Common Plastics and Uses

Plastics are synthetic high-molecular-weight polymers engineered from petrochemical precursors. In basic chemistry, commercial plastics are classified based on their polymerization mechanism—either addition (chain-growth) or condensation (step-growth)—and their thermal behavior, which dictates their industrial applications.

• Thermoplastics: Composed of linear or branched chains held together by weak intermolecular forces. They melt into a viscous liquid when heated and solidify when cooled, allowing for repeatable mechanical recycling.
• Thermosetting Plastics: Formed via extensive cross-linking between polymer chains during heat curing. This establishes permanent primary covalent bonds, meaning they cannot be remelted or reshaped and will instead undergo thermal charring upon reheating.

Commodity Addition Polymers and Their Applications

Addition polymers are formed by the direct linkage of unsaturated monomer units (containing double or triple carbon-carbon bonds) without the elimination of any molecular co-products.

Low-Density Polyethylene (LDPE)

• Chemical Structure: Synthesized via free-radical addition polymerization of ethene (CH₂ = CH₂) under extreme pressures (1,000–3,000 atm) and temperatures (350–570 K) with oxygen or peroxide initiators. This process creates high levels of chain branching, preventing the molecules from packing tightly.
• Key Properties: Low density, high flexibility, excellent chemical inertness, and moisture resistance.
• Major Uses: Squeeze bottles, flexible food packaging films, garbage bags, bubble wraps, and flexible protective coatings for electrical wiring.

High-Density Polyethylene (HDPE)

• Chemical Structure: Synthesized from ethene monomers at lower pressures (6–7 atm) and temperatures (333–343 K) using coordination catalysts, specifically Ziegler-Natta catalysts (a mixture of TiCl₄ and (C₂H₅)₃Al). This results in a highly linear chain structure that allows dense crystalline packing.
• Key Properties: High tensile strength, rigidity, chemical resistance, and a higher melting point than LDPE.
• Major Uses: Heavy-duty milk jugs, chemical containers, household detergent bottles, agricultural drainage pipes, and playground equipment.

Polyvinyl Chloride (PVC)

• Chemical Structure: Produced by the addition polymerization of vinyl chloride (CH₂ = CHCl) monomers. The highly electronegative chlorine atoms generate strong dipole-dipole interactions, making the raw polymer inherently rigid and brittle.
• Plasticizers: To make PVC flexible, chemical compounds called plasticizers (such as phthalates) are added. These insert themselves between the polymer chains, increasing distance and molecular mobility.
• Major Uses: Rigid PVC is used for electrical conduits, construction plumbing pipes, window frames, and gutters. Plasticized PVC is used for blood transfusion bags, intravenous tubing, and raincoats.

Polystyrene (PS)

• Chemical Structure: Formed by the addition polymerization of styrene monomers (vinyl benzene). The bulky pendant phenyl groups restrict chain rotation, making the solid polymer transparent, highly rigid, and brittle.
• Expanded Polystyrene (EPS): Commercially known as Styrofoam, it is produced by introducing blowing agents (like pentane gas) into the polymer matrix during molding, creating a cellular foam that is 95% air.
• Major Uses: Rigid PS is used for clear CD cases, laboratory petri dishes, and disposable plastic cutlery. Expanded PS is used for thermal insulation panels, hot beverage cups, and shock-absorbing protective packaging for electronics.

Polypropylene (PP)

• Chemical Structure: Synthesized from propylene (CH₂ = CH-CH₃) monomers. Commercially utilized in its isotactic form, where all the methyl (-CH₃) groups are aligned on the same side of the polymer backbone, which maximizes crystallinity.
• Key Properties: High melting point (≈ 165°C), high chemical resistance, fatigue resistance, and structural integrity under stress.
• Major Uses: Microwave-safe food containers, Tupperware, medicine bottles, automobile dashboards, disposable syringes, and thermal fabric liners.

Polytetrafluoroethylene (PTFE or Teflon)

• Chemical Structure: Synthesized by heating tetrafluoroethene (CF₂ = CF₂) with a free-radical or persulfate catalyst under high pressure. The carbon-fluorine (C-F) covalent bond is one of the strongest in organic chemistry.
• Key Properties: High thermal stability, near-complete chemical inertness to acids and organic solvents, and an extremely low coefficient of friction.
• Major Uses: Non-stick coatings for cookware, chemically resistant laboratory seals, gaskets, and high-frequency wire insulation.

Engineering Condensation Polymers and Their Applications

Condensation polymers are synthesized through the step-growth reaction of bi-functional or tri-functional monomers, accompanied by the elimination of small molecular co-products such as water (H₂O), alcohol, or hydrochloric acid (HCl).

Polyethylene Terephthalate (PET or PETE)

• Chemical Structure: A thermoplastic polyester synthesized by the condensation copolymerization of ethylene glycol (ethane-1,2-diol) and terephthalic acid (benzene-1,4-dicarboxylic acid) via ester linkages (-CO-O-).
• Key Properties: Exceptional clarity, mechanical strength, and gas barrier efficiency, particularly against carbon dioxide leakage.
• Major Uses: Carbonated soft drink bottles, mineral water jars, synthetic polyester textiles (fleece), and transparent food trays.

Bakelite (Phenol-Formaldehyde Resin)

• Chemical Structure: A classic thermosetting polymer obtained by the cross-linking reaction of phenol with formaldehyde. The reaction initially forms a linear polymer chain called Novolac (used in lacquers and varnishes). Further heating with formaldehyde drives irreversible cross-linking into a rigid, three-dimensional network.
• Key Properties: Infusible, chemically inert, and an excellent electrical insulator.
• Major Uses: Electrical switches, wall sockets, terminal boards, and heat-resistant handles for kitchen utensils.

Melamine-Formaldehyde Resin

• Chemical Structure: A highly cross-linked thermosetting plastic produced by the condensation copolymerization of melamine and formaldehyde.
• Key Properties: High hardness, scratch resistance, structural stability, and fire retardancy.
• Major Uses: Non-breakable, shatterproof dinnerware, domestic laminate countertops, and flame-retardant industrial coatings.

Chemical Summary of Commercial Plastics

UPSC Prelims Applied Chemistry Concepts

Plasticizers and Endocrine Disruption

Raw PVC is hard and brittle due to electrostatic dipole attractions between chlorine atoms. To make it flexible for products like medical tubing, manufacturers add plasticizers, predominantly phthalate esters. Because phthalates are not chemically bonded to the carbon backbone but are held only by weak Van der Waals forces, they easily leach out of the plastic when exposed to fats, heat, or mechanical stress. Once in biological systems, they act as endocrine disruptors by interfering with natural hormonal pathways.

Bisphenol A (BPA) in Polycarbonates

Polycarbonates (grouped under Resin Code 7) are engineered plastics favored for their high impact resistance and optical clarity, making them common in reusable water bottles and food containers. They are synthesized using Bisphenol A (BPA). Over time, exposure to high heat (such as dishwashers or microwaves) or acidic solutions breaks down the ester bonds in the polycarbonate matrix, leaching free BPA. BPA mimics the hormone estrogen, presenting reproductive and developmental risks.

The Mechanical Limits of Thermal Recycling

While thermoplastics can be remelted and reshaped, they cannot be mechanically recycled indefinitely. Each recycling cycle exposes the polymer to high thermal energy and mechanical shearing. This breaks the covalent carbon-carbon bonds along the macromolecular backbone, reducing the average molecular weight of the plastic. This degradation lowers the material’s viscosity and tensile strength, meaning recycled plastic must eventually be “downcycled” into lower-grade applications or blended with virgin petroleum resins to maintain structural integrity.