JP2024076853A - Decorative layer removal method - Google Patents

Abstract

To provide a removal method which can sufficiently remove a decorative layer from a polyolefin base material with a decorative layer where a decorative layer that is a coating film layer or an ink layer is attached to the surface using CO2 in a supercritical state, and is also excellent in work safety and environmental safety.SOLUTION: A decorative layer removal method immerses a polyolefin base material with a decorative layer where a decorative layer that is a coating film layer or an ink layer is attached to the surface in a mixture of CO2 in a supercritical state and an organic solvent, and thereby removes the decorative layer from the polyolefin base material with the decorative layer.SELECTED DRAWING: None

Description

The present invention relates to a method for removing a decorative layer, which is a coating layer or an ink layer, from a polyolefin substrate having the decorative layer attached to the surface.

In order to achieve carbon neutrality, there is a growing demand for chemical and material recycling, even for polyolefin materials, rather than thermal recycling, which emits carbon dioxide.
As part of chemical recycling, decomposition treatment using supercritical fluids has been considered. For example, Patent Document 1 proposes a method in which, when crosslinked polyolefin is decomposed by nitrogen oxide in supercritical CO ₂ , the reaction temperature, pressure, and reaction time are controlled to adsorb nitrogen oxide to the carbon-carbon bond branch point of the crosslinked polyolefin, and the carbon-carbon bond branch point is preferentially oxidized and cut. The crosslinked polyolefin to be treated in Patent Document 1 does not have a coating layer or ink layer attached to the surface, but among the polyolefin products to be recycled, there are polyolefin products (polyolefin substrate with decorative layer) in which a decorative layer, which is a coating layer or ink layer, is attached to the surface of the polyolefin substrate. When considering chemical recycling and material recycling, when recycling polyolefin products, it is desirable to remove the decorative layer and then recycle the plastic substrate portion.

Various materials have been studied as decorative layer removers for removing decorative layers from polyolefin substrates with decorative layers such as coating layers. For example, Patent Document 2 studies a mixture composition consisting of chloropentafluoropropane and 1,1-dichloro-1-fluoroethane, and Patent Document 3 studies a mixture composition consisting of chloropentafluoropropane and dichlorotrifluoroethane as decorative layer removers.
In addition, in Patent Document 4, a stripping agent composition liquid containing a compound having a carbonyl group or its derivative and an alcohol such as monool or a compound having its derivative in a specific ratio is considered as a remover for a coating layer, which is an example of a decorative layer, and in Patent Document 5, a stripping agent containing a compound having an unsaturated bond between different atoms and a monohydric or higher alcohol or its derivative that is liquid at room temperature and has a boiling point of 250° C. or less in a specific ratio is considered as a decorative layer remover.Other decorative layer removers for removing decorative layers from polyolefins include ordinary organic halogen-based solvents such as methylene chloride, strong alkaline solutions, and strong acid solutions.

JP 2008-038006 A Japanese Patent Application Laid-Open No. 02-286795 Japanese Patent Application Laid-Open No. 02-286796 Japanese Patent Application Laid-Open No. 04-293974 Japanese Patent Application Laid-Open No. 05-009419

The mixed solvent compositions disclosed in Patent Documents 2 and 3 have excellent cleaning power for oils and greases, but because the compounds contain many halogen atoms (F, C1), they must be handled with care from an environmental hygiene perspective, and because they use relatively special halogen-based compounds, their industrial applications are limited to electrical component applications, etc.

In addition, with the stripping agent composition liquid described in Patent Document 4 or the stripping liquid of Patent Document 5, depending on the composition of the coating layer, the primer layer containing chlorinated polyolefin and the like provided between the coating layer and a resin substrate such as a polyolefin substrate may not be sufficiently stripped from the resin substrate. If the primer layer is not sufficiently stripped from the resin substrate, problems such as corrosion of the equipment used for processing (e.g., melt-kneading) a processed product (e.g., pulverized product) of the resin substrate under heating conditions may occur due to the chlorinated polyolefin and the like contained in the primer layer.

