1
Q
Why is the ignition temperature of paper difficult to determine?
A
Different types of paper have different additives and binders and manufactured differently so ignition temp is difficult to determine.
2
Q
Simplest solid fuel
A
Carbon
3
Q
Carbon
A
- Most simplest fuel
- Charcoal, coal, wood/paper char is mainly elemental carbon
4
Q
Combustion of elemental carbon
A
- No flame occur (slow, smouldering combustion)
- Happens on the surface of the material
- It will combust by burning with oxygen and for a complete combustion you’ll get CO2.
- You get these incandescently hot regions on the surface of it where the atoms on the surface of the fuel are interacting with the oxygen in the air.
- If you do see some blue flames, that’s carbon monoxide coming off and burning in the air from incomplete combustion.
5
Q
What is most damage in fires caused by?
A
Flaming combustion
6
Q
Different ways solids can produce flames?
A
- A solid melting (liquid) and then evporating (vapour)
- A solid melting (liquid) and evaporating + decomposing (vapour), invovles pyrolysis
- A solid decomposing and melting (liquid) and then decomposing and evporating (vapoue)
- A solid decomposing and evporating (vapour) this is known as sublimation, e,g dry ice.
7
Q
How do plastics produce flames
A
They decompose and melt into a liquid and then decompose and evporate into a vapour
8
Q
How does wax produce a flame?
A
The heat from the flame melts the solid into a liquid and then evaporate.
They can also melt and then decompose + evaporate
9
Q
Pyrolysis
A
- Thermal decomposition of an organic substance to produce lower molecular mass compounds, without involving oxygen
- breaking of chemical bonds, depolymerisation, etc
- Frequently done through radical processes
10
Q
Pyrolysis products
A
- Pyrolysis products can be volatile and flammable. If their concentration is within their flammability range, they may burn at the surface of the solid.
- They can also be carried in the fire plume and ignite elsewhere or settle and be used for trace analysis.
- The solid left behind becomes closer to carbon (char)
- Real substances give complex mixtures of pyrolysis products
- Residues are useful for analysis but can be hard to find origin.
11
Q
Why are pyrolysis products likely to be volatile?
A
You’ve turned a big molecule into a small molecule, the products are often volatile so that they would have a lower boiling point and more likely to be gases.
12
Q
Why are pyrolysis products likely to be flammable?
A
because we’re talking about organic compounds, compounds based on carbon, they also tend to be flammable.
13
Q
What happens to the source material in pyrolysis often?
A
A lot of time the source material will be burnt up so you won’t find it.
14
Q
Cellulose
A
- Wood is made up of cellulose
- Cellulose is a long straight chain polymer
- Sugar rings which make it a carbohydrate.
- Linked via an ether group in the middle of the chain
- Repeating pattern
- Gives wood a lot of it’s strength
- Highly oxygenated which helps it burn
15
Q
Hemicellulose
A
- Carbohydrate polymer
- Much less regular structure
- Complicared polymer
- Aromatic units with an oxygenated alphatic chain
- Comlicated cross-linking
- Because it’s cross linked and branched its harder to burn and much harder to breakd down all the bonds and free the volatile species but it can still be burnt.
16
Q
Wood burning process
A
- It will very slowly pyrolyse and in doing so, it can release ignitable vapours in itself, will turn into charcoal.
- Eventually, that charcoal becomes much more easy to ignite than the wood itself would have been because the wood being cooked and fires can start that way.
17
Q
Hard woods
A
Hard woods are more difficult to ignite, but can cause a hotter and more protracted fire
18
Q
Pyrolysis and burning of wood
A
- Char formation is where pyrolysis has been happening in
- Char layer is where carbonated species are formed and has smouldering combustion.
- Carbon is reacting directly with oxygen.
