CA1190996A - Linear low density polyethylene process - Google Patents

Abstract

ABSTRACT

A process for the production of linear low density poly-ethylene (LLDPE) employing slurry polymerization techniques involving polymerizing ethylene and butene-1 in the presence of an inert C4 liquid diluent and using a catalyst system of an organo aluminum compound and a magnesium halide supported litan-ium halide catalyst component. The process is remarkably flex-ible in that it can be operated with a variety of catalysts with-out sacrifice in operational stability.

Description

LINEAR LOW DENSITY POLYETHYLENE PROCESS

ABST~ACT

A process for the production of linear low density poly-I! ethylene (LLDPE) employing slurry polymeri~ation techniques li involving polymerizing ethylene and butene-l in the presence of an inert C~ liquid diluent and usirlg a catalyst system of an ! organo aluminum compound and a magnesium halide supported titan-! ium halide catalyst component. Tlle process i5 remarkably flex-~ ible in that it can be operated with a variety of catalysts with-I,out sacrifice in operational stability.

., BACKGROUND OF THE INVENTION
!l -! Linear low density polyethylene (hereinafter sometimes re-erred to as LLDPE) can be produced by slurry polymerization lltechniques reacting ethylene and a small amount of another ~ olefin monomer such as butene-l in the presence of a cataly5t mposition and employing a hydrocarbon, e-g., hexane or hep-''tane, as liquid diluent. There are several disadvantages asso-¦lciated with these slurry polymerization processes. For instance, ¦¦the solubility of the copolymer product in the diluent at the 1! operating conditions causes a detrimental increase in the vis~
! cosity of said diluent and inherently adversely limits the con-jlcentration of polymer solids that can be present in the slurry as well as the space time yield of the process. These drawbacks l have been discussed in some detail in U.S. Patent No. 4,~98,713

2~ and said patent provides a process improvement in the production of linear low density polyethylene using a catalyst comprised of a halogen~containing catalyst component supported on a mag-nesium compound and an organo aluminum compound. The slurry ', ~v~

p0lymeri~atl0n is conducted in the presence of an inert rela-tively high boiling diluent such as hexane or heptane. The dis-l~closed improvements in performance is said to be obtained by llcarrying out the process ill at least two steps and limiting the 11 concentration of ethylene in the first stage monomer feed to no more than 10 mole ~. I
; A drawback of the aforementioned process when adapted to llcommercial size continuous operation is the requirement of at l,least two reactors, i-e., one or e~ch sta~e, which adds con- !
I siderably to the cost of the process and product.
U.S. ~atent No. 4,~94,9~7 discloses a one step process for the production of copolymers of eth~71ene and butene-l under slurry polymerization conditions usiny a nonsuppOrted llvanadium-col~taining Zie~ler catalyst- The preferred liquid lidiluent is pure butene-l, however, the scope of the invention ;lincludes the use of a C4 cut contai~ling other C4 componentS -! which have an inert behavior towards the polymerization cata-lyst. The patent shows that inferior yields and catalyst llefficiencies are obtained when the diluent is a mi~ture of 11 butene-l with other inert C4 components A serious disadvantage of using pure butene-l as the d~
uent is the nonflexibility of the process. Because of the re-l,q~ired increase in the ethylene partial pressure in order to ¦iachieve the butene-l/ethylene mole ra-tio required for a desir-¦¦ed density of the polymer product, the operatiny temperature !Imust be low enough to avoid reaching the critical temperature ¦lof the reactant mixture. At tem~eratures above the critical ¦ltemperaturel the slurry conditions cease to exist, the opera-¦~tions become unstable and result in a nonuniform product.
¦ It is, thereforeO an object of the present invention to provide a slurry process for the production of linear low den-Isity polyethylenes wherein stable operations are achieved over _ ~ a wide ran~e of operat1n~ conditions .

