Fibres Flashcards
(53 cards)
1
Q
What are fibres?
A
- basic unit of yarns and threads which then turn into fabrics, garments, textiles
2
Q
Where are fibres found?
A
- clothing
- bedding
- carpets
- curtains
- bandages
- fibreglass
- loft insulation
- seatbelts
3
Q
What are natural fibres?
A
- can be plant, animal or mineral based
- also known as staple fibres
4
Q
What are some examples of natural plant fibres?
A
- cotton
- linen
- hemp
- coir
- flax
5
Q
What aresome examples of natural animal fibres?
A
- silk
- wool
- cashmere
- camel
- angora
6
Q
What is an example of mineral fibre?
A
asbestos
7
Q
What is cotton made from?
A
cellulose (polymer)
8
Q
What is silk made from?
A
sericin/fibroin (polymeric protein)
9
Q
Why is silk shimmery?
A
due to prism-like structure
10
Q
Asbestos
A
- naturally occuring silicate material
- carcinogenic
- sound absorbing, strong, cheap, fire resistant, electrical insulating
11
Q
What are synthetic fibres?
A
- also called filament fibres
- longer than natural fibres
- polymer with very high length to diamter ratio
12
Q
What are some examples synthetic fibres?
A
- polyester
- nylon
- acrylic
- polypropylene
- glass fibre (made from silica which is natural but process of making it isnt)
13
Q
How are synthetic fibres made?
A
- extruded through a spinneret device - polymer is pushed through tiny holes that are differently shaped
- fibres spun into bundles called filaments
- can alter the characteristics with different spinning techniques
14
Q
What are the recovery considerations for fibres?
A
- may become dislodged quickly after deposition
- air dry wet clothing in controlled environment
- store in paper bags to prevent mould growth
- double package
- submit whole item
- druggists fold for small fibres
- never package with debris from the scene
15
Q
What is the analytical workflow for fibres?
A
- gross examination, recovery and collection
- preliminary evaluation of physical characteristics
- physical fit assessment - most probative value
- microscopic techniques
- microspectrophotometry (UV-Vis) - colour determination
- infrared spectroscopy - manufactured fibres
- Raman spectroscopy - dyes and pigments
16
Q
What are some non-routine techniques that arent recommended and are only used after everything else has been exhausted?
A
- thin layer chromatography
- pyrolysis GS-MS
- HPLC
- melting point
- microchemical tests
ALL DESTRUCTIVE METHODS
17
Q
What are some things to look for when analysising fibres?
A
- surface treatments
- dye penetration
- diameter and length
- cross-section
- chemical composition (type, polymer)
- texture
- colourant/dye - colour, type, how applied, weathering
- natural or synthetic and type
- striations (lines down the fibre) and pitting (holes/damage)
- direction of yarn twist
- threat count
- coatings
- scale protrusion on animal fibres
18
Q
What does IR/Raman look at on fibres?
A
- colour - different dyes/pigments
- chemical composition
19
Q
What does dye uptake depend on with fibres?
A
- when the fibre is dyed during the manufacturing process - before, after being spun or after garment construction
20
Q
What does the transfer of fibres depend on?
A
- nature of contact
- multiple association mitigate coincidental transfer
- new fabrics possess loosley adhering fibres
- old/damaged fabrics may shed more
- tightly fibres shed less than staple fibres
- background and persistence considerations
21
Q
What are the issues with fibres as trace evidence?
A
- cant state that it is unique
- few databases for origin
- often overlooked as difficult to locate
- expensive, time-consuming skilled analysis
22
Q
What some microscopic techniques for fibres?
A
- stereoscopic
- comparison
- polarised
- fluorescence
- brightfield
- thermal (destructive)
- SEM and TEM
- crystallography and diffraction
23
Q
What are some physical fit and gross characteristics in fibres?
A
- surface treatments
- dye penetration
- diameter
- chemical composition
- cross section
- texture
- colourant/dye - colour, type, how applied, weathering
24
Q
What is a microscopic technique of cotton?
A
twisting shape
25
What are some microscopic characteristics of flax?
* nodes
26
What are some microscopic characteristics of hemp?
no nodes but irregularities
27
What are some microscopic characteristics of jute?
