Recycled vinyl aromatic polymers decontamination process for food-contact grade applications

Abstract

Process for the production of food-contact grade vinyl aromatic polymers, starting from a post-consumer recycled vinyl aromatic feedstock The process further comprises the falling strand decontamination technology, which, under specific process parameters, significantly reduces the contaminates of the feedstock.

Description

[0001]RECYCLED VINYL AROMATIC POLYMERS DECONTAMINATION PROCESS FOR FOOD-CONTACT GRADE APPLICATIONS Cross-Reference to Related Applications This Patent Application claims priority from Italian Patent Application No. 102024000015028 filed on June 28, 2024, the entire disclosure of which is incorporated herein by reference. Technical Field The present invention relates to a decontamination process for recycled vinyl aromatic polymers for food- contact grade applications. In particular, the present invention relates to a process for the production of vinyl aromatic polymers starting from a recycled vinyl aromatic based raw material. For example, the recycled vinyl aromatic based raw material is coming from post-consumer user. More in particular, the present invention relates to a process for the production of recycled vinyl aromatic polymers that can be used for food-contact applications, for example packaging and containers for fruits, vegetables, meat, fish, cheese and other dietary products. State of the Art In the field of the polystyrene product, it is known that after the polymerization there is the need to remove or reduce at very low levels the unreacted monomers and other contaminants (such as ethyl benzene, oligomers and “waxes”, that are very low molecular weight polystyrene) from the virgin polymers. For food-contact grade applications, the limits on the concentration of contaminants are even lower. Nowadays, the required high levels of decontamination can be reached by using two devolatilization stages: the first stage for removing the major portion of the volatiles and the second stage for further removal of volatiles. In this regard, EP0352727 discloses an innovative solution for the first stage, involving the devolatilization of a polystyrene solution in solvents such as styrene and ethylbenzene. However, this solution is mainly efficient for low viscosity fluids and high content of styrene and other solvents. For the second stage of devolatilization, to obtain commercial grade virgin polystyrene, a different technology shall be used, such as the falling strand devolatilization. According to this technology, in order to remove undesired volatile compounds, such as the monomers and oligomers of styrene, as well as very low molecular weight “waxes” and co-solvent (i.e. ethylbenzene), the polymer coming from the first devolatiliser is heated up and transferred to the top of a degassing vessel, where it is passed through a distributor comprising a large number of very small apertures (typically, small holes of about 0.5 - 6 mm), positioned vertically on the bottom of the distributor. In such a way, the polymer, exiting such holes, produces strands that slowly fall down in the vessel. The vessel is subjected to a very low pressure (few millibars), so that the volatile compounds are removed. The strands have a very large surface to volume ratio, making the process quite effective. The strands collect on the bottom of the vessel, where a polymer pool is generally formed (as they are in the melt state). Hence, the degassed polymer can be withdrawn by means of conventional equipment such as gear pumps. A substantial number of patent and non-patent literature has dealt with the removal of volatile organic compounds (VOCs) from polymers by means of the falling strand devolatilization technology. For example, B. J. Meister and A. E. Platt in “Evaluation of the performance of a commercial polystyrene devolatilizer”, Ind. Eng. Chem. Res. 1989, 28, 11, 1659– 1664, discloses residual styrene, ethylbenzene, dimers and trimers as a function of the devolatilizer tank pressure, which ranged between 4 and 30 torr (precision: + / -0.1 torr). Results show that the viscosity of the obtained polymer (Melt Flow Index, MFI, of 1.2 for product 1 and 7.2 for product 2) had little influence on the devolatilization efficiency. On the contrary, the molecular weight of the molecules, which need to be removed, played a major role over the devolatilization efficiency. Data show a steep reduction of unreacted styrene in the final polymer as pressure is reduced from 30 to 5 torr, but this effect is less evident for molecules having higher molecular weight (such as dimers and trimers of styrene). Hence, the process envisaged by this article is not able to reduce high molecular weight compounds. A. De San Luis et al. in "Removal of volatile organic compounds from bulk and emulsion polymers: a comprehensive survey of the existing techniques" Industrial & Engineering Chemistry Research 58.27 (2019): 11601-11623, evaluates different methods to remove VOCs from polymers such as polystyrene. Among other methods, the falling strand devolatilization is outlined (at pages 11606-11607). This method is described as suitable for viscous polymers (around 1 Pa·s of viscosity) containing a lower concentration of VOCs compared to others devolatilization methods. As an example, it cites polystyrene having 10-40 wt% of styrene treated at 200-280°C and a pressure between 6700 and 26700 Pa in the first devolatilizer tank and 400 to 2700 Pa in the second devolatilizer tank. In this way, the VOCs content was, in most cases, from 0.01 to 0.05 wt.% (i.e. 100-500 wppm). The envisaged process is not able to reduce high molecular weight molecules and semi-volatile organic compounds to the stringent limits of the present process. WO 2023 / 001855 discloses a devolatilization apparatus for removing volatiles from a polymer melt feed, which comprises a nozzle to distribute the polymer, configured to form polymer strands; a collector, configured to receive the polymer strands in the devolatilization vessel; a polymer outlet to remove the degassed polymer from the vessel; and a gas outlet to remove the volatiles. This apparatus is characterized in that the average aperture diameter of the polymer distributor is from 0.5 to 8.0 mm and in that the strand drop height is between 1 and 20 m. This patent application also relates to the process for reducing volatiles in a polymer melt feed. In the examples, the polymer which is fed is polystyrene at from 214 to 235°C, at throughput of from 10 to 600 g / h per aperture, 1.2 to 4 mm aperture, drop height from 3 to 6 m, devolatiliser pressure from 2 to 6 mbar. The best result is obtained in example 2 (carried out at the lowest throughput per aperture, the lowest aperture, the highest polymer inlet temperature), where the outlet residual styrene is 210 wppm. However, the removal of less volatile compounds is even less efficient (at most 5%) and not supported by experiments. Parallel application WO 2023 / 001854 discloses a process, as well as the relative devolatilization apparatus, for reducing volatiles in a polymer melt feed. It differs from WO 2023 / 001855 in few details: the introduction of an additional parameter (such as the ratio of strand drop height / average aperture diameter). It is worth pointing out that neither WO 2023 / 001855 nor WO 2023 / 001854 deal with recycle or post use polymers, let alone food-grade polymers and the process for their production from recycled polymers. GB 1469168 discloses an apparatus for the recovery of an alkenyl aromatic monomer from high molecular weight alkenyl aromatic polymer, alkenyl aromatic oligomers, high boiling point organic compounds and inert hydrocarbons, and for reducing the concentration of alkenyl aromatic oligomers and high boiling point organic compounds by means of a two- phase falling strand devolatilization in two stages, followed by purification of the condensates. From the disclosure, it can be inferred that this process is able to remove oligomers, such as dimers and trimers, as well as other high boiling components. However, the process of GB 1469168 does not involve recycled material and the obtained polymer is not used as food-grade. Beside the apparatus and the related processes involving the falling strand devolatilization technology, US 6,410,683 discloses a process for the removal of impurities from a thermoplastic polymer, comprising mixing with the polymer in a molten state a stripping agent comprising a solution of carbon dioxide in water, passing the mixture to a vessel held at sub-atmospheric pressure so as to allow impurities to diffuse out. In particular, this disclosure is directed to a process for producing styrene polymers having less than 150 ppm of styrene and whose content of oligomeric species, such as styrene dimer, are also minimized. It is in fact stressed that such products are useful for the manufacture of food packaging where migration of residual monomer from the polymer into the food can be troublesome. In example 1, polystyrene at 240°C and 800 ppm of styrene is mixed with a solution of carbon dioxide in water, then it is passed to a low pressure tank thus reducing the styrene monomer to 50 ppm. However, using a stripping agent (such as carbon dioxide) makes the process for the production of polystyrene more complex. Moreover, the use of stripping agents makes the process more complex. Additionally, in the process, disclosed in US 6,410,683, the starting material is not coming from recycled polymers. In a similar context, WO 2023 / 275746 discloses a process for the preparation of vinyl aromatic polymers by means of a continuous stirred tank reactor and a plug flow reactor with partial internal recycle of the produced polymer. Optionally, said internal recycle comprises the fraction of the condensed mixture of the vapours leaving the devolatilization system. Such mixture comprises the so called waxes, defined as oligomers, such as linear and cyclic dimers, and trimers of the monomer (styrene), which derive from the polymerization reaction. Therefore, it could be inferred that the devolatilization system is able not only to remove the solvent and unreacted monomers, but also to reduce the content of high boiling point compounds such as dimers and trimers of styrene. However, no details are given both on the type of devolatilization system and on the devolatilization efficiency. In addition, the process in WO 2023 / 275746 does not deal with treat recycled feedstock. Therefore, there is the long-sought need to provide a process starting from recycled polymer feedstock, in particular from recycled vinyl aromatic polymer, for producing a polymer, in particular a food-contact grade polymer, more in particular a food-contact grade vinyl aromatic polymer. In regard to a process which treats a recycled polymer feedstock, EP2419255 discloses a method and apparatus for recycling plastic materials, which includes, preferably in sequence, the following steps: (a) preparation of the raw material, such as optional comminution and degassing of the (solid) polymer mixture, (b) melting, (c) filtering, (d) homogenizing (preferably by intense shear or tensile stress and by acceleration) and (e) degassing the homogenized melt, (f) discharging and / or subsequently processing the polymer melt (such as by granulation). Preferably, the temperature of the polymer entering the degassing step (e) is the highest of all the steps. The description underlines the importance of comminution and homogenization step, that should be carried out after filtration but before melt degassing, as it makes more efficient the process since the contaminants are more evenly distributed. It is evident that the process of this patent is complex involving many steps. Moreover, this patent does not disclose the production of a food-grade contact polymer. In the same context, US 2021 / 0402651 discloses a method and device for recycling plastics, where the recyclable material is melted using a discharge extruder, filtered using a first filter device and degassed using a degassing device, and discharged using a discharge extruder. The degassing device has at least one filter element and a vacuum chamber at 0.5 to 50 mbar for filtering and degassing. The method has the advantage that the process of filtration and degassing are carried out in one process step. In fact, the filtered plastic melt can be degassed directly by filtration in a vacuum atmosphere. Large-contact area zones between the plastic melt and the vacuum atmosphere can thus be achieved so that the volatile constituents can be released from the plastic melt in a relatively short dwell time, thus minimizing degradation. Moreover, a high degree of purity in the plastic melt can be produced by the multi-stage filtration. However, the filters inside the devolatilizer apparatus are prone to clogging and they are difficult to clean. In continuity with the previous patents, WO 2012 / 119165 discloses a device for removing contaminants from a polymer melt, characterized by one or more chambers where the polymer melt is distributed to a perforated plate having openings less than 1.5 mm and falls down in thin filaments under reduced pressure, more preferably less than 20 mbar, in particular 2-5 mbar, so that impurities diffuse as a result of the temperature and the reduced pressure. Such filaments have diameter less than 1.5 mm and fall freely without contact with the wall of the chamber. The teachings of this patent apply to the production of plastic material or the recycling of plastic material containing water as well as other contaminants, which are reduced by means of a vertical strand devolatilization. However, this patent fails to suggest that this technology is useful for producing food- contact grade polymers and there is no evidence that this process would be able to remove high molecular weight contaminants. In regard to a process for the production of a food- contact grade polymer starting from non-recycled raw materials, US 9,358,547 relates to a process for detoxifying a thermoplastic material or for removing contaminants, wherein the material is heated and mixed, and wherein a rinsing medium is introduced into the container, while keeping the pressure below 100 mbar, preferably below 50 mbar, in particular between 10 and 20 mbar, more preferably under 2 mbar. This disclosure aims to produce plastic that can be used for packages that needs to meet food-contact law rules imposed by FDA, ILSI and EFSA while keeping good efficiency. Even if the rinsing medium is showed to greatly improve the efficacy of the method, nevertheless the required time is quite large. There is no evidence that the process can treat post-consumer recycled material effectively and that it is efficient with the reduction of high molecular weight contaminants. Similarly, EP 1542777 relates to a method for removal of volatiles from