Method for purifying polymer-based materials

Abstract

The invention relates to a method for reducing or completely eliminating foreign compounds and contaminants present in polymer pellets made of materials derived from classified collection waste or industrial waste, such that the recovered polymer material can be used for industrial reuse, and in the recycling process, no odor is generated, and no toxic residues are contained.

Description

Technical Field

[0001] This invention relates to a method for reducing or completely eliminating foreign compounds and contaminants present in polymer pellets made from materials derived from sorted waste, industrial waste, and typically from recycled polymer materials. Background Technology

[0002] Current technologies aim to make the recycling of materials (including polymer-based materials) from the sorted collection of waste from civil and industrial waste more economical and profitable.

[0003] Besides energy recovery through direct incineration (a practice currently discouraged and increasingly discouraged in the future), other technologies for recycling polymer-based materials are known, which can be divided into three main categories:

[0004] 1) Pyrolysis of materials and recovery of gaseous fuels; 2) Depolymerization of raw materials and recovery of monomers used in new polymer production processes; and 3) Clean and remove volatile compounds that cause toxicity and odor in the materials, and then reuse them directly in the production cycle.

[0005] For example, the application of the first method is reported in U.S. Patent Application 2012 / 0261247 A1 and Patent US 4759300; the first of these documents describes a method for processing plastic waste to provide at least one specific fuel product; the second document describes a method in which the waste to be pyrolyzed is effectively dehydrated by microwaves and then irradiated with a very high-intensity laser until methane and other combustible gases are obtained in sufficient quantities to support the pyrolysis reaction in a plant burner.

[0006] For example, patent US10160741B2 describes a depolymerization process of raw materials and a process for recovering monomers used in the production of new polymers, and this patent relates to a method for recovering monomers, particularly from polyurethanes (including thermosetting polyurethanes); patent application WO1997 / 049652A1 relates to a method for recovering depolymerization products from polymers such as polyesters, polyamides, and polyesteramides, particularly when the content of the starting polymer is less than about 98%; and patents US6136869 and US6191177B1 relate to a method (and associated equipment) for recovering polyester base materials that can be used in new products from recycled polyester, the method comprising the stages of: depolymerizing the polyester into its monomers and half-esters, separating the monomers and half-esters from other auxiliary materials, and adding the monomers to the recovered compound to produce low molecular weight polyesters.

[0007] In addition to the pyrolysis and depolymerization technologies mentioned above, there is also increasing potential for the development of technologies to recover polymer-based materials from the plastic portion of separately collected waste. These technologies include cleaning and removing volatile compounds from the materials and reusing them directly during the production cycle.

[0008] The methods for purifying plastics are well known.

[0009] For example, patent EP 2780141 B1 relates to a method for treating polyolefin particles to remove volatile components. The method includes the following stages: preparing a particle bed in a liquid within a treatment container; removing a liquid stream containing hydrocarbons from the treatment container; introducing a first rising steam stream into the treatment container, wherein the steam has a temperature of Tb to Tb+10°C, where Tb is the boiling point of the liquid under applied pressure; removing a second steam stream containing volatile hydrocarbon compounds from the treatment container; and recovering the particles from the treatment container. However, the method in this document only addresses the purification of the virgin polymer material to remove production reagent residues, and does not address the purification of the recovered polymer material. Therefore, the contaminants considered in this document have fixed chemical properties and are limited to a few types, making it easy to design purification processes; conversely, in the washing and purification of post-consumer plastic materials, contaminants encompass a very wide range of substances, often even unknown, making the definition of an effective method for purifying and recovering plastic materials much more complex.

[0010] In the art, U.S. Patent 5,858,105 describes a method for cleaning plastic beverage bottles, which includes pretreating the bottles with a concentrated formulation containing more than 0.5% by weight of an alkaline reagent, and then removing the concentrated solution used for cleaning in one or more subsequent stages.

[0011] Patent application JP-S-5731529 A describes a method in which a plastic product to be recycled is crushed into fine particles, foreign matter is removed from the fine particles, and then the fine particles are washed with alkali.

[0012] Patent application WO 2020 / 245476 A1 describes a method comprising a washing stage of recycled raw material and a stage of stripping the material with a stream of hot air or steam to facilitate the removal of volatile compounds from the treated material prior to a granulation or reuse stage.

[0013] Finally, patent application EP 3501785 A1 describes a method for recycling polymer materials to be recycled, which includes a first stage of washing the raw material with an alkaline aqueous solution, drying the washed material with a hot air stream, and a final extrusion and extraction stage under vacuum, followed by filtering the material obtained in the previous step; these last two steps form the normal operating practice of an extruder, which typically has an exhaust point for discharging the gases and vapors generated during the melting of the material through a suction conduit connected to the extruder screw sleeve.

