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The difference between Degradable, Biodegradable, and Compostable PDF Print E-mail
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Written by Green Plastics   
Friday, 17 August 2012 07:32

There are three terms that get thrown around a lot when people talk about plastics, so it is worth spending a moment to clear up how they are related to one another, and how they are different.

Degradable Plastic.  The word "degradable" just means that something breaks down. Technically, all plastic is degradable plastic.  You can break it with a hammer. You can grind it into a fine powder.  This all counts as "breaking down" the plastic, and therefore (technically) "degrading" the plastic.

This creates a little bit of confusion, because some plastics will add chemicals that will make the plastic break down faster under certain conditions. For example, you can add an additive to normal, petroleum-based plastic that will make it become brittle and crumble in sunlight: this is referred to as making "photodegradable" plastic. Other additives can be put into plastic that will make plastic break down by oxidation: this is referred to as making "oxo-degradable plastic."

These methods will make the bulk of the plastic appear to disappear; however, the small pieces (or even find "sand") that is produced by this effect is still small pieces of plastic.  Nothing has changed. Over a matter of years, it is possible for the pieces to become small enough to be assimilated by microorganisms, but there is still a lot of research that needs to be done to verify how long this might take.  In the mean time, they are just very small pieces of plastic.

So be cautious when you see a plastic product that advertises that it is "degradable" but not "biodegradable" or "compostable," because this is nothing special.  The plastic material does not "return to the earth" in any real way. It just gets really, really small. (You can read more about why this is different from biodegradation in our article: Degrading Dialogue (Achilles and the Tortoise).)

Biodegradable Plastic.  When something is biodegradable, it means it is degradable, but it also means something more: it means that it can be broken down by the metabolism by micro-organisms.  When a plastic is biodegradable, it can be digested, so that the carbon atoms in the chains of the polymer are broken apart and can actually participate in the creation of other organic molecules.  They can be processed by, and become part of, organic living things. This returns them to nature in a very real sense: they become part of the carbon cycle of the ecology of the earth.

Only bioplastics will biodegrade within any reasonable timescale.  Petroleum-based plastic that simply breaks down into a fine sand or small pieces still cannot be digested by microorganisms. Perhaps over the time-span of many years, the pieces may get so small that they can be digested by microorganisms. This is currently the focus of a great deal of research and debate, as different groups try to establish how quickly oxo-degradable plastics can be reduced to a form where they are actually biodegradable.

It is also important to note that even some plastics that are made from renewable resources are processed in a way that makes them non-biodegradable.  They are still "degradable" but they do not return to the earth, and cannot be processed by microorganisms. That is why the difference between biodegradable plastics, and non-biodegradable plastics, is so important.

 

Compostable Plastic.  When something is compostable, it means that it biodegrades, but it also means something more: it will degrade within a certain amount of time, under certain conditions.  For many types of bioplastic, it's possible to say that it will break down "eventually", but if you seal it in an air-tight room, it could take thousands of years.

The standards organizations that regulate materials have come up with a series of tests and benchmarks, saying that if a biodegradable plastic will completely biodegrade fast enough in a certain type of environment, then it can be labelled "compostable."  For more details about what these limits are and how they are measured, check out our article: "Bioplastics Standards 101".

So these three terms aren't really different "classes" of plastic, in the sense of being separate sets. They are subsets of one another: all compostable plastics are biodegradable, and all biodegradable plastics are degradable.  But be wary of people who make claims about the "degradability" of their product: because not all degradable plastics are biodegradable, or compostable.

 

Comments  

 
-2 # Oxobio 2012-08-20 17:13
This article is thoroughly misleading. If you want to know about degradable plastics see www.biodeg.org

The fundamental point is that the oxobiodegradabl e additive causes ordinary plastic to convert after its useful life by an abiotic process in the presence of oxygen into a material with a different molecular structure. It is not necessary for it to be in moist or microbially-active conditions. At the end of that process it is no longer a plastic and has become a material which is inherently biodegradable in the same way as a leaf. Approximate timescales for degradation can be set at manufacture as required. For a video of plastic film degrading, visit: http://degradable.net/play-videos/4

An LCA published by the UK Government in 2011 found that ordinary plastic and oxo had a better LCA than compostable plastic or paper bags.
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-1 # Greg Stevens 2012-08-20 17:52
Hello Oxobio!

Let us be fair to the students that are visiting the site, and paint the whole picture for them. I was neglectful in not mentioning the debate in the article, however the article is NOT (as you say) "misleading." It simply presents one side of an argument.

To anyone reading this article: There is actually a debate within the scientific community on this issue. It has been going on for many years, and it is very heated.

On the one hand, there are the pro-biopolymer people, who are very skeptical of oxo-degradable plastics and do not believe that the science supports the claim that they are truly biodegradable.

On the other hand, there are the pro-oxo people, who support the use of additives to "convert" plastics that are NOT made from biopolymers into degradable (they claim: biodegradable) materials.

