Method for producing recycled styrene monomer

Abstract

In chemical recycling of polystyrene, there has been a problem in that the yield of recycled styrene monomer is low. The purpose of the present invention is to provide a method for producing a recycled styrene monomer, the method comprising a thermal decomposition step for thermally decomposing a waste styrene-based resin composition in the presence of a metal compound to obtain a thermal decomposition product containing the recycled styrene monomer. The metal compound is contained in an amount of 1-25 parts by mass with respect to 100 parts by mass of styrene monomer units in the waste styrene-based resin composition. The metal compound is at least one selected from the group consisting of zinc compounds, iron compounds, potassium compounds, and sodium compounds.

Description

Method for producing recycled styrene monomer

[0001] The present invention relates to a method for producing recycled styrene monomer.

[0002] In recent years, the environmental impact of plastic waste has been increasing, making plastic waste recycling crucial for waste reduction and improved sustainability.

[0003] Styrene is widely used as a raw material for important plastic products such as polystyrene (PS). Styrene is a petroleum-derived monomer, and PS is mainly produced by copolymerization of styrene alone or with other monomers.

[0004] Styrene is a chemically stable molecule, and obtaining high-quality recycled styrene requires removing impurities from discarded styrene-based resin compositions and optimizing reaction conditions, making effective recycling technically challenging.

[0005] Patent Document 1 discloses a method for recycling waste polystyrene products, characterized by including a step of obtaining recycled styrene monomer from waste polystyrene by thermal decomposition treatment of the waste polystyrene products.

[0006] WO2021 / 230312

[0007] However, chemical recycling (CR) of PS had the problem of low yield of recycled styrene monomer.

[0008] The object of the present invention is to provide a method for producing recycled styrene monomer from a waste styrene resin composition that improves the yield of recycled styrene monomer.

[0009] The inventors of this invention have succeeded in increasing the yield of recycled styrene monomer by adding a certain amount of a metal compound such as zinc oxide to a waste styrene resin composition, and thus the present invention has been completed.

[0010] [1] The object of the present invention is to provide a method for producing recycled styrene monomer, comprising a thermal decomposition step of thermally decomposing a waste styrene resin composition in the presence of a metal compound to obtain a thermal decomposition product containing recycled styrene monomer, wherein the metal compound comprises 1 to 25 parts by mass per 100 parts by mass of styrene monomer units in the waste styrene resin composition, and the metal compound is at least one selected from the group consisting of zinc compounds, iron compounds, potassium compounds, and sodium compounds.

[0011] This method can increase the yield of recycled styrene monomer from waste styrene resin compositions.

[0012] [2] In the method described in [1], the yield of the above-mentioned pyrolysis product may be 85% or more.

[0013] [3] In the method described in [1] or [2], the increase in the amount of recycled styrene monomer obtained by thermally decomposing the waste styrene resin composition in the presence of the metal compound compared to the amount of recycled styrene monomer obtained by thermally decomposing the waste styrene resin composition in the absence of the metal compound may be 0.5% or more.

[0014] [4] In the method according to any one of [1] to [3], the waste styrene resin composition may further contain polyethylene terephthalate.

[0015] [5] In the method described in any of [1] to [4], the metal compound may be at least one selected from the group consisting of zinc oxide, iron oxide, potassium carbonate, and sodium bicarbonate.

[0016] [6] In the method described in [5], the metal compound may be zinc oxide and / or iron oxide.

[0017] For the sake of definition, the specific terms used in this application are gathered here. Unless otherwise specified, all technical and scientific terms used in this application have the same meaning as those generally understood by those skilled in the art to which this invention pertains. Unless otherwise specified in the context, the singular forms "a," "an," and "the" include plural references.

[0018] The numerical ranges and parameters shown in this invention are approximations, although the numerical values ​​shown in specific examples are described as accurately as possible. However, all numerical values ​​inherently contain certain errors that inevitably arise from the standard deviation observed in each test measurement. Furthermore, the term "about" as used herein generally means within 10%, 5%, 1%, or 0.5% of a given value or range. Alternatively, the term "about" means that it is within an acceptable standard error, as considered by those skilled in the art.

