Saturday, September 6, 2014

Man-Made Synthetic Fibers - Polyester[1-2]
Art Resource

Marie-Therese Wisniowski

Preamble
This is the thirty-first post in the "Art Resource" series, specifically aimed to construct an appropriate knowledge base in order to develop an artistic voice in ArtCloth.

Other posts in this series are:
Glossary of Terms
Units Used in Dyeing and Printing of Fabrics
Occupational, Health & Safety
A Brief History of Color
The Nature of Color
Psychology of Color
Color Schemes
The Naming of Colors
The Munsell Color Classification System
Methuen Color Index and Classification System
The CIE System
Pantone - A Modern Color Classification System
Optical Properties of Fiber Materials
General Properties of Fiber Polymers and Fibers - Part I
General Properties of Fiber Polymers and Fibers - Part II
General Properties of Fiber Polymers and Fibers - Part III
General Properties of Fiber Polymers and Fibers - Part IV
General Properties of Fiber Polymers and Fibers - Part V
Protein Fibers - Wool
Protein Fibers - Speciality Hair Fibers
Protein Fibers - Silk
Protein Fibers - Wool versus Silk
Timelines of Fabrics, Dyes and Other Stuff
Cellulosic Fibers (Natural) - Cotton
Cellulosic Fibers (Natural) - Linen
Other Natural Cellulosic Fibers
General Overview of Man-Made Fibers
Man-Made Cellulosic Fibers - Viscose
Man-Made Cellulosic Fibers - Esters
Man-Made Synthetic Fibers - Nylon
Man-Made Synthetic Fibers - Polyester
Man-Made Synthetic Fibers - Acrylic and Modacrylic
Man-Made Synthetic Fibers - Olefins
Man-Made Synthetic Fibers - Elastomers
Man-Made Synthetic Fibers - Mineral Fibers
Man Made Fibers - Other Textile Fibers
Fiber Blends
From Fiber to Yarn: Overview - Part I
From Fiber to Yarn: Overview - Part II
Melt-Spun Fibers
Characteristics of Filament Yarn
Yarn Classification
Direct Spun Yarns
Textured Filament Yarns
Fabric Construction - Felt
Fabric Construction - Nonwoven fabrics
A Fashion Data Base
Fabric Construction - Leather
Fabric Construction - Films
Glossary of Colors, Dyes, Inks, Pigments and Resins
Fabric Construction – Foams and Poromeric Material
Knitting
Hosiery
Glossary of Fabrics, Fibers, Finishes, Garments and Yarns
Weaving and the Loom
Similarities and Differences in Woven Fabrics
The Three Basic Weaves - Plain Weave (Part I)
The Three Basic Weaves - Plain Weave (Part II)
The Three Basic Weaves - Twill Weave
The Three Basic Weaves - Satin Weave
Figured Weaves - Leno Weave
Figured Weaves – Piqué Weave
Figured Fabrics
Glossary of Art, Artists, Art Motifs and Art Movements
Crêpe Fabrics
Crêpe Effect Fabrics
Pile Fabrics - General
Woven Pile Fabrics
Chenille Yarn and Tufted Pile Fabrics

The Glossary of Terms, Timelines of Fabrics, Dyes and Other Stuff, A Fashion Data Base, Glossary of Colors, Dyes, Inks, Pigments and Resins, Glossary of Fabrics, Fibers, Finishes, Garments and Yarns and Glossary of Art, Artists, Art Motifs and Art Movements have been updated in order to better inform your art practice.

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Introduction
The polyester fiber was developed in England. Its trade name was Trylene. Du Pont bought the patent rights, did further research and in 1951 produced the first polyester in the USA (which was named Dacron). By 1960 Fortrel had been introduced by the Celanese Corporation and Vycron by the Beaunit Mill Company. Later Tennesse Eastman marketed Kodel. Avlin and Trevira are among others of the polyesters - the fastest growing fiber group.

The word ester is the name given to salts formed from a reaction between an alcohol and an acid. In chemistry an ester is designated the chemical formula of RCOOR’, where the R represents a large array of possible chemical groups and "C" and "O" represents carbon and oxygen, respectively. Esters are therefore organic salts and polyesters mean “many” organic salts. Polyesters are man-made, synthetic polymer, polyester filament or staple fiber. The most common polyester apparel filament or staple fiber is usually composed of terephythalate polymers.

Recycled Polyester.

As with other man-made fibers, the manufacture is a complex chemical process involving many steps. The final step involves forcing a semi-liquid through a spinneret in order to form the filament.

Complex chemical process in order to produce polyester filaments.

Like nylon, there are many types of polyesters, each serving a specific purpose. Polyester fibers are made in staple and filament forms and also as a film called Mylar. This plastic film is used for making "metallic" yarns.


