Polyester: Manufacturing process, chemical composition, and properties

By Eman Abdallah Kamel

Eman is a writer and engineer. She received a bachelor’s degree in textile science from Egypt’s Faculty of Applied Arts.

In this article, you will learn about polyester fibers, including their chemical composition, manufacturing process, properties, and main applications.

Polyester

Polyester is one of the most widely used synthetic fibers in the world, playing a vital role in textile, packaging, and modern industrial applications.

Among the various types of polyester, polyethylene terephthalate (PET) is the most commercially important and is widely used in the production of fibers, fabrics, films, and plastic containers. Polyester fibers were first developed in the early 1940s and have since become a staple in the global textile industry.

Polyester is primarily produced by the polymerization of terephthalic acid and ethylene glycol, resulting in a durable and versatile material.

Polyester’s popularity stems from its cost-effectiveness, ease of maintenance, and compatibility with other fibers such as cotton and wool. In recent years, advances in manufacturing technologies have enabled the production of recycled polyester (rPET) from used plastic bottles, contributing to sustainability initiatives in the textile sector.

Did You Know?

The term “polyester” refers to a class of polymers characterized by the presence of ester functional groups within their molecular structure.

Chemical Composition

Polyester fibers are made from a long-chain synthetic polymer containing at least 85% by weight of a dihydroxy alcohol (HOROH) and terephthalic acid (p-HOOC-C₆H₄COOH) ester.

Polyester fibers are made from two types of terephthalate polymers:

Polyethylene terephthalate (PET).
1,4-cyclohexylene dimethylene terephthalate (PCDT).

The above polymers can be homopolymers or copolymers. Polyester fibers are characterized by tightly packed, well-oriented, straight molecular chains with very strong hydrogen bonds.

Manufacturing Process

Polyester is made in two ways:

Condensation Polymerization
Addition Polymerization

1. Condensation Polymerization

Condensation polymerization is the reaction of two different monomers, each with functional terminal groups at the ends of its molecules. During the condensation reaction, small molecules or monomers combine to form a larger molecule (the polymer), and a small molecule such as water (H₂O) or hydrogen chloride gas (HCl) is released.

In condensation polymerization, a small molecule is lost when the monomers combine. Polyester is made by reacting an acid containing two carbonyl groups (-COOH) with an alcohol containing two hydroxyl groups (-OH).

In common polyester, as in the example above:

The acid is benzene-1,4-dicarboxylic acid (terephthalic acid).
The alcohol is ethane-1,2-diol (ethylene glycol).

Remember,

Esters can be prepared by a condensation reaction between an alcohol and a carboxylic acid, where the alcohol ethanol reacts with the carboxylic acid ethanoic acid to form an ethyl ethanoate ester, and a water molecule is also formed. During the condensation reaction, the -OH group of the carboxylic acid and the -H atom of the alcohol are lost, and then these two groups combine to form a water molecule, and the remaining carboxylic acid and alcohol molecules combine to form an ethyl ethanoate ester molecule.

Polymerization cannot be carried out using these reactants because, once the ester bond is formed, the reaction effectively ends; the alcohol and carboxylic acid cannot react further. However, if we replace the alcohol with a dihydroxyl and the carboxylic acid with a dicarboxylic acid, then, once the ester bond is formed, the reaction can continue because the dihydroxyl and dicarboxylic acid still contain active functional groups at the ends of the molecules.

2. Addition Polymerization

Polyethylene and polyvinyl chloride (PVC) are examples of addition polymers, which use small, unsaturated monomers known as alkenes that are linked together to form long chains. During the addition polymerization, all the monomer is consumed to form a single polymer, and no waste is produced. Furthermore, polymers resulting from addition polymerization have a basic structure composed entirely of carbon atoms.

Properties

1. Physical Properties

Absorbency: This type of fabric has very low absorbency, ranging between 0.4% and 0.8%. Therefore, it is not very comfortable against the skin. It is best blended with cotton fibers. The low absorbency results in increased electrostatic properties.
Flexibility: It has very good elasticity. Its elongation rate ranges from 20 to 48%. It is wrinkle-resistant and, when wrinkled, recovers its shape well, whether wet or dry.
Strength: It is highly durable, offers excellent abrasion resistance, and retains strength when wet. This is due to its high degree of crystallinity and high molecular weight.
Light Effect: Polyester has good light resistance, making it the most important filament for sheer curtains.
Effect of heat: Polyester, like nylon, melts and self-extinguishes. Heating with pressure may lead to glazing.
Mold resistance: Polyester is resistant to mites, fungi, and microorganisms.

2. Chemical Properties

Alkali effect: Polyester is resistant to weak alkalis but is affected by caustic soda solutions. It is harmed by strong alkalis at high temperatures.
Effect of acids: Polyester resists weak acids even at the boiling point. It is unaffected by highly concentrated strong acids at room temperature, but it decomposes in strong acids such as sulfuric acid, hydrochloric acid, and nitric acid.
Dyeability: Polyester’s compact structure and hydrophobic properties make it difficult to dye. It does not swell in aqueous environments.
Effect of bleach: Polyester is resistant to bleach, especially oxidizing bleach.

Applications

Clothing: Polyester fibers are used to make trousers, skirts, dresses, suits, jackets, blouses, and outerwear.

Home furnishings: Polyester is also used in making carpets, curtains, blankets, sheets, pillowcases, wall coverings, and upholstery.

Industry: Polyester fabrics are used in various industries, including conveyor belts, safety belts, coated fabrics, high-energy-absorbing plastic supports, hoses, power transmission belts, ropes and nets, twine, tire cords, automotive upholstery, sails, and floppy disk linings.

Sources

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