JP2006232701A - Method for recovering ester monomer from polyester fiber waste - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for recovering a purified ester monomer from polyester fiber waste in a good efficiency. <P>SOLUTION: This method for recovering the ester monomer from the polyester fiber waste comprises (1) a process of de-polymerizing the polyester fiber waste with ethylene glycol (EG) to make an EG solution containing the ester monomer, (2) a process of filtering insoluble materials in the EG solution at a temperature over 35°C, (3) a process of crystallizing the ester monomer by cooling the EG solution to ≤35°C, (4) performing a solid-liquid separation of the crystallized ester monomer and liquid component to obtain a crude ester monomer, (5) a process of dissolving the crude ester monomer with unused EG to make an EG solution and (6) a process of obtaining the purified ester monomer by purifying the EG solution. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for recovering ester monomers from polyester fiber waste. More specifically, the present invention relates to a method for efficiently recovering a purified ester monomer from polyester fiber waste or a granulated product thereof containing a coloring component of a dye or pigment.

  Polyester, for example, polyethylene terephthalate (hereinafter sometimes abbreviated as PET) is widely used as a fiber, film, resin molded product, etc. due to its excellent properties. Effective use of polyester waste such as resin molded product waste is a problem to be solved from the viewpoint of cost reduction. Moreover, although the fiber, film, and resin molded product used as polyester products are usually discarded after use, this disposal is problematic as it deteriorates the environment. Therefore, as these treatment methods, material recycling, thermal recycling, chemical recycling, and the like have been studied, and various proposals have been made.

Of these, as material recycling, the collected PET bottle waste is pulverized, separated, and purified to produce beautiful PET flakes, and then the flakes are reused as PET molding materials. . However, it is difficult to ensure uniform quality for the fiber waste, and it is extremely difficult to adopt this recycling method.
Thermal recycling, which converts polyester waste into fuel, has the advantage of reusing polyester combustion heat, but there are problems of loss of polyester raw material and generation of carbon dioxide, which is not preferable from the viewpoint of resource saving and environmental conservation. .
Chemical recycling, on the other hand, returns polyester waste to the raw material monomer, which is then subjected to a polycondensation reaction again to produce new polyester, so there is little deterioration in quality associated with the recovery process, and it is suitable for closed-loop recycling. .

  As a recycling method of polyester waste by this chemical recycling, for example, in Patent Document 1, resin-like, fiber-like, or film-like polyester waste generated in a polyester production process is treated with ethylene glycol (hereinafter referred to as EG) with a specific amount ratio. Proposal of a method for obtaining a regenerated polyester by subjecting the resulting aromatic dicarboxylic acid bisglycol ester and its low polymer (BHT) to an unpurified polycondensation reaction again after depolymerization. Has been. However, this method has the advantage that the depolymerization time can be shortened and BHT with less impurities such as diethylene glycol can be obtained. As a result, a polymer having a high quality, particularly a high softening point, can be obtained. In addition, it is not possible to prevent the coloring caused by the ink, and there is a disadvantage that only a special case in which the composition of the polyester waste and the recycled polyester is the same can be adopted.

In addition, when fibers outside the polyester manufacturing process are to be collected, it may be unavoidable to mix fibers different from polyesters such as polyethylene, polypropylene, nylon, and cotton. Furthermore, even if the material is polyester, the one containing a dye is decomposed during a series of reactions such as depolymerization and dispersed in the recovered ester monomer as a coloring component, and the quality is remarkably deteriorated.
Therefore, in Patent Document 2, when recovering terephthalic acid from polyester fiber waste, a polyester fiber waste separated from other fibers is pulverized and granulated into a crude polyester, and in the pretreatment step, The resulting crude polyester is treated by the glycolysis-transesterification method, and the resulting dimethyl terephthalate is hydrolyzed to form terephthalic acid. Further, alkylene glycol is added to and mixed with this terephthalic acid, which is suitable for the polymerization reaction. A method for recovering the slurry as a slurry of terephthalic acid / alkylene glycol ratio has been proposed.
Further, in Patent Document 2, when the crude polyester contains a dye, the crude polyester is treated with a solvent having a temperature of 100 to 190 ° C. (for example, water, alkylene glycol, dimethylformamide, paraxylene, 2-heptanone, etc. It is described that it is transported to the reaction step after passing through a discharging process in which the dye is extracted by putting it into the inside.

However, according to the study of the present inventor, the dye contained in the granulated crude polyester cannot be substantially extracted and removed with water or alkylene glycol, the efficiency is low even with other extractants, and the subsequent processing steps It has become clear that the problem is that the product quality is degraded when the extractant is loaded and the extractant remains.
In addition, as a method for chemically recycling PET bottle waste, the present inventors depolymerized PET bottle waste with excess ethylene glycol (EG), and obtained mixed solution of bis-β-hydroxyethyl terephthalate and EG was activated carbon. A method of obtaining high-purity bis-β-hydroxyethyl terephthalate by subjecting it to a purification treatment combining treatment, ion exchange treatment, crystallization treatment, etc., and further subjecting it to molecular distillation treatment (for example, Patent Literature 3, Patent Literature 4 ).

However, when this method is applied to the chemical recycling of polyester fiber waste, it is clear that coloring components such as dyes contained in the polyester fiber waste overload the refining treatment, and the method cannot be applied as it is. became.
Moreover, it was not clear how much the coloring component contained in the fibrous polyester was removed, and chemical recycling could be efficiently performed without increasing the load of the purification treatment.
JP-A-48-61447 JP 2003-128626 A JP 2000-169623 A JP 2000-255839 A

  This inventor earnestly examined in order to develop the chemical recycling method of polyester fiber waste. As a result, it has been found that according to the following method, a purified ester monomer with high purity can be efficiently obtained. That is, an ester component constituting polyester fiber waste or a granulated product thereof (for example, chips, pellets, etc.) is depolymerized by EG to obtain a glycol solution (depolymerization solution) containing an ester monomer. Next, the crude ester monomer is obtained by subjecting the glycol solution to a treatment for removing coloring components (removal treatment for coarse particles such as pigments by filtration, removal treatment for colored substances such as dyes by crystallization separation). Next, the crude ester monomer is redissolved in unused EG containing no colorant to form an EG solution. Subsequently, the EG solution is subjected to purification treatment (adsorption treatment, ion exchange treatment, crystallization treatment, distillation treatment, or a combination thereof).

