Process for pre-treating recycled polyamides with reduced molecular weight

Abstract

The invention relates to a process for recovering monomers from a polyamide (PA) of the AABB type, comprising the following steps: a) contacting a product (P) comprising a polymer component comprising the polyamide (PA) with a compound (A *) in order to reduce the molecular weight of the polyamide (PA), the polymer component being in molten form and the compound (A *) being selected from the group of C2-C18 carboxylic acids; 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 carried out at a pH of less than 6.0 or higher, respectively, or (ii) by enzymatic hydrolysis; d) recovering a stream (S) comprising the diacid (A), the diamine (B) and the compound (A *) at the end of step c), each of the three compounds being 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), the diamine (B) and the compound (A *).

Description

[0001] This PCT application claims the priority of U.S. Patent Application No. 63 / 405,641 filed on September 12, 2022 and European Patent Application No. 22199325.6 filed on October 3, 2022, the contents of which are hereby incorporated by reference in their entirety for all purposes. If there are any inconsistencies between this PCT application and the two applications that would affect the clarity of terms or expressions, only this PCT application should be referred to.

[0003] This disclosure relates to a method for recycling polyamides, the method comprising a molecular weight reduction pretreatment and comprising depolymerizing the polyamide into its constituent monomers. Background of the Invention

[0005] Plastics are inexpensive and durable materials that can be used to manufacture a wide variety of products with a wide range of applications, such that the production of plastics has increased dramatically over the years since plastics were discovered. It is estimated that approximately 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 one year after manufacture. Due to the durability of the polymers involved, large amounts of plastics are accumulating in landfills and natural habitats around the world, causing increasing environmental problems. Even degradable and biodegradable plastics can persist for decades, depending on local environmental factors (such as UV light exposure levels, temperature, the presence of suitable microorganisms, etc.).

[0006] One solution to reduce the environmental and economic impacts associated with plastic accumulation is closed-loop recycling, in which plastic materials are mechanically reprocessed to manufacture new products. For example, one of the most common closed-loop recycling processes is the recycling of polyethylene terephthalate (PET). PET waste is subjected to a continuous process to obtain food-contact approved recycled PET, which is collected, sorted, baled, shredded, washed, sliced, melted and extruded into pellets and sold. These recycled PETs can then be used to manufacture fabrics for the clothing industry or new packaging (such as bottles or blister packs, etc.).

[0007] However, plastic waste is generally collected together entirely, such that the plastics contain a mixture of different plastics, the composition of which can vary depending on the source and the proportions of which can vary from bale to bale. Therefore, the recycling process requires a preliminary selection to classify it according to the composition, size, resin type, color, functional additives used, etc. of the plastic product.

[0008] Another potential method for recycling plastics involves the chemical recycling of monomers of recycled polymers. The resulting monomers can then be used to remanufacture plastic materials (the same or other plastic materials) or to manufacture other synthetic chemicals. Although the chemical and enzymatic depolymerization processes for PET have been well optimized over the years to recycle ethylene glycol and terephthalic acid in very high yields, there is still a need to similarly develop an efficient and optimized process for polyamides. [Background Art]

[0009] WO 2022 / 058291 discloses an improved method for the acid hydrolysis of polylaurolactam with sulfuric acid at a temperature between 125 °C and 190 °C, preferably at a temperature above 160 °C. The weight ratio of H2SO4 / polyamide used is preferably from 1:0.1 to 1:1.

[0010] US 5,668,277 discloses the depolymerization of nylon 6 or nylon mixtures by reaction with nitrogen-containing compounds such as ammonia or amines. The reaction can be carried out in the molten form. The disclosed method is different from the method of claim 1.

[0011] US 4,620,032 discloses a method for depolymerizing polycondensates such as polyesters or polyamides, which method comprises the steps of: (a) intimately mixing a molten polycondensate selected from the group consisting of polyamides and polyesters with a depolymerizing agent selected from the group consisting of: (i) a product obtained by the complete hydrolysis depolymerization of the molten polycondensate in liquid form, and (ii) water; (b) mixing the molten polycondensate and the depolymerizing agent for a sufficient time to reduce the molecular weight of the polycondensate by at least 50%; and (c) subjecting the treated polycondensate to neutral hydrolysis with a significant excess of water based on the weight of the polycondensate, thereby achieving substantially complete hydrolysis depolymerization of the polycondensate.

[0012] CN 114163622 relates to the technical field of recycling of semi-aromatic polyamide waste and discloses a method for depolymerizing semi-aromatic polyamide by alcoholysis reaction with 1,4-butanediol and a catalyst. The disclosed method is different from the method of claim 1.

[0013] CN 87101541 (D1) does not disclose a compound (A*) selected from the group of C2-C 18 carboxylic acids in contact with polyamide.

[0014] US 4,605,762 (Application No. 563,812) discloses the continuous hydrolysis depolymerization of polycondensates, which comprises subjecting waste materials selected from the group consisting of polyester polymers, polyamide polymers, and polycarbonate polymers to aqueous hydrolysis in a hydrolysis zone at a temperature between 200 °C and 300 °C and at a superatmospheric pressure of at least 15 atm.

