Saturday, October 4, 2014

Man-Made Synthetic Fibers - Acrylic and Modacrylic[1-2]
Art Resource

Marie-Therese Wisniowski

Preamble
This is the thirty-second 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 first acrylic finer, trade-name Orlon, was developed by Du Pont in 1950. Since then others have been produced by different companies: Orlon, Acrilan, Creslan and Zefran are trade names of acrylic fibers.

The term "acrylic" is derived from the Latin word aryl, which means bitter, irritating or pungent and is descriptive of the properties of acrylic acid. Acrylonitrile is chemically related to acrylic acid and the term “acrylic” is a layperson's shortened version of polyacrylonitrile.

Early acrylic fibers were difficult to dye and so often appeared in pastel colors or blended with other fibers that were more easily dyed. Today acrylics come in most colors.

This post will focus on acrylics and modacrylics.


General Overview of Acrylics and Modacrylics
Acrylic fibers are man-made, synthetic polymer based on polyacrylonitrile filaments or staple fibers. They are divided into two camps:
(i) Polyacrylonitrile fibers - generally referred to as the acrylic fibers.
(ii) Modified polyacrylonitrile fibers generally referred to modacrylic fibers. Verel and Dynel are trade names of two modacrylic fibers. These fibers are similar to acrylics but have been modified so that they possess special characteristics that make them suitable for long pile garments, as they look and feel like fur. Wigs, other than those composed from human hair, are made from Dynel.

By definition, acrylic fibers are composed of at least 85% by weight of acrylonitrile units, whereas the modacrylic fibers must be composed of at least 35% but more than 85% by weight of acrylonitrile units. The remaining percentage is made from other polymer(s), which makes the dyeing and printing of these fibers easier.

Acrylic Fibers.

The fiber density of both these fibers averages 1.16 g cm-3, which makes these fibers relatively lightweight, and so makes possible the production of bulky knitwear that is lighter in weight than wool equivalents.

Electron Micrograph Of Arcylic Fibers.

The acrylic fibers appear as regular, translucent, slightly wavy filaments or staple fibers. Hence, in general, acrylics are produced as delustered fibers. The reasons for texturizing, and also crimpling acrylic staple fibers are similar to those give to viscose rayon (see earlier post).

World Wide Production Of Acrylics in 2007.

The diameter of acrylic fibers ranges from about 15 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.

Manufacture of Acrylics.

The main component of the acrylic polymer is the acrylonitrile monomer.

Formation Of Acrylonitrile.
Note: The repeating units; the nitrile (CN) and methylene(CH2)groups.

Polyacrylonitrile or as its commonly known acrylic polymer is a linear polymer with a length of about 500 nm and a thickness ranging from 0.3 nm at the methylene (CH2) group to 0.53 nm at the nitrile (CN) group.

This fiber must be composed of 15% of other polymers, called comonomers (or copolymers). Two such copolymers are acrylic acid and sodium vinylbenzene sulfonate, both of which have anionic groups that attracts the cation of basic dyes.

By definition the modacrylics must contain at least 35%, but not more than 65% of a monomer, other than acrylonitrile. The copolymers for these fibers vary significantly depending on the types of dyes used. For basic dyes, the copolymers all possess anionic groups (see above paragraph). Nevertheless, the copolymers in modacrylics are changed in order that the modacrylics fibers can be dyed with azoic, direct, metal complex, sulfur, vat, chrome and reactive dyes.


Macro Polymer Structure
It is generally accepted due to the non-polar nature of the acrylic polymer system that van der Waals forces are the dominant force that holds the polymer system intact. Furthermore, for van der Waals force to be the main attractive force, excellent alignment or orientation of the long polymer chains are necessary. This indicates a crystalline polymer system, which has been estimated to be about 70 – 80% crystalline and 30 - 20% amorphous. This tends to make the acrylic polymer system very crystalline.

The acrylic fibers show a longitudinal instability when subjected to hot, wet conditions such as immersion in boiling water (conditions used for dyeing). It is believed that during fiber manufacture, the polymer system becomes highly ordered in the lateral direction (i.e. across the fiber width). It further appears that the polymer system does not assume any particular order along the longitudinal direction of the fiber. The difference between the order in the lateral and the disorder in the longitudinal directions permits overstretching during manufacture and so places the polymer system under stress, due to the polymers being forced to adopt unnatural configurations. The intra-polymer forces (i.e. forces within the polymer) are strained in these unnatural configurations and will, if circumstances permit, return the polymers to their natural or relaxed configurations. This relaxation occurs in the presence of wet heat (i.e. during dyeing in boiling water), whereby van der Waals forces of attraction (that is responsible for the polymer to polymer cohesion) are severed, which allows the intra-polymer forces to return the polymer system configurations to their natural state. This is consistent with the observation that the fibers contract in length but increase in bulk under wet heat conditions.

Acrylic fibers hot-stretched.
Note: During a hot wet process, the stretched fibers relax and shrink, pulling the other fibers into loops.


Physical Properties
Tenacity
The fair to strong tenacity of acrylic fibers is due to the crystalline nature of the polymer system because their polymers are extremely long. These two characteristics enable a very efficient and effective environment for van der Waals forces to hold the polymer system intact.

The loss of tenacity that occurs when acrylic fibers are wet is due to the few water molecules that enter the amorphous region of the polymer system, thereby severing van der Waals forces of attraction between the polymer units.

Elastic-Plastic Nature
Acrylics have a soft handle, even though they are crystalline in nature. This is because the polymer system is only weakly held together and so if the acrylic filament or fiber is being crushed or bent, this has sufficient energy to overcome the forces of attraction and so enable the polymers to slide over each other. This slippage is evident on the ease of wrinkling the textile material in response to bending, stretching and/or crushing. The weakness of the forces of attraction, within the polymer system, is exemplified by the low temperature that is required in order to iron out wrinkles and creases etc.

