Note: Descriptions are shown in the official language in which they were submitted.
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MOhDING PRODUCTS
The invention relates to rotational molding and articles
of manufacture produced thereby. Articles of manufacture are
produced from ethylene polymers or copolymers, which over an
extended range of molding temperatures and times, exhibit
ductility at impact.
Rotational molding is frequently the only practical
technique for producing very large molded parts. Rotational
molding is used to fabricate large tanks, up to 10 m3 (greater
than 2600 gallons), complex hollow-shaped objects for which
ZO injection molding is not feasible, hollow spheres, large
pipe, and similar objects.
Resins suitable for rotational molding applications must
display relatively low melt viscosity in order to replicate
the mold surface faithfully. At the same time, many
applications require excellent stress crack resistance.
Resins that meet these requirements display a moderately high
melt index and narrow molecular weight distribution.
A rotational molding hollow mold is charged with resin
in the form of a powder. The powder is made by pulverizing
pellets where the pellets are made by hot compounding an as
synthesized composition which is dry and solvent-free and
comprises spherical particles, which have an average particle
size of 0.015 to 0.035 inches, and a settled bulk density of
from 25 to 36 lb/ft3, and which is a linear polymer or
copolymer of ethylene and an alpha olefin, a MFR of 15 to 20,
and a MW/Mn of from about 2.5 to about 3.0, wherein the
copolymer is further characterized by an MI(I2) of 0.1 to 6Ø
The mold is then transferred into an oven and rotated,
preferably about two axes, to distribute the powder uniformly
over the hot surface of the mold. The heating cycle is
continued until all of the powder has melted and formed a
thick, continuous layer within the mold. The mold is then
removed from the oven and cooled until the resin has fully
solidified, then the part is removed.
For article production, the resin products may contain
any of various additives conventionally added to polymer
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compositions such as lubricants, microtalc, stabilizer,
antioxidants, compatibilizers, pigments, etc. These reagents
can be employed to stabilize the products against oxidation. _
For example, additive packages comprising 400-1200 ppm
hindered phenol(s); 400-2000 ppm phosphites; 1000 to 3000 ppm
UV stabilizers; and 250-1000 ppm stearates, can be
incorporated during pelletization.
Rotational molding is sometimes denoted as '°rotomolding"
in this disclosure. Rotational molding of polyethylene
comprises a process in which a mold is charged with
polyethylene powder, and, while rotating about two axes, is
placed in a hot oven long enough for the powder to melt and
take the shape of the mold; thereafter the mold is removed
from the oven and cooled until the molten polyethylene
solidifies, and then the solidified part is removed from the
mold. Unlike other molding process, no pressure is involved
in rotomolding.
The time which the mold must be kept in the oven depends
on the oven temperature, on the amount of resin in the mold
and on the resin properties. Oven temperatures range from
500° to 700°F. The time in the oven decreases as the
temperature increases and can range from a few hours at 500°F
to a few minutes at 700°F. For a given oven temperature, the
mold must be kept in the oven for a longer time as the amount
of powder in the mold increases. As the amount of powder
that is placed in the mold increases, the wall thickness of
the part increases.
For a given oven temperature and a given amount of
powder, the time which the mold must be kept in the oven
depends on characteristics of the specific resin. Current
commercial rotational molding resins generally have a
relatively narrow range of molding times where parts have
good mechanical integrity without excessive degradation. For
example, a commercial resin could require that the mold be
kept in the oven between 17~ and 28~ minutes to make a good
part with 1/8" wall thickness at 550°F. For longer or
shorter times, the part could have unacceptable properties.
An alternative resin would be particularly desirable for
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rotational molding if it could form a good part (1) in much
less than 17~ minutes or (2) over more than a 1 minute range
, of times.
The resins made with metallocene catalyst used in this
invention form ductile articles when rotationally molded
either for shorter times or over broader range of times than
that which is required to rotationally mold ductile articles
from resins that have similar density and melt index but are
not made with metallocene catalyst. Accordingly, the process
of this invention allows greater process flexibility in the
production of articles of manufacture by rotational molding,
or rotomolding, which exhibit mechanical integrity or impact
resistance.
The article is characterized as having good impact
resistance if it cannot be broken easily by striking it, for
example, with a hammer or by letting an object fall on it.
Frequently, impact resistance is determined by dropping a
dart on the article or on a section taken from the article.
