Production method of (METH)acrylic monomer

Abstract

To provide a production method of a (meth)acrylic monomer for suppressing nonuniformity of a depolymerization system of a (meth)acrylic resin, and making microwaves easily reach the inside of the system efficiently.SOLUTION: A production method of a (meth)acrylic monomer includes steps of: a step 1 of irradiating a charged (meth)acrylic resin with microwaves followed by mixing so as to obtain a molten material containing a (meth)acrylic resin reduced in its molecular weight; a step 2 of adding an additional (meth)acrylic resin to the molten material followed by mixing so as to obtain a mixture containing the additional (meth)acrylic resin in the molten material; and a step 3 of irradiating the mixture with microwaves followed by mixing so as to obtain the (meth)acrylic monomer. With the production method, it is possible to suppress nonuniformity of a depolymerization system of the (meth)acrylic resin, and make microwaves easily reach the inside of the system efficiently.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a (meth)acrylic monomer. More specifically, the present invention relates to a technique for recovering a (meth)acrylic monomer from a (meth)acrylic resin. [Background technology]

[0002] Acrylic resin and methacrylic resin (hereinafter, "acrylic" and "methacrylic" will be collectively referred to as "(meth)acrylic") are amorphous thermoplastics with high transparency and impact resistance. Because (meth)acrylic resin is easy to process and color, it is used as a substitute for inorganic glass in a wide range of applications, including light guide panels for LCD televisions and LCD displays, lighting fixture covers, window materials for buildings and vehicles, motorcycle windshields, aircraft canopies, watch crystals, and aquariums.

[0003] The production volume of (meth)acrylic resin is expected to continue to increase in the future. Meanwhile, from the perspective of resource conservation, technology is being investigated to recycle the monomer by depolymerizing discarded (meth)acrylic resin, without producing the monomer from fossil fuels.

[0004] For example, Patent Document 1 describes a method for decomposing a (meth)acrylic resin by heating a solution in which a (meth)acrylic resin is dissolved to decompose the (meth)acrylic resin, characterized in that heating is performed while supplying an inert gas and / or water vapor into the solution. Specifically, Patent Document 1 describes heating using a jacket-type method in which a heat medium is passed, heating with an electric heater, or a burner method using fuel combustion.

[0005] Patent Document 2 also describes a method for recovering (meth)acrylic esters, which comprises dissolving or swelling a (meth)acrylic resin containing 50% by mass or more of (meth)acrylic ester units in a solvent having a boiling point of 250°C or higher under atmospheric pressure in a resin-containing liquid, irradiating the liquid with microwaves to decompose the (meth)acrylic resin, and separating the resulting (meth)acrylic esters. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-232966 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-230905 Summary of the Invention [Problem to be solved by the invention]

[0007] The technology described in Patent Document 1 requires a heating means and a means for supplying a predetermined gas, resulting in a complex facility. The technology described in Patent Document 2 is a simple depolymerization system that uses microwaves as a heating means, but requires the use of a solvent, which is problematic in that undesired by-products are generated when the solvent is heated.

[0008] (Meth)acrylic resins have a characteristic that their microwave absorption capacity is low at room temperature but increases with increasing temperature. Because their microwave absorption capacity is high near their depolymerization temperature (generally 280°C or higher), it is expected that microwaves can be used to efficiently heat and decompose (meth)acrylic resins. For this reason, the present inventors attempted depolymerization using a molten system of the raw (meth)acrylic resin using microwaves as a heating means and without the use of a solvent. However, upon actual investigation, the present inventors unexpectedly encountered the problem that the high microwave absorption capacity of (meth)acrylic resins, which should be inherently advantageous for depolymerization, resulted in localized microwave absorption at the surface of the melt, preventing microwaves from penetrating deep into the melt. As a result, microwaves were less likely to reach the internal (meth)acrylic resin, making uniform depolymerization difficult. Furthermore, these problems became more pronounced as the scale of the depolymerization system increased.

[0009] Therefore, an object of the present invention is to provide a method for producing a (meth)acrylic monomer that can suppress non-uniformity in the depolymerization system of a (meth)acrylic resin and efficiently promote depolymerization throughout the entire melt. [Means for solving the problem]

[0010] As a result of extensive investigation, the present inventors have found that by adding a new (meth)acrylic resin from outside the system to a melt containing a component with reduced microwave absorption ability, which is obtained by irradiating and mixing a (meth)acrylic resin with microwaves, and then irradiating microwaves to the melt while the (meth)acrylic resin is dispersed therein, the non-uniformity of the depolymerization system of the (meth)acrylic resin is suppressed and microwaves can more efficiently reach the interior of the system. The present invention was completed through further investigation based on this finding.

[0011] That is, the present invention provides the following aspects. Item 1. A step 1 of irradiating a charged (meth)acrylic resin with microwaves and mixing to obtain a melt containing a component having a lower microwave absorption capacity than the charged (meth)acrylic resin at a depolymerization temperature; Step 2: adding and mixing an additional (meth)acrylic resin to the melt to obtain a molten mixture containing the additional (meth)acrylic resin in the melt; and step 3 of irradiating the molten mixture with microwaves and mixing to obtain a (meth)acrylic monomer. Item 2. The method according to Item 1, wherein the amount of the (meth)acrylic resin charged in step 1 is 2.5 kg or more. Item 3. The method according to Item 1 or 2, wherein in step 1, the amount of the (meth)acrylic resin charged is 80 kg or more. Item 4. The method according to any one of Items 1 to 3, wherein steps 2 and 3 are repeated. Item 5. The production method according to any one of Items 1 to 4, wherein in step 2, the additional (meth)acrylic resin has no thermal history of being subjected to a depolymerization temperature. Item 6. The method according to any one of Items 1 to 5, wherein in Step 2, the amount of the additional (meth)acrylic resin added is 5 to 20 parts by weight per 100 parts by weight of the charged (meth)acrylic resin. Item 7. The method according to any one of Items 1 to 6, wherein in Step 2, the additional (meth)acrylic resin is added when the amount of the (meth)acrylic resin charged decreases by 1 to 20 parts by weight from 100 parts by weight. Item 8. The method according to any one of Items 1 to 7, wherein step 3 is carried out at a temperature of 300 to 360°C. Item 9. The method according to any one of Items 1 to 8, wherein the microwave has a frequency of 0.8 to 6 GHz. Item 10. The production method according to any one of Items 1 to 9, wherein steps 1 to 3 are carried out in a vessel having a side wall with an inverted conical bottom. [Effects of the Invention]

