Purification process of bis (2-hydroxyethyl) terephthalate obtained by depolymerizing waste containing polyethylene glycol terephthalate

Abstract

Process for purifying bis (2-hydroxyethyl) terephthalate (BHET) from a crude solution obtained by glycolysis of ethylene glycol (EG) from polyethylene terephthalate (PET) waste containing at least one dissolved barrier polymer, water being added to the crude solution in a predetermined amount as an anti-solvent for the barrier polymer, and controlling the temperature to ensure that the solubility of the barrier polymer is significantly reduced without causing precipitation of BHET and its oligomer. Specifically, water is added to the crude BHET solution in at least two different stages, the weight ratio between EG and H2O is reduced, and the temperature is also reduced. In this way, the barrier polymer can be more effectively removed, and precipitation of BHET and its oligomer is not caused. Furthermore, in this manner, the volume of the liquid to be treated can be maintained within a relatively small value, thereby avoiding equipment defects associated with the treatment of large quantities of liquid by decantation and / or filtration, which may cause the process to be inconvenient from the industrial perspective.

Description

[0001] The invention relates to a purification process for bis(2-hydroxyethyl)terephthalate (BHET) obtained by depolymerizing waste materials containing polyethylene terephthalate (PET).

[0002] Polyethylene terephthalate (PET) is a widely used semi-crystalline thermoplastic polyester with high strength and transparency. Due to its physical and chemical properties, it has a wide range of applications, especially in packaging and fiber production. PET does not present any safety risks, but its increasing consumption and accumulation in waste streams as well as its non-biodegradability raise environmental and economic issues. Therefore, there is a growing interest in PET recycling technologies.

[0003] PET is considered an easily recyclable polymer material and its recycling is the most extensive among polymer materials. These technologies can be divided into two major categories: mechanical recycling and chemical recycling.

[0004] Mechanical recycling mainly consists of obtaining flakes by sorting the waste, removing contaminants, shredding and grinding, which are then fed directly into an extruder to produce new items. The main problem with this technology is the heterogeneity of the solid waste and the low quality of the final product, as the properties of PET deteriorate each time it is recycled.

[0005] Chemical recycling involves decomposing the polyester using agents capable of depolymerizing the PET chains to obtain the starting monomers. Chemical depolymerization of PET is usually achieved by solvolysis, in particular by hydrolysis or methanolysis or glycolysis.

[0006] Hydrolysis depolymerizes PET into terephthalic acid (TPA) and ethylene glycol (EG) (also known as monoethylene glycol - MEG) by reaction with water. Methanolysis degrades PET into dimethyl terephthalate (DMT) and EG by reaction with methanol. Glycolysis causes depolymerization by reaction with EG to form bis(2-hydroxyethyl)terephthalate (BHET), an intermediate formed in the first step of producing PET from the starting monomers.

[0007] Mechanical recycling remains the most widely used technology for processing PET waste to date, with few industrial applications for chemical recycling. However, there is growing interest in chemical recycling technologies, as they are sustainable and provide virgin PET as raw material, which, of course, is of much higher quality than mechanically recycled PET.

[0008] To better understand the basics of chemical recycling, here are the manufacturing basics of PET and glycolysis.

[0009] Industrially, PET is obtained in two separate reaction steps. In the first step, the starting products react to give BHET and oligomers, which are then chain extended in a separate reaction step.

[0010] PET can be produced using two different raw materials: (a) by reacting the acid, TPA and EG to form BHET and water; or (b) by reacting the ester, dimethyl terephthalate (DMT) and EG to form BHET and methanol. Subsequently, the BHET produced by the esterification or transesterification step is polycondensed to form PET; in this step, EG is produced, which can be separated from the reaction mixture:

[0011]

[0012] Polycondensation is an equilibrium reaction, and EG is a byproduct of PET. Therefore, it is necessary to shift the equilibrium to PET by evaporating the generated EG.

[0013] The reverse reaction can be used to produce BHET from PET: the reaction between PET and EG results in chain scission by attacking the ester bonds. Since PET is formed by a reversible polycondensation reaction, the reaction can be turned in the opposite direction by adding EG, converting the polymer into its monomers or oligomers.

[0014] During this reverse reaction (called glycolysis), the polymer chains are transformed by solvolytic chain scission, leading to theoretically complete depolymerization down to monomers (BHET) or partial depolymerization leading to monomers together with oligomers:

[0015]

[0016] The reaction is slightly endothermic and reversible; therefore, it can be pushed toward monomer / oligomer production by using an excess of EG over the starting PET.

