For Peer Review

PLA plasticized with low molecular weight polyesters:

structural, thermal and biodegradability features

Journal: Polymer International

Manuscript ID Draft

Wiley - Manuscript type: Research Article

Date Submitted by the Author: n/a

Complete List of Authors: Cicogna, Francesca; CNR, ICCOM UOs Pisa Coiai, S; CNR , ICCOM UOS pisa De Monte, Cristina; CNR, ICCOM UOS Pisa Spiniello, Roberto; CNR, ICCOM UOS Pisa Fiori, Stefano; Condensia Química S.A., Research and development Braca, Francesca; Laboratori Archa srl , Laboratori Archa srl Franceschi, Massimiliano; Laboratori Archa srl , Laboratori Archa srl Cinelli, Patrizia; National Interuniversity Consortium of Materials of Science and Technology (INSTM), C/O Department of Civil and Industrial Engineering- University of Pisa Lazzeri, Andrea; University of Pisa, Dipartimento di Ingegneria Civile e Industriale fehri, seyedmohammadkazem; University of Pisa, Civil and Industrial engineering PASSAGLIA, ELISA; CNR, ICCOM;

Key Words: PLA, plasticizer, low molecular weight polyesters, compostability

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PLA plasticized with low molecular weight polyesters:

structural, thermal and biodegradability features

Francesca Cicogna1, Serena Coiai

1, Cristina De Monte

1, Roberto Spiniello

1, Stefano Fiori

2, Massimiliano

Franceschi3, Francesca Braca

3, Patrizia Cinelli

4, Seyed Mohammad Kazem Fehri

4, Andrea Lazzeri

4 and Elisa

Passaglia1*

1Istituto di Chimica dei Composti OrganoMetallici (ICCOM) CNR, SS Pisa, Area della Ricerca, via

Moruzzi 1, 56124 Pisa, Italy

2R&D Department, Condensia Química S.A, C/La Cierva 8, 08184 Palau de Plegamans,

Barcelona, Spain

3Laboratori Archa srl Via Tegulaia 10/A - 56121 Pisa, Italy

4Dipartimento di Ingegneria Civile e Industriale, Università di Pisa, Largo Lucio Lazzarino 2,

56126 Pisa, Italy

Corresponding: passaglia@pi.iccom.cnr.it

Abstract

Polylactic acid (PLA) was plasticized with ester oligomers having different structure, molecular weight and

carboxylic acid content (AN) as end-functionalities. In particular PLA oligomers and a low molecular weight

polyester of adipic acid and 1,2-propane diol (an adipate-based derivative) were used and compared. Their

plasticizing capability was tested and the final structural and thermal stability of PLA matrix were evaluated

by correlating the different features to the chemical and physical characteristics of these additives. All the

oligoesters resulted able to decrease the Tg and modulus values providing elongation at break suitable for

flexible packaging applications even if PLA oligomers provided compounds with reduced structural and

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thermal stability. The most performing blend was finally tested for biodegradability to definitely assess the

material suitable for the final application (sustainable packaging).

Keywords

PLA, plasticizer, low molecular weight polyesters, additives, compostability

Introduction

During the last decades, the poly(lactic acid) (PLA), a polymer derived from lactic acid (actually a

copolymer from L and D enantiomers), has become an emergent bio-based polymer since it can be obtained

from renewable resources, it can be processed by injection/blow moulding, it has high strength and modulus,

and it is recyclable and compostable1. Indeed, especially the commercialized grades containing a few

percentages of D-lactide enantiomer can be easily processed by thermoforming, which is the actual

technology in the food packaging sector. In addition PLA has been also recognized as safe in food-contact

articles2. Showing mechanical properties similar to those of polystyrene3, the low deformation at break limits

its application to the rigid packaging field. On the other hand, it has a relatively high cost that can be justified

by its biodegradability in a compost environment which must be maintained in the different final

formulations.

Considerable efforts have been made to improve the PLA flexibility, with the aim to make this polymer

suitable for flexible packaging, particularly by adding chemicals able to act as plasticizers. Among them

citrate4, fatty acid esters5, poly(ethylene glycol)5,6, poly(propylene glycol)7, triacetin even in mixture with

polyadipate8, malonate

9 and malonate esteramides

10, were used, but some issues concerning their

compatibility with the polymer matrix at higher concentrations were not initially addressed. Moreover any

study about biodegradability of resulting blends has been reported.

