Polyolefin-based resin and threaded member containing said polyolefin-based resin

Abstract

Provided is a polyolefin-based resin which has excellent stress crack resistance even if the polyolefin-based resin contains a recycled material having a thermal history due to material recycling or the like, and which is capable of forming a threaded member such as a cap, especially a cap that can be used for a carbonated beverage. The polyolefin-based resin has a density of 0.940 g / cm3 or more, and is characterized in that the values (x = log10Mw and y = log10CL) obtained by calculating the base 10 logarithm of each of the weight average molecular weight Mw and the accumulated chemiluminescent light emission amount (for 5 minutes) CL of the polyolefin-based resin satisfy formula (1) and / or (2).　(1): y ≤ 29.7x - 148.7; (2): y ≤ 5.4x - 21.7

Description

Polyolefin resin and threaded member containing this polyolefin resin

[0001] This invention relates to polyolefin resins, and more particularly to polyolefin resins with excellent stress crack resistance.

[0002] Polyolefin resins such as polyethylene are widely used as packaging materials for beverage containers and caps due to their excellent mechanical strength, transparency, and hygiene. In recent years, recycling of these packaging materials has been promoted in order to reduce the environmental impact of plastic products. Among plastic products, resin caps used in containers such as PET bottles are being collected separately, and there is a demand for material recycling and reuse of these caps. However, recycled materials suffer thermal degradation due to thermal decomposition of the resin during the heat treatment process during recycling, resulting in poor thermal stability and resistance to environmental stress cracking (stress crack resistance). Therefore, it is difficult to reshape and reuse such recycled materials as caps.

[0003] Chemiluminescence (CL method) is known as a method that can sensitively detect heat treatment applied to a resin as an initial thermal history, and polyolefin resins with excellent properties even when containing recycled resin have been proposed using this method. For example, Patent Document 1 below describes a recycled resin film in which the cumulative value of chemiluminescence emission intensity from the start of measurement to 300 seconds is 90,000 counts or less. Patent Document 2 below describes a polyolefin resin composition characterized in that, in accordance with JIS K7351:2018, the cumulative value of chemiluminescence during heating from 50°C at 15°C / min and holding at 200°C for 60 minutes is 700,000 to 15,000,000 counts.

[0004] Japanese Patent Publication No. 2022-155513 Japanese Patent Publication No. 2022-151607

[0005] Patent documents 1 and 2 described above state that even resins containing recycled resin have a certain range of luminescence emission by chemiluminescence, resulting in less thermal degradation and excellent recyclability. However, it is not easy to determine whether a polyolefin resin that has undergone thermal degradation due to material recycling or the like possesses the stress crack resistance (durability) required for screw caps and the like, and there is a demand for a polyolefin resin that can provide the desired stress crack resistance.

[0006] Therefore, the object of the present invention is to provide a polyolefin resin that, even if it contains recycled material having a thermal history due to material recycling or the like, exhibits excellent stress crack resistance and can be used to create screw-type components such as caps, and in particular caps that can be used for carbonated beverages.

[0007] According to the present invention, the density is 0.940 g / cm³. ３ For the polyolefin resin described above, the values ​​obtained by logarithmically transforming the weight-average molecular weight Mw and the chemiluminescence integrated luminescence (5 minutes) CL of the polyolefin resin with base 10 are x = log １０ Mw and y = log １０ A polyolefin resin is provided characterized in that CL satisfies the following formulas (1) and / or (2): y ≤ 29.7x - 148.7 ... (1) y ≤ 5.4x - 21.7 ... (2)

[0008] In the polyolefin resin of the present invention, it is preferable that: (1) the polyolefin resin contains recycled material; (2) the melt flow rate (190°C, 2.16 kg) is 0.8 g / 10 min or more; (3) the weight-average molecular weight Mw is in the range of 120,000 to 500,000; and (4) the chemiluminescence integrated emission amount (5 minutes) CL is 55,000,000 or less.

