CA1068048A - Ethylene polymerization catalysts - Google Patents

Abstract

Abstract of the Disclosure Novel catalysts are provided for the polymer-ization of olefins such as ethylene. The catalysts consist essentially of a chromium compound supported upon an in-organic carrier containing aluminum and phosphorous moieties, at least a portion of the chromium having a valence of less tan 6. The inorganic carrier is an amorphous precipitate of aluminum phosphorus or an amorphous precipitate containing aluminum and phosphorus moieties in an atomic ratio in the range of about 5:1 to 1:1. The catalyses are particularly useful for polymerizing ethylene in a Particle Form Process in that the catalyst has essentially no polymerization induction period and provides ethylene polymers having a desirably broad molecular weight distribution and a de-sirably high melt flow shear ratio.

Description

1~68048 Background of the Invention One of the principal commercial processes employed to manufacture high density linear ethylene polymers is to polymerize ethylene in the presence of a chromium oxide catalyst supported on silica. While the catalysts employed in this process are characterized as being a chromium oxide supported on silica, it is believed that the chromium undergoes at least partial reaction with silicon atoms to form complex molecules whose precise chemical composition has not been established with certainty. It is believed that at least a portion of the chromium is present in the hexavalent state.
In a specific aspect of this process, the poly-I merization is carried out in a liquid hydrocarbon medium having little or no solvent action on the resin being produced, and the resin, as formed, precipitates as fine solid particles.
` For this reason, this particular process is known in the art .
as the Particle Form Process. As used throughout this specification, the term Particle Form Process will be re-stricted to a process carried out in the presence of a chromium catalyst and carried out in a liquid hydro-carbon medium having solubility characteristics such that the resin, as produced, precipitates in the form of fine solid particles.
One of the limitations of the Particle Form Pro- ~ -cess is that the resins produced by the process have a relatively narrow molecular weight distribution and a rela-tively low melt flow shear ratio which conventionally is expressed as the ratio obtained by dividing the high load melt index (ASTM 1238-70, Condition F) by the normal load melt index ( ~ ~1238-70, Condition E). For a number of industrial purposes, it is desirable to have available high density linear ethylene polymers having broad molecular weight distributions and high melt flow shear ratios.
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Many workers have attempted to modify the Particle Form Process to expand its capability to manufacture ethylene ~;
polymers having broader molecular weight distributions and higher melt flow shear ratios. Such efforts have been directed principally to modifying the chromium oxide-supported catalysts employed in the process. The success of such efforts has been marginal, at best, and many workers in the art believe that the Particle Form Process inherently is restricted to the manufacture of ethylene polymers having 10 narrow molecular weight distributions and low melt flow -~
shear ratios.
Summary of the Invention In accordance with the present invention, the applicants have discovered a novel class of catalysts useful in the polymerization of olefins such as ethylene. The catalysts are prepared by depositing a chromium compound -upon an inorganic carrier containing aluminum and phosphorus ` moieties and activating the catalyst by heating the material to a temperature of at least about 350C. The inorganic -~, 20 carrier is selected from the group consisting of:
(a) An amorphous precipitate of aluminum phosphate, (b) An amorphous precipitate containing aluminum and phosphorous moieties in an atomic ratio of 5:1 to 1:1, and ~ c) mixtures of (a) and (b).
Such carriers are prepared by neutralizing a strongly acidic aqueous solution containing Al cations and PO4 anions in a molar ratio in the range of 5:1 to 1:1 to form a solid precipitate containing aluminum and phosphorus moieties.

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These catalysts, when employed to initiate the polymerization of ethylene in the Particle Form Process, have no observable induction period and provide ethylene polymers of significantly broader molecular weight dis-tributions and significantly higher melt flow shear ratios than are obtained in the Particle Form Process with prior art catalysts.

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BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a photomicrograph (at 50,000 diameters~
of the calcined amorphous aluminum phosphate precipitate prepared in Example 1.
Fig. 2 is a photomicrograph (at 50,000 diameters) of the calcined amorphous aluminum-phosphorus containing precipitate prepared in Example 2 and which has an aluminum-phosphorus atomic ratio of 2:1.
Fig. 3 is a photomicrograph (at 50,000 diameters) of the calcined A1203 precipitate prepared in Example 3.
Fig. 4 is a plot of the rheological data of an `~
ethylene polymer prepared with a catalyst of the invention ~ and an ethylene polymer prepared with a prior art catalyst.
,1 Fig. 5 is a plot of certain parameters of the melt v flow properties of ethylene polymers of the invention and , prior art ethylene polymers.

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106~048 _ tailed Description of the Invention In accordance with the present invention, the applicants provide their novel polymerization catalysts by depositing a chromium compound on a particular type of carrier or support and activating the catalyst by heating to a temperature of at least about 350C. For reasons which will be developed, the catalysts of the invention differ significantly from the supported chromium oxide polymerization catalysts of the pr~or art, both with respect to chemical structure and catalytic activity.
The catalysts, when prepared in accordance with the preferred methods hereinafter described, contain a substantial portion of their chromium content in an oxidat-ion state of less than 6. This is evidenced by the fact that the preferred catalysts have a green color as dis-tinguished from the brown to orange color of hexavalent ~ -~
chromium compounds. This is true even if the chromium compound employed in the catalyst preparation is in the hexavalent state. It is known that CrO3, when heated t~ about 20 250C, will generally be converted to Cr2O3 and liberate oxygen. As will be subsequently demonstrated, the catalysts of the invention, when employed in the Particle Form Process, have no observable polymerization induction period.
The carriers or supports for the catalysts of the invention may be of two related and functionally equivalent types. The first type is an amorphous precipitate of alum-inum phosphate. The second type is an amorphous precipi-tate containing aluminum and phosphorus moieties in an atomic ratio in the range of about 5:1 to 1:1 and preferably 30 in the range of about 3.5:1 to 1.1:1.

