Control of molecular weight distribution of polyethylene by external electron donors.

Abstract

The present invention relates to a process for controlling the molecular weight distribution of polyethylene and reducing the amount of smoke during polyethylene formation, wherein the process for preparing polyethylene having a narrow molecular weight distribution comprises the following steps: a) continuously polymerizing ethylene to produce a polymer slurry by exposing an ethylene stream, hydrogen, a solvent, a Ziegler-Natta catalyst containing titanium, a co-catalyst, and an external electron donor selected from an alkoxysilane compound in a reactor; b) removing residual reaction gases from the polymer slurry stream obtained from a); and c) separating the polymer stream of b) from the solvent.

Description

[Technical Field]

[0001] The present invention relates to the field of chemistry, and in particular to the control of molecular weight distribution of polyethylene by external electron donors. [Background technology]

[0002] Polyethylene has become a widely used plastic because its low price has resulted in lower production costs compared to other plastics. Generally, polyethylene can be prepared from the polymerization of ethylene in a solvent suspension stage. As a result, polyethylene prepared by this method cannot be formed in a stretching machine at speeds faster than 250 m / min, causing delays in the industrial production process.

[0003] Another problem commonly encountered in the manufacturing process is smoke generated during the polyethylene fiber molding process, which requires heating to melt the polymer. The polymer is melted and stretched simultaneously, producing smoke during the manufacturing process. Initial analysis found that the smoke was hydrocarbons with a molecular weight of less than 500 g / mol.

[0004] CN106496802A discloses the preparation of ethylene propylene copolymer composite fibers with narrow molecular weight distribution, and reduces the chance of smoke generation during the molding process by controlling the polymerization temperature within 83-85°C and the polymerization pressure at 0.3-0.6 MPa.

[0005] EP172094294 discloses a polypropylene polymerization process comprising propylene, a hydrogen catalyst, and an external electron donor selected from aminosilanes to synthesize polypropylene in a first polymerization medium in a solution or slurry state with a low boiling point or lower. Hydrogen is removed from the first polymerization medium and a separate olefin / polyolefin stage is prepared before the first polymerization medium enters a second polymerization medium in a gas-phase reactor. The resulting product is then reacted with ethylene to obtain an ethylene-propylene copolymer having a melt flow rate of at least 60 g / 10 min. The resulting polymer can be used in automotive parts.

[0006] US8026311B2 discloses a process for the polymerization of propylene and ethylene or other α-olefins by adding cyclohexylmethyldimethoxysilane and diethylaminotriethoxysilane as external electron donors during the polymerization step, which results in an ethylene-propylene copolymer with a higher molecular weight.

[0007] US7531607B2 discloses the preparation of at least two different grades of polypropylene. In this process, isotactic polypropylene is converted while the polymer flow rate is maintained at a predetermined level. The conversion from the primary to the secondary polymer involved at least one polymerization reactor. The propylene polymer can be reacted with a comonomer under polymerization conditions using a Ziegler-Natta catalyst system with a silane group as an external electron donor.

[0008] Although Ziegler-Natta catalysts have been applied with external electron donors in the polymerization of polypropylene, the process is not widely known for the preparation of polyethylene, especially the application of external electron donors in conjunction with the use of catalysts to control molecular weight distribution.

[0009] From the above, the invention aims to discover a method for controlling the molecular weight distribution of polyethylene, including reducing smoke generated during the formation of polyethylene, by using a Ziegler-Natta catalyst together with a silane compound as an external electron donor in the polymerization process of ethylene. Thus, polyethylene prepared by the method of the present invention has a narrow molecular weight distribution and less smoke generated during the production process. Summary of the Invention

[0010] The present invention relates to controlling the molecular weight distribution of polyethylene and reducing smoke generated during the polyethylene production process, and a process for preparing polyethylene having a narrow molecular weight distribution comprises the following steps: a) continuously polymerizing ethylene to produce a polymer slurry by exposing an ethylene stream, hydrogen, a solvent, a titanium-containing Ziegler-Natta catalyst, a co-catalyst, and an external electron donor selected from an alkoxysilane compound in a reactor; b) removing residual reaction gas from the polymer slurry stream obtained from a); and c) separating the polymer stream of b) from the solvent. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention discloses a method for controlling the molecular weight distribution of polyethylene using a Ziegler-Natta catalyst together with a silane compound as an external electron donor. Any embodiment shown herein is meant to include applications to other embodiments of the invention unless otherwise stated.

