Textiles

Question 1

Fibre classification

Answer 1

Fundamentally all fibres are polymers
Fibres used in the manufacturing of textiles are broadly classified as natural or artificial
Animal, vegetable, mineral, artificial

Question 2

Natural Fibres

Answer 2

Cellulose (plants): seed, leaf, stem, miscellaneous
Protein (animals): sheep, specialty/fur, silk, plumage, filament
Mineral (inorganic): asbestos, brucite

Question 3

Artificial Fibres

Answer 3

Natural polymer: cellulose acetate compound, cellulose regenerated
Synthetic: nylon, acrylic, polyester
Inorganic: metallic, glass
Other: biconstituent hybrid

Question 4

Most commonly encountered fibres in forensics

Answer 4

1. Polyester(1/3 of world fibre production)
2. Cotton
3. Polypropylene
4. Polyamide (nylon)
5. Polyacrylonitriles (acrylic)
6. Wool

Question 5

Polyester definition

Answer 5

Any long chain synthetic polymer composed of at least 85% by weight of an ester of substituted aromatic carboxylic acids

Question 6

Polyamide definition

Answer 6

Nylon
Any long chain synthetic polyamide in which less than 85% of the azide linkages are attached directly to 2 aliphatic groups

Question 7

Polyacrylonitriles definition

Answer 7

Acrylic

Any long chain synthetic polymer composed of at least 85% by weight acrylonitrile groups

Question 8

Polypropylene

Answer 8

Subset of polyolefins (C=C); 85% by weight polypropylene units

Question 9

The goal of fibre analysis

Answer 9

• the physical and chemical properties of the fibres in question are used to classify the fibres and compare them with possible donor items to establish their origin
• the more specifically the fibres can be classified, the more salient their evidentiary value becomes

Question 10

General flow of fibre analysis

Answer 10

Fibre -> microscopy -> appearance/fluorescence/colour -> natural vs artificial / microspectrophotometry -> crosssectional/PLM/pleochroism/birefringence/dye analysis/IR

Question 11

White light microscopy and fibres

Answer 11

Fastest method of obtaining classification or determining further analysis needed
Synthetic - featureless
Natural - cuticle pattern, cortex pigment distribution, medulla shape, continuity - animal taxon

Question 12

Cross-sectional analysis of fibres

Answer 12

Cross-sectional shapes important part of classification
Round, flat, trilobal, multilobal

• can sometimes be determined by shading observed in white-light analysis (darker = thicker)

Question 13

Fibre manufacturing additives

Answer 13

Get rid of lustre and shine - introduce delusterants
Titanium dioxide
Diffract light reducing lustre
Discrimination power: size, shape, distribution, concentrations

Question 14

Additional physical features of fibres

Answer 14

Diameter: range for natural fibres more variable
Draw marks: deformation of insoluble materials on fibre surface, when polymer is pulled from surface material
Striations: fine lines running along fibre. Common in rayon as fibre shrinks in solvent removal causing wrinkles
Twists/convolutions/crimps: added in man-made fibres to improve fibre for end use (#crip)
Voids on internal channel: used to produce fibre with specific end use (sports apparel)

Question 15

Polarizing light microscopy (PLM)

Answer 15

Two polarizing filters added to bright light microscope
Isotopic materials (eg glass) have single RI, polarized light remains polarized after passing through, appear dark
Crystalline or psuedocrystalline materials are anisotropic - more than one RI - interact with plane-polarized light

Question 16

How does PLM help in fibre analysis?

Answer 16

Polarized light parallel to crystal axis unchanged, other orientation is split in two perpendicular components. One encounters more atoms and is slowed more
Under crossed polars, the anisotropic fibre will disappear from view when the light propagates long an axis parallel to the crystal axis - phenomenon known as extinction
45 deg between extinction points, maximum brightness occurs because the difference in the magnitude of slow and fast rays is the greatest
Contrast enhancing

Question 17

Birefringence measurements

Answer 17

Numerical difference between parallel and perpendicular refractive indices
Patterns of repeating colours due to thickness - thicker = more retardation
Birefringence = n1-n2 = para-perp

Question 18

Refractive index measurements

Answer 18

Mounting in a medium with a known RI
1. Bring into focus with white light
2. Objective lens raised
3. Line will appear to move into the medium with the higher RI
4. Different media used to narrow down RI
Birefringent - measured parallel and perpendicular

Question 19

Sign of elongation

Answer 19

Determined by comparing para and perp
Perp > para = sign is negative
Para > perp = sign is positive

Question 20

Pleochroism

Answer 20

Result of orientation dependent variations
Selective absorption
Different colours based on different orientations

Question 21

Image enhancement of fibres

Answer 21

Low birefringence or negative elongation -> first order red tint or retardation plate
Produces an increase in retardation distance
Also aid in visualizing pleochroism

Question 22

IR in fibres

Answer 22

Second most important
Identifying the synthetic polymers of the fibre
IR can distinguish between classes of fibres
Copolymers need IR
Raman can be complimentary

Question 23

Additional classifying properties for fibres

Answer 23

Fluorescence - more objective for colour = most important discriminating feature
colour classification - destructive, MSP, Raman
MSP - optical microscope with visible and UV spectra
Raman - further information on dyes that can’t get with MSP (reactive black, flock printing, probabilistic)

Question 24

Fluorescence for fibres

Answer 24

Fibre substrate
Dyes
Pigments
Optical brighteners (white)
Contaminants

Fluor-micro: excitation and emission spectra - comparison

Question 25

Destructive fibre testing

Answer 25

Solubility tests
Melting point test (amorphous - gum; crystalline - gooey)
Flame tests