WO2001000690A2

WO2001000690A2 - Multi-stage process for producing polyethene

Info

Publication number: WO2001000690A2
Application number: PCT/FI2000/000580
Authority: WO
Inventors: Vidar Almqvist; Arild Follestad
Original assignee: Borealis Technology Oy
Priority date: 1999-06-29
Filing date: 2000-06-28
Publication date: 2001-01-04
Also published as: WO2001000690A3; FI991480A0; AU5829700A

Abstract

The invention relates to a multi-stage process for producing ethene homo or copolymer, which multi-stage process comprises, in any order, at least a liquid-phase polymerisation step (a) and a gas-phase polymerisation step (b). The process is characterised in that the polymerisation is carried out in the presence of a chromium catalyst. According to an embodiment of the invention, the molar masses and/or melt flow rates of the ethene homo or copolymers produced in steps (a) and (b) are essentially identical.

Description

Multi-stage process for producing polyethene

The invention relates to a multi-stage process for producing ethene homo or co- polymer, which multi-stage process comprises, in any order, at least a liquid-phase polymerisation step (a) and a gas-phase polymerisation step (b). The invention also relates to the ethene homo or copolymer produced with said process.

Background of the invention

Bimodal polyethene materials produced using Ziegler-Natta catalysts in a multistage process are known to have superior mechanical properties compared to uni- modal materials produced with Ziegler-Natta or chromium catalysts. The bimodal materials also have an excellent processability, which allows them to be run in a variety of machines and machine settings, both in those specifically designed for low density polyethene (PE-LD) produced in a high pressure process and in those specifically designed for linear polymers (PE-LLD and PE-HD) produced in a low pressure process. These materials have the disadvantage, however, that they have a too low melt strength required for certain special applications.

When ethene is polymerised in the presence of a chromium catalysts, the molar mass of the polymer is function of the yield of the polymer on the catalyst particle: the lower the yield, the higher the molar mass. Continuously operating reactors have typically an exponential catalyst particle residence time distribution. This means that a large portion of catalyst particles pass through the reactor very quickly. Thus the yield remains low for these catalyst particles, and consequently the molar mass of the polymer produced by these catalyst particles is high. This large portion of high molar mass polymer particles may cause homogeneity problems in the processing, resulting in gels and/or "fish-eyes" in the final product.

Melt flow rate (MFR), sometimes also called melt index (MI), is a generally used measure of the molar mass of the polymer. The melt flow rate is measured by pressing the polymer melt through a standard cylindrical die at a preset temperature under a specified load. The measurement temperature for polyethene is 190 °C. The load is generally denoted as a subscript. Thus, abbreviation MFR₂₁ denotes the melt flow rate measured under 21.6 kg load and MFR₂ denotes the melt flow rate measured under 2.16 kg load. Melt flow rate can be determined using e.g. ISO 1133 C4, ASTM D 1238 or DIN 53735 standards. The preparation of bimodal polyethene using a Ziegler-Natta catalyst is disclosed e.g. in the applicant's European Patent EP-B-517 868. The publication discloses a process comprising a loop and a gas-phase reactor. The loop reactor may be operated in supercritical conditions above the critical temperature and pressure of the fluid mixture contained within the reactor. While the inventors state that the invention disclosed in EP-B-517 868 is not catalyst specific, the examples they present only disclose the use of a Ziegler-Natta catalyst.

While the above process is able to produce a polyethene material having excellent mechanical properties and processability, it suffers from the fact that the melt strength of the material may be inadequate for certain applications.

One method to overcome the above problem is described in EP-A-829 495, which discloses a process where polyethene is polymerised in a two-stage slurry process, using a chromium catalyst. In the first stage a low molar mass, high density polyethene is produced and in the second stage a high molar mass polyethene having a relatively low density is produced. According to the inventors, the polymerisation stages can thus be controlled independently from each other.

While the above process produces a polyethene material having good mechanical properties and processability with a sufficient melt strength, it has the disadvantage that it cannot produce materials with a low density below 930 kg/m³ due to solu- bility and fouling problems in the polymerisation. It also produces polyethene with a very broad molar mass distribution. Consequently, low molar mass oligomers may be produced during polymerisation and these may deteriorate the organoleptic properties (resulting in bad taste and/or odour).

