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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: [email protected]
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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