FIRE IN COMPOSITE MATERIALS

Fire Retardants

Kenya McNaught

Composite Material DefinitionComposite material. System consisting of at least two phases (matrix and reinforcement) on a macroscopic scale, whose mechanical performance and properties are designed to be superior to those of the constituent materials acting independently.

Matrix Type:• Metal: Thermally stable• Ceramic: High

thermally stable• Carbon: High Thermally

stable• Polymer: Thermally

instable. Largest composite industry due to low material cost and manufacture.

Reinforcement Type:• Glass• Carbon• Aramid Shapes:• Fibers

• Unidirectional• Bidirectional• Choped oriented

or random• Multidirectional

(woven, braided mats)

• Particulates and flakes.

Polymer Composites

Common Polymer Matrix Materials • Polyesters• Epoxies• Vinyl esters• Phenolics

Polymers are most frequently responsible for the propagation of fire after ignition. Since most polymers are hydrocarbon based, the flame is a hydrocarbon flame.

Polymer Combustion

Phases of Polymer combustion:• Polymer heating to a

temperature where decomposition starts.

• Decomposition results in gaseous products usually combustible.

• If there is an ignition source the diffused gas products will undergo combustion generating more heat.

• Heat transfers back to the polymer producing more gaseous products to sustain combustion.

Polymers subjected to ignition sources will generate self-sustained combustion in air/oxygen.

Two types of combustion involved in polymer burning:

1. Flaming combustion: Self propagating combustion reactions, fuel and oxidant are present as gas. Usually hydrocarbon flame present in polymers.

2. Non-flaming combustion: propagates through the polymer by a thermal front or wave

Polymer Combustion

• The primary ignition time controlling variable is heating composite to degradation temperature by conduction.

Ignition time increases with velocity of convective flow like in a vertically oriented sample (vs. horizontal sample as in test)

• Secondary factors is oxygen concentration.

• During heating or combustion the resin vaporizes leaving char residue.

Resin Vaporization allow flame generation in combustion.Char and carbon burn as surface oxidation. Can produce carbon monoxide flame near the oxidizing surface.

Some of the main factors affecting polymer combustion…

Composite vaporization starts

at ~300C after which ignition

occurs.

Heat flux to start ignition

~20kW/m2.

To sustain burning ~50

kW/m2.

• Heat flux increases due to the flame and heated surroundings.Fire in confined space accelerates due to thermal feedback and decelerates due to oxygen depletion.

• Resin vapor escapes through closely spaced carbon fibers generating deformation due to the internal pressures.

Changes in material properties are expected (due to increase in volume, decrease in density, change in structure, resin thermal conductivity decreases and heat degraded properties of both components .)

The major decomposition mechanism is unzipping. The chain depolymerizes by successive release of monomer units from a chain end or at a weak link, which is essentially the reverse of chain polymerization.

The initial breakdown of these polymers is mainly caused by the rupture of the weakest bonds in the polymer chain.

Bond Dissociation of Polymers.

PE PolyethylenePP PolypropylenePS PolystyrenePMMA Polymethyl MetacrylatePET Polyethylene therephthalatePC Polycarbonate

Fire Characteristics of Polymer Composites• Increased composite thickness increases burning time and delays

ignition times for heat fluxes below ~43 kW/m2.

• Complete vaporization of the resin occurs for heat fluxes above 25 kW/m2 in the cone calorimeter.

• Heat of combustion approximately 20 kJ/g-vapor, the yield of CO in flaming is roughly 0.48g CO/g-vapor and the mass fraction of remaining residue roughly 0.74.

• Material can swell to over twice its volume and its porosity after burning is about 65%.

• Flame spread will occur for heat fluxes below 18 kW/m2 after preheating for 4 minutes with upward flame spread occurring for as low as 10 kW/m2 with rates of the order of 1 mm/s.

FIRE TESTING TECHNIQUES

TESTS. Evaluation of Fire behavior is intended to determine:• Ease of Ignition – how readily

material ignites.• Flame spread – how rapidly fire

spreads across a surface.• Fire Endurance – how rapidly fire

penetrates a composite wall.• Rate of heat release- how much and

how quickly heat is released.• Ease of extinction- how rapidly or

easy the flame chemistry will lead to extinction.

• Smoke and toxic gas evolution –Amount, evolution rate and composition of smoke/gases released during stages of fire.

• OXIGEN CONCENTRATION (LOI %)gives the minimum oxygen concentration at ambient temp needed in inert gaseous medium for the material to initiate and sustain burning.• Lowest is for resin only ~21• Self extinguishing material is >26

• HEAT REALEASE RATE is most important variable in fire.

