Polyethylene for coating, application thereof and polyethylene preparation method

Abstract

The invention relates to the field of high-pressure free radical polymerization, and discloses polyethylene for coating, application of the polyethylene and a preparation method of the polyethylene, the polyethylene contains 0.1-2wt% of fractions with the weight-average molecular weight higher than 1000000 g / mol; wherein the polyethylene has a molecular weight distribution width index of 7-18. The polyethylene for coating has wide molecular weight distribution, has a certain amount of ultra-high molecular weight fractions, and has the advantages of high melt strength, good processing fluidity, high long-chain branching degree, high ethylene conversion per pass and good device economy.

Description

Technical Field

[0001] The invention relates to the field of high-pressure free radical polymerization, and in particular to polyethylene for coating, application thereof and a method for preparing polyethylene. Background Art

[0002] Low-density polyethylene has important applications in coating fields such as pipe coating, paperboard coating, and film coating. It is often used as the lining material of packaging materials and is also used as the bonding layer material of composite packaging. In addition to high strength, good toughness, and strong adhesion, low-density polyethylene for coating also requires good processability, such as uniform film formation and fast processing speed.

[0003] At present, polyethylene for coating can be produced by autoclave method and high-pressure tubular method. As is well known, in the high-pressure polymerization process, the reaction process follows the free radical polymerization mechanism, and the backmixing of the polymer is conducive to the preparation of polyethylene with more long chain branches. There is strong polymer backmixing in the autoclave method, and the backmixing creates favorable conditions for the formation of long chain branches. On the one hand, the high content of long chain branches provides melt strength and toughness for polyethylene. Therefore, the polyethylene prepared by the autoclave method is superior to the polyethylene prepared by the high-pressure tubular method in terms of melt strength, minimum coating thickness and film uniformity. On the other hand, a high content of long chain branches is not conducive to improving the processing speed. Therefore, the polyethylene prepared by the high-pressure tubular method is superior to the polyethylene prepared by the autoclave device in terms of processing speed and breaking speed.

[0004] Compared with the kettle process, the tubular process has a higher polyethylene yield. If coating polyethylene with higher melt strength can be produced under high load operation in the tubular process, the economic benefits of the tubular process will be significantly improved.

[0005] CN114502602A discloses a method for preparing a polyethylene composition with a wide melt index ratio and a 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 conversion rate of ethylene, and poor economic benefits of the device.

[0006] CN107075019A discloses a low-density polyethylene for extrusion coating prepared by using oxygen to initiate 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 a large minimum coating thickness, resulting in large trimming losses of the finished product and causing significant waste.

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

[0008] The purpose of the present invention is to overcome the problems of low long-chain branching degree, narrow molecular weight distribution, poor melt strength and processing fluidity of coating polyethylene produced by free radical polymerization device in the prior art, and to provide a coating polyethylene and its application as well as a polyethylene preparation method, wherein the polyethylene has a wide molecular weight distribution, high melt strength, good processing fluidity and high long-chain branching degree.

[0009] In order to achieve the above object, the first aspect of the present invention provides a polyethylene for coating, wherein the polyethylene contains 0.1-2wt% of a fraction having a weight average molecular weight higher than 1000000 g / mol;

[0010] Wherein, 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, which comprises subjecting a reaction material containing an ethylene monomer to a high-pressure free radical polymerization reaction to obtain the polyethylene. 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, which contains a multi-stage reaction zone composed of a plurality of tubular reactors connected in sequence, the first stage reaction zone of the multi-stage reaction zone comprises at least two tubular reactors connected in parallel, and the maximum value of the reaction temperature of at least one tubular reactor in the first stage reaction zone is less than or equal to 290°C, preferably less than or equal to 270°C, and more preferably less than or equal to 250°C.

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

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

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

[0016] (1) The polyethylene provided by the present invention has a relatively 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, high ethylene single-pass conversion rate, and good device economy;

[0018] (3) The polyethylene provided by the present invention preferably achieves regulation of molecular chain structures such as molecular weight distribution (MWD) and long chain branching (LCB) of the polyethylene product in different reaction zones by independently controlling the temperature distribution in the first reaction zone of the multi-stage tubular reactor and the concentration of the chain transfer agent at the feed inlet during the preparation process. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0020] Description of Reference Numerals

[0021] 1. Circulating material 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-circulation circuit

[0024] 11. Low pressure separator 12. Low circulation circuit DETAILED DESCRIPTION

[0025] The endpoints and any values ​​of the ranges disclosed in this article 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 each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0026] The first aspect of the present invention provides a polyethylene for coating, wherein the polyethylene contains 0.1-2wt% of a fraction having a weight average molecular weight higher than 1000000 g / mol;

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

[0028] In the present invention, preferably, the weight average molecular weight M of the polyethylene as a whole is w 80000-400000 g / mol, preferably 100000-200000 g / mol.

