U.S. patent application number 10/022706 was filed with the patent office on 2003-06-19 for polyethylene melt blends for high density polyethylene applications.
Invention is credited to Harris, Michael G., Starita, Joseph M..
Application Number | 20030113496 10/022706 |
Document ID | / |
Family ID | 21811007 |
Filed Date | 2003-06-19 |
United States Patent
Application |
20030113496 |
Kind Code |
A1 |
Harris, Michael G. ; et
al. |
June 19, 2003 |
Polyethylene melt blends for high density polyethylene
applications
Abstract
A polyethylene composition is provided that has a density of
about 0.945 to about 0.960 g/cm.sup.3 and a melt flow index of
about 0.1 to about 0.4. The composition is a melt blend of a linear
low density polyethylene resin and/or a linear medium low density
polyethylene resin, and a high density polyethylene resin. A
feature of the composition is that the resins can independently be
virgin, recycled, scrap and/or wide specification resins, and
mixtures of these. Methods of producing the compositions and
extruded, molded and formed plastic articles manufactured from the
compositions are presented.
Inventors: |
Harris, Michael G.;
(Findlay, OH) ; Starita, Joseph M.; (Marysville,
OH) |
Correspondence
Address: |
STEPHEN D. SCANLON
JONES DAY
901 LAKESIDE AVENUE
CLEVELAND
OH
44114
US
|
Family ID: |
21811007 |
Appl. No.: |
10/022706 |
Filed: |
December 17, 2001 |
Current U.S.
Class: |
428/36.9 ;
523/512; 524/495 |
Current CPC
Class: |
C08L 23/06 20130101;
C08L 23/0815 20130101; Y10T 428/1352 20150115; Y10T 428/31938
20150401; Y10T 428/31855 20150401; C08L 23/0815 20130101; C08L
23/06 20130101; Y10T 428/139 20150115; C08L 2666/06 20130101; C08L
2666/06 20130101 |
Class at
Publication: |
428/36.9 ;
523/512; 524/495 |
International
Class: |
B32B 001/08; C08K
003/04; C08L 067/00; C08L 001/00; C08J 003/00 |
Claims
We claim:
1. A polyethylene composition comprising a melt blend of a high
density polyethylene resin and at least one resin selected from the
group consisting of linear low density polyethylene resins, linear
medium density polyethylene resins, and mixtures thereof, said
resins being present in the melt-blended polyethylene composition
in amounts relative to one another such that the composition has a
density of about 0.945 to about 0.960 g/cm.sup.3, a melt flow index
of about 0.1 to about 0.4, and a stress crack resistance of at
least 24 hours.
2. The composition of claim 1, wherein the resins are independently
selected from the group consisting of virgin, recycled, scrap and
wide specification resins, and mixtures thereof.
3. The composition of claim 1, wherein the at least one resin has a
melt flow index of about 0.1 to about 1.5.
4. The composition of claim 1, wherein the at least one resin has a
density of about 0.920 to about 0.940.
5. The composition of claim 1, wherein the high density
polyethylene resin has a melt flow index of about 0.01 to about
1.5.
6. The composition of claim 1, wherein the high density
polyethylene resin is selected from the group consisting of a
unimodal resin, a bimodal resin, a multimodal resin, and mixtures
thereof.
7. The composition of claim 1, wherein the high density
polyethylene resin is present in an amount of about 50 to about 95
percent by weight.
8. The composition of claim 1, wherein the flow rate ratio of the
melt-blended composition is about 80 to about 130.
9. The composition of claim 1, wherein the flow rate ratio of the
resins is about 20 to about 200.
10. The composition of claim 9, wherein the flow rate ratio of the
resins is about 90 to about 130.
11. The composition of claim 1, wherein the flow rate ratio of the
linear low density polyethylene and the linear medium density
polyethylene is about 20 to about 60.
12. The composition of claim 1, wherein the melt flow index of the
melt-blended composition is about 0.15 to about 0.35.
13. The composition of claim 12, wherein the melt flow index of the
melt-blended composition is about 0.2 to about 0.3.
14. The composition of claim 1, wherein the density of the
melt-blended composition is 0.945 to 0.955 and the melt flow index
is about 0.1 to 0.4.
15. The composition of claim 1, wherein the high density
polyethylene resin is selected from the group consisting of a high
molecular weight high density polyethylene resin having a melt flow
index of about 0.01 to about 0.2, a homopolymer high density
polyethylene resin having a melt flow index of about 0.1 to about
1.5, and mixtures thereof.
16. The composition of claim 15, wherein the high molecular weight
high density polyethylene resin has a density of about 0.941 to
about 0.958 g/cm.sup.3.
17. The composition of claim 15, wherein the homopolymer high
density polyethylene resin has a density of about 0.957 to about
0.970 g/cm.sup.3.
18. The composition of claim 15, wherein the high density
polyethylene resins are independently selected from the group
consisting of a unimodal resin, a bimodal resin, a multimodal
resin, and mixtures thereof.
19. The composition of claim 15, wherein the high density
polyethylene resins are independently selected from the group
consisting of virgin, recycled, scrap and wide specification
resins, and mixtures thereof.
20. A polyethylene composition comprising a melt blend of (a) a
high density polyethylene resin selected from the group consisting
of a high molecular weight high density polyethylene resin having a
density of about 0.941 to about 0.958 g/cm.sup.3 and a melt flow
index of about 0.01 to about 0.2, a homopolymer high density
polyethylene resin having a density of about 0.957 to about 0.970
g/cm.sup.3 and a melt flow index of about 0.1 to about 1.5, and
mixtures thereof; and (b) at least one additional polyethylene
resin having a melt flow index of about 0.1 to about 1.5 and a
density of about 0.920 to about 0.940 g/cm.sup.3, said polyethylene
resins being present in the melt-blended composition in amounts
relative to one another such that the composition has a density of
about 0.945 to about 0.960 g/cm.sup.3, a melt flow index of about
0.1 to about 0.4, and a stress crack resistance of at least 24
hours.
21. The composition of claim 20, wherein the high density
polyethylene resin is present in an amount of about 50 to about 95
percent by weight.
