Purification method for bis(2-hydroxyethyl) terephthalate obtained from depolymerization of polyethylene terephthalate-containing waste materials

Abstract

This process purifies bis(2-hydroxyethyl) terephthalate (BHET) from a crude solution obtained by depolymerizing polyethylene terephthalate (PET) waste containing at least one barrier polymer with ethylene glycol (EG) through glycolysis. The crude solution contains a predetermined amount of water, which acts as a poor solvent for the barrier polymer. The temperature is controlled to significantly reduce the solubility of the barrier polymer without causing precipitation of BHET and its oligomers. Specifically, water is added to the crude BHET solution in at least two distinct steps: a step to reduce the weight ratio of EG to HO and a step to reduce the temperature. This method allows for more effective removal of the barrier polymer and prevents precipitation of BHET and its oligomers. Furthermore, this method allows for the volume of the liquid to be treated to be kept relatively low, thereby avoiding the plant drawbacks associated with processing large volumes of liquid by decantation and / or filtration, which make the process inconvenient from an industrial standpoint.

Description

[Technical Field]

[0001] The present invention relates to a method for purifying bis(2-hydroxyethyl) terephthalate (BHET) obtained from the depolymerization of waste materials containing polyethylene terephthalate (PET). [Background technology]

[0002] Polyethylene terephthalate (PET) is a strong, transparent, and widely used semi-crystalline thermoplastic polyester. Its physical and chemical properties allow for a variety of applications, particularly in packaging and textile manufacturing. While PET poses no safety risks, its increasing consumption, accumulation in waste streams, and lack of biodegradability have raised environmental and economic concerns. Consequently, there has been growing interest in PET recycling technologies.

[0003] PET is considered an easily recyclable polymer material, and its recycling is the most widespread among polymer materials. Techniques can be divided into two broad categories: mechanical recycling and chemical recycling.

[0004] Mechanical recycling primarily consists of sorting the waste, removing contaminants, and crushing and grinding it to obtain flakes that are then fed directly into extrusion moulds to produce new items. The main problems with this technology are the heterogeneity of the solid waste and the poor quality of the final product, as the properties of PET deteriorate each time it is recycled.

[0005] Chemical recycling involves breaking down the polyester using reagents capable of depolymerizing the PET chains to obtain the starting monomers. Chemical depolymerization of PET is usually achieved by solvolysis, especially hydrolysis, or methanolysis or glycolysis.

[0006] In hydrolysis, PET is depolymerized into terephthalic acid (TPA) and ethylene glycol (EG) (also known as monoethylene glycol - MEG) by reaction with water. In methanolysis, PET is decomposed into dimethyl terephthalate (DMT) and EG by reaction with methanol. Glycolysis causes depolymerization by reaction with EG to produce bis(2-hydroxyethyl) terephthalate (BHET), an intermediate product formed from the starting monomer in the first step of PET production.

[0007] To date, mechanical recycling has been the most widely used method for treating PET waste, with chemical recycling seeing only limited industrial application. However, chemical recycling technology is gaining interest as it is in line with the principles of sustainable development and naturally provides virgin PET of much higher quality as a raw material than mechanically recycled PET.

[0008] To better understand the fundamentals of chemical recycling, the basics of PET production and glycolysis are presented below.

[0009] Industrially, PET is obtained from two separate reaction steps: in the first step, the starting products react to produce BHET and oligomers, followed by chain extension in another reaction step.

[0010] Two different raw materials can be used to produce PET: (a) TPA and EG react via an acid to produce BHET and water; or (b) dimethyl terephthalate (DMT) and EG react via an ester to produce BHET and methanol. The BHET produced in an esterification or transesterification step is then reacted to produce PET via a polycondensation reaction. This step produces EG, which can be separated from the reaction mixture:

[0011] [ka]

[0012] Polycondensation is an equilibrium reaction, and EG is produced as a by-product along with PET. Therefore, the equilibrium must be shifted to PET by evaporating the EG produced.

[0013] The reverse reaction can be used to produce BHET from PET: the reaction of PET with EG attacks the ester bond, resulting in chain scission. Because PET is formed by a reversible polycondensation reaction, the polymer can be converted to monomers or oligomers by adding EG to shift the reaction in the opposite direction.

