End-life Prediction of Commercial PLA Used for Food Packaging through Short Term TGA Experiments: Real Chance or Low Reliability?

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1 Chinese Journl of Polymer Science Vol. 32, No. 6, (2014), Chinese Journl of Polymer Science Chinese Chemicl Society Institute of Chemistry, CAS Springer-Verlg Berlin Heidelberg 2014 End-life Prediction of Commercil PLA Used for Food Pckging through Short Term TGA Experiments: Rel Chnce or Low Relibility? Ignzio Blnco * Deprtment of Industril Engineering, University of Ctni, V.le A. Dori 6, Ctni, Itly Abstrct A long-term (bout nine months) isotherml degrdtion experiment of two different commercil polylctide (PLA) smples used for food pckging ws crried out t reltively low temperture (423 K). Thermooxidtive degrdtions of the sme polymers were crried out in thermogrvimetric (TG) nlyser, t higher tempertures (453 T 523 K), under isotherml heting conditions. The obtined set of experimentl TG dt ws used to determine the pprent ctivtion energy (E ) of degrdtion through two isotherml kinetic methods. The results from long-term experiment evidenced considerble mss loss for both PLA smples in the investigted period, but the experimentl dt were not in greement with those from the short-term degrdtions t higher tempertures, thus suggesting different degrdtion kinetics, nd, then low relibility of the lifetime predictions for polymers in service or degrdtion forecsts for the end of their life bsed on experiments t higher tempertures. Keywords: Lifetime prediction; Degrdtion ctivtion energy; Kinetics of polymer degrdtion; Thermogrvimetry. INTRODUCTION Polymers, polymer mtrix composites nd nnocomposites, bio-polymers re incresingly being used in wide rnge of pplictions where long-term service, often in hostile environments, is required. Independently of the nture nd the use of polymer-bsed mteril, there is growing demnd for mnufcturers to gurntee the life expectncy of the product. The problem is very herd in mny polymer derivtive pplictions nd there is gret need to demonstrte tht product, selected for specific pplictions, is fit for service [1]. This is prticulrly importnt for polymeric mterils nd its composites s their mechnicl nd therml properties, like strength, stiffness nd resistnce to therml degrdtion re time/temperture dependent due to the hereditry nture (viscoelsticity) of polymers [2]. On the bsis of the bove considertions the prcticl use of polymeric mterils requires the estimtion of their correct performnces, well known in literture s therml lifetime [3 6]. Lifetime prediction is done on the bsis of the identifiction of the criticl rection which limits the life of mteril [7] nd, since there is strong need for ccelerted lifetime chrcteriztion methodologies, it is evluted from the mesurement of the rection kinetics t high tempertures. Kinetic nlysis cn hve either prcticl or theoreticl purpose: the mjor prcticl purpose is the prediction of process rtes nd mteril lifetimes [8]. A possibility of obtining of this informtion is the use of dt supplied by therml nlysis methods. Prticulrly, thermogrvimetric nlysis (TGA) is very rpid ccurte technique which, given tht the connection between TGA nd long-term life testing cn be estblished [9], should provide proper kinetic expressions nd corresponding pprent ctivtion energy (E ) of degrdtion vlues tht cn be used to predict the therml lifetime of polymeric mterils. Therefore through extrpoltion of the results obtined t higher tempertures, * Corresponding uthor: Ignzio Blnco, E-mil: iblnco@dii.unict.it Received Jnury 16, 2014; Revised Februry 25, 2014; Accepted Mrch 6, 2014 doi: /s