22. The composition of claim 20, wherein the resins are
independently selected from the group consisting of virgin,
recycled, scrap and wide specification resins, and mixtures
thereof.
23. An extruded, molded or formed plastic article comprising a melt
blended polyethylene composition that comprises a melt blend of a
high density polyethylene resin and at least one resin selected
from the group consisting of linear low density polyethylene
resins, linear medium density polyethylene resins, and mixtures
thereof, said resins being present in the melt-blended polyethylene
composition in amounts relative to one another such that the
composition has a density of about 0.945 to about 0.960 g/cm.sup.3,
a melt flow index of about 0.1 to about 0.4, and a stress crack
resistance of at least 24 hours.
24. The article of claim 23, wherein the article is selected from
the group consisting of pipe, pipe fittings, wire insulation
material, cable insulation materials, films, sheets, and
environmental chambers.
25. The article of claim 23, wherein the resins are independently
selected from the group consisting of virgin, scrap, recycled, and
wide specification resins, and mixtures thereof.
26. An extruded, molded or formed plastic article comprising a melt
blended polyethylene composition that comprises a high density
polyethylene resin selected from the group consisting of a high
molecular weight high density polyethylene resin having a density
of about 0.941 to about 0.958 g/cm.sup.3 and a melt flow index of
about 0.01 to about 0.2, a homopolymer high density polyethylene
resin having a density of about 0.957 to about 0.970 g/cm.sup.3 and
a melt flow index of about 0.1 to about 1.5, and mixtures thereof;
and at least one additional polyethylene resin having a melt flow
index of about 0.1 to about 1.5 and a density of about 0.920 to
about 0.940 g/cm.sup.3, said polyethylene resins being present in
the melt-blended composition in amounts relative to one another
such that the composition has a density of about 0.945 to about
0.960 g/cm.sup.3, a melt flow index of about 0.1 to about 0.4, and
a stress crack resistance of at least 24 hours.
27. The article of claim 26, wherein the article is selected from
the group consisting of pipe, pipe fittings, wire insulation
material, cable insulation materials, films, sheets, and
environmental chambers.
28. The article of claim 26, wherein the resins are independently
selected from the group consisting of virgin, scrap, recycled, and
wide specification resins, and mixtures thereof.
29. An extruded, molded or formed pipe and/or pipe fitting
comprising a melt blended polyethylene composition that comprises a
melt blend of a high density polyethylene resin and at least one
resin selected from the group consisting of linear low density
polyethylene resins, linear medium density polyethylene resins, and
mixtures thereof, said resins being present in the melt-blended
polyethylene composition in amounts relative to one another such
that the composition has a density of about 0.945 to about 0.960
g/cm.sup.3, a melt flow index of about 0.1 to about 0.4, and a
stress crack resistance of at least 24 hours.
30. The pipe and/or pipe fitting of claim 29, wherein the resins
are independently selected from the group consisting of virgin,
scrap, recycled, and wide specification resins, and mixtures
thereof.
31. The pipe and/or pipe fitting of claim 29, wherein the
composition further comprises about 1 to about 5 percent by weight
carbon black.
32. The pipe and/or pipe fitting of claim 29, wherein the pipe is
selected from the group consisting of profile pipe, corrugated
pipe, and combinations thereof.
33. The pipe and/or pipe fitting of claim 29, having a density of
0.945 to 0.955, a melt flow index of about 0.1 to 0.4, a minimum
flexural modulus of 110,000 psi and a minimum tensile strength of
3,000 psi.
34. The pipe and/or pipe fitting of claim 33, wherein the pipe is
selected from the group consisting of profile pipe, corrugated
pipe, and combinations thereof.
35. An extruded, molded or formed pipe and/or pipe fitting
comprising a melt blended polyethylene composition that comprises a
high density polyethylene resin selected from the group consisting
of a high molecular weight high density polyethylene resin having a
density of about 0.941 to about 0.958 g/cm.sup.3 and a melt flow
index of about 0.01 to about 0.2, a homopolymer high density
polyethylene resin having a density of about 0.957 to about 0.970
g/cm.sup.3 and a melt flow index of about 0.1 to about 1.5, and
mixtures thereof; and at least one additional polyethylene resin
having a melt flow index of about 0.1 to about 1.5 and a density of
about 0.920 to about 0.940 g/cm.sup.3, said polyethylene resins
being present in the melt-blended composition in amounts relative
to one another such that the composition has a density of about
0.945 to about 0.960 g/cm.sup.3, a melt flow index of about 0.1 to
about 0.4, and a stress crack resistance of at least 24 hours.
36. The pipe and/or pipe fitting of claim 35, wherein the resins
are independently selected from the group consisting of virgin,
scrap, recycled, and wide specification resins, and mixtures
thereof.
37. The pipe and/or pipe fitting of claim 35, wherein the
composition further comprises about 1 to about 5 percent by weight
carbon black.
38. The pipe and/or pipe fitting of claim 35, wherein the pipe is
selected from the group consisting of profile pipe, corrugated
pipe, and combinations thereof.
39. The pipe and/or pipe fitting of claim 35, having a density of
0.945 to 0.955, a melt flow index of about 0.1 to 0.4, a minimum
flexural modulus of 110,000 psi and a minimum tensile strength of
3,000 psi.
40. The pipe and/or pipe fitting of claim 39, wherein the pipe is
selected from the group consisting of profile pipe, corrugated
pipe, and combinations thereof.
41. A method for producing a polyethylene composition, comprising
melt blending together a sufficient amount of a high density
polyethylene resin and a sufficient amount of at least one
additional resin selected from the group consisting of linear low
density polyethylene resins, linear medium density polyethylene
resins, and mixtures thereof, to produce a melt-blended composition
having a density of about 0.945 to about 0.960 g/cm.sup.3, a melt
flow index of about 0.1 to about 0.4, and a stress crack resistance
of at least 24 hours.
42. The method of claim 41, wherein the resins are independently
selected from the group consisting of virgin, recycled, scrap and
wide specification resins, and mixtures thereof.
43. The method of claim 41, wherein the high density polyethylene
resin is present in an amount of about 50 to about 95 percent by
weight.
