Recently we have received a number of comments asking us how people can make their own home-made bioplastics that are waterproof (or at least, water-resistant).
Everyone has seen that there are waterproof biodegradable products out in the world. There are bioplastic coffee cups, there are bioplastic bowls, and there are bioplastic soup spoons. There are even bioplastic wrappers that (one imagines) must be at least a little water resistant to work properly. So you think to yourself: "How can I do this at home?"
Unfortunately, although it is not impossible, the answer is not as easy as you might think.
Let's start by looking at the basic chemical properties of bioplastic, and why the question of waterproofing is difficult. Most of the bioplastics that you can make at home, like the recipes described in the book Green Plastics: An Introduction to the New Science of Biodegradable Plastics, use starch, gelatin and agar as their main polymer bases. We've talked about these recipes a lot on this website. ("Agar," remember, is the technical name for the polymer found in algae plastic.) These biopolymers are hydrophilic, meaning that they interact strongly with water; on their own, and when made into plastic, they are not water resistant.
Some biopolymers are intrinsically more water-resistant, and the most well-known example of this is polylactic acid (PLA). This material is a polyester, is made by the fermentation of sugar feedstocks to produce lactic acid, followed by the polymerization of lactic acid into PLA.This is the type of plastic that is found in most bowls and cutlery that are biodegradable, often referred to as "corn cutlery" because the sugars that they use to create the PLA are taken from corn. Natureworks LLC is a major manufacturer of PLA plastic, and has used it to create not only cups and spoons, but water-proof fibers for clothing, rugs, and other cloth coverings.
Polyhydroxyalkanoates (PHA's) are another naturally produced material, created by microorganisms. The microorganisms are fed with sugar feedstocks, and the PHA is extracted and purified. Both PLA and PHA's are used to produce biodegradable water-proof bottles, bowls, eating utensils, and dishes. However, both materials are relatively expensive, so it is also common to find bioplastic products that blend PLA and PHA's with starch. The PLA or PHA improves water resistance, while the starch lowers the cost. PHA's have also been used to as a coating over pure starch bioplastic foam cups and trays. The coating makes the product more water resistant, while still having the majority of the product made from the (cheaper) starch plastic.
Large manufacturing companies use other strategies, as well. With specialized processes and machinery they can chemically modify even a starch biopolymer to make it more water resistant. For example, starch-based packaging "peanuts" are made water-resistant by acetylating the starch: the hydroxyl (OH) groups on the starch are chemically converted to acetyl (OCOCH3) groups. This process produces packing peanuts that have a higher water resistance, but are still biodegradable. Commercial manufacturers also sometimes blend their polymers: for example, a combination of starch with polycaprolactone has used to make sturdy garbage bags.
However, Natureworks LLC and other large-scale industrial companies have access to processing and machinery that you do not have. You will not be able to manufacture PLA or PHA's in your kitchen, and you will not be able to acetylate your starch polymers. So what can you do?
One strategy used by the "big companies" that you can mimic is using the idea of a coating. Commercial products are often made water resistant with very simple coatings, such as waxes, oils, and even shellac. For example, cellophane, still sometimes used for candy wrappers, cigarette packages, and cigar wrappers, has a base cellulose sheet that absorbs water, but is made moisture proof with a very thin wax coating. Ordinary brown kraft paper can be made water resistant with a thin wax coating and used as an agricultural ground cover. Water-proofing coatings can also be applied through lamination, in which two or more layers of material are bonded together. For example, a starch sheet can be laminated with a water resistant coating of polycaprolactone, a biodegradable polymer made from nonrenewable petroleum-based feedstocks. Unfortunately, this would make the end product biodegradable but less "green" because it is made from non-renewable resources.
So, making water-proof home project bioplastics will require some experimentation. But whether you want to water-proof a solid object, like a gelatin viscose bowl, or a flexible sheet product, the best starting point would be to figure out how to apply a thin water-proof coating onto the surface. If you are working with a porous thin sheet, you may be able to use oil. Otherwise, you may want to experiment with wax or a wax-like substance. If you find even better ideas for ways to coat your bioplastic projects... make sure to leave us a comment here and let us know.