[0015] US 4,620,032 (D2) discloses a method for depolymerizing polycondensates, which method comprises the steps of the following combination: (a) intimately mixing a molten polycondensate selected from the group consisting of polyamides and polyesters with a depolymerizing agent selected from the group consisting of: (i) the product obtained by the complete hydrolysis depolymerization of said polycondensate in liquid form, and (ii) water, said depolymerizing agent being present in said mixture in an amount by weight less than the weight of said polycondensate; (b) mixing said molten polycondensate and said depolymerizing agent for a sufficient time to reduce the molecular weight of said polycondensate by at least 50%; and (c) subjecting said treated polycondensate Significantly excessive water to Neutral hydrolysis thereby achieving substantially complete hydrolysis depolymerization of said polycondensate. D2 discloses the transesterification of PA66 with hexamethylenediamine and subsequent Neutral hydrolysis with water. D2 does not disclose the hydrolysis step of claim 1, which is carried out under acidic or basic conditions (thus, not neutral) or with the aid of enzymes. Furthermore, the only example of polyamide depolymerization is Example 4, in which a compound is added to hexamethylenediamine rather than to a C2-C 18 carboxylic acid.

[0016] Technical Problem to be Solved

[0017] The depolymerization of AABB-type polyamides is well documented and can be carried out under different conditions. AABB-type polyamides are produced by the polycondensation of at least one diamine and at least one diacid.

[0018] There is a need for an efficient and easy-to-implement method that recovers monomers from products based on AABB-type polyamides, especially from products that further contain fillers. To address the lack of water, the method should also be environmentally friendly and use a limited amount of water.

[0019] The method of the present invention aims to solve this technical problem.

[0020] Brief Disclosure of the Invention

[0021] The method of the present invention is disclosed in the appended claims, especially in one of claims 1-28.

[0022] More precise information and details about the method are now provided below.

[0023] General Definition

[0024] wt.% is the percentage by weight. Mol.% is the percentage by mole.

[0025] The proportion of the repeating units in the polymer is expressed in mol% and is given relative to the total amount of the repeating units in the polymer.

[0026] When indicating a numerical range, the range endpoints are included.

[0027] 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 is disclosed elsewhere in this application.

[0028] Polyphthalamide is a polyamide containing more than 50.0 mol% of repeating units (R PPA ), which are formed by the polycondensation of (i) phthalic acid with (i) at least one diamine, and the phthalic acid is selected from the group consisting of isophthalic acid, terephthalic acid, and a combination of isophthalic acid and terephthalic acid, and the proportion in mol% is based on the total amount of the repeating units of the polyamide. Detailed Description

[0029] The present invention relates to a method for recovering monomers from polyamide (PA), especially polyamide (PA) of the AABB type, the method comprising the following steps:

[0030] -a) contacting a product (P) comprising a polymer component containing the polyamide (PA) with a compound (A*) to reduce the molecular weight of the polyamide (PA), wherein the polymer component is in a molten form and the compound (A*) is selected from the group of C2-C 18 carboxylic acids;

[0031] -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;

[0032] -c) the hydrolysis of one or more polyamide molecules present in the mixture obtained at the end of step a) or optional step b) is carried out in an aqueous medium (i) by acidic or basic hydrolysis involving an acid (Ac) or a base (Ba) at a pH below 6.0 or above 8.0 respectively, or (ii) by enzymatic hydrolysis;

[0033] -d) At the end of step c), recycle the stream (S) comprising the diacid (A), the diamine (B) and the compound (A*), each of these three compounds being in its free form or in the form of a salt, and further process this stream in order to separate and recover the diacid (A), the diamine (B) and the compound (A*).

[0034] Product (P)

[0035] The product (P) comprises a polymer component comprising at least one polyamide (PA).

[0036] The product (P) may comprise one or more polyamides (PA). All details provided herein regarding the polyamide (PA) apply to the product (P) comprising more than one polyamide (PA).

[0037] The repeating unit of the polyamide (PA) is formed by the condensation of at least a diacid (A) and at least a diamine (B) into the repeating unit (R PA ), as disclosed in particular below. The repeating unit (R PA ) is typically according to the formula (F):

[0038] -NH-R a -NH-C(O)-R b -C(O)-(F)

[0039] wherein R a and R b are the same or different from each other and are divalent hydrocarbon groups selected from the group of aliphatic, cycloaliphatic, alicyclic and aromatic groups.

[0040] The polyamide (PA) is typically prepared by polycondensation of:

[0041] - at least one diacid (A), which is selected from the group of C2-C 18 aliphatic diacids; diacids having the formula HOOC-Cy-COOH, where Cy is a C3-C6 non-aromatic ring optionally substituted by C1-C 10 alkyl; isophthalic acid and terephthalic acid; and

[0042] - at least one diamine (B), which is selected from the group consisting of C2-C 18 aliphatic diamines, C4-C 18 cycloaliphatic diamines and C8-C 18 arylaliphatic diamines.

[0043] The polyamide (PA) may in particular be prepared by polycondensation of:

[0044] - at least one diacid (A), which is selected from the group of C2-C 18selected from the group consisting of aliphatic diacids, isophthalic acid, and terephthalic acid; and

[0045] - at least one diamine (B), which is selected from the group consisting of C2-C 18 aliphatic diamines, C4-C 18 alicyclic diamines, and C8-C 18 aryl aliphatic diamines.

