Saturday, June 1, 2013

General Properties of Fiber Polymers and Fibers [1-2]
Macro Properties - Part III
Art Resource

Marie-Therese Wisniowski

This is the sixteenth blog in the "Art Resource" series, specifically aimed to construct an appropriate knowledge base in order to develop an artistic voice in ArtCloth.

Today we are continuing to examine general fiber chemistry. 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 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
Fiber Construction - Leather
Fabric Construction - Films
Glossary of Colors, Dyes, Inks, Pigments and Resins
Fabric Construction – Foams and Poromeric Material
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

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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Fibers are either natural or man-made. There are just four major groups of fibers - cellulose, protein, thermoplastic and mineral. It is important to realize that fiber types such as cellulose, protein and mineral can sit in either category.

Courtesy of reference[1].

Fibers can be identified via two processes:
(i) microscopic identification
(ii) by burning.

Microscopic Identification of Fibers
Many common fibers can easily be identified with the aid of a microscope. If a yarn is used, then one end is untwisted and examined. This end if laced in a drop of water or glycerine on a clean glass slide can be inspected with the microscope. Cotton, linen and woolen fibers can easily be identified (see below), whereas for man-made fibers more complicated methods are usually required.

Courtesy of reference[1].

Fiber Identification By Burning
Many fibers can be identified by their burning characteristics. When fibers are blended or have certain finishes added, this test may not be reliable.

A piece of fabric is held with tweezers and the fabric is slowly brought to the flame of a candle, which has been set in a metal plate. A number of features are noted namely:
(a) whether the flame goes out
(b) odor
(c) characteristic of the residue.
Based on these characteristics an assessment can be made about the fiber type.

Courtesy of reference[1].

General Properties of Fiber Polymers and Fibers

Colorful Yarn (54% Nylon, 39% Mohair and 7% Wool).

Fabric fibers are made of molecules called polymers - see Part II. Generally, polymers are extremely long, linear and unreactive, whereas monomers are much smaller and reactive chemical units. Polymers are generally considered to be unreactive. Nevertheless, these bonds can be broken, if polymers are attacked by very reactive chemicals and other degrading agents (such as UV light etc.)

Often we would refer a fabric fiber as say, a polyester polymer system or a cotton polymer system etc. For art and crafters the macro systems (the fibers and the fabrics that these polymers make) are divorced from the micro systems (i.e. the polymer systems and the inter- and intra- bonding mechanisms). Today's post concentrates on the macro properties of fiber polymer and fibers.

Properties of Fiber-Forming Polymers
Ideally fiber-forming polymers that make up fabrics should have the following properties:
(i) Hydrophilic (water loving)
(ii) Chemically resistant
(iii) Linear
(iv) Long in length
(v) Capable of being orientated to create crystalline regions
(vi) Able to form high-melting-point polymer systems.

These would be the ideal properties. Some fibers essentially meet all these requirements (e.g. acetates, acrylics, cotton, flax, nylon, polyester, silk, viscose and wool), whereas some man-made fibers only satisfy requirements (i), (v) and (vi) (e.g. chloro-fibers, polyethylene and polypropylene) and yet their fibers still sell. Natural cellulose fibers such as abaca, coir, hemp, jute, kenaf, ramie and sisal have very restricted apparel use, because they are very stiff. These fibers are examples of requirement (v) being excessive, since their extensive crystalline regions (see below) give these fibers an uncomfortable stiffness against your skin. The polymers of natural fibers - angora, cashmere and mohair - largely meet the above requirements, but because of their expense, they are hardly mass-market fibers.

Hydrophilic Properties
Fiber polymers should be hydrophilic (water loving). This implies that they should possess polar groups (i.e. chemical groups in the polymer system in which parts of the group has a positive charge tendency and another part has a negative charge tendency) since such groups attract water molecules, which themselves are polar (note: “like attracting like”).

