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Quantitative Determination of PA6 and/or PA66 Content in Polyamide-Containing Wastes

Quantitative Determination of PA6 and/or PA66 Content in Polyamide-Containing Wastes This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. pubs.acs.org/journal/ascecg Research Article Quantitative Determination of PA6 and/or PA66 Content in Polyamide-Containing Wastes ̌ ̌ ̌ ̌ ̌ ́ Ema Zagar,* Urska Cesarek, Ana Drincic, Simona Sitar, Igor M. Shlyapnikov, and David Pahovnik Cite This: ACS Sustainable Chem. Eng. 2020, 8, 11818−11826 Read Online Metrics & More Article Recommendations sı Supporting Information ACCESS * ABSTRACT: A fast, robust, and convenient method for quantitative determination of polyamide-6 (PA6) and polyamide- 66 (PA66) in plastic wastes is presented. The method includes a straightforward procedure for complete hydrolysis of polyamides (PAs) into the constituent monomers and their quantitative determination in hydrolyzates by high-performance liquid chromatography on a mixed-mode column, from which the contents of a particular PA are determined. The method was developed on neat PA6 and PA66 as well as their mixture and was further utilized on PA-based composites and postconsumer wastes (carpet and fishing net wastes) containing PA in different amounts. The information on the content of a particular PA type in plastic wastes is important because it determines the maximum recovery of PA constituent monomer(s) by chemical recycling and consequently suitability of waste as a feedstock. Moreover, the proposed method allows for convenient differentiation between PA6 and PA66 in their mixtures, which is difficult to assess with precision by conventional characterization techniques. KEYWORDS: polymer waste, chemical recycling, microwave chemistry, polyamide content, liquid chromatography and the production is estimated to be continuously growing, INTRODUCTION which is associated with increasing waste management issues of Recent trends in plastic waste management are focused on 3,10 these nonbiodegradable polymers. Furthermore, they are improved plastic waste recycling because this could have a often used as PA blends or in combination with other polymers long-term impact on reduced greenhouse gas emissions, like polypropylene or polyethylene as well as composite and dependence on finite petroleum resources, and disposal of multicomponent materials such as carpets, which further plastic wastes in the environment and will finally recover the 7,9 1 complicate PA recycling processes. economic value of plastic wastes. Enhancement of plastic A primary goal of PA6 and PA66 depolymerization is to recycling beyond the current level includes among others produce the constituent monomer(s), these are ε-caprolactam improvements in chemical recycling within a circular economy, (cyclic monomer) or ε-aminocaproic acid (linear monomer), which is promoted by the industry as well. By chemical and hexamethylenediamine (HMDA) and adipic acid (AA), recycling, the polymer waste is converted to the feedstock for respectively, which can be further utilized in the synthesis of monomers, fuel production, or other value-added products and new PA or other polymers because the properties of recycled 2−6 intermediates. Furthermore, chemical recycling is a monomers resemble those of commercial raw materials. promising alternative to recycle composites, containing Several attempts to recycle PA66 and especially PA6 have reinforcement additives, which are otherwise difficult to 11−15 been reported, including depolymerization by hydrolysis, 1,7 recycle by mechanical methods. To accelerate chemical organocatalyzed ring-closing depolymerization in ionic recycling, selective and efficient catalysts are being developed, 16−19 20 21−25 liquids, ammonolysis, alcoholysis/glycolysis, and which at the same time keep the depolymerization process thermal decomposition under pyrolytic conditions in sub-/ 1−4,8 economically viable. 26−38 supercritical fluids. Depolymerization time of PA by acid- Polyamide-6 (PA6) and polyamide-66 (PA66) are versatile semicrystalline polymers with high chemical resistance and Received: June 6, 2020 good thermal and mechanical properties. They are produced Revised: July 6, 2020 from fossil resources, usually in the form of fibers or Published: July 14, 2020 thermoplastics, for the use in a broad scope of applications (textile, automotive, electrical, electronic, construction, pack- aging, coatings, and other industries). Currently, both PA types are produced on an annual worldwide multimillion ton scale, https://dx.doi.org/10.1021/acssuschemeng.0c04190 © 2020 American Chemical Society ACS Sustainable Chem. Eng. 2020, 8, 11818−11826 Downloaded via UNIV OF LJUBLJANA on August 17, 2020 at 05:58:58 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article catalyzed hydrolysis has been significantly shortened by using PA6 and PA66 was examined by analyzing their 1:1 mixture. microwaves as an energy source compared to the processes Furthermore, the method was tested on PA composites and 39,40 using conventional heating. Depolymerization processes postconsumer waste samples such as carpet waste and a reported so far result either in incomplete depolymerization, mixture of waste fishing nets. where final reaction mixtures consist not only of monomer(s) but also of linear and cyclic PA oligomers, or other low- EXPERIMENTAL SECTION molecular-weight chemical compounds of different physico- Chemicals and Materials. All chemicals used were of analytical chemical properties and functionality from PA constituent grade. HCl (18.5 wt %), used for depolymerization of PA samples, monomers(s). Currently, PA6 and PA66 are chemically was prepared by diluting 37 wt % HCl (Merck KGaA). Acetonitrile recycled on an industrial scale in the United States and (ACN) from Honeywell, Riedel-de Haen, Chromasolv, HPLC Europe, where the high energy and decontamination costs are gradient grade, ≥99.9%; trifluoroacetic acid (TFA) from Acros economically balanced by the high price of virgin PA, making Organics, ≥99.5%; or formic acid (FA) from Honeywell, ≥99% were the processes cost-efficient. For chemical recycling of PA, the used as received for preparation of mobile phases. Water was purified information on the content of a particular PA type in different using a Milli-Q reverse osmosis system to obtain MQ-water (MQ) with typically 18.2 MΩ cm resistivity and <4 ppb carbon (Millipore, waste materials is extremely useful because it determines the Watford, UK). Monomer standards ε-aminocaproic acid (or 6- maximum monomer recovery from the waste and consequently aminohexanoic acid, 6-ACA), HMDA, and AA of high purity the price and suitability of waste as a feedstock. (>99.9%) were purchased from Sigma-Aldrich, USA. Salts of amino The overarching objective of this work was to develop a acid (6-ACA·HCl) and diamine (HMDA·2HCl) were prepared by simple and efficient method for quantitative determination of dissolving an appropriate monomer standard in excess amount of PA6/PA66 in different waste types. It also enables convenient 18.5% HCl, then evaporating the solvent on a rotatory evaporator and differentiation of PA6 from PA66 and other PA types in waste drying the resulting residue at 50 °C in a vacuum chamber until mixtures; the information which is difficult to assess with constant weight. PA6 and PA66 polymer standards were purchased precision by conventional characterization techniques such as from Sigma-Aldrich. Glass-fiber reinforced PA66 (PA66-GF) (35 wt Fourier transform infrared (FTIR) spectroscopy, nuclear %) was obtained from Kordsa, Turkey, and glass-fiber reinforced PA6 (PA6-GF) (30 wt %) was purchased from Lanxess, Energizing magnetic resonance (NMR), and differential scanning Chemistry, Germany. Waste carpet and mixture of waste fishing nets calorimetry. Prerequisite for such method development is a as typical examples of postconsumer wastes were obtained from straightforward depolymerization of PAs into the constituent Aquafil, Slovenia. monomers. The depolymerization reaction has to be carried Homogenization of Postconsumer Waste Samples. To out until completion and in the absence of side reactions in a prepare representative and homogeneous waste samples, about 5 g reasonable time frame so that the final reaction mixture does of the sample was homogenized by milling on a ball mill. For this not contain any PA oligomers and side products. To this end, purpose, ∼0.5 g of the waste sample was loaded into a 35 mL stainless we have developed a microwave-assisted hydrolysis of PA steel vessel together with a 25 mm stainless steel ball (Tehtnica catalyzed by hydrochloric acid (HCl), by which complete PA Millmix 20, Domel, Slovenia). The vessel was hermetically closed and depolymerization to the constituent monomer(s) is achieved. cooled in liquid nitrogen at −196 °C for 10 min. Then, the sample was ground for 2 min with a frequency of 30 Hz. After milling, the The obtained depolymerization mixtures (hydrolyzates) were vessel was warmed up to ambient temperature to prevent the sample analyzed by high-performance liquid chromatography from absorbing the moisture. Thus-ground sample (5 g) was (HPLC), where we searched for a robust and simple method homogenized and used for further degradation experiments. to quantitatively determine the content of PA constituent Monitoring the Course of PA Hydrolysis Reaction. Micro- monomers in the hydrolyzates, from which the content of a wave-Assisted Hydrolysis of PA. PA6 or PA66 standard sample (∼4 particular PA in the waste material can be calculated. State-of- g) in the form of pellets was accurately weighed into a borosilicate the-art methods comprise qualitative and semiquantitative glass vial, suitable for a microwave oven Monowave 300 equipped evaluation of water-soluble PA degradation products by HPLC with the camera as well as temperature and pressure sensors (Anton on C18 or C8 reversed phase columns by using evaporative Paar GmbH, Austria). A magnetic stirring bar and 8 mL of 5.56 M HCl were added to the sample. After sealing, the reaction vessel was light scattering (ELS), chemiluminescence, or mass-selective 41−46 placed in a microwave oven, where it was heated to a predetermined detectors. RP-HPLC separates PA monomers as well as temperature of 170 or 190 °C (power of the magnetron automatically linear and cyclic PA oligomers. RP-HPLC was also reported as varied in the range of 0−850 W). Because of the microwave reactor a tool for evaluation of migration of cyclic monomers and design, it was not possible to take aliquots from the reaction mixture oligomers from PA food contact materials into food and food during the course of the reaction. Therefore, the degree of 43,44,47,48 simulants. However, currently available methods in depolymerization was followed by performing identical experiments terms of quantitative determination of PA constituent with different times. The reaction vessel was then cooled to 55 °Cby monomers in hydrolyzates suffer either from nonlinear a stream of compressed air. The solvent was removed from the detector response in a large concentration range or non- reaction mixtures by rotary evaporation and further drying in a conventional detection. Furthermore, determination of the PA vacuum oven at 50 °C until constant weight. The resulting dry solid residues of PA66 were further homogenized by grinding in a mortar. content in the case of incompletely depolymerized PA is Thus-obtained dry PA hydrolyzates were analyzed by H NMR and hampered because the response factors are not known for all HPLC. oligomers due to the unavailability of corresponding standards. 