[0014] The applicant observed that, despite extensive research activity in this field, recycling technologies for polymer-based materials from the plastic portion of sorted waste, including washing and removing volatile compounds from the material and reusing it directly in the production cycle, currently have a number of technical and functional limitations and cannot yield completely satisfactory results.

[0015] Specifically, experiments conducted by the applicant have demonstrated that, according to the law of gas diffusion through poorly permeable solids, volatile compounds present on the surface of granular recycled polymers can be rapidly removed from the granular form using known methods. Simultaneously, the time required to eliminate the trapped portions in the innermost material of the granules increases quadratically with increasing particle diameter. The material obtained using known methods exhibits no odor upon exiting the process; however, when the material is subsequently reprocessed (melted and injected), the trapped gases in the internal portions are released, resulting in a product with a generally unpleasant odor, rendering it ineffective in many applications.

[0016] Furthermore, for current washing systems on the market, and generally all systems that do not involve the depolymerization of materials and subsequent polymer reconstruction, the solid or liquid substances trapped in the polymer undergo chemical changes due to interactions with the environment, subsequently releasing volatile compounds with unpleasant odors or even toxicity that cannot be removed. A typical example is the recycling of the plastic portions of Tetra Pak packaging containers used to sell long-shelf-life milk. The plastic material absorbs milk fats and proteins, which remain in the material during the extrusion stage of the recycling process. The obtained material is then used to produce new objects, but over time, after exposure to air, it interacts with bacteria and yeast (which break down casein, lactose, and fat), giving the product the typical rancid milk odor.

[0017] In particular, the applicant has observed that one of the main problems encountered with using these technologies involves the difficulty of effectively and rapidly removing absorbed residues from recycled materials during and after initial use, especially in the innermost parts of the materials undergoing the recycling process. Indeed, recycling technologies that significantly extend processing time and incur costs to remove these residues are economically unsustainable if, on the one hand, the presence of absorbed residues—even if their amount is generally considered negligible—hinders their use, for example, in the manufacture of packaging for human use.

[0018] Therefore, there remains a need in the field for a technology that can be used to recycle polymer-based materials derived from the plastic portion of separately collected waste, a technology that can overcome the problems of existing technologies.

[0019] The purpose of this invention is to provide a recycling technology for polymer-based materials derived from the plastic portion of sorted waste. This recycling technology can remove residues absorbed by the material during and after initial use in the most efficient and rapid manner possible, thereby ensuring economically sustainable processing time and costs. Summary of the Invention

[0020] This invention achieves this and other objectives. A first aspect of this invention relates to a method for recycling polymer-based materials derived from the plastic portion of sorted waste, the method comprising the following steps: (a) Prepare a certain amount of raw materials, said raw materials comprising at least one polymer material derived from the plastic portion of sorted waste; (b) Adding a foaming agent to the polymer material from the previous step, the foaming agent being selected from: b.1) an alkali metal or alkaline earth metal bicarbonate, wherein the amount of the bicarbonate is 0.1-0.5 wt% relative to the weight of the polymer material; b.2) a pair of materials consisting of a carbonate and an acidified compound, wherein the amount of the carbonate present is 0.2-1 wt% relative to the weight of the polymer material; and b.3) a liquid compound with a boiling point below 100°C, directly injected into the extruder head, wherein the amount of the liquid compound added is 0.3-0.6 wt% relative to the weight of the polymer material, and a mixture is obtained in all three cases; (c) Extruding the mixture from step (b) to obtain granules of recycled material; (d) The granules obtained in step (c) are boiled in an alkaline aqueous solution of an oxidizing compound with a pH of 8 to 13 at a temperature 50°C to 20°C below the softening temperature of the polymer material to perform a first extraction. The oxidizing compound is selected from hydrogen peroxide, sodium perborate, and sodium percarbonate, and the amount of the oxidizing compound relative to the weight of the polymer material is 0.1-2% in the case of hydrogen peroxide, 0.5-5% in the case of sodium perborate, and 0.4-4% in the case of sodium percarbonate. (e) A second liquid extraction is performed on the polymer material treated in step (d) by boiling a solution containing an alkali metal or alkaline earth metal bicarbonate and an oxidizing compound under the same pressure and temperature conditions as in step (d), wherein the oxidizing compound is selected from hydrogen peroxide, sodium perborate, and sodium percarbonate, and the amount of the oxidizing compound relative to the weight of the polymer material is 0.1-0.8% in the case of hydrogen peroxide, 0.4-4% in the case of sodium perborate, and 0.3-3% in the case of sodium percarbonate; (f) Separating the polymer material obtained in step (e) from the current liquid phase; (g) Vacuum extraction of the dried material from step (f) at a residual pressure of less than 10 mbar to remove volatile substances from the recovered polymer material.