[0012] According to the present invention, there is provided a method for producing a (meth)acrylic monomer, which suppresses non-uniformity in the depolymerization system of a (meth)acrylic resin and makes it easier for microwaves to reach the inside of the system efficiently. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a schematic diagram showing an example of an apparatus used in the method for producing a (meth)acrylic monomer of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing another example of an apparatus used in the method for producing a (meth)acrylic monomer of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] 1.Basic process The method for producing a (meth)acrylic monomer of the present invention is characterized by comprising: step 1 of irradiating a charged (meth)acrylic resin with microwaves and mixing to obtain a melt containing a component having a lower microwave absorption ability than the charged (meth)acrylic resin at a depolymerization temperature; step 2 of adding an additional (meth)acrylic resin to the melt and mixing to obtain a molten mixture containing the additional (meth)acrylic resin in the melt; and step 3 of irradiating the molten mixture with microwaves and mixing to obtain a (meth)acrylic monomer.

[0015] Step 2 is carried out after step 1, and step 3 is carried out after step 2. Step 2 can be carried out without or with microwave irradiation, and is preferably carried out with microwave irradiation. The additional (meth)acrylic resin added in step 2 is depolymerized into a low-molecular-weight (meth)acrylic resin (a (meth)acrylic resin having a lower molecular weight than the additional (meth)acrylic resin) in the melt, with a time lag from the (meth)acrylic resin charged in step 1, and is decomposed into (meth)acrylic monomers as the depolymerization progresses.

[0016] It is preferable to repeat steps 2 and 3. In the repetition of steps 2 and 3, for example, a molten mixture to which an additional (meth)acrylic resin has been added is prepared in the previous step 2, and the additional (meth)acrylic resin is depolymerized into a lower molecular weight (meth)acrylic resin, and while the depolymerization progresses and the molten mixture is decomposed into (meth)acrylic monomers, a molten mixture to which a new additional (meth)acrylic resin has been added is prepared in the subsequent step 2. In other words, in the repetition of steps 2 and 3, the state in which a molten mixture containing a new (meth)acrylic resin is dispersed in the depolymerization system can be continuously renewed.

[0017] In a particularly preferred embodiment of the present invention, a (meth)acrylic monomer can be obtained by irradiating a charged (meth)acrylic resin with microwaves and mixing the resulting melt containing a component with low microwave absorption ability, and then continuously or intermittently adding additional (meth)acrylic resin while irradiating and mixing the melt with microwaves, and allowing depolymerization to proceed while mixing.

[0018] The depolymerization system used in the production method of the present invention is usually sealed, and the resulting monomer is in a gaseous state. The resulting monomer is liquefied and recovered by a cooling device provided in communication with the depolymerization system atmosphere.

[0019] In one embodiment of the present invention, from the viewpoint of suppressing an increase in the amount of impurities in the recovered monomer, it is preferable that the method does not further include a step of depolymerizing without adding an additional (meth)acrylic resin and / or a step of depolymerizing without stirring.

[0020] 2.Process 1 In step 1, a charged (meth)acrylic resin is irradiated with microwaves and mixed to obtain a melt containing a component having a lower microwave absorption capacity than the charged (meth)acrylic resin at the depolymerization temperature.

[0021] In step 1, "mixing while irradiating with microwaves" includes an embodiment in which mixing is performed while irradiating with microwaves, and an embodiment in which mixing is performed after irradiating with microwaves.

[0022] The (meth)acrylic resin charge refers to the (meth)acrylic resin as the material initially supplied to the depolymerization system. A specific embodiment of this (meth)acrylic resin is preferably a plastic material containing (meth)acrylic resin, more preferably a waste plastic material containing (meth)acrylic resin. The (meth)acrylic resin charge may be added all at once, or may be added in multiple batches from the viewpoint of temperature uniformity throughout the (meth)acrylic resin charge. When adding in multiple batches, the remaining (meth)acrylic resin charge is added until the target amount is reached when or before the first amount of (meth)acrylic resin charge reaches the depolymerization system temperature. For example, when the (meth)acrylic resin charge is added in two batches, the remaining second amount of (meth)acrylic resin charge is added when or before the first amount of (meth)acrylic resin charge reaches the depolymerization system temperature. For example, when the charged (meth)acrylic resin is added in three or more installments, the remaining charged (meth)acrylic resin is added in multiple installments until the target charge amount is reached when or before the first charged (meth)acrylic resin reaches the depolymerization system temperature. When the remaining charged (meth)acrylic resin is added in multiple installments, the next charged (meth)acrylic resin is added when or before the first charged (meth)acrylic resin and the second and subsequent charged (meth)acrylic resins as a whole reach the depolymerization system temperature.

[0023] Specific examples of the (meth)acrylic resin include polymers having, as structural units, (meth)acrylic monomers selected from the group consisting of acrylic acid, methacrylic acid, and esters thereof. Examples of acrylic acid esters include methyl acrylate, ethyl acrylate, propyl acrylate, and butyl acrylate, while examples of methacrylic acid esters include methyl methacrylate, ethyl methacrylate, propyl methacrylate, and butyl methacrylate. The (meth)acrylic resin may contain a structural unit derived from any one of the above (meth)acrylic monomers alone, or may contain a combination of structural units derived from two or more different types of the above (meth)acrylic monomers.

[0024] The content of structural units derived from (meth)acrylic monomers in the charged (meth)acrylic resin is preferably 70% by weight or more, more preferably 80% by weight or more, even more preferably 90% by weight or more, still more preferably 95% by weight or more, and most preferably 100% by weight. When the charged (meth)acrylic resin contains structural units derived from monomers other than (meth)acrylic monomers, examples of such other monomers include monomers selected from the group consisting of maleic anhydride, styrene, α-methylstyrene, and acrylonitrile.