1 1 HNMR Spectroscopy. HNMR spectraof dried and For the determination of the content of a particular PA homogenized reaction mixtures of PA66 were recorded in DMSO- monomer in hydrolyzates, we have chosen isocratic HPLC on d with added TFA and tetramethylsilane (δ = 0) as an internal a mixed-mode column with refractive index (RI) detection, 1 chemical-shift standard. H NMR spectra of PA6 reaction mixtures which does not require any monomer derivatization. The were recorded directly by using C D as an inset. Measurements were 6 6 proposed method for quantitative determination of PA in performed at room temperature (RT) on a Varian Unity Inova 300 waste materials was verified in terms of accuracy and precision MHz NMR spectrometer (Varian, Inc., USA) with both the relaxation by analyzing neat PA standards, while method selectivity for delay and acquisition time of 5 s. 11819 https://dx.doi.org/10.1021/acssuschemeng.0c04190 ACS Sustainable Chem. Eng. 2020, 8, 11818−11826 ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article Figure 1. Reaction scheme of HCl-catalyzed hydrolysis of PA6 and PA66 into their constituent monomer(s). Figure 2. H NMR spectra of reaction mixtures obtained after 5 min microwave-assisted, acid-catalyzed hydrolysis of PA6 (left) and PA66 (right) at 170 °C. H NMR spectrum of the PA6 reaction mixture was recorded directly by using C D as an inset. PA66 reaction mixture was dried and 6 6 homogenized before H NMR analysis in DMSO-d with added TFA. HPLC Chromatographic Conditions. Dry PA6 or PA66 hydro- was carefully washed with water, and thus-obtained solution was lyzate (∼500 mg) was dissolved in 100 mL of the mobile phase of quantitatively transferred to the rest of the liquid phase. The solvent starting composition, that is, ACN/MQ water containing 1 vol % of and HCl in the filtrate were then evaporated on a rotary evaporator, ACN and 0.1 vol % of FA = 1/99 (v/v) during constant stirring for at and the residue was dried in a vacuum chamber at 50 °C. Afterward, the dried solid was dissolved in an appropriate solvent and diluted to a least 2 h. Afterward, 0.5 mL aliquot was taken from the mixture and final volume of 100 mL to perform HPLC measurements. further diluted to 5 mL to obtain a solution concentration of 0.5 mg/ Quantitative contents of PA constituent monomers in hydrolyzates mL. The presence of oligomers in PA hydrolyzates was assessed by a were determined by isocratic HPLC on a Primesep analytical 100 reversed-phase HPLC on an Agilent Zorbax Eclipse XDB C18 column column (4.6 × 150 mm; 5 μm, SIELC Technologies, USA). The (4.6 × 150 mm; 5 μm). A chromatographic system consisted of an separations were performed on an Agilent Technologies chromato- Agilent 1100 Series binary pump (Agilent Technologies, Inc., USA) graphic system equipped with the isocratic pump, column heater, and equipped with a model 7725i Rheodyne (Bensheim, Germany) Agilent 1260 Infinity Series RI detector (Agilent Technologies, Inc., manual injector and an ELS detector (ELSD) (Agilent Technologies, USA). The mobile phase consisted of a defined mixture of MQ, ACN, Inc., USA). The mobile phase used was a gradient of ACN as solvent and TFA, which was supplied at a flow rate of 1 mL/min. Mobile A and MQ containing 1 vol % of ACN and 0.1 vol % of FA as solvent phase composition consisting of MQ/ACN/TFA was 70/30/0.27 (v/ B. A linear gradient of the mobile phase composition ran from A/B 1/ v/v). The injection volume of sample solutions was 20 μL. The 99 (v/v) to A/B 30/70 (v/v) in the first 20 min, then isocratic elution separations were carried out at 30 °C, while the temperature of the RI was held for 5 min, after which the gradient was restored back to A/B detector was set to 35 °C. The concentrations and contents of 1/99 (v/v). The mobile phase flow rate was 0.5 mL/min, and the monomers in PA hydrolyzates were calculated from the calibration column temperature was 30 °C. ELSD was operated at evaporator and curves, plotted from the peak areas in HPLC chromatograms of the nebulizer temperature of 50 °C. The injection volume of thus corresponding standards (6-ACA, 6-ACA·HCl or HMDA, and prepared sample solutions onto the column was 20 μL. HMDA·2HCl) recorded at different but known solution concen- Quantitative Determination of PA6 and/or PA66 Content. trations (0.5, 1, 1.5, 2, and 2.5 mg/mL) by the same HPLC method. Microwave-Assisted Hydrolysis of PA. A typical depolymerization From the concentration and content of 6-ACA, 6-ACA·HCl or procedure for quantitative determination of PA6 and/or PA66 in PA- HMDA, and HMDA·2HCl in the hydrolyzate, the amount and the based samples is the following: ∼150 mg of the sample in the form of content of PA in the original sample were calculated, according to eqs powder or pellets was accurately weighed into a borosilicate glass vial, 1 and 2 suitable for a microwave oven Monowave 300. Then, a magnetic stirring bar and 6 mL of 5.56 M HCl were added to the sample. The () Ab − m = ××Vk reaction vessel was then sealed and placed into a microwave oven, (1) where it was heated to a predetermined temperature of 170 °C. At that temperature, the sample was treated for 30 min, and then, the %(PA)=× 100 reaction vessel was cooled to 55 °C by a stream of compressed air. If (2) necessary, the liquid phase was filtered through a glass filter with a pore size of 1.0 μm (CHROMAFIL Xtra GF-100/25, Sorbent where m is a determined mass of PA in the weighed sample, m is a Technologies, Inc., USA) to remove reinforcement additives (glass- mass of the sample used for hydrolysis, A is a peak area of the PA fibers) and/or other insoluble sample constituents. The filtrate was constituent monomer in the chromatogram of PA hydrolyzate, a is a then quantitatively transferred into a 100 mL flask. The solid residue slope of the calibration curve, b is an intercept of the calibration curve 11820 https://dx.doi.org/10.1021/acssuschemeng.0c04190 ACS Sustainable Chem. Eng. 2020, 8, 11818−11826 ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article Figure 3. RP-HPLC (C18 column coupled to an ELSD) chromatograms (left) and enlarged H NMR spectra typical for oligomers (right) of PA6 reaction mixtures hydrolyzed at 170 °C for different reaction times. HPLC chromatogram and H NMR spectrum of the reaction mixture hydrolyzed at 170 °C for 30 min show complete PA6 hydrolysis because only 6-ACA was detected. on y-axis, V is a volume of the measuring flask (100 mL), and k is a signals near the amide group of PA oligomers partially overlap recalculation coefficient, taking into account a difference between the with the satellites of methylene signals near the carboxyl and molecular weights of the PA6 or PA66 repeating unit and the amine groups of AA and HMDA. On the contrary, RP-HPLC monomer from which the calibration curve was constructed: M(PA6 coupled to an ELSD undoubtedly detects PA oligomers even repeat unit)/M(6-ACA) = 113.16/131.17 Da = 0.863; M(PA6 repeat when their content is extremely low. For both PA types, unit)/M(6-ACA·HCl) = 113.16 Da/167.63 Da = 0.675 and M(PA66 complete hydrolysis was achieved at a HCl/amide bond molar repeat unit)/M(HMDA) = 226.35 Da/116.21 Da = 1.948; M(PA66 ratio of at least 1.25 in 30 and 10 min reaction time at the repeat unit)/M(HMDA·2HCl) = 226.35 Da/189.13 Da = 1.197. reaction temperatures of 170 and 190 °C, respectively, as RESULTS AND DISCUSSION indicated by the degree of PA degradation monitored by H NMR and absence of oligomer peaks in HPLC chromatograms Complete Hydrolysis of PAs into the Constituent of hydrolyzates (Figure 3, Table S1). Furthermore, H NMR Monomers. For determination of the PA content in plastic and HPLC results show the absence of cyclic ε-caprolactam in waste, we developed a fast degradation method, whereby PA6 the final PA6 degradation products because under the chosen completely depolymerizes into 6-ACA and PA66 into AA and experimental conditions it hydrolyzes into the linear 6-ACA, HMDA (Figure 1). Oligomers and side products in the final which is consistent with the literature data. Complete PA reaction mixtures of PA6 and PA66 were not detected by H hydrolysis was achieved also in the presence of reinforcement NMR (Figure S1). Depolymerization experiments were additives or other sample constituents. After depolymerization, performed by microwave irradiation because direct heating of the hydrolyzate of PA6 standard was a clear solution at RT and the reaction mixtures was shown to be more efficient and that of PA66 was a clear solution at reaction temperature, while easier to control than indirect by conventional heating. HCl AA partially precipitated from the solution during cooling to serves both as a catalyst in PA hydrolysis and is also consumed RT. during the reaction by formation of hydrochloride salts of ACA HPLC Method Development for Quantification of PA and HMDA. Constituent Monomers in Hydrolyzates. PA standards Degree of PA