CN114437297B - Polyethylene powder and preparation method thereof - Google Patents
Polyethylene powder and preparation method thereof Download PDFInfo
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- CN114437297B CN114437297B CN202111669050.5A CN202111669050A CN114437297B CN 114437297 B CN114437297 B CN 114437297B CN 202111669050 A CN202111669050 A CN 202111669050A CN 114437297 B CN114437297 B CN 114437297B
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- C08F255/00—Macromolecular compounds obtained by polymerising monomers on to polymers of hydrocarbons as defined in group C08F10/00
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Abstract
The application relates to a polyethylene powder and its preparation method, this powder granule has core-shell structure, the weight average molecular weight of polyethylene of the core is 10 ten thousand-60 ten thousand, the weight average molecular weight of polyethylene of the core is less than or equal to 3, the weight average molecular weight of polyethylene of the shell is 100 ten thousand-500 ten thousand, the molecular weight of polyethylene of the shell is less than or equal to 5, in the single granule, the core polyethylene accounts for 30-95 parts, the polyethylene of the shell accounts for 5-80 parts, this kind of polyethylene powder of the application prepares through the two-stage polymerization, can use the existing polyethylene polymerization apparatus, the flow is simple; the product prepared by taking the material as the raw material through melt processing has good mechanical property and excellent performance stability, and can be applied to multiple fields such as polyethylene pipes, sheets, plates, fibers and the like.
Description
Technical Field
The application belongs to the technical field of polyethylene materials, relates to polyethylene powder and a preparation method thereof, and in particular relates to polyethylene powder with a core-shell structure and a preparation method thereof.
Background
Polyethylene has low price and excellent performance, and various products of polyethylene have wider application fields. With the improvement of living standard, the performance requirements of people on polyethylene products are continuously improved. Increasing the molecular weight of polyethylene is an effective way to improve the properties of polyethylene materials. When the molecular weight of the polyethylene is higher than 100 ten thousand, the polyethylene may be referred to as ultra-high molecular weight polyethylene. Because the molecular chain length of the ultra-high molecular weight polyethylene is greatly improved compared with that of the traditional polyethylene, the entanglement degree of the chain is also obviously increased, so that special properties such as the wear resistance, the impact resistance and the like of the ultra-high molecular weight polyethylene are greatly improved compared with that of the traditional polyethylene. Products made from ultra-high molecular weight polyethylene are widely used in various industries.
However, the ultra-high molecular weight polyethylene has a high degree of chain entanglement, and even when the temperature is higher than the melting point of the polyethylene or even at a temperature of 250 ℃ far higher than the melting point, the ultra-high molecular weight polyethylene has no fluidity and cannot be melt-processed like conventional polyethylene. This processing challenge greatly limits the use of ultra high molecular weight polyethylene. At present, the ultra-high molecular weight polyethylene plates and castings can be prepared only through a hot pressing mode, then ultra-high molecular weight polyethylene products in various shapes can be prepared through post-processing, the whole preparation efficiency is low, continuous express processing is not possible, and the production cost is greatly increased.
In order to solve the above problems, chinese patent CN108774349a proposes modification of ultra-high molecular weight polyethylene, and then preparing a melt extrusion product such as ultra-high molecular weight polyethylene rod material by a screw extruder, but this method requires compounding raw materials, blending high density polyethylene or low density polyethylene having a molecular weight of 5-50 ten thousand with ultra-high molecular weight polyethylene, and improving melt processability of ultra-high molecular weight polyethylene by utilizing fluidity of high density polyethylene. Chinese patent CN104031305A proposes a special material for ultra-high molecular weight polyethylene pipe, which is prepared by mixing ultra-high molecular weight polyethylene with the molecular weight of 150 ten thousand with metallocene linear low density polyethylene, metallocene polyolefin elastomer and the like, polypropylene and the like to obtain a modified material, and can be used for preparing pipe by melt extrusion, and can be melt extruded at 260 ℃.
