Method for producing epsilon-caprolactam by depolymerization of polycaprolactam (PA6)

Abstract

The present invention relates to a process for producing ε-caprolactam by depolymerization of polycaprolactam (PA6), which comprises: a. contacting a composite material comprising PA6 and at least one fibrous reinforcement with a solvent comprising at least one polyol at a temperature in the range of 130°C to 200°C, thereby obtaining a solution comprising solubilized PA6 and an insoluble fraction comprising at least said fibrous reinforcement; b. separating the insoluble fraction from the solution; c. combining the solution with a coagulation liquid comprising water to obtain coagulated partially depolymerized PA6 dispersed in a liquid phase; d. separating the solidified PA6 from the liquid phase; e. subjecting the coagulated PA6 separated in phase d to further depolymerization by hydrolysis to obtain ε-caprolactam; Includes.

Description

[Technical field]

[0001] The present invention relates to a method for producing ε-caprolactam by depolymerization of polycaprolactam (PA6), in particular the depolymerization of polycaprolactam contained in a composite material comprising at least one fiber reinforcement. [Background technology]

[0002] As is known, polycaprolactam or polyamide-6 (PA6) is a semi-crystalline thermoplastic polymer with very good thermal, mechanical and chemical resistance properties, which allows its use in various application fields such as textiles, automobiles, electrical and electronic components, construction, coatings, etc. In particular, PA6 is widely used in the field of engineering plastics to manufacture composite products, i.e. materials in which said polymers are combined with one or more additional fibrous reinforcements, such as glass fibers, carbon fibers, polymer fibers, etc., to enhance their mechanical properties.

[0003] Recycling of composites containing PA6 is currently done mainly by mechanical means: both post-industrial and post-consumer composites are treated by processes such as crushing, grinding, sieving, washing and densification to obtain material of suitable size for use in new production cycles as partial replacement for virgin polymers.

[0004] However, mechanical recycling has the drawback that the mechanical properties of the polymeric material deteriorate with use, so that new products can only be made of lower quality than the product that generated the waste (downgrading). Furthermore, the presence of coloring compounds prevents the polymer from being reused in all its possible applications. A further problem is the accumulation of additives and contaminants in the polymeric material as the used polymer is reused through repeated cycles.

[0005] In reinforced composites, the fibers are selected according to the specific dimensional and mechanical properties to achieve the desired improvement. For example, for PA6 without and with glass fiber filler, these fibers can improve various mechanical properties, as shown in the table below. [Table 1]

[0006] However, the improvement in mechanical properties that can be achieved by the production of composite materials, especially those made with glass or other inorganic fibers, is accompanied by the difficulty of recycling these materials: in fact, the methods of recycling these products after their end of life are essentially relegated to mechanical recycling, which imposes great limitations on the reuse of recovered polymers, since the mechanical properties of the polymers deteriorate over the use of the product, so that recycled polymers are generally only used in mixtures with virgin polymers, with few exceptions.

[0007] On the other hand, in order to avoid discharging pollutants and non-biodegradable substances into the environment, and to reduce CO from fossil fuels, 2 To meet emissions reduction targets, there is an increasing need to reuse materials, especially plastics, and reuse is often mandated by current regulations that ban certain materials from landfills and require new products to be made using increasing amounts of recycled material.

[0008] While physical-mechanical recycling, although widely used, is still a route that leads to the downgrading of the material, the preferred route is the one that leads to the upgrading of the material. Upgrading by chemical means leads to regaining the starting raw material (in the case of polymers, the starting monomer) that can be used to produce new products of the same quality as those that can be obtained from virgin materials derived from petroleum, using the same methods used to produce the virgin materials.

[0009] Chemical recycling methods described in many studies and patents are based on the dissolution of polymers and composites containing them in solvents, followed by separation of insoluble materials by filtration, removal of residual impurities (e.g. pigments and other additives added to the polymer during the manufacturing process), and subsequent recovery of the polymer, typically in the form of granules, by evaporation of the solvent or other techniques that return the polymer to a solid phase.

[0010] The following are some examples: - the process described in the publication Papaspyrides, CD, JG Poulakis, and CD Arvanitopoulos, "Recycling of glass fiber reinforced thermo-plastic composites. I. Ionomer and low density polyethylene based composites.", Resources, conservation and recycling 14.2 (1995): 91-101), which comprises dissolving glass fiber-filled LDPE in toluene and subsequently separating the polymer from the fibers. - A PVC recycling process by impurity separation proposed by VinyLoop, which involves selectively dissolving PVC in a solvent.

[0011] So far, the techniques of separating the polymers from the fibres and other impurities by dissolving them forming the composite material have not given optimal results, since in any case part of the additives remains in the polymer, altering its appearance, also in terms of colour, and therefore always severely limiting the possible use of recycled polymers. In particular, particles of various chemical compositions, especially inorganic solids, remain in the polymer, preventing the recovered polymers from being used in important production sectors, such as the spinning process in the textile industry, where polyamides and polyesters of synthetic origin are mainly used.

[0012] The above-mentioned disadvantages of mechanical recycling of polymers and recycling via solvent dissolution of polymers, together with the increasingly recognized need to protect the environment by applying the principles of a circular economy, make it highly desirable, for example, to be able to chemically recycle composite materials containing PA6 and remanufacture the starting monomer ε-caprolactam, as is already the case for materials containing PA6 in a mixture with relatively small amounts of foreign matter (e.g. Nylon 6 yarns or textile substrates, or fractions resulting from the separation of polyamides from carpets, tiles, various types of engineering plastics, etc.).