Furthermore, decorative layer removers containing organic halogen-based solvents, as disclosed in Patent Documents 2 and 3, etc., must be used with due consideration given to their operational safety and environmental safety, and depending on the type of solvent, problems with ozone depletion may occur. Also, when using a strong alkaline solution or strong acid solution as a decorative layer remover, it is necessary to use it with due consideration given to operational safety and environmental safety, and after the decorative layer removal process, a post-processing step such as neutralization is essential, making the decorative layer removal process complicated.

Thus, a method for removing a decorative layer that is excellent in work safety and environmental safety has not yet been proposed. Incidentally, since _CO2 in a supercritical state does not require waste liquid treatment, if the decorative layer could be removed using _CO2 in a supercritical state, it would be expected that work safety and environmental safety would be improved, but no attempt has been made to apply _CO2 in a supercritical state to the removal of a decorative layer.

The present invention aims to provide a removal method that can sufficiently remove a decorative layer, which is a coating layer or an ink layer, from a polyolefin substrate having a decorative layer attached to its surface, using _CO2 in a supercritical state, and that is also excellent in terms of operational safety and environmental safety.

As a result of the inventor's research, it has been found that the above-mentioned problems can be solved by the following configuration examples. The configuration examples of the present invention are as shown in the following [1] to [5].
In this specification, the numerical range "A to B" indicates A or more and B or less.

[1] A method for removing a decorative layer, comprising the steps of: immersing a polyolefin substrate having a decorative layer, the decorative layer being a coating layer or an ink layer, adhered to a surface of the polyolefin substrate in a mixture of supercritical _CO2 and an organic solvent, thereby removing the decorative layer from the polyolefin substrate having the decorative layer.

[2] The mixture contains 60 to 99% by mass of CO ₂ in a supercritical state and 1 to 40% by mass of the organic solvent (provided that the sum of the amount of CO ₂ in the supercritical state and the amount of the organic solvent is 100% by mass), the decorative layer removal method described in [1].

[3] The decorative layer removal method described in [1] or [2], in which the polyolefin substrate with the decorative layer is immersed in the mixture while adjusting the temperature of the mixture to 32 to 150°C and the pressure to 7.4 to 30 MPa.

[4] The decorative layer removal method according to any one of [1] to [3], wherein the organic solvent is at least one selected from the group consisting of amide compounds, alcohols, and ionic liquids.

[5] The decorative layer contains at least one resin selected from the group consisting of an acrylic resin, a melamine resin, and a urethane resin,
The decorative layer removal method according to any one of [1] to [4], wherein the polyolefin substrate in the decorative layer-attached polyolefin substrate is made of a polypropylene-based resin.

According to the present invention, a method for removing a decorative layer, which is a coating layer or an ink layer, from a polyolefin substrate having a decorative layer attached to its surface by using supercritical _CO2 is provided, which is excellent in terms of both operational safety and environmental safety.

FIG. 1 illustrates an example of a reaction apparatus.

<How to remove decorative layer>
In the decorative layer removal method according to the present invention, the decorative layer is removed from the polyolefin substrate with a decorative layer by immersing the polyolefin substrate with a decorative layer in a mixture of _CO2 in a supercritical state and an organic solvent.

<Polyolefin substrate with decorative layer>
The polyolefin substrate with a decorative layer is a polyolefin substrate having a decorative layer attached to the surface.
The polyolefin substrate included in the polyolefin substrate with a decorative layer is a molded body obtained by molding a polyolefin. Here, the polyolefin is a polymer having structural units derived from an olefin. Examples of polyolefins include polyethylene, polypropylene, propylene-ethylene copolymers, polypropylene composites in which polypropylene is compounded with an ethylene-based elastomer or an inorganic filler, and 4-methyl-1-pentene polymers.

The ethylene-based elastomer used in the polypropylene composite material is preferably a copolymer of ethylene and an α-olefin having 3 to 10 carbon atoms. Specific examples of α-olefins having 3 to 10 carbon atoms that are copolymerized with ethylene include propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 3-methyl-1-pentene, 1-heptene, 1-octene, 1-nonene, and 1-decene. These α-olefins may be used alone or in combination of two or more.

