Liarou solid wastemanagment

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  • 1. Protection and restoration of the environment XI Solid waste management ENVIRONMENTALLY FRIENDLY CHEMICAL RECYCLING OF POLYESTERS (PET, PPT) USING ALKALINE HYDROLYSIS UNDER MICROWAVE IRRADIATION A. Liarou, G.Z. Papageorgiou and D. S. Achilias* Department of Chemistry, Aristotle University of Thessaloniki, 54124, Thessaloniki, Greece*Corresponding author: E-mail: axilias@chem.auth.gr, Tel +30 2310 997822, Fax: +302310997769 ABSTRACTRecently, a new polyester, namely poly(propylene terephthalate), PPT, has been put on the marketunder the brand name Corterra™ to replace PET mainly in the production of fibers. This polymerhas extensive applications in carpeting, textiles and apparel, engineering thermoplastics, non-wovens, films and monofilaments since it combines the properties of nylon and polyester. In thisstudy an environmentally friendly way to recycle PPT is proposed using alkaline hydrolysis undermicrowave irradiation.Microwave irradiation as a heating technique offers many advantages over the conventional heatingsuch as instantaneous and rapid heating with high specificity without contact with the material to beheated. It is, therefore, a popular technique for heating and drying materials and is utilized in manyhousehold and industrial applications. The main advantage of microwaves over conventionalheating sources is that the irradiation penetrates and simultaneously heats the bulk of the material.Research efforts have thus lead to numerous applications in material processing techniques thathave resulted in shorter reaction times and greater convenience.Recycling of different grades of poly(propylene terephthalate) as well as PET is examined hereusing hydrolytic depolymerization in an alkaline solution, under microwave irradiation. The mainobjective was to provide a recycling method for PPT, using an environmentally friendly way (i.e.microwave irradiation instead of conventional heating) requiring thus lower reaction temperaturesand/or shorter reaction times with substantial energy saving. A final innovative part was theintroduction of a phase transfer catalyst during the depolymerization to facilitate further thereaction.The reaction was carried out in a sealed microwave reactor in which the pressure and temperaturewere controlled. Experiments under constant temperature were carried out at several time intervals.The main products were the monomers terephthalic acid (TPA) (obtained in pure form) andpropylene glycol, which were analyzed and identified. The depolymerised PPT residues were alsoanalyzed using DSC measurements. It was found that depolymerization is favoured by increasingtemperature, time and amorphous phase material.The results of this study confirmed that PTT waste can be successfully converted into usefulproducts using an eco-friendly recycling techniqueKeywordsRecycling; synthetic fibers; alkaline hydrolysis; poly(propylene terephthalate); microwaves. 1140
  • 2. Protection and restoration of the environment XI Solid waste management1. INTRODUCTIONAs it is well-known, the production and consumption of polymer-based materials has recentlyenormously increased. As a result, a large amount of polymers finds its way to wastes everyday. Therecovery of valuable products through the chemical recycling of polymers has been attractingattention in recent years for both environmental and economic reasons. In particular, new methodsare developed for the quantitative recovery of monomers in a short time using environmentalfriendly techniques. One of the major classes of polymers in the waste stream is that of polyesters[such as poly(ethylene terephthalate), PET]. Chemical recycling processes for polyesteres aredivided as follows (Karayannidis and Achilias, 2007; Scheirs, 1998; Sinha et al., 2010): (i)Glycolysis, (ii) Methanolysis, (iii) Hydrolysis and (iv) other processes: Glycolysis involves theinsertion of ethylene glycol units (or diethylene glycol and propylene glycol) in the polyester chainsto give bis (hydroxyalkyl) terephthalate (BHAT) which is a substrate for new polymer synthesis andother oligomers. The production of secondary useful products, such as alkyd resins, has also beenproposed. Methanolysis actually is the degradation of polymers by