Polyethylene resin composition for pipe, preparation method thereof and molded product prepared therefrom

Abstract

Provided are a polyethylene resin composition for pipes, a method for preparing the same, and a molded product comprising the same. The polyethylene resin composition for pipes has a melt index (5 kg load, 190° C.) of 0.16 g / 10 min to 0.24 g / 10 min and is prepared by passing through a two-stage reactor, passing through an extruder, and adding peroxide, wherein the rate of change of the melt index (MI) (5 kg load, 190° C.) caused by the extruder according to Formula 1 is 60% to 90%, and the rate of change of the melt index (MI) (5 kg load, 190° C.) caused by the peroxide according to Formula 2 is 10% to 40%: [Formula 1] {(MI after passing through the two-stage reactor-MI after passing through the extruder) / (MI after passing through the two-stage reactor-final MI)}×100 [Formula 2] {(MI after passing through the extruder-MI after adding peroxide) / (MI after passing through the two-stage reactor-final MI)}×100.

Description

Technical Field

[0001] The embodiment relates to a polyethylene resin composition for pipes, a method for preparing the same, and a molded article prepared therefrom. Background Art

[0002] Pipes made of polyethylene have been used for water pipes, sewage pipes, industrial pipes, etc., and have greater resistance to sagging (also known as "resistance to sagging") and resistance to slow crack growth (also known as "resistance to slow crack growth") to be suitable for large diameter pipes, and their uses are gradually diversifying.

[0003] When processing large diameter pipes, resistance to sagging caused by the weight of the pipe itself is essential, and this can be improved by reducing the melt index (also known as "melt index" or "melt flow index") of the polyethylene resin.

[0004] Methods for reducing the melt index may include changing the physical properties of the polyethylene resin itself, changing the extrusion conditions as a post-treatment method, or adding peroxides during extrusion.

[0005] Lowering the melt index by adding peroxide significantly improves sag resistance, but tends to reduce slow crack growth resistance and increase odor levels. At the same time, simply changing the physical properties of the polyethylene resin itself or lowering the melt index through extrusion conditions has limitations in improving sag resistance. Summary of the invention

[0006] One aspect of the present invention provides a polyethylene resin composition for pipes which is excellent in all aspects of sag resistance, slow crack growth resistance and odor level of the pipes.

[0007] One aspect of the present invention provides a method for preparing the polyethylene resin composition for pipes.

[0008] Another aspect of the present invention provides a molded product prepared from the polyethylene resin composition for pipes.

[0009] According to at least one embodiment, a polyethylene resin composition for pipes has a melt index (under a load of 5 kg at 190°C) of 0.16 g / 10 min to 0.24 g / 10 min, and is prepared by: passing through a two-stage reactor, passing through an extruder, and adding a peroxide, wherein the rate of change in the melt index (MI) (under a load of 5 kg at 190°C) caused by the extruder according to the following formula 1 is 60% to 90%, and the rate of change in the melt index (MI) (under a load of 5 kg at 190°C) caused by the peroxide according to the following formula 2 is 10% to 40%.

[0010] [Formula 1]

[0011] {(MI after passing through the two-stage reactor - MI after passing through the extruder) / (MI after passing through the two-stage reactor - final MI)}×100

[0012] [Formula 2]

[0013] {(MI after passing through the extruder - MI after adding peroxide) / (MI after passing through the two-stage reactor - final MI)}×100

[0014] The polyethylene resin composition may have a zero shear viscosity (η0) of about 8,000,000 poise to about 20,000,000 poise.

[0015] The polyethylene resin composition may have a melt strength of about 200 mN to about 300 mN.

[0016] In the extrusion of the polyethylene resin composition, a reduction rate of dripping time in the presence of sagging relative to dripping time in the absence of sagging may be 0% to 17%, and the dripping time may be measured using a single screw extruder.

[0017] The polyethylene resin composition may have a strain hardening modulus of about 53 MPa to about 80 MPa.

