Polylactic Acid (PLA)

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• Polylactic Acid (PLA)

BSE 4514 Industrial Processing
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Fermentation
Lactic acid is made from fermentation of sugar
with the sugar source usually being starch
because it is cheap and high volume.
acetic acid
CH4 through methanogenic organisms
EtOH
Can have multiple products from a biorefinery
that ferments starch! Changing organisms can
yield H2, butyric acid, acetone, etc.
pyruvic acid
lactic acid
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Process
C6H12O6
2C3H6O3
Lactic acid pathway
Lactobacillus X (Xamylophylus for starch)
bacteria ferments Some Saccharomyces b/c lactic
acid is found in EtOH Fermentation for lactic
acid is an anaerobic (no O2) process used by
cells to obtain energy also known as
glycolysis. 1. Organism splits sugar ring and
removes e- (dehydrogenateremove 2 protons and
2e- or oxidize) 2. e- transfer to organic
molecule (pyruvic acid) 3. Waste product is
lactic acid and excreted from cell
(rehydrogenate)
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Lactic Acid to Lactide Dimer
-2H2O
lactide is a stable form of lactic acid
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Polylactic Acid
Commercial products of PLA like Cargill
NatureWorks usually are formed from the ring
opening polymerization of lactide with
coordination catalysts (i.e., metallic catalysts)
like stannous (tin II) chloride SnCl2, which is
approved for food use. These catalysts are often
used, cheap, and easy.
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PLA Polymerization

• Lactide 150oCSnCl2 forms PLA oligomers (i.e.,
  DP8 or 8 lactic acids linked together)
• Oligomer _at_ 180oC under vacuum (1333 Pa) for 5
  hours PLA of Mw100,000 g/mol
• Solid state above Tc105oC, 66 Pa, 0.5-2 hrs
  followed by annealing at 150oC for 10-30 hours

PLA Mw600,000 and Mw/Mn2 obtained
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Other Reactions

• Lots of research into other catalysts (this is
  the case for many chemical reactions)
• Anionic polymerization w/ alkoxides (aluminum
  triisopropoxide) to give PLA Mw100,000 g/mol
  with Mw/Mn1.4.
• Direct condensation of lactic acid with high
  boiling point solvent and ring opening
  polymerization to give Mw300,000 g/mol
• Tg60oC but Tm(direct)163oC and stable but
  Tm(ring open)178oC and unstable. Instability
  arose from catalyst impurity.

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PLA Properties
Lactobacillus is a very efficient organism and
there is high conversion of glucose to lactic
acid (this has been known for a long time) The
organism forms the L-lactic acid conformation
this is the biologically important
conformation There are researchers trying to
find organisms that can run at higher
temperatures for faster conversions and organisms
that can make the L and D lactic acid
conformations, because producing copolymers of L
and D will yield very different properties than
just having pure L or D.
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PLA Properties
Tg60oC Tm175oC (45-70 xtal dep. on process)
Td190oC
Therefore, brittle at room temperature and will
depolymerize to lactide at high T (usually
transesterification back-biting)
From J. Rheol. 43, 1141-1155, 1999 and Handbook
of Biodegradable Polymers, ed. C. Bastioli,
Rapra, 2005
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PLA Properties
Add D
Lower Tg (still above room T) and Tm, which is
good b/c it is further from Td
Tg100oC

?0.95 g/cm3
From Handbook of Biodegradable Polymers, ed. C.
Bastioli, Rapra, 2005
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PLA Properties
Lyocell and Viscose are regenerated cellulose
or cellulose spun from solution
From Natureworks, LLC
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PLA Industry

• Glossy, clear, good ? and E favor packaging
• 300 million lbs/yr produced in Nebraska by
  Cargill (0.3 of total US plastics market)
  NatureWorks
• Cargill doubled customer base last 2 years (not
  sales, but customers)
• 15,000 grocery stores carry a product packaged in
  PLA
• 18,000 retail stores carry a clothing item made
  with PLA fiber

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An aliphatic polyester
Made with a combination of biotechnology
(fermentation) and chemical technology
(polymerization)!
Polyethylene terephthalate (PETE) is an aromatic
polyester
which is what your soda bottle and polyester
fibers are made of!
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PLA Applications

• Packaging just like PETE but much more
  expensive petroleum prices and larger volume
  production are making PLA more attractive but
  corn prices increasing are eating into this
• Flexible films plasticize (usually with
  oligomeric PLA) but plasticizer migration limits
  apps just like we talked about
• Fibers this is a good app! Can handle price
  because polyester is used in high performance,
  high cost athletic wear like polar fleece
• Biomedical hydrolytic degradation in body just
  like in environment lots of literature b/c off
  the shelf degradation product is lactic acid,
  which can be metabolized by body, BUT lactic
  acid concentration near degradation site changes
  pH and affects biochemistry there

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PLA Degradation

• Two step process hydrolysis (abiotic) and
  enzymatic biodegradation (biotic, i.e., CO2 and
  H2O are produced)
• Under dry conditions, PLA can last decades
• Add H2O (hydrolysis)

?pH can accelerate rxn lactic acid is then
metabolized
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PLA Hydrolysis
This is an amorphous polymer and the lactic acid
oligomer xtallizes!
0 7 14 21 28 35
Neutral conditions (how do they maintain?), body
temperature (above room T)
There is some indirect evidence in the literature
that the entire PLA molecule can be biodegraded
without hydrolysis in the presence of esterase
enzymes but it appears that the two step process
is the most prevalent as these esterases may not
be ubiquitous.
From Handbook of Biodegradable Polymers, ed. C.
Bastioli, Rapra, 2005