DE19930594B4 - Process for the preparation of polyolefin (co) polymers with broadened molecular weight distribution and / or polydispersity of the comonomer content and use of the method - Google Patents

Abstract

Low pressure process for the preparation of polyolefin (co) polymers having a broad molecular weight distribution and / or polydispersity of comonomer content, after which the polymerization reaction within a polymerization reactor at least one process parameter is varied over time, thereby characterized in that for the temperature or the concentration of hydrogen in combination with the comonomer concentration in the reactor, a periodic variation is specified and that the period the variation shorter than half the average reactor residence time.

Description

The Patent application relates to a process for the preparation of olefin (co) polymers with broad molecular weight distribution and / or polydispersity of comonomer content and a use of the method.

polyolefins with broad molecular weight distribution offer the advantage of good processing and to combine mechanical properties. The molecular weight distribution can, for example, in a known manner by the ratio of two Arithmetic means are often characterized by The relationship from mass to number average, the so-called polymolecular index Q. The for the polymolecular index achievable values are in particular of the catalyst system used dependent: with chromium-based catalysts can usually Q-indices ranging from about 8 to 15, with metallocene catalysts usually Q-indices of a maximum of 2 achievable. It is general known in the Trap of polyolefins, especially high-density polyethylenes Q index, the long Chains the good mechanical properties, in particular toughness, Strength and, together with the comonomer content, the ESCR level (= Environmental Stress Crack Resistance, i.e., durability against stress cracking) on the other hand, the short low molecular weight polymer chains decisively for good Processing properties are.

It are already a variety of processes for the preparation of polyolefins known with a broad Q index.

The purely physical mixing of batches with different Molecular weight, for example on an extruder, often results to unsatisfactory results.

Clear better results are generally achieved when components already within the individual polymer particles. in the form of a core-shell construction are almost premixed. Such polymers are characterized by the so-called Cascade technology received.

There are already a number of processes for producing high Q index polyolefins based on cascade technology, ie, using at least two series-connected polymerization reactors. In the cascade process, the reactors connected in series are run at different polymerization conditions, for example at different hydrogen concentrations (corresponding to, for example GB 233,599 A or at different comonomer concentrations.

A very simple variant of a cascade technology describes the EP 0 192 427 B1 according to which the polymer is prepared without any separation of the polymerization media only by different polymerization temperatures of the reactors operated in series.

The EP 0832 905 A1 describes a cascade process in which a high molecular weight polyethylene homopolymer is first prepared in a first reactor at a relatively high temperature (95-110 ° C), then transferred to a second reactor wherein at relatively lower temperature (80-90 ° C) with the addition of more ethylene or α-olefin comonomers, a higher molecular weight homopolymer or copolymer is obtained.

The cascade technology is used equally for the Phillips-loop process, which is carried out in suspension, as well as for gas-phase fluidized-bed polymerizations. To make it more difficult for the practical implementation of a cascade technology with different polymerization media, for example hydrogen concentrations or comonomer contents, that an actual separation of the polymerization media can only be realized with the corresponding expenditure. The direct transfer of a suspension or a gas / solid mixture otherwise causes at least partial mixing of the polymerization. However, extensive independence of the polymerization conditions of the individual stages of the cascade is a prerequisite in order to achieve the desired spread of the products formed and thus the properties of the polymer end product. This extensive independence of the process stages is in the process of already cited EP 0 192 427 B1 achieved by the exiting from a first polymerization reactor mixture of polyolefin powder and reaction gas by decompression to 1/50 to 1/5 of the pressure in the first polymerization reactor separated and the polyolefin powder then a second Polymerization reactor is fed, wherein in this second polymerization reactor, a reaction gas mixture and a catalyst of any composition, that can be added independently of the first polymerization reactor. By observing the same process steps, further polymerization reactors may be provided. However, like any cascade technology, the process requires at least two series-connected polymerization reactors, thus increasing capital expenditure and operating costs.

It is also already known to carry out two-stage processes in a single reactor. Such methods are for example in GB 1,174,542 A . GB 2,020,672 A or BE 883,687 A1 described. However, these are pure batch polymerizations in which different polymerization conditions are set successively, ie in defined successive steps. Amongst other things, batch polymerizations have the disadvantage of higher specific production costs and lower space-time yields over continuous processes.

The cited documents generally describe two-stage polymerization processes, in particular, the hydrogen concentration in the reaction mixture is varied in the reaction stages.

Farther is from Encyclopedia of Polymer Science and Engineering Vol. 6, chap Ethylene Polymers, John Wiley & Sons, Inc. announced that the molecular weight when using Ziegler catalysts preferably over the hydrogen concentration and when using Phillips catalysts preferably over the temperature is regulated.