[0017] The entire glycolysis reaction can be divided into two steps: (i) EG splits the ester bonds in the PET chain to form oligomers and dissolves in EG itself; (ii) the oligomers are in equilibrium with BHET, so by adding excess EG, the reaction progresses towards the formation of BHET.

[0018] Without the use of a catalyst, the glycolysis reaction is slow and it is difficult to completely depolymerize PET to BHET within an acceptable time for industrial processes. Therefore, a transesterification catalyst is usually used to ensure a shorter reaction time at a lower temperature than the non-catalyzed reaction and to achieve an increase in yield by removing or minimizing any side reactions that may affect the entire process. Possible catalysts are described, for example, in US6,630,601 and WO2017 / 111602. The catalytic glycolysis reaction is usually carried out at a temperature of 190°C to 250°C.

[0019] To be used in the production of virgin PET, BHET obtained from the glycolysis of PET waste must be carefully purified because it contains various contaminants that can interfere with the polymerization reaction and / or reduce the clarity and color of the produced PET.

[0020] For example, in patent application WO2021 / 124149 of the same applicant, a process for removing dyes or other soluble organic contaminants from crude BHET obtained by glycolysis of PET is described, which includes an oxidation step and a step of treating the oxidized solution with an adsorbent, the adsorbent adsorbing the oxidation products of the contaminants, which are then removed to obtain a purified BHET solution.

[0021] BHET may also contain contaminants that are insoluble in the medium in which glycolysis is performed, such as aluminum chips, polyolefins, fillers (such as titanium dioxide, carbon black, silicates and other pigments), binders, etc. Insoluble contaminants can be separated by filtration.

[0022] Other contaminants are barrier polymers used to produce multilayer materials. In fact, in order to preserve products that are sensitive to atmospheric oxygen (especially food, beverages, pharmaceuticals or cosmetics), multilayer materials based on PET are used with barrier polymer layers that prevent oxygen penetration. In the case of carbonated beverages, the barrier polymer layer prevents the escape of carbon dioxide, while in the case of food, it preserves the aroma and thus the organoleptic properties of the food itself.

[0023] The most widely used barrier polymers are polyamides (PA), especially nylon-6 (PA-6), nylon-6,6 (PA-6,6), polyvinyl alcohol (PVA), polyvinyl acetate (PVAc), ethylene copolymer / vinyl alcohol (EVOH), etc.

[0024] Typically, these multilayer materials include a polymer barrier layer, a heat-sealable sealing layer (eg polyethylene) and an inner layer of PET which is in direct contact with the product to be preserved.

[0025] The problem faced by the applicant is how to purify bis(2-hydroxyethyl)terephthalate (BHET) obtained by glycolysis of PET waste with EG from the above barrier polymers coupled with PET. The product obtained by glycolytic depolymerization is a crude solution or suspension in which BHET and its oligomers (especially dimers and trimers) are dissolved in EG together with different types of contaminants, which are solid or also dissolved in EG, namely:

[0026] (a) Pollutants that are soluble in EG or that react with EG to dissolve;

[0027] (b) contaminants that remain solid because they are insoluble or only partially soluble in EG at glycolysis temperature;

[0028] (c) Contaminants that do not react with EG but are soluble in EG at glycolysis temperature.

[0029] For example, organic dyes, antistatic agents, stabilizers, UV absorbers, etc. belong to category (a), which can be removed by the process described in the above-mentioned application WO2021 / 124149.

[0030] Solid contaminants (category (b)) at glycolysis temperatures, such as fillers (e.g., titanium dioxide, carbon black, silicon dioxide), binders, pigments, aluminum films or fragments, polyolefins, are separated from the crude BHET solution by filtering the material that accumulates at the bottom of the glycolysis reactor (downward flow) or by skimming off contaminants with a density lower than that of the crude saccharification product (floating on the surface, i.e., in the case of polyolefins). In this regard, see, for example, U.S. Patent 6,410,607.

[0031] The barrier polymers mentioned above belong to class (c). As suggested in the patent application WO2021 / 124149 cited above, the polyamide can be separated from the crude BHET solution by adding water, which favors the precipitation of the polyamide, while the BHET and its oligomers remain in the solution. The precipitated polyamide can then be separated by filtration.

