Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKCTROLTND OF THE INVENTION
Field of the Invention
The invention relates to linear low density ethylene polymer compositions
stabilized
to inhibit crosslinking during melt processing.
Description of the Related Art
Various types of polyethylene are known in the art. Low density polyethylene
("LDPE") is generally prepared at high pressure using free radical initiators
and
typically has a density in the range of 0.915-0.940 g/cm3. LDPE is also known
as
"branched" polyethylene because of the relatively large number of long chain
branches extending from the main polymer backbone.
High density polyethylene {"HDPE") usually has a density in the range of
greater
2 0 than 0.940 to 0.960 g/cm3. HDPE is prepared using a coordination catalyst
e.g.,
Ziegler-Natty type catalysts, at low, moderate or high pressures. HDPE is
generally
linear without any substantial side chain branching, and is a substantially a
crystalline polymer.
2 5 Linear low density polyethylene ("LLDPE") is generally prepared in the
same
. manner as HDPE, but incorporates a relatively minor amount of alpha-olefin
comonomer such as butene, hexene or octene to introduce enough short chain
' branches into the otherwise linear polymer to reduce the density of the
resultant
polymer into the range of that of LDPE. Introducing larger concentrations of
3 0 comonomer can also reduce the density of the ethylene copolymers into the
0.900 to
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0.915 g/cm range of very low density polyethylene (VLDPE) and in the
"plastomer
range" (i.e. 0.88-0.90 g/cm3).
The Ziegler/Natta coordination catalysts used to copolymerize ethylene and the
alpha-olefin generally produce an LLDPE with a relatively broad weight
molecular
weight distribution, i.e., Mw/Mn greater than about 3. Such LLDPE's also have
relatively broad compositions in that the proportion of alpha-olefin comonomer
molecules incorporated into the polymer molecules vanes. Generally, the lower
molecular weight polymer molecules contain a relatively higher proportion of
the
alpha-olefin comonomer than the higher molecular weight polymer molecules.
A polyethylene such as LLDPE having a broad molecular weight distribution is
undesirable in many respects, depending on the desired end use application.
For
example, LLDPE resins known in the prior art containing relatively high
molecular
weight molecules are subject to orientation which results in anisotropic
properties in
the machine versus transverse direction of a fabrication process. On the other
hand,
LLDPE resins containing relatively lower molecular weight molecules, in which
the
comonomer is invariably concentrated, tend to exhibit high block and tackiness
in
fabricated films. These lower molecular weight, highly branched molecules
2 o interfere with the proper function of certain additives compounded in the
resin,
increase the percentage of extractable polymer, and increase fouling in the
polymerization plant. The relatively high alpha-olefin comonomer content of
these
low molecular weight polymer molecules causes such polymer molecules to be
generally amorphous and to exude to the surface of fabricated parts, thereby
2 5 producing an undesirable sticky surface.
Prior art polyethylenes such as LLDPE also generally tend to have a very
broad,
non-uniform distribution of comonomer content, i.e., some polymer molecules
have
a relatively high alpha-olefin comonomer content while others have a
relatively low
3 0 content. Generally, the polymer molecules of low comonomer content are
relatively
r , . . ..
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more crystalline and have a high melting temperature, whereas the high
comonomer
content polymer molecules are more amorphous and melt at a lower temperature.
The presence of a higher melting component is disadvantageous in many
. applications, for example where softness or clarity is desired. On the other
hand, the
presence of a lower melting component frequently results in a high quantity of
extractables, which limit food contact applications.
Recently, a new class of LLDPE polymers has been developed based upon
copolymers of ethylene and a minor content of at least one alpha-olefin
comonomer.
These polymers are prepared preferably using a rnetallocene transition metal
catalyst and exhibit an average molecular weight distribution (Mw/Mn) of < 3
and a
compositional distribution breadth index (CDBI) of at least SO%. These
copolymers
and their method of preparation are more particularly disclosed in US Patent
5,382,631, the complete disclosure of which is incorporated herein by
reference.
