Saturday, December 6, 2014

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

Marie-Therese Wisniowski

Preamble
This is the thirty-fourth 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

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
Elastomeric materials extend when stretched and return to their original size when the pull is released. Textile materials with very low elasticity are either - too rigid and uncomfortable for apparel - or will stretch out of shape during use. Most textile fibers have some common elasticity; but a special group known as SPANDEX or ELASTOMER fibers have been especially designed to return to their original size immediately the pulling stress is released. Such fibers can be pulled to five or six times their original length before breaking.

Australia’s 2014 Prime Minister – Tony Abbot - on a charity bike ride.
Middle-Aged Man in Lycra (MAMIL).

Spandex fibers are the most complex so far developed by man. They are similar to other thermoplastic fibers, but they have an unusual amount of stretch and will return to their original size. Therefore they are called elastic fibers. Lycra, Vyrene, Gospan, Spandelle, Blue “C” and Duraspan are trade names for this fiber, but the fiber is mostly identified by the generic name – Spandex.

Used extensively for foundation and sports garments, spandex fibers are also used in swimwear, and blended with other fibers for many types of stretch fabrics such as hosiery and slacks. The fiber may be used alone (unsupported) or it may be a core around which other fibers are wrapped. Used in this manner, it has the appearance of whatever fiber is wrapped around but it still maintains its stretch properties.


Fiber Classification
The word elastomeric was coined from elastic and polymer, in order to imply an elastic fiber. Chemically the elastomerics are polyurethane-based fibers, whose polymers are characterized by urethane groups: -NH-COO-.

Polyurethane is synthesized from urea: H2NCONH2. Elastomeric are man-made, synthetic polymer based, segmented polyurethane filaments. They are seldom manufactured as staple fibers.

Elastomerics consist of polymers which are at least 85% segmented polyurethanes.

Spandex is a generic name used for elastomeric fibers. This term was coined by reversing syllables in the word expands. The intention was to convey the elastic properties of elastomerics.

Elastomerics have a fiber density of 1.0 g cm-3 making them the lightest apparel fiber in common use. Their lightness in weight and excellent elastic property have revolutionized the construction of women’s foundation garments or lingerie as well as sports apparel.


Fiber Morphology
The Macro-Structure of Elastomerics
Elastomerics are mainly available as fine, regular, off-white strands. Such strands of elastomeric are “quasi-monofilaments” or multifilament yarns of coalesced filaments.

A longitudinal section of a coalesced elastomeric filaments, which form a quasi-monofilament or stand magnified 600 times.

Elastomerics are also available in the form of elastic-tape, produced by coalescing numerous multifilament yarns. Elastomerics are usually produced with a dull luster similar in appearance to white rubber yarn or elastic tape.

Microscopic Appearance of Elastomerics
The longitudinal appearance has distinct striations and specks. The striations are due to the coalesced filaments, where as the specks are minute particles of titanium dioxide used in de-lustering of elastomerics.

The cross-section of the coalesced multifilament yarn has the dumb-bell or dog-bone shape of the individual filaments. The cross-section also reveals the considerable number of air spaces, which exist within the elastomeric multifilament yarn due to the coalesced filaments. Elastomerics cannot be identified by their microscopic appearance.

A cross-section of a coalesced elastomeric filaments showing how the quasi-monofilament is formed. Note: the air spaces between the coalesced, dumbbell shaped cross-sections of the filaments.


The Polymer System
The Elastomeric Polymer
To date this is the most complex textile fiber polymer that has been synthesized. Two types of elastomeric polymers are synthesized. Each is extruded into filaments with excellent elastic properties but differing in their resistance to alkalis.

(a) The polyether type (for example Lycra). This is depicted in the figure below. The repeating unit is about 7.1x10-9 meters (i.e. 7.1 nano meters i.e. nm) long and about 0.7 nm thick. The length of the actual polymer is difficult to determine due to the cross-linking between polymers. The cross-links contribute significantly to the excellent elastic property of the elastomeric filament. The presence of the ether groups greatly contributes to making this type of elastomeric polymer resistant to alkalis.

