Method of removing ink layer from plastic film, method of recycling cleaning liquid, and regenerated plastic pellet and method of manufacturing the same

Abstract

To enhance repetitive utilizability of a cleaning liquid used for removing an ink layer in a method of removing the ink layer from a plastic film.SOLUTION: A method of removing an ink layer from a plastic film comprises the steps of: (A) forwarding the plastic film having the ink layer from a roll of the plastic film and immersing thereof in a cleaning liquid; and (B) processing a surface of the ink layer side of the plastic film in the cleaning liquid by physical means so as to remove the ink layer from the plastic film. The cleaning liquid contains at least one cleaning component selected from a group comprising n-propyl alcohol and monoalkylene glycol mono-n-propyl ether.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for removing an ink layer from a plastic film, a method for recycling a cleaning solution, and recycled plastic pellets and a method for producing the same. [Background technology]

[0002] Plastic films that constitute many plastic products are printed. However, components derived from the ink layer formed by the printing process can cause quality deterioration of recycled plastics (e.g., coloration, deterioration of physical properties, etc.). Therefore, there is a demand for a method of recycling plastic products that does not mix with components derived from the ink layer.

[0003] In response to these problems, Patent Document 1 provides a method for removing ink from a printed film in a rolled state using a cleaning liquid (solvent) and a non-abrasive cloth. Patent Document 2 also provides a method for removing ink from a printed film in a rolled state using a cleaning liquid (solvent), a brush, and a wiper blade. In these conventional methods, a cleaning liquid that easily dissolves ink is selected from the perspective of effectively removing the ink, i.e., from the perspective of achieving good ink removability. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2016-509613 [Patent Document 2] Special Publication No. 2018-514384 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, from the viewpoint of establishing a sustainable film recycling process, the repeated use of the cleaning solution used to remove ink has been considered. When the cleaning solution is used repeatedly, if the components derived from the ink layer removed from the plastic film (hereinafter also referred to as "ink removed matter") remain in the cleaning solution, the ink removed matter will adhere to the plastic film to be cleaned, and the ink removability will decrease. This decrease in ink removability becomes more noticeable as the number of times the recycling process is repeated increases, so when the cleaning solution is used repeatedly, it is desirable to remove the ink removed matter from the used cleaning solution. However, when a cleaning solution that easily dissolves ink is used as in the conventional method, the ink removed matter is finely divided by the cleaning, making it difficult to remove the ink removed matter from the used cleaning solution.

[0006] As described above, the above-mentioned conventional methods focus on ink removability and do not take into consideration the repeated use of the cleaning liquid, so there is room for improvement in terms of the repeated use of the cleaning liquid.

[0007] Therefore, one aspect of the present invention aims to improve the reusability of a cleaning solution used to remove an ink layer from a plastic film in a method for removing the ink layer. [Means for solving the problem]

[0008] Some aspects of the present invention provide the following [1] to [9].

[0009] [1] 1. A method for removing an ink layer from a plastic film, comprising the steps of: (A) unwinding a plastic film having an ink layer from a roll of the plastic film and immersing the plastic film in a cleaning liquid; (B) removing the ink layer from the plastic film by treating the surface of the plastic film on the ink layer side in the cleaning solution by a physical means, The cleaning solution comprises at least one cleaning component selected from the group consisting of n-propyl alcohol and monoalkylene glycol mono-n-propyl ether.

[0010] [2] The method according to [1], wherein in the step (B), the ink layer is removed from the plastic film by rubbing the surface of the plastic film on the ink layer side with a brush in the cleaning solution.

[0011] [3] The method according to [1] or [2], wherein the plastic film in contact with the ink layer is a polyolefin film.

[0012] [4] The method according to any one of [1] to [3], wherein the ink layer is formed of gravure ink.

[0013] [5] The method according to any of [1] to [4], wherein the cleaning solution contains, as the monoalkylene glycol mono-n-propyl ether, at least one selected from the group consisting of monoethylene glycol mono-n-propyl ether and monopropylene glycol mono-n-propyl ether.

[0014] [6] The method according to any one of [1] to [5], wherein the content of the cleaning component is 20 mass % or more based on the total mass of the cleaning solution.

[0015] [7] A method for recycling a cleaning liquid, comprising a step of removing components derived from the ink layer removed from the plastic film from the used cleaning liquid obtained via the method according to any one of [1] to [6].

[0016] [8] A method for producing recycled plastic pellets, comprising a step of forming a plastic film or a processed product thereof from which an ink layer has been removed by the method according to any one of [1] to [6] into a pellet shape.

[0017] [9] A recycled plastic pellet obtained by molding a plastic film from which an ink layer has been removed by the method according to any one of [1] to [6] or a processed product thereof. Effect of the Invention

[0018] According to one aspect of the present invention, in a method for removing an ink layer from a plastic film, it is possible to increase the reusability of a cleaning liquid used for removing the ink layer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Exemplary embodiments of the present invention will be described below. However, the present invention is not limited to the following embodiments. In this specification, a numerical range indicated using "~" indicates a range including the numerical values ​​before and after "~" as the minimum and maximum values, respectively. In addition, unless specifically stated otherwise, the units of the numerical values ​​before and after "~" are the same. In addition, the upper limit and lower limit values ​​individually described can be arbitrarily combined. In addition, specific compounds, products, etc. are exemplified below, but substances other than the exemplified ones can also be used as appropriate.

