Method for producing cyclic lactam

Abstract

Provided is a method for producing a cyclic lactam. A method for producing a cyclic lactam comprising subjecting a thermoplastic resin including a polyamide resin containing a repeating unit represented by formula (a) to a depolymerization reaction with a polyphosphoric acid, wherein the amount of the polyphosphoric acid is 1 part by mass or more relative to 100 parts by mass of the polyamide resin containing the repeating unit represented by formula (a), and the thermoplastic resin contains 6 to 90 parts by mass of a linear polyolefin resin relative to 100 parts by mass of the polyamide resin containing the repeating unit represented by formula (a). In formula (a), n1 is an integer of 3 to 22. The value of n1 may be different for each repeating unit.

Description

Method for producing cyclic lactams

[0001] The present invention relates to a method for producing a cyclic lactam, and more particularly to a method for producing a cyclic lactam by depolymerization of a polyamide resin.

[0002] Depolymerizing polyamide resins and recovering cyclic lactams have been studied for some time. For example, Patent Document 1 discloses a method for recovering ε-caprolactone and ε-caprolactam from a mixture containing an oligomer of ε-oxycaproic acid amide and ε-caprolactam, which comprises contacting the mixture with steam at a temperature of 200 to 360°C in the presence of a phosphoric acid. Furthermore, Patent Document 2 discloses a method for purifying ε-caprolactam, which comprises depolymerizing a thermoplastic material containing nylon 6 as a main component to obtain recovered ε-caprolactam and rectifying the recovered ε-caprolactam.

[0003] JP-A No. 49-132094 JP-A No. 08-217746

[0004] Here, Patent Document 1 describes the use of phosphoric acids such as orthophosphoric acid, orthophosphoric acid, pyrophosphoric acid, and ammonium phosphate as catalysts in the depolymerization of polyamide resins. Patent Document 2 also describes the use of phosphoric acid to depolymerize polyamide resins. However, the inventors' investigations have revealed that the use of such phosphoric acids results in a low yield of the product caprolactam. The present invention aims to solve this problem and to provide a method for producing a cyclic lactam that can produce a cyclic lactam with high selectivity.

[0005] As a result of investigations conducted by the present inventors in light of the above-mentioned problems, the above-mentioned problems have been solved by the following means: <1> A method for producing a cyclic lactam, which comprises using polyphosphoric acid to depolymerize a thermoplastic resin containing a polyamide resin containing a repeating unit represented by formula (a), wherein the amount of polyphosphoric acid is 1 part by mass or more per 100 parts by mass of the polyamide resin containing a repeating unit represented by formula (a), and the thermoplastic resin contains 6 to 90 parts by mass of a linear polyolefin resin per 100 parts by mass of the polyamide resin containing a repeating unit represented by formula (a). (In formula (a), n1 is an integer of 3 to 22. The value of n1 may be different for each repeating unit.) <2> The method for producing a cyclic lactam according to <1>, wherein the linear polyolefin resin contains high-density polyethylene (HDPE) and / or polypropylene. <3> The method for producing a cyclic lactam according to <1> or <2>, wherein the thermoplastic resin contains 6 to 30 parts by mass of a linear polyolefin resin per 100 parts by mass of a polyamide resin. <4> The method for producing a cyclic lactam according to any one of <1> to <3>, wherein 40 parts by mass or more of the polyphosphoric acid is used per 100 parts by mass of the polyamide resin. <5> The method for producing a cyclic lactam according to any one of <1> to <4>, wherein 50 to 10,000 parts by mass of the polyphosphoric acid is used per 100 parts by mass of the polyamide resin. <6> The method for producing a cyclic lactam according to any one of <1> to <5>, wherein the depolymerization reaction is carried out at a pressure of 202.6 kPa or less. <7> The method for producing a cyclic lactam according to any one of <1> to <6>, wherein reactive distillation is carried out while the depolymerization reaction is being carried out. <8> The method for producing a cyclic lactam according to any one of <1> to <7>, wherein distillation is carried out after the depolymerization reaction. <9> The method for producing a cyclic lactam according to any one of <1> to <8>, wherein the depolymerization reaction is carried out at a temperature of 170°C to 300°C. <10> The method for producing a cyclic lactam according to any one of <1> to <9>, wherein superheated steam is not used when the depolymerization reaction is carried out. <11> The method for producing a cyclic lactam according to any one of <1> to <10>, wherein no solvent is used when the depolymerization reaction is carried out, or the amount of the solvent is 10% by mass or less relative to the amount of the polyphosphoric acid. <12> The method for producing a cyclic lactam according to any one of <1> to <11>, wherein the depolymerization reaction is carried out in the presence of a high-boiling point solvent, and the boiling point of the high-boiling point solvent is at least 10°C below the boiling point of the cyclic lactam product. <13> The method for producing a cyclic lactam according to <12>, wherein the high-boiling point solvent is not an alcohol.<14> The method for producing a cyclic lactam according to any one of <1> to <13>, wherein reactive distillation is carried out while the depolymerization reaction is carried out, the depolymerization reaction is carried out at a temperature of 170°C to 300°C, superheated steam is not used when carrying out the depolymerization reaction, and no solvent is used when carrying out the depolymerization reaction, or the amount of the solvent is 10 mass% or less relative to the amount of the polyphosphoric acid. <15> The method for producing a cyclic lactam according to any one of <1> to <14>, wherein reactive distillation is carried out while the depolymerization reaction is carried out, the depolymerization reaction is carried out at a temperature of 170°C to 300°C, superheated steam is not used when carrying out the depolymerization reaction, the depolymerization reaction is carried out in the presence of a high-boiling point solvent, the boiling point of which is at least 10°C lower than the boiling point of the cyclic lactam product, and the high-boiling point solvent is not an alcohol. <16> The method for producing a cyclic lactam according to any one of <1> to <15>, wherein distillation is carried out after the depolymerization reaction, the depolymerization reaction is carried out at a temperature of 170°C to 300°C, superheated steam is not used when carrying out the depolymerization reaction, and no solvent is used or the amount of the solvent is 10 mass% or less relative to the amount of the polyphosphoric acid when carrying out the depolymerization reaction. <17> The method for producing a cyclic lactam according to any one of <1> to <16>, wherein distillation is carried out after the depolymerization reaction, the depolymerization reaction is carried out at a temperature of 170°C to 300°C, superheated steam is not used when carrying out the depolymerization reaction, the depolymerization reaction is carried out in the presence of a high-boiling point solvent, the boiling point of which is at least 10°C lower than the boiling point of the cyclic lactam product, and the high-boiling point solvent is not an alcohol. <18> The method for producing a cyclic lactam according to any one of <1> to <17>, wherein the molecular weight of the polyphosphoric acid is 177 to 10,000. <19> The method for producing a cyclic lactam according to any one of <1> to <18>, wherein the polyamide resin is at least one selected from the group consisting of nylon 4, nylon 5, nylon 6, nylon 11, and nylon 12. <20> The method for producing a cyclic lactam according to any one of <1> to <19>, wherein the polyamide resin is continuously supplied to a reaction system to produce the cyclic lactam. <21> The method for producing a cyclic lactam according to any one of <1> to <20>, wherein the polyphosphoric acid is supplied directly to the reaction system.<22> The method for producing a cyclic lactam according to <21>, wherein the water content of the polyphosphoric acid supplied to the reaction system is 100% by mass or less based on the mass of the polyphosphoric acid. <23> The method for producing a cyclic lactam according to <21> or <22>, wherein polyphosphoric acid (B2) having a higher degree of polymerization than the polyphosphoric acid supplied to the reaction system is hydrolyzed to obtain regenerated polyphosphoric acid (B3). <24> The method for producing a cyclic lactam according to any one of <21> to <23>, wherein polyphosphoric acid (B2) having a higher degree of polymerization than the polyphosphoric acid supplied to the reaction system is hydrolyzed to obtain regenerated polyphosphoric acid (B3) in the reaction system. <25> The method for producing a cyclic lactam according to <23> or <24>, wherein the polyphosphoric acid (B3) is used as the polyphosphoric acid supplied to the reaction system.

