Depolymerization of waste poly(methyl methacrylate) scraps and purification of depolymerized products

Journal of Environmental Management 231 (2019) 1012–1020

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Research article

Depolymerization of waste poly(methyl methacrylate) scraps and purification of depolymerized products

T

Chirag B. Godiyaa,∗, Serena Gabriellia, Stefano Materazzib, Maria Savina Pianesic, Nicola Stefaninia, Enrico Marcantonia,∗∗ a

Chemistry Division, School of Science and Technology, University of Camerino, Via. S. Agostino 1, 62032, Camerino, MC, Italy Department of Chemistry, Sapienza University of Rome, P.le A. Moro 5, 00185, Rome, Italy c Delta Srl, Via Tambroni Armaroli, 2, 62010, Montelupone MC, Italy b

A R T I C LE I N FO

A B S T R A C T

Keywords: PMMA depolymerization Pyrolysis MMA recycling Dissolution/re-precipitation 2,3-butanedione

A big challenge for the civilization in energy saving/waste management can be “the regeneration of monomers from the waste plastics followed by their re-polymerization” using an ideal recycling method. Herein, we investigate the thermal depolymerization of poly(methyl methacrylate) (PMMA) using thermogravimetric analysis coupled with mass spectrometry (TGA-MS). In this process, the polymer chains were decomposed to methyl methacrylate (MMA) in high yield and the degradation species were thoroughly characterized. The obtained MMA contained traces of byproducts. Firstly, the byproducts were found to be nonpolymerizable, secondly, their presence interrupt the polymerization reaction, and thirdly, they reduce the quality of re-polymerized PMMA (rPMMA). This study reclaims that besides the main byproduct (methyl isobutyrate), traces of methyl pyruvate and 2,3-butanedione were also formed during the thermal depolymerization of PMMA. The formed 2,3-butanedione was found to be responsible for the unpleasant smell in the recovered MMA that also found itself in the r-PMMA. Further, the generated byproducts were eliminated from the r-PMMA by a dissolution/re-precipitation method. The structural characterizations of the recycled and purified PMMA were carried out by Fouriertransform-infrared spectroscopy (FT-IR), Hydrogen-1 (1H)- and Carbon-13 (13C)-nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC) and gel permeation chromatography (GPC). The chemical properties of the r-PMMA and purified PMMA proved to be similar to that of the virgin commercial PMMA. This study can provide an effective and practical prototype for the recycling of waste PMMA scraps and thus reduction in pollution caused by the landfilling of waste PMMA scraps.

1. Introduction The worldwide rapid industrialization and population growth have exponentially increased the production and consumption of plastics. Continuous innovation explains that the current plastic production has been increased by ∼62 times (∼311 million tons) on a global basis as compare to the year 1950 (∼5 million tons) (Owusu et al., 2018). This growing production and consumption of plastics has created serious social and environmental arguments in the disposal of their wastes. Then a question arrives what could be done with the plastic wastes. Some countries have better solutions for the plastic wastes management, while, in some countries the plastic wastes end up with landfilling. The landfilling of plastic wastes is considered to be no value. In the modern world, reasonable and more efficient alternatives are

∗

required to effectively manage the plastic wastes from the environmental conservation point of view. Recycling can be a preferred option. Among the different types of plastics, several plastics on the mark are not truly recyclable due to the wide variation in the chemical properties and composition, which makes the recycling problematic. Once those plastics made, they cannot be completely reconstructed into its original monomeric state without forming any unwanted byproducts. For such plastics, energy recovery by pyrolysis can be an alternative option (Anuar Sharuddin et al., 2017). While, some plastics can be transformed into its original monomeric state by chemical or thermal processes, e.g., poly(methyl methacrylate) (PMMA) (Acosta et al., 2015; Braido et al., 2018; Kikuchi et al., 2014). PMMA is a type of thermoplastic used throughout the world in various applications, such as, in the transparent all-weather sheets,

Corresponding author. Corresponding author. E-mail addresses: [email protected] (C.B. Godiya), [email protected] (E. Marcantoni).

∗∗

https://doi.org/10.1016/j.jenvman.2018.10.116 Received 13 August 2018; Received in revised form 21 October 2018; Accepted 31 October 2018 0301-4797/ © 2018 Elsevier Ltd. All rights reserved.