The method comprises the steps of firstly blending the traditional polyethylene (or polyolefin) with relatively low molecular weight with the ultra-high molecular weight polyethylene before melt extrusion, then adding the mixture into a feed inlet of a screw, and driving the ultra-high molecular weight polyethylene to move by the flowability of the traditional polyethylene in the screw to realize melt extrusion processing. The preparation process requires the raw materials to be modified and compounded, the modification cost is high, and the flow is relatively complex.
In addition, it should be noted that the ultra-high molecular weight polyethylene raw materials in the market at present are all powder materials (powder), and the particle diameter of the powder materials is about twenty to one hundred micrometers. The traditional polyethylene and polyolefin in the market are in the form of granules, the diameter of the granules is about 3-5 mm, and the diameter of the granules is far larger than that of the ultra-high molecular weight polyethylene powder. After they are physically blended, the ultra-high molecular weight polyethylene fine particles can move in the gaps of the conventional polyethylene particles and deposit to the bottom under the influence of gravity, so that the ultra-high molecular weight polyethylene powder and the conventional polyethylene (polyolefin) particles cannot be dispersed relatively uniformly. Therefore, even dispersion cannot be achieved before the materials enter the screw, stability and accuracy of the feeding ratio can be greatly influenced, and the difference of the component proportions of extruded parts and the product performance are influenced.
In order to solve the problem of uneven dispersibility, a grinding process is additionally added, and the conventional polyethylene is refined into powder, so that the ultra-high molecular weight polyethylene raw material and the conventional polyethylene raw material are similar in particle size, and a uniformly dispersed mixture is obtained. However, because the glass transition temperature of polyethylene is very low, the polyethylene granules can be mechanically crushed to the size close to that of the ultra-high molecular weight polyethylene after being cooled by liquid nitrogen, and an ultra-low temperature crusher is required, so that the processing cost is greatly increased.
Even though the two are uniformly mixed before entering the screw, when the traditional polyethylene (polyolefin) is heated and melted into a melt in the screw, the ultra-high molecular weight polyethylene cannot be melted due to high entanglement of molecular chains, the particle morphology cannot be changed greatly, the particles exist in the traditional polyethylene melt in the range of tens of micrometers to one hundred micrometers, the outer layer of the ultra-high molecular weight polyethylene can be contacted with the molten traditional polyethylene molecular weight, and the two have a similar chemical structure, so that the two have better interface interaction without phase separation, but the structure inside the ultra-high molecular weight polyethylene particles cannot be changed. The molecular chains of ultra-high molecular weight polyethylene and the molecular chains of conventional polyethylene in the extruded product thereof cannot be uniformly distributed throughout the entire product. This also can significantly compromise the performance of the article.
Therefore, the dispersibility of the ultra-high molecular weight polyethylene in the conventional polyethylene melt must be improved while the melt flowability of the conventional polyethylene is used, so that a melt-extruded product with excellent properties can be obtained, but the melt-extruded product is a bottleneck which cannot be broken through in the prior art.
Disclosure of Invention
The application aims to break the bottleneck of the prior art scheme, overcome the problems of complex production flow, unstable product quality and the like, and provide a polyethylene powder and a preparation method thereof.
The aim of the application can be achieved by the following technical scheme:
the polyethylene powder provided by the application has a core-shell structure (shown in figure 1), the weight average molecular weight of the core polyethylene is 10 ten thousand-60 ten thousand, the molecular weight distribution Mw/Mn of the core polyethylene is less than or equal to 3, the weight average molecular weight of the shell polyethylene is 100 ten thousand-500 ten thousand, the molecular weight distribution Mw/Mn of the shell polyethylene is less than or equal to 5, in the single particles, the mass of the core polyethylene is 30-95 parts, and the mass of the shell polyethylene is 5-80 parts.
Preferably, the diameter of the polyethylene powder particles is 3-100 microns, the diameter of the core polyethylene part is 2-60 microns, and the thickness of the shell polyethylene part is 0.05-35 microns.
