PLA is, in theory, the “environmentally friendly” poster child of plastics as it is made up largely of organic materials and designed to be biodegradable and compostable. You will come across it in all manner of products and is reducing the number of oil-based plastics used.
Wow, we have saved the planet, let’s go home and start composting the stuff and create fantastic finished compost to help our plants thrive, doubly helping the environment! Oh, if it were so simple. There is a problem. You are almost certainly not going to be able to compost PLA. Indeed, if you tried you are likely to end up with a distorted plastic mess that is likely to still be on your compost pile long after you die.
But it is compostable. What is the problem? Well, there are compostable and home compostable and these are two very different processes. In terms of PLA, while it is compostable, it is only such under very specific conditions, conditions that are only found in special industrial facilities. I can almost hear your eyes rolling! So, what is the point? We will look at that in detail below.
What Is PLA and Why Might You Be Able to Compost It?
When looking at PLA in terms of being able to compost it, it helps to understand exactly what it is. PLA stands for Polylactic acid and is a type of plastic that comes from renewable resources like cornstarch or sugarcane and in some applications is an alternative to petroleum-based plastics. It’s considered to be a sustainable material, meaning that it’s environmentally friendly and biodegradable.
A further advantage of PLA is that it is energy efficient in terms of manufacturing. Compared to traditional plastics, PLA requires less energy to produce, reducing the carbon footprint, yet can be produced using much the same tooling as oil-based plastics.
Types of PLA
There are two main types of PLA: crystalline and amorphous. Crystalline PLA is stronger and stiffer than amorphous PLA because of its molecular structure. Like traditional plastics, PLA is very versatile. It can be molded, colored, and even printed just like regular plastic.
Both types of PLA are commonly used in everything from food packaging to medical devices. It’s even used to create toys and clothing. It’s also used to make filaments for 3D printing.
Biodegradability of PLA
The key “selling point” of PLA is that it is completely biodegradable. When exposed to air and water, PLA breaks down into carbon dioxide, water, and lactic acid. The drawback is, that if this process were allowed to happen naturally, it takes a considerable amount of time, upwards of 80 years! This time can be reduced to a matter of months under the right conditions, conditions which, can be reproduced with a highly regulated composting process.
In addition to composting, PLA is in theory, easier to recycle than conventional plastics and can be identified by its labeling and separated from other materials during recycling.
Although PLA is often referred to as a ‘green’ plastic, it is, however, not entirely green as it is actually a combination of both natural and synthetic materials. The addition of synthetic materials is to help reinforce its crystalline structure to improve the plastics’ impact resistance, heat resistance, and tensile strength. This brings it closer, in terms of properties, to traditional oil-based plastics and other bioplastic counterparts, allowing more use case scenarios.
How Does the Inclusion of Synthetic Material in PLA Affect Composting?
It is the addition of synthetic materials in PLA that cause it to need very specific conditions to decompose. It is at this point we have to consider the difference between compostable and home compostable because there are actually two different definitions attached.
Home compostable is a material that you can put on your compost pile and be happy that it will decompose and create finished compost. If however, an item is marked compostable, it means that it needs to be composted in controlled conditions at an industrial composting facility. If you were to casually put PLA on your home composting pile it will be at least two generations after you die that might see it turn into compost.
Industrial Composting of PLA
A great deal of importance is set on being able to refer to a product as biodegradable and compostable, particularly in terms of packaging. To be able to make this claim a product has to be able to pass the American Society for Testing and Materials ASTM 6400 standard, which tests for biodegradability of PLA in industrial facilities. This forces companies to have to use PLA because it is the only standard that allows the packaging to be marked as biodegradable/compostable.
Under What Conditions will PLA Decompose?
The actual conditions that allow for decomposition to take place are fairly exacting:
- Temperature of more than 140 degrees F
- Humidity of 90%
- Plentiful supply of Oxygen
- Maintain conditions for between 60 to 90 days
These are conditions that would be difficult to produce let alone maintain in a domestic setting. As such this means that to compost PLA you need to send it to an industrial composting facility.
Is PLA Compostable at Home and If so How Do You Compost It?
The conditions laid out above mean that there is very little chance that the average, or even experienced composter, will be able to successfully compost PLA. That is not to say that some more adventurous composters might not try.
If you were able to create and maintain these elevated heat levels while creating high humidity and ensuring moisture and oxygen were in plentiful supply you might pull it off.
How You Might Go About Trying To Compost PLA
What you would need to be able to create these conditions is more for an engineer than a lowly gardener. However, in practice, you would need to create an enclosed environment in which temperature and humidity could be regulated. You would be employing an extremely hot composting process to drive the temperature up with high nitrogen content.
As a plentiful supply of oxygen and moisture will also be required to allow the microbes to thrive and maintain the heat a high volume of air would need to be drawn through. This would mean some form of aerating system would be needed, maybe in the form of a series of plastic pipes running vertically and horizontally through the chamber, pulling air from the outside and releasing it through holes drilled in the lengths of the pipes.
All of this is great, in theory, and might work for a short period of time, but whether it is possible to maintain these conditions, for sixty to ninety days, is extremely doubtful, and at the very least would need proactive monitoring.
Why Is PLA Pushed as Biodegradable/Compostable
If PLA decomposes in such a narrow set of circumstances why is it touted widely for its biodegradable/compostable attributes? It all comes down to PLA’s ability to meet the ASTM 6400 standard. As it is the only current test available to determine the composability of plastics only products made from PLA can make the claim. 
Of course, to many amateur gardeners, who do not appreciate that there is a difference between ‘compostable’ and ‘home compostable’, it can be confusing if, when they try to compost PLA, they just end up with a tangled mess of plastic.
There are other bioplastics that are far more compostable than PLA, however, as there is no certification process they can go down, they are unable to legitimately claim to be compostable.
Summary: How Do You Compost PLA
PLA, as far as a typical gardener is concerned, is not compostable. The specific conditions required for this type of plastic to decompose are practically impossible to recreate in a domestic composting environment. If you want to compost PLA your only option is to send it to an industrial composting facility.
This leads to the inevitable conclusion that the idea that PLA is “the green face” of plastics is incredibly misleading. Yes, true if you send the material away to an industrial composting facility, it will become usable finished compost, but how easy is that? In large cities, local governments may have collection points, otherwise, you are reliant on recycling facilities being able to separate it and send it on.
In truth, if you put PLA on your own compost pile or dropped it in the ocean with all the other rubbish that is floating about, it is still likely to be pretty much intact decades later, just as its oil-based counterparts!
Commonly Asked Questions
Can PLA filament really be described as biodegradable?
PLA filament used for 3D printing is considered compostable because it will have met the ASTM 6400 standard for composability. However, this only means that it is compostable in an industrial composting facility, on which the standard is based. You wouldn’t be able to compost PLA on a home compost pile, as it would take upwards of 80 years to decompose.
What actually is bioplastic?
Bioplastics or bio-based plastics are plastics that have been created largely from organic material that will break down when exposed to microbes under specific conditions in a timeframe of fewer than three months. The most commonly known bioplastic is Polylactic acid (PLA).
- ASTM International (American Society for Testing and Materials): Standard Specification for Labeling of Plastics Designed to be Aerobically Composted in Municipal or Industrial Facilities
Keith has been involved in the gardening and landscaping industry fro the past 21 years. From a jobbing gardener to running his own landscaping services. He is now dedicated to bringing you the latest in gardening news.