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2. Experimental

electrical insulation, bathroom units, automotive parts, surface coating, and ion exchange resins. Also PMMA is being the basis of many material families including Perspex®, Plexiglas® and Lucite®. The global market size of PMMA can be exceed to USD 11 billion by 2022 (Pereira et al., 2018). The extensive use, high production cost and to conserve the environment from the landfilling of PMMA, has encouraged the recycling of PMMA from the waste PMMA scraps. Thermal degradation of PMMA can lead to the economic feedstock recycling because of its specific degradation behavior to yield its monomer methyl methacrylate (MMA) (Braido et al., 2018; Rahimi and García, 2017). Recycling of PMMA by pyrolysis has been recognized as the widely applied method in industries, because of its flexibility concerning the quality and relatively low cost (Senthil Kumar et al., 2015; Szabo et al., 2011). The liquid product obtained by pyrolysis of PMMA mainly consists, MMA which can directly be used for the re-polymerization for the production of new materials (Braido et al., 2018). Pyrolysis is a thermally initiated chemical process that generally decompose the macromolecules to monomers by depolymerization in an inert atmosphere (Szabo et al., 2011). Pyrolysis-thermogravimetric analysis-mass-spectrometry (Py-TGA-MS) has made a great stride toward becoming a powerful tool for the structural characterization of polymeric materials and identification of evolved degradation species in both conventional qualitative and quantitative analysis (Chetehouna et al., 2015; Danon et al., 2015; Gianotti et al., 2017). For instance, Wang and co-workers have used the TGA-MS technique to study the pyrolysis behavior and characterization of the green microalgae. Using this technique, they identified the optimal growth phase of the microalgae for the production of large amount of biomass and bio-oil (Wang et al., 2018). Ozsin and co-workers have utilized the TGA-MS technique in the food processing industry for the investigation of the produced bio-waste during the food processing (Özsin and Pütün, 2017). Besides, Gunasee and co-workers have used the TGA-MS technique in the pyrolysis of the municipal solid waste for the investigation and partiallyquantification of the extent of synergistic effects during pyrolysis and combustion process (Gunasee et al., 2016). Also Davies and co-workers have used TGA-MS technique for the qualitative analysis of tobacco biomass by pyrolysis. Using this technique they identified various components in the tobacco biomass (Davies et al., 2018). Herein, we employed the TGA-MS technique to study the depolymerization process of PMMA. The nature of organic byproducts formed during the thermal depolymerization and their effects in the recycled products were thoroughly investigated. Solid phase microextraction-gas chromatography-mass spectrometry (SPME-GC/MS) was employed to determine and characterize the volatile organic compounds (VOCs) in the re-polymerized PMMA (r-PMMA) (PMMA prepared from the recycled MMA). Structural characterizations were carried out by Fouriertransform-infrared spectroscopy (FT-IR), Hydrogen-1 (1H)- and Carbon13 (13C)-nuclear magnetic resonance (NMR) spectra. The glass transition temperatures (Tg) and average molecular weights (Mw) were obtained by the differential scanning calorimetry (DSC) and gel permeation chromatography (GPC), respectively. The r-PMMA contained some undesirable characteristics, such as, higher amount of residual monomer, lower Tg and, lower Mw compare to that of the virgin PMMA. Besides, even more important for an industrial application, the recycled MMA/PMMA contained disagreeable smell. The knowledge of the structure of the molecules responsible for this undesired effect has allowed us to develop a method based on the dissolution/re-precipitation for the elimination of by-products from the r-PMMA. The structural properties of the PMMA obtained after the dissolution/re-precipitation process (purified PMMA) found to be similar with that of the commercial virgin PMMA. Adopting all these data, an effective understanding of the depolymerization process of PMMA is provided. Thus a means of whereby PMMA scrap may be economically recycled in the form of monomeric MMA with high purity.

