Method for purifying a thermoplastic material by selective degradation of impurities

Abstract

The invention relates to a method for purifying a thermoplastic material (22), such as polyethylene terephthalate (PET), comprising the steps of: - supplying pellets or shreds of the thermoplastic material into a melt processing device (20); and - melting the thermoplastic material to form a polymer melt; wherein the method further comprises a step of: - adding a pro-degradant into the pellets or shreds of the thermoplastic material, and / or into the polymer melt, to selectively degrade at least polyvinyl chloride (PVC) and benzene impurities, and / or any intermediate degradation product formed by or leading to the formation of such impurities, contained within said thermoplastic material and / or said polymer melt.

Description

METHOD FOR PURIFYING A THERMOPLASTIC MATERIAL BY SELECTIVE DEGRADATION OF IMPURITIESTechnical field

[0001] The invention relates to the field of thermoplastic materials, such as polyethylene terephthalate (PET).

[0002] In particular, the invention relates to the purification of the thermoplastic materials from impurities.Background art

[0003] Polyethylene terephthalate (PET) is widely used for beverage bottles, packaging trays, blisters, multilayered packaging films, strapping tapes, and textiles. Its popularity as a packaging material is due to its low gas permeability, excellent mechanical properties, high transparency and reflectivity, ease of processing, and affordability. As a result, PET has become the third-largest packaging polymer.

[0004] Mechanical recycling of thermoplastics, such as PET, involves many steps: collecting and sorting plastic waste, shredding, and melt-compounding it into new products, such as PET bottles. This process retains the polymer’s structure, allowing the recycled material to be reprocessed into similar items. It is a widely used, cost-effective, and sustainable method for recycling thermoplastics.

[0005] European regulations set general requirements for all plastic food contact materials. These regulations establish migration limits for NIAS (non- intentionally added substances) such as VOCs (volatile organic compounds) such as benzene.

[0006] In fact, benzene is a VOC that can migrate from PET packaging, and more particularly mechanically recycled PET (r-PET) into food or beverages and can affect the quality and safety of the final product, especially if the benzene levels are high.

[0007] Benzene is a carcinogenic compound, and its presence in r-PET can pose a health risk to consumers if it migrates into the food or beverage contents (risk of blood cancer such as leukemia). Therefore, monitoring andcontrolling benzene levels in r-PET is essential to ensure the safety and quality of the final product. Additionally, benzene contamination in r-PET has significant environmental consequences.

[0008] One known solution to limit NIAS diffusion into food is to design multilayer packaging, where the recycled material is “hidden” in the center of the packaging. This design avoids direct contact between the recycled materials and food or beverages, thus preventing the diffusion of impurities. However, this solution is inefficient, still risky for consumers, and expensive from a processing standpoint.

[0009] Published patent document WO 202100031 A1 discloses a process to counteract the effects of PVC on post-consumer PET flakes during the melt phase of PET resin forming processes by using a stoichiometric mixture containing calcium zinc (or cadmium barium) salts and antioxidants. The document further discloses the elimination of benzene formation by the cyclization of double bonds formed during PVC degradation in the reaction medium.

[0010] However, there is still room for improvement in solution disclosed in the patent document WO 202100031 A1 to further provide a thermoplastic material with a significantly reduced benzene level and a minimized risk of benzene migration into food or beverages.Summary of inventionTechnical problem

[0011] The present invention addresses the above-mentioned deficiencies and aims at providing a solution for effectively purifying a thermoplastic material from molecular impurities, including benzene.Solution

[0012] The above-stated problem is solved by a method for purifying a thermoplastic material, such as polyethylene terephthalate (PET), comprising the steps of:- supplying pellets or shreds of the thermoplastic material into a melt processing device; and- melting the thermoplastic material to form a polymer melt; wherein the method further comprises a step of:- adding a pro-degradant into the pellets or shreds of the thermoplastic material, and / or into the polymer melt, to selectively degrade at least polyvinyl chloride (PVC) and benzene impurities, and / or any intermediate degradation product formed by or leading to the formation of said polyvinyl chloride (PVC) and benzene impurities, contained within said thermoplastic material and / or said polymer melt.

[0013] Advantageously, the pro-degradant is able to selectively degrade impurities e.g. macromolecular secondary materials, with a lower melting point than the polymer melt, such as polyvinyl chloride (PVC) and molecular impurities such as benzene contained within said polymer. The pro-degradant can also selectively degrade intermediate degradation products that contribute to the formation of impurities, or that may form temporarily during the breakdown of impurities. For example, the degradation of polyvinyl chloride (PVC) may occur in stages, with an intermediate degradation product emerging as the PVC begins to decompose, before it fully breaks down into its final byproducts.

