Preparation method of medical-grade low-density polyethylene resin

Abstract

The invention provides a preparation method of medical-grade low-density polyethylene resin. The addition amount of the oxygen initiator is gradually reduced along with the progressive increase of the sequence of the reaction zones, so that the temperature peak of each reaction zone is gradually reduced along with the progressive increase of the sequence of the reaction zones, the temperature of the rear-section reaction zone in which low-molecular-weight components are intensively generated is reduced, the branching reaction rate at low temperature is reduced, and particularly, the branching rate of short-chain branches is reduced; therefore, the branching degree of the low-molecular-weight component is reduced; meanwhile, the adding proportion of the telomeric agent is increased along with the increasing sequence of the reaction zones, so that the reaction of transferring a chain to a polymer in the front reaction zone in which the high molecular weight components are intensively generated is more frequent, the growth of the high molecular weight components is promoted, and the relative content of the low molecular weight components is reduced under the condition of a certain conversion rate. By synthesizing the method, the purpose of inhibiting the generation of low-molecular-weight and high-branching-degree soluble substances is achieved, and the medical-grade low-density polyethylene resin material is prepared.

Description

Technical Field

[0001] The invention belongs to the field of olefin polymerization, and particularly relates to a method for preparing a medical-grade low-density polyethylene resin. Background Art

[0002] Medical polyethylene is a plastic obtained by the polymerization of ethylene. It has excellent mechanical strength, chemical stability and biocompatibility. It is odorless, non-toxic, and has no adverse effects when implanted in the body. It is widely used in the medical field.

[0003] CN108456270A proposes adding a catalyst to a fluidized bed reactor to carry out ethylene polymerization to prepare medical polyethylene, and then performing a devolatilization process on the obtained polyolefin resin. This method uses a metal catalyst, and although the devolatilization process is performed during the post-processing process, a small amount of the metal catalyst will still remain in the polymer, which cannot meet the cleanliness requirements of medical-grade materials.

[0004] In comparison, high-pressure continuous gas-phase bulk free radical polymerization of ethylene does not require the use of a catalyst, and the resulting low-density polyethylene product has a high degree of cleanliness. However, since medical-grade resins are in direct contact with drugs and liquid medicines, it is required that the soluble components in the resin material cannot dissolve into the liquid medicine or the human body. Studies have shown that solubles are a class of low molecular weight, high branching components. Therefore, controlling the content of such low molecular weight, high branching components is a key factor in the use of low-density polyethylene as a medical product.

[0005] The method described in "Zhao Dongbo; Sun Jianmin, Industrial Development of Medical Grade LDPE Resin LD26F. Modern Plastic Processing Applications 2015, 27(1), 29-32" uses propionaldehyde with a stronger chain transfer coefficient as a telogen to reduce the content of soluble matter. The stronger the chain transfer coefficient of the telogen, the lower the branching degree of the polymer. However, the telogen mainly inhibits the transfer of long chain branches and has almost no effect on short chain branches, making it difficult to significantly reduce the branching degree of the soluble matter. Summary of the invention

[0006] Based on this, it is necessary to provide a method for preparing medical-grade low-density polyethylene resin to address the above problems, which can not only reduce the content of low molecular weight components, but also effectively inhibit the degree of branching, thereby achieving the purpose of reducing soluble matter.

[0007] A method for preparing a medical-grade low-density polyethylene resin comprises the following steps:

[0008] (a) conducting a high-pressure free radical polymerization reaction of ethylene monomer in a tubular reactor having at least two reaction zones connected in series;

[0009] (b) using oxygen and a liquid initiator to prepare a mixed initiator, wherein the oxygen initiator is added to the reaction zones other than the last reaction zone, and the amount of the oxygen initiator added decreases as the order of the reaction zones increases;

[0010] (c) injecting a telogen into at least one reaction zone, wherein the addition ratio of the telogen into the reaction zone into which the telogen is injected increases with increasing order of the reaction zones.

[0011] The reaction zones of the ethylene high-pressure free radical polymerization tubular reactor are divided in sequence according to the material flow direction. At the same time, the reaction tube can be divided into reaction zones according to the spatially separated initiator injection points to achieve the division of reaction zones and the sequential connection of all reaction zones.