[0014] During this reverse reaction, called glycolysis, the polymer chains are transformed by solvolytic chain scission, which theoretically leads to complete depolymerization down to the monomer (BHET) or to partial depolymerization giving rise to oligomers along with the monomer:

[0015] [ka]

[0016] Since the reaction is slightly endothermic and reversible; using an excess of EG compared to the starting PET can promote the formation of monomers / oligomers.

[0017] The entire glycolysis reaction can be divided into two steps: (i) EG cleaves the ester bonds of the PET chains, forming oligomers that dissolve in the EG itself; (ii) the oligomers are in equilibrium with BHET, so adding excess EG drives the reaction toward the formation of BHET.

[0018] Without a catalyst, the glycolysis reaction is slow, making it difficult to achieve complete depolymerization of PET to BHET within the timeframe acceptable for industrial processes. Therefore, transesterification catalysts are typically used, which allow for shorter reaction times at lower temperatures than uncatalyzed reactions, thereby increasing yields by eliminating or minimizing side reactions that can affect the overall process. Potential catalysts are described, for example, in U.S. Patent No. 6,630,601 and International Publication No. WO 2017 / 111602. Catalytic glycolysis reactions are typically carried out at temperatures between 190°C and 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 reduce the transparency and color of the resulting PET.

[0020] For example, patent application WO 2021 / 124149 in the name of the same applicant describes a process for removing dyes or other soluble organic pollutants from crude BHET obtained from PET glycolysis, which comprises an oxidation step and a step of treating the oxidized solution with an adsorbent that adsorbs the oxidation products of the pollutants, 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 occurs, such as aluminum flakes, polyolefins, fillers (titanium dioxide, carbon black, silicates, other pigments, etc.), and adhesives. Insoluble contaminants can be separated by filtration.

[0022] Other contaminants include the barrier polymers used in the manufacture of multilayer materials. Indeed, to preserve products sensitive to atmospheric oxygen (especially food, beverages, pharmaceuticals or cosmetics), PET-based multilayer materials are used in combination with a barrier polymer layer that prevents oxygen penetration. In the case of carbonated drinks, the barrier polymer layer prevents the release of carbon dioxide, while in the case of food, it preserves the aroma and therefore 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), and ethylene copolymer / vinyl alcohol (EVOH).

[0024] These multi-layer materials generally include a polymeric barrier layer, a heat-sealable seal layer (such as polyethylene) and an inner layer of PET that is in direct contact with the product to be stored.

[0025] The applicant was faced with the problem of purifying bis(2-hydroxyethyl) terephthalate (BHET), obtained by depolymerization of PET waste by glycolysis with EG, from the barrier polymer to which PET is bonded as described above. The product obtained from depolymerization by glycolysis is a crude solution or suspension containing BHET and its oligomers (especially dimers and trimers) dissolved in EG, and various types of contaminants, either solid or also dissolved in EG, namely: (a) Contaminants that are soluble in EG or that react with EG to become solubilized; (b) contaminants that are insoluble or only partially soluble in EG at glycolysis temperatures and therefore remain solid; (c) It contains contaminants that do not react with EG but are soluble in EG at glycolytic temperatures.

[0026] Category (a) includes, for example, organic dyes, antistatic agents, stabilizers, and UV absorbers, which can be removed by the methods described in the above-mentioned application WO 2021 / 124149.

[0027] Solid contaminants at glycolysis temperatures (category (b)), such as fillers (e.g., titanium dioxide, carbon black, silica), adhesives, pigments, aluminum film or debris, and polyolefins, are separated from the crude BHET solution by filtering the material that accumulates at the bottom of the glycolysis reactor (downflow) or by skimming off the contaminants that float to the surface and are less dense than the crude glycolation product (in the case of polyolefins). See, for example, U.S. Pat. No. 6,410,607.

[0028] The barrier polymers mentioned above belong to category (c). As suggested in the above-mentioned patent application WO 2021 / 124149, the polyamide can be separated from the crude BHET solution by adding water, which promotes precipitation of the polyamide, while BHET and its oligomers remain in solution. The precipitated polyamide can then be separated by filtration.

[0029] Applicants have discovered that effective separation of polyamides and other barrier polymers from crude BHET solutions requires precise dosing of water to act as a poor solvent for the barrier polymer, and temperature control to ensure a significant reduction in the solubility of the barrier polymer without causing precipitation of BHET and its oligomers, which remain in solution.

