High-pressure polymerisation process for the preparation of polyethylene

Abstract

The invention relates to a high-pressure polymerisation process for the preparation of polyethylene. A polymer is added to the extruder via a side feed dosage unit wherein the MFI.sub.sf of the polymer added via the side feed dosage unit has a higher value than the melt flow index (MFL.sub.end) of the polyethylene end product and wherein the Mw/Mn of the end product increases at least 15% compared to the product wherein no polymer is dosed via the side feed.

Claims

1. A polymerisation process for the preparation of polyethylene comprising preparing a homopolymer of ethylene or a copolymer of ethylene and one or more comonomers to form a polymer melt, feeding the polymer melt to one or more hot melt extruders and adding a second polymer to the one or more hot melt extruders via a side feed dosage unit wherein the MFI.sub.sf of the second polymer added via the side feed dosage unit has a higher value than the melt flow index (MFI.sub.end) of the polyethylene end product and wherein the Mw/Mn of the end product increases at least 15% compared to the Mw/Mn of the product wherein no polymer is dosed via the side feed dosage unit.

2. The process according to claim 1, wherein the second polymer is polyethylene.

3. The process according to claim 2, wherein the polyethylene is LDPE, LLDPE or HDPE.

4. The process according to claim 3, wherein the polyethylene is LDPE.

5. The process according to claim 1, wherein the side feed dosage unit is a side feed extruder.

6. The process according to claim 1, wherein the polymerisation process is a tubular polymerisation process.

7. The process according to claim 1, wherein the side feed dosage unit is installed in the extruder.

8. The process according to claim 1, wherein the weight ratio polymer added via side feed: polymer melt produced in a high pressure reactor ranges between 10:1 and 1:10.

9. The process according to claim 1, wherein the MFI.sub.end ranges between 0.1 and 120 dg/minute.

10. The process according to claim 1, wherein the value for Mw/Mn of the end product ranges between 7 and 35.

11. The process according to claim 1, further comprising adding a chain transfer agent to the polymer melt, while simultaneously maintaining a constant melt flow index of the polyethylene end product.

12. The process according to claim 1, wherein the polymerisation takes place in a reactor having an inlet pressure of 50 MPa to 500 MPa.

13. The process according to claim 1, wherein the polymerisation takes place in a reactor having an inlet pressure of 150 MPa to 400 MPa.

Description

(1) Examples of suitable chain transfer agents include cyclopropane, methane, t-butanol, perfluoropropane, deuterobenzene, ethane, ethylene oxide, 2,2-dimethylpropane, benzene, dimethyl sulfoxide, vinyl methyl ether, methanol, propane, 2-methyl-3-buten-2-ol, methyl acetate, t-butyl acetate, methyl formate, ethyl acetate, butane, triphenylphosphine, methylamine, methyl benzoate, ethyl benzoate, N,N-diisopropylacetamide, 2,2,4-trimethylpentane, n-hexane, isobutane, dimethoxymethane, ethanol, n-heptane, n-butyl acetate, cyclohexane, methylcyclohexane, 1,2-dichlorethane, acetronitrile, N-ethylacetamide, propylene, n-decane, N,N-diethylacetamide, cyclopentane, acetic anhydride, n-tridecane, n-butyl benzoate, isopropanol, toluene, acetone, 4,4-dimethylpentene-1, trimethylamine, N,N-dimethylacetamide, isobutylene, n-butyl isocyanate, methyl butyrate, n-butylamine, N,N-dimethylformamide, diethyl sulfide, diisobutylene, tetrahydrofuran, 4-methylpentene-1, p-xylene, p-dioxane, trimethylamine, butene-2, 1-bromo-2-chlorethane, octene-1,2-methylbutene-2, cumene, butene-1, methyl vinyl sulfide, n-butyronitrile, 2-methylbutene-1, ethylbenzene, n-hexadecene, 2-butanone, n-butyl isothiocyanate, methyl 3-cyanopropionate, tri-n-butylamine, 3-methyl-2-butanone, isobutyronitrile, di-n-butylamine, methyl chloroacetate, 3-methylbutene-1, 1,2-dibromoethane, dimethylamine, benzaldehyde, chloroform, 2-ethylhexene-1, propionaldehyde, 1,4-dichlorobutene-2, tri-n-butylphosphine, dimethylphosphine, methyl cyanoacetate, carbon tetrachloride, bromotrichloromethane, di-n-butylphosphine, acetaldehyde, hydrogen and phosphine.

