Narrow molecular weight distribution polypropylene and method of making

Abstract

The embodiment of the application provides a kind of narrow molecular weight distribution polypropylene and preparation method, wherein preparation method is by the screw extrusion part of extrusion equipment to polypropylene raw material melt mixing and stretch shearing, the first screw and the second screw of screw extrusion part differentially rotate, and the temperature of different heating section barrel of screw extrusion part is different;Polypropylene raw material is extruded to cooling part cooling;The polypropylene strip is cut into granules by granulating part to obtain the granules of narrow molecular weight distribution polypropylene;Preparation method is simple and low in cost, without adding chemical auxiliary agent, and the polypropylene granules prepared have narrow molecular weight distribution, high crystallinity, good mechanical properties and good foaming performance.

Description

Technical Field

[0001] The embodiments of this application relate to, but are not limited to, the field of polypropylene materials, and particularly to a narrow molecular weight distribution polypropylene and its preparation method. Background Technology

[0002] Polypropylene possesses excellent mechanical properties and good processing performance. Narrow molecular weight distribution polypropylene exhibits superior filamentation properties, is less prone to chain breakage, and produces filaments with more uniform diameter. This results in meltblown fabrics made from narrow molecular weight distribution polypropylene exhibiting more uniform and superior performance, thus finding widespread application in masks, protective clothing, and related fields.

[0003] Currently, the main methods for preparing narrow molecular weight distribution polypropylene are catalytic and chemical degradation methods. Both methods control the degree of reaction by adjusting the type and content of the added catalysts or degradation aids, thereby obtaining the desired molecular weight distribution. However, catalysts, degradation aids, and byproducts generated during the reaction can easily remain in the final polypropylene product, affecting its purity and quality. Summary of the Invention

[0004] The following is an overview of the subject matter described in detail herein. This overview is not intended to limit the scope of the claims.

[0005] This application provides a narrow molecular weight distribution polypropylene and its preparation method.

[0006] An embodiment of the first aspect of this application provides a method for preparing polypropylene granules, comprising:

[0007] Polypropylene raw materials are melt-blended and stretched by the screw extrusion section of the extrusion equipment to obtain polypropylene strips. The first screw and the second screw of the screw extrusion section rotate at different speeds, and the temperature of the barrel in different heating sections of the screw extrusion section is different.

[0008] The polypropylene strip is extruded into the cooling section of the extrusion equipment for cooling.

[0009] The cooled polypropylene strips are granulated through the granulation section of the extrusion equipment to obtain polypropylene granules with a narrow molecular weight distribution.

[0010] In certain embodiments of the first aspect of this application, the preparation method includes, prior to melt mixing and stretch shearing of the polypropylene raw material through the screw extrusion section of an extrusion device:

[0011] The polypropylene raw material is dried.

[0012] In some embodiments of the first aspect of this application, the drying temperature range for drying the polypropylene raw material is 50°C to 85°C, and the drying time ranges from 2 hours to 3 hours.

[0013] In some embodiments of the first aspect of this application, the speed ratio of the first screw to the second screw ranges from 1:5 to 2:3.

[0014] In some embodiments of the first aspect of this application, the rotational speed of the first screw ranges from 30 rpm to 1500 rpm.

[0015] In some embodiments of the first aspect of this application, the temperature range of the cavity of the screw extrusion section is 80°C to 220°C.

[0016] In certain embodiments of the first aspect of this application, the first screw is provided with a first engagement block, the second screw is provided with a second engagement block, the staggered angle of the first engagement block is in the range of 20 degrees to 180 degrees, and the staggered angle of the second engagement block is in the range of 20 degrees to 180 degrees.

[0017] In some embodiments of the first aspect of this application, the ratio of the length of the first engagement block to the total length of the first screw ranges from 25% to 60%; the ratio of the length of the second engagement block to the total length of the second screw ranges from 25% to 60%.

[0018] According to an embodiment of the second aspect of this application, a polypropylene granule is prepared according to the preparation method described above.