[0012] In the steps, all ethylene monomers are injected from the inlet of the first reaction zone of the tubular reactor. In this case, since the fluid inside the high-pressure tubular reactor is close to an ideal plug flow, the residence time of all ethylene molecules in the reactor can be considered to be the same.

[0013] In the above steps, the ethylene monomer may also be injected from the inlet of the first reaction zone and at least one of the remaining reaction zones. In this case, the residence time of the ethylene monomer injected at different positions is different.

[0014] In the steps, the oxygen initiator is high-purity oxygen or air, and the liquid initiator is one or more of an azo compound or a peroxide. Compared with high-purity oxygen, air is a cheaper oxygen initiator and can achieve the same oxygen initiation effect as high-purity oxygen. When air is used, the gas flow rate is about 5 times higher than that when high-purity oxygen is used. When the liquid initiator adopts a peroxide, peroxyesters, peroxyketals, peroxyketones and peroxycarbonates can be used, such as di(2-ethylhexyl) peroxydicarbonate, tert-butyl pivalate peroxide, dicyclohexyl peroxydicarbonate, diacetyl peroxydicarbonate, tert-butyl peroxyisopropyl carbonate, di-sec-butyl peroxydicarbonate, di-tert-butyl peroxide, di-tert-amyl peroxide, tert-butyl cumyl peroxide, 2,5-dimethyl 2,5-di(tert-butyl peroxy)hex-3-yne, 1,3-diisopropyl monohydroperoxide or tert-butyl hydroperoxide, didecanoyl peroxide, 2,5-dimethyl 2,5-di(2-ethylhexanoyl peroxy)hexane, tert-amyl peroxide 2-ethylhexanoate, peroxy The liquid initiator may be tert-butyl peroxide, ... Preferably, the oxygen initiator is air, and the liquid initiator is any combination of one or more of tert-butyl peroxide pivalate, di-tert-butyl peroxide, tert-butyl peroxyneodecanoate, and tert-butyl perbenzoate.

[0015] In the step, the amount of liquid initiator added is feedback-controlled by the liquid initiator injection pump according to the thermocouples in each reaction zone. A plurality of thermocouples are arranged along the tube of the high-pressure polymerization tubular reactor to monitor the temperature in the reactor. The liquid initiator can be injected into each reaction zone, and the liquid initiator injection pump is feedback-controlled according to the thermocouples in each zone to achieve the peak temperature required by the process.

[0016] In the step, the telogen is any combination of one or more of hydrogen, aliphatic hydrocarbons, olefins, ketones, aldehydes and aliphatic aldehydes. The telogens that can be used are hydrocarbons such as propane, butane, pentane, hexane, cyclohexane, propylene, 1-butene, 1-pentene or 1-hexene; ketones such as acetone, methyl ethyl ketone (2-butyl ketone), methyl isobutyl ketone, methyl isoamyl ketone, diethyl ketone or diamyl ketone; aldehydes such as formaldehyde, acetaldehyde or propionaldehyde; and saturated fatty alcohols such as methanol, ethanol, propanol, isopropanol or butanol. Preferably, the telogen is any combination of one or more of propionaldehyde, propylene, isobutylene, propane and isobutane.

[0017] In the steps, the total mass of the telogen added to each reaction zone does not exceed one thousandth of the total mass of the ethylene monomer injected into each reaction zone. The telogen is also called a molecular weight regulator. The addition of the telogen can transfer a part of the active chains to the telogen molecules, thereby inhibiting the transfer of the active chains to the polymer, reducing the branching degree of the long chain branches and thus reducing the molecular weight. Therefore, the amount of the telogen added should not be too much to avoid affecting the growth of the polymer.

[0018] The method for preparing medical-grade low-density polyethylene resin provided by the present invention has the following beneficial effects: (1) the amount of oxygen initiator added decreases with the increasing order of the reaction zones, so that the temperature peak of each reaction zone decreases with the increasing order of the reaction zones, the temperature of the rear reaction zone where low molecular weight components are concentrated is reduced, the branching reaction rate is reduced at low temperature, especially the short chain branching rate is reduced, thereby reducing the branching degree of the low molecular weight components; (2) the addition ratio of the telogen increases with the increasing order of the reaction zones, so that the chain transfer to the polymer reaction in the front reaction zone where high molecular weight components are concentrated is more frequent, the growth of high molecular weight components is promoted, and the relative content of low molecular weight components is reduced under the condition of a certain conversion rate. The above methods are combined to achieve the purpose of inhibiting the generation of low molecular weight, high branching degree soluble substances, and prepare medical-grade low-density polyethylene resin materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a schematic diagram of a method for preparing a medical-grade low-density polyethylene resin according to an embodiment of the present invention, wherein the number of reactor partitions is 3.

