Poly(ethene)

Disposal of plastic items can be a problem. Poly(ethene is very resilient and may remain intact for well over 100 years. This is much longer than the time between manufacture and disposal, which may be no more than a few months.

Although recycling is feasible for plastics, in packaging applications where poly(ethene) is commonly used the material cannot easily be collected, cleaned and recycled, and so ends up in landfill. This includes items like supermarket produce bags and refuse sacks. Even carrier bags, though potentially reusable, are still eventually thrown away. The situation has become so serious that polythene carrier bags are now outlawed in some countries.

Photo: poly(ethene) litter

In these applications, there is a strong case for degradable polymers. To find out why poly(ethene) is not easily degraded, click here.

Biodegradable Poly(ethene)

Poly(ethene) (PE) eventually becomes brittle in sunlight as some depolymerisation takes place. When buried, however, this cannot happen, and so in landfill the plastic will persist for many years. This is because the microbes in the soil are unable to process the long polymer chains. Sealed PE refuse sacks will also reduce significantly the rate at which their contents degrade when put in landfill tips.

There are several fully degradable polymers that could theoretically replace PE, but the volume of PE produced is so high that it would require massive investment to change to these to produce such volumes. Several companies have instead developed degradable PE.

Degradable Solutions

Six-Pack rings
Starch
d₂w

Six-Packs

Drinks in aluminium cans are often sold as a multi-buy pack. In some areas the plastic structure used to hold the cans together has become a litter problem, and there have been concerns over the effect on wildlife - birds and hedgehogs can become trapped in the rings.

Many are now made from a degradable poly(ethene), which has light-sensitive carbonyl groups inserted into the polymer chain. On exposure to light the chains will begin to disintegrate, leaving short sections that can now be "processed" by microbes.

diagram: chemical structure

Starch Poly(ethene)

100% biodegradable starch-based polymers are in use, but not in the same applications as poly(ethene) (PE) as they have very different properties. Combining PE with up to 20% starch results in a degradable polymer with properties similar to PE. When disposed of by composting, or in landfill, the starch is readily degraded by microbes, leaving smaller fragments of PE that can also be broken down.

Methods for incorporating starch into the polymer structure have been developed that produce a plastic that is as strong as LDPE but uses less petrochemical feedstock, is cheaper (starch costs less than PE), and will degrade when buried.

d₂w

A UK company, Symphony Environmental Ltd, has developed an additive that turns poly(ethene) into a fully biodegradable plastic. With additive levels of no more than 3%, the processing properties of the polymer remain the same. The plastic can be designed to break down in anything between 60 days and 6 years, with the process being started by sunlight, heat or even the stress from tearing.

photo: degradable refuse bag

How it works

The additive catalyses the breaking of carbon-carbon bonds in the poly(ethene), and once favourable conditions exist, the reaction will begin and continue until the long polymer chains are short enough to be digested by bacteria. The final products are carbon dioxide and water, plus a small amount of residual biomass.

This biodegradable poly(ethene), marketed under the d₂w trademark, is already being used for refuse sacks, carrier bags, freezer bags and other consumer products.

image: the d2w logo

For more information about d₂w, see the Symphony Environmental Ltd web site.