Method for recycling an aromatic monomer from a polyamide

Abstract

The invention relates to method of recovery of at least one aromatic monomer from a polyamide (PA) which comprises recurring units (RPA) derived from at least one aromatic diacid (A-ar-A) and / or at least one aromatic diamine (B-ar-B), wherein the method comprises the following steps: - a) a product (P) comprising at least one polyamide (PA) is optionally comminuted; - b) hydrolysis of polyamide (PA) is performed in acidic conditions in the presence of at least one (Ac) having at least one pKa lower than 0 and and selected in the group consisting of the nitrogen-free inorganic acids, microwave radiations being applied to a composition (C) comprising water, the product (P) and the acid (Ac); - c) a stream (S) comprising: ■ the aromatic diacid(s) (A-ar-A); and / or ■ the aromatic diamine(s) (B-ar-B) in the base form or in the form of a salt, notably in the form of a salt with acid (Ac), which is recovered at the end of step b) is further processed in order to recover the aromatic diacid(s) (A-ar-A) and / or the aromatic diamine(s) (B-ar-B), the recovery of these aromatic monomer(s) comprising at least one unit operation selected in the group consisting of liquid-solid separation, crystallization and distillation.

Description

Method for recycling an aromatic monomer from a polyamideThis application claims priority of European patent application No. 23195634.3 filed on 6 September 2023, the content of which being entirely incorporated herein by reference for all purposes. In case of any incoherency between this application that would affect the clarity of a term or expression, it should be made reference to this application only.

[0001] The present disclosure relates to an efficient method for recycling at least one aromatic monomer from a polyamide. The method involves depolymerizing by hydrolysis the polyamide into its constitutive monomers and recovering the aromatic monomer(s).[Context of the invention]

[0002] Plastics are inexpensive and durable materials, which can be used to manufacture a variety of products that find use in a wide range of applications, so that the production of plastics has increased dramatically over the years since their discovery. It is estimated that about 40% of these plastics are used for single-use disposable applications, such as packaging, agricultural films, disposable consumer items or for short-lived products that are discarded within a year of manufacture. Because of the durability of the polymers involved, substantial quantities of plastics are piling up in landfill sites and in natural habitats worldwide, generating increasing environmental problems. Even degradable and biodegradable plastics may persist for decades depending on local environmental factors, like levels of ultraviolet light exposure, temperature, presence of suitable microorganisms, etc.

[0003] One solution to reduce environmental and economic impacts correlated to the accumulation of plastic is closed-loop recycling wherein plastic material is mechanically reprocessed to manufacture new products. For example, one of the most common closed-loop recycling is the polyethylene terephthalate (PET) recycling. PET wastes are subjected to successive treatments leading to a food- contact-approved recycled PET, which is collected, sorted, pressed into bales, crushed, washed, chopped into flakes, melted and extruded in pellets and offered for sale. The recycled PET may be used to create fabrics for the clothing industry or new packaging such as bottles or blister packs, etc.

[0004] However, plastic wastes are generally collected all together, so that plastic bales contain a mixture of different plastics, the composition of which may vary from source to source, and the proportions of which may vary from bale to bale. Consequently, recycling processes require preliminary selection to sort out the plastic products according to their composition, size, resin type, colour, functional additives used, etc.

[0005] Another potential process for recycling plastics consists of a chemical recycling allowing recovering the monomers from the polymer. The resulting monomers may then be used to re-manufacture plastic materials (the same or other plastic materials) or to make other synthetic chemicals. While this chemical and enzymatic depolymerization process has been well optimized over the years for PET with very high yield of recycled ethylene glycol and recycled terephthalic acid, there is still need to develop similarly an efficient and optimized process for polyamides.Background art

[0006] ACS Sustainable Chem. Eng. 2020, 8, 16274-16282 (D1)(https: / / doi.org / 10.1021 / acssuschemeng.0c05706) discloses the treatment of aliphatic polyamides (PA 6,6, PA 10,10, PA 11 , and PA 12) into their constituent monomers through hydrolysis under microwave irradiation using hydrochloric acid as an acid catalyst. The reactor used is a small scale 30 ml glass vial. The temperature applied is higher than 150 °C. D1 does not disclose nor relate to the recovery of an aromatic monomer.

[0007] WO 2022 / 058291 discloses an improved process for the acid hydrolysis of polylaurolactam with sulfuric acid at a temperature between 125 and 190 °C, preferably at a temperature higher than 160 °C. The method does not involve microwave irradiation.

[0008] WO 2013 / 014650 discloses a method for the recycling of polyesters and / or polyamides via a depolymerization process using a solvolytic mixture and a microwave depolymerization reactor.

[0009] WO 2021 / 094949 discloses an apparatus for depolymerization of polymers, in particular polyamides, comprising a microwave depolymerization reactor having a reaction chamber; a microwave generation and transport system to send microwaves into the reaction chamber.

[0010] WO 2023 / 275386 discloses a method of depolymerizing polymers comprising: a) providing a polymer, b) providing an alkali / alcohol reaction mixture c) subjecting the polymer in the reaction mixture to microwave irradiation. The method of the invention is based on the use of an acid and not an alkali. Moreover, in the methodof the invention, the reaction mixture which is irradiated preferably does not comprise an alcohol.

[0011] Polymer 2000, 41 , 4361-4365 discloses a microwave induced depolymerization of polyamide 6 in the presence of H3PO4.

[0012] Global Challenges 2021 , 2000119 discloses a microwave assisted selective hydrolysis of polyamides from multicomponent carpet waste.

[0013] EP 2736968 B1 discloses a method for the recycling of polyesters and / or polyamides via depolymerization process, comprising a depolymerization reaction of the material to treat with a solvolytic mixture. There is no mention of the recovery of aromatic monomers.

[0014] CN 111537661 (D2) discloses a method for identification and quantitative determination of polyamide materials using GC / MS.

[0015] WO 2024 / 002975, WO 2024 / 056584 and WO 2024 / 121309 disclose a method of recovery of the monomers from a polyamide (PA) of the AABB type such as PA66.[Technical problem to be solved]

[0016] There is an interest in recycling the aromatic monomers from polyamides. Indeed, they can be reused in other industries or in the preparation of polymers (polyesters, polyamides). Yet, the recovery of aromatic monomers through an industrial depolymerization of polyamides requires that the method implemented is efficient (with fast kinetics) with a high yield of recovery.

[0017] Moreover, because of nowaday's concerns that the human activities have a minimum impact on the environment, there is a need for an environmentally-safe method of recycling. In particular, there is an interest of a process that uses a limited quantity of water and / or releases a controlled amount of salts.

[0018] The method should also be easily scalable and be implemented under conditions that are not too harsh for the equipment used (therefore also limiting the corrosion of equipment).

[0019] The method of the invention aims at solving these technical problem.

[0020] The method of the invention is disclosed in any one of the claims 1-30.

[0021] More precisions and details about this method are now provided below.

[0022] The following apply to the present disclosure.

[0023] wt.% is a percentage by weight. Mol.% is a percentage by mole.

[0024] The proportions of recurring units in the polyamide (PA) are expressed in mol% and are relative to the total number of recurring units in the polyamide (PA).

[0025] When numerical ranges are indicated, the end-points are included (even for open ranges without lower or upper values).