To overcome the problems related to phases separation, ester oligomers from lactic acid11,12,13

and/or adipic

acid and 1,3 propane diol14,15 as well as sunflower-oil biodiesel-oligoesters16 were employed with the purpose

to decrease the PLA glass transition value (Tg) by increasing the elongation at break, while ensuring (for

processing time and during the final application) an intimate miscibility based on the similar chemical

structure between the components and on the low molecular weight of the additives. Nice results were

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obtained concerning the flexibility improvements11-16

, also after ageing the compounds13,15-16

, at least for

optimized compositions, even if a certain detriment of final thermal stability was evidenced and correlated to

molecular weight and volatility of the oligoesters used as plasticizers12. The biodegradability of final blends

was not investigated.

By taking into account this scenario, the results here reported deal with the use of low molecular weight

polyesters (oligomers of lactic acid and a oligoadipate) having different molar mass and acid number (AN)

that were synthetized and employed as possible plasticizers for PLA (see details in Table 1). The main aim

was to highlight the effect of the chemical structure and end-functionalities on the structural and thermal

features of final blends. In particular the compatibility, the plasticizing extent, the crystallinity degree and the

strain at break were correlated to viscosity and molar mass of additives, while some behaviour at molecular

level were investigated and discussed as depending on the number of carboxylic end groups in the mixtures,

recognised as affecting the molecular weight evolution of PLA during processing17,18

.

Finally by considering that PLA is biodegradable in a compost environment, (bacterium Bacillus

licheniformis is one of the responsible for PLA biodegradation in compost19) when a blend PLA-based is

produced it is important to assess that biodegradability in compost is maintained, following specific

requirements 19,20

. For these reasons the behaviour of the best performing blend (selected on the basis of

processing feasibility, mechanical properties and thermal stability) in compost was assessed in comparison

with pure PLA matrix.

Experimental part

Materials

Commercial poly(lactic acid) (Cargill Dow) PLA2002D (��� =125,000 D, by SEC analysis) and PLA2003D

(��� = 115,000D by SEC analysis) were used to provide plasticized samples after drying under vacuum at

110 °C for 18 hrs.

Different types of oligomers ascribing to D/L-lactic acid series (Glyplast® OLAs, code OLAX_Y where

X=Acid Number (AN) and Y=���) and a polyester of adipic acid and 1,2-propane diol low molecular weight

sample (Glyplast®206/3NL, code Gly05_3400) were supplied by Condensia Química S.A. and used without

any treatment. The structural and thermal characteristics of the different plasticizers are reported in Table 1.

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<Table 1>

Microcrystalline cellulose powder, 20 µm, supplied by Sigma-Aldrich, Milan, Italy was used as received as

standard for biodegradability tests.

Chloroform (>99%, Sigma-Aldrich, stabilized with amylene) was used as received

Samples preparation

Preparation of plasticized PLA was carried out in a discontinuous mechanical mixer Brabender Plastograph

at 180°C for 10 min and setting a rotor speed of 50 rpm. Total amount of samples in all cases was 30g.

PLA/Glyplast OLA series and PLA/Glyplast 206/3NL blends containing 85 wt% of PLA (or 80 wt%) and 15

wt% (or 20 wt%) of plasticizer for a total amount of 30 g have been prepared by introducing the correct

amount of PLA in the Brabender chamber and by adding the plasticizer one minute later. Since plasticizers

are viscous liquids, their addition to the Brabender chamber has been carried out by weighting the exact

quantity of plasticizer inside a PLA cup shaped film and by adding this film to the molten PLA (the weight

of the PLA film was added to the weight of the polymer already introduced in the Brabender chamber thus

reaching the established grams of PLA).

Composites were also prepared by using a MiniLab II HaakeRheomex CTW 5 conical twin-screw extruder

(Thermo Scientific Haake GmbH, Karlsruhe, Germany) with a sample volume of 7 cm3. The materials were

extruded at 190°C, at 90 rpm and injection moulded at 180°C and 650 bar, and cooled in the mould at 35°C

for 15 sec. After extrusion, the molten materials were transferred through a preheated cylinder to the Haake

MiniJet II mini injection moulder (Thermo Scientific Haake GmbH, Karlsruhe, Germany), to obtain, Haake

III type dog-bone specimen that were used for tensile tests.

