Plastic: Properties, Types, Manufacturing, and Applications

Posted on Nov 30, 2024 in Technology

The term plastic, in its most general sense, applies to substances of different natures and structures that lack a fixed boiling point and possess elasticity and flexibility over a temperature range, allowing them to be molded and adapted to different forms and applications. In a narrower sense, however, it denotes certain synthetic materials obtained by polymerization or artificial propagation of carbon atoms in the long molecular chains of organic compounds derived from oil and other natural substances.

The word “plastic” was originally used as an adjective to denote a degree of mobility and ease of acquiring a form, a meaning preserved in the term “plasticity.”

Plastic as an invention is attributed to Leo Hendrik Baekeland, who sold the first one, named Bakelite, in 1907 [citation needed]. Throughout the twentieth century, the use of plastic became extremely popular, replacing other materials in domestic, industrial, and commercial applications.

Plastics are composed of organic macromolecules called polymers. These polymers are large groups of monomers linked by a chemical process called polymerization. Plastics provide a balance of properties not achievable with other materials, such as color, lightweight, pleasant touch, and resistance to environmental and biological degradation.

In fact, “plastic” refers to a material’s condition, not the material itself. Synthetic polymers commonly known as plastics are actually synthetic materials that can reach a plastic state—viscous or fluid, lacking resistance to mechanical stress. This state is usually achieved by heating, making the material ideal for various production processes. In this state, the material can be manipulated into the forms we see today. Thus, “plastic” refers to synthetic materials capable of entering a plastic state, but not all everyday materials referred to as plastic are necessarily in that state.

Most plastics share the following characteristic properties, although some special plastics may not fulfill all of them:

  • They are inexpensive (low market cost).
  • They have low density.
  • Some plastics are permeable, while others are impermeable; diffusion occurs in thermoplastic materials.
  • They are electrical insulators.
  • They provide thermal insulation, but most cannot withstand very high temperatures.
  • They burn very cleanly.
  • They are resistant to corrosion and outdoor exposure.
  • They resist many chemical factors.
  • Some recycle better than others; some are not biodegradable or easy to recycle.
  • They are easy to work with.

The first part of plastic production is the elaboration of polymers in the chemical industry. Today, the recovery of post-consumer plastic is also essential. Part of the industry uses finished plastic directly as grain or resin. More often, various forms of casting (injection, compression, rotation, inflation, etc.) or extrusion of profiles or wires are used. A significant portion of large plastics processing machinery is made by oven operators.

  • Natural: These polymers have monomers derived from natural products with specific characteristics, such as cellulose, casein, and rubber. Two examples give rise to other plastics:
    • Cellulose derivatives: celluloid, cellophane, and Cellon.
    • Rubber products: rubber and ebonite.
  • Synthetic: These originate from man-made products, mainly derived from petroleum.

Classification by Heat Behavior

Thermoplastics

A thermoplastic is a plastic that is deformable at room temperature, melts to a liquid when heated, and hardens into a glassy state when cooled sufficiently. Most thermoplastics are high molecular weight polymers with chains connected by weak Van der Waals forces (polyethylene); strong dipole-dipole interactions and hydrogen bonding; or stacked aromatic rings (polystyrene). Thermoplastic polymers differ from thermosetting polymers in that they can be reheated and remolded after initial heating and molding. Thermosets, or thermodurics, retain their shape after cooling and do not remelt.

Their physical properties change gradually if melted and molded multiple times.

The main thermoplastics are:

  • Cellulosic resins: Derived from cellulose, the constituent material of the woody part of plants. Rayon belongs to this group.
  • Polyethylenes and Derivatives: Employ ethylene obtained from oil cracking as a raw material, which is further processed to allow for different monomers such as vinyl acetate, vinyl alcohol, and vinyl chloride. This group includes PVC, polystyrene, and methacrylate.
  • Protein Derivatives: Nylon and Perlon, obtained from diamide, belong to this group.
  • Rubber Derivatives: Examples include commercially called pliofilmes, rubber hydrochlorides obtained by adding hydrochloric acid to rubber polymers.

Thermosets

Thermosetting plastics are materials that, once heated, melted, solidified, and formed, become rigid and do not melt again. Their production usually involves an aldehyde.

  • Phenol Polymers: These are hard, insoluble, and infusible plastics. However, if an excess of phenol is used during manufacture, thermoplastics can be obtained.
  • Epoxy resins.
  • Melamine resins.
  • Bakelite.
  • Amino: Polymers of urea and derivatives. Melamine belongs to this group.
  • Polyesters: Resins from the esterification of polyols, commonly used in varnishes. If the acid is in excess, thermoplastics are derived.

Classification by Synthesis Reaction

Plastics can be classified according to the reaction that produced the polymer:

Addition Polymers

Always involve breaking or opening a monomer attachment to allow chain formation. As the molecules become longer and heavier, paraffin wax becomes harder and more tenacious. Example:

2n H2C = CH2 → [-CH2-CH2-CH2-CH2-]n

Condensation Polymers

These involve reactions between reactive functional groups present in the monomers. Each monomer must have at least two reactive groups to ensure chain continuity. Example:

R-COOH + R'-OH → R-CO-OR' + H2O

Polymers Formed by Stages

The polymer chain grows gradually as long as monomers are available, adding one monomer at a time. This category includes all Carothers condensation polymers and some other small molecules that do not separate but are gradually formed, such as polyurethanes.

Polymers Formed by Chain Reaction

Each individual polymer chain forms rapidly and then becomes inactive, despite being surrounded by monomer.

Classification by Molecular Structure

Amorphous

Amorphous plastics have molecules arranged randomly without any order. This lack of order creates gaps through which light passes, making amorphous polymers transparent.

Semi-crystalline

Semi-crystalline polymers have ordered regions along with amorphous regions. The ordered regions reduce gaps between chains, preventing light from passing unless the material is very thin.

Crystallizable

The cooling rate can decrease (quenching) or increase (slow cooling) the crystallinity percentage of a semi-crystalline polymer. Amorphous polymers do not exhibit crystallinity, although their cooling rate can be extremely slow.

Classification by Market

Another classification method is based on availability and market sector.

Commodities

These plastics have global manufacturing, availability, and demand, with international pricing and do not require high technology for production and processing.

Engineering Plastics

These materials are created for specific functions, requiring specialized technology for manufacturing or processing and have relatively high prices.

Elastomers or Rubber

Elastomers are characterized by high elasticity and the ability to stretch and rebound, recovering their original shape once the deforming force is removed. They include natural and synthetic rubber, such as neoprene and polybutadiene. Elastomers are materials with large molecules that, after being deformed at room temperature, recover their size and geometry when the deforming force is released.

Common Uses

  • Electrical insulation.
  • Industrial and consumer products (packaging, garbage bags, etc.).
  • Construction, plumbing, insulation foam, polystyrene, etc.
  • Miscellaneous industries: engine parts, body shops, toys, luggage, sporting goods, textiles, etc.
  • Electrical appliances.
  • Computers and furniture.