Process for recycling polyamides comprising pre-treatment step

Abstract

The invention relates to a method for recovering monomers from a polyamide (PA), in particular a polyamide of the AABB type, comprising the following steps: a) contacting a product (P) comprising a polymer component comprising the polyamide (PA) with at least one inorganic salt (A *) in order to remove part or all of the crystallinity of the polyamide (PA), wherein the polymer component is in molten form and the inorganic salt (A *) is selected from the group of alkali metal inorganic salts, alkaline earth metal inorganic salts and inorganic salts of Zn; b) optionally treating the mixture obtained at the end of step a) in order to remove at least one solid material from the molten mixture; c) the hydrolysis of one or more polyamide molecules present in the mixture obtained at the end of step a) or optionally step b) is carried out in an aqueous medium (i) by acidic or basic hydrolysis involving an acid (Ac) or a base (Ba), and preferably at a pH lower than 6.0 or higher than 8.0, respectively, or (ii) by enzymatic hydrolysis; d) at the end of step c), recovering a stream (S) comprising the diacid (A) and the diamine (B), each of the two compounds being in its free form or in the form of a salt, and further processing the stream in order to separate and recover the diacid (A) and the diamine (B).

Description

[0001] This application claims priority from European Patent Application No. 22306819.8, filed on December 8, 2022, the contents of which are incorporated herein by reference in their entirety for all purposes. In the event of any inconsistency between the present applications that would affect the clarity of terminology or expression, reference should be made solely to the present application.

[0002] The present disclosure relates to a process for recycling crystalline polyamides, which comprises a pretreatment step for removing some or all of the crystallinity of the polyamide prior to a depolymerization step for converting the polyamide into its constituent monomers. [Background of the Invention]

[0004] Plastics are cheap and durable materials that can be used to manufacture a wide variety of products with such a wide range of applications that their production has increased dramatically over the years since their discovery. It is estimated that around 40% of these plastics are used in single-use, disposable applications (such as packaging, agricultural films, disposable consumer goods) or in short-lived products that are discarded within a year of manufacture. Due to the durability of the polymers involved, large amounts of plastic are accumulating in landfills and natural habitats around the world, causing a growing environmental problem. Even degradable and biodegradable plastics may last for decades, depending on local environmental factors (such as levels of UV light exposure, temperature, the presence of suitable microorganisms, etc.).

[0005] One solution to reducing the environmental and economic impacts associated with plastic accumulation is closed-loop recycling, in which plastic materials are mechanically reprocessed to make new products. For example, one of the most common closed-loop recycling methods is the recycling of polyethylene terephthalate (PET). PET waste is subjected to continuous processing to obtain food contact-approved recycled PET, which is collected, sorted, pressed into bales, crushed, washed, sliced, melted, extruded into pellets, and sold. This recycled PET can then be used to make fabrics for the apparel industry or new packaging (such as bottles or blister packs).

[0006] However, plastic waste is generally collected together, resulting in a mixture of different plastics, the composition of which can vary from source to source, and the proportions of which can vary from bale to bale. Therefore, recycling processes need to be initially selected to sort plastic products according to their composition, size, resin type, color, functional additives used, etc.

[0007] Another potential method for recycling plastics involves chemical recycling of monomers that allow the recovery of polymers. The resulting monomers can then be used to remanufacture plastic materials (the same or other plastic materials) or to manufacture other synthetic chemicals. While chemical and enzymatic depolymerization processes for PET have been well optimized for many years, producing very high yields of recycled ethylene glycol and recycled terephthalic acid, there remains a need to develop similarly efficient and optimized processes for polyamides. [Background Technology]

[0008] WO 2022 / 058291 discloses an improved process for the acid hydrolysis of polylaurolactam with sulfuric acid at temperatures between 125° C. and 190° C., preferably above 160° C. The weight ratio of H 2 SO 4 / polyamide used is preferably 1:0.1 to 1:1.

[0009] US Pat. No. 5,668,277 discloses the depolymerization of nylon 6 or nylon blends by reaction with nitrogen-containing compounds such as ammonia or amines. The reaction can be carried out in a melt. The disclosed method differs from the method of claim 1.

[0010] Polymer Journal, Vol. 5, No. 3, pp. 248-254 (1973) discloses the dissolution of nylon 6 in a metal halide-alcohol system.

[0011] J.Appl.Polym.Sci.[Journal of Applied Polymer Science]2021,138(40),1-9 "Investigation of the decomplexation of polyamide / CaCl2 complex toward a green, nondestructive recovery of polyamide from textile waste"(https: / / doi.org / 10.1002 / app.51170) discloses the nondestructive dissolution and recovery of polyamide 66 fibers from mixed textile waste by using the solvent system CaCl2 / ethanol / water.

[0012] US 6,214,592 (D1) discloses enzymatic hydrolysis of amide groups of polyamides. D1 discloses CaCl2 in solution and not in contact with the polymer components in molten form.

[0013] WO 2022 / 216681 (D2) discloses a method for chemically recycling a polycondensate, the method comprising: melt processing a mixture comprising a polycondensate and a catalyst to form an amorphous feed material comprising an amorphized polycondensate and the catalyst, wherein the amorphous feed material has a crystalline polymer content of 30 wt.% or less; and

[0014] The amorphous feed material is depolymerized in a reaction medium containing a reactive solvent to form a product mixture containing monomers corresponding to the amorphous polycondensate. ZnAc2 is disclosed in the examples. There are no disclosed examples of methods for depolymerizing polyamides. The method of the present invention does not use a reactive solvent.