A fiber is comfortable to wear if its polymer system contains hydrophilic polymers and the polar system of the fiber itself allows the entry of water molecules. There are of course fibers whose polymer systems are hydrophobic (water hating) – polyester fibers come to mind – and yet these fibers are manufactured for use in apparels. In order to make the fibers of such fabric materials more water friendly and so more comfortable to wear, hydrophobic fibers need to be blended with the hydrophilic fibers. For this reason the water hating nylon and polyester fibers are often blended with the water loving cotton, viscose and wool fibers (e.g. 66% polyester and 34% cotton blend). Acrylics are also hydrophobic fibers and yet the knitted outwear of acrylics is very popular. However, it is usual to wear the non-absorbent acrylics over an absorbent hydrophilic fiber garment to counter the potential discomfort of the acrylic garment.

Fibers consisting of hydrophilic polymers attract water molecules, the latter of which effectively and efficiently dissipates any static electricity build-up under dry atmospheric conditions, due to the former’s very polar nature. Static electricity attracts dirt particles more readily and so soils the fabric material more quickly. Garments that allow static electricity build-up may cling together making them less comfortable to wear.

Chemical Resistance
Fiber polymers should be resistant, for a reasonable length of time, against common degrading agents such as sunlight and weather, common types of soiling, body exudations, laundry liquors and dry cleaning solvents. It is a given that chemical resistant polymers should not be toxic or hazardous to wear against the human skin.

Although fiber polymers should be chemically resistant, generally they should not be inert (chemically unreactive) since fibers that are, such as the chlorofibers, polypropylene and polyethylene, are hydrophobic, making these fibers non-absorbent and so giving them a greasiness and slipperiness handle that is aesthetically an unpleasant “feel”.

Fiber polymers should be linear (like metal chains) and not branched (e.g. like trees). Only linear polymers give adequate alignment and so promote efficient and effective inter-polymer forces of attraction (i.e. the glue that holds the polymer system together). If the glue is not present, the fiber polymer system would readily fall apart or could not even exist.

Linear polymers do have attached to their backbone chemical groups which are important with respect to their reactivity. These are not branches (i.e. they have no large extended side chains) but rather can be thought of as little spatial nodules on the linear polymer backbone.

Long in Length
Fiber polymers should be long. It has been found that the length of the polymers that constitute cloth fibers are in excess of 100 nm (i.e. nm is a nanometer or 10-9 meters).

Generally, fiber polymers that are long will have polymer systems that are more cohesive and so more resistant to breakdown; that is, long polymers produce a stronger fiber. Long polymers can also orientate themselves better with respect to each other and so promote the presence of more and stronger cohesive forces. For example, the figure below is a schematic of long chained polymer system. The forces of cohesion orientate the polymers with respect to each other in order to maximize their attraction to each other.

Schematic of a linear polymer system.
Courtesy of reference[2].

On the other hand, highly branched polymers, as in the figure below, assume more randomly orientated configurations and so minimize somewhat the forces of attraction that glues the polymer system together.

Schematic of a branched polymer system.
Courtesy of reference[2].

Extremely long polymers become very ordered with respect to each other. These polymer systems readily enhance the inter-polymer forces of attraction and so are effectively and efficiently cohesive. To break this fiber network, many bonds that are formed between the polymers, have to be broken. This is represented by the long path needed to break the inter-polymer bonds.

Schematic of a strong fiber polymer system.
Courtesy of reference[2].

Weak fibers are usually composed of "short in-length" polymers. The number of inter-polymer bonds is now much less, which is represented by the shortness of the path, and so the fiber polymer system can be easily broken.

Schematic of a weak fiber polymer system.
Courtesy of reference[2].

Fibers polymers should be capable of being orientated because this is what makes the fiber stronger. This means that the polymers can be arranged or aligned (i.e. orientated) more or less in a parallel fashion along the direction of the longitudinal axis of the fiber or filament. With man-made fibers the operation is called "drawing", which stretches the extruded and coagulated filament in a parallel fashion along the longitudinal direction of the fiber.

Perfect orientation within the polymer system cannot be obtained (or even would be desirable). There are two distinct regions in the fiber polymer system:
(i) The amorphous region - randomly orientated fiber polymers with large voids or gaps
(ii) The crystalline region – well-ordered and orientated fiber polymers with much smaller voids and gaps.

The percentage of crystalline versus amorphous regions within a fiber polymer system is one of the many criteria that needs to be taken into account in coloring or dyeing fabric materials.