[0021] In a second aspect, the present invention relates to an apparatus for recycling polymer-based materials derived from the plastic portion of sorted waste, the apparatus comprising: (A) A unit for extruding polymers and reactants according to step (b) of the method; (B) The unit for the first boiling extraction of step (d) using an alkaline aqueous solution of an oxidizing compound; (C) The unit for the second liquid extraction of step (e) using alkali metal bicarbonate and hydrogen peroxide solution; (D) A unit for separating polymeric materials from the liquid phase used in the previous step by filtration and / or centrifugation (e); (E) Vacuum extraction unit suitable for volatile substances of polymer materials separated in receiving and processing unit (D). Attached Figure Description

[0022] Figure 1 An apparatus for implementing the method of the invention in a first embodiment is illustrated schematically; Figure 2 An apparatus for implementing the method of the invention in a second embodiment is illustrated schematically. Detailed Implementation

[0023] Existing technologies provide for the washing and simple deodorization of polymer-based materials, for example by stripping the material with a stream of hot air or steam. The applicant has surprisingly found that, compared with existing technologies, the method of the present invention allows for the acquisition of particularly pure polymer materials that can be immediately used for industrial reuse without producing odors or containing toxic residues such as benzene in the finished product obtained from said reuse.

[0024] The method of the present invention can significantly reduce these types of unwanted substances by up to 98% or more, resulting in higher quality and therefore more valuable recycled products; to the knowledge of the inventors, this result is impossible to achieve by any other known method of washing and removing volatile compounds from plastic materials.

[0025] The present invention may have one or more preferred features reported below in one or more aspects thereof, which may be combined with each other as needed for the application.

[0026] The method according to the invention includes step (a) preparing a certain amount of raw materials, said raw materials comprising at least one polymer-based material derived from the plastic portion of sorted waste.

[0027] Preferably, the raw material is in the form of flakes obtained by grinding, or an aggregate of multiple flakes.

[0028] The amount of raw materials can be prepared according to any method known to those skilled in the art for this purpose.

[0029] The material in step (a) of the method of the present invention comprises at least one polymer-based material derived from the plastic portion of sorted waste; more preferably, the raw material comprises at least 70% by weight of the at least one polymer-based material relative to the total weight of the grinding material, the at least one polymer-based material being derived from the plastic portion of sorted waste or industrial waste; even more preferably, the raw material is composed of the at least one polymer-based material derived from the plastic portion of sorted waste.

[0030] Polymer-based materials can be derived from the plastic portion of sorted waste from both civil and industrial waste, and typically, but not necessarily or exclusively, include packaging waste such as food trays and containers, vials and bottles.

[0031] Preferably, prior to step (a) of the method according to the invention, the polymer-based material derived from the plastic portion of the sorted waste undergoes one or more preliminary pretreatment and sorting operations. These one or more preliminary operations may include, for example, screening to remove foreign matter (residues of glass, metal, ceramics, wood, etc.) and washing to remove organic residues present on the surface; the preliminary operations may also include opening the pre-prepared packaging of the material and / or directly grinding the material. These pretreatments are typically performed in all plastic material recycling processes and are therefore not part of this invention.

[0032] In step (b) of the method of the present invention, a foaming agent is added to the polymer material of step (a). According to the previously described possibilities (b.1)-b.3), the foaming agent can be one or more alkali metal or alkaline earth metal bicarbonates, a pair of materials consisting of one or more carbonates and one or more acidifying compounds, or one or more liquid compounds. In this specification and claims, "foaming agent" means any compound or composition that acts to generate gas in the extrusion step (c). Hydrogencarbonates are also known in chemistry as the common name bicarbonates, which will be used in the remainder of this specification.

[0033] The foaming agent (or its components, when they consist of a mixture of several compounds) may already be in solution form before being added to the polymer material; however, if they are in solid form, they are preferably added in powder form, which allows for a reduction in the size of the associated storage tank, and thus the size of the equipment in which the method is carried out.

[0034] In cases b.1) and b.2), the amount of bicarbonate or carbonate added relative to the weight of the polymer material is 0.1-0.5% by weight for bicarbonate and 0.2-1% by weight for carbonate. Within these ranges, the higher the molecular weight of the salt, the higher the percentage of bicarbonate or carbonate. When using bicarbonate, sodium bicarbonate is preferred because of its wide commercial availability. When using a carbonate / acidifying compound pair, the preferred carbonate is a carbonate of sodium, potassium, magnesium, or calcium (or a mixture thereof), while the preferred acidifying compound is a solid organic acid, such as >C8 monocarboxylic acids, C2-C7 dicarboxylic acids (i.e., oxalic acid to pimelic acid), and polycarboxylic acids, such as citric acid; the molar ratio of the acidifying agent to the carbonate used is 0.5-1.5.

