Mixed initiator, ethylene-vinyl acetate copolymer and production method thereof

Abstract

The invention relates to the technical field of ethylene-vinyl acetate copolymer production, and discloses a mixed initiator, an ethylene-vinyl acetate copolymer and a production method thereof.The mixed initiator contains 32-50 wt% of a low-temperature initiator A, 15-35 wt% of a secondary low-temperature initiator B and 10-30 wt% of tert-butyl peroxy-2-ethylhexanoate, 0 to 20 weight percent of tert-butyl peroxybenzoate; the 1-hour half-life period temperature TA of the low-temperature initiator A under the normal pressure and the 1-hour half-life period temperature TB of the secondary low-temperature initiator B under the normal pressure meet the condition that TA-TB is more than 0 DEG C and less than or equal to 30 DEG C. According to the technical scheme provided by the invention, the yield of the ethylene-vinyl acetate copolymer can be remarkably increased under the condition that the reaction process is more stable, and the high-pressure polymerization reactor does not need to be expanded. And the economic value is very high.

Description

Technical Field

[0001] The invention relates to the technical field of ethylene-vinyl acetate copolymer production, and in particular to a mixed initiator, an ethylene-vinyl acetate copolymer and a production method thereof. Background Art

[0002] Ethylene-vinyl acetate copolymer resin, referred to as EVA, is produced by copolymerization of ethylene and vinyl acetate. EVA has a wide range of applications and can be used in the photovoltaic industry in the fields of single-sided battery sealing film, shoe foaming, toys, hot melt adhesives, films, etc.

[0003] At present, there are four main EVA production processes in the world: high-pressure continuous bulk polymerization, medium-pressure suspension polymerization, solution polymerization, and emulsion polymerization. Among them, solution polymerization and emulsion polymerization processes are less used. Most EVA resins on the market are produced by high-pressure continuous bulk polymerization. Ethylene and vinyl acetate follow the free radical copolymerization mechanism, and the vinyl acetate (VA) content in the polymerization product is generally 5%-40%.

[0004] The high-pressure continuous bulk polymerization process usually uses an autoclave reactor or a tubular reactor. The autoclave reactor has a lower temperature and pressure, and can produce EVA with a VA content of less than 40%, with a single-pass conversion rate of 10%-20%. The tubular reactor has a higher temperature and pressure, and can produce EVA with a VA content of less than 30%, with a single-pass conversion rate of 25%-35%. The two processes have different reactor types and structures, but the reaction processes they follow are basically the same. The high-pressure free radical copolymerization of ethylene and vinyl acetate is initiated by an initiator, and the competitive polymerization ratio of ethylene and vinyl acetate is ≈1.

[0005] High-pressure free radical copolymerization conditions have an impact on EVA product performance and device capacity. CN107108791A proposes a method that can reduce polymer production while maintaining product quality when reactor production needs to be reduced due to reduced product demand, reduced raw material supply or some other restrictions. The specific operating means is to reduce the inlet pressure and control the total reaction consumption ratio of the maximum temperature initiator system.

[0006] CN110891986A proposes a method to improve the mechanical strength of EVA by controlling the polymerization conditions using an autoclave reactor. A significant temperature difference is created between the upper and lower sections of the reactor by using a low-temperature initiator and a high-temperature initiator respectively. CN110177814A and CN111936533A prepare EVA with a high degree of crosslinking by controlling the temperature and polymerization heat in the autoclave reactor, and by controlling the temperature difference and the input ratio of the initiator in the autoclave reactor respectively. In addition, CN106928385A proposes using a tert-amyl organic peroxide with a half-life temperature of 230°C or lower as a radical initiator to produce EVA under low-temperature and high-pressure conditions, which can inhibit the generation of "fish eyes" on the polymerized product, maintain a high-quality appearance, and improve the processing productivity.

[0007] In the above-mentioned prior arts, there have been many studies on the method of improving the quality of EVA products by initiators and polymerization conditions, but no effective method has been reported for increasing the production. In addition to product quality, the production of EVA is also an extremely important economic factor in the industrial process. The manufacturing cost of high-pressure reactors is expensive, and the reactor scale-up is limited by ultra-high pressure, making it difficult to increase production capacity through simple capacity expansion. Summary of the Invention

[0008] The object of the present invention is to provide a method that does not require the expansion of the high-pressure polymerization reactor, has a more stable reaction process, and can significantly increase the production of ethylene-vinyl acetate copolymer.

