Method for sorting used thermoplastic resin pieces, method for producing recycled styrene-based resin composition, and recycled styrene-based resin composition

Abstract

Provided is a method for sorting used thermoplastic resin pieces, the method comprising a step for preparing a mixture of used thermoplastic resin pieces. In the preparation step, the mixture comprises styrene-based resin pieces and olefin-based resin pieces, and the olefin-based resin pieces include glass-fiber-reinforced polypropylene resin pieces. The method for sorting used thermoplastic resin pieces further comprises: a first sorting step in which, on the basis of the surface-color lightness of the used thermoplastic resin pieces, used thermoplastic resin pieces having a surface-color lightness of 60% or less are removed from the mixture, while used thermoplastic resin pieces having a surface-color lightness exceeding 60% are recovered; and a second sorting step in which, on the basis of a near-infrared reflection spectrum obtained by irradiating the used thermoplastic resin pieces with near-infrared light, the thermoplastic resin pieces in the mixture are sorted into styrene-based resin pieces and non-styrene-based resin pieces.

Description

Method for sorting used thermoplastic resin pieces, method for producing recycled styrenic resin composition, and recycled styrenic resin composition

[0001] The present disclosure relates to a method for sorting used thermoplastic resin pieces, a method for producing a recycled styrenic resin composition, and a recycled styrenic resin composition.

[0002] Japanese Unexamined Patent Application Publication No. 2002-263587 (Patent Document 1) describes a technique for detecting the material of waste plastics using near-infrared rays.

[0003] Japanese Unexamined Patent Application Publication No. 2002-263587

[0004] Examples of thermoplastic resins used in home appliances, OA equipment and the like (hereinafter referred to as home appliances and the like) include styrenic resins and olefinic resins. Examples of the styrenic resins include styrene-acrylonitrile (SAN) resins, acrylonitrile-butadiene-styrene (ABS) resins, and polystyrene (PS) resins. Examples of the PS resins include high impact polystyrene (HIPS) resins, general purpose polystyrene (GPPS) resins, and syndiotactic polystyrene (SPS) resins. Examples of the olefinic resins include polypropylene (PP) resins. The olefinic resins include a plurality of types of glass fiber reinforced PP resins having mutually different surface colors.

[0005] Used thermoplastic resins that have been used in spent home appliances and the like are sorted and recovered from shredder dust (mixture of crushed pieces) of spent home appliances, then melt-kneaded, and reused as recycled thermoplastic resins.

[0006] In order to conform to the required specifications of recycled thermoplastic resins, it is required to sort and recover used thermoplastic resins into styrenic resin pieces and olefinic resin pieces.

[0007] However, conventional sorting methods for used thermoplastic resin fragments make it difficult to reduce the content of glass fiber-reinforced PP resin fragments in the recovered thermoplastic resin fragments, thus making it difficult to efficiently separate styrene-based resin fragments from olefin-based resin fragments.

[0008] For example, in the method for sorting used thermoplastic resin pieces using near-infrared light described in Patent Document 1, near-infrared light is absorbed by thermoplastic resin pieces with a dark surface color. Therefore, when sorting and recovering styrene-based resin pieces, it is difficult to sort and remove olefin-based resin pieces such as glass fiber-reinforced PP pieces with a dark surface color.

[0009] Furthermore, in sorting methods for used thermoplastic resin fragments based on the surface color of the crushed fragments, it is difficult to sort and remove glass fiber reinforced PP resin fragments of various surface colors because the surface colors of the glass fiber reinforced PP resin fragments vary.

[0010] Furthermore, as described later, specific gravity separation is used as an efficient method for separating styrene-based resin pieces from olefin-based resin pieces. In specific gravity separation, the resin pieces to be separated are placed in a tank filled with water, and by agitation or stirring, the difference in specific gravity is used to cause olefin-based resin pieces with a specific gravity of less than 1.0 to float to the top of the tank, and styrene-based resin pieces with a specific gravity of 1.0 or more to sink to the bottom of the tank, thereby separating and recovering them by resin type. However, since glass fiber reinforced PP has a specific gravity of 1.0 or more, and is in the same specific gravity range as styrene-based resin, it is difficult to separate and remove glass fiber reinforced PP from styrene-based resin pieces.

[0011] One objective of this disclosure is to provide a method for sorting used thermoplastic resin pieces that can efficiently separate styrene-based resin pieces from other thermoplastic resin pieces compared to conventional methods for sorting used thermoplastic resin pieces.

[0012] Another object of this disclosure is to provide a recycled styrene resin composition of higher quality than conventional recycled styrene resin compositions and methods for producing the same.

[0013] The method for sorting used thermoplastic resin pieces according to this disclosure comprises a step of preparing a mixture of used thermoplastic resin pieces. In the preparation step, the mixture includes styrene-based resin pieces and olefin-based resin pieces, and the olefin-based resin pieces include glass fiber-reinforced polypropylene resin pieces. The method for sorting used thermoplastic resin pieces comprises a first sorting step of removing used thermoplastic resin pieces with a surface color brightness of 60% or less from the mixture based on the brightness of the surface color of the used thermoplastic resin pieces, and sorting and recovering used thermoplastic resin pieces with a surface color brightness exceeding 60%; and a second sorting step of sorting styrene-based resin pieces and used thermoplastic resin pieces other than styrene-based resin pieces in the mixture based on the near-infrared reflection spectrum measured when the used thermoplastic resin pieces are irradiated with near-infrared light.

[0014] In the method for sorting used thermoplastic resin pieces according to this disclosure, the step of preparing a mixture of used thermoplastic resin pieces may include performing specific gravity sorting on the used thermoplastic resin pieces. In the specific gravity sorting step, for example, resin pieces with a specific gravity of 1.0 or less may be removed. The mixture of used thermoplastic resin pieces prepared may not contain resin pieces with a specific gravity of 1.0 or less.

[0015] The method for producing a recycled styrene-based resin composition according to this disclosure comprises a step of melting and kneading styrene-based resin pieces that have been sorted and recovered by the above-mentioned sorting method for used thermoplastic resin pieces.

[0016] The recycled styrene resin composition relating to this disclosure includes used styrene resin. In the recycled styrene resin composition, the glass fiber content is 0.09% by weight or less, and the transparent resin content, which has a total light transmittance of 80% or more, is 8.0% by weight or less.

[0017] According to this disclosure, styrene resin pieces and other thermoplastic resin pieces can be efficiently separated. According to this disclosure, a high-quality recycled thermoplastic resin composition can be provided.

[0018] This flowchart shows an example of a method for sorting used thermoplastic resin pieces according to Embodiment 1. This flowchart shows an example of a method for producing a recycled styrene-based resin composition according to Embodiment 1. This flowchart shows an example of a method for sorting used thermoplastic resin pieces according to Embodiment 2. This flowchart shows an example of a method for sorting used thermoplastic resin pieces according to Embodiment 3. This flowchart shows a specific example of a method for sorting used thermoplastic resin pieces according to Embodiment 3.