[0015] WO 2023 / 120427 discloses the depolymerization of polyamides, wherein the polyamide is heated in a solvent containing a polyol and an inorganic salt. The process of the present invention is different because the inorganic salt contacted with the polymer component is in molten form and not in a solvent.

[0016] None of these documents discloses or suggests the method of the present invention.

[0017] [Technical issues to be resolved]

[0018] The depolymerization of crystalline polyamides of the AABB type (AABB-polyamides are prepared by reaction of diamines and diacids) is well documented and can be carried out under different conditions.

[0019] There is a need for an efficient and easy-to-implement process for recovering monomers from products based on polyamides of the AABB type, in particular from products which further comprise fillers.

[0020] In particular, there is a need for a recycling process using an amorphization step that requires a smaller proportion of additives to be introduced into the process.

[0021] The method of the present invention aims to solve this technical problem.

[0022] [Brief Disclosure of the Invention]

[0023] The method of the present invention is disclosed in any one of claims 1-23.

[0024] More precise information and details about this method are now provided below.

[0025] [General definition]

[0026] wt.% is percentage by weight.

[0027]

[0046] Herein, when a numerical range is indicated, the endpoints of the range are included (even open-ended ranges such as those including "at least" or "at most").

[0028] The proportion of repeat units in the polymer is expressed in mol % and is given relative to the total amount of repeat units in the polymer.

[0029] In this application, unless otherwise specified, any specific embodiment or technical feature related to the method of the present invention is applicable to and interchangeable with another embodiment or technical feature that is also related to the method of the present invention and disclosed elsewhere in this application.

[0030] Aliphatic diacids are diacids represented by the formula HOOC-Alk-COOH. Aliphatic diamines are diamines represented by the formula H2N-Alk-NH2. Alk represents a linear or branched alkylene group.

[0031] As used herein, the term "inorganic salt" means that the salt contains no carbon atoms in its chemical structure, except that the salt may be a carbonate.

[0032] [Disclosure of the invention]

[0033] The present invention relates to a method for recovering monomers from polyamide (PA), in particular polyamide of the AABB type, comprising the following steps:

[0034] a) contacting a product (P) comprising a polymer component comprising the polyamide (PA) with at least one inorganic salt (A*) in order to partially or completely reduce the crystallinity of the polyamide (PA), step a) being carried out while the polymer component of the product (P) is in molten form and the inorganic salt (A*) is chosen in the group of alkali metal inorganic salts, alkaline earth metal inorganic salts and inorganic salts of Zn;

[0035] -b) optionally treating the mixture obtained at the end of step a) in order to remove at least one solid material from the molten mixture;

[0036] -c) hydrolysis of the polyamide molecule or molecules present in the mixture obtained at the end of step a) or, optionally, step b) is carried out in an aqueous medium (i) by acidic or alkaline hydrolysis involving an acid (Ac) or a base (Ba), preferably at a pH lower than 6.0 or higher than 8.0, respectively, or (ii) by enzymatic hydrolysis;

[0037] -d) recovering, at the end of step c), a stream (S) comprising the diacid (A), the diamine (B), each of these three compounds in its free form or in the form of a salt, and further processing this stream in order to separate and recover the diacid (A) and the diamine (B).

[0038] Details regarding all features of this method are disclosed below and herein.

[0039] Product (P)

[0040] The product (P) comprises a polymer component comprising at least one polyamide (PA), in particular a polyamide of the AABB type. The polyamide of the AABB type results from the polycondensation of at least one diamine and at least one diacid.

[0041] According to an embodiment, the polyamide (PA) comprises at least 50.0 mol % of repeating units (R PA ), in particular as disclosed below. The proportion of the repeating units is expressed in mol % and is based on the total amount of repeating units in the polyamide (PA). According to another embodiment, the repeating units of the polyamide (PA) are repeating units (R) formed by the condensation of at least a diacid (A) and at least a diamine (B). PA ), in particular as disclosed below.

[0042] The repeating unit (RPA) is typically according to the following formula (F):

[0043] -NH-R a -NH-C(O)-R b -C(O)-(F)

[0044] where R a and R b The same as or different from each other are divalent hydrocarbon groups selected from the group consisting of aliphatic, cycloaliphatic, alicyclic and aromatic groups.

[0045] The diacid (A) may be any one of the diacids selected from the group of diacids disclosed herein and / or the diamine (B) may be any one of the diamines selected from the group of diamines disclosed herein.

[0046] Polyamides (PA) are typically prepared by the polycondensation of:

[0047] - at least one diacid (A) having a C2-C 18 aliphatic diacids; diacids having the formula HOOC-Cy-COOH, wherein Cy is optionally replaced by C1-C 10 Alkyl-substituted C3-C6 non-aromatic rings; isophthalic acid and terephthalic acid; and

[0048] - at least one diamine (B) which is present in a C2-C 18 Aliphatic diamine, C4-C 18 Alicyclic diamine and C8-C 18 Select from the group consisting of arylaliphatic diamines.

[0049] Polyamides (PA) are more particularly prepared by polycondensation of:

[0050] - at least one diacid (A) having a C3-C 18 selected from the group consisting of aliphatic diacids, isophthalic acid, and terephthalic acid; and

[0051] - at least one diamine (B) which is present in a C2-C 18 Aliphatic diamine, C4-C 18 Alicyclic diamine and C8-C 18 Select from the group consisting of arylaliphatic diamines.

