GB2119391A - Low density ethylene polymer - Google Patents

Abstract

A process for the production of linear low density ethylene polymer (LLDPE) employing slurry polymerization techniques involves polymerizing ethylene and butene-1 in the presence of an inert C4 liquid diluent using a catalyst system comprising organoaluminum compound and a magnesium halide supported titanium halide catalyst component. The process can be operated with a variety of catalysts without sacrifice in operational stability.

Description

SPECIFICATION Process for preparing a low density ethylene polymer Linear low density ethylene polymer (hereinafter sometimes referred to as LLDPE) can be produced by slurry polymerization techniques reacting ethylene and a small amount of another a-olefin monomer such as butene-1 in the presence of a catalyst composition and employing a hydrocarbon, e.g.

hexane or heptane, as liquid diluent. There are several disadvantages associated with these slurry polymerization processes. For instance, the solubility of the copolymer product in the diluent at the operating conditions causes a detrimental increase in the viscosity of said diluent and inherently adversely limits the concentration of polymer solids that can be present in the slurry as well as the space time yield of the process. These drawbacks have been discussed in some detail in U.S. Patent No.

4,298,713 and the patent provides a process-improvement in the production of linear low density ethylene polymer using a catalyst comprising a halogen-containing catalyst component supported on a magnesium compound and an organoaluminum compound. The slurry polymerization is conducted in the presence of an inert relatively high boiling diluent such as hexane or heptane. The disclosed improvement in performance is said to be obtained by carrying out the process in at least two steps and limiting the concentration of ethylene in the first stage monomer feed to no more than 1 0 mole% A drawback of this process when adapted to commercial size continuous operation is the requirement of at least two reactors, i.e. one for each stage, which adds considerably to the cost of the process and product.

U.S. Patent No. 4,294,947 discloses a one-step process for the production of copolymers of ethylene and butene-1 under slurry polymerization conditions using a nonsupported vanadiumcontaining Ziegier catalyst. The preferred liquid diluent is pure butene-1, however, the scope of the invention includes the use of a C4 cut containing other C4 components which are inert towards the polymerization catalyst. The patent shows that inferior yields and catalyst efficiencies are obtained when the diluent is a mixture of butene-1 with other inert C4 components.

A serious disadvantage of using pure butene-1 as the diluent is the inflexibility of the process.

Because of the required increase in the ethylene partial pressure in order to achieve the butene1/ethylene mole ratio required for a desired density of the polymer product, the operating temperature must be low enough to avoid reaching the critical temperature of the reactant mixture. At temperatures above the critical temperature, the slurry conditions cease to exist, the operations become unstable and result in a nonuniform product.

The present invention provides a slurry process for the production of linear low density ethylene polymers wherein stable operations are achieved over a wide range of operating conditions, using a variety of catalyst compositions.

In accordance with the present invention there is provided a continuous process for the production of a linear low density ethylene polymer resin, which process comprises copolymerizing ethylene and butene-1 in the presence of an inert C4 diluent at a pressure at least sufficient to maintain the C4 diluent in the liquid phase, and up to 500 psig (3.5 MPa gauge), a temperature from 1 300F to 1 700F (54 to 77 0C) and an ethylene partial pressure (pup,;;) of from 50 to 350 psia (0.35 to 2.4 MPa) employing a catalyst having a reactivity index r of from 0.0325 to 0.0500 and composed of (a) an organoaluminum compound, and (b) a titanium catalyst component supported on a magnesium component, maintaining a mol ratio X04 of butene-1 to total C4 hydrocarbon at least the value calculated from the formula: -0.1205 XC = (PPC-) .(10) but not more than 0.8 and recovering a linear low density ethylene polymer having a density of 0.935 and below.

Figures 1 and 2 which are seif-explanatory, show the boundaries of the invention in terms of the operational parameters.

The catalyst composition used in the process can be any one of the recently developed, high activity titanium halide/magnesium compound catalyst components and organoaluminum cocatalyst components disclosed, for example, in U.S. Patents Nos. 3830787, 3953414, 4051313, 4115319, 4149990,4218339, 4220554,4226741, 4252670,4255544, 4263169,4298713 and 4301029, provided that the reactivity index of the catalyst composition is from 0.0325 to 0.0500. The measurement of this property will be discussed in detail below.

