Ultrahigh molecular weight polyethylene having excellent processability and impact resistance

Abstract

Disclosed is an ultrahigh molecular weight polyethylene satisfying both excellent mechanical properties and processability, including abrasion resistance and impact resistance. The present invention provides an ultrahigh molecular weight polyethylene, which is prepared by a Ziegler-Natta catalyst, has a density of 0.925 to 0.940 g / ㎤, a viscosity average molecular weight (Mv) of 3,000,000 g / mol or more, a melting point of 133℃ or lower, and a heat of fusion of 150 J / g or less.

Description

Ultra-high molecular weight polyethylene with excellent processability and impact resistance

[0001] The present invention relates to ultra-high molecular weight polyethylene, and more particularly, to ultra-high molecular weight polyethylene having excellent processability and impact resistance.

[0002] This application claims priority to and the benefit of Republic of Korea Patent Application No. 10-2023-0158242, filed November 15, 2023, which is incorporated herein by reference in its entirety.

[0003] Ultra-high molecular weight polyethylene refers to polyethylene with a molecular weight of at least 1,000,000 g / mol. Since its molecular weight is much higher than that of general-purpose polyethylene, it has excellent characteristics such as rigidity, wear resistance, environmental stress uniformity, self-lubrication, chemical resistance, and electrical properties, and is used for various purposes such as sheets, fibers, and lead acid batteries.

[0004] However, due to its high molecular weight, ultra-high molecular weight polyethylene has very limited flowability when melted. Processing requires high temperatures and pressures, or mixing with oil. High temperatures can cause oxidation and deterioration of the product, degrading its physical properties.

[0005] Meanwhile, the properties required for UHP vary depending on the intended use. If only high mechanical strength (hardness) is required, it may not withstand external impacts and easily break. Furthermore, UHP is frequently used in applications requiring wear resistance, such as bearings, gears, and rollers in machine components.

[0006] Therefore, it is necessary to develop ultra-high molecular weight polyethylene that satisfies a certain level of strength and has excellent processability, impact resistance, and wear resistance.

[0007] Korean Patent No. 10-0822616 discloses a technology for producing polyolefin having a narrow molecular weight distribution using a titanium-containing catalyst for polymerizing ultra-high molecular weight polyolefin containing magnesium, titanium, and halogen, but does not mention the properties of ultra-high molecular weight polyethylene that satisfies both excellent mechanical properties such as wear resistance and impact resistance and processability.

[0008] Korean Patent No. 10-1882110 discloses a technology for producing ultra-high molecular weight polyethylene using a titanium-based catalyst and an alkyl aluminum co-catalyst in a continuous reactor, but does not mention the properties of the ultra-high molecular weight polyethylene that improve processability, wear resistance, and impact resistance.

[0009] The present invention has been devised to solve the above problems, and aims to provide an ultra-high molecular weight polyethylene that satisfies both excellent mechanical properties such as wear resistance and impact resistance, and processability.

[0010] In order to solve the above problem, the present invention provides an ultra-high molecular weight polyethylene produced with a Ziegler-Natta catalyst, having a density of 0.925 to 0.940 g / cm3, a viscosity average molecular weight (Mv) of 3,000,000 g / mol or more, a melting point of 133°C or less, and a heat of fusion of 150 J / g or less.

[0011] In addition, the above ultra-high molecular weight polyethylene has a Charpy impact strength of 65 kJ / m measured by the following method. 2 The present invention provides an ultra-high molecular weight polyethylene characterized by a tensile elongation of 350% or more.

[0012] [Charpy impact strength measurement method]

[0013] Specimens (120×15×10 mm) were manufactured according to ISO 11542-2 standard and the notch (Double V-notch) impact strength was measured;

[0014] [Method for measuring tensile elongation]

[0015] Specimens (115×6×3.2 mm) were manufactured according to ASTM D638 standards and measured under conditions of 50 mm / min.

[0016] According to the present invention, by controlling the molecular weight, melting point and heat of fusion of high-density polyethylene within a specific range to manufacture ultra-high molecular weight polyethylene, ultra-high molecular weight polyethylene can be provided with dramatically improved mechanical property balance of processability, wear resistance and impact resistance.