[0006] According to the present invention, it is possible to provide a method for producing a cyclic lactam that can produce a cyclic lactam with high selectivity.

[0007] FIG. 1 shows a schematic diagram of an example of an apparatus for producing a cyclic lactam according to this embodiment.

[0008] Hereinafter, an embodiment of the present invention (hereinafter simply referred to as "the present embodiment") will be described in detail. Note that the following present embodiment is an example for explaining the present invention, and the present invention is not limited to this embodiment. Note that in this specification, the word "to" is used to mean that the numerical values ​​before and after it are included as the upper and lower limits. Furthermore, any combination of the upper and lower limit values ​​of the numerical values ​​in this specification is cited as an example of the present embodiment.

[0009] In this specification, unless otherwise specified, the number average molecular weight is a value measured by the following method. The number average molecular weight (Mn) of polyphosphoric acid is determined by separating and quantifying the components constituting polyphosphoric acid by ion chromatography. An anion exchange packed column is used as the column, and an aqueous potassium hydroxide solution is used as the eluent. Measurements are performed using an electrical conductivity detector with a sample injection volume of 100 μL, a column temperature of 35°C, an eluent flow rate of 1.0 mL / min, and an electrical conductivity detector. The abundance ratio of each component is determined by calculating the ratio of the area of ​​each peak, assuming the sum of the areas of all peaks to be 100%. In this specification, the boiling point refers to the boiling point at a pressure of 101.33 kPa, unless otherwise specified. In this specification, the melting point refers to a value measured by differential scanning calorimetry (DSC) in accordance with ISO 11357, unless otherwise specified. Using a differential scanning calorimeter, the resin is placed in the measurement pan of the differential scanning calorimeter, heated to a temperature above the melting point at a heating rate of 10 ° C. / min under a nitrogen atmosphere, and then rapidly cooled before measurement. The measurement conditions are to heat the resin to 200 ° C. at a heating rate of 10 ° C. / min, hold at 200 ° C. for 5 minutes, then cool the resin to 20 ° C. at a heating rate of 10 ° C. / min, and measure the melting point. The differential scanning calorimeter used is a "DSC7020" manufactured by Hitachi High-Tech Science Corporation.

[0010] In this specification, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended effect of the process is achieved. If the measurement methods, etc. described in the standards shown in this specification change from year to year, they will be based on the standards as of January 1, 2024, unless otherwise specified. If the measurement methods, etc. described in the standards shown in this specification are abolished as of January 1, 2024, they will be based on the standards at the time of abolition. The scale of Figure 1 may not be consistent with reality.

[0011] The method for producing a cyclic lactam of this embodiment involves using polyphosphoric acid to depolymerize a thermoplastic resin containing a polyamide resin containing a repeating unit represented by formula (a) (hereinafter, may be referred to as "polyamide resin (a)"), wherein the amount of polyphosphoric acid is 1 part by mass or more per 100 parts by mass of the polyamide resin containing a repeating unit represented by formula (a), and the thermoplastic resin contains 6 to 90 parts by mass of a linear polyolefin resin per 100 parts by mass of the polyamide resin containing a repeating unit represented by formula (a). (In formula (a), n1 is an integer of 3 to 22. The value of n1 may be different for each repeating unit.) By using such a structure, it becomes possible to produce a cyclic lactam with high selectivity.

[0012] The present inventors have investigated a method for producing a cyclic lactam, in which a depolymerization reaction is carried out using 1 part by mass or more of polyphosphoric acid per 100 parts by mass of a polyamide resin containing a repeating unit represented by formula (a). (In formula (a), n1 is an integer of 3 to 22. The value of n1 may be different for each repeating unit.)

[0013] In the above-mentioned method for producing cyclic lactams, the depolymerization reaction usually proceeds when polyphosphoric acid is in a liquid state and polyamide resin (a) is dissolved. That is, it is presumed that the depolymerization reaction proceeds not through hydrolysis via water, but through a backbiting reaction in which the terminal amino group or the amide bond in the main chain of polyamide resin (a) attacks the amide bond one unit adjacent to itself. In addition, since polyphosphoric acid has mild acidity, its bonding strength with amino groups is weaker than that of general acids, and it is easy to form a cyclic amide structure during the depolymerization reaction. That is, since polyphosphoric acid serves as both a solvent and an acid catalyst, it is presumed that by carrying out the depolymerization reaction using 1 part by mass or more of polyphosphoric acid per 100 parts by mass of polyamide resin containing a repeating unit represented by formula (a), it becomes possible to produce cyclic lactams with high selectivity.

[0014] Under these circumstances, the present inventors conducted further studies and found that a cyclic lactam can be produced in a higher yield by blending a linear polyolefin resin with the polyamide resin (a) and carrying out the depolymerization reaction. The reasons for this are presumed to be that the linear polyolefin resin acts as a high-boiling point plasticizer, reducing the viscosity of the polyphosphoric acid, and that the molten linear polyolefin resin has lower elasticity than a highly branched polyolefin resin, resulting in better compatibility with polyphosphoric acid. In other words, it is presumed that the use of a linear polyolefin resin rather than a branched polyolefin resin among polyolefin resins results in better compatibility with polyphosphoric acid. The present invention will be described in detail below.

[0015] <Thermoplastic Resin> In this embodiment, the thermoplastic resin includes a polyamide resin (polyamide resin (a)) containing a repeating unit represented by formula (a) and a linear polyolefin resin. In addition, the thermoplastic resin may include other thermoplastic resins within the scope of this embodiment.