Selected formulations were used to produce pellets with a Comac EBC 25HT pilot-scale co-rotating twin-

screw extruder. Extrusion was performed at 10Kg/h flow rate, 200 rpm screws speed, with an eleven heating

zone temperature profile ranging from 160° to 180°C, the fluid plasticizer was added by a peristaltic pump

calibrated for the plasticizer. The pellets were used for the biodegradability test and for the production of

specimen tested by tensile tests.

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Samples Characterization

Thermogravimetric analysis (TGA) was carried out using a Seiko EXSTAR 7200 TGA/DTA instrument.

Analyses were carried out under nitrogen or air flow (200 ml/min) in the 30-700°C range, at 10°C/min

scanning rate, on 5-10 mg samples. From the analysis of the thermograms it was determined the onset

degradation temperature (Tonset), which is the temperature corresponding to 5% mass loss, the rate and

temperature values corresponding to the maximum degradation, and the final residue at 700°C.

Differential scanning calorimetry (DSC) analysis of all the samples was carried out under nitrogen

atmosphere by using a Perkin-Elmer 4000 instrument. The instrument was calibrated with indium and lead as

standards. The analysis was carried out in the temperature range from -40 to 180°C at 10°C/min.

Crystallization and melting enthalpies were evaluated from the integrated areas of melting peaks recordered

during second heating. The glass transition temperature (Tg) was measured from the inflection point in the

second heating curve.

PLA crystallinity of samples containing variable amount of plasticizers was calculated considering the PLA

weight fraction in the blends on the basis of following equation (eq. 1).

%c1 = xPLA-1

(m � cc)

100%c

100 (1)

where ∆Hm is the melting enthalpy of the sample and ∆Hcc is the enthalpy of cold crystallization

∆H100%c is the melting enthalpy of a 100% crystalline PLA (93.0 J/g).

The samples were analysed before and after annealing carried out at 110° in the oven for 12hrs.

Size exclusion chromatography (SEC) analyses were performed in CHCl3 (flux 0.3 ml/min) using an Agilent

Technologies 1200 Series instrument equipped with two PLgel 5 µm MiniMIX-D columns and a refraction

index detector. Monodisperse poly(styrene) samples (Agilent) were used as calibration standards.

Tensile tests were performed at room temperature, at a crosshead speed of 10 mm/min, by means of an

Instron 4302 universal testing machine (Canton MA, USA) equipped with a 10 kN load cell and interfaced

with a computer running the Testworks 4.0 software (MTS Systems Corporation, Eden Prairie MN, USA).

Dog bone specimens, Haake III type produced with the Haake injection moulder, were placed in plastic bags

for vacuum sealing to prevent moisture absorption.

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The biodegradability of a selected blend of PLA with plasticizer was performed under controlled conditions

in accordance with official method21. This method allows the determination of the degree and rate of aerobic

biodegradation of plastic materials on exposure to a controlled-composting environment, in contact with a

mature compost. The goal of this test was to assess at least 90% of mineralization for the plastic material

versus a positive control (i.e. cellulose). Following the procedure biodegradability tests were performed in

batch-scale bioreactors of 3L of volume introduced in a thermostatic incubator at 58°C in the dark for a

maximum period of 6 months. About 600 g (dry weight) of compost was introduced in the vessel with about

100 g (dry weight) of sample which was the tested material, the blank (as compost alone) and the positive

control (a microcrystalline cellulose powder). All the samples were analysed in three replicates. CO2 and O2

concentrations were monitored in the outgoing air by measuring at least twice daily during the first week,

afterwards the measurement frequency was reduced to once per day and finally twice per week. The

produced carbon dioxide was measured by gas-chromatographic determination using thermal conductivity

detector AutoSystem XL-GC produced by Perkin Elmer, Milan, Italy, equipped with a 6-way injection gas

valves, 150 µl loop injection volume. The gas separation was carried out in a packed silica column 60/80

Carboxen 1000 with Length: 4.5 m; inner diameter: 3.2 mm, Supelco. The equipment used for the

biodegradability tests is schematically presented in Figure 1.

Figure 1. Schematic representation of the equipment used for monitoring carbon dioxide production during

biodegradation test.

