About Biodegradability

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compostable plastic bag
Despite what it says, this bag is actually still made from a thermoplastic. It was melted and formed on a conventional processing line, just like any other plastic bag. Because "plastic" has a poor reputation at the moment, there is a growing trend to rebadge these materials as "polymers" instead.

What Are Biodegradable Plastics?

A p-lastic material is one that can be easily deformed into a new shape - like clay or wax (or chocolate!). By contrast, a material that springs back to its orginal shape is called e-lastic.

In common parlance, the "plastics" we use every day are more correctly termed "thermoplastics", which, as the name suggests, means they have to be heated or melted before they can be moulded into the desired shape.

The most common of these thermoplastics (such as polyethylene, polypropylene, polystyrene, etc.) are essentially just very heavy-duty waxes. Whereas candle-wax and beeswax are composed of relatively short hydrocarbon molecules (about 30 atoms long), artificial thermoplastics use very long molecules, typically about 100x longer.

Are These Plastics also Polymers?

waxy plant cutin

Cutin is the naturally occurring polyester that coats leaves and fruit. This barrier layer consists of a web-like 3D network of cutin filaments, into which lower molecular weight waxes are packed.

Yes. These long "macromolecules" are formed by stitching together shorter hydrocarbon fragments, a process termed "polymerisation". If the resulting polymer molecules are made long enough, they have the tendency to get tangled up, and this makes the resulting bulk material much stronger. (Chemically, candle-wax and polythene are exactly the same molecule, the only difference is the length: The wax is very weak, because, like short hair, short molecules don't get tangled.)

Nature also makes extensive use of polymers, common examples being proteins, starches and some polyesters (such as cutin and shellac).

These materials ("biopolymers") have been around for millions of years, and during that time, microbes have evolved enzymes to reverse the process, chopping them back up into smaller building blocks. (Which are then consumed as food.) This mechanism is what makes natural polymers biodegradable in the environment.

With the exception of the PHAs and shellac, very few biopolymers can be used as thermoplastics, because they tend to decompose rather than melt. So over the last century many synthetic alternatives have been developed. In principle, these materials can also be depolymerised into food, but because they have not been around long enough, microbes have not yet evolved the necessary enzymes. Consequently, most industrial plastics don't naturally rot.

However, some synthetic thermoplastics are affected by common existing natural enzymes, and therefore can break down in the environment. These are known as biodegradable plastics, and with a couple of notable exceptions, they are mostly polyesters such as poly lactic acid (PLA), polycaprolactone (PCL), poly butylene adipate terephthalate (PBAT) and poly butylene succinate (PBS).

So What's a Bioplastic?

This is a fairly generic term that refers to a melt-processable material that has undergone or will undergo some form of biological intervention. It could be using a biologically derived ingredient, and/or it could be suitable for a biologically assisted disposal method. Note that the one does not imply the other, and bioplastics are not necessarily biodegradable.

On the ground, wood does not biodegrade in under 1 year

Although undeniably "biodegradable", pieces of wood can linger in the environment for many years.

What Does "Biodegradable" Actually Mean?

The exact meaning can vary between countries, but a robust and widely accepted definition was established by the US FTC green guides in 1998. These influential guidelines stipulate that biodegradable products should "completely break down and return to nature within a reasonably short period of time after customary disposal." This is typically taken as 1 year.more

In practice, only very thin plastic films & sheets are likely to meet this requirement, even under favourable disposal conditions. Nominally this would be via composting, although the requirements also apply to inadvertent loss into the natural enviroment; Recycling, incineration and landfill are not regarded as suitable environments for biodegradation.

Because thicker and larger items can take much longer than 1 year to decompose, they cannot be simply advertised as "biodegradable" according to the guides. (This plain-vanilla catch-all term is reserved for only the most unquestionable cases.) When considering larger plastic products, the term "biodegradable" must be qualified with additional details to explain how long the product may realistically take to degrade, and in what environments.