[0012] Technical or scientific terms used herein have definitions by those of ordinary skill in the art unless otherwise stated.

[0013] Any instrument, device, method, or chemical named herein is meant to be an instrument, device, method, or chemical commonly used by those skilled in the art, unless otherwise specified as an instrument, device, method, or chemical unique only to this invention.

[0014] The use of the singular noun or pronoun in the claims or specification with "comprising" means "one," including "one or more," "at least one," and "one or more than one."

[0015] All compositions and / or methods disclosed in this application, and the claims that follow, are intended to encompass embodiments with any operation, performance, modification, or adjustment without any experimentation significantly different from the present invention, and for purposes not specifically recited in the claims that are practical and that can be obtained by one of ordinary skill in the art with similar results to the present embodiments. Accordingly, all alternative or similar purposes to the present embodiments, including any slight modifications or adjustments that would be obvious to one of ordinary skill in the art, should be construed as remaining within the spirit, scope, and concept of the invention as expressed in the appended claims.

[0016] Throughout this application, the term "about" means that any numerical value expressed or indicated herein can vary or deviate due to the error of either the device, the method, or the individual using the device or method.

[0017] Hereinafter, embodiments of the present invention will be described, but the scope of the present invention is not limited thereto.

[0018] The process for preparing polyethylene with narrow molecular weight distribution according to the invention comprises the following steps:

[0019] a) continuously polymerizing ethylene to produce a polymer slurry by exposing an ethylene stream, hydrogen, a solvent, a titanium-containing Ziegler-Natta catalyst, a co-catalyst, and an external electron donor selected from an alkoxysilane compound in a reactor; b) removing residual reaction gas from the polymer slurry stream obtained from a); and c) separating the polymer stream of b) from the solvent.

[0020] In one aspect of the invention, the Ziegler-Natta catalyst comprises at least one titanium compound.

[0021] In another aspect of the invention, the Ziegler-Natta catalyst is a magnesium chloride supported titanium tetrachloride catalyst.

[0022] In one aspect of the invention, the process according to the invention comprises polymerization in the presence of an organic solvent selected from, but not limited to, propane, butane, isobutane, pentane, hexane, heptane, octane, benzene, and toluene, preferably hexane.

[0023] In one embodiment of the invention, the external electron donor comprises at least one silane compound.

[0024] In one aspect of the invention, the external electron donor is selected from alkoxysilanes as shown in structure (I):

[0025] R′ n R'' m Si(OR'') 4-n-m (I); In the formula, R', R'', and R''' represent substituents independently selected from an alkyl group having 1 to 10 carbon atoms, a cyclic group, or an aromatic group, where n and m are integers of 0 to 4, and when n+m<4.

[0026] In one aspect of the invention, the external electron donor is selected from the group comprising tetraethoxysilane, dimethoxydiphenylsilane, dicyclopentyldimethoxysilane, isobutylisopropyldimethoxysilane, trimethoxypropylsilane, isobutyldimethoxymethylsilane, and trimethoxy-2-methylpropylsilane, or mixtures thereof.

[0027] In one aspect of the invention, the external electron donor is dicyclopentyldimethoxysilane.

[0028] In one embodiment of the invention, the molar ratio of silicon in the alkoxysilane to titanium in the Ziegler-Natta catalyst is in the range of 0.1-20.

[0029] In one embodiment of the invention, the molar ratio of silicon in the alkoxysilane to titanium in the Ziegler-Natta catalyst is in the range of 0.25 to 1.

[0030] In one aspect of the invention, the process according to the invention further comprises an alkylaluminum compound as a co-catalyst. Preferably, the co-catalyst is triethylaluminum.

[0031] In one embodiment of the invention, the catalyst concentration is in the range of 0.005 to 0.1 mmol / L, preferably in the range of 0.03 to 0.05 mmol / L.

[0032] In one embodiment of the invention, the concentration of the co-catalyst is in the range of 0.1 to 2 mmol / L. Preferably, the concentration of the co-catalyst is in the range of 0.2 to 1 mmol / L.

