Process for separating polyurethane from polyamides in waste or recycled products

Abstract

A process for separating polyamide 6 and / or polyamide 66 from elastic polyurethane (spandex) present in a textile production waste or in an end-of-life textile product, and the recovery and recycling of at least one of the polymers thus separated is described.

Description

[0001] “PROCESS FOR SEPARATING POLYURETHANE FROM POLYAMIDES IN WASTE OR RECYCLED PRODUCTS”

[0002] FIELD OF THE INVENTION

[0003] This invention relates to the field of the recovery of raw materials from end-of-life products or production waste. In particular, the invention refers to a process for separating polyurethane from polyamides with the possibility of recovering both polymeric components from textile waste originating from industrial processes or post-consumer waste, currently destined for incineration or landfill.

[0004] STATE OF THE ART

[0005] Polyamides, also generally referred to by the abbreviation PA, are among the most widely used polymers in industry; the most common polyamides are aliphatic ones, in particular PA66, obtained by polycondensation of hexamethylenediamine and adipic acid, and PA6, obtained from caprolactam. In the textile field, aliphatic polyamides (also referred to in the field as nylon, a name that will be used in this invention together with the abbreviations PA6 and PA66) are very often combined with other fibers, such as polyurethane (PU), polypropylene (PP), polyester (in particular polyethylene terephthalate, PET) or cotton (CO), where the combination with these materials provides the product with desired functional or aesthetic properties. One of the most common combinations in the textile field is with spandex (also commonly known as elastane); the latter is a polyurethane where the soft segments are made of a poly ether (typically polytetrahydrofuran) while the rigid segments are made of methylene diphenyl diisocyanate (MDI), using diamines, hydrazine or diols as chain extenders. The resulting polymer is endowed with unique elasticity properties, being able to reach elongations of up to 400-800% with respect to the length at rest, which makes it suitable for the production of fabrics for sports applications, women’s stockings, medical stockings and underwear. The combination of polyamide and polyurethane in the appropriate proportions (65-98% PA and 2-35% PU, by weight) results in fabrics endowed with the elastic properties of spandex and the strength and toughness properties of polyamide. However, the different nature and relative amounts of these two polymers, to which smaller amounts of other fibers (PP, PET, or CO) are often added, make these elastic fabrics particularly difficult to recycle.

[0006] In particular, mechanical recycling, consisting of grinding and subsequent re-extrusion of the resulting mixture, would lead to the loss of the synergistic properties guaranteed by the combination of materials. Furthermore, since the main material is polyamide, a polymer with a high melting point, re-extrusion should be carried out at high temperatures (above 260 °C), an operation that would cause the degradation of polymers with lower thermal and chemical stability with respect to nylon (such as polyurethane), leading to the formation of impurities that lower the quality of the recycled product and limit its downstream applications.

[0007] Another possible form of recycling is chemical recycling. Nylon depolymerization techniques involve the use of very harsh process parameters, with temperatures above 250 °C in the presence of aggressive substances, such as strong acids or bases, sometimes even concentrated ones. These conditions also degrade polymers other than nylon, producing heavily contaminated monomer mixtures that require extensive purification treatments to obtain products suitable for use as raw materials in a new polymerization cycle. Furthermore, the presence of materials other than nylon can poison the catalysts, even reducing the efficiency of the depolymerization process itself.

[0008] For these reasons, mixed PA-PU fabrics are currently generally sent to landfill or incineration, thus minimizing the value of both nylon and polyurethane. The possibility of recycling nylon-based yams, whether from industrial production waste or post-consumer waste, represents therefore a major environmental sustainability challenge, because it would allow, on the one hand, to reduce the impacts related to the production of the intermediates used in the synthesis, and on the other hand, those caused by disposal in landfills.

[0009] One possible way to recycle these materials is offered by selective dissolutionprecipitation (SDP) processes. These technologies are based on the use of specific solvents capable of selectively dissolving one polymeric component while leaving other types of polymers in solid form; the latter are subsequently filtered and separated from the liquid phase, from which the previously dissolved polymer is then precipitated (by varying the operating conditions).

[0010] The scientific and patent literature concerning the recovery of polyamides or polyurethane by solvent recycling describes processes where either the elastomeric component (mainly) or the polyamide component is selectively dissolved.

[0011] As regards the dissolution of polyurethane, solvents typically capable of dissolving this fiber are aprotic polar solvents, such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) or mixtures thereof; amide-amine mixtures and aqueous lactams, where the most suitable are N-methylpyrrolidone (NMP), 2-pyrrolidone and 2-piperidone.

[0012] For example, patent application WO 2024 / 121765 Al describes a process wherein elastomeric PU fibers are selectively dissolved in caprolactam or an aqueous solution thereof, while leaving the other fibers, for example polyamide or polyester, undissolved.

[0013] Nylon (PA6 or PA66) can instead be dissolved in protic polar solvents such as fluorinated solvents, acidic solvents (inorganic or organic with a low number of carbon atoms), alcohols (aliphatic or aromatic) and low molecular weight glycols.