[0032] Applicants have discovered that in order to effectively separate polyamides and other barrier polymers from a crude BHET solution, precise water dosing must be performed to act as an anti-solvent for the barrier polymer and temperature control must be performed to ensure a significant reduction in the solubility of the barrier polymer without causing precipitation of BHET and its oligomers, which remain in solution.

[0033] In particular, the Applicant has found that, in order to remove the barrier polymers substantially completely, it is necessary to add water to the crude BHET solution in at least two different steps, while reducing the weight ratio between EG and HO and reducing the temperature, as described below. In this way, the removal of the barrier polymers is more effective and does not lead to the precipitation of BHET and its oligomers. In other words, at least partial precipitation of the barrier polymers is carried out at a different stage from the separation of the oligomers, which can be recycled to the glycolysis reaction. In addition, the volume of the liquid to be treated can be kept within relatively low values, thereby avoiding the equipment drawbacks associated with the treatment of large amounts of liquid by decantation and / or filtration, which would make the process inconvenient from an industrial point of view.

[0034] Therefore, according to a first aspect, the present invention relates to a process for purifying bis(2-hydroxyethyl)terephthalate (BHET), comprising:

[0035] providing a crude BHET solution obtained by depolymerizing polyethylene terephthalate (PET) waste with ethylene glycol (EG) by glycolysis, the crude BHET solution having a temperature of 160° C. to 220° C., preferably 190° C. to 210° C., and containing at least one dissolved barrier polymer;

[0036] adding a first portion of water to the crude BHET solution in an amount such that the weight ratio of EG:H2O is between 2.0 and 4.0, preferably between 2.5 and 3.5, and cooling the solution thus obtained to a temperature between 60°C and 90°C, preferably between 65°C and 80°C, to obtain a precipitate of the at least one barrier polymer;

[0037] separating said precipitate of said at least one barrier polymer from the BHET solution;

[0038] Adding a second portion of water to the BHET solution obtained in the separation step in an amount such that the weight ratio of EG:H2O is from 0.5 to 2.5, preferably from 1.0 to 2.0, and cooling the solution thus obtained to a temperature of from 5°C to 25°C, preferably from 10°C to 20°C, thereby obtaining crystallization of BHET;

[0039] The crystallized BHET was separated from the water / EG solvent.

[0040] Preferably, the PET waste is a multi-layer product, comprising at least one layer of PET film and at least one layer of polymer barrier film.

[0041] As for the crude BHET solution, it was obtained from the depolymerization of PET waste through a glycolysis reaction with EG.

[0042] The glycolysis reaction is usually carried out in the presence of a heterogeneous transesterification catalyst. The catalyst can be selected, for example, from: carbonates, fatty acid salts or borates (e.g., zinc borate, zinc acetate, sodium carbonate) of Na, Mg, Zn, Cd, Mn, Co, Ca or Ba.

[0043] Preferably, the glycolysis reaction is carried out at a temperature of 170°C to 270°C, more preferably at a temperature of 195°C to 210°C.

[0044] In the glycolysis reaction, EG is generally used in an amount of 1.0 to 10.0 parts by weight, preferably 1.5 to 6.0 parts by weight, based on the weight of the PET waste.

[0045] The duration of the glycolysis reaction can vary over a wide range, depending on the reaction conditions, such as temperature, stirring, reactor type, etc. The reaction time is typically 1 to 8 hours, preferably 1.5 to 3 hours. The reaction can be carried out batchwise or continuously. The reaction pressure is typically atmospheric pressure, but reduced pressure or increased pressure may also be used.

[0046] More details on the glycolysis reaction of PET waste to recover BHET have been reported in, for example, US3222299, US4609680 and EP0723951A1.

[0047] The product obtained from the glycolysis reaction is a crude BHET solution in which BHET is dissolved in EG along with various contaminants from specific components of the PET waste. Typically, the crude BHET solution also contains BHET oligomers, preferably dimers and / or trimers.

[0048] In this specification and the appended claims, the total amount of BHET in the BHET solution is calculated based on BHET (monomer) and all its oligomers.

[0049] At least one barrier polymer is present in the original BHET solution, ie a polymer, usually in the form of a film, which is able to hinder the passage of gases or vapors, in particular oxygen, carbon dioxide and flavors present in food.

