Biodegradation of PLA

1. Seminar
Topic: Environmental degradation of PLA
Submitted by: Sub group 3
Group Members
Ayesha Kanwal (16-Arid-2541)
Sabahat Ali (16-Arid-2569)
Rimsha Tariq (16-Arid-2567)
Tanzeela Noureen
Submitted to: Dr Sadia Mehmood

2. Environmental Degradation of PLA
by Biotic and Abiotic factors.

3. Contents
• Introduction of PLA
• Biodegradation
• Environmental degradation
of PLA
• Biotic degradation
• Bacterial degradation of PLA
• Fungal degradation
• Enzymatic degradation
• Degradation by Abiotic
factors
• Hydrolysis
• Role of pH and UV light
• Degradation by electron
beam irradiation
• Compositing conditions
• Critical analysis

4. POLYLACTIC ACID
Poly(lactic acid) (PLA) is a compostable bioplastic polymer
manufactured by the polymerization of lactic acid monomers
derived from fermentation of starch.
Currently it is estimated that worldwide production will reach
at least 800,000 tons by 2020 with Japan and the USA the two
major producers.
Plastic litter represents a serious environmental problem. This
situation appears to be particularly alarming in developing
countries lacking fully efficient waste management systems.

5. Biodegradation
• Degradable plastics are polymers that undergo changes in
their chemical structure in specific environmental conditions
causing significant loss in physical and mechanical properties
as defined by the American Society for Testing of Materials
(ASTM) and the International Standards Organization (ISO).
• Biodegradable plastics are susceptible to degradation by
enzymes produced by microorganisms including bacteria &
fungi
• During the biodegradation process, biodegradable polymers
are broken down to their simpler constituent components and
redistributed through the carbon and nitrogen cycles and end
products are carbon dioxide, water, biomass and hydrocarbons.

6. Environmental degradation of PLA
 In nature, biotic and abiotic factors exist together as
environmental degradation of PLA. The environmental degradation
process of PLA is affected by its material properties such as:
 Molecular 1st order structure (molecular weight, optical purity)
 Higher order structures (crystallinity, Tg and Tm),
 Environmental factors such as( humidity, temperature ,pH and the
presence of enzymes or microorganisms)Tsuji,H.( 2010)
 During its environmental degradation, ester bonds of PLA must be
cleaved either hydrolytically or by extracellular enzymes to enable
the PLA monomers or oligomers to be assimilated.(Satti et al,. 2017)

7. Contd:
• When the molecular weight is low, PLA is brittle, cloudy and
opaque while at higher molecular weights, PLA is stronger,
more transparent and less susceptible to degradation.(Kale et
al,. 2007)
• Crystalline regions within PLA hydrolyze much more slowly
than the amorphous regions as water diffuses more readily into
the amorphous regions compared to the crystalline regions,
causing greater rates of hydrolysis and biodegradation.
• In semi crystalline PLA, degradation occurs first in the
amorphous regions and more slowly in the crystalline regions.
• Therefore, with time, the proportion of the crystalline regions
within the PLA increases and the rate of degradation
decrease.(Henton et al.,2005,Hoglund et al., 2012)

8. Contd:
• PLA is more difficult to degrade than other biodegradable
plastics. PLA-degrading microorganisms in the environment
are not widely distributed as compared to microorganisms
associated with other types of biodegradable plastics.
• The high molecular weight of PLA retards microbial
degradation activity as compared with low molecular weight
PLA; therefore, high molecular weight PLA benefits from an
initial chemical hydrolysis to reduce its molecular weight prior
to microbial mineralization.

9. Contd:
• The rate of PLA degradation is much greater above the glass
transition temperature (Tg, 55-62 ºC) as polymer chains
become more flexible and water absorption increases,
accelerating both hydrolysis and microbial attachment
.Temperatures at or above Tg (55-62°C) and at high relative
humidity’s (>60%), PLA hydrolysis is rapid.

