Process for the degradation of plastic products containing polyesters

Abstract

Object of the present invention is a novel environmentally friendly process for the degradation of plastic materials containing polyesters. The process provides for a pre-treatment of dissolution / precipitation of the plastic material in appropriate solvents, aimed at facilitating the subsequent depolymerization step of the polyester.

Description

[0001] "PROCESS FOR THE DEGRADATION OF PLASTIC PRODUCTS CONTAINING POLYESTERS"

[0002] ABSTRACT

[0003] Object of the present invention is a novel environmentally friendly process for the degradation of plastic materials containing polyesters. The process provides for a pre-treatment of dissolution / precipitation of the plastic material in appropriate solvents, aimed at facilitating the subsequent depolymerization step of the polyester.

[0004] The process according to the invention enables the recovery of monomers and / or oligomers that can be reused for the ex-novo synthesis of plastic materials with properties unchanged from the virgin material. Such methodology can be easily implemented in industrial settings dedicated to recycling plastic waste, with the potential to produce significant benefits in terms of energy efficiency and environmental sustainability.

[0005] FIELD OF INVENTION AND STATE OF THE ART

[0006] Because of low production costs and excellent thermo-mechanical properties, plastic materials have rapidly become indispensable for the development of modern society. Indeed, the global plastic production has grown substantially in recent years, exceeding 400 million tons in 2019. This upward trend is expected to continue, raising increasing concern about harmful impact on biodiversity, water quality, and especially human health. In particular, polyesters are a broad category of plastic materials widely used in the packaging industry and fiber production, of which polyethylene terephthalate (PET) is undoubtedly the best-known example. To cope with the risks associated with its poor degradability in the natural environment and the release of highly polluting micro-plastics, several solutions have been studied in recent years for recycling PET and other polyesters.

[0007] Mechanical recycling is undoubtedly the most established approach for the treatment of post-consumer plastic materials. This process relies on melting and extruding waste to create recycled granules that can be used as raw material for new applications. The main disadvantage of this technology is that the combined effect of repeated heating and frequent mechanical stresses can lead to a reduction in the molecular weight of the recycled polymer, thus reducing its mechanical and thermal qualities with each reprocessing. Furthermore, mechanical recycling does not allow for the separation of mixed waste in which, in addition to polyester, other polymers and / or additives or contaminants of a different nature are present.

[0008] The limitations of traditional mechanical methods could be overcome by using more sustainable and circular solutions, such as for example, chemical recycling by depolymerization. Such process involves breaking down chemical bonds that constitute the polymer, thus transforming it into monomers that can be reused for the making of plastic manufactured products with unchanged properties from the virgin material. Chemical depolymerization of polyesters can be achieved through different solvolitic processes, the most common of which are glycolysis, methanolysis, neutral, acid and alkaline hydrolysis.

[0009] Despite recent advances in the field, mechanical recycling is still preferable to chemical recycling in terms of economic and environmental impact because of high energy costs associated with the chemical depolymerization process and toxicity and corrosiveness of solvents used in it.

[0010] A strategy that has recently emerged in the literature to significantly accelerate the chemical depolymerization process of polymeric materials recalcitrant to degradation is to pre-treat the polymer by using solvents. The degradation of polymers, and polyesters in particular, is indeed an interface phenomenon that initially involves only the outer surface of the polymer and then extends deep into the material. Such mechanism is significantly hampered by the high level of crystallinity (about 30-40%) of most post-consumer manufactured products. Several studies have shown that through the diffusion of solvent molecules into the polymer matrix, it is possible to weaken the interactions between the polyester chains and increase the specific surface area of the polymer, thus facilitating the access of depolymerizing agents to the reaction sites, resulting in a reduction of depolymerization time. Although pre-treatment of polyesters appears to be a promising solution to reduce time and energy costs associated with the chemical depolymerization process, further efforts are needed to discover more efficient, recyclable and low-toxicity solvents to make chemical recycling competitive with traditional methods.

[0011] OBJECTS OF THE INVENTION

[0012] Object of the present invention is to provide a process for degrading plastic products containing polyesters that is efficient and sustainable.

[0013] Further object of the present invention is to provide a process for degrading plastic products containing polyesters, which uses environmentally friendly "green" solvents widely used in cosmetics and pharmaceuticals.

[0014] Still an object of the present invention is to provide a process for degrading plastic products containing polyesters that uses "green" solvents that are also cost- effective when used on an industrial scale.

[0015] Yet an object of the present invention is to provide a process for degrading plastic products containing polyesters that allows their chemical depolymerization in extremely short time and also allows the use of minimal amounts of catalysts.

[0016] OBJECT OF THE INVENTION

[0017] Object of the present invention is a process for chemical depolymerization of plastic materials containing polyesters, comprising: a pre-treatment step of the plastic material with cyclic alkyl carbonates, resulting in the dissolution and precipitation of said polyesters in the form of pre-treated material, a washing step of said pre-treated material with deionized water and / or at least one organic solvent free of ester functionalities, resulting in a washed pretreated material, a subsequent chemical depolymerization step of said pre-treated washed material, resulting in monomers and / or oligomers.

[0018] The inventors have surprisingly found that the pre-treatment step carried out according to the present invention, by altering the physical structure of the polyesters, allows their subsequent chemical depolymerization in an extremely short time even in the presence of any minimal amounts of catalyst.

