DM

ED

a

ARR2AA

KBBAPCB

1

rripcpee2pho(Raba

(

W

0d

Industrial Crops and Products 33 (2011) 648–658

Contents lists available at ScienceDirect

Industrial Crops and Products

journa l homepage: www.e lsev ier .com/ locate / indcrop

egradation behaviour of poly(lactic acid) films and fibres in soil underediterranean field conditions and laboratory simulations testing

. Rudnik1, D. Briassoulis ∗

epartment of Agricultural Engineering, Agricultural University of Athens, Iera Odos 75, 11855 Athens, Greece

r t i c l e i n f o

rticle history:eceived 25 November 2010eceived in revised form1 December 2010ccepted 23 December 2010vailable online 22 January 2011

a b s t r a c t

Long-term degradation/disintegration behaviour, indicative of the biodegradation in soil behaviour ofpoly(lactic acid) films and fibres, was studied in natural Mediterranean soil environment during aneleven-month trial in the experimental field. In parallel, simulated soil burial experiments were car-ried out under controlled laboratory conditions. The degradation/disintegration behaviour of PLA wasanalysed using visual inspection, mechanical testing, DSC and FTIR analysis. The influence of the thick-ness and the type of the materials (film vs. fibre) on disintegration was investigated under the given mild

eywords:iodegradationiodegradable materialsgricultural filmsoly(lactic acid)omposting

conditions. For comparison purposes, degradation/disintegration of PLA film was also studied under lowtemperature composting conditions (house composting). During long-term exposure under natural soilenvironment dominated by complex and uncontrolled biotic–abiotic conditions and Mediterranean cli-matic conditions and under house composting conditions, PLA film samples of different thicknesses werepartially, to a rather low degree, degraded mechanically or slightly disintegrated. The results showed that

bio-biffere

iodegradation in soil

degradation behaviour ofphenomenon, following d

. Introduction

Poly(hydroxyalkanoates) (PHAs) and poly(lactic acid) (PLA)epresent two families of biodegradable polymers derived fromenewable resources. Polyhydroxyalkanoates (PHAs) are a fam-ly of polyhydroxyesters of 3-,4-,5- and 6-hydroxyalkanoic acids,roduced by a variety of bacterial species under nutrient-limitingonditions with excess of carbon. These water-insoluble storageolymers are biodegradable in general, exhibit thermoplastic prop-rties and can be produced from renewable carbon sources (Ishidat al., 2005; Hazer and Steinbüchel, 2007; Philip et al., 2007; Rudnik,008). The main members of the PHA family are the homopolymersoly(3-hydroxybutyrate) (PHB) and copolymers such as poly(3-ydroxybutyrate-co-3-hydroxyvalerate) (PHBV). Biodegradabilityf these polymers in soil has been investigated in several workse.g. Pickering and Vansco, 2001; Song et al., 2003; Lim et al., 2005;

ychter et al., 2006; Corrêa et al., 2008; Kukade et al., 2010). Itppears that biobased plastics made from PHAs are completelyiodegradable in a wide range of environments – including soil –t the end of useful product life, at various rates.

∗ Corresponding author. Tel.: +30 210 5294011; fax: +30 210 5294023.E-mail addresses: ewa.rudnik@ichp.pl (E. Rudnik), briassou@aua.gr

D. Briassoulis).1 Present address: Industrial Chemistry Research Institute, Rydygiera 8, 01-793arsaw, Poland.

926-6690/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.indcrop.2010.12.031

ased polymers like poly(lactic acid) in a real soil environment is a complexnt patterns regarding morphological changes.

© 2010 Elsevier B.V. All rights reserved.

Poly(lactic acid) (PLA) is the first polymer based on renewableraw materials commercialized at a large scale. It is a compostablematerial, meeting the specifications of international standards(CEN, 2000). However, the biodegradation behaviour of PLA in soilis not clear (Messias de Braganca and Fowler, 2004). There are nostudies available concerning the behaviour of PLA in soil underMediterranean climatic conditions.

The primary mechanism of degradation of PLA is hydrolysis,catalyzed by temperature, followed by bacterial attack on the frag-mented residues. Many studies concern biodegradation of PLAin different environments such as aquatic system, solid, com-post. Some recent reviews are given elsewhere (Rudnik, 2008;Nampoothiri et al., 2010; Shi and Palfery, 2010). It should benoteworthy that most publications report biodegradation studiesperformed at high temperature, typically about 58 ◦C. For exam-ple, Kale et al. (2007) investigated the biodegradation performanceof polylactide (PLA) bottles under simulated composting condi-tions according to ASTM (ASTM, 2003) and ISO (ISO, 2005, 2007)standards and in real composting conditions. However, literatureabout long-term biodegradation behaviour of PLA in real soil envi-ronment is scarce and concerns specific, usually tropical or North,climatic conditions (Ho et al., 1999; Gallet et al., 2001; Urayama

et al., 2002; Shogren et al., 2003; Kamiya et al., 2007). In real soilenvironment the temperature usually does not exceed 30 ◦C. Long-term behaviour is especially important for agricultural applicationswhere biodegradable polymers are intended to biodegrade prefer-ably in soil.