One of the main distinctions which differentiates the newer LLDPE materials
from
the LLDPE of the prior art is that the branching alpha-olefin comonomer tends
to be
more uniformly and randomly distributed along the polymer chain rather than
concentrated in the lower molecular weight fractions of chain molecules as is
the
2 o case with prior art LLDPE described above. Because of this more uniform
comonomer distribution and a narrow molecular weight distribution, the newer
LLDPE materials avoid many of the disadvantages of conventional LLDPE
materials as described above, particularly when used to prepare films for
packaging
applications.
It has been observed, however, that the metallocene-polymerized LLDPE polymers
tend to have a higher susceptibility towards molecular crosslinking when
subjected
' to thermoform shearing forces, e.g. extrusion, than the conventional LLDPE
materials such as prepared using Ziegler/Natta transition metal catalyst
systems.
3 0 This crosslinking phenomena is reflected by gels present in extruded film
and by a
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decrease in the melt index of the polymer after extension. It is believed that
this
phenomena is caused by the organometallic structures of the metallocene
catalyst
residues and their silica supports present in the LLDPE polymer, from which
free
radicals can be generated under the high heat conditions of extrusion. Whereas
thermal degradation processes in conventional LLDPE tends to produce more low
molecular weight species which serve to plasticize the polymer, the thermal
degradation processes in metailocene polymerized LLDPE tends to favor
increased
crosslinking of the molecular chains, likely because the short chain branches
are
more randomly spaced along the polymer chains and thus less susceptible to
1 o scission.
Accordingly, it is a primary object of this invention to provide linear low
density
ethylene polymer compositions which are stabilized to inhibit crosslinking of
the
molecular chains when subjected to shear conditions encountered during
thermoforming.
The invention provides a composition comprising a mixture of (a) a linear low
density ethylene polymer containing from about 1-30 mol% of at least one alpha-
olefin comonomer and having an average molecular weight distribution Mw/Mn of
_< 3 and a compositional distribution breadth index of at least 50%; and (b}
at least
one metal carboxylate of a Ci to C~ saturated or unsaturated carboxylic acid,
said
metal carboxylate present in said composition in an amount sufficient to
inhibit
2 5 crosslinking of said composition when said composition is heated under
conditions
of shear at a temperature above the melting point of said ethylene polymer.
The invention also provides a process for melt processing a polymer
composition
comprising: (a) forming a composition comprising a mixture of a linear low
density
ethylene polymer containing from about 1-30 mol% of at least one alpha-olefin
r ,.
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comonomer and having an average molecular weight distribution Mw/Mn of _< 3
and
a compositional distribution breadth index of at least 50%, and at least about
0.005
wt.%, based on the weight of said ethylene polymer, of a metal carboxylate of
a C,-
Cu carboxylic acid; (b) thermoforming said composition to form a shaped
article at
a temperature above the melting point of said ethylene polymer under mixing
conditions of shear sufficient to cause scission of at least some of the
polymer chains
of said ethylene polymer; and (c) recovering said shaped article.
Shaped articles, e.g. films, prepared by mixing and shaping the polymer
composition
1 o of this invention exhibit a lower degree of crosslinking as evidenced by
less gel
formation in the extruded film and a higher polymer melt index after
thermoforming
than an otherwise identical composition which is free of the metal carboxylate
stabilizer.
DETAILED DESCRIPTION OF THE INVENTION
Prior to a detailed discussion of this invention, it should be pointed out
that it is
known in the art to include minor quantities of various metal carboxylates
into
polyethylene compositions to enhance various physical properties of the
composition or films prepared therefrom. For example, US Patent 4,454,281
discloses low density ethylene copolymer compositions containing a combination
of
metal salt of a fatty acid and an antiblocking agent, present to improve the
optical
properties of extruded film. A similar composition containing a combination of
a
dibenzylidene sorbitol and a metal stearate is disclosed in EPA 0 091 612. In
2 5 addition, molded articles prepared from polyethylene and exhibiting
improved
~ surface gloss and low haze and containing minor amounts of a mixture of a
metal
salt of a C~-C~ fatty acid and a metal salt of an aromatic carboxylic acid are
' disclosed in UK Patent 1,338,142. However, none of these or other prior art
publications disclose the use of metal carboxylates in combination with the
LLDPE
3 o polymers of this invention to inhibit crosslinking of the polymer when
heated under
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conditions of temperature and shear such as encountered during extrusion of
the
polymer composition.