10. Biotic degradation
• When the degradation rates of PLA in a biotic reactor
containing compost were compared with an abiotic reactor
lacking compost and a sterile aqueous system, similar
degradation rates were observed and it was concluded that
there was no evidence for the enhanced degradation of PLA
by microorganisms (Agarwal et al., 1998).

11. Contd:
• Isolation of PLA degraders especially by cultivation methods
have been challenging, for instance, one study aimed to
identify soil bacteria that were able to degrade commercially
available aliphatic polyesters, including PLA, and when
microorganisms were extracted from 3 different soil samples
and screened on agar 33 plates containing different polyester
materials, no PLA degrading organisms were found although
some strains were identified.
• Specialized strains of microorganisms play important role in
degradation of high MW PLA.

12. Bacterial degradation of PLA:
• The first identified bacterium was an actinomycete,
Amycolatopsis HT-32, but was isolated from only 1 out of 45
soil samples. Some actinomycetes efficiently degraded
polyesters including PLA (Hoang et al,. 2007)
• Subsequently, further studies have isolated a number of
different actinomycete and some other bacterial PLA
degraders.
• Enumeration and isolation of Polylactic acid (PLA)-degrading
bacteria from soils and wastewater sludge were performed on
emulsified PLA agar. Two isolates of potent PLA-degrading
bacteria, designated as CH1 and WS3, were selected and

13. Contd:
• identified as S. pavanii and Pseudomonas geniculata,
respectively. PLA was presented as a substrate to stimulate
production of protease and PLA-degrading enzyme by S.
pavanii CH1 and P. geniculata WS3.
• P. geniculata WS3 had a higher percentage of PLA film-
weight loss than that of S. pavanii CH1, corresponding with
reduced molecular weight of PLA.
• P. geniculata WS3, a novel isolated bacterium, has played a
substantial role in PLA biodegradation by producing PLA
degrading enzyme and adhering on PLA surface.

14. Contd:
• The numbers of total viable bacteria were higher than those of
PLA-degrading bacteria because some bacteria cannot use PLA as
sole carbon source for support their growth.
• Number of PLA-degrading bacteria in agricultural soils was lower
than that in samples collected from sanitary landfill sites and
wastewater sludges.
• Both enzymatic and chemical hydrolysis participate in PLA
degradation .(Husarova et al., 2014,: Qi et ai., 2017) Low molecular
weight lactic acid oligomers and monomers can be assimilated by
bacterial cells to serve as a nutrient substrate for their growth.

15. Contd:
• PLA biodegradation is regarded to first occur on the PLA
surface and then spread inside the PLA material to make it
totally degraded.

16. Fungal degradation:
• Tritirachium album was the first fungus reported to be capable
of PLA degradation.(Jarerat and Tokiwa, 2001)
• Saadi et al. (2012) studied fungal degradation of high
molecular weight PLA by inoculating fungi into sterilized
compost and comparing the degradation rate with unsterile
compost.
• This study indicated a synergy between bacteria and fungi in
PLA degradation as the degradation rate was higher in
unsterile compost (Saadi et al, 2012).

17. Contd:
• Phanerochaete chrysosporium white rot fungus was tested for its
ability to biodegrade poly(lactic acid)- based materials .
• Biochemical parameters (superoxide dismutase, catalase,
malondialdehyde) of the fungus were monitored in respect with
soluble protein for 14 days of inoculation.
• The specific activities of both enzymes (catalase and superoxide
dismutase) increased and reached a maximum after 14 days of
incubation.
• The fungus attached itself to polymer film surfaces, continued
growing, and slowly degraded them. The degradation was monitored
by infrared spectroscopy (FTIR), gel permeation chromatography
(GPC), scanning electron microscopy (SEM), and atomic force
microscopy (AFM).

18. Contd:
• Notable structural modifications appear by fungal
biodegradation of PLA showing beginning of the
hydrolytic/fungal action process of degradation. Fungal
biodegradation increases the crystallinity (FTIR determined),
indicating that the degradation occurs mainly in amorphous
regions.
• A significant decrease of the average molecular weight of the
PLA-based materials is noticed after fungus action.
• Surface physical degradation observed by SEM and AFM was
highlighted by an increase of the roughness of the polymeric
surfaces.