l Crop

btomb(yiIpes(p(ctspedidc

sbnTsbcmstbela

2

2

vsspffi(na

ddnuiatwt

E. Rudnik, D. Briassoulis / Industria

The degradation of PLA films in Costa Rican soil was investigatedy Ho et al. (1999). The average soil temperature and moisture con-ent were 27 ◦C and 80%, respectively. The average degradation ratef PLA films in the soil of banana field was 7675 Mw (average-weightolecular weight)/week. Films of poly(l-lactide) (PLLA) were

uried in outdoor environment in south Finland during two yearsGallet et al., 2001). No lactic acid was observed during the firstear of soil burial. Urayama et al. (2002) found only a 20% decreasen molecular weight of PLA (l-rich) plates after 20 months in soil.njection moulded tensile bars composed of native starch (0–70%),oly(d,l-lactic acid) (95% L) (PLA, 13–100%) and poly(hydroxyester-ther) (PHEE, 0–27%) were buried in soil for 1 year in order totudy the effects of starch and PHEE on rates of biodegradationShogren et al., 2003). Rates of weight loss increased in the orderure PLA (0%/year) < starch/PLA (0–15%/year) < starch/PHEE/PLA4–50%/year) and increased with increasing starch and PHEEontents. Studies of the microbial communities responsible forhe decomposition of poly(�-caprolactone) (PCL), poly(butyleneuccinate) (PBS), poly(butylene succinate-co-adipate) (PBSA) andoly(lactide) (PLA), were performed in two Japanese soils (Kamiyat al., 2007). The PCL, PBS and PBSA films were considerablyegraded within 50 days at 25 ◦C under upland dark conditions

n one soil, while the PLA film was not degraded at all after 120ays in soil. In another study (Suyama et al., 1998), no soil bacterialolonies that degrade PLA were found.

The aim of the present research work was to offer a better under-tanding and enrich the existing knowledge about the long-termiodegradation in soil behaviour, of PLA films of various thick-esses and of PLA fibres, under real Mediterranean soil conditions.he biodegradation behaviour in the natural complex biotic–abioticoil environment was studied by full scale field experiments andy simulated soil burial experiments under controlled laboratoryonditions. The biodegradation behaviour was studied throughonitoring the evolution of several important degradation sen-

itive properties along with the disintegration behaviour of theested materials, considered as indicators, but not measures, of theiodegradation process. The results obtained by the full-scale fieldxperiments were compared against the results obtained throughaboratory simulations of PLA degradation/disintegration in soilnd the results of composting of PLA films under farm conditions.

. Experimental studies

.1. Materials

Poly(lactic acid) (PLA) samples in various forms (films witharious thicknesses and fibres) were obtained from commercialuppliers and used in the experimental studies. According to theuppliers, these materials were made out of Nature Works PLAellets [NatureWorks LLC, http://www.natureworksllc.com/] (nourther information was made available though, for reasons of con-dentiality, concerning processing, plasticisers, etc.). Filter paperWhatman) and poly(ethylene) (PE) film were used as a positive andegative reference, respectively. The characteristics of the materi-ls studied are given in Table 1.

Biodegradation under real soil conditions in the field wasetermined indirectly through the study of the degradation andisintegration of the materials, both used as strong indicators (butot as a quantitative measures) of biodegradation. The approachsed is a normal approach adopted by researchers in this RTD area,

ncluding major producers of agricultural biodegradable films. Thispproach is justified based on the fact that the research objec-ive concerns testing biodegradability under real soil environmenthere it is not possible to measure and quantify biodegradation in

he way it is measured in the lab under controlled conditions by

s and Products 33 (2011) 648–658 649

standard respirometric testing methods (Briassoulis and Dejean,2010). The field experimental degradation results were monitoredthrough systematic sampling, measurements and analysis of sev-eral key properties of the materials in the lab and confirmed bysimulated soil burial and composting conditions.

2.2. Full-scale field experiments

Biodegradation studies under real Mediterranean field condi-tions were carried out at the experimental field of the AgriculturalUniversity of Athens in Spata. The latitude of the site is 37◦58′49′′N,longitude 23◦54′49′′E and altitude 119 m. The soil in Spata is clayloam. The detailed soil characteristics of the field at the experimen-tal site are given elsewhere (Rudnik and Briassoulis, 2011).

A first set of materials was studied in the experimental field ofAUA in Spata for a period of 11 months, starting from October 2007till September 2008 (winter–summer period). Samples (size of 1/2A4 in the case of films and approx. 2 g of mass for fibres) were placedin envelopes of plastic nets and buried in the soil 10–15 cm beneaththe surface, in a pattern arranged along 11 lines.

During the test period, the weather and soil parameters wererecorded by a data logger (Campbell) with sensors. The followingparameters were monitored and recorded regularly:

• meteorological conditions (air temperature & humidity, totalirradiation, UV irradiation);

• soil conditions (soil temperature and soil water content).

The following two-day irrigation cycle was applied: one day ofirrigation and one day without irrigation. During rain period theirrigation was interrupted.

Samples were taken off every month, or at appropriately modi-fied time intervals, depending on the evaluation of the results, andanalysed following the experimental protocol.

2.3. Laboratory simulations set-up

In parallel to field testing, laboratory biodegradation studieswere carried out. The laboratory experiments were performed intwo solid environments: in soil using “bioreactors” and under com-posting conditions. For the simulated laboratory experiments soiltaken from the experimental field in Spata was used. A flexibleperiodicity of samples recovery during laboratory and field experi-ments was established according to the degradation/disintegrationprogress, being continuously monitored.

2.3.1. Laboratory experiments for biodegradation in soilThe degradation/disintegration studies on biodegradation were

carried out in “bioreactors” (4 l plastic boxes containing condi-tioned soil). Each “bioreactor” contained about 2 kg of soil collectedfrom the experimental field of AUA in Spata and sieved using 2 mmsize sieve. Studies were performed at room temperature during an11 month-period and water content was periodically adjusted ata constant level (40%). Samples (films and fibres) were placed inenvelops of plastic nets and buried in the conditioned soil contain-ing boxes. Two representative PLA films with thickness 30 �m and75 �m were chosen for the laboratory studies (PLA 30 and PLA 75).Filter paper (Whatman) and polyethylene (PE) film were used as apositive and negative reference, respectively.

Each “bioreactor” contained 1 sample of a given film in theform of 1/2 A4 (for tensile testing) and 5 samples in the form of

2 cm × 2 cm pieces (for mass change measurements).

