Vulcanisation of latex
The point of the experiment is to look at the modification of the properties of a material made from a natural, renewable resource. Sulphur is added to a suspension of natural latex and the mixture is heated in a process known as vulcanisation.
Latex (also known as natural rubber) is produced by many plants, notably the rubber tree. Latex is mostly composed of poly(isoprene), a polymer of the diene isoprene:
When latex is heated with sulphur (which is composed of S8 molecules), cross-links are formed between the polymer chains in a process known as vulcanisation. A heavily cross-linked segment is shown below:
Vulcanisation is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulphur or other equivalent “curatives” or “accelerators”. These additives modify the polymer by forming crosslinks (bridges) between individual polymer chains. Vulcanised materials are less sticky and have superior mechanical properties. They are used to make hard articles such as bowling balls and saxophone mouth pieces.
Uncured natural rubber is sticky, deforms easily when warm, and is brittle when cold.
10cm x 10cm piece of aluminium foil, match stick, sticky label, weighing boat, 2g latex suspension, sulphur powder.
Liquid latex: This can be purchased at low cost from a range of vendors in the UK. You need 2g per person. It is recommended that you transfer the latex into squeezy bottles such as those used to dispense sauces in fast food outlets and cafes for ease of use.
Sulphur: Powdered sulphur should be used and it should be weighed into a weighing boat or small beaker. A glass rod may be needed to crush any small lumps into powder.
Wear safety glasses and apron/labcoat at all times. If you know you have an allergy to latex, you may wish to avoid doing this practical. Liquid latex does not normally cause problems for people with latex allergies.
Gloves are not necessary, but may be used if preferred. Ensure that the room is well ventilated
If Latex vapour is inhaled, provide fresh air, warmth and rest.
If latex touches the skin clean the areas of skin affected with soap and plenty of water.
If latex gets into the eyes, rinse immediately with plenty of water until irritation subsides.
If latex is ingested allow the patient to vomit on his/her own accord. Give copious water to drink and if necessary seek medical advice.
If latex is spilled, absorb onto sand and place in a suitable container for disposal.
Inhaling sulphur dust may irritate the mucous membranes of the respiratory passages.
In some individuals, sulphur dust has an irritant action on the skin, which may be aggravated by perspiration or moisture. Wash with mild soap and water.
Sulphur dust is capable of irritating the inner surfaces of the eyelids so irrigate with copious quantities of water.
Solid sulphur is virtually non-toxic so it can be taken internally in fairly large doses without injury.
Sulphur is flammable in air, so sources of ignition should be avoided.
Disposal of product:
The vulcanised latex can be safely disposed of with normal waste for landfill.
The experiment can be carried out with different masses of sulphur (0.1 g, 0.5 g and 1 g).
1) Shape the piece of aluminium foil into a small bowl approximately 3 cm in diameter.
2) Place the foil bowl on a balance and carefully dispense 2g of latex into it.
3) Weigh the chosen amount of sulphur into a weighing boat and pour it into the foil bowl of latex. Use a glass rod to crush out any small lumps that are present in the sulphur.
5) Mix the contents of the foil bowl thoroughly using a match stick.
6) Place the foil bowl in an oven at 140°C.
7) Remove the foil bowls from the oven after 30 minutes. If there is still liquid present, return them to the oven. Otherwise let them cool for a few minutes.
8) Repeat for the other masses of sulphur.
9) Carefully remove your vulcanised latex sample from the foil. Compare the properties of the samples.
a. Is there any link between the amount of sulphur used in vulcanisation and the properties of the material?
b. What sort of items are made from vulcanised rubber?
c. What are the advantages of using natural latex rather than synthetic material made from crude oil products?
This procedure is based on one developed by Horn, Bader and Buchholz.
The original documentation can be downloaded from:
Tested by David Read (School Teacher Fellow, School of Chemistry, University of Southampton) and Rachel Hadi-Talab (Chemistry Technician, Science Learning Centre London).