[0019] The above-mentioned method has at least the following beneficial effects: the preparation method is simple to operate, low in cost, and easy to promote. It achieves the preparation of polypropylene with a narrow molecular weight distribution without the need for chemical additives, ensuring the purity of the finished polypropylene product and guaranteeing that the prepared polypropylene granules are non-toxic and harmless, thus making the preparation process and product conform to the principles of green manufacturing. The prepared polypropylene granules have a narrow molecular weight distribution, high crystallinity, good mechanical properties, and good foaming properties. Attached Figure Description

[0020] The accompanying drawings are used to provide a further understanding of the technical solutions of this application and constitute a part of the specification. They are used together with the embodiments of this application to explain the technical solutions of this application and do not constitute a limitation on the technical solutions of this application.

[0021] Figure 1 This is a structural schematic diagram of the extrusion equipment;

[0022] Figure 2 This is a schematic diagram of the structure when the first screw and the second screw are engaged together.

[0023] Figure 3This is a step diagram of the method for preparing polypropylene granules provided in the embodiments of this application;

[0024] Figure 4 This is a scanning electron microscope image of the brittle fracture surface of a foamed sample obtained by molding polypropylene raw material into a sheet and then foaming, as described in Comparative Example 1.

[0025] Figure 5 This is a scanning electron microscope image of the brittle fracture surface of the foamed sample obtained by molding polypropylene granules into sheets and then foaming, as described in Comparative Example 2.

[0026] Figure 6 This is a scanning electron microscope image of the brittle fracture surface of a foamed sample obtained by molding polypropylene granules into sheets and then foaming them, as described in Example 1.

[0027] Figure 7 This is a bar chart showing the tensile strength of standard mechanical specimens obtained by pressing and cutting the polypropylene raw material of Comparative Example 1, the polypropylene granules of Comparative Example 2, the polypropylene granules of Example 1, and the polypropylene granules of Example 2 into standard mechanical specimens. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0029] It should be noted that although functional modules are divided in the device schematic diagram and a logical order is shown in the flowchart, in some cases, the steps shown or described may be performed in a different order than the module division in the device or the order in the flowchart. The terms "first," "second," etc., in the specification, claims, or the aforementioned drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0030] The embodiments of this application will be further described below with reference to the accompanying drawings.

[0031] The embodiments of this application show the following schematic diagrams of the extruder structure. Figure 1 As shown.

[0032] Reference Figure 1 The extrusion equipment includes a screw feeding section 100, an extrusion section 200, a cooling section 300, and a granulation section 400.

[0033] The screw extrusion section 200 has a first screw 210 and a second screw 220 inside its cavity. The first screw 210 and the second screw 220 mesh with each other. The first screw 210 is driven by a first driver, and the second screw 220 is driven by a second driver. The cooling section 300 is connected to the first discharge port of the screw extrusion section 200. The granulation section 400 is connected to the second discharge port of the cooling section 300. The granulation section 400 is equipped with a traction roller 410 and a cutter 420.

[0034] The upper end of the screw extrusion section 200 is provided with a feeding hopper 100, through which polypropylene raw materials can be fed into the screw groove of the screw extrusion section 200.

[0035] Reference Figure 2 The first screw 210 is provided with a first engaging block 221, and the second screw 220 is provided with a second engaging block 222. The staggered angle of the first engaging block 221 ranges from 20 degrees to 180 degrees, and the staggered angle of the second engaging block 222 also ranges from 20 degrees to 180 degrees. Specifically, the staggered angle of the first engaging block 221 can be 30 degrees, 45 degrees, 60 degrees, 90 degrees, 120 degrees, or 180 degrees; the staggered angle of the second engaging block 222 can be 30 degrees, 45 degrees, 60 degrees, 90 degrees, 120 degrees, or 180 degrees.

[0036] The ratio of the length of the first engagement block 221 to the total length of the first screw 210 ranges from 25% to 60%; the ratio of the length of the second engagement block 222 to the total length of the second screw 220 ranges from 25% to 60%.

[0037] The first meshing block 221 and the second meshing block 222 cooperate to perform a stretching and shearing action. The stretched and unfolded polypropylene molecular chains (especially long chains) are sheared and broken under the action of shearing force, thereby achieving the purpose of homogenizing the length of the molecular chains.

[0038] The first screw 210 is equipped with a forward conveying rod 224, and the second screw 220 is equipped with a reverse conveying rod 223. The forward conveying element mainly serves to convey materials forward, while the reverse conveying element mainly serves to block the materials conveyed by the forward conveying element. Together with the tensile and shearing action of the first meshing block 221 and the second meshing block 222, a better effect of homogenizing the molecular chain length can be achieved.