Description of the invention

The above cited problems can be overcome by preparing ethene homo or copolymer in a multi-stage process comprising, in any order, at least a liquid-phase polymerisation step (a) and a gas-phase polymerisation step (b). The process is characterised in that the polymerisation is carried out in the presence of a chromium catalyst.

It has thus been found that in comparison to polyethene produced in a multi-stage polymerisation process with Ziegler-Natta catalyst, the polyethene produced in similar process with chromium catalyst exhibits a much higher melt strength at similar molar mass. This is especially advantageous for the blow moulding application, specially for the blow moulding of large containers. In comparison with chromium catalyst used in the conventional single reactor polymerisation process, the advantage of the invention is a better homogeneity of the final polymer product as well as a higher activity of the catalyst. Increased homogeneity results from the fact that the molar mass of the polymer increases with increased yield. Thus, in a two-stage process the catalyst particles have a more even reactor residence time distribution than in a one-stage process, resulting in less polymer particles having a very high molar mass. Consequently, there is a lower risk of inhomogeneities. Also, there are less non-reacted catalyst particles, which would lead to gels in the final product.

According to an embodiment of the invention, the process comprises first a liquid- phase polymerisation step (a) and a subsequent gas-phase polymerisation step (b).

According to another embodiment of the invention, the molar masses and or melt flow rates of the ethene homo or copolymers produced in the steps (a) and (b) are essentially identical. Preferably, the difference between the melt flow rates at a load of 21.6 kg of the polymers produced in steps (a) and (b) is at most 100%, preferably at most 50% of the melt flow rate of the final product.

According to yet another embodiment of the invention, the density of the ethene homo or copolymer produced in step (a) is at least 10 kg/m³, more preferably at least 15 kg/m , most preferably at least 20 kg/m higher or lower than the density of the ethene homo or copolymer produced in step (b). Preferably, the ethene homo or copolymer produced in step (a) has a higher density than the ethene homo or copolymer produced in step (b). The difference in density results in difference in long chain branching and in the branching distribution in the polymer chain.

Materials with different molar masses can be produced by the process according to the invention. The average molar mass of the polymer is controlled by adjusting the operating temperatures of the polymerisation steps, among other process parameters. The molar mass of the material produced using the process according to the invention may vary so that its melt flow rate at a load of 21,6 kg (MFR₂₁, determined at 190 °C according to the standard ISO 1133) ranges from 2,5 to 200 g/lO min.

The process according to the invention also allows the production of materials within a wide density range. The density may be as high as 965 kg/m³, but it is also possible to produce materials having a density as low as 915 kg/m³. The ratio of the amount of polymer produced in step (a) to the amount of polymer produced in step (b) is preferably between 30/70 and 70/30, more preferably between 40/60 and 60/40. If the fraction of relatively high density material produced in step (a) is too low, the mechanical properties of the final product are likely to suffer, since then the relatively low density material produced in step (b) contains too little comonomer. On the other hand, if the fraction of relatively high density material of step (a) is too high, then the final product contains too low an amount of copolymer having a relatively low density, and this again has an adverse effect on the mechanical properties.

According to an especially preferred embodiment of the invention, an ethylene homopolymer having a melt index MFR ₁ (melt flow rate measured at 190 °C under a load of 21.6 kg) of about 0.5-3 g/10 rnin is produced at the first polymerisation step and an ethylene copolymer is produced at the second polymerisation step so that the final polymer composition has an MFR₂₁ of 0.5-3.0 g/10 min and a density of 945-965 kg/m³, preferably of 950-960 kg/m³. The ratio between the first step polymer and the second step polymer ranges from 30/70 to 70/30, preferably from 40/60 to 60/40 and more preferably from 40/60 to 50/50.

The chromium catalyst used in the invention may be any chromium oxide catalyst used in the art. The chromium compound used in the catalyst is any convenient chromium compound that will become an oxide when calcinated. The chromium compound can for example be chromium trioxide, a chromium halide, a chromium oxyhalide, a chromium nitrate, a chromium acetate, a chromium sulphate or a chromium alcoholate. Also various chromates, such as potassium bichromate, ammonium chromate and various alkyl chromates are usable as well as chromium acetylacetonate. Mixtures of at least two chromium compounds can also be used.

