CA1226096A - Linear low density polyethylene process and product - Google Patents

Abstract

LINEAR LOW DENSITY POLYETHYLENE PROCESS AND PRODUCT

ABSTRACT

A process for the production of linear low density poly-ethylene (LLDPE) having a density of 0.930 or lower, employ-ing slurry polymerization techniques involving polymerizing ethylene, butene-l and hexene-l in the presence of an inert C4 liquid diluent and using a catalyst system of an organo aluminum compound and a magnesium halide supported titanium halide catalyst component. The polymer product exhibits improved physical properties particularly advantageous in the production of high clarity films.

Description

~2~96 EP-3016 LINEAR LOW DENSITY POLYETHYLENE PROCESS AND PRODUCT

ABSTRACT

A process for the production of linear low density polyp I
ethylene (LLDPE) having a density of 0.930 or lower, employ-in slurry polymerization techniques involving polymerizing ethylene, buttonhole and hexene-l in the presence of an inert C4 liquid delineate and using a catalyst system of an organ aluminum compound and a magnesium halide supported titanium halide catalyst component. The polymer product exhibits improved physical properties particularly advantageous in the production of high clarity films.

BACKGROUND OF THE INVENTION
Linear low density polyethylene (hereinafter sometimes referred to as LLDPE) can be produced by catalytic polymeric ration techniques involving reaction of ethylene and a small amount of another- alpha-olefin monomer containing from g to 18 carbon atoms in a liquid hydrocarbon delineate, e.g., hexane !
Hutton, cyclohexane, Tulane and other relatively high boil- !
in hydrocarbon liquids.
The reaction can be carried out at temperatures suffix ciently high to dissolve the polymer in the reaction medium.
One such process is disclosed in US. Patent No. 4,076,698 which pertains to copolymers of ethylene and an alpha-olefin monomer in C5-C18 range, prepared by the solution polymerize-lion technique. Copolymers of ethylene and hexene-l and ox-tunnel are commercially available resins produced by this technique.
There are several disadvantages associated with the soul-lion polymerization process. For instance, the polymer con--1- I,'' 22~)9t~ 1 , cent ration in the solution must be maintained at a low level jot prevent operational problems due to an excessive viscosity of the solution. This, in -turn, requires that larger reactor ire used for a constant production rate. Secondly, the recov-' cry of the dissolved product from the delineate requires a number of complicated process steps, which add to the capital !! as well as production costs. Thirdly, high utility require-mints because of the higher temperature, as well as pressures, Indeed to carry out the polymerization in solution add alto the overall cost of the process.
j When lower temperatures are employed to maintain slurry polymerization conditions it is difficult to obtain product loving densities of 0.930 and lower because of the relatively Lehigh ratios required of hexene-l to ethylene causing opera-l tonal problems. Specifically, the volubility of -the Capella-men product in the delineate and in the comonomer, at the operating conditions causes a detrimental increase in the vise costly of the reaction mixture, and inherently adversely limp flits the concentration of polymer solids that can be present in I the slurry as well as the space time yield of the process.
Allis, the polymer particles swell with the liquid hydrocarbons I and the product is difficult to recover because of stickiness and loss of its free flowing state.
Many of the disadvantages of the above-mentioned process uses have been discussed in some detail in US. Patent lo.
~4,298,713, 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 llsupported on a magnesium compound and an organ aluminum come I pound. The slurry polymerization is conducted in the presence of an inert relatively high boiling delineate such as hexane or Litton The disclosed improvements in performance is said to be obtained by carrying out the process in at least two steps Rand limiting the concentration of ethylene in the first stage monomer feed to no more than 10 mole I.
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I` A drawback of the aforementioned process when adapted to Commercial size continuous operation is the requirement of at jlleast two reactors, i.e., one for each stage, which adds con-~lsiderably to the cost of the process and product.
I US. Patent No. 4,294,947 discloses a one step process For the production of a calmer of ethylene and buttonhole under slurry polymerization conditions using a nonsupported ¦lvanadium-containing Ziegler catalyst. The preferred liquid Idiluent is pure buttonhole, however, the scope of the invent Sheehan includes the use of a C4 cut containing owner C4 coupon-lets which have an inert behavior towards the polymerization catalyst. The patent shows that inferior yields and catalyst efficiencies are obtained when the delineate is a mixture of llbutene-l with other inert C4 components.
if It is generally recognized that films made from LLDPE
resins have poorer optical proper-ties, e.g., haze, as come spared to films fabricated from conventional low density polyp ethylene (LPDE3. Good optical properties are required of films used in the packaging of foods and consumer goods which Inked to be fully and clearly visible through the packaging in order to favorably attract the attention of a prospect-jive customer It is, therefore, an object of the present invention to provide an improved slurry process for the production of fin-11 ear low density polyethylene hazing a maximum density of 0 930 g/cc at high catalyst efficiencies.
It is also an object of the present invention to provide a process for the production of a linear low density polyp ethylene having improved clarity.
Al It is a further object to provide a novel linear low den-sty polymer especially useful in the manufacture of food wrap films.
other objects and advantages will become apparent from a reading of the specification and the appended claims 3~2~6C3~