  Accordingly, an object of the present invention is to provide a method for efficiently recovering ester monomers purified from polyester fiber waste by eliminating the problems associated with chemical recycling of conventional polyester fiber waste, particularly PET fiber waste. is there. Other objects and advantages of the present invention will become apparent from the following description.

An object of the present invention is a method of recovering ester monomers from polyester fiber waste,
(1) a depolymerization step in which an ester component constituting polyester fiber waste or a granulated product thereof is depolymerized with ethylene glycol to form an ethylene glycol solution containing an ester monomer;
(2) A filtration step of filtering insoluble matters in the ethylene glycol solution at a temperature exceeding 35 ° C.,
(3) A crystallization step of cooling the ethylene glycol solution obtained by the filtration treatment to 35 ° C. or less to crystallize the ester monomer,
(4) A monomer separation step of obtaining a crude ester monomer by solid-liquid separation of the crystallized ester monomer and the liquid component,
(5) a dissolution step of dissolving the crude ester monomer in unused ethylene glycol to obtain an ethylene glycol solution, and (6) a monomer purification step of purifying the ethylene glycol solution to obtain a purified ester monomer.
This is achieved by a method of recovering ester monomers from polyester fiber waste comprising:

In the present invention, after the monomer separation step (4) and before the dissolution step (5), until the coloring degree of the crude ester monomer is 50 or more in L value,
(A) a step of dissolving a crude ester monomer in unused ethylene glycol to form an ethylene glycol solution;
(B) an intermediate step comprising cooling the ethylene glycol solution to 35 ° C. or lower to crystallize the ester monomer, and (c) solid-liquid separation of the crystallized ester monomer and the liquid component to obtain a crude ester monomer It is preferable to carry out a purification step.

  Moreover, it is preferable that the ester component which comprises a polyester fiber waste is a polyethylene terephthalate, and an ester monomer is bis (2-hydroxyethyl) terephthalate. Furthermore, in the monomer purification step, it is preferable to perform filtration, adsorption, ion exchange, crystallization, distillation or a combination thereof.

  According to the present invention, polyester fiber waste, for example, polyester fiber waste colored with dye, polyester fiber waste colored with dye and pigment, mixed fiber of polyester fiber colored with dye and polyester fiber colored with pigment ( For example, a method of efficiently recovering ester monomers purified from polyester fiber waste by eliminating the problems of chemical recycling of fiber waste made of blended fiber, mixed woven fiber, mixed knitted fiber, mixed nonwoven fabric fiber, etc.) Can be provided.

<Polyester fiber waste>
As the polyester fiber waste in the present invention, first, waste generated in the production process of polyester fibers such as short fibers and long fibers can be mentioned. Moreover, the waste which generate | occur | produced in processing processes, such as textiles using these polyester fibers, a knitted fabric, a nonwoven fabric, and cotton filling, a dyeing process, a sewing process, etc. can be mentioned. Furthermore, polyester fiber (product) scraps such as trainers, jerseys, fleeces and umbrellas collected after being shipped to the market can be mentioned. Among these, polyester fiber (product) waste collected after being shipped to the market is preferable from the viewpoint of closed-loop recycling.

  Polyester fiber waste is discarded in the form of polyester fibers alone or in the form of mixed fibers with other fibers (for example, natural fibers) (for example, mixed spun fibers, mixed woven fibers, mixed knitted fibers, mixed non-woven fibers). There are many cases. When other fibers are mixed, it is preferable that the ratio of the polyester fiber is 50% by weight or more, further 70% by weight, particularly 90% by weight or more in order to increase the efficiency of the recovery treatment. It is preferable to use the UV discrimination method for determining whether or not the fiber waste is mainly made of polyester fibers, or for determining the mixing ratio, but other methods may be used. It is preferable not to use the polyester fiber with a small proportion.

  A matte agent (for example, titanium oxide, silicon oxide, etc.) is usually used for the polyester fiber, and it is white (toned) fiber. And the colored fiber of various colors is manufactured by dyeing the fiber of this color. Also, black polyester fibers such as carbon black may be used instead of the matting agent to form black polyester fibers. In that case, a color tone improving agent (for example, dye etc.) may be used in combination with a black pigment.

  Therefore, the polyester fiber waste in the present invention is intended for white (toned) fibers using this matting agent, black pigments such as carbon black instead of this matting agent, and those colored with other colorants. can do. Moreover, an uncolored thing can also be made into object. Moreover, the mixture of a non-colored thing and the colored thing can also be made into object. The colored polyester fiber is composed of a polyester fiber colored with a dye, a polyester fiber colored with a pigment, or a mixed / woven fiber of a polyester fiber colored with a dye and a polyester fiber colored with a pigment. There are no particular restrictions on the dyes and / or pigments used to color the polyester, for example, direct dyes, acid dyes, basic dyes, mordant dyes, acid mordant dyes, vat dyes, disperse dyes, reactive dyes, fluorescent whitening Examples thereof include dyes, natural organic pigments, natural inorganic pigments, synthetic organic pigments, and synthetic inorganic pigments. Of these, disperse dyes are preferred.

  In addition, in order to impart required properties such as lubricity, bundling property, antistatic property, wettability, and water repellency to polyester fibers, oil agents (eg, mineral oils, animal and vegetable oils, aliphatic esters, aromatic esters) are usually used. In many cases, a sulfur-containing ester, an alkylene oxide copolymer, a lubricant such as silicone oil, a surfactant such as a nonion, an anion, a cation, or an amphoteric surfactant is attached.

  The polyester fiber waste is not particularly limited as long as it has a size and shape that does not hinder the introduction into the depolymerization step. In order to increase the handling efficiency of the input to the depolymerization apparatus, the fiber length is preferably 1 to 20 cm, more preferably 2 to 10 cm if it is fibrous. Moreover, it is preferable that the length of one side of the fabric-like thing is 1-20 cm of the length near the said fiber length, Furthermore, it is 2-10 cm. Further, the polyester fiber waste is preferably subjected to a depolymerization reaction after granulation when the handling property at the time of the depolymerization treatment is increased or the depolymerization apparatus has structural limitations. In such a case, preferable examples of the granulation method include a method of melting and treating with a pelletizer, a method of melting only a part of the surface of the polyester fiber and granulating, a method of compressing and granulating the polyester fiber, and the like. it can. The shape of the granulated product is preferably a cylindrical solid, and the size of the solid is 1 to 20 mm, more preferably 2 to 10 mm, and the length is 1 to 60 mm, further 2 to 50 mm. It is preferable.