[0026] In the present application, unless otherwise indicated, any specific embodiment or technical feature relating to the method of the invention is applicable to and interchangeable with another embodiment or technical feature also relating to the method of the invention and disclosed elsewhere in the application.

[0027] A diacid or a diamine is considered for the purpose of this invention as "aromatic" when it comprises one or more than one aromatic group. Generally, an aromatic diacid or aromatic diamine contains only one aromatic group in its structure. Examples of aromatic monomers are terephthalic acid, isophthalic acid, m-xylylene diamine and p-xylylene diamine. A diacid or a diamine is considered for the purpose of this invention as "non-aromatic" when it does not contain any aromatic group. Examples of non-aromatic monomers are adipic acid, 1 ,4- cyclohexanedicarboxylic acid, hexamethylene diamine, 1 ,3- bis(aminomethyl)cyclohexane and 1 ,4-bis(aminomethyl)cyclohexane.

[0028] To make the invention more understandable, the following notations are used in the present application. An aromatic diacid is noted A-ar-A and an aromatic diamine is noted B-ar-B. Similarly, a non-aromatic diacid is noted A-al-A and a non- aromatic diamine is noted B-al-B.

[0029] For the purpose of the present description, the expression "aromatic polyamide" should be understood as defining a polyamide which comprises more than 50.0 mol% (> 50.0 mol%) of recurring units (RPA iii) formed from the condensation of at least one aromatic diacid (A-ar-A) and at least one aromatic diamine (B-ar-B). Likewise, the expression "semi-aromatic polyamide" should be understood as defining a polyamide which comprises more than 50.0 mol% (> 50.0 mol%) of recurring units (RPA ,) formed from the polycondensation of at least one aromatic diacid (A-ar-A) and at least one non-aromatic diamine (B-al-B) and / or recurring units (RPA U) formed from the condensation of at least one aromatic diamine (B-ar- B) and at least one non-aromatic diacid (A-al-A).

[0030] The method of the invention is disclosed in the appended claims, notably in claim1.

[0031] This method of recovery of at least one aromatic monomer from a polyamide (PA) which comprises recurring units (RPA) derived from at least one aromatic diacid (A- ar-A) and / or at least one aromatic diamine (B-ar-B), comprises the following steps: a) a product (P) comprising at least one polyamide (PA) is optionally comminuted; b) hydrolysis of polyamide (PA) is performed in acidic conditions in the presence of at least one (Ac) having at least one pKa lower than 0 and and selected in the group consisting of the nitrogen-free inorganic acids, microwave radiations being applied to a composition (C) comprising water, the product (P) and the acid (Ac); c) a stream (S) comprising:■ the aromatic diacid(s) (A-ar-A); and / or■ the aromatic diamine(s) (B-ar-B) in the base form or in the form of a salt, notably in the form of a salt with acid (Ac), which is recovered at the end of step b) is further processed in order to recover the aromatic diacid(s) (A-ar-A) and / or the aromatic diamine(s) (B-ar-B), the recovery of these aromatic monomer(s) comprising at least one unit operation selected in the group consisting of liquid-solid separation, crystallization and distillation.

[0032] The method of the invention is applicable to any polyamide (PA) as disclosed in the present disclosure.

[0033] Product (P)

[0034] The product (P) comprises at least one polyamide (PA) as disclosed herein.

[0035] Product (P) may also comprise at least another polymer (p) that is not polyamide (PA). Polymer (p) may be selected in the group of thermoplastic polymers and elastomer materials. Polymer (p) may more particularly be selected in the group consisting of polyamides, polyolefins, fluoropolymers, polyesters, polystyrenes and combinations thereof.

[0036] Polymer (p) may be either blended with polyamide(s) (PA) and / or be physically present with polyamide(s) (PA) in product (P) but not blended. Preferably, polymer (p) is not a polyamide.

[0037] Product (P) usually also comprises at least one polymer additive (A).

[0038] Polyamide (PA) and / or the other polymer (p) (if any) may comprise at least one polymer additive (A), usually blended with said polyamide (PA) and / or with said other polymer (p).

[0039] The polymer additive (A) is generally selected in the group consisting of fillers, colorants, dyes, pigments, lubricants, plasticizers, flame retardants, nucleatingagents, heat stabilizers, UV stabilizers, elastomers, adhesives, antioxidants, processing aids and combinations thereof. The polymer additive (A) may more particularly be selected in the group consisting of fillers, colorants, dyes, pigments, lubricants, elastomers, heat stabilizers and combinations thereof.

[0040] Among the polymer additives cited above, fillers are very frequently used to improve the physico-chemical properties of the polymers. Therefore, product (P) usually also comprises at least one filler and optionally at least one polymer additive (A) selected in the group consisting of colorants, dyes, pigments, lubricants, plasticizers, flame retardants, nucleating agents, heat stabilizers, UV stabilizers, elastomers, adhesives, antioxidants, processing aids and combinations thereof.

[0041] The filler is generally selected in the group consisting of glass fibers, carbon fibers, talc, mica, kaolin, calcium carbonate, calcium silicate, magnesium carbonate, metallic fibers, boron nitride, boron carbide, wollastonite and combinations thereof.

[0042] The proportion of polyamide(s) (PA) in product (P) is generally at least 30.0 wt.%, more particularly at least 40.0 wt.%. This proportion may be at least 50.0 wt.% or even at least 60.0 wt.%. This proportion may be 100 wt.% if product (P) consists of polyamide(s) (PA). Yet, this situation is rare as the method of the invention is meant to apply to products of our day-to-day life for which polymer additives are usually present in combination with the polyamide(s) (PA). The proportion of polyamide (PA) in product (P) is generally less than 99.9 wt.%, more particularly less than 99.5 wt.%.

[0043] Product (P) comprises one or more polyamide (PA). Product (P) preferably comprises only one polyamide (PA).

[0044] About polyamide (PA)

[0045] Polyamide (PA) comprises amide-containing recurring units (RPA) derived from at least one aromatic diacid (A-ar-A) and / or at least one aromatic diamine (B-ar-B).

[0046] Recurring units (RPA) of polyamide (PA) are selected in the group consisting of:■ (i) recurring units (RPA i) derived from the condensation of at least one aromatic diacid (A-ar-A) and at least one non-aromatic diamine (B-al-B);■ (ii) recurring units (RPA ) derived from the condensation of at least one aromatic diamine (B-ar-B) and at least one non-aromatic diacid (A-al-A); and■ (iii) recurring units (RPA hi) derived from the condensation of at least one aromatic diacid (A-ar-A) and at least one aromatic diamine (B-ar-B).

[0047] The total proportion of recurring units (RPA) is typically at least 10.0 mol%. This proportion may be at least 20.0 mol%, at least 30.0 mol%, at least 40.0 mol% or at least 50.0 mol%. This proportion may be between 10.0 and 100 mol%.

[0048] Polyamide (PA) may be an aromatic polyamide or a semi-aromatic polyamide.

[0049] Polyamide (PA) may comprise recurring units distinct from units (RPA), notably selected in the group consisting of units derived from a lactam, units derived from an aminoacid and units derived from the condensation of a non-aromatic diacid and a non-aromatic diamine.

[0050] Polyamide (PA) preferably does not comprise amide recurring units derived from an aminoacid or a lactam.