(2) The chain transfer agent may be used as a solute in a solvent. The solvent may be for example C.sub.5-C.sub.20 normal or iso paraffin or any other solvent suitable in a high-pressure polymerization process.

(3) The polymer melt from the final stage of the product separator will typically be fed to one or more hot melt extruders, for combination with additives, extruding and pelletizing.

(4) LDPE obtained with a high-pressure polymerization process is used for example in the production of films, extrusion coating products, master batches and rotational molding products.

(5) It may be a disadvantage of LDPE obtained with a high pressure tubular or autoclave polymerisation process that the melt elasticity is not sufficient to obtain the properties as required in for example an extrusion coating process.

(6) It is the object of the present invention to provide a high-pressure polymerisation process for the preparation of LDPE with increased melt strength at a specific chosen melt index.

(7) The invention is characterized in that polymer is added to the extruder via a side feed dosage unit wherein the melt flow index of the polymer added via the side feed dosage unit (MFI.sub.sf) has a higher value than the melt flow index of the polyethylene end product (MFI.sub.end) and wherein the Mw/Mn of the end product increases at least 15% compared to the Mw/Mn of the product wherein no polymer is dosed via the side feed.

(8) MFI.sub.sf means MFI of polymer added via the side feed dosage unit.

(9) MFI.sub.end means the MFI of the end product obtained after the dosage of the polymer via the side feed dosage unit.

(10) The melt flow index (melt flow rate) of the polymer added via the side feed dosage unit (MFI.sub.sf) and the melt flow index of the polyethylene end product (MFI.sub.end) are measured according to IS01133:2011 at 190 C. and 2.16 kg.

(11) The Melt Flow Index of LDPE is measured according to IS01133:2011 at 190 C. and 2.16 kg.

(12) MFI.sub.end ranges between 0.1 and 120 dg/minute, preferably between 0.3 and 10.0 dg/minute, more preferably between 4 and 8 dg/minute.

(13) MFI.sub.sf has a higher value than the melt flow index of the polyethylene end product MFI.sub.end.

(14) The value for Mw/Mn of the end product ranges between 7 and 35. Mn and Mw are determined via size exclusion chromatography performed according to Iedema et. al., Polymer 54 (2013) pp. 4093-4104, section 2.2 SEC-MALS on p. 4095.

(15) According to a preferred embodiment of the invention, the Mw/Mn of the end product increases at least 20% compared to the Mw/Mn of the product wherein no polymer is dosed via the side feed.

(16) According to a preferred embodiment of the invention, the Mw/Mn of the end product increases at least 30% compared to the Mw/Mn of the product wherein no polymer is dosed via the side feed.

(17) According to a preferred embodiment of the invention, the polymer is polyethylene.

(18) According to a further preferred embodiment of the invention, the polyethylene is LDPE, HDPE, LLDPE or mixtures thereof.

(19) Most preferably the polyethylene is LDPE.

(20) According to a preferred embodiment of the invention the side feed dosage unit is a side feed extruder or a pump.

(21) Most preferably the dosage unit is a side feed extruder.

(22) The high pressure polymerisation process for the preparation of LDPE may be a tubular or an autoclave process.

(23) Preferably the process is a tubular process.

(24) It is possible to install the side feed dosage unit anywhere in the polymerization reactor system.

(25) Preferably the side feed dosage unit is installed in the extruder.

(26) Preferably the side feed dosage unit is installed as early as possible in the main extruder. This position will result in better mixing results.

(27) It is possible to apply more side feed extruders at different places along the extruder.

(28) The polyethylene end product comprises ethylene homo and/or copolymers.

(29) According to a further preferred embodiment of the invention the weight ratio polymer added via side feed: polyethylene produced in the high pressure reactor ranges between 10:1 and 1:10.

(30) According to a preferred embodiment of the invention the range is between 10:1 and 8:1. at least: 9:1

(31) The process according to the invention results in at least 15% broader molecular weight distribution (MWD) at a same melt index of polyethylene end product.

(32) It is an advantage of the process according to the invention that the melt index of the final product must be constant while the side feed extruder is dosing a higher melt index. The plant conditions for example the CTA concentration will be adjusted in such a way that the material produced in the polymerization section has a lower melt index than the final material. Consequently, the molar mass distribution broadens at the same melt index. Furthermore, the elastic properties can be improved at the same melt index.

