Thermal degradation of poly(methyl methacrylate) polymers Kinetics and recovery of monomers using

Thermal degradation of poly(methyl methacrylate)polymers:Kinetics and recovery of monomers using a fluidized bed reactorBo-Sung Kang a ,Sang Guk Kim b ,Joo-Sik Kim a ,*aFaculty of Environmental Engineering,University of Seoul,90Jeonnong-Dong,Dongdaemun-Gu,Seoul 130-743,Republic of KoreabKorea Institute of Energy Research,Republic of KoreaReceived 20March 2007;accepted 3July 2007Available online 10July 2007AbstractPoly(methyl methacrylate)(PMMA)is a versatile transparent thermoplastic that is used in a wide range of fields and applications,such as in the lenses of exterior lights of automobiles,in electronics,in food containers and in artificial marble.When PMMA is subject to thermal degradation conditions,it degrades mainly into its monomer,methyl methacrylate (MMA).In this study,a virgin PMMA copolymer composed of 97.5%methyl methacrylate,2.5%methyl acrylate (MA)and PMMAwaste plastics were investigated.Kinetic measurements of the thermal degradation of the virgin copolymer were carried out with a TGA.The Chatterjee–Conrad (CC)and Freeman–Caroll (FC)models were applied to determine the pyrolysis kinetic parameters.Both models simulated the experimental DTG curves fairly well.In addition,pyrolysis of the virgin copolymer and the PMMA wastes was carried out using a fluidized bed reactor to recover the monomer.The reaction temperatures ranged from 450to 5008C.In each case,the rate of oil recovery was over 97%,and the content of the MMA in the oils was up to 98wt%.The MMA recovered from the experiments can be used as feedstock for the polymerization of PMMA.#2007Elsevier B.V .All rights reserved.Keywords:Thermal degradation;Fluidized bed;PMMA;Monomer;Kinetics1.IntroductionWaste plastics are usually disposed of in landfill sites or utilized energetically,as for instance,by incineration.Modern society,however,requires reasonable alternatives to cope effectively with needs for energy and environmental conserva-tion.Researchers consider the thermal degradation of waste plastics an alternative to traditional waste plastic management.In thermal degradation,plastic wastes can be converted into chemical feedstock,which can be used to produce valuable new products such as oils and gases of high caloric value.Thermal degradation of various plastics has been extensively investi-gated recently,for which various reactors have been used,such as vessels,autoclaves,rotary kilns and fluidized beds [1–3].Most researches have focused on the thermal degradation ofwidely used plastic wastes,such as polyolefins.There are few commercial applications because of the low heat transfer rates of the melting polymer and the low value of the product oils.In contrast,thermal degradation of PMMA can lead to economic feedstock recycling because of its specific degradation behavior to yield the monomer,methyl methacrylate (MMA).Above 4008C,it will be degraded almost into the monomer [4,5].The recovery of MMA from PMMA has been drawing much attention recently due to its high current cost of above 2000$/ton.Typical reactors for the degradation of PMMA are molten metal baths,extruders and fluidized bed reactors [6,7].The molten bath is composed essentially of tin and lead metal,but it can release hazardous organic metal compounds [8].The annual production of PMMA plastics in Korea amounts to 100,000tons.The PMMA that is mainly produced in Korea is a copolymer composed of MMA and methyl acrylate (MA)or ethyl acrylate (EA),to facilitate the processing of PMMA products.In this study,thermal degradation of a virgin PMMA and waste PMMA plastics is conducted in a fluidized bed/locate/jaapJ.Anal.Appl.Pyrolysis 81(2008)7–13*Corresponding author.Tel.:+82222105621;fax:+82222442245.E-mail address:joosik@uos.ac.kr (J.