Preparation method of medium-melt-index injection molding low-density polyethylene

Abstract

The invention relates to a preparation method of medium-melt-index injection molding low-density polyethylene. The problem that the use effect of the product is not ideal due to high density and hardness of the low-density polyethylene resin product of the conventional fruit wine packaging inner cover is solved. The method comprises the following steps: S1, raw materials comprise a main raw material monomer ethylene, a molecular weight regulator and an initiator; s2, preparation of low-density polyethylene: under the action of a mixed gas of ethylene and a molecular weight regulator as well as an initiator, carrying out polymerization reaction under the conditions that the reaction pressure is 255-270 Mpa and the reaction temperature is 280-300 DEG C, and producing a molten polyethylene product with the melt index of 6.5-8.5 g / 10min and the density of 0.919-0.923 g / cm < 3 > in a reactor; s3, the molten polyethylene product is extruded, granulated and formed through an extruder, and molten polyethylene material granules are obtained. According to the method, the hardness and density of the product are improved, and the performance of the product is more suitable for producing bottle cap products.

Description

Technical Field:

[0001] The present invention relates to the field of low-density polyethylene materials, and particularly to a preparation method of medium melt index injection molding low-density polyethylene. Background Art:

[0002] In existing production, due to the low density and hardness of high-pressure low-density polyethylene resin products, special low-density polyethylene resins are generally used to prepare the inner caps of bottle caps such as fruit wine packaging. For example, Yanshan Petrochemical determined the basic physical properties of the special low-density polyethylene resin LD607BW for the inner cap of fruit wine packaging according to the performance of injection molded bottle caps and the requirements for product processing, combined with the process characteristics of the tubular low-density polyethylene production unit: the density is (0.921 ± 0.002) g / cm 3 , the melt flow rate is (7.5 ± 1.0) g / 10 min, the tensile fracture stress is ≥ 8 MPa, and the nominal strain at break is ≥ 80%. They determined the trial production process conditions according to the basic physical properties, including polymerization temperature, polymerization pressure, and requirements for additives, etc. By increasing the temperature of the fourth and fifth reaction zones at the end of the reactor and reducing the reaction pressure, the long-chain branching and short-chain branching increase, further reducing the density and hardness of the product, and solving the warping problem of injection molded bottle cap products; however, the density and hardness of its products are relatively high, and it is found during use that it is not easy to open when made into a bottle cap, resulting in an unsatisfactory product use effect. Summary of the Invention:

[0003] The present invention aims at the problem that the density and hardness of the existing low-density polyethylene resin products for preparing the inner caps of fruit wine packaging in the background art are relatively high, resulting in an unsatisfactory product use effect, and provides a preparation method of medium melt index injection molding low-density polyethylene. After adjusting the process, the hardness and density of the product are increased, making the product performance more suitable for the production of bottle cap products.

[0004] The present invention can achieve the following technical solutions to solve its problems: The preparation method of the medium melt index injection molding low-density polyethylene includes the following steps:

[0005] S1. Prepare the raw materials for low-density polyethylene: including the main raw material monomer ethylene, molecular weight regulator, and initiator;

[0006] S2. Preparation of low-density polyethylene:

[0007] In a tubular reactor, under the action of a mixed gas of ethylene and molecular weight regulator, and the prepared initiator, a polymerization reaction occurs under the conditions of a reaction pressure of 255 - 270 Mpa and a reaction temperature of 280 - 300 °C, and a melt index of 6.5 - 8.5 g / 10 min and a density of 0.919 - 0.923 g / cm are produced in the reactor3 The molten polyethylene product;

[0008] S3. The molten polyethylene product is extruded and pelletized by an extruder to obtain molten polyethylene material pellets.

[0009] Preferably, the molecular weight regulator is propionaldehyde and propylene.

[0010] Preferably, the weight ratio of each component of the raw materials is as follows: the weight ratio of ethylene, propionaldehyde, propylene, and initiator is 997 - 999:0.54:11 - 12:1.89.