[0014] The pro-degradant can be added inside the melt processing device, or can alternatively be added outside the latter; in this configuration, the output material exiting the melt processing device comprises the polymer melt prior to purification. Preferably, the pro-degradant is added to the polymer melt inside the melt processing device.

[0015] According to a preferred embodiment, the pro-degradant comprises at least one free radical generator such as peroxide and / or at least one bicumenederived molecule and / or ozone gas.

[0016] According to a preferred embodiment, the pro-degradant comprises at least one of the following: zinc peroxide, calcium peroxide, strontium peroxide, magnesium peroxide, p-menthane hydroperoxide, di-cumyl peroxide, di- tert-butyl peroxide, 2,3-dimethyl-2,3-diphenylbutane, ozone, or a combination thereof.

[0017] Preferably, the pro-degradant comprises an activation temperature lower or equal to the processing temperature in the melt processing device.

[0018] According to a preferred embodiment, the polymer melt comprises 0.005 to 10 wt% of the pro-degradant relative to a total weight of the polymer melt.

[0019] According to a preferred embodiment, the pro-degradant remains inside the melt processing device for at least one half-life of said pro-degradant at the temperature of the melting step. Preferably, the half-life is as indicated by the manufacturer of the pro-degradant.

[0020] According to a preferred embodiment, the pro-degradant remains inside the melt processing device for a total duration comprised between 0.1 s and 10 min.

[0021] According to a preferred embodiment, the polymer melt inside the melt processing device comprises a temperature ranging between 120 °C and 350 °C.

[0022] According to a preferred embodiment, said method further comprising a step of devolatilizing or stripping the polymer melt to remove volatile components following the addition of the pro-degradant.

[0023] According to a preferred embodiment, the thermoplastic material comprises neat PET or reclaimed PET or blends thereof, or blends thereof with additional thermoplastics.

[0024] The invention also relates to a purified material comprising a thermoplastic material which has been purified with the method according to the invention.

[0025] Advantageously, said purified material, comprising at most 0.07 ppm of benzene.

[0026] The invention further relates to an end product made from the purified material according to the invention.

[0027] The invention also relates to a melt processing device suitable for executing the steps of the method according to the invention, comprising a proximal end for receiving the pellets or shreds of the thermoplastic material, and a distal end for the exiting of an output thermoplastic material, wherein the melt processing device further comprises a feeding inlet configured forfeeding the pro-degradant into the polymer melt, said feeding inlet being arranged upstream a venting outlet at a distance of at most half of a total extent of the melt processing device.

[0028] According to a preferred embodiment, said melt processing device comprises an extruder, and the distance between the feeding inlet and the venting outlet is 4 to 40 times, preferably 4 to 12 times the diameter of a screw of said extruder.

[0029] The invention also relates to a method for evaluating the level of purification of the material according to the invention, wherein said method comprises the steps of:- heating the purified material to a first temperature in a gas-tight vial;- measuring the benzene released by the purified material after heating at the first temperature;- heating a second sample of the same purified material at a second temperature which is superior to the first temperature, in a second gas-tight vial;- measuring the benzene released by the purified material after heating at the second temperature;- estimating the presence of polyvinyl chloride (PVC) based on the difference between the benzene measured at, respectively, the first temperature and the second temperature.

[0030] Preferably, measurements are performed using gas chromatography (GC) coupled with mass spectrometry (MS) at least in triplicate analyses. More preferably, measuring the benzene released by the purified material is performed by means of HeadSpace SPME coupled to GC-MS / MS.

[0031] According to a preferred embodiment, the first temperature is around 150 °C, and the second temperature is around 200 °C. The “term” around designates ±10 % relative to the corresponding temperature.

[0032] Advantageously, the estimation of the PVC rate can be achieved by evaluation of the amount of benzene after subtracting the amount of benzene emitted at 150 °C from the amount emitted at 200 °C.

[0033] In an advantageous manner, the present invention enables to obtain a purified thermoplastic material having as low as 0.01 ppm of benzene, including the benzene issued from the selective degradation of at least polyvinyl chloride (PVC).

[0034] The addition of the pro-degradant to the thermoplastic polymer enables to selectively degrade at least PVC and benzene impurities and / or any intermediate degradation product formed by or leading to the formation of such impurities, with minor to no degradation of the thermoplastic material. By the selective degradation of at least PVC, the present invention also ensures effective elimination of benzene formation subsequently to the purification of the thermoplastic material. Therefore, the benzene content in the purified material remains unchanged (i.e. , does not increase) when the material is exposed to heat, such as at 200 °C.

