Kinetic Analysis of Thermal Degradation of Polyolefin Mixtures

Transcription

1 International Jornal of Chemical ngineering and Applications, Vol. 5, No., April 14 Kinetic Analysis of Thermal Degradation of Polyolefin Mixtres N. Mrichan and P. Cherntongchai Abstract Plastic wastes are a global concern for their environmental impacts. A proficiency method for plastic waste management is a chemical recycling throgh a pyrolysis process. In this work, kinetic analysis of thermal degradation of polyolefin mixtre between polypropylene (PP) and low density polyethylene (LDP) nder pyrolysis atmosphere at different compositions was investigated sing thermogravimetric analysis (TGA), operated non-isothermally. Besides, Vyazovkin model-free method, together with different temperatre integral approximation approaches, was sed for kinetic parameter analysis from TGA reslts. It was fond that thermal degradation process of polyolefin mixtre was a triple step process and an addition of PP redced degradation temperatre. Besides, different approximation approaches did not affect activation energy vale. For polyolefin mixtre, the activation energy was increased initially, constant intermediately and finally increased again with increasing conversion. Finally, the activation energy of the polyolefin mixtre was lower than those of pre polymers and decreased with increasing PP fraction. Index Terms Thermal degradation, polyolefin, kinetic analysis, thermogravimetric analysis, Vyazovkin model-free method. I. INTRODUCTIONS Problem of plastic wastes is mainly on an environmental isse de to their extremely slow degradability. Since over 6% of the plastic wastes are polyolefin [1], therefore, it shold be sefl, if a particlar attention can be made on handling the polyolefin plastic wastes. Crrently, plastic waste managements are 65 % landfill disposal, 5 % rese and 1 % recycling both mechanically and chemically []. Among these, the chemical recycling method is the most beneficial method for which a prodction of many valable prodcts, sch as gas and liqid fel, petrochemical feedstock and monomer, can be obtained throgh a plastic pyrolysis. In order to design and optimize pyrolytic eqipment and the pyrolysis process appropriately, kinetic information of the plastic pyrolysis is necessary and, hence, scientific investigations in this particlar area are of importance. Thermogravimetric analysis (TGA) is one of widely sed techniqes for the investigation of the kinetics of the thermal degradation of the plastics where, for the kinetic of the plastic pyrolysis, the operation will be carried ot nder nitrogen atmosphere. And, the operation can be operated in both isothermal [3]-[5] and non-isothermal [5]-[11]. For the Manscript received September 9, 13; revised December 1, 13. The athors are with the Department of Indstrial Chemistry, Faclty of Science, Chiang Mai University, Chiang Mai 5, Thailand ( nonglakm5@gmail.com, parimanan@gmail.com). isothermal operation, sample temperatre is initially increased rapidly from room temperatre to a specific temperatre and maintained at that temperatre for an entire degradation process. Then, percent weight losses with respect to time, known as TG crve, are recorded. On the other hand, for the non-isothermal operation, the sample is continosly heated at a certain heating rate. The percent weight losses with respect to temperatre, also known as TG crves, are recorded. The kinetic information derived from the isothermal operation is sefl for flash pyrolysis, while the information from the non-isothermal operation is applicable for a batchwise operation. However, it is noteworthy to address here that the non-isothermal condition is more convenient to operate and is the process applied in this work. Calclation methods for kinetic parameters from the TG crve can be divided into classes, a model-fitting method and a model-free method, which can be sed for the isothermal and the non-isothermal conditions [3]. A featre of the model-fitting method is that, based on rate eqation and Arrhenis eqation, this method reqires an assmption of a reaction model before activation energy and a freqency factor can be interpreted. In addition, the activation energy is varied with only temperatre and is an average vale for an entire conversion level, even thogh it can also be changed with the level of the conversion. Besides, the analysis for the kinetic parameters from the non-isothermal condition ses a single heating rate, which may be not sfficient to obtain the correct activation energy. As a reslt, the kinetic triplets derived from this method are highly ncertain and cannot be compared with the kinetics triplet from the isothermal condition. On the other hands, the model free method has no need for the presmed reaction model. To obtain the accrate activation