EA048833B1 - METHOD FOR PRODUCING EPSILON-CAPROLACTAM BY DEPOLYMERIZATION OF POLYCAPROLACTAM (PA6) - Google Patents

Description

The present invention relates to a method for producing ε-caprolactam by depolymerizing polycaprolactam (PA6). In particular, the present invention relates to depolymerizing polycaprolactam contained in a composite material containing at least one fibrous reinforcing material.

As is well known, polycaprolactam, or polyamide-6 (PA6), is a semi-crystalline thermoplastic polymer with excellent thermal, mechanical and chemical resistance properties, which allows it to be used in various application fields such as textiles, automotive, electrical and electronic components, structures, coatings, etc. In particular, PA6 is widely used in the engineering polymer sector for the production of composite products, i.e. materials in which the aforementioned polymer is used in combination with one or more additional fibrous reinforcing materials such as glass fibers, carbon fibers, polymer fibers and the like, which enhance their mechanical properties.

The processing of composite materials containing PA6 is currently carried out mainly mechanically. That is, composite materials of both industrial and household origin are processed by cutting, crushing, sifting, washing, compaction, etc. in order to obtain a material of a size suitable for use in new production cycles as a partial replacement for the primary polymer.

However, mechanical recycling has the disadvantage that it only produces new products of lower quality (downgrade) than the waste products, since the mechanical properties of the polymer material deteriorate with use. In addition, the presence of coloring compounds does not allow the polymer to be reused in all its potential applications. Another problem is the accumulation of additives and contaminants in the polymer material as cycles of reuse of the polymer with an expired service life are repeated.

In reinforced composites, the fibers are selected according to specific dimensional and mechanical properties to achieve the desired improvement. An example can be PA6 without and with glass fiber fillers, where various mechanical properties can be improved by using these fibers, as shown in the following table.

Properties of PA6 as is and glass fiber reinforced

PA6 as is PA6 glass fiber reinforced Fiberglass % 0 30 Density g/ ^cm3 -1,1 -1.4 Modulus of elasticity MPa -3000 -9000 Payload MPa -80 -180

However, the improvement in mechanical properties that can be achieved by producing composite materials, particularly with glass fibre or other inorganic fibres, is accompanied by complex recycling of these materials. In fact, the recycling methods for these products at the end of their service life are essentially limited to mechanical recycling, which imposes serious limitations on the reuse of recovered polymer, since the mechanical properties of the polymer deteriorate as the products are used: this usually forces the use of recycled polymer almost exclusively in a mixture with virgin polymer.

On the other hand, there is a growing need to recycle materials, especially plastics, to avoid releasing pollutants and non-biodegradable substances into the environment and to meet targets for reducing fossil fuel CO2 emissions. In addition, recycling is often made mandatory by current regulations that prohibit certain materials from being thrown into landfill and require new products to be manufactured using increasing amounts of recycled materials.

Physical-mechanical recycling, although widely used, is still a path that leads to deterioration of the quality of materials, while the preferred path is one that leads to improvement. Renewal by chemical methods leads to the re-obtaining of the original raw material (in the case of polymers, the original monomers), which can then be used to produce new products of the same quality as those obtained from primary raw materials of petroleum origin, using the same methods used to produce the latter.

The chemical recycling method described in numerous studies and patents is based on dissolving the polymer and its containing composites with a solvent, followed by separation of insoluble materials by filtration, removal of remaining impurities (e.g. pigments and other additives added to the polymer during production), and subsequent recovery of the polymer by evaporation of the solvent or other methods that return the polymer to the solid phase, usually in the form of pellets.

- 1 048833

The following examples can be given:

process described in Papaspyrides, C. D., J. G. Poulakis, and C. D. 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 involves dissolving glass fiber-filled LDPE in toluene and then separating the polymer from the fiber;

VinyLoop's process for regenerating PVC by separating impurities involves selectively dissolving PVC in a solvent.

To date, the technology of separating the polymer from fibers and other impurities by dissolving the polymer that forms the composite material has not yielded optimal results, since some of the additives still remain in the polymer, changing its appearance, including color, thereby always imposing severe restrictions on the possibilities of using the recycled polymer. In particular, particles of various chemical compositions, especially inorganic solids, remain in the polymer, which makes the recovered polymer unsuitable for use in important industries, such as spinning processes in the textile industry, where polyamides and polyesters of synthetic origin are mainly used.

The above-mentioned disadvantages of mechanical recycling and recycling by dissolving the polymers in a solvent, together with the increasingly felt need to protect the environment by applying the principles of a circular economy, make the possibility of chemical recycling of composite materials containing PA6 for the re-obtaining of the original monomer ε-caprolactam highly desirable, as is already the case, for example, for materials in which PA6 is contained in a mixture with relatively small amounts of foreign materials (e.g. Nylon 6 yarns or textile substrates, or fractions of polyamide separation from carpets, tiles, various types of engineering plastic, etc.).

In chemical recycling, scrap or waste containing PA6 is subjected to a treatment process in which the polymer chains are depolymerized to yield the original monomer (ε-caprolactam), which after purification has the same quality as the original monomer and can therefore be used without any special restrictions to produce new, high-quality products.