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Another object is to provide a process which is useful -iiwith a variety of catalyst compositions.
A further object is to provide a simple screening of cata-¦¦lysts useful in the process.
Further objects and advantages will become apparent from a reading of the specification and the appended claims.

l`HE INVENTION

In accordance with the present invention ~here is provided !' a continuous process Eor the production of a lirlear low density Ijpolyethylene resin which comprises:
¦, copolymerizing ethylene and butene-l in the presence of an inert C4 diluent at a pressure at least suffi-cient to maintain the C~ diluent in the liquid phase ~ ¦
Il and up to about 500 psig, a temperature from about ~ ¦
130F to about 170F and an ethylene partial pressure PPC- ) of from about 50 to about 350 psia employing a catalyst having a reactivity index r in the range from about 0.0325 to about 0.0500 and containing (a) ¦ an organo aluminum, and ~b) a titanium halide cata-1' lyst component supported on a magnesium halide com-pound, maintaining a mol ratio XC4 of butene-l to total C~ hydrocarbon at least about the value calculated from the formula - 0.1205 l XC4 = (PPC2 ) (10) r l but not more than about 0.8, and recovering a linear il low density polyethylene having a maximum density of about 0.935.

Figures 1 and 2 which are self-explanatory, show the boundaries of the claimed invention in terms of the operation ¦al parameters~
_ j The catalyst composition used in the process can be any 11l . . I
!1 3_ 1 99~

one of ~he recently developed, high activity titanium hal-i ide/magnesium compound catalyst CQmpOnents and organo alum-inum cocatalyst components disclosed, e.g., in ~.S. Patents Il No. 3,830,787, l~o. 3,953,414, No. 4,Q51,313, No. 4,115,319, l! No. 4,149,990, No. 4,218,339, No. 4,220,554, No. 4,226,741, No. q,252,670, No. 4,255,544, No. 4,263,169, No. 4,298,713, Il and No. 4,301,029,provided that the reactivity index of the cata-j~lyst composition falls between about 0.0325 and about 0.0500.
jlThe measurements of this property will be discu5sed in detail ¦Ibelow.
Component (a) of the catalyst composition is an alkyl laluminLtm having from 1 to 8 carbon atoms in the alkyl groups, It is advantageoUSly selected from trialkyl aluminums, dial-'kylaluminum halides or mixtures thereof- The preferred halide ¦
lis chloride. Examples of suitable alkyl aluminums are diethyl-j! aluminum chloride, di-n-butylaluminum chloride, triethyl alum-¦~inum, trimethyl aluminum, tri-n-butyl aluminum, tri-isobut minum~ triisohexyl aluminum, tri-n-octyl aluminum, triiso-octyl aluminum. The alkyl aluminum can, if desired, be com-plexed with an electron donor prior to introduction into the ¦!polymerization reactor. Preferably, the donors are selected ¦Jfrom diamines or esters or carboxylic acids, particularly es-! ters of aromatic acids, ¦ Some typical examples of such compounds are methyl- and !ethylbenzoate, methyl- and ethyl-p-methoxybenzoate/ diethyl-carbonate, ethylacetate, dimethylmaleate, triethylborate, ethyl-O-chlorobenzoate~ ethylnaphthenate, methol-p-toluate, ethyl-toluate~ ethyl-p-butoxy-benzoate, ethylcyclohexanoate, ethyl-pivalate~ N,N,N',N'-tetramethylenediamine, 1~l~4~-trimeth piperazine~ 2,5-dimethylpiperazine and the like. The molar ratio of aluminum alkyl to electron donor should be limited to ~a ran9e between about 2 and about 5. Solutions of the elec-tron donor and the alkyl aluminum compound in a hydrocarbon J
such as nexane or heptane are preferably prereacted for a cer-n period of time generally less than 1 hour prior to feed-ing the mixture into the polymcrization 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 atalyst composition and any of the various techniques known ilin the art may be used. Typically, these techniques involve !the reac-tion of a titanium compound, e-g., a titanium halide llor a titanium oxyhalide with a magnesium compound such as a lQ !Ihalide, alcoholate, haloalco~lolate, carboxylat~, oxide, hy-! droxide or a Gri9nard rea~ent- Other processes include the re-;action of the abovementioned magnesium compounds with an elec-tron donor, a silicon compound, or an organo aluminum compound followed by a further reaction step with tne titanium com-ll pound, sometimes followed by a second reaction step, wherein the product is treated, e.g., with a halogen containing sili-!i con compound, an electron donor, etc.
The nalogen in the respective halides can be chlorine, llbromine or iodine, the preferred halogen being chlorine~
I'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 com-l! pounds are the esters of aromatic carboxylic acids, such as llbenzoic acid, p-methoxybenzoic acid and p-toluic acids and ¦¦ particularly the alkyl esters of said acids; the alkylene dia-! mines, e.g., N~N~N~rNl-tetramethylethylene-diarnine~ The magneS-¦¦ium to electron donor molar ratio are equal to or higher than ~¦1 and preferably between 2 and 10. Generally, the tiranium l!content expressed as titanium me-tal ranges between 0.1 and 1~20 wt % in the supported catalyst component. Treatment steps y also be included in the preparation in order to obtain mponent (b) in spherical or spheroidal form.