From the DE 28 41 646 A1 is the variation of monomer partial vapor pressure, from DE 10 06162 A is the periodic variation of the pressure from the FR 1 476 334 A is the periodic variation of the volume of the reactor charge and from Hochmolekularbericht 1976, ref. H. 9791/76 of SU 427 954 the variation of the catalyst content is known.

It is in contrast Object of the invention, a process for the preparation of olefin (co) - polymers for disposal to put that in an economically advantageous manner in a single Polymerization reactor feasible and that uses the disadvantages of the known methods of a single polymerization reactor does not have. In a Process variant may be the heterogeneity of the cascade available Product further increased become.

The The invention is based on a low-pressure process for the production of olefin (co) polymers with a broad molecular weight distribution and / or polydispersity of comonomer content, after which the polymerization reaction within a polymerization reactor at least one process parameter is varied over time.

The Invention is characterized in that a periodic variation is given.

It was surprising found that through targeted periodic variation of usually kept constant Process variables temperature or Hydrogen concentration in combination with the comonomer concentration clearly heterogeneous products accessible whose properties are between those of products that have been so far were prepared in a single polymerization reactor and targeted bimodal cascade products lie. Usually serve Process control systems keeping parameters constant. However, in the present case, on the contrary by means of modern computer aided Process control systems specified a targeted periodic variation of process parameters.

With the method according to the invention may be a broadening of the polymolecular index depending on of the used catalyst system of up to 30%, up to 70 % or up to 80% over conventional Procedure can be achieved.

The Method according to the invention is basically applicable to all known low-pressure olefin (co) polymers, i.e. on polymers produced by low-pressure polymerization or copolymerization be obtained from olefinic starting materials. Low pressure process are known methods, which are usually in a pressure range carried out from 5 to 50 bar become.

Most preferably, the process is applied to continuous low pressure ethylene polymerization applied. Technically, copolymers of ethylene are also known with small amounts of monomers other than polyethylenes, if these copolymers resemble the homopolymers in their properties. However, the process according to the invention is equally applicable to olefin copolymers, in particular ethylene copolymers containing a larger amount of comonomers.

The Method is not limited to special methods of manufacturing Polyolefins, especially polyethylenes restricted, it is equally applicable to the Phillips loop process, to gas phase fluid bed systems and to solution methods applicable. The process is particularly preferred for low-pressure syntheses of (co) polyethylene can be used.

The inventive method is particularly suitable for the continuous, but also for the batchwise operation (Batch polymerization).

When Polymerization reactor can be any polymerization reactor for the Production of polyolefins, in particular polyethylenes used become. Particularly preferred is the single polymerization reactor a Phillips loop reactor or a fluidized bed reactor.

Within a polymerization reactor according to the invention at least the temperature or the concentration of hydrogen in combination with the comonomer concentration while the polymerization reaction varies periodically targeted. For this become the reactor over a control program setpoints for given the process parameters, which are around an average move, with a given unit, the period, repeated regularly becomes.

The inventive method can in an economically advantageous manner in a single polymerization reactor carried out become. However, it is also possible the process to a cascade of at least two of the reactors apply, reducing the heterogeneity of the cascade obtained Product further increased becomes.

in the In the simplest case, the desired value of at least one method parameter as a jump function between periodically alternating minima and Maxima be specified. The corresponding actual value then fluctuates periodically by one Average.

Prefers becomes a sinusoidal Variation of the process parameters.

Especially preferably a periodic variation is around an average value appropriate selection of the control parameters such that the reactor to be varied (s) Parameters in the form of a sawtooth function extends (run), d. H. the time span from one extreme to the next possible briefly fails.

Advantageous is a process parameter that is periodically varied, the temperature. Temperature fluctuations in the range of 1 to 10 ° C, in particular from 1 to 5 ° C to the Mean value have, for example, the Phillips loop method especially proven.

It but it is also possible alternatively to the temperature, the hydrogen concentration in combination periodically varying with comonomer concentration, that is one subject to periodic fluctuation.

When Comonomers come for it the production of polyethylene in particular linear α-olefins, preferred with 3 to 15 C atoms, particularly preferably propene, 1-butene, 1-hexene and / or 1-octene.

Basically there there are no restrictions in terms of of usable catalysts. Particular preference is given to Phillips-chromium catalysts, Ziegler-Natta catalysts or metallocene catalysts.

The Period of variation of the procedural parameters is preferably shorter than the half the average reactor residence time, more preferably shorter than one third of the mean reactor residence time. Will the process of the invention For example, applied to Phillips-loop method, so is the period of variation preferably ranges from 1 to 60 Minutes, more preferably from 1 to 30 minutes, in particular from 5 to 15 minutes.