[0025] The weight-average molecular weight of the (meth)acrylic resin to be charged is not particularly limited, but may be, for example, 80,000 to 2,000,000, and preferably 100,000 to 2,000,000. The weight-average molecular weight (Mw) is a value measured by gel permeation chromatography (GPC) in terms of standard polystyrene.

[0026] In the plastic material containing the (meth)acrylic resin, the blending ratio of the (meth)acrylic resin is not particularly limited, but is preferably 70% by weight or more, more preferably 80% by weight or more, even more preferably 90% by weight or more, and even more preferably 95% by weight or more. Furthermore, the ratio of the (meth)acrylic resin to the resin contained in the plastic material containing the (meth)acrylic resin is not particularly limited, but is preferably 70% by weight or more, more preferably 80% by weight or more, even more preferably 90% by weight or more, even more preferably 95% by weight or more, and most preferably 100% by weight. In the plastic material containing the (meth)acrylic resin, additives other than the resin may include dyes, pigments, inorganic fillers, etc.

[0027] The amount of (meth)acrylic resin to be charged is not particularly limited. On the other hand, the production method of the present invention can heat the depolymerization system with high uniformity, making it easier for microwaves to reach the interior of the system efficiently. Therefore, the effects of the present invention can be effectively achieved even in large-scale depolymerization systems, where the problem of uneven heating of the depolymerization system and difficulty in reaching the interior of the system becomes significant. From this perspective, preferred examples of the amount of (meth)acrylic resin to be charged in one embodiment of the present invention include 2.5 kg or more, 2.8 kg or more, kg or more, or 3.2 kg or more, more preferably more than 50 kg, even more preferably 80 kg or more, and even more preferably 100 kg or more. The upper limit of the amount of (meth)acrylic resin to be charged in the present invention is not particularly limited, but may be, for example, 2000 kg or less or 1000 kg or less. In addition, when the (meth)acrylic resin is charged in multiple batches, the charge amount of the (meth)acrylic resin refers to the total amount of the (meth)acrylic resin charged from the first batch to the multiple batches (i.e., the target charge amount). When the (meth)acrylic resin is charged in multiple batches, a preferred charge amount is more than 50 kg. In this case, the amount charged in the first batch is 50 kg or less, and then the remaining (meth)acrylic resin can be charged as needed to reach the target charge amount.

[0028] When microwaves are irradiated onto the charged (meth)acrylic resin, the resin is heated and melted. When the temperature reaches the depolymerization temperature, depolymerization proceeds partially at the points irradiated with microwaves, resulting in the generation of low-molecular-weight (meth)acrylic resins in the melt (meth)acrylic resin (i.e., (meth)acrylic resins having a smaller molecular weight than the charged (meth)acrylic resin), which are dispersed in the melt by a mixing operation.

[0029] The microwave irradiation power in step 1 is appropriately selected so as to heat the charged (meth)acrylic resin to a temperature at which depolymerization proceeds. In step 2, the temperature at which depolymerization of the (meth)acrylic resin proceeds is, for example, 280°C or higher, specifically 300 to 390°C, preferably 320 to 390°C, more preferably 325 to 370°C, even more preferably 330 to 360°C, and even more preferably 335 to 350°C.

[0030] The specific irradiation output of the microwaves irradiated to the depolymerization system in step 1 may be appropriately set depending on the set temperature, the amount of (meth)acrylic resin charged, etc., and may be, for example, 1 kW or more, preferably 1.2 kW or more, more preferably 1.5 kW or more in terms of effective output (the value obtained by subtracting the reflected wave (kW) from the incident wave (kW)). The upper limit of the irradiation output (effective output) is not particularly limited, but may be, for example, 200 kW or less.

[0031] The frequency of the microwaves irradiated to the depolymerization system in step 1 is not particularly limited, and examples thereof include 0.8 to 6 GHz. From the viewpoint of making it easier for the microwaves to reach the interior of the melt, the frequency is preferably 0.8 to 2.5 GHz, more preferably 0.8 to 1.5 GHz, and even more preferably 0.8 to 1 GHz or 0.9 to 0.95 GHz. Note that, since the production method of the present invention allows the microwaves to reach the interior of the melt, it is possible to allow the microwaves to reach the interior of the melt even when using microwaves with frequencies of more than 2.5 GHz and not more than 6 GHz or more than 1.5 GHz and not more than 6 GHz, particularly 4 to 6 GHz, which are not inherently advantageous in terms of deep penetration.

[0032] The mixing method in step 1 is not particularly limited, and examples thereof include a method of mixing with a stirrer, a method of rotating the container containing the molten material, a method of combining both of these methods, etc. The degree of mixing may be such that the composition of the molten material becomes uniform.

[0033] The melt obtained in step 1 contains components with lower microwave absorption capacity than the charged (meth)acrylic resin at the depolymerization temperature. The components with lower microwave absorption capacity are components generated by microwave irradiation of the charged (meth)acrylic resin melt, and although the specifics are unknown, they are likely to be the above-mentioned low-molecular-weight (meth)acrylic resin and / or by-products other than the low-molecular-weight (meth)acrylic resin (for example, components that are not converted to (meth)acrylic monomers and ultimately remain as residues). The presence of components with lower microwave absorption capacity than the charged (meth)acrylic resin at the depolymerization temperature can be confirmed by measuring the real part of the complex dielectric constant and the imaginary part of the complex dielectric constant for the melt to be confirmed and the charged (meth)acrylic resin at the same temperature (but within the depolymerization temperature range), and then calculating the microwave half-power depth based on the results.

[0034] 3.Process 2 In step 2, an additional (meth)acrylic resin is added to and mixed with the melt obtained in step 1 to obtain a molten mixture containing the additional (meth)acrylic resin in the melt.

[0035] In step 2, "adding and mixing an additional (meth)acrylic resin" includes an embodiment in which mixing is performed while adding the additional (meth)acrylic resin, and an embodiment in which mixing is performed after adding the additional (meth)acrylic resin.