[0030] In particular, Applicant has discovered that to substantially completely remove the barrier polymer, water must be added to the crude BHET solution in at least two distinct steps: a step to reduce the weight ratio of EG to HO and a step to reduce the temperature, as more clearly defined below. This approach results in more effective removal of the barrier polymer and prevents precipitation of BHET and its oligomers. That is, at least partial precipitation of the barrier polymer occurs in a separate step from the separation of the oligomers, allowing it to be recycled to the glycolysis reaction. Furthermore, the volume of the liquid to be treated can be kept relatively low, thereby avoiding the plant drawbacks associated with the processing of large volumes of liquid by decantation and / or filtration, which make the process inconvenient from an industrial standpoint.

[0031] Thus, according to a first aspect, the present invention relates to a process for purifying bis(2-hydroxyethyl) terephthalate (BHET), the process comprising: providing a crude BHET solution obtained by depolymerizing polyethylene terephthalate (PET) waste by glycolysis with ethylene glycol (EG), 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 the precipitate of the at least one barrier polymer from the BHET solution; adding a second portion of water to the BHET solution obtained from the separation step in an amount such that the weight ratio of EG:HO is 0.5-2.5, preferably 1.0-2.0, and cooling the solution thus obtained to a temperature of 5°C-25°C, preferably 10°C-20°C, to obtain crystals of the BHET; and separating the crystallized BHET from the water / EG solvent.

[0032] Preferably, the PET waste is a multi-layer product combining at least one PET film and at least one polymeric barrier film.

[0033] Crude BHET solution is obtained by depolymerizing PET waste through glycolysis reaction with EG.

[0034] The glycolysis reaction is usually carried out in the presence of a heterogeneous transesterification catalyst, which can be selected from, for example: carbonates, fatty acid salts, or borates of Na, Mg, Zn, Cd, Mn, Co, Ca, or Ba (e.g., zinc borate, zinc acetate, sodium carbonate).

[0035] Preferably, the glycolysis reaction is carried out at a temperature of 170°C to 270°C, more preferably 195°C to 210°C.

[0036] In the glycolysis reaction, EG is usually used in an amount of 1.0 to 10.0 parts by weight, preferably 1.5 to 6.0 parts by weight, per part by weight of PET waste.

[0037] The duration of the glycolysis reaction can vary over a wide range, depending on reaction conditions such as temperature, agitation, and type of reactor. Typically, the reaction time is 1 to 8 hours, preferably 1.5 to 3 hours. The reaction can be carried out batchwise or continuously. The reaction pressure is usually atmospheric, although reduced or increased pressure can also be used.

[0038] Details regarding the glycolysis of PET waste to recover BHET are reported in, for example, US Pat. Nos. 3,222,299, 4,609,680 and EP-A-0,723,951.

[0039] The product of the glycolysis reaction is a solution of crude BHET dissolved in EG along with various contaminants resulting from the specific composition of the PET waste. Typically, the crude BHET solution also contains BHET oligomers, preferably dimers and / or trimers.

[0040] In this specification and the appended claims, the total amount of BHET in a BHET solution is calculated based on BHET (monomer) and all its oligomers.

[0041] In the BHET concentrate, at least one barrier polymer is present, i.e. a polymer generally in the form of a film, which is capable of preventing the passage of gases or vapors, in particular oxygen, carbon dioxide and flavorings present in the food.

[0042] In particular, the barrier polymer is selected from: (a) Polyamides (PA), in particular nylon-6 (PA-6), nylon-6,6 (PA-6,6) and their copolymers (for example, polyamides sold under the brand name Ultramid® by BASF AG); (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 of them.

[0043] In addition to the barrier polymers mentioned above, contaminants present in BHET concentrate include, for example: -dyes, usually organic dyes; -ink; -adhesives and glues; - polyolefins, for example polyethylene or polypropylene, used in the manufacture of caps; -PET-G; -biodegradable polymers, for example PLA; -UV absorbers; - fillers, such as titanium dioxide, carbon black, silica, silicates and other pigments; - Metal sheets and their fragments, for example aluminium sheets.

[0044] After the first portion of water is added and the crude BHET solution is cooled, the barrier polymer present in the solution itself will at least partially precipitate and can therefore be separated from the solution along with other insoluble contaminants, for example, by filtration, which can be carried out, for example, using a filter press.