LOI Limitng Oxigen Index

Aparatus

• Cone Calorimeter: IGNITION, BURNING.• Microscale Combustion Calorimeter: HEAT OF

COMBUSTION OF VOLATILES.• Thermogravimetric Analyzer: DEGRADATION,

MASS LOSS.• Differential Scanning Calorimeter: SPECIFIC

HEAT, HEAT OF DECOMPOSITION FOR COMPOSITE AND CARBON RESIDUE.

• Flame Spread Rig: FLAME SPREAD.

FIRE TESTING TECHNIQUES

TGA chart for Polyester-Glass Fiber Composite 50/50

Cone Calorimeter

Flame Retardants. Suppress combustion acting in the vapor or condensed phase by chemical and/or physical mechanisms.

Flame Retardancy

Class Type What do they do?

Additives

Fillers Dilute Polymer

Hydrated Fillers

Release non-flammmable gases or cool pyrolysis.

Reactive Materials

HalogenInterrupt exothermic process suppressing combustion

Phosphorus

Forms char to inhibit gaseous products/shields polymer

Intumescent

Materials

Swell to form foamed mass acting as barrier to heat, air and pyrolysis products.

Release toxic fumes

Poor compatibi

lity, reduce

properties

Compatible, do not reduce properties

Mixed with polymers during processing.

Applied on surfaces as coating.

Boron Based, Nitrogen based, Silicon based.

Example of Fire Test Results with Fire Retardants

Resin only

Resin+fibers=composite

Composite+4.5%filler in resin

Lowest burning rate

Composite –Biodegradable starch-based resin as matrix and basalt fibre plain fabric as reinforcementAdditive--RP: Red Phosphorus orMH: Metallic Magnesium Hydroxide

Highest strength… decreases with fillers

• Less combustible gas concentration (due to water vapor)

• Blocking of available oxygen• Less burning material• Smoke reduction due to acceleration of

oxidative reaction of sooth caused by emerging MgO

Some Flame Retardants in IndustryFlame Retardant

Categories of Flame Retardants:

1. Minerals such as asbestos, compounds such as aluminum hydroxide, magnesium hydroxide, hydromagnesite, antimony trioxide, various hydrates, red phosphorus, and boron compounds, mostly borates.

2. Tetrakis- phosphonium salts, made by passing phosphine gas through a solution of formaldehyde and a mineral acid such as hydrochloric acid, are used as flame retardants for textiles.

3. Synthetic materials, usually halocarbons. These include organochlorines such as PCBs, chlorendic acid derivates and chlorinated paraffins; organic bromines such as polybrominated diphenyl ether

Ideal Fire Resistant Polymer

• High decomposition temperature.

• Low amount and release rate of volatile fuels.

• Low heat of combustion of volatiles.

• High char yield.

• Endothermic phase transition or decomposition.

• Release of chemical flame retardant molecules (halogen, water, etc.).

SUPPORTING PAPERS

• Bharatkumar Z. Dholakiya. 2009. Use of non-traditional fillers to reduce flammability of polyester resin composites ISSN 0351-187. UDK 678.6:678.01.

• T. Wittek, T. Tanimoto. 2008. Mechanical properties and fire retardancy of bidirectionalreinforced composite based on biodegradable starch resin and basalt fibres.

• Ohlemiller, T., cleary, T. Brow, Shields, J. 1993. Assessing the Flamability of Composite Materials. Journal of Fire Sciences, Vol II.

• Huiqing Z, 2004. Fire Safe Polymers and Polymer Composites. DOT/FAA/AR-04/11.

• Johnston, A., Cole, R., Jodoin, A., MacLauring, J. and Hadjisophocleous, G. Evaluation of fire Performance of Composite Materials for Aircraft Structural Applications.

• Wright, M., Luers, A., Darwin, R. and Scheffey, J. 2003. Composite Materials in Aircraft Mishaps Involving Fire: A Literature Review. NAWCWD TP 8552.

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• Bharatkumar Z. Dholakiya. 2009. Use of non-traditional fillers to reduce flammability of polyester resin composites ISSN 0351-187. UDK 678.6:678.01.

• T. Wittek, T. Tanimoto. 2008. Mechanical properties and fire retardancy of bidirectionalreinforced composite based on biodegradable starch resin and basalt fibres.

• Ohlemiller, T., cleary, T. Brow, Shields, J. 1993. Assessing the Flamability of Composite Materials. Journal of Fire Sciences, Vol II.

• Huiqing Z, 2004. Fire Safe Polymers and Polymer Composites. DOT/FAA/AR-04/11.

• Johnston, A., Cole, R., Jodoin, A., MacLauring, J. and Hadjisophocleous, G. Evaluation of fire Performance of Composite Materials for Aircraft Structural Applications.

• Wright, M., Luers, A., Darwin, R. and Scheffey, J. 2003. Composite Materials in Aircraft Mishaps Involving Fire: A Literature Review. NAWCWD TP 8552.

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