Method for depolymerizing polybutylene terephthalate resin

Abstract

To provide a depolymerization method of a polybutylene terephthalate resin, the method enabling 1,4-butane in particular to be recovered at a sufficient recovery rate in a short time.SOLUTION: A depolymerization method of a polybutylene terephthalate resin includes a step A in which a polybutylene terephthalate resin and methanol are mixed, and a step B in which the methanol is brought into a supercritical state, and reacted with the polybutylene terephthalate resin, and the reactant is depolymerized into dimethyl terephthalate and 1,4-butanediol, wherein in the step A, the polybutylene terephthalate resin and the methanol are mixed so that a molar ratio of the methanol to 1 mol of butylene terephthalate in the polybutylene terephthalate resin is 100 to 500, and wherein in the step B, a reaction temperature is set to be 245-270°C.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for depolymerizing polybutylene terephthalate resin using methanol in a supercritical state. [Background technology]

[0002] Polyester resins have excellent mechanical strength, electrical properties, and other characteristics and are used in a wide range of applications, including automobiles, electrical and electronic devices. On the other hand, effective recycling of polyester resins is required from the viewpoint of efficient resource utilization for protecting the global environment. Known recycling methods include, for example, chemical recycling, material recycling, and thermal recycling.

[0003] Patent Document 1 proposes a method for recovering aromatic dicarboxylic acids and diols, which are monomers of aromatic polyester resins, at high yields by chemical recycling. Specifically, it proposes that aromatic polyester resin and methanol are charged into a reaction vessel, and the mixture is heated in a heating furnace to bring the methanol into supercritical conditions, thereby decomposing (depolymerizing) the aromatic dicarboxylic acid dimethyl and dihydric alcohol for recovery. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 9-249597 Summary of the Invention [Problem to be solved by the invention]

[0005] However, it was found that when depolymerization of polybutylene terephthalate resin was carried out under the reaction conditions described in Patent Document 1, it was not possible to recover monomers, particularly 1,4-butanediol, at a sufficient recovery rate. That is, although polybutylene terephthalate resin is an aromatic polyester resin, the method described in Patent Document 1 did not allow recovery of 1,4-butanediol, in particular, at a sufficient recovery rate.

[0006] The present invention has been made in consideration of the above-mentioned conventional problems, and an object of the present invention is to provide a method for depolymerizing polybutylene terephthalate resin, which is capable of recovering 1,4-butanediol in particular at a sufficient recovery rate and in a short period of time. [Means for solving the problem]

[0007] As a result of extensive research to solve the above problems, the present inventors investigated the concentration and reaction conditions of supercritical methanol for polybutylene terephthalate resin and found that a good recovery rate can be obtained, particularly for 1,4-butanediol, and thus completed the present invention.

[0008] One aspect of the present invention that solves the above problem is as follows. (1) Step A: mixing polybutylene terephthalate resin and methanol; and step B of bringing the methanol to a supercritical state and reacting it with the polybutylene terephthalate resin to depolymerize it into dimethyl terephthalate and 1,4-butanediol, In the step A, the mixture is mixed so that the molar ratio of the methanol to 1 mole of butylene terephthalate units in the polybutylene terephthalate resin is 100 to 500, A method for depolymerizing a polybutylene terephthalate resin, wherein the reaction temperature in step B is 245 to 270°C.

[0009] (2) The method for depolymerizing a polybutylene terephthalate resin according to (1), wherein in step B, the reaction time between the polybutylene terephthalate resin and the methanol in a supercritical state is 30 minutes or less, and the decomposition rate of the polybutylene terephthalate resin is 90% or more. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a method for depolymerizing a polybutylene terephthalate resin, which is capable of recovering 1,4-butanediol in particular at a sufficient recovery rate and in a short period of time. DETAILED DESCRIPTION OF THE INVENTION

[0011] The method for depolymerizing polybutylene terephthalate resin (hereinafter also referred to as "PBT resin") of this embodiment includes step A of mixing polybutylene terephthalate resin with methanol, and step B of bringing the methanol to a supercritical state and reacting it with the polybutylene terephthalate resin to depolymerize it into dimethyl terephthalate and 1,4-butanediol. In step A, the mixing is performed so that the molar ratio of methanol to 1 mole of butylene terephthalate units in the polybutylene terephthalate resin is 100 to 500. In step B, the reaction temperature is set to 245 to 270°C.