methanol at high temperaturesand high pressures with main products: dimethyl terephthalate (DMT) and ethylene glycol (EG).Hydrolysis of polyesters can be carried out in an acid, alkaline or neutral environment to producethe monomers terephthalic acid (TPA) and ethylene glycol (EG). The growing interest in thismethod is connected with the development of new factories for polyester (i.e. PET) synthesisdirectly from TPA and EG. Neutral Hydrolysis is carried out with the use of hot water or steam.Acid hydrolysis is performed most frequently using concentrated sulfuric, nitric or phosphoric acid.Alkaline Hydrolysis of PET is usually carried out with the use of an aqueous alkaline solution ofNaOH, or KOH of a concentration of 4–20 wt-% (Carta et al., 2003). The reaction products are EGand the disodium terephthalate salt TPA-Na2. Pure TPA can be obtained by neutralization of thereaction mixture with a strong mineral acid (e.g. H2SO4). The main advantage of this method is thatit can tolerate highly contaminated post-consumer PET such as magnetic recording tape, metallizedPET film, or photographic (X-ray) film.Concerning the chemical recycling of PET, a number of studies have been published (Sinha et al.,2010; Carta et al., 2003; Karayannidis and Achilias, 2007). However, little work has beenperformed on the depolymerization of a new polyester, poly(propylene terephthalate) (PPT) aimedat replacing PET in fibers’ production. PPT is an aromatic polyester made from thepolycondensation of 1,3-propanediol (1,3-PDO) with either terephthalic acid or dimethylterephthalate. It was first synthesized, just like poly(ethylene terephthalate) (PET) and poly(butyleneterephthalate) (PBT), in 1941 by Winfield and Dickson. However, despite its excellent properties, itbecame commercially available only recently because one of its raw materials (1,3-PDO) was veryexpensive and was available only as a small volume fine chemical. A recent breakthrough in thesynthesis of 1,3-PDO by Shell Chemical Co. at a much lower price via the hydroformylation ofethylene oxide gave also a boost in the production of PPT. PPT has an odd number of methyleneunits between the terephthalate moieties in its chemical structure in comparison with two commonhomologous polyesters, poly(ethylene terephthalate) and poly(1,4-butylene terephthalate) (PBT),and its molecule takes on an extended zigzag shape. Because of this special structure, PPT hasoutstanding resiliency, chemical resistance, and good thermal properties for fibers (mainly carpetfabrics) and engineering thermoplastics. PPT fibers exhibit high elasticity, excellent recovery rate,dye ability and stain resistance, high UV stability, low water absorption and low electrostaticcharging. Global commercial interest in PTT will expand capacity and end uses. An increase in theuses of PPT products will result in a greater amount of waste materials.The repeating unit of this macromolecule has the chemical structure: 1141
  • 3. Protection and restoration of the environment XI Solid waste management O CH2 CH2 CH2 O C C O O nMicrowave-assisted organic synthesis has revolutionized chemical research (Adam, 2003; Lidstromet al., 2001). Microwave irradiation, as a heating technique, offers many advantages overconventional heating, such as instantaneous and rapid heating with high specificity, without contactwith the material to be heated. It is, therefore, a popular technique for heating and drying materialsand is utilized in many household and industrial applications. The main advantage of microwavesover conventional heating sources is that the radiation penetrates and simultaneously heats the bulkof the material. Research efforts have thus led to numerous applications in material processingtechniques that have resulted in shorter reaction times and greater convenience.Although the use of microwave irradiation in chemical reactions is a rather well-establishedtechnique, the papers published on the recycling of polymers are very limited. Some papers havebeen published on the recycling of PET (Nikje and Nazari, 2006; Liu et al., 2005; Li et al., 2008;Krzan, 1998) and none on the use of microwave irradiation in the recycling of PPT. During the pastfew years hydrolysis of waste PET was investigated in our laboratory as potential method for thechemical recycling