Depolymerization of PA6 has been a well-known chemical process for many years and is also used industrially, for example in the processes known as Evergreen (Depolymerization of polyamides, US 5668277, 1997) and Polyamide 2000 (POLYAMIDE 2000-WORLD CONGRESS: The Polyamide Chain Resins, Products, Developments, Technologies, Markets, March 14-15-16, 2000, Zurich, Switzerland).

In the prior art, several processes for the depolymerization of PA6 have been proposed, such as thermal hydrolysis, ammonolysis, solvolysis, and thermal decomposition processes under pyrolysis conditions in supercritical fluids. Among these processes, depolymerization by thermal hydrolysis in the presence or absence of a catalyst is one of the most promising alternatives to chemical recycling due to its possible application and implementation on an industrial scale.

In general, hydrolytic depolymerization involves treating PA6 in the molten state with water in the form of steam at a temperature in the range of 200°C - 450°C and a pressure of 0.2-20.0 bar to form the monomer ε-caprolactam. The monomer is separated in the vapor phase from the depolymerization mixture (e.g. by distillation with a stream of water vapor), condensed and then purified from impurities and undesirable by-products of the reaction.

Examples of hydrolytic depolymerization processes are described, for example, in U.S. Patents 6,087,494 and 5,294,707.

Known depolymerization processes, both hydrolytic and other types, have the practical disadvantage that they cannot directly process PA6-based composites that contain fibrous reinforcement materials in large quantities. In fact, the inorganic fibrous material tends to accumulate in the depolymerization reactor, creating solid residues and contaminants that are very difficult to remove, requiring constant shutdown of the depolymerization plant.

A second problem affecting industrial PA6 depolymerization processes is the duration of the depolymerization reaction, which depends, among other factors, on the molecular weight of the PA6 chains undergoing depolymerization. Although numerous catalysts have been studied in the art to accelerate the depolymerization reaction and/or make it proceed more rapidly, there remains a need to reduce the duration of the depolymerization step, since this will increase the productivity of the ε-caprolactam production process.

A method for recovering polyamide, especially PA66 and PA6, from process waste or post-consumer products containing it in admixture with foreign materials such as fibrous reinforcements such as glass fiber is described in U.S. Patent 5,430,068. The process described in this patent comprises: (1) dissolving the polyamide in an anhydrous polyol or in

- 2 048 833 aliphatic carboxylic acid with 2-6 carbon atoms at high temperature; (2) separating foreign materials from the polyamide solution; (3) combining the polyamide solution with an additional amount of the same solvent at a temperature sufficiently lower than the temperature of the polyamide solution to rapidly cool the solution and cause precipitation of the polyamide; (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 purpose of the process is to recover the polyamide in a substantially undecomposed form, i.e. causing the minimum possible decrease in its molecular weight (monitored by relative viscosity analysis), so that it can be reused in extrusion or molding processes.

PA6, recovered from the composite material by the above-mentioned process, i.e. by dissolution at high temperature and subsequent precipitation of the solid polyamide always and exclusively by contact with the glycol solvent, is however not suitable in itself for carrying out the depolymerization process to obtain ε-caprolactam, especially by hydrolysis, since the treatment by dissolution in EG or PG at high temperature also promotes the reaction of the terminal carboxyl groups of the PA6 chains with the solvent itself and the subsequent formation of ester groups with EG or PG. The presence of these esterified groups in PA6 leads to the introduction of a considerable amount of EG or PG into the depolymerization mixture, which has a negative effect on the yield of the depolymerization reaction and the quality of the ε-caprolactam obtained. Indeed, during the depolymerization stage, EG or PG glycols are released from the esterified terminal groups, which, under the reaction conditions, form light by-products that evaporate together with the caprolactam monomer. By-products include the glycols themselves and their derivatives, such as esters of short-chain organic acids, hydroxy acids, lactones formed by cyclization of the said hydroxy acids, unsaturated ketones, and others. These by-products, which may also be present in relatively high proportions, are difficult to separate from caprolactam by both chemical treatment and various distillation steps, which significantly affects the purification costs and the final quality of the resulting monomer. If present, these by-products also interfere with the polymerization processes of the recycled ε-caprolactam.

Taking into account the above-mentioned prior art, the applicant addressed the problem of developing a process for the depolymerization of PA6 to obtain the monomer ε-caprolactam, which overcomes the disadvantages of the known prior art.

In particular, the object of the present invention is to propose a depolymerization process for obtaining ε-caprolactam, which is applicable to a composite material containing PA6 mixed with at least one fibrous reinforcing material, also in a significant amount.

An additional object of the present invention is to provide a method for producing εcaprolactam in which the depolymerization step is carried out in a relatively short time and with a high conversion yield of PA6, which increases the productivity of the process.

It has now been found that the above and other objects which will be better explained in the following description can be achieved by subjecting the composite material to a pre-treatment for separating the fibrous material (and possibly other foreign materials) which is carried out under conditions which allow PA6 to be isolated in a partially depolymerized form and thus more readily and rapidly hydrolytically depolymerized to ε-caprolactam, resulting in an increase in the overall productivity of the process.