[0011] Preferably, the composition and ratio of the initiator are as follows: the weight ratio of isododecane, peroxide DTBP, peroxide TBPIN, peroxide TBPEH, and peroxide TBPPI is 1.64:0.16:0.04:0.03:0.02.

[0012] Preferably, the tubular reactor is the LUPOTECHT tubular reactor of German Basell Company.

[0013] Preferably, in the design of the reactor temperature distribution, the initiation temperatures in reaction zones 1 to 4 are 170 - 210°C, 180 - 210°C, 180 - 210°C, 170 - 200°C; the peak temperatures in reaction zones 1 to 4 are 290 - 300°C, 280 - 295°C, 280 - 295°C, 280 - 290°C respectively.

[0014] Preferably, the reaction pressure is set to 255.0 - 260.0 MPa.

[0015] Preferably, the reaction pressure is 260 MPa.

[0016] Preferably, the process of extruding and pelletizing the molten polyethylene product by the extruder is as follows:

[0017] The molten polyethylene product enters a low - pressure separation tank, the material goes from the outlet of the low - pressure separator to a single - screw extruder, after being extruded by the extruder, it is pelletized by a pelletizer, and the pelletized product is degassed in a degassing bin for 12 hours and then sent for packaging.

[0018] Preferably, the rotation speed of the extruder is 57 - 58 rpm; the die head pressure of the extruder is 4 - 4.1 MPa, and the current of the extruder is 205 - 215 A.

[0019] The present invention simultaneously uses propylene and propionaldehyde as regulators, making the product have a slightly broader relative molecular mass distribution, increasing the methyl branching degree, reducing the product crystallinity, and ensuring that the medium - melt - index injection molding compound has excellent processing performance and comprehensive performance.

[0020] Increasing the reaction temperature can enhance the mobility of molecular chains, increase the probability of collision with ethylene monomers or comonomers, thereby increasing the degree of short-chain branching and further reducing the product density. Increasing the reaction temperature also raises the rates of the initiation elementary reaction and the chain transfer elementary reaction, increases the degree of short-chain branching, and thus reduces the product density. Additionally, a high concentration of long-chain molecules increases the probability of chain transfer when long-chain molecules collide with active monomers, resulting in more branched molecular chains. As the reaction progresses, the concentration of long-chain molecules in the fourth reaction zone of the reactor is relatively high. According to the requirements of polymerization process control, appropriately increasing the reaction temperature in the fourth reaction zone of the reactor, controlled at 280 - 290 °C, can increase the degree of branching of molecular chains.

[0021] The reaction pressure is an important factor affecting the molecular structure of the product. A high reaction pressure produces more long-chain macromolecules, inhibiting long-chain branching and short-chain branching; when the reaction pressure decreases, the activity of long-chain molecules increases, and long-chain branching and short-chain branching increase. The pressure in the tubular reactor gradually decreases from the front to the back. More long-chain molecules are produced at the front end of the reactor, and more branched molecular chains are produced at the tail end of the reactor. Therefore, appropriately reducing the pressure of the reactor and controlling the reaction pressure at 255.0 - 260.0 MPa can increase the degree of long-chain branching and short-chain branching of the product.

[0022] After adjusting the process, the degree of long-chain branching and short-chain branching of the product increases, long-chain branching and short-chain branching become more numerous, the macroscopic density decreases, the Vicat softening temperature decreases, the methyl branching degree increases, improving the hardness and density of the product, and making the product performance more suitable for the production of bottle caps.

[0023] Determination of the technical indicators of the melt index injection-molded low-density polyethylene in the present invention:

[0024] When LDPE resin is applied to injection-molded bottle caps, it is required to have a short demolding time to improve production efficiency; the resin needs to have good flexibility to ensure good sealing and fastening properties between the bottle cap and other components.