[0009] The present invention also provides a threaded member characterized by containing the above-mentioned polyolefin resin. In the threaded member of the present invention, it is preferable that it be a spout or a cap.

[0010] The present invention further provides a container characterized by containing the above-mentioned polyolefin resin.

[0011] The polyolefin resin of the present invention satisfies formulas (1) and (2) above, and even when it contains recycled material with a thermal history, it has excellent stress crack resistance, with an environmental stress crack resistance (50%) of 4 hours or more, making it applicable to threaded components and containers such as screw caps and threaded spouts. Furthermore, it is possible to use recycled material from polyolefin resin caps that were used on containers such as PET bottles, and even when such recycled material is used, it can still possess stress crack resistance suitable for caps for carbonated beverages.

[0012] This graph shows the relationship between the weight-average molecular weight Mw and the chemiluminescence integrated emission amount (5 minutes) CL of polyolefin resins, after logarithmic transformation to base 10.

[0013] The polyolefin resin of the present invention has a density of 0.940 g / cm³. ３ For the above polyolefin resins, the values ​​obtained by logarithmically transforming the weight-average molecular weight Mw and the chemiluminescence integrated luminescence (5 minutes) CL, respectively, with base 10, are x = log. １０ Mw and y = log １０ An important feature of CL is that it satisfies the following equations (1) and / or (2): y ≤ 29.7x - 148.7 ... (1) y ≤ 5.4x - 21.7 ... (2)

[0014] As described above, it is known to evaluate the degree of oxidative degradation of a resin by the chemiluminescence (CL) method. In the present invention, it has been found that when the weight average molecular weight (Mw) and the CL integrated luminescence intensity (CL) of the polyolefin resin satisfy the above formula (1) and / or (2), excellent stress crack resistance can be exhibited. The above formulas (1) and (2) defined in the present invention are determined based on the graph in Figure 1. That is, in the graph of Figure 1, as will be apparent from the Examples described later, the density is 0.940 g / cm ３ or more of the polyolefin resin (high-density polyethylene), the weight average molecular weight (Mw) and the CL integrated luminescence intensity (CL) were measured, and these values of Mw and CL are converted into logarithmic values with a base of 10 (log １０ Mw and log １０ CL), and the plots are indicated as follows: polyolefin resins having an environmental stress crack resistance (ESCR) (50%) of 4 hours or more are indicated by "●", and polyolefin resins having an environmental stress crack resistance (ESCR) (50%) of less than 4 hours are indicated by "×". As is apparent from this Figure 1, it is understood that polyolefin resins present in the region satisfying the above formula (1) and / or (2) have excellent stress crack resistance with an ESCR of 4 hours or more.

[0015] The above formulas (1) and (2) defined in the present invention are obtained from the results of high-density polyethylene, but even when high-density polyethylene is mixed with another polyolefin resin (for example, polypropylene, low-density polyethylene, elastomer resin, etc.), the density is 0.940 g / cm ３As long as the above-mentioned weight-average molecular weight (Mw) and integrated luminescence (CL) satisfy formulas (1) and / or (2), the ESCR is considered to show a similar trend to that of high-density polyethylene. Furthermore, the above-mentioned ESCR of 4 hours or more, which is the evaluation criterion for the stress crack resistance of polyolefin resins in this invention, is a value that allows for sufficient stress crack resistance even when used in threaded components such as screw caps and screw spouts. This is measured by the method described later, and it measures the time at which the probability of cracking due to environmental stress becomes 50%. In addition, the density of the polyolefin resin is 0.940 g / cm³ ３ If the density is smaller than this, it becomes difficult to impart sufficient rigidity, heat resistance, etc., to the molded product, such as a screw-type component. Therefore, this invention assumes a polyolefin resin having a density equal to or greater than the above value.

[0016] (Polyolefin resin) As described above, the polyolefin resin of the present invention has a density of 0.940 g / cm³ ３ The polyolefin resin described above may consist solely of virgin polyolefins, as long as it satisfies formulas (1) and / or (2) above. However, even if it contains recycled material that has undergone thermal degradation by heat treatment, it is desirable in the present invention to include recycled material, as it can exhibit excellent stress crack resistance with an ESCR of 4 hours or more.