GC-TCD

Air

Splitter

Humidity Bioreactor

CO2

Detection

Climatic Chamber @ 58° C

FM

Condenser

Dryer

FM

Flowmeter

Flowmeter

Thermal Conductivity

Detector

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The compost inoculum derived from composting the organic fraction of municipal waste was provided by an

Italian waste management company, CERMEC, Consorzio Ecologia e Risorse Massa Carrara, Massa, Italy

and it was derived from composting the organic fraction of municipal waste. The used compost had the main

chemical characterizations presented in Table 2: the experimental values were in agreement with those

indicated in the official method, except for the dry solids content (in this case it is allowed by the method21 to

adjust the dry solid content by adding water to the compost to reach the targeted range).

<Table 2>

Finally, all composted samples obtained at the end of the composting process were analyzed, after sieving, in

order to determine the initial values of the same parameters for chemical and chemical-physical evaluation

(total carbon vs. total nitrogen ratio (C/N ratio) and pH, according to national low22

).

Results and Discussion

Different ester oligomers (whose characteristics are reported in Table 1) were melt mixed with PLA and the

effects of their molecular weights and amount of end-carboxylic acid groups onto the structure and the

thermal features of the PLA matrix were investigated (Table 3). Initially torque evolutions of the runs were

registered and a sharp decrease of the melt viscosity owing to the plasticizers addition was observed for all

the samples (Figure 2). After a mixing time period that was variable depending on the type of plasticizer

used (induction period) a torque recovery was observed with a certain stabilization of its value.

In particular, the induction time was longer using Gly0.5_3400 than the plasticizers of OLA series (see for

example Figure 2). The phenomena, being connected with the dispersion of the plasticizer in the molten

polymer mass, can be due to the low viscosity of Gly0.5_3400 thus affecting the melt viscosity at the

beginning of the mixing (900 mPa·s of of Gly0.5_3400 vs 3500 mPa·s of OLA1.7_1400, as example).

Moreover, a major chemical affinity of OLA1.7_1400 with the PLA chains12

than Gly0.5_3400 and some

volatilization effects (later discussed) can be also take into account.

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Figure 2: Torque evolution of the runs PLA_180, PLA/15 OLA1.7_1700, PLA/15 Gly0.5_3400

Nevertheless the final torque values, reported in Table 3 are lower for all the runs carried out with the

oligomeric additives thus suggesting an effective plasticization which increased by raising the content of

plasticizer (it was demonstrated that by using up to 20 wt% of plasticizer the samples are able to maintain

their amorphous nature13

). At the same time, in agreement to the apparent viscosity of the molten polymer,

the higher final torque levels were obtained when plasticizers with higher molecular weight were used:

OLA2.5_2700 and the polyadipate-based oligomer (Gly0.5_3400).

<Table 3>

By evaluating the MW evolution of the PLA matrix after introducing plasticizers, a decrease of both ��� and

��� was observed for all the samples independently of the acid number (AN) and molecular weight of the

plasticizer used (Table 3). Even if the adopted melt-blending procedure (180°C, 50 rpm, 8 min of mixing)

was similar to that reported in literature for analogue samples and it should grant the molecular weight

preservation12,14, this evidence indicates a certain degradation of the matrix likely ascribable to the presence

of the plasticizer as confirmed by the fact that the MWs decrease is more evident by raising the plasticizer

0 100 200 300 400 500 600

0

2

4

6

8

10

12

14

torque (Nm)

time (s)

PLA_180

PLA/15 OLA1.7_1400

PLA/15 Gly0.5_3400

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concentration (PLA/15 OLA1.7_1400 vs PLA/20 OLA1.7_1400 and PLA/15 Gly0.5_3400 vs PLA/20

Gly0.5_3400). By considering that the concentration of (end) acid functionalities can affect the structural

stability of the PLA in the mixtures, the PLA MW evolution for the runs carried out with plasticizers having

similar molar mass and different AN was compared (runs carried out by using OLA0.5_1500, OLA1.0_1600

and OLA1.7_1400). In particular, considering the ��� value, that is more fitting the average kinetic length of

macromolecules, we observed that even the use of the plasticizer having lower AN causes a certain

decrement of the PLA ��� value. Moreover by increasing the AN the deviation is increased (Figure 3), as

counted by the hydrolytic degradation mechanism and its rate which are affected by the functionalities and

by the molecular weight of starting materials17,23,24

. For the same AN value, a better control of the PLA

molecular weight was obtained by using plasticizers having a higher molecular weight (OLA2.5_2700 vs

OLA2.5_1900) as expected on the basis of the total amount of end-functionalities in the mixtures.