44. A method for producing a polyethylene composition, comprising
melt blending together a sufficient amount of a high density
polyethylene resin selected from the group consisting of a high
molecular weight high density polyethylene resin having a density
of about 0.941 to about 0.958 g/cm.sup.3 and a melt flow index of
about 0.01 to about 0.2, a homopolymer high density polyethylene
resin having a density of about 0.957 to about 0.970 g/cm.sup.3 and
a melt flow index of about 0.1 to about 1.5, and mixtures thereof;
and a sufficient amount of at least one additional polyethylene
resin having a melt flow index of about 0.1 to about 1.5 and a
density of about 0.920 to about 0.940 g/cm.sup.3, to produce a
melt-blended composition having a density of about 0.945 to about
0.960 g/cm.sup.3, a melt flow index of about 0.1 to about 0.4, and
a stress crack resistance of at least 24 hours.
45. The method of claim 44, wherein the resins are independently
selected from the group consisting of virgin, recycled, scrap and
wide specification resins, and mixtures thereof.
46. The method of claim 44, wherein the high density polyethylene
resin is present in an amount of about 50 to about 95 percent by
weight.
Description
BACKGROUND OF THE INVENTION
[0001] Plastic pipe, especially for use in drainage, irrigation,
storm sewer and sanitary sewer applications, is produced from high
density polyethylene (HDPE). A typical pipe composition contains a
high density polyethylene copolymer having a melt flow rate of
approximately 0.15 to 0.4 grams per 10 minutes that is blended with
carbon black to minimize the effect of ultraviolet light. The
Departments of Transportation (DOT) of many states of the United
States require plastic pipe used for DOT projects to meet American
Association of State Highway Transportation Officials (AASHTO)
standards, that include American Society of Testing Materials
(ASTM) standards. Current AASHTO standards for corrugated and
profile HDPE pipe require the composition of the pipe to have the
following properties: a minimum carbon black content of 2 percent
by weight; a density of 0.945 to 0.955 grams per cubic centimeter
(g/cm.sup.3); a melt flow index (MFI) maximum of 0.4; a minimum
flexural modulus of 110,000 pounds per square inch (psi); a minimum
tensile strength of 3,000 psi; and a minimum stress crack
resistance of 24 hours determined by a notched constant tensile
load test (NCTL) performed according to ASTM D5397. As used herein,
the melt flow index is intended as an equivalent expression to the
melt flow rate expressed as grams per 10 minutes at 190.degree.
C.
[0002] Many commercially available HDPE resins meeting the
standards for density, MFI, flexural modulus and tensile strength,
fail the NCTL test due to their characteristic broad molecular
weight distribution (MWD) that includes the presence of a low
molecular weight fraction that contributes to failure of the NCTL
test.
[0003] To address this problem, specialized narrow MWD, stress
crack resistant grades of HDPE have been produced by multistage
polymerization to produce a bimodal or multimodal HDPE that when
mixed with, for example, about 2 to about 6 percent by weight of
carbon black, satisfies AASHTO requirements for corrugated and
profile pipe. However, the reactor yield of the specialized HDPE
during polymerization typically varies directly with the breadth of
the molecular weight distribution. As a result, HDPE resins with
narrow MWD are usually sold at a premium.
[0004] In another approach, blending of polyethylene resins has
been used to address the problem of stress crack resistance. For
example, medium density polyethylene pipe blends with improved low
temperature brittleness properties and gloss have been obtained,
that are composed of HDPE and a concentrate mixture of linear low
density polyethylene (LLDPE) and a carbon black, where the LLDPE is
a carrier for the carbon black. This approach has the disadvantage
that the resulting medium density polyethylene pipe blends have
densities (e.g., 0.926 to 0.940 g/cm.sup.3) that are too low to
meet the AASHTO requirements for corrugated and profile HDPE pipe.
Other approaches employ two-stage HDPE polymerization processes to
produce bimodal HDPE that is used as a blending component for a
resulting medium density polyethylene having a density of 0.930 to
0.940 g/cm.sup.3. Similarly, triblends containing a major portion
of LLDPE and minor amounts of HDPEs of low molecular weight or high
molecular weight have also been reported. However, none of the
above methods results in an HDPE having a density of 0.945 to 0.955
g/cm.sup.3 and a MFI maximum of 0.4, required by AASHTO for
corrugated and profile pipe.
SUMMARY OF THE INVENTION
[0005] The invention provides a melt-blended polyethylene
composition that, when used in the manufacture of profile and
corrugated pipe, pipe fittings, and the like, results in products
that meet or exceed AASHTO standards for density, MFI, flexural
modulus, tensile strength and stress crack resistance. An advantage
of using a melt blended polyethylene composition in accordance with
the invention is that, instead of the need for specially
polymerized, premium cost HDPE for pipe, commodity grade resins,
including virgin, recycled, scrap and wide specification resins,
and the like, can be employed, resulting in significant cost
savings. Moreover, by taking advantage of the properties of
polyethylenes, especially density and melt flow index, molecular
weight distribution, modality (i.e., unimodal, bimodal, or
multimodal), and the like, HDPE can be selectively combined with
LLDPE and/or linear medium density polyethylene (LMDPE) in a melt
blend to result in compositions having the desired properties.
[0006] Thus, regardless of the combination of resins employed, the
resulting melt blended polyethylene composition has a density of
about 0.945 to about 0.960 g/cm.sup.3, preferably about 0.945 to
about 0.955 g/cm.sup.3 and, especially, 0.945 to 0.955 g/cm.sup.3,
a melt flow index of about 0.1 to about 0.4, preferably about 0.1
to 0.4, and a stress crack resistance of at least 24 hours. As used
herein, (i) the density of the composition refers to the density
prior to compounding of the composition with other materials, such
as carbon black, and the like, and (ii) the term "polyethylene"
shall admit of (though not require) the presence of small amounts
of propylene, butene, hexene, octene and/or metallacene, and the
like, as is known to those skilled in the art.