The polypropylene that may be included in the polyolefin substrate may contain a structural unit derived from biomass-derived propylene. The propylene that is the raw material of the polypropylene may be only biomass-derived propylene, or may be a mixture of biomass-derived propylene and fossil fuel-derived propylene. Biomass-derived propylene is propylene obtained by using any renewable natural raw material and its residue, such as plant-derived or animal-derived propylene, including fungi, yeast, algae, and bacteria, as the raw material, and contains about ^14C isotope as carbon at a ratio of about ^10-12 , and has a biomass carbon concentration (pMC) of about 100 (pMC) measured in accordance with ASTM D 6866. Biomass-derived propylene is obtained by a conventionally known method. It is preferable that the polypropylene contains a structural unit derived from biomass-derived propylene from the viewpoint of reducing the environmental load (mainly reducing greenhouse gases). If the polymer production conditions, such as the polymerization catalyst, polymerization process, and polymerization temperature, are the same, even if the raw propylene of polypropylene contains biomass-derived propylene, the molecular structure other than the content of ^14C isotopes in a ratio of about ^10-12 to ^10-14 is the same as that of polypropylene whose raw propylene is only fossil fuel-derived propylene. Therefore, the performance of these polypropylenes is said to be the same.

The polypropylene that may be included in the polyolefin base material may contain structural units derived from chemically recycled propylene. The propylene that is the raw material for the polypropylene may be only chemically recycled propylene, or may be a mixture of chemically recycled propylene and fossil fuel-derived propylene and/or biomass-derived propylene. The chemically recycled propylene is obtained by a conventionally known method. It is preferable that the polypropylene contains structural units derived from chemically recycled propylene from the viewpoint of reducing the environmental load (mainly reducing waste). In general, even if the raw material monomer (e.g., propylene) contains a chemically recycled monomer, the chemically recycled monomer is a monomer obtained by depolymerizing or pyrolyzing a polymer such as waste plastic back to a monomer unit such as propylene, and a monomer produced using the monomer as a raw material. Therefore, if the polymer production conditions such as the polymerization catalyst, polymerization process, and polymerization temperature are the same, the molecular structure is the same as that of a polypropylene whose raw material propylene is only fossil fuel-derived propylene. Therefore, it is said that the performance of these polypropylenes is the same.

The inorganic filler contained in the polypropylene composite material is not particularly limited, but examples thereof include heavy calcium carbonate, light calcium carbonate, talc, wollastonite, Mos-Hige (registered trademark), carbon fiber, glass fiber, magnesium carbonate, mica, kaolin, calcium sulfate, barium sulfate, titanium white, white carbon, carbon black; metal oxides such as aluminum oxide, magnesium oxide, silicon oxide, and zinc oxide; metal hydroxides such as aluminum hydroxide and magnesium hydroxide; metal nitrides such as boron nitride and aluminum nitride; metal oxynitrides such as aluminum oxynitride; metal carbides such as silicon carbide; metals or metal alloys such as gold, silver, copper, and aluminum; natural graphite, artificial graphite, and expanded graphite. These may be used alone or in a mixture of two or more. Among these inorganic fillers, talc is preferred.

The decorative layer of the polyolefin substrate with a decorative layer is a coating layer or an ink layer.
The coating layer is generally a layer formed from a paint containing a pigment, a resin, a solvent, and the like. The coating layer is formed on the surface of the polyolefin substrate by applying the paint to the polyolefin substrate and then volatilizing the solvent. Examples of the resin contained in the paint include acrylic resin, melamine resin, and urethane resin. The paint may contain one or more types of resin. That is, the coating layer may contain at least one type selected from the group consisting of acrylic resin, melamine resin, and urethane resin.

