Download 9.Biopolymers

BIOPOLYMERS
Biopolymers are polymers produced by living organisms.
Since they are polymers, biopolymers contain monomeric units that are
covalently bonded to form larger structures.
There are three main classes of biopolymers based on the differing monomeric
units used and the structure of the biopolymer formed:
1.polynucleotides,
nucleotide monomers;
which are long polymers composed of 13 or more
2.polypeptides, which are short polymers of amino acids; and
3.polysaccharides, which are often linear bonded
carbohydrate structures.
Many of the polysaccharides earlier studies
biopolymers since they have repeating units
Cellulose most abundant biopolymer
are
polymeric
also
Synthetic polymers are much simpler and random and molecular
mass.
This fact leads to a molecular mass distribution that is missing
in biopolymers.
All biopolymers of a type (say one specific protein) are all alike:
they all contain the similar sequences and numbers of monomers
and thus all have the same mass.
This phenomenon is called monodispersity in contrast to the
polydispersity encountered in synthetic polymers.
As a result biopolymers have a polydispersity index of 1.
BIOPOLYMERS AS MATERIALS
• Some biopolymers- such as polylactic acid (PLA), naturally
occurring zein, and poly-3-hydroxybutyrate can be used as
plastics, replacing the need for polystyrene or polyethylene based
plastics.
• Some plastics are now referred to as being 'degradable', 'oxydegradable' or 'UV-degradable'. This means that they break
down when exposed to light or air, but these plastics are still primarily
(as much as 98 per cent) oil-based and are not currently certified as
'biodegradable' under certain international laws.
• Biopolymers, however, will break down and some are suitable for
domestic composting.
Zein is a class of prolamine protein found in maize. It is usually manufactured as
a powder from corn gluten meal.
BIOPOLYMER USES
• Biopolymers (also called renewable polymers) are
produced from biomass for use in the packaging industry.
• Biomass comes from crops such as sugar beet, potatoes or
wheat: when used to produce biopolymers, these are classified
as non food crops. These can be converted in the
following pathways:
• Sugar beet > Glyconic acid > Polyglonic acid
• Starch > (fermentation) > Lactic acid > Polylactic acid (PLA)
• Biomass > (fermentation) > Bioethanol > Ethene > Polyethylene
• Many types of packaging can be made from biopolymers: food
trays, blown starch pellets for shipping fragile goods, thin
films for wrapping.
Polylactic acid (PLA)
Poly(lactic acid) or polylactide (PLA) is a
polyester
thermoplastic aliphatic
commonly made from a-hydroxy acids, derived from renewable
resources, such as
• corn starch (in the United States),
• tapioca products (roots, chips or starch mostly in Asia) or
• sugarcanes (in the rest of world).
It can biodegrade under certain conditions, such as the presence of
oxygen, and is difficult to recycle.
PLA is not a polyacid (polyelectrolyte),
but rather a polyester
Polylactic acid (PLA)
• One of the few polymers in which the stereochemical structure can
easily be modified by polymerizing a controlled mixture of L and D
isomers to yield high molecular weight and amorphous or semicrystalline polymers.
• Properties can be both modified through the variation of isomers
(L/D ratio) and the homo and (D, L) copolymers relative contents.
• PLA can be tailored by formulation involving adding plasticizers,
other biopolymers, fillers, etc
Polylactic acid (PLA)
Bacterial fermentation is used to produce lactic acid from corn starch or cane
sugar.
Two lactic acid molecules undergo a single esterfication and then
catalytically cyclized to make a cyclic lactide ester.
PLA of high molecular weight is produced from the dilactate ester by ringopening polymerization.
Polymerization of a racemic mixture of L- and D-lactides usually leads to the
synthesis of poly-DL-lactide (PDLLA) which is amorphous.
Stannous octonate
Or tin(II) chloride
Catalytic and thermolytic ring-opening polymerization of lactide (left) to polylactide (right)
poly-DL-lactide (PDLLA)
PDLA (poly-D-lactide):
Due to the chiral nature of lactic acid, several
distinct forms of polylactide exist: poly-Llactide (PLLA) is the product resulting from
polymerization of L,L-lactide (also known as Llactide). heat resistant PLA can withstand
temperatures of 110C (230F)
optically transparent.
PLA has similar mechanical properties to PETE polymer, but has a significantly
lower maximum continuous use temperature.
PETE: Polyethylene terephthalate, commonly abbreviated PET, PETE, or the obsolete PETP
or PET-P, is a thermoplastic polymer resin of the polyester family and is used in synthetic
fibers; beverage, food and other liquid containers
Polylactic acid (PLA): Biodegradability
• PLA is considered both as biodegradable (e.g. adapted for short-term
packaging) and as biocompatible in contact with living tissues (e.g. for
biomedical applications such as implants, sutures, drug encapsulation,
etc.).
• PLA can be degraded by abiotic degradation (i.e. simple hydrolysis of
the ester bond without requiring the presence of enzymes to catalyze
it). During the biodegradation process, and only in a second step, the
enzymes degrade the residual oligomers till final mineralization (biotic
degradation).
• As long as the basic monomers (lactic acid) are produced from
renewable resources (carbohydrates) by fermentation, PLA complies
with the rising worldwide concept of sustainable development and is
classified as an environmentally friendly material.
APPLICATIONS
• Woven shirts (ironability), microwavable trays, hot-fill
applications and even engineering plastics (in this case, the
stereocomplex is blended with a rubber-like polymer such as
ABS).
• PLA is currently used in a number of biomedical applications,
such as sutures, stents, dialysis media and drug
delivery devices. The total degradation time of PLA is a few
years. It is also being evaluated as a material for tissue
engineering.
