Parameters for the thermal decomposition of epoxy resin/carbon fiber composites in cone calorimeter

4th ICHS Conference, September 14, 2011

D. Quang DaoJ. Luche, F. Richard T. RogaumeC. Bourhy-Weber S. RubanL. Bustamante Valencia

S. Ruban – ICHS2011, Paper No. 182, Sept. 14th 2011 2Public

Epoxy resin/carbon

fiber composite wall (few cm)

ContextThe high-pressure (70 MPa/10.1 kpsi) fully wrapped

epoxy resin/carbon fiber composite cylinder is currently the preferred option for fuel cell electric vehicle

Epoxy resin/carbon fiber composite cylinder

Liner: H2 tightness (few mm)

CostCostGravimetric

capacity

Gravimetric capacity

Volumetric capacity

Volumetric capacity

Light weight Excellent mechanical performance

High capacity of H2 storageGood chemical and electrical resistance

H2 vehicle refilling station

Type 2Type 1 Type 4Type 3Type 2Type 1 Type 4Type 3

Cylinder connector

Fire safety strategy: preventing the cylinder from bursting

Releasing hydrogen through a thermal pressure relief device and/or using a thermal protection

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Objective

To optimize the design of the fire protection of the cylinder by improving the understanding of the thermal behavior of the epoxy resin/carbon fiber composites

The thermal behavior is influenced by (Pilling et al.):Decomposition temperatureCarbon fiber fraction Nature of carbon fiber

Experiments showed:

CF fraction & temperature

Conductivity & decomp. rate

Fire resistance of composite

The thermal parameters such as mass loss, mass loss rate, piloted ignition time, thermal response parameter

and temperature of ignition are investigated

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Materials studied

The epoxy resin/carbon fiber composites are pre-impregnated bands of commercial references

Two representative references are tested:56 vol% Carbon fiber59 vol% Carbon fiber

Results of elementary analysisElement Epoxy resin

Composite 56 vol% CF

Composite 59 vol% CF

C 70.1% 81.6% 84.6%O 17.0% 6.6% 5.4%H 8.7% 4.6% 2.9%N 3.1% 5.1% 5.4%

Water 1.1% 0.5% 0.3%Total 100% 98.4% 98.6%

Thermal property Value

Density (r) 1472 ± 20 kg.m-3

Specific heat capacity (Cp) 0.9 kJ.kg-1.K-1

Thermal diffusivity parallel to CF (α) 2.75 ± 0.10 mm2.s-1

Thermal diffusivity transverse to CF (α) 0.40 ± 0.04 mm2.s-1

Thermal conductivity parallel to CF (λ)a 3.30 ± 0.30 W.m-1.K-1

Thermal conductivity transverse to (λ)a 0.48 ± 0.05 W.m-1.K-1

a Calculated from the heat capacity and the diffusivity

Thermal properties measured

These results are key to understand the fire behavior of the composite samples

The carbon fiber fraction [vol%] is determined experimentally by the acid attack method:

1. Density measurement of the virgin composite

2. Resin dissolution in sulfuric acid +H2O2

3. Mass measurement of the fibers (known density)

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HORIBA PG250 IRTF

13

HORIBA PG250 FTIR

13

Cone calorimeter experiments

Heat fluxes: 14-75 kW.m-2

Spark ignition was usedAtmosphere: airTest procedure: ISO 5660

100 ± 0.5 mm long × 100 ± 0.5 mm wide × 10.1 ± 1.5 mm thick

Composite Masses56 vol% CF 171.2 ± 2.8 g59 vol% CF 148.7 ± 1.8 g

Sample masses

Measurements:Mass lossMasse loss rate (MLR)

Piloted ignition time (tig)In-depth temperature

Two k-type thermocouples in-depth

Composite samples

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Ignition time and critical heat flux (CHF)

Where:

qe: Heat flux [kW.m-2]

Tig, T: Ignition and ambient Temp. [K]: Thermal conductivity [kW.m-1.K-1]r: Density [g.m-3]

Cp: Thermal capacity [kJ.g-1.K-1]

The model of Hopkins and Quintiere (1996) for tig:

