Fire Dynamics I

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Fire Dynamics I

• Lecture 1
• Introduction / Polymeric Fuels
• Jim Mehaffey
• 82.575 or CVG7300

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• Introduction / Polymeric Fuels
• Outline
• Introduction
• Gaseous, Liquid and Solid Fuels
• Nature of Polymeric (Solid) Fuels
• Thermal Decomposition of Polymers

3

• Definitions
• Fire dynamics study of how to describe and
  estimate the growth and intensity of fires
• Fire safety engineering application of
  engineering principles based on a fundamental
  understanding of fire dynamics in order to save
  lives and protect property

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• Fire Dynamics
• Fire involves exothermic chemical reactions
  between combustibles and oxygen
• Physical conditions of fuel environment
  important
• Fire dynamics depend on
• Chemistry
• Heat transfer
• Fluid mechanics

5

• The Nature of Fuels (Fire)
• Gas / Liquid /Solid
• Carbon-based Fuels
• Simple gaseous hydrocarbons (CH4)
• Solids of high molecular weight and great
  chemical complexity (polymers)

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• Gaseous Liquid Fuels Alkanes (CnH2n2)

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• Gaseous Liquid Fuels Others

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• Chemical Formula
• Methane CH4 (actually a tetrahedron)
• n - octane normal octane (C8H8)

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• Molecular Weight
• Mole amount of a substance containing as many
  atoms or molecules as 12 g of Carbon-12
• Mole 6.02 x 1023 atoms or molecules (Avagadro)
• Molecular weight mass of 1 mole of of substance
  (g mol-1)
• Molecular weight sum of atomic weights of
  constituents
• Molecular weight (CH4) 1 x atomic weight of C
  4 x atomic weight of H
• 1 x 12 4 x 1
• 16

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• Solid Fuels

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• Flaming Combustion
• Flaming combustion occurs in the gaseous phase
• Gaseous hydrocarbons (e.g. CH4) can be ignited in
  air to produce flame
• Flaming combustion of liquids solids requires
  conversion to volatiles (vapours)
• For most liquids (e.g. C8H8) volatiles produced
  by evaporation
• Liquids with high boiling points (gt250C) undergo
  chemical decomposition (e.g. cooking oil)

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• Flaming Combustion
• For most solids, chemical decomposition
  (pyrolysis) yields products of low molecular
  weight which volatilize from surface
• Surface temperature of burning solids 400ºC
• Composition of volatiles is complex and depends
  on chemical nature of the solid

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• Polymers
• Combustible solids are polymers of high molecular
  weight
• A polymer consists of long chains of repeated
  units called monomers
• Polymers are classified as
• Natural or synthetic
• Thermoplastic or thermosetting
• Addition or condensation

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• Addition Polymers
• Formed by direct addition of monomer units to a
  growing polymer chain
• Simplest example Generation (polmerization) of
  polyethylene from ethylene
• Ethylene C2H4 is a gas H H
• 
  C C
• H H
• Polymerization commences when double bond is
  broken replaced with a single bond

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• Production of Polyethylene
• Use hydrogen peroxide (H2O2) as initiator
• H H
  H H
• 
  
  
• 1st step H O O H C C
  ? H O H O C C
• 
  
• H H
  H H
• H H
  H H H H H H
• 
  
  
• 2nd step H O C C C C
  ? H O C C C C
• 
  
• H H H H
  H H H H

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• Production of Polyethylene
• Chain reaction proceeds spontaneously
  (exothermic)
• Termination can occur if
• OH group attaches to end, or
• two chains combine
• Length of chain can be controlled by controlling
  amount of initiator

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• Structure of Polyethylene
• n Degree of polymerization
• Also written as
• R - C2H4n - R

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• Structure of Polyethylene
• As n ? gas ? liquid ? wax ? solid (polymer)
• When n 200 material is a solid (polymer)
• For commercial polyethylene n gt 3,000
• For most polymers
• 10,000 lt molecular weight lt 1,000,000
• Polymer chains are twisted intertwined
• Chains are not all the same length

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• Condensation Polymers
• Formed by reactions in which two monomers link
  dropping a small molecular species (H2O)
• Examples Cellulose and nylon 66
• Condensation polymers are often more complex than
  addition polymers

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• Idealized Structure of NYLON 66
• 66 refers to
• 6 carbon atoms in amine unit, and
• 6 carbon atoms in carboxyl acid unit

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• Production of NYLON 66
• ?