[0016] <<Polyamide Resin (a)>> <Polyamide Resin (Polyamide Resin (a)) Comprising a Repeating Unit Represented by Formula (a)>> In the present embodiment, the polyamide resin (a) used in the depolymerization reaction contains a repeating unit represented by formula (a). (In formula (a), n1 is an integer of 3 to 22. The value of n1 may be different for each repeating unit.) n1 is preferably 4 or more, more preferably 5 or more, and preferably 20 or less, more preferably 18 or less, even more preferably 16 or less, even more preferably 14 or less, even more preferably 12 or less, even more preferably 10 or less, even more preferably 8 or less, and particularly preferably 6 or less. The proportion of repeating units represented by formula (a) in polyamide resin (a) is preferably 80% by mass or more of the total repeating units of polyamide resin (a), more preferably 90% by mass or more, even more preferably 95% by mass or more, and even more preferably 99% by mass or more. In addition, polyamide resin (a) may have a terminal group consisting of -NH ２ and / or —COOH, or may be end-capped with an end-capping agent.

[0017] The polyamide resin (a) is preferably at least one selected from the group consisting of nylon 4, nylon 5, nylon 6, nylon 11, and nylon 12, and more preferably nylon 6. The polyamide resin (a) may be synthesized from a cyclic lactam or an aminocarboxylic acid, but is preferably synthesized from a cyclic lactam. The nylon 6 in this embodiment also includes nylon 6 in which a portion (for example, 5% by mass or less, further 3% by mass or less, particularly 1% by mass or less) has been modified with another monomer. The same applies to other nylons.

[0018] In the method for producing a cyclic lactam of this embodiment, only one type of polyamide resin (a) may be used, or two or more types may be used.

[0019] The viscosity average molecular weight of the polyamide resin (a) is preferably 3,000 or more, more preferably 5,000 or more, even more preferably 10,000 or more, and may be 50,000 or more, and is preferably 1,000,000 or less, more preferably 500,000 or less, and even more preferably 100,000 or less. The viscosity average molecular weight is measured in accordance with JIS K7367.

[0020] <<Linear Polyolefin Resin>> The linear polyolefin resin used in this embodiment is not particularly limited in terms of type. Here, linear polyolefin resins include, for example, polyolefin resins with a linear main chain, short side chains, and a small number of side chains. "Linear" means that olefins are polymerized in a linear fashion. However, linear polyolefin resins in this embodiment also include those in which olefins are polymerized in a branched fashion at a ratio of less than 10% by mass, preferably less than 5% by mass, more preferably less than 3% by mass, and even more preferably less than 1% by mass. Examples of linear polyolefin resins include high-density polyethylene (HDPE) and polypropylene. On the other hand, examples of polyolefin resins that do not fall under the category of linear polyolefin resins include low-density polyolefins known as LDPE. Another example of a linear polyolefin resin includes one or more polymers of olefins in which 80% by mass or more (preferably 90% by mass or more) of the olefins (raw material monomers) constituting the linear polyolefin resin have a molecular weight of 28 to 57. The linear polyolefin resin is a polymer of one or more olefins, and may be copolymerized with other monomers in an amount of, for example, less than 10% by mass, preferably less than 5% by mass, more preferably less than 3% by mass, and even more preferably less than 1% by mass of the polymer. The olefin polymer may or may not be terminally modified.

[0021] The linear polyolefin resin preferably contains at least one selected from polyethylene, polypropylene, and an ethylene / propylene copolymer, and more preferably contains polypropylene. The density of the linear polyolefin resin is, for example, 0.940 g / cm in the case of polyethylene. ３ It is preferable that the density is 0.948 g / cm or more. ３ More preferably, it is 0.970 g / cm or more. ３ By setting the density of the polyethylene to the above lower limit or more, it becomes possible to produce cyclic lactams with higher selectivity. High density polyethylene (HDPE) usually has a density of 0.940 g / cm ３ On the other hand, the density of the linear polyolefin resin is, for example, 0.900 g / cm in the case of polypropylene. ３ It is preferable that the viscosity is 0.910 g / cm or more. ３ The density is preferably not more than 100%. The density is measured according to the method described in JIS K 7112-1999.

[0022] Furthermore, the linear polyolefin resin used in this embodiment preferably has a high melting point. A high melting point enables the production of cyclic lactams with higher selectivity. The melting point of the linear polyolefin resin is preferably 125°C or higher, more preferably 126°C or higher, even more preferably 127°C or higher, even more preferably more than 127°C, even more preferably 129°C or higher, and preferably 180°C or lower.

[0023] In the thermoplastic resin of this embodiment, the content of the linear polyolefin resin is 6 parts by mass or more, preferably 9 parts by mass or more, more preferably 12 parts by mass or more, and 90 parts by mass or less, preferably 50 parts by mass or less, more preferably 30 parts by mass or less, and even more preferably 18 parts by mass or less, per 100 parts by mass of the polyamide resin (a). By setting the content at or above the lower limit, the viscosity of the reaction liquid tends to decrease, the depolymerization reaction is promoted, and the yield of the distillation purified product tends to improve. Furthermore, by setting the content at or below the upper limit, distillation inhibition by the linear polyolefin tends to be effectively suppressed. The thermoplastic resin of this embodiment may contain only one type of linear polyolefin resin, or may contain two or more types. When two or more types are contained, it is preferable that the total amount is within the above range.

[0024] <<Other Thermoplastic Resins>> In this embodiment, the thermoplastic resin may include other thermoplastic resins in addition to the polyamide resin (a) and linear polyolefin resin. Examples of other thermoplastic resins include one or more selected from the group consisting of polyester resins such as branched polyolefin resins, cyclic polyolefin resins, and polyethylene terephthalate resins, and polyamide resins other than polyamide resin (a). Examples of other polyamide resins include nylon 66, nylon 666, and semi-aromatic polyamide resins. In this embodiment, the amount of the other thermoplastic resin is preferably less than 5% by mass, more preferably less than 3% by mass, and even more preferably less than 1% by mass, based on 100% by mass of the total amount of the thermoplastic resins.

[0025] <Polyphosphoric Acid> In the method for producing a cyclic lactam of this embodiment, the depolymerization reaction is carried out using 1 part by mass or more of polyphosphoric acid per 100 parts by mass of polyamide resin (a). Polyphosphoric acid acts as a catalyst for the ring-closing reaction of the amide bond in polyamide resin (a) and also dissolves polyamide resin (a), thereby contributing to a decrease in the viscosity of the reaction solution. This makes the reaction solution easier to stir, allowing for efficient production of a cyclic lactam. It is preferable to supply polyphosphoric acid directly to the reaction system (e.g., a reaction vessel).

[0026] The polyphosphoric acid may be a linear polyphosphoric acid or a cyclic polyphosphoric acid, but is preferably a linear polyphosphoric acid. Specifically, it is preferably at least one selected from the group consisting of pyrophosphoric acid, tripolyphosphoric acid, a medium- to long-chain polyphosphoric acid having 4 to 100 phosphate moieties, and trimetaphosphoric acid (six-membered ring), and more preferably at least one selected from the group consisting of pyrophosphoric acid, tripolyphosphoric acid, and a medium- to long-chain polyphosphoric acid having 4 to 100 phosphate moieties.

[0027] More specifically, the polyphosphoric acid preferably contains a compound represented by the following formula (P): (In formula (P), p1 is preferably an integer of 2 to 100.)