A further investigation was made to deepen insight the consequence of different torque recovery times onto

MW evolution. In particular, the PLA/15 OLA1.7_1400 and PLA/15 Gly0.5_3400 runs were repeated by

keeping the material in the mixing chamber for different times (Table 4).

0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8

28

30

32

34

36

38

40

% ∆ Mn = (Mn PLA_180

-Mn PLA_plastizer

)/Mn PLA_180

% ∆ M

n

acid number (mg/KOH g)

Figure 3. Effect of AN of the plasticizers (plasticizers having the similar MW) vs ��� deviation of the PLA

matrix; Error bar calculated on the basis of average medium collected from analyses of PLA sample

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The mixing times were chosen in order to recover the PLA/plasticizer blends at the time preceding the

increase of the torque (3.5 min for both the samples) and at the maximum value of the torque (at 5 and 8 min

for OLA1.7_1400 and Gly0.5_3400, respectively), in addition to those corresponding to the final torque (10

min in both cases) (Figure 2 and Table 4).

<Table 4>

The ��� and ��� changes occurred mainly before the torque recovery, and levelled off to a constant or more

stable value by increasing the time, thus suggesting that the intimate contact between the polymer chains and

the plasticizer was reached at the very beginning of the mixing. In addition, by looking at the data

concerning the final ratios between the PLA and the plasticizers (Table 3), even if these numbers are roughly

estimate by the relative area percentage from chromatogram peaks and then are likely significantly affected

by the different compatibility between the polymer matrix and the type of ester oligomers, we observed that

up to 60% of plasticizer in the series OLA is lost during mixing, while the polyadipate is totally contained in

the final mixture for both percentage (15 and 20 % wt), only partially in agreement with previous results

12,14,15.

This evidence can be preliminary explained by considering the thermal stability, and in particular the Tonset of

TG curves of the different plasticizers (Table 1). It is evident that the oligomers of OLA series are

characterized by a lower thermal stability (or a higher content of volatile fractions) compared to

Gly0.5_3400, with a Tonset really close to the PLA processing temperature with the exception of

OLA2.5_2700. Considering the experimental conditions (180°C, 8 min of blending) a part of mass is lost,

thus confirming the SEC results.

The thermal features of PLA-based samples analyzed by TGA confirmed that the thermal stability of

plasticizers affected that of the corresponding plasticized blends (Table 5). In particular the Tonset was

sensibly reduced for the samples prepared with OLA series oligomers, while the mixtures with polyadipate

resulted more stable in agreement with the intrinsic stability of Gly0.5_3400 (Table 1) and even with the

effect of molecular weight decrease of resulting PLA. Indeed it was previously found that the thermal

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stability of PLA with diminished MW with respect to neat PLA, owing to processing conditions, is generally

reduced23.

The DSC measurements (Table 5) confirmed that PLA/plasticizer blends were miscible at all the

compositions tested since a single Tg was evidenced for each sample; as expected, the addition of all the

plasticizers caused a marked reduction of Tg values whose extent was even more prominent by increasing

the plasticizer amount. For equal starting concentration, lower Tg values were collected for blends

containing polyadipate or OLA-series products having lower molecular weight, which resulted able to shift

the Tg close to room temperature (32-37°C). For OLA series derived blends the Tg increased by increasing

the molar mass of additives in agreement with their higher Tg (see Table 1).

<Table 5>

The cold crystallization peak shifted to lower temperature: the increased chain mobility of PLA due to the

plasticizing effect induced by the additives resulted in a faster crystallization, occurring at temperature

ranging from 90°C to 106°C, sensibly lower than that of un-plasticized sample (118°C). This peak appeared

very sharp and intense with a minimum depending on the structure of plasticizer and its plasticizing

capability (Figure 4); in particular the greater the plasticizing effect the lower the peak temperature

associated with the transition, accounting the lower molar mass and the Tg of plasticizers. The differences

between melting (∆Hm) and crystallization (∆Hc) enthalpies as well as the data concerning the crystallinity

of the sample (see experimental part) showed that the materials were mostly amorphous with the exception

of sample PLA/20 GLY0.5_3400 containing high percentage of polyadipate, in agreement with data already

reported for similar systems15. The curves of plasticized samples were even characterized by the presence of

two melting peaks, due reorganization of lamellae during the cold crystallization generating less perfect

crystals 6,7,25,26

.