hydrolysis was studied as a function of and composite samples were analyzed in the form of pellets as reaction temperature, reaction time, and molar ratio between received, while postconsumer waste samples (i.e., carpet and HCl and amide bonds. The extent of PA hydrolysis was fishing net wastes) were first homogenized by grinding larger followed by H NMR and HPLC on a C18 reversed phase sample amounts on the ball mill in order to analyze the column in a gradient elution mode as both techniques detect representative samples. In further experiments, PA samples the presence of water-soluble oligomers in reaction mixtures. were depolymerized at 170 °C to keep the pressure as low as H NMR spectra of PA hydrolyzates irrespective of a degree of possible (4.6 bar at 170 °C vs 10 bar 190 °C) because a amide group conversion show absence of any foreign signals possible presence of additives, such as commonly used calcium that would indicate the presence of side products, demonstrat- carbonate in carpet waste, additionally contributes to the ing straightforward hydrolysis of PA6 and PA66 into the pressure increase due to carbon dioxide formation. In order to corresponding oligomers and/or monomer(s) (Figure 2). ensure full conversion of PAs into the monomers, the Degree of PA hydrolysis was assessed from H NMR using depolymerization was carried out for 30 min, while the sample eq 3 load was reduced to perform experiments in high excess of HCl PA degradation degree (%) because acid can be consumed also for neutralization of basic additives present in waste materials (e.g., Al O in carpet I[−CH COOH] 2 3 = × 100 waste). II [−CHH COOH] + [−C CONH−] (3) RP-HPLC is an indispensable tool to assess the type and RP-HPLC was found to be superior over H NMR for relative share of water-soluble oligomers in PA hydrolyzates; detection of trace amounts of oligomers (below 1%) in however in combination with an ELSD, it suffers from a hydrolyzates because in H NMR spectra both methylene nonlinear detector response in a large concentration range. 11821 https://dx.doi.org/10.1021/acssuschemeng.0c04190 ACS Sustainable Chem. Eng. 2020, 8, 11818−11826 ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article Furthermore, during cooling of PA66 hydrolyzates to RT, AA partially precipitates from the reaction mixture, while HMDA in the form of hydrochloric salt (HMDA·2HCl) is water soluble. Furthermore, postconsumer wastes can consist of water-insoluble constituents (inorganic reinforcement addi- tives, PP, EVA, PVC, SBR latex, bitumen, etc.), hampering quantification of PA66 via the AA, therefore, we focused on quantitative determination of the HMDA·2HCl monomer in PA66 hydrolyzates. The HMDA·2HCl is difficult to analyze by HPLC because it interacts strongly with silica, while on the reverse-phase column it poorly retains. For direct quantitative determination of HMDA·2HCl and/or 6-ACA·HCl in PA66 and PA6 hydrolyzates, respectively, we developed an HPLC method that does not require monomer derivatization and allows measurements in an isocratic mode and thus the use of Figure 4. HPLC chromatograms obtained on a Primesep 100 mixed- RI as a concentration detector. For this purpose, a mixed-mode mode column coupled to an RI detector for 6-ACA·HCl and HMDA· HPLC column with a stationary phase consisting of hydro- 2HCl in different MQ/ACN/TFA; v/v/v mobile phase compositions; phobic alkyl chains with embedded acid residues (Primesep bottom: 70/30/0.15 optimal for 6-ACA·HCl; middle: 60/40/0.27 100 HPLC column) was chosen. optimal for HMDA·2HCl, and top: 70/30/0.27 optimal for Separation of compounds containing an amine group in the simultaneous quantification of both monomers. structure on the mixed-mode column is governed by 49−52 hydrophobic and cation-exchange mechanisms. The extent of electrostatic interactions of amine-containing monomers of PA6 and PA66 in hydrolyzates is MQ/ACN/ compounds with the column packing material can be easily TFA of 70/30/0.27; v/v/v because both elute from the controlled by a mobile phase pH via the content of TFA, while column in a narrow elution volume range with baseline the extent of hydrophobic interactions is controlled by the separated peaks (Figure 4). In such a mobile phase amount of ACN. Elution order of monomers from the mixed- composition, 6-ACA in the form of salt or not shows a narrow phase column is AA, followed by 6-ACA, and finally HMDA, and symmetric peak in a wide concentration range, while irrespective of the fact whether amine groups of 6-ACA and somewhat broader peak was obtained for HMDA and HMDA· HMDA are initially in the form of amine salt (6-ACA·HCl, 2HCl. Besides, it was noticed that HMDA as a difunctional HMDA·2HCl) or not. Such an elution order is preserved even amine shows an overloading effect at lower injected masses if the contents of TFA and ACN varied and is reversed to that than 6-ACA monofunctional amine as expected but without observed on the C18 column (retention time: HMDA < ACA affecting the linear response of the RI detector. < AA), demonstrating a prevailing electrostatic interaction of At complete exchange of Cl ions of 6-ACA·HCl or HMDA· 6-ACA and especially HMDA with the mixed-mode stationary 2HCl with CF COO of TFA, the determined monomer phase. content in the hydrolyzate and consequently the PA content in Another prerequisite that has to be fulfilled when analyzing the sample should be the same regardless of whether the amines in the form of salts with a counter-ion different from calibration curve is constructed from the corresponding as- that of the acidic component in the mobile phase is rapid and supplied monomers (6-ACA and HMDA) or their hydro- quantitative exchange of the monomer counter-ions (Cl ) with chloric salt. To confirm this, the calibration curves were the acid anions (CF COO ) in the solvent to ensure retention constructedfrom 6-ACA andHMDAaswellastheir of the amine-containing monomers solely in the form of hydrochloric salts. Monomers were dissolved in a suitable trifluoroacetic salts. In this way, distortion of the peak of the solvent to prepare stock solutions, from which lower sample amine-containing monomer in the chromatogram is avoided, concentrations were made by appropriate dilution. Calibration while the specific RI increment (dn/dc) over the monomer curves, representing the area under the monomers’ peaks in peak is uniform, allowing for its quantification by the RI HPLC chromatograms as a function of monomer concen- detector. Our results show that an appropriate peak shape of 6- trations, are linear for all four monomers with correlation ACA·HCl and HMDA·2HCl in the chromatograms is obtained coefficients (R) equal to or higher than 0.9997, indicating when the content of TFA in the solvent used to dissolve the complete monomer mass recovery from the column and − − PA hydrolyzates is at least equimolar to the amine groups of complete Cl /CF COO anion exchange as well (Figure S2). the particular monomer. The most optimal mobile phase Correlation coefficients, approaching unity, together with low composition consisting of MQ/ACN/TFA is 70/30/0.15 and values of intercept of calibration curves thus suggest good 60/40/0.27; v/v/v for 6-ACA·HCl and HMDA·2HCl linearity of the method. respectively; however, the former mobile phase is not Method Accuracy and Precision Evaluated on PA6 appropriate for the HMDA·2HCl because it elutes as a very and PA66 Standards and Their Mixture. Accuracy of the broad unsymmetrical peak between 12.7 and 16 mL (too weak method for quantitative PA determination was verified on PA mobile phase; molar ratio between TFA and −NH is less than standards and their 1:1 mixture. PA6, PA66, and their mixture 1), while the latter mobile phase composition is not were hydrolyzed according to the above-described procedure appropriate for 6-ACA·HCl as it exits the column too early in three parallel weighed samples. For each sample weight, (at 2.75 min) to be reliably quantified (too strong mobile three HPLC measurements were performed to calculate phase; Figure 4). relative standard deviations (RSD) of the peak area in An appropriate mobile phase composition for simultaneous HPLC chromatograms. The contents of PA6 and PA66 in qualitative and quantitative determination of both constituent PA standards and their mixture, calculated from the 11822 https://dx.doi.org/10.1021/acssuschemeng.0c04190 ACS Sustainable Chem. Eng. 2020, 8, 11818−11826 ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article determined contents of the corresponding monomers in PA measurements of the same sample is less than 1 wt % with hydrolyzates based on the calibration curves constructed from high repeatability between the in parallel weighed samples (<1 6-ACA and HMDA or their salts, are highly comparable, when wt %), while mass recoveries of PA6 and PA66 standards the difference between the molecular weights of the PA including their 1:1 mixture are very close to 100 and 50%, repeating unit and the monomer used for the construction of respectively, indicating suitability of the proposed method for the calibration curve was taken into account (eqs 1 and 2, quantitative determination of PA6 and PA66 contents even if Figure 5, Tables 1 and S2). Good accuracy of the method is both of them are the constituents of the same sample (Tables 1 and S2). Determination of PA Content in Composite Samples and Postconsumer Wastes (Carpet Waste and a Mixture of Waste Fishing Nets). The samples were depolymerized according to the above-developed procedure. After reaction completion, the obtained hydrolyzates were filtered to remove glass-fibers and/or other insoluble sample constituents. The solid residues were carefully washed with water, and thus- obtained solutions were quantitatively transferred to the filtrates. The solvent and HCl in filtrates were then evaporated on a rotary evaporator, and the residues were dried in a vacuum chamber until constant weight. Afterward, the dried solids were dissolved in an appropriate solvent and diluted to a final volume to perform HPLC measurements. Figure 5. HPLC−RI chromatograms of hydrolyzates of PA6 and PA66 standards, their 1:1 mixture, PA6-GF and PA66-GF, waste According to the manufacturer’s instruction, PA6-GF and carpet, and waste fishing nets, as obtained on the mixed-mode PA66-GF contain 