If the falling dart has enough energy to pierce the article
and if the deformation is localized around the tip of the
dart, the failure is described as ductile. Ductile failures
indicate that the article was molded well. If the falling
dart causes the article to crack in many directions away from
the point of impact, the failure is described as brittle.
Brittle failures indicate that the article was not left in
the oven long enough (undercure) or that it was left in the
oven too long (overcure). The impact resistance can be
quantified from the dart weight and drop height which cause
failure.
The products of rotational molding in accordance with
the invention exhibit ductility during impact. Specifically,
~ when subjected to dart drop impact sufficient to pierce the
wall of the rotational molded articles, the material of the
wall will not shatter (like glass on impact.)
The articles of manufacture herein are hollow with wall
thicknesses ranging from 3/32" to 1" preferably ranging from
1/8" to 1/2" preferably ranging from 3/16" to 3/8".
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Products which can be made this way include rotationally
molded plastics which are hollow parts. With rotomolding,
parts can be molded economically in a variety of shapes and
sizes, many of them impossible to produce by any other
process. Common rotationally molded products include
shipping drums, storage tanks and receptacles, material
handling bins, fuel tanks and housings. Consumer products
include furniture, light globes, toys, surfboards, and a
marine accessories. Storage containers include, for example,
tanks for storage of solvent (nylon); high purity chemicals
(PDVE), general storage (HDPE) and aggressive chemicals
(XLPE), tanks for may applications, portable tanks, closed-
dome tanks, agricultural and chemical storage tanks, 500
gallon septic tank, toys such as carousel horse, toys storage
container, spring horse, see-saw , rocking horse, picnic
table, play balls, wading pool, hopalong rider bounce toys,
motorcycle fairings and saddle bags, hockey game base, camper
top, video game housing, swimming pool filter, Kayak,
sailboard, canoe, betting station, bicycle trailer, beer keg
cooler, automotive including tool chest for truck, tractor
fuel tank, fuel tank, air ducts, head rest and special
applications, such as salad bar, statue, full service station
island, wonder house, display columns, planter pots, display
globes, kennels, pump island accessories and furniture.
In accordance with the invention, the polyethylene,
preferably polyethylene copolymers described below, have a
wide range of molding times at which parts are ductile during
impact failure. Molders have the opportunity to use shorter
molding cycles. Molders who tend to use less than optimum
molding conditions for resins with a narrow operational
molding window could observe improved properties and improved
quality by using resin with a wide molding latitude. The
resin described below for use in the invention is also
capable of providing a wide molding cycle latitude.
The polyethylene resin, preferably a copolymer, which is
used herein is produced, catalytically, in the gas phase
fluid bed is retrieved as a powder. Additives for
stabilization are incorporated with the reactor powder during
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pelletization, the polyethylene pellets are subjected to
grinding prior to rotational molding.
, The linear copolymer products used herein contain O.1 to
2 ppm of Zr. The product has an average particle size of
5 0.015-0.035 inches, settled bulk density from 25 to 36 lb/ft3.
The particles have spherical shape and are relatively non-
porous.
They are characterized by a density as low as 0.902.
For applications herein, the density is greater than .900,
generally greater than 0.930, preferably ranging from 0.935
to 0.945 g/cm3.
Significantly, the narrow molecular weight distribution
copolymers have been produced with MI of one (1) and less
than 1, down to 0.01, and up to 10. Preferably, products
used in the invention exhibit a MI value which can range from
1 to 7, and most preferably from 2 to 5.
The resins exhibit a melt flow ratio (MFR) range of 15
to 25, preferably from 15 to 20. In products of some of the
Examples, the MFR ranges from 16 to 18. MFR is the ratio
I21/I2 [wherein I21 is measured in accordance with ASTM D-1238,
Condition 190/21.6 and IZ is measured in accordance with ASTM
D-1238, Condition 290/2.16.]
Melting points of the products range from 95C to 130C.
Furthermore, the hexane extractables content is very low,
typically ranging from 0.3 to 1.0 wt.~.
The MW/Mn of these products ranges from about 2.0 to
about 3.5 and from about 2.5 to about 3Ø Mw is the weight
average molecular weight and M" is the number average
molecular weight, each of which is calculated from molecular
weight distribution measured by GPC (gel permeation
chromatography). Products have been produced with MW/M~ lower
than 2.5, in the range of 2.0 to 3.5 preferably in the range
of 2 to 3. In the products of the invention, the numerical
value of Ilo/IZ - 4.63 is less than MW/Mn. I2, or melt index
is
measured in accordance with ASTM D-1238; and Ilo is measured
in accordance with ASTM-D 1238, Condition 190/10. Products
have been made with Ilo/IZ ranging from 5.5 and greater.
