Polyester

Polyester is a broad category of polymers, which are large molecules made of repeating units.

What defines this group is the presence of one or two ester linkages in every repeat unit of the main chain. An ester linkage is a specific chemical bond that holds the polymer together. While many people associate the word with clothing, polyesters are actually a massive family of materials. They include both naturally occurring substances and synthetic ones made in factories. This family is economically vital because of its versatility in science and industry.

To understand how synthetic polyester is made, we must look at the process of polymerization. For the most common type, polyethylene terephthalate (PET), two main raw materials are required. These are purified terephthalic acid (PTA) or dimethyl terephthalate (DMT), and monoethylene glycol (MEG).

The first step is a polycondensation reaction. During this step, the ingredients react at high temperatures to create long polymer chains. This reaction also removes byproducts like water or methanol. Once the reaction is finished, the molten polyester undergoes extrusion. It is pushed into long strands and then cut into small pellets or chips.

To turn these pellets into the fibers used in clothing, a process called spinning is used. The polyester chips are melted and forced through tiny holes called spinnerets. This creates fine strands of polyester filament.

These filaments can be processed into continuous fibers that are then woven or knitted into textiles. Modern production has also evolved to include recycling. Recycled PET, or rPET, is made from post-consumer plastic bottles. This helps reduce the environmental impact of making new polyester.

Polyesters vary greatly based on their chemical structure. They can be classified by their molecular weight and their chemical makeup. Linear aliphatic high molecular weight polyesters have low melting points and are often used in biomedical or pharmaceutical applications. Because they are unstable in water, they are biodegradable. This makes them useful for things like agricultural mulch films or disposable packaging. On the other hand, aliphatic-aromatic polyesters like PET and PBT are high-melting materials. These are used for engineering thermoplastics, fibers, and films.

There are also specialized types like unsaturated polyesters and alkyd resins. Unsaturated polyesters are thermosetting resins used as matrices in composite materials. They are often used in fiberglass laminating or as fillers for car bodies. Some are even used to build the bodies of yachts. Alkyd resins are used widely in coatings because they can be cross-linked in the presence of oxygen. Other versions, called thermoplastic polyester elastomers (TPEE), have a rubber-like quality. The specific structure determines if a polyester is a thermoplastic, which can change shape with heat, or a thermoset.

Nature also uses polyester in surprising ways. In the plant world, a component called cutin forms the plant cuticle. This is made of omega hydroxy acids interlinked via ester bonds. Even insects use it; bees in the genus Colletes are nicknamed "polyester bees." They secrete a cellophane-like polyester lining for their underground brood cells. This shows that the same chemical connections found in our clothes exist in the natural world to provide protection and structure.

Polyester is a massive global industry. In 2019, approximately 30.5 million metric tons of PET were produced worldwide. PET holds an 18% market share of all plastic materials produced. This makes it the third most common plastic, following polyethylene and polypropylene. Its importance comes from the fact that its raw materials are easy to access and its synthesis is well understood.

Because it is a hydrophobic material, it does not absorb water easily. This makes polyester fabrics highly stain-resistant.

Finally, the properties of polyester make it useful for high-performance tasks. Increasing the aromatic parts of the polymer increases its heat resistance and chemical stability. For example, liquid crystalline polyesters are used as abradable seals in jet engines because they can handle extreme heat. In the home, polyester is everywhere, from bed sheets and blankets to the stuffing in pillows and stuffed animals. It can be blended with cotton to create fabrics that are strong and resist wrinkling or shrinking. Whether it is a high-tech jet engine seal or a simple t-shirt, polyester is a fundamental material of modern life.