Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to blends of thermoplastic
ethylene polymers from which medium density grades o pipe
may be formed.
2. Descrip~ion of the Prior Art t
Plas~ic pipe made from thermoplastic e~hylene
polymers is usually characterized, in terms of the density
of the resin stock from which the pipe îs formed, as being
a low, medium or high density grade. The low density
material has a density of about 0.910 to 0.92S grams per
cubic centimeter, the medium density material has a density
of about 0.926 to 0.940 grams per cubic centimeter, and the
high density material has a density of greater than 0.940 ;
to 0.965 grams per cubic centimeter.
Because of the density limitations o each of
these grades of pipe, each grade tends to have limitations ~ ,~
in its other physical properties such as s~ress crack ;~
resistance, low temperature embrittlement point, flexlbility~
burst strength, and upper use temperature.
As a result of these differences in physieal
properties, although there is some overlap, in utility,
between pipe made from each of these three grades of plastic
pipe making material, each grade of material ~ends to be
used for making one or more types of pipe for which the
other two grades of pipe making ma~erial are not suitable.
The mediu~ grade of pipe making material is preferred over
low density polyethylene or high density polyethylene for ~ -
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making pipe for applications such as water distribution,
gas distribution, irrigation and swimming pools because of
a better balance of flexibility and performance character-
istics. These include burst strength and upper use
temperature.
Because low density and high density ethylene
polymers can be more readily made, commercially, than medium
density ethylene polymers, the manufacture of medium density
pipe making compounds has usually been accomplished by blend-
ing low density resin with high density resin so as to be
able to provide a wider range of physical properties with
the resulting blend of resins than could be provided by the
use of individual medium density resins.
Despite this versatility, so to speak, of being
able to provide a wide range of extrudable medium density
pipe making compounds by blending low density resin with
high density resin, it has not been readily possible to
date, prior to the present invention, to provide an extrud-
able medium density pipe making composition from ethylene
polyme-rs which is entirely suitable for making extruded
medium density pipe therefrom under present day high spead
(10 to 100 feet per minute, 100-1000 pounds per hour at
temperatures of ~ 200C. and at pressures of ~ S00 psi) pipe
extrusion conditions and wherein the resulting pipe would
have the following combination of properties:
low temperature embrittlement point of ~ -60C.;
environmental stress-crack resistant (Fso) of
24 hours;
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~ lOOO hours to fail at hoop stress of 1070 psi
during burst testing at 100F;
a design stress rating of 630 psi at 73F; and
smooth, glossy surfaces. Smooth and glossy surfa~es are
important from a fluid ~low considera~ion.
This combination of properties is now essential
for medium density potable water pipe being used under the
following conditions: Pipe specifications according to
ASTM D-2239, PE-2306 classification, and meeting the
requirements of ASTM D-1248, P-23 classification (material
- specifications for PE-2306) and approved by the laboratory
making the evaluation for this purpose.
SUMMARY OF T~E INVENTION
An object of the present invention is to provide
a medium density, thermoplastic, ethylene polymer based
composition that can be readily extrudable into pipe.
An object of the present invention is to provide
such a pipe composition from which extruded pipe having
; high burst strength and stress crack resistance properties
may be prepared.
Another object of the present invention is to
provide such a pipe composition as can be extruded into
such pipe at high production rates.
A further object of the present invention is to
provide such pipe made from such compositions.
These and other objects of the prese~t invention
are achieved with a thermoplastic composition formed from a
selective combination of ethylene polymers.
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DESCRIPTION OF THE PREFERRED EMBODIMENT
The objects of the present i~vention are achieved
by employing as an extrudable, pipe forming, composition
one h~ving the following ormulation, based on a total
weight percent therein of 100;
a) about 40 to 90, and preferably about 60 to 70, :
weight percent of thermoplas~ic high density ethylene poly-
mer;
b) abou~ lO to 60, and preferably about 30 to :.
40, weight percent of thermoplastic low density ethylene
copolymer;
c~ about 0.01 to 0.20, and preerably about 0.05
to 0.10, weight percent of antioxident for such ethylene
polymers.