[0018] According to another embodiment, a method for preparing a polyethylene resin composition for a pipe includes: passing through a two-stage reactor to obtain a polyethylene resin, and adding peroxide to the polyethylene resin and passing through an extruder to obtain the polyethylene resin composition, wherein the extruder is operated at a temperature of 230° C. to 250° C. and a speed of 1,050 rpm to 1,150 rpm.

[0019] The peroxide may be added in an amount of 0.001 to 0.01 parts by weight relative to 100 parts by weight of the polyethylene resin.

[0020] The polyethylene resin may have a melt index (under a load of 5 kg, at 190° C.) after passing through the two-stage reactor in the range of 0.32 g / 10 min to 0.44 g / 10 min.

[0021] The polyethylene resin composition may have a melt index (under a load of 5 kg, at 190° C.) after passing through the extruder and before adding peroxide, of 0.24 g / 10 min to 0.32 g / 10 min.

[0022] The polyethylene resin composition may have a melt index (under a load of 5 kg, at 190° C.) of 0.16 g / 10 min to 0.24 g / 10 min after adding peroxide and passing through an extruder.

[0023] According to another embodiment, provided is a molded product prepared from the polyethylene resin composition for pipes. DETAILED DESCRIPTION

[0024] Hereinafter, embodiments will be described in detail and can be easily performed by a person having common knowledge in the relevant field. However, embodiments may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein.

[0025] The melt index (under a load of 5 kg, at 190° C.) of the polyethylene resin composition for pipes according to one embodiment may be 0.16 g / 10 min to 0.24 g / 10 min, for example, 0.18 g / 10 min to 0.22 g / 10 min. When the melt index of the polyethylene resin composition is within the above range, the processing load and surface roughness of the product are reduced during pipe processing, and the sag resistance of the pipe and its strain hardening modulus can be improved. Therefore, a polyethylene resin composition excellent in both sag resistance and slow crack growth resistance of the pipe can be obtained.

[0026] In polyethylene resin composition, melt index can be changed in stages in the preparation process before obtaining final melt index. According to one embodiment, polyethylene resin is obtained via ethylene polymerization while experiencing by two-stage reactor, then peroxide can be added to polyethylene resin and polyethylene resin composition is prepared together by forcing machine. In this case, the melt index (MI1) of polyethylene resin just after passing through two-stage reactor, the melt index (MI2) of polyethylene resin composition just after passing through forcing machine before adding peroxide and the melt index (MI3) (it corresponds to the melt index of final polyethylene resin composition) after passing through forcing machine with the peroxide added can be different in each stage. That is, the melt index just after passing through two-stage reactor can change its value (changing to MI2 from MI1) after the forcing machine with predetermined operating conditions, and the melt index after forcing machine can also change its value (changing to MI3 from MI2) after passing through the forcing machine with predetermined operating conditions and adding peroxide when not adding peroxide.

[0027] According to one embodiment, the variation rate of the melt index (MI) (under a load of 5 kg, at 190° C.) caused by the extruder according to the following Formula 1, which corresponds to the variation rate from MI1 to MI2, may be 60% to 90%, for example 70% to 80%.

[0028] [Formula 1]

[0029] {(MI after passing through the two-stage reactor - MI after passing through the extruder) / (MI after passing through the two-stage reactor - final MI)}×100

[0030] In addition, the change rate of the melt index (MI) (under a load of 5 kg, at 190° C.) caused by peroxide according to the following Formula 2, which corresponds to the change rate from MI2 to MI3, may be 10% to 40%, for example, 20% to 30%.

[0031] [Formula 2]

[0032] {(MI after passing through the extruder - MI after adding peroxide) / (MI after passing through the two-stage reactor - final MI)}×100

[0033] When the melt index variation rate caused by the extruder and the melt index variation rate caused by the peroxide are each within the above range, a polyethylene resin composition excellent in sag resistance, slow crack growth resistance and odor level of a pipe can be obtained.

[0034] The polyethylene resin composition may have a zero shear viscosity (η0) of 8,000,000 poise to 20,000,000 poise, for example 8,000,000 poise to 12,000,000 poise. The zero shear viscosity may be obtained by measuring the storage modulus and the loss modulus according to the shear rate (in radians per second). When the zero shear viscosity of the polyethylene resin composition is within the above range, the anti-sagging property during pipe processing, especially large diameter pipe processing, may be excellent, and thus the polyethylene resin composition may be usefully used for large diameter pipes.