[0020] The reference numerals in the figure are: 1 low-pressure compressor, 2 / 11 high-pressure compressor, 3 high-pressure free radical polymerization tubular reactor, 4 cooling water jacket, 5 reactor outlet valve, 6 cooler, 7 high-pressure separator, 8 high-pressure circulation loop, 9 low-pressure separator, 10 low-pressure circulation loop. DETAILED DESCRIPTION

[0021] The present invention is described in detail below in conjunction with specific embodiments. It should be noted that the following embodiments are only used to illustrate the invention, but do not limit the scope of the present invention in any form. In addition, it should be noted that those skilled in the art can make various modifications and improvements to the present invention. These all belong to the protection scope of the present invention.

[0022] First, combine Figure 1 The preparation method of the medical-grade low-density polyethylene resin provided by the present invention is described in detail.

[0023] like Figure 1 As shown, a method for preparing a medical grade low-density polyethylene resin comprises the following steps:

[0024] (a) conducting a high-pressure free radical polymerization reaction of ethylene monomer in a tubular reactor having at least two reaction zones connected in series;

[0025] (b) using oxygen and a liquid initiator to prepare a mixed initiator, wherein the oxygen initiator is added to the reaction zones other than the last reaction zone, and the amount of the oxygen initiator added decreases as the order of the reaction zones increases;

[0026] (c) injecting a telogen into at least one reaction zone, wherein the addition ratio of the telogen into the reaction zone into which the telogen is injected increases with increasing order of the reaction zones.

[0027] The ethylene monomer is all injected from the inlet of the first reaction zone of the tubular reactor, or injected from the inlet of the first reaction zone and at least one of the remaining reaction zones.

[0028] The oxygen initiator is high-purity oxygen or air, and the liquid initiator is one or more of an azo compound or a peroxide. The preferred oxygen initiator is air, the preferred liquid initiator is a peroxide, and the more preferred liquid initiator is any combination of one or more of tert-butyl peroxide pivalate, di-tert-butyl peroxide, tert-butyl peroxyneodecanoate, and tert-butyl peroxybenzoate.

[0029] The telogen is one or more of hydrogen, aliphatic hydrocarbons, olefins, ketones, aldehydes and aliphatic aldehydes. The preferred telogen is any combination of one or more of propionaldehyde, propylene, isobutylene, propane and isobutane, and the more preferred telogen is propionaldehyde.

[0030] The total mass of the telogen added to each reaction zone does not exceed 1 / 1000 of the total mass of the ethylene monomer injected into each reaction zone. Preferably, the total mass of the telogen is 1 / 5000 to 1 / 1000 of the total mass of the ethylene monomer injected into each reaction zone.

[0031] The present invention will be further described below by way of examples.

[0032] Example 1

[0033] The present invention utilizes Figure 1 The ethylene high-pressure free radical polymerization tubular reactor and polymerization process shown are used to prepare medical grade low-density polyethylene resin. The reactor is divided into three reaction zones, and the total length of the reactor is 1510m. The first and second reaction zones have ethylene monomer and telogen injection ports, and all reaction zones have initiator injection ports. The ethylene feed rate of the first reaction zone is 27600kg / h, tert-butyl peroxide pivalate (abbreviated as TBPV) is used as a liquid peroxide initiator, the injection rate of TBPV is 2.5kg / h, and air is used as an oxygen initiator, and the air injection rate is 1.0m 3 / h, propionaldehyde was used as a polymerization agent, the propionaldehyde addition amount was 6kg / h, and the initial initiation temperature of the first reactor was set to 170°C. The ethylene feed rate of the second reaction zone was 18400kg / h, the TBPV injection rate was 2.5kg / h, and the air injection rate was 0.85m 3 / h, the amount of propionaldehyde added is 18kg / h, and the initial temperature of the second reactor is set to 180°C. The third reaction zone uses di-tert-butyl peroxide (DTBP for short) as a liquid peroxide initiator, the injection amount of DTBP is 0.05kg / h, the third reaction zone has no telogen injection port, and the initial temperature of the third reactor is set to 277°C. In this embodiment, the total amount of propionaldehyde added is 24kg / h, of which the proportion of propionaldehyde added in the first reaction zone to the total amount is 25%, that is, 6kg / h, and the proportion of propionaldehyde added in the second reaction zone to the total amount is 75%, that is, 18kg / h.