[0050] In particular, the barrier polymer is selected from:

[0051] (a) polyamides (PA), in particular nylon-6 (PA-6), nylon-6,6 (PA-6,6) and copolymers thereof (e.g. manufactured by BASF AG as Polyamide sold under the brand name PTFE);

[0052] (b) polyvinyl alcohol (PVA), polyvinyl acetate (PVAc), ethylene / vinyl alcohol copolymer (EVOH), ethylene / vinyl acetate copolymer (EVA), ethylene / vinyl acetate / vinyl alcohol terpolymer;

[0053] or a mixture thereof.

[0054] In addition to the above-mentioned barrier polymers, contaminants present in the original BHET solution may be, for example:

[0055] - dyes, usually organic dyes;

[0056] - ink;

[0057] - Adhesives and glues;

[0058] - polyolefins, such as polyethylene or polypropylene for the production of bottle caps;

[0059] -PET-G;

[0060] - Biodegradable polymers such as PLA;

[0061] - UV absorbers;

[0062] - fillers, such as titanium dioxide, carbon black, silica, silicates and other pigments;

[0063] - Metal sheets and fragments thereof, such as aluminium sheets.

[0064] After adding the first portion of water and cooling the crude BHET solution, the barrier polymer present in the solution itself at least partially precipitates and can therefore be separated from the solution together with other insoluble contaminants by methods such as filtration. Filtration can be performed, for example, using a filter press.

[0065] If the BHET solution thus obtained contains non-negligible amounts of BHET oligomers which need to be separated from the BHET monomers, the BHET solution can be further cooled to a temperature of 30° C. to 60° C., preferably 35° C. to 55° C., after separation of the barrier polymer and other insoluble contaminants that may be present, in order to precipitate the oligomers while keeping the BHET monomers in solution. At this stage, if the barrier polymer has not been completely separated in the previous stage, the above-mentioned cooling may cause the barrier polymer remaining in the solution to co-precipitate with the oligomers. The precipitated oligomers can then be separated from the solution together with the barrier polymer, for example by filtration or centrifugation, for example by a horizontal axis centrifuge (decanter). The oligomers thus obtained can optionally be recycled to the initial glycolysis reaction together with the co-precipitated barrier polymer.

[0066] To the BHET solution from which the barrier polymer, other insoluble contaminants and possible oligomers have been removed is then added a second portion of water in an amount such that the weight ratio of EG:H2O is from 0.5 to 2.5, preferably from 1.0 to 2.0, and further cooled to a temperature of from 5°C to 25°C, preferably from 10°C to 20°C, thereby obtaining crystallization of BHET.

[0067] The main purpose of adding the second portion of water is to promote the crystallization of BHET and its subsequent separation. In addition, the amount of BHET still dissolved in the water / EG solvent is minimized, which can in any case be subsequently recovered as distillation bottoms of the water / EG mixture.

[0068] The crystallized BHET can then be separated from the water / EG solvent, for example by filtration or other conventional methods, particularly by vacuum filtration.

[0069] The EG present in the water / EG mixture may advantageously be recovered, for example by fractional distillation, while the BHET may be subjected to further purification steps.

[0070] Preferably, the process according to the present invention further comprises:

[0071] Dissolve the crystallized BHET in water;

[0072] treating the solution thus obtained with at least one first adsorbent;

[0073] The at least one first adsorbent is separated from the thus treated solution to obtain a purified BHET first solution.

[0074] The adsorbent has the function of adsorbing any residual barrier polymer or other insoluble contaminants remaining in the crystalline BHET. In this way, the amount of residual contaminants (especially organic dyes) is reduced, thereby reducing the amount of oxidizing agent required for the subsequent purification of the BHET, thereby reducing the amount of its reduction products (e.g., chloride in the case of using hypochlorite as an oxidizing agent).

[0075] An adsorbent is a solid material, usually in powder form, that is capable of adsorbing onto its surface organic molecules dispersed or dissolved in a liquid medium so that these molecules can be isolated and eliminated by separating the adsorbent from the liquid medium.

[0076] The adsorbent is preferably activated carbon or silica. The particle size of the adsorbent is preferably 50 to 500 mesh, more preferably 100 to 350 mesh, measured according to ASTM D2862 / 16 standard.

[0077] Preferably, the surface area (BET) of the adsorbent is from 250 to 5000 m 2 / g, more preferably 500 to 3000 m 2 / g, measured according to ASTM D6556 / 19 standard.