[0019] Embodiments of this disclosure will be described below with reference to the drawings. <Used Thermoplastic Resin Pieces> In embodiments of this disclosure, "used thermoplastic resin pieces" refer to thermoplastic resin pieces recovered from used home appliances, etc., after being used at least once in such appliances. The mixture of used thermoplastic resin pieces includes styrene-based resin pieces and olefin-based resin pieces.

[0020] The styrene resin piece includes at least one selected from the group consisting of PS resin, SAN resin, and ABS resin. The styrene resin piece includes, for example, at least one selected from the group consisting of HIPS resin, GPPS resin, and SPS resin as the PS resin. The styrene resin includes multiple types of styrene resins with different resin flow properties.

[0021] HIPS resin and ABS resin have excellent surface gloss, resin fluidity, and impact resistance. For this reason, HIPS resin and ABS resin are widely used in applications such as air conditioner casings, refrigerator inner boxes, and internal components of other home appliances.

[0022] GPPS resin and SAN resin do not contain rubber particles and have excellent transparency. High molecular weight GPPS resin and high molecular weight SAN resin (hereinafter also referred to as high molecular weight GPPS resin, etc.) also have excellent chemical resistance and high rigidity. Therefore, high molecular weight GPPS resin, etc. is used in applications where transparency, chemical resistance, and high rigidity are required. High molecular weight GPPS resin, etc. is widely used, for example, in internal components of home appliances, specifically in refrigerator shelves, door pockets, water tanks, etc. The weight-average molecular weight (Mw) of GPPS resin used in refrigerator shelves is approximately 200,000 to 350,000. The weight-average molecular weight (Mw) of HIPS used in air conditioner casings or internal components is approximately 150,000 to 250,000. High molecular weight GPPS resin, etc. has a high degree of molecular chain entanglement, resulting in lower resin fluidity compared to HIPS resin and ABS resin.

[0023] Generally, for high molecular weight GPPS resins used in easily removable parts of home appliances such as refrigerator shelves, door pockets, and water tanks, a process has been established for manual dismantling and recovery by workers at home appliance dismantling plants. However, there are cases where manual dismantling and recovery are not possible, and the resins are mixed and crushed together with other parts of home appliances. Furthermore, manual dismantling and recovery of high molecular weight GPPS resins used in parts that are difficult to remove from home appliances is difficult. For these reasons, used thermoplastic resin fragments contain high molecular weight GPPS resins in addition to HIPS resins and ABS resins, and the proportion of these resins varies depending on the time of year and the production lot.

[0024] The olefin-based resin pieces include PP resin. The PP resin includes multiple types of glass fiber reinforced PP resin with different surface colors. The olefin-based resin pieces include glass fiber reinforced PP resin pieces whose surface color brightness exceeds a predetermined threshold, and glass fiber reinforced PP resin pieces whose surface color brightness is below the threshold. "Brightness" refers to a value expressed as a ratio of brightness between white and black to white, with white brightness being 100% and black brightness being 0%. Brightness can be detected, for example, by a CCD (Charge Couple Device) camera attached to a color sorter.

[0025] PP resin is a crystalline resin and has the characteristic of being resistant to damage from various detergents and oils. PP resin is widely used in washing machines, dishwashers, and other appliances. Glass fiber reinforced PP resin is included in parts of home appliances that require high heat resistance, such as parts that come into contact with high-temperature air or water during operation. Glass fiber reinforced PP resin is widely used, for example, in the water tanks of drum-type washing machines with washing and drying functions, or around the pumps of dishwashers.

[0026] <Recycled Styrene Resin Composition> The "recycled styrene resin composition" of the embodiments of this disclosure can be produced from "styrene resin pieces" selected from "used thermoplastic resin pieces" by the used thermoplastic resin piece selection method according to the embodiments of this disclosure, using the method for producing the recycled styrene resin composition according to the embodiments of this disclosure.

[0027] The recycled styrene-based resin composition may contain, as a thermoplastic elastomer, styrene-butadiene-styrene block copolymer (SBS), styrene-isoprene-styrene block copolymer (SIS), styrene-isobutylene-styrene block copolymer (SIBS), and hydrogenated styrene-ethylenebutylene-styrene copolymer (SEBS), styrene-ethylenepropylene-styrene copolymer (SEPS), etc. In addition to the above, the styrene-based resin composition may also contain crosslinked rubbers such as butadiene-based crosslinked rubber and silicone-based crosslinked rubber.

[0028] The recycled styrene resin composition preferably further contains an antioxidant. The recycled styrene resin composition contains, for example, at least one of a phenolic antioxidant, a phosphorus-based antioxidant, and a sulfur-based antioxidant.

[0029] Any phenolic antioxidant can be used, for example, a hindered type, a semi-hindered type, or a less-hindered type. Examples include stearyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, tetrakis[3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]pentaerythritol, 2,2'-dimethyl-2,2'-(2,4,8,10-tetraoxaspiro[5.5]undecane-3,9-diyl)dipropane-1,1'-diyl=bis[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propanoate], 2,4,6-tris(3',5'-di-tert-butyl-4'-hydroxybenzyl)mesitylene, 4,4'-butylidenebis(6-tert-butyl-m-cresol), and 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane. Among these, tetrakis[3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionic acid]pentaerythritol and 2,2'-dimethyl-2,2'-(2,4,8,10-tetraoxaspiro[5.5]undecane-3,9-diyl)dipropane-1,1'-diyl=bis[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propanoate] are preferred from the viewpoint of improving heat resistance.

[0030] Examples of phosphorus-based antioxidants include tris(2,4-di-tert-butylphenyl) phosphite, 3,9-bis(octadecyloxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane, 3,9-bis(2,6-di-tert-butyl-4-methylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane, triphenyl phosphite, tris(4-nonylphenyl) phosphite, and the like. Among these, 3,9-bis(octadecyloxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane and 9-bis(2,6-di-tert-butyl-4-methylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane are preferred from the viewpoint of color stability.

[0031] Examples of sulfur-based antioxidants include bis[3-(dodecylthio)propionic acid]2,2-bis[[3-(dodecylthio)-1-oxopropyloxy]methyl]-1,3-propanediyl, 3,3'-thiobispropionate ditridecyl, 2,4-bis(octylthiomethyl)-6-methylphenol, and 3,3'-thiodipropionate dioctadecyl. Among these, bis[3-(dodecylthio)propionic acid]2,2-bis[[3-(dodecylthio)-1-oxopropyloxy]methyl]-1,3-propanediyl is preferred from the viewpoint of improving long-term heat resistance. <Other Additives> The recycled styrene resin composition according to the embodiments of this disclosure may contain at least one of the following, insofar as it does not hinder the objectives of the embodiments of this disclosure: a metal deactivator, a flame retardant, a flame retardant aid, a plasticizer, a mold release agent, a pigment, an antistatic agent, a conductive agent, and an antimicrobial agent.