[0019] Once the plastic material containing polyesters undergoes pre-treatment with cyclic alkyl carbonates according to the invention, precipitation of the dissolved polyesters in said cyclic alkyl carbonates is induced by cooling, resulting in a "gel", in which the solvent molecules (cyclic alkyl carbonate) remain trapped inside the polymer matrix. The cyclic alkyl carbonate used in the aforementioned pre-treatment step can be extracted from that matrix by repeated washing in water without altering the gel structure of the polymer, which, thanks to the increased specific surface area, is easily degraded in the subsequent chemical depolymerization step. Thanks to the low environmental impact of the solvents used and the significant energy savings related to the reduction in depolymerization time, the process according to the present invention is of considerable interest to chemical industries operating in the recycling sector, thus providing practical help in solving the current crisis caused by the accumulation of plastics.

[0020] Thus, an object of the present invention is a process for the degradation of plastic material containing polyesters, which involves the pre-treatment of said plastic materials by dissolving said plastic materials in an appropriate solvent consisting of cyclic alkyl carbonates and the subsequent precipitation of the polyester in "gel" form, followed by chemical depolymerization of the pre-treated material.

[0021] The process of the present invention will be explained in detail below, also with reference to the attached figures.

[0022] BRIEF DESCRIPTION OF THE FIGURES

[0023] Figure 1 shows bottles containing 1 g PET from the following samples (a) VIRGIN PET, (b) WET PET 25, (c) DRY PET 25.

[0024] Figure 2 shows the PET conversion percentages and TPA yields for (a) virgin (VIRGIN PET), (b) pre-treated (Experiment la) without removing the wash water contained inside the polymer (WET PET 25), (c) pre-treated (Experiment la) and dried in an oven at 70°C (DRY PET 25), high crystallinity PET powder (Poliplast S.r.l. - Bergamo, Italy).

[0025] Figure 3 shows PET conversion percentages and TPA yields at different reaction times (Experiment 1c) at 90°C (Figure 3a) and 25°C (Figure 3b).

[0026] Figure 4 shows PET conversion percentages and TPA yields after 5 minutes of reaction at varying temperature (Experiment 1c).

[0027] Figure 5 shows the relationship between linearized PET conversion (Jander's model) at different temperatures and reaction time (Figure 5a), and the Arrenhius relationship for the alkaline hydrolysis process of PET (Figure 5b).

[0028] Figure 6 shows the PET conversion percentages (Figure 6a) and TPA yields (Figure 6b) for the samples prepared at 25 wt% and 15 wt% at different reaction times (1 g highly crystalline PET powder, 1.2 equivalents of NaOH, 90°C).

[0029] Figure 7 shows the ATR-IR spectra of (a) TPA obtained by depolymerization of highly crystalline PET powder and (b) commercial standard of TPA.

[0030] Figure 8 shows theXH NMR (DMSO-de) spectrum of the reaction product (TPA) isolated from the depolymerization of highly crystalline PET powder.

[0031] Figure 9 shows the recyclability of propylene carbonate, showing the efficiency of four subsequent reaction cycles (Figure 9a), along with the ATR-IR spectrum of propylene carbonate as received and after the fourth recovery cycle (Figure 9b).

[0032] Figure 10 shows the effect of the treatment with activated carbon for decolorization of reaction products.

[0033] Figure 11 shows the TGA curve of water swollen PET (WET PET).

[0034] Figure 12 shows: (a) the SEM image of the surface of virgin PET, (b)-(c) the CRYO-SEM images of the cryo-fractured surface of WET PET 25 after 3 and 10 minutes of ice sublimation at -110 °C, respectively, (d) the SEM image of the surface of DRY PET 25.

[0035] Figure 13 shows the ATR-IR spectra of virgin PET (gray line) and DRY PET 25 (black line). The peaks associated with C=O group and ester stretching vibrations (1090, 1243, 1712 cm'1), benzene ring skeletal vibrations (722, 1410 cm'1), CH stretching (2910, 2970 cm'1), and bending modes (1340, 1370 cm'1) are depicted by the vertical lines.

[0036] Figure 14 shows the TGA curves of virgin and DRY PET 25. The lower panel shows the derivative of the two curves.

[0037] Figure 15 shows the GPC curves depicting the distribution of the mass fraction between the molecular weights of virgin (gray line) and DRY (black line) PET 25.

[0038] Figure 16 shows the multi -peak fit of WAXD spectra of virgin (top panel) and DRY (bottom panel) PET 25. The dark bands depict the crystalline peaks, whereas the enlarged light band is the amorphous material.

[0039] Figure 17 shows the DSC curves of first heating, cooling, and second heating at the rate of 10 °C / min for virgin PET (gray curves) and DRY PET (black curves). The arrows depict the two melting peaks (Tml=237 °C and Tm2=247 °C) in the first heating scan of virgin PET, and the crystallization peak during cooling in virgin PET (Tcv=159 °C) and DRY PET (Tcd=179 °C).

[0040] Figure 18 shows the band decomposition of the ATR-IR spectra in the 1320- 1425 cm'1region for virgin (gray) and DRY (black) PET 25. The absorption bands of the glycol segments in trans-extended (1340 cm'1) and gauche-twisted (1370 cm'1) conformations are highlighted with a darker color.

