comparison of the degradability of poly(lactide) packages in composting and ambient exposure...

PACKAGING TECHNOLOGY AND SCIENCEPackag. Technol. Sci. 2007; 20: 49–70Published online 4 August 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pts.742

Comparison of the Degradability of Poly(lactide) Packages in Composting and Ambient Exposure Conditions

By Gaurav Kale, Rafael Auras* and Sher Paul SinghSchool of Packaging, Michigan State University, East Lansing, MI-48823-1223, USA

The adoption of biodegradable polymeric materials is increasing in food andconsumer goods packaging applications, due to concerns about the disposal ofpetroleum-based polymers and the increasing cost of petroleum-based polymerresins. Currently, poly(lactide) (PLA) polymers are the biggest commerciallyavailable bio-based polymeric packaging materials. As the main motivation foradopting biopolymers is environmental, there is a need to address the degradabilityand environmental performance of biodegradable packages. The aim of this studywas to investigate and compare the degradation of two commercially availablebiodegradable packages made of PLA under real compost conditions and underambient exposure, using visual inspection, gel permeation chromatography,differential scanning calorimetry and thermal gravimetric analysis. A noveltechnique to study and track the degradability of these packages under realcompost conditions was used. Both packages were subjected to composting andambient exposure conditions for 30 days, and the degradation of the physicalproperties was measured at 1, 2, 4, 6, 9, 15 and 30 days. PLA bottles made of 96%L-lactide exhibited lower degradation than PLA delicatessen (‘deli’) containersmade of 94% L-lactide, mainly due to their highly ordered structure and, therefore,their higher crystallinity. The degradation rate changed as the initial crystallinityand the L-lactide content of the packages varied. Temperature, relative humidityand pH of the compost pile played an important role in the rate of degradation ofthe packages. First-order degradation kinetics and linear degradation trends wereobserved for both packages subjected to composting conditions. Copyright © 2006John Wiley & Sons, Ltd.Received 7 December 2005; Revised 31 March 2006; Accepted 7 April 2006

KEY WORDS: biopolymers; compostability; degradation; GPC, hydrolysis; poly(lactide)

* Correspondence to: R. A. Auras, 140 Packaging Building, School of Packaging, Michigan State University, East Lansing, MI 48823-1223, USA.E-mail: [email protected]

Copyright © 2006 John Wiley & Sons, Ltd.

INTRODUCTION

There is an increasing demand for biopolymer-based packaging materials that are easily renewable, providing enhanced environmentalperformance. Until recently the majority of the

production of packaging plastics was based onnon-renewable materials. Plastic packaging mate-rials are often landfilled due to their content offoodstuffs and other biological substances, makingphysical recycling of these materials impractical.Use of biopolymer-based packaging materials

reduces concerns such as landfilling, sorting andreprocessing by taking advantage of their uniquefunctionality, i.e. compostability. Hence, com-postability has been the main focus of applicationsof biobased packaging materials, which is thenatural outcome for a vast amount of food andpharmaceutical packaging materials and waste.Composting permits disposal of biodegradablepackages and is not as energy intensive as sortingand reprocessing for recycling, although it requiresmore energy than landfilling. For example, incountries like the USA, where landfilling is predominant, composting at this time is moreexpensive.1

Composting is the controlled and naturaldecomposition of organic materials by microor-ganisms. The organic materials are decomposedinto a soil-like substance called humus. The majorgroups of microorganisms involved in compostingare fungi, bacteria and actinomycetes. Microor-ganisms need food in the form of carbon, nitrogen,oxygen and water. Organisms decompose theorganic matter by utilizing carbon as a source ofenergy and nitrogen for building cells. A 30 :1carbon:nitrogen ratio is an ideal proportion for thereproduction of thermophilic microorganisms.2 Acompost pile goes through two composting stages.In the first stage, the temperature rises up toaround 60°C, so long as oxygen, carbon and nitro-gen are available in the ideal proportions, and promotes strong microbial activity. In the second‘curing’ stage, the decomposition continues at a slower rate and the last remaining nutrients are consumed by microorganisms and until almostall the carbon has been converted to carbondioxide.

The compostability of a compostable plastic (i.e. ‘aplastic that undergoes degradation by biologicalprocesses during composting to yield CO2, water,inorganic compounds, and biomass at a rate con-sistent with other known compostable materialsand leaves no visible, distinguishable or toxicresidue’3) is commonly evaluated in simulatedcompost conditions and by assessing the finalquality of the compost. While evaluating the com-postability, plastics are subjected to mechanical,thermal and chemical degradation, of which chem-ical degradation is the most important. Biodegrad-able polymers first become susceptible to waterattack and chemical degradation initiates the

polymer erosion as a result of hydrolysable func-tional groups in the polymer backbone. Standardshave been developed for evaluating the degrada-tion and compostability of a biopolymer by theAmerican Society for Testing and Materials(ASTM), the International Standards Organization(ISO) and the European Committee for Standard-ization. The ASTM standards (i.e. ASTM D5338-98,4 D6003-96,5 D6954-04,6 D6400,3 and D 6002-96(reapproved 2002)e1,7) developed by subcommittee20.96 for assessing compostability, are laboratory-scale and limited to the evaluation of plastic mate-rials.3,5–7 Similarly ISO standards, such as ISO14851,8 148529 and 14855,10 allow evaluation ofmaterials under laboratory conditions and arebased on measuring the carbon dioxide evolutionand oxygen demand during degradation. The EN13432 :200011 standard developed by the EuropeanCommittee for Standardization addresses com-postability referring to ISO standards and evalu-ates the compost quality and toxicity. Theabove-mentioned standards mainly focus onaddressing the compostability of a polymer or amaterial, but not that of a package in real condi-tions. The degradation time of an entire package asencountered in the case of full-scale facilities thatdo not grind feedstock may be much longer thanwhen the polymer pieces are ground, representinga worst-case scenario for compostability. More-over, poor representation of actual compostingconditions is a major negative aspect, since mistaken conclusions could easily be drawn as biodegradation mechanisms vary among substrates.