The linear low density ethylene (LLDPE) polymer component of the present
invention is a copolymer (interpolymer) of from about 70-99 mol% of ethylene
and
from about 1-30 mol% of one or more alpha olefin comonomers, said polymer
having a density in the range of from about 0.9 to about 0.94 g/cm3. The
preferred
alpha-olefin content of the ethylene polymer lies in the range of from about 2-
15
mol%.
The molecular weight of the LLDPE component may range from 1,000 to 1,000,000
or more depending on the particular end use, preferably 104-105, and
especially
2x104-5x105. As used herein, the terms "average molecular weight" and
"molecular
weight" refer to weight average molecular weight unless otherwise indicated.
The
linear polyethylene component preferably has a narrow molecular weight
distribution (MWD). By "narrow MWD" is meant that the ratio of the weight
average molecular weight (MW) to the number average molecular weight (M~ is
less
than or equal to 3Ø Particularly preferred are the linear polyethylene
components
having a very narrow MWD, i.e. M,ylM" less than or equal to 2.5, and
especially less
2 o than or equal to 2Ø Molecular weight distributions of ethylene
interpolymers are
readily determined by techniques known in the art, such as, for example, size
exclusion or gel permeation chromatography.
The linear polyethylene component preferably has a composition distribution
(CD)
such that the composition distribution breadth index (CDBI) is at least 50%,
more
preferably greater than about 60% and most preferably greater than about 70%.
CDBI is a measure of composition distribution, and is defined as the weight
percent
of the copolymer molecules having a comonomer content within 50% (that is, 25%
3 0 on each side) of the median total molar comonomer content. The CDBI of a
~.~.. 1 , _.
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copolymer is readily determined utilizing well known techniques for isolating
individual fractions of a sample of the copolymer. One such technique is
Temperature Rising Elution Fraction (TREF), as described in Wild et al., J.
Poly.
Sci., Poly. Phys. Ed., vol. 20, p. 441 (1982), which is incorporated herein by
reference.
To determine CDBI, a solubility distribution curve is first generated for the
copolymer. This may be accomplished using data acquired from TREF techniques
described above. This solubility distribution curve is a plot of the weight
fraction of
the copolymer that is solubilized as a function of temperature. This is
converted to a
weight fraction versus composition distribution curve. For the purpose of
simplifying the correlation of composition with elution temperature, the
weight
fractions less than 15,000 are ignored. These low weight fractions generally
represent a trivial portion of the polymer. The remainder of this description
and the
appended claims maintain this convention of ignoring weight fractions below
15,000
in the CDBI measurements.
From the weight fraction versus composition distribution curve the CDBI is
deternined by establishing what weight percent of the sample has comonomer
2 0 content within 25% each side of the median comonomer content. Further
details of
determining CDBI of a copolymer are known to those skilled in the art, see,
for
example, PCT Patent Application WO 93/03093, published February 18, 1993.
Unless otherwise indicated, terms such as "comonomer content", "average
2 5 comonomer content" and the like refer to the bulk comonomer content of the
indicated copolymer on a molar basis.
' The linear polyethylene of the invention may be prepared by the use of
activated
catalyst systems of the metallocene type known to provide narrow CD/MWD
resins.
3 0 Cyclopentadienylide catalyst systems using a metallocene complex in
conjunction
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with an alumoxane cocatalyst or reaction product thereof are suitable for
preparing
the polymer components utilized in the invention. Similarly, catalyst systems
with
ionizing cocatalysts capable of providing non-coordinating anions will also be
suitable in this regard. The metallocene catalyst may be represented by the
general
formula (CY)mMR"R'p wherein CY is a substituted or unsubstituted
cyclopentadienyl ring; M is a Group IVB, or VB transition metal; R and R' are
independently selected from halogen, hydrocarbyl groups, or hydrocarboxyl
groups
having 1-20 carbon atoms; m = 1-3, n = 0-3, p = 0-3, and the sum of m+n+p
equals
the oxidation state of M. Various forms of the catalyst system of the
metallocene
type may be used for polymerization to prepare the polymer components of the
present invention including those of the homogenous or the heterogenous,
supported
catalyst type wherein the catalyst and aiumoxane cocatalyst are together
supported
or reacted together onto an inert support for polymerization by gas-phase,
high
pressure, slurry, or solution polymerization.
