Degradation of Poly(lactic acid) under Simulated Landfill Conditions

Abstract

In this study, the physical and chemical properties change of poly(lactic acid) after burying in the mixture of soil and sludge under thermophilic (61 °C) oxygen limited conditions were investigated using various analytical techniques. The environmental factors under these setting conditions and microbial activities accelerated the degradation process of PLA. Under tested conditions, PLA loss their weight about 90% at the burying time of 90 days. During the degradation process, PLA samples were continuously broken to small fragile fragments and showed the size less than 1 mm at the end of degradation test. Change of the surface morphology change was revealed by scanning electron microscopy (SEM). Many pores, cracks and irregular roughness were presented on the PLA surface. Thermal decomposition was decreased from 387.8 to 289.2 °C. The percentage of carbon content in molecular structure decreased from 49.46% to 45.42%. In addition, the Fourier transformed infrared spectroscopy (FTIR) revealed the change of ester bonds. This study can be used for developing PLA waste management process.

Full text

Environment and Natural Resources J. Vol 14, No.2, July-December2016:1-9 1
*Corresponding author: ISSN 1686-5456(Print)
E-mail: chomnutcha.b@gmail.com ISSN 2408-2384(Online)
Degradation of Poly (lactic acid) under Simulated Landfill Conditions
Chomnutcha Boonmee 1*, Charnwit Kositanont 2 and Thanawadee Leejarkpai 3
1 Inter-department of Environmental Science, Graduate School, Chulalongkorn University, Bangkok 10330, Thailand
2Department of Microbiology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
3National Metal and Materials Technology Center, Pathumthani 12120, Thailand
Abstract
In this study, the physical and chemical properties change of poly(lactic acid) after burying in
the mixture of soil and sludge under thermophilic (61 °C) oxygen limited conditions were investigated
using various analytical techniques. The environmental factors under these setting conditions and
microbial activities accelerated the degradation process of PLA. Under tested conditions, PLA loss their
weight about 90% at the burying time of 90 days. During the degradation process, PLA samples were
continuously broken to small fragile fragments and showed the size less than 1 mm at the end of
degradation test. Change of the surface morphology change was revealed by scanning electron
microscopy (SEM). Many pores, cracks and irregular roughness were presented on the PLA surface.
Thermal decomposition was decreased from 387.8 to 289.2 °C. The percentage of carbon content in
molecular structure decreased from 49.46% to 45.42%. In addition, the Fourier transformed infrared
spectroscopy (FTIR) revealed the change of ester bonds. This study can be used for developing PLA
waste management process.
Keywords: Poly(lactic acid)/ PLA/ Degradation/ Physical Properties
Received: 17 June 2016 Accepted:1 August 2016
DOI: 10.14456/ennrj.2016.8
1. Introduction
The worldwide plastic consumption has
been increased continuously according to the
population and the economic growth (Biron,
2014). Plastic wastes are the large portion (16%)
of municipal solid waste that regarded as non-
degradable materials (Muenmee et al., 2015).
Therefore, they are accumulated and caused
environmental problems. Biodegradable plastics
are expected to be the solution for the problems.
Anyhow, at high volume of biodegradable plastics
waste generated, the proper means for waste
management are needed. Therefore, the
understanding of their degradation behavior is
necessary. Sanitary landfill is widely used as a
common waste disposal method in many countries
including Thailand. However, there is limited
information on the biodegradable plastic
degradation under the landfill conditions. The poly
(lactic acid) or PLA is promoted as the
environmental friendly material. Because of its
property as a biodegradable plastics, PLA is
designed to be used in applications such as
medical devices, agricultural tools as well as
packaging materials (Avérous, 2008; Gupta and
Kumar, 2007; Sin et al., 2013). The good
degradation rates of PLA was found at/or above
glass transition temperature (Tg ~55-60°C)
(Itävaara et al., 2002; Yagi et al., 2013). In our
previous study, the optimal conditions of PLA
degradation under simulated landfill conditions
were 61 °C under oxygen limited conditions
(unpublished data). In this study, the degradation
behavior of PLA under these conditions was
examined within 90 days of burying test.
2. Methodology
2.1 Poly(lactic acid) sheet casting
The poly(lactic acid) (PLA) that used in
this study was obtained from National Metal and
Materials Technology Center, Thailand (MTEC).
PLA was molded by the cast sheet extrusion
machine (HAAKELR 8906-02F). The average
thickness of the sample was 0.5 mm. Before using
in the experiments, the sheet of PLA was cut into
square shape (2×2 cm2) as demonstrated in Fig.
1A. The plastic shape and size are set to follow the
International standard ISO 15985, 2004 (Plastics-
Determination of the ultimate anaerobic
biodegradation and disintegration under high-
solids anaerobic-digestion conditions). The
chemical structure of PLA was showed in Fig. 1B.
2.2 Medium preparation
The mixture of land filling soil and
sludge was used as medium. Soil was obtained
from Suphanburi province municipal solid waste
disposal landfill site. Sludge was received from
the anaerobic wastewater treatment plant of fruit
juice factory, Malee Sampran Public Company
Limited, Nakhon Pathom province, Thailand. Both
materials were collected and preserved at 4 °C
until experimental used.