The adoption of biodegradable polymeric mate-rials is increasing in food and consumer goodspackaging applications, due to concern about thedisposal of petroleum-based polymers. Currently,poly(lactide) (PLA) polymer developed by CargillDow LLC (Blair, NE) and at this time under thename of NatureWorks® LLC, is the biggest com-mercially available bio-based polymeric packag-ing material. NatureWorks® LLC is producing 300million lb PLA annually for a variety of packagingand fibre applications. Eastman ChemicalCompany (Hartlepool, UK), has developed EastarBio aliphatic co-polyester, which is being used inlawn and garden bags, food packaging and horti-cultural applications worldwide. Similar to Eastar,Proctor and Gamble Co. (P&G) (Cincinnati, OH)

G. KALE, R. AURAS AND S. P. SINGHPackaging Technologyand Science

Copyright © 2006 John Wiley & Sons, Ltd. 50 Packag. Technol. Sci. 2007; 20: 49–70DOI: 10.1002/pts

has produced an aliphatic co-polyester (Nodax)line of polymers that are biodegradable in aerobicand anaerobic conditions. The Nodax polymers areproduced by microorganisms through a fermenta-tion process, and the plastics are extracted from thebiomass. DuPont has a 200 million lb/year production facility in Tennessee for its Biomaxpolyethylene terephthalate co-polymer hydro/biodegradable polyester, which is available bothoverseas and in the USA.12

PLA polymer, the predominant biopolymer inthe market for packaging applications, can be man-ufactured by carbohydrate fermentation or chem-ical synthesis. Lactic acid (2-hydroxypropionicacid) is the simplest hydroxyl acid with an asym-metric carbon atom, and it exists in two opticallyactive configurations, the l(+) and d(−) isomers.The majority of lactic acid is made by bacterial fer-mentation of carbohydrates. The fermentationprocesses to obtain lactic acid can be classifiedaccording to the type of bacteria used. High mole-cular weight PLA can be obtained using differentmethods: (a) direct condensation polymerization;13

(b) azeotropic dehydrative condensation polymer-ization, currently used by Mitsui Toatsu;13 and (c)polymerization through lactide formation, devel-oped by Cargill Inc. in 1992.13 The properties ofhigh molecular weight PLA are determined by thepolymer architecture (i.e. the stereochemical make-up of the backbone) and the molecular mass,which is controlled by addition hydroxylic com-pounds.13 The ability to control the stereochemicalarchitecture permits precise control over the speedof crystallization and finally the degree of crys-tallinity, the mechanical properties and the pro-cessing temperature of the material.13 In addition,the degradation behaviour strongly depends onthe crystallinity of the PLA.13 The glass transitiontemperature (Tg) is in the range 50–80°C, while the melting temperature (Tm) is in the range130–180°C. PLA can be processed by injectionmoulding, sheet extrusion, blow moulding, ther-moforming and film forming. PLA is approved bythe Food and Drug Administration for its intendeduse in fabricating articles in contact with food.13

Life-cycle assessment (LCA) and economic studiesindicated that PLA polymers are more energy-efficient than PP and PS polymers,14–17 which ismainly because PLA consumes almost no feed-stock energy.

Currently, PLA is being commercialized andused as a food packaging polymer for short shelf-life products with common applications, such ascontainers,18 drinking cups, sundae and saladcups, overwrap and lamination films, blister pack-ages, and bottles. As the PLA consumption isincreasing, there is a need to address its com-postability in real composting conditions and itspotential recyclability. In 2003 in the USA,1 15 full-scale solid waste composting facilities (i.e. ‘onesthat include the residential waste stream thatarrives at the plant as mixed waste or source sep-arated fractions’1) were in operation. Hence forPLA to be considered as an alternative to conven-tional polymers, a wide range of composting facil-ities needs to be developed, or PLA will need to becomposted with general yard waste. Looking atPLA’s potential recyclability, NatureWorks® LLCinstituted a large volume ‘buy-back’ programme inNorth America for post-consumer treatment ofPLA bottles in mixed plastic waste recyclingstreams.19 PLA can be sorted from other plasticsusing near-infrared technology. However, asalready mentioned, recyclability is not an alterna-tive for containers with foodstuff contents.

As previously described, the standards devel-oped so far mainly address the compostability ofplastic in simulated conditions and correlated toevolution of CO2. There are several parameterswhich differentiate the real and simulated or controlled composting conditions. According toASTM D 6002-96(2002)e1,7 and the Federal TradeCommission (FTC),20 ‘compostable claims wouldbe appropriate on products or packages that willbreak down, or become part of usable compost, insafe and timely manner in home compost piles’,20

where ‘timely manner’ means the time necessaryfor leaves, grass and foodstuffs to compost. Someof the commercially available ‘biopolymer materi-als’ comply with the standards of compostability,but generally packages or containers made fromthese materials were not evaluated. According toASTM D 6400-04,3 ‘products and finished articlesshould be tested in the same form as they areintended to be used’. Therefore, if the packagesshow different chemical composition or structure,it is necessary to test them to evaluate their compostability.