It is possible, the continuously emerging from a polymerization reactor product mixture in a downstream apparatus using mixing means, for example by means of special Entga homogenize further by means of appropriately sized double cone mixers.

Prefers is doing at least during the a fluctuation period of the at least one method parameter homogenizing product quantity homogenized in a mixing device and in each case marketed as a separate batch.

These Homogenised semolina batch may, if appropriate after addition of Aggregates, stabilizers, dyes, etc. on extruders be granulated.

In addition to temporal variation of the process parameters, it is possible at least a process parameter to spatially to vary different locations of a polymerization reactor. For example, it is possible at different points of a loop reactor to the same Time to specify different polymerization temperatures.

The inventive method is particularly suitable for the production of low-pressure (co) polyolefins, especially low pressure (co) polyethylene.

The Method is explained in more detail below with reference to exemplary embodiments:

catalyst

The catalyst used was a chromium catalyst containing 1% by weight of chromium on silica gel. As a suitable element source for chromium, a methanolic Cr (NO ₃ ) ₃ × 9 H 2 O solution was initially charged. The catalyst was heated in a fluidized bed for 6 hours at a temperature of 830 ° C and then cooled again. From 140 ° C, the fluidized bed was purged with nitrogen to remove traces of oxygen that would interfere with subsequent polymerization.

Example (not according to the invention)

A continuous suspension polymerization was carried out using isobutane as a suspending agent in the presence of the chromium catalyst described above in a 0.2 m ³ loop reactor. The internal reactor pressure was 39 bar. The circulation pump for mixing the reactor contents was operated at 2100 rpm. The mean reactor temperature was 101 ° C. Within a residence time of 2 hours, 12 equal periods of 10 minutes each were run through, in each case a maximum temperature of about 102.5 ° C and a minimum temperature of about 99.5 ° C were reached. An ethylene stream of 25 kg / h and a hexene stream of 450 kg / h were metered into the reactor.

The obtained polymer showed the compiled in the table below Properties:

Table 1

M_w

= Gewichtsmittel der Molmasse und

M_n

= Zahlmittel der Molmasse.

The determination of the material properties was carried out by the following measurement methods:
Density according to ISO1183
Melt flow rate, HLMI (High Load Melt Index) according to ISO1133, and a load of 21.6 kg Polymolecular index Q = M w / M n . in which

M _w

= Weight average molecular weight and

M _n

= Number average molecular weight.

The Molar masses were determined by gel chromatography.

resistance against stress cracking, ESCR (Environmental Stress Crack Resistance) with disc-shaped specimens a diameter of 40 mm and a thickness of 2 mm, one-sided carved with a notch of 20 mm length and 0.1 mm depth at 50 ° C in a commercial Nekanil solution d. H. a surfactant solution from 5% Lutensol, immersed and loaded with a pressure of 3 bar. The time was measured in hours until the occurrence of stress cracks.

to Determination of surface roughness Tubes were made from the polymer and their surface visually assessed.

Comparative example

It was a continuous polymerization with the same process parameters as carried out in Example 1, deviating, however, the reactor temperature was kept constant at 101 ° C.

The polymer obtained showed the measured values determined in accordance with the measurement methods described above and summarized in Table 2: TABLE 2

By the inventive method Thus, a broadening of the polymolecular index Q achieved by 30%, creating a variety of processing and product properties the resulting polymer were simultaneously improved: density, Viscosity, stress cracking and surface roughness.

Claims (8)

Low-pressure process for the preparation of polyolefin (co) polymers having a broad molecular weight distribution and / or polydispersity of the comonomer content, according to which at least one process parameter is varied during the polymerization reaction within a polymerization reactor, characterized in that the temperature or the hydrogen concentration in combination with the comonomer concentration in the reactor is given a periodic variation and that the period of the variation is shorter than half of the average reactor residence time. Method according to claim 1, characterized in that that the Variation sinusoidal runs around an average. Method according to claim 2, characterized in that that the Process parameters hydrogen concentration and / or catalyst dosage be varied periodically. Method according to one of claims 1 to 3, characterized that the Period of variation of the process parameter (s) shorter than is one third of the average reactor residence time. Method according to one of claims 1 to 4, characterized that at least a process parameter in addition in spatially varies different sites of the same polymerization reactor becomes. Method according to one of claims 1 to 5, characterized that at least a polymerization reactor is a loop reactor. Method according to one of claims 1 to 6, characterized that this product emerging from at least one polymerization reactor is homogenized in a downstream apparatus. Use of the process according to any one of claims 1 to 7 for the preparation of lower pressure (co) polyolefins, in particular low pressure (co) polyethylene.