[0041] Figure 19 shows the WAXD spectrum of the recovered TPA. The reference model for TPA (PDF 00-031-1916) is shown with gray bars.

[0042] DETAILED DESCRIPTION OF THE INVENTION

[0043] By the term "polymers" are meant molecules with a high molecular weight consisting of a large number of repeated structural units called "monomers", covalently linked together to form a long chain. Some common examples of polymers are polyethylene terephthalate (PET), polyethylene (PE), polystyrene (PS) and polypropylene (PP). To further clarify the meaning of these terms, by way of example, polyethylene terephthalate is the polymer resulting from the polycondensation reaction of two monomers, terephthalic acid (TPA) and ethylene glycol (EG), according to the following structure:

[0044] Polyethylene terephthalate

[0045] (PET)

[0046] The term "oligomers" refers to a class of molecules intermediate in size between monomers and polymers. For example, oligomers derived from polyethylene terephthalate typically contain 2 to 20 monomers.

[0047] The term "polyesters" means polymers that contain at least one ester group in the main chain. Some of the most common examples of polyesters are: polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN) and polylactic acid (PLA).

[0048] The term "plastic product containing polyesters", or equivalently "plastic article containing polyesters", means a plastic manufactured product or article consisting of at least one polymer, which is polyester. Common examples of plastic objects containing polyesters according to the present definition include, for example, water bottles, textile fibers, shopping bags, packaging films or plastic components for electronics. The plastic product containing polyesters may also further contain contaminants or additives, including plasticizers, pigments, dyes, organic and inorganic fillers or other polymers.

[0049] The term "mixed plastic waste", or simply "mixed waste", refers to plastic products containing two or more different polymers, or one polymer and additives and / or contaminants, combined in a way that is extremely difficult to mechanically separate and recycle. Typical examples of mixed plastic waste are, for example, multilayer packaging or polymer blends commonly used in carpet or textile production.

[0050] The terms "degradation" and "depolymerization" of polyester are used equivalently to refer to the process of fragmentation of polyester polymer chains into smaller units such as monomers or oligomers.

[0051] The term "(chemical) recycling process" means a process during which at least one of the polymers in the starting material undergoes a depolymerization reaction. Through this process, it is possible to recover monomers and / or oligomers that can be reused for the synthesis of new plastic materials.

[0052] The term "gel" means a two-phase material consisting of a liquid phase embedded in a solid matrix. According to the present invention, cyclic alkyl carbonate is the liquid phase whereas polyester is the solid phase.

[0053] To calculate the concentration of a particular substance in a solution, in the description of the present invention the percent mass / mass concentration (in English wt%) defined according to the following formula (1) will be used:

[0054] < Solute Mass Solute Mass

[0055] Concentration (wt%) ~ : — r; — x 100 ~ 7-5 - n - jr-x - r: — x 100 (1)

[0056] Solution Mass Solute Mass +• oolvent Mass

[0057] The term "crystallinity" means the degree of order with which, when aligned together, the polymer chains are arranged in ordered structures. Polymers can exist in forms characterized by several degrees of crystallinity ranging from fully amorphous (randomly oriented chains) to semi-crystalline (ordered regions coexisting with amorphous regions), or fully crystalline. The degree of crystallinity of a polymer can be estimated by several analytical techniques, of which, the most commonly used are differential scanning calorimetry (DSC) and X-ray diffraction (XRD). According to the present invention, the degree of crystallinity of polyester inside a plastic material product corresponds to that measured by DSC according to following formula (2): 100 (2) where:

[0058] • AHf is the melting enthalpy

[0059] • AHcc is the cold crystallization enthalpy

[0060] • wt is the mass fraction of polyester to the total mass of the plastic object

[0061] • AHfioo% is the melting enthalpy of the examined polyester in the fully crystalline state

[0062] The value of AHfioo%, to be substituted in formula II for given polyester, can be deduced from the scientific literature. For example, fully crystalline PET has a AHfioo% of 140.1 J / g. The level of crystallinity significantly affects the recyclability of polymers. Highly crystalline polymers are generally more difficult to recycle than their amorphous homologues.

[0063] The term "dissolution / precipitation process" means a procedure used to alter the physical properties of a polymer and consists of dissolving this polymer in a solvent and then precipitating it from the solution. In the context of the invention, this process is used to separate the polyester from other polymers and additives and / or contaminants and to facilitate the subsequent depolymerization step.

[0064] The expression "cyclic alkyl carbonates", or simply "alkyl carbonates", is used in the text to refer to a class of molecules with the following 5-membered cyclic structure: where possible substituents were denoted by the letters Ri, R2, R3, and R4, wherein Ri, R2, R3, and R4 can be the same or different from each other and selected from: H and C1-C4 alkyl chains, preferably selected from H, CH3, C2H5, C3H7 or C4H9. According to the present invention, cyclic alkyl carbonates can also be different and in mixture with each other. Thanks to their important chemical and physical properties, most notably their low vapor pressure, low flammability and low toxicity, cyclic alkyl carbonates are frequently used in industrial applications as solvents, lubricants or electrolytes. Furthermore, they have recently attracted significant interest in the context of "green chemistry" or "eco-friendly chemistry", since they can be synthesized by cycloaddition of CO2 to epoxides, a process that allows highly polluting greenhouse gases to be transformed into high value-added materials.