  As the polymer constituting the polyester fiber waste, polyethylene terephthalate is particularly preferable. The polyethylene terephthalate is preferably a homopolymer, but may be a copolymer obtained by copolymerizing the third component in a small proportion (for example, 20 mol% or less with respect to the total acid component). Further, it may be a blend polymer in which other condensed resin is mixed in a small proportion (for example, 20% by weight or less based on the total weight). These polymers usually contain modifiers (for example, pigments, stabilizers, etc.) for imparting properties required for fibers. Examples of the pigment include titanium oxide, carbon black, bengara, silica gel, and the like. Moreover, as a stabilizer, a phosphorus compound, a hindered phenol type compound, a thioether type compound, etc. can be illustrated.

<Depolymerization step (1)>
The depolymerization step is a step of obtaining an EG solution containing an ester monomer by depolymerizing the polyester fiber waste or the ester component constituting the granulated product using EG to obtain an ester monomer. In this depolymerization treatment, the polyester fiber waste is usually subjected to the depolymerization as it is, but depending on the case, the handling property at the time of the depolymerization treatment is increased or the depolymerization apparatus has structural restrictions. Is preferably granulated and then subjected to a depolymerization reaction. The shape of the granulated product is preferably a cylindrical solid, and the diameter of the solid is 1 to 20 mm, more preferably 2 to 10 mm, and the length is 1 to 60 mm, further 2 to 50 mm. Is preferred.

  The ester component constituting the polyester is a unit in which a dicarboxylic acid and a diol component are bonded via an ester bond, and includes a polymer having a polymerization degree of 70 to 200 to an oligomer having a polymerization degree of 2 to 10. Therefore, according to the present invention, the ester component is depolymerized and decomposed into ester monomers exemplified by bis (2-hydroxyethyl) terephthalate (hereinafter sometimes abbreviated as BHET) having one dicarboxylic acid unit. .

  The EG used in the depolymerization step preferably has a purity of 80% by weight or more. The EG can contain a solid content as an impure component, for example, an ester solid content such as BHET, preferably 10% by weight or less. Further, a liquid component as an impure component, for example, water, diethylene glycol and the like can be preferably contained in an amount of 10% by weight or less. There is no particular limitation as long as the EG satisfies this condition. For example, EG obtained from the market, EG of filtrate or evaporation generated in the monomer purification step described later (recovered EG), and EG (distilled and purified) obtained by distillation purification of these Any of EG) can be used. Moreover, although these EG can be used independently, 2 or more types of EG can be mixed and used (for example, collection | recovery EG and distillation refinement | purification EG are mixed). Of these, it is preferable to use an EG obtained from the market (particularly, an EG that satisfies the provisions of JIS K 1527).

  The depolymerization temperature is preferably 160 to 270 ° C, more preferably 170 to 250 ° C. The amount ratio of polyester and EG at the time of depolymerization is preferably 1: 9 to 3: 7 by weight. Preferred examples of the depolymerization catalyst include sodium hydroxide, sodium carbonate, sodium methylate, and zinc acetate. Further, the amount of the catalyst is preferably 0.05 to 0.50% by weight, more preferably 0.15 to 0.40% by weight, based on the polyester. The depolymerization reaction time is preferably 0.5 to 7.0 hours, more preferably 0.5 to 5.0 hours.

As the matting agent, particularly the titanium oxide flocculant, an alkaline earth metal compound, particularly calcium hydroxide is preferably mentioned. Moreover, as an addition amount, it is preferable that it is 0.05-0.50 weight% with respect to polyester, Furthermore, it is preferable that it is 0.15-0.40 weight%.
The concentration of the solid content in the obtained EG solution is preferably 10 to 30% by weight, more preferably 15 to 25% by weight. This solid content is mainly composed of an ester monomer, and the ratio of the oligomer in the solid content is 1 to 30% by weight, and further 2 to 20% by weight.
When the ester component constituting the polyester fiber waste is an ethylene terephthalate unit, BHET is obtained as an ester monomer by depolymerization.

  In this case, if the amount of polyester fiber waste is too small relative to EG, the amount of BHET produced will be less than the saturation solubility of EG and less than the maximum yield obtained for the total liquid volume obtained in the depolymerization step. BHET can be obtained only in an amount, and the efficiency of the crystallization step (3) and monomer separation step (4) described later is not economical. On the other hand, if the amount of the polyester fiber product is too large relative to the EG, the oligomer of BHET increases and the yield of BHET decreases. Further, if BHET is present exceeding the saturation solubility of EG, BHET is precipitated, so that deionization treatment cannot be performed.

  The depolymerization is preferably performed while a rectifying column is provided in the reaction apparatus and water is distilled from the reaction solution to the outside of the system. At that time, the evaporated EG is preferably returned to the system. Since the amount of water in the EG solution obtained by performing the depolymerization reaction in this manner can be reduced, the hydrolysis reaction of the ester monomer can be suppressed. It is preferable to adjust the amount of water contained in the EG solution to 0.5% by weight or less. The amount of moisture can be obtained by measuring the solution with a Karl Fischer moisture meter MKC-510N manufactured by Kyoto Electronics Industry Co., Ltd.

(Preliminary depolymerization)
In the depolymerization step, before depolymerization by EG, ester components constituting polyester fiber waste are preliminarily depolymerized using polyester monomers and / or oligomers, and then the predepolymerized product is depolymerized by EG ( This depolymerization can also be carried out. When the polyester is PET, the polyester monomer is preferably BHET and the oligomer is preferably a BHET oligomer.

  The preliminary depolymerization is preferably carried out by kneading the polyester fiber product with its monomers and / or oligomers (degree of polymerization 2 to 10) at a temperature of 200 to 300 ° C, preferably 230 to 280 ° C. In addition, the preliminary depolymerization time is preferably 1.0 minute to 5.0 hours, more preferably 1.0 minute to 1.5 hours. At that time, the amount of the monomer and / or oligomer (degree of polymerization 2 to 10) with respect to the polyester is 0.1 to 1.0 part by weight, and further 0.2 to 0.8 part by weight per 1 part by weight of the polyester. It is preferable. The resulting pre-depolymerized product is composed of an ester component. The molecular weight is preferably 2 to 20, more preferably 3 to 10 in terms of polymerization degree.