The method according to the present invention also allows the use of composite materials based on PA6 and reinforcing fibers, for example from the engineering plastics sector, in processes for the chemical processing of PA6.

The separation of the fibrous material is achieved by treating the composite material with heating (130-200°C) with a solvent containing at least one polyol, preferably a glycol, to obtain an initial suspension containing solubilized PA6 and an insoluble fraction containing fibrous material, which is easily separated from the PA6 solution. The PA6 solution is then contacted with an aqueous coagulating liquid to obtain coagulated PA6 in a partially depolymerized form. Surprisingly, it has been found that coagulation of PA6 solubilized in a polyol with water, rather than with the polyol itself, as described in U.S. Patent 5,430,068, inhibits the esterification of the terminal carboxyl groups of the polyamide with the polyol, thereby facilitating the formation of higher quality ε-caprolactam in the subsequent step of completing the depolymerization by hydrolysis.

By removing the fibrous material and recovering the partially depolymerized PA6, substantially devoid of esterified end groups, the method of the present invention makes it possible to effectively use hydrolytic depolymerization for the chemical recycling of PA6 contained in composite materials, thereby making it possible to use these

- 3,048,833 materials still dumped or mechanically recycled as raw material for the production of ε-caprolactam monomer.

Furthermore, since the partial degradation of the recovered PA6 is advantageous for the subsequent completion of the depolymerization to ε-caprolactam, the steps of dissolution of PA6, separation of the fibrous material and coagulation of the solubilized PA6 can also be carried out in the presence of oxygen, in particular air, which is known to lead at high temperature to oxidation (and therefore chemical change) of PA6, which simplifies the process control in the plant, since there is no need to adopt conditions of an absolute inert atmosphere (where the oxygen concentration is limited, in the worst case, to a few ppm), but only to guarantee the necessary coating conditions in order to avoid flammable or explosive solvent conditions.

Another advantage of the present invention is that the recovered fibrous reinforcing material is of high quality, substantially free of polymer residues and is therefore in a form suitable for reuse in the same or another production process, which allows for savings in raw materials and energy, especially in the case of glass or carbon fibers, the production processes of which are very expensive.

Thus, according to a first aspect, the present invention relates to a method for producing ε-caprolactam by depolymerizing polycaprolactam (PA6), comprising:

a) contacting a composite material comprising PA6 and at least one fibrous reinforcing material with a solvent comprising at least one polyol at a temperature in the range of 130-200°C to obtain a solution comprising solubilised PA6 and an insoluble fraction comprising at least said fibrous reinforcing material;

b) separating said insoluble fraction from said solution;

c) combining said solution with a coagulation liquid containing water to obtain partially depolymerized coagulated PA6 dispersed in the liquid phase;

d) separating said coagulated PA6 from said liquid phase;

e) further depolymerization of said coagulated PA6 isolated in said step d by hydrolysis to obtain ε-caprolactam.

The composite material used for the purposes of the present invention comprises PA6 and at least one fibrous reinforcing material.

The fibrous reinforcing material comprises inorganic or organic fibers that are insoluble in the polyol used in the dissolution step a, such as glass fiber, carbon fiber, polymer fiber from polymers other than PA6, and mixtures of the above-mentioned fibers. In one embodiment, the fibrous reinforcing material comprises at least glass fibers.

The composite material may also include one or more additional materials other than PA6 and reinforcing fibres. Examples of additional materials that may be present in the composite material include: colourants, UV absorbers, fillers, additives of various types (e.g. flame retardants, antistatic agents, antibacterial agents, crystallisation nuclei, etc.) and other contaminants arising from the use of the product or its recovery (e.g. additives that accumulate in the material as a result of previous mechanical recycling cycles).

Preferably, PA6 is present in the composite material in an amount in the range of 50-95 wt.% based on the total weight of PA6 and the fibrous reinforcing material, more preferably in the range of 55-90 wt.%, even more preferably in the range of 60-85 wt.%.

Preferably, the fibrous reinforcing material is present in the composite material in an amount in the range of 5-50 wt.% of the total weight of PA6 and the fibrous reinforcing material, more preferably in the range of 10-45 wt.%, even more preferably in the range of 15-40 wt.%.

In general, the composite material comprises additional materials in an amount of up to 15 wt.% of the total weight of PA6 and a fibrous reinforcing material, preferably in an amount of up to 10 wt.%, for example in the range of 1-5%.

Preferably, the composite material comprises household waste (expired products) and/or industrial waste (waste from composite manufacturing processes).

The composite material can be fed into the process of the present invention in various shapes and sizes. Preferably, the composite material is fed in fragments of approximately 1-3 mm in size, which can be obtained, for example, by pre-processing by cutting or grinding the composite material.

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 glycerol. The polyols can be used individually or in the form of

- 4 048833 mixtures of two or more polyols.

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.

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

In step a), the composite material is contacted with a solvent to dissolve PA6 at a temperature in the range of 130-230°C, preferably in the range of 150-180°C.

