Transcript Fire Retardants

Fire Retardants
By: Mark Bryson and Nate Craft
Overview
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Basics of Combustion
Theories of Fire Retardancy
Fire Retardant Polymers
Traditional Chemicals
Green Alternatives
History
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First century,
– Romans used solutions of alum and vinegar to protect their boats.
– In 1820, Gay-Lussac advocated the use of ammonium phosphates and borax for
treating cellulosic material.
– World War II was another dominant milestone in the creation of flame retardants.
Troops slept in canvas tents made flame retardant and waterproof by the use of
chlorinated paraffin, antimony oxide and a binding agent.
Early Studies
– 1930-1935
• Comprehensive study of 130 fire retardant chemicals
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1.Diammonium phosphate
2. Monoammonium phosphate
3. Ammonium chloride
4. Ammonium sulfate
5. Borax
6. Zinc chloride.
Combustion
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Process
– 1. heating of the fuel source.
– 2. decomposition of the fuel source into combustible and noncombustible materials.
– 3. ignition of the combustible fuel and air mixture to produce a flame.
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The formation of water (ΔHf = -57.75 kJ/mol; ΔG = -56.69 kcal/mol) and
carbon dioxide (ΔHf = -93.99 kcal/mol; ΔG = -94.254 kcal/mol) are
exothermic reactions that feed energy back into the system. This energy
leads to further breakdown of the fuel source to continue the combustion
cycle.
Fire Retardant Methods
• Chemical
– To interfere with free radical reactions which occur
during combustion (gas phase)
– To insulate the underlying material from temperature
rises via char formation (solid phase)
• Physical
– To lower temperatures by endothermic reactions
– To slow down the spread of fire by diluting oxygen
with non-combustible gases
– To provide protective impervious surface layer
Theories of Fire Retardancy
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Barrier
– Formation of a glassy layer
• Insulates and prevents oxygen from reaching the substrate.
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Thermal
– Increase thermal conductivity of the wood
– Physical or chemical change that results in the heat being absorbed by the
additive thereby preventing combustion
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Dilution by Non-Combustible Gases
– Noncombustible gases dilute the combustible gases formed
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Free Radical Trap
– Fire retardant chemicals release free radical inhibitors that interrupt the chain
propagation mechanism of flammability.
Theories of Fire Retardancy (cont.)
• Barrier
– Sodium Silicates
– Coatings that intumesce
(puff and form a cellular
structure that remains
attached to substrate)
• Dilution of
Noncombustible
Gases
– Dicyandiamide and urea
release non-combustible
gases.
– Borax releases water in
high quantities.
• Thermal
– Impregnate wood with a
metal alloy
Free Radical Trap Theory
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This theory involves:
– Chain Branching Reaction
• The decomposition of the fuel source, i.e. breaking the chemical bonds into high energy
free radicals, is initially done by an external ignition source that starts the cycle.
• Heating of combustible materials results in the generation of hydrogen, oxygen,
hydroxide, and peroxide radicals that are subsequently oxidized with flame.
• If the resulting compound is less readily oxidized than the radical that is removed, the
result is reduced flammability.
Free Radical Trap Theory(Cont’d)
– Bromine and Chlorine are good free radical inhibitors
• Complex with free radicals and stop the combustion mechanism.
– Large amounts are required for practical fire retardancy
• 15-30% by weight
• Halide is regenerated to continue the reaction
– Routes with oxygen are also possible
Major Chemicals
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The main fire retardant chemical classes together with some representative examples
are listed below:
– Inorganics compounds
• Aluminium trihydroxide, ammonium polyphosphate, antimony trioxide.
– Halogenated organic compounds
• Brominated and chlorinated compounds. These include chlorinated paraffins,
tetrabromobisphenol-A, decabromodiphenyl ether.
– Organic phosphorous compounds
• Phosphate esters such as triphenyl phosphate, others combined with
halogen compounds.
– Nitrogen based compounds
• Melamines
Brominated Flame Retardants
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The family of Brominated Flame Retardants (BFRs) contain more than 75
different chemicals.
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Three Classes:
– 1) Aromatics - including tetrabromobisphenol-A, (TBBA), polybrominatedethers
(PBDEs) and Polybrominated biphenyls (PBBs)
– 2) Aliphatics - which tend to have limited use and
– 3) Cycloaliphatics - such as hexabromocyclododecane (HBCD)
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The major types used in the Flame Retardant industry are:
– Poly Brominated Diphenyl Ethers (PBDEs)
– Hexabromocyclododecane (HBCD)
– Tetrabromobisphenol-A (TBBA)
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Synergistic Chemicals –
– Antimony Trioxide - (Sb2O3). The synergist acts to improve the activity
of the additive in the polymer, thus lowering the amount of halogenated
additive needed. Antimony oxide acts as a halogen shuttle bringing
SbX3 into the vapor phase
Nitrogen
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Nitrogen based FRs result in char formation by causing the material to swell as
flammable gases are evolved.
Nitrogen decomposes in the vapor phase to form non-flammable gases such as HNO2
and HNO3 and also prevents the phosphorous compounds from being pyrolyzed in the
vapor phase.
NH2
HN
H2N
MDF(Multiple Density
Fiberboard) with
Melamine Coating
NH
N
NH2
H
Melamime
Phosphorus Compounds
• Monoamonium and Diamonium phosphates.
– Presence of Nitrogen produces a synergistic effect
– Increased Flame Spread Resistance with lower chemical loading levels.
• Organophosphours and Polyphosphate
• Phosphorous containing FRs influence chemical reactions taking
place on the surface. Upon heating they decompose to phosphoric
acid which when condensed causes the material to char. Some
phosphorous FRs can also act in the gas phase as radical traps but
it is less common.