The pipe forming compositions of the present
invention are used in thermoplastic form, that is, they are
not cross-linked. The pipe forming compositions of the
pres.ent invention have the following properties: :
Density (ASTM D-1505) of 0.926 to 0.940 grams per
cubic centimeter and preferably of about 0.935 to 0.940
grams per cubic centimeter;
Melt index (ASTM D-1238) of about 0.10 to 2.0
decigrams per minute and preferably of about 0.50 to 1.0
decigrams per minute.
They can be extruded in pipe at the rate of about 100 to 1000
pounds p~r hour under ~ypical pipe extrusion conditions o~
a stock t~mperat~re of about 200~C.
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The extruded compositions have the Eollowing
properties in extruded form:
burst test properties (ASTM D-2239) at 100F. at
least 1000 hours to Eailure at a hoop stress of 1070 pounds
per square inch.
A minimum instant burst hoop stress of 2520 psi
at 730F. conforms to the specified pipe diameter and wall
thickness in ASTM D-2239 or other appropriate pipe
specifications.
High Density Ethylene Polymer
The high density ethylene polymer which is to be
used in the compositions of the present invention is a
normally solid (i.e., solid at 23C.) thermoplastic resin
having
a denslty of greater than 0.940 to 0.965 grams
per cubic centimeter and preferably of about 0.948 to 0.952
grams per cubic centimeter (as measured by ASTM 1505 with
preparation as in ASTM D-1928, Procedure C),
at a melt index of about 0.10 to 2.0 decigrams per
20 minute and preferably of about 0.8 to 1.4 decigrams per
minute (as neasured by ASTM D-1238 at 44 psi test pressure).
The high density polyethylene can contain C3 to
C6 interpolymerized mono-alpha-olefins such as propylene,
butene-l and hexene-l.
The high density ethylene polymers may be used
individually or in combination with each other in the com-
positions of the present invention.
The high density ethylene polymers have a cyclo-
hexane extractables (24 hours, at reflux temperature at
atmospheric pressure) content o~ 1 to about 20 weight
percent.
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The high density ethylene polymers may be made
under low pressure conditions of about 150 ~o 300 psl with
supported chromium compound based catalysts such as
chromium oxide ~as disclosed in U.S. 2,825,721)
silyl chromate (as disclosed in U.S. 3,023,203)
bis (cyclopentadiene) chromium (as disclosed in
U.SO 3,687,920 or U.S. 3,709,853).
As disclosed in these patents, the high density
ethylene polymers may be homopolymers, or copolymers of a
major portion of ethylene and a minor portion of the C3 to
C6 alpha mono-olefin comonomer.
Low Density Ethylene Coeo~ymer
The low density ethylene copolymer which is to be
used in the compositions of the present invention is a
normally solid (i.e. solid at 23C.) thermoplastic resin
having
a density of about O.glO to 0.925, grams per
cubic centimeter and preferably of about 0.918 to 0.922,
grams per cubic centimeter,
a melt index of about 0.10 to 2.0 decigrams per
minute and preferably of about 0.70 to 0.90 decigrams per
minute.
The low density ethylene copolymer contains C3
to C6 interpolymerized mono-alpha-olefins such as propylene,
butene-l and hexene-l.
The low density ethylene copolymers may be used
individually or in combination with each other in the
compositions of the present invention.
The low density ethylene copolymers may be made
under low pressure conditions of about 150 to 300 psi with
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supported chromi~ oxide based catalysts that are modified
with titanium and, optionally, fluorine, as disclosed in
U.S. 3,606,736 and 4,011,382.
As disclosed in these patents the low density
ethylene polymers are formed from a major portion of
ethylene and a minor portion of the C3 to C6 alpha-mono-
olefin comonomers.
Carbon Black
The composition of the present invention also can
advantageously include carbon black.
The carbon black which may be used herein includes
all reinforcing carbon blacks, including furnace blacks,
acetylene blacks and channel blacks. The carbon black, for
weather resistant end-use applications, should have a
particle size of the order of about 15 to 25 millimicrons.