[0025] ① Density

[0026] The physical properties of polyethylene are determined by its macromolecular chain structure and the aggregated state structure. Different conformational arrangements of macromolecular chains determine its aggregated state structure. The density of LDPE resin macroscopically reflects the molecular chain structure and crystalline morphology inside the material. The density of the resin has a linear relationship with the crystallinity and Shore hardness. As the density increases, the crystallinity and Shore hardness of the resin increase, and the demolding time of the injection-molded bottle cap products shortens, but the flexibility and sealing performance of the resin will be reduced. Considering the comprehensive performance of the products and processing requirements and the limitations of the polymerization process conditions, the density of the medium melt index injection-molded low-density polyethylene 2445L is finally determined to be (0.921 ± 0.002) g / cm 3 .

[0027] ②Melt flow rate (MFR)

[0028] For injection - molded bottle caps, the shorter the injection cycle, the better. This requires that the molten material is easy to flow under a certain pressure. The faster it flows, the more quickly it can fill the cavity, improving production efficiency. Especially for molds with complex cavities, narrow runners, large projected areas, and thin - sheet parts, more attention is paid to the processing fluidity of the molten material. From the perspective of the product, on the premise of ensuring molding, it is also necessary to meet a shorter demolding time and better mechanical properties (including the flexibility and sealing performance of the bottle cap). Considering the above requirements comprehensively and combining with the polymerization process, the MFR of medium - melt - index injection - molded low - density polyethylene 2445L is determined to be (7.5 ± 1.0) g / 10min.

[0029] ③Mechanical properties

[0030] For injection - molded bottle caps, it is also necessary to ensure a certain stiffness and flexibility during the matching process with other components to ensure the smoothness of the assembly process. Appropriate mechanical properties can ensure the safety of the contained substances during the encapsulation, storage, and transportation of the bottle cap. Considering the requirements of injection - molded bottle caps for resin, the information feedback from the manufacturer, and the requirements of the polymerization process, the tensile fracture stress of medium - melt - index injection - molded low - density polyethylene 2445L is determined to be ≥8 MPa, and the nominal strain at break is ≥80%.

[0031] The present invention can have the following beneficial effects compared with the above - mentioned background technology:

[0032] The medium - melt - index injection - molded low - density polyethylene product of the present invention has a relatively high melt index, low density, slightly wider relative molecular mass distribution, high methyl branching degree, low product crystallinity, good mechanical properties, and excellent processing performance and comprehensive performance. The performance of the product is more suitable for the inner cap of the production of bottle caps. The medium - melt - index injection - molded low - density polyethylene product of the present invention meets the quality standards of the benchmark product. Specific embodiments:

[0033] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0034] The experimental methods used in the following embodiments are all conventional methods unless otherwise specified; the materials, reagents, etc. used in the following embodiments can be obtained from commercial channels unless otherwise specified.

[0035] Control of the medium - melt - index injection - molded low - density polyethylene polymerization process of the present invention:

[0036] The density of the medium melt index injection molded low density polyethylene product is (0.921±0.002) g / cm 3 , and propylene and propionaldehyde are used as the relative molecular mass regulators in the device.

[0037] 1) Polymerization temperature

[0038] For injection molded bottle cap products, it is required that the resin has a more suitable MFR, relative molecular mass and a wider relative molecular mass distribution, so that the products have better toughness and strength. In the production process of LDPE, the lower the reaction pressure, the higher the reaction temperature, and the longer the residence time of the raw materials in the reactor, the easier it is to form long chain branching. The degree of long chain branching in LDPE has an impact on the melt viscosity, elongation and elasticity. The long branched chains affect the weight average molecular weight. The increase in long branched chains makes the weight average molecular weight increase, and then makes the relative molecular mass distribution wider. In order to obtain the desired weight average molecular weight, degree of long chain branching and relative molecular mass distribution, in the design of the reactor temperature distribution, it is necessary to focus on the temperature setting at the head and tail of the reactor. According to the process design requirements and past production operation experience, the initiation temperatures of the first to fourth zones of the reactor are determined to be 170 - 210°C, 180 - 210°C, 180 - 210°C, 170 - 200°C respectively; the peak temperatures are 290 - 300°C, 280 - 295°C, 280 - 295°C, 280 - 290°C respectively.

