Kinetic Analysis of the Thermal Degradation of Polystyrene-Montmorillonite Nanocomposite

Nanocomposites exhibit a combination of unique properties, such as increased heat distortion temperature, reduced permeability, reduced flammability and improved mechanical properties. In this work, a polystyrene (PS) clay nanocomposite was prepared via bulk polymerization using a novel organically modified montmorillonite (MMT). The organic-modifier is the N,N-dimethyl-n-hexadecyl-(4-vinylbenzyl) ammonium chloride (VB16). The thermal stability of PS–VB16 compared to pure PS is examined in pyrolytic and thermo-oxidative conditions. It is then studied using a kinetic analysis. It is shown that the stability of PS is significantly increased in the presence of clay. The thermal behavior of PS and PS nanocomposite is modeled and simulated. A very good agreement between experimental and simulated curves both in dynamic and isothermal conditions is observed. Using kinetic analysis associated to the reaction to fire of PS nanocomposite simulated in a cone calorimeter, the peak of heat release rate is half that of virgin PS, it is suggested that the clay acts as a char promoter slowing down the degradation and providing a protective barrier to the nanocomposite. The combination of these two effects is an important factor lowering the HRR

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

Quantification of thermal barrier efficiency of intumescent coatings on glass fibre-reinforced epoxy composites

In this work, the thermal barrier efficiency of three commercial intumescent coatings of varying thicknesses on glass fibre-reinforced epoxy (GRE) composites has been studied using cone calorimetric parameters and temperature profiles through the thicknesses, obtained by inserting thermocouples in the sample during the experiment. The methodologies developed to measure char expansion of the three coatings during the cone experiment as well under slow heating conditions using an advanced rheometric expansion system have been discussed. While the expansion ratios in the two experiments were different, the trends were similar. Thermal conductivities of the chars as a function of time were measured, which could be related to the intumescence steps of respective coatings. The accurate measurements of these parameters are important in predicting the surface requirements of an ideal coating that can enable a given composite structure to survive a defined thermal threat for a specified period of time

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

Investigation of nanodispersion in polystyrene-montmorillonite nanocomposites by solid state NMR

Nanocomposites result from combinations of materials with vastly different properties in the nanometer scale. These materials exhibit many unique properties such as improved thermal stability, reduced flammability, and improved mechanical properties. Many of the properties associated with polymer–clay nanocomposites are a function of the extent of exfoliation of the individual clay sheets or the quality of the nanodispersion. This work demonstrates that solid-state NMR can be used to characterize, quantitatively, the nanodispersion of variously modified montmorillonite (MMT) clays in polystyrene (PS) matrices. The direct influence of the paramagnetic Fe3, embedded in the aluminosilicate layers of MMT, on polymer protons within about 1 nm from the clay surfaces creates relaxation sources, which, via spin diffusion, significantly shorten the overall proton longitudinal relaxation time (T1 H). Deoxygenated samples were used to avoid the particularly strong contribution to the T1 H of PS from paramagnetic molecular oxygen. We used T1 H as an indicator of the nanodispersion of the clay in PS. This approach correlated reasonably well with X-ray diffraction and transmission electron microscopy (TEM) data. A model for interpreting the saturation-recovery data is proposed such that two parameters relating to the dispersion can be extracted. The first parameter, f, is the fraction of the potentially available clay surface that has been transformed into polymer–clay interfaces. The second parameter is a relative measure of the homogeneity of the dispersion of these actual polymer–clay interfaces. Finally, a quick assay of T1 H is reported for samples equilibrated with atmospheric oxygen. Included are these samples as well as 28 PS/MMT nanocomposite samples prepared by extrusion. These measurements are related to the development of highthroughput characterization techniques. This approach gives qualitative indications about dispersion; however, the more time-consuming analysis, of a few deoxygenated samples from this latter set, offers significantly greater insight into the clay dispersion. A second, probably superior, rapid-analysis method, applicable to oxygen-containing samples, is also demonstrated that should yield a reasonable estimate of the f parameter. Thus, for PS/MMT nanocomposites, one has the choice of a less complete NMR assay of dispersion that is significantly faster than TEM analysis, versus a slower and more complete NMR analysis with sample times comparable to TEM, information rivaling that of TEM, and a substantial advantage that this is a bulk characterization method. © 2003 Wiley Periodicals, Inc.* J Polym Sci Part B: Polym Phys 41: 3188–3213, 200

Development of Bioepoxy Resin Microencapsulated Ammonium-Polyphosphate for Flame Retardancy of Polylactic Acid

Ammonium-polyphosphate (APP) was modified by microencapsulation with a biobased sorbitol polyglycidyl ether (SPE) type epoxy resin and used as a flame retardant additive in polylactic acid (PLA) matrix. The bioresin encapsulated APP (MCAPP) particles were characterized using Fourier transform infrared (FTIR) spectroscopy and Raman mapping, particle size distribution was determined by processing of scanning electron microscopic (SEM) images. Interaction between the APP core and the bioresin shell was revealed by combined thermogravimetric analysis (TGA) FTIR spectroscopy. The APP to SPE mass ratio of 10 to 2 was found to be optimal in terms of thermal, flammability and mechanical properties of 15wt% additive containing biocomposites. The bioresin shell effectively promotes the charring of the APP loaded PLA composites, as found using TGA and cone calorimetry, and eliminates the flammable dripping of the specimens during UL-94 tests. Thus V-0 rating, increased limiting oxygen index and by 20% reduced peak of heat release rate were reached compared to the effects of neat APP. Furthermore, better interfacial interaction of the MCAPP with PLA was indicated by differential scanning calorimetry and SEM observation; the stiff interphase resulted in increased modulus of these composites. Besides, microencapsulation provided improved water resistance to the flame retardant biopolymer system