mass processable polymer by means of a multi-chambered devolatilizer. The disclosure aims to produce the highest quality product by reducing the residual concentration of volatiles in the finished polymer as much as possible. For many commercial applications, such as food packaging, very high standards are imposed on the acceptable levels of volatiles in the polymer product. The method of this patent is carried out by means of falling strand devolatilization on a multi-chambered devolatilizer. For instance, a first devolatilizer is followed by a second devolatilization carried out in a second devolatilizer, which can be split into two sections, where the polymer exiting the first section is fed to the second section, plus an optional third devolatilizer. This patent discloses a polystyrene sample (named CX7200), which was treated in different ways. Firstly, the sample is processed in a conventional devolatilization, obtaining about 190 ppm of residual volatiles. According to this disclosure, the polystyrene sample was then passed through the multi- chambered devolatilizer configured to allow both partial and full recycling of the polymer stream, leading about 148 ppm and about 57 ppm of residual volatiles, respectively. However, the method of this disclosure is complex since it requires multiple devolatilizers and recycles. Moreover, this method does not include the feeding with recycled polymer, nor even with recycled vinyl aromatic polymer raw material. Regarding a process which includes the recycled polymer feedstock and the production of a food-contact grade polymer, WO 2001 / 021373 discloses a process for preparing food-contact grade PET from a waste stream containing PET and non-PET materials. The process involves the steps of sorting, washing, dewatering, drying, and optionally removing contaminants by heating at 170-220°C in vacuum (2 to 7 mbar of pressure) for 30-60 minutes. The decontaminated PET flakes are then fed to a screw extruder, such as the extruder used in US 9,358,547, with an extrusion screw designed for the processing of PET and capable of applying vacuum venting to the PET melt at 280 to 290°C. The process is conducted in a twin or multi-screw extruder with vacuum venting. The key requirement is the capability to melt the PET (melt temperature in the range 265-300°C, preferably 280-290°C) and to apply one or more stages of vacuum venting (at preferably 1 millibar or less) without applying excessive shear to the PET. According to this disclosure, the application of the high temperatures to the melt, coupled with the vacuum venting, allows removal of the least volatile contaminants that may have been absorbed into the PET. In the examples of this disclosure, PET flakes were contaminated with known quantities of toluene (10%), chloroform (10%), benzophenone (1%), methyl stearate (1%) and copper octoate (1%). It is apparent that extruded PET pellets have much lower content of contaminants if compared with PET flakes after wash. In fact, the process to reduce such contaminants was very efficient for toluene (-98%) and methyl stearate (- 94%), good for benzophenone (-69%) and chloroform (-53%), and not so effective for copper octanoate (-38%). According to this process, the food simulations after the migration test in PET bottles were very good too (<0.01 ppm for any contaminant). Apparently, vacuum venting during extrusion of PET at 280-290°C (while keeping this temperature for less than 60 seconds) was a key step that allowed the proposed process to produce food-contact grade PET from heavily contaminated PET flakes. However, it can be assumed that the process of this disclosure cannot be reproduced with a polystyrene feed, as the melting temperature of polystyrene is quite lower, thus rendering impossible to carry out the optional heating and mixing step, at least at the same temperatures used for PET. Such optional heating and mixing step require also a rather long period of time (about 1 hour). Moreover, vacuum venting was carried out bringing the temperature of the melt to 280-290°C for less than 60 seconds. That temperature range would be impractical for polystyrene as it would degrade quickly at such temperature. In connection with the previous patent application, WO 2023 / 148026 discloses an apparatus and method for reprocessing post-consumer plastic waste that is shredded and washed, comprising a “melting extruder” to homogenize the feed and a “degassing extruder” for degassing the plastic melt and a granulating device. An ozone generator is connected so that ozone can be supplied at any point along the path (i.e. in any or both extruders or between them). Such a process offers an efficient and at the same time environmentally friendly process for processing post- consumer waste. The cleaning efficiency is so high that the produced material is suitable for food and is odorless. However, ozone is harmful and requires the use of very special materials (especially gaskets) since ozone is extremely oxidizing. Similarly, EP 2507022 discloses a process for producing food-contact grade pellets from post-consumer recycle (PCR) polyolefin feedstocks which is “odoriferous”. The process comprises extruding the PCR polyolefin into pellets and contacting the resulting pellets with hot air at 50-125°C for 3-10 hours, with the flow of countercurrent air to the pellets, so as to remove the odors. In this disclosure, “suitable for food contact” meant having less than 320 ppb of limonene (the threshold for approved food contact according to FDA). Limonene concentration of the pellets, produced by the process of this disclosure, is in the range 40-240 ppb. Therefore, this disclosure can provide not only FDA acceptable pellets for food and beverage container applications, but also it provides fragrance free pellets for household product containers. However, this disclosure does not deal with recycled vinyl aromatic based raw material. It is thus evident from the analysed prior art that processes based on falling strand decontamination for removing volatile compounds from virgin polystyrene are known. However, such documents do not disclose that the same process could be used to treat recycled polystyrene comprising various contaminants (volatile, semi-volatile and non-volatile). Moreover, some processes disclosed in previous prior art documents use harmful or corrosive reagents to remove the contaminants. In parallel, some documents address the problem of decontamination of polymers from post-consumer recycled (PCR) feedstocks, in particular some provide a process to produce food-contact grade polymers from PCR feedstocks. However, such processes typically are complex, involve several steps and units and do not deal with recycled polystyrene. F. Welle in "Recycling of post-consumer polystyrene packaging waste into new food packaging applications—Part 1: Direct food contact." Recycling 8.1 (2023), discloses the state of the art of the regulations on this subject and define the evaluation criteria for process developers. Depending on the country and regional entities (such as Europe), and depending on the material (PET, PE, vinyl aromatic materials), regulations on purification processes to produce polymers suitable for food-contact applications starting from recycled materials are different and at different stage of development. The regulatory framework in Europe is currently in development: according to EC regulation 282 / 2008, recycled plastic in direct contact with food can only be obtained from processes that have obtained a "safety assessment" by the European Food Safety Authority (EFSA) followed by an authorization from the European Commission. This regulation was recently replaced by 2022 / 1616, which introduces the "suitable" and "novel" recycling categories. As far as it is known, the processes for production of food-contact grade polymer from recycled vinyl aromatic feedstock to date are all "novel" i.e. they must be notified to the authority and a list of all the contaminants must be provided. EFSA's evaluation criteria for post-consumer plastics in direct contact with food are very conservative, ensuring a high level of consumer protection and, consequently, very strict criteria, in particular for concentration of contaminants, for use of polymers derived from post-consumer recycled material. Polystyrene, being a low diffusion polymer similar to PET, is, thus, a promising candidate for recycling. However, to date there is no clear guidelines for acceptance limits for recycled polystyrene for food contact grade applications. Indeed, as mentioned above, while for some plastics such as PET there are large experimental datasets, in the field of food-contact grade vinyl aromatic polymers there is lack of data and only few processes are under development. Accordingly, it is desirable to process recycled vinyl aromatic polymer for producing vinyl aromatic polymer for food-contact grade applications. Summary of the Invention To address the deficiencies of the prior art, the present invention provides a new process for the production of vinyl aromatic polymers for food-contact grade applications. In particular, the present invention combines cost and environmental benefits with improved decontamination efficiency over the known art. This is achieved by a process described in claim 1 and further by the subject-matters of the further independent claims. The invention further relates to food-contact grade vinyl aromatic polymers obtained according to the present process and to the use of the said food-contact grade vinyl aromatic polymers obtained according to the present process as claimed by claim 11 and 12, respectively. Further features and advantages of the disclosed subject matter, whether explicitly mentioned or not, will become apparent in view of the disclosure provided below. In brief, a decontamination process for recycled vinyl aromatic polymers for food-contact grade applications is disclosed, whose decontamination technology has been selected for achieving a level of decontamination such that the resulted vinyl aromatic polymer is used for food-contact applications. Indeed, differently to the known processes employing similar decontamination technology, the present invention applies this technology over a different and more contaminated feeding material. Said decontamination technology is, thus, a suitable super-clean technology showing high removal efficiency of most contaminants, which are present in the feeding material (such as polar and non- polar, volatile, semi-volatile and non-volatile compounds having boiling point at 0.1 bar(a) up to 300 °C, optionally compounds with metals). The decontamination technology according to the present invention is also a highly efficient and environmentally friendly process. The process according to the present invention is able to produce recycled vinyl aromatic polymers for food-contact grade applications. Surprisingly, the present process is not only able to remove contaminants with low boiling point (such as styrene and ethylbenzene) but also with high boiling point (such as benzophenone and methyl stearate). Other advantages are also achieved by the present process. For example, this process is: - simple and easy in terms, for example, of decontamination phase. This means that the present process achieves very good reduction in contaminants content by means of a single decontamination step, performed in one single decontamination apparatus; - quick: the decontamination step takes place in seconds; this also ensures little or no degradation of the treated polymer, even at elevated treatment temperatures; - efficient: the reduction of contaminants is very high, even for high boiling point and high molecular weight compounds. This means that the concentration of contaminants corresponds to values below the limits, where the limits are the values that would cause excessive migration of the contaminant into the food products that the package prepared from the treated recycled material would contain; - no need of aiding fluids: the present process does need neither injection of liquids or gases into the polymer to be decontaminated, nor stripping gases inserted in the decontamination apparatus; - able to remove or reduce a large spectrum of contaminants: not only high boiling point, but also both polar (such as chlorobenzene) and non-polar compounds (such as phenyl cyclohexane) are removed from the polymer. By the way, it can be considered surprising that also polar compounds can be removed without the use of polar stripping agents. Moreover, the proposed process has low energy requirements, requires low maintenance, can be easily cleaned and can be run very easily. Brief Description of the Drawings Further characteristics and advantages of the present invention will become clear from the following description, from practical and comparative non-limiting examples and with reference to the drawings attached, in which: Figure 1 shows a decontamination device used to carry out some of the process steps according to the present invention; Figure 2 shows a particular apparatus used to carry out the process according to the present invention and that was used for the Examples, comprising an extruder, a static mixer and a decontamination device according to Figure 1; and Figure 3 is the Gas Chromatography (GC) chromatogram of the material obtained from Inventive Example 6, which is according to the process of the present invention. Detailed Description In more details, the subject-matter of the present invention is a process for the production of food-contact grade vinyl aromatic polymer from a feedstock based on post- consumer recycled vinyl aromatic polymer, which can be used in high value-added applications. For instance, such high value-added applications for food contact are packaging and containers for fruits, vegetables, meats, fishes, cheese and other dietary products. Exemplary final products are, for example, yogurt jars, meat trays, hot drink cups. Indeed, the food-contact grade vinyl aromatic polymers, obtained according to the process of the present invention, are used for producing: food trays, for example, for fruit, vegetable, meat, fish, dairy, baking, sweet, deli, cereals, cheese, eggs; cups and containers for food, for example, for hot and cold beverages, coffee, soup, yogurt, ice cream and takeaway food; sheets, for example, for packaging fresh food products, for covering trays and plates and for making food pouches; bottles, for example, for cold beverage, water and fruit juices; blister packs for pre-cut fruits, vegetables, cheese, backed goods and snacks; plastic cutlery and plates. Preferably, the food-contact grade vinyl aromatic polymers obtained according to the process of the present invention, are used for producing: food trays; cups and containers for food; and sheets for packaging fresh food products. Since the vinyl aromatic