* nodes
* tapering on the ends and is more narrow
* colouration changes as it is rotated in polaried light
28
What is a microscopic characteristic of silk?
ribbon shape
29
What is a microscopic characteristic of wool?
scales
30
What is a microscopic characteristic of synthetic fibres?
* longtiudinal appearance - texture, crimping, pigment (can be unique to manufacturing process or machine)
* cross-sectional appearance - cut and observe ((can be unique to manufacturing process or specifically engineered)
* dye penetration
* presence of crystalline regions - will refract light
* gas voids
31
What is the most common shape in nylon synthetic fibres?
tri-lobal
32
How do we characterise nanostructures of the surface or bulk?
* surface: microscopy (SEM, TEM, atomic force)
* bulk: diffraction (X-ray power, optical)
33
What are the benefits of electron microscopy for analysing fibres?
* non-destructive analysis - beam damage can occur for sensitive samples
* rapid accumulation of results
* can give elemental composition
* higher resolutions are achieved by use of electrons instead of light
34
Light vs SEM vs TEM
* light: low depth of focus, good field of view, easy and rapid, cheap, worst resolution
* SEM: high depth of focus, good field of view, easy and rapid, quite expensive
* TEM: medium depth of focus, limited field of view, skilled and slow, very expensive, best resolution
35
What does SEM do for fibre analysis?
* elemental analysis
* surface features like scales
* fibre-end fracture morphology
* even more expensive
36
What are the two types of microscopic analysis?
* refractive index
* birefringence
37
What is refractive index?
* when two refractive indices are equal, the light passing through the particle does not deviate at all and the particle remains invisible
* when the refractive indicies are far apart, the light passing through will change diretion substantially
* if refracted sufficiently, they miss the objective lens and these areas of the particle become dark, resulting in high contrast
38
How is the refractive index measured?
* Becke Line Test
39
What is the Becke Line Test?
* refracting light depends on the relative RI values of the particle and mounting medium
* a particle with a higher RI mounted on a medium or lower RI, will focus axial illuminating rays towards a point above the particle
* lower RI particle in a higher RI medium will direct light in opposite direction, moving the line outside the particle
40
How are Becke Line immersions measurements made?
* mounting the substance in a media of varying RI's until little change is observed
41
What is the limitation of the Becke Line Test?
will only be true for one wavelength of light at a time - wavelength dependent
* need a more variation method for a precise measurement
42
Normal light vs linearly polarised light
* normal: waves vibrating in every direction perpendicular to the direction of travel
* polarised: waves vibrating in one direction
43
What is anisotropic?
different physical properties in different planes
43
Why and how is a polarised light microscope used?
* light will go through a polariser to become plane polarised light
* as the plane polarised light passes through the sample if it is crystalline or anisotropic it will change the light as only particular polarisations of light will be transmitted
44
What is a uniaxial material?
allows rays to vibrate in two axes, ω (blue) and ε′ (red) depending on it’s orientation
45
How to measure refractive index in uniaxial materials when the light has been polarised so that it is vibrating in the E-W plane?
* when the epsilon (ε′) plane is aligned with the E-W polarised light use Becke line or variation methods- RI that we measure will correspond to that physical feature of that crystalline structure
* when the omega (ω) plane is aligned with the E-W plane use Becke line or variation methods
* in between the two orientations there is an intermediate where there is a linear response or where they look very similar in terms of the RI
46
What is pleochroism?
* property in anisotropic materials
* causes the sample to show different absorption colours when exposed to polarised light coming from different directions
* can see the difference of the omega plane and epsilon plane
47
What is retardation?
when full white light passes through a particle, it wont pass through at the same speed, the slow ray is said to be more retarded than the other
* the exact distance that the slow ray falls behind is the retardation
48
What is birefringence?
* full white light is shone at a particle
* one light ray will be more retarded than the other (slower)
* this will cause a change in colour from the white light
49
How are the velocities of the two rays determined in birefringence?
by the values of ω and ε′
50
What is the equation to work out the birefringence of a material?
R (nm) = B x T (um) x 1000 (nm/um)
* R = retardation
* B = birefringence
* T = thickness
51
How is the retardation measured?
by rotating the analyser relative to the polariser
52
How many polarised rays does light split into when passing through a birefringent material?
2