[0017] Figure 1 is a diagram showing the results of differential scanning calorimeter (DSC) analysis for Example 1 and Comparative Examples 1 and 2.

[0018] Hereinafter, preferred embodiments of the present invention will be described in detail. In describing the present invention, detailed descriptions of related known technologies will be omitted if they are deemed to obscure the gist of the present invention. Throughout the specification, when a part is said to "include" a component, unless otherwise specifically stated, this does not exclude other components but rather implies the inclusion of additional components.

[0019]

[0020] The present inventors have faced the fact that conventional ultra-high molecular weight polyethylene manufactured using a Ziegler-Natta catalyst has focused only on controlling the uniform particle size distribution, and has not clearly presented the properties of the resin for improving the balance of mechanical properties such as processability, wear resistance, and impact resistance. As a result of repeated research, they have discovered that when ultra-high molecular weight polyethylene is manufactured by controlling the molecular weight, melting point, and heat of fusion within a specific range as a high-density polyethylene having a specific density range, the mechanical property balance of wear resistance and impact resistance can be dramatically improved while having excellent processability, and this has led to the present invention.

[0021] Accordingly, the present invention discloses an ultra-high molecular weight polyethylene produced with a Ziegler-Natta catalyst, having a density of 0.925 to 0.940 g / cm3, a viscosity average molecular weight (Mv) of 3,000,000 g / mol or more, a melting point of 133°C or less, and a heat of fusion of 150 J / g or less.

[0022] In the present invention, the ultra-high molecular weight polyethylene is high-density polyethylene and has a density of 0.925 to 0.940 g / cm3, preferably 0.930 to 0.935 g / cm3. When the density is less than 0.925 g / cm3, impact resistance is reduced, and when it exceeds 0.940 g / cm3, wear resistance is reduced.

[0023] In addition, the ultra-high molecular weight polyethylene according to the present invention has an ultra-high molecular weight of 3,000,000 g / mol or more in viscosity average molecular weight (Mv), preferably 3,000,000 to 10,000,000 g / mol, and more preferably 5,000,000 to 8,000,000 g / mol. When the viscosity average molecular weight (Mv) is less than 3,000,000 g / mol, impact resistance is reduced, and when it is too excessive, problems such as process load may occur during resin manufacturing.

[0024] Even if the molecular weight of polyethylene is the same or similar, the rheological behavior of the melt and the resulting processability and final mechanical properties vary greatly depending on the melting point and heat of fusion (crystallization degree) compared to the molecular weight. If the melting point and heat of fusion are low, not much heat is required during processing, and the processability and flowability are excellent under the same molding conditions. The resin exhibits elastic properties due to a high proportion of amorphous portions, so it satisfies a certain level of strength and has the advantage of excellent wear resistance and impact strength. Here, in the present invention, it was confirmed that when the melting point and heat of fusion are lowered to a specific range in ultra-high molecular weight polyethylene having a viscosity average molecular weight (Mv) of 3,000,000 g / mol or more manufactured using a Ziegler-Natta catalyst, not only the processability and flowability are excellent, but also the impact resistance and wear resistance are significantly improved.

[0025] That is, in the present invention, the ultra-high molecular weight polyethylene has a melting point of 133°C or lower and a heat of fusion of 150 J / g or lower, and preferably, the melting point may be 130 to 133°C and the heat of fusion may be 140 to 150 J / g. When the melting point exceeds 133°C or the heat of fusion exceeds 150 J / g, not only the processability but also the impact resistance and wear resistance begin to rapidly deteriorate, and thus, for example, a high processing temperature is required when forming a fiber using the ultra-high molecular weight polyethylene.

[0026] Ultra-high molecular weight polyethylene manufactured with a Ziegler-Natta catalyst and having the density, molecular weight, melting point and heat of fusion characteristics described above can have excellent processability, wear resistance and impact resistance, and in the present invention, a Charpy impact strength of 65 kJ / m measured by the following method is a preferable characteristic. 2 It is possible to provide ultra-high molecular weight polyethylene having a tensile elongation of 350% or more and a Charpy impact strength of 70 kJ / m 2And, it is possible to provide ultra-high molecular weight polyethylene having a tensile elongation of 380% or more.