j THE INVENTION
., .
In accordance with the present invention there is pro-lvided a continuous process for the production of a linear allow density polyethylene resin which comprises:
,, copolymerizing ethylene, buttonhole and hexene-l in the presence of an inert C3-C4 hydrocarbon delineate at a pressure at least sufficient to maintain the delineate in liquid phase and up to about 500 I prig, a temperature from about 130F to about 190F, if an ethylene/butene-l mow ratio in the vapor space of the reaction zone between about 2 and about 20, and a hexene-l/butene-l mow ratio in the feed to the reaction zone between about 0.05 and about 10, em-I! plying a catalyst having a reactivity index r in ¦ the range from about 0.0325 to about 0.0500 and con twining (a) an organ aluminum, and (b) a titanium l halide catalyst component supported on a magnesium ¦ halide compound, and recovering a linear low den-l sty polyethylene having a maximum density of about 1 0.93~.

I The catalyst composition used in the process can be any zone of the recently developed, high activity titanium hat-iide/magnesium compound catalyst components and organ alum-Linus cocatalyst components disclosed, e.g., in US. Patents No. 3,830,787, No. 3,953,414, No. 4,051,313, No. 4,115,319, No. YO-YO, No. 4,218,339, No. 4,220,554, No. 4,225,741, No. 4,252,S70, No. 4,255~544, No. 4,263,169, No. 4,298,713, No. 4,301,029, and No. 4,331,561, The reactivity index of the gala-Lucite composition should fall between about 0.0325 and about l0.05. The measurements of this property will be discussed in detail below i Component (a) of the catalyst composition is an alkyd aluminum having from 1 to 8 carbon atoms in the alkyd groups.
Kit is advantageously selected from trialkyl aluminum, dial-i if -4- 1 ~ylaluminum halides or mixtures thereon. The preferred halide l is chloride. Examples of suitable alkyd aluminums are deathly-I; aluminum chloride, di-n-butylaluminum chloride, triethyl alum-11 inum, trim ethyl aluminum, tri-n-butyl aluminum, tri-isobutyl 11 aluminum, triisohexyl aluminum, tri-n-octyl aluminum, Tracy-octal aluminum. The alkyd aluminum can, if desired, be come j flexed with an electron donor prior to introduction into the polymerization reactor. Preferably, the donors are selected I from dominoes or esters of carboxylic acids, particularly en-I lens of aromatic acids.
, some typical examples of such compounds are methyl- and I ethylbenzoater methyl- and ethyl-p-methoxybenzoate, deathly-Al carbonate ethyl acetate, dimethylmaleate, triethylborate, ethyl-11 o-chlorobenZoater ethylnaphthenate, methol-p-toluate, ethyl-I twilight ethyl-p-butoxy-benzoate, ethylcyclohexanoate, ethyl-¦pivalate, N,N,N',N'-tetramethylenediamine, lo -trimeth ¦piperazinel 2,5-dimethylpiperazine and the like. The molar I ratio of aluminum alkyd to electron door should be limited to ¦ a range between about 2 and about 5. Solutions of the elect I iron donor and the alkyd aluminum compound in a hydrocarbon such as hexane or Hutton are preferably prereacted for a con- !
lain period of time generally less than 1 hour prior to feed-in the mixture into the polymerization reaction zone.
j 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 o'er a titanium oxyhalide with a magnesium compound such as a halide, alcohol ate, haloalcoholate, carboxylate, oxide, ho-dioxide, or a Grignard reagent. Other processes include the no-action ox the above mentioned magnesium compounds with an elect iron donor, a silicon compound, or an organ aluminum compound followed by a further reaction step with the titanium come !
Pond s~etimes hollowed by a second reaction step, wherein I

