Saturday, June 7, 2014

Man-Made Cellulosic Fibers – Viscose[1-3]
Art Resource

Marie-Therese Wisniowski

Preamble
This is the twenty-eighth 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
Rayon occurs as two types, both of which are made from cellulose; cuprammonium and viscose. In recent years several new rayons called polynosic rayon have been developed with greater wet strength. Rayon is one of the cheapest "synthetics" and it is easily blended.

Generally, rayon is made primarily from wood pulp, which is chemically converted into a soluble compound. It is then dissolved and forced through a spinneret to produce filaments which are chemically solidified, resulting in synthetic fibers of nearly pure cellulose. As rayon is manufactured from naturally occurring polymers, it is considered a semi-synthetic fiber. Specific types of rayon include viscose, cuprammonium, polynosic rayon (modal and lyocell) each of which differs in manufacturing process and properties of the finished product.

Today we shall concentrate on only one form of rayon called viscose.

Viscose Fibers.


Source and Production of Rayon
A regenerated cellulose finer means that it is cellulose in a different physical but same chemical form. Most rayons today are made by the viscose process; that is, wood pulp or cotton linters are soaked in caustic soda and treated with other chemicals so that the liquid can be forced through holes of a spinneret into a bath of sulphuric acid, which hardens the newly formed fibers. For smooth appearing fabrics, the filaments are wound directly on bobbins or cones. For cotton or wool-like fabrics, the filaments are cut into short lengths and spun into yarn.

Spun Viscose.
Note: Viscose rayon fiber polymer solution is first de-aerated and then pumped under pressure through a metering pump unit, then through the filter and finally through the spinneret.

Cuprammomium is another process which is similar to viscose, but usually results in a finer yarn.

Other more recently developed rayons are polynosic and high-modulus. Common trade names are Avril and Zantrel: these do not shrink in laundering and have a fine silky feel. They are stronger than regular rayons. In many ways they are similar to cotton.


General Properties of Rayons
Microscopic Appearance
The finer appears like a smooth rod. Therefore fibers tend to shed dirt easily and yarns will fray unless made from staple lengths and into spun rayon fabrics.

Burning Behaviour
Burns fast like other cellulose fibers with an door of paper and a soft greyish ash.

Strength
Most rayons are moderately strong when dry but are weaker when wet (see below). They need special care during washing. High-modulus rayons are strong and not weakened by moisture.

Resiliency
Wrinkles easily (see below) and so needs special finishing treatments.

Washability
May be washed like other cellulose fibers except some rayons lose strength when wet and need a short cycle when washed by machine. May be dried at any temperature and pressed with a medium-hot iron (almost as hot as for cotton). Will shrink unless given a shrink-resistant finish. Note: wrinkle-resistant finishes are also used to add body as well as to improve resistance to wrinkling and to minimize ironing.

Uses
Rayon is often known as the great imitator since it can be made to resemble fabrics of silk, cotton, linen and wool. It is often blended with other man-made fibers to add absorbency, to cut costs and because it takes certain dyes so beautifully. Most "paper" fabrics are made from rayon fibers.


Viscose
Viscose is a man-made, natural polymer, cellulosic or regenerated cellulose filament or stable fiber. The International Organization for Standardization defines viscose as: regenerated cellulose obtained by the viscose process. The name viscose was derived from the word "viscous", which describes the liquid state of the spinning solution. A viscous solution is thick and flows slowly, like honey or syrup.

The fiber density of viscose is 1.49 g cm-3, which makes it a heavy weight fiber, similar to cotton and flax.

Electron Micrograph of Viscose Fibers.

Viscose is a fine, regular filament or staple fiber that is usually manufactured in the crimped configuration in order to overcome the very regular, smooth and slippery nature of the uncrimped equivalent.

Viscose is extruded in fiber diameters of 12 to 22 microns, depending on its end-use requirements. The fiber length to breadth ratio is in excess of 2000:1, which ensures that even shorter staple fibers will spin satisfactorily into yarn.

Schematic Cross-Section of Viscose Fiber.
Courtesy reference[1].

The color of the extruded viscose filaments tends to be slightly off white, which is due to its translucency, and which permits some light to pass through the filaments before it is reflected. Although, most of the incident light is reflected, some of it is transmitted and so viscose appears off-white.

Most of the incident light upon viscose is reflected with considerable intensity from the staple fibers’ smooth and regular surface, resulting in a harsh bright luster. Hence a delustering agent, such as titanium oxide, is added to the spinning solution.