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

Typically the dissolution stage lasts in the range of 0.5-6 hours, more preferably in the range of 2-4 hours.

After dissolution of PA6, a two-phase mixture is present in the reactor, consisting of a solution containing solubilized PA6 (liquid phase) and an insoluble fraction (solid phase) containing fibrous reinforcing material and any other materials or substances (e.g. small amounts of polymer additives) insoluble in the solvent under the operating conditions of step a).

The insoluble fraction is then separated from the PA6 solution (step b), for example by filtration or decanting. Preferably, the separation is carried out hot, for example at a temperature in the range of 130-200°C, more preferably at the same temperature at which the polymer was dissolved in step a).

The viscosity of the solution containing solubilized PA6 varies with temperature and PA6 concentration. Typically, the PA6 solution is a clear, slightly viscous solution at the temperature at which step a) is carried out. If left to cool to room temperature, it forms a very viscous paste-like mass with no phase separation between solid and liquid.

The fibrous reinforcement material separated from the PA6 solution can be advantageously washed, for example with the same solvent used for solubilization, to remove any associated residual polymer.

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

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

Preferably, the mixture of the PA6 solution with the coagulation liquid is cooled to room temperature with the simultaneous gradual formation of a coagulated PA6 polymer, which remains dispersed in a liquid phase containing a solubilizing solvent and water. The coagulated polymer may be in the form of, for example, irregularly shaped particles of approximately 1-5 mm in size.

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.

Preferably, in one embodiment, the water stream used to wash the coagulated PA6 and also containing a portion of the solvent is collected together with the solvent/coagulation liquid phase stream formed during the separation of the coagulated PA6.

In one embodiment, at least a portion of the mixture of the two aforementioned streams is returned directly to step c), where it is used as coagulation liquid.

In a further embodiment, at least a portion or all of the mixture of these two streams is subjected to a treatment to separate the solvent from the coagulation liquid (e.g. water).

For example, a mixture of the two above-mentioned streams containing a solvent and a coagulation liquid, such as glycol and water, is subjected to a separation process by distillation. The evaporated phase containing the coagulation liquid (water) and small amounts of solvent can be reused in the coagulation step c), while the bottom liquid phase of the distillation, essentially containing the solvent, can be reused in step a) for processing another composite material.

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

The amount of carboxyl groups that are esterified as a result of solubilization and coagulation treatment of PA6 can be determined by NMR analysis as described in the examples.

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By means of the process of the present invention it is possible to obtain a coagulated and partially depolymerized PA6 at the end of step d), which has a quantity of terminal carboxyl groups esterified by solvent molecules, in particular ethylene glycol, preferably less

2.0 wt.% based on the weight of dry coagulated PA6.

The coagulated and partially depolymerized PA6 obtained as described above can be fed to the next hydrolytic depolymerization stage (stage e) to complete the depolymerization and obtain the ε-caprolactam monomer.

The hydrolytic depolymerization step can be carried out according to methods known to a person skilled in the art.

In general, coagulated and partially depolymerized PA6 can be reacted with water in the form of a steam stream, preferably in the presence of an acid catalyst.

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

For this purpose, PA6 is fed, preferably in a molten state, for example via an extruder/pre-melter, into the depolymerization reactor. However, PA6 can also be fed into the reactor in the form of solid granules, for example via a hopper and/or a dosing screw.

The steam, which is superheated to prevent its re-condensation inside the reactor, is preferably distributed in the melt to maintain a continuous uniform distribution and promote capillary contact with the melt, while ensuring its continuous mixing and homogenization.

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

The ε-caprolactam monomer formed during depolymerization is separated in the gas phase from the depolymerization mixture, for example, by a stream of water vapor, and then condensed.

For example, the vapors leaving the reactor can be sent to a condensation system by direct contact with water, which allows two streams to be separated: a first stream consisting essentially of vapor only; a second stream consisting of an aqueous solution containing crude ε-caprolactam (however already having a high purity) in a concentration of 50% or more.

The condensed monomer is then purified using methods known to those skilled in the art, such as solvent or solventless extraction (as described, for example, in US 20190382339), hydrogenation, treatment with potassium permanganate, and distillation to separate light and heavy like products.

An example of a possible embodiment of the method according to the present invention is described below with reference to the following figure.

The figure is a schematic representation of the process for producing ε-caprolactam according to the present invention, where: PA6 with GF means a composite material containing polyamide and glass fiber as a reinforcing material; EG means ethylene glycol.

In the figure, the composite material 1 is fed to the crushing stage 2 to obtain small-sized fragments, for example, 1-3 mm. The crushed composite material 3 is then subjected to the dissolution stage 4 in the solvent EG at a temperature of 130-200 °C, at which a two-phase mixture 5 is formed, containing PA6 solubilized in EG, and an insoluble fraction in suspension containing glass fibers. The solvent EG used for dissolution (stream 37) is taken from storage 28.