[0013] In chemical recycling, scrap or waste containing PA6 undergoes a treatment process in which the polymer chains are depolymerized to obtain the original monomer (ε-caprolactam), which after purification has the same quality as virgin monomer and can therefore be used without any particular restrictions to manufacture new high-quality products.

[0014] The depolymerization of PA6 is a chemical process that has been known for many years and is also applied industrially in processes known as, for example, Evergreen (Depolymerization of polyamides, U.S. Pat. No. 5,668,277, 1997) and Polyamide 2000 (POLYAMIDE 2000-WORLD CONGRESS: The Polyamide Chain Resins, Products, Developments, Technologies, Markets, March 14-15-16, 2000, Zurich, Switzerland).

[0015] In current technology, several processes have been proposed to depolymerize PA6, including pyrolysis, ammonolysis, solvolysis, and pyrolysis processes under pyrolysis conditions in supercritical fluids. Among these processes, depolymerization by pyrohydrolysis in the presence or absence of a catalyst is one of the most promising chemical recycling options due to its feasibility of application and implementation on an industrial scale.

[0016] In general, hydrolytic depolymerization involves treating PA6 in the molten state with water in the form of steam at temperatures in the range of 200° C.-450° C. and pressures of 0.2-20.0 bar to form the monomer ε-caprolactam, which is separated from the depolymerization mixture in the vapor phase (e.g., by stripping with a steam stream), condensed, and then purified to remove impurities and unwanted reaction by-products.

[0017] Examples of hydrolytic depolymerization processes are described, for example, in US Pat. No. 6,087,494 and US Pat. No. 5,294,707.

[0018] Known depolymerization processes, whether of hydrolytic or other type, have the practical disadvantage that they are unable to directly process PA6-based composites containing a large amount of fiber reinforcement. In fact, inorganic fiber materials tend to accumulate in the depolymerization reactor, generating solid residues and adherent contamination that are very difficult to remove, making it necessary to periodically shut down the operation of the depolymerization plant.

[0019] The second problem affecting the PA6 depolymerization process industrially is the duration of the depolymerization reaction, which depends, among other factors, on the molecular weight of the PA6 chain undergoing depolymerization. Although numerous catalysts have been investigated in the art to accelerate the depolymerization reaction and / or make it run faster, there remains a need to reduce the duration of the depolymerization step, as this would improve the productivity of the ε-caprolactam manufacturing process.

[0020] A process for recovering polyamides, particularly PA66 and PA6, from process wastes or post-consumer products containing polyamides in a mixture with foreign bodies such as fiber reinforcements like glass fibers is described in US Pat. No. 5,430,068. The process described in this patent includes (1) dissolving the polyamide in an anhydrous polyol or in an aliphatic carboxylic acid having 2 to 6 carbon atoms at high temperature, (2) separating the foreign bodies from the polyamide solution, (3) combining the polyamide solution with an additional amount of the same solvent at a temperature sufficiently lower than that of the polyamide solution so as to rapidly cool the solution and precipitate the polyamide, and (4) recovering the precipitated polyamide. In one embodiment, the solvent used to dissolve PA66 or PA6 is a glycol, such as ethylene glycol (EG) or propylene glycol (PG). The process aims to recover the polyamide in an essentially undegraded form, i.e. with as little degradation of its molecular weight as possible (controlled by relative viscosity analysis) to allow its reuse in extrusion or molding processes.

[0021] However, PA6, which can be recovered from the composites by the above-mentioned process, i.e. by dissolving at high temperature and subsequently precipitating the solid polyamide only by contact with the solvent glycol, is not suitable as such to undergo a depolymerization process to obtain ε-caprolactam, in particular by hydrolysis, since said dissolution treatment at high temperature in EG or PG also promotes the reaction of the terminal carboxyl groups of the PA6 chains with the solvent itself and the resulting formation of ester groups with EG or PG. The presence of these esterified groups in PA6 results in the introduction of non-negligible amounts of EG or PG into the depolymerization mixture, which has a negative effect on the yield of the depolymerization reaction and on the quality of the ε-caprolactam obtained. In fact, during the depolymerization step, the glycols EG and PG are liberated from the esterified end groups, which form light by-products that evaporate under the reaction conditions together with the caprolactam monomer. These by-products include the glycol itself and its derivatives, such as esters of short-chain organic acids, hydroxy acids, lactones formed by cyclization of the hydroxy acids, unsaturated ketones, etc. These by-products, which may be present in relatively high proportions, are difficult to separate from the caprolactam, either by using chemical treatments or by carrying out various distillation steps, and have a significant impact on the purification costs and the final quality of said monomer obtained. These by-products, if present, also interfere with the polymerization process of the recycled ε-caprolactam. Summary of the Invention [Problem to be solved by the invention]

[0022] In view of the above-mentioned current state of the art, the Applicant has addressed the problem of providing a method for the depolymerization of PA6 to produce the monomer ε-caprolactam, which overcomes the drawbacks of the known art.

[0023] In particular, it is an object of the present invention to provide a depolymerization process for producing ε-caprolactam that is applicable to composite materials comprising PA6 also blended with a significant amount of at least one fiber reinforcement.