2.1. Materials Poly(methyl methacrylate) (PMMA) (bulk density ∼ 1.20 g/cm3) and methyl methacrylate (MMA) were purchased from Madreperla S.p.A., Spain. The solvents, such as, toluene, xylene, methanol, dichloromethane (DCM), cyclohexane, acetone, n-hexane and deuterated dichloromethane (CD2Cl2) were reagent grade and purchased from Sigma Aldrich. The re-polymerized PMMA (r-PMMA) used in this study was prepared by the free radical polymerization of the pyrolyzed MMA as per the reported method (Zhang et al., 2011). Potassium bromide (KBr) and the MS standards: methyl methacrylate, methyl isobutyrate, methyl pyruvate, 2,3-butanedione were > 99% pure and purchased from Sigma Aldrich. 2.2. Methods 2.2.1. Depolymerization of PMMA by pyrolysis-thermogravimetric analysismass spectrometry (Py-TGA-MS) The depolymerization of PMMA experiment was conducted by PyTGA-MS [PerkinElmer TGA7 equipment, STD 2960 simultaneous DTATGA apparatus (TA Instruments Inc., USA) using sealed crucibles with a pinhole on the top] as per the previously described method (Rusu et al., 2015). Briefly, the solid PMMA sample (7–8 mg) was placed in a small platinum crucible with a circular base (6 mm diameter x 3 mm height) in the TGA-MS and heated up to 450 °C at the heating rate 10 °C/min, under N2 atmosphere (flow rate 100 mL min−1) and a scanning rate of 5 °C min−1. The solid weight loss, together with other process variables such as temperature and gaseous species detected by the MS were continuously monitored by a quadrupole mass spectrometer equipped with Channeltron detector (EI, 70 eV), through a heated 100% methyl deactivated fused silica capillary tube. 2.2.2. Purification of r-PMMA The byproducts remained unpolymerized in the r-PMMA were eliminated by the widely applied dissolution/re-precipitation technique using DCM as a solvent and n-hexane as a non-solvent (Stejskal et al., 2014; Zhao et al., 2018). Briefly, the r-PMMA (2.0 g) was dissolved in CH2Cl2 (10 mL) and this solution was slowly poured into n-hexane (30 mL) with continuous stirring. After the re-precipitation of PMMA in an acceptable form, the phase separated solvents were analyzed by GCMS to detect the presence of any organic molecules separated from the r-PMMA. The purified PMMA was washed several times with n-hexane, filtered and dried overnight in an oven at 70 °C. 2.2.3. Other characterizations The extraction of the VOCs was performed by SPME using the divinylbenzene/carboxen/polydimethylsiloxane fiber (DVB/CAR/PDMS, 50/30 μm, grey fiber) purchased from Supelco (Bellefonte, PA, USA). The analyses were performed on PMMA (50 mg) samples in a 4 mL screw cap vial (Agilent Technologies, Santa Clara, CA, USA), at 70 °C, by equilibrating and extracting the samples for 30 min, respectively. Then, the analyses were accomplished by GC-MS (GC-MS 6850, Agilent Technologies, equipped with a single 5973 quadrupole mass spectrometer detector). The capillary chromatographic column [30 mt. (length) × 0.25 mm (inside diameter) × 0.25 μm (film thickness)] was HP-5MS from Agilent Technologies. Desorption was performed in a split-less mode for 4 min at 250 °C. GC-MS analysis: Oven temperature was held at 40 °C for 3 min, then raised to 300 °C at 15 °C min−1 and held for 8 min. The initial carrier gas (helium) flow rate was 2 mL min−1. The mass analyses were performed in the scan mode in the range of 29–400 Da. No solvent delay was set. The transfer line, ion source and quadrupole were maintained at 300 °C, 230 °C and 150 °C, respectively. The detected VOCs in the PMMA samples were identified by comparing their mass spectra with those of authentic standards, with 1013

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presented a small thermal event (∼2 wt% loss) between 0-100 °C. This event was indicative of the inherent moisture within the material being released (Nampoothiri et al., 2017). The proceeding largest peak was the most significant representing the greatest weight loss. The virgin PMMA and r-PMMA exhibited a single peak, denoting the thermal decomposition in the temperature range 344–430 °C and 310–430 °C, respectively. Earlier it was reported that PMMA exhibits two or three smaller steps of degradation at ∼200 °C and ∼280 °C, respectively (Hirata et al., 1985; Nampoothiri et al., 2017; Xu et al., 2004). The MS inlet was closed until 200 °C to keep the CO2, CO, CH4 and other gas compositions out from the MS. Fig. 2 shows the mass spectra of the products collected when a PMMA sample was heated to 450 °C. The Mass spectra clearly showed a large amount of MMA (m/z 100 Da), together with traces of organic species with greater and lower masses than 100 Da. Earlier it was reported that during the thermal degradation of PMMA, traces of byproducts forms with m/z greater than 100 Da along with MMA (Grause et al., 2006; Kang et al., 2008; Ohno et al., 1987). In particular, the byproduct with m/z 102 Da was recognized as methyl isobutyrate (Kang et al., 2008). The depolymerization reaction of PMMA is a free radical chain process, which is well-established and the formation of methyl isobutyrate is related to the terminal part of the chain (Ohno et al., 2014). As mentioned before the MMA/r-PMMA contained unpleasant smell, which could be directly correlated to the formed impurities during the depolymerization. However, the presence of methyl isobutyrate cannot be considered to be responsible for the unpleasant smell in the recycled MMA/r-PMMA, because it is a naturally occurring compound of the strawberry aroma and it is a member of the fragrance structural group aryl, alkyl, alcohols, acid and, esters (Garlapati and Banerjee, 2013). Furthermore, we also focused our attention to the products in the chromatogram of m/z lower than 100 Da. The most abundant peak was m/z 86 Da, which was assigned to 2,3butanedione (diacetyl) (Liu et al., 2015). Also its structural assignment was confirmed against standard sample. Earlier it was reported that 2,3butanedione contains unpleasant smell (Díaz et al., 2004). Recently 2,3butanedione showed useful application in trickling the mosquito olfactory receptors for mosquito control strategies (Potter, 2014; Tauxe et al., 2013). Similarly, in a very small amount it has been used as an additive in food as well as food flavouring compound (Lima Jedlicka et al., 2015; Song and Liu, 2018). For example, 2,3-butanedione was found in the gluten-free breads which was formed during the fermentation process of dough (Pico et al., 2017). The identification of 2,3-butanedione was an interesting result, and it was important to understand its formation during the pyrolysis of PMMA. The intermediates formed during the degradation were in trace amounts and may have short lifetimes. In this regard, the analytical technique for the detection and identification of such intermediates needs to be rapid and sensitive, in order to avoid the loss of valuable information on the transformation pathways. Significant improvement in both speed and sensitivity can be obtained by the solid-phase microextraction (SPME) technique. This technique allows identification of VOCs with unambiguous detections of degradation products of polymers including a variety of low molecular weight compounds (Curran and Strlič, 2015; Grafit et al., 2018).