[0029] According to the present invention, preferably, the polyethylene contains 0.15-1 wt% of a fraction having 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 breadth index of 8-14.

[0031] In the present invention, the molecular weight distribution curve of polyethylene is measured by using a differential detector according to high temperature gel permeation chromatography, and the number average molecular weight M of polyethylene is obtained according to the definition of different types of molecular weights. 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 the present invention, preferably, the melt index of the polyethylene is measured according to GB / T-3682-2000, and the measuring conditions are: temperature 190° C., load 2.16 kg.

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

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

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

[0036] The second aspect of the present invention provides a method for preparing polyethylene, which comprises subjecting a reaction material containing an ethylene monomer to a high-pressure free radical polymerization reaction to obtain the polyethylene. 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 the present invention, preferably, the olefin free radical polymerization device includes 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-circulation loop 10, a low-pressure separator 11 and a low-circulation loop 12 connected in sequence.

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

[0039] In the present invention, preferably, the inlet temperature of the multi-stage reaction zone of the olefin free radical polymerization device is 120-260° C., preferably 130-250° C., and the lower the inlet temperature, the greater the molecular weight of the generated polyethylene.

[0040] In the present invention, as long as the purpose of the present 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 flow channels is 2-4.

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

[0042] In the present invention, when the ultra-high molecular weight polyethylene produced in the first reaction zone adheres to the wall and scales, causing the ethylene conversion rate of the first reaction zone to drop by at least 10%, the parallel tubular reactors in the first reaction zone are switched to each other.

[0043] In the present 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, and 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 scales, the heat transfer coefficient and production load of the reactor are reduced. The switching is achieved by changing the type of cooling medium and the injection amount of initiator of the parallel tubular reactors. The reaction material flow channel for online descaling and the reaction material flow channel for normal production are switched to ensure the production rate of polyethylene, and at the same time, the quality indicators of the product brand will not be significantly affected.

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

[0045] In the present invention, the reaction material containing ethylene monomer is divided into multiple streams and enters the parallel tubular reactor of the first reaction zone respectively. The multiple reaction zones are connected in sequence, wherein the material at the outlet of the previous reaction zone is fed to the inlet of the next reaction zone, and each undergoes a high-pressure free radical polymerization reaction.

[0046] In the present invention, as long as the purpose of the present invention can be achieved, the order of polymerization in each reaction zone is not limited, and they can be carried out simultaneously or at different times, as long as the polymerization product of the previous reaction zone flows into the next reaction zone as a raw material for high-pressure free radical polymerization.

[0047] In the present invention, the initiator solution has a common meaning in the art, and the initiator solution is used to initiate olefin free radical polymerization reaction.

[0048] In the present invention, different initiator solutions are added to multiple reaction zones respectively, and by matching the decomposition law of the initiator with the reaction temperature distribution law, different initiators whose decomposition temperatures can cover the reaction temperature range are selected according to the reaction temperature range of each reaction zone in the multiple reaction zones. Suitable initiator formulas are selected in different reaction zones to adapt to the temperature distribution of the corresponding reaction zones, thereby improving the use efficiency of the initiator, and the prepared coating polyethylene has higher melt strength.

[0049] According to the present invention, as long as the purpose of the present invention can be achieved, the amount of the initiator added can be selected as needed. According to a preferred embodiment of the present invention, the mass ratio of the total mass of the initiator to the ethylene monomer in the mixed material obtained by mixing the initiator into 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, preferably 10-50 wt %.

[0051] In the present invention, preferably, the amount of initiator added to the initiator solution in each parallel tubular reactor in the first stage reaction zone accounts for no less than 15 wt % of the total amount of initiator added in the first stage reaction zone.

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

[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 wt %, preferably 70-85 wt %, based on the total weight of the initiator solution.

[0055] According to the present invention, preferably, the initiator is an organic peroxide, preferably at least one selected from peroxyester, peroxyketal, peroxyketone and peroxycarbonate.

