Method for recycling pet

Abstract

The present invention relates to a novel method for recycling polyethylene terephthalate (PET) using a glycolysis process. This method comprises depolymerising the PET via a transesterification reaction using ethylene glycol (EG) and specific catalysts. The method comprises subsequently synthesising an unsaturated polyester (UPE) from the glycolysis products and mixing the UPE with styrene, which produces an unsaturated polyester resin (UPER). Lastly, the UPER is chemically crosslinked, thereby obtaining a thermosetting material with mechanical properties comparable to commercial resins, which is suitable for use in the chemical polymer industry.

Description

[0001] METHOD FOR RECYCLING PET

[0002] TECHNICAL FIELD

[0003] The present invention finds use in the field of polymer recycling, more specifically, PET recycling.

[0004] BACKGROUND AND PREVIOUS ART

[0005] Globally, the accumulation of fossil polymers represents a significant problem. In the specific case of polyethylene terephthalate (PET), global production is estimated at approximately 32 million tons annually. Therefore, recycling these used materials presents an opportunity both to improve environmental conditions and to utilize waste products made from PET. The results of a prior art search are summarized below, showing that the present invention has improved features compared to the documents summarized below.

[0006] WO2016096768A1 describes a method for recycling PET using glycolysis reactions. It specifies a PET / ethylene glycol (EG) ratio of 1 / 50 for the reaction, at a temperature between 100 and 300 °C. Finally, it mentions that the process produces the PET monomer, Bis(2-hydroxyethyl) terephthalate (BHET). WO2016096767A1 describes a process for recycling PET to BHET. This process is carried out via glycolysis in the presence of EG using spinel (ZxAl2O(3+x)) as a catalyst, at a temperature between 100 and 300 °C, with an EG / PET ratio of 1 / 50. WO2013025186A1 describes a process for producing polyesters with a high recycled content. Specifically, it mentions PET recycling through glycolysis and also mentions EG as one of the components for carrying out the recycling. W00047659A1 describes a process for polyester recycling.The process is carried out via glycolysis and includes the step of contacting a glycol with PET under certain conditions. A 1:5 EG:PET ratio and a process temperature between 150 and 300 °C are specified. MX2007004429A describes a process for recycling PET by means of glycolysis. It mentions the glycolysis of PET in the presence of EG at a molar ratio of 1.5:1 EG:PET, at a temperature between 210 and 250 °C.

[0007] BRIEF DESCRIPTION OF FIGURES FIG. 1: HPLC chromatogram of the PETEG formulation = 0.25:1.00 (A) and of the PETEG formulation = 1.00:1.00 (B), where 4 main peaks associated with the BHET monomer, dimer, trimer, and tetramer are identified. FIG. 2: HNMR spectrum of the unsaturated polyester (UPE) obtained using a PET:EG:MA molar ratio of 0.5:1, 0:1, 1. The values ​​above the different peaks represent the peak position in ppm. A molecular representation of UPE is shown at the top of the figure.

[0008] FIG. 3: Diagram of the polycondensation reaction between the reagents presented from glycolysis, and maleic anhydride (MA) producing UPE. The subscript n means the number of units of each component of the final product: n1: BHET or PET oligomers from PET glycolysis; n2: Number of units of the reaction product between MA and n1; n3: Number of units of the reaction product between MA and EG; and n4 number of units of the resulting macrooligomer.

[0009] FIG. 4: Infrared (FTIR) spectrum of UPE and unsaturated polyester resin (UPER), between 4000 and 400 cm-1.

[0010] FIG. 5: Effect of PET:EG molar ratio (0.25:1.00 and 0.50:1.00) using different EG:MA:Styrene (St) molar ratios on the mechanical properties of the synthesized UPER: (A): Young's modulus; (B): Tensile strength; and (C): Elongation at rest.

[0011] FIG. 6: Comparison of the mechanical properties between a commercial resin and two formulations (PET:EG:AM:St) with similar properties. (A): Young's modulus. (B): Tensile strength. (C): Elongation at rest.