the reference spectra available in the database of US National Institute of Standards and Technology (NIST), and with the linear retention indices available from the NIST Chemistry Web Book (NIST2011). The FT-IR spectra were recorded on the FT-IR spectrophotometer (Perkin-Elmer, UATR-2) in the range 450 cm−1 to 4000 cm−1. Each sample was scanned in triplicate. A commercial software Spectrum quant v10.5.0.560 (Perkin-Elmer FT-IR C102493) was used to process and calculate the wavenumber from the spectra. The 1H and 13C-NMR spectra were recorded on a Varian Mercury plus 400 system at 400 MHz and 100 MHz frequency, respectively, in CD2Cl2 at an ambient probe temperature (ca. 25 °C). The following abbreviations were used; s = singlet, d = doublet, t = triplet, q = quartet, quint. = quintet, dd = double doublet, dt = double triplet, tt = triple triplet, and m = multiplet. Residual monomer analyses were determined on a GC [HewlettPackard (HP) 5890A] equipped with a MS (HP 5971A). GC-MS program: ∼1.0 μL sample was injected using a splitless injection. The oven temperature was held at 40 °C for 3.0 min, then increased up to 300 °C at 15 °C min−1 rate and held for 8.0 min. No solvent delay was set. For the MS detection, the transfer line, ion source, and quadrupole were maintained at 300 °C, 230 °C and 150 °C, respectively. The mass fragments were detected between 29-400 Da. After each analysis a blank sample (acetone) was recorded to flush the column. The thermal properties were measured on a differential scanning calorimetry (DSC) (DSC 200 F3 Maia, from NETZSCH). The temperature axis and measured enthalpy were calibrated by using pure indium following the universal calibration technique. The sample (10–15 mg) was kept in an aluminium crucible and placed at the appropriate position in the instrument. The heat release was recorded at a temperature interval 25–250 °C by the scan rate of 10 K min−1 to ensure the complete polymerization of the traces of residual monomer. After the sample was cooled down to 25 °C, was again heated up to 250 °C at a scan rate of 10 K min−1. The data obtained from the second heating/ scanning were accepted for the measurement of glass transition temperature (Tg). The Tg was considered at the point where a change in the slope of the curve was observed. The average molecular weight (Mw) and polydispersity index (PDI) were determined by the Gel Permeation Chromatography (GPC) (Thermo Knauer) using a differential refractive index detector, and three columns (5 μm, 500 Å, 7.5 cm x 300 mL; 5 μm, 1000 Å, 7.5 cm x 300 mL; 5 μm, 10000 Å, 7.5 cm x 300 mL) in a series. All the samples were dissolved in 7:3 = THF:DMF solution at a constant concentration of 0.2 wt%. After filtration, 25 μL of each sample was injected into the chromatograph. The elution solvent was THF at a constant flow rate of 1.0 mL min−1. Calibration of GPC was carried out using standard polystyrene samples following the universal calibration technique. 3. Results and discussion 3.1. Depolymerization of PMMA The depolymerization experiments of the PMMA samples were carried out from 25 to 450 °C (heating rate 10 and 20 °C min−1) under an inert atmosphere on a Py-TGA-MS system. Fig. S1 of the supporting information illustrates the flow diagram of the Py-TGA/MS system, where the PMMA samples were heated in an electric oven and the solid weight loss was monitored by TGA. The released gaseous phase volatiles were transferred to the MS via a heated line. Thermogram of both the virgin PMMA and r-PMMA were represented with their derivative weight loss in Fig. 1a-b. The mass loss rates of the PMMA samples showed a similar trend under both the pyrolyzing heating rates employed. The TGA-MS thermogram for the virgin PMMA showed major thermal events occurring between 344 and 430 °C, with the representative peak of weight loss at ∼404 °C. While the r-PMMA showed degradation at slightly lower temperature, between 310-430 °C, with the representative peak of weight loss at ∼384 °C. Both the PMMAs