[0009] To achieve the above object, a first aspect of the present invention provides a mixed initiator for producing ethylene-vinyl acetate copolymer, which contains 32-50 wt% of a low-temperature initiator A, 15-35 wt% of a sub-low-temperature initiator B, 10-30 wt% of tert-butyl peroxy-2-ethylhexanoate, and 0-20 wt% of tert-butyl peroxybenzoate;

[0010] The low-temperature initiator A and the sub-low-temperature initiator B are respectively the initiators with the shortest and the second shortest half-lives in the mixed initiator under normal pressure and normal temperature conditions;

[0011] The low-temperature initiator A is 1,1,3,3-tetramethylbutyl peroxyneodecanoate or bis(2-ethylhexyl) peroxydicarbonate;

[0012] The 1-hour half-life temperature T of the low-temperature initiator A under normal pressure A and the 1-hour half-life temperature T of the sub-low-temperature initiator B under normal pressure B satisfy: 0°C < T A - T B ≤ 30°C.

[0013] The second aspect of the present invention provides a method for producing ethylene-vinyl acetate copolymer, which comprises:

[0014] In the presence of an initiator, carrying out a free radical copolymerization reaction of ethylene monomer and vinyl acetate monomer in a high-pressure polymerization reactor containing at least two reaction zones to obtain an ethylene-vinyl acetate copolymer;

[0015] Wherein, the initiator used in the first reaction zone in the high-pressure polymerization reactor is the mixed initiator described in the foregoing first aspect; the initiators used in the remaining reaction zones are the same as or different from that in the first reaction zone.

[0016] The third aspect of the present invention provides an ethylene-vinyl acetate copolymer prepared by the method described in the foregoing second aspect.

[0017] The technical solution provided by the present invention has at least the following advantages:

[0018] The technical solution provided by the present invention does not need to adjust the reactor volume, the maximum reaction pressure and the maximum reaction temperature, strictly controls the content of the low-temperature initiator in the mixed initiator not to exceed 50 wt%, and particularly limits the difference in the half-life temperature at normal pressure for 1 hour between the low-temperature initiator and the sub-low-temperature initiator in the mixed initiator within a specific range. While conveniently and quickly increasing the yield of the ethylene-vinyl acetate copolymer, it can also make the polymerization reaction proceed more smoothly.

[0019] The present invention optimizes the initiator formulation, so that the polymerization reaction starts at a lower temperature, while ensuring that the peak temperature (the highest temperature) in each reaction zone remains unchanged. On the premise that the peak temperature remains unchanged, by reducing the initial initiation temperature of the mixed initiator, the yield of the ethylene-vinyl acetate copolymer is significantly increased. Description of the Drawings

[0020] Figure 1 is the high-pressure tubular reactor used in the embodiment of the present invention.

[0021] Description of the Reference Numerals

[0022] 1 Low-pressure compressor; 2 and 16 are both high-pressure compressors; 3 High-pressure tubular reactor; 4 Cooling water jacket; 5, 6, 7, 8, 9 are all initiator injection pumps; 10 Reactor outlet valve; 11 Cooler; 12 High-pressure separator; 13 High-pressure circulation loop; 14 Low-pressure separator; 15 Low-pressure circulation loop. Detailed Embodiments

[0023] The endpoints and any values in the ranges disclosed in this document are not limited to the exact ranges or values. 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, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in this document.

[0024] The normal pressure described in the present invention is 0.1 MPa, and the normal temperature described in the present invention is 25 °C.

[0025] The "wt%" described in the present invention all represents the mass percentage content.

[0026] In the present invention, the substances corresponding to the respective letter codes are as follows:

[0027] TBPND: tert-butyl peroxyneodecanoate, CAS No.: 26748-41-4;

[0028] TBPV: tert-butyl peroxypivalate, CAS No.: 927-07-1;

[0029] EHP: bis(2-ethylhexyl) peroxydicarbonate, CAS No.: 16111-62-9;

[0030] TOPND: 1,1,3,3-tetramethylbutyl peroxyneodecanoate, CAS No.: 51240-95-0;

[0031] DBPO: diisobutyryl peroxide, CAS No.: 3437-84-1;

[0032] TBPEH: tert-butyl peroxy-2-ethylhexanoate, CAS No.: 3006-82-4;

[0033] TBPB: tert-butyl peroxybenzoate, CAS No.: 614-45-9.

