A Stovetop Bioplastic Experiment

A little while ago I wrote that you could make bioplastic, and that it was simple to do so. There are plenty of websites that tell you this, and countless chirpy teenagers demonstrating various techniques on YouTube… but I decided I’d better put the whole thing to the test.

For my first experimental bioplastic , I tried polylactic acid (PLA) – simply because I had some milk left over and it wasn’t going to last much longer. I used a recipe provided by the folks at the Smithsonian, which appealed to me because of its simplicity. Later, I’d try some things that required me to buy ingredients, but for this one all I needed was milk and a little vinegar.

Selecting an old saucepan (just in case something horrible was produced – it wasn’t) I warmed up the milk to the point where velvety bubbles were just beginning to appear. Meanwhile, I mixed in some food colouring, to make things more interesting. Then I added vinegar, in a ratio of one tablespoon of vinegar to one cup of milk. (Why do American recipes always use the ‘cup’ as a unit of measure? Which cup? They’re all different sizes…)

Pan of milk, vinegar and food colouring.

Bioplatic. It’s what’s for dinner.

I kept on stirring, and pretty soon the mixture polymerised… which is a nice way to say that it curdled, just as if I had added lemon juice to cream when cooking. The result was a vile-looking mixture of thin, clear liquid and small rubbery chunks with the consistency of cottage cheese. The chunks were what I was after.

I poured the whole lot through a sieve, to retain the chunks. Some of the smaller chunks slipped through and I had a momentary panic that I was pouring plastic down the drain, where it might clog… then I remembered that this is bioplastic, so it can be depended upon to rot away quite quickly. Score one point for home-made bioplastic!


I would estimate that I got about 10% chunks, 90% waste liquid.

I scooped the chunks out of the sieve and dabbed at the mixture with kitchen roll to remove excess liquid. Then I thought, what on Earth am I going to do with this bioplastic anyway? A quick search of the kitchen revealed an ice cube tray (a promotional freebie that makes ice cubes in the shape of VW Polos) so I decided to make some bioplastic Polos. I spooned the chunks into the cavities and pressed it down as best I could: the process was nothing like any plastic moulding I’ve done before, and that was a disappointment: I’d read that bioplastics could be substituted for oil-based plastics in existing processes. Not this one… or at least, not with this recipe.

Ice cube tray

Who knew you could make VW Polos from bioplastic?

One of my concerns was that this experiment would smell really bad. I don’t like milk very much at the best of times, and heating it and then leaving it around the house for several days really didn’t seem like a good idea. I’m pleased to report, however, that the only smell coming from the experiment after two days was a faint smell of vinegar.

Now, something that has surprised me in many of the bioplastic recipes I’ve read is a requirement to let them dry. They set over time, it seems. That’s another disappointment, because it reduces their utility a little… but perhaps I could make some bioplastic, let it ‘dry’ and then soften it with heat and mould it like a regular thermoplastic? Well… we shall see. I suppose that’s my end goal in these experiments: to identify an easily-made, biodegradable plastic that can be substituted directly for something like polystyrene, or PET. Imagine the benefit if an existing waste material such as food industry byproducts could be used to make packaging, reducing dependency upon oil imports and simply turning back into soil at the end of life!

The ‘milk plastic’ exhibited a tremendous amount of shrinkage as it dried. The ‘ice cube’ VW Polos became a lot narrower (around 25%) as they dried. Clearly, there was a lot of liquid left in the mixture, and it all had to evaporate away. This is a problem because working with PLA in this way seems to be a race against time: will the plastic set before it biodegrades? Again, I don’t really want rotting milk products around the house.

Mould shrinkage

Nasty shrinkage – at least 25% – during drying.

The ice cube tray proved to be a poor choice of mould, as it inhibited drying. After two days, I tried to dig the first Polo out of the tray, and found that it was still gooey inside. A better result came from some surplus bioplastic that I had left on a porous surface; it dried much more thoroughly, and shows no sign of decomposing. Best of all, once dry, it had no smell.

Dried bioplastic

To quote Lord Percy Percy: “A nugget of purest green!”

The strength of the material was nothing spectacular: I would estimate it was about as strong as a wax crayon: not much use for an industrial application, then. You might manage to make plant pots out of the stuff (so that seedlings can be put in the ground without removing them from the pot) but you can do that with fibrous pots made from pressed peat anyway. Another issue with this bioplastic recipe is that it isn’t really ‘green’ enough – milk is produced by farming, which may not use sustainable methods. Also, it involves using a food in a non-food application, which isn’t really ethical while not everybody has enough.

All in all, ‘milk plastic’ was a disappointment, although a useful learning experience. It took time, effort and energy to produce something that had only limited practical application. Meanwhile, the milk bottle that I washed out and put in the recycling consisted of 37 grams of virgin HDPE (an oil-based plastic) that I expect will be burnt for energy recovery. I would have come closer to my goal by grinding up the milk bottle into granules, and using that in a moulding process (exactly as some 3D printing enthusiasts are now doing). The HDPE is easier to work with, stronger, chemically more stable, and simply better at virtually everything except rotting. What a waste!


One thought on “A Stovetop Bioplastic Experiment

  1. Pingback: Another day, another bioplastic | Capacify

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