energy, the TG data of different heating rates are sed. In addition, this method allows a variation of the activation energy with respect to both the temperatre and the conversion to be revealed. Moreover, this later method is compatible with a complex reaction kinetic [6]. Therefore, the model-free method can be recommended as a trstworthy way to obtain the reliable and consistent kinetic information from the non-isothermal condition. And, it can help to reveal a complexity of mltiple reactions de to the dependencies of the activation energy on the extent of the conversion [3], [6], [1]. Considering the non-isothermal model-free method, there are different types of the temperatre integral nmerical approximation approaches for example Ozawa, Flynn and Wall (OFW) method sing a linear approximation (less accrate) tilizing Doyle s approximation [9] and Vyazovkin (VYZ) method sing a non-linear approximation (more accrate) [3] tilizing a direct nmerical integration method DOI: /IJCA.14.V

2 International Jornal of Chemical ngineering and Applications, Vol. 5, No., April 14 [11], Cai et al. approximation [5], Coats & Redfern approximation, Gorbachev approximation or Agrawal and Sivasbramanian approximation [6]. Between OFW method and VYZ method, VYZ model-free method has become poplar becase it can bring ot the more reliable and consistent kinetic information from the non-isothermal TG data of the polymer degradation [11]. And, this method was applied in this work. There were many previos works investigated the kinetics of the thermal degradation of the plastics sing TGA nder the pyrolysis atmosphere (Nitrogen atmosphere) and the non-isothermal condition together with an application of the VYZ model-free method for the kinetic parameter interpretation. Saha et al. [5] stdied the isothermal and the non-isothermal degradation kinetics of PT waste sing varios temperatre integral approximation approaches sch as Coats and Redfern, Gorbachev, and Agrawal and Sivasbramanian and the direct nmerical integration to analyze the decomposition kinetics. The reslts showed that the activation energy was a weak bt increasing fnction with the conversion in case of the non-isothermal condition bt was a decreasing fnction with the conversion in case of the isothermal condition. In addition, the activation energy obtained for the non-isothermal data showed similar reslts for all types of the approximation approaches (Coats and Redfern, Gorbachev, and Agrawal and Sivasbramanian). Saha and Ghoshal [6] stdied the thermal degradation kinetics of polyethylene (P) waste. In this work, Cai et al. temperatre integral approximation approach was firstly sed along with other approximation approaches which were Coats and Redfern, Gorbachev, Agrawal and Sivasbramanian and the direct nmerical integration. They reported that the activation energy increased continosly with increasing conversion. Frthermore, they also addressed that the limiting step of the thermal degradation shifted toward the degradation initiated by random scission leading to the higher activation energy at the higher conversion. In addition, they fond that Cai et al. approximation approach was the best method among other approximation approaches. Chowl et al. [11] stdied the non-isothermal degradation of PP and LDP mixtres of different ratios (:8, 35:65, 5:5, 65:35, 8:) sing the direct nmerical integration for the temperatre integral approximation. They reported that, the activation energy weakly increased with the conversion dring the initial degradation stage bt, then, became strongly increased with increased the conversion at the later stage ntil the end of the degradation process. They, finally, proposed that the ratio of PP: LDP eqal to 65:35 was the most preferable composition becase of its low activation energy and high amont of light hydrocarbons ( C1), delivered. In this work, we investigated the kinetics of the thermal degradation of pre polypropylene (PP), pre low density polyethylene (LDP) and the mixtres of polyolefin (PP/LDP), nder the pyrolysis atmosphere, sing thermogravimetric analysis (TGA). In addition, nder the non-isothermal operation, the Vyazovkin model-free method was sed to interpret the kinetic parameters from the TG crves. Finally, the kinetic parameters obtained sing varios temperatre integral approximation approaches, which were Coats and Redfern, Gorbachev, Agrawal and Sivasbramanian and Cai et al. approximation, were also compared. II. XPRIMNTS A. qipment and Materials Thermogravimetric analyser (TGA) sed in this work was Rigak TGA (TG-DTA81). Scanning electron microscope was sed to observe images of polymer powders. In addition, sieve analysis was sed for particle size analysis. Polymers sed in this work were polypropylene copolymer (PP, Grade PP151PC) and low density polypropylene