Polyethylene for Coating and Its Applications and Methods for Preparing Polyethylene

Abstract

This invention relates to the field of high-pressure free radical polymerization, and discloses a coating polyethylene, its application, and a method for preparing polyethylene. The polyethylene contains 0.1-2 wt% of a fraction with a weight-average molecular weight higher than 1,000,000 g / mol; wherein the polyethylene has a molecular weight distribution width index of 7-18. The coating polyethylene has a wide molecular weight distribution, contains a certain amount of ultra-high molecular weight fractions, and has advantages such as high melt strength, good processing fluidity, high long-chain branching degree, high ethylene single-pass conversion rate, and good equipment economy.

Description

Technical Field

[0001] This invention relates to the field of high-pressure free radical polymerization, specifically to a coating polyethylene and its application, as well as a method for preparing polyethylene. Background Technology

[0002] Low-density polyethylene (LDPE) has important applications in coating fields such as pipe coating, cardboard coating, and film coating. It is often used as an inner lining material for packaging materials and also as an adhesive layer material for composite packaging. In addition to high strength, good toughness, and strong adhesion, LPE for coating also requires good processability, such as uniform film formation and fast processing speed.

[0003] Currently, both autoclave and tubular high-pressure polymerization (APPC) processes can be used to produce coated polyethylene. As is well known, during high-pressure polymerization, the reaction follows a free radical polymerization mechanism, and polymer backmixing is beneficial for producing polyethylene with more long-chain branches. Strong polymer backmixing occurs in autoclave processes, creating favorable conditions for the formation of long branches. On the one hand, the high content of long branches provides polyethylene with melt strength and toughness; therefore, polyethylene produced by the autoclave process is superior to polyethylene produced by the tubular high-pressure process in terms of melt strength, minimum coating thickness, and film uniformity. On the other hand, a high content of long branches is detrimental to increasing processing speed; therefore, polyethylene produced by the tubular high-pressure process is superior to polyethylene produced by the autoclave process in terms of processing speed and breaking speed.

[0004] Compared to batch reactors, tubular reactors offer higher polyethylene yields. If coated polyethylene with higher melt strength can be produced under high-load operation in tubular reactors, the economic benefits of the tubular reactors will be significantly improved.

[0005] CN114502602A discloses a method for preparing a polyethylene composition with a wide melt index ratio and narrow molecular weight distribution in a tubular reactor. In this method, the peak temperature in the upstream reaction zone is significantly reduced, which will lead to a reduction in the production load of the tubular reactor, a low single-pass ethylene conversion rate, and a decrease in the economic efficiency of the plant.

[0006] CN107075019A discloses a method for preparing low-density polyethylene for extrusion coating by using oxygen-initiated ethylene free radical polymerization in a tubular reactor. The low-density polyethylene prepared by this method has the advantages of low odor, good sensory properties and high processing speed, but poor film-forming properties and large minimum coating thickness, resulting in large edge loss of the finished product and significant waste.

[0007] Therefore, there is a need for a coating polyethylene with high melt strength and good processing fluidity, which has good economic benefits. Summary of the Invention

[0008] The purpose of this invention is to overcome the problems of low long-chain branching, narrow molecular weight distribution, poor melt strength and processing fluidity of coating polyethylene produced by existing free radical polymerization devices. The invention provides a coating polyethylene, its application, and a method for preparing polyethylene, which has a wide molecular weight distribution, high melt strength, good processing fluidity, and high long-chain branching.

[0009] To achieve the above objectives, the first aspect of the present invention provides a coating polyethylene containing 0.1-2 wt% of a fraction with a weight-average molecular weight higher than 1,000,000 g / mol;

[0010] The polyethylene has a molecular weight distribution width index of 7-18.

[0011] The second aspect of the present invention provides a method for preparing polyethylene, comprising preparing polyethylene by subjecting reactants containing ethylene monomers to a high-pressure free radical polymerization reaction, wherein the polymerization reaction conditions include: a reaction pressure of 150-250 MPa, preferably 180-240 MPa, and a maximum reaction temperature of 290-320°C, preferably 295-310°C.

[0012] According to the present invention, preferably, the high-pressure free radical polymerization reaction is carried out in an olefin free radical polymerization device, the olefin free radical polymerization device having a multi-stage reaction zone composed of a plurality of tubular reactors connected in sequence, the first stage of the multi-stage reaction zone including at least two tubular reactors connected in parallel, the maximum reaction temperature of at least one tubular reactor in the first stage of the reaction zone being less than or equal to 290°C, preferably less than or equal to 270°C, more preferably less than or equal to 250°C.

