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Bioplastics or natural polymers are a form of plastics derived from renewable biomass sources, such as vegetable fats and oils, corn starch, pea starch, or microbiota, rather than fossil-fuel plastics (synthetic polymers) which are derived from petroleum. Some bioplastics are designed to biodegrade, conditions (temperature, ...) under which biodegration occurs vary.
Most bioplastic making projects start off from starch, which is either bought or made DIY from a crop (ie potatoes, ...). Several crops can thus be used, we'll discuss potatoes here.
- Get a potato, and wash it.
- Use a peeler to take the skin off.
- Cut the naked potato up into cubes, put them in the mortar, add 1 cup of water and then crush it using the pestle. Alternatively, you can use a blender (grind for a minute or two).
- Use a coffee filter to strain off the cloudy water.
- If you plan on making the plastic right away, drying the mixture is not completely necessary, but if you plan on storing it for a while, spread it out on wax paper in a sunny area for it to dry (it could get moldy otherwise). 
- 7 parts water
- 1,5 parts starch
- 1,5 parts glycerin (=plasticizer)
- 1 part vinegar
- 5 drops of food coloring (optional)
The combined ingredients are put in the pan and are stirred until we get a smooth, milk-like consistency. We then put the pan on medium heat and stir continually. The mixture will 'turn' suddenly and become glue-like; at this point we stir and fold vigorously to keep residue from sticking to the pan. The material will start to bubble, wait until large bubbles form. Cook for approximately 3 minutes in total or until the material is nearly completely transparent.
Note that more glycerine makes for a harder plastic. More starch makes for a denser and less viscous plastic. An 8 parts water, 1 vinegar, 1 gycerine, 1.5 starch variant worked best for filling molds and was much easier to work with. A recipe with 4 tablespoons water, 1 starch and 1 teaspoon of both gylcerine and vinegar made for a bit tougher but also less uniform result.
Depending on humidity, it should take about 1 day to dry in a sunny place. You can dry it faster by putting it in an oven set to 65°C for 1-2 hours. Another option is to use a modified pressure cooker. This will create a higher-strength bioplastic (called Hylon VII). See http://www.instructables.com/id/Starch-Plastic-20-Pressure-Cooker-Hacking/