Making Cornstarch Bioplastic
Many plastics are made using petroleum, a product of crude oil. There is a very limited supply of these fossil fuels left. Global oil reserves are estimated at 120 gigatons (1 gigaton = 109 tons). If worldwide consumption of oil is roughly 30 billion barrels per year, and the average barrel of oil weighs 0.13 tons, then the supply will run out in about 30 years.
30 x E9 barrels year-1 x 0.13 tons/barrel = 3.9 x E9 tons year-1. 120 x E9 tons/3.9 x E9 tons year^-1 = ~30 years supply left – You will see the impact of this in your lifetime! (even now with rapidly increasing fuel prices).
It is therefore of huge importance for us to find sustainable ways of making the products we take for granted.
This practical experiment is designed to be used at GCSE or A-Level chemistry classes. Green plastics are relevant to the AQA, OCR and Edexcel chemistry curricula, but also bridge environmental issues in other subjects such as physics, biology and geography. You are probably aware of the concept of green chemistry but you will not necessarily see the importance.
The practical entails combination of a biodegradable (sustainable) polymer – starch, and a plasticiser – glycerine. The net result is an environmentally friendly plastic that you can take home and offers scope for further projects or homework. There are numerous media articles concerning sustainable materials to give you some context for this experiment.
Plasticisers are additives that increase the plasticity or fluidity of a material.
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined by glycosidic bonds. Its formula is (C6H10O5)n– where n means repeat units of glucose. ~20% amylose and 80% amylopectin.
Amylose is a linear polymer made up of D-glucose units and it is an ideal polymer for plastics.
Amylopectin is a soluble polysaccharide and highly branched polymer of glucose found in plants. It not as useful as it results in a brittle plastic.
Glycerine is a simple polyol (sugar alcohol) compound. It is a colourless, odourless, viscous liquid that is widely used in pharmaceutical formulations. Its formula is C3H5(OH)3 and it has the role of the plasticiser. It is a molecular lubricant, allowing polymer chains to slide past each other.
The role of vinegar (ethanoic acid) in acid hydrolysis is to break up the branches of the amylopectin which would otherwise have made it brittle.
Hydrolysis usually means the breaking of chemical bonds by the addition of water.
Equipment List for bioplastic practical
Cornflour 15ml (1tbs);
Hot plate or Bunsen;
Chopping board/aluminium foil.
1. You need to think about what sort of plastic you want. Do you want it to be strong or brittle? Do you want it to be stiff or flexible? It is the ratio of glycerin/starch that will dictate the properties. If you want a brittle plastic – add more starch. For a more flexible plastic, add more glycerin. Otherwise follow the amounts used on the equipment list.
2. Measure out your starch, water, glycerine and vinegar and add them to your pot. Make a note of what you use if you don’t stick to the original list.
3. Stir your mixture thoroughly before turning on the heat—you should have a milky-white solution. Continually stir the mixture until it is a clear gel. This should take a few minutes.
4. When your mixture is a clear gel, you can scrape the plastic onto a flat surface or into a mould. You can spread it as thin or as thick as you like. Remember to turn off the heat! You can experiment with the shape and even make a mould to pour the plastic into.
In the picture below the mixture is being heated on a hot plate at a temperature of about 100oC and it is being stirred by a magnetic stirrer. It was stirred manually as well. Some green colour was added for a green plastic.
Setting of plastic should take around 24 hours.
An instructional video showing the exact practical can be found here: http://www.youtube.com/watch?v=LskPNBvbuqM&feature=related
Further information and alternative recipes can be found here:
Design a product utilising bioplastic such as mobile phone cases, school stationary (pens, rulers, setsquares etc) or plates/mugs.
Measure the Young’s modulus of a variety of green plastics by measuring tensile stress/strain.
Record the rate at which the bioplastic decomposes compared to a range of differently-sourced plastics over a period of weeks.