[0012] FIG. 7: Thermogravimetric analysis (TGA) of representative UPER resins. Three UPER formulations and a commercial resin are compared. The corresponding PET:EG:AM:St molar ratio is: 0.25:1, 0:0.9:0.5 for flexible UPER; 0.5:1, 0:1, 1:1.0 for brittle UPER; and 0.5:1, 0:1, 1:0.75 for a UPER with similar mechanical behavior compared to the commercial resin.

[0013] SUMMARY OF THE INVENTION

[0014] The invention relates to a method of recycling PET considering glycolysis reactions.

[0015] DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention relates to a method for recycling PET. The method comprises at least the following steps:

[0017] a. Depolymerize PET to be recycled by means of glycolysis;

[0018] b. Synthesize unsaturated polyester (UPE); c. Prepare a mixture of UPE and styrene to generate an unsaturated polyester resin (UPER);

[0019] d. Perform a chemical crosslinking of the UPER.

[0020] In one specific embodiment, the depolymerization of the PET to be recycled is carried out via a transesterification reaction, with EG reacting with the PET to be recycled. In this specific configuration, a PET:EG molar ratio of between 0.11:1 and 1:1 is used.

[0021] In another embodiment, a catalyst selected from heterolite, nanomaghemite, zinc acetate, manganese acetate, titanium tetrabutoxide (TITB), potassium sulfate, sodium sulfate, and others is used. In a more specific embodiment, the catalyst concentration is between 0.1 and 0.8 wt% relative to the mass of PET.

[0022] In another embodiment, the glycolysis reaction is carried out at a temperature between 180 and 250 °C for a period of between 2 and 8 hours.

[0023] In another embodiment, the synthesis of unsaturated polyester (UPE) from the depolymerization products is carried out in the same reactor by adding maleic anhydride at a rate of between 0.25 and 2 g per gram of PET, so as to achieve polycondensation at temperatures between 140 and 180 °C and for a period of between 2 and 8 hours.

[0024] In another embodiment, the UPE and styrene mixture is prepared by blending UPE with styrene, where the styrene acts as a crosslinking agent, at a ratio of 15 to 40% by mass of the resin. To prevent crosslinking prior to the addition of the initiator, hydroquinone (HQ) is added at concentrations of 0 to 0.1% by weight of the UPE+styrene mixture. The UPE+styrene mixture is then stirred until a homogeneous mixture is obtained. Optionally, a temperature of 50 to 80 °C is used to prevent styrene evaporation and crosslinking of the thermal breaks in the styrene double bond.

[0025] Regarding crosslinking, the curing of the UPE+styrene mixture is carried out at room temperature using appropriate catalysts selected from among aluminum trimethyl phosphite, titanium trimethyl phosphite, dimethylamine, cobalt octoate, dimethyl-p-toluene, diethylaniline, and phenyldiethanolamine, among others, and an initiator. More specifically, the initiator is selected from among benzoyl peroxide, propylbenzene peroxide, isopropyl peroxide, acetyl peroxide, methyl ethyl ketone peroxide, lauroyl peroxide, and cyclohexanone peroxide, among others. As for reaction time, gel times range from 5 to 20 minutes, and the time to achieve final rigidity is between 6 and 24 hours.

[0026] The curing reaction is carried out using catalyst concentrations between 0.05 and 0.5% by weight and initiator concentrations between 0.1 and 3% by weight. The curing reaction is performed by progressively adding the catalyst and initiator, following these steps:

[0027] a. Add an initial amount of the catalyst to the UPE+styrene mixture and allow stirring until a homogeneous mixture is achieved;

[0028] b. Add starter to the homogeneous mixture obtained in the previous step;

[0029] c. Repeat steps a and b until you obtain a gel-like mixture.

[0030] Specifically, between 0.01 and 10% of catalyst and between 0.1 and 3% of initiator are added.

[0031] EXAMPLES

[0032] Materials

[0033] All reagents and materials listed below were used as received. Recycled polyethylene terephthalate (PET) bottle flakes, ranging in size from 2 to 8 mm and 0.6 mm thick, were used as raw material for the synthesis and were supplied cleaned and ground. Ethylene glycol (EG) 98%, titanium tetrabutoxide (TITB) 97%, maleic anhydride (MA) 99%, styrene (St) 99%, hydroquinone (HQ) 99%, and methyl ethyl ketone peroxide (MEKP) 35% w / w were purchased from Sigma Aldrich. A commercially available polyester orthophthalic resin, Polipol 332, from Poliya Composite Resins and Polymers Inc., was used for comparison purposes.