The aim of this paper is to provide informationabout the comparison of degradability of two


DEGRADABILITY OF POLY(LACTIDE) PACKAGES Packaging Technologyand Science

commercially available biodegradable packages inreal composting and ambient environments,further correlating and comparing their degrada-tion through visual inspection and analysis ofphysical properties. The physical propertiesanalysed were: molecular weight, using gel per-meation chromatography (GPC); glass transition(Tg) and melting temperature (Tm), using differen-tial scanning calorimetry (DSC); and decomposi-tion temperature (TD), using a thermogravimetricanalysis (TGA).

MATERIALS AND METHODS

Packages

Poly(lactide) bottles were obtained from Nature-Works® LLC (Blair, NE) and commercialized byBiota brands of America (Telluride, CO) with 96%l-lactide and bluetone additive, height = 0.2m and base diameter = 0.0065m (volume = 500ml).Delicatessen (‘deli’) containers were obtained fromWilkinson Manufacturing Company (FortCalhoun, NE) with 94% l-lactide; height = 0.07mand base diameter = 0.09m (volume = 600ml).Figure 1a, b shows these containers.

Compost pile

A compost pile used for this study was composedof cow manure and wood shavings prepared at theMichigan State University Composting Facility(East Lansing, MI) and was used for the study. Ini-tially, 11.6m3 cow manure and 7.8m3 wood shav-ings were mixed. This mixture was combined withwaste feed (i.e. the feed that the cows do not eatbetween feedings) in a proportion of 2 :1. Themixture allowed a carbon:nitrogen ratio of 30 :1.The mixture was kept in a rectangular bay of 36.5× 3.6 × 1.8m, which was turned using a Marvelmodel of a commercial turner manufactured byGlobal Earth (Ontario, Canada) 3 days/week for 3weeks. Due to turning, the mixture was heated to60°C in the presence of aeration. This temperaturewas enough to kill weed seeds and pathogens.Later, the mixture was pulled out of the bay, and apile of 24 × 6 × 3m was built up on an asphalt pad.

Initially the compost parameters, such as temper-ature, moisture and pH, were measured and deter-mined. A temperature of 65 ± 5°C, moisture of 63± 5% and pH of 8.5 ± 0.5 was observed. Figure 2shows a two-dimensional graph of the tempera-ture distribution inside the compost pile at thebeginning of the testing.

Box

Wooden boxes of dimensions 0.6 × 0.3 × 0.1m weremanufactured using treated wood and were usedfor subjecting packages into the compost pile.Those boxes facilitated the exact location and



(a)

(b)

Figure 1. PLA containers: (a) bottle and (b) deli container.

identification of the package in the compost pile;and also the removal of the package and portionof compost for analysis. A mesh of 0.011 gauge was fitted at the bottom of the boxes. A three-dimensional image of a wooden box is shown inFigure 3.

Plastic containers

Foldable reusable plastic containers obtained fromCHEP-USA (Orlando, FL) of dimensions 0.6 × 0.4× 0.4m were used to contain and expose the pack-ages to ambient conditions.

Placement of packages

Composting exposure. Both bottles and deli con-tainers were placed in duplicate sets in thecompost pile with the help of boxes, as mentionedabove, at approximately 1.2m above the groundand 1m inside the compost pile, where a uniformcomposting temperature was obtained during the experiment. Initially, the compost was placedover the mesh in the box; later, a package wasplaced with the addition of compost completelyover the box. The handle on the box facilitatedidentification of the exact location of the boxes inthe pile.



Figure 2.Temperature distribution inside the compost pile at thebeginning of the testing.

Ambient exposure. The packages were placed inthe plastic containers mentioned above in dupli-cate sets, as in case of composting exposure. Thepackages were taken out at 1, 2, 4, 6, 9, 15 and 30 days from both the compost pile and plasticcontainers.

Compost parameters

Temperature. Temperatures were recorded everytime the packages were removed from the compostpile, using a A60FR fast response windrow stain-less steel thermometer (±1°C) obtained fromReotemp (San Diego, CA). Temperatures were con-tinuously recorded using H12 Type J HOBO®

brand battery-operated data loggers obtained fromOnset Computers (Pocasset, MA) at 6h intervalsfor the duration of the study.

Moisture content. The wet weight moisturecontent of the compost was measured using amodified version of ASTM D4643-0021 (previouslyvalidated using a traditional vacuum oven).22 Asample of compost was obtained whenever pack-ages were taken out of the compost pile andchecked immediately for the moisture content. Ini-tially the wet weight of compost was recorded andthen heated in a 600W microwave, ModelMW8625W, obtained from Emerson Radio Corpo-ration (Parisppany, NJ) for 3min. The weightreduction in the compost due to evaporation ofmoisture was recorded, and the sample was againsubjected to microwave heating for 1min. The

cycle of recording the weight and heating for 1minwas continued until constant weight was obtained.The percentage wet weight moisture content wasdetermined by the ratio of the difference betweenthe weight of the moist and oven-dried specimensto the total weight of the moist specimen.

pH. The protocol for measuring pH of compostwas originally obtained from Cornell Compost-ing.22 After the compost was dried through themicrowave heating process, 5g of the specimenwas added to 25ml deionized water. This mixturewas stabilized for 5min before the pH of the solu-tion was recorded using a calibrated pH paper(150AB pHydrion paper dispenser), obtained fromMicro Essential Laboratory Inc. (Brooklyn, NY).