[0028] In the compound represented by formula (P), p1 is preferably 3 or more, more preferably 4 or more, and may be 5 or more. By setting it to the lower limit or above, there is a tendency for the depolymerization reaction to be promoted due to high catalytic activity and for the selectivity for cyclic lactam to be further improved. Furthermore, in the compound represented by formula (P), p1 is preferably 90 or less, more preferably 70 or less, even more preferably 50 or less, even more preferably 30 or less, even more preferably 20 or less, and even more preferably 10 or less. By setting it to the upper limit or below, there is a tendency for the viscosity of the reaction solution to be more appropriate, and the dissolution of polyamide resin (a) and the efficiency of the depolymerization reaction to be further improved. Polyphosphoric acid may be composed of only one compound represented by formula (P), or may be a mixture of two or more compounds represented by formula (P). Furthermore, polyphosphoric acid may contain a compound in which p1 in formula (P) is 1, or may contain a compound in formula (P) in which p1 is greater than 100.

[0029] When polyphosphoric acid contains a compound represented by the above formula (P) but does not contain a compound in which p1 in formula (P) is 1 or more and / or a compound in which p1 in formula (P) is more than 100, or when it does not contain both a compound in which p1 in formula (P) is 1 or more and a compound in formula (P) in which p1 is more than 100, the average value of p1 in formula (P) is preferably within the range of p1 in the compound represented by formula (P) described above. Furthermore, when the polyphosphoric acid contains a compound represented by the above formula (P) and contains a compound in which p1 in formula (P) is 1 or more and / or a compound in which p1 in formula (P) is more than 100, or further when it contains both a compound in formula (P) in which p1 is 1 or more and a compound in formula (P) in which p1 is more than 100, the average value of p1 in formula (P) is preferably 1.5 or more, more preferably 2.0 or more, even more preferably 2.5 or more, still more preferably 3.0 or more, or may be 5.0 or more, and is preferably 100 or less, more preferably 70 or less, still more preferably 50 or less, still more preferably 30 or less, still more preferably 20 or less, and still more preferably 10 or less.

[0030] The molecular weight of the compound represented by formula (P) is preferably 177 or more, more preferably 257 or more. By setting it to the lower limit or more, the high catalytic activity tends to more effectively promote the depolymerization reaction, and cyclic lactams tend to be produced with higher selectivity. Furthermore, the molecular weight of the compound represented by formula (P) is preferably 10,000 or less, more preferably 7,000 or less, even more preferably 5,000 or less, even more preferably 3,000 or less, and even more preferably 1,000 or less. By setting it to the upper limit or less, the viscosity of the reaction solution becomes more appropriate, and the dissolution of polyamide resin (a) and the efficiency of the depolymerization reaction tend to be further improved.

[0031] The number-average molecular weight of polyphosphoric acid is preferably 137 or more, more preferably 177 or more. By setting it to the lower limit or more, the high catalytic activity tends to more effectively promote the depolymerization reaction, and cyclic lactams tend to be produced with higher selectivity. Furthermore, the number-average molecular weight of polyphosphoric acid is preferably 10,000 or less, more preferably 8,017 or less, even more preferably 5,617 or less, even more preferably 4,018 or less, even more preferably 2,418 or less, and even more preferably 818 or less. By setting it to the upper limit or less, the viscosity of the reaction solution becomes more appropriate, and the dissolution of polyamide resin (a) and the efficiency of the depolymerization reaction tend to be further improved.

[0032] In the method for producing a cyclic lactam of this embodiment, the amount of polyphosphoric acid used (amount added to the reaction system) per 100 parts by mass of polyamide resin (a) is 1 part by mass or more, preferably 10 parts by mass or more, more preferably 40 parts by mass or more, more preferably 50 parts by mass or more, even more preferably 70 parts by mass or more, even more preferably 90 parts by mass or more, and even more preferably 100 parts by mass or more. By setting the amount at or above the lower limit, the depolymerization reaction tends to be more effectively promoted due to high catalytic activity, and the cyclic lactam tends to be produced with higher selectivity. The amount of polyphosphoric acid used (amount added to the reaction system) per 100 parts by mass of the polyamide resin (a) is preferably 10,000 parts by mass or less, more preferably 1,000 parts by mass or less, even more preferably 500 parts by mass or less, even more preferably 300 parts by mass or less, and even more preferably 200 parts by mass or less. By setting the amount at or below the upper limit, raw material costs and energy required for distillation purification of the cyclic lactam can be reduced. When the polyamide resin (a) is continuously supplied to the reaction system to produce a cyclic lactam, it is preferable that the amount of polyphosphoric acid present in the reaction system satisfies the above range.

[0033] In this embodiment, the water content of the polyphosphoric acid (polyphosphoric acid itself without water) supplied to the reaction system is preferably 100% by mass or less, more preferably 50% by mass or less, even more preferably 10% by mass or less, even more preferably 1% by mass or less, even more preferably 0.1% by mass or less, and even more preferably 0.001% by mass or less, relative to the mass of polyphosphoric acid. By setting it below the upper limit, selective production of cyclic lactam and simple distillation purification thereof can be achieved. The lower limit of the water content of the polyphosphoric acid supplied to the reaction system may be 0% by mass, but, for example, even if it is 0.0001% by mass or more, the required performance will be sufficiently satisfied.

[0034] <Solvent> In the method for producing a cyclic lactam of this embodiment, polyphosphoric acid serves as a solvent, and therefore a solvent (reaction solvent) may or may not be used.

[0035] In the first form of the solvent in the method for producing a cyclic lactam of this embodiment, no solvent is used during the depolymerization reaction of polyamide resin (a), or the amount of solvent is 10% by mass or less relative to the amount of polyphosphoric acid. In the first form of the solvent, even when substantially no solvent is used, polyphosphoric acid still serves as the solvent. In the first form of the solvent, the amount of solvent is preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 1% by mass or less, and even more preferably 0.1% by mass or less relative to the amount of polyphosphoric acid. The first form of the solvent allows for easy purification of the cyclic lactam by reactive distillation or distillation after the reaction. It also tends to reduce energy costs. In the first form of the solvent, it is preferable that 95% by mass or more (preferably 99% by mass or more) of the materials supplied to the reaction system (e.g., reaction vessel) be composed of polyphosphoric acid and a thermoplastic resin. However, it goes without saying that during the reaction, in addition to the target cyclic lactam, the reaction system also contains produced water, unreacted polyamide resin (a), and intermediate polyamide oligomers.