[0078] The amount of adsorbent to be added depends mainly on its nature and on the concentration of the pollutants to be separated. For example, in the case of activated carbon, it is preferably added in an amount of 0.05 wt% to 3 wt%, more preferably 0.1 wt% to 1 wt%, relative to the weight of BHET present in the solution to be purified. The purification step is preferably carried out at a temperature of 40°C to 100°C, more preferably 70°C to 90°C.

[0079] After treatment with the first adsorbent, separation of the first adsorbent from the solution is performed to obtain a purified BHET first solution. The separation can be performed according to known techniques, preferably by filtration.

[0080] In order to eliminate further contaminants, in particular organic dyes and other low molecular weight organic contaminants, from the thus obtained first purified solution of BHET, the process according to the invention preferably further comprises:

[0081] treating the solution obtained after separating the first adsorbent with at least one oxidizing agent at a temperature of 30°C to 100°C, preferably 50°C to 90°C, to obtain an oxidized solution;

[0082] treating the oxidizing solution with at least one second adsorbent to obtain a purified oxidizing solution;

[0083] The at least one second adsorbent is separated from the purified oxidized solution to obtain a second purified BHET solution.

[0084] The second adsorbent is the same as or different from the first adsorbent and has characteristics selected from those of the first adsorbent described above.

[0085] The oxidizing agent is preferably selected from inorganic oxidizing agents, such as: alkali metal or alkaline earth metal chlorites or hypochlorites; alkali metal or alkaline earth metal persalts, preferably persulfates; potassium permanganate; hydrogen peroxide; ozone; gaseous chlorine; or mixtures thereof. Preferably, the alkali metal is sodium or potassium, and the alkaline earth metal is magnesium, calcium or barium.

[0086] The following are particularly preferred: sodium or potassium hypochlorite or sodium or potassium chlorite; hydrogen peroxide; or mixtures thereof.

[0087] To increase the effectiveness of hydrogen peroxide, it can be combined with ultraviolet radiation. To this end, the treated solution is irradiated with an ultraviolet lamp, usually radiating a wavelength of 150nm to 400nm.

[0088] The amount of oxidant used is preferably 0.005 wt % to 5 wt %, preferably 0.01 wt % to 3 wt %, based on the weight of BHET in the solution.

[0089] More details about the purification of BHET solutions by treatment with oxidizing agents can be found in patent application WO 2021 / 124149 already mentioned above.

[0090] BHET and its oligomers, if present, can be recovered from the first or second BHET solution, preferably purified by precipitation. To this end, the solution can be cooled to a temperature of 5° C. to 30° C., more preferably 10° C. to 20° C. BHET and its oligomers can be precipitated in an amorphous state or in at least partially crystalline form, depending mainly on the conditions used for precipitation.

[0091] The purified BHET thus obtained can then be washed with water and finally dried.

[0092] The following examples are intended only to illustrate the present invention and should not be considered as limiting the scope as defined by the appended claims.

[0093] Examples 1a-1b-1c

[0094] A series of tests were conducted on PET scrap obtained from bottles (r-PET) with the addition of various polyamides used as barrier polymers in food packaging. The r-PET was in the form of clear, clean flakes and was essentially free of insoluble contaminants.

[0095] In each sample, the amount of polyamide added corresponded to 5 wt% of the weight of r-PET. The amount of polyamide in the suspension subjected to glycolysis corresponded to 1 wt%.

[0096] The following polyamides were used:

[0097] PA-6 produced by AquafilS.pA (Example 1a);

[0098] PA-6,6 produced by AquafilS.pA (Example 1b);

[0099] Produced by BASFAG Class B (Example 1c).

[0100] PET waste to which PA was added was dissolved in EG at 200°C under reduced pressure (p = 0.4 MPa). The glycolysis reaction was carried out in the presence of zinc acetate (ZnAc) as a heterogeneous catalyst (0.1 wt% of the weight of PET). The glycolysis temperature was 207°C for example 1a and 204°C for examples 1b and 1c. For all examples, the glycolysis was carried out in two stages with different PET / EG ratios: the first stage was 40 minutes with a PET / EG ratio equal to 1; the second stage was 30 minutes with a PET / EG ratio equal to 4.

[0101] Table 1 shows the amounts of products used in the three glycolysis reactions.

