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Chapter 15: Textile Fibers
15.1: Textile Fibers
Textile Fibers – a ubiquitous type of evidence. They are “common” in the sense that textiles surround us in our homes, offices, and vehicles. We are in constant contact with textiles daily.
- Textile fibers are also among the most neglected and undervalued kinds of forensic evidence.
- It is a unit of matter that forms the basic element of fabrics and other textiles and has a length of at least 100 times its diameter.
Natural Fiber – any fiber that exists as a fiber in its natural state.
Manufactured Fiber – any fiber derived by a process of manufacture from any substance that, at any point in the manufacturing process, is not a fiber.
Fibers can also be designated by their chemical makeup:
- Protein fibers – composed of polymers of amino acids
- Cellulosic fibers – are made of polymers formed from carbohydrates.
- Mineral (inorganic) fibers – may be composed of silica obtained from rocks or sand.
- Synthetic fibers – are made of polymers that originate from small organic molecules that combine with water and air.
Filaments – a type of fiber having indefinite or extreme length, such as silk or a manufactured fiber
Staple Fibers – natural fibers (except silk) or cut lengths of filament, typically being 7/8–8in (2.2–28.5cm) in length.
Denier – a direct numbering system in which the lower numbers represent the finer sizes and the higher numbers the larger sizes.
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Federal Trade Commission Textile Products Identification Act, 1954, Definitions
- Acetate – the fiber-forming substance is cellulose acetate. Where not less than 92% of the hydroxyl groups are acetylated, the term “triacetate” may be used as a generic description of the fiber.
- Acrylic – the fiber-forming substance is any long-chain synthetic polymer composed of at least 85% by weight of acrylonitrile units.
- Anidex – the fiber-forming substance is any long-chain synthetic polymer composed of at least 50% by weight of one or more esters of a monohydric alcohol and acrylic acid.
- Aramid – the fiber-forming substance is any long-chain synthetic polyamide in which at least 85% of the amide linkages are attached directly to two aromatic rings.
- Glass – the fiber-forming substance is glass.
- Lyocell – composed of precipitated cellulose and produced by a solvent extrusion process where no chemical intermediates are formed.
- Metallic – composed of metal, plastic-coated metal, metal-coated plastic, or a core completely covered by metal.
- Modacrylic – the fiber-forming substance is any long-chain synthetic polymer composed of less than 85% but at least 35% by weight of acrylonitrile units.
- Novoloid – the fiber-forming substance is any long-chain synthetic polymer composed of at least 85% of a long-chain polymer of vinylidene dinitrile where the vinylidene dinitrile content is no less than every other unit in the polymer chain.
- Nylon – the fiber-forming substance is any long-chain synthetic polyamide in which less than 85% of the amide linkages are attached directly to two aromatic rings.
- Olefin – the fiber-forming substance is any long-chain synthetic polymer composed of at least 85% by weight of ethylene, propylene, or other olefin units.
- Polyester – the fiber-forming substance is any long-chain synthetic polymer composed of at least 85% by weight of an ester or a substituted aromatic carboxylic acid, including but not restricted to substituted terephthalate units and para-substituted hydroxybenzoate units.
- Rayon – composed of regenerated cellulose, as well as manufactured fibers composed of regenerated cellulose in which substituents have replaced not more than 15% of the hydrogens of the hydroxyl groups.
- Saran – the fiber-forming substance is any long-chain synthetic polymer composed of at least 80% by weight of vinylidene chloride units.
- Spandex – the fiber-forming substance is any long-chain synthetic polymer composed of at least 85% of a segmented polyurethane.
- Vinyl – the fiber-forming substance is any long-chain synthetic polymer composed of at least 50% by weight of vinyl alcohol units and in which the total of the vinyl alcohol units and any one or more of the various acetal units is at least 85% by weight of the fiber.
- Vinyon – the fiber-forming substance is any long-chain synthetic polymer composed of at least 85% by weight of vinyl chloride units.