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The copolymers are produced with ethylene and optionally
one or more C3-Clo alpha-olefins, in accordance with the
invention. The copolymers contain at least 80 weight
ethylene units. The comonomers used in the present invention
preferably contain 3 to 8 carbon atoms. Suitable alpha
olefins include propylene, butene-2, pentene-1, hexene-1, 4-
methylpentene-I, heptene-1 and octene-1. Preferably, the
alpha-olefin comonomer is 1- butene, 1-hexene, and 1- octene.
The most preferred alpha olefin is hexene-1. Thus,
copolymers having two monomeric units are possible as well as
terpolymers having three monomeric units. Particular
examples of such polymers include ethylene/1-butene
copolymers, ethylene/1-hexene copolymers, ethylene/4-methyl-
1-pentene copolymers, ethylene/1-butene/1-hexene terpolymers,
35 ethylene/propylene/1-hexene terpolymers and
ethylene/propylene/1-butene terpolymers.
Hydrogen, frequently used as a chain transfer agent in
the polymerization reaction, is not necessary for the present
invention. Any gas inert to the catalyst and reactants can
also be present in the gas stream.
These products are prepared in the presence of catalyst,
preferably under either slurry or fluid bed catalytic
polymerization conditions described below. When made in the
gas phase fluid bed process, on pilot plant scale, the
product is dry and solvent-free and comprises spherical, non-
porous particles, which has an average particle size of 0.015
to 0.035 inches and a settled bulk density of from 25 to 36
lb/ft3. For the production of ethylene resins in the process
of the present invention an operating temperature of 60° to
115°C is preferred, and a temperature of 75° to 95°C is
most
preferred.
The fluid bed reactor is operated at pressures of about '
150 to 350 psi, with operation at the higher pressures in
such ranges favoring heat transfer since an increase in '
pressure increases the unit volume heat capacity of the gas.
A °'diluent'° gas is employed with the comonomers. It is
nonreactive under the conditions in the polymerization
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reactor. The diluent gas can be nitrogen, argon, helium,
methane, ethane, and the like.
In fluidized bed reactors, the superficial gas velocity
of the gaseous reaction mixture through the bed must exceed
the minimum flow required for fluidization, and preferably is
at least 0.2 feet per second above the minimum flow.
Ordinarily the superficial gas velocity does not exceed 5.0
feet per second, and most usually no more than 2.5 feet per
second is sufficient. The feed stream of gaseous monomer,
with or without inert gaseous diluents, is fed into the
reactor at a space time yield of 2 to 10 pounds/hour/cubic
foot of bed volume.
The catalysts used to form the polyethylene resins
preferably polyethylene copolymers, comprise a carrier, an
aluminoxane and at least one metallocene.
The carrier material is a solid, particulate, porous,
inorganic or organic materials, but preferably inorganic
material, such as an oxide of silicon and/or of aluminum.
The carrier material is used in the form of a dry powder
having an average particle size of from about 1 micron to
about 250 microns, preferably from about 10 microns to about
150 microns. If necessary, the treated carrier material may
be sieved to insure that the particles have an average
particle size of preferably less than 150 microns. This is
highly desirable in forming narrow molecular weight LLDPE, to
reduce gels. The surface area of the carrier is at least 3
square meters per gram (m2/gm), and preferably at least 50
m2/gm up to 350 m2/gm. When the carrier is silica, it is
heated to preferably 100° to about 850°C and most preferably
at about 250°C. The carrier material must have at least some
active hydroxyl (OH) groups to produce the catalyst
- composition of this invention.
In the most preferred embodiment, the carrier is silica
which, prior to the use thereof in the first catalyst
synthesis step, has been dehydrated by fluidizing it with
nitrogen and heating at about 250°C for aproximately 4 hours
to achieve a surface hydroxyl group concentration of about
1.8 millimoles per gram (mmols/gm). The silica of the most
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preferred embodiment is a high surface area, amorphous silica
(surface area = 300 m2/gm: pare volume of 1.65 cm3/gm), and it
is a material marketed under the tradenames of Davison 952-
1836, Davison 952 or Davison 955 by the Davison Chemical
Division of W. R. Grace and Company. The silica is in the
form of spherical particles, e.g., as obtained by a spray-
drying process.