[0035] The polyethylene resin composition may have a melt strength of 200 to 300 mN, for example, 200 to 250 mN. The melt strength may be obtained by measuring the force for stabilization as the pulling rate of the resin increases at a resin temperature of 200° C. and a chamber temperature of 180° C. When the melt strength of the polyethylene resin composition is within the above range, anti-sagging property at the time of pipe processing, especially large-diameter pipe processing, may be excellent, and thus the polyethylene resin composition may be usefully used for large-diameter pipes.

[0036] In the polyethylene resin composition, during extrusion, the reduction rate of the dripping time in the presence of sagging relative to the dripping time in the absence of sagging can be 0% to 17%, for example, 10% to 17%. The dripping time can be measured using a single screw extruder. Specifically, during extrusion, under the conditions of a temperature of 200°C, an extrusion amount of 38.1g / min and an initial linear velocity of the extruded resin of 0.546cm / s, when the height from the ground is 90.5cm and the extruded resin does not sag, for the case where the time to reach the ground is 165.7 seconds, the time for the polyethylene resin composition to reach the ground can be measured to determine the reduction rate relative to the case where the above-mentioned sagging is not caused. When the reduction rate of the dripping time is within the above range, the anti-sagging property during pipe processing, especially large-diameter pipe processing, can be excellent, and thus the polyethylene resin composition can be usefully used for large-diameter pipes.

[0037] The polyethylene resin composition may have a strain hardening modulus of 53 MPa to 80 MPa, for example 53 MPa to 70 MPa. When the strain hardening rate of the polyethylene resin composition is within the above range, the slow crack growth resistance is excellent, thereby allowing long-term use of the pipe and providing a large diameter pipe with excellent durability.

[0038] Hereinafter, a method for preparing a polyethylene resin composition for a pipe according to another embodiment will be described.

[0039] The polyethylene resin composition can be prepared by passing through a two-stage reactor to obtain a polyethylene resin, and adding a peroxide to the polyethylene resin and passing through an extruder. In this case, the extruder can be operated at a temperature of 230° C. to 250° C. (e.g., a temperature of 240° C. to 250° C.) and a speed of 1,050 rpm to 1,150 rpm (e.g., a speed of 1,080 rpm to 1,140 rpm).

[0040] When the polyethylene resin composition is prepared by an extruder operating within the temperature and rate range, a variation rate of the melt index (MI) (under a load of 5 kg, at 190° C.) of the polyethylene resin composition caused by the extruder in the range of 60% to 90% can be obtained, and accordingly, a polyethylene resin composition excellent in sag resistance, slow crack growth resistance, and odor level of a pipe can be obtained. The variation rate of the melt index caused by the extruder can be obtained according to Formula 1 as described above.

[0041] Specifically, the melt index (under a load of 5 kg, at 190° C.) of the polyethylene resin after passing through the two-stage reactor may be 0.32 g / 10 min to 0.44 g / 10 min, such as 0.35 g / 10 min to 0.42 g / 10 min. In addition, the melt index (under a load of 5 kg, at 190° C.) of the polyethylene resin composition after passing through the extruder before adding peroxide may be 0.24 g / 10 min to 0.32 g / 10 min, such as 0.24 g / 10 min to 0.28 g / 10 min.

[0042] In addition, when a polyethylene resin composition is prepared by passing through an extruder together with an added peroxide, a polyethylene resin composition having a change rate of the melt index (MI) (under a load of 5 kg, at 190°C) of the polyethylene resin composition caused by the peroxide in the range of 10% to 40% can be obtained, and accordingly, a polyethylene resin composition excellent in terms of anti-sagging property, slow crack growth resistance and odor level of the pipe can be obtained.

[0043] Specifically, the melt index (under a load of 5 kg, at 190° C.) of the polyethylene resin composition after passing through an extruder before adding peroxide and the melt index of the polyethylene resin composition after passing through an extruder together with the added peroxide (i.e., the melt index of the final polyethylene resin composition) are each as described above.

