Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
O.Z. 31,713
10 8 ~ ~
MOLDING COMPOSITIONS WITH IMPROVED STRESS CRACK RESISTANCE
AND GOOD LOW TEMPERATURE BEHAVIOR
The present invention relates to molding compositions com-
prising polyethylene, an ethylene copolymer and a styrene-buta-
diene-styrene block copolymer.
The improved stress crack resistance o~ mixtures of poly-
ethylene and styrene-butadiene-styrene block-copolymers, as com-
pared to pure polyethylene,has been disclosed. However, the said
mixtures 108e their good stress crack resistance under high shear
stresses, such as may arise, for example, during extrusion of
the mixtures.
Mixtures of polyethylene and~an ethylene/vinyl acetate co-
:
polymer are-disclo~ed in British Paeent ljO58,670.
The stress crack resistance of~these mixtures i~ substantially
better than that of polyethylene. Howeverj moldin~s produced
from such compositions have a reduced low temperature resistance
.
compared to polyethylene.
; German Laid-Open Application D~S 2,053,067 discloses molding
compositions, comprising poiyethylene, an ethylene copolymer and
from 0.1 to 5 per cent by weight of N,N'-distearoylethylenediamine,
which have improved-stress crack resistance and good low tempera-
ture behavior. However, moidings produced from these mixtures are
difficult to weld, so that failure can easily occur at the welds.
It is an object of the present invention to provide molding
composieions which have good weldability, are resistant to high
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shear stresses, and at the same time exhibit improved low
temperature behavior, without suffering from a lower stress -
crack resistance.
We have found that this object is achieved, ac-
cording to the invention, by molding compositions which
comprise
(A) from 84 to 95 parts by weight of polyethylene which has
a density of from 0.915 to 0.935 g/cm3 and a melt index of
from 0.005 to 1 [g/10 mi~ , measured by the method of ASTM D
; 10 1238-70 at 190C under 2.16 kg load,
(B) from 3 to 10 parts by weight of a polystyrene-polybuta-
diene-polystyrene block copolymer containing from 10 to 60
per cent by weight of styrene and from 40 to 90 percent by
weight of butadiene and having a molecular weight, determined
from its solution viscosity, of from 50,000 to 1,000,000 and
(C) from 2 to 6 parts by weight of an çthylene/vinyl acetate
copolymer which contains from 25 to 50 percent by weight of
vinyl acetate as copolymerized units and has a melt index of
from 0.5 to 200 [g/10 mi~ , measured by the method of
ASTM D 1238-70 at 190C under 2.16 kg load,
with or without the conventional amounts of conventional
fillers and stabilizers.
The polyethylene used to manufacture the molding
compositions may have been manufactured by the high pressure
or low pressure polymerization process. A polyethylene having
a density of from 0.915 to 0.935 g/cm3 is used. The melt index
of the polyethylene - measured by the method of ASTM D 1238-70
will be from 0.005 to 1, preferably from 0.05 to 0.5.
The second component of the molding compositions
of the invention is a polystyrene-polybutadiene-polystyrene
block copolymer, which has a styrene o~ntent of from 10 to 60,
preferably from 20 to
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_ O.Z. 31,713
50, per cent by weight, and a butadiene content of from 40 to 90,
preferably from 80 to 50, per cent by weight. The molecular weight
of the block copolymer, determined from its solution viscosity, is
from 50,000 to 1,000,000 and the proportion by weight of the block
copolymer in 100 parts by weight of the molding composition is from
3 to 10, preferably from 3 to 8,-parts bei weight.
The third component is a copolymer of ethylene and vinyl acetate,
which contains from 25 to 50, preferably from 30 to 40, per cent by
weight of vinyl acetate as copolymerized units. The melt~ndex of the
copolymer, measured by the method of AST~ D 1238-70 at 190C under
2.16 kg load Will be from 0.5 to 200, preferably from 5 to
100 Lg/10 min].
j 100 parts by weight of the moiding composition of the invention
contain from 2 to 6, preferably from 3 to 5, parts by weight of the
copolymer of ethylene and vinyl acetate. However, in place of an
ethylene/vinyl acetate copolymer it is also possible to employ co-
polymers of ethylene and esters of acrylic acid or methacrylic acid
with alcohols of 1 to 8 carbon atoms. In that casej 100 parts by
weight of the molding composition of the invention contain from 2
to 6 parts by weight of the copolymer. The melt index o~ the
e~hylene/acrylic acid ester or ethylene/methacrylic acid ester
copolymer is also from 0.5 to 200 Cg/10 mi~ , and-the proportion of
copolymerized units of acrylic acid ester or methacrylic acid
ester in the copolymer is from 20 to 60, preferably from 25 to 50,
;~ per cent by weight,
The molding compositions of the invention may also contain
oonventional 5tabilizers and/or conventional fillers, e.g. chalk
or carbon black. The use of the stabilizer 4,4'-thio-bis-(3-methyl- -
6-tert.-butyl-1-phenol), in amounts of up to one per cent by weight,
preferably of from 0.05 to 0.3 per cent by weight, based on the ` -
molding oomposition5, is prsrerred.