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AmOrphous precipitates of aluminum phosphate are known in the art. These precipitates are prepared by neu-tralization of a strongly acidic aqueous medium containing aluminum cations and PO4 anions in a substantially equal molar ratio. Such acidic solutions are prepared by dis-solving in water a highly soluble aluminum salt and a source of PO4 ions, usually or'cho-phosphoric acid. The aluminum salt employed is not critical, provided only that it does not contain an anion which will form a precipitate in the subsequent precipitation step. Aluminum nitrate and alum~
inum halides, particularly aluminum chloride, are the aluminum ;
salts of choice for use in the invention. While certain phosphate salts such as triammonium ortho-phosphate can be used as the source of the PO4 ions, ortho-phosphoric acid is the source of choice for providing the PO4 ions.
The amorphous aluminum phosphate precipitate is prepared by neutralizing the acidic medium containing aluminum cations and phosphate anions. When the pH is increased to 6 or higher, the aluminum and phosphorus moieties precipitate 20 from the aqueous medium. While in theory the neutralization :;
- can be carried out by mixing the acidic solution with an appropriate alkali in any manner, it is preferred to simul-taneously add the acidic medium and the neutralizing alkali to a stirred aqueous medium. The two solutions should be added at controlled rates so that the pH is continuously maintained at a preselected pH in the range of about 6.0 -10.0~ While a wide variety of bases can be used to neutralize the acidic medium, it is preferred to use ammonium hydroxide or an ammonium salt such as ammonium carbonate so that the aluminum-phosphorus precipitate will be free of metallic ions that might be incorporated into the precipitate, if inorganic . . ,. ~

1(~680~8 bases such as sodium carbonate or sodium hydroxide were used in the process. While the precipitation reaction can be carried out over a wide range of temperatures, ambient temperature usually is employed, as no significant advantages are obtained by heating or cooling.
After the precipitation is completed, the pre-cipitate is filtered, washed one or more times to free the precipitate of occluded ions, and dried. Thereafter, the precipitate is calcined in a conventional manner at a suitable temperature, typically in a range of about 125-500 C. No advantages are obtained by calcining at higher temperatures and it is preferred to avoid calcining the product at tempera-tures above about 1100C., as some crystallization takes place at these higher temperatures. A product calcined for 4 hours at 1100C. appeared to be crystalline and to have a ;~
rudimentary tridymite-type structure.
Tbe calcined aluminum phosphate product is amorphous, and usually has a bulk density in the range of about 0.25 to 0.5 grams/cm3, and has the appearance of a compacted mass of spherical granules having a diameter in the 1-5 micron range.
The second type of aarrier for the catalysts of the invention consists of amorphous precipitates containing aluminum and phosphorus moieties in which the aluminum and phosphorus are present in an atomic ratio within the range previously described. While it is possible to prepare pre-cipitates having an aluminum/phosphorus atomic ratio of greater than 5:1, the ultimate chromium containing catalysts prepared therefrom give polymerization rates lower than desired in commercial practice. ~
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The aluminum-phosphorus containing precipitates are prepared by the same procedures employed to prepare the ~ -aluminum phosphate precipitates, except that the molar ratio of aluminum cations to PO4 anions is adjusted to a range from about 5:1 to substantially 1:1, and preferably to a range from about 3.5:1 to about 1.1:1.
In physical appearance and gross physical properties, the calcined amorphous aluminum-phosphorus containing pre-cipitates are virtually indistinguishable from the calcined ~-amorphous aluminum phosphate precipitates previously described.
The similarity of the physical appearance of the two types of calcined amorphous precipitates is seen by an examination of Fig. 1 and 2. Eig. 1 is a photomicrograph (at 50,000 diameters) of the calcined amorphous aluminum phosphate pre-cipitate prepared in Example 1. Fig. 2 is a photomicrograph (at 50,000 diameters) of the calcined amorphous aluminum-phosphorus containing precipitate prepared in Examp~e2 which has an aluminum-phosphorus atomic ratio of 2:1.
The calcined amorphous aluminum-phosphorus contain- -ing precipitates -- over the entire range of aluminum-phosphorus atomic ratios operable in the present invention --show none of the characteristics of calcined A12O3 precipi-tates. This fact is established by an examination of Figs.

2 and 3. Fig. 2 has been previously described and Fig. 3 is a photomicrograph (at 50,000 diameters) of the calcined A12O3 precipitate prepared in Example 3.
Xray diffraction data also show that calcined aluminum-phosphorus precipitates do not have any of the characteristics of calcined A12O3 precipitates. Two lots of ' 30 (1) the aluminum phosphate precipitate of Examples 1, (2) the A12O3 precipitate of Example 3, and (3) an .

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aluminum-phosphorus precipitate having the aluminum and phosphorus moieties present in a 4:1 atomic ratio were calcined for, respectively, 16 hours at 900C and 16 hours at 1100C. The A12O3 calcined at 900C was a mixture of the gama and alpha crystalline structure, whereas the A12O3 calcined at 1100C was entirely in the alpha crystalline structure.
The Xray diffraction of the other two samples that were calcined at 900C showed no evidence of any type of crystalline structure. The Xray diffraction pattern of the product of Example 1 that was calcined at 1100C showed evidence of a rudimentary tridymite-type crystalline structure, which was quite different from the pattern of the A12O3 product. The Xray diffraction pattern of the aluminum-phosphorus precipitate (4:1 Al/P ratio) calcined at 1100C showed evidence of partial crystallization. The pattern was consistent with (1) an incompletely crystallized tridymite type structure (typical of certain crystalline AlPO4 structures) and (2) a gamma type A12O3 crystalline structure.
As certain aluminum salts, ortho-phosphoric acid - 20 and ammonium hydroxide are soluble in certain polar solvents ~ -such as methanol, it is possible to prepare the previously described inorganic carriers by carrying out the indicated synthesis steps in such polar solvents or in mixtures of water and such polar solvents.
The catalysts of the invention are prepared by depositing a chromium compound on a carrier or support of the type previously described. The concentration of the chromium compound deposited upon the carrier is not critical, but ordinarily will be in the range of about 0.1 - 4.0~ and preferably in the range about 0.2 - 3.0% and more especially about 1.5 - 2.5%, expressed as free chromium. Thereafter, the catalysts are activated by being heated as subsequently described.
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106~30~8 The chromium may be deposited on the carrier in almost any chemical form such as chromic anhydride or a salt ; such as chromium chloride, chromium nitrate, chromium ace-tate, and the like. Upon being heated in the activation step, the chromium is probably converted to a different chemical form. The precise chemical form in which the chromium exists after activation is not known with certainty, but it may exist as an oxide or a phosphate or may be incor-porated into the structure of the support.
In one embodiment of the invention, the catalysts are prepared by depositing chromic anhydride on the carrier.
This can be done by simply admixing appropriate quantities of chromic anhydride and the carrier and tumbling the mate-rials together in a suitable vessel at an elevated tempera-ture under reduced pressure. Under these conditions, the -chromium deposits itself substantially uniformly over the entire surface of the carrier.
n another embodiment of the invention, the chromic anhydride or a water soluble chromium salt in an appropriate ~.