[0052] Diacid (A) can be an aliphatic diacid represented by the general formula (I) HOOC-Alk-COOH, wherein Alk is C1-C 16 Straight or branched alkylene. Alk is usually C1-C 16 For example, the aliphatic diacid (A) may be adipic acid, azelaic acid or sebacic acid.

[0053] Diacid (A) may be a diacid having the formula HOOC-Cy-COOH, wherein Cy is optionally replaced by C1-C 10 Alkyl substituted C3-C6 non-aromatic ring. For example, the diacid can be 1,4-cyclohexanedicarboxylic acid or 1,3-cyclohexanedicarboxylic acid.

[0054] The diamine (B) may be an aliphatic diamine represented by the general formula (II) H2N-Alk-NH2, wherein Alk is C2-C 18 For example, the aliphatic diamine may be hexamethylenediamine, 2,2,4-trimethyl-1,6-hexanediamine, 2,4,4-trimethyl-1,6-hexanediamine, 2-methyl-1,5-diaminopentane, 1,5-diaminopentane or 1,9-diamino-nonane.

[0055] Diamine (B) can also be C4-C 18 Cycloaliphatic diamines. Cycloaliphatic diamines are diamines containing at least one cycloaliphatic group between two NH2 groups. The cycloaliphatic diamines can more particularly be selected from the group consisting of isophoronediamine, norbornanediamine, 1,3-bis(aminomethyl)cyclohexane (1,3-BAC), 1,4-bis(aminomethyl)cyclohexane (1,4-BAC) and diamines of formula (III):

[0056] wherein R1, R2, R3 and R4 are independently selected from the group consisting of H and C1-C6 alkyl, and X is C1-C 10Alkylene. In formula (III), X is more particularly methylene. In formula (III), R1, R2, R3 and R4 are more particularly independently selected from the group consisting of H and CH3.

[0057] The cycloaliphatic diamine may more particularly be chosen from the group consisting of isophoronediamine, norbornanediamine, 1,3-BAC, 1,4-BAC, p-bis(aminocyclohexyl)methane (PACM) and bis-(3-methyl-4-aminocyclohexyl)methane (MACM).

[0058] The diamine (B) may be C8-C 18 Aryl aliphatic diamines, especially of formula (IV):

[0059] wherein Alk is a C1-C6 linear or branched alkylene group. The diamine having formula (IV) may be, for example, meta-phenylenediamine (MXDA) or para-phenylenediamine (PXDA).

[0060] The polyamide (PA) is a semicrystalline polyamide. The heat of fusion (Hm) of the polyamide (PA) is generally at least 5.0 J / g, preferably at least 10.0 J / g.

[0061] The polyamide (PA) can more particularly be chosen in the group of polyamides XY, wherein X is an aliphatic diamine represented by formula (II) or as listed herein, and Y is an aliphatic diacid represented by formula (I) or as listed herein.

[0062] The polyamide (PA) can more particularly be chosen in the group consisting of polyamide 6.6, polyamide 6.9, polyamide 6.10, polyamide 6.12, polyamide 10.10, polyamide 5.10, polyamide 5.6, polyamide 5.9, polyamide 4.6, polyamide 4.9, polyamide 4.10, polyamide 12.12, polyamide 10.12, polyamide XT (wherein X is C4-C 12 -diamine); polyamide MXD6, polyamide MXD6 / PXD6, polyamide MXD6 / MXDI, polyamide 6T / 66, polyamide 6T / 6I / 66, polyamide 6T / 6I and mixtures thereof (Note: MXD represents a structural unit with MXDA (m-phenylenediamine) as a diamine; PXD represents a structural unit with PXDA (p-phenylenediamine) as a diamine; I represents a structural unit with isophthalic acid as a diacid; T represents a structural unit with terephthalic acid as a diacid).

[0063] The polyamide (PA) may more particularly be polyamide 6.6, polyamide 6.10 or MXD6.

[0064] The polymer component may also comprise another polymer that is not a polyamide (PA) as defined above. This polymer may be blended with the polyamide (PA) and / or physically present with the polyamide (PA) in the product (P) but not blended. Preferably, the other polymer is not a polyamide. For example, the other polymer may be a polyester, such as polyethylene terephthalate, polypropylene terephthalate, polybutylene terephthalate, polycyclohexanedimethylene terephthalate; a polyolefin, such as polyethylene or polypropylene; or a polyphenylene ether.

[0065] The product (P) typically further comprises at least one polymer additive (Add). The polymer additive (Add) may be selected from the group consisting of fillers, colorants, dyes, pigments, lubricants, plasticizers, flame retardants, nucleating agents, heat stabilizers, UV stabilizers, elastomers, core-shell particles, adhesives, antioxidants, and processing aids. More particularly, the polymer additive (Add) may be selected from the group consisting of fillers, colorants, dyes, pigments, lubricants, elastomers, and heat stabilizers.

[0066] The product (P) to be processed can be in various forms. In fact, the product (P) can be, for example, in the form of a pellet, powder, film, sheet, molded or extruded or 3D-printed part, tube, filament, yarn, textile, fabric or any type of geometric shape. More particularly, the product (P) can be in the form of a film comprising at least one layer comprising polyamide (PA) or made thereof. The product (P) can be more particularly in the form of a multilayer film comprising at least one layer comprising polyamide (PA) or made thereof. For example, the product (P) can be in the form of a multilayer film comprising a layer comprising polyamide (PA) (especially MXD6) or made thereof between two layers comprising polyethylene terephthalate (PET).

