Natural Rubber

Natural rubber is usually derived from the sap of the Brazilian Pará rubber tree (Hevea brasiliensis). In order to make this material, the milky white fluid that fills the tree’s cells (latex) is ‘tapped’ by cutting a gash in the bark of the rubber tree so that it oozes out freely. This flowing vegetable fluid is collected in cups, coagulated by adding an acid (usually formic acid), passed through heavy rollers to squeeze out any excess water, and then dried. This caramel-coloured chunk of rubber has been dried on a textured surface, and has taken on its criss-cross pattern.

The resulting rubber material is waterproof and very stretchy, but in its raw state it also has a tendency to quickly rot and ‘perish’. Natural rubber loses its ability to stretch and becomes hard and brittle when exposed to cold temperatures, light and air (see Perished Rubber Band). The inevitable deterioration of natural rubber can be halted by a process called vulcanization (see Vulcanized Rubber Cube).

Rubber as a material, and rubber bands in particular, are often used to explain entropy (a measure of disorder used in physics) and the second law of thermodynamics, which states that entropy in the universe will either stay constant or increase. When it is tapped from the tree as a liquid, the major constituent of liquid rubber is a small carbon-based molecule called isoprene. As the liquid latex dries, the isoprene molecules begin to bind to one another and form long chains called polyisoprene. When a rubber band is relaxed (i.e. not stretched), these long chains are intertwined and entangled like a plate of cooked spaghetti. They are quite happy to sit coiled up together inside the rubber band due to an electrostatic attraction between them. When the rubber band is stretched, the individual polymer chains are untangled and line up (like a pack on uncooked spaghetti). The molecules inside the rubber therefore become much more ordered, and the entropy of the system is decreased. Because entropy always tends to increase, as soon as you release the rubber band, all the polyisoprene chains pull themselves back into their more disordered, tangled, plate-of-spaghetti formation, and the rubber band pings back to its original shape.

This particular piece was collected during the filming of the BBC TV series ‘Secret World of Materials’, featuring Institute of Making Director Mark Miodownik.