~ 20 quantity may be admixed with the aqueous slurry of the - carrier as it is prepared. Thereafter the slurry may be dried in any desired manner. One of the preferred methods for preparing the catalysts is to add the chromium compound to the aqueous slurry of the carrier and to then spray dry the slurry. This spray drying technique has the advantage that the catalyst is recovered with a particle size dis-tribution that is convenient for use in the polymerization of ethylene. Typically, the catalyst prepared by the spray drying technique will have particle sizes in the range of about 50 - 150 microns. Particles outside of this desired range can be removed by screening, but proper spray drying techniques can largely eliminate any need for screening.

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The catalysts pre~ared as described above will be activated by being heated to an elevated temperature in the range of about 350 - 950C and preferably in the range of about 700 - 925C. The activation is conveniently carried out by the same techniques employed to activate the prior art catalysts previously described, as by being suspended and fluidized in a stream of heated oxygen containing gas.
In yet another embodiment of the invention, a ~ater-soluble chromium compound will be incorporated into the acidic solution employed to prepare the aluminum-phosphorus containing carrier. The precipitation of the aluminum and phosphorus moieties also precipitates the chromium compound which becomes intimately admixed with the ~ aluminum and phosphorous moieties. When this carrier is ., ~ -.
heated to the activation temperatures previously described, highly active catalysts are obtained.
As earlier noted herein, the preferred catalysts of the invention have a substantial portion of the chromium -~ compound in a valence state of less than 6. This presents ; 20 no problems so long as appropriate control of temperature is -maintained in activating the catalysts. At temperatures ' above about 250C. any hexavalent chromium oxides present decompose with the liberation of oxygen. When activation temperatures above about 950C are employed, however, for reasons which are not fully understood, the polymerization activity of the catalyst declines.
In another embodiment of the invention, organo-chromium compounds can be deposited upon the supports of the type previously described. Examples of suitable organo-chromium compounds include dicyclopentadienyl chromium (II)and triphenylsilyl chromate. Other organochromium compounds ~
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that can be employed are those disclosed in the following-issued U.S. patents:

3,157,712 3,709,954 3,324,095 3,756,998 3,324,101 3,757,002 3,687,920 3,806,500 3,709,853 Such chromium compounds are dissolved in an appropriate sol-vent which then is used to impregnate the support, after ~' which the solvent is removed by evaporation. With catalysts of this type, it is not necessary to heat activate the finished ::
catalyst. The support will be calcined to temperatures within ' 10 the range previously discussed before the organochromium -~
compound is deposited thereon.
While the polymerization catalysts of this invention are employed in the conventional manner in the polymeri-zation of ethylene, unexpected benefits are obtained by use j~ ,~
of the catalysts of the invention. Specifically, when the :: :
catalysts of the invention are employed in the polymeri-~ zation of the ethylene by the Particle Form Process, no j observable,induction period is encountered. By contrast, ,l the prior art catalysts in which chromic anhydride is de-posi'ted on silica have a substantial induction period.
Moreover, the ethylene polymers produced by the use of the ' catalysts of the invention in the Particle Form Process provide ethylene polymers having desirably broad molecular weight distributions and desirably high melt flow shear ratios.
' In carrying out the Particle Form Process with the catalysts of the invention, the process can be controlled and/or modified by techniques similar to those used with other catalysts in the Particle Form Process. By way of example, increasing the temperature of polymerization, other .,, ' ~, ' ,;

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conditions being held constant, lowers the molecular weight of the polymer being produced. Similarly, the inclusion of hydrogen in the reaction zone lowers the molecular weight of the polymer being produced. The inclusion of higher mono alpha-olefins such as propylene, hexane and the like in the reaction zone produces copolymers having lower densities t~n~ the ethylene homopolymers otherwise produced under the prevailing polymerization conditions.
The catalysts of the invention also can be employed to polymerize ethylene in a vapor phase, fluidized bed process. The ethylene polymers produced by such processes have desirably broad molecular weight distributions and desirably high melt flow shear ratios similar to those of the "~
ethylene polymers produced by a Particle Form Process.
The following examples are set forth to illustrate -~
more clearly the principle and practice of this invention to those skilled in the art. Where references are made to percentages and parts, such percentages and parts are expressed ~
on a weight basis unless otherwise indicated. ~-Example 1 This example will illustrate the preparation of a calcined amorphous aluminum phosphate precipitate.
A strongly acidic solution containing aluminum cations and ortho-phosphate anions in an equal molar ratio was prepared by dissolving 242 grams (1 mol) of aluminum chloride (AlCl3.6H20) in 1 liter of distilled water and then ~ adding 117 grams (1 mol) of an 85~ solution of ortho-; phosphoric acid. Water was added to bring the volume of this solution up to 3 liters. A second solution was prepared by diluting 300 ml of concentrated 28% ammonium hydroxide with 300 ml of distilled water. This solution contained approximately 2.4 mols of ammonium hydroxide. -::

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A stirred reaction vessel was charged with 1000 ml of distilled water. To this distilled water was added the previously described acidic solution at a rate of approxi-mately lO0 ml per minute. The ammonium hydroxide solution was added simultaneously at a rate sufficient to maintain the pH of the stirred reaction mixture at a constant value of 8Ø After the addition of the acidic solution was com-pleted, the reaction mixture was stirred for an additional - half hour. A total of 580 ml of the ammonium hydroxide solution was used. The precipitated aluminum phosphate then was filtered, washed with 3000 ml of distilled water and dried overnight at 120C. The oven-dried granular material was calcined in air at 500C in a muffle furnace.
Example 2 This example will illustrate the preparation of a calcined amorphous aluminum-phosphorus containing precipitate having an aluminum-phosphorus atomic ratio of 2:1. The precipitate was made in the identical manner set forth in Example 1, except that the acidic solution was prepared by dissolving 750 grams (2 mols) of aluminum nitrate (Al(NO3)3.9H2O) in the 5 liters of distilled water before adding the 117 grams tl mol) of 85~ ortho-phosphoric acid thereto.
Example 3 This example will illustrate the preparation of a calcined precipitate of Al2O3, which precipitate was pre-pared as a control to illustrate certain differences in the physical properties of calcined aluminum-phosphorus precipitates as compared with calcined Al2O3 precipitates. The Al2O3 precipitate was prepared in exactly the same manner as set forth in Example 1, except that no phosphoric acid was added to the acidic aqueous solution of aluminum chloride.

