Understanding Plastics: Types, Manufacturing, and Environmental Impact
Plastic is a synthetic material developed in the 20th century, derived from organic compounds like petroleum and other natural polymers through chemical reactions. These reactions create large molecules containing carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. Plastics are versatile materials found in nearly all types of everyday objects. Many plastics share characteristics such as being electrical insulators, lightweight, and easy to work with.
Obtaining Plastics
Plastics are obtained from various sources:
- Natural Plant Materials: Cellulose, latex
- Animal Materials: Artificial casein
- Derived from Coal
- Derived from Natural Gas
- Derived from Petroleum
Ethylene, a molecule with two carbon atoms and four hydrogen atoms (C2H4), is a monomer. Monomers combine to form larger molecules called polymers through a process called polymerization. Polymerization is achieved by using chemical agents and initiators called catalysts. The plastic is then formed into small pellets to achieve the desired shape.
Types of Plastic
- Thermoplastics: Macromolecules arranged freely without interlacing. Heat allows them to be shaped and retain that shape upon cooling.
- Thermosets: Macromolecules intersect, forming a closed-loop network. They can only be heated and shaped once.
- Elastomers: Macromolecules are arranged in a mesh network with few links, resulting in high elasticity.
Forming of Plastics
Extrusion Molding
In extrusion molding, a worm-like machine pushes plastic granules into a hot zone where they melt. The molten plastic then passes through a shaped die to fabricate a profile. Materials used include polyethylene, PVC, and nylon.
Injection Molding
Injection molding is similar to metal casting. Molten plastic is injected under pressure into a mold. Materials used include polyurethane, polystyrene, polypropylene, and nylon.
Blow Molding and Compressed Air
This method is used to manufacture hollow pieces like toys. A plastic tube from an extruder is enclosed between two mold pieces. Hot air is injected under pressure, expanding the tube to take the shape of the mold. Materials used include polyethylene, polypropylene, and PVC.
Vacuum Forming
A thin thermoplastic sheet is placed over a mold, heated, and then vacuumed to adhere to the mold’s shape. This is used for shallow vessels like yogurt containers. Materials used include polystyrene, PVC, and acrylic plastic.
Calendering
Plastic mass is passed through rollers to flatten it into a sheet. This process is used to produce PVC sheets.
Compression Molding
Similar to forging but with plastic, this involves compressing plastic powder between two heated mold halves to take the shape of the mold. This is used to manufacture electrical equipment, toilet seats, and pot handles.
Techniques of Machining Plastics
Cutting
Use a cutter and ruler for scoring, scissors for thin plastics, thick scissors for thicker sheets, and a saw for arcs. If the plastic has a protective layer, do not remove it before sawing.
Drilling
Use a punch for soft, reinforced plastic. For hard plastics, use a drill. Holes larger than 6mm should be made in 2 or 3 steps.
Joining
There are several ways to join plastics:
- Adhesion: Using special glues for different types of plastic.
- Cohesion: Merging two plastic pieces with solvent or heat.
- Mechanical Bond: Using rivets, screws, or nuts and bolts.
Plastics and the Environment
Plastic is a stable material that degrades slowly, making plastic waste a significant environmental hazard. The best solution is to reduce the use of hazardous plastics like PVC and to recycle. Employ the 3 R’s: reduce consumption, reuse, and recycle.