BIOPOLYMERS

Many of the polysaccharides earlier studies are alsobiopolymers since they have repeating units

Cellulose most abundant biopolymer

Biopolymers are polymers produced by living organisms.

Since they are polymers, biopolymers contain monomeric units that arecovalently bonded to form larger structures.

There are three main classes of biopolymers based on the differing monomericunits used and the structure of the biopolymer formed:

1.polynucleotides, which are long polymers composed of 13 or morenucleotide monomers;

2.polypeptides, which are short polymers of amino acids; and

3.polysaccharides, which are often linear bonded polymericcarbohydrate structures.

Synthetic polymers are much simpler and random and molecularmass.This fact leads to a molecular mass distribution that is missingin biopolymers.

All biopolymers of a type (say one specific protein) are all alike:they all contain the similar sequences and numbers of monomersand 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), naturallyoccurring zein, and poly-3-hydroxybutyrate can be used asplastics, replacing the need for polystyrene or polyethylene basedplastics.

• Some plastics are now referred to as being 'degradable', 'oxy-degradable' or 'UV-degradable'. This means that they breakdown 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 fordomestic composting.

Zein is a class of prolamine protein found in maize. It is usually manufactured as a powder from corn gluten meal.

• Biopolymers (also called renewable polymers) areproduced from biomass for use in the packaging industry.

• Biomass comes from crops such as sugar beet, potatoes orwheat: when used to produce biopolymers, these are classifiedas non food crops. These can be converted in thefollowing 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: foodtrays, blown starch pellets for shipping fragile goods, thinfilms for wrapping.

BIOPOLYMER USES

Polylactic acid (PLA)

Poly(lactic acid) or polylactide (PLA) is a thermoplastic aliphaticpolyester commonly made from a-hydroxy acids, derived from renewableresources, 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 ofoxygen, and is difficult to recycle.

PLA is not a polyacid (polyelectrolyte), but rather a polyester

• One of the few polymers in which the stereochemical structure caneasily be modified by polymerizing a controlled mixture of L and Disomers to yield high molecular weight and amorphous or semi-crystalline 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)

Polylactic acid (PLA)

Bacterial fermentation is used to produce lactic acid from corn starch or cane sugar.

Catalytic and thermolytic ring-opening polymerization of lactide (left) to polylactide (right)

Stannous octonate

Or tin(II) chloride

Two lactic acid molecules undergo a single esterfication and thencatalytically cyclized to make a cyclic lactide ester.

PLA of high molecular weight is produced from the dilactate ester by ring-opening polymerization.

Polymerization of a racemic mixture of L- and D-lactides usually leads to thesynthesis of poly-DL-lactide (PDLLA) which is amorphous.

poly-DL-lactide (PDLLA)

Due to the chiral nature of lactic acid, severaldistinct forms of polylactide exist: poly-L-lactide (PLLA) is the product resulting frompolymerization of L,L-lactide (also known as L-lactide). heat resistant PLA can withstandtemperatures of 110C (230F)

PDLA (poly-D-lactide): 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 PETPor PET-P, is a thermoplastic polymer resin of the polyester family and is used in syntheticfibers; beverage, food and other liquid containers

• PLA is considered both as biodegradable (e.g. adapted for short-termpackaging) and as biocompatible in contact with living tissues (e.g. forbiomedical applications such as implants, sutures, drug encapsulation,etc.).

• PLA can be degraded by abiotic degradation (i.e. simple hydrolysis ofthe ester bond without requiring the presence of enzymes to catalyzeit). During the biodegradation process, and only in a second step, theenzymes degrade the residual oligomers till final mineralization (bioticdegradation).

• As long as the basic monomers (lactic acid) are produced fromrenewable resources (carbohydrates) by fermentation, PLA complieswith the rising worldwide concept of sustainable development and isclassified as an environmentally friendly material.

Polylactic acid (PLA): Biodegradability

• Woven shirts (ironability), microwavable trays, hot-fillapplications and even engineering plastics (in this case, thestereocomplex is blended with a rubber-like polymer such asABS).

APPLICATIONS

• PLA is currently used in a number of biomedical applications,such as sutures, stents, dialysis media and drugdelivery devices. The total degradation time of PLA is a fewyears. It is also being evaluated as a material for tissueengineering.

• Because it is biodegradable, it can also be employed in the

preparation of bioplastic, useful for producing loose-fillpackaging, compost bags, food packaging, and disposabletableware. In the form of fibers and non-woven textiles, PLAalso has many potential uses, for example as upholstery,disposable garments, awnings, feminine hygiene products, anddiapers.

PLA is more expensive than many petroleum-derived commodityplastics, but its price has been falling as production increases.

The demand for corn is growing, both due to the use of corn forbioethanol and for corn-dependent commodities, including PLA.

• PLA is a sustainable alternative to petrochemical-derivedproducts, since the lactides from which it is ultimatelyproduced can be derived from the fermentation of agriculturalby-products such as corn starch or other carbohydrate-richsubstances like maize, sugar or wheat.

• PLA can be an alternative to high-impact polystyrene by usingas much as 1 wt% non-PLA due to creating co-polymers whichcan strengthen PLA plastic.

The Korean research center KAIST has announced that they have found a way to produce PLA using bio-engineered Escherichia coli.

As of Jun 2010, NatureWorks was the primary producer of PLA (bioplastic) inthe United States.

Plastics are resistant to biodegradation accumulating at the rate of 25milliontonnes per year. Much disposed in landfill sites. Possibility of recycling plastics islimited and incineration yields toxic compounds

Due to PLA's relatively low glasstransition temperature, PLA cupscannot hold hot liquids. However,much research is devoted todeveloping a heat resistant PLA

Mulch film made of polylacticacid (PLA)-blend bio-flex

Biodegradable cups at a restaurant

Degradable plastics can be biodegradable or photodegradablePhotodegradable plastics can break down to small fragments and losestructure but small fragments are not degradable.Biodegradable plastics can be metabolized by MO

Semidegradable plastics contain starch, cellulose and polyetheneFor complete degradation 50% mix is required which compromises structural properties

Biodegradable plasticsPolyhydroxy alkanoates (PHAs): PHBPolyactidesAliphatic polyestersPolysaccharidesBlends of above

Bioplastics from Microorganisms

Degradable polymers that are naturally degraded by the action ofmicroorganisms such as bacteria, fungi and algae

Several legislations enacted but demand for bioplastics have not increased

Benefits

• 100 % biodegradable

• Produced from natural, renewable resources

• Able to be recycled, composted or burned without producing toxic byproducts

• 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

IMPORTANCE

Carbon Cycle of Bioplastics

CO2

H2OBiodegradation

CarbohydratesPlastic Products

Plants

Fermentation PHA Polymer

Photosynthesis

Recycle

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

� 2 different types:� Short-chain-length 3-5 Carbons

� Medium-chain-length 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 megaterium80 different types of PHAs formed from 3-hydroxyalkanoate acid monomers3-14 carbons in length

Energy store when nutrient is limited

Alcaligenes eutrophus (Ralstnia etropha) to produce PHB

Polymer had low thermal stability and brittleAddition of propionate to culture produced P (3HB-co-3HV)and polymer was flexible and tough

HB: hydroxybutyrate HV: hydroxyvalerate

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$/kgPHVB: 3-5$/kg

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 withSudan black or Nile blue

• Cells separated out by centrifugation orfiltration

• 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

� How many of you would be willing to pay 2-3 timesmore for plastic products because they were“environmentally friendly”?