2.3.2. Composting studiesThe composting studies were performed outdoors using a com-

post bin of 100 l volume with perforated wall for improved air

650 E. Rudnik, D. Briassoulis / Industrial Crops and Products 33 (2011) 648–658

Table 1Characteristics of the materials studied for biodegradation in soil.

Material (commercial name) Producer Form Characteristics Abbreviationused in thepaper

PLA (EarthFirst®PLA)a P1 (Plastic Suppliers Inc.; Sidaplax v.o.f.) Film Thickness: 20 �m transparent PLA 20PLA (EarthFirst®PLA) P1 (Plastic Suppliers Inc.; Sidaplax v.o.f.) Film Thickness: 30 �m transparent PLA 30PLA (EarthFirst®PLA) P1 (Plastic Suppliers Inc.; Sidaplax v.o.f.) Film Thickness: 40 �m transparent PLA 40PLA (EarthFirst®PLA) P1 (Plastic Suppliers Inc.; Sidaplax v.o.f.) Film Thickness: 50 �m transparent PLA 50PLA (EarthFirst®PLA) P1 (Plastic Suppliers Inc.; Sidaplax v.o.f.) Film Thickness: 75 �m transparent PLA 75

)

csowc[ltp2c1

2

bpss

2

rsdb

2

6

PLA (NatureWorksTM PLA polymer 2002D) P2 (FOLIETECHNIEK International BVPLA (PLSTD-013NRR-1950) P3 (MiniFIBERS, Inc.)

a Based on IngeoTM resin of NatureWorks® LLC.

irculation. The composting medium was prepared by blendingheep manure and sawdust in a ratio of 5:1 (w/w). Temperaturesutside and inside the compost bin were monitored, as well asater content. Initial water content was 52%. The composting pro-

ess was evaluated systematically by Solvita respiration testingSolvita, http://solvita.com/]. For composting studies Whatman cel-ulose paper was chosen as positive reference. PLA film of 30 �mhickness (PLA 30) and PLA fibres (PLA fib), were chosen for com-osting studies. Film samples of 1/2 A4 size and fibres of approx.g were placed in envelopes of plastic nets and buried in theomposting medium. The period of the composting studies was1 months.

.4. Methods of analysis and testing

The degradation/disintegration process in soil was followedy visual inspection (digital images), as well as by mechanicalroperties testing (tensile properties for films) and FTIR analy-is. Morphological changes were investigated using the differentialcanning calorimetry (DSC) method.

.4.1. Mechanical testingTensile properties were measured using an Instron 4204 mate-

ials testing machine. Cross-speed rate was 10 mm/min. Fivepecimens were tested for each film sample. Samples were con-itioned at 23 ◦C and 50% humidity in a climate chamber for 24 h

efore testing.

.4.2. DSC methodThermal analysis was carried out using a Perkin Elmer Pyris DSC

Differential scanning calorimeter. The sample (≈3 mg) was heated

0

5

10

15

20

25

30

Oct 2007

Nov2007

Dec 2007

Jan 2008

Feb 2008

Mar 2008

A2

Te

mp

era

ture

, oC

Temperature

Fig. 1. Temperature and air humidity data during the period Oc

Film Thickness: approx. 400 �m transparent PLA 400Fibres Length, 19 mm 1.3 denier per filament; white PLA fib

at a rate of 10 ◦C min−1 from 20 ◦C to 200 ◦C, then was held for 1 minat 200 ◦C and cooled at a cooling rate of 10 ◦C min−1 to 20 ◦C. Theprocedure was repeated twice.

For the polylactic samples (PLA) the glass transition, crystal-lization, and melting temperatures (Tg, Tc and Tm, respectively),as well as the enthalpy of fusion (�Hf), the enthalpy of crystal-lization (�Hc), and crystallinity (Xc) were determined. Tm and �Hfwere determined by the first scan, while Tg and Tc were determinedby the second scan. The repeatability of DSC measurements waschecked for selected samples.

The Xc values were calculated according to the following equa-tion:

Xc(%) = 100 × �Hc + �Hf

93(1)

where �Hf is the melting enthalpy, �Hc is the crystallizationenthalpy and 93 (J/g) the melting enthalpy of 100% crystalline PLAsample (Ahmed et al., 2009).

2.4.3. FTIR analysisA qualitative surface analysis by attenuated total reflectance

(ATR) infrared spectroscopy was carried out using a Bruker Tensor27 instrument. Resolution: 1 cm−1, range; 600–4000 cm−1.

3. Results and discussion

3.1. Meteorological data

Fig. 1 presents some of the weather data (temperature and rel-ative humidity) recorded in Spata during the field experiments forthe period October 2007–September 2008. Total rainfall ranged

pr 008

May2008

Jun 2008

Jul 2008

Aug2008

Sep 2008

0

10

20

30

40

50

60

70

80

Hu

mid

ity

, %

Humidity

tober 2007–September 2008 in Spata (monthly averages).

E. Rudnik, D. Briassoulis / Industrial Crops and Products 33 (2011) 648–658 651

F egrad2 arch 2

ftuMai

3

3

sf

FO

ig. 2. Photos of PLA 20 (a), PLA 40 (b), PLA 75 (c) and PLA 400 (d) films during biod008), at different stages of degradation (left to right: November 2007 1-month, M

rom 10 mm (February 2008), 8–6 mm (October–December 2007)o 2 mm (March, April, September 2008) and nearly zero mm (Jan-ary, May, July, August 2008). Soil water content during the perioday 2007–October 2008 was in the range 34–47% (monthly aver-

ges). Soil temperature measured at 30 cm below soil surface variedn the range 16–21 ◦C.