[0102] Table 1

[0103]

[0104] According to the present invention, the crude BHET solution obtained in the above glycolysis process is purified. The different steps of the process and the specific conditions used are as follows:

[0105] (a) The crude solution obtained during glycolysis was cooled to 110 °C and then water was added (EG:H2O ratio = 3);

[0106] (b) cooling the solution obtained in step (a) to 75° C. to obtain partial precipitation of PA;

[0107] (c) filtering the solution obtained in step (b) through a laboratory filter to separate the precipitated PA;

[0108] (d) cooling the filtered solution obtained in step (c) to 50° C. to obtain a precipitate of BHET oligomers, which is separated by a horizontal axis centrifuge (decanter); together with the BHET oligomers, the residual part of the PA remaining in the solution is co-precipitated and also separated together with the oligomers;

[0109] (e) After separating the oligomers and PA according to step (d), water is added to the resulting solution in an amount such that EG:H2O=1.5;

[0110] (f) cooling the solution obtained in step (e) to a temperature of 15° C. to obtain a crystalline precipitate of BHET;

[0111] (g) filtering with a laboratory filter to separate the BHET precipitate from the mother liquor (EG / H2O mixture);

[0112] (h) BHET was dissolved in water and activated carbon (PAC) was added in an amount equivalent to 0.25 wt% of the weight of BHET (PAC characteristics: particle size: 325 mesh; BET surface area: about 2000 m 2 / g);

[0113] (i) stirring the BHET solution containing the suspended PAC at a temperature of 70° C. to 75° C. for 30 minutes; and then removing the residual portion of PAC and PA by filtration;

[0114] (j) then oxidizing the first purified BHET solution obtained from step (i) with sodium hypochlorite (17 wt % aqueous solution) in an amount equivalent to 0.25 wt % of the weight of BHET at a temperature of 70° C. to 75° C. for a reaction time of 40 minutes;

[0115] (k) treating the oxidized solution obtained from step (j) with the same PAC under the same conditions as steps (h) and (i) to obtain a second solution of purified BHET, from which BHET is separated by crystallization by cooling to 15° C.;

[0116] (l) The recrystallized BHET was filtered, washed with water and dried.

[0117] The process flow of this process is as follows Figure 1 shown.

[0118] Analysis to verify and quantify polyamide removal

[0119] To verify the effectiveness of the process in isolating PAs, two different analytical techniques were used: (i) FT-IR spectroscopy, verifying the presence of PAs through their characteristic absorption bands, and (ii) gravimetric measurement of the amount of PA removed in the various steps of the process (mass balance).

[0120] These analyses were performed on the following products obtained at different steps of the process (e.g. Figure 1 As shown in the process diagram):

[0121] (A) the filtered solid obtained from step (c);

[0122] (B) the filtered solid obtained from step (d);

[0123] (C) the mother liquor obtained from step (g);

[0124] (D) the filtered solid obtained from step (i);

[0125] (E) Final BHET obtained from step (1).

[0126] The table below shows the characteristic IR absorption bands (ν, cm -1 )’s location.

[0127] Table 2

[0128] PA-6 PA-66 PA-6ULTRAMID NH amide stretching 3291 3293 3296 C=O amide stretching 1634 1630 1633 NH Amide Bending 1535 1534 1534

[0129] The presence of these bands makes it possible to verify, within the sensitivity range of the instrument, the presence of polyamide in the various products (A) to (E) described above.

[0130] In order to quantitatively evaluate the polyamide isolated at each stage of the above process, the procedure was as follows.

[0131] As for the filtered solid (A), this consists essentially of PA, since the r-PET used contains essentially no insoluble contaminants.

[0132] The filtered solid (B) essentially contains the oligomers and a portion of the previously unseparated residual PA. After separation of the oligomers after dissolution with a water / methanol mixture, the difference from the initial weight is determined as the weight of PA.

[0133] The mother solution (C) consisted of a small amount of BHET and its oligomers and PA dissolved in a mixture of EG / H2O. To determine the amount of PA present in the mother solution (C), a solute precipitation was performed, separated and weighed by a thermobalance. A sample of the precipitate was weighed, BHET and oligomers were dissolved, and the solution was analyzed by HPLC to determine its composition. The amount of PA was calculated by the difference.

[0134] As for the filtered solid (D), it was dried and weighed: since the amount of PAC added was known, the amount of PA was determined by difference.

[0135] In the final BHET(E), PA is no longer present, as shown by the IR spectrum of the product, which shows no PA characteristic bands.