[0067] The proportion of polyamide (PA) in the product (P) is generally at least 30.0 wt.%, more particularly at least 40.0 wt.%. The proportion can be at least 50.0 wt.% or even at least 60.0 wt.%. If the product (P) consists of polyamide (PA), the proportion can be 100 wt.%. However, this is rare because the method of the present invention is intended to be applied to products in our daily life, in which polymer additives are usually present in combination with polyamide (PA). The proportion of polyamide (PA) in the product (P) is generally less than 99.9 wt.%, more particularly less than 99.5 wt.%.

[0068] The number-average molecular weight (Mn) of polyamide (PA) is generally between 500 and 50,000 g / mol, preferably between 7,000 and 35,000 g / mol, and even more preferably between 10,000 and 20,000 g / mol. Mn is determined by the equation Mn = 2,000,000 / [EG], where [EG] is the concentration of the end groups of the polyamide (PA) in meq (milliequivalents) / kg. Common end groups in polyamides are -NH2 and -COOH. However, these end groups can be partially or completely converted into other end groups in some processes by reaction with end-capping agents. Examples of end-capping agents are monofunctional molecules containing amines or carboxylic acids (e.g., acetic acid, benzoic acid, propionic acid).

[0069] Optional pre-treatment of the product (P)

[0070] The product (P) may be pretreated prior to step a). This pretreatment step may comprise mechanical or physical modification of the product, such as cutting, crushing, grinding or fractionation.

[0071] The product (P) is conveniently in the form of particles having a size of less than 10.0 mm, preferably less than 5.0 mm, even more preferably less than 3.0 mm.

[0072] Step a)

[0073] In step a), the product (P) is brought into contact with an inorganic salt (A*) in order to partially or completely reduce the crystallinity of the polyamide (PA).

[0074] Step a) is carried out with the polymer components of the product (P) in molten form.

[0075] Step a) can be carried out in any melt mixing apparatus designed to mix polymers in molten form.By carrying out step a) in molten form without the need to use any solvents, the environmental footprint of the process is improved.

[0076] Step a) can be carried out in a kneader (e.g. a Banbury mixer), in a static mixer (e.g. the SMX static mixer commercialized by Sulzer) or in an extruder. The static mixer needs to be adapted to high viscosity fluids such as molten polymers. The skilled person is aware of several types of static mixers adapted to high viscosity fluids, in particular those disclosed in Progress in Polymer Science, 37(10), 1333-1349.

[0077] Step a) is preferably carried out in an extruder, in particular a single-screw extruder or a twin-screw extruder.

[0078] Inorganic salt (A*) and product (P) can be introduced into the melt mixing device in different ways. According to an embodiment, they are introduced separately. For example, inorganic salt (A)* can be introduced into the extruder at the side port of the extruder. According to another embodiment, inorganic salt (A*) and product (P) are introduced into the melt mixing device in the form of a premix. For example, inorganic salt (A*) and product (P) can be contacted, both of which are in solid form. In order to ensure close contact between the particles of inorganic salt (A*) and the particles of product (P), the average size of the particles is preferably low. As another example, inorganic salt (A*) and product (P) can contact when product (P) is in molten form.

[0079] The temperature profile is adapted to the polyamide (PA) and the other polymer(s), if any, of the polymer components.

[0080] In step a), the temperature of the polymer components is strictly above 210°C (>210°C). It is generally at least 220°C.

[0081] The inorganic salt (A*) is selected from the group consisting of an inorganic salt of an alkali metal, an inorganic salt of an alkaline earth metal, and an inorganic salt of Zn. The anion of the inorganic salt may be a chloride ion or a nitrate. The anion of the inorganic salt is advantageously a chloride ion because it is more environmentally friendly than a nitrate.

[0082] The inorganic salt (A*) may be in anhydrous or hydrated form. The inorganic salt (A*) is advantageously in hydrated form.

[0083] The inorganic salt (A*) can be selected from the following group: CaCl2, CaCl2,xH2O, Ca(NO3)2, Ca(NO3)2,xH2O, LiCl, ZnCl2, MgCl2 (wherein x is an integer between 1 and 6), and a combination of two or more of the inorganic salts.

[0084] The inorganic salt (A*) may more particularly be any one of the inorganic salts (A*) tested in the examples of the experimental part.

[0085] The reduction in the crystallinity of the polyamide (PA) can be monitored and quantified by techniques known to the skilled person, such as differential scanning calorimetry (DSC), X-ray diffraction (XRD) or densitometry.

[0086] In step a), the proportion of inorganic salt (A*) is generally less than 60.0 wt.-%, preferably less than 50.0 wt.-%, the proportion in wt.-% being the weight of inorganic salt (A*) relative to the total weight of polyamide (PA) + inorganic salt (A*). This proportion is typically at least 5.0 wt.-%, preferably at least 10.0 wt.-%.

[0087] The proportion of inorganic salt (A*) may be between 5.0 and 50.0 wt % or between 5.0 and 30.0 wt %, the proportion in wt % being expressed as the weight of inorganic salt (A*) relative to the total weight of polyamide (PA) + inorganic salt (A*).

[0088] At the end of step a), the crystallinity of the polyamide (PA) is preferably reduced by a ratio r of at least 25.0%, preferably at least 50.0%. r is defined as: r=(Hm of the polyamide (PA) - Hm at the end of step a)) / Hm of the polyamide (PA)×100, Hm being the melting enthalpy determined by DSC.