The ink layer is generally a layer formed from an ink containing a dye or pigment, a resin, a solvent, and the like. The ink layer is formed by printing the surface of a polyolefin substrate with the ink. Typical examples of resins contained in the ink include acrylic resins, melamine resins, urethane resins, silicone resins, rosin-modified phenolic resins, and alkyd resins. The ink may contain one or more types of resin. In other words, the ink layer may contain at least one type selected from the group consisting of acrylic resins, melamine resins, urethane resins, silicone resins, rosin-modified phenolic resins, and alkyd resins.

The coating layer or ink layer that will become the decorative layer may be formed on the surface of the polyolefin substrate via a primer layer. The primer layer is usually formed of a resin, and examples of the resin used for the primer layer include chlorinated polyethylene, chlorinated polypropylene, and acrylic-modified chlorinated polypropylene. According to the decorative layer removal method of the present invention, the layers formed on the surface of the polyolefin substrate, including these primer layers, can be sufficiently removed.

Examples of polyolefin substrates with decorative layers include bumpers made of polyolefin substrates on which a coating layer has been formed, packaging materials made of polyolefin substrates on which an ink layer has been formed, and containers such as bottles.

<Mixture>
In the decorative layer removal method according to the present invention, the polyolefin substrate with the decorative layer is immersed in a mixture of supercritical _CO2 and an organic solvent. The mixture may contain other components in addition to the supercritical _CO2 and the organic solvent.

Since the mixture contains _CO2 in a supercritical state, it is presumed that the supercritical _CO2 penetrates into the polyolefin substrate from the exposed portion of the polyolefin substrate with the decorative layer, causing the polyolefin substrate to swell, thereby promoting peeling of the decorative layer.

In addition, _CO2 in a supercritical state tends to improve the diffusibility of the organic solvent in the mixture and its permeability into polyolefin. As a result, it is presumed that the organic solvent in the mixture acts more strongly on the interface between the decorative layer and the polyolefin substrate than when the same organic solvent is placed at normal temperature and pressure, thereby reducing the adhesive strength between the decorative layer and the polyolefin substrate. Therefore, in the decorative layer removal method according to the present invention, the amount of organic solvent used is relatively small, so that the environmental load can be reduced (i.e., environmental safety can be improved).

_CO2 in a supercritical state is at a temperature and pressure higher than the critical point of _CO2 (31.1°C, 7.4 MPa). The means for bringing _CO2 into a supercritical state is not particularly limited as long as it is a heating and pressurizing means capable of adjusting the temperature and pressure to higher than the critical point of _CO2 . For heating, for example, an electric heater, a heating coil, a high-frequency heating device, etc. can be used. For pressurization, for example, a pressure-resistant sealed container is preferably used.

The content of supercritical _CO2 in the mixture is preferably 60 to 99% by mass, more preferably 85 to 99% by mass, since the swelling of the polyolefin substrate by supercritical _CO2 facilitates peeling of the decorative layer (provided that the total of the amount of supercritical _CO2 and the amount of the organic solvent is 100% by mass). Furthermore, by setting the content of supercritical _CO2 within the above range, the diffusibility and permeability of the organic solvent in the mixture are improved, so that the organic solvent acts on the interface between the decorative layer and the polyolefin substrate, and the adhesive strength between the decorative layer and the polyolefin substrate is easily reduced.

<Organic solvent>
Examples of the organic solvent contained in the mixture include amide compounds, alcohols, ionic liquids, etc. The organic solvent contained in the mixture may be one type or two or more types. That is, the organic solvent contained in the mixture may be at least one type selected from the group consisting of amide compounds, alcohols, and ionic liquids.

Examples of the amide-based compound contained in the mixture include N,N-dimethylformamide (DMF), N-methyl-2-pyrrolidone, and N,N-dimethylacetamide.
The alcohols contained in the mixture include propylene glycol, isopropyl alcohol, ethylene glycol, and the like.

In the present invention, an ionic liquid is a liquid that is liquid at 100° C. or below, typically around room temperature (25° C.), and is composed of a compound composed only of anion and cation ions, or a mixture thereof.
Examples of the ionic liquid used in the present invention include 1-ethyl-3-methylimidazolium methylphosphonate, 2-hydroxyethyl-trimethylammonium acetate, N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium=2-methoxyacetic acid, and 1-ethyl-3-methylimidazolium acetate.
Among these compounds that can be used as ionic liquids, 1-ethyl-3-methylimidazolium methylphosphonate and N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium=2-methoxyacetic acid are preferred from the viewpoint of more thorough removal of the decorative layer.