[0136] The results are summarized in Tables 3A-3B-3C below, where the amount of isolated polyamide in each sample is reported relative to the total amount added to the r-PET, expressed as weight percent relative to the total weight added to the r-PET subjected to glycolysis. The presence or absence of characteristic IR peaks is also reported (FT-IR spectra were not determined for the liquid C product). It should be noted that the positions of the characteristic peaks measured on the pure polymer were slightly shifted (<1%) due to the hydrogen bonds established between H2O and EG and the polar groups of the polymer.

[0137] Table 3A

[0138]

[0139] Table 3B

[0140]

[0141] Table 3C

[0142]

[0143] It can be seen that the polyamides are eliminated in a quantitative manner. While the polyamides PA-6 and PA-6,6 are largely eliminated in step (c), i.e. after the first precipitation and subsequent filtration, the Ultramid PA-6 polyamide is only partially removed in step (c), which is completed in subsequent steps, in particular after the first treatment with PAC (step (i)).

[0144] Analysis to determine the composition of BHET and its oligomers

[0145] The composition of BHET and its oligomers at different steps of the process was also determined. Quantitative analysis was performed by HPLC on samples collected from the process flow of the following products:

[0146] (1) Crude solution obtained from glycolysis;

[0147] (2) the filtered solution obtained from step (c);

[0148] (3) the filtered solution obtained from step (d);

[0149] (4) the BHET precipitate obtained from step (g) was dissolved in a water:methanol mixture with a weight ratio of 65:35;

[0150] (5) the first purified BHET solution obtained in step (h);

[0151] (6) The crystallized, washed and dried BHET obtained in step (1) was dissolved in the same water:methanol mixture as above.

[0152] Tables 4A, 4B and 4C report the weight percentages of BHET, dimers, trimers and other oligomers obtained in the different products (1) to (6) of amide PA-6, PA-66 and Ultramid PA-6.

[0153] Table 4A

[0154]

[0155] Table 4B

[0156]

[0157] Table 4C

[0158]

[0159] It can be seen that this process can ensure the gradual improvement of BHET purity.

[0160] Final BHET Color Analysis

[0161] In order to verify the overall effectiveness of the process in purifying BHET, the final BHET obtained from step (1) was subjected to color analysis (by UV-visible spectroscopy and according to Cie LAB colorimetry).

[0162] UV-Vis spectrophotometry

[0163] To evaluate the color properties of the final BHET, the total absorbance of the sample at a predetermined wavelength (Σabs0) can be used, measured by a spectrophotometer operating in the desired wavelength range.

[0164] Usually, in order to cover the visible light range, the selected wavelengths are: 475, 510, 570, 590, 650nm, so:

[0165] Σabs0=abs 475 +abs 510 +abs 570 +abs 590 +abs 650

[0166] The measurement process is as follows: the BHET to be analyzed is weighed and dissolved in dimethyl sulfoxide (DMSO) to obtain a BHET:DMSO weight ratio of 1: 1. The solution thus obtained is placed in an optical glass cuvette (optical path: 1.5 cm) and inserted into a spectrophotometer to measure the UV-visible spectrum.

[0167] Cie LAB Color

[0168] The color of the dry BHET powder was measured using the Hunter Lab color scale according to the Hunter Lab L, a, b color space (details of this measurement can be found at https: / / support.hunterlab.com).

[0169] The measurements were performed on cylindrical samples of dry BHET powder (diameter: 50 mm; height: 10 mm) obtained by compressing the powder in a cylindrical container at 400 bar.

[0170] The results are shown in Table 5.

[0171] Table 5

[0172]

[0173] The obtained values ​​are well within the specification limits (Σabs0<0,15; L>95; a=-0,2 / +0,3; b<3).

[0174] Example 2

[0175] A series of tests were conducted on PET scrap obtained from bottles (r-PET) to which ethylene / vinyl alcohol copolymer (EVOH) used as a barrier polymer for food packaging was added (commercial product EVOHFIOIB 32% B-Pack TM EVAL - 32 mol% ethylene). The amount of EVOH added is equivalent to 5% by weight of r-PET.

[0176] The PET waste to which EVOH was added was subjected to glycolysis reaction under the same conditions as in Example 1. The composition of the reaction mixture was as follows:

[0177]

[0178] The crude BHET solution obtained during glycolysis was subjected to the purification process according to the invention, the process being the same as reported in Example 1 (steps (a)-(l)).

[0179] In order to verify the effectiveness of this process in EVOH removal, a qualitative process by detecting the characteristic infrared absorption peak of EVOH could not be used because the characteristic peak of EVOH is located at about 3000 nm and overlaps with the absorption peak of BHET (located in the range of 2800-3300 nm).