[0089] According to an embodiment of the present disclosure, r>0.

[0090] The ratio r may be between 25.0% and 100.0%, more particularly between 25.0% and 98.0%, even more particularly between 50.0% and 95% or between 75.0% and 90.0%.

[0091] At the end of step a), the number average molecular weight (Mn) of the polyamide (PA) is preferably reduced by a ratio r* of at least 20.0%, preferably at least 40.0%, more preferably at least 60.0%, more preferably at least 75.0%. r is defined as: r*=(Mn of the polyamide (PA) - Mn at the end of step a)) / Mn of the polyamide (PA) x 100. Mn can advantageously be determined by size exclusion chromatography (SEC).

[0092] The ratio r* may be between 20.0% and 100.0%, more particularly between 20.0% and 98.0%, even more particularly between 40.0% and 95% or between 75.0% and 90.0%.

[0093] Step b)

[0094] In step b), the mixture obtained at the end of step a) is optionally treated in order to remove at least one solid material from the molten mixture. A convenient treatment is filtering the mixture in molten form.

[0095] The solid material that can be removed can be one of the one or more polymer additives (Add). For example, the solid material that can be removed in step b) can be one or more fillers or one or more elastomers.

[0096] Solid materials that can be removed can also be degradation products of the polymer(s) or polymer(s) additive(s) produced in step a).

[0097] Step c)

[0098] In step c), the hydrolysis of the polyamide molecule(s) present in the mixture obtained at the end of step a) or optionally step b) is carried out. The polyamide molecule(s) to be hydrolyzed are in particular the polyamide molecule(s) initially present in the product (P) and the polyamide molecule(s) resulting from the reaction with the inorganic salt(s) (A*).

[0099] With decreasing crystallinity of the polyamide (PA) obtained at the end of step a), the molecules of the polyamide (PA) are more susceptible to hydrolysis, in particular by enzymatic hydrolysis.

[0100] The hydrolysis of the polyamides depends on a number of parameters and is carried out in an aqueous medium. Step a) makes it possible to increase the dispersibility of the polyamide or polyamides in the aqueous medium.

[0101] Acidic or alkaline hydrolysis (option (i))

[0102] Under option (i), the hydrolysis is an acidic or basic hydrolysis involving at least one acid (Ac) or at least one base (Ba), respectively, and carried out in an aqueous medium at a pH lower than 6.0 or higher than 8.0, respectively.

[0103] The acidic hydrolysis is preferably carried out at a pH below 6.0, preferably below 4.0. The acid (Ac) used for the acidic hydrolysis is preferably a strong acid with a pKa below 2.0. The acid (Ac) can be selected from the group consisting of HCl, H2SO4, or a combination of two or more of these acids. The acid (Ac) is preferably HCl.

[0104] The hydrolysis under acidic conditions of step c) of option (i) is advantageously carried out in an aqueous medium with the following initial ratios:

[0105] - Proportion of polyamide (PA): at least 15.0 wt.%;

[0106] - water content: between 20.0 and 70.0 wt.%;

[0107] - the remainder is acid (Ac), provided that the proportion of acid (Ac) is at least 2.0 wt.%, preferably at least 2.5 wt.%.

[0108] These proportions are expressed in wt.% and are based on the total weight of polyamide (PA), water and acid (Ac) in the liquid medium. If the product (P) contains more than one polyamide (PA), these proportions are based on the total weight of polyamide (PA), water and acid (Ac).

[0109] The initial proportion of polyamide (PA) in the aqueous medium is advantageously at least 15.0 wt.%. This proportion may be between 15.0 wt.% and 55.0 wt.%, preferably between 15.0 wt.% and 35.0 wt.%, and more preferably between 15.0 wt.% and 30.0 wt.%. This proportion is calculated taking into account the proportion of polyamide (PA) in the product (P). The higher the proportion of polyamide (PA) in the liquid medium, the better the productivity. However, this proportion is limited by the fact that the viscosity of the liquid medium increases over time and a sufficient amount of acid (Ac) must be maintained to maintain appropriate depolymerization kinetics.

[0110] The proportion of water in the aqueous medium is typically between 20.0 wt.% and 70.0 wt.%. The proportion may be between 30.0 wt.% and 70.0 wt.% or between 35.0 wt.% and 65.0 wt.%. For the sake of clarity, it should be noted that the proportion of water used in the context of the present invention takes into account both the added water and the water that may originate from the solution of the acid (Ac), unless otherwise stated.

[0111] The initial proportion of acid (Ac) corresponds to a 100 wt.% replenishment. In other words, the proportion of acid (Ac) in wt.% = 100% - the proportion of PA in wt.% - the proportion of water in wt.%. The minimum proportion of acid (Ac) in the liquid medium is preferably at least 2.0 wt.%, preferably at least 2.5 wt.%.

[0112] The initial proportion of acid (Ac) is generally between 2.0 wt.% and 55.0 wt.%. The proportion may be between 5.0 wt.% and 55.0 wt.%.

[0113] For the sake of clarity, it should be noted that, unless otherwise stated, the proportion of acid (Ac) in the liquid medium used in the context of the present invention is given as the proportion of pure acid. For example, a proportion of 20.0 wt.% refers to 20.0 wt.% pure HCl, regardless of the strength of the solution (e.g., a 37 wt.% HCl solution). Also for the sake of clarity, if the acid (Ac) corresponds to a combination of two or more of the acids defined above, the proportion of acid (Ac) given herein corresponds to the total proportion of these acids.