Component (a) of the catalyst composition is an organoaluminum compound, preferably an alkylaluminum having from 1 to 8 carbon atoms in the alkyl groups. It is advantageously selected from trialkylaluminums, dialkylaluminum halides or mixtures thereof. The preferred halide is chloride.

Examples of suitable alkylaluminums are diethylaluminum chloride, di-n-butylaluminum chloride, triethylaluminum, trimethylaluminum, tri-n-butylaluminum, tri-isobutylaluminum, triisohexylaluminum, tri-n-octylaluminum and triisooctylaluminum. The organoaluminum can, if desired, be complexed with an electron donor before its introduction into the polymerization reactor. Preferably, the donors are selected from diamines or esters of carboxylic acids, particularly esters of aromatic acids.

Some typical examples of such electron donors are methyl and ethylbenzoate, methyl and ethyl pmethoxybenzoate, diethylcarbonate, ethyl acetate, dimethyl maleate, triethylborate, ethyl ochlorobenzoate, ethyl naphthenate, methyl p-toluate, ethyl toluate, ethyl p-butoxy-benzoate, ethyl cyclohexanoate, ethyl pivalate, N,N,N',N'-tetramethylenediamine, 1,1 ,4-trimethylpiperazine and 2,5dimethylpiperazine. The molar ratio of organoaluminum compound to electron donor should be limited to between 2:1 and 5:1. Solutions of the electron donor and the organoaluminum compound in a hydrocarbon such as hexane or heptane are preferably prereacted for a certain period of time, generally less than 1 hour, before the mixture is fed into the polymerization reaction zone.

It is not critical to the process of the present invention what method is used in the preparation of component (b) of the catalyst composition and any of the various techniques known in the art may be used. Typically, these techniques involve the reaction of a titanium compound, e.g. a titanium halide or a titanium oxyhalide, with a magnesium compound such as a halide, alcoholate, haloalcoholate, carboxylate, oxide, hydroxide, or a Grignard reagent. Other processes include the reaction of a magnesium compound with an electron donor, a silicon compound, or an organoaluminum compound followed by a further reaction step with the titanium compound, sometimes followed by a second reaction step, wherein the product is treated, for example with a halogen-containing silicon compound or an electron donor.

The halogen in the respective halides can be chlorine, bromine or iodine, the preferred halogen being chlorine. The electron donor, if it is used in forming a complex, is suitably selected from the esters of inorganic and organic oxygenated acids and the polyamines. Examples of such compounds are the esters of aromatic carboxylic acids, such as benzoic acid, p-methoxybenzoic acid and p-toluic acids and particularly the alkyl esters of said acids; and the alkylene diamines, e.g. N,N,N',N'-tetramethylethylene- diamine. The magnesium to electron donor molar ratio are equal to or higher than 1:1 and preferably 2:1 and 10:1. Generally, the titanium content expressed as titanium metal is between 0.1 and 20 wtSo in the supported catalyst component.Treatment steps may also be included in the preparation in order to obtain component (b) in spherical or spheroidal form.

Methods suitable for the preparation of the magnesium-supported titanium halide catalyst component are disclosed in detail in the patents listed above.

The determination of the reactivity index of the catalyst is suitably carried out in an autoclave reactor provided with a bladed agitator, a cooling jacket for at least partial reactor temperature control, inlet ports for organoaluminum and supported titanium halide catalyst components, inlets for supplying of ethylene, hydrogen, butene-1 and butane diluent to the reactor, a vapor line provided with a condenser and return conduits for separate recycling of condensate and of cooled gases. After compression the gases are introduced below the liquid surface in the reactor. Product slurry is withdrawn through a valve located in a conduit at or near the bottom of the autoclave.A small conduit is also provided at or near the top of the reactor for withdrawing a small vapor stream to a gas chromatograph for continuous monitoring of the concentrations of the components ethylene, hydrogen, butene-1 and butane in the vapor space of the autoclave. The conditions to be maintained in the reactor at steady state conditions are as follows: Temperature, OF (OC) 150 (66) Ethylene, psia (MPa) 200 (1.4) Hydrogen, mol% 15 Mol ratio C-41Tot. C41s 0.1: :1 Residence time-hrs 2 Ti Catalyst rate suff. for 30% polymer in reactor slurry Organoaluminum-wt% 0.1 basis total reactor content weight After a brief drying step, the density do 1 of the polymer product is determined and finally the reactivity index r is determined from the relationship: r=0.3029 (d01 -0.8145) If the measured density do 1 equals or exceeds 0.945 g/cc, it is recommented that the test be carried out under siightly modified conditions, i.e. the Xc4 ratio is increased to 0.5, and the reactivity index r is then determined from the relationship:: r = 0.3843 (do - 0.8145) It is not necessary to condense and recycle the overhead vapors from the reactor, provided that the conditions in the autoclave vapor space are maintained at the conditions set forth above.