30 and 35 wt % glass-fibers, respectively, the Primesep 100 column. values which were also confirmed by thermogravimetric analysis (TGA) (Figure S3). The determined contents of PA6 and PA66 in PA6-GF and PA66-GF samples are 68.9 and indicated by the calculated values of sample mass recovery, while good precision is indicated by the calculated value of 62.9 wt %, respectively, which perfectly agree with samples’ RSD between the parallel measurements. RSD of three specifications when a content of additives (up to 1−2 wt %), Table 1. Contents of PA6 and PA66 in PA Standards, as Determined by Microwave-Assisted, Acid-Catalyzed PA Hydrolysis and Further Determination of Monomer Contents in Hydrolyzates by HPLC Using a Primesep 100 Column and Calibration Curves from 6-ACA, 6-ACA·HCl and HMDA, HMDA·2HCl, Respectively (See Eqs 1 and 2) 1st parallel 2nd parallel 3rd parallel 1st parallel 2nd parallel 3rd parallel calibration curve from 6-ACA calibration curve from HMDA weight 147.6 147.6 147.6 147.5 147.5 147.5 peak area 4000 3945 3963 2347 2330 2349 3999 3940 3964 2340 2360 2341 3991 3953 3960 2357 2347 2361 average 3997 3946 3962 2348 2346 2350 determined mass, mg 171.4 169.0 169.8 76.5 76.0 76.6 171.3 168.8 169.8 76.3 77.0 76.4 171.0 169.3 169.6 76.9 76.6 77.0 average, mg 171.2 169.0 169.7 76.6 76.5 76.7 mass recovery, wt % 100.2 98.8 99.2 101.1 100.4 101.2 100.2 98.7 99.3 100.8 101.7 100.9 100.0 99.0 99.2 101.5 101.1 101.7 average, wt % 100.1 ± 0.1 98.8 ± 0.2 99.2 ± 0.1 101.2 ± 0.4 101.1 ± 0.6 101.3 ± 0.4 average between parallel samples 99.4 ± 0.6 wt % of PA6 101.2 ± 0.1 wt % of PA66 calibration curve from 6-ACA·HCl calibration curve from HMDA·2HCl determined mass, mg 220.8 217.8 218.8 122.8 121.9 122.9 220.7 217.5 218.8 122.5 123.5 122.5 220.3 218.2 218.6 123.3 122.8 123.6 average, mg 220.6 217.8 218.7 122.9 122.8 123.0 mass recovery, wt % 101.0 99.6 100.0 99.7 99.0 99.8 100.9 99.4 100.0 99.3 100.2 99.4 100.7 99.8 99.8 100.1 99.7 100.3 average, wt % 100.9 ± 0.1 99.6 ± 0.2 99.9 ± 0.1 99.7 ± 0.4 99.6 ± 0.6 99.8 ± 0.4 average between parallel samples 100.2 ± 0.6 wt % of PA6 99.7 ± 0.1 wt % of PA66 RSD between parallel samples, % 0.6 0.1 MQ/ACN/TFA = 70/30/0.27; v/v/v was used as the solvent and mobile phase. The content of water, as determined by TGA, was subtracted from PA weights. 11823 https://dx.doi.org/10.1021/acssuschemeng.0c04190 ACS Sustainable Chem. Eng. 2020, 8, 11818−11826 ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article Table 2. Contents of PA6 and PA66 in Glass-Fiber Reinforced Composites and Postconsumer Waste Carpet and a Mixture of Waste Fishing Nets, as Determined from the Contents of Monomers in Hydrolyzates by HPLC Using a Primesep 100 Column and Calibration Curves from 6-ACA and HMDA·2HCl, Respectively (See Eqs 1 and 2) 1st parallel 2nd parallel 3rd parallel 1st parallel 2nd parallel 3rd parallel PA6-GF PA66-GF mass recovery, wt % 69.2 68.6 69.1 62.9 62.9 62.9 69.4 68.3 69.1 63.1 63.0 62.5 average of mass recovery, wt % 69.3 ± 0.2 68.4 ± 0.3 69.1 ± 0.02 63.0 ± 0.3 63.0 ± 0.1 62.7 ± 0.3 average between parallel samples 68.9 62.9 RSD between parallel samples, % 0.7 0.2 waste fishing nets waste carpet mass recovery, wt % 96.0 96.8 96.5 56.4 57.5 57.2 96.5 96.6 96.4 56.7 57.1 57.1 average of mass recovery, wt % 96.3 ± 0.4 96.7 ± 0.2 96.5 ± 0.1 56.6 ± 0.4 57.3 ± 0.4 57.2 ± 0.1 average between parallel samples 96.5 57.0 RSD between parallel samples, % 0.2 0.7 MQ/ACN/TFA = 70/30/0.27; v/v/v was used as the solvent and mobile phase. The content of water, as determined by TGA, was subtracted from sample weights. observed also in HPLC chromatograms of composite hydro- dissolved in a suitable solvent for HPLC analysis, which is lyzates as the small intensity signals at 3.8 mL, is taken into performed in an isocratic mode on a mixed-mode HPLC account (Table 2, Figure 5). This signal most likely belongs to column using RI as a concentration detector. The monomer water-soluble degradation products of ester- or amide-based content in the hydrolyzate is determined from a calibration additive(s) commonly used in PA-based composites as curve constructed by plotting peak area as a function of processing auxiliaries and stabilizers. Waste fishing nets contain concentration of a suitable monomer standard. The content of a high amount of PA6 (96.5 wt %, Table 2, Figure 5), while the PA in the sample is then calculated from the monomer content rest of the sample consists mainly of bitumen as a hydrophobic in the hydrolyzate by taking into account the differences coating present on the surface of fishing nets, as indicated by between the molecular weights of PA repeating units and the FTIR and H NMR spectra of 2−3 wt % solid residues monomer used for the construction of the calibration curve. obtained after filtration of hydrolyzates (Figure S4). On the The developed analytical method is fast, robust, accurate (mass other hand, carpet waste contains only 57 wt % of PA6 (Table recoveries of PA6 and PA66 standards and their 1:1 mixture 2, Figure 5). The residue after carpet hydrolysis represents ∼13 are close to 100 and 50%, respectively), and repeatable (RSD < wt % and consists mainly of styrene butadiene (SB) latex ±1.0%) and was demonstrated to be applicable for PA-based (Figure S5), which is commonly used a binder for fusing the composites and postconsumer PA-containing plastic wastes face yarn to backing. Aluminum trihydroxide (Al(OH) ) was such as carpet and fishing net wastes. added to polymer backing as a fire-retardant filler, as demonstrated by FTIR and TGA results of carpet (Figure ASSOCIATED CONTENT S6). Because Al(OH) reacted with HCl during PA hydrolysis sı * Supporting Information forming a water-soluble product, its content was assessed from The Supporting Information is available free of charge at the residue after TGA analysis of carpet, which coincided with https://pubs.acs.org/doi/10.1021/acssuschemeng.0c04190. a mass loss because of decomposition of Al(OH) into Al O 3 2 3 and water at 283 °C(Figure S6). Its amount in the carpet was PA hydrolysis as a function of reaction time at 170 and estimated to be ∼29 wt % and is thus a significant contributor 190 °C; quantification of PA6 and PA66 in their to the total carpet weight. In summary, the waste carpet mixture; experimental conditions for FTIR and TGA consists of 57 wt % PA6, 13 wt % SB latex, and 29 wt % measurements; TGA characterization of glass-fiber Al(OH) , a sum of which is close to 100 wt %, indicating 1 reinforced PA composites; FTIR and H NMR spectra robustness of the method despite the presence of various of residue obtained after microwave-assisted hydrolysis constituents and additives in complex waste samples. of waste fishing nets; and FTIR spectrum of carpet backing and TGA curve of waste carpet (PDF) CONCLUSIONS We present a method for quantitative determination of PA6 AUTHOR INFORMATION and/or PA66 in waste materials, which allows us to accurately evaluate the maximum recovery of PA constituent monomers Corresponding Author from the waste and consequently the waste price and its Ema Zagar − Department of Polymer Chemistry and suitability as a feedstock. A typical procedure for quantitative Technology, National Institute of Chemistry, SI-1000 Ljubljana, PA determination includes straightforward microwave-assisted Slovenia; orcid.org/0000-0002-2694-4312; hydrolysis of PA using HCl as a catalyst under the conditions Email: [email protected] allowing for complete PA degradation into the constituent Authors monomer(s). After hydrolysis, the reaction mixture is filtered if necessary, followed by evaporation of water and HCl from the Urskǎ Cesarek ̌ − Department of Polymer Chemistry and hydrolyzate on a rotary evaporator and further drying the solid Technology, National Institute of Chemistry, SI-1000 residue in a vacuum oven. Thus-obtained dry hydrolyzate is Ljubljana, Slovenia 11824 https://dx.doi.org/10.1021/acssuschemeng.0c04190 ACS Sustainable Chem. Eng. 2020, 8, 11818−11826 ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article (12) Hocker, S.; Rhudy, A. K.; Ginsburg, G.; Kranbuehl, D. E. Ana Drincič ́ − Department of Polymer Chemistry and Polyamide hydrolysis accelerated by small weak organic acids. Polymer Technology, National Institute of Chemistry, SI-1000 2014, 55, 5057−5064. Ljubljana, Slovenia (13) Wang, X. C.; Wang, P. Y.; Ren, L. F.; Qiang, T. T. Moderate Simona Sitar − Department of Polymer Chemistry and hydrolysis of nylon 6 fabric with different acids. Adv. Mater. Res. 2011, Technology, National Institute of Chemistry, SI-1000 331, 347−351. Ljubljana, Slovenia (14) Shukla, S. R.; Harad, A. M.; Mahato, D. Depolymerization of Igor M. Shlyapnikov − Department of Polymer Chemistry and Nylon 6 Waste Fibers. J. Appl. Polym. Sci. 2006, 100, 186−190. Technology, National Institute of Chemistry, SI-1000 (15) Nemade, A. M.; Mishra, S.; Zope, V. S. Chemical Recycling of Ljubljana, Slovenia Polyamide Waste at Various Temperatures and Pressures Using High David Pahovnik − Department of Polymer Chemistry and Pressure Autoclave Technique. J. Polym. Environ. 2011, 19, 110−114. Technology, National Institute of Chemistry, SI-1000 (16) Kamimura, A.; Yamamoto, S. An Efficient Method To Depolymerize Polyamide Plastics: A New Use of Ionic Liquids. Org. Ljubljana, Slovenia; orcid.org/0000-0001-8024-8871 Lett. 2007, 9, 2533−2535. Complete contact information is available at: (17) Kamimura, A.; Yamamoto, S. A Novel Depolymerization of https://pubs.acs.org/10.1021/acssuschemeng.0c04190 Nylons in Ionic Liquids. Polym. Adv. Technol. 2008, 19, 1391−1395. (18) Yamamoto, S.; Kamimura, A. Preparation of Novel Function- Author Contributions alized Ammonium Salts that Effectively Catalyze Depolymerization of Nylon-6 in Ionic Liquids. Chem. Lett. 2009, 38, 1016−1017. The manuscript was written through contributions of all (19) Kamimura, A.; Shiramatsu, Y.; Kawamoto, T. Depolymerization authors. All authors have given approval to the final version of of Polyamide 6 in Hydrophilic Ionic Liquids. 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H.; Euerby, M. R.; McCalley, D. V. A Study of Retention and Overloading of Basic Compounds with Mixed-mode Reversed-phase/Cation-exchange Columns in High Performance Liquid Chromatography. J. Chromatogr. A 2007, 1138,65−72. 11826 https://dx.doi.org/10.1021/acssuschemeng.0c04190 ACS Sustainable Chem. Eng. 2020, 8, 11818−11826 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png ACS Sustainable Chemistry & Engineering Unpaywall