[0045] If the BHET solution thus obtained contains a significant amount of BHET oligomers that must be separated from the BHET monomers, after separating any possible barrier polymers and other insoluble contaminants, the BHET solution can be further cooled to a temperature of 30°C to 60°C, preferably 35°C to 55°C, to precipitate the oligomers while maintaining the BHET monomers in solution. At this stage, if the barrier polymer was not completely separated in the previous step, the cooling may cause the remaining barrier polymer to co-precipitate with the oligomers. The precipitated oligomers can then be separated from the solution together with the barrier polymer by, for example, filtration or centrifugation (e.g., in a horizontal axis centrifuge (decanter)). The oligomers thus obtained, along with the co-precipitated barrier polymer, can optionally be recycled to the initial glycolysis reaction.

[0046] The BHET solution from which the barrier polymer, other insoluble contaminants, and optionally oligomers have been removed is then added with a second portion of water in an amount such that the weight ratio of EG:HO is 0.5 to 2.5, preferably 1.0 to 2.0, and the mixture is further cooled to a temperature of 5°C to 25°C, preferably 10°C to 20°C, to obtain crystals of BHET.

[0047] The main purpose of adding the second portion of water is to facilitate the crystallization and subsequent isolation of BHET. Furthermore, the amount of BHET remaining dissolved in the water / EG solvent is minimized, and in any case can be subsequently recovered as the distillation residue of the water / EG mixture.

[0048] The crystallized BHET can then be separated from the water / EG solvent, for example, by filtration or other conventional methods, especially vacuum filtration.

[0049] The EG present in the water / EG mixture can be advantageously recovered, for example by fractional distillation, while the BHET can be subjected to further purification steps.

[0050] Preferably, the method of the present invention further comprises: Dissolving the crystallized BHET in water; treating the solution thus obtained with at least a first adsorbent; and separating the at least one first adsorbent from the thus treated solution to obtain a purified first BHET solution.

[0051] The adsorbent functions to adsorb residual barrier polymer or other insoluble contaminants remaining in the crystallized BHET, thus reducing the amount of residual contaminants (especially organic dyes) and the amount of oxidizing agent and, therefore, its reduction products (e.g., chlorides when hypochlorite is used as the oxidizing agent) required for the subsequent purification of the BHET.

[0052] Adsorbents are solid substances, usually powdered, that adsorb organic molecules dispersed or dissolved in a liquid medium onto their surface, allowing these molecules to be sequestered and removed by separating the adsorbent from the liquid medium.

[0053] Preferably, the adsorbent is activated carbon or silica. Preferably, the adsorbent has a particle size of 50 to 500 mesh, more preferably 100 to 350 mesh, measured according to ASTM D2862 / 16 standard.

[0054] Preferably, the adsorbent has a surface area (BET) measured according to the ASTM D6556 / 19 standard of 250 to 5000 m 2 / g, more preferably 500 to 3000m 2 / g.

[0055] The amount of adsorbent added depends mainly on its nature and the concentration of the contaminants to be separated. For example, activated carbon is added in an amount of preferably 0.05% to 3% by weight, more preferably 0.1% to 1% by weight, based on the weight of BHET present in the solution to be purified. The purification step is preferably carried out at temperatures between 40°C and 100°C, more preferably between 70°C and 90°C.

[0056] After treatment with the first adsorbent, the first adsorbent is separated from the solution to obtain a first solution of purified BHET, which separation can be carried out by known techniques, preferably by filtration.

[0057] In order to remove further contaminants, in particular organic dyes and other low molecular weight organic contaminants, from the first purified solution of BHET thus obtained, the method of the invention preferably further comprises: treating the solution obtained after separation of the adsorbent with at least one oxidizing agent at a temperature between 30°C and 100°C, preferably between 50°C and 90°C, to obtain an oxidized solution; treating the oxidized solution with at least one second adsorbent to obtain a purified oxidized solution; and separating the at least one second adsorbent from the purified oxidized solution to obtain a second purified BHET solution.

[0058] The second adsorbent may be the same as or different from the first adsorbent and may have characteristics selected from those set forth above for the first adsorbent.

[0059] 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, chlorine gas, or mixtures thereof. Preferably, the alkali metal is sodium or potassium, and the alkaline earth metal is magnesium, calcium, or barium.

