Composites on fire at reduced scale: evaluation, characterization and modeling

Composite materials are increasingly being used in the design of aircraft, train, ship and buildings. They are very often structural parts and they must meet the difficult challenge of having adequate structural fire protection. In fire scenarios of particular relevance according to the targeted applications, suitable strategies to control fire hazards are needed for composite structures. There are three main methods available to design composite structures with improved fire resistance behavior: (i) use “normal” structural materials and add surface protection, (ii) use fire retarded versions of “normal” structural materials, and (iii) use structural materials with inherently good fire retardant properties. The first approach is of interest since it does not modify the intrinsic properties of the structural composites and does not lead to processing problems (e.g. incorporation of fillers in the material). It can be achieved with intumescent coatings: when heating beyond a critical temperature, the intumescent material begins to swell and then to expand forming an insulative coating limiting heat and mass transfers. Intumescence will be used in this work On the other hand, the evaluation of fire resistance of intumescent coatings protecting structural composite requires large scale equipment. Due to the complexity of fire phenomenon, full-scale tests are still the main and the most credible tool for investigating fire-related issues but they are however very costly, and generally, the cost significantly increases with scale. For those reasons we have developed reliable, repeatable and fast small scale tests including: (i) a furnace delivering temperature/time curves such as ISO 834, UL-1709 and other curves depending on specific fire conditions (curves ‘on demand’), (ii) a jet fuel fire test (according to ISO 2685 or NextGen) devoted to evaluate the fire resistance of components, equipment and structure located in ‘fire zones’ in aircraft (e.g. compartments containing main engines and auxiliary power units) and (iii) a mini Steiner tunnel (according to ASTM E84). It then permits the ‘high throughput’ development of intumescent coatings protecting composites. Examples using the mini Steiner tunnel and the reduced jet fuel fire test will be presented in the talk. The first example deals with the fire protection carbon fiber reinforced polymer (CFRP) in aircraft structure. Intumescent silicone based-coatings (low and high intumescing coatings) are evaluated on CFRP using a bench mimicking a jet fuel fire occurring at high heat flux (200 kW/m²) (Figure 1). It is shown the development of large intumescence (high intumescing coating) associated with appropriate thermal properties of the coating (heat conductivity measured as low as 0.3 W/m.K) provides efficient protection for the CFRP at the jet fire test. On the other hand, the formation of cohesive ceramic (low intumescing coating) with low heat conductivity (constant heat conductivity as a function of temperature of 0.35 W/m.K) also provides protection but its efficiency is lower than that of intumescent char. It is evidenced that intumescent silicone-based coatings are materials of choice for protecting CFRP in the case of jet fuel fire. Figure 1 – Jet fuel fire at reduced scale on CFRP protected by an intumescent coating In the second example, different intumescent coatings protecting polyethylene terephthalate (PET) rigid foams used in roofing structure are evaluated using the mini Steiner tunnel. Results show good correlation between the two scales and the approach developed at the small scale permits the fast screening of intumescent paints to predict their fire behavior at the large scale. Finally, mechanistic aspects of intumescence based on our small scale tests will be investigated including the chemistry, the physic, the rheology and the modeling of the intumescenc

Innovative polyelectrolyte treatment to flame-retard wood

Fire protection has been a major challenge in wood construction for many years, mainly due to the high flame spread risk associated with wood flooring. Wood fire-retardancy is framed by two main axes: coating and bulk impregnation. There is a growing need for economically and environmentally friendly alternatives. The study of polyelectrolyte complexes (PECs) for wood substrates is in its infancy, but PECs’ versatility and eco-friendly character are already recognized for fabric fire-retardancy fabrics. In this study, a new approach to PEC characterization is proposed. First, PECs, which consist of polyethyleneimine and sodium phytate, were chemically and thermally characterized to select the most promising systems. Then, yellow birch (Betula alleghaniensis Britt.) was surface-impregnated under reduced pressure with the two PECs identified as the best options. Overall, wood fire-retardancy was improved with a low weight gain of 2 wt.% without increasing water uptake