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(A)
(B)
Figure 1: (A) PLA sheet. (B) The chemical structures of PLA
2.3 Experimental set-up
In this experiment, landfill soil and
anaerobic sludge were mixed at the ratio of 50:50
(W/W) and used as the medium. 330 g of medium
was weighted and placed in a 660 ml glass bottle.
PLA samples were placed in a plastic net bag at
1.5% weight by weight of the medium then, bury
in the medium. To achieve oxygen limited
conditions, the glass bottles were capped with a
rubber bung to prevent air transferring in the glass
bottles. The tested bottles were incubated at 61 °C
in dark conditions for 90 days. The experiment
was conducted in triplicate.
2.4 Weight loss determination
For weight loss determination, the
remaining of PLA samples was recovered from
the tested bottles at 15, 30, 45, 60, 75 and 90 days
of burying test. The PLA samples were rinsed
with sterile water to washout the soil and sludge
particles. PLA samples were kept in a desiccator at
room temperature and weighted until the weight
was constant. The percentage of PLA weight loss
was calculated by following the equation (1).
 

Where Wi represented the initial dry
weight of plastic material (g) and Wf represented
the dry weight of the recovery fragment of plastic
material after the degradation (g). The percentage
of weight loss was taken from the average of three
samples.
2.5 pH analysis
The pH of medium samples was
monitored by using pH meter (Consort, C862,
Belgium). Ten g of soil was suspended in 50 mL
deionized water. The solution was stirred during
measurement.
2.6 Surface change and particle size analysis
The surface PLA samples before and
after burying test were investigated by visual
observations and by using Scanning Electron
Microscopy (SEM) (model JSM-5410LVJEOL,
Japan) at 2000x. For particle size analysis, the
fragmented residuals of PLA samples were
divided into three fraction sizes including greater
than 2 mm, 2-1 mm, and less than 1 mm by using
sieving method. Each size of PLA samples was
separately collected and weighted for determining
the percentage of weight fraction.
2.7 Thermal gravimetric analysis (TGA)
The thermal property of PLA samples
before and after incubation were studied by TGA
analysis, using Thermal Gravimetric Analyzer
(NETZSCH thermos gravimetric balance, model
TG 209F3, NETZSCH, Germany). The samples
decomposition temperature (Td) in the range of
28 °C to 600 °C were determined under nitrogen
atmosphere using 8-11 mg PLA.
2.8 Elemental analysis
The C, H, O contents of PLA samples
before and after degradation were determined with
an Elemental Analyzer (PerkinElmer model
PE2400 Series II, PerkinElmer, USA).
2.9 Chemical characteristic changes
The chemical characteristic changes of
the PLA samples were studied by using Fourier
Transform Infrared Spectrometer (Spectrum One,
PerkinElmer, USA). The IR wave number
frequency range was 650 cm-1 to 4000 cm-1.
3. Results and Discussion
3.1 Weight loss of PLA under tested conditions
The PLA degradation during 90 days of
tested conditions is showed in Fig. 2. The result
indicated that the weight of PLA samples was
decreased when increased burying time. It was
found that weight loss was approximately 90% at
90 days. The result was in accordance with the
study of Yagi et al. (2009) and Arrieta et al.
(2014) reported the rapid disintegration of PLA at
90% of weight loss under thermophilic conditions.
Generally, PLA was degraded via
hydrolytic and microorganism degradation
process. The hydrolytic process depend on the
temperature and humidity level (Lunt, 1997;

