In a box of childhood rubbish, I discovered a rubber pencil eraser and a plastic disc 鈥� once fired out of a toy Klingon spaceship 鈥� that over the years had somehow welded themselves together (see Photo). There was some transitionary rubber/plastic substance at the join. How did this happen?
鈥� I found details of the eraser on the manufacturer鈥檚 internet site and it is composed of PVC not rubber. It is not obvious what the disc is made of, but it is probably a commodity polymer such as PVC or polyethylene. To make the eraser flexible it would be necessary to add a 鈥� a low-molecular-weight substance which is dissolved in the polymer.
Over time the plasticiser can leach out to the surface. On contact with the disc it would have acted as a solvent, causing the two substances to stick together as parts of the polymer chains diffuse across the interface between them. This 鈥渋nterdiffusion鈥� leads to entanglements, physically bonding the two materials together.
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The level of penetration of the ends of the polymer chains depends on precisely which polymers are involved and the contact time between the two materials. A similar method is used to assemble plastic model kits: applying a suitable solvent to polystyrene surfaces slightly dissolves the polymer, after which the parts are pressed together, the solvent evaporates and a bond is formed.
In the case of the eraser, the solvent, in the form of the plasticiser, does not evaporate but instead migrates from the eraser into the disc, and this is what forms the transitory substance at the join.
Michael Nugent, Polymer Engineering Department, Athlone Institute of Technology, Westmeath, Ireland