[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-amyl peroxide, 2,5-dimethyl-2,5-di-tert-butyl peroxyhexane, tert-butyl cumyl peroxide or didecanoyl peroxide, 2,5-dimethyl-2,5-di(2-ethylhexanoyl peroxy)hexane, tert-amyl peroxy-2-ethylhexanoate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxydiethyl acetate, di-tert-butyl peroxide At least one of tert-butyl ethyl isobutyrate, tert-butyl peroxy-3,5,5-trimethylhexanoate, 1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-amyl peroxyneodecanoate, tert-amyl peroxypivalate, tert-butyl peroxyneodecanoate, tert-butyl permaleate, tert-butyl peroxypivalate, tert-butyl peroxyisononanoate, 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 perbenzoate, tert-butyl peroxy-2-ethylhexanoate, tert-amyl peroxy-2-ethylhexanoate and tert-butyl peroxypivalate.

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

[0059] According to the present invention, preferably, a chain transfer agent is added to the polyethylene during the preparation process to adjust the molecular weight of the polyethylene, and the chain transfer agent is added through the reactant feed port 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 alkene, preferably at least one selected from ethane, propane, butane, pentane, hexane, cyclohexane, propylene, 1-butene, 1-pentene, 1-hexene and 1-octene.

[0061] In the present invention, it is further preferred that the chain transfer agent is at least one selected from 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 reaction material is 0.001-0.5:1, preferably 0.002-0.05:1.

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

[0064] In the present invention, preferably, the concentration of the chain transfer agent is controlled by changing the feed amount per unit time of the chain transfer agent 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 the parallel tubular reactors in the first reaction zone, and the polyethylene finally obtained has a wider molecular weight distribution.

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

[0066] According to the present invention, preferably, the ratio of the maximum and minimum values ​​of the chain transfer agent concentration in the reaction material 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 the range consisting of any of the above values, preferably 1.2-4:1.

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

[0068] In the present invention, preferably, the polyethylene prepared by the above method contains 0.1-2wt% of a fraction having a weight average molecular weight higher than 1000000 g / mol, preferably 0.15-1wt%;

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

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

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

[0072] In the present invention, the minimum coating thickness of the polyethylene is 6-20 μm, preferably 8-12 μm, the minimum coating thickness is low, the processing fluidity is good, the melt strength is high, and it is suitable for various coating fields such as pipeline coating, paperboard coating, and film coating.

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

[0074] In the present invention, preferably, the reaction material containing ethylene monomer enters through the primary compressor 2 and the secondary compressor 3, is fully mixed and divided into multiple streams and enter the tubular reactor 5a through the preheater 4a, and enters the tubular reactor 5b through the preheater 4b. The reaction monomer flow feed port ends of the tubular reactor 5a and the tubular reactor 5b are both provided with initiator feed ports. An initiator solution is added through the initiator feed port to carry out high-pressure polymerization reaction with the reaction material containing olefins. The reaction products of the tubular reaction zones 5a and 5b flow into the tubular reactors 6a, 6b and 6c connected in series to continue high-pressure free radical polymerization. The feed ports of the tubular reactors 6a, 6b and 6c are each provided with an initiator feed port. The initiator solutions of the second and subsequent reaction zones are respectively introduced through the initiator feed ports to participate in the corresponding high-pressure free radical polymerization reaction.

[0075] The high-pressure polymerization product passes through the high-pressure relief valve 7 and the cooler 8 in sequence and flows into the high-pressure separator 9 to be separated into a gaseous fraction A and a liquid fraction A. The liquid fraction A flows into the low-pressure separator 11 to be separated into a gaseous fraction B and a polymer product. The gaseous fraction A is cooled and separated by the cooler and separator on the high circulation loop 10 to obtain unreacted monomers, which flow into the suction side of the secondary compressor. The gaseous fraction B is cooled and separated by the cooler and separator on the low circulation loop 12 to obtain unreacted monomers, which are compressed by the circulating material compressor 1 on the low 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, if no specific conditions are specified, the conditions are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be obtained through commercial channels.

[0077] Example 1

[0078] exist Figure 1 In the olefin free radical polymerization apparatus shown, ethylene free radical polymerization is carried out to produce polyethylene.

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

[0080] The chain transfer agent is fed to the upstream of the first stage reactor A and B and fully mixed with the ethylene heated by the preheater, and the outlet materials of the two parallel first stage reactors A / B are fed to the inlet of the second stage reaction zone. The initiator solution is fed to the inlet of the first stage reactor A / B and the inlet of the second to fourth stage reaction zones respectively.

[0081] The inlet ethylene throughputs of the two parallel first stage reactors A / B are: 21750 kg / h;

[0082] Propylene feed as chain transfer agent to the first stage reactor A: 63 kg / h;

[0083] Propylene feed as chain transfer agent to the first stage reactor B: 162 kg / h;

[0084] The inlet pressure of the first stage reactor A / B is 220 MPa.