[0034] A 500 mL glass bulb was used as a closed reactor, and a cooling column was used to maintain reflux of the vapors in the reaction. The reactor was immersed in a thermal oil bath heated by an electric hot plate to control the reaction temperature.

[0035] PET glycolysis

[0036] For the depolymerization of PET via glycolysis, a mass of EG between 0.3 and 3.0 grams per gram of PET was used, equivalent to a PETEG molar ratio between 1.0:1.0 and 0.11:1.0, respectively. The PETEG molar ratio was calculated considering the repeating unit of PET as the basis for calculating the number of moles. For example, a PETEG molar ratio of 0.25:1.0 means that for each repeating unit of PET, there are 4 EG molecules. The upper limit (PETEG = 0.11:1.0) corresponds to the concentration that optimizes the BHET yield, and the lower limit (PET:EG = 1.0:1.0) corresponds to the minimum amount of EG according to the reaction stoichiometry, as previously reported.

[0037] TITB was used as a catalyst for the PET transesterification reaction at a concentration of 0.3% w / w with respect to PET mass.

[0038] The reaction temperatures and times were determined within the range of 180 to 250 °C and 2 to 8 hours, respectively. These temperatures correspond to the temperature of the oil in the reactor's thermal bath. A temperature difference of 30 °C was measured between the oil and the reaction system.

[0039] Synthesis of Unsaturated Polyester

[0040] The synthesis of unsaturated polyester (UPE) from the depolymerization reaction products was carried out in the same reactor by adding maleic anhydride, between 0.25 and 2 grams per gram of PET, in order to carry out polycondensation at temperatures between 140 and 180 °C for a period of time between 2 and 8 hours according to previous results.

[0041] At the end of the reaction time, the system was allowed to cool to a temperature of 150 °C and the reactor was opened, maintaining the temperature for one hour to remove the water produced by means of a dry air flow.

[0042] Preparation of the UPE and styrene mixture to generate UPER

[0043] The UPE was physically mixed with St acting as a crosslinking agent, in quantities between 15 and 40% relative to the resin mass. HQ was also added at concentrations of 0%, 0.01%, 0.05%, and 0.1% by weight relative to the UPE+St mixture to prevent crosslinking prior to the addition of the initiator.

[0044] The UPE and styrene mixture was stirred manually until a homogeneous mixture was obtained. If the UPE at room temperature has a very viscous consistency that prevents mixing, the solution is heated below 80 °C, preventing evaporation of St and avoiding crosslinking of thermal breaks in the St double bond.

[0045] Chemical cross-linking of the UPER

[0046] The curing of the UPER was carried out at room temperature following standard chemical crosslinking protocols for commercial polyester resins using the cobalt octoate catalyst and methyl ethyl ketone peroxide initiator.

[0047] During curing, gel times ranged from 5 to 20 minutes, and the time it took for the resin to reach final rigidity varied between 6 and 24 hours. The curing reactions were carried out with cobalt octoate (Co) concentrations between 0.05 and 5% w / w and MEKP concentrations between 0.1 and 3% w / w. The tests were performed by progressively adding Co and MEKP according to the following protocol: 1) 0.05% Co was added to the UPER and stirred until homogenized; 2) 0.10% MEKP was added to the UPER and stirred until homogenized; and 3) after 5 minutes, the condition of the mixture was confirmed. If it continued to behave as a liquid, steps 1), 2), and 3) were repeated. If not, either because it turned into a gel or because stirring was not possible, it was allowed to stand.

[0048] Successful curing tests were repeated to corroborate the concentration required to have a gel time between 5 and 10 minutes and complete curing in 24 hours (or less).