The compounds constituting the ionic liquid used in the present invention may be used alone or in combination of two or more.

The ionic liquid used in the present invention is flame retardant, and preferably has a flash point measured according to JIS K 2265-4 of more than 150° C. Therefore, when the ionic liquid is used as an organic solvent, it is excellent in both work safety and environmental safety.
Furthermore, the ionic liquid used in the present invention does not usually have a GHS (Globally Harmonized System of Classification and Labelling of Chemicals) classification indicating health hazards, and therefore, when the ionic liquid is used as an organic solvent, it is possible to prevent health hazards to workers, and it is also excellent in terms of work safety and environmental safety.

Of the amide compounds, alcohols, and ionic liquids listed above, N,N-dimethylformamide (DMF), propylene glycol, isopropyl alcohol, and N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium = 2-methoxyacetic acid are preferred as organic solvents from the viewpoint of more thorough removal of the decorative layer.

The content of the organic solvent in the mixture is preferably 1 to 40% by mass, and more preferably 1 to 15% by mass, because the organic solvent acts on the interface between the decorative layer and the polyolefin substrate to reduce the adhesive strength between the decorative layer and the polyolefin substrate, and because a reduction in the adhesive strength between the decorative layer and the polyolefin substrate is expected due to swelling of the polyolefin substrate by supercritical _CO2 (however, the total of the amount of _CO2 in the supercritical state and the amount of the organic solvent is 100% by mass).

When the content of the organic solvent in the mixture is within the above range, a relatively small amount of organic solvent can be used. Therefore, although the amide compounds and alcohols among the organic solvents that can be contained in the mixture may pose a health hazard, the probability of health damage to workers and the environmental load can be reduced (i.e., environmental safety can be improved). Furthermore, when an ionic liquid is used as the organic solvent, health damage to workers can be prevented, and work safety and environmental safety are also excellent.

<Other ingredients that may be included in the mixture>
In order to promote the peeling of the decorative layer, glass beads, alumina, zirconia, silica, diatomaceous earth, etc. may be added to the mixture of supercritical _CO2 and organic solvent. That is, at least one selected from the group consisting of glass beads, alumina, zirconia, silica, and diatomaceous earth may be included in the mixture as an optional component. When glass beads, alumina, zirconia, silica, or diatomaceous earth is included in the mixture, these substances are agitated together with the polyolefin substrate with the decorative layer to be treated during the decorative layer removal process and collide with the polyolefin substrate with the decorative layer, thereby promoting the peeling of the decorative layer from the polyolefin substrate.

<Method for removing decorative layer>
In the decorative layer removal method of the present invention, the polyolefin substrate with the decorative layer is immersed in a mixture of supercritical _CO2 and an organic solvent. The immersion method of the polyolefin substrate with the decorative layer is not particularly limited as long as the object of the present invention can be achieved, and may be, for example, a continuous method in which the polyolefin substrate with the decorative layer is continuously transferred and the polyolefin substrate with the decorative layer is continuously brought into contact with the mixture, or a batch method.

In addition, for an article that is a polyolefin substrate with a decorative layer, the decorative layer may be removed while it is still in the state in which it was used as an article, but the decorative layer may also be removed from the polyolefin substrate with a decorative layer after the article is pulverized. When a polyolefin substrate with a decorative layer is pulverized, the area of contact between the interface between the decorative layer and the polyolefin substrate and the mixture increases, making it easier to remove the decorative layer efficiently.

Below, we will explain how to remove a decorative layer from a polyolefin substrate with a decorative layer using the reaction apparatus shown in Figure 1.

