Amorphization process for a material comprising a polymer

Abstract

The present invention relates to a method for amorphizing a material comprising at least one polymer, comprising the following steps: a) adding the material to a solution S1 containing at least one salt and having a pH less than or equal to 7; b) exposing the solution S1 to a temperature T1 greater than or equal to 20°C; c) maintaining the material in said solution S1 until the polymer is amorphized; d) adjusting the pH of the solution S1 to a pH greater than or equal to 4 or transferring the material obtained into a solution S2 containing at least one salt and having a pH greater than or equal to 4 and in which the mass concentration of salt is greater than or equal to 10% by weight relative to the total weight of the solution S2.

Description

Title of the invention: Amorphization process for a material comprising a polymer technical field

[0001] The present invention relates to the field of materials comprising at least one polymer. More particularly, the present invention relates to a method for amorphizing a material comprising at least one polymer. Previous techniques

[0002] In the current climate context, numerous measures are being implemented to improve the durability and recyclability of materials, such as common plastics. This environmental issue is part of a broader effort to reduce carbon dioxide emissions. The use of recycled materials is indeed one way to achieve this. Therefore, current and future environmental requirements, particularly in the automotive sector, mandate a minimum percentage of post-consumer recycled (PCR) material to be incorporated into new vehicles, as well as the integration of materials from end-of-life vehicle recycling.

[0003] Thus, the materials concerned are polymers in general, and in particular polyamides (PA).

[0004] Polyamides (PA) are a family of thermoplastic polymers containing amide groups and are generally semi-crystalline. They can be synthesized either by polycondensation of diacids and diamines or by chain opening of lactams. They are named according to the number of carbon atoms present in the carbon chain of the monomers for homopolymers and according to the number of carbon atoms in the amine and then the acid for copolymers.

[0005] Polyamides are generally both strong and ductile: the hydrogen bonds of the amide groups are responsible for the rigidity, resistance to organic compounds, and high melting point of PAs, while the aliphatic parts influence hardness and water resistance. The exact combination of properties depends on the type of polyamide.

[0006] A wide variety of polyamides are commercially available: examples include polyamides 6, 6.6, 4.6, 4.10, 6.10, 6.12, 11, 12, etc. They have widespread applications in the textile and transport industries, including the automotive industry, but also in construction, upholstery, kitchen utensils, machinery, desalination membranes, etc.

[0007] Recycled polyamides are often highly degraded. Consequently, mechanical recycling does not yield sufficient quality for reuse in demanding industries such as the automotive sector. Therefore, a depolymerization process that allows for repolymerization and achieves qualities identical to virgin materials is of great interest.

[0008] Various depolymerization routes for polyamides have been studied. However, the semi-crystalline nature of polyamides presents a disadvantage for depolymerization. Indeed, the presence of crystallites hinders the mobility of catalysts. Consequently, surface degradation has been observed, but no bulk degradation has been demonstrated. Thus, the various routes studied involve the use of harsh conditions for efficient depolymerization since strong acids are generally employed and / or high-temperature heating, above the melting point of the material, is applied. These different routes are therefore energy-intensive.

[0009] It would therefore be desirable to find an alternative route to eventually consider less expensive depolymerizations of polymers, in particular polyamides, for example by bio-degradation, in particular by enzymatic pathways. Description of the invention

[0010] The invention therefore relates to a process for amorphizing a material comprising at least one polymer, comprising the following steps:

[0011] a) add the material into an SI solution containing at least one salt and having a pH less than or equal to 7;

[0012] b) expose the SI solution to a temperature Tl greater than or equal to 20°C;

[0013] c) maintain the material in said SI solution until the polymer amorphizes;

[0014] d) adjust the pH of the SI solution to a pH greater than or equal to 4 or transfer the material obtained into a solution S2 containing at least one salt and having a pH greater than or equal to 4 and in which the mass concentration of salt is greater than or equal to 10% by weight relative to the total weight of the solution S2.

[0015] The amorphization process for a material comprising at least one polymer according to the invention not only renders the polymer contained in the material amorphous but also stabilizes the amorphous character of the polymer. Consequently, the amorphization process represents an extremely advantageous prerequisite insofar as it overcomes significant technological barriers, opening up prospects for the depolymerization of materials comprising a polymer, particularly polyamide. Indeed, the material obtained at the end of the amorphization process according to the invention can be used in gentler and more environmentally friendly depolymerization methods than those currently used, for example by enzymatic degradation.