[0065] The examples below are given for illustrative purposes only and not limiting the scope of the present invention.

[0066] The application field of the invention is not limited solely to the specific examples set forth.

[0067] In one of the preferred embodiments, the cyclic alkyl carbonates used for said pre-treatment step of the plastic material according to the present invention are selected from: ethylene carbonate (EC, RI-4=H), propylene carbonate (PC, RI-3=H, R4=CH3) or mixtures thereof.

[0068] Elhytehe Carbonate Propylene Carbonate

[0069] Still according to a preferred embodiment of the present invention, the plastic material product subjected to the process of the invention contains polyethylene terephthalate (PET).

[0070] According to alternative embodiments of the present invention, the plastic material product subjected to the process of the invention contains at least one of the following polymers: polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polyethylene naphthalate (PEN), polyethylene isophthalate (PEI), polycaprolactone (PCL), polyglycolic acid (PGA), poly-lactic acid (PLA), polybutylene succinate (PBS), polyhydroxyalkanoate (PHA), polyethylene furanoate (PEF). The plastic material product can also consist of an appropriate blend of the aforementioned polymers.

[0071] According to one of the preferred embodiments, the plastic material product undergoing the process may contain, in addition to at least one polyester, other polymers including: polyamide 6 (PA6), polyamide 66 (PA66), polyethylene (PE), polypropylene (PP) or cellulose.

[0072] Still according to the present invention, the plastic material product subjected to the process of the invention may also contain contaminants or additives such as, for example, greases, lubricants, plasticizers, flame retardants, pigments, dyes, antistatic agents, organic and inorganic fillers, even in mixtures with each other.

[0073] Still according to the present invention, the plastic material product subjected to the process of the invention can be pulverized by various techniques, including: mechanical crushing, dry or cryogenic grinding, ultrasonic crushing, micronization or heat treatment. The purpose of the pulverization process is to reduce the size of the material to be treated and increase its surface area.

[0074] Still according to the present invention, the plastic material product subjected to the process of the invention is appropriately preliminarily treated by washing and disinfecting to remove possible contaminants.

[0075] Still according to the present invention, the plastic material product is properly dried beforehand to remove any residual moisture. Indeed, the material to be subjected to dissolution / precipitation treatment must not have residual moisture, which could lead to even partial hydrolysis of the cyclic alkyl carbonate and thus a reduction in its solubilizing power. This drying step preferably occurs in an oven under an inert atmosphere (e.g., under nitrogen) or, alternatively, by vacuum drying. Preferably, the drying temperature is between 70°C and 180°C.

[0076] The process according to the present invention is carried out by loading cyclic alkyl carbonate, which serves as the solvent in this pre-treatment step of the starting plastic material, into a thermostated reactor that is gradually heated to a temperature between 100°C and the boiling point of the cyclic alkyl carbonate, in order to remove any residual moisture retained in the solvent.

[0077] The plastic material product containing polyesters is then added to the preheated cyclic alkyl carbonate inside the reactor, respecting a weight percentage ratio between 5 and 70 wt% of polymer to total polymer-solvent. Preferably, the weight percentage ratio of polymer to total polymer-solvent is between 15 and 30 wt%.

[0078] To facilitate the dissolution process, appropriate agitation is provided by a mechanical stirrer.

[0079] If the plastic material product to be treated contains impurities, additives or polymers other than polyesters, which do not dissolve in the cyclic alkyl carbonate, an operation can be carried out to separate and remove these impurities from the polyester, for example, by hot filtration of the solution.

[0080] Still according to the invention, once the polyester component of the plastic material product to be treated is completely dissolved, its precipitation is caused by cooling the reactor. It is preferred that the reactor temperature in this step is brought to a range between 140°C and 20°C; the "cooling" temperature will depend on the cyclic alkyl carbonate selected as the solvent in the pre-treatment step.

[0081] The pre-treated material, obtained from the pre-treatment step of the process according to the present invention, appears as an easily deformable uniform solid consisting of a porous polymer matrix inside which cyclic alkyl carbonate solvent is retained. The consistency and deformability of the resulting solid strongly depend on the polymer-solvent ratio selected for dissolution. Higher proportions of polymer to the solvent result in greater resistance to deformation of the resulting pre-treated material or precipitated solid.

[0082] According to a preferred aspect of the present invention, the solid obtained from the pre-processing step is suitably pulverized by known techniques such as, for example: mechanical crushing, dry or cryogenic grinding, ultrasonic crushing, micronization or heat treatment. Pulverization of the pre-treated material obtained from said pre-treatment step facilitates subsequent washing and removal of the solvent from the pre-treated polymer.

[0083] The resulting powder is repeatedly washed to remove residual solvent, for example, the powder is washed in deionized water. Alternatively, organic solvents such as acetone, ethanol, methanol or other organic solvents can be used to wash the pre-treated material.

[0084] This last step plays a key role in the next step of depolymerization. Indeed, the solvent used for washing becomes intercalated directly into the polymer, replacing the cyclic alkyl carbonate used for dissolution, which otherwise could interfere with the depolymerization process, since it is also has ester functionalities.

[0085] It is of paramount importance that once washed, the pre-treated polymer directly undergoes the next step of chemical depolymerization, without preliminary drying procedures. Indeed, the presence of the washing solvent inside the polymer matrix allows it to swell and ensures greater accessibility to the inside of the polymer matrix for depolymerizing agents. The impact of intercalated solvent on the degradability of polyester appears evident from the significant drop in depolymerization yields observed when the treated material is dried before depolymerization treatment, as shown in Figure 2.