of soft drink bottles (Kosmidis et al., 2001). In addition, PET recycling undermicrowave irradiation was examined using hydrolysis, glycolysis and aminolysis (Achilias et al.,2010; 2011; Siddiqui et al., 2010).In this study, depolymerization of PPT, taken from a commercial product (i.e. Corterra™ from ShellChemicals), was subjected to alkaline hydrolysis in a lab-scale microwave reactor, in order to studythe effect of microwave irradiation on its degradation. This polymer has extensive applications incarpeting, textiles and apparel, engineering thermoplastics, non-wovens, films and monofilamentssince it combines the properties of nylon and polyester. The reaction was carried out in a sealedmicrowave reactor, in which pressure and temperature were controlled and recorded. The mainproducts were the monomers terephthalic acid (obtained in pure form) and propylene glycol, whichwere analyzed and identified. The effect of several process parameters, including the degree ofcrystallinity of the original polymer on the amount of PPT depolymerized and TPA recovery, wasinvestigated. The main objective was to provide a recycling method for PPT, using anenvironmentally friendly way (i.e. microwave irradiation instead of conventional heating), thusrequiring lower reaction temperatures and/or shorter reaction times with substantial energy saving.A final innovative part was the introduction of a phase transfer catalyst during the depolymerizationto facilitate further the reaction.2. EXPERIMENTAL2.1 MaterialsPPT used was supplied by Shell Co., Houston, TX under the Trade name CorterraTM. The chemicalsused were reagent grade. Amorphous PPT films were prepared by melt-pressing with an Otto WeberPW 30 hydraulic press at 250°C and under a load of 6 kN on a ram of 110 mm, followed byquenching in cold water. In addition, PET flakes were prepared from used clear PET bottles, fromwhich the labels and glue had been removed. The bottles were cut and fed to a rotary cutterproducing flakes with a maximum size of 6 mm. The phase transfer catalyst Hexadecyl TriMethylAmmonium Bromide (HDTMAB) was obtained from Aldrich. The chemical structure of thecatalysts is: 1142
  • 4. Protection and restoration of the environment XI Solid waste management CH3 H3C N+ C16H33 Br- CH32.2 Hydrolytic depolymerizationPPT and PET decomposition reaction was conducted in a microwave reactor (model Discover fromCEM corporation), equipped with a digital temperature control system and pressure sensors inserteddirectly into the 10 mL PTFE reaction tube. Pellets of sodium hydroxide (10 g) were dissolved in100 mL of distilled water and the resultant NaOH solution (2.5 M, 10%w/v) was used for theexperiments. Polyester flakes (0.5 g) together with 5 mL of NaOH solution were added into thereactor, sealed under inert atmosphere (N2) and the heat-up period to the desired set-point started. Inmost experiments 0.01 g of HDTMAB was also fed into the reactor. When the set temperature wasachieved the reaction time began and the polymer decomposition was followed for a specified timeperiod. After that time period, the reaction vessel was automatically cooled and the reaction mixturewas filtered to remove the unreacted polyester residues. The final unreacted polymer was measuredupon filtration of the final mixture through a G3 glass filter, washing with water, drying in avacuum oven at 40oC and weighing. The experiments were repeated using a second PPT sampleobtained in amorphous condition by initially melting the original commercial PPT sample followedby quenching.When PET is hydrolyzed in sodium hydroxide the disodium salt and ethylene glycol are produced,according to the following chemical reaction: O O H O CH2CH2 O C C n OH 2n NaOH O O n NaO C C ONa + n HOCH2CH2OH + H2OThe same reaction hold for PPT except that propylene glycol is produced instead of EG.The TPA-Na2 salt was continuously acidified with sulfuric acid, H2SO4 (10%) to a pH of 2.5 toprecipitate the TPA monomer. Finally, the mixture was filtered and washed with absolute ethanol.The solid TPA produced was dried in a vacuum oven at 40oC and weighed. O O O O NaO C C ONa + H2SO4 HO C C OH + Na2SO42.3 Analysis of the ResultsThe % yield in TPA was calculated using the formula: NTPATPA Yield (%)  100 (1) NTPA,0 1143