Characterisation of the dispersion in polymer flame retarded nanocomposites

Flame retardant nanocomposites have attracted many research efforts because they combine the advantages of a conventional flame retardant polymer with that of polymer nanocomposite. However the properties obtained depend on the dispersion of the nanoparticles. In this study, three types of polymer flame retarded nanocomposites based on different matrices (polypropylene (PP), polybutadiene terephtalate (PBT) and polyamide 6 (PA6)) have been prepared by extrusion. In order to investigate the dispersion of nanoparticles in the polymer containing flame retardant, conventional methods used to characterise the morphology of composites have been applied to FR composites containing nanoclays. XRD, TEM and melt rheology give useful information to describe the dispersion of the nanofiller in the flame retarded nanocomposite. In the PA6-OP1311 (phosphorus based flame retardant) materials, the clay is well dispersed unlike in PBT and PP materials where microcomposites are obtained with some intercalation. The poor dispersion is also highlighted by NMR measurements but the presence of flame retardant particles interferes in the quantitative evaluation of nanoclay dispersion and underestimations are made

Protection mechanism of a flame-retarded polyamide 6 nanocomposite

Investigations have been performed to determine the causes of the synergistic effect observed when a combination of flame retardants (FR; aluminum phosphinate/melamine polyphosphate) and organomodified montmorillonite (o-MMT) is used in polyamide 6 (PA6). Structures obtained at different stage of degradation in cone calorimeter experiments for the PA6, PA6/FR, PA6/o-MMT, PA6/FR/o-MMT are compared. The evolution of the chemical species formed according to the stages of degradation together with the characterization of the resistance of the char have enabled to elucidate the mechanism of protection involved for PA6/FR/o-MMT. Infrared characterizations have demonstrated that the species formed are degradation products of the components of the formulation that are also observed for PA6/FR and PA6/o-MMT and do not explain the improved performances. However, the structure of the char with clay distinguishes itself with a very specific small closed-cells foamed structure exhibiting an enhanced char strength. A detailed comparison of the mode of protection involved when using FR and a combination of FR/o-MMT when submitted to fire is proposed. </jats:p

A facile technique to extract the cross-sectional structure of brittle porous chars from Intumescent coatings

Intumescent coatings are part of passive fire protection systems. In case of fire, they expand under thermal stimuli and reduce heat transfer rates. Their expansion mechanisms are more or less recognized, but the fire testing data shall be interpreted as function of coating morphology. Expansion ratios are examined together with the inner structures of specimens submitted to fire. Bare cutting techniques damage the highly porous and fibrous specimens because they become very crumbly due to charring. So far, absorption contrasted X-ray computed microtomography (CT) was used as a non-destructive technique. Nevertheless, access to X-ray platforms can be relatively expensive and scarce for regular use. Also, it has some drawbacks for carbon rich specimens strongly adhering on steel substrates because it leads sometimes to noisy images and lost data due to resolution limits on specimens reaching ten of centimeters. Therefore, we propose an inexpensive and more accessible experimental approach to observe those specimens with minimized structural damage under visible lighting. To that end, charred specimens were casted into pigmented epoxy resin. After surface treatments, color contrasted cross-sections could be observed under optical digital microscopy thanks to high level of interconnectivity of pores. Subsequent image treatments confirmed that the structural integrity was kept when compared to previous CT data. The proposed method is practical, cheaper and more accessible for the quantitative assessment of inner structure of charred brittle specimens