[0114] The initial molar ratio (H / N), which represents the amount of H from the acid (Ac) to the amount of N from the amide bonds of the polyamide (PA), is preferably between 1.1 and 7.0. This ratio is preferably at least 2.0. This ratio may preferably be between 2.5 and 7.0.

[0115] The alkaline hydrolysis is preferably carried out at a pH above 8.0, preferably above 9.0. The base (Ba) used for the alkaline hydrolysis is preferably a strong base with a pKa above 8.0. The base (Ba) can be selected from the group consisting of NaOH, KOH, or a combination of two or more of these bases. The base (Ba) is preferably NaOH.

[0116] Step c) under option i) may be carried out at a temperature between 20°C and 150°C. The temperature is preferably between 100°C and 150°C.

[0117] Enzymatic hydrolysis (option (ii))

[0118] Under option (ii), the hydrolysis is an enzymatic hydrolysis. This type of hydrolysis is carried out in the presence of at least one enzyme.

[0119] The enzyme is selected from the group of enzymes suitable for breaking the -NH-CO- bonds of one or more polyamides. The enzyme may be an amidase, such as an aryl-acyl amidase. The enzyme may, for example, be one of the enzymes disclosed in US Pat. No. 6,214,592 or US Pat. No. 6,180,388 to Rhone Poulenc Fibres et polymeres SA, in particular an enzyme having the sequence provided in said US Pat. No. 6,214,592 or US Pat. No. 6,180,388. Other enzymes are also disclosed in Appl. Microbiol. Biotechnol. (2014) 98: 8751-8761 (DOI 10.1007 / s00253-014-5885-2) or in Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, 2749-2753 (2005) (DOI: 10.1002 / pola.20739). The other enzymes are as follows: NylA, NylB and NylC from Arthrobacter species KI72; NylA and NylB derived from Pseudomonas species NK8; NylB and NylC derived from Agromyces species KY5R; NylA, NylB and NylC derived from Kocuria species KY2.

[0120] Step c) under option ii) is preferably carried out at a temperature that is not detrimental to the activity of the enzyme or enzymes. The temperature is preferably below 60°C.

[0121] Step c) can advantageously be carried out with limited amounts of water.Step c) is advantageously carried out with a weight ratio of water used in step c) / weight of product (P) lower than 5.0.

[0122] Step c) can be carried out in continuous mode or preferably batchwise.The skilled person may refer to the Dictionary of Chemical Engineering of Professor Carl Schaschke, ISBN 978-0-19-965145-0 for an explanation of these terms.

[0123] Step c) is preferably carried out batchwise in a batch reactor which is initially charged with the product (P) and into which no further product (P) is transferred during the hydrolysis.

[0124] Step c) is preferably carried out at a pressure strictly less than 15.0 bar (<15.0 bar), preferably less than and equal to 10.0 bar (≤10.0 bar), preferably less than and equal to 5.0 bar (≤5.0 bar), preferably less than and equal to 3.0 bar (≤3.0 bar).

[0125] Conversion R: The degree of conversion R achieved at the end of step c) 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 reduction in the amount of reactants divided by their initial amount (IUPAC definition).

[0126] R can be easily calculated by taking into account the weight of the polyamide(s) (PA) initially present in the product (P) and the amount of polyamide(s) (PA) remaining at the end of step c) by mass conversion. If the product (P) contains more than one polyamide (PA), R is calculated by taking into account the total weight of the polyamide(s) (PA).

[0127] Using the process of the present invention, it is possible to achieve at least 90.0 mol %, or even at least 95.0 mol % of R within a duration of step c) of less than 9.0 hours, preferably less than 8.0 hours.

[0128] Step d)

[0129] In step d), a stream (S) comprising the diacid (A) and the diamine (B), each of these two compounds in its free form or in the form of a salt, is recovered at the end of step c), and this stream is further processed in order to separate and recover the diacid (A) and the diamine (B).

[0130] Stream (S) generally also comprises one or more inorganic salts (A*) still present in this step of the process. If step b) is carried out, the one or more inorganic salts (A*) are already partially or completely removed.

[0131] Depending on the conditions of the hydrolysis step c), the diacid (A) on the one hand and the diamine (B) on the other hand may be in their free form (acid form or base form, respectively) or in the form of a salt.

[0132] Thus, if the hydrolysis is an acidic hydrolysis, the stream (S) comprises the diacid (A), the diamine (B) in salt form, in particular in the form of a salt with an acid (Ac). Likewise, if the hydrolysis is a basic hydrolysis, the stream (S) comprises the salt of the diacid (A) with a base (Ba) and the diamine (B).

[0133] The further processing of the stream (S) is conveniently based on crystallization and distillation steps. According to embodiments of the present disclosure, the further processing of the stream (S) is based on at least one crystallization step and / or at least one distillation step. Different methods combining crystallization and separation steps can be used to further process the stream (S).

[0134] As an example, in the case of acidic hydrolysis, the salts of the diacid (A) and diamine (B) present in the stream (S) are separated. The separated salt of the diamine (B) can then be reacted with an inorganic base having the formula XOH, X being Li, Na, K or a combination of two or more of these cations, to release the diamine (B). XOH is preferably NaOH.