To form the catalysts, all catalyst precursor components
can be dissolved with aluminoxane and reacted with a carrier.
The carrier material is reacted with an aluminoxane solution,
preferably methylaluminoxane, in a process described below.
The class of aluminoxanes comprises oligomeric linear and/or
cyclic alkylaluminoxanes represented by the formula:
R-(A1(R)-O)n-A1R2 for oligomeric, linear aluminoxanes and
(-A1(R)-O-)m for oligomeric cyclic aluminoxane
wherein n is 1-40, preferably 10-20, m is 3-40, preferably 3-
and R is a C1-CB alkyl group and preferably methyl.
Methylaluminoxane (MAO) is a mixture of oligomers with a very
wide distribution of molecular weights and usually with an
20 average molecular weight of about 1000. MAO is typically
kept in solution in toluene.
In a preferred embodiment of aluminoxane incorporation
into the carrier, one of the controlling factors in the
aluminoxane incorporation into the carrier material during
catalyst synthesis is the pore volume of the silica. In this
preferred embodiment, the process of impregnating the carrier
material is by infusion of the aluminoxane solution, without
forming a slurry of the carrier material, such as silica, in
the aluminoxane solution. The volume of the solution of the
aluminoxane is sufficient to fill the pores of the carrier
material without forming a slurry in which the volume of the
solution exceeds the pore volume of the silica; accordingly "
and preferably, the maximum volume of the aluminoxane
solution is and does not exceed the total pore volume of the
carrier material sample. That maximum volume of the
aluminoxane solution insures that no slurry of silica is
formed. Accordingly, if the pore volume of the carrier
material is 1.65 cm3/g, then the volume of aluminoxane will be
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equal to or less than 1.65 cm3/gram of carrier material. As a
result of this proviso, the impregnated carrier material will
appear dry immediately following impregnation although the
pores of the carrier will be filled with inter solvent.
Solvent may be removed from the aluminoxane impregnated
pores of the carrier material by heating and/or under a
positive pressure induced by an inert gas, such as nitrogen.
If employed, the conditions in this step are controlled to
reduce, if not to eliminate, agglomeration of impregnated
carrier particles and/or crosslinking of the aluminoxane. In
this step, solvent can be removed by evaporation effected at
relatively low elevated temperatures of above about 40° and
below about 50°C. Although solvent can be removed by
evaporation at relatively higher temperatures than that
defined by the range above 40° and below about 50°C, very
short heating times schedules must be employed.
In a preferred embodiment, the metallocene is added to
the solution of the aluminoxane prior to reacting the carrier
with the solution. Again the maximum volume of the
aluminoxane solution also including the metallocene is the
total pore volume of the carrier material sample. The mole
ratio of aluminoxane provided aluminum, expressed as A1, to
metallocene metal expressed as M (e.g. Zr), ranges from 5o to
500, preferably 75 to 300, and most preferably 100 to 200.
An added advantage of the present invention is that this
Al:Zr ratio can be directly controlled. In a preferred
embodiment the aluminoxane and metallocene compound are mixed
together at a temperature of 20° to 80°C, for 0.1 to 6.0
hours, prior to reaction with the carrier. The solvent for
the metallocene and aluminoxane can be appro-priate solvents,
such as aromatic hydrocarbons, halogenated hydrocarbon or
halogenated aromatic hydrocarbons, preferably toluene.
The metallocene compound has the formula CpmMAnBp in
which Cp is an unsubstituted or substituted cyclopenta-dienyl
group, M is zirconium or hafnium and A and B belong to the
group including a halogen atom, hydrogen or an alkyl group.
In the above formula of the metallocene compound, the
preferred transition metal atom M is zirconium. In the above
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formula of the metallocene compound, the Cp group is an
unsubstituted, a mono- or a polysubstituted cyclopenta-dienyl
group. The substituents on the cyclopentadienyl group can be
preferably straight-chain or branched C1-C6 alkyl groups. The
5 cyclopentadienyl group can be also a part of a bicyclic or a s
tricyclic moiety such as indenyl, tetrahydroindenyl,
fluorenyl or a partially hydrogenated fluorenyl group, as
well. as a part of a substituted bicyclic or tricyclic moiety.