polymer, obtained by means of the present process, is used in such specific and sensitive applications, it is required to reduce contaminants concentrations below specific limits, where the limits are the values that would cause excessive migration of the contaminants into the food from the packaging made of the vinyl aromatic polymer, obtained by means of the present process. Indeed, as disclosed below, the process of the present invention is able to reduce contaminants below the limits that ensure that the maximum dietary exposure of such contaminants is not more than 0.0025 μg / kg body per day for a toddler of 10 kg of weight, with all the contaminant migration scenarios disclosed by F. Welle in "Recycling of post-consumer polystyrene packaging waste into new food packaging applications—Part 1: Direct food contact." Recycling 2023, 8, 26. https: / / doi.org / 10.3390 / recycling8010026. Optionally, the vinyl aromatic polymer, produced through the present process, can undergo standard further processing, before being used for the above-mentioned applications. For standard further processing are meant all processes that transform plastic, typically in pellets, into the final product. This can be carried out by means of methods that are already used for virgin plastics, such as extrusion, molding or thermoforming, with or without expanding agents and / or other additives that are added for specific purposes, such as nucleating agents, inks, fillers, athermanous agents, and so on. The present process comprises many steps. It starts from step (a) of providing a feedstock based on post-consumer recycled vinyl aromatic polymer. Preferably, the post-consumer recycled vinyl aromatic polymer is post-consumer recycled polystyrene. In the following, feedstock or feedstock based on post- consumer recycled vinyl aromatic polymer indicate the same concept. For the purpose of the present description and the following claims, the expression “post-consumer recycled vinyl aromatic polymer” means a vinyl aromatic based plastic material obtained from a recycling process, said plastic material in particular coming from waste plastic materials derived from post-consumer consumption. For the definition of “pre-consumer”, “post-consumer”, “recycling”, and “waste”, reference is made to “Standard Guide for Waste Reduction, Resource Recovery, and Use of Recycled Polymeric Materials and Products” (ISO 15270:2008(E)). In the present document, the adjective “recycled” refers to a material that had been subjected to recycling. Waste plastic materials derived from post-consumer encompasses packaging for solid and liquid products based on vinyl aromatic polymer, such as food trays, for example, for fruit, vegetable, meat, fish, dairy, baking, sweet, deli, cereals, cheese, eggs; cups and containers for food, for example, for hot and cold beverages, coffee, soup, yogurt, ice cream and takeaway food; sheets, for example, for packaging fresh food products, for covering trays and plates and for making food pouches; bottles, for example, for cold beverage, water and fruit juices; blister packs for pre-cut fruits, vegetables, cheese, backed goods and snacks. In addition, this expression also encompasses the plastic cutlery, plates and food containers. Notably, the feedstock used in step (a) of the present process may or may not be certified as based on recycled material. Certification authorities include the APR in USA and the EuCertPlast in Europe, the latter being recognized by other recycling organizations such as the Italian National Consortium for the Collection and Recycling of Plastic Packaging (COREPLA) and Citeo. Some recycled products may also be labelled, such as “The Blue Angel” ecolabel in Germany. The feedstock based on post-consumer recycled vinyl aromatic polymer is obtained from the waste of end consumers, including both household users and commercial and / or industrial structures or institutes that are end users of the product that, preferably, was in direct contact with food. For instance, in canteens and other restaurants, plastic cutlery, plates, food containers thrown in the garbage bins by the final customers are included. Oppositely, waste from commercial or industrial structures or institutes that are not end users of the product are not included. For instance, plastic waste from a fridge manufacturer is not included as the fridge manufacturer is not an end user of the product. According to some embodiments, the feedstock based on post-consumer recycled vinyl aromatic polymer is obtained from the waste of household users. The feedstock of step (a) based on post-consumer recycled vinyl aromatic polymer comprises: (i) post-consumer recycled non-rubber reinforced vinyl aromatic polymer, preferably selected from the group consisting of general purpose polystyrene (GPPS), such as radical and anionic polymerized polystyrene, and styrene- alkyl-substituted styrene copolymers, such as styrene-alpha- methylstyrene copolymers, more preferably general purpose polystyrene (GPPS), and / or (ii) post-consumer recycled rubber reinforced vinyl aromatic polymer, preferably selected from the group consisting of high impact polystyrene (HIPS) and styrene-butadiene block copolymers, more preferably high impact polystyrene (HIPS); (iii) optionally, polymeric impurities, as defined here below; (iv) optionally, non-polymeric impurities, as defined here below; (v) contaminants, defined as compounds having molecular weight ranging from 50 to 400 Da and boiling point ranging from 0°C to 300°C at 0.1 bar(a), ranging from 0.0001 wt% to 0.5 wt%; and (vi) water ranging from 0 wt% to 9.9999 wt%; with the provision that the sum of the quantities (i), (ii), (iii), (iv), (v) and (vi) is 100%; with the additional provision that the sum of the quantity of (i) and (ii) is at least 90 wt%, preferably at least 95 wt%; with the further provision that the sum of the quantity of (iii) and (iv) ranges from 0 wt% to 9.9999 wt%, preferably from 0.1 wt% to 4.9999 wt%, even more preferably from 0.2 wt% to 2 wt%. Notably, the weight percentages are computed with respect to the total mass of the feedstock based on post- consumer recycled vinyl aromatic polymer. Preferably, the rubber, contained in (ii) the post- consumer recycled rubber reinforced aromatic vinyl polymer, such as HIPS, ranges from 0.1 wt / wt% to 9.0 wt / wt%, where the percentage is computed with respect to the total mass of the post-consumer recycled rubber reinforced vinyl aromatic polymer. Preferably, the weight average molecular weight of the post-consumer recycled non-rubber reinforced and rubber reinforced vinyl aromatic polymers, in particular GPPS and HIPS polystyrene, is in the range from 100 kDa to 400 kDa, particularly preferably in the range from 150 kDa to 300 kDa, measured by means of gel permeation chromatography against polystyrene standards. For the purpose of the present description and the following claims, “polymeric impurity” (iii) are selected from the group consisting of acrylonitrile-butadiene styrene polymers (ABS), styrene-acrylonitrile polymer (SAN), acrylonitrile-styrene-acrylate (ASA), styrene acrylates, such as styrene-methyl acrylate and styrene-methyl methacrylate (SMMA), styrene maleic anhydride (SMA), methyl methacrylate- butadiene styrene (MBS), methyl methacrylate- acrylonitrile-butadiene-styrene (MABS) polymers, styrene-N- phenylmaleimide copolymers (SPMI), polyamides (PA), polyolefins, e.g. polypropylene (PP) or polyethylene (PE), polyacrylates, e.g. polymethyl methacrylate (PMMA), polycarbonate (PC), polyesters, e.g. polyethylene terephthalate (PET) or polybutylene terephthalate (PBT), polyphenylene ethers (PPE), polyether sulfones (PES), polyetheretherketones (PEEK), polyether sulfides, , rubbers, e.g. polydienes, biodegradable aliphatic and aliphatic / aromatic copolyesters, and a mixture thereof. In any case, polymers as defined in point (i) and (ii) of the present invention do not include polymeric impurities as defined in point (iii). For the purpose of the present description and the following claims, “non-polymeric impurity” (iv) comprises anti-oxidants, UV stabilizers, nucleating agents, pigments, inorganic additives. Inorganic additives comprise inorganic carbon, in particular carbon black, graphene, coke, graphite; metal salts such as calcium carbonate, calcium sulphate; perovskites and ilmenites such as metal titanates; metal oxides such as titanium oxide, aluminum oxide, iron oxide; metal dichalcogenides; silicates and especially phyllosilicates such as hydrated magnesium silicate, serpentine, chlorite, micas, clay. The sum of polymeric (iii) and non-polymeric (iv) impurities in the feedstock ranges from 0 wt% to 9.9999 wt%, preferably from 0.1 wt% to 4.9999 wt%, even more preferably from 0.2 wt% to 2 wt%. For the purpose of the present description and the following claims, term “contaminant” (v) comprises polar, non-polar, volatile, non-volatile compounds different from water, regardless of their origin, and having molecular weight ranging from 50 to 400 Da and boiling point ranging from 0°C to 300°C at 0.1 bar(a). Polymeric impurities (iii) and non-polymeric impurities (iv) in any case do not include contaminants (v) as defined above. That means, for instance, that for the purpose of the present invention, lauric acid (C12H24O2, molecular weight 200.3 Da, boiling point 220.5°C at 0.1 bar(a)) or phenyl acetic acid (C8H8O2, molecular weight 136.15 Da, boiling point 191.8°C at 0.1 bar(a)) shall be considered as contaminants (v) and not non-polymeric impurities (iv). Viceversa, for instance, sodium chloride (molecular weight 58.44 Da and boiling point at 0.1 bar(a) equal to 1185°C), or 1-phenyl- 2-benzyl-2-heptadecene (having molecular weight 404.68 and boiling point 347.8°C at 0.1 bar(a)) shall be considered as non-polymeric impurities (iv) and not contaminants (v). All compounds as defined above (having molecular weight ranging from 50 to 400 Da and boiling point ranging from 0°C to 300°C at 0.1 bar(a)), that are comprised in the feedstock of step (a) based on post-consumer recycled vinyl aromatic polymer, shall be considered contaminants, irrespectively of their origin. The feedstock based on post-consumer recycled vinyl aromatic polymer, provided during step (a) of the present process, can be produced according to any recycling process known in the art. Examples of recycling processes include the sorting process that can separate plastics based on their properties, such as density (e.g. by flotation) and real- time sorting based on optical properties, such as near infrared spectroscopy based techniques. Alternatively, high- vinyl aromatic content feedstocks can be produced from application-specific post-consumer sources that are known to use vinyl aromatic based plastics by a large amount, such as seed trays, meat and vegetable trays, hot drinks cups, and so on. Optionally, the feedstock can be pre-treated before step (a) for concentrating the vinyl aromatic content and / or for removing organic residues that are detrimental for its final use in food-contact applications. This pre-treatment step can be carried out by single or multiple washing steps in water, and / or by adding specific additives that can interact with this raw material. For instance, sodium, potassium and calcium oxide or hydroxides, as well as carbonates can be added to the post-consumer recycled vinyl aromatic based polymer for this scope. In addition, the feedstock can be optionally pre-treated to reduce the odors, in particularly odors that are produced by fermentation of the organic material. Some techniques include, for instance, treatments with ozone, hydrogen peroxide, sodium hypochlorite. At the same time, the pre-treatment step can be performed for preliminarily reducing the content of some volatile organic (and inorganic) compounds by means of conventional techniques. Such techniques include, for instance, low temperature (such as in the range of 70 to 150°C) degassing, as disclosed in US 9,914,156, or high temperature (such as in the range of 200 to 280°C) melt extrusion degassing. Degassing can include stripping agents such as nitrogen, nitrous oxide or carbon dioxide, directly added or produced in-situ by decomposition of carbonic compounds such as sodium hydrogen carbonate or azodicarbonamide. Alternatively, the pre-treatment step can be carried out in an extruder. Such pre-treatment may be useful in particular when high quantities of volatile organic compounds are present in the feedstock; however, it is not mandatory as the present process is very effective even without other conventional VOC-reduction treatments. Optionally, the pre-treatment step is followed by a mixer for homogenizing the pre-treated feedstock. In regard to step (a) of the present process, there are no particular limitations. Therefore, any process known in the art can be used to this scope. For example, the feedstock can be provided in solid state, for instance, in form of granules, or already in the melt state. The latter case (melt state) is typical when the feedstock is pre- treated in the melt state, e.g. by extrusion, for instance for preliminary reduction of volatiles. Following step (a), the present process optionally includes the step of melting the feedstock and, thus, obtaining a polymer melt feed. Obviously, this step is omitted when the feedstock, provided in step (a), is already in a melt state, for example, as a result of the previous pre-treatment step by means of the devolatilizing extruder. In detail, the step of melting the feedstock is carried out at the temperature ranging from 160°C to 240°C, preferably from 170°C to 220°C. Also, for this step, there are no particular limitations. For example, it can be carried out by employing conventional means, for instance heat exchangers or extruders. The latter means is particularly effective in melting feedstock based on post-consumer recycled vinyl aromatic polymer. Other means include heating by heating jackets where suitable thermal oil flows, gas heating by means of flow of suitable inert gases such as carbon dioxide, nitrogen, argon; or optical heating such as infrared or microwave heating. Regardless of the presence or absence of the optionally step of melting the feedstock, the feedstock in the form of a polymer melt feed is brought to a decontamination device, step (b) of the present process, where the polymer melt feed is passed through at least one aperture (step (c)), thus forming at least one polymer strand that falls by gravity in a closed vessel (step (d)) and will continue in the subsequent steps of the present process. With reference to figure 1, an example of a decontamination device 1 is depicted, which is used for performing the falling strand decontamination steps according to the present invention, starting with step (b). The decontamination device 1 comprises at least one inlet section 3, for receiving the polymer melt feed