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Figure 4: DSC melting thermograms of neat PLA and plasticized samples obtained by adding oligomers of

OLA series having different molecular weight and AN.

After the annealing all the plasticized samples evidenced an increased crystallinity and a lower melting

temperature with respect to the un-plasticized sample (PLA_180), especially those compounds provided by

using plasticizers having lower MW or by increasing their content. Both the effects can be rationalized by

considering that the growth of crystalline domains is likely facilitated if the chain mobility is increased, even

if with a less ordered packing 25,26

.

The most performing samples in terms of thermal stability and plasticizing effects PLA/15 GLY0.5_3400

and PLA/20 GLY0.5_3400 were further processed in the minilab extruder and the specimen produced by

injection moulding were used to evaluate their mechanical properties by tensile tests, in comparison with

those of samples plasticized with oligoester from OLA series (PLA/15 OLA1.7_1700 and PLA/20

OLA1.7_1700).

From the mechanical tests (Table 6) it was found that both OLA1.7_1400, and GLY0.5_3400 were very

efficient as plasticizers. By adding 15 wt% of either OLA1.7_1400 or GLY 05_3400 the effect on the

mechanical properties of PLA was significant with a light plasticizer effect for 15% of OLA1.7_1400 in

terms of stress at yield. In the blends containing 20 wt% of the plasticizers a considerable reduction in the

0 40 80 120 160

heat flow

Temperature (°C)

PLA2002D_180

PLA2002D/15 OLA0.5_1500

PLA2002D/15 OLA2.5_1900

PLA2002D/15 OLA2.5_2700

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elastic modulus and strength both at yield and at break with a substantial increase in elongation at break were

evidenced.

<Table 6>

The present results are particularly valuable since the processing conditions and time of residence of molten

material in the minilab extruder are very similar to the common processing in industrial extruder applied for

production of PLA based blends. Therefore the blends selected can be applied for an industrial production of

PLA based materials. Even if the performance of OLA1.7_1400 and GLY 05_3400 were both very good as

plasticizers we developed further formulations based on GLY 05_3400 because of its higher thermal stability

and stabilization exerted in the compounds (see Tables 1 and 5). Moreover its easier dosing and feeding in

the extruder due to lower viscosity compared to OLA1.7_1400, whose high viscosity made dosing and

feeding quite challenging, play a key role in technological application. The materials obtained as pellets were

further used for the determination of the percentage of biodegradation during composting tests.

Composting is a natural process that involves the aerobic biological decomposition of organic materials

under controlled conditions 27-29

. Compost is a nutrient rich soil-like material created by the biological

decomposition of organic materials such as vegetative debris and livestock manures 30, it can improve soil

fertility, extent fertilizers, save water, suppress plant diseases, and boost soil tilt 30

. During composting

organic matter from the biodegradable wastes is microbiologically degraded, resulting in final product

containing stabilized carbon, nitrogen and other nutrients in the organic fraction, the stability depending on

the compost maturity 31

.

The aim of the test of mineralization in compost was to assess that the rate of biodegradability of the

developed PLA based blends under controlled composting conditions can meet requirements according to

official method21

. The samples based on PLA and GLY 05_3400 resulted more performing both for thermal

stability than for mechanical properties thus these blends were selected for evaluating the compostability of

PLA based blend with this plasticizer and it was compared with the pristine PLA plastic material, to evaluate

the effects of the presence of plasticizer.

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Figure 5: Biodegradation curves of cellulose reference material, PLA and PLA plasticized with

Gly0.5_3400.

During the test, after 10 days, the compost (blank samples) had to produce amounts of carbon dioxide in the

range 50 – 150 mg CO2/volatile solids; and after 45 days the microcrystalline cellulose (reference sample)

had to be biodegraded more than 70%, as required by the standard method.

