The Science of Plastics: Properties, Types, and Recycling

What Are Plastics? Origin and Composition

Plastics originate from fossil fuels (coal, oil, natural gas). They were formed millions of years ago by the decomposition of organic matter. The main chemical elements composing plastics are carbon, hydrogen, and oxygen.

Properties of Plastics

• Lightweight
• Impermeable and waterproof
• Thermal insulator
• Electrical insulator
• Corrosion resistant
• Resistant to many chemical substances
• Easily formable/moldable
• Tough (not so fragile)
• Little resistance to strain (low tensile strength)
• Photodegradable (in some types)

Advantages of Plastics

• As they are lightweight, significant energy is saved in the transport of plastic objects or products packaged in plastics.
• Having a low melting point means less energy is consumed during the manufacturing process.
• As thermal insulators in homes, they help save energy on heating and cooling.

Disadvantages and Environmental Impact

• They are not biodegradable like wood or natural fibers; therefore, as waste, they do not naturally disappear until after hundreds of years.
• Some types of plastics can be very toxic if they are eliminated by burning them in incinerators.
• Due to the great variety of plastics placed into recycling containers, sorting and recycling is more difficult than for other materials such as glass, paper, or metals.

Plastic Recycling Methods

Selective Recycling Process

In selective recycling, plastics are separated by type. The process involves:

1. Crushing the plastics.
2. Cleaning them.
3. Melting them down.
4. Manufacturing new objects with almost the same quality as the originals.

Non-Selective Recycling

This process is similar to selective recycling but without separating plastics by type. It is the cheapest method, but the resulting plastic obtained is of lower quality than the original and often cannot be used for high-grade applications.

Main Categories of Plastics

• Thermoplastics
• Thermosets (Thermoestable plastics)
• Elastomers

Common Plastic Types and Uses (Resin Codes)

1. PET (Polyethylene Terephthalate): Plastic bottles
2. HDPE (High-Density Polyethylene): Buckets, containers
3. PVC (Polyvinyl Chloride): Plastic tubes, pipes
4. LDPE (Low-Density Polyethylene): Plastic bags, films
5. PP (Polypropylene): Stoppers, caps, food containers
6. PS (Polystyrene): Yogurt packs, disposable cutlery
7. PC (Polycarbonate): Helmet visors, CDs
8. PA (Polyamide/Nylon): Guitar strings, textiles
9. PMMA (Polymethyl Methacrylate/Acrylic): Car lights, transparent sheets
10. PTFE (Polytetrafluoroethylene/Teflon): Non-stick coatings (e.g., skillets)

Why Do Thermoplastics Have Numeric Codes?

They have numeric codes (Resin Identification Codes 1-7) to differentiate them, which is crucial for effective sorting and recycling.

Bioplastics: Definition and Concerns

Bioplastics are plastics obtained from vegetable sources (biomass).

Associated Concerns/Disadvantages

The production of bioplastics raises concerns regarding the potential lack of fertile soil (due to land use competition) and the overall environmental impact of large-scale agricultural production.

General Steps in Plastic Object Manufacturing

1. Place the plastic material (sheet, granules, etc.) into the mold.
2. Heat the plastic until it is melted.
3. Shape the material using the mold.
4. Cool the object.
5. Remove the finished object from the mold.

Specific Manufacturing Techniques

• Compression Molding
• Extrusion
• Injection Molding
• Blow Molding
• Vacuum Forming
• Calendering
• Rotational Molding
• Immersion Molding