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• Production of NYLON 66
• Polymerization can continue at both ends of the
  new molecule by the same reaction
• An endothermic reaction, so heat must be added
  for reaction to continue
• Reaction continues until almost all of one of
  reactants is used up

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• Basic Structure of Polymers (Plastics)
• Monomer A with two reactive groups can form
  linear chains
• A A A A A A A
• Properties of polymer can be modified by
• Changing chain length (control amount of
  initiator)
• Changing the monomer A
• Introducing branching (vary conditions of
  reaction)

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• Polyethylene
• Branching in Polyethylene

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• Branching in Polyethylene
• Low density polyethylene (LDPE)
• 8 to 40 branch points per 1000 main chain C atoms
• Density 940 kg m-3
• High density polyethylene (HDPE)
• 5 or less branch points per 1000 main chain C
  atoms
• Density 970 kg m-3
• Polymethylene (PM)
• Polyethylene with no branch points
• LDPE is less thermally stable than HDPE which is
  less thermally stable than PM

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• Branching in Polymers
• Branching occurs in linear polymers
  (polyethylene)
• Branching can also be achieved by adding a small
  amount of a monomer B with 3 reactive groups
• A
• A
• A A A B A A A

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• Cross-Linking in Polymers
• Branching can produce cross-linked structures
  with altered physical and chemical properties
• A A A B A A A B A A A A
  A B A A A
• 
  
  
• A
  A
  A
• 
  
  
• A
  A
  A
• 
  
  
• B A A A
  B A A B A A B A B A
• 
  
  
• A
  A
  A
• 
  
  
• A B A A B A A A A A A
  B A A A A B A A
• A

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• Cross-Linked Polymer
• Example Polyurethane (PU)
• A copolymer of tolylene di-isocyanate and a
  polymer diol
• B trihydric alcohol
• Flexible PU foam - low degree of cross-linking
• Rigid PU foam - high degree of cross-linking

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• Thermoplastics
• Flexible linear chains held together by weak
  (van der Waals) forces
• Produced by addition or condensation reactions
• Fire Performance
• Melting temperature lower than ignition
  temperature
• Generally do not produce char
• Fire spread may be enhanced by burning droplets
  or spread of a pool of molten polymer
• Fire spread may reduced if plastic melts and
  falls away from fire (NBCC 1995 / 3.1.13.4. Light
  Diffusers and Lenses)

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• Thermosetting Plastics
• Linear chains bonded together via cross-linking
  into a rigid 3-D network
• Cross-linking produced by condensation reactions
• Fire Performance
• Do not melt. Decompose to give volatiles char.
• Volatile yield decreases as cross-linking
  increases (e.g. phenolic resins heated to 500C
  yield 60 char)
• Thermosets burn much like wood
• Lightly cross-linked thermosets may behave like
  thermoplastics (e.g. flexible polyurethane foam)

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• Formation of Volatiles from a Solid

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• Formation of Volatiles from a Solid

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• Products of Thermal Decomposition
• Volatiles range from simple molecules to
  relatively large molecules
• Volatiles (and rate of production) effect
  ignitability, flammability and toxicity
• In flaming combustion most of volatiles consumed
• In pyrolysis without combustion liquids tars
  condense forming aerosol smoke
• Char regulates heat transfer to deeper
  pyrolysing levels hence effects flammability
• Char may undergo oxidation

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• Mechanisms of Thermal Decomposition
• 1. Unzipping or end-chain scission
• For some addition polymers, thermal decomposition
  occurs by successive removal of monomer units
  from end of polymer backbone

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• Mechanisms of Thermal Decomposition
• 2. Random chain Scission
• Main chain bonds are broken at random locations
  along polymer backbone
• Process continues until sections small enough to
  volatilize are generated
• Products of thermal decomposition include wide
  range of molecular species
• Example polyethylene

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• Mechanisms of Thermal Decomposition
• 2. Random chain scission
• Main chain bonds are broken at random locations
  along polymer backbone
• Process continues until sections small enough to
  volatilize are generated
• Products of thermal decomposition include wide
  range of molecular species
• Example polyethylene

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• Mechanisms of Thermal Decomposition
• 3. Chain stripping
• Backbone remains intact but molecular species
  which are not part of main chain break away
• Example Polyvinyl chloride (PVC) loses HCl at
  about 250C leaving a char-like residue

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• Flammability of PVC
• HCl is an effective combustion inhibitor
• Like most plastics, PVC has additives
• (plasticizer / filler / pigment / fire
  retardant)
• Rigid PVC grades used for electrical insulation
  often have low flammability
• Flexible PVC often contains plasticizer which
  makes it more flammable
• Caution HCl is an acid gas that is both
  corrosive and toxic
• Char residue burns at high temperatures

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• Mechanisms of Thermal Decomposition
• 4. Cross-linking
• Some polymers undergo cross-linking during
  pyrolysis generating a lot of char
• This reduces volatiles and flammability
• Cross-linking during pyrolysis usually occurs
  together with other mechanisms
• Effect not usually significant for thermoplastics
• For thermosets, like phenolic resin (slide 1-30),
  which are highly cross-linked, further
  cross-linking can occur during pyrolysis

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• Thermal Stability of Polymers
• Th Temperature at which half-life of polymer is
  30 min

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• Factors Affecting Thermal Stability
• Increasing molecular weight strengthens polymer
• Increasing chain branching weakens polymer
• Increasing cross-linking strengthens polymer
• Introducing double bonds in polymer backbone
  weakens polymer
• Introducing aromatic rings in polymer backbone
  strengthens polymer
• Introducing oxygen in polymer backbone weakens
  polymer

