Polyethylene Terephthalate (PET): A Widely Used Plastic Material

Polyethylene terephthalate (PET) is a widely used synthetic fiber and plastic resin of the polyester family. PET is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food, and other liquid containers; thermoforming applications; and engineering resins often in combination with glass fiber. In terms of quantity produced, PET is the largest of the commercially used thermoplastics. PET is a desirable material as it is lightweight, strong, and possesses relatively high resistance to heat and chemicals. Due to these properties, PET has become the primary choice for plastic soft-drink and water bottles.

Chemical Structure and Properties

PET is a linear polyester derived from polyethylene glycol and terephthalic acid or dimethyl terephthalate. The chemical formula of PET is (C10H8O4)n. PET has a glass transition temperature of approximately 70-80°C. It has excellent tensile strength, rigidity, and electrical insulation properties. Additionally, PET offers good impact strength, shatter resistance, and thermal stability. Some key properties of PET include: density of 1.3-1.4 g/cm3, melting point of approximately 250°C, and a glass transition temperature of 75°C. The inertness and stability of PET make it suitable for packaging food and beverages where maintaining the integrity and shelf life of contents is important.

Production and Processing

PET is produced by direct esterification of terephthalic acid (TPA) with ethylene glycol under heat. This is followed by polycondensation under reduced pressure to achieve the desired molecular weight and intrinsic viscosity. Stabilizers and additives such as brighteners, flexibilizers, and colorants are blended into PET to improve its performance characteristics.

Injection molding is one of the most common thermoplastic processing techniques used for PET. Bottles and food containers are commonly made through stretch blow molding where PET preforms or parisons are quickly stretched into a mold by air pressure. Extrusion is used to produce PET films, sheets, profiles, and tubes. Thermoforming is also utilized to shape PET sheets into packaging trays, clamshell containers, and other formed parts. Fibers can be produced from PET through melt spinning or direct spinning processes.

Uses and Applications

Bottled beverages: Due to its clarity, safety, recyclability and affordability, PET is the dominant plastic used for water, soft drinks, juices, and other packaged beverage containers globally. In 2019, approximately 71 billion PET bottles were sold in the US alone.

Food packaging: Polyethylene Terephthalate (PET) is utilized for packaging numerous food items including peanut butter, salad dressings, honey, as well as fresh and frozen foods. Its barrier properties help maintain product freshness and quality.

Thermoforming sheet: PET sheet is employed for packaging goods with rigid and semi-rigid container solutions produced through thermoforming. This includes bakery and produce packaging, berry clamshells, and take-out food containers.

Engineered parts: Reinforced with glass fibers, PET provides durable engineered parts for automotive components, plastic cooling fans, pool liners, and electronic housings. The chemical resistance and rigidity of PET produce quality technical parts.

Fibers: Staple fibers and continuous filament yarns made from PET are commonly utilized for clothing, home furnishings like curtains and upholstery, industrial fabrics, and nonwovens such as furniture padding.

Films: Thin PET films are widely used in packaging foodstuffs and consumer goods. Specialty applications include magnetic audio and video tapes as well as photgraphic films.

Sustainability and Recycling of PET

While PET offers desirable properties for packaging, its widespread use and slow degradation in landfills or as litter pose environmental concerns. However, PET is one of the most recyclable plastics due to its desirable properties even after multiple recycling cycles. PET containers, films, sheets and fibers can be mechanically recycled by washing, drying, and remelting the material to manufacture new products. Additionally, chemical recycling has potential to break PET down into its base monomers for reuse by depolymerization.

In the US and Europe, PET recycling rates have increased significantly in recent decades aided by bottle deposit initiatives, curbside collection programs, and educational efforts. However, more work remains as millions of tons of PET still end up as trash annually. Future opportunities lie in using more recycled PET content in packaging and products to create a truly circular plastics economy for this ubiquitous material. Standardization of recycling processes and technologies globally would also further advance PET sustainability efforts.

PET has emerged as one of the most extensively produced synthetic polymers globally due to its desirable properties and performance advantages over alternative materials. Key attributes such as clarity, flexibility, strength and durability have made PET a preferred choice for beverage containers, food packaging, engineered parts and other consumer goods applications. While initiatives are underway, continued progress on PET recycling infrastructure and rates is crucial to reduce reliance on virgin resins and enable a more sustainable plastics future through circular material flows. With innovation, PET can continue delivering value to industries and consumers in an environmentally-friendly manner.