Quantitative Determination of PA6 and/or PA66 Content in Polyamide-Containing Wastes

Žagar, Ema; Češarek, Urška; Drinčić, Ana; Sitar, Simona; Shlyapnikov, Igor M.; Pahovnik, David
ACS Sustainable Chemistry & Engineering – Jul 14, 2020
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Abstract

This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. pubs.acs.org/journal/ascecg Research Article Quantitative Determination of PA6 and/or PA66 Content in Polyamide-Containing Wastes ̌ ̌ ̌ ̌ ̌ ́ Ema Zagar,* Urska Cesarek, Ana Drincic, Simona Sitar, Igor M. Shlyapnikov, and David Pahovnik Cite This: ACS Sustainable Chem. Eng. 2020, 8, 11818−11826 Read Online Metrics & More Article Recommendations sı Supporting Information ACCESS * ABSTRACT: A fast, robust, and convenient method for quantitative determination of polyamide-6 (PA6) and polyamide- 66 (PA66) in plastic wastes is presented. The method includes a straightforward procedure for complete hydrolysis of polyamides (PAs) into the constituent monomers and their quantitative determination in hydrolyzates by high-performance liquid chromatography on a mixed-mode column, from which the contents of a particular PA are determined. The method was developed on neat PA6 and PA66 as well as their mixture and was further utilized on PA-based composites and postconsumer wastes (carpet and fishing net wastes) containing PA in different amounts. The information on the content of a particular PA type in plastic wastes is important because it determines the maximum recovery of PA constituent monomer(s) by chemical recycling and consequently suitability of waste as a feedstock. Moreover, the proposed method allows for convenient differentiation between PA6 and PA66 in their mixtures, which is difficult to assess with precision by conventional characterization techniques. KEYWORDS: polymer waste, chemical recycling, microwave chemistry, polyamide content, liquid chromatography and the production is estimated to be continuously growing, INTRODUCTION which is associated with increasing waste management issues of Recent trends in plastic waste management are focused on 3,10 these nonbiodegradable polymers. Furthermore, they are improved plastic waste recycling because this could have a often used as PA blends or in combination with other polymers long-term impact on reduced greenhouse gas emissions, like polypropylene or polyethylene as well as composite and dependence on finite petroleum resources, and disposal of multicomponent materials such as carpets, which further plastic wastes in the environment and will finally recover the 7,9 1 complicate PA recycling processes. economic value of plastic wastes. Enhancement of plastic A primary goal of PA6 and PA66 depolymerization is to recycling beyond the current level includes among others produce the constituent monomer(s), these are ε-caprolactam improvements in chemical recycling within a circular economy, (cyclic monomer) or ε-aminocaproic acid (linear monomer), which is promoted by the industry as well. By chemical and hexamethylenediamine (HMDA) and adipic acid (AA), recycling, the polymer waste is converted to the feedstock for respectively, which can be further utilized in the synthesis of monomers, fuel production, or other value-added products and new PA or other polymers because the properties of recycled 2−6 intermediates. Furthermore, chemical recycling is a monomers resemble those of commercial raw materials. promising alternative to recycle composites, containing Several attempts to recycle PA66 and especially PA6 have reinforcement additives, which are otherwise difficult to 11−15 been reported, including depolymerization by hydrolysis, 1,7 recycle by mechanical methods. To accelerate chemical organocatalyzed ring-closing depolymerization in ionic recycling, selective and efficient catalysts are being developed, 16−19 20 21−25 liquids, ammonolysis, alcoholysis/glycolysis, and which at the same time keep the depolymerization process thermal decomposition under pyrolytic conditions in sub-/ 1−4,8 economically viable. 26−38 supercritical fluids. Depolymerization time of PA by acid- Polyamide-6 (PA6) and polyamide-66 (PA66) are versatile semicrystalline polymers with high chemical resistance and Received: June 6, 2020 good thermal and mechanical properties. They are produced Revised: July 6, 2020 from fossil resources, usually in the form of fibers or Published: July 14, 2020 thermoplastics, for the use in a broad scope of applications (textile, automotive, electrical, electronic, construction, pack- aging, coatings, and other industries). Currently, both PA types are produced on an annual worldwide multimillion ton scale, https://dx.doi.org/10.1021/acssuschemeng.0c04190 © 2020 American Chemical Society ACS Sustainable Chem. Eng. 2020, 8, 11818−11826 Downloaded via UNIV OF LJUBLJANA on August 17, 2020 at 05:58:58 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article catalyzed hydrolysis has been significantly shortened by using PA6 and PA66 was examined by analyzing their 1:1 mixture. microwaves as an energy source compared to the processes Furthermore, the method was tested on PA composites and 39,40 using conventional heating. Depolymerization processes postconsumer waste samples such as carpet waste and a reported so far result either in incomplete depolymerization, mixture of waste fishing nets. where final reaction mixtures consist not only of monomer(s) but also of linear and cyclic PA oligomers, or other low- EXPERIMENTAL SECTION molecular-weight chemical compounds of different physico- Chemicals and Materials. All chemicals used were of analytical chemical properties and functionality from PA constituent grade. HCl (18.5 wt %), used for depolymerization of PA samples, monomers(s). Currently, PA6 and PA66 are chemically was prepared by diluting 37 wt % HCl (Merck KGaA). Acetonitrile recycled on an industrial scale in the United States and (ACN) from Honeywell, Riedel-de Haen, Chromasolv, HPLC Europe, where the high energy and decontamination costs are gradient grade, ≥99.9%; trifluoroacetic acid (TFA) from Acros economically balanced by the high price of virgin PA, making Organics, ≥99.5%; or formic acid (FA) from Honeywell, ≥99% were the processes cost-efficient. For chemical recycling of PA, the used as received for preparation of mobile phases. Water was purified information on the content of a particular PA type in different using a Milli-Q reverse osmosis system to obtain MQ-water (MQ) with typically 18.2 MΩ cm resistivity and <4 ppb carbon (Millipore, waste materials is extremely useful because it determines the Watford, UK). Monomer standards ε-aminocaproic acid (or 6- maximum monomer recovery from the waste and consequently aminohexanoic acid, 6-ACA), HMDA, and AA of high purity the price and suitability of waste as a feedstock. (>99.9%) were purchased from Sigma-Aldrich, USA. Salts of amino The overarching objective of this work was to develop a acid (6-ACA·HCl) and diamine (HMDA·2HCl) were prepared by simple and efficient method for quantitative determination of dissolving an appropriate monomer standard in excess amount of PA6/PA66 in different waste types. It also enables convenient 18.5% HCl, then evaporating the solvent on a rotatory evaporator and differentiation of PA6 from PA66 and other PA types in waste drying the resulting residue at 50 °C in a vacuum chamber until mixtures; the information which is difficult to assess with constant weight. PA6 and PA66 polymer standards were purchased precision by conventional characterization techniques such as from Sigma-Aldrich. Glass-fiber reinforced PA66 (PA66-GF) (35 wt Fourier transform infrared (FTIR) spectroscopy, nuclear %) was obtained from Kordsa, Turkey, and glass-fiber reinforced PA6 (PA6-GF) (30 wt %) was purchased from Lanxess, Energizing magnetic resonance (NMR), and differential scanning Chemistry, Germany. Waste carpet and mixture of waste fishing nets calorimetry. Prerequisite for such method development is a as typical examples of postconsumer wastes were obtained from straightforward depolymerization of PAs into the constituent Aquafil, Slovenia. monomers. The depolymerization reaction has to be carried Homogenization of Postconsumer Waste Samples. To out until completion and in the absence of side reactions in a prepare representative and homogeneous waste samples, about 5 g reasonable time frame so that the final reaction mixture does of the sample was homogenized by milling on a ball mill. For this not contain any PA oligomers and side products. To this end, purpose, ∼0.5 g of the waste sample was loaded into a 35 mL stainless we have developed a microwave-assisted hydrolysis of PA steel vessel together with a 25 mm stainless steel ball (Tehtnica catalyzed by hydrochloric acid (HCl), by which complete PA Millmix 20, Domel, Slovenia). The vessel was hermetically closed and depolymerization to the constituent monomer(s) is achieved. cooled in liquid nitrogen at −196 °C for 10 min. Then, the sample was ground for 2 min with a frequency of 30 Hz. After milling, the The obtained depolymerization mixtures (hydrolyzates) were vessel was warmed up to ambient temperature to prevent the sample analyzed by high-performance liquid chromatography from absorbing the moisture. Thus-ground sample (5 g) was (HPLC), where we searched for a robust and simple method homogenized and used for further degradation experiments. to quantitatively determine the content of PA constituent Monitoring the Course of PA Hydrolysis Reaction. Micro- monomers in the hydrolyzates, from which the content of a wave-Assisted Hydrolysis of PA. PA6 or PA66 standard sample (∼4 particular PA in the waste material can be calculated. State-of- g) in the form of pellets was accurately weighed into a borosilicate the-art methods comprise qualitative and semiquantitative glass vial, suitable for a microwave oven Monowave 300 equipped evaluation of water-soluble PA degradation products by HPLC with the camera as well as temperature and pressure sensors (Anton on C18 or C8 reversed phase columns by using evaporative Paar GmbH, Austria). A magnetic stirring bar