GB1604708A - Ethylene polymerisation process - Google Patents

Description

(54) ETHYLENE POLYMERISATION PROCESS (71) We, NATIONAL PETRO CHEMICALS CORPORATION, a Corporation organised and existing under the laws of the State of Delaware, having a place of business at 99 Park Avenue, City and State of New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to the production of polyethylenes, especially single reactor blow molding polyethylenes.

Molded articles, and particularly blow molded structures such as bottles, are commonly formed from polymers of l-olefins such as polyethylene. It is important to the commercial utilization of a given polymer system that the converted product such as a bottle exhibit an optimized balance of properties, including, for example, acceptable stress crack resistance and fiexural stiffness. In addition, and in a contrbuting sense, it is necessary that the polymer exhibits suitable processability, i.e. satisfactory rheological behavior under flow and deformation during fabrication. Although the viscoelastic behavior of polymer melts has been the subject of considerable study, it has not proven possible to translate performance during fabrication to end use articles in such manner as to selectively determine polymerization and particularly catalyst requirements. Moreover catalyst performance must also be measured in terms of efficiency or productivity and stability over a sensible life.

The use of chromium compounds in the polymerization of olefins is well-known.

U.S. Pat. Nos. 2,825,721 and 2,951,816 teach the use for olefin polymerization of CrO8 supported on an inorganic material such as silica, alumina or combinations of silica and alumina and activated by heating at elevated temperatures. When these catalyst systems are used in various polymerization processes such as the well-known particle-form process, the resins produced, while useful in many applications, are unsatisfactory for others because of a deficiency in certain properties suh as melt index.

Improved chromium based supported catalysts are known, particularly those disclosed and claimed in U.S. 3,984,351 and 3,985,676. Such catalysts permit the production of resins of improved flow properties and shear response, but have been found difficult to employ on a commercial scale without product segregation or resin blending because of variation in rheological properties of polymer produced, relative to its use in fabrication and especially blow molding e.g. in accumulator, or accumulator ram equipment.

An examination of this phenomenon utilizing now classic measures of resin shear response (HLMI/MI values determined according to ASTM-D-1238, Conditions F/E) evidenced no apparent reason for differential performance of polyethylenes in fabrication equipment. Empirical studies suggested to us that a more exacting viscosity analysis was required to isolate polyethylene candidates adapted to afford shortened cycle times or otherwise improved performance in selected blow molding equipment It has been found that such a determination may be made using a viscosity ratio of Etal/EtatO"0, broadening the range covered and expressly including the range 1 to 1000 reciprocal seconds. These values provide a correlatable measure of critical performance in end use as more fully described hereinafter, and permit the selection of polyethylene candidates particularly adapted to use in such blow molding equipment as the accumulator ram equipment aforementioned.

Further studies of chromium-catalysed polyethylene variability in terms of this viscosity ratio characteristic permitted identification of production factors critical to the controlled production of polyethylenes of the desired characteristics.

The present invention provides a process for the production of polyethylene comprising polymerizing ethvlene at elevated temperature and pressure in the presence of a catalytic amount of ethylene polymerization catalyst prepared by depositing a chromium compound on a porous inorganic oxide support, depositing a hydrolysabla aluminum compound on the support when it has a water content in the range of 0.25 to 6% by wt. of the support whereby deposited aluminum compound is hydrolysed in situ by the said water content, and thereafter heat activating in a non-reducing atmosphere. Tt also provides such a process for the production of polyethylene having a predetermined ratio of melt viscosities measured at shear rates of 1 and 1000 reciprocal seconds, wherein said water content is selected to be within 0.15% (by wt. of support) of a value in the said range which is predetermined to result under the other conditions obtaining in polyethylene having said melt viscosity ratio.

It has been discovered that level and type of aluminum values deposited upon a supported catalyst control its characteristics, and may be employed in a chromium catalyst system as a direct means to achieve selective polyethylene production in a controlled polymerization. The same effects can be achieved indirectly by establishing and maintaining a fixed ratio of water to aluminum compound during catalyst preparation.