[0014] Patent applications JP 2008-031127 A and JP 2011-088943 A describe depolymerization processes of nylon 6 to yield caprolactam, in which dissolution steps of polyurethane fibers are carried out with solvents such as DMF or DMSO and aqueous lactams, respectively; the removal of the elastomeric fraction is of fundamental importance for obtaining a fiber-grade monomer (caprolactam). In these processes, the selective dissolution of PU is carried out at temperatures > 80 °C with contact times of between 6 minutes and 5 hours. However, the solvents used are not suitable for industrial use as they are considered toxic to humans and dangerous to the environment. Furthermore, since they are selective towards PU only, the process can only be applied to fabrics exclusively containing PA / PU, as they would leave on the polyamide other polymeric components (cotton, polypropylene, polyester) that may possibly be present.

[0015] The article “Simple process for separation and recycling of nylon 6 and polyurethane components from waste nylon 6 / polyurethane debris”, Gong C. el al., Textile Research Journal 2020, Vol. 91(1-2) 18-27, describes a recycling process for PA6 / PU-based textile waste by dissolving the elastomeric component in DMF, filtering the resulting solution, and subsequent precipitation by adding water as an antisolvent. This technique also suffers from the same development limitations listed above.

[0016] In patent application CH 711187 A2, it is possible to confer selectivity to the dissolution of polymers of different natures by using dimethyl sulfoxide as a solvent and varying the process temperature. For values between 40 < T < 120 °C, selective dissolution of polyurethane fibers occurs, for temperatures between 140 < T < 175 °C, synthetic polyamide and polyester fibers dissolve, while no dissolution is observed for natural fibers such as cotton. Depending on the type of mixed fabric, by varying the temperature it is possible to selectively dissolve polyurethane or polyamide, while leaving polyamide or cotton, respectively, undissolved on the filter. Although the technology offers the possibility of recovering the different polymeric components, the use of this solvent does not allow, for example, to dissolve polyamide without first dissolving the polyurethane fibers as well.

[0017] DMSO-based solvents are also used in the article “Solvent blends for selective elastane dissolution and recovery from mixed polyamide fabrics”, L. Vonbriil et al., Resources, Conservation & Recycling 200 (2024) 107302; as described in the article, the addition of ethers (the most suitable being tetrahydrofuran, cyclopentyl methyl ether, and methyl tetrahydrofuran) improves the solvent power of DMSO towards spandex, making it even superior to that of DMF at room temperature. The main disadvantage of using ethers lies in their high flammability, which requires specific precautions; furthermore, the mixture is not capable of dissolving polyamide but only the elastomeric component.

[0018] To overcome the issue of toxic or hazardous solvents mentioned in the previously cited documents, and following the recent trend of using natural substances and less impactful processes whenever possible, according to the principles of green chemistry, the article “Analysing the potential of the selective dissolution of elastane from mixed fiber textile waste”, K. Phan et al., Resources, Conservation & Recycling 191 (2023) suggests the use of new naturally derived compounds, such as cyrene, y-valerolactone and tetrahydrofuryl alcohol, for the dissolution of polyurethane fibers aimed at recycling textile waste; these compounds are able to solubilize PU with results similar to those of traditional solvents (DMF, NMP, DMAc). In addition to cyrene, patent application WO 2021 / 165531 A2 suggests 2,5-dimethylisosorbide as a naturally derived solvent, carrying out the dissolution at high temperature (140-200 °C) with contact times ranging from 2 to 8 hours. The issue associated with these new solvents is their still very high cost: despite the processes being closed-loop, losses due to purges would make the process economically unsustainable if the costs of the products are compared with similar virgin products.

[0019] As regards the selective dissolution of PA (leaving the elastomer undissolved), few processes are reported, since the solvents commonly used to dissolve polyamide are very aggressive and often tend to solubilize or degrade the polyurethane as well, also due to the more severe process conditions (pressures, temperatures, contact times) needed to bring a highly crystalline polymer like nylon into solution. Solvents commonly used for this type of separation are those based on calcium chlorides and alcohols. As reported in the previously cited article by L. Vonbriil el al., a mixture of CaCh, ethanol, and water is used to quantify the nylon content in stretch fabrics by selectively dissolving the polyamide. However, the use of high concentrations of chlorides is extremely problematic due to the corrosion of stainless steels, as they cause pitting corrosion. Furthermore, the salt is very difficult to remove from the recycled polymer, requiring high water consumption.

[0020] US Patent Application 2015 / 0031778 Al describes a process wherein the polyamide component of a waste material is selectively dissolved, while leaving the other polymers that compose it, including polyurethane, undissolved. The process in this document is primarily directed to tires recycling treatment, and the solvent used in this document is glacial acetic acid, while the text of the application states that acetic acid aqueous solutions do not yield useful results because they do not allow the recovery of nylon by precipitation.

[0021] Other processes for recycling textile waste containing polyurethane which use solvents are described in the international Patent Application WO 2013 / 032408 Al. In the reported process, solvent extraction is preceded by a thermal degradation step in an inert environment at T > 200 °C for 2 hours, wherein polyurethane, which is chemically and thermally less stable than polyamide, is selectively degraded / depolymerized. The resulting fabric is then “washed” with ethyl alcohol at a temperature of between 20 and 78 °C for 30 minutes, which selectively dissolves all the products resulting from the thermal degradation of spandex, leaving a chemically pure nylon. However, thermal degradation of polyurethane can lead to the formation of very toxic products, such as isocyanates, benzene, ammonia, carbon monoxide, and hydrocyanic acid. Furthermore, the use of ethanol, although it is a non-toxic solvent compared to DMF, presents critical issues related to flammability. The reported process allows the recovery of polyamide, but not that of polyurethane, which is degraded but not recovered as a valuable waste product.

