Plastics are made of polymers, long, spaghetti-like chains of molecules made of organic material, which are based on carbon, oxygen, and hydrogen atoms. For many polymer mixtures, sharkskin forms, marring the desired appearance and texture of the plastic. Several fixes for sharkskin exist, but none are entirely satisfactory for industry. Sharkskin can be prevented by pushing the polymer slowly through a die, but this is too inefficient for companies wishing to make plastics quickly. It can be prevented by manufacturing the plastic through highly controlled conditions, but this is expensive and impractical for mass-production.
Manufacturers often use anti-sharkskin additives, but until now it was unclear why they work.
Sending polyethylene through a transparent sapphire tube, Migler and colleagues used a high-speed video microscope to watch what happens to polyethylene as it forms sharkskin. With these direct observations, researchers have seen that the polymer undergoes extreme stretching as it passes through the exit hole of the tube, causing the material to rupture. The polymer splits into two parts, one consisting of the surface of the polymer, and the other consisting of its buried, inner core. The surface of the polymer actually passes slowly, as it sticks to the wall of the tube, but the polymer core passes through quickly. The surface of the polymer accumulates near the wall of the tube and then peels off.