3.2. Formation of methyl pyruvate during the depolymerization of PMMA Thermal degradation of PMMA has widely been studied due to its industrial value (Braido et al., 2018; Kang et al., 2008). It is exclusively advanced mechanism followed via radial intermediates and has been investigated even at 95 °C (Bennet et al., 2010). It was generally accepted that the degradation of PMMA initiates by homolytic scission of a methoxycarbonyl side group by a random scission degradation (Bhargava et al., 2016; Manring, 1991). The main chain scissions were kinetically inhibited and were related to the side group or chain end scissions due to the efficient recombination of caged radicals (Manring et al., 1989). Regardless of the initial step of the mechanism, the 1014

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Fig. 1. TGA/DTG thermogram of (a) pure PMMA and (b) recycled PMMA.

equivalent amount of tertiary radicals (Scheme 1). During the degradation process in the system a number of small radicals can be formed via disproportionation, evaporation and integration with other radicals, as shown in Scheme 2. The second primary radical formed does not depolymerize efficiently. Therefore, different mechanisms have been proposed to describe the degradation of those radicals and to predict the different products obtained during the process (Fateh et al., 2016; Holland and Hay, 2001; Kashiwagi et al., 1989). The depolymerization by chain and end chain scission, formed MMA until the chain terminus reached as well as formed byproducts by disproportionation (Manring, 1991). Also the mechanism of PMMA depolymerization and generation of byproducts during, depends on the degradation temperature, initial molecular weight of PMMA and presence of oxygen. (Smolders and Baeyens, 2004). The second order reactions of carbon centred terminal radicals with traces of O2 present in the system can form different products at a high temperature (Scheme 2). Later the PMMA units can undergo to OeO bond forming scission to form methyl pyruvate which was the major oxidation product (Chiantore et al., 1989; Mukundan and Kishore, 1987) (Scheme 2). Finally, during the pyrolysis process methyl pyruvate can be transformed into the corresponding 2,3-butanedione as shown in Scheme 3. The first step of this pathway was the condensation of two methyl pyruvate molecules and concomitant decarboxylation to form methyl 2-acetolactate. The subsequent oxidative decarboxylation leads to the formation of 2,3-butanedione (Zhang et al., 2015) without passing through the formation of acetoin. The later step of the reaction can

Fig. 2. MS spectra of PMMA degradation obtained from TGA-MS.

exclusive degradation product obtained was MMA monomer. At high temperature (above 300 °C) PMMA becomes thermally instable, leading to the fragmentation of tertiary alkyl radicals to yield monomer and

Scheme 1. Thermal degradation of PMMA. 1015

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Scheme 2. Reaction of carbon centred terminal radicals with oxygen.

be the catalysis by the presence of trace metals (Mohr et al., 1997).

Table 1 SPME fibers behavior: VOCs detected by GC-MS in PMMA.

3.3. Qualitative analysis of volatile compounds by SPME-GC/MS

Product

Further analysis of the VOCs in the virgin PMMA, r-PMMA and purified PMMA was determined by SPME-GC-MS, by selecting the major analytes presented in the samples. The purpose of the SPME-GCMS analysis was to confirm the compound obtained (m/z 102 Da) during the depolymerization of PMMA, which was believed to be methyl isobutyrate. All the principal detected compounds through the extraction technique are summarized in Table 1. It is interesting to note that the analysis of VOCs of all the three polymer samples, showed the presence of organic compounds, including the monomer. Among all the three PMMAs, in r-PMMA the presence of 2,3-butanedione was observed, which confirmed the obtained compound at m/z 86 Da was 2,3butanedione during the thermal degradation. The gas chromatogram of r-PMMA also showed the presence of another molecular ion with mass m/z 102 Da. The electron ionization mass spectra fragmentation study of this compound (m/z 102) confirmed unambiguously that the present compound was methyl pyruvate. It is also important to note that the presence of methyl pyruvate was observed only in r-PMMA, and not in virgin and purified PMMAs. The SPME-GC-MS analysis confirmed that during the thermal depolymerization of PMMA, 2,3-butanedione and methyl pyruvate were formed together as byproducts along with MMA. It can also be assume that the unpleasant acrid smell which was a typical characteristic of the pyrolyzed MMA and r-PMMA, was due to the presence of 2,3-butanedione, that formed during the depolymerization of PMMA.