The reaction apparatus shown in Fig. 1 includes a reactor 14 in a furnace 11 equipped with a heating coil 12 used for heating. The reactor 14 is excellent in heat resistance and pressure resistance to the extent that it can hold a mixture of _CO2 in a supercritical state and an organic solvent. There are no particular limitations on the material of the reactor 14, but since it is the part that comes into direct contact with _CO2 in a supercritical state, it is preferable to use one made of a corrosion-resistant material such as Hastelloy (an alloy made of Ni, Cr, Mo, etc.) or SUS.

The reaction vessel 14 shown in FIG. 1 is equipped with an agitator 15. The agitator 15 agitates the contents in the reaction vessel 14, so that the mixture and the polyolefin substrate 16 with the decorative layer are agitated. The agitator 15 is optional, and the reaction vessel does not necessarily have to include the agitator 15.

_CO2 is supplied from a _CO2 cylinder 1 to the reaction vessel 14 through a valve 2, a cooling device 3, a _CO2 pump 4, a valve 5, and a pipe connecting the reaction vessel 14. The _CO2 passing through the pipe is heated when passing through a heating coil 12, and is heated to a desired temperature. Meanwhile, the temperature in the reaction vessel (i.e., the temperature of the mixture) is monitored by a thermocouple 13. The pressure in the reaction vessel 14 is monitored by a pressure gauge 6 connected to the reaction vessel 14 by a pipe without a valve.

The organic solvent is supplied from the organic solvent pump 7 to the reaction vessel 14 via the valve 5. The organic solvent to be mixed into the mixture is supplied to the organic solvent pump 7. The organic solvent is also supplied to the reaction vessel 14 through a pipe, and is heated as it passes through the heating coil 12, raising the temperature to the desired level.

A pipe extends from the reaction vessel 14, connecting the filter 21, the exhaust valve 22, and the exhaust tank 24. The exhaust valve 22 is equipped with a heating device 23 to prevent freezing.

An example of the decorative layer removal process will be described below in chronological order.
First, the polyolefin substrate 16 with a decorative layer is placed in a reaction vessel 14. The amount of the mixture (a mixture of _CO2 in a supercritical state and an organic solvent) used is not particularly limited as long as the polyolefin substrate with a decorative layer can be immersed in the mixture.

Next, the valve 2 is opened, and the CO ₂ supplied from the CO ₂ cylinder 1 is cooled by the cooling device 3, and then the valve 5 is opened, and the pressure in the reaction vessel 14 is increased to a desired pressure by the CO ₂ pump 4. Furthermore, a valve (not shown) installed in the pipe between the organic solvent pump 7 and the valve 5 is opened, and the organic solvent is supplied into the reaction vessel 14 by the organic solvent pump 7 while the organic solvent is heated by the heating coil 12. The mixture of CO ₂ and the organic solvent is brought to a desired pressure and temperature, so that the CO ₂ in the reaction vessel 14 is brought to a supercritical state. This process results in a state in which a mixture of CO ₂ and the organic solvent in a supercritical state is introduced into the reaction vessel 14. In addition, this process causes the mixture in the reaction vessel 14 to be pressurized and heated to conditions suitable for the treatment of the polyolefin substrate 16 with a decorative layer. After the mixture is introduced into the reaction vessel 14, the valve 5 is closed.

Thereafter, stirring is performed by the stirring device 15 so that the polyolefin substrates 16 with the decorative layers rub against each other and the decorative layers are easily peeled off.
Here, the conditions for immersing the polyolefin substrate with a decorative layer in a mixture of supercritical _CO2 and an organic solvent are conditions under which _CO2 is in a supercritical state, and the mixture temperature is preferably 32 to 150°C and the pressure is 7.4 to 30 MPa, more preferably the mixture temperature is 50 to 150°C and the pressure is 8.0 to 30 MPa, and even more preferably the mixture temperature is 70 to 150°C and the pressure is 8.0 to 20 MPa.

The polyolefin substrate 16 with the decorative layer is immersed in the mixture for preferably 0.5 to 2 hours, more preferably 0.5 to 1.0 hour.