[0046] The diacid (A) can be an aliphatic diacid represented by the general formula (I) HOOC-Alk-COOH, where Alk is a C1-C 16 linear or branched alkylene group. Alk is usually a C1-C 16 linear alkylene group. For example, the aliphatic diacid (A) can be adipic acid, azelaic acid, or sebacic acid.

[0047] The diacid (A) can be a diacid having the formula HOOC-Cy-COOH, where Cy is a C3-C6 non-aromatic ring optionally substituted with a C1-C 10 alkyl group. For example, the diacid can be 1,4-cyclohexanedicarboxylic acid or 1,3-cyclohexanedicarboxylic acid.

[0048] The diamine (B) can be an aliphatic diamine represented by the general formula (II) H2N-Alk-NH2, where Alk is a C2-C 18 linear or branched alkylene group. For example, the aliphatic diamine can 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-diaminononane.

[0049] The diamine (B) can also be an alicyclic diamine. An alicyclic diamine is a diamine containing at least one alicyclic group between two NH2 groups. The alicyclic diamine can be more particularly 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 a diamine having the formula (III):

[0050] where R1, R2, R3, and R4 are independently selected from the group consisting of H and C1-C6 alkyl groups, and X is a C1-C 10 alkylene group. In formula (III), X is more particularly a methylene group. In formula (III), R1, R2, R3, and R4 are more particularly independently selected from the group consisting of H and CH3.

[0051] The alicyclic diamine can be more particularly selected from the group consisting of isophorone diamine, norbornane diamine, 1,3-BAC, 1,4-BAC, p-bis(aminocyclohexyl)-methane (PACM), and bis-(3-methyl-4-aminocyclohexyl)-methane (MACM).

[0052] The diamine (B) can be a diamine having the formula (IV):

[0053] wherein Alk is a C1-C6 straight-chain or branched alkylene. The diamine having the formula (IV) can be, for example, meta-xylylenediamine (MXDA) or para-xylylenediamine (PXDA).

[0054] The polyamide (PA) can be an amorphous or semi-crystalline polyamide.

[0055] The polyamide (PA) can be a polyphthalamide.

[0056] The polyamide (PA) can be more particularly selected from 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 (where X is a C4-C12-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 (meta-xylylenediamine) as the diamine; PXD represents a structural unit with PXDA (para-xylylenediamine) as the diamine; I represents a structural unit with isophthalic acid as the diacid; T represents a structural unit with terephthalic acid as the diacid).

[0057] The polyamide (PA) can be more particularly polyamide 6.6, polyamide 6.10, or MXD6.

[0058] According to the embodiment, the polyamide (PA) is not PA 6 or PA66.

[0059] According to another embodiment, the product (P) does not contain PA 6 or PA66.

[0060] The polymer component may also comprise another polymer (p) which is not a polyamide (PA) as defined above. The polymer (p) may be blended with the polyamide (PA) and / or physically present in the product (P) with the polyamide (PA) but not blended. Preferably, the other polymer (p) is not a polyamide. For example, the other polymer (p) may be a polyester such as polyethylene terephthalate, polypropylene terephthalate, polybutylene terephthalate, cyclohexanedimethanol terephthalate; a polyolefin such as polyethylene or polypropylene; or a polyphenylene ether.

[0061] The product (P) generally also 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, binders, antioxidants and processing aids. The polymer additive (Add) may more particularly be selected from the group consisting of: fillers, colorants, dyes, pigments, lubricants, elastomers and heat stabilizers.

[0062] The product (P) to be treated may be in various forms. In fact, the product (P) may, for example, be in the form of granules, powders, films, sheets, molded or extruded or 3D printed parts, tubes, filaments, yarns, textiles, fabrics or any type of geometric shape. More particularly, the product (P) may be in the form of a film which comprises at least one layer comprising or made of polyamide (PA). The product (P) may more particularly be in the form of a multilayer film which comprises at least one layer comprising or made of polyamide (PA). For example, the product (P) may be in the form of a multilayer film which comprises a layer comprising or made of polyamide (PA) (in particular MXD6) between two layers comprising polyethylene terephthalate (PET).

[0063] 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.

[0064] The proportion of polyamide (PA) in the 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.%. If the product (P) consists of polyamide (PA), this proportion may be 100 wt.%. However, this situation is rare since the method of the present invention is intended to be applied to products of our daily life where 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.%.

[0065] The number-average molecular weight Mn of the 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 g / mol 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 / kg. Common end groups in polyamides are -NH2 and -COOH. However, those end groups may be partially or completely converted into other end groups by reaction with a capping agent in some processes. Examples of capping agents are monofunctional molecules containing amines or carboxylic acids such as acetic acid, benzoic acid, and propionic acid.

[0066] In the experimental section, Mn was measured according to size-exclusion chromatography (SEC).

[0067] Optional pretreatment of product (P)

[0068] The product (P) can be pretreated before step a). This pretreatment step can include mechanical or physical modification of the product, such as cutting, crushing, grinding, or fractionation.

[0069] Step a)

[0070] In step a), the product (P) is contacted with the compound (A*) to reduce the molecular weight of the polyamide (PA).

[0071] Step a) is carried out in a molten form. The polymer component containing the one or more polyamides (PA) is in a molten form. The temperature T of the polymer component is typically greater than the melting point of the semi-crystalline polyamide (PA) or greater than the glass transition temperature of the amorphous polyamide (PA). If the product (P) contains more than one polyamide (PA) and / or any polymer (p), the respective melting points or glass transition temperatures of all the polymers of the product P need to be considered. The temperature T is typically at least 200 °C or even at least 250 °C.

