Post on 29-Jan-2016
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Fiber AnalysisFiber Analysis
Fiber EvidenceFiber Evidence• Fiber
– The smallest unit of a textile material that has a length many times greater than its diameter
– Can be spun with other fibers to form a yarn (rope) that can be woven or knitted to form a fabric
• Can be natural (plant or animal) or man-made– Man-made fibers now account for over
½ of all textile production
Forensic ValueForensic Value• Are considered class evidence• Are common trace evidence at a
crime scene• Can be characterized based on
comparison of both physical and chemical properties
Fiber ClassificationFiber Classification• Natural fibers are classified
according to their origin– Vegetable or cellulose– Animal or protein– Mineral
• Some natural fibers can be altered into artificial ones– Cotton and rayon– Rayon is chemically-altered cellulose
Chemical AlterationChemical Alteration
Note the differences between the cotton fiber (left) and rayon (right).
Fiber EvidenceFiber Evidence• Important characteristics
– Type and length of fiber– Spinning method– Fabric construction
• Above characteristics can greatly affect transfer of fibers and significance as evidence
• Can originate from more than clothing– Furniture, upholstery, etc.
Fiber EvidenceFiber Evidence• Significance
– Based upon clear relationships between objects associated with crime
– Relative value of evidence can be based upon several factors• Type of fiber• Number of fibers• Color or variation of color• Location of fibers• Number of different fibers• Likelihood of transfer based on fiber
construction
Fiber EvidenceFiber Evidence• Useful in similar crimes to hairs• Since DNA is not found, often limited
to class characteristics due to mass production
• Transfer usually found through physical contact
• Routinely found in vehicle accidents or on glass/screens
Animal FibersAnimal Fibers•Silk
–Technically a protein secretion–No longer found in nature–Among the most expensive fibers–Fibers are altered by changing diet of silkworm–Does not resemble typical non-human hair –Highly light reflective
Mineral FibersMineral Fibers•Fiberglass
–Artificially produced fiber
•Asbestos–Natural fiber from the mineral serpentine
Plant FibersPlant Fibers•Cotton
–Most common plant fiber–Virtually worthless as evidence if white
•If dyed, the dye is more valuable than the cotton
–Distinctive twisted, ribbon-like shape
Plant FibersPlant Fibers• Flax
– Used to make linen– Distinctive lateral
structures through fiber– Can be expensive
Plant FibersPlant Fibers
Ramie fiber, common in SE Asia
Hemp fiber, the strongest natural fiber
Jute fiber, also common in Asia
Plant fibers are based on the polymer cellulose, the chemical that forms the cell wall of plant cells
Synthetic FibersSynthetic Fibers• First introduced in 1911 (rayon)
followed by nylon in 1939• Hundreds of names currently used for
essentially the same artificial fibers– Generally grouped into generic names
• Example: Polyester - Sold under 28 different names
Synthetic Fiber ChemistrySynthetic Fiber Chemistry• Based on polymers
– Long chain molecules• Come in natural and synthetic varieties• Synthetic polymers were first produced in
1909
– Originally discovered by accident• When a glass rod was removed from contact
with polymer, it stretched and stuck to the rod. It hardened when cooled and would stretch into long filaments
Synthetic Fiber ChemistrySynthetic Fiber Chemistry• Synthetic polymers
– Basic chemical substance of all artificial fibers
– Also include most paints, plastics, adhesives, rubbers
• Polymers are routinely formed from thousands of individual atoms
• Composed of repeating units called monomers that link like chains
Synthetic Fiber ChemistrySynthetic Fiber Chemistry
“Classic” polymer structure of the synthetic fiber nylon. Note the long chain of repeating atoms.
Synthetic FibersSynthetic Fibers• In spite of common names, practically
no two of the same type of fiber are manufactured the same way all the way through the production process
• Positive ID almost always involves microscopy and chemical analysis
Three different scanning electron microscope images of nylon carpet fibers.
Note clear structural differences at this magnification.
Synthetic Fiber AnalysisSynthetic Fiber Analysis• Identification and Comparison• Color
– Based on introduced combinations of dyes
– Different dyes may identify manufacturer
• Diameter– Typically very little variation due to
precise machinery– Cross section usually helpful as well
• Production characteristics– Striations - almost always lengthwise– “Pitting” - occurs from particles added
to fiber to reduce shine
Typical cross section of synthetic
carpet fibers.
Synthetic Fiber AnalysisSynthetic Fiber Analysis•Production method can be single largest identifier•Cross sections are exceptionally important
–Synthetic fibers are forced out of a nozzle when they are hot–Holes of the nozzle are not always round
Synthetic Fiber AnalysisSynthetic Fiber Analysis• Burning
• Chemistry of fibers can cause fiber to burn in different ways
• Odor, color of flame, smoke and the appearance of the residue can also be an indicator
• Thermal decomposition• Gently heating can break down polymers to
basic monomers
• Chemical testing• Solubility and decomposition
Synthetic Fiber AnalysisSynthetic Fiber Analysis• Chemical composition
– Most companies have different “formulas” for product
– Product “fingerprint” can be determined if chemicals are identified• Require spectroscopy
– Fibers can also be melted down into crystals and then identified• Light reflectance will be different
for unique crystals• Known as crystallography
Common nylon (above) and rayon
(below) fibers under polarized
light
Synthetic Fiber AnalysisSynthetic Fiber Analysis
Spectroscopic analysis of two apparently identical red, cotton fibers. Note the clear chemical
differences between the two dyes.
Synthetic Fiber ChemistrySynthetic Fiber Chemistry• Polymers
– Well known natural polymers• Starch• Cellulose - nature of polymer provided
structure to plants• Proteins - form animal hairs
– Well known synthetic polymers• Nylon• Teflon• Silicone• Vulcanized rubber