[0007] In one embodiment of the invention, the polyethylene
composition comprises a melt blend of HDPE and at least one resin
selected from the group consisting of LLDPE, LMDPE, and mixtures of
these, the resins being present in the melt-blended polyethylene
composition in amounts relative to one another such that the
composition has a density of about 0.945 to about 0.960 g/cm.sup.3,
a melt flow index of about 0.1 to about 0.4, and a stress crack
resistance of at least 24 hours. In a preferred embodiment, the
HDPE can be present in an amount of about 50 to about 95 percent by
weight. Correspondingly, the LLDPE and/or LMDPE can be present in
an amount of about 5 to about 50 percent by weight. The LLDPE
and/or LMDPE preferably have a melt flow index of about 0.1 to
about 1.5 and a density of about 0.920 to about 0.940 g/cm.sup.3.
The HDPE preferably has a melt flow index of about 0.01 to about
1.5 and a density of about 0.941 to about 0.970 g/cm.sup.3.
[0008] In an embodiment of the invention, the HDPE can be selected
from the group consisting of a high molecular weight high density
polyethylene resin (HMW-HDPE) having a melt flow index of about
0.01 to about 0.2, a homopolymer high density polyethylene resin
(H-HDPE) having a melt flow index of about 0.1 to about 1.5, and
mixtures of these. The HMW-HDPE can have a density of about 0.941
to about 0.958 g/cm.sup.3, preferably about 0.945 to about 0.955
g/cm.sup.3. The H-HDPE can have a density of about 0.957 to about
0.970 g/cm.sup.3, preferably about 0.959 to about 0.965
g/cm.sup.3.
[0009] In the foregoing embodiments, the resins can be
independently selected from the group consisting of virgin,
recycled, scrap and wide specification resins, and mixtures
thereof.
[0010] The invention also provides methods for producing the melt
blended polyethylene compositions according to embodiments of the
invention, and extruded, molded or formed products, especially
pipes and/or pipe fittings, comprising the composition.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 illustrates a molecular weight distribution (MWD)
curve for a typical prior art HDPE copolymer pipe resin having a
low molecular weight tail.
[0012] FIG. 2 illustrates a MWD curve for a prior art polymerized
bimodal narrow molecular weight distribution HDPE copolymer
overlaid with the curve of FIG. 1.
[0013] FIG. 3 illustrates component MWD curves for a blend of
linear low density polyethylene (LLDPE) and a homopolymer high
density polyethylene (H-HDPE), according to an embodiment of the
invention, overlaid with the curve of FIG. 1.
[0014] FIG. 4 illustrates the resulting MWD curve for the melt
blend of LLDPE and H-HDPE illustrated in FIG. 3, overlaid with the
curve of FIG. 1.
[0015] FIG. 5 illustrates component MWD curves for a blend of LLDPE
and a HMW-HDPE, according to an embodiment of the invention,
overlaid with the curve of FIG. 1.
[0016] FIG. 6 illustrates the resulting MWD curve for the melt
blend of LLDPE and HMW-HDPE illustrated in FIG. 5, overlaid with
the curve of FIG. 1.
[0017] FIG. 7 illustrates component MWD curves for a terblend of
LLDPE, H-HDPE and HMW-HDPE, according to an embodiment of the
invention.
[0018] FIG. 8 illustrates the resulting MWD curve for the melt
blend of LLDPE, H-HDPE and HMW-HDPE illustrated in FIG. 7, overlaid
with the curve of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
[0019] A polyethylene composition in accordance with the invention
is a melt blend of high density polyethylene resins especially for
use in the manufacture of pipe and pipe fittings, such as, but not
limited to, those used for drainage, irrigation, storm sewer and
sanitary sewer applications. The composition is particularly useful
for extruded, injection molded and blow molded profile and
corrugated pipe and pipe fittings. The composition is also useful
in the manufacture of other extruded, molded or formed plastic
articles such as, but not limited to, smooth or corrugated conduit
pipe for electrical, fiber-optic and telecommunication
applications, wire and cable insulation materials, injection molded
parts, extruded films and sheets (e.g., geomembranes and
environmental films, such as those used for pond liners, landfill
liners, and the like), environmental chambers, and the like,
especially for applications in which good stress crack resistance
is desired.
[0020] As referred to herein, density, MFI and stress crack
resistance measurements are obtained according to ASTM D1505, ASTM
D1238, and ASTM D5397, respectively. Flexural modulus and tensile
strength are measured according to ASTM D790 and ASTM D638,
respectively. Other tests that may be conducted for stress crack
resistance include, but are not limited to, the standard bent strip
Environmental Stress Crack Resistance test (ESCR), according to
ASTM D1693, the Notched Constant Ligament Stress Test (ASTM D5397),
and the Bottle ESCR test (ASTM D2561).
[0021] Enhancement of the environmental and long term stress crack
resistance of polyethylene molded articles is dependent on
increasing the number of tie molecules connecting the crystalline
lamellae of the semicrystalline high density polyethylene material.
The number of tie molecules is inversely related to the low
molecular weight faction of the polyethylene that forms the molded
article. In other words, the low molecular weight polyethylene
molecules associated with broad molecular weight distribution HDPE
diminish the number of tie molecules between lamellae, with the
effect of decreasing the stress crack resistance. Until the present
invention, pipe manufacturers have had to rely on specially
polymerized and expensive HDPE to satisfy standards for the
physical properties of pipe. Conventional commodity HDPE has been
unsatisfactory for use because of its broad molecular weight
distribution, which includes a low molecular weight tail that
contributes to failure of the NCTL test for stress crack resistance
over a 24 hour period.
[0022] To address this problem, one embodiment of the invention
provides a polyethylene composition in which LLDPE and/or LMDPE,
and HDPE are melt blended together, for example in an extruder or
other mixer (e.g., Banbury, Henschel, and the like), in amounts
relative to one another such that the resulting melt-blended,
moldable or otherwise formable polyethylene composition has a
density of about 0.945 to about 0.960 g/cm.sup.3, preferably about
0.945 to about 0.955 g/cm.sup.3, a MFI of about 0.1 to about 0.4,
preferably about 0.1 to 0.4, and a stress crack resistance of at
least 24 hours. Preferably, the HDPE resin is present in the
composition in an amount of about 50 to about 95 percent by weight
and, correspondingly, the LLDPE and/or the LMDPE is present in an
amount of about 5 to about 50 percent by weight.