[0035] The purification method of this invention is simple and efficient. This method effectively limits benzene levels in thermoplastic materials, and particularly in mechanically recycled thermoplastics, such as PET, and end products, such as packaging and bottles for food contact application, thereby contributing to the preservation of public health and the environment.Brief description of the drawings

[0036] Figure 1 shows a comparison graph of benzene rates measured at 150 °C and 200 °C for three samples of the thermoplastic material made of different polyethylene terephthalate (PET);

[0037] Figure 2 highlights the increase in the benzene rates between the measures performed at 150 °C and 200 °C for the three PET samples of figure 1 ;

[0038] Figure 3 illustrates the steps of a method for purifying a thermoplastic material according to the invention;

[0039] Figure 4 represents a melt processing device according to a first embodiment of the present invention;

[0040] Figure 5 represents a melt processing device according to a second embodiment of the present invention;

[0041] Figure 6 shows a comparison graph of benzene rates measured for r-PET- 2 as a reference, and for purified PET thermoplastic materials that have been purified from at least polyvinyl chloride (PVC) and benzene impurities by means of a selective degradation performed according to the method of figure 3, using a first variant comprising 4 wt% of different pro-degradants added to the corresponding PET polymer melt, with a residence duration of 5 min in the melt processing device of the invention;

[0042] Figure 7 is a continuation of the comparison graph of figure 5, showing additional PET thermoplastic materials whose polymer melts have been mixed with various other pro-degradants;

[0043] Figure 8 shows a comparison graph of benzene rates measured for r-PET- 2 as a reference, and for purified PET thermoplastic materials using a second variant of the invention comprising 2 wt% of different pro-degradants added to the corresponding PET polymer melt, with a residence duration of 2 min in the melt processing device of the invention;

[0044] Figure 9 shows a similar comparison graph as figure 7, except using a third variant of the invention comprising 1 wt% of different pro-degradants added to the corresponding PET polymer melt, with a residence duration of 2 min in the melt processing device of the invention;Figure 10 shows a similar comparison graph as figure 8, except using a fourth variant of the invention comprising 1 wt% of different pro-degradants added to the corresponding PET polymer melt, with a residence duration of 1 min in the melt processing device of the invention;

[0045] Figure 11 shows a similar comparison graph as figure 9, except using a fifth variant of the invention comprising 0.5 wt% of different pro-degradants added to the corresponding PET polymer melt, with a residence duration of 1 min in the melt processing device of the invention.Detailed description of the drawings

[0046] Figure 1 shows a comparison graph of benzene rates measured at 150 °C and 200 °C for three samples of the thermoplastic material made of different polyethylene terephthalate (PET), including neat PET, recycled PET type 1 (r-PET-1 ), and recycled PET type 2 (r-PET-2).

[0047] The benzene rates have been measured using gas chromatography coupled with mass spectrometry (GC-MS). It can be observed that by increasing the measurement temperature in GC-MS, the benzene rate increases for the three PET samples, and increases considerably for r-PET- 2 for which a temperature increase of 50 °C is associated with a multiplication of the benzene rate to approximately 8 times the initial rate. Figure 2 highlights this increase.

[0048] The origin of the subsequent presence of benzene in recycled PET is due to PVC degradation. In fact, a correlation has been established between chlorine present in PET bottles and benzene migration from said bottles to water after 10 days at 40 °C.

[0049] PET and PVC cannot be separated easily due to close densities of around 1.30. At a PET processing temperature of around 270 °C, PVC impurities can generate hazardous products, such as benzene, which can pose a risk of leukemia if present in water at a content of at least 1 ppb.

[0050] It can be stated that increasing the measurement temperature in GC-MS allows for the detection of in situ degradation of PVC, as traces of PVC remain in r-PET during thermal treatment.

[0051] The commercial r-PET pellets available in the market can have various levels of benzene content depending notably on the price. The most expensive PET thermoplastic materials have the lowest amounts of benzene. However, those materials seem to contain significant amounts of PVC which can lead to further formation of benzene during the processing of rPET (injection of preforms or melt-blowing of bottles).

[0052] Advantageously, the present invention provides a solution to minimize subsequent benzene formation in PET materials, particularly as temperatures increase.

[0053] For that matter, the invention relates to a method 100 for purifying a thermoplastic material, preferably PET, such as neat PET and / or reclaimed PET (e.g., post-industrial, and / or post-consumer PET) and / or blends thereof, comprising the steps (represented in figure 3) of:- S102 supplying pellets or shreds of the thermoplastic material into a melt processing device (visible in figures 4 and 5);- S103 melting the thermoplastic material to form a polymer melt; the method 100 further comprises a step S104 of adding a pro-degradant into the pellets or shreds of the thermoplastic material, and / or into the polymer melt, to selectively degrade at least polyvinyl chloride (PVC) and benzene impurities and / or any intermediate degradation product formed by or leading to the formation of such impurities, contained within said thermoplastic material and / or said polymer melt.

