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Fires and Explosions > Lecture 18 > Flashcards

Lecture 18 Flashcards

1
Q

what can change the ignition temperature of different paper types

A

paper is a long chain polymeric material so different properties such as secondary structure and bonding can affect the ignition temperature

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2
Q

how does a smouldering combustion go to a flaming combustion

A

when gases begin the evaporate from the solid fuel

these gases make up the flame

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2
Q

what is the equation for the combustion of carbon

is this a complete or incomplete combustion - how can we tell?

where does this combustion occur in materials

A

C(s) + O2(g) –> CO2(g)

generally an incomplete combustion shown by the blue flame due to carbon monoxide production

occurs on the surface of a material

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2
Q

what is the simplest solid fuel

A

carbon

coal, charcoal, wood/paper is primarily carbon

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3
Q

what is the name of the combustion where no flame occurs

A

a smouldering combustion

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4
Q

what is pyrolysis

A

the thermal decomposition of an organic substance to produce lower molecular mass compounds without the involvement of oxygen

involving the breaking of chemical bonds and depolymerisation

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4
Q

do smouldering or flaming combustions do more damage

A

flaming combustions

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5
Q

what are the ways a solid fuel and produce vapour that can lead to flames (5)

A
  1. sublimation
  2. melting then evaporation = candle wax
  3. melting then decomposition and evaporation
  4. decomposition and melting then decomposition and evaporation = plastics
  5. decomposition an evaporation
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5
Q

why are pyrolysis product residues often hard to analyses to find the origin

A

as real substances are complex mixtures so give complex mixtures of pyrolysis products

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5
Q

what are two characteristics of pyrolysis products

what two other things can they be

A

they are volatile and flammable

but can be toxic and corrosive

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6
Q

where can pyrolysis products be carried

A

in the fire plume and be ignited elsewhere

particularly if the material is lightweight

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6
Q

when will a pyrolysis product burn at the surface of the solid they came from

A

if their concentration is within their flammability range

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6
Q

what does the solid left behind after pyrolysis products have been carried away become closer to

A

carbon (char) so hard to find the source of the fuel

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7
Q

what are the main two components of wood and how do their structures differ

A

cellulose = a long chain polymer
lignin = a branched polymer

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8
Q

what can charring of a surface mis lead to an investigator

A

the surface temperature of the fuel

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9
Q

what types of wood are easy to ignite

what types are harder

A

resinous woods e.g pine

hard woods - but can cause hotter and more protraced fires

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10
Q

what three things are manufactured wood products also likely to have that impact its ignition temperature

A

adhesives holding layers together

surface coatings

varnishes

11
Q

what are the 5 layers observed when wood is burnt/burning

A

char layer
char base
pyrolysis zone
pyrolysis zone base
normal wood = high heat capacity

the reaction propagates along this until there is not fuel left

12
Q

what does the char layer act as in the burning of wood

A

the smouldering combustion

13
Q

what does the pyrolysis zone act as in the burning of wood

A

creates volatiles for the flaming combustion

14
Q

what is paper made of

what makes paper easier to burn than wood

A

cellulose

paper has a lower heat capacity and is much thinner, better surface area exposed and air flow

15
Q

why is a large stack of paper hard to burn

A

there is no air flow

16
Q

how is paper often used as an arson set

why

A

a flammable liquid poured into paper

this is easily ignitable, the paper absorbs the liquid = high surface area of evaporation of gases to be ignited

17
Q

can can a fire including paper spread easily

A

paper is lightweight so can be carried away from the original fire in convection currents and start a fore elsewhere

18
Q

name 4 natural fabrics that are used in starting fires and give some info about their use for starting a fire

A

cotton
linen
- both cellulose based, large SA:V, smoulders and easy to burn

wool
silk - real silk
- both protein based, high ignition temp, hard to burn, self extinguishes, gives off HCN when it does burn (toxic)

19
Q

what are synthetic fabrics made of and why are they good for starting a fire

A

plastic

high SA:V

20
Q

what is a
polymer
oligomer
monomer

A
  • a large molecule composed of many monomers joined together
  • a small portion of a polymeric chain
  • the smallest subunit of a polymer
21
Q

what are plastics made of

what are the two main classes of polymeric plastics

A

polymers with hydrocarbon chains

thermoplastics and thermosetting plastics

22
Q

what is a thermoplastic

give three examples

A

a structure of linear chains
undergo reversible melting without appreciable chemical decomposition

polyethylene, polystyrene and nylon

23
Q

what is a thermosetting plastic

give two examples

A

cross-linked structures
so do not melt but do chemically degrade to give solid char

polyesters and rigid polyurethane foam

24
Q

why do thermosetting plastics not melt like thermoplastics

A

due to the cross links in their structure being covalent bonds that are hard to break

giving strength and resistance to melting

thermoplastic = melts
thermoset = chars

25
Q

why do thermoplastics melt before it burns

A

their melting temperature is lower than the ignition temperature

so if we have a thermoplastic on a shelf this can melt and drop onto the floor where a fire cant be started then on the floor also

26
Q

what can thermoplastic drops be confused with at a crime scene (2)

A

the splashing of a low viscosity accelerant

the pyrolysis products of plastics and arson accelerants are similar as they are made of similar things

27
Q

what do the combustion properties of plastics depend on (2)

A

polymer chain length

chemical structure (linear or branched/crosslinked)

27
Q

what do thermosetting plastics burn like

A

wood - pyrolysis gives volatile molecules leaving a solid char

28
Q

is the production of volatile gases more in lower or highly cross linked polymers

A

more gases in lower cross linked polymers

29
Q

what are the 4 decomposition mechanisms in pyrolysis of thermosetting plastics

briefly explain each

A

end-chain scission = removal of monomer units from end of polymer chain = produced monomers

random scission = main chain bonds broken at random locations along polymer chain so sections are small enough to volatilise = produces oligomers

chain stripping = polymer chain is intact by species not a part of the main chain break away

cross-linking = the further cross linking producing more char

30
Q

what is modern furniture more resistant to

but what do these have little to no resistance to

A

common accidental ignition e.g cigarettes and other glowing sources

due to the introduction of new legislation reducing the chances of smouldering combustions occurring

little to no resistance to flaming sources

31
Q

where might we find plastics in quantities that can cause a fire (6)

A

carpets = polypropylene yarn and backing over a polyurethane underlay

curtains = synthetic fabrics

sofas/cushions/mattresses = polyurethane foams

window = polyvinylchloride

flooring = melamine

paint = latex, acrylic and polyvinylacetate

32
Q

despite flaming combustions only happening in the gas phase, when may a flaming combustion of a metal be seen

what metals is this seen in and in what kind if setting

A

if the metal is thin enough and has a large SA:V ratio e.g magnesium ribbon or dust

metals oxidise in air so if thin enough this can lead to

seen in aluminium, magnesium and iron in industrial settings

32
Q

why are solid fuels often relevant in forensic settings

A

because they are everywhere

33
Q

what does of combustion does carbon undergo to form char

A

smouldering combustion

Fires and Explosions (20 decks)

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