Integrated HP

To degrade, or not

Our project started with one image: a PLA plastic bottle on the ground, which was shown to be degradable in 80 days (see Figure 1). This image was catchy, and remained in our minds for a long time. However, when going into detail, it was mentioned in small letters that this happened in particular soil conditions. If Googling “PLA degradability”, a large set of controversial blogs appeared. Thus, we decided to take a little experiment on PLA degradation.

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Figure 1. Image showing PLA degrading in 80 days. From [1]

Science and skepticism

Science is unpopular for good reason. It caused the extinction of the faeries. It caused magic powers to fail inexplicably on The Amazing Randi’s show. It kidnapped every child’s imaginary friends and won’t tell us where it’s hiding them.
— Scott Cunningham, in Respectful Insolence.

In our daily life, we are exposed to a big set of statements, that we tend to – consciously or unconsciously – classify as real or unreal facts. In science, we use the scientific method to design experiments and generate conclusions from them. These conclusions can come from the interpretation of data that we have measured or observed in first person, and thus, we believe. If provided by a third-person, science demand the methodology and protocols necessary to replicate the experience. This, together with the peer-reviewing system, is what make us trust on published scientific papers.

When applying the scientific method, science takes skepticism as basic attitude. By dictionary definition, skepticism is "the attitude of doubt or a disposition to incredulity either in general or towards a particular object." (from Merriam Webster Dictionary) We can see skepticism as the act of looking for reasons that critically support a fact, making us think of it as feasible, or valid.

In the case of PLA, if we would see it degrade quickly, we would believe it, right? Spoiler alert: we didn't.

The Plastic Cup experience

To start, we needed a PLA plastic object. We took plastic cups as example of daily cutlery that can finish thrown in environment. We used 3 different cups:

PLA plastic cup
Polystyrene (PS) plastic cup
Polypropylene (PP) plastic cup

We would use PS and PP plastic cups as control to compare degradability. The design was simple:

There would be 2 different experimental conditions: garden soil and sea water.
Weight would be the quantifiable measure that would allow us get degradability data.
We would weight the plastic cups every forthnight during the month of September, and every week during October (until the Wikifreeze).

During a curious trip to Normandy, we visited the beach and we took sea water sample back to Paris. For garden soil, we would use a spot near the iSSB, our lab. Figure 2 shows the plastic cups located on its environmental conditions.

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Figure 2. PLA cups in garden soil conditions.

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Figure 3. PLA cups in sea water conditions.

Results

Table 1 shows the weights measured during the experiment weeks. Variations observed were both positive and negative, and in all cases <0.2g. Thus, we considered them as non significant and we concluded there was no degradation.

Table 1. PLA cups weight changes in ground and sea water conditions.

	PP (g)		PS (g)		PLA (g)
Date	Ground	Sea water	Ground	Sea water	Ground	Sea water
02/09/2016	7,0471	7,0285	6.7652	6,7403	10,1361	9,9935
16/09/2016	7,0722	7,1144	6,8408	6,791	10,1866	10,0816
28/09/2016	7,1074	7,1066	6,8525	6,8409	10,3106	10,1683
09/10/2016	7,03	7,0752	6,77	6,7949	10,18	10,1501
16/10/2016	7,0502	7,0291	6,7661	6,7903	10,1435	10,0017

Discussion and insights

From our empirical observations on this little experiment, we can conclude that the degradation shown in the Figure 1 is not representative of daily environmental conditions.

In our experiment, soil and water conditions did not have a tight control of paramenters such as temperature, humidity or composition, neither the sea water had the physical movement that it would in the open sea. However, we aimed to observe the degradation in open daily conditions, for which it was enough.

Overall, we wanted to point out that we have to be critical with the informations that we find. We have to know well the sources, and we have to go deep to see if experimental data is supporting the provided statements. For instance, most of the controversies - both in favour or against - came from blogs such as [1] or [2] of which scientific fiability is doubtful. As scientists we have to compare informations with databases such as PubMed [3], and in our case, look for articles on PLA degradation that are reviewed. For instance [4].

Concerning PLA degradation, after this experience, we would rather believe statements such as the one made by E. Royte in the Smithsonian [5], specifying that it may break down into its constituent parts (carbon dioxide and water) within three months in a “controlled composting environment,” referring to an industrial composting facility at 140ºF and with digestive microbes on the soil.

PLA, for its chemical characteristics, can biodegrade and a part from that, has several other characteristics such as coming from renewable resources that make it a good alternative to traditional plastics. Thus, we will keep on fostering its bioproduction and defending its usability as bioplastic, but this experiment has been useful to emphasize the need of critical point of view and contrast of sources when being given information.

References

3Dp Nexus. Understanding PLA 3D printing filaments. Retrieved from: http://3dpnexus.com/pla-3d-printing-filament-explained/
About News: pros and cons of the corn based plastic PLA. Retrieved from: http://environment.about.com/od/greenlivingdesign/a/pla.htm
PubMed. US National Library of Medicine National Institutes of Health Available at https://www.ncbi.nlm.nih.gov/pubmed
Agarwal M, Koelling KW, Chalmers JJ. Characterization of the degradation of polylactic acid polymer in a solid substrate environment. Biotechnol Prog. 1998 May-Jun;14(3):517-26.
Royte, E. Corn Plastic to the Rescue. Smithsonian (2006) Retrieved from: http://www.smithsonianmag.com/science-nature/corn-plastic-to-the-rescue-126404720/?no-ist