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Examples 4 - 7 Calcined amorphous precipitates containing the aluminum and phosphorus moieties in varying atomic ratios were prepared in the same manner as described in Example 2 except that the quantities of the aluminum nitrate and the ortho-phosphoric acid were varied to give the desired aluminum- -phosphorous atomic ratios. The ratios employed are set forth below:
Example No.Al-P Atomic Ratio

4 1.5:1 3:1 6 4:1 7 9:1 Example 8 An acidic solution was prepared by dissolving 1000 g ;~
Al(N03)3.9H20 (2.67 mols) in 7 liters of distilled water. To this solution was added 153 g of H3P04 (85%), (1.33 mols) with thorough mixing. A stock solution of NH40H was prepared -by mixing 1 liter of NH40H (28%) with 2 liters of distilled water. This NH40H solution was added slowly, with vigorous ` stirring, to the acidic solution until the pH of the acidic ; solu~ion reached 8Ø A precipitate was formed during the addition of the NH40H solution. After the pH of 8.0 was reached, stirring was continued for 10 minutes and the slurry was then allowed to stand for 2 hours. The precipitate was separated from the liquid by filtration and was washed on the filter with 10 liters of distilled water. The moist filter cake was stored in a sealed plastic container until -ready for the addition of chromium.
Example 9 An acidic solution was prepared by dissolving 100 g of Al(N03)3.9H20 (0.27 mol) in 2 liters of distilled water. To this solution was added 31 g of H3P04 (85%) (0.27 mol) with stirring. A stock solution of ammonium carbonate ~ 16 -~ _ _ _ _ . . . .. .. . . .
- - - . . ~ - , was prepared by dissolving 119 g of NH411C03 in 1000 ml of distilled water and NH40H was added to adjust the pH of this solution to 10.7. A reaction vessel was charged with 10~0 . .
ml of distilled water to provide a stirring medium. The acidic solution was added to this reaction vessel at a rate of approximately 15 cc per minute with vigorous stirring, and the ammonium carbonate solution was added simultaneously at a rate to maintain a pH of 10 in the reaction vessel.
After all of the acidic solution was added, stirring was continued for 10 minutes, followed by filtration. The filter cake was washed on the filter with 3000 ml of dis-- tilled water and the moist filter cake was stored in a sealed container until ready for the addition of chromium.
Example 10 ' Example 9 was duplicated except for two modifi-.:~
cations. First, aluminum chloride was employed in lieu of aluminum nitrate. Second, the aluminum chloride and phosp-!
horic acid were employed in a ratio to provide an Al:P
atomic ratio of 1.2:1.
Example 11 An acidic solution was prepared ~y dissolving lO,OOa g Al(N03)3.9H20 (26.7 molsj in 50 liters of distilled water: To this was added 1555 g H3P04 (13.3 mols) with stirring. A stock solution of NH40H was prepared by di-`~ luting ammonium hydroxide (28~) with an equal volume of water and mixing. A reaction vessel was charged with 10 liters of distilled water to provide a stirring medium, and the acidic solution was added to this at a rate of approxi-mately S00 ml per minute, with vigorous stirring. The ammonium hydroxide solution was added simultaneously at a rate sufficient to maintain a pH of 8~.~. After all of the acidic solution was added, stirring was continued for 10 minutes, after which the slurry was filtered. The filter - :
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cake was washed on the filter with 120 liters of distilled water. The moist filter cake, when removed from the filter, had a solids content of 12% by weight.
The above procedure was repeated, and the two moist filter cakes were combined in a mixing vessel and reslurried with a quantity of distilled water sufficient to reduce the solids content to 9 wt ~. This slurry was spray dried and the product was collected in two fractions, a coarse fraction which remained in the collector at the bottom of the spray dryer and the fines which were carried overhead to a second collector. The total yield of product was 5117 g.
Example 12 7500 g (20 mols) of Al(N03)3.9H20 was dissolved in 50 liters of H20. To this solution was added 1150 g of H3P04 (10 mols) with stirring. A stock solution of NH40H
was prepared by mixing 3 liters of concentrated 28% ammonium hydroxide with 3 liters of distilled water.
A stirred reaction vessel was charged with 10 liters of distilled water. To this distilled water was added the acidic solution of aluminum nitrate and phosphoric acid at a rate of approximately 500 ml per minute. The ammonium hydroxide solution was added simultaneously at a rate sufficient to maintain the pH at 8. The reaction zone was stirred vigorously to maintain good mixing of the solu-tions. After the addition of the acidic solution was com-pleted, the reaction mixture was stirred for 10 minutes.
The precipitated aluminum phosphate then was filtered and washed with 100 liters of water. The filter cake as removed 30 from the filter contained 8.4 weight % solids. -A chromic acid solution was prepared by dissolving ; -40 g of CrO3 in 300 cc water. This solution was mixed thoroughly with the above wet filter cake until a homogeneous slurry was obtained. The chromium content corresponds to .. .. ~

approximately 1.2 wt ~ Cr based on the solids content. This homogeneous slurry was dried in a spray dryer and the dried product yield was 1469 g of powder.
Examples 1, 2 and 4 - 11 above illustrate the preparation of the aluminum-phosphorus supports for the catalysts of the invention. The catalysts of the invention are prepared by impregnating such supports with an appro- `
priate chromium compound and activating the catalyst by heat.
One method of preparing the catalyst is to first calcine the support, impregnate the support with chromic anhydride, and then activate the catalyst by fluidizing the catalyst in a stream of heated dry air. A typical procedure it~
is illustrated in Example 13 below.
Example 13 A l-liter round bottom flask was charged with 2 parts of chromium anhydride and 98 parts of the calcined amorphous aluminum phosphate prepared in Example 1. The charged round bottom flask then was attached to a Buchi Rotovapor. The pressure within the Rotovapor apparatus was ; reduced to 12 kilopascals (approximately 0.12 atmosphere) and the temperature was raised to 70C and maintained at this temperature for 1 hour. The temperature then was raised to 120C. At the end of the second hour, the pres-sure was lowered to 8 kilopascals (approximately 0.08 atmos-phere) and the temperature was increased to 170C. At the end of the third hour, the temperature was increased to 180C. At the end of the fourth hour, the pressure was lowered to 0.4 kilopascal (approximately 0.004 atmosphere) and the temperature was raised to 205C. Heating was con-tinued for an additional three hours under these conditions.
The catalyst then was cooled to ambient temperature and air ;