[0039] The cooling section 300 is provided with a water inlet and a water outlet, and the material can be cooled by liquid cooling in the cooling section 300.

[0040] An embodiment of this application provides a green preparation method for polypropylene with a narrow molecular weight distribution, using the extrusion equipment described above.

[0041] Reference Figure 3 The preparation methods of polypropylene granules include:

[0042] Step S100: Dry the polypropylene raw material;

[0043] In step S200, the polypropylene raw material is melt-blended and stretched by the screw extrusion section 200 of the extrusion equipment to obtain polypropylene strips. The first screw 210 and the second screw 220 of the screw extrusion section 200 rotate at different speeds, and the temperatures of different barrel sections of the screw extrusion section 200 are different.

[0044] In step S300, the polypropylene strip that has undergone stretching and shearing to break the chain is drawn to the cooling section 300 of the extrusion equipment for cooling.

[0045] In step S400, the cooled polypropylene strip is granulated through the granulation section 400 of the extrusion equipment to obtain polypropylene granules.

[0046] The above-described method for preparing polypropylene granules is simple to operate, low in cost, and easy to promote. It achieves the preparation of polypropylene with a narrow molecular weight distribution without the need for chemical additives, ensuring that the resulting polypropylene granules are non-toxic and harmless, thus conforming to green manufacturing principles in both the preparation process and the final product. The prepared polypropylene granules exhibit a narrow molecular weight distribution, high crystallinity, good mechanical properties, and good foaming performance.

[0047] For step S100, the drying temperature range for drying the polypropylene raw material is 50℃ to 85℃, and the drying time range is 2h to 3h.

[0048] In step S200, the polypropylene raw material is melt-blended and subjected to stretch shearing to break the chains through the screw extrusion section 200 of the extrusion equipment. The speed ratio between the first screw 210 and the second screw 220 ranges from 1:5 to 2:3. Specifically, the first screw 210 is a low-speed screw, and the second screw 220 is a high-speed screw; the speed of the second screw 220 is higher than that of the first screw 210.

[0049] The rotational speed of the first screw 210 ranges from 30 rpm to 1500 rpm. The rotational speed of the second screw 220 can be calculated based on the rotational speed ratio between the first screw 210 and the second screw 220.

[0050] The barrel temperature range of the screw extrusion section 200 is 80°C to 220°C.

[0051] Because the first screw 210 and the second screw 220 rotate at different speeds, a tensile force is generated from the lower-speed screw to the higher-speed screw, causing some polypropylene molecular chains to orient under the action of the tensile force. When passing through the meshing blocks, under the meshing action of the first meshing block 221 and the second meshing block 222, the unoriented polypropylene molecular chains are more disordered than the oriented polypropylene molecular chains, with long and short chains being randomly sheared. The oriented polypropylene molecular chains have longer chains that are more easily sheared. The fact that longer-chain polypropylene is easier to cut than shorter-chain polypropylene reduces the proportion of long-chain polypropylene, ultimately resulting in polypropylene with a narrow molecular weight distribution. The shortening of long chains increases the number-average molecular weight and crystallinity, ultimately increasing the tensile strength of the polypropylene.

[0052] By setting different temperatures in different barrel sections, it is beneficial to stretch and shear the polypropylene molecular chains to break them.

[0053] For step S300, since the polypropylene raw material undergoes high-temperature melt mixing in the screw extrusion section 200, the polypropylene strip leaving the screw extrusion section 200 has a high temperature and needs to be cooled. The polypropylene strip that has undergone shearing and chain breaking in the screw extrusion section 200 is extruded to the cooling section 300 of the extrusion equipment for cooling by liquid cooling.

[0054] In step S400, the cooled polypropylene strip is granulated through the granulation section 400 of the extrusion equipment to obtain polypropylene granules.

[0055] An embodiment of this application provides a polypropylene granule, prepared according to the preparation method described above.