[0032] Examples of metal deactivators used include N-(2H-1,2,4-triazole-5-yl)salicylamide, bis[N2-(2-hydroxybenzoyl)hydrazide] dodecanediate, and N,N'-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyl]hydrazine.

[0033] As flame retardants, for example, bromine-based flame retardants such as ethylenebispentabromophenyl and ethylenebistetrabromophthalimide, phosphorus-based flame retardants such as triphenyl phosphate and tricresyl phosphate, inorganic flame retardants such as magnesium hydroxide and aluminum hydroxide, and silicone-based flame retardants can be optionally used.

[0034] As a flame retardant additive, any of the following can be used: antimony trioxide, zinc borate, zinc stannate, zinc sulfide, etc.

[0035] Examples of plasticizers include phthalic acid-based, aliphatic dibasic acid-based, polyester-based, chlorinated paraffin-based, epoxy-based, phosphate ester-based, trimellitic acid-based, pyromellitic acid-based, and ether ester-based plasticizers.

[0036] Examples of fluidity modifiers include liquid paraffin, paraffin wax, polyethylene wax, stearic acid, behenic acid, stearyl alcohol, methylenebisstearate, ethylenebisstearate, zinc stearate, butyl stearate, and stearyl stearate.

[0037] The recycled styrene resin compositions according to the embodiments of this disclosure may also contain, in addition to the above, inorganic fillers such as carbon black, titanium black, titanium dioxide, barium carbonate, zinc oxide, calcium carbonate, red iron oxide, alumina, silica, zinc oxide, aluminum oxide, magnesium oxide, antimony oxide, aluminum hydroxide, magnesium hydroxide, zeolite, talc, mica, glass fiber, carbon fiber, sepiolite, potassium titanate, boron nitride, hydrotalcite, montmorillonite, barium sulfate, and shirasu balloons.

[0038] The following describes embodiments of the sorting method for used thermoplastic resin pieces related to this disclosure, and the recycled styrene-based resin composition and its manufacturing method related to this disclosure.

[0039] Embodiment 1. <Method for sorting used thermoplastic resin pieces> As shown in Figure 1, the method for sorting used thermoplastic resin pieces according to Embodiment 1 is a method for sorting styrene-based resin pieces and olefin-based resin pieces from used thermoplastic resin pieces.

[0040] As shown in Figure 1, the method for sorting used thermoplastic resin pieces according to Embodiment 1 comprises a preparation step (S10) and a sorting step (S02).

[0041] <Preparation Step (S10)> In the preparation step, a mixture of used thermoplastic resin pieces is prepared. The mixture of used thermoplastic resin pieces includes styrene resin pieces and olefin resin pieces. The olefin resin pieces include glass fiber reinforced PP resin pieces.

[0042] The mixture of used thermoplastic resin pieces is recovered, for example, from a mixture of crushed pieces of used home appliances, etc. The crushed piece mixture may contain crushed pieces of rubber and metal, in addition to used thermoplastic resin pieces. Preferably, the preparation step includes removing the crushed pieces of rubber and metal, etc. from the crushed piece mixture and separating and recovering the mixture of used thermoplastic resin pieces from the mixture containing styrene-based resin pieces and olefin-based resin pieces. Any sorting method may be performed in the step of sorting and recovering the mixture of used thermoplastic resin pieces. In the step of sorting and recovering the mixture of used thermoplastic resin pieces, for example, at least one of the sorting methods listed below, or a combination of two or more, may be performed.

[0043] <Specific Gravity Sorting> Specific gravity sorting is roughly divided into wet type and dry type. In wet specific gravity sorting, liquids with multiple specific gravity zones are used, so that used thermoplastic resin pieces can be sorted and separated from metal pieces, wires and the like having different specific gravities therefrom. For example, since the specific gravity of PP resin is 1.0 or less, the PP resin can be easily separated and recovered as resin crushed pieces that float on water. Dry specific gravity sorting is based on the principle of sorting by the action of air blowing from the lower part and vibration of the table, and is used for sorting metal pieces, wires, ores and the like. The sorting objects with large specific gravity move to the upper part of the inclined table under the action of vibration, and the sorting objects with small specific gravity float from the inclined table and move to the lower part of the inclined table, so that the sorting objects with different specific gravities can be separated and recovered. The step of sorting and recovering the mixture of used thermoplastic resin pieces may include sorting and recovering resin pieces that do not contain resin pieces with a specific gravity of 1.0 or less by specific gravity sorting.

[0044] <Air Sorting> Air sorting is a method that utilizes the property that sorting objects are thrown into a region where strong wind is blown by a blower, and the sorting objects float or fall due to differences in shape and specific gravity. Air sorting is used, for example, to remove foreign matters such as polyurethane foam and dust.

[0045] <Friction Sorting> Friction sorting is a method that utilizes the difference in friction coefficient of materials. In friction sorting, for example, sorting objects are dropped onto a rotating roll, and an object with a large friction coefficient relative to the roll material is flicked far away by the rotation of the roll, while an object with a small friction coefficient relative to the roll material can be separated and recovered by utilizing the property of being flicked nearby. In some cases, the roll is made of rubber to increase the generated friction coefficient. In home appliance recycling, it is effective for the purpose of removing silicone-based foreign matters and rubber mixed in during mixed crushing.

[0046] <Electrostatic Sorting> Electrostatic sorting is a method in which sorting objects are charged using static electricity, the charged sorting objects are dropped between electrodes applied with a high DC voltage, and the falling trajectory is changed by the generated Coulomb force to perform separation and recovery. Electrostatic sorting is effective, for example, for separating and recovering PS resin, ABS resin, and filler-reinforced PP resin after specific gravity sorting. The sorting object is not particularly limited.

[0047] <Color Sorting> Color sorting is a technique of irradiating a target material to be sorted with visible light using LEDs or fluorescent lamps as light sources, and separating and recovering the material into recovered material and objects to be removed by using a preset threshold based on the surface tint of the target material and the presence or absence of foreign matter as the judgment criterion. Color sorting apparatuses that perform color sorting are widely used as a technique for improving quality by removing defective products such as polished rice and resin pellets, and are mainly composed of a part for charging the target material to be sorted, a part for conveying the material, a part for photographing the appearance of the material, a part for judging a non-defective product or a defective product from the photographed image of the appearance of the material, a part for jetting air to the material based on the judgment result of the material, and a part for recovering the recovery or removal target. The throughput per hour of the color sorting apparatus reaches 1 ton or more, and is used for stably supplying high-quality products by excluding defective products.