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~ ethods fox the preparation of the magnesium supported l! titanium halide catalyst component are disclosed in detail in the patents listed above~
I

The determination of the reactivity index of the catalyst ii is suitably carried out in an autoclave reactor provided with a bladed a~itator, a cooling jacket for at least partial reactOr temperature control, inlet ports for triethylaluminum ¦~and supported titanium halide catalyst components, inlets for j~ supply of ethylene, hydro~en, butene-l and butane diluent to the reactor, a vapor line provided with a condenser and return ¦lconduits for separate recyclin~ of condensate and of cooled l,~ases- After compression the gases are introduced below the ~jliquid surface in the reactor. Product slurry is withdrawn rough a valve located in a conduit at or near the bottom of ¦Ithe autoclave~ A small condui~ is also provided at or near the ¦¦top of the reactor for withdrawing a small vapor stream to a l! gas chromatograph for continuous monitoring of the concentra-- I! tions of tne components ethylene, hydrogen, butene-l and butane lin the vapor space of the autoclave. The conditions to be main-tained in the reactor at steady state conditions are as follows Temperature, F 150 ; Ethylene, psia 200 ¦ Hydrogen, mol~ 15 I ~5O1 ratio C4/Tot. C4's0.1:1 ¦ Residence time hrs 2 Ti Catalyst ratesuff. for 30% polymer i in reactor slurry Alkyl aluminum - wt% basis 0.1 total reactor content weight After a brief dryin~ step, the density do 1 of the polymer ¦product is determined and finally the reacti~ity index r is determined from the relationship: ¦
.
r = 0.3029 ~dD 1 ~ 0.8145) If the measured density do lequals or exceeds 0.945 g/cc, it-¦iis recommended that the test be carried out under slightly odified conditions, i.e., the Xc ratio is increased to O.S, and lithe reactivity index r is then determined from the relationship I r = 0.3843 (do 5 ~ 0.8145) It is not necessary to conden~e and recycle the overhead vapors from the reactor, provided that the conditions in the autoclave vapor space are maintained at the conditions set llforth above.
ll The test can a]so be carried out, if convellient, by batch polymerization at the ~iven conditions.
The catalyst components (a) and (b) are separately fed to ¦the reaction zone. The aluminum alkyl is provided in amounts ¦ranging from about 0.025 to about 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 in the range 50,000 and 1,500,000. The preferred reactivity index of the catalyst should range between about 0.0325 and about 0.0425.
I Temperatures at which the LLDPE formation should be car-!ried out are to be in the narrow range of from about 130F to about 170 F, and preferably between about 145F and about 155E
IThe pressures should be sufficient to provide the proper ethy-lene partial pressure and to maintain the C4 hydrocarbon dilu-l ent and butene-l mol~omer in the liquid phase, usually from l about 275 psig and up to about 500 psig, preferably between about 300 psig and about 450 psig. The ethylene partial pres-sure, which can be maintained as high as 350 psia at reactor pressures in the upper region of the total pressure range, preferably ranges between about 150 and about 275 psia.
The average residence time in the re~ctor can be from about l/i2 to about 10 hours and preferably between about 1.0 and about 4 hours. The polymer solids content of the reactor slurry is usually maintained from about 15 to about 50 wt % and prefer-al`y bet~een Z0 and dbO~t 40 ' t ~. ¦

