What is polyester?
Polyester is the name for a group of substances. They are all polymers of chemicals called esters. A polymer is a very long molecule, made by sticking together many copies of a smaller molecule, like a necklace made of popper beads or the many carriages that make up a train. Because their molecules are very long, certain polymers (like polyester) make good strong fibres.
Is polyester natural or man-made?
Naturally occurring polyesters do exist, but the name is usually used for synthetic polyesters made in a chemical process from substances extracted from oil.
The first polyesters
Naturally occurring polyesters were first discovered in about 1830 but the first synthetic polyester was invented just before the first World War by W.H. Carruthers, a research scientist working for Du Pont in the USA. He didn’t follow up his work on this new chemical, however, as he invented Nylon at about the same time and developed that instead.
Some years later a group of British scientists, working for the Calico Printer’s Association in Manchester, did start working with the new substance. In 1941 this group created the first polyester fibre, a form of polyethylene terephthalate (PET, still the most common type of polyester) which they named Terylene.
How is polyester fibre made?
The polyester molecules themselves are made by the condensation polymerisation of a carboxylic acid and an alcohol in a vacuum at very high temperature. The exact details of the process vary between forms of polyester and between manufacturers and tend to be closely guarded trade secrets.
The newly-formed polymer is then forced out of the machine in the form of a ribbon and cools. Once it has cooled the polymer is solid. The solid ribbon is then cut into chips.
Polyester is naturally hygroscopic, which means that it absorbs water from its surroundings. If the polyester is heated to high temperatures with the water present then the polymer will start to break down (basically, the condensation polymerisation will happen in reverse), so the chips need to be thoroughly dried before they can be processed any further.
The chips therefore are put through a complex machine which loops air through them and then through a drying agent and then back again. Hot air is passed through the chips, absorbing some of the moisture. The air is then cooled and passed through the drying agent. The cool dry air which results is then heated again and passed through the polyester chips once more. This process is repeated until the level of water in the chips is less than 40 parts per million. Drying takes four hours or more – it has to be done very gradually and slowly because to do it faster would mean using hotter air, and if the air goes above 160ºC then it will start causing the breakdown reaction that all this is designed to avoid!
The dry chips are then put into the next machine to be melted and spun. Polyester is a ‘melt spun’ fibre, meaning that it is spun by being heated until it melts and then forced out of spinnerets in the form of thin fibres which set as they cool. The silkworm, too, makes its thread by forcing a quick-setting liquid through a fine spinneret. The cross-section of the spinnerets can be varied to give polyesters of different properties. Other ways of spinning man-made fibres include wet spinning and dry spinning.
The fine fibres are heated again and stretched to five times their length, or more, in a process called drawing. This makes them even finer. These long drawn fibres (called filaments) are then made into yarn. This can be by twisting together whole filaments (this is called filament yarn), or by cutting the filaments into shorter lengths, called staples, and spinning these into yarn in the same way that wool or cotton yarn is made (this is called spun yarn).
Filament yarns tend to have a high lustre, and be a thicker, harder yarn. Fabrics like satin and taffeta are typical filament yarn fabrics. Spun yarns tend to have a duller, matt finish, be finer yarns and have a softer feel, this is because of the many tiny filament ends poking out. If you could look at a spun yarn down a microscope it would look as if it had many fine hairs all along its length. Our fabrics are mostly spun yarn fabrics.
Once the polyester has been made into yarn it may be blended with other fibres to make fabrics which combine the qualities of both. Popular blends include polyester/wool, polyester/rayon and polyester/cotton (often abbreviated to polycotton).
Polycotton is probably the most popular blend. The polyester fibres make the cotton more resilient, so it keeps its shape and doesn’t stretch or wrinkle, or stain so easily. The cotton makes the fabric more absorbent and may make it feel more comfortable. Many people still think that polyester doesn’t have a nice feel, mainly due to the bad press it got in the seventies, but modern polyester fabrics are miles away from those seventies double-knits and can feel even better than natural fabrics.
Also, because they are less absorbent, modern polyester fabrics don’t hang onto your sweat and leave you standing in damp kit in the British weather the minute you stop playing and start to cool down! This is why most of our kit is made of modern high-performance polyesters, although we include hardwearing polycotton in our range for those who prefer that.
Yarns into fabrics
Yarns are then sent to textile mills where they are either knitted or woven into fabrics. Knitted fabrics have a lot more stretch, while woven fabrics are more rigid and, depending on the weave, may be far less porous (i.e. they have less gaps). This is why we use knitted fabrics for our playing kits, socks, t-shirts, etc, but woven microfibre fabrics for our outerwear.
Knitting is one way to turn yarn into fabric. The earliest known examples date from the 14th Century, but some claim that it dates from several centuries BC.
Knitting is more complex than weaving. Each row of yarn is looped through the row above and the row below, with no straight line of yarn anywhere. This means that knitted fabrics are stretchy in all directions. This makes them very different to woven fabrics, which are generally only stretchy in one direction (although modern elastic yarns mean this isn’t always the case).
Weaving is another way of making fabric from yarn. Weaving has been known since Neolithic times, possibly even since the Palaeolithic. Simply put, two sets of fibres, at right angles to each other, are interwoven together. The vertical fibres are called the warp and are stretched out and held parallel by the loom. The weft fibres are added between them, one row at a time.
Woven fabrics are only stretchy in the bias direction (i.e. at 45 degrees to the warp and weft), unless they are made of yarn with a lot of elasticity. They generally have far smaller holes than knitted fabrics and so are used for windproof and waterproof fabrics, as well as fabrics where stretch is not desirable.
Want to know more?
If after all this you still want to know more about polyester (really?), try the polyester entry in Wikipedia.