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was bled into the Rotovapor apparatus to bring the pressure -~
back to atmospheric pressure. The oranye-brown product then was transferred to a fluidized bed apparatus for activation. -The catalyst was activated by being heated to 760C for a period of 5 hours while maintaining the catalyst in a fluidized condition by the passage of air through the, fluidized bed. The air used for this purpose had been treated so that it had a dew point of less than -50C. The finished catalyst was green in color. The color indicated lO that the valence state of the chromium has been reduced to `
less than 6.
Another method of preparing the catalysts of the -`;
invention is to impregnate the carrier with the chromium , compound before the carrier is dried. This procedure is illustrated in Example 12 above. The chromium impregnated car~ier must be dried before being activated. The drying may be carried out in any manner as by simply heating in an , oven, removing the residual water by mixing with an azeotroping 1 solvent such as ethanol and distilling off the water as an ', 20 azeotrope, or, preferably, by spray drying. The catalyst is activated as previously described. , , . The catalysts of the invention were evaluated in a standardized Particle Form Process that was run on a batch '~
basis. In this standardized method a stirred polymerization ' reaction vessel was maintained in a heated jacket maintained , ; at a temperature of about llOC. The polymerization vessel ;
was charged with the catalyst to be evaluated. A small quantity of dry, oxygen-free isobutane then was charged to the reactor, allowed to vaporize, and vented from the reactor `~
30 to remove all traces of oxygen from the reactor. The reactor ,, then was charged with 500 parts of isobutane and attached to ~' a reservoir of polymerization grade ethylene gas maintained at a pressure of 3.5 megapascals (approximately 35 atmospheres).

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The reactor was in continuous open communication with the reservoir of ethylene gas with a flow meter being maintained in the ethylene line to measur~ the flow of gas to the reactor. During the charging period, the temperature of the vessel fell below 110C, but normally the temperature was reestablished at about 110C within a few minutes after the isobutane was charged to the reactor.
With the catalysts of the invention, polymeri-zation started almost immediately with no observable in-duction period.* Each polymerization was run for 90 minutesand the flow meter was read at 10 minute intervals to deter-mine if there was any change in the rate of polymerization with time over the 90 minute period of the polymerization.
At the end of the 90 minute period, the flow of the ethylene gas was discontinued, the reactor was vented, and the polyethylene was recovered and weighed. -Example 14 ; A series of four catalysts were prepared from the aluminum-phosphorus carrier described in Example 11. The carrier was calcined for 5 hours at 450C in a muffle fur-nace with a slow stream of dry air (dew point less than -50C.) being passed through the furnace. Chromic anhydride in an amount equivalent to 1% or 2~ elemental chromium was deposited on the carrier by the technique described in , Example 13. The catalysts were activated in a fluidized bed at a temperature of 540C or 760C for a period of 5 or 14 hours. Details of the catalyst preparations and the polymeri-zation results are set forth in Table I.

* - With a commercial grade chromia catalyst supported on silica, induction periods of up to 50 minutes are frequently observed.

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Table I

Polymerization Catalyst Preparation_ _ Results Run ~ctivation Activation Grams of Polymer Identification % Chromium Temp C Time, hours Polymer Rate (1) _ _ . .

10 (1) Grams of polymer/gram of catalyst/hour.
Each of the catalysts gave good rates of polymerization and there was no induction period. The rate of polymerization was constant throughout each of the runs.
Example 15 Example 14 was repeated employing two levels of -chromium with the aluminum-phosphorus carrier being calcined at 750~C. Details of the catalyst preparations ;
and the polymerization data are set forth in Table II.
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Table II -- 20 Polymerization ~ CatalYst Pre~aration Results !~ Run Activation Activation Grams of Polymer.
Identification % Chromium Temp C Time, hours Polymer. Rate (1) A 0.2 760 14 520 347 B 1.0 760 5 681 454 ~

(1) Grams of polymer/gram of catalyst/hour. ~;

Again it will be noted that good rates of polymerization ` were obtained.

Example 16 31 The chromium containing spray dried catalyst of . . . .
Example 12 was calcined for 5 hours at 500C. in a muffle -furnace in the presence of dry air having a dew point of less than -50C. The calcined product was divided into several aliquots which were activated in a fluidized bed with dry air - ;
for varying time periods at varying activation temperatures.

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The d~tails of the catalyst preparations and the polymerization data are shown in Table IIL.

Table III

CatalYst Prepara-tion Polymerization Results Run Actlvation Activation Grams of Poiymerization Identification Temp- C Time, hours Polymer Rate (1) .

(1) Grams of polymer/gram of catalyst/hour.

Example 17 Several carriers having varying Al:P atomic ratios were prepared as described in Examp';e 11. Chromic anhydride was added to the slurries to provide 2 wt ~ chromium in the i; .
finished catalyst. The catalysts were dried and calcined for 5 hours at 500C. The dried catalysts were ground and the fractions having a particle size range of 43 to 149 ~-`
microns were activated for 5 hours at 500C. in a fluidized bed.
. Ethylene polymers were prepared with each of the catalysts. The polymers' melt viscosities at 190C., certain calculated HLMI/MI ratios, and rates of polymerization are shown in Table IV. The melt viscosities were determined as described in Example 19.