[0048] <Near-Infrared Sorting> Near-infrared sorting is a technique of irradiating a target resin to be sorted with near-infrared rays output from a near-infrared light source, acquiring a reflection spectrum reflected from the material, identifying the material, and separating and recovering the material into recovered material and removed material using the identification result as a judgment criterion. In near-infrared sorting, the characteristic that each material exhibits a unique reflection spectrum due to different absorbance of near-infrared wavelengths depending on the material is utilized, and this technique is widely used for sorting resin crushed pieces, container and packaging-derived plastics, inorganic materials, and the like. Near-infrared sorting can be performed by a near-infrared sorting apparatus. The near-infrared sorting apparatus includes, for example, a charging unit that charges a mixture of crushed pieces that are sorting targets, a near-infrared light source that irradiates the sorting targets with near-infrared rays, a measurement unit that measures a reflection spectrum from the sorting targets, a determination unit that identifies the material type of the sorting targets based on the reflection spectrum and determines whether the sorting targets are recovery targets or removal targets, a jetting unit that jets air to the sorting targets determined to be removal targets by the determination unit, and a recovery unit that recovers the recovery targets and the removal targets respectively. Preferably, a near-infrared sorting apparatus having a throughput of 1 ton or more per hour is used.

[0049] <Sorting Process (S20)> The sorting process (S20) includes a first sorting process (S21) and a second sorting process (S21). The order in which the first sorting process and the second sorting process are executed is not particularly limited. For example, the second sorting process is executed after the first sorting process. The first sorting process may be executed after the second sorting process. The object to be sorted in the first sorting process may be a mixture of used thermoplastic resin pieces sorted and recovered in the second sorting process. The second sorting process may be executed before the first sorting process. The object to be sorted in the second sorting process may be a mixture of used thermoplastic resin pieces prepared in the preparation process (S10).

[0050] <First sorting step (S21)> The target of sorting in the first sorting step is, for example, a mixture of used thermoplastic resin pieces prepared in the preparation step (S10). In the first sorting step, based on the brightness of the surface color of the used thermoplastic resin pieces, used thermoplastic resin pieces with a surface color brightness of 60% or less are removed from the mixture, and used thermoplastic resin pieces with a surface color brightness exceeding 60% are sorted and recovered. As described above, "brightness" is a value expressed as a ratio of brightness between white and black to white, with white brightness being 100% and black brightness being 0%. Brightness can be detected, for example, by a CCD camera.

[0051] Used thermoplastic resin pieces with a surface color brightness exceeding 60% include light-colored styrene-based resin pieces with a surface color brightness exceeding 60% and light-colored olefin-based resin pieces with a surface color brightness exceeding 60%. Used thermoplastic resin pieces with a surface color brightness of 60% or less include dark-colored styrene-based resin pieces with a surface color brightness of 60% or less and dark-colored olefin-based resin pieces with a surface color brightness of 60% or less.

[0052] <Second sorting step (S22)> The sorting target in the second sorting step is, for example, a mixture of used thermoplastic resin pieces with a surface color brightness exceeding 60%, which were sorted and recovered in the first sorting step (S21). In the second sorting step, styrene-based resin pieces and used thermoplastic resin pieces other than styrene-based resin pieces are sorted from the mixture based on the near-infrared reflection spectrum measured when the used thermoplastic resin pieces are irradiated with near-infrared light (wavelength range 800 nm to 2500 nm). In the second sorting step, for example, used thermoplastic resin pieces other than styrene-based resin pieces are sorted and removed from the mixture. In the second sorting step, styrene-based resin pieces may also be sorted and recovered from the mixture.

[0053] <Method for producing recycled styrene resin composition> As shown in Figure 1, the method for producing a recycled styrene resin composition according to Embodiment 1 is a method for producing a recycled styrene resin composition using styrene resin pieces sorted and recovered by the sorting method for used thermoplastic resin pieces according to Embodiment 1 as raw materials.

[0054] As shown in Figure 2, the method for producing a recycled styrene-based resin composition according to Embodiment 1 comprises a method for sorting used thermoplastic resin pieces according to Embodiment 1, and a step (S30) of melting and kneading the styrene-based resin pieces sorted and recovered by the sorting method.

[0055] In the melt-mixing process, antioxidants, pigments, etc., may be added as appropriate to the styrene resin pieces selected and recovered by the above selection method, and then the mixture may be melt-mixed.

[0056] The equipment used for melt mixing is not particularly limited. Examples of equipment used for melt mixing include extruders, kneaders, Banbury mixers, Laboplast mills, bead mixers, and roll mills. The equipment used for melt mixing is operated continuously or batch-wise. The resin composition melt-mixed in the above equipment may be pelletized by a single-screw, multi-screw, or tandem extruder.

[0057] A recycled styrene resin composition produced by the method for producing recycled styrene resin compositions can be processed into a molded product by any molding method. Examples of molding methods for recycled styrene resin compositions include injection molding and compression molding.

[0058] The inventors have found that the sorting method for used thermoplastic resin pieces according to Embodiment 1 can efficiently sort styrene-based resin pieces and other thermoplastic resin pieces, including olefin-based resin pieces. According to the sorting method for used thermoplastic resin pieces according to Embodiment 1, when sorting and recovering styrene-based resin pieces, the respective content of glass fiber-reinforced PP resin pieces and glass fibers in the sorted and recovered styrene-based resin pieces can be reduced. Glass fiber-reinforced PP resin pieces with a surface color brightness of 60% or less are difficult to sort using only the second sorting step based on near-infrared reflection spectroscopy, but can be removed in the first sorting step. Glass fiber-reinforced PP resin pieces with a surface color brightness exceeding 60% can be removed in the second sorting step based on near-infrared reflection spectroscopy.

[0059] The inventors have found that glass fibers that reduce the aesthetic appeal of molded articles made from recycled styrene resin compositions are caused by glass fiber-reinforced PP resin contained in a mixture of used thermoplastic resin fragments. Since the recycled styrene resin composition produced by the manufacturing method of the recycled styrene resin composition according to Embodiment 1 has a low glass fiber content, this recycled styrene resin composition is suitable as a raw material for aesthetic parts. The inventors have confirmed that the glass fiber content can be reduced to 0.09% by weight or less with the recycled styrene resin composition produced by the manufacturing method of the recycled styrene resin composition according to Embodiment 1.

[0060] The inventors confirmed that when the brightness threshold in the first sorting step is set to 70%, the recovery yield of styrene-based resin pieces recovered by the sorting method for used thermoplastic resin pieces is less than 60% by weight. The inventors also confirmed that when the brightness threshold in the first sorting step is set to 50%, the content of glass fiber-reinforced PP resin in the styrene-based resin pieces recovered by the sorting method for used thermoplastic resin pieces exceeds 0.1% by weight.

