Greg Macleod, of King's High School, asks :-

What substances are in rubber?

John Packer, a chemist at the University of Auckland and author of the book "Chemical Processes in New Zealand", responded.

Natural rubber comes from the "latex" (sticky liquid) from the rubber tree "Hevea brasiliensis". Synthetic rubbers use man-made polymeric substances instead.

The main ingredient in natural rubber is a polymer (giant molecule) made up of chains of between 1500 and 60 000 isoprene units joined together. Each isoprene unit contains five carbon atoms and eight hydrogen atoms such that each unit has a carbon-carbon double bond which partly accounts for rubber's elasticity and its reactivity in vulcanisation. By itself "polyisoprene" hardens below zero degrees Celsius and becomes a sticky mess above 80 degrees Celsius. Vulcanisation is a process whereby sulphur is added and on heat treatment the chains are joined together (cross-linked) by bridges of sulphur atoms to improve its properties.

In addition rubber contains "fillers", inert substances to impart desirable properties, (cheapen it by making it go further; colour it, e.g. china clay for white or carbon black for black; reinforcers to increase tensile strength and abrasion resistance, plasticisers to soften the rubber), "activators and accelerators" to control the vulcanisation process, and "antioxidants" to prevent the rubber deteriorating in air.

A typical mix for tyres (relative weights) is: natural rubber (100); zinc oxide (3) and stearic acid (5) - activators; carbon black (50) - reinforcer; process oil (5) - self adhesion; antioxidant (1); accelerator (0.5); sulfur (2.5) - vulcanising agent. This mixture is "cured" by heating to 148 degrees Celsius for 30 minutes.