[0085] The initiator feed amount, reaction zone 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 perbenzoate, 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 80wt%.

[0087] Table 1

[0088] The i-th reaction zone 1A 1B 2 3 4 Initiator feed rate [kg / h] 4.07 10 6.03 5.87 5.52 Inlet temperature [℃] 140 170 201 219 240 Maximum temperature [℃] 240 305 295 295 295

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

[0090] The mass ratio of initiator: DTBP / TBPB in the second reaction zone is 1:1;

[0091] The mass ratio of initiator: DTBP / TBPB in the third reaction zone is 1:0.1;

[0092] The initiator of 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, except that:

[0095] The initiator feed amount, reaction zone inlet temperature and maximum temperature of each reaction zone are adjusted as shown in Table 2.

[0096] Table 2

[0097] The i-th reaction zone 1A 1B 2 3 4 Initiator feed rate [kg / h] 4.45 9.99 5.4 5.7 4.79 Inlet temperature [℃] 170 170 187 223 239 Maximum temperature [℃] 260 305 295 295 295

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

[0099] The mass ratio of initiator: DTBP / TBPB in the second reaction zone is 1:1;

[0100] The mass ratio of initiator: DTBP / TBPB in the third reaction zone is 1:0.1;

[0101] The initiator in the fourth reaction zone is DTBP, and low-density polyethylene is prepared. The solvent in the initiator solution is isododecane, and the mass fraction of the solvent is 80wt%, and low-density polyethylene is prepared.

[0102] Example 3

[0103] Polyethylene was prepared according to the method of Example 1, except that: the initiators of the four reaction zones were the same;

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

[0105] Table 3

[0106] The i-th reaction zone 1A 1B 2 3 4 Initiator feed rate [kg / h] 4.07 10.01 15.66 25.27 23.11 Inlet temperature [℃] 140 170 201 217 241 Maximum temperature [℃] 240 305 295 295 295

[0107] The mass ratio of initiator: DTBP / TBPB / TBPEH in the first to fourth reaction zones is 1:2:2, the solvent in the initiator solution is all isododecane, and the mass fraction of the solvent is 80wt%, so as to prepare low-density polyethylene.

[0108] Example 4

[0109] Polyethylene was prepared according to the method of Example 1, except that:

[0110] Propylene feed as chain transfer agent to the first stage reactor A: 113 kg / h;

[0111] Propylene feed as chain transfer agent to the first stage reactor B: 113 kg / h;

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

[0113] Table 4

[0114] The i-th reaction zone 1A 1B 2 3 4 Initiator feed rate [kg / h] 4.09 9.92 6.50 5.87 5.52 Inlet temperature [℃] 140 170 212 219 240 Maximum temperature [℃] 240 305 295 295 295

[0115] Comparative Example 1

[0116] Polyethylene was prepared according to the method of Example 1, except that:

[0117] The first reaction zone of the multi-stage tubular reactor has only one jacketed reaction pipeline with an inner diameter of 0.045m and a length of 560m. The multi-stage tubular reactor is provided with one reactant feed port and one polymer discharge port. The second reaction zone starts 560m downstream of the inlet of the multi-stage tubular reactor. The lengths of the second to fourth reaction zones are all 400m, and the inner diameters are all 0.045m. The chain transfer agent is fed to the downstream of the secondary compressor and fully mixed with the fresh ethylene, and then fed into the tubular reactor after being heated by the preheater;

[0118] Propylene as chain transfer agent fed to the inlet of the multi-stage tubular reactor: 225 kg / h;

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

[0120] Table 5

[0121] The i-th reaction zone 1 2 3 4 Initiator feed rate [kg / h] 13.55 6.32 6.59 7.58 Inlet temperature [℃] 170 204 218 220 Maximum temperature [℃] 305 295 295 295

[0122] Comparative Example 2

[0123] Polyethylene was prepared according to the method of Comparative Example 1, except that:

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

[0125] Table 6

[0126] The i-th reaction zone 1 2 3 4 Initiator feed rate [kg / h] 5.74 2.76 6.48 7.12 Inlet temperature [℃] 170 209 217 221 Maximum temperature [℃] 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, output, 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 by high temperature gel permeation chromatography using a differential detector;

[0130] The melt index of polyethylene is measured according to GB / T-3682-2000, and the measurement conditions are: temperature 190°C, load 2.16 kg;

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

[0132] The coating thickness of polyethylene was measured by 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] It can be seen from the results of Table 7 and Table 8 that the polyethylene for coating prepared in Examples 1-4 of the present invention, while ensuring substantially equivalent production load conditions for the polymerization unit, independently controls the temperature distribution in the first reaction zone and the concentration of the chain transfer agent to control the molecular weight distribution (MWD) and the amount of long chain branching (LCB) of the polyethylene product, thereby improving the melt strength of the product, reducing the minimum coating thickness, and increasing the degree of long chain branching, thereby achieving better economic benefits.