[0019] The embodiments of the present invention will now be described. The following embodiments are illustrative, and the scope of the present invention is not limited to those shown in the following embodiments. In order to avoid repetition and complexity, explanations of similar content will be omitted as appropriate.

[0020] <Method for producing recycled styrene monomer> The method for producing recycled styrene monomer according to this embodiment includes a thermal decomposition step of thermally decomposing a waste styrene resin composition in the presence of a metal compound to obtain a thermal decomposition product containing recycled styrene monomer.

[0021] In one embodiment, a method for producing recycled styrene monomer further comprises a distillation purification step of distilling and purifying the above-mentioned thermal decomposition product.

[0022] Pyrolysis Process In the pyrolysis process, for example, it is preferable to supply the above-mentioned waste styrene resin composition into a pyrolysis furnace and heat it to a temperature of 400 to 800°C in a non-oxidizing atmosphere. Specifically, the heating temperature is, for example, 400, 410, 420, 430, 440, 450, 460, 470, 480, 490, 500, 550, 600, 650, 700, 750, and 800°C, and may be within the range of any two of the values ​​exemplified here. Specifically, the time for the pyrolysis reaction is, for example, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, and 5.0 hours, and may be within the range of any two of the values ​​exemplified here. The non-oxidizing atmosphere is preferably an inert gas atmosphere such as nitrogen, water vapor, carbon dioxide, or argon, or a negative pressure atmosphere. The gas produced by the pyrolysis process is sent to a condenser where it is condensed into an oily pyrolysis product (pyrolysis oil), which is then sent to a distillation and refining process to further refine the pyrolysis product.

[0023] In the distillation purification process, it is preferable to use two distillation columns, for example, separating compounds with a lower boiling point than styrene in the first distillation column and separating compounds with a higher boiling point than styrene in the second distillation column.

[0024] <Recycled Styrene Monomer> Recycled styrene monomer is styrene monomer (pyrolytic styrene monomer) recovered from the pyrolysis products obtained by thermally decomposing waste styrene resin compositions. Before the distillation purification process, recycled styrene monomer is contained in the pyrolysis products and is purified by distillation from the pyrolysis products in the distillation purification process.

[0025] <Waste Styrene Resin Composition> The waste styrene resin composition is a styrene resin product that is no longer needed, regardless of whether it is usable or not. A styrene resin product refers to a resin product containing styrene resin, and refers to a resin product manufactured using styrene resin or partially using styrene resin. A styrene resin product may have any shape, such as its shape when unused, when in use, or when discarded, and may be partially missing or crushed. In one embodiment, the waste styrene resin composition further contains polyethylene terephthalate (PET). <Pyrolysis Products> The pyrolysis products are oily pyrolysis products derived from the pyrolysis products of the waste styrene resin product and containing recycled styrene monomer. Recycled styrene monomer can be used as a raw material for manufacturing recycled styrene resin in conventional styrene resin manufacturing methods such as bulk polymerization and solution polymerization.

[0026] In one embodiment, the yield of the pyrolysis product is 85% or higher (for example, 85% or higher, 86% or higher, 87% or higher, 88% or higher, 89% or higher, 90% or higher, 91% or higher, 92% or higher, 93% or higher, 94% or higher, 95% or higher, 96% or higher, 97% or higher, 98% or higher, 99% or higher, 99.5% or higher, and 99.8% or higher). If the yield of the pyrolysis product is 85% or higher, recycled styrene monomer can be recovered efficiently. The yield (%) of the pyrolysis product is calculated based on [mass of pyrolysis product obtained by pyrolysis of the waste styrene resin composition] / [mass of waste styrene resin composition].