homopolymer (LDP, Grade LD6K). All plastics were shredded into a powder form as illstrated in Fig. 1 for SM image of PP and Fig. 1 for SM image of LDP within a size range of 18-5 µm. From the sieve analysis, particle size distribtions of PP and LDP were presented in Fig.. Fig. SM images of PP and LDP. Fig.. Particle size distribtions of PP and LDP. B. xperimental Procedres Non-isothermal degradation experiments were carried ot nder ltra-high prity nitrogen atmosphere. Temperatre range was from 98 to 173 K. Nitrogen flow rate was maintained at 3 mlmin -1 and different heating rates sed were 1, 15, and 5 Kmin -1. Total sample mass was 4-5 mg for each rn. In this work, the percent ratios of PP to LDP were 1%:%, %:8%, 35%:65%, 5%:5%, 65%:35%, 8%:% and %:1%. III. THORY Vyazovkin (VYZ) model-free method is started from rate eqation and Arrhenis eqation and based on an assmption that reaction model is independent on temperatre and 17

3 International Jornal of Chemical ngineering and Applications, Vol. 5, No., April 14 heating rate [13]. The rate eqation can be written as in (1): d = k(t)f() dt where is conversion, t is time, k(t) is rate constant and f() is reaction model. The temperatre dependence of k(t) is described by the Arrhenis eqation. - k( T) A exp RT where A and are freqency factor and activation energy, respectively, and R is niversal gas constant. Sbstittion of () into (1) gives: d = A exp f() dt RT For non-isothermal conditions, sing chain rle, where: qation (3) becomes: d d dt =. dt dt dt d A = exp f() dt RT where β =dt/dt is heating rate (Kmin 1 ) and d/dt is rate of conversion with respect to temperatre (K 1 ). Integration of (5) gives: T (1) () (3) (4) (5) A A g( ) exp dt I(, T ) (6) β RT β g() is an integral term of f(), as written in (7). 1 g( ) f ( ) d (7) Since f() is independent of temperatre and heating rate, this is likewise for its integral term. Hence, I(, T ) is β constant, where I(, T) is a temperatre integral and; T exp dt I (, T ) (8) RT For a set of for experiments, carried ot at different heating rates (1, 15, and 5 Kmin -1 ), the activation energy can be determined at any particlar vale of by finding the vale of, for which fnction is minimm. n n, T β j Ω I (9) i1 j1, T β j i The temperatre integral in (9) can be evalated by several nmerical approximation approaches. The typical one is the direct nmerical integration method, when: T exp - dt = P() (1) RT R where is the activation energy at each conversion, T i is the temperatre at each conversion for each heat rate, is RT, and exp(-) exp (-) (11) P()= d = - i() where exp(-) and therefore: i()= d exp(-) I(,T )== - i() R (1) For Coats and Redfern approximation, the temperatre integral term can be calclated sing (13) [5]. RT RT - I(,T )= 1- exp (13) RT For Gorbachev approximation, the temperatre integral term can be calclated sing (14) [5]. RT 1 - I(, T )= exp RT RT 1+ (14) For Agrawal and Sivasbramanian approximation, the temperatre integral term can be calclated sing (15) [5]. RT 1- RT - I(, T )= exp RT RT 1-5 (15) For Cai et al., approximation, the temperatre integral term can be calclated sing (16) [6]. RT RT I(, T )= exp RT RT IV. RSULTS AND DISCUSSIONS A. Thermogravimetric Analysis of LDP and PP (16) TG crves of LDP and PP were shown in Fig. 3 and Fig. 4, respectively. Thermal degradation of PP occrred at a lower temperatre range than that of LDP de to non-stabilization effect of tertiary carbons dring the thermal degradation of polymer chain. In addition, LDP degradation was observed to be a single step, beginning at 6 K and ending at 75 K. Likewise, for PP degradation, the single step observation was also fond and the degradation was begn at abot 58 K and ended at 73 K. In addition, the variation of the temperatre degradation range with respect to heating rate was also fond. For all samples, the TG crves were shifted toward the higher degradation temperatre range with increasing heating rate from 1 to 5 Kmin -1. However, for PP (Fig. 3), the shifting of the TG crves with increasing heat rate also tended to approach a limit vale. Fig. 3 and Fig. 4 illstrated corresponding derivative thermogravimetric crves (DTG crves) of LDP and PP, 171