.2. Field test results

.2.1. Visual observationsPhotos of PLA samples recovered during the biodegradation

tudies in the experimental field of AUA in Spata are shown in Fig. 2or selected PLA 20, PLA 40, PLA 75 and PLA 400 film samples.

ig. 3. Photo of filter paper after 1 month (a), PE film (b) and PLA fibres (c) after 11 monctober 2007–September 2008).

ation studies in the experimental field of AUA (period 1: October 2007–September008 3-months, May 2008 7-months and September 2008 11-months).

During 11 months of burial in soil, fragmentation and disin-tegration of PLA film samples was observed. The first significantchanges were observed after 7 months. It seems that the increasein temperature in May 2008 correlates with the onset of fragmen-tation of the PLA film samples. Then the extent of fragmentationand disintegration proceeded faster. At the end of 11 months burialsamples disintegration of the materials could be clearly observed(a strong indication of the associated stage of the biodegradation ofthe material process). However, the degree of disintegration of the

material was still rather low. Obviously, thickness influences theextent of degradation of PLA films. The thinner the films, the morepronounced changes can be observed, in general.

All PLA films became brittle after 1 month of soil burial. Manycracks could be seen in the samples of thin PLA films. It is notewor-

ths of soil burial in the experimental field of AUA in Spata, respectively (period 1:

652 E. Rudnik, D. Briassoulis / Industrial Crop

F2(

tot

cPow

ig. 4. Tensile properties after 1 month of burial in Spata: tensile strength for PLA0 and PLA 50 films (top figure); elongation at break for PLA 20 and PLA 50 filmsbottom figure).

hy that during burial in the real soil environment worms and rootsf plants clearly contributed to degradation (mechanical fragmen-ation and disintegration) of samples.

Filter paper (cellulose) used as positive reference biodegradedompletely after 1 month of burial (Fig. 3a). On the contrary for theE film used as negative control sample no degradation signs werebserved during the field experiments (Fig. 3b). The PE film surfaceas not found to be broken even by the activity of plant roots.

Fig. 5. FTIR-ATR spectra of PLA 75 film: initial (top) and

s and Products 33 (2011) 648–658

As far as the fibres are concerned, no visual changes wereobserved (PLA fib), during 11 months soil burial at the experimentalfield in Spata (Fig. 3c).

The degree of disintegration of the studied PLA fibres under realfield conditions was shown to be even lower (non-detectable) thanthat of the films. This is attributed to the limited contact of PLAfibres with soil, as they were put in the form of a mass of fibres inthe plastic net envelope. Further, it is known that the thickness andthe form of tested material (i.e. fibres) influences the biodegrada-tion rate (Yang et al., 2005; Starnecker and Menner, 1996) It has alsobeen reported that the degradation rate of PLA can be affected bymany factors, including crystallinity (Auras et al., 2004; MacDonaldet al., 1996). Thus, the higher crystallinity of PLA fibres in compari-son to PLA films (cf. DSC analysis) may also have contributed to theslow degradation/disintegration process of these materials in theMediterranean soil environment.

In a relevant work, Tokiwa and Calabia (2006) explained thatPLA-degrading microorganisms are not widely distributed in thenatural environment and thus, PLA is less susceptible to micro-bial attack in the natural environment than other microbial andaliphatic polyesters. Thus, the slow degradation/disintegration rateof PLA films and fibres in soil observed in the present studysuggests a scarcity of PLA-degrading microorganisms also in theMediterranean natural environment as it has been reported alreadyfor tropical and north soil climatic conditions. Another factorcontributing to this behaviour is the relatively low temperatureprevailing in soil under real field conditions in all these climaticconditions (as compared to industrial composting conditions).

3.2.2. Testing of mechanical propertiesAfter one month burial in the experimental field, PLA film sam-

ples of 30, 40, 75 and 400 �m thickness were too brittle and theywere broken when trying to cut specimens as a result of the firststage of the material degradation. Only for PLA 20 and PLA 50 sam-ples it was possible to cut specimens for tensile testing and this onlyfor the early period of soil burial (first month). The results for the

tested PLA films are presented in Fig. 4 (the initial values were foundto be close to those reported for PLA (98% l-lactide) in Auras et al.(2004)). The tensile properties of both PLA samples with differentthicknesses decreased significantly after a burial period of just onemonth. The tensile strength for the PLA 20 film decreased almost

after 11 months of soil burial (bottom) in Spata.

E. Rudnik, D. Briassoulis / Industrial Crops and Products 33 (2011) 648–658 653

46

48

50

52

54

56

58

60

62

64

0 1 3 5 7 9 11

Gla

ss t

ran

siti

on

Tg

,ºC

Time, months

PLA

durin

tt5mPbl

tfitserttsT

PLA 20 PLA 30 PLA 40 PLA 50

Fig. 6. Glass transition temperatures for PLA films

o one-third its initial value, from 53.8 to 14.9 MPa, whereas for thehicker PLA 50 film, the decrease was half the initial value, i.e. from3.2 to 24.0 MPa. The elongation at break values was decreased dra-atically for both tested PLA films. This explains the fact that most

LA film samples were found to be too brittle, after one month ofurial, already with cracks. The tensile properties of the polyethy-

ene film used as negative control were not affected, as expected.The early drastic decrease of the tensile strength and elonga-

ion at break values of the PLA films exposed to Mediterraneaneld soil burial conditions (one month exposure) suggests that,he first stage of the PLA biodegradation in soil process results inevere degradation of the mechanical properties of the material (i.e.mbrittlement), leading to gradual slow disintegration of the mate-

ial. Analogous results were obtained with the PLA films exposedo the laboratory simulated soil burial conditions. The decrease ofhe elongation at break was found to be higher than that of the ten-ile strength in all cases tested under both soil burial conditions.his confirms similar research results reported earlier under dif-

158

159

160

161

162

163

164

165

166

167

168

0 1 3

Mel

tin

g t

emp

erat

ure

, oC

Time,

PLA 20 PLA 30

PLA 75 PLA fibers

Fig. 7. Melting temperatures of PLA films and fibres durin

75 PLA 400 PLA 30-lab PLA 75-lab

g soil burial in the field in Spata and at laboratory.

ferent climatic conditions. Thus, for example, it has been reportedby Maharana et al. (2009) that non-enzymatic hydrolysis of PLAis preceded by an apparently inert first stage of degradation with-out weight loss but resulting in random cleavage of polymer chainbackbone (endo-type degradation) with a concomitant substantialdecrease in molecular weight, leading to a decrease in mechanicalproperties such as tensile strength and elongation at break.