The invention reflects the discovery that polyethylene property variations may be traced to system water, or moisture level, present in the course of catalyst preparation. Silica gel and other inorganic catalyst supports are known to be active adsorbents, and readily pick up significant quantities of water. (Dried silica gel with less than 0.5% water content, simply poured through 12 inches of 80"F. air at 50% RH, will reach a moisture content of 2%). Their exposure to a humid atmosphere is accordingly conventionally controlled in handling, and the level of adsorbed water is ordinarily within such low limits, i.e. a few percent by weight, as not to constitute a noticeable element in the system. However, such handling procedures as have customarily been employed permit considerable variation within the range.

In the course of preparation of polyethylenes for general use, such moisture variation in the catalyst preparation system shows no meaningful differentiation in resin performance. However, it has now been found that heretofore undiscerned d;fferences in resin rheological properties can become important to efficiency in certain critical end uses such as the blow molding of bottles utilizing accumulator ram equipment. Surprisingly, it has been discovered that such resin variations are traceable to and may be controlled by moisture level in the catalyst preparation system.

While not wishing to be bound by an essentially hypothetical elucidation, it is presently believed that the selective performance of the catalyst systems used herein may be attributed to the morphology or stereoconfiguration of the activated catalyst surface due to the aluminum species formed in situ by controlled hydroIysis. That is, a series of compositional modifications are believed to result from the hydrolysis reactions with residual moisture in the system, which modifications range through varying steric configurations of the aluminum-containing moieties, and appear individually to permanently and selectively control polymerization performance upon heat activation. Thus, a range of resin properties may be selectively and controllably produced by control of water and aluminum levels employed in catalyst preparation.

In formation of a catalyst for use in the present invention an organophosphoryl chromium reaction product can be deposited upon a high surface area silica gel of controlled water level with an aluminum compound reactive with water and the catalyst intermediate so produced heat activated.

The inorganic support materials useful in the present invention include those normally employed in supported chromium catalysts used in olefin polymerizations such as those discussed in U.S. Pat. No. 2,825,721. Typically, these support materials are inorganic oxides of silica, alumina, silica-alumina mixtures, thoria, zirconia and comparable oxides which are porous, have a medium surface area, and have surface hydroxyl groups. Preferred support materials are silica xerogels or xerogels containing silica as the major constituent. Especially preferred are the silica xerogels described in U.S. Pat. Nos. 3,652,214--6 which silica xerogels have a surface area in the range of 200 to 500 m2/g. and a pore volume greater than about 2.0 cc/g. a major portion of the pore volume being provided by pores having diameters in the range of 300 to 600 A.

Such supports are provided with a regulated water content in the range of 0.25 to 6.0 weight percent based upon the support. The support material may be dried or moisturized, as by equilibration with the atmosphere, to the selected water level. In general, levels of water much above 3.5% by weight will have little additional effect upon the results achieved at low aluminum levels e.g., 3.7%, and accordingly lower levels are preferred for best system control. Higher water levels may be necessary at higher aluminum levels e.g. 10%. Preferably the water rontent is regulated to within +0.15 /, (by weight of the support) of a predetermined value in the said range.

The aluminum-containing compound deposited on the support is reactive with water, i.e. it undergoes a controlled hydrolysis ranging through stages of partial hydrolysis (depending upon levels of available moisture in the system relative to aluminum compound charged) corresponding to different aluminum species and admixtures thereof. The aluminum compounds are also reactive with the surface hydroxyl groups of the inorganic support material, as are the reaction products with water.

Preferred aluminum compounds may be presented by the formula: A1(X)a(Y)b(Z)e wherein X is R, Y is OR, and Z is H or a halogen; a is 0--3, b is Q3, c is 0-3, and a +b + c equals 3; and R is an alkyl or aryl group having from one to eight carbon atoms.