[0036] The melt obtained in step 1 contains a component that has a lower microwave absorption capacity than the charged (meth)acrylic resin at the depolymerization temperature, and therefore the melt itself has a lower microwave absorption capacity than the charged (meth)acrylic resin at the depolymerization temperature. In step 2, additional (meth)acrylic resin is added to the melt with a lower microwave absorption capacity, mixed, and dispersed in the melt, allowing microwaves to reach the interior of the molten mixture.

[0037] In the present invention, the "additional (meth)acrylic resin" specified as being added in step 2 is added to a melt that has a lower microwave absorption ability than the melt of the charged (meth)acrylic resin, but it is also permissible to add another additional (meth)acrylic resin in a step other than step 2, separate from the "additional (meth)acrylic resin" added in step 2. An example of adding "another additional (meth)acrylic resin" in a step other than step 2 is adding "another additional (meth)acrylic resin" once or multiple times at a timing before obtaining "a melt containing a component having a lower microwave absorption ability than the charged (meth)acrylic resin at the depolymerization temperature" in step 1.

[0038] A specific embodiment of the additional (meth)acrylic resin is preferably a plastic material containing a (meth)acrylic resin, more preferably a waste plastic material containing a (meth)acrylic resin.

[0039] The type of additional (meth)acrylic resin, the content ratio of structural units derived from (meth)acrylic monomers, the type of other monomers, and the weight-average molecular weight can be selected from the examples given for the charged (meth)acrylic resin. The blending ratio of the (meth)acrylic resin in the plastic material containing the additional (meth)acrylic resin and the additional components that may be contained in addition to the resin can also be selected from the examples given for the plastic material containing the charged (meth)acrylic resin. The additional (meth)acrylic resin or the plastic material containing it may be the same as or different from the charged (meth)acrylic resin or the plastic material containing it.

[0040] The additional (meth)acrylic resin may be in a solid or molten state.

[0041] Furthermore, from the viewpoint of further suppressing non-uniformity in the depolymerization system of the (meth)acrylic resin and more efficiently promoting depolymerization throughout the melt, it is preferable that the additional (meth)acrylic resin does not have a thermal history of being exposed to a depolymerization temperature. In other words, from this viewpoint, it is preferable that the additional (meth)acrylic resin is not one that has been once subjected to the depolymerization system but one that has not been subjected to the depolymerization system is newly added.

[0042] Specific examples of the timing and amount (per addition) of the additional (meth)acrylic resin include adding the additional (meth)acrylic resin in an amount corresponding to the amount of decrease when the amount of the (meth)acrylic resin added decreases by preferably 20 parts by weight or less, more preferably 1 to 20 parts by weight, even more preferably 2 to 18 parts by weight, and still more preferably 3 to 15 parts by weight per 100 parts by weight of the added amount of (meth)acrylic resin.

[0043] More specifically, when the amount of (meth)acrylic resin charged is 1 kg or more but less than 50 kg, it is preferable to replenish the (meth)acrylic resin in an amount equivalent to the amount of decrease when the amount of charge has decreased by 1 to 15 parts by weight, preferably 3 to 15 parts by weight, from 100 parts by weight; when the amount of (meth)acrylic resin charged is 50 kg or more but less than 80 kg, it is preferable to replenish the (meth)acrylic resin in an amount equivalent to the amount of decrease when the amount of charge has decreased by 1 to 5 parts by weight, preferably 2.5 to 5 parts by weight, from 100 parts by weight. <In the case where the amount of (meth)acrylic resin to be charged is 80 kg or more but less than 400 kg, it is preferable to replenish additional (meth)acrylic resin in an amount equivalent to the amount of decrease when the amount of charge has decreased from 100 parts by weight by 1 to 4 parts by weight, preferably 2 to 4 parts by weight, and more preferably 3 to 4 parts by weight; and in the case where the amount of (meth)acrylic resin to be charged is 400 kg or more, it is preferable to replenish additional (meth)acrylic resin in an amount equivalent to the amount of decrease when the amount of charge has decreased from 100 parts by weight by 1 to 3 parts by weight, preferably 1.5 to 3 parts by weight.

[0044] Step 2 can be carried out without or with microwave irradiation, but is preferably carried out with microwave irradiation in order to avoid the complexity of control in the manufacturing process.

[0045] The temperature in step 2 may be any temperature at which the additional (meth)acrylic resin is at least melted, and may also be a temperature at which depolymerization can proceed, as mentioned in step 1. From the viewpoint of avoiding the complexity of control in the production process, the temperature conditions may be the same as those in step 1. Temperature control can be performed by irradiating microwaves.

[0046] The mixing method in step 2 is not particularly limited, and examples thereof include a method of mixing with a stirrer, a method of rotating the container containing the molten mixture itself, a method combining both of these methods, etc. The degree of mixing may be such that the composition of the molten mixture becomes uniform, that is, such that the additional (meth)acrylic resin is uniformly dispersed in the molten mixture.

[0047] 4.Process 3 In step 3, the molten mixture obtained in step 2 is irradiated with microwaves and mixed to obtain a (meth)acrylic monomer.

[0048] In step 3, "mixing while irradiating with microwaves" includes an embodiment in which mixing is performed while irradiating with microwaves, and an embodiment in which mixing is performed after irradiating with microwaves.

[0049] In the molten mixture obtained in step 2, the additional (meth)acrylic resin is dispersed in the molten mixture, where local microwave absorption on the surface of the molten mixture is suppressed and microwaves can reach the interior, which is thought to allow the depolymerization reaction to proceed throughout the molten mixture. Note that the depolymerization of the additional (meth)acrylic resin proceeds with a time lag relative to the charged (meth)acrylic resin. Furthermore, since step 3 is performed while mixing, the occurrence of locally excessively heated areas is suppressed, thereby reducing the amount of undesired by-products in the recovered monomer.

[0050] The microwave irradiation power in step 3 is appropriately selected so as to heat the mixture to a temperature at which depolymerization can proceed. The temperature at which depolymerization can proceed in step 3 is, for example, 280°C or higher, specifically 300 to 390°C, and preferably 320 to 390°C.

