Understanding plastic 'sharkskin': Results may pave way for eliminating problems in low-cost plastics

Now, polymer physicists at the National Institute of Science and Technology have made new insights into the causes -- and solutions -- for sharkskin. They reported their findings at last week's meeting of the Society of Rheology in Hilton Head Island, South Carolina. These insights may help manufacturers to control -- and eventually eliminate -- the problem. Studied since the World War II, sharkskin is actually a problem in many plastic manufacturing techniques. Plastics are made of polymers, long, spaghetti-like chains of molecules made of organic material, which are based on carbon, oxygen, and hydrogen atoms. Workers make many plastic products in a process similar to pushing pasta through a pasta maker: they force molten polymer mixtures through small holes in a die. 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.

Kalman Migler and his colleagues have performed new experiments that investigate how sharkskin forms -- and how it can be prevented. 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. Migler and colleagues conclude that this peeling-off of the surface polymer creates the ridges. This picture supports a 25-year-old theory, advanced by Frederic Cogswell, then at ICI (a leading manufacturer of paints and other specialized materials), but very difficult to test directly until now.

In addition, the researchers directly observed why one particular anti-sharkskin additive is good at preventing sharkskin. The additive, known as a fluoropolymer, was mixed with the polyethylene as the combination traveled through the sapphire tube. (A fluoropolymer is a polymer in which some of the hydrogen atoms are replaced by fluorine atoms.) In their observations, the researchers found that the polyethylene slips over the fluoropolymer, rather than sticking at the walls. This dramatically reduces the extreme stretching at the exit, thus inhibiting the formation of sharkskin.

"This is a long-standing and very interesting problem," says Mort Denn, professor of chemical engineering at the City University of New York and editor of the Journal of Rheology. "Kalman is doing nice work, as are others."