[0024] A further object of the present invention is to provide a process for the preparation of ε-caprolactam, in which the depolymerization step is carried out in a relatively short time and with a high conversion yield of PA6, thus increasing the productivity of said process. [Means for solving the problem]

[0025] It has now been found that the above-mentioned and other objects, which will be better explained in the following description, can be achieved by subjecting the composite material to a pretreatment for separating the fibre material (and possibly other foreign matter), which pretreatment is carried out under conditions which make it possible to recover the PA6 in a partially depolymerised form and therefore to more easily and quickly depolymerise it by hydrolysis to ε-caprolactam, as a result of which the overall productivity of the process is increased.

[0026] The method according to the invention also makes it possible to use composites based on PA6 and reinforcing fibres, for example from the engineering plastics sector, in the chemical recycling process of PA6.

[0027] The separation of the fibre material is achieved by treating the composite material with a solvent comprising at least one polyol, preferably a glycol, under heating (130°C to 200°C) so as to obtain an initial suspension containing solubilized PA6 and an insoluble fraction containing fibre material that can be easily separated from the PA6 solution. The PA6 solution is then contacted with an aqueous coagulation liquid to produce coagulated PA6 in a partially depolymerized form. Surprisingly, it has been found that the coagulation of PA6 solubilized in a polyol by water, but not by the polyol itself as described in US Pat. No. 5,430,068, inhibits the esterification of the carboxyl end groups of the polyamide by the polyol, thus favouring the production of higher quality ε-caprolactam at the subsequent stage of completion of the depolymerization by hydrolysis.

[0028] As a result of the removal of the fiber material and the recovery of the partially depolymerized PA6 substantially free of esterified end groups, the method of the invention makes it possible to effectively apply hydrolytic depolymerization to chemically recycle said PA6 contained in composite materials, thus allowing these materials, previously disposed of in landfills or mechanically recycled, to be used as raw material for the production of the monomer ε-caprolactam.

[0029] In addition, since the partial decomposition of the recovered PA6 is favourable for the subsequent completion of the depolymerisation to ε-caprolactam, the steps of dissolving PA6, separating the fibre material and solidifying the solubilised PA6 can also be carried out in the presence of oxygen, in particular air, which, as is known, at high temperatures leads to the oxidation (and therefore chemical transformation) of PA6, thus simplifying the plant management of the process, since it is not necessary to adopt absolute inert atmospheric conditions (oxygen concentration limited to a few ppm under worst case conditions) but only to ensure blanket conditions necessary to avoid flammable or explosive conditions of the solvent.

[0030] A further advantage of the present invention lies in the fact that the recovered fiber reinforcement is of high quality, substantially free of polymer residues and therefore in a form suitable for reuse in the same or another manufacturing process, thus saving raw materials and energy, especially in the case of glass or carbon fibers, whose manufacturing processes are very expensive.

[0031] Thus, according to a first aspect, the present invention relates to a process for the preparation of ε-caprolactam by depolymerization of polycaprolactam (PA6), the process comprising the steps of: a. contacting a composite material comprising PA6 and at least one fiber reinforcement with a solvent comprising at least one polyol at a temperature in the range of 130°C to 200°C, thereby obtaining a solution comprising solubilized PA6 and an insoluble fraction comprising at least said fiber reinforcement; b. separating the insoluble fraction from the solution; c. combining said solution with a coagulation liquid comprising water to obtain coagulated partially depolymerized PA6 dispersed in a liquid phase; d. separating the solidified PA6 from the liquid phase; e. subjecting the coagulated PA6 separated in phase d to further depolymerization by hydrolysis to obtain ε-caprolactam; Includes.

[0032] The composite material usable for the purposes of the present invention comprises PA6 and at least one fiber reinforcement.

[0033] The fibrous reinforcement comprises inorganic or organic fibers insoluble in the polyol used in the dissolution phase a, such as glass fibers, carbon fibers, polymeric fibers of polymers other than PA6, and mixtures of the above mentioned fibers. In one embodiment, the fibrous reinforcement comprises at least glass fibers.

[0034] The composite material may also include one or more additional materials other than PA6 and reinforcing fibers. Examples of additional materials that may be present in the composite material include dyes, UV absorbers, fillers, various types of additives (e.g., flame retardants, antistatic agents, antibacterial agents, nucleating agents, etc.), and other contaminants resulting from the use of the product or its recovery (e.g., additives accumulated in the material as a result of previous mechanical recycling cycles).

[0035] Preferably, PA6 is present in the composite in an amount in the range of 50% to 95% by weight, more preferably in the range of 55% to 90% by weight, even more preferably in the range of 60% to 85% by weight, based on the total weight of PA6 and the fiber reinforcement.

[0036] Preferably, the fiber reinforcement is present in the composite material in an amount in the range of 5% to 50% by weight, more preferably in the range of 10% to 45% by weight, even more preferably in the range of 15% to 40% by weight, relative to the total weight of PA6 and the fiber reinforcement.

[0037] Typically the composite material will contain the additional material in an amount of up to 15% by weight, preferably up to 10% by weight, for example in the range 1% to 5%, based on the total weight of PA6 and the fibre reinforcement.

[0038] Preferably, the composite material comprises post-consumer waste (end-of-life products) and / or post-industrial waste (waste from the composite manufacturing process).