[0034] As described above, the first aspect of the present invention provides a mixed initiator for producing ethylene-vinyl acetate copolymer. The mixed initiator contains 32-50 wt% of low-temperature initiator A, 15-35 wt% of sub-low-temperature initiator B, 10-30 wt% of tert-butyl peroxy-2-ethylhexanoate, and 0-20 wt% of tert-butyl peroxybenzoate;

[0035] The low-temperature initiator A and the sub-low-temperature initiator B are respectively the initiators with the shortest and the second shortest half-lives in the mixed initiator under normal pressure and normal temperature conditions;

[0036] The low-temperature initiator A is 1,1,3,3-tetramethylbutyl peroxynoate or bis(2-ethylhexyl) peroxydicarbonate;

[0037] The 1-hour half-life temperature T of the low-temperature initiator A under atmospheric pressure A and the 1-hour half-life temperature T of the sub-low-temperature initiator B under atmospheric pressure B satisfy: 0 °C < T A - T B ≤ 30 °C.

[0038] Preferably, the mixed initiator contains 40-50 wt% of the low-temperature initiator A, 20-30 wt% of the sub-low-temperature initiator B, 15-25 wt% of tert-butyl peroxy-2-ethylhexanoate, and 5-15 wt% of tert-butyl peroxybenzoate. The inventors of the present invention found in the research that using the mixed initiator under the preferred conditions to participate in the free radical copolymerization reaction can further increase the yield of ethylene-vinyl acetate copolymer.

[0039] Preferably, the 1-hour half-life temperature T of the low-temperature initiator A under atmospheric pressure A and the 1-hour half-life temperature T of the sub-low-temperature initiator B under atmospheric pressure B satisfy: 0 °C < T A - T B ≤ 25 °C.

[0040] More preferably, the 1-hour half-life temperature T of the low-temperature initiator A under atmospheric pressure A and the 1-hour half-life temperature T of the sub-low-temperature initiator B under atmospheric pressure B satisfy: 10 °C ≤ T A - T B ≤ 20 °C. The inventors of the present invention found in the research that under the preferred conditions, the mixed initiator provided by the present invention can increase the yield of ethylene-vinyl acetate copolymer to a greater extent when used in the preparation of ethylene-vinyl acetate copolymer, and make the polymerization reaction proceed more smoothly.

[0041] The "1-hour half-life temperature" of the initiator described in the present invention refers to the temperature corresponding to the half-life of 1 hour of the initiator under atmospheric pressure.

[0042] Preferably, the sub-low-temperature initiator B is an organic peroxide.

[0043] More preferably, the sub-low-temperature initiator B is tert-butyl peroxypivalate.

[0044] Particularly preferably, in the mixed initiator provided in the present invention, the low-temperature initiator A is 1,1,3,3-tetramethylbutyl peroxyneodecanoate, and the sub-low-temperature initiator B is tert-butyl peroxy pivalate. The inventors of the present invention have found in their research that using the mixed initiator under the preferred matching conditions to participate in the free radical copolymerization reaction can further increase the yield of ethylene-vinyl acetate copolymer.

[0045] As described above, the second aspect of the present invention provides a method for producing ethylene-vinyl acetate copolymer, which method comprises:

[0046] Performing a free radical copolymerization reaction of ethylene monomer and vinyl acetate monomer in a high-pressure polymerization reactor containing at least two reaction zones in the presence of an initiator to obtain an ethylene-vinyl acetate copolymer;

[0047] Wherein, the initiator used in the first reaction zone in the high-pressure polymerization reactor is the mixed initiator described in the first aspect above; the initiators used in the remaining reaction zones are the same as or different from those in the first reaction zone.

[0048] It should be noted that the "first reaction zone" described in the present invention refers to: among the multiple reaction zones separated by the initiator injection point in the direction of reactor material flow, the reaction zone directly connected to the main feed port of the high-pressure polymerization reactor is called the first reaction zone.

[0049] Preferably, the initiators used in the remaining reaction zones are different from those in the first reaction zone.