- Yarn – continuous strands of textile fibers, filaments, or material in a form suitable for weaving, knitting, or otherwise entangling to form a textile fabric.
- Z-twist / S-twist – no twist at all.
- Plied Yarn – formed from several smaller single yarns twisted together.
15.2: Fabric Construction
- Fabric – a textile structure produced by interlacing yarns, fibers, or filaments with a substantial surface area about its thickness.
- Woven Fabrics – those fabrics composed of two sets of yarns, called warp and weft, and are formed by the interlacing of these sets of yarns.
- Knitted Fabrics – constructed of interlocking series of loops of one or more yarns and fall into two major categories: warp knitting and weft knitting.
- Courses – rows of loops across the width of the fabric.
- Wales – rows of loops along the length of the fabric.
- Nonwoven Fabrics – an assembly of textile fibers held together by mechanical interlocking in a random web or mat, by fusing of the fibers, or by bonding with a cementing medium.
15.3: Fiber Characteristics
- Crimp – the waviness of a fiber expressed as crimps per unit length.
- Cross-sectional shape – the shape of an individual filament when cut at a right angle to its long axis, is a critical characteristic of fiber analysis.
- Color – is introduced to manufactured fibers with dyes or pigments, while natural fibers may be originally white, off-white, or a shade of brown.
Natural Fibers
- Animal Fibers – come either from mammals (hairs) or from certain invertebrates, such as the silkworm. These are from wool-bearing animals, such as sheep and goats, or fur-bearing animals, like rabbits, mink, and fox.
- Plant Fibers
- Three major sources of fiber are derived from plants: seed, stem, and leaf.
- Technical Fiber – used in cordage, sacks, mats, etc., or individual cells, as in fabrics or paper.
- The examination of technical fibers should include a search for internal structures such as lumen, spiral vessels, or crystals and the preparation of a cross-section.
- Technical fibers should be mashed, fabrics teased apart, and paper repulped for the examination of individual cells.
- The most common plant fibers encountered in casework are cotton, flax, jute, hemp, ramie, sisal, abaca, coir, and kapok.
Various Natural Fibers and Their Microscopic Characteristics
| Kind | Plant | Genus and Species | Characteristics |
|---|---|---|---|
| Bast (stem) fibers | Flax (linen) | Linum usitatissimum | The ultimates (individual fiber cells) are polygonal in cross-section, with thick walls and small lumina. Microscopically, the fibers have dark dislocations, which are roughly perpendicular to the long axis of the fiber. |
| Jute | Corchorus capsularis | This fiber appears bundled microscopically and may have a yellowish cast. The ultimates are polygonal but angular with medium-sized lumina. It can be distinguished easily from flax by its counterclockwise twist. The dislocations appear as angular X or v’s and may be numerous. | |
| Ramie | Boehmeria nivea | Ramie has very long and very wide ultimates. The walls are thick and, in cross-section, appear flattened. Ramie has frequent, short dislocations, and longer transverse striations. In cross-section, radial cracks may be present. | |
| Hemp | Cannabis sativa | With the ultimates more bundled, a wider lumen, and fewer nodes, hemp is easy to distinguish from flax. Cross-sectioning hemp helps in distinguishing it from jute because hemp’s lumina are rounder and more flattened than jute’s. Hemp may also have a brownish cast to it. | |
| Leaf fibers | Sisa | Agave siciliana | Sisal is relatively easy to identify due to its irregular lumen size, crystals, spiral elements, and annular vessels. In cross-section, sisal looks somewhat like cut celery |
| Abaca | Musa textilis | Although potentially difficult to distinguish from sisal, abaca’s ultimates have a uniform diameter and a waxy appearance; often it is darker than sisal. Its ultimates are polygonal in cross-section and vary in size. Abaca may present spiral elements but often will have small crown-like structures | |
| Seed fibers | Cotton | Genus Gossypium | Mature cotton has a flat, twisted, ribbon-like appearance that is easy to identify. Cotton fibers are made up of several spiraling layers around a central lumen. |
| Kapok | Ceiba pentandra | Kapok fiber is used primarily for life preservers and upholstery padding because the fibers are hollow, producing very buoyant products. But they are brittle, which prevents spinning or weaving. | |
| Coir | Coco Nucifera | Coir comes from the husk of the coconut and, accordingly, is a very dense, stiff fiber easily identified microscopically. On a slide mount, coir appears very dark brown or opaque with very large, coarse ultimates |
Manufactured Fibers
- Manufactured fibers – the various families of fibers produced from fiber-forming substances, which may be synthesized polymers, modified or transformed natural polymers, or glass.