3.2.3. FTIR-ATR analysisFTIR-ATR spectroscopy was used to evaluate chemical modifi-

cations occurring on film surface. The FTIR-ATR spectra of PLA 75before its use and after 11 months burial in the soil under real fieldconditions are presented in Fig. 5. The two spectra seem similar.

No shifts of characteristic bands or formation of new bands areobserved. However, the intensity of absorbance at 1748 cm−1, cor-responding to carbonyl C = 0 stretching band, decreased after 11months of soil burial. Similar tendencies are observed in the spec-tra obtained for PLA film of various thicknesses. After 11 months

5 7 9 11

months

PLA 40 PLA 50

PLA 30-lab PLA 75-lab

g soil burial in the field in Spata and at laboratory.

654 E. Rudnik, D. Briassoulis / Industrial Crops and Products 33 (2011) 648–658

0

10

20

30

40

50

60

70

0 1 3 5 7 9 11

Cry

stal

linit

y, %

Time, months

F

ofc1(nct

3

ubgssphftDatfi1

ttl

big

ib

PLA 20 PLA 30 PLA 40 PLA 50 PLA 75 PLA fib

ig. 8. Crystallinity of PLA films and fibres during soil burial in the field in Spata.

f soil burial the carbonyl index decreased from 3.24 to 2.44 androm 3.6 to 3.5, respectively for PLA 30 and PLA 75 films. Thearbonyl index was calculated as the ratio of the intensity of the748 cm−1 carbonyl peak in the FTIR spectrum to a reference peak1451 cm−1). These changes, i.e. no significant shifts or formation ofew bands suggest that while slow hydrolysis of ester bonds pro-eeds, no degradation by-products were emerged on the surface ofhe specimens.

.2.4. DSC analysisMorphological changes during degradation/disintegration

nder real conditions in the experimental field were determinedy the DSC method. An endothermic event superimposed on thelass transition Tg was observed during the first heating of PLA filmample. This endothermic relaxation is considered to result fromecondary molecular re-ordering undergone in the amorphoushase of semicrystalline polymers (Auras et al., 2003). The secondeating curve showed a glass transition temperature of 57.4 ◦C

or PLA 30 film. Peak crystallization temperature and meltingemperature for PLA 30 were at 94.2 ◦C and 163.7 ◦C, respectively.SC curves of all PLA samples were similar except for PLA 400 filmnd PLA fibres. PLA 400 film is the thickest film investigated inhis study. It exhibited only strong glass transition at 59.8 ◦C. PLAbres are highly crystalline materials with melting temperature at63.5 ◦C.

Changes in characteristic thermal properties, i.e., glassransitions and melting temperatures during degrada-ion/disintegradation in soil in the experimental field (and ataboratory) are given in Figs. 6 and 7.

As far as glass transition temperature is concerned after soilurial under real soil conditions and laboratory conditions, an

ncrease generally was observed during the first 3 months then thelass transition remains almost stable.

As far as the melting temperature is concerned, this alsoncreased slowly for the first 3 months of soil burial, during theiodegradation in soil (hydrolysis) process of the PLA films under

Fig. 9. Photos of PLA 30 (a), PLA 75 (b) and PE film (c) sam

Fig. 10. Tensile properties during simulated burial at laboratory: tensile strengthfor PLA 30 and PLA 75 films (top figure); elongation at break for PLA 30 and PLA 75films (bottom figure).

real field conditions, and then stabilized. This behaviour may beexplained by a possible thickening of the PLA crystallites probablydue to a decrease in chain mobility in the amorphous region.

It is noteworthy that the tendencies under real field conditionsare consistent with those observed or measured during the simu-lated soil burial tests at laboratory (Fig. 6; refer to next section).

Crystallinity changes based on DSC analysis during soil burialin Spata are given in Fig. 8 for both PLA films and PLA fibres.Crystallinity of PLA films did not change significantly during degra-dation/disintegration under real field conditions in Spata (Fig. 8).Some fluctuations are observed. However, in general, crystallinityremained at the same level. An exception is noticed with the PLAfibres for which a clear decrease in crystallinity was observed

(Fig. 8).

Morphological changes observed during soil burial of PLA filmand fibres follow different patterns, reflecting slow degradation innatural soil environment dominated by complex and uncontrolled

ples after 11 months of soil burial at the laboratory.

E. Rudnik, D. Briassoulis / Industrial Crops and Products 33 (2011) 648–658 655

0

5

10

15

20

25

30

35

40

45

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49

Time, days

Tem

per

atu

re,º

C

peratu

profil

bt(motds

3

sp1oO

Temperature outside Tem

Fig. 11. Temperature

iotic–abiotic conditions and mild Mediterranean climatic condi-ions. For north climatic conditions, it was observed by Gallet et al.2001) that thermal properties of PLA films (glass transition and

elting temperatures) were affected only during the second yearf soil burial in south Finland. This behaviour may be explained byhe phenomenon observed by Shi and Palfery (2010) that when theegrading temperature is below PLA glass transition temperature,ample degradation is slowed.