  The preliminary depolymerization is usually carried out under normal pressure, but at that time, 30% by weight or less, and further 1 to 25% by weight of low-boiling components such as free EG and water are supplied into the system together with the polyester fiber product. In such a case, it is preferable to carry out the process while evaporating and removing them out of the system. Moreover, you may implement under reduced pressure of the grade which the evaporate from a reaction system does not inhibit progress of reaction. In some cases, the evaporation may be performed under pressure, preferably not more than the vapor pressure of EG (for example, 1.3 MPa or less) while preventing the evaporated product from returning to the system.

  Next, the pre-depolymerized product is removed from foreign matters (carbides, etc.) that have not been pre-depolymerized with a metal strainer having a pore diameter of 0.1 to 2 mm, for example, and then in the presence of a depolymerization catalyst using EG. The depolymerization (main depolymerization) is preferably performed at a temperature of 180 to 250 ° C. Preferred examples of the depolymerization catalyst include sodium hydroxide, sodium carbonate, sodium methylate, and zinc acetate. Further, the amount of the catalyst is preferably 0.05 to 0.50% by weight, more preferably 0.15 to 0.40% by weight, based on the value obtained by converting the preliminary depolymerized product to polyester. The depolymerization reaction time is preferably 0.5 to 5.0 hours, more preferably 0.5 to 4.0 hours. At that time, preferred examples of the titanium oxide flocculant include alkaline earth metal compounds, and further calcium compounds (for example, calcium hydroxide, calcium carbonate, etc.), and calcium hydroxide is particularly preferred. The amount of EG is preferably 2 to 10 parts by weight, more preferably 3 to 7 parts by weight per part by weight of the pre-depolymerized product. The solid content of the obtained EG solution (depolymerization solution) is mainly composed of a polyester ester monomer, and the proportion of the oligomer is reduced to 1 to 30% by weight, and further to 2 to 20% by weight. The concentration of the solid content is preferably 10 to 30% by weight, more preferably 15 to 25% by weight.

<Filtering step (2)>
The filtration step is usually a step for filtering out the insoluble material from the EG solution because the EG solution (depolymerization solution) contains an insoluble material (a material insoluble in EG). The filtration step is performed at a temperature exceeding 35 ° C, preferably 38 to 270 ° C, more preferably 40 to 250 ° C. In the filtration step, it is preferable to perform the following first-stage filtration treatment, second-stage filtration treatment, or both. When both the first stage filtration process and the second stage filtration process are performed, it is preferable to perform the second stage filtration process after the first stage filtration process.

(First stage filtration)
The first-stage filtration process is a process for removing a coarse substance by filtering the EG solution (depolymerization solution). This filtration treatment is preferably performed to remove coarse substances that have not been decomposed by the depolymerization reaction among the insoluble substances, for example, solid foreign matters such as natural fiber waste. For example, it is preferable to pass the EG solution through a metal strainer having a pore diameter of 0.1 to 2 mm. At that time, the solution is maintained at a temperature exceeding 35 ° C., preferably 38 to 270 ° C., more preferably 40 to 250 ° C.

(Second stage filtration)
The second-stage filtration process is a process for filtering the EG solution to remove relatively large solid particles. This filtration treatment preferably removes solid particles that have not been decomposed by the depolymerization reaction, such as solid foreign matters having an average particle diameter of about 1 to 500 μm (such as pigments such as titanium oxide and carbon black). For example, it is preferable to pass the EG solution through a filtration device in which a filter medium made of 3 to 20 dtex fibers (fibrous polyester, polypropylene fibers, etc.) is filled with a porosity of 70 to 98%. At that time, the solution is filtered while being cooled and maintained at a temperature exceeding 35 ° C., preferably 38 to 150 ° C., more preferably 40 to 90 ° C.

  In addition, relatively large solid particles such as glycol-insoluble pigments and titanium oxide are aggregated to about 0.5 to 10 μm by adding an aggregating agent in the depolymerization step, and exceed 35 ° C. It is preferable to remove by a second-stage filtration treatment with a filter such as a fiber filter medium in the temperature range of 38 to 150 ° C., particularly 40 to 90 ° C.

(Liquid separation process)
When the oil agent component is present in the EG solution, it is preferable to separate the oil agent component liquid-liquid. The liquid-liquid separation process is a process for separating and removing the oil agent component because it exists in a liquid phase different from that of the EG solution and cannot be removed in the filtration process when the oil agent component is present in the EG solution. . In order to impart required properties such as lubricity, bundling properties, antistatic properties, wettability, water repellency to polyester fibers, usually oil agents (for example, mineral oils, animal and vegetable oils, aliphatic esters, aromatic esters, Lubricants such as sulfur-containing esters, alkylene oxide copolymers, silicone oils, and surfactants such as nonions, anions, cations, and amphoteric surfactants are attached. In order to remove this oil agent, it is preferable to separate the liquids using an apparatus such as a separatory funnel, a decanter, a separation plate type centrifuge, a basket type centrifuge. This separation process can be performed either before the first stage filtration process, between the first stage filtration process and the second stage filtration process, or after the second stage filtration process. It is particularly preferred to carry out between the first stage filtration process and the second stage filtration process. At that time, the solution is preferably treated at a temperature exceeding 35 ° C., further at a temperature of 38 to 150 ° C., particularly 40 to 90 ° C.

<Crystalling step (3), monomer separation step (4)>
In the crystallization step (3), the EG solution treated in the filtration step is cooled to 35 ° C. or lower, preferably −10 to 30 ° C. to crystallize the ester monomer which is a solid component. In the monomer separation step (4), the EG solution containing a crystallization product mainly containing an ester monomer as a solid obtained in the crystallization step is maintained at a low temperature to perform the pressure filtration, vacuum filtration, and centrifugation. Solid-liquid separation is performed by separation. This crystallization / monomer separation step is a step of leaving a colored component (particularly a dye) in a liquid component mainly composed of EG and separating an ester monomer component which is a solid component as a crude ester monomer cake. The crude ester monomer cake usually contains 30 to 60% by weight, more preferably 35 to 55% by weight of a solvent component, although it depends on the performance of the solid-liquid separator.

  Components of synthetic fibers such as nylon that are decomposed by depolymerization and dissolved in the EG solution by performing crystallization treatment (for example, glycol ester of 5-aminocaproic acid, hexamethylenediamine, glycol ester of adipic acid, etc. ) Is also dissolved in the EG and can be separated from the crude ester monomer.

