Method for removing impure substance from plastic

Abstract

The present invention provides a method for removing impurities from plastics, characterized in that: impurities are removed from plastics by using water heated to a level where the relative permittivity is below 60 as a cleaning medium.

Description

Technical Field

[0001] This invention relates to a method for removing impurities from plastics. Prior Technology

[0002] In recent years, considering environmental protection, the recycling of various plastic molded products such as films and containers has been pursued. When recycling plastics, in addition to directly reusing them without damaging their shape, there are also cases where they are crushed and recycled in the form of granules or pellets as recycled plastic for reuse in molding.

[0003] In any form of plastic reuse, it is necessary to remove impurities from the plastic that has become soiled due to use. In terms of methods for removing impurities from plastics, the common method is to use organic solvents, water, etc. for cleaning.

[0004] For example, Patent Document 1 discloses a method for removing impurities by contacting polyethylene with an organic solvent and purifying the solution. Furthermore, Patent Document 2 discloses that after the packaging material (plastic film) is crushed, it is chemically cleaned using acid or alkali and then recycled. Patent Document 3 discloses that water washing is used to separate foreign matter from plastic. Furthermore, Patent Document 4 discloses the reuse of recycled material (plastic) with impurities removed as a material for film formation. [Previous Technical Documents] [Patent Literature]

[0005] Patent Document 1: Japanese Patent Publication No. 2020-511560 Patent Document 2: Japanese Patent Application Publication No. 6-31733 Patent Document 3: Japanese Patent Application Publication No. 6-173182 Patent Document 4: Japanese Patent Application Publication No. 2001-122985 Summary of the Invention

[0006] [The problem that the invention aims to solve]

[0007] However, using organic solvents as a medium to remove impurities from plastics presents environmental pollution problems, as well as significant costs associated with exhaust systems and wastewater treatment of organic solvents. Conversely, using acids or alkalis as cleaning media avoids environmental pollution, but requires larger equipment and increases costs. On the other hand, when using water as a cleaning medium, there is virtually no waste liquid disposal issue. However, from a cleaning performance perspective, it is significantly inferior to organic solvents. It is insufficient for cleaning grease and other oil stains adhering to the surface of plastics. Furthermore, it is almost impossible to remove adsorbed substances absorbed into the interior of the plastic. In reality, organic solvents are ultimately still required.

[0008] Therefore, the purpose of this invention is to provide a method for removing impurities from plastics that does not cause environmental pollution, reduces the burden of exhaust and waste liquid treatment, and can effectively remove impurities such as grease adhering to or adsorbed on the plastic surface to the same extent as when using organic solvents as cleaning media. [Methods for solving problems]

[0009] According to the present invention, a method is provided for removing impurities from plastic by using water heated to a level where the relative permittivity is below 60 as a cleaning medium.

[0010] In the method of this invention, the following means are preferably used. (1) The aforementioned cleaning medium is superheated steam. (2) The aforementioned cleaning medium is subcritical or supercritical water heated by a pressure of 0.5 MPa or above and a temperature of 150°C or above. (3) The aforementioned cleaning medium is supercritical water. (4) The aforementioned plastic polyolefins. (5) The aforementioned plastics are plastic molded products that have been used. (6) The aforementioned impurities dissolve into the aforementioned plastic. [Effects of the Invention]

[0011] This invention uses water as a cleaning medium, which can thoroughly remove not only grease and other oil stains adhering to the surface of plastics, but also oil stains (i.e., adsorbed components) and other impurities that have penetrated into the plastic, just as it can when using organic solvents as a cleaning medium. In other words, because water is used, it does not have an adverse impact on the environment and does not require large-scale exhaust equipment or wastewater treatment equipment. This is a major advantage of this invention, making it highly useful in industrial applications. Simple Explanation of the Diagram

[0012] [Figure 1] is a state diagram of water used as the cleaning medium. Implementation

[0013] <Cleaning Medium> In this invention, water is used as the cleaning medium when removing impurities from plastics. Importantly, this water is used after its dielectric constant has been reduced. Specifically, in this invention, water is heated to a relative dielectric constant of 60 or less, preferably 40 or less, and more preferably 30 or less, and the plastics are cleaned using this water with a reduced dielectric constant.

[0014] In other words, the relative permittivity of water at room temperature is significantly higher than that of organic solvents, making it a particularly polar solvent. This is the reason why it lacks affinity (oil-repelling properties) for organic components (such as greases) and is difficult to clean. On the other hand, it is known that the relative permittivity decreases as temperature increases. In this invention, this characteristic is utilized to achieve a low permittivity due to heating, and water is used for cleaning in a state close to that of organic solvents. For example, if water is heated to about 200°C, its relative permittivity becomes about 35, close to that of methanol at room temperature. If heated to about 300°C, the relative permittivity becomes about 20, close to that of acetone at room temperature. When heated further to near the critical point, it becomes approximately equivalent to that of non-polar organic solvents such as chloroform and ether.

