JPH0423645B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to improvements in the process of copolymerizing ethylene and other olefins. In particular, the present invention
This invention relates to an improved method for copolymerizing ethylene and other olefins to obtain linear low density polyethylene copolymers. U.S. Pat. No. 3,087,817 to JSScoggin, published April 30, 1963, discloses the polymerization of ethylene in the presence of a liquid reaction medium and a catalyst to form a solid polymer product which is dispersed in the liquid reaction medium as it forms. A method for obtaining it in particle form is disclosed. This type of reaction process is known to those skilled in the art as a "slurry" polymerization process and is widely used in the production of high density polyethylene. Such slurry methods typically employ isobutane,
Pentane, hexane or similar saturated aliphatic hydrocarbons are used. However, the so-called “particle shape”
Conventional slurry methods involving ethylene/olefin copolymers exhibit certain drawbacks when applied to the production of ethylene/olefin copolymers, particularly of the type known as "linear low density polyethylene" (LLDPE). .
For example, the reaction temperatures that can be operated are limited because at high temperatures linear low density polyethylene liquefies and adversely affects heat transfer and production rates. Moreover, complete separation of comonomer and reaction medium is difficult, and the copolymer products generally have a relatively broad molecular weight distribution and contain undesirably high amounts of wax. Copolymers made by conventional slurry methods tend to be low in bulk density. One aim of the invention is to overcome one or more of the above problems. According to the invention, copolymers or terpolymers are prepared in solid particle form by copolymerizing ethylene and at least one other 1-olefin in the presence of a catalyst and liquid propane. These products generally have densities ranging from about 0.90 to 0.945 g/cc. The method of the present invention is particularly suitable for producing so-called linear low-density polyethylene copolymers. The process of the present invention is more economical than slurry processes using isobutane, hexane or other liquid diluents and provides copolymer products with more desirable physical properties. Other objects and benefits will be apparent to those skilled in the art from the following description. The present invention will be explained in detail below. According to the invention, ethylene and about 3 to 18 carbon atoms
olefin comonomers are copolymerized in the presence of a polymerization catalyst and liquid propane.
The present invention provides linear low density polyethylene which is an ethylene/1-olefin copolymer having a density in the range of about 0.90 to 0.945 g/cc, which is substantially free of long chain branching and has a high degree of short chain branching. Particularly suitable for the production of copolymers. Various benefits are derived from the improved method of the present invention. For example, propane is less expensive and has a lower heat of vaporization than liquid diluents such as isobutane and hexane that have been used in traditional slurry polymerization processes. Additionally, in the synthesis of linear low-density polymers, the use of propane in the so-called "particle form" allows reactors to be operated at higher temperatures than isobutane without blockage or fouling of the reactor. Make it. Furthermore, since propane is more volatile, there is more residual sensible heat, thereby reducing the
More efficient flash separation of polymer products is possible. Butene and hexene are ideal comonomers for the copolymerization of ethylene according to the present invention. In the case of butene, simple flash evaporation allows almost complete separation of the polymer product from comonomer and reaction medium. In the case of 1-hexene, additional drying may be required, but the load on the dryer is much less than when using isobutane or other conventional diluents. Since propane is a relatively poor solvent for linear low density polyethylene, the viscosity of the product/reaction medium slurry is low, thus making the reaction mixture easy to mix and allowing immediate removal of the heat of polymerization. The use of propane allows the polymerization reaction to be carried out at relatively high temperatures without clogging the reactor, and at the same time at such high temperatures (e.g. 75-58 °C, preferably 76-82 °C).