Flame retardancy of microcellular poly(lactic acid) foams prepared by supercritical CO2-assisted extrusion

Flame-retardant-treated cellulose (FR-cell) was used as bio-based charring agent in combination with ammonium polyphosphate (APP) based intumescent flame retardant (IFR) system to reduce the flammability of poly(lactic acid) (PLA) foams produced by supercritical carbon dioxide (sc-CO2) assisted extrusion. FR-cell was obtained by surface treatment of cellulose with diammonium phosphate (DAP) and boric acid (BA). To enhance foamability, the inherently low melt strength and slow crystallization rate of PLA was increased by adding epoxy-based chain extender (CE) and montmorillonite (MMT) nanoclay, respectively. The morphology of the foams was examined using water displacement method, scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS). Thermal properties were assessed using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Flammability was evaluated by limiting oxygen index (LOI) measurements, UL-94 tests and pyrolysis combustion flow calorimetry (PCFC). The continuous extrusion foaming technique allowed the preparation of low density PLA foams with uniform microcellular structure and void fractions higher than 90% accompanied with increased crystallinity of up to 19%. Despite the high expansion ratios (i.e. high surface area), the PLA foams showed excellent flame retardancy, UL-94 V-0 rate and LOI value of 31.5 vol% was achieved with an additive content as small as 19.5%. However, the flame retardant synergism evinced between IFR and MMT proved to be less pronounced in the expanded foams compared to bulk materials with identical additive contents

Fire retardancy of polymeric materials: a journey through the mechanisms

International audienc

FIRE BEHAVIOR OF CABLE SHEATHING AT REDUCED SCALE

International audienceEthylene vinyl acetate (EVA) is an essential polymer in halogen-free flame retardant (HFFR) cable materials because they exhibit enhanced filler loading and appropriate mechanical properties such as flexibility. Usual flame retardants include aluminum trihydroxide (ATH) and magnesium dihydroxide (MDH). The purpose of this work is to create numerical models describing the fire behavior of such materials (i.e. cable sheathing made in EVA/ATH). Pyrolysis modeling of materials involve the resolution of mass and energy conservation equations. The energy conservation equation was solved taking into account conduction, heat generation from reactions, and convection from adjacent elements. The mass conservation equation was solved accounting for the production of components due to reactions, advection of gaseous components. The measurement of input data is essential to get both accurate and comprehensive model. Novel experimental protocols were developed to determine the thermophysical parameters as a function of temperature and of decomposition state (e.g. heat capacity, heat conductivity). The model also includes kinetic aspect and the formation of the different species according to the decomposition state of the material. The resolution of the equations permits to completely describe the fire behavior of the material with a reasonable accuracy capturing the physics of the phenomena. Cables have been included in the EN 13501-6 classification standard within the Construction Products Regulation (CPR) and it is required to test them according to the EN50399 standard: a large scale test for classification of vertically mounted cables. Based on the simulation of the fire behavior of cable sheathing and on dimensional analysis, bench scale test was developed and correlations were found between the large scale and reduced scale tests

3D PRINTING AND FIRE RETARDANCY: CONVENTIONAL WAY OR NEW OPPORTUNITIES

Additive manufacturing (AM) alias 3D printing translates computer-aided design (CAD) virtual 3D models into physical objects. By digital slicing of CAD, 3D scan, or tomography data, AM builds objects layer by layer without the need for molds or machining. The ongoing transition from rapid prototyping to rapid manufacturing prompts new challenges for materials scientists. Many AM techniques are now available and it includes vat photopolymerization (stereolithography), powder bed fusion (SLS), material and binder jetting (inkjet and aerosol 3D printing), sheet lamination (LOM), extrusion (FDM, 3D dispensing, 3D fiber deposition, and 3D plotting), and 3D bioprinting [1].The range of polymers used in AM encompasses thermoplastics, thermosets, elastomers, hydrogels, functional polymers, polymer blends, composites, and biological systems. With this wide range of materials, new parts or repaired parts can be made rapidly with the appropriate design. It saves time and money. Some of the parts must be flame retarded (FR) polymers and the influence of the printing method and/or how the material is printed on the FR performance of the materials is a main concern. As an example, the printing orientation of a FR Acrylonitrile Butadiene Styrene (ABS) modifies the UL-94 classification [2]. Comparaison between printed FR materials and injection molded FR materials is made in the talk based on published papers [3-5] and on our work [6]. 3D printing gives the possibility to examine new design of polymeric materials easily (Figure 1). It is also the goal of this talk to investigate new design and new multi-materials to achieve the highest level of performance