[0013] According to the present invention, preferably, at least two of the feed inlets of the multi-stage tubular reactor in the first stage reaction zone have different concentrations of chain transfer agent in the reactants.

[0014] A third aspect of the present invention provides an application of the aforementioned polyethylene in the field of coating, preferably in the fields of pipe coating, paperboard coating or film coating.

[0015] The beneficial effects of the present invention through the above technical solution are as follows:

[0016] (1) The polyethylene provided by the present invention has a wide molecular weight distribution and a certain amount of ultra-high molecular weight fraction.

[0017] (2) The polyethylene provided by the present invention preferably has the advantages of high melt strength, good processing fluidity, high long-chain branching degree, high ethylene single-pass conversion rate, and good equipment economy.

[0018] (3) The polyethylene provided by the present invention preferably achieves the control of molecular weight distribution (MWD) and long chain branching degree (LCB) of polyethylene products in different reaction zones by independently controlling the temperature distribution of the first reaction zone of the multi-segment tubular reactor and the concentration of chain transfer agent at the feed inlet during the preparation process. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of an olefin free radical polymerization apparatus.

[0020] Explanation of reference numerals in the attached figures

[0021] 1. Recycled feed compressor; 2. Primary compressor; 3. Secondary compressor

[0022] 4a / 4b, Preheater 5a / 5b / 6a / 6b / 6c, Tubular Reactor 7, High-Pressure Relief Valve

[0023] 8. Cooler 9. High-pressure separator 10. High-pressure circulation loop

[0024] 11. Low-pressure separator 12. Low-pressure circulation loop Detailed Implementation

[0025] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0026] The first aspect of the present invention provides a coating polyethylene, wherein the polyethylene contains 0.1-2 wt% of a fraction with a weight-average molecular weight higher than 1,000,000 g / mol;

[0027] The polyethylene has a molecular weight distribution width index of 7-18.

[0028] In this invention, preferably, the weight-average molecular weight M of the polyethylene as a whole is... w The concentration is between 80,000 and 400,000 g / mol, preferably between 100,000 and 200,000 g / mol.

[0029] According to the present invention, preferably, the polyethylene contains 0.15-1 wt% of a fraction with a weight-average molecular weight higher than 1,000,000 g / mol.

[0030] According to the present invention, preferably, the polyethylene has a molecular weight distribution width index of 8-14.

[0031] In this invention, the molecular weight distribution curve of polyethylene is measured using a differential detector based on high-temperature gel permeation chromatography, and the number-average molecular weight M of polyethylene is obtained according to the definitions of different molecular weight types. n Weight-average molecular weight M w Then calculate the molecular weight distribution width index (PDI). The relationship between the molecular weight distribution width index, weight-average molecular weight, and number-average molecular weight is PDI = M w / M n .

[0032] In this invention, preferably, the melt index of the polyethylene is measured according to GB / T-3682-2000, and the measurement conditions are: temperature 190℃ and load 2.16kg.

[0033] According to the present invention, preferably, the melt index of the polyethylene at 190°C and a load of 2.16 kg is 4-9 g / 10 min, more preferably 5-8 g / 10 min.

[0034] According to the present invention, preferably, the polyethylene is low-density polyethylene with a density of 0.91-0.93 g / cm³. 3 The preferred value is 0.918-0.925 g / cm³. 3 .

[0035] In this invention, preferably, the density of the polyethylene is measured according to the method of GB / 1033-1986.

[0036] The second aspect of the present invention provides a method for preparing polyethylene, comprising preparing polyethylene by subjecting reactants containing ethylene monomers to a high-pressure free radical polymerization reaction, wherein the polymerization reaction conditions include: a reaction pressure of 150-250 MPa, preferably 180-240 MPa, and a maximum reaction temperature of 290-320°C, preferably 295-310°C.

[0037] In this invention, preferably, the olefin free radical polymerization apparatus comprises, in sequence, a circulating feed compressor 1, a primary compressor 2, a secondary compressor 3, a preheater 4a / 4b, a tubular reactor 5a / 5b, a tubular reactor 6a / 6b / 6c, a high-pressure relief valve 7, a cooler 8, a high-pressure separator 9, a high-pressure circulation loop 10, a low-pressure separator 11, and a low-pressure circulation loop 12.