-S.Kim).0165-2370/$–see front matter #2007Elsevier B.V .All rights reserved.doi:10.1016/j.jaap.2007.07.001pyrolysis plant to obtain the optimal reaction temperature for a high yield of monomer.In addition,the kinetic characteristics of the thermal degradation of a virgin PMMA copolymer were also investigated using a TGA.Some researchers have already carried out kinetic experiments on the degradation of PMMA [9,10].In this study,the Chatterjee–Conrad (CC)and Freeman–Caroll (FC)models were applied to determine the kinetic parameters.2.Experimental methodology 2.1.MaterialsThe virgin PMMA copolymer was supplied by the company LG MMA.It was the bead type and was composed of 97.5%methyl methacrylate (MMA)and 2.5%methyl acrylate (MA).It was radically polymerized in emulsion and its weight-average molecular weight was about 100,000g/mol.The waste PMMA plastics used in the experiments were automobile taillight lenses and light guide plates.They were collected from scrapped cars and an electronic waste separation facility,respectively.They were then pretreated with a grinder to transform them into the proper size for the feeding system.The size of the PMMA plastics ranged from 1to 1.5mm.The specifications of the PMMA plastics are listed in Table 1.2.2.Kinetic experiment conditionsThe thermogravimetric analyzer used in the thermal degradation kinetic experiment was a TA instrument SDT 2960,and the experiments were conducted at room tempera-tures of up to 6008C and at heating rates of 5,10,15and 208C/min.The sample was 14–21mg of a virgin PMMA bead,and nitrogen gas was used as the purge gas,at a rate of 100ml/min.For the kinetic work,we used TGA curves and DTG curves.A TGA curve measures the change in weight of a sample as a function of temperature,and a DTG curve obtained shows the change in weight with time as a function of temperature or time.2.3.Pyrolysis plant and reaction conditionsThe pyrolysis experiments were carried out in a plant that had a fluidized bed with a capacity of up to 1kg/h.A schematic diagram of the plant is given in Fig.1.The key part of the plant was the fluidized bed,which was made up of sus-304.It was heated indirectly by an electric heater.The PMMA entered the reactor directly into the sand that was used as the bed material,with the help of two screw feeders,after which it was pyrolyzed.There were three thermocouples in the reactor.One was in the freeboard and the others were in the sand.The reaction temperature is defined as the average value taken from these thermocouples.A cyclone and a hot filter constituted the solid particle separation system.A series of quenching columns,operated with cool water and ethanol to a minimum temperature of À308C,was applied to cool the product gas efficiently and quickly.Part of the product gas was either burned in a stack to regulate the pressure in the plant or was used for gas sampling.Most of the product gas was circulated into the fluidized bed by a compressor.In each experiment,the product gas served as a fluidizing medium.Quartz sand (1.5kg)was used as the fluidized bed material.It had a mean particle size of ca.0.4mm.The feed rate of each experiment was around 200g/h.The calculated residence time of the product vapor in the reactor was around 0.5s in each of the experiments.Because of the relatively uniform residence time and feed rate in each of the experiments,it was possible to relatively easily compare the influence of the reaction temperature on the product spectrum.The reaction conditions are summarized in Table 2.2.4.AnalysisIn each of the experiments,gas,oil and solid residues were obtained as products.A fraction of the product oil from each run was distilled in a bench-scale apparatus under reduced pressure (2108C and 13kPa)to obtain a light fraction of oil and a distillation residue.The pyrolysis gas was analyzed with GCs using TCD and FID,and with helium as the carrier gas.The applied columns were a Carboxen 1000(TCD)and an HP-plot Al 2O 3/KCl (FID).The light fraction of oil from the distillation was analyzed with a GC–MS system using a column HP-5.3.Results and discussion3.1.Thermal degradation of the virgin PMMA and waste PMMA plastics in a TGAThe TGA curve for the thermal degradation of the virgin PMMA is shown in Fig.2;of the automobile taillight lens,in Fig.3;and of the waste light guide plate,in Fig.4.The fractional conversion is as follows:x =(m 0Àm )/(m 0Àm 1),in which m 0is the initial sample weight,m 1is the residual weight at infinite time,and m is the residual sample weight at time t .As the heating rate increased,the temperature range at which weight loss took place moved to a higher-temperature region.The main thermal degradation took place at the temperature range of 350–4008C,but as the heating rate increased,the thermal degradation continued up to slightly over 4008C.The TGA curve of the automobile taillight lens at the heating rate of 208C/min shows ingredients that are degradable at a temperature range of 450–5508C.In Fig.2,the onset ofTable 1Specifications of the PMMA plastics Compositions (wt%)Virgin PMMA Automobile taillight lens Light guide plate V olatiles 99.9899.9699.94Ashes 0.020.040.06Elemental analysis Virgin Automobile taillight lens Light guide plate C 59.759.459.6H 8.08.08.0O32.332.632.4B.