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Mathew et al., 2005). In this case, high
temperature conditions enhanced the break down
of polymeric chain. Mixing the sludge in burial
medium was not only increase in microorganism
number but also the humidity in the system. The
water absorption of PLA has been reported to
accelerate the hydrolytic degradation,
disintegration, and cleavage the ester linkages of
polymeric chain resulting in weight loss of PLA
(Lunt, 1997; Copinet et al., 2004; Mathew et al.,
2005 ; Wang et al., 2008).
3.2 pH change
During the burying test, the pH of media
was measured (Fig. 3). The pH of media was
decreased from 7.3 ± 0.1 to 5.0 ± 0.2 within 90
days. The evidence of pH decreasing was also
reported by Dong et al. (2013) who suggested that
the carboxyl group (−COOH) and hydroxyl group
(−OH) that occurred after PLA hydrolytic
degradation had the effect on decreasing the pH of
media.
Figure 2: Weight loss (%) determination of PLA during 90 days of burying test
Figure 3: Change of pH in burial medium during PLA degradation test

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3.3 Changes in physical appearance and size of
PLA after burying test
The degradation of PLA was confirmed
by visual physical appearance observation and size
measurement as a function of time. The sample
size before and after the burying test were showed
in Fig. 4. The color of PLA in the image before
and after degradation process was changed from
the clear color to opaque white. In this study, the
disintegration/fragmentation was started after the
day 15th and continuously disintegrated into small
fragile size until 90 days. In the Fig. 5, from 15 to
30 days, the separation of PLA fragments started
breaking into size greater than 2 mm and other
smaller fractions. During 45 to 90 days, the
number of smaller fraction was increased by the
time of burying. At the end of burying test, the
fraction greater than 2 mm was not founded and
the fraction of less than 1 mm was the largest
percentage. The increasing of smallest fraction
(less than 1 mm) indicated the continuous
degradation process of the test system.
The color change after the burying test
related to the absorption of water from medium to
the PLA matrix during the stage of hydrolysis
(Arrieta et al., 2014). The similar result was
reported by Ghorpade et al. (2001)
Lunt (1997) summarized the hydrolytic
degradation of PLA as a function of time. At
thermophilic temperature around 60 °C, the onset
of disintegration started at 8.5 days. This
temperature was close to the glass transition
temperature (Tg) of PLA resulting in increasing
the speed of the PLA degradation/disintegration.
The slower disintegrations were found at the
incubation temperature below its Tg (Rudnik and
Briassoulis, 2011, Weng et al., 2013).
Figure 4: Appearance of PLA (A) before burying test; (B) after 15; (C) 30; (D) 45; (E) 60; (F) 75;
and (G) 90 days of burying test at 61 °C