[0017] The polyolefin resin of the present invention preferably has a melt flow rate (MFR: 190°C, 2.16 kg) of 0.8 g / 10 min or more, and more preferably in the range of 0.8 to 2.5 g / 10 min. If the MFR is smaller than the above range, the moldability will be inferior compared to when it is within the above range, and if it is larger than the above range, the stress crack resistance will be inferior compared to when it is within the above range, and there is a risk that the resulting molded product will not have sufficient rigidity. Furthermore, the weight-average molecular weight (Mw) of the polyolefin resin preferably has a range of 120,000 to 500,000, and more preferably in the range of 200,000 to 500,000. By having Mw within the above range, the generation of low molecular weight components due to resin degradation (thermal decomposition) due to heat treatment is suppressed, and a molded product with excellent rigidity can be obtained. Furthermore, the polyolefin resin preferably has a chemiluminescence integrated emission amount (CL) of 55,000,000 or less, and particularly preferably 10,000,000 or less. As mentioned above, the CL integrated emission amount is an indicator of the degree of oxidative degradation of the resin due to heat treatment, etc., and the lower the CL integrated emission amount, the less oxidative degradation of the resin is, making it possible to have excellent stress crack resistance.

[0018] As mentioned above, polyolefin resins have a density of 0.940 g / cm³. ３As long as the above conditions are met, there are no limitations, but examples include ethylene homopolymers, random or block copolymers of ethylene with 3 to 20 carbon atoms α-olefins or vinyl compounds such as styrene that can copolymerize with ethylene, and cyclic olefin copolymers. The polyolefin resin may also be a blend of the above polymers or copolymers. The polyolefin resin may contain various additives such as antioxidants, heat stabilizers, lubricants, and pigments as needed. High-density polyethylene is preferred as the polyolefin resin of the present invention. Examples of recycled materials include materials that have undergone one or more melting heat histories (melting → cooling), such as regrind resins containing scrap such as burrs generated during molding using the above-mentioned polyolefin resins (post-industrial material recycled (PIR) material), or recycled materials (post-consumer recycled (PCR) material) recycled from separately collected containers and caps. Recycled materials differ in properties due to differences in resin type, thermal history, etc., and consequently in the amount of low molecular weight components. However, according to the present invention, as long as the density described above and formulas (1) and / or (2) are satisfied, the material may consist solely of recycled material, be a blend with virgin material, or be a blend of PIR material, PCR material, or both. The proportion of recycled material contained in the polyolefin resin is not limited, depending on the properties of the recycled material, but for example, a lower limit of 10% by mass or more is desirable.

[0019] In this invention, the density is 0.940 g / cm³. ３ By determining whether the above polyolefin resins satisfy formulas (1) and / or (2), it is possible to determine whether the polyolefin resins have excellent stress crack resistance without performing ESCR measurements. In particular, recycled materials do not have a clear thermal history, and their properties differ from one recycled material to another, but according to the present invention, it is possible to easily identify recycled materials with excellent stress crack resistance. That is, when the density is 0.940 g / cm³ ３For the polyolefin resins described above, if their weight-average molecular weight and chemiluminescence value are determined and the above formulas (1) and / or (2) are satisfied, it can be seen that the polyolefin resin has stress crack resistance of 4 hours or more. Therefore, this is a useful method for easily determining the suitability of stress crack resistance not only for virgin polyolefin resins with stable physical properties, but also for recycled materials whose thermal history is unknown and whose physical properties are not stable.

[0020] (Threaded Components) As described above, the polyolefin resin of the present invention has excellent stress crack resistance and can therefore be suitably used for threaded components such as spouts and caps. In other words, in threaded components, excessive tightening can cause large pressures to be applied to the threaded portion, seal portion, and cap skirt portion. If such large pressures are applied over a long period of time, there is a risk that cracks may form in the stressed areas. Furthermore, in the case of threaded components such as screw caps, if they are used with contents that have self-sustaining pressure, such as carbonated beverages, or if the contents spoil, internal pressure will act on the top surface of the cap over a long period of time, which may cause cracks to form on the top surface of the cap. However, the polyolefin resin of the present invention has excellent stress crack resistance, so it can effectively prevent the occurrence of cracks even in such cases.