  • 5. Protection and restoration of the environment XI Solid waste managementwhere, NTPA and NTPA,0 refer to the number of moles weighed and the theoretical number of TPAmoles that will be produced upon complete decomposition of PET, respectively.The percent degradation of PPT (or similarly of PET) was calculated using the following equation: W PPT ,0  W PPT , fPPT Degradation (%)  100 (2) WPPT ,0where, WPPT,0 and WPPT,f refer to the initial and final weight of PPT, respectively.2.4 Product characterizationThe determination of purity of terephthalic acid was performed by titration with 0.5 N NaOHsolution. About 1 g of TPA is weighed to the nearest milligram into a 250 mL conical flask. Todissolve the sample 25 mL of analytical grade pyridine is added by pipette and the suspension isheated with a reflux condenser until a clear solution is obtained. The condenser is then washed outby the addition of about 5 mL of pure pyridine through the top and the content of the flask is titratedwith approximately 0.5 N standard sodium hydroxide solution to the phenolphthalein endpoint.The chemical structure of the TPA separated, was confirmed by recording its IR spectra. Theinstrument used was an FTIR spectrophotometer of Perkin-Elmer, Spectrum One. The resolution ofthe equipment was 4 cm-1. The recorded wavenumber range was from 450 to 4000 cm-1 and 32spectra were averaged to reduce the noise. A commercial software Spectrum v5.0.1 (Perkin ElmerLLC 1500F2429) was used to process and calculate all the data from the spectra. The KBr pellettechnique was used.Thermal characteristics of the original PPT samples and those obtained after degradation wereobtained using a Differential Scanning Calorimeter (Perkin-Elmer, Pyris Diamond DSC). Theinstrument was calibrated using high purity Indium and Zinc standards. Samples of about 5 mg wereused. The samples sealed in aluminum pans were initially heated from 0 to 270oC at a rate of10oC/min. Subsequently cooled to 0oC at a rate of 20oC/min and reheated to 270oC. Tests wereperformed under a nitrogen atmosphere.3. RESULTS AND DISCUSSION3.1 Degradation kineticsDepolymerization experiments were carried out using either amorphous or crystalline PPT and PETfor comparison. Three temperatures, i.e. 120, 150 and 180 oC were selected for isothermal tests.The effect of the type of polymer used on the alkaline hydrolysis at different depolymerization timeperiods is shown in Figure 1a - c for the experiments carried out at 120, 150 and 180 oC,respectively. It was observed that at all experimental conditions, the amorphous material lead tohigher degradation values compared to the crystalline. Crystallinity, since it reflects to condensedmaterial, poses some resistance during degradation due to reduction of diffusion rates of the alkalisolution into the bulk of the polyester. In addition, at low temperatures degradation was morepronounced in PPT due to its lower Tg compared to PET (Tg of PPT is 47oC compared to 80oC forPET) which means an increased polymer chain mobility and permeability. This observation wastransverse at the highest temperature used (i.e. 180 oC) as PPT crystallizes much more than PET atsuch temperatures during the experiments. Moreover, as expected, an increase in the reactiontemperature leads to an augmentation in the decomposition of both PPT and PET. Temperature is avery crucial factor, since as it can be observed from this Figure the degradation of amorphous PPTat 180oC is almost 90% in only 30 min, while at 120oC even after 60 min the polymer degradation isless than 80%. 1144
  • 6. Protection and restoration of the environment XI Solid waste management 100 o 120 C PET 80 PPT-crystaline Reacted polymer (%) PPT-amorphous 60 40 20 0 0 20 40 60 Irradiation Time (min) (a) 100 80 Reacted polymer (%) 60 40 o 150 C 20 PET PPT-crystaline PPT-amorphous 0 0 20 40 60 Irradiation Time (min) (b) 100 80 Reacted polymer (%) 60 40 o 180 C 20 PET PPT-crystaline PPT-amorphous 0 0 10 20 30 40 50 60 70 Irradiation Time (min) (c)Figure 1. Amount of polymer reacted versus irradiation time during alkaline hydrolysis of PET, crystalline PPT and amorphous PPT at 120 (a), 150 (b) and 180 oC (c). 1145