[0135] It should be noted that before reacting with the inorganic base (e.g., after step c) and before step d), the method optionally includes partially or completely removing unreacted acid (Ac) from the reaction mixture. When the acid (Ac) is volatile, this removal is easier. For example, HCl, HBr or methanesulfonic acid are acids that can be easily removed from aqueous media, especially by distillation. The acid (Ac) can be partially or completely removed from the reaction mixture by heating and / or applying a vacuum to the reaction mixture. This makes it possible to use a reduced amount of inorganic base for the reaction (neutralization) and thus reduces the amount of the salt of X released.

[0136] The salts of the diacid (A) and diamine (B) can be conveniently separated by crystallization. Crystallization is a separation technique that exploits the differences in solubility of components present in an aqueous medium. For example, in the case of PA 66, most of the diacid (A) (adipic acid) can crystallize at ambient temperature, while the salt of the diamine (B) (hexamethylenediamine) can remain in the aqueous medium. If the aqueous medium is cooled to below ambient temperature, more adipic acid crystallizes, which helps to increase the recovery yield of adipic acid. After neutralization with the inorganic base XOH, the diamine (B) is released and can be recovered, for example, by distillation. The one or more inorganic salts (A*) can then remain in the boiler of the distillation column along with the residue.

[0137] As another example, in the case of alkaline hydrolysis, the salt of the diacid (A) and the diamine (B) present in stream (S) are separated. The separation can conveniently be carried out by crystallization. The diacid (A) can then be recovered after reaction of the salt with an acid (e.g., hydrochloric acid).

[0138] The recovered compound can be further purified to the purity level required by the application.

[0139] [Experimental part]

[0140] Various blends were prepared using a DSM microcompounder by melt mixing virgin polyamide 66 resin and an adjusted amount of inorganic salt (A*) at 280-285°C at 50 rpm for 3 minutes until homogenized. The blends were then recovered, quenched, and analyzed.

[0141] Table I

[0142]

[0143]

[0144] The degree of plasticization is easily monitored by following the residual melting enthalpy Hm via DSC.

[0145] Therefore, samples with reduced crystallinity provide easier access to amide bonds via H-bonding interactions that are no longer involved in the dense crystal structure.

[0146] SEC results also show that amorphization can be accompanied by a decrease in molecular weight, especially if the inorganic salt (A*) is in a hydrated form (e.g., E7). This MW reduction improves the water dispersibility of the polymer. This, combined with amorphization, increases accessibility to the amide bond, which is necessary for complete hydrolysis, either chemically or enzymatically.

[0147] Comparative Example Using ZnAc2, 2H2O

[0148] In the second round of experiments, various blends were prepared using a DSM microcompounder by melt-blending virgin polyamide 66 resin with adjusted amounts of ZnAc2,2H2O (ranging from 2 wt% to 30 wt%) at 280-285°C at 50 rpm for 3 minutes until homogenized. The blends were then recovered, quenched, and analyzed.

[0149] Table II

[0150]

[0151] As can be seen from the results disclosed in Table II, ZnAc2,2H2O is less effective than the inorganic salts (A*) listed in Table I for reducing the crystallinity of polyamide: CE16 (65% amorphization, containing 30 wt% additive) is compared with E2 (100% amorphization, containing 30 wt% anhydrous CaCl2); E3-E6 (75% amorphization, containing 20 wt% CaCl2,6H2O); E8 (69% amorphization, containing 20 wt% ZnCl2); E9 (100% amorphization, containing 20 wt% MgCl2) and E10 (55% amorphization, containing 20 wt% Ca(NO3)2,4H2O). Therefore, the method of the present invention is more effective than the method of D2.

Claims

1. A method for recovering monomers from polyamide (PA), in particular AABB-type polyamide, comprising the following steps: a) contacting a product (P) comprising a polymer component comprising the polyamide (PA) with at least one inorganic salt (A*) in order to partially or completely reduce the crystallinity of the polyamide (PA), step a) being carried out while the polymer component of the product (P) is in molten form and the inorganic salt (A*) is chosen in the group of alkali metal inorganic salts, alkaline earth metal inorganic salts and inorganic salts of Zn; -b) optionally treating the mixture obtained at the end of step a) in order to remove at least one solid material from the molten mixture; -c) hydrolysis of the polyamide molecule or molecules present in the mixture obtained at the end of step a) or, optionally, step b) is carried out in an aqueous medium (i) by acidic or alkaline hydrolysis involving an acid (Ac) or a base (Ba), preferably at a pH lower than 6.0 or higher than 8.0, respectively, or (ii) by enzymatic hydrolysis; -d) recovering, at the end of step c), a stream (S) comprising the diacid (A), the diamine (B), each of these two compounds in its free form or in the form of a salt, and further processing this stream in order to separate and recover the diacid (A) and the diamine (B).

2. The method according to claim 1, wherein: The polyamide (PA) comprises at least 50.0 mol % of repeating units (R) formed by the condensation of at least a diacid (A) and at least a diamine (B) PA ), the ratio being based on the total amount of repeating units in the polyamide (PA); or The repeating units of the polyamide (PA) are repeating units (R) formed by the condensation of at least a diacid (A) and at least a diamine (B) PA )composition.

3. The method according to claim 2, wherein: These repeating units (R PA ) is according to the following formula (F): -NH-R a -NH-C(O)-R b -C(O)-(F) where R a and R b The same as or different from each other are divalent hydrocarbon groups selected from the group consisting of aliphatic, cycloaliphatic, alicyclic and aromatic groups.