Structure

Methyl methacrylate

Dimethyl 2-methyl-5-oxohexanedioate

2-Hydroxyethyl methacrylate

Methyl pyruvate

2,3-butanedione

Ethane-1,2-diyl bis(2-methylacrylate)

agglomeration form and was analyzed by the SPME-GC/MS for the confirmation of VOCs/byproducts. Also the filtrate was tested by GCMS for the analysis of soluble components of r-PMMA, as well as the residual monomer. In the phase-separated solvent (mixture of DCM and n-hexane) obtained after the dissolution/re-precipitation treatment, 569.2 ppm residual monomer was detected (Table S2 of the supporting information). When we applied the dissolution/re-precipitation treatment on the purified PMMA, and analyzed the phase separated solvent by GC-MS, no presence of residual monomer was detected (Table S2 of the supporting information), confirming the high-efficiency of the dissolution/re-precipitation technique. The comparison of the efficiency of the dissolution/re-precipitation purification technique with other reported techniques used for polymers purification in literature are listed in Table 2. Various purification techniques, such as protonation/deprotonation (Humpolicek et al., 2012), membrane filtration (Brocken et al., 2017), diafiltration (Tishchenko et al., 2001), recycling size exclusion chromatography (Ashraf et al., 2013), and Soxhlet extraction (Mitchell et al., 2016) were proved to be highly efficient for the purification of various polymers. However, herein our experiments, due to the specific properties of PMMA and the best availability of facility, we choose the dissolution/re-precipitation technique for the purification of r-PMMA. Based on further structural characterization and the results of the residual monomer analysis of the r-PMMA and purified PMMA available in the following sections, the dissolution/re-precipitation technique proved to be highly efficient for the purification of r-PMMA.

3.4. Purification of r-PMMA by dissolution/re-precipitation The purification process of r-PMMA, involved the dissolution/reprecipitation in an appropriate solvent/non-solvent system. Approach to the purification of r-PMMA included no change in the structure of purified PMMA. Solvents studied for the dissolution of PMMA include toluene, xylene, dichloromethane and acetone, whereas non-solvents, methanol, n-hexane and cyclohexane were tested. The main criteria for the selection of solvents and non-solvents were their solubility parameter, availability, cost, toxicity, color, and viscosity. The physical properties of the solvents and non-solvents were shown in Table S1 of the supporting information. The solvent/non-solvent combination of toluene/methanol, xylene/cyclohexane, and acetone/hexane had limitation based on the partial solubility of PMMA or the re-precipitation of PMMA was as lumps. It is worthwhile to mention here that the preliminary experiments were aimed to achieve the re-precipitation of the r-PMMA in an acceptable form i.e., to exclude the formation of jelly polymer lumps which can prohibit the recovery of the purified PMMA as well as the solvents. On the basis of the above-mentioned criteria DCM/n-hexane proved to be the most satisfactory solvent/non-solvent system. The PMMA obtained after the dissolution/re-precipitation in the DCM: n-hexane solvent: non-solvent system was in an

Scheme 3. Stepwise transformation of methyl pyruvate to 2,3-butanedione.

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Table 2 Purification method for the polymers in this work in comparison to the published methods. Purification method

Efficiency

Purified polymer(s)

Reference

Dissolution/re-precipitation Protonation/deprotonation

High High

PMMA Polyaniline

Membrane filtration Dialysis Diafiltration

High Low High

Poly(acrylic acid) Poly(acrylic acid) Polymer nanoparticles emulsions

Recycling size exclusion chromatography Soxhlet extraction

High

Indacenodithiophene- co –benzothiadiazole and germaindacenodithiophen- co –benzothiadiazole polymers Poly(3-hexylthiophene)-b- poly(2,7-(9′,9′-dioctylfluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′benzothiadiazole) (P3HT-b-PFTEGTBT) and P3HT-b-PFTEGT6BT polymers

This work Humpolicek et al., (2012) Brocken et al., (2017) Brocken et al., (2017) Tishchenko et al., (2001) Ashraf et al., (2013)

