins allows the generation of polymer/polymer
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composite materials with enhanced mechanical and barrier
properties.
This type of organic composite materials with lamellar
structures can be easily processed using the conventional
machinery developed for thermoplastic materials.
The major differences between the previous work disclosed in
the article of FAISANT et al. and the present invention are
as follows.
TABLE 6
Prior Work
Aspect Details Properties
Materials Blends of PP-g-MAH/EVOH Good mechanical
dispersed in PPlv properties
PP-g-MAH/(HDPE-EVOH) Enhanced barrier
Processing Two steps operation properties
conditions followed by film drawing Preservation of
at the exit of the die cristallinity and
mechanical properties
Lamellar structure of PP
obtained in solid state
Easy processing
conditions
Versatility to tailor
desired material for
specific application
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TABLE 7
Invention
Aspect Details Properties
New PP-g-DEM,(1) (1)-Lamellar morphology
compatibilizers (1)-Enhanced barrier
SEBS-g-DEM,(2) properties
SEBS-g-MAH(3) (1)-Lower elongation at
break
Processing Film drawing
conditions using cheap (2)-Lamellar morphology
equipment (2)-Enhanced barrier
Lamellar properties
structure (2)-Good mechanical
obtained in properties
solid state (2)-High elongation at
break
(2)-Enhanced rheological
and thermomechanical
properties
(2)-Improved interfacial
adhesion that insures
better stress
transfer between the
phases
(3)-Mainly fibrillar
morphology
(3)-Good mechanical
properties but lower
barrier properties
-Easy processing
conditions
-Variety of
compatibilizing
strategies
-Versatility to tailor
desired material for
specific application
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CONCLUSION
The above examples clearly emphasize the influence of the
compatibilizers' structures and of the processing conditions
(film stretching) on properties of the extruded films of
PPE/EVOH blend. In particular, stretching of the polymer
films after extrusion substantially improves the barrier
properties of the film as compared to those of non-stretched
films.
The principal aim of compatibilization and film stretching is
to obtain a morphology that shows the highest barrier
property and facility of processing.
The results of the above tests show that PP-g-MAH and
SEBS-g-MAH extensively reduce the size of the EVOH phase and
improve interfacial adhesion. These compatibilizers also
improve mechanical properties but they increase the
permeability of the blends to oxygen.
The diethyl maleate (DEM) counterparts of these polymers
improve mechanical properties but do not change the
morphology with respect to that of unmodified PP/EVOH blends.
DEM grafted polymers extensively improve barrier properties
of their blends.
It is worth noting that enhanced mechanical and barrier
properties can also be obtained by dispersing the first blend
via a second extrusion or injection molding at temperatures
lower than the melting temperature of EVOH lamellae. Unlike
classical composites, the organic composite materials with a
lamellar structure according to the invention can be easily
processed using the conventional equipments used for
thermoplastic materials.
