Understanding Plastics: Types, Properties, and Recycling

Posted on Mar 29, 2025 in Technology

Plastics are materials made of polymers consisting of long chains of atoms that contain carbon.

Types of Plastics According to Their Origin:

  • Natural: Derived from vegetable or animal raw materials.
  • Synthetic or Artificial: A compound derived from petroleum, natural gas, or coal.

Physical Properties of Plastics

These include hardness, elasticity, rigidity, toughness, and flexibility, which are specific properties of certain plastics and vary from each other.

Environmental Properties of Plastics

Most plastics are not biodegradable and are often cremated to dispose of them. However, there are also biodegradable plastics that break down naturally, such as biopol.

Plastic Recycling Processes

Plastic materials can be subjected to a recycling process. There are two main types:

  • Chemical Recycling: This involves recovering the original constituents to obtain new materials.
  • Mechanical Recycling: In this process, the materials are crushed to obtain granules to be used to produce new plastic.

Types of Plastics by Their Response to Heat

Thermoplastics

Most thermoplastic compounds are obtained from petroleum. They consist of chains weakly bound together.

These materials soften when heated and can be molded, giving them new shapes upon cooling. This process can be repeated as many times as desired.

Examples:

  • PVC (polyvinyl chloride): pipes, shoe soles, gloves, waterproof suits, hoses
  • Polystyrene: transparent films for packaging, food packaging
  • Polyethylene: household appliances, bags, glasses, plates
  • Methacrylate: headlights, car drivers, windows, illuminated signs, goggles, watches
  • Teflon: kitchen utensils, countertop surfaces
  • Cellophane: packaging, reached base, packing
  • Nylon: tissues, toothbrushes, shoe strings

Thermosets

They consist of tightly linked chains in different directions. When subjected to heat, they become rigid and can only be heated once and not deformed.

In general, they are hard and extremely durable and are more brittle than thermoplastics.

Examples: Polyurethane, bakelite, and melamine.

Elastomers

This type of plastic is produced by vulcanization. This process, invented by American Charles Goodyear (1800-1860), consists of mixing sulfur and rubber at 160 ยบ C, giving the latter material elastic properties.

Elastomers are laterally attached by chains and folded in on themselves, like a ball. When force is applied, the chains are stretched, which gives these materials great elasticity, adhesion, and hardness.

Technical Conformation: Plastic Processing Techniques

Extrusion

  1. The thermoplastic material is introduced in the form of granules through the funnel or hopper of the extruder, which falls on a pre-heated cylinder.
  2. The cylinder consists of a spindle or large screw that moves the material, melts it, and forces it through a nozzle to mold exit.
  3. The material, while forming, slowly cools and solidifies in a cooling bath.
  4. Finally, the pieces obtained are picked up by a drive system.

Calendering

It consists of passing the thermoplastic between rotating rollers in order to obtain continuous sheets and plates. You can obtain surfaces with different finishes (matte, glossy), depending on the coating applied on the top roller.

Vacuum Forming

This technique is used, especially with large plastic sheet surfaces.

Steps:

  1. The thermoplastic material is subjected to a mold.
  2. The film is heated with a heater to soften the material.
  3. Then the air is sucked from underneath the sheet, creating a vacuum, so the material fits the mold walls and takes the desired shape.
  4. Once cooled, the mold is opened to remove the piece.

Blow Molding

Material in the form of a tube is inserted into a hollow mold whose bottom surface corresponds to the shape of the object to be produced.

  1. After closing the mold, compressed air is injected inside the tube so that the material meets the walls of the mold and takes shape.
  2. After cooling, the mold is opened and the piece is removed.

Injection Molding

Thermoplastic molten material is injected into a mold.

  1. If the material has cooled and solidified, the mold opens and the piece is removed.

Compression Molding

Thermosetting material is introduced in powder form in a female mold.

  1. A male mold is tapped against the female mold, and heated to soften the material.
  2. The material takes the form of the internal cavity in both molds.
  3. After it cools, the piece is removed.