[0050] Preferably, the initiators in the remaining reaction zones are each independently selected from at least one of organic peroxides and azo compounds.

[0051] More preferably, the organic peroxide is selected from at least one of peroxy esters, peroxy ketals, peroxy ketones, and peroxy carbonates.

[0052] More preferably, the azo compound is selected from at least one of azoalkanes, azodicarboxylates, and azodicarbonitrile.

[0053] According to a particularly preferred specific embodiment 1: the initiator used in the remaining reaction zones is initiator complex A, and the initiator complex A contains: 30-40 wt% of TBPND, 30-40 wt% of TBPV, 20-30 wt% of TBPEH, 5-15 wt% of TBPB.

[0054] According to a particularly preferred specific embodiment 2: the initiator used in the remaining reaction zones is initiator complex B, and the initiator complex B contains: 70-80 wt% of TBPEH, 20-30 wt% of TBPB.

[0055] According to a particularly preferred specific embodiment 3: The free radical copolymerization reaction of the present invention is carried out in a high-pressure polymerization reactor containing three reaction zones. Along the direction of material flow, the initiator used in the first reaction zone is the mixed initiator described in the first aspect above, the second reaction zone uses initiator complex A, and the third reaction zone uses initiator complex B.

[0056] According to a particularly preferred specific embodiment 4: The free radical copolymerization reaction of the present invention is carried out in a high-pressure polymerization reactor containing four reaction zones. Along the direction of material flow, the initiator used in the first reaction zone is the mixed initiator described in the first aspect above, the second reaction zone uses initiator complex A, and both the third reaction zone and the fourth reaction zone use initiator complex B.

[0057] According to a particularly preferred specific embodiment 5: The free radical copolymerization reaction of the present invention is carried out in a high-pressure polymerization reactor containing five reaction zones. Along the direction of material flow, the initiator used in the first reaction zone is the mixed initiator described in the first aspect above, the second reaction zone uses initiator complex A, and the third reaction zone to the fifth reaction zone all use initiator complex B.

[0058] The inventors of the present invention found in the research that by using the initiator complex and the mixed initiator that meet the particularly preferred specific embodiment, without expanding the capacity of the high-pressure polymerization reactor, the yield of ethylene-vinyl acetate copolymer can be increased to a greater extent, and the production process can be made more stable, and the fluctuations in the reaction process can be controlled at a low-risk level.

[0059] Preferably, the pressure at the inlet of the high-pressure polymerization reactor is 100 - 300 MPa, more preferably 120 - 250 MPa.

[0060] Preferably, the initial initiation temperature at the inlet of the first reaction zone is not lower than 125 °C, more preferably not lower than 130 °C.

[0061] According to a preferred specific embodiment, in the high-pressure polymerization reactor, an injection pump is used to inject the initiator into each reaction zone, and a reactor temperature feedback control system is used to control the initiator flow rate in the injection pump, thereby controlling the reaction temperature in each reaction zone.

[0062] In the present invention, the reactor temperature feedback control system is an automatic control system known to those skilled in the art based on the feedback principle. The output variable is the temperature measured by the thermocouple inside the reactor, and the input variable is the initiator injection pump flow rate. This reactor temperature feedback control system is connected to the distributed control system (DCS). The present invention will not elaborate further here, and those skilled in the art should not consider it as a limitation to the present invention.

[0063] Preferably, the maximum temperature of each reaction zone in the high-pressure polymerization reactor is independently not more than 260 °C, more preferably not more than 250 °C.

[0064] In the present invention, the mass percentage content of each initiator component in the mixed initiator is calculated based on the total weight of each initiator component in the mixed initiator. During actual use or sales, the mixed initiator can be mixed with a solvent to form an initiator stock solution or an initiator solution and then sold or used. These simple variations and combinations should also be regarded as the content disclosed in the present invention and all fall within the protection scope of the present invention.

[0065] The present invention has no particular limitation on the type and amount of the solvent. Those skilled in the art can select according to the solvents known in the art, and the present invention will not elaborate herein. Those skilled in the art should not understand this as a limitation of the present invention.

[0066] As described above, the third aspect of the present invention provides an ethylene-vinyl acetate copolymer prepared by the method described in the second aspect above.