Ambient parameters

Hourly data for ambient parameters such as tem-perature, relative humidity and solar radiationwere obtained from the Michigan AutomatedWeather Network (East Lansing, MI) located at42.6734° latitude, −84.4870° longitude and 264melevation, for the complete duration of the study.The air temperature measurements were taken 1.5m above ground level. Figure 4a shows themaximum and minimum ambient temperatures;Figure 4b shows the maximum and minimum rel-ative humidity during the 30 day testing period;and Figure 4c shows the average daily solar radi-ation during the same period.

Visual inspection

Every time that the packages were removed fromthe compost and ambient exposure conditions theywere visually inspected by the authors. A SonyCybershot DSC-P150 7.2 MegaPixel digital camerawas used to take pictures. The packages wereinspected for colour, texture, shape and changes indimensions.

Physical properties

Thickness. The thickness of the packages wasdetermined using a Magna Mike 8000 thickness



Figure 3. 3D view of the box.

gauge, which utilizes a magnetic method manu-factured by Panametrics (Japan) according toASTM D4166-99 (2004)e1.23

Molecular weight. The molecular weight wasdetermined using a standard GPC technique. A 600Multisolvent delivery system equipped with 717

autosampler and 2410 RI detector from Waters(Milford, MA) was used to determine the molecu-lar weight of samples after extraction. Inhibitor-free tetrahydrofuran (THF) solution obtained fromSigma Aldrich (Milwaukee, WI) was transferred to2ml vials containing 2mg of specimen. The vialswith the specimens were manually shaken for 2min. The dissolved samples were filtered with 0.2mm pore size, 13mm disposable PTFE (polytetra-fluoroethylene) filters obtained from Whatman(Florham Park, NJ). Diluted solution was trans-ferred to the 1ml clear glass shell vials used in theautosampler and capped using polyethylene snapcaps; both obtained from Waters (Milford, MA).Two PL gel 10mm MIXED-B 300 × 7.5mm i.d.columns from Polymer Laboratories (Amherst,MA) in series were used, giving a detection rangeof 1000–10000000Da. Polystyrene obtained fromSigma Aldrich (Milwaukee, WI) was used as astandard for calibration purposes. Experimentswere run at 35°C. Sample concentrations for poly-styrene and PLA samples were 1mg/ml at a flowrate of 1ml/min.

Glass transition and melting temperature,enthalpy of fusion and crystallinity. The glasstransition temperature, melting temperature andcrystallinity were determined using a DSC Q-100made by TA Instruments (Newcastle, DE) in accor-dance with ASTM D 3418-03.24 The DSC standardcalibration procedure was performed according toASTM E967-0325 and ASTM E968-02.26 Analyses ofthe results were made using Universal analysissoftware (version 3.9A). The percentage crys-tallinity was determined according to ASTMD3417-9927 and equation 1:

(1)

where ∆Hc is the enthalpy of cold crystallization,∆Hm is the enthalpy of fusion, and ∆Hc

m is theheating of melting of purely crystalline PLA, 135J/g.28,29

Decomposition temperature. The decompositiontemperature was obtained using a TGA TA 2950made by TA Instruments (Newcastle, DE) in accor-dance with ASTM E1131-03.30 The specimens wereheated at a rate of 20°C/min from 23°C to 500°C

xH H

Hc

c m

mc

%( ) = × +100

∆ ∆∆



Sola

r R

adia

tion

103 , k

J/m

2

Figure 4. (a) 30 days maximum and minimumtemperature data. (b) 30 days maximum and minimumrelative humidity data. (c) 30 days average total solar

radiation.

in the presence of inert gas (N2) and oxidative gas(O2), both > 90p.s.i. The results were analysedusing Universal analysis software (version 3.9A).

Statistical analysis

All treatments were conducted in duplicate. Sta-tistical analyses were carried out using the GeneralLinear Models procedure in JMP software (SASInstitute Inc., SAS Campus Drive, Cary, NC).

RESULTS AND DISCUSSION

Poly(lactide) bottles and deli containers were sub-jected to composting and ambient exposure condi-tions for a period of 30 days. Table 1 shows theinitial physical properties of these packages. ThePLA bottles, made of 96% l-lactide, are a morehighly ordered structure, resulting in a highercrystallinity than the deli containers, with 94% l-lactide. PLA derived from >93% l-lactic acid canbe semi-crystalline.13,31 Meso- and d-lactide inducetwists in the otherwise very regular poly(l-lactide)molecular architecture. Molecular imperfectionsare responsible for the decrease in both the rateand extent of poly(l-lactide) crystallization. In thisstudy, the deli container had a higher molecularweight than the bottle and a lower initial polydis-persity index (PDI).

The containers were introduced and located inthe compost pile as described above. The temper-ature, relative humidity and pH to which the three

packages were exposed during the compostingconditions are shown in Figure 5a, b. pH is one ofthe most important factors of hydrolytic polymerdegradation, since pH variations can changehydrolysis rates by a few orders of magnitude.32–35

In this study, there was a slight alkalization of thepile after the second day of testing, although thisdifference was not statistically significant at the a = 0.05 level (p = 0.91) during the 30 days of composting.