[0027] [Charpy impact strength measurement method]

[0028] Specimens (120×15×10 mm) were manufactured according to ISO 11542-2 standard and the notch (Double V-notch) impact strength was measured;

[0029] [Method for measuring tensile elongation]

[0030] Specimens (115×6×3.2 mm) were manufactured according to ASTM D638 standards and measured under conditions of 50 mm / min.

[0031] In order to produce ultra-high molecular weight polyethylene according to the present invention, one reactor can be used, or two or more reactors can be connected in series or parallel. In the case of a method using one reactor, ultra-high molecular weight polyethylene satisfying the above properties can be produced by controlling the type and amount of catalyst. In addition, in the case of a method using two or more reactors connected in series, a polyethylene resin satisfying the above properties can be produced by controlling the amount of hydrogen vented between the reactors. In addition, in the case of a method using two or more reactors connected in parallel, ultra-high molecular weight polyethylene satisfying the above properties can be produced by controlling the amounts of hydrogen and catalyst used for polymerization and the difference in viscosity of the resin formed in each reactor.

[0032] Meanwhile, the Ziegler-Natta catalyst used in the production of ultra-high molecular weight polyethylene according to the present invention is composed of magnesium, titanium, and a halogen as essential components, and is derived from a metal hydride and a metal alkyl, an organoaluminum compound, and a transition metal halide such as titanium, chromium, or vanadium as a cocatalyst. In addition, a donor may be additionally added to control the stereoregularity that affects the physical properties of the polymer when producing the magnesium-supported Ziegler-Natta catalyst. By appropriately selecting the donor, an olefin polymer having the desired stereoregularity and physical properties can be obtained. In addition, an external electron donor may be included to increase the molecular weight.

[0033] The ultra-high molecular weight polyethylene according to the present invention can be used to manufacture molded articles, and is not particularly limited to the molded articles, but the ultra-high molecular weight polyethylene according to the present invention is preferably used for various sheet and rod molding purposes.

[0034] Hereinafter, specific examples according to the present invention will be described.

[0035]

[0036] Manufacturing example: Preparation of Ziegler-Natta catalyst

[0037] (1) Preparation of magnesium compound solution

[0038] A 300 ml pressure-resistant glass reactor equipped with a stirrer and an oil circulation heater was used to charge 3 g of magnesium chloride, 14.7 g of decane, and 16.7 g of ethyl hexanol under a nitrogen atmosphere and stirred at a rotation speed of 400 rpm at 80°C. The temperature was increased to 135°C to completely dissolve the magnesium compound, and after a homogeneous solution was obtained, it was aged for 2 hours and cooled to 20°C, and 16.5 g of hexane was added to prepare a homogeneous magnesium compound solution.

[0039] (2) Preparation of solid titanium catalyst

[0040] The magnesium compound solution prepared above was slowly added to a mixed solution of 16.3 ml of hexane and 12 ml of titanium tetrachloride solution at a rotation speed of 500 rpm for 2 hours. At this time, the temperature of the reactants was maintained at 0°C. After the addition was completed, the mixture was aged for 30 minutes, and then the temperature of the reactor was increased from 0°C to 20°C at a rate of 0.5°C / min. After the temperature increase was completed, the mixture was aged at 20°C for 30 minutes, and then the temperature of the reactor was increased to 74°C at a rate of 1°C / min and aged for 2 hours. Thereafter, the temperature of the reactor was cooled to 40°C. Stirring was stopped, precipitation was allowed, the supernatant was removed, and the mixture was washed six times with 200 ml of 40°C hexane. The final slurry was vacuum-dried for 30 minutes to obtain a catalyst.

[0041]

[0042] Examples and Comparative Examples

[0043] A bimodal process using two reactors connected in parallel with the Ziegler-Natta catalyst manufactured above was used, and in order to have uniform properties, the two reactors were operated under the same conditions, and the amounts of hydrogen and catalyst used for polymerization and the viscosity of the resin formed in each reactor were controlled to manufacture ultra-high molecular weight polyethylene having the material properties (molecular weight, melting point, heat of fusion, and particle size) shown in Table 1 below. The criteria for measuring the resin properties are as follows.

[0044] (1) Density

[0045] Measured according to ASTM D 1505 density gradient method.