In the case when m in the above formula of the metallocene
10 compound is equal to 2, the cyclopentadienyl groups can be
also bridged by polymethylene or dialkylsilane groups, such
as -CH2-, -CH2-CHZ-, -CR ° R°°- arid -CR °
R°°-CR ° R°°- Where R' arid R'°
are short alkyl groups or hydrogen, -Si(CH3)2-, Si(CH3)2-CHZ-
CHZ-Si(CH3)2- and similar bridge groups. If the A and B
substituents in the above formula of the metallocene compound
are halogen atoms, they belong to the group of fluorine,
chlorine, bromine or iodine. If the substituents A and B in
the above formula of the metallocene compound are alkyl or
aromatic groups, they are preferably straight-chain or
branched C1-C$ alkyl groups, such as methyl, ethyl, n-propyl,
isopropyl, n-butyl, isobutyl, n-pentyl, n-hexyl or n-octyl.
Suitable metallocene compounds include
bis(cyclopentadienyl)metal dihalides,
bis(cyclopentadienyl)metal hydridohalides,
bis(cyclopentadienyl)metal monoalkyl monohalides,
bis(cyclopentadienyl)metal dialkyls and bis(indenyl)metal
dihalides wherein the metal is titanium, zirconium or
hafnium, halide groups are preferably chlorine and the alkyl
groups are C1-C6 alkyls. Illustrative, but non-limiting
examples of metallocenes include
bis(cyclopentadienyl)zirconium dichloride,
bis(cyclopentadienyl)hafnium dichloride,
bis(cyclopentadienyl)zirconium dimethyl,
bis(cyclopentadienyl)hafnium dimethyl,
bis(cyclopentadienyl)zirconium hydridochloride,
bis(cyclopentadienyl)hafnium hydridochloride,
bis(pentamethylcyclopentadienyl)zirconium dichloride,
bis(pentamethylcyclopentadienyl)hafnium dichloride, bis(n-
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butylcyclopentadienyl)zirconium dichloride, bis(iso-
butylcyclopentadienyl) zirconium dichloride,
cyclopentadienyl-zirconium trichloride, bis(indenyl)zirconium
dichloride, bis(4,5,6,7-tetrahydro-1-indenyl)zirconium
dichloride, and ethylene-[bis(4,5,6,7-tetrahydro-1-indenyl)]
zirconium dichloride. The metallocene compounds utilized
within the embodiment of this art can be used as crystalline
solids, as solutions in aromatic hydrocarbons or in a
supported form. The catalyst comprising a metallocene
compound and an aluminoxane in particulate form is fed to the
fluid bed reactor for gas phase polymerizations and
copolymerizations of ethylene and higher alpha olefins.
The Process Conditions
The following Examples further illustrate the features
of the invention. However, it will be apparent to those
skilled in the art that the specific reactants and reaction
conditions used in the Examples do not limit the scope of the
invention.
Example I
Polyethylene having a 6.0 melt index, 16 melt-flow-ratio
and 0.936 density was produced with a metallocene catalyst
and hexene comonomer in a gas phase reactor. Conditions for
the pilot plant Rx1 were:
Temperature 84°C
Ethylene 126 psi
Hexene/Ethylene ratio 0.0044
Fluidization velocity 1.7 ft/sec
Residence time 2.5-3.2 hr
Ash 100-180 ppm
The metallocene produced polyethylene was (1) melt
compounded on a 25-pound Banbury mixer with 750 ppm Irganox
1010, 400 ppm Irgafos 168, 500 ppm Calcium Stearate and 2000
ppm Tinuvin 622 and (2) pulverized on a semi-works scale
J
Wedco pulverizing mill. As a control for the melt
compounding, pulverizing and rotational molding processes,
commercial as-polymerized polyethylene particles (Mobil
3559B-M4HN) were selected having a 6 melt index, 25 melt-
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flow-ratio and 0.936 density. The commercial polyethylene
particles were melt compounded on the same equipment with the
same additives as the metallocene catalyzed polyethylene.
Both polyethylenes were pulverized on the same semi-works
scale Wedco pulverizing mill. 4
The powders from the commercial polyethylene and from
the metallocene catalyzed polyethylene were molded side-by-
side in a rotating twin-cube mold at 550°F and each of
several molding times from 12 to 20 minutes. The molds were
charged with 8 1/4 pounds of polyethylene powder, which
produced walls approximately 1/8 inch thick.