The application also provides a preparation method of the polyethylene powder, which comprises the steps of preparing the polyethylene powder by ethylene through a two-stage polymerization process, polymerizing the ethylene under the action of a catalyst to prepare the core polyethylene in the first stage, and polymerizing the ethylene on the core polyethylene under the action of the catalyst to form the shell polyethylene in the second stage.
Preferably, the catalyst used in the first stage polymerization comprises a metallocene catalyst, or a magnesium-titanium-based catalyst, or a chromium-based catalyst, and the catalyst used in the second stage polymerization comprises a metallocene catalyst, or a chromium-based catalyst, or a late transition metal catalyst.
Preferably, the reaction temperature of the first stage polymerization is 30 to 90℃and the reaction temperature of the second stage polymerization is 35 to 95 ℃.
Preferably, the polyethylene powder prepared has a melt index of 0.05-50g/10min and a density of 0.93-0.96g/cm3 at 190℃under a 21.6kg load.
The application prepares the polyethylene particles with core-shell structure under different reaction conditions by two-stage ethylene polymerization, and the diameter of the particles is 3-40 microns. The single particle comprises two polyethylenes with different molecular weight properties, namely the core polyethylene has a molecular weight relatively low, which is close to that of the traditional high-density polyethylene, while the shell polyethylene has a molecular weight greater than 100 ten thousand, which belongs to the category of ultra-high molecular weight polyethylene. By the method provided by the application, two polyethylenes with larger molecular weight difference can be compounded together in the process of polyethylene polymerization, so that uniform dispersion of the polyethylenes in micrometer scale and even nanometer scale is realized. The powder can be directly used for screw melt extrusion, and in the screw, the core polyethylene is heated and extruded and sheared under the action of relatively low molecular weight to form a polyethylene melt with good fluidity; the shell layer is ultra-high molecular weight polyethylene, and the molecular chains are highly entangled and limited in movement ability. The polymer powder particles are heated, extruded and sheared in the pushing process of the screw, and the shell layer ultra-high molecular weight polyethylene layer is broken under the shearing and extrusion actions to form the ultra-high molecular weight polyethylene layer with the micrometer or even nanometer size; the polyethylene melt of the core breaks through the shell layer and carries the ultra-high molecular weight polyethylene layer to flow together and mix more uniformly under the shearing action of the screw, so that the uniform dispersion of two polyethylenes with different molecular weight characteristics in a product formed after cooling is ensured, the ultra-high molecular weight polyethylene part can exist in a layered structure, and compared with the ultra-high molecular weight polyethylene particle form in the existing blending technology, the layered structure has larger specific surface area, can form more interfaces with the high-density polyethylene, also contributes to the disentanglement of the ultra-high molecular weight polyethylene molecular chain, leads the interaction force of two phases to be larger, contributes to the mutual interpenetration of the two polyethylene molecular chains and contributes to the improvement of the overall mechanical property.
Compared with the prior art, the application has the beneficial effects that:
the two polyethylenes with different molecular weight characteristics are compounded in the powder particles in a core-shell structure form, so that the relative uniform distribution and mixing of microscopic dimensions are realized on the raw material level, the polyethylene can be directly used for melt extrusion without a blending step, the process is greatly simplified, and the uniform mixing of the two polyethylenes is realized; the formed conventional molecular weight polyethylene melt can provide fluidity for the ultra-high molecular weight polyethylene by being heated and extruded by a screw, and the ultra-high molecular weight polyethylene exists in a lamellar form with a large surface, can form more interfaces with the conventional molecular weight polyethylene, and is also beneficial to disentangling of the ultra-high molecular weight polyethylene molecular chains, so that the interaction force of two phases is larger, the mutual interpenetration of the two polyethylene molecular chains is facilitated, and the improvement of the overall mechanical property is facilitated.