[0051] Polyamide (PA) typically comprises recurring units formed by the condensation of a mixture of monomers (MM) comprising or consisting of:- at least one aromatic diacid (A-ar-A) and / or at least one aromatic diamine (B-ar- B);- optionally at least one non-aromatic diacid (A-al-A);- optionally at least one non-aromatic diamine (B-al-B); with the condition that (MM) comprises at least one diacid and at least one diamine.

[0052] (MM) typically comprises or consists of:- at least one aromatic diacid (A-ar-A);- optionally at least one non-aromatic diacid (A-al-A);- at least one non-aromatic diamine (B-al-B).

[0053] If (MM) comprises at least one aromatic diacid (A-ar-A), polyamide (PA) comprises amide-containing recurring units (RPAi) and / or recurring units (RPA).

[0054] If (MM) comprises at least one aromatic diamine (B-ar-B), polyamide (PA) comprises amide-containing recurring units (RPA U) and / or recurring units (RPA ).

[0055] Polyamide (PA) may be amorphous or semi-crystalline.

[0056] Typical examples of polyamides (PA) are now provided:- polyamide (PA) obtained from a mixture of monomers (MM) consisting of one aromatic diacid (A-ar-A), at least one non-aromatic diacid (A-al-A), notably an aliphatic diacid of formula (III), and at least one non-aromatic diamine (B-al-B), notably an aliphatic diamine of formula (V);- polyamide (PA) obtained from a mixture of monomers (MM) consisting of one aromatic diacid (A-ar-A) and one non-aromatic diamine (B-al-B), notably an aliphatic diamine of formula (V);- polyamide (PA) obtained from a mixture of monomers (MM) consisting of one aromatic diacid (A-ar-A), one non-aromatic diacid (A-al-A), notably an aliphaticdiacid of formula (III), and one non-aromatic diamine (B-al-B), notably an aliphatic diamine of formula (V);- polyamide (PA) obtained from a mixture of monomers (MM) consisting of two aromatic diacids (A-ar-A), one non-aromatic diacid (A-al-A), notably an aliphatic diacid of formula (III), and one non-aromatic diamine (B-al-B), notably an aliphatic diamine of formula (V);- polyamide (PA) obtained from a mixture of monomers (MM) consisting of one non-aromatic diacid (A-al-A), notably one aliphatic diacid of formula (III), and one aromatic diamine (B-ar-B);- polyamide (PA) may also be a polyamide as disclosed generically in Example A, Example B or Example C as defined below or in the illustrative examples of each of those Examples.

[0057] Further details about and examples of monomers are provided below.

[0058] Aromatic diacid (A-ar-A)

[0059] According to an embodiment, the method of the invention makes it possible to recover one or more aromatic diacids (A-ar-A). The aromatic diacid (A-ar-A) may be any one of the aromatic diacids disclosed herein.

[0060] An aromatic diacid (A-ar-A) comprises an aromatic ring Ar and two carboxylic acid groups -COOH.

[0061] More particularly, the aromatic diacid (A-ar-A) may be represented by generic formula (I): HOOC-XrAr-X2-COOH where Xi and X2denote independently a bond or a (Ci-Ce)-alkylene group and Ar denotes an aromatic ring. The aromatic ring may be more particularly a phenyl, a naphtyl or a pyridyl group ring. The aromatic ring may more particularly be a phenyl ring.

[0062] The aromatic diacid (A-ar-A) may more particularly be selected in the group consisting of phthalic acids, such as terephthalic acid and isophthalic acid; naphthalenedicarboxylic acids such as naphthalene-2,6-dicarboxylic acid; 4,4’- bibenzoic acid; 2,5-pyridinedicarboxylic acid; 2,4-pyridinedicarboxylic acid; 3,5- pyridinedicarboxylic acid and combination of two or more of said diacids.

[0063] The aromatic diacid (A-ar-A) may more particularly be selected in the group consisting of terephthalic acid, isophthalic acid and combination of terephthalic acid and isophthalic acid. These two aromatic diacids are very frequently used in the industry, so that there is a particular interest in recovering them.

[0064] According to an embodiment of the present invention, the method of the invention is a method for recovery of terephthalic acid and / or isophthalic acid.

[0065] Aromatic diamine (B-ar-B)

[0066] According to an embodiment, the method of the invention makes it possible to recover one or more aromatic diamines (B-ar-B). The aromatic diacid (B-ar-B) may be any one of the aromatic diamines disclosed herein.

[0067] An aromatic diamine (B-ar-B) comprises an aromatic ring Ar and two diamine groups -NH2.

[0068] The aromatic diamine (B-ar-B) may be selected in the group of diamines of formula (II):(II) wherein Aik is a CrCe linear or branched alkylene group.

[0069] The aromatic diamine (B-ar-B) may be selected in the group consisting of m- xylylene diamine (MXDA), p-xylylene diamine (PXDA), 2,5-diamino pyridine, 2,3- or 2,4- or 2,6- or 3,4-diamino toluene, 1 ,2- or 1 ,3- or 1 ,4- diamino benzene, p-di- (2-aminopropyl)benzene, 3,3'-(1 ,4-phenylene)di(1-propanamine), 3, 3'-(1 ,3- phenylene)-di(l-propanamine), 3,3'-(1 ,2-phenylene)-di(1-propanamine), 2,2'-(1 ,4- phenylene)diethanamine, 2,2'-(1 ,3-phenylene)diethanamine and 2,2'-(1 ,2- phenylene)-diethanamine.

[0070] The aromatic diamine (B-ar-B) may more particularly be selected in the group consisting of m-xylylene diamine, p-xylylene diamine (PXDA), 3,3'-(1 ,4- phenylene)-di(l-propanamine), 2,2'-(1 ,4-phenylene)diethanamine and combination thereof.

[0071] The aromatic diamine (B-ar-B) may more particularly be selected in the group consisting of m-xylylene diamine, p-xylylene diamine (PXDA) and combination thereof.

[0072] According to an embodiment of the present invention, the method of the invention is a method for recovery of m-xylylene diamine and / or p-xylylene diamine.

[0073] Non-aromatic diacid (A-al-A)

[0074] The aromatic polyamide (PA) may comprise recurring units derived from at least one non-aromatic diacid (A-al-A). The non-aromatic diacid (A-al-A) may be any one of the non-aromatic diacids disclosed herein.

[0075] The non-aromatic diacid (A-al-A) may be selected in the group consisting of aliphatic diacids of formula (III) HOOC-Alk-COOH where Aik is a C2-C18 linear or branched alkylene group; cyclic diacids of formula (IV) HOOC-Cy-COOH where Cy is a C3-C6 non-aromatic cycle optionally substituted by C1-C10 alkyl groups and combination thereof.

[0076] The aliphatic diacid may be selected in the group consisting of adipic acid, 2,4,4- trimethyl adipic acid, pimelic acid, suberic acid, sebacic acid, undecanedioic acid and dodecanedioic acid.

[0077] The cyclic diacid may be selected in the group consisting of 1 ,4- cyclohexanedicarboxylic acid, cyclopropane- 1 , 2-dicarboxylic acid, 1- methylcyclopropane-1 , 2-dicarboxylic acid, cyclobutane- 1 , 2-dicarboxylic acid, tetrahydrofuran-2,5-dicarboxylic acid and 1 ,3-adamantanedicarboxylic acid.

[0078] Non-aromatic diamine (B-al-B)

[0079] The aromatic polyamide (PA) may comprise recurring units derived from at least one non-aromatic diamine (B-al-B). The non-aromatic diamine (B-al-B) may be any one of the non-aromatic diamines disclosed herein.