[0072] Step a) can be carried out in any melt mixing device designed to mix polymers in a molten form. The mixing in step a) ensures good dispersion of the compound (A*).

[0073] Step a) can be carried out in a kneader (such as a Banbury mixer), in a static mixer (such as the SMX static mixer commercialized by Sulzer), or in an extruder.

[0074] The static mixer needs to be adapted to highly viscous fluids such as molten polymers. Those skilled in the art know that there are several types of static mixers adapted to highly viscous fluids, especially those disclosed in Progress in Polymer Science, 37(10), 1333-1349.

[0075] Step a) is preferably carried out in an extruder, especially a single-screw extruder or a twin-screw extruder.

[0076] Compound (A*) is selected from the group of C2-C 18 carboxylic acids.

[0077] Compound (A*) can more particularly be selected from the group consisting of: C2-C 18 monocarboxylic acids, C2-C 18 dicarboxylic acids and combinations of two or more of said acids.

[0078] Compound (A*) can more particularly be selected from the group consisting of: straight-chain or branched C2-C 18 monocarboxylic acids, straight-chain or branched C2-C5 dicarboxylic acids, straight-chain or branched C7-C 18 dicarboxylic acids and combinations of two or more of said acids.

[0079] Compound (A*) can more particularly be selected from the group consisting of: carboxylic acids having the formula CH3-(CH2) n -COOH (V) (n is an integer between 0 and 16), carboxylic acids having the formula HOOC-(CH2) m -COOH (VI) (m is an integer between 0 and 16), isophthalic acid, terephthalic acid and combinations of two or more of said acids. m can more specifically be an integer between 1 and 16. m can more specifically be an integer between 1 and 16, provided that m is not 2.

[0080] Compound (A*) can more particularly be adipic acid, sebacic acid, azelaic acid, terephthalic acid or isophthalic acid.

[0081] According to an embodiment, compound (A*) is advantageously the same as the dicarboxylic acid (A) of polyamide (PA), especially polyamide AABB, or one of the dicarboxylic acids (A). This embodiment makes it possible to avoid introducing an acid different from the dicarboxylic acid (A) into the stream (S) to be further processed, which makes step d) of separation and recovery easier to implement.

[0082] According to an embodiment, compound (A*) is not adipic acid.

[0083] According to a specific embodiment (E), the polyamide is a polyphthalamide and the compound (A*) is terephthalic acid and / or isophthalic acid.

[0084] One or more compounds (A*) can be used in the process of the present invention. However, in order to avoid having too many components in the stream (S) to be further processed in step d), it is preferred to use only one compound (A*).

[0085] In step a), the proportion of the compound (A*) is generally less than 50.0 wt%, preferably less than 40.0 wt%, and this proportion in wt% is expressed as the weight of the compound (A*) relative to the total weight of [polyamide (PA) + compound (A*)]. This proportion is typically at least 5.0 wt%, preferably at least 10.0 wt%. For the sake of clarity, if the product (P) contains more than one polyamide (PA), then this proportion is relative to the total weight of [polyamide (PA) + compound (A*)].

[0086] In 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 polyamide (PA) - Mn at the end of step a) / Mn of polyamide (PA) × 100. Mn can advantageously be determined by size exclusion chromatography (SEC).

[0087] The ratio r can 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%.

[0088] Step b)

[0089] 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. Step a) makes it possible to reduce the viscosity of the molten mixture, which helps to manipulate and process the molten mixture. See the experimental section.

[0090] The solid material that can be removed can be one of one or more polymer additives (Add). For example, the solid material that can be removed in step b) can be one or more fillers, one or more elastomers or degradation products of the product (P).

[0091] The solid material that can be removed can also be degradation products of one or more polymers or one or more polymer additives produced in step a).

[0092] Step c)

[0093] In step c), 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. The one or more polyamide molecules to be hydrolyzed are in particular one or more polyamide molecules initially present in product (P) and one or more polyamide molecules resulting from the reaction with compound (A*).

[0094] The hydrolysis of the polyamide depends on many parameters and is carried out in an aqueous medium. Step a) makes it possible to increase the dispersibility of one or more polyamides in the aqueous medium, which helps to reduce the hydrolysis time.

[0095] Step c) can be carried out in continuous mode or preferably batchwise. Those skilled in the art can refer to the Dictionary of Chemical Engineering of Professor Carl Schaschke, ISBN 978-0-19-965145-0 to illustrate these terms.

[0096] Step c) is preferably carried out batchwise in a batch reactor in which product (P) is initially charged and no additional product (P) is introduced during hydrolysis.

[0097] Under option (i), the hydrolysis is acidic or basic hydrolysis carried out in an aqueous medium involving at least one acid (Ac) or at least one base (Ba) respectively, at a pH below 6.0 or above 8.0 respectively. Additional details related to option (i) are provided below.

[0098] Hydrolysis under acidic conditions

[0099] The acidic hydrolysis is carried out at a pH below 6.0, preferably below 4.0. The acid (Ac) used for 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 said acids. The acid (Ac) is preferably HCl.

