Saturday, July 5, 2014

Man-Made Cellulosic Fibers - Esters[1-3]
Art Resource

Marie-Therese Wisniowski

This is the twenty-nineth 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 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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Textile fibers, which are composed of ester-cellulose, are called acetate fibers. There are two types: acetate and triacetate. Since acetate fibers have very similar properties both will be considered in this post. Note: In chemistry an ester is designated the chemical formula of "RCOOR", where "C" stands for carbon, "O" for oxygen and the "R" represents a large array of possible chemical groups. The term "acetate" is derive from "acet" and "ate", with the former derived from acetic acid (the acid of vinegar) and the latter donates a chemical salt. Acetate therefore means a salt of the acid of vinegar (i.e. a salt of acetic acid).

Acetate was developed about the same time as rayon and it was initially named as acetate rayon until 1952 when the US Federal Trade Commission recognised that it had a different chemical composition and properties to rayon and so ruled it could not be labeled as rayon. It therefore became known as acetate.

Acetate fibers are frequently used for women's dresses and exhibits their beauty in the form of women's formal wear, suits, coats or knitwear.

Triacetate was introduced in 1954 to serve slightly different needs, as it can be more easily washed. Anrnel is a trade name for triacetate.

This post will focus on acetate and triacetate fibers.

Source and Production
Acetate is made from cellulose materials such as wood pulp, but the manufacturing process results in a fiber that is chemically different from rayon. The cellulose is combined with acetic acid to create a new chemical compound, cellulose acetate. When a solvent, acetone, is added, the cellulose acetate dissolves into a honey-like consistency, ready to be forced through the holes of a tiny spinneret. Fine filaments are produced and twisted together, then wound on a bobbin in the form of a yarn.

To make a cellulose salt of acetic acid, such a salt is known as an ester in Organic Chemistry parlance. As a result, acetate and triacetate fibres are also referred to as cellulose esters or ester-cellulose fibers.

In the manufacture of ester-cellulose fibers, triacetate is produced first; it is known as the primary cellulose acetate fiber since it is fully acetylated which means that the six –OH groups of the cellobiose unit is converted to six –OCOCH3 (or acetate groups). The secondary cellulose acetate fiber (acetate) is hydrolyzed (reacted with water) so that theoretically only 2.3 or 2.4 acetyl or acetate groups per the glucose unit occur and so it is only partially acetylated.

Manufacture Of Acetate Polymers.
Courtesy of Japan Chemical Fiber Association.

Hence there are two types of acetate fibersthat can be formed in the above process:
(i) Acetate – a man-made natural polymer base that is called a secondary cellulose filament or staple fiber.
(ii) Triacetate – a man-made natural polymer base that is called a primary cellulose acetate filament or staple fiber.

World consumption of acetate cellulose fibers.

General Properties of Acetate and Triacetate Fibers
Fiber Density
The fiber density of these fibers is 1.32 g cm-3 which makes them medium weight fibers.

Micro Structure of Acetate and Triacetate Fibers
Both of the polymer systems of acetate and triacetate are linear, but the acetyl groups form bulky side groups. The acetate or secondary cellulose acetate polymer is about 160 nm long and about 2.3 nm thick, whereas the triacetate or primary cellulose acetate polymer is 240 nm long and about 2.6 nm thick.

Triacetate Fibers.

The important chemical groupings of the acetate polymers are the hydroxyl and acetate groups. The hydroxyl groups tend to indicate the existence of hydrogen bonding. However, these hydrogen bonds are few in number, and cannot be significant in maintaining the polymer system structure. The acetate groups of the secondary cellulose acetate polymers are essentially non-polar and out number the hydroxyl groups. This explains the lack of polarity of the acetate polymers. In regard to the triacetate, its polymers must have less than 8% of hydroxyl groups by definition, and so are too few in number to cause significant hydrogen bonding. The triacetate polymers also lack polarity for this reason. Hence both polymer systems rely on weak van der Waals forces for their structural integrity.

Acetylation changes hydrophilic cellulose into hydrophobic cellulose acetate.
Courtesy reference[1].

Macro Polymer Structure
The acetate polymer system is estimated to be 40% crystalline and 60% amorphous, whilst the triacetate is considered somewhat more crystalline. Both fibers are therefore very amorphous. The inter-polymer system is held together via hydrogen bonding, but mostly by weak van der Waals forces.

Since both types of acetate fibers have a polymer backbone of hexagonal units, their polymer systems could be visualized as a disarranged roll of chicken wire, the disarranged portions being the amorphous regions of the polymer systems, whilst the more orderly sections are the crystalline region of the polymer systems.