A 20 pound dart, having a 1 inch diameter hemi-spherical
tip, was dropped on 4"x4"x1/8" specimens which had been kept
overnight in a freezer at -40°F. For molding times from 12
to 18 minutes, the polyethylene from the metallocene catalyst
had mean failure energy ranging from 53 to 69 ft-lbs, and the
failures were ductile. The commercial polyethylene had 100
c3t~c~t_i l_rs f~ilyrnr.'-~. ;~~th mQ~n f,~i3'4'r~ ene~'gy--of-_~r9-
f~ai~t'.rs'u--bialy
at molding time of 15 minutes. For molding times from 12 to
14 minutes and from 16 to 18 minutes, the commercial
polyethylene had 20-100 brittle failures. For molding times
from 19 to 20 minutes, both types of polyethylene had 100
' brittle failures.
Example 2
Polyethylene having a 3.8-4.4 melt index, 16 melt-flow-
ratio and 0.936 density was produced with a metallocene
catalyst and hexene comonomer in a gas phase reactor.
Conditions for the pilot plant Rx1 were:
Temperature 84°C
Ethylene 146 psi
Hexene/Ethylene ratio 0.0048
Fluidization velocity 1.7 ft/sec
Residence time 2.5-3.2 hr
Ash 100-180 ppm
The metallocene catalyzed polyethylene was (1) melt
compounded on a 25-pound Banbury mixer with 750 ppm Irganox
1010, 400 ppm Irgafos 168, 500 ppm Calcium Stearate and 2000
ppm Tinuvin 622 and (2) pulverized on a semi-works scale
Wedco pulverizing mill. As a control for pulverizing and for
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rotational molding evaluations, commercial polyethylene
pellets, Mobil NRA-235, were selected having a 5 melt index,
24 melt-flow-ratio and 0.939 density and containing the same
additives as the metallocene catalyzed polyethylene. The
commercial pellets were pulverized on the same semi-works
scale Wedco pulverizing mill.
The powders from the commercial polyethylene pellets and
from the metallocene catalyzed polyethylene were molded side-
by-side in a rotating twin-cube mold at 550F at each of
three molding times with increasing amounts of resin being
charged to the mold for each molding time. For the shortest
molding time, 17 minutes, the mold was charged with 8 1/4
pounds and the wall thickness was approximately 1/8 inch.
For molding times of 20 and 24 minutes, the mold was charged
with 16 and 24 pounds which produced walls approximately 1/4
and 3/8 inch thick respectively.
A 30 pound dart, having a 1 inch diameter hemi-spherical
tip, was dropped on 4"x4" specimens which had been kept
overnight in a freezer at -40F. For the 8 1/4 pound charge,
the polyethylene from the metallocene catalyst and the
commercial polyethylene had similar mean failure energy
ranging from 50 to 60 ft-lbs, and the failures were ductile.
For the 16 and 24 pound charges, the polyethylene from the
metallocene catalyst had mean failure energy of 130 to 200
ft-lbs, respectively, and the failures were ductile. For the
16 and 24 pound charges, the commercial polyethylene had mean
failure energy of only 60 and 80 ft-lbs, respectively, and
the failures were brittle.
Esaiap3e 3
Polyethylene having a 3.2-3.8 melt index, 17 melt-flow-
ratio and 0.939 density was produced with a metallocene
- catalyst and hexene comonomer in a gas phase reactor.
Conditions for the pilot plant Rx2 were:
Temperature 84C
Ethylene 282 psi
Fiexene/Ethylene ratio 0.0044
Fluidization velocity 1.7 ft/sec
Residence time 2.5-3.2 hr
Ash 100-180 ppm
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The metallocene catalyzed polyethylene was (1) melt
compounded on a 25-pound Banbury mixer with 75o ppm Irganox
1010, 400 ppm Irgafos 168, 500 ppm Calcium Stearate and 2000
ppm Tinuvin 622 and (2) pulverized on a Wedco pulverizing
mill. As a control for rotational molding evaluations, a
commercial polyethylene powder, Mobil HRP-134, was selected
having a 3.4 melt index, 24 melt-flow-ratio and 0.939 density
and containing the same additives as the metallocene
catalyzed polyethylene.
The polyethylene powder from the metallocene catalyst
and the commercial powder were molded side-by-side in a
rotating twin-cube mold at 550°F at each of several molding
times from 17 to 25 minutes. Each cube was charged with 16
pounds of polyethylene powder, which produced a wall
thickness of approximately 1/4 inch.