[0067] For a better understanding of the purpose, process technology and functions of the present invention, the following combines Figure 1 to describe in detail the preferred specific embodiments of the present invention, but does not limit the present invention thereby. Figure 1 The high-pressure tubular reactor 3 shown is composed of five reaction zones, which are, from left to right, the first reaction zone, the second reaction zone, the third reaction zone, the fourth reaction zone and the fifth reaction zone. The method of the present invention includes:

[0068] Fresh ethylene is compressed successively through a low-pressure compressor 1 and a high-pressure compressor 2 and enters the first reaction zone through the main feed port. At the same time, ethylene enters the second reaction zone and the third reaction zone through the ethylene side feed ports of the second reaction zone and the third reaction zone. The initiator enters the first reaction zone to the fifth reaction zone through the initiator injection pumps 5, 6, 7, 8, and 9 respectively from the starting points of each reaction zone to participate in the free radical copolymerization reaction. A cooling water jacket 4 is provided outside the reactor of each reaction zone to control the reaction temperature;

[0069] After the reaction is completed, open the reactor outlet valve 10. The mixed material containing ethylene-vinyl acetate copolymer enters the cooler 11 for cooling, and then enters the high-pressure separator 12 to separate a part of the high-pressure circulating material and the polymer-rich phase. The part of the high-pressure circulating material is recycled through the high-pressure circulation loop 13 and participates in the free-radical copolymerization reaction together with fresh ethylene and vinyl acetate; the polymer-rich phase is introduced into the low-pressure separator 14 for further separation to obtain a part of the low-pressure circulating material and the polymer phase. The part of the ethylene monomer is introduced into the high-pressure compressor 16 through the low-pressure circulation loop 15 for compression, and then participates in the free-radical copolymerization reaction together with fresh ethylene. The polymer phase is led out of the low-pressure separator 14 for further post-treatment.

[0070] The present invention will be described in detail below through examples. In the following examples, unless otherwise specified, the raw materials are all commercially available products.

[0071] The "half-life temperature at normal pressure for 1 hour" of the initiator in the following examples is taken from the product manual data of the initiator supplier Akzo Nobel.

[0072] In the following examples, Figure 1 The free-radical copolymerization reaction of ethylene and vinyl acetate is carried out in the high-pressure tubular reactor shown. The reactor consists of five reaction zones. From left to right, they are the first reaction zone, the second reaction zone, the third reaction zone, the fourth reaction zone, and the fifth reaction zone. After fresh ethylene is compressed by a low-pressure compressor and a high-pressure compressor in sequence, it enters the first reaction zone through the main feed port, and at the same time enters the second reaction zone and the third reaction zone through the ethylene side feed ports of the second reaction zone and the third reaction zone; there are initiator injection ports in all five reaction zones, and the corresponding initiators are injected from the starting point of each reaction zone.

[0073] Example 1

[0074] In the presence of an initiator, the ethylene monomer and vinyl acetate monomer are subjected to a free-radical copolymerization reaction Figure 1 in the high-pressure tubular reactor shown to obtain ethylene-vinyl acetate copolymer;

[0075] Among them, the initiator used in the first reaction zone of the high-pressure tubular reactor is shown in Table 1;

[0076] Among them, in the high-pressure tubular reactor: the initiator used in the second reaction zone is 32 wt% TBPND, 32 wt% TBPV, 26 wt% TBPEH, and 10 wt% TBPB;

[0077] The initiator used in the third reaction zone is 75 wt% TBPEH and 25 wt% TBPB;

[0078] The initiator used in the fourth reaction zone is 75 wt% TBPEH and 25 wt% TBPB;

[0079] The initiator used in the fifth reaction zone is 75 wt% TBPEH and 25 wt% TBPB;

[0080] Since EHP is used as the low-temperature initiator, under the initiation of EHP, the initial initiation temperature at the inlet of the first reaction zone is 139 °C, and the pressure at the inlet of the high-pressure tubular reactor is 250 MPa;

[0081] The reactor temperature feedback control system controls the flow rate of the initiator injection pump, and the maximum temperatures of the first to fifth reaction zones are 239 °C, 250 °C, 242 °C, 241 °C and 231 °C respectively;

[0082] Under these operating conditions, the start-up is successful, and the output is 22.65 tons per hour.