<Intermediate purification process>
If the target dye or the like cannot be removed by a single crystallization / monomer separation process due to the ratio of the ester monomer component, the type and concentration of the dye, etc., the monomer is dissolved after the monomer separation step (4). Before the step (5), until the coloring degree of the crude ester monomer is 50 or more in L value,
(A) a step of dissolving a crude ester monomer in unused ethylene glycol to form an ethylene glycol solution;
(B) an intermediate step comprising cooling the ethylene glycol solution to 35 ° C. or lower to crystallize the ester monomer, and (c) solid-liquid separation of the crystallized ester monomer and the liquid component to obtain a crude ester monomer It is preferable to carry out a purification step.

  The number of times of performing the intermediate purification treatment is preferably 1 or more, and more preferably 1 to 2 times. The coloring degree of the crude ester monomer thus obtained is preferably 50 or more, more preferably 60 or more in terms of L value. The degree of coloration of the crude ester monomer is determined by directly filling a cake in a round cell for reflection measurement and measuring with a color measuring instrument ZE-2000 manufactured by Nippon Denshoku Industries Co., Ltd.

  The unused EG used in this step includes EG obtained by distillation purification of a filtrate or an evaporate generated in a monomer purification step described later, in addition to EG obtained from the market. The EG obtained from the market usually satisfies the quality defined in JIS K 1527. As can be understood from the above, this unused EG has a colorless and transparent appearance and is free of suspended matters. Particularly, the purity is 99% by weight or more, further 99.5% by weight or more, and the coloration degree (APHA) is 100 or less. Further, it is preferably 50 or less. The APHA of the EG is obtained by comparing the sample with the standard colorimetric liquid (standard color) with the naked eye using a colorimetric tube (inner diameter 25 mm, height 250 mm), and this is used as the coloring degree of the EG. . A standard colorimetric solution is prepared by adding pure water to a mixture of 2.492 g of potassium chloroplatinate, 2.038 g of cobalt chloride and 10 ml of special grade hydrochloric acid to prepare 1 liter of standard colorimetric solution. 1, 2, 3, 4, 5, 10 and 20 ml of this undiluted solution are collected, and pure water is added to make 100 ml each, which is used as a standard colorimetric solution. The APHA of this standard colorimetric solution is 10, 20, 30, 40, 50, 100, and 200 in order from the lowest concentration of the stock solution, and the APHA of pure water only is 0.

  By using EG of this quality in the intermediate purification step, the coloring degree (L value) of the crude ester monomer can be efficiently increased to a desired value or more. Further, by obtaining the coloring degree of the crude ester monomer in this way, the degree of removal of the coloring substance can be known, and thereby it can be known whether or not the crude ester monomer can be subjected to the monomer purification step.

<Dissolution step (5)>
In the dissolution step (5), the crude ester monomer cake thus obtained is used in an unused EG (coloring) so that the content of the ester monomer component is preferably 10 to 30% by weight, more preferably 15 to 25% by weight. In EG) that does not contain an agent, and again into a crude ester monomer EG solution. The dissolution temperature at this time is preferably 60 to 95 ° C, more preferably 70 to 90 ° C.
The unused EG used in this step includes EG obtained by distillation purification of a filtrate or an evaporate generated in a monomer purification step described later, in addition to EG obtained from the market. The EG obtained from the market usually satisfies the quality defined in JIS K 1527. As can be understood from the above, this unused EG has a colorless and transparent appearance and is free of suspended matters. Particularly, the purity is 99% by weight or more, further 99.5% by weight or more, and the coloration degree (APHA) is 100 or less. Further, it is preferably 50 or less.

<Monomer purification step (6)>
The monomer purification step (6) is a step of obtaining a purified ester monomer by subjecting the EG solution obtained in the dissolution step to a purification treatment. The monomer purification step includes an adsorption treatment, an ion exchange treatment, a crystallization treatment, a distillation treatment, or a combination thereof.
In order to remove trace amounts of impurities, side reaction promoting components, etc. contained in the EG solution, it is preferable to remove them by adsorption treatment, ion exchange treatment, or a combination thereof. In other words, fine particles such as a small amount of pigment that could not be removed by the previous color component removal treatment can be removed by adsorption treatment with activated carbon, and ionic substances such as catalyst residues can be removed by ion exchange treatment for purification. preferable.

Moreover, it is preferable to remove the EG soluble impurities and by-products that could not be removed by the treatment so far by crystallization treatment. Further, it is preferable to remove high-boiling impurities such as residual low-boiling substances and oligomers that cannot be removed by crystallization treatment by distillation treatment.
Therefore, in the monomer purification step of the present invention, it is preferable to perform the adsorption treatment, ion exchange treatment, crystallization treatment, and distillation treatment in this order.

(Adsorption treatment)
The adsorption process is a process in which impurities in the EG solution are adsorbed by an adsorbent such as activated carbon or bone charcoal. In the adsorption treatment, the EG solution is maintained at a temperature of 60 to 95 ° C., preferably 70 to 90 ° C., and is passed through an adsorption tower packed with activated carbon at a space velocity (SV) of 0.1 to 5.0 hr ^−1. Preferably it is done. This adsorption treatment also removes a colorant such as a fine pigment and can be called a decolorization treatment. Examples of the activated carbon include “Dia Hope 006” and “Dia Hope 008” manufactured by Mitsubishi Chemical Corporation.

(Ion exchange treatment)
The ion exchange treatment is a treatment for removing an ionic substance such as a catalyst residue in the EG solution or an ionic substance derived from an amine component constituting a synthetic fiber such as nylon (for example, hexamethylenediamine). In the ion exchange treatment, the EG solution is maintained at a temperature of 60 to 95 ° C., preferably 70 to 90 ° C., and passed through a decation tower packed with a cation exchanger at a space velocity (SV) of ¹ to 12 hr ^−1. Te cation exchange treatment, then, connecting the pipe to the de-anion column were allowed to pass filled with an anion exchanger at 3 seconds to 10 minutes at a space velocity (SV) 0.5~10hr ^-1 and liquid passing anions Exchange processing is preferable. Preferable examples of the cation exchanger include cation exchange resin “Amberlite IR-120B” manufactured by Rohm and Haas. Moreover, as an anion exchanger, the mixture etc. of anion exchange resin "Amberlite IRA96SB" by Rohm and Haas company and a cation exchange resin "Amberlite IR-120B" etc. can be mentioned preferably, for example. The mixing ratio (volume ratio) of the anion exchange resin and cation exchange resin is preferably 1: 3 to 5: 1.