[0036] A second embodiment of the solvent in the method for producing a cyclic lactam of this embodiment is one in which the depolymerization reaction of polyamide resin (a) is carried out in the presence of a high-boiling solvent, and the boiling point of the high-boiling solvent is at least 10°C below the boiling point of the cyclic lactam product. The use of a high-boiling solvent reduces the viscosity of the reaction solution, accelerates the depolymerization reaction, and tends to improve the yield of the distillation purified product (cyclic lactam). The boiling point of the high-boiling solvent is preferably at least 5°C below the boiling point of the cyclic lactam product, more preferably at least -3°C above the boiling point, even more preferably at least -1°C above the boiling point, still more preferably at least the boiling point, even more preferably at least +1°C above the boiling point, and even more preferably at least +3°C above the boiling point. The upper limit of the boiling point of the high-boiling solvent is preferably at most +200°C above the boiling point of the cyclic lactam product. Furthermore, when two or more high-boiling solvents are used, the boiling point of the solvent with the greatest content is used. The viscosity of the high-boiling point solvent at 25° C. is preferably 1 cSt or more, more preferably 10 cSt or more, and may be 20 cSt or more, and is preferably 500 cSt or less, more preferably 300 cSt or less, and even more preferably 100 cSt or less. The viscosity of the high-boiling point solvent is measured in accordance with JIS Z8803.

[0037] Examples of high-boiling point solvents include polysiloxanes and end-capped polyethylene glycols, with polyalkylsiloxanes (wherein the alkyl group has 1 to 3 carbon atoms and the average number of alkyl groups bonded to one Si atom is 1 or 2) being preferred, and polydimethylsiloxane being more preferred. Furthermore, it is preferable that the high-boiling point solvent is not an alcohol. By not using an alcohol, it is possible to effectively prevent a decrease in catalytic activity due to alcoholysis of polyphosphoric acid.

[0038] In the second embodiment of the solvent in the method for producing a cyclic lactam of this embodiment, the amount of the high-boiling solvent used is preferably 10 parts by mass or more, more preferably 50 parts by mass or more, and even more preferably 100 parts by mass or more, relative to 100 parts by mass of polyphosphoric acid. By setting the amount at or above the lower limit, the depolymerization reaction is promoted, and the yield of the distillation purified product (cyclic lactam) tends to be improved. Furthermore, the upper limit of the amount of the high-boiling solvent used is preferably 1,000 parts by mass or less, more preferably 500 parts by mass or less, even more preferably 400 parts by mass or less, even more preferably 300 parts by mass or less, and even more preferably 200 parts by mass or less, relative to 100 parts by mass of polyphosphoric acid. Setting the amount at or below the upper limit tends to prevent the reaction solution from becoming too dilute. More specifically, raw material costs can be reduced and high depolymerization reactivity can be maintained. In the method for producing a cyclic lactam of this embodiment, only one high-boiling solvent may be used, or two or more high-boiling solvents may be used. When two or more high-boiling solvents are used, it is preferable that the total amount be within the above range.

[0039] In the second form of the solvent, it is preferable that 95% by mass or more (preferably 99% by mass or more) of the materials supplied to the reaction system (for example, reaction vessel) are composed of polyphosphoric acid, polyamide resin (a), and a high-boiling solvent. However, it goes without saying that during the reaction, in addition to the target cyclic lactam, the reaction system also contains produced water, unreacted polyamide resin (a), and polyamide oligomers as intermediate products.

[0040] A third embodiment of the solvent in the method for producing a cyclic lactam of this embodiment is one in which, when carrying out the depolymerization reaction of polyamide resin (a), no solvent other than a high-boiling solvent is used, or the amount of solvent other than a high-boiling solvent is 10% by mass or less relative to the amount of polyphosphoric acid. In the third embodiment of the solvent, it is preferable that 95% by mass or more (preferably 99% by mass or more) of the materials supplied to the reaction system (e.g., reaction vessel) are composed of polyphosphoric acid, polyamide resin (a), and a selectively blended high-boiling solvent. However, it goes without saying that during the reaction, in addition to the target cyclic lactam, the reaction system also contains produced water, unreacted polyamide resin (a), and intermediate polyamide oligomers. The details of the high-boiling solvent are the same as those of the second embodiment of the solvent, and when a high-boiling solvent is used, the preferred ranges for its type, amount, etc. are also the same.

[0041] <Depolymerization Reaction> Next, the details of the depolymerization reaction will be described. In this embodiment, the pressure when the depolymerization reaction is carried out is preferably 202.6 kPa or less, more preferably 150.0 kPa or less, even more preferably less than 101.3 kPa, even more preferably 13.3 kPa or less, and even more preferably 4.0 kPa or less. By setting the pressure at or below the upper limit, the cyclic lactam tends to be obtained in a higher yield by reactive distillation. The lower limit of the pressure when the depolymerization reaction is carried out is preferably 0.01 kPa or more, for example. The pressure when the depolymerization reaction is carried out means the pressure when the pressure in the reaction system is adjusted by reducing or increasing the pressure and reaches a steady state. Note that in this embodiment, the reaction may be carried out under two or more pressure stages, for example, by carrying out a reaction at normal pressure for a certain period of time and then under reduced pressure for a certain period of time. In this case, it is preferable that the pressure in each stage satisfies the above range.

[0042] The temperature when carrying out the depolymerization reaction is preferably 170°C or higher, more preferably 190°C or higher, even more preferably 210°C or higher, and even more preferably 230°C or higher. By setting the temperature at or above the lower limit, the depolymerization reaction is promoted, enabling more efficient distillation of the cyclic lactam. Furthermore, the temperature when carrying out the depolymerization reaction is preferably 300°C or lower, more preferably 280°C or lower, and even more preferably 260°C or lower. By setting the temperature at or below the upper limit, the energy required for producing the cyclic lactam can be reduced, and the generation of by-products tends to be reduced. The temperature when carrying out the depolymerization reaction means the temperature at which the temperature in the reaction system is increased and reaches a steady state. In this embodiment, the reaction may be carried out at two or more temperature stages, such as by reacting at a specific temperature for a certain period of time, and then further increasing or decreasing the temperature and continuing the reaction for a certain period of time. In this case, it is preferable that the reaction temperature in each stage satisfies the above range.

[0043] The depolymerization reaction may be carried out without stirring, but is preferably carried out with stirring. Stirring allows the reaction to proceed more effectively. In this embodiment, stirring can be carried out by using polyphosphoric acid. The stirring speed is adjusted appropriately depending on the reaction scale (apparatus scale) and the apparatus structure, and is, for example, preferably 10 rpm or more, more preferably 30 rpm or more, even more preferably 100 rpm or more, even more preferably 150 rpm or more, even more preferably 200 rpm or more, and may be 300 rpm or more, or 500 rpm or more. By setting the stirring speed at or above the lower limit, the polyamide resin (a) and polyphosphoric acid tend to be mixed more uniformly, and the depolymerization reaction tends to proceed efficiently. In addition, the stirring speed is preferably 1500 rpm or less, more preferably 1200 rpm or less, even more preferably 1000 rpm or less, even more preferably 800 rpm or less, even more preferably 600 rpm or less, and may be 550 rpm or less. By setting the temperature at or below the upper limit, the reaction liquid can be prevented from scattering onto the side or top surfaces of the reaction apparatus, effectively suppressing a decrease in yield. Furthermore, clogging of the distillation flow path can be effectively prevented. The stirring speed refers to the temperature at which the stirring speed in the reaction system is adjusted and a steady state is reached. Stirring may be performed at two or more stirring speed stages, such as at a specific speed for a certain period of time and then at a different stirring speed. In this case, it is preferable that the stirring speed at each stage satisfies the above range. Furthermore, the equipment used for stirring can be equipment typically used in the production of chemical products, and the shape of the stirring blades can be appropriately selected, such as paddle type, anchor type, or helical ribbon type.

