Biodegradation of Plastics
Plastic? Synthetic polymers can be classified as plastics, elastomers and synthetic fibers. Plastics is the general term for a wide range of synthetic or semi synthetic polymerization products. They are composed of organic condensation or addition polymers and may contain other substances to improve performance or reduce costs.
Plumbing, textiles, paint, floor coverings at home are mostly made of synthetic polymers, some of which are described as plastics. The term ‘polymer’ are derived from Greek words meaning “many parts”. Polymers are macromolecules built up from linked smaller molecules called monomers.
Plastics Benefits of using plastics: Low
cost Strength (physically strong): lightweight and do not break like glass Durability Printability
Plastic Uses PLASTIC
USES
Polyethylene
Packaging, electrical insulation
Polyvinyl chloride (PVC)
Credit cards, floor coverings, rainwear
Polyvinyl acetate (PVA)
Latex paints
Polybutadine (BUNA) rubbers
Car tires, hoses
Poly (methyl methacrylate)
Transport equipment
Some common synthetic polymers chemical structures
The Problem Public concern about the long term effects of plastics technology. Barry Commoner in late 1960s suggested that the earth surface would be buried ten feet deep in plastic waste because there were no biological organisms capable of destroying it.
Plastics are the most common man-made object sighted at sea. During a 1998 survey, 89% of the trash observed floating in the North Pacific Ocean was plastic. Plastic sheeting has been documented in the stomachs of sperm whales, round -toothed dolphins and a Curvier beaked whale. Many sea turtles frequently swallow plastic bags when they mistake them for jelly fish, which is one of their favorite foods.
The lightweight nature of plastics which can make them deadly in the marine environment, as plastic items can float on the surface of the ocean or within the water column. Plastics are composed of long chains of hydrocarbons, which are hydrogen and carbon atoms that are bound together very tightly. Microorganisms such as plankton do not have the ability to break down these bonds and therefore plastic does not decompose easily.
Biodegradation (Biotic degradation) Chemical degradation from the action of naturally occurring microorganism such as bacteria, fungi and algae.
Biodegradable Plastic A degradable plastic in which the degradation results from the action of naturally occurring microorganisms such as bacterial, fungi, and algae.
Biodegradation of Plastics Biodegradation of polymers involves breakdown by microorganisms or higher organisms, using enzyme-catalyzed pathways. Biological systems tend to degrade natural polymers e.g. starch by hydrolysis then oxidation.
Aliphatic e.g. nonaromatic polyesters are among most biodegradable. Example, the degradable and absorbable sutures used in surgical procedures are often made of poly (glycolic acid). Thermoplastic materials made from polycaprolactone have been shown to almost completely degrade on burial in soil for 12 months. Microorganisms used the polymer as a growth substance and converting it into microbial biomass, CO2 and H2O.
Polyethylene, polypropylene, polyvinyl chloride and polystyrene are most widely used plastics. These polymers are very bioresistant because it involves only carbon atoms in their main chain, with no hydrolyzable functional group. These compounds are degraded by microorganisms such as fungi up until the alkane molecular weight reaches 500. For larger alkanes, biodegradation becomes effectively zero. Chain branching inhibits biodegradation (a phenomenon observed with surfactants).
Influence of chain length and extent of branching of alkanes on biodegradation by fungi mixture MOLECULAR WEIGHT
RELATIVE BIODEGRADABILITY (0=poor, 4=good)
Hexadecane (C16H34)
226
4
Octadecane (C18H38)
255
4
Hexatriacontane (C36H74)
507
0
Tetracontane (C40H82)
563
0
2,6,11-Trimethyldocane (C15H32)a
212
0
COMPOUND
a
Branched chain
Materials with low molecular weights with a higher proportion of chain 535 carbons and less branching would be susceptible to biodegradation. Most biodegradable plastics are technically unsuitable for uses such as packaging and thus cannot be readily substituted for the less biodegradable substances.
Solution? Biodegradation capacity can be improved by: Mixing with natural biodegradable substances such as starch and proteins e.g. conversion of cellulose into cellulose nitrate and cellulose acetate;
Cellulose nitrate has been used for movie films and old films using this material are often in degraded state
Solution?
Incorporating hydrolyzable functional groups such as esters, amides and urethanes; Ensuring that the main chain is flexible and thus able to facilitate binding to active sites of enzyme.
Compostable Plastic A plastic that undergoes degradation by biological processes during composting to yield CO2, H2O, inorganic compounds and biomass at a rate consistent with other known compostable materials (cellulose) and leaves no visually distinguishable or toxic residues.
Composting is an effort of developing biodegradable plastics for waste management and agricultural application.