[0038] According to the present invention, preferably, the high-pressure free radical polymerization reaction is carried out in an olefin free radical polymerization device, the olefin free radical polymerization device having a multi-stage reaction zone composed of a plurality of tubular reactors connected in sequence, the first stage of the multi-stage reaction zone including at least two tubular reactors connected in parallel, the maximum reaction temperature of at least one tubular reactor in the first stage of the reaction zone being less than or equal to 290°C, preferably less than or equal to 270°C, more preferably less than or equal to 250°C.

[0039] In this invention, preferably, the inlet temperature of the multi-stage reaction zone of the olefin free radical polymerization device is 120-260℃, more preferably 130-250℃. The lower the inlet temperature, the larger the molecular weight of the generated polyethylene.

[0040] In this invention, as long as the purpose of this invention can be achieved, there is no limit to the number of parallel tubular reactors in the first reaction zone. In some preferred embodiments, the number of parallel reaction material channels is 2-4.

[0041] According to the present invention, preferably, the multi-segment reaction zone contains at least two reaction zones, more preferably 3-7 segments, and more preferably 3-5 segments.

[0042] In this invention, when the ultra-high molecular weight polyethylene generated in the first reaction zone adheres to the wall and forms scale, causing the ethylene conversion rate in the first reaction zone to decrease by at least 10%, the parallel tubular reactors in the first reaction zone are switched to each other.

[0043] In this invention, the parallel tubular reactors in the first reaction zone are divided into a reaction material flow channel for online descaling and a reaction material flow channel for normal production. The temperature distribution of the ethylene-containing reaction material in the first reaction zone is controlled. When the ultra-high molecular weight polyethylene generated in the first reaction zone adheres to the wall and forms scale, it will lead to a decrease in the heat transfer coefficient and production load of the reactor. The switching is achieved by changing the type of cooling medium and the amount of initiator injected into the parallel tubular reactors, switching between the reaction material flow channel for online descaling and the reaction material flow channel for normal production, ensuring the production rate of polyethylene, while not significantly affecting the quality indicators of the product grade.

[0044] According to the present invention, preferably, the polyethylene preparation process includes: adding reactant material containing ethylene monomer through the reactant inlet of a tubular reactor connected in parallel with the first reaction zone; adding an initiator solution to each of the multi-stage reaction zones to initiate the ethylene polymerization reaction of the reactant material; wherein the initiator solution added to each reaction zone contains a solvent and at least one initiator.

[0045] In this invention, the ethylene monomer-containing reactants are divided into multiple streams and fed into parallel tubular reactors in the first reaction zone. The multiple reaction zones are connected in sequence, and the material from the outlet of the first reaction zone is fed into the inlet of the next reaction zone to carry out high-pressure free radical polymerization.

[0046] In this invention, as long as the purpose of this invention can be achieved, there is no restriction on the order of polymerization in each reaction zone. They can be carried out simultaneously or not simultaneously, as long as the polymerization product of the previous reaction zone flows into the next reaction zone as raw material for high-pressure free radical polymerization.

[0047] In this invention, the initiator solution has the conventional meaning in the art, and the initiator solution is used to initiate olefin free radical polymerization.

[0048] In this invention, different initiator solutions are added to multiple reaction zones. By matching the initiator decomposition rules with the reaction temperature distribution rules, different initiators whose decomposition temperatures can cover the reaction temperature range are selected according to the reaction temperature range of each reaction zone. By selecting appropriate initiator formulations for different reaction zones and adapting them to the temperature distribution of the corresponding reaction zones, the efficiency of initiator use can be improved, and the prepared coating polyethylene has higher melt strength.

[0049] According to the present invention, the amount of initiator added can be selected as needed as long as the purpose of the present invention can be achieved. According to a preferred embodiment of the present invention, the mass ratio of the total mass of initiator to the mass of ethylene monomer in the mixture obtained after the initiator enters the reactor is 0.00005-0.001:1, preferably 0.0001-0.0005:1.

[0050] According to the present invention, preferably, the amount of initiator added to the initiator solution in each reaction zone accounts for 5-50 wt% of the total amount of initiator added, and more preferably 10-50 wt%.

[0051] In this invention, preferably, the amount of initiator added to the initiator solution in each parallel tubular reactor in the first reaction zone is not less than 15 wt% of the total amount of initiator added in the first reaction zone.