[0012] In the method for depolymerizing a PBT resin according to the present embodiment, in step A, mixing is performed so that the molar ratio of methanol to 1 mole of butylene terephthalate units in the polybutylene terephthalate resin is 100 to 500. In addition, in step B, the reaction temperature is set to 245 to 270° C. By performing steps A and B in this manner, 1,4-butanediol in particular can be recovered at a sufficient recovery rate. Each step of the method for depolymerizing a PBT resin according to this embodiment will be described in detail below.

[0013] [Step A] In step A, PBT resin and methanol are mixed. In this embodiment, the PBT resin to be depolymerized is a resin obtained by polycondensation of a dicarboxylic acid component containing at least terephthalic acid or its ester-forming derivative (C1-6 alkyl ester, acid halide, etc.) and a glycol component containing at least alkylene glycol having 4 carbon atoms (1,4-butanediol) or its ester-forming derivative (acetylated product, etc.). Examples of PBT resins include homopolyesters (polybutylene terephthalate) and / or copolyesters (butylene terephthalate copolymers, polybutylene terephthalate copolyesters, or modified PBT resins) containing butylene terephthalate as the main component. The PBT resin may also contain various known additives, such as stabilizers, lubricants, nucleating agents, surfactants, colorants, polymer modifiers, different polymers, and inorganic or organic fibrous, powdery, or plate-like fillers, of which one or more may be blended. In the method for depolymerizing a PBT resin according to the present embodiment, the PBT resin is depolymerized into dimethyl terephthalate and 1,4-butanediol.

[0014] In step A, the PBT resin to be depolymerized and mixed with methanol may include used PBT resin products, unused PBT resin pellets, resin waste generated in the manufacturing process of PBT resin pellets and PBT resin products, etc. Such PBT resin is preferably cut or pulverized as necessary before being mixed with methanol.

[0015] In step A, when PBT resin and methanol are mixed, they are mixed so that the molar ratio of methanol to 1 mole of butylene terephthalate units in the PBT resin is 100 to 500. If the molar ratio is less than 100, the recovery rate of the monomer cannot be increased, and even if it exceeds 500, no further effect can be expected and the amount becomes excess. The molar ratio is preferably 100 to 350, more preferably 120 to 250. In other words, the butylene terephthalate unit in the PBT resin means one repeating unit in the PBT resin.

[0016] There are no particular limitations on the mixing method for mixing the PBT resin and methanol in step A. Examples include a method in which the PBT resin is filled into a mixing container and then methanol is added and mixed, a method in which the PBT resin is filled into a mixing container and then methanol is added and mixed, and a method in which the components are continuously joined in a pipe and then mixed in a container. Furthermore, there are no particular limitations on the mixing temperature or time for mixing the above components.

[0017] After completion of the above step A, i.e., after mixing the PBT resin and methanol, the process proceeds to step B. Step A and step B may be performed consecutively, or there may be any time between the end of step A and the start of step B. The mixing vessel used in step A and the reaction vessel used in step B may be the same or different. However, from the viewpoint of being able to proceed to step B in the same state after completing step A, it is preferable that the vessels used in steps A and B are the same.

[0018] [Step B] In step B, methanol is brought to a supercritical state and reacted with the PBT resin to depolymerize it into dimethyl terephthalate and 1,4-butanediol. Here, methanol is brought into a supercritical state by setting the temperature to 239°C or higher and the pressure to 8.1 MPa or higher. In step B, the temperature is set to 245 to 270°C, and the supercritical methanol is reacted with the PBT resin. If the temperature is lower than 245°C or higher than 270°C, the recovery rate of the monomer becomes insufficient. The reaction temperature in step B is preferably 250 to 270°C, and more preferably 250 to 265°C. Note that the pressure is not particularly limited as long as the methanol is brought into a supercritical state, and can be set appropriately to 8.1 MPa or higher, but is preferably 8.5 to 20 MPa.

[0019] In step B, a reaction vessel that can withstand high temperatures and pressures is used to set the reaction temperature and pressure so that methanol reaches a supercritical state, and the shape and volume of the reaction vessel are optional. The heat source for maintaining the temperature within the above range is preferably one that can control the temperature, and examples thereof include a heat medium used in a liquid state, a salt bath, a fluidized sand bath, a metal bath, a heater, an oven, and microwave heating.