General Properties of Polyesters
Polyester fiber density is 1.39 g cm-3, which makes these fibers relatively medium weight. To circumvent this, polyesters are manufactured as “thin” fabrics, thereby making them more lightweight.

The polyester filaments or staple fibers are fine, regular and translucent. Both filament and staple fibers are usually crimped or textured for the same reason as outlined for viscose (see an earlier post).

The diameter of polyester fibers ranges from about 12 to 25 microns, depending on the end-use requirements. The fiber length to breadth ratio is usually in excess of 2000:1. This ensures that even the shortest staple fibers will satisfactorily spin into yarn.

The main component of the acrylic polymer is the ethylene glycol terephthalate repeat unit.

A section of the polyester polymer.
Note: The oxygen atom (O) of the carbonyl group (CO) gives rise to weak hydrogen bonding with the hydrogen atoms (H) of the methylene (CH2) group.

The polymer is linear and is usually based on the polyethylene terephthalate repeat unit. The polymer lengths are about 120 to 150 nm, with a thickness of about 0.6 nm.

Microscopic photograph of surface and cross-section of Dacron polyester.

The most important chemical groups in the polyester polymer are the methylene groups (-CH2-), the slightly polar carbonyl groups (-CO-) and the ester groups (-OCO-). As the polarity of the polyester polymer is slight it is considered to be held together mainly by van der Waals forces of attraction and to a lesser extent by some very weak hydrogen bonds.


Macro Polymer Structure
The polymer system is estimated to be about 65-85% crystalline and 35-15% amorphous, thereby it can be described as very crystalline. This is consistent with the hydrophobic nature, poor dyeing ability, but good overall chemical resistance.

Very weak hydrogen bonding is thought to occur between the oxygen atom of the CO group, and with the hydrogen atoms of the adjacent CH2 groups (see above). However, van der Waals forces of attraction are considered to be the dominant cohesive mechanism of this polymer system. For both these forces to be efficient and effective, excellent polymer orientation is required (which occurs since it is crystalline) and moreover, the inter polymer-polymer distances should be within 0.3nm (which is thought to be the case in the polyester polymer system).


Physical Properties
Tenacity
Polyester filaments and staple fibers are strong to very strong, because of their extremely crystalline natures, allowing the formation of effective and efficient van der Waals forces of attraction as well as promoting weak hydrogen bond formation. Both of these characteristics result in a very good tenacity. The tenacity of the polyester filament or staple fibers remains unaltered when wet, which is attributed to their hydrophobic and crystalline natures, both of which act in concert to prevent much water uptake into the voids of the polyester polymer system.

Elastic-Plastic Nature
The stiffness and hard handle of polyester filaments or staple fibers is due to the their extremely crystalline nature, which resists bending or flexing of the filament or staple fiber. This also explains their resistance to wrinkling or creasing.

Polyester filaments or staple fibers are about as plastic as they are elastic – as is observed on distortion by repeated stretching and straining. This is because weak van der Waals forces of attraction, which are responsible for the polyester polymer system cohesion, cannot withstand stretching or straining and so are severed, thus allowing polymer slippage to occur. The weak hydrogen bonds are also readily broken and are unable to prevent polymer slippage.

The distinctly waxy handle of polyester textile materials is due to the presence of methylene (CH2) and phenyl (C6H5) groups in the polyester polymer system.

Hygroscopic Nature
Polyester filaments or staple fibers are hydrophobic. The lack of polarity and the extremely crystalline structure resist entry of water molecules in this textile material. The insignificant amount of moisture can only exist as a molecular film on the surface of the filaments or staple fibers.

The lack of water uptake of the polyester polymer system results in the lack of dissipation of static electricity build up in dry atmospheric conditions.

The hydrophobic nature of the polyester polymer system attracts fats, grease, oils and greasy dirt. In other words, the polyester polymer system is oleophilic (oil loving). The water insolubility of greasy soils and the hydrophobic nature make it almost impossible to remove greasy soils from dirty polyester textile materials. This is further complicated by the fact that the polyester polymer system easily develops static electricity, which further attracts airborne dust and grease particles, leading to rapid soiling of polyester fabrics. It is only via the use of organic solvents, as used in dry cleaning processes, that greasy soils can be effectively removed from polyester fabrics.

Thermal Properties
The degree of polymerization ranges from about 115 to 140 for the polyester polymer system, which indicates they have small length polymers (when compared to cotton, with a ratio of 5000). In any polymer unit, there are 3N-6 degrees of vibrational freedom, where N is the number of atoms in the polymer chain. Applied heat to the polymer system causes the polymer units to vibrate. As the modes of vibration is restricted by the length of the polyester polymer chain, it cannot dissipate the heat within the polymer chain and so violent vibrations of the polymers sever more van der Waals forces than cause the creation of “new” links. Hence, polyester polymer systems show poor heat conductivity and have a low heat resistance.