.3. Laboratory test results

Photos of samples taken during biodegradation under simulated

oil burial conditions are given in Fig. 9. First changes for filteraper (cellulose) used as positive reference were seen already aftermonth. Some small fragments remained in the soil. After 3 monthsf soil burial at the laboratory cellulose disintegrated completely.n the contrary PLA 30 and PLA 75 films were not disintegrated

Fig. 12. Tensile properties changes du

re inside 1 Temperature inside 2

e during composting.

after 11 months of burial. However, the PLA films become brittle asin the case of the field experiments and creased with some cracksindicating initiation of mechanical degradation (hydrolysis stage)of the material. Similarly, small samples (size 2 cm2) destined todetermine mass loss remained optically unchanged. No mass losswas observed for PLA films during the soil burial trial at the labora-tory. No visible degradation traces were seen for the studied fibreseither. Obviously, PE film used as negative reference did not changeat all.

Fig. 10 shows the effects of burial time on the mechanicalproperties of PLA 30 and PLA 75 films. Both, tensile strength andelongation at break decreased during soil burial. For PLA 30 film

the tensile strength decreased up to 1/3 the initial value (16.1 MPa)after 7 months, while the tensile strength of the thicker PLA 75sample decreased to a smaller extent, i.e. up to 5/6 the initial value(48.4 MPa), exhibiting a large variability during its burial in soilperiod. The elongation at break of the PLA samples suffered a more

ring composting for PLA 30 film.

656 E. Rudnik, D. Briassoulis / Industrial Crops and Products 33 (2011) 648–658

nd PL

dtnaodropebstcbtditaee

raaas

3

oo(ep(

12

Fig. 13. Photos of cellulose (positive control) (a; 1, 3, 7 weeks) a

rastic reduction, a behaviour analogous to the one observed withhe samples exposed under real field conditions. From the begin-ing of the laboratory simulated soil burial the values of elongationt break of both PLA films decreased dramatically. After 1 monthf burial the elongation at break decreased already from 17.5%own to 1.3% and from 3.5% down to 1.8%, for PLA 30 and PLA 75,espectively. For the thinner PLA 30 film the effect of soil burialn the degradation, measured by the tensile properties, is moreronounced as compared with thicker PLA 75 film. The tensile prop-rties of the PE film were not affected during the simulated soilurial laboratory experiments. It is noteworthy that after 7 monthsimulated burial under laboratory conditions PLA samples wereoo brittle to determine mechanical properties. It was difficult tout the specimens for mechanical testing. In general, PLA samplesuried in soil under real field conditions degraded faster than thoseested under laboratory conditions. This is indicated by the fasterecrease of the elongation at break values of most PLA films buried

n the field under Mediterranean conditions that become too brittleo cut specimens even after only one month of burial. This explainslso the higher degree of disintegration observed for the samplesxposed under real filed conditions as compared to the samplesxposed to the simulated laboratory conditions.

The measurement of biodegradation through the BOD basedespirometric method confirmed that the tested PLA materials werelmost not degraded at 30 ◦C after 20 days of incubation (Rudniknd Briassoulis, 2011). Biodegradation of PLA films and fibres wast a rather low level of about 10%. Further, PLA fibres degraded morelowly in comparison with PLA films.

.4. Composting studies results

Composting refers to a biodegradation process of a mixture ofrganic substrates carried out by a microbial community composedf various populations in aerobic conditions and in solid phaseRudnik, 2008; Diaz et al., 2007). The exothermic process producesnergy in the form of heat, which results in an increase of the tem-erature in the mass. Composting process proceeds in three phases

Rudnik, 2008; Diaz et al., 2007):

. Mesophilic phase.

. Thermophilic phase.

A 30 (b; 3 weeks, 8, and 11 months) during composting studies.

3. Cooling (called second mesophilic phase) and maturation phase.

The highest temperature is reached during the thermophilicphase (35–65 ◦C), when thermophilic bacteria and fungi take over,and the degradation rate of the waste increases. After the easilydegradable carbon sources have been consumed, the compost startsto cool. After cooling, the compost is stable. Mesophilic bacteria andfungi reappear, and the maturation phase follows.

Temperature profiles, i.e. temperature outside as well as tem-perature inside the compost bin (2 sensors) measured duringbiodegradation of PLA 30 films under home/farm type Mediter-ranean composting conditions in the present study are shownin Fig. 11 (daily averages). Water content was in the range45–60%. The highest temperature was recorded during thefirst week of composting then the second mesophilic phaseproceeded.

Solvita respirometric tests confirmed the temperature profileof the compost medium. Solvita compost maturity test index at thebeginning of the experiments was 3, indicating active compost withthe fresh ingredients. After 7 weeks of composting the condition ofthe compost changed and the compost was moving past the activephases of the decomposition and was ready for curing (value ofSolvita maturity index increased to 5).

The tensile properties of PLA 30 film samples taken during vari-ous composting phases are presented in Fig. 12. The tensile strengthof the PLA 30 film did not change significantly during 7 weeks ofcomposting except during the last week when the tensile strengthreduced to 2/3 the initial value. However, a major decrease in elon-gation at break, down to 1/8 the initial value, was observed alreadyafter 1 week of composting. Then the low elongation at break valuereaches a plateau.

The exposure of the PLA films to home composting conditionsrevealed that the elongation at break values decrease drasticallyand abruptly already after 1 week of exposure, while the tensilestrength values decrease gradually and only after one month ofexposure. It is noteworthy that the high sensitivity of the elon-gation at break property follows the same pattern under both

Mediterranean soil and composting environments, with a dras-tic reduction of the elongation at break during the early stage ofbiodegradation. It appears that during the first stage, the prelimi-nary steps of the hydrolysis mechanism induce degradation of themechanical properties of the material leading to serious embrit-

l Crop

tetbtmu

clolcc

aeiosepdt2ra6

4

ettece

ahtm

pcp

rPeimacatfi2ccntc

E. Rudnik, D. Briassoulis / Industria

lement under both, soil and composting conditions. This earlymbrittlement phenomenon, affecting predominantly the elonga-ion at break value, has also been observed and reported for severaliodegradable materials exposed to open field conditions duringheir useful life-time, through monitoring of the evolution of their

echanical properties before their burial in soil (i.e. degradationnder UV radiation, etc.) (Briassoulis, 2006, 2007).