[0016] Other advantages and features of the invention will become more apparent upon examination of the detailed description and accompanying drawings, in which:

[0017] [Fig-1] is a graph representing the evolution of the mass gain of a sample of a polyamide 6.6 material as a function of the immersion time in different saline solutions;

[0018] [Fig.2] is a graph representing the evolution of the mass gain of a sample of a polyamide 6.6 material as a function of the immersion time in a ZnCl2 solution;

[0019] [Fig.3] is a graph representing the evolution of the mass gain of a sample of a polyamide 6.6 material as a function of the immersion time in a ZnSO4 solution;

[0020] [Fig.4] is an XRD spectrum of different samples of a polyamide 6.6 material after immersion in a ZnCl2 solution;

[0021] [Fig.5] is an XRD spectrum of different samples of a polyamide 6.6 material obtained from an amorphization process according to the invention or a comparative process.

[0022] It is specified that the expression "from... to..." used in this description of the invention should be understood as including each of the limits mentioned.

[0023] As indicated above, the process for amorphizing a material comprising at least one polymer according to the invention comprises:

[0024] a) add the material into an SI solution containing at least one salt and having a pH less than or equal to 7.

[0025] Advantageously, the polymer is a polyamide, preferably selected from polyamide 6, polyamide 6-6 and their mixture.

[0026] According to a particular embodiment, the material comprising at least one polyamide can be any part made of polyamide, for example any part made of polyamide 6, polyamide 6-6, but also any material comprising polyamide and capable of containing a filler, such as glass fibers, mineral fillers, such as talc, mineral dust, montmorillonite, carbon fibers, solid glass beads, hollow glass beads, and / or an inorganic filler and / or an organic nodule.

[0027] The material comprising at least one polyamide may be a part comprising two polyamides of different types, for example, a part obtained after mixing PA 6.6 with PA 6 or vice versa. This initial material is advantageous since, with transamidification between the polyamides present, the crystallinity of the mixture The resulting loss is further reduced. The part can be recovered by micro-granulation before being used in the process according to the invention.

[0028] The material comprising at least one polymer, in particular a polyamide, may be in any form.

[0029] Advantageously, the material is a part with a high surface area to volume ratio. Such a surface area to volume ratio can be obtained by grinding the part before it is used in the process according to the invention. Thus, the material, comprising at least one polymer, can be a ground part.

[0030] Another form of said material may be a film having a thickness ranging from 50 to 500 pm.

[0031] Advantageously, the salt of the SI solution is a salt chosen from CuCl2, ZnBr2, ZnCl2 and their mixtures.

[0032] In a particularly preferred manner, the salt of the SI solution is the salt ZnCl2.

[0033] In a particularly preferred manner, when the salt is ZnCl2 salt, the pH of the Solution S1 is advantageously less than or equal to 2.

[0034] Advantageously, the mass concentration of the salt cation is greater than or equal to 20% by weight relative to the total weight of the solution.

[0035] More preferably, the mass concentration of the zinc salt is greater than or equal to 20% by weight relative to the total weight of the solution.

[0036] Advantageously, the SI solution containing at least one salt is a saturated solution. The use of a saturated SI solution is advantageous because the amorphization time is rapid. However, any mass concentration can be used, but the amorphization time will be longer. Those skilled in the art know how to adjust the mass concentration of such a solution.

[0037] Advantageously, the pH of the SI solution is less than or equal to 2, preferably less than or equal to 1. More preferably, the pH of the SI solution is preferably equal to 0. It may be advantageously adjusted, if necessary, by adding an acid to the solution.

[0038] Advantageously, the SI solution comprises a solvent which may be water. In one embodiment, the solvent may be water in combination with at least one other solvent other than water. The solvent other than water may be chosen from alcohols, such as ethanol, methanol, propanol, butanol; acetic acid; hydrochloric acid; sulfuric acid; DMSO; THF; pyridine, etc.

[0039] Thus, according to one embodiment, the SI solution comprises a mixture of solvents, for example water and at least one other solvent other than water. In particular, the mixture of solvents can be a mixture of 2 solvents, 3 solvents, 4 solvents, etc.

[0040] As previously stated, the method according to the invention comprises:

[0041] b) expose the SI solution to a temperature Tl greater than or equal to 20°C;

[0042] c) maintain the material in said SI solution until the polymer amorphizes.