[0052] According to the present invention, preferably, the solvent is a C4-C25 saturated hydrocarbon and / or cycloalkanes, more preferably a C6-C15 saturated hydrocarbon and / or cycloalkanes.

[0053] According to a preferred embodiment of the present invention, the solvent of the initiator solution is selected from at least one of octane, decane and isododecane.

[0054] According to a preferred embodiment of the present invention, the solvent content in the initiator solution is 60-90% by weight, preferably 70-85% by weight, based on the total weight of the initiator solution.

[0055] According to the present invention, preferably, the initiator is an organic peroxide, and more preferably selected from at least one of peroxide ester, peroxide ketal, peroxide ketone and peroxide carbonate.

[0056] In some preferred embodiments of the present invention, the initiator includes di(2-ethylhexyl) peroxydicarbonate, tert-butyl peroxyisopropyl carbonate, di-tert-butyl peroxide, di-tert-pentyl peroxide, 2,5-dimethyl-2,5-di-tert-butylperoxyhexane, tert-butylcumyl peroxide or didecanoyl peroxide, 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane, tert-pentyl peroxy-2-ethylhexanoate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxydiethylacetate, and so on. At least one of the following: tert-butyl ethyl isobutyrate, tert-butyl peroxy-3,5,5-trimethylhexanoate, 1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-pentyl peroxyneodecanate, tert-pentyl peroxyneodecanate, tert-butyl peroxyneodecanate, tert-butyl permaleate, tert-butyl peroxyneodecanate, tert-butyl peroxyisonononate, tert-butyl peroxybenzoate, methyl isobutyl ketone hydroperoxide, 3,6,9-triethyl-3,6,9-trimethyltriperoxycyclononane, and 2,2-di(tert-butylperoxy)butane.

[0057] According to a preferred embodiment of the present invention, the initiator is selected from at least one of di-tert-butyl peroxide, tert-butyl peroxybenzoate, tert-butyl peroxy-2-ethylhexanoate, tert-pentyl peroxy-2-ethylhexanoate, and tert-butyl peroxyneoplastate.

[0058] According to the present invention, preferably, the initiator content in the initiator solution is 10-40% by weight, based on the total weight of the initiator solution, for example, 10%, 15%, 20%, 25%, 30%, 35%, 40%, and any range of any combination thereof, preferably 15-30% by weight.

[0059] According to the present invention, preferably, a chain transfer agent is added during the preparation of the polyethylene to adjust the molecular weight of the polyethylene. The chain transfer agent is added through the reactant inlet of the parallel tubular reactor in the first reaction zone.

[0060] According to the present invention, preferably, the chain transfer agent is a C2-C12 alkane and / or olefin, preferably selected from at least one of ethane, propane, butane, pentane, hexane, cyclohexane, propylene, 1-butene, 1-pentene, 1-hexene and 1-octene.

[0061] In this invention, more preferably, the chain transfer agent is selected from at least one of ethane, propane, butane, propylene, 1-butene, 1-hexene, and 1-octene.

[0062] According to the present invention, preferably, the mass ratio of the chain transfer agent to the ethylene monomer in the reactants is 0.001-0.5:1, more preferably 0.002-0.05:1.

[0063] According to the present invention, preferably, at least two of the feed inlets of the multi-stage tubular reactor in the first stage reaction zone have different concentrations of chain transfer agent in the reactants.

[0064] In this invention, preferably, the concentration of the chain transfer agent is controlled by changing the feed rate of the chain transfer agent per unit time in the tubular reactor, and the chain transfer agent is added to change the molecular weight of the polymer to be prepared. By controlling the concentration of the chain transfer agent, polyethylene with significantly different molecular weights can be prepared in parallel tubular reactors in the first reaction zone, and the final polyethylene has a wide molecular weight distribution.

[0065] According to the present invention, preferably, the concentration of chain transfer agent in the reactants at the feed inlet of each tubular reactor in the first reaction zone is different.

[0066] According to the present invention, preferably, the ratio of the maximum to the minimum concentration of the chain transfer agent in the reactant at the feed inlet of each tubular reactor in the first reaction zone is 1.01-6:1, for example, 1.01:1, 1.2:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, 5:1, 5.5:1, 6:1, and any range of the above values, preferably 1.2-4:1.