[0044] The peroxide may be added in an amount of 0.001 to 0.01 parts by weight, for example, 0.001 to 0.005 parts by weight or 0.001 to 0.003 parts by weight, relative to 100 parts by weight of the polyethylene resin. When the peroxide is added within the above amount range, a polyethylene resin composition excellent in sag resistance, slow crack growth resistance and odor level of the pipe may be obtained.

[0045] In the stage of obtaining the polyethylene resin composition, an additive may be further included, the additive including an antioxidant, a neutralizer, or a combination thereof, and the additive may be included in an amount of 0.01 to 0.5 parts by weight relative to 100 parts by weight of the polyethylene resin composition.

[0046] The antioxidant may include 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene, 1,6-bis[3-(3,5)-di-tert-butyl-4-hydroxyphenyl)propionamido]hexane, 1,6-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionamido]propane, tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane, bis(2,6-di-tert-butyl-4-methylphenyl)pentaerythritol-di-phosphite, bis(2,4-di-tert-butylphenyl)pentaerythritol-di-phosphite, or a combination thereof.

[0047] The antioxidant may be included in an amount of 0.01 to 0.5 parts by weight, for example, 0.2 to 0.4 parts by weight, relative to 100 parts by weight of the polyethylene resin composition. When the antioxidant is added within the above amount range, a polyethylene resin composition excellent in sag resistance, slow crack growth resistance, and odor level of the pipe may be obtained.

[0048] The neutralizing agent may include calcium stearate, zinc stearate, magnesium aluminum hydroxycarbonate, zinc oxide, magnesium hydroxystearate, or a combination thereof.

[0049] The neutralizer may be included in an amount of 0.01 to 0.3 parts by weight, for example, 0.1 to 0.2 parts by weight, relative to 100 parts by weight of the polyethylene resin composition. When the neutralizer is added within the above amount range, a polyethylene resin composition excellent in sag resistance, slow crack growth resistance, and odor level of the pipe may be obtained.

[0050] According to another embodiment, a molded article prepared from the above polyethylene resin composition is provided.

[0051] The molded article may be a pipe for water pipes, sewage pipes, industrial pipes, etc., particularly a large-diameter pipe having a large diameter.

[0052] The polyethylene resin composition for pipes is excellent in sag resistance, slow crack growth resistance and odor level of pipes, and can therefore be usefully used for large-diameter pipes.

[0053] Hereinafter, specific embodiments of the present invention are provided. However, the following examples are only used to illustrate or describe the present invention in more detail and should not be considered as limiting the present invention. In addition, the content not described herein can be fully understood by those skilled in the art with knowledge of the art, and is therefore omitted.

[0054] (Preparation of polyethylene resin composition)

[0055] Example 1

[0056] Ethylene polymerization was carried out using a Ziegler-Natta catalyst and 1-hexene comonomer by a tandem polymerization process in which two reactors were connected in series.

[0057] The melt index (under a load of 5 kg, at 190° C.) of the powder type polyethylene resin obtained after the two-stage reactor was 0.40 g / 10 min.

[0058] 0.15 parts by weight of Irganox 1010 and 0.15 parts by weight of Irgafos-168 (as antioxidants), 0.15 parts by weight of calcium stearate (as a neutralizer), and 0.0015 parts by weight of Trigonox 101 (as a peroxide) were mixed into 100 parts by weight of the powdered polyethylene resin obtained above using a Henschel mixer to prepare a polyethylene resin composition in the form of pellets using a twin-screw extruder. In this case, as the twin-screw extruder, a screw having a diameter of 608 mm and an L / D (screw length / diameter) of 8 was used, and the twin-screw extruder was operated at a temperature of 240° C. and a speed of 1,090 rpm.

[0059] The melt index (under a load of 5 kg, at 190° C.) of the pellet-form polyethylene resin composition prepared therefrom was 0.25 g / 10 min before the addition of peroxide and 0.21 g / 10 min after the addition of peroxide. This is shown in Table 1 below.

[0060] Example 2

[0061] Ethylene polymerization was carried out using a Ziegler-Natta catalyst and 1-hexene comonomer by a tandem polymerization process in which two reactors were connected in series.

