Biodegradable polythene film

Polyethylene or polythene film biodegrades naturally, albeit over a long period of time. Methods are available to make it more degradable under certain conditions of sunlight, moisture, oxygen, and composting and enhancement of biodegradation by reducing the hydrophobic polymer and increasing hydrophilic properties.[1]

If traditional polyethylene film is littered it can be unsightly, and a hazard to wildlife. Some people 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 degradation 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 will naturally fragment and biodegrade, but it can take many decades to do this.[2] There are two methods to resolve this problem. One is to modify the carbon chain of polyethylene with an additive to improve its degradability and then its biodegradability; the other is to make a film with similar properties to polyethylene from a biodegradable substance such as starch. The latter are however much more expensive.

Starch based or biobased (hydrodegradable) film

This type 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 90 days or less at 140 degrees F. However, actual products made with this type of film may not meet those standards.

Examples of polymers made from starch

The heat, moisture and aeration in an industrial composting plant are required for this type of film to biodegrade, so it will not therefore readily degrade if littered in the environment.

Pros & cons of starch based film/bag

Pros
Cons

Typical applications

Carrier bag, refusal sacks, vegetable bags, food films, agricultural films, mailing films. However, these applications are still very limited compared to those of petroleum based plastic films.

Oxodegradable

These films are made by incorporating an additive within normal polymers to provide an oxidative and then a biological mechanism to degrade them. This typically takes 6 months to 1 year in the environment with adequate exposure to oxygen Degradation is a two-stage process; first the plastic is converted by reaction with oxygen (light, heat and/or stress accelerates the process but is not essential) to hydrophilic low molecular-weight materials and then these smaller oxidized molecules are biodegraded, i.e. converted into carbon dioxide, water and biomass by naturally occurring microorganisms. Commercial competitors and their trade associations allege that the process of biodegradation stops at a certain point, leaving fragments, but they have never established why or at what point. In fact Oxo-biodegradation of polymer material has been studied in depth at the Technical Research Institute of Sweden and the Swedish University of Agricultural Sciences. A peer-reviewed report of the work was published in Vol 96 of the journal of Polymer Degradation & Stability (2011) at page 919-928. It shows 91% biodegradation in a soil environment within 24 months, when tested in accordance with ISO 17556. This is similar to the breakdown of woody plant material where lignin is broken down and forms a humus component improving the soil quality. If put into an in-vessel composting system this type of plastic will rapidly break down, but it is not currently marketed as compostable because the market has been conditioned to expect compliance with ASTM D6400 or EN13432 - although these are not legal requirements.

Enhancing Hydrophilicity of the Polymer

These films are inherently biodegradable over a long period of time. Enhancement of the polymer by adding in additives to change the hydrophobic nature of the resin to slightly hydrophilic allows microorganisms to consume the macromolecules of the product, these products often are confused with oxobiodegradable products, but work in a different way. Enhancing of the hydrophilicity of the polymer allows fungus and bacteria to consume the polymer at a faster rate utilizing the carbon inside the polymer chain for energy. These additives attract certain microorganisms found in nature and many tests have been completed on the mixing of synthetic and biobased materials which are inherently biodegradable for enhancing the biodegradability of synthetic polymers that are not as fast to biodegrade.[3]

Pros & cons of additive based film/bag

Pros
Cons

Typical applications

Trash Bags, Garbage Bags, Compost Bags, Carrier bag, Agricultural Film, Mulch Film, produce bags, - in fact all forms of short-life plastic film packaging

References

  1. ↑ "Biodegradable Plastic enhancing hydrophilic properties.". Biosphere Plastic. Retrieved 2011-06-30.
  2. ↑ "Microbes biodegrade aromatic hydrocarbons". Biosphere Plastic. Retrieved 2011-05-20.
  3. ↑ Biodegradable Polymers. By Jasim Ahmed, Brijesh K. Tiwari, Syed H. Imam. Retrieved 2009-03-10.
  4. ↑ "RECYCLING OF PLASTICS". Oxobiodegradable Plastics Association. Archived from the original on 19 January 2010. Retrieved July 9, 2012.

See also

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