19. Enzymatic degradation:
• Enzymes involved in PLA:
• Microbial enzymes potentially involved in the degradation of
PLA.
• Ebeling et al., (1974) identified a serine protease, proteinase
K, from the filamentous fungus Tritirachium album and
Williams (1981) identified proteinase K from T.album as an
enzyme capable of degrading PLA.
• However, PLA degradation by T. album ATCC 22563 only
occurred in the presence of gelatin which induced protease
production. No degradation was observed in the absence of
gelatin.
• whereas in the presence of 0.1% gelatin, 76% of the high
molecular weight PLA film was degraded by the fungus .

20. Contd:
The presence of gelatin in the culture medium has been
reported to enhance microbial degradation of high molecular
weight PLA films which are not usually attacked by
microorganisms.
• Three enzymes lipases,estrase,and alcalases are known to
hydrolyze PLA fibers effectively.
• Alcalases was more efficient than lipases and estrases.
• Crystalline regions are highly resistant to protease degradation
compared to amorphous regions.

21. Degradation by CLE
• An enzyme referred as cutinase-like enzyme (CLE) purified
from the yeast Cryptococcus sp. completely degraded high
molecular weight PLA and at a faster rate than proteinase K
(Masaki et al., 2005).
• It has also been reported that Amycolatopsis orientalis ssp.
orientalis produced three serine-like proteases capable of
degrading PLA as a sole carbon source .
• Another serine protease isolated from an actinomycete,
Actinomadura, was also reported to be capable of degrading
PLA.

22. Degradation by Abiotic factors
• PLA degradation is generally accepted to be a two-step
mechanism involving first abiotic factors then biotic
factors.
• The abiotic process, which is the chemical hydrolysis of
PLA in the presence of water at elevated temperatures, is
followed by biotic degradation in which microorganisms
decompose polymer break-down products generating
carbon dioxide, water and biomass hydrocarbons
conditions (Kale et al., 2007; Sangwan and Wu, 2008;
Copinet et al., 2009; Saadi et al., 2012).

23. Contd:
• However degradation mechanisms of PLA and understanding
the association and role of microorganisms during the
environmental degradation are still poorly understood
(Sangwan and Wu, 2008).
• The most commonly suggested mechanism is that
microorganisms can degrade PLA only after high molecular
weight PLA goes under hydrolysis and the molecular weight
of PLA falls 10 000 Da or less (Copinet et al., 2009; Saadi et
al., 2012)

24. Hydrolysis
• Abiotic hydrolysis is the main degradation step as high
humidity and temperature enables the cleavage of the
ester linkages by water uptake causing reduction in
molecular weight; with microorganisms assimilating acid
lactic oligomers, 32 releasing carbon dioxide and (Copinet
et al., 2009; Saadi et al., 2012)
• Abiotic hydrolysis is critically influenced by temperature ,
where its rates differ considerably in the range 20 ◦C to 60
◦C. Crystallinity with the preference of amorphous regions
seems to play a role as well.
• The conditions, especially during the thermophilic phase of
the composting process, appear favorable in spite of the
presence of degrading microorganisms and the promotion
of hydrolysis under elevated temperature.

25. Contd:
• It has been suggested that abiotic hydrolysis represents a first
step in PLA biodegradation.
• Nevertheless, experimental data have indicated highly rapid
and efficient decomposition of PLA by some enzymes as well
as the presence of hydrolytic enzymes produced by PLA-
utilizing microorganisms.

26. Role of pH & UV light
• PLA degradation by pH and UV light. PLA degrades faster in
alkaline conditions because during hydrolysis, cleavage of
ester groups is catalyzed by hydroxide ions, therefore, the high
concentration of hydroxide ions in alkaline media enhances
PLA degradation . (Cam et al., 1995; Tsuji and Ikada, 1998).
• UV light exposure was also found to affect PLA degradation .
When PLA films were exposed to UV light for 8 weeks under
a low relative humidity (10%, which restricts the rate of PLA
hydrolysis), UV light decreased the physical integrity and
enhanced PLA degradation . (Ho and Pometto, 1999).

