- Email: [email protected]
Introduction to textile fiber identification
1 Introduction to textile fiber identification M M HOUCK, West Virginia University, USA
Abstract: The identification of fibers is critical to a number of industries, including textiles, forensic science, fashion, and design. The actual identification, however, varies with industry and method. Changes in textile technology create a constant need to improve identification methodology. The old methods – despite the increased pace of new technology – are often the best. Microscopy still dominates the field for analytical methods and provides a range of analysis barely possible with any other method. Combined with spectroscopy, microscopy is the quintessential fiber identification tool. Key words: fiber identification, microscopy, methodology, spectroscopy.
Manufacturers use set methods to ensure a quality product fit for purpose. This implies a market-based taxonomy with a company-product orientation, a supply chain of raw and processed materials, and explicit rules on categories. For example, the American Association of Textile Chemists and Colorists (AATCC) lists the following specified methods: • colorfastness to commercial laundering and to domestic washing • flammability of clothing textiles • smoothness of seams in fabrics after repeated home laundering • electrostatic propensity of carpets • wrinkle recovery of fabrics: appearance method • dimensional changes in textiles other than wool. The titles to these methods indicate what is important to define in their products. AATCC lists microscopy as a method to identify fibers but notes that it should be used with caution since manufactured fibers are frequently produced in a number of modifications which alter their appearance.1 The Association also lists ‘reaction to flame’ (Table III) as a test method with the following diagnostics: melts near flame, shrinks from flame, burns in flame, etc. By contrast, ASTM International lists the following in their methods: • • •
flame resistant materials used in camping tentage pile retention of corduroy fabrics elastic properties of textile fibers 3
4 • •
Identification of textile fibers performance specifications for underwear fabrics, woven, men’s and boys’ commercial moisture regains for textile fibers.
While the view is similar to that of AATCC, ASTM International lists infrared spectroscopy as the preferred method for fiber identification: ‘additional physical properties of the fibers such as density, melting point, regain, refractive indices, and birefringence . . . are useful for confirming the identification’ (Volume 7.01, D276). Both organizations treat the samples as bulk for purposes of the possible identifying tests; after all, if you make them, sample size should not be a problem. By comparison, the forensic sciences treat microscopy as the primary method for fiber identification: Microscopic examination provides the quickest, most accurate, and least destructive means of determining the microscopic characteristics and polymer type of textile fibers. Additionally, a point-by-point, side-by-side microscopic comparison provides the most discriminating method of determining if two or more fibers are consistent with originating from the same source.2
Forensic fiber examiners use microscopy first and then other methods, such as infrared spectroscopy, as confirmatory techniques. Why not start with infrared spectroscopy, as do the manufacturers? A company that spins fibers knows what products it makes; therefore, the universe of possible answers is sufficiently limited. For a forensic scientist, however, fibers found at a crime scene could conceivably be from nearly any source and you can’t make assumptions. A rayon fiber and a cotton fiber will both show up as ‘cellulose’ on an infrared spectrum; a microscope easily distinguishes between them. Two nylon 6,6 fibers may be chemically identical but have different diameters, cross-sections, or birefringences. The orientation of the analysis is different (comparison), although the goal is partially the same (identification). Forensic fiber analysis routinely has minimal samples with which to work and this structures what tests can be used. Forensic fiber analysis is also interested in traits of which the manufacturers may not be aware. For example, delustrants, typically titanium dioxide, are added to deluster or dull otherwise bright fibers. The manufacturers add the delustrants during the fiber spinning process at a certain rate to achieve the desired end goals with little regard for the distribution of the granules within the fiber. For a forensic scientist, however, that distribution can be a significant comparator between two otherwise similar samples: one fiber with large, aggregated granules is dissimilar to one with small, evenly-distributed granules, all other factors being the same. The difference could be batch-to-batch variation, plant-to-plant variation, or some other node along the supply chain. Suffice it to say, the fiber manufacturer did not intentionally distribute the delustrant in such a way as to aid the forensic scientist. Forensic scientists, therefore, add additional infor-
Introduction to textile fiber identification
5
Production analytical methods Market taxonomy company-product orientation supply web explicit rules on categories Forensic analytical methods After-market taxonomy end use as used implicit rules on categories
Sharing methodologies Different approaches due to different goals – quality at lowest price for manufacturing, reconstruction and product tracking for forensics
Manufacturing
Forensics
1.1 Analytical methods used by manufacturers and forensic scientists.
mation to the market-based taxonomy with their own forensic taxonomy of products. As the physicist P.W. Bridgman succinctly said, ‘The concept is synonymous with the corresponding set of operations’,3 meaning, the methods you use frame the orientation of your analysis from the start. The analytical schemes used by both manufacturers and forensic scientists have value, although they may have different goals (Fig. 1.1). Some methods are used exclusively by one group, others are shared, while some shared methods have greater or lesser utility for the analyst. This book strives to provide a broader perspective about the methods available for fiber identification to create a fuller toolbox for the fiber analyst, regardless of their scientific orientation.
1.1
References
1. American Association of Textile Chemists and Colorists, AATCC Technical Manual, Research Triangle Park, NC: AATCC, 2008. 2. Scientific Working Group for Materials Analysis, Forensic Fiber Examination Guidelines, 1999, Forensic Science Communications 1(1), online at www.fbi.gov. 3. Bridgman PW. The Logic of Modern Physics. New York, NY: McMillan Publishers, 1928; page 23.