[0015] As can be understood from the above description, the water used in this invention becomes nonpolar due to the reduction of its dielectric constant, thus becoming close to that of an organic solvent. Furthermore, through this reduction of the dielectric constant, the water reaches a high temperature. If it comes into contact with water at such a high temperature, the polymer chains constituting the plastic will loosen, and the water that has undergone the reduction of its dielectric constant can easily penetrate into the plastic. According to the present invention, as described above, water, which is reduced to a near-nonpolar state similar to an organic solvent by heating at high temperatures to lower its dielectric constant, is used as a cleaning medium. This effectively removes impurities adhering to the surface of plastics and impurities adsorbed within the plastic. For example, heating to 90°C or higher under a pressure of 0.1 MPa can reduce the relative dielectric constant to 60 or lower. Under the same pressure, heating to 180°C or higher is required to reduce the relative dielectric constant to 40 or lower, heating to 230°C or higher is required to reduce the relative dielectric constant to 30 or lower, and heating to 300°C or higher is required to reduce the relative dielectric constant to 20 or lower.

[0016] Furthermore, in this invention, the water used as the cleaning medium is preferably heated to achieve a low dielectric constant, as described above, and thus becomes what is referred to as superheated steam, subcritical water, or supercritical water.

[0017] For example, refer to the state diagram of water shown in Figure 1. Due to changes in intermolecular interactions and kinetic energy accompanying pressure changes, water changes through solid, liquid, and water vapor (gas) states. As shown in Figure 1, point A on the gas-liquid coexistence line is the critical point (374°C, 22.1 MPa). Water in this state above the temperature and pressure is supercritical water, possessing properties as both a liquid and a gas. Subcritical water refers to water in a temperature and pressure range slightly lower than the critical point (e.g., pressure above 0.5 MPa, temperature above 150°C). Such subcritical or supercritical water has a relatively low dielectric constant and the property of dissolving or hydrolyzing nonpolar organic compounds. That is, by using water heated to such a state and thus having a low dielectric constant, impurities adhering to the surface of plastics or adsorbed inside plastics can be effectively removed.

[0018] Furthermore, regarding methods for determining the relative permittivity, there are the coaxial probe method, the lumped parameter capacity method, the transmission line method, the free-space reflection method, and the cavity resonator method (see Japanese Patent No. 5499379, etc.). The methods for determining the relative permittivity vary depending on the frequency of measurement. The coaxial probe method is used in a wide measurement band from MHz to GHz and can also be used for liquids. In addition, in the following documents (1) to (3), some have proposed methods for determining and calculating the relative permittivity of water in the temperature and pressure range above the critical point. (1)M. Yao, K. Okada and Y. Imashu ku, Rev. High Pressure Sci. Techno l., 7, 1118 (1998) (2)DP Fernandez, ARH Goodw in, EW Lemmon, JMH Levelt Sengers and RC Williams, J. Phy s. Chem. Ref. Data., Vol.26, No.4 (1997) (3) Yasuhiro Shibue, Research Notes on Hyogo University of Education, Vol. 47 (2015)

[0019] <Cleaning Target: Plastic> In this invention, any plastic (used molded plastic article) that is to be cleaned can be used, as long as it will not decompose when cleaned with heated water as described above. Olefin resins are preferred, such as low-density polyethylene, linear low-density polyethylene, medium- or high-density polyethylene, polypropylene, poly(1-butene), and poly(4-methyl-1-pentene). Of course, random or block copolymers between α-olefins such as ethylene, propylene, 1-butene, and 4-methyl-1-pentene can also be used. Furthermore, cyclic olefin copolymers disclosed in Japanese Patent Application Publication No. 2007-284066 are also ideally suitable. This is because such olefin resins are chemically very stable, and the risk of hydrolysis is relatively small when cleaned with hot water that has been converted to a low dielectric constant.

[0020] Cleaning The aforementioned cleaning of plastics can be performed, for example, by immersing the object to be cleaned (the plastic) in high-temperature water that has undergone low dielectric constant treatment, such as subcritical or supercritical water. Specifically, this can be done by immersing the object to be cleaned (the plastic) in a container containing the cleaning medium (water) and stirring. Alternatively, it can be performed by blowing high-temperature water vapor with a low dielectric constant, or by immersing the object to be cleaned (the plastic) in a container containing superheated water vapor and bringing it into contact with the cleaning medium.