-S.Kang et al./J.Anal.Appl.Pyrolysis 81(2008)7–138degradation appears around3008C.It is difficult to observe typical two small mass losses below3008C in the thermal degradation of radical polymerized PMMA[11].This can indicate very small amount of head to head linkage and terminal vinyl bond in the virgin PMMA.It is also interesting that Fig.2 shows(for108C/min)a half degradation temperature of about 360–3708C,whereas Fig.3gives nearly at4008C(at the same heating rate)and Fig.4at3808C.This tendency may be either due to the difference of the initial molecular weight between the virgin PMMA and PMMA wastes produced by variousfirms or due to the effect of thermal stabilizers in PMMA wastes.In Fig.4we can see two mass losses at about300–3508C and thereafter.The degradation region about300–3508C may be the degradation of additives in the PMMA waste.The earlier onset of degradation with PMMA wastes may result from the difference in initial molecular weight between the virgin PMMA and the PMMA wastes,because PMMA with the highest molecular weight degrades more at lower temperatures than PMMA with the lowest molecular weight[12].3.2.Kinetics of the thermal degradation of the PMMA copolymerThe differential method–the Chatterjee–Conrad(CC) method–and the difference method–the Freeman–Caroll(FC) method–were used to obtain the kinetic parameters with the virgin PMMA.The simulation of the DTG curves was based on the kinetic law commonly used for a single weight loss,as follows:d xd t¼bd xd T¼A eÀE=RT fðxÞ(1)Fig.1.Schematic diagram of the pyrolysis plant.Table2Pyrolysis conditionsVirgin PMMA Automobile taillight lens Light guide plateRun1Run2Run3Run4Run5Run6Run7Run8Run9Run10Run11 Temperature(8C)440450460470450470500450470480500 Flow rate(NL/min)3131303031302931303029 Input(g)95599197379599498910001000100010001000 Duration(min)240290290220190290310290260290270B.-S.Kang et al./J.Anal.Appl.Pyrolysis81(2008)7–139in which A is the pre-exponential factor,E the activation energy,R the gas constant,T the temperature,t the time,and f (x )is the model function.If b is the constant heating rate,d T =b d t .The pyrolysis kinetic parameters of the PMMA obtained using the CC and FC methods are shown in Table 3.The FC method was chosen to compare the experimental DTG curve and the kinetic model prediction DTG curve,since this method can provide three kinetic parameters (n ,A and E ).To obtain the DTG curve via model prediction,it is necessary tointegrate the kinetic equation (1):Zx0d xð1Àx Þn¼ZTA bexpÀE RTd T (2)For convenience,u =ÀE /RT .The initial temperature,T 0,can be assumed to be zero,since it is very low compared to the pyrolysis temperature.Eq.(2)thus becomes (1/(1Àn ))(1Àx )1Àn =(AE /b R )p (u ),in which p ðu Þ¼R ðe u =u 2Þd u .To calculate p (u ),for which there is no analytical solution,Lyon’s approximation,p (u )=e u /(u (u À2)),is applied.As the initial temperature can be assumed to be zero without appreciable error,the p (u )at low temperatures can be ignored,since the p (u )at low temperatures is quite small,compared to the p (u )at pyrolysis temperatures.When Lyon’s approximation is applied,the fractional conversion,x ,is as follows:x ¼1ÀAE b R e uu ðu À2Þðn À1Þþ11=1Àn(3)Figs.5and 6show the experimental DTG curve and the model-predicted DTG curve at the heating rates of 108C/min and 208C/min,respectively.There is fairly good agreement between the experimental and model-predicted DTG curves.Table 3Apparent kinetic parameters for the thermal degradation of the virgin PMMA MethodHeating rate (8C/min)Activation energy (J/mol)ln A (min À1)Reaction order,n Chatterjee–Conrad5102,46118.11110118,49421.34115103,82218.91120139,50225.361Freeman–Caroll5331,59461.73 1.6710206,33437.75 1.2515263,48448.40 1.4220256,94847.001.39parison of the experimental and model-predicted DTG curves at the heating rate of 108C/min.Fig.4.The TGA curve for the thermal degradation of the light guideplate.Fig.3.The TGA curve for the thermal degradation of the automobile taillightlens.Fig.2.The TGA curve for the thermal degradation of the virgin PMMA.B.