High

Mitchell et al., (2016)

respectively (Erbas Kiziltas et al., 2015). The CeC stretching bands were appeared between 981-744 cm−1 (Erbas Kiziltas et al., 2015). The FT-IR results suggested that the chemical structure of r-PMMA and purified PMMA were intact and same as the virgin PMMA. 3.6. 1H and

13

C-NMR

The compositions of the PMMAs were deduced from their 1H and C-NMR spectra by comparing the integral areas of appropriately assigned signals. The 1H and 13C-NMR resonances of virgin PMMA, rPMMA and purified PMMA in CD2Cl2 with probable assignments were depicted in Fig. 4A and 4B, respectively. Various characterization data for these polymers from NMR spectroscopy were listed in Table 3. As shown in Fig. 4A, in the 1H NMR, for all the PMMAs, the resonance band at 3.58–3.63 ppm was attributed to the methyl proton connected to the ester group (Crispim et al., 2000). The proton resonance absorptions of the methylene and substituted methyl groups were appeared at 1.45–1.89 ppm and 0.8–1.18 ppm, respectively (Crispim et al., 2000). The r-PMMA and purified PMMA demonstrated similar spectrum to that of the virgin PMMA. In the 13C NMR (Fig. 4B), five carbons of the PMMA were appeared at 16.6–19.0, 44.6–45.0, 51.83–53.9, 54.3–54.5 and 177.1–178.2 corresponding to the C-1, C-2, C-3, C-4 and C-5 of the PMMA molecule, respectively (Endo et al., 2012). The chemical shifts of the r-PMMA and purified PMMA were identical to that of the virgin PMMA and in good agreement with the obtained values. Thus, 1H and 13C-NMR results, further suggested that the structures of the r-PMMA and purified PMMA were intact and same as that of the virgin PMMA. 13

Fig. 3. FT-IR spectra of (a) virgin PMMA, (b) r-PMMA, and (c) purified PMMA.

3.5. Fourier transform infrared spectroscopy In order to confirm the chemical structure of r-PMMA and purified PMMA compared with the virgin PMMA, their FT-IR spectra were recorded as presented in Fig. 3. The main characteristic peaks were also listed in Table 3. The appearance of the absorption bands at ∼3000 cm−1 and ∼2957 cm−1 were assigned to the stretching vibrations of the eCH3 groups (Bergamonti et al., 2018; Godiya et. al., 2019). The absorption frequency at ∼1722 cm−1 was a characteristic band for the carbonyl stretching (C]O) (Anju and Narayanankutty, 2017). The bands at ∼1431 cm−1 and ∼1385 cm−1 were attributed to the CeH asymmetric and symmetric stretching modes of α-CH3, respectively (Anju and Narayanankutty, 2017; Bergamonti et al., 2018). The bands at ∼1240 cm−1 and ∼1142 cm−1 were assigned to the CeO stretching and skeletal vibrations coupled to the CeH deformations, Table 3 FT-IR, 1H and

13

3.7. Differential scanning calorimetry (DSC) The results obtained of the structure and properties of virgin and rPMMAs, were further confirmed by their glass transition temperature (Tg). The results of Tg were shown in Table S2 of the supporting information. No residue was found in the pan after the complete degradation of PMMA samples. The DSC heating curves of all the PMMAs

C-NMR assignments of PMMAs. 1

FT-IR

H-NMR

13

C-NMR

Vibration

Wavenumber (cm−1)

Assignments

Resonance (ppm)

Assignments of protons

Resonance (ppm)

Off resonance splitting

Stretching Stretching Stretching

∼3000, ∼2957 ∼1722 ∼1431

Methyl groups Carbonyl CeH asymmetric

0.98, 0.8 (S, 3 H) 1.45-1.89 (S, 2 H) 3.58-3.63 (S, 3 H)

Substituted methyl Methylene proton Methyl (connected to ester group)

16.67, 18.87 (s, 1 C) 44.75-45.1 (d, 1 C) 51.91 (m, 1 C)

α- CH3 -C-OCH3

Stretching Stretching

∼1385 ∼1240 and ∼1142

54.67 (s, 1 C) 177.1–178.31 (dd, 1 C)

-CH2C=O

Stretching

981–744

CeH symmetric CeCeO and skeletal coupled to CeH deformations CeC

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Fig. 4. (A) 1H- and (B)

13

C-NMR spectra of a) pure PMMA, b) r-PMMA and c) purified PMMA.