Both samples PLA and PLA/15 GLY0.5_3400 passed the threshold limit imposed by the standard21, because

the percentage of biodegradability reached over 90%, versus cellulose, within six months, as defined for a

biodegradable attribution of the material. Thus as reported in Figure 5 PLA reached 99.6% of mineralization

and PLA/15 GLY0.5_3400 reached 94.9% in 105 days accounting the claimed requirements.

<Table 7>

Finally the chemical and physical determinations on final composted materials both for PLA and PLA/15

GLY0.5_3400, showed the perfect compliance with the optimal values for fertilizer as defined by the

National Regulation22 (see Table 7 and Table 2 for comparison)

Conclusions

Plasticized PLA-based materials were prepared by using oligoesters derived either from adipate or lactide.

Viscosity, volatility, molecular weight and chemical reactivity of end-functionalities of the additives used as

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plasticizers as well as their starting thermal stability really affected the processability and the thermal

features of ultimate plasticized products. Especially referring to oligomers of lactide series it was found that

they promote the hydrolytic degradation during the processing, even if the use of plasticizers having lower

acidic number and higher MW can partially keep under control this effect. Even if all the plasticizers can

produce flexible compounds, the adipate-based product resulted more thermally stable and efficient, even by

increasing its quantity. In addition thanks to its low viscosity, both dosing and feeding in extruder were

easier and thus this oligomer appeared more suitable among all those tested for a possible industrial

application. In addition, the mineralization test of plasticized PLA evidenced that this last compound

accounts for the regulation, definitely assessing that the present material has the potentiality to meet standard

for getting the compostability logos, and accordingly it can be directed to a bio-recycling facility as end of

life option with a significant benefit for the environment which is supported also by the high content of bio-

based components.

Acknowledgements

FP7 Large Cooperation Grant Agreement (DIBBIOPACK) number 280676 were acknowledged for funding

the research.

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Table 1: Structural characteristics and thermal stability of plasticizing additives

Code Composition and

Structure

Acid number

(mgKOH/g)1 ���

2

(g/mol)

Viscosity

(mPa s)

Tonset3 Tg

4

Gly0.5_3400

Low MW polymers based

on adipic acid and

propylene glycol

OO

O

O

n

0.5 3400 900 (25°C) 258 -67.9

OLA1.7_1400 Low MW D/L-lactic acid

polymers

1.7 1400 3500 (25°C) 198 -52.9 OLA0.5_1500 0.5 1500 4200 (25°C) 184 n.d

OLA2.5_1900 2.5 1900 24 (100°C) 194 -35.7

OLA2.5_2700 2.5 2700 50 (100°C) 212 -24.6

OLA1.0_1600 1.0 1600 n.d 195 n.d 1Determined by titration methodology 2Determined by SEC analysis

3Onset degradation temperature (Tonset), which is the temperature corresponding to 5% mass loss, determined by TGA

analysis carried out under nitrogen atmosphere 4Determined by DSC measurements

Table 2: Chemical and physical parameters for the used compost (before sample composting tests)

Sample Dry material

(% wt/wt)

Volatile solids (% wt) C/N pH

Initial compost 59.6 28.4 15.3 8.2

Optimised values22 50-55 <30 10 - 40 7.0 – 9.0

Table 3: Composition, torque evolution and molecular weights of samples provided by melt blending the

PLA with low molecular weight polyesters (Plast)

* determined by the relative area collected from SEC chromatograms

Sample Plast. code Content

(% wt)

Final

Torque

(Nxm)

Recover

y time

(sec)

��� (kg/mol)

��� (kg/mol)

PLA/Plast.

relative

content*

PLA_180 - - 4.1 - 110.2 192.3 -

PLA/15 OLA1.7_1400 OLA1.7_1400 15 1.1 250 68.9 133.8 94/6

PLA/20 OLA1.7_1400 OLA1.7_1400 20 0.6 250 58.2 111.1 91/9

PLA/15 Gly0.5_3400 Gly0.5_3400 15 2.1 560 89.6 152.2 84/16

PLA/20 Gly0.5_3400 Gly0.5_3400 20 0.9 560 79.1 130.3 76/24

PLA/15 OLA0.5_1500 OLA0.5_1500 15 1.0 330 76.8 135.2 92/8

PLA/15 OLA2.5_1900 OLA2.5_1900 15 1.5 270 89.1 154.2 91/9

PLA/15 OLA2.5_2700 OLA2.5_2700 15 2.1 350 108.7 175.1 91/9

PLA/15 OLA1.0_1600 OLA1.0_1600 15 1.0 280 75.9 148.5 91/9

HO

O

O R

n

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Table 4: Molecular weight evolution (��� and ���) versus time for runs PLA/15 OLA1.7_1400 and PLA/15