and 8 mL of 5.56 M HCl were added to the sample. After sealing, the reaction vessel was light scattering (ELS), chemiluminescence, or mass-selective 41−46 placed in a microwave oven, where it was heated to a predetermined detectors. RP-HPLC separates PA monomers as well as temperature of 170 or 190 °C (power of the magnetron automatically linear and cyclic PA oligomers. RP-HPLC was also reported as varied in the range of 0−850 W). Because of the microwave reactor a tool for evaluation of migration of cyclic monomers and design, it was not possible to take aliquots from the reaction mixture oligomers from PA food contact materials into food and food during the course of the reaction. Therefore, the degree of 43,44,47,48 simulants. However, currently available methods in depolymerization was followed by performing identical experiments terms of quantitative determination of PA constituent with different times. The reaction vessel was then cooled to 55 °Cby monomers in hydrolyzates suffer either from nonlinear a stream of compressed air. The solvent was removed from the detector response in a large concentration range or non- reaction mixtures by rotary evaporation and further drying in a conventional detection. Furthermore, determination of the PA vacuum oven at 50 °C until constant weight. The resulting dry solid residues of PA66 were further homogenized by grinding in a mortar. content in the case of incompletely depolymerized PA is Thus-obtained dry PA hydrolyzates were analyzed by H NMR and hampered because the response factors are not known for all HPLC. oligomers due to the unavailability of corresponding standards. 1 1 HNMR Spectroscopy. HNMR spectraof dried and For the determination of the content of a particular PA homogenized reaction mixtures of PA66 were recorded in DMSO- monomer in hydrolyzates, we have chosen isocratic HPLC on d with added TFA and tetramethylsilane (δ = 0) as an internal a mixed-mode column with refractive index (RI) detection, 1 chemical-shift standard. H NMR spectra of PA6 reaction mixtures which does not require any monomer derivatization. The were recorded directly by using C D as an inset. Measurements were 6 6 proposed method for quantitative determination of PA in performed at room temperature (RT) on a Varian Unity Inova 300 waste materials was verified in terms of accuracy and precision MHz NMR spectrometer (Varian, Inc., USA) with both the relaxation by analyzing neat PA standards, while method selectivity for delay and acquisition time of 5 s. 11819 https://dx.doi.org/10.1021/acssuschemeng.0c04190 ACS Sustainable Chem. Eng. 2020, 8, 11818−11826 ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article Figure 1. Reaction scheme of HCl-catalyzed hydrolysis of PA6 and PA66 into their constituent monomer(s). Figure 2. H NMR spectra of reaction mixtures obtained after 5 min microwave-assisted, acid-catalyzed hydrolysis of PA6 (left) and PA66 (right) at 170 °C. H NMR spectrum of the PA6 reaction mixture was recorded directly by using C D as an inset. PA66 reaction mixture was dried and 6 6 homogenized before H NMR analysis in DMSO-d with added TFA. HPLC Chromatographic Conditions. Dry PA6 or PA66 hydro- was carefully washed with water, and thus-obtained solution was lyzate (∼500 mg) was dissolved in 100 mL of the mobile phase of quantitatively transferred to the rest of the liquid phase. The solvent starting composition, that is, ACN/MQ water containing 1 vol % of and HCl in the filtrate were then evaporated on a rotary evaporator, ACN and 0.1 vol % of FA = 1/99 (v/v) during constant stirring for at and the residue was dried in a vacuum chamber at 50 °C. Afterward, the dried solid was dissolved in an appropriate solvent and diluted to a least 2 h. Afterward, 0.5 mL aliquot was taken from the mixture and final volume of 100 mL to perform HPLC measurements. further diluted to 5 mL to obtain a solution concentration of 0.5 mg/ Quantitative contents of PA constituent monomers in hydrolyzates mL. The presence of oligomers in PA hydrolyzates was assessed by a were determined by isocratic HPLC on a Primesep analytical 100 reversed-phase HPLC on an Agilent Zorbax Eclipse XDB C18 column column (4.6 × 150 mm; 5 μm, SIELC Technologies, USA). The (4.6 × 150 mm; 5 μm). A chromatographic system consisted of an separations were performed on an Agilent Technologies chromato- Agilent 1100 Series binary pump (Agilent Technologies, Inc., USA) graphic system equipped with the isocratic pump, column heater, and equipped with a model 7725i Rheodyne (Bensheim, Germany) Agilent 1260 Infinity Series RI detector (Agilent Technologies, Inc., manual injector and an ELS detector (ELSD) (Agilent Technologies, USA). The mobile phase consisted of a defined mixture of MQ, ACN, Inc., USA). The mobile phase used was a gradient of ACN as solvent and TFA, which was supplied at a flow rate of 1 mL/min. Mobile A and MQ containing 1 vol % of ACN and 0.1 vol % of FA as solvent phase composition consisting of MQ/ACN/TFA was 70/30/0.27 (v/ B. A linear gradient of the mobile phase composition ran from A/B 1/ v/v). The injection volume of sample solutions was 20 μL. The 99 (v/v) to A/B 30/70 (v/v) in the first 20 min, then isocratic elution separations were carried out at 30 °C, while the temperature of the RI was held for 5 min, after which the gradient was restored back to A/B detector was set to 35 °C. The concentrations and contents of 1/99 (v/v). The mobile phase flow rate was 0.5 mL/min, and the monomers in PA hydrolyzates were calculated from the calibration column temperature was 30 °C. ELSD was operated at evaporator and curves, plotted from the peak areas in HPLC chromatograms of the nebulizer temperature of 50 °C. The injection volume of thus corresponding standards (6-ACA, 6-ACA·HCl or HMDA, and prepared sample solutions onto the column was 20 μL. HMDA·2HCl) recorded at different but known solution concen- Quantitative Determination of PA6 and/or PA66 Content. trations (0.5, 1, 1.5, 2, and 2.5 mg/mL) by the same HPLC method. Microwave-Assisted Hydrolysis of PA. A typical depolymerization From the concentration and content of 6-ACA, 6-ACA·HCl or procedure for quantitative determination of PA6 and/or PA66 in PA- HMDA, and HMDA·2HCl in the hydrolyzate, the amount and the based samples is the following: ∼150 mg of the sample in the form of content of PA in the original sample were calculated, according to eqs powder or pellets was accurately weighed into a borosilicate glass vial, 1 and 2 suitable for a microwave oven Monowave 300. Then, a magnetic stirring bar and 6 mL of 5.56 M HCl were added to the sample. The () Ab − m = ××Vk reaction vessel was then sealed and placed into a microwave oven, (1) where it was heated to a predetermined temperature of 170 °C. At that temperature, the sample was treated for 30 min, and then, the %(PA)=× 100 reaction vessel was cooled to 55 °C by a stream of compressed air. If (2) necessary, the liquid phase was filtered through a glass filter with a pore size of 1.0 μm (CHROMAFIL Xtra GF-100/25, Sorbent where m is a determined mass of PA in the weighed sample, m is a Technologies, Inc., USA) to remove reinforcement additives (glass- mass of the sample used for hydrolysis, A is a peak area of the PA fibers) and/or other insoluble sample constituents. The filtrate was constituent monomer in the chromatogram of PA hydrolyzate, a is a then quantitatively transferred into a 100 mL flask. The solid residue slope of the calibration curve, b is an intercept of the calibration curve 11820 https://dx.doi.org/10.1021/acssuschemeng.0c04190 ACS Sustainable Chem. Eng. 2020, 8, 11818−11826 ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article Figure 3. RP-HPLC (C18 column coupled to an ELSD) chromatograms (left) and enlarged H NMR spectra typical for oligomers (right) of PA6 reaction mixtures hydrolyzed at 170 °C for different reaction times. HPLC chromatogram and H NMR spectrum of the reaction mixture hydrolyzed at 170 °C for 30 min show complete PA6 hydrolysis because only 6-ACA was detected. on y-axis, V is a volume of the measuring flask (100 mL), and k is a signals near the amide group of PA oligomers partially overlap recalculation coefficient, taking into account a difference between the with the satellites of methylene signals near the carboxyl and molecular weights of the PA6 or PA66 repeating unit and the amine groups of AA and HMDA. On the contrary, RP-HPLC monomer from which the calibration curve was constructed: M(PA6 coupled to an ELSD undoubtedly detects PA oligomers even repeat unit)/M(6-ACA) = 113.16/131.17 Da = 0.863; M(PA6 repeat when their content is extremely low. For both PA types, unit)/M(6-ACA·HCl) = 113.16 Da/167.63 Da = 0.675 and M(PA66 complete hydrolysis was achieved at a HCl/amide bond molar repeat unit)/M(HMDA) = 226.35 Da/116.21 Da = 1.948; M(PA66 ratio of at least 1.25 in 30 and 10 min reaction time at the repeat unit)/M(HMDA·2HCl) = 226.35 Da/189.13 Da = 1.197. reaction temperatures of 170 and 190 °C, respectively, as RESULTS AND DISCUSSION indicated by the degree of PA degradation monitored by H NMR and absence of oligomer peaks in HPLC chromatograms Complete Hydrolysis of PAs into the Constituent of hydrolyzates (Figure 3, Table S1). Furthermore, H NMR Monomers. For determination of the PA content in plastic and HPLC results show the absence of cyclic ε-caprolactam in waste, we developed a fast degradation method, whereby PA6 the final PA6 degradation products because under the chosen completely depolymerizes into 6-ACA and PA66 into AA and experimental conditions it hydrolyzes into the linear 6-ACA, HMDA (Figure 1). Oligomers and side products in the final which is consistent with the literature data. Complete PA reaction mixtures of PA6 and PA66 were not detected by H hydrolysis was achieved also in the presence of reinforcement NMR (Figure S1). Depolymerization experiments were additives or other sample constituents. After depolymerization, performed by microwave irradiation because direct heating of the hydrolyzate of PA6 standard was a clear solution at RT and the reaction mixtures was shown to be more efficient and that of PA66 was a clear solution at reaction temperature, while easier to control than indirect by conventional heating. HCl AA partially precipitated from the solution during cooling to serves