[0054] The melt processing device can be an extruder, an internal mixer, a kneader, an injection molding machine, a blow-molding machine, a compression molding machine, a thermoforming machine, a rotational molding machine, a calendaring machine, a pultrusion machine. Preferably, the melt processing device corresponds to an extruder, and more preferably to a twin-screw extruder.

[0055] The purification method 100 of the invention preferably corresponds to a mechanical recycling process (extrusion) of the thermoplastic material, performed right after a drying process and before a solid-state polycondensation.

[0056] The pro-degradant comprises a substance that can be solid, liquid or gaseous. Preferably, the pro-degradant comprises at least one free radical generator capable of generating radical species such as at least one peroxide and / or at least one bicumene-derived molecule, and / or a blend of fluids including ozone gas.

[0057] The free radical generator, though its type may not be critical, has to be active at the temperature of the process i.e. , have an activation temperature lower or equal to the processing temperature in the melt processing device or in the process following it (e.g., drying, solid-state polycondensation and / or injection molding).

[0058] Preferred free radical generators for food contact applications, comprise at least one of the following: zinc peroxide, calcium peroxide, strontium peroxide, magnesium peroxide, p-menthane hydroperoxide, di-cumylperoxide, di-tert-butyl peroxide, 2,3-dimethyl-2,3-diphenylbutane (known as bicumene), ozone, or a combination thereof.

[0059] Preferred free radical generators for non-food contact applications, at least one of the following: diaryl peroxides, ketone peroxides, peroxydicarbonates, peroxyesters, dialkyl peroxides, hydroperoxides, benzoyl peroxides, lauroyl peroxides, di-t-butyl peroxide, dicumyl peroxide, t-butylcumyl peroxide, t-butyl peroctoate, p-di(t- butylperoxy)diisopropylbenzene, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 1 , 1 -di(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2 , 5-d im ethy I-2 , 5-d i(t- butylperoxy)hexene-3, t-butyl-peroxy-(cis-3-carboxy)propenoate, 1 , 1 -di(t- amylperoxy)cyclohexane, t-amyl-(2-ethylhexyl)peroxycarbonate, t- butylperoxy-3,5,5-trimethylhexanoate, t-butyl peroxybenzoate, and mixtures thereof. In particular, the organic peroxide may be one or more of benzoyl peroxide, methyl ethyl ketone peroxide, tert-butyl peroxy benzoate, tert-butyl perbenzoate, tert-butylperoxy acetate, tert-butyl(2- ethylhexyl)monoperoxy carbonate, n-butyl-4,4-di-(tert-butyl peroxy )valerate, 1 , 1 -bis(tert-butylperoxy)cyclohexane, 1 , 1 -bi s(tert- butylperoxy)-3,3,5-trimethylcyclohexane, 2,2-bis(tert-butylperoxy)butane, tert-butylcumylperoxide, bis-(tert-butylperoxy isopropylbenzene, di-tert- butylperoxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, cyclohexanone peroxide, t-butylperoxyisopropyl carbonate, di-ti-butyl terphthalate, 2,5- dimethyl-2,5-di(t-butylperoxy)hexene, 2,5-dimethyl-2,5-di(t- butylperoxy)hexyne-3, di-(tert-amyl)peroxide, bis(alpha- methylbenzyl)peroxide, benzoyl peroxide, 3,6,9-triethyl-3,6,9-trimethyl- 1 ,4,7-triperoxonane, bis(tert-butylperoxy)-diisopropylbenzene, di-tert-butyl peroxide, cumene hydroperoxide, tert-butyl hydroperoxide, hydroperoxides, dilauryl peroxide, dicumyl peroxide, derivatives thereof, or combinations thereof.

[0060] Other suitable initiators include at least one of: azodicarboxylic esters, azodicarboxylic dinitriles and 1 ,1 ,2,2-tetramethylethane derivatives, and other components capable of forming free radicals in the desired operating temperature range. For higher operation temperatures (of the polymer melt) in the range from 300 °C to 350 °C, the free radical generator can be at leastone of: peresters, perketals, peroxy ketones, percarbonates and cyclic multifunctional peroxides can be used.

[0061] The pro-degradant containing the free radical generator can be added into the thermoplastic to be purified before the melting step S103 (the thermoplastic material being in a solid state). Alternatively, the prodegradant can be added during said melting step S103 inside the melt processing device (the thermoplastic material being in a liquid and / or solid state i.e. polymer melt). In another alternative, the pro-degradant can be added both before and during the melting step S103.