[0060] Particularly preferred are the following: sodium or potassium hypochlorite or chlorite; hydrogen peroxide; or mixtures thereof.

[0061] To enhance the effectiveness of hydrogen peroxide, it can be used in combination with ultraviolet light. For this purpose, the treated solution is usually irradiated with ultraviolet lamps with a radiation wavelength of 150 nm to 400 nm.

[0062] The oxidizing agent is preferably used in an amount of 0.005% to 5% by weight, preferably 0.01% to 3%, based on the weight of BHET in the solution.

[0063] Details of the purification of BHET solutions by treatment with oxidizing agents can be found in patent application WO 2021 / 124149 already mentioned above.

[0064] BHET, and possibly its oligomers, can be recovered from the purified first or second solution of BHET, preferably by precipitation. To this end, the solution can be cooled to a temperature between 5°C and 30°C, more preferably between 10°C and 20°C. BHET and its oligomers can precipitate in an amorphous state or in an at least partially crystalline state, depending primarily on the conditions applied for the precipitation.

[0065] The purified BHET thus obtained can then be washed with water and finally dried.

[0066] The following examples are provided purely for illustrative purposes of the present invention and should not be construed as limiting the scope defined by the appended claims. [Example]

[0067] Examples 1a-1b-1c A series of tests were carried out on PET waste (r-PET) obtained from bottles doped with various polyamides used as barrier polymers in food packaging. The r-PET was found to be clear, clean flake-like, and virtually free of insoluble contaminants.

[0068] In each sample, the amount of polyamide added was equal to 5 wt.% relative to the weight of r-PET. The amount of polyamide in the suspension subjected to glycolysis was equal to 1 wt.%.

[0069] The following polyamides were used: PA-6 from Aquafil Corporation (Example 1a); PA-6,6 from Aquafil Corporation (Example 1b); PA-6 Ultramid® B grade manufactured by BASF Ltd. (Example 1c).

[0070] The PA-added PET waste 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) (0.1 wt % relative to the PET weight) as a heterogeneous catalyst. The glycolysis temperature was 207°C in Example 1a and 204°C in Examples 1b and 1c. In all cases, glycolysis was carried out in two stages with different PET / EG ratios: a first stage lasting 40 min with a PET / EG ratio equal to 1, followed by a second stage lasting 30 min with a PET / EG ratio equal to 4.

[0071] Table 1 shows the amounts of products used in the three glycolytic reactions.

[0072] [Table 1]

[0073] The crude BHET solution obtained from the glycolysis process described above was subjected to a purification process according to the invention. The different steps of the process under the specific conditions used are given below: (a) The crude solution obtained from glycolysis was cooled to 110°C, and then water was added (EG:H2O ratio = 3); (b) The solution obtained in step (a) was cooled to 75°C to obtain partial precipitation of PA; (c) The solution obtained in step (b) was filtered through a laboratory filter to separate the precipitated PA; (d) The filtered solution obtained in step c) was cooled to 50°C to precipitate the BHET oligomers, which were then separated in a horizontal axis centrifuge (decanter); together with the BHET oligomers, the remaining portion of the PA remaining in the solution also coprecipitated and was also separated together with the oligomers; (e) To the solution obtained after separating the oligomers and PA according to step (d), water was added in an amount such that the EG:H2O ratio was 1.5; (f) cooling the solution obtained in step (e) to a temperature of 15°C in order to obtain a precipitate of BHET in crystalline form; (g) The BHET precipitate was separated from the mother liquor (EG / H20 mixture) by filtration through a laboratory filter; (h) BHET was dissolved in water, and activated carbon (PAC) was added in an amount equivalent to 0.25 wt.% relative to the weight of BHET (PAC characteristics: particle size: 325 mesh; BET surface area: approximately 2000 m 2 / g); (i) The BHET solution containing the suspended PAC was stirred at 70-75°C for 30 minutes; then, the residual PAC and PA were removed by filtration; (j) Next, the first purified BHET solution obtained in step (i) was oxidized by reacting it with sodium hypochlorite (aqueous solution of 17% by weight) in an amount equivalent to 0.25% by weight based on the weight of BHET at a temperature of 70°C to 75°C for 40 minutes; (k) treating the oxidized solution obtained in step (j) with the same PAC under the same conditions as in steps (h) and (i) to obtain a second purified BHET solution, from which BHET was isolated by cooling to 15°C and crystallization; (l) The recrystallized BHET was filtered, washed with water and dried.

