Fibres Flashcards
What are fibres?
- basic unit of yarns and threads which then turn into fabrics, garments, textiles
Where are fibres found?
- clothing
- bedding
- carpets
- curtains
- bandages
- fibreglass
- loft insulation
- seatbelts
What are natural fibres?
- can be plant, animal or mineral based
- also known as staple fibres
What are some examples of natural plant fibres?
- cotton
- linen
- hemp
- coir
- flax
What aresome examples of natural animal fibres?
- silk
- wool
- cashmere
- camel
- angora
What is an example of mineral fibre?
asbestos
What is cotton made from?
cellulose (polymer)
What is silk made from?
sericin/fibroin (polymeric protein)
Why is silk shimmery?
due to prism-like structure
Asbestos
- naturally occuring silicate material
- carcinogenic
- sound absorbing, strong, cheap, fire resistant, electrical insulating
What are synthetic fibres?
- also called filament fibres
- longer than natural fibres
- polymer with very high length to diamter ratio
What are some examples synthetic fibres?
- polyester
- nylon
- acrylic
- polypropylene
- glass fibre (made from silica which is natural but process of making it isnt)
How are synthetic fibres made?
- 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
What are the recovery considerations for fibres?
- 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
What is the analytical workflow for fibres?
- 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
What are some non-routine techniques that arent recommended and are only used after everything else has been exhausted?
- thin layer chromatography
- pyrolysis GS-MS
- HPLC
- melting point
- microchemical tests
ALL DESTRUCTIVE METHODS
What are some things to look for when analysising fibres?
- 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
What does IR/Raman look at on fibres?
- colour - different dyes/pigments
- chemical composition
What does dye uptake depend on with fibres?
- when the fibre is dyed during the manufacturing process - before, after being spun or after garment construction
What does the transfer of fibres depend on?
- 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
What are the issues with fibres as trace evidence?
- cant state that it is unique
- few databases for origin
- often overlooked as difficult to locate
- expensive, time-consuming skilled analysis
What some microscopic techniques for fibres?
- stereoscopic
- comparison
- polarised
- fluorescence
- brightfield
- thermal (destructive)
- SEM and TEM
- crystallography and diffraction
What are some physical fit and gross characteristics in fibres?
- surface treatments
- dye penetration
- diameter
- chemical composition
- cross section
- texture
- colourant/dye - colour, type, how applied, weathering
What is a microscopic technique of cotton?
twisting shape
What are some microscopic characteristics of flax?
- nodes
What are some microscopic characteristics of hemp?
no nodes but irregularities
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
What is a microscopic characteristic of silk?
ribbon shape
What is a microscopic characteristic of wool?
scales
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
What is the most common shape in nylon synthetic fibres?
tri-lobal
How do we characterise nanostructures of the surface or bulk?
- surface: microscopy (SEM, TEM, atomic force)
- bulk: diffraction (X-ray power, optical)
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
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
What does SEM do for fibre analysis?
- elemental analysis
- surface features like scales
- fibre-end fracture morphology
- even more expensive
What are the two types of microscopic analysis?
- refractive index
- birefringence
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
How is the refractive index measured?
- Becke Line Test
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
How are Becke Line immersions measurements made?
- mounting the substance in a media of varying RI’s until little change is observed
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
Normal light vs linearly polarised light
- normal: waves vibrating in every direction perpendicular to the direction of travel
- polarised: waves vibrating in one direction
What is anisotropic?
different physical properties in different planes
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
What is a uniaxial material?
allows rays to vibrate in two axes, ω (blue) and ε′ (red) depending on it’s orientation
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
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
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
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
How are the velocities of the two rays determined in birefringence?
by the values of ω and ε′
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
How is the retardation measured?
by rotating the analyser relative to the polariser
How many polarised rays does light split into when passing through a birefringent material?
2