[0080] The non-aromatic diamine (B-al-B) may be selected in the group consisting of aliphatic diamines represented by the general formula (V) H2N-Alk-NH2, wherein Aik is a C2-C18 linear or branched alkylene group; cycloaliphatic diamines comprising at least one cycloaliphatic group between two NH2 groups and combination thereof.

[0081] The aliphatic diamine may be selected in the group consisting of hexamethylene diamine, 2,2 ,4-tri methyl- 1 ,6-hexanediamine, 2,4,4-tri methyl- 1 ,6-hexanediamine, 1 ,3-diaminopentane, 1 ,5-diaminohexane and 1 ,8-diamino-nonane.

[0082] The cycloaliphatic diamine may be selected in the group consisting of isophorone diamine, norbornane diamine, 1 ,3-bis(aminomethyl)cyclohexane (1 ,3-BAC), 1 ,4- bis(aminomethyl)cyclohexane (1 ,4-BAC) and the diamines of formula (VI):(VI), wherein R1, R2, R3 and R4 are independently selected in the group of H and Ci-Ce alkyl groups and X is a C1-C10 alkylene groups. In formula (VI), X is more particularly a methylene group. In formula (VI), R1, R2, R3 and R4 are more particularly independently selected in the group consisting of H and CH3.

[0083] The cycloaliphatic diamine may more particularly be selected in group consisting of isophorone diamine, norbornane diamine, 1 ,3-BAC, 1 ,4-BAC, para- bis(aminocyclohexyl)-methane (PACM) and bis-(3-methyl-4-aminocyclohexyl)- methane (MACM).

[0084] Polyamide (PA) may more particularly be selected in the group consisting of PA 9T; PA 10T; PA 6T / 9T; PA 6T / 10T; PA 6T / 4T; 11 / 10T; PA 6T / 12; PA 6.T / 6.l / MXDA.T / MXDA.I ; PA MXD6; PA MXD6 / PXD6; PA MXD6 / MXDI; PA 6T / 66; PA 6T / 6I / 66; PA 6T / 6I; PA 6T / 10T / BACT; PA 6T / 10T / 6.CHDA / 10.CHDA and PA 6T / 9T / 6.CHDA / 9.CHDA. (MXD designates a unit derived from MXDA as the diamine; PXD designates a unit derived from PXDA as the diamine; I designates a unit derived from isophthalic acid as the di acid; T designates terephthalic acid as the diacid; 11 designates units derived from 11-aminoundecanoic acid; 12 designates units derived from lauryl lactam; BAC designates 1,3- bis(aminomethyl)cyclohexane and / or 1,4-bis(aminomethyl)cyclohexane; CHDA designates 1 ,4-cyclohexanedicarboxylic acid).

[0085] The number average molecular weight Mnof the polyamide (PA) is generally between 500 and 50,000 g / mol, preferentially between 1 ,000 and 35,000 g / mol, even more preferentially between 2,000 g / mol and 20,000 g / mol. The Mnis determined by the equation Mn= 2,000,000 / [EG], with [EG] being the concentration in meq / kg of the end-groups of the polyamide (PA). The usual end-groups in polyamides are -NH2 and -COOH. Yet, those end-groups may in some processes be converted, partly or totally, into other end-groups by reaction with an endcapping agent. Examples of end-capping agent are monofunctional molecules containing an amine or a carboxylic acid such as acetic acid, benzoic acid, propionic acid.

[0086] Optional step a)

[0087] Product (P) to be treated may be in various forms. Indeed, product (P) may for instance be in the form of pellets, powders, films, flakes, molded or extruded or 3D printed parts, tubes, filaments, yarns, textiles, fabrics or under any type of geometry.

[0088] Yet, the size of product (P) is preferably reduced to decrease the time of depolymerization step b).

[0089] Step b): in step b), hydrolysis of polyamide (PA) is performed in acidic conditions in the presence of at least one (Ac) having at least one pKa lower than 0 and and selected in the group consisting of the nitrogen-free inorganic acids, microwave radiations being applied to a composition (C) comprising water, the product (P) and the acid (Ac).

[0090] In step b), the polyamide (PA) is depolymerized into its constitutive monomers. The depolymerization of polyamide (PA) is performed thanks to the combination of microwave radiations and the presence of at least one acid (Ac). As can be seen in the experimental section, the use of microwaves enables a fast kinetics of depolymerization.

[0091] As mentioned above, the size of product (P) is preferably reduced in step a) before step b). Therefore, product (P) is in the form of particles with a size lower than 10.0 mm, preferably lower than 5.0 mm, even preferably lower than 3.0 mm.

[0092] The frequency of the microwave radiations may be between 300 MHz and 300 GHz. The frequency of the microwave radiations may be between 4 GHz and 18 GHz.

[0093] The frequency may for instance be about 915 MHz, about 2.45 GHz, about 5.8 GHz, about 24 GHz.

[0094] The power of the microwave generator is adapted to the volume of the composition (C) to be irradiated. The power be from 200 W to 100 kW.

[0095] Examples of available generators that can be used are detailed below:

[0096] The microwaves are generated by a microwave unit including a microwave oscillator and at least one waveguide that transmits the microwaves.

[0097] The microwave radiations may be applied in different ways. At the laboratory scale, one can use a microwave reactor comprising a magnetron delivering microwave radiations at a fixed frequency (e.g. 2.455 GHz). For instance, one can use the monomode microwave reactor Monowave 300 commercialized by Anton Paar, Graz, Austria, equipped with a single magnetron (fixed frequency 2.455 GHz), delivering up to 850 W microwave radiations in unpulsed mode.

[0098] At a larger scale, one can use an equipment comprising (1) a reaction chamber (or reactor) in which step b) is performed and (2) one or more microwave units. The volume of the reaction chamber in which step b) is performed is typically at least 10 liters. The generator(s) can be positioned outside of the reaction chamber andthe waveguide(s) is / are associated with the generator(s) and housed in the reaction chamber to convey and distribute the microwave radiations in the reaction chamber. An example of such equipment can be found in WO 2021 / 094949.

[0099] The microwave waveguide(s) is / are advantageously positioned in the reactor chamber. According to an embodiment of the present invention, the microwave waveguide(s) transmit the microwaves to the unfilled space in the reactor chamber.

[0100] The method of the invention is based on the use of at least one acid (Ac). The acid (Ac) has at least one pKa lower than 0 and selected in the group consisting of the nitrogen-free inorganic acids.

[0101] The acid (Ac) may be more particularly selected in the group consisting of HCI, HBr, H2SO4, and combinations of two or more of these acids. All these acids exhibit at least one pKa lower than 0: HCI: -6.3; HBr: -8.7; H2SO4: -3 for the first acidity.

[0102] The acid (Ac) is preferably HCI or H2SO4. The acid (Ac) is more preferably HCI

[0103] HNO3 and other nitrogen-containing inorganic acids are not suitable as they tend to release NO2 and be harmful for the environment. Weak acids are also avoided. The method of the present invention therefore does not use HNO3 and other nitrogen-containing inorganic acids. For the same reasons, the method of the present invention therefore does not need the addition of any nitroso compounds as in DE 69309709. The liquid medium thus preferably does not comprise any nitroso compounds.