[0023] FIG. 1 illustrates the broad shape of the MWD of a
conventional HDPE copolymer pipe resin (10) including the low
molecular weight tail. A MWD curve for a specially polymerized
bimodal narrow MWD HDPE copolymer (20) having a MFI in the same
range as that of HDPE (10) is illustrated in FIG. 2, with the curve
of FIG. 1 (dotted line) superimposed for comparison. The specialty
HDPE (20) has superior stress crack resistance because it does not
contain the low molecular weight tail present in conventional HDPE
(10). The specialty HDPE (20) also has good processing
characteristics based on its bimodal MWD.
[0024] A narrow MWD (i.e., the absence of a low molecular weight
tail) is also characteristic of polymerized LLDPE and LMDPE resins.
Because of this narrow MWD, LLDPE and LMDPE have a high stress
crack resistance. However LLDPE and LMDPE have a high melt flow
rate (MFI), which can make processing difficult during extrusion
and molding applications. The invention takes advantage of the high
stress crack resistance characteristic of LLDPE and/or LMDPE, by
melt blending LLDPE and/or LMDPE having a density of about 0.920 to
about 0.940 g/cm.sup.3 with a higher density conventional
homopolymer HDPE (H-HDPE) or high molecular weight HDPE (HMW-HDPE)
to increase processability. As the average molecular weight of the
HDPE employed in the composition is increased, the more the MWD of
the composition shifts to higher molecular weight. For example, an
LLDPE and/or LMDPE having a density of about 0.920 to about 0.940
g/cm.sup.3 can be melt blended with a H-HDPE having a density
greater than 0.959 g/cm.sup.3, to lift the blended density into the
preferred density range of about 0.945 to about 0.960 g/cm.sup.3.
The MWD curves of the individual blend components of LLDPE (30) and
a unimodal H-HDPE (40), of a composition according to the invention
are illustrated in FIG. 3. The resulting bimodal MWD curve (50) of
the melt blend containing these components is illustrated in FIG.
4. The curve of the conventional HDPE resin (10) (dotted line) from
FIG. 1 is superimposed on each of the curves, for comparison. As
illustrated in FIG. 4, the low molecular weight tail of the
resulting melt blend compositions is greatly reduced compared to
the low molecular weight tail of the conventional HDPE resin.
[0025] FIG. 5 illustrates the individual MWD curves of a melt blend
composition according to the invention containing LLDPE (30) and a
unimodal HMW-HDPE (60). As used in the context of the melt blended
polyethylene compositions according to the invention, a HMW-HDPE is
one that has a sufficiently high molecular weight that it, by
itself, has a NCTL stress crack resistance that exceeds 24 hours.
The HMW-HDPE (60) employed in the compositions also has a molecular
weight that is sufficiently high that it does not contribute
molecules of lower molecular weight than those of the LLDPE (30).
FIG. 6 illustrates the bimodal MWD curve (70) of the resulting
polyethylene melt blend, with the curve of the conventional HDPE
resin (10) (dotted line) from FIG. 1 superimposed for comparison.
In comparison with the conventional HDPE resin, the low molecular
weight tail in this embodiment of the invention melt blend has
disappeared.
[0026] In a preferred embodiment of the invention illustrated in
FIG. 7, the LLDPE (30) is melt blended with both a unimodal H-HDPE
(40) and a HMW-HDPE (60) to produce the trimodal MWD curve
illustrated in FIG. 8. In comparison with the conventional HDPE
resin, the low molecular weight tail in this embodiment of the melt
blend is greatly reduced.
[0027] Another property that contributes to the selection of resins
for the melt blend compositions according to the invention is the
melt flow index, which is a measure of the viscosity of the
component and affects the processability of the melt blend. For
example, LLDPE having a high MFI can be melt blended with a HDPE
having a lower MFI to achieve the desired stiffness and, therefore,
the desired processability. The melt flow index is a general
indicator of the weight average molecular weight of the resins.
[0028] Yet another property that contributes to the selection of
resins for the melt blend compositions of the invention is the Flow
Rate Ratio (FRR), such as that defined in ASTM D1238, which is a
good indicator of the weight average molecular weight and a
generally accepted test method for the polydispersity of
polyethylene resin grades. Polydispersity is the ratio of the
weight average molecular weight to the number average molecular
weight, and the lower the polydispersity (and the FRR), the
narrower is the MWD. Polydispersity can be measured by gel
permeation chromatography (GPC), although this is not generally
recommended for polyethylene resins, which have poor solubility
unless special solvents are employed. GPC is also difficult to
conduct as a quality control test on, for example, recycled and/or
scrap resin products for use in the invention compositions. For
example, such recycled or scrap products may include, but are not
limited to, such usuable resins obtained as "milk jugs" produced
from H-HDPE, "T-shirt bags" constructed from bimodal HMW-HDPE,
recycled 55 gallon drums constructed from HMW-HDPE, plastic dry
cleaning bag material made of LLDPE, and the like. Because it is a
simple test to conduct, the FRR is, therefore, preferred to GPC for
estimating the polydispersity of the resins, where the
polydispersity is not specified, prior to their use in the melt
blend compositions.
[0029] The FRR is the ratio of the high load melt index (HLMI,
condition F at 21.6 kg at 190.degree. C.) to the melt index (MI,
condition E at 2.16 kg and 190.degree. C.). For example, an LLDPE
or a LMDPE with a nominal MI of 0.7 and a HLMI of 21.0 would have
an FRR (HLMI/MI=21.0/0.7) of about 30. HMW-HDPE having an HLMI of
4.5 and an MI of 0.05 would have an FRR of about 90. Both of these
materials would be considered to have a narrow molecular weight
distribution and low polydispersity. Polyethylene resins suitable
for use in the invention compositions can have a FRR of about 20 to
about 200, preferably about 90 to about 130. The LLDPE and LMDPE
resins suitable for use in the compositions according to the
invention have a very narrow MWD and an FRR of about 20 to about
60.