After the polyolefin substrate 16 with the decorative layer is immersed in the mixture, the _CO2 is exhausted through the filter 21 and the exhaust valve 22. Then, the peeled decorative layer, the polyolefin substrate, and the organic solvent remain in the reaction vessel 14. Therefore, the obtained polyolefin substrate can be used as a recycled material after appropriate cleaning. In addition, since the organic solvent used to prepare the mixture remains in the reaction vessel 14, it can be reused by removing the peeled pieces of the decorative layer and the components dissolved from the decorative layer as appropriate. In addition, the _CO2 used to prepare the mixture can be recovered in the exhaust tank 24.

Although an example of a reaction apparatus and an example of removing a decorative layer using the reaction apparatus have been described above with reference to FIG. 1, the reaction apparatus that can be used in this invention is not limited to the apparatus shown in FIG. 1, and may be appropriately modified in design within the scope of the method of this invention. For example, the reaction apparatus does not need to be equipped with the stirring device 15. Also, even if a reaction apparatus equipped with the stirring device 15 is used, the polyolefin substrate 16 with the decorative layer may be left immersed in the mixture for the aforementioned treatment time without using the stirring device 15.

Furthermore, even if the reaction apparatus used to remove the decorative layer is not that shown in Figure 1, the preferred ranges of processing conditions such as mixture temperature, pressure, and immersion time when removing the decorative layer are the same as the preferred ranges described with reference to Figure 1.

In addition, in the present invention, for example, a polyolefin substrate with a decorative layer that has become waste material is used as a raw material, which is crushed as necessary, and then a polyolefin substrate from which the decorative layer has been removed can be obtained by the decorative layer removal method described above, so this is also a method for producing recycled materials using the decorative layer removal method described above. The polyolefin substrate from which the decorative layer has been removed is useful as a raw material for chemical recycling or material recycling.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

<Ingredients>
In the examples and comparative examples, the following raw materials were used.
<Polyolefin substrate with decorative layer>
"Paint-layer-attached bumper piece": a bumper piece obtained by crushing a paint-layer-attached bumper (the surface area of the paint layer is about ¹ cm2, the thickness is about 75 μm, and the polyolefin base material is either polypropylene, propylene-ethylene copolymer, or polypropylene composite, but this can vary depending on the bumper piece).

<Organic Solvents Used>
"DMF": N,N-dimethylformamide (Kanto Chemical Co., Ltd.)
・ "IPA": Isopropyl alcohol (manufactured by Sankyo Chemical Co., Ltd.)
"Ionic liquid A": N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium = 2-methoxyacetic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)

Example 1
The apparatus shown in FIG. 1 was used. Ten bumper pieces with a coating layer were put into the reaction vessel 14. Then, CO 2 and DMF were supplied into the reaction vessel 14 while heating and pressurizing so that the amount of DMF in the mixture in the reaction vessel 14 was 10 mass% (the sum of the amount of DMF and the amount of CO ₂ in a supercritical state was 100 mass%), to prepare a mixture of CO ₂ and DMF in a supercritical state. The conditions for heating and pressurizing were adjusted so that the pressure in the reaction vessel ₁₄ was 20 MPa and the temperature in the reaction vessel 14 was 140°C. Then, stirring was performed with the stirring device 15. This state was maintained for 60 minutes, and then CO ₂ was exhausted. The presence or absence of a coating was visually confirmed for each of the bumper pieces remaining in the reaction vessel 14, and the number of particles from which the coating was peeled off cleanly was defined as the number of particles from which the coating was removed, and the ratio of the number of bumper pieces from which the coating was removed was calculated using the following formula and evaluated according to the following evaluation criteria. The results obtained are shown in Table 1.
Number of bumper pieces with paint removed/10 pieces x 100
= Percentage of bumper pieces with paint removed
(Evaluation criteria)
◎: 60% or more of the bumper pieces had the paint removed. ◯: 20% or more but less than 60% of the bumper pieces had the paint removed. ×: Less than 20% of the bumper pieces had the paint removed.

Example 2
Except for adjusting the pressure inside the reaction vessel 14 as shown in Table 1, the coating layer removal treatment and evaluation were carried out in the same manner as in Example 1. The results are shown in Table 1.