[0056] Example 1

[0057] Polypropylene raw material with a melt index of 50 was dried in a forced-air drying oven at 80℃ for 2 hours. The barrel temperatures of each section of the extruder along the extrusion direction were set to 120℃-150℃-170℃-180℃-180℃-190℃-185℃. Once the temperature reached the set value and stabilized, the dried polypropylene granules were fed into the extrusion equipment feed port. Melt extrusion, cooling traction, and granulation were performed at a low-speed screw speed of 400 rpm, a screw speed ratio of 1:2 between the first screw 210 and the second screw 220, and a production rate of 16 kg / h. During this process, the polypropylene molecular chains oriented under the tensile force generated by the two screws with different speeds. After orientation, the longer polypropylene molecular chains are more easily sheared. The easier it is to cut long-chain polypropylene compared to short-chain polypropylene reduces the proportion of long-chain polypropylene, resulting in polypropylene granules with a narrow molecular weight distribution. After stabilizing extrusion for 10 minutes, the obtained polypropylene granules were dried and hot-pressed to obtain polypropylene sheets.

[0058] Example 2

[0059] Polypropylene raw material with a melt index of 50 was dried in a forced-air drying oven at 80℃ for 2 hours. The barrel temperatures of each section of the extruder along the extrusion direction were set to 120℃-150℃-170℃-180℃-180℃-190℃-185℃. Once the temperature reached the set value and stabilized, the dried polypropylene granules were fed into the extrusion equipment feed port. Melt extrusion, cooling traction, and granulation were performed at a low-speed bar rotation speed of 500 rpm, a screw speed ratio of 1:2 (first screw 210 to second screw 220), and a production rate of 16 kg / h. During this process, the polypropylene molecular chains oriented under the tensile force generated by the two screws with different rotation speeds. After orientation, the longer polypropylene molecular chains are more easily sheared. The easier it is to cut longer-chain polypropylene compared to shorter-chain polypropylene reduces the proportion of long-chain polypropylene, resulting in polypropylene granules with a narrow molecular weight distribution. After stabilizing extrusion for 10 minutes, the obtained polypropylene granules were dried and hot-pressed to obtain polypropylene sheets.

[0060] Comparative Example 1

[0061] Polypropylene raw materials with a melt flow index of 50 are dried and hot-pressed to obtain polypropylene sheets.

[0062] Comparative Example 2

[0063] Polypropylene raw material with a melt index of 50 was placed in a forced-air drying oven and dried at 80℃ for 2 hours. The temperatures of each section of the extruder along the extrusion direction were set to 120℃-150℃-170℃-180℃-180℃-190℃-185℃. After the temperature reached the set temperature and stabilized, the dried polypropylene granules were fed into the extrusion equipment. Melt extrusion, cooling, and traction granulation were carried out at a low-speed screw speed of 400 rpm, a screw speed ratio of 1:1 between the first screw 210 and the second screw 220, and a production rate of 16 kg / h. After stabilizing the extrusion for 10 minutes, the resulting polypropylene granules were dried and hot-pressed to obtain polypropylene sheets.

[0064] The number-average molecular weight and molecular weight distribution coefficient of the polypropylene granules obtained in Examples 1, 2, 1, and 2 were measured. The number-average molecular weight and distribution coefficient were determined by gel permeation chromatography (GPC) using 1,2,4-trichlorobenzene as the solvent.

[0065] Table 1 Comparison of Number Average Molecular Weight and Molecular Weight Distribution Coefficient of Polypropylene Granules

[0066] Number-average molecular weight (g / mol) Distribution coefficient Example 1 45342 3.9871 Example 2 48605 3.78204 Comparative Example 1 42240 4.56529 Comparative Example 2 43790 4.35405

[0067] As shown in Table 1, compared with Comparative Examples 1 and 2, the polypropylene granules obtained in Examples 1 and 2 have smaller molecular weight distribution coefficients, indicating that the latter two have more uniform molecular weight distributions. The reasons are as follows: The dried polypropylene raw material encounters shearing and stretching action from the interlocking blocks in the extruder. Under stretching, the polypropylene is oriented, and long chains have a greater chance of being sheared, turning one long chain into multiple short chains, resulting in molecular weight aggregation and a smaller distribution coefficient. In Comparative Example 2, the two screws of the extrusion equipment rotate at equal speeds, and only the interlocking blocks shear and break the polypropylene molecular chains. Compared with Example 1, there is less stretching action, resulting in a lower degree of molecular orientation, with both long and short chains being randomly sheared, leading to a relatively wider molecular weight distribution of the obtained polypropylene. Compared with Example 1, Example 2 uses a higher screw speed, therefore the differential screw exhibits a stronger stretching effect, which, combined with the chain-breaking effect of the interlocking blocks, results in a narrower number-average molecular weight of the polypropylene granules obtained in Example 2.