[0021] Examples of threaded components include a threaded spout consisting of a spout body and a threaded cap, a threaded cap used for containers such as PET bottles, or a container such as a bottle with a threaded opening. The polyolefin resin of the present invention used for the threaded spout and threaded cap has a density of 0.940 g / cm³ from the viewpoint of moldability, rigidity, and hygiene. ３ In particular, 0.945–0.968 g / cm³ ３ It is preferable that the material is high-density polyethylene within the specified range, and that the MFR (190°C, 2.16 kg) is 0.8 g / 10 min or more, and particularly preferable that it is in the range of 0.8 to 2.5 g / 10 min.

[0022] In the threaded component of the present invention, particularly in the threaded cap, recycled material obtained by material recycling of caps made of high-density polyethylene used in PET bottles and the like can be suitably used. By measuring the weight-average molecular weight (Mw) and chemiluminescence integrated emission amount (CL) of such recycled material, estimating whether the ESCR is 4 hours or more from the above-mentioned formulas (1) and / or (2), and blending in virgin material as necessary, a threaded cap with excellent stress crack resistance can be molded.

[0023] The polyolefin resin of the present invention, used in containers having a screw portion, has a density of 0.940 g / cm³ from the viewpoint of moldability, rigidity, etc. ３ In particular, 0.945–0.968 g / cm³ ３ It is preferable that the MFR (190°C, 2.16 kg) be within the specified range, and that the MFR (190°C, 2.16 kg) be 0.8 g / 10 min or more, particularly in the range of 0.8 to 2.5 g / 10 min. High-density polyethylene is preferred as the polyolefin resin, and in the container, as with the screw-on cap, virgin material can be blended with recycled material. The container having a screw portion may be a single-layer structure made of the polyolefin resin of the present invention, or it may be a multi-layer structure, and is not limited thereto, but it may also include other layers made of known thermoplastic resins, such as a gas barrier layer as an intermediate layer. The method of manufacturing the container is not limited, and it can be molded by various molding methods such as extrusion molding, injection molding, compression molding, and direct blow molding, depending on the MFR of the polyolefin resin.

[0024] <Preparation of Initial Raw Materials> Five types of high-density polyethylene, A, B, C, D, and E, were supplied to an extruder as raw materials, and PET bottle caps for beverages were molded by compression molding. The molding temperature was 200°C and the molding speed was 800 pieces / minute. Next, the caps were crushed in a plastic crusher equipped with an outlet filter with a mesh size of 8 mm to produce polyolefin resin compositions A-0, B-0, C-0, D-0, and E-0, which will serve as initial recycled raw materials.

[0025] <Preparation of Recycled Material> The polyolefin resin composition A-0 was supplied to a co-rotating twin-screw extruder (Shibaura Machinery Co., Ltd.: TEM-26SS) equipped with a pellet granulation device, and extruded and pelletized under the conditions of a molding temperature of 280°C, a screw rotation speed of 150 rpm, and a discharge rate of 7.0 kg / h to form pellet A-1, which had been extruded once. Furthermore, pellet A-2, which had been extruded twice, was formed by extruding and pelletizing pellet A-1 in the same manner. By repeating this operation up to five times, pellets A-n (n = 1, 2, 3, 4, 5, where n indicates the number of extrusions) were obtained by repeatedly extruding polyolefin resin composition A-0 one to five times using it as the initial raw material. Similarly, the same procedure was performed on polyolefin resin compositions B-0, C-0, D-0, and E-0 to obtain pellets B-n (n=1, 2, 3, 4, 5), C-n (n=1, 2, 3, 4, 5), D-n (n=1, 2, 3, 4, 5), and E-n (n=1, 2, 3, 4, 5). Next, these pellets, either individually or mixed by the dry blending method, were supplied to an extruder to form PET bottle caps for beverages by compression molding. Furthermore, these caps were crushed in a plastic crusher equipped with an outlet filter with an 8 mm mesh size to produce polyolefin resin compositions A-n (n=1, 2, 3, 4, 5), B-n (n=1, 2, 3, 4, 5), C-n (n=1, 2, 3, 4, 5), D-n (n=1, 2, 3, 4, 5), E-n (n=1, 2, 3, 4, 5), a molten blend composition made by blending B-4 and D-2 in a 50:50 ratio, and a molten blend composition made by blending B-5 and C-3 in a 50:50 ratio, respectively, as recycled materials.