- Synthetic fibers – those manufactured fibers that are synthesized from chemical compounds.
15.4: Fiber Manufacture
- Spinning Dope – synthetic fibers are formed by extruding a fiber-forming substance, through a hole or holes in a shower head-like device called a spinneret.
- It is created by rendering solid monomeric material into a liquid or semiliquid form with a solvent or heat.
- Optical properties, such as refractive index, birefringence, and color, are those traits that relate to a fiber’s structure or treatment revealed through observation.
- The fluorescence of fibers and their dyes is another useful point of comparison. Thermal properties relate to the softening and melting temperatures for manufactured fibers and the changes the fiber exhibits when heated.
- Based on a fiber’s polymer composition, it will react differently to various instrumental methods, such as Fourier transform infrared spectroscopy (FTIR) or pyrolysis-gas chromatography (PGC), and chemicals, such as acids or bases.
15.5: Microscopic Characteristics
Polarized light microscope – the primary tool for the identification and analysis of manufactured fibers.
- It’s an easy, quick, and non-destructive way to determine the generic polymer class of manufactured and synthetic textile fibers.
Delustrants – finely ground particles of materials, such as titanium dioxide, that are introduced into the spinning dope.
The examination of the optical properties of manufactured fibers can yield a tremendous amount of information about their chemistry, production, end-use, and environment.
If a fiber has a higher refractive index than the medium in which it is mounted, it acts as a converging lens, concentrating light within the fiber. If the fiber has a lower refractive index than the mounting medium, it acts as a “diverging” lens and the light rays diverge from the fiber.
Becke Line – a thin bright line that appears when light rays slightly converge or diverge.
One of the more distinctive traits of a fiber is its birefringence. The interference colors seen after crossing the polarizing filters relate to a fiber’s material nature, orientation, and crystallinity.
Many dyes used to color textiles have fluorescent components, and their response to certain wavelengths of light can be useful in comparing textile fibers.
15.6: Colors in Textiles
Color is one of the most critical characteristics in a fiber comparison.
Simultaneous Contrast – the tendency of color to induce its opposite in hue, value, and intensity upon an adjacent color and be mutually affected in return.
Dye – an organic chemical that can absorb and reflect certain wavelengths of visible light.
Pigments – are microscopic, water-insoluble particles that are either incorporated into the fiber at the time of production or are bonded to the surface of the fiber by a resin.
Three main methods of analyzing the color of fibers:
- Visual Examination – this method is subjective and because of day-to-day and observer-to-observer variations, it is not always a repeatable method.
- Metameric Colors – those that appear to match in one set of lighting conditions but do not in another.
- Chemical Analysis – involves extracting the dye and characterizing or identifying its chemistry. It addresses the type of dye or dyes used to color the fiber and may help to sort out metameric colors.
- Instrumental Analysis – is objective and repeatable, the results are quantitative, and the methods can be standardized. Importantly, it is not destructive to the fiber, and the analysis may be repeated.
- Microspectrophotometer – an instrument that allows for the color measurement of individual fibers. It compares the amount of light passing through the air with the amount of light transmitted through or reflected off a sample.
Analyzing the fibers chemically offers not only a confirmation of the microscopic work but also may provide additional information about the specific polymer type or types that make up the fiber.
FTIR and PGC are both methods of assessing the chemical structure of polymers. FTIR is the preferred method because it is not destructive to the fibers.
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