at a target temperature T, at least one distributor 5 with at least one aperture 6, a closed vessel 10, which is connected with the distributor 5 and is provided with at least one polymer outlet 12 and with at least one vapour outlet 8. Preferably, the decontamination device 1 receives the polymer melt feed, via the inlet section 3, at the target temperature, i.e. at the temperature ranging from 210°C to 300°C, preferably from 220°C to 290°C, more preferably from 225°C to 280°C, even more preferably from 230°C to 270°C. If not yet at that temperature, the polymer melt feed can be brought to said target temperature T, for instance by means of a heat exchanger driven by a flow of athermanous fluid. The inlet section 3 can be a duct with one aperture or several apertures from which the polymer melt feed is brought into the decontamination device 1 in step (b). Once the polymer melt feed has brought into the decontamination device 1 within the step (b) of the present step, this polymer melt feed is then passed through the at least one aperture 6 in step (c) of the present process, thereby forming at least one polymer strand. In details, the inlet section 3 is in communication with at least one distributor 5. However, the decontamination device 1 can also comprise multiple distributions 5: a first distributor may split the polymer melt feed in few polymer channels, a second distributor may distribute the polymer melt feed inside each polymer channel into the at least one aperture 6 of step (d). The shape of the distributor 5 is not limited to the linear one. In particular, circular, ellipsoid or polygonal shapes can be used. The length of each polymer channel is not restricted to be the same for all polymer channels. For instance, the distributor 5 can be in the shape of a spider net, where, for example, a first distributor 5 distributes the polymer melt feed to a second concentrical distributor 5, resulting in annuli having different radiuses and, thus, different circumferences. Alternatively, the shape of the distributor 5 may be not linear but circular or with circular hollow profile, for instance forming an annulus. Hollowed or non-hollowed polyhedral, such as rectangular, hexagonal or square-shaped distributors are also possible. There are no particular limitations to the material of the distributor 5. Typically, the distributor 5 is made in metal, in particular in steel, inconel, incoloy, brass, bronze or titanium alloys. To ensure a good distribution of the polymer melt feed (that is, almost the same flow rate for each aperture 6), the pressure drop of the polymer melt feed from the inlet section 3 to the distribution aperture 6 should be as low as possible. In particular, it may be advantageous that the pressure drop of the polymer melt feed from the inlet section 3 to the distribution aperture 6 is no more than 1 / 10th of the pressure drop of the polymer melt feed passing through the apertures 6 in step (c). That typically means that the equivalent diameter of the distributor 5 and possibly of its polymer channels is much larger than the equivalent diameter of the apertures 6, for instance the ratio of said equivalent diameters is at least 10, more preferably from 20 to 400, even more preferably from 30 to 300. In the present disclosure, the equivalent diameter is meant as the ratio of the area of the aperture by its perimeter, multiplied by four. For instance, for a circular aperture, its equivalent diameter is equal to the diameter of the circle. The equivalent diameter at the aperture exit, defined as Dex, is the equivalent diameter of the apertures 6 at the aperture exit. In case the apertures 6 have different equivalent diameters, Dex is computed as the number-averaged equivalent diameters of the apertures 6 at the aperture exit. The number of apertures 6 is at least one. When the number of apertures is more than one, the number of apertures per the internal sectional area of the closed vessel 10 is preferably from 10000 [m-2] to 100000 [m-2], even more preferably from 20000 [m-2] to 50000 [m-2], where the sectional area is the area of the internal section of closed vessel which is perpendicular to the vertical direction. When the number of apertures is more than one, the distance between the apertures 6, one another, is preferably between 1.1 and 10 times the equivalent diameter of the apertures at the exit Dex, more preferably between 2 and 5 times. An optional booster polymer pump can be used between step (a) and (b). Such pump can be useful to increase the polymer pressure up to the value required for the polymer melt feed to pass through at least one aperture 6 of step (c). Optionally the pump speed can be regulated so as to ensure that the mass flow rate of the polymer melt feed through this at least one aperture 6 in step (c) is within specified ranges. In case such a pump is not foreseen, the same result can be obtained by an upstream pressurizing unit (such as the above mentioned degassing extruder for pre- treatment step). However, in such case there is generally less flexibility, in particular if it is required to vary the mass flow of the polymer melt to ensure that the mass flow per aperture of step (c) stays within the specified ranges. Regardless the shape and the number of distributors 5, each distributor 5 has a number of apertures 6, which can be carried out in any way known in the art. For instance, the distribution apertures 6 can be produced by means of drilling. To obtain apertures 6 of specific shape, or for low diameter holes, or, for non-circular apertures, such as slots, when the minor size of the aperture is small (that is, less than about 1 mm), or to increase the accuracy of the aperture dimensions, other methods can be used. For instance, water cutting or electroerosion cutting (electrical discharge machining). Alternatively, the distributor 5 itself can be produced with the apertures 6. For instance, it can be produced by molding, such as by lost wax technique, or with additive manufacturing, such as metal 3D printers. The at least one aperture 6 is substantially vertically oriented with respect to the closed vessel 10. By substantially vertically oriented aperture 6 is meant that the angle between the axis of the aperture and the direction of gravity is low, preferably no more than 60°, more preferably no more than 45°, even more preferably no more than 30°, and even more preferably no more than 15°. The at least one aperture 6 of step (c) can be of any shape. In particular, it can be tapered or not-tapered bore or slot. For slots, the section can be rectangular or polyhedral, and in the section, the major dimension is at least 5 times the minor dimension, preferably from 10 to 1000 times. For bores, the section can be for instance polyhedral, circular or elliptical or star-shaped. Preferably it is circular or elliptical, even more preferably circular. The tapering can increase or reduce the sectional area of the aperture 6 from inlet to outlet. Typically, the tapering reduces the sectional area as the polymer melt flows from the inlet to the outlet. If tapering is present, the angle between the axis of the aperture and the lateral wall of the bore is no more than 60°, preferably between 1° and 45°, even more preferably between 2° and 30°, where for angles “°” denotes the sexagesimal degree. Preferably, the bores have circular symmetry. Preferably, the bores are not tapered. For the present process, it is essential that the apertures 6 have an equivalent diameter at their exit (Dex) that is at least 0.3·10-3m, preferably at least 0.8·10-3m, even more preferably at least 1.0·10-3m. Beside Dex, the apertures 6 are also defined based on another parameter, namely the aperture ratio (Dr), which is the ratio between Dex and the critical diameter Dcrit. Notably, for the computation of Dr, the equivalent diameter of the apertures 6 at the exit (Dex) of the aperture towards the closed vessel 10 shall be considered (in fact, for tapered apertures, said diameter is not constant along the axis of the aperture). Specifically, in case of apertures 6 having different diameters, as disclosed before, the equivalent diameter (Dex) to be considered for the computation of the aperture ratio Dr is the number-averaged equivalent diameter of the apertures. Critical diameter (Dcrit) of the distributor aperture 6 corresponds to the maximum diameter of the aperture 6 which, when the polymer melt feed passes through it, does lead to the formation of a stable polymer strand within step (c). Therefore, Dcrit is linked with the formation of a stable polymer strand. To ensure the production of food-contact grade vinyl aromatic polymer according to the present process, Dr needs to be at most 1.0, preferably at most 0.8, even more preferably at most 0.7. In fact, it was found that when the Dex, as well as Dr, of the apertures was too high, polymer strands can not be formed, as dripping or intermittent jetting can be observed instead. It was found that the dripping and intermittent jetting regimes are not as efficient as the melt strand regime in removing the contaminants, as it will be shown below in the examples. In detail, Dcrit is defined according to following equation (1) 5 1 1 2 Dcrit = 5.8 ∙ ^^6 ∙ ^^−3 ∙ (g ∙ μ)6 ∙ ^^−3 (1) wherein: - ^^ (kg / s) is the mass flow of the polymer melt feed flowing out of the apertures divided by the number of apertures 6 (i.e. it is the average mass flow per aperture), - ^^ (kg / m3) is the polymer density of the polymer melt feed, calculated according to the following equation (2) ^^ =1257 (2) exp (0.000 -= 9.8^^ is gravitational acceleration on Earth, - μ (Pa·s) is the dynamic viscosity, assumed to be the viscosity of the polymer melt feed at the temperature T at the inlet section 3 and at shear rate of 0.1 s-1, - ^^ (kg / s2) is the surface tension of the polymer melt feed, calculated according to the following equation (3) ^^ = 0.0407 − 0.000072 ∙ (^^ − 293.15[^^]) (3)- ^^ (K) is the temperature of the polymer melt feed at the inlet section 3. The dynamic viscosity μ can be measured, for instance with a capillary rheometer, or can be computed if a suitable model to compute dynamic viscosity is known (for instance, for a Cross-WLF model, see e.g. M.A. Peydró et al in “Study of rheological behaviour during the recovery process of high impact polystyrene using cross-WLF model”, J. Appl. Polym. Sci., 120: 2400-2410). In case viscosity is measured with said capillary rheometer, as in the following Examples, the capillary measurements are performed using a Göttfert Rheograph 2002 with a 12 mm diameter barrel using a tungsten carbide capillary with length L=30 mm and diameter D= 1mm (entry angle=90°). However, if the dynamic viscosity of the post-consumer recycled vinyl aromatic polymer based feed is unknown, the following relationship (equation (4)), which is derived from cross-WLF model, shall be applied to estimate its value (in Pa.s) : where: - T (°C) is the temperature of the polymer melt feed at the inlet section 3; - MFI is the melt flow index of the post-consumer recycled vinyl aromatic polymer measured at 200°C, with a load of 5kg, expressed in g / 10’, and -the values of the numeric constants are: ^^0 = 675 ∙ 103; ^^1 = 4.8; ^^2 = −42; ^^3 = 4.7[s]; ^^4 = 180; ^^ = 0.7; ^̇^=0.1[s-1]Equation (1) was defined by the Applicant but is based on the theory of the stability of the jetting regime, as opposite to dripping and intermittent jetting, as developed by Buggisch and Sauter (Sauter, Buggisch, Stability of initially slow viscous jets driven by gravity. Journal of Fluid Mechanics. 2005; 533:237-257. Doi: 10.1017 / S002211200500412X). More in particular, it is derived from the expression for the line of marginal stability of a long liquid jet subjected to forces corresponding to viscosity, inertia, surface tension and gravity. The marginal line, which is a relationship between adimensionalized nozzle cross section area and adimensionalized exit velocity, separates the jet pulsation regime with the stable jet regime. The relation by Sauter and Buggisch was not developed for polymers in the present process conditions; however, the Applicant, based on experimental results, found that it could be adapted to equation (1), which showed to be able to predict stable formation of polymer strands in the conditions of the present invention. Following step (c), the at least one formed polymer strand is let it drop into the closed vessel 10 within step (d) of the present process, thereby obtaining a decontaminated vinyl aromatic polymer in closed vessel 10 and removed contaminants. Indeed, the at least one aperture 6 is above the closed vessel 10 in such a way that passing through this at least one aperture 6 within step (d), the polymer melt feed flows from the distributor 5 into the closed vessel 10 in the downward direction and in the form of polymer strands. Therefore, in step (d), the polymer strands drop by gravity inside the closed vessel 10. Optionally, the polymer strands, dropped into the closed vessel 10, form a polymer strand pool. In such a case the height of the polymer strands is lower, since the closed vessel 10 is filled with the decontaminated vinyl aromatic polymer making the drop in step (d) from the at least one aperture 6 to the closed vessel 10 shorter. This reduction of the height of the polymer strands, generally, reduces the efficacy of the decontamination. However, according to the present invention, increasing the height of the strands does not always increase the efficacy of the treatment. This is because, depending on the viscosity and mass flow rate per aperture, the melt polymer strands become unstable and break. In particular, it was found that the minimum mass flow ratio ^^^^^^^^^^, defined as the ratio between the mass flow ^^ (as defined above, for equation (1)) and the critical minimum mass flow rate needs to be at least 1.0, preferably at least 1.1, even more preferably at least 1.2. Specifically, the critical minimum mass flow ^^^^^^^^corresponds to the mass flow value below which the polymer strand breaks (due to excessive thinning and growth of capillary instability) and takes into account the height of the polymer strand. Therefore, the minimum mass flow ratio ^^^^^^^^^^is another parameter representing the decontamination efficiency, which needs to be considered for using the vinyl aromatic polymer, obtained within the present process, in food-contact grade applications. The critical minimum mass flow ^^^^^^^^is calculated according to the following equation (5): 5 5^^ ^^^^^^^^ = 134.0 ∙ ^^ ∙ ^^2+^^ ∙ (^^2+^^^^)∙ (5) wherein: 4 - ^^ is a numeric constant having value equal to 3, - ^^ (kg / m3) is the density of the polymer melt feed, calculated according to equation (2), -^^ = 9.8^^ 2 is gravitational acceleration on Earth, ^^ - ^^ (Pa.s) is the dynamic viscosity, assumed to be the viscosity of the polymer melt feed at the temperature T (see below) at the inlet section 3 (see below and Figure 1) and at shear rate of 0.1 s-1, - ^^ (kg / s2) is the surface tension of the polymer melt feed, calculated according to equation (3), - ^^ (m) is essentially the maximum length of the polymer strand, - ^^ (K) is the temperature of the polymer melt feed at the inlet section 3. In regard to ^^ within equation (5), when there is the polymer strand pool inside the closed vessel 10, it is the maximum vertical-axis-projected distance (that is, the distance measured along the axis of gravity) between the exit of the apertures 6 and the underlying polymer strand pool. Therefore, in this case ^^ is the maximum difference between the height of the exit of the apertures 6 and the height of the polymer melt pool. However, when no polymer strand pool is present, or it is of unknown height (for instance, if the polymer strand pool height is not measured or its height cannot be estimated otherwise), ^^ is computed as the maximum vertical-axis-projected distance (that is, the distance measured along the axis of gravity) between the exit of the apertures 6 and the polymer outlet 12 on the bottom of the closed vessel 10. This is a safe criterium as the allowed polymer mass flow range narrows. In some embodiments, the maximum length of the polymer strand, H, is at most 5 m, preferably between 0.5 m and 4 m. As well as for equation (1), if the viscosity ^^ in equation (5) of the post-consumer recycled vinyl aromatic polymer based feed is unknown, it can be calculated by equation (4). Equation (5) was defined by the Applicant but is based on the theory of the breakup of liquid jets, first described by Plateau and Rayleigh. Starting from that theory, Javadi et al., "Delayed Capillary Breakup of Falling Viscous Jets", Physical Review Letters 110 (2013) 10.1103 / PhysRevLett.110.144501, in the following referred as Javadi, developed a model to predict liquid jet breakup for viscous silicon oils, based on adimensional analysis. More in particular, Buckingham’s PI theorem implies that the adimensional number corresponding to strand breakup length Π^^is a function of the adimensional number of viscosity Π^^, the adimensional number of surface tension Π^^and the adimensional number of the aperture equivalent diameter Π^^: Π^^ = Π^^). For viscous fluids, Π^^ is a function ofonly according to equation (6): Π= ^^^ ^^ ^ ∙ Π^^(6) where q is 4 / 3 (see equation 4.39 in Javadi). Equation (5) derives from the algebraic development of equation (6) and by experimental fitting of constant C. The experimental relationship by Javadi was developed for viscous silicon fluids, which do not behave like polymers. However, it was found by the Applicant of the present invention that the equation (5) was in good agreement with the experiments on polymeric falling strands. Together with the ^^^^^^^^^^parameter, another parameter needs to be taken into account for ensuring an efficient decontamination. This parameter is the maximum mass flow ratio ^^^^^^^^^^, defined as the ratio between the mass flow rate ^^ (as defined above, in equation (1)) and the critical maximum mass flow rate ^^^^^^^^. Specifically, the critical maximum mass flow ^^^^^^^^corresponds to the mass flow value above which the residence time, for the process conditions employed, is insufficient for removing the contaminations to values below the limits, where the limits are the values that would cause excessive migration of the contaminant into the food products that the package prepared from the vinyl aromatic polymer, obtained according to the present process. It turns out that also ^^^^^^^^^^is a parameter representing the decontamination efficiency of the present process, which needs to be at most 1.0, preferably at most 0.8, even more preferably at most 0.7. The critical maximum mass flow ^^^^^^^^is calculated according to the following equation (7): ^^ = 3.9 ∙ 10−19 ∙ 9.366^^^^^^ ^^ ∙ ^^ ∙ exp [(0.088 − 2.945 wherein: - ^^ (m) is essentially the maximum length of the polymer strand. All the above-reported considerations on ^^ regarding the presence or absence of the polymer strand pool are still valid; - ^^ (°C) is the temperature of the polymer melt feed at the inlet section 3; - ^^^^ (Da) is the value of the molecular weight of the contaminant (v) having the highest molecular weight. In absence of specific information, the ^^^^ is set to the default value of 400 [Da]. In fact, among the contaminants that were considered by EFSA and Welle, the one having the highest molecular weight is methyl stearate, having molecular weight equal to 298.5 [Da]. Therefore, it was assumed safe to define ^^^^=400 [Da]. Though not bound to any specific theory, it is believed that the ratio of ^^^^^^^^ / ^^ is inversely proportional to the residence time of a polymer molecule and contaminants on the strand. Temperature has a large effect both on vapor pressure and diffusion coefficient, increasing both when its value is increased. Therefore, it is somewhat reasonable that a smaller time could be required when operating at higher temperatures. Molecular weight has a large effect on the diffusion coefficient, and therefore its influence on the required decontamination time can also be reasonably expected. However, in the prior art no indication at all is found that the above parameter should preferably hold so as to obtain a decontamination level that is adequate to the present goal. Together with ^^^^^^, ^^^^, ^^^^^^^^^^and ^^^^^^^^^^, another essential parameter is the pressure inside the closed vessel 10, which needs to range from 1 to 8000 Pa (abs), preferably from 10 to 6000 Pa (abs), more preferably from 50 to 3500 Pa (abs), even more preferably from 100 to 2000 Pa (abs). In regard to the closed vessel 10, where step (d) of the present process has occurred, it can be any apparatus, equipment or device that is able to contain polymer strands and decontaminated vinyl aromatic polymer, at the specified conditions of temperature and pressure. For instance, such closed vessel 10 can be made of metal, such as steel, especially stainless steels such as austenitic or duplex steels, aluminium, titanium, bronze and so on. The distributor 5 and the closed vessel 10 can be a single and integrated unit or can be two separated units. In any case, the apertures 6 of the distributor 5, where the polymer melt feed exits to form strands, shall reside in the closed vessel 10. If the distributor 5 is separated from the closed vessel 10, it can be located on the top of the closed vessel 10. Advantageously, in this way it is possible to produce the apertures 6 directly in the shell that closes the closed vessel 10 on the top. If the distributor 5 is integrated into the closed vessel 10, typically the closed vessel 10 has just at least one inlet for the polymer that is fluidly connected with said distributor, that typically resides completely at the interior of the closed vessel 10, in its top part. The shape of the closed vessel 10 is not particularly limited. For instance, it can be a cylindrical shaped shell, with flat or dished heads on the top or bottom. The ratio between the height H and the closed vessel equivalent diameter ^^^^^^^^^^^^^^is not particularly limited to any range. For instance, it can be between 0.1 and 8, preferably between 0.2 and 5. The closed vessel 10 can be provided with devices and equipment, shown in Figure 2, which facilitate a smooth and efficient running such as sensors for measuring polymer melt temperature 4, vessel pressure 2 and vessel level, and sight glasses 9 or cameras for inspection. Vessel pressure transmitter can be located also outside the closed vessel 10, for instance in the vapor outlet 8 or even in the condenser (if present) which recovers the condensable vapours, as long as the pressure drop is negligeable, and they are fluidly connected to the closed vessel 10. The vessel level transmitter can be of any type, such as differential ones (DP-cell type), radar-based, ultrasound-based or radiometric-based ones. Following step (d), the obtained decontaminated vinyl aromatic polymer is recovered in step (e) of the present process by means of the at least one polymer outlet 12. For example, the at least one polymer outlet is an exit nozzle positioned at the bottom of the closed vessel 10. Alternatively, the decontaminated vinyl aromatic polymer can be recovered by a retractable spoon made, for example, in copper. This spoon can be inserted laterally on the bottom of the closed vessel 10. For example, the decontaminated vinyl aromatic polymer is recovered in step (e) by means of a suitable device, such as a gear pump. Optionally, the recovered decontaminated vinyl aromatic polymer can then be pelletized. Together with the recovering of the decontaminated vinyl aromatic polymer within step (e), the removed contaminants present in vapours (possibly comprising also other gases such as nitrogen) are also recovered in step (f) of the present process by means of the at least one vapor outlet 8, which is kept under vacuum by means, for example, of vacuum pumps or ejectors. The at least one vapor outlet 8 is also in communication with the closed vessel 10. Optionally, the recovered volatiles are condensed prior to pass through vacuum pump. There are no particular limitations on the condenser that can be used. For instance, the condenser can be a reflux condenser. When the condenser is used, the vacuum pump can be positioned on the duct removing the uncondensed gases. With or without condenser, there are no particular limitations on the type of vacuum pump that can be used. Both wet and dry vacuum pumps can be used. Example of vacuum pumps that can be used to the purpose include ejectors, liquid ring vacuum pump, rotary claw vacuum pump, rotary screw pumps, side channel blowers. Although a single polymer outlet 12 and vapor outlet 8 are drown in figure 1, it is intended that more vapour and polymer outlets can be provided. A particular apparatus which is suitable to carry out the process according to the present invention is shown in Figure 2. With reference to Figure 2, it is shown an apparatus 20 used to carry out the process steps of the present invention, and that was used for the Examples. The apparatus 20 comprises elements, related to the melting of the feedstock based on post-consumer recycled vinyl aromatic polymer prior to step (b) of the present process, and a specific decontamination device according to the decontamination device 1 of Figure 1. More in detail, polymer in solid state is fed to a hopper 14 of an extruder 15. The extruder comprises 4 zones (Z1, Z2, Z3 and Z4) of thermoregulation. An outlet 16 of the extruder 15 is flanged to a vertical SMX static mixer 17, heated at the target temperature by a thermal fluid regulation, that is aimed to homogenize the polymer temperature. The static mixer 17 is connected to the inlet section 3 of decontamination device 1. A temperature transmitter 4 located at the exit of the static mixer 17 measures the temperature T of the polymer melt. The polymer melt coming from the static mixer 17 is fed to the decontamination device 1. As shown in Figure 1, the decontamination device 1 comprises: - one inlet section 3, for receiving the polymer melt feed at a target temperature T; - one distributor 5 with one aperture 6. The distributor is connected to a closed vessel 10. The closed vessel 10 consists of two parts, a top part 18 and a bottom part 13, that are flanged each other. The closed vessel 10 is provided with: - to the polymer outlet 12'; - at least one vapour outlet 8, where the pressure of the closed vessel 10 is measured by the pressure transmitter 2. In addition, the decontamination device 1 of Figure 2 further comprises: - a heating jacket 7 to ensure that the temperature of the decontamination device 1 is kept to the temperature T measured by temperature transmitter 4; - a polymer outlet 12’, located in the bottom part 13, corresponding to the polymer outlet 12 of Figure 1, and consisting in the receiving portion of the retractable spoon 11, which is able to collect the falling polymer strand; and - a sight glass 9, which allows the visual inspection of the falling polymer strand. EXEMPLARY MODES OF CARRYING OUT THE TEACHING OF THE PRESENT DISCLOSURE Inventive and comparative examples are reported here by way of illustration and are not intended to limit the invention. ANALYTICAL TECHNIQUE USED FOR ANALYSING THE EXAMPLES Gas chromatography used to quantify the total concentration of contaminants: The determination of the concentration of the contaminants in the specifically contaminated vinyl aromatic polymer (i.e. the surrogates in the challenge test) were measured by gas chromatography (GC). The measured contaminants were: toluene, chlorobenzene, methyl-salicilate, 1,3-di-isopropylbenzene, benzophenone, methyl-stearate. Response factor for each Additive i: A response factor ^^^^ was calculated from a solution with a known concentration of additive i. In particular, a solution was prepared, in a 3 / 8 w / w CHCl3 / EtOH mixture, containing 45 ppm w / w of each additive i and 45 ppm w / w of n-dodecane (the internal standard). The solution thus obtained was analyzed through the GC HP8890 (Agilent Technologies) equipment, equipped with "split / splitless" injector, flame ionization detector (FID detector), and autosampler. The following were the instrumental parameters used for the analysis: - Capillary column: Stationary phase: HP-1, length 30 m, inner diameter 0.53 mm, thickness 2.65 μm; - Carrier gas: H2, start at 3.75 ml / min, stay at 3.75 ml / min for 30 min, then rise with slope 3 ml / min per minute up to reaching 7 ml / min, then keep at 7 ml / min until end of run; - Injection type: splitless (40 ml / min for 0.5 min); - Liner type: Ultra Inert with glass wool; - Injector temperature: 280°C; - Detector temperature: 300°C; - Oven program: start at 40°C, isotherm at 40°C for 4.8 min, increase 8.3°C / min up to 250°C, increase 40°C / min up to 260°C, isotherm at 260°C for 15 min; - Injection volume: 1 μl. The response factor ^^^^ for each analyte i was defined as follows: where: ^^^^ is additive i concentration in ppm w / w, ^^^^^^^^ is n-dodecane concentration in ppm w / w, ^^^^ is additive i peak area, ^^^^^^^^^^ is n-dodecane peak area. Sample preparation: sample was homogenized through cryogenic grinding (liquid N2). 