In the next filtration stage 6, the insoluble fraction 7 containing glass fibers is separated from the solubilized PA6 8, for example by hot filtration at the same temperature as in the dissolution stage 4. The glass fibers of the insoluble fraction 7 are then subjected to a washing stage 9 using a stream of recycled glycol 10 from the storage 11, and are then withdrawn from the process (stream 12). The glycol used for washing is recycled (stream 13) partly to the coagulation unit 15 (stream 13a) and partly to the storage 11 (stream 13b), to which the make-up glycol stream 14 is also fed.

The solubilized PA6 stream 8 is subjected to a coagulation stage 15 by contacting it with a water stream 17. The coagulation produces PA6 granules in partially depolymerized form, dispersed in the liquid phase of water and EG (stream 16), which are fed to the next filtration stage 21. The water stream 17 used for coagulation comes from a water reservoir 18, to which a water stream 19 is also fed from a condensation stage 33 after a distillation stage 30 of the water/EG mixture and a make-up water stream 20.

In the filtration step 21, the PA6 granule dispersion stream 16 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 (stream 35) to a storage facility 27, where a mixture is also fed

- 6 048833 water/EG 23 from filtration stage 21.

The stream 29 of the water/EG mixture contained in the storage 27 is subjected to a distillation stage 30, which produces a stream EG 31 and a stream of steam 32, which, after passing through a condensation stage 33, forms a stream of water 34 containing traces of EG, separated from the cycle and the main stream 19, which is sent to the storage 18 for reuse in the coagulation stage 15.

The EG 31 stream, recovered during distillation, is collected in storage 28, where the make-up EG 36 stream is also fed.

The following examples are presented solely for the purpose of illustrating the present invention and should not be construed as limiting the scope of protection defined by the appended claims.

In the examples, the relative viscosity (RV) is measured using the ISO 307 method (1% in H ₂ SO ₄ ).

Examples

Reference example (comparative): direct hydrolytic depolymerization of a composite material (PA6 GF30).

A pilot reactor equipped with a heating jacket and a volume of approximately 100 l is loaded with 30 kg of granules of a commercial composite material called PA6 GF30, containing 70 wt.% PA6 and 30 wt.% glass fibre, a total of 21 kg PA6 and 9 kg glass fibre (relative viscosity (RV) of PA6: 2.7; fibre diameter: 11 μm; fibre length: 4.5 mm; glass: E-Glass (DIN 1259)).

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

As depolymerization continues, which in fact cannot be carried out for more than 20-30% of the loaded material, a gradual accumulation of glass fibers is observed inside the reactor, which makes it impossible to discharge the residual depolymerization mixture through the bottom valve. In order to restore the installation, it is necessary to dismantle the reactor and break up the resulting solid mass mechanically, replacing some of the clogged parts.

This test shows that it is impossible to depolymerize a composite material containing inorganic fibrous reinforcement.

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) are loaded into an apparatus equipped with a condenser with a reflux condenser, a thermometer for temperature control and a heating system. PA6 has a relative viscosity of RV = 2.7, which corresponds to polymer chains with an average molecular weight of 19,500. The mixture is gradually heated with continuous stirring to approximately 135 °C, at which point complete dissolution of the polymer is observed, forming a clear yellow solution. The PA6 solution is heated to 150 °C and maintained at this temperature for approximately 1 h, without observed sediment formation. The mixture is then allowed to cool to room temperature, at which point it becomes a homogeneous, unfilterable pasty mass (without separation of the solid and liquid phases).

Examples 2 and 3: Testing the maximum solubilization of PA6 in ethylene glycol (EG) at temperatures below the boiling point of EG.

Ethylene glycol has a boiling point of 195°C. For operation of the equipment at atmospheric pressure, concentrations are tested at which complete dissolution of the polymer can be achieved at below 195°C. For this purpose, following the same process as in Example 1, dissolution tests are carried out using the quantities of PA6 and EG indicated in Table 1. Table 1 also shows the temperatures at which complete dissolution of PA6 is observed (dissolve temp) and the maximum temperature (max temp) at which the solution is maintained (1 h) after complete dissolution of PA6.

Table 1

Example RA6 (RV 2.7) EG Ratio Solution temp. Max. temp. Precipitation No. G G EG/PA6 °С °С 1 50 1000 20 135 170-180 No sediment 2 200 1000 5 160 170-180 No sediment 3 250 1000 4 170 180 No sediment

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

The PA6 solutions from Examples 2 and 3 also form a homogeneous, non-filterable paste-like mass after cooling to room temperature.

Example 4. Dissolution of PA6 GF30 composite material with a high solvent/polymer ratio.

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The dissolution test from Example 1 for the composite material PA6 GF30 is repeated under the conditions shown in Table 2.

Table 2

Example РА6 GF3 RA6 GF EG EG/PA6 ratio 30GF EG/PA6 ratio Solution temp. Tmax Time at Tmax No. G G G G °С °С minutes 4 50 35 15 1000 20 28,5 150 170- 180 120

At the end of the test a suspension of white solid in a yellowish solution is obtained. The suspension is filtered on a filter maintained at 170°C and then the operations described below are carried out: the white solid remaining on the filter is washed with 50 g of hot ethylene glycol (temperature 150-170°C) and then with small amounts of H ₂ O to obtain a friable-looking solid, which is then dried at 70°C;

the clear solution obtained by filtration at 170°C (filtrate), in an amount of 1035 g, mixed with 50 g of water used to wash the solid (total 1085 g), is poured into 4000 g of H2O at a temperature of 90°C with stirring, to obtain a mixture with coagulated PA6 suspended in the liquid phase.