[0083] Compared with Comparative Example 1, in Example 1, a lower-temperature initiator EHP was used in the first reaction zone. The difference in the 1-hour half-life temperature at atmospheric pressure between it and TBPV is larger, and the polymerization reaction can be initiated at a lower initial initiation temperature. Finally, the initial initiation temperature at the inlet of the first reaction zone is reduced to 139 °C, and the difference in the 1-hour half-life temperature at atmospheric pressure between the low-temperature initiator A and the sub-low-temperature initiator B in the first reaction zone is 12 °C, which can successfully initiate the polymerization reaction, ensure the reaction is stable, the start-up of the device is successful, and compared with Comparative Example 1, the output is increased by 0.34 tons per hour.

[0084] Example 2

[0085] In the presence of an initiator, ethylene monomer and vinyl acetate monomer are subjected to free radical copolymerization reaction in the Figure 1 shown high-pressure tubular reactor to obtain ethylene-vinyl acetate copolymer;

[0086] Among them, the initiator used in the first reaction zone in the high-pressure tubular reactor is shown in Table 1;

[0087] Among them, the initiators used in the second, third, fourth and fifth reaction zones are the same as those in Example 1.

[0088] Since TOPND is used as the low-temperature initiator, under the initiation of TOPND, the initial initiation temperature at the inlet of the first reaction zone is 130 °C, and the pressure at the inlet of the high-pressure tubular reactor is 250 MPa;

[0089] The reactor temperature feedback control system controls the flow rate of the initiator injection pump, and the maximum temperatures of the first to fifth reaction zones are 239 °C, 249 °C, 242 °C, 241 °C and 230 °C respectively;

[0090] Under these operating conditions, the start-up was successful, and the production rate was 23.17 tons per hour.

[0091] Compared with Example 1, in Example 2, a lower-temperature initiator TOPND was used. The difference in the 1-hour half-life temperature at atmospheric pressure between it and TBPV was greater, and it could initiate the polymerization reaction at a lower initial initiation temperature. Eventually, the initial initiation temperature at the inlet of the first reaction zone was reduced to 130°C, and the difference in the 1-hour half-life temperature at atmospheric pressure between the low-temperature initiator A and the sub-low-temperature initiator B in the first reaction zone was 18°C, enabling the initiator polymerization reaction to succeed. The start-up of the device was successful, and the production capacity was increased.

[0092] Example 3

[0093] In the presence of an initiator, ethylene monomer and vinyl acetate monomer were subjected to free radical copolymerization reaction in the Figure 1 high-pressure tubular reactor shown to obtain ethylene-vinyl acetate copolymer;

[0094] Among them, the initiator used in the first reaction zone in the high-pressure tubular reactor is shown in Table 1;

[0095] Among them, the initiators used in the second, third, fourth, and fifth reaction zones are the same as those in Example 1.

[0096] Due to using TOPND as the low-temperature initiator, under the initiation of TOPND, the initial initiation temperature at the inlet of the first reaction zone was 130°C, and the pressure at the inlet of the high-pressure tubular reactor was 250 MPa;

[0097] The reactor temperature feedback control system controlled the flow rate of the initiator injection pump, and the maximum temperatures in the first to fifth reaction zones were 240°C, 250°C, 241°C, 241°C, and 230°C respectively;

[0098] Under these operating conditions, the start-up was successful, and the production rate was 23.20 tons per hour.

[0099] Compared with Example 2, in Example 3, the dosages of the initiators in the mixed initiator used in the first reaction zone were adjusted, and the mass fraction of the low-temperature initiator TOPND was increased from 44 wt% to 50 wt%. It could also successfully initiate the polymerization reaction. The start-up of the device was successful, and the production capacity was increased.

[0100] Comparative Example 1

[0101] In the presence of an initiator, ethylene monomer and vinyl acetate monomer were subjected to free radical copolymerization reaction in the Figure 1 high-pressure tubular reactor shown to obtain ethylene-vinyl acetate copolymer;

[0102] Among them, the initiator used in the first reaction zone of the high-pressure tubular reactor is shown in Table 1;

[0103] Among them, the initiators used in the second, third, fourth, and fifth reaction zones are the same as those in Example 1.