obtained by GPC. The results of GPC analysis were shown in Table S2 of the supporting information. The number average molecular weights (Mn) of the virgin PMMA, r-PMMA and purified PMMA were 6.6 × 104, 6.1 × 104, and 6.08 × 104 g/mol, respectively. The Mn of the r-PMMA and purified were found to be slightly lower than that of the virgin PMMA. Therefore, the decreased Tg (as mentioned in the previous section) for the r-PMMA and purified PMMA could be correlated to the lower Mw of the r-PMMA and purified PMMA compared to the virgin PMMA. This in turn can be explained that the presence of impurities into the spaces makes hindrance for the movement of active macroradicals and thus interrupts the polymerization process, which leads to the production of macromolecules with lower Mw.

were depicted in Fig. S2 of the supporting information. The Tg of virgin PMMA was appeared between 104-108 °C, which was close to the reported value of Tg −106.5 °C for the virgin PMMA (Porter and Blum, 2000). While the Tg of the r-PMMA was appeared between 95-98 °C, which was ∼10 °C lower than that of the virgin PMMA. The Tg of the purified PMMA was also similar to that of the r-PMMA. It can be explain that during the repolymerization of recycled MMA the present byproducts inhibits the polymerization reaction and leads to the production of macromolecules with lower average molecular weight (Mw) and consequently lowers the Tg of the PMMA.

3.8. Gel permeation chromatography (GPC) The molecular weight distribution and Mw of the r-PMMA and purified PMMA were compared with that of the virgin PMMA as 1018

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Table 4 Economic parameters.

Cost for collecting waste PMMA scraps Cost for electricity Cost for analytical facility Total cost of solvents Cost of solvents recovery Working time Direct manpower cost Overall manpower cost

Value

Unit

Ref.

– ∼21.4 – ∼312.5 – 8 ∼12.6 ∼3030

– Euro/kWh – Euro – h/day Euro/h Euro/month

Collected from our partner industry Price in Italy in the year 2018 We used all analytical facility in our lab or our partner industry Sigma Aldrich All the solvents were recycled in our lab – Average total cost for the manpower worked in this study The whole framework of this study was completed within a month

byproducts. The physical and chemical properties of the obtained rPMMA and purified PMMA were found to be similar to that of the commercial virgin PMMA as characterized by FT-IR, 1H-NMR, 13CNMR, DSC and GPC. We believe this investigation can encourage the PMMA recycling from the waste scraps in the organizations that manufacture PMMA based products.

3.9. Residual monomer analysis The determination of the amount of unreacted monomer in all the PMMA samples was carried out by following the aforementioned dissolution/re-precipitation technique. The results of the obtained residual monomer were listed in Table S2 of the supporting information. The amount of residual monomer in the pure and r-PMMAs were 102.9 and 569.2 ppm respectively, while no residual monomer was observed in the purified PMMA. The absence of residual monomers in the purified PMMA confirmed the high efficiency of the dissolution/re-precipitation purification process.

Acknowledgement The work was carried out under the frame work of strategic University FAR project (UNICAM). The authors are thankful to Delta, Montecassiano (MC) and Marche Region for the doctoral fellowship for C.G., and N. S. gratefully acknowledges Elantas Europe Ascoli Piceno Plant for a doctoral fellowship.

3.10. Economical analysis The demand of recycled materials can be increased in industries if there are economic advantages of using so. First, the recycled materials must be available in enough quantities and, second, the adequate quality of the recycled materials, which must not change or negatively effect on the properties of the final products. Considering these factors, it is important to obtain and provide a sufficient quantity of highquality recycled materials to the manufacturers at a price that is competitive with the virgin materials. Therefore, the value of the recycled material and the transaction costs of recycling along with the value chain is an important factor. As mentioned before, the PMMA is largely used polymer and can be recycled in a great purity which can directly be applied for the production of new materials. So, using the recycled PMMA, the above mentioned criteria may well be satisfied. This study is also concerned with the economical aspects associated with the recycled PMMA. Firstly, the recycling method presented here needs high-temperature in an inert atmosphere which may cheaper and faster manner to perform the recycling of PMMA. Secondly, we study how the primer purity of the recycled MMA influences on the quality of r-PMMA. Thirdly, we eliminated the impurities from the r-PMMA. Table 4 shows the total estimated cost of the recycling process of PMMA up to the stage we obtained the purified PMMA. The major costs for the PMMA recycling can be the electricity or fuel for the heating apparatus, analytical facility, and the organic solvents for the purification of rPMMA. All the organic solvents used in this study were effectively recycled for the further use. We believe that by the optimization of this study in a large scale, the cost of the recycled PMMA can be competitive with the commercially available virgin PMMA.

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