• Because it is biodegradable, it can also be employed in the
preparation of bioplastic, useful for producing loose-fill
packaging, compost bags, food packaging, and disposable
tableware. In the form of fibers and non-woven textiles, PLA
also has many potential uses, for example as upholstery,
disposable garments, awnings, feminine hygiene products, and
diapers.
• PLA is a sustainable alternative
to petrochemical-derived
products, since the lactides from which it is ultimately
produced can be derived from the fermentation of agricultural
by-products such as corn starch or other carbohydrate-rich
substances like maize, sugar or wheat.
• PLA can be an alternative to high-impact polystyrene by using
as much as 1 wt% non-PLA due to creating co-polymers which
can strengthen PLA plastic.
PLA is more expensive than many petroleum-derived commodity
plastics, but its price has been falling as production increases.
The demand for corn is growing, both due to the use of corn for
bioethanol and for corn-dependent commodities, including PLA.
Plastics are resistant to biodegradation accumulating at the rate of 25million
tonnes per year. Much disposed in landfill sites. Possibility of recycling plastics is
limited and incineration yields toxic compounds
As of Jun 2010, NatureWorks was the primary producer of PLA (bioplastic) in
the United States.
The Korean research center KAIST has announced that they have found a way
to produce PLA using bio-engineered Escherichia coli.
Due to PLA's relatively low glass
transition temperature, PLA cups
cannot hold hot liquids. However,
much research is devoted to
developing a heat resistant PLA
Biodegradable cups at a restaurant
Mulch film made of polylactic
acid (PLA)-blend bio-flex
Degradable plastics can be biodegradable or photodegradable
Photodegradable plastics can break down to small fragments and lose
structure but small fragments are not degradable.
Biodegradable plastics can be metabolized by MO
Semidegradable plastics contain starch, cellulose and polyethene
For complete degradation 50% mix is required which compromises
structural properties
Biodegradable plastics
Polyhydroxy alkanoates (PHAs): PHB
Polyactides
Aliphatic polyesters
Polysaccharides
Blends of above
Bioplastics from Microorganisms
Degradable polymers that are naturally degraded by the action of
microorganisms such as bacteria, fungi and algae
Benefits
•
100 % biodegradable
•
Produced from natural,
renewable resources
•
Able to be recycled, composted
or burned without producing
toxic byproducts
Several legislations enacted but demand for bioplastics
have not increased
IMPORTANCE
• 2003- North America
– 107 billion pounds of
synthetic plastics
produced from petroleum
– Take >50 years to degrade
– Improper disposal and
failure to recycle 
overflowing landfills
Carbon Cycle of Bioplastics
Photosynthesis
Plants
CO2
H2O
Biodegradation
Recycle
Carbohydrates
Fermentation
Plastic Products
PHA Polymer
Polyhydroxyalkanoates (PHAs)
• Polyesters accumulated inside microbial cells
as carbon & energy source storage
Ojumu et al., 2004
Polyhydroxyalkanoates (PHAs)
• Produced under conditions of:
– Low limiting nutrients (P, S, N, O)
– Excess carbon
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2 different types:
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Short-chain-length
Medium-chain-length
3-5 Carbons
6-14 Carbons
~250 different bacteria have been found to
produce some form of PHAs
Polyhydroxybutyrate (PHB)
• Example of short-chain-length
PHA
• Produced in activated sludge
• Found in Alcaligenes eutrophus
• Accumulated intracellularly as
granules (>80% cell dry weight)
Lee et al., 1996
PHA Biosynthesis
Ojumu et al., 2004
PHB: polyhydroxybutyrate
Intracellular microbial plastic first found in Bacillus megaterium
80 different types of PHAs formed from 3-hydroxyalkanoate acid monomers
3-14 carbons in length
Energy store when nutrient is limited
Alcaligenes eutrophus (Ralstnia etropha) to produce PHB
Polymer had low thermal stability and brittle
Addition of propionate to culture produced P (3HB-co-3HV)
and polymer was flexible and tough
Marketed as BIOPOLTM used to make films, coated paper, compost bags,
disposable foodwares , bottles, razors
COST is still HIGHER than chemically synthesized polymers
Propylene: 1$/kg
PHVB: 3-5$/kg
HB: hydroxybutyrate HV: hydroxyvalerate
phbC-A-B Operon in A. eutrophus
• Structural genes encoded in single operon
– PHA synthase
– b-ketothiolase
– NADPH-dependent acetoacetyl-CoA reductase
Lee et al., 1996
Recovery of PHAs from Cells
• PHA producing microorganisms stained with
Sudan black or Nile blue
• Cells separated out by centrifugation or
filtration
• PHA is recovered using solvents (chloroform)
to break cell wall & extract polymer
• Purification of polymer
Bioplastic Properties
• Some are stiff and brittle
– Crystalline structure  rigidity
• Some are rubbery and moldable
• Properties may be manipulated by blending polymers or
genetic modifications
• Degrades at 185°C
• Moisture resistant, water insoluble, optically pure,
impermeable to oxygen
• Must maintain stability during manufacture and use but
degrade rapidly when disposed of or recycled
Biodegradation
• Fastest in anaerobic sewage and slowest in seawater
• Depends on temperature, light, moisture, exposed surface
area, pH and microbial activity
• Degrading microbes colonize polymer surface & secrete PHA
depolymerases
• PHA  CO2 + H2O (aerobically)
• PHA  CO2 + H2O + CH4 (anaerobically)
Biodegradation by
PHA depolymerases
Conclusions
• Need for bioplastic optimization:
– Economically feasible to produce
– Cost appealing to consumers
– Give our landfills a break
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How many of you would be willing to pay 2-3 times
more for plastic products because they were
“environmentally friendly”?