Crystalline and amorphous regions of a fiber polymer system.
Courtesy of reference[2].

Heat Resistance
Heat resistant fiber polymer systems are systems that have a high melting point. Low melting point fiber systems would not be able to withstand heat treatments with respect to fabric finishes, cloth manufacture, laundry, pressing or ironing of a fabic during the lifetime of its use. A rough rule-of-thumb is that if the polymer systems melting point is above 225oC, it is useful for fabric manufacture and apparel use.

Generally, the longer the polymer and the more orientated that the polymer system is, the greater are the inter-polymer forces holding it together (i.e. stronger is the glue) and the higher the melting point.

You should be aware that long polymeric systems, when heat is applied to them, have 3N-6 degrees of vibrations readily available to them (where N is the number of atoms in the polymeric system). Hence they have many channels to dissipate the applied heat and so, for a given quantity of heat, do not vibrate as violently as shorter polymers, which have far fewer channels available to them. The slow wriggling of these long length polymers, with respect to applied heat, means that as many bonds that are broken by vibrations, others are formed, which makes them conducive to resisting heat. On the other hand, shorter polymer, for the same quantity of applied heat, they wiggle or vibrate so violently that much fewer bonds are formed than those broken, and so they degrade far more quickly on heating. Nevertheless, massive amounts of applied heat to long length polymers make them also vibrate violently and so they will also degrade.

The melting of a fiber polymer system occurs that when the heat applied is so large, it severs the glue (attractive forces) that holds these polymer units together, and so the individual polymers are now free to move independently of each other. Gravity causes them to move to the lowest point on the plane they are situated, which melds them into a mass at that point. This is when we recognize that the fiber has melted.

Continued exposure of a heat source, will not only melt the polymer system (breaking of the inter-polymer forces of attraction), but will break up each polymer (breaking of the covalent bonds that hold individual polymers together, that is, severing the intra-polymer bonds). Breaking up the individual polymers will provide feedstock to accelerate the flames and so cause what we would observe as fiber combustion.

The heat resistance and heat conductivity of any fiber polymer system is related. For example, heat resistant fibers such as cotton or viscose will also conduct heat, whereas fiber polymer systems that are poor in resisting heat, such as wool or nylon, will be a non-conductor or an insulator of heat. The ability of fibers to conduct heat (i.e. passing it along the backbone of its polymers and at the same time not severing inter-polymer links) is a function of its length (see above paragraph) and a function of the degree of polarity of its polymers. Polar sites along the polymer backbone may provide pathways for heat to be dissipated by electronic means (that is not too dissimilar in concept to the conduction of an electric current travelling through copper wires).

The thermoplastic nature of a fiber polymer system centers on whether inter-polymer forces can be broken at the same time, under controlled conditions of heating, which then can be induced, under another set of controlled conditions, to adopt new positions in the polymer system of the fiber, which will be then create new inter-polymer bonds and so on cooling, hold the new set position. The fiber is then said to be heat set. Thus temporarily melting the fiber polymer system is necessary for heat setting to occur and so the fiber polymer system is said to be thermoplastic (i.e. heat changeable). Thermoplastic fibers do not loose their heat-setting properties and so they can be made changeable more than once, and are often used in the manufacture of apparels.

Amorphous Fibers versus Crystalline Fibers
In general, the more amorphous fibers are:
(a) The more absorbent
(b) The weaker in strength
(c) The less durable
(d) The more easily degraded by chemicals
(e) The more easily dyed
(f) The more pliable and softer handling
(g) The more plastic and so the more easily distorted.

In general, the more crystalline fibers are:
(a) The less absorbent
(b) The stronger the fiber
(c) The more durable
(d) The less easily degraded by chemicals
(e) The less easily dyed
(f) The less pliable, the stiffer the handling
(g) The less plastic, and so the more resistant to distortion.

[1] E.J. Gawne, Fabrics For Clothing Chas. A. Bennett Co. Inc., Illinois (1973).
[2] E.P.G. Gohl and L.D. Vilensky, Textile Science, Longman Cheshire, Melbourne (1989).

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