[0049] Characterization

[0050] The glycolysis products were identified using an HPLC 1100 (Agilent Technologies Inc., CA, USA) coupled to an Esquire 4000 ESI-IT electrospray-ion trap mass spectrometer (Bruker Daltonik GmbH, Germany). A 4.6 mm Chromolith RP-100 column (Merck KGaA, Germany) and a UV detector at 254 nm were used. A 1.5 mg / mL stock solution in dichloromethane was prepared for the working solution, and methanol was used as the mobile phase at a flow rate of 1 mL / min.

[0051] The UPE was analyzed by 1 HNRM (BRUKER AVANCE III HD-400) using 99.9% deuterated acetone (acetone-D6) from Sigma-Aldrich as the solvent and TMS as a reference. Changes in the chemical structure of the UPE during the curing process were studied by FTIR (Agilent Cary 630).

[0052] The UPER samples were subjected to tensile testing based on ASTM D638. A commercial resin was also used as a reference for the expected UPER values ​​and performance. The tests were performed using a Jinan WDW-S50 universal tensile and compression testing machine. The UPER was thermally analyzed using a TGA-DTA analyzer (NETZSCH TG 209 F1 Libra) under 20 mL / min of 99.9% nitrogen at a heating rate of 10 °C / min.

[0053] Results and discussion

[0054] PET depolymerization via glycolysis

[0055] Due to the wide range of temperatures and reaction times reported for PET glycolysis, a preliminary study was conducted on the effect of these two variables using PET:EG molar ratios of 0.25:1.0 and 0.11:1.0 and a catalyst concentration of 0.3% w / w. PET particles could be identified at temperatures below 200 °C and reaction times of up to 4 hours, indicating an incomplete reaction. A predominantly white liquid mixture was observed at temperatures between 200 and 220 °C and reaction times of up to 12 hours. Finally, at temperatures above 230 °C and reaction times of at least 4 hours, the product obtained was translucent with no solid particles observed. This last result is considered complete PET depolymerization, and a temperature of 230 °C and a reaction time of 4 hours were subsequently established for the following reactions.

[0056] To further confirm PET depolymerization, the previously obtained reaction products were analyzed by HPLC and mass spectrometry, as shown in Figure 1. The presence of peaks at 19, 29, 34, and 38 minutes is associated with BHET and dimer, trimer, and tetramer oligomers from PET glycolysis, respectively. It is relevant that BHET is the main component of the obtained products. Figure 1 also shows that as the PET:EG ratio increases, the intensity of the peaks associated with the oligomers increases relative to the peak associated with the BHET monomer (peak 1).

[0057] Table 1 shows the composition of these compounds as measured by integrating each peak, normalizing the total sum of the areas under the peaks to 1, under different PET:EG ratios and catalyst concentrations. The highest BHET yield was 71% using a PETEG molar ratio of 0.25:1.00, which is lower than the yields reported in the prior art, ranging from 80% to 85%. Higher-than-optimal PETEG ratios were also used to reduce the amount of residual EG in the product mixture. This is relevant because no separation process was performed to remove unreacted products and oligomers.

[0058] Table 1. Relative composition of the BHET and PET oligomers produced by glycolysis determined by HPLC.

[0059] Catalyst

[0060] PET (6) :EG Peak 1 < 2 > Peak 2 < 3 > Peak 3 < 4 > Peak 4 < 5 > (TITB)

[0061] Peak Area Ratio Peak Area Peak Area Peak Area % w / w < 1 >

[0062] molar normalized normalized normalized normalized 0.25:1 .0 0.300 (0.23) 0.71 0.24 0.05 0.00 0.50:1 .0 0.300 (0.46) 0.59 0.28 0.11 0.03 0.67:1 .0 0.300 (0.63) 0.52 0.31 0.13 0.03 1 .00:1 .0 0.300 (0.93) 0.43 0.32 0.19 0.06 0.50:1 .0 0.150 (0.23) 0.57 0.30 0.10 0.03 1 .00:1 .0 0.075 (0.23) 0.46 0.31 0.18 0.05(1): wt % with respect to PET and, in parentheses, with respect to EG. (2): Elution time of BHET is 19.3 minutes; (3): Elution time of BHET dimer is 29.8 minutes; (4): Elution time of BHET trimer is 34.8 minutes; (5): Elution time of BHET tetramer is 37.7 minutes. (6): Using 10 g of PET as the basis for the experiments.