2 682 I. Blnco one cn estimte firly ccurtely the service life of mteril. Before you engge in lborious kinetic nlysis of complex process, like polymer degrdtion, which could be performed in some fvourble cses by employing softwre pckges vilble for commercil use [10 14], it is good to sk whether isotherml or constnt heting rte experiments re better. Probbly the correct nswer is tht both isotherml nd non-isotherml methods hve dvntges nd disdvntges. As reported by Vyzovkin et l. strictly isotherml experiments re not possible, becuse there is lwys finite non-isotherml het-up time [8] (usully few minutes) nd nywy these experiments re rther time consuming. By contrst the inherent experimentl errors, the reltive high number of djustble kinetic prmeters, nd the existence of compenstion effect of ctivtion prmeters [15] mke impossible the discrimintion of the correct mechnism of this complex process only by using nonisotherml dt [16]. For this reson in the pst, together with my co-uthors, we studied by using both isotherml nd dynmic condition, the degrdtions of different sets of polymers: very thermlly stble polyetherketones (PEK) nd polyethersulfones (PES) [17 19] nd well known polymer s polyethylene (PE), polystyrene (PS), polycrbonte (PC) nd poly(methyl methcrylte) (PMMA) [20, 21]. The dt obtined through literture kinetic methods, showed greement ech other only when single degrdtion step occurred [17] while significnt difference in the kinetic prmeters were found in the cse of multiple degrdtion stges (like for instnce in the cse of polyethylene degrdtion) [20]. Even though the biggest disdvntge of isotherml experiments is limited temperture rnge, especilly t lower tempertures it my be very difficult to rech complete conversion over resonble time period, we crried out, lso, long-term (bout 3 yers) isotherml degrdtions. The results of these experiments were compred with those obtined by short-term degrdtions nd suggested tht kinetic prmeters extrpolted from mesurements t higher tempertures pper not sufficiently relible, s well s the lifetime predictions so determined [17, 20, 21]. In this work long-term (nine months) isotherml degrdtion of two different commercil polylctide (PLA) smples used for food pckging ws crried out in sttic ir tmosphere, t 423 K. A set of short-term isotherml degrdtion experiments were lso crried out in sttic ir tmosphere, in the K temperture rnge. The degrdtion ctivtion energy vlues were determined through n isotherml literture method nd the results obtined were compred with those obtined with n isotherml method set up in our lbortory. PLA ws selected becuse it is, in these two forms, one of the most used polymeric mterils in food pckgings nd represents one of the mjor domestic wste mterils, but bove ll PLA hs low glss trnsition temperture, in respect to the polymers exmined in the pst, which suggests the possibility of sizble degrdtion during the long-term mesurements. The ims of this work were: (1) to find possible correltion between the dt obtined from the long-term isotherml degrdtion performed t 423 K (temperture lower thn the melting temperture of PLA nd closer to the glss trnsition temperture) with those obtined from the short-term degrdtions t higher tempertures nd then to verify the vlidity of lifetime predictions of polymers in service mde by degrdtion experiments t higher tempertures; (2) to hve informtion bout the time needed to rech pprecible mss loss ner the melting temperture of PLA, in order to explore the possibility of performing degrdtion experiments below the glss trnsition temperture (nd then ner the service temperture) in resonble time; nd (3) to extrpolte, in cse of positive correltion between the dt obtined from the long-term isotherml degrdtion nd those from the short-term experiments, these vlues t room temperture nd then predict, eventully, the end lifeservice nd, mostly, the end life-expirtion of the mteril in order to llow, in the next future, better mngement of wste from food pckging. EXPERIMENTAL Mterils Two polylctide smples used for commercil food pckging, one in the form of film nd the other in the form of pn (Fig. 1), were studied s purchsed without ny specil preliminry tretments.

End-Life Prediction of Commercil PLA 683 Fig. 1 Imge of PLA smple 1 (), smple 2 (b).

onset ) of our smples. Instrument clibrtion ws mde following the mnufcturer instructions reported in previous work [22]. Smples of bout 6.

Long-term Degrdtion For the long-term oxidtive isotherml degrdtion, weighed quntities of the two PLA smples in lumin open crucibles were put into n oven t (423 ± 1) K nd there kept in isotherml

Smples were weighed once dy (first two months of heting), then three times week (for three months) nd successively once week, using Mettler AE 240 electronic blnce (± 1 10 5 g).