both as a catalyst in PA hydrolysis and is also consumed RT. during the reaction by formation of hydrochloride salts of ACA HPLC Method Development for Quantification of PA and HMDA. Constituent Monomers in Hydrolyzates. PA standards Degree of PA hydrolysis was studied as a function of and composite samples were analyzed in the form of pellets as reaction temperature, reaction time, and molar ratio between received, while postconsumer waste samples (i.e., carpet and HCl and amide bonds. The extent of PA hydrolysis was fishing net wastes) were first homogenized by grinding larger followed by H NMR and HPLC on a C18 reversed phase sample amounts on the ball mill in order to analyze the column in a gradient elution mode as both techniques detect representative samples. In further experiments, PA samples the presence of water-soluble oligomers in reaction mixtures. were depolymerized at 170 °C to keep the pressure as low as H NMR spectra of PA hydrolyzates irrespective of a degree of possible (4.6 bar at 170 °C vs 10 bar 190 °C) because a amide group conversion show absence of any foreign signals possible presence of additives, such as commonly used calcium that would indicate the presence of side products, demonstrat- carbonate in carpet waste, additionally contributes to the ing straightforward hydrolysis of PA6 and PA66 into the pressure increase due to carbon dioxide formation. In order to corresponding oligomers and/or monomer(s) (Figure 2). ensure full conversion of PAs into the monomers, the Degree of PA hydrolysis was assessed from H NMR using depolymerization was carried out for 30 min, while the sample eq 3 load was reduced to perform experiments in high excess of HCl PA degradation degree (%) because acid can be consumed also for neutralization of basic additives present in waste materials (e.g., Al O in carpet I[−CH COOH] 2 3 = × 100 waste). II [−CHH COOH] + [−C CONH−] (3) RP-HPLC is an indispensable tool to assess the type and RP-HPLC was found to be superior over H NMR for relative share of water-soluble oligomers in PA hydrolyzates; detection of trace amounts of oligomers (below 1%) in however in combination with an ELSD, it suffers from a hydrolyzates because in H NMR spectra both methylene nonlinear detector response in a large concentration range. 11821 https://dx.doi.org/10.1021/acssuschemeng.0c04190 ACS Sustainable Chem. Eng. 2020, 8, 11818−11826 ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article Furthermore, during cooling of PA66 hydrolyzates to RT, AA partially precipitates from the reaction mixture, while HMDA in the form of hydrochloric salt (HMDA·2HCl) is water soluble. Furthermore, postconsumer wastes can consist of water-insoluble constituents (inorganic reinforcement addi- tives, PP, EVA, PVC, SBR latex, bitumen, etc.), hampering quantification of PA66 via the AA, therefore, we focused on quantitative determination of the HMDA·2HCl monomer in PA66 hydrolyzates. The HMDA·2HCl is difficult to analyze by HPLC because it interacts strongly with silica, while on the reverse-phase column it poorly retains. For direct quantitative determination of HMDA·2HCl and/or 6-ACA·HCl in PA66 and PA6 hydrolyzates, respectively, we developed an HPLC method that does not require monomer derivatization and allows measurements in an isocratic mode and thus the use of Figure 4. HPLC chromatograms obtained on a Primesep 100 mixed- RI as a concentration detector. For this purpose, a mixed-mode mode column coupled to an RI detector for 6-ACA·HCl and HMDA· HPLC column with a stationary phase consisting of hydro- 2HCl in different MQ/ACN/TFA; v/v/v mobile phase compositions; phobic alkyl chains with embedded acid residues (Primesep bottom: 70/30/0.15 optimal for 6-ACA·HCl; middle: 60/40/0.27 100 HPLC column) was chosen. optimal for HMDA·2HCl, and top: 70/30/0.27 optimal for Separation of compounds containing an amine group in the simultaneous quantification of both monomers. structure on the mixed-mode column is governed by 49−52 hydrophobic and cation-exchange mechanisms. The extent of electrostatic interactions of amine-containing monomers of PA6 and PA66 in hydrolyzates is MQ/ACN/ compounds with the column packing material can be easily TFA of 70/30/0.27; v/v/v because both elute from the controlled by a mobile phase pH via the content of TFA, while column in a narrow elution volume range with baseline the extent of hydrophobic interactions is controlled by the separated peaks (Figure 4). In such a mobile phase amount of ACN. Elution order of monomers from the mixed- composition, 6-ACA in the form of salt or not shows a narrow phase column is AA, followed by 6-ACA, and finally HMDA, and symmetric peak in a wide concentration range, while irrespective of the fact whether amine groups of 6-ACA and somewhat broader peak was obtained for HMDA and HMDA· HMDA are initially in the form of amine salt (6-ACA·HCl, 2HCl. Besides, it was noticed that HMDA as a difunctional HMDA·2HCl) or not. Such an elution order is preserved even amine shows an overloading effect at lower injected masses if the contents of TFA and ACN varied and is reversed to that than 6-ACA monofunctional amine as expected but without observed on the C18 column (retention time: HMDA < ACA affecting the linear response of the RI detector. < AA), demonstrating a prevailing electrostatic interaction of At complete exchange of Cl ions of 6-ACA·HCl or HMDA· 6-ACA and especially HMDA with the mixed-mode stationary 2HCl with CF COO of TFA, the determined monomer phase. content in the hydrolyzate and consequently the PA content in Another prerequisite that has to be fulfilled when analyzing the sample should be the same regardless of whether the amines in the form of salts with a counter-ion different from calibration curve is constructed from the corresponding as- that of the acidic component in the mobile phase is rapid and supplied monomers (6-ACA and HMDA) or their hydro- quantitative exchange of the monomer counter-ions (Cl ) with chloric salt. To confirm this, the calibration curves were the acid anions (CF COO ) in the solvent to ensure retention constructedfrom 6-ACA andHMDAaswellastheir of the amine-containing monomers solely in the form of hydrochloric salts. Monomers were dissolved in a suitable trifluoroacetic salts. In this way, distortion of the peak of the solvent to prepare stock solutions, from which lower sample amine-containing monomer in the chromatogram is avoided, concentrations were made by appropriate dilution. Calibration while the specific RI increment (dn/dc) over the monomer curves, representing the area under the monomers’ peaks in peak is uniform, allowing for its quantification by the RI HPLC chromatograms as a function of monomer concen- detector. Our results show that an appropriate peak shape of 6- trations, are linear for all four monomers with correlation ACA·HCl and HMDA·2HCl in the chromatograms is obtained coefficients (R) equal to or higher than 0.9997, indicating when the content of TFA in the solvent used to dissolve the complete monomer mass recovery from the column and − − PA hydrolyzates is at least equimolar to the amine groups of complete Cl /CF COO anion exchange as well (Figure S2). the particular monomer. The most optimal mobile phase Correlation coefficients, approaching unity, together with low composition consisting of MQ/ACN/TFA is 70/30/0.15 and values of intercept of calibration curves thus suggest good 60/40/0.27; v/v/v for 6-ACA·HCl and HMDA·2HCl linearity of the method. respectively; however, the former mobile phase is not Method Accuracy and Precision Evaluated on PA6 appropriate for the HMDA·2HCl because it elutes as a very and PA66 Standards and Their Mixture. Accuracy of the broad unsymmetrical peak between 12.7 and 16 mL (too weak method for quantitative PA determination was verified on PA mobile phase; molar ratio between TFA and −NH is less than standards and their 1:1 mixture. PA6, PA66, and their mixture 1), while the latter mobile phase composition is not were hydrolyzed according to the above-described procedure appropriate for 6-ACA·HCl as it exits the column too early in three parallel weighed samples. For each sample weight, (at 2.75 min) to be reliably quantified (too strong mobile three HPLC measurements were performed to calculate phase; Figure 4). relative standard deviations (RSD) of the peak area in An appropriate mobile phase composition for simultaneous HPLC chromatograms. The contents of PA6 and PA66 in qualitative and quantitative determination of both constituent PA standards and their mixture, calculated from the 11822 https://dx.doi.org/10.1021/acssuschemeng.0c04190 ACS Sustainable Chem. Eng. 2020, 8, 11818−11826 ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article determined contents of the corresponding monomers in PA measurements of the same sample is less than 1 wt % with hydrolyzates based on the calibration curves constructed from high repeatability between the in parallel weighed samples (<1 6-ACA and HMDA or their salts, are highly comparable, when wt %), while mass recoveries of PA6 and PA66 standards the difference between the molecular weights of the PA including their 1:1 mixture are very close to 100 and 50%, repeating unit and the monomer used for the construction of respectively, indicating suitability of the proposed method for the calibration curve was taken into account (eqs 1 and 2, quantitative determination of PA6 and PA66 contents even if Figure 5, Tables 1 and S2). Good accuracy of the method is both of them are the constituents of the same sample (Tables 1 and S2). Determination of PA Content in Composite Samples and Postconsumer Wastes (Carpet Waste and a Mixture of Waste Fishing Nets). The samples were depolymerized according to the above-developed procedure. After reaction completion, the obtained hydrolyzates were filtered to remove glass-fibers and/or other insoluble sample constituents. The solid residues were carefully washed with water, and thus- obtained solutions were quantitatively transferred to the filtrates. The solvent and HCl in filtrates were then evaporated on a rotary evaporator, and the residues were dried in a vacuum chamber until constant weight. Afterward, the dried solids were dissolved in an appropriate solvent and diluted to a final volume to perform HPLC