-S.Kang et al./J.Anal.Appl.Pyrolysis 81(2008)7–13103.3.Pyrolysis of the PMMA plastics in the plantThe mass balance of each of the experiments is shown in Table 4.The main product was oil,the yield of which amounted to up to 99wt%of the product.In the experiments with thewaste PMMA plastics,the amount of oil was slightly lower.Some additives of the plastic wastes seem to have been converted into gas.It can be seen that an increase in the reaction temperature leads to a decrease in oil production.A strong decrease in oil production,however,was not observed in the experiments.The gas and char production in each of the experiments was very small,with values under 3wt%.A strong increase in gas production has been observed at reaction temperatures of above 5008C [13].Reaction temperatures of around 4508C seem to be optimal for a high oil yield,but the operation of the pyrolysis process at these temperatures was not easy in this study due to the coagulation of sand and the melted PMMA.Regarding facile operation and high oil yield,reaction temperatures of 470–5008C would be appropriate for the fluidized bed pyrolysis of PMMA plastics.The product gas was composed mainly of CO,CO 2,CH 4and other hydrocarbons.The CO and CO 2originated mainly from the oxygen constituent in the PMMA.Table 5shows the quantitative and qualitative analysis of the product oil obtained from the GCs and a GC–MS.The main component of the product oils was MMA,the yield of whichTable 4Mass balance of the pyrolysis experiments with different PMMA polymers wt(%)Virgin PMMA Automobile taillight lens Light guide plate Run1(4408C)Run2(4508C)Run3(4608C)Run4(4708C)Run5(4508C)Run6(4708C)Run7(5008C)Run8(4508C)Run9(4708C)Run10(4808C)Run11(5008C)Oil 99.399.298.198.397.898.497.398.497.598.096.8Gas 0.50.6 1.8 1.6 2.1 1.5 2.4 1.5 2.4 1.9 3.1Char0.20.20.10.10.10.10.30.10.10.10.1Table 5Analysis of product oil wt(%)Virgin PMMA Automobile taillight lens Light guide plate Run1(4408C)Run2(4508C)Run3(4608C)Run4(4708C)Run5(4508C)Run6(4708C)Run7(5008C)Run8(4508C)Run9(4708C)Run10(4808C)Run11(5008C)MA 1.76 1.41 1.83 1.69 1.39 1.31 1.670.96 1.27 1.07 1.05MMA97.3496.7897.2396.3096.5995.6596.5397.2996.3896.9696.76Methyl isobutyrate 0.200.180.230.200.240.250.360.170.180.160.22Ethyl acrylate ––––0.570.770.16––––Butyl acrylate–––––––0.180.240.230.252-Butenoic acid methyl ester –––––0.20–––––Methylene-Butanedioic acid dimethyl ester––––0.180.33–––––1,2-Cyclopentanedicarboxylic acid dimethyl ester0.300.270.31–0.290.400.37–0.300.200.222-Methyl-3-furancarboxylic acid methyl ester 0.220.30––0.22––––––2,5-Diethylthiophene––––0.14––––––Methacrylic acid ethyl ester –0.19–0.21–––0.180.190.17–Diethyl phthalate–0.27–0.30–0.33–––––1-Naphthalenecarboxylic acid methyl ester –––––––0.26–––Cyclopropanecarboxylic acid methyl ester––––––––0.29–0.202-Naphthalenecarboxylic acid methyl ester –––––––––0.22–Benzene ––––––––––0.15(–):notdetected.parison of the experimental and model-predicted DTG curves at the heating rate of 208C/min.B.-S.Kang et al./J.Anal.Appl.Pyrolysis 81(2008)7–1311amounted to up to 98wt%of the oil.MA,which is commonly used as comonomer of PMMA plastics in Korea,was also identified in each of the experiments.The amount of MA in the oils reached up to about 2wt%of the product oil.The identification of the EA in the pyrolysis of the automobile taillight lens is unique in the oil analysis,which exhibits that the material contained EA as a comonomer.In the pyrolysis of the light guide plate,butyl acrylate (BA)was also solely identified,which indicates that the material can be a copolymer of MMA and BA.In the pyrolysis of pure PMMA carried out by Kaminsky [7],the content of MMA in the pyrolysis oil was 99wt%.Our work with the MMA content of 98wt%confirms the Kaminsky’s work.The high yield of MMA is based on the radial degradation mechanism of PMMA.At first,a PMMA molecule is broken by random scission,and then two radicals,the isobutyryl macromolecule radical and the primary macro-molecule radical,are formed [14].The isobutyryl macro-molecule radical is then degraded easily to yield the monomer MMA.On the other hand,the primary macromolecule radical istransformed into the isobutyryl macromolecule radical via a PMMA molecule with a methallyl end group,and is then degraded to produce an MMA unit [15].Along with the characteristic