[0051] From the viewpoint of further improving the (meth)acrylic monomer recovery rate, the temperature at which depolymerization proceeds in step 3 is preferably 325 to 390°C, more preferably 330 to 390°C, even more preferably 335 to 390°C, still more preferably 340 to 390°C, still more preferably 345 to 390°C, and particularly preferably 350 to 390°C.

[0052] On the other hand, from the viewpoint of further improving the purity of the target (meth)acrylic monomer in the recovered monomer liquid, the temperature at which depolymerization proceeds in step 3 is preferably 320 to 380°C, more preferably 320 to 370°C, even more preferably 320 to 360°C, even more preferably 320 to 355°C, even more preferably 320 to 345°C, particularly preferably 320 to 342°C, and most preferably 320 to 340°C. The improvement in the purity of the target (meth)acrylic monomer in the recovered monomer liquid can also be confirmed by an increase in the weight ratio of the (meth)acrylic monomer in the recovered monomer liquid and / or a decrease in the amount of minor components that may not be easily reflected in the weight ratio. Examples of such minor components include colored components that can be confirmed visually or by chroma measurement, and / or methyl isobutyrate, which can be confirmed by chromatography.

[0053] From the viewpoint of avoiding the complexity of control in the production process, the temperature at which depolymerization proceeds in step 3 can be the same as the temperature conditions in step 1 and / or step 2. The temperature control can be carried out by irradiating microwaves.

[0054] The specific irradiation output of microwaves to the depolymerization system in step 3 may be appropriately set depending on the set temperature, the scale of the depolymerization system, etc., and may be, for example, 1 kW or more, preferably 1.2 kW or more, more preferably 1.5 kW or more in terms of effective output (the value obtained by subtracting the reflected wave (kW) from the incident wave (kW)). The upper limit of the irradiation output (effective output) is not particularly limited, but may be, for example, 200 kW or less.

[0055] The frequency of the microwaves irradiated to the depolymerization system in step 2 is not particularly limited, and examples thereof include 0.8 to 6 GHz. From the viewpoint of making it easier for the microwaves to reach the interior of the molten mixture, the frequency is preferably 0.8 to 2.5 GHz, more preferably 0.8 to 1.5 GHz, and even more preferably 0.8 to 1 GHz or 0.9 to 0.95 GHz. Note that, since the production method of the present invention allows the microwaves to reach the interior of the molten mixture, it is possible to allow the microwaves to reach the interior of the molten mixture even when using microwaves with frequencies of more than 2.5 GHz and not more than 6 GHz or more than 1.5 GHz and not more than 6 GHz, particularly 4 to 6 GHz, which are not inherently advantageous in terms of deep penetration.

[0056] The mixing method in step 3 is not particularly limited, and examples thereof include a method of mixing with a stirring blade, a method of rotating the container containing the molten mixture, a method combining both of these methods, etc. The degree of mixing may be such that the composition of the molten mixture becomes uniform.

[0057] 5. Repeat steps 2 and 3 In the embodiment in which steps 2 and 3 are repeated, the state in which a molten mixture containing an additional (meth)acrylic resin is dispersed in the molten material in a state in which local microwave absorption is suppressed and microwaves can reach the interior can be continuously renewed. That is, in the embodiment in which steps 2 and 3 are repeated, depolymerization can be maintained in which microwaves can reach the interior of the system uniformly and efficiently, so that metabolism of the depolymerization target occurs efficiently throughout the entire molten material, enabling efficient depolymerization even in a large-scale depolymerization system.

[0058] In the embodiment in which steps 2 and 3 are repeated, the timing of step 2 in which additional (meth)acrylic resin is added again is not particularly limited.

[0059] For example, the timing for adding additional (meth)acrylic resin in Step 2 may be the timing when an amount of monomer corresponding to the amount of additional (meth)acrylic resin added in the preceding Step 2 is recovered; specifically, when x parts by weight of additional (meth)acrylic resin is added per 100 parts by weight of the charged amount of (meth)acrylic resin in the nth Step 2, x parts by weight of additional (meth)acrylic resin per 100 parts by weight of the charged amount of (meth)acrylic resin can be added again in the (n+1)th Step 2 at the timing when a weight of monomer roughly corresponding to x parts by weight has been liquefied and recovered.

[0060] Furthermore, for example, from the viewpoint of improving the accuracy of temperature control in the vessel and making it easier to proceed with decomposition at a desired depolymerization temperature, the timing and additional addition amount of step 2 in which additional (meth)acrylic resin is added again is preferably such that, assuming that the total amount of (meth)acrylic resin in the vessel when the previous additional (meth)acrylic resin was added is 100 parts by weight, the additional (meth)acrylic resin is replenished in an amount equivalent to the amount of decrease when the total amount has decreased by 1 to 20 parts by weight from 100 parts by weight. More specifically, if the total amount of (meth)acrylic resin in the vessel when the previous additional (meth)acrylic resin was added is 1 kg or more but less than 50 kg, the additional (meth)acrylic resin is replenished in an amount equivalent to the amount of decrease when the total amount has decreased by 1 to 15 parts by weight from 100 parts by weight; if the total amount of (meth)acrylic resin in the vessel when the previous additional (meth)acrylic resin was added is 50 kg or more but less than 80 kg, the additional (meth)acrylic resin is replenished in an amount equivalent to the amount of decrease when the total amount has decreased by 1 to 5 parts by weight from 100 parts by weight. When the total amount of (meth)acrylic resin in the container when the previous additional (meth)acrylic resin was added was 80 kg or more but less than 400 kg, it is preferable to replenish the container with additional (meth)acrylic resin in an amount equivalent to the amount of decrease when the total amount has decreased by 1 to 4 parts by weight from 100 parts by weight; when the total amount of (meth)acrylic resin in the container when the previous additional (meth)acrylic resin was added was 400 kg or more, it is preferable to replenish the container with additional (meth)acrylic resin in an amount equivalent to the amount of decrease when the total amount has decreased by 1 to 3 parts by weight from 100 parts by weight.