Photos taken during composting are shown in Fig. 13. Firsthanges due to degradation can be observed for filter paper (cellu-ose) already after 1 week of composting. Then degradation processf cellulose proceeds. After 3 weeks a few small fragments of cel-ulose remained and after 7 weeks the cellulose is disintegratedompletely. However, PLA 30 film was not disintegrated during theomposting experiments.

The rather low degree of disintegration of PLA films waslso observed under the composting conditions. This may bexplained by the fact that the performed composting stud-es reflect conditions usually achieved for gardening and houser farm composting. These conditions are closer to those ofoil burial rather than to industrial composting (Briassoulist al., 2010). PLA is a material compostable at industrial com-osting facilities, but will not disintegrate sufficiently fast inomestic composting piles since the minimum required condi-ions are typically not met (Messias de Braganca and Fowler,004; Kale et al., 2007). This is verified now under Mediter-anean farm composting conditions as well. The temperaturechievable under industrial composting conditions is about5 ◦C.

. Conclusions

The soil burial tests show that the degradation of PLA in soilnvironment under Mediterranean real field conditions is slow andhat it takes a long time for the material disintegration to start, evenhough degradation of the mechanical properties appears to startarlier. Laboratory simulated soil burial results were found to beonsistent with those obtained under real field conditions in thexperimental field.

Besides, thickness and also form of material was shown to playcrucial role in the biodegradation process under soil burial andome composting conditions (i.e. surface area of material exposedo soil or composting medium; film vs. fibres; single specimens vs.

ass of material).The results showed that degradation behaviour of bio-based

olymers like poly(lactic acid) in a real soil environment is aomplex phenomenon, following different patterns regarding mor-hological changes.

In general, during long-term biodegradation under Mediter-anean natural soil environment and home composting conditionsLA of different thicknesses became brittle very early during theirxposure, while they were partially degraded mechanically and dis-ntegrated to a low degree at the end of 11 months trials. These

aterials need a much longer time and/or higher temperatures forcomplete biodegradation to be achieved, taking into account thatomposting of PLA at industrial facilities proceeds at higher temper-ture (usually at 60 ◦C during thermophilic phase). It is noteworthyhat during soil burial under real conditions in the experimentaleld the air and the soil temperatures never exceeded 40 ◦C and1 ◦C, respectively. Taking into account that the field studies were

arried under the favourable conditions of the warm Mediterraneanlimate of Greece, higher temperatures under real conditions mayot be realistically achievable, and so PLA materials, regardless ofhe form, need longer time to biodegrade under Mediterraneanonditions.

s and Products 33 (2011) 648–658 657

Acknowledgments

The work has been carried out under the project BIODESOPOfunded by the European Commission (Marie Curie Fellowship ofDr. Ewa Rudnik).

Dr. Ewa Rudnik expresses her thanks to Mr. D. Giannopoulos forhis help during experiments.

References

Ahmed, J., Varshney, S.K., Zhang, J.X., Ramaswamy, H., 2009. Effect of high pressuretreatment on thermal properties of polylactides. J. Food Eng. 93, 308–312.

ASTM D 5338-98(2003), 2003. Standard Test Method for Determining AerobicBiodegradation of Plastic Materials Under Controlled Composting Conditions,Edition.

Auras, R.A., Harte, B., Selke, S., Hernandez, R.J., 2003. Mechanical, physical, and bar-rier properties of poly(lactide) films. J. Plast. Film Sheet. 19, 123–135.

Auras, R., Harte, B., Selke, S., 2004. An overview of polylactides as packaging mate-rials. Macromol. Biosci. 4, 835–864.

Briassoulis, D., 2006. Mechanical behaviour of biodegradable agricultural filmsunder real field conditions. Polym. Degrad. Stabil. 91, 1256–1272.

Briassoulis, D., 2007. Analysis of the mechanical and degradation performancesof optimised agricultural biodegradable films. Polym. Degrad. Stabil. 92,1115–1132.

Briassoulis, D., Dejean, C., 2010. Critical review of norms and standards forbiodegradable agricultural plastics. Part I. Biodegradation in soil. J. Polym. Env-iron. 18, 384–400.

Briassoulis, D., Dejean, C., Picuno, P., 2010. Critical review of norms and standardsfor biodegradable agricultural plastics. Part II. Composting. J. Polym. Environ. 18(3), 364–383.

Comite Europeen de Normalisation CEN. CEN EN 13432: 2000/AC:2005, 2005.Packaging–Requirements for Packaging Recoverable Through Composting AndBiodegradation-Test Scheme and Evaluation Criteria for the Final Acceptance ofPackaging, European Standard. European Committee for Standardization, Brus-sels, Belgium.

Corrêa, M.C.S., Rezende, M.L., Rosa, D.S., Agnelli, J.A.M., Nascente, P.A.P., 2008. Surfacecomposition and morphology of poly(3-hydroxybutyrate) exposed to biodegra-dation. Polym. Test. 27, 447–452.

Diaz, L.F., De Bertoldi, M., Bidlingmaier, W., Stentiford, E., 2007. Compost Scienceand Technology. Elsevier, Amsterdam.

Gallet, G., Lempiänen, R., Karlsson, S., 2001. Characterisation by solid phasemicroextraction-gas chromatography–mass spectrometry of matrix changes ofpoly(l-lactide) exposed to outdoor soil environment. Polym. Degrad. Stabil. 71,147–151.

Hazer, B., Steinbüchel, A., 2007. Increased diversification of polyhydroxyalkanoatesby modification reactions for industrial and medical applications. Appl. Micro-biol. Biotechnol. 74, 1–12.