27. Degradation by Electron Beam
Irradiation
• In addition, electron beam irradiation has also been shown to
effect PLA integrity.
• Pre-treating PLA by electron beam irradiation increased PLA
brittleness and decreased molecular weight during compost
degradation compared to non-irradiated samples .
• In another study, treatment of films by electron-beam
irradiation decreased the average molecular weight, stress at
break and percentage elongation . (Vargas et al., 2009).

28. Contd:
• Cairns et al., also showed that depending on the beam energy
used, physical properties, molecular weight were affected
increasing the degradation rate of PLA.
• PLA degradation is generally accepted to be a two-step
mechanism involving first abiotic factors then biotic factors
Cairns et al.,(2011)

29. Composting condition
• PLA can be degraded in a composting environment after 45–
60 days at 50– 60°C by microorganisms in the compost .
• Compost is a humic, organic-rich, biological environment
where the environmental degradation of organic matter occurs
• When biodegradation of PLA film and fabrics was studied in
composting system, PLA fabrics were degraded after 40 days
and PLA sheets after 20 days
• . Commercially available PLA bottles and PLA delicatessen
containers degraded visibly in 30 days under composting
conditions with PLA bottles having a lower degradation rate
due to a higher degree of crystallinity.

30. Contd:
• PLA degradation in soil is much slower than compost medium
because compost has a higher moisture content and
temperature range encouraging PLA hydrolysis and
assimilation of PLA by thermophilic microorganisms .
• A 20 month PLA soil burial trial caused 20% and 75%
degradation of PLA100 (crystalline PLA) and PLA75
(amorphous PLA), respectively.

31. Critical Analysis of PLA
biodegradation
 Some studies have reported that microorganisms do not
enhance PLA degradation other studies have suggested that
microbial enzymes exist that are capable of directly degrading
high molecular weight PLA.
 Degradation mechanisms of PLA and understanding the
association and role of microorganisms during the
environmental degradation are still poorly understood. The
most commonly suggested mechanism is that microorganisms
can degrade PLA only after high molecular weight PLA goes
under hydrolysis and the molecular weight of PLA falls 10000
Da or less.

32. Critical Analysis of PLA
biodegradation
• Environmental factors do not only influence polymer to be
degraded they also have crucial influence on the microbial
population and on the activity on different microorganisms
themselves.
• Parameters such as humidity, absence and presence of oxygen,
and supply of different nutrients have important effect on
degradation of polymer and so these conditions must be
considered when the biodegradability is tested.
• PLA degradation has been found to be dependent on range of
factors such as molecular weight, temperature, pH, purity.
Analysis shows that PLA biodegradation is specifically related to
the presence of microorganisms and environmental conditions

33. Contd:
Breakthroughs in this field is are still very difficult to achieve
before the following crucial problems are solved…
 Analysis of enzymatic families involved in PLA degradation
 Exploration of high efficient degradation methods based on
PLA degrading microorganisms is crucial for accelerating PLA
degrading process.
 The simulated system based on aerobic biodegradation of
PLA is currently limited due to lack of available information
on process parameters.
How to construct more robust system for PLA biodegradation
brings new challenge to us

34. References
Sangwan, P., & Wu, D. Y. (2008). New insights into polylactide
biodegradation from molecular ecological techniques. Macromolecular
Bioscience, 8(4), 304–31
Saadi Z, Rasmont A, Cesar G, Bewa H, Benguigui L. Fungal degradation
of poly(l-lactide) in soil and in compost. J Polym Environ 2012;20:273e82.
Agarwal M, Koelling KW, Chalmers JJ. Characterization of the
degradation of polylactic acid polymer in a solid substrate environment.
Biotechnol Prog 1998;14:517e26.
Jarerat, A., & Tokiwa, Y. (2001). Degradation of poly(l-lactide) by a
fungus. Macromolecular Bioscience, 1(4), 136–140.