The chromium compound is advantageously supported on a support, that is preferably silica. The carrier may also contain minor amounts of other components than silica, like oxides of aluminium, titanium or zirconium. Optionally, the carrier is formed in a precipitation step together with the chromium compound. The carrier may also consist primarily of aluminophosphate. The surface area of the catalyst is preferably greater than 300 m²/g, and the pore volume is greater than 1 cmVg.

Typically, the catalyst needs to be activated prior to use. The activation is performed at an elevated temperature in an oxidising atmosphere, and is called oxidat- ive calcination. Thus, the catalyst may be activated in an air stream at a temperature between 400-950 °C, preferably between 500-900 °C. If after the oxidative calci- nation a prereduction of the catalyst is desired, it is preferably done at a somewhat lower temperature than the activation, e.g. between 250-500 °C.

An increase of the oxidative calcination temperature results in a increase of the melt flow rate (MFR) of the polymer. However, if the polymerisation temperature is reduced to achieve the same MFR, then is seen that the processability in terms of MFR₂₁/MFR₂ increases (is improved), environmental stress cracking corrosion resistance (ESCR) is lowered (worsened), and die swell is reduced. The choice of the calcination temperature is thus a compromise between the desired properties.

Advantageously, the catalyst comprises about 0.5-5.0 wt-% of chromium. Mainly, the chromium is, after the oxidative oxidation stage, at an oxidation state of +VI and it is reduced in the reactor by ethene to oxidation state +11. The catalyst may also be reduced prior to its introduction into the reactor by using a convenient reduction agent, like carbon monoxide.

According to the invention, the catalyst used in the process is preferably only slightly sensitive to hydrogen, since use of hydrogen results in a decreased content of long chain branches. This again has a negative effect on the melt strength of the polymer.

Preferably, the catalyst is used without cocatalyst. If however specifically desired, a cocatalyst may be used. An example of a cocatalyst that may be used in combi- nation with a chromium catalyst is triethylboron (TEB), which would lead to increased MFR₂₁/MFR₂, or triemylaluminium (TEA) which, when used in small amounts, primarily acts as an activity booster, probably by acting as a poison scavenger.

According to one embodiment of the invention, the polymerisation step (a) is carried out in a loop reactor, in the presence of a reaction medium, the monomer and optionally one or several comonomers as well as hydrogen. Preferably, no comonomer is used. The ratio of hydrogen to ethene in step (a) is within the range of 0-1 mol/mol, preferably within 0-1.5 mol/mol. Most preferably, the polymerisation is carried out in the absence of hydrogen.

In the loop reactor the polymer slurry is circulated by means of a circulation pump. The reactor is surrounded by a cooling jacket to control the temperature and to remove the heat of polymerisation. Non-polar hydrocarbons can be used as reaction medium in loop reactor. Alkanes form one suitable group of hydrocarbons that can be used as the reaction medium. Examples of suitable alkanes are propane, i-butane, n-butane, pentane, hexane and heptane, as well as mixtures of at least two of them. It is especially suitable to use propane as the reaction medium, since then the critical temperature of the fluid mixture is sufficiently low to allow operation in supercritical conditions at a temperature below the softening point of the polymer.

According to an embodiment of the invention, the polymerisation step (a) is performed in supercritical conditions, whereby the polymerisation temperature and pressure are above the corresponding critical points of the reaction mixture formed by the reaction medium, monomer and optionally hydrogen and comonomer and the polymerisation temperature is lower than the melting point of the polymer formed.

The polymerisation in supercritical conditions in loop reactor is advantageous, because there is no risk of formation of gas bubbles in the reactor, which would lead to problems in the operation of the loop pump. Also, the separation of hydrocarbons from the polymer is economical since no additional energy is needed to evaporate the medium.

The molar mass of the material produced in the polymerisation step (a) is primarily controlled by adjusting the operating temperature of the reactor. Thus, the catalyst should be able to produce the desired molar mass at a temperature which exceeds the critical temperature of the reaction mixture. A suitable range of the operating temperature, when propane is used as medium, is 90-110 °C, preferably 95-105 °C. The operating pressure preferably exceeds the critical pressure of the mixture. A suitable range, when propane is used as medium, is between 50-75 bar, preferably between 55-70 bar.