[0086] Still according to a preferred embodiment of the present invention, to minimize process costs on an industrial scale, the cyclic alkyl carbonate is then separated by distillation from the washing solvent.

[0087] Once washed, the pre-treated material is directly subjected to the depolymerization step without being dried first.

[0088] According to the present invention, the depolymerization step consists of a hydrolysis reaction, preferentially an alkaline hydrolysis. Alternatively, depolymerization can be carried out according to any of the known solvolitic processes of polyester degradation. In order to avoid subsequent separation operations and optimize the reaction yield, the solvent selected for washing should correspond to the solvolitic agent used for depolymerization (for example, water for hydrolysis, ethylene glycol for glycolysis, ethanol or methanol for alcoholysis, etc...).

[0089] According to a preferred embodiment, said depolymerization step is carried out in the presence of a catalyst selected, for example, from the group comprising: NaOH, Na2CCh, KOH, K2CO3 and LiOH. Furthermore, phase-transfer catalysts (for example, quaternary ammonium or phosphonium salts) can be used to facilitate the interaction between the polyester and the catalyst.

[0090] Still according to a preferred aspect of the present invention, the depolymerization step is a biological depolymerization carried out by using depolymerase capable of hydrolyzing the ester bond. Depolymerase preferably belongs to the cutinase, lipase, esterase, petase or carboxylesterase groups.

[0091] According to the present invention, the pre-treated material is directly loaded into a thermostated reactor together with the catalyst. Alternatively, to speed up the reaction, reduced amounts of water or organic solvent can be added, depending on the solvolitic process selected. During the depolymerization step, stirring is provided by mechanical stirrer.

[0092] The duration of the depolymerization process, as well as the optimal temperature, may depend on the nature of the product to be degraded, the type and amounts of catalyst that may be used, and the type of solvolitic process selected.

[0093] Once the depolymerization reaction is completed, the monomers and / or oligomers obtained can be separated and purified by using different methodologies, including, for example: solvent extraction, filtration, distillation, column chromatography or precipitation.

[0094] Still according to the present invention, the monomers and / or oligomers obtained from the depolymerization step can be used to re-synthesize the starting polyester with unaltered chemical and physical properties compared to the virgin material. Alternatively, the monomers and / or oligomers obtained from the depolymerization step can be used for the synthesis of new polymers different from the starting polyester, or for the synthesis of other high value-added substances.

[0095] Still according to the invention, the non-depolymerized portion of the material undergoing degradation can be separated from the reaction products by filtration, centrifugation or solvent extraction.

[0096] For illustrative and non-limiting purpose only, some examples of practical implementation of the present invention are shown below in the experimental part set forth below.

[0097] The process according to the present invention allows a rapid depolymerization step to be carried out, for example, by alkaline hydrolysis under mild conditions, subsequent to the pre-treatment step of the starting plastic material.

[0098] One of the highly innovative aspects of the process subject-matter of the present invention is the washing step of the material obtained by precipitation during the pre-treatment step with cyclic alkyl carbonates, which allows the dissolution solvent intercalated in the precipitated material to be completely replaced with water, thus preventing re-compaction of the solid polymer matrix and promoting the diffusion of alkalis to the ester bonds.

[0099] Consequently, the washed pretreated material remains swollen with water and, for example, by alkaline hydrolysis, completely depolymerizes into monomers within 5 minutes at 90 °C and before 2 hours at room temperature (25 °C), producing significantly improved results compared with conventional hydrolysis methods.

[0100] In other words, the process according to the present invention allows chemical depolymerization of plastic materials containing at least one polyester that is complete under unburdened conditions to be achieved and, in the case of depolymerization by alkaline hydrolysis with NaOH, with minimal NaOH consumption.

[0101] Furthermore, the process according to the present invention, for example, involves the use of propylene carbonate (PC), one of the most environmentally friendly and widely available solvents on the market.

[0102] The process exploits, in the pre-treatment step, the high solubility of PET in PC at high temperature for complete dissolution, whereas the subsequent cooling to room temperature triggers thermally induced phase separation, thus forming a porous solid matrix that retains the dissolution solvent. The intercalated PC molecules can then be replaced by water, through said washing step, thus avoiding the recompaction of the polymer chains into a hydrophobic solid structure.

[0103] The resulting PET swollen with water is capable of retaining water up to about three times its mass. Increased interfacial water stabilizes the separation of polymer chains and contributes to their depolymerization by significantly promoting the diffusion of alkali to ester bonds, thus minimizing NaOH consumption.

[0104] The depolymerization step, for example, by alkaline hydrolysis reaction, is greatly enhanced because the problem posed by the hydrophobicity of PET is overcome thanks to the almost complete substitution by water of the pre-treatment solvent (cyclic alkyl carbonate - for example, PC). This substitution occurs without causing any chemical alteration to the polymer, but rather significantly contributing in facilitating the breakage of the polymer chains in the subsequent chemical depolymerization step, such as for example by alkaline hydrolysis.