[0035] In case b.3), the blowing agent is selected from CO2, hydrofluoroolefin refrigerants (referred to in the art by the abbreviation HFO) such as trans-1-chloro-3,3,3-trifluoro-1-propene (referred to as R-1233zd) or trans-1,3,3,3-tetrafluoro-1-propene (referred to as R-1234ze), hydrocarbons with a boiling point below 100°C, or mixtures thereof. In the case of liquid blowing agents, these blowing agents are injected directly into the extruder head at an amount of 0.3-0.6% by weight relative to the polymer to be treated.

[0036] In all three cases b.1) to b.3), in the first two cases, the mixture is obtained at the extruder inlet, and in the third case, the mixture is obtained directly at the material outlet head.

[0037] The advantage of using bicarbonates over liquid foaming agents (such as HFO or CO2) emerges in the case of polyolefin recycling. If these materials are subjected to high stress during extrusion, they begin to degrade in the presence of oxygen at temperatures above 200°C, producing large amounts of acetic acid. This acetic acid is trapped within the material and released over time, giving it a pungent and unpleasant odor. The presence of a base derived from the bicarbonate decomposition process (e.g., NaOH in the case of sodium bicarbonate) stabilizes the acid in sodium acetate, preventing the release of odor over time.

[0038] By keeping within the above-mentioned range of expansion agent additions, it is possible to verify the optimal amount of the compound to be used with a specific portion of the recycled plastic to be treated through preliminary tests; these tests consist of: extrude a small amount of plastic in a laboratory extruder and verify the reduction of volatile substances in the product obtained after steps (d)-(g), and then conduct them on a laboratory scale.

[0039] In step (c), the mixture obtained in step (b) is extruded. This step is carried out in a single-screw or twin-screw extruder, or with other mechanical mixing systems that can bring the material above its softening point and initiate the decomposition reactions of the reagents in cases b.1) and b.2), or bring the compound in case b.3) to a gaseous state, operating at temperatures above 110°C, and depending on the specific polymer or polymer mixture being processed.

[0040] When the material is known, the softening temperature of the polymer is known in the literature; alternatively, if the material is unknown, the temperature can be estimated by indicative testing on a small sample of the same material using the method defined in UNI EN 1427, which relates to the polymer material.

[0041] For clarity, step (c) is referred to here as a separate step, but it can be performed in conjunction with step (b); in other words, the component selected for step (b) (bicarbonate, carbonate / acidifier or foaming compound) can be fed into the feed hopper of the extruder, where the polymer material to be recycled has already been introduced, and both components are processed continuously immediately after the addition of the component from step (b).

[0042] According to methods well known in the art, the mixture is extruded at the outlet of an extruder into particles with a diameter of 3 mm to 10 mm and a length of 4 mm to 10 mm; the particles are then introduced into a reactor for the next step.

[0043] Step (d) consists of a first boiling extraction of the mixture obtained in step (c) with an alkaline aqueous solution of an oxidizing compound as defined above. This step typically lasts 30–120 minutes.

[0044] To perform this step, the pellets are introduced into a reactor containing preheated water, for example at a temperature 50°C to 20°C below the material softening temperature, where the volume of water is approximately equal to the volume of the pellets themselves.

[0045] In the case of hydrogen peroxide, the amount is 0.1-2% by weight relative to the weight of the polymer material, depending on the polymer type and the amount of contaminant. Hydrogen peroxide (H2O2) is not added all at the start of the reaction, but continuously at a relatively low flow rate, primarily to avoid runaway reaction caused by the generated oxygen; the inventors also observed that adding H2O2 over time at a constant flow rate can actually halve reagent consumption by reducing the dispersed active oxygen portion.

[0046] Hydrogen peroxide is preferably used in the form of a 35-40% by weight aqueous solution.

[0047] The amount of alkali added makes the pH of the solution 8-13, preferably 8-11. The alkali is preferably sodium bicarbonate, NaHCO3, calcium hydroxide, Ca(OH)2, or even more preferably a mixture of these two compounds.

[0048] An alkaline but not too high pH value makes it possible to achieve an optimal balance between the need for the oxidation of contaminants in plastic materials by H2O2 without impairing the reaction and the need to slow down their decomposition to give the oxidant time to reach the molecules to be decomposed.

[0049] Another effect of using Ca(OH)₂ is the ability to chelate elements (in their respective calcium salt forms), such as fluorine and phosphorus, derived from the decomposition of toxic compounds present in the polymer. These toxic compounds can be pesticides or insecticides, often found in recycled polymers previously used in agriculture. This effect can also be achieved, if desired, by adding other compounds to the extract, such as aluminum hydroxide, Al(OH)₃, which does not contribute to achieving an alkaline pH.

[0050] In the case of sodium perborate, the operating conditions are similar to those described above for H2O2, and the relationship between the amount of the component and the polymer material is as described above.

[0051] On the other hand, in the case of sodium percarbonate, alkaline components (NaHCO3 and / or Ca(OH)2) are not necessary because these oxidizing compounds are also alkaline, and are sufficient in the amounts described above to keep the pH of the solution within the range suitable for carrying out step (d).