[0104] The initial molar ratio (H / N) representing the amount of H from the acid (Ac) over the amount of N from the amide bonds of the polyamide (PA) is between 1.1 and 7.0. This ratio may be between 1.1 and 5.0 or between 1.1 and 4.0 or between 1.1. and 3.0.

[0105] In step b), the composition (C) is advantageously at a temperature T greater than or equal to 80 °C. T may be between 80 °C and 250 °C, preferably between 100 °C and 250 °C, preferably between 150 °C and 250 °C.

[0106] A temperature T > 80 °C can be reached by heating and / or applying the microwave radiations. For instance, according to an embodiment, temperature T > 80 °C can be reached by first heating the composition (e.g. by classical means by using an electrical furnace) before stopping the heating and then applying the microwave radiations. The microwave radiations can then be powerful enough to maintain the temperature T > 80 °C. According to another embodiment, temperature T > 80 °C can be reached by simultaneously heating the composition (eg by classical means such as by using an electrical furnace) and applying the microwave radiations. According to another embodiment, temperature T > 80 °C can be reached by applying the microwave radiations only.

[0107] The pressure in the tank in which step b) is performed is the natural pressure developed by the heated liquid medium. The pressure inside the tank is generally between 1.0 and 60.0 bar. The pressure inside the tank may be between 1.0 and 25.0 bar.

[0108] During step b), there is an evolution of the composition (C) with the effect of the microwaves and of the hydrolysis of the polyamide (PA) into its constitutive monomers. Moreover, if not already dissolved, polyamide (PA) may progressively dissolve.

[0109] Step b) is performed in batch, in semi-continuous or in continuous. It is preferably performed in batch to control effectively the conditions and the proportions of the ingredients of the reaction mixture. Moreover, working in batch also ensures an easier industrial implementation of the method.

[0110] The duration of step b) depends on the operative conditions and the degree of hydrolysis R targeted. Because of the use of microwaves, the duration of step b) can be significantly shortened. The duration of step b) is generally between 1 minute and 120 minutes, preferably between 1 minute and 60 minutes, even more preferably between 1 minute and 30 minutes.

[0111] At the end of step b), one obtains a stream (S) which comprises in an aqueous medium:- the aromatic diacid(s) (A-ar-A) and / or the aromatic diamine(s) (B-ar-B) to be recovered;- the non-aromatic diacid(s) (A-al-A) (if any);- the non-aromatic diamine(s) (B-al-B) (if any); the diamine(s) being in the base form of in the form of a salt, notably in the form of salt with acid (Ac).

[0112] The aqueous medium is water.

[0113] In addition to the molecules mentioned above, stream (S) may comprise oligomers having the same recurring units as polyamide (PA) and a lower molecular weight.

[0114] In addition to the molecules mentioned above, stream (S) generally also comprises solid materials dispersed in stream (S). These solid materials are typically: -the additives (A) that may be present in product (P) (e.g. the fillers); and / or- the particles of the polyamide (PA) which have not been dissolved and / or been depolymerized; and / or- the particles of the other polymer(s) (p) that may be present in product (P); and / or- the products of degradation of the other polymer(s) (p) that may be present in product (P).

[0115] The pH of stream (S) at the end of step b) is at most 7.0. It is generally at most 6.0.

[0116] Removal of used acid (Ac)

[0117] According to an embodiment advantageous for the environment, after step b), the free acid (Ac) such as HCI or HBr is removed partially or totally from the aqueous medium by heating and / or applying vacuum to the reaction mixture. The term "free acid” refers to the acid (Ac) which is not present in the aqueous medium in the form of a salt.

[0118] This embodiment which is detailed in claim 25 makes it possible makes it possible to reduce the amount of salt generated by the reaction of acid (Ac) with the base (e.g. XOH) during the release of the aromatic diamine (B-ar-B) and / or the nonaromatic diamine (B-al-B).

[0119] The removal is preferably performed after step b) and before step c).

[0120] The removed acid (Ac) may be recycled to be reused in step b) of the method. Before being reused, the removed acid (Ac) may optionally be concentrated.

[0121] Degree of hydrolysis R: the degree of hydrolysis R achieved at the end of step b) is preferably at least 90.0 mol%, preferably at least 95.0 mol%. This ratio corresponds to the degree of conversion of the hydrolysis of the polyamide (PA). The degree of conversion is defined as the decrease in the quantity of a reactant divided by the initial quantity thereof (IUPAC definition).

[0122] The degree of hydrolysis R can be followed easily through the use of a suitable analytical technique. For instance, the degree of hydrolysis R can be monitored by 1H NMR of samples of the reaction mixture collected over time. For instance, dried samples may be analysed by1H NMR in deuterated trifluoroacetic acid or hexafluoroisopropanol. Liquid chromatography is another technique which makes it possible to determine the quantity of one or more of the monomers released.

[0123] The skilled person can refer to ACS Sustainable Chem. Eng. 2020, 8, 16274-16282 or Journal of Applied Chemistry, Volume 2014, "Kinetics and Thermodynamic Studies of Depolymerization of Nylon Waste by Hydrolysis Reaction" (htp: / / dx.doi.Org / 10.1155 / 2014 / 286709) for a method of measurement of the degree of hydrolysis.

[0124] Optional pretreatment of stream (S)

[0125] It is preferable to have a stream (S) substantially free of the solid materials mentioned above to perform subsequent step c). This is why the method of the invention advantageously comprises a pretreatment step after step b) and before step c) in order to remove partly or totally the solid materials mentioned above. The pretreatment step may comprise a liquid-solid separation (e.g. filtration).

[0126] After the pretreatment step, stream (S) is advantageously substantially free of these solid materials.

[0127] Step c): in step c), the aqueous stream (S) obtained after step b) and optionally pretreated to remove partly or totally the solid materials mentioned above, is processed in order to recover the aromatic diacid(s) (A-ar-A) and / or the aromatic diamine(s) (B-ar-B). The recovery of these aromatic monomer(s) of interest comprises at least one unit operation selected in the group consisting of liquid-solid separation (e.g. filtration), crystallization and distillation.

[0128] More particularly, the recovery of these aromatic monomer(s) of interest comprises:(i) - at least one unit operation selected in the group consisting of liquid-solid separation (e.g. filtration), crystallization and distillation; and(ii) - if polyamide (PA) comprises at least one aromatic diamine (B-ar-B), a step in which the salt of the diamine (B-ar-B) reacts with a base in order to release the diamine (B-ar-B). The base used in step (ii) is advantageously an inorganic base of formula XOH, X being Li, Na, K or a combination of two or more of these cations.

[0129] According to an embodiment, the method of the invention comprises in step c), at least one liquid-solid separation to separate a diacid from a diamine which is either in the base form or in the salified form. The diamine that is separated stays substantially in the liquid phase. The diacid that may be separated may be an aromatic diacid (A-ar-A) or a non-aromatic diacid (A-al-A).

[0130] In stream (S), the diamine(s) (e.g. the aromatic diamine (B-ar-B) and / or the non- aromatic diamine(s) (B-al-B)) are typically in the salified form. They are typically associated with the acid (Ac). As the acid (Ac) is typically HCI or HBr, they are then in the form of chlorhydrates.