The polyether-type repeating unit of the elstomeric polymer. The value of m depends on the actual polyether type of the elastomeric polymer being polymerized. The degree of polymerization (n) is not known.

(b) The polyester type (for example, Vyrene). The repeating unit of this type of elastomeric is about 37.2 nm long and about 0.5 nm thick. It is even more complex than the polyether elastomeric polymer and is so long that were its formula to be reproduced in the typeface used here, it would extend over about 4 pages in an A4 size book. It is for this reason that its detailed formula is not reproduced. The most relevant parts of this type of elastomeric polymer are the ester groups. These hydrolyze in alkaline solutions, such as laundry liquors.

Polyester type repeating unit of elastomeric polymer. The values of m and p depend on the actual polymer type of elastomeric polymer being polymerized. The degree of polymerization (n) is not known.

Each type of elastomeric polymer is linear and consists of rigid and flexible segments.

Rigid Segments
These are also called hard segments. They consist of a diphenyl methyl group with a urethane group at each end (see both diagrams above). The aromatic or benzene structure of the diphenyl methyl groups, with their urethane groups impart a certain degree of rigidity to the elastomeric polymer. The polarity of the urethane groups causes hydrogen bond formation with adjacent urethane groups. The rigid polymer segments are chemically more inert and thereby contribute to the stability of the elastomeric polymer.

The Flexible Segments
These are also called the soft segments. They may consist of long polyethylene glycol segments, as shown in the two diagrams above and/or long polypropylene adipate segments. These long segments polymerize in a linear as well as multi-directional fashion (see figure below). The flexible segments are responsible for the amorphous nature of the elastomerics when their polymer system is in a relaxed state (see figure below).

The urethane groups are polar and should be able to attract water molecules. However, as these groups are usually well aligned, water molecules are not attracted in significant numbers.

(a) The unstretched or amorphous state of the polymer system of an elastomeric.
(b) The stretched or more crystalline state of the polymer system of an elastomeric.

The Polymer System
The elastomeric polymer system is held together by a significant number of cross-links, which are formed in the final stages of quite an involved polymerization of the elastomeric polymers. Such polymerization is often only completed at the moment when the elastomeric filament has been extruded. When the filament is at rest (i.e. not stretched or extended), its polymer system is predominately amorphous. This is largely due to the flexible segments, which are folded upon themselves and generally present a random arrangement. The rigid segments tend to be more aligned. This causes the polar groups of the urethane groups in the segments to form hydrogen bonds, enhancing rigidity of these sections of the elastomeric polymer system. The alignment of the rigid segments, with their polar groups, is considered sufficient to exclude the entry and attraction of a significant number of water molecules – hence the hydrophobic nature of elastomerics.

When the elastomeric filament is stretched its polymers unfold their flexible segments. At the same time, the rigid segments tend to align themselves so that the polymer system becomes quite crystalline (see above figure (b)). Stretching the filament beyond its extendable limit will result in polymer rupture, and cause the breakdown of the excellent elastic recovery of the elastomeric polymer system (see below).


Elasticity of Elastomers
When elastomers are extended, some special bonds which hold the molecules in their normal, relaxed shape are distorted. A certain amount of energy is needed to distort each one of these bonds. A 5 cm length of a wide rubber band contains more such bonds than a 5 cm length of a narrow rubber band.

This means the wider elastic needs more force to distort all its bonds than the narrow elastic does. The energy that you put into pulling the elastic is stored up in each of the distorted bonds (e.g. it feels warm). When you let go, the stored up energy is released, and pulls the bonds back into their original, relaxed shapes (e.g. it now feels quite cold).

Elastomers extended and contracted.

The force with which an elastomer resists extension and with which it tends to return to its original state thus depends on the number and kinds of bonds that are distorted by the stretching process.

Power Stretch
Those materials, which pull back strongly, are called “Power Stretch” elastomers. Those that are elastic, but not as “powerful” create - “Comfort Stretch” - when used in apparel. Textured yarn (e.g. pantyhose thread) provides comfortable ease of movement at places where a lot of extension is regularly needed, such as the knees. Power stretch materials are used for figure control. Lycra has more “power” than rubber. A figure control garment made of rubber would need to be much thicker than an equally effective one made Lycra.