Thermoplasticity
Polyester textile materials can be permanently heat set. Textile fibers classed as thermoplastic are acetate, triacetate, nylon and polyester. Polyesters retain a heat set permanently, whereas the acetate fibers do not hold a heat set as satisfactorily.

The extent to which a thermoplastic fiber will retain its heat set will depend entirely on its second order transition temperature.

A first order transition temperature would be the temperature at which you convert a liquid, such as water, into a gas, such as steam. A second order transition temperature would be the temperature at which say a stiff garden hose made from rubber becomes softer, more limp and more manageable. In terms of thermoplasticity, the second order transition temperature would then be defined as the temperature at which fibers will retain their heat set. If fibers have a low in magnitude second order transition temperature, then on applying a small amount of heat the textile material will be set, but by applying a greater amount of heat from an iron at a later date, the textile material that was set, will relax and so the set will only be temporary. However, if the second order transition temperature is large in magnitude for it to be set, then not even the heat from the hottest iron at a later date can reach this temperature and so the heat set of the textile material will be permanent.

A complication is that the second transition temperature of a textile material cannot be measured absolutely. However, it can be measured in a relative sense and so thermoplastic fibers can be ranked according to their relative second order transition temperature. Hence of the four fibers listed above, polyester is ranked one (highest second order transition temperature) and acetate the lowest (smallest second order transition temperature).


Chemical Properties
Effect of Acids
The ester groups of the polyester polymers are resistant to acid hydrolysis, as are the other chemical groups of the fiber system. This resistance is further complemented by the extreme crystallinity of the polyester polymer system, which prevents entry of any acid or water molecules into the filament or the staple fiber.

Effect of Alkalis
Alkaline conditions (such as encountered in laundering) may hydrolyze the polyester polymers at the ester groups. The extreme crystalline nature restricts the hydrolysis (or saponification) to the surface of the polyester filament or staple fibers.

As the hydrolysis of polyesters is restricted to the surface, polyester textile materials retain their whiteness during laundering. The surface polymers of the polyester filaments or staple fibers are hydrolyzed as shown in the figure below.

With time, the polyester textile material will become fiber and silkier with regular laundering and continued hydrolysis.

Effect of Bleaches
Normally polyester textile materials do not need to be bleached. As explained above, white polyester tends to retain its whiteness during normal domestic laundering. If bleaching is desired it is normally achieved using sodium chlorite.

Sunlight and Weather Resistance
The acid resistance of polyesters helps to protect polyester textile materials from slightly acidic conditions arising because of polluted atmospheres. The benzene rings of the polyester polymer provide electronic stability to the whole polymer, enabling the polymers to withstand the detrimental effects of UV light. This explains why polyester textile materials are only second to acrylics in terms of very good sunlight and weather resistance.

Color-Fastness
It is very difficult for dye molecules to penetrate the extremely crystalline regions of the fiber, which constitutes the majority of the polyester polymer system. Furthermore, the hydrophobic nature prevents water molecules entering the amorphous regions. Only the relatively small molecules of disperse dyes can enter both regions: that is, the crystalline and amorphous regions.

Disperse Dyes
The hydrophobic nature of disperse dye molecules makes them substantive with respect to the polyester polymer system. Only pastel-colored polyester textile materials are obtained under conventional dyeing techniques, even with the dye liquor at the boil. The limited dye uptake, which occurs using conventional techniques is illustrated in the figure below, which shows the cross-section of such dyed polyester filaments or staple fibers. To obtain deeper shades, pressure dyeing is necessary. Under conditions of about 130oC to 140oC and pressures of 1 kg.cm-2 to 1.5 kg.cm-2, medium and even deep shades can be obtained on polyesters.

Cross-Section of a polyester fiber showing dyeing under atmospheric pressure.

Pressure dyeing opens up the voids in the polyester polymer, enabling the dye molecules to penetrate, and so generating greater dye uptake. When the pressure is removed, these voids get smaller in size, thereby trapping the dye molecules within the crystalline and amorphous regions of the polyester polymer system.

The fair to good wash-fastness of the disperse dyed and printed polyester textile materials is due to their hydrophobic nature, the insolubility of the dyed molecules with respect to water and because of the extreme crystalline nature of the polyester polymer system.

The fair to good light-fastness of disperse dyed and printed polyester textile materials is due to the electronic stability of the benzene ring system in the polyester polymer system to withstand detrimental effects due to UV light.

Marie-Therese Wisniowski, Autumn Bolt (disperse dyed polyester artwork).


References:
[1] A Fritz and J. Cant, Consumer Textiles, Oxford University Press, Melbourne (1986).
[2] E.P.G. Gohl and L.D. Vilensky, Textile Science, Longman Cheshire, Melbourne (1989).

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