Fire Retardants: A Comprehensive Overview

1. Fire Retardants By: Mark Bryson and Nate Craft

2. Overview Basics of Combustion Theories of Fire Retardancy Fire Retardant Polymers Traditional Chemicals Green Alternatives

3. History 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 1.Diammonium phosphate 2. Monoammonium phosphate 3. Ammonium chloride 4. Ammonium sulfate 5. Borax 6. Zinc chloride.

4. Combustion Process 1. heating of the fuel source. 2. decomposition of the fuel source into combustible and non- combustible materials. 3. ignition of the combustible fuel and air mixture to produce a flame. The formation of water ( H f = -57.75 kJ/mol; G = -56.69 kcal/mol) and carbon dioxide ( H f = -93.99 kcal/mol; G = -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.

5. 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

6. Theories of Fire Retardancy Barrier Formation of a glassy layer Insulates and prevents oxygen from reaching the substrate. Thermal Increase thermal conductivity of the wood Physical or chemical change that results in the heat being absorbed by the additive thereby preventing combustion Dilution by Non-Combustible Gases Noncombustible gases dilute the combustible gases formed Free Radical Trap Fire retardant chemicals release free radical inhibitors that interrupt the chain propagation mechanism of flammability.

7. 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

8. Free Radical Trap Theory 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.

9. Free Radical Trap Theory(Contd) 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

10. Major Chemicals 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

11. Brominated Flame Retardants The family of Brominated Flame Retardants (BFRs) contain more than 75 different chemicals. 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) The major types used in the Flame Retardant industry are: Poly Brominated Diphenyl Ethers (PBDEs) Hexabromocyclododecane (HBCD) Tetrabromobisphenol-A (TBBA) 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

12. Nitrogen 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 HNO 2 and HNO 3 and also prevents the phosphorous compounds from being pyrolyzed in the vapor phase. MDF(Multiple Density Fiberboard) with Melamine Coating

13. 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

14. Boron Compounds 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. Polybor Na2B8O13 4H2O Borax Na2B4O7 10H2O 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.

15. Aluminum Trihydrate 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.

16. Fire Retardant Polymers 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. Economically Feasible Polymers Kevlar ,Polyether imides, Polyetheretherketone (PEEK), Teflon

17. Fire Retardant Polymers 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. 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 Duponts Nomex, forms a more stable benzoxazole structure on heating

18. 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.

19. 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

20. Bromine Free Alternatives Most commonly used bromine free alternatives Inorganic Aluminium Trihydroxide Magnesium Hydroxide Ammonium Polyphosphate Red Phosphorous Zinc Borate Organophosphorous Triphenyl Phosphate Tricresyl Phosphate Resorcinol bis(diphenyl phosphate) Phosphonic Acid (2-((hydroxymethyl) carbamyl)ethyl)- dimethyl ester Phosphorous and Nitrogen containing thermosets Nitrogen Containing Melamine

21. Bibliography 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-Jan- 09reduced.pdf Janssen, Sarah. Halogenated Fire Retardants. Green Science Policy Institute- The Fire Retardant Dilemma: Part VII, January 30, 2009. http://greensciencepolicy.org/assets/Uploads/Jan- 30-09-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

22. Major Chemicals Nitrogen Barrier Phosphorus Barrier Boron Thermal Dilution of combustible gases Aluminum Trihydrate. Thermal Dilution of combustible gases