Preparing a polyurethane foam from castor oil
There are a huge range of different polymers in use around today with all sorts of useful properties. This has allowed us to create a more comfortable life for ourselves as well as helping us to make all kinds of useful materials at a low cost. One very useful class of polymers are polyurethanes, which are commonly found in foams which are used in upholstery, insulation and even in shoes. They are very useful indeed, and life wouldn’t be the same without them. Polyurethanes are typically made from precursors which are derived from crude oil, but there is growing interest in the use of renewable starting materials e.g. castor oil.
The –OH groups on the chains in the castor oil react with the MDI, which can form links between two triglyceride units. Each triglyceride has 3 –OH groups, so it is possible for cross-links to be formed between a large number of units. This results in the formation of a polyurethane foam. DABCO is an amine base which acts as a catalyst by removing protons from the –OH groups.
Points to consider:
a. Can the foam produced in the experiment genuinely be considered to be ‘renewable’?
b. What needs to be done to prepare foams with different properties?
The experiment takes very little time if you have the materials weighed out beforehand and the samples only need to be placed in an oven for half an hour.
Notes on materials needed:
Diphenylmethane-4, 4′-diisocyanate (MDI) This is fairly cheap and is available from most mainstream chemical suppliers.
1, 4-diazabicyclo [2.2.2] octane. This is fairly cheap and is available from most mainstream chemical suppliers.
The gas given off during the reaction is carbon dioxide, and there should be no toxic fumes given off (unless you overcook the foam!). Nevertheless, you may prefer to carry out the demonstration in a fume cupboard.
MDI is harmful by inhalation or ingestion. It may be harmful through skin contact. It is an eye, skin and respiratory irritant. It may cause allergic sensitization. It is stable but combustible. It is incompatible with strong oxidizing agents. It reacts violently with alcohols.
DABCO is harmful if swallowed (it is very destructive of mucous membranes). It is stable, but very hygroscopic (it absorbs water readily so store in a desiccator). It is incompatible with strong oxidizing agents and strong acids. It is highly flammable.
Wear safety glasses, gloves and ensure good ventilation.
1) Place 4g of castor oil, 3.3g of diphenylmethane-4, 4′-diisocyanate (MDI), a micro-spatula load of 1,4-diazabicyclo[2.2.2]octane (DABCO) and 6 drops of water in a boiling tube and mix thoroughly with a glass rod.
2) Heat the reaction mixture using a Bunsen burner for 30 seconds and stir until a foam is formed. The test-tube is removed from the heat at this point.
3) You will be able to observe the properties of the foam at this point, although it won’t be possible to remove it from the tube.
The tube will be unusable after this and can be disposed of with household waste, with normal precautions applying.
The activity is based on the original by Born et al http://www.rsc.org/education/teachers/learnnet/green/docs/plastics.doc. Tested by David Read (School Teacher Fellow, School of Chemistry, University of Southampton) and Rachel Hadi-Talab (Chemistry Technician, Science Learning Centre London).
These documents have been written by David Read with reference to the original material by Born, Bader and Buchholz. David Read’s role is supported by Strand 3.1 of the Royal Society of Chemistry’s ‘Chemistry For Our Future’ programme (http://www.rsc.org/Education/CFOF/).