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Dear Green Plastics,

Is there anything we can substitute for glycerin?

See, my group in school is building a parachute to hold an egg safely to ground... and we get parked up for making plastic. however we have to use materials from da vinci's time... so is there any alternative to glycerin that was around in da vinci's time? thanks!

RESPONSE FROM GREEN-PLASTICS.NET:

Hello!  Thank you for your question, and it sounds like an interesting challenge for your project.

Leonardo da Vinci was alive in the late 1400's and early 1500's.  Technology was fairly simple back then, but they did make soap!

This is important for you, because glycerol is actually a by-product of soap-making.  For a somewhat technical discussion of this relationship, check out the Wikipedia article on Soap.  The basic idea is that you take some kind of oil and mix it with a chemical called lye (both of these have been around for centuries), and the chemical reaction that results (called saponification) converts the triglycerides in the fat into fatty acid salt and glycerol.

I don't recommend that you actually try to make your own glycerol with this process.  Lye is very toxic and the whole thing can be quite messy.  However, I would recommend you approach your teacher and ask whether you can use store-bought glycerol, since chemically it is the same substance that could easily be found in the 1400's as a by-product of soap-making.

Then, I would recommend that you make your bioplastic using gelatin as the polymer and glycerol as the plasticizer. Gelatin is actually easier to work with than starch and will produce some nice, strong pieces of solid plastic.  In da Vinci's time, they would get gelatin by boiling pigs' feet for several hours.  (Again, instead of boiling pigs feet, I would suggest buying the gelatin from the store.)

What is the actual recipe?

Unfortunately, you were a little vague about what you want the plastic to be used for in your device, so I don't really know what kind of plastic you want.  I will give you two recipes for a gelatin-glycerol plastic: one will produce a thin flexible sheet that can be folded and cut with scissors, and the other will produce hard solid pieces of plastic that can be molded into buttons or other inflexible solid shapes.

THIN SHEET RECIPE

Combine 6.0 grams (2 tsp) gelatin with 320 ml (1 1/3 cup) of 1% glycerol solution and 160 ml (2/3 cup) water.

THICK SOLID OBJECT RECIPE

Combine 3.0 grams (1/2 tsp) glycerol and 12.0 g gelatin (4 tsp) with 60 ml (1/4 cup) hot water.

You will notice that the big difference in the recipes is simply the proportions of each part: the thin sheet needs more plasticizer, the hard pieces need much less.

For both recipes, mix all of the ingredients together in the amounts above, and stir.  Keep mixing until there are no clumps and it is as dispersed as it's going to get.  Then heat the mixture to 95 C or to when it starts to froth (whichever comes first). Stir the mixture while you are heating it, and once it is at the right temperature (or starts to froth), remove the heat and keep stirring.  Scoop out excess froth with a spoon, and make sure there are no clumps.  Carefully pour the mixture into a drying pan, and make sure to spread it out to let it dry. How long it takes will depend on the temperature and humidity in the room, and it may take several days (depending on your formulation). You won't be able to remove the plastic from the drying sheet easily until it is completely dry, so be patient!  If your first batch turns out too sticky or slimy, you can try it again with slightly less plasticizer.

Finally, one word of caution: These recipes are good for flexible sheets or solid parts, but if you are planning on making a mechanical object with moving parts, you are probably better off using traditional old-fashioned wood.

Good luck, and let us know how it turns out!