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• Increasing mol wt strengthens polymer
• Polymethylmethacrylate (PMMA)
• PMMA A (MW 1.5 x 105) Th 283 ?C
• PMMA B (MW 5.1 x 106) Th 327 ?C
• Monomer - C5H8O2 -

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• Increasing branching weakens polymer
• Polymethylene (PM) Th 415 ?C
• Polyethylene (PE) Th 406 ?C
• Polypropylene (PP) Th 387 ?C
• Polyisobutylene Th 348 ?C

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• Increasing branching weakens polymer
• Monomers

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• Impact of Products of Decomposition
• on Polymer Flammability
• Production of char reduces production of volatile
• Char insulates unburned polymer from heat of
  flame
• Volatiles affect flame chemistry
• Volatiles affect soot production
• Soot controls thermal radiation
• Aromatic volatiles (benzene) yield flames of high
  emissivity (e.g. polystyrene)
• Thermal radiation affects rate of burning

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• Impact of Products of Decomposition
• on Polymer Flammability
• Volatiles (product of decomposition) affect
  toxicity HCl from PVC HCN from wool or
  polyurethane)
• Principal toxicant is CO (carbon monoxide), a
  product of combustion (not decomposition). Rate
  of generation CO depends on conditions of burning
  and supply of air

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• Behaviour of Individual Polymers
• The properties of a plastic depend sensitively on
  the degree of polymerization (molecular weight)
  of the main polymer, its degree of branching and
  the presence of additives. Values given below
  may not apply to all commercial plastics.
• Additives can comprise 1/2 or more of the weight
  of commercially available plastics.
• Additives fillers, plasticisers, lubricants,
  anti-aging agents, fire retardants, colorants,
  blowing agents, impurities

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• Definitions
• Tm melting temperature
• Td temperature at which molecular weight
  decreases (depolymerization commences)
• Tv temperature at which volatilization
  commences
• Th half-life temperature (30 minutes)
• Tig ignition temperature
• Mech primary mechanism of thermal decomposition
• Prod principal products of thermal
  decomposition

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• Polyethylene (PE)
• Applications waste baskets, bags, plastic chairs
• Type thermoplastic
• Tm 133C 406 K
• Td 292C 565 K Note PE is recycable
• Tv 372C 645 K
• Th 406C 679 K
• Tig 363C 636 K
• Mech random chain scission
• Prod alkanes alkenes with 2 or 3 carbons

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• Polypropylene (PP)
• Applications carpet backing, TV computers
• Type thermoplastic
• Tm 186C 459 K
• Td 237C 510 K
• Tv 302C 575 K
• Th 387C 660 K
• Tig 334C 607 K
• Mech random chain scission
• Note PP is recycable

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• Polyvinyl Chloride (PVC)
• Applications piping, siding, flooring,
  upholstery
• Type thermoplastic, but does produce char
• Td Tv 250C 523 K (generation of HCl)
• Mech chain stripping
• Note H2 is evolved from char 452C 725 K
• Monomer -C2H3Cl-

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• Polystyrene (PS)
• Applications insulation, foam cups
• Type thermoplastic
• Tm 240C 513 K
• Tv 302C 575 K
• Th 364C 637 K
• Tig 366C 639 K
• Mech random chain scission
• Prod monomer, dimer, trimer tetramer
• Note PE is recycable

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• Polystyrene (PS)
• Monomer -C8H8-

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• Polymethyl Methacrylate (PMMA)
• Applications plexiglass, glazing
• Type thermoplastic, but does not flow well
• Tm 160C 433 K
• Tv Td 272C 545 K
• Th 327C 600 K
• Tig 310C 583 K
• Mech end chain scission
• Prod monomer
• Note Th Tig are for high molecular weight PMMA
  B

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• Polyurethane (PU)
• Applications flexible foam cushions, mattresses
  rigid foam insulation
  
• Type thermosetting, but does melt
• Td gt 202C 475 K
• Mech very complex decomposition process many
  steps and many products (including HCN)

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• References
• D. Drysdale, An Introduction to Fire
  Dynamics,Wiley, 1999, Chap 1
• F.W. Billmeyer, Textbook of Polymer Science,
  Wiley, 1984, Chap 1
• Donald R. Askeland, Science and Engineering of
  Materials, Chapman Hall, 1990, Chapter 15
• C.F. Cullis and M.M. Hirschler, The Combustion of
  Organic Polymers, Oxford Science Publications,
  1981, Chapter 1
• C.L. Beyler and M.M. Hirschler, "Thermal
  Decomposition of Polymers" Section 1 / Chapter 7,
  SFPE Handbook, 2nd Ed. (1995)
• C.F. Cullis and M.M. Hirschler, The Combustion of
  Organic Polymers, Oxford Science Publications,
  1981, Chapter 1