[0052] The temperature for this step is 50°C to 20°C lower than the softening temperature of the polymer material. Depending on the type of polymer to be recycled, the softening temperature can vary from about 40°C for polycaprolactone (PCL) to over 200°C for some polyamides; therefore, the highest temperature at which this step can be performed can vary between about 20°C and 200°C, preferably between 60°C and 150°C. Once the temperature is determined, the pressure for step (d) is selected to allow the extraction process to proceed under boiling conditions with the aqueous solution; by referring to the phase diagram of water, a technician can easily determine the pressure value of boiling water that may be obtained at the selected operating temperature.

[0053] The solution in the reactor can be kept under stirring using a known technique, such as mechanical stirring, extractant circulation pump, or ultrasound; a combination of techniques can also be used, particularly combining ultrasound with one of the other techniques, to improve extraction efficiency.

[0054] In this step, as described above, contaminants present in the recycled polymer material are decomposed through oxidation.

[0055] At the end of the first liquid extraction operation, the liquid phase is removed, the polymer is rinsed with water, and then sent to the next step.

[0056] In step (e), the polymer material treated in step (d) is subjected to a second liquid extraction with a boiling solution under the same pressure and temperature conditions.

[0057] In this case, an alkaline solution of the oxidizing compound is used, which may contain hydrogen peroxide or sodium perborate and an alkali metal bicarbonate (preferably sodium bicarbonate), or a sodium percarbonate solution. The amount of the oxidizing compound used in step (e) is less than that in step (d); when using a hydrogen peroxide solution, its amount is 0.1-0.8% by weight, preferably about 0.5% by weight, relative to the weight of the polymer material; when using sodium perborate or sodium percarbonate, their amounts are 0.4-4% by weight and 0.3-3% by weight, respectively, relative to the weight of the polymer material.

[0058] This step completes the extraction of the polymer, which is derived from the recovery and conversion of residual contaminants into compounds that are more easily eliminated in downstream steps. Specifically, the following items are performed: - Fat saponifies and then dissolves; - Ester decomposition due to an alkaline environment; - Replace halide functional groups with OH groups; - Reducing the size of complex contaminants facilitates subsequent stripping operations from the liquid phase.

[0059] Step (e) lasts 60-120 minutes.

[0060] Following the second liquid extraction step (e), the liquid phase is removed, the polymer is rinsed with water, and then proceeded to the next step (f) to remove the rinsing water. This step can be performed according to various methods known in the art, which do not need to be described in detail, such as filtration, pressure filtration, or preferably centrifugation. In the preferred case of centrifugation, this is typically carried out at a speed of 100-900 rpm for 1-10 minutes.

[0061] Finally, in step (g), the polymer material recovered at the end of step (f) is subjected to a vacuum extraction operation to extract volatile substances still present in the material itself, which are referred to in the art as VOCs (volatile organic compounds).

[0062] The still-wet material from the previous step is transferred to one or more stripping towers, heated to a temperature approximately 20°C lower than the softening temperature of the plastic material, and vacuum extracted for 30–120 minutes.

[0063] The final vacuum achieved in this operation is less than 10 millibars, preferably less than 1 millibars.

[0064] In the subsequent stages, this vacuum level is gradually reached.

[0065] In the first stage, a low-vacuum pump is used, typically a liquid ring pump with a water-liquid ring, which achieves a pressure of approximately 950 mbar; this value is highly dependent on the temperature and the flow rate of residual contaminants. This type of pump is better able to withstand the strong contaminant flow that occurs in the first part of this step.

[0066] When the pressure in the system reaches the equilibrium pressure of the first pump, the next pump is started. This pump is preferably also of the liquid ring type, but has an oil ring for high vacuum.

[0067] This second pump can reduce the pressure in the extraction chamber containing the polymer to a residual pressure of less than 1 millibar.

[0068] The extraction system can be equipped with sampling of the extraction gas to verify the actual completion of the process based on gas chromatography or alternative infrared spectroscopy analysis of some target compounds.

[0069] At the end of this step, the polymer material is recycled and ready to be sent to a reuse process for the production of new products.

[0070] In a second aspect, the present invention relates to apparatus for carrying out the above-described method. The system includes: (A) Unit 3 for extruding polymer and reactants according to step (b) of the method; (B) The unit for the first boiling extraction of step (d) using an alkaline aqueous solution of an oxidizing compound; (C) The unit for the second liquid extraction of step (e) using alkali metal bicarbonate and hydrogen peroxide solution; (D) A unit for separating polymeric materials from the liquid phase used in the previous step by filtration and / or centrifugation (e); (E) Vacuum extraction unit suitable for volatile substances of polymer materials separated in receiving and processing unit (D).