[0021] Furthermore, in this invention, water with a low dielectric constant is preferably used as the cleaning medium for cleaning. However, without compromising the advantages of this invention, hydrophilic organic solvents such as acids, alkalis, or ethanol may also be added to the water with the low dielectric constant. Alternatively, before cleaning with water of low dielectric constant as described above, cleaning can be performed using conventionally known methods, thus performing cleaning in a multi-stage manner.

[0022] Furthermore, in principle, the method of this invention can also directly supply used plastic, maintaining the shape of the molded article, for cleaning, and then reuse the cleaned molded article as a recyclable product. However, it is generally advisable to pulverize the plastic into small, easily cleanable granules for cleaning. This is because cleaning becomes difficult when the plastic is large, and heat-induced morphological changes occur, making it difficult to reuse as a molded article. In addition, pulverization allows the water in the cleaning medium to penetrate the plastic and more effectively removes adsorbed impurities.

[0023] Plastics that have had impurities (organic impurities such as grease and oil) removed through the aforementioned cleaning process can be melt-extruded alone using an extruder, or melt-blended and melt-extruded together with virgin plastic to form granules, which can then be used as recycled plastic for re-molding and reused as packaging containers. Alternatively, the cleaned plastic can be directly reused as a recyclable product.

[0024] The method of this invention, by using water as the cleaning medium, avoids environmental pollution and reduces the burden of exhaust and wastewater treatment. Furthermore, it not only removes oil stains from plastics with the same effectiveness as methods using organic solvents, but also avoids the problem of solvent residue in the plastic.

[0025] [Example] The present invention is further illustrated by the following embodiments and comparative examples, but the present invention is not limited to these embodiments. In the following examples and comparative examples, five types of samples were used for cleaning the plastic objects, and various measurements and evaluations are described below.

[0026] <Impurity Determination> The impurities contained in the cleaned plastic were pretreated by solid-phase microextraction as shown below and determined by gas chromatography-mass spectrometry. For the gas chromatography quality analysis apparatus, Agilent Technologies' GC / MS (GC-7890A, MSD-5975C) was used. Furthermore, regarding the adsorbent, SPME fiber (Supelco CA R / PDMS 85μm) was used.

[0027] The pretreatment using solid-phase microextraction was carried out by the following method: 2g of sample was sealed in a headspace sampling vial, and the adsorbent, namely SPME fiber, was placed in it. The vial was then kept at 40°C for 15 minutes to allow impurities to be adsorbed onto the SPME fiber. The SPME fiber is installed in the aforementioned gas chromatography mass analysis device to desorb the impurities adsorbed on the SPME fiber, and the separated components are detected and analyzed to perform qualitative and quantitative analysis of the impurities.

[0028] The above measurement conditions are as follows. Column: Agilent Technologies J&W DB WAX (60m × 0.25mm, 5μm film thickness) Carrier gas: Helium Detector: MSD Ionization method: EI Oven temperature: Increase from 45℃ to 220℃ at a rate of 7℃ / min and hold for 15 minutes. Injection inlet temperature: 250℃, injection mode: splitless, mass spectrometry range: 27~600.

[0029] From the graph obtained by measurement, the sum of the area values ​​of all components except hydrocarbons is taken as the amount of impurity components. By comparing the samples before and after the cleaning treatment, the removal efficiency (%) of the amount of impurity components obtained by the treatment is calculated using the following formula. Removal efficiency (%) = (1 - X / Y) × 100 Area value of the sample after X-series cleaning treatment Y represents the area of ​​the untreated sample that has not undergone cleaning.

[0030] <Evaluation of yellowing of samples due to cleaning process> Yellowing of samples caused by cleaning is evaluated by using the b* value of the CIE1976 (L*,a*,b*) color space. In other words, a colorimeter manufactured by Suga Test Instruments Co., Ltd. was used to evaluate the yellowing of samples caused by cleaning treatment, and this served as a criterion for judging the inhibition of resin degradation. The smaller the change in b* value before and after cleaning treatment, the more suppressed the resin degradation; the larger the change in b* value, the more the resin deteriorated due to cleaning treatment.

[0031] <Relative permittivity of water> The relative permittivity of the water used for cleaning was calculated based on references (2) and (3).