A more complete description of these metallocene catalysts and their method of
preparation may be found in the above referenced US Patent 5,382,631, and also
in
US Patents 5,408,017, 5,470,927, 5,483,014 and WO 96/04319.
2 0 Metal carboxylates which are suitable as crosslinking retarders in this
invention
include metal salts of carboxylic acids having from 1 to about 22 carbon
atoms,
more preferably from 2 to about 18 carbon atoms. The number of carbon atoms
includes the carboxylic acid group. Typical acids are monocarboxylic saturated
or
unsaturated acids such as formic, acetic, heptylic, caprylic, capric, lauric,
pahnitic,
2 5 stearic and behenic acids as well as their unsaturated analogs such as
oleic and
ricinoleic acids. The carboxylic acid may also include aromatic acids such as
benzoic or naphthenic acid and their derivatives.
Suitable salt-forming cations include zinc, calcium, copper, cadmium,
aluminum,
3 0 sodium, potassium, nickel, magnesium, barium, lead and iron, most
preferably
~ i
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cations of Group I to Group III metals of the Periodic Table. The most
preferred
carboxylates include zinc acetate and zinc stearate.
a The quantity of metal carboxylate added to the ethylene polymer composition
to
hinder polymer crosslinking may generally range from about 0.005 up to about 1
wt.%, more preferably from about 0.01 up to about 0.5 wt.%, and most
preferably
from about 0.01 up to about 0.25 wt.%, based on the weight of the ethylene
polymer
present in the composition.
The composition of the invention may also include a blend of the LLDPE of the
invention with up to about 50 wt.%, based on total polymer content, of one or
more
different olefin polymers such as other low, medium or high density
polyethylenes,
polypropylene, copolymers of ethylene and propylene and like thermoplastics.
In a preferred embodiment of the invention, the composition also contains one
or
more antioxidant (stabilizer) materials, such as phenolic, phosphite or
phosphonite
antioxidants.
Suitable phenolic antioxidants which may be used include polyalkyl-substituted
2 0 phenols such as 2,6-di-t-butyl-p-cresol, octadecyl-3-(3',5'-di-t-butyl-4'-
hydroxyphenyl} propio-nate, octadecyl-(3,5-di-tert-butyl-4-hydroxyhydro-
cinnamate), 2,6-di-tert-butyl-4-me-thylphenol, tetrakis (methylene-3(3', 5'-di-
t-butyl-
4-hydroxyphenyl)propionate} meth-ane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-
butyl-4-
hydroxybenzyl) benzene, 1,3,5-tris(3,5-di-t-butyl-4-hydroxybenzyl)
isocyanurate,
2 5 and n-octadecyl-8-(4'-hydroxy-3',5'-di-t-butylphenyl) propionate. These
phenolic
. antioxidants may be used either individually or in combination of two or
more
thereof.
Suitable phosphite and phosphonite stabilizers which can be used include alkyl
and
3 0 aryl phospites such as tri-n-octyl, tri-n-decyl and tri (mixed mono and
dinonyl
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phenyl) phosphate, distearyl pentaerythritol diphosphite; tetrakis {2,4-t-
butylphenyl)-
4,4' biphenylene diphosphite; bas(2,4-di-t-butylphenyl)-pentaerythritol
diphosphite;
tris (2,4-di-t-butylphenyl) phosphate; and like materials. The most preferred
stabilizes are those having the formula:
c~
Ro-~a~ ~~-oK
0
wherein R is C2-C18 alkyl or alkyl-substituted phenyl. A particularly
preferred
phosphate is of the above formula wherein R is -CigH3~, marketed by Borg
Warner
l0 under the tradename WESTON~6I9, or WESTON~399. Mixtures of phenolic
and phosphate antioxidants may also be used.
The antioxidant is preferably added to the composition at a level of from
about 100
to about 5,000 parts per million, more preferably 150 to 2500 parts per
million,
15 based on the polymer content of the composition.