[0020] <How to remove the ink layer> One embodiment of the present invention is a method for removing an ink layer from a plastic film (hereinafter simply referred to as the "removal method"), which includes the following step (A) and the following step (B). (A) A step of unwinding a plastic film having an ink layer from a roll of the plastic film and immersing the plastic film in a cleaning solution. (B) A process for removing the ink layer from the plastic film by treating the surface of the plastic film on the ink layer side by physical means in a cleaning solution.

[0021] The cleaning solution used in the above-mentioned removal method contains at least one cleaning component selected from the group consisting of n-propyl alcohol and monoalkylene glycol mono-n-propyl ether.

[0022] The above-mentioned removal method can improve the reusability of the cleaning liquid. That is, the above-mentioned removal method can suppress the break-up of the components (ink removed matter) derived from the ink layer removed from the plastic film in the above-mentioned step (B), and can coarsen the ink removed matter mixed into the used cleaning liquid. Since the coarse ink removed matter can be removed by a simple operation such as filtration, the above-mentioned removal method can suppress the deterioration of the ink removability due to the repeated use of the cleaning liquid by a simple operation, even when the cleaning liquid is used repeatedly.

[0023] The above-mentioned removal method may include the following steps (C) to (E) in addition to the above steps (A) and (B). (C) A process of washing away the cleaning solution adhering to the plastic film after the process (B) (i.e., the plastic film removed from the cleaning solution) with a rinsing solution. (D) A process for removing any liquid adhering to the plastic film after process (B) or (C). (E) A process of winding up the plastic film from which the ink layer has been removed after the process (D) (hereinafter referred to as the "treated plastic film") to obtain a roll of the treated plastic film.

[0024] In the following, first, the apparatus used in the above-mentioned removal method will be described, and then steps (A) to (E) will be described.

[0025] (Device) An apparatus used in the removal method of one embodiment includes, for example, a first section, a second section, a third section, and a fourth section.

[0026] The first section is a section for supplying the plastic film to be processed to the second section, and includes, for example, a frame on which the roll of the plastic film is rotatably arranged.

[0027] The second section is a section where the steps (A) to (C) are performed. The second section includes, for example, a tank (washing tank) containing a washing liquid, an auxiliary frame for transporting the plastic film from the first section into the washing tank, and a physical means (washing means) for treating the surface of the ink layer side of the plastic film to remove the ink layer, and in some cases, further includes a rinsing means (for example, a tank containing a rinsing liquid) for rinsing the plastic film that has come out of the washing tank with a rinsing liquid. A part of the auxiliary frame is positioned in the above-mentioned washing tank, and a washing device as a physical means (washing device) is disposed at a position of the auxiliary frame located in the tank. The washing device is, for example, a scraper, and is preferably a rotary scraper that rotates at a speed that is matched to the speed at which the washed plastic film comes out of the washing tank. The rotary scraper is, for example, a roller (brush roller) covered with a brush. From the viewpoint of efficiently removing the ink layer from the plastic film, the washing device preferably has a mechanism for rubbing the plastic film with a brush, and is more preferably the above-mentioned brush roller. The material of the brush is, for example, a plastic such as nylon or polypropylene, and is preferably polypropylene. The physical means (cleaning means) provided in the second section may be a physical means such as ultrasonic waves, micro / nano bubbles, or a water flow.

[0028] The third section is a section where step (D) is carried out. The third section includes, for example, a means for removing a liquid (cleaning liquid or rinsing liquid) attached to the plastic film after cleaning (i.e., after step (B)) or rinsing (i.e., after step (C)). Examples of the means for removing the liquid include a means for wiping the liquid attached to the plastic film (e.g., a wiper blade, etc.) and a means for drying the plastic film (e.g., an air knife, a drying chamber, an infrared heater, an oven, a hot air dryer, a microwave, etc.), and these can also be combined.

[0029] The fourth section is a section where the step (E) is carried out. The fourth section includes, for example, a motor for winding up the plastic film after drying (i.e., after the step (D)).

[0030] An example of the device described above is the device described in Patent Document 2 (a device for cleaning printed bands). A specific example of the device is a film cleaner manufactured by Polymount. The device that can be used in the above removal method is not limited to the above device, and for example, the system described in Patent Document 1 (a system for removing ink from film) can also be used.

[0031] (Process:(A)) In the step (A), the plastic film having an ink layer is unwound from a roll of the plastic film and immersed in a cleaning liquid.

[0032] [Plastic film] The plastic film having an ink layer has at least a resin film layer and an ink layer provided on the resin film layer. The plastic film may be, for example, a plastic film that has become waste (so-called waste plastic film). As the plastic film, films that are generally distributed as packaging materials for food packaging or daily necessities, films having various types of resin film layers that have been discarded by recycling, etc., can be used without any particular limitation, and these can also be used in combination.