[0039] The composite material can be provided to the method according to the invention in various shapes and sizes. Preferably, the composite material is provided in pieces of approximately 1-3 mm in size, which can be obtained, for example, by pre-treatment by crushing or grinding the composite material.

[0040] The solvent used to dissolve the PA6 contained in the composite material comprises at least one polyol. Examples of polyols that can be used for the purposes of the present invention are ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, butylene glycol, and glycerin. The polyol can be used alone or as a mixture of two or more polyols.

[0041] In a preferred embodiment, the solvent comprises or consists of at least one glycol, preferably selected from ethylene glycol, propylene glycol, and mixtures thereof. In a particularly preferred form, the solvent is ethylene glycol.

[0042] Preferably, in step a, the weight ratio of composite material to solvent is in the range of 1:1 to 1:50, preferably in the range of 1:2 to 1:25.

[0043] In step a, the composite material is contacted with the solvent to solubilize the PA6 at a temperature in the range of 130°C to 230°C, preferably in the range of 150°C to 180°C.

[0044] Step a can be carried out, for example, in a reactor by keeping the mixture of solvent and composite material under stirring until the PA6 is completely dissolved.

[0045] Generally, the dissolution step lasts for a period ranging from 0.5 to 6 hours, more preferably from 2 to 4 hours.

[0046] After dissolution of the PA6, there is a two-phase mixture in the reactor consisting of a solution (liquid phase) containing the solubilized PA6 and an insoluble fraction (solid phase) containing the fiber reinforcement and any other materials or substances that are insoluble in the solvent at the operating conditions of step a (e.g. small amounts of polymer additives).

[0047] The insoluble fraction is then separated from the PA6 solution (step b), for example by filtration or decantation. Advantageously, said separation is carried out at elevated temperature, for example at a temperature in the range of 130° C. to 200° C., more preferably at the same temperature at which the polymer dissolution in step a was carried out.

[0048] The viscosity of the solution containing the solubilized PA6 varies depending on the temperature and the PA6 concentration. Generally, the PA6 solution appears as a clear, slightly viscous solution at the temperature where step a is carried out. Upon cooling to room temperature, it forms a highly viscous, paste-like material without undergoing solid-liquid phase separation.

[0049] The fibre reinforcement separated from the PA6 solution may advantageously be washed, for example with the same solvent used for solubilisation, to remove residual polymer associated with it.

[0050] In the coagulation step c, the PA6 solution is combined with a coagulation liquid comprising water to obtain coagulated and partially depolymerized PA6. For this purpose, for example, the PA6 solution can be poured into a vessel containing the coagulation liquid. Preferably, the coagulation liquid is maintained at a temperature in the range of 50°C to 95°C, more preferably at a temperature in the range of 80°C to 95°C.

[0051] Preferably, in the coagulation step c) the weight ratio between the solution containing solubilized PA6 and the coagulation liquid is in the range of 1:1 to 1:10, preferably in the range of 1:1 to 1:5.

[0052] Preferably, the mixture of the PA6 solution and the coagulation liquid is cooled to room temperature while the coagulated PA6 polymer is gradually formed, which is maintained in a dispersed state in a liquid phase comprising the solubilizing solvent and water. The coagulated polymer may appear in the form of irregularly shaped particles, for example of a size of about 1-5 mm.

[0053] In step d, the coagulated PA6 is separated from the liquid phase "solvent / coagulation liquid", for example by filtration. Preferably, the coagulated PA6 is washed with water to remove any residual solvent present.

[0054] Advantageously, in one embodiment, the water stream used to wash the coagulated PA6 and which also contains a proportion of solvent is recovered together with the liquid phase solvent / coagulation liquid stream produced by separation of the coagulated PA6.

[0055] In one embodiment, at least a portion of the mixture of the two streams is recycled directly to step c, where it is used as coagulation liquid.

[0056] In a further embodiment, at least a portion or all of the mixture of these two streams is subjected to a process to separate the solvent from the coagulation liquid (eg, water).

[0057] For example, the mixture of the two streams, comprising the solvent and the coagulation liquid, e.g. glycol and water, is subjected to a separation process by distillation: the evaporated phase, comprising the coagulation liquid (water) and a modest amount of solvent, can be reused in the coagulation step c, while the bottom liquid phase of the distillation, comprising essentially the solvent, can be reused in step a for further processing of the composite material.

[0058] The degree of partial depolymerization of the coagulated PA6 can be assessed by determining the relative viscosity value (Relative Viscosity - ISO 307 or ASTM 789).

[0059] The amount of carboxyl groups esterified as a result of the solubilization and coagulation treatment of PA6 can be determined by NMR analysis, as described in the Examples.

[0060] By means of the process according to the invention, it is possible, after completion of step d, to obtain a coagulated, partially depolymerized PA6 in which the amount of terminal carboxyl groups esterified with solvent molecules, in particular with ethylene glycol, is preferably less than 2.0% by weight, relative to the weight of the dried, coagulated PA6.

[0061] The coagulated, partially depolymerized PA6 obtained as described above can be fed to a subsequent hydrolytic depolymerization step (step e) to complete the depolymerization and obtain the monomer ε-caprolactam.

[0062] The hydrolytic depolymerization step may be carried out according to processes known to those skilled in the art.

[0063] Generally, the coagulated, partially depolymerized PA6 can be reacted with water, preferably in the form of a vapor stream, in the presence of an acid catalyst.