[0039] 2) Polymerization pressure

[0040] As the polymerization pressure decreases, the density decreases; the lower the reaction pressure, the easier it is to form long chain branching. In order to obtain the desired degree of long chain branching and density, a lower reaction pressure should be set. However, the setting of the minimum operating pressure of the reactor should ensure that the materials at the tail end of the reactor remain in the same phase and there should be no liquid phase. Therefore, the reaction pressure cannot be lower than 240.0 MPa. When producing the medium melt index injection molded low density polyethylene 2445L, the reaction pressure is set at 255.0 - 270.0 MPa.

[0041] 3) Additives

[0042] Considering the short processing time of the bottle cap material and its application fields, no additives are added when producing the medium melt index injection molded low density polyethylene, which not only reduces the production cost but also increases the safety of the products used in the food field.

[0043] The present invention provides a preparation method of medium melt index injection molded low density polyethylene, comprising the following steps:

[0044] S1, preparing the raw materials for low density polyethylene: monomer ethylene as the main raw material, propionaldehyde and propylene as the molecular weight regulators, and peroxide as the initiator;

[0045] The weight ratio of each component of the raw materials is as follows: the weight ratio of ethylene, propionaldehyde, propylene, and initiator is 997 - 999:0.54:11 - 12:1.89.

[0046] The composition and ratio of the initiator are as follows: the weight ratio of isododecane, peroxide DTBP, peroxide TBPIN, peroxide TBPEH, and peroxide TBPPI is 1.64:0.16:0.04:0.03:0.02.

[0047] S2. Preparation of low - density polyethylene:

[0048] Use the LUPOTECHT tubular reactor of Basell Company in Germany. In the tubular reactor, under the action of the mixed gas of ethylene, propylene, and propionaldehyde, and the prepared initiator, a polymerization reaction occurs under the conditions of a reaction pressure of 255 - 270 Mpa and a reaction temperature of 280 - 300 °C, and a molten - state polyethylene product with a melt index of 6.5 - 8.5 g / 10min and a density of 0.919 - 0.923 g / cm 3 is produced;

[0049] S3. The molten polyethylene product is extruded and pelletized by an extruder to obtain pelletized molten polyethylene material.

[0050] It enters the high - pressure separator through the reactor pulse valve. After gas - liquid separation, the molten polyethylene product enters the low - pressure separation tank. The material goes from the outlet of the low - pressure separator to the single - screw extruder. After extrusion by the extruder, it is pelletized by a pelletizer. The rotation speed of the extruder is 57 - 58, the die head pressure of the extruder is 4 - 4.1 MPa, and the current of the extruder is 205 - 215 A; the product after pelletization is degassed in a degassing bin for 12 hours and then sent for packaging.

[0051] Example 1

[0052] The preparation method of medium - melt - index injection - molding low - density polyethylene is specifically illustrated by producing the medium - melt - index injection - molding low - density polyethylene 2445L product in Daqing Petrochemical. The method includes the following steps:

[0053] Step 1. Preparation of the raw materials for low - density polyethylene: Use the patented technology of the LUPOTECHT tubular reactor of Basell Company in Germany. Use ethylene as the main raw material (polymerization monomer), propionaldehyde and propylene as molecular weight regulators, and peroxide as the initiator for the reaction.

[0054] For our company, the unit consumption for producing one ton of medium melt index injection molding low density polyethylene 2445L product is as follows: ethylene unit consumption is 998.2 kg / t, isododecane unit consumption is 1.64 kg / t, dicumyl peroxide (DTBP) unit consumption is 0.16 kg / t, tert-butyl peroxyisopropyl carbonate (42S) unit consumption is 0.04 kg / t, tert-butyl peroxy-2-ethylhexanoate

[0055] (21S) unit consumption is 0.03 kg / t, tert-butyl peroxypivalate (C75) unit consumption is 0.02 kg / t, propionaldehyde unit consumption is 0.54 kg / t, propylene unit consumption is 12 kg / t.