[0033] The plastic film may be a so-called surface-printed film. The plastic film may be a film having an ink layer on the outermost surface (i.e., the ink layer is provided exposed on the outermost surface), or may be a film having a functional coating layer covering the ink layer provided on the surface of the laminated film. In other words, the layer forming at least one outermost surface of the plastic film may be an ink layer or a functional coating layer covering the ink layer. In particular, when the plastic film has an ink layer on the outermost surface, the ink removability is better.

[0034] -Resin film layer- The resin film layer can be classified according to the required role as a base film layer (F1), a sealant layer (F2) that becomes a heat-sealed portion when forming a packaging material, or the like.

[0035] Examples of resin films that can be used as the base film layer (F1) include polyolefin films made of low-density polyethylene, high-density polyethylene, linear low-density polyethylene, OPP (biaxially oriented polypropylene), CPP (non-oriented polypropylene), etc.; polyester films made of polyethylene terephthalate (PET), polybutylene terephthalate, etc.; polyamide films made of nylon 6, nylon 6,6, metaxylene adipamide (N-MXD6), etc.; biodegradable films made of polylactic acid, etc.; polyacrylonitrile films, poly(meth)acrylic films, polystyrene films, polycarbonate films, saponified ethylene-vinyl acetate copolymer (EVOH) films, polyvinyl alcohol films, etc. These films may contain pigments. A transparent deposition layer formed by depositing alumina, silica, etc. may be formed on the surface of these films. Various surface treatments such as flame treatment, corona discharge treatment, or chemical treatment with a desorption primer may be performed on the surface of these films.

[0036] The resin film to be the sealant layer (F2) is, for example, a flexible polymer film. Examples of the flexible polymer film include polyolefin-based films such as polyethylene films, polypropylene films, and ethylene-vinyl acetate copolymer films, as well as films composed of ionomer resins, EAA resins, EMAA resins, EMA resins, EMMA resins, biodegradable resins, and the like. In terms of generic names, they include CPP (unoriented polypropylene) films, VMCPP (aluminum-vapor-deposited unoriented polypropylene) films, LLDPE (linear low-density polyethylene) films, LDPE (low-density polyethylene) films, HDPE (high-density polyethylene), and VMLDPE (aluminum-vapor-deposited non-low-density polyethylene) films. These films may contain pigments. The surfaces of these films may be subjected to various surface treatments such as flame treatment, corona discharge treatment, or chemical treatments such as desorption primers.

[0037] From the viewpoint of reuse as a recycled substrate, the resin film layer preferably contains a film (polyolefin film) formed of a polyolefin resin such as polyethylene or polypropylene, and more preferably contains a polypropylene film. Among them, from the viewpoint of achieving a higher compatibility between ink removability and repeated use of the cleaning liquid, it is preferable that the film in contact with the ink layer is a polyolefin film. The polyolefin film may be a stretched film (for example, a uniaxially stretched or biaxially stretched film) or a non-stretched film.

[0038] The thickness of the resin film layer is preferably 5 μm or more and 200 μm or less, more preferably 10 μm or more and 100 μm or less, and further preferably 10 μm or more and 50 μm or less.

[0039] -Ink layer- The ink layer is a layer containing ink, and may be a layer having a function of displaying any design, pattern, letter, symbol, etc. for the purpose of providing decoration or aesthetics, or displaying the contents, expiration date, manufacturer or seller, etc. The ink layer may be a solid ink layer having no design, pattern, letter, symbol, etc.

[0040] The ink layer is formed by printing using, for example, a gravure printer, a flexographic printer, an offset printer, an inkjet printer, or the like. That is, the ink used to form the ink layer may be an ink for gravure printing (gravure ink), an ink for flexographic printing (flexo ink), an ink for offset printing (offset ink), or an ink for inkjet printing (inkjet ink). Among these, it is preferable that the ink forming the ink layer is a gravure ink, from the viewpoint of being easily removed by the method of the present embodiment.

[0041] The ink used to form the ink layer may be, for example, an organic solvent-based printing ink, a water-based ink, or an active energy ray-curable ink. The ink layer may be formed of one type of ink, or may be formed of multiple types of ink. That is, the ink layer may be a monochromatic ink layer, or a multicolor ink layer.

[0042] The ink contains, for example, a binder resin and an organic solvent as main components. The ink may contain a pigment and / or a dye as a colorant. The ink may contain a pigment derivative and / or a resin-type dispersant as a dispersant for the colorant. These materials may be used alone or in combination. The pigment derivative and the resin-type dispersant are used in combination to further improve the dispersion stability and stability over time.

[0043] The ink may contain a compound having an acidic group. The compound having an acidic group can be used without any particular limitation, but a compound (e.g., a resin or a low molecular weight compound) that can be easily mixed with the binder resin or organic solvent, which are the main components of the ink, is preferably used.