[0044] In this embodiment, superheated steam can be avoided when carrying out the depolymerization reaction. By not using superheated steam, it is possible to effectively reduce energy consumption and waste during purification. Examples of superheated steam include superheated steam of water, amines, and alcohols. More specifically, examples include superheated steam, methanol vapor, and ammonia vapor. This embodiment is particularly advantageous in that the depolymerization reaction can proceed without using superheated steam.

[0045] The depolymerization reaction in this embodiment is not usually a hydrolysis reaction mediated by water. Therefore, in this embodiment, it is preferable to carry out the depolymerization reaction while removing water. Furthermore, it is preferable that the depolymerization reaction in this embodiment proceeds as a backbiting reaction in which a terminal amino group or an amide bond in the main chain of the polyamide resin (a) attacks an amide bond one unit adjacent to itself.

[0046] <Distillation> In the method for producing a cyclic lactam of this embodiment, distillation is preferred. By distillation, the target cyclic lactam can be separated and purified. Furthermore, water may be present in the reaction system due to moisture absorption by the raw material polyamide resin (a) or condensation of a portion of the polyphosphoric acid during the reaction, but distillation can remove the water. Distillation may be performed by reactive distillation while the depolymerization reaction is being carried out, or by distillation after the depolymerization reaction. Of course, reactive distillation may be performed while the depolymerization reaction is being carried out, and then distillation may be performed after the depolymerization reaction.

[0047] The pressure during distillation after the depolymerization reaction is preferably less than 101.3 kPa, more preferably 13.3 kPa or less, even more preferably less than 4.0 kPa, even more preferably 2.7 kPa or less, and even more preferably 1.4 kPa or less. By setting the pressure at or below the upper limit, the cyclic lactam tends to be isolated in a higher yield. The lower limit of the pressure during distillation is preferably, for example, 0.01 kPa or more.

[0048] <Feeding to the reaction system> In the method for producing a cyclic lactam of this embodiment, a thermoplastic resin containing a polyamide resin containing a repeating unit represented by formula (a) and polyphosphoric acid are usually supplied as raw materials to the reaction system to produce a cyclic lactam. The thermoplastic resin containing polyamide resin (a) may be supplied to the reaction system all at once, or may be supplied to the reaction system continuously. When the thermoplastic resin containing polyamide resin (a) is supplied to the reaction system continuously, the amount of polyphosphoric acid present in the reaction system is preferably 1 part by mass or more per 100 parts by mass of polyamide resin (a). In addition, the polyamide resin (a) and the linear polyolefin resin may be added simultaneously or separately.

[0049] In the method for producing a cyclic lactam of this embodiment, polyphosphoric acid is supplied to the reaction system. In this case, it may be supplied to the reaction system all at once or may be supplied continuously to the reaction system. From the viewpoint of industrial production, continuous supply is preferred. It is also preferable to reuse polyphosphoric acid.

[0050] As the production of a cyclic lactam progresses, the polyphosphoric acid supplied to the reaction system may have a higher degree of polymerization than the polyphosphoric acid supplied to the reaction system. Therefore, the method for producing a cyclic lactam of this embodiment may include hydrolyzing polyphosphoric acid (B2) that has a higher degree of polymerization than the polyphosphoric acid supplied to the reaction system to obtain regenerated polyphosphoric acid (B3). In this embodiment, in particular, polyphosphoric acid (B2) that has a higher degree of polymerization than the polyphosphoric acid supplied to the reaction system may be hydrolyzed to obtain regenerated polyphosphoric acid (B3) in the reaction system. Furthermore, the regenerated polyphosphoric acid (B3) can also be used as the polyphosphoric acid supplied to the reaction system. Here, the polyphosphoric acid (B2) that has a higher degree of polymerization than the polyphosphoric acid supplied to the reaction system preferably includes, for example, a compound represented by the following formula (P2): (In formula (P2), p2 is preferably 3 to 130.) Polyphosphoric acid (B2) is usually a mixture of compounds represented by formula (P2) having different values ​​for p2.

[0051] When polyphosphoric acid contains a compound represented by formula (P1) and polyphosphoric acid (B2) contains a compound represented by formula (P2), the average value of p2 of the compound represented by formula (P2) contained in polyphosphoric acid (B2) is the difference from the average value of p1 of the compound represented by formula (P1) contained in the polyphosphoric acid supplied to the reaction system, p2 - p1, and is, for example, 1 or more, 2 or more, or 3 or more, and is, for example, 30 or less, 20 or less, 10 or less, or 5 or less.

[0052] <Embodiments> A preferred embodiment of the method for producing a cyclic lactam will be described below. It goes without saying that the present invention is not limited to this embodiment.

[0053] A first embodiment of the method for producing a cyclic lactam is a method for producing a cyclic lactam in which reactive distillation is carried out while a depolymerization reaction is carried out, the depolymerization reaction is carried out at a temperature of 170°C to 300°C, no superheated steam is used during the depolymerization reaction, and no solvent is used during the depolymerization reaction, or the amount of the solvent is 10 mass% or less relative to the amount of polyphosphoric acid. By using the first embodiment, the production time of the cyclic lactam can be shortened, and the cyclic lactam can be obtained efficiently.

[0054] A second embodiment of the method for producing a cyclic lactam comprises carrying out a reactive distillation while carrying out a depolymerization reaction, carrying out the depolymerization reaction at a temperature of 170°C to 300°C, not using superheated steam, and carrying out the depolymerization reaction in the presence of a high-boiling solvent, the boiling point of which is at least 10°C below the boiling point of the cyclic lactam product, and the high-boiling solvent is not an alcohol. By employing the second embodiment, the viscosity of the reaction solution can be reduced, thereby accelerating the depolymerization reaction and shortening the time required for producing the cyclic lactam by reactive distillation.

[0055] A third embodiment of the method for producing a cyclic lactam comprises carrying out a polymerization reaction followed by distillation, followed by a depolymerization reaction at a temperature of 170° C. to 300° C., wherein the depolymerization reaction is carried out without using superheated steam, and wherein the depolymerization reaction is carried out without using a solvent or in which the amount of the solvent is 10% by mass or less relative to the amount of polyphosphoric acid. By employing the third embodiment, distillation purification can be carried out at a temperature lower than that during depolymerization.

[0056] A fourth embodiment of the method for producing a cyclic lactam comprises carrying out a depolymerization reaction followed by distillation at a temperature of 170°C to 300°C, without using superheated steam, and carrying out the depolymerization reaction in the presence of a high-boiling solvent, the boiling point of which is at least 10°C below the boiling point of the cyclic lactam product, and the high-boiling solvent is not an alcohol. By employing the fourth embodiment, the viscosity of the reaction solution can be reduced, thereby accelerating the depolymerization reaction, and distillation purification can be carried out at a temperature lower than that during depolymerization.