Examples of such aluminum compounds include aluminum alkoxides such as aluminum sec-butoxide, aluminum ethoxide, aluminum isopropoxide; alkyl aluminum alkoxides such as ethyl aluminum ethoxide, methyl aluminum propoxide, diethyl aluminum ethoxide, diisobutyl aluminum ethoxide, etc.; alkyl aluminum compounds such as triethyl aluminum; triisobutyl aluminum, etc.; alkyl or aryl aluminum halides such as diethyl aluminum chloride; aryl aluminum compounds such as triphenyl aluminum, aryloxy aluminum compounds such as aluminum phenoxide and mixed aryl, alkyl and aryloxy, alkyl aluminum compounds.

The chromium containing compounds useful in the present invention comprise any chromium containing compound capable of reacting with the surface hydroxyl groups of an inorganic support. Examples of such compounds include chromium trioxide, chromate esters such as the hindered di-tertiary polyalicyclic chromate esters, silyl chromate esters and phosphorus containing chromate esters disclosed in U.S. Pat.

Nos. 3,642,749; and 3,704,287, and organophosphoryl chromium compounds such as those disclosed in U.S. Patent No. 3,985,676 (to which reference is directed for detail) which comprise the reaction product of chromium trioxide with an organophosphorus compound having the formula:

wherein R is alkyl, aralkyl, aryl, cycloalkyl or hydrogen, but at least one R is other than hydrogen. The preferred organophosphorus compounds are trialkyl phosphates such as triethyl phosphate.

The catalyst used in the present invention may be prepared by depositing the chromium containing compound and the aluminum compound on the inorganic support in any suitable manner such as by vapor coating or by impregnating the support with solutions of the chromium containing compound and the aluminum compound in a suitable inert solvent which is normally an anhydrous organic solvent. Such organic solvents include aliphatic, cycloalkyl, and alkylaryl hydrocarbons and their halogenated derivatives. A preferred organic solvent is dichloromethane. The chromium and aluminum compounds may be applied together or individually.

In applicant's usual method of catalyst preparation, the support is impregnated first with the chromium containing compound and then the aluminum compound.

Most preferred for optimum reproducibility is anhydrous organic solvent application by impregnation, employing about 1 to 2 pore volumes of a solvent such as methylene chloride.

When an organophosphoryl chromium compound of the type disclosed in the aforesaid U.S. Patent No. 3,985,676 is utilized in the practice of the present invention, it is preferred to employ the particular catalyst preparation techniques described in that specification, to which reference is directed for detail. In such instance the organo- aluminum compound may be applied to the catalyst support under conditions similar to those utilized for deposition of the organophosphoryl chromium compound.

The most effective catalysts have been found to be those containing the chromium compound in an amount such that the amount of Cr by weight based on the weight of the support is from about 0.25 to 2.5% and preferably is from about 0.5 to 1.25%, although amounts outside of these ranges still yield operable catalysts. The aluminum compound should be added in sufficient amounts to provide from about 0.1 to 10% of aluminum by weight based on the weight of the aluminum-containing sUpport and preferably from about 0.5 to 5.5% although other amounts outside of these ranges can be used to prepare operable catalysts.

After the chromium containing compound and the aluminum compound have been deposited on the inorganic support, the support is heated in a non-reducing atmosphere, preferably in an oxygen containing atmosphere, e.g. at a temperature above about 200"F up to the decomposition temperature of the support. Typically, the supported compositions are heated at a temperature of from 800"F to 20000F.

The heating time may vary, for example, depending on the temperatures used, from i hour or less to 50 hours or more. Normally the heating is carried out over a period of 2 to 12 hours. The non-reducing atmosphere which is preferably air or other oxygen-containing gas should be dry and preferably should be dehumidified down to a few parts per million (ppm) of water to obtain maximum catalyst activity. Typically, air used in the procedure described in this application is dried to less than 2-3 ppm of water.