Sol–Gel Treatments to Flame Retard PA11/Flax Composites

International audienceThis work investigates the efficiency of sol-gel treatments to flame retard flax fabric/PA11 composites. Different sol-gel treatments applied to the flax fabrics were prepared using TEOS in combination with phosphorus and/or nitrogen containing co-precursors (DEPTES, APTES) or additives (OP1230, OP1311). When the nitrogen and the phosphorus co-precursors were used, two coating methods were studied: a 'one-pot' route and a successive layer deposition method. For the "one-pot" method, the three precursors (TEOS, DEPTES, and APTES) were mixed together in the same solutions whereas for the different layers deposition method, the three different treatments were deposited on the fibers successively, first the TEOS, then a mix of TEOS/DEPTES, and finally a mix of TEOS/APTES. After deposition, the sol-gel coatings were characterized using scanning electron microscope, electron probe microanalyzer, and 29 Si and 31 P solid-state NMR. When only TEOS or a mix of TEOS and DEPTES is used, homogeneous coatings are obtained presenting well-condensed Si units (mainly Q units). When APTES is added, the coatings are less homogenous and agglomerates are present. A lower condensation rate of the Si network is also noticed by solid-state NMR. When additives are used in combination with TEOS, the TEOS forms a homogenous and continuous film at the surface of the fibers, but the flame retardants are not well distributed and form aggregates. The flame retardant (FR) efficiency of the different treatments on flax fabrics was evaluated using horizontal flame spread test. The following ranking of the different systems is obtained: TEOS + Additives > TEOS > TEOS + DEPTES~TEOS + DEPTES + APTES > multilayers. All the sol-gel coatings improve the flame retardant properties of the flax fabric, except the multilayer treatment. Based on these results, the three most efficient sol-gels were selected to prepare sol-gel-modified flax/PA11 composites. The composite modified with only TEOS showed the best FR properties. Surprisingly, the composite modified with the phosphorus-based flame retardant (AlPi) did not exhibit improved FR properties. This effect was attributed to the fact that the amount of the FR additive deposited on the fabrics was too low

A Facile Technique to Extract the Cross-Sectional Structure of Brittle Porous Chars from Intumescent Coatings

Intumescent coatings are part of passive fire protection systems. In case of fire, they expand under thermal stimuli and reduce heat transfer rates. Their expansion mechanisms are more or less recognized, but the fire testing data shall be interpreted as function of coating morphology. Expansion ratios are examined together with the inner structures of specimens submitted to fire. Bare cutting techniques damage the highly porous and fibrous specimens because they become very crumbly due to charring. So far, absorption contrasted X-ray computed microtomography (CT) was used as a non-destructive technique. Nevertheless, access to X-ray platforms can be relatively expensive and scarce for regular use. Also, it has some drawbacks for carbon rich specimens strongly adhering on steel substrates because it leads sometimes to noisy images and lost data due to resolution limits on specimens reaching ten of centimeters. Therefore, we propose an inexpensive and more accessible experimental approach to observe those specimens with minimized structural damage under visible lighting. To that end, charred specimens were casted into pigmented epoxy resin. After surface treatments, color contrasted cross-sections could be observed under optical digital microscopy thanks to high level of interconnectivity of pores. Subsequent image treatments confirmed that the structural integrity was kept when compared to previous CT data. The proposed method is practical, cheaper and more accessible for the quantitative assessment of inner structure of charred brittle specimens

Intumescent Polymer Metal Laminates for Fire Protection

Intumescent paints are applied on materials to protect them against fire, but the development of novel chemistries has reached some limits. Recently, the concept of &ldquo;Polymer Metal Laminates,&rdquo; consisting of alternating thin aluminum foils and thin epoxy resin layers has been proven efficient against fire, due to the delamination between layers during burning. In this paper, both concepts were considered to design &ldquo;Intumescent Polymer Metal Laminates&rdquo; (IPML), i.e., successive thin layers of aluminum foils and intumescent coatings. Three different intumescent coatings were selected to prepare ten-plies IPML glued onto steel substrates. The IPMLs were characterized using optical microscopy, and their efficiency towards fire was evaluated using a burn-through test. Thermal profiles obtained were compared to those obtained for a monolayer of intumescent paint. For two of three coatings, the use of IPML revealed a clear improvement at the beginning of the test, with the slopes of the curves being dramatically decreased. Characterizations (expansion measurements, microscopic analyses, in situ temperature, and thermal measurements) were carried out on the different samples. It is suggested that the polymer metal laminates (PML) design, delays the carbonization of the residue. This work highlighted that design is as important as the chemistry of the formulation, to obtain an effective fire barrier