Gly0.5_3400, compared to PLAD_180

Table 5: Thermal data obtained by TGA and DSC analyses of neat PLA and samples obtained by adding the

different plasticizers

Sample Tg

(°C)

Tcc

(°C) ∆Hcc

(°C)

Tm

(°C) ∆Hm

(J/g) %c1

* Tonset

(°C)

Tinf

(°C)

Tm§

(°C) %c1

§*

PLA_180 57.1 118.5 -21.3 150.8 26.1 5.2 316 354 148.8/156.4 30.2

PLA/15 OLA1.7_1700 37.2 97.9 -24.3 137.9/151.9 25.2 0.9 266 319 138.4/151.6 42.7

PLA/20 OLA1.7_1700 20.0 96.7 -24.6 135.6/151.1 26.8 2.3 248 308 143.5 43.1

PLA/15 GLY0.5_3400 32.0 106.5 -27.9 143.4/153.5 28.2 0.3 302 366 140.0/154.0 43.7

PLA/20 GLY0.5_3400 19.4 100.3 -21.8 142.6/155.1 29.1 6.6 287 367 153 48.3

PLA/15 OLA0.5_1500 37.8 96.2 -22.8 135.8/153.3 23.3 0.2 272 358 140.1/152.4 47.1

PLA/15 OLA2.5_1900 43.1 101.5 -22.8 139.3/152.5 22.9 ~0 264 360 140.4/153.1 46.0

PLA/15 OLA2.5_2700 47.3 107.3 -22.7 143.3/152.9 21.7 ~0 270 350 142.3/153.6 38.3

PLA/15 OLA1.0_1600 37.0 89.4 -25.0 135.6/151.8 24.6 ~0 264 360 139.2/151.6 43.9

* See experimental part

§ after annealing

Table 6: Results of mechanical tests on PLA plasticized compounds.

Sample

Stress at

yield

(MPa)

Elongation at

yield

(%)

Stress at break

(MPa)

Elongation at

break

(%)

E

(MPa)

PLA 2002D 61.6±1.8 2.2±0.1 53.3±1.7 4.9±0.5 3.100±340

PLA/15 OLA1.7_1700 32.4±3.1 6.4±0.3 26.7±1.4 518.8±15 280±30

PLA/15 GLY0.5_3400 23.6±2.1 7.4±0.2 28.3±2.1 481.8±13 270±35

PLA/20 OLA1.7_1700 nd n.d. 20.8±2.1 634.3±15 240±35

PLA/20 GLY0.5_3400 7.9±0.5 9.3±0.3 21.7 491.9±32 340±43

Table 7: Analysis of compost after mineralization test of PLA and plasticized PLA sample.

Sample Dry Material

(%)

Volatile

solids (%)

C/N* pH

PLA 53.1 14.9 13.8 8.3

PLA/15 GLY0.5_3400 53.4 16.1 14.4 8.1

���

(kg/mol) ���

(kg/mol)

Run time (sec)

PLA_180 PLA/15

OLA1.7_1400 PLA/15

Gly0.5_3400 PLA_180

PLA/15 OLA1.7_1400

PLA/15 Gly0.5_3400

210 - 75.0 105.8 - 140.0 182.8

300 - 73.0 - - 146.6 -

510 - - 90.7 - - 156.7

600 110.5 68.9 89.6 192.3 133.8 152.2

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Graphical Table of Content

PLA plasticized with low molecular weight polyesters: structural, thermal and biodegradability

features

Francesca Cicogna, Serena Coiai, Cristina De Monte, Roberto Spiniello, Stefano Fiori, Massimiliano

Franceschi, Francesca Braca, Patrizia Cinelli, Seyed Mohammad Kazem Fehri, Andrea Lazzeri and Elisa

Passaglia*

Flexible materials were obtained by melt mixing PLA with oligoesters having different structure and

molecular weight whose effect on structural and thermal stability of blends was deeply investigated together

with the biodegradability performance of the most promising blend.

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