Material of the month – Polyester
This month, Anna Ploszajski discovers the history behind the miracle fibre.
The term polyester is the name given to the family of polymers that contain an ester functional group. Polyesters can be classified as saturated where the polymer is a thermoplastic, such as polyethylene terephthalate (PET) or unsaturated, in the case of thermosetting polyester resins. Their different properties lead to a range of diverse applications, from the clothes on our backs to the food we eat.
It was Wallace Carothers, the inventor of nylon (Material of the month, March 2014 Materials World), who laid the foundations for polyester’s development, when he devised a method of combining alcohols and carboxyl acids to make fibres while working for DuPont in the early 1940s. He famously decided to direct his efforts towards nylon research, but tragically did not live long enough to enjoy the abounding success of his synthetic fibre inventions. Carothers’ work was taken on by British scientists Whinfield and Dickson at the Calico Printers’ Association laboratories as they went on to gain the rights to PET (otherwise known as terylene) in 1941. DuPont subsequently bought the legal rights from Britain in 1946 and resumed research into polyester, which led to its trademarking of Dacron in 1950, and Mylar in 1952.
These polymer fibres could be woven to make fabrics and, in the subsequent decades, polyester textiles surged in popularity thanks to their easy care, low cost, and superior durability compared with natural fibres. In fact, garments from polyester fabric were introduced in the USA and described as a ‘miracle fibre that could be worn for 68 days straight without ironing and still look presentable’.
The popularity of polyester fabrics has been up and down since its conception. Sales plummeted in the 1970s as a result of a negative public image of the infamous polyester double knit fabric, which became synonymous with bad taste. A revival effort by polyester manufacturers in the early 1980s saw various TV and radio campaigns attempt to alter its image from a cheap cloth to a superior, easy-to-care-for textile. Nowadays, consumers are embracing synthetic fibres once more, thanks to improved processing technologies providing better quality materials and at half the price of cotton.
Polyester production is not environmentally friendly, since the raw materials are crude oil derivatives and the multiple processing steps are energy intensive. PET is produced by reacting ethylene glycol with dimethyl terephthalate at 150–200oC, a process called transesterification, which creates a monomer. This monomer is further heated to 280oC and it reacts to form PET, with excess ethylene glycol as a by-product. The product is extruded and cooled, and these long ribbons are cut into chip and completely dried to ensure uniform consistency.
To make the fibres, the chips are melted and spun at 270oC and forced through tiny holes in a spinneret. At this stage, other chemicals can be added to make the product flame retardant, antistatic and/or easier to dye. Ester groups in the polyester chain are polar, since the carbonyl oxygen atom has negative charge and the carbonyl carbon has the opposite. This polarity favours the alignment of ester groups to form crystals during the spinning process, which gives the polyester fibres their strength, tenacity and resilience. This is the secret to the success of polyester textiles, as well as their resistance to permanent deformation, abrasion and biological damage, and their ability to retain creases. These advantages outweigh the main disadvantage for a fabric in the domestic setting – its high flammability.
Behind the scenes
The textiles industry was not the only one to be revolutionised by the discovery of polyester. In 1967, Nathaniel Wyeth, a materials engineer working for DuPont, began to develop the use of PET for fizzy drinks bottles, and the finished product was patented in 1973. Many independent processing plants sprung up, but during the 1980s, Coca-Cola started to manufacture the bottles in-house. This move prompted the independent factories to quickly diversify PET applications, since they could no longer depend on Coca-Cola’s custom. New, alternative avenues for PET packaging included food, personal care products and household chemicals. Today, the PET industry is worth US$48.1bln and is estimated to reach US$60bln in the next five years.
To produce plastic bottles from raw material, the molten PET is injected into a long, thin mould called a parison. It is transferred into a bottle-shaped mould and a thin steel rod called a mandrel is inserted. Highly pressurised air is injected through the mandrel, which causes the parison to adopt the shape of the mould. This is cooled quickly to set the bottle, which is then removed and trimmed of excess material. The rapid cooling results in an amorphous microstructure and transparent product. In this way, each machine is capable of producing hundreds, if not thousands, of bottles per hour.