[0100] The hydrolysis in acidic conditions of step c) is advantageously carried out in an aqueous medium in the following initial proportions:

[0101] - Proportion of polyamide (PA): at least 15.0 wt.%;

[0102] - Proportion of water: between 20.0 and 70.0 wt.%;

[0103] - The remainder is acid (Ac), provided that the proportion of acid (Ac) is at least 2.0 wt.%, preferably at least 2.5 wt.%.

[0104] These ratios are expressed in wt.% and are based on the total weight of polyamide (PA), water and acid (Ac) in the liquid medium. As mentioned above, if the product (P) contains more than one polyamide (PA), these ratios are based on the total weight of polyamide (PA), water and acid (Ac).

[0105] The initial ratio of polyamide (PA) in the aqueous medium is advantageously at least 15.0 wt.%. This ratio can be between 15.0 wt.% and 55.0 wt.%, preferably between 15.0 wt.% and 35.0 wt.%, preferably between 15.0 wt.% and 30.0 wt.%. This ratio is calculated by considering the ratio of polyamide (PA) in the product (P). The higher the ratio of polyamide (PA) in the liquid medium, the better the productivity. However, this ratio is limited considering the increase in the viscosity of the liquid medium over time and the need to maintain a sufficient amount of acid (Ac) to sustain proper depolymerization kinetics.

[0106] The ratio of water in the aqueous medium is typically between 20.0 wt.% and 70.0 wt.%. This ratio can 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 unless otherwise stated, the ratio of water used in the context of the present invention takes into account the added water and the water that may originate from the solution of acid (Ac).

[0107] The initial ratio of acid (Ac) corresponds to a complement of 100 wt.%. In other words, the ratio of acid (Ac) in wt.% = 100% - the ratio of PA in wt.% - the ratio of water in wt.%. The minimum ratio of acid (Ac) in the liquid medium is preferably at least 2.0 wt.%, preferably at least 2.5 wt.%.

[0108] The initial ratio of acid (Ac) is generally between 2.0 wt.% and 55.0 wt.%. This ratio can be between 5.0 wt% and 55.0 wt%.

[0109] For the sake of clarity, it should be noted that unless otherwise stated, the ratio of acid (Ac) in the liquid medium used in the context of the present invention is given as the ratio of pure acid. For example, a ratio of 20.0 wt.% refers to 20.0 wt.% of pure HCl, regardless of the strength of the solution (e.g., a 37 wt.% HCl solution). Also for the sake of clarity, if acid (Ac) corresponds to a combination of two or more of the acids as defined above, the ratio of acid (Ac) given herein corresponds to the total ratio of these acids.

[0110] The initial molar ratio (H / N) of the amount of H from the acid (Ac) to the amount of N of 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 can preferably be between 2.5 and 7.0.

[0111] Specific conditions for hydrolysis under acidic conditions that can be used are now provided below:

[0112] Specific conditions (I)

[0113] - Proportion of polyamide (PA): between 15.0 and 55.0 wt.%;

[0114] - Proportion of water: between 20.0 and 70.0 wt.%;

[0115] - Proportion of acid (Ac): between 10.0 and 55.0 wt.%;

[0116] - H / N is between 2.0 and 7.0.

[0117] Specific conditions (II)

[0118] - Proportion of polyamide (PA): between 15.0 and 35.0 wt.%;

[0119] - Proportion of water: between 20.0 and 55.0 wt.%;

[0120] - Proportion of acid (Ac): between 12.0 and 50.0 wt.%;

[0121] - H / N is between 2.5 and 3.5.

[0122] Specific conditions (III)

[0123] - Proportion of polyamide (PA): between 15.0 and 30.0 wt.%;

[0124] - Proportion of water: between 30.0 and 45.0 wt.%;

[0125] - Proportion of acid (Ac): between 12.0 and 50.0 wt.%;

[0126] - H / N is between 2.5 and 3.5.

[0127] In addition to water, product (P) and acid (Ac), the aqueous medium may further comprise an organic component selected from the group consisting of alcohols, ketones and combinations thereof. The organic component is typically liquid at ambient temperature. The organic component preferably comprises less than 10 carbon atoms. The alcohol may more particularly be selected from the group consisting of methanol, ethanol, propanol, butanol and combinations thereof. The ketone may more particularly be selected from the group consisting of acetone, propanone, butanone and combinations thereof.

[0128] Hydrolysis under alkaline conditions

[0129] Alkaline hydrolysis is carried out at a pH above 8.0, preferably above 9.0. The base (Ba) for alkaline hydrolysis is preferably a strong base with a pKa above 8.0. The base (Ba) may be selected from the group consisting of NaOH, KOH or a combination of two or more of said bases. The base (Ba) is preferably NaOH.

[0130] Step c) under option i) can be carried out at a temperature between 20 °C and 190 °C or between 20 °C and 150 °C. The temperature is preferably between 100 °C and 150 °C.

[0131] Under option (ii), the hydrolysis is enzymatic hydrolysis. This type of hydrolysis is carried out in the presence of at least one enzyme.

[0132] The enzyme is suitable for breaking the -NH-CO- bonds of one or more polyamides. The enzyme may be an amidase, such as aryl-acylamidase. The enzyme may for example be one of the enzymes disclosed in US 6,214,592 of Rhone Poulenc Fibres et polymeres SA, in particular an enzyme having the sequences provided in said US patent.