[0062] The melt index (under a load of 5 kg, at 190° C.) of the powder type polyethylene resin obtained after the two-stage reactor was 0.40 g / 10 min.

[0063] 0.15 parts by weight of Irganox 1010 and 0.15 parts by weight of Irgafos-168 (as antioxidants), 0.15 parts by weight of calcium stearate (as a neutralizer), and 0.002 parts by weight of Trigonox 101 (as a peroxide) were mixed into 100 parts by weight of the powdered polyethylene resin obtained above using a Henschel mixer to prepare a polyethylene resin composition in the form of pellets using a twin-screw extruder. In this case, as the twin-screw extruder, a screw having a diameter of 608 mm and an L / D (screw length / diameter) of 8 was used, and the twin-screw extruder was operated at a temperature of 240° C. and a speed of 1,090 rpm.

[0064] The melt index (under a load of 5 kg, at 190° C.) of the pellet-form polyethylene resin composition prepared therefrom was 0.25 g / 10 min before the addition of peroxide and was 0.19 g / 10 min after the addition of peroxide. This is shown in Table 1 below.

[0065] Comparative Example 1

[0066] A polyethylene resin composition was prepared in the same manner as in Example 1, except that no peroxide was added.

[0067] The melt index (under a load of 5 kg, at 190° C.) of a polyethylene resin composition in the form of pellets prepared therefrom was 0.25 g / 10 min.

[0068] Comparative Example 2

[0069] The melt index (under a load of 5 kg, at 190°C) of the powdered polyethylene resin obtained after the two-stage reactor was 0.35 g / 10 min instead of 0.40 g / 10 min, and a polyethylene resin composition was prepared in the same manner as in Example 1 except that no peroxide was added.

[0070] The melt index (under a load of 5 kg, at 190° C.) of a polyethylene resin composition in the form of pellets prepared therefrom was 0.21 g / 10 min.

[0071] Comparative Example 3

[0072] A polyethylene resin composition was prepared in the same manner as in Example 1, except that no peroxide was added in Example 1, and the twin-screw extruder was operated at a temperature of 252° C. and a speed of 1,140 rpm.

[0073] The melt index (under a load of 5 kg, at 190° C.) of a polyethylene resin composition in the form of pellets prepared therefrom was 0.21 g / 10 min.

[0074] Comparative Example 4

[0075] Ethylene polymerization was carried out using a Ziegler-Natta catalyst and 1-hexene comonomer by a tandem polymerization process in which two reactors were connected in series.

[0076] The melt index (under a load of 5 kg, at 190° C.) of the powder type polyethylene resin obtained after the two-stage reactor was 0.41 g / 10 min.

[0077] 0.15 parts by weight of Irganox 1010 and 0.15 parts by weight of Irgafos-168 (as antioxidants), 0.15 parts by weight of calcium stearate (as a neutralizer) and 0.02 parts by weight of Trigonox 101 (as a peroxide) were mixed into 100 parts by weight of the powdered polyethylene resin obtained above using a Henschel mixer to prepare a polyethylene resin composition in the form of pellets using a single screw extruder. In this case, as the single screw extruder, a screw having a diameter of 40 mm and an L / D (screw length / diameter) of 33 was used, and the single screw extruder was operated at a temperature of 200° C. and a speed of 90 rpm.

[0078] The melt index (under a load of 5 kg, at 190° C.) of the pellet-form polyethylene resin composition prepared therefrom was 0.36 g / 10 min before the addition of peroxide and 0.20 g / 10 min after the addition of peroxide. This is shown in Table 1 below.

[0079] Evaluation: Measurement of physical properties of polyethylene resin compositions

[0080] The following physical properties were measured for the polyethylene resin compositions prepared in Examples 1 and 2 and Comparative Examples 1 to 4, and the results are shown in Tables 1 and 2 below.

[0081] Melt Index (MI)

[0082] The melt index is measured at 190° C. under a load of 5 kg according to ASTM D1238.

