CN114437297A - Polyethylene powder and preparation method thereof - Google Patents
Polyethylene powder and preparation method thereof Download PDFInfo
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- CN114437297A CN114437297A CN202111669050.5A CN202111669050A CN114437297A CN 114437297 A CN114437297 A CN 114437297A CN 202111669050 A CN202111669050 A CN 202111669050A CN 114437297 A CN114437297 A CN 114437297A
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- C08F255/00—Macromolecular compounds obtained by polymerising monomers on to polymers of hydrocarbons as defined in group C08F10/00
- C08F255/02—Macromolecular compounds obtained by polymerising monomers on to polymers of hydrocarbons as defined in group C08F10/00 on to polymers of olefins having two or three carbon atoms
Abstract
The invention relates to a polyethylene powder and a preparation method thereof, the powder particles have a core-shell structure, the weight average molecular weight of polyethylene of the core is 10-60 ten thousand, the weight average molecular weight distribution Mw/Mn of the polyethylene of the core is less than or equal to 3, the weight average molecular weight of polyethylene of the shell is 100-500 ten thousand, the weight average molecular weight distribution Mw/Mn of the polyethylene of the shell is less than or equal to 5, the weight of the polyethylene of the core accounts for 30-95 parts and the weight of the polyethylene of the shell accounts for 5-80 parts in a single particle, the polyethylene powder is prepared by two-stage polymerization reaction, the existing polyethylene polymerization device can be used, and the process is simple; the product prepared by melting and processing the raw material has good mechanical property and excellent performance stability, and can be applied to the fields of polyethylene pipes, sheets, plates, fibers and the like.
Description
Technical Field
The invention belongs to the technical field of polyethylene materials, relates to polyethylene powder and a preparation method thereof, and particularly relates to polyethylene powder with a core-shell structure and a preparation method thereof.
Background
The polyethylene has low price and excellent performance, and various products of the polyethylene have wide 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 polyethylene has a molecular weight higher than 100 ten thousand, the polyethylene may be referred to as ultra high molecular weight polyethylene. Because the chain length of the ultra-high molecular weight polyethylene molecules is greatly improved compared with the traditional polyethylene, the entanglement degree of the chains is 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 the traditional polyethylene. Products made from ultra-high molecular weight polyethylene have been widely used in various industries.
However, the ultrahigh molecular weight polyethylene has a high degree of chain entanglement, and even at temperatures above the melting point of polyethylene, even at temperatures of 250 ℃ which are much higher than the melting point, the ultrahigh molecular weight polyethylene does not have flowability, 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 only be prepared in a hot pressing mode, and then ultra-high molecular weight polyethylene products in various shapes can be prepared through post-processing, so that the whole preparation efficiency is low, continuous express processing cannot be realized, and the production cost is greatly increased.
In order to solve the above problems, chinese patent CN108774349A proposes to modify ultra-high molecular weight polyethylene and then prepare melt extrusion products such as ultra-high molecular weight polyethylene rods by a screw extruder, but this method needs to compound raw materials, blend high density polyethylene or low density polyethylene with molecular weight of 5-50 ten thousand with ultra-high molecular weight polyethylene, and use the fluidity of high density polyethylene to improve the melt processability of ultra-high molecular weight 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 molecular weight of 150 ten thousand with metallocene linear low density polyethylene, metallocene polyolefin elastomer, polypropylene, etc. to obtain modified material, and the modified material is used for preparing pipe by melt extrusion and can be melt extruded at 260 ℃.
The method is that before melt extrusion, the traditional polyethylene (or polyolefin) with relatively low molecular weight and the ultra-high molecular weight polyethylene are blended, then the mixture is added into a screw feeding port, and the ultra-high molecular weight polyethylene is driven to move by means of the fluidity of the traditional polyethylene in the screw, so that the melt extrusion processing is realized. The preparation process needs to modify and compound the raw materials, the modification cost is high, and the flow is relatively complex.
In addition, the ultra-high molecular weight polyethylene raw materials in the current market are 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 all 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 ultrahigh molecular weight polyethylene powder. After the materials are physically blended, the ultra-high molecular weight polyethylene fine particles can move in the gaps of the traditional polyethylene particles and are deposited to the bottom under the influence of gravity, so that the ultra-high molecular weight polyethylene powder and the traditional polyethylene (polyolefin) granules cannot be relatively uniformly dispersed. Therefore, the two materials cannot be uniformly dispersed before entering the screw, which greatly influences the stability and accuracy of the feeding proportion, influences the different component proportions of extruded parts and influences the product performance.
In order to solve the problem of uneven dispersibility, a pulverization process is additionally required to refine the traditional polyethylene into powder, so that the particle sizes of the ultra-high molecular weight polyethylene raw material and the traditional polyethylene raw material are close to each other, 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 a size close to that of ultra-high molecular weight polyethylene only after being cooled by liquid nitrogen, an ultra-low temperature crusher is needed, and the processing cost is greatly increased.