[0034] The polymer situation at the outlet was obtained by simulating the deterministic model of an ethylene high-pressure free radical polymerization tubular reactor. Table 1 shows the number average molecular weight (Mn), weight average molecular weight (Mw), molecular weight distribution index (PDI), low molecular weight component content (Mw content below 1500 and below 2500), high molecular weight component content (Mw content above 60000 and above 70000) and ethylene conversion of the low density polyethylene product.

[0035] Example 2

[0036] The present invention utilizes Figure 1 The ethylene high pressure free radical polymerization tubular reactor and polymerization process shown are used to prepare medical grade low density polyethylene resin. The reactor is divided into three reaction zones, and the total length of the reactor is 1510m. The first and second reaction zones have ethylene monomer and telogen injection ports, and all reaction zones have initiator injection ports. The ethylene feed rate of the first reaction zone is 27600kg / h, TBPV is used as a liquid peroxide initiator, the injection rate of TBPV is 2.5kg / h, and air is used as an oxygen initiator, and the air injection rate is 1.0m 3 / h, the amount of telogen added to the first reaction zone was 0, and the initial initiation temperature of the first reactor was set to 170°C. The ethylene feed rate of the second reaction zone was 18400 kg / h, the injection rate of TBPV was 2.5 kg / h, and the air injection rate was 0.85 m 3 / h, the amount of propionaldehyde added is 24kg / h, and the initial temperature of the second reactor is set to 180°C. DTBP is used as a liquid peroxide initiator in the third reaction zone, the injection amount of DTBP is 0.05kg / h, there is no telogen injection port in the third reaction zone, and the initial temperature of the third reactor is set to 277°C. In this embodiment, the total amount of propionaldehyde added is 24kg / h, wherein the proportion of propionaldehyde added in the first reaction zone to the total amount is 0, and the proportion of propionaldehyde added in the second reaction zone to the total amount is 100%, that is, 24kg / h.

[0037] The polymer situation at the outlet was obtained by simulating the deterministic model of an ethylene high-pressure free radical polymerization tubular reactor. Table 1 shows the number average molecular weight (Mn), weight average molecular weight (Mw), molecular weight distribution index (PDI), low molecular weight component content (Mw content below 1500 and below 2500), high molecular weight component content (Mw content above 60000 and above 70000) and ethylene conversion of the low density polyethylene product.

[0038] Example 3

[0039] The difference between Example 3 and Example 1 is that isobutylene is used as the telogen. In this example, the total amount of propylene added is 24 kg / h, of which the amount of propylene added to the first reaction zone accounts for 25% of the total amount, i.e., 6 kg / h, and the amount of propylene added to the second reaction zone accounts for 75% of the total amount, i.e., 18 kg / h.

[0040] The polymer situation at the outlet was obtained by simulating the deterministic model of an ethylene high-pressure free radical polymerization tubular reactor. Table 1 shows the number average molecular weight (Mn), weight average molecular weight (Mw), molecular weight distribution index (PDI), low molecular weight component content (Mw content below 1500 and below 2500), high molecular weight component content (Mw content above 60000 and above 70000) and ethylene conversion of the low density polyethylene product.

[0041] Example 4

[0042] The difference between Example 4 and Example 1 is that propylene is used as the telogen. In this example, the total amount of propylene added is 24 kg / h, of which the amount of propylene added to the first reaction zone accounts for 25% of the total amount, i.e., 6 kg / h, and the amount of propylene added to the second reaction zone accounts for 75% of the total amount, i.e., 18 kg / h.

[0043] The polymer situation at the outlet was obtained by simulating the deterministic model of an ethylene high-pressure free radical polymerization tubular reactor. Table 1 shows the number average molecular weight (Mn), weight average molecular weight (Mw), molecular weight distribution index (PDI), low molecular weight component content (Mw content below 1500 and below 2500), high molecular weight component content (Mw content above 60000 and above 70000) and ethylene conversion of the low density polyethylene product.