[0026] <Molding of ESCR Test Specimens> Using all 30 types of polyolefin resin compositions A-0 to E-5, a dry blend composition obtained by blending B-4 and D-2 in a 50:50 ratio, and a dry blend composition obtained by blending B-5 and C-3 in a 50:50 ratio as raw materials, ESCR test specimens measuring 38 mm in length, 13 mm in width, and 1.96 mm in thickness were molded using a fully electric small injection molding machine (Sumitomo Heavy Industries, Ltd.: SE75EV-A). The molding temperature was set to 215°C, and the mold cooling temperature to 40°C.

[0027] <Measurement of ESCR (50%)> In accordance with ASTM D1693 Condition B, the time at which the crack initiation rate due to environmental stress in the ESCR test specimens reached 50% was measured using a stress crack testing apparatus (manufactured by Toyo Seiki Seisakusho). The ESCR test specimens were pre-notched in accordance with ASTM D1693 Condition B. Ten of these test specimens were fixed in a test specimen holder in a U-shape along their length. Furthermore, the holder with the fixed test specimens was placed inside a test tube, and a 10 wt% aqueous solution of Igepal CO-630 (manufactured by SIGMA-ALDRICH) was added to a level where the entire test specimen was in contact with the water, and then sealed with a rubber stopper. This test tube was immersed in a 50°C water bath with an observation window, and the presence or absence of cracks on the surface of the test specimens was checked every hour. The time at which five test specimens showed cracks was defined as ESCR (50%) (h).

[0028] <Measurement of Weight-Average Molecular Weight (Mw)> The polyolefin resin composition and the molten blend composition were wrapped in a 26 μm stainless steel mesh and placed in a screw-cap vial. 10 mL of a solution of o-dichlorobenzene (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) with 0.05 wt% BHT added as a stabilizer was injected, and the mixture was shaken and stirred at a set temperature of 140°C and 70 rpm using a dissolution filtration device (Tosoh Corporation: DF-8321H) to prepare a measurement sample. The measurement was performed using a high-temperature GPC device (Tosoh Corporation: HLC-8321GPC / HT) under the conditions of o-dichlorobenzene with 0.05 wt% BHT added as the eluent, a temperature of 145°C, a concentration of 0.1 wt / vol%, and a flow rate of 1.0 mL / min. Two TSKgelGMHHR-H(20)HT2 columns (Tosoh Corporation) were used, and a guard column was set up separately. A differential refractometer (RI) was used as the detector. The weight-average molecular weight (Mw) in polystyrene equivalent was calculated using analysis software (Tosoh Corporation: 8321GPC-WS).

[0029] <Measurement of Chemiluminescence Integrated Emission (CL)> Using a chemiluminescence analyzer (manufactured by Tohoku Electronics Industry Co., Ltd.: main unit: CLA-FS5, sample chamber: CLS-ST5), 200 mg each of the above-mentioned polyolefin resin composition and molten blend composition were placed in 20 mm diameter aluminum cups to a uniform thickness, and the chemiluminescence emission (counts) was measured at a temperature of 160°C and under a nitrogen gas atmosphere of 50 mL / min. The sampling interval was 1 sec. The background value was measured and subtracted, and the integrated value from immediately after the start of measurement to 5 minutes was taken as the chemiluminescence integrated emission (CL) (counts).