[0105] EXPERIMENTAL PART

[0106] Materials and reagents:

[0107] PET with a high level of crystallinity (42% C.I.) in granules and powder form (sieved at 1200 pm) was supplied by Poliplast S.p.a. (Bergamo, Italy) and used as received. Post-consumer PET manufactured products such as bottles, textiles and containers were purchased from local retailers, washed with water and detergents, cut into approximately 2 cm x 2 cm fragments, and dried in an oven at 70°C. A mix of different PET waste products was provided by a local landfill in the form of ground product. The material received was washed with water and detergents, and dried in an oven at 70°C prior to use.

[0108] Propylene carbonate (PC, 99.7% purity), ethylene carbonate (EC, 99% purity), NaOH (>97.0% purity), H2SO4 (>96.0% purity) and terephthalic acid (TPA, 99% purity) were purchased from Sigma Aldrich. Milli-Q water (resistivity 18.2 MQ / cm at 25°C) was used in all experiments. All reagents were used as supplied.

[0109] Fourier transform infrared spectra in attenuated total reflectance (ATR-IR) were acquired by using a Bruker Alpha spectrometer. All spectra were recorded in the range 4000-400 cm'1, with a resolution of 2 cm'1, accumulating 30 scans for each measurement. Nuclear magnetic resonance (NMR) spectra were recorded with a 600 MHz Bruker-Advance spectrometer (Bruker, Germany) at 25 °C, by dissolving all samples in DMSO-de in a capillary with an inner diameter of 1 mm. Differential scanning calorimetry (DSC) measurements were performed using a DSC 2500 calorimeter (TA instruments USA), in the 30-300°C range with a heating progression of 10°C / min in a nitrogen atmosphere, by sealing the samples in aluminum crucibles. Thermogravimetric analyses (TGA) were performed with a Netzsch STA 2500 Regulus analyzer, in the 30-600 °C range with a heating progression of 5°C / min, under nitrogen atmosphere. SEM images were acquired with a LEO 1525 ZEISS FE- SEM microscope, Cryo-SEM images were obtained with a Leo Crossbeam 1540 XBFIB-SEM ZEISS microscope. Gel permeation chromatography (GPC) measurements were performed using an Agilent 1260 Infinity II Multi Detector Suite equipped with three distinct types of detectors. Wide-angle X-ray diffraction (WAXD) measurements were performed using a Bruker D8 Advance diffractometer in Bragg-Brentano geometry, equipped with a Lynxeye XE-T fast detector and using Cu-Ka radiation.

[0110] Example 1 Depolymerization of commercial powdered PET with high crystallinity

[0111] Experiment la Pre-treatment of polymer by dissolution and precipitation

[0112] Powdered PET, supplied by Poliplast srl, with a high level of crystallinity (42% CI, measured by DSC) was used as the reference standard for performing the experiments set forth in this example. The pre-treatment step in cyclic alkyl carbonates was performed on the received powder according to the following protocol. 60 g propylene carbonate was weighed and added to a 100-ml round- bottomed flask with two necks, equipped with a thermometer and mechanical stirrer. The flask was immersed in a silicone oil bath preheated to a temperature of about 200°C. When thermal equilibrium was reached, 20 g powdered PET, pre-dried in an oven at 100°C, was poured into the flask (25% polymer-to-solvent mass ratio). The solution was kept under mechanical stirring (250 rpm) for 10 minutes until the powder was completely dissolved. When all the powder was dissolved, the flask was taken out of the hot bath to allow cooling to room temperature and subsequent precipitation of the polymer in the form of a gelatinous matrix. The resulting material was pulverized while still hot by using a blender, and then washed thoroughly with deionized water until all residual solvent (propylene carbonate) was completely removed and replaced by water. This produces a wet porous powder (referred to in the rest of the Experimental Section as "WET PET 25") consisting of about 70% of its weight of water (Figure 11), with a recovery rate of 98%. The procedure was also repeated using 15 wt% and 40 wt% as the polymer-solvent ratio, taking 6 and 20 minutes for dissolution, respectively. Since the prepared 40 wt% solution is extremely viscous and difficult to stir, for reasons of operational convenience, only the samples prepared at 25 and 15 wt% were used for the subsequent experiments.

[0113] Experiment lb Effectiveness of pre-treatment

[0114] The effectiveness of the treatment in increasing polymer degradability was evaluated by subjecting virgin PET powder, and material obtained after the treatment described in Experiment la prepared at a concentration of 25 wt% (polymer-solvent ratio), to an alkaline hydrolysis process under the same conditions. The material obtained as described in Experiment la was used both in the form of wet powder ("WET PET 25") and after being dried in an oven for 24 h at 70°C ("DRY PET 25"), as shown in Figure 1.

[0115] For the depolymerization step according to alkaline hydrolysis, 5 g PET powder (in the case of the "wet PET" sample, an amount of wet powder corresponding to 5 g polymer) was weighed and transferred to a 100-ml round-bottomed flask. The flask was immersed in a silicone oil bath preheated to a temperature of 90°C. 2.5 g (1.2 equivalents) NaOH was dissolved in 15 ml of deionized water in a separate vial.

[0116] The prepared solution was added to the hot flask. The mixture formed, having the consistency of slurry because of the high polymer / solvent ratio, was kept under mechanical stirring at 250 rpm for 10 minutes.

[0117] In an alkaline environment, the hydrolysis reaction proceeds according to the following scheme: providing disodium terephthalate (Na2TP) and ethylene glycol (EG) as soluble products.