These two groups disagree. I have never hidden the fact that this website has a pro biopolymer slant to it... although, in all fairness to my friends and colleagues on the other side, I should probably have mentioned in this article that this debate exists.

For more about this debate, check out the commentary and links over on the article called "The Oxo Debate" on the Green Plastics Wiki here:

http://greenplastics.com/wiki/The_Oxo_Debate
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-1 # Del Andrus 2012-08-22 16:31
I too would agree that the article is misleading. There are more sides to the discussion than the degradable or compostable plastic technologies.

The article states that petroleum based plastics cannot biodegrade. This is not technically accurate. Microorganisms definitely convert the carbon backbone into biogas or biomass - it just takes a very long time due to the long chains. But the process of assimilation is definitely happening.

Technologies do exist which enhance the biodegradation of standard polymers (microorganisms converting the carbon to biogases and biomass).

We would love to be included in the discussions your students are having regarding degradable, biodegradable, and compostable plastics. Even the term compostable plastic is not as clearly defined as mentioned in the article; home compost and industrial compostable plastics are very different and require different environments.
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+1 # Greg Stevens 2012-08-28 10:29
Hi Del!

Just so you know, Danny Clark wrote an article for us.

http://green-plastics.net/news/45-science/77-whats-in-the-word-biodegradable

I mention of this because both of you, being associated with Enso Plastics, are clear "additive" advocates. I've made an attempt, across the website (even if not in every article) to get a distribution of views from different sides.

However, I also want to address a couple of issues with your comment here.

You say that petroleum can biodegrade but "it just takes a very long time due to the long chains." Technically you are correct. Eventually, when the molecular weight of polyolefins is reduced enough (and there is no sharply defined point here) the chain segments can be assimilated by microorganisms and biodegradabilit y can occur.

But it's all a matter of time scale. Claims of manufacturers will have to be confirmed by independent scientific studies... and to my knowledge, they have NOT to this point done this.

You also talk about composting. The fact that oxo-degradable plastics don't meet the ASTM 6400 requirements of compostability is still very relevant. As Professor Ramani Narayan, of Michigan State Unviersity, has said (C&E News, June 13, 2011) "Composting is meant to accelerate the rate of biodegradation, " he says."If something doesn't go away in compost, the probability of it going (away) in soil, which is at room temperature and has a low availability of microorganisms, is very negligible."

I'm very open to having lively discussions about this issue. Also, if you have specific research addressing the timescale of biodegradation of petroleum plastic, or an update on the relationship between oxo- plastics and ASTM 6400, please send them my way! I'm always happy to learn things that I didn't know before.

Thanks
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-3 # Oxobio 2012-08-26 06:40
I still say that your website is misleading, and I note the admission that it has a pro biopolymer slant to it.
You are doing the students no favours by paraphrasing the facts and getting them wrong. For example “you can add an additive to normal, petroleum-based plastic that will make it become brittle and crumble in sunlight. This will appear to reduce the amount of waste in landfills; however, the small pieces (or even find "sand") that is produced at the end of this process is still small pieces of plastic. Nothing has changed.”
On the contrary, everything has changed. Polymer scientists know that when molecular weight has reduced to 10,000 the material is no longer a plastic, and has become biodegradable. You are also confusing photo-degradable plastic with oxo-biodegradable plastic. Oxo is not designed to degrade in anaerobic conditions in landfill – it is designed to deal with plastic waste which escapes into the open environment.
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+1 # Greg Stevens 2012-08-28 10:32
Hello again!

With all due respect, and at least based on the research that I'm aware of, petroleum based plastics cannot biodegrade on any reasonable time scale. They undergo oxidative degradation on a time scale of many decades. Eventually the chains become short enough for biodegradation to occur but, again, this occurs only after many decades.

I will go back to the above article and soften the language to reflect the fact that biodegradabilit y can "eventually" happen, but that does not change the real core of the argument. Timescale matters. Technologies exist that use additives to enhance the oxidative degradation of petroleum based plastics (oxo-degradable plastics) and eventually the chains will become short enough to be attacked by microorganisms but the time scale is still on the order of years. Proponents of oxo-degradable plastics are continually being pressed to cite the relevant scientific literature to indicate the time scale of biodegradation, as opposed to citing manufacturers' general unspecific and unquantified claims.
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+4 # Greg Stevens 2012-08-28 10:43
P.S. have now updated the article to reflect some of the points that you have made. Still presenting "one side", but correcting inaccuracies.

Would you like to write an article for us? A kind of "counter-point"? I absolutely would welcome it. Let me know.
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0 # fayyaz ashfaq 2013-11-15 10:05
hello everyone,
I am working on my project which is "Bio-degradable additive for plastic " ,can any one from all of you suggest me on behave of experiences that all the statement as we read above is correct?I want lots of reliable notes on it to study and make an excellent presentation on bio ,photo and oxo drgdble.
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