Compostable bags, bio polyethylene or degradable polymers – what’s the difference?

Polyethylene has been a mainstay of British industry for many years. We use it to package, and wrap, and also to manufacture the bags we use to dispose of products when they are no longer needed.

In recent years, the focus has been on greening the supply chain – making sure that bags and plastic wrap are made from the most sustainable materials possible, and preventing used carrier bags from littering the environment. However, there is a great deal of confusion around the different options – from compostable bags, to bio polyethylene and degradable polymers.

Bio polyethylene
Polyethylene is one of our most common polymers, used in a wide range of applications, from carrier bags and wrapping for food, to refuse sacks and other film applications.

The building block for all types of polyethylene is ethylene. Standard polyethylene comes from fossil-based sources, while bio polyethylene is manufactured from renewable sources, such as sugar cane. Once the raw materials are polymerized, the resulting products are the same, so bio polyethylene can be blended with standard polyethylene, and holds exactly the same properties.

Since it is identical to standard polyethylene, bio-based polymers are 100 per cent recyclable with conventional waste streams. This gives them a huge advantage, and means they can go back into the loop many times; each time reducing the carbon footprint required for manufacture.

The appeal of bio polyethylene comes from the use of renewable materials, which results in a lower carbon footprint during the manufacturing process. However, the inclusion of the term ‘bio’ can be deceptive, since bio polyethylene is not biodegradable or compostable. Like any polymer, it will degrade over time, but this might take as long as 50 years.

There is currently only one supplier for bio polyethylene, which is based in South America. With no competitive markets, the product has been expensive, and transportation from South America also has an impact on the carbon footprint.

South America has undergone significant deforestation, and bio polyethylene has faced criticism for utilizing land that could have been used for growing food. Although the capacity of the producer remains static, additional land still needs to be cultivated to replace cropped canes.

Although the term ‘compostable’ might suggest that bags come from plant-based materials such as eucalyptus, sunflower plants, or sugar beet, compostable bags can be made from either renewable sources or petrochemicals. Typically, they are manufactured from fossil-based polymers.

At the end of its life, a compostable bag will degrade, leaving no visually distinguishable or toxic residues. This does not mean that it will entirely disappear – biological processes will result in carbon dioxide, water, and inorganic compounds – but the remnants will be too small for the eye to see and will blend in with the soil.

Like bio polyethylene, compostable bags made from renewable sources are grown in land that could have been used to produce food. However, unlike bio polyethylene, the polymers are largely manufactured in Europe and the Middle East, so transportation issues are reduced. Renewable and standard polyethylene versions are both made in the same regions, so the transportation footprint is relatively equal, even though compostable materials have a lower melting temperature, and therefore need less energy, to convert them into a useful product.

Compostable bags come in two types – those suitable for home composting, and those designed for large commercial systems. Most bags are certified under an EU harmonised standard for compostable and biodegradable packaging - EN 13432. Bags are certified by an independent organisation, which tests their rate of degradation and assesses the raw materials used. Those suitable for domestic use degrade at ambient temperatures and come with an ‘OK Compost’ mark. Those intended for local authority collections and industrial composting feature the ‘Compost’ mark.

Degradable polymers are those that can be broken down by micro-organisms into water, carbon dioxide and other remnants, which may create toxic end products. These are frequently confused with compostable products, but their properties differ significantly.

Unlike compostable bags, degradable ones usually include an additive, which is sensitive to heat and light. This chemical, which makes the product ‘oxy biodegradable’, can leave a toxic residue.

Degradable bags are not recyclable, and can be extremely detrimental if they find their way into a standard polyethylene recycling system. For example, a damp course or other construction membrane made from recycled polyethylene will be deployed underground and may be expected to last 50 years. The degradable polymer will have a much shorter life span, which can seriously impact the effectiveness of the product if the two types of polymer are unintentionally mixed.

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