degradation mechanism of PMMA,the fluidized bed reactor also contributes to the high yield of MMA reducing side reactions via its inherently excellent heat transfer and the short residence time of the product gases in the reactor.The existence of MA or EA in the coploymer is likely to block the unzip degradation reaction of the pure PMMA to produce a somewhat lower MMA yield.This can be mainly due to the less stable secondary radicals formed on the MA or EA unit,compared with the tertiary radicals formed in the degradation of the pure PMMA.From the experiments,however,the extent of the unzip reaction from the tertiary radicals did not strongly diminish with a small amount of MA or EA in the molecular backbones.Fig.7shows the boiling point distribution of a light fraction of the product oil obtained after the distillation.The boiling point of the main component was just above 1008C,which means that MMA (bp:1018C)was the main component of the oil,comprising at least more than 96wt%of it.Table 6shows the mass balance of the experiments,which refers to the total organic input.The table also shows the distillation residue,the amount of which was only up to 1wt%of the organic products.The MMA yield from the virgin PMMA ranged from 95to 97wt%.In contrast,the pyrolysis of the PMMA waste plastics produced MMA in the range of 94and 96wt%.The total yield of the typical comonomers (MMA,MA and EA)ranged from 97to 98wt%in the case of the virgin PMMA,and from 95to 97wt%in the case of the PMMA waste plastics.4.ConclusionThe thermogravimetric analysis shows that the main thermal degradation took place at the temperature range of 350–4008C.At a higher heating rate,the temperature range at which weight loss took place moved to a higher-temperatureTable 6Organic mass balance of the pyrolysis experiments wt(%)Virgin PMMA Automobile taillight lens Light guide plate Run1(4408C)Run2(4508C)Run3(4608C)Run4(4708C)Run5(4508C)Run6(4708C)Run7(5008C)Run8(4508C)Run9(4708C)Run10(4808C)Run11(5008C)MA 1.7 1.4 1.8 1.7 1.4 1.3 1.6 1.0 1.2 1.1 1.0MMA96.796.095.495.094.494.193.995.793.995.093.6Methyl isobutyrate 0.20.20.20.20.20.30.30.20.20.20.2Ethyl acrylate ––––0.60.80.2––––Butyl acrylate –––––––0.20.20.30.2Other oils0.50.90.30.50.8 1.30.30.40.80.60.6Distillation residue 0.20.60.40.9 1.00.70.90.9 1.10.7 1.1CO 0.20.20.50.40.50.30.60.60.80.7 1.1CO 20.20.30.70.90.20.7 1.00.40.80.60.9CH 40.040.10.20.10.20.10.20.20.30.30.5Other gases 0.10.10.40.20.60.30.70.30.60.40.7Char 0.20.20.10.10.10.10.30.10.10.10.1Total100100100100100100100100100100100(–):notdetected.Fig.7.Boiling point distribution of a light fraction of the product oil from Run2.B.-S.Kang et al./J.Anal.Appl.Pyrolysis 81(2008)7–1312region.The Freeman–Carroll method was used to compare the experimental DTG curve and the kinetic-model-predicted DTG curve.Lyon’s approximation was chosen to calculate the fractional conversion.There was fairly good agree-ment between the experimental results and the model prediction.A virgin PMMA copolymer and waste PMMA plastics were pyrolyzed in afluidized bed reactor to obtain a high monomer yield.The reaction temperatures selected ranged from440to5008C,based on the TGA experiments.The influence of the reaction temperature in the region on the oil yield was not so strong,but a higher reaction temperature showed a negative effect.The oil yield ranged from97to 99wt%of the product.The gas and char yields were very small,with values of almost under3wt%,taken together.The main component of the product oil was MMA.With MA and EA,the total monomer yield reached up to98wt%on the organic input base.The high yield of MMA in the experiments is generally coincident with results of other previous works on the pyrolysis of PMMA.The experiments show that the pyrolysis of PMMA seems to be an economical route to the recycling of PMMA.One prerequisite,however, for the recycling of PMMA wastes via pyrolysis is the sorting out of PMMA from other plastic wastes.The electrostatic sorting method would be efficient for this due to the+ charging ability of the PMMA.Furthermore,if PMMA wastes can be collected efficiently from engineering plastic waste streams,the pyrolysis of PMMA will be a more attractive tool for the treatment of PMMA wastes.AcknowledgmentThis study is supported by the Ministry of Environment of Korea as‘‘The 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