About 70% of polyester produced globally is destined for use in textiles, and the majority of the remainder is used in packaging. As well as clothing, polyester fibre is used in upholstery, bed sheets and blankets, as well as cushioning and insulation. Carpets made from polyester are resistant to biological damage, are hypoallergenic and resist fading. Although these materials feel less natural than cotton, silk or wool, polyester fabrics have improved wrinkle resistance, and better durability and colour retention compared to their natural fibre counterparts. Polyester fibres can be spun together with natural fibres to produce textile blends with intermediate properties. Other industrial applications for polyester fibres include yarns and ropes used for tyre reinforcements, fabric for conveyor belts and safety belts, and plastic reinforcements to produce composites with high energy absorption.
Discoveries for use
If a single product could be said to have revolutionised the reputation of polyester, it would be microfibre cloth, discovered in 1991. Consumers had a hard time distinguishing the new fabric from silk, since microfibre textiles are exceptionally soft. It can hold up to eight times its weight in water, absorbs oils and doesn’t scratch, so microfibre is an ideal candidate for cleaning products, towels, and cloths for sensitive optical surfaces. The fibres in the highest quality products are split during manufacture to produce multi-stranded fibres. These have an asterisk-like cross-section and are positively charged. This structure and charge greatly enhances its ability to trap negatively charged dirt and absorb liquids.
As early as 1955, polyester films were used in specialised photography, but in the 1990s their popularity surged for motion picture prints. It was considered preferable for post-production, exhibition and archival purposes compared with cellulose-based film, thanks to its flexibility, stability and strength. However, its strength was also to be a disadvantage, since it was more likely to damage the expensive camera equipment were it to jam.
Polyester as an electrical insulator is suitable for use as the dielectric material in thin-film capacitors. PET film capacitors have the advantage of being cheap and mass-producible, and can be small with relatively high capacitance. Another polyester, polyethylene napthalate, can be used as a dielectric to produce capacitors with better stability at high temperatures.
The high mechanical stability at elevated temperatures of engineering polyesters such as PET and polybutylene terephthalate (PBT) as well as their low moisture absorption makes them well-suited to high-temperature and environmentally abrasive applications. This includes door handles, fog lamps and windscreen wipers in cars, and as an abradable seal in jet engines. Thermosetting polyester resins are used as the matrix material in fibreglass composites, such as in cars and yachts.
Recycling keeps non-biodegradable polymers out of landfill and reduces toxic waste, and the numbers are impressive. In 2011, 7.5 million tonnes of PET were collected globally and in 2012, 81% of the PET bottles sold in Switzerland were recycled due to its ability to be melted and re-formed. This means that thermoplastic polyester products could form a closed recycling loop, being continuously re-made into new products. Although PET drinks bottles must be made from virgin plastic because the material absorbs the molecules of its contents, textiles are well-suited for production from recycled polyester, as the Patagonia outdoor clothing company has succeeded in doing since 1993. But how exactly is it done?
Post-consumer PET is sorted into different colours–transparent, blue and green, and mixed. Every time the industry introduces more colours, this stage becomes more complex. The sorted PET waste is crushed and pressed into bales, and sold to recycling companies. Next, the waste is washed, separated, dried and shredded. Thorough dehydration is essential, since PET is extremely sensitive to hydrolytic degradation. After shredding, waste such as paper labels and plastic caps are removed to produce a collection of pure PET flakes, which can be used as raw materials for polyester fibres. During the extrusion of fibres, a process called melt filtration removes contaminants, producing high-quality, high-performance polyester fibres.
In its short lifetime, polyester has transformed two major industries, bringing major innovations in packaging and textiles. Its recyclability has saved millions of tonnes of waste from landfill. As a cheap, lightweight, adaptable material, polyester has become indispensable, and will no doubt continue to mould itself into new applications in the future.