[0064] The PA6 is preferably subjected to hydrolytic depolymerization using a superheated steam stream at a temperature in the range of 200° C. to 450° C., for example at a pressure of 0.2 to 20 bar.

[0065] For this purpose, the PA6 is preferably fed to the depolymerization reactor in the molten state, for example via an extruder / pre-melter, but it may also be fed to the reactor in the form of solid granules, for example via a hopper and / or a dosing auger.

[0066] The steam, which is superheated to prevent recondensation inside the reactor, is preferably dispersed in the melt with the aim of maintaining a continuous uniform distribution and promoting capillary contact with the melt while ensuring continuous stirring and homogenization thereof.

[0067] The acid catalyst may be one of those known to those skilled in the art, for example selected from inorganic acids (e.g. orthophosphoric acid and boric acid), organic acids (e.g. p-toluenesulfonic acid), and ammonium phosphate salts.

[0068] The monomer ε-caprolactam formed during the depolymerization is separated from the depolymerization mixture in the gas phase, for example by a steam stream, and subsequently condensed.

[0069] For example, the vapours leaving the reactor may be sent to a condensation system by direct contact with water, making it possible to separate two streams: a first stream consisting essentially of vapours only; a second stream consisting of an aqueous solution containing crude ε-caprolactam at a concentration of more than 50% (but which is already of high purity).

[0070] The condensed monomer is then subjected to purification using methods known to those skilled in the art, for example, by solvent or solventless extraction (e.g., as described in U.S. Patent Application Publication No. 20190382339), hydrogenation, treatment with potassium permanganate, and distillation to separate light and heavy by-products.

[0071] One possible embodiment of the method according to the invention is described below with reference to the following figures: [Brief description of the drawings]

[0072] [Figure 1] FIG. 1 is a schematic diagram of a method for producing ε-caprolactam according to the present invention, in which “PA6 with GF” means a composite material containing polyamide and glass fiber as a reinforcement, and “EG” is ethylene glycol. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0073] In Figure 1, composite material 1 is fed to a grinding step 2 to obtain small size fragments, for example 1-3 mm. The ground composite material 3 is then subjected to a dissolution step 4 in EG solvent at a temperature of 130-200°C, in which a two-phase mixture 5 is produced comprising PA6 solubilized in EG and a suspended insoluble fraction comprising glass fibres. The solvent EG used for dissolution (stream 37) is taken from a storage unit 28.

[0074] In a subsequent filtration step 6, the insoluble fraction 7 containing the glass fibres is separated from the solubilized PA 6 at 8, for example by hot filtration at the same temperature as in the dissolution step 4. The glass fibres of the insoluble fraction 7 are then subjected to a washing step 9 with a stream 10 of glycol recycled from a storage unit 11 and subsequently extracted from the process (stream 12). The glycol used for washing (stream 13) is recycled partly to a coagulation unit 15 (stream 13a) and partly to a storage unit 11 (stream 13b) which is also supplied with a make-up glycol stream 14.

[0075] The solubilized PA6 stream 8 is subjected to a coagulation step 15 by contacting it with a water stream 17. Said coagulation produces PA6 granules in partially depolymerized form dispersed in a liquid phase of water and EG (stream 16) which are fed to a subsequent filtration step 21. The water stream 17 used for the coagulation is obtained from a water storage unit 18, which is also fed with a water stream 19 from the condensation phase 33 following the distillation phase 30 of the water / EG mixture and a make-up water stream 20.

[0076] In the filtration step 21, the stream 16 of the PA6 granule dispersion is separated into a stream 22 of coagulated PA6 granules and a stream 23 containing a water / EG mixture. The coagulated PA6 granules 22 are then subjected to a washing and drying step 24 to obtain dried granules 25. For washing, a water stream 26a is used, which is then partially recycled (water stream 35) to a storage unit 27, which is also supplied with the water / EG mixture 23 from the filtration step 21.

[0077] The water / EG mixture stream 29 contained in the storage unit 27 is subjected to a distillation step 30 from which an EG stream 31 and a vapor stream 32 are obtained, the vapor stream 32 being subjected to a condensation step 33 before producing a water stream 34 containing traces of EG separated from the cycle and the main stream 19 which is sent to the storage unit 18 for reuse in the coagulation step 15.

[0078] The EG stream 31 recovered from the distillation is collected in a storage unit 28 to which a make-up EG stream 36 is also provided.

[0079] The following examples are provided solely for the purpose of illustrating the present invention and should not be considered as limiting the scope of protection defined by the appended claims.

[0080] In the examples, the relative viscosity (RV) is measured according to the ISO 307 method (H 2 SO 4 The measurements were performed according to the following standard deviation: EXAMPLES

[0081] Reference Example (Comparative Example): Direct hydrolytic depolymerization of composite material (PA6 GF30) A pilot reactor with a volume of approximately 100 liters and equipped with a heating jacket was charged with 30 kg of granules of a commercial composite material called "PA6 GF30" containing 70 wt. % PA6 and 30 wt. % glass fibers, resulting in a total of 21 kg PA6 and 9 kg glass fibers (PA6 relative viscosity (RV): 2.7, fiber diameter: 11 μm, fiber length: 4.5 mm, glass: E-Glass (DIN 1259)).

[0082] The composite material was subjected to hydrolytic depolymerization in the presence of an acid catalyst operating at atmospheric pressure and at a temperature in the range of 250-300° C. to obtain caprolactam monomer.