Visual inspection

Pictures showing the degradation process of thebottles and deli containers in the compost pile are



Figure 5. (a) Temperature and relative humidity of thecompost pile at time of package removal. (b) pH of the

compost pile at time of package removal.

Table 1. Physical properties of the poly(lactide) bottles and deli containers

Properties Bottle Deli

L-Lactide (%) 96 94Molecular weight (kDa) 209.3 ± 1.06 222.7 ± 9.20PDI 1.72 1.66Tg (°C) 60.6 ± 0.3 62.6 ± 4.3Tm (°C) 151.0 ± 0.1 149.0 ± 1.1Crystallinity (%)a 12.2 ± 1.4 1.4 ± 0.3a Percentage crystallinity was calculated according to equation 1.

presented in Figures 6, 8. Figure 6 shows the degra-dation of the PLA bottles over the 30 days. Initiallythe bottles decreased in size. The change in shapeis because of distortion due to the higher temper-atures in the compost pile. A similar degradationprocess can be seen in Figure 8 for deli containers.From the first day of being in the compost pile,shape changes were observed in both packages.The dimensions of the containers before and aftercomposting until the bottles and deli containersstarted to fragment were calculated by measuringthe variations in width, length, height and thick-ness of the containers. Variation in thicknesses andshapes were observed on both packages from day1, when the bottle dimensions reduced to 90% and

those of the deli container reduced to 22.22% oftheir original volumes. Colour changes wereobserved in the deli containers, which becamewhite at the bottom. On day 4, the bottle structuresseemed the same as on the first day, but withshorter dimensions by approximately 63.4% of theoriginal volume, whereas the deli containersshowed toughness in the material. On day 6, bottlebreakdown at the neck was observed and bottlethreads were already separated. Colour changeand brittleness were also observed. The deli con-tainers showed a similar rate of degradation. Onday 9, the bottle colour showed white, blue andyellow shades and a powdery and more brittletexture; the deli containers started breaking apart,



Figure 6. Pictorial view of the PLA bottles exposed at day 30 ofcompost conditions.

had powdered structure, and were very brittle. Byday 15, the bottle walls and necks were almostdegraded except for the parts of the bottle threadsand bottom that still had some residues, whereasthe deli containers showed some residues in theform of a yellowish film, although it could not beidentified whether the residue was from the wallsor bottom of the containers. Some residuals fromthe bottles were still observed on day 30. The resid-uals were mostly part of bottle threads and in theform of string-like structures. Bottles and deli con-tainers exposed to ambient conditions were alsoexamined by visual inspection. Figures 7, 9 showthe degradation process of the bottles and deli con-tainers in ambient environments. The packagesfaced different atmospheric conditions such as

solar radiation, rain, snow, wind and variableatmospheric pressures (Figure 4a–c). No visibledifference was observed in either of the packagesexposed to ambient exposure conditions for theduration of the study.

Physical properties

Molecular weight. The molecular weight wasmonitored using the standard GPC technique, aspreviously described. The molecular weightdegradation gives information about the mainfragmentation which occurs in a polymer. PLApolymers, by having —C—O— ester linkages inthe polymer backbone that are hydrolysable func-



Figure 7. Pictorial view of the PLA bottles exposed for 30 days underambient conditions.

tional groups (see Scheme 1), are susceptible tohydrolysis.

Initially, random non-enzymatic chain scissionof the ester groups leads to a reduction in molecu-lar weight and is accelerated by acids or bases,affected by temperature and moisture levels.13

Embrittlement of polymers occurs with reduction

of molecular weight to around 50000Da. The PLAdegradation is driven by the hydrolysis and cleav-age of the ester linkages in the polymer backbone,autocatalysed by the carboxylic acid end-groups.This part of the process follows first-order kinetics.

Secondly, low molecular weight PLA (Mn < 10000) or low molecular weight oligomers are metab-



Figure 8. Pictorial view of the PLA deli containers exposed at day 30 ofcompost conditions.

olized by microorganisms to yield carbon dioxideand water. In general, high temperature andhumidity (50–60°C and RH > 60%) will cause PLAto degrade rapidly.36 Mainly, the polymer degra-dation rate is determined by the nature of the func-tional group and the polymer reactivity with waterand catalysts. Although the degradation process inPLA is a simple hydrolysis, any factor that affectsthe reactivity and accessibility, such as particle sizeand shape, temperature, moisture, crystallinity,isomer percentage, residual lactic acid concentra-tion, molecular weight, molecular weight distribu-tion, water diffusion and metal impurities from thecatalyst, will affect the PLA degradation rate.13,37,38

The variation in molecular weight of bottles anddeli containers is shown in Figure 10. PLA bottlesand deli containers exposed to ambient conditionsfor 30 days did not show significant Mw variationas a function of time at the a = 0.05 level (p > 0.01)(Figure 10a, c). Also, the variations of PDI for thebottles and deli containers were not statisticallysignificant at the a = 0.05 level (p > 0.01 for thebottle and deli containers). Kai-Lai and Pometto

(1999) showed that the exposure of poly(lactide)films to ultraviolet light (UV) for 8 weeks enhancesthe degradation rate and the deterioration of themechanical properties of these polymers.39 In thisstudy, the bottles and deli containers were exposedfor 4 weeks to ambient conditions and comparedwith the time taken by the packages to degrade inthe compost environment.