[0046] (2) Molecular weight

[0047] After sufficiently dissolving the sample in Decalin (decahydronaphthalene) at a high temperature of 150℃, the intrinsic viscosity (η) was obtained according to ISO 1628-3 using an automatic viscosity measuring device (AUTO IV Meter), and the viscosity average molecular weight (Mv) was calculated according to the Margolies equation of the following mathematical formula 1.

[0048] [Mathematical Formula 1]

[0049] Mv = 5.3 × 10 4 [η] 1.49

[0050] (3) Melting point and heat of fusion

[0051] According to the ISO 11357 standard, the melting point and heat of fusion were obtained by measuring the difference in thermal history (endothermic and exothermic) between the sample and the reference sample using a differential scanning calorimeter (DSC), and the DSC analysis results are shown in Figure 1.

[0052] (4) Average Particle Size (APS)

[0053] According to the ISO 13320 standard, the MS 3000, a powder particle size analysis device, was used to drop 10 g of the sample and measure the particle diameter and distribution of the sample using Laser Diffraction.

[0054]

[0055] Exam example

[0056] Compression specimens for test evaluation were manufactured using the ultra-high molecular weight polyethylene manufactured above, and the impact resistance (Charpy impact strength), abrasion resistance, and tensile elongation were measured using the following method, and the results are shown in Table 1 below.

[0057] [measurement method]

[0058] (1) Charpy impact strength

[0059] Specimens (120×15×10 mm) were manufactured according to the ISO 11542-2 standard and the notch (Double V-notch) impact strength was measured.

[0060] (2) Wear resistance

[0061] Specimens (76.2 × 25.4 × 6.35 mm) were manufactured according to the ISO 15527 standard and worn at the specified rotation speed and time. The weight loss before and after the test was expressed as a percentage and then expressed relatively based on Example 1. At this time, the weight loss of the sample was set to be at least 50 mg.

[0062] (3) Tensile elongation

[0063] Ultra-high molecular weight polyethylene powder was formed into a sheet using a press molding machine, and then specimens (115 × 6 × 3.2 mm) were produced according to ASTM D638 standards and measured under conditions of 50 mm / min.

[0064]

[0065]

[0066]

[0067] Referring to Table 1, it can be confirmed that when ultra-high molecular weight polyethylene is manufactured by controlling the density, molecular weight, melting point, and heat of fusion within specific ranges as high-density polyethylene manufactured using a Ziegler-Natta catalyst according to the present invention (Examples 1 and 2), not only is the processability excellent, but also the elastic properties are excellent, and the mechanical property balance of wear resistance and impact resistance is very excellent.

[0068] In this regard, it can be seen that when the melting point and heat of fusion exceed a certain limit at a similar molecular weight level (Comparative Examples 1 to 4), the elastic properties deteriorate, and in particular, the impact resistance and tensile elongation are significantly reduced.

[0069]

[0070] The preferred embodiments of the present invention have been described in detail above. The description of the present invention is provided for illustrative purposes only, and those skilled in the art will readily appreciate that other specific modifications can be readily made without altering the technical spirit or essential features of the present invention.

[0071] Accordingly, the scope of the present invention is indicated by the claims described below rather than the detailed description above, and all changes or modifications derived from the meaning, scope and equivalent concepts of the claims should be interpreted as being included in the scope of the present invention.

Claims

1. Ultra-high molecular weight polyethylene manufactured with a Ziegler-Natta catalyst and having a density of 0.925 to 0.940 g / cm3, a viscosity-average molecular weight (Mv) of 3,000,000 g / mol or more, a melting point of 133°C or less, and a heat of fusion of 150 J / g or less.

2. In paragraph 1, The above ultra-high molecular weight polyethylene has a Charpy impact strength of 65 kJ / m as measured by the following method. 2 Ultra-high molecular weight polyethylene characterized by a tensile elongation of 350% or more: [Charpy impact strength measurement method] Specimens (120×15×10 mm) were manufactured according to ISO 11542-2 standard and the notch (Double V-notch) impact strength was measured; [Method for measuring tensile elongation] Specimens (115 × 6 × 3.2 mm) were manufactured according to ASTM D638 standard and measured under the condition of 50 mm / min.

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