[0044] Examples of resins having an acidic group include resins having an acid value such as rosin-modified maleic acid resins and rosin-modified fumaric acid resins, polymerizable monomers having a carboxyl group such as acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, cinnamic acid, or acid anhydrides thereof, polymerizable monomers having a sulfonic acid group such as sulfonated styrene, polymerizable monomers having a sulfonamide group such as vinylbenzenesulfonamide, and other resins that are radical copolymers such as (meth)acrylic resins, styrene-(meth)acrylic resins, styrene-maleic acid (anhydride) resins, and terpene-maleic acid (anhydride) resins, and acid-modified polyolefin resins. These can be used alone or in combination.

[0045] Examples of low molecular weight compounds having an acidic group include saturated fatty acids, unsaturated fatty acids, hydroxy acids, aromatic carboxylic acids, dicarboxylic acids, tricarboxylic acids, oxocarboxylic acids, carboxylic acid derivatives, acid anhydrides, etc. These can be used alone or in combination.

[0046] Examples of saturated fatty acids include lauric acid, myristic acid, palmitic acid, margaric acid, and stearic acid. Examples of unsaturated fatty acids include oleic acid, linoleic acid, linolenic acid, arachidonic acid, eicosapentaenoic acid, docosahexaenoic acid, and sorbic acid. Examples of hydroxy acids include lactic acid, malic acid, and citric acid. Examples of aromatic carboxylic acids include benzoic acid, phthalic acid, isophthalic acid, terephthalic acid, salicylic acid, gallic acid, mellitic acid, and cinnamic acid. Examples of dicarboxylic acids include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, fumaric acid, and maleic acid. Examples of tricarboxylic acids include aconitic acid. Examples of oxocarboxylic acids include pyruvic acid and oxaloacetic acid. Examples of carboxylic acid derivatives include amino acids and nitrocarboxylic acids. Examples of acid anhydrides include trimellitic anhydride and pyromellitic anhydride. These may be used alone or in combination of two or more.

[0047] The content of the compound having an acidic group may be appropriately determined within a range that does not impair the printability of the ink, but is preferably in the range of 0.5 to 50 mass % and more preferably in the range of 1.0 to 30 mass % relative to the solid content of the ink.

[0048] The ink layer may be of a single layer or multi-layer construction. The thickness of the ink layer is preferably 0.1 to 10 μm, more preferably 1 to 5 μm.

[0049] The ink layer to be removed by the removal method of the present embodiment is preferably provided in contact with a resin film layer, more preferably in contact with a polyolefin film, and is preferably provided on the outermost surface of a plastic film (i.e., the outermost layer) as described above.

[0050] -Functional coating layer- The functional coating layer may be a layer formed for the purpose of, for example, hard coating, silicone-based peeling, IR cut, waterproof and moisture proof, antibacterial, UV cut, heat dissipation, photocatalysis, weather resistance, anti-fogging, fingerprint and stain resistance, self-repair, water and oil repellency, etc. The functional coating layer may be colorless or colored.

[0051] The functional coating layer can be formed by coating the surface of the resin film layer with various coating agents such as surface modifiers such as hard coating agents, self-repellent coating agents, fingerprint-resistant and stain-resistant coating agents, anti-fog coating agents, silicone-based release agents, non-silicone-based release agents, waterproof and moisture-proof coating agents, water-repellent and oil-repellent coating agents, photocatalyst coating agents, weather-resistant coating agents, and IR-cut coating agents, optical pressure sensitive adhesives, polyimide varnish, liquid crystal alignment film materials, electromagnetic wave shielding coating agents, fine wiring pastes, antistatic coating agents, high refractive index coating agents, and coating agents for optical lenses, and other electronic materials.

[0052] The functional coating layer may be a metal layer formed of a metal material. The metal layer may be a layer made of a metal foil, or may be a metal deposition layer formed by deposition of a metal or metal oxide. Examples of the metal foil include foils of metals with excellent ductility, such as gold, silver, copper, zinc, iron, lead, tin, and alloys thereof, steel, stainless steel, and aluminum. Examples of the metal deposition layer include layers made of one or more of aluminum, aluminum oxide, silica, zinc oxide, and the like.

[0053] The functional coating layer may be an oxygen absorbing layer, an anchor coating layer, a detachment primer layer (e.g., a layer containing a compound having an acidic group as described above) provided to facilitate peeling of the ink layer, etc.

[0054] The thickness of the functional coating layer is preferably 0.1 μm or more and 100 μm or less, more preferably 0.1 μm or more and 10 μm or less, and further preferably 1 μm or more and 5 μm or less.

[0055] The functional coating layer may be a single layer or may be a laminate of different types of layers. For example, the functional coating layer may have a layer formed of one of the above-mentioned various coating agents and the above-mentioned metal vapor deposition layer.

[0056] The plastic film may further include layers other than the above-mentioned resin film layer, ink layer, and functional coating layer. For example, the plastic film may include a paper layer made of natural paper or synthetic paper.

[0057] The thickness of the plastic film is preferably 5 μm or more and 200 μm or less, more preferably 10 μm or more and 100 μm or less, and further preferably 10 μm or more and 50 μm or less.

