1. What are the environmental benefits of using degradable bags?
Normal plastic degrades very, very slowly. As it was invented less than 100 years ago, we still don’t know how long it might take to totally break down: possibly hundreds of years. That means that standard plastic bags take up space in precious landfill sites. Bio-degradation reduces the enormous waste volume from 100% down to 15% to 20% as the plastic is converted into CO2, water and biomass.
2. What are degradable carrier bags made of?
Normal plastic bags are made from HDPE (high-density polyethylene), LDPE (low-density polyethylene), colouring and chalk. To make them degradable, all we have done is include trace amounts of a special additive. This additive causes the bags to oxidise, meaning that the structure of the plastic is attacked by oxygen causing it to disintegrate more rapidly (i.e. it acts as a catalyst, speeding up a process that happens extremely slowly naturally).Chalk (or calcium carbonate) is added as a modifier to the polyethylene during the film producing process in order to minimise the use of plastic, speed up the production process and improve the physical strength of the bag.
3. When does degradation begin?
The additive has a built-in time release, which has been set to activate approximately 12 months after the bag is made. This allows plenty of time for the bags to be shipped to stores, given away at tills and used by the customer – possibly more than once – before the degradation process begins.The amount of time it takes for the degradable bag to disappear depends on how it is disposed of: it can take as little as 6 weeks but will typically take a couple of years. Estimates suggest that a standard, non-degradable plastic bag could take hundreds of years to disintegrate – if ever.
4. How does the additive work?
The system operates by thermo-oxidation, which means that once the additive is activated, the presence of oxygen causes the bag to disintegrate. Heat, sunlight and stress (i.e. movement) trigger and accelerate this process. If all four components are present – for instance a bag stuck in a tree being buffeted by the wind during summer – it can degrade in as little as 6 weeks. However, as long as there is oxygen, “thermal” degradation can occur at any temperature (even at less than 0*C, although it may take several years).
5. And if there isn’t any oxygen?
The bag will not degrade. This is a big advantage over direct biodegradable materials that, in the absence of oxygen, begin anaerobic decomposition, the product of which is methane – a greenhouse gas 20 times more damaging than CO2.
6. What exactly happens – and what is left – when the bag degrades?
The system we are using is a two step process: initially there is a chemical process where oxygen attacks the carbon chains in the HDPE, turning long chains into smaller and smaller chains which now incorporate oxygen in their chemical make-up. At this point, the plastic has been turned into organic functional groups (ketones, carboxylic acids, alcohols etc) which attract water. These transformations (smaller chains and the presence of oxygen and water) create the conditions for a potential second stage: microbial digestion. If micro organisms are present (as they will be in compost or landfill) then these small fragments will be included in their tropic chain, as food. These fragments therefore decompose further into simple elements: carbon dioxide, water and biomass (biomass is just the organic waste of microbe cells). This process is similar to the degradation of lignin in wood.
7. Are they, therefore, biodegradable?
If the conditions are right and microbes are present – such as in active landfill or in maintained compost – then, yes, the carbon chains do indeed biodegrade.
8. You say the process releases CO2 but, as it is a greenhouse gas, isn't it bad for the environment?
CO2 is unavoidably created in all natural breakdown processes. The alternative is to keep the carbon locked up as plastic but that means the bag lasts hundreds of years - potentially longer. CO2 is an integral part of the photosynthetic process in plants, a by-product of which is O2 or oxygen.
9. So what happens in a typical landfill site?
Landfills vary – if they are well maintained and active (compost plants are being turned over) then the bags will rapidly disappear. Estimates suggest that carrier bags take up as much as 1% of landfill space so by making ours degradable we are freeing up a considerable amount of space. Carrier bags are often recycled in the home as bin liners so a secondary benefit is that any contents of a sealed plastic bag will also be released to degrade.
10. I heard the degradable additive is a heavy metal – doesn’t that mean it pollutes the environment?
The active substances in the additive are metal salts that are not classified as heavy metals.The metals used are essential parts of the minerals used in living systems (like our own bodies).
11. Can you put them in domestic compost heaps?
We would not advise this, simply because home compost does not usually create enough heat to cause the breakdown to occur quickly. While it will eventually disintegrate, the lower temperature means it could take several years.
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