[0030] The FRR is also a good indicator of the processability at
higher shear rates of the final melt blended compound. Therefore,
an advantage of the methods of the invention, is that the FRR of a
final composition can be predetermined by selecting resins having
FRR values that will achieve desired processing and final product
considerations, such as processability, melt strength, die swell
ratio, forming, wall thickness, and the like. It is preferred that
the final melt-blended composition have an FRR of about 80 to about
130, with about 90 to about 110 being more preferred. It has been
found that melt-blended compositions with FRR greater than 150 risk
failing the 24 hour stress crack resistance test (NCTL) and may be
difficult to process.
[0031] The LLDPE, LMDPE and HDPE resins used in the composition,
methods, and articles according to the invention, can be unimodal,
bimodal, multimodal, or mixtures of these types.
[0032] By the "modality" of the resins, is meant the number of
peaks in a molecular weight distribution curve.
[0033] In one embodiment of the invention, a polyethylene
composition comprises a melt blend of HDPE and at least one resin
selected from the group consisting of LLDPE, LMDPE, and mixtures
thereof, the resins being present in the melt-blended polyethylene
composition in amounts relative to one another such that the
composition has a density of about 0.945 to about 0.960 g/cm.sup.3,
a melt flow index of about 0.1 to about 0.4, and a stress crack
resistance of at least 24 hours. The resins are independently
selected from the group consisting of virgin, recycled, scrap and
wide specification resins, and mixtures thereof. The LLDPE and/or
the LMDPE can be present in the composition in an amount of about 5
to about 50 percent by weight, preferably about 15 to about 45
percent by weight and, more preferably, about 20 to about 35
percent by weight. The LLDPE and/or the LMDPE resin can have a melt
flow index of about 0.1 to about 1.5, preferably about 0.4 to about
1.0. The density of the linear resins can range from about 0.920 to
about 0.940 g/cm.sup.3, preferably about 0.925 to about 0.935
g/cm.sup.3. As known to those skilled in the art, the density of
LLDPE is about 0.910 to 0.925 g/cm.sup.3, and the density of LMDPE
is about 0.926 to about 0.940 g/cm.sup.3. However, suitable LLDPE
for use in the compositions according to the invention has a
density of about 0.920 to about 0.925 g/cm.sup.3.
[0034] The HDPE resin can be selected from the group consisting of
HMW-HDPE resin having a melt flow index of about 0.01 to about 0.2,
preferably about 0.05 to about 0.15, and H-HDPE having a melt flow
index of about 0.1 to about 1.5, preferably about 0.3 to about 1.0,
and mixtures thereof. The HMW-HDPE can have a density of about
0.941 to about 0.958 g/cm.sup.3, preferably about 0.945 to about
0.955 g/cm.sup.3 and the H-HDPE can have a density of about 0.957
to about 0.970 g/cm.sup.3, preferably about 0.959 to about 0.965
g/cm 3. The HDPE component can be present in the composition in an
amount of about 50 to about 95 percent by weight, preferably about
55 to about 85 percent by weight. The H-HDPE can be present in an
amount of about 50 to about 95 percent by weight. More preferably,
the H-HDPE is present in an amount of about 55 to about 85 percent
by weight.
[0035] A suitable HMW-HDPE for use in the melt blends according to
the invention has a weight average molecular weight of about
100,000 to about 1,000,000 daltons. As is known to those skilled in
the art, the melt flow index of the polymer varies inversely with
the molecular weight. According to the invention, the HMW-HDPE is
preferably selected based on its melt flow index and density,
rather than its particular molecular weight.
[0036] Any or all of the LLDPE and/or the LMDPE and the HDPE resins
in the embodiments of the invention melt blended compositions can
be recycled, wide specification, scrap and/or virgin resin, with
mixtures of these source resins being typical. In particular, the
use of recycled, wide specification and/or scrap resins is very
economical in comparison to the use of virgin resins. Suitable
virgin, scrap, recycled and wide specification LLDPE, H-HDPE, and
HMW-HDPE are known in the art. Virgin resins are commercially
available from, for example, Exxon Mobil Corporation (Irving,
Tex.), Chevron Phillips Chemical Company LP (Houston, Tex.), Dow
Chemical Company (Midland, Mich.), Ipiranga Quimica (Porto Alegre,
Brazil), Samsung General Chemicals Co., Ltd. (Seosan, Korea), and
SABIC Plastic Products (Riyadh, Saudi Arabia).
[0037] Exemplary recycled and/or scrap LLDPE and LMDPE can be, but
are not limited to, for example, printed plastic dry cleaning bag
material, off-color plastic bags and the like. The visual
characteristics of the printed or off-color plastic bags are not
apparent when used in compositions for applications, including pipe
and pipe fittings, where the addition of carbon black or another
colorant masks the off-color material and/or the printing dye.
Exemplary recycled and/or scrap HMW-HDPE film used to make plastic
bags, such as grocery bags or "T-shirt bags" for the retail
clothing industry, and the like, may be recycled for use in the
melt blends in accordance with the invention. For example, such
plastic bags can be constructed of bimodal HMW-HDPE. Other examples
of recyclable materials constructed of HMW-HDPE include, but are
not limited to, 55 gallon plastic drums. H-HDPE used to make milk
jugs or other such containers, for example, may be recycled for use
in the melt blends in accordance with the invention.
[0038] Wide specification resins differ from on-specification
resins in that a wide specification resin is out of the desired
specification range for at least one physical property including,
but not limited to, density, melt flow index and FRR. Because
on-specification resins are desirable and sometimes necessary for
particular applications, they can command a premium price.