[0032] <The plastic used for cleaning> Sample A: The cooking oil was drained from a high-density polyethylene bottle that had been filled with cooking oil and kept at 25°C for 3 months, and the used bottle was then crushed to obtain thin flakes. Area value from impurities: 1.1 × E+08 b* value: -3.57 Sample B: Edible oil is drained from a high-density polyethylene bottle that has been filled with edible oil and kept at 25°C for 3 months. The used bottle is then crushed, and the edible oil adhering to the surface is removed and cleaned as much as possible to obtain a thin sheet. Area value from impurities: 2.9 × E+07 b* value: -1.93 Sample C: Mayonnaise was drained from low-density polyethylene bottles that had been filled with mayonnaise and kept at 25°C for 3 months, and the resulting used bottles were then crushed to obtain flakes. Area value from impurities: 4.0 × E+07 b* value: -0.34 Sample D: Virgin pellets of high-density polyethylene. Area value from impurities: 6.1 × E+06 b* value: -4.14 Sample E: Thin sheets are obtained by crushing high-density polyethylene bottles that are not filled with edible oil. Area value from impurities: 1.1 × E+07 b* value: -3.31

[0033] <Example 1> For the plastic object being cleaned, sample A is used. Sample A was placed in a test treatment device filled with superheated steam at 240°C for 5 minutes. After treatment, the test treatment device was opened and sample A was removed directly at the treatment temperature. That is, the treatment conditions are as described below. Processing temperature: 240℃ Processing time: 5 minutes The relative permittivity of the water used in the cleaning process is 28. Furthermore, the experimental treatment device, filled with superheated steam, is not a completely closed system; the superheated steam is continuously supplied at a fixed flow rate and sequentially discharged. The internal dimensions of the experimental treatment device are a square with dimensions of 200 mm square, and the flow rate of the superheated steam is 20 kg / h. Additionally, a stainless steel mesh basket with a base area of ​​50 mm × 50 mm was used for settling within the experimental treatment device. A 10 g sample was placed in the mesh basket for treatment. For the cleaned samples, solid-phase microextraction was used as a pretreatment method, and gas chromatography was used for quality analysis to determine the impurity components and a colorimeter was used to determine the b* value. The evaluation results are shown in Table 1.

[0034] <Example 2> The processing time was set to 10 minutes. Otherwise, sample A was cleaned in the same manner as in Example 1, and the same measurements were performed. The various evaluation results are shown in Table 1.

[0035] <Example 3> The processing time was set to 30 minutes. Otherwise, sample A was cleaned in the same manner as in Example 1, and the same measurements were performed. The various evaluation results are shown in Table 1.

[0036] <Comparative Example 1> Sample A was cleaned using room temperature (25°C) water instead of superheated steam. Otherwise, the cleaning process was performed in the same manner as in Example 1, and the same measurements were taken. Furthermore, the cleaning process using room temperature water is carried out as follows. 10g of sample A was placed in a beaker and stirred for 10 minutes with 500mL of room temperature water. Then, the water was replaced, and the same procedure was repeated three times, for a total cleaning time of 40 minutes. The relative permittivity of the water used for the cleaning process was 78. The evaluation results are shown in Table 1.

[0037] <Example 4> Regarding the plastic object to be cleaned, Sample B was used, the treatment temperature was set to 200°C, and water with a relative permittivity of 35 was used for cleaning. Otherwise, the same tests and measurements were performed as in Example 1. The various evaluation results are shown in Table 1.

[0038] <Example 5> The processing time was set to 10 minutes. Otherwise, sample B was cleaned in the same manner as in Example 4, and the same tests and measurements were performed. The various evaluation results are shown in Table 1.

[0039] <Example 6> The processing time was set at 30 minutes. Otherwise, sample B was cleaned in the same manner as in Example 4, and the same tests and measurements were performed. The various evaluation results are shown in Table 1.

[0040] <Example 7> The processing temperature was set at 240°C, and the sample was cleaned using water with a relative permittivity of 28. Otherwise, the sample B was cleaned in the same manner as in Example 4, and the same tests and measurements were performed. The various evaluation results are shown in Table 1.

[0041] <Example 8> The processing time was set to 10 minutes. Otherwise, sample B was cleaned in the same manner as in Example 7, and the same tests and measurements were performed. The various evaluation results are shown in Table 1.

[0042] <Example 9> The processing time was set at 30 minutes. Otherwise, sample B was cleaned in the same manner as in Example 7, and the same tests and measurements were performed. The various evaluation results are shown in Table 1.

[0043] <Example 10> The flow rate of superheated steam was set to 10 kg / h. Otherwise, sample B was cleaned in the same manner as in Example 7, and the same tests and measurements were performed. The various evaluation results are shown in Table 1.

[0044] <Example 11> The flow rate of superheated steam was set to 5 kg / h. Otherwise, sample B was cleaned in the same manner as in Example 7, and the same tests and measurements were performed. The various evaluation results are shown in Table 1.