  • 7. Protection and restoration of the environment XI Solid waste managementOn comparing the present results on PET degradation with corresponding values obtained withoutthe use of microwave irradiation (Kosmidis et al., 2001), it can be postulated that at all temperaturesinvestigated, the time required for the degradation of PET at a certain level has been considerablyshortened when using microwave irradiation. Specifically, the time required to achieve 98 wt-%TPA yield at 180oC has been shortened from 1 h to 0.5 h. Correspondingly at 150oC, the timerequired to achieve 76 wt-% TPA yield has been shortened from 5 h to 1h and at 120oC the timerequired for 33 % TPA yield from 7 h to 1.5 h. This difference is even greater if we take intoaccount also the pre-heating period, which in conventional depolymerization is between 20 to 40min, while under microwave irradiation only 2 min. Moreover, after 1h hydrolytic depolymerizationat 150oC the conversion to TPA using microwave irradiation has increased from 35% to 76% and at120oC has almost been tripled from 6% to 20%.3.2. Product characterizationSubsequently the polymer recovered after filtration was characterized using differential scanningcalorimetry. Results comparing the polymer recovered after degradation of crystalline PPT at 120 or150 oC at 30 min are compared to corresponding of the original material in Figure 2. As it can beseen the original material presents a high endothermal peak at 230 oC which is the melting point ofthe polymer. The solid recovered after degradation at 120 oC exhibits almost the same melting point,meaning that besides the mass loss it is the same material that has been recovered. However, at thedegradation experiment at higher temperatures (i.e. 150 oC or 180 oC) the endothermal peak(melting point of the polymer) is shifted to lower values (i.e.140 oC) and even a bimodal peakappears. This means that the material recovered is no longer a polymer like the original one butrather an oligomer or a mixture of oligomers. Almost the same phenomena have been observedwhen amorphous PPT was used (shown in Figure 3). The original melting point of almost 227 oC isshifted to 145 oC after degradation at 150 oC for 30 min. PPT crystaline original PPT crystaline-120oC-30min PPT crystaline -150oC-30min Heat flow endo Up (a.u) 0 25 50 75 100 125 150 175 200 225 250 o Temperature ( C)Figure 2. DSC traces of original crystalline PPT and the material remained after alkaline hydrolysis under microwave irradiation at 120 and 150 oC for 30 min. 1146
  • 8. Protection and restoration of the environment XI Solid waste management PPT amorphous original PPT amorphous-150oC-30min Heat flow endo Up (a.u) 0 25 50 75 100 125 150 175 200 225 250 o Temperature ( C)Figure 3. DSC traces of original amorphous PPT and the material remained after alkaline hydrolysis under microwave irradiation at 150 oC for 30 min.3.3 Characterization of the purity of TPA receivedThe purity of TPA received was investigated by titration as it is reported in the experimental part, aswell as by FTIR analysis in order to detect any PET oligomers. The purity of TPA based oncarboxyl content was found always to be greater than 99% and on average equal to 99.4%.Furthermore, characteristic FTIR spectra of the product obtained from different experimentalconditions appear in Figure 4. From these spectra the following comments can be made.  The main peak at 1689 cm-1, is due to the existence of a C=O stretching band in the carboxyl group. If this absorption is in values greater than 1700 cm-1 then it denotes the existence of a carbonyl group in an ester. However, (as in our case) if it is in less than 1700cm-1 it is characteristic of carbonyl groups present in a carboxyl acid. Therefore, it seems that total depolymerization to monomer TPA is achieved.  The peak at 1285 cm-1 shows the existence of an C-O bond present in TPA.  The absorption peaks at 1510 and 1575 cm-1 prove the existence of a benzene ring.  The broad peak between 2500 and 3000 cm-1 is indicative of an –OH (hydroxyl group) in terephthalic acid.  The absorption peak at 783 cm-1 proves the para- position of the carboxyl groups in the benzene ring.  All spectra taken were similar which means that the same product is always produced.Therefore, it was concluded that the solid produced was pure monomer terephthalic acid.The suitability of TPA received for direct polymerization to PET was also investigated bypolymerizing it with ethylene glycol using tetrabutyl titanate as catalyst. This terephthalic acid whenesterified and polycondensed with ethylene glycol gave a pure white polymer which showed anintrinsic viscosity, [ ] = 0.53 dL g-1. 1147