3 End-Life Prediction of Commercil PLA 683 Fig. 1 Imge of PLA smple 1 (), smple 2 (b). DSC Mesurements Differentil Scnning Clorimetry (DSC) experiments were performed by Mettler DSC 20 clorimeter in order to determine glss trnsition temperture (T g ) nd onset melting temperture (T onset ) of our smples. Instrument clibrtion ws mde following the mnufcturer instructions reported in previous work [22]. Smples of bout g, held in seled luminium pns, heting rte of 10 K min 1 nd sttic ir tmosphere were used for mesurements. Long-term Degrdtion For the long-term oxidtive isotherml degrdtion, weighed quntities of the two PLA smples in lumin open crucibles were put into n oven t (423 ± 1) K nd there kept in isotherml conditions for bout nine months (261 dys). Smples were weighed once dy (first two months of heting), then three times week (for three months) nd successively once week, using Mettler AE 240 electronic blnce (± g). To this im, crucibles were extrcted from oven, cooled in desicctor t room temperture for 1 h, weighed nd then immeditely put gin into the oven. Short-term Degrdtions A Mettler TA 3000 thermogrvimetric nlyser coupled with Mettler TC 10A processor s control nd evlution unit ws used for short-term degrdtions. The temperture clibrtion of thermoblnce ws mde ccording the procedure suggested by the supplier nd reported in previous works [23 25], bsed on the chnge of mgnetic properties of three metl smples (istherm, nickel nd trfoperm) t their Curie points (415.6, nd K, respectively). Isotherml experiments were performed in sttic ir tmosphere s follows: smples ( g), held in lumin open crucibles, were quickly heted (20 K min 1 ) into the thermoblnce from room temperture to the selected one nd then mintined t this temperture for 900 min. The weight of smple t the strt of isotherml heting ws considered the initil one. The short-term isotherml degrdtions of polymers were repeted three times, nd the D verge vlues, where D = (W o W)/W o, nd W o nd W were the weights t the strting point nd during scnning, t vrious times were in greement with the experimentl ones within ± 3% in every cse. All methods, used to clculte the degrdtion ctivtion energy of the studied polymers, ssume tht degrdtion occurs through single stge kinetics, nd then the process is chrcterized by single kinetic triplet. RESULTS AND DISCUSSION Glss trnsition tempertures (T g ) nd onset melting tempertures (T onset ) of PLA smples were t first clorimetriclly determined, nd the results re reported in Tble 1. The long-term isotherml degrdtions of smples 1 nd 2 were performed for nine months t 423 K, temperture slightly higher thn the melting ones of the smples, nd the weight losses were determined t vrious heting times. Tble 1. Glss trnsition temperture (T g ) nd onset melting temperture (T onset ) for PLA smples Smples T g (K) T onset (K)

4 684 I. Blnco In Fig. 2 the percentge of undegrded smple (1 D)% for both polymers re plotted s function of heting time. Smple 1 strted to degrde immeditely differently thn compound 2, which showed n initil induction period (τ) of bout 10 dys, during which only 1% of mss loss ws observed compred with 10% of compound 1 in the sme time. An exponentil degrdtion trend, much more pronounced for smple 1, followed by liner one ws observed for both polymers. The overll mss losses in bout 261 dys of the two PLA smples, in isotherml heting conditions (423 K) nd in sttic ir tmosphere, were 98% nd 71% for compounds 1 nd 2 respectively. Due to their simplicity nd to the consistent time sving, TGA experiments in the scnning mode re more frequently used thn those in isotherml heting conditions for determining kinetic prmeters ssocited with the degrdtion of polymers. Fig. 2 Percentge of undegrded smple (1 D)% t 423 K s function of heting time (t) for PLA smples 1 nd 2 Since my purpose ws to compre the dt from long-term experiments with those from the short-term ones, in order to verify if they were coherent with ech other, I ws forced to perform short-term degrdtions in isotherml heting conditions. TGA experiments were crried out t vrious tempertures, in the rnge K, in ten degrees intervl, in order to obtin E vlues for the degrdtion degrees (D%) included in the 5% 15% rnge bout. This rnge ws chosen becuse bove this percentge bout, polymer is usully not suitble to be used. The mss losses in 900 min of investigted polymers t the vrious tempertures re listed in Tble 2. The isotherml degrdtion t 453 K nd 463 K were not considered for smple 2 becuse of the low mss loss observed t these tempertures. The set of short-term experimentl isotherml dt ws used to estimte the degrdtion ctivtion energy of PLA smples through the McCllum [26] nd the Abte- Blnco [17,18] isotherml methods. The McCllum method, extensively reported by Hill et l [27], is bsed on the following liner eqution: lnt = + b/t iso (1) where t = time employed to rech fixed degree of degrdtion D, = ln[f(1 D)] lna, b = E /R, T iso = temperture of isotherml degrdtion nd F(1 D) is function of degree of degrdtion. Thus the pprent ctivtion energy, t ech degree of degrdtion, cn be obtined by the slope of the lnt versus 1/T iso stright line. The experimentl isotherml dt (lnt nd 1/T iso ) obtined in the investigted temperture rnge ( K) were treted ccording to the eqution (1), giving rise, for ech polymer, to liner lnt = f (1/T iso ) reltionships t the vrious selected degrees of degrdtion. The corresponding liner regression coefficients re reported in Tble 3 for smples 1 nd 2 respectively, together with the clculted degrdtion E vlues.