[0061] 6. (Meth)acrylic monomer manufacturing equipment The configuration of the apparatus that can be used in the method for producing a (meth)acrylic monomer of the present invention is not particularly limited as long as it has a configuration that enables the above steps 1 to 3 to be carried out.

[0062] An example of an apparatus that can be used in the method for producing a (meth)acrylic monomer of the present invention is shown in Fig. 1. The apparatus shown in Fig. 1 includes a vessel 10 that provides a site for the depolymerization reaction; a stirring device 20 for stirring the reaction melt inside the vessel 10 with stirring blades 211, 212; a waveguide 30 that is provided in communication with the interior of the vessel 10 and sealed with an airtight window 31 for irradiating microwaves in the R1 direction; a vessel 40 that is provided in communication with the interior of the vessel 10 and for containing a plastic material to be added to the vessel 10 (in the R2 direction); a thermometer 50 for measuring the temperature of the reaction melt inside the vessel 10; a cooling device 60 (specifically, a spiral condenser was used, and the chiller temperature was set to 0°C) that is provided in communication with the interior of the vessel 10 and for cooling the monomer vaporized inside the vessel 10 via a flexible hose 61; and a vessel 70 for recovering the liquefied monomer cooled by the cooling device 60. In the example of Fig. 1, the bottom of the vessel 10 is rounded (dish-shaped with a curved shape that is convex in the depth direction), and the stirring blades 211, 212 are paddle-shaped on both sides (however, the surface direction of the paddle on one side and the paddle on the other side is parallel to the rotation axis direction in the stirring blade 211, and the surface directions of the paddle on the other side are orthogonal to each other in the stirring blade 212), and are arranged at different heights in the vertical direction (the up and down direction in Fig. 1), and are arranged orthogonal when viewed in the vertical direction. The lower stirring blade 211 has a shape that follows the shape of the bottom inner wall when viewed in the horizontal direction, enabling efficient stirring, while the upper stirring blade 211 has an arbitrary shape (for example, a rectangular flat plate shape).

[0063] Another example of an apparatus that can be used in the method for producing a (meth)acrylic monomer of the present invention is shown in Figure 2. The apparatus shown in Figure 2 differs from the apparatus shown in Figure 1 at least in the shapes of the container 10a and the stirring blade 211a. The container 10a has a bottom with an inverted conical sidewall. By having such a bottom shape, compared to the container 10 in the apparatus of Figure 1, charges are concentrated at the bottom (tip) of the container 10a, and the electric field strength is increased in that vicinity, allowing microwaves to penetrate deeper and enabling a more uniform reaction. [Example]

[0064] The present invention will be explained in more detail below by showing examples and comparative examples, but the present invention is not limited to these.

[0065] [Example 1] (1) Plastic materials Polymethyl methacrylate (hereinafter, also referred to as PMMA) was used as the plastic material containing the (meth)acrylic resin to be subjected to depolymerization. This PMMA is a polymer obtained by polymerizing methyl methacrylate (MMA) and methyl acrylate (MA) in a weight ratio of MMA:MA = 98:2, and its weight-average molecular weight was 114,190 (measured by GPC using tetrahydrofuran as the developing solvent and converted into standard polystyrene). This plastic material was composed of 100% PMMA. This plastic material was also used as the charged (meth)acrylic resin and the additional (meth)acrylic resin.

[0066] (2) Monomer production equipment A schematic diagram of the apparatus used to produce monomer from plastic material is shown in Figure 1. The apparatus includes: a vessel 10 that provides a site for the depolymerization reaction; a stirring device 20 for stirring the reaction melt inside the vessel 10 with stirring blades 211, 212; a waveguide 30 that is provided to communicate with the interior of the vessel 10 and sealed with an airtight window 31 for irradiating microwaves in the R1 direction; a vessel 40 that is provided to communicate with the interior of the vessel 10 and for containing plastic material to be added to the vessel 10 (in the R2 direction); a thermometer 50 for measuring the temperature of the reaction melt inside the vessel 10; a cooling device 60 (specifically, a spiral condenser was used, with the chiller temperature set to 0°C) that is provided to communicate with the interior of the vessel 10 and for cooling the monomer vaporized inside the vessel 10 via a flexible hose 61; and a vessel 70 for recovering the liquefied monomer cooled by the cooling device 60. The bottom of the vessel 10 is rounded (dish-shaped with a curved shape that is convex in the depth direction), and the agitating blades 211, 212 are paddle-shaped on both sides (however, the surface directions of the paddles on one side and the paddles on the other side are parallel to the rotation axis direction in the agitating blade 211, and are orthogonal to each other in the agitating blade 212), and are arranged at different heights in the vertical direction (the up and down direction in FIG. 1), and are arranged orthogonal when viewed in the vertical direction. The paddle of the lower agitating blade 211 has a shape that follows the shape of the inner wall of the bottom when viewed in the horizontal direction, and the paddle of the upper agitating blade 211 has a rectangular flat plate shape.

[0067] (3) Operation procedure 2.8 kg of PMMA (first batch of PMMA) was placed into an empty container 10. While stirring with a stirrer 20, microwaves (2.45 GHz microwaves; the same applies below) generated by a microwave oscillator (not shown) were applied to the PMMA in container 10 via a waveguide 30, melting the PMMA. Stirring and microwave irradiation were continued while measuring the temperature with a thermometer 50, and the temperature rose to 340°C. While stirring and microwave irradiation were continued, 0.4 kg of PMMA (second batch of PMMA) was added from container 40 to container 10. The monomer gas generated in container 10 was cooled and liquefied by a cooling device 60 via a flexible hose 61 and collected in container 70. While stirring and microwave irradiation were continued, approximately 0.4 kg of PMMA (additional PMMA) was added from container 40 every time the weight of the monomer collected in container 70 increased by approximately 0.4 kg. The monomer recovered in vessel 70 was removed (aliquoted) in approximately 0.4 kg increments when additional PMMA was added, and then pooled after analysis (5) described below. The additional PMMA was added approximately every 10 minutes, continuing until approximately 300 minutes after the initial PMMA melting. The total amount of additional PMMA added was 11.2 kg. Continuous stirring throughout microwave irradiation maintained a uniform temperature within vessel 10 and facilitated microwave penetration into the interior of the reaction melt. The effective microwave power during depolymerization was 1.5–2.3 kW (average 1.7 kW). Furthermore, because additional PMMA was added at a fairly constant frequency of approximately every 10 minutes from the initial PMMA melting until approximately 300 minutes after the initial PMMA melting, good reaction efficiency was maintained throughout.