[0133] Step c) under option ii) is preferably carried out at a temperature that is not harmful to the activity of one or more enzymes. The temperature is generally below 60 °C.

[0134] For both options i) and ii), step c) is preferably carried out at a pressure strictly less than 15.0 bar (< 15.0 bar), preferably less than or equal to 10.0 bar (≤ 10.0 bar), preferably less than or equal to 5.0 bar (≤ 5.0 bar), preferably less than or equal to 3.0 bar (≤ 3.0 bar).

[0135] Step c) can advantageously be carried out with a limited amount of water. Step c) is advantageously carried out at a weight ratio r 水 of less than 5.0, where r 水 is defined as the weight of water in the aqueous medium used in step c) / the weight of product (P).

[0136] Degree of conversion R: The conversion degree 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 conversion degree of the hydrolysis of polyamide (PA). The conversion degree is defined as the reduction in the amount of the reactant divided by its initial amount (IUPAC definition).

[0137] R can be easily calculated by mass conversion by considering the weight of one or more polyamides (PA) initially present in the product (P) and the amount of one or more polyamides (PA) remaining at the end of step c). If the product (P) contains more than one polyamide (PA), then R is calculated by considering the total weight of the one or more polyamides (PA).

[0138] Using the method of the present invention, an R of at least 90.0 mol% or even at least 95.0 mol% can be achieved within a duration of step c) of less than 9.0 hours, preferably less than 8.0 hours.

[0139] Step d)

[0140] In step d), a stream (S) containing the diacid (A), the diamine (B), and the compound (A*) is recovered at the end of step c), each of these three compounds being in its free form or in the form of a salt, and the stream is further processed to separate and recover the diacid (A), the diamine (B), and the compound (A*).

[0141] Depending on the conditions of the hydrolysis step c), on the one hand the diacid (A) and the compound (A*), and on the other hand the diamine (B) can be in their free form (correspondingly the acid form or the base form) or in the form of a salt. Thus, if the hydrolysis is an acid hydrolysis, the stream (S) contains the diacid (A), the diamine (B) in the form of a salt (in particular in the form of a salt with an acid (Ac)), and the compound (A*). Similarly, if the hydrolysis is a base hydrolysis, the stream (S) contains the salt of the diacid (A) with a base (Ba), the diamine (B), and the salt of the compound (A*) with a base (Ba).

[0142] The recovered monomers can be further purified to the purity level required by the present application.

[0143] Optionally, the recovered compound (A*) can also be recycled and used again in step a).

[0144] Step d) after hydrolysis under acidic conditions includes:

[0145] (i) - at least one unit operation selected from the group consisting of: liquid-solid separation (e.g., filtration), crystallization, and distillation; and

[0146] (ii) – The step in which the salt of the diamine (B) is reacted with a base to release the diamine (B). The base used in step (ii) is advantageously an inorganic base of the formula XOH, where X is Li, Na, K or a combination of two or more of these cations.

[0147] Different methods of combining the crystallization and separation steps can be used for the further processing of stream (S). As an example, in the case of acid hydrolysis, the salt of the diacid (A), the compound (A*), and the diamine (B) is separated. The separated salt of the diamine (B) is reacted with an inorganic base of the formula XOH to release the diamine (B), where X is Li, Na, K or a combination of two or more of these cations. This neutralization reaction is as follows: salt of diamine (B) + XOH -> diamine (B). For example, if HCl is used for the acid hydrolysis of PA66, the salt of the diamine (B) is NH2-(CH2)6-NH2,2HCl, and the neutralization reaction is: NH2-(CH2)6-NH2,2HCl + 2XOH -> NH2-(CH2)6-NH2 + 2XCl + 2H2O. XOH is preferably NaOH.

[0148] Separation can be conveniently carried out by crystallization. Crystallization is a separation technique that utilizes the solubility differences of the 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 reaction with the inorganic base XOH, the diamine (B) is released and can be recovered, for example, by distillation.

[0149] Experimental Section

[0150] Example 1: Polyamide 66, where A* = adipic acid

[0151] Step a) was carried out using a DSM micro mixer under the following conditions: 13.5 g of recycled virgin polyamide 66 was mixed with 1.5 g of adipic acid (corresponding approximately to a proportion of 10.0 wt% of the compound (A*)), and the micro mixer was run at 285 °C and 100 rpm for 3 minutes. The mixture obtained at the end of step a) was characterized by SEC and showed a decrease in Mn.

[0152] Step a) was carried out again with two other proportions of adipic acid: 20% wt. and 30% wt.: see Table 1.

[0153] Table I

[0154]

[0155]

[0156] Measure the melt viscosity of polyamide PA66 and the product after step a) using an ARES rheometer (for the starting PA66) or a coquette module (for the products of Ex1-2, 1-3, and 1-4). It can be seen that the viscosity of the products of Ex1-2, 1-3, and 1-4 is well reduced. This low viscosity in the molten form facilitates the subsequent steps of the method of the present invention.

[0157] Compared with polyamide PA66, the three products of Ex1-2, 1-3, and 1-4 show improved water dispersibility in deionized water. When in contact with water, the particles are misaligned, and when the dispersion is shaken, the particles stay in suspension for a longer time.

[0158] Example 2: Polyamide 66, where A* = terephthalic acid

[0159] Step a) is carried out using a DSM micro mixer under the following conditions: 9.1 g of polyamide 66 is mixed with 3.9 g of terephthalic acid (ratio = 30 wt%), and the micro mixer is run at 280 °C and 100 rpm for 3 minutes.