(33) It is advantageous that better elastic properties for example melt strength, neck-in and balloon stability at the same MFI are obtained which results in an economical advantage.

(34) Another advantage is that more flexibility in tuning melt properties of product over the whole MFI range is obtained.

(35) A further advantage is that during the production of blown films a higher MI can be applied while maintaining balloon stability (increased throughput or for multilayer purposes).

(36) The elastic properties of the final polyethylene may be characterized by neck-in. Neck-in is disclosed during the presentation Statistical models to describe the correlations between the molecular mass distribution and the extrusion coating process ability by Marcel Neilen on the 2003 9.sup.th European PLACE Conference, May 12-14, 2003 in Rome and in WO2006094723.

(37) Generally the density of LDPE ranges between 915 kg/m.sup.3 and 930 kg/m.sup.3 (according to IS01183) and the melt index ranges between 0.1 dg/minute and 120 dg/minute (according to ISO1133:2011 at 190 C. and 2.16 kg).

(38) LDPE can be used alone, blended or coextruded for a variety of packaging, construction, agricultural, industrial and consumer applications.

(39) The LDPE obtained with the process according to the invention is suitable to be used in extrusion coating applications for coatings on various substrates such as for example paper, board, cloth, aluminium and other materials. The coatings provide for example a very good adhesion, heat sealing performance and moisture barrier to the substrate. Suitable fields of application are for example liquid packaging cartons, aseptic packaging, food packaging, tapes, paper board cups, food carton stock, frozen food and dual oven able trays, pouches, multi wall bags, release papers and photographic papers such as for example ink jet papers. The extrusion coating process is described for example by Harold Giles in Extrusion: The Definitive Processing Guide and Handbook (ISBN 0-8155-1473-50) in the FIG. 47.2 at page 465.

(40) The present invention is also directed to an extrusion coating composition comprising polyethylene obtained with the process according to the present invention. These compositions may also comprise other additives being dependent on the desired application.

(41) The polymer obtained with the process according to the invention can also be applied in for example the film segment, for extruded products, in the cast film segment, in packaging applications, in moulding applications for example closures and medical and consumer bottles, in wire and cable coating applications for electrical and communication cables, in foams, in master batches and in blown films.

(42) Master batches may also be added via a side extruder. Generally, a master batch is a concentrated mixture of pigments and/or additives encapsulated during a heat process into a carrier resin, which is then cooled and cut into a granular shape. However, the addition of a master batch does not result in the desired broadening of the molecular weight distribution. When applying a master batch the Mw/Mn of the end product does not increase at least 15% compared to the product wherein no master batch is dosed via the side feed. It is possible to add additives (for example flame retardants, colourants and stabilizer) up to 1000 ppm in the polymer.

(43) WO2008008835 discloses a bimodal polymer comprising Ziegler-Natta catalyzed polyethylene, having a density of from 0.930 g/cc to 0.960 g/cc, and a molecular weight distribution of from 10 to 25, wherein an article formed from the polymer has a PENT of at least 1500 using test ASTM F 1473. The polymer is applied in the production of pipes.

(44) WO2014190036 discloses a composition comprising a first ethylene-based polymer formed by a high pressure, free-radical polymerization process comprising specific properties and a second ethylene-based polymer having a specific melt index.

(45) WO2009085922 discloses a composition comprising a blend, which comprises a high molecular weight ethylene-based polymer and a low molecular weight ethylene-based polymer. The high molecular weight ethylene-based polymer has a density less than, or equal to, 0.955 g/cm.sup.3. The blend has a high load melt index greater than, or equal to, 15 g/10 min, and a molecular weight distribution greater than, or equal to, 15.

(46) WO2008008835, WO2014190036 and WO2009085922 do not disclose and do not indicate a high pressure polymerisation process for the preparation of polyethylene with an additional step wherein a polymer is added to the extruder via a side feed dosage unit.

(47) WO2008008835, WO2014190036 and WO2009085922 do not disclose and do not indicate a process wherein the plant conditions such as the CTA concentration will be adjusted in such a way that the material produced in the polymerization section has a lower melt index than the final material and wherein the molar mass distribution broadens at the same melt index.