[0022] There is still a need in the industry for an industrial process for separating polyamide and polyurethane (including other possible components) that is both economical and environmentally sustainable, to obtain a nylon that can replace virgin polymer.

[0023] The object of the present invention is to provide a process for separating mixed fabrics based on polyamide and PU, whether derived from industrial or post-consumer waste, which is free from the drawbacks of the prior art, allowing for an effective recycling treatment of these fabrics and expanding the range of potentially recyclable waste materials.

[0024] SUMMARY OF THE INVENTION

[0025] This and other objects are achieved according to the present invention with a process for separating PA6 and / or PA66 from PU present in a textile production waste or in an end-of-life textile product, wherein PA6 and / or PA66 and PU are present in the form of fibers, which comprises the following steps:

[0026] A) treating said waste or textile product with acetic acid or a mixture of acetic acid and water, at a temperature equal to or higher than the dissolution temperature of one of PA6, PA66 and PU but lower than the dissolution temperature of one or both of the other two components, obtaining a liquid phase and a solid phase, wherein said dissolution temperatures are determined by the following pairs of equations: 0.4315 [H2O]2- 4.1911 [H2O] + 47.327 0 < [H2O] < 5 (1) 0.1107 [H2O]2+ 1.7756 [H2O] + 27.098 5 < [H2O] < 20 (1’) 0.0157 [H2O]2- 2.2592 [H2O] + 75.492 0 < [H2O] < 10 (2)

[0027] - 0.0066 [H2O]2+ 1.2974 [H2O] + 41.011 10 < [H2O] < 55 (2’)

[0028] 0.2601 [H2O]2- 5.003 [H2O] + 99.34 0 < [H2O] < 10 (3) 0.013 [H2O]2+ 0.3794 [H2O] + 70.545 10 < [H2O] < 37 (3’) wherein Tpu, TPA6 and TPA66 are, respectively, the dissolution temperature of PU, PA6 and PA66 expressed as °C, [H2O] is the concentration of water as percent by weight in the mixture of acetic acid and water, each pair of equations (1) / (T), (2) / (2’) and (3) / (3’) defines a dissolution curve for one of the three polymers, and in each pair the first equation is valid for water concentrations lower than the minimum of the curve and the second equation is valid for water concentrations higher than the minimum of the curve;

[0029] B) separating the liquid and solid phases obtained in step A;

[0030] C) drying the solid phase obtained in step B to recover a solid polymer selected from a polyamide and a polyurethane.

[0031] The process of the invention may also optionally include one or more of the following additional steps:

[0032] D) causing the precipitation in solid form of the polymer dissolved in the liquid phase separated in step B;

[0033] E) separating the polymer precipitated in solid form from the liquid phase in step D;

[0034] F) drying the solid polymer obtained in step E;

[0035] G) solid / liquid extraction of the solid polymer obtained in step C to remove residual solvents and impurities derived from said textile waste or product;

[0036] H) solid / liquid extraction of the polymer obtained in step F to remove residual solvents and impurities derived from said textile waste or product;

[0037] I) distillation of the solvent recovered in steps D and E, and recirculation of the solvent distilled in step A of the process.

[0038] BRIEF DESCRIPTION OF THE FIGURES

[0039] The invention will be described in detail below with reference to the figures, in which: - Fig. 1 shows the necessary steps and some optional steps of the process according to the invention in the form of a flow chart;

[0040] - Fig. 2 is a graph showing the trend of the minimum dissolution temperature of PU, PA6 and PA66 at atmospheric pressure as a function of the amount of water contained in an acetic acid / water solvent mixture;

[0041] - Figs. 3 and 4 are graphs corresponding to that of Fig. 2, but showing the trend of the minimum dissolution temperature of PU and, respectively, PA6 only and PA66 only;

[0042] - Figs. 5 to 12 reproduce FT-IR spectra of products recovered with the method of the invention and, for comparison, spectra of pure products, as detailed in the Examples.

[0043] DETAILED DESCRIPTION OF THE INVENTION

[0044] In the following description, the following conventions and abbreviations are adopted:

[0045] - all component percentages are by weight unless otherwise indicated;

[0046] - “atmospheric pressure” is intended to mean a pressure of between 980 and 1030 hPa;

[0047] - although various types of polyurethanes exist with different characteristics depending on the formulation and production process, in the following description and in the claims, the term polyurethane or its abbreviation PU are intended to refer exclusively to the elastic polyurethane used in textiles, for which the common name spandex will also be used;

[0048] - the products to which the method of the invention can be applied, whether they are end- of-life products or production waste, will be cumulatively referred to as “textile products”.

[0049] The invention is based on the observation made by the inventors that PA6, PA66 and PU can be selectively dissolved in acetic acid, or mixtures of acetic acid and water comprising a maximum of 55% by weight of water, operating in temperature ranges that depend on the water content of the mixture.

[0050] Compared to the solvents used in the selective dissolution methods of the prior art, acetic acid is not dangerous for the environment and for humans, and has reduced flammability when mixed with water, in addition to being economical.