like product is treated, e.g., with a halogen containing sift-Leon compound, an electron donor, etc.
The halogen in the respective halides can be chlorine, Ibromine or iodine, the preferred halogen being chlorine.
Tithe electron donor, if it is used in forming a complex, is suitably selected from the esters of inorganic and organic oxygenated acids and the polyamides. Examples of such come pounds are the esters of aromatic carboxylic acids, such as Ibenzoic acid, p-methoxybenzoie acid and talk acids and Particularly the alkyd esters of said acids; the alkaline die-mines, e.g., N,N,N',N7-tetramethylethylene-diamine. The manges-I'm to electron donor molar ratio are equal to or higher than I and preferably between 2 and 10. Generally, the titanium Content expressed as titanium metal ranges between 0.1 and I 20 wit % 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 for the preparation of the magnesium supported Titanium halide catalyst component are disclosed in detail in Tithe patents listed above, which are hereby incorporated by reference into this application.
¦ The determination of the reactivity index of the catalyst is suitably carried out in an autoclave reactor provided with lo blazed agitator, a cooling jacket for at least partial Reactor temperature control, inlet ports for triethylaluminum Rand supported titanium halide catalyst components, inlets for supply of ethylene, hydrogen, buttonhole and butane delineate 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 chromatography for continuous monitoring of the concentra ' -6-ISLE
lions of the components ethylene, hydrogen, buttonhole and butane in the vapor space of the autoclave. The conditions to be main-twined in the reactor at steady state conditions are as follows:
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Temperature, OF 150 Ethylene, Asia 200 Hydrogen, molt 15 Mow ratio C4-/Tot- C4's 0.1:1 Residence time - his 2 To Catalyst rate surf. for 30% polymer in reactor slurry Alkyd aluminum - wit% 0.1 ¦-basis total reactor content weight After a brief drying step, the density do lo the polyp men product is determined and finally the reactivity index r is determined from the relationship:
r = 0.3029 (do 1 - 0.81~5) .
If the measured density do 1 equals or exceeds 0.945 `20 g/cc, it is recommended that the test be carried out under slightly modified conditions, i.e., the Xc ratio is in-creased to 0.5, and the reactivity index r is then determined from the relationship:

r = Q.3843 (do 5 0.8145) ! .

It is not necessary to condense and recycle the over-head 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 (by are separately fed to the reaction zone. The aluminum alkyd is provided in amounts ranging from about 0.025 to about 0.3 wit based on the total weight of monomers and delineate fed to the reaction - I

)96 zone. The monomer feed to To metal weight ratio is usually - I
in the range of 50,000 and 1,500,000. The preferred reactive ¦fly index of the catalyst should range between about 0.0325 and about 0.0425.
Temperatures at which the LLDPE formation should be carried out are to be in the narrow range of from about 130F to about 190F, and preferably between about 145F and about 165F. The pressures should be sufficient to maintain hydrocarbon delineate and comonomers in liquid phase usually from about 275 prig and up to about 500 prig, preferably between about 300 prig and about 450 prig. The ethylene/
buttonhole mow ratio (in the vapor phase) should be maintained between about 2 and about 20, preferably between about 3 and about 10. The hexene-l/butene-l mow ratio in the feed to the reaction zone should be maintained in the range from about 0.05 to about 10, preferably from about 0.06 to about 3.
The average residence time in the reactor can be from about 1/2 to about 10 hours and preferably between about 1.0 and about 4 hours. The polymer solids content of the no-actor slurry is usually maintained from about 15 to about So wit and preferably between 20 and about 40 wit %.
The reaction is continuous and monomer feeds, delineate and catalyst components are continuously fed to the reactor and a slurry of polymer product 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 mod-liens and their use are well known in the art and need not be discussed in any detail. When hydrogen is employed to in-crease the melt index of the product, its concentration is us-ally maintained between about 5 and about 50 mole percent based on the composition of the vapor phase in the reactor.
he inert delineate is preferably a C4 hydrocarbon usually normal butane, but can also be other inert hydrocarbons in the C3-C4 hydrocarbon range such as propane, isobutane, ox Rex-~L~26c~