The preferred minimum butene-l/total C4 ratio (Xc4) is Il calculable from the relationship:

Xc~ = (PPC2) (10) ~ r 1, At ratiOS above thi5 minimum the LLDPE products usually have ji ~ensities less than or about 0.920 gm/cc.
The reaction is continuous and monomer feeds, diluent and j catalyst components are continuously fed to the reactor and a slurry of polymer product and liquid C~'s is withdrawn, pref-I! erablY through a cyclic discharge valve which simulates contin-1! uous operation. Various modifiers sllch as hydJAoyen may be added l ~o alter the properties of the polym~r product. Such modi-iers and their use are well known in the art and need not he ¦I discussed in any detail- When ~lydrogen is employed to in-¦l crease the melt flow of the product, its concentration is usu-¦i ally maintained between about 5 and about 40 mole percent based ¦¦ on the composition of the vapor phase in the reactor. `
ll The C~ diluent is usually normal butane but can also be ¦j any other inert C4 hydrocarbon such as isobutane, or mixtures f Il such inert compounds.
2~ 1~ BecauSe of the generally high productivity of the support- j Il ed ca~alyst system expressed in terms of pounds of polymer pro-¦lduced per pound of titanium metal, there is no need to remove lvst residues from the polymer in a deashing step as is the ! case with conventional catalyst.
¦ ~hen a titanium halide catalyst component of spherical or ¦splleroidal shape is employed, the resulting polymer product is also recovered in such form5 obviating the need for further granulation or pelletizing of the polymer product hefore ship-Iment to the user.
Various additives can, if desired, be incorporate~ into the LLDPE resin, such as fibers, fillers, antioxidaJlts, metal deactivating agents, heat and liyht stabilizers, dyes~ pig-!l ~

I ments, lu~ricants an~ the like. -~
¦! The LLDPE product of this process has excellent physical roperties which makes it llseful for a variety of applications, lle.y., in the manufacture of cast and blown film, cable and wire - jicoatings~ molded housewaresr etc. The LLDPE product may be used alone or as a blend with other polymers such as convention ¦lal low ~ensity polyethylene, ethylene-vinyl acetate copolymer and many others.
jl One major advantage of this invention is the remarkable llease with which the contilluous process can be controlled to ob-¦ltain a desi~ed density product at Lavorable prociuctivity rates.
!! ~ny significant drift in the density away flom -the target value ¦ican easily be corrected by a simple adjustment in the butene-l/
l¦total C4 feed ratio, e-g-, with too low densities the ratio ¦should be decreased and vice versa- Also, by appropriate ad-¦justment on the aforementioned ratio, a change in product lines from one density product ~o another is also easily accomplished with a minimum of process condition changes and without significantly affecting productivity rates. Specifi-¦cally, total pressure, ethylene feed rate and partial pressure, hydrogen feed rate (if used), catalyst rate and reactor temp-erature can be maintained at substantially constant conditions while the desired density change is achieved by an adiustment of the butene-l concentration in the total C~ hydrocarbon feed~
Another~ probably even more important advantage of the process of this invention is its superb flexibility with re-gard to the use of a variety of catalysts from many different , sources. Wnen changing from one catalyst system to another, it is merely required to determine the reactivity index of the new catalyst and then making a corresponding adiustment in the butene-l/total C4 ratios while maintaining other opera~-ting conditions substantially at their respective previous levels. _ .
_9_ 9~ ~

further advantaqe of the process of the inVCntion is l the hiyh productivity rates per unit volume of reactor, i.e., j~ space~time-yield~ which c~n b~ obtained because of the lowe~r il solubilitY of polymer ln C4 hydrocarbons relative to tilose ll of hexane and heptane. T}lis, in turn, enables the process ji to be conducted at high solid polymer concentrations in the slurry.
The followiny examples further illustrate the advantag~s I obtained by the invention.