[0045] <Example 12> After the treatment was completed, the test treatment device was not opened directly at the treatment temperature. Instead, it was cooled to 150°C in a superheated steam environment before being opened and removed. Otherwise, sample B was cleaned in the same manner as in Example 7, and the same tests and measurements were performed. The various evaluation results are shown in Table 1.

[0046] <Comparative Example 2> Instead of cleaning with superheated steam, sample B was cleaned using a heated oven. Otherwise, the same cleaning process as in Example 4 was performed, and the same tests and measurements were conducted. Furthermore, the treatment using a heated oven involves placing the prepared thin slices in an oven heated to 200°C for 5 minutes. After the treatment, the oven is opened and the slices are removed. The various evaluation results are shown in Table 1.

[0047] <Comparative Example 3> The processing temperature was set at 240°C. Otherwise, sample B was cleaned in the same manner as in Comparative Example 2, and the same tests and measurements were performed. The various evaluation results are shown in Table 1.

[0048] <Example 13> For the plastic object being cleaned, sample C was used, and otherwise, the same tests and measurements were performed as in Example 1. The various evaluation results are shown in Table 1.

[0049] <Example 14> Regarding the plastic object to be cleaned, sample D was used, and otherwise the same tests and measurements were performed as in Example 12. The various evaluation results are shown in Table 1.

[0050] <Example 15> For the plastic object to be cleaned, use sample A. Fill the reaction fixture with the prepared sample A sheet and pure water, and place it in a salt bath at 200°C for 10 minutes. After treatment, let it cool to room temperature and remove the sample. At this time, the processing temperature was 200℃, the processing time was 10 minutes, and the cleaning process was carried out using water with a relative permittivity of 35. Furthermore, the amount of pure water added was set at 3.805g relative to 1g of the sheet, and the reaction pressure was the saturated water vapor pressure, i.e., 1.55MPa. The cleaned samples were pretreated using solid-phase microextraction and subjected to gas chromatography for impurity analysis, and b* values ​​were determined using a colorimeter. The evaluation results are shown in Table 1.

[0051] <Example 16> The processing temperature was set at 240°C, and water with a relative permittivity of 28 was used for cleaning. The amount of pure water added was 3.579 g relative to 1 g of the sheet. The reaction pressure was the saturated water vapor pressure, i.e., 3.35 MPa. Otherwise, sample A was cleaned in the same manner as in Example 15, and the same tests and measurements were performed. The various evaluation results are shown in Table 1.

[0052] <Example 17> Regarding the plastic object to be cleaned, Sample B was used, and otherwise the same tests and measurements were performed as in Example 15. The various evaluation results are shown in Table 1.

[0053] [Table 1] Impurity removal Resin yellowing inhibition Total area value Removal efficiency b*value b*difference Example 1(A) 3.9.E+07 65% -1.92 1.66 2(A) 3.7.E+07 66% 0.50 4.08 3(A) 3.3.E+07 70% 5.58 9.16 Comparative example 1(A) 7.3.E+07 34% -2.35 1.22 Example 4(B) 1.4.E+07 51% -0.24 1.69 5(B) 1.1.E+07 61% -1.02 0.90 6(B) 1.4.E+07 51% 1.89 3.82 7(B) 9.7.E+06 66% -0.36 1.56 8(B) 1.2.E+07 59% 3.71 5.64 9(B) 1.1.E+07 60% 2.55 4.48 10(B) 1.2.E+07 59% -0.85 1.08 11(B) 1.2.E+07 58% 1.96 3.88 12(B) 8.3.E+06 71% -2.58 0.66 Comparative example 2(B) 2.7.E+07 8% -2.64 0.72 3(B) 2.4.E+07 18% -2.22 0.30 Example 13(C) 2.0.E+07 50% 1.95 2.30 14(D) 3.0.E+06 50% -5.14 1.01 Example 15(A) 5.6.E+07 50% 6.76 10.34 16(A) 4.4.E+07 61% 11.11 14.68 17(B) 1.2.E+07 59% 3.79 5.72

[0054] none

Claims

1. A method for removing impurities from plastic, characterized by: cleaning with water heated to 180°C to 300°C to a level where the relative permittivity is below 40, thereby removing impurities from the plastic; the cleaning medium is superheated steam, and the cleaning is carried out in a non-closed system where superheated steam is continuously supplied and sequentially discharged; after cleaning, the temperature is lowered to 150°C in the superheated steam environment; the plastic is a polyolefin; the plastic is used plastic, and the impurities are dissolved in the plastic.

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