[0030] <Measurement of Melt Flow Rate (MFR)> The melt flow rate (MFR) of the above-mentioned polyolefin resin composition and melt blend composition was measured using a melt flow rate tester (Toyo Seiki Seisakusho Co., Ltd.: Melt Indexer F-F01) at a temperature of 190°C and a load of 2.16 kg. The average value of N=10 of the MFR extruded samples obtained in the test was defined as MFR (g / 10min).

[0031] <Density Measurement> The above-mentioned polyolefin resin composition and molten blend composition were immersed in boiling pure water for 30 minutes, then allowed to cool for 1 hour, and vacuum-dried at 40°C. The density of the samples was measured using a dry automatic densimeter (Shimadzu Corporation: AccuPic 1330). Ten measurements were taken at 23°C using a 10cc cell, and the average value of N=10 was used to determine the density (g / cm³). ３ )

[0032] <Examples 1-20, Comparative Examples 1-12> For the polyolefin resin compositions and melt blend compositions prepared by the above method, ESCR, Mw, CL, MFR, and density were measured according to the above method. Table 1 shows the initial raw materials and recycled materials used and the corresponding measurement results. Figure 1 shows the values ​​of Mw and CL as base 10 logarithmic values ​​(log １０ Mw and log １０is plotted against CL, and in the display of the plot, polyolefin resins with an ESCR (50%) of 4 hours or more are indicated by "●", and polyolefin resins with an ESCR (50%) of less than 4 hours are indicated by "×". As shown in Figure 1, in Examples 1 to 20, x=log １０ Mw and y=log １０ CL satisfies the following formula (1) and / or (2), has an ESCR of 4 hours or more, and exhibits sufficient environmental stress crack resistance even when formed into a threaded member such as a screw cap or a threaded spout. y≦29.7x−148.7 ・・・(1) y≦5.4x−21.7 ・・・(2) On the other hand, Comparative Examples 1 to 12 satisfy neither formula (1) nor (2) above, have an ESCR of less than 4 hours, and are inferior in environmental stress crack resistance.

[0033]

[0034] The polyolefin resin of the present invention has excellent environmental stress crack resistance even when it contains a recycled material, and can be suitably used for threaded members such as screw caps, threaded spouts, or bottles having a threaded portion. In addition, the present invention is not limited to threaded members, and since recycled materials excellent in environmental stress crack resistance can be effectively utilized, the polyolefin resin of the present invention can also be used for various packaging materials such as films, trays, and cups. Furthermore, by satisfying the above formula (1) and / or (2) defined in the present invention, it can be inferred that the polyolefin resin has an ESCR of 4 hours or more, so suitability assessment when using recycled materials can also be easily performed.

Claims

1. Density is 0.940 g / cm³ ３ For the polyolefin resin described above, the values ​​obtained by logarithmically transforming the weight-average molecular weight Mw and the chemiluminescence integrated luminescence (5 minutes) CL of the polyolefin resin with base 10 are x = log １０ Mw and y = log １０ A polyolefin resin characterized in that CL satisfies the following formulas (1) and / or (2): y ≤ 29.7x - 148.7 ... (1) y ≤ 5.4x - 21.7 ... (2) 2. The polyolefin resin according to claim 1, wherein the polyolefin resin contains recycled material.

3. The polyolefin resin according to claim 1 or 2, wherein the melt flow rate (190°C, 2.16 kg) is 0.8 g / 10 min or more.

4. The polyolefin resin according to claim 1 or 2, wherein the weight-average molecular weight Mw is in the range of 120,000 to 500,000.

5. The polyolefin resin according to claim 1 or 2, wherein the chemiluminescence integrated emission amount (over 5 minutes) CL is 55,000,000 or less.

6. A screw-type member characterized by containing the polyolefin resin described in claim 1.

7. The threaded member according to claim 6, which is a spout or a cap.

8. A container characterized by containing the polyolefin resin described in claim 1.

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