[0043] Advantageously, the two steps b) and c) are simultaneous.

[0044] Thus, the material is maintained in the SI solution, with exposure to a temperature Tl greater than or equal to 20°C, until the amorphization of the polymer, in particular the polyamide.

[0045] The material is thus maintained in the SI solution for a time necessary to obtain the amorphized polymer, in particular the amorphized polyamide. This time depends on the temperature Tl. The time required to obtain the amorphized polymer, in particular the amorphized polyamide, will be longer if the temperature Tl is equal to 20°C than if this temperature Tl is higher, for example 50°C or even 80°C, at atmospheric pressure.

[0046] For example, at a temperature of 20°C, the time to obtain the amorphized polymer, in particular the amorphized polyamide, can be several weeks.

[0047] For a temperature Tl equal to 80°C, at atmospheric pressure, the time required to obtain the amorphized polymer, in particular the amorphized polyamide, can range from a few minutes to a few days.

[0048] The higher the temperature, the faster the amorphization. Thus, using the highest possible temperature is advantageous.

[0049] Advantageously, at atmospheric pressure, the temperature Tl can range from 20°C to 80°C, preferably from 50 to 80°C, more preferably from 70 to 80°C, particularly preferably, the temperature Tl is equal to 80°C.

[0050] According to another embodiment, the material can be maintained in the SI solution in a pressurized environment. Advantageously, the maximum pressure that can be applied is that at which the solvent changes state, taking into account the melting temperature of the polymer. For example, in the case of water as the solvent and polyamide 6.6 as the polymer, the melting temperature of polyamide is 260°C; the material can then be maintained at a pressure strictly greater than 1000 hPa and less than or equal to 50,000 hPa.

[0051] Advantageously, when the material is maintained in the SI solution under pressure, the maximum temperature Tl that can be applied is that at which boiling occurs. This can be determined by reading the phase diagrams of solvents known to those skilled in the art.

[0052] According to a preferred embodiment, when the material is kept in the SI solution under pressure, the temperature Tl can range from 20°C to the melting temperature of the polymer, in particular from 20 to 260°C.

[0053] When the temperature Tl is above 20°C, the SI solution, in which the material comprising at least one polymer, in particular a polyamide, is located, can be heated by any means known to a person skilled in the art, for example by means of an oven, a wall, a heating collar, etc.

[0054] Advantageously, the temperature Tl is greater than or equal to the glass transition temperature of the polymer, in particular of the polyamide.

[0055] The glass transition temperature of the polymer, in particular of polyamide, can be determined by any method known to those skilled in the art. In particular, it can be measured in a known manner by DSC (Differential Scanning Calorimetry), on the second pass, for example, and unless otherwise specified in this application, according to ISO 11357-2018 (Mettler Toledo "822-2" DSC apparatus; nitrogen atmosphere). It can also be measured by the following method: samples preheated from -50°C to 285°C (10°C to 20°C / min), then rapidly cooled to -50°C, before final recording of the DSC curve from -50°C to 285°C, according to a ramp of 10°C to 20°C / min).

[0056] As previously stated, the material is maintained in said SI solution until the polymer, in particular the polyamide, undergoes amorphization.

[0057] A person skilled in the art can determine when the polymer, in particular polyamide, is amorphized.

[0058] The amorphization of the polymer, in particular polyamide, can be monitored throughout the process by measurements well known to those skilled in the art, for example by DSC or by X-ray diffraction measurements. DSC measurements can be carried out by any method known to those skilled in the art, for example according to the method described above. X-ray diffraction measurements can be carried out by any method known to those skilled in the art, for example according to the method described below.

[0059] Thus, throughout the process, samples can be taken at regular intervals and the degree of crystallinity can thus be determined.

[0060] Advantageously, the polymer, in particular polyamide, is considered to be sufficiently amorphized when the degree of crystallinity is less than 5%, preferably less than 1%.

[0061] Within the framework of the present invention, it can be considered that the polymer, in particular the polyamide, is sufficiently amorphized when it is in compatible conditions to consider depolymerization by bio-degradation, in particular by enzymatic degradations.

[0062] As stated above, the method according to the invention also comprises:

[0063] d) adjust the pH of the SI solution to a pH greater than or equal to 4 or transfer the material obtained in a solution S2 containing at least one salt and having a pH greater than or equal to 4 and in which the mass concentration of salt is greater than or equal to 10% by weight relative to the total weight of the solution S2.