(Crystal treatment)
The crystallization process is a process of cooling the EG solution, precipitating solid components such as ester monomers, and solid-liquid separation. In the crystallization treatment, the solution after the deionization treatment (deionization treatment solution) is cooled in the crystallization tank from a temperature equal to or higher than the saturation solubility to a temperature in the range of 15 to 30 ° C. It is preferable to deposit the ester monomer so that the average particle size of the precipitate is 40 to 200 μm (measured by diluting 10 times with EG using a Shimadzu SALD-200VER) while maintaining for 12 hours. When the deionized solution is cooled from a temperature equal to or higher than the saturation solubility, it is preferably slowly cooled, for example, at a rate of 0.1 to 0.5 ° C./min.

The obtained precipitate is separated into solid components by solid-liquid separation. Solid-liquid separation is preferably performed while maintaining the temperature during the crystallization treatment, that is, the temperature in the range of 15 to 30 ° C. The method for solid-liquid separation is not particularly limited, and preferred examples include pressure filtration, vacuum filtration, and centrifugation. For example, when pressure filtration is performed, the precipitate is preferably filtered by a filter press using a filter cloth having an air permeability of ^{3 to 30} cm ³ / min · cm ² . It is preferable to filter using 30 μm filter paper or filter cloth. Furthermore, recrystallization purification treatment may be performed using an organic solvent such as isopropyl alcohol.

  In the crystallization of the EG solution, components derived from synthetic fibers such as nylon, which are included in the crude ester monomer cake obtained in the monomer separation step (4), are brought into the EG solution, and are dissolved in the solution. A part of (for example, 5-aminocaproic acid glycol ester, adipic acid glycol ester, etc.) is dissolved in the filtrate and thus separated from the solid content.

(Distillation process)
The distillation treatment is a treatment for obtaining a purified ester monomer by distilling and purifying a solid content such as an ester monomer obtained by a crystallization treatment. In the distillation treatment, a filter cake such as ester monomer obtained in the crystallization treatment is dissolved at 70 to 120 ° C., and the melt is introduced into the first evaporator, and the temperature is 130 to 170 ° C. and the pressure is 300 to 1,000 Pa. The low boiling point component is evaporated under the conditions (first evaporation), and then the melt that has undergone the first evaporation is introduced into the second evaporator, and the low boiling point component is evaporated under the conditions of a temperature of 130 to 170 ° C. and a pressure of 50 to 250 Pa. (Second evaporation) is preferred. Next, it is preferable to introduce the melt obtained through the second evaporation into a falling film molecular distillation apparatus, and perform distillation (molecular distillation) under conditions of a temperature of 180 to 220 ° C. and a pressure of 25 Pa or less to obtain an ester monomer as a fraction.

By purifying the ester monomer by the above-described operation, a high-quality ester monomer, preferably bis (2-hydroxyethyl) terephthalate (BHET) can be obtained efficiently. The purity of the purified BHET is preferably 90% by weight or more, more preferably 95% by weight or more, and the optical density (OD ₃₈₀ ) at ₃₈₀ nm is 0.000 to 0.010, more preferably 0.000 to 0.008, and particularly preferably 0.00. It is preferable that it is 000-0.006.
This purified BHET can be produced in the presence of a polymerization catalyst to produce high-quality (for example, excellent color tone) polyethylene terephthalate.

  Hereinafter, the present invention will be described more specifically with reference to examples. Needless to say, the present invention is not limited to this embodiment. Further, “part” in the example represents “part by weight”, and “capacity part” is a unit that can be converted into 1 part by weight, for example, in the case of water. The characteristics in the examples are measured by the following method.

<UV discrimination method>
1 g of sample fiber was dissolved in 500 ml of a mixed solution of phenol / tetrachloroethane = 3/2. The insoluble matter remaining after suction filtration with 5A filter paper is washed with methanol and dried to determine the insoluble matter contamination rate. Subsequently, the absorbance at 310 nm of the filtrate was measured with an ultraviolet-visible spectrophotometer “UVmini-1240” (manufactured by Shimadzu Corporation), and the height of the peak corresponding to the waveform of polyethylene terephthalate as a standard substance was compared. The concentration of dissolved polyethylene terephthalate was calculated. The polyester content in the sample fiber is measured from the mixing ratio and the concentration.

<Optical density measurement method (OD ₃₈₀ )>
Dissolve 5 g of the sample in methanol to make a 10 wt% methanol solution. The cell length is 10 mm by UVmini-1240 (manufactured by Shimadzu Corporation), the blank is zero-point corrected using methanol, and the absorbance of these solutions at 380 nm Measure.

<Purity of BHET>
A 50 mg sample is precisely weighed, a 1,000 ppm solution is prepared using chloroform / ethanol = 1/1 (volume ratio), and this is developed on a high performance liquid chromatogram LC-2010HT (manufactured by Shimadzu Corporation). The ratio of each component in the sample was determined from the peak area, and the content of BHET was calculated therefrom.

<Coloring degree (L value)>
The coloring degree of the crude ester monomer is represented by the L value of the obtained cake, and the L value is determined by the following method. The cake was directly filled in the round cell for reflection measurement, and the L value was determined by measuring with a color measuring device ZE-2000 manufactured by Nippon Denshoku Industries Co., Ltd.

[Example 1]
The light blue fleece (tag display: 100% polyester) fastener, tag, and the like were removed and recovered as fiber waste. Moreover, it confirmed that fabric consisted of 100% of polyester fiber by UV method. Subsequently, 102 parts of the fiber waste cut into about 3 cm square were collected. The L value of this cut product was 40.

<Depolymerization step (1)>
560 parts of No. 1 ethylene glycol (EG), 0.25 part of sodium hydroxide (NaOH) and 0.25 part of calcium hydroxide (Ca (OH) ₂ ) specified in JIS K 1527 Into an autoclave equipped with a reflux apparatus and a stirrer, depolymerization was performed for 3 hours at normal pressure and at the boiling point of EG. As a result, a light blue depolymerization solution (EG solution) containing coarse particles of titanium oxide was obtained.