[0063] Synthesis of unsaturated polyester (UPE)

[0064] The products obtained from PET glycolysis were directly mixed with maleic anhydride (AM) to synthesize unsaturated polyester (UPE). Since the products were not purified, the effect of oligomers could be studied. Preliminary tests were conducted to determine suitable PETAM and EG:AM molar ratios by analyzing the formation of a solid resin after adding styrene, Co, and MEKP to crosslink the UPE samples. In this case, a PET:EG molar ratio of 0.25:1.0 and a catalyst concentration of 0.3% w / w were used to test molar ratios of 0.2:1.0 to 1.0:1.0 and 1.0:0.125 to 1.0:1.0 for PETAM and EG:AM, respectively. A solid UPER resin was obtained using an EG:AM molar ratio of around 1.0:1.0 in the synthesis of UPE, regardless of the PET:EG ratio used during glycolysis.Based on this study, the glycolysis products presented in Table 1 were used to synthesize UPE using EG:AM molar ratios of 1.0:0, 9, 1.0:1.0, and 1.0:1.1. All the compounds in Table 1 produced UPE with the same appearance: a very homogeneous, white, viscous liquid. Therefore, the presence of oligomers and unreacted EG did not affect UPE production, meaning they can be incorporated into the macromolecule. This was confirmed by ¹H-NMR, as shown in Figure 2. The expected structures from the reactions between AM and all the glycolysis compounds were identified: aromatic ring hydrogens at 8.14 ppm; and hydrogens adjacent to the non-saturations, where the trans and cis isomers were identified, at 6.80 ppm and 6.41 ppm, respectively. Between 3.5 and 5 ppm, the signals associated with CH2 were observed mainly with multiple peaks due to the different adjacent structures.

[0065] The ratio between the identified structures was also quantified from the spectrum by integrating the area under the signals. The signal at 8.14 ppm is used as a reference, with a value of 4 indicating 4 hydrogen atoms (H), corresponding to 1 aromatic ring. The signals associated with unsaturations correspond to 6.41 ppm and 6.80 ppm, and integrate a value of 3.97, indicating 2 double bonds. The signals associated with CH2 integrate a value of 12.21, indicating 6 ethyl groups. To calculate the PET:EG:AM ratio, PET is assumed to be 1, since it is the only structure with aromatic rings, while AM ​​is assumed to be 2, since it is the only compound incorporating unsaturations in its structure. Of the 6 CH2 groups identified, 2 are part of the repeating unit of PET, the remaining 4 CH2 are provided by 2 EG (4 CH2 of 2 x HO-CH2 CH2-OH).In summary, the PET:EG:AM ratio obtained from the ¹H NMR spectrum is 1:2:2, or equivalent to 0.5:1.0:1.0, which was similar to the theoretical formulation (0.5:1.0:1.1) in the amount of AM. The difference in the AM molar ratio may arise from the process at the end of the polycondensation, with AM molecules being removed by convection due to the airflow. Based on these results, a proposed scheme of the polycondensation reaction and the products obtained in the UPE synthesis is shown in Figure 3. The inclusion of BHET, oligomers, and residual EG during the reaction with AM is proposed, where the latter reacts indiscriminately with all the reagents present.

[0066] The UPE produced from the formulation PET:EG:AM = 0.5:1, 0:1,1 was characterized by FTIR (FIG. 4) to confirm the structure of FIG. 3. The FTIR spectrum of the UPE showed the bands associated with the proposed structure at: 3372 cm -1 , for the vibration of the OH bond; 1711 cm-1 , for the vibration of the C=O bond of the ester group; 1246 cm -1 , for the vibration of the C-0 bond of the ester group, and 1099 cm -1 , for the vibration of the C-O bond in the -O-CH2- group. Bands also appear at 1408 cm -1 and 727 cm - 1 of aromatic CH vibrations in para-disubstituted structures. The peaks at 1645 cm -1 and 1580 cm -1 These correspond to C=C bond vibrations and the peaks at 977 cm -1 and 630 cm -1 These are vibrations of CH bonds in trans and cis alkenes, respectively.