The test can also be carried out, if convenient, by batch polymerization at the given conditions.

The catalyst components (a) and (b) are separately fed to the reaction zone. The organoaluminum is provided in amounts ranging from 0.025 to 0.3 wt% based on the total weight of monomers and diluent fed to the reaction zone. The monomer feed to Ti metal weight ratio is usually between 50,000:1 and 1,500,000:1. The preferred reactivity index of the catalyst is between 0.0325 and 0.0425.

Temperatures at which the LLDPE formation should be carried out are in the narrow range of from 1 300F to 1 700F (54 to 770C), and preferably between 1 450F and 1 550F (63 and 680C). The pressures should be sufficient to provide the proper ethylene partial pressure and to maintain the C4 hydrocarbon diluent and butene-1 monomer in the liquid phase, usually from 275 psig to 500 psig (1.9 to 3.5 MPa gauge), preferably between 300 psig and 450 psig (2.07 to 3.10 MPa gauge). The ethylene partial pressure, which can be maintained as high as 350 psia (2.4 MPa) at reactor pressure in the upper region of the total pressure range, preferably is between 150 and 275 psia (1.04 and 1.90 MPa).The average residence time in the reactor can be from 0.5 to 10 hours and preferably between 1.0 and 4 hours. The polymer solids content of the reactor slurry is usually maintained from 1 5 to 50 wt% and preferably between 20 and 40 wt%.

The preferred minimum butene-1/total C4 ratio (Xc4) is calculable from the relationship: - 0.1 205 XC4 = (ppC2) .(10) r At ratios above this minimum the LLDPE products usually have densities less than or about 0.920 gm/cc.

The reaction is continuous and monomer feeds, diluent and catalyst components are continuously fed to the reactor and a slurry of polymer product and liquid C4,s is withdrawn, preferably through a cyclic discharge valve which simulates continuous operation. Various modifiers such as hydrogen may be added to alter the properties of the polymer product. Such modifiers and their use are well known in the art and need not be discussed in any detail. When hydrogen is employed to increase the melt flow of the product, its concentration is usually maintained between 5 to 40 mole percent based on the composition of the vapor phase in the reactor.

The C4 diluent is usually normal butane but can also be any other inert C4 hydrocarbon such as isobutane, or mixtures of such inert compounds.

Because of the generally high productivity of the supported catalyst system expressed in terms of unit mass of polymer produced per unit mass of titanium metal, there is no need to remove catalyst residues from the polymer in a deashing step as is the case with conventional catalyst.

When a titanium halide catalyst component of spherical or spheroidal shape is employed, the resulting polymer product is also recovered in such forms obviating the need for further granulation or pelletizing of the polymer product before shipment to the user.

Various additives can, if desired, be incorporated into the LLDPE resin, such as fibers, fillers, antioxidants, metal deactivating agents, heat and light stabilizers, dyes, pigments and lubricants.

The LLDPE product produced by the process of the invention has excellent physical properties which makes it useful for a variety of applications, for example in the manufacture of cast and blown film, cable and wire coatings and molded housewares. The LLDPE product may be used alone or as a blend with other polymers such as conventional low density polyethylene, ethylene-vinyl avetate copolymer and many others.