Where comfort stretch is needed in apparel.

The percentage of extension of each item is given by the following formula:
% Extension = 100 x (extended length – original length)/(original length)


The Molecular Structure of Spandex
Synthetic snap-back fibers are known by the collective name of “Spandex”. Spandex fibers are polyurethanes (see above for more detailed explanation). They are what is called block ploymers, with urethane groups providing hydrogen-bonded, ordered regions between long, coiled often amorphous structures, where there is little bonding between adjacent molecular chains.

Hydrogen bonds between hard polyurethane segments provide the bonding between the polymer chains, in both the stretched and unstretched state.

Hard polyurethane segments represented by square blocks, whereas hydrogen bonds represented by coils.

When the fiber is stretched, the amorphous “soft” segments uncoil; these, however, contain bulky side-groups, which do not allow them to pack in an ordered linear form. When the pulling force is released, the soft segments coil once more to relieve the distortion in the bulky side-groups.

When the fiber is extended, the soft segments of the molecule uncoil. The straightened-out length is much greater than the coiled, amorphous length, and so the fiber can be stretched considerably.

The stretched molecules cannot slip past each other, because they are held firmly by the hydrogen bonds in the “hard” urethane segments. If molecular slippage occurred, it would cause irreversible extension.

The natural state of linear polymers is a coiled, random form. When the molecules are lined up by the drawing and stretching process in production, inter-chain attractions act to keep them in this ordered state. The soft segments of the polyurethanes, however, contain bulky side-groups, which make them difficult to pack into linear arrangement. Not only do the bulky side-groups prevent close packing, and so prevent interchain, they are often distorted when the molecules are pulled straight. So, although the amorphous regions straighten out when the fibers are pulled, they return to their natural, random state as soon as the stress is removed.

Lycra
Lycra is a polyurethane, which has polybutylene ether as the “soft” segment, with amine groups to provide the hydrogen bonding in the hard, crystalline segment. Lycra is soluble in dimethyl formamide, and is dry spun from its solution.

Lycra has a polybutyl ether (shown as the repeat unit) in its soft segments.

Vyrene is a polyester-type segmented polyurethane. The polyester segment forming the soft areas has the methyl (-CH3) side-groups which help to keep it coiled amd amorphous.

In Vyrene, the soft segments are polyesters (repeat unit) made from two different dialcohols.

There are a variety of Spandex yarns manufactured in different ways from a variety of raw materials. Because of their amorphous non-polar soft segments, they are all able to be dyed, and therefore are especially suitable for apparel end-uses.


Physical Properties
Tenacity
Elastomeric filaments are weak. Their excellent elastic recovery properties may give the impression that elastomerics are stronger than they actually are. Because of the hydrophobic nature of the fibers, the tenacity is unaffected whether wet or dry.

Elastic-Plastic Nature
This is the most important property of the elastomeric filaments. It is also the property which makes them technologically and commercially so important. The excellent recovery property of the elastomerics is due to a relatively small amount of cross-linking between the polymers in their polymer system. The presence of the flexible segments which form the amorphous regions of the elastomeric polymer system enables the filaments to be stretched.

When the filaments are released, the multi-directional bonding of the flexible segments and the cross-links pull the polymers back into their original configuration. Elasticity is relatively permanent because on extension of the elastomeric filament, few if any intra-polymer bonds are broken. Intra-polymer and inter-polymer bonds are only strained, and the polymers will readily return to their resting configuration whenever the load causing the extension is released.

The elastic properties of the elastomeric polymer system are unaffected by moisture because of its hydrophobic nature. The rubbery, wax-like handle of elastomeric textile materials is due to the alkane or saturated hydrocarbon consituents of long, flexible segments; polyethylene glycol and/or polypylene adipate.