The mass ratio of H2O to PA6 solution in EG is 4000:1085=3.7; the ratio of _H2O :PA6 is 4000:35=114; the ratio of _H2O :EG is 4000:1050=3.8.

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 determinations indicated in Table 3.

Table 3

Example 4

Taste-smell Results Notes FTIR Polymer PA6 Ash Absent RV viscosity at 1% in H2SO4 1,8 RV PA6 composite PA6 GF30=2.7 NH2 amine groups, meq/kg 13.8 Initial _NH2 = 35 meq/kg Carboxyl groups COOH, meq/kg 34.6 Initial COOH=50 meq/kg

The amount of amine and carboxyl groups in the polymer is determined by potentiometric titration.

The data in Table 3 show that the PA6 polymer, completely separated from the glass fiber, underwent partial depolymerization of the polymer chains (the RV value decreased from 2.7 to 1.8). Partial depolymerization promotes the subsequent chemical reduction of the εcaprolactam monomer. Indeed, the average chain length of the polymer with an RV of 2.7 corresponds to approximately 19500/113 = 170 units, whereas the chain length of the coagulated polymer with an RV of 1.8 corresponds to approximately 9200/113 = 80 (the ratio between the average molecular weight and the RV coefficient is Mw = 11500 (RV-1), the molecular weight of a repeating unit of PA6 is 113).

Examples 5 and 6. Dissolution of PA6 GF30 at a reduced solvent/polymer ratio.

The process of Example 4 is repeated, increasing the amount of composite material treated, using the same volumes of EG and coagulation water.

The composition of the mixture and the dissolution conditions adopted in the tests are given in Table 4.

Examples 5 and 6 differ in the time during which the PA6 solution is kept at the maximum dissolution temperature (2 hours for example 5; 4 hours for example 6) in order to test the possible influence of these operating conditions on the degree of esterification of the PA6 end groups with EG, all other things being equal.

Table 4

Example RA6 30 GF RA6 GF EG EG/PA6 ratio 30GF EG/PA6 ratio Solution temp. Tmax Time at Tmax No. G G G G °С °С minutes 5 250 175 75 1000 4 5.7 170 170-180 120 6 250 175 75 1000 4 5.7 170 170-180 240

At the end of the test, a suspension of white solid in a yellowish solution is obtained. Suspension

- 8 048833 is filtered in a filter maintained at a temperature of 180°C, and then the operations described below are carried out:

the white solid remaining on the filter is washed with 50 g of hot ethylene glycol and then with small amounts of H2O to give a friable-looking solid, which is then dried at 70°C;

The clear solution obtained by filtration at 180°C (filtrate), in an amount of 1175 g, mixed with 50 g of water used for washing the solid (total 1225 g), is poured into 4000 g of H2O at a temperature of 90°C with stirring, resulting in a mixture with coagulated PA6 suspended in the liquid phase.

The mass ratio of H2O and PA6 solution in EG is 4000:1225=3.3; the ratio of _H2O :PA6 is 4000:155=22.8; the ratio of _H2O :EG is 4000:1050=3.8.

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 the analytical determinations indicated in Table 4.1 for Example 5 and 4.2 for Example 6.

Table 4.1

Example 5

Taste-smell Results Notes FTIR Polymer PA6 Ash Absent RV viscosity at 1% in H2SO4 1.58 RV PA6 composite PA6 GF30=2.7 NH2 amine groups, meq/kg 57.5 Initial _NH2 = 35 meq/kg Carboxyl groups COOH, meq/kg 47.6 Initial COOH=50 meq/kg

Even at significantly higher concentrations of the PA6-30GF composite material in the EG than in Example 4, it is possible to separate the PA6 polymer from the composite material and recover PA6 in a partially depolymerized form (RV=1.58). The average chain length of the polymer changes from a value of 19500/113=170 units of the original polymer, having an RV=2.7, to approximately 6670/113=60 units of the recovered polymer, having an RV=1.58.

Table 4.2

Example 6

Taste-smell Results Notes FTIR Polymer PA6 Ash Absent RV viscosity at 1% in H2SO4 1.62 RV PA6 composite PA6 GF30=2.7 NH2 amine groups, meq/kg 73.15 Initial _NH2 = 35 meq/kg Carboxyl groups COOH, meq/kg 52.7 Initial COOH=50 meq/kg

Also in Example 6, again at significantly higher concentrations of the PA6 30GF composite material in the EG than in Example 4, it is possible to separate the PA6 polymer from the composite material and recover PA6 in a partially depolymerized form (RV=1.62). Increasing the time until the maximum dissolution temperature is reached does not cause esterification of the end groups. In this case, the average chain length of the polymer changes from a value of 19500/113=170 units of the original polymer, having an RV=2.7, to approximately 7130/113=63 units of the recovered polymer, having an RV=1.62.