[0058] [Cleaning solution] The cleaning liquid is a liquid containing at least one cleaning component selected from the group consisting of n-propyl alcohol (1-propanol) and monoalkylene glycol mono-n-propyl ether. Hereinafter, n-propyl alcohol and monoalkylene glycol mono-n-propyl ether are collectively referred to as the "first cleaning component". The first cleaning component may be used alone or in combination of two or more.

[0059] Monoalkylene glycol mono-n-propyl ether is a compound represented by the formula: CH3CH2CH2OROH. In the formula, R represents an alkylene group (alkanediyl group). The number of carbon atoms in the alkylene group is, for example, 1 to 4, and preferably 1 to 3. Examples of the alkylene group include a methylene group, an ethylene group, a propylene group, and a butylene group. The alkylene group may be linear or branched. For example, the propylene group may be an n-propylene group or an isopropylene group.

[0060] As the monoalkylene glycol mono-n-propyl ether, from the viewpoint of achieving a high compatibility between the ink removability and the reusability of the cleaning solution, and from the viewpoint of the safety of the cleaning solution, it is preferable to use at least one selected from the group consisting of monoethylene glycol mono-n-propyl ether and monopropylene glycol mono-n-propyl ether, and it is more preferable to use propylene glycol mono-n-propyl ether. Note that propylene glycol mono-n-propyl ether is generally a compound (1-propoxy-2-propanol) in which R in the above formula is an isopropylene group, but a mixture containing the compound and an isomer such as 2-isopropoxy-1-propanol may also be used. The content of 1-propoxy-2-propanol in the mixture is preferably 60% by mass or more, and may be 80% by mass or more, 90% by mass or more, or 100% by mass.

[0061] The cleaning liquid may be, for example, a liquid consisting of a liquid cleaning component, or may be a liquid containing water and a cleaning component (aqueous cleaning liquid). When the cleaning liquid contains water, the operational stability and environmental stability of the step (A) can be improved.

[0062] The content of the first cleaning component in the cleaning liquid may be 20% by mass or more (e.g., 20 to 100% by mass), 40% by mass or more, or 60% by mass or more, based on the total mass of the cleaning liquid, from the viewpoint of improving ink removability. The content of the first cleaning component in the cleaning liquid may be 80% by mass or less, 60% by mass or less, or 40% by mass or less, based on the total mass of the cleaning liquid, from the viewpoint of increasing the particle size of the ink removed matter to make it easier to remove the ink removed matter from the cleaning liquid.

[0063] The water content in the cleaning solution may be adjusted so that the content of the first cleaning component falls within the above range, and may be, for example, 0 to 60% by mass based on the total mass of the cleaning solution. From the viewpoint of improving the operational stability and environmental stability of the (A) step, the water content in the cleaning solution may be 20% by mass or more, 40% by mass or more, or 60% by mass or more based on the total mass of the cleaning solution.

[0064] The cleaning solution may contain a cleaning component (second cleaning component) other than n-propyl alcohol and monoalkylene glycol mono-n-propyl ether as long as the effect of the present invention is not impaired. The second cleaning component may be, for example, an alcohol other than n-propyl alcohol, or a glycol ether other than monoalkylene glycol mono-n-propyl ether. The second cleaning component may be a known surfactant such as an anionic surfactant, a nonionic surfactant, an amphoteric surfactant, or a cationic surfactant.

[0065] The content of the second cleaning component may be 0 to 10 mass %, 0 to 5 mass %, or 0 to 1 mass %, based on the total mass of the cleaning liquid.

[0066] The cleaning liquid may contain an antifoaming agent. The antifoaming agent is usually used together with water. As the antifoaming agent, a water-soluble organic solvent, a nonionic surfactant having an HLB value in the range of 1 to 3, or the like may be used, or a silicone-based compound may be used. In terms of high antifoaming ability, a silicone-based compound is preferred, and among them, an emulsion-type or self-emulsifying silicone-based compound is more preferred. These antifoaming agents may be used alone or in combination of two or more types.

[0067] Specific examples of self-emulsifying defoamers include X-50-1176, KS-530, and KS-537 manufactured by Shin-Etsu Chemical Co., Ltd. Specific examples of emulsion-type defoamers include KM-7750D, KM-7752, and KM-98 manufactured by Shin-Etsu Chemical Co., Ltd., and FS Antifoam 025, FS Antifoam 80, FS Antifoam 92, FS Antifoam 93, DKQ1-1183, and DKQ1-1247 manufactured by Nagase Chemspec Corporation. However, the present invention is not limited to these.

[0068] The content of the antifoaming agent may be, for example, 0 to 5 mass %, 0.01 to 5 mass %, 0.02 to 4 mass %, or 0.03 to 3 mass %, based on the total amount of the cleaning liquid (eg, aqueous cleaning liquid).

[0069] [Immersion conditions] The temperature of the cleaning solution when the plastic film is immersed in the cleaning solution in step (A) is not particularly limited as long as the cleaning solution can maintain its liquid state, but is preferably -5 to 40°C.