A 20 pound or a 30 pound dart, having a 1 inch diameter
hemi-spherical tip, was dropped on 4"x4"x1/4'° specimens which
had been kept overnight in a freezer at -40°F. The
polyethylene from the metallocene catalyst had a mean failure
energy ranging from 108 to 153 ft-lbs for molding times from
17 to 25 minutes, and the failures were ductile. The
commercial polyethylene had a mean failure energy ranging
from 47 to 78 ft-lbs, and the failures were brittle.
8xample 4
Polyethylene having a 2.6 melt index, 16 melt-flow-ratio
and 0.939 density was produced with a metallocene catalyst
and hexene comonomer in a gas phase reactor. Conditions for
the pilot plant Rx2 were:
Temperature 84°C
Ethylene 215 psi
Hexene/Ethylene ratio 0.0045
Fluidization velocity 1.7 ft/sec
Residence time 2.5-3.2 hr
Ash 100-180 ppm
The metallocene catalyzed polyethylene was (1) melt
compounded on a 25-pound Banbury mixer with 750 ppm Irganox
2010, 400 ppm Irgafos 168, 500 ppm Calcium Stearate and 2000
ppm Tinuvin 622 and (2) pulverized on a Wedco pulverizing
mill. As a control for rotational molding evaluations, a
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commercial polyethylene powder, Mobil HRP-134, was selected
having a 2.9 melt index, 24 melt-flow-ratio and 0.939 density
and containing the same additives as the metallocene
catalyzed polyethylene.
5 The polyethylene powder from the metallocene catalyst
and the commercial powder were molded side-by-side in a
rotating twin-cube mold at 550F at each of several molding
times from 16 to 20 minutes. Each cube was charged with 8
1/4 pounds of polyethylene powder, which produced a wall
10 thickness of approximately 1/8 inch.
A 20 pound dart, having a 1 inch diameter hemi-spherical
tip, was dropped on 4"x4"x1/8" specimens which had been kept
overnight in a freezer at -40F. The polyethylene from the
metallocene catalyst had a mean failure energy ranging from
15 52 to 68 ft-lbs for molding times from 16 to 19 minutes, and
the failures were ductile. The commercial polyethylene had
90-100 ductile failures with mean failure energy of 39-56
ft-lbs at molding times of 17-19 minutes. For molding time
of 16 minutes, the commercial polyethylene had 80~ brittle
failures. For molding time of 20 minutes, both types of
polyethylene had 100 brittle failures.
The properties of the polymers produced in the Examples
were determined by the following test methods:
Density ASTM D-1505 - a plaque is made and
conditioned not less than 40 hours at
23C, 50~RH to approach equilibrium
crystallinity. Measurement for density
is then made in a density gradient
column; reported as gms/cc.
Melt Index ASTM D-1238 - Condition 190C/2.16 kg
(MI), I2 Reported as grams per 10 minutes.
High Load ASTM D-1238 - Condition 190C/21.6 kg
Melt Index
(HLMI ) , I2i
Melt Flow I2z
Ratio (MFR)
Iz
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16
Catalyst F"xamplP 1
The steps of the metallocene catalyst preparation for
production of the PE used in the foregoing Examples are set r
forth below:
Raw materials used in catalyst preparation included
505 g of Davison 952-1836 silica, 698 g of
methylaluminoxane
in
toluene
solution
(30 wt.~ MAO), 7.148 g of bis(n-
butylcyclopentadienyl)
zirconium
dichloride.
1. Dehydrate the 955 silica at 250C for 4 hours using
air to purge. Then purge with nitrogen on cooling.
2. Transfer the silica to a mix-vessel.
3. Add 7.148 g of bis(n-butylcyclopentadienyl)
zirconium dichloride and 698 g of methylaluminoxane
to a bottle.
4. Agitate the catalyst solution in the bottle until
the metallocene dissolves in the MAO solution.
5. Transfer the MAO and metallocene solution into the
mix-vessel containing the dehydrated 955 silica
slowly while agitating the silica bed vigorously to
make sure that the catalyst solution is well
dispersed into the silica bed.
6. After the addition, continue to agitate the catalyst
for 1/2 hours.
7. Start drying the catalyst by purging with nitrogen
for 5 hours at 45C.
8. Sieve the catalyst to remove particles larger than
150 micron.
9. The catalyst has the following analysis:
Yield = 914 g catalyst (from 500 g of silica)
A1 = 10 wt.~
Zr = 0.2 wt.~