The e~periments were conducted in large scale continuous ¦~ pilot plant operations using a method and equipment essential-¦lly similar to those described in connection with the deter-!~ mination of the catalyst reactivity index. The catalyst sys- -lltem which had a reactivity index of 0.0400, consisted of tri-¦iethyl aluminum and a titanium chloride magnesium chloride cata-¦¦lyst containing about 15 wt % titanium and prepared according ito the method of U.S. Patent No. 4,218,339. In each of the Iruns, the recycle gas rate was about 1800 SCFH and the resl-¦Idence time was 2 hours- The other pertinent data of the runs are shown in Table I. Comparative Examples l and 2 were car-! ried out in the absence of inert diluent, and as a result of ¦,the required increase in ethylene reactant, the system became lunstable at the moderate reactor temperature of 140 F and severe operational difficulties were encounteredO
Examples 3-6 were carried out within the limits of the invention and resulted in excellent operational stability~
IThe effect of increasing the butene-l concentration in the ¦total C~'s upon the density is amply illustrated by these exam-Iples.

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These experiments were carried essentially as before ex-l cept that the catalyst system had a reactivity index of 0.0358, li the titanium chloride magnesium chloride component had a titan-1l ium content of 2.5~ and was prepared by the method of ~.S.
Il Patent No. 3,953,414. The recycle gas rate in Exarnple 10 !! was 3100 SCFH and in Example 11 5900 SCFH. The data from these ! runs are summarized in Table II.
It is obvious to those skilled in the art that many varia-li tions and modifications can be made to the process of tl~is i' invention- All such departures from the fo.reyoing specifica-tion and consider~d within the scope of this invention as de-fined by the specification and the appended claim5, l l ,, .

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Claims (15)

CLAIMS:

1. A method of reducing unwanted fluctuations around a predetermined polymer density target value, said fluctuations occuring periodically in a continuous process for the pro-duction of a linear low density polyethylene resin having a density of about 0.935 and below in which process ethylene and butene-1 are copolymerized 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 about 500 psig, at a temperature from about 130°F to about 170°F and at an ethylene partial pressure of from about 50 to about 350 psia employing a catalyst having a reactivity index r in the range from about 0.0325 to about 0.0500 and being composed of (a) an organo aluminum, and (b) a titanium catalyst component supported on a magnesium component, which method comprises:
establishing the limits of a range of a mol ratio of butene-1 to total C4 hydrocarbon in which the lower limit is about the value calculated from the formula:
and the upper limit is about 0.8, increasing the process mol ratio within said range responsive to fluctuations of the polymer density above the density target value and decreasing said mol ratio within said range respon-sive to fluctuations of the polymer density below the density target value.

2. The process of claim 1 wherein the minimum ratio is calculated from the formula:

3. The process of claim 1 wherein the reactivity ratio ranges between about 0.0325 and about 0.0425.

4. The process of claim 1 wherein the temperature is maintained between about 145°F and about 155°F.

5. The process of claim 1 wherein the ethylene partial pressure is maintained between about 150 psia and about 275 psia.

6. The process of claim 1 wherein the total pressure is maintained between about 300 psig and about 450 psig.

7. The process of claim 1 wherein the residence time is between about 1.0 and about 4 hours.

8. The process of claim 1 wherein the polymer solids content of the reactor slurry ranges between about 15 and about 50 wt %.

9. The process of claim 8 wherein the polymer solids content is between about 20 and about 40 wt %.

10. The process of claim 1 wherein hydrogen is present at a concentration of from about 5 to about 40 mol %
in the vapor phase.

11. The process of claim 1 wherein the diluent is n-butane.

12. The process of claim 1 wherein com-ponent (b) is a titanium chloride.

13. The process of claim 1 wherein the organo aluminum is a trialkyl aluminum.

14. The process of claim 11 wherein the trialkyl alum-inum is triethyl aluminum.

15. The process of claim 1 wherein the component (b) is in spherical or spheroidal form.