Chemical Composition of Fibers

Chemical Composition of Fibers

Fibers are polymers:long chains made of simple

molecules

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Polymers Synthetic fibers are made of polymers which are

long chains of repeating chemical units. The word polymer means many (poly), units (mer). The repeating units of a polymer are called

monomers. By varying the chemical structure of the monomers

or by varying the way they are joined together, polymers are created that have different properties.

As a result of these differences, forensically they can be distinguished from one another.

Natural Fibers1. Wool

Polymer= Polypeptide chains (forming the protein keratin)

Monomer= amino acids (one of which is cysteine . . .cysteine contains Sulfur)

http://www.leydenglenlamb.com/photos.htm

Natural Fibers1. Wool

Polymer= Polypeptide chains (forming the protein keratin)

Monomer= amino acids (one of which is cysteine . . .cysteine contains Sulfur)

2. Silk Polymer= protein Monomer= amino acids (but less of a

variety than wool)

http://entomology.unl.edu/images/silkworm/silkworm1.jpg

Natural Fibers1. Wool

Polymer= Polypeptide chains (forming the protein keratin)

Monomer= amino acids (one of which is cysteine . . .cysteine contains Sulfur)

2. Silk Polymer= protein Monomer= amino acids (but less of a variety

than wool)3. Cotton

Polymer= cellulose (plant) Monomer= glucose

http://www.texaspolicecentral.com/Texas-Patriotism.php

Natural Fibers1. Wool

Polymer= Polypeptide chains (forming the protein keratin)

Monomer= amino acids (one of which is cysteine . . .cysteine contains Sulfur)

2. Silk Polymer= protein Monomer= amino acids (but less of a variety than wool)

3. Cotton Polymer= cellulose (plant) Monomer= glucose

4. Linen Polymer= cellulose (flax plant)

Longer chains than cotton which makes it brittle Monomer= glucose http://www.flowers-cs.com/flax.html

Man-made Fibers1. Rayon

• Cellulose fiber, then chemically altered, burns like cotton

2. Acetate• Made from a reaction between cellulose

and acetic acid

Synthetic Fibers:Generic classification of synthetic fibers1. Polyamides

—linked by amino group• Example: Nylon• Stronger and more chemically inert than

natural fibers

Image from: http://www.imageenvision.com/collection/fitness.html

Synthetic Fibers:Generic classification of synthetic fibers1. Polyamides—linked by amino group

• Example: Nylon• Stronger and more chemically inert than

natural fibers2. Polyesters

—linked by ester group• Stronger linkage than polyamides• Linkage can be linear (flexible) or cross-

linked (rigid)

Image from: http://www.imageenvision.com/collection/fitness.html

Synthetic Fibers:Generic classification of synthetic fibers1. Polyamides—linked by amino group

• Example: Nylon• Stronger and more chemically inert than natural fibers

2. Polyesters—lined by ester group• Stronger linkage than polyamides• Linkage can be linear (flexible) or cross-linked (rigid )

3. Spandex—structurally similar to polyamide

• Made of segments of polyurethane connected to long segments of polyesters

• Resulting spandex can stretch up to 600 times and recover original strength

http://www.fingerpainrelief.com/rubber-band-therapy-for-finger-pain /

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Testing for Identification Microscopic observation Burning—observation of how a fiber burns,

the odor, color of flame, smoke and the appearance of the residue

Thermal decomposition—gently heating to break down the fiber to the basic monomers

Chemical tests—solubility and decomposition

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Dyes Components that make up dyes can be

separated and matched to an unknown. There are more than 7000 different dye

formulations. Chromatography is used to separate dyes for

comparative analysis. The way a fabric accepts a particular dye

may also be used to identify and compare samples.

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Collection ofFiber Evidence

Bag clothing items individually in paper bags. Make sure that different items are not placed on the same surface before being bagged.

Make tape lifts of exposed skin areas of bodies and any inanimate objects

Removed fibers should be folded into a small sheet of paper and stored in a paper bag.

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Fiber EvidenceFiber evidence in court cases can be used to connect the suspect to the victim or to the crime scene. In the case of Wayne Williams, fibers weighed heavily on the outcome of the case. Williams was convicted in 1982 based on carpet fibers that were found in his home, car and on several murder victims.

Video clips on slide two—click on the two pictures within the

chain Clip 1

› http://www.youtube.com/watch?v=cmlunCbBV_o&feature=related

› If link is broken, go to YouTube and search “Oil Absorbing Polymer”

Clip 2› http://www.youtube.com/watch?

v=YWoQ3D6RMXs&feature=related› If link is broken, go to YouTube and search “Spangler's

Polymer Experiment on The Ellen Show”