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O.Z. 31,713
The molding compositions of the invention may be manufactured
by conventional methods, eOg. by mixing the three stated components `
at from 100 to 300C, preferably from 150 to 200C, in kneaders,
mills or twin-screw or single-screw extrudersO
; The molding compositions of the invention may be used for
the manufacture of shaped articles, e.g. pipes,-cables, profiles,
hollow bodies and injection moldings. They may be processed on
conventional molding equipment.
The Examples and Comparative Examples which follow illustrate
1l 10 the invention.
,! EXAMPLE 1
8908 parts by weight of polyethylene which has a density of
0.918 g/cm3 and a melt index of 0018 g/10 mins (measured by the
method of ASTM D 1238-70 at 190C under 2.16 kg load), 0.2 part
by weight Or the stabilizer 4,4'-thio-bis-(3-methyl-6-tert.-butyl-
1-phenol) (I) and 10 parts by weight of a copolymer of 72 per
cent by weight of ethylene and 28 per cent by weight of vinyl
acetate, having a melt index of 30 g/lO mins, are-homogenized at
154 C in a roll kneader (Brabender Plastographj running at 125 rpm.
The homogeneous mixture is then-kneaded for one hour under the
above conditions~ 3 mm thick sheets are pressed from the mixture
~1 which has been subjected to these stresses and are chilled. Test
specimens according to ASTM 1693-70 are produced from these sheets,
and are conditioned for 18 hours at 70C and then tested. In the
brittleness temperature test by the method of ASTM-D 746, of test
specimens obtained from pressed sheets produced as described in
ASTM.; D 1928-70, procedure A, 50% failure is observed at -60C. On
testing the specimens by the method of ASTM D 1693-70 for stress
cracking, no failures are detectable after 1,000 hours. The con-
centration of the reagent used was 10%, representing a modifica-
tion of the ASTM.`D 1693-70 procedure.
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-- OOZo 31,713
EXAMPLE 2 (Compa~ive)
Test specimens are produced, in accordance with the descrip-
tion of Example 1, from 91o 8 parts by weight of polyethylene,
0.2 part by weight of stabilizer I and 8 parts by weight of a
polystyrene-polybutadiene-polystyrene block copolymer having a
styrene content of 30 per cent by weight and a butadiene content
of 70 per cent by weight, the molecular weight, determined from
the solution viscosity, being 120,000. The stress crack resistance
of the specimens is tested as indicated in Example 1 and it is
found that after only four hours 50% of the samples have cracked.
EXAMPLE 3
Test specimens are produced, using the method described in
Example 1, from 9008 parts by weight of polyethylene having a
density of 0,918 g/cm3 and a melt index of 0018 g/10 mins~ 0.2
part by weight of stabilizer I, 3 parts by weight of an ethylene~
vinyl acetate copolymer which contains 40 per cent by weight of
vinyl acetate as copolymerized units and has a melt index of
40 g/10 mins and 6 parts by weight of the polystyrene-poly-
butadiene-polystyrene block copolymer defined in Example 2. The ~ -
stress crack resistance of the specimens is tested by the method
described in Example 1, representing a modification of the
ASTM-D 1693-70 procedureO No failures are detectable after
1,000 hours. On testing the low temperature behavior by the method
of ASTM~D 746, as described in Example 1, no fracture is found
at -72C,
EXAMPLE 4
Test specimens are produced by the method described in
Example 1 from 91.3 parts by weight of polyethylene (D = 0.918
g~cm3, MFI = 0018 g/10 mins), 0.2 part by weight of stabilizer
I, 405 parts by weight of the polystyrene-polybutadiene-poly-
styrene block copolymer defined in Example 2 and 4.5 parts by
weight of a copolymer of 70 per cent by weight of ethylene and
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O.ZO 31,713
30 per cent by weight of vinyl acetate, and their stress crack
resistance is testedO Even after 1,000 hours, no failures are
~ discernibleO On testing the low temperature behavior by the
- method of ASTM-D 746, no ~ailures are found at -72C.
EXAMPLE 5 (Comparative)
Using the method described in Example 1,-test specimens
according to ASTM 1,693 are produced from 96.6 parts by weight
of polyethylene (D - 0.918 g/cm3, MFI = 0.18 g/10 mins), 0.1 part
~/ by weight of stabilizer I, 2 parts by weight of an ethylene/vinyl
l~ acetate copolymer which contains 40 per cent by-weight Or vinyl
acetate as copolymerized units and has a-melt index of 40 g/10 mins,
and one part by weight Or NjN'-distearoylethylenediamine. Pairs
o~ these test specimens are half overlapped and weided at 150C
under a pressure of about 2 kp/cm2. These welded samples are bent
in a U-shape and kept at 50C in the 10% strength reagent solution
used in Example 1 (ASTM 1693-70)0 After 48 hours, more than 70
o~ the welds had split openO - -
Test specimens of mixtures obtained according to Examples 3
~ or 4 are overlapped and welded as described above. After a test
;~ o~ 200 hours in the detergent solution, the weld has not split
open on any of the test specimens.
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