[0051] The process subject matter of this second aspect of the invention is described below with reference to the flow diagram in Fig. 1. The temperature ranges for operating at atmospheric pressure in order to carry out step A of the process of the invention are graphically illustrated in Figures 2-4, and are determined by the relationship between the dissolution temperature curves described by the equation pairs (1) / (1’), (2) / (2’) and (3) / (3 ’) reported above. When the exact composition of the textile product to be treated is not known, and in particular it is not known whether the polyamide component is PA6, PA66 or a mixture of the two, the graph to refer to is that of Fig. 2; if, on the other hand, it is known for certain that the textile product contains, as a polyamide component, only PA6 or only PA66, it is possible to operate with the guidance of Figures 3 and 4, respectively, which represent a simplified version of the graph in Fig. 2 and allow for greater freedom of operation.

[0052] As can be noted in Fig. 2, the minimum solubilization temperature of PU, PA6 and PA66 in acetic acid, or mixtures thereof with water, varies with the percentage of water in said mixtures; the complete curve for each of the three polymers has a minimum, at a concentration of 5% water in acetic acid for PU and at a concentration of 10% water in acetic acid for PA6 and PA66. Using pure acetic acid, or mixtures thereof with a maximum water content of 10%, PU has a lower solubilization temperature than both PA6 and PA66; in particular, the solubilization temperature of PU is 48 °C in pure acetic acid, it decreases to about 36 °C as the water content in the acetic acid / water mixture increases up to 5%, and then rises again to the boiling point of the mixture (approximately 105 °C) at a water content of 20%; the descending and ascending branches of the PU dissolution temperature curve as a function of increasing water content are described by equations (1) and (1’) reported above, respectively. The dissolution curve of PA6 shows that this polymer has a dissolution onset temperature of approximately 75 °C in pure acetic acid, which decreases with increasing water content in the mixture with the acid until it reaches a value of approximately 55 °C at [H2O] = 10%, and then it increases again with increasing water content up to 90 °C for [H2O] = 55%. Finally, the dissolution curve of PA66 shows that this polymer has a dissolution onset temperature of approximately 100 °C in pure acetic acid, which decreases with increasing water content in the acid mixture until it reaches a value of approximately 75 °C at [H2O] = 10%, and then it increases again with increasing water content up to the boiling point T of approximately 102 °C for [H2O] =37%.

[0053] Using the data in Fig. 2, it is therefore possible to define treatment conditions for the textile product that selectively dissolve only the polyamide component or only the polyurethane; for example, by operating at a temperature of between 48 and 75 °C in pure acetic acid, or between 36 and 65 °C in an acetic acid / water mixture containing 5% water, selective dissolution of the PU component is obtained, while any PA6 or PA66 fibers present remain unaltered; from the solution obtained with this treatment, it is then possible to separate the polyamide fibers by filtration as a solid fraction and a solution containing dissolved PU, which can then be recovered in subsequent treatments. Other pairs of T values at which it is possible to operate for separating PU can be obtained from the graph; for example, using a mixture containing 2% water in acetic acid, it is possible to operate between 40 and 72 °C.

[0054] At approximately 10% water, the curves of the minimum dissolution temperature of PU and PA6 intersect; consequently, operating, for example, at 15% water, it is possible to selectively dissolve PA6 while leaving PU fibers unaltered by operating at a temperature between approximately 58 and 74 °C, while the same result can be achieved by operating with any solvent mixture with a water content greater than 20%, beyond which it is not possible to obtain PU dissolution.

[0055] Other conditions for selective dissolution and separation of individual polymers from the textile product will be immediately apparent to those skilled in the art from the graph in Fig. 2.

[0056] The graphs in Figures 3 and 4 can instead be used to define a selective dissolution / separation treatment when it is known whether the polyamide component of the textile product is only PA6 or only PA66. In these cases, the operating temperature ranges become wider, and therefore step A of the process of the invention is easier to implement. For example, comparing Figure 2 and Figure 3, it can be noted that to selectively dissolve PA6, using, for example, a solvent mixture containing 20% water, it is necessary to operate at a temperature of between approximately 64 and 105 °C, while if PA66 is also present (or its presence cannot be excluded), it is necessary to operate at a temperature above 84 °C, at which PA66 also begins to dissolve, and up to 105 °C. Similarly, comparing Figure 2 and Figure 3, it can be noted that to selectively dissolve PU, in the case where PA6 is also present (or its presence cannot be excluded), it is necessary to operate, for example with a solvent mixture containing 5% water, at a T between approximately 36 and 65 °C, the temperature at which PA6 begins to dissolve; if, however, it is known that the textile product contains only PU and PA66, Figure 4 shows that to obtain the selective dissolution of PU, the treatment temperature with this solvent mixture can be raised up to approximately 80 °C; furthermore, comparing Figures 2-4, it can be noted that to selectively dissolve PU, without knowing whether PA6 is present or not, it is possible to work only with solvent mixtures containing a maximum of 10% water, while if it is known that the textile product contains only PU and PA66, there is more freedom of choice, being possible to operate with solvent mixtures containing at least up to 14% water.