5 End-Life Prediction of Commercil PLA 685 Tble 2. Mss loss % observed in 900 min for PLA smples t the vrious tempertures of isotherml degrdtions (T iso ) T iso (K) Tble 3. Regression coefficients nd pprent ctivtion energies by McCllum eqution for the isotherml degrdtion of PLA smples t vrious degrees of conversion (D%) 1 2 D% b 10 3 E r b 10 E r (K) (kj/mol) (K) (kj/mol) (± 1.3) 10.8 (± 0.6) (± 5) 19.7 (± 1.9) b 12.1 (± 0.9) b b 101 (± 7) b (± 1.3) 10.7 (± 0.6) (± 5) 19.9 (± 2.1) b 12.3 (± 1.0) b b 102 (± 8) b (± 1.3) 10.7 (± 0.6) (± 5) 19.7 (± 3.4) c 12.4 (± 1.7) c c 103 (± 14) c (± 1.3) 10.8 (± 0.6) (± 5) 19.7 (± 3.6) c 12.5 (± 1.8) c c 104 (± 15) c (± 1.4) 10.8 (± 0.7) (± 6) 19.1(± 2.2) c 12.2 (± 1.1) c c 101 (± 9) c Determined in the rnge K; b Determined in the rnge K; c Determined in the rnge K The experimentl vlues t 423 K of studied polymers, t the sme selected degrees of degrdtion, were lter considered for liner regressions. As shown in Fig. 3 the vlues t lower temperture, included into the corresponding single McCllum stright lines, gve rise to mrked devition of linerity, thus suggesting higher degrdtion pprent ctivtion energy t 423 K. The sme set of dt ws used to derive degrdtion E vlues ccording to the Abte-Blnco method. The experimentl weight loss dt t vrious times of isotherml degrdtions were simply trnsformed ll together in D vlues nd then fitted into pproprite D = f(t) equtions. The corresponding degrdtion curves t 473 K, in the considered degrdtion rnges (5% 15%), re reported s exmples in Fig. 4. Fig. 3 McCllum stright lines for smple 1 () nd smple 2 (b) t vrious D vlues: 5% ( ), 7.5% ( ), 10% ( ), 12.5% ( ) nd 15% (Δ)

6 686 I. Blnco Fig. 4 Experimentl D vlues s function of heting time (t) nd corresponding smoothed D = f (t) degrdtion stright line t 473 K for smple 1 () nd smple 2 (b) D = D o + βt (2) Liner reltionships were obtined for both polymers nd the regression coefficients t vrious tempertures re reported in Tble 4. The coefficient of time β, whose dimensions re t 1, then representing mesure of decomposition rte, incresed exponentilly s function of 1/T iso, thus giving rise to Arrhenius type equtions. ln β = lna B/T iso (3) Tble 4. Regression coefficients of the D = D o + β t equtions t the vrious tempertures of isotherml experiments for the (5% D 15 %) degrdtion stge of PLA smples 1 2 T iso (K) D o β (min 1 ) r D o β (min 1 ) r In Figs. 5() nd 5(b) were reported the Arrhenius plots for both 1 nd 2 smples, respectively, where the used β vlues were those obtined by Abte-Blnco method in the short-term degrdtion experiments t higher tempertures (453 K T 523 K) nd tht obtined by the sme method in the long-term degrdtion t 423 K. Differently from wht ws observed for the McCllum reltionships (Figs. 4 nd 4b), the point t 423 K fell on the sme stright line obtined with the points t higher tempertures, thus suggesting the sme kinetics of degrdtion. The corresponding liner regression coefficients for the considered degrdtion degree rnges re reported in Tble 5. The E vlues clculted both excluding nd including the point t 423 K seem to confirm unique kinetic of degrdtion only for the compound 2, for which n ctivtion energy of degrdtion of 106 (± 13) kj mol 1 ws obtined on considering only the β vlues from short-term experiments compred to E vlue of 109 (± 7) kj mol 1 clculted by including the coefficient chieved t 423 K. By contrst the E vlues of smple 1 were quite different, 90 (± 6) kj mol 1 on considering only the β vlues from short-term experiments nd