The composition may also contain one or more adjuvant materials which are
commonly employed in ethylene polymer-based extrudable compositions, including
plasticizers, fillers, pigments, lubricants, slip agents, processing aids,
dyes, pigments
2 0 and like materials.
The composition may also contain additional decomposition inhibitors or free
radical scavengers such as zinc or magnesium oxide, polyakylene glycol anti-
gelation agents such as polyethylene glycol or polypropylene glycol. These
2 5 components or combinations thereof may be generally present in the
composition at
Levels in the range of from about 0.01 to about 1 wt%, based on the weight of
the
polymer component of the composition.
These and other adjuvants, as well as the metal carboxylate, may be
incorporated
3 0 into the composition either at the time of polymer composition is
pelletized or by the
r i.
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user as a separate additive package prior to the thermoforming of the
composition to
form shaped articles.
The polymer composition of this invention may be thermoformed to form shaped
articles such as films, containers and molded three dimensional articles by
well
known techniques such as blown film or cast film extrusion, uni- or biaxial
orientation, blow molding, injection molding or rotomolding. In these methods,
the
polymer composition is first mixed under conditions of shear in a suitable
mixing
device such as a screw extruder, Banbury mixer or Brabender~ plasticorder and
heated to a temperature above the melting point of the polymer components of
the
composition, generally in the range of about 140 °C up to about 350
°C, more
preferably from about 150 °C to about 300 °C. Tubular film may
be prepared using
an extruder/rnixer by passing the extrudate through an annular die in an
upward or
downward direction and the resulting tubular film expanded to the desired
extent
using a pressurized gas, cooled and flattened, followed by slitting to form a
film.
Other shaped articles such as bottles, lids and other molded shapes may be
prepared
by subjecting extrudate or molten polymer to well known injection molding,
blow
molding or rotomolding techniques.
2 0 The following examples are illustrative of the invention. Materials
identified in the
examples are as follows:
ECD 103 - a metailocene/alumoxane polymerized copolymer
of ethylene and about 3 mol% hexene - MI of 1.04
2 5 dg./min, density of 0.9169 g/cm3, and an ash of 274 ppm.
ECD 103' - a metallocenelalumoxane polymerized copolymer
of ethylene and about 3 mol% hexene - MI of 1.13
' dg./min, density of 0.9161 g/cm3, and an ash of 544 ppm.
LLDPE 3001 - ,~. Ziegler/Natta polymerized copolymer of
3 0 ethylene and about 3 mol% hexene - MI of
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0.88 dg/min, density of 0.9187 g/cm3, and an ash of 367 ppm.
IRGANOX~ 1076 - octadecyl - 3-(3',5'-ditert-butyl-4'-
hyroxyphenyl) propionate.
W-399 - phosphite
PEG - polyethylene glycol (anti gelation agent)
PPA - fluoroelastomer (processing agent)
l0 A series of 12 different formulations as described in Table 1 were prepared
by
mixing the indicated ingredients in a small scale Brabenderd plasticorder at
250 °C
for the times indicated in Table 1. The resulting blends were evaluated for
Melt
Index (MI - ASTM 1238 - Cond. E), Flow Index (HIvuJZ - ASTM 1238 - Cond F),
Melt Index Ratio (MIR - Flow Index/Melt Index) and Swell Ratio (SR), which is
a
measure of the diameter of the polymer strand extruded in the MI measurement
and
is inversely proportional to the melt index.
..