[0067] In this invention, by independently controlling the reaction temperature distribution and chain transfer agent concentration in the first reaction zone of the multi-stage reaction zone, the molecular weight distribution and long-chain branching degree of polyethylene products in different reaction zones can be regulated. The polyethylene prepared has higher melt strength, wider molecular weight distribution, high ethylene single-pass conversion rate, and good equipment economy.

[0068] In this invention, preferably, the polyethylene obtained by the above method contains 0.1-2 wt% of a fraction with a weight-average molecular weight higher than 1,000,000 g / mol, more preferably 0.15-1 wt%.

[0069] The polyethylene has a molecular weight distribution width index of 7-18, preferably 8-14.

[0070] In this invention, preferably, the polyethylene obtained by the above method has a melt index of 4-9 g / 10 min at 190°C and a load of 2.16 kg, more preferably 5-8 g / 10 min. The density of the polyethylene is 0.91-0.93 g / cm³. 3 The preferred value is 0.918-0.925 g / cm³. 3 .

[0071] A third aspect of the present invention provides an application of the aforementioned polyethylene in the field of coating, preferably in the fields of pipe coating, paperboard coating or film coating.

[0072] In this invention, the minimum coating thickness of the polyethylene is 6-20 μm, preferably 8-12 μm. The low minimum coating thickness results in good processing fluidity and high melt strength, making it suitable for various coating applications such as pipe coating, paperboard coating, and film coating.

[0073] In this invention, the coating thickness of the polyethylene is measured using a high-precision thickness gauge.

[0074] In this invention, preferably, the reactants containing ethylene monomers are fed into a primary compressor 2 and a secondary compressor 3, and after thorough mixing, they are divided into multiple streams that pass through a preheater 4a and into a tubular reactor 5a, and then through a preheater 4b and into a tubular reactor 5b. Both tubular reactors 5a and 5b have initiator inlets at their monomer feed ports. An initiator solution is added through the initiator inlets to react with the olefin-containing reactants under high pressure polymerization. The reaction products from tubular reactors 5a and 5b flow into tubular reactors 6a, 6b, and 6c, which are connected in series, to continue high-pressure free radical polymerization. Each of the tubular reactors 6a, 6b, and 6c has an initiator inlet at its feed port. The initiator solutions from the second stage and subsequent reaction zones are introduced through their respective initiator inlets to participate in the corresponding high-pressure free radical polymerization reactions.

[0075] The high-pressure polymerization product flows into the high-pressure separator 9 after passing through the high-pressure relief valve 7 and the cooler 8 to separate into gaseous fraction A and liquid fraction A. Liquid fraction A flows into the low-pressure separator 11 to separate into gaseous fraction B and polymer product. Gaseous fraction A is cooled and separated by the cooler and separator on the high-pressure circulation loop 10 to obtain unreacted monomers that flow into the suction side of the secondary compressor. Gaseous fraction B is cooled and separated by the cooler and separator on the low-pressure circulation loop 12 to obtain unreacted monomers that are compressed by the circulating feed compressor 1 on the low-pressure circulation loop and flow into the suction side of the primary compressor.

[0076] The present invention will be described in detail below through examples and comparative examples. In the following examples and comparative examples, unless specific conditions are specified, conventional conditions or conditions recommended by the manufacturer were followed. Reagents or instruments used, unless the manufacturer is specified, are all conventional products that can be obtained commercially.

[0077] Example 1

[0078] exist Figure 1 The olefin radical polymerization apparatus shown is used to carry out ethylene radical polymerization to prepare polyethylene.

[0079] The diagram shows four reaction zones connected in series. The first reaction zone contains two parallel tubular reactors (Reactor A and Reactor B). The length of the first reaction zone is 560m and the inner diameter is 0.032m. The lengths of the second to fourth reaction zones are all 400m and the inner diameter is 0.045m.

[0080] The chain transfer agent is fed upstream of reactors A and B in stage 1 and thoroughly mixed with the ethylene obtained by preheating. The effluent from the two parallel stage 1 reactors A / B is fed into the inlet of the reaction zone in stage 2. The initiator solution is fed into the inlet of reactors A / B in stage 1 and the inlet of the reaction zones in stages 2 to 4, respectively.

[0081] The inlet ethylene throughput of the two parallel first-stage reactors A / B is 21750 kg / h, respectively.

[0082] Propylene as a chain transfer agent is fed into reactor A of stage 1 at a rate of 63 kg / h.