Although anhydrous solvents in the deposition procedure, and dehumidified air in drying or heat activation are normally employed, in practice control of moisture on the support is found sufficient to achieve the objects of the invention. It is of course also possible at constant support water level to adjust by moisture present in the solvent treatment systems. Time of reaction or interaction of the aluminum compound does not appear to be critical, and deposition is normally effected under ambient conditions, as in a conventional blender-coater apparatus.

Also, the catalyst may be prepared by separately activating the catalyst after the addition of each separate component.

The absolute level of aluminum and the ratio of water to aluminum is considered important to the controlled polymerization of the present invention. Preferably the proportion of aluminum ranges from about 0.1 to 10% by weight, based on the aluminum-containing support, e.g. from 0.35 to 5.5% and will, at constant moisture level in the preparation, evidence in use decreasing molecular weight with aluminum level and increasing molecular weight distribution as measured by melt index values on resin produced. The water to aluminum molar ratio may vary from an almost anhydrous system to about 4.0, preferably 0.5 to 2.0, with lower values correlating with lower molecular weight and intermediate shear response, or molecular weight distribution.

Proportions of aluminum have been expressed herein in terms of weight percent based upon the aluminum-containing support and water/aluminum ratios as the molar ratio of water on aluminum-containing support to aluminum. While these values may be readily calculated, for ease of conversion, reference may be had to the following Table I.

Control of these variables under otherwise equivalent operating conditions accordingly offers control over resin characteristics. However, as noted hereinabove, the differentiation of resin characteristics, although critical to fabricators, may be ascertainable only through the use of specialized melt viscometry measurements. Certain fabrication equipment such as accumulator ram blow molding equipment is responsive to melt rheology correlatable with higher shear rates and different shear rate response than may be determined utilizing conventional melt index measurements.

Accordingly, for the purpose of this invention melt vsicosity is measured directly as 'Eta', at shear rates of 1 and 1000 reciprocal seconds, and shear response is expressed as the viscosity ratio Etal/Etal,00. This measurement provides a reliable tool for correlating reproducibly resin rheology with fabrication requirements. Lower water to aluminum ratios evidence lower Eta values (lower molecular weight) and intermediate viscosity ratios (broadest shear response, or molecular weight distribution, is evident at an intermediate water to aluminum ratio); and lower absolute aluminum levels evidence increased Eta values and lower viscosity ratios at constant water to aluminum ratios.

Specific resins may accordingly be tailored for use, e.g., in respect of shear level and response by control of water to aluminum ratios and aluminum coating levels.

Best results have been achieved with organophosphoryl chromium reaction product on Polypor silica gel for accumulator ram blow molding equipment at an absolute aluminum level of .5% and a water to aluminum molar ratio of .5, the resins so produced under standard conditions (1 ppm TEB, about 1 wgt.% Cr and H2 .3-.7 mol%) evidencing a viscosity ratio of about 37 to 43. Obviously, the shape of the molecular weight distribution curve and therefore shear response at a given average molecular weight may be controlled by the artisan in accordance with the invention.

The activated supported chromium- and aluminum-containing catalyst may be used in the present invention in combination with metallic and/or non-metallic reducing agents. Examples of metallic reducing agents include trialkyl aluminums, such as triethyl aluminum, triisobutyl aluminum, alkyl aluminum halides, alkyl aluminum alkoxides, dialkyl zinc, dialkyl magnesium, and metal borohydrides including those of the alkali metals, especially sodium, lithium and potassium, and of magnesium, beryllium and aluminum. The non-metal reducing agents include alkyl boranes such as triethyl borane, triisobutyl borane, and trimethyl borane and hydrides of boron such as diborane, pentaborane, hexaborane and decaborane.