lures of such inert C3-C4 compounds. The mow ratio of inert hydrocarbon to the total of inert hydrocarbon, buttonhole and hexene-l in the liquid phase, is generally maintained between about 0.1 and about 0.9.
Because of the generally high productivity of the support-Ed catalyst system expressed in terms of pounds of polymer pro-duped per pound of titanium metal, there is no need to remove catalyst residues from the polymer in a dashing 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 poulticing of the polymer product before ship-mint to the user.
The polymer of this invention generally contains from about 3 to about 13 wit % of polymerized buttonhole and hexene-l, I the remaining being ethylene derived units (from about 87 to about 97 wit %).
Various additives can, if desired, be incorporated into 20 - the LLDPE resin, such as fibers, fillers, antioxidant, metal deactivating agents, heat and light stabilizers, dyes, pig-mints, lubricants and the live.
- ,' One major advantage of this invention is the ease with which the continuous process can be controlled to obtain a desired low density product. Another advantage is the improve-mint apparently had in productivity rates with the process of this invention as compared to the results obtained when pro-paring either ethylene/butene or ethylene/hexene LLDPE resins at similar conditions.
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 volubility of polymer in C3-C4 hydrocarbons relative to those of hexane and Hutton. This, in turn, enables the pro- ,, ~2~9~i , I
cuss to be conducted at high solid polymer concentrations in the slurry.
The most important advantage of the invention is that LLDPE polymers having densities below 0.930 and simultaneously ' exhibiting improved optical properties can be produced by the slurry process of this invention. In fact, it was completely us-expected to find that the terpolymer produced by the process had significantly better haze properties than either an ethylene/
buttonhole or an ethylene/hexene-l LLDPE resin of comparable density. It was also found that unexpectedly the % hexane extractibles (FDA method was considerably lower in the ton-polymer LLDPE than in an LLDPE copolymer of comparable density and fractional melt index. The terpolymer is, therefore, an excellent resin for the fabrication of food film wraps, since films can be produced having good optical properties as well as a hexane extractable concentration below the maximum level permitted by the Food and Drug Administration. A preferred terpolymer product of this invention is one having a melt in- , dew of about 0.5 to about 1.0, a density of about 0.915 to about 0.925, and when in film form, having a maximum haze of about 12~ and a maximum hexane extractable concentration of about 2.5%.
The LLDPE product of this process has excellent physical properties which also makes it useful for a variety of apply-cations other than film fabrication, e.g., in the manufacture of cable and wire coatings, molded housewares, etc. The LLDPE
product may be used alone or as a blend with other polymers such as conventional low density polyethylene, ethylene-vinyl acetate copolymer and many others.
The following examples further illustrate the advantages obtained by the invention.
1, i 6~6 I¦ COMPARATIVE EXP~IPLES 1 4 AND EXAMPLE 5 The experiments were conducted in large scale continue ooze pilot plant operations in liquid butane delineate using a method and equipment essentially similar to those described in connectio~with the determination of the catalyst reactive fly index, except for the nature of the comonomer feed.
Hexene-l was used in Examples 1-4, while in Example 5, the comonomer feed was a mixture of buttonhole and hexene-1. The catalyst system which had a reactivity index of 0.0400, con-sited of triethyl aluminum and a titanium chloride manges-j I'm chloride catalyst containing about 15 wit titanium and prepared according to the method ox US. Patent No. 4,218,339, In each of Examples 1-3, the recycle gas rate was about 2400 l' SKIFF and the residence time was 2 hours, while in Examples 4 and 5, the gas rate was 1800 SKIFF and the residence time lo about 3 hours. The pertinent data of the runs are shown - l in Table I.
In the series of runs consisting of Comparative Exam-if lies 1-4 using hexene-l as the comonomer, the reactor condo-I lions were deliberately varied to promote the production of low density product. For instance, the total pressure was ¦¦ successively lowered to decrease the ethylene partial pressure, and the hexene-l feed rate was successively increased to in-lookers the hexene-l ethylene mow ratio in the vapor phase.
11 O . 928 g/cc was the lowest density obtained on the LLDPE prod-if vats in these series of runs, which were plagued with operation-flat problems, such as plugging, poor temperature control and sometimes runaway conditions. In similar experiments using propane as delineate the same difficulties were encountered and Tithe lowest density of the ethylene/hexene-l copolymer was 0.930 g/cc.
With the addition of 20 wit buttonhole in the comonomer feed n Example 5, the density was dramatically lower-d to !