0.5 grams of ground sample (precisely weighted and defined as ^^^^) were then dissolved in 3 mL of a an "internal standard solution" of n-dodecane (the internal standard). This "internal standard solution" consisted of a solution at 50 ppm concentration of n-dodecane (the internal standard) in CHCl3. Both the exact concentration of the "internal standard solution" (defined as "^^^^^^^^^^^^^^^^") and the exact weight of the 3 ml of the "internal standard solution" (defined as "^^^^^^^^^^") were recorded. The resulting solution was then added dropwise to 8 ml of ethanol under magnetic stirring, which resulted in slow precipitation of the clean polymer. The sample thus obtained was filtered by syringe filter to separate traces of solid from the liquid phase and the latter was also analyzed through the GC HP8890 (Agilent Technologies) equipment, equipped with "split / splitless" injector, flame ionization detector (FID detector), autosampler, according to the same instrumental parameters defined above. The concentration of each additive i (^^^^) in the sample, expressed in ppm, was then calculated as follows: ^^^^ =(^^^^∙^^^^^^∙^^^^) (9) (^^^^^^^^^^∙^^^^) where: ^^^^ is Additive i peak area, ^^^^^^^^^^ is n-dodecane peak area (retention time 10.690 min), ^^^^ is the response factor for Additive i, ^^^^ is the weight in g of the ground sample, ^^^^^^ is weight in μg of the n-dodecane used. ^^^^^^ was defined as the weight in μg of the n-dodecane in the final solution. It was computed by multiplying the concentration of the internal standard solution for the exact weight of the 3 ml of that same solution used in the sample preparation. Therefore: ^^^^^^ = ^^^^^^^^^^^^^^^^ ∙ ^^^^^^^^^^ (10)Figure 3 shows the chromatogram of Inventive Example 6, obtained as explained below. EXAMPLE 1 PREPARATION OF THE FEEDSTOCK FOR EXAMPLES 2 TO 13 The feedstock used for the Examples 2 to 13 is not based on post-consumer recycled vinyl aromatic polymer, instead it has been prepared a surrogates-into-virgin polymer, following the Welle’s approach in Welle, F., Recycling 2023, 8, 26, https: / / doi.org / 10.3390 / recycling 8010026 "Recycling of post-consumer polystyrene packaging waste into new food packaging applications—Part 1: Direct food contact.". This approach has been adopted for the following reason: the feedstock based on post-consumer recycled vinyl aromatic polymer is very variable and, thus, the content of contaminants is not constant. A first advantage of using a virgin polymer is that the resulting feedstock is homogeneous, therefore allowing high reproducibility and reliability of results. Secondly, using a virgin polymer it is possible to evaluate the performances of the decontamination of the selected contaminants, that is without the interference due to other possible compounds present in the recycled material. The surrogates that were chosen are representative of the different possible categories of contaminants (more or less volatile, polar or non-polar) and following the above- mentioned Welle’s approach. Following this approach, instead of using actual feedstock based on post-consumer recycled vinyl aromatic polymer, decontamination tests were carried out on a virgin vinyl aromatic based feedstock comprising a defined choice of contaminants in a defined quantity, that are used as model compounds, i.e. compounds that can mimic the real contaminants, and that, thus, will be called “surrogates” in the here below. The virgin vinyl aromatic based feedstock that was chosen for this purpose was high-impact polystyrene (HIPS), as generally it is the vinyl aromatic material mostly present in the post-consumer recycled vinyl aromatic polymer. Moreover, for all the Examples the polymer used was Edistir® R850 produced by Versalis (simply R850 in the following), having a melt flow index (200°C, 5kg / 10’) of 6, Vicat softening temperature Vicat B / 50 (50N – 50°C / h) equal to 88°C and Izod impact strength notched 23°C – 4mm equal to 5.5 kJ / m2. The surrogates that were chosen for the experiments, all bought from Merck, were: toluene (MW 92.14 Da, Sigma Aldrich 320552), chlorobenzene (MW 112.6 Da, Sigma Aldrich 319996), methyl salicylate (MW 152.15 Da, Supelco 76631), phenyl cyclohexane (MW 160.3 Da, Sigma Aldrich C104809), methyl stearate (MW 298.5 Da, Supelco 85769) and benzophenone (MW 182.22 Da, Sigma Aldrich 8.01801). The choice of these surrogates, that correspond to the ones chosen by both EFSA (in EFSA Journal 2011; 9(7):2184) and by Welle (in "Recycling of post-consumer polystyrene packaging waste into new food packaging applications—Part 1: Direct food contact." Recycling 8.1, 2023), reflects the need to represent both polar and non-polar compounds, as well as both low and high boiling point compounds. The chosen surrogates have relatively low melting point (the highest is benzophenone, having melting point of 48.5°C). Therefore, it was chosen to mix the surrogates in the liquid state. To this task, 600 parts of toluene; 1000 parts of chlorobenzene; 1000 parts of methyl salicylate; 600 parts of phenyl cyclohexane; 1000 parts of methyl stearate and 800 parts of benzophenone (total: 5000 parts, here and in the following parts are by weight) were put in a beaker, and then brought to 60°C in a magnetic stirrer with heating while mixing with a magnetic bar. 995000 parts of R850 were brought to 70°C in a ventilated oven for at least 24 hours. A 4 liters rotary evaporator (Rotavapor® by Buchi), that has a thermal regulation unit that can be set precisely to the required temperature and that ensures that the content of the rotary flask is kept at the same temperature, was used to mix the polymer to the surrogates. The Rotavapor was pre-heated at 70°C, while the rotation speed was set to 20 rpm. The vapor outlet was closed so as to try to avoid loss of surrogates by evaporation. The rotary evaporator flask was loaded up to about 60% of its volume with the R850 polymer and then the mixture of surrogates is added. Care was taken to preserve the temperature of both polymer and surrogate before the loading to the evaporator flask. Therefore, both polymer and surrogates were taken immediately from the oven and the heated container respectively to preserve the temperature. After 2 hours, the thermal regulation unit was cooled and it was waited till the temperature reduced to 30°C. Following Welle in "Recycling of post-consumer polystyrene packaging waste into new food packaging applications—Part 1: Direct food contact." Recycling 8.1, 2023, extrusion was carried out to mix the surrogates to the polymer in the melt state, so as to homogenize them (instead of having the surrogates only on the surface). To this task, the composition prepared in the Rotavapor was fed to the hopper of a twin-screw extruder (Baker Perkins, model MPC / V30, 1985). The screw diameter D was 30mm and the length to diameter ratio (L / D) was 16. The degassing vent was located at about 11 diameters from the hopper. The extruder was operated with the degassing vent closed, speed 200 rpm and with a temperature profile of 168°C in the first heating zone, 185°C in the second and 190°C in the third and fourth heating zones, flow rate 5 kg / hr. The extruder was connected with a die having a single 3 mm diameter hole. The polymer melt coming out from the die was immediately cooled by immersion in cold water and granulated. Then, the collected granules were put in a dry mixer for further homogenization, for 20 minutes, followed by a new extrusion in the same twin-screw extruder with the same conditions. The granules were analysed by gas chromatography (GC) as described above. In this way the evaluation of the quantity of surrogates has been done. The obtained results were the following (quantity in weight ppm): Toluene 540 Chlorobenzene 880 Methyl salicylate 991 Phenyl cyclohexane 512 Methyl stearate 934 Benzophenone 709 Compared with the parts of polymer and contaminants that were initially used, a slight difference is found as the final quantities are a little smaller than expected; however, this is not surprising, as there were different preparation steps where a little part of the contaminants was likely lost. PREPARATION OF THE VINYL AROMATIC POLYMER ACCORDING TO THE PROCESS OF THE INVENTION The apparatus used for the Inventive Examples 2 to 9 and Comparative Examples 10 to 13 is depicted in Figure 2. The surrogates-into-virgin polymer feedstock (in the following, the feedstock), prepared as described in Example 1, has been provided as required by step (a) of the present process. The feedstock was fed to a single-screw extruder (model TR12 / 240M, 1992), having screw diameter 12 mm and L / D=24, produced by Gimac at Castronno (Italy), and mounted vertically (the material flowing from top to bottom), where it was melted. The temperature profile of the extruder was 170°C on the first zone (Z1) while the other three zones (Z2, Z3, Z4) were set to the target temperature T as shown in table 1. For each Example, the speed of the extruder (revolutions per minute, RPM) was regulated so as to obtain a constant mass flow of polymer, correspondent to the target value m, as shown in table 1. The extruder was equipped with a pressure transmitter and a valve that could be used to regulate its outlet pressure. The extruder outlet was flanged to a vertical SMX static mixer 17, heated at the target temperature T by a thermal fluid regulation unit, to homogenize the polymer temperature. The static mixer 17 was connected to the decontamination device 1, as depicted in Figure 2. A temperature transmitter 4 located at the exit of the static mixer 17 measured the polymer melt temperature to ensure that the actual polymer melt temperature corresponded to said target temperature T + / - 3°C. According to step (b), the melt surrogates-into-virgin polymer feedstock is fed to the decontamination device 1, which comprised a closed vessel 10 that was an AISI 304L steel thick circular tube (3’’ MPS80 sch.80S, length 95cm). The closed vessel 10 comprised vertical positioned sight glass 9, which allowed the visual inspection of the falling polymer strand over almost the entire length of the closed vessel 10. The decontamination device 1 comprised a heating jacket where silicon oil thermal fluid was flowing to ensure that the temperature of the decontamination device 1 was kept to the same target temperature T used in the static mixer 17 and the last zones of the extruder 15. On the top of the decontamination device 1, an inlet section 3, in the form of a threaded tube, was connected with the static mixer for receiving the melt surrogates- into-virgin polymer. Further, the decontamination device 1 comprises a distributor 5, which was connected to the inlet section 3. The distributor 5 had one aperture 6. The aperture 6 was circular and cylindrical, with diameter Dex according to Table 1, while the hole thickness was constant and equal to 4mm. According to step (c), the melt surrogates-into-virgin polymer was passed through this aperture 6, and, thus, a polymer strand was formed. This polymer strand was then let drop into the closed vessel 10 (step (d)), obtaining a decontaminated polymer. According to step (e), the decontaminated polymer was recovered with the retractable spoon made in copper, positioned in correspondence with the bottom part 13 of the closed vessel 10. The vertical distance between the receiving portion 12’ of the retractable spoon 11 and the exit of the distributor aperture 6 (that is, the maximum length of the polymer strand H of expression (5)) was 83 cm. Such retractable spoon 11, when fully inserted in the closed vessel 10, was located centrally and aligned to the distributor 5, and therefore receives the polymer strand which is falling from the aperture 6. At the start and at the end of the experimentation, the spoon 11 was fully retracted, so that the polymer strand dropping from the aperture was not collected on the receiving part 12’ of the spoon 11 but on the bottom 13 of the closed vessel 10. In such a way, the spoon was fully inserted only when operating conditions were stable and to the set values. After collecting the sample (few grams) on the receiving part 12’ of the spoon 11, the spoon was immediately fully retracted. Before collecting the sample, the spoon 11 was retracted and kept cold by applying ice on the part of the spoon which stayed outside the closed vessel 10. Copper ensured good thermal transmission to the part of the spoon used for the collection of the polymer strand 12’, so as to quickly cool the polymer strand (when collecting it), ensuring that decontamination could not continue once the polymer strand reached the spoon. In such a way, the decontamination was carried out only on the melt polymer dropping from the aperture 6 to the receiving part 12’ of the spoon 11. In parallel to the collection of the decontaminated polymer, the removed contaminants (comprising also other gases) were also recovered through the vapor outlet 8. To this task, the vacuum system connected to vapor outlet consisted in: - a two-staged vacuum pump, Edwards model E2M18, which had volumetric flow rate of 18 m3 / h. The maximum attainable pressure was 1 mBar (abs) in the closed vessel 10. - a piezoelectric vacuum meter, MKS model BARATRON 127A, measuring pressures from 0.1 to 1000 mBar(abs), with absolute error of 0.15%, read on the digital indicator MKS model PR-2000, located in proximity of the closed vessel 10. - a butterfly valve located on the vacuum pump inlet, controlled by the regulator MKS model 252A. - a condenser to condense condensable gases, required to reduce the contamination of the oil of the pump, operated at -30°C by means of thermoregulating bath HAAKE model 001- 0385. A specifically built gasket ensured full tightness so that full vacuum could be applied to the closed vessel 10. After collection of the sample, the extruder and vacuum pump were stopped, the flange between upper part 18 and bottom part 13 of the closed vessel was opened and the material collected on the receiving part 12’ of the spoon 11 was immediately recovered and refrigerated at 5°C. Mass flow rate m was computed by dividing the polymer mass recovered on the receiving part 12’ of the spoon 9 by the time the spoon was inserted. EXAMPLES 2 TO 13: DECONTAMINATION TESTS As will be shown in tables 1 to 3 reported below, the examples according to the invention work, while the comparative examples having parameters outside the range of the invention lead to non-working results. By “working” it is meant that the decontamination efficiency is adequate to produce a food-contact grade vinyl aromatic polymer starting from a surrogates-into-virgin polymer feedstock, which simulates a feedstock based on a post-consumer recycled vinyl aromatic polymer. Adequate decontamination efficiency means the reduction of the contaminants’ concentration to values below the limits, where the limits are the values that would cause excessive migration of the contaminant, from the package prepared using the treated recycled polymer into the food products. Such limits are described in detail in F. Welle (“Recycling of Post-Consumer Polystyrene Packaging Waste into New Food Packaging Applications—Part 1: Direct Food Contact. Recycling 2023, 8, 26), where it is given a summary of the evaluation of safety as provided by the European Food Safety Authority (EFSA). The latter was based on the Threshold of Toxicological Concern (TTC) Concept (see e.g. EFSA (European Food Safety