[0057] For the purposes of this invention, it may be advantageous to work with water / acetic acid mixtures containing at least 0.5%, and preferably at least 1%, of water, because these mixtures significantly lower the dissolution temperature of the PU, thus facilitating its separation from the polyamides.

[0058] Naturally, the T values indicated above, which define the extremes of the ranges in which it is possible to work to obtain the different separations, represent the limit values of these ranges, wherein either the dissolution is ineffective or slow (when the values are close to the minimum dissolution temperature of a component), or they are critical values because they could also cause the dissolution, at least in part, of the second polymer; consequently, for each range of T values indicated above, or those inferable from the inspection of the graphs in Figs. 2-4, for a given concentration of water in acetic acid, it is preferable to work in narrow ranges, where the minimum T is 5 °C higher than the theoretical minimum temperature, and the maximum T is 2 °C lower than the theoretical maximum temperature, more preferably in ranges where the minimum T is 10 °C higher than the theoretical minimum temperature and the maximum T is 4 °C lower than the theoretical maximum temperature (where theoretical temperatures are intended to be those derived from equations (l)-(3)).

[0059] By repeating the procedure of the method operating sequentially under suitable conditions derived from equations (l)-(3) and Figs. 2-4, it is possible to separate in a solution form all three indicated polymers, which represent the largest amount of polymers present in the selected textile products, so that it is also possible to separate from them the insoluble components such as cotton, polyester, polypropylene or other impurities.

[0060] In order to maximize the separation of the different polymers, the selective dissolution described above may also consist of multiple operations, wherein a liquid phase obtained from a dissolution step subsequent to the first one is used as a solvent in one of the preceding dissolution steps. Operating in this way, the liquid phase obtained from the first dissolution step, which is the one containing the maximum concentration of the polymer intended to be dissolved, is sent to recovery of this polymer (for example by precipitation with an antisolvent); the solid phase separated in the first dissolution step is washed in one or more subsequent steps with the same solvent (z.e., a solution containing the same acetic acid / water ratio), which may be fresh solvent or the liquid phase derived from one of the washings of the solid residue from one of the dissolution steps. The number of steps to be adopted according to this operating mode depends on the process conditions (the closer the operating temperature to the minimum dissolution temperature, the greater the number of steps required) and the nature of the textile product to be treated (the thicker the fabric, the greater the number of steps required, preferably in countercurrent, to completely extract the polymer to be dissolved). The operation may be carried out batchwise or, preferably, continuously, according to methods known to those skilled in the art, using machines that are scale-ups of laboratory equipment (e.g., industrial reactors and filters) or using dedicated equipment such as carousels.

[0061] For the dissolution in step A, an amount of solvent mixture equal to or greater than 2.5 L per kg of textile product is used; below this value, the dissolution efficiency decreases due to the excessive concentration of the resulting solution. In addition, the solvent mixture is preferably used in an amount not exceeding 50 L per kg of textile product, in order not to unnecessarily increase the solution volumes to be subsequently treated for the recovery of the dissolved polymers.

[0062] Downstream to the selective dissolution step, the solid phase is separated from the liquid phase (the one richer in dissolved polymer) in step B of the process (Fig. 1), which can be carried out using any technique known in the field, such as, for example, using centrifuges, filter presses, rotary drum filters, candle filters, etc. The separation may be carried out with one or more operations, starting with a coarse filtration to remove insoluble polymeric material and a finer filtration (<10 pm) to separate pigments such as TiCh, carbon black, various powders, etc., possibly using several of the techniques mentioned for the various operations.

[0063] The solid polymeric fraction obtained from the first separation step can be washed with a solvent having the same composition and temperature as that used in dissolution step A, in order to maximize the recovery of the polymer solution remaining impregnated in the solid fraction.

[0064] The solid polymer remaining on the filter, now containing only residual solvent, is sent to drying (Fig. 1, step C) for the recovery of the polymer not dissolved in step A. This step can be carried out with known techniques such as, for example, fluid bed dryers, vibrated bed dryers, belt dryers, etc. The residual solvent separated in this step is recirculated to the selective dissolution step A. The recovered polymer can be used as such in downstream applications (molding or spinning), or it can be subjected to a further optional step (Fig. 1, step G) which involves a solid-liquid extraction aimed at removing degradation products, oligomers or dyes from the fabric. This step is preferably carried out with hot demineralized water at a temperature of between 50 and 100 °C, or with a polar organic solvent such as an alcohol, a ketone, an ester or an ether.

[0065] In addition to the necessary steps A, B and C, which form the minimum and simplest form of carrying out the process of the invention, this process may include one or more further steps D to I, which are optional but preferably performed.

[0066] In step D, the clarified solution containing the polymer dissolved in step A and separated in step B is sent to the precipitation section in order to obtain again as a solid phase the polymer dissolved in step A (Fig. 1, step D). This operation can be carried to out by evaporation of the solvent, by cooling the solution to room temperature, or by adding an antisolvent, the most suitable of which are water itself or acetic acid esters, such as ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, isobutyl acetate, tert-butyl acetate, sec-butyl acetate, etc., which can be added at any temperature, preferably at the same temperature as the polymer solution to be added to. The addition can be normal (antisolvent added to the solution) or inverse, where the polymer solution is added to the antisolvent.

[0067] In step E, the solid thus precipitated is separated from any mother liquors; if the separation of the solid from the liquid occurs by evaporation of the latter in step D, steps D and E coincide.