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NO. MIXTURE WEIGHT TEMP. TIME MI HMI MIR SR
(G) C (MIN)
1 ECD (50+.05)250 10 0.07 8.46 120.85 0.09
103 + Zn0
2 ECD (50+.05)250 75 0.24 30.80 126.75 0.12
103 + Zn0
3 ECD (50+.05)250 10 0.34 20.20 59.41 0.10
103 + ZnAc
4 ECD (50+.05)250 75 0.61 48.66 79.77 0.12
103 + ZnAc
5 ECD (50+.05)250 10 0.55 18.81 33.59 0.10
103'+ Zn0
6 ECD (50+.05)250 75 0.55 36.33 66.05 0.13
103'+ Zn0
7 ECD (50+.05)250 10 1.31 90.60 69.16 0.10
103'+ ZnAc
B ECD (50+.05)250 75 1.51 99.66 66.00 0.14
103'+ ZnAc
9 LLDPE (50+.05)250 10 0.31 26.64 85.93 0.10
3001 +
Zn0
10 LLDPE (50+.05)250 75 2.20 106.08 48.60 0.12
3001 +
Zn0
11 LLDPE (50+.05)250 10 0.71 44.16 62.18 0.11
3001 +
2nAc
12 LLDPE (50+.05)250 75 2.17 106.08 48.88 0.11
3001+ZnAc
Formulations 1-2, 5-6 and 9-10 contain zinc oxide (Zn0) which is a known
additive
for minimizing the formation of free radical groups, while formulations 3-4, 7-
8 and
11-12 contain zinc acetate, an additive within the scope of the invention.
Comparison of the met index and melt flow properties show that the
compositions
based on the metallocene polymerized ethylene polymer containing the zinc
acetate
additive exhibit an increase in melt index and melt flow properties which is
indicative of a reduced crosslinking propensity under the conditions of shear
mixing.
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This effect is less predominant with respect to samples 11 and 12 as compared
with
samples 9 and 10 which each contain a Ziegler/Natta polymerized ethylene
polymer.
A series of 19 different formulations each containing 50 g of ECD-103
polyethylene
and other components in the quantities listed in Table 2 were processed in a
Brandender~ plasticorder at 250 °C for the times indicated in Table 2.
Sample 13
reflects the properties of unprocessed virgin ECD-103, whereas samples 14 and
15
are processed controls containing no additives. The Irganox~ 1076, W-399 and
indicated zinc salts were added as an additive package in the quantities
listed in the
table prior to mixing. Samples 16 and 17 also contained 200 ppm of PEG and 800
ppm of PPA.
The data showed a synergistic effect with respect to the use of the additive
package
components Irganox~ 1076 and W 399 used in combination with zinc acetate or
zinc stearate as compared with formulations which did not contain this
combination
of ingredients or which contained ZnO, as reflected by a comparison of the
melt
index values of samples 18, 19, 28 and 29 with samples 16 and 17, as well as a
2 0 comparison of the melt index values of samples 22, 23, 30 and 31 with
samples 26-
27.
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No. Irganox W399 Zinc Time MI HIHI MFR SR
1076 ppm Salt mins g/ming/min
ppm
ppm
13 -- -- - -- 1.05 18.6 17.7 0.0895
14 -- -- - 5 0.12 9.3 77.5 0.095
15 -- -- - 10 0.06 8.7 145.0 0.087
,
16 1500 1500 - 5 0.96 16.16 16.83 0.087
17 1500 1500 - 10 0.59 16.12 27.32 0.105
18 1500 1500 ZnAc: 5 1.37 23.58 17.21 0.09
1000
19 1500 1500 ZnAC: 10 1.49 25.34 17.01 0.09
1000
20 500 500 ZnAC: 5 1.12 21.92 19.57 0.10
1000
21 500 500 ZnAC: 10 1.39 26.44 19.02 0.10
1000
22 500 500 ZnAc: 5 1.24 22.16 17.87 0.10
500
23 500 500 ZnAc: 10 1.27 22.06 20.85 0.10
500
24 1500 1500 ZnO: 5 1.27 21.36 17.07 0.088
1000
25 1500 1500 ZnO: 10 1.25 21.06 16.85 0.096
1000
26 500 500 ZnO: 5 1.12 20.87 18.62 0.102
500
27 500 500 ZnO: 10 0.76 20.24 26.63 0.107
500
28 1500 1500 ZnSt: 5 1.35 23.46 17.38 0.087
1000
29 1500 1500 ZnSt: 10 1.32 23.86 18.08 0.087
loco
30 500 500 ZnSt: 5 1.2 21.92 18.27 0.088
500
31 500 500 ZnSt: 10 1.3 26.54 20.42 0.096
500
note : All blends contained 50 g of ECD-103 copolymer and were processed at
250 °C.