4. A method according to any one of the preceding claims, wherein The polyamide (PA) is prepared by polycondensation of: - at least one diacid (A) having a C2-C 18 aliphatic diacids; diacids having the formula HOOC-Cy-COOH, wherein Cy is optionally replaced by C1-C 10 Alkyl-substituted C3-C6 non-aromatic rings; isophthalic acid and terephthalic acid; and - at least one diamine (B) which is present in a C2-C 18 Aliphatic diamine, C4-C 18 Alicyclic diamine and C8-C 18 Select from the group consisting of arylaliphatic diamines.

5. A method according to any one of the preceding claims, wherein The polyamide (PA) is prepared by polycondensation of: - at least one diacid (A) having a C3-C 18 selected from the group consisting of aliphatic diacids, isophthalic acid, and terephthalic acid; and - at least one diamine (B) which is present in a C2-C 18 Aliphatic diamine, C4-C 18 Alicyclic diamine and C8-C 18 Select from the group consisting of arylaliphatic diamines.

6. A method according to any one of the preceding claims, wherein The polyamide (PA) is selected from the group of polyamides XY, wherein X is an aliphatic diamine represented by the general formula (II) H2N-Alk-NH2, wherein Alk is C2-C 18 A linear or branched alkylene group, and Y is an aliphatic diacid represented by the general formula (I) HOOC-Alk-COOH, wherein Alk is C1-C 16 Straight-chain or branched-chain alkylene.

7. A method according to any one of the preceding claims, wherein The polyamide (PA) is selected from the group consisting of polyamide 6.6, polyamide 6.9, polyamide 6.10, polyamide 6.12, polyamide 10.10, polyamide 4.6, polyamide 4.9, polyamide 4.10, polyamide 12.12, polyamide 10.12, polyamide MXD6, polyamide MXD6 / PXD6, polyamide MXD6 / MXDI, polyamide 6T / 66, polyamide 6T / 6I / 66, polyamide 6T / 6I and mixtures thereof.

8. A method according to any one of the preceding claims, wherein The product (P) further comprises at least one polymer additive (Add), which is in particular selected from the group consisting of fillers, colorants, dyes, pigments, lubricants, plasticizers, flame retardants, nucleating agents, heat stabilizers, UV stabilizers, elastomers, core-shell particles, adhesives, antioxidants and processing aids.

9. A method according to any one of the preceding claims, wherein The proportion of polyamide (PA) in the product (P) is at least 30.0 wt.%, more particularly at least 40.0 wt.%.

10. A method according to any one of the preceding claims, wherein Step a) is carried out in an extruder or in a static mixer.

11. A method according to any one of the preceding claims, wherein The anion of the inorganic salt (A*) is chloride or nitrate.

12. A method according to any one of the preceding claims, wherein The inorganic salt (A*) is selected from the group consisting of CaCl2, CaCl2,xH2O, Ca(NO3)2, Ca(NO3)2,xH2O, LiCl, ZnCl2, MgCl2 - wherein x is an integer between 1 and 6 - and combinations of two or more of said inorganic salts.

13. A method according to any one of the preceding claims, wherein The ratio of inorganic salt (A*) is: - less than 60.0 wt. %, preferably less than 50.0 wt. %, the proportion in wt. % being expressed as the weight of inorganic salt (A*) relative to the total weight of polyamide (PA) + inorganic salt (A*); and / or at least 5.0 wt. %, preferably at least 10.0 wt. %, the proportion in wt. % being expressed as the weight of the inorganic salt(s) (A*) relative to the total weight of polyamide (PA) + inorganic salt (A*).

14. A method according to any one of the preceding claims, wherein At the end of step a), the crystallinity of the polyamide (PA) is reduced by at least 25%, preferably by at least 50%, by a ratio r, r being defined as: r=(Hm of polyamide (PA) - Hm at the end of step a)) / Hm of polyamide (PA)×100, Hm being determined by DSC.

15. A method according to any one of the preceding claims, wherein At the end of step a), the number average molecular weight (Mn) of the polyamide (PA) is reduced by a ratio r* of at least 20.0%, preferably at least 40.0%, more preferably at least 60.0%, more preferably at least 75.0%, r* being defined as: r=(Mn of polyamide (PA) - Mn at the end of step a)) / Mn of polyamide (PA))×100.

16. A method according to any one of the preceding claims, wherein The degree of conversion R of the polyamide (PA) achieved at the end of the hydrolysis step c) is at least 90.0 mol %, preferably at least 95.0 mol %.

17. A method according to any one of the preceding claims, wherein Step c) is an acidic hydrolysis, preferably carried out at a pH below 6.0, preferably below 4.

0.

18. The method according to claim 17, wherein Step c) is an acidic hydrolysis involving HCl or H2SO4 as acid.

19. The method according to any one of claims 1 to 16, wherein: Step c) is an alkaline hydrolysis, preferably carried out at a pH above 8.0, preferably above 9.

0.

20. The method according to any one of claims 1 to 16, wherein Step c) is an enzymatic hydrolysis carried out in the presence of at least one enzyme.

21. The method according to claim 20, wherein The enzyme is selected from the group of enzymes suitable for breaking the -NH-CO- bonds of one or more polyamides.

22. A method according to any one of the preceding claims, wherein The further processing of this stream (S) is based on at least one crystallization step and / or at least one distillation step.

23. The method according to claim 17 or 18, wherein The hydrolysis is an acidic hydrolysis and wherein the diacid (A) and the salt of the diamine (B) present in the stream (S) are separated and wherein the salt of the diamine (B) that has been separated is reacted with an inorganic base of formula XOH, X being Li, Na, K or a combination of two or more of these cations, to release the diamine (B).

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