[0104] Since TBPND is used as the low-temperature initiator, under the initiation of TBPND, the initial initiation temperature at the inlet of the first reaction zone is 145 °C, and the pressure at the inlet of the high-pressure tubular reactor is 250 MPa;

[0105] The reactor temperature feedback control system controls the flow rate of the initiator injection pump, and the maximum temperatures of the first to fifth reaction zones are 240 °C, 250 °C, 243 °C, 241 °C, and 230 °C respectively;

[0106] Under these operating conditions, the startup is successful, and the output is 22.31 tons per hour.

[0107] Comparative Example 2

[0108] In the presence of an initiator, ethylene monomer and vinyl acetate monomer are subjected to free radical copolymerization reaction in the Figure 1 shown high-pressure tubular reactor to obtain ethylene-vinyl acetate copolymer;

[0109] Among them, the initiator used in the first reaction zone of the high-pressure tubular reactor is shown in Table 1;

[0110] Among them, the initiators used in the second, third, fourth, and fifth reaction zones are the same as those in Example 1.

[0111] Since DBPO is used as the low-temperature initiator, under the initiation of DBPO, the initial initiation temperature at the inlet of the first reaction zone is 120 °C, and the pressure at the inlet of the high-pressure tubular reactor is 250 MPa;

[0112] The reactor temperature feedback control system controls the flow rate of the initiator injection pump. However, since the initial initiation temperature at the inlet of the first reaction zone is only 120 °C, the maximum temperature of the first reaction zone only reaches 155 °C, while the temperature of the second reaction zone soars above 260 °C, and the temperature interlock device alarms, and the device stops.

[0113] Compared with Example 2, Comparative Example 2 uses an initiator DBPO with a 1-hour half-life temperature far lower than that of TBPND under normal pressure. Although it can initiate the polymerization reaction at 120 °C, since the 1-hour half-life temperature of diisobutyryl peroxide under normal pressure differs from that of the sub-low-temperature initiator TBPV by more than 30 °C under normal pressure, a "cascading relay" cannot be formed, resulting in the maximum temperature of the first reaction zone only reaching 155 °C, and a large amount of initiator remaining in the subsequent reaction zones, leading to a "temperature runaway" in the second reaction zone, the temperature interlock device alarms, and the device stops.

[0114] Comparative Example 3

[0115] In the presence of an initiator, ethylene monomer and vinyl acetate monomer are subjected to a free radical copolymerization reaction in the high-pressure tubular reactor shown in Figure 1 to obtain an ethylene-vinyl acetate copolymer;

[0116] Among them, the initiator used in the first reaction zone in the high-pressure tubular reactor is shown in Table 1;

[0117] Among them, the initiators used in the second, third, fourth, and fifth reaction zones are the same as those in Example 1.

[0118] The initial initiation temperature at the inlet of the first reaction zone is 139 °C, and the pressure at the inlet of the high-pressure tubular reactor is 250 MPa;

[0119] The reactor temperature feedback control system controls the flow rate of the initiator injection pump, and the highest temperatures in the first to fifth reaction zones are 202 °C, 250 °C, 242 °C, 241 °C, and 231 °C respectively;

[0120] Under these operating conditions, the unit is started up successfully, and the output is 20.26 tons per hour.

[0121] Compared with Example 1, the low-temperature initiator used in the first reaction zone in Comparative Example 3 does not meet the requirements of the present invention, resulting in a lower temperature peak in the first reaction zone. Although the unit can be started up, the output is reduced.

[0122] Comparative Example 4

[0123] In the presence of an initiator, ethylene monomer and vinyl acetate monomer are subjected to a free radical copolymerization reaction in the high-pressure tubular reactor shown in Figure 1 to obtain an ethylene-vinyl acetate copolymer;

[0124] Among them, the initiator used in the first reaction zone in the high-pressure tubular reactor is shown in Table 1;

[0125] Among them, the initiators used in the second, third, fourth, and fifth reaction zones are the same as those in Example 1.

[0126] Since TBPND is used as the low-temperature initiator, under the initiation of TBPND, the initial initiation temperature at the inlet of the first reaction zone is 145 °C, and the pressure at the inlet of the high-pressure tubular reactor is 250 MPa;

[0127] The reactor temperature feedback control system controls the flow rate of the initiator injection pump, and the highest temperatures in the first to fifth reaction zones are 241 °C, 250 °C, 243 °C, 241 °C, and 230 °C respectively;

[0128] Under these operating conditions, the startup was successful and the production rate was 22.32 tons per hour.