Monoamonium Phosphate
Diamonium Phosphate
Boron Compounds
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Borax and Boric Acid
– Low Melting Points
– Form glassy films at high
temperature
– Borax
• Inhibits surface flame
spread
• Can promote smoldering
and glowing
– Boric Acid
• Reduce smoldering and
glowing
• Little effect on flame
spread
Oftentimes both chemicals are
used together.
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Polybor
– Na2B8O13 • 4H2O
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Borax
– Na2B4O7 • 10H2O
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Solubility increases when borax is
added to boric acid.
Polymerization of polyborates
remove boric acid and borates
from solution. This allows more
boric acid and borax to dissolve.
The resulting solution is polybor
Aluminum Trihydrate and Boron
produce a synergistic effect similar
to that of Nitrogen and
Phosphorus.
OH
HO
B
OH
Boric Acid
Aluminum Trihydrate
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Retardancy Based Upon
– Endothermic dehydration to aluminum oxide and water.
– In absorbing some of the heat of combustion and lowering the
temperature of the substrate near the flame, the hydrate functions as a
chemical heat sink. The water vapor provided by such action dilutes the
gaseous reactants in the flame until all the water of crystallization is
exhausted
– Only fire-retardant ingredient in fiberboard.
Fire Retardant Polymers
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Requirements
– 1. Compounds with strong covalent bonds should be used.
– 2. There should be no easy pathway for the molecular rearrangements.
– 3. Resonance stabilization of aromatic polymer rings should be used to maximize
the bonding energy.
– 4. All of the rings in the structure should have normal bond angles, i.e. no bond
strain or weak points.
– 5. Multiple bonding to several centers should be utilized, i.e. ladder polymer
would be the most stable, para linkages are most stable.
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Economically Feasible Polymers
– Kevlar ,Polyether imides, Polyetheretherketone (PEEK), Teflon
Fire Retardant Polymers
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An important feature in some of these polymers is having unsaturation in
the polymer backbone or various leaving groups, which will crosslink in a
fire to form char, i.e. poly (styryl pyridine) or PSP.
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The final way in which these polymers can form char during the burning
process is to form additional rings either by Claisen-Cope or Diels Alder
rearrangement. The following polyamide, similar to Dupont’s Nomex, forms
a more stable benzoxazole structure on heating
Issues with Traditional Retardants
• Persistence
– Resist breakdown in the environment
• Toxicity
– Firefighters at risk
• Dioxins and furans produced at high levels
– European Union banned the use of all polybrominated diphenyl ethers
(PBDEs) and polybrominated biphenyls (PBBs) in electronic products
starting in 2006.
– Halogenated Organic Compounds are considered Persistent Organic
Pollutants(POP)
– Antimony Oxide – possible link to Sudden Infant Death Syndrome.
Green Alternatives
• Non-Halogenated Fire Retardants
– Carbon Nanotubes and Clay
• Synergistic effects improve the flame retardancy of
polymeric materials without the use of toxic
chemicals
• Barrier properties of clay and tensile strength of
carbon nanotubes
– Expectation of high performance characteristics with
reduced use of potentially toxic chemicals
– Researchers have been able to modify the
flammability properties of polymers with
carbon nanotubes
Bromine Free Alternatives
• Most commonly used bromine free alternatives
– Inorganic
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Aluminium Trihydroxide
Magnesium Hydroxide
Ammonium Polyphosphate
Red Phosphorous
Zinc Borate
– Organophosphorous
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Triphenyl Phosphate
Tricresyl Phosphate
Resorcinol bis(diphenyl phosphate)
Phosphonic Acid (2-((hydroxymethyl) carbamyl)ethyl)- dimethyl ester
Phosphorous and Nitrogen containing thermosets
– Nitrogen Containing
• Melamine
Bibliography
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LeVan, Susan L. Chemistry of Fire Retardancy. U.S. Department of Agriculture, Forest Service,
Forest Products Laboratory.
Berger, Michael. “Flame-retardant materials with more nanotechnology and less toxic chemicals.”
Nanowerk, 8/30/2007. Accessed 4/13/09. http://www.nanowerk.com/spotlight/spotid=2445.php
Blum, Arlene. “Review of The Fire Retardant Dilemma Parts I-VI" (part 2). Green Science Policy
Institute- The Fire Retardant Dilemma: Part VII, January 30, 2009. Accesed 4/14/09.
http://greensciencepolicy.org/assets/Uploads/Jan-30-09-Conf-Presentations/Blum-FRDVII-Jan09reduced.pdf
Janssen, Sarah. “Halogenated Fire Retardants.” Green Science Policy Institute- The Fire
Retardant Dilemma: Part VII, January 30, 2009. http://greensciencepolicy.org/assets/Uploads/Jan30-09-Conf-Presentations/Janssen-FRD-presentation01-09.pdf
Hepburn, C. Rubber Compounding Ingredients – Need, Theory, and Innovation, Part II. Vol 9 Num
1. 1997.
Flame Retardants: A General Introduction, Enviornmental Health Criteria 192. United Nations
Enviornment Programme. 1997. http://www.inchem.org/documents/ehc/ehc/ehc192.htm.
Federal Aviation Association. U.S Department of Transportation. Development and Testing of
Flame Retardant Additives and Polymers. 04/07. 04/17/09.
http://www.tc.faa.gov/its/worldpac/techrpt/ar0725.pdf
Major Chemicals
• Nitrogen
– Barrier
• Phosphorus
– Barrier
• Boron
– Thermal
– Dilution of combustible gases
• Aluminum Trihydrate.
– Thermal
– Dilution of combustible gases