Plastics

Plastic is found everywhere in our society today. Almost all plastic is made of oil that’s pumped up from the ground. Oil is a mixture of different substances. The substances can be separated in a refinery. Then you get: ethylene and propylene.

This is what their molecules look like. It’s possible to reassemble the molecules into giant molecules, polymers. Poly means many and .. ..mer means part. Many parts. Ethylene becomes polyethylene and propylene becomes polypropylene.

These are two common examples of polymers that can be shaped - plastics. There are two main groups of plastics. Those that can be melted and reshaped - thermoplastics .. ... and those that don’t melt, and can’t change shape once they have solidified - thermosetting plastics. Polyethylene and polypropylene are thermoplastics.

When produced, they are first formed into small grains, granulates. When we want to make something in plastic, there are various ways of melting and shaping the granulates. One way is to use a barrel with a large screw inside. When the screw moves, the granulates are forced forward. Further into the barrel, the screw gets wider.

This causes the granulates to be pressed against the wall of the barrel. The friction, the pressure, and the heat cause the granulates to melt. At the end of the barrel is a die with a certain shape. The molten plastic is extruded through the shaping die and takes on that shape. This method is called extrusion.

With this method you can produce round things like rods and tubes, or flat things like sheets, or other shapes. Plastic bags are made with a die shaped like a ring.. .. through which air is blown, so that the plastic becomes a long thin tube, which is rolled up, and can then be divided into bags of the size that’s needed. Another method is injection moulding. This works in a similar way to extrusion.

But instead of the plastic being extruded through a die it’s injected into a mold. When the plastic solidifies, the mold is opened. This method is used to manufacture, chairs, toys, shells for mobile phones, disposable cutlery, lego pieces, and more.. Another variant is blow molding where a tube of plastic is extruded.. ..into a mold. And then air is blown into the mold so that the plastic is pressed against the walls.

This is how you make plastic bottles, for example. Instead of extruding a plastic, you can press it into the shape needed. First, a plastic sheet is heated so that it becomes easy to shape. Then it’s common to do one of these two things: Either put the plastic sheet in a mold, and suck the air out of the mold, so that the soft plastic is pressed against the walls of the mold. Or push the mold against the plastic sheet, and then suck the air out.

Both techniques are called thermoforming. This method can be used to manufacture, for example, ice cream cartons and yoghurt pots. Since it has been so easy to shape so many different, useful things from plastic, it’s not surprising that we have produced so much - so much that it has now become a problem.