Burning Behaviour of Fibers
Continues to burn when removed from source of flame. May actually melt. Has a vinegar or acetic acid odour. Forms a hard, black plastic bead.

Fiber Strength
Not too strong, lacks abrasion resistance. For this reason, it is considered a beauty fiber. It is weaker wet than dry.

Moisture Absorbency
Absorbs less moisture than cellulose and protein fibers, but more than other thermoplastics. Because of this it is less likely to shrink and will dry more quickly. It is more resistant to soiling than cellulose and protein fibers, but less than other thermoplastics. Note: As a thermoplastic fiber it is sensitive to heat and so a hot iron will make the fabric soften, glaze, and even fuse and develop holes.

Effect of Acetone
Acetone is a solvent. Fingernail polish remover contains acetone and so will cause holes if spilled on triacetate and acetate fibers.

Color Fastness
Blue and green dyes on acetate fabrics will often fade due to the gases contained in air (primarily oxygen). Solution dyed fabrics are permanently colorfast.

Specific Triacetate Properties
Triacetate was primarily developed since it is stronger, can withstand higher temperatures and is less likely to shrink or stretch. Originally used for tennis dresses because it washes so well, stays white, and has a good body, it is well liked for knitted fabrics and for many other easy-care garments.

Clothing made of acetate should be hand washed in luke warm water and rinsed well. Press on the wrong side while damp, with an iron at a low setting. Triacetate fabrics can be machine laundered and bleached if necessary. They can stand ironing at higher temperatures, preferably the wrong side. Many acetate garments should be dry cleaned.

Considered the beauty fiber, acetate has a pleasing hand and drape so it is often used for evening gowns and party frocks.

Triacetate is especially popular for jersey dresses, for travel wear, for summer pleated skirts, and other easy care needs.

Physical Properties
Both types of acetate fibers are weak due to the amorphous nature of their polymer systems, which limits the number of inter-polymer forces of attraction, which can occur. In addition, the predominant forces of attraction between polymer units are the weak van der Waals forces.

Acetate and triacetate become weaker when wet, which occurs because the water molecules enter the amorphous region of the polymer systems, thereby pushing the polymer units apart (i.e. swells the region), and so weakening the short range attraction of the weak van der Waals forces. This causes a loss in tenacity of the filament or staple fiber.

Elastic-Plastic Nature
Both acetate and triacetate are plastic because their amorphous regions being the dominant region, and because of weak van der Waals forces of attraction that occur between the polymer units. With such weak forces and because of their amorphous nature, slippage readily occurs, even under slight strains, causing these textile materials to readily distort and/or wrinkle.

Both acetate and triacetate fibers become more plastic when wet, since water entering the amorphous regions break a significant number of inter-polymer forces of attraction. The disruption of the van der Waals forces makes slippage even easier, under the slightest strain and so these fabrics are more likely to distort and/or wrinkle when wet or just damp.

Acetate filaments and staple fibers have the softest handling of all textiles in common use, which once again is due to the amorphous nature of the polymer system and the weak bonding between polymer units. When pressure, such as handling, is applied to these secondary cellulose textile materials, weak inter-polymer forces present little resistance, creating a soft handling sensation.

Triacetate fibers tend to have a stiffer handle, particularly after heat setting. It is considered that after heat setting, the triacetate fibers move closer together (see thermal properties below) providing a more rigid polymer structure due to van der Waals forces of attraction increasing significantly in magnitude (because of the smaller distance) and so, triacetates offer greater resistance to any pressure such as handling.

Hygroscopic Nature
Despite their amorphous nature, both acetate and triacetate fibers have only fair moisture absorbency, mainly due to their low polarity and so have little attraction to the water molecules, which are polar.

Acetate polymer contains about two hydroxyl groups per cellobiose unit, which makes these polymers slightly more polar and the filaments and staple fibers more absorbent than those of triacetate. The latter polymer system contains six ester groups, which impart very little, if any, polarity making it more hydrophobic (water hating) than acetate. The triacetate fiber can even become more water hating on heat setting. This is due to the triacetate polymers being forced closer together, thereby even reducing the size of the inter-polymer spaces. This restricts the entry of water molecules into the polymer system. In effect, heat setting reduces the moisture absorbency of the triacetate polymer system by about half.

Finally, the limited hygroscopic nature of both types of acetate fibers, makes them prone to a build up of static electricity in dry atmospheric conditions. The limited hygroscopic nature is due to the lack of polarity of the polymer systems, thereby preventing attraction of water molecules into the amorphous regions of the polymer system that would dissipate static electricity in dry climates.