For example, based upon a catalyst composition containing about 1% by weight of Cr based upon the weight of the support, the preferred amount of an organometallic reducing agent for use therewith, e.g. triisobutyl aluminum (TIBAL), is about 11.4% by weight and equivalent to an Al/Cr atomic ratio of about 3/1. The preferred range of atomic ratios of Al to Cr is from about 0.5/1 to about 8/1, or from about 1.9% to about 30% by weight TIBAL. The overall practicable limits of TIBAL in terms of the Al/Cr atomic ratio are from about 0.1/1 to 20/1, and in terms of weight are from about 0.4% to about 75% by weight.

The heat-treated, supported chromium- and aluminum-containing catalyst may be combined with the metallic or non-metallic reducing agent prior to being fed to an ethylene polymerization reactor or they may be fed separately to the reactor.

In proportioning the amount of metallic or non-metallic reducing agent to the amount of chromium used in the present invention, fairly wide latitude is available, but some guidelines have been established consistent with good yield, favorable polymer properties and economic use of materials, and the values set forth below are representative. The atomic ratios are based upon a calculation of the metal in the metallic reducing agent and/or the non-metal in the non-metallic reducing agent versus the chromium content present in the chromium compound on the support.

Thus for use with a catalyst containing about 1% by weight of Cr based upon the weight of the support, the preferred amount of triethyl aluminum (TEA) is about 6.6% by weight based upon the weight of the support giving an Al/Cr atomic ratio of about 3/1. The preferred range of atomic ratios of Al to Cr is from about 0.1/1 to 20/1, or from about 0.22% to about 44% by weight TEA, most preferably from about 0.5/1 to about 8/1, or from about 1.1% to about 18% by weight of TEA.

Triethyl boron (TEB) is a preferred non-metallic reducing agent for use with the catalyst in the present invention. Again for use with a catalyst containing about I% by weight of Cr based upon the weight of the support, the preferred amount of TEB is about 5% by weight based upon the weight of the support giving a B/Cr atomic ratio of about 2.7/1. The preferred range of atomic ratios of B to Cr is from about 0.1/1 to 10/1, or from about 0.19 to about 19% TEB. A wider range, in terms of a B/Cr ratio, is from about 0.01/1 to about 20/1, and in terms of weight, from about 0.02% to about 38% by weight based upon the weight of the support.

The polymerisation process may be effected under temperature and pressure conditions generally employed in the art, e.g. temperatures of from about 400 to about 2000C and preferably from about 700 to 1100 C, and pressures of from 200 to 1000 psig and preferably from 300 to 800 psig, as are used in slurry or particle form polymerizations. Hydrogen may be employed in the polymerization zone, and in these cases the relation of product molecular weight and molecular weight distribution to hydrogen demand is modified, as compared to when there are used catalysts in which water level is not controlled. Thus in the process of the invention hydrogen response evident at low water to aluminum mol ratios permits wider latitude in hydrogen levels while maintaining acceptable productivity. In addition the shape of molecular we:ght distribution curves can be distinguished, with implications to shear response and die swell properties.

These capabilities translate into resin performance. Heretofore, it had been common practice and a commercial necessity to provide the necessary rheological properties for certain extrusion and forming operations, notably blow molding, by blending together resin products of more than one reactor, e.g. solution and particle form resins. The process of the present invention can provide a single resin, i.e. the product of a 'single reactor', which provides the necessary rheological properties directly.

The following Examples illustrate preferred modes of preparing the catalyst and using it according to the invention for the preparation of polyethylenes of controlled rheological properties. It will be understood that the Examples are illustrative only and that various modifications may be made in the specified parameters without departing from the scope of the invention. The Reference Example illustrates the deposition of chromium and aluminum compounds on the support.

The melt viscosities reported are determined with an "Instron" (Trade Mark) Capillary Rheometer at 1900C and at shear rates of 1 to 1000 reciprocal seconds. The Viscosity Ratio expressed is the ratio of Etal/Etal000, referring to the 1 and 1000 reciprocal second values, respectively. Absolute values are reported in poises.

Melt indices where recorded, are determined in accordance with ASTM-D- 1238, Conditions E(MI) and F(HLMI), HLMI/HI ratios are measured only over a 10X shear rate range.