[0070] The time for which the plastic film is immersed in the cleaning solution in step (A) (immersion time) needs to be adjusted appropriately depending on the type of ink and the type of cleaning solution, but from the viewpoint of cleaning efficiency, it is preferably 5 seconds or more and 5 minutes or less. The immersion time can be adjusted by the speed at which the plastic film is sent out (transport speed). The immersion time means the time from when the plastic film is immersed in the cleaning solution until when step (B) is carried out.

[0071] (Process:(B)) In step (B), the ink layer is removed from the plastic film by treating the surface of the plastic film on the ink layer side in a cleaning solution with a physical means. From the viewpoint of efficiently removing the ink layer from the plastic film, it is preferable that the physical means is a brush. That is, in step (B), it is preferable to remove the ink layer from the plastic film by rubbing the surface of the plastic film on the ink layer side with a brush in the cleaning solution. Preferred examples of the brush are as described above.

[0072] (Process:(C)) In the step (C), the cleaning solution adhering to the plastic film after the step (B) (i.e., the plastic film removed from the cleaning solution) is washed away with a rinse solution. The rinse solution is not particularly limited as long as it is a liquid capable of washing away the cleaning solution (particularly, the cleaning components in the cleaning solution), and may be, for example, water. The rinse solution may contain the cleaning components at a concentration lower than that of the cleaning solution.

[0073] The step (C) can be carried out, for example, by spraying a rinse liquid onto the plastic film after the step (B), or by immersing the plastic film after the step (B) in a tank containing the rinse liquid. In the method of immersing in a tank containing the rinse liquid, the surface of the plastic film (the surface on which the ink layer was formed) may be treated by a physical means, as in the step (B). In either method, the temperature of the rinse liquid is preferably around room temperature (specifically, −5 to 40° C.), more preferably 0 to 30° C., from the viewpoint of efficiently washing away the cleaning liquid.

[0074] (Process:(D)) In step (D), the liquid adhering to the plastic film after step (B) or (C) is removed. The specific means are as described above, but it is preferable that step (D) includes a step of wiping the liquid (cleaning liquid or rinsing liquid) adhering to the plastic film, for example, with a wiper blade, and a step of drying the plastic film, for example, with a hot air dryer, after wiping the liquid from the plastic film. The drying conditions are not particularly limited, but in the case of heat drying, the drying temperature is usually 70 to 150°C, and the drying time is about 5 seconds to 5 minutes.

[0075] (Process:(E)) In step (E), the treated plastic film after step (D) is wound up to obtain a roll of the treated plastic film. The treated plastic film recovered in step (E) can be a plastic film that does not have an ink layer or has a sufficiently small amount of ink layer attached thereto, and therefore can be used as a recycled plastic material, for example, in the manufacturing method of recycled plastic pellets described below. Therefore, it can be said that the removal method of this embodiment is suitable as a process for manufacturing high-quality recycled plastic raw materials (e.g., recycled plastic pellets).

[0076] The ink removability in the above-mentioned removing method can be evaluated based on the removal rate of the ink layer, which can be determined, for example, by the following method. First, the outermost surface of the plastic film obtained by the above method (the plastic film after washing and drying) is photographed with an optical microscope, and the area of ​​the part where the ink layer is attached (ink layer attachment area) is calculated by image processing of the photograph. Next, the ink layer removal rate is calculated using the following formula. Ink layer removal rate (%) = (1-[ink layer area after cleaning] / [ink layer area before cleaning]) x 100

[0077] According to the removal method of the above embodiment, the removal rate of the ink layer can be, for example, 50% or more. Also, according to a preferred embodiment of the removal method, the removal rate of the ink layer can be 75% or more or 90% or more.

[0078] The used cleaning liquid obtained after the above-mentioned removal method contains ink-removed matter (components derived from the ink layer removed from the plastic film). The above-mentioned removal method can coarsen the ink-removed matter. The volume-based median diameter (D50, cumulative 50% diameter of the volume-based particle size distribution of the ink-removed matter) of the ink-removed matter is, for example, 5 μm or more. According to the above-mentioned removal method, the volume-based median diameter (D50) of the ink-removed matter can also be set to 10 μm or more, 15 μm or more, or 20 μm or more. In addition, in the above-mentioned method, the span value A of the ink-removed matter represented by the following formula tends to be small. The span value A of the ink-removed matter can be 10 or less, 8 or less, or 5 or less. The smaller the span value A, the narrower the particle size distribution width. Span value A = (D90-D10) / D50 D10 represents the cumulative 10% diameter of the volumetric particle size distribution of the ink-removed product, and D90 represents the cumulative 90% diameter of the volumetric particle size distribution of the ink-removed product. Note that D10, D50, and D90 are values ​​measured using a laser diffraction particle size distribution analyzer.

[0079] <How to recycle cleaning fluid> Another embodiment of the present invention is a method for recycling cleaning liquid, which includes a step of removing ink removed matter (components derived from the ink layer removed from the plastic film) from used cleaning liquid obtained through the removal method of the above embodiment.