Even if the two are uniformly mixed before entering the screw, when the traditional polyethylene (polyolefin) is heated and melted in the screw to form a melt in the screw, the ultrahigh molecular weight polyethylene still cannot be melted due to high entanglement of molecular chains, and the particle morphology cannot be greatly changed. The molecular chains of the ultra-high molecular weight polyethylene and the molecular chains of the traditional polyethylene in the extruded product can not be uniformly distributed in the whole product. This also results in a significant compromise in the performance of the article.
Therefore, the melt flowability of the conventional polyethylene must be assisted, and the dispersibility of the ultrahigh molecular weight polyethylene in the conventional polyethylene melt must be improved, so as to obtain a melt-extruded product with excellent performance, which is a bottleneck that cannot be broken through by the prior art.
Disclosure of Invention
The invention 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 purpose of the invention can be realized by the following technical scheme:
according to the polyethylene powder provided by the invention, single powder particles have a core-shell structure (shown in figure 1), the weight average molecular weight of polyethylene in a core is 10-60 ten thousand, the weight average molecular weight of polyethylene in the core is less than or equal to 3, the weight average molecular weight of polyethylene in a shell is 100-500 ten thousand, the weight average molecular weight of polyethylene in the shell is less than or equal to 5, and in a single particle, the weight of polyethylene in the core accounts for 30-95 parts, and the weight of polyethylene in the shell accounts for 5-80 parts.
Preferably, the polyethylene powder particles have a diameter of 3 to 100 microns, the core polyethylene fraction has a diameter of 2 to 60 microns, and the shell polyethylene fraction has a thickness of 0.05 to 35 microns.
The invention also provides a preparation method of the polyethylene powder, which comprises the steps of preparing the polyethylene powder from ethylene by 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 for the first stage polymerization is 30 to 90 ℃ and the reaction temperature for the second stage polymerization is 35 to 95 ℃.
Preferably, the polyethylene powder produced has a melt index of 0.05 to 50g/10min at 190 ℃ and a density of 0.93 to 0.96g/cm3 under a 21.6kg load.
The invention prepares the polyethylene particles with the core-shell structure through two-stage ethylene polymerization reaction under different reaction conditions, and the diameter of the particles is 3-40 microns. The single particle comprises two polyethylenes with different molecular weight properties, i.e. the polyethylene of the core has a relatively low molecular weight, close to the molecular weight of the conventional high density polyethylene, while the polyethylene of the shell has a molecular weight of more than 100 ten thousand, belonging to the category of ultra high molecular weight polyethylene. The method provided by the invention can compound two polyethylenes with large molecular weight difference together in the polyethylene polymerization process, and realizes the uniform dispersion of the two polyethylenes in micrometer scale or even nanometer scale. The powder can be directly used for screw melt extrusion, and in the screw, the polyethylene at the core is heated, extruded and sheared to form polyethylene melt with good fluidity because of relatively low molecular weight; and the shell layer is made of ultra-high molecular weight polyethylene, molecular chains of the ultra-high molecular weight polyethylene are highly entangled, and the movement capability is limited. The polymer powder particles are heated, extruded and sheared in the pushing process of the screw, and the shell layer ultrahigh molecular weight polyethylene layer is broken under the shearing and extruding action to form a micrometer or even nanometer sized ultrahigh molecular weight polyethylene layer; the polyethylene melt of the core breaks through the shell layer, bears the ultra-high molecular weight polyethylene layer to flow together, and is more uniformly mixed under the shearing action of the screw, so that the uniform dispersion of two types of polyethylene with different molecular weight characteristics in a product formed after cooling is ensured, and the ultra-high molecular weight polyethylene part can exist in a layered structure.
Compared with the prior art, the invention 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 relatively uniform distribution and mixing of micro-scale are realized on the raw material level, the polyethylene can be directly used for melt extrusion without blending, the process is greatly simplified, and the uniform mixing of the two polyethylenes is realized; the conventional molecular weight polyethylene melt formed by heating and extruding the screw can provide fluidity for the ultra-high molecular weight polyethylene, and the ultra-high molecular weight polyethylene exists in a large-surface lamellar form, can form more interfaces with the conventional molecular weight polyethylene, is also beneficial to unwrapping of molecular chains of the ultra-high molecular weight polyethylene, enables the interaction force of two phases to be larger, is beneficial to the mutual interpenetration of the two molecular chains of the polyethylene, and is beneficial to the improvement of the overall mechanical property.
The polyethylene powder can be added with other various auxiliary agents such as an antioxidant, an antistatic agent, a flame retardant, a pigment, glass fiber, inorganic powder and the like according to the property requirements of the product during processing. Suitable application fields include injection molding, die pressing, cast films, fibers and the like.