[0068] (4) Analysis of decomposition rate The decomposition rate (%), that is, (AR) / R×100, was calculated when the total weight of the charged PMMA and the added PMMA was A (g) and the weight of the residue in the container 10 was R (g). The decomposition rate in this example was 80.4%.

[0069] (5) Analysis of recovered materials The monomer liquid collected in container 70 was diluted 100-fold with acetone solvent to a concentration of 1.0 wt %, and the resulting diluted liquid was subjected to an absolute calibration curve method using a gas chromatograph (GC-FID, Shimadzu GC-2010) to measure the content of methacrylic monomer (methyl methacrylate; MMA) in the collected monomer liquid. The MMA content of the first collected (fractionated) monomer liquid in container 70 was 99.19 wt %. As the decomposition progressed, the MMA content in the monomer liquid collected (fractionated) in container 70 decreased, and at a decomposition rate of 80.4%, the MMA content of the monomer liquid collected (fractionated) in container 70 was 96.35 wt %.

[0070] Furthermore, in the GC analysis using the above-mentioned diluted solution, the peak area value of methyl isobutyrate (MIBA), one of the by-products in the monomer liquid recovered in container 70, was read as the MIBA content. Of the MIBA contents in the monomer liquid recovered in container 70, the MIBA content in the monomer liquid recovered (fractionated) initially was 625 in terms of peak area value. As the decomposition progressed, the MIBA content in the monomer liquid recovered (fractionated) in container 70 increased, and at the time when the decomposition rate reached 80.4%, the MIBA content in the monomer liquid recovered (fractionated) in container 70 was 1,184 in terms of peak area value.

[0071] In addition, 0.40 g of the residue obtained by depolymerization to a decomposition rate of 80.4% was used under the following measurement conditions to calculate the real part e' of the complex dielectric constant, the imaginary part e'' of the complex dielectric constant, and the microwave half-power depth derived from them. The results are shown in Table 1. Table 1 also shows the results of measurements of the charged PMMA under the same conditions for comparison. The weight-average molecular weight of the residue was 27,263.

[0072] (Measurement conditions) Equipment used: Cavity resonator (Linghe Electronics) Frequency: 915MHz Measurement temperature: 340℃ Quartz tube: f 10 mm × 8 mm N2: 100 mL / min

[0073] [Table 1]

[0074] As shown in Table 1, it was confirmed that the residual components generated by depolymerization had a lower imaginary part e'' of the complex dielectric constant and a deeper microwave half-power depth compared to the starting PMMA. From this, it can be reasonably inferred that, when any of the additional PMMA added repeatedly to the depolymerization system was added, the residual components with low microwave absorption capacity accumulated in the melt, reducing the microwave absorption capacity of the melt itself. This enabled microwaves to penetrate deeper into the melt, suppressing the heterogeneity of the depolymerization system and enabling efficient depolymerization.

[0075] [Example 2] The same materials and equipment as in Example 1 were used, and the same amount of PMMA was charged as in Example 1. The heating temperature using microwaves (frequency 2.45 GHz) was 350°C, the effective microwave power during depolymerization was 2.1 to 2.5 kW (average 2.2 kW), and additional PMMA (0.4 kg per addition) was added approximately every 8 minutes, continuing until approximately 250 minutes after the start of melting the charged PMMA. The total amount of additional PMMA added was 12.8 kg. The decomposition rate and recovered materials were analyzed as in Example 1. The temperature of the reaction melt in the vessel 10 was kept uniform, allowing microwaves to reach the interior of the reaction melt. Furthermore, since additional PMMA was added at a fairly constant frequency of approximately every 8 minutes from the start of melting the charged PMMA until approximately 250 minutes after the start of melting, good reaction efficiency was maintained throughout.

[0076] The decomposition rate in this example was 80.0%. Of the MMA contents in the monomer liquid recovered in container 70, the MMA content in the first recovered (separated) monomer liquid was 98.81 wt%. As the decomposition progressed, the MMA content in the monomer liquid recovered (separated) in container 70 decreased, and the MMA content in the monomer liquid recovered (separated) in container 70 at the time when the decomposition rate was 80.0% was 95.34 wt%. Furthermore, when all of the monomer liquids recovered from the first recovered monomer liquid to the time when the decomposition rate was 80.0% were combined, the average MMA content was 97.26 wt%.

[0077] Of the MIBA contents in the monomer liquid recovered in container 70, the MIBA content in the first recovered (fractionated) monomer liquid was 672. As the decomposition progressed, the MIBA content in the monomer liquid recovered (fractionated) in container 70 increased, and the relative MIBA content in the monomer liquid recovered (fractionated) in container 70 at a decomposition rate of 80.0% was 1,475. Furthermore, when all of the monomer liquids from the first recovered monomer liquid to the monomer liquid recovered at a decomposition rate of 80.0% were combined, the average MIBA content was 1,063.

[0078] [Example 3] In Example 2, additional PMMA was added until approximately 250 minutes after the start of melting of the charged PMMA, and then depolymerization was continued without adding additional PMMA while continuing stirring and microwave irradiation (frequency 2.45 GHz) until a decomposition rate of 95.4% was reached. The decomposition rate and recovered materials were analyzed in the same manner as in Example 1.

[0079] In this example, the MMA content in the monomer liquid recovered (fractionated) in the container 70 at the time when the decomposition rate reached 95.4% was 78.28% by weight. Furthermore, when all of the monomer liquids recovered from the first recovered monomer liquid to the time when the decomposition rate reached 95.4% were combined, the average MMA content was 96.25% by weight.