Abstract: The identification of fibers is critical to a number of industries, including textiles, forensic science, fashion, and design. The actual identification, however, varies with industry and method. Changes in textile technology create a constant need to improve identification methodology. The old methods – despite the increased pace of new technology – are often the best. Microscopy still dominates the field for analytical methods and provides a range of analysis barely possible with any other method. Combined with spectroscopy, microscopy is the quintessential fiber identification tool. Key words: fiber identification, microscopy, methodology, spectroscopy.
Manufacturers use set methods to ensure a quality product fit for purpose. This implies a market-based taxonomy with a company-product orientation, a supply chain of raw and processed materials, and explicit rules on categories. For example, the American Association of Textile Chemists and Colorists (AATCC) lists the following specified methods: • colorfastness to commercial laundering and to domestic washing • flammability of clothing textiles • smoothness of seams in fabrics after repeated home laundering • electrostatic propensity of carpets • wrinkle recovery of fabrics: appearance method • dimensional changes in textiles other than wool. The titles to these methods indicate what is important to define in their products. AATCC lists microscopy as a method to identify fibers but notes that it should be used with caution since manufactured fibers are frequently produced in a number of modifications which alter their appearance.1 The Association also lists ‘reaction to flame’ (Table III) as a test method with the following diagnostics: melts near flame, shrinks from flame, burns in flame, etc. By contrast, ASTM International lists the following in their methods: • • •
flame resistant materials used in camping tentage pile retention of corduroy fabrics elastic properties of textile fibers 3
4 • •
Identification of textile fibers performance specifications for underwear fabrics, woven, men’s and boys’ commercial moisture regains for textile fibers.
While the view is similar to that of AATCC, ASTM International lists infrared spectroscopy as the preferred method for fiber identification: ‘additional physical properties of the fibers such as density, melting point, regain, refractive indices, and birefringence . . . are useful for confirming the identification’ (Volume 7.01, D276). Both organizations treat the samples as bulk for purposes of the possible identifying tests; after all, if you make them, sample size should not be a problem. By comparison, the forensic sciences treat microscopy as the primary method for fiber identification: Microscopic examination provides the quickest, most accurate, and least destructive means of determining the microscopic characteristics and polymer type of textile fibers. Additionally, a point-by-point, side-by-side microscopic comparison provides the most discriminating method of determining if two or more fibers are consistent with originating from the same source.2
Forensic fiber examiners use microscopy first and then other methods, such as infrared spectroscopy, as confirmatory techniques. Why not start with infrared spectroscopy, as do the manufacturers? A company that spins fibers knows what products it makes; therefore, the universe of possible answers is sufficiently limited. For a forensic scientist, however, fibers found at a crime scene could conceivably be from nearly any source and you can’t make assumptions. A rayon fiber and a cotton fiber will both show up as ‘cellulose’ on an infrared spectrum; a microscope easily distinguishes between them. Two nylon 6,6 fibers may be chemically identical but have different diameters, cross-sections, or birefringences. The orientation of the analysis is different (comparison), although the goal is partially the same (identification). Forensic fiber analysis routinely has minimal samples with which to work and this structures what tests can be used. Forensic fiber analysis is also interested in traits of which the manufacturers may not be aware. For example, delustrants, typically titanium dioxide, are added to deluster or dull otherwise bright fibers. The manufacturers add the delustrants during the fiber spinning process at a certain rate to achieve the desired end goals with little regard for the distribution of the granules within the fiber. For a forensic scientist, however, that distribution can be a significant comparator between two otherwise similar samples: one fiber with large, aggregated granules is dissimilar to one with small, evenly-distributed granules, all other factors being the same. The difference could be batch-to-batch variation, plant-to-plant variation, or some other node along the supply chain. Suffice it to say, the fiber manufacturer did not intentionally distribute the delustrant in such a way as to aid the forensic scientist. Forensic scientists, therefore, add additional infor-
Introduction to textile fiber identification
5
Production analytical methods Market taxonomy company-product orientation supply web explicit rules on categories Forensic analytical methods After-market taxonomy end use as used implicit rules on categories
Sharing methodologies Different approaches due to different goals – quality at lowest price for manufacturing, reconstruction and product tracking for forensics
Manufacturing
Forensics
1.1 Analytical methods used by manufacturers and forensic scientists.
mation to the market-based taxonomy with their own forensic taxonomy of products. As the physicist P.W. Bridgman succinctly said, ‘The concept is synonymous with the corresponding set of operations’,3 meaning, the methods you use frame the orientation of your analysis from the start. The analytical schemes used by both manufacturers and forensic scientists have value, although they may have different goals (Fig. 1.1). Some methods are used exclusively by one group, others are shared, while some shared methods have greater or lesser utility for the analyst. This book strives to provide a broader perspective about the methods available for fiber identification to create a fuller toolbox for the fiber analyst, regardless of their scientific orientation.
1.1
References
1. American Association of Textile Chemists and Colorists, AATCC Technical Manual, Research Triangle Park, NC: AATCC, 2008. 2. Scientific Working Group for Materials Analysis, Forensic Fiber Examination Guidelines, 1999, Forensic Science Communications 1(1), online at www.fbi.gov. 3. Bridgman PW. The Logic of Modern Physics. New York, NY: McMillan Publishers, 1928; page 23.