<Examples 3 to 5>
The pressure and temperature in the reaction vessel 14 were adjusted as shown in Table 1, and stirring was not performed, but the coating layer was removed and evaluated in the same manner as in Example 1. The results are shown in Table 1.

Example 6
Except for changing the organic solvent put into the reaction vessel 14 to IPA, the coating layer removal treatment and evaluation were carried out in the same manner as in Example 3. The results are shown in Table 1.

Example 7
The coating layer was removed and evaluated in the same manner as in Example 6, except that the pressure and temperature in the reaction vessel 14 were adjusted as shown in Table 1. The results are shown in Table 1.

Example 8
The coating layer was removed and evaluated in the same manner as in Example 3, except that the organic solvent placed in the reaction vessel 14 was changed to ionic liquid A. The results are shown in Table 1.

<Comparative Example 1>
DMF was placed in a beaker at normal pressure and heated to 80° C. while stirring with a stirrer. When the temperature reached 80° C., 10 pieces of the bumper with the coating layer were placed in the beaker and held for 60 minutes while stirring. Thereafter, each of the bumper pieces was visually inspected for the presence or absence of a coating, and was evaluated according to the same evaluation criteria as in Example 1. The results are shown in Table 2.

<Comparative Example 2>
Ionic liquid A was placed in a beaker at normal pressure and heated to 100°C while stirring with a stirrer. When the temperature reached 100°C, 10 bumper pieces with a coating layer were placed in the beaker and held for 60 minutes while stirring. Thereafter, each bumper piece was visually inspected for the presence or absence of a coating, and was evaluated according to the same evaluation criteria as in Example 1. The results are shown in Table 2.

<Comparative Example 3>
Except for adjusting the temperature as shown in Table 2, the coating layer was removed and evaluated in the same manner as in Comparative Example 2. The results are shown in Table 2.

<Comparative Example 4>
Supercritical _CO2 was used instead of the mixture, the pressure and temperature of the reaction vessel 14 were adjusted as shown in Table 2, and stirring was not performed, but the coating layer was removed and evaluated in the same manner as in Example 1. The results are shown in Table 2.

<Comparative Example 5>
Except for using supercritical water instead of the mixture, adjusting the pressure and temperature of the reaction vessel 14 as shown in Table 2, and not performing stirring, the coating layer removal treatment and evaluation were performed in the same manner as in Example 1. The results are shown in Table 2.

Reference Signs List 1: _CO2 cylinder 2, 5: valve 3: cooling device 4: _CO2 pump 6: pressure gauge 7: organic solvent pump 11: furnace 12: heating coil 13: thermocouple 14: reaction vessel 15: stirring device 16: polyolefin substrate with decorative layer 21: filter 22: exhaust pressure valve 23: heating device 24: exhaust tank

Claims (5)

A method for removing a decorative layer, comprising the steps of: immersing a polyolefin substrate having a decorative layer, the decorative layer being a coating layer or an ink layer, adhered to a surface thereof, in a mixture of supercritical _CO2 and an organic solvent, thereby removing the decorative layer from the polyolefin substrate having the decorative layer. The mixture contains 60 to 99% by mass of CO ₂ in a supercritical state and 1 to 40% by mass of the organic solvent (provided that the total amount of the CO ₂ in the supercritical state and the organic solvent is 100% by mass). The decorative layer removal method according to claim 1. The decorative layer removal method according to claim 1, in which the polyolefin substrate with the decorative layer is immersed in the mixture while the temperature of the mixture is adjusted to 32 to 150°C and the pressure is adjusted to 7.4 to 30 MPa. The decorative layer removal method according to claim 1, wherein the organic solvent is at least one selected from the group consisting of amide compounds, alcohols, and ionic liquids. the decorative layer contains at least one resin selected from the group consisting of an acrylic resin, a melamine resin, and a urethane resin;
The decorative layer removing method according to any one of claims 1 to 4, wherein the polyolefin substrate in the decorative layer-attached polyolefin substrate is made of a polypropylene-based resin.