[0080] The MIBA content in the monomer liquid recovered (fractionated) in the container 70 when the decomposition rate in this example reached 95.4% was 24,916. Furthermore, when all of the monomer liquids recovered from the first recovered monomer liquid to the monomer liquid recovered when the decomposition rate reached 95.4% were combined, the average MIBA content was 1778.

[0081] [Example 4] 50 kg of PMMA (first batch of PMMA), the same material as in Example 1, was placed into an empty container 10. While stirring with a stirrer 20, microwaves (915 MHz microwaves) generated by a microwave oscillator (not shown) were irradiated through a waveguide 30 onto the PMMA in the container 10, melting the PMMA. Stirring and microwave irradiation were continued while measuring the temperature with a thermometer 50, and the temperature was raised to 350°C. While continuing stirring and microwave irradiation, 40 kg of PMMA (second batch of PMMA) was added from container 40 to container 10, bringing the total amount of PMMA charged to 90 kg (target amount). The monomer gas generated in the container 10 was cooled and liquefied by a cooling device 60 via a flexible hose 61 and collected in a container 70. While continuing stirring and microwave irradiation, PMMA (additional PMMA) was added from vessel 40 in approximately 3 kg increments each time the weight of PMMA in vessel 70 decreased by approximately 3 kg. This process was repeated. The monomer collected in vessel 70 was removed (aliquoted) every time approximately 40 kg accumulated, and after the analysis described in (5) above, it was pooled. The operation of adding additional PMMA was carried out approximately once every 5 minutes and continued until approximately 360 minutes after the initial PMMA melting. The total amount of additional PMMA added was 250 kg. Stirring was continued throughout microwave irradiation to maintain a uniform temperature in vessel 10 and facilitate microwave penetration into the interior of the reaction melt. The effective microwave power during depolymerization was 20 to 24 kW (average 22 kW). Furthermore, from the time of melting of the charged PMMA until about 360 minutes later, additional PMMA was added at a nearly constant frequency of about once every 5 minutes, so good reaction efficiency was maintained throughout. Furthermore, the decomposition rate and recovered materials were analyzed in the same manner as in Example 1.

[0082] The decomposition rate in this example was 95.1%, and the MMA content in the monomer liquid recovered (separated) initially in the monomer liquid recovered in container 70 was 91.67% by weight. As the decomposition progressed, the MMA content in the monomer liquid recovered (separated) in container 70 decreased, and the MMA content in the monomer liquid recovered (separated) in container 70 at the time when the decomposition rate reached 95.1% was 89.46% by weight.

[0083] In this example, the MIBA content in the monomer liquid initially collected (separated) in the container 70 was 0.06 wt % of the total MIBA content in the monomer liquid collected (separated) in the container 70. As the decomposition progressed, the MIBA content in the monomer liquid collected (separated) in the container 70 increased, and when the decomposition rate reached 95.1%, the MIBA content in the monomer liquid collected (separated) in the container 70 was 0.14 wt %.

[0084] [summary] The conditions and results of Examples 1 to 4 above are shown in Table 2 below.

[0085] [Table 2]

[0086] As shown in Examples 1 to 4, a high monomer recovery rate was achieved by depolymerizing PMMA by microwave heating while adding additional PMMA. Furthermore, as shown in Example 3, in which a depolymerization step without adding additional PMMA was additionally performed, compared to Example 2, in which additional PMMA was constantly added until the end of depolymerization, adding a depolymerization step without adding additional PMMA promoted PMMA decomposition but increased the amount of by-products. In other words, it was found that by performing PMMA depolymerization by microwave heating while adding additional PMMA, the heterogeneity of the PMMA depolymerization system was suppressed and microwaves were more efficiently able to reach the interior of the system, thereby suppressing the generation of by-products and improving the yield of the target product. Since the effect of performing the step of depolymerizing PMMA by microwave heating while adding additional PMMA is so remarkably excellent, even when an additional depolymerization step without adding additional PMMA was added, as shown in Comparative Example 3, an excellent yield of the target product was achieved overall (although not as good as in Example 2).Furthermore, as shown in Example 4, even when the reaction scale was extremely increased, the amount of by-products was kept extremely constant considering the reaction scale, and an excellent yield of the target product was achieved.

[0087] [Example 5] The same operations as in Example 1 were carried out except that the same materials as in Example 1 were used, the container was a 100 mL container, the amount of PMMA charged was 10 g, the amount of additional PMMA added was 1 g per addition, the total number of additions was 100, and the heating temperature for depolymerization using microwaves (frequency 915 MHz) was changed in the following order: 350°C (at the 1st to 60th additions), 375°C (at the 61st to 90th additions), and 360°C (at the 91st to 100th additions). The monomer liquids recovered (aliquoted) in container 70 at the 40th, 50th, and 60th additions, the 70th, 80th, and 90th additions, and the 100th addition were measured for saturation c as follows. * was measured.

[0088] The monomer liquid collected in the container 70 was measured using a 1 cm quartz cell with an ultraviolet, visible, and infrared spectrophotometer (V-600 manufactured by JASCO Corporation) to determine the chromaticity value (L * a * b * The degree of yellow coloration was calculated as chroma c * Saturation c * The larger the value, the stronger the coloring. The results are shown in Table 3.

[0089]

number

[0090] [Table 3]

[0091] As is clear from Table 3, a significant decrease in saturation was observed when the depolymerization temperature was 360° C. compared to 375° C. Furthermore, a decrease in saturation was also observed when the depolymerization temperature was 350° C. compared to 360° C. In other words, it was observed that coloration decreased in the order 375° C., 360° C., and 350° C., and therefore it was observed that the production of minor components related to coloration was suppressed in this order.

Claims

Claim 1 Step 1 of obtaining a melt containing a component having a lower microwave absorption ability than the charged (meth)acrylic resin at the depolymerization temperature by irradiating and mixing the charged (meth)acrylic resin with microwaves; Step 2 of adding and mixing an additional (meth)acrylic resin to the melt to obtain a molten mixture containing the additional (meth)acrylic resin in the melt; Step 3 of irradiating and mixing the molten mixture with microwaves to obtain a (meth)acrylic monomer, and A method for producing a (meth)acrylic monomer, wherein the said Step 2 and Step 3 are repeated.

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