  • 9. Protection and restoration of the environment XI Solid waste management 100 80 Transmittance (%) 60 40 20 PPT crystaline 120 C, 30 min 0 PPT crystaline 150 C, 30 min PPT crystaline 180 C, 30 min 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm ) Figure 4. FTIR spectra of the solid received after microwave depolymerization of crystalline PPT with 10% NaOH at 180 oC, 30 min (a); 150oC, 30 min (b); 120oC, 30 min (c).4. CONCLUSIONHydrolytic depolymerization of crystalline and amorphous PPT in alkaline solution undermicrowave irradiation was investigated as an effective technique for the chemical recycling of PPTand recovery of its monomers TPA and PG. Results were compared to corresponding from PETrecycling. Microwave irradiation shortens very much the time needed to achieve a specificdegradation of polymer. Degradation is favored by increased temperature, irradiation time and useof amorphous instead of crystalline material. High depolymerization degrees (near 90%) occurred in30 min at 180oC. The solid material remained after degradation had characteristics similar to theoriginal polymer when degradation took place at low temperatures (120 C), while at higherdegradation temperatures rather a mixture of oligomers was received. Finally, the purity of themonomer recovered was checked by three different methods.Finally, it should be stressed that all this research has been carried out and proved valid for benchscale experiments. Further scaled up experiments are needed if this method could be employed inindustrial scale, where tons of polymers need to be tackled.REFERENCES1. Achilias D. S., Antonakou E.V., Koutsokosta E.E, Lappas A.A.(2009) “Chemical Recycling of Polymers from Waste Electric and Electronic Equipment” Journal of Applied Polymers Science, Vol.114, pp. 212-221.2. Achilias D.S., Karayannidis G.P. (2004) “The chemical recycling of PET in the framework of sustainable development” Water Air and Soil Pollution, Vol.4, pp.385-396.1. Scheirs J. (1998) Recycling of PET, in: Polymer Recycling. Wiley series in Polymer Science; J. Wiley & Sons, W. Sussex, UK.2. Karayannidis G.P., Achilias D.S. (2007) “Chemical recycling of PET” Macromol. Mater. Eng. 292, 128-146. 1148
  • 10. Protection and restoration of the environment XI Solid waste management3. Sinha V., Patel M.R., Patel J.V. (2010) “PET waste management by chemical recycling: A review” J. Polym. Envirom. 18, 8-25.4. Carta D., Cao G., D’Angeli C. (2003) “Chemical recycling of PET by hydrolysis and glycolysis” Environ. Sci. & Pollut. Res. 10, 390-394.5. Kosmidis V.A., Achilias D.S., Karayannidis G.P. (2001) “PET recycling and recovery of pure terephthalic acid. Kinetics of a phase transfer catalysed alkaline hydrolysis” Macromol. Mater. Eng. 286, 640-647.6. Adam D. (2003) “Out of the kitchen” Nature 421, 571-572.7. Lidström P., Tierney J., Wathey B., Westman J. (2001) “Microwave assisted organic synthesis: A review” Tetrahedron 57, 9225-9283.8. Nikje M.A., Nazari F. (2006) “Microwave-assisted depolymerization of PET at atmospheric pressure” Adv. Polym. Technol. 25(4), 242-246.9. Liu L., Zhang D., An L., Zhang H., Tian Y. (2005) “Hydrolytic depolymerization of PET under microwave irradiation” J. Appl. Polym. Sci. 95, 719-723.10. Li K., Song X., Zhang D. (2008) “Depolymerization of PET with catalyst under microwave irradiation” J. Appl. Polym. Sci. 109, 1298-1301.11. Krzan A. (1998) “Microwave irradiation as an energy source in PET solvolysis” J. Appl. Polym. Sci. 69(6), 1115-1118.12. Siddiqui M.N., Achilias D.S., Redhwi H.H., Bikiaris D.N., Katsogiannis K.-A.G., Karayannidis G.P. (2010) “Hydrolytic Depolymerization of PET in a Microwave Reactor” Macromol. Mater. Eng. 295, 575–584.13. Achilias D.S., Redhwi H.H., Siddiqui M.N., Nikolaidis A.K., Bikiaris D.N., Karayannidis G.P. (2010) “Glycolytic Depolymerization of PET Waste in a Microwave Reactor” J. Appl. Polym. Sci. 118(5), 3066–3073.14. Achilias D.S., Tsintzou G.P., Nikolaidis A.K., Bikiaris D.N., Karayannidis G.P. (2011) “Aminolytic Depolymerization of poly(ethylene terephthalate) Waste in a Microwave Reactor” Polym. Int. 60, 500–506. 1149