[0129] Compared with Example 1, in Comparative Example 4, the mass fraction of the low-temperature initiator TBPND in the first reaction zone was the same as that of the low-temperature initiator EHP in the first reaction zone of Example 1, both being 41 wt%, but the initial initiation temperature of TBPND was still 145°C. Although the device could be started up, the production rate of Example 1 could not be achieved.

[0130] Table 1

[0131]

[0132] T A 、T B 、T TBPND respectively represent the half-life temperature of the low-temperature initiator A, the sub-low-temperature initiator B, and TBPND under normal pressure for 1 hour.

[0133] From the above results, it can be seen that by adopting the technical solution provided by the present invention, the production rate of ethylene-vinyl acetate copolymer can be significantly increased while the reaction process is more stable, and there is no need to expand the high-pressure polymerization reactor. It has high economic value.

[0134] 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 technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A mixed initiator for producing ethylene-vinyl acetate copolymer, characterized in that, The mixed initiator contains 32 - 50 wt% of low-temperature initiator A, 15 - 35 wt% of sub-low-temperature initiator B, 10 - 30 wt% of tert-butyl peroxy-2-ethylhexanoate, and 0 - 20 wt% of tert-butyl peroxybenzoate; The low-temperature initiator A and the sub-low-temperature initiator B are respectively the initiators with the shortest and the second shortest half-lives in the mixed initiator under normal pressure and normal temperature conditions; The low-temperature initiator A is 1,1,3,3-tetramethylbutyl peroxyneodecanoate or bis(2-ethylhexyl) peroxydicarbonate; The temperature T of the 1-hour half-life of the low-temperature initiator A under atmospheric pressure A and the temperature T of the 1-hour half-life of the sub-low-temperature initiator B under atmospheric pressure B satisfy: 0°C < T A - T B ≤ 30°C.

2. The hybrid initiator according to claim 1, characterized in that, The 1-hour half-life temperature T of the low-temperature initiator A under atmospheric pressure A and the 1-hour half-life temperature T of the sub-low-temperature initiator B under atmospheric pressure B satisfy: 0 °C < T A - T B ≤ 25 °C.

3. The hybrid initiator according to claim 1 or 2, characterized in that, The sub-low-temperature initiator B is an organic peroxide; Preferably, the sub-low-temperature initiator B is tert-butyl peroxy pivalate.

4. A method for producing ethylene-vinyl acetate copolymer, characterized in that, This method includes: In the presence of an initiator, carrying out a radical copolymerization reaction of ethylene monomer and vinyl acetate monomer in a high-pressure polymerization reactor containing at least two reaction zones to obtain an ethylene-vinyl acetate copolymer; Among them, the initiator used in the first reaction zone in the high-pressure polymerization reactor is the mixed initiator described in any one of claims 1 - 3; the initiators used in the remaining reaction zones are the same as or different from that in the first reaction zone.

5. The method according to claim 4, characterized in that, The initiators in the remaining reaction zones are each independently selected from at least one of organic peroxides and azo compounds; Preferably, the organic peroxide is selected from at least one of peroxy esters, peroxy ketals, peroxy ketones, and peroxy carbonates; Preferably, the azo compound is selected from at least one of azoalkanes, azo dicarboxylates, and azo dicarboxylic dinitriles.

6. The method according to claim 4 or 5, characterized in that, The pressure at the inlet of the high-pressure polymerization reactor is 100 - 300 MPa, preferably 120 - 250 MPa.

7. The method according to any one of claims 4-6, characterized in that, The initial initiation temperature at the inlet of the first reaction zone is not lower than 125 °C, preferably not lower than 130 °C.

8. The method according to any one of claims 4 to 7, characterized in that In the high-pressure polymerization reactor, an injection pump is used to inject the initiator into each reaction zone, and a reactor temperature feedback control system is used to control the initiator flow rate in the injection pump, thereby controlling the reaction temperature in each reaction zone.

9. The method according to claim 8, wherein The maximum temperature in each reaction zone in the high-pressure polymerization reactor is each independently not more than 260 °C, preferably not more than 250 °C.

10. An ethylene-vinyl acetate copolymer prepared by the method according to any one of claims 4 - 9.

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