[0131] According to an embodiment, the recovery of the aromatic monomers comprises the following steps:• step (i1): starting with stream (S), at least one liquid-solid separation is performed to obtain a stream (S1) comprising the diacid(s) in the solid form and a stream (L1) in the form of a solution comprising the diamine(s) in the base form or in the salified form;• optional step (i2): stream (S1) is further processed in order to recover the aromatic diacid (A-ar-A) (if any) or to separate and recover the aromatic diacids (A-ar-A) (if any);• optional step (i3): stream (L1) is further processed in order to recover the aromatic diamine (B-ar-B) (if any) or to separate and recover the aromatic diamines (B-ar-B) (if any); wherein at least one step among (i2) and (i3) is present.

[0132] The reaction of the diamine(s) in the salified form with the inorganic base may be performed before step (i1) or preferably after step (i1).

[0133] Liquid-solid separation is an operation of separation of a solid and a liquid phase from a suspension. Filtration is a liquid-solid separation which uses a filter medium. Crystallization is a physical operation that involves the formation of a solid from a solution or a suspension. Distillation is an operation of separation of liquids by virtue of their difference in boiling points. The distillation may be performed at atmospheric pressure or under reduced pressure.

[0134] Liquid-solid separation is well suited for the recovery of the diacid(s) since in step (i 1 ) , the diamine(s) either in the salified form or in the base form stay predominantly in the aqueous medium. As an example, hexamethylene diamine or a salt thereof is readily soluble in water whereas terephthalic acid and isophthalic acid both exhibit a limited solubility of respectively 17-19 mg / L and 120 mg / L in water at 25 °C and adipic acid exhibits a limited solubility of 23 g / L in water at 25 °C.

[0135] Liquid-solid separation is also well suited for the separation of the diacids in step (i2). The difference of solubilities of the diacids in the aqueous medium makes it possible to separate them.

[0136] Crystallization may be performed before a liquid-solid separation. Indeed, crystallization makes it possible to increase the proportion of a diacid in the solid form in an aqueous medium.

[0137] In step (i3), stream (L1) is further processed in order to recover the aromatic diamine (B-ar-B) (if any) or to separate and recover the aromatic diamines (B-ar- B) (if any).

[0138] Any recovered aromatic diacid (A-ar-A) and / or any recovered aromatic diamine (B- ar-B) may be further purified by traditional purification techniques, such as by crystallization and / or by distillation.

[0139] The types and number of operations depend on the composition of polyamide (PA). Examples which form part of the invention are now provided to illustrate various embodiments of step c).

[0140] Example A: polyamide (PA) based on a mixture of monomers (MM) consisting of one aromatic diacid (A-ar-A) and one non-aromatic diamine (A-al-B), notably an aliphatic diamine of formula (V)

[0141] Polyamides 9T and 6T are illustrative of this embodiment.

[0142] - step (i1) : starting with stream (S), at least one liquid-solid separation is performed to obtain a stream (S1) comprising the aromatic diacid (A-ar-A) in the solid form and one stream (L1) in the form of a solution comprising the non- aromatic diamine (B-al-B) in the base form or in the salified form;- step (i2) - stream (S1 ) is further processed in order to recover the aromatic diacid (A-ar-A).

[0143] Example B: polyamide (PA) based based on a mixture of monomers consisting of one aromatic diacid (A-ar-A), one non-aromatic diacid (A-al-A) and one nonaromatic diamine (B-al-B)

[0144] More particularly, the non-aromatic diacid (A-al-A) may be an aliphatic diacid of formula (III) and / or the non-aromatic diamine (B-al-B) may be an aliphatic diamine of formula (V).

[0145] Polyamide 6T / 66 is illustrative of this embodiment.

[0146] - step (i1) : starting with stream (S), at least one liquid-solid separation is performed to obtain a stream (S1) comprising the aromatic diacid (A-ar-A) and the non-aromatic diacid (A-al-A) in the solid form and one stream (L1) in the form of a solution comprising the non-aromatic diamine (B-al-B) in the base form or in the salified form;- step (i2) : stream (S1) is further processed in order to separate and recover the aromatic diacid (A-ar-A) and the non-aromatic diacid (A-al-A).

[0147] Example C: polyamide (PA) based on a mixture of monomers consisting of one non-aromatic diacid (A-al-A), notably one aliphatic diacid of formula (III), and one aromatic diamine (B-ar-B)

[0148] Polyamide MXD6 is illustrative of this embodiment.

[0149] - step (i1) : starting with stream (S), at least one liquid-solid separation is performed to obtain a stream (S1) comprising the non-aromatic diacid (A-al-A) in the solid form and a stream (L1) in the form of a solution comprising the aromatic diamine (B-ar-B) in the base form or in the salified form;- step (i3) : stream (L1) is further processed in order to recover the aromatic diamine (B-ar-B).

[0150] In step (i3), the salt of the diamine (B-ar-B) reacts with a base in order to release the diamine (B-ar-B). The diamine (B-ar-B) may then be recovered by distillation.

[0151] Recovery of the non-aromatic monomers (A-al-A) and / or (B-al-B)

[0152] The non-aromatic monomers that may be present in polyamide (PA) may also be recovered. As in step c), the aqueous stream (S) is processed in order to recover the aromatic monomer(s), the treatment involves a separation of the monomers. Step c) may therefore comprise also the optional recovery of the non-aromatic monomers (A-al-A) and / or (A-al-B).

[0153] The separation techniques disclosed above for the aromatic monomers apply similarly to the non-aromatic monomers. Thus, similarly to what is disclosed above for the recovery of the aromatic monomer(s), the recovery of the non-aromaticmonomer(s) (if any) comprises at least one unit operation selected in the group consisting of liquid-solid separation (e.g. filtration), crystallization and distillation.

[0154] The recovery of the non-aromatic monomer(s) may more particularly comprise:(i*) - at least one unit operation selected in the group consisting of liquid-solid separation (e.g. filtration), crystallization and distillation; and(ii*) - if polyamide (PA) comprises at least one non-aromatic diamine (B-al-B), a step in which the salt of the diamine (B-al-B) reacts with a base in order to release the diamine (B-al-B).

[0155] The base used in step (ii*) is advantageously an inorganic base of formula XOH, X being Li, Na, K or a combination of two or more of these cations.

[0156] Polyamides used- MXD6: semi-aromatic polyamide from Solvay; melting temperature around 235 °C;- PA66 (35 mol%) 1 6T (65 mol%): semi-aromatic polyamide from Solvay; melting temperature around 325 °C.

[0157] Appliance used for the production of micro-waves

[0158] Monowave 300 commercialized by Anton Paar, Graz, Austria, equipped with a single magnetron (fixed frequency 2.455 GHz), delivering up to 850 W microwave radiations in unpulsed mode.

[0159] General method used

[0160] Pellets of polyamide and the appropriate volume of HCI solution to reach the desired mole ratio between HCI I amide groups in the polyamide backbone. To prevent overheating, the reaction mixtures are heated to a predetermined temperature gradually in 5 min. When the targeted temperature of step b) is reached, it is kept constant for the whole duration of the experiment by a minimal microwave power input required. Afterwards, the reaction vessel is rapidly cooled down by a stream of compressed air.

[0161] Examples 1 and 2 demonstrate it is possible to use the method of the invention to recover efficiently (high yield + with a good kinetics) a purified aromatic diamine (example 1) or a purified aromatic diacid (example 2). It is also possible to recover a purified non-aromatic diamine (example 1) or a purified non-aromatic diacid (example 2).