Thermal Properties
The degree of polymerization of viscose is 175 and that of acetate and triacetate polymers is 130 and 225 respectively. This suggests that the acetate and triacetate polymer lengths are similar to viscose and much smaller than cotton. Hence the explanation of viscose poor heat resistance and conductivity can be applied to both the acetate and triacetate filaments, and to their staple fibers.

Both fibers are thermoplastic, which means that they may be shaped, set, creased or pleated by the application of heat. When acetate textile materials are heated, the inter-polymer forces of attraction between the polymer units are severed. This permits the acetate and triacetate polymers to assume the configuration required of them by the set that is being applied to their textile material. On cooling, the inter-polymer forms of attraction reform to hold the polymers in this new position.

Secondary cellulose acetate cannot be heat set satisfactorily because it has relatively few hydrogen bonds. When the system is subject to the slightest strains, the new inter-polymer configuration can be altered since this will severe the van der Waals forces of attraction between polymer units and so allow the acetate polymers to be readily displaced from their new configuration.

On the other hand, primary cellulose acetate will retain a heat set more satisfactorily than secondary cellulose acetate, even though the polymer system is held together by weak van der Waals forces. Generally, a setting of a thermoplastic polymer system is thought to occur via a rearrangement of the polymer system configuration, resulting in an increase in tenacity. However, in the case of triacetate, the increase in tenacity does not occur on heat setting. A rearrangement of the triacetate polymers does occur, since the triacetate material will display a stiffer more paper-like handle, after heat setting. In addition, heat setting reduces the moisture absorbency by about half.

The handle observation is consistent with the process of recrystallization, which implies that on heating, the polymer units of triacetate get closer together increasing the strength of the van der Waals forces of attraction between the polymer units, and on cooling this renewed crystallization allows for resistance to straining against the new set positions of their filaments and staple fibers, thereby giving the fabric a more paper-like handle. In addition, the triacetate polymer system is thought to contract, and so reduce in size the inter-polymer spaces, and so prevent water uptake (i.e. causing a significant reduction in moisture absorbency).

Chemical Properties
Effect Of Acids
As both of the acetate polymers have essentially a cellulose backbone, acids hydrolyze them, causing polymer degradation, and resulting in weakening and eventual destruction of textile materials.

The triacetate polymer system is somewhat more crystalline than that of acetate, and so textile materials of triacetate will be more resist to acid degradation.

Effect Of Alkalis
The celloluse backbone makes acetate and triacetate polymers more resistant to alkalis rather than to acids. However, the acetyl, acetate or ester side groups will be hydrolyzed or saponified (i.e. hydrolysis of an ester to an alcohol) on exposure to alkaline conditions. The effect of this is the conversion of the acetate groups to hydroxyl groups as are found in the original cellulose polymer.

Such alkaline hydrolysis occurs in the first instance on the surface of the filaments or staple fibers, resulting in a yellowing of white or dulling of color acetate and triacetate textile materials.

Effect Of Bleaches
Bleaches in general have the same effect on acetate fibers as they do on cotton fibers.

Effect of Sunlight and Weather
The relatively lack of polarity of the acetate and triacetate polymers is one of the factors contributing to fair to good sunlight and weather resistance. The lack of polarity protects the polymers from destruction since in general most of the pollutants and degrading agents possess polarity.

Finally, acetate textile materials have fair to good sunlight resistance because of their electronic configurations are stable and so resist the degrading effect of UV light.

The acetate fibers are not easy fibers to dye or print, due to the relative lack of polarity. Disperse dyes have been specifically developed for this purpose (see below). Using these dyes, both polymer systems show good light- and wash-fastness.

Disperse Dyes
Disperse dyes are relatively non-polar and so they are also known as non-ionic dyes. Their lack of polarity makes them compatible with the relatively non-polar acetate and triacetate polymer systems.

The fair to good light-fastness of disperse dyed and printed acetate and triacetate textile material is due to their stable electronic arrangement of the chromophores of the disperse dye molecules, which can withstand the degrading effects of UV light.

The good wash-fastness of disperse dyed and printed acetate and triacetate textile materials is due to the non-polar nature of the disperse dye molecules, which are insoluble in water. These dye molecules are water hating and so the weak van der Waals forces of attraction together with the dye molecule entanglement in the amorphous regions of the polymer systems all contribute, albeit in different order of significance, in preventing water to rinse out of the disperse dye molecule from the acetate textile materials.

Acetate Satin Fabric.
Note: It tends to look like silk.

[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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