Figure 10a, c, shows the molecular weightchange of the packages that were exposed tocompost conditions for 30 days. Figure 10a showsthat the molecular weight variation of the PLAbottles at the first 15 days of composting is muchlower than the PLA deli containers, as shown inFigure 10c. Both high molecular weight polymersare reduced to low molecular weight polymer bya combination of chain scission and removal ofrepeat units from the chain ends of the polymer.This leads to fragmentation or dissolution of thepolymer. Major fragmentation with Mw < 10000 ofbottles and deli were observed at day 15. At day30 it was not possible to locate any residues of thedeli containers. Further enzymatic action yields



Figure 9. Pictorial view of the PLA deli containers exposed for 30 daysunder ambient conditions.



Scheme 1. PLA hydrolysis and molecular weight loss.

oligomeric fragments and simple organic com-pounds that are intermediates of the biodegrada-tion process. The PLA bottles (Figure 10a) show asmall increase in the molecular weight after beingexposed to the compost pile for 1 day. This increasein molecular weight could mainly be attributed tocross-linking or recombination reactions. In thecase of PLA, the slow degradation rate produces aloss of molecular weight over the polymer cross-section, following first-order kinetics.40 Therefore,by fitting the data of the variation of the molecu-lar weight as a function of time (equation 2), wecan observe that the Mw degradation of the bottles

and deli containers correlated well with a first-order kinetic process:

(2)

Table 2 shows the estimate of a and b values fromequation (2) and their statistical level of signifi-cance for the PLA bottles and deli containers. Theb (the pre-exponential factor of equation 2) valuesindicate the degradation rate. Higher b valuesdenote higher degradation rates. We can observethat PLA bottles have a higher b value (b = 0.18 ±0.05) than deli containers (b = 0.15 ± 0.04) (solid linein Figure 10c). However, if the Mw values for the

Mw a b t= −* exp *

deli containers at days 4 and 6 are not taken in con-sideration when fitting equation 2, the degradationrate of the deli containers (b = 0.29 ± 0.04) is higherthan that of the bottles (b = 0.18 ± 0.05). Also, ahigher adjusted R value is obtained in the secondcase for the deli containers (Adj RsqrDeli = 0.991).The higher Mw values obtained at days 4 and 6 forthe deli containers could be mainly attributed tothe temperature variations on those days in theposition that the deli containers were located, andto local variations in the compost pile. PLA poly-mers in a slightly alkaline medium follow a first-order hydrolysis process mainly affected by theinitial crystallinity, thickness and the shape of the

samples, as previously demonstrated by otherresearchers.32

Figure 10b, d, shows the variation in PDI forboth bottles and deli containers subjected to com-posting and ambient conditions. Longer PLAchains are more susceptible to cleavage than theshorter ones, as the hydrolysis of PLA occurs ran-domly. Hence, an initial rise in PDI for both pack-ages subjected to compositing conditions wasobserved on day 4, which could be correlated to the fragmentation process, which producesdecomposition of the macromolecules into shorteroligomer chains and monomers. After day 15, nar-rowing of the molecular weight distribution was



Mol

ecul

ar w

eigh

t × 1

03, D

a

Time, days Time, days

Mol

ecul

ar w

eigh

t × 1

03, D

a

Figure 10.Variation of the molecular weight and polydispersity index(PDI) as a function of time for (a, b) bottles exposed to composting

(�) and ambient (�) conditions for 30 days and (c, d) deli containersexposed to composting (�) and ambient (�) conditions for 30 days.

observed with decrease in PDI until completedegradation. At this point, only oligomers of thePLA chains are present. When both packages weresubjected to ambient conditions, no significantchanges of Mw at the a = 0.05 level (p > 0.01) wereobserved.

Glass transition and melting temperature. Thesecond run DSC plots for PLA bottles and deli con-tainers exposed to composting conditions areshown in Figure 11. A decrease of 28°C wasobserved in bottles on day 30, whereas it was 8°Cfor deli containers on day 15. Initially, a shortincrease of Tg is observed, which can be attributedto the short span increment of the molecularweight. The later reduction in Tg is associated withthe reduction of the molecular weight for bothpackages. Since the hydrolysis of PLA polymersoccurs at a higher rate in the amorphous region, theoverall crystallinity of the containers increased asdegradation of the polymer chains took place. Bythe preferential degradation of amorphous areas,an increase in total crystallinity was observedduring the degradation process of the crystallinePLA polymers in aqueous media by otherresearchers.41 In this study, the initial crystallinityof the bottles (cc = 12.2 ± 1.4) increased to values of16% until the last degradation day, and in the caseof the deli containers cc = 1.40 ± 0.3 it was observedto increase to 27_ higher (first run of the DSC, not

shown). During the second run, the crystallinity ofthe samples decreased because the heating of thesamples over the melting temperature erased allthe previous thermal history of the samples and thecooling cycle did not allow recrystallization.

The variation in Tg and Tm for packages sub-jected to both composting and ambient environ-ments is shown in Figure 12. Figure 12a, b, showsthe Tg and Tm variations, respectively, for bottlesand Figure 12c, d, shows Tg and Tm variations,respectively, for deli containers. A slight increase inTg for both bottles and deli containers wasobserved for the samples exposed to composting,which can be correlated to the increase in molecu-lar weight in the early stages, due to recombina-tion reactions.