[0067] Preparation of unsaturated polyester resin (UPER) and crosslinking

[0068] The UPE was physically blended with styrene (St) to obtain a liquid, homogeneous pre-resin at room temperature. The relative concentrations used are shown in Table 2, where 0.01% w / w hydroquinone was additionally used as a stabilizer. For PET:EG molar ratios of 1.0:1.0 and 0.67:1.0, a liquid mixture was not obtained at room temperature because the high amount of PET oligomers altered the liquid / viscous characteristic of the pre-resin. Therefore, these resins were not cross-linked.

[0069] The resulting UPE / St mixtures were crosslinked by first adding 0.15% w / w cobalt octoate and then 0.45% w / w methyl ethyl ketone peroxide, and stirring until homogenized. The mixture was then poured into molds to obtain specimens with dimensions according to ASTM D638. Crosslinking was carried out at room temperature, and after 24 hours the samples were removed from the molds for tensile testing a couple of days later. Table 2: Formulations used in the UPER development study and corresponding mechanical property values ​​obtained.

[0070] Resistance Elongation in Modulus of PET^EG EG : MA EG : St St

[0071] to traction the rest Young Relationship Relationship Relationship

[0072] % p / p < 1 > MPa % GPa molar molar molar

[0073] 1.0:0, 9 1.0:0, 5 20.8 2.95 40.81 0.01 1, 0:1.0 1.0:0, 5 20.0 11.48 3.16 0.42 0.25:1.0 .

[0074] 1.0:0, 5 19.3 13.32 7.79 0.29 1 01 1 _

[0075] 1, 0:1,0 32,3 14,05 5,51 0,32 1,0:0, 5 17,4 14,21 5,41 0,38 1 009 .

[0076] 1, 0:1,0 29,7 34,37 3,52 0,94 1,0:0, 5 16,9 10,97 6,30 0,30 1, 0:1,0 1,0:0,75 23,4 12,57 2,95 0,57 0,50:1,0 _

[0077] 1, 0:1,0 28,9 13,66 2,31 0,73 1,0:0, 5 16,4 22,30 2,10 1,34 1, 0:1,1 1,0:0,75 22,7 29,63 3,56 0,89

[0078] 1, 0:1,0 28,1 9,72 0,77 1,69 0,67:1,0 1, 0:1,1 . . .

[0079] 1, 0:1,0 1, 0:1,0 . . .

[0080] (1): Percentage by mass of styrene with respect to the total mass of resin; (2): Using 10 g of PET as the basis for the experiments.

[0081] Table 2 shows the tensile strength, elongation at break, and Young's modulus values ​​obtained from tests performed on the produced UPER. The values ​​show high variation depending on the specific formulation, with differences as high as an order of magnitude. The wide range of mechanical properties confirmed that the behavior depends on the relative concentrations of PET, EG, MA, and St. Therefore, the mechanical properties of a UPER can be tailored according to the relative compositions of EG, oligomers, AM, and St.

[0082] Figure 5 shows the effect of the PET:EG ratio (0.25:1.0 and 0.5:1.0) grouped according to the EG:MA:St molar ratio. In general, an increase in the PET:EG ratio implies an increase in tensile strength and Young's modulus, and a decrease in elongation at break. This indicates that a higher amount of PET relative to EG during glycolysis, generating a higher fraction of oligomers (see Table 1) and a lower amount of residual EG, produced a material with greater stiffness and toughness. The residual EG produces saturated linear segments during polycondensation, giving flexibility to the UPER structure. Furthermore, the presence of oligomers in the formulation can contribute to both toughness and brittleness due to their aromatic structure derived from the PET chemical structure. In Figure 5, the PET:EG ratio is shown in Figure 5.Figure 6 shows a comparison between a commercial IIPER and two of our formulations, showing that the process used to produce IIPER from PET allows obtaining a thermoset material with properties comparable to those of a commercial resin without any separation process.