[0162] Example 1 : hydrolysis of MXD6 under microwaves and recovery of the aromatic diamine MXDA (B-ar-B)

[0163] 6 g of MXD6 are suspended in 11 mL of 5.56 M HCI (n(HCI) : n(amide bonds) = 1.25) at room temperature. The reaction is then heated up to 200 °C using microwave irradiation, where the microwave power output is automatically adjusted to reach the temperature of 200 °C in 5 min, without overheating the reaction mixture. The reaction mixture is then kept at 200 °C for 10 min to achieve complete hydrolysis (no oligomers left in the reaction mixture as evidenced by1H NMR).

[0164] Recovery and purification of MXDA (B-ar-B): the reaction mixture is then first cooled down to ~0 °C to precipitate the adipic acid (A-al-A). The adipic acid is isolated by a liquid-solid separation (filtration) and separated from the salt of MXDA (B-ar-B) that remains in the filtrate. The filtrate is first concentrated on a rotary evaporator (Rotavapor R210, Buchi) to remove approximately 2 / 3 of the liquid, before it is neutralized with concentrated NaOH solution to reach a final pH > 12. After the neutralization, the sample is further concentrated with the Rotavapor and then transferred to a Kugelrohr vacuum distillation apparatus (Buchi), where complete removal of the water is first achieved at 110 °C and 290 mbar (gradually lowered to 160 mbar), followed by isolation of recycled MXDA by vacuum distillation at 150 °C and 15 mbar with a 87% recovery yield. The1H NMR spectrum of the recycled MXDA also indicates high purity, which was further confirmed by HPLC.

[0165] Recovery and purification of adipic acid (A-al-A): the precipitate is washed with cold water and recrystallized from water to yield the final recycled acidic acid with a 87% yield. The1H NMR spectrum of purified recycled adipic acid shows the typical signals of adipic acid and no additional signals due to the MXDA or other impurities, indicating a high purity of the diacid, which was further confirmed by HPLC.

[0166] Example 2: hydrolysis of polyamide 66 / 6T under microwaves and recovery of the aromatic diacid terephthalic acid T (A-ar-A)

[0167] Due to the very poor solubility of the formed T, the hydrolysis of PA 66 / 6T copolymer yields turbid reaction mixture, even when hot. To improve the stirring of the reaction mixture and the hydrolysis yield of PA 66 / 6T amide groups, the amount of copolymer is decreased to 2.5 g per 9.6 mL of 5.56 M HCI (ratio of n(HCI) : n(amide bonds) of 2.5). To achieve good conversion yields, the reaction mixture is kept at 200 °C for 30 min.

[0168] Recovery and purification of T (A-ar-A): the reaction mixture is cooled down to ~90 °C degrees and the precipitated T is isolated by filtration and washed with hot water (~90 °C) and dried to recover the recycled T (filtered) with 97% crude yield. This sample is further purified by dissolving it in 1 M NaOH to filter away undissolvedoligomeric particles. The clear liquid is then neutralized by 0.5 M HCI to further precipitate T, which is then isolated by filtration and washed with hot water before drying to recover T with 97% yield (94% total yield). The1H NMR analysis shows the presence of TPA-HMDA-TPA trimers but only in limited quantity (<1 %). To further purify T, 0.75 g of sample denoted as recycled T is suspended in 10 mL of 2.23 M HCI and heated up in a microwave reactor to 200 °C in 5 minutes. The reaction mixture is kept at 200 °C for 30 min. T is then isolated by filtration to recover the recycled T (2X) in 95% yield (89% total yield). The1H NMR spectrum of the recycled T (2X) does not show any oligomeric signals, indicating a high purity, which was further confirmed by HPLC.

[0169] Recovery and purification of adipic acid (A-al-A) and HMDA (B-al-B): The remaining reaction mixture, containing the adipic acid and HMDA in the form of HCI salt, is concentrated on a rotary evaporator (Rotavapor R210, Buchi) to remove approximately 80% of the solvent and then cooled down to ~0 °C to precipitate the adipic acid, which is isolated by filtration. The1H NMR spectrum shows that a small amount of residual HMDA is still present. Precipitated adipic acid is recrystallized from water to recover the recycled diacid in 78% yield. The recycled diacid after recrystallization from water still contains ~0.1 mol% of T since they cannot be removed by recrystallization from water. T can be removed from AA by additional recrystallization from ethanol, however at the expense of lower isolation yield.

[0170] The remaining reaction mixture is then neutralized with NaOH solution to pH>12 and dried to remove majority of the water. HMDA is then distilled on a Kugelrohr vacuum distillation apparatus (Buchi), where complete removal of the water is first achieved at 110 °C and 290 mbar (gradually lowered to 160 mbar), followed by distillation of recycled HMDA at 140 °C and 80 mbar to recover the recycled HMDA in 65% yield. The1H NMR spectrum shows the typical signals of HMDA and no additional signals due to the impurities, indicating high HMDA purity, which was further confirmed by HPLC.