According to yet another embodiment of the invention, the density of the polymer formed in the polymerisation step (a) is lower than the density of the final polymer. In this case comonomer is added into the loop reactor. The comonomer however increases the critical temperature of the reaction mixture and thus the operation requires more energy since the polymerisation temperature has to be increased.

The polymer slurry is discharged from the loop reactor either continuously or discontinuously by using any method known in the art. An especially suitable method is to employ settling legs, where the slurry is allowed to concentrate before it is discharged from the reactor. In this way, a higher polymer concentration is reached at the outlet than is present in the reactor.

The slurry is transferred from the polymerisation step (a) into a separation step where the hydrocarbons are separated from the polymer. The medium and unreacted monomer are recycled back into the polymerisation step (a). The polymer containing the active catalyst is transferred into the polymerisation step (b).

According to the invention, the polymerisation step (b) is carried out in one or more gas phase reactors. Preferably one gas-phase reactor is used, and most preferably said step (b) is carried out in a fluidised bed reactor. Said fluidised bed reactor may also be equipped with an agitator. The polymer bed is fluidised by the circulation gas, which is introduced to the bottom of the reactor, and has a sufficient velocity to fluidise the polymer particles. The unreacted monomer and eventually comonomer is collected at the top of the reactor, compressed and cooled to remove the heat of polymerisation, and finally reintroduced to the reactor. Other types of gas phase reactors, like mixed bed reactors or fast fluidised bed reactors, may also be used.

Ethene, optionally one or more comonomers and one or more inert gases are introduced into the reactor so that a desired production rate and product density are reached. When comonomer is used, it is preferably a C₃-C₁₂ α-olefin. The operating temperature is set to such a level that the desired molar mass of the polymer is reached. Typically, the operating temperature is 75-110 °C, preferably 85-105 °C. The operating pressure is typically 5-50 bar, preferably 10-30 bar.

The polymer produced in the polymerisation step (b) has a molar mass which is essentially equal to that of the material produced in the polymerisation step (a). The density of the polymer is controlled so that the final product has the predefined target density. According to a preferred embodiment of the invention, the density of the polymer produced in step (b) is lower than the density of the polymer produced in step (a).

The most important process variables to control the molar mass and consequently the melt flow rate of the polymer produced in each step are the polymerisation temperatures. The MFR is increased by decreasing the polymerisation temperature. Furthermore, MFR is also increased by increasing the ratio between comonomer and ethene. However, this ratio is also used for controlling the density of the polymer. The invention also relates to ethene homo or copolymer, which is characterised in that is comprises at least two fractions, which fractions have essentially the same molar mass and/or melt flow rate and essentially different densities. The difference between the melt flow rates at a load of 21.6 kg of the first and second fractions is preferably at most 15%, more preferably at most 10% of the melt flow rate of the final product, and the difference of density of the at least two fractions is at least 10 kg/m³, more preferably at least 15 kg/m³, most preferably at least 20 kg/m³.

The polymer may be compounded and extruded into pellets using any method known in the art. Especially suitable devices are twin screw extruders, which homogenise the material to a certain extent before pelletising. The extruder may be of either corotating or counterrotating type.

Experimental section

Example 1

A chromium catalyst was activated at 650 °C for 6 hours. The thus activated catalyst was used in polymerisation as follows:

Into a loop reactor of 500 dm was continuously introduced propane diluent, ethylene and the above polymerisation catalyst. The reactor was operated at 82 °C and 60 bar, and the feeds were adjusted so that the production rate of ethylene homopolymer was 25 kg/h and the average residence time was 0.8 hours. The MFR₂₁ of the ethylene homopolymer resin was 1.8 g/10 min and density of 962 kg/m³.

The polymer slurry was continuously withdrawn from the loop reactor, hydrocarbons were removed and the polymer was introduced into a fluidised bed gas phase reactor operated at 85 °C and 20 bar. Additional ethylene and 1-butene comonomer were introduced into the reactor so that 53 kg/h of copolymer of ethylene and 1-butene having MFR₂₁ of 2.0 g/10 min and density of 954 kg/m³ was recovered from the reactor.