[0071] More specifically, the device includes a unit in which operations corresponding to the operation and conversion steps of the present invention are performed, a storage tank for reagents used in the method, a valve for proper movement of the reagents in the device, and a metering unit for the valve; valves and metering units known to those skilled in the art are not mentioned in the following description.

[0072] Figure 1 A first possible embodiment of the device of the present invention is shown in schematic form, wherein the above method can be performed when the oxidizing compound is hydrogen peroxide or sodium perborate.

[0073] refer to Figure 1The first unit (A) of the equipment includes a first storage tank 1 for the polymer to be treated, a second storage tank 2, and an extruder 3. The second storage tank 2 contains an alkali metal or alkaline earth metal bicarbonate for step (b) according to mode b.1, or a carbonate / acidifying agent pair for step b.2, or finally a liquid compound for step b.3; in the case of b.3, the foaming agent is supplied to the extruder by adjusting the flow rate using a variable valve or flow meter or by a pump.

[0074] By appropriately adjusting the distributor (not shown) downstream of tank 1 and the valves, flow meters, or pumps downstream of tank 2, the polymer and reagents required for step (b) of one of methods b.1-b.3 are fed into extruder 3 in the desired proportions through pipelines L1 and L2, respectively, to carry out step (c) of the method of the present invention. At the outlet of extruder 3, the mixture is converted into granules and then solidified by forced cooling.

[0075] Unit (B) comprises reactor 4 and two storage tanks 5 and 6, containing aqueous solutions of hydrogen peroxide or sodium perborate (in powder form or as a solution) and an alkaline compound (in solid or solution form), respectively, to achieve a pH of 8 to 11 for the reagent system. The mixed granules exiting extruder 3 are fed into reactor 4 via pipeline L3 for a first boiling extraction (step (d) of the method). The hydrogen peroxide or sodium perborate solution in storage tank 5 and the alkaline solution in storage tank 6 are fed into reactor 4 via pipelines L4 and L5, respectively.

[0076] The pellets that have undergone the first liquid extraction in unit (B) are sent to the second liquid extraction in unit (C) via pipeline L6, which exits reactor 4; the washing liquid phase exiting reactor 4 is then sent to the recycling treatment via pipeline L7, which flows into the collection pipeline L8 shared by all units of the system.

[0077] Unit (C) includes a second reactor 7, a storage tank 5 which is also part of unit (B), and a second storage tank 8. Storage tanks 5 and 8 are respectively filled with an oxidant (hydrogen peroxide solution or sodium perborate, solid or solution) and an alkaline compound, an alkali metal or alkaline earth metal bicarbonate (also solid or solution), and are respectively connected to reactor 7 through pipelines L9 and L10.

[0078] The granules that have undergone a second liquid extraction in unit (C) are sent to separation unit (D) via pipeline L11, while the wash water is sent to collection pipeline L8 via pipeline L12 for post-processing.

[0079] The separation unit (D) consists of an instrument shown as a centrifuge (9) in the figure, but as mentioned above, it can be a filter or a filter press. The liquid separated in unit (D) is sent to a post-processing unit via collection line L8, while the particulate material is sent via line L13 to one or more stripping towers 10 (only one is shown in the figure), where vacuum removal is performed in the stripping tower 10 to remove volatile components still present in the recovered polymer material particulate material. The particulate material thus purified from VOCs is recovered via line L14, while VOCs are extracted from tower 10 via lines L15 and L16 connected to an intermediate vacuum pump and a high vacuum pump (not shown in the figure).

[0080] exist Figure 2 In the alternative embodiments shown, when the oxidizing compound is sodium percarbonate, the apparatus is suitable for carrying out the method of the invention, as previously described, sodium percarbonate is also an alkaline and soluble compound; Figure 1 and Figure 2 In this context, elements with the same reference numerals have the same function. Figure 2 In this system, there is no need for a separate storage tank for adding alkaline compounds, and the storage tank 5' containing sodium percarbonate (in this case, either in solid or solution form) performs the operation. Figure 1 The functions of storage tanks 5, 6 and 8.

[0081] The present invention will be further illustrated by the following embodiments.

[0082] Materials, experimental conditions and methods Example 1 10 kg of sheets of PE, PP and a mixture of the two polymers (selected from waste materials collected by sorting plastic materials) are pre-screened to remove foreign matter (residues of glass, metal, ceramics, wood, etc.) and preliminarily washed to remove organic residues present on the surface.

[0083] The processed material is placed into a 20-liter metering hopper to feed it into the extrusion unit.

[0084] 4 kg of 99.3% pure NaHCO3 was loaded into the second 5-liter metering hopper.

[0085] The material was extruded using a laboratory extruder (model MD30, Bausano & Figli SpA, Rivaro Locanavese (TO), Italy) under the following conditions: - Polymer feed: 2 kg / min; - NaHCO3 feed: 8 g / min; - Material inlet temperature: room temperature (T); - Material outlet temperature before pellet cutting: 240°C; - Air-cooled head cutting.