The carbon black is commonly used in the form of
a low density polyethylene based master batch to facilitate
the admixture of the carbon black with the other components
of the composition of the present invention. The master-batch
is made using conventional technology and is prepared from
about 25 to 35 parts by weight oE the carbon black and
about 75 to 65 parts by weight of polyethylene having a
density of about 0.90 + 0.02 which is made with free
radical catalysts in tubular reactors under pressures of
about ~20,000 psi. ~;
Antioxidant
The compositions of the present invention also
advantageously contain one or more suitable high temperature ~ ; -
antioxidants for the polymer systems.
The antioxidants are preferably sterically
hindered phenols. Such compounds would include
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1,3,5-trimethyl-2,4,6-trisC3,5-ditertiary butyl-
4-hydroxy benzyl)benzene;
1,3,5-tris(3,5-ditertiary butyl-4-hydro~y benzyl)-
S-triazine-2,4,6-(IH,3H,SH)trione;
tetrakis-[methylene-3-(3',5-di-t-butyl-4'-hydroxy
phenyl)-propionate] methane; and
di(2-methyl-4-hydroxy 5-t-butyl phenyl)sulfide.
Polymerized 2,2,4-trimethyl dihydroquinoline may
also be used.
The antioxidants may be used individually, or in
combination with one another.
Adjuvants for~ position
In addition to the high density ethylene polymer
and low den=ity ethylene copolymer, the compositions of the
present invention may also contain one or more adjuvaDt
materials of the types normally used in resin based pipe or
tubing compositions.
These other adjuvants would include carbon black;
antioxidants; water-proofing fillers; inorganic fillers such
as clay, talc and calcium carbonate; lubricants; stabilizers;
and processing aids.
These adjuvants would be used in amounts designed
to provide their intended effect in the resulting composit-
ion. The total amount of such adjuvants will range from 0
to about 10 weight percent based on the total weight of the
composition.
ProcessiDL~ ~ . ieL~ns
.
All of the components o the compositions of the
present invention are usually blended or compounded ~ogether
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prior to their introduction into the extrusion device from
which they are to be extruded into the form o pipe or tub-
ing. The e~hylene polymer and alkylene copolymer of the
composition, and the other desired const~tuents thereof> may
be blended together by any of the techniques used in the art
to blend and compound thermoplastics to homogeneous masses.
For instance, the components may be fluxed on a variety of
apparatus including multi-roll mills, screw mills, continuous
mixers, compounding extruders and Banbury mixers, or dis-
solved in mutual or compatible solvents.
When all the solid components of the composition
are available in the form of a powder, or as small particles,
the compositions are most conveniently prepared by first
making a blend of the components, say in a Banbury mixer or
a continuous extruder, and then masticating this blend on a
heated mill, ~or instance on a two-roll mill, and the mill-
ing continued until an intimate mixture of the components is
obtained. AlternativelyJ as previously described, a master
batch containing the base polymers and the carbon black and/
or the antioxidant and, if desired, some or all of the other
components, may be added to a mass of polymer, Where the base
polymers are not available in powder form, the compositions
may be made by introducing the polymers to the mill, ;
masticating un~il they form a band around o~e roll, after
which a blend of the remai~ing components is added and the I ;~
milling continued until an inti~ate mix~ure is obtained. I
The rolls are prefera~ly maintained at a temperature which
is within the range of about 80C. to 150C. The composit-
ion, in the form o~ a sheet, is removed from the mill and
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then brought into a form, typically dice-like pieces, suit- -
able for subsequent processing.
Aft~r the various co~ponen~s of the compositions
are uniformly admixed and blended tog~ther, they are further
processed, in accordance with the present invention, in a
convention pipe or tubing extrusion apparatus at about
175 to 235C.
The pipe or tubing is usually made with walls that
are about 0.060 to 0.50 inches thick, and the inner diameter
of the tubing may be of the order of 0.60 to 6.0 inches.
Examples
The following examples are merely illustrative of
the present invention and are not intended as a limitation
on the scope thereof.
