MaterialsInMotion

I don’t know why I assumed that everything made of plastic was produced in a similar way, but I was shocked to find out that I was very wrong. In fact, depending on the end product’s size, shape, purpose, and other factors, the polymers used to produce these products will need to be processed in different ways. Today, Professor Kolodka introduced five of the most common methods of polymer processing: extrusion, injection molding, blown film extrusion, blow molding, and rotomolding.

Extrusion uses a long machine called an extruder that uses a rotating screw to melt polymers into continuous shapes like pipes, tubing, and wire insulation. Injection molding is best for small oddly-shaped parts, and it forces molten polymers under high pressure into steel molds where they cool into the shape of the mold cavities. Blown film extrusion inflates melted polymers into giant bubbles to create thin film products like trash bags and saran wrap. Blow molding is a similar process but instead makes rigid 3D products like plastic bottles and jars. Rotomolding involves melting pellets inside a large hollow mold that rotates along two axes to create uniform, durable products like water tanks and kayaks.

The most interesting thing I learned was that most plastic food containers have several layers that are cured with different additives and that the layer that directly contacts the food has to be virgin (freshly produced) rather than recycled. For example, a plastic bottle is made of three layers: one that resists oxygen from diffusing into the bottle to keep the drink fresh, one that resists carbon dioxide from diffusing out of the bottle to keep the drink carbonated, and the virgin layer that directly contacts the drink. What I don’t understand, though, is why the layer contacting the food has to be virgin. Professor Kolodka explained that it became a regulation because producing new plastic is cheaper than recycling and because people don’t want to drink from plastic that was potentially from another person’s drink, but to me, the second reason seems flawed. The temperature necessary to melt plastic during recycling kills most, if not all, germs that could’ve originally been on the plastic, so how would cross-contamination even occur?