The polyethylene powder of the application can be added with other various auxiliary agents such as antioxidant, antistatic agent, flame retardant, pigment, glass fiber, inorganic powder and the like according to the property requirements of products during processing. Suitable fields of application include injection molding, compression molding, cast films, fibers, and the like.
Drawings
The application and its features, aspects and advantages will become more apparent from the detailed description of non-limiting embodiments with reference to the following drawings. Like numbers refer to like parts throughout. The drawings are not intended to be drawn to scale, emphasis instead being placed upon illustrating the principles of the application.
Fig. 1 is a schematic diagram of the internal structure of a polyethylene particle with a core-shell structure according to embodiment 1 of the present application.
Detailed Description
The application will be further illustrated with reference to specific examples. It is to be understood that these examples are illustrative of the present application and are not intended to limit the scope of the present application. Furthermore, it should be understood that various changes and modifications can be made by one skilled in the art after reading the teachings of the present application, and such equivalents are intended to fall within the scope of the application as defined in the appended claims.
Example 1
Triethylaluminum is used as a catalyst, normal hexane is used as a diluent, ethylene monomers are introduced, and the first-stage polymerization reaction is carried out under the nitrogen atmosphere. The polymerization temperature was 40℃and the reaction was stopped after 2 hours of polymerization time. Then loading magnesium chloride/titanium tetrachloride on the polyethylene particles in the first stage, and then introducing ethylene to carry out the polymerization in the second stage, wherein the polymerization temperature is 60 ℃, and the polymerization time is 2 hours. The polyethylene obtained in the first stage has a weight average molecular weight of 30 ten thousand and a molecular weight distribution Mw/Mn of 2.1; the shell layer polyethylene has a weight average molecular weight of 170 ten thousand and a molecular weight distribution of 3.8. The average radius of the core polyethylene in the powder particles is 20 microns, and the average thickness of the shell polyethylene part is 2 microns. 75 parts of core polyethylene and 25 parts of shell polyethylene. The density of the polyethylene powder is 0.949g/cm 3 Melt index at 190℃under a 21.6kg load of7.5g/10min. The polyethylene powder is injection molded into sheets by an injection molding machine at 250 ℃, the nozzle temperature is 180 ℃, the mold temperature is 50 ℃, the injection pressure is 220MPa, the holding pressure is 245MPa, and the holding time is 1min. The surface of the obtained sheet is flat and smooth. The performance indexes of the sheet are shown in Table 1.
Example 2
Zirconium dichloride is used as a catalyst, normal hexane is used as a diluent, ethylene monomer is introduced, and the first-stage polymerization reaction is carried out under the nitrogen atmosphere. The polymerization temperature was 50℃and the reaction was stopped after 3 hours of polymerization time. Then loading titanium tetrachloride on the polyethylene particles in the first stage, and then introducing ethylene to carry out a second-stage polymerization reaction, wherein the polymerization reaction temperature is 50 ℃, and the polymerization reaction time is 2 hours. The weight average molecular weight of the polyethylene obtained in the first stage is 20 ten thousand, and the molecular weight distribution Mw/Mn is 2.5; the shell layer polyethylene has a weight average molecular weight of 200 ten thousand and a molecular weight distribution of 4. The average radius of the core polyethylene in the powder particles is 25 microns, and the average thickness of the shell polyethylene part is 3 microns. 71 parts of core polyethylene and 29 parts of shell polyethylene. The density of the polyethylene powder is 0.951g/cm 3 The melt index was 5.3g/10min at 190℃under a 21.6kg load. The polyethylene powder is injection molded into sheets by an injection molding machine at 255 ℃, the nozzle temperature is 175 ℃, the mold temperature is 50 ℃, the injection pressure is 220MPa, the holding pressure is 245MPa, and the holding time is 1min. The surface of the obtained sheet is flat and smooth. The performance indexes of the sheet are shown in Table 1.
