[-O-(C₆H₄)-C(CH₃)₂-(C₆H₄)-O-CO-]_n — Polycarbonate

Polycarbonate (PC) is a durable, transparent thermoplastic polymer characterized by carbonate linkages \((-O-CO-O-)\) in its backbone. It was first developed in the 1950s by researchers at Bayer and General Electric. Owing to its combination of high impact resistance, thermal stability, and optical clarity, polycarbonate has become one of the most versatile engineering plastics.

Unlike glass, which is brittle, polycarbonate is lightweight yet nearly unbreakable, making it ideal for safety glasses, bulletproof windows, compact discs, and automotive parts. It can be easily molded, machined, and thermoformed, which enhances its industrial usability across a range of applications.

The molecular structure of polycarbonate contains repeating units with aromatic rings and carbonate groups that give it rigidity, toughness, and high transparency. The general repeating unit is represented as:

The most common type of polycarbonate is produced from Bisphenol A (BPA) and phosgene (COCl2). The presence of aromatic phenyl rings imparts rigidity, while the carbonate linkages contribute to flexibility and resistance to deformation under stress. This unique structure allows the material to exhibit both high strength and elasticity.

Polycarbonate is considered an amorphous polymer, meaning it lacks a crystalline structure. This property contributes to its high optical clarity, making it suitable for lenses and transparent panels.

Polycarbonate is primarily synthesized by a condensation polymerization reaction between bisphenol A (BPA) and phosgene (COCl2). The process involves the formation of carbonate linkages with the release of hydrochloric acid (HCl) as a byproduct:

This reaction can occur via two main industrial methods:

• Interfacial Polymerization: The reaction takes place at the interface of an aqueous phase containing bisphenol A and an organic phase containing phosgene. A base such as sodium hydroxide is used to neutralize the HCl formed.
• Transesterification Process: In this solvent-free method, diphenyl carbonate is used instead of phosgene, reacting with bisphenol A at elevated temperatures to form polycarbonate and phenol as a byproduct:

The transesterification route is considered safer and environmentally friendlier, as it avoids the use of toxic phosgene gas.

Polycarbonate exhibits a unique balance of toughness, transparency, and dimensional stability, making it superior to many other thermoplastics:

• Transparency: Polycarbonate transmits over 90% of visible light, comparable to glass but without brittleness.
• Impact Resistance: It is 200–250 times stronger than glass and 30 times stronger than acrylic.
• Thermal Stability: Retains mechanical properties between –40°C and 140°C, with a glass transition temperature of about 150°C.
• Electrical Insulation: Excellent dielectric strength, making it suitable for electrical housings and components.
• Weather Resistance: UV-stabilized polycarbonate grades resist degradation under sunlight.
• Chemical Resistance: Resistant to dilute acids and alcohols but sensitive to strong alkalis and organic solvents like acetone.

Because of its toughness and optical properties, polycarbonate is often used as a replacement for glass in demanding environments.

Polycarbonate’s combination of transparency, heat resistance, and impact strength makes it invaluable across multiple sectors:

• Automotive Industry: Used in headlamp lenses, instrument panels, and safety glazing.
• Electronics: Used in compact discs (CDs), DVDs, optical lenses, and electrical enclosures due to its clarity and insulating properties.
• Construction: Used in roofing sheets, skylights, and safety barriers due to its light weight and impact strength.
• Medical Devices: Used in blood oxygenators, surgical instruments, and safety goggles because it is sterilizable and biocompatible.
• Consumer Products: Used in water bottles, eyewear lenses, kitchenware, and protective gear.
• Aerospace and Defense: Used in bulletproof windows, visors, and cockpit canopies for its transparency and durability.

Additionally, polycarbonate blends (such as PC/ABS) are used in electronic housings and structural components for improved toughness and thermal stability.

Polycarbonate is recyclable and identified by the recycling code #7 (Other). It can be reprocessed into lower-grade products or chemically depolymerized to recover bisphenol A and carbonate monomers. However, the presence of Bisphenol A (BPA) in traditional polycarbonate has raised environmental and health concerns due to its potential endocrine-disrupting effects.

To address this, industries are developing BPA-free polycarbonates using alternative monomers like isosorbide, a bio-based compound. Despite these challenges, polycarbonate remains a sustainable and versatile plastic when managed properly through recycling and responsible production methods.

Moreover, the material’s long life span and durability contribute to energy savings in applications such as construction and transportation by reducing replacement frequency.