chapter 21 - fibers, paints, and other polymers

what is a polymer?

long chain of repeating molecular units

what is a polymer made of?

monomers

what we look for when examining fibers

type - most important charcteristics

size - natural fibers measured in micrometers, synthetic fibers measured in denier (weight unit)

cross section - round, bilobal, trilobal

color - natural, bleached, or dyed

crimp - natural wave or twist

fiber shapes

oval, elongated, bilobal, trilobal

types of paints

automotive paints, structural paints, artistic paints

what is automotive paint used for?

cars; most encountered in forensic work

structural paints used for?

houses or structures

artistic paints used for?

long lasting and made from naturally occurring oils

how are cars painted

rustproofing - 2 coats

primer - minimize corrosion

topcoat - give the car its color

clearcoat - acrylic protective layer

how paint evidence in collected

proper collection of known samples - compare known and unknown samples

collect paint from undamaged areas of car as near as possible to damaged area

if possible, entire object should be removed and sent to lab

analysis of paint evidence

physical properties: Paint Layer Sequence

chemical properties: solubility, chromatography, infrared micro spectrophotometry (FTIR)

clear coat

the final layer of paint applied to an automobile that adds durability and ultraviolet protection to the colored paint layer

dye

generally organic substances that bind to fiber and absorb various wavelengths of light, producing a colored appearance

fracture match

matching broken objects to the original source

metamerism

the quality of some colors that causes them to appear differently under different light sources

monomer

single repeating units of molecules

natural fiber

a fiber that exists in nature

pigment

mostly inorganic particles, small in size and insoluble; they suspend in the chemical mixture of a fiber and absorb different wavelengths of light

polymer

long chains of repeating molecular units or long chains of monomers

primer

the coating layer applied to the unpainted automobile - the first layer applied after the bare metal

semi-synthetic fiber

a fiber that is sourced to natural, nonfibrous material but undergoes chemical processing to produce a fiber

refractive index

a number that represents the ratio between the speed of light in air and the speed of light in the medium analyzed

synthetic fiber

fibers that are man-made chemical compounds formed into predetermined fiber shapes by extrusion from a machine

tear match

two materials that have been ripped in pieces that can be shown to match based on examination (usually microscopically) of the patterns

topcoat

the coating layer for an automobile that imparts the color to the vehicle

THC

Tetrahydrocannabinol