[0027] <Metal Compounds> The metal compound is contained in the waste styrene resin composition in an amount of 1 to 25 parts by mass (for example, within the range of any two values ​​selected from 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, and 25 parts by mass) per 100 parts by mass of styrene monomer units.

[0028] The metal compound is at least one selected from the group consisting of zinc compounds, iron compounds, potassium compounds, and sodium compounds. In one embodiment, the metal compound is zinc oxide (ZnO), iron oxide (e.g., FeO, Fe ３ O ４ and Fe ２ O ３ ), potassium carbonate (K ２ CO ３ ) and sodium bicarbonate (NaHCO3) ３ It is at least one selected from the group consisting of ). In another embodiment, the metal compound is zinc oxide and / or iron oxide.

[0029] <Recycled Styrene Monomer Yield> In one embodiment, the increase in the amount of recycled styrene monomer obtained by thermally decomposing a waste styrene resin composition in the presence of a metal compound compared to the amount of recycled styrene monomer obtained by thermally decomposing a waste styrene resin composition in the absence of a metal compound is 0.5% or more, preferably 1.0% or more, and more preferably 2.0% or more.

[0030] The recycled styrene monomer yield is the mass percentage of recycled styrene monomer obtained by thermal decomposition when the amount of styrene monomer units in the waste styrene resin composition is set at 100 parts by mass, and can be calculated as [yield of thermal decomposition product × styrene concentration in thermal decomposition product].

[0031] <Styrene-based resin> A styrene-based resin (styrene-based resin composition) is a polymer having styrene monomer units, obtained by polymerizing a styrene monomer composition, a recycled styrene monomer composition, or a combination thereof. The styrene-based resin preferably contains more than 15% by mass and less than or equal to 100% by mass of styrene monomer units in 100% by mass of the polymer having styrene monomer units. In one embodiment, the styrene-based resin may be a polymer of styrene monomers (styrene-based polymer). In another embodiment, the styrene-based resin may be a copolymer having styrene monomer units and (meth)acrylic monomer units (styrene-(meth)acrylic copolymer). The styrene-based polymer may be a homopolymer of styrene monomers obtained by polymerizing only one type of styrene monomer, or it may be a copolymer obtained by polymerizing only two or more types of styrene monomers. Furthermore, the styrene-based polymer preferably contains 95% by mass or more, and more preferably (substantially) 100% by mass of styrene monomer units in 100% by mass of the styrene-based polymer. The content of styrene monomer units is specifically, for example, 95, 96, 97, 98, 99, 99.5, or 100% by mass of 100% by mass of the styrene polymer, and may be within the range of any two of the values ​​exemplified here. Note that this content is the total mass if two or more types of styrene monomer units are used. From the viewpoint of minimizing dimensional changes, the styrene polymer is preferably a homopolymer of styrene.

[0032] (Meth)acrylic monomer units are (meth)acrylic acid monomer units or (meth)acrylic acid ester monomer units. Styrene-(meth)acrylic copolymers are styrene-(meth)acrylic acid copolymers or styrene-(meth)acrylic acid ester copolymers.

[0033] <Method for producing styrene-based resin> The method for producing styrene-based resin according to this embodiment includes a polymerization step of polymerizing the above-mentioned styrene monomer composition.

[0034] Polymerization methods for producing styrene-based resins include known styrene polymerization methods such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. In terms of quality and productivity, bulk polymerization and solution polymerization are preferred, and continuous polymerization is preferable. Examples of solvents that can be used include alkylbenzenes such as benzene, toluene, ethylbenzene, and xylene, ketones such as acetone and methyl ethyl ketone, and aliphatic hydrocarbons such as hexane and cyclohexane.

