Green Plastics: Biosynthesis and Biodegradation of Polyesters

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Green PlasticsBiosynthesis and Biodegradation
of Polyesters
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Petroleum-derived plastics

• 270 million tons of oil and gas converted into
  plastics each year
• Polymer molecules too large and too tightly bound
  together for biodegradation
• Disposal options
• Landfill
• Incineration
• Recycling

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Aliphatic polyester a green plastic?

• What is PHA?
• Properties similar to those of polyethylene and
  polypropylene
• Processed on conventional equipment
• Biodegradable but often lack good thermal and
  mechanical properties

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PHA Production

• Bacteria accumulate insoluble energy reserves in
  cytoplasm
• Up to 90 of a single cell's mass
• Type depends on nutrient imbalance (in the
  presence of surplus carbon) and genotype

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Who makes PHA?

• aerobic and anaerobic heterotrophic bacteria (Liu
  et al 2002)
• aerobic and anaerobic phototrophs (Rothermich et
  al 2000)
• archaebacteria (Rehm 2003)
• transgenic plants (slower growing) (Glazer 1995)

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Under What Conditions?

• Nutrient limitation, e.g. nitrogen
• Wide range of substrata (Glucose, Acetate,
  Glutamate, Pyruvate, Methane, Methanol, Mannitol,
  Carbon dioxide, etc.)
• Toxic compounds (e.g. phenol, sodium benzoate)
• Agriculture Waste
• Dairy Waste
• Food waste
• Multiple substrata produce multiple different
  PHAs (Kocer 2003)

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Biodegradation of PHAs

• More dense than petroleum-derived counterparts PE
  and PP
• Aerobic (Glazer 1995)
• Anaerobic (Federle et al 2002)

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Scanning electron micrographs of PTMS films
(2000x magnification) (A) without innoculation
(B) after cultivation with M. rosea subsp. aerata
IFO 14046 at 50 degrees C for 14 days (scale bar
10 microns)
Source Jarerat and Tokiwa, 2001
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Innovation Timeline

• PHB discovered in 1926 by Lemoigne
• Patents filed 1962
• Biodegraders discovered - 1963
• Industrial production 1982

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Future Development

• Production
• PHAs costs 2 to 10 times more than petro-plastics
  and require a significant amount of energy to
  produce
• Transgenic plant large scale production possible
  in near future
• Disposal
• Landfilling may result in slow degradation due to
  absence of moisture and oxygen
• Rapid anaerobic degradation observed in municipal
  sludge
• Degradation rate of PHA can be controlled by
  altering its monomer compositions