[0086] The obtained material was placed in a 30-liter container along with 12 liters of water, 8 g of Ca(OH)2 and 28 g of 35% H2O2 aqueous solution, and added uniformly over the first 30 minutes of the temperature treatment.

[0087] The solution is heated to boiling by electric heating and maintained at 98-100°C and 990-1020 hPa for 60 minutes with continuous mechanical stirring. The container is equipped with a lid, which is not airtight in this case, and water evaporated during the process.

[0088] At the end of the previous stage, remove the porous basket containing the granular material, rinse it with water, and place the basket into a second container similar to the previous one. Add 12 liters of water, 6 g of NaHCO3 and 20 g of H2O2 aqueous solution to the container, and add them evenly and quantitatively over the first 30 minutes of treatment at this temperature.

[0089] The solution is heated to boiling by electric heating and maintained at 98-100°C and 990-1020 hPa for 120 minutes with continuous mechanical stirring. The container is equipped with a lid, which is not airtight in this case, and water evaporated during the process.

[0090] At the end of this stage, remove the porous basket containing the granules, rinse it with water and dry it to drain excess water.

[0091] The material is then transferred to a heated, airtight hopper, where it is first dried at 70°C for 120 minutes with an airflow of 10 liters / minute to remove moisture, followed by vacuum stripping in a two-phase operation under the following conditions: - Phase 1: Maintain at 80°C for 60 minutes, with residual pressure in the room equal to 50-100 millibars; - Phase 2: Maintain at 80°C for 60 minutes, with residual pressure in the room equal to 1-4 millibars.

[0092] The indicated pressure levels were obtained in stage 1 using a liquid ring pump with a water ring (model TMR, Pompetravaini SpA, Castano Primo (MI), Italy) and in stage 2 using a two-stage liquid ring pump for high vacuum (model RC4M, DVP Vacuum Technology SpA, San Pedro, Casale (BO), Italy).

[0093] Example 2 For comparison, samples were prepared from the same initial materials according to the same procedure described in Example 1, but with simple extrusion of the recycled materials, without adding any compounds (oxidizing and basic compounds) that constitute the characteristics of this invention. Liquid phase extraction was also not performed.

[0094] Example 3 Gas release tests were performed on the samples prepared in Examples 1 and 2.

[0095] These tests were performed using GC-MS measurements at 100°C using solid-phase microextraction (SPME).

[0096] The following equipment is used for testing: - An analytical dial accurate to one ten-thousandth of a gram; - TD GC-MS / MS System Center: Agilent Technologies - 7820A; Agilent Technologies - MDD 5977B; - SUPELCO SPB-624 column, inner diameter (ID) 20m×0.18 mm, film thickness (df) 1.00 μm; - Divinylbenzene / formaldehyde / polydimethylsiloxane (DVB / CAR / PDMS) SPME system for autosamplers, needle size 23 ga, metal alloy fiber, length 1 cm.

[0097] The analysis procedure includes preliminary sampling of the instrument headspace under the following conditions: - Sample incubation: 5 min at 100℃; - Headspace sampling volume: 5 mL; - Cryogenic trap: Material discharge, Markes International: Loading temperature 10°C; Desorption temperature 250°C; Overall segmentation ratio: 13.5:1.

[0098] The conditions for performing chromatographic tests are as follows: - Initial column temperature: 40℃ for 5 minutes; - Temperature program: 10℃ / min to 80℃, 25℃ / min to 220℃, isothermal at 220℃ for 5 min; - Carrier gas flow rate (He): 1.2 mL / min.

[0099] The parameters of the mass spectrometer are as follows: - Source temperature: 230°C; - Interface temperature: 150°C; - Solvent delay time: 3.0 min; - Acquisition mode: Full scan (quality range 50-450 m / z).

[0100] Eight of the most prominent compounds were identified by evaluating the measurable peak areas in the GC-MS chromatograms obtained from the analysis of treated samples (generated in Example 1) and untreated samples (Example 2). A control blank was also analyzed to verify the presence of any spurious contributions to the recorded signal. The compounds were identified by comparing the experimentally obtained mass spectra with those available in the NIST database (version 2.0f, April 2009).

[0101] The results of the GC-MS analysis are shown in the table below:

[0102] The data in the table show that after treatment, almost all compounds that cause the material to release odors are reduced in quantity and will not be released as volatile compounds during reprocessing.