[0035] During polymerization of the styrene monomer composition, polymerization initiators, chain transfer agents, crosslinking agents, and other polymerization aids may be used as needed. As polymerization initiators, radical polymerization initiators are preferred, including well-known and conventional peroxyketals such as 1,1-di(t-butylperoxy)cyclohexane, 2,2-di(t-butylperoxy)butane, 2,2-di(4,4-di-t-butylperoxycyclohexyl)propane, and 1,1-di(t-amylperoxy)cyclohexane; hydroperoxides such as cumene hydroperoxide and t-butyl hydroperoxide; alkyl peroxides such as t-amylperoxyisononanoate; and dialkyl peroxides such as t-butylcumylperoxide, di-t-butylperoxide, dicumylperoxide, and di-t-hexylperoxide. Examples include peroxides, peroxyesters such as t-butyl peroxyacetate, t-butyl peroxybenzoate, and t-butyl peroxyisopropyl monocarbonate, peroxycarbonates such as t-butyl peroxyisopropyl carbonate and polyethertetrakis(t-butyl peroxycarbonate), and N,N'-azobis(cyclohexane-1-carbonitride), N,N'-azobis(2-methylbutyronitrile), N,N'-azobis(2,4-dimethylvaleronitrile), and N,N'-azobis[2-(hydroxymethyl)propionitrile]. One or more of these can be used in combination. Examples of chain transfer agents include aliphatic mercaptans such as n-dodecyl mercaptan and tert-dodecyl mercaptan, aromatic mercaptans, thiocarboxylic acids such as thioglycolic acid and mercaptopropionic acid, polyhydric alcohols such as ethylene glycol, tetraethylene glycol, neopentyl glycol, trimethylolpropane, pentaerythritol, dipentaerythritol, tripentaerythritol, and sorbitol, polyfunctional mercaptans obtained by esterifying the hydroxyl group of polyhydric alcohols with thioglycolic acid or mercaptopropionic acid, pentaphenylethane, α-methylstyrene dimer, and terpinolene. Among these, aliphatic mercaptans, aromatic mercaptans, thiocarboxylic acids, and polyfunctional mercaptans are preferred because their molecular weight can be easily adjusted.

[0036] In the case of continuous polymerization, the styrene resin can be produced by a method comprising a polymerization step, a devolatilization step, and a granulation step.

[0037] First, in the polymerization step, a known completely mixed tank type stirring tank, a tower type reactor, or the like is used, and the polymerization reaction is controlled by adjusting the polymerization temperature or the like so as to achieve the target molecular weight, molecular weight distribution, and reaction conversion rate.

[0038] The polymerization solution containing the polymer discharged from the polymerization step is transferred to the devolatilization step, and unreacted monomers and the polymerization solvent are removed. The devolatilization step is composed of a vacuum devolatilization tank with a heater, a devolatilization extruder with a vent, or the like. The polymer in the molten state discharged from the devolatilization step is transferred to the granulation step. In the granulation step, the molten resin is extruded in a strand shape from a porous die and processed into a pellet shape by a cold cut method, an air hot cut method, or a water hot cut method.

[0039] Hereinafter, the present invention will be described in more detail with reference to examples. These are all illustrative and do not limit the content of the present invention.

[0040] [Production Examples of Styrene Resin Compositions A-1, A-2, and A-3] Styrene resin compositions A-1, A-2, and A-3 were produced based on the compositions and processes shown in Table 1. As follows, styrene resin compositions A-1, A-2, and A-3 were prepared based on the conditions in Table 1. A polymerization step was constituted by connecting a first reactor, which is a completely mixed type stirring tank, a second reactor, and a third reactor, which is a plug flow type reactor with a static mixer, in series, and the production of the styrene resin composition was carried out under the conditions shown in Table 一. The capacity of each reactor was set such that the first reactor was 39 liters, the second reactor was 39 liters, and the third reactor was 16 liters. A raw material solution was prepared with the raw material composition described in Table 1, and the raw material solution was continuously supplied to the first reactor at the flow rate described in Table 1. The polymerization initiator was added to the raw material solution so as to have the addition concentration (concentration based on mass with respect to the total amount of the raw material monomers (styrene)) described in Table 1 at the inlet of the first reactor and uniformly mixed.