One major advantage of the process of this invention is the remarkable ease with which the continuous process can be controlled to obtain a desired density product at favourable productivity rates. Any significant drift in the density away from the target value can easily be corrected by a simple adjustment in the butene-1/total C4 feed ratio, e.g. with too low densities the ratio should be decreased and vice versa. Also, by appropriate adjustment on the aforementioned ratio, a change in product lines from one density product to another is also easily accomplished with a minimum of process condition changes and without significantly affecting productivity rates.Specifically, total pressure, ethylene feed rate and partial pressure, hydrogen feed rate (if used), catalyst rate and reactor temperature can be maintained at substantially constant conditions while the desired density change is achieved by an adjustment of the butene-1 concentration in the total C4 hydrocarbon feed.

Another, probably even more important, advantage of the process of this invention is its superb flexibility with regard to the use of a variety of catalysts from many different sources. When changing from one catalyst system to another, it is merely required to determine the reactivity index of the new catalyst and then to make a corresponding adjustment in the butene-1/total C4 ratios while maintaining other operating conditions substantially unchanged.

A further advantage of the process of the invention is the high productivity rates per unit volume of reactor, i.e. space-time-yield, which can be obtained because of the lower solubility of polymer in C4 hydrocarbons relative to those of hexane and heptane. This, in turn, enables the process to be conducted at high solid polymer concentrations in the slurry.

The following Examples further illustrate the advantages obtained by the invention.

EXAMPLES 1-6 The experiments were conducted in large scale continuous pilot plant operations using a method and equipment essaentially similar to those described in connection with the determination of the catalyst reactivity index. The catalyst system which had a reactivity index of 0.0400, consisted of triethylaluminum and a titanium chloride-magnesium chloride catalyst containing about 1 5 wt% titanium and prepared according to the method of U.S. Patent No. 4,218,339. In each of the runs, the recycle gas rate was about 1800 SCFH (51 m3/hour) and the residence time was 2 hours. The other pertinent data of the runs are shown in Table I.Comparative Examples 1 and 2 were carried out in the absence of inert diluent, and as a result of the required increase in ethylene reactant, the system became unstable at the moderate reactor temperature of 1 400F (600 C) and severe operational difficulties were encountered.

Examples 3-6 were carried out within the limits of the invention and resulted in excellent operational stability. The effect of increasing the butene-1 concentration in the total C4,s upon the density is amply illustrated by these examples.

Table I

EXAMPLE Comp. 1 Comp. 2 3 4 5 6 Temperature, OF (OC) 140 (60) 150(66) 145 (63) 151(66) 150(66) 149 (65) Pressure, psig (MPa 596 (4.11) 593 (4.09) 380 (2.62) 380 (2.62) 379 (2.61) 371(2.56) gauge) Ethylene, Ibs/hr (kg/hr) 49.7(22.5) 29.8(13.5) 17.5(7.94) 32.7 (14.8) 31(14.1) 25(11.3) Butene-1, Ibs/hr (kg/hr) (1) (1) 44 (20) 16.6 (7.53) 15.8(7.17) 17.5(7.94) Butane, lbs-hr (kg/hr) - - 37.5 (17.0) 16.1(7.30) 13.4(6.08) 14.4(6.53) Hydrogen, mol % 13.4 11.6 16.6 15.7 15.8 18.9 Ethylene pp, psia (MPa) 426 (2.94) 390(2.69) 228(1.57) 261 (1.80) 256(1.77) 244 (1.68) Butene-1/Total C4,s-mol ratio 1.0 1.0 0.268 0.537 0.574 0.605 Al/Ti Ratio 48 46 57 94.6 97.5 48 Solids Concentration-wt% (1) (1) 13.9 29.6 20.7 26 Productivity-kg polymer/kg Ti 90,000 67,600 64,250 116,400 118,600 76,600 PrnductRate,lbs/hr(kg/hr) (1) 19.7(8.94) 14.9(6.76) 36.5 (16.6) 25(11.3) 30(13.6) Melt Index gms/1 0 min 0.7 0.6 0.8 0.94 0.9 1.6 Densitygms/cc 0.927 0.920 0.931 0.923 0.921 0.919 (1) No accurate data available due to unstable conditions EXAMPLE 7-11 These experments were carried out essentially as before except that the catalyst system had a reactivity index of 0.0358, the titanium chloride magnesium chloride component had a titanium content of 2.5% and was prepared by the method of U.S. Patent No. 3,953,414. The recycle gas rate in Example 10 was 3100 SCFH (88 hour) and in Example 11 5900 SCFH (170 m3/hour). The data from these runs are summarised in Table II.