Therefore, by using wide specification resins in the melt blends of
the embodiments of the invention composition, it is possible to
achieve a cost savings in comparison to using resins having
specified properties. As a non-limiting example, when wide
specification LLDPE and/or LMDPE resin is employed, the physical
properties of the melt blend compositions are obtained by
compensating for the out-of-range specification characteristic with
a formulation change that can include one or more HDPEs having a
corresponding off-setting characteristic. For example, to
compensate for LLDPE having a melt flow index in a range of 0.4 to
1.0, HDPE having a low melt flow index of, for example, 0.01 to 0.1
can be employed, or the proportion of HDPE with a melt flow index
range of, for example, 0.05 to 0.1 can be increased in the
composition. As another example, to compensate for LLDPE having a
density of, for example, 0.920 g/cm.sup.3, a 50:50 mixture of the
LLDPE and an H-HDPE having a density of 0.965 g/cm.sup.3 can be
employed, to bring the density of the final product to 0.945
g/cm.sup.3. However, because LLDPE has good stress crack
resistance, it is desirable to use a only small amount of, for
example, H-HDPE which has a low stress crack resistance. In
general, as the density of the HDPE increases, the less is the
amount of the HDPE required in the composition to achieve the
desired density, melt flow index and other physical properties.
[0039] A feature of the invention compositions is that more than
one LLDPE, LMDPE, H-HDPE, and/or HMW-HDPE, each having individual
ranges of density, MFI and/or FRR and/or modalities, can be
employed to increase the flexibility by which components can be
melt-blended together in order to achieve the desired physical
characteristics of the resulting composition. For example, it is
possible to use combinations of resins that include, for example
but not limited to, one to about 6 or more individual LLDPEs,
LMDPEs, H-HDPEs, and/or HMW-HDPEs.
[0040] Once in possession of the teachings herein, including the
examples below, of components which can be utilized to achieve the
desired physical properties of the melt blend compositions, the
selection of suitable components, not limited to those expressly
disclosed, will be within the ordinary skill in the art. The
invention practitioner will be able to adjust the components of the
composition for specification variations without undue
experimentation.
[0041] The melt-blended compositions of the invention can be used
to produce an extruded, molded or formed plastic article having a
density of about 0.945 to about 0.960 g/cm.sup.3, a melt flow index
of about 0.1 to about 0.4, and a stress crack resistance of at
least 24 hours. Exemplary articles include, but are not limited to,
pipe, including conduit pipe, pipe fittings, wire insulation
material, cable insulation materials, films, sheets, and
environmental chambers, especially for applications described
above.
[0042] The melt-blended compositions of the invention are
particularly useful to produce profile or corrugated pipe and/or
pipe fittings, having physical properties that conform to
applicable standards. As a non-limiting example of pipe and pipe
fitting applications, the compositions can be used to produce
profile and corrugated pipe having a density of 0.945 to 0.955, a
MFI of about 0.1 to 0.4, a minimum flexural modulus of 110,000 psi,
a minimum tensile strength of 3,000 psi and a minimum stress crack
resistance of at least 24 hours, as required by current AASHTO
standards.
[0043] Generally, a small diameter extruded pipe (e.g., about 2
inches to about 12 inches) is easier to extrude and form. Thus, a
small diameter pipe, for example, can be formed from a melt blend
composition of the invention having a MFI of about 0.3 to less than
0.4; whereas a large diameter extruded pipe (e.g., about 36 inches
to about 72 inches) should be stiffer for extrusion and forming.
Therefore a large diameter pipe can be formed from a melt blend
composition having a MFI of about 0.15 to about 0.2, for example.
Medium diameter pipes (e.g., about 15 inches to about 30 inches)
can be formed from a melt blend composition having a moderate MFI
of about 0.2 to about 0.3, for example.
[0044] Similarly, it is generally understood that the
processability of corrugated pipe is improved by an increased
stiffness of the melt blend, in comparison to the stiffness of the
melt blend used to produce profile pipe. Accordingly, one of
ordinary skill in the art could produce pipe according to the
invention by varying the proportions of the melt blend components
in the melt blend until the desired melt flow index and density of
the composition was achieved, without undue experimentation.
[0045] Natural ethylene polymers have a detrimental property in
that they slowly degrade in the presence of oxygen (air), and the
degradation is known to be accelerated by the presence of heat
and/or ultraviolet radiation. Preferably, pipes or pipe fittings
comprising the melt blended composition are compounded with small
amounts of carbon black, or other photo- and thermal-oxidation
retarders to minimize the effects of heat and ultra violet light.
For example, the composition can comprise carbon black (about 1
percent to about 5 percent, preferably about 2 percent to about 3
percent by weight). The carbon blacks can include any of the
commonly available, commercially-produced carbon blacks including,
but not limited to, furnace blacks, acetylene blacks, channel
blacks and lamp blacks.
[0046] The compositions according to the invention may also
comprise other additives customary for use in resin-based
compositions, according to the application for which they are being
used. Such additives include, but are not limited to, antioxidants,
antiozonants, lubricants, stabilizers, processing aids,
water-proofing fillers, inorganic fillers, colorants, curatives,
and the like. These additives are used in amounts designed to
provide their intended effect in the resulting composition. The
total amount of such additives can range from zero to about 10
percent by weight based on the total weight of the composition.
[0047] In one embodiment, a method for producing a polyethylene
composition according to the invention comprises melt blending
together a sufficient amount of a HDPE resin and a sufficient
amount of at least one additional resin selected from the group
consisting of LLDPE resins, LMDPE resins, and mixtures thereof, to
produce a melt-blended composition having a density of about 0.945
to about 0.960 g/cm.sup.3, a melt flow index of about 0.1 to about
0.4, and a stress crack resistance of at least 24 hours. The HDPE
resin can be present in an amount of about 50 to about 95 percent
by weight.
[0048] In another embodiment, a method for producing a polyethylene
composition according to the invention comprises melt blending
together a sufficient amount of a HDPE resin selected from the
group consisting of a HMW-HDPE resin having a density of about
0.941 to about 0.958 g/cm.sup.3 and a melt flow index of about 0.01
to about 0.2, a H-HDPE resin having a density of about 0.957 to
about 0.970 g/cm.sup.3 and a melt flow index of about 0.1 to about
1.5, and mixtures thereof; and a sufficient amount of at least one
additional polyethylene resin having a melt flow index of about 0.1
to about 1.5 and a density of about 0.920 to about 0.940
g/cm.sup.3, to produce a melt-blended composition having a density
of about 0.945 to about 0.960 g/cm.sup.3, a melt flow index of
about 0.1 to about 0.4, and a stress crack resistance of at least
24 hours. The HDPE resin can be present in an amount of about 50 to
about 95 percent by weight.