[0041]

[0042] The polymerization initiators described in Table 1 are as follows. Polymerization Initiator - 1: 2,2 - bis(4,4 - t - butylperoxycyclohexyl)propane (Perhexa A manufactured by NOF Corporation was used.) Polymerization Initiator - 2: 1,1 - bis(t - butylperoxy)cyclohexane (Perhexa C manufactured by NOF Corporation was used.)

[0043] In the third reactor, a temperature gradient was created along the flow direction, and the temperature was adjusted so that the temperature in the middle part and the outlet part was the same as that in Table 1. Subsequently, the solution containing the polymer continuously taken out from the third reactor was introduced into a vacuum devolatilization tank with a preheater composed of two stages in series. The temperature of the preheater was adjusted so that the resin temperature was the same as that described in Table 1, and the pressure was adjusted to the pressure described in Table 1. After separating unreacted styrene and ethylbenzene, it was extruded in a strand shape from a porous die, and the strand was cooled and cut by a cold - cut method to be pelletized.

[0044] The MMA content described in Table 1 represents the content of methyl methacrylate monomer units, and the MAA content represents the content of methacrylic acid monomer units.

[0045] <Mass - average molecular weight (Mw)> The mass - average molecular weight (Mw) of the styrene - based resin composition was measured under the following conditions using gel permeation chromatography (GPC). GPC model: Shodex GPC - 101 manufactured by Showa Denko K.K. Column: PLgel 10μm MIXED - B manufactured by Polymer Laboratories Mobile phase: Tetrahydrofuran Sample concentration: 0.2 mass% Temperature: Oven 40°C, injection port 35°C, detector 35°C Detector: Differential refractometer The molecular weight was calculated as the molecular weight in terms of polystyrene by calculating the molecular weight at each elution time from the elution curve of monodisperse polystyrene.

[0046] <Melt flow rate (MFR) g / 10 min> The obtained pellets were measured based on JIS K7210. Test temperature: 200°C Test load: 49 N

[0047] <Measurement of Vicat softening temperature (VST)> The measurement of the Vicat softening temperature was carried out in accordance with JIS K 7206, with a heating rate of 50°C / hr and a test load of 50 N.

[0048] <Polyethylene terephthalate B-1 and metal compounds> The polyethylene terephthalate and metal compounds described in Tables 2 and 3 are as follows. B-1: Polyethylene terephthalate (TRN-8550FF manufactured by Teijin Chemicals Ltd. was used.) C-1: Zinc oxide (manufactured by Junsei Chemical Co., Ltd. was used.) C-2: Iron(III) oxide (Fe ２ O ３ ) (manufactured by Junsei Chemical Co., Ltd. was used.) C-3: Potassium carbonate (manufactured by Junsei Chemical Co., Ltd. was used.) C-4: Sodium hydrogen carbonate (manufactured by Junsei Chemical Co., Ltd. was used.) C-5: Magnesium oxide (manufactured by Junsei Chemical Co., Ltd. was used.) C-6: Calcium hydroxide (manufactured by Junsei Chemical Co., Ltd. was used.)

[0049] [Examples 1 to 10 and Comparative Examples 1 to 9] The metal compounds and their amounts shown in Tables 2 and 3 were added to the corresponding styrene resin compositions A-1, A-2 and A-3 and B-1, and raw materials (waste styrene resin compositions) according to Examples 1 to 10 and Comparative Examples 1 to 9 were produced. Each of the produced raw materials was subjected to pyrolysis treatment, and the yield of the pyrolysis product, the recycled styrene monomer yield, the increase in the recycled styrene monomer yield compared to the case where no additive was added, and the adhesion to the pyrolysis gas pipe were confirmed.

[0050] Pyrolysis conditions The raw materials according to Examples 1 to 10 and Comparative Examples 1 to 9 were charged into a reactor and heated to 440 °C, and a pyrolysis reaction was carried out for 3.5 hours.

