Lecture 9: Fibres Flashcards
1
Q
What are fibres?
A
- Basic unit of yarns and threads, which then turn into fabrics, garments, textiles, etc.
- Length significantly exceeds width, flexible.
- All fibres are based on polymers, bar a few.
- Fibres are the smallest unti used to make a fabric.
2
Q
Natural fibres
A
- Can be plant, animal or mineral based
- Often termed “staple” fibres
- Plant → cotton, linen, hemp, jute, flax, sisal, coir
- (can originate from seed, stem, leaf or fruit)
- Animal → silk, wool, cashmere, angora (bunnies), camel
- Mineral → asbestos only
3
Q
Cotton
A
- Most common fibre encountered in forensic labs
- Cellulose is the base unit of cotton – polymer!
- Shrub that grows ~3-4 feet high produces pink flowers.
- Once the pink flowers fall off we get a seed pod which guves us the fibres we need to harvest into cotton.
- Raw cotton harvested from boll ~2” long fibres
- Fibres then raked to clean, pull and thin out.
- A lot of stuff used in labs is cotton eg the tip of a swab.
4
Q
Hemp
A
- Derives from the cannabis sativa plant
- Cellulose based bast fibre
- Low THC and high CBD content
- Not the same as sisal – agave plant
- Woven into fibres ~50,000 years ago!
- Hemp fibres are traditionally used for ropes but they can be used to mark garments.
- Hemp was one of the first fibres used as a weave to make material and fabric.
5
Q
Bast fibre
A
Comes from a part of the stem which is underneath the bark.
6
Q
Silk
A
- Made from a combination of sericin and fibroin which are polymeric proteins.
- Comes from the cocoons of the larvae of moths
- H-bonding therefore strong fibres
- Produced industrially by silkworms
- Other animals produce these proteins and produce similar materials of silk
- Spider produce several types of silk
- Shimmery due to prism-like structure which can’t be exploited for forensic analysis.
- Prism causes light to bounce of in different directions giving it a silky shimmeryness.
- Animal based fibre
7
Q
Asbestos
A
- Naturally occurring silicate mineral based on silica
- Compised of thin fibrous crystals called fibrils which is why it is dangerous.
- The fibrils are vert thin and birrle so they are prone to snapping allowing for tiny fibres to get into the atmosphere which is dangerous if inhaled.
- Sound absorbing, strong, cheap, fire resistant, insulating
- Carcinogenic and now largely banned
8
Q
Synthetic fibres
A
- Polymer with very high length to diameter ratio
- Often termed “filament” fibres
- Polyester, nylon, polypropylene, acrylic, etc.
- Rayon (viscose) semi-synthetic, derived from wood
- Raw polymer converted into fibres via “spinning”
- Very high length to diameter ratio
- Much longer than staple fibres
- With synthetic fibres we can determine the length
- Rayon (viscose) is man made but it is derived from cellulose from wood so it is semi-synthetic
- Glass fibre is made from silica which is natural but the process at which its made makes it synthetic.
- Mineral wool
9
Q
How are synthetic fibres made?
A
- Extruded through a spinneret device which comes in different shapes and sizes.
- Fibres spun into bundles called filaments
- Various spinning techniques → alter characteristics.
- The spinnerets are added onto manufacturing process and polymer is pushed through these tiny holes which causes characteristics and differences between fibres.
- Characteristics and properties are more forensically useful than chemical composition which is why microscopy is important.
10
Q
Fibre recovery considerations
A
- Fibres will be dislodged quickly after deposition, particularly if they are moving or outside.
- If we’re collecting wet clothing from a scene it must be air dried in a controlled environment prior to packing. Something has to be put underneath it bc as it dries lose fibres fall and we want to collect them.
- Put them in paper bags as we don’t want mold!
- Never package then with debris from the scene.
- Can be lost during emergency service intervention.
- Preferable to submit an entire item to a lab.
- Druggists fold is the best way to store small fibres and then label them.
- They shoykd be double packaged into an evidence bag.
- Control smaples should be packaged separately.
11
Q
Analytical workflow for fibres
A
- Gross examination, recovery and collection
- Preliminary evaluation of physical characteristics
- Physical fit assessment – most probative value, show how a bundle of fibres originated as one and has then been split.
- All microscopic techniques – Indication of the cross sectional profile of synthetic fibres is useful and relates to the spinnerets used.
- Microspectrophotometry (UV-Vis) – colour determination with values.
- Infrared spectroscopy – gives you information about the fibres themselves which is useful for manufactured fibres
- Raman spectroscopy is good for dyes and pigments.
12
Q
Non routine technique used for fibres
A
- Thin layer chromatography
- Pyrolysis gas chromatography mass spectrometry
- High performance liquid chromatography
- Melting point
- Microchemical tests, e.g. solubility
- “non-destructive methods must be exhausted before subjecting the sample to any destructive tests” - SWGMAT fibre analysis guidelines 2011
- Not recommended unless absolutely needed, it destroys the same and must only be used after the other methods have been used.
- Can dissolve it in acids or alkalis to tests its solubility
13
Q
Things to look for in fibre analysis
A
- Surface treatments / delustering agents (TiO2)
- Dye penetration
- Presence of mercerisation
- Coatings (teflon, waterproof cooatings on car seals)
- Direction of yarn twist (left or right)
- Thread count
- Length and Diameter
- Chemical composition type and polymer
- Cross section profile (round, flat, trilobal, dumbbell)
- Texture (striations and pitting)
- Colourant/dye (colour, type, application method, weathering like fading and discolouration) This can be determined through visual inspection and through MSP / FTIR/Raman
- Natural or synthetic and type
- Scale protrusion if its an animal fibre
14
Q
Higher thread count
A
Higher thread count = the denser and better quality the material
15
Q
Mercerisation
A
- Mainly done to cotton and flax
- Chemically altering the fibre to avoid shrinkage
- Creates a sheen effect
- Improves durability
16
Q
Delusturants
A
These take of the shine of fabrics to make them look matt and uniform, titanium dioxide is the most common.
17
Q
Fibre analysis
Colours
A
- Can be dyed before being spun into yarn, after spinning, after garment construction
- The dye uptake will vary depending on when it was dyed.
- The method of adding colour to a garment can be done in several ways.
- Surface printing onto fabric
- Visual colour may be achieved using a variety of different dyes/pigments → IR/Raman spectroscopy
- Visual colour is best determined through IR or Raman bc the colour can appear the same but be based on different chemicals or the relative intensities fo the chemicals could be different.
- Chemometrics can be used to discriminate the differences
- Can also look at chemical composition of the fibre itself – at least 8 different types of nylon!
18
Q
Fibre interpretations
A
- Rarity of fibre increases probative value
- Number and location of fibres found is important.
- Substrate considerations (absence ≠ absence)
- Multiple associations mitigate coincidental transfer
- Nature of contact – damage increases transfer
- More fibres of different types that you can link between the suspect, scene and object, the less likely you can say it was coincidental and the more you can say it was true contact.
- Old and damaged fibres may shed more.
- Filament fibres are longer and have more durability so they shed less.
- New fabrics possess loosely adhering fibres.
- Tightly knit or woven fabrics shed less/
- Background and persistance considerations need to be taken into account.
19
Q
Class characteristics
A
- Traits common to a group
- e.g all cotton fibres have afinite length
20
Q
Individual characteristics
A
Traits that define and ID an item as different to others in the class
21
Q
Caveats of fibres
A
- Can never state fibre is “unique”!
- Few databases for origin and given rapidly changing area, databases unlikely in future
- Often overlooked as difficult to locate
- Once located, expensive, time-consuming, skilled analysis.