A linear degradation trend in case of PLA pack-ages subjected to composting conditions wasobserved. Values in Figure 12 for equation 3 areshown in Table 3:

(3)

where Tg(0) is the glass transition temperature attime zero, and d is the reduction of the Tg as a func-tion of time. Table 3 and Figure 12 show that areduction of Tg = 0.97°C/day took place in thebottles exposed to composting. For the deli con-tainers, a reduction of Tg = 0.46°C/day wasobserved, but a good correlation was not obtained

T T d tg g= +( )0 *



Table 2. First-order equation [Mw × 103 = a*exp(−b*t)] of the degradation process of poly(lactide)bottles and deli containers

L-Lactide AdjProperties (%) cc a3 pa

1 b3 pb1 Rsqr2

Bottle 96 12.2 ± 1.4 229.7 ± 28.4 0.0002 0.18 ± 0.05 0.01 0.867Deli 94 1.4 ± 0.3 209.6 ± 27.8 0.0010 0.15 ± 0.04 0.03 0.823Deli (-.-)4 94 1.4 ± 0.3 221.2 ± 11.7 0.0003 0.29 ± 0.04 0.008 0.9911 pa and pb are the probability of being wrong in concluding that there is an association between the dependent and independent vari-ables.The smaller the p value, the greater the probability that there is an association. For this study, a = 0.05.2 Adj Rsqr is the R2 which measures the proportion of the variation in the dependent variable accounting for the number of explanatoryvariables.3 a and b values refer to equation 2.These values are shown with their 95% confidence levels.4 Second line fitted in Figure 10c.



Tg

Tg

Tg

Tg

Figure 11. Second run of DSC showing the glass transition and meltingtemperature variations of the PLA (a) bottle and (b) deli container

exposed to composting conditions for 30 days.



Tm

, °C

Time, days Time, days

Tm

, °C

Tg,

°C

Tg,

°C

Figure 12.Variation of the glass transition and melting temperature as afunction of time for (a, b) bottles exposed to composting (�) and

ambient (�) conditions for 30 days and (c, d) deli containers exposedto composting (�) and ambient (�) conditions for 30 days.

Table 3.Variation of glass transition Tg = Tg(0) + d*t as a function of time for poly(lactide) bottlesand deli containers

L-Lactide, AdjProperties % cc Tg(0)

3 PTg(0)1 d3 pd

1 Rsqr2

Bottle 96 12.2 ± 1.4 60.86 ± 1.01 <0.0001 −0.97 ± 0.08 <0.0001 0.95Deli 94 1.4 ± 0.3 59.90 ± 1.30 <0.0001 −0.46 ± 0.18 0.0516 0.481 pTg(0) and pd are the probability of being wrong in concluding that there is an association between the dependent and independent vari-ables.The smaller the p value, the greater the probability that there is an association. For this study, a = 0.05.2 Adj Rsqr is the R2 which measures the proportion of the variation in the dependent variable accounting for the number of explanatoryvariables.3 Tg(0) and d values refer to equation 3.These values are shown with their 95% confidence levels.

(Adj Rsqr = 0.47). Higher adjustment R values wereobserved for bottles than for deli containers.

No significant changes in Tg and Tm wereobserved in case of both packages subjected toambient conditions at a = 0.05 level (p > 0.01).However, a slight reduction of the Tg values isgraphically observed (Figure 12c).

Decomposition temperature. Decompositiontemperature of PLA package material wasanalysed using TGA. A plot obtained from TGAshowing the variation of weight and the derivativeweight as function of temperature for bottles isshown in Figure 13a, b, and for deli containers inFigure 13c, d.

The variation of TD as a function of time for bothpackage and for both environments is shown inFigure 14a, b. A linear variation was observed forpackages subjected to compositing conditions,whereas no significant changes were observed forthe packages subjected to ambient conditions at a= 0.05 level (p > 0.01).

Table 4 shows the values obtained from fittingequation 4 to the data shown in Figure 14:

(4)

where TD0 is the decomposition temperature at dayt = 0, and e is the variation of TD as a function oftime. Table 4 also shows the variation of TD vs. Mn,number average molecular weight for equation 5:

(5)

where TD(∞) is the TD for very high Mn, and B is aconstant term. The Adj Rsqr values obtained forfitting equation 5 to both packages are shown inTable 4. An inverse decay (as shown in equation 5)was observed in correlation of TD with the numberaverage molecular weight for both Rsqr packages.

A graphical decrease in TD was observed in caseof both packages subjected to ambient conditions.TD for bottles decreased from 395°C to 378°C;however, no statistical significance was observedat the a = 0.05 level (p = 0.0130). Similarly thedecrease in case of deli containers was from 396°Cto 378°C and no statistical significance wasobserved at the a = 0.05 level (p = 0.0118).