[0062] Preferably, the method 100 of the invention further comprises a step S105 of removing volatile organic compounds following the addition of the prodegradant, notably those obtained by the selective degradation of impurities by the free radical generator. This removal can be performed by venting, devolatilization, heating, stripping (i.e., the injection of a fluid which can then be pumped out) or a combination thereof. At step S105, the main thermoplastic matrix can be in the melt sate (i.e., polymer melt in the melt processing device) and / or in a solid state i.e., after cooling from the melt state in the following processing steps such as, in the case of PET, during a solid-state polycondensation or during an injection molding.

[0063] Devolatilization preferably takes place in the melt processing device, more particularly in a twin-screw extruder, a single-screw extruder or an injection molding machine. Reduced pressures can be obtained in different areas by the combined effect of screw-speed, temperature, adapting the design of the screws and the use of various pumps ensuring optimized vapors removal. A vacuum gradient can be applied to the devolatilization areas to drive the volatiles out of the polymer melt. The vacuum levels can range from 0.1 to 800 mbar, preferably from 0.1 to 10 mbar. When the length of the extruder allows it, multiple vacuum outlets can be used along the extruder to enhance devolatilization efficiency. This staged approach allows for gradual removal of volatiles.

[0064] The stripping process also takes place in the melt processing device. It involves feeding a fluid (e.g., solvents and / or gas and / or gas in their supercritical state; water should be avoided when the main thermoplastic matrixis a polyester, like in the case of PET) under pressure to the main polymer matrix melt as an entrainer to discharge the impurities out. Preferably, the fluid’s proportion to the polymer melt is at least 3 wt%. At the end of the extraction process, the fluid enriched with the impurities is preferably discharged from the polymer melt.

[0065] Preferably, the stripping fluid is fed in the polymer melt at a high pressure, higher than the fluid’s vapor pressure; at the process temperature, the fluid in the extraction process is maintained at a pressure of 100 - 200 %, the fluid vapor pressure at the polymer stream in the extraction process. The fluid may stay in the polymer melt for a period of more than 1 second, preferably more or equal to 5s and less or equal to 600s, preferably less than 300s, in particular less than 120s. The fluid discharge may take place under vacuum which is preferably 1 - 800 mbar, preferably 300 - 800 mbar.

[0066] For that matter, the melt processing device according to the present invention is depicted in figures 4 and 5 according to two different embodiments, depending on whether the polymer melt undergoes devolatilization or stripping.

[0067] Figure 4 represents the melt processing device 20 according to the first embodiment of the present invention, which is designed to ensure devolatilization of the polymer melt.

[0068] The melt processing device 20, being preferably a twin-screw extruder, comprises a proximal end 20.1 for receiving the thermoplastic material 22 as an input material in the form of pellets, shreds, powders or liquid, and a distal end 20.2 provided with a die 24 for the exiting of a purified thermoplastic material 26 (as the output material).

[0069] The device 20 comprises a melting zone 27 adjacent to the proximal end 20.1 equipped with a heating device and where screws may apply enough shearing to melt the thermoplastic matrix to form the polymer melt.

[0070] Preferably, the melt processing device 20 further comprises a feeding inlet 28 configured for feeding the free radical generator containing prodegradant into the polymer melt. For this purpose, a suitable pump 28.1 and a tank 28.2 may be arranged.

[0071] The feeding inlet 28 is preferably arranged upstream a venting outlet 30 at a distance d of at most half of a total extent L of the melt processing device 20. The distance d enables to set the residence duration of the prodegradant in the polymer melt. The inlet 28 can be positioned anywhere along the total length L, provided it is upstream of the venting outlet 30.

[0072] In an alternative embodiment of the invention, the pro-degradant can be added in a solid state along with the thermoplastic material 22 directly in the proximal end region 20.1.

[0073] In another alternative, the pro-degradant can be added in a liquid or gaseous state to the exiting output thermoplastic material 26 (corresponding to an unpurified polymer melt) outside the melt processing device 20. In this configuration, the extraction of volatile components can be performed subsequently to the melt-compounding step (downstream the die 24).

[0074] The devolatilization can be performed on a length of 2 to 40 times the diameter of the screws, and preferably 4 to 12 times, more preferably 4 times the diameter of the screws, possibly in different location along the screws, preferably as close as possible from the die 24.

[0075] Preferably, a fluid removal device 30.1 , such as a vacuum, is directly connected to the venting outlet 30, allowing for the extraction of volatile components in either fluid or gas form from the polymer melt. The fluid removal device 30.1 may further comprise a condenser (not shown) acting as a cold trap to assist in the collection of VOCs.

[0076] The melt processing device 20 may include an additional inlet 32 for additives to help compounding of the main thermoplastic material. Various additives can be added ranging from colorants, flame-retardants, antioxidants, heat stabilizers, impact modifiers, lubricants, nucleating agents, optical brightening agents and / or UV stabilizers or mixtures thereof.