Drawings
The invention and its features, aspects and advantages will become more apparent from reading the following detailed description of non-limiting embodiments with reference to the accompanying drawings. Like reference symbols in the various drawings indicate like elements. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
Fig. 1 is a schematic view of an internal structure of a core-shell polyethylene particle provided in embodiment 1 of the present invention.
Detailed Description
The invention will be further illustrated with reference to the following specific examples. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present invention. Further, it should be understood that various changes or modifications of the present invention may be made by those skilled in the art after reading the teaching of the present invention, and such equivalents may fall within the scope of the present invention as defined in the appended claims.
Example 1
Triethyl aluminum 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 in the nitrogen atmosphere. The polymerization temperature was 40 ℃ and the reaction was stopped after a polymerization time of 2 hours. Then, after loading magnesium chloride/titanium tetrachloride on the polyethylene particles in the first stage, ethylene was introduced to carry out polymerization in the second stage at a polymerization temperature of 60 ℃ for 2 hours. The weight average molecular weight of the polyethylene obtained in the first stage is 30 ten thousand, and the molecular weight distribution Mw/Mn is 2.1; the weight average molecular weight of the polyethylene of the shell layer is 170 ten thousand, and the molecular weight distribution is 3.8. The average radius of the core polyethylene in the powder particles was 20 microns and the average thickness of the polyethylene portion of the shell was 2 microns. The core polyethylene accounts for 75 parts by mass, and the shell polyethylene accounts for 25 parts by mass. The polyethylene powder had a density of 0.949g/cm3The melt index at 190 ℃ under a load of 21.6kg was 7.5g/10 min. The polyethylene powder is subjected to injection molding at 250 ℃ by an injection molding machine to form a sheet, the nozzle temperature is 180 ℃, the mold temperature is 50 ℃, the injection pressure is 220MPa, the pressure maintaining pressure is 245MPa, and the pressure maintaining time is 1 min. The obtained sheet has a smooth surface. The sheet materials have the performance indexes shown in Table 1.
Example 2
Introducing ethylene monomer with zirconocene dichloride as catalyst and n-hexane as diluent to perform the first-stage polymerization reaction in nitrogen atmosphere. The polymerization temperature was 50 ℃ and the reaction was stopped after 3 hours of polymerization time. Then, titanium tetrachloride was loaded on the polyethylene particles in the first stage, and ethylene was introduced to carry out polymerization in the second stage at a polymerization temperature of 50 ℃ for 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 weight average molecular weight of the polyethylene of the shell layer is 200 ten thousand, and the molecular weight distribution is 4. The average radius of the core polyethylene in the powder particles was 25 microns and the average thickness of the polyethylene portion of the shell was 3 microns. The core polyethylene accounts for 71 parts by mass, and the shell polyethylene accounts for 29 parts by mass. The density of the polyethylene powder is 0.951g/cm3The melt index at 190 ℃ under a 21.6kg load was 5.3g/10 min. The polyethylene powder is formed by injection molding at 255 ℃ through an injection molding machineThe temperature of a nozzle of the sheet is 175 ℃, the temperature of a mould is 50 ℃, the injection pressure is 220MPa, the pressure maintaining pressure is 245MPa, and the pressure is maintained for 1 min. The obtained sheet has a smooth surface. The sheet materials have the performance indexes shown in Table 1.
Example 3
Triethyl aluminum 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 in the nitrogen atmosphere. The polymerization temperature was 45 ℃ and the reaction was stopped after a polymerization time of 2.5 hours. Then, titanium tetrachloride was loaded on the polyethylene particles in the first stage, and ethylene was introduced to carry out polymerization in the second stage at a polymerization temperature of 75 ℃ for 1.5 hours. The weight average molecular weight of the polyethylene obtained in the first stage is 33 ten thousand, and the molecular weight distribution Mw/Mn is 2.0; the weight average molecular weight of the polyethylene of the shell layer is 350 ten thousand, and the molecular weight distribution is 4.3. The average radius of the core polyethylene in the powder particles was 19 microns and the average thickness of the polyethylene portion of the shell was 3 microns. The core polyethylene accounts for 64 parts by mass, and the shell polyethylene accounts for 36 parts by mass. The polyethylene powder had a density of 0.955g/cm3The melt index at 190 ℃ under a load of 21.6kg was 1.3g/10 min. The polyethylene powder is subjected to injection molding at 265 ℃ by an injection molding machine to form a sheet, the nozzle temperature is 170 ℃, the mold temperature is 50 ℃, the injection pressure is 220MPa, the pressure maintaining pressure is 245MPa, and the pressure is maintained for 1 min. The obtained sheet has a smooth surface. The sheet materials have the performance indexes shown in Table 1.
