Clever solutions
We know that plastic bags are bad for the environment - but what alternatives are being
offered as clever solutions?
Degradable, biodegradable and compostable bags
Some time when you’re out shopping you might be handed a bag marked ‘biodegradable’. But what this means for the environment can depend on what the bag is made of. There are biodegradable bags coming in and out of the market all the time. And not all of them are as good as we’d like to hope.
Biodegradable bags have radically different life spans and effects on the environment. Plain old polyethylene supermarket bags are biodegradable – they just take up to 1000 years to fully biodegrade. Just because a bag is biodegradable doesn’t mean you can throw it away – it may cause just as many environmental problems as an ordinary plastic bag.
The story of biodegradable bags is by no means simple. Currently, even what it means for plastic to be ‘biodegradable’ is under debate. Biodegradable simply means something that breaks down through the actions of microorganisms. But this can take many years. In landfill, where materials decay without the presence of oxygen, this process can take even longer.
Degradable bags
Degradable materials includes biodegradable materials, but also materials that break down into smaller pieces in the presence of heat (thermoplastics) and light (photodegradable).
Photodegradable and thermoplastics break down into smaller and smaller pieces of plastic, which eventually biodegrade. They are made of modified polyethylene with various additives including some heavy metals.
Degradable plastic bags can reduce the degrading time of polyethylene to 18 months by weakening the bonds between plastic polymers.
Biodegradable bags
There are biodegradable bags available including cornstarch bags, starch-polymer blends, and microbial and other polyesters.
Biodegradable bags have many benefits over plastic bags. Some of them can be good for compost – they increase nutrients, and water retention capabilities. They may increase the rate organics degrade in landfill, decreasing the amount of methane, a potent greenhouse gas. Most biodegradable bags require less energy in manufacturing than polyethylene bags, again reducing greenhouse gases emissions.
But there can be a down side to the use of biodegradable bags. Most of them cost more to make. While some biodegradable bags break down into simple molecules such as carbon dioxide and water, others break down into smaller and smaller pieces of plastic. Some biodegradable bags can persist in marine environments for up to six months, and if swallowed will still harm or kill wildlife, including small baby reef turtles.
There are other problems with biodegradable bags. Plastics recyclers are concerned about contamination of the recycling stream, and reject recycling streams that are even remotely likely to contain biodegradable plastic bags. Another problem is that biodegradable bags don’t break down in landfill, because landfills are ‘deadzones’ when it comes to microorganisms.
There are basically two main types of biodegradable bags, made of either starch-based polymers or polyesters.
Biodegradable starch-based polymers
Starch bags can contain from 10 to 90% starch blended with high performance polymers. How quickly they break down depends on how much starch is present. If it’s more than 60% starch, it breaks down into sugars quite quickly, and it’s also water soluble. (Examples are Plantic products and thermoplastic starch.)
If the plastic contains less than 60% starch, the bag will break down first into plastic polymers and then eventually biodegrade fully. (Examples are PCL polymers, 45% starch, starch-polymer blends with added plasticiser, Bioflex, Biobag)
Bags made of 100% cornstarch, with vegetable oil added for elasticity, break down in ten days to a month in soil, compost or water. Even if they are eaten during this time by marine animals they pass harmlessly through the digestive system.
Biodegradable polyesters
There are a variety of biodegradable plastics made from polyesters, including polyhydroxyalkanoate (PHA), which is produced from genetically engineering microbes grown in a sugar-based medium (such as molasses!). The plastics polymer accumulates in the microbes cell during growth. PHA can be expensive and lack mechanical strength, but degrades completely in compost in 10 weeks.
Scientists have even taken genes from the microbes and stitched them into corn plants, which then manufacture the plastic in their own cells.
Compostable bags
Biodegradable bags that are compostable break down within 12 weeks into pieces able to fit through a sieve with two millimetre holes. They eventually decompose into carbon dioxide, water and biomass.
Bags marked compostable are the only bags that have passed a standard test to break down with a specified time span. But the bags need to be treated like any other compost – turned over regularly and kept well-aerated. When they are well composted, these bags add yummy nutrients to the soil to help worms, bacteria and other helpful organisms grow.
Not all biodegradable bags are compostable, so it’s worth checking before you bury your shopping bags in the backyard compost – some of them could still be there next year.
No matter what your biodegradable bag of choice, there are still many issues to consider. While biodegradable bags might stick around less than plastic bags, they have a range of life spans and some may still get out into the environment. It’s important to remember that they don’t make the plastic problem just disappear.
|
clever solutions 