[0044] Example 5

[0045] The difference between Example 5 and Example 1 is that isobutane is used as the telogen. In this example, the total amount of propane added is 24 kg / h, of which the amount of propane added to the first reaction zone accounts for 25% of the total amount, i.e., 6 kg / h, and the amount of propane added to the second reaction zone accounts for 75% of the total amount, i.e., 18 kg / h.

[0046] The polymer situation at the outlet was obtained by simulating the deterministic model of an ethylene high-pressure free radical polymerization tubular reactor. Table 1 shows the number average molecular weight (Mn), weight average molecular weight (Mw), molecular weight distribution index (PDI), low molecular weight component content (Mw content below 1500 and below 2500), high molecular weight component content (Mw content above 60000 and above 70000) and ethylene conversion of the low density polyethylene product.

[0047] Example 6

[0048] The difference between Example 6 and Example 1 is that propane is used as the telogen. In this example, the total amount of propane added is 24 kg / h, of which the amount of propane added to the first reaction zone accounts for 25% of the total amount, i.e., 6 kg / h, and the amount of propane added to the second reaction zone accounts for 75% of the total amount, i.e., 18 kg / h.

[0049] The polymer situation at the outlet was obtained by simulating the deterministic model of an ethylene high-pressure free radical polymerization tubular reactor. Table 1 shows the number average molecular weight (Mn), weight average molecular weight (Mw), molecular weight distribution index (PDI), low molecular weight component content (Mw content below 1500 and below 2500), high molecular weight component content (Mw content above 60000 and above 70000) and ethylene conversion of the low density polyethylene product.

[0050] Example 7

[0051] The difference of embodiment 7 and embodiment 1 is that the first reaction zone and the second reaction zone use tert-butyl peroxyneodecanoate (abbreviated as TBPND) as liquid peroxide initiator, and the third reaction zone uses tert-butyl peroxybenzoate (abbreviated as TBPB) as liquid peroxide initiator.For reaching the required temperature peak of the same transformation efficiency as embodiment 1, the addition of TBPND and TBPB is feedback controlled by the liquid initiator injection pump according to the thermocouple of each reaction zone.Finally, the injection amount of TBPND in the first reaction zone is 8.33kg / h, and the injection amount of TBPND in the second reaction zone is 8.33kg / h, and the injection amount of TBPB in the third reaction zone is 0.08kg / h, and the total conversion rate of the reactor reaches 18.70%, which is suitable with embodiment 1 transformation efficiency.

[0052] The polymer situation at the outlet was obtained by simulating the deterministic model of an ethylene high-pressure free radical polymerization tubular reactor. Table 1 shows the number average molecular weight (Mn), weight average molecular weight (Mw), molecular weight distribution index (PDI), low molecular weight component content (Mw content below 1500 and below 2500), high molecular weight component content (Mw content above 60000 and above 70000) and ethylene conversion of the low density polyethylene product.

[0053] Example 8

[0054] The difference between Example 8 and Example 1 is that the ethylene feed rate of the first reaction zone is 46000 kg / h, and the ethylene feed rate of the second reaction zone is 0. In order to achieve the temperature peak required for the same conversion rate as Example 1, the amount of initiator added to each zone is feedback controlled by the liquid initiator injection pump according to the thermocouple of each reaction zone. Finally, the injection rate of TBPV in the first reaction zone is 4.22 kg / h, the injection rate of TBPV in the second reaction zone is 1.91 kg / h, and the injection rate of DTPB in the third reaction zone is 0.05 kg / h. The total conversion rate of the reactor reaches 18.71%, which is equivalent to the conversion rate of Example 1.

[0055] The polymer situation at the outlet was obtained by simulating the deterministic model of an ethylene high-pressure free radical polymerization tubular reactor. Table 1 shows the number average molecular weight (Mn), weight average molecular weight (Mw), molecular weight distribution index (PDI), low molecular weight component content (Mw content below 1500 and below 2500), high molecular weight component content (Mw content above 60000 and above 70000) and ethylene conversion of the low density polyethylene product.

[0056] Comparative Example 1

[0057] The difference between Comparative Example 1 and Example 1 is that the total amount of propionaldehyde added is 24 kg / h, of which the proportion of propionaldehyde added in the first reaction zone accounts for 50% of the total amount added, i.e. 12 kg / h, and the proportion of propionaldehyde added in the second reaction zone accounts for 50% of the total amount added, i.e. 12 kg / h.