measurements. Figure 5. HPLC−RI chromatograms of hydrolyzates of PA6 and PA66 standards, their 1:1 mixture, PA6-GF and PA66-GF, waste According to the manufacturer’s instruction, PA6-GF and carpet, and waste fishing nets, as obtained on the mixed-mode PA66-GF contain 30 and 35 wt % glass-fibers, respectively, the Primesep 100 column. values which were also confirmed by thermogravimetric analysis (TGA) (Figure S3). The determined contents of PA6 and PA66 in PA6-GF and PA66-GF samples are 68.9 and indicated by the calculated values of sample mass recovery, while good precision is indicated by the calculated value of 62.9 wt %, respectively, which perfectly agree with samples’ RSD between the parallel measurements. RSD of three specifications when a content of additives (up to 1−2 wt %), Table 1. Contents of PA6 and PA66 in PA Standards, as Determined by Microwave-Assisted, Acid-Catalyzed PA Hydrolysis and Further Determination of Monomer Contents in Hydrolyzates by HPLC Using a Primesep 100 Column and Calibration Curves from 6-ACA, 6-ACA·HCl and HMDA, HMDA·2HCl, Respectively (See Eqs 1 and 2) 1st parallel 2nd parallel 3rd parallel 1st parallel 2nd parallel 3rd parallel calibration curve from 6-ACA calibration curve from HMDA weight 147.6 147.6 147.6 147.5 147.5 147.5 peak area 4000 3945 3963 2347 2330 2349 3999 3940 3964 2340 2360 2341 3991 3953 3960 2357 2347 2361 average 3997 3946 3962 2348 2346 2350 determined mass, mg 171.4 169.0 169.8 76.5 76.0 76.6 171.3 168.8 169.8 76.3 77.0 76.4 171.0 169.3 169.6 76.9 76.6 77.0 average, mg 171.2 169.0 169.7 76.6 76.5 76.7 mass recovery, wt % 100.2 98.8 99.2 101.1 100.4 101.2 100.2 98.7 99.3 100.8 101.7 100.9 100.0 99.0 99.2 101.5 101.1 101.7 average, wt % 100.1 ± 0.1 98.8 ± 0.2 99.2 ± 0.1 101.2 ± 0.4 101.1 ± 0.6 101.3 ± 0.4 average between parallel samples 99.4 ± 0.6 wt % of PA6 101.2 ± 0.1 wt % of PA66 calibration curve from 6-ACA·HCl calibration curve from HMDA·2HCl determined mass, mg 220.8 217.8 218.8 122.8 121.9 122.9 220.7 217.5 218.8 122.5 123.5 122.5 220.3 218.2 218.6 123.3 122.8 123.6 average, mg 220.6 217.8 218.7 122.9 122.8 123.0 mass recovery, wt % 101.0 99.6 100.0 99.7 99.0 99.8 100.9 99.4 100.0 99.3 100.2 99.4 100.7 99.8 99.8 100.1 99.7 100.3 average, wt % 100.9 ± 0.1 99.6 ± 0.2 99.9 ± 0.1 99.7 ± 0.4 99.6 ± 0.6 99.8 ± 0.4 average between parallel samples 100.2 ± 0.6 wt % of PA6 99.7 ± 0.1 wt % of PA66 RSD between parallel samples, % 0.6 0.1 MQ/ACN/TFA = 70/30/0.27; v/v/v was used as the solvent and mobile phase. The content of water, as determined by TGA, was subtracted from PA weights. 11823 https://dx.doi.org/10.1021/acssuschemeng.0c04190 ACS Sustainable Chem. Eng. 2020, 8, 11818−11826 ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article Table 2. Contents of PA6 and PA66 in Glass-Fiber Reinforced Composites and Postconsumer Waste Carpet and a Mixture of Waste Fishing Nets, as Determined from the Contents of Monomers in Hydrolyzates by HPLC Using a Primesep 100 Column and Calibration Curves from 6-ACA and HMDA·2HCl, Respectively (See Eqs 1 and 2) 1st parallel 2nd parallel 3rd parallel 1st parallel 2nd parallel 3rd parallel PA6-GF PA66-GF mass recovery, wt % 69.2 68.6 69.1 62.9 62.9 62.9 69.4 68.3 69.1 63.1 63.0 62.5 average of mass recovery, wt % 69.3 ± 0.2 68.4 ± 0.3 69.1 ± 0.02 63.0 ± 0.3 63.0 ± 0.1 62.7 ± 0.3 average between parallel samples 68.9 62.9 RSD between parallel samples, % 0.7 0.2 waste fishing nets waste carpet mass recovery, wt % 96.0 96.8 96.5 56.4 57.5 57.2 96.5 96.6 96.4 56.7 57.1 57.1 average of mass recovery, wt % 96.3 ± 0.4 96.7 ± 0.2 96.5 ± 0.1 56.6 ± 0.4 57.3 ± 0.4 57.2 ± 0.1 average between parallel samples 96.5 57.0 RSD between parallel samples, % 0.2 0.7 MQ/ACN/TFA = 70/30/0.27; v/v/v was used as the solvent and mobile phase. The content of water, as determined by TGA, was subtracted from sample weights. observed also in HPLC chromatograms of composite hydro- dissolved in a suitable solvent for HPLC analysis, which is lyzates as the small intensity signals at 3.8 mL, is taken into performed in an isocratic mode on a mixed-mode HPLC account (Table 2, Figure 5). This signal most likely belongs to column using RI as a concentration detector. The monomer water-soluble degradation products of ester- or amide-based content in the hydrolyzate is determined from a calibration additive(s) commonly used in PA-based composites as curve constructed by plotting peak area as a function of processing auxiliaries and stabilizers. Waste fishing nets contain concentration of a suitable monomer standard. The content of a high amount of PA6 (96.5 wt %, Table 2, Figure 5), while the PA in the sample is then calculated from the monomer content rest of the sample consists mainly of bitumen as a hydrophobic in the hydrolyzate by taking into account the differences coating present on the surface of fishing nets, as indicated by between the molecular weights of PA repeating units and the FTIR and H NMR spectra of 2−3 wt % solid residues monomer used for the construction of the calibration curve. obtained after filtration of hydrolyzates (Figure S4). On the The developed analytical method is fast, robust, accurate (mass other hand, carpet waste contains only 57 wt % of PA6 (Table recoveries of PA6 and PA66 standards and their 1:1 mixture 2, Figure 5). The residue after carpet hydrolysis represents ∼13 are close to 100 and 50%, respectively), and repeatable (RSD < wt % and consists mainly of styrene butadiene (SB) latex ±1.0%) and was demonstrated to be applicable for PA-based (Figure S5), which is commonly used a binder for fusing the composites and postconsumer PA-containing plastic wastes face yarn to backing. Aluminum trihydroxide (Al(OH) ) was such as carpet and fishing net wastes. added to polymer backing as a fire-retardant filler, as demonstrated by FTIR and TGA results of carpet (Figure ASSOCIATED CONTENT S6). Because Al(OH) reacted with HCl during PA hydrolysis sı * Supporting Information forming a water-soluble product, its content was assessed from The Supporting Information is available free of charge at the residue after TGA analysis of carpet, which coincided with https://pubs.acs.org/doi/10.1021/acssuschemeng.0c04190. a mass loss because of decomposition of Al(OH) into Al O 3 2 3 and water at 283 °C(Figure S6). Its amount in the carpet was PA hydrolysis as a function of reaction time at 170 and estimated to be ∼29 wt % and is thus a significant contributor 190 °C; quantification of PA6 and PA66 in their to the total carpet weight. In summary, the waste carpet mixture; experimental conditions for FTIR and TGA consists of 57 wt % PA6, 13 wt % SB latex, and 29 wt % measurements; TGA characterization of glass-fiber Al(OH) , a sum of which is close to 100 wt %, indicating 1 reinforced PA composites; FTIR and H NMR spectra robustness of the method despite the presence of various of residue obtained after microwave-assisted hydrolysis constituents and additives in complex waste samples. of waste fishing nets; and FTIR spectrum of carpet backing and TGA curve of waste carpet (PDF) CONCLUSIONS We present a method for quantitative determination of PA6 AUTHOR INFORMATION and/or PA66 in waste materials, which allows us to accurately evaluate the maximum recovery of PA constituent monomers Corresponding Author from the waste and consequently the waste price and its Ema Zagar − Department of Polymer Chemistry and suitability as a feedstock. A typical procedure for quantitative Technology, National Institute of Chemistry, SI-1000 Ljubljana, PA determination includes straightforward microwave-assisted Slovenia; orcid.org/0000-0002-2694-4312; hydrolysis of PA using HCl as a catalyst under the conditions Email: [email protected] allowing for complete PA degradation into the constituent Authors monomer(s). After hydrolysis, the reaction mixture is filtered if necessary, followed by evaporation of water and HCl from the Urskǎ Cesarek ̌ − Department of Polymer Chemistry and hydrolyzate on a rotary evaporator and further drying the solid Technology, National Institute of Chemistry, SI-1000 residue in a vacuum oven. Thus-obtained dry hydrolyzate is Ljubljana, Slovenia 11824 https://dx.doi.org/10.1021/acssuschemeng.0c04190 ACS Sustainable Chem. Eng. 2020, 8, 11818−11826 ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article (12) Hocker, S.; Rhudy, A. K.; Ginsburg, G.; Kranbuehl, D. E. Ana Drincič ́ − Department of Polymer Chemistry and Polyamide hydrolysis accelerated by small weak organic acids. Polymer Technology, National Institute of Chemistry, SI-1000 2014, 55, 5057−5064. Ljubljana, Slovenia (13) Wang, X. C.; Wang, P. Y.; Ren, L. F.; Qiang, T. T. Moderate Simona Sitar − Department of Polymer Chemistry and hydrolysis of nylon 6 fabric with different acids. Adv. Mater. Res. 2011, Technology, National Institute of Chemistry, SI-1000 331, 347−351. Ljubljana, Slovenia (14) Shukla, S. R.; Harad, A. M.; Mahato, D. Depolymerization of Igor M. Shlyapnikov − Department of Polymer Chemistry and Nylon 6 Waste Fibers. J. Appl. Polym. Sci. 2006, 100, 186−190. Technology, National Institute of Chemistry, SI-1000 (15) Nemade, A. M.; Mishra, S.; Zope, V. S. Chemical Recycling of Ljubljana, Slovenia Polyamide Waste at Various Temperatures and Pressures Using High David Pahovnik − Department of Polymer Chemistry and Pressure Autoclave Technique. J. Polym. Environ. 2011, 19, 110−114. Technology, National Institute of Chemistry, SI-1000 (16) Kamimura, A.; Yamamoto, S. An Efficient Method To Depolymerize Polyamide Plastics: A New Use of Ionic Liquids. Org. Ljubljana, Slovenia; orcid.org/0000-0001-8024-8871 Lett. 2007, 9, 2533−2535. Complete contact information is available at: (17) Kamimura, A.; Yamamoto, S. A Novel Depolymerization of https://pubs.acs.org/10.1021/acssuschemeng.0c04190 Nylons in Ionic Liquids. Polym. Adv. Technol. 2008, 19, 1391−1395. (18) Yamamoto, S.; Kamimura, A. Preparation of Novel Function- Author Contributions alized Ammonium Salts that Effectively Catalyze Depolymerization of Nylon-6 in Ionic Liquids. Chem. Lett. 2009, 38, 1016−1017. The manuscript was written through contributions of all (19) Kamimura, A.; Shiramatsu, Y.; Kawamoto, T. Depolymerization authors. All authors have given approval to the final version of of Polyamide 6 in Hydrophilic Ionic Liquids. 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