[0033] In one embodiment of the invention, the polymerization of ethylene in step a) is operated at a temperature in the range of 60 to 90° C. and a pressure in the range of 1 to 8 bar.

[0034] In one embodiment of the invention, the process further comprises in step-a) adding an alpha olefin having 3 to 10 carbon atoms to the reactor, wherein the concentration of the alpha olefin is in the range of 0.1 to 10 wt% polyethylene.

[0035] In one embodiment of the invention, the molecular weight of the polyethylene prepared by the process according to the invention is in the range of 40,000-300,000 g / mol and the molar molecular weight distribution (Mw / Mn) is in the range of 4-8.

[0036] In one aspect of the invention, the density of the polyethylene prepared from the process according to the invention is 0.940 to 0.965 g / cm 3 The melt flow rate (2.16 kg / 190° C.) is in the range of 0.1 to 30 g / 10 min.

[0037] In one aspect of the invention, the polymerization of ethylene further comprises adding an additive to the polymer mixture selected from a processing aid, a mold release agent, an antioxidant, a light stabilizer, a heat stabilizer, or a mixture thereof.

[0038] In one aspect of the invention, the polyethylene prepared by the process according to the invention can be formed into products by injection molding processes, extrusion blow molding, and rotational molding processes.

[0039] In one aspect of the invention, the polyethylene may be applied to form products or articles including, but not limited to, rope, fiber, or nonwoven fabrics.

[0040] The following examples are intended only to demonstrate one aspect of the present invention and are not intended to limit the scope of the invention in any way.

[0041] Tests to select suitable external electron donors Preparation of comparative samples Hexane (1,000–3,000 mL) was added to the reactor. Triethylaluminum was added at a controlled concentration ranging from 0.2–1.0 mmol / L. PZ-type Ziegler-Natta catalyst (produced by Mitsui Chemicals Inc.) was added at a controlled concentration ranging from 0.01–0.05 mmol / L. Hydrogen gas and ethylene gas were then fed into the reactor. The reaction temperature and pressure were controlled at 60–90°C and 1.0–8.0 bar, respectively, and the reaction time was 2–3 hours. The temperature was then reduced to room temperature. The prepared polymer was subjected to a drying process and extruded into pellets.

[0042] Preparation of samples according to inventions 1 to 12 The samples were prepared in the same manner as described in the comparative samples by adding silane compounds as external electron donors as shown in Table 1.

[0043] Table 1: Testing of external electron donors from comparative and inventive samples [Table 1] *Area under the curve analyzed by gas chromatography-flame ionization detector (GC-FID)

[0044] Molecular weight distribution test The molecular weight distribution was analyzed by gel permeation chromatography (GPC-IR) using three detectors (Polymer Char) according to the following steps: The polymer (4.0–8.0 mg) was added to a vial. Then, 8.0 mL of 1,2,4-trichlorobenzene was added. The sample was injected into a high-pressure liquid chromatograph and heated to 150–160 °C.

[0045] Smoke volume test The amount of smoke was tested by extracting low molecular weight polyethylene from the samples by Soxhlet extraction with hexane (Soxhlet extractor BUCHI B-S11).The low molecular weight polyethylene was then analyzed by gas chromatography (Intuvo 9000 GC system, Agilent Technologies) according to the following steps:

[0046] Approximately 10 g of polymer was added to the thimble, and hexane was added to the Soxhlet for extraction. The extract was analyzed by filtration and chromatography on a DB-5MS UI column (30 m x 0.32 mm x 0.25 micron) at a flow rate of 6.0 mL / sec and a temperature of 300°C. The area under the curve was collected to quantify the amount of smoke.

[0047] From Table 1, it was found that the silane compounds can reduce the molecular weight distribution. Also, the use of dimethoxydiphenylsilane, dicyclopentyldimethoxysilane, isobutylisopropyldimethoxysilane, trimethoxypropylsilane, isobutyldimethoxymethylsilane, tetraethoxysilane, and trimethoxy-2-methylpropylsilane can reduce the amount of smoke generated in the process.

[0048] Testing the appropriate amount of external electron donor Preparation of samples according to inventions 13 to 16 The samples were prepared in the same manner as the samples according to invention 2, and dicyclopentyldimethoxysilane was added to the samples in the concentrations shown in Table 2.