Authority) and WHO (World Health Organization), 2016, Review of the Threshold of Toxicological Concern (TTC) Approach and Development of New TTC Decision Tree, EFSA Supporting Publication: EN-1006. p. 50.). The TTC approach was used by EFSA for safety evaluation of other plastics such as recycled PET. Reference is made to: EFSA CEF Panel (EFSA Panel on Food Contact Materials, Enzymes, Flavourings and Processing Aids), Scientific Opinion on the criteria to be used for safety evaluation of a mechanical recycling process to produce recycled PET intended to be used for manufacture of materials and articles in contact with food, EFSA J. 2011, 9, 2184. Up to now, EFSA did not produce similar documentation for vinyl aromatic based polymers, such as polystyrene and rubber reinforced vinyl aromatic polymers (such as HIPS). The assumed scenario is that, according to the TTC concept, an exposure of 0.0025 μg of a substance per kg body weight per day is not critical to human health. In absence of specific information on vinyl aromatic based polymers, it was assumed the same TTC concept of recycled PET in the present invention. That means, the above cited excessive migration of the contaminant into the food products is meant as the migration of the contaminant that would cause a daily ingestion in a human being of a quantity of such contaminant such that said exposure of 0.0025 μg of a substance per kg body weight would be exceeded. There are different possible scenario and applications (i.e. the food contamination scenarios). For the present invention, the chosen scenarios were the same of Welle (see Table 2 at page 7 of Welle). That means, the human being is the toddler of 10 kg of weight and the food consumption per day as well as the applications were the same as reported by Welle in Table 2 at page 7. Correspondingly, in the present invention the “maximum migration with overestimation factor 5” were the assumed targets. Considering a maximum content of any single contaminant in the recycled vinyl aromatic based feedstock equal to 3 mg / kg (therefore, following Welle and also EFSA assumptions for PET in the above references), it is possible to estimate the so-called “minimum cleaning efficiency”, that is the minimum value that, in the post-consumer recycled vinyl aromatic polymer, a given contaminant initially present at 3 mg / kg should be reduced so as to ensure that migration in the food in the above scenarios is below the limit disclosed above. The computation was carried out following the European Technical Report “Practical Guidelines on the Application of Migration Modelling for the Estimation of Specific Migration”; EU Report 27529 EN; Publications Office of the European Union: Luxembourg, 2015; ISBN 9789279527906, with particular reference to the analytical solution to the diffusion equation at section 9.1 and the ranges of parameters for the applicability of the migration model for PS, HIPS and SBS as reported at section “3.2.2 Polystyrenes”, Table 2, and more in particular choosing Ap’*=1.0 and tau=0 (i.e., the data for HIPS) and the diffusion coefficients as estimated by equations 1 and 2 as reported at section 3.1. Specifically, the cleaning efficiency (^^^^) is the quantity of surrogate removed, expressed in the following equation (11): wherein ^^^^^^^^^^^^and ^^^^^^^^^^^^^^^^are the mass fraction of the surrogate in the polymer after and before the decontamination treatment, respectively. To pass the food-contact grade test, the cleaning efficiency (CE) shall be equal or higher than the Minimum Cleaning Efficiency (MCE), for all the used surrogates. Minimum cleaning efficiencies (^^^^^^) depends on the specific application of the decontaminated polymer in contact with food. The minimum cleaning efficiencies were computed with the analytical method used by EFSA disclosed in the already cited articles, method that was briefly summed up also in Welle at paragraph 3.4 page 9-10. In particular, the ^^^^^^ is the value required to a post-consumer recycled vinyl aromatic polymer feedstock, having an initial contamination level equal to the maximum initial contamination (MIC) level that is considered possible for that typology of recycled polymer, to reduce its content to the maximum safe concentration (MSC). According to Welle, the maximum initial contamination (MIC) level can be safely assumed to be 3 milligrams of contaminant per kg of polymer. The maximum safe concentration (MSC) is the concentration of contaminant in the plastic package that would lead to an intake of the contaminant, by a toddler of 10 kg of body weight, corresponding to the maximum safe exposure of the contaminant (MSEC). The MSEC was set by Welle and EFSA to be 2.5 nanograms of contaminant per day and per kg of body weight. For some applications and for some contaminants, the maximum safe concentration (MSC) can be higher than the maximum initial contamination (MIC) level. In such cases, the minimum cleaning efficiency (MCE) is zero. That means: (12) The ^^^^^^ values were reported by Welle in Table 4 at page 10 in “Recycling of post-consumer polystyrene packaging waste into new food packaging applications—Part 1: Direct food contact.” Recycling 8.1, 2023. The ^^^^^^ values are the highest for the application “trays for food and vegetables” for all surrogates. This was not surprising as this application is critical for large food quantity, large storage time and relatively high temperature (i.e. consumption is 500g / day, storage time of 30 days at 25°C). Following the indications disclosed in the articles of EFSA and of Welle mentioned above, the ^^^^^^ was computed by analytically solving the previously cited diffusion equation (EU Report 27529, section 9.1, equations 3 and 11). More precisely, such equations derive from equation 4.37 of Crank (Crank, J. (1975) The Mathematics of Diffusion. Second Edition, Oxford University Press, London, at page 57), which can be reformulated in terms of MCE as per the following equation (13): (13) Where OEF is the overestimation factor (4.8), MSEC themaximum safe exposure to contaminants (2.5 ∙ 10−12kgC / (kgBW ⋅day)), MIC the maximum initial contamination (3 ∙ 10−6kgC / kgP),FC the food consumption per day and mass of body weight(0.05kgF / kgBW ⋅ day), Kpf the partition coefficient (1.0), ρF thefood density (1040 kg / m3), ρPthe polymer density (1040kg / m3), α = V^^ / (Kpf ⋅ A ⋅ d^^) = 55.6 where A is the food-plasticinterface area (0.06 m2), V^^the food volume (0.001 m3), d^^is the plastic thickness (0.0003 m), DP(^^,^^^^) is the ′ diffusion coefficient, computed with constants Ap=1.0 and^^ = 0, and MW the molecular weight of the contaminant, in Dalton: with T in Kelvin. The number of terms n in the series was chosen to be at least 100 (minimum 100 terms, but even more in case the subsequent term would change MCE by more than 0.01%). Finally, Qnare the non-zero positive roots of the following equation (Crank, eq. 4.38, reported also as equation 5 in EFSA report, section 9.1): ^^^^^^(qn) = −^^ ∙ qn = −55.6 ∙ qn (15)The roots can be computed solving: ^^(qn) = ^^^^^^(qn) + ^^ ∙ qn = 0 (16)with high degree of precision by numerical methods. The Newton-Raphson algorithm was used, with maximum error tolerance on both abscissas qnand ordinates ^^(qn) equal to 0.0001. Replacing the known values of the parameters into the MCE equation reported above, the following final equation is obtained: 1.0 ⋅ 1040^^^^ ⋅ ^^3exp (−q2n ⋅DP(^^,^^^^) (−3 2 2 ⋅ 30 ^^^^^^ ⋅ 86400 ^^ / ^^^^^^)1040 1 ^^3 ∙ 10 ) ^^ 1 + 55.6 1 1 + 55.6 + 55.62 ⋅ q2n] (17) Instead of the analytical solution, to compute MCE Welle used a finite element solvent software AKTS SML (see Welle, par.4.4). Also, Welle chose the over-estimation factor to be 5, instead of 4.8 (almost equivalent results are obtained when OEF=5 and food density is 1000 instead of 1040 kg / m3; food density was not reported by Welle). The above-reported equation (17) was used to compute MCE also for methyl salicylate, not reported in Welle article. For the application “Trays for food and vegetables”, which is the most demanding in terms of cleaning efficiency, the computed ^^^^^^ values were the following: Toluene 93.7% Chlorobenzene 92.5% Methyl salicylate 90.0% Phenyl cyclohexane 89.4% Methyl stearate 76.1% Benzophenone 87.8% The differences with respect to the ^^^^^^ values reported in Welle’s article was methyl stearate (76.1% instead of 76.2%) and the value for methyl salicylate that was not reported by Welle. The above values were the MCE values used in the present disclosure, to assess whether a decontamination test was passed or not: the test was passed if, for all surrogates, the attained decontamination value (the cleaning efficiency CE) was not less than the tabulated MCE values reported here above: Toluene 93.7%, Chlorobenzene 92.5%, Methyl salicylate 90.0%, Phenyl cyclohexane 89.4%, Methyl stearate 76.1%, Benzophenone 87.8%. For each Example and Comparative Example, the density was computed following equation (2), the surface tension following equation (3), and the viscosity was computed from the MFI (that was 6 g / 10’) following equation (4). The vertical strand length H was 0.83 [m] as previously specified, while the MW used in equation (7) was 400 Da. Table 1 reports, for each Example and Comparative Example, the process parameters (polymer melt temperature T at section 3, mass flow per aperture ^^, pressure of the closed vessel P, aperture equivalent diameter ^^^^^^) and the computation of specific parameters (^^^^^^^^^^, ^^^^^^^^^^and ^^^^) and if all the parameters fall in the required ranges (that is, whether all criteria were passed): Table 1 Ex. Inv / ^^^^^^T ^^ P ^^^^^^^^^^^^^^^^^^^^^^^^Criteria Comp [mm] [°C] [g / h] [mbar(a)] [-] [-] [-] passed? [yes / no] 2 INV 0.3 250 25 13 2.8 0.2 0.15 YES 3 INV 0.3 230 20 12 8.82 0.34 0.17 YES 4 INV 0.3 260 30 12 1.87 0.16 0.14 YES 5 INV 0.3 260 20 12 1.24 0.11 0.19 YES 6 INV 1 230 40 12 17.63 0.69 0.32 YES 7 INV 3 250 65 11 7.27 0.51 0.69 YES 8 INV 1 260 40 12 2.49 0.22 0.36 YES 9 INV 1 230 40 20 17.63 0.69 0.32 YES 10 COMP 1 250 60 86 6.71 0.47 0.25 NO 11 COMP 1 275 30 12 0.86 0.1 0.48 NO 12 COMP 3 250 130 11 14.55 1.02 0.39 NO 13 COMP 5 250 65 12 7.27 0.51 1.16 NO Table 2 reports, for the same Inventive and Comparative Examples, the decontamination results, and whether or not the decontamination test passed. Above the heading of the surrogate names, the initial concentration is reported to make easier the evaluation of the decontamination effectiveness. Notably, the concentrations in wppm of the surrogates are given by the GC analytical technique. To pass the decontamination test, for all surrogates the attained cleaning efficiency (CE) shall be not less than the tabulated MCE reported above, as reported in headings of Table 3. Table 2 Initial concentration 540 880 991 512 934 709 Ex. Inv / Toluene Cholo Methylsa Phenylcy Methyls Benzophen Test Comp benzene licilate clohexan tearate one passed? e [wppm] [wppm] [wppm] [wppm] [wppm] [wppm] [YES / NO] 2 INV 10 10 11 20 33 23 YES 3 INV 27 48 39 48 116 67 YES 4 INV 10 10 10 24 83 42 YES 5 INV 10 22 27 39 116 58 YES 6 INV 6 17 29 25 86 41 YES 7 INV 33 62 76 52 190 83 YES 8 INV 10 12 20 30 126 50 YES 9 INV 10 14 37 41 153 69 YES 10 COMP 75 140 237 227 676 361 NO 11 COMP 57 99 151 162 380 208 NO 12 COMP 157 274 331 299 469 378 NO 13 COMP 131 237 264 219 402 311 NO Table 3 reports the same results but in terms of cleaning efficiency (CE) compared with the computed Minimum Cleaning Efficiency (MCE), which were both computed as described above. Table 3 MCE93.70% 92.50% 90% 89.40% 76.10 87.80% Ex. Inv / Toluene Cholo Methylsa Phenylcyc Methyls Benzophe Test Comp benzene licilate lohexane tearate none passed? [CE] [CE] [CE] [CE] [CE] [CE] [YES / NO] 2 INV 98.1% 98.9% 98.9% 96.1% 96.5% 96.8% YES 3 INV 95.0% 94.5% 96.1% 90.6% 87.6% 90.6% YES 4 INV 98.1% 98.9% 99.0% 95.3% 91.1% 94.1% YES 5 INV 98.1% 97.5% 97.3% 92.4% 87.6% 91.8% YES 6 INV 98.9% 98.1% 97.1% 95.1% 90.8% 94.2% YES 7 INV 93.9% 93.0% 92.3% 89.8% 79.7% 88.3% YES 8 INV 98.1% 98.6% 98.0% 94.1% 86.5% 92.9% YES 9 INV 98.1% 98.4% 96.3% 92.0% 83.6% 90.3% YES 10 COMP 86.1% 84.1% 76.1% 55.7% 27.6% 49.1% NO 11 COMP 89.4% 88.8% 84.8% 68.4% 59.3% 70.7% NO 12 COMP 70.9% 68.9% 66.6% 41.6% 49.8% 46.7% NO 13 COMP 75.7% 73.1% 73.4% 57.2% 57.0% 56.1% NO Notably, in all the inventive examples, according to the present invention, the criteria on the three specific parameters (^^^^^^^^^^, ^^^^^^^^^^and ^^^^) were met. The residual quantity of the surrogates was such that the decontamination efficiency (CE) of each surrogate was equal or better than the tabulated MCE. On the contrary, in all the comparative examples, which are not according to the present invention, as at least one criteria on the three specific parameters (^^^^^^^^^^, ^^^^^^^^^^and ^^^^) was not met, resulted in cleaning efficiency (CE) that was below the minimum cleaning efficiency (MCE) for at least one surrogate. In particular, inventive examples from 1 to 9 show that the cleaning efficiency (CE) is equal or higher than the MCE for all surrogates in a variety of conditions of pressure (11-20 mbar), mass flow rate per aperture (20-65 g / h) and temperature of the polymer melt (230-260°C), when all the criteria defined in the present invention were met. Comparative example 10 shows that when the pressure inside the closed vessel is too high, the MCE was not met for all surrogates. Comparative Example 11 shows that when the mass flow per aperture is below the value of ^^^^^^^^(^^^^^^^^^^<1), the MCE was not met for all surrogates. Comparative example 12 shows that when the mass flow per aperture is above the value of ^^^^^^^^(^^^^^^^^^^>1), the MCE was not met for all surrogates. Comparative example 13 shows that when the aperture equivalent diameter is above the critical equivalent diameter dcrit, the MCE was not met for all surrogates. From the above working examples, it is clear that the inventive examples fulfil the requirements for being used as in food-contact grade applications. Since the concentrations of surrogates are typically higher than the level of the real life contamination, then the decontaminated vinyl aromatic polymers obtained from feedstock based on a post- consumer recycled vinyl aromatic polymer treated with the process according to the invention will be suitable for food- contact grade applications. In fact, the expert of the art knows that the diffusion coefficient of the contaminant decreases when its concentration in the polymer increases. Therefore, decontamination of polymers having high concentration of a given contaminant is more critical than real life post- consumer recycled vinyl aromatic polymers, where the maximum contamination level is just 3 mg / kg. In addition, the present process has been shown to satisfies all the aims, such as that it is simple, environmentally friendly and highly efficient for removing a large spectrum of contaminants.