Example 3
Triethylaluminum is used as a catalyst, normal hexane is used as a diluent, ethylene monomers are introduced, and the first-stage polymerization reaction is carried out under the nitrogen atmosphere. The polymerization temperature was 45℃and the reaction was stopped after 2.5 hours of polymerization time. Then loading titanium tetrachloride on the polyethylene particles in the first stage, and then introducing ethylene to carry out a second-stage polymerization reaction, wherein the polymerization reaction temperature is 75 ℃, and the polymerization reaction time is 1.5 hours. The polyethylene obtained in the first stage has a weight average molecular weight of 33 ten thousand and a molecular weight distribution Mw/Mn of 2.0; the shell layer polyethylene has a weight average molecular weight of 350 ten thousand and a molecular weight distribution of 4.3. Powder particle coreThe average radius of the core polyethylene was 19 microns and the average thickness of the shell polyethylene portion was 3 microns. The mass of the core polyethylene is 64 parts, and the mass of the shell polyethylene is 36 parts. The density of the polyethylene powder is 0.955g/cm 3 The melt index was 1.3g/10min at 190℃under a 21.6kg load. The polyethylene powder is injection molded into sheets by an injection molding machine at 265 ℃, the nozzle temperature is 170 ℃, the mold temperature is 50 ℃, the injection pressure is 220MPa, the holding pressure is 245MPa, and the holding time is 1min. The surface of the obtained sheet is flat and smooth. The performance indexes of the sheet are shown in Table 1.
Example 4
Triethylaluminum modified magnesium chloride is used as a catalyst, normal hexane is used as a diluent, ethylene monomers are introduced, and the first-stage polymerization reaction is carried out under the nitrogen atmosphere. The polymerization temperature was 65℃and the reaction was stopped after 3 hours of polymerization time. Then loading titanium tetrachloride on the polyethylene particles in the first stage, and then introducing ethylene to carry out a second-stage polymerization reaction, wherein the polymerization reaction temperature is 55 ℃, and the polymerization reaction time is 2.5 hours. The polyethylene obtained in the first stage has a weight average molecular weight of 21 ten thousand and a molecular weight distribution Mw/Mn of 2.2; the shell layer polyethylene has a weight average molecular weight of 230 ten thousand and a molecular weight distribution of 4.1. The average radius of the core polyethylene in the powder particles was 25 microns and the average thickness of the shell polyethylene portion was 6 microns. The mass of the core polyethylene is 52 parts, and the mass of the shell polyethylene is 48 parts. The density of the polyethylene powder is 0.957g/cm 3 The melt index was 2.9g/10min at 190℃under a 21.6kg load. The polyethylene powder is injection molded into sheets by an injection molding machine at 268 ℃, the nozzle temperature is 170 ℃, the mold temperature is 50 ℃, the injection pressure is 220MPa, the holding pressure is 245MPa, and the holding time is 1min. The surface of the obtained sheet is flat and smooth. The performance indexes of the sheet are shown in Table 1.
Comparative example 1
Selecting ultra-high molecular weight polyethylene powder with weight average molecular weight of 200 ten thousand and traditional high-density polyethylene with weight average molecular weight of 20 ten thousand, carrying out high-speed mechanical blending according to a mass ratio of 20:80, and carrying out injection molding on the obtained mixture at 280 ℃ by an injection molding machine to obtain a sheet, wherein the nozzle temperature is 170 ℃, the mold temperature is 50 ℃, the injection pressure is 220MPa, the pressure maintaining pressure is 245MPa, and the pressure maintaining time is 1min. The resulting sheet had a rough surface and a grainy feel. The performance indexes of the sheet are shown in Table 1.