7 End-Life Prediction of Commercil PLA (± 7) kj mol 1 by including the point t 423 K, mking questionble the linerity between β vlues t higher nd lower tempertures reported in Fig. 5(), nd then indicting different (lower) degrdtion rte vlues thn those predictble on the bsis of the experiments t higher tempertures. In Tble 6 the degrdtion E vlues determined by McCllum nd Abte-Blnco methods re listed. Fig. 5 Arrhenius plot of β vlues obtined by Abte-Blnco method for smple 1 () nd smple 2 (b) from short- nd long- term experiments Tble 5. Correltion coefficients of the lnβ = lna B/T iso Arrhenius type eqution obtined ccording to the Abte-Blnco method for PLA smples, nd corresponding degrdtion E vlues Smple lna B 10 3 r E (kj/mol) (± 1.5) 10.8 (± 0.7) (± 6) 1 b 24.1 (± 1.8) 12.9 (± 0.8) (± 7) (± 3.0) 12.8 (± 1.6) (± 13) 2 b 23.5 (± 1.8) 13.1 (± 0.9) (± 7) Determined in the rnge K; b Determined in the rnge K Tble 6. Degrdtion E vlues (kj mol 1 ) for PLA smples determined by Abte-Blnco nd McCllum (verge vlues) methods Smple Abte-Blnco McCllum Abte-Blnco b 1 90 (± 6) 90 (± 5) 107 (± 7) (± 13) 102 (± 11) 109 (± 7) Determined in the rnge K; b Determined in the rnge K The degrdtion E vlues reported for the McCllum method were verged from those obtined t vrious degrees of conversion in the considered D rnge. It ppers correct becuse the differences mong the vlues t vrious degrees of degrdtion re substntilly ll within the stndrd devitions. The dt in Tble 6 concerning the different smples of PLA indicte tht the degrdtion E vlues obtined through the two methods, in the sme temperture rnge, re in good greement with ech other, confirming the vlidity of the simple nd fst Abte-Blnco method to obtin degrdtion E vlues of polymers exclusively by fitting the experimentl TGA curve in the desired degrdtion rnge. In Tble 6 is then highlighted the difference between the degrdtion E vlues of smples 1 nd 2, obtined with the Abte-Blnco method, clculted by including or less the point t 423 K. On the one hnd the ctivtion energy of smple 2, clculted in the whole experimentl temperture rnge, lmost equl to tht obtined by short-term mesurements would seem to suggest n greement, nd then the possibility to extrpolte the degrdtion behvior of the polymer t lower tempertures nd lifetime prediction. By contrst the difference of 17 kj mol 1 found, for smple 1, in the two different temperture rnge mkes chncy this type of prediction.