[0056] Step 2: Preparation of low density polyethylene:

[0057] Inject propylene into the inlet of the primary compressor at a flow rate of 310 kg / h and a pressure of 2.5 MPa; propylene is mixed with ethylene at a flow rate of 25.4 t / h and a pressure of 2 MPa at the inlet of the primary compressor to form a process gas, which is compressed to 4 - 5 MPa by the first-stage cylinder of the primary compressor. Adjust the stroke of the on-site propionaldehyde pump to 12, so that propionaldehyde is injected into the outlet of the first-stage cylinder of the primary compressor (4 - 5 MPa) at a flow rate of 15 kg / h. At this time, the mixed gas of ethylene, propylene, and propionaldehyde is compressed to 24 - 25 MPa by the primary compressor, and then compressed to 255 - 260 MPa by the secondary compressor;

[0058] The gas compressed to 255 - 260 MPa is heated to 160 - 165 °C using hot water and high-pressure steam. Use a peroxide pump to inject the peroxides in the four injection zones that have been prepared into the four reaction zones of the reactor. Control the peak temperature of reaction 1 at 290 - 295 °C, the peak temperature of reaction 2 at 290 - 295 °C, the peak temperature of reaction 3 at 289 - 293 °C, and the peak temperature of reaction 4 at 285 - 290 °C by controlling the stroke of the peroxide pump (pump 1 stroke 62, pump 2 stroke 65, pump 3 stroke 52, pump 4 stroke 66). Under the conditions of ultra-high pressure (255 - 260 Mpa), initiation temperature (160 - 165 °C), and the action of the prepared peroxides as initiators, a free radical polymerization reaction occurs, and a molten polyethylene product with a melt index of 6.5 - 8.5 g / 10 min and a density of 0.919 - 0.923 g / cm 3 is produced in the reactor.

[0059] The preparation process of the initiator peroxide: The polymerization reaction needs to be initiated by an appropriate chain initiator, and a peroxide is used as the initiator.

[0060] The peroxide injection unit dilutes the peroxide to a specified concentration with an organic solvent (isododecane) to prepare a peroxide initiator. This unit provides an appropriate initiator to each reaction zone to initiate the polymerization reaction.

[0061] The composition and proportion of the initiator are as follows: The weight ratio of isododecane, peroxide DTBP, peroxide TBPIN, peroxide TBPEH, and peroxide TBPPI is 1.64:0.16:0.04:0.03:0.02.

[0062] Step 3: The molten polyethylene product is extruded and pelletized by an extruder to obtain pelletized molten polyethylene material.

[0063] The produced molten polyethylene product enters the high-pressure separator through the reactor pulse valve. After gas-liquid separation, the molten polyethylene product enters the low-pressure separation tank. The material goes to the single-screw extruder from the outlet of the low-pressure separator. After extrusion by the extruder, it is pelletized by a pelletizer. The rotational speed of the extruder is 57 - 58 rpm, the die head pressure of the extruder is 4 - 4.1 MPa, and the current of the extruder is 205 - 215 A. The product after pelletization is degassed in a degassing bin for 12 hours and then sent for packaging.

[0064] Test Example 1

[0065] Perform performance tests on the medium-density polyethylene resin for melt injection molding prepared in the examples. The test indicators are shown in Table 1.

[0066] Table 1

[0067]

[0068] The product of the present invention was tested in Sichuan Yibin Pulasi Packaging Materials Co., Ltd. for condiment processing. At an injection pressure of 1000 bar, an injection speed of 110 m / s, a holding pressure of 58 bar, and a holding time of 1.8 s, the functionality of the product is qualified and the hygienic performance is also qualified. The plasticizer and the material meet the requirements.