[0068] Subsequently, in step F, the solid obtained in the form of a wet powder in step E is dried, recovering the liquid fraction of solvent (and any antisolvent) with which it is impregnated. This step can be carried out using techniques known in the art, such as fluid bed dryers, plate dryers, drum dryers, etc. The residual solvent is recovered and added to the stream of mother liquors exiting step D.

[0069] The crystallization mother liquors obtained in step D are recirculated as such, if the composition is suitable for subsequent use, or brought back to grade, preferably by distillation, or using other techniques such as liquid-liquid extraction or membrane filtration (Fig. 1, step I) if they are mixed with antisolvent or have a water concentration unsuitable for subsequent dissolution.

[0070] Similarly to the solid obtained in step C, should subsequent applications make it necessary, the solid obtained in step F may optionally be subjected to a further solid-liquid extraction step H to remove impurities such as oligomers, solvent / antisolvent residues, dyes and finishing products used in the textile industry, using the same solvents and operating conditions described above for step G.

[0071] The entire process (steps A to I) can be repeated should the fabric contain any further contaminations of polyester, cotton, polypropylene, etc., which remain in the solid component filtered in step B and subsequently dried in step C. In this case, the process is repeated and in step A (bis) the appropriate acetic acid composition will be selected to dissolve PA6, PA66 or PU depending on the filtered component, by repeating the entire procedure described.

[0072] The invention will be further described through the following experimental section.

[0073] MATERIALS, INSTRUMENTS AND METHODS

[0074] Acetic acid, ethyl acetate and isopropanol used in the Examples were purchased from Clean Consult International S.p.A. di Lodi Vecchio (LO). FT-IR spectra were obtained with a Bruker FT-IR Alpha spectrophotometer.

[0075] EXAMPLE 1

[0076] This example refers to a procedure for solubilizing PA6, wherein PU is left undissolved.

[0077] The waste treated in this example consisted of white stretch socks, made of 90% PA6 and the remaining 10% spandex.

[0078] 150 g of ground white socks were added to 850 g of a solution comprising 60% by weight of acetic acid and 40% by weight of demineralized water; the resulting suspension was heated to 100 °C and maintained at this temperature for 20 minutes, under stirring, to promote the dissolution of PA6. These treatment conditions fall within area 3 of Fig. 3.

[0079] At the end of the dissolution, the slurry was filtered through a porous glass septum (porosity 100-160 pm), and the undissolved polyurethane was separated from the recovered liquid phase. 100 g of solvent at 100 °C (60% by mass acetic acid) were added to the filtered polyurethane to wash the solid; the resulting liquid was added to the filtered polymer solution and sent to the polyamide recovery step. 850 g of ethyl acetate at 70 °C were slowly added under vigorous stirring to the polymer solution, inducing the precipitation of PA6. After filtration of the solid phase, the mother liquors were sent to the distillation step for solvent recovery, while the solid was sent to drying. This last operation was carried out in a vacuum oven under a nitrogen stream at 75 °C for 48 hours.

[0080] The dried PA6 was then subjected to a solid-liquid extraction step with 500 g of demineralized water at 80 °C, to remove any traces of residual solvent and oligomers (polymer chain degradation products) resulting from the process. The resulting polyamide powder was then filtered and further dried to remove any residual water. An FT-IR analysis was performed on the solid thus obtained, whose result is shown in Fig. 5; the upper spectrum in the figure is the one obtained on the powder recovered as described, while the lower spectrum refers to a standard reference sample of PA6; the substantial identity of the two spectra confirms the presence of the polyamide component alone in the precipitated solid, and therefore the successful separation.

[0081] The polyurethane recovered on the filter was subjected to a vacuum drying step to recover the solvent, and subsequently to a solid-liquid extractor with 100 g of water at 70 °C to remove traces of solvent and any degradation products formed during the separation. The polyurethane, once dried, was subjected to FT-IR analysis, thus obtaining the spectrum in the upper part of Fig. 6, in which the spectrum obtained on a standard reference sample of PU is shown in the lower part for comparison; also in this case, the substantial identity of the two spectra confirms the presence of PU alone in the solid separated after the selective dissolution step.

[0082] EXAMPLE 2

[0083] This example refers to a PU solubilization procedure, wherein PA66 is left undissolved.

[0084] The waste treated in this example was a stretch fabric, made of 75% PA66 and the remaining 25% spandex.

[0085] 50 g of ground fabric were added to 950 g of 100% acetic acid maintained at 80 °C (conditions corresponding to area 4 of Fig. 4). In order to completely extract polyurethane, three countercurrent extraction steps of 30 min each were carried out at the same temperature and with the same solid / liquid ratio, while keeping the solid under vigorous stirring. At the end of the extraction, the resulting fabric was filtered and washed with 100 g of fresh 100% acetic acid at 80 °C to remove the soaked liquid. The resulting solid was dried to recover the solvent; FT- IR analysis, compared with a PA66 standard, shows only the characteristic peaks of this polymer (Fig. 7, the spectrum of the reference standard sample is shown in the upper part of the figure, the one of the sample obtained in the test is shown in the lower part of the figure).

