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