Understanding Plastics: Types, Properties, and Forming
Plastics consist of long chains of molecules of hydrogen and carbon. These chains are called polymers, which is another name *for* plastics. Plastics can be divided into two types depending on their origin.
Natural Plastics
Natural plastics are obtained from vegetable or animal sources. Examples include:
- Latex and cellulose from plants
- Casein, a protein found in milk
Synthetic Plastics
Synthetic plastics are made with compounds that come from petroleum, natural gas, and carbon. Most plastics that we use today are synthetic.
Transformation of Plastics
The industrial production of plastic from raw materials is called polymerization.
During the manufacturing process, fillers may be added to reduce the cost of production and enhance certain properties of the raw materials. Examples include:
- Fiberglass
- Paper
- Sawdust
Chemical additives can also be used, such as plasticizers, which increase the flexibility and resistance of the final product.
Properties of Plastics
Depending on their composition, plastics can be:
- Resistant
- Insulators
- Ductile
- Malleable
- Light
- Impermeable
- Recyclable
Recycling Plastics
Plastics can be recycled in many ways:
- Chemical recycling: Breaks plastic down into its chemical components.
- Mechanical recycling: Breaks plastic into small pieces that can be melted.
- Energy recovery: Burns plastic to produce heat, energy, or electricity.
Biodegradability of Plastics
Non-biodegradable plastics: These are made from petroleum products. Most of the plastics we use today are non-biodegradable.
Biodegradable plastics: These are becoming more common. Some biodegradable plastics are broken down by bacteria and other biological agents. An example is Biopol.
Types of Plastics Based on Thermal Behavior
- Thermoplastics: These are composed of polymer chains that are weakly connected to each other.
- Thermosetting: These are composed of polymer chains that are strongly connected. They can be heated and shaped *only* once.
- Elastomers: These are composed of polymer chains that are laterally connected. They can be folded or rolled into a ball, like string.
Plastic Forming Techniques
Extrusion
- Thermoplastic material is added to an extrusion machine. These go into a heated cylinder where they melt and form a thick liquid.
- Inside the cylinder, there is a screw that turns and pushes the plastic out through a die.
- When the plastic leaves the machine, it is hardened in a bath of cool water.
Examples: Tubes, water pipes, insulation for cables.
Calendering
Thermoplastic material is passed between rollers to make thin sheets. The last set of rollers ensures the plastic has the proper thickness. These rollers may also give the surface a shiny or matte finish.
Examples: Kitchen countertops and cupboards.
Vacuum Forming
- A sheet of thermoplastic is placed over a mold.
- The sheet of plastic is heated until it becomes soft.
- The air under the sheet is sucked out to create a vacuum. This vacuum pulls the sheet into the mold, which gives it the shape.
- The sheet is cooled and removed from the mold.
Examples: Shop signs, ice trays, car dashboards.
Molding
Blow Molding
- A tube of thermoplastic material is produced by an extrusion machine. Then, this tube is introduced into a mold.
- The mold is filled with compressed air, which pushes the plastic against the sides of the mold. This gives the plastic its shape.
- When the plastic cools, it is removed from the mold.
Examples: Bottles and hollow toys.
Injection Molding
- Melted thermoplastic is introduced into a mold.
- After the material cools down and hardens, it is removed.
Examples: Buckets and household items.
Compression Molding
Compression molding is done with a machine called a press.
- Thermosetting plastics are introduced into a cavity mold.
- Heating elements soften the plastic while it is compressed by the other side.
- The plastic is shaped by the molds on both sides.
- After the molds cool, the plastic object is removed.
Examples: Containers and outer casings.