[0086] 600 g of demineralized water, maintained at 80 °C under vigorous stirring, were added to the hot- filtered liquid from the first extraction, in which polyurethane was dissolved; this water acted as an antisolvent, causing the PU to precipitate. The liquids from the second and third extractions were used as solvents, respectively, for the first and second extractions of a subsequent treatment not described herein. The precipitated polymer was filtered and dried at 70 °C for 48 hours under vacuum and under a nitrogen stream. Fig. 8 reproduces the FT-IR spectra obtained on the PU sample recovered in the test and on a standard reference sample, in the upper and lower parts of the figure, respectively; the comparison of the two spectra in the figure confirms the presence of the polyurethane component alone in the solid recovered in the test.

[0087] EXAMPLE 3

[0088] This example refers to a PA66 solubilization procedure, wherein PU is left undissolved.

[0089] The waste treated in this example consisted of a blend of ground stockings and pantyhose made, on average, of 12% spandex, 80% PA66, and the remaining 8% polyester (PET).

[0090] A solvent solution consisting of 75% acetic acid and 25% water was prepared; the solution was heated to 102 °C (condition corresponding to area 6 in Fig. 4).

[0091] 150 g of the ground stockings and pantyhose blend were added to 850 g of the solvent solution described above.

[0092] To completely extract PA66, two countercurrent extraction steps of 20 min each were carried out at the same temperature and with the same solid / liquid ratio, while maintaining the suspension under vigorous stirring. At the end of each extraction, the slurry was filtered through a Buchner filter and washed with 100 g of solvent having the same temperature and composition as that used in the dissolution, which was added to the previously filtered mother liquor. The first extraction solution was rapidly cooled to room temperature, thus inducing the precipitation of PA66. The second extraction solution was instead reused as the solvent for the first extraction of a subsequent treatment not described herein. The solid obtained by precipitation was then sent to drying in a vacuum oven at 75 °C for 48 h to remove the solvent impregnated in the powder. An FT-IR analysis was performed on a sample of the powder thus obtained, the result of which is reproduced in the upper part of Fig. 9; a spectrum of a standard reference sample of PA66 is reproduced in the lower part of the figure; comparing the two spectra, it is noted that the sample recovered as a solid in the test consists solely of PA66 with no traces of PU.

[0093] The filtered solid component, containing PU and, to a lesser extent, PET, was also dried in a vacuum oven to recover the solvent. An FT-IR test was performed on a sample of this solid component; the resulting spectrum is shown in the upper section of Fig. 10; the intermediate and lower spectra in Fig. 10 are spectra of standard reference samples of PET and PU, respectively; the figure highlights that both PU and PET peaks are present.

[0094] EXAMPLE 4 The test in this example was carried out on the PU and PET residue obtained in Example 3 as a solid residue in the separation step after selective dissolution; the purpose of this test was to extract PU from the PET residue.

[0095] A solvent solution comprising 95% acetic acid and 5% demineralized water was prepared; the solvent solution was heated to 80 °C.

[0096] 25 g of the mixed PU / PET solid residue obtained in Example 3 were added to 800 g of the solvent solution.

[0097] To maximize the extraction, four extraction steps were performed, each with 200 g of solvent solution each, for 30 minutes at the same temperature and with the same solid / liquid ratio, while keeping the suspension under vigorous stirring. The suspension obtained from each extraction was filtered, and the resulting solid was washed with 50 g of the same solvent, which was added to the extraction liquid and sent to polyurethane precipitation. To recover PU, 500 g of demineralized water at 80 °C were added to the solution, while maintaining the liquid under vigorous stirring; this operation causes PU to precipitate.

[0098] The recovered PU was filtered and then sent to drying, which was carried out in a vacuum oven at 70 °C under a nitrogen stream. The crystallization mother liquor was sent to the distillation section for re-grading. The polyester remaining on the filter was dried for 48 hours in a vacuum oven, and then extracted with 100 g of demineralized water for 20 min at 60 °C; after the final drying, the fibers were analyzed by FT-IR; the resulting spectrum is shown in Fig. 11, along with a spectrum of a standard PU sample (intermediate spectrum in the figure) and a standard PET reference sample (upper spectrum in the figure); comparison of the three spectra confirms that the sample recovered in this example consists almost entirely of PET, with traces of PU.

[0099] The PU obtained by precipitation after selective dissolution was finally washed with 150 g of isopropanol at 50 °C for 20 min to remove the PU degradation products. After drying the solvent, the resulting powder was analyzed by FT-IR, which confirmed the presence of PU alone in the precipitated solid (Fig. 12: the spectrum of the recovered PU is in the upper part of the figure, the spectrum of the standard PU reference sample is in the lower part of the figure).