[0058] The polymer situation at the outlet was obtained by simulating the deterministic model of an ethylene high-pressure free radical polymerization tubular reactor. Table 1 shows the number average molecular weight (Mn), weight average molecular weight (Mw), molecular weight distribution index (PDI), low molecular weight component content (Mw content below 1500 and below 2500), high molecular weight component content (Mw content above 60000 and above 70000) and ethylene conversion of the low density polyethylene product.

[0059] Comparative Example 2

[0060] The difference between Comparative Example 2 and Example 1 is that the total amount of propionaldehyde added is 24 kg / h, of which the proportion of propionaldehyde added in the first reaction zone to the total amount added is 75%, i.e. 18 kg / h, and the proportion of propionaldehyde added in the second reaction zone to the total amount added is 25%, i.e. 6 kg / h.

[0061] The polymer situation at the outlet was obtained by simulating the deterministic model of an ethylene high-pressure free radical polymerization tubular reactor. Table 1 shows the number average molecular weight (Mn), weight average molecular weight (Mw), molecular weight distribution index (PDI), low molecular weight component content (Mw content below 1500 and below 2500), high molecular weight component content (Mw content above 60000 and above 70000) and ethylene conversion of the low density polyethylene product.

[0062] Comparative Example 3

[0063] The difference between Comparative Example 3 and Example 2 is that the total amount of propionaldehyde added is 24 kg / h, wherein the proportion of propionaldehyde added in the first reaction zone to the total amount added is 100%, i.e. 24 kg / h, and the proportion of propionaldehyde added in the second reaction zone to the total amount added is 0.

[0064] The polymer situation at the outlet was obtained by simulating the deterministic model of an ethylene high-pressure free radical polymerization tubular reactor. Table 1 shows the number average molecular weight (Mn), weight average molecular weight (Mw), molecular weight distribution index (PDI), low molecular weight component content (Mw content below 1500 and below 2500), high molecular weight component content (Mw content above 60000 and above 70000) and ethylene conversion of the low density polyethylene product.

[0065] Comparative Example 4

[0066] The difference between Comparative Example 4 and Example 1 is that, in addition to injecting 0.05 kg / h of di-tert-butyl peroxide into the third reaction zone, 0.70 m 3 / h air.

[0067] The polymer situation at the outlet was obtained by simulating the deterministic model of the ethylene high-pressure free radical polymerization tubular reactor, and it was found that ethylene decomposition reaction had occurred in the third reaction zone, posing a serious safety hazard.

[0068] Table 1 Low density polyethylene product indicators

[0069]

[0070] Comparison between Example 1, Example 2, and Comparative Examples 1 to 3 shows that under the same monomer feeding method, the same initiator type and amount, the same telogen type and amount, and the same ethylene conversion rate, the telogen addition ratio in the first reaction zone gradually increases from 100%, 75%, 50%, 25%, and 0%, and the telogen addition ratio in the second reaction zone gradually decreases from 0%, 25%, 50%, 75%, and 100%, so that the Mn, Mw, and PDI of the polymer continue to increase, the content of low molecular weight components with Mw below 1500 and below 2500 continues to decrease, and the content of high molecular weight components with Mw above 60000 and above 70000 continues to increase. The results of Example 1, Example 2, and Comparative Examples 1 to 3 show that the addition ratio of the telogen increases with the increasing order of the reaction zone, that is, the later the telogen is added, the less the relative content of the low molecular weight component can be, the generation of soluble matter can be suppressed, and it is beneficial to the preparation of medical-grade low-density polyethylene resin materials.

[0071] Comparison of Example 1 and Examples 3 to 6 shows that the order of chain transfer ability among the telogens propionaldehyde, isobutylene, propylene, isobutane, and propane used in Examples 1 and Examples 3 to 6 is propionaldehyde>isobutylene>propylene>isobutane>propane. The weaker the chain transfer ability, the less chain transfer reaction to the telogen, and the more free short-chain oligomers in the system, which gradually increases the content of low molecular weight components in Examples 1 and Examples 3 to 6. In comparison, the use of propionaldehyde is more conducive to the preparation of medical-grade low-density polyethylene resin materials.

