Plastic pollution is one of the modern world’s most recognized environmental problems. Scientists have been working on solutions for a long time, and one of their most promising solutions is creating biodegradable plastic derived from biological substances - otherwise known as bioplastics. One particular source for these plastics is a material found in crustaceous shells: chitin. Crustacean shells also happen to be waste products and seafood processing plants. For this project, I experimented with extracting chitin from shrimp shells and producing bioplastics from it.
Extracting the chitin: For the chitin extraction, I had to put the shells through the following processes:
- Demineralization: Removing the minerals in the shells, by treating them in HCl (hydrochloric acid) solution.
- Deproteinization: Removing the protein in the shells, by treating them in NaOH (sodium hydroxide) solution.
- Decolorization: Removing the colour from the demineralized and deproteinized shells, by soaking them in ethanol.
- Deacetylation: Removing the acetyl groups from the chitin in order to get chitosan - a material similar to chitin, but which dissolves more easily - by soaking the chitin in a bath of stronger NaOH.
Bioplastic production: Taking the chitosan I extracted, I mixed it with vinegar and water to get a basic solution, and added a percentage of plasticizer (glycerin) in order to keep the plastic from becoming flaky. I added the required additives to get the right consistency for the type of plastic I was aiming for. For example, I used wood flour (very fine sawdust) as a solidifying agent for solid plastics, or a higher percentage of glycerin as a plasticizer for more flexible plastics.
- Dissolving chitin: When it came to first making plastics, I needed to dissolve the chitosan into a solution that I could pour out and leave to dry for it to become a plastic. To do that, I experimented with proportions of chitosan, vinegar and water in order to find a solution that left me with a good compromise between dissolution time and drying time.
- Experimenting with additives: With my first plastic samples, I was mainly experimenting with vinegar to water ratios. When I finally got something I thought was right, the plastic was still flaky and fragile, so I started to experiment with plasticizers (namely glycerin), in order to give the plastic samples more plastic-like qualities. And when I started to try and make solid plastics, I used solidifiers (wood flour- fine sawdust) as another additive.
Extracting the chitin: The first time I tried this experiment (last year, with lobster shells) I was able to successfully demineralize and deproteinize the shells, but I was not able to grind it into a fine enough powder for it to be usable for bioplastic production. At the time, many of the papers I was able to find did not give full details on the conditions used in their experiments.
The second time around (this year, with the shrimp shells), I got the chitin ground down into a fine powder and was actually able to make a bioplastic sample with it. I also had carried out more research and found more details on conditions (such as concentrations in the acid and sodium hydroxide, times and temperatures). The sample was not as consistent as the samples I had created using commercially available chitosan. I suspect that this was a result of insufficient deacetylation. Unfortunately I did not have access to a spectrophotometer which would be used to measure the degree of deacetylation. However, there is a lot of research into ways to improve deacetylation that are more complicated than the simple deacetylation step I used.
Bioplastic production: Through lots of experimentation, no matter what sort of plastic you are aiming for (even solids), you always need to add a plasticizer (glycerin, in my case). Through trial and error, I ended up with satisfactory recipes for cups, solids, and plastic sheets.
- Dissolving chitin: I found that a 33% vinegar solution works best for dissolving chitosan. That means 1 part vinegar to 2 parts water. When I was making the solution, I always used 50 mL vinegar and 100 mL water to 5 g of chitosan.
- Experimenting with additives: Through lots of trial and error, I found that 2-3% glycerin is a good average for plastic sheets - not too sticky, but still flexible. 10% glycerin gives you a cling-wrap consistency, but anything above that is too flimsy and ripped when I tried to remove it from the petri dish. And for solids, 10% glycerin is good for getting the shape of the mold, but it’s best to use a solidifying agent (wood flour, from my experience) to keep the solid from shrinking too quickly.
- I also tested the plastics for biodegradability. There are some advantages to the plastics that I made, such as that they do biodegrade quickly, and some disadvantages, such that they can dissolve in water, which may limit their practicality.
Extracting the chitin: Extracting chitosan is a lengthy process, and without the right tools, it’d be hard to get consistent batches that were good for making bioplastics. So I found it was better to use industrial chitosan for the production of my bioplastics, but it was nice to go through the whole process from start to finish and have a satisfactory end result.
Bioplastic production: It took lots of trial and error, but I was able to observe the different qualities and properties that plastics had with different additives. I think I found recipes for pretty satisfactory results, even if they are not perfect. For example, I found that around 2-3% glycerin was good for average plastic bags, then increasing up to 10% was good for plastic with properties resembling cling wrap, and 10% glycerin with wood flour as a solidifying agent was able to be used for solids.
This project focused on demonstrating the lifecycle from crustacean shells to chitosan based bioplastics (and even their biodegradability). There is a lot of research on each of the steps involved in the process, and a science project could be done on any one of those steps.
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And special thanks to my parents for supervising my experiments and to The Captain's Boil and Black Forest Wood Company for providing me with materials to carry out my experiments!