[0074] The process scheme is shown in Figure 1. [Brief explanation of the drawings]

[0075] [Figure 1] Figure 1 shows a scheme of the process.

[0076] Analysis to verify and quantitatively determine polyamide removal To verify the effectiveness of the process to separate the polyamide, two different analytical techniques were used: (i) FT-IR spectroscopy to verify the presence of polyamide by its characteristic absorption bands; (ii) the amount of PA removed at different steps of the process was determined gravimetrically (mass balance).

[0077] These analyses were performed on the following products obtained at the various steps of the process (shown in the process diagram in Figure 1): (A) The filtered solid obtained from step (c); (B) the filtered solid obtained from step (d); (C) mother liquor obtained from step (g); (D) the filtered solid obtained from step (i); (E) The final BHET obtained from step (l).

[0078] The following table shows the IR absorption bands (ν, cm) specific to the polyamides used. -1 ) is shown.

[0079] [Table 2]

[0080] The presence of these bands allowed us to confirm, within the sensitivity limits of the instrument, the presence or absence of polyamide in the various products (A) to (E) described above.

[0081] As reported above, the procedure for quantitative evaluation of polyamides separated at different stages of the process was as follows.

[0082] As for the filtered solid (A), it consisted essentially of PA, since the r-PET used was practically free of insoluble contaminants.

[0083] The filtered solid (B) contained essentially oligomers and some of the residual PA that had not been separated. The weight of the PA was determined as the difference with respect to the initial weight after separating the oligomers by dissolving it in a water / methanol mixture.

[0084] The mother liquor (C) consisted of small amounts of BHET and its oligomers and PA dissolved in the EG / HO mixture. To determine the amount of PA present in the mother liquor (C), precipitation of the solutes was performed, and they were separated and weighed on a thermoscale. A sample of the precipitate was weighed, and the BHET and oligomers were solubilized. The solution was analyzed by HPLC to determine its composition. The amount of PA was calculated as the difference.

[0085] The filtered solid (D) was dried and weighed; since the amount of PAC added was known, the amount of PA was determined by the difference.

[0086] In the final BHET(E), PA was no longer present and no PA characteristic bands were shown, as shown in the IR spectrum of the product.

[0087] The results are summarized in the following Tables 3A-3B-3C, where the amount of isolated polyamide present in each sample is reported relative to the total amount added to the r-PET, expressed both by weight and as a weight percent of 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 for the pure polymer were slightly shifted (<1%) due to hydrogen bonds established between HO and EG and the polar groups of the polymer.

[0088] [Table 3]

[0089] [Table 4]

[0090] [Table 5]

[0091] As can be seen, the polyamides were quantitatively removed. The polyamides PA-6 and PA-6,6 were largely removed after step (c), i.e., the first precipitation and subsequent filtration, whereas in the case of the Ultramid PA-6 polyamide, only partial removal was achieved during step (c) and was complete in the subsequent steps, especially after the first treatment with PAC (step (i)).

[0092] Analysis to determine the composition of BHET and its oligomers The composition of BHET and its oligomers at various steps in the process was also determined. Quantitative analysis by HPLC was performed on samples taken from the process flow of the following products: (1) Crude solution obtained from glycolysis; (2) the filtered solution obtained from step (c); (3) the filtered solution obtained from step (d); (4) the BHET precipitate obtained in step (g) dissolved in a water:methanol mixture in a weight ratio of 65:35; (5) the first purified BHET solution obtained from step (h); (6) The crystallized, washed, and dried BHET obtained in step (1) was dissolved in the same water:methanol mixture as above.

[0093] The weight percentages of BHET, dimers, trimers and other oligomers obtained in the various products (1)-(6) using amide PA-6, PA-66 and Ultramid PA-6 are reported in Tables 4A, 4B and 4C.

[0094] [Table 6]

[0095] [Table 7]

[0096] [Table 8]

[0097] As can be seen, this process ensures a gradual improvement in the purity of BHET.

[0098] Final BHET color analysis To verify the overall effectiveness of the process in purifying BHET, color analysis (by UV-Vis spectroscopy and Cie LAB colorimetry) was carried out on the final BHET obtained in step (1).