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Table IV

Al/P Ratio Polymerization Melt Viscosity ~LMI/MI
In Carrier Rate (1)@ 190C ~2) Ratio 1.2 300 ~6 900 1.5 3~0 84 730 2.0 390 129 2500 3.0 350 172 480 4.0 228 204 821 9.0 10 (3) (3) (I) Grams of polymer/gram of catalyst/hour (2) Poises x 103 @ 10 sec. ] shear rate (3) Too little polymer was recovered to obtain rheological - data. !`
From the above data and additional data not included in Table IV, it has been noted that the Al/P ratio of the supports has several effects on the catalysts of the invention and ethylene polymers prepared therefrom by the Particle Form Process. First, the melt viscosity of the polymer at 190C. `-increases with ~l/P ratio. Second, the rate of polymerization 20, decreases as the A1/P ratio is increased to above a ratio of about 4:1.
Although the anhydrous aluminum phosphate carriers having precisely a 1:1 atomic ratio can be employed in the practice of the invention, it has been observed that more consistent and reproducible results are obtained when the carriers of the invention have an Al/P ratio of at least about 1.1:1Ø In particular, the finished catalysts prepared from such carriers have a bright green color and give high rates of polymerization.

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10~ 48 .
Example ]8 Ethylene polymer was produced by a Particle Form Process by a continuous process employing a catalyst of the in-vention. The catalyst was prepared by depositing 2.2 weight ~ chromium on an aluminum-phosphorus precipitate prepared by a process as illustrated in Example 2. The chromium was ` deposited upon the support as chromium oxide by a process as illustrated in Example 12. The catalyst was activated by being heated for 15 hours at about 890C. in a fluidized bed -~
of dry air.
A continuous polymerization was carried out in a pressurized, circulating loop reactor having a volume of 55 gallons. The reactor was fitted with inlets for isobutane, ethylene, and catalyst slurry. The reactor also was fitted with a discharge outlet. The reactor was initially chàrged with isobutane containing a small percentage of ethylene. A ~ -catalyst charge of about 0.3 gram was added to the reactor to initiate polymerization. The reactor was heated to a temperature of 105C. and, under these conditions, the -reactor pressure was about 540-550 psig.
The polymerization began without an induction period.
Throughout the duration of the run, isobutane was continuously charged to the reactor at a rate of 53 pounds per hour and ethylene was charged to the reactor at a rate of 20 pounds per hour. Fresh catalyst slurry was charged to the reactor , at a rate sufficient to maintain the temperature constant. ;
' This required an average of 32 injections of 0.3 gram o~
;` catalyst per hour. Product slurry consisting of ethylene polymer, isobutane, unpolymerized ethylene and catalyst was ;
continuously discharged from the reactor at a rate of about 75 pounds per hour.

Under the above-described operating conditions, the average residence time of ethylene in the , ~ ~

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106804~

reactor was about 3.5 hours. The product slurry recovered from the reactor had a bulk density of about 32 pounds per cubic foot. The reaction was carried out over a period of 24 hours and ethylene polymer was produced at a rate of just over 20 pounds per hour. The polymer had a melt index (ASTM
1238-70 Condition E) of 0.03.
Ethylene polymers produced employing the catalysts of the present invention differ in important respects from ethylene polymers of identical melt index (or identical molecular weight) produced by alternate prior art polymeri-zation processes, e.g., ethylene polymers prepared by a Particle Form Process with a commercial chromia catalyst , ~-supported on silica. While such ethylene polymers may have identical melt indexes, the ethylene polymers have sub-,; .
stantially different processing characteristics. Speci- ~
., fically, with the ethylene polymers made with the catalysts of the invention, the change of melt viscosity with applied shear is much greater than is the case with the prior art ethylene polymers. One significance of this fact is that ethylene polymers of very low melt index made with the cata-lysts of the invention can be extruded in conventional extruders by carrying out the extrusion at high applied shear rates. By contrast, prior art ethylene polymers of comparable low melt indexes simply cannot be extruded in conventional extruders. The difficulties in extruding the prior art very high molecular weight ethylene polymers are discussed by L. V. Cancio and R. S. Joyner in their paper GAINS ARE MADE IN EXTRUDING HMW PE POWDERS, Plastics Technology, February 1975, pp. 40-44.

.

Example 19 The melt flow characteristics of two resins were determined at 190C. in a rheometer. The apparent melt viscosities of the resins at varying apparent shear rates were determined and are shown in Table V.
Table V

Apparent Apparent PolymerMelt Viscosity (1)Shear Rate (2) Prior Art Resin 164 1.5 7.4 -~
48 14.8 33 29.6 74.1 . . 13 148 Resin of Invention 181 3.0 .
98 7.4 , 20 63 14.8 :' 38 29.6 .
22 74.1 :. 14 148 ~:

(1) Poise x 103 @ 190C.

~2) Sec.

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:, The ethylene polymer of the invention was prepared by the standard procedure previously described. The support for the catalyst was prepared as described in Example 12 and had an Al:P atomic ratio of 2:1. One percent chromium was deposited on the carrier which then was calcined at 500C.
The catalyst was activated with dry air in a fluidized bed for 14 hours at 760C. A commercially available prior art :. , linear ethylene polymer was employed for comparison purposes. - .
The approximate normal load melt index (MI) (ASTM ..
10 1238-70 Condition E) and the approximate high load melt index (HLMI) (ASTM 1238-70 Condition F) of the two polymers can be calculated from the data of Table V. The HLMI/MI
ratio for the prior art polymer is approximately 130. The corresponding ratio for the polymer made with t~e catalyst of the present invention was approximately 760. The higher HLMI~MI ratio of the ethylene polymer of the invention indi-cates that the catalysts of the invention provide ethylene polymers which have an unusually broad molecular weight dis- -.: . .
tribution. Such polymers inherently have better processing 20 characteristics. ~ -The data of Table V are plotted in Fig. 4 on log -log paper to show the change in apparent viscosity (in : poises) with apparent shear rate (in reciprocal seconds).
The curves for both resins are approximately straight lines, but it will be noted that the curve for the ethylene polymer of the invention has a much steeper slope. The ~-curve for the ethylene polymer of the invention has a slope of -0.65 (measured at 10 sec. 1), whereas the curve for the prior art ethylene polymer has a slope of -0.54. These negative slopes will be characterized as S values in the subsequent discussion.