Hygroscopic Nature
Acrylic fibers are hydrophobic (water hating) because the polymer system is highly crystalline, and so because the crystalline regions have too small voids, water molecules are prevented to enter this region, thereby restricting their entry into only the amorphous regions (which by definition is comparatively small). Furthermore, the slight polarity of the of the nitrile groups in the acrylic polymer and the stronger polarity of the anionic groups as copolymers, is not as dominant as its crystalline properties.

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

Thermal Properties
Acrylics are the most heat sensitive of the commonly used synthetic fibers. The weaker forces of attraction, which are the cohesive force between polymers units, is primarily responsible for their heat sensitivity. Body heat in conjunction with the stresses and strains of day-to-day wear, can provide sufficient energy to reduce cohesive effectiveness of the van der Waals forces of attraction in the acrylic polymer system. This causes the ease of wrinkling and/or distortions of acrylic fabrics under such conditions.

When near a naked flame, acrylic fibers tend to ignite immediately with a smoky luminous flame, rather than melt and then burn - as do nylon and polyester fibers. Acrylic fibers are the most flammable fibers in common use.

The ease at which acrylics burn is not applicable to modacrylic fibers, which have been copolymerized with chlorine containing monomers. Modacrylics will not burn, but will melt, char and disintegrate. Generally, polymer systems that contain chlorine-carbon bonds yield a two fold effect: (i) the chlorine-carbon bond severing process requires heat to be absorbed, thereby lowering the temperature of the flame; (ii) the liberation of chlorine radicals captures other radicals in the flame and so renders these as non-combustible materials, starving the flame of its “feedstock”. Countering this effect is the severing of carbon-oxygen and carbon-hydrogen bonds, which provides heat and “feedstock” for the flame to intensify. In general, the former processes out weigh the latter, since there are more carbon-chlorine bonds than carbon-hydrogen and carbon–oxygen bonds in these fabrics. Hence, in chlorine containing modacrylics, such as Teklan, contact with a flame will not readily produce combustion.

Chlorine-containing modacrylics, however, are more sensitive than other modacrylics or acrylics, since they have weaker van der Waals forces holding their polymer system intact, and so will soften and distort, at lower temperatures than conventional acrylic fibers.


Chemical Properties
Effect Of Acids
The acrylic fibers are resistant to acids, because their polymer systems do not contain any chemical groups, which will attract or react with acidic ions. Hence they are used for clothing of people who work with common chemicals.

Effect Of Alkalis
The very crystalline nature of the acrylic polymer system provides too small gaps for entry of alkali substances. However, surface alkaline hydrolysis (or surface saponification) will occur. Thus any nitrile group (CN) and/or anionic or basic group on the surface of the fiber will react with sodium or the cation of the alkali. It should be noted that the anionic or basic groups were introduced via the copolymerization process, while sodium is a major constituent of such common alkalis as soap, laundry detergent powder or liquids, washing soda etc. This fiber saponification is gradual; it will eventually lead to surface discoloration, yellowing, and/or dulling of the acrylic textile.

Effect of Bleaches
Bleaches in general have the same effect on acetate fibers as they do on cotton fibers (see earlier post).

Effect of Sunlight and Weather
Acrylic fibers are the most sunlight and weather-resistant fibers in common use. Hence they are used for awnings. Their resistance to atmosphere, which is slightly acidic, is due to their innate resistance to acids in general.

Acrylic textiles, when subjected to sunlight, will initially suffer a small loss of tenacity, which then levels off. The levelling off is due to a slight internal polymer arrangement that causes particular sections of the polymer system to form stable ring structures, which is fuelled energy-wise, by the continual exposure to sunlight. This enables the polymer system to resist the influence of UV light and other degrading agents.

Color-Fastness
The acrylic and modacrylic fibers are mostly dyed and printed with basic and disperse dyes. Hence, their color-fastness will be discussed below.

Basic Dyes
When dyes were originally developed for acrylic fibers, they were referred to as “modified” basic dyes. Since the original basic dyes that were used on cellulose fibers were no longer in use, these altered basic dyes have had the descriptor “modified” dropped and so they were labeled as the “basic dyes”.

Basic dyes are also known as cationic dyes, since the colored portion of the basic dye is cationic or the positively charged part of the dye molecule. The cationic (or basic) radical is attracted to the anionic (or acidic) radical on the acrylic or modacrylic copolymers.

The very good wash-fastness of basic dyed and/or printed acrylic textile materials is due to the hydrophobic and very crystalline nature of the acrylic polymer system. These properties minimize the entry of water molecules into the amorphous regions, which are water hating, and by far the largest region. The voids in this region are too small for entry of water molecules, and so there is little opportunity for water up-take in both the crystalline and amorphous region of the acrylic polymer system. Hence there is little tendency for the dye molecule to be rinsed from the fabric.

Basic dyed and/or printed acrylic textile have also very good light-fastness. The crystalline structure produces very efficient and effective van der Waals forces, which hold the polymer system intact. The chromophores in the basic dye molecule resist degradation from UV light.

Basic Dyed Acrylics.

Disperse Dyes
Acrylic fibers, which are hydrophobic, are readily dyed with non-ionic disperse dyes. The fair to good light-fastness of disperse dyed and/or printed acrylic textile materials is attributed to the dyed molecule’s non-ionic nature, which tends to indicate that its chromophores are resistant to UV light. Nevertheless, prolonged exposure to UV light will adversely affect coloration.

Disperse dyed and/or printed acrylic textile materials have good wash-fastness for the same reasons attributed to basic dyes


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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