Is there a way to make bioplastic without glycerin?
Thank you so much, Shemuwel, for this question! Since we have received so many questions from people about glycerin, and have heard from many people who have difficulty buying glycerin, I thought would take this chance to write something with a little more detail about glycerin in bioplastic: exactly why it is used, what the alternatives are for, and so on.
Most of these answers are taken from Green Plastics: An Introduction to the New Science of Biodegradable Plastics. which is a fantastic resource for more information, if you are interested finding out more details like this about home-made bioplastic.
First, let’s talk about glycerol (p.166 in the book):
“Glycerol, also called glycerin, makes a very useful plasticizer. Glycerol is produced by the fermentation of sugar, or from vegetable and animal oils and fats, as a by-product in the manufacture of soaps and fatty acids. Glycerol is often available in drugstores. It is liquid at room temperature.”
So the function of glycerol in bioplastic is to act as a plasticizer: it is a material that makes the polymer chain molecules bend and slide past each other more easily, which adds to the flexibility of the plastic.
What else can act as a plasticizer in bioplastics? The book goes on to say:
“The water used in the following recipes also acts as a plasticizer. Also, teachers or students in high school or college will have access to a much wider range of materials through chemical supply houses. Sorbitol is also useful… Sorbitol, like glycerol, acts as a plasticizer.”
If you are interested in exactly how glycerol and other plasticizers work, the book also contains this description (pp 166-167):
“The mechanism by which glycerol and other small molecules, like sorbitol, increase the mechanical plasticity of cast films and sheets is still not completely understood. In recent research on starch-sorbitol films cast from water, the local mobility of the starch molecules has actually been found to be reduced by the addition of sorbitol… The observed increase in mechanical plasticity, therefore, is pictured as arising from the formation of clusters of sorbitol molecules. Glycerol may act in a similar fashion…”
So should you use sorbitol or glycerol or what? The book says (p.167):
“Glycerol is an effective plasticizer and inexpensive, and it tends to make the resulting plastic flexible even at the very low temperatures of a freezer, as might be required for a freezer wrap. On the other hand, too much of it makes the plastic curl up in a microwave oven and turn into gum. Even more important, glycerol tends to lose its effectiveness as a plasticizing agent over time, leading to a slow increase in brittleness.”
“Sorbitol produces better resistance to microwave radiation and does not lose its effectiveness as a plasticizer as quickly as glycerol. On the other hand, it typically leads to brittleness at freezer temperatures, and if there is too much of it, chalking tends to occur: the sorbitol crystallizes out, producing a white spotty appearance.”
So, because glycerol is simply easier to get for most people than sorbitol, most of the recipes on this website (and in the book) focus on glycerol as a plasticizing agent. However, if you can get your hands on sorbitol, you can use that instead.
In the Green Plastics book there are a number of recipes that use different combinations, sometimes combining both glycerol and sorbitol in different amounts, to produce different effects.
What if I can’t get glycerol or sorbitol?
According to the book (p.180), both glucose and sucrose “have been used as plasticizers.” Unfortunately, it does not provide any further details about how much to use, or whether you need to perform any other special instructions if you substitute the glycerin in a recipe with glucose instead. I personally have no tried it.
If you have the time, curiosity, and inclination, please do some experiments and try making bioplastic using glucose or sucrose as a plasticizer… and leave your results in a comment below! If you are successful, I may ask you to write up an article for this website!