CA2247916C - Molding products - Google Patents

Abstract

This invention relates to polyethylene resins which have wide molding latitu de in the rotational molding process when produced with a metallocene catalyst.

Description

WO 97/32707 , PCT/US97/03366 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 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 WO 97!32707 PCT/(JS97/03366 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".

WO 97/32707 , PCT/LTS97/03366 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 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.

WO 97/32707 PCTlUS97/03366 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 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 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 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 WO 97/32707 PCTlUS97/03366 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-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-WO 9?/32707 PCT/US97/03366 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 WO 97!32707 PCT/ITS97/03366 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 WO 97/32707 PCT/iJS97/03366 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 WO 97!32707 , PCT/US97/03366 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 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.~

Claims (6)

WHAT IS CLAIMED IS:

1. ~In a process for producing a hollow article of manufacture by rotomolding at an oven temperature in the range from 500° to 700°F and for a first period of time, wherein said temperature and said period of time, together, constitute a critical window effective to provide an article exhibiting impact resistance and ductility, the improvement comprising charging a mold with an ethylene polymer or copolymer powder which contains 0.1 to 2 ppm Zr, which has a melting point of 95 to 130° C., and which exhibits an I10/I2-4.63 which is less than Mw/Mn;
rotating the mold about at least one of its axes, in a hot oven at said temperature for a second period of time which exceeds that of said critical window, to allow the ethylene polymer or copolymer powder to melt and take the shape of the mold, which second period of time differs from the first period time of said critical window, removing the mold from the oven and cooling it until the molten polyethylene solidifies and recovering a solidified hollow part which exhibits ductility at impact.

2. The process of Claim 2, wherein the second period of time is less than that of said first period of time.

3. The process of Claim 1, wherein the second period of time is greater than that of said first period of time.

4. The process of Claim 1, wherein the solidified hollow part has a wall thickness which ranges from 3/32 inch to one (1) inch.

5. The process of Claim 2, wherein the solidified hollow part has a wall thickness which ranges from 3/32 inch to one (i) inch.

6. The process of Claim 3, wherein the solidified hollow part has a wall thickness which ranges from 3/32 inch to one (1) inch.