Water determinations were made in standard manner employing a titration technique in pyridine using Karl Fisher reagent, and are calculated by weight based upon the aluminum-containing support.

The "single reactor' resins produced according to the invention can exhibit a melt index of up to about .4, a viscosity ratio of 35 to 45 and melt strength, die swell, and parison extrusion time equivalent to otherwise comparable resin blends, rendering them particularly well adapted for use in forming operations including employment as the sole resin in blow molding utilizing accumulator or accumulator ram equipment, known for its critical acceptance of resin candidates with particular regard for cycle time, and wall distribution, trimming characteristics, top load values and surface properties of fabricated articles.

For blow molding, polyethylene resin may be prepared having a density of 0.953 to .955, a melt index of 0.25 to 0.40, a melt viscosity of 3.65 to 3.85X103 poise at 1000 sec-l, a viscosity ratio of 35 to 45, and a die swell at 1000 sec of 170-185%.

In the Examples, the inorganic oxide support, silica xerogel having a pore volume of about 2.5 cc/g prepared in accordance with the disclosure in U.S. Patent No.

3,652,215, is regulated to within 10.15% of a selected moisture level in the range of 0.25 to 6.0 percent by weight of the support by drying in nitrogen at an elevated temperature under vacuum or moisturizing in contact with a humid atmosphere, and thereafter coated with the aluminum and chromium compounds.

In the case of spray coating, the aluminum compound represented here by aluminum sec-butoxide, is suitably diluted with one pore volume (relative to silica gel) of methylene chloride and sprayed onto the neat or chromium coated support at 900F over a period of one hour (during which 3 bed turnovers are accomplished). The coated catalyst is dried at 235"F for two to six hours at 10-15 in. Hg. vacuum to remove volatiles. In a preferred modification of this procedure, the aluminum compound is slurried with two pore volumes of anhydrous methylene chloride, and the solvent removed by drying as aforesaid.

As in the Reference Example, however, the gel may also be simply slurried in a suitable anhydrous solvent such as methylene chloride with the aluminum compound and the chromium compound, and thereafter dried to remove volatiles. The dried catalyst may be blended with untreated silica gel, silica gel coated with chromium compound or other support material where further adjustment in water to aluminum ratio is desired, especially to calculated levels of water based upon the aluminumcontaining support of below 0.4 weight percent.

To heat activate the catalyst in the Examples, the supported catalyst is fluidized with dry air at 0.20 feet per minute lineal velocity while being heated to a temperature of 9000C and held at this temperature for 6 hours. The activated supported catalyst is recovered as a powder.

REFERENCE EXAMPLE.

Silica gel having a pore volume of about 2.5 cc/g prepared in accordance with the disclosure in U.S. Pat. No. 3,652,215 is added to a 2000 ml, three-neck round bottom flask equipped with a stirrer, nitrogen inlet and y-tube with water condenser.

A nitrogen atmosphere is maintained during the coating operation. Dichloromethane is then added to the flask containing the silica gel and stirring is commenced to ensure uniform wetting of the gel. A dichloromethane solution of the reaction product of CrO3 and triethyl phosphate prepared as described in U.S. Patent 3,985,676 is then added to the flask in sufficient quantity to provide a dry coated catalyst containing about 1% by weight of Cr. The supernatant liquid is removed by filtration and the coated gel is dried in a rotary evaporator at 60"C and with 29 inches of Hg vacuum.

Dichloromethane is added to a similar flask as prepared above and while maintaining a nitrogen atmosphere stirring is commenced. To the flask is added the supported chromium composition as prepared above. A solution of dichloromethane and aluminum sec-butoxide is prepared in a pressure equalizing dropping funnel and the funnel attached to the stirred flask. The aluminum sec-butoxide solution is gradually added to the flask at the rate of 10 grams of solution per minute. After the addition of the solution is complete the slurry in the flask is stirred for about 1 hour. The supernatant liquid is removed by filtration and the coated gel is dried in a rotary evaporator at temperatures up to about 60"C and 29 inches Hg vacuum.

