Material Properties of Rubber
Rubber is a unique material which behaves very differently to other engineering materials such as plastics and metals. It is very “bouncy” and has a high elasticity – meaning if it is stretched it will return to its original shape. Other important properties of rubber are its flexibility, strength, permeability to water (not soluble in water) and high abrasion resistance. Rubbers also tend to have high melting and boiling points.
The behaviour of a material is governed by its chemical structure and the behaviour of its molecules. Normally, elastomers require vulcanisation before they can be used. Unvulcanised rubber has a similar consistency to chewing gum – it is not strong; it does not return to its shape after a large deformation and becomes sticky when hot. As rubber warms up, the polymer chains flow over each other. This is a useful property as it allows us to easily shape the rubber. This quality however is usually less desirable in the final rubber product and so the rubber can be cured to change its material properties. Vulcanisation causes crosslinks to form between molecules and stops the flow of chains over each other.
Vulcanised rubber is very elastic. Figure 1 illustrates the behaviour of the crosslinked rubber under tension. The crosslinks prevent the chains from flowing relative to each other and will pull the polymer chains back into their original position once the load is removed. Some rubbers may be stretched up to 10 times their original length and will still return to their original size and shape once the external force is removed. Another very important quality of elastomers is that they have a very low compression set. This means that an elastomer will return to its original dimensions once the force is removed even if it has been under compression for a long period of time.