The viscose cellulose polymer is linear like cotton, but it does not have the spiral configuration of cotton. The viscose polymer system is similar to cotton, however, unlike cotton the amorphous region constitutes 65 - 60%, whereas the crystalline region is 35 - 40% (the reverse of the cotton polymer system). The viscose polymer is only composed of 175 celloboise units, and not the 5000 associated with the cotton polymer. This is due to the relative short viscose polymers making it difficult for a large ordered crystalline region.

The Cellobiose Unit of Viscose.
Note: Like cotton, the viscose polymer contains important chemical groups such as the hydroxyl group (-OH) and the methylol group (-CH2OH). Their polarity gives rise to hydrogen bonds between the OH groups of adjacent viscose polymers, yielding a structural integrity to the viscose polymer system. It should be also noted the van der Waals forces are also present, but these forces are much weaker than hydrogen bonds.


Physical Properties of Viscose
Tenacity
The viscose polymer system is very amorphous and so its filaments or staple fibers are weaker than cotton and only have a fair tenacity. The shorter, more poorly aligned viscose polymers give rise to fewer hydrogen bonds than would otherwise be possible. When wet, viscose is only half as strong as dry, which once again is attributed to its very amorphous nature, which readily permits the entry of water molecules into its polymer system. On entry the water molecules push the polymers apart, breaking significant numbers of hydrogen bonds, and rendering a weaker fiber.

Length of Polymer Chains of Viscose.
Note: Cotton has long polymer chains, whereas viscose has much shorter polymer chains. Longer polymer chains leads to larger crystalline regions (cotton ca. 65%), whereas shorter polymer chains leads to a less crystalline structure (viscose ca. 35%).
Courtesy of reference[1].

Elastic-Plastic Nature
Viscose is a limp handling fiber, because its polymer system is very amorphous. Its polymers are not sufficiently long for a more satisfactory alignment, and so do not allow the formation of more hydrogen bonds, which would result in a more rigid polymer system, and thus yielding a crisper handle to the viscose fiber and its textile materials.

The very amorphous nature and fewer hydrogen bonds of viscose when compared with cotton, enable the polymers to slide pass each other when the filament or staple fiber is put under strain. When the strain is removed, the polymers do not return to the original position they occupied in the polymer system. Thus, the polymer system of the filaments will be disarranged and the viscose fabric will become distorted, stretched, wrinkled and or creased.

Viscose and other regenerated cellulose fibers, become more plastic when wet, for similar reasons used to explain its reduced wet tenacity.

Hygroscopic Nature
The very amorphous polymer system of viscose, as well as the polarity of its polymer system, make viscose the most absorbent fiber in common use.

With regard to other hygroscopic properties (such as crispness and static electricity) the explanations given to cotton apply to viscose.

Thermal Properties
Viscose has similar thermal properties to cotton. The explanations offered for cotton also applies to this fiber. The poorer heat resistance and heat conductivity of viscose compared to cotton rests with the shortness of its polymer length. As heat is applied the small crystalline region polymers vibrate, but since in viscose the polymer chain is short, its vibrations cannot be dissipated along the length of its short chain when compared to the situation for longer chained cotton polymer. As a result, the vibrational energy per polymer unit is much more disruptive and so hydrogen bonds, which are severed, cannot be countered by those that are formed (the latter is larger than the former in the cotton polymer system).

Viscose is the most common man-made fiber that is not thermoplastic, due to its hygroscopic nature, which enables it to absorb water molecules very readily into its large amorphous region of the viscose polymer system. These water molecules sever a significant number of hydrogen bonds, which prevent the retention of any heat-set.


Chemical Properties of Viscose
The chemical properties of cotton and all the regenerated cellulose fibers are similar and so explanations given for the chemical properties of cotton apply also to viscose (as well as to all regenerated cellulose fibers). However, in general the shorter polymers and the very amorphous nature of the regenerated cellulose fibers (such as viscose) are responsible for the much greater sensitivity of these fibers to acids, alkalis, bleaches, sunlight and weather - when compared to cotton.

With regard to dyeing and printing, the regenerated cellulose fibers will generally color more brightly, even when delustered, than their mercerised cotton equivalents. This is due to the greater amount of incident light reflected by viscose, even when delustered. The reflected light brightens, or increases the value and chroma of the dyed or printed viscose fabrics.

Luster of Dyed and Printed Viscose Fabric.


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).

[3] E. J. Gawne, Fabrics for Clothing, 3rd Edition, Chas. A. Bennett Co., Peoria (1973).

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