Table II

EXAMPLE 7 8 9 10 11 Temperature, OF (OC) 150(66) 150(66) 150 (66) 145 (63) 145 (63) Pressure, psig (MPa gauge) 370 (2.55) 370 (2.55) 370 (2.55) 290 (2.00) 290 (2.00) Ethylene, Ibs/hr (kg/hr) 30.8 (14.0) 32.2 (14.6) 28.4 (12.9) 31(14.1) 32.9 (14.9) Butene-1, Ibs/hr (kg/hr) 3.5 (1.6) 6.7 (3.0) 3.0 (1.4) 11.1(5.03) 8.9 (4.04) Butane, Ibs/hr (kg/hr) 7.8 (3.5) 2.9 (1.3) 7.2 (3.3) 14.4 (6.53) 8.9 (4.04) Hydrogen, mol % 12.9 27.7 35.4 9.6 9.7 Ethylene pp, psia (MPa) 245 (1.69) 192 (1.32) 162 (1.12) 185(1.28) 174(1.20) Butene-1/Total C4,s-mol ratio 0.265 0.234 0.096 0.175 0.238 Al/Ti Ratio 624 395 302 377 290 Solids Concentration-wt% 31 32.3 29.2 34.4 32.5 Productivity-kg polymer/kg/Ti 952,000 628,000 420,000 706,000 - 536,000 Product Rate, Ibs/hr (kg/hr) 37.9 (17.2) 39.5(17.9) 34.6(15.7) 41.3 (18.7) 40(18.1) Melt Index gms/1 0 min 1.0 15 21 0.9 1.4 Density gms/cc 0.920 0.920 0.935 0.921 0.196

Claims (14)

1. A continuous process for the production of a linear low density ethylene polymer resin, which process comprises copolymerizing ethylene and butene-1 in the presence of an inert C4 diluent at a pressure at least sufficient to maintain the C4 diluent in the liquid phase, and up to 500 psig (3.5 MPa gauge), a temperature from 1 300F to 1 700F (54 to 77 C) and an ethylene partial pressure (PPc2) of from 50 to 350 psia (0.35 to 2.4 MPa) employing a catalyst having a reactivity index r of from 0.0325 to 0.0500 and composed of (a) an organoaluminum compound, and (b) a titanium catalyst component supported on a magnesium component, maintaining a mol ratio Xc4 of butene-1 to total C4 hydrocarbon at least the value calculated from the formula:: -0.1205 Xc4= (ppC2) .(10) r but not more than 0.8 and recovering a linear low density ethylene polymer having a density of 0.935 and below.

2. A process according to claim 1 wherein the minimum X04 ratio is calculated from the formula: -0.1205 Xc4 = (PPc2) .(10) r

3. A process according to claim 1 or 2 wherein the reactivity ratio is between 0.0325 and 0.0425.

4. A process according to claim 1, 2 or 3 wherein the temperature is maintained between 1 450F and 1 550F (63 and 680C).

5. A process according to any one of the preceding claims wherein the ethylene partial pressure is maintained between 150 psia and 275 psia (1.04 and 1.90 MPa).

6. A process according to any one of the preceding claims wherein the total pressure is maintained between 300 psig and 450 psig. (2.07 to 3.10 MPa gauge).

7. A process according to any one of the preceding claims wherein the residence time is between 1.0 and 4 hours.

8. A process according to any one of the preceding claims wherein the polymer solids content of the reactor slurry is between 1 5 and 50 wt%.

9. A process according to claim 8 wherein the polymer solids content is between 20 and 40 wt%.

10. A process according to any one of the preceding claims wherein hydrogen is present at a concentration of from 5 to 40 mol % in the vapor phase.

11. A process according to any one of the preceding claims wherein the diluent is n-butane.

12. A process according to any one of the preceding claims wherein the halide of component (b) is chloride.

13. A process according to any one of the preceding claims wherein the organoaluminum is a trialkylaluminum.

14. A process according to claim 11 wherein the trialkyl aluminum is triethyl aluminum.

1 5. A process according to any one of the preceding claims wherein component (b) is in spherical or spheroidal form.

1 6. A process according to claim 1 substantially as described with reference to any one of Examples 3 to 6.