[0083] Propylene as a chain transfer agent is fed into reactor B of stage 1 at a rate of 162 kg / h.

[0084] The inlet pressure of reactors A and B in the first stage is 220 MPa.

[0085] The initiator feed rate, inlet temperature, and maximum temperature of each reaction zone are shown in Table 1.

[0086] In the following initiators, DTBP is di-tert-butyl peroxide, TBPB is tert-butyl peroxybenzoate, and TBPEH is tert-butyl peroxy-2-ethylhexanoate. The initiator feed amount is the mass of the pure initiator. The solvent in the initiator solution is isododecane, and the mass fraction of the solvent is 80 wt%.

[0087] Table 1

[0088] Initiator feed rate [kg / h] 4.07 10 6.03 5.87 5.52 Inlet temperature [°C] 140 170 201 219 240 Maximum temperature [°C] 240 305 295 295 295

[0089] The initiator in the first reaction zone has a mass ratio of DTBP / TBPB / TBPEH of 1:2:2;

[0090] The initiator in the second reaction zone has a DTBP / TBPB mass ratio of 1:1.

[0091] The initiator in the third reaction zone has a DTBP / TBPB mass ratio of 1:0.1.

[0092] The initiator in the fourth reaction zone is DTBP, which is used to prepare low-density polyethylene.

[0093] Example 2

[0094] Polyethylene was prepared according to the method of Example 1, with the following difference:

[0095] The initiator feed rate, inlet temperature, and maximum temperature adjustment for each reaction zone are shown in Table 2.

[0096] Table 2

[0097] Initiator feed rate [kg / h] 4.45 9.99 5.4 5.7 4.79 Inlet temperature [°C] 170 170 187 223 239 Maximum temperature [°C] 260 305 295 295 295

[0098] The initiator in the first reaction zone has a mass ratio of DTBP / TBPB / TBPEH of 1:2:2;

[0099] The initiator in the second reaction zone has a DTBP / TBPB mass ratio of 1:1.

[0100] The initiator in the third reaction zone has a DTBP / TBPB mass ratio of 1:0.1.

[0101] The initiator in the fourth reaction zone is DTBP, which is used to prepare low-density polyethylene. The solvent in the initiator solution is isododecane, with a solvent mass fraction of 80 wt%, which is also used to prepare low-density polyethylene.

[0102] Example 3

[0103] Polyethylene was prepared according to the method of Example 1, except that the initiator was the same in all four reaction zones.

[0104] The initiator feed rate, inlet temperature, and maximum temperature adjustment for each reaction zone are shown in Table 3.

[0105] Table 3

[0106] Initiator feed rate [kg / h] 4.07 10.01 15.66 25.27 23.11 Inlet temperature [°C] 140 170 201 217 241 Maximum temperature [°C] 240 305 295 295 295

[0107] The initiator in the reaction zones 1 to 4 has a mass ratio of DTBP / TBPB / TBPEH of 1:2:2. The solvent in the initiator solution is isododecane, with a solvent mass fraction of 80wt%, to prepare low-density polyethylene.

[0108] Example 4

[0109] Polyethylene was prepared according to the method of Example 1, with the following difference:

[0110] Propylene as a chain transfer agent is fed into reactor A of stage 1 at a rate of 113 kg / h.

[0111] Propylene as a chain transfer agent is fed into reactor B of stage 1 at a rate of 113 kg / h.

[0112] The initiator feed rate, inlet temperature, and maximum temperature of each reaction zone are adjusted as shown in Table 4. The composition of the initiator solution in each section is the same as in Example 1, and low-density polyethylene is prepared.

[0113] Table 4

[0114] Initiator feed rate [kg / h] 4.09 9.92 6.50 5.87 5.52 Inlet temperature [°C] 140 170 212 219 240 Maximum temperature [°C] 240 305 295 295 295

[0115] Comparative Example 1

[0116] Polyethylene was prepared according to the method of Example 1, with the following difference:

[0117] The first reaction section of the multi-stage tubular reactor has only one jacketed reaction pipe with an inner diameter of 0.045 m and a length of 560 m. The reactor has one reactant inlet and one polymer outlet. The second reaction section begins 560 m downstream of the reactor inlet. The second to fourth reaction sections are each 400 m long and have an inner diameter of 0.045 m. The chain transfer agent is fed downstream of the secondary compressor and thoroughly mixed with fresh ethylene. After being heated in a preheater, it is fed into the tubular reactor.