[0077] Figure 5 represents a melt processing device 20’ according to the second embodiment of the present invention, which is designed to ensure stripping of the polymer melt.

[0078] Preferably, the melt processing device 20’ comprises a stripping inlet 34 configured for enabling a stripping fluid injection into the polymer melt. Forthis purpose, a suitable fluid pressure pump 34.1 and a tank 34.2 may be arranged. Stripping can be performed on a length of 4 to 20 times the diameter of the screws, preferably 4 to 10 times the diameter of the screws.

[0079] The melt processing device 20’ preferably comprises a first venting outlet 36 connected to a pump 36.1 , which is arranged downstream the feeding inlet 28 and the stripping inlet 34, enabling extraction of at least a part of the volatile components and the removal of the stripping fluid. The device 20’ preferably further comprises a second venting outlet 38 and a corresponding pump 38.1 arranged downstream the inlet 32 for additives.

[0080] Inside the melt processing device 20, 20’ of the invention, the main polymer matrix to be purified is melted by being submitted to temperature and shearing. The temperature can be comprised between 100 °C and 400 °C, and preferably between 120 °C and 350 °C, and more preferably between 200 °C and 300 °C, typically at the processing temperature of the main thermoplastic material. It is to note that the processing temperature is typically 10 to 50 °C above the melting point onset of the main thermoplastic matrix, as measured by DSC (at 10 K / min under air) and below its degradation temperature onset as measured by TGA (at 10 K / min, under air). In the case of PET based compounds, this temperature can be between 240 and 300 °C.

[0081] With reference to figure 3, in step S105 of adding the pro-degradant, the selective degradation is advantageously performed on the impurities i.e., macromolecular secondary materials, with a lower melting point than the main thermoplastic matrix, and molecular impurities. The added free radical generator reaches a temperature above the activation of its decomposition temperature triggering the generation of radical species, with or without the combination of specific catalysts. The temperature of this step can even be higher than the boiling point of the free radical generator.

[0082] Preferably, the pro-degradant remains inside the melt processing device for at least a half-life reaction of said pro-degradant at the temperature of the melt-compounding (step S103). The duration of radical generation (residence duration of the pro-degradant) can range from 0.01 s to 240 min. Preferably, when in the polymer melt, it ranges from 0.1 s to 20 min, morepreferably from 0.1 s to 10 min, and when in the solid state, from 60 to 240 min.

[0083] Advantageously, adding the pro-degradant containing the free radical generator into the polymer melt may further selectively degrade:- secondary materials, i.e., other polymers present in smaller quantities such as thermoplastics and / or thermosets and / or combinations thereof, in particular those melting at lower temperature than the main thermoplastic to be purified. In the case of reclaimed PET based thermoplastic PVC is a well-known secondary material melting at lower temperature.- other secondary polymers, which are contained in the main thermoplastic matrix, such as at least high-density polyethylene, low- density polyethylene, polyvinyl chloride, polypropylene, polystyrene, all types of polyamides, acrylonitrile butadiene styrene, and and / or various types thermoplastic elastomer including thermoplastic and / or copolymers thereof and / or mixtures thereof.- impurities mainly volatile organic components such as at least benzene, HCI, CO, CO2, acetaldehyde, benzoic acid, toluene, xylene, phenols, limonene, phthalate esters, acetaldehyde, bisphenol A, hexanal, benzaldehyde, methyl salicylate, carvacrol, ethanol, 2-butanone, monomers, oligomers, solvents, inks derivates and / or, glue derivates and / or mixtures thereof.- any intermediate degradation product formed by or leading to the formation of PVC and benzene impurities.

[0084] Preferably, the thermoplastic material to be purified comprises 0.005 to 10 wt% of the pro-degradant relative to a total weight of the polymer melt, and more preferably comprises from 0.01 to 2 wt%.

[0085] The effectiveness of selectively degrading PVC and benzene impurities using various types of pro-degradants has been tested, and the results are presented in this description along with figures 6 to 11 . The most preferable types of pro-degradants used in the tests are listed in Table 1 below.

[0086] Table 1 :

[0087] Figures 6 and 7 show a comparison graph of benzene rates measured for r-PET-2 as a reference, and for purified PET thermoplastic materials that have been purified from at least PVC and benzene impurities by means of the selective degradation performed according to the method of the invention, using a first variant comprising 4 wt% of different pro-degradants added to the corresponding PET polymer melt, with a residence duration of 5 min in the melt processing device.

[0088] It should be noted that, volatile components were removed from the thermoplastic material during each of the tests of figures 6 to 11 by means of an extruder degassing system (no devolatilization or stripping step was carried out).