In summary, PLA polymers break down due tothe absorption of water, resulting in hydrolysis of

T T B MD D n= −∞( )

T T e tD D= +0 *



Table 4.Variation of decomposition temperature TD = TD0 + e*t as a function of time (equation 4)and molecular number for poly(lactide) bottles and deli containers.Variation of decomposition

temperature TD = TD(∞) − B/Mn as a function of Mn (equation 5)

Equation 4: TD = TD0 + e*t

L-Lactide AdjProperties (%) cc TD0

3 pTD01 e3 pe

1 Rsqr2

Bottle 96 12.2 ± 1.4 403.4 ± 2.7 <0.0001 −2.8 ± 0.2 <0.0001 0.96Deli 94 1.4 ± 0.3 404.1 ± 4.8 <0.0001 −5.6 ± 0.6 <0.0001 0.91

Equation 5: TD = TD(∞) − B/Mn

L-Lactide AdjProperties (%) cc TD(∞)

3 pTD(∞)1 B3 pB

1 Rsqr2

Bottle 96 12.2 ± 1.4 402.2 ± 2.3 <0.0001 −327501.0 ± 21647.0 <0.0001 0.97Deli 94 1.4 ± 0.3 394.0 ± 2.5 <0.0001 −403956.8 ± 30589.1 <0.0001 0.971 pTDO, pe, TD(∞) and pB are the probability of being wrong in concluding that there is an association between the dependent and indepen-dent variables.The smaller the p value, the greater the probability that there is an association. For this study, a = 0.05.2 Adj Rsqr is the R2 which measures the proportion of the variation in the dependent variable accounting for the number of explanatoryvariables.3 TD0 refers to equation 4; and TD(∞) and B refer to equation 5.These values are shown with their 95% confidence levels.

the ester linkages. The rate of degradation is highlyaffected due to the temperature, moisture and pHconditions of the compost. Considering the PLApackage, the rate of degradation was affected bythe initial crystallinity and the percentage l-lactidecontent. The 94% l-lactide content packages dis-

appear more rapidly from the compost than the96% l-lactide packages. Hence, the initial crys-tallinity and l-lactide content of the PLA packagesshould be considered for estimation of the timerequired for decomposition in the compostingenvironment. Similar compost studies, but with



Figure 13. (a) Percentage weight reduction vs. temperature; (b) firstderivative vs. temperature of PLA bottles; (c) percentage weight

reduction vs. temperature; and (d) first derivative weight vs. temperatureof PLA deli containers exposed to composting conditions for 30 days.

PLA samples and incomplete packages, werecarried out by Weber42 by storing PLA samples inbiodegradation chambers. Weber42 recommendedthat a maximum of 10% PLA be used in compostpiles to prevent pH reduction of the pile. In thisstudy, this concern was not a problem, due to theratio of polymer to compost. Some comparisonsbetween laboratory and field exposure degrada-

tion have been carried out in variousstudies.36,39,43–45 They exposed PLA films in bananafields in Costa Rica, and found that these PLAplastic films lost mechanical properties anddegraded faster in composting conditions thanduring exposure in soil conditions (3 weeks and 6months, respectively).44 These studies also foundthat degradation of PLA is enhanced by an increasein temperature and relative humidity.36 Previousstudies have been based on assessing the degrad-ability of plastic samples and not complete pack-ages. This study has assessed and addressed thedegradation time, physical properties and com-parison of two commercially available PLA pack-ages. It also gives information about thecompostability and reduction of the physical prop-erties under real compost as well as ambient expo-sure conditions. Packages made of PLA willcompost in municipal/industrial facilities, butthey may be difficult to completely compost inbackyard composting, since PLA degradation isdriven by hydrolysis, which needs higher temper-atures in order to take place (T > 50°C). Furtherresearch is being carried out to simulate the realdegradation process in simulated or controlledconditions in order to establish a standard labora-tory-scale test for the evaluation of packages underreal compost conditions. Furthermore, futureresearch is necessary to find methods and tech-niques that can assess the degradability ofbiodegradable packages under real compostingconditions before they are introduced anddegraded in commercial composting operations.

CONCLUSIONS

Two PLA packages, a bottle and a deli container,were used to determine the degradation process ofPLA under ambient exposure and compost condi-tions. A novel method was used to identify andtrack of the degradation of the PLA packages in areal compost facility. The degradation of the PLAcontainers was monitored by visual inspection,GPC, DSC and TGA. PLA deli containers degradedin <30 days under composting conditions (T >60°C, RH > 65%, pH ≈ 7.5). First-order degradationkinetics were observed for the bottles. A Tg reduc-tion of 1°C/day was found for PLA bottles with



TD

, °C

Time, days

TD

, °C

Figure 14.Variation in decomposition temperature as afunction of time for (a) bottles exposed to composting

(�) and ambient (�) conditions for 30 days and (b) delicontainers exposed to composting (�) and ambient (�)

conditions for 30 days.

96% l-lactide. A method to study the compostabil-ity of biodegradable packages under real compostconditions has been outlined. Further studies willaid in the development of a reliable laboratory testthat can address the compostability of biodegrad-able packages resembling real conditions.

ACKNOWLEDGEMENTS

The authors would like to thank John Biernbaum andAndy Fogiel for their help on addressing aerobic com-posting conditions; Anthony Melvin Boughton, theMSU composting facility manager, for allowing us touse the MSU composting facility; Gregory Baker andXuwei Jiang for their help with the molecular weightdetermination; NatureWorks TM LLC (Blair, NE) for thePLA bottles; Wilkinson Manufacturing Co. (FortCalhoun, NE) for the PLA trays and deli containers; andSusan Selke and Bruce Harte for many valuable com-ments and suggestions.

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comparison of the degradability of poly(lactide) packages in composting and ambient exposure...

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