Last updated on October 2nd, 2023 at 11:03 pm
Polyester are polymers made by a condensation reaction taking place between small molecules, in which the linkage of the molecules occurs through the formation of ester groups.
Polyesters are commonly made by interaction of a dibasic acid with a dihydric alcohol:
HOOC-X-COOH + HO-Y-OH → …OC-X-COO-Y-OCO-X-COO-Y-OCO…
The formation of polyester was studied by Wallace H. Carothers of du Point during the investigation of polyesters which lead eventually to the discovery of nylon. Development of the polyesters was overshadowed, however, by the polyamide research, and it was not until 1941 that a valuable polyester fibre was discovered. in that year, J. T. Dickson and J. R. Winfield of the Calico Printers’ Association England made a synthetic fibre from polyethylene terephthalate by condensing ethylene glycol with terephthalic acid.
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After the war, development of the fibre was carried out under license by I.C.I. Ltd. in the UK and du du Pont in USA, resulting in the fibres known respectively as ‘Terylene’ and ‘Dacron’.
Today, polythylene terepthalate fibres are being made in many countries, and modified forms of this fibre are also produced. Other polyesters have become of commercial importance.
What is Polyester?
According to Federal Trade Commission, a manufactured fibre in which the fiber-forming substance is any long chain synthetic polymer composed of at least 85 per cent by weight of an ester of an substituted aromatic carboxylic acid, including but not restricted to substituted terepthalate unites p(-R-O-CO-C6H4-CO-O-) and para-substituted hydroxybenzoate unites p(-R-O-C6H4-CO-O-).
Types of Polyester Fibre
Polyester fibres are subdivided into the following types:
- Polyethylene Terepthalate Fibre (PET Polyester Fibres)
- Poly-1, 4-Cyclohexylene-Dimethylene Terepthalate Fibres
- Other types of Polyester Fibre.
Among them Polyethylene Terepthalate Fibre (PET Polyester Fibres) is mostly produced and used in daily life. So, we are discussing on it.
Polyethylene Terepthalate Fibres (PET Polyester Fibres) are produced as multifilament yarn, staple fibre and tow, in a wide range of counts and staple lengths to suit virtually all textile requirements.
Types and Sizes
The fibres are available in bright, semi-dull and dull lustres. The properties of the fibre may be modified over a range which is limited by the inherent characteristics of the polymer, each manufacturer controlling his process to produce fibres that will meet specific requirements. In general, commercial PET polyester fibres fall into two main classes:
- Regular tenacity
- High tenacity.
PET polyester fibres are produced commonly in round cross-section, but fibres of special cross-section are now available from a number of manufacturers. PET polyester fibres are thermoplastic, and lend themselves well to physical modification associated with this property. Crimped and textured yarns of all familiar types are available.
Production Process of Polyester
Polyethylene terephthalate is made by the condensation of terephthalic acid, or a derivative such as dimethyl terephthalate, with ethylene glycol.
Ethylene Glycol: It is made by the catalytic oxidation of ethylene, which is obtained from petroleum craching. Ethylene oxide is produced1. Hydration of this yields ethylene glycol2.
Terephthalic Acid; Dimethyl Terephthalate: Para-xylene obtained from petroleum is oxidized3, for example with nitric acid or with air in the presence of catalyst. Terephthalic acid is esterified with methyl alcohol4 to form dimethyl terephthalate.
Polymerization: Polyethylene terephthalate is made by condensing ethylene glycol with either terephthalic acid itself or with dimethyl terephthalate.
Condensation of ethylene glycol with terephthalic acid1 is an esterification reaction, water being eliminated as the reaction takes place. Condensation of ethylene glycol with dimethyl terephthalate2 is an ester interchange reaction, methyl alcohol being eliminated as the reaction takes place. The polymer obtained in this way would be expected to have ester end group instead of the carboxilic acid end group in the case of the polymer obtained by the terephthalic acid route.
In either case, the condensation is carried out by heating the ethylene glycol and terephthalic acid or dimethyl terephthalete and removing the water or the methyl alcohol in vacuo. When the desired degree of polymerization has been reached, the clear, colourless polyester is extruded through a slot on to a casting wheel. The polymer solidifies into an endless ribbon, which is fed to a cutter and cut into chips in the form of cubes with 3-6mm sides.
Polyethylene terepthalate melts at about 2600C, and the molten polymer is stable so long as oxygen is rigorously excluded. Every care is taken during melt spinning, as in the polymerization process, to prevent air coming into contact with the molten polymer.
In the spinning building, the chips of polymer are dried to remove traces of moisture, and then passed to strong hoppers. From the hoppers the chips are fed as required to the spinning machines.
The molten polymer being pumped through holes in a spinneret. As the filaments emerge, they solidify and are wound into packages of undrawn yarn.
The undrawn yarn is stretched to about five times its original length on draw-twist machines, the stretching being carried out usually at elevated temperature. If high tenacity yarn is being made, the filaments are drawn to higher degree than in the manufacture of regular tenacity yarn.
It is normal practice for PET polyester yarns to be drawn hot, as this gives a more uniform product that could draw. The stretching of heavy denier yarns and monofilaments may, however, be carried out at room temperature, as the poor heat conductivity of the fibre makes for irregularities in thick filaments which are drawn hot.
End Usage of Polyester
PET polyester is used in different ways for its various properties. Some of them discussed bellow:
- Apparel Fabrics: PET is used alone or blended with wool, cotton, flax and other fibres provides an extensive range of hard-wearing, comfortable, easily-looked-after garments with high dimensional stability and wrinkle resistance.
- Curtains: The good resistance to sunlight which is a feature of PET polyester fibres, especially behind the glass, has enable these fibres to establish an important outlet in the curtain trade. The fast dyes used in dyeing PET polyester fabrics have contributed generally to success in this field.
- Floor Coverage: It is also used for floor coverage, notably in the production of sliver knit rugs and tufted carpeting.
- Laundry Equipment: PET polyester fibre has heat resistance property, so for this reason PET is used as laundry equipment.
- Conveyor Belt: PET polyester fibres are friction resistance, strong and soft properties, that’s why it is used for conveyor belt.
- Ropes, Net, Sailcloth: PET polyester fibres can take a large amount of load and have drape properties. For this reason it is used to produce ropes, net, sailcloth.
- Filling: PET polyester staple is used as filling in pillow, mattrace, quilts etc. for it soft feeling property.
- Sewing Thread: PET polyester fibre is used as core of core-spun yarns for its strength and the outer layer of cotton is used. But the comfort remains like cotton.
- Electrical Insulation: PET polyester has great heat and electric resistance power, also has the resistance to outer weather. That is why it is used for insulation.
- Tyres: PET polyester has great property of rubbing fastness and to heat. It also remains in shape for long time. For this reason it is used as raw material of tyres.
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