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Fires & Explosions > Lecture 7 > Flashcards

Lecture 7 Flashcards

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.

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2
Q

Simplest solid fuel

A

Carbon

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3
Q

Carbon

A
  • Most simplest fuel
  • Charcoal, coal, wood/paper char is mainly elemental carbon
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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.
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5
Q

What is most damage in fires caused by?

A

Flaming combustion

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6
Q

Different ways solids can produce flames?

A
  1. A solid melting (liquid) and then evporating (vapour)
  2. A solid melting (liquid) and evaporating + decomposing (vapour), invovles pyrolysis
  3. A solid decomposing and melting (liquid) and then decomposing and evporating (vapoue)
  4. A solid decomposing and evporating (vapour) this is known as sublimation, e,g dry ice.
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7
Q

How do plastics produce flames

A

They decompose and melt into a liquid and then decompose and evporate into a vapour

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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

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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
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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.
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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.

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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.

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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.

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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
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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.
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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.
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17
Q

Hard woods

A

Hard woods are more difficult to ignite, but can cause a hotter and more protracted fire

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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
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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.
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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.
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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
Q

Oligomer

A

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
Q

Monomer

A

the smallest subunit of the polymer.

27
Q

Plastics

A

Plastics are polymers containing hydrocarbon chains and other structural units. Most are combustible.

28
Q

Thermoplastics

A
  • 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
Q

Thermosetting plastics

A
  • 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
Q

Thermoplastic melting temp

A
  • 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
Q

Thermosetting plastics burning

A
  • 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
Q

Combustion properties of plastic

A
  • The detailed combustion properties of plastics depend on the polymer chain length as well as their chemical structure
33
Q

Why are thermoplastics misleading in an investigation?

A
  • 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
Q

What C containing compound makes up a lot of different arsenic products?

A

C8 containing compounds make up a lot of different arsenic products

35
Q

Thermoplastic decomposition mechanisms?

A
  1. End-chain scission
  2. Random scission
  3. Chain stripping
  4. Cross-linking
36
Q

End chain scission

Thermoset decomposition

A
  • Successive removal of monomer units from the end of the polymer backbone.
  • They char and break up to produce monomers from polymers
37
Q

Random scission

Thermoset decomposition

A
  • 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
Q

Chain stripping

Thermoset decomposition

A
  • The polymer backbone remains intact, but molecular species which are not part of main chain break away
  • Bridges are removed more easily
39
Q

Cross-linking

Theremoset decomposition

A
  • Some thermosetting polymers undergo further cross-linking during pyrolysis, generating a lot of char
  • Gets stronger bc of the cross-linking
40
Q

Cellulosic polymers

A

These are effectively carbohydrates. The main pyrolysis product is water and oxygenated aliphatics.

41
Q

Polyolefins

A

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
Q

Polyesters

A
  • Commercial polyesters are generally polymers of terephthalic acid and ethylene glycol.
  • PET is Polyethylene terephthalate.
  • Pyrolysis products: carbon
  • monoxide (highly toxic) and water.
43
Q

Polyurethanes

A

Polymers of isocyanates (-N-C=O group) and alcohols. Pyrolysis products include carbon monoxide, hydrogen cyanide (HCN)
and isocyanates. All are
highly toxic.

44
Q

nylons

A

Polymers of acids and amines. Pyrolysis products: carbon monoxide and hydrogen cyanide. Highly toxic.

45
Q

Polumethylmethacrylates

A
  • 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
Q

Polyvinyl chloride (PVC)

A
  • 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
Q

Polystyrene

A
  • 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
Q

where might you find plastics in quantities of consequence in a fire?

A
  • 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
Q

Metals

A
  • 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
Q

Magnesium

A
  • 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

Fires & Explosions (18 decks)

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