[0057] <Apparatus for Producing Cyclic Lactam> This embodiment further discloses an apparatus for producing a cyclic lactam that utilizes the method for producing a cyclic lactam of this embodiment. FIG. 1 is a schematic diagram of an example of an apparatus for producing a cyclic lactam. In FIG. 1, 1 indicates a reactor, 2 indicates a container for storing a cyclic lactam, 3 indicates a flow path, 4 indicates a pressure regulator, 5 indicates a heating section, 6 indicates a device for removing water, and 7 indicates a container for recovering water. Hereinafter, the apparatus for producing a cyclic lactam of this embodiment will be described with reference to FIG. 1. However, it goes without saying that the apparatus for producing a cyclic lactam of this embodiment is not limited to FIG. 1.

[0058] Specifically, this embodiment discloses an apparatus for producing a cyclic lactam, which comprises the following facilities for carrying out the method for producing a cyclic lactam of this embodiment: (1) a reactor 1 for heating a mixture A containing a polyamide resin (a), a linear polyolefin resin, and polyphosphoric acid to carry out a depolymerization reaction to produce a cyclic lactam and obtain a mixture B containing polyphosphoric acid and the cyclic lactam; (2) a container 2 for containing the cyclic lactam evaporated from the mixture B in the reactor 1; and (3) a flow path 3 connecting the reactor 1 and the container 2 and for feeding the evaporated cyclic lactam from the reactor 1 to the container 2.

[0059] Mixture B typically also contains a polyamide resin containing a repeating unit represented by formula (a) and / or a polyamide oligomer containing a repeating unit represented by formula (a). In Mixture A containing polyamide resin (a), a linear polyolefin resin, and polyphosphoric acid, and Mixture B containing polyphosphoric acid and a cyclic lactam, the polyphosphoric acid is synonymous with the polyamide resin (a) and polyphosphoric acid supplied to the reaction system described above. The ratios and preferred ranges thereof can be found in the descriptions of <<Polyamide Resin Containing a Repeating Unit Represented by Formula (a) (Polyamide Resin (a))>>, <<Linear Polyolefin Resin>>, and <<Polyphosphoric Acid>> above. Furthermore, Mixture A and Mixture B may contain a solvent, the details of which can be found in the description of <Solvent>>. The details of the depolymerization reaction can be found in the description of <<Depolymerization Reaction>> above.

[0060] The reactor 1 may be, for example, a glass-lined or anodic-protected reactor made of stainless steel or tantalum.

[0061] The vessel 2 for containing the cyclic lactam evaporated from the mixture B in the reactor 1 can be exemplified by vessels made of at least one material selected from the group consisting of glass-lined stainless steel, stainless steel (SUS), tantalum titanium, Hastelloy, Teflon (registered trademark) lining, nickel, zirconium, copper, and carbon steel (CS).

[0062] The flow path 3 (shown by a dotted line in FIG. 1 for convenience) that connects the reactor 1 and the vessel 2 and transports the evaporated cyclic lactam from the reactor 1 to the vessel 2 is exemplified by a flow path made of stainless steel and / or titanium. Valves, joints, etc. may be provided between the reactor 1 and the flow path 3 and / or between the vessel 2 and the flow path 3. The provision of a valve makes it possible to adjust the pressure and flow rate. A valve may also be provided midway along the flow path 3 to adjust the pressure and flow rate.

[0063] Furthermore, the cyclic lactam production apparatus of this embodiment preferably includes a pressure regulator 4 for maintaining a pressure of 202.6 kPa or less within the reactor 1, vessel 2, and flow path 3. Examples of the pressure regulator 4 include a vacuum pump such as a reciprocating piston pump or a rotary pump. In the cyclic lactam production apparatus of this embodiment, the pressure in the entire area through which the raw materials or products pass, including the reactor 1, vessel 2, and flow path 3, may typically be regulated by a single pressure regulator 4 installed in either the reactor 1, vessel 2, or flow path 3, or may be regulated by a pressure regulator 4 installed in each of the reactor 1, vessel 2, and flow path 3. The pressure regulator 4 may be connected to any portion of the reactor 1, vessel 2, or flow path 3, but is preferably connected to the flow path 3. This configuration allows for efficient separation of compounds with a boiling point lower than that of the cyclic lactam, such as water, which may be mixed in a small amount. A valve, a joint, or the like may be provided between the flow path 3 and the pressure regulator 4. It is also preferable that the pressure regulator 4 is connected between the flow path 3 and a water removal device 6, which will be described later. Furthermore, in this embodiment, it is preferable to have a heating unit 5 for maintaining the temperature in the flow path 3 at or above the melting point of the cyclic lactam (preferably at or above the melting point + 1°C and at or below the melting point + 50°C). By providing the heating unit 5, blockage due to precipitation of the cyclic lactam can be effectively suppressed. Examples of the heating unit 5 include a heat transfer medium such as hot water or steam at 50°C or above and a heat exchanger for circulating the heat transfer medium. Furthermore, in this embodiment, it is preferable to have a device 6 for removing water from the reactor 1. By providing the water removal device 6, it is possible to effectively suppress hydrolysis of the cyclic lactam and polyphosphoric acid. Examples of the water removal device 6 include a condenser and a molecular sieve column. Furthermore, the cyclic lactam production apparatus of this embodiment preferably has a container 7 for recovering the removed water.

[0064] The cyclic lactam obtained by the cyclic lactam production method or production apparatus of this embodiment can be reused as a raw material for polyamide resins and fibers, or can be used as a raw material for medical polymers.

[0065] The present invention will be explained in more detail below with reference to examples. The materials, amounts used, ratios, treatment contents, treatment procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the specific examples shown below. If the measuring instruments used in the examples are difficult to obtain due to discontinuation or the like, measurements can be made using other instruments with equivalent performance.

[0066] Raw materials Polyamide 6 (nylon 6): Manufacturer: Sigma Aldrich, Product No.: 181110, Viscosity average molecular weight: 10,000 Polypropylene: Linear polypropylene, manufactured by Japan Polypropylene Corporation, FY6, Density 0.900 g / cm ３ HDPE: High-density polyethylene (linear), manufactured by Japan Polyethylene Corporation, HJ491, density 0.955 g / cm ３ LDPE: Low-density polyethylene (long-chain branched), manufactured by Japan Polyethylene Corporation, LC8001, density 0.917 g / cm ３ Polyphosphate: Manufacturer: Fujifilm Wako Pure Chemical Industries, Part Number: 166-03035

[0067] Example 1 A distillation apparatus was prepared, comprising a pear-shaped flask (reactor) equipped with a stirrer, a pear-shaped flask (container for containing cyclic lactam) for recovering the distilled purified product, and a Liebig tube (flow path). 1 part by weight of polyamide, 0.1 parts by weight of polypropylene, and 1 part by weight of polyphosphoric acid were mixed into the pear-shaped flask reactor. After mixing, the mixture was stirred at 300 rpm for 6 hours in an oil bath at 230 ° C., and the pressure was reduced to 20 mmHg (2.6645 kPa) using a diaphragm vacuum pump (pressure regulating device) equipped with a pressure controller. Trace amounts of water were collected using a cold trap (water removal device) installed between the distillation apparatus and the diaphragm vacuum pump.