Example 7. Dissolution of PA6 GF20 (PA6 with other 20 wt.% glass fiber).

The process of Examples 5 and 6 is repeated using a PA6 GF20 composite material (80 wt.% PA6, 20 wt.% glass fiber).

The composition of the mixture and the dissolution conditions adopted in the test are given in Table 5.

Table 5

Example RA6 20 GF RA6 GF EG EG/PA6 ratio 20GF EG/PA6 ratio Solution temp. Tmax Time at T ^A max No. G G G G °С °С minutes 7 250 200 50 1000 4 5 170 170- 180 120

At the end of the test, a suspension of white solid in a yellowish solution is obtained. The suspension is filtered and the separated coagulated solid is washed as described in Example 5. After drying, the solid is subjected to the analytical determinations indicated in Table 6.

This example shows that even at other glass fiber concentrations it is possible to separate the PA6 polymer from the composite material and recover PA6 in a partially depolymerized form (RV=1.61). The average polymer chain length changes from the value of 19500/113=170 units of the original

- 9,048,833 polymer having RV=2.7, to approximately 7015/113 =62 units of recovered polymer having

RV=1.61.

Table 6

Example 7

Taste-smell Results Notes FTIR Polymer PA6 Ash Absent Viscosity RV at 1 _% in _H2SO4 1.61 RV PA6 composite PA6 GF30=2.7 NH2 amine groups, meq/kg 76.7 Initial NH2=35 meq/kg Carboxyl groups COOH, meq/kg 53.2 Initial COOH=50 meq/kg

Example 8. Dissolution of PA6 GF30 with decreasing solvent/polymer ratio.

The process of Examples 5 and 6 is repeated using the composite material PA6 GF30 (30 wt.% glass fiber).

The composition of the mixture and the dissolution conditions adopted in the tests are given in Table 7. In particular, in this example, the amount of glycol used is reduced compared to the PA6 GF30 composite.

Table 7

Example RA6 30 GF RA6 GF EG EG/PA6 ratio 30GF EG/PA6 ratio Solution temp. Tmax Time at T _max No. G G G G °С °С minutes 8 250 175 75 750 3 4.3 170 170- 180 80

After dissolution, the resulting solution is subjected to the coagulation, filtration and washing process described in Example 5. After drying, the solid substance is subjected to the analytical determinations given in Table 8.

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

The average chain length of the polymer varies from 19500/113=170 units of the original polymer, having RV=2.7, to approximately 10925/113=97 units of the recovered polymer, having RV=1.95.

Table 8

Example 8

Taste-smell Results Notes FTIR Polymer PA6 Ash Absent RV viscosity at 1% in H2SO4 1.95 RV PA6 composite PA6 GF30=2.7 NH2 amine groups, meq/kg 43.8 Initial NH2=35 meq/kg Carboxyl groups COOH, meq/kg 29.1 Initial COOH=50 meq/kg

Example 9. Suitability of PA6 isolated from a composite material for depolymerization to ε-caprolactam.

The suitability of PA6 recovered from the composite material for depolymerization to the monomer ε-caprolactam was studied by NMR spectroscopy. For the purposes of the present invention, PA6 is considered suitable for hydrolytic depolymerization if the conditions in Table 9 are met.

Table 9

Parameter Maximum permissible limit of the depolymerization process Contents PA6 >90% Inorganic substances 3% max Other polymers 6% max Other non-hazardous substances* 1% max Halogenated compounds <50 ppm

*Additives present in the recovered polymer (e.g. heat stabilizers, antioxidants, dyes, etc.)

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

The coagulated PA6 granules from Examples 4 (RV=1.8), 5 (RV=1.58) and 6 (RV 1.62), after separation, washing and drying, were analyzed by 1D and 2D NMR spectroscopy (1H and 13C). For this purpose, 51 mg PA6 from Example 4, 54 mg PA6 from Example 5 and 52 mg from Example 6 were each dissolved in 600 ml

- 10 048833 deuterated trifluoroethanol (CF3CD2OD) after heating for several minutes. From the spectra

NMR of three samples prepared in this way determines the parameters shown in Table 10.

Table 10

Ethylene glycol RA6 PA6 with glycol esterified ends Total Example 4 0.6% >98.9% <0.5% 100% Example 5 0.3% 98.3% 1.4% 100% Example 6 0.1% 97.9 2.0% 100%

The data in Table 10 confirm the suitability of PA6 recovered from the composite material for hydrolytic depolymerization; it can be seen that the useful content of PA6 significantly exceeds the specification limit set at 90%.

Example 10. Depolymerization of PA6 isolated from a composite material.

In order to recover the PA6 polymer separated from the glass fibre for depolymerisation testing purposes, the process described in Example 4 was repeated on a sample of the PA6 30GF composite material using a 50-litre reactor equipped with a reflux condenser, a thermometer for temperature control and an oil bath heating system. The reactor was charged with 20 kg of ethylene glycol (EG) and 5 kg of PA6 30FV granules, for a total of 25 kg.