[0083] Melt Index (MI) Change Rate

[0084] The change rate of the melt index caused by the extruder is obtained by the following Formula 1, and the change rate of the melt index caused by the peroxide is obtained by the following Formula 2.

[0085] [Formula 1]

[0086] {(MI after passing through the two-stage reactor - MI after passing through the extruder) / (MI after passing through the two-stage reactor - final MI)}×100

[0087] [Formula 2]

[0088] {(MI after passing through the extruder - MI after adding peroxide) / (MI after passing through the two-stage reactor - final MI)}×100

[0089] Zero shear viscosity (η0)

[0090] The storage modulus G′ and loss modulus G″ were measured according to the shear rate (in radians per second) using an Advanced Rheology Extension System (ARES, 190°C), and the zero shear viscosity η0 was calculated by the Carreau model.

[0091] Melt Strength

[0092] The force for stabilization was measured as the pulling rate of the resin increased at a resin temperature of 200°C and a chamber temperature of 180°C.

[0093] Drop time reduction rate

[0094] Using HAAKE TM and Rheomex OS single screw extruder, and a rod-shaped die with a diameter of 12 mm was used. The temperature was set to 200°C, the extrusion rate was maintained at 38.1 g / min, and the initial linear velocity of the extruded resin was 0.546 cm / s. The height from the ground was 90.5 cm, and the time to reach the ground when the extruded resin did not sag was 165.7 seconds. By measuring the time for each polyethylene resin composition to reach the ground, the reduction rate relative to the case of no sag was calculated.

[0095] Strain Hardening Modulus (SHM)

[0096] The strain hardening modulus was measured according to ISO 18488.

[0097] odor

[0098] 100 g of the polyethylene resin composition and 5 mL of water were put into an airtight container, left in an oven at 80° C. for 3 hours, and then left at room temperature for 1 hour to evaluate the odor level. The worse the odor level, the higher the score, and the score was rated on a scale of 1 to 5.

[0099] Impact strength

[0100] Impact strength was measured by Charpy impact test at -30°C according to ISO 179-1.

[0101] Oxidation Induction Time (OIT)

[0102] The oxidation induction time was measured at 210°C according to ISO 11357-6.

[0103] [Table 1]

[0104]

[0105]

Table 2

[0106] Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Zero shear viscosity (poise) 8,930,000 9,849,000 3,023,000 6,445,000 6,803,000 15,380,000 Melt strength (mN) 212 229 172 181 185 238 Dropping time(s) 139 142 127 132 133 151 Dropping time reduction rate (%) 16 14 23 20 20 9 Strain hardening modulus (MPa) 61 58 64 78 68 44 odor 2.2 2.4 2.1 2.2 3.1 4.0 <![CDATA[Impact strength (kJ / m 2 )]]> 16 18 14 22 20 24 Oxidation induction time (min) 55 55 56 55 44 42

[0107] Table 1 shows that the final melt index, the rate of change of the melt index caused by the extruder, and the rate of change of the melt index caused by the peroxide satisfy the predetermined range according to the embodiment in the polyethylene resin compositions according to Examples 1 and 2. For Examples 1 and 2, it can be seen that the sag resistance, slow crack growth resistance, and odor level of the pipe are all better than those of Comparative Examples 1 to 4.

[0108] On the other hand, in Comparative Example 1 in which no peroxide was added, the final melt index was out of the range according to the embodiment, and in this case, the slow crack growth resistance and the odor level were slightly superior, but the zero shear viscosity and melt strength were greatly reduced, and the reduction rate of the drip time was greatly increased, indicating that the sag resistance of the pipe was reduced. In addition, in Comparative Example 2, the slow crack growth resistance was improved, but since no peroxide was added, it was seen that the sag resistance of the pipe was reduced.

[0109] For Comparative Example 3, which is the case of not including peroxide and passing through the extruder outside the predetermined temperature and rate conditions, it is seen that the sag resistance of the pipe is slightly improved, but the odor level is increased due to the high shear stress and the oxidation induction time is reduced.