[0118] The feed rate of propylene as a chain transfer agent to the inlet of the multi-stage tubular reactor is 225 kg / h.

[0119] The initiator feed rate, inlet temperature, and maximum temperature of each reaction zone are adjusted as shown in Table 5. The composition of the initiator solution in each section is the same as in Example 1, and low-density polyethylene is prepared.

[0120] Table 5

[0121] Initiator feed rate [kg / h] 13.55 6.32 6.59 7.58 Inlet temperature [°C] 170 204 218 220 Maximum temperature [°C] 305 295 295 295

[0122] Comparative Example 2

[0123] Polyethylene was prepared according to the method of Comparative Example 1, with the following difference:

[0124] The initiator feed rate, inlet temperature, and maximum temperature of each reaction zone are adjusted as shown in Table 6. The composition of the initiator solution in each section is the same as in Example 1, and low-density polyethylene is prepared.

[0125] Table 6

[0126] Initiator feed rate [kg / h] 5.74 2.76 6.48 7.12 Inlet temperature [°C] 170 209 217 221 Maximum temperature [°C] 260 270 295 295

[0127] Test Example 1

[0128] The number-average molecular weight, weight-average molecular weight, molecular weight distribution index, melt index, long-chain branching degree, density, yield, minimum coating thickness, and processing rate of the prepared polyethylene were tested.

[0129] The number-average molecular weight, weight-average molecular weight, and molecular weight distribution width of polyethylene were measured and calculated using a differential detector in high-temperature gel permeation chromatography.

[0130] The melt flow index of polyethylene is measured according to GB / T-3682-2000, under the following conditions: temperature 190℃ and load 2.16kg.

[0131] The density of polyethylene was measured according to the method in GB / 1033-1986, and the test results are shown in Table 7.

[0132] The coating thickness of polyethylene was measured using a thickness gauge;

[0133] The maximum processing speed of polyethylene was measured by the processing equipment, and the results are shown in Table 8.

[0134] Table 7

[0135]

[0136] Table 8

[0137]

[0138] As can be seen from the results in Tables 7 and 8, the polyethylene for coating prepared in Examples 1-4 of the present invention, under the condition of ensuring a basically equivalent production load of the polymerization unit, can improve the melt strength, reduce the minimum coating thickness, and increase the degree of long chain branching of the polyethylene product by independently controlling the temperature distribution and chain transfer agent concentration in the first reaction zone. This results in better economic benefits.

[0139] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A polyethylene coating, characterized in that, The polyethylene contains 0.15-1 wt% of a fraction with a weight-average molecular weight higher than 1,000,000 g / mol; The polyethylene has a molecular weight distribution width index of 8-14. The method for preparing polyethylene includes preparing it by high-pressure free radical polymerization of reactants containing ethylene monomers. The polymerization reaction conditions include: a reaction pressure of 150-250 MPa and a maximum reaction temperature of 290-320 °C. The high-pressure free radical polymerization reaction is carried out in an olefin free radical polymerization device, which contains multiple tubular reactors connected in sequence to form a multi-stage reaction zone. The first stage of the multi-stage reaction zone includes at least two tubular reactors connected in parallel, and the maximum reaction temperature of at least one tubular reactor in the first stage of the reaction zone is less than or equal to 290°C.

2. The polyethylene according to claim 1, wherein, The polyethylene has a melt index of 4-9 g / 10 min at 190°C and a load of 2.16 kg.

3. The polyethylene according to claim 2, wherein, The polyethylene has a melt index of 5-8 g / 10 min at 190°C and a load of 2.16 kg.

4. The polyethylene according to any one of claims 1-3, wherein, The polyethylene is low-density polyethylene with a density of 0.91-0.93 g / cm³. 3 .

5. The polyethylene according to claim 4, wherein, The polyethylene is low-density polyethylene with a density of 0.918-0.925 g / cm³. 3 .

6. A method for preparing polyethylene according to claim 1, wherein, The method involves preparing the product by high-pressure free radical polymerization of reactants containing ethylene monomers. The polymerization reaction conditions include: a reaction pressure of 150-250 MPa and a maximum reaction temperature of 290-320 °C. The high-pressure free radical polymerization reaction is carried out in an olefin free radical polymerization device, which contains multiple tubular reactors connected in sequence to form a multi-stage reaction zone. The first stage of the multi-stage reaction zone includes at least two tubular reactors connected in parallel, and the maximum reaction temperature of at least one tubular reactor in the first stage of the reaction zone is less than or equal to 290°C.

