8.3 Notes Fibers

Objectives Understand the differences between natural and manufactured fibers List the properties of fibers that are more useful for forensic comparisons Describe proper collection of fiber evidence

Types of Fibers Fibers important evidence in incidents that involve personal contact May be fixed on vehicle of hit and run May be fixed in screens, broken glass Must be able to narrow their origin to a limited number of sources May be natural or synthetic

Natural Fibers Derived from animal or plant sources Hair coverings Most common fiber is cotton Has a ribbon-like shape with irregular interval twists

Man Made Fibers First was rayon Grouped into major generic fibers with common trade names and characteristics 1.Regenerated fibers Manufactured from natural raw materials and include rayon, acetate, triacetate Manufactured from natural raw materials and include rayon, acetate, triacetate Produced from regenerated cellulose Produced from regenerated cellulose

Man Made Fibers cont. 2.Synthetic fibers Include nylons, polyesters, acrylics Include nylons, polyesters, acrylics Composed of synthesized polymers Composed of synthesized polymers (pages ) 13 major generic fiber groups with characteristics and domestic and industrial uses 13 major generic fiber groups with characteristics and domestic and industrial uses

Polymer basic chemical substance of all synthetic fibers – plastics, paints, adhesives, synthetic rubber Often referred to a macromolecules – thousands or millions of atoms linked as large molecules Often referred to a macromolecules – thousands or millions of atoms linked as large molecules Monomers are repeating units that make up a polymer Monomers are repeating units that make up a polymer

Identification and Comparison of Man Made Fibers 1.most cases have limited number of fibers for identification and comparison and therefore the criminalist must use comparison of standard and crime scene fibers 2.microscopic comparison for color and diameter using a comparison microscope 3.lengthwise striations on the surface of some fibers and the pitting of the surface

Identification cont. 4.dye composition is essential using a microspectrophotometer to show the same chemical composition 5.may use analysis of fiber dye through chromatographic separation and then a TLC 6.both fibers (victim/suspect) must have same chemical composition down to the types of fibers within each group

Identification cont. 7.most useful physical property of fibers is their double refraction due to the crystallinity of the finished fiber after processing 8.light that passes through a synthetic fiber emerges polarized, perpendicular and parallel to the length of the fiber. This can be used to compare it to a refractive index of known fibers. 9.Using a microspectrophotometer the infrared analysis can also be used to determine generic class

Collection and Preservation of Fiber Evidence Must identify and preserve potential carriers of fiber evidence Relevant articles of clothing are packaged carefully in paper bags separately No contact between articles of clothing (included the surface) Some fibers removed at the scene must be laid on paper