[0083] Thermal analysis

[0084] Figure 7 shows the results of the thermogravimetric analysis (TGA) performed on three representative IIPER formulations and on the commercial resin. The IIPER selected for this analysis represents resins that exhibit flexible (molar ratios of 0.25:1, 0:0.9:0.5), brittle (molar ratios of 0.5:1, 0:1, 1:1.0), and similar (molar ratios of 0.5:1, 0:1, 1:0.75) behaviors compared to the commercial resin. At least three relevant characteristics were identified in the TGA analysis: a slight mass loss between 100 and 280 °C; a significant mass loss at 380 °C; and the residual mass at the end of the experiment. Table 3 summarizes the values ​​associated with these observations.

[0085] Table 3: Temperatures and percentages of degraded and residual mass in the stages identified from the TGA analysis.

[0086] 10% temperature Maximum temperature

[0087] Residual mass Sample of weight loss degradation rate

[0088] (°C) (°C) ( / o)

[0089] CR 312.2 395.5 1 .6 0.25:1 :0.9:0.5 241 .3 395.1 13.6

[0090] 0, 5:1 :1 , 1 :1 ,0 285,6 395,5 15,0 0,5:1 :1 ,1 :0,75 311 ,0 397,8 14,5 CR: Commercial resin. PET:EG:MA:St formulation molar ratio.

[0091] The first observed mass loss is associated with the volatilization of residual reactants (monomers) present in the polymer matrix, primarily EG, which has a boiling point of 198 °C; maleic anhydride, with a boiling point of 202 °C; cis-butanedioic acid and trans-butanedioic acid (products of the reaction between maleic anhydride and water during polycondensation), with boiling points of 135 °C and 275 °C, respectively; and styrene, with a boiling point of 145 °C. It should be noted that both the commercial resin and the resin with the best mechanical properties exhibit mass losses of around 10% by mass between 0 and 315 °C, while the other two resins show greater mass decreases, which could indicate less monomer incorporation into the polymer matrix. The latter is consistent with the formation of a weaker network or lower mechanical properties.

[0092] Although there are significant differences in temperature for 10% mass loss, the temperature at which maximum degradation occurs is very similar for all four resins and corresponds to the main decomposition stage. Several studies have concluded that primary degradation in polyesters and polyester resins occurs with the rupture of C-O bonds adjacent to the ester groups through a P-CH2 hydrogen ion transfer process. Therefore, the similarities in this temperature across all samples corroborate that the lattice is composed of covalent bonds between polyester chains, which characterize the thermal stability of the resins. Styrene chains or styrene oligomers degrade thermally through relatively weak breaks in the C-C bonds adjacent to the polyester.

[0093] Regarding the residual mass obtained, for orthophalic polyester resins, after the decomposition of CO bonds, italic acid is produced, a compound that, upon dehydration, forms italic anhydride and volatilizes. On the other hand, the phenomenon of intumescence is reported, where carbon-rich esterified compounds decompose through dehydration and / or decarboxylation, resulting in the formation of a carbonization residue. This occurs in terephthalic polyester resins where, after the decomposition of CO bonds, terephthalic acid is produced, which does not have a boiling point and degrades at high temperatures through dehydration and / or decarboxylation, resulting in a carbonization residue (thermal stability studies of PET also show the formation of residues).In summary, the differences between the residues of the commercial resin versus the synthesized IIPER are due to the fact that the commercial resin is of the orthophalic type, whose thermal decomposition produces phthalic anhydride that volatilizes, and IIPER has parasubstituted aromatic groups (terephthalate group) that decompose by dehydration and decarboxylation forming a carbon structure.

[0094] INDUSTRIAL APPLICATION

[0095] The present invention has application in the chemical polymer industry, more specifically, the recycling of polymers.

Claims

CLAIMS 1. Method for recycling polyethylene terephthalate (PET), CHARACTERIZED in that it comprises at least the following steps: a. depolymerize PET for recycling by means of glycolysis; b. synthesize unsaturated polyester (UPE); c. prepare a mixture of UPE and styrene to generate an unsaturated polyester resin (UPER); d. perform a chemical crosslinking of the UPER.

2. Method for recycling polyethylene terephthalate PET according to claim 1, CHARACTERIZED in that the depolymerization of the PET to be recycled is carried out by a transesterification reaction, reacting ethylene glycol (EG) with the PET to be recycled.