[0139] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A polyethylene for coating, characterized in that: The polyethylene contains 0.1-2wt% of a fraction having 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.

2. The polyethylene according to claim 1, wherein The polyethylene contains 0.15-1wt% of a fraction having a weight average molecular weight higher than 1,000,000 g / mol; Preferably, the polyethylene has a molecular weight distribution breadth index of 8-14.

3. The polyethylene according to claim 1 or 2, wherein The polyethylene has a melt index of 4-9 g / 10 min, preferably 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 to 3, wherein The polyethylene is low-density polyethylene with a density of 0.91-0.93 g / cm 3 , preferably 0.918-0.925 g / cm 3 .

5. A method for preparing polyethylene, wherein: The method comprises subjecting the reaction materials containing ethylene monomer to high pressure free radical polymerization, wherein the polymerization conditions include: the reaction pressure is 150-250MPa, preferably 180-240MPa, and the maximum reaction temperature in the polymerization reaction is 290-320°C, preferably 295-310°C.

6. The preparation method according to claim 5, wherein: The high-pressure free radical polymerization reaction is carried out in an olefin free radical polymerization device, which comprises a multi-stage reaction zone composed of a plurality of tubular reactors connected in sequence, wherein the first stage reaction zone of the multi-stage reaction zone comprises at least two tubular reactors connected in parallel, and the maximum reaction temperature of at least one tubular reactor in the first stage reaction zone is less than or equal to 290° C., preferably less than or equal to 270° C., and more preferably less than or equal to 250° C.; Preferably, the multi-stage reaction zone contains at least two reaction zones, preferably 3-7 stages, more preferably 3-5 stages.

7. The preparation method according to claim 6, wherein: The polyethylene preparation process comprises: adding a reaction material containing ethylene monomer through a reactant feed port of a tubular reactor connected in parallel to the first reaction zone, and respectively adding an initiator solution to each reaction zone of the multiple reaction zones to initiate the reaction material to perform an ethylene polymerization reaction, wherein the initiator solution added to each reaction zone respectively contains a solvent and at least one initiator; Preferably, the mass ratio of the total mass of the initiator to the ethylene monomer in the mixed material obtained by mixing the initiator into the reactor is 0.00005-0.001:1, preferably 0.0001-0.0005:1; Preferably, the amount of initiator added to the initiator solution in each reaction zone accounts for 5-50wt% of the total amount of initiator added; Preferably, the solvent is a C4-C25 saturated hydrocarbon and / or cycloalkane; Preferably, the initiator is an organic peroxide, preferably at least one selected from peroxyesters, peroxyketals, peroxyketones and peroxycarbonates; Preferably, the content of the initiator in the initiator solution is 10-40 wt %, preferably 15-30 wt %, based on the total weight of the initiator solution.

8. The preparation method according to claim 6 or 7, wherein: A chain transfer agent is also added during the preparation of the polyethylene, and the chain transfer agent is added through the reactant feed port of the parallel tubular reactor in the first reaction zone; Preferably, the chain transfer agent is a C2-C12 alkane and / or alkene, preferably at least one selected from ethane, propane, butane, pentane, hexane, cyclohexane, propylene, 1-butene, 1-pentene, 1-hexene and 1-octene, more preferably at least one selected from ethane, propane, butane, propylene, 1-butene, 1-hexene and 1-octene; Preferably, the mass ratio of the chain transfer agent to the ethylene monomer in the reaction mass is 0.001-0.5:1, preferably 0.002-0.05:

1.

9. The preparation method according to claim 8, wherein: In the first reaction zone of the multi-stage tubular reactor, the concentration of chain transfer agent in the reaction materials at the feed inlets of at least two tubular reactors is different; Preferably, the concentration of chain transfer agent in the reaction material at the feed inlet of each tubular reactor in the first stage reaction zone is different; Preferably, the ratio of the maximum value to the minimum value of the chain transfer agent concentration in the reaction material at the feed inlet of each tubular reactor in the first stage reaction zone is 1.01-6:1, preferably 1.2-4:

1.

10. Use of the polyethylene according to any one of claims 1 to 4 in the field of coating, preferably in the field of pipe coating, paperboard coating or film coating.

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