In this study, an alternative composting method for biodegradable PLA was proposed, capable of reducing the molecular weight by 80% in 90 minutes. The poster was presented at the GENIUS Olympiad, an international high school environmental conference, at SUNY Oswego.

1. Effect of UV Treatment on Degradation of Biodegradable PLA
By Catherine Zhang, Shrewsbury High School, Shrewsbury, MA, USA
Introduction:
Environmental Issues:
Though many biodegradable plastics have recently
entered the market in aims of reducing the growing
landfill problem, only a minority of these plastics
ends up in an industrial composting facility and
fulfills its primary purpose of degrading quickly.
Hence, there is a need to develop a new alternative
composting method to accelerate degradation of
biodegradable polymers. Polylactic acid (PLA) was
selected for this study due to its potential in
packaging, textiling, and medical applications to
replace non-bio and non-biodegradable plastics,
such as polyethylene.
Polymer Degradation:
Packaging containers made up 31% of total solid waste (after recycling) in 2005, and this percentage is
increasing annually (Dell, 2010).
Increasing need for a more efficient as well as economically viable method of plastic waste treatment
Increasing
plastic waste
in landfills
PLA Synthesis and Properties:
Synthesis
Created in a 2-step process
1) Production of lactic acid: Extracting sugar from corn, which is then fermented by microorganisms
2) Production of PLA from lactic acid by direct condensation of the lactic acid or ring-opening
polymerization of cyclic lactide dimer.
PLA is both bio-based (made from feedstock), as well as biodegradable (Shen et al., 2009, p. 60).
Develop an
alternative
composting
method to
accelerate
degradation
Though PLA
is both bio-based
and biodegradable,
there are
challenges in
expanding its
usage
Goals:
The aims of this study were: 1) to evaluate the
1. UV Treatment
effectiveness of UV treatment on the degradation of 2. Mechanical
PLA; 2) to examine the influence of mechanical
Chopping
chopping on the degradation of PLA consequently to
propose an alternative composting method to
accelerate the degradation of PLA.
Advantages
PLA can be shaped into transparent films, fibers, bottles, and containers. Its properties can be
improved by fillers or layering it with silicate nano-composites (Pandey et al., 2005). PLA will degrade
in a industrial compositing facility by hydrolysis method and its by-products are non-toxic.
Disadvantages
At temperatures above Tg, it loses its stiffness significantly. PLA is 20% more expensive compared
with traditional plastics, and there could be a potential shortage of feedstock (Groot et al., 2010)
Applications:
PLA is also used in many food packaging applications such as in cups, bottles, food bags, etc.
Coca-Cola®
used in clothing applications
When a polymer degrades, it becomes brittle, limiting its lifespan
Photodegradation: Process in which UV light oxidizes polymeric structure, causing
mechanical and molecular breakage into small pieces (Brenndorfer, n.d.)
UVC Light has an energy per photon of 4.43 to 12.4 eV
Exposure to UV light causes the breakage of bonds in polymers leading to photoPolymers can be
oxidation
UVA: 315-400 nm
degraded through
UVB: 280-315 nm
photodegradation
UVC: 100-280 nm
and hydrolysis
Hydrolysis: Chemical process in which a water molecule is added to a polymer resulting in the
break down of that polymer
GPC (Gel Permeation Chromatography)
An analytic technique that measures relative molecular weight, consisting of passing a dilute
polymer solution through a column filled with polymeric gel beads
During GPC, a sample of the solution with the PLA and without it is injected into the
chromatograph (column)
Difference in molecular weight results measured in a difference in the time it takes the polymer
to pass through the column (Beaucage, 2005)
GPC can be used
to measure relative
molecular weights
of polymers
PLA has
been used in
multiple
packaging
applications
Experimental Design:
During experimentation, the PLA bag was cut into ten 6
cm by 6 cm squares. Half of the PLA cut films was
chopped into small pieces (mechanical degradation)
and half was left whole. The PLA (chopped and
unchopped) was treated for 30, 60, and 90 minutes in
the UV Chamber at the UMass Food Science
Department. At 30 minute intervals, the PLA was taken
out to be massed and observed visually. At the end of
the total treatment time, the treated PLA was sent to the
UMass Polymer Science Department for GPC testing.
Treated
Unchopped and
Chopped PLA for
30, 60, 90
minutes: Massed
samples and sent
for GPC
Qualitative Results and Discussion:
Increased
Darkening and brittleness of the PLA were observed
discoloration
on both unchopped and chopped PLA after UV
and brittleness
treatment. There is increased discoloration over
over time: Sign
time. This phenomenon signifies further oxidation
of oxidation
and degradation.
and
Chopped PLA pieces were stacked in a small pile,
degradation
which may lead to non-uniform UV exposure.