[0110] The polyethylene resin composition of Comparative Example 4 was extruded using a single screw extruder under low shear stress extrusion conditions to reduce the change rate of the melt index caused by the extruder to 24%, and the change rate of the melt index caused by the peroxide was greatly increased to 76% by adding an excessive amount of peroxide. Accordingly, the sag resistance of the pipe was greatly improved, but the slow crack growth resistance and oxidation induction time were greatly reduced due to the addition of a large amount of peroxide, and the odor level was also greatly increased.

[0111] The polyethylene resin composition for pipes according to the embodiment is excellent in all aspects of sag resistance, slow crack growth resistance, and odor level of pipes, and thus can be usefully used for pipes, particularly large diameter pipes.

[0112] Although the preferred embodiments of the present invention have been described above, the present invention is not limited thereto and can be implemented by being modified in various ways within the scope of the claims and detailed description. Such modifications belong to the scope of the present invention.

Claims

1. A polyethylene resin composition for pipes, wherein the polyethylene resin composition for pipes has a melt index (MI) of 0.16 g / 10 min to 0.24 g / 10 min at 190° C. and a load of 5 kg, and is prepared by: using a two-stage reactor, using an extruder, and adding a peroxide, Wherein the change rate of the melt index MI at 190°C and a load of 5 kg caused by the extruder according to the following formula 1 is 60% to 90%, and the change rate of the melt index MI at 190°C and a load of 5 kg caused by the peroxide according to the following formula 2 is 10% to 40%: [Formula 1] {(MI after passing through the two-stage reactor - MI after passing through the extruder) / (MI after passing through the two-stage reactor - final MI)}×100; [Formula 2] {(MI after passing through the extruder - MI after adding peroxide) / (MI after passing through the two-stage reactor - final MI)}×100. 2 . The polyethylene resin composition for pipes according to claim 1 , wherein the zero shear viscosity η0 of the polyethylene resin composition is 8,000,000 poise to 20,000,000 poise. 3 . The polyethylene resin composition for pipes according to claim 1 , wherein the melt strength of the polyethylene resin composition is 200 to 300 mN.

4. The polyethylene resin composition for pipes according to claim 1, wherein during extrusion of the polyethylene resin composition, a reduction rate of dripping time in the presence of sagging relative to dripping time in the absence of sagging is 0% to 17%, and The drip time is measured using a single screw extruder. The polyethylene resin composition for pipes according to claim 1 , wherein the polyethylene resin composition has a strain hardening modulus of 53 MPa to 80 MPa.

6. A method for preparing a polyethylene resin composition for pipes, the method comprising: Through a two-stage reactor to obtain polyethylene resin; and Add peroxide to the polyethylene resin and pass through an extruder to obtain a polyethylene resin composition, wherein the extruder is operated at a temperature of 230°C to 250°C and a speed of 1,050 rpm to 1,150 rpm, The variation rate of the melt index MI at 190° C. and a load of 5 kg caused by the extruder according to the following formula 1 is 60% to 90%, and the variation rate of the melt index MI at 190° C. and a load of 5 kg caused by the peroxide according to the following formula 2 is 10% to 40%: [Formula 1] {(MI after passing through the two-stage reactor - MI after passing through the extruder) / (MI after passing through the two-stage reactor - final MI)}×100; [Formula 2] {(MI after passing through the extruder - MI after adding peroxide) / (MI after passing through the two-stage reactor - final MI)}×100. 7 . The method according to claim 6 , wherein the peroxide is added in an amount of 0.001 to 0.01 parts by weight relative to 100 parts by weight of the polyethylene resin.

8. The method of claim 6, wherein the melt index of the polyethylene resin at 190°C and a load of 5 kg after passing through the two-stage reactor is 0.32 g / 10 min to 0.44 g / 10 min.

9. The method of claim 6, wherein the melt index of the polyethylene resin composition at 190°C under a load of 5 kg after passing through the extruder and before adding the peroxide is 0.24 to 0.32 g / 10 min. 10 . The method of claim 6 , wherein the melt index of the polyethylene resin composition at 190° C. and a load of 5 kg is 0.16 to 0.24 g / 10 min after adding the peroxide and passing through the extruder.

11. A molded article prepared from the polyethylene resin composition for pipes according to any one of claims 1 to 5.

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