Step 5: The Science behind it.
First, lets look at how regular petroleum based plastics are formed. According to Wikipedia plastics are:
polymers: long chains of atoms bonded to one another. Common thermoplastics range from 20,000 to 500,000 in molecular weight, while thermosets are assumed to have infinite molecular weight. These chains are made up of many repeating molecular units, known as "repeat units", derived from "monomers"; each polymer chain will have several 1000's of repeat units. The vast majority of plastics are composed of polymers of carbon and hydrogen alone or with oxygen, nitrogen, chlorine or sulfur in the backbone. The backbone is that part of the chain on the main "path" linking a large number of repeat units together.
If you want a really good visual explanation of polymers go here to polymer planet.
So now that we understand polymers and how regular plastics are formed, lets look at how this happens with starch.
Starch is mixture of two polymers called Amylose and Amylopectin, in a ratio of about 1:4 ( 4 being the Amylopectin). Amylose is a linear polymer of glucose linked with mainly a(1 --> 4) bonds. It can be made of several thousands of glucose units , where as Amylopectin is a highly branched polymer of glucose.
What does all this mean? Well to get something that resembles plastic, we need to have LONG STRAIGHT chains of polymers. Since starch has both straight and branched polymers, we need to remove or alter the amylopectin, so we are left with long straight chains. Since going through and picking out all the individual amylopectin molecules with an electron microscope is not an option, we had to alter the starch. We used vinegar (acetic acid) to break down the amylopectin into straight but short molecules of dextrin ( commercially dextrin is produced from amylopectin by Hydrochloric acid hydrolysis: the amylopectin is roasted in vessels jacketed in hot oil - the oil is heated to 450C - at the same time HCl is added to the amylopectin - the acid plus the heat causes the molecular structure of the starch to change/polymerize ).
So now we are left with amylose and dextrin to which we added glycerin. Why did we do this? Well, not all normal plastics are soft and flexible. To achieve this property something called a plasticiser is added to the plastic resin. From wikipedia, Plasticisers for plastics are additive, most commonly phthalates, that give hard plastics like PVC the desired flexibility and durability. They are often based on esters of polycarboxylic acids with linear or branched aliphatic alcohols of moderate chain length. Plasticisers work by embedding themselves between the chains of polymers, spacing them apart (increasing of the "free volume"), and thus significantly lowering the glass transition temperature for the plastic and making it softer. For plastics such as PVC, the more plasticiser added, the lower its cold flex temperature will be. This means that it will be more flexible, though its strength and hardness will decrease as a result of it.
Imagine that you have a bunch of cooked spaghetti noodles laid out lengthwise. When the water in the spaghetti dries out they begin to stick to each other. If you were to grab a large handful of it and bend it, the spaghetti would most likely break. However, if we added some olive oil or butter to the spaghetti, it would be more flexible and not break. The butter or oil is getting in between the individual spaghetti strings and lubricating them.
This same concept applies to our plastic. The glycerin gets in between the amylose and dextrin molecules and keeps them from sticking together. Without the glycerin in our plastic, it would crack and shatter if we bent it, or put stress on it, but since we added glycerin- a plasticiser to it, the starch plastic is flexible.
Now that you understand how to make potato plastic, and the science behind it, go forth and spread the good news to all, so they can enjoy it themselves, and save our planets resources at the same time.