Fiber Analysis

1
Fiber Analysis

• Hair and Fiber 3

2
Fiber 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

3
Forensic 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

4
Fiber 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

5
Chemical Alteration
Note the differences between the cotton fiber
(left) and rayon (right). Can you detect what
happens in the chemical transformation process?
6
Fiber 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.

7
Fiber 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

8
Fiber 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

9
Animal 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

10
Mineral Fibers

• Fiberglass
• Artificially produced fiber
• Asbestos
• Natural fiber from the mineral serpentine

11
Plant 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

12
Plant Fibers

• Flax
• Used to make linen
• Distinctive lateral structures through fiber
• Can be expensive

13
Plant Fibers
Ramie fiber, common in SE Asia
Hemp fiber, the strongest natural fiber
Plant fibers are based on the polymer cellulose,
the chemical that forms the cell wall of plant
cells
Jute fiber, also common in Asia
14
Synthetic 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

15
Synthetic Fiber Chemistry

• Based on polymers
• Long chain molecules
• Come in natural and man-made 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

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Synthetic Fiber Chemistry

• Polymers
• Well known natural polymers
• Starch
• Cellulose - nature of polymer provided structure
  to plants
• Proteins - form animal hairs
• Well known man-made polymers
• Nylon
• Teflon
• Silicone
• Vulcanized rubber

17
Synthetic Fiber Chemistry

• Man-made 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

18
Synthetic Fiber Chemistry
Classic polymer structure of the synthetic
fiber nylon. Note the long chain of repeating
atoms.
19
Synthetic 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.
20
Synthetic 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.
21
Synthetic 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

22
Synthetic 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

23
Synthetic 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
24
Synthetic Fiber Analysis
Spectroscopic analysis of two apparently
identical red, cotton fibers. Note the clear
chemical differences between the two dyes.