[0064] Thus, at the end of step c), the pH of the SI solution is adjusted to a pH greater than or equal to 4. Alternatively, the material obtained can be transferred into a solution S2 as indicated above.

[0065] Advantageously, when the pH of the SI solution is strictly less than 4, at step a), preferably less than or equal to 2, then, at step d), the pH of the SI solution is adjusted upwards to a pH greater than or equal to 4.

[0066] This step d) is particularly advantageous because it allows the amorphous character of the polymer, in particular of the polyamide, to be stabilized, i.e. to be preserved, which is essential for the present invention.

[0067] Advantageously, during step d), the pH of the SI solution goes from 4 to 5, preferably from 4 to 4.5.

[0068] The pH of the solution can be adjusted by any means known to a person skilled in the art, for example by adding NaOH.

[0069] According to a preferred embodiment, the salt of solution S2 is a salt chosen from CuCl2, ZnBr2, ZnCl2 and their mixtures, particularly preferably the salt of solution S2 is the salt ZnCl2.

[0070] Advantageously, the pH of solution S2 ranges from 4 to 5, preferably from 4 to 4.5.

[0071] Preferably, during step d), the SI solution, or the S2 solution, is heated at a temperature ranging from 40 to 60°C.

[0072] The process according to the invention may further include, before step a), heating the material, preferably to a temperature greater than or equal to 50°C.

[0073] This preliminary step to step a) allows the material comprising at least one polymer, in particular a polyamide, to dry. The higher the temperature, the faster the material dries. Thus, using the highest possible temperature during this step is advantageous since it reduces the drying time.

[0074] The temperature applied during this step can advantageously range from 50 to 110°C, preferably from 80 to 110°C. At these temperatures ranging from 80 to 110°C, the drying time can typically be 4 hours.

[0075] At the end of the process according to the invention, the material obtained is in conditions compatible for considering depolymerization by bio-degradation, in particular by enzymatic degradations.

[0076] The present invention is illustrated in a non-limiting manner by the following examples. Examples Measurement equipment and methods

[0077] Polyamide 6.6 (PA 6.6) films with a thickness of 500 µm, purchased in 1m x 20cm rolls from Goodfellow, were used (reference AM32-FM-000200). The films were manufactured by extrusion followed by stretching on metal calenders, the spacing of which determines the film thickness. They therefore have an orientation in the direction of extrusion that can easily be determined by heating between the glass transition temperature (Tg) and the temperature (Tm).

[0078] Rectangular samples measuring 1 x 10 cm were cut with scissors, and ISO527 1BA type samples were cut with a punch. PA 6.6 ISO527 5A type samples, 2 mm thick, were also supplied by Celanese (ZYTEL 101L NC010). The aqueous saline solutions used, detailed below, were mixed from salts purchased from Sigma Aldrich Merck. Since the salts are highly hygroscopic, special care was taken with their storage: the salt containers were unsealed just before the solutions were prepared.

[0079] Mass gain measurement

[0080] The initial conditions are defined after drying the sample at 80°C in a ventilated oven for a minimum of 24 hours. The sample mass is then Wo. The sample is then immersed in a closed bath of saline solution at 50°C placed in a ventilated oven. No mechanical stress was applied. The sample is regularly removed from the bath, rinsed with distilled water, dried, and weighed (Wt) to an accuracy of 0.1 mg. The evolution of the % mass gain is defined as follows:

[0081] % mass gain = (lOO*(Wt-Wo)) / Wo.

[0082] The mass gain measurements were stopped when the curve reached a well-defined plateau, after a few hundred hours of immersion.

[0083] X-ray diffraction (XRD)

[0084] Sample squares adapted to the substrate were cut from a 500 µm thick PA 6.6 film. They were subjected to the various conditions described above and below, then glued to the substrate and equilibrated at room temperature. The XRD spectrum was acquired for 1h on a Philips X'Pert diffractometer, under a voltage of 40 kV and a current of 40 mA, for angles between 10 and 60°. Example 1

[0085] Samples of PA 6.6 of type ISO527 5A with a thickness of 2 mm were added to different saline solutions, the solutions being heated to 50°C in a ventilated study.