[0049] The LLDPE, LMDPE and HDPE resins can be in pellet, powdered,
flake or regrind form, or the like. The methods are not intended to
be limited to any one method of melt blending the components. For
example, mixing or melt blending of components, including any
additives if used, can be by batch compounding, such as in a
Banbury or Henschel type mixer, or can be continuous compounding in
an extruder. For example, in one embodiment of the method, the
components of the composition can be dry-blended prior to melt
blending by single screw or twin screw extrusion. In another
embodiment, the dry components can be separately fed through
separate ports into an extruder for melt blending. In yet another
embodiment, the two or more components can be pre-combined in a
mixer, such as a Banbury or Henschel mixer, preferably under high
intensity blending, to form a hot melt which then can be combined
with a third component (e.g., a let down resin) in an extruder. For
example, a sufficient amount of LLDPE and H-HDPE can be preblended
in a Banbury mixer, and the resulting composition blended with a
sufficient amount of HMW-HDPE in an extruder to produce the desired
percentages of each of the three types of components in the final
composition. Moreover, any of the components can be mixed with, for
example, carbon black or other colorants and/or other additives, as
a master batch, which is then added to a let down resin comprising
one or more of the remaining components, to produce the desired
percentages of the components in the final composition. As a
non-limiting example, 25% of a master batch composition comprising
90% LLDPE and 10% carbon black can be combined with 75% HDPE let
down resin to provide the desired amount of LLDPE, HDPE and carbon
black in the final composition to provide the desired properties of
density and melt flow index. The temperatures and other variables
required for dry blending, hot melts, and melt blending are well
known to those skilled in the art.
[0050] Following melt blending of the components, the composition
can be injection molded, blow molded, rolled, milled, sheet
extruded, film extruded, pipe extruded, or formed or fabricated in
any manner whatsoever to form the desired product by known methods.
Once the teachings herein are in hand, the skilled practitioner of
this invention will be able to adapt conventional methods of
forming material, such as injection molding and other techniques
mentioned above, to the production of desired articles of
manufacture using the polyethylene composition of the invention.
This adaptation can be implemented on an empirical basis, without
undue experimentation.
EXAMPLES
[0051] The following examples illustrate methods of preparation of
melt blended polyethylene compositions of the invention. However,
the examples are not intended to be limiting, as other methods for
preparing these compounds and different compounding formulations
may be determined by those skilled in the art. Further, the blend
components are not limited to the specific polyethylenes shown.
Thus, it is believed that any of the variables disclosed herein can
readily be determined and controlled without departing from the
scope of the invention herein disclosed and described.
[0052] Below are three examples of preferred embodiments in which
the mix ratios of LLDPE, H-HDPE and HMW-HDPE components differ with
respect to each other. In each of the following examples, the
exemplary polyethylene was prepared under industry standard
conditions using melt blending techniques as known in the art. Dry
mixtures of pelletized LLDPE and HDPEs were introduced directly
into a profile extruder to produce HDPE pipe.
[0053] The H-HDPE used in all of the following examples was Grade
GD 4960, supplied by Ipiranga Quimica (Porto Alegre, Brazil),
having a melt flow index of 0.80 and a density of 0.962 g/cm.sup.3.
The HMW-HDPE used in all of the examples was grade F120A, supplied
by Samsung General Chemicals Co., Ltd. (Seosan, Korea), having a
melt flow index of 0.044 and a density of 0.956 g/cm.sup.3. The
LLDPE used in all of the examples was Grade 726N, supplied by SABIC
Plastic Products (a division of Saudi Arabia Basic Industries
Corporation, Riyadh, Saudi Arabia), having a melt flow index of
0.70 and a density of 0.926 g/cm.sup.3.
Example 1
[0054] The percentages by weight of the polyethylene components
used to produce a melt-blended polyethylene composition suitable
for a producing a pipe having a diameter of about 24 to about 30
inches are listed below. The physical properties of the resulting
melt-blended polyethylene composition are illustrated in Table
1.
[0055] 35% H-HDPE
[0056] 20% HMW-HDPE
[0057] 45% LLDPE
Example 2
[0058] The percentages by weight of the polyethylene components
used to produce a melt-blended polyethylene composition suitable
for a producing a pipe having a diameter of about 12 to about 18
inches are listed below. The physical properties of the resulting
melt-blended polyethylene composition are illustrated in Table
1.
[0059] 40% H-HDPE
[0060] 40% HMW-HDPE
[0061] 20% LLDPE
Example 3
[0062] The percentages by weight of the polyethylene components
used to produce a melt-blended polyethylene composition suitable
for a producing a pipe having a diameter of about 36 to about 72
inches are listed below. The physical properties of the resulting
melt-blended polyethylene composition are illustrated in Table
1.
[0063] 35% H-HDPE
[0064] 55% HMW-HDPE
[0065] 10% LLDPE
1TABLE 1 Physical properties of the melt-blended polyethylene
compositions Test Method Exam- Exam- Exam- Property Units (ASTM)
ple 1 ple 2 ple 3 Density g/cm.sup.3 D1505 0.945 0.952 0.955 MFR
(190.degree. C.) g/10 min D1238 0.4 0.25 0.15 NCTL hours D5397
>24 >24 >24 Tensile Strength psi D638 3000 3200 3500
Flexural Modulus psi D790 110,000 135,000 160,000 Notched Izod
ft-lb/in D256 5 4 3 Cell Classification n/a D3350 335400 335400
335400 Flow Rate Ratio n/a D1238 80 90 110
[0066] This written description uses examples to disclose the
invention, including the best mode, and also to enable any person
skilled in the art to make and use the invention. The patentable
scope of the invention is defined by the claims, and may include
other examples that occur to those skilled in the art. Such other
examples are intended to be within the scope of the claims if they
have elements that do not differ from the literal language of the
claims, or if they include equivalent elements with insubstantial
differences from the literal language of the claims.
* * * * *