Ho, K.-L., Pometto III, A.L., Gadea-Rivas, A., Briceno, J.A., Rojas, A., 1999. Degradationof polylactic acid (PLA) plastic in Costa Rican soil and Iowa State Universitycompost rows. J. Environ. Polym. Degrad. 7, 173–177.

Ishida, K., Asakawa, N., Inoue, Y., 2005. Structure, properties and biodegradation ofsome bacterial copoly(hydroxyalkanoate)s. Macromol. Symp. 224, 47–58.

ISO 14855-1:2005, 2005. Determination of the Ultimate Aerobic Biodegradability ofPlastic Materials Under Controlled Composting Conditions—Method by Analysisof Evolved Carbon Dioxide—Part 1: General Method, Edition.

ISO 14855-2:2007, 2007. Determination of the Ultimate Aerobic Biodegradability ofPlastic Materials Under Controlled Composting Conditions—Method by Analysisof Evolved Carbon Dioxide—Part 2: Gravimetric Measurement of Carbon DioxideEvolved in a Laboratory-scale Test, Edition.

Kamiya, M., Asakawa, S., Kimura, M., 2007. Molecular analysis of fungal commu-nities of biodegradable plastics in two Japanese soils. Soil Sci. Plant Nutr. 53,568–574.

Kale, G., Auras, R., Singh, S.P., Narayan, R., 2007. Biodegradability of polylactide bot-tles in real and simulated composting conditions. Polym. Test. 26, 1049–1061.

Kukade, P.P., McCarthy, S., Krishnaswamy, R.K., Sun, X., Shabtai, Y., 2010. Biodegra-dation of poly(hydroxy butanoic acid) copolymer mulch films in soil. In: AnnualTechnical Conference—ANTEC, Conference Proceedings, vol. 3 , pp. 2049–2052.

Lim, S.-P, Gan, S.-N., Tan, I.K.P., 2005. Degradation of medium-chain-length poly-hydroxyalkanoates in tropical forest and mangrove soils. Appl. Biochem.Biotechnol. 126, 23–33.

MacDonald, R., McCarthy, S., Gross, R., 1996. Enzymatic degradability ofpoly(lactide): effects of chain stereochemistry and material crystallinity. Macro-molecules 29, 7356–7361.

Maharana, T., Mohanty, B., Negi, Y.S., 2009. Melt-solid polycondensation of lacticacid and its biodegradability. Prog. Polym. Sci. 34, 99–124.

Messias de Braganca, R., Fowler, P., 2004. Industrial Markets for Starch. The Biocom-

posites Center, University of Wales, Bangor.

Nampoothiri, K.M., Nair, N.M., John, R.P., 2010. An overview of the recent develop-ments in polylactide (PLA) research. Bioresour. Technol. 101, 8493–8501.

Philip, S., Keshavarz, T., Roy, Y., 2007. Review. Polyhydroxyalkanoates: biodegrad-able polymers with a range of applications. J. Chem. Technol. Biotechnol. 82,233–247.

6 l Crop

P

RR

R

S

S

S

Appl. Microbiol. Biotechnol. 72, 244–251.Urayama, H., Kanamori, T., Kimura, Y., 2002. Properties and biodegradability of poly-

58 E. Rudnik, D. Briassoulis / Industria

ickering, J.P., Vansco, G.J., 2001. Use of in-situ atomic force microscopy to moni-tor the biodegradation of polyhydroxyalkanoates (PHAs). Macromol. Symp. 167,139–151.

udnik, E., 2008. Compostable Polymer Materials. Elsevier, Oxford.udnik, E., Briassoulis, D., 2011, Comparative biodegradation in soil behavior of

two biodegradable polymers based on renewable resources, J. Polym. Environ.,doi:10.1007/s10924-010-0243-7 Online FirstTM.

ychter, P., Biczak, R., Herman, B., Smytta, A., Kurcok, P., Adamus, G., Kowalczuk,M., 2006. Environmental degradation of polyester blends containing atacticpoly(3-hydroxybutyrate). Biodegradation in soil and ecotoxicological impact.Biomacromolecules 7, 3125–3131.

hi, B., Palfery, D., 2010. Enhanced mineralization of PLA meltblown materials due

to plasticization. J. Polym. Environ. 18, 122–127.

hogren, R.H.L., Doane, W.M., Garlotta, D., Lawton, J.W., Wilett, J.L., 2003. Biodegra-dation of starch/polylactic acid/poly(hydroxy ester ether) composite bars in soil.Polym. Degrad. Stabil. 79, 405–411.

ong, C., Wang, S., Ono, S., Zhang, B., Shimasaki, C., Inoue, M., 2003.The biodegradation of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB/V)

s and Products 33 (2011) 648–658

and PHB/V-degrading microorganisms in soil. Polym. Adv. Technol. 14,184–188.

Starnecker, A., Menner, M., 1996. Assessment of biodegradability of plastics undersimulated composting conditions in a laboratory test system. Int. Biodeter.Biodegrad. 37, 85–92.

Suyama, T., Tokiwa, Y., Ouichanpagdee, P., Kanagawa, T., Kamagata, Y., 1998.Phylogenetic affiliation of soil bacteria that degrade aliphatic polyesters avail-able commercially as biodegradable plastics. Appl. Environ. Microbiol. 64,5008–5011.

Tokiwa, Y., Calabia, B., 2006. Biodegradability and biodegradation of poly(lactide).

mer blends of poly(l-lactide)s with different optical purity of the lactate units.Macromol. Mater. Eng. 287, 116–121.

Yang, H.-S., Yoon, Y.-S., Kim, N.-M., 2005. Dependence of biodegradability of plasticsin compost on the shape of specimens. Polym. Degrad. Stabil. 87, 131–135.