- Pyrolysis zone creates volatile species that vaporise and are used for flaming combustion
- Afterwars is the normal wood. This is protected by high heat capacity so it has a layer of protection
19
Q
paper
Cellulose
A
- Made from cellulose
- Has different properties to wood as it has been reformatted
- A free sheet of paper can be lit easily
- A large stack of paper has no air flow and is hard to burn because they are densely packed together so its hard for oxygen to access.
- Depending on distribution, can spread fire rapidly
- Flammable liquid poured onto paper is a common arson set – look for unburnt traces of accelerant.
- Has a low heat capacity has it is thin
- Surface area and relative air flow determine how fast a fire can burn/spread.
20
Q
Flammable liquid poured onto paper
A
- Common amongst arsonists
- Flammable liquid permeeates paper and travel through it so we get high surface evaporation and lots of vapour being produced so its easily set alight.
- Bits of paper get carried off into convective air stream so we might get some unburn traces of acclerant and paper which is good for sampling.
21
Q
Cotton & linen
A
- Composed of cellulose, like wood
- Large surface area:volume ratio
- Easy to burn with flame
- Lots of pyrolysis products coming off it which burn a lot easier than wood
- Smoky smouldering for extended time
- They’ve been turned into these very fine threads and they will be a certain degree of air circulation going between these threads, which means that it’s much easier to get the right ratio of volatiles to oxygen to get a flame.
- Even if there is enough oxygen, it’s not enough to make the volatiles for the flame, it will still smoulder for a prolonged period.
22
Q
Wool and silk
A
- Composed of protein, mainly keratin
- Amide linkages and side chains (long and complex)
- High ignition temp, low heat of combustion
- Difficult to burn, self-extinguishes
- Gives off HCN when the amides burn
- If they do ignitre they don’t release much energy so it tends to go out by itself. Heat becomes the limiting factor
23
Q
Synthetic plastics
A
If we have synthetic plastics, then they will have a high surface: volume ratio. Because they have essentially been reformatted.
24
Q
Polymer
A
A large molecule composed of many subunits (monomers) joined together
25
Oligomer
A small portion of a polymeric chain (i.e. a couple of subunits or monomers long) as little of 2-3 or 7-8
26
Monomer
the smallest subunit of the polymer.
27
Plastics
Plastics are polymers containing hydrocarbon chains and other structural units. Most are combustible.
28
Thermoplastics
* Have structures of linear chains.
* Bonds are tangled and overlapped but no covalent bonds
* They undergo reversible melting without appreciable chemical decomposition.
* Examples: polyethylene, polystyrene, nylon
29
Thermosetting plastics
* Have cross-linked structures.
* They do not melt, but decompose chemically and leave a solid char.
* Lotss of different polymeric species bonded together so they are resistant to melting
* Covalently bonded to one another that gives them strength
* Examples: polyesters, rigid polyurethane foam
30
Thermoplastic melting temp
- The melting temperature is lower than the ignition temperature.
- Fire may be spread by burning droplets, or a pool of molten polymer.
- Lightly cross-linked polymers may behave more like thermoplastics as there are fewer links which can be broken easy and resemble thermoplastics. (e.g. flexible polyurethane foam).
31
Thermosetting plastics burning
* Pyrolysis gives volatile molecules, leaving a solid char.
* The yield of volatiles is lower in more highly cross-linked polymers.
* Pyrolysis products, as well as being flammable, may be highly toxic and corrosive.
* Solid goes straight into gas phase
32
Combustion properties of plastic
* The detailed combustion properties of plastics depend on the polymer chain length as well as their chemical structure
33
Why are thermoplastics misleading in an investigation?
* If you've got a thermoplastic, then the plastic is going to melt. It may melt while being on fire.
* So you may have droplets of burning plastic or a pool of molten burning polymer which could flow around. These droplets can get hot and ignite which can get confused with satellitle droplets from an arsenic accelerant.
* This is going to provide another way of spreading fire around the room.
34
What C containing compound makes up a lot of different arsenic products?