[0049] Table 2: Testing the amount of external electron donor with comparative and inventive samples [Table 2] *Area under the curve analyzed by gas chromatography-flame ionization detector (GC-FID) From Table 2, it can be seen that the molecular weight distribution becomes narrower when the amount of external electron donor is higher. The catalyst concentration is preferably in the range of 0.005 to 0.1 mmol / L, preferably 0.03 to 0.05 mmol / L. The amount of silicon in the silane compound in terms of the molar ratio of titanium in the Ziegler-Natta catalyst (Si / Ti determination) is in the range of 0.1 to 20.0 mol / mol, preferably 0.25 to 1.0 mol / mol.

[0050] From the above, the use of a silane compound as an external electron donor is beneficial for narrowing the molecular weight distribution of polyethylene. Consequently, the polyethylene prepared by the process according to the present invention can be drawn at a faster rate. It can also reduce the amount of smoke generated during the process. Therefore, the polymer according to the present invention is suitable for use in the preparation of industrial-scale fibers. [Preferred embodiment of the invention] The preferred embodiments of the present invention are as set forth in the description of the invention.

Claims

1. 1. A process for preparing polyethylene having a narrow molecular weight distribution, comprising the steps of: a) continuously polymerizing ethylene to produce a polymer slurry by exposing an ethylene stream, hydrogen, a solvent, a titanium-containing Ziegler-Natta catalyst, a co-catalyst, and an external electron donor selected from an alkoxysilane compound in a reactor; b) removing residual reaction gas from the polymer slurry stream obtained from a); and c) separating the polymer stream of b) from said solvent. Equipped with the alkoxysilane compound is selected from the group consisting of tetraethoxysilane, dimethoxydiphenylsilane, dicyclopentyldimethoxysilane, isobutylisopropyldimethoxysilane, trimethoxypropylsilane, isobutyldimethoxymethylsilane, and trimethoxy-2-methylpropylsilane, or a mixture thereof; The molar ratio of silicon in the alkoxysilane to titanium in said Ziegler-Natta catalyst ranges from 0.25 to 1.

2. The external electron donor is selected from alkoxysilanes shown in structure (I): R′ ｎ R′′ ｍ Si(OR′′′) ４－ｎ－ｍ (I) 2. The process of claim 1, wherein R′, R″, and R′″ represent substituents independently selected from alkyl groups having 1 to 10 carbon atoms, or cyclic groups, or aromatic groups, where n and m are integers from 0 to 4, and when n+m<4.

3. 2. The process of claim 1, wherein the external electron donor is dicyclopentyldimethoxysilane.

4. 4. The process of claim 1, wherein the cocatalyst is an alkylaluminum compound.

5. 5. The process of claim 4 wherein the cocatalyst is triethylaluminum.

6. 6. The process of any one of claims 1 to 5, wherein the concentration of the Ziegler-Natta catalyst is in the range of 0.005 to 0.1 mmol / L.

7. 7. The process of claim 6, wherein the concentration of the Ziegler-Natta catalyst ranges from 0.03 to 0.05 mmol / L.

8. 8. The process of any one of claims 1 to 7, wherein the concentration of the co-catalyst is in the range of 0.1 to 2 mmol / L.

9. 9. The process of any one of claims 1 to 8, wherein the concentration of the co-catalyst is in the range of 0.2 to 1 mmol / L.

10. 10. The process of any one of claims 1 to 9, wherein the polymerization of ethylene in step a) is operated at a temperature in the range of from 60 to 90°C and a pressure in the range of from 1 to 8 bar.

11. 11. The process of any one of claims 1 to 10, further comprising adding an alpha olefin having from 3 to 10 carbon atoms into the reactor of step (a).

12. 12. The process of claim 11, wherein the concentration of the alpha olefin ranges from 0.1 to 10% by weight of the polyethylene.

13. 13. The process of any one of claims 1 to 12, wherein the polyethylene prepared from the process has a molecular weight in the range of 40,000 to 300,000 g / mol and a molecular weight distribution (Mw / Mn) in the range of 4 to 8.

14. The density of the polyethylene prepared by the process is 0.940 to 0.965 g / cm ３ and a melt flow rate (2.16 kg / 190°C) in the range of 0.1 to 30 g / 10 min.

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