[0083] As the depolymerization continued, although in practice it was not possible to depolymerize more than 20-30% of the input material, a gradual accumulation of glass fibers was observed inside the reactor, which made it impossible to discharge the remaining depolymerized mixture through the bottom valve. In order to restore the plant, it was necessary to dismantle the reactor, break down the formed solids by mechanical means and replace some of the blocked parts.

[0084] This test shows that it is not possible to depolymerize composite materials containing inorganic fiber reinforcement.

[0085] Example 1: Testing the solubility of PA6 in ethylene glycol 1000 g of ethylene glycol (EG) and 50 g of PA6 granules (PA6:EG ratio 1:20) were charged into an apparatus equipped with a reflux condenser, a temperature control thermometer, and a heating system. The PA6 had a relative viscosity RV=2.7, corresponding to a polymer chain with an average molecular weight of 19500. The mixture was gradually heated to approximately 135° C. under continuous stirring, whereupon complete dissolution of the polymer was observed, forming a clear yellow solution. The PA6 solution was heated to 150° C. and held at this temperature for approximately 1 hour without any formation of precipitate being observed. The mixture was then cooled to room temperature, resulting in a viscous, unfilterable, paste-like material (no solid-liquid phase separation).

[0086] Examples 2 and 3: Testing the maximum solubilization of PA6 in ethylene glycol (EG) at temperatures below the boiling point of EG The boiling point of ethylene glycol is 195° C. In order to operate the device at atmospheric pressure, the concentrations at which complete dissolution of the polymer could be achieved below 195° C. were investigated. For this purpose, dissolution tests were carried out following the same process as in Example 1, with the amounts of PA6 and EG shown in Table 1. Table 1 also shows the temperature at which complete dissolution of PA6 was observed (dissolution temperature) and the maximum temperature at which the solution was maintained (1 h) after said complete dissolution of PA6 (max temperature). [Table 2]

[0087] Based on the data obtained in Examples 1-3, the dissolution of PA6 in ethylene glycol below the boiling point of the solvent (195° C.) and at atmospheric pressure shows a substantially linear trend.

[0088] The PA6 solutions of Examples 2 and 3 also form a viscous, unfilterable, paste-like material after cooling to room temperature.

[0089] Example 4: Dissolution of PA6 GF30 composites at high solvent / polymer ratios The dissolution test of Example 1 for the composite material PA6 GF30 was repeated under the conditions shown in Table 2. [Table 3]

[0090] At the end of the test, a suspension of a white solid in a yellowish solution was obtained, which was filtered through a filter maintained at 170° C., followed by the procedure described below. The white solid remaining on the filter was washed with 50 g of hot ethylene glycol (temperature 150-170°C), followed by washing with a small amount of H 2 O to give a fluffy solid which was then dried at 70°C. The clear solution (filtrate) obtained by filtration at 170°C in an amount of 1035 g was mixed with 50 g of water used to wash the solid (total of 1085 g) and stirred in 4000 g of H2O at 90°C. 2 0 to give a mixture in which the solidified PA6 was suspended in the liquid phase.

[0091] H 2 The weight ratio of O to the PA6 solution in EG was 4000:1085=3.7, and H 2 The ratio of O:PA6 is 4000:35=114, and H 2 The O:EG ratio was 4000:1050 = 3.8.

[0092] The mixture containing the coagulated PA6 was filtered at room temperature to obtain a yellowish solid, which was then washed with a small amount of water to remove residual glycol on the surface. After drying, the solid was subjected to the analytical characteristics in Table 3. [Table 4]

[0093] The amount of amine and carboxyl groups in the polymer was determined by potentiometric titration.

[0094] The data in Table 3 show that the PA6 polymer, completely separated from the glass fibers, has undergone partial depolymerization of the polymer chains (RV value drops from 2.7 to 1.8). Partial depolymerization facilitates the subsequent chemical recovery of the ε-caprolactam monomer. In fact, the average chain length of the polymer with RV of 2.7 corresponds to approximately 19500 / 113=170 units, while the average chain length of the coagulated polymer with RV of 1.8 corresponds to approximately 9200 / 113=80 units (the relationship between the average molecular weight and the RV index is Mw=11500 (RV-1), the molecular weight of the repeating unit of PA6 is 113).

[0095] Examples 5 and 6: Dissolution of PA6 GF30 at low solvent / polymer ratios The process of Example 4 was repeated with the same amounts of EG and coagulation water, but with increasing amounts of composite material being processed.

[0096] The composition of the mixture and the dissolution conditions used in the test are shown in Table 4.

[0097] Examples 5 and 6 differ with respect to the time that the PA6 solution was held at the maximum dissolution temperature (2 h in Example 5 and 4 h in Example 6) in order to examine the possible effect of this operating condition on the degree of esterification of the PA6 end groups with EG, all other conditions being the same. [Table 5]

[0098] At the end of the test, a suspension of a white solid in a yellowish solution was obtained, which was filtered through a filter maintained at 180° C., followed by the procedure described below. The white solid remaining on the filter was washed with 50 g of hot ethylene glycol, followed by a small amount of H 2 O to give a fluffy solid which was then dried at 70°C. The clear solution (filtrate) obtained by filtration at 180°C in an amount of 1175 g was mixed with 50 g of water used to wash the solid (total of 1225 g) and stirred in 4000 g of H2O at 90°C. 2 0 to give a mixture in which the solidified PA6 was suspended in the liquid phase.