<Filtering step (2)>
The obtained depolymerization solution was passed through a filter medium having a pore diameter of 1 mm at 190 ° C., and a first-stage filtration treatment was performed. Next, this solution was cooled to 85 ° C., and while maintaining this temperature, it was filtered through a filter medium having a retained particle diameter of 6 μm to remove suspended substances in the depolymerization solution, and the filtrate was received in a receiver.

<Crystalling step (3), monomer separation step (4)>
The filtrate obtained in the filtration step was rapidly cooled from 85 ° C. to 40 ° C., and then gradually cooled to 25 ° C. with stirring to perform crystallization to obtain a crystal slurry. Next, the obtained crystal slurry was filtered through a filter medium having a retained particle diameter of 6 μm while maintaining 25 ° C., and a cake was obtained as a filtrate.
The obtained cake was 189 parts, having a solid content of 63% by weight and a liquid component of 37% by weight. The composition in the solid content was 87% by weight of bis (2-hydroxyethyl) terephthalate (BHET), 5% by weight of dimer, 3% by weight of mono (2-hydroxyethyl) terephthalate (MHET), and 5% by weight of others. This cake was light blue. The L value of this cake was 75.

<Dissolution step (5)>
To 150 parts of the obtained cake, 322 parts of EG No. 1 defined in JIS K 1527 was added and dissolved by heating at 85 ° C. to obtain an EG solution.

<Monomer purification step (6)>
(Adsorption treatment)
This EG solution was passed through an 85 ° C. heat-retaining column packed with 30 parts of activated carbon (manufactured by Mitsubishi Chemical Corporation, “Diahope 006”) at a space velocity (SV) of 3.0 hr ⁻¹ and adsorbed with activated carbon ( (Decolorization treatment) was performed.

(Ion exchange treatment)
Next, this EG solution was applied to an 85 ° C. heat retention column packed with 30 parts by volume of a cation exchanger (Rohm and Haas, strong acid cation exchange resin “Amberlite IR-120B”) at a space velocity of 5.0 hr ⁻¹ . The solution was passed through to perform cation exchange treatment. Further, 30 parts by volume of the EG solution (Rohm and Haas, strong acid cation exchange resin “Amberlite IR-120B” and Rohm and Haas, weakly basic anion exchange resin “Amberlite” were used. A mixture of "IRA-96SB" in a volume ratio of 1: 2) is passed through an 85 ° C. heat retention column at a space velocity (SV) of 5.0 hr ⁻¹ to perform anion exchange treatment to obtain a deionized solution. It was.

(Crystal treatment)
The resulting deionized solution was cooled at a rate of 85 ° C. to 0.2 ° C./min while stirring, and maintained at 25 ° C. for 2 hours for crystallization to obtain a crystal slurry. Next, the obtained crystal slurry was filtered through a filter medium having a retained particle diameter of 6 μm while maintaining 25 ° C. to obtain a cake. The filtrate generated at this time was 353 parts.
The obtained cake was 119 parts, having a solid content of 63% by weight and a liquid component of 37% by weight. The composition in the solid content was BHET 89% by weight, dimer 4% by weight, MHET 2% by weight, and other 5% by weight.

(Distillation process)
The obtained cake was heated at 100 ° C. to obtain a melt. In order to distill off low-boiling substances mainly composed of EG, this melt was introduced into a falling film evaporator having a vaporization surface temperature of 150 ° C. and a pressure of 500 Pa to perform first evaporation. Further, the melt obtained by the first evaporation was introduced into a falling thin film evaporator having an evaporation surface temperature of 150 ° C. and a pressure of 133 Pa to perform second evaporation to obtain a concentrate of crude BHET. The obtained concentrated liquid was 79 parts, and the solid content concentration mainly containing BHET was 99.6% by weight. Moreover, the low boiling point substance (evaporated liquid) which generate | occur | produced at this time was 40 parts.
The obtained concentrated liquid was introduced into a falling film type molecular distillation apparatus having an evaporation surface temperature of 195 ° C. and a pressure of 3 Pa to perform molecular distillation to obtain purified BHET. The purified BHET obtained was 55 parts, and the purity was 97.2% by weight and the optical density at 380 nm (OD ₃₈₀ ) was 0.004.

[Example 2]
The fasteners, umbrella bones, and tags of dark red training pants (tag display: 100% polyester) and water-repellent dark blue rain umbrella (tag display: 100% polyester) were removed and recovered as fiber waste. About these, it confirmed that fabric consisted of 100% of polyester fiber by UV method. Next, 51 copies each of which was cut to about 3 cm square were collected. Among these cut products, the L value derived from the dark red training pants was 33, and the L value derived from the dark blue umbrella was 17.

<Depolymerization step (1)>
353 parts of the filtrate generated in the monomer purification step of Example 1, 40 parts of the evaporation liquid, and 167 parts of EG No. 1 defined by JIS K 1527 were mixed to prepare 560 parts of EG having a purity of 97% by weight. 102 parts of this EG mixed with the above-mentioned cut product, 0.25 part of sodium hydroxide (NaOH) and 0.25 part of calcium hydroxide (Ca (OH) ₂ ) were added to an autoclave equipped with a reflux device and a stirrer. The depolymerization was conducted for 3 hours at normal pressure and the boiling point of EG. As a result, a dark red depolymerization solution (EG solution) containing coarse particles of titanium oxide was obtained.

<Filtering step (2)>
The obtained depolymerization solution was passed through a filter medium having a pore diameter of 1 mm at 190 ° C., and a first-stage filtration treatment was performed. Subsequently, this solution was cooled to 85 ° C., and an oily substance floating on the liquid surface was separated by a liquid-liquid separator. Thereafter, while maintaining this temperature, filtration was performed with a filter medium having a retained particle diameter of 6 μm to remove suspended substances in the depolymerization solution, and the filtrate was received in a receiver.

<Crystalling step (3), monomer separation step (4)>
The filtrate obtained in the filtration step (2) was rapidly cooled from 85 ° C. to 40 ° C., and then gradually cooled to 25 ° C. while stirring to obtain a crystal slurry. Next, the obtained crystal slurry was filtered through a filter medium having a retained particle diameter of 6 μm while maintaining 25 ° C., and a cake was obtained as a filtrate.
The obtained cake was 189 parts, having a solid content of 63% by weight and a liquid component of 37% by weight. The composition in the solid content was 87% by weight of BHET, 4% by weight of dimer, 3% by weight of MHET, and 6% by weight of others. This cake was reddish green. The L value of this cake was 45.