[0061] <Modified Examples> The sorting method for used thermoplastic resin pieces according to Embodiment 1 may be performed as a method for sorting and recovering specific styrene-based resin pieces from a mixture of used thermoplastic resin pieces. In the second sorting step, thermoplastic resin pieces other than the specific styrene-based resin pieces may be removed based on the near-infrared reflection spectrum. Alternatively, in the second sorting step, specific styrene-based resin pieces may be selectively recovered based on the near-infrared reflection spectrum. For example, the sorting method for used thermoplastic resin pieces according to Embodiment 1 may be performed as a method for sorting and recovering PS resin pieces or ABS resin pieces. In the second sorting step, thermoplastic resin pieces other than PS resin pieces or ABS resin pieces may be removed based on the near-infrared reflection spectrum.

[0062] Embodiment 2. The method for sorting used thermoplastic resin pieces according to Embodiment 2 has the same configuration and effects as the method for sorting used thermoplastic resin pieces according to Embodiment 1, unless otherwise specified, and can be similarly modified. The method for producing a recycled styrene-based resin composition according to Embodiment 2 has the same configuration and effects as the method for producing a recycled styrene-based resin composition according to Embodiment 1, unless otherwise specified, and can be similarly modified. Therefore, the same reference numerals are used for components identical to those in Embodiment 1, and the explanations are not repeated.

[0063] As shown in Figure 3, the method for sorting used thermoplastic resin pieces according to Embodiment 2 further comprises a third sorting step (S23). The sorting step (S20) includes the first sorting step (S21) and the second sorting step (S22), in addition to the third sorting step (S23). In the sorting step (S20), the third sorting step (S23) is performed, for example, after the first sorting step (S21) and before the second sorting step (S22).

[0064] The execution order of the first sorting step, the second sorting step, and the third sorting step is not particularly limited. The third sorting step may be performed before the first sorting step or after the second sorting step. The object to be sorted in the third sorting step may be a mixture of used thermoplastic resin pieces prepared in the preparation step (S10), or a mixture of used thermoplastic resin pieces sorted and recovered in the second sorting step. Both the third sorting step and the first sorting step can utilize a color sorting step. From the viewpoint of work efficiency of the sorting method for used thermoplastic resin pieces, it is preferable that the third sorting step (S23) is performed before or after the first sorting step (S21) and in succession with the first sorting step (S21).

[0065] In the third sorting step (S23), used thermoplastic resin pieces with a total light transmittance of less than 80% are sorted and recovered from the mixture based on their total light transmittance. The total light transmittance of used thermoplastic resin pieces is measured when the used thermoplastic resin pieces are irradiated with visible light (wavelength range 400 nm to 800 nm). The total light transmittance of used thermoplastic resin pieces is measured in the visible light range (400 nm to 800 nm) in accordance with, for example, the provisions of JIS K 7361 (1997).

[0066] The inventors have found that GPPS resin and SAN resin, which have low resin fluidity, exist as transparent crushed pieces with a total light transmittance of 80% or more in a mixture of used thermoplastic resin pieces, and that therefore, the sorting method for used thermoplastic resin pieces according to Embodiment 2 can efficiently sort and recover styrene-based resin pieces with high resin fluidity compared to the sorting method for used thermoplastic resin pieces according to Embodiment 1.

[0067] The surfaces of GPPS resin pieces and SAN resin pieces (hereinafter also referred to as GPPS resin, etc.) in a mixture of used thermoplastic resin pieces are prone to dirt adhering due to friction with other resin pieces or the crusher during use of home appliances, etc., or during the crushing of used home appliances, etc., and it may be difficult to efficiently remove GPPS resin pieces, etc. by the first sorting step based on the brightness of the surface color alone. Furthermore, as described above, high molecular weight GPPS resin, etc. exists in the mixture of used thermoplastic resin pieces as transparent crushed pieces with a total light transmittance of 80% or more. Therefore, in the second sorting step using near-infrared light, which has the property of penetrating transparent materials, it is difficult to efficiently remove GPPS resin pieces, etc., unless a reflector can be installed on the opposite side (back side) from the light source to the used thermoplastic resin pieces to be sorted. The inventors have confirmed that GPPS resin pieces and SAN resin pieces can be efficiently removed by a third sorting step that sorts and recovers used thermoplastic resin pieces with a total light transmittance of less than 80% from the mixture. The inventors have confirmed that, with the recycled styrene-based resin composition produced by the method for producing a recycled styrene-based resin composition according to Embodiment 2, the glass fiber content can be reduced to 0.09% by weight or less, and the transparent resin content with a total light transmittance of 80% or more can be reduced to 8.0% by weight or less.

[0068] The present inventors have found that the recycled styrene-based resin composition produced by the method for producing the recycled styrene-based resin composition according to Embodiment 2 has a low content of high molecular weight GPPS resin and other materials that may cause molding defects such as short shots, sink marks, and burrs in molded products, and therefore the recycled styrene-based resin composition is suitable as a raw material for molded products (especially design parts where molding defects are strictly controlled).

[0069] The inventors confirmed that when the threshold for total light transmittance in the third sorting step is set to 90%, the content of transparent resin pieces in the styrene-based resin pieces recovered by the sorting method for used thermoplastic resin pieces becomes 10% by weight or more.

[0070] Embodiment 3. The method for sorting used thermoplastic resin pieces according to Embodiment 3 has the same configuration and effects as the method for sorting used thermoplastic resin pieces according to Embodiment 1 or 2 described above, unless otherwise specified, and can be similarly modified. The method for producing a recycled styrene-based resin composition according to Embodiment 3 has the same configuration and effects as the method for producing a recycled styrene-based resin composition according to Embodiment 1 or 2 described above, unless otherwise specified, and can be similarly modified. Therefore, the same reference numerals are used for components identical to those in Embodiment 1 or 2, and the descriptions are not repeated.

[0071] As shown in Figure 4, the method for sorting used thermoplastic resin pieces according to Embodiment 3 further comprises a fourth sorting step (S24). The sorting step (S20) further includes the fourth sorting step (S24) in addition to the first sorting step (S21) and the second sorting step (S22). The sorting step (S20) further includes, for example, the fourth sorting step (S24) in addition to the first sorting step (S21), the second sorting step (S22), and the third sorting step (S23). As shown in Figure 4, in the sorting step (S20), the fourth sorting step (S24) is performed, for example, after the first sorting step (S21).

[0072] The execution order of the fourth sorting step and the other sorting steps is not particularly limited. As shown in Figure 5, for example, the first sorting step (S21), the second sorting step (S22), and the third sorting step (S23) may be executed in this order, and the fourth sorting step (S24) may be executed in parallel with at least one of the second sorting step (S22) and the third sorting step (S23).