[0080] The method of removing the ink removed material from the cleaning liquid includes, for example, filtering the cleaning liquid. The filter used for filtering the cleaning liquid may have a mesh size large enough to remove the ink removed material (for example, a mesh size of about 5 μm).

[0081] In the above process, it is not necessary to completely remove the ink removed matter from the cleaning liquid, but the content of solid matter such as ink removed matter in the cleaning liquid after the above process (cleaning liquid to be reused) is preferably 5% by mass or less based on the total mass of the cleaning liquid. If the content of solid matter in the cleaning liquid exceeds 5% by mass, the cleaning liquid may be distilled after the above process to obtain a cleaning liquid for reuse. Depending on the type of ink, the distillation temperature may be limited. For example, when removing ink containing a flammable substance such as nitrocellulose, the distillation temperature must be 160°C or less. Therefore, it is preferable that the boiling point at 1 atmospheric pressure of the cleaning liquid used in the above removal method is 160°C or less.

[0082] <Recycled plastic pellets and their manufacturing method> Another embodiment of the present invention is recycled plastic pellets obtained by molding a plastic film (treated plastic film) from which the ink layer has been removed by the above-mentioned removal method, or a processed product thereof. Another embodiment of the present invention is a method for producing recycled plastic pellets, which includes a step of molding a plastic film (treated plastic film) from which the ink layer has been removed by the above-mentioned removal method, or a processed product thereof, into a pellet shape. The processed product of the treated plastic film refers to, for example, a product obtained by subjecting the treated plastic film obtained by the removal method of the above-mentioned embodiment to a crushing process, melt-kneading process, or the like.

[0083] The method of forming the treated plastic film or the treated product into pellets is not particularly limited, and may be a known method. For example, various additives, master batches (generally, a mixture of thermoplastic resins such as polyethylene resins and polypropylene resins and colorants) are added to the treated plastic film or the treated product as necessary, and mixed with a Henschel mixer, tumbler, disperser, etc., and the resulting mixture is melt-kneaded using a kneader, roll mill, twin-screw extruder, single-screw extruder, rotor-type twin-screw kneader, etc., and formed into pellets, thereby producing recycled plastic pellets.

[0084] The recycled plastic pellets obtained by the above manufacturing method may be recycled plastic pellets that do not contain components derived from the ink layer of the plastic film material, or the content of such components is sufficiently reduced. Therefore, by using the above recycled plastic pellets as a material, it is possible to manufacture high-quality recycled plastic products. Therefore, the above recycled plastic pellets can be used in various fields such as home appliances, stationery, automobile parts, toys, sports goods, medical use, and building and construction materials. The recycled plastic product may be, for example, a molded body obtained by hot molding (e.g., injection molding, extrusion molding, blow molding, compression molding, etc.) the above recycled plastic pellets. EXAMPLES

[0085] The present invention will be described in more detail below using examples and comparative examples, but the present invention is not limited to the following examples.

[0086] <Examples 1 to 6 and Comparative Examples 1 to 5> (Preparing the plastic film) The following materials (resin film and gravure ink) were prepared. [Resin film] OPP: Biaxially oriented polypropylene film 20μm ·CPP: Non-oriented polypropylene film 35μm [Gravure ink] INK1: DIC Graphics Glossa 507 Primary Indigo S2 INK2: DIC Graphics Finart R507 primary blue INK3: DIC Graphics Finart R794 White S

[0087] Using any one of the above resin films and at least one of the above gravure inks, plastic films 1 to 3 having the layer configurations shown in Table 1 below were produced by spreading the gravure ink on the resin film using a proofer.

[0088] [Table 1]

[0089] (Preparing cleaning solution) The following cleaning ingredients were prepared: NPA: n-propyl alcohol (manufactured by Sankyo Chemical Co., Ltd.) PGPE: Monopropylene glycol propyl ether (Tokyo Chemical Industry Co., Ltd.) GBL: γ-butyrolactone (Mitsubishi Chemical) EGBE: Monoethylene glycol butyl ether (Tokyo Chemical Industry Co., Ltd.) IPA: Isopropyl alcohol (manufactured by Sankyo Chemical Co., Ltd.) DPM: Dipropylene glycol methyl ether (manufactured by Toho Chemical Industry Co., Ltd.) PGM: Monopropylene glycol methyl ether (manufactured by Yamaichi Chemical Industry Co., Ltd.)

[0090] Using at least one of the above cleaning components, cleaning solutions (CL1 to CL9) were prepared with the compositions shown in Table 2. The values ​​shown in Table 2 are the blending amounts of each component based on the total mass of the cleaning solution, and the units are mass %.