[0099] -UV-Vis spectrophotometric analysis To evaluate the color properties of the final BHET, the total absorbance of the sample at a given wavelength (Σabs0) measured with a spectrophotometer operating in the desired wavelength range can be used.

[0100] Typically, the wavelengths chosen to cover the visible range are 475, 510, 570, 590, and 650 nm, thus:

number

[0101] The method used for this measurement is as follows: weigh the BHET to be analyzed and dissolve it in dimethyl sulfoxide (DMSO) so that the weight ratio of BHET:DMSO is 1:1. The solution thus obtained is placed in an optical glass test tube (path length: 1.5 cm) and inserted into a spectrophotometer to measure the UV-Vis spectrum.

[0102] -Cie LAB color The color of the dried BHET powder was measured using the Hunter Lab color scale according to Hunter Lab's L, a, b color space (details of this measurement can be found at https: / / support.hunterlab.com).

[0103] The measurements were carried out on cylindrical samples (diameter: 50 mm; height: 10 mm) of dry BHET powder obtained by compressing the powder in a cylindrical container at 400 bar.

[0104] The results are shown in Table 5.

[0105] [Table 9]

[0106] The obtained values are well within the specification limits (Σabs0<0,15; L>95; a=-0,2 / +0,3; b<3).

[0107] Example 2 A series of tests were carried out on PET waste (r-PET) obtained from bottles to which ethylene / vinyl alcohol copolymer (EVOH), used as a barrier polymer in food packaging (commercial product EVOH FIOIB 32% B-Pack™ EVAL - 32% moles of ethylene), was added. The amount of EVOH added was equivalent to 5% by weight of the r-PET.

[0108] The PET waste to which EVOH was added was subjected to a glycolysis reaction under the same conditions as in Example 1. The composition of the reaction mixture was as follows:

[0109] Weight (g) Weight % r-PET 285 19 EG 1200 80 ZnAc 0.3 0 EVOH 15 1 Total 1550.3 100

[0110] The crude BHET solution obtained from the glycolysis process was subjected to a purification process according to the present invention carried out in the same manner as reported in Example 1 (steps (a) to (l)).

[0111] To verify the effectiveness of the EVOH removal process, it was not possible to use a qualitative method based on detecting the IR absorption peak characteristics of EVOH, because the characteristic peak of EVOH, located at approximately 3000 nm, overlaps with the absorption peak of BHET (located in the range of 2800–3300 nm).

[0112] The analyses already described above for Example 1 were then carried out to determine the quantitative removal of EVOH at the various stages of the process and the composition in terms of BHET and its oligomers, as well as the color analysis of the final BHET obtained from step (1), according to the methods described above. The results are reported in the following Tables 6 to 8:

[0113] [Table 10]

[0114] [Table 11]

[0115] [Table 12]

[0116] As can be seen at a glance, the method according to the invention proved to be effective and allowed the EVOH to be quantitatively removed. As with the polyamide Ultramid PA-6, only partial removal was achieved in step (c), being complete in the following stages, in particular after the first treatment with PAC (step (i)).

[0117] Furthermore, this process can guarantee a gradual improvement in the purity and color of the BHET.

Claims

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

2. 10. The process of claim 1, wherein the PET waste is a multi-layer product combining at least one PET film and at least one barrier polymer film.

3. The at least one barrier polymer comprises: (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; 3. The process according to claim 1, wherein the hydroxybenzoate is selected from the group consisting of hydroxybenzoates, ...

4. 4. The process according to any one of claims 1 to 3, wherein after the step of separating the precipitate of the at least one barrier polymer from the BHET solution, the BHET solution thus obtained is further cooled to a temperature of from 30°C to 60°C, preferably from 35°C to 55°C, to precipitate the oligomers and optionally further portions of the barrier polymer.

5. The process according to any one of claims 1 to 4, wherein: After separating the crystallized BHET from the water / EG solvent, dissolving the crystallized BHET in water; treating the resulting solution with at least one first adsorbent; and separating said at least one first adsorbent from the thus treated solution to obtain a first purified BHET solution.

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

7. 7. The process of claim 6, wherein the at least one oxidizing agent is an inorganic oxidizing agent selected from: an alkali metal or alkaline earth metal chlorite or hypochlorite; an alkali metal or alkaline earth metal persalt, preferably a persulfate; potassium permanganate; hydrogen peroxide; ozone; chlorine gas; or a mixture thereof.

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

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