Terminology(2), Spontaneous Combustion, Investigating the Scene Flashcards
1
Q
What is Pyrolysis?
A
- Thermal decomposition of an organic substance to produce lower molecular mass compounds, without involving oxygen
- Pyrolysis products can be volatile and flammable. If their concentration is within their flammability range, they may burn at the surface of the solid.
- Solid left behind becomes closer to carbon.
- Real substances give complex mixtures of pyrolysis products
- Pyrolysis products of plastics can be highly toxic.
2
Q
What is smoldering combustion?
A
- Smoulder - to burn and smoke without flame; it is a form of flameless combustion.
- Occurs in substances that can form a solid char by pyrolysis e.g. wood, fabrics, packaging etc.
- Takes place at surface of material: fuel-air interface; not in gas phase like flaming combustion.
- Can occur at very low O2 concentrations, especially if the molecular structure contains oxygen.
3
Q
What is spontaneous combustion?
A
- Some substances undergo natural exothermic reactions
- Heat is lost from the external surface of the material
- Temperatures rise internally. The reaction accelerates, but heat is lost more rapidly, so an equilibrium temperature is reached.
- In a large quantity of material, the surface area is relatively small and the equilibrium temperature is higher. If the temperature exceeds auto-ignition, a fire can start within the bulk of the material.
4
Q
What is characteristic of spontaneous combustion?
A
- Common in sawdust, coal dust, hay, straw etc.
- Happens in unsaturated oils soaked into rags etc.
- May resemble an arson attack
- Highly reactive chemicals can also ignite spontaneously in air.
5
Q
What is explosive combustion?
A
- Occurs when vapours, gases or dusts are premixed with an appropriate amount of air (stoichiometric) and ignited.
- Premixing allows entire combustion to occur in very rapid time
- Mechanical effects are recognised as an explosion.
6
Q
What are dust explosions?
A
- Occurs when finely divided solid fuel is mixed with air and ignited.
- High risk comes from coal dust, sawdust, flour and chemical dusts.
- Flammability/explosion limits are defined in terms of mass
- Finer dusts have a lower explosion limit
7
Q
What is wood?
A
- Wood is made up of cellulose (~50%), hemicellulose (~25%) and lignin (~25%)
- Cellulose is a polymer (C6-H10-O5) of about 1500 chains.
- Wood differs in density, water content, voltatile resin content etc.
- Ignition and combustion depends on formation of combustible pyrolysis products and their mixing with air.
- These products can be ignited by a flame at flash point temperature or ignite spontaneously at ignition temperature.
8
Q
How easily ignitable is wood?
A
- Ignition temperatures as low as 230oC
- Resinous woods such as pine are easily ignited.
- Hard woods are more difficult to ignite, but produce a hotter and more protracted fire
- Many manufactured wood products contain inorganic adhesives, holding layers or pieces of wood together
- May also have surface coatings or varnishes
9
Q
What are plastics?
A
- Plastics are polymers containing hydrocarbon chainsand other structural units.
- Thermoplastics: have structural linear chains. Undergo reversible melting without appreciable chemical decomposition e.g. polyethylene, polystyrene. Tm < ignition temperature
- Thermosetting plastics: cross-linked structures. Do not melt but decompose chemically and leave a solid char.
10
Q
What are the different decomposition mechanisms of thermosetting plastics?
A
- end-chain scission: removal of monomers from end of polymer backbone. Produces monomers
- Random scission: main chain bonds broken at random locations until the sections small enough to volatise. Produces range of oligomeric molecules.
- Chain stripping: polymer backbone intact, but molecular species not part of bb break away.
- Cross linking: some undergo further cross-linking during pyrolysis, generating further char.
11
Q
What are the pyrolysis products of cellulosic polymers, polyolefins and polyesters?
A
Cellulosic polymers: Main pyrolysis products are water
Polyolefins: Hydrocarbons such as polyethylene and polypropylene. Products are oligomers of the chain and hydrogen.
Polyesters: pyrolysis products include carbon monoxide and water.