[0089] The r-PET-2 reference highlights the increase of the benzene rate due to the presence of PVC. The element “0” corresponds to a thermoplastic material which was melt-compounded in the melt processing device, but without the addition of the pro-degradant. The other elements numbered 1 to 8 correspond to a respective pro-degradant type form Table 1 above.

[0090] It can be observed that the pro-degradants 1 (Zn peroxide), 5 (p-Menthane hydroperoxide) and 7 (Di-tert-butyl peroxide) seem to be giving the best results i.e. , effective selective degradation of PVC and benzene, highlighting an overall reduction of up to 100 % of impurities. However, in those conditions the PET thermoplastic is too degraded in the process.

[0091] In figures 8 to 11 , the same pro-degradants 1 , 5 and 7 were tested with different amounts in the thermoplastic material to be purified and with different residence durations in the melt processing device.

[0092] In figure 8, the comparison was performed using a second variant of the invention comprising a content of 2 wt% of the pro-degradant added to the corresponding PET polymer melt, and with a residence duration of 2 min.

[0093] With this second variant, only a minor degradation was observed on the purified PET, while achieving good results, for instance, with the prodegradant 7, r-PET-2 went from having 5.717 ppm of benzene at 200 °C, to 0.273 ppm, thus achieving a reduction of up to 95 % of the benzene rate.

[0094] In the third variant of figure 9, only the amount of the pro-degradant was changed to 1 wt% relative to the total weight of the polymer melt. Where this time, the pro-degradant 5 shows the best results (getting the r-PET-2 to 0.119 ppm of benzene at 200 °C) while causing minor to no degradation to PET.

[0095] In the fourth variant presented in figure 10, the added pro-degradant is in the amount of 1 wt%, and the residence duration was of 1 min. This time both pro-degradants 5 and 7 exhibited the best results.

[0096] Further reducing the content of the pro-degradant to 0.5 wt% in the fifth variant achieved the best results, as can be seen in figure 11 . Where the three pro-degradants 1 , 5 and 7 were able to significantly reduce / elim inate the benzene rate from r-PET-2, while causing minor to no degradation to the latter.

[0097] Advantageously, the purified material obtained according to the method of the invention comprises a content of benzene as low as 0 and as high as 0.1 ppm, and preferably equal to at most 0.07 ppm.

[0098] The disclosed results in figures 6 to 11 , were obtained using a new method developed by the inventors. In fact, as part of the present invention, and besides developing a method to analyze the free benzene contained in r- PET, inventors also developed a simple method for evaluating the level of purification of the thermoplastic matrix enabling to analyze the amount of remaining PVC and the amount of benzene that could be emitted by itsdegradation. Advantageously, this new method helps to demonstrate that not only the pro-degradant is degrading / removing benzene but also degrading / removing PVC, thus ensuring a better closed loop recycling of PET materials.

[0099] The method for evaluating the level of purification of a thermoplastic matrix, notably of the purified material according to the invention, specifically when the impurities are macromolecular secondary materials such as PVC and molecular impurities such as benzene, comprises measurements performed by gas chromatography (GC) coupled with mass spectrometry (MS) at least in triplicates. The method preferably comprising one measurement performed at a first temperature of 150 °C to evaluate the amount of molecular impurities such as benzene, and two measurements performed at two different temperatures 150 and 200 °C are needed to evaluate the amount of benzene that can be emitted by the thermal degradation of macromolecular impurities such as PVC.

[0100] For the valuation of molecular impurities such as benzene, the following steps can be performed:- Aliquots of samples of the thermoplastic matrix can be precisely weighed in gas-tight glass vials adapted to a GC headspace sampler, immediately capped with the adequate crimped cap after weighing; and- Vials are preferably heated at the temperature of 150 °C for 60 min allowing the thermal desorption of benzene without degrading the thermoplastic matrix and the PVC impurities. The vial can then be cooled down at room temperature and maintained at 10 °C; and- Vials can be heated at a temperature of 60 °C while solid phase microextraction (SPME) fibers are exposed to the headspace for 30 min; and- SPME fibers can be inserted in the GC injection port to transfer the adsorbed volatile compounds to the GC capillary column. The benzene may be detected by triple-quadrupole mass spectrometry and quantified using external calibration based on certified benzene standards.