Claims

1. Multi-stage process for producing ethene homo or copolymer, which multistage process comprises, in any order, at least a liquid-phase polymerisation step (a) and a gas-phase polymerisation step (b), characterised in that the polymerisation is carried out in the presence of a chromium catalyst.

2. A process according to claim 1, characterised in that the process comprises a liquid-phase polymerisation step (a) and a subsequent gas-phase polymerisation step (b).

3. A process according to claim 1, characterised in that - ethene homo or copolymer is produced in said liquid-phase polymerisation step (a), and

- ethene homo or copolymer having essentially identical molar mass and/or melt flow rate than the ethene homo or copolymer produced in step (a), is produced in said one or more gas-phase polymerisation step (b).

4. A process according to claim 3, characterised in that the difference between the melt flow rates at a load of 21,6 kg of the polymers produced in step (a) and in step (b) is at most 100%, preferably at most 50% of the melt flow rate of the final product.

5. A process according to claim 1, characterised in that the density of the ethene homo or copolymer produced in step (a) is at least 10 kg/m³, more preferably at least 15 kg m³, most preferably at least 20 kg/m³ lower or higher than the density of the ethene homo or copolymer produced in step (b).

6. A process according to claim 1, characterised in that the density of the ethene homo or copolymer produced in the polymerisation step (a) is higher than the density of the ethene homo or copolymer produced in the polymerisation step (b).

7. A process according to claim 1, characterised in that the density of the final product is 915-965 kg/m³.

8. A process according to claim 1, characterised in that the melt flow rate at a load of 21.6 kg of the final product is 2.5-200 g/10 min.

9. A process according to claim 1, characterised in that the ratio of the amount of polyethene produced in the polymerisation step (a) to the amount of the poly- ethene produced in the polymerisation step (b) is 30/70-70/30, preferably 40/60- 60/40.

10. A process according to claim 1, characterised in that said chromium catalyst comprises 0.5-5.0 wt-% of a chromium compound and a carrier.

11. A process according to claim 1, characterised in that the polymerisation step (a) is carried out in a loop reactor.

12. A process according to claim 11, characterised in that the polymerisation step

(a) is carried out in a reaction medium which is preferably a non-polar hydrocarbon, more preferably selected from the group consisting of propane, i-butane, n- butane, pentane, hexane, heptane and the mixture of at least two of them, most preferably said medium is propane.

13. A process according to claim 11, characterised in that polymerisation step (a) is carried out in supercritical conditions, whereby the polymerisation temperature and pressure are above the corresponding critical points of the reaction mixture formed by reaction medium, monomer, hydrogen and optional comonomer and the polymerisation temperature is lower than the melting point of the polymer formed.

14. A process according to claim 1, characterised in that the polymerisation step

(b) is carried out in one or more fluidised bed reactors.

15. A process according to claim 1, characterised in that at least one comonomer is used in the polymerisation step (b).

16. A process according to claim 15, characterised in that said comonomer is selected from the group consisting of C₃-C₁₂ α-olefϊns and their mixtures.

17. A process according to claim 15, characterised in that the reaction temperature in the polymerisation step (b) is 75-110 °C, preferably 85-105 °C.

18. A process according to claim 15, characterised in that the reaction pressure in the polymerisation step (b) is 5-50 bar, preferably 10-30 bar.

19. A process according to claim 1, characterised in that it comprises one or more liquid-phase polymerisation steps (a) and one or more gas-phase polymerisation steps (b).

20. Ethene homo or copolymer, characterised in that it has been prepared according to any of claims 1-19.

21. Ethene homo or copolymer, characterised in that it comprises at least two fractions, which fractions have essentially the same molar mass and/or melt flow rate and essentially different densities.

22. Ethene homo or copolymer according to claim 21, characterised in that the difference between the melt flow rates at a load of 21.6 kg of the first and second fractions is at most 100%, preferably at most 50% of the melt flow rate of the final product.

23. Ethene homo or copolymer according to claim 21, characterised in that the difference of density of the at least two fractions is at least 10 kg/m³, more preferably at least 15 kg/m³, most preferably at least 20 kg/m³.

24. Ethene homo or copolymer according to any of claims 21-23, characterised in that it has been prepared in a multi-stage polymerisation process.