Curves of the type shown in Fig. 4 graphically illustrate the shear thinning or pseudoplastic flow of ethylene polymers. When curves of this type are _ .............. . , . . . .. . , _ ., _ . . .. . . . .

prepared for two ethylene polymers having identical or simi-lar melt indexes, the polymer having the curve with the greater slope will exhibit greater shear thinning, or pseudo-plastic flow, and will flow more readily at high applied shear rates.
For purposes of the present specification, we will designate the slope of such curves as a "slope parameter,"
which we will represent by S, and which is the value of the negative slope of a plot of the natural logarithm of the polymer's apparent melt viscosity versus the natural logarithm of the apparent shear rate; such slope being measured at 10 sec. at 190C. A comparison of such S values for the ethylene polymers of the present invention with the S values of prior art linear ethylene polymers, both values being determined on polymers of similar melt index, always demon-strates that the ethylene polymers of the present invention have larger S values.
For any family of ethylene polymers, the absolute value of S is a function of the polymer's melt viscosity level. The change of S with melt viscosity level can be characterized by use of a "viscosity reference parameter"
which will be represented by Ao and which is the natural logarithm of the polymer's apparent melt viscosity at l sec.
; Linear ethylene polymers heretofore available to the art and prepared with chromia catalysts supported on silica have a relationship between their weight average molecular weight and their number average molecular weight such that the ratio MW/Mn is approximately 10. For such prior art ethylene polymers, the approximate relationship between S and Ao is defined by Formula l: ~ -(l) S = 0.0813 Ao - 0.47 The best curve for the relationship between S and Ao for the ethylene polymers of the present invention . . ,.. . .__ _ 10~;804~ ~
for S values within a range of about 0.61 to about 0.90 and Ao values within a range of about 12.3 to about 14.25, is -~
defined by Formula 2: ~
(2) S = 0.106Ao -0.71 + 0.2 : .
Typical experimental S and Ao values for several ethylene polymers of the invention having Ao values from about 12.5 to 14.2 are set forth in Table Vl.
; Table VI

`~ Polymer Ao S
.~ 10 Identification . A 12.53 0.68 .
B 12.65 0.65 C 12.80 0.66 D 13.07 0.67 ~ 13.33 0.71 : F 13.52 0.73 G 13.78 0.75 . H 14.20 0.79 :::
. Where experimentally determined S values depart from Formula 2, the experimentally determined values are nearly ~ always larger than the predicted values. Such departures : from Formula 2 occur most frequently when the Ao values are below about 13.5.
A formula for the relationship between S and Ao ; for the ethylene polymers of the invention; valid for S
values within a range of about 0.61 to about 0.90 and Ao values within a range of about 12.0 to about 14.5, which -includes all presently determined experimental values is defined by Formula 3:
(3) S ~ 0.0830Ao - 0.442 Fig. 5 sets forth a graphic representation of Formula 1 and Formula 2. It will be observed that the two .~ 30 .

~)6~3~48 curves are substantially parallel to each other, with the curve for Fomula 2 lying to the right of the curve for Formula 1. The relationship between the two curves indicates that, when an ethylene polymer of the invention and a prior art ethylene have identical Ao values, the ethylene polymer of the invention always will have a significantly higher S
value. Fig. S also sets forth the limiting value of Formula 3 when S is defined by the formula:
S = 0.0830Ao - 0.442 The area defined by the lines joining A, B, C, D, and E
includes the S and Ao values for the preferred ethylene polymers of the invention. To the best of the applicants' knowledge, no ethylene polymer reported in the the prior art, or tested by them, has S and Ao values lying within this area of Fig. 5.
The importance of the S values of ethylene polymers results from the fact that the physical properties of most articles fabricated from ethylene polymers are improved as the molecular weight of the polymer is increased. However, the melt viscosity of ethylene polymers also increases with molecular weight. The melt fabrication of ethylene polymers becomes increasingly more difficult as the polymer's melt viscosity increases. Polymer fabrication apparatus presently available cannot process prior art ethylene polymers of very high molecular weight for reasons discussed below.
-` When an attempt is made to extrude a prior art linear ethylene polymer having an Ao value of the order of 12.0 or higher, it is necessary to extrude the polymer at a minimum shear rate of about 300 sec. 1 to obtain extru-sion rates approaching the design output rate of the extruder.
At these shear rates, the shear stress on the polymer at the die orifice exceeds about 3 x 106 dynes/cm2. At these .. . .. . -, - ~ . -. .

- ~0~8048 levels of shear stress, the quality of the extruded article is quite poor and its physical properties are poor. This results from a phenomenon known in the art as melt fracture, or melt instability.
By reason of the relationship existing between their S values and Ao values, high molecular weight ethylene polymers of this invention can be more readily fabricated with conventional fabricating apparatus to provide polymer articles of excellent quality and physical properties. The ethylene polymers of the invention having the optimum prop-erties desired by the art have the following characteristics:
(1) Melt flow properties conforming to the S - A relationship of formula 3 previously set forth, (2) An Ao value in the range of about 12.0 to about 14.5, (3) An S value in the range of about 0.61 to about 0.90.
Especially preferred ethylene polymers of the invention are those having an Ao value in the range of about 12.25 to 14Ø
By reason of their high melt flow shear ratios and the considerations discussed above, the ethylene polymers of the invention can be fabricated into articles of manufacture having significantly superior physical propertiec as compared 'h~
to corre0ponding articles fabricated from linear ethylene polymers heretofore available to the art. These differences are particularly noticeable in the manufacture of film from ~ ethylene polymers having very low melt indexes.
; Example 20 Blown film of 2.5 mil gauge was prepared from the ethylene polymer of Example 18. The film was extruded through a 1-3/4 inch extruder employing a melt temperature of about 295C. a pressure of about 5,600 psi, and a screw speed of 60 rpm. A blow-up ratio of 3.8 was employed.
As a control, 2.5 mil gauge film was preparea from :........................................ . .

la6so4s a prior art ethylene polymer prepared by a Particle Form Process employing a chromia on silica catalyst~ This polymer had a melt index (ASTM 1238~70, Condition E), of 0.6. The extrusion conditions employed were those pre-viously established as being optimum for this polymer.
Several physical properties of the two films were measured and are set forth in Table VII. In the table ~ .
MD signifies a measurement in the machine direction, while TD signifies a measurement in the transverse direction.