8 688 I. Blnco Fig. 6 Rel nd expected (extrpolted from McCllum equtions) percentge of degrdtion for smple 1 () nd smple 2 (b) In order to quntify the uncertinty in the lifetime prediction of the studied polymers, in Fig. 6 the trends of the rel degrdtion nd tht extrpolted from the McCllum equtions, obtined through the short term experiments t the sme degree of degrdtion, re shown for smples 1 nd 2 respectively. It is quite evident the difference between the rel nd the expected trend of degrdtion, it is worth to note, t 30% of degrdtion degree, difference of bout 12 dys for smple 1 (Fig. 6) nd bout 16 dys for smple 2 (Fig. 6b) t 20 % of degrdtion degree. CONCLUSIONS The exmined commercil smples of PLA hve reched degree of degrdtion of 15% t temperture ner their melting fter pproximtely 12 (Smple 1) nd 30 (Smple 2) dys respectively, thus suggesting the impossibility of performing degrdtion experiments below the glss trnsition temperture in resonble time. Without going into the merits of wht could be the predominnt mechnism of degrdtion in the temperture rnge investigted, the results of the present study demonstrte, for commercil PLA polymers used in food pckging, the questionbility of degrdtion E vlues extrpolted to lower tempertures from mesurements t higher ones nd then of degrdtion rtes nd lifetime predictions bsed on mesurements t higher tempertures. In the future, through rel mesurements of degrdtion, crried out t reltively low temperture, it could be possible to crete dtbse of the degrdtion times of the most commercilly used polymers, so s to obtin series of correction fctors for the kinetic prmeters chieved through short-term mesurements. REFERENCES 1 Mxwell, A.S., Broughton, W.R., Den, G. nd Sims, G.D., Ntionl Physicl Lbortory Report DEPC MPR 016: Middlesex, Guedes, R.M., Compos. Prt A-Appl. S., 2006, 37(5): Akbş, A., Aksoy, S. nd Hsırcı, N., Polymer, 1994, 35(12): Hmid, S.H. nd Amin, M.B., J. Appl. Polym. Sci., 1995, 55(10): Flynn, J.H., J. Therm. Anl. Clorim., 1995, 44(2): Dobkowski, Z. nd Rudnik, E., J. Therm. Anl. Clorim., 1997, 48(6): Gupt, Y.N., Chkrborty, A., Pndey, G.D. nd Setu, D.K., J. Appl. Polym. Sci., 2004, 92(3): 1737

9 End-Life Prediction of Commercil PLA Vyzovkin, S., Burnhm, A.K., Crido, J.M., Pérez-Mqued, L.A., Popescu, C. nd Sbirrzzuoli, N., Thermochim. Act, 2011, 520(1 2): 1 9 Toop, D.J., IEEE T. Electr. Insul., 1971, EI-6(1): 2 10 Anderson, H.L., Kemmler, A. nd Strey, E., Thermochim. Act, 1996, 271: Burnhm, A.K. nd Brun, R.L., Energ. Fuel., 1999, 13(1): 1 12 Opfermnn, J., J. Therm. Anl. Clorim., 2000, 60(2): Roduit, B., Thermochim. Act, 2000, 355(1 2): Roduit, B., Borget, C., Berger, B., Folly, P., Alonso, B., Aebischer, J.N. nd Stoessel, F., J. Therm. Anl. Clorim., 2005, 80(1): Vyzovkin, S. nd Linert, W., Chem. Phys., 1995, 193(1 2): Budrugec, P., Thermochim. Act, 2013, 558: Abte, L., Blnco, I., Mott, O., Pollicino, A. nd Recc, A., Polymer Degr. Stbil., 2002, 75(3): Abte, L., Blnco, I., Pollicino, A. nd Recc, A., J. Therm. Anl. Clorim., 2002, 70(1): Abte, L., Blnco, I., Orestno, A., Pollicino, A. nd Recc, A., Polym. Degrd. Stbil., 2005, 87(2): Blnco, I., Abte, L. nd Antonelli, M.L., Polym. Degrd. Stbil., 2011, 96(11): Blnco, I., Abte, L., Antonelli, M.L. nd Bottino, F.A., Polym. Degrd. Stbil., 2013, 98(11): Abte, L., Bde, E., Blnco, I. nd Dell Gtt, G., J. Chem. Eng. Dt., 2008, 53(4): Abte, L., Blnco, I., Ppplrdo, A. nd Pollicino, A., J. Therm. Anl. Clorim., 2001, 65(2): Abte, L., Blnco, I., Cicl, G., Recc, A. nd Restucci, C.L., Polym. Degrd. Stbil., 2006, 91(12): Blnco, I., Oliveri, L., Cicl, G. nd Recc, A., J. Therm. Anl. Clorim., 2012, 108(2): McCllum, J.R., Comprehensive Polymer Science Vol. 1; Pergmon Press: Oxford, Hill, D.J.T., Dong, L., O'Donnell, J. H., George, G. nd Pomery, P., Polym. Degrd. Stbil., 1993, 40(2): 143

Determination of the activation energy of silicone rubbers using different kinetic analysis methods

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