4 International Jornal of Chemical ngineering and Applications, Vol. 5, No., April K/min K/min 1 K/min K/min K/min 1 K/min K/min -15 K/min K/min Temperatre(K) 8 9 Fig. 5.. TG and DTG crves of PP/LDP mixtre (PP/LDP ratio = %:8%) at different heating rate K/min -4 1 K/min 5 K/min respectively. It can be seen that maximm degradation rate of PP was higher than that of LDP and its corresponding degradation temperatre of PP was lower than that of LDP. In addition, the maximm degradation rate and the degradation temperatre at the maximm degradation rate (Tm) also increased with increasing heating rate and, for PP, this trend was fond to approach the limit vale. The possible explanation is becase of a shorter soaking time at the higher heating rate and, hence, the actal sample temperatre was in fact lower than a frnace temperatre Fig. 3.. TG and DTG crves of LDP PP(%)+LDP(8%) PP(35%)+LDP(65%) PP(5%)+LDP(5%) PP(65%)+LDP(35%) PP(8%)+LDP(%) PP(5%)+LDP(5%) K/min K/min 1 K/min PP(35%)+LDP(65%) -1 PP(65%)+LDP(35%) PP(8%)+LDP(%) PP(%)+LDP(8%) Fig. 6.. TG and DTG crves of different PP/LDP ratios at 1 K min-1. 1 K/min -1 B. Thermogravimetric Analysis of PP/LDP Mixtres The thermal degradation of PP/LDP mixtres was also carried ot for for heating rates (1, 15, and 5 K min-1). The trend of the TG crves and the DTG crves of PP/LDP mixtres (PP/LDP ratio = %:8%) at the different heating rate was illstrated in Fig. 5. From Fig. 5, it can also be observed that the thermal degradation was a triple step process, where a beginning step was proximately at 35-6 K from =.1 to.1, an intermediate step was at 6-7 K from =.1 to.9 and a final step was at 7-1 K from = In addition, it was also fond that the TG crves - -3 K/min K/min Fig. 4.. TG and DTG crves of PP. 17

5 International Jornal of Chemical ngineering and Applications, Vol. 5, No., April 14 were shifted toward the higher degradation temperatre range with increasing heating rate. This was the same for all mixtre compositions. For the effect of the heating rate on the maximm degradation rate and T m, it was fond from Fig. 5 that the maximm degradation rate and T m increased with increasing heating rate. Considering Fig. 6, it can be seen that PP/LDP compositions did not affect the TG crves (Fig. 6) bt the DTG crves (Fig. 6). Considering DTG crve (Fig. 6), in comparison with Fig. 3 and Fig. 4, the maximm degradation rates for the polyolefin mixtres were mch less than that of the pre polymers. From TG and DTG crves (Fig. 6), in comparison with Fig. 3 and Fig. 4, boarder and lower ranges of the degradation temperatre for the polyolefin mixtres than those of the pre polymers were fond (see also Table I). From the TG and DTG crves of all PP/LDP compositions, temperatre at =.1 (T d ) and T m were reported in Table I, as well as Fig. 7 for T m and Fig. 8 for T d for each heating rate. According to Fig. 7, it can be observed that, by adding PP in LDP, T m dropped from the vale of 1% LDP. Besides, with increasing a percentage of PP, the vale tended to decreased frther below T m of 1%PP. This means the mixed polyolefin lowered T m at the maximm degradation rate. In addition, T m was also fond to increase with increasing heating rate for the polyolefin mixtre. For the change of T d with the addition of PP, it can be observed from Fig. 8 that the T d of the polyolefin mixtre was lower than those of the pre polymers. However, with increasing percent PP, T d tended to slightly increase. And the heating rate showed no clear effect on T d. Tm (K) Heating rate 1 K/min Heating rate Heating rate K/min Heating rate 5 K/min of PP Fig. 7. Variation of T m with different PP/LDP ratios at different heating rate. Td (K) Heating rate 1 K/min Heating rate Heating rate K/min Heating rate 5 K/min of PP Fig. 8. Variation T with different PP/LDP ratios at different heating rate. C. Kinetic Parameters Analysis for Thermal Degradation of Polyolefin Mixtre Following the calclation method conclded in the theoretical part, sing different temperatre integral approximation approaches, which were Coats and Redfern, Gorbachev, Agrawal and Sivasbramanian and Cai et al. approximation, it can be observed that the different approximation approaches delivered the same vales of the activation energy (Fig. 9 and Fig. 1). The same finding was also reported by Saha et al. [5]. Fig. 9 and Fig. 1 showed a dependency of the activation energy with respect to the conversion for LDP and PP, respectively. From Fig. 9 and Fig. 1, it was pointed ot that the activation energy of LDP was higher than that of PP for the entire conversion range. It was also observed that, for LDP (Fig. 9), the activation energy increased strongly with the conversion from the activation energy eqal to 9- kj/mole for the conversion eqal to Likewise, for PP (Fig. 1), the activation energy increased from 6 to 98 kj/mole for the conversion eqal to.-.7. A slightly different relation was fond at the initial degradation process ( =.1-.) and at the very end of the degradation process ( =.7-.8) where the activation energy was constant over these two conversion ranges. TABL I: DGRADATION TMPRATUR RANG, T D AND T M OF POLYOLFIN MIXTUR FROM TGA RSULTS Plastics Heating rate (K-min -1 ) Degradation temperatre range (K) T d /T m (K) ( =.1-.99) PP(%)+LDP(1%) / / / / / / / /745.7 PP(%)+LDP(8%) / / / / / / / /741.1 PP(35%)+LDP(65%) / / / / / / / /737.6 PP(5%)+LDP(5%) / / / / / / / /684.3 PP(65%)+LDP(35%) / / / / / / / /694.5 PP(8%)+LDP(%) / / / / / / / /683.4 PP(1%)+LDP(%) / / / / / / / /75.4 Fig. 11 represented the relation between the activation energy and the conversion for PP/LDP mixtre at different PP/LDP ratios (%:8%, 35%:65%, 5%:5%, 65%:35% and 8%:%). It can be approximated that, with increase fraction of PP, the activation energy decreased. In addition, 173