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Figure 5: Particle size fractions of PLA after burying test.
3.4 Change of surface morphology
The change of surface morphology
during the degradation process was examined by
using scanning electron microscopy (SEM). The
appearance of PLA surface after 30, 60 and 90
days of degradation were revealed at 2000x
magnification as demonstrated in Fig. 6. Before
the burying test, the PLA sample had smooth and
clear surface (Fig. 6A). After 30 days, the
roughness and small pores were appeared on the
PLA surface as showed in Fig 6B. The drastic
change on PLA surface was found after 60 days of
degradation process showing big cracks and small
pores as present in Fig. 6C. At the end of the
burying test (90 days) PLA surface became totally
roughed with many micropores (Fig. 6D).
Under tested conditions, PLA not only
reduced in small size (Fig. 5.) but also changed on
the surface morphology (Fig. 6.) However, the
disintegration of PLA was found after 15 days
while the crack on surface was obviously observed
after 60 days of burying test. These evidences
could be indicate that the degradation profile of
PLA was started with the disintegration/
fragmentation and followed by the degradation on
the surface.
3.5 Thermal gravimetric analysis (TGA)
Thermal gravimetric analysis (TGA)
provided information on the structure of the PLA
samples by the mass loss steps. The result in Fig. 7
demonstrated the curves of PLA samples before
and after burying test at 30, 60 and 90 days. At
initial time of study, the maximum temperature for
the decomposition was 387.8 °C, but after 90 days
of burying test, the thermal stability of PLA is
decreased to 289.2 °C. The decreasing of thermal
stability of PLA during degradation time could be
originate from the degradation of the long chain
polymer to short chain and reduced molecular
weight of PLA (Al-Itry et al., 2012).
3.6 Element content before and after degradation
The results of element analysis before
and after degradation were summarized in Table 1.
The carbon content in samples was continuously
decreased while oxygen content in samples was
increased due to the degradation process. The
sample before degradation showed 49.46% of
carbon content. After degradation at 30 60 and 90
days, the percentages of carbon content were
decreased to 46.33, 46.35 and 45.42%,
respectively. The similar tendency in decreasing in
carbon content after degradation was reported by
Weng et al. (2013).
3.7 Fourier Transform Infrared (FTIR)
examination
The changes of PLA functional groups
before and after degradation were shown in Fig. 8.
The peaks presented at wave number 2800-3000
cm-1 which associated with the C–H groups. The
distinctive peak at 1722-1759 cm-1 indicated the
C=O group of active ester bond for hydrolysis
(Ndazi and Karlsson, 2011). Peaks located around
1300-1500 cm-1 related to the vibration of C-H in
CH3 groups stretching vibrations. The intensive of
peaks according to 1050-1250 cm-1 due to the C-O
stretching vibration (Al-Itry et al., 2012; Pamula et
al., 2001). The FTIR is useful for characterize the
PLA base material, this result indicated that there
were some peak around 1602 -1635 cm-1 increased
after 30 days and accumulated until the end of
burying test. This peak associated with the
formation of carboxylate ions. Arrieta et al. (2014)
found this peak after PLA degradation and
summarized that the carboxylate ion was resulted
from the degradation of lactic acid by

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microorganisms and leaved the carboxylate ion at
the end of the chain.
Moreover, in this study, the shape of
peaks according to C-O was slightly changed
which indicated that the degradation of PLA could
be occurred at this point. Our results were
complied with the study of Weng et al. (2013).
Figure 6: SEM micrographs (2,000x) of PLA (A) surface before burying test; (B) after 30; (C) 60; (D)
after 90 days of burying test
Figure 7: TGA curves of PLA

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Table 1: Elements content of %C, %H and %O of PLA before and after degradation
Sampling date
%C
%H
%O
Original PLA
49.46
5.54
45.00
30 days
46.33
5.54
48.13
60 days
46.35
5.54
48.11
90 days
45.42
5.44
49.04
Figure 8: FTIR spectra for PLA before and after degradation
4. Conclusions
In this study, the physical and chemical
changes of PLA sheets during burying test under
simulated landfill conditions were investigated.
Landfill soil and anaerobic sludge was used as
burying medium which have a diversity of
microorganisms. Under setting conditions, the
hydrolytic degradation process was occurred

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resulting in the change of PLA properties. These
conditions enhanced PLA degradation up to 90%
of weight loss in 90 days. The degradation of PLA
was confirmed by visual examination and particle
size analysis. PLA samples were reduced to small
brittle fragments as a function of degradation time.
After the end of burying test, most of remained
fragmented sizes were less than 1 mm. The
occurrence of pores, cracks and irregular
roughness was increased through the buried time.
Moreover, the strength of PLA was continuously
decreased during burying test as seen from lower
decomposition temperature. Meanwhile, the
percentage of carbon content in molecular
structure also decreased during the degradation
test. In addition, the change in chemical properties
of PLA was tested by FTIR. The FTIR graph
presented some peaks and changed in the shape of
peak area.
5. Acknowledgments
This study was supported by the 90th
Anniversary of Chulalongkorn University fund
(Ratchadaphiseksomphot Endowment Fund).
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