[0091] [Table 2]

[0092] (Ink layer removal) The ink layers of plastic films 1 to 3 were removed. Specifically, the plastic films of each Example and Comparative Example were set in a roll-to-roll type film cleaning device, and the cleaning device was operated to remove the ink layers from the plastic films. The film cleaning device used was equipped with a frame that enables the roll of plastic film to rotate, a tank (cleaning tank) containing a cleaning liquid, a tank (rinsing tank) containing a rinsing liquid, a liquid removal section including a wiper blade and a hot air dryer, and an auxiliary frame that transports the film sent out from the roll of plastic film to the cleaning tank, rinsing tank, and liquid removal section, and is an apparatus that cleans plastic films by the following operations (1) to (4). (1) A roll of plastic film is unwound and immersed in a cleaning solution, and the surface of the plastic film is brush-cleaned in the cleaning solution. (2) Next, the washed plastic film is immersed in a rinsing liquid, and the surface of the plastic film is brush-washed in the rinsing liquid. (3) Next, the liquid adhering to the plastic film is wiped off with a wiper blade, and the plastic film is then dried with hot air. (4) Finally, the dried plastic film is wound into a roll to produce a roll of treated plastic film.

[0093] The cleaning liquid used was one of the cleaning liquids shown in Table 2. Water was used as the rinsing liquid. The combinations of plastic films and cleaning liquids used in each Example and Comparative Example were as shown in Tables 3 and 4.

[0094] <Evaluation> (Ink removability) The ink removability of the methods of each Example and Comparative Example was evaluated by the following method.

[0095] First, the treated plastic film obtained by the above method was crushed into plastic film pieces, and the obtained plastic film pieces were extruded at 200°C using a twin-screw extruder, and the extruded strands were pelletized to obtain recycled plastic pellets. Next, the recycled plastic pellets were hot-pressed at 200°C using a heat press to produce a 1 mm thick press plate (recycled press plate). In addition, a resin film (OPP in the example using plastic films 1 and 3, CPP in the example using plastic film 2) that is the raw material of the plastic film was crushed in the same manner as above, molded into plastic pellets, and hot-pressed to produce a 1 mm thick press plate (raw material press plate) as a standard sample.

[0096] The color value L of the recycled press plate and raw press plate obtained above * , a * , b * The color difference ΔE between the recycled press plate and the raw press plate was calculated using the following formula. ΔE=((L * xL * y) 2 +(a * xa * y) 2 +(b * x-b * y) 2 ) 1 / 2 x: Raw material pellets y: Recycled pellets

[0097] The smaller the color difference ΔE, the smaller the amount of components derived from the ink layer remaining in the recycled pellets, and the better the ink removability. In this evaluation, a color difference ΔE of 10 or less was evaluated as having good ink removability. The results are shown in Tables 3 and 4.

[0098] (Reusability of cleaning solution) The particle size distribution of the cleaning liquid (cleaning liquid containing ink removal material) used in the method of each Example and Comparative Example was measured, and the reusability of the cleaning liquid was evaluated. The particle size distribution of the cleaning liquid was measured using a particle size distribution measuring device (Shimadzu Corporation, SALD-7000) using the laser diffraction / scattering method. The dispersion medium had the same composition as the cleaning liquid being measured.

[0099] The larger the median diameter (D50) obtained from the volume frequency particle size distribution curve, the easier it is to remove ink residue from the cleaning liquid, and the better the reusability of the cleaning liquid. In this evaluation, a cleaning liquid with a median diameter (D50) of 5 μm or more was evaluated as having good reusability. The results are shown in Tables 3 and 4.

[0100] [Table 3]

[0101] [Table 4]

Claims

1. 1. A method for removing an ink layer from a plastic film, comprising the steps of: (A) unwinding a plastic film having an ink layer from a roll of the plastic film and immersing the plastic film in a cleaning liquid; (B) removing the ink layer from the plastic film by treating the surface of the plastic film on the ink layer side in the cleaning solution by a physical means; The cleaning solution comprises at least one cleaning component selected from the group consisting of n-propyl alcohol and monoalkylene glycol mono-n-propyl ether.

2. The method according to claim 1 , wherein in the step (B), the ink layer is removed from the plastic film by scrubbing the surface of the plastic film on the ink layer side with a brush in the cleaning solution.

3. 3. The method according to claim 1, wherein the plastic film in contact with the ink layer is a polyolefin film.

4. 3. The method according to claim 1, wherein the ink layer is formed from a gravure ink.

5. 3. The method according to claim 1, wherein the cleaning solution contains, as the monoalkylene glycol mono-n-propyl ether, at least one selected from the group consisting of monoethylene glycol mono-n-propyl ether and monopropylene glycol mono-n-propyl ether.

6. The method according to claim 1 or 2, wherein the content of the cleaning component is 20 mass % or more based on the total mass of the cleaning solution.

7. A method for recycling a cleaning liquid, comprising a step of removing components derived from the ink layer removed from the plastic film from the used cleaning liquid obtained through the method according to claim 1 or 2.

8. 3. A method for producing recycled plastic pellets, comprising a step of forming a plastic film or a processed product thereof from which an ink layer has been removed by the method according to claim 1 or 2 into pellets.

9. 3. A recycled plastic pellet obtained by molding a plastic film from which an ink layer has been removed by the method according to claim 1 or 2, or a processed product thereof.

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