Conor Buckley started our bioplastic journey to develop an art material for use in the Trinity Trees Project by explaining about the cellular make up of the different base materials we would be using. This was a great help for me to understand how the material would act and react depending on what you added to it.
We stared by talking about how one would make a starch based bioplastic.
Starch bioplastic: the recipe:
9.95grams of corn starch
60ml distilled water
5ml Acetic acid (vinegar) 5% solution
5ml glycerol (a light sensitive animal fat)
Cornstarch is a complex carbohydrate called a polysaccharide. It has a polymer chain with branches. When we add vinegar to the starch and water solution it cleaves the branches from the polymer chain separating them into whats called an amalose mix i.e. the starch and vinegar solubalise in water. When you add glycerol (the plasticiser), which is like an oil, it allows the chains to slide over each other. The stiffness or flexibility of the final product depends on the amount of glycerol we add to it. The more glycerol the more plasticised it becomes.
Starch bioplastic: the method:
Add water and corn starch to a graduated beaker. Add vinegar to create amalose. Add glycerol to plasticise. Put the beaker with all the ingredients over the heat and gradually increase the temperature from 100 to 140 and up to 160 degrees over about 10-15 minutes all the time stirring the mixture.
When the mixture changes from opaque to clear it is the time to scoop it out and spread it out on the chosen dish to dry. In our case we used petri dishes. Then the dish was placed in an oven set at 65 degrees to dry. For this starch how you dry it is important. If you leave it on bench to dry over a long period of time the top layer will dry faster and form a skin. Drying in the oven in @65 would allow for the material to dry more evenly.
Conor and I were delighted with the result, which was much more opaque than the samples I had created the year before. As most of last years samples ended up white in colour when I tried to add colour I ended up with varying shades of pastel colours. I had also tried many different ways of adding colour to this bioplastic. But now thanks to Conor’s knowledge I realise that what I thought was a good idea, adding acrylic (plastic based) paint to the mix, was in fact reacting with the cellular chains and making the material more brittle.
On the down side this and last years cornstarch bioplastics were very thick, lumpy and hard to spread evenly. If I were to use it to cover elements of my sculptures we would have to spread it when it was at its most liquid form when it was really hot and sticky As it looses heat quite quickly cools and becomes more unworkable it would be a tall order to use it as a material. A really fantastic matieral but unfortunately not one that I could use.
Two handy pieces of information that Conor imparted to me that I thought useful to share are:
1. When you are trying to optimise the mechanical properties of a material change one thing at a time. You don’t change the base amount of starch, water and vinegar components only the glycerol.
2. If you want to colour a bioplastic materials it is best to use three drops of food colouring and only after all the ingredients have been solubalised. If you add the colour earlier you might not be able to see the point at which the material changes from opaque to clear indicating that it is ready to use.
The second type of bioplastic we said we would explore was an Agar based one.
Agar is a generic term for seaweed and alginate is a made by processing a type of seaweed. The algae base material that we are using is called agarose. This is a thermoreversable material i.e. it melts in water when you heat it up, sets when cool and can be reheated to liquid form again. It is important to note that the more times one heats and cools the material the weaker the material becomes. Conor explained what happens when the agarose is heated up in water. As it is made of nano scale helical shaped chains, they unravel when heated and straighten out and then return to the helical shape when they cool down.
In our initial experiment we just used the pure agarose.
Agar bioplastic: recipe 1:
1.5 grams agar
50 ml of water
Agar bioplastic: recipe 2: agarose and glycerol version
1.5 grams agar
50ml of distilled water
4ml of glycerol
Agar bioplastic: the method:
Measure out the 1.5 grams of agarose. Boil distilled water. Add agarose. Stir continuously over heat for 10-15 minutes until all the agarose strands have disappeared. Pour solubalised agrose into petri dish and leave to set. If glycerol is to be added do this after the agar has solubalised and before pouring into the petri dish.
Interestingly after describing what I was looking for from a bioplastic Conor suggested that by using a paint brush when this material was hot we could paint the bioplastic onto the fabric, which would then set and dry over time.
I also wondered if this material would be flexible enough to be able to move with the fabrics i use or would it be too brittle and tear. Conor decided to try and add some gylcerol to the mixture to act like a plastisier. He decided to start off with 2ml of glycerol, which we added to the solubalised mix. We planned to leave it over night to dry to see how that would look and work.
Another really interesting idea that Conor suggested on this first day is that he wanted to try adding bubbles to the mix. By adding bubbles to the recipe without glycerol he hoped to add an extra element of flexibility. So I was asked to bring in some alka selser for our next batch of experiments.