[0118] After 10 minutes, the reaction was stopped by rapidly cooling the flask in an ice-water bath and adding about 50 ml of deionized water to the mixture. The nondepolymerized PET residues and any insoluble oligomers were separated by filtering, repeatedly washed in water, dried in an oven at 70°C and finally weighed. The filtrate was acidified by adding H2SO4 until the pH was brought to a value of about 2.5-3, causing terephthalic acid (TP A) to precipitate. The precipitate was then isolated by filtration, properly washed in deionized water and dried in an oven at 70°C, until it reached constant weight. The results of the process for the three samples are shown in

[0119] Figure 2, calculating PET conversion percentages ("PET conversion") and terephthalic acid yields ("TPA yield") as: wherein 192.16 and 166.13 are the molecular weights of the repeating unit of polyethylene terephthalate and terephthalic acid, respectively. From the graph shown in Figure 2, it is evident that whereas virgin powder is poorly affected by the alkaline hydrolysis process, the depolymerization reaction reaches completion in only 10 minutes for wet powder ("WET PET"), significantly outperforming in efficiency conventional alkaline hydrolysis processes that require high temperature and pressure conditions (T>200°C and P>3 Atm) and longer times. The dramatic drop in yields observed for the dried sample ("DRY PET") highlights the key role played by water intercalated in the polymer during the washing step that precedes the depolymerization step.

[0120] Experiment 1c Temperature effect and reaction kinetics

[0121] The depolymerization step by alkaline hydrolysis described in the previous experiment was repeated on the "WET PET 25" sample on a reduced scale (1 g PET, 0.5 g NaOH (1.2 eq.), 3 mL FEO), varying the temperature and stopping the reaction at different time intervals. The results obtained are summarized in Figures 3a and 3b. At 90°C, the depolymerization reaction is almost complete in only 5 minutes and, surprisingly, is active even at room temperature (25°C), reaching completion in about 2 h.

[0122] The kinetics of the reaction was studied by modeling the hydrolysis process through the following Jander equation:

[0123] [1 - (1 - a)5]2= kt (5) commonly used to describe diffusion-controlled reactions with three- dimensional geometry, where a, k and t refers to the percentage of PET conversion, the kinetic constant of the reaction and the elapsed time from the start of the reaction, respectively. The effect of temperature on the kinetic constant k, is shown in Figure 5a. All the data collected show a good fit to the proposed model (R>0.98), confirming the diffusion-controlled nature of the reaction. The activation energy of the process can be estimated by linear fit, using the following Arrehnius equation: wherein A, Ea, R and T are the pre-exponential factor, activation energy, gas constant (8.31 J / k mol) and absolute temperature expressed in Kelvin, respectively. The Arrehnius plot of the collected data is shown in Figure 5b. From the linear fit of the data (R=0.9987), it is possible to estimate an activation energy of 41.57 kJ / mol, significantly lower than that set forth in the literature for conventional alkaline hydrolysis processes.

[0124] Experiment Id Effect of the amount of NaOH

[0125] The depolymerization step by alkaline hydrolysis on the "WET PET 25" sample was repeated at room temperature, by placing an amount of wet powder corresponding to 1 g together with 1 g (2.4 equivalent) NaOH in tablets into a 50 mL flask. Then 3 ml deionized water was added to the flask and the mixture was kept under stirring for 15 minutes. Thanks to the combined effect of the doubled amount of NaOH and the heat developed by its dissolution, a PET conversion of 95±2% and a TPA yield of 91±2% could be achieved at room temperature (25°C).

[0126] Experiment le Polymer-solvent ratio

[0127] In order to evaluate the influence that the polymer-solvent ratio used for the pretreatment step exerts on depolymerization yields, hydrolysis at 90°C was also tested on the sample prepared at a polymer-solvent concentration of 15 wt%, according to the methods described in Experiment lb. The comparison of the yields obtained for the sample prepared at 25 wt% and the sample prepared at 15 wt% is shown in Figure 6. In both cases, the depolymerization process is completed in about 10 minutes. The small differences found can be attributed to the higher amount of water (~85 wt%) retained after washing the sample prepared at 15 wt%, which reduces the concentration of NaOH in the mixture.

[0128] Experiment If Characterization of reaction products

[0129] The chemical identity of the products obtained from the alkaline hydrolysis step was analyzed by FTIR and ’H NMR spectroscopic investigations, and WAXD measurements. The results shown in Figures 7, 8 and 19 confirm that the product obtained is terephthalic acid (TPA).

[0130] Experiment 1g Recycle of solvent

[0131] 40 g powdered PET was dissolved in 120 g propylene carbonate and then precipitated from the solution according to the protocol described in Experiment la. The solvent (propylene carbonate) was extracted from the precipitated gel by consecutive washes in acetone and then recovered from the solvent mixture by removing and recovering acetone by distillation with a rotary evaporator. The recovered propylene carbonate was again used to dissolve powdered PET at a polymer-solvent concentration of 25 wt% for 4 additional cycles. The dissolved PET at each cycle was washed thoroughly with water to completely remove residual acetone and subjected to depolymerization by alkaline hydrolysis (see Experiment lb). As shown in Figure 9, the solvent can be recovered, without significant losses in process efficiency, up to 4 times.

