[-O-CH₂-CH₂-O-CO-(C₆H₄)-CO-]_n — Polyethylene Terephthalate

Polyethylene Terephthalate (PET) is one of the most commonly used synthetic polymers belonging to the polyester family. It is formed by the condensation polymerization of ethylene glycol (HOCH2CH2OH) and terephthalic acid (HOOC–C6H4–COOH). The resulting polymer has repeating ester linkages that give PET its strength, flexibility, and resistance to chemicals.

PET is known for its clarity, high tensile strength, impact resistance, and recyclability. It is widely used to manufacture beverage bottles, food containers, fibers for textiles (polyester), and engineering resins. As a thermoplastic material, PET can be melted and reshaped multiple times without significant degradation, making it highly sustainable for recycling programs.

The structural unit of PET consists of alternating ethylene glycol and terephthalate units linked through ester bonds. The general formula of the repeating unit can be represented as:

Each repeating unit contains an aromatic ring (benzene) that contributes to rigidity and chemical resistance, while the ethylene group provides flexibility. PET has a semi-crystalline structure, meaning it exhibits both amorphous (transparent) and crystalline (opaque) regions, depending on the cooling rate during processing.

The presence of ester linkages (–COO–) in the backbone is responsible for its typical polyester properties — durability, resistance to stretching, and resilience under varying environmental conditions.

PET is produced by condensation polymerization between ethylene glycol and terephthalic acid or dimethyl terephthalate (DMT). The process proceeds in two main steps:

1. Esterification: Terephthalic acid reacts with ethylene glycol to form bis(2-hydroxyethyl) terephthalate (BHET) and water as a by-product:

   \( n\,HOCH_2CH_2OH + n\,HOOC-C_6H_4-COOH \rightarrow n\,HOCH_2CH_2OOC-C_6H_4-COOH + n\,H_2O \)

2. Polycondensation: The intermediate BHET further reacts at high temperatures (250–270°C) to form long PET chains, releasing ethylene glycol as a by-product:

   \( n\,HOCH_2CH_2OOC-C_6H_4-COOH \xrightarrow[]{heat} [-O-CH_2CH_2-OOC-C_6H_4-CO-]_n + n\,HOCH_2CH_2OH \)

The polymer is then extruded and cooled into pellets, which can later be melted and molded into fibers, films, or bottles.

PET exhibits an exceptional balance of physical, mechanical, and chemical properties that make it suitable for a wide variety of industrial applications:

• Strength and Durability: PET has excellent tensile and impact strength, making it ideal for packaging and fibers.
• Thermal Stability: It maintains dimensional stability up to 150°C, with a melting point around 260°C.
• Transparency: Amorphous PET is clear and glass-like, allowing it to be used in bottles and films.
• Chemical Resistance: PET resists moisture, weak acids, and alcohols but is attacked by strong alkalis and phenols.
• Electrical Insulation: It has good dielectric strength, useful in electronic components.
• Barrier Properties: Excellent barrier to gases like oxygen and carbon dioxide, extending shelf life of packaged goods.

These attributes make PET one of the most preferred materials in packaging and textiles globally.

PET’s versatility allows it to be used in multiple industries:

• Packaging Industry: Used for water bottles, soda containers, and food packaging due to its strength, transparency, and gas barrier properties.
• Textile Industry: When drawn into fibers, PET is known as polyester, widely used in clothing, upholstery, and carpets.
• Film and Sheet Production: Used in photographic films, X-ray films, magnetic tapes, and solar panels.
• Engineering Applications: Reinforced PET composites are used in automotive components, electrical housings, and machine parts.
• 3D Printing and Manufacturing: PETG (glycol-modified PET) is used for its ease of molding and reduced brittleness.

Globally, PET accounts for a significant share of total plastic consumption due to its cost-effectiveness, recyclability, and superior performance across industries.

PET is one of the most recyclable plastics in the world, identified by the recycling code #1. It can be recycled by mechanical or chemical processes to produce new PET bottles, fibers, or films. Recycled PET is often known as rPET and is used in textiles, carpets, and food-grade packaging.

However, improper disposal of PET contributes to plastic pollution and microplastic contamination in the environment. To address this, advanced recycling technologies like depolymerization are being employed to recover monomers for reuse:

Efforts are also underway to develop bio-based PET alternatives derived from renewable sources like biomass. These innovations are driving the transition toward a circular economy in the plastics industry.