4th ICHS Conference, September 14, 2011 Parameters for the thermal decomposition of epoxy resin/carbon fiber composites in cone calorimeter D. Quang Dao J. Luche, F. Richard T. Rogaume C. Bourhy-Weber S. Ruban L. Bustamante Valencia Good afternoon ladies & gentlemen. It's a pleasure for me to be here and be able to give you this presentation. Actually I will have two presentations during this session on the same subject: the safety of the high pressure composite cylinder in fire. This first presentation will give you an idea of the decomposition of epoxy resin and carbon fiber composite subjected to thermal flux. It is a middle scale approach giving a good idea of real fire conditions. The general intention is to better understand the behaviour of high pressure composite hydrogen cylinder in fire but the results of this particular study could also be used in other application area such as CNG storage , submarine industry or space industry. (The second presentation will present real size tests of composite cylinder in fire). The first authors are part of the PPRIME institute who actually performed the tests with cone calorimeter apparatus. I also would like to give a special thank Mr. Bustamante who recently arrived in Air Liquide and prepared this presentation 4th ICHS Conference, September 14, 2011

Context The 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 Light weight Excellent mechanical performance High capacity of H2 storage Good chemical and electrical resistance Cost Gravimetric capacity Volumetric capacity H2 vehicle refilling station Cylinder connector Fire safety strategy: preventing the cylinder from bursting Coming back to hydrogen application, the high pressure composite cylinder is currently considered as the preferred option for fuel cell electric vehicle and is also studied for on-site large capacity storages. Their characteristics are light weight, excellent mechanical performance, high capacity H2 storage and good chemical and electrical resistance. But when exposed to fire, the feared risk is the burst of the cylinder. So the generally accepted safety strategy consists in adding a release device and sometimes even a thermal protection. Releasing hydrogen through a thermal pressure relief device and/or using a thermal protection Epoxy resin/carbon fiber composite wall (few cm) Liner: H2 tightness (few mm)

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 temperature Carbon fiber fraction Nature of carbon fiber Experiments showed: CF fraction & temperature Conductivity & decomp. rate Fire resistance of composite While the main objective of our study is mainly to characterize thermal parameters such as ignition time, mass loss or temperature of ignition of the composite, we also studied the influence of the carbon fiber fraction because it happened to be different from one series of samples to the other. At the end, the objective it to fully understand the decomposition parameters in order to build a relevant model of it. The thermal parameters such as mass loss, mass loss rate, piloted ignition time, thermal response parameter and temperature of ignition are investigated

Thermal properties measured Materials studied The epoxy resin/carbon fiber composites are pre-impregnated bands of commercial references Two representative references are tested: 56 vol% Carbon fiber 59 vol% Carbon fiber Results of elementary analysis 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: Density measurement of the virgin composite Resin dissolution in sulfuric acid +H2O2 Mass measurement of the fibers (known density) Thermal properties measured The characteristics of the materials have been checked by elementary analysis, which have been repeated 3 times with a maximal deviation for each element of ±0.4 wt%. The 1.4 to 1.6% missing in composite analysis may be due to minor compounds such as ash. Thermal properties were measured for the 59% sample only at ambiant temperature. The specific heat capacity (Cp) was determined by differential scanning calorimetry (DSC). The thermal diffusivity (a) was measured by using a laser source and an infrared camera. Finally, the thermal conductivity (l) was calculated from the heat capacity and the diffusivity

Cone calorimeter experiments Composite samples HORIBA PG250 IRTF 13 FTIR Two k-type thermocouples in-depth 100 ± 0.5 mm long × 100 ± 0.5 mm wide × 10.1 ± 1.5 mm thick Sample masses The cone calorimeter was used to measure the mass loss (with thermobalance). It uses a piezo electric ignitor inducing a continuous spark in order to ignite th sample as soon as a combustible atmosphere is released from the sample. The heater can be adapted to produce a constant heat flux to a maximum of 75 kW/m2. 2 thermocouples were added to the sample in order to measure the temperature at the surface of the sample and in depth in the middle of the thickness of the sample Measurements: Mass loss Masse loss rate (MLR) Piloted ignition time (tig) In-depth temperature Heat fluxes: 14-75 kW.m-2 Spark ignition was used Atmosphere: air Test procedure: ISO 5660