Claims

ClaimsClaim 1. Method of recovery of at least one aromatic monomer from a polyamide (PA) which comprises recurring units (RPA) derived from at least one aromatic diacid (A-ar-A) and / or at least one aromatic diamine (B-ar-B), wherein the method comprises the following steps: a) a product (P) comprising at least one polyamide (PA) is optionally comminuted; b) hydrolysis of polyamide (PA) is performed in acidic conditions in the presence of at least one acid (Ac) having at least one pKa lower than 0 and and selected in the group consisting of the nitrogen-free inorganic acids, microwave radiations being applied to a composition (C) comprising water, the product (P) and the acid (Ac); c) a stream (S) comprising:■ the aromatic diacid(s) (A-ar-A); and / or■ the aromatic diamine(s) (B-ar-B) in the base form or in the form of a salt, notably in the form of a salt with the acid (Ac), which is recovered at the end of step b) is further processed in order to recover the aromatic diacid(s) (A-ar-A) and / or the aromatic diamine(s) (B-ar-B), the recovery of these aromatic monomer(s) comprising at least one unit operation selected in the group consisting of liquid-solid separation, crystallization and distillation.Claim 2. The method of claim 1 , wherein in step c), the recovery of the aromatic monomer(s) comprises:■ at least one unit operation selected in the group consisting of liquid-solid separation, crystallization and distillation; and■ a step in which the salt of the diamine (B-ar-B) reacts with a base in order to release the diamine (B-ar-B).Claim 3. Method according to claim 1 or claim 2, wherein the recurring units (RPA) are selected in the group consisting of:■ (i) recurring units (RPA i) derived from the condensation of at least one aromatic diacid (A-ar-A) and at least one non-aromatic diamine (B-al-B); and■ (ii) recurring units (RPA) derived from the condensation of at least one aromatic diamine (B-ar-B) and at least one non-aromatic diacid (A-al-A);■ (iii) recurring units (RPAhi) derived from the condensation of at least one aromatic diacid (A-ar-A) and at least one aromatic diamine (B-ar-B).Claim 4. The method according to any one of the preceding claims, wherein the polyamide (PA) comprises recurring units formed by the polycondensation of a mixture of monomers (MM) comprising or consisting of:- at least one aromatic diacid (A-ar-A) and / or at least one aromatic diamine (B-ar- B);- optionally at least one non-aromatic diacid (A-al-A);- optionally at least one non-aromatic diamine (B-al-B); with the condition that (MM) comprises at least one diacid and one diamine.Claim 5. Method according to any one of the preceding claims, wherein the aromatic diacid (A-ar-A) is represented by generic formula (I): HOOC-Xi-Ar-X2-COOH where Xi and X2 denote independently a bond or a (Ci-Ce)-alkylene group and Ar denotes an aromatic ring.Claim 6. Method according to any one of the preceding claims, wherein the aromatic diacid (A-ar-A) is selected in the group consisting of phthalic acids, such as terephthalic acid and isophthalic acid; naphthalenedicarboxylic acids such as naphthalene-2,6-dicarboxylic acid; 4,4’-bibenzoic acid; 2,5-pyridinedicarboxylic acid; 2,4-pyridinedicarboxylic acid; 3,5- pyridinedicarboxylic acid and combination of two or more of said diacids; is preferably selected in the group consisting of terephthalic acid, isophthalic acid and combination of terephthalic acid and isophthalic acid.Claim 7. Method according to any one of the preceding claims, wherein the aromatic diamine (B-ar-B) is selected in the group consisting of m-xylylene diamine (MXDA), p- xylylene diamine (PXDA), 3,3'-(1 ,4-phenylene) di(l-propanamine), 2,2'-(1 ,4- phenylene)diethanamine and combination thereof; is preferably selected in the group consisting of m-xylylene diamine, p-xylylene diamine (PXDA) and combination thereof.Claim 8. Method according to any one of the preceding claims, wherein the non-aromatic diacid (A-al-A) is selected in the group consisting of aliphatic diacids of formula (III) HOOC- Alk-COOH where Aik is a C2-C18 linear or branched alkylene group; cyclic diacids of formula(IV) HOOC-Cy-COOH where Cy is a C3-C6 non-aromatic cycle optionally substituted by C Cw alkyl groups and combination thereof; and / or the non-aromatic diamine (B-al-B) is selected in the group consisting of aliphatic diamines represented by the general formula(V) H2N-Alk-NH2, wherein Aik is a C2-C18 linear or branched alkylene group; cycloaliphatic diamines comprising at least one cycloaliphatic group between two NH2 groups and combination thereof.Claim 9. Method according to any one of the preceding claims, wherein the polyamide (PA) is selected in the group consisting of 9T; 10T; 6T / 9T; 6T / 10T; 6T / 4T; MXD6; MXD6 / PXD6; MXD6 / MXDI; 6T / 66; 6T / 6I / 66; 6T / 6I; 6T / 10T / BACT; 6T / 10T / 6.CHDA / 10.CHDA and 6T / 9T / 6.CHDA / 9.CHDA.Claim 10. Method according to any one of the preceding claims wherein the proportion of the polyamide (PA) in product (P) is at least 30.0 wt.%, more particularly at least 40.0 wt.%, more particularly at least 50.0 wt.% or at least 60.0 wt.%.Claim 11. Method according to any one of the preceding claims wherein product (P) is in the form of pellets, powders, films, flakes, molded or extruded or 3D printed parts, tubes, filaments, yarns, textiles or fabrics.Claim 12. Method according to any one of the preceding claims wherein the acid (Ac) is selected in the group consisting of HCI, HBr, H2SO4, methane sulfonic acid and combinations of two or more of these acids, preferably is HCI or H2SO4.Claim 14. Method according to any one of the preceding claims wherein the initial molar ratio (H / N) representing the amount of H from the acid (Ac) over the amount of N from the amide bonds of the polyamide (PA) is between 1.1 and 7.0.Claim 15. Method according to any one of the preceding claims, wherein in step b), composition (C) is at a temperature T greater than or equal to 80 °C, preferably between 80 °C and 250 °C, preferably between 100 °C and 250 °C, preferably between 150 °C and 250 °C.Claim 16. Method according to any one of the preceding claims, wherein the degree of hydrolysis R achieved at the end of step b) is at least 90.0 mol%, preferably at least 95.0 mol%, this ratio corresponding to the degree of conversion of the hydrolysis of the polyamide(s) (PA).Claim 17. Method according to any one of the preceding claims, wherein the duration of step b) is between 1 minute and 120 minutes, preferably between 1 minute and 60 minutes, even more preferably between 1 minute and 30 minutes.Claim 18. Method according to any one of the preceding claims, wherein the method comprises a pretreatment step after step b) and before step c) in order to remove partly or totally the solid materials dispersed in stream (S), these solid materials being selected in the group comprising:-the additives (A) that may be present in product (P); and / or- the particles of the polyamide (PA) which have not been dissolved and / or been depolymerized and / or- the particles of the other polymer(s) (p) that may be present in product (P); and / or- the products of degradation of the other polymer(s) (p) that may be present in product (P).Claim 19. Method according to any one of the preceding claims, wherein step b) is performed in a reaction chamber having a volume of at least 10 liters.Claim 20. Method according to any one of the preceding claims, wherein the method comprises in step c), at least one liquid-solid separation to separate a diacid from a diamine which is either in the base form or in the salified form.Claim 21. Method of claim 20, wherein the diacid is an aromatic diacid (A-ar-A) or a nonaromatic diacid (A-al-A).Claim 22. Method according to any one of the preceding claims, wherein in step c), the recovery of the aromatic monomers comprises the following steps:• step (i1): starting with stream (S), at least one liquid-solid separation is performed to obtain a stream (S1) comprising the diacid(s) in the solid form and a stream (L1) in the form of a solution comprising the diamine(s) in the base form or in the salified form;• optional step (i2): stream (S1) is further processed in order to recover the aromatic diacid (A-ar-A) (if any) or to separate and recover the aromatic diacids (A-ar-A) (if any);• optional step (i3): stream (L1) is further processed in order to recover the aromatic diamine (B-ar-B) (if any) or to separate and recover the aromatic diamines (B-ar-B) (if any); wherein at least one step among (i2) and (i3) is present.Claim 23. The method according to any one of the preceding claims, wherein the recovered aromatic diacid(s) (A-ar-A) and / or the recovered aromatic diamine(s) (B-ar-B) are further purified by crystallization and / or by distillation.Claim 24. The method according to any one of the preceding claims, wherein the free acid (Ac) such as HCI or HBr is removed partially or totally from the reaction mixture by heating and / or applying vacuum to the aqueous medium.Claim 25. The method according to claim 24, wherein the removal of the acid (Ac) is performed after step b) and before step c).Claim 26. The method according to any one of the preceding claims, wherein step b) is performed in batch.Claim 27. The method according to any one of the preceding claims, wherein in step c), the non-aromatic monomer(s) (if any) recovered by at least one unit operation selected in the group consisting of liquid-solid separation, such as filtration, crystallization and distillation.Claim 28. The method according to any one of the preceding claims, wherein in step c), one or more non-aromatic monomer(s) is / are recovered and step c) comprises:(i*) - at least one unit operation selected in the group consisting of liquid-solid separation (e.g. filtration), crystallization and distillation; and(ii*) - if polyamide (PA) comprises at least one non-aromatic diamine (B-al-B), a step in which the salt of the diamine (B-al-B) reacts with a base in order to release the diamine (B-al-B).Claim 29. The method according to any one of the preceding claims, wherein the aromatic monomer recovered from polyamide (PA) is terephthalic acid and / or isophthalic acid.Claim 30. The method according to any one of the preceding claims, wherein the aromatic monomer recovered from polyamide (PA) is m-xylylene diamine and / or p-xylylene diamine.

Images