[0086] Aqueous saline solutions of NaCl, CaCl2, ZnCl2 and ZnSO4 were used, in particular:

[0087] - three NaCl solutions having a mass concentration of 4% (curve B), 13.5% (curve C) and 27% (curve D) by weight, respectively, relative to the total weight of the solution;

[0088] - three CaCl2 solutions having a mass concentration of 8.7% (curve E), 29% (curve F) and 58% (curve G) by weight, respectively, relative to the total weight of the solution;

[0089] - three solutions of ZnCl2 having a mass concentration of 12% (curve H), 41% (curve I) and 82% (curve J) by weight, respectively, relative to the total weight of the solution; and

[0090] - three solutions of ZnSO4 having a mass concentration of 6.3% (curve M), 21% (curve L) and 42% (curve K) by weight, respectively, relative to the total weight of the solution.

[0091] Furthermore, a sample of PA 6.6 of type ISO527 5A was added to water to serve as a reference (curve A).

[0092] The evolution of mass gain as a function of immersion time was monitored, as can be seen in Figures 1 to 3.

[0093] As can be seen in figures 1 and 3, a mass gain is observed as a function of time with an increase in mass until reaching a plateau (see curves B to G and curves K to M).

[0094] In contrast, the behavior is different for samples immersed in ZnCl2 saline solutions. Indeed, as can be seen in [Fig. 2], an increase in mass is observed, but no plateau is reached. [Fig. 2] shows immersion times of up to just over 300 hours, but further measurements were carried out with immersion times of up to thousands of hours. Even after these immersion times, no plateau was observed.

[0095] Thus, these figures clearly demonstrate that not only zinc salt but also chloride ions have an impact on mass gain. Thus, it is demonstrated that ZnCl2 salt has an impact.

[0096] Measurements were also made for samples of PA 6.6 of type ISO527 5A with a thickness of 2 mm which were added to different saline solutions, the solutions being heated not to 50°C in a ventilated study but to 25°C in a ventilated oven.

[0097] The above observations also apply to "tests at 25°C". Example 2

[0098] In this example, a sample square adapted to the support was cut from a PA 6.6 film with a thickness of 500 µm, then after heating the samples at a temperature of 80°C in a ventilated oven for 48 hours, The sample was added to various aqueous saline solutions, the solutions being heated to 50°C in a ventilated study.

[0099] Aqueous saline solutions of NaCl, CaCl2, ZnCl2 and ZnSO4 were used, in particular:

[0100] - a NaCl solution having a mass concentration of 27% by weight relative to the total weight of the solution;

[0101] - a CaCl2 solution having a mass concentration of 57% by weight relative to the total weight of the solution;

[0102] - a ZnSO4 solution having a mass concentration of 42% by weight ratio to the total weight of the solution; and

[0103] - a ZnCl2 solution having a mass concentration of 82% by weight ratio to the total weight of the solution (having a pH of 0); and

[0104] Furthermore, a sample square cut from a PA 6.6 film with a thickness of 500 µm was heated at a temperature of 80°C in a ventilated oven for 48 hours. This sample is used as a reference (N curve).

[0105] Then, the XRD spectra were acquired under the conditions previously described.

[0106] As can be seen in [Fig.4], three curves are present:

[0107] - curve N corresponds to the reference sample;

[0108] - the curve O corresponding to the sample that has been immersed in a solution of ZnCl2 as described above for a period of 24 hours;

[0109] - the curve P corresponding to the sample that has been immersed in a solution of ZnCl2 as described above for a period of 48 hours.

[0110] This is the same sample throughout the experiment with the immersion time varying.

[0111] It is clear that the crystalline character of PA 6.6 has disappeared, by observing the O and P curves. This means that an amorphization of PA 6.6 has been achieved thanks to the ZnCl2 solution.

[0112] The XRD spectra of the samples that were immersed in the other saline solutions (NaCl, CaCl2 and ZnSO4) are not shown in [Fig.4] because they are similar to that of the reference sample, and therefore the curves obtained are similar to that of the N curve.

[0113] Thus, [Fig.4] clearly demonstrates that the ZnC12 salt allows PA 6.6 to be amorphized. Example 3

[0114] In this example, we repeat the procedure of example 2 but we replace PA 6.6 with PA 6. We observe at the same time the same transformation thus demonstrating the amorphization. Example 4

[0115] In this example, the conditions of example 2 as described have been repeated.

