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Fibers

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Fibers

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  1. Fibers Chem-195

  2. Fibers • Fiber: is the smallest unit of a textile material woven or twisted together to form a thread or yarn. • Fibers (like hair) are among the most common items left at a crime scene. • Class evidence- mass produced by textile difficult to trace back • Probative value- can create connection btwn a victim and suspect. (evidence which is sufficiently useful to prove something important in a trial) Chem-195

  3. Types of Fabric Weave In a weave, the lengthwise yarn is called the warp. The crosswise yarn is called the weft or woof. Types include: • Plain • Twill • Satin Chem-195

  4. Woven Fabric • PLAIN • Simplest and most common weave • Warp and weft pass under each other alternately • Design resembles a checkerboard Chem-195

  5. Woven Fabric • TWILL • Create by passing the warp yearn over one to three weft yearns before going under one • Makes a diagonal weave • Design resembles a stair steps • Denim is the most obvious example Chem-195

  6. Woven Fabric • SATIN • The yarn interlacing is not uniform • Creates long floats • Interlacing weave passes over four or more yarns • Satin is the most obvious example Chem-195

  7. Knitted Fabric Knitted fabrics are made by interlocking loops into a specific arrangement. It may be one continuous thread or a combination. Diagram: Chem-195

  8. Blendsuse of different fabrics & colors to create the warp and weft patterns. Chem-195

  9. Fibers All fibers are made of polymers which are long chains of repeating units. The word polymer means many (poly) units (mer). The repeating units of a polymer are called monomers. Chem-195

  10. Fiber Morphology Investigators use: 1. fiber cross section 2. chemical structure 3. synthetic polymers for forensic analysis Chem-195

  11. Fiber Cross Section • The cross section of a man-made fiber can be manufacturer-specific • Unusual cross sections increase fiber association. Chem-195

  12. Synthetic Polyester Rayon Nylon Acetate Acrylic Spandex Natural Silk Cotton Wool Mohair Cashmere Types of Fibers Chem-195

  13. Natural Fibers Many different natural fibers that come from plants and animals are used in the production of fabric. Cotton fibers are the plant fibers most commonly used in textile materials The animal fiber most frequently used in the production of textile materials is wool, and the most common wool fibers originate from sheep. Chem-195 http://www.fireflydiapers.com/articles/diaperarticle_naturalfibersabsorb.htm

  14. Natural Fibers • Wool--animal fiber from sheep, may also be goat (mohair), rabbit (angora), camel, mink, beaver • wool: composed of polypeptide keratin chains of amino acid. • main bond is sulfur which accounts for the smell of wool when burned Wool Fibers (400X) Chem-195

  15. Chemical structure: Natural fibers • Silk: composed of proteins, only 2 amino acids • Silk--animal fiber produced by silkworms (cocoon) Chem-195

  16. Cotton • #polymers is glucose # smell like burning leaves # 40% of all fabric is cotton # 60% of all clothes and furniture # low probative value because its too common Chem-195

  17. Linen: made from flax plant (genus (Linum of the family Linaceae, the flaxfamily) # Contains cellulose (fibers are longer than cotton) # often blended with other fibers because it is brittle. Chem-195

  18. Fibers under a microscope Synthetic Fibers More than half of all fibers used in the production of textile materials are synthetic or man-made. Nylon, rayon, and polyester are all examples of synthetic fibers. *nylon and polyester are the most common Cross-section of a man-made fiber Chem-195 Images: http://www.trashforteaching.org/phpstore/product_images/YarnWS.JPG http://www.fbi.gov/hq/lab/fsc/backissu/july2000/deedric3.htm#Fiber%20Evidencehttp://www.jivepuppi.com/images/fiber_evidence.jpg

  19. Synthetic Fibers Made from cellulose • Rayon--first man-made fiber; • Acetate:– made from a reaction with acetic acid. Chem-195

  20. Synthetic Fibers(Made from derivatives of petroleum, coal and natural gas) • Acrylic- used in carpets • Spandex(a type of material that stretches and is used especially for making clothes fit tightly) elastic properties Chem-195

  21. Forensics of Fiber Analysis • Cross transfers of fiber often occur in cases in which there is person-to-person contact • More contact= more fiber transferred Chem-195

  22. Forensic Analysis • What can be used to identify and compare fibers as forensic evidence? 1. fiber cross section • View through microscope • Burn test • Thermal decomposition • Refractive index • Chemical test Chem-195