Claims

CLAIMS 1. Process for the production of food-contact grade vinyl aromatic polymers, comprising the steps of: (a) providing a feedstock based on post-consumer recycled vinyl aromatic polymer comprising: (i) post-consumer recycled non-rubber reinforced vinyl aromatic polymer, and / or (ii) post-consumer recycled rubber reinforced vinyl aromatic polymer; (iii) optionally, polymeric impurities; (iv) optionally, non-polymeric impurities; (v) contaminants, having molecular weight ranging from 50 to 400 Da and boiling point ranging from 0°C to 300°C at 0.1 bar(a), ranging from 0.0001 wt% to 0.5 wt%; and (vi) water ranging from 0 wt% to 9.9999 wt%; with the provision that the weight percentages (wt%) are calculated with respect to the total mass of the feedstock based on post-consumer recycled vinyl aromatic polymer; with another provision that the sum of the quantities (i), (ii), (iii), (iv), (v) and (vi) is 100%; with the additional provision that the sum of the quantity of (i) and (ii) is at least 90 wt%, preferably at least 95 wt%; with the further provision that the sum of the quantity of (iii) and (iv) ranges from 0 wt% to 9.9999 wt%; preferably from 0.1 wt% to 4.9999 wt%, even more preferably from 0.2 wt% to 2 wt%; (b) bringing the feedstock to a decontamination device (1) in the form of a polymer melt feed, the decontamination device (1) comprising at least an inlet section (3) receiving the polymer melt feed at a temperature T, at least one distributor (5) with at least one aperture (6), a closed vessel (10) connected to the distributor (5) and provided with at least one polymer outlet (12) and with at least onevapor outlet (8); (c) passing the polymer melt feed through said at least one aperture (6), thereby forming at least one polymer strand; (d) letting the at least one polymer strand drop into the closed vessel (10), thereby obtaining a decontaminated vinyl aromatic polymer in the closed vessel (10) and the removed contaminants; (e) recovering and optionally pelletizing the decontaminated vinyl aromatic polymer through the at least one polymer outlet (12); and (f) recovering and optionally condensing the removed contaminants through the at least one vapor outlet (8); wherein: - said at least one aperture (6) has an equivalent diameter (Dex) at the exit that is at least 0.3·10-3m, preferably at least 0.8·10-3m, even more preferably at least 1.0·10-3m, - the aperture ratio (Dr), defined as the ratio between the equivalent diameter at the exit (Dex) and the critical diameter (Dcrit), is at most 1.0, preferably at most 0.8, even more preferably at most 0.7, wherein Dcrit is defined according to the following equation (1) 5 1 1 2 Dcrit = 5.8 ∙ ^^6 ∙ ^^−3 ∙ (g ∙ μ)6 ∙ ^^−3 (1) wherein: - ^^ (kg / s) is the mass flow of the polymer melt feed flowing out of the apertures (6) divided by the number of apertures (6), - ^^ (kg / m3) is the density of the polymer melt feed, calculated according to the following equation (2)-= 9.8^^is gravitational acceleration on Earth, - μ (Pa·s) is the dynamic viscosity, assumed to be the viscosity of the polymer melt feed at the temperatureT at the inlet section (3) and at shear rate of 0.1 s-1, - ^^ (kg / s2) is the surface tension of the polymer melt feed, calculated according to the following equation (3) ^^ = 0.0407 − 0.000072 ∙ (^^ − 293.15[^^]) (3)wherein ^^ (K) is the temperature of the polymer melt feed at the inlet section (3), - the pressure in the closed vessel (10) ranges from 1 to 8000 Pa (abs), preferably from 10 to 6000 Pa (abs), more preferably from 50 to 3500 Pa (abs), even more preferably from 100 to 2000 Pa (abs), - the minimum flow ratio (mrmin), defined as the ratio between the mass flow (m) and the critical minimum mass flow rate (mmin), is at least 1.0, preferably at least 1.1, even more preferably at least 1.2, wherein mmin is calculated according to the following equation (5)wherein: 4 - ^^ is a numeric constant having value equal to 3, - ^^ (kg / m3) is the density of polymer melt feed, calculated according to equation (2), -^^ = 9.8^^ ^^2is gravitational acceleration on Earth, - ^^ (Pa.s) is the dynamic viscosity, assumed to be the viscosity of the polymer melt feed at the temperature T at the inlet section (3) and at shear rate of 0.1 s-1, - ^^ (kg / s2) is the surface tension of the polymer melt feed, calculated according to equation (3), - ^^ (m) is essentially the maximum length of the polymer strand, - the maximum flow ratio (mrmax), defined as the ratio between the mass flow (m) and the critical maximum mass flow rate (mmax), is at most 1.0, preferably at most 0.8, even more preferably at most 0.7, wherein mmax is calculatedaccording to the following equation (7) ^^ −19 9.366^^^^^^ = 3.9 ∙ 10 ∙ ^^ ∙ ^^ ∙ exp [(0.088 − 2.945(7) wherein: - ^^ (m) is essentially the maximum length of the polymer strand, - ^^ (°C) is the temperature of the polymer melt feed at the inlet section (3), - ^^^^ (Da) is the value of the molecular weight of the contaminant (v) having the highest molecular weight.

2. Process according to claim 1, wherein the post-consumer recycled vinyl aromatic polymer is post-consumer recycled polystyrene.

3. Process according to any previous claims, wherein (i) is selected from the group consisting of general purpose polystyrene (GPPS), such as radical and anionic polymerized polystyrene, and styrene-alkyl-substituted styrene copolymers, such as styrene-alpha-methylstyrene copolymers, preferably (i) is general purpose polystyrene (GPPS).

4. Process according to any previous claims, wherein (ii) is selected from the group consisting of high impact polystyrene (HIPS) and styrene-butadiene block copolymers, preferably (ii) is high impact polystyrene (HIPS).

5. Process according to any one of the previous claims, wherein the (i) post-consumer recycled non-rubber reinforced vinyl aromatic polymer and (ii) post-consumer recycled rubber reinforced vinyl aromatic polymer do not include (iii) polymeric impurities.

6. Process according to any one of the previous claims, wherein the (iii) polymeric impurities are selected from the group consisting of acrylonitrile-butadiene styrene polymers (ABS), styrene-acrylonitrile polymer (SAN), acrylonitrile- styrene-acrylate (ASA), styrene acrylates, such as styrene-methyl acrylate and styrene-methyl methacrylate (SMMA), styrene maleic anhydride (SMA), methyl methacrylate- butadiene styrene (MBS), methyl methacrylate-acrylonitrile- butadiene-styrene (MABS) polymers, styrene-N- phenylmaleimide copolymers (SPMI), polyamides (PA), polyolefins, e.g. polypropylene (PP) or polyethylene (PE), polyacrylates, e.g. polymethyl methacrylate (PMMA), polycarbonate (PC), polyesters, e.g. polyethylene terephthalate (PET) or polybutylene terephthalate (PBT), polyphenylene ethers (PPE), polyether sulfones (PES), polyetheretherketones (PEEK), polyether sulfides, , rubbers, e.g. polydienes, biodegradable aliphatic and aliphatic / aromatic copolyesters, and a mixture thereof.

7. Process according to any one of the previous claims, wherein the (iv) non-polymeric impurities are selected from the group consisting of anti-oxidants, UV stabilizers, nucleating agents, pigments, inorganic additives and mixture thereof.

8. Process according to any one of the previous claims, wherein a pre-treatment step is carried out prior to step (a).

9. Process according to any one of the previous claims, wherein the temperature T in step (b) ranges from 210°C to 300°C, preferably from 220°C to 290°C, more preferably from 225°C to 280°C, even more preferably from 230°C to 270°C.

10. Process according to any one of the previous claims, wherein the dynamic viscosity in equations (1) and (5) of the polymer at the temperature T of the polymer melt feed at the inlet section (3) at shear rate (^̇^) of 0.1 s-1is calculated by the equation (4):wherein:- T (°C) is the temperature of the polymer melt feed at the inlet section (3), - MFI is the melt flow index of the post-consumer recycled vinyl aromatic polymer measured at 200°C, with a load of 5kg, expressed in g / 10’, and -the values of the numeric constants are: ^^0 = 675 ∙103; ^^1 = 4.8; ^^2 = −42; ^^3 = 4.7[s]; ^^4 = 180; ^^ = 0.7; ^̇^ =0.1[s-1].

11. The food-contact grade vinyl aromatic polymers obtained according to any one of claims from 1 to 10.

12. Use of the food-contact grade vinyl aromatic polymers according to claim 11, for food trays, for example, for fruit, vegetable, meat, fish, dairy, baking, sweet, deli, cereals, cheese, eggs; cups and containers for food, for example, for hot and cold beverages, coffee, soup, yogurt, ice cream and takeaway food; sheets, for example, for packaging fresh food products, for covering trays and plates and for making food pouches; bottles, for example, for cold beverage, water and fruit juices; blister packs for pre-cut fruits, vegetables, cheese, backed goods and snacks; plastic cutlery and plates.

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