[0160] Measure the molecular weight reduction from Mn = 10,100 g / mol to Mn = 1,090 g / mol (r = 89% reduction) by SEC using a light scattering detector.

[0161] Example 3: Polyamide 66, where A* = sebacic acid

[0162] Step a) is carried out using a DSM micro mixer under the following conditions: 9.1 g of polyamide 66 is mixed with 3.9 g of sebacic acid (ratio 30 wt%), and the micro mixer is run at 280 °C and 100 rpm for 3 minutes.

[0163] Measure the molecular weight reduction from Mn = 10,100 g / mol to Mn = 2,870 g / mol (r = 72% reduction) by SEC using a light scattering detector.

[0164] Example 4: Acidic hydrolysis of the product of Ex1-6 obtained at the end of step b)

[0165] Charge 30 g of the solid flakes recovered from Ex 1-4 (0.186 mol of amide bonds) and a 37 wt% hydrochloric acid solution into a 300 ml glass reactor equipped with a reflux condenser and a mechanical stirrer. Immerse the reaction mixture in an oil bath at 120 °C and keep it under reflux for 24 hours.

[0166] The solid flakes are easily dispersed in the hydrochloric acid solution.

[0167] Example 5: Polyamide 66 and polyamide 66 / glass fiber, where A* = adipic acid

[0168] Ex.5-1: Step a) was carried out using a DSM micro mixer under the following conditions: 13.5 g of PA66 (A45 from Radici Group, where Mn = 25,400 g / mol) was mixed with 1.0 g of adipic acid, and the micro mixer was run at 290 °C and 200 rpm for 3 minutes (Ex5-1). The mixture obtained at the end of step a) was characterized by SEC and showed a decrease in the average Mn. See Table II. A45, where Mn = 25,400 g / mol) was mixed with 1.0 g of adipic acid, and the micro mixer was run at 290 °C and 200 rpm for 3 minutes. The mixture obtained at the end of step a) was characterized by SEC and showed a decrease in the average Mn. See Table II.

[0169] Ex.5-2: A compound made of PA66 (A45 from Radici Group, where Mn = 25,400 g / mol) and 56 wt% chopped glass fibers was prepared by twin-screw extrusion. Step a) was carried out with the following compound: 13.5 g of the compound was mixed with 0.4 g of adipic acid, and the micro mixer was run at 290 °C and 200 rpm for 3 minutes. The mixture obtained at the end of step a) was characterized by SEC and similarly showed a decrease in the average Mn. See Table II. A45, where Mn = 25,400 g / mol) and 56 wt% chopped glass fibers was prepared by twin-screw extrusion. Step a) was carried out with the following compound: 13.5 g of the compound was mixed with 0.4 g of adipic acid, and the micro mixer was run at 290 °C and 200 rpm for 3 minutes. The mixture obtained at the end of step a) was characterized by SEC and similarly showed a decrease in the average Mn. See Table II.

[0170] Table II

[0171] Ex. Proportion of (A*) Mn (g / mol) Ratio r Ex5-1 6.9％ 9,800 61％ Ex5-2 6.3％ 8,400 67％

Claims

1. A method for recovering monomers from polyamide (PA), especially polyamide of the AABB type, the method comprising the following steps: -a) Contacting a product (P) comprising a polymer component containing the polyamide (PA) with a compound (A*) to reduce the molecular weight of the polyamide (PA), wherein the polymer component is in a molten form and the compound (A*) is selected from the group of C2-C 18 carboxylic acids; - 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 one or more polyamide molecules present in the mixture obtained at the end of step a) or optionally step b) in an aqueous medium (i) by acidic or basic hydrolysis involving an acid (Ac) (hydrolysis in acidic conditions) or a base (Ba) (hydrolysis in basic conditions) and at a pH below 6.0 or above 8.0 respectively, or (ii) by enzymatic hydrolysis; - d) Recovering at the end of step c) a stream (S) comprising a diacid (A), a diamine (B) and a compound (A*), each of these three compounds being 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), the diamine (B) and the compound (A*).

2. The method according to claim 1, wherein The repeating unit of polyamide (PA) consists of a repeating unit (R PA ) formed by the condensation of at least a diacid (A) and at least a diamine (B).

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

4. The method according to any one of the preceding claims, wherein, The polyamide (PA) is prepared by polycondensation of: - At least one diacid (A), which is selected from the group of aliphatic diacids; diacids having the formula HOOC-Cy-COOH, where Cy is a C3-C6 non-aromatic ring optionally substituted by C1-C 18 alkyl groups; isophthalic acid and terephthalic acid; and 10 ​ - at least one diamine (B), which is selected from the group consisting of C2-C 18 aliphatic diamines, C4-C 18 alicyclic diamines and C8-C 18 aralkyl diamines.

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

6. The method according to claim 1 or 2, wherein The polyamide (PA) is a polyphthalamide comprising more than 50.0 mol% of repeating units (R PPA ), which are formed by the polycondensation of (i) phthalic acid with (i) at least one diamine, the phthalic acid being selected from the group consisting of isophthalic acid, terephthalic acid, and combinations of isophthalic acid and terephthalic acid, and the proportions in mol% being based on the total amount of the repeating units of the polyamide (PA).