Claims

CLAIMS1. Process for separating PA6 and / or PA66 from PU present in a textile production waste or in an end-of-life textile product, wherein PA6 and / or PA66 and PU are present in the form of fibers, comprising the following steps:A) treating said waste or textile product with acetic acid or a mixture of acetic acid and water, at a temperature equal to or higher than the dissolution temperature of one of PA6, PA66 and PU but lower than the dissolution temperature of one or both of the other two components, obtaining a liquid phase and a solid phase, wherein said dissolution temperatures are determined by the following pairs of equations: 0,4315 [H2O]2- 4.1911 [H2O] + 47.327 0 < [H2O] < 5 (1)0.1107 [H2O]2+ 1.7756 [H2O] + 27.098 5 < [H2O] < 20 (P) 0.0157 [H2O]2- 2.2592 [H2O] + 75.492 0 < [H2O] < 10 (2)- 0.0066 [H2O]2+ 1.2974 [H2O] + 41.011 10 < [H2O] < 55 (2’) 0.2601 [H2O]2- 5.003 [H2O] + 99.34 0 < [H2O] < 10 (3)0.013 [H2O]2+ 0.3794 [H2O] + 70.545 10 < [H2O] < 37 (3’) wherein Tpu, TPA6 and TPA66 are respectively the dissolution temperature of PU, PA6 and PA66 expressed as °C, [H2O] is the concentration of water as percent by weight in the mixture of acetic acid and water, each pair of equations (l) / ( 1’), (2) / (2’) and (3) / (3’) defines a dissolution curve for one of the three polymers, and in each pair the first equation is valid for water concentrations lower than the minimum of the curve and the second equation is valid for water concentrations higher than the minimum of the curve;B) separating the liquid and solid phases obtained in step A;C) drying the solid phase obtained in step B) to recover a solid polymer selected from a polyamide and a polyurethane.

2. Process according to claim 1, further comprising one or more of the following additional steps:D) causing the precipitation in solid form of the polymer dissolved in the liquid phase separated in step B;E) separating the polymer precipitated in solid form in step D from the liquid phase;F) drying the polymer in solid form obtained in step E;G) solid / liquid extraction of the solid polymer obtained in step C to remove residual solvents and impurities derived from said textile waste or product;H) solid / liquid extraction of the polymer obtained in step F to remove residual solvents and impurities derived from said textile waste or product;I) distillation of the solvent recovered in steps D and E, and recirculation of the distilled solvent in step A of the process.

3. Process according to any one of claims 1 or 2, wherein PU is separated from both PA6 and PA66 polymers, by selectively dissolving PU with acetic acid, or an acetic acid / water mixture containing less than 10% water, and a temperature higher than that determined by equations (1) and (1’) and lower than that determined by equation (2), or by treating said waste or textile product with an acetic acid / water mixture comprising between 15 and 37% water at a temperature higher than that determined by equation (3’) and lower than that determined by equation (1’).

4. Process according to any one of claims 1 or 2, wherein the textile production waste or the end-of-life textile product contains PU and PA6 but does not contain PA66, and PU is separated from PA6 by selectively dissolving PU with acetic acid, or an acetic acid / water mixture containing less than 10% water, at a temperature higher than that determined by equations (1) and (1’) and lower than that determined by equation (2), or by treating said textile waste or product with an acetic acid / water mixture comprising between 10 and 55% water at a temperature higher than that determined by equation (2’) and lower thanthat determined by equation (1’).

5. Process according to any one of claims 1 or 2, wherein the textile production waste or the end-of-life textile product contains PU and PA66 but does not contain PA6, and PU is separated from PA66 by selectively dissolving PU with acetic acid, or an acetic acid / water mixture containing less than 15% water, at a temperature higher than that determined by equations (1) and (U) and lower than that determined by equations (3) and (3’), or by treating said textile waste or product with an acetic acid / water mixture comprising between 15 and 37% water at a temperature higher than that determined by equation (3’) and lower than that determined by equation (1’).

6. Process according to claim 3, wherein PA6 and PA66 are further separated by selectively dissolving PA6 with acetic acid or an acetic acid / water mixture containing up to 55% water, at a temperature higher than that determined by equations (2) and (2’) and lower than that determined by equations (3) and (3’).

7. Process according to any one of the preceding claims, wherein said acetic acid / water mixture contains at least 0.5% water.

8. Process according to claim 7, wherein said acetic acid / water mixture contains at least 1% water.

9. Process according to any one of the preceding claims, wherein, for each given concentration of water in acetic acid, the process is carried out in a temperature range in which the minimum T is 5 °C higher than the theoretical dissolution temperature of the component to be dissolved, and the maximum T is 2 °C lower than the theoretical dissolution temperature of the component with the lowest dissolution temperature between the other two components or the theoretical dissolution temperature of the onlyother component, wherein said theoretical temperatures are those obtained from equations (!), (!’), (2), (2’), (3) and (3’).

10. Process according to claim 9 wherein, for each given concentration of water in acetic acid, the process is carried out in a temperature range in which the minimum T is 10 °C higher than the theoretical dissolution temperature of the component to be dissolved, and the maximum T is 4 °C lower than the theoretical dissolution temperature of the component with the lowest dissolution temperature between the other two components or the theoretical dissolution temperature of the only other component, wherein said theoretical temperatures are those obtained from equations (1), (1’), (2), (2’), (3) and (3’).

11. Process according to any one of the preceding claims, wherein the treatment of said textile production waste or end-of-life textile product is carried out by solid-liquid extraction consisting of several steps wherein a liquid phase obtained from a dissolution step following the first one is used as a solvent in one of the preceding dissolution steps.

12. Process according to claim 11, wherein said treatment is carried out in a continuous manner.

13. Process according to any one of the preceding claims, wherein an amount of solvent or solvent mixture equal to or greater than 2.5 L, and not greater than 50 L, is used per kg of textile production waste or end-of-life textile product.

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