Obtaining this information requires testing the actual finished product: Although material testing is useful to establish an item's intrinsic biodegradability, it cannot accurately determine how long it will take in practice. As noted in ISO 17556, the time required for full decomposition depends greatly on the size of the item.more

Ideally, the exact composition of the product should also be made available, since there is considerable scientific research available concerning most biodegradable polymers. The FTC guidelines specifically state that any unqualified claims must be supported by such evidence.more

In practice (especially outside the US), these guidelines are not rigorously adhered to, and many products are promoted as "biodegradable" with little or no supporting evidence. Such claims cannot be substantiated without additional testing.

PHB microbial ot

A close up image of PHB decomposing. This material is produced naturally by many bacterial species, and has been available industrially since the 1980s. It degrades very rapidly even at cold temperatures. (Picture shown after 2 years ambient soil burial, 1750µm width.)

How Effective Are Biodegradable Plastics?

They can be quite good, but the key is choosing the best material for the intended disposal environment.

For example, PLA needs heat to decompose. It is very durable in the (much cooler) natural environment, and is therefore mostly suited to horticultural applications where the waste-stream already goes to a (hot) industrial composting facility.

By contrast, the PHAs are naturally produced by bacteria and consequently will degrade very quickly even in cold, ambient conditions. However, these materials are difficult to work with and are only suitable for niche applications.

Other commonly used biodegradable plastics generally lie somewhere between these two extremes. They degrade best in hot compost heaps, but most will also slowly break down in soils at ambient temperatures, though it can take a few years depending on the specifics. A few are known to degrade slowly in the oceans, but none will rapidly biodegrade in a typical landfill, which is why the FTC green guides discount them. (Landfills are not quite "sealed tombs", but they can be quite dry and anaerobic environments; Even food waste can take over 20 years to fully decompose.)

polycaprolactone microbial rot

A piece of polycaprolactone (PCL) showing extensive microbial penetration and biodegradation. Large portions of the material have been eroded away leaving a rounded, pitted appearance. Note the complete lack of sharp edges. (Picture shown after 2 years ambient soil burial, 1750µm width.)

For outdoor applications, PCL and PBAT are reasonably strong & flexible materials, and for rigid or film applications, cellulose acetate makes a good choice. Studies have shown these plastics can be intrinsically biodegradable, but end users should ideally evaluate their anticipated disposal conditions with test samples to confirm suitability.

For dry (mainly indoor) applications, "PVA" (properly PVOH) is an interesting alternative. Essentially it's an ingredient in common glue, and is used extensively in the paper and cardboard industries (mostly as a sizing agent); Large quantities are therefore routinely recycled through existing waste-streams.

Certain types of PVOH can dissolve quickly (12-24hrs) in cold water, and once dispersed, these specific versions have been shown to biodegrade in most environments.

Amongst a multitude of other applications, PVOH is also used in the food industry as a thickener (E1203) and medically for the coating of tablets. However, it is unsuitable for damp or wet conditions, and this somewhat limits its use for making solid plastic items.

Fragmentation of a slow biodegradable compound

This biodegradable material was custom made for us, and although it is clearly disintegrating, it is not showing typical signs of decay. Note how the edges of the cracks are sharp and well defined. (Compare this with the PCL and PHB images above.)

Sharp edges would normally be the first portions consumed by microbes. Their presence suggests this material contains PLA and is therefore only suitable for disposal via hot composting. (Picture shown after 2 years ambient soil burial, 1750µm width.)

Do These Materials Produce Microplastics As They Break Down?

No. The European Chemicals Agency (ECHA) defines microplastic as "insoluble and nonbiodegradable solid particles measuring less than 5mm". Water soluble and biodegradable materials therefore cannot produce microplastics.

These materials will, of course, disintegrate as they decompose, and smaller fragments will therefore be released. This is not a problem if the material is intrinsically biodegradable in the appropriate disposal environment; In fact it can be beneficial, since smaller pieces present a greater surface area, and their biodegradation rate therefore tends to accelerate.more

However, if an unsuitable biodegradable plastic is used, then the released fragments could well be more persistent. It is therefore important to match a product's composition with its disposal requirements to avoid this pitfall.

In particular, compounds containing poly lactic acid (PLA) should be carefully analysed for potential disposal issues. This material is known to biodegrade rapidly in hot composting facilities, but only very slowly at lower temperatures.

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