Hygroscopic Nature
Elastomerics are hydrophobic, despite the fact that the urethane groups, which form part of the rigid segments of the elastomeric polymer are polar. Of course, some water molecules are attracted by the polarity of the urethane groups. However, the number attracted is insufficient to make the elastomeric polymer system hydrophilic (water loving), because the rigid segments are always sufficiently aligned to prevent entry of water molecules in significant numbers. The amorphous regions formed by the non-polar flexible segments do not attract water molecules.

Because of their hydrophobic nature, elastomeric textiles readily develop static electricity. A build up of static electricity may lead to severe soiling, and explains the need to launder, or at least rinse, elastomeric garments after each wearing.

Thermal Properties
Elastomerics, like all other synthetic textile fibers, are thermoplastic. The application of heat to elastomeric polymers may cause the development of sufficient kinetic energy to rupture the covalent cross-links of the polymer system. This may result in elastic properties of elastomerics being adversely affected. Excessive application of heat may result in a complete loss of the excellent elastic properties of elsatomerics.


Chemical Properties
Effect of Acids
Elastomeric textile materials in general are resistant to acids. Acid radical have little effect on the polymer system. This is further assisted by the hydrophobic (water hating) nature of the fiber minimizing the entry into the polymer system of acid radicals.

Effect of Alkalis
The elastomerics, whose polymers contain ester groups, are sensitive to alkalis such as laundry liquors. The detrimental effect of such alkalis upon the elastomerics with ester-containing polymers is first seen as a yellowing of the white or a dulling of the colored elastomeric textile material. This occurs as a result of the alkaline hydrolysis of the ester groups, which is explained in more detail in the section on “Polyesters”. Elastomerics of the polymer type are alkali resistant.

Effect of Bleaches
Hydrogen peroxide is the only bleach that can be used safely on elastomeric textile materials.

Effect of Sunlight and Weather
The relative inertness of the elastomeric polymers, as evidenced by their hydrophobic nature and resistance to acids, is probably the main reason for their good resistance to the degrading effects of the sun’s ultraviolet radiation. It is probable that the stability of the electron arrangement in the aromatic or di-isocynate segments of the elastomeric polymers also contributes towards their sunlight and weather resistance. As a result, it requires prolonged exposure to the usually slightly acidic atmosphere to have a significant and detrimental effect on the elastomeric polymer.

Color-Fastness
Elastomeric textile materials tend to be difficult to dye owing to the hydrophobic and very crystalline of their polymer system. Disperse dyes, acid dyes and metal complex dyes are used for elastomeric materials. The fastness to washing and light is only fair.


Care
Elastomeric materials appear in our everyday lives in many forms. The single largest outlet for polyurethanes is elastic foam, used for mattresses, pillows, stuffed toys and carpet underlays.

Elastomers are rarely used in direct contact with the skin (except for Lycra). To allow the elastomer to slide smoothly over the skin, it is usually covered with by fibers (e.g. core-spun yarns) or by another yarn (e.g. covered yarn) or by the knitted or woven structure of cloth.

Elastomers in Use.

Elastomers are produced as monofilaments, partly joined multifilaments, or as sheets cut into filaments with square cross-section.

In consumer use, rubber as an elastomer has the disadvantages of not being dyeable, of being affected by perspiration, cosmetic oils and ageing. Spandex fibers have mostly overcome these problems. They are resistant to acids, alkalis, dry-cleaning solvents, perspiration, oils, insects, mildew, light and ageing. Their one remaining sensitivity is to sodium hypochlorite. This means not only that Spandex garments should never be bleached with chlorine bleach, but also that swimming pool chlorine can attack such garments if not thoroughly rinsed out. They will have reduced life in areas of chlorinated water supply.

Spandex garments have low moisture affinity – the hydrogen bonds are only in the crystalline part of the molecules – and have low electric conductivity. Reaction to heat varies among different brands, but they are generally thermoplastic. This has little importance: because of their resilience, Spandex garments rarely need ironing. Should the occasion arise however, ironing temperature should not exceed 120oC because of the danger of yellowing. Care should be taken also to keep washing temperatures below 60oC, and tumble-drying temperatures below 80oC.


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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