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,l0.907. 0~9 mix films were prepared from the products of Come -parative Example 3 and Example 4 and analyzed. The pertinent properties are shown in Table 2.
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EXAMPLE NO. 3 5 Haze - % 70.9 26 Hexane Extractibles % - 3.5 As seen from the data, the haze of the butene-l/hexene-l terpolymer of Example 5 was significantly better than that of the Helene polymer of Example 3.

1.
This set of experiments were carried out using the goner-- at method and catalyst system of Examples 1-5, except that the residence time and the recycle rate were respectively maintained - at 2 hours and 1800 SKIFF in each of the runs. Buttonhole was used as comonomer in Comparative Example 6 while a mixture of buy ' tunnel and hexene-l were employed in the remaining Examples ; which were carried out according to the preferred conditions of the invention. The operations were smoother and no difficult ties were encountered in producing the butene-l/hexene-l ton-; polymer resins according to this invention. The pertinent data ; are presented in Table 3.
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11 Films were prepared from samples or products produced I under conditions similar to those of Examples 6 and 9. Some 'l minor variation in melt index and density occurred and are ; , noted in Table 4 below, which also list results of analyses performed on the films.
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m Jo (1) (2) EX~lPLE NO. 11 12 Melt index, g~10 min. 0.9 0.8 Density, gag 0.9172 0.9175 Film Gauge, miss 1.0 1.0 Hexane Extractibles, % 2.9 1.0 Haze, % 17.5 11.1 (1) Copolymer ethylene/butene-l . ;

(2) Terpolymer ethylene/butene-l/hexene-l The data shows that the haze properties and the hexane extractable values were significantly improved in the terpoly-men as compared to the copolymer of ethylene/butene-l of essentially the same melt index and density Thus, the results of the data in Tables 2 and 4 con-Jo elusively show that an LLDPE terpolymer of ethylene/butene-l/
hexene-l has a surprisingly lower percent haze than either an ethylene/butene-l or an ethylene/hexene-l LLDPE copolymer.

It is obvious to those skilled in the art that many variations and modifications can be made to the process of . this invention. All such departures from the foregoing specie - ligation are considered within the scope of this invention as dew nod by the specification and the appended claims.

1.

Claims (17)

What is claimed is:

1. A continuous process for the production of a linear low density polyethylene resin of improved clarity which comprises:
copolymerizing ethylene, butene-l and hexene-l in the presence of an inert C3-C4 hydrocarbon dil-uent at a pressure at least sufficient to main-tain the diluent in liquid phase, and up to about 500 psig, a temperature from about 130°F to about 190°F, an ethylene/butene-l mol ratio in the vapor space of the reaction zone between about 2 and about 20 and a hexene-l/butene-l mol ratio in the feed to the reaction zone between about 0.05 and about 10, 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 catalyst component supported on a magnesium halide com-ponent and recovering a linear low density poly-ethylene having a maximum density of about 0.930.

2. The process of claim 1 wherein the reactiviy ratio ranges between about 0.0325 and about 0.0425.

3. The process of claim 1 wherein the temperature is maintained between about 145°F and about 165°F.

4. The process of claim 1, wherein the ethylene/butene-mol ratio is maintained between about 3 and about 10.

5, The process of claim 1, wherein the hexene-l/butene-l mol ratio is between about 0.06 and about 3.

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 50 mol % in the vapor phase.

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

12. The process of claim 1 wherein the halide of component (b) is 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.

16, An LLDPE terpolymer of ethylene, butene-l and hexene-l having a melt index of about 0.5 to about 1.0, a density of about 0.915 to about 0.925 and when in film form having a-maximum haze of about 12% and a maximum hexane extractibles con-centration of about 2.5%.

17. An LLDPE terpolymer produced by the process of claim 1 and having a melt index of about 0.5 to about 1.0, a density of about 0.915 to about 0.925 and when in film form having a maximum haze of about 12% and a maximum hexane extractibles concen-tration of about 2.5%.