[0073] In the fourth sorting step (S24), used thermoplastic resin pieces with a surface color brightness of 30% or less are selected and recovered from among the used thermoplastic resin pieces with a surface color brightness of 60% or less, based on the surface color observed when visible light is irradiated onto the used thermoplastic resin pieces with a surface color brightness of 60% or less that were removed in the first sorting step (S21).

[0074] Used thermoplastic resin pieces with a surface color brightness of 30% or less, which are sorted and recovered in the fourth sorting step (S24), are recovered together with used thermoplastic resin pieces sorted and recovered through the first sorting step (S21) and the second sorting step (S22), or used thermoplastic resin pieces sorted and recovered through the first sorting step (S21), the second sorting step (S22), and the third sorting step (S23) (see, for example, the recovery step (S25) in Figure 5). The used thermoplastic resin pieces sorted and recovered in the fourth sorting step (S24) may be added to the used thermoplastic resin pieces sorted and recovered in the first sorting step (S21) and then subjected to other sorting steps. Furthermore, the used thermoplastic resin pieces sorted and recovered in the fourth sorting step (S24) may be added to the used thermoplastic resin pieces sorted and recovered in the third sorting step (S23), for example, before being subjected to the second sorting step (S22).

[0075] In the method for sorting used thermoplastic resin pieces according to Embodiment 3, the step of preparing a mixture of used thermoplastic resin pieces (S10) preferably includes a specific gravity sorting step for the used thermoplastic resin pieces. In the specific gravity sorting step, it is preferable to remove resin pieces with a specific gravity of 1.0 or less. Preferably, the mixture of used thermoplastic resin pieces prepared in step (S10) does not contain resin pieces with a specific gravity of 1.0 or less.

[0076] In the method for producing a recycled styrene-based resin composition according to Embodiment 3, a mixture of the used thermoplastic resin pieces sorted and recovered in the fourth sorting step (S24) and the used thermoplastic resin pieces sorted and recovered through other sorting steps is melt-kneaded.

[0077] <Modification> The fourth sorting step (S24) may be performed simultaneously with the first sorting step (S21). In the first sorting step (S21), used thermoplastic resin pieces with a surface color brightness of more than 30% and 60% or less may be removed from the mixture, and used thermoplastic resin pieces with a surface color brightness of 30% or less and used thermoplastic resin pieces with a surface color brightness of more than 60% may be sorted and recovered. However, in a method of sorting based on two thresholds for surface color brightness, there may be concerns about the occurrence of missed recovery or erroneous removal. In such cases, a sorting method for used thermoplastic resin pieces comprising a fourth sorting step in addition to the first sorting step is preferred.

[0078] The inventors have found that in a mixture of used thermoplastic resin fragments recovered from shredder dust of home appliances, etc., by specific gravity separation, the minimum brightness of the surface color of glass fiber reinforced PP resin is higher than 30%, and the majority of used thermoplastic resin fragments with a surface color brightness of 30% or less are dark-colored styrene-based resin fragments.

[0079] Furthermore, the inventors confirmed that by recovering used thermoplastic resin pieces with a surface color brightness of 30% or less, which are separated and recovered in the fourth sorting step (S24), together with used thermoplastic resin pieces recovered through other sorting steps, the recovery yield of styrene-based resin pieces can be improved compared to the sorting method for used thermoplastic resin pieces according to Embodiments 1 to 3, which do not include the fourth sorting step (S24).

[0080] The present inventors have confirmed that by recovering used thermoplastic resin pieces with a surface color brightness of 30% or less, which are separated and recovered in the first separation step (S21), together with used thermoplastic resin pieces recovered through other separation steps, the recovery yield of styrene-based resin pieces can be improved compared to the method for separating used thermoplastic resin pieces according to Embodiments 1 to 3, which do not include a fourth separation step (S24).

[0081] The embodiments of this disclosure will be described in more detail below with reference to examples. However, these embodiments are not limited to the following examples.

[0082] The used thermoplastic resin fragments from samples 1 to 16 were sorted and recovered using the following sorting method for used thermoplastic resin fragments. Table 1 shows the sorting method for used thermoplastic resin fragments from samples 1 to 16. The numbers 1 to 3 assigned to each step in Table 1 indicate the order in which the steps were performed in the sorting method for used thermoplastic resin fragments from samples 1 to 16.

[0083]

[0084] <Samples 1-4> A mixture of used thermoplastic resin fragments recovered from shredder dust of home appliances was prepared. The mixture of used thermoplastic resin fragments consisted of resin fragments that did not contain resin fragments with a specific gravity of 1.0 or less, which had been recovered through specific gravity separation. Next, used thermoplastic resin fragments with a surface color brightness of 60% or less were removed from the prepared mixture, and used thermoplastic resin fragments with a surface color brightness exceeding 60% were separated and recovered. Next, from the separated and recovered used thermoplastic resin fragments with a surface color brightness exceeding 60%, used thermoplastic resin fragments with a total light transmittance of less than 80% were separated and recovered. Next, from the separated and recovered used thermoplastic resin fragments with a total light transmittance of less than 80%, used PS resin fragments were separated and recovered based on the near-infrared reflection spectrum measured when the used thermoplastic resin fragments were irradiated with near-infrared light.

[0085] Samples 1 to 4 were all sorted and recovered using the sorting method described above. Note that the mixture of used thermoplastic resin fragments recovered from shredder dust of home appliances contains some variation in the mixing ratio of thermoplastic resins. Therefore, samples 1 to 4 were sorted and recovered from mixtures of used thermoplastic resin fragments collected at different times.

[0086] <Sample 5> Similar to Samples 1-4, a mixture of used thermoplastic resin fragments recovered from shredder dust of home appliances was prepared. The mixture of used thermoplastic resin fragments consisted of resin fragments that did not contain resin fragments with a specific gravity of 1.0 or less, which had been recovered through specific gravity separation. From the prepared mixture, used thermoplastic resin fragments with a surface color brightness of 60% or less were removed, and used thermoplastic resin fragments with a surface color brightness exceeding 60% were selected and recovered as Sample 5. In the selection method for Sample 5, only the first selection step based on surface color brightness was performed.

[0087] <Sample 6> Similar to Samples 1-4, a mixture of used thermoplastic resin fragments recovered from shredder dust of home appliances was prepared. The mixture of used thermoplastic resin fragments consisted of resin fragments that did not contain resin fragments with a specific gravity of 1.0 or less, which had been recovered through specific gravity separation. From the prepared mixture, used thermoplastic resin fragments with a surface color brightness of 70% or less were removed, and used thermoplastic resin fragments with a surface color brightness exceeding 70% were selected and recovered as Sample 6. In the selection method for Sample 6, only a selection process based on surface color brightness was performed, and the brightness threshold was set to 70%.