<Intermediate purification process>
To 150 parts of the obtained cake, 360 parts of No. 1 EG defined in JIS K 1527 was added and heated at 85 ° C. to completely dissolve the crystals to obtain an EG solution. The EG solution was rapidly cooled from 85 ° C. to 40 ° C., and then cooled to 25 ° C. with stirring to recrystallize to obtain a recrystallized slurry. Thereafter, while maintaining the temperature at 25 ° C., the mixture was filtered with a filter medium having a reserved particle diameter of 6 μm to obtain a cake.
The obtained cake was 120 parts, with a solid content of 68% by weight and a liquid component of 32% by weight. The composition of the solid content was 91% by weight of BHET, 4.5% by weight of dimer, 1.3% by weight of MHET, and 3.2% by weight of others. This cake was light reddish green. The L value of this cake was 80.

<Dissolution step (5)>
To 120 parts of the cake obtained, 290 parts of No. 1 EG defined in JIS K 1527 was added and dissolved by heating at 85 ° C. to obtain an EG solution.

<Monomer purification step (6)>
(Adsorption treatment)
This EG solution was passed through an 85 ° C. heat-retaining column packed with 30 parts of activated carbon (manufactured by Mitsubishi Chemical Corporation, “Diahope 006”) at a space velocity (SV) of 3.0 hr ⁻¹ and adsorbed with activated carbon ( (Decolorization treatment) was performed.

(Ion exchange treatment)
Next, this EG solution was applied to an 85 ° C. thermal insulation column packed with 30 parts by volume of a cation exchanger (manufactured by Rohm and Haas, strong acid cation exchange resin “Amberlite IR-120B”) at a space velocity (SV) of 5.0 hr. ^The cation exchange treatment was performed by passing the solution at ^-1 . Further, 30 parts by volume of the EG solution (Rohm and Haas, strong acid cation exchange resin “Amberlite IR-120B” and Rohm and Haas, weakly basic anion exchange resin “Amberlite” were used. A mixture of "IRA-96SB" in a volume ratio of 1: 2) is passed through an 85 ° C. heat retention column at a space velocity (SV) of 5.0 hr ⁻¹ to perform anion exchange treatment to obtain a deionized solution. It was.

(Crystal treatment)
The resulting deionized solution was cooled at a rate of 85 ° C. to 0.2 ° C./min while stirring, and kept at 25 ° C. for 2 hours for crystallization to obtain a crystal slurry. Next, the obtained crystal slurry was filtered through a filter medium having a retained particle diameter of 6 μm while maintaining 25 ° C. to obtain a cake.
The obtained cake was 114 parts, having a solid content of 63% by weight and a liquid component of 37% by weight. The composition in the solid content was 91% by weight of BHET, 4% by weight of dimer, 1% by weight of MHET, and 4% by weight of others.

(Distillation process)
The obtained cake was heated at 100 ° C. to obtain a melt. In order to distill off low-boiling substances mainly composed of EG, this melt was introduced into a falling film evaporator having a vaporization surface temperature of 150 ° C. and a pressure of 500 Pa to perform first evaporation. Further, the melt obtained by the first evaporation was introduced into a falling thin film evaporator having an evaporation surface temperature of 150 ° C. and a pressure of 133 Pa to perform second evaporation to obtain a concentrate of crude BHET. The obtained concentrated liquid was 72.5 parts, and the solid content concentration mainly containing BHET was 99.5% by weight.
The obtained concentrated liquid was introduced into a falling film type molecular distillation apparatus having an evaporation surface temperature of 195 ° C. and a pressure of 3 Pa to perform molecular distillation to obtain purified BHET. The purified BHET obtained was 51 parts, and the purity was 97.3% by weight, and the optical density (OD ₃₈₀ ) at ₃₈₀ nm was 0.003.

[Comparative Example 1]
<Depolymerization step (1), filtration step (2)>
The same method as in Example 2 was used.

<Crystalling step (3), monomer separation step (4), dissolution step (5)>
For the obtained filtrate (EG solution), the steps of crystallization, monomer separation, and dissolution were omitted and subjected to the next step.

<Monomer purification step (6)>
(Adsorption treatment)
The EG solution obtained in the previous step had a dark red color at 85 ° C. When this EG solution was subjected to an adsorption treatment in the same manner as in Example 2, breakthrough of activated carbon occurred immediately, and the liquid exiting from the activated carbon packed tower became dark red immediately after the start of treatment.

The present invention is a method for efficiently recovering high-purity ester monomers from polyester fiber scraps, particularly colored polyester fiber scraps, and can greatly contribute to the recycling industry.

Claims (4)

A method for recovering ester monomers from polyester fiber waste,
(1) a depolymerization step in which an ester component constituting polyester fiber waste or a granulated product thereof is depolymerized with ethylene glycol to form an ethylene glycol solution containing an ester monomer;
(2) A filtration step of filtering insoluble matters in the ethylene glycol solution at a temperature exceeding 35 ° C.,
(3) A crystallization step of cooling the ethylene glycol solution obtained by the filtration treatment to 35 ° C. or less to crystallize the ester monomer,
(4) A monomer separation step of obtaining a crude ester monomer by solid-liquid separation of the crystallized ester monomer and the liquid component,
(5) a dissolution step of dissolving the crude ester monomer in unused ethylene glycol to obtain an ethylene glycol solution, and (6) a monomer purification step of purifying the ethylene glycol solution to obtain a purified ester monomer.
A method for recovering ester monomers from waste polyester fiber. After the monomer separation step (4) and before the dissolution step (5), until the coloring degree of the crude ester monomer is 50 or more in L value,
(A) a step of dissolving a crude ester monomer in unused ethylene glycol to form an ethylene glycol solution;
(B) an intermediate step comprising cooling the ethylene glycol solution to 35 ° C. or lower to crystallize the ester monomer, and (c) solid-liquid separation of the crystallized ester monomer and the liquid component to obtain a crude ester monomer The method according to claim 1, wherein a purification step is performed. The method according to claim 1 or 2, wherein the ester component constituting the polyester fiber waste is polyethylene terephthalate and the ester monomer is bis (2-hydroxyethyl) terephthalate. The method according to any one of claims 1 to 3, wherein the monomer purification step comprises filtration, adsorption, ion exchange, crystallization, distillation or a combination thereof.