  23. Microscopic Examination • A compound microscope: uses light • The comparison microscope (two compound microscopes joined by an optical bridge) is used for more precise identification.  • phase-contrast microscope, reveals fiber structure • Scanning electron microscope converts the emitted electrons into a photographic image for display.  This affords high resolution and depth of focus. Chem-195

  24. Fiber Comparison Can you tell the difference(s) between the cotton on the left and the rayon on the right? Chem-195

  25. Spectrometer • The spectrometer, which separates light into component wavelengths.  • By passing light through something to produce a spectrum, the analyst can read the resulting lines, called "absorption lines."  • specific wavelengths are specific to molecules of the substance.  Chem-195

  26. Fiber Evidence Collection • Fibers are gathered at a crime scene with tweezers, tape, or a vacuum.  • They generally come from clothing, drapery, wigs, carpeting, furniture, and blankets.  • For analysis, they are first determined to be natural, manufactured, or a mix of both. Chem-195

  27. Steps of Fiber Analysis • The first step in fiber analysis is to compare color and diameter.  • Dyes can also be further analyzed with chromatography, which uses solvents to separate the dye's chemical constituents.  Chem-195

  28. Fiber Color • Color influences the value given to a particular fiber identification. • Often several dyes are used • How color is applied and absorbed along the length of the fiber are important comparison characteristics. • Color-fading and discoloration can also lend increased value to a fiber association. Chem-195

  29. Fiber Analysis Chem-195

  30. Testing Fibers • Burn Test: look @ how a fiber burns, its odor, and appearance of ash. • Thermal decomposition: how a fiber breaks down when heated. *it returns to its monomer (initial building block) * ex: Acetate acetic acid litmus test will turn paper blue to red Chem-195

  31. Thermal Decomposition of fiber activity Chem-195

  32. 3) Chemical test: test the solubility and decomposition of a fabric using a strong acids (Hydrochloric acid or sulfuric acid) or strong bases (NaOCl, acetone, NaOH) – this determines the fabrics polymers. 4) Density: m/v (pg. 148). Density of water is 1.00 g/ml. Olefin is the only fabric that will float in water. Chem-195

  33. 5) Refractive index: bending of light as it passes from air into a solid or liquid -- investigators measure the refractive index of an unknown sample with liquids of a known refractive index. --place sample in different liquids until Becke line is no longer visible liquid same index as fabric liquid w/ w/ high index low refractive index A B C Chem-195

  34. 6) Fluorescence: some fibers will fluoresce when exposed to UV light --- laundry soap and some bleach has whiting agents that cause blue light to be reflected making it appear whiter. 7)Dyes: investigators use a fabric to see if it accepts a particular dye to identify and compare it to an unknown sample Chem-195

  35. 8)Chromatography: separation of dyes by thin layer chromatography (TLC) Rfvalue = distance of pigment distance of solvent front Chem-195

  36. Other test used in Fiber Analysis 9) FTIR: Fourier Transform Infrared • based on the absorption and wavelength of light in a fabrics polymer. • can be used on a single fiber • Non-destructive 10) PGS-MS: Pyrolysis Gas Chamber-Mass Spectrometry • Burns and separates each combustion product of sample • Match results of chromatogram & products to known • Can be used in short length fibers but is destructive. Chem-195

  37. Fiber Forensics FYI • analyst gets only a limited number of fibers to work with—sometimes only one. • Fibers are sent to the lab for analysis. • Fibers from scene are compared to victim Chem-195

  38. Fiber Transfer and Persistence • Fibers can be used as trace evidence due to fiber transfer. • How easy the fiber is transferred is affected by: • Area of contact • the amount of pressure used • Any friction due to side to side contact • Number of passes or contacts • Kind of clothing donor/recipient was wearing • Fiber type, length and texture • History of the garment. Chem-195

  39. How long transfer fibers remain on victim is fiber persistence. • Time of wear and movement • What is covering the fabric • Type of activity • Weather condition ** fiber persistence decreases exponentially (with reference to an increase) more and more rapidly. with time of wear. Chem-195

  40. Chem-195

  41. Types of Fibers - Key A B C Acrylic Yarn Cotton Yarn Nylon Rope D E F Polyester Yarn Rayon Rope Wool Yarn Chem-195