[0088] <Sample 7> Similar to Samples 1-4, a mixture of used thermoplastic resin fragments recovered from shredder dust of home appliances was prepared. The mixture of used thermoplastic resin fragments consisted of resin fragments that did not contain resin fragments with a specific gravity of 1.0 or less, which had been recovered through specific gravity separation. From the prepared mixture, used thermoplastic resin fragments with a surface color brightness of 50% or less were removed, and used thermoplastic resin fragments with a surface color brightness exceeding 50% were selected and recovered as Sample 7. In the selection method for Sample 7, only a selection process based on surface color brightness was performed, and the brightness threshold was set to 50%.

[0089] <Sample 8> Similar to Samples 1-4, a mixture of used thermoplastic resin fragments recovered from shredder dust of home appliances was prepared. The mixture of used thermoplastic resin fragments consisted of resin fragments that did not contain resin fragments with a specific gravity of 1.0 or less, which had been recovered after specific gravity separation. From the prepared mixture, used thermoplastic resin fragments with a total light transmittance of less than 80% were selected and recovered as Sample 8. In the selection method for Sample 8, only the third selection step based on total light transmittance was performed.

[0090] <Sample 9> Similar to Samples 1-4, a mixture of used thermoplastic resin fragments recovered from shredder dust of home appliances was prepared. The mixture of used thermoplastic resin fragments consisted of resin fragments that did not contain resin fragments with a specific gravity of 1.0 or less, which had been recovered after specific gravity separation. From the prepared mixture, used thermoplastic resin fragments with a total light transmittance of less than 90% were selected and recovered as Sample 9. In the selection method for Sample 9, only a selection process based on total light transmittance was performed, and the threshold for total light transmittance was set at 90%.

[0091] <Sample 10> Similar to Samples 1-4, a mixture of used thermoplastic resin fragments recovered from shredder dust of home appliances was prepared. The mixture of used thermoplastic resin fragments consisted of resin fragments that did not contain resin fragments with a specific gravity of 1.0 or less, which had been recovered after specific gravity separation. From the prepared mixture, thermoplastic resin fragments other than PS resin fragments were selected and recovered as Sample 10 based on near-infrared reflection spectra. In the selection method for Sample 10, only the selection step based on near-infrared reflection spectra was performed.

[0092] <Sample 11> Similar to Samples 1-4, a mixture of used thermoplastic resin fragments recovered from shredder dust of home appliances was prepared. The mixture of used thermoplastic resin fragments consisted of resin fragments that did not contain resin fragments with a specific gravity of 1.0 or less, which had been recovered after specific gravity separation. From the prepared mixture, thermoplastic resin fragments identified as PS resin fragments based on near-infrared reflection spectra were selected and recovered as Sample 11. In the selection method for Sample 11, only the selection step based on near-infrared reflection spectra was performed.

[0093] <Sample 12> Used thermoplastic resin pieces with a surface color brightness exceeding 70% were sorted and recovered, and the remaining PS resin pieces, sorted and recovered using the same sorting method as Samples 1 to 4, were designated as Sample 12.

[0094] <Sample 13> Used thermoplastic resin pieces with a surface color brightness exceeding 50% were sorted and recovered, and PS resin pieces sorted and recovered using the same sorting method as Samples 1 to 4 were excluded and designated as Sample 13.

[0095] <Sample 14> Sample 14 was obtained by sorting and recovering thermoplastic resin pieces using the same sorting method as Samples 1 to 4, with the exception of the second sorting step described above.

[0096] <Sample 15> Used thermoplastic resin pieces with a surface color brightness exceeding 70% were sorted and recovered, and the second sorting step described above was not performed. The thermoplastic resin pieces sorted and recovered using the same sorting method as Samples 1 to 4, excluding these, were designated as Sample 15.

[0097] <Sample 16> Sample 16 was obtained by sorting and recovering thermoplastic resin pieces using the same sorting method as Samples 1 to 4, with the exception of the third sorting step described above.

[0098] <Evaluation Method> Samples 1 to 16 were evaluated as follows.

[0099] <Content of Glass Fiber Reinforced PP Resin Pieces and Glass Fibers> The content of glass fiber reinforced PP resin pieces (unit: weight %) in samples 1 to 16 was calculated by determining the resin type of 100g of thermoplastic resin pieces from each sample using near-infrared and FT-IR analysis. For the PP resin pieces, elemental analysis was further performed using fluorescent X-ray analysis to evaluate the presence or absence of glass fibers. The content of glass fibers (unit: weight %) in samples 1 to 16 was measured according to JIS K7052 "Glass fiber reinforced plastics - Prepregs, molding materials and molded products - Method for determining glass fiber and inorganic filler content - Sintering method". Here, the inorganic components obtained by sintering were dispersed in alcohol, and glass fibers were selectively recovered by suction filtration using a resin filter, thereby obtaining a highly accurate glass fiber content.

[0100] Samples with a glass fiber reinforced PP resin content of less than 1.0% by weight were judged as acceptable, while samples with a glass fiber reinforced PP resin content of 1.0% by weight or more were judged as unacceptable. Samples with a glass fiber content of less than 0.1% by weight were judged as acceptable, while samples with a glass fiber content of 0.1% by weight or more were judged as unacceptable.

[0101] <Content of Transparent Resin Pieces> The content of transparent resin pieces (unit: weight %) in samples 1 to 16 was calculated based on the transmittance of 100 g of thermoplastic resin pieces from each sample, measured using a haze meter. Transparent resin pieces were defined as thermoplastic resin pieces with a total light transmittance of 80% or more.

[0102] Samples with a transparent resin fragment content of less than 10% by weight were judged as "good," and samples with a transparent resin fragment content of more than 10% by weight were judged as "acceptable."

[0103] <Recovery Yield> For each of samples 1 to 16, the recovery yield was calculated as the ratio of the total weight of the thermoplastic resin pieces (used PS resin pieces) that were ultimately sorted and recovered to the total weight of the mixture of used thermoplastic resin pieces prepared initially.

[0104] Samples with a recovery yield of 60% by weight or more were judged as acceptable, and samples with a recovery yield of less than 60% by weight were judged as unacceptable.

[0105] <Overall Judgment> Samples that passed three of the above four evaluation items—the content of glass fiber reinforced PP resin pieces, the content of glass fibers, and the recovery yield—and whose content of transparent resin pieces was rated "Good" were given an overall judgment of A (Pass). Samples that passed three of the above evaluation items—the content of glass fiber reinforced PP resin pieces, the content of glass fibers, and the recovery yield—and whose content of transparent resin pieces was rated "Acceptable" were given an overall judgment of B (Pass). Samples that failed even one of the three evaluation items—the content of glass fiber reinforced PP resin pieces, the content of glass fibers, and the recovery yield—were given an overall judgment of F (Fail).

[0106] <Results> The evaluation results for samples 1 to 16 are shown in Table 2.