℃) results in relatively high catalytic reactivity. In addition, surprisingly, copolymers and terpolymers made by slurry processes using propane as the liquid diluent typically have L _s values of less than 3.0, and less than 50. It shows significantly lower elasticity than conventional methods, and Polymer
Engineering and Science, Vol.11, No.2, p124
128 (1971), it was found to exhibit a fairly narrow molecular weight distribution when measured by rheological dispersion (R _D ), as described by Shida et al. The R _D values of products made according to the invention are typically below 4.0, preferably below 3.5, compared to conventional process values of >4.0 (often >5.0). Polymers made in accordance with the present invention desirably exhibit high impact properties. The copolymer and terpolymer products of the present invention are
The products of conventional processes are produced as discrete particles with substantially uniform size and shape characteristics, in contrast to the products of conventional processes, which are highly heterogeneous in size and shape and often in aggregated form. Comonomers useful in the process of this invention consist of one or more 1-olefins having about 3 to 18 carbon atoms. Suitable 1-olefin comonomers include propylene, 1-butene, 1-pentene, 1
-hexene, 4-methyl-1-pentene, 1-octene, 1-decene, 1-dodecene and 1-octadecene, but the invention is not limited thereto. Preferably, 1-butene, 1-hexene or 1-octene is used to produce linear low density polyethylene. As is well known in the art, the comonomer/ethylene ratio can be varied to provide a copolymer product with a desired density. For example, by selecting the proportions of butene, hexane or octene comonomers, linear low density polyethylene can be made. 1
- In the case of butenes, the proportion of butenes that should be present in the copolymer product is about 1 to 10 mol %. In the case of 1-hexene, the corresponding proportions are approximately 0.5-10 mol%, 1
- about 0.5 to 5 mol % in the case of octene. Linear low density polyethylene typically has substantially no long chain branching and a high degree of short chain branching, from about 0.90 to
Ethylene/1- with density in the range 0.945g/cc
Defined as an olefin copolymer. The polymerization reaction can be carried out in any convenient type of reactor, such as a loop reactor, autoclave or tubular reactor. The reaction is carried out at polymerization conditions of temperature and pressure such that the propane remains liquid. The critical temperature of propane is approximately 96.8°C and is independent of pressure. It is therefore preferable to carry out the reaction below this critical temperature. Although the reaction can be carried out at lower temperatures, approximately
By reaction at 75-85°C, preferably 76-82°C,
It was found that considerably high catalytic activity was obtained. Generally, the reaction is carried out by dispersing the catalyst in liquid propane in the presence of ethylene and comonomer. The copolymer or terpolymer product is made in particulate form that is dispersed as a slurry in liquid propane. Some conventional methods for producing low density ethylene copolymers require a prepolymerization step.
In the process, ethylene (up to about 20% of the total amount of monomers ethylene and 1-olefin to be polymerized) is dispersed in a slurry in a liquid diluent in the presence of a catalyst before copolymerization with the 1-olefin comonomer. Prepolymerization is performed with This prepolymerization results in a catalyst slurry mixed with catalyst and ethylene homopolymer. According to the present invention, such a prepolymerization step is neither necessary nor desirable. In the process of the present invention, propane diluent, catalyst, and ethylene and 1-olefin comonomers can be introduced simultaneously into the reactor. Also advantageously, the copolymerization reactions of the present invention are carried out in one step, as opposed to the two-step copolymerization reactions of conventional methods. After the reaction is complete, the polymer is physically separated from the liquid propane and comonomer by any suitable means. Because propane is considerably more volatile than conventional diluents, it is well suited for flash evaporative separation of polymers from diluents and comonomers. If the comonomer has a relatively high carbon number, additional drying may be necessary, but the load on the dryer is much lower than in conventional systems. If desired, separation of the product from the diluent can be accomplished by mechanical means, such as centrifugation. Propane's high volatility also aids in separating propane from higher molecular weight comonomers. Since propane is a fairly poor solvent for the product, relatively high polymerization temperatures can be used without significant reactor blockage. Blockage of the reactor causes costly process interruptions and reactor downtime. Usually called "linear low density polyethylene",
Also, ethylene/1 of the type described herein
- Any polymerization catalyst suitable for producing an olefin copolymer can be used in the present invention. Examples of such catalysts include modified Ziegler catalysts, but modified chromium catalysts such as chromium oxide catalysts commonly referred to as "Phillips" catalysts are not suitable for producing linear low density polyethylene copolymers. This is because it results in products with an undesirably broad molecular weight distribution. Suitable catalysts include Pullukat et al.
U.S. Patent Application No. 112560 dated March 16, 1984 and corresponding British Patent No. 2068007 (published June 22, 1983), U.S. Patent Application No. 590157 of Pullukat et al.
No. 4,499,198 to Pullukat et al. (February 1985)
12), so please refer to these documents as necessary. Preferred catalysts are so-called Ziegler catalysts containing transition metals selected from groups and groups of the periodic table, which are used in conjunction with alkyl aluminum promoters. More preferred is a titanium-containing catalyst. Even if the catalyst useful in the present invention is supported,
Alternatively, it may not be supported. Other preferred catalysts include Pullukat et al.
