Lecture 18 Flashcards
what can change the ignition temperature of different paper types
paper is a long chain polymeric material so different properties such as secondary structure and bonding can affect the ignition temperature
how does a smouldering combustion go to a flaming combustion
when gases begin the evaporate from the solid fuel
these gases make up the flame
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
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
what is the simplest solid fuel
carbon
coal, charcoal, wood/paper is primarily carbon
what is the name of the combustion where no flame occurs
a smouldering combustion
what is pyrolysis
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
do smouldering or flaming combustions do more damage
flaming combustions
what are the ways a solid fuel and produce vapour that can lead to flames (5)
- sublimation
- melting then evaporation = candle wax
- melting then decomposition and evaporation
- decomposition and melting then decomposition and evaporation = plastics
- decomposition an evaporation
why are pyrolysis product residues often hard to analyses to find the origin
as real substances are complex mixtures so give complex mixtures of pyrolysis products
what are two characteristics of pyrolysis products
what two other things can they be
they are volatile and flammable
but can be toxic and corrosive
where can pyrolysis products be carried
in the fire plume and be ignited elsewhere
particularly if the material is lightweight
when will a pyrolysis product burn at the surface of the solid they came from
if their concentration is within their flammability range
what does the solid left behind after pyrolysis products have been carried away become closer to
carbon (char) so hard to find the source of the fuel
what are the main two components of wood and how do their structures differ
cellulose = a long chain polymer
lignin = a branched polymer
what can charring of a surface mis lead to an investigator
the surface temperature of the fuel
what types of wood are easy to ignite
what types are harder
resinous woods e.g pine
hard woods - but can cause hotter and more protraced fires
what three things are manufactured wood products also likely to have that impact its ignition temperature
adhesives holding layers together
surface coatings
varnishes
what are the 5 layers observed when wood is burnt/burning
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
what does the char layer act as in the burning of wood
the smouldering combustion
what does the pyrolysis zone act as in the burning of wood
creates volatiles for the flaming combustion
what is paper made of
what makes paper easier to burn than wood
cellulose
paper has a lower heat capacity and is much thinner, better surface area exposed and air flow
why is a large stack of paper hard to burn
there is no air flow
how is paper often used as an arson set
why
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
can can a fire including paper spread easily
paper is lightweight so can be carried away from the original fire in convection currents and start a fore elsewhere
name 4 natural fabrics that are used in starting fires and give some info about their use for starting a fire
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)
what are synthetic fabrics made of and why are they good for starting a fire
plastic
high SA:V
what is a
polymer
oligomer
monomer
- a large molecule composed of many monomers joined together
- a small portion of a polymeric chain
- the smallest subunit of a polymer
what are plastics made of
what are the two main classes of polymeric plastics
polymers with hydrocarbon chains
thermoplastics and thermosetting plastics
what is a thermoplastic
give three examples
a structure of linear chains
undergo reversible melting without appreciable chemical decomposition
polyethylene, polystyrene and nylon
what is a thermosetting plastic
give two examples
cross-linked structures
so do not melt but do chemically degrade to give solid char
polyesters and rigid polyurethane foam
why do thermosetting plastics not melt like thermoplastics
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
why do thermoplastics melt before it burns
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
what can thermoplastic drops be confused with at a crime scene (2)
the splashing of a low viscosity accelerant
the pyrolysis products of plastics and arson accelerants are similar as they are made of similar things
what do the combustion properties of plastics depend on (2)
polymer chain length
chemical structure (linear or branched/crosslinked)
what do thermosetting plastics burn like
wood - pyrolysis gives volatile molecules leaving a solid char
is the production of volatile gases more in lower or highly cross linked polymers
more gases in lower cross linked polymers
what are the 4 decomposition mechanisms in pyrolysis of thermosetting plastics
briefly explain each
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
what is modern furniture more resistant to
but what do these have little to no resistance to
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
where might we find plastics in quantities that can cause a fire (6)
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
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
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
why are solid fuels often relevant in forensic settings
because they are everywhere
what does of combustion does carbon undergo to form char
smouldering combustion