[0132] Experiment Ih Depolymerization with K2CO3

[0133] The depolymerization step by alkaline hydrolysis of the "wet PET 25%" sample, according to the methods described in Experiment lb, was repeated by replacing the NaOH solution with a solution prepared by dissolving 25 g K2CO3 in 20 ml deionized water. In this case, the process achieves a PET conversion of 93±3% and a terephthalic acid yield of 90±2% in 5 h.

[0134] Experiment li Pre-treatment and depolymerization with ethylene carbonate

[0135] The procedure described in Experiment la was repeated in the same manner by using ethylene carbonate instead of propylene carbonate as cyclic alkyl carbonate. The resulting material, still containing the wash water (WET PET), was subjected to the depolymerization step by alkaline hydrolysis described in Experiment lb, resulting in a PET conversion of 98.3±0.7% and a terephthalic acid yield of 96±2%.

[0136] Example 2 Depolymerization of post-consumer PET products

[0137] Different post-consumer products such as bottles, fabrics and containers were purchased from local vendors, washed with water and detergents, cut into fragments and dried in a vacuum oven at 75°C. All materials were treated according to the protocol described in Experiment la of the previous example, at a concentration of 25 wt%. The depolymerization process by alkaline hydrolysis was carried out following the methods described in Experiment lb of the previous example. The results obtained are summarized in the following Table 1. *A11 alkaline hydrolysis experiments (conditions of Experiment lb) were carried out in triplicate. Yields obtained for post-consumer materials that did not undergo any pretreatment are shown in parentheses. Example 3 Separation of PET from other polymers in mixed post-consumer products

[0138] To evaluate the effectiveness of the pretreatment step in enabling the separation of polyesters from any polymers of different nature, the process of the present invention was applied, in the manner described in Experiments la and lb of Example 1, to the following mixed post-consumer materials:

[0139] 1) Blue PET bottle + polyethylene cap

[0140] 2) Transparent PET container + polypropylene cap

[0141] Since propylene carbonate (cyclic alkyl carbonate solvent) is unable to dissolve polyaddition polymers such as polyethylene and polypropylene, these were easily removed with tweezers from the solution once the PET was completely dissolved. The recovered PET was then subjected to the depolymerization process by alkaline hydrolysis according to the methods described in Experiment lb of Example 1. The results obtained are shown in following Table 2.

[0142]

[0143] **A11 alkaline hydrolysis experiments (conditions of Experiment lb) were carried out in triplicate. Yields obtained for post-consumer materials that did not undergo any pretreatment are shown in parentheses. Example 4 Discoloration of reaction products

[0144] 1 g PET from a green fabric (see Example 2) was treated according to the protocol described in Experiment la of Example 1 and subjected to depolymerization by alkaline hydrolysis at 90°C for 10 min with 1.2 equivalents NaOH. After 10 minutes, 1 g activated carbon was added to the mixture, which was kept under stirring for additional 20 minutes. The activated carbon was then removed by filtration and the reaction products precipitated from the filtrate by acidification (see Example 1). The difference in the coloration of the final product can be observed in Figure 10, from which the bleaching action of activated carbon is clearly evident.

Claims

CLAIMS1. A chemical depolymerization process of plastic materials containing at least one polyester, which comprises: a pre-treatment step of said plastic material with at least one cyclic alkyl carbonate, resulting in the dissolution and precipitation of said polyesters in the form of pre-treated material,- a washing step of said pre-treated material with deionized water and / or at least one organic solvent free of ester functionalities, resulting in a washed pretreated material, a subsequent chemical depolymerization step of said pre-treated washed material, resulting in monomers and / or oligomers.

2. The process according to claim 1, characterized in that said pre-treated material is in gel form.

3. The process according to claim 1, characterized in that said cyclic alkyl carbonate has the following formula:where Ri, R2, R3 and R4 can be the same or different from each other and are selected from: H and C1-C4 alkyl chains, preferably selected from H, CH3, C2H5, C3H7 or C4H9.

4. The process according to claim 3, characterized in that said cyclic alkyl carbonate is selected from: ethylene carbonate (EC, R1-4 = H), propylene carbonate (PC, Ri-3 = H, R4 = CH3) or mixtures thereof.

5. The process according to claim 1, characterized in that said polyester is polyethylene terephthalate (PET).

6. The process according to claim 1, characterized in that said plastic material contains at least one polymer selected from: polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polyethylene naphthalate (PEN), polyethylene isophthalate (PEI), polyglycolic acid (PGA), polylactic acid (PLA), polybutylene succinate (PBS), polyhydroxyalkanoate (PHA), polyethylenefuranoate (PEF), polyamide 6 (PA6), polyamide 66 (PA66), polyethylene (PE), polypropylene (PP) or cellulose.

7. The process according to claim 1, characterized in that said pre-treatment step is carried out respecting a weight percentage ratio of polymer to the total polymer-solvent between 5 and 70 wt%, preferably between 15 and 30 wt%.

8. The process according to claim 1, characterized in that said depolymerization step is carried out by hydrolysis, preferably alkaline hydrolysis.

9. The process according to claim 8, characterized in that said alkaline hydrolysis is carried out in the presence of at least one compound selected from: NaOH, Na2CO3, KOH, K2CO3, LiOH.

10. Use of cyclic alkyl carbonates for the chemical depolymerization of plastic materials containing at least one polyester.

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