[0107]

[0108] In samples 1-4 and 16, which were sorted and recovered through the first and second sorting processes described above, the content of glass fiber-reinforced PP resin pieces was less than 1.0% by weight, and the content of each type of glass fiber was less than 0.1% by weight. In samples 1-4 and 16, the content of glass fiber-reinforced PP resin pieces was 0.6% by weight or less, and the content of each type of glass fiber was 0.09% by weight or less. It is considered that by performing at least the first and second sorting processes, light-colored glass fiber-reinforced PP resin pieces that could not be removed by the first sorting process alone, and dark-colored glass fiber-reinforced PP resin pieces that could not be removed by the second sorting process alone, were removed from the sorted and recovered samples 1-4 and 16.

[0109] From the results for samples 5 to 10, it was confirmed that when sorting and recovering using only one of the first, second, or third sorting processes, the content of glass fiber-reinforced PP resin pieces was 1.0% by weight or more, and the content of glass fibers was 0.1% by weight or more.

[0110] For samples 13 to 15, it was confirmed that the content of glass fiber reinforced PP resin pieces was 1.0% by weight or more, and the content of glass fibers was 0.1% by weight or more. From the results for samples 1 and 13, it is considered that if the threshold for surface color brightness is set to 50% in the first sorting step, glass fiber reinforced PP resin pieces with a surface color brightness exceeding 50% but not exceeding 60% cannot be removed, compared to sample 1, where the threshold for surface color brightness was set to 60% in the first sorting step. For the results for samples 14 and 15, it is considered that the glass fiber reinforced PP resin pieces contained in the used thermoplastic resin pieces sorted and recovered in the first sorting step cannot be removed because the second sorting step was not performed.

[0111] In samples 1-4 and 16, the recovery yield of PS resin fragments was 60% by weight or more. On the other hand, from the results for sample 11, when only the process of sorting and recovering thermoplastic resin fragments identified as PS resin fragments based on near-infrared reflection spectra was performed, the recovery yield of PS resin fragments was 46% by weight, which was significantly lower than that of samples 1-4 and 16. It is thought that the process of sorting and recovering thermoplastic resin fragments identified as PS resin fragments based on near-infrared reflection spectra alone is insufficient to sort and recover dark-colored PS resin fragments that absorb near-infrared light.

[0112] Based on the results for Sample 1 and Sample 12, it is considered that when the surface color brightness threshold was set to 70% in the first sorting step, the recovery yield decreased compared to Sample 1, where the surface color brightness threshold was set to 60% in the first sorting step, because PS resin pieces with a surface color brightness exceeding 60% but not exceeding 70% could not be recovered.

[0113] The results for samples 1-4 and sample 16 confirmed that by performing a third sorting step in addition to the first and second sorting steps, the content of transparent resin fragments could be reduced to less than 10% by weight. In samples 1-4, the content of transparent resin with a total light transmittance of 80% or more was 8.0% by weight or less. The significantly higher content of transparent resin fragments in sample 16 compared to the content of each transparent resin fragment in samples 1-4 is thought to be because transparent resin fragments were not removed in the first sorting step, and because near-infrared light was not easily absorbed by the transparent resin fragments in the second sorting step, resulting in the recovery of transparent PS resin fragments without removal.

[0114] Furthermore, from the results for samples 5-7 and 9-11, it is considered that when the third sorting step is not performed, and when samples with a total light transmittance of less than 90% are recovered in the third sorting step, transparent resin fragments with a total light transmittance of 80% or more but less than 90% are recovered without being removed, compared to when samples with a total light transmittance of less than 80% are recovered in the third sorting step, resulting in a transparent resin fragment content of 10% by weight or more.

[0115] The embodiments and examples disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of this invention is indicated by the claims rather than by the embodiments and examples described herein, and all modifications within the meaning and scope equivalent to the claims are intended to be included.

Claims

1. A method for sorting used thermoplastic resin pieces, comprising the steps of: preparing a mixture of used thermoplastic resin pieces, wherein in the preparation step, the mixture comprises styrene-based resin pieces and olefin-based resin pieces, the olefin-based resin pieces comprises glass fiber-reinforced polypropylene resin pieces; a first sorting step of removing used thermoplastic resin pieces from the mixture whose surface color brightness is 60% or less based on the brightness of the surface color of the used thermoplastic resin pieces, and sorting and recovering used thermoplastic resin pieces whose surface color brightness exceeds 60%; and a second sorting step of sorting the mixture from the styrene-based resin pieces and the used thermoplastic resin pieces other than the styrene-based resin pieces based on the near-infrared reflection spectrum measured when the used thermoplastic resin pieces are irradiated with near-infrared light.

2. The method for sorting used thermoplastic resin pieces according to claim 1, further comprising a third sorting step of sorting and recovering used thermoplastic resin pieces from the mixture whose total light transmittance is less than 80% based on the total light transmittance of the used thermoplastic resin pieces.

3. The method for sorting used thermoplastic resin pieces according to claim 2, wherein the third sorting step includes reducing the content of used thermoplastic resin pieces having a total light transmittance of 80% or more in the mixture to less than 10% by weight.

4. The method for sorting used thermoplastic resin pieces according to claim 3, wherein in the preparation step, the styrene resin piece comprises at least one of high impact polystyrene (HIPS) resin, general-purpose polystyrene (GPPS) resin, crystalline polystyrene (SPS) resin, styrene-acrylonitrile (SAN) resin, and acrylonitrile-butadiene-styrene (ABS) resin, and in the third sorting step, the content of the GPPS resin and the SAN resin in the mixture is reduced.

5. A method for sorting used thermoplastic resin pieces according to any one of claims 1 to 4, further comprising a fourth sorting step of sorting and recovering used thermoplastic resin pieces having a surface color brightness of 30% or less from among the used thermoplastic resin pieces having a surface color brightness of 60% or less, based on the surface color observed when visible light is irradiated onto the used thermoplastic resin pieces having a surface color brightness of 60% or less that were removed in the first sorting step.

6. The method for sorting used thermoplastic resin pieces according to claim 5, wherein the preparation step includes removing resin pieces with a specific gravity of 1.0 or less by specific gravity sorting.

7. A method for producing a recycled styrene-based resin composition, comprising: a method for sorting used thermoplastic resin pieces according to any one of claims 1 to 5; and a step of melting and kneading the styrene-based resin pieces sorted and recovered by the method for sorting used thermoplastic resin pieces.

8. A method for producing a recycled styrene-based resin composition, comprising: a method for sorting used thermoplastic resin pieces according to claim 6; and a step of melt-kneading the styrene-based resin pieces sorted and recovered by the second sorting step and the used thermoplastic resin pieces sorted and recovered by the fourth sorting step having a surface color brightness of 30% or less.

9. A recycled styrene-based resin composition containing used styrene-based resin, wherein the glass fiber content is 0.09% by weight or less, and the transparent resin content with a total light transmittance of 80% or more is 8.0% by weight or less.

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