[0068] The enthalpy of melting of polypropylene granules obtained in Examples 1, 2, Comparative Example 1, and 2 was measured. The enthalpy was obtained using differential scanning calorimetry (DSC) under nitrogen atmosphere. The test started at 30°C, increased at a rate of 10°C / min to 220°C, and held for 10 min. The temperature was then decreased to 30°C at a rate of 10°C / min and held for 10 min. The temperature was then increased again to 220°C at a rate of 10°C / min. The enthalpy obtained during the second heating cycle was recorded. The enthalpy of the polypropylene granules reflects the degree of crystallinity.

[0069] Table 2 Comparison of enthalpy values ​​of polypropylene granules

[0070] Enthalpy (joules per gram) Example 1 92.42 Example 2 95.45 Comparative Example 1 91.52 Comparative Example 2 92.32

[0071] As shown in Table 2, compared with Comparative Example 1, the enthalpy values ​​of the polypropylene granules in Comparative Example 2, Example 1, and Example 2 all increased, indicating that the polypropylene granules in Comparative Example 2, Example 1, and Example 2 had greater crystallinity. The increased crystallinity is because the number-average molecular weight of the samples is still within a relatively small range; within this small range, the crystallinity increases slightly with the increase of the number-average molecular weight.

[0072] Reference Figure 4 , Figure 5 and Figure 6 As can be seen, the polypropylene granules of Example 1 have better foaming properties than the polypropylene granules of Comparative Example 1 and Comparative Example 2.

[0073] Reference Figure 7 Compared with Comparative Example 1 and Comparative Example 2, the polypropylene granules obtained in Example 1 and Example 2 have higher tensile strength because the molecular weight distribution is narrower and the crystallinity is slightly increased, which ultimately results in greater tensile strength.

[0074] Although embodiments of this application have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of this application, the scope of which is defined by the claims and their equivalents.

[0075] The above is a detailed description of the preferred embodiments of this application, but this application is not limited to the embodiments. Those skilled in the art can make various equivalent modifications or substitutions without departing from the spirit of this application, and these equivalent modifications or substitutions are all included within the scope defined by the claims of this application.

Claims

1. A method for preparing polypropylene with a narrow molecular weight distribution, characterized in that, include: Polypropylene raw material is melt-mixed and stretched and sheared through the screw extrusion section of the extrusion equipment to obtain polypropylene strips. The first screw and the second screw of the screw extrusion section rotate at different speeds, and the temperature of the barrel in different heating sections of the screw extrusion section is different. The polypropylene strip is extruded into the cooling section of the extrusion equipment for cooling. The cooled polypropylene strips are granulated through the granulation section of the extrusion equipment to obtain polypropylene granules with a narrow molecular weight distribution. The first screw is provided with a first engagement block, and the second screw is provided with a second engagement block. The staggered angle of the first engagement block is in the range of 20 degrees to 180 degrees, and the staggered angle of the second engagement block is in the range of 20 degrees to 180 degrees.

2. The method for preparing narrow molecular weight distribution polypropylene according to claim 1, characterized in that, The preparation method includes the following steps, prior to the melt mixing and stretch shearing of the polypropylene raw material through the screw extrusion section of an extrusion device: The polypropylene raw material is dried.

3. The method for preparing narrow molecular weight distribution polypropylene according to claim 2, characterized in that, The drying temperature range for the polypropylene raw material is 50°C to 85°C, and the drying time range is 2 hours to 3 hours.

4. The method for preparing narrow molecular weight distribution polypropylene according to claim 1, characterized in that, The speed ratio between the first screw and the second screw ranges from 1:5 to 2:

3.

5. The method for preparing narrow molecular weight distribution polypropylene according to claim 1, characterized in that, The rotational speed of the first screw ranges from 30 rpm to 1500 rpm.

6. The method for preparing narrow molecular weight distribution polypropylene according to claim 1, characterized in that, The barrel temperature range of the screw extrusion section is 80°C to 220°C.

7. The method for preparing narrow molecular weight distribution polypropylene according to claim 1, characterized in that, The ratio of the length of the first engagement block to the total length of the first screw ranges from 25% to 60%; the ratio of the length of the second engagement block to the total length of the second screw ranges from 25% to 60%.

8. A narrow molecular weight distribution polypropylene, characterized in that, It is prepared according to the preparation method described in any one of claims 1 to 7.

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