[0020] In the method for depolymerizing a PBT resin according to this embodiment, the reaction time between the PBT resin and methanol in a supercritical state can be set to 30 minutes or less, and the decomposition rate of the PBT resin can be set to 90% or more. That is, depolymerization can be performed in a short reaction time of 30 minutes or less, with a sufficient recovery rate. Furthermore, even if the reaction time between the PBT resin and methanol in a supercritical state is set to 20 minutes or less, the decomposition rate of the PBT resin can be set to 90% or more. The lower limit of the reaction time is arbitrary, as long as it is the time at which the decomposition rate of the PBT resin is 90% or more. The reaction time is the time elapsed since the methanol reached a supercritical state.

[0021] The reaction in step B may be carried out with or without stirring.

[0022] In the method for depolymerizing a PBT resin according to this embodiment, the depolymerization of the PBT resin may be carried out in a batch manner or a continuous manner.

[0023] Furthermore, in the method for depolymerizing a PBT resin according to this embodiment, depolymerization can be carried out in the presence of an alkaline additive to promote depolymerization. Examples of the alkaline additive include compounds such as hydroxides, carbonates, hydrogencarbonates, and carboxylates of alkali metals and alkaline earth metals, and amines, and one or more of these can be used. [Example]

[0024] The present embodiment will be described in more detail below with reference to examples, but the present embodiment is not limited to the following examples.

[0025] [Examples 1 to 5, Comparative Examples 1 to 15] As the PBT resin to be depolymerized, a PBT resin (Duranex (registered trademark) PBT 300FP) manufactured by Polyplastics Co., Ltd. was prepared, and the following operation was carried out. In each example and comparative example, PBT resin and methanol were mixed in the proportions shown in Tables 1 to 3, and the mixture was placed in a tubular reaction vessel made of SUS316 (volume: 8.8 cm 3 ) into the reaction vessel. The tube-type reaction vessel was then placed in a salt bath (KNO3, NaNO2) equipped with a stirrer and thermometer and heated to the temperatures shown in Tables 1 to 3. After the reaction times shown in Tables 1 to 3 had elapsed, the reaction was stopped by rapid cooling. After the reaction was stopped, the decomposition product was recovered with chloroform and subjected to suction filtration to separate the filtrate and residue. The amounts of dimethyl terephthalate and 1,4-butanediol in the filtrate were then measured by GC / FID analysis, and the recovery rate of each monomer was calculated. The filtration residue of the decomposition product was dried and then mass-measured to calculate the decomposition rate of the PBT resin. The recovery rate of each monomer and the decomposition rate of PBT were calculated based on the following formulas.

[0026]

number

[0027]

number

[0028]

number

[0029] The number of butylene terephthalate units in the PBT resin can be calculated using the following formula.

[0030]

number

[0031] [Table 1]

[0032] [Table 2]

[0033] [Table 3]

[0034] Tables 1 to 3 show that in all of Examples 1 to 5, the recovery rates of both dimethyl terephthalate and 1,4-butanediol were higher than those of all of the comparative examples. In particular, 1,4-butanediol was recovered at a sufficient recovery rate. Moreover, in all of Examples 1 to 5, the decomposition rate of PBT resin was 90% or higher in a short reaction time of 30 minutes or less, demonstrating that depolymerization of PBT resin can be carried out in a short period of time. In other words, it can be seen that the method for depolymerizing PBT resin of this embodiment makes it possible to recover 1,4-butanediol in particular at a sufficient recovery rate and in a short period of time.

Claims

1. Step A: mixing polybutylene terephthalate resin and methanol; and step B of bringing the methanol to a supercritical state and reacting it with the polybutylene terephthalate resin to depolymerize it into dimethyl terephthalate and 1,4-butanediol; In step A, the mixture is mixed so that the molar ratio of the methanol to 1 mole of butylene terephthalate units in the polybutylene terephthalate resin is 100 to 500; In the step B, the reaction temperature is 245 to 270°C.

2. 2. The method for depolymerizing a polybutylene terephthalate resin according to claim 1, wherein in step B, the reaction time between the polybutylene terephthalate resin and the methanol in a supercritical state is 30 minutes or less, and the decomposition rate of the polybutylene terephthalate resin is 90% or more.

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