Conclusion:
Quantitative Results and Discussion:
Mass Loss
Mass loss of both unchopped and chopped PLA increases linearly with increasing the UV
exposure time
Greater mass loss in unchopped PLA film than chopped PLA pieces at same UV treating
conditions
As treatment
time increased,
mass
decreased, and
unchopped lost
more mass than
chopped
GPC (Gel Permeation Chromatography)
From Highest Molecular Weight To Least (less degradation to more):
a) Unchopped: 0 min, 30 min, 90 min, 60 min
b) Chopped: 0 min, 30 min, 60 min, 90 min
c) 30 Minute UV Treated: Unchopped, Chopped
d) 90 Minute UV Treated: Chopped, Unchopped
UV treatment reduced the molecular weight significantly even after only 30 minutes UV
exposure time (decreased about 80%)
Further UV treatment can further reduce the molecular weight
Chopping only accelerated PLA degradation at 30 minutes
Proposed Alternative Composting Process Flow Chart:
As treatment
time increased,
molecular
weight
decreased,
inconsistent
comparison of
chopped and
unchopped
Unchopped PLA at Different Stages of UV Treatment:
0 Minutes
30 Minutes
60 Minutes
UVC light can rapidly degrade PLA due to the correlation between
increased treatment time and decreased molecular weight. However,
the results of mechanical degradation (chopping) were inconclusive.
Increased discoloration and brittleness seen over time,
UVC light can
signifying oxidation and degradation
significantly
The mass decreased as treatment time increased.
and rapidly
The unchopped film lost twice as much mass as the chopped
degrade PLA,
pieces.
results of
Molecular weight decreased as treatment time increased with
chopping vs.
the exception of the 60 minute unchopped
unchopping
Results of unchopped vs. chopped are inconclusive due to:
are
Only 3 to 5 mg of the PLA were taken from UV treated
inconclusive
samples for GPC
Location of the PLA could have been altered some of
the chopped PLA used for GPC received more direct
UV treatment than others (such as: on the top of pile
vs. the bottom) due to the shielding effect of the pile
UV light can introduce different groups with oxygen (carboxylic acid)
in them into the PLA, leading to a drop in pH
UV light can degrade PLA to an extent where it becomes water soluble
Mass Loss of PLA Over Time in UV Chamber
Combine all steps of experiment to create a process to accelerate
PLA degradation
PLA Waste
90 Minutes
Composting
a)
b)
c)
Mechanical
Chopping
UV Treatment
Conveyor Belt
Oven
(60 min)
Water Soaking
Bath
Proposed
Alternative
Process:
Combination of
Chopping, UV
Treatment,
Water Bath
d)
Chopped PLA at Different Stages of UV Treatment:
0 Minutes
30 Minutes
60 Minutes
90 Minutes
a)
b)
c)
d)
References:
a)
b)
c)
d)
Additional Experimental Results:
Acknowledgements:
I would like to thank Professor Julie Goddard, Professor
Shaw Ling Hsu, Sahas Rathi, and Fang Tian for their
guidance and suggestions throughout the study. I would
especially like to thank Sahas Rathi for conducting the GPC
testing, as well as Professor Julie Goddard for allowing me
to use her lab for my study. I would also like to thank Allen
King from NatureWorks for donating the PLA samples used
in this study. I would also like to thank Ms. Constantine and
Mr. Collins for their guidance throughout the study.
Hypothesis:
If the UV treated PLA is soaked in water, it will result in further mass reduction and the pH of water solution will drop (more acidic) because the water
solution will dissolve low molecular weight oligomers.
Soaking in water
Independent Variables:
leads to additional
15 min ultrasonic soaking time in 25 mL de-ionized water of 0.2648 g 60 minute treated chopped PLA
mass loss of 31.6%
Dependent Variables:
and decrease in pH
pH values of de-ionized water solution and masses of the UV treated PLA pieces before and after 15 min ultrasonic water soaking.
Results:
Mass of 60 Minute UV Treated Chopped PLA: 0.2648 g, Mass Loss: 0.0066 g
pH of Untreated Chopped PLA in Water: 7.71
Mass of 60 Minute Treated Chopped PLA After 15 Min. Soaking: 0.1812 g, Mass Loss: 0.0836 g
pH of 60 Minute Treated Chopped PLA in Water: 3.87
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