[0099] H 2 The weight ratio of O to the PA6 solution in EG was 4000:1225=3.3, and H 2 The ratio of O:PA6 is 4000:155=22.8, and H 2 The O:EG ratio was 4000:1050 = 3.8.

[0100] The mixture containing the coagulated PA6 was filtered at room temperature to obtain a yellowish solid, which was then washed with a small amount of water to completely remove the glycol remaining on the surface. After drying, the solid was subjected to analytical specifications in Table 4.1 for Example 5 and Table 4.2 for Example 6. [Table 6]

[0101] It is possible to separate the PA6 polymer from the composite and recover the PA6 in a partially depolymerized form (RV=1.58), even though the concentration of PA6 30GF composite in EG is significantly higher than in Example 4. The average chain length of the polymer changes from a value of 19500 / 113=170 units for the initial polymer with RV=2.7 to approximately 6670 / 113=60 for the recovered polymer with RV=1.58. [Table 7]

[0102] In Example 6, where the concentration of PA6 30GF composite in EG is also significantly higher than in Example 4, it is also possible to separate the PA6 polymer from the composite and recover the PA6 in a partially depolymerized form (RV=1.62). Extending the time to the maximum dissolution temperature does not result in esterification of the end groups. In this case, the average chain length of the polymer changes from a value of 19500 / 113=170 units for the initial polymer with RV=2.7 to approximately 7130 / 113=63 units for the recovered polymer with RV=1.62.

[0103] Example 7: Dissolution of "PA6 GF20" (PA6 filled with different 20% glass fibers by weight) The process of Examples 5 and 6 was repeated using the composite material "PA6 GF20" (80% by weight PA6, 20% by weight glass fibre).

[0104] The composition of the mixture and the dissolution conditions used in the test are shown in Table 5. [Table 8]

[0105] At the end of the test, a suspension of white solid in a yellowish solution was obtained. The suspension was filtered and the separated coagulated solid was washed as described in Example 5. After drying, the solid was submitted for analytical properties in Table 6.

[0106] This example shows that even with different concentrations of glass fibers it is possible to separate the PA6 polymer from the composite and recover it in a partially depolymerized form (RV=1.61). The average chain length of the polymer changes from a value of 19500 / 113=170 units for the initial polymer with RV=2.7 to approximately 7015 / 113=62 units for the recovered polymer with RV=1.61. [Table 9]

[0107] Example 8: Dissolution of PA6 GF30 with reduced solvent / polymer ratio The process of Examples 5 and 6 was repeated using the composite material "PA6 GF30" (30% by weight glass fibre).

[0108] The mixture compositions and dissolution conditions used in the tests are shown in Table 7. Notably, in this example, the amount of glycol used was reduced for the PA6 GF30 composite. [Table 10]

[0109] After dissolution, the resulting solution was subjected to the coagulation, filtration, and washing processes described in Example 5. After drying, the solid was subjected to analytical characteristics in Table 8.

[0110] This example shows that even when the amount of glycol is reduced, it is possible to separate the PA6 polymer from the composite and recover the PA6 in a partially depolymerized form (RV=1.95).

[0111] The average chain length of the polymer changes from a value of 19500 / 113=170 units for the initial polymer with RV=2.7 to approximately 10925 / 113=97 units for the recovered polymer with RV=1.95. [Table 11]

[0112] Example 9: Suitability of PA6 separated from composites for the depolymerization to ε-caprolactam The suitability of the PA6 recovered from the composite for depolymerization to ε-caprolactam monomer was investigated by NMR spectroscopy. For the purposes of this invention, a PA6 is considered suitable for hydrolytic depolymerization if the conditions in Table 9 are met. [Table 12]

[0113] Feeding PA6 that does not meet one or more of the requirements in Table 9 to the hydrolytic depolymerization may affect the activity of the catalyst or the quality of the resulting ε-caprolactam (e.g., presence of undesirable by-products, generation of corrosive inorganic acids, etc.).

[0114] After separation, washing and drying, the solidified PA6 granules of Example 4 (RV=1.8), Example 5 (RV=1.58) and Example 6 (RV=1.62) were analyzed by 1D and 2D NMR spectroscopy (1H and 13C). For this purpose, 51 mg of PA6 from Example 4, 54 mg of PA6 from Example 5 and 52 mg from Example 6 were each dissolved in 600 mL of deuterated trifluoroethanol (CF 3 CD 2 The NMR spectra of the three samples thus prepared gave the following parameters, as shown in Table 10. [Table 13]

[0115] The data in Table 10 confirm that the PA6 recovered from the composite is suitable to undergo hydrolytic depolymerization, with the useful PA6 content being well above the specification limit set at 90%.

[0116] Example 10: Depolymerization of PA6 separated from composites The process described in Example 4 was repeated on a sample of PA6 30GF composite using a 50 liter reactor equipped with a reflux condenser, a thermometer for temperature control, and an oil bath heating system with the goal of recovering the PA6 polymer separated from the glass fibers for depolymerization testing purposes. 20 kg of ethylene glycol (EG) and 5 kg of PA6 30FV granules were charged into the reactor for a total of 25 kg.