[0068] Example 2, Example 3, Comparative Example 1, Comparative Example 2, Comparative Example 3, Reference Example 1 As shown in Table 1, the types of other resins and their amounts added were changed, and the rest were carried out in the same manner as in Example 1.

[0069]

[0070] As is clear from the above results, the cyclic lactam production method of the present invention produced cyclic lactam with higher selectivity (Examples 1 to 3) compared to the case where no linear polyolefin resin was added (Reference Example 1). In contrast, the selectivity was low when a branched polyethylene resin was added (Comparative Example 1). Furthermore, even when a linear polyolefin resin was added in large amounts (Comparative Examples 2 and 3), the selectivity to cyclic lactam was low.

[0071] REFERENCE SIGNS LIST 1 Reactor 2 Container for storing cyclic lactam 3 Flow path 4 Pressure regulating device 5 Heating section 6 Device for removing water 7 Container for recovering water

Claims

1. A method for producing a cyclic lactam, which comprises depolymerizing a thermoplastic resin containing a polyamide resin containing a repeating unit represented by formula (a) using polyphosphoric acid, wherein the amount of polyphosphoric acid is 1 part by mass or more per 100 parts by mass of the polyamide resin containing a repeating unit represented by formula (a), and the thermoplastic resin contains 6 to 90 parts by mass of a linear polyolefin resin per 100 parts by mass of the polyamide resin containing a repeating unit represented by formula (a). (In formula (a), n1 is an integer of 3 to 22. The value of n1 may be different for each repeating unit.) 2. The method for producing a cyclic lactam according to claim 1, wherein the linear polyolefin resin comprises high density polyethylene (HDPE) and / or polypropylene.

3. The method for producing a cyclic lactam according to claim 1 or 2, wherein the thermoplastic resin contains 6 to 30 parts by mass of a linear polyolefin resin per 100 parts by mass of a polyamide resin.

4. The method for producing a cyclic lactam according to claim 1 or 2, wherein the polyphosphoric acid is used in an amount of 40 parts by mass or more per 100 parts by mass of the polyamide resin.

5. The method for producing a cyclic lactam according to claim 1 or 2, wherein 50 to 10,000 parts by mass of the polyphosphoric acid is used per 100 parts by mass of the polyamide resin.

6. The method for producing a cyclic lactam according to claim 1 or 2, wherein the depolymerization reaction is carried out at a pressure of 202.6 kPa or less.

7. The method for producing a cyclic lactam according to claim 1 or 2, wherein reactive distillation is carried out while the depolymerization reaction is carried out.

8. The method for producing a cyclic lactam according to claim 1 or 2, wherein the depolymerization reaction is followed by distillation.

9. The method for producing a cyclic lactam according to claim 1 or 2, wherein the depolymerization reaction is carried out at a temperature of 170°C to 300°C.

10. The method for producing a cyclic lactam according to claim 1 or 2, wherein the depolymerization reaction is carried out without using superheated steam.

11. The method for producing a cyclic lactam according to claim 1 or 2, wherein the depolymerization reaction is carried out without using a solvent or with an amount of the solvent of 10 mass % or less relative to the amount of the polyphosphoric acid.

12. The method for producing a cyclic lactam according to claim 1 or 2, wherein the depolymerization reaction is carried out in the presence of a high-boiling solvent, and the boiling point of the high-boiling solvent is at least 10°C lower than the boiling point of the cyclic lactam product.

13. The method for producing a cyclic lactam according to claim 12, wherein the high boiling point solvent is not an alcohol.

14. The method for producing a cyclic lactam according to claim 1 or 2, wherein reactive distillation is carried out while the depolymerization reaction is being carried out, the depolymerization reaction is carried out at a temperature of 170°C to 300°C, no superheated steam is used when the depolymerization reaction is being carried out, and no solvent is used when the depolymerization reaction is being carried out, or the amount of the solvent is 10 mass% or less relative to the amount of the polyphosphoric acid.

15. The method for producing a cyclic lactam according to claim 1 or 2, wherein reactive distillation is carried out while the depolymerization reaction is carried out, the depolymerization reaction is carried out at a temperature of 170°C to 300°C, no superheated steam is used when the depolymerization reaction is carried out, the depolymerization reaction is carried out in the presence of a high-boiling point solvent, the boiling point of which is at least 10°C below the boiling point of the cyclic lactam product, and the high-boiling point solvent is not an alcohol.

16. The method for producing a cyclic lactam according to claim 1 or 2, wherein after the depolymerization reaction, distillation is carried out; the depolymerization reaction is carried out at a temperature of 170°C to 300°C; no superheated steam is used when the depolymerization reaction is carried out; and no solvent is used when the depolymerization reaction is carried out, or the amount of the solvent is 10 mass% or less relative to the amount of the polyphosphoric acid.

17. The method for producing a cyclic lactam according to claim 1 or 2, wherein distillation is carried out after the depolymerization reaction, and the depolymerization reaction is carried out at a temperature of 170°C to 300°C, and wherein superheated steam is not used when carrying out the depolymerization reaction, and the depolymerization reaction is carried out in the presence of a high-boiling point solvent, the boiling point of which is at least 10°C below the boiling point of the cyclic lactam product, and the high-boiling point solvent is not an alcohol.

18. The method for producing a cyclic lactam according to claim 1 or 2, wherein the molecular weight of the polyphosphoric acid is 177 or more and 10,000 or less.

19. The method for producing a cyclic lactam according to claim 1 or 2, wherein the polyamide resin is at least one selected from the group consisting of nylon 4, nylon 5, nylon 6, nylon 11, and nylon 12.

20. A method for producing a cyclic lactam according to claim 1 or 2, wherein the polyamide resin is continuously supplied to the reaction system to produce the cyclic lactam.

21. The method for producing a cyclic lactam according to claim 1 or 2, wherein the polyphosphoric acid is directly supplied to the reaction system.

22. The method for producing a cyclic lactam according to claim 21, wherein the water content of the polyphosphoric acid supplied to the reaction system is 100% by mass or less based on the mass of the polyphosphoric acid.

23. The method for producing a cyclic lactam according to claim 21, wherein polyphosphoric acid (B2) having a higher degree of polymerization than the polyphosphoric acid supplied to the reaction system is hydrolyzed to obtain regenerated polyphosphoric acid (B3).

24. The method for producing a cyclic lactam according to claim 21, wherein polyphosphoric acid (B2) having a higher degree of polymerization than the polyphosphoric acid supplied to the reaction system is hydrolyzed to obtain regenerated polyphosphoric acid (B3) in the reaction system.

25. The method for producing a cyclic lactam according to claim 23, wherein the polyphosphoric acid (B3) is used as the polyphosphoric acid supplied to the reaction system.

Images