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Ethylene Polymer Property Present Invention Prior Art Film Density, g/Cm3 (1) 0.954 0.959 Crystallinity, % (2) 78.6 70 ;

Tensile at yield, psi, MD (3) 3870 4400 " " " " , TD (3) 4125 3600 ; Elongation at break, ~, MD (4) 475 200 " " " , %, TD (4) 500 2 10Elmendorf Tear, g/mil, MD (5) 54 18 -~

, TD (5) 49 39 Dart Impact at 26", g/mil (6) 38 25 (1) ASTM D 1509 (2) Determined by Differential Thermal Analysis "
(3) ASTM D882 - i (4)

(5) " D1922 $~

(6) " D1709 The above data demonstrate that the film prepared from the ethylene polymer of the invention is remarkably superior to its prior art counterpart. It will be specifi-cally noted that its measured properties in the machine and transverse directions are quite close to each other. This is a highly desirable feature in a film. The film of the invention is much superior to its prior art counterpart with -~¦ respect to elongation at break, Elmendorf tear, and dart impact, all of which are important film properties.
Blown film prepared from ethylene polymers of the i B invention has~ a significantly lower gel content ~ blown film prepared from linear ethylene polymers of the prior art.
Moreover, what few gels are occasionally observed are smaller in size than the gels present in the prior art film.

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~068~48 The gel content of blown film is determined by counting the gels in 240 in.2 of the film and measuring the diameter of the gels observed. The film of the invention prepared in Example 20 contained fewer than 10 gels. Only one of these gels had a diameter between l/64" and l/32", with the remaining gels having diameters smaller than 1/64". ~ -The gel content of this film was actually lower than that observed with film prepared from good quality low density polyethylene resins. This observation is quite significant as it is recognized in the art that film prepared from low density polyethylene resins usually is substantially freer of gels than film prepared from linear ethylene polymers.
For comparison purposes, film prepared from linear ethylene polymer prep.ared by a Particle Form Process employing a chromia catalyst supported on silica typically will have well in excess of 40 gels per 240 in . Typically five of these gels will have diameters of 1/32" or more, 15 of these gels will have diameters between 1/64" and 1/32", with the ~ ~ balance having diameters smaller than 1/64". ~el~ el ;, '20 Ethylene polymers of the invention having L~la~l v~ .:
low molecular weights and relatively high melt indexes oP the order of 0.5 or more (ASTM 1238-70, Condition ~), by reason of their high S values, have extremely low apparent melt ; viscosities under the apparent shear rates employed in commercial extruders. As compared with prior art linear ethylene polymers of these melt lndexes, the~ethylene po`lymers of the invention can be extruded at significantly higher rates and with lower extruder power consumption. These factors significantly reduce the cost of preparing such extruded products.

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Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for preparing an olefin polymerization catalyst which consists essentially of depositing a chromium compound upon an inorganic carrier containing aluminum and phosphorus moieties, the inorganic carrier being selected from the group consisting of:
(a) an amorphous precipitate of aluminum phosphate, (b) an amorphous precipitate containing aluminum and phosphorus moieties in an atomic ratio of about 5:1 to 1:1, and (c) mixtures of (a) and (b);
said carrier having been prepared by neutralizing a strongly acidic solution containing Al+++ cations and PO4--- anions in a molar ratio in the range of 5:1 to 1:1 to form a solid precipitate containing aluminum and phosphorus moieties, and recovering the precipitate.

2. The process of claim 1 in which the catalyst is activated by being heated to a temperature in the range of about 350°C. to 950°C.

3. The process of claim 1 which includes the steps of:
(a) preparing an aqueous slurry of the inorganic carrier by neutralizing the acidic aqueous solution containing Al+++ and PO4--- ions, (b) admixing an inorganic chromium compound with the slurry of (a) to provide about 0.1 - 4.0% by weight chromium based on the total solids, (c) spray drying the slurry of (b), and (d) activating the dry catalyst of (c) by heating to a temperature of about 350 -950°C.

4. The process of claim 1 which includes the steps of:
(a) preparing an acidic solution containing chromium ions in addition to Al+++ and PO4--- ions, (b) precipitating an inorganic material con-taining aluminum, phosphorus and chromium moieties by neutralizing the acidic solution of (a), (c) drying the precipitate of (b), and (d) activating the dry catalyst of (c) by heating to a temperature of about 350° -950°C.
the chromium added in step (a) being sufficient to constitute about 0.1 - 4.0% by weight of the precipitate of step (b).

5. The process of claim 1 in which the support is an amorphous precipitate containing aluminum and phosphorus moieties in an atomic ratio of about 3.5:1 to about 1.1:1.

6. A process for preparing an olefin polymerization catalyst which consists essentially of:
(a) preparing a strongly acidic solution con-taining Al+++ cations and PO4--- anions in a molar ratio in the range of 5:1 to 1:1;
(b) neutralizing the solution of (a) to form a precipitate;
(c) calcining the precipitate of (c); and (d) impregnating the calcined precipitate of (c) with an organochromium compound.

7. An olefin polymerization catalyst prepared by the method of claim 1.

8. In a Particle Form Process for the polymeri-zation of ethylene; the improvement which comprises employing the olefin polymerization catalyst of claim 7 to initiate the polymerization of the ethylene.

9. An ethylene polymer:
(a) having melt flow properties such that the relationship between its slope parameter S
and its apparent melt viscosity in poises at 1 sec.-1 Ao is defined by the formula:
S ? 0.0830Ao - 0.442 where S is the negative slope of the curve obtained from a plot of the logarithm of the polymer's apparent melt viscosity in poises versus the logarithm of the apparent shear rate in sec.-1, said slope measured at 10 sec.-1; and where Ao is the natural logarithm of the polymer's apparent viscosity in poises measured at 1 sec.-1 at 190°C;
(b) having an Ao value in the range of about 12.0 to about 14.5; and (c) having an S value in the range of about 0.61 to about 0.90.

10. An ethylene polymer of claim 9 where the Ao value of the polymer is in the range of about 12.25 to about 14Ø

11. Film prepared from an ethylene polymer of claim 9, said film being characterized by:
(a) having values for elongation at break measured in the machine direction and in the transverse direction that are similar, and (b) having a low gel count, nearly all of such gels having diameters of less than 1/32 inch.