Example 4
And (2) taking magnesium chloride modified by triethyl aluminum as a catalyst, taking normal hexane as a diluent, introducing an ethylene monomer, and carrying out a first-stage polymerization reaction in a nitrogen atmosphere. The polymerization temperature was 65 ℃ and the reaction was stopped after 3 hours of polymerization time. Then, titanium tetrachloride was loaded on the polyethylene particles in the first stage, and ethylene was introduced to carry out polymerization in the second stage at a polymerization temperature of 55 ℃ for 2.5 hours. The weight average molecular weight of the polyethylene obtained in the first stage is 21 ten thousand, and the molecular weight distribution Mw/Mn is 2.2; the weight average molecular weight of the polyethylene of the shell layer is 230 ten thousand, and the molecular weight distribution is 4.1. The average radius of the core polyethylene in the powder particles is 25 microns, and the polyethylene of the shell layerThe average thickness of the sections was 6 microns. The core polyethylene accounts for 52 parts by mass, and the shell polyethylene accounts for 48 parts by mass. The polyethylene powder had a density of 0.957g/cm3The melt index at 190 ℃ under a load of 21.6kg was 2.9g/10 min. The polyethylene powder is subjected to injection molding at 268 ℃ by an injection molding machine to form a sheet, the temperature of a nozzle is 170 ℃, the temperature of a mold is 50 ℃, the injection pressure is 220MPa, the pressure maintaining pressure is 245MPa, and the pressure maintaining time is 1 min. The obtained sheet has a smooth surface. The sheet materials have the performance indexes shown in Table 1.
Comparative example 1
The preparation method comprises the steps of selecting ultra-high molecular weight polyethylene powder with the weight-average molecular weight of 200 ten thousand and traditional high-density polyethylene with the weight-average molecular weight of 20 ten thousand to carry out high-speed mechanical blending according to the mass ratio of 20:80, carrying out injection molding on the obtained mixture at 280 ℃ through 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 is carried out for 1 min. The obtained sheet had a rough surface and a granular feel. The sheet materials have the performance indexes shown in Table 1.
Comparative example 2
The preparation method comprises the steps of 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 to carry out high-speed mechanical blending according to the mass ratio of 35:65, carrying out injection molding on the obtained mixture at 265 ℃ through 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 is carried out for 1 min. The obtained sheet had a rough surface and a granular feel. The sheet materials have the performance indexes shown in Table 1.
The material properties of examples 1 to 4 and comparative examples 1 to 2 are shown in Table 1.
TABLE 1
Note: in the table, the tensile strength and the impact strength are the average values of the measured values of 20 samples in the same processing technology, and each coefficient of variation is calculated by the measured value and the average value of each sample.
The above table shows that the tensile strength and impact strength of the sheet prepared by the polyethylene powder through a specific injection molding process are significantly improved compared with those of the existing ultrahigh molecular weight product, particularly the stability of the performance index is significantly improved, and the coefficient of variation is generally low, which indicates that the core-shell structure of the polyethylene powder prepared by the invention can effectively improve the dispersion uniformity of two polyethylenes with different molecular weight characteristics, and is beneficial to the stability of the material product.
Claims (6)
1. The polyethylene powder is characterized in that powder particles have a core-shell structure, the molecular weight of polyethylene in a core is 10-60 ten thousand, the molecular weight distribution Mw/Mn of polyethylene in the core is less than or equal to 3, the molecular weight of polyethylene in a shell is 100-500 ten thousand, the molecular weight distribution Mw/Mn of polyethylene in the shell is less than or equal to 5, and in a single particle, the mass of polyethylene in the core accounts for 30-95 parts, and the mass of polyethylene in the shell accounts for 5-80 parts.
2. A polyethylene powder according to claim 1 wherein the polyethylene powder particles have a diameter of from 3 to 100 microns, the core polyethylene fraction has a diameter of from 2 to 60 microns and the sheath polyethylene fraction has a thickness of from 0.05 to 35 microns.
3. The method for preparing polyethylene powder according to any one of claims 1 to 2, wherein the preparation process comprises a two-stage polymerization process, wherein the first stage comprises polymerizing ethylene under the action of a catalyst to obtain the core polyethylene, and the second stage comprises polymerizing ethylene on the core polyethylene under the action of a catalyst to form the shell polyethylene.
4. The method of claim 4, 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 late transition metal catalyst.
5. The process for preparing polyethylene powder according to claim 5, wherein the reaction temperature of the first stage polymerization is 30-90 ℃ and the reaction temperature of the second stage polymerization is 35-95 ℃.
6. A polyethylene powder according to claim 1 having a melt index of 0.05 to 50g/10min at 190 ℃ and a density of 0.93 to 0.96g/cm under a 21.6kg load3。
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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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Patent Citations (3)
| 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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