The supported catalyst is placed in a cylindrical container and fluidized with dry air TABLE II Wt % Water Viscosity Ratio, Hydrogen Response % Die Swell (H2O/Al Molar) MI HLMI HLMI/MI Eta/Eta1000 H2/C2H4 mol ratio 1000 sec .1 0.5 (0.45) .20 28 146 55.4 0.07 172.5 0.6 (0.50) .17 29 151 56.1 0 176 1.3 (1.1) .16 24 142 60.6 0.033 165 2.5 (2.1) .21 27 129 53.8 0.35 162 EXAMPLE II.

A further polymerization of ethylene is conducted at a temperature of 220 F, a hydrogen level of 0.5-0.8 mol% (ethylene=6-8 mol%) to secure target M.I. of .30#0.5, and 1 ppm triethylborane utilizing a heat-activated catalyst composed of organophosphoryl chromium reaction product (1% Cr) on Polypor silica xerogel (manufactured by National Petro Chemicals Corp.) at 0.5 to 0.6 water level, coated (2 pore volumes, methylene chloride) with 1.85 weight percent aluminium as aluminium sec-butoxide, blended at about a 4:1 weight ratio with the same chromium-containing catalyst without added aluminium compound (nominal water level 0.5-0.6 weight percent) to provide a water (calculated on aluminium-containing support) to aluminium molar ratio of 0.5 and an aluminium level (calculated on total support) of 0.5 weight percent.

The resin produced is of about .953 density, 0.3 melt index, a melt viscosity of 3.75X10 poise at 1000 sec-1 and a viscosity ratio of about 40. The resin exhibits a die swell of 171% at 1000 sec-1 and may be run successfully without blending on an accumulator ram blow molding device to produce e.g., a 22 oz. detergent bottle with equivalent parison extrusion and cycle times as compared to blended resins.

Claims (11)

1. WHAT WE CLAIM IS:1. A process for the production of polyethylene comprising polymerizing ethylene at elevated temperature and pressure in the presence of a catalytic amount of ethylene polymerization catalyst prepared by depositing a chromium compound on a porous inorganic oxide support, depositing a hydrolysable aluminium compound on the support when it has a water content in the range of 0.25 to 6% by wt. of the support whereby deposited aluminium compound is hydrolysed in situ by the said water content, and thereafter heat activating in a non-reducing atmosphere.
2. 2. A process according to claim 1 wherein the catalyst is used in combination with a metallic and/or non-metallic reducing agent.
3. 3. A process according to claim 2 wherein the reducing agent comprises triethyl borane.
4. 4. A process according to any of claims 1 to 3 wherein the polymerization is conducted in the presence of hydrogen.
5. 5. A process according to any of claims 1 to 4 wherein the support is a silica xerogel having a surface area in the range of 200 to 500 m2/g and a pore volume greater than 2.00 cc/g, a major portion of the pore volume being provided by pores having diameters in the range of 300 to 600 A.
6. 6. A process according to any of claims 1 to 5 wherein the catalyst is one heat activated in a non-reducing atmosphere at a temperature of from 200"F up to the decomposition temperature of the support.
7. 7. A process according to any of claims 1 to 6 wherein the chromium compound deposited comprises an organophosphoryl chromium-containing compound.
8. 8. A process according to claim 7 wherein the organophosphoryl compound is the reaction product of CrO3 and triethylphosphate.
9. 9. An ethylene polymerization process according to claim 1 and substantially as hereinbefore described in any run of Example I or II.
10. 10. A process according to any of claims 1 to 8 for the production of polyethylene having a predetermined ratio of melt viscosities measured at shear rates of 1 and 1000 reciprocal seconds, wherein said water content is selected to be within 0.15% (by wt. of the support) of a predetermined value in the said range.
11. 11. Polyethylene obtained by a process according to any of claims 1 to 10.