The compositions that were evaluated had the
following compositions:
Composition A
68.5 parts by weight of high density polyethylene
virgin powder of an average diamter of 0.01 to 0.02 inches,
having a density of 0.952, and a melt index of 1.4 decigrams
per minute and a cyclohexane extractableR content of 6.8
percent;
25.0 parts by weight of low density polyethylene
pellets of cylindrical shape and an average size of l/8
inch diameter and 1/8 inch length having a density of 0.918
and a melt index of 0.2V decigrams per minute;
6.5 parts by weight of carbon black (35 percent
carbon black in low density-h~gh pressurQ-polyethylene);
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0.1 part by weight o~ an antioxidant (which ~as
4,4'-thio-bis (6-tertiary butyl meta cresol)).
Composition B
68.5 parts by weight of high density polyethylene
virgin powder of an average diameter of 0.01 to 0.02 inches,
having a density of 0.952, a melt index of 1.4 decigrams per
minute and a cyclohexane extractables cont~nt of 6.8 percent;
25.0 parts by weight of an ethylene-butene-l co-
: polymer virgin powder of an average diameter of 0.03 to 0.04
inches having a density of 0.918 and a melt index of 0.80
decigrams per minute;
; 6.5 parts by weight of carbon black (35 percent
carbon black in low density-high pressure-polyethylene); .
0.1 part of an antioxidant (which was 4,4'-thio-
bis (6-tertiary butyl meta cresol?).
: Composition C
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68.5 parts by weight of high density polyethylene i;:
pellets of cylindrical shape and an average size of 1/8 inch
diameter and 1/8 inch length having a density of 0.952, a
melt index of 0.97 decigrams per minute and a cyclohexane
extractables content of 3.7%;
25.0 parts by weight of low density polyethylene ;
pellets of cylindrical shape and an average size of 1/8 ~ ;
inch diameter and 1/8 inch length having a density of 0.918
and a melt of 0.20 decigrams per minute; :~ .
6.5 parts by weight of carbon bla~k (35 percent
carbon black in 1QW density-high pressure-polyethylene);
~.~ part by weight of an antio~idant ~which was
4,4'-thio-bis (6-tertiary butyl meta cresol)~
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Com~osition D
68.5 parts by weight of high density polyethylene
peilets of cyllndrical shape and an average size of 1/8 inch
diameter and 1/8 inch length having a density of 0.952, a
melt index of 0.97 decigrams per min.u~e and a cylohexane
extractables conten~ of 3.7 percent;
25.0 parts by weight of an ethylene butene-l co-
polymer virgin powder of an average diameter of 0.03 to 0.04 :
inches having a density of 0.918 and a melt index of 0.80
decigrams per minute;
6.5 parts by weight of carbon black (35 percent
carbon black in low density-high pressure- polyethylene);
0.1 part of an antioxidant (which was 4,4'-thio-
bis (6-tertiary butyl meta cresol)~.
These compositions were subjected to the following
tests:
Melt Index ASTM D-1238; Stress crack resistance,
- ASTM 1693; Fso is the time in hours for 50 percent of the ~ :
samples to ~ail; Extrusion rate is measured on pipe under
extrusion conditions of 45 revolutions per minute in a
2-lt2" diameter - 16/1 length to diameter ratio extruder,
for example; Burst testing according to ASTM D-1598 by
extruding the compositions under conventional extrusion
techniques into pipe of 1 inch inside diameter and a wall
thickness of ~ inches.
The results are set forth in the following Table:
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Table
Composition A ~ C D
Melt Index 0.80 1.0 0.51 0.74
Stress Crack
Resistance, F50, hours 48 216 192 504
Extrusion Ra~e, lbs./hr71 7~ 69 72
Burst Testing at 100F~
hours to fail at Hoop
Stress of 1070 psi 696 1328 1312 1903
The results show that there is a significant in-
crease in the failure time with Compositions B and D of the
present invention and thus they are stress crack resistant ~ .
as compared with Compositions A and C of the prior art. .-
The extrusion rate shows a slight improvement for the com-
positions o~ the present invention. Additionally, the
compositions of the present invention have increased failure ,~
time in burst testing asi.compared with the compositions of
the prior art.
Obviously, other modifications and variations of
the present invention are possible in light of the above
teachings. It is, therefore, to be understood that changes
may be made in the particular embodiments o this invention
described which are within the ~ull intended scope o~ the
invention as defined by the appended c:laus.
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