[0101] For the valuation of macromolecular impurities such as PVC, the following steps can be performed:- Aliquots of samples of the thermoplastic matrix can be precisely weighed in gas-tight glass vials adapted to a GC headspace sampler, immediately capped with the adequate crimped cap after weighing; and- Vials can be heated at the temperature of 200 °C for 60 min allowing both the in situ thermal degradation of PVC impurities and the thermal desorption of benzene without degrading the thermoplastic matrix. The vial is then cooled down at room temperature and maintained at 10 °C; and- Vials can be heated at a temperature of 60 °C while solid phase microextraction (SPME) fibers are exposed to the headspace for 30 min; and- SPME fibers can be inserted in the GC injection port to transfer the adsorbed volatile compounds to the GC capillary column. The benzene may be detected by triple-quadrupole mass spectrometry and quantified using external calibration based on certified benzene standards; and- The presence of PVC can be evaluated by the one of benzene when subtracting the amount of benzene emitted at 200 °C by the one emitted at 150 °C. Advantageously, the present invention ensures that the difference between the two benzene amounts is zero, thereby demonstrating that the PVC was completely degraded by the pro-degradant.

Claims

Claims1. Method (100) for purifying a thermoplastic material (22), such as polyethylene terephthalate (PET), comprising the steps of:- (S102) supplying pellets or shreds of the thermoplastic material into a melt processing device (20; 20’); and- (S103) melting the thermoplastic material to form a polymer melt; characterized in that the method (100) further comprises a step of:- (S104) adding a pro-degradant into the pellets or shreds of the thermoplastic material (22), and / or into the polymer melt, to selectively degrade at least polyvinyl chloride (PVC) and benzene impurities, and / or any intermediate degradation product formed by or leading to the formation of said polyvinyl chloride (PVC) and benzene impurities, contained within said thermoplastic material (22) and / or said polymer melt.

2. Method (100) according to claim 1 , wherein the pro-degradant comprises at least one free radical generator such as peroxide and / or at least one bicumenederived molecule and / or ozone gas.

3. Method (100) according to any of claims 1 and 2, wherein the pro-degradant comprises at least one of the following: zinc peroxide, calcium peroxide, strontium peroxide, magnesium peroxide, p-menthane hydroperoxide, di-cumyl peroxide, di-tert-butyl peroxide, 2,3-dimethyl-2,3-diphenylbutane, ozone, or a combination thereof.

4. Method (100) according to any of claims 1 to 3, wherein the polymer melt comprises 0.005 to 10 wt% of the pro-degradant relative to a total weight of the polymer melt.

5. Method (100) according to any of claims 1 to 4, wherein the pro-degradant remains inside the melt processing device (20; 20’) for at least one half-life of said pro-degradant at the temperature of the melting step (S103).

6. Method (100) according to any of claims 1 to 5, wherein the pro-degradant remains inside the melt processing device (20; 20’) for a total duration ranging from 0.1 s to 10 min.

7. Method (100) according to any of claims 1 to 6, wherein the polymer melt inside the melt processing device (20; 20’) comprises a temperature ranging between 120 °C and 350 °C.

8. Method (100) according to any of claims 1 to 7, further comprising a step (S105) of devolatilizing or stripping the polymer melt to remove volatile components following the addition (S104) of the pro-degradant.

9. Method (100) according to any of claims 1 to 8, wherein the thermoplastic material comprises neat PET or reclaimed PET or blends thereof, or blends thereof with additional thermoplastics.

10. Purified material comprising a thermoplastic material which has been purified with the method (100) according to any of claims 1 to 9.11 . End product made from the purified material according to claim 10.

12. Melt processing device (20; 20’) suitable for executing the steps of the method (100) according to any of claims 1 to 9, comprising a proximal end (20.1 ) for receiving the pellets or shreds of the thermoplastic material (22), and a distal end (20.2) for the exiting of an output thermoplastic material (26), characterized in that the melt processing device (20; 20’) further comprises a feeding inlet (28) configured for feeding the pro-degradant into the polymer melt, said feeding inlet (28) being arranged upstream a venting outlet (30, 36, 38) at a distance (d) of at most half of a total extent (L) of the melt processing device (20; 20’).

13. Melt processing device (20; 20’) according to claim 12, wherein said melt processing device (20; 20’) comprises an extruder, and the distance (d) betweenthe feeding inlet (28) and the venting outlet (30, 36, 38) is 4 to 40 times, preferably 4 to 12 times the diameter of a screw of said extruder.

14. Method for evaluating the level of purification of the material according to claim 10, characterized in that said method comprises the steps of:- heating the purified material to a first temperature in a gas-tight vial;- measuring the benzene released by the purified material after heating at the first temperature;- heating a second sample of the same purified material at a second temperature which is superior to the first temperature, in a second gas-tight vial;- measuring the benzene released by the purified material after heating at the second temperature;- estimating the presence of polyvinyl chloride (PVC) based on the difference between the benzene measured at, respectively, the first temperature and the second temperature.

15. Method according to claim 14, wherein the first temperature is around 150 °C, and the second temperature is around 200 °C.

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