7. The 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 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 where X is a C4-C12-diamine, polyamide MXD6, polyamide MXD6 / PXD6, polyamide MXD6 / MXDI, polyamide 6T / 66, polyamide 6T / 6I / 66, polyamide 6T / 6I and mixtures thereof.

8. The method according to any one of the preceding claims, wherein, The product (P) further comprises at least one polymer additive (Add), which is especially 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. The 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. The method according to any one of the preceding claims, wherein, Step a) is carried out in an extruder or in a static mixer.

11. The method according to any one of the preceding claims, wherein, The polymer component is in a molten form at a temperature T of at least 200 °C or at least 250 °C.

12. The method according to any one of the preceding claims, wherein, Compound (A*): -Select from the group consisting of: C2-C 18 monocarboxylic acids, C2-C 18 dicarboxylic acids, and combinations of two or more of said acids; or - Select from the group consisting of linear or branched C2-C 18 monocarboxylic acids, linear or branched C2-C5 dicarboxylic acids, linear or branched C7-C 18 dicarboxylic acids, and combinations of two or more of said acids.

13. The method according to any one of the preceding claims, wherein, Compound (A*): - Select from the group consisting of: carboxylic acids having the formula CH3-(CH2) n -COOH (V) - where n is an integer between 0 and 16, carboxylic acids having the formula HOOC-(CH2) m -COOH (VI) - where m is an integer between 0 and 16, isophthalic acid, terephthalic acid, and combinations of two or more of said acids; or -Select from the group consisting of: carboxylic acids having the formula CH3-(CH2) n -COOH (V) - where n is an integer between 0 and 16, carboxylic acids having the formula HOOC-(CH2) m -COOH (VI) - where m is an integer between 0 and 16, provided that m is not 2, isophthalic acid, terephthalic acid, and combinations of two or more of said acids.

14. The method according to any one of the preceding claims, wherein, Compound (A*) is identical to the diacid (A) of the polyamide (PA) or to one of these diacids (A).

15. The method according to any one of the preceding claims, wherein, The compound (A*) is terephthalic acid and / or isophthalic acid, and one of the polyamides (PA) or the polyamide (PA) is a polyphthalamide comprising more than 50.0 mol% of repeating units (R PPA ), which are formed by the polycondensation of (i) a phthalic acid with (i) at least one diamine, the phthalic acid being selected from the group consisting of isophthalic acid, terephthalic acid and combinations of isophthalic acid and terephthalic acid, the proportion in mol% being based on the total amount of the repeating units of the polyamide (PA).

16. The method according to any one of the preceding claims, wherein, The proportion of compound (A*) is less than 50.0 wt%, preferably less than 40.0 wt%, this proportion in wt% being expressed as the weight of compound (A*) relative to the total weight of [polyamide (PA) + compound (A*)].

17. The method according to any one of the preceding claims, wherein, The proportion of compound (A*) is at least 5.0 wt%, preferably at least 10.0 wt%, this proportion in wt% being expressed as the weight of compound (A*) relative to the total weight of [polyamide (PA) + compound (A*).

18. The method according to any one of the preceding claims, wherein, 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%, even more preferably at least 75.0%, where r is defined as: r = (Mn of the polyamide (PA) at the end of step a) - Mn of the polyamide (PA)) / Mn of the polyamide (PA) × 100.

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

20. The method according to any one of the preceding claims, wherein, Step c) is an acid hydrolysis, which is preferably carried out at a pH below 4.

0.

21. The method according to claim 20, wherein, Step c) is an acid hydrolysis involving HCl as the acid (Ac).

22. The method according to claim 20 or 21, wherein, The hydrolysis under acidic conditions in step c) is carried out in an aqueous medium in the following initial proportions: - Proportion of polyamide (PA): at least 15.0 wt.%; - Proportion of water: between 20.0 and 70.0 wt.%; - The remainder is acid (Ac), provided that the proportion of acid (Ac) is at least 2.0 wt.%, preferably at least 2.5 wt.%; These proportions are expressed in wt.% and are based on the total weight of polyamide (PA), water and acid (Ac) in the liquid medium.

23. The method according to any one of claims 20-22, wherein, The stream (S) contains the diacid (A), the diamine (B) in the form of a salt, in particular in the form of a salt with the acid (Ac), and the compound (A*).

24. The method according to any one of claims 20-23, wherein, Step d) after hydrolysis under acidic conditions includes: (i) - at least one unit operation selected from the group consisting of liquid-solid separation (e.g., filtration), crystallization and distillation; and (ii) - a step in which the salt of the diamine (B) is reacted with a base in order to release the diamine (B).

25. The method according to any one of the preceding claims, wherein, Step c) is carried out at a weight ratio r lower than 5.0 水 where r 水 is defined as the weight of water in the aqueous medium used in step c) / the weight of the product (P).

26. The method according to any one of the preceding claims, wherein, The further processing of the stream (S) is based on crystallization and distillation steps.

27. The method according to any one of the preceding claims, wherein, Step d) includes at least one unit operation selected from the group consisting of liquid-solid separation (e.g., filtration), crystallization and distillation.

28. The method according to any one of the preceding claims, wherein, Step c) is carried out batchwise in a batch reactor in which the product (P) is initially charged and no additional product (P) is introduced during the hydrolysis.

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