Biodegradable polythene film


Biodegradable polythene film

Polyethylene or polythene film is usually stable and resistant to degradation. Methods are available to make it more degradable under certain conditions of sunlight, moisture, oxygen, and composting.

If traditional polyethylene film is littered it can be unsightly, and a hazard to wildlife. Some believe that making plastic shopping bags biodegradable is one way to try to allow the open litter to degrade.

Plastic recycling improves usage of resources. Biodegradable films need to be kept away from the usual recycling stream to prevent contaminating the polymers to be recycled.

If disposed of in a sanitary landfill, most traditional plastics do not readily decompose. The sterile conditions of a sealed landfill also deter degredation of "biodegradable" polymers. Polyethylene is a polymer consisting of long chains of the monomer ethylene (IUPAC name ethene). The recommended scientific name polyethene is systematically derived from the scientific name of the monomer. [1] [2] In certain circumstances it is useful to use a structure–based nomenclature. In such cases IUPAC recommends poly(methylene). [2] The difference is due to the opening up of the monomer's double bond upon polymerisation.

In the polymer industry the name is sometimes shortened to PE in a manner similar to that by which other polymers like polypropylene and polystyrene are shortened to PP and PS respectively. In the United Kingdom the polymer is commonly called polythene, although this is not recognised scientifically.

The ethene molecule (known almost universally by its common name ethylene) C2H4 is CH2=CH2, Two CH2 groups connected by a double bond, thus: Polyethylene is created through polymerization of ethene. It can be produced through radical polymerization, anionic addition polymerization, ion coordination polymerization or cationic addition polymerization. This is because ethene does not have any substituent groups that influence the stability of the propagation head of the polymer. Each of these methods results in a different type of polyethylene.

Types of biodegradable polythene film

Polythene or Polyethylene film cannot biodegrade naturally. There are two methods to resolve this problem. One is to modify the carbon chain of polyethylene to improve its degradability and at some point its biodegradability; the other is to make a film with similar properties to polyethylene from a biodegradable substance such as starch.

tarch based or biobased (hydrodegradable) film

This is made from corn (maize), potatoes or wheat. This form of biodegradable film meets the ASTM standard (American Standard for Testing Materials) and European norm EN13432 for compostability as it degrades at least 90% within 180 days or less under specified conditions.

Examples of polymers with which starch is commonly used

*Polycaprolactone (PCL)
*Polyvinyl alcohol (PVA)
*Polylactic acid (PLA)The heat, moisture and aeration one gets in a compost pile are required for this type of biodegradable film to biodegrade.

Pros & cons of starch based film/bag

Pros

*Litter is biodegradable and compostable. Biodegradable means that, under certain conditions, the material will be degraded into small pieces that can be absorbed by microorganisms and transformed into CO2, H2O, energy and neutral residue.
*Reduced fossil fuel content (depending on loading of filler)
*Faster degradation of litter
*No net increase of carbon dioxide in global ecosystem Fact|date=November 2007

Cons

*Source of starch can be problematic (competition against food use, rainforests being cleared to grow crops for bioplastics)
*Poorer mechanical strength than additive based example – filling a starch bag with wet leaves and placing it curbside can result in the bottom falling out when a hauler picks it up. However, some biodegradable and compostable films are now very close to polyethylene or polypropylene, depending on the starch used.
*Degradation in a sealed landfill takes at least 6 months.
*Limited Shelf life. Some conditions must be respected for stockage.
*Some need to be composted in industrial facilities because the temperature of the compost needs to be at 58°C. Others ( OK-compost) are home composting (temperature 20°C).
*If mixed with other plastics for recycling, the value of recycling is reduced.
*For plastic recycling, resin identification code 7 is applicable.

Typical applications

Carrier bag, refusal sacks, vegetable bags, food films, agricultural films, mailing films. The list of applications is not exhaustive as new developments constantly.

However, these applications are still very limited compare to those of petroleum based plastic films.

Additive based (oxodegradable/photodegradable)

These films are made by blending an additive to provide a UV / oxidative and/or biological mechanism to degrade them. This typically takes 6 months to 2 years in a landfill site. According to people who develop this process, degradation is a two stage process; first the plastic is converted by reaction with oxygen (light, heat and/or stress) to molecular fragments that water can wet, and then these smaller oxidized molecules are biodegraded, i.e. converted into carbon dioxide, water and biomass by microorganisms. "But complete biodegradation, i.e. transformation by micro-organisms in energy, CO2 and H2O, has never been established." Indeed, at the end of degradation, the small particles are still too big to be absorbed and transformed by micro-organisms. This is similar to the break down of woody plant material where lignin is not readily completely broken down and persists as a humus component improving the soil quality (water and nutrient retention / release properties). If put in a compost, the oxo degradables [ [http://www.kaysons.in/articles/biodocs.htm Law suit on Oxo-degradable bags] ] which persist could be considered a form of pollution (cf International Biodegradable Polymers Association and Working groups).

Pros & cons of additive based film/bag

Pros

*Much cheaper than starch-based plastics
*Materials are well known
*Does not compete against food use
*These films look, act and perform just like their non-degradable counterparts, except that they are weaker and break down after being discarded.

Cons

*Made using fossil fuel (contributes to global warming through the release of carbon dioxide)
*Degradation in a sealed landfill is very slow.
*Degradation depends on conditions of heat, light, stress, air etc.
* No biodegradability established, which means it cannot be absorbed by micro-organisms. Pollution is not visible but exists.
*They do not comply with European Norms on compostable products
*If mixed with other plastics for recycling, the value of recycling is reduced.
*For plastic recycling, resin identification code 7 is applicable.
*Proprietary additives often include Cobalt Stearate, which dissociates into toxic cobalt free metal during the degradation process.

Typical applications

Trash Bags, Garbage Bags, Compost Bags, Carrier bag, Agricultural Film, Mulch Film

References

BBC News: "All Tesco bags 'to be degradable dt. 10th May’06" http://news.bbc.co.uk/1/hi/uk/4758419.stm

BBC News: "Degradable carrier bags launched dt. 2nd Sep’02" http://news.bbc.co.uk/1/hi/uk/2229698.stm

ee also

*Biodegradable plastic
*Bioplastic
*Plastic bag
*Plastic recycling
*Packaging
*Photodegradation
* [http://www.kaysons.in/articles/bionorms.htm Biodegradable plastic norms]


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