C8 containing compounds make up a lot of different arsenic products
35
Thermoplastic decomposition mechanisms?
1. End-chain scission
2. Random scission
3. Chain stripping
4. Cross-linking
36
End chain scission
| Thermoset decomposition
* Successive removal of monomer units from the end of the polymer backbone.
* They char and break up to produce monomers from polymers
37
Random scission
| Thermoset decomposition
* Main chain bonds are broken at random locations along the polymer backbone until sections small enough to volatilise are generated.
* Produces a range of oligomeric molecules
38
Chain stripping
| Thermoset decomposition
- The polymer backbone remains intact, but molecular species which are not part of main chain break away
- Bridges are removed more easily
39
Cross-linking
| Theremoset decomposition
- Some thermosetting polymers undergo further cross-linking during pyrolysis, generating a lot of char
- Gets stronger bc of the cross-linking
40
Cellulosic polymers
These are effectively carbohydrates. The main pyrolysis product is water and oxygenated aliphatics.
41
Polyolefins
Hydrocarbons, general formula CnH2n+2 . Examples: Polyethylene and polypropylene. The main pyrolysis products are oligomers of the chain (from random scission), and hydrogen; all highly flammable.
42
Polyesters
* Commercial polyesters are generally polymers of terephthalic acid and ethylene glycol.
* PET is Polyethylene terephthalate.
* Pyrolysis products: carbon
* monoxide (highly toxic) and water.
43
Polyurethanes
Polymers of isocyanates (-N-C=O group) and alcohols. Pyrolysis products include carbon monoxide, hydrogen cyanide (HCN)
and isocyanates. All are
highly toxic.
44
nylons
Polymers of acids and amines. Pyrolysis products: carbon monoxide and hydrogen cyanide. Highly toxic.
45
Polumethylmethacrylates
- Polymers of methacrylic acid. Pyrolysis products include the monomer (from end-chain scission), with methanol and formaldehyde - all flammable and very toxic.
- Highly toxic but less flammable than many other pyrolysis gases
46
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Polyvinyl chloride (PVC)
- The main pyrolysis products are the vinyl chloride monomer (CH2=CHCl), and HCl.
- Highly toxic but less flammable than many other pyrolysis gases
- Quench radical reactions just not as well as HALON1211
- In fact, HCl acts to inhibit combustion. HCl is produced by chain stripping, leaving a char. At high temperatures, the char in turn gives off H2 radical.
- Not a good fuel, it slwos donw fires but gives off toxic byproducts
47
Polystyrene
- The main pyrolysis product is the styrene monomer, which is flammable and highly toxic.
- Styrene is a C8 molecule, and can be confused with residues from petrol used as an arson accelerant
- Polystyrene foams are difficult to ignite,
because in melting they shrink away from the heat source.
- Once ignition has occurred, flames spread rapidly, producing thick dense smoke.
- Thermoplastic
- Less radiation will occur as its actively moving away from source
48
where might you find plastics in quantities of consequence in a fire?
* Carpets – polypropylene yarn and backing over polyurethane underlay
* Curtains – synthetic fabrics
* Sofas/cushions/mattresses – polyurethane foams
* Window – polyvinylchloride
* Flooring – melamine
* Paint – latex, polyvinylacetate, acrylic
* Modern furniture is resistant to common accidental ignition (cigarette or other glowing sources)
* Little to no resistance to flaming sources!
49
Metals
* Metals which oxidise in air (e.g. Fe, U) can be pyrophoric when finely divided
* So the oxidation with air is exothermic and it happens sufficiently quickly then you get a chain reaction and you get a flame.
* More commonly substantial ignition required, even for metal dusts
* Aluminium can burn, but due to formation of an oxide layer, is very rare
* Usually only relevant in industrial settings
* Can also happen with iron or uranium (much more powerful)
50
Magnesium
* Magnesium burns with a bright white flame, and cannot be extinguished with water – will react with water when hot to produce hydrogen which burns massively