6 International Jornal of Chemical ngineering and Applications, Vol. 5, No., April 14 for all mixtre compositions, it was fond that the activation energy was slowly increased with the conversion dring the initial degradation stage ( =.1-.). Then, the activation energy became nearly constant or a very weak fnction with the conversion dring the middle stage of thermal degradation (=.-.6). It, finally, increased with increasing the conversion again dring the final degradation stage (=.6-.8). This finding was in contrast with the report of Chowl et al. [11] who also stdied the kinetic analysis of the non-isothermal degradation of PP/LDP mixtre. Based on VYZ model-free method and sing direct nmerical integration method instead of Cai et al. approximation approach for the temperatre integral approximation, they reported that, the activation energy always increased with the conversion, where the gradal increase of the activation energy was fond for the initial degradation process and the strong increase of the activation energy was fond for the remaining degradation process. (kj/mol) Coats & Redfern Gorbachev Agrawal & Sivasbramanian Cai et al Fig. 9. Dependency of activation energy on conversion dring non-isothermal degradation of LDP sing different temperatre integral approximation approaches. (kj/mol) Coats & Redfern 7 Gorbachev 65 Agrawal & Sivasbramanian Cai et al Fig. 1. Dependency of activation energy on conversion dring non-isothermal degradation of PP sing different temperatre integral approximation approaches. (kj/mol) PP(%)+LDP(8%) 4 PP(35%)+LDP(65%) PP(5%)+LDP(5%) PP(65%)+LDP(35%) PP(8%)+LDP(%) Fig. 11. Dependency of activation energy on conversion dring non-isothermal degradtion of different PP/LDP ratios sing Cai et al. temperatre integral approximation approach. Another important point, which can be drawn from or work, was that the polyolefin mixtre had the lower activation energy than those of the pre polymers (Fig. 11). The lowering in the activation energy for all mixtres compositions were corresponding to the lower degradation temperatre, mentioned previosly. This was also reported by Miranda et al. [14] that for the PP/P mixtre, PP cased a redction of the activation energy. They explained in their work that free radicals were initially formed from the less stable polymer, which was PP in this case, de to a transfer of hydrogen. Then, these free radicals destabilized the more stable polymer (LDP), reslting in the lower activation energy of the polyolefin mixtre. And, the same explanation was also addressed by Chowl et al. [11]. V. CONCLUSIONS Kinetic analysis of thermal degradation of pre and mixed polyolefin nder non-isothermal condition was carried ot sing thermogravimetric analysis (TGA) and Vyazovkin (VYZ) model-free method. It can be conclded that: TG crves of pre polyolefin followed a single step process bt this was a triple step process for polyolefin mixtre. Addition of percent PP lowered degradation temperatre range, maximm degradation rate, degradation temperatre at the maximm degradation rate and degradation temperatre at conversion of.1, in comparison with those of the pre polymers. Different nmerical temperatre integral approximation approach showed no effect of the interpreted vales of activation energy. The activation energy increased continosly with increasing conversion for pre LDP. On the other hand, for PP, the activation energy increased with increasing conversion dring an intermediate degradation stage, bt was constant dring an initial and a final degradation stage. For polyolefin mixtre, with increasing conversion, the activation energy was increased dring the initial degradation stage, constant dring the intermediate degradation stage and increased again dring the final degradation stage. The polyolefin mixtres had the lower activation energy than those of the original pre polymers. In addition, the activation energy decreased with increasing ratio of PP. ACKNOWLDGMNT We wold like to express or sincere thanks to nergy Policy and Planning Office (PPO), Ministry of nergy, Thailand and Gradated School, Chiang Mai University, Chiang Mai, Thailand for their financial spport of this research project. RFRNCS [1] S. M. A. Salem, P. Lettieri, and J. Baeyens, The valorization of plastic solid waste (PSW) by primary to qaternary rotes: From re-se to energy and chemicals, Progress in nergy and Combstion Science, vol. 36, pp ,

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