No. 4,499,198 and US patent application Ser. No. 590,157, filed March 16, 1984. Such catalysts are described in U.S. Pat. No. 4,383,119 to Pullukat et al. (1983).
One or more magnesium silylamide compounds (please refer to this document as necessary) as disclosed in the publication published on May 10, 2015) and Groups B and B of Periods 4 and 5 of the Periodic Table. , B group and B group, and at least one transition metal compound selected from B group and B group of the fourth period. The catalyst is
In addition to the magnesium and titanium transition metal compounds, one or more of Lewis acids, Lewis bases, hydrogen halides, or zirconium tetrachloride can optionally be included. The transition metal compound of the catalyst used in the present invention is
It is important that it is selected for its ability to produce a linear low density polyethylene product with a molecular weight distribution commonly referred to in the art as "narrow".
According to the invention, products with a narrow molecular weight distribution can be obtained, characterized in that they have a rheological dispersity (R _D ) of less than 4. The symbols used herein are as follows: HMDS - Hexamethyldisilazane LLDPE - Linear low density polyethylene L _s - Elastic scale MI - Melt index R _D - Rheological dispersibility TIBAL - Triisobutylaluminum Example: The examples illustrate the method of the invention and its advantages over conventional methods, but are not to be considered as limiting the invention in any manner. All polymerization tests in the following examples were performed using hexamethyldisilazane (HMDS)-
Treated silica, titanium and triisobutylaluminum (TIBAL) at a concentration of 1.88mM per gram of silica
It was carried out with a catalyst system made with a cocatalyst (aluminum/titanium ratio approximately 7.0). The procedure for making the catalyst is illustrated below: To a flask purged with nitrogen was added 1-2 g of Davison 952 brand silica treated with hexamethyldisilazane. After 30 minutes of nitrogen purge, 20 ml of heptane was added to form a slurry. To this slurry, add 10% 6.5E in heptane.
Added Magala (Texas Alkyls) solution (6.5E
_Magala =[(n- _C4H9 ) _{2Mg ] 6.5.Al} ( _C2H5 ).
Addition of Magala resulted in 1.88mM Mg/g silanized silica. The slurry was stirred for 30 minutes and then fresh titanium tetrachloride (TiCl ₄ ) was added. The amount added was such as to yield 1.88mM Ti/g silanized silica. Therefore, the magnesium/titanium molar ratio was 1.0. The slurry was then stirred for 30 minutes. The catalyst was dried at 90°C with a nitrogen flush. The catalyst is
It was a smooth dark brown powder with a titanium concentration of approximately 5.6% by weight. The polymerization test procedure is described below: 2. Purge the reactor with nitrogen and bring it to the desired temperature. The catalyst and cocatalyst solutions were then added to the reactor under a nitrogen atmosphere. The reactor was closed and 1000 ml of diluent (propane or isobutane) was pressurized into the reactor. Hydrogen gas was then added (10-50 psi) as a molecular weight regulator. Then, at the same time, 1-hexene was measured by sight glass and ethylene was added to the predetermined reactor pressure. This pressure was maintained by adding ethylene to the reactor as needed. The temperature was adjusted to ±1.0°C during the 1 hour polymerization period.