[0180] The analysis of Example 1 above was then performed according to the above process to determine the quantitative removal of EVOH and the composition of BHET and its oligomers at each stage of the process, as well as a color analysis of the final BHET obtained from step (1). The results are recorded in the following Tables 6-8:

[0181] Table 6

[0182]

[0183] Table 7

[0184]

[0185]

[0186] Table 8

[0187] Absorbance <![CDATA[abs 475 ]]> 0.006 <![CDATA[abs 510 ]]> 0.006 <![CDATA[abs 570 ]]> 0.003 <![CDATA[abs 590 ]]> 0.003 <![CDATA[abs 650 ]]> 0 <![CDATA[Σabs0]]> 0.018 Cie LAB Color L 98.46 a -0.14 b 0.84

[0188] It can be seen that in this case, the process according to the invention also proves to be effective and allows quantitative elimination of EVOH. Similar to the polyamide Ultramid PA-6, in step (c), only a partial removal is obtained, which is completed in subsequent stages, in particular after the first treatment with PAC (stage (i)).

[0189] In addition, the process can ensure that the purity and color of BHET gradually improve.

Claims

1. A process for purifying bis(2-hydroxyethyl)terephthalate (BHET), comprising: providing a crude BHET solution obtained by depolymerizing polyethylene terephthalate (PET) waste with ethylene glycol (EG) by glycolysis, the crude BHET solution having a temperature of 160° C. to 220° C., preferably 190° C. to 210° C., and containing at least one dissolved barrier polymer; adding a first portion of water to the crude BHET solution in an amount such that the weight ratio of EG:H2O is between 2.0 and 4.0, preferably between 2.5 and 3.5, and cooling the solution thus obtained to a temperature between 60°C and 90°C, preferably between 65°C and 80°C, to obtain a precipitate of the at least one barrier polymer; separating said precipitate of said at least one barrier polymer from the BHET solution; Adding a second portion of water to the BHET solution obtained in the separation step in an amount such that the weight ratio of EG:H2O is from 0.5 to 2.5, preferably from 1.0 to 2.0, and cooling the solution thus obtained to a temperature of from 5°C to 25°C, preferably from 10°C to 20°C, to obtain crystallization of BHET; The crystallized BHET was separated from the water / EG solvent.

2. The process according to claim 1, wherein The PET scrap is a multilayer product comprising at least one PET film and at least one barrier polymer film.

3. A process according to any one of the preceding claims, wherein The at least one barrier polymer is selected from: (a) polyamide (PA), in particular nylon-6 (PA-6), nylon-6.6 (PA-6.6) and copolymers thereof; (b) polyvinyl alcohol (PVA), polyvinyl acetate (PVAc), ethylene / vinyl alcohol copolymer (EVOH), ethylene / vinyl acetate copolymer (EVA), ethylene / vinyl acetate / vinyl alcohol terpolymer; or a mixture thereof.

4. A process according to any one of the preceding claims, wherein After the step of separating said precipitate of at least one barrier polymer from the BHET solution, the BHET solution thus obtained is further cooled to a temperature of 30 to 60°C, preferably 35 to 55°C, to precipitate oligomers and possibly another part of said barrier polymer.

5. The process according to any one of the preceding claims, further comprising: After the step of separating the crystallized BHET from the water / EG solvent, the crystallized BHET is dissolved in water; treating the solution thus obtained with at least one first adsorbent; At least one first adsorbent is separated from the solution thus treated, thereby obtaining a first purified solution of BHET.

6. The process according to claim 5, further comprising: treating the solution obtained after separating the first adsorbent with at least one oxidizing agent at a temperature of 30°C to 100°C, preferably 50°C to 90°C, to obtain an oxidized solution; treating the oxidizing solution with at least one second adsorbent to obtain a purified oxidizing solution; The at least one second adsorbent is separated from the purified oxidized solution to obtain a second purified BHET solution.

7. The process according to claim 6, wherein: The at least one oxidizing agent is an inorganic oxidizing agent selected from: alkali metal or alkaline earth metal chlorites or hypochlorites; alkali metal or alkaline earth metal persalts, preferably persulfates; potassium permanganate; hydrogen peroxide; ozone; gaseous chlorine; or mixtures thereof.

8. The process according to claim 5 or 6, wherein: The at least one first adsorbent and the at least one second adsorbent are the same as or different from each other and are activated carbon or silica.

Images