[0051] Recycled styrene monomer yield The recycled styrene monomer yield was taken as the mass % of the recycled styrene monomer obtained by pyrolysis (calculated from the mass yield of the pyrolysis product × the styrene concentration in the pyrolysis product) when the styrene monomer unit in each raw material was 100 parts by mass.

[0052] Adhesion to the pyrolysis gas pipe The adhesion of the pyrolysis gas to the pipe was evaluated visually as follows. A: No adhesion B: Some adhesion C: Large amount of adhesion

[0053] Increase in recycled styrene monomer yield compared to no additives: Examples 1-6, Comparative Example 1, and Comparative Examples 5-8 were calculated based on [recycled styrene monomer yield when each raw material composition is thermally decomposed - recycled styrene monomer yield of Comparative Example 1]. Examples 7 and Comparative Example 3 were calculated based on [recycled styrene monomer yield when each raw material composition is thermally decomposed - recycled styrene monomer yield of Comparative Example 3]. Examples 8 and Comparative Example 4 were calculated based on [recycled styrene monomer yield when each raw material composition is thermally decomposed - recycled styrene monomer yield of Comparative Example 4]. Examples 9-10, Comparative Example 2, and Comparative Example 9 were calculated based on [recycled styrene monomer yield when each raw material composition is thermally decomposed - recycled styrene monomer yield of Comparative Example 2].

[0054]

[0055]

[0056] The increase in the recycled styrene monomer yield of the raw materials in Examples 1 to 6, which include styrene-based resin composition A-1 (polystyrene), was higher than that of the corresponding Comparative Examples 1 and 5 to 8.

[0057] The increase in the recycled styrene monomer yield of the raw materials in Example 7, which includes styrene-based resin composition A-2 (styrene-(meth)acrylate copolymer), was higher than that of the corresponding Comparative Example 3.

[0058] The increase in the recycled styrene monomer yield of the raw materials in Example 8, which includes styrene-based resin composition A-3 (styrene-(meth)acrylic acid copolymer), was higher than that of the corresponding Comparative Example 4.

[0059] The increase in the recycled styrene monomer yield of the raw materials in Examples 9 and 10, which included styrene-based resin composition A-1 and polyethylene terephthalate, was higher than that of the corresponding Comparative Examples 2 and 9. Furthermore, no adhesion was observed inside the pyrolysis gas piping with the raw material in Example 9, and only slight adhesion was observed inside the pyrolysis gas piping with the raw material in Example 10. On the other hand, with the raw materials in the corresponding Comparative Examples 2 and 9, a large amount of terephthalic acid and benzoic acid, which are pyrolysis products of polyethylene terephthalate, adhered inside the pyrolysis gas piping, potentially causing blockage of the piping.

Claims

1. A method for producing recycled styrene monomer, comprising a thermal decomposition step of thermally decomposing a waste styrene resin composition in the presence of a metal compound to obtain a thermal decomposition product containing recycled styrene monomer, wherein the metal compound comprises 1 to 25 parts by mass per 100 parts by mass of styrene monomer units in the waste styrene resin composition, and the metal compound is at least one selected from the group consisting of zinc compounds, iron compounds, potassium compounds, and sodium compounds.

2. The method according to claim 1, wherein the yield of the thermal decomposition product is 85% or more.

3. The method according to claim 2, wherein the increase in the amount of recycled styrene monomer obtained by thermally decomposing the waste styrene resin composition in the presence of the metal compound is 0.5% or more compared to the amount of recycled styrene monomer obtained by thermally decomposing the waste styrene resin composition in the absence of the metal compound.

4. The method according to claim 1, wherein the waste styrene resin composition further comprises polyethylene terephthalate.

5. The method according to any one of claims 1 to 4, wherein the metal compound is at least one selected from the group consisting of zinc oxide, iron oxide, potassium carbonate, and sodium bicarbonate.

6. The method according to claim 5, wherein the metal compound is zinc oxide and / or iron oxide.

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