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
