Home Discussion Student's Corner Q&A: Can I make waterproof bioplastic?
Q&A: Can I make waterproof bioplastic? PDF Print E-mail
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Student's Corner
Written by Green Plastics   
Sunday, 28 August 2011 11:44

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.

Happy experimenting!

 

Comments  

 
0 # Janko Gall 2011-11-07 12:14
could you add a bit of parafin to the mixture before you mold it?
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+1 # Greg Stevens 2011-11-28 19:14
That's a very interesting idea! You might get better results, however, using a parafin coating over the main bioplastic material/object, rather than mixing the parafin in with the polymer itself.

Parafin, wax, and oil are all good optins for coating a final product to increase water resistance.
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0 # Maya 2012-12-02 18:54
If I change the plasticizer (I was using glycerol), to something hydrophobic, would that make the bioplastic waterproof? If so, is there any such plasticizer that is easily accessible? I'm trying to make bioplastic waterproof, for my science fair experiment. I read that glycerol, sorbitol and camphor are all hygroscopic...the opposite of what I need.
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0 # Greg Stevens 2012-12-15 11:29
Maya,

This is such a good question, that I thought we would write an article for the website with an answer. We have posted it here:

http://green-plastics.net/discussion/54-student/147-qaa-will-hydrophobic-plasticizer-make-waterproof-bioplastic

Thanks so much for your question!
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0 # Maya 2013-01-05 12:26
Thank you for answering my previous questions, but I still have more. :)
For my science experiment I've now decided to substitute the polymers in the bioplastic recipe. I was going to use shellac, chitin, and lignin and corn starch. But both lignin and chitin are really hard to find. I read that peanuts have a large amount of lignin.
Could I extract the lignin? And if I do extract something, how do I know it's lignin?
Is there a place/website to buy a small amount of chitin for a low price (I didn't see one)?
If not, could I use natural rubber as a polymer? Would doing this affect the biodegradabilit y of the plastic? Finally, you suggested the use of shellac as a coating, but it is also a polymer. Could it be the polymer in a bioplastic?
Thanks for your help!
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0 # Greg Stevens 2013-01-20 11:57
Thanks for your question again, Maya!

We have written an article to answer it here:

http://green-plastics.net/discussion/55-science/149-what-polymers-can-i-use
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0 # Baodan 2013-05-09 09:05
Would the plastic then become water resistant if in the process of making the plastic shellac was added to it? Would this make the entire piece of plastic water resistant?
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0 # Chidharth 2013-04-25 12:31
i ve tried producing bioplastic from agar. the results were good.
addition of PLA made it waterproof but still the bioplastic produced was very sticky.
are there any suggestions regarding its improvement?
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0 # inocencio 2014-01-23 07:17
..if i will make a starch based bio-plastic what is a good agricultural waste product that is very appropriate in making such thing?.
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