The mixture is gradually heated with continuous stirring to about 170-180°C, where complete dissolution of the polymer is observed, and then maintained at this temperature for about 2 hours. From the bottom valve, the suspension, which is kept warm all the time, is directly filtered through a pilot filter (Nutsche type) under a slight vacuum, and then flows into a 120-liter mobile reactor equipped with an air stirrer, in which 80 liters of demineralized water at room temperature are present as coagulation liquid. PA6 is drained from the reactor in such a way that the glass fiber inside the filter system remains wet on the surface with a light layer of PA6 solution in glycol.

The coagulated solid substance in the form of irregularly shaped yellowish granules is filtered, washed through a filter with water and, after unloading, dried in an oven at 70°C under a slight vacuum until the moisture content is less than 0.5 wt%.

Approximately 3.0 kg of PA6 with an RV of 1.6 is obtained, which is also observed in the tests in Examples 4, 5 and 6. In particular, in the dried PA6, ash is essentially absent, which confirms the absence of residual glass fibre.

The above pre-treatment for glass fiber separation is repeated with four aliquots of PA6 GF30 until 12.34 kg of dried PA6 is collected.

The PA6 polymer recovered in this manner, free of glass fibre, is loaded into a pilot reactor (volume of approximately 50 l) made of an acid-resistant alloy similar to the alloy of the Reference Example, equipped with a heating jacket and a superheated steam supply system.

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

The PA6 polymer is fed directly into the reactor in the form of solid pellets via a hopper and/or a dosing screw system. The reactor is then also fed with an acid catalyst and steam superheated to above 300°C. The steam leaving the reactor is fed into a condensation system by direct contact with water, separating a first fraction containing only steam and a second fraction containing an aqueous solution of crude ε-caprolactam (concentration of approximately 50 wt.%). The crude ε-caprolactam solution is then subjected to a conventional purification process to obtain a final product meeting commercial specifications (in terms of acidity, alkalinity, volatile bases, APHA, optical density and permanganate index).

During depolymerization, no fouling phenomenon is observed as described in the comparative example, and the rate of the depolymerization process is higher than the rate observed for the depolymerization of PA6 waste in which the polymer retains an RV of 2.7, such as recovered from carpet separation.

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

Table 11

PA6 from carpet waste (90 wt.% PA6, RV 2.7) PA6 coagulated Ex. 10 (98 wt.% PA6, RV 1.6 Quantity loaded Kg 12 (equivalent to 10.8 kg 12.34, (equivalent to 12.2 kg PA6)

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Claims (10)

1.6 The coagulated PA6 obtained after separation of glass fibres proved to be as well depolymerisable as the best PA6 waste obtained during separation of polyamide from carpets, but with higher yields and final solution concentrations under the same experimental test conditions. CLAUSE OF THE INVENTION 1. A method for producing ε-caprolactam by depolymerization of polycaprolactam (PA6), which comprises: a) contacting a composite material comprising PA6 and at least one fibrous reinforcing material with a solvent comprising at least one polyol at a temperature in the range of 130-200°C to obtain a solution comprising solubilised PA6 and an insoluble fraction comprising at least said fibrous reinforcing material; b) separating said insoluble fraction from said solution; c) combining said solution with a coagulation liquid containing water to obtain partially depolymerized coagulated PA6 dispersed in the liquid phase; d) separating said coagulated PA6 from said liquid phase; e) further depolymerization of said coagulated PA6 isolated in said step d) by hydrolysis to obtain ε-caprolactam. 2. The method according to claim 1, characterized in that said coagulation liquid has a temperature in the range of 50-95°C, preferably in the range of 80-95°C. 3. The method according to any one of claims 1, 2, characterized in that said PA6 is present in the composite material in an amount in the range of 50-95 wt.% relative to the total weight of PA6 and the fibrous reinforcing material, more preferably in the range of 55-90 wt.%, even more preferably in the range of 60-85 wt.%. 4. The method according to any one of claims 1 to 3, characterized in that said fibrous reinforcing material is present in the composite material in an amount in the range of 5-50 wt.% relative to the total weight of PA6 and fibrous reinforcing material, more preferably in the range of 10-45 wt.%, even more preferably in the range of 15-40 wt.%. 5. The method according to any one of claims 1 to 4, characterized in that said solvent contains at least one glycol. 6. The method according to claim 5, characterized in that said solvent is selected from ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, butylene glycol and mixtures thereof. 7. The method according to any one of paragraphs 1-6, characterized in that the said solubilization stage a) is carried out at a temperature in the range of 150-170°C. 8. The method according to any one of claims 1 to 7, characterized in that in step a) the weight ratio between said composite material and said solvent is in the range from 1:1 to 1:50, preferably in the range from 1:2 to 1:25. 9. The method according to any one of claims 1 to 8, characterized in that the weight ratio between said solution containing said solubilized PA6 and said coagulation liquid is in the range from 1:1 to 1:10, preferably in the range from 1:1 to 1:5. 10. The method according to any one of claims 1 to 9, characterized in that said depolymerization by hydrolysis is carried out in the presence of at least one acid catalyst. -