Ignition time and critical heat flux (CHF) t = 0 s is the exposition to external heat flux CF fraction & temperature tig & critical heat flux Fire resistance of composite On this figure, you can see the ignition time measured for different incident heat fluxes. The critical heat flux, which is the minimum heat flux allowing the igntion of the sample is slightely higher for the 56% volume fraction than fot the 59% volume fraction. The ignition time decreases with the increase of the heat flux until 40 s and 32 s respectively for the 56% and 59%. The model of Hopkins and Quintière allows to calculate the ignition time and is valid of thermally thick materials and high heat fluxes,   Where: qe: Heat flux [kW.m-2] Tig, T: Ignition and ambient Temp. [K] : Thermal conductivity [kW.m-1.K-1] : Density [g.m-3] Cp: Thermal capacity [kJ.g-1.K-1] The model of Hopkins and Quintiere (1996) for tig:

Allows calculation of TRP & P Thermal parameters TRP: Thermal response parameter characterizes the material resistance to generate a gas combustible mixture P: Thermal inertia is a measurement of a material ability to resist to a temperature variation Using this model, we can calculate the thermal parameters such as the thermal response parameter (TRP) or the thermal inertial (P) Allows calculation of TRP & P

The ignition temperatures of samples are between 240 °C and 300 °C In-depth temperature 56 vol% 59 vol% The ignition temperatures of samples are between 240 °C and 300 °C Summary of the thermal parameters We can check that the model can be applied because when we plot the temperature measured at the surface and in depth in the sample, we get a temperature gradient between the two, with a maximum difference of about 100°C. The time of ignition can be estimated by the sudden rise in temperature. It gives a 240°C for 56% and 300°C for 59% carbone fiber composite respectively, at a heat flux of 40 kW/m2. This time of ignition is slightely longer than the time of ignition observed. Compared to epoxy resin, we can check that the thermal response decreases with the fiber increase.

Mass loss and mass loss rate 56 vol% 59 vol% Heat flux Ignition time Mass loss 56 vol% 59 vol% Here you can see the influence of different heat flux  on the ignition time and mass loss. When derivating these curves we obtain the mass loss rates which are much more interesting to understand the material degradation. It allows us to define 4 major steps.

Decomposition stages The thermal decomposition of the epoxy resin / carbon fiber composite takes place in four stages The four stages are: 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 Talk to Sidonie: Difference between degradation and decomposition. Heat flux = 50 kW.m-2

Sample combustion in cone calorimeter Stage 2 Stage 1 Stage 3 stage 1 corresponds to resin devolatilisation Stage 2 to decomposition and production of liuquid residue Stage 3 acceleration of the decomposition rate and cobustion of the liquid residue Stage 4 corresponds to char pyrolisis and oxidization. ay that few mass felt down (by dripping) from the sample holder. This does not affect the interpretation of the decomposition mechanism. Stage 4 Heat flux = 50 kW.m-2

Mass loss rate MLR Heat flux MLR peak width MLR amplitude 56 vol% 59 vol% Heat flux MLR peak width MLR amplitude Thermal resistance In accordance to the observations of Pilling et al. Warning: MLR amplitude increases with the increase of heat flux MLR amplitude decreases with the increase of carbon fiber volume fraction  The mass loss rate increases with the increase of heat flux but decreases with the increase of carbon fiber volume fraction.   The increase of carbon fiber fraction leads to the MLR peak amplitude decrease at a given external heat flux

Heat of gasification CF vol% TRP (volatile production resist) 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]   The heat of gasification can also be defined as the inverse of the slope of the curve linking the mass loss rate and the heat flux. It allows to calculate its value, which is much higher than for other known materials. CF vol% TRP (volatile production resist) L (energy to produce volatiles)

Conclusion The 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 material It 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 Worse thermal resistance means faster ignition As a conclusion these tests allowed us to better understand the decomposition steps of carbon/fiber - epoxy composite subjected to heat flux. This is a first step and it will help to model the behaviour of type IV or type III cylinders. The cone calorimeter also allows to compare different types of composites. The time to ignition is relatively low for high fluxes and the more carbon fiber we have, the less thermally resistant will be the cylinder.   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

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

Thank you! Sidonie Ruban sidonie.ruban@airliquide.com