[0069] This product has passed the certification of SGS-CSTC Standards Technical Services Co., Ltd. (China National Accreditation Service for Conformity Assessment TESTING CNAS L2774). The ignition residue, loss on drying, sensory properties (appearance: normal color, no abnormal odor, no impurities, etc.), and n-hexane extract of the product have been tested and certified, and the results all show compliance.

[0070] Those of ordinary skill in the art will realize that the embodiments described herein are for helping readers understand the implementation methods of the present invention, and it should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those of ordinary skill in the art can make various other specific deformations and combinations that do not deviate from the essence of the present invention based on the technical revelations disclosed in the present invention, and these deformations and combinations are still within the protection scope of the present invention.

Claims

1. A preparation method of medium melt index injection-molded low-density polyethylene, characterized in that: It includes the following steps: S1. Prepare the raw materials for low-density polyethylene: including the main raw material monomer ethylene, molecular weight regulator and initiator; S2. Prepare low-density polyethylene; In a tubular reactor, a polymerization reaction occurs under the action of a mixed gas of ethylene and a molecular weight regulator, as well as a prepared initiator, under the conditions of a reaction pressure of 255 - 270 Mpa and a reaction temperature of 280 - 300 °C, and a molten polyethylene product with a melt index of 6.5 - 8.5 g / 10 min and a density of 0.919 - 0.923 g / cm 3 is produced; S3. The molten polyethylene product is extruded and pelletized by an extruder to obtain pelletized molten polyethylene material.

2. The preparation method of medium melt index injection-molded low-density polyethylene according to claim 1, characterized in that: The molecular weight regulator is propionaldehyde and propylene.

3. The preparation method of medium melt index injection-molded low-density polyethylene according to claim 2, characterized in that: The weight ratio of each component of the raw materials is as follows: the weight ratio of ethylene, propionaldehyde, propylene and initiator is 997-999:0.54:11-12:1.

89.

4. The preparation method of medium melt index injection-molded low-density polyethylene according to claim 1, characterized in that: The composition and ratio of the initiator are as follows: the weight ratio of isododecane, peroxide DTBP, peroxide TBPIN, peroxide TBPEH, peroxide TBPPI is 1.64:0.16:0.04:0.03:0.

02.

5. The preparation method of medium melt index injection-molded low-density polyethylene according to claim 1, characterized in that: The tubular reactor adopts the LUPOTECHT tubular reactor of Basell Company, Germany.

6. The preparation method of medium melt index injection-molded low-density polyethylene according to claim 1, characterized in that: In the design of the reactor temperature distribution, the initiation temperatures in reaction zones 1 to 4 are 170-210°C, 180-210°C, 180-210°C, 170-200°C; the peak temperatures in reaction zones 1 to 4 are 290-300°C, 280-295°C, 280-295°C, 280-290°C respectively.

7. The preparation method of medium melt index injection-molded low-density polyethylene according to claim 1, characterized in that: The reaction pressure is set at 255.0-260.0 MPa.

8. The preparation method of medium melt index injection-molded low-density polyethylene according to claim 7, characterized in that: The reaction pressure is 260 MPa.

9. The preparation method of medium melt index injection-molded low-density polyethylene according to claim 1, characterized in that: The process of extruding and pelletizing the molten polyethylene product by an extruder is as follows: The molten polyethylene product enters a low-pressure separation tank, the material goes from the outlet of the low-pressure separator to a single-screw extruder, after extrusion by the extruder, it is pelletized by a pelletizer, and the pelletized product is degassed in a degassing bin for 12 hours and then sent for packaging.

10. The preparation method of medium melt index injection-molded low-density polyethylene according to claim 9, characterized in that: The rotation speed of the extruder is 57-58 rpm; the die head pressure of the extruder is 4-4.1 MPa, and the current of the extruder is 205-215 A.

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