Tuesday, May 13, 2014

The simple maths for replacing petroleum-based plastic

Somehow Harvard University was able to recycle old news as new-news last week, reintroducing "Shrilk" as a novel plastic. (Shrilk was first described in 2011.) And since it is based on biowaste, specifically chitan chitin (the shells of crabs, shrimp and other shellfish), it is a green option.

The fly in the ointment that is not mentioned of course, is that only about 60,000 tonnes of chitan chitin are produced each year, and worldwide plastic production is 4666 times larger, about 280 million tonnes. So we better be eating more shellfish. A lot more. I hope the ocean can keep up with our hunger.

While I mention this with regards to Shrilk today, tomorrow there will be another novel material and the same claims will be made over and over again, about how this will replace petroleum-based plastics. No it won't. The simple math above tells the same story over and over again and again. To completely replace petroleum-based plastics, you better have 280 million tonnes of starting material - per year. Globally, that is about 25 kg per person.

So if you want to take a whack at this problem, go right ahead. But before you trumpet your brilliance to the ends of the earth, think long and hard about your starting material and also about that number - 25 kg per person per year. If you can find a bio-based waste source that large to use as a feedstock, you will have passed the first hurdle and you might just succeed. Anything short of that smells awfully fishy.

Previous Years
May 13, 2010 - Followup on "The World's Smallest Rheologist"

May 13, 2010 - Airbornre Polymers


Anonymous said...

Isn't it chitin?

The base hydrolyzed derivative is chitosan. (deacetylation via wikipedia)

I feel like I get those mixed up every time I see them in a journal article.

Bend said...

I like to imagine giant cricket farms for the harvesting of chitin. It would be a carbon sink also! Cricket feces and innards could be used to make ethanol! Favor a cocktail right about now?

John said...


Chitin it is. Spelling is never a strong point for me, although I very much understand its importance.

I do know that Shrilk is made from chitosan, but I saw that as a niggly detail, albeit the deacetylization reduces the mass of the starting material some, meaning we need to eat even more shrimp.


A drink? Make mine a grasshopper