CF fraction & temperature

tig & critical heat flux Fire resistance of composite

t = 0 s is the exposition to external heat flux

Heat flux

[kW.m-2]56 vol%

CF59 vol%

CF56 vol%

CF59 vol%

CF

14 - 1515 - 1417 - 1413 - 2118 830 1089 22 -20 750 659 26 2430 390 238 30 2540 152 150 32 2850 94 64 33 3060 51 40 36 3375 40 32 35 36

Ignition time, tig [s]

Mass loss [%]

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Thermal parameters

TRP: Thermal response parameter characterizes the material resistance to generate a gas combustible mixtureP: Thermal inertia is a measurement of a material ability to resist to a temperature variation

Allows calculation of TRP & P

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In-depth temperature

Summary of the thermal parameters

The ignition temperatures of samples are between 240 °C and 300 °C

56 vol% 59 vol%

CHF Tig TRP P

[kW.m-2] [°C] [kW.s1/2.m-2] [kW2.s.m-4.K-2]56 vol% CF 18 240 435 5.07 This work59 vol% CF 14 300 370 2.25 This work

Nylon 14 380 275 0.87 Hopkins, 1996Polyethylene (PE) 9 200 310 1.80 Hopkins, 1996

Polypropylene (PP) 5 210 227 2.20 Hopkins, 1996PMMA 4 180 198 2.10 Hopkins, 1996

Epoxy resin - - 457 - Scudamore, 1991

Material Reference

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Mass loss and mass loss rate

Heat flux Ignition time Mass loss

56 vol% 59 vol%

59 vol%56 vol%

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1) Resin devolatilization

2) Resin decomposition and production of liquid residue

3) Acceleration of the decomposition rate & combustion of the liquid residue

4) Char pyrolysis and oxidization

The four stages are:

Heat flux = 50 kW.m-2

Decomposition stages

The thermal decomposition of the epoxy resin / carbon fiber composite takes place in four stages

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Stage 1Stage 2

Stage 3

Stage 4

Sample combustion in cone calorimeter

Heat flux = 50 kW.m-2

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Mass loss rate MLR

The increase of carbon fiber fraction leads to the MLR peak amplitude decrease at a given external heat flux

Heat flux MLR peak width

MLR amplitude Thermal resistance

In accordance to the observations of Pilling et al.

56 vol% 59 vol%

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Heat of gasification

Where: L: Heat of gasification [kJ.g-1]

qfl: Heat flux of the flame [kW.m-2]m: Specific MLR (SMLR) [g.s-1.m-2]

Tig, T: Ignition and ambient Temp. [K]: Emissivity [-]σ: Stefan-Boltzmann constant [W.m-2.K-4]

Tv: Vaporisation temperature [K]

TRP L

[kW.s1/2.m-2] [kJ.g-1]56 vol% CF 435 48 This work59 vol% CF 370 44 This work

Nylon 275 3.80 Hopkins, 1996Polyethylene (PE) 310 3.60 Hopkins, 1996

Polypropylene (PP) 227 3.10 Hopkins, 1996PMMA 198 2.80 Hopkins, 1996

Material Reference

CF vol% TRP (volatile production resist)

L (energy to produce volatiles)

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ConclusionThe influence of the carbon fiber volume fraction on fire behavior of two epoxy resin (56 and 59 vol% CF) composites was assessed:

The increase of the carbon fiber fraction in the composites leads to a lower thermal resistance of the materialIt was found that all the parameters that characterize the material thermal resistance such as piloted ignition time, thermal response parameter, heat of gasification, thermal inertia and critical heat flux for ignition, decrease when the carbon fiber volume fraction increases from 56 to 59 vol%The thermal decomposition of the composite occurs in four stages: devolatilisation, solid to liquid transition, combustion of liquid residue and char formation

The choice of an optimal carbon fiber fraction is critical to maintain simultaneously good mechanical and thermal resistance properties for epoxy resin/carbon fiber composites

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Acknowledgements

To OSEO for the funding for this projectTo all the team of the Pprime Institute for the laboratory work and their scientific support

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Thank you!

Sidonie Rubansidonie.ruban@airliquide.com