The reaction was stopped by turning off the ethylene gas and venting the reactor to the catch pot via a remote controlled bottom valve. reaction medium that can be used;
Depending on the amount of 1-hexene and the temperature, the polymer may be very "wet" with diluent and 1-hexene, so it is strained to remove excess 1-hexene and allowed to dry overnight. . This time was sufficient to obtain a dry polymer. The concentration of ethylene was controlled by changing the ethylene partial pressure in the system. The vapor pressure for propane and isobutane is determined by the Physical Properties of
Hydroearbons (written by RW Gallant, published by Gulf Publishing, Hyuston, Texas). Examples A-F (Comparative) Several ethylene/1-hexene copolymers were conducted using isobutane as a diluent. The table below details the reactor temperature, ethylene partial pressure, hydrogen partial pressure and 1-hexene concentration. Examples AC were carried out at 54.4°C and Examples D-F at 65.5°C. In each case, the polymer separated from the reaction mixture was filtered to remove excess 1-hexene. Despite this procedure, the polymer still has a significant 1-
Contains hexene. Upon final drying, the polymer made at 65.5°C was very sticky and had poor particle size and shape properties (as shown in Figure 1 for the product of Example D). On the other hand, the polymer made at 54.4 °C has better particle size and shape properties,
It wasn't very sticky. However, at this temperature, the reactivity was quite low and large amounts of isobutane and 1-hexene remained in the polymer. The polymer rheology for the three samples shows that these samples have fairly high elasticity, as evidenced by the high L _s values (a measure of elasticity). In addition, the polymer (in the case of Ziegler catalyst)
It has a fairly broad molecular weight distribution. A polymerization temperature of about 54.4° C. using this catalyst results in a density of about 0.920 and a melt index (MI) of about 1 to
This is about the maximum polymerization temperature at which polymer No. 2 can be made without fear of clogging the reactor. Above this temperature, the particle size and shape properties of the polymer rapidly deteriorate. EXAMPLES GK (Invention) Several ethylene/1-hexene copolymerizations were carried out using propane as the liquid diluent. The table below details the reactor temperature, ethylene partial pressure, hydrogen partial pressure and 1-hexene concentration. The product of Example I is shown in FIG. Example L (Invention) Ethylene/1-butene copolymerization was carried out in liquid propane diluent using the catalyst described above. The reaction conditions are shown in the table below. The polymer had a density of about 0.901-0.911 and was not sticky. The polymer particles had uniform particle size and shape characteristics. Examples GL of the present invention demonstrate that relatively high polymerization temperatures can be used without clogging the reactor. For example, polymerization can be carried out at 82.2° C. with 30% by weight 1-hexene in the reactor without blockage. All of the resulting polymers have desirably uniform particle size and shape characteristics. Polymers made using liquid propane as a diluent are
It showed an additional and unexpected benefit of improved polymer properties. Polymers made in liquid propane diluent according to the present invention have lower elasticity and narrow molecular weight distribution (lower R _D ) as evidenced by lower L _s values compared to polymers made using isobutane diluent.
has. Both are highly desirable properties for LLDPE film resins.

[Table] The above description is for mere illustration of the present invention,
It is not intended to limit the invention thereby, and it will be obvious to those skilled in the art that modifications may be made within the scope of the invention.

[Brief explanation of drawings]

FIG. 1 is a photograph showing the particle structure of an ethylene/1-hexene copolymer (Example D) having a density of about 0.922 g/cc, prepared by a conventional slurry process using an isobutane diluent. FIG. 2 is a photograph showing the particle structure of an ethylene/1-hexene copolymer (Example I) having a density of about 0.922 g/cc made by the slurry process of the present invention using a propane diluent.

Claims (1)

[Claims] 1. Polymerization in which ethylene and at least one 1-olefin comonomer having about 3 to 18 carbon atoms are dispersed in liquid propane at selected temperatures, pressures, and 1-olefin/ethylene ratios. copolymerized in a single step in the presence of a catalyst, thereby producing a
A linear low density polyethylene product having a density in the range of 0.945 g/cc is dispersed in said liquid propane and formed as discrete solid particles having a narrow molecular weight distribution of 4 or less as measured by R _D value. A method for producing linear low density polyethylene. 2. The method according to claim 1, wherein the molecular weight distribution is 3.3 or less as measured by R _D value. 3. Process according to claim 1, in which the copolymerization of ethylene and one comonomer is carried out at a temperature below the critical temperature of propane, and the solid particles of the product exhibit low elastic properties. 4 The comonomer is propylene, 1-butene, 1
- selected from the group consisting of pentene, 1-hexene, 4-methyl-1-pentene, 1-octyne, 1-decene, 1-dodecene and 1-octadecene, according to any one of claims 1 to 3. the method of. 5. The method of claim 4, wherein the comonomer is 1-butene. 6. A process according to claim 5, wherein the 1-butene/ethylene ratio is selected to be about 1 to 10 mole % 1-butene in the product. 7. The method of claim 4, wherein the comonomer is 1-hexene. 8. Process according to claim 6, in which the 1-hexene/ethylene ratio is selected to be about 0.5 to 10 mol % 1-hexene in the product. 9. The method of claim 4, wherein the comonomer is 1-octene. 10. A process according to claim 9, wherein the proportion of 1-octene/ethylene is selected to be about 0.5 to 5 mol % 1-octene in the product. 11. Claims 1 to 3, wherein the product is separated from the diluent after the reaction by evaporation of the diluent.
The method described in any of the sections. 12. The method of claim 11, wherein the evaporation is carried out by flash drying under reduced pressure conditions.