Comparative example 2
Selecting ultra-high molecular weight polyethylene powder with the weight average molecular weight of 150 ten thousand and traditional high-density polyethylene with the weight average molecular weight of 15 ten thousand, carrying out high-speed mechanical blending according to the mass ratio of 35:65, and carrying out injection molding on the obtained mixture at 265 ℃ by an injection molding machine to obtain a sheet, wherein the nozzle temperature is 170 ℃, the mold temperature is 50 ℃, the injection pressure is 220MPa, the pressure maintaining pressure is 245MPa, and the pressure maintaining time is 1min. The resulting sheet had a rough surface and a grainy feel. The performance indexes of the sheet are shown in Table 1.
The properties of the materials related to examples 1 to 4 and comparative examples 1 to 2 are shown in Table 1.
TABLE 1
Note that: the tensile strength and impact strength in the table are average values of 20 sample measured values under the same processing technology, and each variation coefficient is calculated from each sample measured value and the average value.
As shown in the table, the tensile strength and the impact strength of the sheet prepared by the specific injection molding process of the polyethylene powder disclosed by the application are obviously improved compared with those of the existing ultra-high molecular weight product, particularly, the stability of performance indexes is obviously improved, and the variation coefficient is generally very low, so that the core-shell structure of the polyethylene powder disclosed by the application can effectively improve the dispersion uniformity of two polyethylenes with different molecular weight characteristics, and is beneficial to the stability of material products.
Claims (5)
1. The polyethylene powder is characterized in that powder particles are of a core-shell structure, the molecular weight of core polyethylene is 10-60 ten thousand, the molecular weight distribution Mw/Mn of the core polyethylene is less than or equal to 3, the molecular weight of shell polyethylene is 100-500 ten thousand, the molecular weight distribution Mw/Mn of the shell polyethylene is less than or equal to 5, in a single particle, the mass of the core polyethylene is 30-95 parts, and the mass of the shell polyethylene is 5-80 parts;
the preparation method of the polyethylene powder comprises the following steps: a two-stage polymerization process, wherein ethylene is polymerized under the action of a catalyst in the first stage to prepare the core polyethylene; and in the second stage, ethylene is polymerized on the core polyethylene under the action of a catalyst to form the shell polyethylene.
2. The polyethylene powder according to claim 1, wherein the polyethylene powder particles have a diameter of 3 to 100 microns, the core polyethylene portion has a diameter of 2 to 60 microns, and the shell polyethylene portion has a thickness of 0.05 to 35 microns.
3. The polyethylene powder according to claim 1, wherein the catalyst used in the first stage polymerization comprises a metallocene catalyst, or a magnesium-titanium-based catalyst, or a chromium-based catalyst, and the catalyst used in the second stage polymerization comprises a metallocene catalyst, or a chromium-based catalyst, or a post-transition metal catalyst.
4. The polyethylene powder according to claim 1, wherein the reaction temperature of the first stage polymerization is 30 to 90℃and the reaction temperature of the second stage polymerization is 35 to 95 ℃.
5. The polyethylene powder according to claim 1, wherein the polyethylene powder has a melt index of 0.05-50g/10min and a density of 0.93-0.96g/cm under a 21.6kg load at 190 ℃ 3 。
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Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN109486040A (en) * | 2018-10-29 | 2019-03-19 | 宁波大学 | A method of preparing polyolefin blends |
| CN110283281A (en) * | 2019-06-26 | 2019-09-27 | 深圳聚石新材料科技有限公司 | A kind of preparation method of ultra-high molecular weight polyethylene and polypropylene in-situ blending object |
| CN112638958A (en) * | 2018-07-19 | 2021-04-09 | 博里利斯股份公司 | Process for preparing UHMWPE homopolymer |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112638958A (en) * | 2018-07-19 | 2021-04-09 | 博里利斯股份公司 | Process for preparing UHMWPE homopolymer |
| CN109486040A (en) * | 2018-10-29 | 2019-03-19 | 宁波大学 | A method of preparing polyolefin blends |
| CN110283281A (en) * | 2019-06-26 | 2019-09-27 | 深圳聚石新材料科技有限公司 | A kind of preparation method of ultra-high molecular weight polyethylene and polypropylene in-situ blending object |
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