The term ‘plastic’ derives from the Greek word ‘plastikos’ meaning ‘capable of being shaped or molded’. Plastic is a substance made up of synthetic or non-synthetic organic compound i.e., chains of carbon atoms, ‘pure’ or with the addition of oxygen, nitrogen, or sulfur. Each polymer chain comprises many repetitive units, formed from monomers (the chemical building blocks which form polymers). These different types of plastic are used daily for many purposes.
Thermoset or Thermosetting Plastics
It is formed by hardening a soft solid or a viscous liquid prepolymer or resin. It is hardened by curing the soft solid. Curing is induced by heat or suitable radiation and it can be enhanced by applying high pressure, or mixing with a catalyst. It results in extensive cross-linking between polymer chains to produce an infusible and insoluble polymer network. Once cooled and hardened, these plastics form permanent shapes and cannot return to their original form. They are hard and durable. Thermosets are used for auto parts, aircraft parts, and tires. Examples include polyurethanes, polyesters, epoxy resins, phenolic resins, etc.
Thermoplastics are less rigid than thermosets. These are chemically reversible. Thermoplastics can soften upon heating and return to their original form. It can be solidified on cooling. They are molded easily and extruded into films, packaging, and fibres. It includes examples like polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC).
Polyethylene Terephthalate (PETE or PET)
PETE is formed by esterification between terephthalic acids and ethylene glycol. These are thermoplastics. They are semicrystalline and are transparent to visible light and microwaves. This type of plastics is lightweight, durable, and has moisture barriers properties. PET is widely used in the world. It is used predominantly as a fibre and for bottling or packaging. PET is the plastic used for bottled water and is highly recyclable.
It is a widely used and durable thermoplastics which is formed by addition or radical polymerization of ethylene (olefin) monomer. Ziegler-Natta is used as a catalyst commonly to carry out polymerization. There are different variants of polyethylene. Low-density polyethylene (LDPE) and high-density polyethylene (HDPE) are the two most commonly used. The material properties vary across the different variants.
- LDPE: It used for plastic bags in grocery stores. It has high ductility but low tensile strength.
- HDPE: This rigid plastic is used for packagings like laundry detergent containers, construction applications, or trash bins.
- UHMW: It is an extremely strong plastic that can exceed steel in strength. It is used for applications like medical devices (e.g. artificial hips).
Polyvinyl Chloride (PVC)
It is the world’s third most-produced synthetic polymer after polyethylene and polypropylene. It is widely used for construction applications. Polyvinyl Chloride is made from one of three emulsion processes:
- Suspension polymerization
- Emulsion polymerization
- Bulk polymerization
PVC is very dense compared to most plastics and are readily available. It has two forms: rigid and flexible. Rigid PVC has good tensile strength. These are also used in 3D printing, CNC machining, and injection molding. Flexible PVC is used for plumbing, insulation of electrical wires, and vinyl siding, etc.
Polypropylene is made from the polymerization of propylene monomer by Ziegler-Natta polymerization process. These are lightweight, rigid, and semi-crystalline thermoplastics and are highly flammable. It is semi-transparent, has a low-friction surface, and doesn’t react well with liquids. It has good electrical resistance. Polypropylene is used in a variety of applications like consumer products packaging, plastic parts for the automotive industry, special devices like living hinges, and textiles.
Polystyrene is amorphous, colorless, and transparent commodity thermoplastic. It is rigid, brittle, and relatively hard. It has good electrical properties. It can also be radiation sterilized. These are produced with the help of styrene monomers. Polystyrene is used widely under the trade name “Styrofoam” in packaging. It is also available as a naturally transparent solid commonly used for consumer products like medical devices including test tubes or Petri dishes.
Polylactic Acid (PLA)
Polylactic Acid is unique in relation to the other plastics as it is derived from renewable resources. It biodegrades much quicker than traditional plastic materials. It is used in food packaging and medical implants that biodegrade within the body over time. Like most plastics, it has the potential to be toxic if inhaled or absorbed into the skin or eyes as a vapour or liquid.
Polycarbonate is a transparent material known for its particularly high impact strength compared to other plastics. The polycarbonate is manufactured by condensation polymerization between bisphenol A and either carbonyl chloride or diphenyl carbonate. Polycarbonates used in engineering are strong and tough materials. And some grades are optically transparent. They are easily worked, molded, and thermoformed. It is used in greenhouses insulation where high transmissivity and high strength are both required.
Acrylic is best known for its application in optical devices. Acrylic plastic polymers are formed by using a monomer methyl methacrylate. It is a tough, highly transparent material with excellent resistance to ultraviolet radiation and weathering. It can be molded, cut, drilled, colored, and formed. It is used in many applications like aeroplane windshields, shelves, automobile taillights, indoor and outdoor signs, etc.
Acetal (Polyoxymethylene, POM)
Acetals are formed when a ketone or aldehyde reacts with an alcohol in the presence of an acid catalyst. Acetal is a very high tensile strength plastic with significant creep resistant properties i.e. there is a gradual deformation of its shape under mechanical stress. It is known for its high resistance to heat, abrasion, water, and chemical compounds. It is widely used in the automotive and electronic industries.
Nylon is synthesised via a condensation polymerization reaction between the appropriate monomers. It is used for various purposes like clothing, ropes or threads, reinforcement in rubber material like car tires, and for a number of injection molded parts for vehicles and mechanical equipment. It is often used as a substitute for low strength metals in applications like car engines because of its high strength, high-temperature resilience, and high chemical compatibility. It is very useful for applications that utilize gears.