[0116] Then, the pH of the solution containing the sample was adjusted to pH 4 by adding NaOH, the solution being heated to 50°C in a ventilated environment with varying immersion times. At the end of the experiment, the solution was filtered.

[0117] Another sample which was previously added (and therefore immersed) in a ZnCl2 solution having a mass concentration of 82% by weight relative to the total weight of the solution (having a pH of 0), was washed by immersing it in water.

[0118] Therefore, the sample, which is immersed in the solution whose pH has been adjusted to a pH of 4, is a sample obtained by a process according to the invention.

[0119] The sample that was washed was obtained as a result of a comparative process.

[0120] Then, the XRD spectra were acquired under the conditions previously described.

[0121] As can be seen in [Fig.5], four curves are present:

[0122] - curve N corresponds to the reference sample;

[0123] - the curve Q corresponding to the sample that has been immersed in a solution of ZnCl2 as described above for a period of 24 hours;

[0124] - the curve R corresponding to the sample that has been immersed in a solution of ZnCl2 as described above for a period of 48 hours;

[0125] - the curve S corresponding to the sample that has been immersed in a solution of ZnCl2 as described above for a period of 336 hours.

[0126] This is the same sample throughout the experiment with the immersion time varying.

[0127] It is clear that the crystalline character of PA 6.6 did not reappear after observation of the Q to S curves. However, a very slight recrystallization (< 1%) can be noted. This means that the amorphization of PA 6.6 was stabilized by the ZnCl2 solution, which had a higher pH following pH adjustment.

[0128] Recrystallization of the washed sample, i.e., the sample obtained at the end of the comparative process, was observed by DSC measurement. This means that the amorphous character of the polyamide was not stabilized.

[0129] Thus, without step d) of the process according to the invention, a simple washing, for example by immersion in water, of the material obtained immediately leads to the recrystallization of the material, which is to be avoided. The polyamide then regains its semi-crystalline character.

[0130] Thus, [Fig.5] clearly demonstrates that the process according to the invention makes it possible to amorphize a material comprising at least polyamide but also to stabilize the amorphous character of the polyamide.

[0131] Consequently, the amorphization process represents an extremely advantageous prerequisite insofar as it makes it possible to overcome significant technological barriers, opening up new possibilities in the degradation of materials comprising a polymer, in particular a polyamide. Indeed, the material obtained at the end of the amorphization process according to the invention can be subjected to gentler depolymerization pathways than those currently used, in particular biodegradation processes, especially enzymatic pathways.

Claims

Demands

1. A process for amorphizing a material comprising at least one polymer comprising the following steps: a) adding the material to a solution SI containing at least one salt and having a pH less than or equal to 7; b) exposing the solution SI to a temperature Tl greater than or equal to 20°C; c) maintaining the material in said solution SI until amorphization of the polymer; d) adjusting the pH of the solution SI to a pH greater than or equal to 4 or transferring the material obtained into a solution S2 containing at least one salt and having a pH greater than or equal to 4 and in which the mass concentration of salt is greater than or equal to 10% by weight relative to the total weight of the solution S2.

2. A method according to claim 1, characterized in that the polymer is a polyamide, preferably selected from polyamide 6, polyamide 6.6 and their mixture.

3. A method according to claim 1 or 2, characterized in that the salt of the SI solution is a salt selected from CuCl2, ZnBr2, ZnCl2 and mixtures thereof, particularly preferably the salt is the ZnCl2 salt.

4. A method according to any one of the preceding claims, characterized in that the pH of the SI solution is less than or equal to 2, preferably less than or equal to 1.

5. A method according to any one of the preceding claims, characterized in that the temperature Tl is greater than or equal to the glass transition temperature of the polymer.

6. A method according to any one of the preceding claims, characterized in that, during step d), the pH of the SI solution goes from 4 to 5, preferably from 4 to 4.

5.

7. A method according to any one of the preceding claims, characterized in that the salt of solution S2 is a salt selected from CuCl2, ZnBr2, ZnCl2 and mixtures thereof, particularly preferably the salt of solution S2 is the salt ZnCl2.

8. A method according to any one of the preceding claims, characterized in that the pH of the solution S2 ranges from 4 to 5, preferably from 4 to 4.

5. 14

9. A method according to any one of the preceding claims, characterized in that it further comprises, before step a), heating the material, preferably to a temperature greater than or equal to 50°C.

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