3. Method for recycling polyethylene terephthalate PET according to claim 2, CHARACTERIZED in that a molar ratio of PET:EG of between 0.11:1 and 1:1 is used.

4. Method for recycling polyethylene terephthalate PET according to any of claims 1, 2 and 3, CHARACTERIZED in that a catalyst selected from heterolite, nanomaghemite, zinc acetate, manganese acetate, titanium tetrabutoxide (TITB), potassium sulfate, sodium sulfate, inorganic salts of acetates or sulfates, metal oxides, nanomaghemite and metal alkoxides is used.

5. Method for recycling polyethylene terephthalate PET according to claim 4, CHARACTERIZED in that the concentration of the catalyst is between 0.1 and 0.8% by weight with respect to the mass of PET.

6. Method for recycling polyethylene terephthalate PET according to any of claims 1 to 5, CHARACTERIZED in that the glycolysis reaction is carried out at a temperature of between 180 and 250 °C for a period of between 2 and 8 hours.

7. Method for recycling polyethylene terephthalate PET according to any of claims 1 to 6, CHARACTERIZED in that the synthesis of unsaturated polyester (UPE), from the depolymerization products, is carried out in the same reactor by adding maleic anhydride at a ratio of between 0.25 to 2 grams per gram of PET, so as to achieve polycondensation at temperatures between 140 and 180 °C and for a period of between 2 and 8 hours.

8. Method for recycling polyethylene terephthalate PET according to any of claims 1 to 7, CHARACTERIZED in that the preparation of the UPE and styrene mixture is achieved by mixing the UPE with styrene, wherein the styrene acts as a crosslinking agent, in a ratio of between 15 and 40% with respect to the mass of resin.

9. Method for recycling polyethylene terephthalate PET according to any of claims 1 to 8, CHARACTERIZED in that, to avoid crosslinking prior to the addition of the initiator, hydroquinone (HQ) is added in concentrations of between 0.001 and 0.1% by weight with respect to the UPE mixture with styrene.

10. Method for recycling polyethylene terephthalate PET according to any of claims 1 to 9, CHARACTERIZED in that the UPE mixture with styrene is physically stirred until a homogeneous mixture is obtained.

11. Method for recycling polyethylene terephthalate PET according to claim 10, CHARACTERIZED in that a temperature of between 50 and 80 °C is considered to avoid evaporation of styrene and to prevent cross-linking of the thermal breaks of the styrene double bond.

12. A method for recycling polyethylene terephthalate (PET) according to any one of claims 1 to 11, CHARACTERIZED in that, to achieve crosslinking, the UPE mixture is cured with styrene, the reaction is carried out at room temperature, using appropriate catalysts selected from aluminum trimethyl phosphite, titanium trimethyl phosphite, dimethylamine, cobalt octoate, dimethyl-p-toluene, diethylaniline, phenyldiethanolamine, among others, and the initiator. More particularly, the initiator is selected from benzoyl peroxide, propylbenzene peroxide, isopropyl peroxide, acetyl peroxide, methyl ethyl ketone peroxide, lauroyl peroxide, and cyclohexanone peroxide.

13. Method for recycling polyethylene terephthalate PET according to claim 12, CHARACTERIZED in that for the curing reaction time, the gel times are between 5 and 20 minutes and the time to achieve final rigidity is between 6 and 24 hours.

14. A method for recycling polyethylene terephthalate (PET) according to any of claims 12 and 13, CHARACTERIZED in that the curing reaction is carried out considering catalyst concentrations of between 0.05 and 0.5% by weight and initiator concentrations of between 0.1 and 3% by weight, wherein the curing reaction is carried out by progressively adding the catalyst and initiator considering the following steps: a. Add an initial amount of the catalyst to the UPE mixture with styrene and allow stirring until a homogeneous mixture is achieved; b. Add initiator to the homogeneous mixture obtained in the previous step; c. Repeat points a and b until a gel-type mixture is obtained, where the concentration of the catalyst is between 0.01 and 10% and the concentration of the initiator is between 0.1 and 3%.

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