[0117] The mixture was gradually heated under continuous stirring to approximately 170-180°C and kept at this temperature for approximately 2 hours, ensuring complete dissolution of the polymer. From the bottom valve, the constantly warmed suspension was directly infiltrated onto a pilot filter (Nutsche type) under slight vacuum and then into a 120 liter mobile reactor equipped with an air stirrer, in which 80 liters of demineralized water at room temperature were present as coagulation liquid. The PA6 was discharged from the reactor in such a way that the glass fibers were left inside the filter system with a thin layer of the glycol solution of PA6 on the surface.

[0118] The coagulated solid, in the form of irregular yellowish appearing granules, was filtered, washed through the filter with water, drained and then dried in an oven at 70° C. under slight vacuum to a moisture content of less than 0.5% by weight.

[0119] Approximately 3.0 kg of PA6 was obtained with an RV of 1.6, as observed in the tests of Examples 4, 5, and 6. In particular, it was confirmed that there was essentially no ash in the dried PA6, and therefore no residual glass fibers.

[0120] The above-mentioned glass fiber separation pretreatment was repeated with four aliquots of PA6 GF30 until the recovery of dry PA6 was 12.34 kg.

[0121] The thus recovered glass fiber-free PA6 polymer was charged into a pilot reactor (volume: approximately 50 liters) made of an acid-resistant alloy similar to that of the Reference Example, equipped with a heating jacket and a superheated steam supply system.

[0122] The material was subjected to hydrolytic depolymerization in the presence of an acid catalyst operating at atmospheric pressure and at a temperature in the range of 250-300° C. to obtain caprolactam monomer.

[0123] The PA6 polymer was fed directly into the reactor in the form of its solid granules via a hopper and / or a metered Auger loading system. The acid catalyst and steam superheated to over 300° C. were then fed into the reactor. The steam leaving the reactor was fed into a condensation system by direct contact with water and separated into a first fraction containing only steam and a second fraction containing an aqueous solution of crude ε-caprolactam (concentration of approximately 50% by weight). The aqueous solution of crude ε-caprolactam was then passed through a conventional purification process to obtain a final product in line with commercial specifications (in terms of acidity, alkalinity, volatile base, APHA, optical density, and permanganate index).

[0124] During the depolymerization, no clogging phenomena were observed as described in the comparative example and the rate of the depolymerization process was higher than that observed in the depolymerization of PA6 waste, e.g. polymer recovered from carpet separation, where the RV of the polymer was maintained at 2.7.

[0125] Table 11 below shows the depolymerization data performed on the coagulated PA6 of Example 10 compared to a reference material consisting of post-consumer carpet waste with the following composition: 90 wt. % PA6 (RV=2.7), 10 wt. % other materials (mainly polypropylene, polyethylene terephthalate, adhesives, and additives). [Table 14]

[0126] The coagulated PA6 obtained after glass fiber separation proved to be fully depolymerizable with higher final solution yields and concentrations, similar to the best PA6 rejects recovered by separation of polyamide from carpets, but under the same experimental test conditions.

Claims

1. 1. A process for producing ε-caprolactam by depolymerization of polycaprolactam (PA6), comprising: a. contacting a composite material comprising PA6 and at least one fibrous reinforcement with a solvent comprising at least one polyol at a temperature in the range of 130°C to 200°C, thereby obtaining a solution comprising solubilized PA6 and an insoluble fraction comprising at least the fibrous reinforcement; b. Separating the insoluble fraction from the solution; c) combining the solution with a coagulation liquid comprising water to obtain coagulated partially depolymerized PA6 dispersed in a liquid phase; d. Separating the solidified PA6 from the liquid phase; e. subjecting the coagulated PA6 separated in phase d to further depolymerization by hydrolysis to obtain ε-caprolactam; The process includes:

2. 2. The process of claim 1, wherein the coagulation liquid is at a temperature in the range of 50°C to 95°C, preferably in the range of 80°C to 95°C.

3. 3. The process of claim 1 or claim 2, wherein the PA6 is present in the composite material in an amount in the range of 50 wt.% to 95 wt.%, more preferably in the range of 55 wt.% to 90 wt.%, even more preferably in the range of 60 wt.% to 85 wt.%, relative to the total weight of PA6 and the fibrous reinforcement.

4. 3. The process of claim 1 or claim 2, wherein the fibrous reinforcement is present in the composite material in an amount in the range of 5 wt.% to 50 wt.%, more preferably in the range of 10 wt.% to 45 wt.%, even more preferably in the range of 15 wt.% to 40 wt.%, relative to the total weight of PA6 and the fibrous reinforcement.

5. 3. The process of claim 1 or claim 2, wherein the solvent comprises at least one glycol.

6. 6. The process of claim 5, wherein the solvent is selected from ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, butylene glycol, and mixtures thereof.

7. 3. The process of claim 1 or claim 2, wherein the solubilization step a) is carried out at a temperature in the range of 150 to 170°C.

8. The process according to claim 1 or claim 2, wherein in step a, the weight ratio between the composite material and the solvent is in the range of 1:1 to 1:50, preferably in the range of 1:2 to 1:

25.

9. 3. The process according to claim 1 or claim 2, wherein the weight ratio between the solution containing the solubilized PA6 and the coagulation liquid is in the range of 1:1 to 1:10, preferably in the range of 1:1 to 1:

5.

10. 3. The process of claim 1 or claim 2, wherein the depolymerization by hydrolysis is carried out in the presence of at least one acid catalyst.

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