Claims

1. A method for recycling polymer-based materials, said polymer-based materials being derived from the plastic portion of sorted waste, said method comprising the following steps: (a) Prepare a certain amount of raw materials, said raw materials comprising at least one polymer material derived from the plastic portion of sorted waste; (b) Adding a foaming agent to the polymer material from the previous step, the foaming agent being selected from: b.1) an alkali metal or alkaline earth metal bicarbonate, wherein the amount of the bicarbonate is 0.1-0.5 wt% relative to the weight of the polymer material; b.2) a pair of materials consisting of a carbonate and an acidified compound, wherein the amount of the carbonate is present is 0.2-1 wt% relative to the weight of the polymer material; and b.3) a liquid compound with a boiling point below 100°C, directly injected into the extruder head, wherein the amount of the liquid compound added is 0.3-0.6 wt% relative to the weight of the polymer material, and a mixture is obtained in all three cases; (c) Extruding the mixture from step (b) to obtain granules of recycled material; (d) A first extraction is performed by boiling the granules obtained in step (c) in an alkaline aqueous solution of an oxidizing compound with a pH of 8 to 13 at a temperature 50°C to 20°C below the softening temperature of the polymer material. The oxidizing compound is selected from hydrogen peroxide, sodium perborate, and sodium percarbonate, and the amount of the oxidizing compound relative to the weight of the polymer material is 0.1-2% in the case of hydrogen peroxide, 0.5-5% in the case of sodium perborate, and 0.4-4% in the case of sodium percarbonate. (e) A second liquid extraction is performed by boiling the polymer material treated in step (d) in a solution containing an alkali metal or alkaline earth metal bicarbonate and an oxidizing compound under the same pressure and temperature conditions as in step (d), wherein the oxidizing compound is selected from hydrogen peroxide, sodium perborate, and sodium percarbonate, and the amount of the oxidizing compound relative to the weight of the polymer material is 0.1-0.8% in the case of hydrogen peroxide, 0.4-4% in the case of sodium perborate, and 0.3-3% in the case of sodium percarbonate; (f) Separating the polymer material obtained in step (e) from the current liquid phase; (g) Vacuum extraction of the dried material from step (f) at a residual pressure of less than 10 mbar to remove volatile substances from the recovered polymer material.

2. The method according to claim 1, wherein in step (b): - In the case of b.1), the bicarbonate is sodium bicarbonate; - In case b.2), the carbonate is selected from sodium carbonate, potassium carbonate, magnesium carbonate, and calcium carbonate, or mixtures thereof, and the acidifying compound is selected from >C8 monocarboxylic acids, C2-C7 dicarboxylic acids, citric acid, and mixtures thereof, and is used in a molar ratio of 0.5-1.5 relative to the carbonate; and - In case b.3), the foaming agent is selected from CO2, hydrofluoroolefin refrigerants, hydrocarbons with boiling points below 100°C, and mixtures thereof.

3. The method according to any one of claims 1 or 2, wherein step (c) is performed after step (b) or simultaneously with step (b).

4. The method according to any one of the preceding claims, wherein step (d) is carried out at a pH of 8-11, with stirring by mechanical stirring, stirring with a circulating pump, ultrasonication, or a combination of these techniques.

5. The method according to any one of the preceding claims, wherein step (d) is carried out with a 35-40% by weight aqueous solution of hydrogen peroxide, wherein sodium bicarbonate, calcium hydroxide or a mixture thereof is added, and the aqueous solution of hydrogen peroxide is added under continuous flow conditions.

6. The method according to any one of claims 1-4, wherein step (d) is carried out with an aqueous solution of sodium perborate, wherein sodium bicarbonate, calcium hydroxide or a mixture thereof is added.

7. The method according to any one of the preceding claims, wherein step (f) is performed by filtration, pressure filtration or centrifugation.

8. The method according to any one of the preceding claims, wherein step (g) is carried out in one or more stripping towers at a temperature about 20°C lower than the softening temperature of the plastic material, and at a pressure of less than 10 mbar is reached in at least two subsequent stages, the first stage being carried out with a low vacuum pump to bring the pressure to a value of about 950 mbar, and the second stage being carried out with a high vacuum pump, the second stage being activated when the pressure obtained in the system with the low vacuum pump reaches an equilibrium value.

9. The method according to any one of the preceding claims, wherein one or more preliminary operations are performed prior to step (a) for pretreatment of the polymer-based material derived from the plastic portion of the sorted waste, said one or more preliminary operations being selected from screening to remove foreign matter and washing to remove organic residues present on the surface of said polymer-based material.

10. An apparatus for recycling polymer-based materials derived from the plastic portion of sorted waste, the apparatus comprising: (A) A unit for extruding polymers and reactants according to step (b) of the method; (B) Unit (4) for the first boiling extraction of step (d) using an alkaline aqueous solution of an oxidizing compound; (C) Unit (7) for the second liquid extraction of step (e) using a solution of alkali metal bicarbonate and hydrogen peroxide. (D) A unit (9) for separating polymeric materials from the liquid phase used in the previous step by filtration and / or centrifugation (e); and (E) Vacuum extraction unit (10) suitable for receiving and processing volatile substances of polymer materials separated in unit (D).

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