[0072] Comparison of Example 1 with Example 7 shows that the liquid peroxide initiator used in the first and second reaction zones of Example 7 is TBPND, and the liquid peroxide initiator used in the third reaction zone is TBPB. TBPND has lower activation energy than TBPV, and TBPB has lower activation energy than DTBP. When TBPND is used at an initial initiation temperature of 170 to 180°C, the initiator efficiency is very low; when TBPB is used at an initial initiation temperature of 277°C, the initiator efficiency is also low. The above reasons lead to an increase of 11.69 kg / h in the total initiator dosage of Example 7 compared with Example 1 at the same conversion rate, a high unit consumption of initiators, low economic benefits, and an increase in the content of low molecular weight components. In comparison, for the process of this embodiment, the use of TBPV and DTBP is more conducive to the preparation of industrial-grade medical-grade low-density polyethylene resin materials.

[0073] Comparison of Example 1 with Example 8 shows that in Example 8, all ethylene monomers are fed from the first reaction zone, and the ethylene feed amount of the second reaction zone is 0. At the same conversion rate as Example 1, high molecular weight components are more concentrated in the front section of the reactor. However, due to the large amount of monomer injection in the first reaction zone and the large amount of initiator added, the local initiator concentration is high, the initiator efficiency decreases, and Example 8 increases the total initiator dosage by 1.13kg / h compared with Example 1. The Mn, Mw and PDI of the polymer obtained in the final Example 6 are larger than those in Example 1, and the content of low molecular weight components with Mw below 1500 and below 2500 increases. In comparison, in order to reduce the initiator unit consumption and reduce the low molecular weight components at the same time, feeding ethylene monomers from the first reaction zone and the second reaction zone separately is more conducive to the preparation of industrial medical grade low density polyethylene resin materials.

[0074] Finally, it should be noted that the above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and the technical solutions recorded in the above-mentioned embodiments can be modified, or some of the technical features can be replaced by equivalents, which all belong to the protection scope of the present invention.

Claims

1. A method for preparing a medical grade low-density polyethylene resin, It is characterized in that The steps include: (a) conducting a high-pressure free radical polymerization reaction of ethylene monomer in a tubular reactor having at least two reaction zones connected in series; (b) using oxygen and a liquid initiator to prepare a mixed initiator, wherein the oxygen initiator is added to the reaction zones other than the last reaction zone, and the amount of the oxygen initiator added decreases as the order of the reaction zones increases; (c) injecting a telogen into at least one reaction zone, wherein the addition ratio of the telogen into the reaction zone into which the telogen is injected increases with increasing order of the reaction zones.

2. The preparation method according to claim 1, It is characterized in that The ethylene monomer is all injected from the inlet of the first reaction zone of the tubular reactor.

3. The preparation method according to claim 1, It is characterized in that The ethylene monomer is injected from the inlet of the first reaction zone and at least one of the remaining reaction zones.

4. The preparation method according to claim 1, It is characterized in that The oxygen initiator is high-purity oxygen or air, and the liquid initiator is any combination of one or more of azo compounds or peroxides, wherein the azo compound is any combination of one or more of azoalkanes (diazenes), azodicarboxylates, and azodicarboxylic acid dinitriles; and the peroxide is any combination of one or more of peroxyesters, peroxyketals, peroxyketones, or peroxycarbonates.

5. The preparation method according to claim 4, It is characterized in that The oxygen initiator is air, and the liquid initiator is any combination of one or more of tert-butyl peroxide pivalate, di-tert-butyl peroxide, tert-butyl peroxyneodecanoate, and tert-butyl perbenzoate.

6. The preparation method according to claim 1, It is characterized in that The amount of the liquid initiator added is feedback-controlled by a liquid initiator injection pump according to the thermocouples of each reaction zone.

7. The preparation method according to claim 1, It is characterized in that The telogen is any combination of one or more of hydrogen, aliphatic hydrocarbon, olefin, ketone, aldehyde and aliphatic aldehyde.

8. The preparation method according to claim 7, It is characterized in that The telogen is any combination of one or more of propionaldehyde, propylene, isobutylene, propane and isobutane.

9. The preparation method according to claim 1, It is characterized in that The total mass of the telogen added to each reaction zone shall not exceed one thousandth of the total mass of the ethylene monomer injected into each reaction zone.

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