7. The preparation method according to claim 6, wherein, The polymerization reaction conditions include: a reaction pressure of 180-240 MPa and a maximum reaction temperature of 295-310℃.

8. The preparation method according to claim 6, wherein, The maximum reaction temperature of at least one tubular reactor in the first reaction zone is less than or equal to 270°C.

9. The preparation method according to claim 8, wherein, The maximum reaction temperature of at least one tubular reactor in the first reaction zone is less than or equal to 250°C.

10. The preparation method according to claim 6, wherein, The multi-stage reaction zone contains at least two reaction stages.

11. The preparation method according to claim 10, wherein, The multi-stage reaction zone contains 3-7 reaction stages.

12. The preparation method according to claim 11, wherein, The multi-stage reaction zone contains 3-5 reaction stages.

13. The preparation method according to claim 6, wherein, The polyethylene preparation process includes: adding ethylene monomer-containing reactant material through the reactant inlet of a tubular reactor connected in parallel with the first reaction zone; adding an initiator solution to each of the multi-stage reaction zones to initiate the ethylene polymerization reaction of the reactant material; and the initiator solution added to each reaction zone contains a solvent and at least one initiator.

14. The preparation method according to claim 13, wherein, The ratio of the total mass of the initiator to the mass of the ethylene monomer in the mixture obtained after the initiator enters the reactor is 0.00005-0.001:

1. And / or, the amount of initiator added to the initiator solution in each reaction zone accounts for 5-50 wt% of the total initiator added.

15. The preparation method according to claim 14, wherein, The ratio of the total mass of the initiator to the mass of the ethylene monomer in the mixture obtained after the initiator enters the reactor is 0.0001-0.0005:

1.

16. The preparation method according to claim 13, wherein, The solvent is a C4-C25 saturated hydrocarbon and / or cycloalkanes; And / or, the initiator is an organic peroxide.

17. The preparation method according to claim 16, wherein, The initiator is selected from at least one of peroxy ester, peroxy ketal, peroxy ketone and peroxy carbonate.

18. The preparation method according to claim 13, wherein, The initiator content in the initiator solution is 10-40% by weight, based on the total weight of the initiator solution.

19. The preparation method according to claim 18, wherein, The initiator content in the initiator solution is 15-30% by weight, based on the total weight of the initiator solution.

20. The preparation method according to claim 13, wherein, A chain transfer agent is also added during the preparation of the polyethylene. The chain transfer agent is added through the reactant inlet of the parallel tubular reactor in the first reaction zone.

21. The preparation method according to claim 20, wherein, The chain transfer agent is a C2-C12 alkane and / or olefin; And / or, the mass ratio of the chain transfer agent to the ethylene monomer in the reactants is 0.001-0.5:

1.

22. The preparation method according to claim 21, wherein, The chain transfer agent is selected from at least one of ethane, propane, butane, pentane, hexane, cyclohexane, propylene, 1-butene, 1-pentene, 1-hexene, and 1-octene; And / or, the mass ratio of the chain transfer agent to the ethylene monomer in the reactants is 0.002-0.05:

1.

23. The preparation method according to claim 22, wherein, The chain transfer agent is selected from at least one of ethane, propane, butane, propylene, 1-butene, 1-hexene, and 1-octene.

24. The preparation method according to claim 6, wherein, In the first stage of the multi-stage reaction zone, at least two tubular reactor inlets have different concentrations of chain transfer agent in the reactants.

25. The preparation method according to claim 24, wherein, In the first reaction zone, the concentration of chain transfer agent in the reactants at the feed inlet of each tubular reactor is different.

26. The preparation method according to claim 24, wherein, The ratio of the maximum to the minimum concentration of chain transfer agent in the reactants at the feed inlet of each tubular reactor in the first reaction zone is 1.01-6:

1.

27. The preparation method according to claim 26, wherein, The ratio of the maximum to the minimum concentration of chain transfer agent in the reactants at the feed inlet of each tubular reactor in the first reaction zone is 1.2-4:

1.

28. The application of the polyethylene according to any one of claims 1-5 in the field of coating.

29. The application according to claim 28, wherein, The application of the aforementioned polyethylene in the fields of pipe coating, paperboard coating, or film coating.

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