Science Forums 2

[HydrogenBond]

Plastics are polymers or long molecules made up of repeat units. For example, polyethylene is long molecule made of ethylene units. The repeat unit is called a monomer. The long chain of monomers, like cars in a train, is called a polymer. Some plastics have two or more different repeat or monomer units allowing a wide range of properties. Proteins are natural polymers, sort of natural plastics, using animo acids as monomers.

There are different ways to make common plastics. Many use a thing called an initiator, which reacts to or grabs one of the monomers and turns the other end into an active site. This allows the next monomer unit to attach, with it then ending up with the active site, etc. The chain reaction is like cars being added to a train.

One way to adjust some properties of any given polymer is by adjusting the concentration of initiator. If there is only a little there will only be a few very long chains forming. This version is very viscous tough plastics. If we add extra initiator, there are more little chains forming at the same time before we use up all the monomer. The result are more smaller chains. This makes the plastic less viscous or more pliable. Depending on what kind of properties we need to make we can adjust the plastic this way.

One of the most used plastics is acrylic. This is polymethymethacrylate. This is made with a process called emulsion polymerization. One of the initiators often used, is the active ingredient in some acne medications; Benzoyl peroxide. The monomer unit is soluble in water. But once the acrylic plastic forms it is not soluble and begin to separate out. You need to stir real good so the little active ends can find the monomers. The result is like mixing water and oil (plastic) to get an emulsion. Typically, you add the monomer and water. Then carefully calculate the amount of initiator needed to get the chains long enough to give the properties you want. Once you add that you stir the heck out of it and its gets thicker and thicker until it is done. With paint they will add other ingredients. Contact lenses like it pure.

Once a plastic is made if often dried and transported as a powder. Next, we need to shape it into something useful like a plastic cup. One technique is called extrusion. The plastic powder is fed into a machine that has worm screws that convey, grid and pressurize the powder until it becomes a fluid. At the end of this machine are molds in the shape you need to make the product. The plastic is squeezed under pressure into the mold, the mold is then cooled and the part released. The result can be a plastic cup, dish, pipe, or computer case, depending on mold and plastic chosen.

The extruder sounds simple, but is complicated because plastics are weird because they are not newtonian fluids. As you add pressure the material doesn't act normally but has strange compression properties. It takes the study called rheology to figure out how to balance the feed rate and the pressure to make the material flow out right for zero defects.