12
Q
What are the pyrolysis products of polyurethane and nylon?
A
Polyurethanes are polymers of isocyanates and alcohols. Pyrolysis products include carbon monoxide, hydrogen cyanide and isocyanates.
Nylons: polymers of acids and amines. Pyrolysis products include carbon monoxide and hydrogen cyanide.
13
Q
What are the pyrolysis products of polymethylmethacrylates and PVC?
A
- Polymethylmethacrylates are polymers of methacrylic acid. Pyrolysis products include the monomer, with methanol and formaldehyde.
- PVC produces vinyl chloride (CH2=CHCl) and HCl. Highly toxic but less flammable than other pyrolysis gases. HCl acts to inhibit combustion. HCl is produced by chain stripping, leaving a char.
14
Q
What are the pyrolysis products of polystyrene?
A
- Main pyrolysis product is the styrene monomer, which is flammable and highly toxic.
- This is a C8 molecule which can be confused with residues from petrol
15
Q
How is heat propagated by conduction?
A
- Conduction is the heat transfer through a material by direct atomic or molecular contact.
- Heat energy in form of atomic vibrations.
- Most important in solids where atoms are in direct contact.
- A fire spreads through a solid fuel by conduction of the activation energy into adjacent unreacted areas of the fuel.
- Materials with low thermal conductivity heat up most rapidly and can soon reach temperatures for pyrolysis and ignition.
16
Q
How is heat propagated by convection?
A
- Convection is transfer of heat in gas or liquid by circulation of molecules, caused by temperature differences.
- Regions of high temperature are less dense and rise.
- In burning fires, this sets up air currents which draw in oxygen at the base of the fire, increasing ventilation.
- Walls and ceilings are dried and heated by hot gases rising from a fire. Even if the flames do not reach them directly, they can auto-ignite if hot enough.
17
Q
How is heat propagated by radiation?
A
- Radiation is emission of heat energy as electromagnetic radiation (infra-red energy)
- Intensity is proportional to the fourth power of the absolute temperature; Stefan-Boltzmann Law
- Radiation energy falls of as inverse square of the distance from the source.
- Radiation from flames plays a major role in spread of fire.
18
Q
How does ‘flameover’ lead to radiation-induced flashover?
A
- If a fire plume cannot escape, it will spread a layer of hot gases underneath the ceiling.
- Flames can spread horizontally at great speed; flameover
- Enormous amounts of heat is radiated downwards.
- At these heats, all flammable gases and fuels in the room begin to decompose. Within seconds they reach ignition temperatures and catch fire.
- This is radiation induced flashover and represents a downward spread of fire.
19
Q
What evidence and information may be obtained at a fire scene?
A
- Where did the fire start? How did it start? How did it spread?
- Physical evidence includes smoke records, temperature indications, debris layer sequences and remains of ignition devices.
- Chemical evidence include chemical analysis of trace residues and presence of accelerants.
- Circumstantial evidence: evidence of forced entry, witness statements etc.
20
Q
Before the forensic investigation can begin, what must be completed?
A
- The fire scene must be made safe.
- Risk of structural collapse
- Live electrical cables
- Broken sewers/drainage - biohazard
- dust hazards etc.
- Risk assments must be dynamic.
21
Q
How is the point of ignition located?
A
- Fire burns upwards. The lowest point of burning generally indicates the seat of the fire.
- Rapid upward circulation of fire gives a V shaped smoke patter on walls, pointing downwards to the seat.
- Directional indicators: typically, the face of the material that is directly exposed to the fire will show greatest damage.
- Glass and plastics melt towards the fire
22
Q
What should the forensic scientist look for after finding the seat of the fire?
A
- Trails of streamers from accelerants, trails of paper etc.
- Containers capable of holding accelerants
- Ignition devices: matches, lighters, timers etc.
- Signs of forced entry.
- May need to excavate debris layers and reconstruct scene.
- Notes and sketches are highly important.
- If a fire has more than one seat, that is evidence of arson.
